Nitrogen and phosphorus co-doped ultrathin carbon nanosheets as superior cathode catalysts for Zn–air batteries

Abstract

Exploring efficient metal-free electrocatalysts for the oxygen reduction reaction (ORR) plays a significant role in a variety of storage technologies and renewable energy conversion. In this work, nitrogen (N) and phosphorus (P) co-doped ultra-thin nanosheets (CNP-ns) were firstly synthesized through high temperature pyrolysis utilizing starch, dicyandiamide and triphenylphosphine as carbon, nitrogen and phosphorus sources respectively. According to the optimum N/P ratio among the all CNP-ns catalysts, a mesoporous nitrogen–phosphorus co-doped nanosheet (CNP₁₀₀-ns-NaCl) was then obtained further using a NaCl hard template method, which displays an ultra-thin thickness of 1.29 nm. Among the prepared catalysts, the catalyst CNP₁₀₀-ns-NaCl exhibits the outstanding ORR electrocatalytic performance, with the ORR onset potential (E_onset) of 1.01 V (vs. RHE), half-wave potential (E_1/2) of 0.95 V (vs. RHE), limiting diffusion current of 7.45 mA cm⁻² and excellent resistance to methanol in alkaline media, and its overall performance is superior to that of the Pt/C catalyst. The self-assembled Zn–air battery with CNP₁₀₀-ns-NaCl as a cathode electrocatalyst shows excellent discharge performance and superior durability. The CNP₁₀₀-ns-NaCl battery delivers an open circuit voltage of 1.52 V and a maximum power density of 302 mW cm⁻². In addition, the constant current discharge curves of the CNP₁₀₀-ns-NaCl battery under different current densities show higher voltage plateaus and stability than those of the Pt/C battery, and it delivers a high specific capacity of 801 mA h g_Zn⁻¹. The results show that the catalyst of the present investigation has potential application prospects in alkaline zinc–air batteries.