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Cite this: Catal. Sci. Technol., 2014,
4, 1644

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Photocatalytic conversion of carbon dioxide intomethanol in reverse fuel cells with tungsten oxide
and layered double hydroxide photocatalysts forsolar fuel generation†
Motoharu Morikawa,a Yuta Ogura,b Naveed Ahmed,b Shogo Kawamura,bGaku Mikami,b Seiji Okamotob and Yasuo Izumi*b
The phenomena of the photocatalytic oxidation of water and photocatalytic reduction of CO2 werecombined using reverse photofuel cells, in which the two photocatalysts, WO3 and layered doublehydroxide (LDH), were separated by a polymer electrolyte (PE) film. WO3 was used for thephotooxidation of water, whereas LDH, comprising Zn, Cu, and Ga, was used for the photoreduction of
CO2. For this process, photocatalysts pressed on both sides of the PE film were irradiated with UV-visible
light through quartz windows and through the space in carbon electrode plates and water-repellent
carbon paper for both gas flow and light transmission. 45% of the photocatalyst area was irradiatedthrough the windows. The protons and electrons, which were formed on WO3 under the flow of helium
and moisture, transferred to the LDH via the PE and external circuit, respectively. Methanol was the major
product from the LDH under the flow of CO2 and helium. The observed photoreduction rates of CO2 tomethanol accounted for 68%–100% of photocurrents. This supports the effectiveness of the combined
photooxidation and photoreduction mechanism as a viable strategy to selectively produce methanol.
In addition, we tested reverse photofuel cell-2, which consisted of a WO3 film pressed on C paper and
LDH film pressed on Cu foil. The photoelectrodes were immersed in acidic solutions of pH 4, with thePE film distinguishing the two compartments. Both the photoelectrodes were completely irradiated by
UV-visible light through the quartz windows. Consequently, the photocurrent from the LDH under CO2
flow to WO3 under N2 flow was increased by 2.4–3.4 times in comparison to photofuel cell-1 tested

Received 23rd November 2013,
Accepted 18th February 2014
DOI: 10.1039/c3cy00959a
www.rsc.org/catalysis

under similar conditions. However, the major product from the LDH was H2 rather than methanol using
photofuel cell-2. The photogenerated electrons in the irradiated area of the photocatalysts were obliged
to diffuse laterally to the unirradiated area of photocatalysts in contact with the C papers in photofuelcell-1. This lateral diffusion reduced the photocatalytic conversion rates of CO2, despite the advantagesof photofuel cell-1 in terms of selective formation and easy separation of gas-phase methanol.

Introduction
The photocatalytic conversion of CO2 into fuels has emerged
as an attractive option,1 in terms of both reducing theincreased concentration of atmospheric CO2 as well as generating renewable hydrocarbon fuels that can directly be supplied to our present energy infrastructure. Analogous tophotosynthesis, the photocatalytic conversion of CO2 involves(1) the photooxidation of water and (2) the photoreduction ofa

Department of Nanomaterial Science, Graduate School of Advanced Integration
Science, Chiba University, Yayoi 1-33, Inage-ku, Chiba 263-8522, Japanb
Department of Chemistry, Graduate School of Science, Chiba University,
Yayoi 1-33, Inage-ku, Chiba 263-8522, Japan. E-mail: yizumi@faculty.chiba-u.jp† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c3cy00959a

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CO2 corresponding to photosystem II, and subsequent dark
reactions to incorporate CO2 into carbohydrates utilizing
NADPH formed in photosystem I.2,3
Thus far, several researchers have reported photocatalytic
cells for water oxidation2 and proton reduction4–6 by using
Pt–WO3 and Pt–SrTiO3,7 as well as TiO2 and Pt–TiO2 separated by a polymer electrolyte (PE) film.8 The combinationof steps 1 and 2 of the photocatalytic conversion of CO2
has been realized using metal complexes and enzymatic
photocatalysts.9 To the best of our knowledge, there are no
studies reporting on the combination of semiconductor
photocatalysts for the photocatalytic conversion of CO2. In
principle, the combination of steps 1 and 2 can be regarded
as reverse fuel cell operation. This process is considered to bea reverse mechanism because the products generated byThis journal is © The Royal Society of Chemistry 2014

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conventional fuel cells, i.e., CO2 and H2O, react to form methanol and O2, which are the reactants in the conventional fuel cell.
In fact, the original concept of reaction between CO2 and H2O
was proposed10–12 and demonstrated to be a reverse fuel cell
operation,13 rather than a photocatalytic conversion process.
In this paper, we report the fabrication of a polymer
electrolyte fuel cell (PEFC; Fig. 1) comprising tungsten oxide(WO3) and layered double hydroxide (LDH) photocatalysts,
and a photofuel cell consisting of acid solutions separated bya PE film (Fig. 2). In addition, we experimentally demonstratesolar fuel generation,10,14–17 in which the photooxidation ofwater results in the transfer of protons and electrons to form
fuel combined with CO2.
The primary advantage of using photofuel cells is theband potential of two photocatalysts, which is tunable incomparison with the oxidation and reduction potential ofindependent reactions, by the choice of the elemental composition of the semiconductor catalysts. Traditionally, WO3 hasbeen the catalyst of choice for the photooxidation of water(E° = 1.23 − 0.0591 × pH V, 298 K), primarily because of the relatively positive band potential.18 Similarly, LDH is an unusualclass of layered materials comprising positively charged cation
layer and interlayer anionic species such as CO32−.19 Recently,
researchers have demonstrated the photocatalytic conversion
of CO2 into CO and methanol in water20 or hydrogen21–23 byusing LDHs comprising Ni, In, Zn, Cu, and Ga. From thesereports, we chose LDH compounds comprising Zn, Cu, and Ga
for the photoreduction of CO2 in this study.
In addition, the added advantage of using a photofuel cellis that the photocatalysts used are earth-abundant elements

Fig. 1 Fabricated photofuel cell-1. The figures show the different
components (A) and a photocatalytic test with flowing moisture + He(balance) (front side) and circulating CO2 + He (balance) (rear side) (B).
A1 depicts photofuel cell-1 at the center of B. A2 and A3 depict components of the photocathode and photoanode, respectively. A4 is a viewof the window of photofuel cell-1. A5 depicts the assembled quartz
window, silicon gasket, C electrode plate, C paper, silicon gasket, and
WO3–Nafion–LDH1 assembly.

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Fig. 2 Fabricated photofuel cell-2. The figures show the different
components (A) and LDH1 mounted onto Cu foil and WO3 mounted
onto water-repellent C paper (B, left to right). The smaller cell-2 was also
used in which the LDH1 side (right) was circulated with CO2 gas (C).

and are relatively inexpensive. Furthermore, the products
formed as a result of photocatalysis can be readily separated
in the gas phase. This is a unique advantage of using aphotofuel cell based on PEFC (Fig. 1), as reported for theelectrocatalytic cell separated by Nafion.24

Experimental
Synthesis of WO3 and LDH consisting of Zn, Cu, and Ga
WO3 was synthesized by the calcination of ammonium
paratungstate pentahydrate (NH4)10+(W12O41)10−·5H2O (Aldrich,>99.99%) in air at 973 K for 4 h.
Nitrogen adsorption isotherm measurements were
performed at 77 K within the pressure range 1.0–90 kPa in a vacuum system connected to diffusion and rotary pumps (10−6 Pa)
and equipped with a capacitance manometer (Models CCMT1000A and GM2001, ULVAC). The Brunauer–Emmett–Teller
surface area (SA) was calculated on the basis of eight-point
measurements between 10 and 46 kPa (P/P0 = 0.10–0.45) on theadsorption isotherm. The samples were evacuated at 383 K for2 h before the measurements. The specific SA of WO3 thus
obtained was estimated to be 20 m2 g−1.
In this study, we synthesized LDH1 [Zn1.5Cu1.5Ga(OH)8]2+[Cu(OH)4]2−·mH2O starting from nitrates of Zn, Cu, and Ga
and ammonium tetracholorocupurate dihydrate at pH 8.
Details of the experimental procedure adopted for the synthesis of LDH1 have been reported in the literature.22,23 The
exact molecular formula of LDH1, as determined by using
extended X-ray absorption fine structure analysis, was foundto be Zn3Cu3Ga2(OH)13[(μ-O)3Cu(OH)(H2O)2]·mH2O, with thedehydration of three water molecules.21 Furthermore, thespecific SA of LDH1 was estimated to be 62 m2 g−1.
Design of photofuel cell-1
For this process, 95 mg of WO3 or 45 mg of LDH1 wassuspended in 5% of Nafion dispersion solution (DE521,
Wako Pure Chemical; 0.2 mL) and 1-propanol (0.1 mL) andCatal. Sci. Technol., 2014, 4, 1644–1651 | 1645

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separately mounted onto water-repellent carbon paper(C paper, TGP-H-060H, Chemix) in an area of 4 cm2. Subsequently, WO3 mounted onto the C paper was pressed with
LDH1 mounted onto the C paper and separated using a PE
film (Nafion, NR-212, Dupont) of thickness 50 μm. The layers
were pressed using a tabletop press (Model SA-302, Tester
Sangyo Co.) by applying a pressure of 2.0 MPa at 393 K for
10 min. Then the C papers on both sides were carefully
removed, resulting in the formation of the WO3–Nafion–
LDH1 assembly.
The electronic conductivity of WO3 and LDH1 was
improved by mixing WO3 (95 mg) and LDH1 (45 mg) with aminimum amount of C black (0.2 mg; Vulcan XC72, specific
SA 250 m2 g−1) using a mortar and pestle. Subsequently, theobtained WO3 and LDH1 were pressed together with a Nafion
film separator to form a WO3/C–Nafion–LDH1/C assembly, byusing a procedure similar to that adopted for the fabricationof WO3–Nafion–LDH1.
Photofuel cell-1 designed in this study is shown in Fig. S1(ESI†). The prepared WO3–Nafion–LDH1 assembly was first
sandwiched by a water-repellent C paper and then by C
electrode plates, both via a serpentine route (the space section for gas flow and window for light; Fig. 1A2, A3). Subsequently, this assembly was sandwiched by quartz window
plates. It should be noted that the C paper and C plate were
fixed in an area of 2.2 cm2 and the area of the serpentine
route was 1.8 cm2 out of the total 4 cm2 of each quartz window (Fig. 1). Therefore, among a catalyst area of 4 cm2, thearea of 1.8 cm2 (45%) was irradiated through the serpentine
route whereas the area of 2.2 cm2 (55%) was blocked by the
C paper and C plate from light (Fig. S1†).
Photofuel cell-1 offers the advantage of easy separation ofgas-phase products, including methanol. However, a disadvantage associated with the design of the photofuel cell isthat only 45% of the photocatalysts get irradiated.

CO2 conversion tests using photofuel cell-1
The fabricated photofuel cell-1 was further tested for its CO2
conversion efficiency. As the first step, all solvents includedin the Nafion dispersion solution were removed by flowing
N2 gas at a rate of 100 mL min−1 through a water bubbler
maintained at 343 K, independently to WO3 and LDH1 for
10 h. Subsequently, WO3 in the system was purged with He
gas at a rate of 50 mL min−1 through the water bubbler at
323 K. CO2 (3.5%) with the remaining He gas (total 101 kPa)
was circulated in a glass line at a rate of 450 mL min−1
through LDH1 (Fig. 1A1).
WO3 and LDH1 on the Nafion film were irradiated by
UV-visible light from a 500 W xenon arc lamp (Ushio, ModelSX-UID502XAM) via a two-way quartz fiber light guide of 1 m(San-ei Electric Co., Model 5Φ-2B-1000 L) for 10 h (Fig. 1B).
The distance between fiber light exit and photofuel cell window was maintained at 50 mm. The light intensity on thephotocatalyst was 33 mW cm−2. With the onset of irradiation,
the photocatalyst temperature reached as high as 315 K.

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The gas circulating through LDH1 was analyzed by an
inline gas chromatograph equipped with a thermal conductivity detector (GC-TCD; Shimadzu, Model GC-8A), using
He as the carrier gas. The columns were packed with Molecular Sieve 13X-S and polyethylene glycol-6000 supported on
Flusin P (GL Science). The current between the two electrodes(Fig. 1A, B) was monitored simultaneously.
Design of photofuel cell-2
We also designed photofuel cell-2, as shown in Fig. 2. Bothphotofuel cells were compared for their performance.
For the fabrication of photofuel cell-2, 95 mg of WO3 wassuspended in 5% of Nafion dispersion solution (0.24 mL)
and 1-propanol (0.16 mL) and mounted onto water-repellent
C paper in an area of 4 cm2. WO3 mounted onto the C paper
was then covered with a Kapton film (200H, Dupont) of thickness 50 μm and pressed using a tabletop press (SA-302) byapplying a pressure of 2.0 MPa at 393 K for 10 min. Then,the Kapton film was carefully removed to obtain the WO3/C
photoelectrode (Fig. 2B, right). In a separate process, 45 mg
of LDH1 was suspended in 5% of Nafion dispersion solution(0.24 mL) and 1-propanol (0.16 mL) and mounted onto Cu
foil (2.3 × 2.5 cm2) of thickness 30 μm in an area of 4 cm2.
LDH1 mounted onto the Cu foil was then covered with awater-repellent C paper and pressed using a tabletop press(SA-302) by applying a pressure of 2.0 MPa at 393 K for 10 min.
Following that, the C paper was carefully removed to obtain
the LDH1/Cu photoelectrode. In addition, the LDH1/Cu
photoelectrode was prepared by dispersing LDH1 in
1-propanol (0.4 mL) instead of Nafion dispersion solution in amanner similar to that for LDH1/Cu. LDH1 is sticky andattached to Cu foil even in the absence of Nafion (Fig. 2B, left).
CO2 conversion tests using photofuel cell-2
As the next step, we analyzed the CO2 conversion performance of photofuel cell-2. For this, the prepared WO3/C andLDH1/Cu photoelectrodes were immersed in aqueoushydrochloric acid solution (50 mL at each electrode) at pH 4.0,
separated by a 50 μm-thick Nafion film at the center of the cell(Fig. 2A). Subsequently, N2 and CO2 were bubbled at a rate of
50 mL min−1 through the sides of WO3 and LDH1, respectively.
Both photocatalysts were irradiated using a 500 W Xe arc lamp
via a two-way quartz fiber light guide. The photocatalysts
were irradiated from both sides (Fig. 2A) for 30 min and kept
in the dark for 30 min. The light on/off cycle was repeated fivetimes. The distance between the fiber light exit and thephotoelectrode was maintained at 50 mm. The light intensity
on the photocatalyst at this position was 33 mW cm−2.
Moreover, a smaller photofuel cell-2 was also fabricated(Fig. 2C) to make the product concentration relatively higher
for the GC analysis. 15 mL of HCl solution was introduced ateach electrode. CO2 was circulated in a glass line at a rate of
450 mL min−1 through the side of LDH1. The other conditions
were similar to that for Fig. 2A. The gas circulated throughLDH1 was analyzed by the inline GC-TCD (Model GC-9A) using

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Ar as the carrier gas and also a GC-flame ionization detector(FID; Shimadzu, Model GC-18A) equipped with a capillarycolumn Ultra ALLOY-5 (Frontier Laboratories; inner diameter
250 μm, length 30 m) using He as the carrier gas.

Results

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CO2 to methanol conversion in photofuel cell-1
WO3 on one side of the Nafion film was irradiated with
UV-visible light under the flow of He and moisture, while
LDH1 on the other side of the Nafion film was irradiated with
UV-visible light under the flow of CO2 (3.5%) (Fig. 1B). The
main product of the reaction was methanol, whose amountmonotonously increased as a function of time irradiatedunder light (Fig. 3, entry a). Other gases generated during thereaction, including CO and methane, were not detected, probably because the amount was below the detection limit of GC.
Similarly, H2 generation could not be monitored, as He wasthe carrier gas. In this process, the methanol formation rate
was estimated to be 0.045 μmol h−1 gcat−1, considering that the
LDH1 photoelectrode was irradiated through the serpentine
route and the effective illumination area is 45% (Fig. 1A4).
During the total illumination period of 10 h, the current
from the LDH1 electrode to the WO3 electrode was stabilized
to a constant value of 0.22 μA in 1.6 h (Fig. 4a). The current
corresponds to an electron flow rate of 0.40 μmol-e− h−1 gLDH−1
from WO3 to LDH1 (Table 1a). Here, 68% of the photocurrent
was accounted for by the six-electron reduction reaction(6 × 0.045 μmol-CH3OH h−1 gcat−1 = 0.27 μmol-e− h−1 gcat−1)
from CO2 to methanol.
Furthermore, we analyzed the effects of mixing C blackwith the photoelectrodes by comparing the CO2 conversion
performance of the WO3/C–Nafion–LDH1/C assembly withthat of the WO3–Nafion–LDH1 described above. The CO2 conversion results of the WO3/C–Nafion–LDH1/C assembly in thephotofuel cell are plotted in Fig. 3, entry b. As seen from thefigure, the methanol formation rate is 0.029 μmol h−1 gcat−1.
This indicates a reduction in the methanol formation rateby 36% with the addition of C black to LDH1 and WO3.
The lower formation rate should be due to lower light

Fig. 4 Time evolution of the photocurrent generated during CO2
conversion tests using photofuel cell-1 consisting of WO3 and LDH1 (a)and WO3 and LDH1 both mixed with C black (b). The photocatalysts
were in the dark for 10 h and UV-visible light was irradiated in the next
10 h (a, b).

penetration depth into both photocatalyst layers mixed
with C rather than the effect of improved electron conductivity by mixing the photocatalysts with C. During the test,
the photocurrent from the LDH electrode to the WO3electrode stabilized to a constant value of 0.10 μA in 1.6 h(Fig. 4b). The constant current corresponds to an electron
flow rate of 0.18 μmol-e− h−1 gLDH−1 from WO3/C to LDH1/C(Table 1b). Most of the current was accounted for by the sixelectron reduction reaction (6 × 0.029 μmol-CH3OH h−1 gcat−1 =0.17 μmol h−1 gcat−1) from CO2 to methanol.
The generated photocurrents decreased in the initial 1.6 hof irradiation both in Fig. 4a and b. In comparison to therather constant formation of methanol (Fig. 3), a major partof the initial photocurrents may not be due to photocatalysis.
In this study, we performed two blank tests. One blanktest was performed at 323 K, which is higher than the maximum temperature (315 K) of kinetic tests irradiated by light,
under dark conditions and flow of He and moisture to WO3
and flow of CO2 (3.5%) and the remaining He to LDH1. The
other blank test was performed with light irradiation under
the flow of He and moisture to WO3, and the flow of He to
LDH1. For both tests, we could not detect products above
the detection limit of GC. The photocurrent was negligible
after 2 h of reaction.

CO2 to methanol conversion using photofuel cell-2

Fig. 3 Time evolution of the formation of methanol in photofuel cell-1
consisting of WO3 and LDH1 (a) and WO3 and LDH1 mixed with C black(b). The photocatalysts were in the dark for 10 h and UV-visible light
was irradiated in the next 10 h (a, b).

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The CO2 photoconversion performance of photofuel cell-1 wascompared with that of photofuel cell-2 containing WO3 andLDH1 immersed in acid solutions, and fully exposed to light.
For the photofuel cell containing 95 mg of WO3 and45 mg of LDH1 separately mounted onto C paper and Cu foil,
respectively (Fig. 2A, B), the photocurrent gradually increased
within 15–20 min of irradiation under UV-visible light andgradually decreased to background levels within 10–15 minin the dark (Fig. 5a).
The maximum photocurrent from the LDH1 electrodeto the WO3 electrode was 1.15 μA, as determined from five
on/off cycles of light irradiation. The photocurrent corresponded

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Table 1 Comparison of photocurrents generated during the CO2 conversion tests using the photofuel cells designed in this study

Entry

Cell type

WO3a support

LDH1b support

Current (μA)

e− flow rate
(μmol h−1 gLDH−1)

MeOH or H2 form rate
(μmol h−1 gLDH−1)abPhotofuel cell-1

0.40d
0.18d

0.045 (MeOH)d0.029 (MeOH)dPhotofuel cell-2

Nafion film
Nafion film
(C black mixedc )Cu foilCu foil (no Nafion disp sol usede )

0.22
0.10

c
d

Nafion film
Nafion film
(C black mixedc )C paperC paper

1.15
1.63

0.96
1.36

0.49 (H2)0.67 (H2)

95 mg. b 45 mg. c 0.2 mg. d Divided by exposed LDH1 sample to light (45 mg × 0.45). e LDH1 was dispersed in 1-propanol only and mounted
on Cu foil.

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ato an electron flow rate of 0.96 μmol h−1 gLDH−1, a factor of
2.4 times greater than that using photofuel cell-1 (Table 1c, a).
In this comparison, the electron flow rates were estimated by
the area of photocatalyst exposed to light irradiation, e.g., 45%
and 100% for photofuel cell-1 and photofuel cell-2, respectively.
In addition, we performed the CO2 photoconversion test
using photofuel cell-2 consisting of WO3 (95 mg) on C paperand LDH1 (45 mg) on Cu foil, which was prepared without
the Nafion dispersion solution. The change in photocurrent
during the five on/off cycles of light is depicted in Fig. 5b. As
seen from the figure, the photocurrent increases gradually
under light and decreases gradually to background level inthe dark. This trend is very similar to that shown in Fig. 5a.
However, the maximum photocurrent (1.63 μA) was found toincrease by a factor of 1.42 times with the elimination of
Nafion in the LDH1 photoelectrode mounted on Cu foil.
The photocurrent corresponded to an electron flow rate of
1.36 μmol h−1 gLDH−1, which was 3.4 times greater than thatusing photofuel cell-1 (Table 1d, a).
Both the gas circulated through LDH1 and the acidsolution for LDH1 were analyzed in a separate test using asmaller photofuel cell-2 (Fig. 2C). The time evolution isdepicted in Fig. 6. When WO3 on C paper was purged with
N2 gas and LDH1 on Cu foil was circulated with CO2, bothprepared using a Nafion dispersion solution, hydrogen wasa major product from LDH1 (Fig. 6, entry a) at a formation

rate of 0.49 μmol-H2 h−1 gLDH−1. H2 formation was also
observed in dark cycles. This fact is related to the slow decreaseof photocurrent in dark cycles (Fig. 5a), suggesting diffusion
control. Most of the current (0.96 μmol-e− h−1 gLDH−1)
was accounted for by the two-electron reduction reaction(2 × 0.49 μmol-H2 h−1 gLDH−1 = 0.98 μmol h−1 gLDH−1; Table 1c).
When WO3 on C paper was purged with N2 gas andLDH1 on Cu foil was circulated with CO2, prepared without
the Nafion dispersion solution, the hydrogen formation
under UV-visible light irradiation became faster, at a rateof 0.67 μmol-H2 h−1 gLDH−1 (Fig. 6b). Most of the current(1.36 μmol-e− h−1 gLDH−1; Fig. 5b) was accounted for by thetwo-electron reduction reaction (2 × 0.67 μmol-H2 h−1 gLDH−1 =1.34 μmol h−1 gLDH−1; Table 1d).
In both tests in Fig. 6a and b, other gases generated
during the reaction through LDH1 were not detected above
the detection limit of GC-TCD. The acid solution for LDH1
in photofuel cell-2 under the conditions of Fig. 6b (Fig. 2C)
irradiated under UV-visible light for 10 h was analyzed by
GC-FID, but methanol was not detected above the detection
limit (4.83 pmol in 1.0 μL injection). The detection limit
corresponded to the formation rate of 0.16 μmol-CH3OH h−1
gLDH−1. Taking the six-electron reduction into account, thedetection limit was 0.96 μmol-e− h−1 gLDH−1 versus the observed
current of 1.36 μmol-e− h−1 gLDH−1 (Fig. 5b). Therefore methanol was not the major product in photofuel cell-2.

Fig. 5 Time evolution of photocurrent generated during CO2
conversion tests using photofuel cell-2 comprising WO3 on C paper,
prepared using Nafion suspension solution (a, b) and LDH1 on Cu foil,
prepared using Nafion suspension solution (a) and prepared using
1-propanol without Nafion (b).

Fig. 6 Time evolution of the formation of hydrogen during CO2
conversion tests using photofuel cell-2 comprising WO3 on C paper,
prepared using Nafion dispersion solution (a, b) and LDH1 on Cu foil,
prepared using Nafion suspension solution (a) and prepared using
1-propanol without Nafion (b).

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Discussion
It is well known that the mechanism underlying the photocatalytic splitting of water using WO3 under UV irradiation is
via the following equation.25,26

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2H2O → O2 + 4H+ + 4e−(1)

In this study, WO3 exhibited constant oxygen generation
upon irradiation by UV-visible light in the presence of a sacrificial oxidant (Ag+).27 The potentials for the photoreduction
reactions, e.g., H2 formation, are beyond the bandgap energyof WO3.18 Thus, the protons generated in this process transferred to LDH1 via Nafion film in the case of photofuel cell-1,
and via the acid solution and Nafion film in the case ofphotofuel cell-2. On the other hand, the electrons generated
in this process transferred to LDH1 via the external circuit(Scheme 1).
On comparing the photocurrent generated in photofuelcell-1 and photofuel cell-2 with the same types and amounts ofphotocatalysts, it was observed that photofuel cell-2 generated
a higher photocurrent. This suggests that proton diffusion in
the HCl solutions (pH 4) of photofuel cell-2 (Table 1c, d) is sufficiently high and that the diffusion of protons in 50 μm-thick
Nafion film is relatively critical. The conductivity of Nafion film ata relative humidity of 10–100% was reported to be 79 mS cm−1
based on electrochemical impedance spectroscopy.28 If theapplied voltage was assumed to be 1 V, the conductivity
corresponded to proton conductivity of the order of 10−5 mol h−1
for Nafion film of thickness 50 μm. In contrast, the proton diffusion rate in acid solution (pH) can be estimated based on the
catalytic rate of the diffusion-limited reaction.29 The hydrogenformation rate from an Fe2S2(CO)4[P(OCH3)3]2 complex was ofthe order of 10−1 mol h−1. Apparently, proton conduction in
Nafion film is more critical compared to that in acid solution,
but it is somewhat faster compared to the charge flow in thecells of this study (3.6–61 nmol h−1; Table 1), suggesting rate
control primarily by surface reactions.
Fig. 5a and b show a gradual increase in the backgroundcurrent. This could primarily be attributed to the slow supplyof protons to LDH1, leading to a concentration gradient of

Scheme 1 Schematic illustration of the flow of materials and electrons
during photocatalytic CO2 conversion in reverse photofuel cell-1
consisting of WO3 and LDH1.

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solutions between the WO3 side and LDH1 side. Alternatively,
the pH of the solution in the LDH1 side would have increased
with the increase in temperature from 290 to 310 K due to thesolubility dependence of CO2 during the five on/off irradiation
cycles. Consequently, to compensate the pH change, the background current might have increased gradually.
Reaction (1) in the presence of the WO3 catalyst isfollowed by reaction (2) over LDH1.
CO2 + 6H+ + 6e− → CH3OH + H2O(2)

The mechanism underlying reaction (2) seems to be similar to that of reaction (2′), which has been reported for LDH1
and [Zn1.5Cu1.5Ga(OH)8]2+CO32−·mH2O photocatalysts.21–23
More specifically, the importance of protonation to the oneelectron-reduced CO2 species has been discussed while
explaining the formation of formic acid30 and methane31 in
the presence of TiO2 and Zn–Ge oxynitride.32 Specific reaction pathways have been proposed for reaction (2) or reaction(2′), which result in methanol (or CO/methane) via the formation of formaldehyde,21,23,33 carbene,33 and glyoxal33 by multiple protonation steps and electron supply.
CO2 + 3H2 → CH3OH + H2O(2′)

Similar to the reports for reaction (2′),21–23 methanol was
the major product formed in photofuel cell-1. 68%–100% ofelectron production/consumption during the steady-state
period, between 2 and 10 h, in photofuel cell-1 (Fig. 3 and 4)
can be attributed to the conversion of CO2 to methanol. We
evaluated the photocatalytic performance starting from H2O
and CO2 according to the photocurrent from LDH1 to WO3electrodes in the photofuel cells (Table 1).
The excessive photocurrents observed in the initial 1.6 h
in photofuel cell-1 corresponded to 0.022 μmol of electrons(Fig. 4a, b). A major part of the initial photocurrents are notdue to photocatalysis. One of the possibilities is that WO3proceeded reaction (1) whereas the supplied electrons from
WO3 to the photocathode were consumed non-catalytically,e.g. the partial reduction of CuII sites in LDH1 under irradiation. In this study, the amount of Cu used in the cell was
180 μmol. We tried to detect reduced CuI sites among major
CuII sites in LDH1 used for the tests in Fig. 5b & 6b by synchrotron Cu K-edge X-ray absorption spectroscopy at KEK-PF& SPring-8.21,23,34 However, 120 ppm of CuI at maximum was
hard to detect experimentally.
The major generation of hydrogen from the photocathode(LDH1) was directly monitored in photofuel cell-2 (Fig. 6). In
contrast, the part of the photocurrent that was not attributed
to the conversion of CO2 to methanol, which is less than 32%
of total photocurrents (Table 1), may be ascribed to the formation of hydrogen in photofuel cell-1. The reduction potential for reaction (2) (E° = −0.32 − 0.0591 × pH V), in whichmethanol concentration is negligible compared to proton
concentration, is similar to that for reaction (3) (E° = 0 −0.0591 × pH V).35 In the blank test with light irradiation

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under the flow of He and moisture to WO3, and He to LDH1
in photofuel cell-1, the photocurrent was negligible, after 2 hof reaction. Thus, H2 formation was negligible but further
photocatalytic reduction to methanol should proceed at theinterface of LDH1 and the gas (CO2 & He) in photofuel cell-1
whereas reaction (3) predominantly proceeded at the interface of LDH1 and the acid solution of pH 4.0 in photofuelcell-2. The observed clear contrast in selectivity is due to gaseous CO2 (3.5%) in photofuel cell-1 (methanol formation) versus the limited solubility of CO2 in water in photofuel cell-2(hydrogen formation).
2H+ + 2e− → H2

(3)

In summary, efficient CO2 conversion to methanol can beobserved over LDH1 in photofuel cell-1, which is analogous
to the CO2 photoconversion to methanol in CO2 + H2 mixed
gases,21–23 whereas hydrogen was preferably formed inphotofuel cell-2.
Although the major photocatalytic products (methanol orH2) were different, the electron flow rates in photofuel cell-2(0.96–1.36 μmol h−1 gLDH−1; Table 1c, d) were observed to besignificantly higher, by 2.4–3.4 times, when compared tothose in photofuel cell-1 (0.40 μmol h−1 gLDH−1; Table 1a). A
reason for this is the longer electron diffusion path from theWO3 layer to the C electrode and that from the C electrode to
the LDH1 layer for photofuel cell-1. In particular, the protontransfer in the serpentine area (Fig. 1A2–5) should be good,while the electrons separated from holes in the irradiated serpentine area of photocatalysts must laterally diffuse to theunirradiated area of photocatalysts, C paper, and then to theC electrode (anode) connected to the external circuit. For
the photocathode, the electron flow from the C paper to theunirradiated area of LDH1 and then laterally to the irradiatedserpentine area of LDH1 is needed. For photofuel cell-2, theelectrons separated under irradiation easily diffuse into thethin WO3 photocatalyst vertically to the C paper,36 and fromthe C paper vertically to the thin LDH1 (Fig. 2A, C).
Of the two photofuel cells analyzed in this study, the maximum photocurrent was observed (Fig. 5b) in photofuel cell-2
consisting of WO3 and the LDH1 photoelectrode on Cu foil prepared without Nafion dispersion solution (Fig. 2B). The current
was quantitatively in accord with dominant H2 formation(Fig. 6b). Nafion dispersion solution used to mount LDH1 on
Cu foil had a negative effect in covering the active photoreduction sites (Table 1c, d). Furthermore, X-ray diffraction studiesof the LDH1 sample indicate that the layered structure of
LDH1 is not destroyed during the photoreduction test for 5 h.
In spite of this, LDHs as hydroxide-based materials needing to
be in close contact with Nafion dispersion solution/film would
be unstable in the longer-term application of photofuel cell-1.

Conclusions and future prospects
A reverse photofuel cell to form O2 and methanol was
designed using WO3 as the photooxidation catalyst of water

1650 | Catal. Sci. Technol., 2014, 4, 1644–1651

and a LDH, Zn3Cu3Ga2(OH)13[(μ-O)3Cu(OH)(H2O)2]·mH2O, as
the photoreduction catalyst of CO2. This cell could be used toenable solar fuel generation if optimum photooxidation/
reduction catalysts are chosen independently.
Photofuel cell-1, which was designed on the basis of PEFC,
was advantageous in terms of the easy separation of methanol gas as the major product. On the other hand, solar fuelgeneration was limited, especially from the photoelectron
flow efficiency viewpoint. The electrons generated in WO3 by
the photooxidation of water were obliged to move laterally tothe unirradiated area of WO3 and then to carbon, and fromcarbon to the unirradiated area of LDH1 and then laterally tothe irradiated area of LDH1.
Photofuel cell-2, consisting of independent photoelectrodes
immersed in acid solution (pH 4), had higher photocurrents,
by 2.4–3.4 times. However, the reaction on LDH1 caused H2formation and methanol was not found above the detection
limit of GC-FID. Future studies are required to improve
photofuel cell-1 and to control the potential of the harder step,i.e., CO2 photoreduction at the photocathode by using sustainable sources such as solar cells.27

Acknowledgements
The authors are grateful for the financial support from theGrant-in-Aid for Scientific Research C (proposal no. 22550117)
from Monbukagakusho and the Feasibility Study Stage of
A-STEP (proposal no. AS231Z01459C and AS251Z00906L) fromthe Japan Science and Technology Agency. The X-ray absorption
experiments were performed with the approvals of the Photon
Factory Proposal Review Committee (no. 2011G033 and2009G552) and the grant of the Priority Program for DisasterAffected Quantum Beam Facilities (2011A1977, SPring-8 & KEK).

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