Electrochemical, Structural, and Hyperspectral Imaging Investigation of Bifunctional Zn-Air Battery Gas-Diffusion Electrodes
Authors: Alleva Alessandro , Ciancio Regina , Emanuele Elisa , Gianoncelli Alessandra , Kourousias George , Bozzini Benedetto 
Published: August 2024

Rechargeable Zinc-air batteries (RZAB) emerge as highly promising solutions for electrochemical energy storage (EES) applications. Nevertheless, a noteworthy drawback in the existing generation of these batteries lies in the air gas-diffusion electrode (GDE). This component grapples with issues related to both efficiency and durability. In response to this challenge, our research is concentrated on the creation of a novel GDE, accompanied by an in-depth examination of its electrochemical, structural, and compositional characteristics. The primary objective is to comprehensively assess the physico-chemical factors that exert an influence on its efficiency and durability. In the case of the present research, two different catalysts were chosen for the ORR (discharge) and OER (charge) electrocatalysis, namely α-MnO2 nanorods (NR) and Ni nanoparticles (NP), respectively. The GDEs were produced by applying a water-based catalyst ink, augmented with PTFE and C-black, through spray-coating onto a carbon paper support, which serves as the gas diffusion layer (GDL). Initially, the ideal proportion of OER/ORR catalyst was determined by conducting electrochemical tests on both the individual material (via RRDE studies) and the gas diffusion electrode using a rapid screening protocol. The material extracted from the GDEs, in pristine state and after ageing under operating conditions, was then characterized with electrochemical methods, SEM microscopy, HRTEM/SAED microscopy/electron diffraction, as well as with direct- and Fourier-space soft x-ray transmission microscopy (STXM) at the Mn, Ni and Zn L-edges. The electrochemical tests conducted in this study demonstrated a progressive improvement in performance as the nickel nanoparticle content increased, until a plateau is reached after 23% wt. Specifically, lower overpotentials were observed alongside increased cyclic stability for the combined ORR/OER processes. These assessments were conducted in two distinct electrolyte environments: pure KOH and KOH with the inclusion of Zn2+. The inclusion of Zn2+ aimed to replicate and understand the potential impact of anode chemistry on the cathode's overall performance. The investigation employed HRTEM and SAED analyses to meticulously monitor alterations in the crystal structure of the α-MnO2 electrocatalyst under diverse aging conditions. Notably, during ORR cycling, the manganese nanorods underwent amorphization. However, cyclic processes between oxygen OER and ORR revealed the recovering of the original crystalline phase. The introduction of Zn2+ into the system resulted in the formation of a partially amorphous Zn-Mn mixed oxide. To delve further into the nuanced changes, soft-X ray hyperspectral imaging, both in direct and Fourier space was employed. By spectral imaging at the Mn and Ni L-edges, we could assess the space-dependent valence alterations of Manganese and Nickel at different stages of aging with space resolution down to a few tens of nanometers. The integration of diverse analytical techniques, including electrochemical assessments, STXM, HRTEM/ SAED imaging, along with space-resolved structural analyses, has provided a comprehensive, molecular-level understanding of the performance and degradation of bifunctional ORR/OER electrocatalysts within the realistic context of gas diffusion electrodes. Of particular significance is the capability of this combined approach to meticulously trace and correlate the changes in the chemical state of the manganese ORR electrocatalyst. This shift, notably away from the optimal 3+/4+ mixed valence state, was observed in response to aging and Zn2+ contamination. The correlation of this chemical state drift with electrochemical conditions and the evolution of crystal structure adds a nuanced layer of insight into the intricate interplay of factors influencing the performance and degradation of these electrocatalysts. In particular, the control of the oxidative stress under OER, enabled by addition of Ni NPs, was correlated with preservation of α-MnO2 crystallinity, GDE operation in the presence of Zn2+ was proved to give rise to the formation of ZnMn2O4, poisoning the electrocatalyst. Lastly, the role of hydrophobicity of the active layer of the GDE was proven to be of crucial importance in the stability of the Mn-based catalyst, as PTFE-surrounded clusters were observed to be less affected by cycling with respect to others which clearly underwent electrolyte flooding.

Figure 1