A new technique developed at Imperial College London measures oxygen production in real time, revealing hidden losses in solar water splitting.
Researchers at Imperial team found that electrical current did not always correspond to oxygen generation in hematite, one of the most widely studied materials in the field, revealing a previously unknown limitation in the reaction.
The study was led by Dr Flurin Eisner, formerly a Research Fellow in Imperial's Department of Materials and now at Queen Mary University of London, together with Dr Daniele Benetti from Imperial's Department of Chemistry. Collaborators included Professor Ifan Stephens from the Department of Materials and Professor James Durrant from the Department of Chemistry.
Revealing a hidden stage in the reaction
The team identified what they described as a "selectivity switch". At low current levels, much of the electrical charge was diverted into competing surface reactions rather than oxygen production. Once enough positive charge accumulated at the material's surface, oxygen generation became far more efficient, with oxygen-production efficiency rising to around 80%.
The same behaviour was observed when the reaction was driven electrically in the dark, suggesting it is a fundamental property of hematite rather than something specific to light-driven reactions.
The findings challenge the widespread assumption that electrical current always reflects how much oxygen is being produced, giving researchers a more accurate way to identify and improve materials for future solar fuel and green hydrogen technologies.
Royce-enabled instrument development
The discovery was made using a custom photoelectrochemical mass spectrometry (PEC-MS) platform developed at the Royce at Imperial facility in White City.
Access to the EC-MS equipment at Royce Imperial, together with support to develop a light-compatible cell, made this study possible. Dr Flurin Eisner Former Research Fellow, Department of Materials at Imperial
Supported by the Royce Small Equipment Fund, the researchers adapted an electrochemical mass spectrometry system into a platform capable of operating under illumination and measuring oxygen production in real time with exceptional sensitivity. The resulting PEC-MS platform enabled the team to simultaneously illuminate a sample, measure photocurrent and directly monitor oxygen generation as the reaction proceeded, making it possible to compare oxygen production with electrical current and reveal behaviour that would otherwise have remained hidden.
Dr Eisner said: "Access to the EC-MS equipment at Royce Imperial, together with support to develop a light-compatible cell, made this study possible. Its sensitivity allowed us to measure oxygen at reaction rates where current alone would have given an incomplete picture."
Professor Ifan Stephens said: "I am very proud to have played a small role in this paper. It is amazing to see how multiple techniques together, in this case electrochemistry, operando optical spectroscopy and electrochemistry mass spectrometry, can make us rethink our mechanistic understanding of a reaction." He added, "It is also very pleasing to see how well Flurin used the Royce Small Equipment Fund."
Improving solar fuel technologies
The findings could help researchers develop and evaluate new materials for solar fuel production more accurately.
The team now plans to apply the approach to other metal-oxide materials and explore its potential in a wider range of energy-conversion reactions, including carbon dioxide and nitrogen reduction.
The paper is published in Journal American Chemical Society. Link to publication: https://doi.org/10.1021/jacs.6c11949
