Nafion-induced reduction of manganese and its impact on the electrocatalytic properties of a highly active MnFeNi oxide for bifunctional oxygen conversion

TL;DR

This study reveals that Nafion, commonly used as an electrode binder, significantly affects the electrochemical properties and manganese valence state in MnFeNi oxide catalysts for oxygen reactions, influencing their performance.

Contribution

It demonstrates the substantial impact of Nafion on catalyst properties and highlights the need to consider binder effects in electrocatalyst evaluations.

Findings

Nafion improves mechanical stability and particle connectivity.

Nafion decreases Mn valence state, affecting ORR selectivity.

Binder effects can alter catalyst performance assessments.

Abstract

Electrocatalysts for bifunctional oxygen reduction (ORR) and oxygen evolution reaction (OER) are commonly studied under hydrodynamic conditions, rendering the use of binders necessary to ensure the mechanical stability of the electrode films. The presence of a binder, however, may influence the properties of the materials under examination to an unknown extent. Herein, we investigate the impact of Nafion on a highly active ORR/OER catalyst consisting of MnFeNi oxide nanoparticles supported on multi-walled carbon nanotubes. Electrochemical studies revealed that, in addition to enhancing the mechanical stability and particle connectivity, Nafion poses a major impact on the ORR selectivity, which correlates with a decrease in the valence state of Mn according to X-ray absorption spectroscopy. These findings call for awareness regarding the use of electrode additives, since in some cases the extent of their impact on the properties of electrode films cannot be regarded as negligible.