Implications of Cation-Dependent Proton Affinity on Electrocatalytic Nitrate Reduction

Abstract

Perturbation of the nitrogen cycle via Haber-Bosch nitrogen fixation and concomitant fertilizer amendment has manifested in alarmingly increased groundwater nitrate concentrations, with global human-health and ecological ramifications. Correspondingly, the denitrification of drinking water resources represents a growing area of energy consumption, though is still outpaced by the carbon-intensive Haber-Bosch process. Electrocatalytic nitrate reduction therefore offers a competitive and distributable green alternative to traditional ammonia production approaches, using renewably-sourced electrons and protons sourced from water as reducing agents without the need for added chemicals or elevated temperatures and pressures.While considerable progress has been made in understanding the effects of electrocatalyst material parameters on nitrate reduction activity and ammonium selectivity, these studies are conducted in well-controlled and highly purified electrolytes that lack the complexity typical of expected nitrate-rich waste source streams. Recent literature on other electrocatalytic hydrogenation reactions (hydrogen evolution and carbon dioxide reduction reactions) has identified cation-dependent activity and selectivity, indicating the significance of these nominally simple ions in facilitating proton abstraction from water—of particular importance in the proton-hungry conversion of nitrate to ammonium (10 H+ per nitrate converted). Here, we investigate cation-dependent (Li+, Na+, K+) nitrate reduction activity and selectivity in (phosphate) buffered neutral electrolytes across a series of transition metals known to exhibit varying ammonium selectivities (Ag, Cu, Co). We frame our discussion of observed cation-dependent activity and selectivity within empirical literature on cation hydration free energy to understand the role of cations in mediating proton transfer during electrocatalytic hydrogenation reactions.