Renewable energy conversion and storage are pressing topics in applied research today, as the use of non-fossil energy sources is essential to reduce CO₂ emissions. Solar and wind energy harvesting combined with chemical energy storage in batteries or fuels such as hydrogen are essential processes, that need to be improved in terms of efficiency, long-term stability, and scalability. Although significant progress has been made, many proposed solutions and devices show satisfying short-term efficiencies in the laboratory, but are too cost-intensive for large-scale applications, because they rely on rare or non-sustainable materials and/or they are not fabricated using scalable processes. Our long-term goal is to contribute to the development of scalable solutions for energy conversion and storage while respecting sustainability criteria: On the one hand, through the responsible development of functional materials and devices based on scalable (solid-state chemical) fabrication techniques. On the other hand, by exploring structure/morphology-property relationships in energy conversion and storage materials with the aim of obtaining efficient, long-lived materials.

One of the challenges of solar hydrogen production is related to the high overpotentials encountered for the oxygen evolution reaction at photoanodes and for photocatalyst particles. Materials such as oxynitrides and bismuth vandate show promise but are still limited in their performance and stability. The development of stable and efficient photocatalytically active materials based on a mechanistic process understanding is an important part of our work. Comprehensive, in-depth characterisation is key, as structure, composition, crystallinity, porosity and transport properties influence the efficiency and stability of active materials and devices. We have already made progress through detailed material understanding based on a wide range of analytical techniques (XRD, TEM, SEM, photoelectrochemical and photocatalytic studies) and through improved synthesis design, such as better control of morphology (see Figure above) or modification of carrier density by substitution. The task of developing active materials for photoelectrodes therefore involves the desigen of suitable photocatalysts, including additives such as protective layers and photocatalysts.

The shift towards renewable energy sources rather than fossil fuels is linked to our ability to store electricity in devices such as rechargeable batteries. Today, the leading technology used in both stationary and mobile applications is the lithium-ion battery (LIB). Although LIBs present numerous advantages, primarily in terms of performance, the limited availability of lithium and safety concerns related to the reactivity of lithium provide the motivation to enhance existing concepts and develop alternative solutions. Promising post-LIB chemistries rely on more abundant elements such as sodium or magnesium. In addition, the use of aqueous electrolytes replacing flammable, sometimes toxic organic electrolytes has made significant progress in the last years. Nevertheless, post-LIB concepts need further improvement with respect to recyclability and sustainability while maintaining or, in many cases, reaching competitive performance levels. We aim for responsible development of competitive electrode/electrolyte combinations with a focus on post-LIBs, while respecting sustainability criteria. Consequently, we focus our research on a selected number of materials containing elements such as silicon or vanadium allowing for a trade-off between functionality, abundance, criticality, and sustainability. In order to adapt the selected materials and to improve performance, we do not limit our investigations to pre- and post-mortem studies, but observe electrochemical processes during operation (operando XRD, see Figure above).

In addition to the active materials, the cell design, such as the combination of electrodes and electrolyte and/or the fabrication of electrodes containing the active materials. plays an important role in energy storage systems. Li-ion batteries are a well-studied example, where numerous combinations have been commercialized after careful adjustments. Consequently, in order to develop a high-performance electrode, not only the active material itself is important, but also the way in which an electrically conductive connection is established between the (usually insulating) oxidic particles. While active materials development for energy conversion and storage is often concerned with morphology and interface design at the nanoscale, the implementation of these active materials into functioning devices is determined on the micrometre-scale. One important question is how to establish conductive connections or networks between active materials particles and substrates while maintaining or even improving the properties of the active material. Conductive networks/connections are formed by several material classes and address different length scales. Inorganic or ceramic bridges are often local, limited to the nanoscale, while layered systems such as reduced graphene oxide or carbon nanotubes form long-range connections in the micrometer range. The synthesis and subsequent deposition of hybrid or composite materials is one of the routes we are pursuing to fabricate electrodes containing conducting networks and enhance the performance of our active materials (see Figure above).process helps to better understand potential degradation processes.