Chemical-state evolution of Ni in Mn[sbnd]Ni/polypyrrole nanocomposites under bifunctional air electrode conditions, investigated by quasi-in situ multi-scale soft X-ray absorption spectroscopy

Abstract

Rechargeable metal-air batteries are highly prospective energy conversion and storage devices with enormous potential impact on broad deployment of renewable energies as well as on transportation. Nevertheless, efficient and durable systems are not yet available, mainly owing to limited chemical stability and durability of bifunctional air electrodes, that undergo poorly understood changes in chemical state and space distribution of the electrocatalytic materials. Manganese oxides (MnOx) are popular non-precious oxygen-reduction reaction (ORR) catalysts due to their low cost and relatively high activity. Unfortunately, the stability of MnOx under ORR conditions and its performance in the oxygen-evolution reaction (OER) are not completely satisfactory. Previous work in our group has shown that mixed MnNi oxides exhibit a notably improved ORR activity and durability, strongly enhanced by dispersing the oxides in a polypyrrole (PPy) matrix. Moreover, PPy exhibits high stability as well as catalytic activity in the OER. This study thus focuses on the ORR/OER behaviour of MnNi/PPy nanocomposite and is aimed at following the changes in chemical state distribution of dopant Ni at the sub-microscale. These changes result from the cathodic and anodic operation of this material under conditions relevant to the oxygen electrode of Zn-air batteries. Our study is based on quasi-in situ identical-location soft-X ray absorption microspectroscopy at the Ni L-edge, complemented by spectroscopy performed through ptychography for the case of ORR. Electrochemical operation results in a space- and potential-dependent rearrangement of the distribution of elemental and oxidized Nickel and in a change of their ratios. Operation under ORR conditions is accompanied by an increase of the oxidized Nickel fraction owing to the presence of an oxidizer in the electrolyte. As expected, oxidizing OER conditions cause an increase of the oxidized Nickel fraction.