Single-Atomic Co-N₄ Sites with CrCo Nanoparticles for Metal-Air Battery-Driven Hydrogen Evolution

Designing highly active and robust earth abundant trifunctional electrocatalysts for energy storage and conversion applications remain an enormous challenge. Herein, we report a trifunctional electrocatalyst (CrCo/CoN₄@CNT-5), synthesized at low calcination temperature (550 °C), which consists of Co-N₄ single atom and CrCo alloy nanoparticles and exhibits outstanding electrocatalytic performance for the hydrogen evolution reaction, oxygen evolution reaction, and oxygen reduction reaction. The catalyst is able to deliver a current density of 10 mA cm^-2 in an alkaline electrolytic cell at a very low cell voltage of ∼1.60 V. When the catalyst is equipped in a liquid rechargeable Zn-air battery, it endowed a high open-circuit voltage with excellent cycling durability and outperformed the commercial Pt/C+IrO₂ catalytic system. Furthermore, the Zn-air battery powered self-driven water splitting system is displayed using CrCo/CoN₄@CNT-5 as sole trifunctional catalyst, delivering a high H₂ evolution rate of 168 μmol h^-1. Theoretical calculations reveal synergistic interaction between Co-N₄ active sites and CrCo nanoparticles, favoring the Gibbs free energy for H₂ evolution. The presence of Cr not only enhances the H₂O adsorption and dissociation but also tunes the electronic property of CrCo nanoparticles to provide optimized hydrogen binding capacity to Co-N₄ sites, thus giving rise to accelerated H₂ evolution kinetics. This work highlights the importance of the presence of small quantity of Cr in enhancing the electrocatalytic activity as well as robustness of single-atom catalyst and suggests the design of the multifunctional robust electrocatalysts for long-term H₂ evolution application.