Cobalt phosphide (CoP) is a promising alternative to platinum for producing hydrogen from water. However, scientists have struggled to explain why it performs so well. Computer models have suggested that CoP should become unstable under the conditions used to produce hydrogen, yet experiments show that it remains active and durable in acidic, neutral and alkaline environments.

A new study offers an explanation: CoP changes its surface as it produces hydrogen, helping it maintain its performance while protecting itself from damage. Rather than remaining chemically unchanged, the material responds to the electrical voltage applied during water electrolysis. The chemical species that form on its surface help drive hydrogen production and prevent the catalyst from breaking down.

The study was published in the Journal of the American Chemical Society on October 29, 2026.

At lower voltages, hydroxyl groups (P-OH) form on the phosphorus sites of the catalyst. These groups help neighboring cobalt atoms activate water and bind hydrogen more effectively. As the voltage increases, the surface changes again, becoming covered with hydrogen-containing species (Co-H and P-H). This adjustment allows the catalyst to maintain an effective balance between binding and releasing hydrogen across a wide range of pH conditions.

Importantly, these surface changes also help protect the material. The adsorbed hydroxyl and hydrogen species act as a protective layer, limiting oxidation and preventing cobalt and phosphorus from dissolving into the surrounding solution.

To investigate this process, the researchers combined artificial intelligence (AI)-assisted analysis of previous studies, computer simulations and laboratory experiments. Their analysis covered 2,282 observations from approximately 886 cobalt phosphide-based catalysts reported in around 700 scientific publications. They then used computer models to predict how the catalyst's surface would change during operation and tested these predictions using specially prepared CoP nanosheets.

Experiments confirmed the predicted surface changes, with advanced spectroscopic techniques detecting the chemical species involved. Additional tests using deuterium, a heavier form of hydrogen, provided further evidence that hydrogen was forming bonds with phosphorus. The catalyst also demonstrated strong performance in all three environments, requiring relatively little additional voltage to produce hydrogen. It remained stable for 24 hours even at very high current densities exceeding 1 A cm⁻².

These findings could help researchers develop more affordable and versatile catalysts for green hydrogen production. More broadly, they highlight an important principle for designing future catalysts: rather than focusing only on the material as it is initially made, scientists should consider how its surface changes under actual operating conditions.

The discovery shows how cobalt phosphide can effectively protect itself while it works, helping explain why it remains an effective hydrogen-producing catalyst across a wide range of conditions.