The effect of the crystallographic form of MnO2 on the kinetics of oxygen reduction and evolution reaction

The performance of electrically rechargeable zinc-air batteries depends on the electrocatalysts. MnO2 is one of the most extensively studied electrocatalysts for oxygen reduction reaction (ORR) and exhibits polymorphism. Though there are a few reports on the effect of the crystal structure of MnO2 on ORR, the studies were restricted to only two crystallographic forms, or the textural properties of various forms of MnO2 are not similar. However, the textural properties influence the electrocatalytic activity of MnO2. Herein, five crystallographic forms of MnO2 with similar textural properties are synthesized by a hydrothermal method. The effect of crystal structure on the kinetics of ORR and oxygen evolution reaction (OER) is reported. The crystal structure-dependent catalytic activity of MnO2 towards ORR and OER is established by systematic electrochemical investigations coupled with X-ray diffraction, X-ray photoelectron spectroscopic, microscopic and sorption studies. The electrochemical results reveal that α-MnO2 is the best candidate for ORR and OER. [Display omitted] •α-, β-, γ-, δ- and λ- MnO2 are synthesized with similar textural properties.•The ORR activity follows the order: α-MnO2 > δ-MnO2 > β-MnO2 > λ-MnO2 > γ-MnO2.•The OER activity follows the order: α-MnO2 > δ-MnO2 > γ-MnO2 ≈ λ-MnO2 > β-MnO2.•The ORR activity of α-MnO2 is comparable to benchmark catalyst, Pt/C.•α-MnO2 with wide tunnel and higher oxygen-based functionalities provide more active sites for ORR and OER.