The high energy densities promised by metal-air batteries for the next-generation of electrochemical energy storage devices has prompted large research incentives for the development of more efficient and more durable bifunctional gas diffusion electrodes (GDEs). At the air-cathode, the electrode material has to exhibit high reactivity and selectivity in reaction products for both the oxygen reduction reaction (ORR, discharge) and the oxygen evolution reaction (OER, charge), and, additionally, it has to demonstrate high stability. One of the main constituents commonly employed for GDEs is porous carbon, which provides both mechanical and chemical support to the catalyst component. However, the high potentials at which OER takes place cause gradual degradation of the carbon support [1], leading to a rapid decrease of the battery’s performance and, eventually, to its failure. Nonetheless, carbon support also shows significant activity for the 2 e- ORR process in alkaline media [2], thus synergistically enhancing the ORR activity of the electrode material [3, 4] upon further reduction of the hydrogen peroxide at the supported catalyst component (typically a particulate transition metal oxide). A number of approaches attempt to enhance the support’s stability, either by, e.g., doping or graphitizing the carbon [5], or by substituting it entirely with other elements [6], but a satisfying alternative to the porous carbon support has yet to be developed.

In this communication, we will present a systematic and comparative study about the influence of different electronically conductive support materials for the promotion of the ORR activity in alkaline electrolyte. We investigated the properties of α-MnO₂ supported on different materials, namely nickel (α-MnO₂/Ni), carbon (α-MnO₂/C) and gold (α-MnO₂/Au), including also a mixture of supports. We will detail synergetic effects taking place on the different materials by comparing their properties towards the ORR to those exhibited by the supports alone by means of rotating disk electrode (RDE) and rotating ring disk electrode (RRDE) experiments. This will include a depicting of the intrinsic kinetic limitations for the reaction rate, reactant mass-transport limited current characteristics and of the potential-dependent selectivity in hydrogen peroxide formation. While the purpose of Au-based supports is to resolve trends in synergetic effects between catalyst and support, Ni-based supports offer interesting perspectives due to their stability in alkaline solutions within a broad potential window and their high intrinsic catalytic towards OER being favorable for fast battery charging. In addition, X-ray diffraction (XRD) and scanning electron microscopy (SEM) techniques provided detailed insights on the structural characteristics of the catalyst-support mixtures. Finally, we will aim at the establishment of straightforward structure / reactivity / selectivity relationships to pave the way towards a rational design of more efficient and more stable cathode materials for metal-air batteries.

References:

[1] N. Staud et al., J. Electrochem. Soc., 136, 3570, 1989.

[2] C. Guo-Liang et al., J. Phys. Chem. C., 121, 14524, 2017.

[3] A. S. Ryabova et al., ChemElectroChem, 3, 1667, 2016.

[4] C. M. Lousada et al., J. Phys. Chem. C., 116, 9533, 2012.

[5] S. Müller et al., Journal of New Materials for Electrochemical Systems, 2, 227, 1999.

[6] D. U. Lee et al., Adv. Energy. Mater., 4, 1301389, 2014.