Operando XAS of a Bifunctional Gas Diffusion Electrode for Zn-Air Batteries under Realistic Application Conditions

Emanuele Marini,Francesca Rossi,Giuliana Aquilanti,Michael Liebert,Benedetto Bozzini,Danilo Oliveira De Souza

doi:10.3390/app112411672

Abstract

In this study, operando X-ray absorption spectroscopy (XAS) measurements were carried out on a newly developed O2 bi-functional gas diffusion electrode (GDE) for rechargeable Zn-air batteries, consisting of a mixture of α-MnO2 and carbon black. The architecture and composition of the GDE, as well as the electrochemical cell, were designed to achieve optimum edge-jumps and signal-to-noise ratio in the absorption spectra for the Mn K-edge at current densities that are relevant for practical conditions. Herein, we reported the chemical changes that occur on the MnO2 component when the GDE is tested under normal operating conditions, during both battery discharge (ORR) and charge (OER), on the background of more critical conditions that simulate oxygen starvation in a flooded electrode.

Highlights

In order to apply strategies for the rational design of bi-functional oxygen reduction reaction (ORR—discharge mode) and oxygen evolution reaction (OER—charge mode) catalysts for metal-air batteries, it is important to improve our understanding of the chemical and structural characteristics of the active material under reaction conditions
OER experiments after complete O2 starvation were not performed because the interest of the experimental protocol was to assess the chemical state change of Mn resulting from extensive O2 starving, rather than checking the anodic performance after the gas diffusion electrode (GDE) has been exposed to such stress conditions
Operando X-ray absorption spectroscopy (XAS) was employed to probe structural and chemical changes of the αMnO2 electrocatalyst embedded into a bifunctional GDE for metal-air batteries

Summary

Introduction

In order to apply strategies for the rational design of bi-functional oxygen reduction reaction (ORR—discharge mode) and oxygen evolution reaction (OER—charge mode) catalysts for metal-air batteries, it is important to improve our understanding of the chemical and structural characteristics of the active material under reaction conditions In this field, in recent years, X-ray absorption spectroscopy (XAS) is becoming increasingly popular because it can be combined with electrochemistry to elucidate the properties of catalytic materials in situ and operando. We conducted in situ XAS measurements of a newly developed O2 bi-functional gas diffusion electrode (GDE) made of α-MnO2 nanowires (NWs), obtained from a facile synthesis method, combined with commercially available carbon black Both the architecture and composition of the electrode, as well as the configuration of the electrochemical setup, have been designed to achieve optimal edgejumps and signal-to-noise ratio in the absorption spectra for the XAS Mn K-edge, without sacrificing the current densities applied during testing (electrode performance). We compared the chemical changes that occur on the MnO2 component when the GDE is tested under normal operating conditions, on the background of more critical conditions that simulate oxygen starvation in a flooded electrode, where the consumption of oxygen is faster than the rate at which oxygen is transported from the gas phase to the catalyst surface

Experimental

Fabrication of the GDE

GDE Testing during Operando XAS

Results and Discussion

Chronovoltammetry

Conclusions