Binder-free carbon electrode driven from biomass for selective electroreduction of nitrate to ammonia

Abstract

Electrochemical nitrate reduction to ammonia (NO3RR) has emerged as a viable process for addressing nitrate pollution and producing ammonia (NH3) as a renewable hydrogen carrier and fertiliser component. Previous studies explored noble and transition metal cathodes, but their high cost, limited durability, and potential toxicity restrict practical applications. Carbon-based materials such as biochar are promising cathode candidates due to their electrical conductivity and porous structure. However, conventional binders like polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF) used in electrode fabrication have the potential to hinder biochar electrode performance by blocking pores and impeding electron transfer. This study evaluates the influence of material properties and binders on the electrochemical reactivity towards NO3RR. Electrochemical reduction experiments were conducted in a dual-chamber reactor at a 20 mA cm−2 current density. The study compared the activity of Ti/IrO2, platinum, and graphite cathodes, then verified binder effects by comparing the bulk charcoal electrode to those made with PTFE and PVDF. The results indicate that cathode material properties are critical for NO3RR. Graphite exhibited the highest nitrate removal efficiency (55.3%), faradaic efficiency (40.5%), and ammonia yield (0.72 mg h−1 cm−2). Regarding the binder effect study, the bulk charcoal electrode achieved a nitrate removal efficiency of 39.1%, compared to PTFE (16.6%) and PVDF (4.2%), thus outperforming the two electrodes with a binder. The performance degradation using binders is due to their hydrophobic and insulating nature, which prevents NO−3 from reaching active sites and hinders electron transfer. The bulk charcoal electrode consistently achieved the highest ammonia yield (0.37 mg h−1 cm−2) and faradaic efficiency (23.5%). This study demonstrates that traditional binders during biochar electrode fabrication hinder NO3RR reaction. Therefore, binder-free electrode fabrication is an important strategy for enhancing the electrochemical performance of biochar-based electrodes and finding more effective and sustainable means of nitrate reduction and ammonia generation.