On the deactivation mechanisms of MnO2 electrocatalyst during operation in rechargeable zinc-air batteries studied via density functional theory

Due to its abundance and ecological friendliness, alpha-manganese dioxide (α-MnO2) is recognized as a cost-effective bifunctional electrocatalyst for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). The α-MnO2 is widely used in rechargeable zinc-air batteries (ZABs). However, its performance gradually deteriorates upon charge-discharge cycling. Thus, its deactivation mechanisms must be understood to tackle such an issue. Herein, the deactivation mechanisms of MnO2 are duly investigated via the density functional theory–based analysis, where the MnO2 electrocatalysts are modeled based on the X-ray diffraction (XRD) profiles of the fresh MnO2 dominated by the α-MnO2 (211) and the spent one dominated by the β-MnO2 (110). Based on the simulated event of ORR, the *OOH species exhibited the strongest chemisorption to all of the surfaces, followed in order by *O and *OH, respectively. Regarding the surface distortion due to the species presented during ORR, it was investigated by the Bader charge and charge density difference analysis. It was found that all surface experienced some degrees of distortion confirmed by the calculated strain value, where the most distorted surface is the deactivated MnO2, the β-MnO2 (110). As the MnO2 phase transformation from α to β during ORR generates a less active surface, the low activity found on β-MnO2 (110) confirmed by the ORR overpotential (η) derived from its Gibbs free energy diagram is due to the limiting elementary step converting *OOH to *O, which blocks the propagation towards the final product. While the OER performance for the fresh and spent catalysts are similar. Hence, the deactivation of the MnO2 electrocatalyst occurring during ORR is due to (1) the phase change from α-MnO2 to β-MnO2 and (2) the limiting reaction, which leads to the formation of *OOH species rather than the H2O product. As a design guideline, the prevention of phase transformation is one of the key factors towards high-performance MnO2-based ORR/OER electrocatalysts.