Buffer Effects on Nitrite Reduction Electrocatalysis

Abstract

The Haber-Bosch process has provided an energy-intensive way to produce ammonia for over 100 years. However, alternative methods are required to lower pollution and enhance energy efficiency. Unfortunately, key mechanistic insights into the heterogeneous reduction of nitrogen and its intermediates are lacking. The nitrite reduction reaction (NO₂RR) is an important electrochemical reaction in the nitrogen cycle, playing a significant role in ammonia-based energy storage and wastewater remediation. Although the NO₂RR involves the transfer of multiple protons competing with the hydrogen evolution reaction (HER), the effect of the proton donor has not been investigated in heterogeneous electrocatalysis. We now present an electrochemical study of nitrite reduction in four buffer systems acting as proton donors: citrate, phosphate, 2-(N-morpholino)ethanesulfonic acid, and borate buffers. The chosen catalyst was a typical iron- and nitrogen-codoped carbon (FeNC) with atomically dispersed FeN₄ sites. All buffers except borate enhanced the NO₂RR considerably, while the reduction mechanism was independent of buffer identity. The kinetics of the reaction depended more strongly on buffer concentration than on the NO−2${\mathrm{NO}}_2^{-}$${\mathrm{NO}}_2^{-}$ concentration. Furthermore, we propose a double role for the protonated buffer species: a crucial proton donor during the rate-determining reduction of an NO_x intermediate and an efficient pH regulator near the electrode. These key mechanistic insights into heterogeneous nitrite reduction help to understand proton-coupled electrocatalysis and contribute to the development of alternative nitrogen-based fuels.