Single-Atomic Co-N 4 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/CoN ₄ @CNT-5), synthesized at low calcination temperature (550 °C), which consists of Co-N ₄ 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+IrO ₂ catalytic system. Furthermore, the Zn-air battery powered self-driven water splitting system is displayed using CrCo/CoN ₄ @CNT-5 as sole trifunctional catalyst, delivering a high H ₂ evolution rate of 168 μmol h ^-1 . Theoretical calculations reveal synergistic interaction between Co-N ₄ active sites and CrCo nanoparticles, favoring the Gibbs free energy for H ₂ evolution. The presence of Cr not only enhances the H ₂ O adsorption and dissociation but also tunes the electronic property of CrCo nanoparticles to provide optimized hydrogen binding capacity to Co-N ₄ sites, thus giving rise to accelerated H ₂ 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 H ₂ evolution application.