Electrically rechargeable alkaline zinc-air batteries (RZAB) hold immense promise for future energy storage, offering a sustainable and cost-effective solution for both stationary and mobile applications. Zinc-air batteries operate on the coupled electrochemistry of zinc and oxygen. Reversible oxygen redox is enabled by a bifunctional gas-diffusion-electrode (GDE), that drives oxygen reduction during discharge and oxygen evolution during recharge. With present-day technologies, the alternation of these processes leads to the accumulation of damage, causing durability issues that still hamper implementation in real-life devices.
The aim of the present research is to fabricate a durable, efficient and sustainable bifunctional GDE. To achieve this objective, an insightful understanding of the electrode, jointly addressing molecular-level out-of-equilibrium electrochemistry and mesoscale architecture geometry evolution is required. The novel bifunctional GDE features a-MnO2 nanowires as oxygen reduction electrocatalyst and Ni@NiO core-shell nanoparticles as oxygen evolution electrocatalyst. The fabrication process consists in microwave-assisted hydrothermal synthesis of α-MnO2 nanowires, formulation of an ink with different contents of Ni/NiO nanoparticles, and spray-coating onto carbon paper, followed by thermal treatment.
Electrochemical performance is assessed using voltammetry, galvanostatic sequences representative of realistic operating conditions, and electrochemical impedance spectroscopy (EIS) in half-cell configuration. The novel GDEs exhibit remarkable oxygen reduction current densities, in excess of 200 mA cm-2, with improved stability during successive charge-discharge cycles. The addition of Ni@NiO nanoparticles lowers anodic overvoltages, mitigating carbon-support corrosion and enhancing overall GDE stability. However, the presence of Zn2+, released to the electrolyte by the anodic process, accelerates GDE failure due to the formation of inactive Zn-Mn-containing phases: this degradation mode is however mitigated by the Ni-based electrocatalyst, showing an anodic contribution also to poisoning.
Electrochemical measurements, combined with morphological SEM and TEM observations and STXM spectromicroscopy, performed at Elettra’s TwinMic beamline, allowed to pinpoint the degradation mechanisms, providing concrete guidance to overcome them. Specifically, electrochemical ageing, on the one hand, targets catalyst stability, triggering cathodic dissolution of Mn and anodic redeposition of MnO2 in less active forms, and, on the other hand, high anodic overvoltages, due to insufficient Ni-contaning electrocatalyst, favour oxygen bubble formation in the bulk of the active layer architecture, leading to cracking. Chemical degradation of the electrocatalysts causes nanorod agglomeration, growth of amorphous phases and Ostwald ripening of the Ni nanoparticles. Figures 1a and 2a display, respectively, ADHUC Mn L-edge spectra of a selection of samples tested in this study, accompanied by a typical chemical-state map, representative electrochemical results and TEM images. The alteration in the valence state of Mn and its space distribution can be readily inferred from stacks of absorption maps.
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Image: (a) Space-averaged spectra for indicated electrode conditions. (b) Corresponding (colour-coded) TEM micrographs and schematics of MnO2-evolution process. Elaborated with permission from the reference reported below.