  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.
