Origin of the superior oxygen reduction activity of zirconium nitride in alkaline media

Abstract

The anion exchange membrane fuel cell (AEMFC), which can operate in alkaline media, paves a promising avenue for the broad application of earth-abundant element based catalysts. Recent pioneering studies found that zirconium nitride (ZrN) with low upfront capital cost can exhibit high activity, even surpassing that of Pt in alkaline oxygen reduction reaction (ORR). However, the origin of its superior ORR activity was not well understood. Herein, we propose a new theoretical framework to uncover the ORR mechanism of ZrN by integrating surface state analysis, electric field effect simulations, and pH-dependent microkinetic modelling. The ZrN surface was found to be covered by ∼1 monolayer (ML) HO* under ORR operating conditions, which can accommodate the adsorbates in a bridge-site configuration for the ORR. Electric field effect simulations demonstrate that O* adsorption on a 1 ML HO* covered surface only induces a consistently small dipole moment change, resulting in a moderate bonding strength that can account for the superior activity. Based on the identified surface state of ZrN and electric field simulations, pH-dependent microkinetic modelling found that ZrN reaches the Sabatier optimum of the kinetic ORR volcano model in alkaline media, with the simulated polarization curves being in excellent agreement with the experimental data of ZrN and Pt/C. Finally, we show that this theoretical framework can lead to a good explanation for the alkaline oxygen electrocatalysis of other transition metal nitrites such as Fe₃N, TiN, and HfN. In summary, this study proposes a new framework to rationalize and design transition metal nitrides for alkaline ORR.