9834
J. Phys. Chem. B 1998, 102, 9834-9843
Photoelectrochemical Reduction of CO2 in a High-Pressure CO2 + Methanol Medium at
p-Type Semiconductor Electrodes
Kouske Hirota,† Donald A. Tryk,† Toshio Yamamoto,† Kazuhito Hashimoto,‡
Masafumi Okawa,§ and Akira Fujishima*,†
Department of Applied Chemistry, School of Engineering, The UniVersity of Tokyo, 7-3-1 Hongo, Bunkyo-ku,
Tokyo 113-8656, Japan, Research Center for AdVanced Science and Technology, The UniVersity of Tokyo,
4-6-1 Komaba, Meguro-ku, Tokyo 153-8904, Japan, and Electric Power DeVelopment Company, Limited,
6-15-1, Ginza, Chuo-ku, Tokyo 104-8165, Japan
ReceiVed: May 19, 1998; In Final Form: October 10, 1998
Photoelectrochemical CO2 reduction was examined in a high-pressure (40 atm) CO2 + methanol medium
using the p-type semiconductor electrodes p-InP, p-GaAs, and p-Si. With the p-InP photocathodes, current
densities up to 200 mA cm-2 were achieved, with current efficiencies of over 90% for CO production, while
hydrogen gas evolution was suppressed to low levels. At high current densities and CO2 pressures, the CO2
reduction current was found to be limited principally by light intensity. Of the various factors that were
found to influence the product distribution, including the concentrations of added water and strong acid, CO2
pressure was the most critical factor. We propose that the adsorbed (CO2)2•- radical anion complex reaches
high coverages at high CO2 pressures and is responsible for both the high current efficiencies observed for
CO production and the low values observed for H2 evolution. Furthermore, we propose that this adsorbed
complex is responsible for stabilizing all three semiconductor electrode materials at high CO2 pressures, even
at current densities as high as 100 mA cm-2.
Introduction
To produce even a small impact on the buildup of CO2 in
the global atmosphere using electrochemical approaches, it will
be necessary to develop systems that combine high current
density, high current efficiency, and low input voltage.1-3 The
two main approaches toward achieving high current density,
which involve either the use of gas-diffusion electrodes4-10 or
the use of high-pressure CO2,9,11-23 have yielded encouraging
results, with a value of -3.0 A cm-2 being obtained for a gasdiffusion electrode under high pressure.9 Much of the effort to
achieve high current efficiencies has focused on the use of metal
electrodes that have either high hydrogen overpotentials or
specific catalytic activity for the production of CO, formate,
alcohols, or hydrocarbons, either in aqueous25-33 or nonaqeous
electrolyte.13,34,35 In our own laboratory, we have examined
the use of high-pressure CO2-methanol as a solvent system in
order to achieve high CO2 mass transport, using principally
copper as an electrode material, as well as other metals.19-24
Current densities as high as 0.2 A cm-2 were achieved for
electrochemical CO2 reduction using a methanol catholyte and
aqueous anolyte in order to produce oxygen gas at the anode.24
It is also tempting to imagine scenarios in which the electrical
power for CO2 reduction can be supplied by solar radiation,
via a kind of artificial photosynthesis, as envisioned by
Professors Bard and Fox,36 using either separate or integrated
systems for power generation and electrochemistry.37 Since the
first report of photoelectrochemical reduction of CO2 in 1978
by Halmann,38 electrochemists have always considered the
* To whom correspondence should be addressed.
† Department of Applied Chemistry, The University of Tokyo.
‡ Research Center for Advanced Science and Technology, The University
of Tokyo.
§ Electric Power Development Company, Limited.
integrated approach to be a worthy challenge. Various p-type
semiconductors have been examined,38-55 including pGaP,38-40,42,44,45 p-CdTe,40-43 p-Si,40,41,46-49 p-GaAs,40,44,50,51
p-InP,40,42,52-54 and p-SiC.55 However, the photocurrent densities for CO2 reduction have been limited to relatively low values
(<10 mA cm-2), even at higher light intensities.56 The possible
reasons for the low current densities include CO2 mass transport
limitations and competition with H2 production.
To combine the advantages of high-pressure CO2 systems,
i.e., enhanced mass transport and product selectivity, with the
possibly decreased input voltages of photoelectrochemical
systems, we have recently begun to examine the use of the highpressure CO2-methanol system with p-type semiconductor
photoelectrodes. We have made preliminary reports of current
densities for photoelectrochemical CO2 reduction that have
reached values of 100-200 mA cm-2 using p-InP photocathodes.57-59 CO2 reduction to CO proceeds with high current
efficiency even at such high current densities. We also found
that photoelectrochemical CO2 reduction in the high-pressure
CO2 (40 atm)-methanol solution is not limited by CO2 mass
transport, but by light intensity. In addition to the highly
selective CO2 reduction at high current densities, another
important advantage of the CO2 (40 atm)-methanol solution
was found to be greatly enhanced stability of the p-InP
electrodes, which can only partly be explained on the basis of
the generally recognized enhancement of stability in nonaqueous
electrolytes.60,61
In the present study, we conducted a detailed comparison
study of three different p-type materials, p-InP, p-GaAs, and
p-Si. The effect of varying the CO2 pressure, current density,
water concentration, and proton concentration upon the product
distribution and current-potential behavior was examined. The
major aspects of the behavior were similar, demonstrating the
10.1021/jp9822945 CCC: $15.00 © 1998 American Chemical Society
Published on Web 11/10/1998
Photoelectrochemical Reduction of CO2
generality of the high-pressure methanol-CO2 approach for
various types of semiconductor materials.
Experimental Section
A p-type InP wafer [(100), Cd-doped, 2 × 1018 cm-3,
Atramet] was cut into ∼ 0.4 cm × 0.5 cm electrodes. Ohmic
contact was made by means of successive vapor deposition of
Zn (30 nm) and Au (100 nm), with subsequent annealing at
425 °C in Ar.62 The electrodes were mounted with Torr-Seal
(Varian) epoxy resin. The p-GaAs [(100), Zn-doped, 1-5 ×
1017 cm-3, Atramet] and p-Si [(100), 10-50 Ω, Silicon Quest
International] electrodes were prepared in the same way. A
silver wire (0.8 mm diameter) covered with epoxy, with only
the end exposed, was used as a quasi-reference electrode (Ag
QRE, ∼+80 mV vs SCE and -310 mV vs Fc/Fc+). A Pt wire
(0.8 mm diameter) was used as the counter electrode. The p-InP
and p-GaAs electrodes were etched in hot aqua regia for ∼5 s
before each experiment. The p-Si electrodes were etched first
in a mixture of 5% HF, acetic acid, and 10% nitric acid, and
finally in 10% HF-ethanol solution, each for ∼30 s.
The equipment and the procedures used in the experiments
with the CO2-methanol medium have been described in
previous papers.19,20 A modified cell was used for illumination,
which consists of a stainless steel pressure vessel equipped with
a 2-cm thick quartz window. The electrolyte solution [3 cm3,
0.3 M tetrabutylammonium perchlorate (TBAP, reagent grade,
Tokyo Kasei) in CH3OH (reagent grade, Nacalai Tesque)] was
placed in a glass cell liner in the stainless steel vessel. CO2
was introduced into the pressure vessel and was allowed to
equilibrate for 1 h at the designated pressures (1-40 atm). In
control experiments, 1 atm Ar was used.
A 500-W xenon lamp (Ushio, UXL-500D-O, in a UI-501C
housing) was used to illuminate the semiconductor photocathode. Wavelengths shorter than 370 nm were filtered out using
a UV-blocking filter (UV-37, Toshiba) in order to avoid
photodecomposition of the supporting electrolyte. After filtering, the intensity of the lamp output was found to be 480 mW
cm-2 at the electrode surface. The current-potential curve
measurements were performed without ohmic loss compensation
for solution resistance. Ohmic loss measurements (and simultaneous correction) were carried out during electrolysis using
an Toho Technical Research model 2024 IR Compensator, and
corrections were later performed mathematically on the currentpotential curves.
During the photoelectrolyses, total amounts of charge from
1 to 10 C were passed galvanostatically at 1-100 mA cm-2
using a potentiostat-galvanostat (Hokuto Denko, model HA501). After the electrolysis, the gas- and liquid-phase products
were analyzed using gas chromatography with either a flame
ionization detector (Shimadzu, model GC-8A, Porapak Q
column) or a thermal conductivity detector (Hitachi, model GC163, molecular sieve type 13X column).
X-ray photoelectron spectroscopic measurements were carried
out using a Perkin-Elmer PHI 5600 monochromated XPS
spectrometer, using Mg KR radiation (400 W) at a pass energy
of 187.85 eV, after argon ion sputtering for 1 min at an
acceleration voltage of 3 kV.
Results and Discussion
Current-Potential Curves. Typical current-potential curves
for the three different p-type semiconductor photocathodes in
CO2 (40 atm)-methanol are shown in Figure 1. Under
illumination, the p-InP photocathode exhibited large photocurrent densities, exceeding -150 mA cm-2, for potentials more
J. Phys. Chem. B, Vol. 102, No. 49, 1998 9835
negative than -1.8 V vs Ag QRE (Figure 1A, curve a), while
the dark current was negligibly small (less than -1 mA cm-2)
at potentials down to -2.0 V (curve b).
To determine the ohmic loss-free behavior, the potential was
corrected for the solution resistance, which was measured during
a separate galvanostatic electrolysis, using an IR compensator.
The high-pressure CO2-methanol solution, because it contains
a high concentration of nonconductive CO2, has a high solution
resistance.20 The actual resistance in the present setup varied
in the range from 50 to 70 Ω, as measured during constant
current electrolysis (see Figure 1 caption). Both the corrected
and uncorrected potential curves are shown in Figure 1, the latter
shown as curve c in parts A, B, and C. The initial part of the
curve is characterized by an exponentially increasing current.
At the midpoint (∼-50 mA cm-2), the curve no longer increases
so quickly. As shown in our previous report, the Tafel plot
deviates from linearity at higher current densities, which
suggests either mass transport or light intensity limitations.59
To check the latter, a neutral density filter with an absorbance
of 0.3 was used (halving the light intensity); the initially
measured photocurrent current density of -180 mA cm-2
decreased to approximately -90 mA cm-2. Thus, even at
higher current densities, the reaction is seen to be limited by
light intensity and not by mass transport.
The overall reaction, shown later to be
2CO2 + 2e- ) CO + CO32has an E° of -0.555 V vs NHE in aqueous solution, based on
1 atm gas pressures and 1 M CO32- concentration63 (an
erroneous value of -1.07 V has been quoted in the literature3,40).
We assume for the moment that this value can also be used in
methanol solution. Correcting for the CO2 pressure (40 atm)
yields a value of -0.46 V, and making a further correction to
the Ag QRE scale yields a value of -0.78 V, as indicated in
Figure 1. At this potential, we observe a small but measurable
current density (-2 to -3 mA cm-2). To put this result in
the perspective of a high-performance hydrogen-evolving PEC,
this would correspond to a short-circuit condition, for which
the maximum current density at this illumination intensity (white
light, 480 mW cm-2) would be greater than -100 mA cm-2.64
However, given the intrinsic activation overpotential that would
be expected from an overall two-electron reaction, the present
result may represent at least a base level, on which further
improvements could be made. One of the best results reported
thus far in the literature for photoelectrochemical CO2 reduction
has achieved approximately the same current density (∼-3 mA
cm-2) for an Au-deposited p-Si photocathode in aqueous
solution, but using a smaller illumination intensity (100 mW
cm-2).49
As a further comparison, a current-potential curve is also
shown for the dark electrochemical reaction on a copper
electrode measured under essentially the same conditions (data
taken from ref 20). The current onset is ∼0.4 V more negative
compared to that for the p-InP photoelectrode (with a larger
shift, ∼0.7 V for higher current densities), showing that, in
principle, the bias voltage of a CO2-producing photoelectrochemical cell based on such an electrode would be significantly
smaller than the corresponding nonilluminated electrochemical
cell. Other workers have also shown similar comparisons.40,43
The conduction band energy for p-InP (based on data obtained
by Kohl and Bard in acetonitrile65) can be taken to be ∼-1.12
V vs SCE, or ∼-1.20 V vs Ag QRE. We can see that there is
a significant current at this potential, ∼-70 mA cm-2, but the
current has not yet saturated. Again, it is obvious that this result
9836 J. Phys. Chem. B, Vol. 102, No. 49, 1998
Figure 1. Current-potential curves for (A) p-InP, (B) p-GaAs, and
(C) p-Si electrodes in 0.3 M TBAP in methanol (40 atm CO2) under
illumination (a, c). The c curves correspond to the original, uncorrected
data, and the a curves were corrected for ohmic losses according to
the measured resistance. The b curves were obtained without illumination (dark). In part A, a current-potential curve for electrochemical
reduction at a metallic Cu electrode (curve d) is also shown for
comparison. The solid and dashed parts of this curve were taken from
Figures 6 and 9 of ref 20.
does not compete with high-performance PECs, due mostly to
kinetic losses.
The p-GaAs and p-Si photocathodes (Figure 1, B and C,
respectively) also exhibited large photocurrents under illumination, and the current flow without illumination was negligible.
However, for these two photoelectrodes, the performance was
significantly poorer than that for p-InP. At the reversible
potential, there was no measurable current, and even at the
conduction band edges [∼-1.12 V vs SCE (-1.20 V vs Ag
QRE) for p-GaAs66 and ∼-0.85 V vs SCE for p-Si67,68 (-0.93
V vs Ag QRE), also based on data for acetonitrile], the currents
were small (∼-8 mA cm-2 and less than -1 mA cm-2,
respectively. For the p-GaAs electrode, a slight tapering off of
the current increase was observed, while the p-Si electrode
exhibited an exponential current increase up to -150 mA cm-2.
This can be understood in terms of the range of absorbable
wavelengths in the white light source. Because the band gap
for Si (1.1 eV) is smaller than that for InP and GaAs (1.25 and
1.43 eV, respectively), the range of the wavelengths absorbable
for the p-Si electrode is wider than that for the other materials,
and consequently the absorbed light intensity is higher.69 Thus,
for the p-Si electrode, a limitation due to light intensity was
not evident in this current region.
For p-InP electrodes, the photocurrent onset potentials
(defined as the point at which the current density exeeded 0.2
mA cm-2) were approximately -0.6 V. For p-GaAs and p-Si,
Hirota et al.
the onset potentials were approximately -1.0 V vs Ag QRE.
The relative ordering of the semiconductor performances is
consistent with results of Taniguchi et al., who compared various
p-type semiconductor materials for CO2 reduction in DMF
containing 5% water.40
Photoelectrolysis Products. Galvanostatic electrolyses were
conducted under illumination, and the results are summarized
in Table 1. In Ar-saturated (1 atm) solution, hydrogen was
produced via the decomposition of methanol and was the only
product obtained in both gas and liquid phases. The total current
efficiency was 91%, which may be due to reoxidation of the
evolved hydrogen at the counter electrode in the one-compartment cell.
In the CO2-saturated (40 atm) solution, photoelectrolyses were
carried out at various current densities using the three different
electrode materials. The photocurrents were stable regardless
of the material, and the photopotentials were also stable, varying
less than 100 mV during the photoelectrolysis. No visible
damage of the electrode surface was observed after electrolysis.
The principal product was CO for all three types of semiconducting materials. The source of the CO can be rather
confidently stated to be CO2 via reduction at the semiconductor
electrode rather than methanol via oxidation at the Pt counter
electrode, on the basis of two separate results. First, as stated
in the previous paragraph, the sole product in the Ar-saturated
solution was hydrogen, with no CO or methyl formate detected
(Table 1). This result is essentially the same as that previously
reported by Saeki et al., in which N2-saturated solution was
used.20 In that work, CO was not detected, but a small amount
of methyl formate (4.6% current efficiency) was produced,
probably due to rereduction of CO2 produced via methanol
oxidation at the Pt counter electrode. Second, in the same work
just cited, it was shown using isotopic labeling (13CO2) that 99%
of the CO generated at a Cu working electrode, under conditions
very similar to those reported in the present work, originated
from the labeled CO2.20 The remaining 1% of the CO (detected
as 12CO) was thought to have been an impurity in the 13CO2
gas used. In that work, the only difference in the experimental
conditions was that the supporting electrolyte was tetrabutylammonium tetrafluoroborate instead of tetrabutylammonium
perchlorate, which was used in the present work. However,
this difference is not expected to be significant.
Methyl formate was also formed, but to a much lesser degree,
the current efficiencies varying between 6 and 19%. On the
basis of additional isotopic labeling studies reported in the
previously cited work carried out in our laboratory, the bulk of
this compound is also produced via CO2 reduction rather than
methanol oxidation.20 That work showed that the formyl group
of methyl formate is produced via reduction of CO2 to formate,
which then reacts with methanol to produce the ester. In the
present work, methyl formate is assumed to be produced in the
same way. Insignificant amounts of hydrocarbons were detected.
For the p-InP electrodes at 40 atm CO2 pressure, the current
efficiencies for CO production were 87-93% in the current
range from 50 to 100 mA cm-2, compared to 60% at 1 atm,
while hydrogen production was suppressed at 40 atm (<5%),
compared to that at 1 atm (33%). Thus, a very high selectivity
for CO2 reduction was obtained in CO2 (40 atm)-methanol at
high current densities. This result is somewhat similar to those
for certain metal electrodes such as Ag and Au.26,28,33
We also found that this situation was maintained at even
higher current densities. For example, using a different p-InP
electrode, from another manufacturer (Electronics and Materials
Photoelectrochemical Reduction of CO2
J. Phys. Chem. B, Vol. 102, No. 49, 1998 9837
TABLE 1: Product Distributions for Photoelectrolysis
cathode
gas
pressure
(atm)
photocurrent
(mA cm-2)
p-InP
p-InP
p-InP
p-InP
p-InP
Ar
CO2
CO2
CO2
CO2
1
1
40
40
40
5.0
50
50
100
200
p-GaAs
p-GaAs
p-GaAs
CO2
CO2
CO2
1
40
40
p-Si
p-Si
CO2
CO2
1
40
potential (V)
corr.a
uncorr.b
-1.4
H2
current efficiency (%)
CO
HCOOCH3
total
-1.1
-1.4
(-2.3)c
-1.5
-2.1
-1.6
-2.5
-4.3
91
33
4.5
2.9
2.1
60
89
93
94
16
6.1
11
3.7
91
109
100
107
100
50
50
100
-2.0
-1.6
-1.9
-2.2
-2.1
-2.9
76
16
12
11
74
82
19
10
6.0
106
100
100
50
50
-1.7
-1.8
-2.1
-2.5
82
9.1
17
75
19
10
119
94
a
Potential vs Ag QRE, corrected according to the measured ohmic loss. b Potential vs Ag QRE, uncorrected. c Potential was corrected on the
basis of an estimated resistance of 60 Ω for a similar geometry.
Figure 2. Current efficiencies for production of (a) b CO, (b) 0 H2,
and (c) ] HCOOCH3, as a function of current density at (A) 1 atm
and (B) 40 atm CO2 on a p-InP electrode.
Corp.), the electrolysis resulted in 94% current efficiency for
CO production at -200 mA cm-2.57 When the electrolysis was
carried out at 1 atm CO2, a galvanostatic electrolysis at -50
mA cm-2 with a p-InP electrode resulted in CO production at
a current efficiency of 60% and hydrogen production at 33%.
The decrease in CO production efficiency can be attributed to
a lack of available CO2 at the electrode surface, as discussed
later.
To compare the product distributions at 1 atm vs 40 atm CO2
in more detail, the product distributions obtained as a function
of current density are compared in Figure 2 for 1 atm CO2 (part
A) and 40 atm CO2 (part B). At 40 atm, the current efficiencies
for CO production increased as a function of current density
and reached very high values, exceeding 90%. On the contrary,
at 1 atm, the current efficiencies for CO production initially
increased and then quickly decreased as the current density
increased. At -100 mA cm-2, the CO current efficiency was
approximately 45%, but that for H2 reached 40%. These results
clearly indicate that the high-pressure CO2 (40 atm)-methanol
system provides high selectivity for CO2 reduction at high
current densities.
The p-GaAs and p-Si photocathodes also exhibited high
current efficiencies for CO2 reduction in the high-pressure CO2methanol medium (Table 1). The CO current efficiencies at
-50 mA cm-2 were 70-80% for these materials. Hydrogen
production current efficiencies for p-GaAs and p-Si (9-16%)
were slightly greater than that for p-InP (2-5%). For these
materials, in contrast with p-InP, the main electrolytic product
at 1 atm CO2 was hydrogen. Thus, we can see that the effect
of pressure was even more significant for these two semiconductors than that for p-InP.
As expected from the current-potential curves (Figure 1),
the required electrode potentials for the galvanostatic electrolyses
were dependent upon the type of semiconductor. The potentials
Figure 3. Partial current densities for various products for a p-InP
photocathode at (A) 1 atm CO2 and (B) 40 atm CO2: (a) b CO; (b) 0
H2, and (c) ] HCOOCH3.
for the electrolyses using p-GaAs and p-Si were more negative
by 0.5-0.6 V than that for p-InP at the same current density.
Thus, p-InP appears to be a promising material for photoelectrochemical CO2 reduction in the high-pressure CO2-methanol
solvent system compared with p-GaAs and p-Si because CO2
reduction proceeds both selectively and with a lower input
potential at high current densities.
Potential Dependence. Figure 3 compares the partial current
densities for the electrolytic reduction products vs ohmic losscorrected potential for a typical p-InP electrode at 1 atm (part
A) and 40 atm (part B). The behavior at 40 atm was distinctly
different from that at 1 atm, particularly in the potential region
more negative than -1.1 V vs Ag QRE. At 40 atm, the partial
current density for CO production increased steadily as the
potential became more negative, showing no tendency to
saturate. It reached approximately -95 mA cm-2 at -1.4 V.
The current efficiency for CO production reached over 90%,
but that for hydrogen production was less than 5% in this
potential region. At 1 atm CO2, in contrast, the current density
for CO production showed a tendency to saturate at approximately -25 mA cm-2. Even at more negative potentials,
e.g., -1.9 V, the current for CO production was limited to
approximately -40 mA cm-2. These results are consistent with
9838 J. Phys. Chem. B, Vol. 102, No. 49, 1998
Hirota et al.
Figure 4. Partial current densities for various products for a p-GaAs
photocathode at 40 atm CO2: (a) b CO; (b) 0 H2, and (c) ]
HCOOCH3.
the fact that the current efficiencies for CO production decreased
with increasing current density at 1 atm CO2 but approached
95% at 40 atm (Figure 2).
In contrast, the partial current densities for hydrogen production at 1 atm CO2 (Figure 3B) continued to increase as the
potential became more negative, whereas those for 40 atm
(Figure 3A) remained very low, reaching less than -5 mA cm-2
in the whole potential range (to ∼-1.5 V). Thus, we can see
a sort of mirror-image relationship between the current efficiencies for CO and H2 production.
It is interesting to note that at the “toe” of both currentpotential curves, i.e., from -0.8 to -1.1 V, the partial current
densities were very similar at CO2 pressures of 1 and 40 atm
(Figure 3A,B). For both pressures, the current efficiencies (not
shown) for CO production increased with increasing negative
potential, while those for H2 production decreased in this
potential region. This result implies that the environment at
the electrode surface in this potential region does not depend
greatly on the bulk CO2 concentration.
For the p-GaAs photoelectrode at 40 atm CO2 pressure
(Figure 4), behavior similar to that for p-InP was observed. The
current density for CO increased steeply as the potential became
more negative, while that for hydrogen production remained
very low (∼-10 mA cm-2). The current efficiency for CO
production (not shown) reached 75%, whereas that for hydrogen
production decreased to less than 20% at potentials more
negative than -1.7 V. For given values of current density, the
potentials were approximately 0.5 V more negative compared
with p-InP, consistent with the current-potential curves (Figure
1).
Effect of CO2 Pressure. The effect of CO2 pressure on the
product distribution was examined for the p-InP and p-GaAs
electrodes (Figure 5). The CO2 concentration in methanol
approximately obeys Henry’s Law as a function of CO2 pressure
over the 1-50 atm range, with the value at 40 atm being ∼ 8
M or 0.33 mole fraction (Figure 5A).70 Galvanostatic electrolyses at various CO2 pressures were conducted at a current
density of -50 mA cm-2, and the current efficiencies for various
products were measured. As the CO2 pressure (and thus the
CO2 concentration) was increased, the current efficiency for CO
production increased, while that for H2 production decreased
for both p-InP and p-GaAs. Particularly in the region of CO2
pressure from 1 to 25 atm, changes in product distribution were
significant. Current efficiencies for CO production on p-InP
increased from 59% to 83% and on p-GaAs from 11% to 49%.
At pressures greater than 25 atm, current efficiencies for H2
production were very low, less than 20% for p-GaAs and less
than 5% for p-InP. At -50 mA cm-2 for CO2 pressures greater
than 25 atm, CO2 reduction proceeds efficiently on both
electrode materials.
Figure 5. (A) Pressure-concentration isotherms for CO2 dissolution
in methanol: (a) O CO2 concentration, (b) 9 CO2 mole fraction at 25
°C. (B) Current efficiencies for reduction products at various CO2
pressures for a p-InP photocathode: (a) b CO, (b) 0 H2, and (c) ]
HCOOCH3. (C) Current efficiencies for reduction products at various
CO2 pressures for a p-GaAs photocathode: (a) b CO, (b) 0 H2, and
(c) ] HCOOCH3.
Figure 6. Tafel plots of the partial current densities for the production
of (a) b CO, (b) 0 H2, and (c) ] HCOOCH3 for a p-InP photocathode
at 40 atm CO2 pressure. Curve d, O, shows the total current density.
On the other hand, the current efficiency for methyl formate
production did not change significantly with increasing CO2
pressure. For a typical p-InP electrode, the current efficiency
decreased slightly as the CO2 pressure increased from 1 to 40
atm.
Tafel Plots for Various Products. The partial current
density Tafel plots for each of the measured products at 40 atm
CO2 pressure are shown in Figure 6. It can be seen that the
current densities for CO (curve a) and methyl formate (curve
b) production exhibited Tafel linearity up to approximately -50
mA cm-2, while that for H2 evolution (curve c) saturated at
lower current densities (approximately -3 mA cm-2). At
Photoelectrochemical Reduction of CO2
J. Phys. Chem. B, Vol. 102, No. 49, 1998 9839
Figure 7. Current efficiencies for various products as a function of
added water concentration at -50 mA cm-2 under 40 atm CO2 for a
p-GaAs electrode: (a) b CO, (b) 0 H2, and (c) ] HCOOCH3. Curve
d, O, shows the total of the partial current efficiencies.
Figure 9. Schematic representation of (top) a close-packed layer of
adsorbed CO2 and (bottom) the same layer after reduction of half of
the molecules to the CO2•- radical anion form (based on proposed
packing arrangements of CO2 and CO2•- on Ni(110), taken from refs
83 and 84, respectively). The negative charges are thought to be
delocalized on the two oxygen atoms of the CO2 moieties.
Figure 8. X-ray photoelectron spectra for (A) a virgin p-InP (100)
wafer, (B) a p-InP electrode after being used in a CO2 reduction
electrolysis experiment in CO2 (40 atm)-methanol, and (C) a p-InP
electrode after being in an electrolysis experiment in CO2 (1 atm)methanol. For (B) and (C), the current density was -50 mA cm-2 and
the total charge was ∼5 C.
current densities greater than -50 mA cm-2, the current
densities for CO and methyl formate production also exhibited
saturation, and the total current density saturated as well. The
Tafel slope for CO production was approximately -120 mV
decade-1 at lower current densities. This value is different from
that obtained for the total photocurrent (curve d), owing to the
contribution of the partial current densities for hydrogen and
methyl formate production. The slope for CO production in
our system (∼-120 mV decade-1) is in agreement with those
for electrochemical CO2 reduction to CO reported by several
authors for aqueous media71 and nonaqueous media72 as well
as that for the photoelectrochemical reaction, reported by
Bockris and co-workers using p-CdTe in DMF containing 5%
water.40,43
Effect of Added Water and Strong Acid. The effect of
added water was examined using a typical p-GaAs electrode
(Figure 7). In the methanol solution used in the present
experiments, water exists as an impurity at a concentration
estimated by gas chromatography to be approximately 60 mM.
To determine the effect of water, various amounts of water were
added to the electrolyte, and galvanostatic electrolyses were
conducted at -50 mA cm-2. As can be seen, the product
distribution did not change significantly until the water concentration reached 0.4 M. Thus, at high CO2 pressures, the
presence of low-to-moderate water concentrations does not
greatly affect the CO2 reduction. At higher water concentrations
(>1 M), however, the product distribution changed (Figure 7).
The main product of the electrolysis became hydrogen, and the
current efficiency for CO production decreased.
The effect of added strong acid on the distribution of
reduction was also examined. Various amounts of concentrated
perchloric acid were added to the electrolyte, yielding concentrations in the 3.1 × 10-7 to 7.2 × 10-3 M range. Galvanostatic
electrolyses were conducted at 50 mA cm-2 using a p-GaAs
electrode. The product distribution did not change significantly
with increasing proton concentration until a value of 3.1 × 10-4
M was reached. Even at the highest concentration (3.1 × 10-3
M), however, the current efficiency for CO production only
dropped to ∼68%, while that for H2 production only rose to
∼15%.
Electrode Stability. Regarding electrode stability, the results
obtained for all three p-type semiconductor electrodes were quite
encouraging. At 40 atm CO2 pressure, no surface degradation
was visually observed for any of the three electrode materials,
even after electrolysis at -100 mA cm-2. These results are in
contrast to those obtained in our previous work on CO2 reduction
on p-type semiconductors in aqueous media, in which photocorrosion was observed.55,73 Other workers have also reported
stability problems for both p-InP40,42 and p-GaAs40 during CO2
reduction in both aqueous and water-containing nonaqueous
electrolytes. The key to the stability reported here appears to
lie in the use of a nonaqueous solvent plus high-pressure CO2.
In the case of 1 atm CO2 pressure, a visually observable white
film was observed on the surface of the p-InP electrodes after
electrolysis, even at relatively low current densities, e.g., -20
9840 J. Phys. Chem. B, Vol. 102, No. 49, 1998
Hirota et al.
mA cm-2. X-ray photoelectron spectroscopic (XPS) measurements indicated that the white film was pure indium. The
presence of a peak in the O 1s region (see Supporting
Information) indicates that the film was at least partly oxidized.
A section of the survey spectrum for a p-InP electrode obtained
after galvanostatic electrolysis under 40 atm CO2 pressure
(Figure 8B) showed that the surface was essentially identical
with the virgin surface (Figure 8A). Neither surface exhibited
an O 1s peak (see Supporting Information). However, after
electrolysis under 1 atm CO2 pressure (Figure 8C), the peaks
for phosphorus completely disappeared, indicating that only
indium remains on the surface.
In the case of p-GaAs, a white film was observed on the
electrode surface after electrolysis in the water-containing
solutions, whereas film formation was not observed in the
absence of added water. This white material was probably either
metallic Ga or an oxide or hydroxide, analogous to the case of
p-InP. In the case of the higher acid concentrations, a small
extent of surface corrosion was observed, but no white films
were observed.
Electrode Reaction Mechanism. To explain the production
of CO in the electrochemical and photoelectrochemical reduction
of CO2 in nonaqueous solution, several different mechanistic
schemes have been proposed. On the basis of in situ reflectance
measurements using a lead electrode in 0.4 M tetramethylammonium perchlorate in dry propylene carbonate, Aylmer-Kelly
et al. proposed the following mechanism:72
slow
CO2 + e- f CO2•-
(2)
CO2 + CO2•- f (CO2)2•-
(3)
(CO2)2•- + e- f (COO-)2
(4)
The first-order dependence of CO2 pressure and the -107 mV
Tafel slope were taken as evidence for eq 2 being ratedetermining.
Amatore and Saveant examined CO2 reduction on Hg in
dimethylformamide (DMF) with and without added water.74
Three competing pathways were found: (1) oxalate production,
as in 4 above; (2) CO production via reaction of CO and CO2•-;
and (3) formate production via protonation of CO2•- by water.
The CO production mechanism was thought to involve the
coupling of CO2 and CO2•- radical anion, as in eq 3 above,
where the complex radical anion can be represented as
This species then can undergo either electrochemical reduction,
as in eq 4, or chemical reduction via CO2•, and the resulting
dianion can then decompose into CO and carbonate:
(CO2)22- f CO + CO32-
(5)
The competing oxalate pathway involves a carbon-carbonbonded dimeric radical anion.
On the basis of studies on Hg, Pb, Sn, In, and Pt electrodes
in tetraethylammonium perchlorate/acetonitrile, Vassiliev et al.
have proposed a similar mechanism, except that the intermediates are considered to be adsorbed:35
fast
CO2•-(ads) + CO2(ads) 98 (CO2)2•-(ads)
slow
(CO2)2•-(ads) + e- 98 (COO-)2
(6)
(7)
On Pt, the mechanism is thought to involve
2CO2•-(ads) f CO + CO32-
(8)
To explain the existence of two Tafel slopes (-140 to ∼-180
mV and -600 to ∼-700 mV) for all of the metals except Pt,
reaction 7, i.e., the second electron transfer, is considered to be
rate-determining in the lower slope region and reaction 2 to be
rate-determining in the higher slope region. The reason that
the slopes expected at 298 K (i.e., ∼-40 and ∼-120 mV,
respectively) are not observed is proposed to be due to the
potential dependences of the coverages of CO2 and CO2•-, the
coverages decreasing with increasing negative potential. Hori
et al. have also invoked a similar scheme to explain the
production of CO on a number of different metals in nonaqueous
solution.33
For photoelectrochemical CO2 reduction, on p-CdTe in 0.1
M tetrabutylammonium perchlorate/DMF containing 5% water,
Taniguchi et al. have proposed a similar scheme, in which the
first electron transfer (reaction 2) can explain the -120 to
∼-150 mV Tafel slopes observed.40 Bockris and Wass later
proposed a more complex mechanism in order to explain the
catalytic effect of the tetraalkylammonium cation (TAA+) in
CO2 reduction on p-CdTe in acetonitrile containing 1% water.43
This scheme involves two types of reaction sites: on A sites,
the catalytic step (redox mediation by the TAA+ cation to
produce the CO2•- radical anion) occurs, while on the B sites,
these radical ions undergo photoassisted electron transfer
CO2•- + H+ + e-(photoexcited) f CO + OH-
(9)
In the present work, we would like to explain the following
results: (1) a partial current density Tafel slope for CO
production of ∼-120 mV; (2) the onset of current at a potential
(-0.5 V vs Ag QRE) that is much more positive, with respect
to the E° for the CO2/CO2•- redox couple (-1.90 V vs SHE,
or -2.22 V vs Ag QRE), than could be expected due to
photoassistance; and (3) the high current efficiencies for CO
production, particularly at high CO2 pressure and negative
potentials (or high current densities). This latter result is one
of the more intriguing aspects of the present work, although
similar effects have been observed for electrochemical CO2
reduction on Ag.9
The ∼-120 mV Tafel slope, (i.e., -2.3RT/RnF, where the
transfer coefficient R ) 0.5) is usually a good indication of a
first electron-transfer step being rate-determining.75,76 The fact
that slopes of about this value have been obtained for both
electrochemical and photoelectrochemical experiments for
several different types of electrodes that produce mostly CO in
nonaqueous solution therefore suggests a high likelihood for
the first electron transfer to CO2 (reaction 2) being ratedetermining. This step might involve either CO2 from solution
or CO2 as an adsorbed species. If the adsorbed species is at
low coverage, the same Tafel slope would be expected. For
higher coverages, larger slopes would be expected owing to
repulsive interactions between the adsorbates.25,35 Indeed, we
observe a much higher slope for CO production at more negative
Photoelectrochemical Reduction of CO2
J. Phys. Chem. B, Vol. 102, No. 49, 1998 9841
potentials (Figure 6), but we have shown that this effect is due
almost completely to the light intensity becoming a limiting
factor. In any case, we would expect the CO2 coverage to
decrease with increasingly negative potential owing to competition with adsorbed cations (TAA+), as normally expected for
the adsorption of a neutral species and as shown spectroscopically by Chandrasekaran and Bockris for Pt.77 This kind of
effect appears to be important in explaining the present results
obtained at 1 atm CO2 pressure, i.e., the saturation of current
seen in Figure 3A. However, at 40 atm, the absence of
saturation out to -1.4 V vs Ag QRE (Figure 3B) is striking.
This effect cannot be explained on the basis of a simple
isotherm, for which the expected shifts in the coverage vs
potential behavior would tend to quickly overwhelm the
difference in concentration.
Furthermore, reaction 2 cannot be rate-determining simply
as written, because of point 2, i.e., the early onset of current (a
shift of ∼+1.7 V, of which at most 0.7 V could be accounted
for by photoassistance). To explain such a shift, we must
consider that the CO2•- radical anion is somehow stabilized on
the surface, i.e., a kind of catalytic effect.
To explain all of the present results, we must examine more
carefully the properties of the (CO2)2•- radical ion complex I.
On the basis of theoretical calculations, this species is stabilized
with respect to dissociation into CO2 and CO2•- by 0.58 eV.78
On the basis of molecular beam experiments, the electron
affinity for I was found to be positive (+0.9 eV), whereas that
for CO2•- was negative (-0.6 eV).76 Therefore, even with no
solvation or adsorption effects, one might expect a shift of 1.5
V for the one-electron reduction of CO2 by simply including
an additional CO2 molecule:
2CO2 + e- f (CO2)2•-
(10)
putting the E° for this reaction in the neighborhood of -0.4 V
vs SHE or -0.72 V vs Ag QRE. Of course, any solvation or
adsorption effects would modify this. This value appears to
be reasonable if we examine the current-potential behavior for
p-InP (Figure 1A). We must also consider the adsorption
equilibrium
CO2(sol) f CO2(ads)
(11)
coupling with reaction 10, which we could write in terms of
the adsorbed species:
2CO2(ads) + e- f (CO2)2•-(ads)
(10′)
We therefore propose the latter as the rate-determining step in
the photoelectrochemical production of CO in CO2-CH3OH.
A proposed surface arrangement for the adsorbed product of
eq 10′ is given in Figure 9.
Thus, it also becomes easier to understand point 3, i.e., high
CO yield at high CO2 pressures and negative potentials, because
the extraordinary stability of I would be enhanced at high CO2
pressure. In the absence of such stabilization, the increasingly
strong adsorption of TAA+ cations with increasing negative
potential would tend to inhibit CO2 adsorption.25,35,77,80 In
addition, the CO2•- radical anion would tend to be electrostatically repelled to an increasing extent.
Comparisons of Semiconductor Behavior. In comparing
the CO2 reduction behavior for the different semiconductor
materials, it is clear that we must consider both bulk semiconductor properties and surface catalytic properties.81,82 It has
already been suggested by Hori et al. that CO production on
various metals, e.g., in particular, Ag, Au, and Pt, is favored
by strong adsorption of CO2•-.33 These latter metals appear to
adsorb CO2•- even in aqueous solution. However, other metals,
e.g., In, appear to adsorb CO2•- better in nonaqueous solution,33
possibly owing to the lack of competition with water adsorption.
A similar situation also probably exists for semiconductor
surfaces. The adsorption of both CO2 and CO2•- is also
probably not as strong as on the metals mentioned above, and
this may be why high-pressure CO2 is necessary to enhance
the CO2 reduction to CO.
Differences in the nature of the adsorption of CO2 on the
three semiconductor materials, p-InP, p-GaAs, and p-Si, may
be at least partly responsible for the observed differences in
the photoelectrochemical behavior. Both the lattice matching
and the strength of the adsorption are likely to be involved.
Figure 9 shows schematically the structures that have been
proposed for adsorbed layers of CO2 and (CO2)2•- on Ni(110).83,84 Subject to the constraints of lattice matching, such
layers could also occur on semiconductor surfaces.
Another possible explanation for some of the differences in
the CO2 reduction behavior for these three materials is the
hydrophobicity of the electrode surface, as proposed by Yoneyama and co-workers.42 These authors explained the differences in the product distributions for CO2 reduction on p-CdTe
and p-InP in terms of differences in the hydrophobicities. A
hydrophobic surface is thought to allow hydrophobic cations
such as TBA+ to adsorb on the electrode surface, thus producing
a hydrophobic environment. CO2 reduction to CO can be
expected to proceed more efficiently in a hydrophobic environment, compared with methyl formate formation or hydrogen
evolution, both of which involve protons as the reactants.
Yoneyama and co-workers proposed the free energy of oxide
formation of the metal ∆Gf° making up the semiconductor as a
measure of the hydrophobicity. In the present work, p-InP
would be considered to be more hydrophobic than p-GaAs,
because the ∆Gf° value for In2O3 (-410.9 kJ mol-1)85 is less
negative compared to that for Ga2O3 (-496.2 kJ mol-1).86 Si
would be considered to be highly hydrophilic owing to the very
negative value (-798.7 kJ mol-1).87
In light of the involvement of adsorbed CO2 proposed here,
the ease of oxide formation could also indicate the tendency of
the surface to adsorb methanol compared to CO2. We propose
that the adsorption of methanol leads to enhanced hydrogen
evolution at negative potentials in the case of 1 atm CO2. The
hydroxyl proton appears to be sufficiently acid to support this
reaction because, as already shown in this work, it is possible
to electrolytically evolve hydrogen in dry methanol electrolyte.
Rationale for the Effects of Water and Acid. The fact that
the CO2 reduction behavior for p-GaAs is not highly sensitive
to moderate concentrations of added water and strong acid in
the case of 40 atm CO2 pressure is further evidence for a surface
protective effect, which is consistent with the proposed layer
of adsorbed (CO2)2•-. As already noted, the p-GaAs surface is
somewhat hydrophilic, while, at 1 atm pressure, CO2 is
apparently not adsorbed strongly enough to counteract the effects
of the adsorbed water. Therefore, high pressure is required to
enhance the adsorption.
Rationale for Electrode Stability. As already mentioned,
p-InP undergoes photocorrosion during electrolysis in the CO2
(1 atm)-methanol solution, and p-GaAs does also in the
presence of water. Cathodic photocorrosion of InP is known
to occur in aqueous solution, producing a layer of metallic In
on the surface:88-90
9842 J. Phys. Chem. B, Vol. 102, No. 49, 1998
Hirota et al.
InP + 3e- + 3H+ f In + PH3
(12)
InP + 3e- + 3H2O f In + PH3 + 3OH-
(13)
or
It is not clear whether this reaction is taking place only with
trace water or if protons can be supplied by methanol. In an
analogous fashion, GaAs can also undergo corrosion in aqueous
solution:91,92
GaAs + 3e- + 3H+ f Ga + AsH3
(14)
or
GaAs + 3e- + 3H2O f Ga + AsH3 + 3OHAccording to the present results, the latter reactions take place
at a slower rate compared to those for InP.
The stabilization of these two materials in the presence of
the high-pressure CO2 solution is consistent with the proposed
adsorption of CO2 or (CO2)2•-. The adsorbed species could
act in the same way that strongly adsorbed redox couples act
to protect chalcogenide semiconductor surfaces in the case of
aqueous electrolytes.93-95 These redox couples can compete
rather successfully with the semiconductor surface itself as a
trap for photogenerated holes, thus preventing the oxidative
attack of the surface. In the present case, with high-pressure
CO2, the adsorbed species appear to be able to compete
successfully with the semiconductor in trapping photogenerated
electrons.
Conclusions
The present work shows that CO2 can be photoelectrochemically reduced to CO at potentials of at least 0.5 V more positive
than those for a representative metallic electrode (Cu) under
similar experimental conditions in the dark. The onset of
measurable photocurrent was also significantly positive with
respect to the estimated reversible potential for the CO2/CO,
CO32- couple. CO2 was also photoelectrochemically reduced
to CO on p-GaAs and p-Si electrodes at potentials that were
significantly positive compared to the Cu electrode. For all three
of the electrode materials, p-InP, p-GaAs, and p-Si, we were
able to reach relatively high photocurrent densities, -100 to
∼-200 mA cm-2 for p-InP, -100 mA cm-2 for p-GaAs, and
-50 mA cm-2 for p-Si. Comparing the potentials at the same
current density, e.g., -50 mA cm-2, the ranking was p-InP
(-1.1 V) > p-GaAs (-1.6 V) > p-Si (-1.8 V), where these
potentials have been corrected for solution resistance. The
differences in behavior may be ascribed to differences in both
the semiconductor bulk properties and surface properties,
including the adsorption strengths of methanol, TAA+, CO2,
and CO2•-.
On the basis of the Tafel slope for CO production for the
p-InP electrode, we conclude that the first electron transfer is
rate-determining, which indicates that the CO2•- radical anion
is produced as an intermediate. The fact that the photocurrent
onset is shifted ∼1.7 V with respect to the standard redox
potential for this couple, only 0.5-0.7 V of which can be
attributed to the photoexcitation of electrons into the conduction
band, suggests a strong catalytic effect. Such an effect can be
rationalized by the stabilization of the CO2•- species by
complexation with CO2 itself, as predicted by published
theoretical and molecular beam studies.
The present work shows that the high-pressure CO2methanol-TBAP electrolyte system offers several attractive
features in terms of photoelectrochemical CO2 reduction at
p-type semiconductors, including (1) high current densities, (2)
high selectivity for CO2 reduction to CO, and (3) high electrode
stability. The high current densities are facilitated by the high
CO2 solution concentration. The high current efficiencies for
CO2 reduction to CO are thought to stem from a highly stable
adsorbed layer of (CO2)2•-, particularly at high CO2 pressures,
which would block access to the electrode surface by methanol.
Such an adsorbed layer appears to also be able to block access
of water and strong acid in the case of p-GaAs, even at water
concentrations as high as 0.4 M and proton concentrations as
high as 3 × 10-4 M.
Finally, regarding the electrode stability, there appears to be
interplay between the current density (and therefore the potential) and the CO2 pressure, such that, at high current densities,
the CO2 pressure must also be high in order to avoid reductive
degradation of the electrode material. The proposed presence
of an adsorbed layer of (CO2)•-, at potentials negative of the
photocurrent onset, may be instrumental in protecting the
semiconductor electrode surface, both by providing a sink for
photogenerated electrons and by blocking access of methanol.
Acknowledgment. This work was partially supported by a
Grant-in-Aid for Scientific Research from the Ministry of
Education, Science, Sports and Culture of Japan and was also
partially supported by the International Joint Research Program
of New Energy and Industrial Technology Development Organization (NEDO).
Supporting Information Available: X-ray photoelectron
survey spectra for the virgin p-InP (100) surface and the p-InP
(100) surface after CO2 photoelectrolysis in 1 and 40 atm CO2
(4 pages). See any current masthead page for ordering and access
information.
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