UC San Diego

The electrocatalytic reduction of CO2 represents a possible means of disrupting current petrochemical reliance by leveraging renewably-generated electricity to manufacture commodity chemicals and liquid fuels. Such an approach may enable economic feasibility of anthropogenic climate change mitigation by catalytic conversion of atmospheric CO2 to value-added products. However, many of the most active catalysts consist of rare metals whose cost renders them prohibitively expensive for scalability. Additionally, a wide distribution of reduction products is often possible and poor selectivity decreases overall efficiency and complicates downstream purification processes. Fundamental understanding of elementary steps involved in catalytic pathways may therefore allow for improved catalyst design from inexpensive, earth-abundant materials.

One such elementary step consists of hydride transfer from a catalyst intermediate to CO2, yielding the two-electron reduction product, formate. The investigation, targeting, and tuning of the thermodynamics of such hydride intermediates to enable improved catalyst design represents the overarching aim of the work presented herein, which is underpinned by three distinct objectives. Firstly, a powerful scaling relationship is elucidated which relates the thermodynamic propensity of hydride transfer, hydricity (ΔG0H−), and the first reduction potential of the parent metal complex (E1/2(Mn+/(n-1)+)). This relationship not only establishes a mechanism for estimating hydricity based on E1/2(Mn+/(n-1)+) but also provides a platform for rationally targeting and tuning hydride intermediates for reactivity towards CO2 and proton sources.

This scaling relationship is subsequently utilized to target and design reactive hydrides at nickel and the validity of the relationship is established. Installation of highly electron-donating ligand frameworks demonstrates successful tuning of hydricity at first-row metal complexes and additional thermodynamic arguments enable for the rational selection of mild conditions for both electrocatalytic hydrogen evolution at extremely low overpotentials and the complete suppression thereof.

Finally, the electrocatalytic reactivity of these species with CO2 is described. While development of nickel-based CO2-to-formate catalysts using these methods is ultimately unsuccessful, this work provides insight on the utility and limitations of thermodynamic scaling relationships in catalyst design. Furthermore, the findings herein underscore the necessity of divergence from such scaling relationships and shed light on strategies by which that may be accomplished.