Assessing Electrocatalyst-Ionomer Interactions: HOR/HER Activity of Pt/C Catalyst Layers incorporating Poly(imidazolium)s

In this work, a specifically designed C2-protected poly(imidazolium) with a limited number of freely rotating phenyl groups was synthesized and studied as a binder for a commercial Pt/C electrocatalyst. The ionomer-electrocatalyst interactions were extensively investigated in terms of kinetic and limiting current for the hydrogen oxidation reaction (HOR) and hydrogen evolution reaction (HER). A non-monotonic evolution of the electrochemically active surface area (ECSA) was observed with the increase of the ionomer-to-carbon ratio. The addition of ionomer to the catalyst layer leads to an increase in stability and activity of the electrocatalyst during HOR, while substantially increasing the ionomer content leads to only a marginal decrease in ECSA. Potential phenyl adsorption and cation-hydroxide ion-water interactions which impart deleterious impact on electrochemical activity are minimized owing to the sterically encumbered design of the polyimidazolium which (i) limits direct molecular interaction of the imidazolium cationic ring with Pt and (ii) encumbers phenyl and imidazolium ring rotation to hinder parallel aromatic ring adsorption onto the electrocatalyst surface, hence mitigating strong adsorption/blocking interactions. A poly(imidazolium) ionomer with a limited number of freely rotating phenyl groups was investigated as a binder in a Pt/C catalytic layer. The ionomer-electrocatalyst interactions and their impact onto the hydrogen oxidation and evolution reactions were assessed, evidencing a marginal effect of the ionomer onto the active surface area, owing to the sterically-designed ionomer that limits interactions between the phenyl/imidazolium rings and Pt.+image