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The sluggish kinetics of oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are the bottleneck in the development of electrically rechargeable zinc-air batteries (erZABs), making the choice of electrocatalyst crucial. Among the electrocatalysts, MnO2 is attractive for its economic and environmental attributes. Nevertheless, MnO2 faces obstacles such as low conductivity, degradation on repeated cycling, and lower activity compared to benchmark catalyst. The strategies explored to enhance the catalytic performance of MnO2 are often energy-intense and/or costly. One of the promising avenues is tuning the surface oxygen vacancies (OVs). Though there are a few reports on tuning the surface OVs of MnO2, they are restricted to ORR activities or use inert or reductive atmosphere for creating surface OVs. In this work, the bifunctional catalytic activities of α-MnO2 are improved by engineering its surface OVs through a simple heat treatment in air. The surface OVs are quantified by Raman, XPS and EPR spectroscopic studies. MnO2 heated at 500 °C in air exhibits better ORR (onset potential: 802 mV, current density: -6.67 mA cm−2) and OER (overpotential of 489 mV to reach 10 mA cm−2) activities. [ABSTRACT FROM AUTHOR]

Understanding the charge storage mechanism of MnCO3 is essential to improve its capacitance performance. Herein, we report the charge storage mechanism of MnCO3 in aqueous Na2SO4 and Mg(ClO4)2 electrolytes studied systematically by using ex‐situ X‐ray diffraction and X‐ray photoelectron spectroscopy. Theoretical specific capacitance of MnCO3.H2O in the potential window of 0.1 to 1.0 V is 806 F g−1, however, it delivers a specific capacitance value of only 95 and 66 F g−1 in 0.1 M Mg(ClO4)2 and 0.1 M Na2SO4 electrolytes, respectively, which suggests that only a limited fraction of MnCO3 is participating in the charge storage. The ex‐situ X‐ray diffraction and X‐ray photoelectron spectroscopic studies reveal that the insertion and extraction of Mg2+‐ions into/from MnCO3 accompanied by redox reaction between Mn2+ and Mn1+ during charge/discharge are reversible and do not result in phase transformation in aqueous 0.1 M Mg(ClO4)2 electrolyte. In contrast, lattice expansion and contraction by insertion and extraction of Na+‐ions into/from MnCO3 result in a gradual transformation of MnCO3 into α‐MnO2 in aqueous 0.1 M Na2SO4 electrolyte. [ABSTRACT FROM AUTHOR]