Publication date: 7th June 2020
Electroreduction of CO2 (CO2RR) to valuable fuels or chemicals is a promising approach to alleviate the dependence on fossil fuels. Due to the high activation barrier and thus significant overpotentials for C=O dissociation, an efficient catalyst is necessary to improve the kinetics of the process. Among the reported catalysts, Au, Ag, Pd and Cu are the most studied. Alternative to noble metal catalysts, the earth-abundant Zn possesses a relatively low CO binding energy and thus is a promising candidate for electrochemically reducing CO2 to CO.
Most of the reported ZnO catalysts in CO2RR are fully or partially reduced to metallic Zn when they are involved in CO2 reduction. Often ZnO-based catalysts present particular morphologies, and expose peculiar facets, which greatly influence the CO2RR performance. Although it is known that the structure of the reduced phase can have a strong influence on the selectivity of the catalysts, [1] the influence of the initial structure of the metal oxide phase and the reducing environment on the structure, and hence performance, of the metallic phase has not been yet studied in detail.
To explore these aspects, we prepared ZnO nanorod catalysts with a hexagonal shape using a traditional low-temperature hydrothermal method. ZnO nanorods with different aspect ratios were grown on carbon paper and electrochemically reduced in different electrolytes. This led to reduced phases with different structural properties; a sponge-like Zn, Zn nanorods and Zn nanoplates. The sponge-like Zn produced predominantly syngas with H2:CO = 2 but also some formate, the Zn nanorods produced only syngas with H2:CO = 1, while Zn nanoplates exhibited 85% selectivity towards CO selectivity. In this study, we show for the first time that the initial structure of the metal oxide phase and the electrolyte medium have a profound impact on the structure of the catalytic active Zn metal phase, and thereby influence significantly the activity but especially the selectivity of the catalysts.
This work was supported by the European Research Council; project number ERC-2014-CoG 648991. Marisol Tapia Rosales, Francesco Mattarozzi and Jan Willem de Rijk are acknowledged for useful discussions on the electrochemical set-up and measurements.
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