Nitrogen dopants in nickel nanoparticles embedded carbon nanotubes promote overall urea oxidation

Abstract

Urea electro-oxidation is an attractive and alternative anodic reaction in the electrochemical generation of hydrogen using wastewater ascribing to the low theoretical voltage and non-precious metal (nickel) catalyst for urea oxidation reaction (UOR); however, the sluggish UOR and poisoning of catalyst impede the practical application. Here, in this work, we synthesize a series of nickel nanoparticles embedded nitrogen doped carbon nanotubes (Ni@NCNT) and study the effect of nitrogen dopants on UOR catalytic activity. The nitrogen dopants can weaken the binding strength between CO2 species and active sites resulting in alleviation of CO2 poisoning; simultaneously, nitrogen dopants also promote the in-situ conversion of Ni3+ species facilitating UOR catalysis; as a result, electrocatalytic current density of 45.8 mA cm−2 is recorded for Ni@NCNT in 1 M KOH electrolyte with 0.5 M urea at 1.5 V vs. RHE, which is 3.8 fold better than commercial Pt/C (11.8 mA cm−2). Moreover, Ni@NCNT, due to the more nitrogen dopants, exhibits a comparable overpotential to commercial Pt/C for driving hydrogen evolution reaction (HER) catalysis in 1 M KOH electrolyte at high current density (400 mA cm−2). Subsequently, 1.56 V is demanded for overall UOR catalysis on Ni@NCNT with current density of 10 mA cm−2. This work offers useful information for designing a stable and efficient electrocatalyst for not only UOR but also electrochemical generation of H2 from wastewater.