Near-complete suppression of oxygen evolution for photoelectrochemical H2O oxidative H2O2 synthesis

Article abstract of DOI:10.1021/jacs.9b13410

Solar energy-assisted water oxidative hydrogen peroxide (H₂O₂) production on an anode combined with H₂ production on a cathode increases the value of solar water splitting, but the challenge of the dominant oxidative product, O₂, needs to be overcome. Here, we report a SnO₂-_x overlayer coated BiVO₄ photoanode, which demonstrates the great ability to near-completely suppress O₂ evolution for photoelectrochemical (PEC) H₂O oxidative H₂O₂ evolution. Based on the surface hole accumulation measured by surface photovoltage, downward quasi-hole Fermi energy at the photoanode/electrolyte interface and thermodynamic Gibbs free energy between 2-electron and 4-electron competitive reactions, we are able to consider the photoinduced holes of BiVO₄ that migrate to the SnO₂-_x overlayer kinetically favor H₂O₂ evolution with great selectivity by reduced band bending. The formation of H₂O₂ may be mediated by the formation of hydroxyl radicals (OH·), from 1-electron water oxidation reactions, as evidenced by spin-trapping electron paramagnetic resonance (EPR) studies conducted herein. In addition to the H₂O oxidative H₂O₂ evolution from PEC water splitting, the SnO₂-_x/BiVO₄ photoanode can also inhibit H₂O₂ decomposition into O₂ under either electrocatalysis or photocatalysis conditions for continuous H₂O₂ accumulation. Overall, the SnO₂-_x/BiVO₄ photoanode achieves a Faraday efficiency (FE) of over 86% for H₂O₂ generation in a wide potential region (0.6-2.1 V vs reversible hydrogen electrode (RHE)) and an H₂O₂ evolution rate averaging 0.825 μmol/min/ cm² at 1.23 V vs RHE under AM 1.5 illumination, corresponding to a solar to H₂O₂ efficiency of ～5.6%; this performance surpasses almost all previous solar energy-assisted H₂O₂ evolution performances. Because of the simultaneous production of H₂O₂ and H₂ by solar water splitting in the PEC cells, our results highlight a potentially greener and more cost-effective approach for “solar-to-fuel” conversion.

Full text of DOI:10.1021/jacs.9b13410

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Article
Near-Complete Suppression of Oxygen Evolution for
Photoelectrochemical H2O Oxidative H2O2 Synthesis
Kan Zhang, Jiali Liu, Luyang Wang, Bingjun Jin, Xiaofei Yang, Shengli Zhang, and Jong Hyeok Park
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b13410 • Publication Date (Web): 11 Mar 2020
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Near-Complete Suppression of Oxygen Evolution for
Photoelectrochemical H2O Oxidative H2O2 Synthesis
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Kan Zhang,*,† Jiali Liu,† Luyang Wang,§Bingjun Jin,‖Xiaofei Yang, ‡ Shengli Zhang,† and Jong
Hyeok Park*,‖
†
Institute of Optoelectronics & Nanomaterials, MIIT Key Laboratory of Advanced Display Material and Devices,
College of Materials Science and Engineering, Nanjing University of Science and Technology, Nanjing 210094, China
‡ College of Science, Nanjing Forestry University, Nanjing 210037, PR China
§
College of New Materials and New Energies, Shenzhen Technology University, Shenzhen, Guangdong 518118, PR
China
‖
Department of Chemical and Biomolecular Engineering, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul
120-749, Republic of Korea
KEYWORDS: Water oxidation, Hydrogen peroxide, Fermi level pinning, BiVO4 photoanode
ABSTRACT: Solar energy-assisted water oxidative hydrogen peroxide (H2O2) production on an anode combined with H2
production on a cathode increases the value of solar water splitting, but the challenge of the dominant oxidative product,
O2, needs to be overcome. Here, we report a SnO2-x overlayer coated BiVO4 photoanode, which demonstrates a great ability
to near-completely suppress O2 evolution for photoelectrochemical (PEC) H2O oxidative H2O2 evolution. Based on the
surface hole accumulation measured by surface photovoltage, downward quasi-hole Fermi energy at the
photoanode/electrolyte interface and thermodynamic Gibbs free energy between 2-electron and 4-electron competitive
reactions, we are able to consider the photoinduced holes of BiVO4 that migrate to the SnO2-x overlayer kinetically favour
H2O2 evolution with great selectivity by reduced band bending. The formation of H2O2 may be mediated by the formation
of hydroxyl radicals (OH•), from 1-electron water oxidation reactions, as evidenced by spin-trapping electron paramagnetic
resonance (EPR) studies conducted herein. In addition to the H2O oxidative H2O2 evolution from PEC water splitting, the
SnO2-x/BiVO4 photoanode can also inhibit H2O2 decomposition into O2 under either electrocatalysis or photocatalysis
conditions for continuous H2O2 accumulation. Overall, the SnO2-x/BiVO4 photoanode achieves a Faraday efficiency (FE) of
over 86% for H2O2 generation in a wide potential region (0.6~2.1 V vs. reversible hydrogen electrode (RHE)) and an H2O2
evolution rate averaging 0.825 μmol/min/cm-2 at 1.23 V vs. RHE under AM 1.5 illumination, corresponding to a solar to H2O2
efficiency of ~5.6%; this performance surpasses almost all previous solar energy-assisted H2O2 evolution performances.
Because of the simultaneous production of H2O2 and H2 by solar water splitting in the PEC cells, our results highlight a
potentially greener and more cost-effective approach for “solar-to-fuel” conversion.
INTRODUCTION
production is, therefore, an alternative strategy since H2
production by water reduction does not need to be
sacrificed to generate dual-energy carriers through the
redox coupling of water. 10, 11 Unfortunately, the production
of H2O2 via an oxidative water process is a formidable
challenge, which requires the two-electron transfer from
the photoanodes. H2O2 evolution is thermodynamically
less favourable than O2 evolution, as shown in the
following equations: 12
As an important chemical in industrial chemistry, the
market size for hydrogen peroxide (H2O2) has reached 3.5
billion USD in 2015 with a compound annual growth rate
of more than 5% from 2016 to 2024. Because traditional
anthraquinone oxidative H2O2 production requires large
1
2
plants and suffers from inconvenient transportation,
photocatalytic or electrochemical (EC) H2O2 production
has attracted considerable attention, partially due to on-
site production and low cost. Currently, photocatalytic or
EC O2 reductive H2O2 production through the two-electron
pathway has been widely studied.
2H2O + 4e-/h+ → O2 + 2H2 (E0= +1.23 V vs. RHE)
(1)
2H2O + 2e-/h+ → H2O2 + H2 (E0= +1.77 V vs. RHE) (2)
3-8
However, the
reduction reaction needs to continuously supply
a
Therefore, there is no significant concern for H2O2
evolution by EC water oxidation.
feedstock of O2 gas, which sacrifices H2 production due to
9
water reduction. Catalysing water oxidation for H2O2
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Recently, Fuku and Sayama demonstrated that BiVO4- the BiVO4 photoanode significantly modulates the surface
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based photoanodes can selectively catalyse H2O to H2O2 in
the presence of HCO3-. 13 Norskov and Zheng’s groups have
subsequently exhibited the EC oxidation of water to H2O2
in a HCO3--containing electrolyte using various kinds of
photoanode materials, such as BiVO4, SnO2, and WO3;
these authors demonstrated that the free energy of
hole oxidation reaction kinetics. This process turns the
complete reaction of H2O2 and O2 evolution at the
photoanodes into H2O2 evolution and OH radical (OH)
generation during the water oxidation process,
accompanied by the suppression of H2O2 decomposition in
the hole re-oxidation process. As a result, the O2 evolution
is almost completely suppressed by substituting OH,
achieving an average FE for H2O2 evolution of 86% at a
widely applied bias ranging from 0.6 to 2.1 V (vs. RHE).
absorbed OH is a key factor in thermodynamically
14, 15
favourable H2O2 evolution.
Despite the numerous
efforts regarding this advanced water oxidation process,
the EC approach resulted in an unsatisfactory Faraday
efficiency (FE) ( ≤ 80%) and required a high operation
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RESULTS AND DISCUSSION
potential with a narrow potential window between 2.7 and
The BiVO4 electrodes were prepared by electrodeposition
based on a previous report. The SnO2 overlayer with
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3.2 V (vs. reversible hydrogen electrode (RHE)).
In
addition to the thermodynamic issue regarding the
competitive oxidation reaction between oxygen evolution
and H2O2 production, the decomposition of highly reactive
H2O2 readily occurs at the photoanodes (Eq 3) 2
controllable oxygen vacancies was electrochemically
deposited on the porous BiVO4 electrodes in a mildly acidic
plating solution using SnCl2 as a precursor, followed by
annealing at 450 °C under air or argon (Ar) conditions (see
the experimental section in detail). The oxygen vacancies
were determined by electron paramagnetic resonance
(EPR) spectra, as shown in Figure 1a. The strong EPR signal
H2O2 → O2 + 2H+ + 2e- E° = +0.68 V vs. RHE
(3)
Therefore, competitive O2 evolution may occur in two ways:
4-electron transfer or stepped 2-electron transfer via a
H2O2 intermediate, which highlights a key counterbalance
between suppressing H2O2 decomposition and boosting 2-
electron water oxidation.
with g=2.25 for SnO2 annealed at 450 °C in Ar can be
22
assigned to ionized oxygen vacancies (Vo).
The X-ray
photoelectron spectroscopy (XPS) Sn 3d spectra of SnO2
annealed at 450 °C in Ar shows an ~0.4 eV blueshift
compared to the sample annealed at 450 °C in air (Figure
S1), which is ascribed to fewer O neighbours around Sn on
Here, we report a photoelectrochemical (PEC) approach
for highly selective water oxidative H2O2 production using
23
a
surface-modified
BiVO4
photoanode.
An
average. The X-ray diffraction (XRD) pattern of SnO2
electrodeposited SnO2 overlayer with oxygen vacancies on
annealed at 450 °C in Ar also displays relatively weaker
Figure 1 (a) EPR spectra of electrodeposited SnO2 annealed at 450 °C in Ar and air, (b) top-view and (c) cross-sectional
SEM image, (d) and (e) HR-TEM image with the corresponding electron diffraction spot and (f) HAADF-STEM-EDX
elemental mapping of the SnO2-x/BiVO4 photoanode.
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diffraction peaks compared to SnO2 annealed at 450 °C in photocurrent density compared to the BiVO4 and
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air (Figure S2), and no diffraction peaks can be assigned to
SnO, indicating that neither Ar nor air annealing affects
the crystal structure of SnO2. Herein, the SnO2 overlayer
obtained by this means on the BiVO4 photoanode can be
labeled as SnO2-x/BiVO4 and SnO2/BiVO4. The scanning
electron microscopy (SEM) image of a typical SnO2-
x/BiVO4 electrode is shown in Figure 1b, which displays a
similar porous morphology to the pure BiVO4 electrode
(Figure S3) because of the very thin SnO2 (<6 nm) layer on
the BiVO4 surface. The total thickness of the SnO2-x/BiVO4
electrode is approximately 800 nm, as shown in Figure 1c,
and elemental mapping shows the homogeneous
distribution of Sn in the entire electrode (Figure S4).
Additionally, the XRD patterns and Raman shifts of the
SnO2-x/BiVO4 or SnO2/BiVO4 photoanodes do not show an
obvious effect on the BiVO4 crystals by the SnO2 coating
(Figure S5). Note that an overly negative potential for
electrodepositing SnO2 can reduce Bi3+ to Bi0, 24 although it
seems to form a thick SnO2 overlayer (Figure S6). The
thickness of the SnO2-x overlayer is determined by high-
resolution transmission electron microscopy (HR-TEM),
which demonstrates a 6 nm thick SnO2 layer that
conformally covers the BiVO4 particles (Figure 1d). The fast
SnO2/BiVO4 photoanodes, along with an obvious cathodic
shift of the onset potential. The results clearly indicate that
the SnO2-x overlayer can efficiently improve the PEC
performance of the BiVO4 photoanode. However, the SnO2
overlayer plays a negative role with respect to the
photocurrent density. A more in-depth discussion and
analysis regarding PEC performance and dark current are
provided in Figure S11. Because the band edge potentials of
SnO2 straddle that of BiVO4, the decreased photocurrent
density of SnO2/BiVO4 photoanode indicates that the Type
I band edge configuration somewhat affects the number of
surface-reaching holes, even though the thickness of SnO2
is only 6 nm. 26 Compared with SnO2, the valance band (VB)
edge of SnO2-x is shifted up by ~0.5 eV, as determined by
VB-XPS (Figure S12), presumably ascribing to the presence
of Vo. As evidenced by first-principles calculations, a new
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band state appears and up-shifts the VB edge for SnO2-x
,
which is consistent with the results of the valence band
XPS spectra (Figure S13). The upward VB edge will reduce
the energy barrier of the hole migration from BiVO4 to
SnO2-x to improve the water oxidation photocurrent. The
possible hole migration can be further demonstrated by
the transient-state surface photovoltage (TS-SPV)
responses, in which the photoanodes exposed to the air
atmosphere give rise to upward band bending that leads to
Fourier transform (FFT) pattern exhibits
a highly
crystalline nature with characteristic crystallographic
orientations of (100), (110), and (010) for monoclinal BiVO4,
which is simultaneously accompanied by crystallographic
orientations of (110) for tetragonal SnO2. A HR-TEM image
taken from the SnO2-x/BiVO4 interface region reveals a less
than 2 nm channel in addition to the lattice spacings of
0.467 and 0.475 nm corresponding to the (110) and (011)
planes of monoclinic BiVO4, respectively, and the lattice
spacing of 0.23 nm is consistent with the d value of the (1-
1-1) plane of SnO2 (Figure 1e). The Bi 4f and V 2p XPS
spectra peaks of BiVO4 are shifted to higher binding
energies after coating with SnO2 or SnO2-x, which might be
caused by the interactions of Sn4+ with Bi and V atoms in
the interfacial region (Figure S7).25 The high-angle annular
dark-field scanning transmission electron microscopy
(HAADF-STEM) image with elemental maps in Figure 1f
solidly demonstrates the surface coverage of SnO2 with a
maximum thickness of 6 nm. The HAADF-STEM-EDS
elemental mapping for cross-view demonstrated the
conformal coating of SnO2-x on BiVO4 surface (Figure S8).
As the O 1s XPS spectra of annealed pure BiVO4 at 450 °C
in either Ar or air overlap, excluding the possible Vo
induced by this process (Figure S9). The optical absorption
spectra show no significant change in the band gap of
BiVO4 with either SnO2 or SnO2-x coverages (Figure S10).
the surface migration of photoinduced holes due to the
27
surface absorbed water.
As shown in Figure 2b, the
surface migration behaviour of the photoinduced hole
enables positive SPV signals for all photoanodes. Clearly,
the SnO2-x/BiVO4 photoanode has the highest signal
intensity, indicating that the photoinduced holes that
reach the photoanode surface are dramatically promoted
by the SnO2-x overlayer. Moreover, both the SnO2/BiVO4
and SnO2-x/BiVO4 photoanodes favour long-lived holes at
the surface, as the delay times of the photovoltage signals
are significantly prolonged as a result of the reduction of
28
surface recombination. The PEC water oxidative H2O2
evolution was quantified by the N,N-diethyl-1,4-
phenylene-diamine (DPD) method, which was determined
by calibrating the standards of the H2O2 solution (Figure
S14). The real-time FE for H2O2 production is displayed in
Figure 2c. In the PEC reaction region, the real-time FE for
all photoanodes can be maintained at a steady level, and
either the SnO2 or SnO2-x overlayers significantly increase
the real-time FE of the BiVO4 photoanode from an average
of 23% to an average of 61% and 86% in the applied bias
ranging from 0.6 to 2.1 V (vs. RHE). The enhanced real-
time FE by either the SnO2 or SnO2-x overlayers is suddenly
dropped from 88% to 66% and from 81% to 39%,
respectively, when the applied bias is larger than 2.1 V (vs.
RHE) where the EC reaction occurs. The phenomenon is in
good agreement with their dark current behaviours in
which the SnO2/BiVO4 and SnO2-x/BiVO4 photoanodes
show a larger overpotential and a lower current density. In
most cases, beyond EC water splitting, not only one but
several EC products (e.g., H2O2, O2, or OH) are generally
thermodynamically favoured. 29-31 From an EC viewpoint,
the applied bias is associated with the Fermi level at the
PEC H2O2 generation was performed in 1 M NaHCO3
solution with a pH of 8.3 under AM 1.5 illumination (100
mW/cm2). As shown in Figure 2a, the dark currents of
BiVO4, SnO2-x/BiVO4, and SnO2/BiVO4 photoanodes
demonstrate that either SnO2 or SnO2-x coverages are
responsible for suppressing the surface reaction kinetics of
BiVO4. Although it is consistent with the activity trends in
EC oxidative H2O to H2O2 predicted by Nørskov and Zheng
et al.,14 the SnO2-x/BiVO4 photoanode exhibits an enhanced
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Figure 2. (a) J–V curves of BiVO4, SnO2/BiVO4 and SnO2-x/BiVO4 photoanodes in 1 M NaHCO3 electrolyte (pH = 8.3)
under AM 1.5 illumination; (b) the TS-SPV responses of the BiVO4, SnO2/BiVO4 and SnO2-x/BiVO4 photoanodes. The
wavelength and intensity of the excitation pulse are 355 nm and 50 μJ, respectively; (c) calculated real-time FE of the
H2O2 evolution at various applied biases of the BiVO4, SnO2/BiVO4 and SnO2-x/BiVO4 photoanodes; (d) A simplified
comparison depicting the fundamental differences in interfacial charge transfer energetics between EC and PEC WOR;
(e) the actual quantities of the O2 evolution amounts of the BiVO4, SnO2/BiVO4 and SnO2-x/BiVO4 photoanodes at 1.23
V vs. RHE in 1 M NaHCO3 electrolyte (pH = 8.3) under AM 1.5 illumination and corresponding overall FE values of both
O2 and H2O2 evolution (navy, green and dark yellow columns are the FE values of H2O2 evolution of the BiVO4,
SnO2/BiVO4 and SnO2-x/BiVO4 photoanodes, respectively); and (f) Mott-Schottky plots measured in under dark and 218
electrode/reactant interface; either an increase or decrease
will cause a change in the potential drop across the EC
double layer, thereby varying the activation energies and
rate constants for all possible interfacial charge transfer
processes. This process is hypothesized to be an essential
reason for the applied bias depending on the EC product
selectivity.32 In contrast to the EC reaction, the quasi-Fermi
level formed at the electrode/reactant interface in the PEC
reaction is pinned; therefore, the activation energies and
rate constants for the interfacial charge transfers are less
affected 33-35. Different mechanisms are responsible for the
dramatic FE changes in water oxidative H2O2 generation
across the EC and PEC regions (Figure 2d).
(Figure S16), which is consistent with characteristic IPCE
curve of BiVO4 for PEC water splitting 36, 37. Clearly, the
SnO2/BiVO4 and SnO2-x/BiVO4 photoanodes display better
stability than the BiVO4 photoanode during the H2O2
evolution process, ascribing to the passivation effect of the
overlayers that suppresses the photo-corrosion of BiVO4. 38,
39
Importantly, the O2 evolution over the SnO2-x/BiVO4
photoanode is almost undetectable, whereas the BiVO4
and SnO2/BiVO4 photoanodes show total O2 amounts of
2.628 and 0.773 μmol, respectively, during a H2O2 evolution
process of 600 s. The calculated overall real-time FE for
H2O2 and O2 evolution is exhibited in Figure 2e.
Considering the total H2O2 and O2 amounts, the overall
real-time FE of the BiVO4 photoanode is near 100%,
corresponding to competitive H2O2 and O2 evolution
processes. However, for SnO2/BiVO4 and SnO2-x/BiVO4
photoanodes, the overall real-time FE is less than 100%,
suggesting products other than H2O2 and O2 are generated
by PEC water oxidation. In particular, the SnO2-x/BiVO4
photoanode is not capable of O2 evolution, which is an
important indication of the near-complete suppression of
O2 evolution that occurs by neither water oxidation nor
H2O2 decomposition.
Since the accumulation amount of H2O2 was measured by
the DPD method, the estimated amount that solely
corresponds to the H2O2 yield might not be accurate due
to another possibility of the decomposition of H2O2 by Eq
3. To explore the underlying mechanism, these
photoanodes were continuously illuminated for 600 s at
1.23 V vs. RHE in a 1 M NaHCO3 electrolyte, and the time
courses of the PEC reaction are shown in Figure S15. The
corresponding incident photon-to-electron conversion
efficiency (IPCE) at 1.23 V vs RHE show that all
photoanodes exhibit a maximum value of 65% at 460 nm
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Figure 3. (a) Free energy diagram of water oxidation plotted at U=1.77 V vs. RHE on BiVO4 (111), SnO2 (110) and SnO2-x
(110). (b) Disk-ring currents recorded at 1600 rpm in a 1 M NaHCO3 electrolyte. EPR responses of OHgeneration by
BiVO4, SnO2 and SnO2-x under visible light illumination in 1 M NaHCO3 (c) and 1 M phosphate buffer (d) solutions,
respectively without applied bias.
In the photo-oxidative water process, the H2O2 evolution
reaction can compete with the O2 evolution reaction if
photogenerated holes in the VB edge of the photoanode
determine the O2 or H2O2 evolution. For the BiVO4
photoanode, the reaction step from absorbed OH* to H2O2
evolution is downhill in the free energy, suggesting much
more favourable thermodynamics for H2O2 evolution.
Remarkably, the ∆GOH* of the SnO2 and SnO2-x surfaces
leads to an uphill reaction for H2O2 evolution. However,
compared with the O2 evolution, the H2O2 evolution on the
SnO2 surface is still favourable because of its smaller free
are deeper than the overpotential required by H2O2
40, 41
evolution.
Because of the same VB edge originating
from BiVO4, the activity trends between the H2O2 and O2
evolution processes can be considered the different surface
thermodynamic processes as well as the hole quasi-Fermi
level for BiVO4, SnO2, and SnO2-x to drive the water
oxidation kinetics. The signatures of the quasi-Fermi level
changes are displayed in Figure 2f. Notably, the Fermi level
of BiVO4 is pinned as observed at an unchanged flat-band
potential under dark and illumination conditions.
Significantly, the flat-band potential shifts anodically upon
the SnO2-x coverage, and the positive shift does not
contribute to the cathodic shift of the photocurrent onset
energy
barrier.
In
contrast,
SnO2-x
exhibits
thermodynamically unfavourable H2O2 evolution as a
result of a much higher free energy barrier for H2O2
formation. The results are consistent with their EC
performances but inconsistent with their PEC activity
trends for H2O2 generation, as shown in Figure 2a.
Therefore, the reduced band bending in SnO2-x/BiVO4 that
is associated with the hole quasi-Fermi level seems to be
responsible for PEC H2O2 generation with high selectivity.
potential in any case because it represents less upward
42, 43
band bending.
Thus, we conjecture that the cathodic
In addition to the water oxidative H2O2 evolution, the
H2O2 decomposition performances on the BiVO4, SnO2,
and SnO2-x surfaces were monitored in real time using the
ring-disk electrode technique. As shown in Figure S19, the
reaction on the disk electrode is H2O2 evolution by BiVO4,
while the reaction on the ring electrode is H2O2
decomposition by BiVO4, SnO2, and SnO2-x. To avoid other-
side reactions, the applied bias for the ring electrode is
established at 0.7 V vs. RHE, at which the only possible EC
reaction is H2O2 decomposition. Figure 3b shows the linear
shift of the photocurrent for the SnO2-x/BiVO4 photoanode
may be the cause of the changing water oxidation reaction
from O2 evolution to H2O2 evolution. The mechanism
diagram is shown in Figure S17. Figure 3a shows the
calculated free energy diagram for the 2-electron and 4-
electron oxidation processes of water on the BiVO4, SnO2,
and SnO2-x surfaces from the thermodynamic models in
Figure S18. At a theoretical potential of 1.77 V vs. RHE for
H2O2 evolution, it can be seen that the free energy for
absorbed OH* (∆GOH*) is the key bifurcating point to
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Figure 4. (a) SnO2-x overlayer associated with the surface states that are energetically shifted away from the intra-
bandgap region, which overcomes the O2 evolution reaction. (b) Plots of the theoretical charge number obtained
from the J–t curves collected at 1.23 V vs. RHE and the actual quantities of H2 and H2O2 generated by SnO2-x/BiVO4
photoanode under AM 1.5 illumination. (c) A summary of PEC H2O2 production with various parameters under AM
1.5 illumination. The open-circuit voltage is converted applied bias vs RHE based on pH value of electrolyte.
sweep voltammetry (LSV) curves of different electrodes
recorded at
scan rate of 10ꢀmVꢀs−1 in
1-electron transfer by SnO2 and SnO2-x overlayers.
a
a
1
M
Furthermore, Figure 3d shows the EPR spectra of the
BiVO4, SnO2/BiVO4, and SnO2-x/BiVO4 photoanodes in a
0.5 M phosphate buffer solution, which is a general
electrolyte for O2 evolution. Note that without light
illumination, no signals associated with OHcan be
detected (Figure S20); moreover, H2O2 would not be
excited to OHin the absence of ultraviolet (UV) light. The
trends of OHgeneration in the phosphate buffer solution
are similar to that in HCO3- containing the electrolyte,
suggesting essentially a complete reaction of 1-electron
transfer during the water oxidation process. The
photocatalytic H2O2 degradation over the BiVO4,
SnO2/BiVO4, and SnO2-x/BiVO4 photoanodes was
investigated under AM 1.5 illumination, which is direct
evidence of H2O2 accumulation during PEC water
oxidation (Figure S21). The H2O2 concentration is gradually
and rapidly decreased on the illuminated BiVO4
photoanode, which becomes undetectable for 60 min. In
sharp contrast, the H2O2 can be preserved by 79% and 81%
in illuminated SnO2/BiVO4 and SnO2-x/BiVO4 photoanode
solutions, respectively, which is slightly lower than the
photocatalysis of H2O2 (87%) during 60 min of
illumination. The above results demonstrate that the SnO2-
NaHCO3 electrolyte solution at room temperature at 1600
rpm. Under the same disk current generated by water
oxidative H2O2 evolution, the ring currents that arise from
oxidative H2O2 decomposition can reflect the equilibrium
of the reaction of H2O2 evolution and decomposition. 44, 45
Remarkably, the ring current of SnO2-x approaches zero in
the whole applied bias, indicating negligible H2O2
decomposition on the SnO2-x/BiVO4 photoanode.
Based on these results, the function of the SnO2-x
overlayer can be considered to suppress O2 evolution from
both the water oxidation and H2O2 decomposition
processes. The lower than real-time 100% FE of H2O2
evolution, therefore, implies another completive reaction
other than the O2 evolution reaction. To unmask the
underlying reaction, EPR detection was used to record
hydroxyl radical (OH) generation as a result of 1-electron
transfer during the water oxidation process:
H2O → OH+ H+ + e- E° = +2.73 V vs. RHE
(4)
As shown in Figure 3c, four characteristic peaks of
DMPO- OHappear in the HCO3--containing electrolyte in
the presence of SnO2/BiVO4 and SnO2-x/BiVO4
photoanodes under visible light irradiation. The signal
intensities of OH by the SnO2-x/BiVO4 photoanode are
stronger than those of the SnO2/BiVO4 photoanode,
whereas the OHsignal in the spectrum of the BiVO4
photoanode is negligible. The results disclose the favoured
overlayer is responsible for the thermodynamic
x
suppression of O2 evolution upon the PEC water oxidation
and H2O2 oxidation processes.
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The SnO2-x overlayer regulates the surface reaction
selectivity for PEC water oxidative H2O2 generation using a
BiVO4 photoanode under simulated solar light. Our
investigations on the charge exchange mechanism
demonstrated the near-suppression of O2 evolution during
PEC water splitting, due to the photogenerated holes of
BiVO4 migrating to the SnO2-x overlayer; this process
regulated the competitive 2-electron/4-electron transfer to
a 2-electron/1-electron transfer by interface energetics and
the concomitant suppression of H2O2 decomposition. As a
result, the FE for H2O2 generation reached ca. 86% over a
wide potential range from 0.6 to 2.1 V vs. RHE with an
average H2O2 evolution rate of 0.825 μmol/min/cm-2 at 1.23
V vs. RHE. Because the thermodynamically less favourable
H2O oxidative H2O2 reaction, this study highlighted the
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kinetics of the BiVO4 photoanode from the completive
reactions of the 2-electron and 4-electron transfers to
completive reactions of the 1-electron and 2-electron
transfers. The processes of water oxidation are
schematically illustrated in Figure 4a. It is well known that
the interfacial energetics at semiconductor photoanode is
associated with surface state, in general, intra-gap states.
Especially for BiVO4 photoanode, the surface state enables
significant regulation of hole transfer kinetic constant for
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O2 evolution.42, The passivating surface state is of the
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essence, even in case of co-catalyst deposition.46 The
mechanism of such passivation typically is strong
interacted by the orbitals of the new surface component to
modulate surface states, resulting in formation of new
bonding and antibonding orbitals that are energetically
shifted out of the bandgap.47, 48 Considering the interface
fusion (Figure 1e) and quasi-Fermi level change (Figure 2f),
the reducing band bending is hypothesized to be
independent of the thermodynamic process for H2O2
evolution. Since the PEC water splitting over the SnO2-
x/BiVO4 photoanode involves the H2O2 and H2 evolution
processes, the time courses of PEC water oxidative H2O2
production and water reductive H2 production are
investigated. At an applied bias of 1.23 V vs. RHE, the
obtained H2 and H2O2 quantities as a function of the
theoretical electron number calculated on the basis of its
photocurrent density are shown in Figure 4b. It can be
clearly seen that the H2 amounts are well-matched to their
theoretical value, while the measured H2O2 amounts
deviate from the stoichiometric 2-electron transfer with an
average production rate of 0.825 μmol/min/cm-2. The
stable photocurrent density and constant FE produce a
solar to H2O2 efficiency of ~5.6% at 1.23 V vs RHE under
AM 1.5 illumination. As mentioned above, the
photocurrent for water oxidation involves OH generation,
significance of
a surface state modulation on the
photoanode/electrolyte interface, opening the possibility
of highly selective H2O2 synthesis from PEC water splitting
by Fermi level pining effect.
ASSOCIATED CONTENT
Supporting Information. Detailed experimental sections;
figures; tables. This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
* zhangkan@njust.edu.cn
* lutts@yonsei.ac.kr
Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENT
This work was supported by NSFC (51802157, 21902104), the
Natural Science Foundation of Jiangsu Province of China
which is mainly
a side reaction to consume the
(BK20180493),
NRF
Korea
(NRF-2019R1A2C3010479,
photogenerated holes during H2O2 evolution. Nevertheless,
to our knowledge, our SnO2-x/BiVO4 photoanode
demonstrates superior PEC H2O2 evolution relative to
other photoanodes for both PEC water oxidative H2O2
evolution and PEC O2 reductive H2O2 evolution (Table S1
and Figure 4c).12, 13, 19, 49, 50 Furthermore, the SEM, XRD and
XPS results of these photoanodes after the long-term H2O2
evolution show no detectable change in morphology and
component (Figure S22), indicating a well structure
stability for PEC H2O2 evolution. The HAADF-STEM-EDX
images of the SnO2-x/BiVO4 photoanode taken after the
long-term H2O2 evolution confirm the uniform SnO2-x
2019M1A2A2065612, 2019M3E6A1064525, 2019R1A4A1029237),
the Fundamental Research Funds for the Central Universities
(No.30918012202). LY. Wang acknowledges the support by
Natural Science Foundation of Top Talent of SZTU (grant no.
2019106101003).
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