This document is the accepted manuscript version of a published work that appeared in final form in Sustainable Energy Fuels,
2018, 2, 589, DOI: 10.1039/c7se00512a
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Cobalt hexacyanoferrate supported on Sb-doped
SnO2 as a non-noble catalyst for oxygen evolution
in acidic medium
Barbara Rodr´guez-Garc´a, A
ı
ı ´lvaro Reyes-Carmona, Ignacio Jimenez-Morales,
´
´
Marta Blasco-Ahicart, Sara Cavaliere, Marc Dupont, Deborah Jones,
Jacques Rozi` re, Jose Ramon Galan-Mascaros and Frederic Jaouen
e
´
´
´
´
´ ´
This study investigates the activity and stability of a Prussian blue analogue (PBA) as an inexpensive anode
catalyst for Polymer Electrolyte Membrane Water Electrolysis (PEMWE). While some PBAs have recently
been reported to catalyze the oxygen evolution reaction (OER) in acidic electrolytes, the present study
focuses on their integration in a PEMWE device. Cobalt hexacyanoferrate nanoparticles were interfaced
with an electrically conductive support that withstands the PEMWE anodic conditions, namely Sb-doped
SnO2. The OER activity of the composite materials was first verified in liquid electrolytes and then in
PEMWE. A promising current density of 50–100 mA cm-2 was reached at 2 V cell voltage. The PBA/Sb–
SnO2 anode was stable up to 1.9 V, but showed more and more instability at higher potentials. Increasing
leaching rates of Sn and Sb observed above 1.9 V suggest that the material instability above 1.9 V can
mainly be assigned to Sb-doped SnO2 conductive support. These results are overall promising for the
use of PBAs as catalytic sites at the anode of PEMWE. The study also identifies the need for more active
PBAs in order to reach a higher current density at a cell voltage of 1.6–1.9 V, a potential range necessary
for an acceptable energy efficiency of the PEMWE.

1 Introduction
Water electrolysis is a promising approach to convert electricity
into hydrogen via water splitting, with energy-conversion efficiency up to 75%.1 The increase of electric power produced from
solar and wind energy will require efficient and affordable
energy storage solutions. Clean hydrogen produced via water
electrolysis is a promising energy vector with key applications
for transportation and electricity storage. Polymer Electrolyte
Membrane Water Electrolysis (PEMWE) is a particularly
appealing electrolysis technology with key advantages of fast
response, production of ultrapure and electrochemicallypressurized H2 and low footprint. However, its current high
cost could limit its widespread application in the near future. Its
operating conditions (acidic medium, ca. 80 o C and high

a

Institute of Chemical Research of Catalonia (ICIQ), The Barcelona Institute of Science
and Technology (BIST), Av. Pa¨
ısos Catalans, 16. Tarragona, E-43007, Spain. E-mail:
jgalan@iciq.es
b

Institute Charles Gerhardt Montpellier (ICGM), UMR 5253, Universit´ de Montpellier,
e
CNRS, ENSCM, F-34095 Montpellier Cedex 5, France. E-mail: frederic.jaouen@
umontpellier.fr
c

ICREA, Pg. Llu´ Companys, 23, E-08010, Barcelona, Spain
ıs

† Electronic supplementary information (ESI) available: SEM and TEM images of
Sb–SnO2 tubes and of the composite catalyst, photograph of the membrane
electrode assembly and graph showing the anodic current of Sb–SnO2 during
the electrochemical corrosion test. See DOI: 10.1039/c7se00512a

electrochemical anode potential) strongly limit the choice of
materials that may be used for cell housing, bipolar plates and
electrode catalysts and supports. The high electrochemical
anode potential during operation sets particularly stringent
requirements on materials. Titanium is currently used as
a bipolar plate material in commercial PEMWE,1,2 while bulk
IrO2–RuO2 supported on Ti prevails as the anode catalyst.3
Iridium is expensive and the rarest metal in Earth's crust, with
a current production of less than 9 tons a year.4 The main
ongoing lines of research for reducing the Ir loading while
maintaining the same performance of PEMWE anodes are (i)
nanostructuring of IrO2,5 (ii) dispersion on high surface area
supports, 6 (iii) formation of solid solutions with less expensive
elements such as Ru and Sn,7 (iv) core–shell approaches,8 and
(v) amorphous IrO2.9 The higher Oxygen Evolution Reaction
(OER) activity of amorphous IrO2 has however recently been
reported to be accompanied by poor stability.9
Substitution of IrO2 and RuO2 by a low-cost alternative for
OER catalysis in acidic medium would thus have a tremendous
impact on the future large-scale deployment of PEMWE technology. While earth-abundant and inexpensive transition metal
oxides such as Ni, Fe and Co catalyze the OER in alkaline
medium, none of these materials that might be active toward
OER in acid are thermodynamically stable in strongly acidic
medium in the potential range of 1.0–1.7 V vs. RHE. While the
literature on non-platinum group metals (PGM) catalysts for

OER in acidic medium is scarce, recent studies have investigated stabilized metal oxides of earth-abundant metals.10–12
Early transition metals from the rst row of 3d elements form
more stable oxides than late transition metals from the same
row, and MnO2 sits at the threshold of required stability at pH
0–1, with expected stability in the range of 1.0–1.6 V vs. RHE.13
The OER activity of MnO2 in acidic medium is however much
lower than that in alkaline medium, and its stability is also
insufficient.10 The Mn leaching rate during OER at 1.8 or 1.9 V
vs. RHE could be reduced by ca. a factor two via the preparation
of co-sputtered Mn and Ti oxides, following density-functionaltheory (DFT) insights on possible stabilization paths.10 Co3O4
has also recently been reinvestigated for OER in acid medium. A
cobalt lm was deposited with electron-beam evaporation on
uoride-doped tin oxide (FTO) and annealed in air to form
a non-smooth but crystalline Co3O4 surface.11 In 0.5 M H2SO4,
the 300 nm thick Co3O4/FTO lm showed a current density of
ca. 8 mA cm-2 at 1.8 V vs. RHE, vs. ca. 1.58 V and 1.55 V for
RuO2/FTO and IrO2/FTO lms at the same current. With various
control experiments, the authors could conclude that the key to
observe a similar high OER activity was a proper interface
between CoOx and the conductive substrate, including a small
thickness of cobalt oxide. The leaching rate of cobalt during
OER was also measured in galvanostatic conditions and reported to be 6 mgCo h-1 at 10 mA cm-2 (ca. 1.8 V vs. RHE
applied). This relatively mild leaching rate at pH <1 is surprising
considering the Pourbaix diagram of cobalt and a previous
investigation of the pH-dependency of active cobalt species for
OER.14 In another attempt to stabilize OER active cobalt sites in
acidic medium, Nocera and co-workers recently investigated
decoupled activity and stability concepts, including CoMnOx
and CoFePbOx lms.12 In that study, the CoFePbOx lm was the
only one resulting in a nearly constant potential (ca. 1.65 V vs.
RHE) over 12 h at a xed current density of 1 mA cm-2 in pH 2.5
solution, extended to 50 h in pH 2. These studies show that
cobalt ions in mixed metal oxides may be relatively stable in
acidic conditions, even at high electrochemical potential.
Cobalt ions coordinated by nitrogen and/or carbon atoms
have also been predicted by DFT to be OER active,15 and pyrolyzed Co–N–C catalysts have shown promising high initial OER
activity, at least in alkaline medium.16 The underlying major
issue with Co(Metal)–N–C catalysts for OER is the recognized
thermodynamic and kinetic instability of amorphous as well as
graphitic carbon at OER potentials, further exacerbated at
a temperature higher than ca. 50 o C.17,18 In this work, we
investigate the OER activity and stability of a Prussian-blue
analogue (PBA) based on Co, Fe and cyano-ligands, free of
weak C–C bonds. Despite the fact that PBAs are a well-known
class of materials with interesting photocatalytic, electrochemical, and electrochromic properties, their catalytic activity
towards OER was not reported until some years ago, when we
reported it.19–21 Among such materials, cobalt hexacyanoferrate
(CoHFe) has hitherto shown the highest OER activity in neutral
and acidic electrolyte22 and, remarkable for precious-metal-free
materials, has displayed strong stability even a er prolonged
immersion in pH 1 solution, or a er OER operation in pH 2
electrolyte.20 While CoHFe was recently proposed as a potential

anode catalyst for PEMWE, it has hitherto not yet been integrated in a Membrane Electrode Assembly (MEA) and characterized in a PEMWE device, in relevant conditions (80 o C,
proton-conducting polymer electrolyte). For MEA integration,
a major drawback of CoHFe, and PBA in general, is their low
electronic conductivity. Hence, such catalysts need to be properly interfaced with an electron-conductive support that also
tolerates the anode operating conditions of PEMWE. While the
preparation of composites of CoHFe or PBA deposited on
carbon allotropes have previously been investigated,23 carbon
allotropes are unsuitable for PEMWE anodes, as mentioned
earlier. Composites of CoHFe and TiO2 were recently prepared
and characterized,24 but not interrogated for their OER activity.
Here, we report the rst interfacing of CoHFe nanoparticles25
with a conductive oxide that withstands the PEMWE anode
conditions, namely antimony-doped tin oxide (ATO). The latter
has recently been investigated as a carbon-free and therefore
corrosion-resistant support for platinum particles at the
cathode of PEM fuel cells26 and also as a stable support for IrO2
at the anode of PEM electrolyzers.27,28 The ATO@CoHFe
composite catalyst shows promising activity and short term
stability for at least 20 h in PEMWE operating conditions (80 o C,
Na on® polymer electrolyte), while the upper limit of ATO
stability itself is shown to be ca. 1.9 V vs. RHE.

2 Experimental
2.1 Synthesis of CoHFe and ATO
CoHFe nanoparticles were prepared by a modi ed literature
protocol.29 Vo et al. reported a correlation between decreasing
CoHFe particle size and increasing formamide content in
formamide–water solutions used for PBA crystal growth. To
prepare nanosized CoHFe, we used undiluted formamide as to
achieve the minimum size accessible by this technique. A
formamide solution of K3Fe(CN)6 (0.03 M, 50 mL, SigmaAldrich) was instantly poured into a formamide solution of
Co(NO3)2 (0.02 M, 50 mL, Sigma-Aldrich) immediately forming
a dark purple dispersion. The solution was kept under stirring
for 2 h and collected by ltration. The solid material was washed
by redispersion in deionized water and centrifuged at 6000 rpm
and decanted. This washing/puri cation process was repeated
three times to collect the product as a ne red powder. Finally,
the CoHFe nanoparticles were dried at 60 o C. Antimony tin
oxide (ATO) loose tubes were prepared by electrospinning, as
previously reported by us.30 In summary, 0.1 g SbCl3 (99%,
Sigma-Aldrich) was added to a solution containing 0.78 g SnCl2
(98%, Sigma-Aldrich) and 0.8 g polyvinylpyrrolidone (PVP, Mw
rv 1 300 000, Sigma-Aldrich) dissolved in a 1.8 : 1 ratio mixture
of ethanol and N,N-dimethylformamide. The obtained mixture
was stirred overnight and electrospun at room temperature on
a rotating drum (Linari Biomedical) at 15 kV, a needle-collector
distance of 10 cm and a ow rate of 0.3 mL h-1. The electrospun
SbCl3–SnCl2/PVP bers were calcined at 600 o C for 4 h in air
with a heating rate of 5 o C min-1 in order to remove the carrier
polymer and to form crystalline ATO loose tubes. The synthesis
described above results in 10 at% Sb in SnO2, a composition

that was previously
conductivity.30

shown

to result

in high

electronic 2.5 Characterization methods

2.2 CoHFe and ATO mixing and ink preparation for liquidelectrolyte electrochemistry
CoHFe nanoparticles and ATO were mixed in deionized water by
a hydrothermal method. ATO (20 mg) and the desired mass of
CoHFe (from 2 to 16 mg, corresponding to 9–44 wt% CoHFe on
ATO) were dispersed in 10 mL of deionized water (pH 6.2).
Then, the solution was heated under autogenous pressure at
100 o C for 20 h. A er this, the solid material was separated from
the solvent by centrifugation, washed with deionized water, and
dried via successive ethanol/ether washings.
The catalyst ink for the anode preparation in RDE experiments was prepared by adding the ATO@CoHFe sample (5 mg)
to a mixture of ethanol (440 mL), H2O (100 mL) and Na on
solution (10 mL, 5 wt% Na on dispersed in lower alcohols,
Sigma-Aldrich). The suspension was sonicated for 30 min. An
ink aliquot (4.7 mL) was deposited on a titanium support electrode of 0.07 cm2 geometric area, for a total ATO@CoHFe
loading of 610 mg cm-2. Blank inks were prepared with pure
ATO or pure CoHFe at the same loadings.
2.3 Ink preparation and catalyst layer preparation for
PEMWE anodes
ATO@17% CoHFe (e.g. obtained from 39.7 mg ATO and 8 mg
CoHFe) was selected on the basis of its OER activity measured in
liquid electrolyte. The catalyst ink for the PEMWE anode was
obtained by mixing ATO@17% CoHFe (37.5 mg), Na on solution (215 mL of 5 wt% Na on solution, Sigma-Aldrich), 3 mL
isopropanol and 1 mL deionized water in a 5 mL vial. This
results in 20 wt% dry Na on to the total mass of solids. The ink
was sonicated for 45 min and entirely sprayed with an aerograph (Badger Air-brush, universal®) on a Te on-glass ber
fabric masked with a second, thicker, Te on-glass ber fabric
´
(Plastiques Elastom`res). The targeted catalyst load was 3 mg
e
cm-2. Both masks were placed on a heating pad at 80 o C. The
exact catalyst loading deposited on the active area was deduced
from the weight change of the Te on sheet before and a er the
spraying and drying.
2.4 MEA preparation
The Na on® 115 membrane (Ion Power inc.) was rst cleaned
with 3% H2O2 solution at 80 o C for 1 h, then with 1 M H2SO4 at
80 o C for 1 h and nally with deionized water at 80 o C for 1 h.
The membrane was dried at 80 o C in an oven and stored. The
anode catalyst layer deposited on Te on glass- ber was transferred onto the membrane by hot pressing. The membrane and
anode/Te on sheet were sandwiched between two metallic
plates and pressed at 8.1 MPa and 80 o C. Then the temperature
was increased to 135 o C and once reached, the pressure
was increased to 15.7 MPa for 5 min. In a second step, the
catalyst coated cathode gas diffusion layer (Sigracet 10BC,
0.5 mgPt cm-2, Baltic Fuel Cells) was hot pressed onto the
anode-membrane assembly in the same conditions as described
above, to complete the MEA fabrication.

Powder X-ray diffraction (XRD) data were collected in Bragg–
Brentano con guration using a PANAlytical X'pert diffractometer, equipped with a hybrid monochromator, operating with
˚
Cu Ka radiation (l ¼ 1.514 A), and using a step size of 0.1o
within the 2q domain starting from 10o angle. Fourier Transform Infra-Red (FT-IR) measurements were carried out on
a Thermo Nicolet iS50 spectrometer equipped with a DTGS
detector, KBr beam splitter at 4 cm-1 resolution and were
collected in absorbance mode on pellets of KBr and of the
material of interest. Scanning Electron Microscopy (SEM)
images were obtained on CoHFe nanoparticles with an environmental microscope JEOL-JMS6400. ATO loose tubes were
observed by eld emission-scanning electron microscopy (FESEM) using a Hitachi S-4800 microscope and by transmission
electron microscopy (TEM) using a JEOL 1200 EXII microscope
operating at 120 kV equipped with a CCD camera SIS Olympus
Quemesa. For TEM analysis, the samples were suspended in
ethanol and sonicated before deposition onto carbon-coated
copper grids. Nitrogen sorption measurements on ATO tubes
were conducted at 77 K with a Micromeritics ASAP 2020 apparatus a er outgassing overnight under vacuum (10-5 Torr) at
200 o C. The speci c surface area was calculated using the Brunauer Emmett Teller (BET) equation. The elemental composition of ATO was analyzed with X-ray uorescence, previously
calibrated for an appropriate range of Sb and Sn contents.
2.6 Electrochemical characterization in liquid electrolyte
For OER activity measurements on ATO@CoHFe composites,
cyclic voltammetry experiments were performed at room
temperature with a RDE setup, cycling the potential from 1 V vs.
RHE to 2.2 V vs. RHE at 50 mV s-1 with a Bio-Logic SP-150
Potentiostat. The reference electrode was an Ag/AgCl (3.5 M
KCl) electrode, the counter electrode a Pt mesh (2 cm2
geometric area) and the working electrode support was a titanium rotating disk electrode (RDE) with 0.07 cm2 geometric
area and with a rotating speed of 1600 rpm. All potentials were
converted to the reversible hydrogen electrode (RHE) scale
using the equation E (V vs. RHE) ¼ E (V vs. Ag/AgCl) + 0.264 V.
The electrolyte was an aqueous 0.1 M H2SO4 solution. The
faradaic current density was obtained from the total current
density by subtracting the plateau of current density observed at
1.2 V vs. RHE on the positive-going scan.
To investigate the electrochemical corrosion of ATO at
anodic potentials, chronoamperometry was carried out in
a three-electrode cell comprising a titanium foil coated with an
ATO ink as a working electrode (200 mm thick, geometric area 4
cm2), a RHE reference electrode and a platinum wire counter
electrode. An ATO loading of 2.8 mg cm-2 was deposited from
an ink prepared by mixing 11.2 mg of ATO support with 55 mL of
5% Na on solution, 1800 mL of ethanol and 600 mL of deionized
water. A er 10 min of sonication, the ink was sprayed using an
aerograph onto the Ti foil, the foil itself being placed on
a heating plate at 80 o C. Chronoamperometry measurements
were performed with a potentiostat (Pine Instruments AFCBP1)
by holding the working electrode for 4 h at a constant potential

(OCP, 1.9, 2.0 or 2.2 V vs. RHE) in 0.5 M H2SO4 at 80 o C. In order frequencies from 20 KHz to 50 mHz with a wave amplitude of
to quantify the amount of metal leached from ATO, 10 mL of 10% of the steady-state current.
electrolyte were withdrawn at the end of the potentiostatic
control and analyzed using an Inductively-Coupled Plasma 3
Results and discussion
Mass Spectrometer (Agilent ICP-MS 7900).
3.1 Structure and morphology of CoHFe nanoparticles, ATO
tubes and ATO@17% CoHFe
2.7 PEMWE measurements
The CoHFe nanoparticles were rst characterized by TEM.
MEAs were assembled in a square 6.25 cm2 active-area single
cell, having Ti and Au-coated stainless steel anode and cathode
end plates, respectively. A Ti-sintered mesh (Bekaert, ST/Ti/20/
450/70, 0.3 mm) gas diffusion layer was used on the anode.
Deionized water (18 MOhm) was pumped through the anode
with a ow rate of 200 mL h-1. The cathode was previously
ooded with deionized water. The MEA was assembled in the
cell with
uorinated ethylene propylene gaskets to give
a compression of 43 ± 2% and assembled with a torque of
10 Nm. To characterize the cell, a Bio-Logic SP-150 Potentiostat
with a 20 A booster was used. The cell and MEA were conditioned at 80 o C for 24 hours at open circuit voltage (OCP) to
ensure full hydration of the membrane. Polarization curves with
the ATO@CoHFe anode were recorded by stepping the current
density from 0 to 100 mA cm-2 keeping the current density
constant until the variation of the steady-state potential was less
than 1 mV min-1. Electrochemical impedance spectroscopy
(EIS) was performed in galvanostatic mode, using scanning

Fig. 1A shows a few particles with the typical anisotropic shape
with characteristic length and width of ca. 100 and 25–50 nm,
respectively. This is a smaller size than the CoHFe cubic particles of ca. 250 nm edge previously investigated by us that were
electrochemically prepared.19 The smallest particle size of ca.
25 nm is in agreement with the original report of the preparation of CoHFe nanoparticles in formamide.29 The XRD pattern
in the 2q range 10–40o (Fig. 1B) shows diffraction lines assigned
to the (111), (200), (220), (400) and (420) crystal planes of the
face-centered cubic PBA crystal structure.31 The ATO
morphology was evaluated by electron microscopies. The FESEM micrograph shown in Fig. 1C indicates ber-in-tube
structures with outer diameter of ca. 170 nm. The outer
surface of the loose tubes is homogeneously covered with grains
having an average diameter of 15 nm (Fig. 1C and ESI Fig. S1†).
The XRD pattern of the ATO tubes (Fig. 1D) displays diffraction
peaks corresponding to the rutile SnO2 structure. The SnO2
crystallite size estimated using the Scherrer equation was

Characterisation of pristine CoHFe and ATO. (A) TEM micrograph of CoHFe nanoparticles, (B) XRD of CoHFe, (C) SEM micrograph of ATO
tubes (some fibre-in-tube structures are seen for tubes that are oriented nearly normal to the image plane, as for example on the lower right
handside corner), (D) XRD pattern of ATO tubes. Indexes for the standard rutile structure of SnO2 are indicated.
Fig. 1

16.5 nm, in agreement with the grain size observed in the SEM
micrograph (ESI Fig. S1†). The nitrogen sorption isotherm of
ATO was typical of a mesoporous material and its speci c
surface area obtained by the BET equation was 35 m2 g-1. The
enhanced surface area compared to SnO2 materials with lower
Sb content is attributed to a smaller grain size.32
As reported later, ATO@17% CoHFe leads to the highest OER
activity. Its morphology and structure were thus investigated in
detail. Fig. 2A shows a high magni cation TEM image, revealing
the partial coverage of the outer surface of ATO tubes by CoHFe
particles (light grey). Due to the transparency of CoHFe in TEM,
CoHFe particles located on top of the ATO tube may not be all
visible. Also, any presence of CoHFe inside the ATO tubes
cannot be assessed. While some CoHFe particles seem to be
loosely connected to the ATO tubes (ESI Fig. S2†), FT-IR analyses
presented later suggest a modi ed electronic state for all CoHFe
particles in ATO@17% CoHFe relative to pure CoHFe, implying
that most of the CoHFe particles are electrically connected to
the ATO support.
The XRD pattern of ATO@17% CoHFe (red curve in Fig. 2C)
reveals the most intense peaks of the SnO2 rutile structure (2q ¼
27, 34, 38o , see Fig. 1D), with very weak peaks assigned to CoHFe
(see Fig. 1B). These peaks appear at higher angles than for pure
CoHFe, indicating a decreased interplanar distance. This is
possibly due to a different charge distribution in the metal
centers. A charge transfer mechanism between CoHFe and TiO2

anatase was recently reported by Berrettoni and co-workers.33,34
The study concluded that TiO2 stabilizes the low spin (LS)
electronic con guration CoIII(LS)–N–C–FeII(LS) vs. the high spin
(HS) con guration CoII(HS)–N–C–FeIII(LS), typically observed in
the bulk. This results in a shortened Co–N bond. This is in good
˚
agreement with the decreased unit cell parameter from 10.34 A
˚
(CoHFe particles) to 10.07 A (TiO2@CoHFe composite) observed
by Berrettoni.34 Decrease of the unit cell dimension as a result of
a change in the Co(II)/Co(III) oxidation state is well known for
pure CoHFe compounds, with the most stable con guration
being controlled by stoichiometry, counter-cations, solvent or
temperature.35
In order to further investigate whether such charge transfer
also occurs for ATO@17% CoHFe, the electronic state and
coordination of Fe and Co ions were investigated by FT-IR
spectroscopy (Fig. 2D). The spectrum in the 2000–2300 cm-1
range is characteristic for vibration bands of the cyanide triple
bond, that are highly sensitive to the nature, spin and oxidation
states of linked metal cations.36 In the infrared spectrum of
bulk CoHFe (blue curve in Fig. 2C), two bands centered at 2170
and 2140 cm-1 are observed, as well as a shoulder around
2080–2110 cm-1. The vibration mode at 2170 cm-1 can be
assigned to Co(II)–N–C–Fe(III) bridges (2156–2166 cm-1 reported in ref. 36 and 37) and Co(III)–N–C–Fe(III) bridges
(2190 cm-1, ref. 37). The band at 2140 cm-1 can be assigned to
Co(III)–N–C–Fe(II) bridges (2120–2130 cm-1, ref. 36 and 37). The

Characterisation of ATO@17% CoHFe before and after PEMWE operation. (A) TEM micrograph of ATO@17% CoHFe, (B) TEM micrograph of
ATO@17% CoHFe after 2 h at 2 V in PEMWE, (C) XRD of ATO@17% CoHFe anode before and after PEMWE for 2 h at 2 V, (D) FT-IR spectra of
CoHFe, ATO@17% CoHFe, and an anode comprising ATO@17% CoHFe after PEMWE for 2 h at 2 V.
Fig. 2

shoulder around 2080–2110 cm-1 is attributed to Co(II)–N–C–
Fe(II) bridges or to non-bridging cyano groups on the surface or
at defect sites.35,37,38 The most intense infrared peaks of ATO are
observed at 1613, 1396, 1130 and 701 cm-1 (assigned to SnO2),
while three weak peaks are observed for ATO at 1950, 2120 and
2270 cm-1 (ESI Fig. S3†).39,40 For ATO@CoHFe (red curve in
Fig. 2D), a single band is observed at 2100 cm-1, overlapping
with the shoulder seen in the spectrum of CoHFe alone. This
clear spectral change unambiguously demonstrates a longrange electronic effect of ATO on CoHFe, since they were
separately synthesized before being mixed. The same type of
spectral modi cation was previously observed for TiO2@
CoHFe composites. While two vibration bands at 2157 and
2119 cm-1 were visible for pure CoHFe, a single band at 2119–
2133 cm-1 (depending on the ratio CoHFe : TiO2) was visible
for TiO2@CoHFe composites.24 From FT-IR but also X-ray
photoelectron and X-ray absorption spectroscopies, the
authors concluded that the composites with 1 : 1 and 10 : 1
molar ratios of TiO2 : CoHFe do not contain ferric species. In
the FT-IR spectrum of ATO@17% CoHFe, we observe an even
stronger shi to lower frequencies relative to CoHFe. The
single band observed for ATO@17% CoHFe located at the lower
wave number of 2100 cm-1 possibly corresponds to the vibration mode of Co(II)–N–C–Fe(II). This may be related to the
higher energy of formation of TiO2 vs. SnO2 (higher stability of
TiO2). This red shi from ca. 2150 to 2100 cm-1 of the FT-IR
spectra from CoHFe to ATO@17% CoHFe perfectly matches
the spectro-electrochemical red shi observed when electrochemically cycling CoHFe lms from 0.9 to 0.0 V vs. SCE
(converting CoII 3[FeIII(CN)6]2 into K2CoII3[FeII(CN)6]2).37
In
summary, XRD and FT-IR show that CoHFe nanoparticles were
not denatured by the hydrothermal treatment used in preparation of the supported catalyst (XRD pattern typical for
CoHFe), that the unit cell parameter is slightly decreased and
the FT-IR is red-shi ed, both changes being explained by
different Co–N–C–Fe electronic states existing in CoHFe, with
respect to a single dominant Co(II)–N–C–Fe(II) state in
ATO@17% CoHFe.

3.2 Electrocatalytic OER activity in sulfuric acid electrolyte
We investigated the OER activity of ATO@CoHFe materials by
RDE experiments. First we optimized the ATO/CoHFe ratio. As
seen in Fig. 3A, ATO@17% CoHFe results in the highest OER
activity, for a 610 mg cm-2 loading.
Also a er normalization by the mass of CoHFe, the catalyst
ATO@17% CoHFe results in the highest mass activity, at par
with that of ATO@9% CoHFe (inset of Fig. 3A). Higher CoHFe
contents are detrimental, probably due to poor electronic
conductivity, because of the insulating character of CoHFe. This
results in loss of electron percolation through the active layer.
Fig. 3A also shows the negligible OER activity for both ATO
(black curve) and CoHFe alone (not shown, because it almost
perfectly superimposed on the ATO curve), highlighting the
synergy between OER catalyst and ATO support.
We performed cyclic voltammetry on ATO@17% CoHFe to
investigate the presence of redox peaks and the capacitive

Fig. 3 (A) OER activity in acidic electrolyte of ATO@x% CoHFe and

pure ATO. Linear scan voltammetry with a scan rate 50 mV s-1,
1600 rpm, 0.1 M H2SO4. The inset shows the faradaic current density
after normalization by the mass of CoHFe only. Total catalyst loading
610 mg cm-2. (B) Cyclic voltammetry for CoHFe, ATO and ATO@17%
CoHFe. Scan rate 20 mV s-1, 0.1 M H2SO4. A titanium RDE tip was used
for all experiments.

current. In Fig. 3B, we observe a single apparent pair of redox
process, with a reduction peak at ca. 0.55 V vs. RHE and an
oxidation peak at ca. 0.75 V vs. RHE. This redox pair is also
observed on unsupported CoHFe, and can be assigned to the
Fe(III)/Fe(II) redox switch.41 The position of the peaks, though, is
shi ed to lower potentials for ATO@17% CoHFe, indicating the
electronic interaction between catalyst and support. Possibly,
the shoulders at 0.4–0.6 V vs. RHE (oxidation peak) and 0.7–
0.9 V vs. RHE (reduction peak) belong to a second pair of redox
peaks, as observed for CoHFe in ref. 41 (and references therein).
Both pairs of redox peaks where assigned to Fe(III)/Fe(II) redox
switch, with a redox position slightly depending on the countercation and exact CoHFe stoichiometry. Fig. 3B also shows that
the electrochemical signal for ATO@17% CoHFe, to a rst
approximation, results from the superimposition of the capacitive current of ATO and of the characteristic CoHFe redox
peaks. The fact that the double layer current of ATO@17%
CoHFe is as high as that of ATO alone (compare the regions
0.25–0.40 V and 0.95–1.15 V vs. RHE, where CoHFe redox peaks'
contribution is negligible) indicates that the ATO surface is
mostly unblocked by CoHFe nanoparticles. FT-IR analysis
suggest that all CoHFe particles are in electric contact with ATO,
implying that the CoHFe content in ATO@17% CoHFe is
sufficiently low to avoid the formation of a thick layer of CoHFe
particles on ATO tubes.

3.3 Investigation of ATO@17% CoHFe in PEMWE
Na on content lower than 20 wt% was insufficient to allow
proper decal transfer of the sprayed catalyst layer onto the
Na on membrane, while Na on contents higher than 30 wt%
lead to overly high cell ohmic resistance during break-in of the
PEMWE, as determined by EIS. Following this initial screening,
ATO@17% CoHFe was mixed with 20 wt% Na on (see Experimental section), sprayed and transferred onto a Na on® 115
membrane at a catalyst loading of ca. 3 mg cm-2 (ESI Fig. S4†).
Following the break-in procedure, the initial polarization curve
was recorded (Fig. 4A). The onset potential is ca. 1.75 V, in line
with the RDE results (Fig. 3A). The low ohmic resistance (high
frequency intercept in the Nyquist plot, Fig. 4B) is comparable
to the ohmic-resistance value expected for the Na on
membrane only. This con rms that the ATO@17% CoHFe
composite with 20 wt% Na on leads to an electrode with
percolation for electrons.
To investigate the stability, the cell voltage was xed at 2 V for
22 h, and the current density recorded (Fig. 4C). A decrease of
only 10% of the current density was observed during this time,
attesting to the relative stability of the anode catalyst in acid
medium even at high electrochemical potential. To further
explore the potential-dependence of this apparent instability,
we gradually increased the cell potential from 1.5 to 2 V,
maintaining the new potential a er each step for 4 h, and
completing the experiment with a 2.2 V potentiostatic control
for 20 h (Fig. 4D). The current density was found constant (or
even slowly increasing) at each step up to 1.9 V. A small decrease

was recorded during the 4 h step at 2 V, and a strong decrease
was observed during the nal step at 2.2 V.
This apparent performance loss at high potentials was
surprising. CoHFe has been reported as stable in acidic media,
and at applied potential differences well above 2 V.20 On the
other hand, ATO has been reported to be stable in anodic
conditions in PEMWE up to 1.8 V. However, its long term
stability in the range of 1.9–2.2 V has not been studied yet, to the
best of our knowledge. While instability of ATO has been reported a er galvanostatic control leading to potentials as high
as 3.5 V,42,43 such high potentials would mean very low energy
efficiency of water electrolysis. In order to better assign the
origin of this instability at potentials above 1.9 V but up to 2.2 V
maximum, we investigated the fate of ATO and of ATO@CoHFe
anodes a er electrolysis.
To further investigate the stability of ATO to high potential in
acid medium, an ATO-coated Ti electrode was potentiostatically
controlled at OCP, 1.9, 2.0 or 2.2 V vs. RHE for 4 h. Signi cant
leaching of Sn and Sb was detected by ICP-MS at 1.9 and 2.0 V vs.
RHE (Table 1) and massive leaching at 2.2 V vs. RHE. These
results suggest that the main degradation mechanism leading
to the small decrease in current density observed at 2 V in
PEMWE with the ATO@17% CoHFe anode and the much faster
decrease observed at 2.2 V is the electrochemical corrosion of
the ATO support.
The leaching from ATO is clearly associated with the applied
electrochemical potential (see Table 1, no leaching at OCP). The
preferential loss of Sb (Table 1) must have as a consequence
a reduction in the electronic conductivity of ATO, especially at

(A) PEMWE polarization curves before (filled circles) and after the 22 h control at 2 V (empty triangles), (B) EIS Nyquist plot recorded at
50 mA cm-2 before the 22 h potentiostatic control at 2 V, (C) current vs. time during the potentiostatic control at 2 V, (D) series of potentiostatic
controls for 4 h from 1.5 to 2.0 V and ended with 20 h at 2.2 V. (A–C) recorded with a first MEA and (D) recorded on a second MEA. All experiments
were performed at 80 o C, anode ATO@17% CoHFe (3 mg cm-2), cathode Pt/C (0.5 mgPt cm-2) and Nafion® 115 membrane.
Fig. 4

Relative amounts of Sn and Sb leached during ATO electrochemical corrosion testing (4 hours at indicated potential, 80 o C, 0.5 M
H2SO4) as measured by ICP-MS and resulting calculated Sb content in
the corroded ATO material
Table 1

Potential/V
vs. RHE

Sn loss/%

Sb loss/%

Remaining Sb in
corroded material/at%

OCP
1.9
2.0
2.2

<10-5
0.14
0.16
22.6

<10-4
7.7
8.5
52.7

10.0
9.5
9.4
6.0

the surface. Thus, disrupting the conductive network of the
anode layer is detrimental to its performance, even if CoHFe
itself is stable in such conditions. Sn is already fully oxidized in
ATO, and therefore cannot lead to an oxidation current (ESI
Fig. S5†). It has been recently reported that for OER-active metal
oxides such as RuO2 in acidic medium and MnOx in alkaline
electrolyte, electrochemically-induced degradation cannot, in
practice, be detected by recording the anodic dissolution
current that is orders of magnitude lower than the OER
current.44
In contrast, ICP-MS can quantitatively determine very small
amounts of dissolved elements, and allows off-line and online
monitor of the dissolution of metals as a function of the electrochemical potential.45,46
The end of test ATO@17% CoHFe anode was also characterized (a er a potentiostatic control for 2 h at 2 V) by TEM, XRD
and FT-IR spectroscopy. The TEM image a er PEMWE shows
smaller ATO grain size on the surface (Fig. 2B vs. Fig. 2A).
However, similar width of the SnO2 peaks in XRD (Fig. 2C,
brown and red curves) suggests that either the grain size in the
bulk of ATO bres did not change, or that the small grains in
Fig. 2B do not correspond to the domain sizes that coherently
diffract. The low intensity diffraction lines given by the CoHFe
component in the XRD pattern is less marked a er PEMWE
(Fig. 2C, brown curve) and no peak related to CoHFe is distinct.
It is therefore difficult to discuss from this result possible
electronic or structural changes. FT-IR (Fig. 2D) of the used ATO
reveals two bands. The original one located at 2100 cm-1,
assigned to Co(II)–N–C–Fe(II), that con rms the robustness of
the OER catalysts, preserving its initial state. There is also a new
band at ca. 2060 cm-1, which has been associated with the same
Co(II)–N–C–Fe(II) pairs, with higher occupancy of cations in
tetrahedral sites.47 This suggests that protons may be integrated
in these sites during OER, promoting the appearance of the new
IR band. Still, these bands can only be assigned to bridging CN
moieties, and are experimental evidence that the PBA structure
is conserved, subject to amorphization and partial restructuring. Operando characterization is needed to further investigate
evolution in Fe and Co coordination during OER.

Conclusions
The OER activity of the optimized ATO@CoHFe composite was
ca. 6 mA cm-2 at 1.8 V in PEMWE conditions for a CoHFe

loading of 0.5 mg cm-2. This compares favorably with another
non-PGM OER catalyst based on Ti-stabilized MnO2 that
showed ca. 3 mA cm-2 at 1.8 V in 0.05 M H2SO4 (ref. 10) and with
an electrodeposited lm of CoFePbOx that showed ca. 1 mA
cm-2 at 1.8 V vs. RHE (corresponding to 1.65 V vs. NHE at pH
2.5).12 Recently reported CoOx lms and CoOx nanoparticles
physically deposited as metallic cobalt on FTO and calcined in
air reached 6–20 mA cm-2 at 1.8 V vs. RHE in 0.5 M H2SO4.11
Improved coverage of CoHFe on ATO or other conducting
nanostructured oxides could lead to further improvement in
OER activity. The direct growth of CoHFe on the support could
be a promising approach, while detailed end of test and operando characterization is required to better understand the
coordination of cobalt in CoHFe when it catalyzes the OER.
While the OER activity of such non-PGM catalysts is so far too
low to rival IrO2 for utilization in PEMWE operating at high
current density, their application in photo-electrochemical
water splitting devices with maximum photocurrent of ca. 10–
14 mA cm-2 can be foreseen.48 A PEM-based electrolyzer with
a PGM-free anode can thus be designed, the acidic polymer
electrolyte being more favorable than alkaline electrolytes for
the hydrogen evolution reaction at the cathode, on both PGM
and non-PGM catalysts.49,50

Conflicts of interest
The authors declare no con ict of interest.

Acknowledgements
Funding from the European Research Council under the
European Union's Seventh Framework Programme (FP/20072013)/ERC Grant Agreement No. 306682 SPINAM and under
the H2020 Programme/ERC StG Grant Agreement No. 279313
CHEMCOMP is gratefully acknowledged. This work was also
partially supported by the Spanish Ministerio de Econom´ y
ıa
Competitividad (MINECO) through project CTQ2015-71287-R,
and the Severo Ochoa Excellence Accreditation 2014-2018 SEV2013-0319; and the Generalitat de Catalunya (2014-SGR-797,
and the CERCA program). ARC thanks the Marie Curie
COFUND Action from the European Commission for conancing his postdoctoral fellowship. MBA thanks the
Generalitat Catalana (AGAUR) for a predoctoral fellowship.

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