The capabilities of soft x-ray absorption (XAS), soft x-ray emission spectroscopy (XES), resonant inelastic soft x-ray scattering (RIXS), and their application to characterization of solar energy materials are presented. We have discussed some recent developments of in-situ soft x-ray spectroscopy cells, which enable the investigation of critical solid/liquid or solid/gas interfaces in chemical and electrochemical processes under real-world practical conditions. In particular, in-situ soft x-ray spectroscopies can be utilized to probe the formation of intermediate species, charge separation upon illumination of sunlight and electron transfer to the interfacial reactions. A number of examples using soft x-ray spectroscopies to study these solar energy and catalytic materials are discussed to demonstrate how these powerful characterization tools could be helpful to further understand the energy conversion and energy storage systems.

Nature Communications 7:10804 Article number 10804 (2016); Published 23 February 2016; Updated 18 March 2016 The financial support for this Article was not fully acknowledged. The Acknowledgements should have included the following: The authors gratefully acknowledge research support from the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, Fuel Cell Technologies Office, under Contract No.

Developing efficient photoreversible color switching systems for constructing rewritable paper is of significant practical interest owing to the potential environmental benefits including forest conservation, pollution reduction, and resource sustainability. Here we report that the color change associated with the redox chemistry of nanoparticles of Prussian blue and its analogues could be integrated with the photocatalytic activity of TiO₂ nanoparticles to construct a class of new photoreversible color switching systems, which can be conveniently utilized for fabricating ink-free, light printable rewritable paper with various working colors. The current system also addresses the phase separation issue of the previous organic dye-based color switching system so that it can be conveniently applied to the surface of conventional paper to produce an ink-free light printable rewritable paper that has the same feel and appearance as the conventional paper. With its additional advantages such as excellent scalability and outstanding rewriting performance (reversibility >80 times, legible time >5 days, and resolution >5 μm), this novel system can serve as an eco-friendly alternative to regular paper in meeting the increasing global needs for environment protection and resource sustainability.

The evolving oxygen state plays key roles in the performance and stability of high-energy batteries involving oxygen redox reactions. Here, high-efficiency full energy range O-K mapping of resonant inelastic X-ray scattering (mRIXS) was collected from O₂ (O⁰) and CO₂ (O^2- with strong covalency) molecules and compared directly with Li₂O₂ (O^-) and the oxidized oxygen state in representative Na/Li-ion battery electrodes. Our results confirm again that the critical mRIXS feature around the 523.7 eV emission energy is from intrinsically oxidized oxygen, but not from the highly covalent oxygen state (CO₂). The comparison of the mRIXS profile of the four different oxygen states, i.e., O^2-, O^-, O^n- (0 < n < 2), and O⁰, reveals that oxygen redox states in batteries have distinct widths and positions along the excitation energy compared with Li₂O₂ and O₂. The nature of the oxidized oxygen state in oxide electrodes is thus beyond a simple molecular configuration of either peroxide or O₂.

Recent advances in synchrotron based x-ray spectroscopy enable materials scientists to emanate fingerprints on important materials properties, e.g., electronic, optical, structural, and magnetic properties, in real-time and under nearly real-world conditions. This characterization in combination with optimized materials synthesis routes and tailored morphological properties could contribute greatly to the advances in solid-state electronics and renewable energy technologies. In connection to this, such perspective reflects the current materials research in the space of emerging energy technologies, namely photocatalysis, with a focus on transition metal oxides, mainly on the Fe2O3- and TiO2-based materials.

The interfaces between electrode and electrolyte play a vital role in various electrochemical systems. However, the understanding of such embedded interfaces and interfacial processes is still limited partly because of the short of proper characterizations tools, especially in-situ/operando techniques. In the past decade, substantial effort has been devoted in the development of in-situ/operando techniques to investigate the solid/liquid interfaces. One successful approach is the combination of in-situ liquid cells and soft x-ray absorption spectroscopy (sXAS). In this review, we showcased several recent examples using such liquid-cell-based in-situ/operando sXAS method to study different aspects of the electrode/electrolyte interface under reaction conditions, including the adsorption and deposition of solute species on electrode surface, morphological and chemical changes of electrode surface, and potential-dependent orientations of interfacial solvent molecules. These examples demonstrated capability of such in-situ/operando sXAS techniques in providing element- and chemical-sensitive interfacial information. This in-situ/operando sXAS technique also opens the way to the investigation of various solid/liquid interfaces in other important heterogeneous reactions.

Interest in hydrogen fuel is growing for automotive applications; however, safe, dense, solid-state hydrogen storage remains a formidable scientific challenge. Metal hydrides offer ample storage capacity and do not require cryogens or exceedingly high pressures for operation. However, hydrides have largely been abandoned because of oxidative instability and sluggish kinetics. We report a new, environmentally stable hydrogen storage material constructed of Mg nanocrystals encapsulated by atomically thin and gas-selective reduced graphene oxide (rGO) sheets. This material, protected from oxygen and moisture by the rGO layers, exhibits exceptionally dense hydrogen storage (6.5 wt% and 0.105 kg H2 per litre in the total composite). As rGO is atomically thin, this approach minimizes inactive mass in the composite, while also providing a kinetic enhancement to hydrogen sorption performance. These multilaminates of rGO-Mg are able to deliver exceptionally dense hydrogen storage and provide a material platform for harnessing the attributes of sensitive nanomaterials in demanding environments.

Abstract: Many energy storage and energy conversion systems are based on the complexity of material architecture, chemistry, and interfacial interactions. To understand and thus ultimately control the energy applications calls for in situ/operando characterization tools. Over the years, in situ/operando soft x-ray spectroscopy has been developed for the studies of gas molecules, molecular liquids, catalytic, and electrochemical reactions. Soft x-ray spectroscopy offers electronic structure characterization of materials in energy conversion, energy storage, and catalysis regarding functionality, complexity of material architecture, and chemical interactions. It has been shown how to use the powerful in situ/operando soft x-ray spectroscopy characterization techniques of interfacial phenomena and how to reveal the mechanism of charge transfer and chemical transformation of solid/gas and solid/liquid interfaces of energy storage and catalytic materials in a realistic environment. It has been demonstrated how to overcome the challenge that soft x-rays cannot easily peek into the high-pressure catalytic or liquid electrochemical reactions. The unique design of in situ/operando soft x-ray spectroscopy instrumentation and fabrication principle and examples of experiments are presented. Graphic Abstract: [Figure not available: see fulltext.]

The observation of a spontaneous electron enrichment at the apexes of bismuth vanadate (BiVO4) nanopyramidal arrays is achieved by performing synchrotron-based angular dependent X-ray absorption spectroscopy. In accordance with density function theory calculations, the formation of (112) facets enriched apexes is proposed. Such intrinsically electron-enriched (112) facets at the apexes produce a potential facilitating the diffusion of photoexcited holes, which not only mitigates the recombination with the photoexcited electrons, but also provide an active reaction site for the photoassisted selective growth of cobalt phosphate (CoPi), a well known and highly active co-catalyst for water oxidation at the apex of BiVO4 nanopyramids. Benefiting from the element-resolved properties of synchrotron-based X-ray spectroscopy, 3d electrons on the vanadium site are directly resolved by resonant inelastic X-ray scattering measurements. Due to this unique morphology, the charge at the apex is expected to induce a strong interaction with CoPi, as a result of the metal-to-ligand charge transfer spectroscopy feature observed from the cobalt site. Substantial evidences are collected by means of comprehensive soft X-ray spectroscopy techniques with high spectral resolution revealing the atomic-scale origin and the nature of the synergy as well as the strong correlation between its unique structural properties and the electronic structure of this novel highly-ordered hybrid photocatalyst and its significantly improved photoelectrochemical performance.

Design of interfaces with thermodynamic and kinetic specificity is of great importance for hydrogen storage from both an applied and fundamental perspective. Here, in order to destabilize the metal hydride and protect the dehydrogenated products from oxidizing, a unique core-shell structure of porous Mg(BH₄ )₂ -based framework with a thin layer (no more than 5 nm) of MgCl₂ additives on the surface, has been proposed and synthesized via a wet-chemical method. The local structure and electronic state of the present complex system are systematically investigated to understand the correlation between the distribution of additives and dehydrogenation property of Mg(BH₄ )₂ . A significant improvement is achieved for hydrogen desorption with chlorides: initial hydrogen release from MgCl₂ decorated γ-phase Mg(BH₄ )₂ particles commences at 100 °C and reaches a maximum of 9.4 wt% at 385 °C. Besides the decreased decomposition temperature, an activation barrier of about 76.4 kJ mol^-1 lower than that of Mg(BH₄ )₂ without MgCl₂ is obtained. Moreover, MgCl₂ decoration can also prevent the whole decomposed system (both Mg- and B- elements) from oxidizing, which is a necessary condition to reversibility.