DE
-
SC0022087
Final Technical Report
DOE
-
CALTECH
-
22087
-
1
202
1
August
1
–
2023 Ju
ly
3
1
Fundamental Science for Enhancing the Durability of Photoelectrodes for
Solar Fuels Production
Nathan S. Lewis, Principal Investigator
California Institute of Technology
1.
Overview
During the award period,
we performed a series of investigations to address a key
bottleneck in solar
-
driven fuel production, which is materials stability. Regardless of the activity
of electrocatalysts or selectivity of products, or efficiency of the light absorbers, if the systems are
not stable then
there is no
ultimate technological relevance for solar fuels production. Degradation
of semiconductor photoelectrodes is a well
-
known, long
-
recognized impediment to
implementation of practical stable solar fuels systems. Recent devel
opments however provide
d
a
n
opportunity to more fully understand, control, and mitigate failure of a variety of photoelectrodes
for solar fuels production.
The Materials Project produced a plethora of
theoretical Pourbaix
diagrams
for photoelectrodes that set stage for both guiding experimental design and validation or
modification by experimental observations.
Protection layers
were
developed that yield
remarkable enhancements in stability of photoelectrodes, but a comprehensive description of
short
-
term and long
-
term failure modes ha
d
not yet been clearly defined in most systems.
Thus,
we
aimed to define the thermodynamics and kinetics of the (electro)chemical processes that
underpin corrosion of semiconductor photoelectrodes
. We a
lso sought to
develop protective
coatings and kinetic control strategies
to extend durability of semiconductor photoelectrodes.
Our
work towards these efforts was disseminated
the form of six peer
-
reviewed publications
, the details
of which are summarized in the sections below.
1
-
6
2.
Investigations of the Stability of GaAs for Photoelectrochemical H
2
Evolution in Acidic
or Alkaline Aqueous Electrolytes
1
The lack of long
-
term durability of light absorbers in aqueous media under extreme
-
pH
conditions continues to be an impediment to the development of efficient, safe, and practical
hydrogen
production via water splitting.
7,8
III
-
V semiconductors are a useful class of materials for
water splitting owing to their band gaps being favorable for pairing with Si to make high
-
efficiency
devices.
9
-
14
To understand the impact of each of these components on the reactivity of the surface, we
systematically investigated the stability of n
-
type GaAs, p
-
type GaAs, InP, and GaInP
photoelectrodes in alkaline and acidic media. Stoichiometric dissolution of GaAs
was observed at
the open
-
circuit potential,
E
oc
, under N
2
in both acidic (1.0 M H
2
SO
4
) and alkaline (1.0 M KOH)
environments.
1
The dissolution rate of GaAs was higher in alkaline solution than in acidic aqueous
solutions (~7 nm/h and ~0.16 nm/h, respectively). Contrary to expectations from the Pourbaix
diagram, minimal As
0
was observed spectroscopically on the electrode surface.
15,16
Under
illumination at negative potentials, p
-
GaAs exhibited very little corrosion, consistent with
operation in a cathodic protection regime. However, the photocurrents were low in both
electrolytes, which is ascribable to a low barrier height between the
Fermi level of p
-
GaAs and
RHE.
DE
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SC0022087
Final Technical Report
DOE
-
CALTECH
-
22087
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1
202
1
August
1
–
2023 Ju
ly
3
1
Pt was then electrodeposited on p
-
GaAs, and XPS indicated the formation of As
0
in either
1.0 M acid or 1.0 M base. In base, the formation of As
0
was accompanied by GaO
x
species,
evidenced by XPS and cross
-
sectional TEM, whereas in acidic conditions Ga preferentially
dissolved (
Fig.
1
).
Figure
1
.
Results of p
-
GaAs/Pt electrodes evaluated in acidic and alkaline electrolytes. (a and b) Comparison of the
J
–
E behavior over time of illuminated p
-
GaAs/Pt electrodes (1 sun) during CA at E = −0.2 V vs. RHE in (a) 1.0 M
H
2
SO
4
(aq) and (b) 1.0 M KOH(aq). (c
–
e) XP spectra in the (c) Ga 2p
3/2
, (d) Ga 3d and (e) As 3d regions for p
-
GaAs/Pt
electrodes before and after CA in (a and b).
Sputtered Pt caused the formation of As
0
without exposure to solution. The surficial As
0
shunted the photoelectrodes and lead to loss of photoactivity over time. This behavior is consistent
with mid
-
gap surface states originating from As providing a recombination pathway that competes
with the HER (
Fig.
2
). In contrast to the behavior with Pt, GaAs coated with a CoP HER
electrocatalyst did not form deleterious As
0
, consistent with the lack of reaction due to the higher
resistivity of the CoP catalyst slowin
g or preventing interfacial reactions of the GaAs.
Thus, although Ga plating can be avoided by judicious control over the operating potential,
the durability of p
-
GaAs photocathodes is limited fundamentally by both dissolution and corrosion
processes in aqueous electrolytes. Furthermore, the performance of
p
-
GaAs photocathodes in
aqueous conditions is limited by interfacial As
0
formation, which leads to shunt pathways that limit
the photovoltage and photocurrent of the resulting photoelectrodes.
DE
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SC0022087
Final Technical Report
DOE
-
CALTECH
-
22087
-
1
202
1
August
1
–
2023 Ju
ly
3
1
Figure
2
.
(a
–
c) Schematic illustration of the changes of surface conditions for (a) etched p
-
GaAs and (b and c) p
-
GaAs/Pt electrodes before and after operation of the HER under illumination at pH = 0 and pH = 14. (d) Galvanic
corrosion of GaAs forming surficial As
0
and GaO
x
species assisted by Pt catalyst. (e) Comparison of band energy
diagram of illuminated p
-
GaAs electrodes with a stoichiometric or a non
-
stoichiometric surface under cathodic bias;
a stoichiometric GaAs surface promotes effective charge se
paration, whereas charge recombination occurs for a non
-
stoichiometric GaAs surface due to surface states (SS).
3.
Investigations
of the Stability of Etched or Platinized p
-
InP(100) Photocathodes for
Solar
-
driven Hydrogen Evolution in Acidic or Alkaline Aqueous Electrolytes
2
We also systematically investigated the corrosion behavior of InP electrodes.
2
The
Pourbaix diagram predicts that InP is unstable in aqueous solutions at all potential and pH
conditions.
16
Mott
-
Schottky analysis for n
-
and p
-
InP in both 1.0 M H
2
SO
4
and 1.0 M KOH
indicated that the flat
-
band potentials for n
-
and p
-
InP (~
-
0.3 V and 0.9 V, respectively) were
essentially invariant with pH. Under the conditions studied, In is predicted to be most stable as In
0
metal. Consistently, in the dark at
E
<
-
0.6 V vs RHE, In plated at the photocathode and constituted
a primary failure mode due to indium’s optical opacity and slow kinetics for the HER. In
0
plated
DE
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SC0022087
Final Technical Report
DOE
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CALTECH
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22087
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1
202
1
August
1
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2023 Ju
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1
under illumination even at potentials positive of RHE due to the photopotential produced by the
illuminated p
-
type semiconductor.
However, addition of Pt to the surface inhibited the formation of In
0
, indicative of a kinetic
effect in which Pt reduced the overpotential for HER, preventing a high surface potential that
would otherwise result in cathodic corrosion (
Fig.
3
).
Figure
3
. Comparison of the band energy diagram and the direction of photogenerated electron (e
−
) flow for (a) an
etched p
-
InP
electrode, (b) a p
-
InP/Pt electrode and (c) a p
-
InP/InOx/Pt electrode under illumination in contact with
1.0 M KOH(aq).
E
app
and
E
surf
represent the applied potential at the back contact and the surface potential of
photogenerated minority carriers (e−), respectively. SS represents the surface states produced by an In
-
rich surface of
p
-
InP,
∼
0.4 eV below the conduction
-
band minimum (CBM). The horizontal lines marked InP/In
0
and RHE represent
the thermodynamic equilibrium redox potentials for the cathodic c
orrosion of p
-
InP and for the HER, respectively,
which are
∼
−
0.31 V vs. RHE and 0 V vs. RHE at both pH 0 and pH 14.
The proposed corrosion reactions for the p
-
InP/Pt electrodes therefore as follows:
Scheme 1
푝퐻
=
0
:
퐼푛푃
(
푠
)
+
3
퐻
+
(
푎푞
)
+
3
푒
−
→
퐼
푛
3
+
(
푎푞
)
+
퐻
3
푃
푂
4
(
푎푞
)
+
4
퐻
2
(
푔
)
Scheme 2
푝퐻
=
14
:
2
퐼푛푃
(
푠
)
+
6
푂
퐻
−
(
푎푞
)
+
5
퐻
2
푂
(
푙
)
→
퐼
푛
2
푂
3
(
푠
)
+
2
푃
푂
4
3
−
(
푎푞
)
+
8
퐻
2
(
푔
)
In acidic solution, dissolved In
3+
species were observed by inductively coupled plasma
-
mass spectrometry (ICP
-
MS), whereas dissolved In
3+
was not observed in alkaline solution,
consistent with expectations that insoluble In
2
O
3
formed, as supported by XPS analysis. In both
cases, the driving force for corrosion of InP is proposed to be the oxidation of P
3
-
coupled with
reduction of either water or protons.
In long
-
term PEC experiments in acid, the InP surface was initially stoichiometric, but after
25 h, the surface became P
-
rich as In
3+
leached from the electrode (
Fig.
4
). In base, the formation
of InO
x
species passivated the surface, leading to rapid degradation in
J
-
E
behavior. Upon
DE
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SC0022087
Final Technical Report
DOE
-
CALTECH
-
22087
-
1
202
1
August
1
–
2023 Ju
ly
3
1
exposure to acid to remove the surface oxide, the p
-
InP/Pt electrodes recovered their
J
-
E
behavior,
further supporting the hypothesis.
Thus, in contrast to GaAs, InP offers an opportunity to avoid mid
-
gap states derived from
elemental As formation. The key reactions that limit the durability of p
-
InP photocathodes are In
0
plating on the surface and InO
x
formation, both of which lead to rapid recombination and parasitic
light absorption and thus over time reduce the photovoltage and photocurrent. In
0
plating can be
impeded kinetically in acidic media by a low
-
overpotential catalyst, whereas in alkaline solution
formation of InO
x
is an impediment to the long
-
term durability of p
-
InP photocathodes.
Figure
4
. (a) Chronoamperometry of a p
-
InP/Pt photoelectrode at 0 V vs. RHE in 1.0 M H
2
SO
4
(aq) under 1 Sun
illumination. (b) Comparison of
J
–
E
behaviors of a p
-
InP/Pt electrode (scan rate: 50 mV s
−
1
) and (c) the corrosion
thickness of InP over time measured during the CA in (a). (d) SEM image and (e and f) XP spectra in the (e) In 3d
and (f) P 2p regions, and (g
–
i) cross
-
sectional TEM images of the p
-
InP/Pt electrode after the 285 h of CA in (a).
4.
Understanding the Stability of Etched or Platinized p‑GaInP Photocathodes for Solar
-
Driven H
2
Evolution
3