Storage of solar and other sources of renewable electricity may be enabled by the catalytic production of fuels such as H₂ or reduced carbon-containing compounds via the electro-chemical reduction of H₂O or CO₂, respectively. The renewable production of reduced carbon compounds is especially attractive, as liquid fuels can directly address energy needs associated with transportation, which account for nearly 30% of the US energy demand. An attractive strategy for the synthesis of carbon-based fuels is the marriage of a robust electrocatalyst for CO₂ reduction with a photoelectrochemical device or a conventional electrolyzer powered by a renewable source of electrical current.

...Bismuth is an attractive material for development of heterogeneous CO₂ reduction catalysts, as this metal is nontoxic and has a very small environmental impact. Moreover, Bi is a byproduct of Pb, Cu, and Sn refining, and has few significant commercial applications, resulting in the price of Bi being low and stable. Despite these attractive qualities, there has been only one report of electrocatalytic CO₂ reduction using Bi, and the ability of this metal to drive CO production has not been demonstrated. As such, development of Bi-based cathodes for conversion of CO₂ to CO would represent an important development for the fields of CO₂ electrocatalysis and renewable energy conversion.

—DiMeglio and Joel Rosenthal

The researchers electro-deposited a material containing Bi⁰ and Bi³⁺onto an inert electrode substrate to produce the Bi-CMEC catalyst. Used in conjunction with ionic liquids, it effected the electrocatalytic conversion of CO₂ to CO with appreciable current density at overpotentials below 0.2 V.

Under these conditions, Bi-CMEC is selective for production of CO, operating with a Faradaic efficiency (FE) of approximately 95%. When taken together, these correspond to a high-energy efficiency for CO production, on par with that which has historically only been observed using expensive silver and gold cathodes.

...this Bi⁰/Bi³⁺ assembly catalyzes the conversion of CO₂ to CO with high selectivity and FE. At present, the importance of the Bi³⁺ ions to the efficacy of Bi-CMEC is unclear. It is possible that the interface between Bi⁰ and Bi³⁺ sites serves to stabilize CO₂^•− intermediates at the electrode surface in a fashion similar to that proposed for metal oxides on etched electrodes.

Alternatively, the exceptional activity of Bi-CMEC may be derived from in situ reduction of the Bi⁰/Bi³⁺ material to generate metastable surfaces with enhanced catalytic properties. The use of imidazolium ILs may be especially critical in this regard, as recent work has demonstrated that interplay between adsorbed imidazolium cations and CO₂ at bulk platinum electrodes can significantly enhance electrocatalytic CO₂ reduction. Future efforts to uncover the atomic-level structure of Bi-CMEC will be critical to understanding the mechanism by which this system activates CO₂ and, more generally, may provide a rational route for the development of additional inexpensive electrode materials for CO evolution.

—DiMeglio and Joel Rosenthal

Resources

• John L. DiMeglio and Joel Rosenthal (2013) Selective Conversion of CO2 to CO with High Efficiency Using an Inexpensive Bismuth-Based Electrocatalyst. Journal of the American Chemical Society 2013 135 (24), 8798-8801doi: 10.1021/ja4033549