Role of Surface Chemistry on Catalyst/Ionomer Interactions for Transition Metal–Nitrogen–Carbon Electrocatalysts

Abstract

The role of the interaction between doped carbon-based materials and ionic conductors is essential in multiple technologies, from fuel cells and energy storage devices to conductive polymer composites. In this paper, we report how the surface chemistry of transition metal−nitrogen−carbon (MNC) electrocatalysts affects catalyst−ionomer interaction and the resulting structure of cathodes. The cathode structure resulting from these interactions is directly related to the performance in membrane electrode assembly (MEA) fuel cells. To advance the development of platinum group metal (PGM)free electrodes for the oxygen reduction reaction it is necessary to understand the structure of the catalyst layers with focus on chemistry and distribution of active sites and ionomer morphology. To assess catalyst interaction with an ionomer, X-ray photoelectron spectroscopy is applied to study the chemistry of catalyst layers while density functional theory (DFT) is used to calculate adsorption energies of the ionomer side chain on different nitrogen species. We report that a high surface concentration of hydrogenated nitrogen at the surface of MNC catalysts causes inefficient ionomer morphology, while an abundance of surface oxides promotes both an efficient distribution of active sites and an optimal ionomer−catalyst interface. The critical role of protonation of nitrogen within catalytic layers in inhibiting proton transport during fuel cell operation is also suggested. This is the first report of the effect the surface chemistry of MNC catalysts, in the presence of the ionomer, has on the structure and performance of MEA electrodes.

Nitride or Oxynitride? Elucidating the Composition–Activity Relationships in Molybdenum Nitride Electrocatalysts for the Oxygen Reduction Reaction

“…15,20,21 Specifically, Fe-N-C and Co-N-C catalysts have shown activities approaching that of Pt and in-depth characterization has been used to provide insight to design more active M-NxC moieties. [22][23][24][25][26] While Pt-like performance has been achieved in a PEMFC configuration, their lower ORR 4eselectivity (Equation 1) and concerns regarding catalyst stability have complicated commercial application to date. 27,28 Carbon corrosion has been identified as one of the major degradation mechanisms and carbon oxidation at higher potentials means that the more active catalysts are more susceptible to rapid degradation.…”

Single Cobalt Sites Dispersed in Hierarchically Porous Nanofiber Networks for Durable and High‐Power PGM‐Free Cathodes in Fuel Cells

“…[25] The lack of large connected pores leads to the poor dispersion of the ionomer into the interior of primary catalyst particles and thus results in lower ionic conductivity within electrodes. [26][27][28] Therefore, advancements in MEA performance require innovative electrode design with tailored micro-, meso-, and macroporosity, aiming at maximizing the density of effective TPBs and improving mass and charge transport. [29][30][31][32][33] Electrospinning has proven a practical approach to introducing large meso-and macropores in nanomaterials.…”

Current challenges related to the deployment of shape-controlled Pt alloy oxygen reduction reaction nanocatalysts into low Pt-loaded cathode layers of proton exchange membrane fuel cells

“…The study highlighted that the ionomer morphology itself is an important performance parameter of the catalytic layer. In another study of the same year, Artyushkova emphasized the orientation and morphology of the ionomer spreading over the entire catalysts' surface in non-noble metal catalysts [54]. Herein, N-moieties influence the ionomer orientation, thus the physical properties such as hydrophobicity that in turn is essential for mass transport and water management.…”