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.