Abstract
Rigorous electrokinetic results are key to understanding the reaction mechanisms in the electrochemical CO reduction reaction (CORR), however, most reported results are compromised by the CO mass transport limitation. In this work, we determined mass transport-free CORR kinetics by employing a gas-diffusion type electrode and identified dependence of catalyst surface speciation on the electrolyte pH using in-situ surface enhanced vibrational spectroscopies. Based on the measured Tafel slopes and reaction orders, we demonstrate that the formation rates of C2+ products are most likely limited by the dimerization of CO adsorbate. CH4 production is limited by the CO hydrogenation step via a proton coupled electron transfer and a chemical hydrogenation step of CO by adsorbed hydrogen atom in weakly (7 < pH < 11) and strongly (pH > 11) alkaline electrolytes, respectively. Further, CH4 and C2+ products are likely formed on distinct types of active sites.
Highlights
Mass transport limitations could be partially alleviated by using flow-type reactors[12,34,35,36]; these configurations introduce additional complexities to electrode/electrolyte interfaces and flow patterns for the benefit of high current densities, making them unsuitable for mechanistic investigations[37,38,39,40]
In summary, we determined mass transport-free CO reduction reaction (CORR) kinetics, in a standard H-type electrochemical cell with a three-electrode setup, by employing a gas-diffusion-type polycrystalline copper powder electrode and identified dependence of catalyst surface speciation on the electrolyte pH using in situ surface-enhanced vibrational spectroscopies
Through combined electrokinetic and in situ spectroscopic investigations, we provide compelling experimental evidence that the formation rate of C2+ products in the CORR on Cu does not depend on the electrolyte pH and is limited by the first electron transfer without involving a proton
Summary
Mass transport limitations could be partially alleviated by using flow-type reactors[12,34,35,36]; these configurations introduce additional complexities to electrode/electrolyte interfaces and flow patterns for the benefit of high current densities, making them unsuitable for mechanistic investigations[37,38,39,40]. We systematically determined Tafel slopes and CO reaction orders in forming C2+ products and CH4 at electrolyte pH from 7 to 14 using our recently developed polycrystalline Cu electrode with a gas-diffusion mechanism in a standard threeelectrode H-cell (Supplementary Fig. 1)[28]. We show that rates of C2+ products are limited by the first electron transfer process and rates determined at different electrolyte pH essentially overlap at the SHE scale. Methane production rates determined in different electrolytes overlap in neither the SHE nor the RHE scale. Together with in situ surface-enhanced infrared and Raman spectroscopic results, we conclude that methane and C2+ products are produced on sites with distinct properties. Possible reaction pathways and associated RDS are discussed in the context of electrokinetic and spectroscopic results
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