Effective Ion Concentration as a Descriptor for the Local Reaction Environment at Nanoparticle-Based Electrocatalysts

Electrocatalyst nanoparticles, attached to an electronically conductive support material, are key components that determine the performance and lifetime of electrochemical devices like fuel cells and electrolyzers. Differences in electronic and electrochemical properties between nanoparticles and support induce phenomena subsumed as electro-ionic metal-support interactions. These phenomena are responsible for heterogeneously distributed electron densities and electrical double-layer properties over the surface. The resulting local reaction environment (LRE), qualitatively different from that of single-crystalline extended surfaces, remains poorly understood. In an effort to address this shortcoming, the current work introduces the effective ion concentration as a quantitative descriptor for the LRE around supported nanoparticles. This property is defined as the average ion concentration over the reaction plane. Using gold-supported silver nanoparticles immersed in acidic solutions as a model system, we investigate how the effective proton concentration depends on the size and the packing density of nanoparticles, Fermi levels of nanoparticle and support materials, bulk electrolyte concentrations, and electrode potential. To further rationalize its impact on electrocatalytic activity, we define a complementary LRE descriptor that incorporates the effect of the local electrostatic potential. Based thereon, an activity descriptor is introduced by combining the two reaction-agnostic LRE descriptors with two reaction-specific kinetic parameters, viz., reaction order and transfer coefficient. Results are discussed in view of the suitability of the descriptors to be used in the design and optimization of nanoparticle-based electrocatalysts for electrochemical applications.