# Spin Balance Over Janus Ir-Co Magnetic Atoms for Efficient Acidic Water Oxidation

## TL;DR

This paper introduces a new method to enhance water oxidation using a Janus Ir–Co structure that balances spin states for better performance.

## Contribution

A novel spin balance strategy using a Janus Ir–Co structure to regulate Ir4+ spin and improve acidic OER performance.

## Key findings

- The 0.2Co–CIO catalyst achieves an ultrasmall overpotential of ~200 mV at 10 mA cm−2 for water oxidation.
- The catalyst shows a high mass activity of 1110 A gIr−1 and turnover frequency of 2050 h−1 under 300 mV overpotential.
- The superoxide path mechanism is favored due to optimized eg1 and reduced t2g orbital occupancy in the Janus Ir–Co structure.

## Abstract

Monodisperse and substitute Co were doped into edge-sharing [IrO6] octahedra of Ca2IrO4 model catalyst, which usually present the intrinsic and strong stability for acid oxygen evolution reaction (OER)The optimized Janus Co–Ir local structure triggers spin balance effect with optimal eg1 orbital and uneven t2g orbital despite the large crystal field of Ir, which co-promote the OER activity with a relatively stable crystal structure.Different from the slowly kinetics of adsorbates evolution mechanism on Ca2IrO4, superoxide path mechanism occurs on Co doped Ca2IrO4 based on the assignment of *OO on dual active sites of Ir and Co.

Monodisperse and substitute Co were doped into edge-sharing [IrO6] octahedra of Ca2IrO4 model catalyst, which usually present the intrinsic and strong stability for acid oxygen evolution reaction (OER)

The optimized Janus Co–Ir local structure triggers spin balance effect with optimal eg1 orbital and uneven t2g orbital despite the large crystal field of Ir, which co-promote the OER activity with a relatively stable crystal structure.

Different from the slowly kinetics of adsorbates evolution mechanism on Ca2IrO4, superoxide path mechanism occurs on Co doped Ca2IrO4 based on the assignment of *OO on dual active sites of Ir and Co.

The online version contains supplementary material available at 10.1007/s40820-026-02082-2.

Herein, spin regulation of the low-spin Ir4+ in Ca2IrO4 is realized via a novel spin balance strategy through a Janus Ir–Co structure using high-spin Co3+ dopants, achieving the intermediate-spin state of Ir and Co atoms and enhancing the acidic oxygen evolution reaction (OER) performance of the obtained catalysts (Co–CIO). The optimized 0.2Co–CIO catalyst, with a nominal Co/(Co + Ir) metal atom percentage of 20%, displays exceptional electrochemical water oxidation activity with an ultrasmall overpotential of ~ 200 mV at 10 mA cm−2, ultralarge mass activity of 1110 A gIr−1, and high turnover frequency of 2050 h‒1 under an overpotential of 300 mV in 1 M HClO4, outperforming most recently reported Ir–based oxides catalysts. Molecular and atomic characterizations via in situ X-ray absorption near-edge and Raman spectroscopy demonstrate the acceleration of bridged O‒O formation over the Janus Ir–Co units, indicating a preference for the superoxide path mechanism for Co–CIO. Furthermore, density functional theory calculations rationalize the promotion of the superoxide *O‒O intermediate over the spin-regulated Ir‒O‒Co units, thanks to optimized eg1 orbital and reduced t2g orbital occupancy. The study presents a rare example of Ir spin regulation via a Janus Ir–Co magnetic structure, thereby promoting acidic OER activity.

The online version contains supplementary material available at 10.1007/s40820-026-02082-2.