Abstract
AbstractLow cost and abundant catalysts demonstrating high activity and stability towards the oxygen reactions, i. e., the oxygen reduction (ORR) and oxygen evolution reaction (OER), are crucial for the development of electrically rechargeable zinc‐air batteries. Herein, the facile synthesis and systematic characterisation of two highly active and stable oxygen electrocatalysts, i. e., high surface area α‐MnO2 microspheres and nanoparticulate Co3O4, are reported. α‐MnO2 exhibits low half‐wave potential and potential of −0.197 and −0.226 V (vs. Ag/AgCl) at −3 mA cm−2, respectively, that are only marginally higher compared to commercial Pt/C (E1/2=−0.161 V, Ej=‐3=−0.171 V) for ORR. Meanwhile, Co3O4 needs a potential of 0.601 V (vs. Ag/AgCl) to drive 10 mA cm−2 being competitive to commercial Ir/C (Ej=10=0.60 V) for OER. In order to create a bifunctional catalyst, two approaches were pursued: i) Co3O4 nanoparticles were homogeneously grown on the surface of α‐MnO2 microspheres yielding a radial hybrid composite catalyst material in the form of a core (α‐MnO2) shell (Co3O4) structure and ii), much simpler, individual α‐MnO2 microspheres and Co3O4 nanoparticles were physically mixed in a powder blend. The powder blend demonstrates superior overall bifunctional catalytic properties such that the individual catalysts still dominate their respective oxygen reaction and, due to synergistic interactions between both catalysts, an improved ORR activity could be achieved.
Highlights
IntroductionLow cost and abundant catalysts demonstrating high activity and stability towards the oxygen reactions, i
While primary (i. e. not rechargeable) zinc-air batteries (ZAB) is a mature technology well established in consumer products such as hearing aids, their use as secondary batteries, has so far been impeded due to challenges associated with the reversibility of the zincanode as well as the air-cathode
As-synthesized α-MnO2 catalyst secondary particles can be best described as isolated highly uniform spheres with an average diameter of 2.5 μm (Figure 1b)
Summary
Low cost and abundant catalysts demonstrating high activity and stability towards the oxygen reactions, i. E. concentrated aqueous potassium hydroxide (KOH)) and a porous air/O2 breathing cathode utilizing O2 from ambient air as the positive active material and enabling higher energy density (1353 Wh kg 1) compared to LIB and other battery types.[3] In addition to that, ZAB hold sufficient material availability (Zn is the 24th most abundant element in Earth’s crust and can be fully recycled), low cost (< $100 kW 1 h 1), inherent safety (metallic Zn has very low reactivity and can be handled in humid air) and environmental friendliness as well as stand out from other MAB in terms of better corrosion stability in aqueous electrolyte, low self-discharge, long shelf life and a reasonably high theoretical working voltage.[1,4] While primary Air-cathodes, so-called gas diffusion electrodes (GDE), for secondary ZAB require electrocatalysts demonstrating high (bifunctional) activity and (electro)chemical stability to efficiently enable both the discharge and charge reaction, i. Air-cathodes, so-called gas diffusion electrodes (GDE), for secondary ZAB require electrocatalysts demonstrating high (bifunctional) activity and (electro)chemical stability to efficiently enable both the discharge and charge reaction, i. e. the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), respectively.[5,6,7] precious metal catalysts, i. e. carbon supported nanoscale Pt, Ru,
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