of 23
Article
Addressing solar photochemistry durability
with an amorphous nickel antimonate
photoanode
Harvesting solar energy to sustainably synthesize fuels requires the identification
of materials that can operate durably for many years. Inspired by the corrosion
resistance of amorphous oxides, Zhou et al. report an amorphous nickel
antimonate with visible light photoactivity and lower corrosion than analogous
materials, paving a pathway for further technology development.
Lan Zhou, Elizabeth A. Peterson,
KarunK.Rao,...,JeffreyB.
Neaton, Michal Bajdich, John M.
Gregoire
gregoire@caltech.edu
Highlights
High-throughput experiments
enable the discovery of an
amorphous Ni-Sb photoanode
Favorable Pourbaix energetics
and amorphous surface provide
excellent durability
The broad spectral response
stands in contrast to end members
Sb
2
O
5
and NiO
Electronic and chemical structure
characterization pave the way for
future study
Zhou et al., Cell Reports Physical Science
3
,
100959
July 20, 2022
ª
2022 The Author(s).
https://doi.org/10.1016/j.xcrp.2022.100959
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Article
Addressing solar photochemistry durability
with an amorphous nickel antimonate photoanode
Lan Zhou,
1
,
2
Elizabeth A. Peterson,
3
,
8
Karun K. Rao,
4
,
5
Yubing Lu,
6
Xiang Li,
7
,
8
Yungchieh Lai,
1
,
2
Sage R. Bauers,
9
Matthias H. Richter,
1
,
2
Kevin Kan,
1
,
2
Yu Wang,
1
Paul F. Newhouse,
1
Junko Yano,
6
,
8
Jeffrey B. Neaton,
3
,
8
,
10
,
11
Michal Bajdich,
4
and John M. Gregoire
1
,
2
,
12
,
*
SUMMARY
Renewable generation of fuels using solar energy is a promising
technology whose deployment hin
ges on the discovery of materials
with a combination of durability a
nd solar-to-chemical conversion
efficiency that has yet to be demonstrated. Stable operation of pho-
toanodes hasbeen demonstrated with wide-gap semiconductors, as
well as protected visible gap semiconductors. Visible photores-
ponse from electrochemically stable materials is quite rare. In this
paper, we report the high-throughput discovery of an amorphous
Ni-Sb (1:1) oxide photoanode that meets the requirements of oper-
ational stability, visible photoresp
onse, and appreciable photovolt-
age. X-ray absorption character
ization of Ni and Sb establishes a
structural connection to rutile NiSb
2
O
6
, guiding electronic structure
characterization via X-ray photoe
lectron experiments and density
functional theory. This amorphous photoanode opens avenues for
photoelectrode development due to the lack of crystal anisotropy
combined with its operational stability, which mitigates the forma-
tion of an interphase that disrupts the semiconductor-electrolyte
junction.
INTRODUCTION
Solar photoelectrochemical (PEC) generation of chemical fuels using sunlight, H
2
O,
and CO
2
is a promising renewable energy technology whose technology readiness
would be vastly improved by the identification of an efficient and robust photoanode
for the oxygen evolution reaction (OER).
1
,
2
Multiphysics modeling has demonstrated
that a variety of factors underlie the solar-to-chemical (STC) energy conversion effi-
ciency, most centrally the band gaps of the 1, 2, or 3 absorbers.
3
The STC along with
the cost of the source materials and the device durability are the central factors in
determining the price of sustainably generated fuel, for which the US Department
of Energy has established technology targets.
4
In this context, photoanode discov-
ery research entails co-optimization concerning a breadth of performance criteria,
including (1) operational PEC stability, (2) a band gap suitable for efficient utilization
of the solar spectrum, (3) valence band energy (versus vacuum level) sufficiently
negative to catalyze the OER, and (4) con
duction band and Fermi energies suffi-
ciently positive to perform the fuel-forming reaction or more commonly to couple
to a photocathode in a tandem absorber configuration. Commensurate with these
requirements are the energy conversion ef
ficiency considerations of (5) maximizing
external quantum efficiency (EQE), characterized as the fraction of incident photons
that give rise to photoanodic current, and (6) maximizing photovoltage, which is
typically characterized as minimizing the turn-on potential, i.e., the lowest potential
1
Division of Engineering and Applied Science,
California Institute of Technology, Pasadena, CA
91125, USA
2
Liquid Sunlight Alliance, California Institute of
Technology, Pasadena, CA 91125, USA
3
Department of Physics, University of California,
Berkeley, Berkeley, CA 94720, USA
4
SUNCAT Center for Interface Science and
Catalysis and Liquid Sunlight Alliance, SLAC
National Accelerator Laboratory, Menlo Park, CA
94025, USA
5
Department of Chemical Engineering, Stanford
University, Stanford, CA 94305, USA
6
Molecular Biophysics and Integrated
Bioimaging Division, Lawrence Berkeley National
Laboratory, Berkeley, CA 94720, USA
7
SLAC National Accelerator Laboratory, Menlo
Park, CA 94025, USA
8
Liquid Sunlight Alliance, Lawrence Berkeley
National Laboratory, Berkeley, CA 94720, USA
9
Materials Science Center, National Renewable
Energy Laboratory, Golden, CO 80401, USA
10
Materials Sciences Division, Lawrence Berkeley
National Laboratory, Berkeley, CA 94720, USA
11
Kavli Energy NanoSciences Institute, University
of California, Berkeley, Berkeley, CA 94720, USA
12
Lead contact
*Correspondence:
gregoire@caltech.edu
https://doi.org/10.1016/j.xcrp.2022.100959
Cell Reports Physical Science
3
, 100959, July 20, 2022
ª
2022 The Author(s).
This is an open access article under the CC BY license (
http://creativecommons.org/licenses/by/4.0/
).
1
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at which anodic photocurrent is generated.
5–8
These efficiency considerations
encompass the need for the photoanode to
support the transport of photocarriers
while mitigating carrier recombination. Regarding the chemical space of interest
(7) earth-abundant elements are desirable f
or scalability of the resulting technology
and (8) the metal elements (or more generally all elements other than O and H) and
their equilibrium dissolved metals concen
trations during operation must satisfy the
requirements for device-level durability.
We recently discussed this last requirement
in the context of catalyst codesign.
9
For discovery research that is not constrained by
a specific device architecture, requirements (1) and (8) correspond to the demonstra-
tion of a consistent photoactivity with low equilibrium dissolved metals concentra-
tions, i.e., the dissolved metals concentrat
ions at which the Pourbaix decomposition
energy of the operational photoanode surf
ace is zero. This last requirement alone
limits the search space to metal oxides or semiconductors that form a metal oxide
passivation layer. Many metal oxides also meet the band alignment and non-
precious metal requirements, motivating
the community’s concerted effort to iden-
tify metal oxide phases that meet additional
requirements. The recent proliferation
in photoanode discovery has produced a compendium of metal oxides that each
meet several but not all of these criteria.
10–12
Combinatorial synthesis and screening
has been an effective strategy for discovering these photoanodes.
13–16
Hence the
search continues with an increased focu
s on the two criteria that have been least
commonly demonstrated: low turn-on potential and stable operation with low dis-
solved metals concentrations.
The majority of discovered visible light active phases (sub-2.8 eV band-gap energy)
are V-, W-, and Fe-based oxide photoanodes, such as WO
3
,
a
-Fe
2
O
3
,BiVO
4
, several
copper vanadates, CuWO
4
,
a
-SnWO4, FeWO
4
,andBiFeO
3
.
5
,
6
Among these candi-
date photoanodes, performance limitations include insufficient utilization of the so-
lar spectrum, poor charge carrier separation and transport, and poor operational
stability. For example, WO
3
exhibits high operational stability in acidic electrolytes
but suffers from a >2.7 eV band gap that substantially limits its solar conversion ef-
ficiency. Conversely,
a
-Fe
2
O
3
shows a relatively low band-gap energy near 2 eV and
high stability in alkaline media but is limited by its low carrier mobility and short hole
diffusion length. A group of copper vanadates were identified with a 2 eV band gap
and excellent stability in pH 9–10 electrolytes due to surface self-passivation but suf-
fer from poor carrier transport and high turn-on potential.
17–22
CuWO
4
has a band
gap between 2.2 and 2.4 eV and is stable under acidic and oxidizing conditions
but exhibits poor photoactivity due to fast recombination of charge carriers, low
electronic conductivity, and the presence of midgap electronic states on its sur-
face.
23
Recent investigations of
a
-SnWO
4
have demonstrated favorable charge car-
rier transport properties and a suitable band gap of 1.9 eV, but the PEC performance
could be limited by rapid surface passivation by SnO
2
.
24
,
25
BiVO
4
is the most widely
studied photoanode material, and the intensive effort to optimize its performance
has resulted in nearly perfect carrier separation.
5
Its 2.4 eV band gap limits the
STC efficiency
3
and its propensity for corrosion at all values of pH compromises de-
vice durability.
5
The thermodynamic driving force toward cor
rosion is characterized by the Pourbaix
decomposition energy, which is typically calculated at the concentration of a dis-
solved metal at or below 10

5
M.
26
AphotoanodewithanappreciablePourbaix
decomposition energy under operating conditions poses substantial challenges
for attaining operational durability.
27
One strategy is to saturate the electrolyte
with dissolved metals, as was recently demonstrated for BiVO
4
by using a high V
5+
concentration to suppress corrosion.
28
We note in the
discussion
that this strategy
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is often incompatible with device-level dur
ability. Alternatively, the photoanode can
‘‘self-passivate’’ if a Pourbaix-stable phase forms on the surface to mitigate further
corrosion. This interfacial layer plays a critical role in the photoanode efficiency,
but its electronic properties cannot be rationally designed to optimize those prop-
erties because the chemistry and structure are dictated by corrosion processes.
Identifying photoanodes with favorable P
ourbaix decomposition energies is thus a
promising avenue to overcome the hurd
les in metal oxide photoanode perfor-
mance, which we address in this work through combinatorial experiments akin to
the theory-guided tiered screening workflow we reported previously.
11
Metal antimonates, especially transitio
n metal rutile structures of the form MSb
2
O
6
(M = Mn, Co, Ni), have been studied for various applications, such as propane am-
moxidation,
29
gas sensors,
30–32
lithium storage,
33
and visible light photocatalyst, for
degradation of water pollutant.
34
,
35
More recently, using computational Pourbaix
diagrams, these rutile antimonates were identified as Pourbaix-stable in potential
ranges relevant for oxygen electrochemistry in strong acid electrolytes.
36
,
37
Several
transition-metal antimonates have been investigated experimentally as electrocata-
lysts for OER,
38–41
the chlorine evolution reaction,
40
,
42
and oxygen reduction reac-
tion.
43
,
44
This family of materials includes examples of visible light absorption, as
well as activity and stability for OER, motiva
ting our combinatorial investigation of
Ni-Sb oxide library as visible light photoanodes, which, to the best of our knowl-
edge, has not been previously reported. While we successfully synthesized one
target phase, NiSb
2
O
6
, its underwhelming photoactivity hinders its further develop-
ment as a photoanode, despite its favorable Pourbaix energetics. By virtue of the
combinatorial nature of our experiments,
we identified a more promising material
that exhibits similarities to NiSb
2
O
6
but has a Ni:Sb ratio near 1 and is amorphous.
This photoanode is referred to herein as am-NiSbO
z
and its appreciable EQE,
despite its lack of crystallinity, is uniq
ue among known photoanodes. Our combined
experiment and theory characterization of this material demonstrates that it per-
forms quite well concerning the requirements for broad spectral response, opera-
tional stability, and turn-on potential.
RESULTS
High-throughput experimentation
Combinatorial exploration of the Ni-Sb oxide system commenced with the synthesis
of a Ni
x
Sb
1–x
oxide composition spread thin film on F-doped SnO
2
(FTO)/Tec7 glass
substrate, which was subsequently annealed at 610

C in air. A series of compositions
were characterized f
or photoactivity in a three-electrode scanning drop cell (SDC)
(
Figure 1
A) using toggled illumination chronoamperometry (CA) at 1.23 V versus
RHE at 12 combinations of electrolyte and photon energy. A new series of as-synthe-
sized compositions were used for each of the three electrolytes, which included pH
10 and pH 7 electrolytes with 0.01 M sodium
sulfite (SLF) as well as pH 1 electrolyte
with 0.1 M methanol (MET), as shown in
Table S1
. Each composition in each electro-
lyte was measured under 3.2, 2.7, 2.4, and 2.06 eV illumination (
Figure 1
B). Sulfite
and methanol served as sacrificial hole acceptors to characterize photoactivity
without requiring the photoanode surfaces to support the OER. The results are sum-
marized in
Figure 1
CusingtheEQE(
Note S1
) to adjust the measured photocurrent
by the different illumination intensities (
Tables S2
and
S3
). While all Ni
x
Sb
1–x
oxide
compositions exhibited photo activity in at least one of these 12 conditions, the
compositions with
x
< 0.35 were only photoactive under 2.7 and 3.2 eV illumination.
More Ni-rich compositions exhibited photoactivity at low photon energies with sub-
stantial sensitivity to composition, especially at pH 10, where increasing the nickel
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concentration from
x
= 0.35 to 0.5 resulted in a nearly 10-fold increase in EQE. With
increasing
x
above 0.5, the EQE decreases slowly and then more dramatically,
enabling the identification of the
x
= 0.5 composition as a primary target for further
characterization as well as the
x
= 0.33 composition as a valuable point of
comparison.
The selection of these compositions of in
terest is further supported by the phase
behavior of the oxide composition spread. X-ray diffraction (XRD) measurements
in
Figure 2
revealed the presence of two crys
talline phases. The rutile NiSb
2
O
6
struc-
ture exhibits appreciable intensity for
x
%
0.33, the formula unit value, with
decreasing intensity as
x
increases such that the signal is near the detectability limit
at
x
=0.5.When
x
increases above 0.7 a weak NiO signal is observed with intensity
increasing as
x
increases. Collectivel
y the results indicate that the film contains an
X-ray amorphous component for all compositions with
x
> 0.33, which corresponds
to all compositions where photoactivity at 2.06 or 2.4 eV illumination was observed
(
Figure 1
C). This X-ray amorphous phase is in high phase purity from
x
=0.5to0.6,
which corresponds to the compositions where the highest photoactivity was
observedforeachphotonenergy.Inourexperiencewithmetaloxidedeposition,
many compositions are X-ray amorphous as-deposited, but only the most refractory
AB
C
Figure 1. Combinatorial photoelectrochemical characterization of Ni-Sb oxides
(A) Schematic of the scanning droplet cell used in the combinatorial photoelectrochemical
measurements.
(B) Photocurrent density under four illumination sources at the applied potential of 1.23 V versus
RHE for each photoanode thin film sample in the Ni-Sb oxide library in the pH 10 electrolyte with
0.01 M sodium sulfite (SLF10), as shown here for sample Ni
0.52
Sb
0.48
O
z
.
(C) The resulting EQE values calculated from phot
ocurrent density for each illumination source in
SLF10, sulfite-containing pH 7 (SLF7), and methanol-containing pH 1 (MET1) electrolytes shown as a
function of Ni composition,
x
. The corresponding LED energies are indicated by colors shown in (B):
black, blue, green, and orange refer to photon energies of 3.2, 2.7, 2.4, and 2.06 eV, respectively.
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oxides resist crystallization upon annealing at 610

C. We have not found literature
characterization of amorphous oxides in this composition region and herein refer
to this phase as am-NiSbO
z
.
Computational and experiment
al assessment of durability
While the lack of a structural model for am-NiSbO
z
limits the ability to computa-
tionally characterize its formation energy, we estimate the energetics of this
A
B
Figure 2. Structural and phase distribution characterization of the Ni-Sb oxide library
(A) XRD intensity heatmap of as-synthesized Ni
x
Sb
1–x
O
z
binary composition spread annealed at 610

C with
Ni concentration
x
from 0.22 to 0.88 without background subtraction, where the FTO substrate contributes
to the peaks that are observed at all compositions. The increased intensity of the broad features with
2-theta < 40

for
x
> 0.38 is consistent with scattering from an amorphous phase.
(B) The corresponding XRD intensity curves after background subtraction. Below Ni concentration
of 0.32, all the diffraction peaks besides
ones from FTO substrates (ICDD no. 01-070-6153) are in
good agreement with the rutile structure of NiSb
2
O
6
(tetragonal with space group of P4
2
/mnm, and
lattice constants of a = 4.662 A
̊
and c = 3.068 A
̊
. The red stick patterns were modified using rutile
MnSb
2
O
6
phase with ICDD no. 04-011-4962). With increasing Ni concentration
x
,rutileNiSb
2
O
6
intensity drops, resulting in an X-ray a
morphous composition region between
x
= 0.52–0.65, and
NiO (blue sticks, ICDD no. 00-044-1159) appears in the thin film while Ni concentration
x
>0.72.
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phase by first calculating the Ni-Sb-O grand potential phase diagram (
Figure 3
A).
Using an oxygen chemical potential of

5.9 eV (½ O
2
from Materials Project [MP]
plus

1 eV correction for experimental synthesis conditions relative to standard
conditions) and considering all phases in the MP, the only stable ternary oxide
phase occurs at a Ni composition of 0.33 (NiSb
2
O
6
, mp-505271). At the compo-
sition
x
= 0.5, a pyrochlore phase Ni
2
Sb
2
O
7
(mp-1190650) appears at 0.88 eV
per metal atom above the solid-state free energy hull. This phase has been syn-
thesized
45
but is not observed in this work. Since the exact formation energy of
am-NiSbO
z
is unknown, we approximate that this phase lies on the free energy
hull on the coexistence line between NiSb
2
O
6
and NiO (
Figure 3
A), which is
likely the lower limit of the true formation energy and serves as a non-arbitrary
way to place this phase on the phase diagram. Under this approximation, the
Pourbaix energetics of am-NiSbO
z
can be evaluated using the MP Pourbaix mod-
ule, resulting in the Pourbaix diagram and corresponding map of Pourbaix
decomposition energy (
G
pbx
)in
Figure 3
B. These bulk thermodynamics indicate
that am-NiSbO
z
may self-passivate with a NiSb
2
O
6
layer in neutral to acidic
OER conditions. In pH 10 electrolyte, am-NiSbO
z
is thermodynamically stable un-
der OER conditions. The Pourbaix diagram of NiSb
2
O
6
is qualitatively similar
(
Figure S1
,
Note S2
), especially under mild alkaline conditions. We note that, if
the true formation energy of am-NiSbO
z
corresponds to the above-hull energy
in
Figure 3
A, the minimum
G
pbx
of am-NiSbO
z
in
Figure 3
B would become
this same above-hull energy due to the thermodynamic preference for decom-
posing into NiSb
2
O
6
and NiO, although for OER-relevant potentials in pH 10
the predicted lack of corrosion would remain unchanged.
At lower potentials, am-NiSbO
z
and NiSb
2
O
6
are predicted to undergo cathodic
corrosion at potentials of 0.5, 0.35, and 0.3 V versus RHE for pH 1, 7, and 10, respec-
tively (
Figure 3
B). This prediction is consistent with the experimental observation of
AB
Figure 3. Grand potential phase diagram and Pourbaix stability analysis of Ni-Sb oxide system
(A) Calculated formation energy variation with th
e Ni concentration under the chemical potential of
oxygen
m
O2
=

5.9 eV to approximate synthesis conditions. All phases above-hull are labeled with
red squares, while the on-hull tri-rutile NiSb
2
O
6
and two end oxides Sb
2
O
5
and NiO are indicated
with green circles. The experimentally observed phase am-NiSbO
z
(blue circle at
x
=0.5)is
considered to exist on the tie line between NiSb
2
O
6
and NiO.
(B) Computationally predicted Pourbaix diag
ram of Ni-Sb-O using MP data augmented with the
am-NiSbO
z
phase. The ion concentration for metal is fixed at 10

6
M. The Gibbs free energy of am-
NiSbO
z
decomposition with respect to the Pourbaix-
stable phases is superimposed as a heatmap
on the diagram. The thermodynamic stability window of am-NiSbO
z
(yellow outline) extends from
approximately 0.2 to 1.7 V versus RHE and from pH 9 to 13. The two red dashed lines denote 0 and
1.23 V versus RHE, respectively.
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