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www.chemelectrochem.org
DOI: 10.1002/celc.201402268

Water oxidation at electrodes modified with Earth-abundant
transition metal catalysts
José Ramón Galán-Mascarós[a,b]

1. Introduction
The development of a real alternative to fossil fuels is one of
the most important challenges that scientists are facing at
present.[1] Without a viable renewable energy source able to
match global energy consumption, it will not be possible to
maintain technological progress and living standards in a
viable environment -- at least as we know it. The retrieval and
use of fossil fuels has a deleterious impact on the environment
that cannot be neglected, and it reaches beyond the
controversial climate change. Toxic enissions and greenhouse
gases, mainly CO2, are changing the atmospheric equilibrium
where human life has evolved in the last hundred thousand
years, with uncertain consequences.
Artificial photosynthesis (Scheme 1),[2] as a process in which
solar energy will be used to reduce a substrate into an energy
rich chemical (H+ to H2, or CO2 to CO or CH3OH, for example),
while oxidizing water to molecular oxygen, is a dream that may
come true if the technology becomes robust enough and costeffective.[ 3 - 11 ] Solar water splitting is not a strictly scientific
problem. Water can be readily split into oxygen and hydrogen
using as power source a commercial photovoltaic cell and a
pair of electrodes. Indeed, there are several large scale
projects working on the combination of renewable energy
sources with commercially available water electrolyzers.[ 12 ]
Furthermore, the wireless artificial leaf [13] already exists, and
works. The problem is to make the process competitive.[ 14 ]
Fossil fuels are so cheap, that the transformational drivingforce cannot be exclusively environmental issues and policymakers. Only a financially attractive green alternative will be
able to promote a true societal change.
In this regard, the search for a robust, efficient and
inexpensive water oxidation catalyst (WOC) appears to be one
of the main challenges. This does not mean that the other
processes of an artificial photosynthesis scheme cannot be

[a]

[b]

Prof. J. R. Galan-Mascaros
Institute of Chemical Research of Catalonia (ICIQ)
Av. Paisos Catalans, 16, E-43007, Tarragona, Spain
E-mail: jrgalan@iciq.es
Prof. J. R. Galan-Mascaros
Catalan Institution for Research and Advanced Studies (ICREA)
Passeig Lluis Companys, 23, E-08010, Barcelona, Spain

“JR” Galán-Mascarós received his
PhD from University of Valencia,
working with Prof. E. Coronado. After
a post-doctoral stay with Prof. Kim R.
Dunbar at Texas A&M University, he
returned to Valencia as a Research
Fellow (Ramon y Cajal program).
Currently, he is ICREA Research
Professor at the Institute of Chemical
Research of Catalonia (ICIQ), where
he joined in 2009. His interests are
centered in the application of
coordination chemistry tools to solve chemical challenges in
such fields such as solar fuels, and materials.

improved.[ 15 ] Particularly the membrane responsible for fast
proton exchange, while efficiently separating products, is
another bottleneck.[ 16 ] However, the lack of a stable
inexpensive water oxidation catalyst able to work under mild
conditions is, arguably, the main reason for the low commercial
impact of water electrolysis, which accounts for less than 2% of
the total worldwide H2 production. Some 96 % of the world
hydrogen production still comes from fossil fuels reforming.
Proton-exchange membrane (PEM) electrolyzers need noble
metals as catalysts, and ultra-pure water as feedstock. Noblemetal-free commercial water electrolyzers depend on highly
alkaline conditions, and work at low efficiencies. In the
absence of these technologies, a photovoltaic plus electrolyzer
scheme is far from becoming economically relevant. This
needs to be changed.

Scheme 1. An artificial photosynthesis scheme.

This review intends to be a broad and critical description of
the state-of-the-art regarding the oxygen evolving reaction
(OER) in water oxidation electrocatalysts (Eq. 1) obtained from
earth-abundant metals -- in other words, those technologically
relevant for energy production. In this direction, it is clear that
the OER in a solar to fuels scheme will be required to occur at
heterogeneous electrodes. Furthermore, mass production will
require abundant material sources. Our analysis of their pros
and cons will be complementary to more general recent
reviews, focused on another aspects of this crucial
challenge.[17-23]
2H2 O → O2 + 4H + + 4e− ; 𝐸 = 1.229 V − 0.059pH

2. Heterogeneous electrocatalysts
2.1. Metal oxides

(1)

www.chemelectrochem.org
Electrocatalytic properties of metal oxides have been studied
for decades.[24-26] Regarding water electrolysis, metal oxides
have been heavily investigated since the 1960s and, indeed,
they form part of commercially available electrolyzers.[27] There
are two main drawbacks to first-row transition metal oxides as
WOCs:
i) Difficult characterization and benchmarking. The activity
of metal oxides depends on many parameters such as
preparative conditions, aging, thermal treatment, precursors,
processing, surface morphology, blending with supports, etc.
Experimental conditions need to be carefully controlled to
obtain consistent results
ii) Strict working conditions. First row transition metal
oxides are excellent WOC in alkali media, but underperform in
neutral conditions, and are inactive in acidic media. Higher
versatility would be desirable.
The lack of exact structural and electronic information on
the active sites make it difficult to model and optimize these
WOCs,[ 28 , 29 ] although some interesting results have been
obtained albeit with some uncertainties.[30-32]
Several reviews have been published from different
perspectives,[33-36] although the literature on the use of first row
transition metal oxo/hydroxides for water oxidation is so vast
that it is an impossible task to classify and put into context all
the results. After decades of research, there are still many
open controversies regarding function, stability and
performance, with contradictory data available. It is also
surprising to find many redundancies, with analogous results
published by different groups, in multiple journals.
A comprehensive review of first row transition metal oxide
catalysts would be a very useful contribution. Nevertheless, the
present article will focus in the main certainties and in some of
the open questions about the state-of-the-art in this field.
2.1.1. Cobalt oxides
Co3O4 is a typical electrocatalyst with many applications.[ 37 ]
Indeed, electrochemical deposition of CoOx from a buffered
solution was first reported over a century ago.[38,39] Along with
other metal oxides, CoOx was commonly used as a cathode for
electrochemical water splitting since the 1970s. [40,41] Currently,
cobalt is the element of choice in WOC research since it has
yielded the richest family of WOCs (and also catalysts for the
hydrogen evolution reaction, HER).[42]
The discovery of a self-repairing mechanism operating at
neutral pH when the CoOx cathodes are used in a phosphate
(Pi) electrolyte renewed the interest in metal oxides as viable
WOCs.[43,44] To highlight the distinct robustness in this media,
these CoOx catalysts were referred to as "CoPi", although the
role of the Pi anions is exclusively structural, in stabilizing the
CoOx catalytic domains to avoid Co leaching.[ 45 ] This CoPi
catalyst can be better described as layered CoOx with
molecular dimensions, stabilized at neutral pH by the PO4
groups.[46] The Pi electrolyte also plays a role by facilitating
rapid proton transfer,[ 47 ] and improving the kinetics of the
process.[48]
CoPi has been extensively studied, including EQCM
formation/stability,[49]electronic structure[50 ] and its similarities
with the PSII cluster.[51,52] The growth mechanism of CoPi films
depends on many parameters, including pH, applied voltage
and electrolyte. Better performance has been claimed when
grown from acetate buffered solution,[53] or using cobaloximes

as cobalt source.[54] Self-repair was confirmed to occur at pH
values > 6.[ 55 ] Mechanistic studies have also been
completed.[48] The fast kinetics of CoPi were empirically
determined to depend on the catalytic rate constant but also on
proton-electron transfer.[56] Thus, the current density increases
with film thickness until reaching saturation as a result of
mutual compensation between catalytic rate and protonelectron transfer. An optimal film thickness of 1.44 mm was
defined in high buffer concentration solution. When
electrodeposited on large surface area electrodes, currents
over 100 mA cm–2 have been reached, working in buffered
natural waters.[57]
Another attractive characteristic of CoPi is its bifunctionality, since it can be reversibly catalyze OER or the
hydrogen evolving reaction (HER) through a self-repairing
equilibrium with Co2+ species in solution.[ 58 ] In this line,
mesoporous Co3O4 has proved to be stable achieving high
current densities both for OER and for the oxygen reduction
reaction (ORR),[59] showing potential to replace noble metalbased catalysts in air-metal batteries.
Bard et al. compared Co3O4 and CoPi using dynamic
potential-pH diagrams.[ 60 ] The overall higher activity of CoPi
was attributed to higher hydration, leading to an increase in the
number of accessible sites.[61] On the other hand, CoPi showed
a significant decrease of current at high current densities.
Although this could be due to oxygen bubble formation, the
authors ruled out this explanation. CoPi did not show any
current decrease at pH 14, reaching over 10 mA cm –2. Thus,
current drop, occurring up to pH 12, is probably due to the selfhealing process, where cobalt ions need to be re-oxidized from
solution to the electrode. This instability, however, was not
observed for analogous Co3O4 electrodes, probably due to
their better mechanical strength and better adhesion to the
support. This demonstrates that electrode preparation is key in
reaching high current densities that can show significant
differences. Substrate support showed no significant influence,
and high phosphate concentrations allowed higher currents.
Since CoOx preparation controls performance, multiple
processing techniques have been tested.[62-65]. Aging also has
a strong effect on activity.[ 66 ] Inelastic X-ray scattering and
absorption support an active CoOOH environment, with
significant presence of Co4+ sites as potential increases in this
non-stoichiometric catalyst.[67] Lithium and lanthanide doping
has also been tested.[68-70]
Nanostructured Co3O4 improves catalytic performance, by
increasing the surface to volume ratio.[71,72] Nanoparticles can
be prepared by pulsed laser ablation, and drop-casted on
pyrolitic graphite,[73] exhibiting higher turnover frequency (TOF).
A single cobalt monolayer has been deposited on gold
electrodes to control mass loading and surface area. Under
these conditions, phosphate in solution still enhanced the
current, four times larger than in the absence of PO43–
electrolyte.[74 ] The activity of nanoparticles of different CoOx
have been shown to be very similar under basic conditions at
identical size and shape, independent of stoichiometry. [75] This
indicates that, during water oxidation, the same active species
evolve in all cases.
One of the most comprehensive studies on the CoOx
catalysts was carried out by Gerken et al.[76] The activity of the
Co2+(aq)/CoOx system was analyzed in terms of pH,
preparation and electrolyte. The catalytic activity follows the
Pourbaix diagram derived from the thermodynamic properties

of CoOx structures, assigning the structure of the CoOx
catalysts as prepared in situ on an electrode to that of layered
double-hydroxides. This layered structure can support
oxidation state change at the cobalt sites, while retaining the
same coordination geometry. Proton transfers would
compensate charge differences. As a function of pH, the
resting state was determined to be Co4+. The authors also
argued that the catalytic activity observed in spinel type Co3O4
arises from structural remodeling of the latter to generate a
layered double-hydroxide surface, the genuine active
structure.[77]
This surface transformation as the origin of the high
catalytic activity is also supported by theoretical DFT
calculations,[28] which confer low activity on the regular sites of
CoO(OH) and CoO2, the thermodynamically stable CoOx
phases under water oxidation conditions. This is also the case,
for example, in the WOC activity found in Co(PO3)2 modified
anodes.[78] Nanocrystals of Co(PO3)2 were grown on Ni foam
electrodes by thermal decomposition of the polymeric
precursor {Co(O2P(OtBu)2]2}n.[ 79 ] These electrodes were
claimed to improve the activity of Co3O4, although surface
differences and the use of Ni foam make this conclusion
difficult to corroborate. Nevertheless, the formation of a COx
surface species as the active site was confirmed, reducing the
role of Co(PO3)2 to that of a catalyst support.
Without a doubt, CoOx is the WOC that has been more
often combined with photoanodes including TiO2, -Fe2O3,
BiVO4, WO3, thallium oxynitride and carbon nitride to promote
enhanced photocatalytic water oxidation.[80-102] The success of
CoOx (or CoPi) has been attributed to its hole-scavenging
capabilities, 103 able to partially suppress surface
recombination.[104,105]
Cobalt oxide activity has also been investigated on
semiconducting supports such as silicon,[ 106 ] amorphous
carbon,[107] graphene,[108] or single-wall carbon nanotubes,[109]
resulting in enhanced OER activity.
2.1.3. Nickel oxides
Although nickel oxides[ 110 ] are used in commercially
available alkaline electrolyzers,[111,112] research wise, their use
has been much more limited when compared with CoOx.[113]
One possible explanation is the need for a highly basic
medium,[114,115] and a complex evolution of the catalyst with
aging.[ 116 ] Starting from the stable α-NiO(OH), it dehydrates
into β-NiO(OH). Under an applied oxidation potential in alkali
media evolves (at least partially) into a mixture β- and γNiO(OH).[113,117 ] However, the presence of another unknown
phase cannot be discarded.[118] Contradictory reports indicate
that NiOx activity can improve or decrease upon aging.[113,119]
The latter has been more commonly accepted with the high
activity assigned to the β-NiO(OH) phase, and its deactivation
allegedly due to the formation of Ni 4+ during OER in alkaline
media at high overpotentials.[113, 120 , 121 ] The only apparent
conclusion is that OER NiOx activity depends heavily on subtle
differences in preparation and working conditions.[110,122-125]
NiOx Tafel behavior is unique, exhibiting a ca. 60
mV/decade slope at low overpotentials, and a larger 120
mV/decade at high overpotentials, indicating a different
mechanism.[ 126 ] This Tafel slope can be reduced through
nanostructuration reaching 42 mV/decade, and very high
current densities, always in alkaline media. [ 127 ] The different

slope indicates a different mechanism, thus it cannot be due
only to nanostructuration. Different active species must be
present in these nanostructures.
Analogous to the CoPi work, NiOx films can also be
electrodeposited in a controllable fashion using a borate
electrolyte.[128,129] These films operate at higher pH (9.2) and
require a pre-electrolysis treatment in order for their efficiency
to increase two orders of magnitude. [130] This has been initially
related to an increase in the oxidation state of Ni centers, from
Ni3+ to Ni4+, at least on the surface of the films, although Ni4+
was thought to be inactive. HER activity has also been
reported for these NiBi amorphous films.[131]
NiOx have also been used as catalysts on photoanodes.[132135 ]
On single-crystal TiO2 electrodes it has shown good
performance through a unique "adaptive" evolution of the
catalyst-semiconductor junction.[ 136 ] Thin NiOx films have
shown electrochromic properties, with increased water
oxidation currents under solar radiation. [ 137 ] A combined
TiO2/NiOx photocatalyst has been processed on a WO3
photoelectrode from a single Ti/Ni precursor.[138]
2.1.4. Iron oxides
Iron oxide films are very attractive as they are the cheapest
possible catalyst (rust).[139] They were studied in detail by Doyle
et al.[ 140 ] including kinetic and mechanistic data.[ 141 , 142 ] In
addition to their catalytic activity, FeOx films could also function
as photoelectrochemical anodes since the hematite band gap
should be good enough to promote water oxidation (2.1
eV).[ 143 , 144 ] However, the mismatch between hole diffusion,
slow interfacial electron transfer, and optical penetration yields
very poor efficiency.[145-147] As a WOC, though, FeOx is less
active than Ni or Co, although Fe content dramatically
improves the activity of NiOx, and maybe CoOx (vide infra).
In general, photoelectrodes can be improved by
nanostructuration.[ 148 , 149 ] For example, remarkable activities
were found by deposition of antimony-doped tin oxide colloids
on FTO surfaces, as transparent support for incorporation of
nanostructured hematite.[ 150 - 152 ] Ti-doped Fe2O3 films have
shown enhanced electrical conductivity, facilitating charge
separation,[ 153 - 155 ] which can be further optimized through
design of the heterojunction in contact with the
electrolyte.[156,157] A reduction pretreatment in the dark (1 M
NaOH) has also yielded enhanced photocurrent. [158] As with the
rest of the metal oxides, enhanced activity has been reported
when FeOx is combined with solid state supports, such as
Nafion, or single-walled carbon nanotube composites, showing
a decrease in overpotential up to 50 mV.[159] Remarkably, when
hematite photoanodes are covered with a thin film of
amorphous FeOOH, the photocurrent doubles, 160 exhibiting a
catalytic activity and long-term stability comparable to the best
CoOx catalysts.[161]
2.1.5. Manganese oxides
Manganese is one of the most studied metals in water
oxidation catalysis,[24, 162 - 164 ] being the element selected by
photosynthetic organisms to split water. [165,166] In addition to the
fascination of mimicking life, Mn is earth-abundant, but also
non-toxic with in comparison with the other more active
catalysts. The identification of the active MnOx forms is
particularly difficult. WOC activity has been attributed to

www.chemelectrochem.org
layered oxides,[ 167 - 169 ] rutile phases,[ 170 ] spinels[ 171 - 173 ] and
perovskites.[ 174 , 175 ] In general, MnOx catalysts exhibit lower
activity than CoOx or NiOx, which has been attributed to the
stable formation of inactive MnO2 domains.[ 176 ] Dau et al.
disclosed a clever solution to avoid this deactivation by
potential cycling during electrodeposition of MnOx films. During
reduction potential sweeping the already formed MnO2 is
modified into a mixed-valency Mn3+/4+ oxide,[177] which remains
highly active even in mildly acidic solutions. However, stability
is still a concern since rapid dissolution cannot be avoided at
neutral pH. Evolution of a nanolayered MnOx structure at
neutral pH structure has been related to enhanced activity.[178]
The presence of Mn3+ species has been described as
crucial to promote water oxidation.[179] Since Mn3+ is unstable
at pH < 9, due to the disproportionation reaction into Mn2+ and
Mn4+, additional strategies are needed to improve the long term
catalytic activity. One possibility is the use of electrolytes that
stabilize the conversion of MnOx into layered structures, i.e.
Li+.[178] Layered MnOx has been interleaved by organic
molecules to tune the catalytic activity.[180] Coordinating amine
groups on the surface of MnO2 stabilize Mn3+, shifting
electrocatalytic activity by ~500 mV.[181,182] Oxygen vacancies
enhance WOC activity.[ 183 ] Interestingly, photocurrents have
been found in birnessite-type MnO2 nanostructures,[169,184] with
a heavy dependence on the selected precursors.
One of the advantages of MnOx electrodes is that they can
even be screen-printed.[ 185 ] Additionally, although it shows
lower catalytic activity, it exhibits a unique self-repairing
mechanism in acidic media, where Mn2+ ions from solution are
re-oxidized at OER potentials.[186] The activity of MnOx WOCs
has been enhanced in hybrid nanocomposites on gold or
CoSe2 surfaces,[187,188] and as Ca2+-doped nanostructures.[189]
2.1.6. Mixed oxides
Metal oxide catalysts have traditionally been prepared by
electrodeposition and this makes difficult to obtain and study
desirable mixed-metal compositions that can improve catalytic
features, as described for the case of NiOx. Preparation from
co-precipitation or alloys does not yield well-defined
materials.[190,191]
Boettcher et al. disclosed a clever approach to obtain a
variety of nanostructured mixed metal oxides by simple
solution casting.[ 192 ] This easy procedure allows for the
preparation of a few nanometer thick oxide thin films of
controlled composition and good reliability. They found
optimized performance in Ni0.9Fe0.1Ox films, exhibiting a 30
mV/decade Tafel slope, and reaching 1 mA cm–2 at just 300
mV overpotential at pH 14. These thin films have the
advantage
of
being
perfectly
compatible
with
photoanodes.[136,193]
The better performance of films containing Ni was
attributed to the in situ structural transformation from oxides to
layered double hydroxides, as already proposed by Gerken et
al.[76] Fe doping improved the performance of NiOx, whereas
Co doping did not produce any synergic effect, when CoO x is
just second best after NiOx.
The improved catalytic activity of NiOx when doped by Fe
had already been described by Corrigan, [194] who observed an
enhancement of the OER activity of NiOx films after several
cycles, correlated with the incorporation of Fe impurities from
the KOH electrolyte. Tafel slopes down to 25 mV/decade were

found. The positive effect of Fe upon the catalytic features of
NiOx were also corroborated by experiments performed by Bell
et al.[ 195 ] Kitchin et al. identified the presence of a mixed
NiO/NiFe2O4 phase at low Fe concentrations as a key
component for the enhanced activity.[196] In a complementary
study, by doping Fe3O4 with Ni, the NiFe2O4 was the most
active phase, with Tafel slopes of 41 mV/decade.[197] Low Fe
doping also improves the electrocatalytic properties of other
oxides, such as Cr2(MO4)3[198,199] The lower overpotentials with
Fe doping were studied over 20 years ago, relating the better
performance to the Fe centers increasing the oxidation
potential of the active sites.[200]
Very recently, experiments in Fe-free KOH solutions
demonstrated that NiOx is not as active as it was supposed,[201]
with a real onset catalytic activity above 400 mV. In the
absence of Fe impurities (coming from the electrolyte), the
OER activity of NiOx films actually decreases upon anodization.
Thus, the increased activity upon aging, observed even in NiBi
films, is suggested to come from Fe incorporation, which
occurs even without an applied potential. Ni(OH)2 uptakes Fe
when suspended in KOH solutions of different purity.
Structurally, the presence of Fe3+ facilitates the formation of a
layered double hydroxide structure. Aging of NiFeO x films did
not show significant changes, suggesting that the OER activity
is not related to defects, but inherent to the Fe/Ni structure.
The lack of OER activity up to very positive potentials in the
absence of Fe3+ also reveals an electronic contribution. Fe
increases the conductivity of the NiOx films, diminishing the
potential drop. More importantly, Fe has an electron
withdrawing effect upon Ni centers, giving more oxidizing
power to the Ni3+/4+ sites. These findings also call into question
most studies (if not all) reported with NiOx (in section 2.1.3) or
even those for other oxides in alkali media, regarding the
composition of the true catalysts.
In the case of photocatalytic currents, "optocatalytic"
efficiency has also been defined taking into account
electrocatalytic performance, but also considering photon
absorption. An optimal thickness of 0.4 nm (2 monolayers) has
been proposed for Ni0.9Fe0.1Ox.[193]
Berlinguette et al. described another technique for the
preparation of amorphous mixed-metal oxides, through the
photochemical
decomposition
of
metal-organic
complexes.[202,203] Using this strategy the authors completed
another systematic study of binary and ternary amorphous
metal oxides. They found that amorphous FeNi4Ox offered the
best performance at pH 13 (0.1 M, KOH solution), in very good
agreement with the previous work by Boettcher.
Surface modified fluoride-doped tin oxide (FTO) with Co[204]
gave a distinct catalytic activity, with very fast TOF (4 s –1 at pH
7.2 and 800 mV overpotential) although current densities were
very low due to the low Co content. This could also maximize
experimental TOF if compared with bulk or even
nanostructured catalysts.
Layered double hydroxides (LDH), supposed to be the
active site of most oxide-based WOCs, are efficient catalysts in
base,[205-207] but also at neutral pH. The latter depends greatly
on preparation and electrode architecture. Although good
current densities are reached at relatively low overpotential,
the very high Tafel slopes observed indicate a kinetic reaction
problem that has been assigned to the formation of an
insulating layer of anions.[208] The highest efficiency has been

assigned to derivatives with higher Fe content in the layered
structure.[209]
Double perovskites have been described as highly active
WOCs and stable in alkaline solution,[210-213] although the need
to include rare earths precludes their extended use.[214,215] For
example, Ba-Sr-Co-Fe perovskite proved to be 10 times faster
than IrOx.[216] Some of these perovskites have shown structural
changes during the water oxidation reaction, going from
crystalline to amorphous, which could be due to conversion of
the local structure from corner sharing octahedral to edge
sharing octahedral, associated with enhanced activity. [217,218 ]
Low-temperature annealing optimizes the performance
promoting less concentration of charged cations, and
increased OH surface coverage.[219] It is worth mentioning that
perovskites are bi-functional catalysts, being active also for
ORR.[220]
Mixed spinels are also active WOCs,[210, 221 - 225 ] including
molybdates[226-229] and Cr-doped ferrites.[230,231] Zinc-doping in
the Co3O4 spinel led to the preparation of nanostructured
hierarchical arrays exhibiting better performance due to their
high porosity.[ 232 ] After a detailed combinatorial survey of
thousands of trimetallic oxides, Stahl et al. have discovered
that the inverse spinel NiFeAlO4 shows activity exceeding all
other Ni/Fe mixed oxides.[ 233 ] NiCo2O4 improved its
electrocatalytic behavior when prepared on a graphene-MnO2
3D framework.[234] Lithiated LixCoO2 spinels (x ≈ 0.5) have
been proposed as bifunctional catalysts able to promote
oxygen reduction and oxygen evolution reaction. [235]

window (~80 mV).[243] So it is apparent that neither kinetics nor
stability makes one candidate significantly better in alkali
media. None of them is active at low pH, though. Beyond the
presence of impurities in the electrolyte, other interesting
trends have been found, for example that smaller alkali cations
lead to lower activity.[244]
Metal-oxide WOCs have also shown enhanced
performance when coupled with conducting substrates, such
as nanocarbon,[108,245247] or gold.[188] Copper nanowires are a
particularly attractive support because they are transparent to
visible light, exhibiting promising features.[248]

2.1.7. Comparison and benchmarking

Table 1. Comparison of some catalytic parameters for heterogeneous WOCs


(mV)


(mV)

NiFeOx

-

-

297

NiOx

-

-

NiOx

>400

NiOx

a

b


(mV)
c

Tafel slope
(mV/decade)

pH

ref.

30

14

192

-

67

14

110

-

>1000

-

14

201

-

-

300

29

14

192

CoOx

-

-

381

42

14

192

NiCoOx

-

-

312

33

14

192

FeOx

345

-

445

40

14

140

FeOx

-

-

405

51

14

192

Fe2O3

<350

-

430

64

14

151

MnOx

320

-

514

49

14

192

Fe3Ni2Ox

The lack of a standardized method to evaluate and
compare the activity and stability of WOCs makes it difficult to
derive final conclusions from the available data. This is
particularly relevant for metal oxides. Table 1 summarizes
some representative catalytic parameters reported for oxides
and other heterogeneous WOCs. In general, most
experimental data indicate that catalytic performance
decreases in the order Ni > Co > Fe > Mn in the MOx family.[236]
For this reason, most alkali electrolyzers work with Ni alloys,
and its superior activity has been rationalized on the basis of
the HO–M bond strength.[ 237 ] Indeed, the experimental data
available shows a good relationship between activity and the
binding energy of HOO* and HO*.[238]
A detailed comparison of the electrocatalytic activity of
multiple metal oxides, taking into account their preparation, has
recently been reported by Doyle and co-workers.[33] The Tafel
slope decreases down to 40 mV/decade with increasing
charge capacity in the case of FeOx suggesting that hydrated
oxide formation is proportional to charge capacity.[ 239 ]
Dehydration of FeOx, which has a characteristic 40 mV/decade
Tafel slope, results in higher slopes above 60
mV/decade.[140,240] The opposite has been found in NiOx films.
However, catalysts transformation due to impurities could be
crucial and has not been studied in detail until very recently,[241]
casting doubt on common belief. Novel in operando
characterization techniques will be very valuable in determining
the genuine performance of metal oxides.[242]
Peters et al. developed a strategy for comparing WOC
activity of all metal oxides under strictly identical conditions,
concluding that all of them exhibit similar activity, reaching
comparable current densities within a very short potential

compound

270

-

-

41

13

233

FeNiOx

211

-

-

24

13

202

Fe2Ni3Ox

190

230

250

34

13

203

NiOx

191

250

280

73

13

203

NiOx

295

-

-

54

13

233

CoFeOxd

397

-

-

13

207

CoOx

<200

-

<250

54

13

76

FeOx

320

380

410

40

13

203

CoOx

210

260

270

42

13

203

CoOx

295

-

-

75

13

233

FeCoOx

181

-

-

33

13

202

FeCoNiOx

191

-

-

31

13

202

Ni2FeAlOx

270

-

-

30

13

233

NiFeMo3Ox

250

-

-

36

13

233

Ni2FeCr2Ox

240

-

-

38

13

233

NiFeGa3Ox

240

-

-

35

13

233

CoSe2

373

-

380

66

13

249

NG-CoSe2

294

-

320

40

13

249

MnOx

<300

-

>1000

60

>11.5

186

FeOOH

300

-

420

11

161

NiBi

300

-

425

9.2

128

59

www.chemelectrochem.org
MnOx

<300

-

>1000

127

8.5-5.5

186

CoOx

<200

-

<300

65

7

76

MnOx

390

-

590

60-80

7

177

MnOx

441

-

600

120

7

250

CoFePBA

291

-

> 600

88

7

303

MnOx

150

-

>1000

-

7

184

CoPi

281

-

410

60

7

43

MnOx

>700

-

>1000

109

7

176

LixMnP2O7

500

-

-

120

7

251

Co(PO3)2

313

-

320

74

6.4

78

MnOx

<300

-

>1000

653

3.5

186

Co2+(1M)

<580

-

600

118

1

76

PBAs appear to be a viable WOC alternative to metal
oxides for promoting water oxidation in artificial photosynthesis
devices. They present competitive kinetics, have better stability
in neutral and acidic media, are obtained from Earth-abundant
metals, and can easily be processed as powders, thin-films or
nanoparticles, with classic coordination chemistry tools.
Furthermore, they are active at neutral or acidic pH, without the
need for additional electrolytes, since their stability resides in
the strong cyanide bridge in the solid state, with no
participation of oxo or hydroxo groups in their skeleton.

[a] onset overpotential. [b] @ 0.5 mA cm–2. [c] @ 1 mA cm–2. [d] LDH.

2.2. Prussian blue derivatives
The catalytic activity of Prussian blue analogs (PBAs) in redox
chemistry has been well documented for many years.[252] PBs
have been used for the preparation of modified electrodes [253262]
for multiple applications: from electrochemical sensors[263271 ]
to electrochromic displays.[ 272 - 274 ] In the field of
electrocatalysis, different derivatives have been used for the
oxidation or reduction of organic materials. [275,276] In oxidation
catalysis,[277,278] PB-modified electrodes have been developed
to detect and quantify a variety of substrates, such as
hydrazine,[ 279 - 283 ] vitamins,[ 284 - 287 ] amino acids,[ 288 - 291 ]
nucleobases,[ 292 ] glucose[ 293 ] and other biologically relevant
molecules. [ 294 - 299 ] They have also been postulated for
electrocontrolled drug delivery.[300]
In the field of energy applications, Prussian white was
reported as an efficient proton reduction catalyst for hydrogen
evolution, exhibiting higher activity than noble metals, such as
Pt.[301] Ruthenium purple also showed HER activity.[302] Despite
being heavily used as oxidation catalysts, the activity of PBAs
for the OER was not disclosed until 2013, when we reported
the electrocatalytic activity of K2xCo(3-x)[Fe(CN)6]2 (CoFePB)[303]
At neutral pH this PBA exhibits kinetics comparable (if not
superior) to those of metal oxides, with unparalleled long term
stability. Electrodes coated with CoFePB maintain a persistent
catalytic activity for weeks at neutral pH and ambient
conditions. At high overpotentials current densities deviate
from Tafel behavior, which has been attributed to poor
connectivity between PBA crystallites and the electrode
surface.
Although kinetics and poisoning studies are still needed to
be completely definitive about the nature of the active sites in
these PBAs, we were able to gather experimental evidences of
several types, which permit us to rule out the participation of
metal oxide nanostructures evolved during the catalysis.
Detailed structural characterization, including surface-sensitive
techniques such as Raman spectroscopy or XPS, confirmed
the absence of any trace of a transition metal oxide in the "as
used" PBAs.[304] In addition, they remain active in acid media,
where CoOx participation cannot be claimed.

2.3. Other heterogeneous catalysts
Apart from oxides and cyanides, no other type of
heterogeneous WOC has been systematically studied,
although there are some promising examples.
Manganese pyrophosphate exhibits superior catalytic
performance to that of the corresponding Mn oxides at neutral
pH,[251] opening novel future perspectives for Mn-based WOCs,
active and robust under mild conditions.
Cobalt MOFs with carboxylate or polypyridyl ligands have
also been claimed to promote electrocatalytic oxygen evolution
from water,[305] where high surface area could be an additional
advantage.
Amorphous cobalt potassium phosphate has been reported
to exhibit slightly lower activity than CoOx in the dark, but also
with a unique photocurrent, which could have further
interest.[306]
CoWO4 has also shown WOC activity in neutral
electrolytes.[ 307 ] Amorphous phases resemble CoPi activity,
whereas crystalline phases, annealed at high temperatures,
show different behavior, with significantly higher Tafel slopes
(–110 mV/decade), indicating a different mechanism and a
different active catalyst.
One of the most interesting candidates is CoSe2. This
material has been used as co-catalyst or promoter for classic
oxide WOCs. Very recently, its genuine activity has been
revealed,[249] offering very promising performance in alkaline
media. When supported on nitrogen-doped graphene, Tafel
slopes as low as 40 mV/decade, and very high current
densities at low overpotentials (10 mA cm–2 @  = 366 mV)
have been achieved.

3. Supported homogeneous electrocatalysts
Homogeneous and heterogeneous catalysts have their
respective intrinsic advantages.[308,309 ] For an electrocatalytic
cycle, heterogeneous conditions are clearly preferred in order
to maximize the efficiency of electron transfer. However,
homogeneous catalysts offer superior processability,
optimization and kinetics.[310 - 312 ] Therefore, immobilization of
homogeneous catalysts onto (high surface area) solid state
supports appears to offer the best of both worlds. At the
nanoscale, multiple strategies are available to process
homogeneous catalysts on electrodes.[313-315]
Stability, though, is a main issue for homogeneous
catalysts, since they are typically not as robust as their
heterogeneous counterparts. Under water oxidation conditions
this is even more crucial, because of the easy evolution of
metal oxides in situ from decomposition of molecular
precursors.[308,316-318] As metal oxides are highly efficient WOCs,

some homogeneous WOCs have been mistakenly reported,
when the activity of minor oxide impurities has been wrongfully
assigned to the homogeneous species. The determination of
the true active site in catalytic reactions is a clear challenge,
particularly in water oxidation due to the extreme working
conditions. Actually, great effort is needed to obtain
experimental evidence to identify the genuine catalyst, beyond
any
reasonable
doubt
-if
indeed
this
is
feasible.[308,309,319320321322323324325
Genuine homogeneous WOCs have been fixed into a
polymeric matrix or onto surfaces to produce modified
electrodes, although in most cases with noble-metal
catalysts.[ 326 - 332 ] In the case of Earth-abundant WOCs, few
examples have been reported.
A plausible strategy to immobilized homogeneous WOC
onto electrodes is the use a Nafion-containing solution to be
drop casted on the electrode surface. This approach has been
used successfully with cobalt cubanes.[333] The incorporation of
these pyridine-based WOCs improves the current and
photocurrent of hematite electrodes, while maintaining the
molecular structure. The same strategy with [Mn4O4L6]+ (L =
diaryl-phosphinate) cubanes[ 334 , 335 ] was not equally reliable.
Although solar-to-hydrogen conversions of up to 1% were
reported,[336] it was later found that the cluster was only a precatalyst for the formation of high surface area MnO x.[167]
Iron complexes of tetra-amido macrocyclic ligands (TAML)
have been mixed with carbon black and immobilized with
Nafion to prepare electrocatalytic anodes. [337] These electrodes
are robust, although with limited selectivity for O2 evolution.
Modified cobalt hangman corroles have been tailored to
optimized oxidation catalysis, conveniently supported on
Nafion films.[ 338 ] Reasonably good activity and remarkable
stability at any pH were achieved through chemical
modification of the corrole framework, demonstrating how
subtle differences can greatly affect performance. This was
analyzed through theoretical modeling. [339]
Insoluble Cs+ salts of high nuclearity Co-containing POMs
are able to promote catalytic water oxidation reaching high
currents at low overpotentials when incorporated into a carbon
paste blend.[340] Remarkably, catalytic activity of these POMcarbon blends is maintained at any pH range, since the
insolubility of these ionic salts is not affected by OH–
concentration. If these insoluble POM salts can be
incorporated into large surface area electrodes, they would
constitute a unique opportunity to prepare anodes able to work
at any pH conditions. [Co4(H2O)2(PW 9O34)2]10–[341,342] has also
been immobilized into a mesoporous carbon nitride matrix,
resulting in improved catalytic activity and durability.[343]
Cobalt porphyrins can be processed in organic solvents to
prepare water-insoluble thin films on FTO working electrodes.
These films have shown very fast catalytic cycles although only
at high pH.[ 344 ] When combined with a perylene derivative,
visible-light-induced dioxygen evolution has been detected.[345]
Water-insoluble cobalt corroles have also been processed
as thin films by simple drop casting of organic solutions. [346]
Bifunctional action has been observed in these films,
promoting water oxidation at neutral or basic pH, and proton
reduction in acidic media. The dynamics of these molecular
species at the water interface are not completely understood
yet, since ligand substitution from the solution-organic film
equilibrium needs to occur.

4. Summary and Outlook
Although many problems remain to solve, in our opinion, one
may derive some reasonable conclusions from the presently
available data.
At this moment, metal oxides are the only WOC solution in
alkaline media. Some of them can also be used in neutral
and/or acidic media, with the help of electrolytes (phosphate)
or favorable disproportionation (as with Mn). However, the
selection of the best performers is still difficult due to the
contradictory data available. This is probably a result of the
different experimental environments in which experiments have
been carried out. Comparison of catalytic parameters (Table 1)
is only meaningful under exact identical working conditions. It
is surprising that better performance is claimed in some
relevant recent publications comparing parameters obtained
even under different conditions of pH. It is well known that the
water oxidation mechanism changes with pH, along with the
thermodynamic potential.[43,76] Thus, such claims should be put
into context.
Careful processing and optimization of mixed metal oxides
has been very valuable, although it is not easy to determine if
the benefit is worth the extra cost. Alkali media electrolyzers
are a very mature technology, and Ni alloys already seem to
perform well enough, when evolving into the highly active
mixed Fe/Ni LDHs under working conditions, through Feuptake from the alkali media.
Cyanides are a promising solution in neutral and acidic
media. Compared with the oxide WOCs, PBs have already
proved to be competitive, with many possibilities still open for
improvement in processing. The main difference between
these two WOC types is their stability under working conditions.
Oxides are greatly affected by pH in water, changing structure,
activity and solubility. PBs remain identical in structure and
function through the entire pH range at which they are stable
(pH < 9), being inert to most electrolytes. Their stability in acid
media could also be exploited.
In acid media the most appealing alternative to noble-metal
oxides appear to be modified electrodes with homogeneous
catalysts. Cobalt seems the element of choice in this approach,
and several promising examples with organic and inorganic
ligands stabilizing the active sites are available.
Although outside the scope of this review, it is worth
mentioning here that metal-free doped-carbon nanostructures
could also become important players in oxygen evolution
electrocatalysis.[347]
With so many examples of available of WOCs, with
excellent Tafel slopes, and more-than-acceptable current
densities, one wonders if the challenge is still of a chemical
nature. In our opinion, the next most crucial step is electrode
design and engineering. Many efforts are being made to find
the newest, fastest, most rugged, precisely designed WOC.
However, how will this be technologically relevant? In any
introduction to a WOC manuscript, including this one, one
encounters a discussion of the future of the energy challenge,
often with a reverie of green chemistry and fuels production.
Unfortunately, later on, what we read about is often a
discussion about a remarkably good WOC, available only in
milligrams, after a huge investment, and with a relatively short
lifetime. Such a WOC is of great interest, to us and to a wide
audience. Fundamental chemistry is not only useful but key in
pushing the limits of knowledge. Nevertheless, we may be in

www.chemelectrochem.org
danger of oversell when the results do not match all the initial
promises from the so-called "introduction section". If
technologically relevant requirements are not taken into
account, seems appropriate not to claim technological
relevance. A closer, honest and open-minded collaboration
between electrochemists, chemists and engineers will be
needed to define viable solutions to the solar fuels production
challenge as society demands.
Another subject that remains open for discussion is the
incorporation of WOCs onto photoanodes.[103,348] This is again
a very interesting chemical and photophysical problem from a
fundamental perspective. Great efforts are being made, and
the scientific challenge is clearly worthy. Nevertheless, on the
technological road to solar fuels production, why would this
approach be preferred to one of using photovoltaics plus an
electrolyzer? The latter should be considered, also from a
basic science perspective. Sometimes, the easier approach
can also be the most effective and convenient.

[17]
[18]
[19]
[20]
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[22]
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[25]
[26]
[27]
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[30]

Acknowledgements
The author gratefully acknowledges helpful assistance and
discussion with Prof. Larry R. Falvello (Universidad de
Zaragoza). The financial support of the EU (ERC Stg grant
279313 - CHEMCOMP), the Spanish MINECO (Grant
CTQ2012-34088, and Severo Ochoa Excellence Accreditation
2014-2018, SEV-2013-0319), and the ICIQ Foundation is also
acknowledged.

[31]
[32]
[33]
[34]
[35]
[36]
[37]

Keywords: water electrolysis • heterogeneous catalysis •
oxygen evolution • oxides • cyanide

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Entry for the Table of Contents

REVIEWS
J. R. Galan-Mascaros*
Page No. – Page No.
Water oxidation at electrodes
modified with earth-abundant
transition metal catalysts
A critical description of the state-of-the art in water oxidation electrocatalysts in
heterogeneous conditions, highlighting their technological relevance towards a solar
fuels production scheme.