Energy & Fuels 2004, 18, 285-286
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Novel CO2 Electrochemical Reduction to Methanol for H2 Storage Takeshi Kobayashi* and Hiroshi Takahashi Department of Chemical System Engineering, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, 113-8656, Tokyo, Japan Received May 29, 2003. Revised Manuscript Received September 30, 2003 A utilization of a sustainable energy is one of the effective technologies for a mitigation of the CO2 accumulation in the atmosphere. Because sustainable energies such as sunlight or wind power are intermittent and their energy densities are low, they must be converted to a chemical energy that can be stored and transported for efficient utilization. H2 production has been actively investigated for storing energy produced sustainably. However, there are many challenges to be solved in the use of H2. The storage and transportation of H2 is one of the largest issues due to its low energy density per volume. For better handling of the fuel, converting H2 to liquid organic compounds such as methanol is one of the potential strategies. Methanol synthesis using H2 with catalytic hydrogenation has already been industrialized. Although methanol is a liquid fuel with easy handling, an energy efficiency in the conversion is at most approximately 65%.1 Furthermore, the process requires high temperature and high pressure, and it is difficult to obtain such conditions from sustainable energy. If renewable energy can be used for methanol synthesis, H2 is converted to liquid fuel without increasing CO2 emission. Therefore, we have investigated methanol synthesis with low-density energy using a novel electrochemical cell. Here we report the design of the cell and its use for methanol synthesis. The architecture of the designed cell is drawn in Figure 1a. A Pt-modified carbon catalyst (Pt/C) and a Cu-base methanol synthesis catalyst with carbon powder are applied to each side of a polymer electrolyte membrane (Dupont, Nafion 117). While a chamber of the Pt/C catalyst side is supplied with H2 (anode chamber), the opposite chamber is filled with catholyte (cathode chamber) and CO2 is bubbled to the catholyte. Proton generated on the Pt/C transfers to the cathode through the membrane, while an electron transfers to the cathode through an external circuit. CO2 is reduced by the transferred proton and electron on the cathode. In the past twenty years, many papers relating to electrochemical reduction of CO2 have been published.2-15 Hori et al. reported that CO2 and CO effectively yielded organic hydrocarbons and * Corresponding author. Tel: +81-3-5841-7783. Fax: +81-3-58417771. E-mail:
[email protected]. (1) Ando, Y. Nikkei Mechanical 2000, 555, 39-42. (2) Hori, Y.; Kikuchi, K.; Suzuki, S. Chem. Lett. 1985, 1695-1698. (3) Hori, Y.; Murata, A.; Takahashi, R.; Suzuki, S. J. Am. Chem. Soc. 1987, 109, 5022-5023. (4) Hori, Y.; Murata, A.; Takahashi, R. J. Chem. Soc., Faraday Trans. 1989, 85 (8), 2309-2326. (5) Hori, Y.; Murata, A.; Yoshinami, Y. J. Chem. Soc., Faraday Trans. 1991, 87 (1), 125-128. (6) Yamamoto, T.; Tryk, D. A.; Hashimoto, K.; Fujishima, A.; Ohkawa, M. J. Electrochem. Soc. 2000, 147 (9), 3393-3400. (7) Summers, D. P.; Leach, S.; Frese, K. W., Jr. J. Electroanal. Chem. 1986, 205, 219-232. (8) Watanabe, M.; Shibata, M.; Kato, A.; Azuma, M.; Sakata, T. J. Electrochem. Soc. 1991, 138, 3382-3389. (9) Bandi, A.; Kuhne, H.-M. J. Electrochem. Soc. 1992, 139 (6), 16051610. (10) Popic, J. P.; Avramov-Ivic, M. L.; Vukovic, N. B. J. Electroanal. Chem. 1997, 421, 105-110.
Figure 1. Designed electrochemical cell architecture.
alcohols in aqueous electrolytes with metallic electrodes by electrochemical reduction.2-5 Subsequently, many groups investigated the electrochemical reduction of CO2 and methanol synthesis was reported.7-15 However, those studies have faced a competition between CO2 reduction and H2 evolution and low selectivity in products on conventional cells.15 These problems originate in low flexibility of electrocatalyst design, because CO2 reduction and hydrogen generation must simultaneously occur on the cathode as shown in Figure 1b. In many previous works, metal foil was used as a cathode and a modification of the electrocatalyst was limited. The flexibility of catalyst design is improved by introduction of a gas-diffusion electrode; however, the competition between CO2 reduction and H2 evolution still remains.3,8,15 In contrast to conventional methods, CO2 reduction and hydrogen formation can be separated in the designed cell (Figure 1a). Consequently, an improvement of the flexibility in an electrocatalyst design is expected. The cathode catalyst was prepared in the following manner.16 An aqueous solution of sodium carbonate (0.2 M) and an aqueous solution of copper nitrate, zinc nitrate, and aluminum nitrate (Cu: 40 mM, Zn: 23 mM, Al: 37 mM) were prepared individually (Wako Pure Chemical Inc.). The Milli-Q water (Millipore) and reagent grade chemicals were used in all experiments. Sodium carbonate solution (100 mL) and 100 mL of metal nitrate solution were dropped into 40 mL of water simultaneously with vigorous agitation. The obtained slurry was filtered and the cake was washed sufficiently. The cake was dried at 110 °C and calcined in air at 400 °C for 2 h. The composition of the resultant oxide was approximately 45 wt %-CuO/27.5 wt %-ZnO/27.5 wt %-Al2O3. A 200-mg quantity of 5 wt % Nafion-aliphatic alcohol solution (Aldrich) was poured into 100 mg of butyl (11) Ohkawa, K.; Noguchi, Y.; Nakayama, S.; Hashimoto, K.; Fujishima, A. J. Electroanal. Chem. 1994, 367, 165-173. (12) Schwanrtz, M.; Cook, R. L.; Kehoe, V. M.; MacDuff, R. C.; Patel, J.; Sammells, A. F. J. Electrochem. Soc. 1993, 140 (3), 614-618. (13) Li, J.; Prentice, G. J. Electrochem. Soc. 1997, 144 (12), 42844288. (14) Frese, K. W., Jr.; Leach, S. J. Electrochem. Soc. 1985, 132 (1), 259-260. (15) Sanchez-Sanchez, C. M.; Montiel, V.; Tryk, D. A.; Aldaz, A.; Fujishima, A. Pure Appl. Chem. 2001, 73 (12), 1917-1927. (16) Japanese patent laid-open 1994, tokkai-hei 6-179632.
10.1021/ef030121v CCC: $27.50 © 2004 American Chemical Society Published on Web 11/20/2003
286 Energy & Fuels, Vol. 18, No. 1, 2004
Figure 2. Correlation between polarization and current efficiency of methanol yield.
acetate as a dispersant (Wako Pure Chemical Inc.). A 50mg quantity of the prepared catalyst and 50 mg of carbon black (Ketjen black EC carbon black) were added to the dispersant with ultrasonic agitation.17 The paste of the anode catalyst was prepared in the same manner by using purchased a Pt/C catalyst (Tanaka Kikinzoku Kogyo Co., 46.5 wt % Pt, TEC10E50E). The resultant catalyst pastes were uniformly applied to a PTFE sheet to obtain Pt and CuO loadings of 2.0 and 1.0 mg/cm2, respectively. After the applied catalysts were dried, the catalyst films were transferred from the PTFE sheet to the membrane.18 The MEA was mounted on the electrolysis cell with a current collector (Au) and a seal. The cathode chamber was filled with 0.1 M KHCO3 aqueous solution. 100 mL/min of Ar, and 20 mL/min of H2 were fed for 2 h as a pretreatment. Then, Ar was switched to 50 mL/min of CO2. After 2 h additional pretreatment, the potentiostatic electrolysis was conducted for 5 h at 15 ( 2 °C. The cathode was polarized at -20, -50, -100, and -300 mV for the anode. The electrolyte was degassed with agitation by a magnetic stirrer. Reaction product in the electrolyte was analyzed by gas chromatograph (Shimazu GC-14A, FID detector, J&W Scientific DB-WAX). After the 5 h electrolysis, only methanol was detected as shown in Figure 2 and other reaction products could not be found in the catholyte. The current efficiency (CE) of methanol yield is summarized in Figure 2. Here, the CE was defined as CE ) (methanol yield/reaction charge) × 6 because six electrons are associated with the CO2 reduction to methanol (Reaction 1). The methanol which might have been stripped by CO2 bubbling is estimated to be 0.1% or less of the amount of production and which is negligible. It is a notable result that the CE was close to 1.0 up to -100 mV electrolysis. On the other hand, the CE drastically dropped at -300 mV electrolysis. It is thought that the H2 evolution was accelerated and the CE decreased at the polarization. The large difference in the CE is discussed. During the electrolysis, the cathodic current asymptotically closed in on the steady state. The current densities in the steady state at -20, -50, and -100 mV electrolysis were almost equivalent and the current densities were approximately 250 µA/ cm2 as shown in Figure 3. Also the current density at -300 mV electrolysis approached the steady state, 600 µA/cm2. Furthermore, gas evolution was observed on the cathode in this case. On the basis of those results, it is considered that H2 evolved and consequently the CE drastically dropped at -300 mV polarization. There was no difference in the current densities below the -100 mV electrolysis. The fact indicates that the rate-determining step is not a reaction with charge transfer but CO2 diffusion from bulk to the electrocatalyst surface. In most previous works, the electrolysis was conducted at larger polarization for proton or H2 formation, (17) Uchida, M.; Aoyama, Y.; Eda, N.; Ohta, A. J. Electrochem. Soc. 1995, 142 (2), 463-469. (18) Wei, Z. B.; Wang, S. L.; Yi, B. L.; Chen, L. K.; Zhou, W. J.; Li, W. Z.; Xin, Q. J. Power Sources 2002, 106 (1-2), 364-369.
Communications
Figure 3. Galvanometric measurement during electrolysis. Table 1. Comparison of Cathode Catalyst, Electrolysis Potential, and Current Efficiency cathode catalyst Cu/Zn/Al alloy
aCu/Ni
bCu/RuO
2/TiO2
cRu/Cd dCu/Pd eperovskite
a
cell architecture
potential/ V vs SHE
CE of methanol yield
MEA electrodes in electrolyzer v v v v
-0.02 to -0.1 -0.9
0.84 to 0.97 0.07
-0.7 to -0.75 -0.56 -1.36 -2.1 to -2.4
0.06 to 0.30 0.20 to 0.38 0.15 0.02
Ref 8. b Ref 9. c Ref 10.
d
Ref 11. e Ref 12.
however, which led to H2 evolution on the cathode and lower current efficiency. In contrast with the conventional method, proton or atomic hydrogen could be generated at the lower polarization on the designed cell architecture. The low polarization would bring suppression of the H2 evolution and the high CE. Table 1 shows a comparison of cathode catalyst, electrolysis potential, and current efficiency of methanol production between the present and previous works. High current efficiencies are obtained in this study as compared to the previous works. Methanol formed even at small polarization, which demonstrated that the overpotential in the reaction process was decreased by the designed cell architecture. The change of the Gibbs energy (∆Gf 0) in the methanol formation from CO2 and H2 (reaction 1) is -10 kJ mol-1.
CO2 + 3H2 f CH3OH + H2O
-10 kJ mol-1
(1)
Therefore, the reaction progresses with small polarization if the reaction overpotential is lowered. The open circuit potential (Voc) is approximately 17 mV estimated by the ∆Gf 0. The measured Voc, however, was approximately 400 mV. The measured Voc indicates that the cell behaves as a H2 concentration cell based on the difference in the proton activity between the anode and the cathode.19 The difference in the proton activity acts as a driving force for the reaction. When the CO2 was switched to Ar, the Voc was decreased to approximately 360 mV. H2 evolution occurs on the metal or metal oxide rather than on the carbon surface. It is suggested that the CO2 adsorbed on the catalyst and suppressed H2 evolution. The facts indicate that the proton or atomic hydrogen is generated on the same site as the CO2 adsorption. The intimate spatial relationship would lead to the suppression of the H2 evolution and the high CE. Methanol is commercially produced under conditions of high temperature and high pressure. These severe conditions are difficult to achieve using sustainable energy. The electrochemical reduction of CO2 has been limited by H2 evolution and finite mass transport limitation rates for CO2. The former problem can be solved by the cell architecture described here; the latter still remains. It is expected that the limitation of CO2 may be overcome through the direct CO2 feed without electrolyte. EF030121V (19) Sasaki, T.; Miura, M.; Saito, Y.; Ando, Y.; Tanaka, T. Shokubai (catalysts) 2003, 45 (2), 80-82.
