Hydrogen Bubble Templated Electrodeposited Cu As CO2 Reduction Catalyst: Substrate Effect
Authors: Kottakkat Tintula , Klingan Katharina , Jovanov Zarko P , Bergmann Arno , Kubella Paul , Strasser Peter , Dau Holger , Roth Christina 
Published: February 2020

Selective conversion of CO2 into high quality chemical feedstock and fuels represents an emerging energy storage technology1. The electrochemical reduction of carbon dioxide using renewable sources of energy is a promising way of utilizing the only intermittently available renewable energies to produce chemical feedstocks and fuels for later use. Presently, copper is the one metal in focus because of its ability to catalyze CO2 reduction towards energy-dense hydrocarbon products. However, the electrochemical reduction of CO2 on copper results in many different products at a wide range of overpotentials. The selectivity and efficiency of the Cu-based catalysts towards specific products can be improved by fine tuning the structure and composition2-4. In this work, high surface area porous Cu films were electrodeposited either on Cu or Pt substrates at various deposition currents and durations using the dynamic hydrogen bubble template method5.
During electrodeposition at high current values, the size and residence time of H2 bubbles vary based on the substrate and on the in-situ deposited Cu, leading to significantly different catalyst morphologies. Consequently, well-defined dendritic structures with large pores are observed (inset of Fig. 1) for Cu deposited on Cu substrate (CuCat/Cu) as compared to Cu deposited on Pt substrate (CuCat/Pt). Interestingly, product quantification by gas chromatography during electrochemical measurements showed that the product selectivity observed for all variations of the catalysts deposited on a particular substrate does not depend significantly on the differences in pore size and film thickness of the material. However there is a difference in product selectivity for the respective catalysts deposited on different substrates as represented in Fig. 1, where ethylene and carbon-monoxide selectivities are shown at two different potentials. The formation of methane is significantly suppressed (<1%) on both substrates in the potential range where the catalysts show high selectivity for ethylene. From X-ray absorption measurements it is observed that the catalysts are composed of metallic Cu and CuI
2O species with no evidence of CuIIO species. The morphological changes, atomic state and composition of the catalyst may have a dialectic role in deciding the selectivity. In the follow-up work, the selectivity of various Cu films electrodeposited on both the substrates will be correlated with respect to surface area and CuI
2O content after comprehensive quantification of the gaseous and liquid reaction products by gas chromatography, liquid gas chromatography and HPLC. These studies are expected to reveal strategies by which catalysts can be designed for higher selectivity towards a specific hydrocarbon product.

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Figure 1