On the role of carbon additives in the electrochemical reduction of nitrate to ammonia in energy storage

Electrochemical electrode design has traditionally focused on metal-based active sites while treating conductive carbon additives as passive conductivity enhancers. Such conventional approaches fundamentally underestimate the direct catalytic contribution of carbon materials to electrochemical processes. The present research redefines this perspective by demonstrating that commonly used conductive carbons exhibit significant intrinsic activity for the electrochemical nitrate reduction reaction (eNO3RR). Through a systematic evaluation of three commercially available carbon catalysts, Black Pearl 2000 (CB), multi-walled carbon nanotubes (MWCNT), and Vulcan XC-72 (VCX) we reveal their distinct catalytic behaviour towards eNO3RR. At -0.45 V vs. RHE, CB yields Faradaic efficiencies (FE) of 50% for NH3, 12% for NO2⁻ and 19% for NH2OH, while MWCNT delivers 24% for NH3, 12% for NO2⁻ and 16% for NH2OH. In comparison, VCX achieves 45% for NH2OH, 11% for NO2⁻ and 24% for NH3. These findings indicate that CB, possessing the high surface area, enables more complete NO3− reduction to NH3 with higher FE. Conversely, MWCNT, despite its high electrical conductivity, shows comparatively lower activity for eNO3RR. VCX, with moderate conductivity and surface area, favours partial NO3− reduction to intermediates, a behaviour attributed to its pyrrolic nitrogen functionalities that enhance selectivity and stabilize reaction intermediates.