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Cite this: DOI: 10.1039/d0cs00458h

A comparative perspective of electrochemical

and photochemical approaches for catalytic

production

Yanyan Sun,

Lei Han*

and Peter Strasser *

Hydrogen peroxide (H

) has a wide range of important applications in various fields including

chemical industry, environmental remediation, and sustainable energy conversion/storage. Nevertheless,

the stark disconnect between today’s huge market demand and the historical unsustainability of the

currently-used industrial anthraquinone-based production process is promoting extensive research on

the development of eﬃcient, energy-saving and sustainable methods for H

production. Among

several sustainable strategies, H

production via electrochemical and photochemical routes has

shown particular appeal, because only water, O

, and solar energy/electricity are involved during the

whole process. In the past few years, considerable eﬀorts have been devoted to the development of

advanced electrocatalysts and photocatalysts for eﬃcient and scalable H

production with high

eﬃciency and stability. In this review, we compare and contrast the two distinct yet inherently closely

linked catalytic processes, before we detail recent advances in the design, preparation, and applications

of diﬀerent H

catalyst systems from the viewpoint of electrochemical and photochemical

approaches. We close with a balanced perspective on remaining future scientific and technical

challenges and opportunities.

1 Introduction

Hydrogen peroxide (H

) has been widely used as an eco-

friendly oxidant in chemical industry and environmental treatment

such as organic/inorganic chemical synthesis, pulp and paper

Department of Chemistry, Technical University of Berlin, 10623 Berlin, Germany.

E-mail: pstrasse[email protected]

College of Materials Science and Engineering, Hunan University, 410082,

Changsha, Hunan, China. E-mail: hanlei@hnu.edu.cn

Yanyan Sun

Yanyan Sun received her PhD

degree in Physical Chemistry

and Electrochemistry from

Technical University of Berlin

(Germany) under the supervision

of Prof. Peter Strasser in 2018.

Currently, she is continuing her

postdoctoral research with Prof.

Peter Strasser. Her research

focuses on the design and fabri-

cation of carbon-based functional

materials for the oxygen reduc-

tion reaction in fuel-cells and

hydrogen peroxide production. Lei Han

Lei Han is a professor at the

School of Materials Science and

Engineering, Hunan University,

P. R. China. He received his PhD

degree from Changchun Institute

of Applied Chemistry, Chinese

Academy of Sciences, under the

supervision of Prof. Shaojun

Dong in 2015. After that, he did

postdoctoral research at the

Nanyang Technological University,

University of Waterloo, and Free

University of Berlin before joining

Hunan University in 2018. His

research interests include the design and application of

nanostructured materials for solar energy conversion and storage

systems including photocatalysis, photoelectrochemical devices

and oxygen electrochemistry.

Received 28th April 2020

DOI: 10.1039/d0cs00458h

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bleaching, medical disinfection and wastewater treatment.

1–5

Moreover, H

could also be utilized as an ideal oxidant and

sustainable energy carrier alternative to oxygen and hydrogen

in fuel cells since it possesses the advantages of easy storage

and safe operation, high oxidation potential, and only water

as a by-product.

6,7

Currently, the well-established industrial

anthraquinone-based method for H

production involves

the hydrogenation and oxidation of the anthraquinone mole-

cule, and extraction, purification and concentration of H

solution.

This method is very beneficial for the large-scale

production of highly concentrated H

, but there are still

some serious sustainability challenges to be solved. First, this

method involves a sequential multi-step procedure including

many purification steps, thus consuming a significant amount

of energy and resources. Moreover, the use of a large amount of

during the hydrogenation step leads to difficulty in handling

and storage. The by-products of anthraquinone/anthrahydro-

quinone require the use of a large amount of organic solvents

during the process, which will become waste at the end.

Furthermore, highly concentrated H

is a hazardous chemical

and requires great caution in transport, handling and storage,

leading to an increase in cost and some safety issues. Alterna-

tively, considerable efforts have been dedicated to developing the

direct production of H

from a mixture of H

and O

using

chemical catalysts.

8–11

However, to catalyze this reaction, noble-

metal-based catalysts such as Pd, Au and their alloys are needed,

which is a potential barrier in view of scale-up practical applica-

tions owing to their high cost and low abundance. Moreover, the

activity and selectivity toward H

production are also quite low,

because water production is thermodynamically more favorable

than H

production. Last but not least, the incoming feed of

and O

constitutes an explosive and potentially hazardous

mixture. To avoid this last issue, hydrogen-permeable mem-

branes possessing catalytic activity have also been developed,

which are generally made of thin Pd and Pd–Ag alloy membranes,

Pd-coated V, Nb, or Mo metal membranes, and porous inorganic

membranes. These bi-functional membranes enable the direct

production of H

withoutmixingH

and O

.However,they

lack sufficient stability and reproducibility, and have thus

remained unattractive for industrial-scale production of H

the moment. Another important aspect that needs to be consi-

dered in the development of a future, sustainable synthetic H

process is down-scalability of the process and product, that is, the

direct production of dilute H

from O

or H

O locally and

on smaller scales. Dilute H

is more suitable for a wide range

of applications, and also addresses the safety issue and largely

decreases the cost mentioned above.

The electricity- and light-driven oxygen reduction reaction

(ORR) and/or water oxidation reaction (WOR) through proton-

coupled electron transfer (PCET) have been considered to be

appealing processes for direct eﬃcient and economic H

production, chiefly because they merely require water, O

, and

solar/electric energy,

13–22

and enable scalable H

production

without H

gas explosion risks, thus dramatically decreasing

the cost of transport, storage and handling of highly concen-

trated H

. The ORR process plays critical roles in various

sustainable energy conversion/storage devices such as recharge-

able fuel cells and rechargeable metal–air batteries.

23–26

Generally,

there are three kinds of reaction pathway during the ORR process

(eqn (1)–(3)): (1) the four-electron ORR for water production; (2) the

two-electron ORR for H

production; and (3) the one-electron

ORR to OOH. It should be mentioned that all the reduction

potentials in the present work are relative to the standard hydrogen

electrode (SHE) without a special statement. Nevertheless, massive

efforts have been made to develop highly advanced four-electron

ORR catalysts for boosting this reaction and thus maximizing

the energy conversion efficiency of devices, probably because

production could result in low energy conversion efficiency

and cause instability issues such as catalyst corrosion and

chemical degradation of the polymer electrolyte membrane in

a proton-exchange membrane fuel cell (PEMFC) resulting from

the self-decomposition of the produced H

Moreover, the

four-electron ORR process (+1.23 V

SHE

)isthermodynamically

favored compared to the two-electron ORR process (+0.68 V

SHE

)

due to its more positive equilibrium potential. Recently, the

direct electrochemical two-electron ORR for H

production

began to receive increasing attention from the perspective of

chemical synthesis purposes. H

production through the

electrochemical two-electron ORR process can be performed in

several types of electrochemical devices such as electrolytic

cells,

fuel cells,

28,29

and microbial fuel cells.

30,31

In an aqueous

electrolytic device, H

production can be achieved through the

electrolysis of water and O

, where the anodic half-cell reaction

proceeds by water oxidation and the cathodic half-cell reaction

proceeds by the ORR. It should be here highlighted that on-site

production of H

has been achieved at the point of use

Peter Strasser

Peter Strasser is the chaired

professor of ‘‘Electrochemistry

and Electrocatalysis’’ in the

Department of Chemistry at the

Technical University of Berlin.

Prior to his appointment, he was

Assistant Professor at the

Department of Chemical and

Biomolecular Engineering at the

University of Houston, after he

served as Postdoctoral Scientist

and later Senior Member of staﬀ

at Symyx Technologies, Inc.,

Santa Clara, USA. He earned his

PhD in Physical Chemistry and Electrochemistry from the Fritz-

Haber-Institute of the Max-Planck-Society in Berlin under the

direction of Gerhard Ertl. He studied chemistry at Stanford

University, the University of Tu

¨bingen, Germany, and the

University of Pisa, Italy. He was awarded the ISE ‘‘Brian Conway

Prize’’ in Physical Electrochemistry, the IAHE ‘‘Sir William Grove’’

award, the ‘‘Otto-Roelen’’ medal by the German Catalysis Society,

the ‘‘Ertl Prize’’, as well as the ‘‘Otto-Hahn Research Medal’’ by the

Max-Planck Society.

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through electrochemical generation devices by the HPNow

company.

Moreover, electrolyte-free H

could also be

obtained in the case of using a solid polymer electrolyte.

1,15

Besides, in combination with intermittent renewable power

sources like solar or wind, H

could be produced in an eco-

friendly fashion without additional energy input. What’s more,

electrochemical devices also enable in situ, on-demand H

production for some practical applications, thus avoiding the

cost of storage and transportation. In particular, if the two-

electron ORR process is carried out in fuel cells and microbial

fuel cells, additional chemical electricity could be recovered

along with H

production. Meanwhile, in the fuel cell system,

the potential for explosion could be avoided due to the separa-

tion of O

and H

by an ion-exchange membrane. Additionally,

in the microbial fuel cell system, some organic compounds in

wastewater could be directly oxidized by electrochemically active

bacteria on the anode, thus achieving simultaneously wastewater

treatment. In contrast, differently from the two-electron ORR

process, the direct two-electron WOR has recently also attracted

growing interest for electrochemical on-site production of H

recently since it requires only the use of water as a raw material

and enables the simultaneous production of valuable H

at the

cathode by a single water electrolysis device. Similarly, there are

generally three reaction pathways during the WOR process

(eqn (4)–(6), e.g. in acid electrolyte),

including the one-electron

pathway to form OH, the two-electron pathway for H

production and the four-electron pathway for O

evolution.

Among these three water oxidation routes, currently, most

research has mainly focused on the development of oxygen

electrocatalysts for promoting the four-electron WOR (also

called the oxygen evolution reaction, OER) since the OER is

an important half-reaction in electrochemical water splitting

and rechargeable metal–air batteries.

So, improvement of the

catalyst is necessary to achieve efficient energy conversion and

storage. In contrast, two-electron water oxidation to H

production has been long overlooked. This may be because

the equilibrium potential of the two-electron WOR (+1.77 V

SHE

)

is thermodynamically higher than that of the four-electron

WOR (+1.23 V

SHE

). Therefore, the critical challenge for this

route lies in the development of effective WOR catalysts capable

of significantly promoting the two-electron pathway and

suppressing the thermodynamically favorable four-electron

pathway owing to its more negative potential.

+4H

+4e



-2H

= +1.23 V

SHE

(1)

+2H

+2e



-H

= +0.68 V

SHE

(2)



-OOH E

=0.13 V

SHE

(3)

O-OH + H



= +2.73 V

SHE

(4)

O-H

+2H

+2e



= +1.77 V

SHE

(5)

O-O

+4H

+4e



= +1.23 V

SHE

(6)

On the other hand, H

production could also be achieved

through the photocatalytic process in aqueous solution,

5,20,21,35,36

which mainly involved light absorption, photo-generated carrier

separation (also called electrons and holes), and interfacial carrier

transfer to drive the corresponding reactions over the surface of

semiconductor photocatalysts. During the process, the photo-

generated holes from the valence band could oxidize water into

, whereas the photo-generated electrons from the conduction

band perform the two-electron ORR into H

37–39

Nevertheless,

thekeychallengefortheachievementofsolar-to-H

conversion

lies in the design and development of highly efficient, stable, low-

cost photocatalysts with appropriate band structure capable of

promoting both water oxidation and a selective two-electron

ORR under moderate reaction conditions. Besides, the photo-

electrochemical (PEC) two-electron WOR has also been developed

as an efficient and sustainable route for H

production,

especially significantly improving the overall energy conversion

efficiency of PEC devices by coupling with other cathodic

reduction reactions like the hydrogen evolution reaction (HER)

and two-electron ORR.

5,40

In addition to this, careful observation

demonstrates an interesting internal connection between the

electrochemical and photochemical routes, that is, both routes

involve a two-electron ORR process to achieve H

produc-

tion stimulated by external energy like solar energy/electricity.

Therefore, it could be deduced that the incorporation of two-

electron ORR electrocatalysts into photocatalytic systems as

co-catalysts will also be beneficial to improve the efficiency of

production by promoting the surface charge transfer on the

surface of semiconductor photocatalysts.

Since the ORR is an inherently sluggish process involving

proton-coupled multi-electron transfer and multiple reaction

paths,

1,18,24

a highly eﬃcient and selective ORR for H

production via the electrochemical and photochemical methods

remains a significant challenge and faces many fundamental

scientific and technical hurdles.

1,13

The first study on electro-

chemical production of H

came from Traube in 1887, who

produced H

from O

at a Hg–Au electrode.

Subsequently,

Berl et al. reported the electrochemical performances of a carbon

electrode for H

production in 1939.

After that, there was a

long time in which most research mainly focused on the

investigation of experimental influencing parameters on the

production in electrochemical devices, such as high inter-

nal resistance and a low concentration of oxygen dissolved in

the electrolyte.

43,44

Some methods have also been developed to

solve these issues above, including the usage of gas diffusion

electrodes,

an applied external voltage,

and buffered electro-

lytes, and reducing the distance between the anode and the

cathode.

In contrast, little attention was paid to the develop-

ment of cost-effective advanced two-electron ORR catalysts with

high catalytic activity, selectivity, and stability, which is actually

also very important for H

production. Recently, various ORR

catalysts toward H

production have sprung up including

noble-metal-based materials, transition-metal-based materials,

and metal-free carbon-based materials. Besides, some metal

oxides such as ZnO, WO

,SnO

, BiVO

, and TiO

have been

both theoretically estimated and experimentally investigated as

potential catalyst candidates for H

production from two-

electron water oxidation. On the other hand, the history of

photocatalytic H

production can be traced back to the 20th

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century. Baur and Neuweiler first observed the photo-driven ORR

for H

production over ZnO in the presence of glycerine and

glucose under light illumination in 1927.

In subsequent years,

various photocatalysts were developed to catalyze H

produc-

tion through the two-electron ORR, including metal oxides,

metal–organic frameworks or coordination polymers, and

metal-free graphitic carbon nitride (g-C

). Meanwhile, some

photoelectrochemical systems based on electrocatalysts and

photocatalysts have been designed and developed for H

production. In the past few years, considerable research efforts

were reported with a focus on the electrocatalytic and photo-

catalytic production of H

, and many exciting results have

been achieved that merit a careful review to establish the

conceptual links between the recent advances in materials and

methodologies.

This review begins with a brief introduction of the funda-

mentals principles and some important parameters for evalu-

ating the catalytic performances of electricity/photo-driven ORR

and WOR catalysts designed for H

production. Thereafter,

we summarize recent progress in the design, preparation, and

applications of diﬀerent catalysts for H

production from the

perspective of electrochemical and photochemical routes.

Finally, the critical challenges and opportunities for future

development in this field are also presented. We hope that this

review could provide guidance for the design and preparation

of novel electricity/photo-driven ORR and WOR catalysts with

high performance, and promote their practical applications in

energy conversion/storage systems.

2 Fundamentals of electrocatalytic

and photocatalytic H

production

Electrocatalytic and photocatalytic H

production through

the ORR and WOR processes are both thermodynamically

uphill reactions. What complicates the H

production

chemistry further is the fact that there are several competing

reduction products (e.g. H

O, H

, and O

2

) formed via

distinct reaction pathways of the ORR and WOR processes with

one-, two- and four-electron transfer. The dominant reaction

pathway is mainly determined by the nature of the electro-

catalysts and photocatalysts as well as the experimental

conditions.

47–49

Asaresultofthesecompetingreactionpathways,

electrocatalytic and photocatalytic H

production face severe

challenges in terms of low energy efficiency and limited Faradaic

efficiency (chemical selectivity).

2.1 Fundamentals of electrocatalytic H

production

The mechanism of the ORR process has been widely investi-

gated. Following a popular 4-step proton-coupled electron

transfer mechanism from the early 2000s, the process involves

three reaction intermediates (HOO*, O*, and HO*) and proceeds

as follows in acid medium:



+*-HOO* (7)

HOO* + H



-O* + H

O (8)

O* + H



-HO* + H

O (9)

HO* + H



-*+H

O (10)

By contrast, only one reaction intermediate (HOO*) is involved

in the two-electron process as the following steps in acid

medium:



+*-HOO* (11)

HOO* + H



-*+H

(12)

where * denotes a bare surface active site. It can be seen that

the two kinds of ORR reaction pathway both involve the

reaction intermediate HOO*, so balanced binding of HOO*

on the surface of catalysts is very necessary to produce H

through preventing the breakage of the O–O bond during the

ORR process.

2,51

In practice, there are some important para-

meters to be measured and/or calculated for the evaluation and

comparison of the catalytic performances of new ORR catalysts,

including the catalytic activity and selectivity of the desired

production reaction, the competing catalytic activity

toward the H

reduction reaction, the H

productivity

and Faradaic efficiency, and finally the catalyst stability.

The catalytic activity and selectivity toward H

production

can be evaluated using the Rotating Ring-Disk Electrode

(RRDE) technique. For the RRDE technique, linear sweep

voltammetry (LSV) was generally performed on the disk elec-

trode with a constant applied potential on the ring electrode

(e.g. +1.2 V

RHE

) for detecting the H

produced on the disk

electrode. So, the current assigned to the production of H

H2O2

, in units of mA) can be calculated from the ring current

ring

, in units of mA) and the electrode geometry-dependent

collection eﬃciency (N) of the ring electrode as follows:

IH2O2¼Iring

N:(13)

Whereas, the disk current (I

disk

, in units of mA) is the sum of

the two-electron I

H2O2

and the four-electron I

H2O

(the current

attributed to the production of H

O), that is,

disk

H2O2

H2O

(14)

The H

selectivity with respect to the conversion of molecular

oxygen, assuming negligible electroreduction of H

to H

can be calculated from the molar flux rates of O

according to

the following equations:

18,52

nH2O2¼IH2O2

2F:(15)

nH2O¼IH2O

4F:(16)

H2O2%¼nH2O2

nH2O2þnH2O

¼2Iring

NIdisk

jjþIring

:(17)

Under certain assumptions, the accurate number of transferred

electrons (n) during the ORR process can be calculated

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according to the Koutechy–Levich equation:

52,53

j¼1

þ1

0:62nFC0D0

ðÞ

2=3n1=6o1=2



:(18)

Here jis the measured current density (mA cm

2

), j

is the

kinetic current density (mA cm

2

), nis the number of electrons

transferred, Fis the Faraday constant (96 485 C mol

1

), D

is the

diffusion coefficient of oxygen in the electrolyte (cm

1

), C

the bulk concentration of oxygen in the electrolyte (mol cm

3

nis the kinetic viscosity of the electrolyte (cm

1

), and ois the

angular velocity of the disk (o=2pN,Nis the linear rotation

speed (rpm)).

The produced H

may be further catalytically reduced

to H

O by the ORR catalyst, which is detrimental for overall

production. Starting from this point, the catalytic activity

toward the H

reduction reaction (H

RR) also needs to be

investigated for accurately evaluating the actual H

produc-

tion capacity. Typically, the evaluation of the H

RR was

performed under the same conditions as the ORR measure-

ment except the addition of H

in the nitrogen-saturated

electrolyte.

The evaluation of the H

productivity in various reaction

media has been typically performed in membrane-separated

two-compartment H-cells, H

fuel cells or electrolysis cells

by measuring the concentration of the produced H

at the

cathode within defined time intervals. During this process, one

of the biggest problems facing ORR routes to H

production

is the low solubility of O

in various electrolytes, such as the

bulk concentration of O

up to 1.2 mM in 0.1 M KOH and

1.1 mM in 0.5 M H

Moreover, the solubility of O

decreases with increasing the temperature and the concen-

tration of the electrolyte but increases as the pressure increases.

In order to solve this issue eﬃciently, gas diﬀusion layers

(GDLs) modified with ORR catalysts (then referred to as gas

diﬀusion electrodes, GDEs) have been recently deployed to

promote the mass transport of O

. Enhanced mass transport

and larger local O

concentrations at the catalyst surface

prolonged the contact time of O

with the catalyst surface at

a 3-phase interface (solid catalyst, liquid solvent/electrolyte,

and gaseous oxygen), further supported by unique porous and

hydrophobic structures, ultimately resulting in enhanced space

time yields of H

by the ORR.

18,43

To date, most of the

investigated ORR catalysts as cathodes in these systems were

commercial carbon-based materials (e.g. active carbon and

vapor-grown-carbon-fiber)

and metal complexes (e.g. iron

and cobalt-phthalocyanine).

56,57

Moreover, some researchers

demonstrated that electrolyte-free neutral H

solution can

also be obtained through the use of a solid polymer electrolyte

(SPE), which is more useful and flexible for practical

applications.

29,58

Besides, continuous electrolytic flow reactors,

also called flow-through electrolyzer cells, overcame the above

issues of mass transport by continuously circulating the

reactants and products to and away from the electrodes, and

inhibit the subsequent decomposition of H

due to the

accumulation of H

, thus resulting in an increased amount

of produced H

18,43,59

The H

concentration within certain time intervals can be

determined by the UV-vis spectrophotometric method, and

chemical titration of potassium permanganate or iodometry

solution,

60,61

as well as by the chemiluminescence method

based on the catalytic oxidation of luminol by hydrogen

peroxide.

Among these methods, the UV-vis spectrophoto-

metric method has been well developed and only requires the

use of a spectrophotometer with easy operation and low-cost.

Moreover, several indicators have been proposed for the

measurement of the H

concentration including ammonium

molybdate/KI solution,

potassium titanium(IV) oxalate,

TiOSO

and a commercial hydrogen peroxide test.

Never-

theless, this method is only suitable for the determination

of low-concentration hydrogen peroxide, so in order to ensure

accuracy, further dilution is generally necessary prior to

measurement. Besides, standard solutions of H

also need

to be prepared and measured to obtain the calibration curve.

In contrast, the operation of chemical titration is relatively

simple without the need for a calibration curve and suitable for

high-concentration hydrogen peroxide, whereas the accuracy

may be inferior to the spectrophotometric method. Diﬀerently,

additional flow-injection is needed during the measurement

for the chemiluminescence method, which makes the overall

operation complex.

The H

production Faradaic eﬃciency (H

FE) can then

be calculated by dividing the charge transferred to the pro-

duced H

by the total charge passed through the circuit as

the following equation:

H2O2FE ð%Þ¼2CVF

Q:(19)

Here Cis the H

concentration (mol L

1

), Vis the volume of

electrolyte (L), Fis the Faraday constant (96 485 C mol

1

), and

Qis the amount of charge passed (C).

In addition to the catalytic activity and selectivity above,

long-term stability is also of importance for any H

produc-

tion catalysts in practical applications. In this regard, there are

two reported methods for the evaluation of the catalytic stability of

catalysts including chronoamperometry (CA) or chronopotentio-

metry (CP), and accelerated durability tests (ADTs).

Similar to the ORR process, the WOR process also involves

three oxygenated reaction intermediates on the surface of

catalysts including O*, HO*, and HOO*.

33,64

The activity and

selectivity of the WOR process are related to the adsorption free

energies of these relevant intermediates (DG

,DG

HO*

, and

HOO*

), and theoretical results demonstrated that the proper

range of DG

HO*

from 1.6 eV to 2.4 eV is beneficial for H

production, especially with zero theoretical over-potential

under the condition of DG

HO*

E1.76 eV. Generally, there are

also some parameters for evaluating the performances of WOR

catalysts toward H

production, including the onset

potential, selectivity or Faradaic eﬃciency, and stability. For

the onset potential, there are two kinds of diﬀerent definition,

that is, the experimental potential at a current density of

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0.2 mA cm

2

and the experimental potential for the H

concentration to go up to 1 ppm after testing for 10 min,

respectively.

The Faradaic eﬃciency for H

production via

the WOR process could be calculated in a similar manner to the

ORR process, that is, the ratio between the amount of experi-

mentally produced H

and the theoretically calculated H

according to the current density.

Also, the stability could be

evaluated by the CA or CP method.

2.2 Fundamentals of photocatalytic H

production

Typically, there are five continuous steps over a semiconductor

photocatalyst during the photocatalytic H

production process,

including light harvesting, photo-generated carrier separation,

adsorption, surface redox reactions and H

desorption.

Electrons and holes are firstly generated from the valence band

and conduction band within the semiconductor photocatalyst

under illumination with incident photons with energy identical

to its band gap. In order to better utilize solar energy, it is thus very

necessary to develop visible-light-driven semiconductor photo-

catalysts since about 40% of the solar spectrum is located in the

visible light region. Besides, the use of UV light also induces

the decomposition of the produced H

on the surface of the

semiconductor photocatalyst.

Prior to participation in the

subsequent redox reactions, photo-generated electrons and

holes first need to be separated with the minority carrier being

transferred to the surface of the semiconductor photocatalyst.

Charge recombination is a competing detrimental process,

which is influenced by several factors such as the crystallinity,

morphology, and surface properties of the photocatalyst.

36,39

The proper modification of material structures is the most

eﬀective way to improve the overall photocatalytic eﬃciency

by means of regulating the separation and recombination

processes of photo-generated carriers. After transferring to

the surface, photo-generated electrons and holes can drive

the ORR to either H

or the superoxide radical (OOH), while

catalyzing water oxidation to molecular oxygen, respectively.

37,67

Conduction band edge positions located between the redox

potential of oxygen reduction to H

(+0.68 V

SHE

) and OOH

(0.13 V

SHE

) are most favorable for H

production.

37,68

At the

same time, regardless of H

and OOH production, the

valence band edge position should be more positive than

the redox potential of water oxidation (+1.23 V

SHE

), which is

energetically favorable for water oxidation. In practice, kinetic

over-potentials are inevitable during these electrochemical

processes. In this regard, considerable eﬀorts have been devoted

to engineering the band structure of semiconductor photocatalysts

by means of heteroatom-doping, surface modification, and the

formation of hetero-junctions, aiming at meeting the ideal require-

ments. Especially, the incorporation of two-electron ORR or water

oxidation catalysts as co-catalysts could significantly accelerate the

kinetic process of the interfacial redox reaction, and thus facilitate

charge transfer and separation, clearly demonstrating the close

connection between electrocatalysis and photocatalysis.

Photocatalytic H

production is usually performed with

the photocatalyst suspended in a mixture of water and an

electron donor such as ethanol, methanol, formic acid, and

2-propanol under illumination. It should be noted that the use

of electron donors leads to diﬃculties in the subsequent

separation of H

, which is why it is highly desirable

to develop a photocatalytic system without the need for an

electron donor. In addition, semiconductor photocatalysts have

also been used to fabricate photoelectrodes for photoelectro-

chemical production of H

via the two-electron ORR or two-

electron WOR. The catalytic performance of the photocatalyst is

usually expressed using the H

production rate in terms of

mmol h

1

or mmol L

1

under given conditions including

temperature and light intensity. The concentration of the

produced H

can be determined by potassium permanganate

(KMnO

) and iodometric titration or HPLC in conjunction with

an electrochemical analyzer.

The H

selectivity over semi-

conductor photocatalysts can also be evaluated by the RDE

technique according to Koutecky–Levich plots.

In addition,

from the viewpoint of practical application, the behavior of

decomposition over the semiconductor photocatalyst also

needs to be investigated by dispersing the semiconductor

photocatalyst into nitrogen-saturated H

solution under

illumination.

Besides, there are two additional important

parameters for evaluating the photocatalytic performances of

photocatalysts including the apparent quantum yield (AQY)

and solar-to-chemical conversion (SCC) efficiency. In the

reported literature, the AQY is defined as the ratio of the

number of electrons transferred toward H

production rela-

tive to the incident photons at a given wavelength, and thus

could be calculated according to the following equations:

37,69

fAQY ð%Þ¼2nH2O2

Naph

100%:(20)

Naph ¼EAd

:(21)

Ul¼hcNA

l:(22)

Here n

H2O2

is the molar amount of produced H

aph

is the

number of incident photons entering the reaction vessel, Eis

the measured difference in the light intensity transmitted before

and after being absorbed by the photocatalyst (mW cm

2

is the area of the light collector part of the radiometer (cm

is the mole photon energy of the given wavelength l

(J mol

photon1

), his the Planck constant (6.626 10

34

J s),

cis the speed of light in a vacuum (3 10

1

), N

is the

Avogadro number (6.022 10

mol

1

), and lis the given

wavelength of incident photons (nm). AQY measurements are

generally carried out in a borosilicate glass bottle using an air

mass (AM) 1.5 solar simulator combined with wavelength-

dependent band-pass filters.

Similarly, the SCC efficiency is

measured under similar conditions to those of AQY measure-

ments except for the use of a l4420 nm cutoff filter,

which

is used to suppress the subsequent decomposition of the

produced H

by UV light. The SCC efficiency was calculated

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based on the following equation:

SCC efficiency ð%Þ¼DGnH2O2

Pt100%:(23)

Here DGis the free energy for H

generation (117 kJ mol

1

Pis the power of the incident photons (W), and tis

the illumination time (s). The number of incident photons

and average intensity of irradiation can be measured by a

radiometer.

3 Electrocatalytic materials for H

production

ORR electrocatalysts are considered to be one of the most

important components in electrochemical devices for H

production. To date, the developed ORR electrocatalysts can

be categorized into noble-metal-based materials, transition-

metal-based materials, and metal-free carbon-based materials.

All these developed ORR electrocatalysts will be discussed in

detail below.

3.1 Noble-metal-based materials

Platinum-based catalysts are considered to be the state-of-

the-art ORR electrocatalysts, and have also been widely used

in commercialized PEMFCs due to their high catalytic activity,

selectivity, and durability in acidic environments.

Generally,

the ORR process over platinum-based catalysts mainly proceeds

by a four-electron reaction pathway for selective production of

water, which is also highly desirable from the perspective of the

achievement of highly eﬃcient energy conversion. Besides,

during the ORR process, the reaction pathway is usually con-

sidered to be strongly dependent on the ability of the catalyst

for the dissociation of the O–O bond, whereas the cleavage of

the O–O bond is undesirable for H

production. In this

regard, two configurations have been proposed for O

adsorp-

tion on the surface of these platinum-based catalysts, including

a ‘‘side-on’’ adsorption configuration and an ‘‘end-on’’ adsorp-

tion configuration (Fig. 1a and b).

For the ‘‘side-on’’ adsorp-

tion configuration, the four-electron ORR reaction pathway is

dominant because the adjacent noble metal atoms favor the

dissociative adsorption of O

and the continual reduction of

through providing separate binding sites for the two

oxygen atoms in O

and H

, which was also called ensemble

effects. By contrast, the elimination of accessible ensembles of

noble metal active sites (site isolation strategy) could induce the

‘‘end-on’’ adsorption configuration of O

on the surface of

the catalyst, thus making the ORR process occur through both

four-electron and two-electron reaction pathways, which was

beneficial for the selective production of H

. On the basis of

these considerations above, Choi’s group coated amorphous

carbon layers on the surface of a commercial Pt/C catalyst

(Johnson–Matthey, 60 wt% Pt) through chemical vapor deposi-

tion (CVD) in order to eliminate accessible Pt ensemble sites,

thus resulting in the increased selective production of H

during the ORR process. Furthermore, the amorphous carbon

layers also efficiently suppressed the consecutive chemical

decomposition of H

by a disproportionation reaction or

electrochemical reduction reaction, and also made the catalyst

possess excellent stability.

Theoretically, according to Sabatier’s principle, the binding

of the reaction intermediates on the surface of the catalyst

should be neither too strong nor too weak for the ideal catalyst,

that is, a moderate interaction between the catalyst surface and

the reaction intermediates.

2,51

Therefore, as the sole reaction

intermediate during the H

production process, HOO*

should be bound on the surface of the catalyst within a

moderate range, which is thus conducive to the preservation

of the O–O bond. Nevertheless, for conventional Pt-based

catalysts, the HOO* dissociation to O* and HO* (the four-

electron ORR intermediates) is generally preferred over the

HOO* hydrogenation to H

due to their strong binding of

HOO* on their surface. Rossmeisl and Stephens recently

reported the tunable binding of the reaction intermediates

through regulating the composition of the catalysts, and screened

a series of oxygen electrocatalysts based on noble metal alloys

for H

production using density functional theory (DFT)

calculations (Fig. 1c–f).

In their work, the binding of HOO*

could be optimized by the modification of reactive noble metals

including Pt, Pd, and Ag with inactive metals such as Au and

Hg, in order to regulate the catalytic activity and selectivity

toward H

production. Electrochemical results demonstrated

that the resultant Pt–Hg, Ag–Hg, and Pd–Hg exhibited high

selectivity toward H

production owing to the elimination of

the accessible active metal ensemble sites through the isolation

of active sites using Hg. Besides, starting from the viewpoint of

both composition and morphology optimization, Mahata et al.

also studied three cuboctahedral core–shell nanoclusters of

@Pt

,Co

@Pt

and Au

@Pt

by performing DFT

calculations to get better understanding of the influence of

the core metal (Co and Au) on improving the selectivity and

efficiency of H

production.

The optimal binding of HOO*

can be achieved by the formation of a mixed alloy consisting

of Co and Au due to the synergetic effect of Co and Au

with different oxygen binding energies. Among them, the

@Pt

NCs showed the lowest over-potential and high-

est selectivity for H

production.

The particle size of nanostructured materials has been

reported to show significant influences on the catalytic perfor-

mances including the catalytic activity and reaction pathway.

For instance, Anderson’s group prepared diﬀerent size Pt

clusters supported on indium tin oxide (ITO) films in an

ultrahigh vacuum, and found size-dependent catalytic activity

and selectivity toward H

production,

where the smallest

clusters were most beneficial for H

production. Other

typical examples of the size eﬀect are single-atom catalysts

(SACs) with the advantages of minimal noble metal usage and

unique catalytic properties. Extensive eﬀorts have been devoted

to the preparation of Pt SACs dispersed on various substrates

such as MgO, FeO

, zeolites, TiO

, and MoS

, showing high

catalytic activity for diﬀerent catalytic applications such as pro-

pane combustion,

CO oxidation,

nitroarene hydrogenation,

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and photocatalytic hydrogen production.

77,78

Besides, the elimi-

nation of Pt ensemble sites in these Pt SACs also induced the

pathway change of some chemical reactions such as formic acid

oxidation.

Generally, formic acid oxidation proceeds by the

indirect dehydration pathway over conventional Pt nanoparticles.

However, Pt SACs can catalyze formic acid oxidation through the

direct dehydration pathway. Recently, Pt SACs have also been

employed as ORR electrocatalysts for selective H

production.

For instance, Lee’s group demonstrated high mass activity and

selectivity toward H

production over single-atom Pt supported

on TiN nanoparticles because of the absence of Pt ensemble

active sites,

and also observed the support eﬀect on the catalytic

performances of Pt SACs toward the electrochemical ORR,

indicating that the substrate was not only used as anchoring

sites to stabilize the single metal atoms, but also may participate

in the surface catalytic reaction.

Meanwhile, the composition

Fig. 1 (a and b) ORR pathways on Pt surfaces: (a) on the pristine Pt/C, molecular O

prefers to adsorb on the Pt surface as a side-on configuration and

then reduces to H

O through (1) dissociative, (2) associative, and (3) non-dissociative mechanisms; (b) on the carbon-coated Pt, molecular O

absorbs on

the Pt surface as an end-on configuration and then produces H

OandH

via competitive dissociation and desorption steps, respectively. The

produced H

is not further reduced on the Pt surface due to hindrance by the carbon layer. (c–f) Trends in activity and selectivity for H

production:

on a model Pd

(001) surface. Palladium atoms are represented in green, mercury in blue,

oxygen in red, and hydrogen in yellow. (d) Partial kinetic current density to H

as a function of the applied potential, corrected for mass transport

losses. (e) Potential required to reach 1 mA cm

2

kinetic current density to H

on polycrystalline catalysts as a function of the calculated HOO* binding

energy. The solid lines represent the theoretical Sabatier volcano. The dotted line represents the thermodynamic potential for oxygen reduction to H

(f) H

selectivity for different catalysts at 2.5 mA cm

2

total current density. All electrochemical experiments were performed at 50 mV s

1

and

1600 rpm in O

-saturated 0.1 M HClO

at room temperature with corrections for ohmic drop. The surface area was normalized to the geometrical value.

(a and b) Reprinted with permission.

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and structure of the substrate have significant eﬀects on the

loading of atomically dispersed Pt. A high loading of atomically

dispersed Pt up to 5 wt% can be achieved using sulfur-doped

zeolite-templated carbon as a support with a high sulfur content

of 17 wt% and highly curved three dimensional networks of

graphene nanoribbons, which was attributed to the abundant

S-functionalities and unique carbon structure.

Very recently, the

loading of single atom Pt sites was further improved to 24.8% by

atomically dispersing platinum on the surface of an amorphous

CuS

hollow nanosphere support due to the strong Pt–S inter-

action (Fig. 2),

which exhibited a high H

selectivity of

92–96% over a wide potential range as well as excellent stability

even after 10 000 cyclic voltammetry cycles within a potential

range from +0.1 to +0.8 V

RHE

in HClO

electrolyte. In addition, the

loading amount of the Pt-based catalyst on the electrode showed

a significant influence on the H

production during the ORR

process, and showed an inverse correlation with the H

selectivity.

A similar phenomenon was also observed for some

non-Pt-based ORR electrocatalysts like Fe–N–C and Se/Ru/C

catalysts.

83,84

Au has been demonstrated to be a promising ORR electro-

catalyst for H

production, and its selectivity is strongly

influenced by the crystallographic orientation and experi-

mental conditions.

However, the weak binding of HOO* on

its surface resulted in a low coverage of O

and thus limited the

catalytic activity.

Recently, rational design of alloying with

other metals, such as Pd, Pt, and Rh, has been proposed to

optimize the catalytic properties of Au. The typical example

reported is the Au–Pd alloy system,

which has been investi-

gated as a heterogeneous catalyst for direct catalytic production

of H

from H

and O

For instance, the Au–Pd alloy with

variable Pd content has also been investigated as an ORR

electrocatalyst for H

production, and increased H

selec-

tivity was observed at the optimal content of Pd owing to the

ensemble effect arising from the presence of finely dispersed

Pd within Au. In contrast, excess content of Pd resulted in a

decrease of H

selectivity due to the strong binding of HOO*

on the surface of Pd. In addition to Pd, some DFT calculation

results demonstrated that the combination of Ni with Au resulted

in increased coverage of O

, maintaining simultaneously weak

Fig. 2 (a) Schematic illustration of the structure evolution of h-Pt

–CuS

(blue, purple and white balls represent Cu, Pt, and S atoms, respectively); and

(b–g) characterization of h-Pt

–CuS

nanoparticles: (b) TEM image of h-Pt

–CuS

(the inset is the photograph of the nanoparticles dispersed in

cyclohexane); (c) high-magnification TEM image (the inset is the SAED pattern); (d) XRD pattern of the Cu

1.94

S nanoparticle seeds and the h-Pt

–CuS

nanoparticles; (e) EDS elemental mapping of Pt, S, and Cu; and (f and g) AC-HAADF-STEM images of h-Pt

–CuS

. Reprinted with permission.

2019, Elsevier Inc.

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dissociation of the O–O bond. Inspired by this point, Amal’s group

reported the designed synthesis of Au–Ni core–shell nanorods

though the epitaxial growth method,

which exhibited improved

catalytic activity and selectivity toward H

production. The

enhanced results could be attributed to the following aspects:

(1) the decreased tendency for the dissociation of the O–O bond

resulting from Au–Ni; and (2) the complex electron interaction

and the lattice strain effect resulting from the epitaxial growth

process. What’s more, a Pt layer was also further introduced to

improve the catalytic activity and selectivity toward H

produc-

tion by tuning the electrical properties of the Ni shell and the

degree of lattice strain caused by the high work function of Pt.

Similarly, the substrate has a significant influence on selective

production over Au-based ORR electrocatalysts. Camargo’s

group reported the synthesis of hybrid materials consisting of

Au and TiO

with different morphologies including spheres and

wires.

Interestingly, the TiO

spheres had a positive effect on the

selectivity for H

production whereas the H

selectivity

decreased for the TiO

wires.

3.2 Transition-metal-based materials

Transition-metal-based materials, mainly including transition-

metal complexes, metal–N–C materials, metal/C materials, and

metal oxides, showed good catalytic activity and selectivity

toward the ORR to H

via the two-electron reaction pathway.

The most popular examples of transition-metal complex

ORR electrocatalysts for H

production are cobalt-based

porphyrins and iron-based phthalocyanines.

56,57

Their unique

geometric configuration, consisting of reactive metal atoms

surrounded by supporting and coordinating atoms such as N

and C, could efficiently restrain the dissociation of the O–O

bond, thus realizing high selectivity toward two-electron H

production. Besides, DFT calculation results revealed that

the neither too strong nor too weak binding of the reaction

intermediate HOO* on the surface of these catalysts proved

most beneficial for H

production.

However, the high

cost and unsatisfactory durability of these catalysts largely

prohibited their practical applications. Nevertheless, some

research studies also reported the significant improvement of

their catalytic activity and durability through heat treatment.

Inspired by this, extensive efforts have been made for the

synthesis of Co–N–C catalysts from various nitrogen-containing

precursors, carbon supports, and Co salts. Research demon-

strated that the catalytic activity and selectivity of these Co–N–C

catalysts were largely related to the nature of the carbon supports

and nitrogen-containing precursors. For instance, bidentate

N-ligands were found to be optimal nitrogen-containing carbon

precursors because of the favorable formation of such structures

with Co-coordinated Co–N

–C sites during the heat treatment

process, which were found to be active sites for the selective ORR

for H

production.

In addition to the formation of Co–N

–C

sites, metal Co nanoparticles coated with Co oxides were also

generally produced during the heat treatment process, where the

produced H

was further electrochemically reduced to H

Besides, there have also been conflicting opinions on the roles

of Co–N

–C sites (xrepresents the coordination number), that is,

Co–N

–C sites are considered to be active sites for H

produc-

tion through the two-electron ORR, whereas Co–N

–C sites

promote the four-electron ORR for H

O production.

Recently,

our group observed the activity–selectivitytrendsforelectro-

chemical H

production over single-site metal–N–C (metal =

Mn, Fe, Co, Ni, and Cu) by the combination of computational

calculations and experimental results. The as-prepared Co–N–C

catalyst was found to possess the most optimal binding energy of

the HO* intermediate near the top of the volcano of the two-

electron ORR, which well explained its outstanding H

produc-

tivity with high ORR activity, highest H

selectivity, and lowest

reduction reaction activity (Fig. 3a–e).

Moreover, industrial

productivity over the prepared Co–N–C catalyst in a micro

flow cell was achieved with a production rate of more than

4molperoxideg

catalyst1

1

at a current density of 50 mA cm

2

Later, the introduction of oxygen functional groups (OFGs) into

carbon-based materials possessing Co–N

–C active sites could

further improve the catalytic activity and selectivity toward

electrochemical H

production, where Co–N

–C sites mainly

contributed to the catalytic ORR reactivity, whereas the H

selectivity was attributed to OFGs (Fig. 3f and g).

Very recently,

Hyeon’s group also observed nearly the same phenomenon,

that is, Co–N

moieties surrounded by oxygen species (C–O–C

epoxides) as an ideal system for highly active H

production

from both electrochemical results and DFT calculations.

Besides, Wang’s group also reported Fe–O–C as an efficient

ORR catalyst for H

production with a relative positive onset

potential of +0.822 V

RHE

and high selectivity of more than 95% in

both alkaline and neutral medium,

which was completely

different from the previously reported Fe–N–C ORR catalysts.

Therefore, the fundamental understandingofthenatureofthe

active sites of these M–N–C catalysts toward the ORR remains

important yet challenging, and more work is still needed.

3.3 Metal-free carbon-based materials

In the past few decades, metal-free carbon-based materials have

been widely used as supports in many energy-related applica-

tions due to their advantages of earth-abundance, low-cost,

facile preparation, large specific surface area, excellent electrical

conductivity, chemical resistance, and mechanical stability.

23,25,93

Recently, metal-free carbon-based materials, such as activated

carbon fibers and graphite, have also shown great promise as

alternative catalysts to noble metal materials for electrochemical

production of H

through the two-electron ORR process in

alkaline solution.

14–16,42,94,95

It has been well accepted that the

electrochemical performances of metal-free carbon-based materials

are strongly determined by their geometric and electronic

structures. Starting from these two aspects, considerable efforts

have been made for the synthesis of advanced metal-free

carbon-based materials with high activity, selectivity, and

stability for electrochemical H

production.

One of the most promising geometric structures for H

production is reported to be porous structures since such

structures with large surface area and high pore volume are

beneficial for mass transfer and also help in exposing more

catalytically active sites. Metal–organic frameworks (MOFs) as a

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typical porous multi-functional material have received increasing

attention due to their high surface area and uniform porosity, and

have been proposed as promising precursors for the fabrication

of porous carbon materials. For instance, hierarchically porous

carbon (HPC) was fabricated under an atmosphere of H

through

the direct pyrolysis of MOF-5, which was synthesized using zinc

nitrate and terephthalic acid.

The resultant porous carbon exhi-

bited high catalytic activity and selectivity for electrochemical

reduction of O

to H

production in a wide range of pH from 1

to 7. These outstanding performances can be attributed to the

high amount of sp

-C and defects, high surface area, and

favorable mass transfer. In addition, the pore size also showed

a significant influence on the catalytic properties of porous carbon

materials. Two kinds of porous carbon materials with mesopore-

dominant porous carbon (Meso-C) and micropore-dominant

porous carbon (Micro-C) have been investigated for electro-

chemical production of H

, where Meso-C exhibited superior

ORR performance for H

production compared to Micro-

94,96

This result may be explained by the fact that the

favorable mass transport within the mesoporous structure

resulted in the fast release of the produced H

from the

surface of the catalyst, thus avoiding the subsequent reduction

of H

Heteroatom doping, such as with oxygen, nitrogen, sulphur,

and fluorine, has been reported as another promising strategy

to regulate the catalytic activity and selectivity of metal-free

carbon materials through tailoring their electronic structures.

16,94,97

Santos’s group compared the electrochemical performance

of Vulcan XC-72R and Printex carbon supports for H

production.

They found that there are more oxygenated

Fig. 3 (a) Linear sweep voltammetry (LSV) in a rotating ring-disk electrode (RRDE) setup with the Pt ring held at +1.2 V

RHE

.(b)H

selectivity (H

and the number of electrons (n)at+0.1V

RHE

derived from RRDE data. (c) Background-corrected H

RR performance in N

-saturated 0.5 M H

electrolyte containing 1 mM H

. (d) Thermodynamic relations (volcano) lines for the two- (green solid line) and four-electron ORR (black solid line).

The DFT calculated ORR onset potential values (circles) are on the left y-axis, while the experimental current densities (crosses and triangles), reported as

ln(|j|), are on the right y-axis. Both are shown as functions of the chosen reaction descriptor, the DFT calculated HO* binding free energy (G

(e) Scheme of the micro-flow cell setup. (f) Schematic of the synergistic strategy of atomic Co–N

–C sites and OFGs for H

electrosynthesis on noble-

metal-free electrocatalysts. (g) Performance comparison in regard to reactivity and selectivity for H

electrosynthesis on Co–POC–O, Co–POC–R,

and POC–O electrocatalysts. The inset in (g) shows the mechanism scheme for synergistic H

electrosynthesis. The carbon, nitrogen, oxygen, and

cobalt atoms are marked with black, blue, yellow, and red, respectively. (a–e) Reprinted with permission.

(f and g) Reprinted with permission.

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functional groups on the surface of Printex L6 compared to

Vulcan XC-72R, resulting in improved H

selectivity. More-

over, the same group also further investigated the influence

of diﬀerent surface modification on the selectivity of these

two carbon supports toward H

production through pre-

treatment with nitric acid and ammonia.

A similar phenom-

enon was also observed that acid-treated Printex L6 exhibited

the highest selectivity toward H

production due to the

largest concentration of oxygenated functional groups. Very

recently, Cui and co-workers also reported the increased activity

and selectivity of carbon materials toward H

production

through surface oxidation treatment.

Moreover, a linear corre-

lation of the catalytic activity and selectivity with the oxygen

content was observed, and DFT results demonstrated that the

existence of the –COOH functional group in the armchair edge

and the C–O–C functional group in the basal plane of graphene

resulted in the high activity and selectivity toward H

production. These results implied that the introduction of

oxygenated functional groups plays an important role in regu-

lating the chemical selectivity.

In addition to oxygen functionalization, nitrogen doping has

also been reported to be an eﬃcient means for improving the

ORR performances of metal-free carbon materials since the

incorporation of nitrogen with higher electronegativity could

induce the charge redistribution of the pconjugated system of

the carbon frameworks and thus tailor the adsorption proper-

ties of carbon materials for ORR reactive intermediates.

100

Nevertheless, most of the reported nitrogen-doped carbon

materials led to the four-electron ORR to H

O, and only a few

samples were reported to exhibit high selectivity toward H

production.

63,97,101–104

For example, Anderson’s group investi-

gated the ORR performance of nitrogen-doped Ketjenblack for

production from both experimental and theoretical cal-

culation viewpoints.

105

Experimental results demonstrated that

the nitrogen-doped Ketjenblack showed a lower onset potential

and mainly followed the two-electron ORR process for H

production. Meanwhile, DFT results also indicated that the

formed carbon radical sites neighboring substitutional N in

graphite were active for the electrochemical ORR to H

In addition, Iglesias et al. also reported the synthesis of

nitrogen-doped graphitized single wall carbon nanohorns (CNHs)

by coating polydopamine (PDA) followed by annealing,

106

which

also exhibited high catalytic activity with a very positive onset

potential, high Faradaic efficiency, and excellent stability in a wide

pH range from 1.0 to 13.0. In their work, the outstanding perfor-

mances were assigned to their specific N

pyridinic

pyrrolic

ratios and

microporosity. Besides, our group also reported improved ORR

activity and H

selectivity induced by nitrogen doping and

observed the different roles of nitrogen doping species during the

ORR process by means of ex situ XPS, where pyridinic-N contri-

buted to the catalytic ORR process in acid medium while graphitic-N

groups appeared to be catalytically active moieties in neutral

and alkaline conditions.

63,107

Further, our group investigated

the effect of graphene precursors with different in-plane carbon

lattice defect density on the subsequent nitrogen doping and the

catalytic performances of their derived nitrogen-doped graphene.

The resultant nitrogen-doped graphene derived from oxo-G with

lower carbon lattice defect density exhibited the highest H

selectivity. Similar to nitrogen, fluorine doping can also create

active sites through inducing the polarization of adjacent carbon,

which is beneficial for improving the ORR performances. More-

over, the fluorine doping content and different fluorine species

were also found to influence the catalytic activity and selectivity

toward H

production.Forinstance,Zhaoet al. reported the

synthesis of fluorine-doped hierarchically porous carbon (FPC)

from an aluminum-based MOF.

108

Electrochemical measure-

ments and DFT calculations demonstrated that the incorporation

of CF

2,3

atoms into carbon frameworks could facilitate the

adsorption of O

and desorption of reactive intermediate

HOO*, thus resulting in significantly increased activity and

selectivity toward H

production.

In addition to the two-electron ORR process, the two-

electron WOR process has also been widely investigated theo-

retically and experimentally for H

production recently.

111

For instance, Nørskov’s group presented a thermodynamic

picture of the adsorption free energies of the intermediates

(HO*) on the surface of diﬀerent catalysts for the three diﬀerent

reaction pathways using thermodynamic analysis based on

DFT calculations, and SnO

and TiO

were identified as pro-

mising candidate materials with good selectivity for H

production.

33,112

Subsequently, they further adopted DFT calcu-

lations to investigate the trends in activity of four diﬀerent

oxides including WO

,SnO

,TiO

and BiVO

for water oxidation

toward H

production.

109

BiVO

was identified theoretically and

experimentally as the best catalyst for H

production with a high

FE of 70% in the dark and 98% under 1 sun illumination under

the condition of the optimal bias range (Fig. 4a). Nevertheless,

there also exist some limitations of the BiVO

catalyst, including

high over-potential and poor stability from VO

43

dissolution.

Recently, Zheng’s group developed an eﬃcient method to improve

the activity and stability of BiVO

for H

production by doping

rare earth element gadolinium (Gd).

Theoretical calculation

results demonstrated that Gd doping not only makes several facets

of BiVO

more active for H

production, but also results in the

increase of the energy barrier of VO

43

dissolution. Experimentally,

the Gd-doped BiVO

exhibited a significant decrease of the onset

potential for H

production by around 110 mV with a high FE of

around 99.5% and a substantially prolonged catalytic lifetime

under illumination. Meanwhile, the same group also employed

DFT calculations to identify another two eﬃcient and selective

WOR catalysts for H

production, including CaSnO

and ZnO,

which were also verified experimentally (Fig. 4b–d).

64,110

4 Photocatalytic materials for H

production

Semiconductor photocatalysts have been widely applied in the

field of various solar energy storage and conversion systems

including dye-sensitized solar cells (DSSCs), photocatalytic

water splitting and CO

reduction since Fujishima and Honda first

demonstrated the photocatalytic application of TiO

in 1972.

113–115

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Recent studies also began to focus on the design and development

of advanced semiconductor photocatalysts with novel compositions

and structures for photocatalytic H

production. To date,

the reported semiconductor photocatalysts mainly include metal

oxides, metal–organic complexes, and metal-free graphitic carbon

nitride (Table 1). Subsequently, recent progress on these photo-

catalysts will be discussed in detail below.

4.1 Metal oxides

Metal oxide semiconductors have been proposed as promising

catalysts for photocatalytic H

production because of their

general superior stability to liquid electrolytes and facile

preparation.

60,116

Among the various metal oxide semiconductors,

TiO

is the most widely investigated but usually showed unsatis-

factory eﬃciency for H

production within the micro molar

range. This is largely a consequence of the intrinsic properties of

TiO

related to its band gap width and band gap structure, coupled

with the sluggish intrinsic surface reaction kinetics of water

oxidation, low selectivity toward the two-electron ORR for H

production, and the decomposition of the produced H

the absorbed UV light and photo-generated electrons/holes,

thus significantly hampering the overall photocatalytic perfor-

mances.

In order to mitigate the problems above, various

strategies have been explored to improve the photocatalytic

performances for H

production over TiO

-based photo-

catalysts, including the incorporation of noble metal

60,66,117

and carbon materials,

48,118

and reaction medium optimi-

zation.

47,119

The introduction of noble metals is one powerful route for

the design and preparation of advanced TiO

-based photo-

catalysts for H

production. The most commonly used noble

metals include Au,

the Au–Ag bimetallic alloy,

and Pd.

120

For instance, Tada’s group demonstrated a significant improve-

ment in H

production through loading Au nanoparticles on

the surface of TiO

On one hand, the formation of a junction

between Au and TiO

was beneficial for efficient photo-induced

Fig. 4 (a) Activity volcano plots based on calculated limiting potentials as a function of calculated adsorption energies of HO* (DG

) for two-electron

oxidation of water to hydrogen peroxide evolution (black) and four-electron oxidation to oxygen evolution (blue). The corresponding equilibrium

potentials for each reaction are shown as dashed lines. (b) Volcano plots showing the calculated limiting potential UL for four-electron H

O oxidation to

(blue dashed line) and two-electron H

O oxidation to H

(black solid line) as a function of DG

. The convention is such that a low value of UL

(a high value of UL) corresponds to a low overpotential and thus a high rate. The full lines correspond to the trend model described in the text, and DFT

calculated values for diﬀerent catalyst materials are included. The 0.5 and 1 ML H ter. of (0001) ZnO indicate two diﬀerent terminations at the bottom of

the slabs. (c) Overall current density J–Vcurves for (10%

10) ZnO, (0001) ZnO, and several reported metal oxides. (d) Corresponding J

–Vcurves for the

current density toward H

formation, for which J

was calculated by multiplying the overall current J

overall

by the Faraday efficiency at each

potential. (a) Reprinted with permission.

109

110

Chemical Society.

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interfacial electron transfer from TiO

to Au. On the other

hand, these transferred electrons were favorable to selectively

follow the two-electron ORR process toward H

production

over the Au nanoparticles. These two effects both resulted in

efficient H

production. Nevertheless, Au particles were also

found to promote the decomposition of H

by reduction with

the transferred electrons due to their strong adsorption for

molecules.

In this regard, the introduction of Au–Ag

bimetallic alloy particles as a substitute for Au was reported to

address this dilemma and showed improved efficiency for H

production by means of the decreased adsorption of H

onto

the Au atoms (Fig. 5a and b).

Recently, Kim’s group reported

the synthesis of electronically tuned Pd nanoparticles loaded

on a TiO

substrate by coordinating organic ligands on the

surface of Pd.

120

The increased negative charge on the surface

of Pd nanoparticles induced by electron donation from amine

groups of the coordinated ligands was found to facilitate

enhanced catalytic activity and selectivity for the two-electron

ORR for H

production (Fig. 5c–e). In short, the introduction

of noble metals plays several different vital roles in enhancing

the photocatalytic performances: (1) promoting the separation

of photo-generated electron–hole pairs through the formation

of a hetero-junction between the metal and metal oxide semi-

conductor; (2) reducing the decomposition of the produced

through suppressing the adsorption of H

on the

surface of photocatalysts; and (3) improving the H

selectivity

by promoting the desired two-electron ORR. Nevertheless,

despite improved H

efficiencies, there still exist other

challenges that need viable solutions, such as the low quantum

efficiency resulting from UV illumination and wide band gap

semiconductors, and the continuous need for a suitable

electron donor molecule in the liquid phase.

In addition to noble metals, carbon materials were consi-

dered as promising candidates for the fabrication of high

performance metal-oxide-based photocatalysts due to low cost,

large surface area, and high conductivity.

48,118

In particular,

Table 1 Summary of the reported photocatalysts for H

production

Photocatalyst Electron donor Light wavelength/nm H

/mMh

1

AQY

/%@420 nm Ref.

TiO

Benzyl alcohol 4280 3300 29.1@334 nm 116

Au/TiO

Ethanol 4300 291.7 — 60

Au/TiO

–CO

32

Formic acid 4430 640 5.4@530 nm 117

Au–Ag alloy/TiO

Ethanol 4280 283.3 — 66

Pd/APTMS/TiO

/ — 150 — 120

SN-GQD/TiO

2-Propanol Z420 451 — 118

rGO/TiO

2-Propanol Z320 1512 — 48

HTNT–CD / 4420 2118 0.7 121

Co@TiO

Methanol =400 1650 — 122

Au/BiVO

Ethanol 4420 40.2 0.24 37

Ni/MIL-125-NH

Benzyl alcohol 4420 986.8 — 123

Alkylated MIL-125-R Benzyl alcohol 4420 766.7 — 124

)

Methanol Z420 2200 — 125

)

/rGO Methanol Z420 297.6 3.5@450 nm 126

g-C

Ethanol 4420 1641 — 127

Mesoporous g-C

Ethanol 4420 742.8 — 67

g-C

with C vacancies / 4420 91.3 — 128

IO g-C

with C vacancies Ethanol Z420 162.9 — 36

–carbon 2-Propanol — 317.8 — 129

g-C

with N vacancies Ethanol 4400 288.9 — 130

Holey defective g-C

2-Propanol 4420 80 10.7 131

Reduced g-C

/4420 170 4.3 132

O-Enriched g-C

2-Propanol Z420 1200 10.2 21

K, P, O co-doped g-C

Ethanol Z420 241.3 8.0 38

KPF

-modified g-C

Ethanol 4420 312.5 24.3 133

K, P co-doped g-C

Ethanol Z420 507 — 134

g-C

/PDI / 4420 20.8 — 135

g-C

/PDI/graphene 2-Propanol 4420 40.3 5.68 20

g-C

/PDI/BN-rGO 2-Propanol 4420 3095 7.3 136

g-C

/MTI / 4420 27.8 6.3 39

g-C

/BDI 2-Propanol 4420 29.2 4.5 70

Perylene imide/g-C

/Z420 1200 3.2 137

g-C

–SiW

Methanol — 152 6.7 138

3DOM g-C

–PW

/Z320 350 — 139

g-C

–PWO / Z420 1007 — 140

g-C

–CoWO / Z420 97 14.6 141

Metal oxide/g-C

/ — 42 — 142

g-C

–CNT Formic acid Z400 487 — 143

Au/g-C

2-Propanol — 330 3.63@400 nm 144

Au/g-C

Ethanol 4420 63.8 — 145

CoP/g-C

Ethanol Z420 70 — 146

Note: APTMS is aminopropyltrimethoxysilane. HTNT–CD is proton-form titania nanotubes with carbon dots. IO g-C

means inverse opal g-C

‘‘/’’ means without the use of an electron donor.

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carbon quantum dots (CQDs), a kind of amorphous carbon

along with sp

hybridized graphitic carbon, have received

considerable attention in the fields of bio-imaging, biosensors,

and other opto-electrical devices due to their unique features

such as fluorescence properties with excitation wavelength-

dependent multi-color emission.

147

This fascinating feature

made CQDs a promising concept in order to expand the

absorption capability of photocatalysts into smaller band gaps,

thereby harvesting photons in the range of the UV-visible

spectral region. Recently, Xiao’s group reported a hybrid photo-

catalyst of protonated TiO

nanotubes and CQDs (PTNT–CQD)

with an H

productivity of 3.42 mmol g

cat1

1

, a solar-to-

efficiency of 5.2%, and good continuous cyclic stability

under visible-light illumination.

121

The protons on PTNT–CQD

were found to play critical roles in boosting the two-electron

ORR for H

production and suppressing the decomposition

of the produced H

. Long’s group also utilized highly lumi-

nescent sulfur and nitrogen co-doped graphene quantum dots

(SN-GQDs) that were loaded on the surface of TiO

in order to

extend the light absorption into the visible light region and

promote the migration of photo-generated electrons.

118

More-

over, the introduction of SN-GQDs resulted in a promoted two-

electron ORR accompanied by the suppressed decomposition

of H

, thus achieving photocatalytic performances with an

productivity of 451 mmol L

1

. Besides, a similar role of

graphene was also observed in the photocatalytic systems of

and TiO

/WO

composites for enhanced performances for

photocatalytic H

production.

148,149

The photocatalytic performance of a semiconductor photo-

catalyst is usually dependent on the bulk (e.g. light absorption,

separation and recombination of photo-generated electron and

hole pairs, and band edge position) and surface (e.g. structural

defects and reconstruction, and surface charge) properties.

119

However, the optimization of the entire catalytic solid–liquid

interface is of the essence to arrive at a viable photocatalytic

reaction system. For instance, a recent study examined the role

of the reaction medium in the regulation of the surface proper-

ties of semiconductor photocatalysts for H

production.

More specifically, the study focused on the electrolyte’s pH,

the addition of inorganic anions and cations, and the nature of

Fig. 5 (a) Photocatalytic formation and decomposition of H

on TiO

and Au/TiO

catalysts; (b) time-dependent change in H

concentration

during the photoreaction with the respective catalysts; (c) solvent-dispersed metal nanoparticles that are electronically tuned by coordinating with

dissolved organic ligands; (d) preparation of electronically tuned Pd nanoparticles on TiO

by coordinating with immobilized organic ligands; and

(e) mechanisms of oxygen reduction on the surface of Pd nanoparticles. (a and b) Reprinted with permission.

Society. (c–e) Reprinted with permission.

120

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the sacrificial electron donor. The authors highlighted how

surface passivation through complexation with metal cations

or non-metal anions leads to blocking of the trapping sites

(RTi–OH) on the surface of TiO

for photo-generated electrons

and holes, and thereby suppresses the formation of RTi–OOH

complexes for the decreased decomposition of H

. For

instance, the introduction of Cu ions and Zn ions into the

photocatalytic system resulted in an increased H

yield over

the TiO

photocatalyst and the surface coverage of Zn ions was

also observed to be strongly dependent on the electrolyte’s

pH.

119,150

In addition, the introduction of F



also achieved

enhanced efficiency for photocatalytic H

production

through surface fluorination to prohibit the surface complexa-

tion of superoxide/peroxide species and the decomposition of

the produced H

These results demonstrated the important

roles of surface speciation during the photocatalytic process and

also provided insights on improving the photocatalytic H

production efficiency by the combined effect of pH and surface

passivation. On the other hand, ethanol is usually used as an

electron and proton donor in photocatalytic systems for H

production in order to promote the separation of photo-generated

electrons and holes by replacing water oxidation with sluggish

kinetics. Recently, the use of benzylic alcohols as a substitute

for ethanol was found to further improve the H

yield up to

ca. 40 mM, which was attributed to the efficient formation of

side-on coordinated peroxo species on the surface of the TiO

photocatalyst through the reaction between benzylic alcohol and

116

The formed peroxo species can be easily converted into

, thus facilitating highly efficient H

production.

Direct production of H

without the need for sacrificial

organic electron donors in the photocatalytic system is desir-

able from the viewpoint of green chemistry and sustainability.

To that eﬀect, Choi’s group reported enhanced H

produc-

tion up to a milli molar level over reduced graphene oxide

(rGO)/TiO

composites in the presence of phosphate and cobalt

ions in the absence of organic electron donors.

On the one

hand, the improvements were attributed to the synergistic

eﬀect of RGO, phosphate and cobalt ions. The introduced

RGO can be used as an electron mediator to facilitate the

separation of photo-generated electron–hole pairs thanks to

the lower Fermi level compared to the conduction band edge of

TiO

. Furthermore, RGO can also be used as a co-catalyst to

promote the two-electron ORR for H

production. On the

other hand, two different roles of phosphate were observed in

their work: (1) phosphate can be used as a surface passivation

agent to retard the adsorption and decomposition of the

produced H

within a wide range of pH; and (2) phosphate

reacts with cobalt ions for in situ formation of cobalt phosphate

complexes on the surface of the RGO/TiO

composite as a water

oxidation co-catalyst to promote water oxidation.

Given that visible light is dominant in the solar spectrum

and UV light induces the decomposition of the produced H

the development of visible-light-driven photocatalysts is highly

desirable for eﬃcient and sustainable H

production. In this

regard, Shiraishi’s group reported the design and synthesis of

an advanced inorganic photocatalyst consisting of Au nanoparticles

loaded on BiVO

for highly eﬃcient H

production from water

and O

under visible light illumination (Fig. 6).

In their work,

BiVO

possesses not only a narrow band gap for a visible-light

response, but also a proper position of the CB edge (+0.02 V

SHE

)

between the one-electron ORR potential (0.13 V

SHE

) and the

two-electron ORR potential (+0.68 V

SHE

), thus facilitating selec-

tive H

production. Besides, the formation of a junction

between Au and BiVO

was also favorable for the separation of

photo-generated electron–hole pairs and the two-electron ORR

for H

production.

4.2 Metal–organic-frameworks or metal-coordination

polymers

Metal–organic-frameworks (MOFs), a family of coordination

materials consisting of secondary building units interconnected

by organic linkers, have been widely investigated in various fields

of gas storage/separation, drug delivery, sensors, and catalysis due

to their significant advantages of facile preparation, high surface

area, and adjustable composition and structure.

151–155

Recently,

MOFs with amine-functionalization of terephthalic acid as an

organic linker have been demonstrated to catalyze various

photocatalytic reactions such as water splitting, CO

reduction

and oxidation of organic compounds.

156–159

For instance,

Yamashita’s group reported photocatalytic H

production

over a MOF photocatalyst consisting of Ti

(OH)

clusters

and 2-aminoterephthalic acid linkers (MIL-125-NH

) in the

presence of triethanolamine (TEOA) and benzyl alcohol under

visible-light illumination.

123

In this case, Ti(IV) within the

(OH)

clusters was first reduced to Ti(III) by ligand-to-

cluster electron transfer (LCET), which can be used to further

reduce O

to produce the O

2

intermediate with subsequent

fast disproportionation for H

production, whereas benzyl

alcohol was oxidized to benzaldehyde by photo-generated

holes. Nevertheless, the resultant mixture of H

and benz-

aldehyde dissolved in acetonitrile brought difficulty in the

separation process and thus resulted in high energy consumption.

Later, they applied a two-phase system consisting of benzyl

alcohol and water to achieve efficient separation between H

and benzaldehyde accompanied by enhanced photocatalytic H

production, which mainly benefited from the hydrophobization of

the MOF by the modification of alkyl chains (Fig. 7).

124

Moreover,

the concentration of the obtained H

solution can be easily

tuned by the volume of the aqueous phase.

In addition, metal coordination polymers have also been

investigated as robust photocatalysts for H

production,

which was mainly inspired by the structure of superoxide

dismutases (SODs) in nature systems consisting essentially of

late transition metal ions as central atoms and proteins as

organic ligands.

160

Long’s group developed an octahedral

)

coordination polymer as a kind of novel noble-

metal-free photocatalyst for photocatalytic H

production in

a mixture of methanol and water solution, where high H

productivity of ca. 110.0 mmol L

1

at pH = 2.8 could be

achieved under visible-light illumination.

125

In order to further

improve the H

production eﬃciency, they introduced RGO

as a 2D support to modulate the growth and formation of

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)

, leading to accelerated photo-generated charge

transfer and thus achieving 2.5-fold enhancement.

126

4.3 Metal-free graphitic carbon nitride

Graphitic carbon nitride (g-C

) possesses a graphitic stacking

structure of C

layers comprising tri-s-triazine units con-

nected through planar amino groups.

67,127,135,161

To date,

g-C

-based materials as metal-free organic polymeric semi-

conductor photocatalysts have received wide attention in the

fields of water splitting,

162–164

the ORR,

165

reduction,

166,167

organic photosynthesis,

168

and organic pollutant degra-

dation,

169

due to their facile preparation, environmentally

friendly nature, low-cost, reliable thermal and chemical stabi-

lity and, more importantly, appropriate band gap structure for

a visible light response and target reactions,

165

since the first

report about the excellent performances in photocatalytic

hydrogen production by Antonietti’s group in 2009.

161

Recently,

Shiraishi’s group extended the application of the g-C

Fig. 7 (a) Digital photographs of two-phase systems composed of an aqueous phase and a benzyl alcohol phase containing MIL-125-NH

(left) and

MIL-125-Rn(right); (b) photocatalytic H

production utilizing the two-phase system; and (c) time courses of H

production under photoirradiation

(l4420 nm) of the two-phase system composed of benzyl alcohol (5.0 mL) and water (2.0 mL) catalyzed by 5.0 mg of MIL-125-NH

(blue), MIL-125-R4

(green), and MIL-125-R7 (orange). Reprinted with permission.

124

Fig. 6 (a) Energy diagrams for Au/TiO

and Au/BiVO

and the reduction potential of O

; (b) time-dependent changes in the H

concentrations during

the photoreaction on Au

0.2

/TiO

(l4300 nm) and Au

0.2

/BiVO

prepared by calcination at 673 K (l4420 nm); (c) linear-sweep voltammograms of

0.2

/TiO

and Au

0.2

/BiVO

prepared by calcination at 673 K, measured on a rotating-disk electrode at diﬀerent rotating speeds and (d) Koutecky–Levich

plots of the data obtained at a constant potential (0.3 V). Reprinted with permission.

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photocatalyst into photocatalytic H

production.

127

They

pointed out that the efficient formation of 1,4-endoperoxide

species on the surface of g-C

could suppress the one-

electron ORR to produce OOH and promote the selective

two-electron ORR to produce H

, thus resulting in high

selectivity. Nevertheless, the overall photo-conversion

efficiency is still low because of the limited absorption and

utilization in the visible-light region, the poor separation

efficiency of photo-generated electrons and holes, and the

sluggish kinetics for water oxidation, and thus needs to be

further improved prior to practical applications. In this regard,

various strategies have been actively developed in order to

improve the photocatalytic performances of g-C

for H

production such as self-modification, chemical modification,

and hybridization with other materials including noble metals

and metal phosphides.

In recent years, self-modification of g-C

could be achieved

by the introduction of carbon vacancies, nitrogen vacancies,

and heteroatom doping. Wang’s group demonstrated improved

photocatalytic H

production eﬃciency over g-C

through the

incorporation of carbon vacancies.

128

Two important roles of

carbon vacancies were proposed: (1) reducing the symmetry and

band gap of C

facilitated the extension of the visible-light

absorption and the increase of photo-generated electrons; and

(2) increasing the catalytic sites enhanced the adsorption and

activation of O

molecules and induced the transformation of the

production pathway from the two-step one-electron ORR to

the one-step two-electron ORR. An inverse opal (IO) structure was

also introduced into g-C

with carbon vacancies to further

improve the eﬃciency for H

production through the enhanced

absorption and utilization of visible light, and increased surface

area, which benefited from the unique structural advantages of

the IO structure such as the slow photon eﬀect, Bragg diﬀraction

and scattering.

Besides, a series of g-C

with tunable doping

of foreign carbon was also prepared by a facile hydrothermal

reaction of glucose with subsequent thermal treatment.

129

The

energy levels of the resultant carbon-doped g-C

were found to

vary with the doping content of carbon, accompanied by the

positive shift of the conduction and valence band, which was

favorable for a selective two-electron ORR and achieved two-fold

enhancement of H

production.

In addition to carbon vacancies, nitrogen vacancies have

also been developed to modify the intrinsic electronic structure

to narrow the band gap and create more catalytically active sites

for photocatalytic reactions. For instance, Zhang’s group devel-

oped the thermal reduction treatment of g-C

in the presence

of NaBH

to introduce nitrogen vacancies through the for-

mation of CRN functional groups.

132

This structural change

resulted in the narrowing of the band gap and the positive shift

of the band edge, thus extending the light absorption within

the visible-light region, and facilitating water oxidation and a

selective two-electron ORR for H

production. The optimal

reduced g-C

photocatalyst exhibited an H

productivity of

170 mmol L

1

with a solar-to-H

chemical conversion

eﬃciency of 0.26% and AQY of 4.3% under visible-light illumi-

nation without organic electron donors. Besides, nitrogen

vacancies have been successfully in situ embedded into g-C

moieties through dielectric barrier discharge (DBD) plasma under

an H

atmosphere.

130

Compared with the direct annealing treat-

ment under an H

atmosphere, H

plasma treatment can achieve

more nitrogen vacancies embedded in g-C

moieties, which

facilitated the adsorption of O

molecules and the transfer of

photo-generated electrons for the subsequent two-electron ORR.

Nevertheless, these methods have inherent shortcomings such as

high energy consumption and explosive potential. In this case,

Ye’s group reported a novel photo-assisted thermal reduction

route for the preparation of holey defective g-C

photocatalysts

(Fig. 8).

131

The developed method could achieve the introduction

of abundant nitrogen vacancies and holey structure within the

g-C

photocatalyst simultaneously, thus facilitating the increase

of the visible-light absorption range, the separation of photo-

generated electrons and holes, and the accessibility of reactants to

the surface active sites. The optimal holey defective g-C

(DCN)

photocatalysts show ten-fold enhanced photocatalytic activity for

production compared to pristine bulk g-C

(BCN).

Recent research demonstrated that the introduction of

oxygen function groups (–COOH and C–O–C) and other hetero-

atom doping into carbon frameworks can significantly improve

the selective electrochemical two-electron ORR for H

production.

Similarly, the doping of earth-abundant hetero-

atoms (potassium, phosphorus, oxygen, and fluorine) into

the C

framework was reported to improve the efficiency of

photocatalytic H

production through the formation of

photo-generated charge trapping sites on the surface of the

g-C

photocatalyst. Zhu’s group prepared oxygen-enriched

g-C

(OCN) using ammonium para-tungstate and dicyan-

diamide,

which exhibited a 3.5-fold higher AQY for H

production of 10.2% at 420 nm compared to pristine bulk

g-C

due to the relatively easy formation of 1,4-endoperoxide

speciesandthehighlyselectiveORRtoH

. Potassium-,

phosphorus-, and oxygen-doped g-C

can also be successfully

prepared through the one-pot thermal polymerization of urea or

melamine in the presence of phosphates.

The incorporation of

these heteroatoms resulted in the narrowing of the band gap, the

increase of the generation, transfer, and lifetime of photo-

generated electrons and holes, and the inhibition of the decom-

position of the produced H

, thus achieving high AQY for H

production.

Chemical modification with electron-deficient or p-conjugated

organic monomers was another eﬃcient way to improve the

photocatalytic performances of g-C

for H

production.

For instance, the incorporation of pyromellitic diimide (PDI)

with high electron aﬃnity into the lattice network of g-C

which was achieved by a facile thermal condensation of melem

and pyromellitic dianhydride (PMDA), resulted in the simulta-

neous positive shift of the valence and conduction band edge,

thus promoting water oxidation and maintaining a highly

selective two-electron ORR for H

production.

135

Subse-

quently, graphene with high charge carrier mobility was also

introduced into the g-C

/PDI moiety by a hydrothermal-

calcination process (Fig. 9a–c).

The introduction of graphene

facilitated the transfer of photo-generated electrons from

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g-C

/PDI to graphene and then promoted the selective two-

electron ORR for H

production on the graphene moiety.

In order to further improve the photocatalytic activity for H

production, layered boron nitride (BN) was also introduced into

the above hybrid catalyst of g-C

/PDI/graphene to promote

the transfer of photo-generated electrons and holes to graphene

and BN, thus promoting the two-electron ORR for H

production on the surface of graphene and water oxidation

on the surface of BN, respectively.

136

Meanwhile, they also

utilized three-directional mellitic triimide (MTI) and biphenyl

diimide (BDI) units as a substitute for PDI to incorporate into

the g-C

moiety to promote H

production from the

viewpoint of enhanced electrical conductivity and charge

separation.

39,70

Polyoxometalates (POMs), consisting of cations and poly-

anion clusters, were reported to produce a hole center (O



) and

trapped electron center (M

(n1)+

) pair as electron acceptors and

donors through the charge transfer between O

2

and M

(n=5,6)

under light illumination, making them promising guest

molecules for chemical modification of the g-C

host. Based

on this point, Zhu’s group demonstrated the covalent combi-

nation of a POM cluster of [PW

]

7

(PW

) with three

dimensionally ordered macroporous (3DOM) g-C

through

the reaction between the amine groups of the g-C

frame-

work and (triethoxysilyl)-propyl isocyanate (Fig. 9d).

139

The

resultant 3DOM g-C

/PW

hybrid photocatalyst exhibited

eﬃcient photocatalytic H

production without the use of

organic electron donors, which was attributed to the suppressed

one-electron ORR to OOH and the promoted charge separation

and two-electron ORR to H

. Later, they applied a similar

organic linker strategy to fabricate the hybrid catalyst of a POM

cluster of [SiW

]

8

(SiW

) possessing a more negative CB

edge and g-C

containing more amine groups obtained by

thermal decomposition of urea.

138

Recently, Zhao’s group

reported the incorporation of POM-derived metal oxides into

the g-C

framework by the thermal treatment of the g-C

precursor and the POM precursor.

140,141

The incorporation of

POM-derived metal oxides resulted in the negative shift of the

CB edge of g-C

and thus promoted the one-electron ORR to

OOH, whereas it is also thermodynamically favored to oxidize

OOH by photo-generated holes to singlet oxygen (

), both of

which can promote photocatalytic H

production. A similar

eﬀect was also achieved through the covalent combination of

carbon nanotubes (CNTs) with g-C

because of their unique

p-conjugated structure capable of accepting, transporting and

storing electrons.

143

The hybridization of g-C

with catalytically active metal

nanoparticles (NPs) as a co-catalyst has also been used to

improve the photocatalytic performances for H

production

by promoting the separation eﬃciency of photo-generated

electrons and holes. For instance, loading Au nanoparticles

on the surface of g-C

resulted in highly eﬃcient and stable

production because of the favorable two-electron ORR

and the inert nature for catalyzing the decomposition of the

produced H

over the hybrid catalyst.

144,145

As a noble-metal-

free co-catalyst, CoP has also been loaded on the surface of

Fig. 8 (a) Illustration of the preparation process of holey DCN, and the insets are the simulated structure (H, C, and N atoms are represented by the gray,

red, and blue balls) and (b) schematic of mechanisms underlying the photoexcited dynamics involved in photocatalytic H

evolution over BCN and

DCN-15A. Reprinted with permission.

131

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g-C

to improve the photocatalytic H

production eﬃ-

ciency under visible-light illumination by extending the light

absorption range and promoting charge separation and elec-

tron transfer.

146

Last but not least, a new type of photoelectrochemical (PEC)

system without an external bias has also been developed to

simultaneously achieve the generation of H

and electricity

in recent years.

174,175

Generally, the PEC configuration for H

production can be divided into three categories based on where

was produced, at the cathode, anode, or both. Early

research demonstrated the indirect ORR for H

production

through the photoelectrochemical reduction of anthraquinone

derivative molecules followed by reaction with O

(Fig. 10a).

170,176,177

Nevertheless, for this kind of configuration, the use of organic

solvents is unavoidable and the energy conversion eﬃciency is

low, arising from sluggish electrode kinetics, which hinders wide

application from the viewpoint of sustainability. Recently, the direct

ORR to H

has also been achieved with high Faradaic eﬃciency

and decent photocurrent on a dye-sensitized NiO photocathode

in aqueous electrolyte,

171,178

where photo-generated electron–hole

pairs from the excited dye upon illumination performed fast hole

injection into NiO and one-electron transfer to O

for the formation

of OOH, followed by reaction with a proton and disproportionation

into hydrogen peroxide in protic electrolytes (Fig. 10b).

It should be highlighted here that the design of dye molecules

has a significant effect on the energy conversion efficiency and

electrode stability. Currently, the investigated dye molecules

mainly include porphyrin, coumarin, and ruthenium dyes,

and BH4 dye (a kind of hydrophobic donor–double-acceptor

dye). In addition, some organic polymeric semiconductors,

like polyterthiophene (pTTh),

179

polymeric metal salen-type

complexes,

180

poly-tetrakis-5,10,15,20-(4-aminophenyl)porphyrin

(pTAPP) and its cobalt derivative (pCoTAPP),

173

have been directly

explored as efficient photocathodes without dye sensitization for

production via the two-electron ORR (Fig. 10c and d).

In order to further optimize the performances, an organic

hetero-junction photocathode comprising phthalocyanine and

tetracarboxylic perylenediimide has been developed, which

exhibited continuous generation of high concentrations of

peroxide with the Faradaic efficiency remaining at around

70%.

174

Besides, cathodic H

production has also been

performed in such a PEC configuration, where semiconductor

photocatalysts were used as photoanodes to drive water oxidation,

and electrocatalysts as cathodes to promote the selective two-

electron ORR for H

production.

5,172,181,182

For instance,

Fukuzumi’s group systematically investigated WO

and BiVO

Fig. 9 (a–c) g-C

/PDI structures and mechanism for photocatalytic H

production: (a) three-dimensional structure, (b) electronic band structure

of g-C

/PDI (containing 51% PDI units), and (c) mechanism of photocatalytic H

production. (d) Preparation process of 3DOM g-C

–PW

(a–c) Reprinted with permission.

139

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as a durable photoanode with a cobalt chlorine complex

supported on a glassy carbon substrate as a cathode to construct

a two-compartment PEC cell separated by a Nafion membrane

for efficient H

production from water and O

under solar

illumination.

5,172

Especially, when iron(III) oxide(hydroxide)

(FeO(OH)) was modified as a water oxidation catalyst on the

surface of BiVO

,theproducedH

with a high concentration

up to 61 mM can be used directly as a fuel to generate electricity in

an H

fuel cell.

172

On the other hand, H

production has also been reported

through two-electron water oxidation over semiconductor

photoanodes.

40,185–189

For instance, Sayama’s group reported

the production and accumulation of H

through two-electron

water oxidation using a WO

/BiVO

photoanode with simulta-

neous H

production on a Pt cathode in an aqueous solution of

KHCO

at an applied voltage far lower than the theoretical

electrolysis voltage under simulated solar light.

Particularly,

formation was proposed through the hydrolysis of

percarbonate intermediates (HCO

4

and C

62

) from the

oxidation of HCO

3

by the photo-generated holes within BiVO

Moreover, highly concentrated HCO

3

could effectively sup-

press the oxidation degradation of the produced H

. In order

to further improve the selective H

production by two-electron

water oxidation, the surface modification of the WO

/BiVO

photoanode with an Al

layer has also been developed by

the metal–organic decomposition method and chemical vapor

deposition method to suppress the oxidative decomposition of the

produced H

, thus leading to high Faradaic efficiency for H

production.

68,185

Further, the introduction of Au-supported

fluorine-doped tin oxide (FTO) glass or ordered mesoporous

carbon as a substitute for the Pt cathode into the above system

successfully achieved the simultaneous production of H

and

electricity without the need for an external bias from both sides of

the PEC cell through the two-electron ORR and two-electron water

oxidation (Fig. 11), respectively.

183,184

5 Conclusions and perspectives

plays an essential role in the fields of chemical industry,

environmental treatment, and sustainable energy conversion/

storage. Therefore, the development of eﬃcient, energy-saving

and sustainable methods for H

production is of great

significance and urgency to address the contradiction between

Fig. 10 (a) Illustration of the working principle of dye sensitized photo-electrochemical cells (DSPECs) for H

production by using AQ redox

mediators (a, top), and the corresponding schematic representation (a, bottom). (b) Schematic representation of the DSPECs for H

production in

protic electrolyte and nucleophilic substitution in aprotic electrolyte. (c) Schematic reaction diagram of a photoelectrochemical cell for hydrogen

peroxide production. (d) Band gap diagram showing photoresponse behaviors in the photosynthetic reduction of oxygen by pTAPP. (a) Reprinted with

permission.

170

171

published by The Royal Society of Chemistry. (c) Reprinted with

permission.

172

173

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the growing H

demand and market, on one side, and

the severe unsustainability of today’s industrial production

methods, on the other. Future H

production pathways

involving either electrochemical or photochemical approaches

are currently considered most promising and sustainable,

because only water, O

, solar energy or electricity from renew-

able power sources are involved during the entire process. The

key challenge lies in the development of new scalable catalysts

with low-cost, high eﬃciency and excellent electrochemical

stability. In the past few years, various catalysts have been

widely studied from the viewpoint of both electrochemical

and photochemical H

production, including ORR electro-

catalysts such as noble-metal-based materials, transition-metal-

based materials, and metal-free carbon-based materials, and

semiconductor photocatalysts such as metal oxides, metal–

organic complexes, and metal-free graphitic carbon nitride.

Recent advances have been summarized herein. Importantly,

upon comparing various ORR electrocatalysts and semiconductor

photocatalysts, it becomes obvious that most strategies that have

been explored to improve the catalyst activity and selectivity for

production largely follow two aspects, that is, geometric

structural engineering and electronic structure engineering.

Geometric structure engineering of catalysts can be achieved

through accurately controlling the morphology of both ORR

electrocatalysts and semiconductor photocatalysts. Electronic

structure engineering of ORR electrocatalysts, on the other

hand, can be achieved through surface passivation or alloying

with inactive elements, single-atom dispersion, and surface

heteroatom doping, whereas for semiconductor photocatalysts,

electronic structure engineering can be achieved through self-

modification by the introduction of carbon or nitrogen vacan-

cies and heteroatom doping, chemical modification with

electron-deficient or p-conjugated organic monomers and

POMs, hybridization with other materials such as noble metal

and carbon materials, and surface passivation by anion and

cations. Besides, the simultaneous generation of H

and

electricity can also be achieved by the combination of semi-

conductor photocatalysts as photoanodes with ORR electro-

catalysts as cathodes in photoelectrochemical systems.

Generally, there are three diﬀerent reaction intermediates

during the ORR and WOR processes for H

production,

including HOO*, O*, and HO*. According to Sabatier’s principle,

the optimal catalyst for H

production should possess balanced

bindingofHOO*onthesurfaceofthecatalystduringtheORR

Fig. 11 (a) Diagram of the photoelectrode system for producing only H

by using a two-electron oxidation system of H

OonaWO

/BiVO

photoanode under solar light irradiation; (b) energy diagram of the photoelectrode systems; (c) schematic illustration of the design of a light-driven fuel

cell with spontaneous H

generation; and (d) the band diagram of the system. The conduction band (CB) and valence band (VB) edge positions of

BiVO

straddle the redox potentials of O

and H

O/H

, suggesting the possibility of unassisted H

production. The theoretical V

for the

light-driven fuel cell is 0.693 V, estimated from the CB of BiVO

and the O

redox potential. (a and b) Reprinted with permission.

183

Wiley-VCH. (c and d) Reprinted with permission.

184

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process, whereas balanced binding of HO* during the WOR

process. In reported studies, most of them mainly focused on

tuning the binding of the reaction intermediates through coating

a carbon layer on the surface of catalysts, forming single metal

atoms dispersed on the support, or regulating the composition of

the catalyst like the formation of alloys comprising reactive metals

with inactive metals, making the overall reaction pathway favor-

able for the two-electron pathway. In addition, optimizing the

morphological structure of catalysts has been reported as an

eﬃcient method to promote H

production. For instance, the

mass transport within a mesoporous structure favored the fast

release of the produced H

from the surface of the catalyst, thus

avoiding the subsequent reduction of H

. Besides, for photo-

catalytic systems, the experimental conditions have a certain eﬀect

on the reaction intermediates during the H

production pro-

cess. For instance, using benzylic alcohols as electron donors was

reported to result in the formation of new reaction intermediates,

coordinated peroxo species, which facilitated H

production.

Even though there are some significant advances in the

development of both electrocatalytic and photocatalytic H

production over various nanostructured heterogeneous cata-

lysts, there remains a grand challenge to further improve their

performances prior to commercialization becoming indust-

rially viable and economical. Future work in this area should

focus on the following aspects.

5.1 Exploiting further improved catalytic materials

Seeking materials with new compositions and structures is still

at the heart of research work on electrocatalytic and photo-

catalytic H

production. To date, the developed noble-metal-

based materials are considered to be most eﬃcient ORR

electrocatalysts for H

production, but their inherent dis-

advantages of high cost and scarcity significantly hamper their

practical industrial application. In view of cost reduction, it is

thus urgently needed that ORR electrocatalysts with low-cost

and high eﬃciency are developed. Recently, H

production

by two-electron water oxidation over metal oxides has also

shown great potential application in the field of water electro-

lysis, capable of the simultaneous generation of two important

valuable products of H

and H

in a single electrochemical

system using water as the only raw material.

109,110,190

Never-

theless, very few kinds of metal oxides were explored, such as

ZnO, WO

, SnO

, BiVO

, and TiO

, and thus considerable

eﬀorts need to be made for the development of other metal

oxides and beyond. For photocatalytic H

production, con-

sidering that UV light only accounts for ca. 4% and also induces

decomposition, it is thus essential to design and develop

visible-light-driven photocatalysts with highly selective promo-

tion of the two-electron ORR to produce H

and eﬃcient

suppression of subsequent decomposition of the produced

, thus enhancing the SCC eﬃciency. Nevertheless, the

reported visible-light-driven photocatalysts such as BiVO

and

g-C

still have some critical issues including sluggish kinetics of

water oxidation and unsatisfactory stability. In addition, careful

observation demonstrates that the only diﬀerence between photo-

catalytic H

production and photocatalytic water splitting lies in

the surface redox reaction process. This point provides a possi-

bility for the integration of the existing photocatalysts for water

splitting with two-electron ORR and water oxidation electrocata-

lysts as co-catalysts to achieve highly eﬃcient H

production.

5.2 Mechanistic investigations

The introduction of porous structure and heteroatom doping

has been reported to play an important role in improving the

capability of carbon-based materials for electrochemical H

production through the two-electron ORR. However, the precise

role of the pore size and heteroatom doping species is still

unclear and even conflicting. Similarly, the introduction of metal

catalytically active sites into nitrogen-doped carbon materials

can further improve the catalytic performances through the

formation of metal-coordinated metal–N

–C moieties, but there

have also been conflicting opinions on which coordination

number is more favorable for the two-electron ORR for H

production. For photocatalytic H

production, most studies

demonstrated the negative effect of the one-electron ORR on the

efficiency of the ORR and H

production, and thus various

methods have been developed to suppress the one-electron ORR

by the positive shift of the CB edge within semiconductor

photocatalysts. Nevertheless, recent research also reported the

negative shift of the CB edge of g-C

-based catalysts to

enhance the one-electron ORR for OOH formation and thus

promote the sequential two-step one-electron ORR for H

production. Besides, the type of electron donor used has been

found to play distinct roles in influencing the ORR reaction

pathways in photocatalytic systems, but the detailed reason is

still unknown and thus requires further study. All in all, more

studies are needed to understand the nature of active sites and

the reaction pathways, and how they contribute to the high

catalytic performances for H

production, which provides

guidance for the design of novel catalytic systems for electro-

chemical and photochemical H

production.

5.3 Scalable photo-/electro-chemical interfaces and devices

Most of the recently reported research work mainly evaluated

the electrochemical performances of ORR electrocatalysts for

selectivity by the RRDE technique, whereas only a few

studies about the practical H

productivity in home-made

two-compartment H-cells and flow cells were reported. Never-

theless, it should be highlighted that flow cells may be more

relevant to a commercial scale H

production system com-

pared to H-cells, providing a key stepping stone for the transla-

tion of fundamental lab discoveries into practice. In addition,

the electrochemical systems discussed above generally produce

a mixture of H

and solutes in traditional liquid electrolytes,

thus requiring extra separation processes for the purification of

the produced H

solutions. Recently, Wang’s group produced

a new strategy to achieve the direct electrochemical production

of pure H

solutions by using a porous solid electrolyte, and

various concentrations of pure H

solutions could be easily

achieved by the change of the water flow rate.

Based on this

point, we expect more widespread use of solid electrolytes in

PEC systems for simultaneous generation of H

and electricity.

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Anyway, the research based on such a thought is still in its

infancy, and more work is needed.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

P. S. and Y. S. acknowledge financial support from FCH Joint

Undertaking (CRESCENDO project, Grant Agreement No.

779366) and the German Ministry of Economics and Energy

(BMWi) through project ‘‘ChemEFlex’’ (FKN 0350013A). L. H.

gratefully acknowledges financial support from Fundamental

Research Funds for the Central Universities (No. 531118010232)

and Huxiang High-Level Talent Gathering Project of Hunan

Province (No. 2019RS1012).

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