MnO<sub>2</sub> nanosheets@Ni-N doped graphene aerogel for Zn-air battery

Abstract

Zinc-air battery has the advantages of low cost and high theoretical specific capacity (1084 Wh kg−1). The sluggish kinetic of oxygen reduction reaction (ORR) of air cathode limits the reaction rate of zinc-air battery and restricts its practical applications. Moreover, scarce precious metal-based catalysts such as Pt/C are exprensive. Therefore, it is necessary to obtain low-cost ORR catalysts. The conductivity of catalysts affects electron transfer rate in ORR process and consequently affects the catalytic activity. MnO2 has been studied as a promising ORR catalyst for its advantages of low cost, abundant resources and environmental friendliness. However, the poor electronic conductivity of MnO2 limits its applications in ORR related energy storage devices including zinc-air battery. The conductivity can be improved by assembling MnO2 with highly conductive metals and carbon materials. Graphene and its derivatives are often used as substrates of electrochemical materials due to its excellent electronic conductivity and good electrochemical stability. By doping N atoms into graphene skeleton, electronic state can be improved. Doped N atoms make the nearby C atoms become active adsorption sites. Besides, the introduction of oxygen vacancies (OVs) can also improve electronic conductivity of the materials. In this work, a coassembly of MNSs@Ni-NGA showing high ORR catalytic activity was fabricated containing Ni nanoparticles of 10−15 nm in diameter, MnO2 nanosheets (MNSs) of 5−10 nm in sheet thickness at the ratio of exposed surficial to total unit cells of 37.4%, the ratio of OVs to total O atoms of 46.9at% and NGA about 300 nm in wall thickness. Ni nanoparticles, MNSs and NGA all have ORR activity, providing plenty of active sites for ORR. Ni nanoparticles, NGA and OVs improve the conductivity of coassembly and facilitate electron transfer. At the same time, NGA prevents Ni nanoparticles from falling off and aggregating. The big specific surface area (208.0 m2 g−1) of MNSs@Ni-NGA benefits sufficient exposure of active sites. Plenty of mesoporous pores in MNSs@Ni-NGA benefit fast mass transfer during electrochemical process. The basic ORR was characterized by rotating disk electrode (RDE) in electrochemical workstation. Performances of Zn-air battery were studied by electrochemical workstation and LAND battery test system. Through synergistic effects of Ni nanoparticles, MNSs, OVs and NGA, MNSs@Ni-NGA has an onset potential of 0.98 V and a half wave potential of 0.84 V in alkaline electrolyte, close to those of commercial Pt/C (0.98 and 0.85 V). To demonstrate practical application, a Zn-air battery with MNSs@Ni-NGA as air cathode material was assembled, showing an open circuit voltage of 1.53 V, a maximum power density of 114.8 mW cm−2, a specific capacity of 801.9 mAh gZn−1 and a specific energy of 962.3 Wh kgZn−1 (88.8% of the theoretical specific energy), higher than those of commercial Pt/C+IrO2. This work provides a strategy for designing and assembling high performance ORR catalysts with non precious metallic nanoparticles and OVs-containing MnO2 nanostructures for energy storage devices.