# Multimodal Operando Characterization of Cation Effects at the Iridium Oxide–Electrolyte Interface for Alkaline Water Oxidation

## TL;DR

This study explores how different cations affect the performance of iridium oxide in water oxidation reactions by using advanced spectroscopy and simulations.

## Contribution

The paper introduces a multimodal operando approach to reveal how cation size influences interfacial solvent structure and oxygen evolution reaction kinetics.

## Key findings

- OER activity increases with larger cation size (TMAOH > KOH > NaOH > LiOH).
- Interfacial solvent disorder, driven by cation size, enhances OH– reactivity and O–O bond formation.
- Larger cations shift the potential of maximum entropy closer to the water oxidation potential.

## Abstract

Understanding the electrode/electrolyte interface is
essential
for tuning electrocatalyst activity. Here, we combine operando optical
spectroscopy, laser-induced current transient (LICT) measurements,
and surface-enhanced infrared absorption spectroscopy (SEIRAS) to
investigate the origin of cation-dependent oxygen evolution reaction
(OER) activity on electrodeposited iridium oxide in 0.1 M MOH (M =
TMA+, K+, Na+, and Li+). We find that OER activity increases with increasing cation size
(TMAOH > KOH > NaOH > LiOH). Operando optical spectroscopy
reveals
that the energetics of the redox transitions and the population of
the redox-active species are independent of the electrolyte. Instead,
the intrinsic turnover frequency varies strongly with the nature of
the cation. LICT, SEIRAS, and quantum mechanics/molecular mechanics
(QM/MM) simulations suggest that the interfacial solvent structure
is the origin of this difference. With increasing cation size, the
fraction of isolated water molecules and cation-coordinated water
molecules increases, producing a more disordered interfacial environment.
LICT measurements confirm that the potential of maximum entropy shifts
closer to the water oxidation potential in the presence of larger
cations in the electrolyte. We propose that a more disordered interface
results in more isolated and reactive OH– ions and
faster reorganization of the interfacial solvent structure during
the rate-determining O–O bond formation step, thereby accelerating
the OER kinetics. Through our work, using multimodal operando spectroscopy
and molecular simulations, we highlight how interfacial solvent structure,
controlled by electrolyte cations, governs reactivity at complex electrochemical
interfaces.