Amorphous outperforms crystalline nanomaterials: surface modifications of molecularly derived CoP electro(pre)catalysts for efficient water-splitting [original]

Amorphous outperforms crystalline nanomaterials:
surface modi ﬁ cations of molecularly derived CoP
electro(pre)catalysts for e ﬃ cient water-splitting †
Rodrigo Beltr ´
an-Suito, Prashanth W. Meneze s * and Matthias D riess *
The single source precursor (SSP) approach was used to prepare highly active CoP bifunctional electro(pre)
catalysts for the oxygen evolution reaction (OER), hydrogen evolution reaction (HER) and overall water
splitting (OWS) reaction starting from a molecular b -diketiminato Co( I ) cyclo-P
4
complex. Crystalline or
amorphous CoP particles were attained depending on the preparation route. Notably, the amorphous
CoP displayed higher activity compared to the crystalline CoP on nickel foam (NF) and ﬂ uorinated tin
oxide (FTO) substrates due to its unique electronic properties and surface characteristics. During the
OER, severe oxidation to Co-oxy(hydroxides)/oxide s by the loss of P was found to be crucial to increase
the concentration of CoO
x
active sites. Interestingly, complete leaching of surface P from CoP and
surface Co enrichment occurred during the HER. Finally, an OWS device was fabricated where the
amorphous CoP outperform ed the crystalline CoP with respect to low OWS cell voltage (with
ad i ﬀ erence of 130 mV) and enhanced stability for 5 days.
1. Introduction
The rapid growth of human population in the 21
st
century has
increased the demand for energy and search for cleaner and
sustainable energy sources.
1
Hydrogen (H
2
) energy is a prom-
ising alternative to replace fossil fuels as it is contaminant free
and has a high-energy density and zero CO
2
release.
2 – 4
In this
respect, electrochemical water splitting can e ﬃ ciently produce
H
2
and oxygen (O
2
).
3,5
However, this already high energy
consuming process requires the application of an overpotential
( h ).
6,7
At present, the noble metal oxides of ruthenium or
iridium for the OER and elemental platinum for the HER still
represent the benchmark catalysts for practical applications of
water splitting.
8
Their high cost and scarce availability limit
their use in large-scale applications. Consequently, it is imper-
ative to use electrocatalysts that reduce the overpotentials and
increase the energy-e ﬃ ciency of the system based on low-cost,
environmentally benign and earth-abundant transition
metals,
9
which is the ultimate focus of our research group.
10 – 12
Transition metal phosphides (TMPs) have emerged as high-
performance catalysts for electrochemical water splitting
because of their low hydrogen adsorption energies, high elec-
trical conductivity and promising chemical resistance.
3,13,14
Among them, cobalt phosphides (CoP or Co
2
P) have received
attention recently due to their high HER electrocatalytic
activity.
15
In addition, follow-up studies revealed that negatively
charged P atoms can trap protons and promote H
2
liberation.
16
E ﬀ orts have also been devoted to using these materials as OER
catalysts for enabling OWS.
15,17
In this case, the positively
charged Co
d +
sites can act as hydroxyl acceptors, simultaneously
facilitated by the negatively charged P
d 
centers, favouring O
2
evolution by discharging and desorption.
18
Recently, several novel synthetic strategies have been
employed to prepare crystalline and amorphous CoP electro-
catalysts and enhance their activity, such as aerosol spray from
Co – P precursors,
19
electrodeposition,
20
MOF-derived TMPs
21 – 23
and the combination with carbon nanostructures
24,25
or other
transition metals.
26,27
The commonly applied synthetic routes
are usually based on conventional solid-state syntheses which
require highly reactive and pyrophoric reagents or high
temperatures leading to a random distribution of aggregates
and an in  nite number of nanostructures.
28
To prevent this,
new synthetic strategies like the low-temperature molecular SSP
approach are used, showing several advantages, foremost
a better control of the composition and size distribution of the
resulting nanomaterial, which can be varied depending on the
experimental conditions.
29
Recently, this synthetic method has
been applied to access a broad range of high-performance
electrocatalytic OER, HER and OWS materials, including chal-
cogenides
30
and pnictides.
31
Examples of preparation of crys-
talline cobalt phosphides by the SSP approach either for the
HER or for the OER and/or OWS electrocatalysts are limited
32 – 34
and access to amorphous cobalt phosphide phases by the SSP
approach towards OWS remains unexplored. Moreover, the
Department of Chemis try: Metalorgan ics and Inorganic Materials, Technische
Universit ¨
at Berlin, Straße des 17 Juni 135, Sekr. C2, 10623 Berlin, Germany. E-mail:
matthias.driess@tu-be rlin.de; prashanth.men [email protected]
† Electronic supplementary inform ation (ESI) available . See DOI:
10.1039/c9ta04583j
Cite this: J. Mater. Chem. A ,2 0 1 9 , 7 ,
15749
Received 2nd May 2019
Accepted 2nd June 2019
DOI: 10.1039/c9ta04583j
rsc.li/materials-a
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striking structural di ﬀ erence between amorphous and crystal-
line cobalt phosphides in  uencing the net catalytic activity and
their structural transformation during bifunctional electro-
chemical OER and HER catalysis is currently unknown.
Herein, we present a novel molecular-based approach to
synthesizing amorphous and crystalline CoP electro(pre)cata-
lysts through hot injection and pyrolysis of a unique molecular
b -diketiminato cyclo-P
4
dicobalt( I ) complex with a Co
2
P
4
core.
The substantial di ﬀ erence between amorphous and crystalline
CoP structures with respect to electrocatalytic OER and HER
activities has been systematically investigated on distinct elec-
trode substrates under alkaline conditions. In addition, their
surface structures and the nature of the active species have also
been elucidated by means of advanced characterization tech-
niques. Finally, a two-electrode alkaline electrolyser was fabri-
cated to demonstrate the practical advantage of employing
amorphous vs. crystalline CoP.
2. Experimental
2.1. General considerations and instrumentation
All synthetic procedures were done under inert conditions using
standard Schlenk techniques or a M. Braun dry box containing
an atmosphere of inert puri  ed nitrogen. Solvents were dried by
standard methods.
1
H NMR spectra were recorded on a Bruker
Spectrometer APX 200 at room temperature and the solvent
residual signals were referenced to the internal standard.
Fourier transform infrared (FTIR) spectra were recorded on
a Thermo Fisher Nicolet iS5 IR spectrometer (ATR-Diamond)
under inert conditions. Elemental analysis was carried out
with a Thermo Flash EA 1112 Organic Elemental Analyzer by
dynamic  ash combustion at 1020  C. Inductively coupled
plasma atomic emission spectroscopy (ICP-AES) was carried out
using a Thermo Jarrell Ash Trace Scan analyser. The samples
were digested in aqua regia HCl : HNO
3
3 : 1 v/v (nitric acid,
SUPRA-Qualit ¨
at ROTIPURAN® Supra 69% and hydrochloric
acid, SUPRA-Qualit ¨
at ROTIPURAN® Supra 30%) and the
average of three reproducible independent experiments is re-
ported. The digestion volume (2.5 mL) was diluted with Milli-Q
water up to 15 mL. Calibration curves were recorded for both
cobalt and phosphorus with concentrations between 1 mg L
 1
and 100 mg L
 1
from standard solutions (1000 mg L
 1
Single-
Element ICP-Standard Solution ROTI®STAR). Powder X-ray
di ﬀ raction (PXRD) patterns were obtained on a Bruker AXS D8
advanced automatic di ﬀ ractometer equipped with a position-
sensitive detector (PSD) and a curved germanium (111)
primary monochromator using Cu K a radiation ( l ¼ 1.5418 ˚
A) .
The determination of the surface area was performed by
nitrogen sorption using the BET method. Measurements were
performed with a Nova 4000e from Quantachrome Instruments.
Scanning electron microscopy (SEM) was carried out on a LEO
DSM 982 microscope integrated with an EDX (EDAX, Apollo
XPP). Data handling and analyses were performed with the
so  ware package EDAX. The most abundant elements were
selected from the EDX spectra. Transmission electron micros-
copy (TEM) was accomplished on an FEI Tecnai G2 20 S-TWIN
transmission electron microscope (FEI Company, Eindhoven,
Netherlands) equipped with a LaB
6
source at 200 kV accelera-
tion voltage. Energy dispersive X-ray (EDX) analyses were per-
formed with an EDAX r-TEM SUTW detector (Si (Li) detector),
and the images were recorded with a GATAN MS794 P CCD
camera. The SEM and TEM experiments were conducted at the
Zentrum f ¨
ur Elektronenmikroskopie (ZELMI) of the TU Berlin.
2.2. X-ray photoelectron spectroscopy (XPS)
The X-ray photoelectron spectroscopy (XPS) measurements were
conducted on a Kratos Axis Ultra X-ray photoelectron spec-
trometer (Kratos Analytical Ltd., Manchester, U.K.) using an Al
K a monochromatic radiation source (1486.7 eV) with a 90  take
o ﬀ angle (normal to analyser). The vacuum pressure in the
analysis chamber was kept at 2  10
 9
Torr. The XPS spectra
were collected for O 1s, Co 2p, and P 2p levels with a pass energy
of 20 eV and a step of 0.1 eV. The binding energies were cali-
brated relative to the C 1s peak energy position at 285.0 eV. Data
analyses were carried out using Casa XPS (Casa So  ware Ltd.)
and the Vision data processing program (Kratos Analytical Ltd.).
2.3. Synthesis of amorphous CoP by hot injection
To a three-necked round bottom Schlenk  ask  tted with
a temperature sensor and a condenser, 25 mL oleic acid (Fisher
Scienti  c) was added. The solvent was degassed by a 3-cycle
freeze-pump method. The whole set-up was degassed using
vacuum followed by re  lling with nitrogen three times and then
the  ask was heated to 300  C. The precursor (0.47 g, 0.46
mmol) was dissolved in 5 mL of dry oleic acid at 30  Ci n
another  ask. The solution was transferred to the three-necked
 ask at 300  C by injection under inert conditions. The reaction
temperature was maintained at 300  C for one more hour and
then the mixture was allowed to cool down naturally to room
temperature. The whole reaction mixture was transferred into
a centrifuge tube and centrifuged along with an additional
20 mL methanol at 9000 rpm to produce a black solid. Washing
with methanol was repeated thrice more to remove any excess
ligand and oleic acid. The precipitate was then washed with
acetone and dried to store for further use.
2.4. Synthesis of crystalline CoP by pyrolysis
0.20 g (0.20 mmol) of the precursor was introduced into a quartz
tube under an inert atmosphere and subsequently introduced
into a tube furnace SR-A60-300/12 GERO (Neuhausen). The
sample was then heated up to 600  Ca t1 0  C min
 1
from room
temperature and held at that temperature for 2 h under Ar. A  er
cooling to room temperature, a black powder was collected and
washed with acetone several times. Furthermore, the use of
a lower temperature (300 and 450  C) leads to the formation of
materials with a large amount of carbon due to the incomplete
decomposition of the molecular precursor.
2.5. Preparation of  lms
The synthesized materials  lms were prepared by electropho-
retic deposition (EPD) on FTO and NF. 40 mg of the obtained
material and 7 mg of I
2
(Merck, double sublimated) were
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suspended in acetone by sonication for 1 h. In this way,
generated protons get adsorbed onto the particles of the
materials.
35
A 10 V potential di ﬀ erence on a 1  1c m
2
area was
applied on a two electrode system using a PS 303 Pro Conrad
power supply. The applied potential forces the migration of the
charged particles to the negative electrode (cathode). The
deposition time was 5 min for each  lm and the respective  lms
were washed with acetone. The sample loading was  0.4 mg
cm
 2
, determined by weighing the FTO or NF substrate before
and a  er EPD. A similar procedure was also followed to deposit
commercial IrO
2
(Alfa Aesar, 99%) and compare its activity with
that of the synthesized materials.
2.6. Electrochemical measurements
Several electrochemical measurements were performed on the
as-prepared catalysts. A typical catalytic run was carried out
using a three-electrode setup, consisting of a reference elec-
trode (RE), a counter electrode (CE) and the catalyst-modi  ed
working electrode (WE). The three electrodes were immersed
in an aqueous electrolyte (1 M KOH, Sigma Aldrich). The pres-
ence of Fe in it was ruled out by purifying the electrolyte before
its use following a literature procedure.
36
A potentiostat (SP-200,
BioLogic Science Instruments) controlled with the EC-Lab
v10.20 so  ware package was used for all the experiments. The
electrodes with samples deposited served as the WE, Pt wire (or
a graphite rod for the HER) as the CE and Hg/Hg
2
SO
4
(HER) or
Hg/HgO (OER) as the RE. Linear sweep voltammetry (LSV) and
cyclic voltammetry (CV) experiments were carried out with an iR
compensation of 85%, applied before each experiment. The
potential ranges were 1.1 to 1.8 V vs. RHE for OER LSV and 0.0 to
 0.5 V for HER LSV. The overpotential for the OER or HER was
determined from the resulting polarization curves. CP
measurements were done in 1 M KOH at a constant current
density of  10 mA cm
 2
, depending on OER or HER experi-
ments. The overall water splitting LSV and CP experiments were
performed in a two-electrode setup with the catalysts deposited
on NF electrodes. The potentials were referenced to the refer-
ence hydrogen electrode (RHE) by using the following equations
in 1 M KOH: E (RHE) ¼ E (Hg/Hg
2
SO
4
, 1 M KOH) + 1.476 V and
E (RHE) ¼ E (Hg/HgO, 1 M KOH) + 0.924 V. The polarization
curves were replotted as overpotential ( h ) vs. the logarithm of
current density (log j ) to obtain Tafel plots. The double layer
capacitance ( C
dl
) was determined to calculate the active surface
area of the materials and the substrate.
37
From the already
measured LSV, a potential range in which no faradaic process
(no catalysis) occurs was selected. CVs were recorded at
di ﬀ erent scan rates (5 mV s
 1
to 200 mV s
 1
). The half of the
value of the slope of the plot of the capacitive current (the
di ﬀ erence between anodic and cathodic current density) vs. the
scan rate provides the double layer capacitances, C
dl
, of the
layer and the  lms. The ECSA can be calculated from the
following equation: ECSA ¼ C
dl
/ C
s
, where C
s
is the speci  c
capacitance of the sample or the capacitance of an atomically
smooth planar surface of the material per unit area under
identical electrolyte conditions. The following value was used:
C
s
¼ 1.7 mF cm
 2
for 1 M KOH on NF.
38
EIS measurements were
performed over a frequency range from 100 kHz to 10 mHz at
0.7 V vs. Hg/HgO for materials deposited on NF. A sinusoidal
potential was applied, and the frequency-dependent complex
impedance is measured. A Nyquist plot was constructed and the
resistance of the electrolyte solution ( R
s
) and the resistance of
the charge transfer ( R
CT
) were calculated from the  t of the data
to a Randles circuit.
3. Results and discussion
The precursor was prepared according to the reported literature
procedure and well characterized (Fig. S1 – S7 and Tables S1 and
S2, ESI † ).
39
Both hot injection and pyrolysis of the molecular
precursor a ﬀ orded black powders (Scheme 1). The powder X-ray
di ﬀ raction (PXRD) analysis of the materials revealed an amor-
phous phase in the hot injection product and a crystalline CoP
phase in the pyrolysis product (Fig. S8 and S9, ESI † ). Scanning
electron microscopy (SEM) revealed agglomerated materials,
composed of smaller particles, and transmission electron
microscopy (TEM) of the hot-injected product showed that the
agglomerates were rather composed of particles of size # 10 nm
(Fig. 1a, b, S10 and S11b, ESI † ) that do not show any crystalline
fringes con  rming the amorphous nature of the material, as
supported by the selected area electron di ﬀ raction (SAED,
Fig. 1c). The pyrolysis product consists of agglomerates of
particles (Fig. 1d and S12a, ESI † ) with clear crystalline fringes
associated with the lattice spacing of 0.19 nm corresponding to
the (211) plane of the CoP phase (Fig. 1e and S12b, ESI † ). In
addition, the SAED pattern displayed sharp di ﬀ raction rings
representative of the crystalline CoP phase (JCPDS 29-0497,
Fig. 1f). The phase composition and uniform distribution of the
constituent elements were con  rmed by inductively coupled
plasma atomic emission spectroscopy (ICP-AES) (Table S3 † ) and
energy-dispersive X-ray spectroscopy (EDX) (Fig. S13 – S16, ESI † ).
Surface-bonded oxygen (O) due to air exposure was veri  ed by
Fourier-transform infrared spectroscopy (FTIR) (Fig. S17 † ).
Scheme 1 Schematic representation of synthetic routes applied to
form amorphous and crystalline CoP by the molecular SSP approach.
The crystal structure of crystalline CoP was determined being identical
with that reported in JCPDS 29-0497, whereas the structure of
amorphous CoP was drawn schematica lly only for a graphical
di ﬀ erentiation.
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Elemental analysis revealed the presence of a small amount of
carbon (Table S4 † ), derived from the decomposition of the
diketiminate ligand into unidenti  ed organic species.
Furthermore, the Brunauer – Emmett – Teller (BET) surface area
determination using nitrogen (N
2
) adsorption – desorption
isotherms evidenced almost similar speci  c surface areas for
both materials (Table S5 † ). X-ray photoelectron spectroscopy
(XPS) was performed to gain information on chemical compo-
sition and electronic states. The details of Co 2p, P 2p and O 1s
XPS of both materials are given in Fig. S18 (ESI † ).
Th e as -p re p ar ed ma te r ia l s were  rs t e l ec t ro ph or e ti c al ly
de p os i te d on NF to ev al ua t e the i r OER a ct i vi ty by pe rf o rm i ng
linear sweep voltammetry (LS V) in a 1 M KOH electrolyte and then
compar ed to the state-of-the-art catalyst IrO
2
, Pt w ir e , an d ba r e
NF. The att ained electrocatalytic activi ty of amorphous CoP for
th e O ER ( Fi g . 2 a) w as v er y h i gh, ac hi evi ng a n h
10 mA
¼ 284 mV,
which surpassed the activ ity of the crystalline CoP ( h
10 mA
¼ 305
mV), commer cial IrO
2
( h
10 mA
¼ 287 mV) and ev en the best per-
fo rmi ng TMP e le ctr oc ata l ys ts re port ed to da te in th e li ter at ure
(Tables S6 – S8 , ES I † ). A low Tafe l slope of 45 m V dec
 1
was
ob serv ed fo r the am orph ou s CoP wh ic h w as si gni  cantly smaller
than that of the crystalline C oP (82 mV dec
 1
)a n dI r O
2
(118 mV
dec
 1
), indicating a m uch fast er r eaction kine tics (Fig . 2c).
40
A
small rever sible redox couple was detec ted prior to the catalyti c
wa t er o xi dati on , wh ic h cou ld be a ttr ibu ted to th e pr esen ce o f Co
II I
and Co
IV
(Fig. S 19 † ). Both mater ials were found to be very stable at
least for the period of 24 h by chronopotentiome tric (CP) experi-
ments at 10 mA cm
 2
(Fig. S20b † ). In additi on to the NF, the
ca ta ly st s were a lso de pos ite d o n a F TO su bs tra te and me a su red
un der s imil ar c ond iti ons , re sul tin g in a si mil a r tr en d o f ac tiv it y,
lo ng- ter m s ta bil it y and Ta fel sl ope (F ig. S21 a nd S2 2, ES I † ).
Similar experiments were carried out to explore the catalytic
activity towards the HER. The overpotential of the amorphous
CoP was 143 mV at a current density of  10 mA cm
 2
when
deposited on NF (Fig. 2b). The crystalline CoP and commercial
IrO
2
were clearly less active ( h
 10 mA
¼ 261 mV and 209 mV,
respectively). However, the lowest overpotential was achieved
with Pt ( h
 10 mA
¼ 39 mV). In addition, Tafel plots for the HER
were determined (Fig. 2d) from which a Tafel slope of 63 mV
dec
 1
was obtained for the amorphous CoP which was lower
than that of the crystalline CoP (80 mV dec
 1
) and even Pt wire
(73 mV dec
 1
). The attained values of Tafel slopes fall in the
range of 40 – 120 mV dec
 1
which indicates that the HER reac-
tion proceeds via the Volmer – Heyrovsky mechanism on the
surface.
41
The long-term experiments (Fig. S23b † ) showed that
both materials have a stable activity under operating conditions
over 24 h. When FTO was used instead of NF, a similar trend
was observed in activity, stability and the Tafel slope (Fig. S24
and S25, ESI † ). A detailed comparison of the activity of the
prepared materials to that of other non-noble metal-based
catalysts, non-noble TMP catalysts and Co – P-based catalysts is
given in Tables S9 – S11, † respectively (ESI † ). Throughout the
electrochemical OER and HER measurements, the amorphous
CoP activity was found to be superior to that of the crystalline
phase. This di ﬀ erence arises from the surface and electronic
characteristics of the materials. As the BET surface areas were
similar (Table S5 † ), the electrochemically active surface area
(ECSA) was used to compare the active area available for catal-
ysis.
42
The ECSAs of the materials were determined from the
calculation of the double layer capacitance ( C
dl
) (see details in
the ESI † ). The ECSA of the amorphous CoP is about  2.7 times
larger than that of the crystalline CoP (Table S12 † ). A  er the
OER and HER, the ECSA increases  1.5 times in the amorphous
CoP (  0.49 cm
2
) and  2.1 times in the crystalline CoP (  0.24
cm
2
) owing to their increased surface transformation.
43
This
proves that the amorphous CoP has a higher density of surface
defects, randomly oriented bonds, higher structural  exibility,
44
and coordinative unsaturated surface metal sites available for
reaction.
16,45
In addition, the ECSA normalized LSV curves also
con  rmed the higher electrocatalytic activity of the amorphous
CoP (Fig. S28 † ). Electrochemical impedance spectroscopy (EIS)
measurements were carried out to probe the charge transfer
properties of the materials, which determine their activity. A
lower resistance to charge transfer ( R
CT
) was found on the
amorphous CoP relative to the crystalline CoP from the Nyquist
plots of the materials (Fig. S29 † ). Moreover, the R
CT
decreases
on both materials a  er the OER and HER, indicating a highly
e ﬃ cient electron transfer during catalysis (Table S13 † ). The use
of carbon nanomaterials to improve the charge transfer e ﬃ -
ciency and conductivity has been shown to be a common
strategy for structure and electronic tuning of nanomaterials.
46
However, we assume that the amount of carbon species attained
was very low to in  uence the catalytic performance of the
prepared CoP, as demonstrated for metal – organic framework
(MOF) derived TMP electrocatalysts.
47 – 50
The post-catalysis characterization revealed the trans-
formation in the structure and composition. A  er the OER, the
amorphous CoP did not show any drastic changes in its overall
structure as well as in the SAED pattern (Fig. S30 † ). In the case
of the crystalline CoP, an amorphous layer encapsulating the
crystalline core is formed, which was also demonstrated by the
Fig. 1 TEM, HR-TEM and SAED of the as-prepared amorphous CoP
(a – c) and crystalline CoP (d – f). The TEM and HR-TEM (a and b) of the
amorphous CoP exhibited smaller agglomerated particles with no
obvious di ﬀ raction rings in the SAED pattern (c). However, although
the particle features were similar (d and e), the crystalline CoP dis-
played a lattice spacing of 0.19 nm corresponding to the (211) plane
(inset e) and clear crystalline di ﬀ raction rings associated with CoP in
the SAED pattern (f).
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SAED pattern (Fig. S30 † ). Elemental mapping shows that Co, O,
and P are present in both materials (Fig. S31 and S32, ESI † ).
Phosphate formation also enhances the OER activity because it
can act as a promoting ligand in the Co
II
/Co
III
/Co
IV
redox
process and also facilitate the four-electron proton-coupled
transfer steps during the OER.
51
The structural trans-
formations at the near-surface were further con  rmed by the
XPS analysis (Fig. 3). The peaks corresponding to Co
d +
(778.7 eV
2p
3/2
and 793.9 eV 2p
1/2
) completely disappeared during the
OER forming new peaks: the ones associated with Co
2+
and Co
3+
which indicate the formation of cobalt oxidized species (Fig. 3a,
S33a and b, ESI † ).
52
Additionally, the P 2p spectra displayed the
diminishing P
d 
signal at 129.0 eV and the oxidised P
5+
(phos-
phate) a  er LSV and CP experiments (133.7 eV) (Fig. 3b, S33c
and d, ESI † ).
52,53
In the O 1s spectra (Fig. 3c, S33e and f, ESI † ),
the deconvolution resulted in one major (largely hydroxylated)
and two minor peaks (formation of an oxide and adsorbed water
molecules at the surface).
54,55
Similar transformations were
observed for the crystalline CoP (Fig. 3d – f) and the detailed
information on its deconvoluted spectra is given in Fig. S34
(ESI † ). The FTIR spectra (Fig. S35 † ) additionally con  rmed the
surface hydroxylation. Therefore, it can be deduced that during
the OER in alkaline media, the surface of CoP undergoes
oxidation to polyphosphate and oxo(hydroxo) containing
species (CoO
x
(OH)
y
).
56 – 58
Since the polyphosphates are highly
soluble in alkaline solution,
58,59
the surface becomes enriched
with CoO
x
(OH)
y
which is the true active electrocatalyst for the
OER.
15
This oxidation process is likely to go deeper beyond the
nanoparticle surface under further prolonged electrolysis and
may completely transform the original CoP structure to CoO
x
(-
OH)
y
. Thus, the higher leaching of P as polyphosphate (85%)
anions from the amorphous CoP con  rms greater structural
rearrangements in the defect-enriched (CoO
x
(OH)
y
) surface.
60,61
On the other hand, the P dissolution rate in the crystalline CoP
is only moderate (11%) (Table S14 † ), which con  rms the larger
number of arbitrarily oriented bonds and higher structural
 exibility of the amorphous CoP relative to its crystalline
counterpart, which intensi  es the structural modi  cation and
translates into the formation of a larger number of surface
unsaturated sites for facile reactant adsorption, as con  rmed by
the ECSA di ﬀ erence of the materials and its change during
catalysis.
43
Fig. 2 Polarization curves of the (a) OER and (b) HER of di ﬀ erent CoP materials and commercial noble metal-based catalysts deposited on NF
with a scan rate of 10 mV s
 1
in 1 M KOH ( iR compensation: 85%) and corresponding Tafel slopes obtained for the (c) OER and (d) HER. Selected
regions of the polarization curves are shown in Fig. S20a and S23a (ESI † ).
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Likewise, the characterization a  er the HER was also carried
out. TEM and SAED of the amorphous CoP (Fig. S36a and b,
ESI † ) showed no apparent change in the phase a  er catalysis.
An amorphous layer was formed over the crystalline CoP but
retaining its crystalline core (Fig. S36c and d, ESI † ). Elemental
mapping and EDX of the amorphous material and crystalline
CoP (Fig. S37 and S38, ESI † ) revealed that Co, O, and P are
present. In both cases, the HER gave rise to the loss of P into
solution by 13% and 5% for the amorphous and crystalline CoP,
respectively (Table S14 † ). Post-HER XPS spectra of the amor-
phous CoP revealed oxidation of Co to Co
2+/3+
(Fig. 3a and S39d,
ESI † ) and the absence of P signals (Fig. 3b). O 1s deconvolution
a  er LSV and CP results in three deconvoluted peaks with
assignments as in the case a  er the OER (Fig. 3c, S39e and f,
ESI † ). Similar assignments were observed in the XPS spectra of
the crystalline CoP and FTIR a  er the HER (Fig. 3d – f, S40 and
S41, ESI † ). Two important phenomena contribute to the surface
transformation during the HER. Initially, a similar phenom-
enon as in the OER occurs and results in the outer surface
transformation to oxy(hydroxide) species induced by the phos-
phorus dissolution. At the same time, the application of
a negative potential leads to the electrochemical reduction of
the oxidized species generated in the surface.
58
In consequence,
phosphorus remains as P
d 
a  er catalysis in the XPS spectrum
(Fig. S39c † ). However, a  er the CP 24 h experiment, a Co-
enriched surface is produced since all surface phosphorus is
dissolved in solution, and hence no signal in the P 2p spectrum
appears. During the application of the negative potential, in situ
generated thermodynamically favourable Co
0
species are
formed (Fig. S42 † ) which can act as binding sites for protons.
Similar surface transformations and formation of in situ Co
0
at
the surface under reductive conditions have been shown before
for several phosphorus-based materials.
33,43
Although phos-
phorus was lost on the outer surface of CoP, the presence of P in
the bulk also contributes to catalysis. It has been shown previ-
ously that more electronegative P atoms can withdraw electron
density from the metal by acting as a base to trap protons,
whereas metal atoms can behave as a hydride acceptor.
62,63
The
metallic-character CoP core also contributes to catalysis by
accelerating the charge transfer from the active catalyst site on
the surface to the electrode substrate to e ﬃ ciently accomplish
the HER, which was veri  ed by EIS measurements (Table S13 † ).
Inspired by the outstanding OER and HER activities of
amorphous CoP on NF, we assembled an OWS device in a two
electrode con  guration using both CoP/NF as both the anode
and cathode (CoP/NF k CoP/NF) in 1 M KOH (Fig. S43a † ). For
comparison, crystalline CoP and bare NF electrodes were also
measured under similar conditions. A cell voltage of 1.65 V is
needed to reach a current density of 10 mA cm
 2
for the
amorphous CoP, whereas for the crystalline CoP, a potential of
1.79 V was required (LSV, Fig. 4a), which were superior to the
bare NF. Finally, the long-term CP of both CoP for OWS was
measured resulting in exceptional stability over 5 days for both
materials (Fig. 4b). Moreover, an inverted electrochemical cell
(graduated) was used in which H
2
and O
2
could be collected
separately at atmospheric pressure (Fig. S44 † ) and the H
2
:O
2
volume ratio was  2 : 1, showing an e ﬃ cient selectivity of the
catalysts for each half-cell reaction (Fig. S45 † ). The calculated
faradaic e ﬃ ciency (FE) also was nearly 100% for each half-cell
reaction (see Table S15 † ) with amorphous CoP.
Fig. 3 XPS spectra of the amorphous CoP: (a) Co 2p, (b) P 2p and O 1s (c) and the crystalline CoP: (d) Co 2p, (e) P 2p and (f) O 1s before and after
catalysis. Deconvoluted spectra and details on assignments are shown in the ESI (Fig. S18, S32, S33, S38 and S39, ESI † ).
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4. Conclusions
In sum mary , stru ctur ally d i ﬀ er ent a mo rpho us and c rysta llin e
Co P have b een pre par ed th rou gh two d i ﬀ er ent th ermo lytic
app roa ches a nd ut iliz ed as elec tro( pre) cat alys ts for the OE R, HER
and OWS in alk al ine me dia . The am orph ous Co P mate ria l
ex hibi ts rema rka ble ca taly tic ac tivi ty for bot h the OER an d the
HE R with lo ng- term s tabil ity a nd a con sid er abl y sma ll Tafe l slo pe
in c om pari son to the c rys ta ll ine Co P. Du ri ng th e OE R in al kal in e
sol uti on, Co oxy (hyd roxi de) /o xi de enri chme nt occu rs on the
surf ace o f b ot h cat alys ts as a res ult of P diss olut ion f rom t he pr e-
ca ta lyst fo rmin g poly pho spha te spe cies th at d iss olve in the
el ec trol yte . Two sim ult aneou s pro cess es occu r duri ng the HE R:
th e init ial o xida tion o f CoP by th e cont act wit h the st ro ngl y
alk alin e elec tr ol yt e and co ncom ita nt p hosp horu s los s into th e
el ec trol yte , form ing a Co- rich su rf ac e that en caps ula tes the Co P
co ndu ctiv e co re . Se cond , the reduc tion of th e oxi dize d Co s pe cies
to in situ gene rate d Co
0
act ive site s, whic h ser ve for pro to n
re du ctio n. More over , the me tall ic char acter of th e CoP core al so
co ntr ib utes to ca tal ysi s by its hi gh con du ctiv ity to acco mpl is h
ch arg e trans fer fr om the a ctiv e cat al yst s urf ace to th e elec tr od e
sub str ate, as det ermi ned by th e EIS ex peri me nts . The ob ser ved
di ﬀ er en ce in act ivit y is att ri bute d to di ﬀ er ent di ssol ut io n rate s of
P as pho sp ha te, di ve rse mo rphol ogi ca l, sur face a nd elec tron ic
ch ara cter isti cs an d h ig her act ive su rfa ce ar ea of amor phou s Co P
th an th at of cr ysta ll ine, whi ch is a lso c onsis tent wit h th e lo wer
R
CT
of th e amo rph ous Co P. More over , the ov erpo te nti als at ta in ed
fo r amor phou s CoP in OE R and HE R cata lysi s are fa r bett er tha n
th ose of mo st of the re port ed Co – Pc a t a l y s t  lms to dat e. Fina lly,
th e bif uncti ona l ca tal yti c act ivit y of bot h mate rial s wa s test ed
usi ng a tw o- elec tro de cel l. As antic ipat ed, th e amo rpho us Co P
ex hibi ted a l ow vo lta ge and stri king ly over 100 h st abi lit y for
OW S. Fu rthe rmor e, the pr es ente d pre par ati on me th od cou ld be
ea sil y ex te nd ed to pre par e othe r bif unc tion al elec tr oca taly sts
sta rtin g from mo lec ula r sin gl e-s ou rce or gano me ta lli c prec urs ors .
Con ﬂ icts of interest
The authors declare no competing interests or other interests
that might be perceived to in  uence the results and discussion
reported in this paper.
Acknowledgements
The authors are indebted to Dr Vitaly Gutkin (Hebrew Univer-
sity, Jerusalem) for XPS, Mr Christoph Fahrenson (ZELMI, TU
Berlin) for SEM, Jan Niklas Haussmann (TU Berlin) for TEM,
and the group of Prof. Martin Lerch (TU Berlin) for PXRD
measurements. We thank the Deutsche For-
schungsgemeinscha  (DFG, German Research Foundation)
under Germany's Excellence Strategy – EXC 2008/1 – 390540038
(UniSysCat) for  nancial support.
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Why institutions use Plag.ai for originality review, entry 3

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