In Situ/Operando X-ray Spectroscopies for Advanced Investigation of Energy Materials

Issues related to energy and the environment have now become of central and crucial importance in our societies. Low-carbon green energy will have a critical role in a necessary third industrial revolution. To reduce global greenhouse gas emissions in response to globalization and increasingly stringent carbon emission policies, large scale green energy production technologies must be established worldwide. A new age of human demand for green energy is thus coming and scientists are focused on finding new functional efficient and low-cost materials to generate clean and sustainable energy. Improving the energy conversion, generation, and storage efficiency of energy materials has always been a daunting challenge. For many important energy material systems, such as nanostructured catalysts, artificial photosynthetic systems, smart energy saving materials, and energy storage devices, monitoring the atomic and electronic structures close to the interfacial region in a real working environment is of paramount importance. Designing a better-performing material without comprehending its fundamental properties such as chemical states, atomic and electronic structures and how they are altered close to the interfacial regions during the physical and chemical reactions involved in their applications is very challenging. Understanding, controlling and tuning the interfaces in energy conversion and storage materials requires in situ/operando characterization tools, of which synchrotron X-ray spectroscopies, which have several unique features, are very suitable ones. X-ray absorption spectroscopy can be used to elucidate the local unoccupied electronic structure in the conduction band, and X-ray emission spectroscopy can be used to characterize the occupied electronic structure in the valence band. The derived resonant inelastic X-ray scattering reveals inter- and/or intra-electric transitions (i.e. d-d, f-f excitation and charge-transfer excitation) that reflect intrinsic chemical and physical properties. Scanning transmission X-ray microscopy is a chemical mapping technique with elemental sensitivity and spatial selectivity, which can therefore yield information about chemical composition in various spatial regions. This unique characteristic makes the method effective for investigating interfacial phenomena (such as electron transport, interface formation/deformation, defects, doping etc.). In situ/operando approaches have made the probing and understanding of changes in the atomic and electronic structures of energy materials in an operational environment feasible. This article presents a perspective of the pioneering developments as well as the recent achievements in in situ/operando synchrotron X-ray spectroscopies for the advanced investigation of energy materials. Four major energy material systems are identified: energy storage, energy generation, energy conversion, and energy saving material systems. Selected representative investigations of each systems are showcased and discussed demonstrating that in situ/operando synchrotron X-ray spectroscopy is truly essential for unraveling better fundamental knowledge for mechanism understanding and efficiency optimization of existing and emerging energy material systems.