X-ray absorption spectroscopy (XAS) has found applications in a range of fields including materials, physics, chemistry, biology and earth science. XAS can probe the local electronic and geometric structure, such as the average oxidation state, coordination environment and interatomic distances, surrounding an element of interest. Thus, XAS is a valuable tool to inform catalyst design by tracking catalyst evolution under operating conditions, for example, via providing dynamic snapshots of the essential information.
Given the broad application of operando XAS in the characterization of materials, we place the focus on electrocatalytic water splitting, a technology that is of growing interest to the community of sustainability and relies heavily on XAS for the understanding of the structure–activity relationships and thus the development of more efficient and cost-effective electrocatalysts. However, challenges persist in performing reliable operando XAS experiments, particularly due to the complexity of the oxygen evolution reaction (OER), the anodic half reaction in water splitting. In this context, operando hard (h-)XAS provides unprecedented insight into the physicochemical properties of OER catalysts under operating condition, being the ‘working catalyst’ often distinctly different from the as-prepared catalyst powder. When a liquid electrolyte is involved, and the reactants or products are gases, an extra layer of difficulty is added to the design and interpretation of the operando h-XAS experiments.
In this Comment, we formulate a set of guidelines for the use of operando h-XAS in a typical area of application, electrochemical water splitting. The proposed principles can not only allow for maximized understanding of underlying reaction mechanisms and better OER electrocatalyst designs, but more importantly are extensible to other fields. Time-resolved operando XAS will be essential for understanding key OER catalyst properties, establishing structure–activity relationships and formulating design principles for novel catalyst materials. Here we discussed the most pertinent points for properly designing an operando h-XAS study. Future research should move from half-reaction to full device evaluation, testing OER catalysts in a membrane electrode assembly configuration. Although the half-cell configuration allows for understanding key catalyst properties, we also need to study the long-term stability or degradation mechanism of the catalyst under technical operating conditions. Considering operando experiments generate a considerable amount of data, researchers should devote resources to develop machine learning tools to effectively analyse such XAS data.
Contact
Dr. Emiliana Fabbri
Co-Group Leader, Electrocatalysis and Interfaces Group
Paul Scherrer Institut PSI, 5232 Villigen PSI, Switzerland
Telephone: :+41 56 310 5775
E-mail: emiliana.fabbri@psi.ch
Original Publications
Best practices for operando hard X-ray absorption spectroscopy
Adam H. Clark, Thomas J. Schmidt & Emiliana Fabbri
DOI: 10.1038/s41893-024-01322-w
Acknowledgement to Funding Agency
E.F. gratefully acknowledges the Swiss National Science Foundation through its PRIMA grant (grant no. PR00P2_193111). T.J.S. thanks the Swiss Center of Excellence on Net Zero Emissions.