Analysis of Products from Photoelectrochemical Reduction of

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Analysis of Products from Photoelectrochemical Reduction of by GaN-Si Based Tandem Photoelectrode

13

CO2

Takeyuki Sekimoto,*,† Hiroshi Hashiba,† Shuichi Shinagawa,‡ Yusuke Uetake,§ Masahiro Deguchi,† Satoshi Yotsuhashi,† and Kazuhiro Ohkawa§ †

Advanced Research Division, Panasonic Corporation, Seika, Kyoto 619-0237, Japan Panasonic Semiconductor Solutions Co., Ltd., Hioki, Kagoshima 899-2595, Japan § Department of Applied Physics, Tokyo University of Science, Katsushika, Tokyo 125-8585, Japan ‡

ABSTRACT: Gallium nitride (GaN) has been shown to be a good photocatalyst for not only water splitting but also carbon dioxide (CO2) reduction. To verify the catalytic reaction involved in CO2 reduction, it is essential to confirm that the reaction products only come from dissolved CO2. Here, we report the results of 13CO2-labeling experiments on a GaN-Si based photoelectrochemical system for each reduction product. It was found that the 13 C-based CO2 was almost completely converted to formic acid (HCOOH), methane, and ethylene when KCl was used as the electrolyte. In contrast, HCOOH from 12C was observed when KHCO3 electrolyte was used, meaning that HCO3− in the electrolyte partly contributed to CO2 reduction. The energy conversion efficiency and Faradaic efficiency of the respective reduction processes in the present system are also discussed.

line silicon (μc-Si) p-i-n17 as the photoabsorption layer and Si p−n junction device, respectively. In the case of CO2 reduction using organic catalysts, 13CO2labeling experiments are often conducted2−5 to clarify whether the CO2 reduction products are derived from catalytic reaction or from the dissociation of organic impurities. In the case of CO2 reduction using inorganic catalysts, although there is almost no possibility that CO2 reduction products are derived from the dissociation of organic impurities, it is still preferable to carry out 13CO2-labeling experiments to exclude the possibility of experimental contaminants. In this paper, using 13CO2-saturated cathode electrolytes, we demonstrate that the hydrocarbons (methane [CH4] and ethylene [C2H4]) and HCOOH produced in our system are derived from PEC CO2 reduction only. On the basis of this confirmation, we report that the energy conversion efficiency of hydrocarbon and alcohol production was highly improved using the InGaN photoabsorption layer and a-Si p-i-n/μc-Si p-i-n device.

1. INTRODUCTION Global concerns continue to grow regarding the increase of carbon dioxide (CO2) in the atmosphere as a greenhouse gas and the future depletion of fossil fuels. Artificial photosynthesis technologies, which can convert CO2 and water to various fuels using solar energy, are the ultimate solution to solve both of these concerns at the same time. Until now, artificial photosynthesis has been mainly investigated for systems based on organic catalysts,1−5 and only a few results on artificial photosynthesis using inorganic catalysts have been reported.6,7 The authors have previously shown that gallium nitride (GaN) photoelectrodes (PEs) can reduce CO2 using the photoelectrochemical (PEC) system shown in Figure 1a8,9 and that the heterostructure of the unintentionally doped (uid-) AlGaN (photoabsorption layer)/highly conductive (n+-) GaN on n-GaN substrate10−13 enhanced the efficiency of the reduction through a piezoelectric effect. However, AlGaN can absorb only the UV region of solar light, and therefore almost all of the visible light passed through the device without being absorbed. To efficiently use the wide wavelength range of solar light, we connected Si p−n junction devices14 to the back of the GaN PE as shown in Figure 1b15 and then used GaN and InGaN, which have a narrower bandgap, instead of AlGaN as the photoabsorption layer.16 The energy regions of light absorption in the solar light spectrum in each layer are shown in Figure 1c. As a result, we achieved an energy conversion efficiency for formic acid (HCOOH) production of 0.97%16 using InGaN and amorphous silicon (a-Si) p-i-n/microcrystal© 2016 American Chemical Society

2. EXPERIMENTAL SECTION PEC CO2 reduction was carried out using an H-type cell as shown in Figure 1a. The tandem PEs (TPEs) consisted of 2 in. diameter circular photoabsorption layer/n+-GaN layer/n-type GaN substrate and Si p−n junctions as shown in Figure 1b. GaN and InGaN (In: 6 at. %) layers were adopted as the Received: April 15, 2016 Revised: June 15, 2016 Published: June 27, 2016 13970

DOI: 10.1021/acs.jpcc.6b03840 J. Phys. Chem. C 2016, 120, 13970−13975

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The Journal of Physical Chemistry C

Figure 2. (a) TEM image of an AlGaN PE surface, and the number “1” represents the position where the EDX spectrum in (b) was measured. (c) TEM image of a NiO cocatalyst particle on the AlGaN PE.

spectrum shown in Figure 2b and the TEM image of a NiO cocatalyst particle on the AlGaN PE shown in Figure 2c revealed that NiO cocatalysts were solidly supported on the AlGaN PE surface. Silicon heterojunction (SHJ) solar cell14 or a-Si/μc-Si tandem solar cell17 designs were used for the Si p−n junction devices in this study, which were positioned on the backside of the n-type GaN substrate. An ohmic electrode of titanium (Ti)/aluminum (Al)/gold (Au) was deposited by electron beam evaporation on part of the backside of the n-type GaN substrate. The n-type GaN substrate and Si p−n junction were electrically connected via the ohmic electrode15 for the SHJ solar cell or an electrical lead16 for the a-Si/μc-Si solar cell. The surface of the GaN PE was directly exposed to the anode electrolyte during the experiments. A 20 mm diameter circular copper cathode was used for hydrocarbon and alcohol production, while a 2 in. diameter circular indium (99.9999%, Nilaco, Japan) cathode was used for HCOOH production. The surface of the cathode was exposed to the cathode electrolyte during the experiments. It has already been shown that copper and indium cathodes can reduce CO2 to hydrocarbons19−22 and alcohols,19−21 and HCOOH19,23 with high selectivity, respectively. For hydrocarbon and alcohol production experiments using the copper cathode, aqueous 5.0 M NaOH and 0.5 M KCl solutions were used as the anode and cathode electrolytes, respectively. For HCOOH production experiments using the indium cathode, aqueous 0.1 M KHCO3 solution was used for both electrolytes, or 0.5 M KHCO3 and 0.5 M KCl were respectively used as the anode and cathode electrolytes. The anode and cathode compartments were separated by a cation exchange membrane (Nafion 424, DuPont). An Ag/AgCl reference electrode (RE) was immersed near the cathode as shown in Figure 1a, and the potential between the RE and cathode was measured using a voltmeter. After purging any impurity gases from the cathode electrolyte using N2 bubbling, CO2 was bubbled through the cathode electrolyte until its concentration reached close to saturation. Labeled 13CO2 gas with chemical purity of >99.9% and isotopic purity of 99% and/or unlabeled 12CO2 gas was used for the

Figure 1. Schematic illustrations of the (a) PEC reactor and (b) structure of the tandem photoelectrode used in this study and (c) the energy region of light absorption in each layer with the solar light spectrum of air mass 1.5.

photoabsorption layers, and the respective TPEs are hereafter abbreviated as GaN-Si and InGaN-Si TPEs. The gallium nitride layers were grown by metal−organic vapor-phase epitaxy, and their crystallinity was confirmed by X-ray diffraction analysis. Here, the thickness of GaN photoabsorption layer was 100 or 200 nm, while that of InGaN photoabsorption layer was 100 nm. Nickel oxide (NiO) cocatalyst particles were deposited on the photoabsorption layers by the same method as that described in the literature.9,18 NiO cocatalysts improve the water oxidation reaction and prevent degradation of the GaN surface during the reaction. The transmission electron microscopy (TEM) image of an AlGaN PE surface is shown in Figure 2a. The energy-dispersive X-ray spectroscopy (EDX) 13971

DOI: 10.1021/acs.jpcc.6b03840 J. Phys. Chem. C 2016, 120, 13970−13975

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The Journal of Physical Chemistry C experiments in which the CO2 reduction products were confirmed using mass spectrometry. The anode electrolyte was purged by Ar bubbling. The anode and cathode compartments were sealed before photoelectrolysis was begun. PEC CO2 reduction was carried out under light illumination of the TPE with zero bias between the cathode and anode. The spectrum of the light used in this experiment was customized using two Xe lamps (MAX-303, Asahi Spectra) to almost completely reproduce the visible and ultraviolet (UV) regions of solar light, as shown in ref 15 with pseudo solar spectrum (air mass 1.5). The power of the light was calibrated using a reference Si photodiode with a power meter (Model 1928-C, Newport). The irradiated area on the surface of the TPE was 3 cm × 3 cm. The gaseous and liquid products of the experiments were analyzed by gas and liquid chromatography, respectively. H2 and carbon monoxide (CO) were determined using a thermal conductivity detector and a flame ionization detector, respectively, attached to a gas chromatograph (GC; GC-4000, GL Science). The total amount of HCOOH produced was determined using a liquid chromatograph (LC; LC-2010, Shimadzu) equipped with a UV detector. Alcohol and aldehyde components were detected by gas chromatography (GC-17A, Shimadzu) equipped with a headspace sampler (Turbomatrix HS40, PerkinElmer). To distinctively detect CH4 and C2H4 gases with 12C or 13C, total ion current chromatograms were obtained using a gas chromatogram mass spectrometer (GC-MS; Shimadzu), where HP-PLOT Q (Agilent Technologies) was used for the separation columns. The produced amounts of H12COO− and H13COO− were quantitatively determined using an ion chromatograph (IP-25, DIONEX) interfaced with a time-of-flight mass spectrometer (IC-TOFMS; LC/MSD TOF system, Agilent Technologies). IonPac AG23 and A23 separation columns (DIONEX), 4.5 mM Na2CO3/0.8 mM NaHCO3 eluent, and a negative conductivity detector (ESI) were used. The sum of the H12COOH and H13COOH determined by IC-TOFMS was confirmed to correspond to the total amount of HCOOH determined by LC. The Faradaic efficiencies were calculated from the charge amount for the reaction products divided by the total charge amounts during the experiment. The energy conversion efficiency (η) for each product in the whole system was calculated by dividing the thermodynamic energy of product formation, for example 890.67 kJ/mol for CH4, by the power of AM 1.5 light (100 mW/cm2).

Table 1. Cathode Electrolyte, Amount of Light, Reaction Voltage and Current, and Produced Amounts of Hydrocarbons for PEC Hydrocarbon Production with GaN 200 nm Photoabsorption Layer, SHJ/SHJ Si p−n Junction Device, Copper Cathode, and 5.0 M NaOH Anode Electrolyte (Total Electric Charge Was 70 C) 12

cathode electrolyte amount of light (mW/cm2) reaction voltage (V) reaction current (mA) CH4 (μmol/(cm2 h)) C2H4 (μmol/(cm2 h))

CO2-saturated 0.5 M KCl

13

CO2-saturated 0.5 M KCl

592

606

−1.67 −20.00 0.28 0.17

−1.66 −21.00 0.22 0.24

Figure 3. Gas chromatograms of gas phases after CO2 reduction with 12 CO2- and 13CO2-saturated KCl cathode electrolyte.

C2H4 peaks did not overlap with the N2, O2, and CO2 peaks. The mass spectra of (a) CH4 and (b) C2H4 components in the gas phases obtained when 12CO2 and 13CO2 were used are compared in Figure 4. In the case of 12CO2, a 12CH4 peak at m/ z = 16 and 12C2H4 peak at m/z = 28 with fragmentary peaks of 12 CHx and 12C2Hx (x = 0−3) were observed. In contrast, when 13 CO2 was used, the 13CH4 and 13C2H4 peaks were shifted to higher m/z compared with those observed when 12CO2 was used, and a 13CH4 peak at m/z = 17 and 13C2H4 peak at m/z = 30 were detected. Moreover, when 13CO2 was used, the intensity of the fragmentary 12C peak at m/z = 12 and the 12C2 peak at m/z = 24 was almost zero. This means that the hydrocarbons produced by our system derived from PEC CO2 reduction and that there were no carbon contaminants with which their chemical reactions occurred. Next, PEC CO2 reduction experiments were conducted using the indium cathode and 13CO2-saturated 0.1 M KHCO3 and 0.5 M KCl aqueous solutions to confirm whether the HCOOH component in liquid phase produced using our system derived from CO2 reduction. The experimental conditions and results are summarized in Table 2. The results of the analysis of the liquid phases using IC-TOF/MS are shown in Figure 5. When 0.1 M KHCO3 was used as the cathode electrolyte, in spite of the low concentration of KHCO 3 and short-time CO 2 reduction until 3 C, a H12COO− peak (m/z = 45.00) was detected in addition to the H13COO− peak (m/z = 46.00). It is thought that this H12COO− peak originated from the PEC reduction of 12CO2 ionized from KH12CO3. In contrast, when 0.5 M KCl was used as the cathode electrolyte, H12COO− was

3. RESULTS 3.1. Confirmation of Reduction Products Using 13CO2. To confirm that the detected hydrocarbons were derived from CO2 reduction, PEC CO2 reduction was carried out with a copper cathode and 12CO2- and 13CO2-saturated KCl as cathode electrolytes. In these experiments, the photoabsorption layer was 200 nm GaN and the Si p-n device had the SHJ/SHJ structure. The top SHJ cell was hollowed in the center to produce a ring shape, and the illumination areas of the top and bottom SHJ cells were arranged so that their generated photocurrents were equal. The other experimental conditions and the results are summarized in Table 1. Figure 3 shows examples of gas chromatograms obtained from the GC-MS measurements. For all PEC CO2 reduction experiments using 12 CO2- and 13CO2-saturated cathode electrolytes, the CH4 and 13972

DOI: 10.1021/acs.jpcc.6b03840 J. Phys. Chem. C 2016, 120, 13970−13975

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Figure 5. IC-TOFMS spectra of H12COO− (m/z 45.00) and H13COO− (m/z 46.00) for (a) 0.1 M KHCO3 and (b) 0.5 M KCl cathode electrolytes.

potentiostat placed between the cathode and anode, and the potential difference between them was controlled to 0 V using the potentiostat. The experiment was carried out until a total electric charge of 50 C. The amount of light incident on the InGaN-Si was 204 mW/cm2 (∼2 sun). The detailed results are summarized in Table 3. The time course of the measured Figure 4. GC-MS spectra for (a) CH4 and (b) C2H4 in the gas phase after CO2 reduction with 12CO2- and 13CO2-saturated KCl cathode electrolyte.

Table 3. Amount of Light, Reaction Voltage and Current, and Rates of Hydrocarbon and Alcohol Production per 1 sun Illumination on InGaN-Si TPEa

Table 2. Anode and Cathode Electrolytes, Si p−n Junction Device, Amount of Light, Reaction Voltage and Current, Electric Charge, and Produced Amounts of HCOOH for PEC HCOOH Production with GaN 100 nm Photoabsorption Layer, Indium Cathode, and 13CO2Saturated Cathode Electrolyte anode electrolyte cathode electrolyte

0.1 M KHCO3 0.1 M KHCO3

0.5 M KHCO3 0.5 M KCl

Si p−n junction device amount of light (mW/cm2) reaction voltage (V) reaction current (mA) electric charge (C) H12COOH (μM) H13COOH (μM) total HCOOH (μM)

a-Si/μc-Si 204 −1.45 −5.01 3 42 80 122

SHJ/SHJ 205 −1.42 −6.28 7 0 245 245

photoabsorption layer

InGaN

Si p−n junction device

a-Si/μc-Si

amount of light (mW/cm2) reaction voltage (V) reaction current (mA) production rate of hydrocarbon (μmol/(cm2 h)) production rate of alcohol (μmol/(cm2 h))

204 −1.63 −15.45 0.99 0.21

a

Anode and cathode electrolytes were 5.0 M NaOH and 0.5 M KCl, respectively. The total electric charge was 50 C. The irradiation area was 9 cm2.

photocurrent is shown in Figure 6. The almost constant photocurrent meant that the PEC CO2 reduction in our system was stable. The products detected for the CO2 reduction were H2, CO, HCOOH, hydrocarbons (CH4, C2H4), alcohols (C2H5OH, CH2CHCH2OH, n-C3H7OH), and aldehydes (CH3CHO, C2H5CHO). The Faradaic efficiency for each reaction product is shown in Figure 7. The total Faradaic efficiency for hydrocarbon and alcohol production was 31.43% and 9.24%, respectively. The sum Faradaic efficiency of all reaction products was less than 100%, at 76.47%. One of the reasons for this was the generation of unmeasurable liquid products such as acetate, glycolaldehyde, and ethylene glycol, as previously reported in the literature.21 In general, the reaction products of CO2 reduction depend on the reaction voltage.20,21 It is thought that the low reaction voltage of −1.63 V for this

not detected within the limit of measurement, and almost only H13COO− was detected. These results indicate that the carbon source for HCOOH production came from CO2 in the electrolysis solution. The above 13CO2-labeling experiments revealed that our system was able to produce fuels not through chemical reactions with carbon contaminants, but from PEC CO2 reduction. 3.2. CO2 to Fuel Conversion. The photocurrent in the InGaN-Si TPE under light illumination was measured via a 13973

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KCl cathode electrolyte. We revealed that the CO2 pressure and stirring speed contributed to the CO2 supply and that the reaction temperature significantly affected the maximum Faradaic efficiency of CH4 production.22 Zhong et al. have reported the effect of CO2 bubbling for various electrolytes.25 They suggested that the activity of CO2 reduction was affected by a ratio of active species (H2CO3*), in addition to the conductivity of an electrolyte solution and the buffer effect against local pH changes. The influence of cathode electrolyte on the CH4 selectivity is to be discussed elsewhere in the near future,26 and we believe that η for fuel production should be improved based on these findings.

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AUTHOR INFORMATION

Corresponding Author

Figure 6. Time course of photocurrent measured in InGaN-Si TPE (2 sun) with copper cathode. The anode and cathode electrolytes were 5.0 M NaOH and 0.5 M KCl, respectively. The irradiation area was 9 cm2.

*E-mail [email protected] (T.S.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Dr. M. Taguchi, S. Shimada, K. Matsuyama, M. Matsumoto, Dr. A. Terakawa, and Dr. W. Shinohara of Eco Solutions Company, Panasonic Corporation, and Dr. S. Yata of Panasonic Eco Solutions Amorton Co., Ltd., for providing the Si p−n junction devices and for their fruitful discussions.

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REFERENCES

(1) Berardi, S.; Drouet, S.; Francás, L.; Surinach, C. G.; Guttentag, M.; Richmond, C.; Stoll, T.; Llobet, A. Molecular Artificial Photosynthesis. Chem. Soc. Rev. 2014, 43, 7501−7519. (2) Luca, O. R.; McCrory, C. C. L.; Dalleska, N. F.; Koval, C. A. The Selective Electrochemical Conversion of Preactivated CO2 to Methane. J. Electrochem. Soc. 2015, 162, H473−H476. (3) Nakada, A.; Koike, K.; Nakashima, T.; Morimoto, T.; Ishitani, O. Photocatalytic CO2 Reduction to Formic Acid Using a Ru(II)-Re(I) Supramolecular Complex in an Aqueous Solution. Inorg. Chem. 2015, 54, 1800−1807. (4) Arai, T.; Sato, S.; Morikawa, T. A monolithic device for CO2 photoreduction to generate liquid organic substances in a singlecompartment reactor. Energy Environ. Sci. 2015, 8, 1998−2002. (5) Nakada, A.; Koike, K.; Maeda, K.; Ishitani, O. Highly efficient visible-light-driven CO2 reduction to CO using Ru(II)-Re(I) supramolecular photocatalyst in an aqueous solution. Green Chem. 2016, 18, 139−143. (6) Sayama, K.; Arakawa, H. Photocatalytic decomposition of water and photocatalytic reduction of carbon dioxide over zirconia catalyst. J. Phys. Chem. 1993, 97, 531−533. (7) Iizuka, K.; Wato, T.; Miseki, Y.; Saito, K.; Kudo, A. Photocatalytic reduction of carbon dioxide over Ag cocatalyst-loaded ALa4Ti4O15 (A = Ca, Sr, and Ba) using water as a reducing reagent. J. Am. Chem. Soc. 2011, 133, 20863−20868. (8) Yotsuhashi, S.; Deguchi, M.; Zenitani, Y.; Hinogami, R.; Hashiba, H.; Yamada, Y.; Ohkawa, K. Photo-induced CO2 Reduction with GaN Electrode in Aqueous system. Appl. Phys. Express 2011, 4, 117101. (9) Yotsuhashi, S.; Deguchi, M.; Zenitani, Y.; Hinogami, R.; Hashiba, H.; Yamada, Y.; Ohkawa, K. CO2 Conversion with Light and Water by GaN Photoelectrode. Jpn. J. Appl. Phys. 2012, 51, 02BP07. (10) Yotsuhashi, S.; Deguchi, M.; Hashiba, H.; Zenitani, Y.; Hinogami, R.; Yamada, Y.; Ohkawa, K. Enhanced CO2 Reduction Capability in an AlGaN/GaN Photoelectrode. Appl. Phys. Lett. 2012, 100, 243904. (11) Yotsuhashi, S.; Hashiba, H.; Deguchi, M.; Zenitani, Y.; Hinogami, R.; Yamada, Y.; Deura, M.; Ohkawa, K. Highly efficient photochemical HCOOH production from CO2 and water using an inorganic system. AIP Adv. 2012, 2, 042160.

Figure 7. Comparison of Faradaic efficiencies for each reaction product generated by CO2 reduction on InGaN-Si TPE with copper cathode. The anode and cathode electrolytes were 5.0 M NaOH and 0.5 M KCl, respectively.

experiment compared with the −1.66 to −1.67 V in Table 1 was mainly caused by the decrease in open circuit voltage resulting from the smaller amount of light used. As a result, the η for hydrocarbon production was greatly improved from 0.046% in the literature15 to 0.316% for the present InGaN-Si TPE. This efficiency is greater than the average efficiency of global biological photosynthesis.24 Simultaneously, a η of 0.092% for alcohol production was obtained.

4. CONCLUSIONS PEC 13CO2 reduction using KCl aqueous solution as the cathode electrolyte resulted in almost no detection of reduction products containing 12C in both gaseous and liquid phases, revealing that for our system the carbon source of the reduction products clearly originated from the CO2 in the cathode electrolyte. Based on this confirmation, a total solar-to-fuels efficiency of 0.408% (0.316% for hydrocarbons and 0.092% for alcohols) was achieved for the InGaN-Si tandem photoelectrode and copper cathode. Recently, our group has reported the systematic analysis of electrochemical CO2 reduction using Cu cathode and 0.5 M 13974

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The Journal of Physical Chemistry C (12) Deguchi, M.; Yotsuhashi, S.; Hashiba, H.; Yamada, Y.; Ohkawa, K. Enhanced Capability of Photoelectrochemical CO2 Conversion System Using an AlGaN/GaN Photoelectrode. Jpn. J. Appl. Phys. 2013, 52, 08JF07. (13) Hashiba, H.; Yotsuhashi, S.; Deguchi, M.; Yamada, Y.; Ohkawa, K. Selectivity Control of CO2 Reduction in an Inorganic Artificial Photosynthesis System. Appl. Phys. Express 2013, 6, 097102. (14) Taguchi, M.; Yano, A.; Tohoda, S.; Matsuyama, K.; Nakamura, Y.; Nishiwaki, T.; Fujita, K.; Maruyama, E. 24.7% Record Efficiency HIT Solar Cell on Thin Silicon Wafer. IEEE J. Photovoltaics. 2014, 4, 96−99. (15) Yotsuhashi, S.; Deguchi, M.; Yamada, Y.; Ohkawa, K. Effect of inserted Si p−n junction on GaN-based photo-electrochemical CO2 conversion system. AIP Adv. 2014, 4, 067135. (16) Sekimoto, T.; Shinagawa, S.; Uetake, Y.; Noda, K.; Deguchi, M.; Yotsuhashi, S.; Ohkawa, K. Tandem photo-electrode of InGaN with two Si p-n junctions for CO2 conversion to HCOOH with the efficiency greater than biological photosynthesis. Appl. Phys. Lett. 2015, 106, 073902. (17) Terakawa, A. Review of thin-film silicon deposition techniques for high-efficiency solar cells developed at Panasonic/Sanyo. Sol. Energy Mater. Sol. Cells 2013, 119, 204−208. (18) Hayashi, T.; Deura, M.; Ohkawa, K. High Stability and Efficiency of GaN Photocatalyst for Hydrogen Generation from Water. Jpn. J. Appl. Phys. 2012, 51, 112601. (19) Hori, Y. Electrochemical CO2 Reduction of Metal Electrodes. In Modern Aspects of Electrochemistry; Vayenas, C. G., White, R. E., Gamboa-Aldeco, M. E., Eds.; Springer: New York, 2008; pp 89−189. (20) Noda, H.; Ikeda, S.; Oda, Y.; Ito, K. Potential Dependencies of the Products on Electrochemical Reduction of Carbon Dioxide at a Copper Electrode. Chem. Lett. 1989, 289−292. (21) Kuhl, K. P.; Cave, E. R.; Abram, D. N.; Jaramillo, T. F. New insights into the electrochemical reduction of carbon dioxide on metallic copper surfaces. Energy Environ. Sci. 2012, 5, 7050−7059. (22) Hashiba, H.; Yotsuhashi, S.; Deguchi, M.; Yamada, Y. Systematic Analysis of Electrochemical CO2 Reduction with Various Reaction Parameters using Combinatorial Reactors. ACS Comb. Sci. 2016, 18, 203−208. (23) Detweiler, Z. M.; White, J. L.; Bernasek, S. L.; Bocarsly, A. B. Anodized Indium Metal Electrodes for Enhanced Carbon Dioxide Reduction in Aqueous Electrolyte. Langmuir 2014, 30, 7593−7600. (24) Barber, J. Photosynthetic energy conversion: natural and artificial. Chem. Soc. Rev. 2009, 38, 185−196. (25) Zhong, H.; Fujii, K.; Nakano, Y.; Jin. Fangming. Effect of CO2 Bubbling into Aqueous Solutions Used for Electrochemical Reduction of CO2 for Energy Conversion and Storage. J. Phys. Chem. C 2015, 119, 55−61. (26) Hashiba, H.; et al. Manuscript in preparation.

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DOI: 10.1021/acs.jpcc.6b03840 J. Phys. Chem. C 2016, 120, 13970−13975