Doping Effect on Electronic and Electrochemical Properties of High-Entropy Perovskite Oxide Thin Films

Electrochemical water splitting is a promising clean technology for generating hydrogen gas from water, but its practical use is largely hindered by the anodic oxygen evolution reaction (OER), which suffers from slow reaction kinetics associated with its multiple proton-coupled electron transfer reactions. To overcome this challenge, there is an urgent need to develop more efficient and stable electrocatalysts. Recently, high entropy oxides (HEOs), which feature at least five atoms at the same lattice site in near-equimolar amounts, have shown great potential to combine high performance and stability for applications such as energy storage and catalysis. However, the specific roles of individual cations in promoting OER activity and enhancing chemical stability remain unclear. This project will utilize epitaxial high entropy perovskite oxide thin films with precise control over composition, phases, and strain states as a platform to yield mechanistic insights into the effect of element doping on the electronic and electrochemical properties of HEOs. We have synthesized a series of epitaxial La(5B)_1-xNi_xO₃ (where 5B denotes Cr_0.2Mn_0.2Fe_0.2Co_0.2Ni_0.2 and x = 0, 0.125, 0.25, 0.375, 0.5, 1) thin films using pulsed laser deposition. Electrochemical measurements show that OER activity drops significantly as x increases from 0 to 0.125, then rises at x = 0.25, followed by a gradual decrease with further increasing x. X-ray absorption spectroscopy reveals that Cr and Ni oxidation states increase with Ni doping, while Co shows a slight increase, and Mn and Fe remain relatively unchanged. Notably, the trend in Cr oxidation state aligns with the observed OER activity trend, indicating that Cr cations play a key role in determining the OER performance of La5BO₃. Compared to lower-valent Cr cations, the higher-valent Cr cations may exhibit higher absorption energy of OH^-, making the initial step of OER more difficult, which in turn decreases overall OER activity. The insights gained from this study will accelerate the design of novel functional materials for water oxidation, advancing the development of scalable, efficient, and stable electrocatalysts.