Etosha R. Cave

We report new insights into the electrochemical reduction of CO2 on a metallic copper surface, enabled by the development of an experimental methodology with unprecedented sensitivity for the identification and quantification of CO2 electroreduction products. This involves a custom electrochemical cell designed to maximize product concentrations coupled to gas chromatography and nuclear magnetic resonance for the identification and quantification of gas and liquid products, respectively. We studied copper across a range of potentials and observed a total of 16 different CO2 reduction products, five of which are reported here for the first time, thus providing the most complete view of the reaction chemistry reported to date. Taking into account the chemical identities of the wide range of C1–C3 products generated and the potential-dependence of their turnover frequencies, mechanistic information is deduced. We discuss a scheme for the formation of multicarbon products involving enol-like surface intermediates as a possible pathway, accounting for the observed selectivity for eleven distinct C2+ oxygenated products including aldehydes, ketones, alcohols, and carboxylic acids.

Fuels and industrial chemicals that are conventionally derived from fossil resources could potentially be produced in a renewable, sustainable manner by an electrochemical process that operates at room temperature and atmospheric pressure, using only water, CO2, and electricity as inputs. To enable this technology, improved catalysts must be developed. Herein, we report trends in the electrocatalytic conversion of CO2 on a broad group of seven transition metal surfaces: Au, Ag, Zn, Cu, Ni, Pt, and Fe. Contrary to conventional knowledge in the field, all metals studied are capable of producing methane or methanol. We quantify reaction rates for these two products and describe catalyst activity and selectivity in the framework of CO binding energies for the different metals. While selectivity toward methane or methanol is low for most of these metals, the fact that they are all capable of producing these products, even at a low rate, is important new knowledge. This study reveals a richer surface chemistry for transition metals than previously known and provides new insights to guide the development of improved CO2 conversion catalysts.

Increases in energy demand and in chemical production, together with the rise in CO2 levels in the atmosphere, motivate the development of renewable energy sources. Electrochemical CO2 reduction to fuels and chemicals is an appealing alternative to traditional pathways to fuels and chemicals due to its intrinsic ability to couple to solar and wind energy sources. Formate (HCOO–) is a key chemical for many industries; however, greater understanding is needed regarding the mechanism and key intermediates for HCOO– production. This work reports a joint experimental and theoretical investigation of the electrochemical reduction of CO2 to HCOO– on polycrystalline Sn surfaces, which have been identified as promising catalysts for selectively producing HCOO–. Our results show that Sn electrodes produce HCOO–, carbon monoxide (CO), and hydrogen (H2) across a range of potentials and that HCOO– production becomes favored at potentials more negative than −0.8 V vs RHE, reaching a maximum Faradaic efficiency of 70% at −0.9 V vs RHE. Scaling relations for Sn and other transition metals are examined using experimental current densities and density functional theory (DFT) binding energies. While *COOH was determined to be the key intermediate for CO production on metal surfaces, we suggest that it is unlikely to be the primary intermediate for HCOO– production. Instead, *OCHO is suggested to be the key intermediate for the CO2RR to HCOO– transformation, and Sn’s optimal *OCHO binding energy supports its high selectivity for HCOO–. These results suggest that oxygen-bound intermediates are critical to understand the mechanism of CO2 reduction to HCOO– on metal surfaces.

The electrochemical reduction of CO2 could allow for a sustainable process by which renewable energy from wind and solar are used directly in the production of fuels and chemicals. In this work we investigated the potential dependent activity and selectivity of the electrochemical reduction of CO2 on metallic silver surfaces under ambient conditions. Our results deepen our understanding of the surface chemistry and provide insight into the factors important to designing better catalysts for the reaction. The high sensitivity of our experimental methods for identifying and quantifying products of reaction allowed for the observation of six reduction products including CO and hydrogen as major products and formate, methane, methanol, and ethanol as minor products. By quantifying the potential-dependent behavior of all products, we provide insights into kinetics and mechanisms at play, in particular involving the production of hydrocarbons and alcohols on catalysts with weak CO binding energy as well as the formation of a C-C bond required to produce ethanol.