A research team led by Prof. ZENG Jie from the University of Science and Technology of China (USTC) of the Chinese Academy of Sciences found that the stability of iridium (Ir) single-atom catalysts introduced in the lattice of spinel cobalt oxides in acidic oxygen evolution reaction is strongly correlated with the distance between adjacent Ir single atoms. The study was published in Nature Communications. 

Spinel cobalt oxides have high electrochemical oxygenation activities and are considered a very promising catalyst of non-precious oxygen evolution reaction. However, the cobalt atoms in cobalt-based spinel oxides are susceptible to dissolution under the acidic conditions of proton exchange membrane water electrolysis, leading to the structural collapse of spinel oxides, which severely limits their application in proton exchange membrane water electrolyzers. 

The stability of cobalt-based spinel oxides can be improved by introducing Ir single atoms into the lattice of cobalt spinel oxides. However, the stability of cobalt-based spinel oxides is closely related to the distance between the nearest Ir single atoms. Therefore, there is an urgent need to explore the single-atom distance effect in the stability study of cobalt-based spinel oxides on an atomic scale. 

Researchers revealed the distance effect in stabilizing Cu_0.3Co_2.7O₄ by introducing Ir single atoms of different distances into the process. It was shown that the stabilization effect of Ir single atoms on Cu_0.3Co_2.7O₄ is localized. When the distance between the adjacent Ir single atoms is large, the stabilization effect of the atoms on Cu_0.3Co_2.7O₄ is relatively independent, and it cannot stabilize the whole carrier. When the Ir-Ir distance is close (d = 0.6 nm), the stabilization effect of neighboring Ir single atoms on Cu_0.3Co_2.7O₄ overlaps, at which time the Ir single atoms can stabilize the whole carrier. 

Then, researchers established a structural model of Ir single atoms at different distances and investigated the effect of the atoms at different distances on the migration energy of cobalt atoms in Cu_0.3Co_2.7O₄. It was found that the migration energy of cobalt atoms increases from 1.63 eV to 1.83 eV as the distance between Ir single atoms gradually decreases from 1.14 nm to 0.56 nm. The results indicated that the stabilizing effect on Cu_0.3Co_2.7O₄ is weak when the distance between Ir single atoms is large, and when Ir-Ir distance is close enough, the stabilizing effect of the adjacent Ir atoms on cobalt atoms is superimposed on each other, thus stabilizing the whole Cu_0.3Co_2.7O₄ catalyst. 

To investigate the stabilizing effect of Ir single atoms at different distances on Cu_0.3Co_2.7O₄, researchers evaluated the stability of the series of catalysts. The results showed that the stability of the catalysts gradually increases as the distance between Ir single atoms is shortened from 1.1 nm to 0.6 nm. When the Ir-Ir distance is 0.6 nm, the performance of the catalyst remains stable after 1,000 laps of testing, and there was almost no dissolved cobalt species in the electrolyte. The catalyst could be operated stably for 60 h at a current density of 10 mA cm^-2, indicating its excellent stability. 

The study demonstrated that the stabilizing effect of Ir single atoms on cobalt spinel oxides is closely related to the distance between the adjacent Ir single atoms, which offers inspirations for the design of proton exchange membrane water electrolyzers.