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

## TL;DR

This paper introduces a new way to describe the local reaction environment around nanoparticles in electrochemical devices, which could help improve their performance and longevity.

## Contribution

The paper introduces effective ion concentration as a novel descriptor for the local reaction environment at nanoparticle-based electrocatalysts.

## Key findings

- Effective ion concentration depends on nanoparticle size, packing density, and Fermi levels.
- A new activity descriptor combines LRE descriptors with kinetic parameters for electrocatalytic activity.
- The model system used gold-supported silver nanoparticles in acidic solutions.

## Abstract

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.