A critical challenge in electrocatalytic CO 2 reduction to renewable fuels is product selectivity. Desirable products of CO 2 reduction require proton equivalents, but key catalytic intermediates can also be competent for direct proton reduction to H 2 . Understanding how to manage divergent reaction pathways at these shared intermediates is essential to achieving high selectivity. Both proton reduction to hydrogen and CO 2 reduction to formate generally proceed through a metal hydride intermediate. We apply thermodynamic relationships that describe the reactivity of metal hydrides with H + and CO 2 to generate a thermodynamic product diagram, which outlines the free energy of product formation as a function of proton activity and hydricity (ΔG H− ), or hydride donor strength. The diagram outlines a region of metal hydricity and proton activity in which CO 2 reduction is favorable and H + reduction is suppressed. We apply our diagram to inform our selection of [Pt(dmpe) 2 ](PF 6 ) 2 as a potential catalyst, because the corresponding hydride [HPt(dmpe) 2 ] + has the correct hydricity to access the region where selective CO 2 reduction is possible. We validate our choice experimentally; [Pt(dmpe) 2 ](PF 6 ) 2 is a highly selective electrocatalyst for CO 2 reduction to formate (>90% Faradaic efficiency) at an overpotential of less than 100 mV in acetonitrile with no evidence of catalyst degradation after electrolysis. Our report of a selective catalyst for CO 2 reduction illustrates how our thermodynamic diagrams can guide selective and efficient catalyst discovery. electrocatalysis | CO 2 reduction | solar fuel | formate production | hydride T he emerging availability of inexpensive renewable electricity has motivated interest in using electrolytic methods to generate sustainable fuels. Electrocatalytic CO 2 reduction provides an entry to carbon-neutral fuels, but product selectivity remains a significant challenge (1, 2). Nearly all reductive reactions of interest involve protons as well as electrons, introducing the complication of direct proton reduction to H 2 under electrolytic conditions. Diversion of electron equivalents into proton reduction results in lower Faradaic efficiency for the desired CO 2 reduction reaction. Various strategies to inhibit or suppress H 2 evolution for heterogeneous (3-7) and homogeneous (8,9) catalysts have been explored.To understand the factors that determine selectivity between CO 2 and H + reduction, we have been investigating the reactivity of metal hydrides. Selectivity for formate production is particularly challenging because metal hydride intermediates are common to both reaction pathways (Fig. 1). As a result, very few heterogeneous (10-13) or homogeneous (14-16) catalysts have been reported with high (>90%) Faradaic efficiency for formate production. Understanding the reactivity of metal hydrides is key to controlling the bifurcating reaction pathways that ultimately determine selectivity. Most selective catalysts for CO 2 reduction utilize kinetic inhibition (or a high-...

The hydricity (ΔG) of a newly synthesized nickel hydride was experimentally determined in acetonitrile (50.6 kcal mol), dimethyl sulfoxide (47.1 kcal mol), and water (22.8 kcal mol). The hydricity values indicate hydride transfer from [HNi(TMEPE)][BF] (TMEPE = 1,2-bis[di(methoxyethyl)phosphino]ethane) to CO is exergonic in water and endergonic in the organic solvents.