The oxygen reduction reaction (ORR) is a key process in fuel cells and metal-air batteries, technologies expected to play a central role in a low-carbon energy future. However, ORR proceeds slowly on most materials, limiting efficiency and increasing costs. Finding catalysts that can speed up this reaction is therefore a major challenge in reducing our energy footprint.

Two-dimensional (2D) topological materials have recently attracted attention as potential electrocatalysts. Their unusual electronic properties arise from spin-orbit coupling (SOC), which creates robust topological surface states (TSSs) that can enhance charge transport. Until now, most studies have assumed these surfaces remain clean and unchanged during reactions.

But the reality is different. In real electrochemical environments catalyst surfaces are far from pristine. They constantly interact with the surrounding electrolyte and reaction intermediates, forming so-called electrochemical surface states (ESSs). Understanding how these realistic surfaces affect the topological properties and catalytic performance is necessary if scientists are to utilize 2D topological materials.

To address this issue, researchers at Tohoku University examined monolayer platinum bismuthide (PtBi₂), an atomically thin two-dimensional material, as a model topological electrocatalyst. By combining quantum-level calculations with models that capture how reactions depend on pH, the team determined the catalyst's true working surface under oxygen reduction conditions.

Their results revealed that PtBi₂ is stabilized at ORR-relevant potentials with nearly one monolayer of hydroxyl (HO*) species covering its surface. This means the active surface is not the idealized topological surface, but an HO*-induced electrochemical surface state formed during operation.

Importantly, this surface reconstruction does not erase the material's topological nature. Instead, it reshapes the electronic landscape, creating localized SOC-enabled surface states and a flat-band-like feature with a high density of electronic states near the Fermi level. These features enhance electronic coupling to ORR intermediates and reduce sensitivity to interfacial dipoles.

Much like roads can guide crowded traffic, the topological framework steers electron flow in beneficial ways despite adsorbates covering the catalyst surface.

By explicitly accounting for pH effects, the researchers further predict that PtBi₂ achieves near-peak ORR activity in alkaline environments. This highlights the importance of evaluating catalytic performance under realistic electrochemical conditions rather than relying on idealized surface models.

"Our findings show that topological surface states can survive, and even be optimized by, electrochemical reconstruction," says Hao Li, a Distinguished Professor at Tohoku University's WPI-AIMR. "This provides a practical design principle for next-generation electrocatalysts, where quantum topology and electrochemical surface chemistry must be considered together."

Additionally, all the computational results have been uploaded to the Digital Catalysis Platform (DigCat), the world's first and largest experimental + computational catalysis database to date, developed by the Hao Li Lab.

Details of their findings were published in the Journal of Physical Chemistry Letters on December 9, 2025.

Publication Details:

Title: 2D Topological Electrocatalysts with Spin−Orbit Coupling: Interplay between the "Electrochemical" and "Topological" Surface States

Authors: Heng Liu, Tran Ba Hung, Yuan Wang, Di Zhang, Yiming Lu, and Hao Li

Journal: The Journal of Physical Chemistry Letters

DOI: 10.1021/acs.jpclett.5c03589