Molecular electrocatalysts for rapid and selective reduction of nitrogenous waste to ammonia

Abstract

Electrocatalysis of the nitrate (NO₃⁻) reduction reaction (NO₃RR) or nitrite (NO₂⁻) reduction reaction (NO₂RR) is a sustainable route for removing nitrogenous pollutes and producing ammonia (NH₃). Developing electrocatalysts that can exhibit high NH₃ selectivity at high current densities is important for their practical applications, but still remains challenging. Here we demonstrate that molecularly dispersed electrocatalysts (MDEs) of metal phthalocyanines on carbon nanotubes can enable rapid and selective NH₃ production from the NO₃RR and NO₂RR. The copper phthalocyanine (CuPc) MDE shows a much lower overpotential requirement for the NO₃RR than the aggregated CuPc sample, delivering a high NH₃ partial current density of ∼1 A cm⁻² with a faradaic efficiency (FE) of NH₃ over 98%. Due to the favorable NO₃⁻ absorption, the cobalt phthalocyanine (CoPc) MDE shows higher activity than the CuPc MDE for the NO₃RR, but FE(NH₃)s of the CoPc MDE at high current densities are limited by its enhanced hydrogen evolution activity. It is further found that the CoPc MDE is more active in the NO₂RR than in the NO₃RR, which is opposite to the CuPc MDE. Thus, the CoPc MDE employed for the NO₂RR can overcome the limitation of the hydrogen evolution reaction, showing a high NH₃ partial current density of 466 mA cm⁻² with FE(NH₃) >97%.