Synthesis, characterization, functional testing and ageing analysis of bifunctional Zn-air battery GDEs, based on α-MnO2 nanowires and Ni/NiO nanoparticle electrocatalysts

Abstract

Electrically rechargeable alkaline zinc air batteries (RZAB) – currently still at the R&D stage –, have great potential for stationary, as well as prospectively mobile, electrochemical energy storage applications. Their chief appeal is that they are made of abundant, environmentally friendly, intrinsically safe, and cheap materials, with established recycling concepts and auspicious life-cycle costs. One of the key weak points of present-generation RZAB programs is the air gas-diffusion electrode (GDE). In fact, on the one hand, GDE fabrication and testing are generally based on poorly understood protocols, and, on the other hand, performance is challenged by efficiency and durability issues. This work is centred on the fabrication of a novel bifunctional GDE for the air side of RZABs, on the assessment of its electrochemical performance and on the identification of factors impacting its efficiency and durability. The electrocatalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are α-MnO2 nanowires and Ni/NiO nanoparticles, respectively. The composition of the active layer was optimized with rotating ring-disc electrode (RRDE) electrocatalysts tests. The GDEs were fabricated by spray-coating an ink, formulated with the electrocatalysts and the PTFE binder in an aqueous matrix. Fabrication and functional performance of GDEs – in pristine form and after ageing under realistic RZAB conditions – are rationalized on the basis of Scanning Electron Microscopy (SEM), Scanning Transmission X-ray SpectroMicroscopy (STXSM) at the Mn l-edge and Transmission Electron Microscopy (TEM) analyses. Imaging and spectral imaging disclosed the morphological and chemical-state evolution, brought about by electrochemical cycling. Special attention was devoted to the understanding of the role played by the presence of zincate in the electrolyte on the performance and ageing of the reversible air electrodes.