Electrochemical Reduction of CO2 to Useful Fuels on Molybdenum and Molybdenum Oxide Thin-Film Catalysts

Conversion of CO₂ into useful carbon neutral fuels using renewable energy sources is a very attractive way to sustainably reduce CO₂ accumulation in the atmosphere that could eventually threaten the environment¹. Using the excesses of electricity generated from renewable sources (such as wind and solar) and coupling it with an electrolytic device is an ideal mean of achieving high-density renewable energy storage. However, CO₂ reduction into a desirable fuel is very challenging at present due to the multiple proton-coupled electron transfer steps needed², and finding an efficient and selective catalyst is essential to achieve a cost-effective process³. Cu is the most heavily investigated metal catalyst, since it can produce hydrocarbons with reasonable Faradaic efficiencies⁴. However, among the 16 different CO₂ reduction products detected on Cu, low efficiencies and poor selectivities have been reported for the production of methanol⁵, which can be used as a liquid fuel for direct methanol fuel cells (DMFCs) and modified diesel engines. On the other hand, formation of methanol on Mo and Mo oxides has been observed in several studies^6,7 (with Faradic efficiencies of up to 80% at around -0.7 V vs. RHE at pH 4.2⁶), yet selectivities of these materials (e.g. H₂ and CO yields) were not described. Additionally, recent experimental studies on Cu and Au electrodes have shown that higher CO₂ and CO reduction efficiencies could be achieved by modifying these metallic electrodes with oxide species^8,9. For example, severely-roughened Cu films prepared by electrochemically reducing thermally grown Cu oxide (Cu₂O) layers exhibit dramatically improved selectivity and up to 50% efficiency towards ethanol at -0.35 V⁹.

Inspired by these studies, this contribution will explore the behavior of model thin film electrodes of Mo and Mo oxides fabricated by reactive sputter deposition¹⁰. Electrochemical reduction of CO₂ is performed in 0.5 M KHCO₃ using a 3 electrode configuration and the reaction products are analyzed by differential electrochemical mass spectrometry, nuclear magnetic resonance spectroscopy and ionic chromatography. The surface chemistry and crystal structure of these model electrodes is also studied before and after electrochemistry via X-ray photoelectron spectroscopy and grazing angle X-ray diffraction, respectively. To the best of our knowledge, such a detailed investigation of the catalytic properties of these materials for the electroreduction of CO₂ has never been presented.

References:

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²   Kumar, B., Llorente, M., Froehlich, J., Dang, T., Sathrum, A., Kubiak, C. P., Annu. Rev. Phys. Chem. 63, 541-569 (2012).

³   Jhong, H.-R. M., Ma, S. & Kenis, P. J. A., Curr. Opin. Chem. Eng. 2, 191–199 (2013).

⁴   Hori, Y. in Modern Aspects of Electrochemistry Vol. 42, Ch. 3, 89–189, Springer, New York (2008).

⁵   Kuhl, K. P., Cave, E. R., Abram, D. N. & Jaramillo, T. F., Energy Environ. Sci. 5, 7050–7059 (2012).

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⁷   Bandi, A., J. Electrochem. Soc. 137, 7, 2157 – 2160 (1990).

⁸   Chen, Y., Li, C. W. and Kanan, M. W., J. Am. Chem. Soc.134, 19969 – 19972 (2012).

⁹   Li, C. W., Ciston, J. and Kanan, M. W., Nature, 508, 504 – 507, (2014).

¹⁰ Rabis, A., Kramer, D., Fabbri, E., Worsdale, M., Kötz, R., Schmidt, T.J., J. Phys. Chem. 118, 11292 (2014).