2, Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California, United States
3, Department of Chemistry, University of California, Berkeley, Berkeley, California, United States
An efficient electrocatalyst for the reduction of CO2 (CO2R) would enable the conversion of a major contributor to global warming into valuable carbon-based products. Cu surfaces are unique in that they are the only single-metal system to catalyze the formation of multicarbon products from CO2. However, the lack of selectivity of these Cu surfaces necessitates investigation into how their catalytic behavior may be adjusted.
The complexity of the catalyst/electrolyte interface poses challenges in identifying how a change in an electrocatalyst affects the observed CO2R selectivity. Therefore, we employ a methodical experimental approach that focuses upon identifying which parameters play an important role in determining CO2R selectivity.
First, we examined the effect of a series of organic modifiers on the CO2R selectivity of Cu. Oxide-derived Cu surfaces were functionalized with molecular and polymeric modifiers featuring a wide variety of structural characteristics, including neutral and cationic species, protic and aprotic species, and species bearing various functional groups. The product distribution of these surfaces in the CO2R reaction was characterized at -0.7 V vs. RHE, focusing upon the selectivity between CO, formic acid and H2. This allowed classification of the organic structures based upon the promoted product. Finally, each class of modifiers was examined and key, common structural characteristics of the modifiers were identified.
Through this systematic study, we demonstrate that the CO2R selectivity of a non-precious metal catalyst may be improved for CO, formic acid or H2 by changing the organic modifier applied. Selectivities of up to 76% CO or 62% formic acid were observed, and H2 selectivity was tuned from 97% down to 2%. In this presentation, we describe the structural characteristics of these modifiers that are key to changing the observed selectivity. These common features offer insights into the mechanism by which organic modifiers influence CO2R selectivity at the metal surface. We expect that the identified structure-reactivity relationships will illuminate important design principles for novel, selective CO2R electrocatalysts and organic structures for CO2R devices.