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 ( 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. References and Notes (1) Halmann, M. Chemical Fixation of Carbon Dioxide: Methods for Recycling CO2 into Useful Products; CRC Press: Boca Raton, FL, 1993. (2) Electrochemical and Electrocatalytical Reactions of Carbon Dioxide; Sullivan, B. P., Krist, K., Guard, H. E., Eds.; Elsevier Science Publishers B.V.: Amsterdam, 1993. (3) Taniguchi, I., In Modern Aspects of Electrochemistry; Bockris, J. O’M., White, R. E., Conway, B. E., Eds.; Plenum Publishing: New York, 1989; No. 20, Chapter 5. (4) Cook, R. L.; MacDuff, R. C.; Sammells, A. F. J. Electrochem. Soc. 1990, 137, 607-608. (5) Schwartz, M.; Vercauteren, M. E.; Sammells, A. F. J. Electrochem. Soc. 1994, 141, 3119-3127. (6) Furuya, N.; Koike, S. J. Electroanal. Chem. 1989, 271, 181-191. (7) Ikeda, S.; Ito, T.; Azuma, K.; Nishi, N.; Ito, K.; Noda, H. Denki Kagaku 1996, 64, 69-75. (8) Hara, K.; Sakata, T. J. Electrochem. Soc. 1997, 144, 539-545. (9) Hara, K.; Sakata, T. Bull. Chem. Soc. Jpn. 1997, 70, 571-576. (10) Hara, K.; Kudo, A.; Sakata, T.; Watanabe, M. J. Electrochem. Soc. 1995, 142, L57-L59. (11) Ito, K.; Ikeda, S.; Okabe, M. Denki Kagaku 1980, 48, 247-252. (12) Ito, K.; Ikeda, S.; Iida, T.; Niwa, H. Denki Kagaku 1981, 49, 106112. (13) Ito, K.; Ikeda, S.; Iida, T.; Niwa, H. Denki Kagaku 1982, 50, 463469. (14) Nakagawa, S.; Kudo, A.; Azuma, M.; Sakata, T. J. Electroanal. Chem. 1991, 308, 339-343. (15) Kudo, A.; Nakagawa, S.; Tsuneto, A.; Sakata, T. J. Electrochem. Soc. 1993, 140, 1541-1545. (16) Hara, K.; Tsuneto, A.; Kudo, A.; Sakata, T. J. Electrochem. Soc. 1994, 141, 2097-2103. (17) Hara, K.; Kudo, A.; Sakata, T. J. Electroanal. Chem. 1995, 391, 141-147. (18) Todoroki, M.; Hara, K.; Kudo, A.; Sakata, T. J. Electroanal. Chem. 1995, 394, 199-203.
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