2480 |Energy Environ. Sci., 2020, 13, 2480--2500 This journal is ©The Royal Society of Chemistry 2020
Cite this: Energy Environ. Sci.,
2020, 13,2480
Establishing reactivity descriptors for platinum
group metal (PGM)-free Fe–N–C catalysts
for PEM fuel cells†
Mathias Primbs, ‡
a
Yanyan Sun,‡
a
Aaron Roy,
b
Daniel Malko,
c
Asad Mehmood,
c
Moulay-Tahar Sougrati,
b
Pierre-Yves Blanchard,
b
Gaetano Granozzi,
d
Tomasz Kosmala,
d
Giorgia Daniel,
d
Plamen Atanassov,
e
Jonathan Sharman,*
f
Christian Durante, *
d
Anthony Kucernak, *
c
Deborah Jones,*
b
Fre
´de
´ric Jaouen *
b
and Peter Strasser *
a
We report a comprehensive analysis of the catalytic oxygen reduction reaction (ORR) reactivity of four
of today’s most active benchmark platinum group metal-free (PGM-free) iron/nitrogen doped carbon
electrocatalysts (Fe–N–Cs). Our analysis reaches far beyond previous such attempts in linking kinetic
performance metrics, such as electrocatalytic mass-based and surface area-based catalytic activity with
previously elusive kinetic metrics such as the active metal site density (SD) and the catalytic turnover
frequency (TOF). Kinetic ORR activities, SD and TOF values were evaluated using in situ electrochemical
NO
2
reduction as well as an ex situ gaseous CO cryo chemisorption. Experimental ex situ and in situ Fe
surface site densities displayed remarkable quantitative congruence. Plots of SD versus TOF (‘‘reactivity
maps’’) are utilized as new analytical tools to deconvolute ORR reactivities and thus enabling rational
catalyst developments. A microporous catalyst showed large SD values paired with low TOF, while
mesoporous catalysts displayed the opposite. Trends in Fe surface site density were linked to molecular
nitrogen and Fe moieties (D1 and D2 from
57
Fe Mo
¨ssbauer spectroscopy), from which pore locations of
catalytically active D1 and D2 sites were established. This cross-laboratory analysis, its employed
experimental practices and analytical methodologies are expected to serve as a widely accepted
reference for future, knowledge-based research into improved PGM-free fuel cell cathode catalysts.
Broader context
Polymer electrolyte membrane fuel cells (PEMFC) have reached the commercial stage and ever wider deployment is imminent. To further reduce the loading
of platinum group metal (PGM) catalysts in PEMFC electrodes, PGM-free, iron and nitrogen-doped carbon oxygen reduction (ORR) electrocatalysts (Fe–N–C)
were developed over past decades. Recent advances in activity and stability of Fe–N–C are impressive, yet methods to evaluate the number of catalytic active
Fe sites at the surface and intrinsic turn over frequency remained elusive. This changed with the advent of CO cryo-sorption and in situ nitrite
stripping techniques that yielded these intrinsic reactivity descriptors. Never before, however, have these two complementary specific adsorption/stripping
techniques been compared and combined with other chemical and spectroscopic analytics for an in-depth analysis of catalytic reactivity of Fe–N–C ORR
electrocatalysts. The present study addresses this issue and presents a comprehensive analysis of the reactivity of the four state-of-the-art Fe–N–C PEMFC
electrocatalysts. The study provides a deeper understanding of the origin and difference in catalytic performance through the combination of a host of
different surface sensitive and bulk analysis methods. The methodologies and analyses of this benchmark catalyst study will benefit future developments in
Fe–N–C catalysis.
a
Department of Chemistry, Chemical Engineering Division, Technical University of Berlin, 10623 Berlin, Germany. E-mail: pstrasse[email protected]
b
ICGM, Univ., Montpellier, ENSCM, Montpellier, France. E-mail: Deborah.Jones@umontpellier.fr, frederic.jaouen@umontpellier.fr
c
Department of Chemistry, Imperial College London, South Kensington, SW7 2AZ, London, UK. E-mail: anthony@imperial.ac.uk
d
Department of Chemical Sciences, University of Padova, Via Marzolo 1, 35131 Padova, Italy. E-mail: christia[email protected]
e
Department of Chemical & Biomolecular Engineering and National Fuel Cell Research Center, University of California, Irvine, CA 92697, USA
f
Johnson Matthey Technology Center, Blount’s Court, Sonning Common, Reading RG4 9NH, UK. E-mail: jonathan.sharman@matthey.com
†Electronic supplementary information (ESI) available. See DOI: 10.1039/d0ee01013h
‡These authors contributed equally.
Received 31st March 2020,
Accepted 24th June 2020
DOI: 10.1039/d0ee01013h
rsc.li/ees
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1. Introduction
Currently, proton exchange membrane fuel cells (PEMFCs) are
on the verge of massive deployment, in the transport sector in
particular, but still require expensive and scarce platinum-
group-metal (PGM)-based electrocatalysts to promote the slug-
gish oxygen reduction reaction (ORR) occurring at the cathode
of PEMFCs.
1–8
This is the reason why much international effort
is now being devoted to a rational design and development
of lower-cost PGM-free ORR electrocatalysts. Large research
consortia, such as the ElectroCat network
9
funded by the US
Department of Energy and the EU projects CRESCENDO
10
and
PEGASUS
11
funded by the Fuel Cells and Hydrogen Joint
Undertaking (FCH-JU), are working to meet specific perfor-
mance targets. The latter are defined so that fuel cell stacks
with PGM-free ORR catalysts become cost- and performance-
competitive with PGM-based catalysts, even for the highly
demanding automotive application.
4–6,12–22
The most promi-
nent example of PGM-free ORR electrocatalysts for acidic
medium is the family of iron- (or cobalt-) and nitrogen-doped
high surface area carbon matrix, typically referred as ‘‘Fe–N–C’’
catalysts, with atomically-dispersed Fe cations coordinated with
nitrogen atoms as the recognized most active sites.
5,23–32
Unlike
PGM-based single atom catalysts, where the atoms exist in a
carbon matrix as a sole atoms
33,34
or dimeric compounds,
35
iron generally has to be coordinated with hetero atoms. Several
general approaches have been established in order to control
the carbon micro and/or meso-porosity in M–N–C catalysts, a
key for high performance: functionalisation of microporous
carbon blacks with metal and N precursors,
4
hard-templating
of C and N precursors with e.g. silica,
36
adding porogens before
pyrolysis,
37
using reactive gases such as ammonia or CO
2
during pyrolysis,
38
and last but not least by soft templating
with e.g. metal organic frameworks
39,40
or porous organic
polymers.
36,40–60
Despite the impressive achievements in the
catalytic performance of Fe–N–C catalysts, further improve-
ments in their ORR activity and, in particular, durability are
needed before their large-scale deployment in commercial
PEMFCs becomes a reality.
12,26,61–63
Over the past decades, studies to identify more active
Fe–N–C catalysts have largely relied on empirical approaches
involving the systematic variation of elemental precursors
and/or synthesis conditions to prepare Fe–N–C materials and
their correlation with the resulting kinetic current density ( J
kin
)
and other lump performance metrics of ORR catalysts.
7,36,64–66
While this approach has had some success in the early stages of
Fe–N–C materials development, it now seems to have reached
its limitation, with stalled progress in the power and durability
performance of Fe–N–C cathodes in PEMFCs in the last years,
despite intense international efforts. Novel and more rational
approaches are needed in order to deconvolute the overall
activity and durability of Fe–N–C catalysts into the contribu-
tions arising from different Fe-based active sites, in order to
identify the most active and/or most durable sites and to
develop synthetic strategies to selectively optimize the number
of such sites.
31,67
The first step towards this goal implies the
development of experimental methods that evaluate the
number of Fe-based catalytic sites that are located at the surface
ofthecatalyst(sitedensity,SD).TheSDvalueisthencombined
with the kinetic current density, J
kin
, and elemental electric
charge, e, in order to extract the average intrinsic turn over
frequency (TOF) of the Fe-based active sites in a given Fe–N–C
catalyst, according to
68
J
kin
[A g
1
] = TOF [electron site
1
s
1
]SD [site g
1
]
e[C electron
1
] (1)
TOF and SD are fundamental descriptors of catalytic reactivity
and can provide guidelines for the synthesis of more active
catalysts. Efforts to improve the overall activity of a catalyst
may now focus on synthetic strategies to increase, separately or
combined, the SD value or to enhance the intrinsic TOF value of
the active sites.
Theoretical–computational research has offered a much
clearer, albeit not fully resolved, picture of the chemical
structure of favorable, catalytically active Fe–N
x
single metal
sites.
16,69
Advanced experimental analytical techniques such as
57
Fe Mo
¨ssbauer spectroscopy and high resolution STEM-EELS
microscopy have now qualitatively proven the existence of such
sites in active Fe–N–C materials.
13–15,26,60,70–73
A serious hurdle
in the rational improvement of the catalytic activity of Fe–N–C
catalysts, however, has been the lack of suitable methods that
accurately enumerate the electrochemically accessible Fe–N
x
sites (SD). Even for model Fe–N–C materials comprising only
Fe–N
x
sites, the SD value cannot be accessed with the sole
knowledge of the total Fe content, due to the location of a
significant fraction of Fe–N
x
sites not only on the surface but
also in the bulk of the carbon matrix. This issue results from
the pyrolytic process employed to form such active sites.
A range of spectroscopic methods based on X-rays and g-rays
have been applied in order to probe and quantify bulk and/or
surface Fe-based sites, namely X-ray photoelectron spectro-
scopy (XPS), X-ray absorption spectroscopy (XAS) and
57
Fe
Mo
¨ssbauer spectroscopy.
7,13,15,31,63,70,71
However, there exist
inherent shortcomings for each of these analysis methods. XAS
and
57
Fe Mo
¨ssbauer spectroscopy are inherently bulk methods, so
they identify both electrochemically accessible and inaccessible
Fe-based sites. X-ray photoelectron spectroscopy (XPS) are element
specific but not surface sensitive for carbon-based materials with
high surface area, due to the escape path of several nm of
photoelectrons throughthecarbonmatrix.
74
Synchrotron-based
XPS with tuned energy of the X-rays has improved the surface
sensitivity for carbon-based materials, and been successfully
applied to study Fe–N–C materials.
75,76
While synchrotron-based
XPS can give information on surface elemental composition, it
however cannot yield absolute numbers of metal-based sites in the
overall sample. In addition, while XPS successfully distinguishes
the presence of different oxidation states of a metal, it is not
powerful at discriminating between different environments. For
example, iron in ferric oxide and Fe(III)N
x
sites cannot be distin-
guished with XPS, and the root for this is that the detected photo-
electrons come from the core.
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Adsorption/desorption techniques involving probe molecules
are inherently well suited to count sites on the surface, yet often
lack chemical specificity.
77
Probe molecules such as CO,
78
NO,
79
CN
(ref. 80) or SCN
have been explored as surface probes for
Fe–N–C materials under electrochemicalconditions, however
none was successfully applied for a quantitative evaluation
of surface sites.
81
For example, both CN
and tris(hydroxy-
methyl)aminomethane (Tris) have been successfully employed
in partial poisoning studies of Fe–N
x
surface sites of Fe–N–C
catalysts.
80,82
This implies that counting the number of Tris
molecules or cyanide ions irreversibly adsorbed on Fe–N–C (after
washing the surface with electrolyte free of such probe species)
would underestimate the total number of surface-located Fe-based
sites, due to too weak adsorption on some sites. Recently, new
complementary adsorption/desorption techniques were speci-
fically developed for Fe–N–C materials and validated, one
based on low-temperature CO adsorption
83
and the other on
ambient-temperature NO
2
/NO adsorption.
84
The ex situ, low-
temperature CO cryo pulse chemisorption/desorption techni-
que featured good specificity to Fe sites and resulted in
reproducible SD values for different single metal active sites,
in particular for Fe–N–C materials.
68,85,86
The technique relies
on rapid adsorption rates and strong binding at 80 1C
between CO molecules and atomically dispersed single Fe–N
x
sites embedded in a carbon framework. Possible pitfalls of this
technique include overestimation, because it is not possible to
show that ORR is blocked by CO and due to the possibility of
single sites to bind more than one CO molecule. Also, initial
poisoning of a fraction of the single Fe sites may alter the
subsequent CO uptake amount, leading to undersampling.
A careful pretreatment procedure is therefore necessary to
desorb oxygenates quantitatively from all surface Fe-based sites
site prior to CO uptake. A standardized thermal pretreatment
protocol of Fe–N–C now ensures reproducible CO uptake values
on oxygen-free Fe(II)N
x
sites.
85,86
Second, a complementary in situ electrochemical nitrite
adsorption/NO electrostripping technique was put forward by
Kucernak’s group.
77
The method relies on the very specific and
strong interaction of Fe–N
x
sites with nitrite anions resulting in
NO adsorption, followed by electrochemical reductive stripping
of NO into ammonia.
87
Thus, a quantification of Fe–N
x
sites is
achieved by means of the stripping charge of the five-electron
process. Issues related to this method include the fact that
it requires a moderately acidic pH of about 5, which is less
acidic than the conditions prevailing at a PEMFC cathode.
Furthermore, although the majority of ORR current is blocked
by NO adsorption (470%), some ORR current remains suggesting
thepresenceofmultipletypesofFe–N
x
sites. NO may poison only
a fraction of the exposed sites due to its very high chemical
specificity, which leads to undersampling. Together, the ex situ
CO cryo probe technique and the in situ NO probe technique offer
a powerful pair of complementary physico-chemical strategies to
quantify the number of Fe–N
x
sites on the surface of Fe–N–C
catalysts. Together, both methods may yield a balanced and
reliable range of quantitative values for (i) the SD and (ii) after
combination with ORR activity measurements, for the TOF.
This enables a rational, knowledge-driven improvement of the
reactivity of Fe–N–C catalysts. However, hitherto these two SD
probe techniques have never been combined to study and analyze
the catalytic ORR reactivity of a same set of PGM-free catalysts to
extract their SD and TOF values and to cross-compare the values
obtained with the two techniques. Likewise, no study has hitherto
attempted to draw useful correlations between the composition
and structural or morphological characteristics of Fe–N–C cata-
lysts and their fundamental reactivity parameters such as TOF
and SD. The objectives of this contribution are to compare the SD
and TOF values determined for several Fe–N–C catalysts with the
nitrite stripping and CO cryo chemisorption techniques, as well as
to establish novel structure–reactivity correlations, deconvoluting
the reactivity into SD and TOF values, moving beyond the lump
ORR activity descriptor used hitherto.
Here, we present the first comprehensive analysis of trends
in the two fundamental descriptors of the electrocatalytic
reactivity of today’s state-of-art Fe–N–C catalysts, namely SD
and TOF, as measured with the ex situ CO cryo probe technique
and the in situ NO probe techniques. We then establish novel
correlations between SD and/or TOF descriptors and several
descriptors of the structure, morphology and/or elemental
composition of Fe–N–C catalysts. What sets this study apart is
not only the fact that the catalytic ORR reactivity of four of the
most active Fe–N–C catalysts is deconvoluted into SD and TOF
contributions, but also that the presented data, trends and
conclusions are based on the combination of the independent
analyses of four different laboratories. Furthermore, outcomes
include both new and in part quite surprising correlations
between the SD data resulting from ex situ CO and in situ NO
techniques, as well as and more importantly previously unavail-
able fundamental insights into the origin of the catalytic ORR
reactivity of these four benchmark catalysts.
More specifically, starting from the rotating ring-disk elec-
trode (RRDE) based ORR mass activity (MA), we derive quanti-
tative values for (i) SD for each benchmark catalyst, on a mass-
basis and/or surface-area basis, and (ii) TOF values. In parallel,
the Fe–N
x
coordination environment and elemental composition
in the bulk of the sample were determined by
57
Fe Mo
¨ssbauer
spectroscopy and XPS, respectively. The pore structure and
specific surface area were evaluated with nitrogen sorption
isotherms. Previously inaccessible mass activity maps were
established from the knowledge of the SD and TOF values of
the catalysts, on the one hand, and between SD or TOF values
and the experimentally determined type and quantity of
Fe–N
x
sites, on the other hand. Our analyses offer rational
guidelines how to achieve further improvements in the PGM-
free ORR activity in order to reach future targeted performance
characteristics.
2. Experimental section
The present cross-laboratory study was carried out at the
University of Padua, Imperial College London, the Institut
Charles Gerhardt (CNRS – University of Montpellier – ENSCM),
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and at the Technical University Berlin. Some of the analyses were
duplicated at different laboratories using distinct equipment. The
experimental details are described below, by method and/or
laboratory.
2.1 Benchmark catalysts
The four benchmark Fe–N–C catalysts investigated here were
sourced from different laboratories and were used as received.
Their detailed synthesis have been published in the literature.
They are currently considered best-in-class Fe–N–C catalysts
for PEMFC applications. They include a ZIF-derived catalyst
from CNRS/University of Montpellier (henceforth referred to
as CNRS),
88,89
a polymerized di-amino naphthalene based
catalysts from Imperial College London (ICL),
77
two catalysts
synthesized via hard templating with fumed silica, one from
the University of New Mexico (UNM) and another one from
Pajarito Powder Inc. (PAJ). The latter can be obtained as a
commercial product under the name PMF-011904.
2.2 Physicochemical characterization
Nitrogen physisorption. At one laboratory, nitrogen physi-
sorption was performed in a Micromeritics ASAP 2020 instrument.
100 to 150 mg of the catalyst was inserted in a sample tube with
glass wool and filling rods on top. Before the measurement, the
samples were pre-treated under vacuum (300 1C, 20 h) to remove
any species adsorbed on the sample. After cooling to room
temperature, helium was backfilled into the sample tube. During
the measurements, the sample was cooled to 77 K (liquid nitrogen).
The Brunauer–Emmett–Teller (BET) equation was used to estimate
the total surface area. Non-local density functional theory (2D-
NLDFT) was used to model isotherms to calculate pore size
distributions of microporous carbon materials with pores from
0.35 to 25 nm. For the analysis, an assumption of 2D model of
finite slit pores having a diameter-to-width aspect ratio of 4–6–12
was made. At another laboratory, nitrogen physisorption was
conducted on a Micromeritics Tristar II 3020. The analysis tem-
perature was 77 K and the BET equation was also used to estimate
thetotalsurfacearea.Thebestregionforthelinearfitwas
determined by the Rouquerol method.
90
Samples were degassed
and dried overnight at 300 1C under flowing nitrogen prior to the
measurement. Gases used were nitrogen (BIP plus-X47S) for
drying and adsorption and He (BIP plus-X47S) for free-space
measurement. Pore volume was determined as per NLDFT as
implemented in the software Micromeritics ‘‘Microactive for
Tristar II’’. The model was based on a slit shaped pore.
X-ray photoelectron spectroscopy. The XPS measurements
were carried out in a custom-designed UHV system equipped
with an EA 125 Omicron electron analyzer ending with a five
channeltron detector, working at a base pressure of 10
10
mbar.
The photoemission spectra were collected at room temperature
using the Mg K
a
line (hn= 1253.6 eV) of a non-monochromatised
dual-anode DAR400 X-ray source. The survey spectra were
acquired using 0.5 eV energy step, 0.5 s collection time, and
50 eV pass energy. Additionally, single components (C 1s, O 1s,
N1s,Fe2p
3/2
) were acquired with the same parameters in order to
increase accuracy of the calculation of surface composition
(i.e. Fe 2p
3/2
line was acquired 60 times). High resolution spectra
were acquired using 0.1 eV energy steps, 0.5 s collection time,
and20eVpassenergyforthecurvesfitting.
57
Fe Mo
¨ssbauer spectroscopy.
57
Fe Mo
¨ssbauer spectra were
measured with a Rh matrix
57
Co source. The measurements
were performed keeping both the source and the absorber at
room temperature, unless otherwise mentioned. The spectro-
meter was operated with a triangular velocity waveform, and a
gas filled proportional counter was used for the detection of the
g-rays. Velocity calibration was performed with an a-Fe foil. The
spectra were fitted individually with appropriate combinations
of Lorentzian lines. In this way, spectral parameters such as the
isomer shift (IS) and the electric quadrupole splitting (QS), and
the relative resonance areas (A) of the different components were
determined. Isomer shift values are reported relative to a-Fe.
Elemental analysis (EA). Elemental analysis was carried out
using a Thermo Scientific Flash 2000 analyser.
Inductively coupled plasma-mass spectrometry (ICP-MS).
An Agilent Technologies 7700x ICP-MS was employed for
inductively coupled plasma mass spectroscopy analysis. The
samples (15 mg) for ICP analysis were treated with 2 mL of nitric
acid (69% w/w) and heated at 100 1Cfor1h.Themixtureswere
diluted up to 40 g with Milli-Q water and after filtration, 2 mL of
the solutions were analyzed. For ICP analysis, another protocol was
tested using a microwave system CEM EXPLORER SP.D PLUS at a
heating rate of 40 1Cmin
1
from room temperature to 220 1Cwith
a pressure of 400 psi and a power a 300 W. In the latter method the
samples were dispersed in 2 mL of nitric acid, 6 mL of hydrochloric
acid (37% w/w) and 3 mL of sulfuric acid (93–98% w/w).
2.3 Electrochemical measurements
The electrochemical measurements consisted of the determi-
nation of the catalytic ORR activity and selectivity using rotating
ring-disk electrode (RRDE) set-ups at two different geometric
catalyst loadings of 0.2 and 0.8 mg cm
2
on the disk electrode,
in order to study the influence of layer thickness on the catalyst
performance. All laboratories involved in this study performed
RRDE testing, and error bars originated from the variations of
data across the laboratories.
Ink formulations. The catalyst ink consisted of a slurry of
the catalyst, isopropanol and ultrapure water in a water to
isopropanol mass ratio of 1 : 1, and Nafion (5 wt%, Sigma-
Aldrich). The catalyst content was either 0.5 wt% (0.2 mg cm
2
loading) or 2.0 wt% (0.8 mg cm
2
loading) of the total ink with
a mass ratio of water to catalyst of 1 : 10 and 1 : 40 respectively.
The ionomer to catalyst ratio is 1 : 2. The suspension was ultra-
sonicated until a stable suspension was reached.
Electrochemical set-ups. The electrolyte was 0.5 M H
2
SO
4
(ANALR grade or EMSURE Merck Millipore, as available to all of
the project partners). All the measurements were performed in
a glass jacket cell at 25 1C with a reversible hydrogen electrode
(RHE) reference electrode, a graphite counter electrode, and a
glassy carbon disk with a platinum or gold ring as working
electrode. The ring-disk electrodes were polished and cleaned
in an ultra-sonication bath with isopropanol and ultrapure
water. The cleaned electrodes were dried in nitrogen and the
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ink was deposited on the disk surface and dried at room
temperature or in an oven at 50 1C.
Break-in procedures. The activation of the catalyst was
performed in N
2
-saturated electrolyte via cyclic voltammetry
(0.0–1.0 V
RHE
,10mVs
1
) with a minimum of five cycles until
the change in capacitance in the 0.95–1.0 V
RHE
region between
two successive scans was less than 2%.
ORR activity and selectivity measurements. Cyclic voltam-
metry was performed in an O
2
-saturated electrolyte (0.925–0.00
V
RHE
, 1–2 mV s
1
, rotation rate 1600 rpm, ring potential 1.5
V
RHE
) starting from open circuit potential (OCP) to the lower
potential of 0.0 V
RHE
and a back scan to 0.925 V
RHE
. The scan
rates are low enough to neglect non-faradaic currents.
Accelerated stress tests (AST). AST was performed with a
catalyst loading of 0.2 mg cm
2
in sequence with the oxygen
reduction reaction (ORR) activity measurement. The electrolyte
was saturated with nitrogen and cyclic voltammetry applied
(0.60–0.925 V
RHE
, 100 mV s
1
, 10 000 cycles).
Data analysis. For the determination of the kinetic current
density J
kin
the forward and backward scans of the cyclic
voltammetry of the disc current densities, J, were first averaged
to correct for minimum interfacial capacitance at 1–2 mV s
1
and/or memory effects due to the direction of the scan. Then,
the Koutecky´–Levich equation was used to calculate the kinetic
current density (J
kin
) from the averaged geometric current
density, J, at 0.80 and 0.85 V
RHE
, according to
1
J¼1
Jkin
þ1
Jlim
(2)
Jkin ¼JJlim
Jlim J(3)
where J
lim
is the diffusion-limited current density, measured at
0.20 V
RHE
. The following formula was used for quantifying the
H
2
O
2
production, with N being the collection efficiency of the
ring-disk-electrode:
H2O2%¼2IRing=N
IDisk þIRing=N100 (4)
2.4 Ex situ and in situ evaluation of Fe surface site density
(SD) and turnover frequency
CO cryo chemisorption measurements. CO pulse chemi-
sorption and temperature programmed desorption (TPD) were
performed in a Thermo Scientific TPD/R/O 110 instrument.
A weighed mass of 100 to 150 mg of catalyst was inserted
between two pieces of quartz wool at the bottom of the internal
quartz bulb. Before the measurement, the catalyst was pre-treated
to remove any species strongly adsorbed on the metal-based sites
on the surface, in particular O
2
. Pre-treatment of the catalyst
begins with cleaning of the lines with helium (20 cm
3
min
1
,
30 min) and a consecutive ramp heating from 30 to 600 1C
(10 1Cmin
1
, 15 min hold time at 600 1C)andfollowedby
cooling to room temperature. Pulse chemisorption at 80 1C
(dry ice and acetone) consisted of 10 min line flushing (helium,
20 cm
3
min
1
), followed by six consecutive CO pulses injected by
the automated sample loop (helium as a carrier gas,
20 cm
3
min
1
, loop volume was determined to be 0.341 mL) in
intervals of 25 min.
68,85
Prior to TPD analysis, three consecutive
CO pulses are performed to ensure the saturation of the active
centres with CO. Thereafter TPD (80 1Cto6001C, 10 1Cmin
1
,
hold time 10 min, He as carrier, 20 cm
3
min
1
) with a consecutive
cooling to 30 1C(201min
1
) were performed.
For the catalyst surface areas and masses employed in this
study, the CO cryo adsorption reached saturation after 3 pulses.
The difference in peak areas (DA), corresponding to the
adsorbed molar CO amount, can be calculated from the six
individual baseline-corrected integral pulse areas A
1,sample
to
A
6
,
sample
(formal physical unit of the integrated detector signal
is [mV s]) according to:
DA¼A4;sample þA5;sample þA6;sample
3X
3
k¼1
Ak;sample (5)
Using the injection of a known volume of CO gas, a calibration
constant c
f
E4.14 10
7
mmol per unit area was derived. The
calibration factor was henceforth used for the conversion
between integral peak areas and molar CO amounts. In particular,
the molar amount of adsorbed CO (N
CO,ad
), also referred to as the
molar CO uptake, is the product of c
f
and DA. The mass-based
molaramountofadsorbedCO,n
CO
, was then calculated by
dividing by the mass of the catalyst sample inserted in the quartz
tube of the chemisorption reactor, m
cat
, according to
N
CO,ad
[nmol] = c
f
DA10
6
(6)
nCO nmol mgcat1
¼NCO;ad
mcat
(7)
The mass-based site density with CO chemisorption (SD
mass
(CO))
was then calculated from n
CO
via Avogadro’s constant (N
A
)
according to
SD
mass
(CO) [sites g
cat1
]=n
CO
[nmol mg
cat1
]N
A
[site mol
1
]
10
6
(8)
BET surface area-based SD values, SD
BET
(CO), with units of
[site m
2
], were obtained by dividing SD
mass
(CO) by the mass-
specific surface area, A
BET
[m
2
g
cat1
].
The turnover frequency TOF(CO) was calculated from the
catalyst mass-based kinetic current, J
kin,mass
[A g
cat1
], and the
CO uptake-derived catalyst mass-based surface site density,
SD
mass
(CO),ortheadsorbedmolaruptakeofCO,n
CO
,accordingto
TOF electronsite1s1
¼Jkin;mass NA
SDmass F
¼Jkin;mass NA
NCO;ad mcat1NA106F
¼Jkin;mass
nCO F
(9)
J
kin,mass
was evaluated from the ratio between the mass-transport
corrected geometric current density, J
kin
[mA cm
2
]andthe
Energy & Environmental Science Paper
Open Access Article. Published on 24 June 2020. Downloaded on 8/13/2020 4:01:56 PM.
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