Cobalt nanoparticle-embedded nitrogen-doped carbon/carbon nanotube frameworks derived from a metal–organic framework for tri-functional ORR, OER and HER electrocatalysis

Abstract

Developing active and stable electrocatalysts of earth-abundant elements towards the oxygen reduction reaction (ORR), oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) still remains a crucial challenge. Herein, a cobalt-containing metal–organic framework using adenine as a ligand was synthesized and pyrolyzed without any other precursors, forming a cobalt nanoparticle-embedded nitrogen-doped carbon/carbon nanotube framework (Co@N-CNTF). Due to the abundant active sites of homogeneously distributed cobalt nanoparticles within nitrogen-doped graphitic layers, the resultant Co@N-CNTF catalysts exhibit an efficient and stable electrocatalytic performance as a tri-functional catalyst towards the ORR, OER and HER, including a high half-wave potential of 0.81 V vs. RHE for the ORR, and a low overpotential at 10 mA cm⁻² for the OER (0.35 V) and HER (0.22 V). As a proof-of-concept, the Co@N-CNTF as an OER/HER bifunctional catalyst for full water splitting affords an alkaline electrolyzer with 10 mA cm⁻² under a stable voltage of 1.71 V. Moreover, an integrated unit of a water-splitting electrolyzer using the Co@N-CNTF catalysts, which is powered with a rechargeable Zn–air battery using the Co@N-CNTF as an ORR/OER bifunctional catalyst on air electrodes, can operate under ambient conditions with high cycling stability, demonstrating the viability and efficiency of the self-powered water-splitting system.