Insights on degradation and regeneration of Ca2Fe2O5 photocathodes

A new study published in RSC Applied Interfaces explores why calcium iron oxide (Ca₂Fe₂O₅) thin films, considered by some researchers as promising photocathodes for photoelectrochemical water splitting, often degrade during operation. The material’s band gap and earth-abundant composition make it an attractive candidate for solar hydrogen production, but its stability has remained a major obstacle. Researchers in the LIMNO lab at EPFL found that performance loss arises from two degradation pathways: reduction of iron within the film and leaching of calcium into the electrolyte. Surprisingly, illumination alone caused little damage; instead, applying potential—even in the dark—was enough to trigger structural and functional decline. The research also showed that in some cases the material could be regenerated: after short exposure, thermal treatment restored its performance and even improved stability by forming a thin iron-oxide overlayer. However, longer operation led to calcium depletion, which could not be reversed and resulted in the permanent transformation of the material into iron oxide, effectively ending its functionality as a photocathode. Attempts to prevent degradation with protective coatings such as TiO₂ and Ga₂O₃ were largely ineffective, underscoring the challenge of safeguarding this otherwise promising material.