Zn-air batteries: spectral STXM shows how working conditions impact cathode stability

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.

Figure 1: (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.

This study emphasizes the importance of balanced Mn(III) and Mn(IV) contents and the space correlation of these two species for optimal electrocatalytic activity. Instead, a high fraction of Mn(II) and a spatially uncorrelated distribution of Mn(III) and Mn(IV) witness the formation of inactive phases and Zn-containing poisoning species (Figure 2). This study highlights the impact of spectromicroscopy for a deeper, molecular-level understanding of the performance of metal-air battery oxygen electrodes, that stringently depends on the synergy between chemical nature and morphology of the electrocatalysts and active-layer architecture.

Figure 2: (a) Space distribution of the ratio of Mn(III) and Mn(II) fractions in electrocatalyst cluster aged under ORR conditions. (b) Electrochemical response of GDE in electrolyte without (black) and with (red) added Zn²⁺. (c) Electrochemical impedance spectra measured during aging under ORR conditions. Elaborated with permission from the reference reported below.

This research was conducted by the following research team:

Yawar Salman¹, Sheharyar Waseem¹, Alessandro Alleva¹, Pritam Banerjee2,3, Valentina Bonanni⁴, Elisa Emanuele¹, Regina Ciancio^2,5, Alessandra Gianoncelli⁴, George Kourousias⁴, Andrea Li Bassi¹, Andrea Macrelli¹, Emanuele Marini⁶, Piu Rajak^2,3, Benedetto Bozzini^1
1 Department of Energy, Politecnico di Milano, Milano, Italy
² Istituto Officina dei Materiali IOM-CNR, Trieste, Italy
³ Abdus Salam International Centre for Theoretical Physics, Trieste, Italy
⁴ Elettra - Sinctrotrone Trieste S.C.p.A., Trieste, Italy
⁵ Area Science Park, Padriciano, Trieste, Italy
⁶ Zentrum für Sonnenenergie- und Wasserstoff-Forschung Baden-Württemberg (ZSW), Ulm, Germany

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