Operando X-ray absorption spectroscopy of phosphomolybdic acid redox mediators for electrochemical conversion of biomass to green hydrogen

Tools

Price, Robert and MacDonald, Lewis and Keenan, Luke Lawrence and Johnson, William and Lyth, Stephen Matthew and Li, Jun and Brightman, Edward (2026) Operando X-ray absorption spectroscopy of phosphomolybdic acid redox mediators for electrochemical conversion of biomass to green hydrogen. Catalysis Science and Technology, 16 (4). pp. 1444-1456. ISSN 2044-4761 (https://doi.org/10.1039/D5CY00763A)

[thumbnail of Price-etal-2026-Operando-X-ray-absorption-spectroscopy-of-phosphomolybdic-acid-redox]
Preview
 Text. Filename: Price-etal-2026-Operando-X-ray-absorption-spectroscopy-of-phosphomolybdic-acid-redox.pdf
Final Published Version
License: Creative Commons Attribution 3.0 logo
Download (2MB)| Preview

Abstract

Green hydrogen production is limited by the thermodynamic energy requirement of water splitting. By decoupling the anodic and cathodic reactions using redox mediators, alternative feedstocks such as lignocellulosic biomass can yield comparable hydrogen purity with around half the total electrical energy input of conventional water electrolysis. Keggin-type heteropolyacids such as phosphomolybdic acid (H3PMo12O40) are increasingly of interest as mediators in decoupled electrochemical energy conversion applications, due to their reversible redox characteristics. However, the redox behaviour of heteropolyacids in aqueous solution during this process is still poorly understood. X-ray absorption spectroscopy (XAS) techniques offer the opportunity to directly probe changes in oxidation state and local structure of the mediator in-operando, allowing unprecedented insight into redox processes in a flow cell environment. Here, for the first time, we demonstrate operando XAS of reduced phosphomolybdic acid as it undergoes oxidation in a proton exchange membrane electrolyser with bespoke 3D-printed flow-fields. Changes in the time-resolved XAS were correlated to structural changes in the Keggin anion, which were closely related to the state-of-charge of the mediator, laying the groundwork for future studies of solution-phase Keggin-type heteropolyacid redox states. Furthermore, reduction of the phosphomolybdic acid via mild thermal digestion of either pure lignin or real agricultural biowaste is compared, in a step towards real-world applications. This work contributes significantly to the efficient and low-cost production of green hydrogen from waste biomass feedstocks, as well as providing insights into similar decoupled electrolysis processes.

ORCID iDs

Price, Robert, MacDonald, Lewis ORCID logoORCID: https://orcid.org/0000-0002-4883-6419, Keenan, Luke Lawrence, Johnson, William, Lyth, Stephen Matthew ORCID logoORCID: https://orcid.org/0000-0001-9563-867X, Li, Jun ORCID logoORCID: https://orcid.org/0000-0002-7685-8543 and Brightman, Edward ORCID logoORCID: https://orcid.org/0000-0002-5352-7936;
CORE (COnnecting REpositories)