Advanced ceramic supported oxygen carriers

In the future, fuel cells with hydrogen as fuel will be used for environmentally friendly and efficient electricity generation. The Reformer Steam Iron Cycle (RESC) developed at TU Graz enables the production of high-purity hydrogen in small plants directly at the consumer by using a chemical looping process. In this process, the oxygen carrier iron oxide is reduced to iron in the reduction step by releasing oxygen into the biogas supplied, thus oxidising the biogas to carbon dioxide and water vapour. In the oxidation step, the iron is oxidised again by the oxygen contained in the supplied water vapour and highly pure hydrogen is produced as a product gas for use in fuel cells. The research work in the project focuses on increasing the lifetime and stability of the iron oxide and other oxygen carrier materials developed for the process. The oxygen carrier materials are repeatedly reduced and oxidised at temperatures of 600-1000C. Due to the phase transformations and sintering, the performance of the material for hydrogen production continuously decreases. In the project, both new materials and the influence of innovative manufacturing methods on the service life of the oxygen carrier materials are being investigated. These include, for example, electron- and ion-conducting high- temperature ceramics, which are used as a stable support structure for the oxygen carrier. The newly developed oxygen carriers are tested and evaluated in the laboratory under conditions close to the application and can then finally be used in plants for hydrogen production from biogas.

The ACCEPTOR research project focused on advancing chemical looping hydrogen (CLH) technology as a safe, efficient, and scalable solution for green hydrogen production and storage. CLH uses iron-based materials as reversible hydrogen carriers: a chemical potential (energy) is temporarily stored in metallic iron and later released by reaction with steam, producing high-purity hydrogen without the need for additional purification. This makes the technology attractive for decentralized hydrogen production, long-distance transport, and large-scale storage. A major barrier to industrial implementation of CLH is the limited lifetime of iron-based oxygen carriers. Under the high temperatures and repeated redox cycling required in fixed-bed reactors, conventional materials suffer from sintering, agglomeration, and chemical deactivation, which significantly reduce hydrogen productivity over time. The central objective of ACCEPTOR was therefore to understand these degradation mechanisms and to develop improved materials that remain stable over many operating cycles. In the first phase of the project, ACCEPTOR investigated microscopic and structural phenomena that govern the performance of iron-based oxygen carriers. The research demonstrated that phase transitions, crystal structure changes, and chemical interactions between iron and common support materials play a decisive role in material degradation. In particular, some widely used supports form inactive compounds with iron, permanently reducing hydrogen yield. Zirconia-based supports, especially when stabilized with yttrium or magnesium, were identified as highly promising due to their chemical inertness and ability to suppress damaging phase transitions. Building on these insights, ACCEPTOR developed advanced oxygen carrier materials tailored for realistic reactor operation. A key innovation was the design of structured pellets with controlled internal architectures, including core-shell concepts. These structures physically separate reactive iron regions while maintaining open pore networks for gas transport. As a result, sintering and pellet agglomeration were strongly reduced. Fixed-bed reactor tests with a few hundert grams of material showed stable hydrogen production over up to 100 cycles while retaining more than 80% of the oxygen exchange capacity. In a further development step, the project integrated iron-based oxygen carriers into ceramic-structured environments, such as zirconia-based foams. These structures act as a mechanical exoskeleton, stabilizing the active material and improving gas flow and pressure behavior. The use of mixed ionic-electronic conducting ceramics significantly enhanced hydrogen productivity and enabled stable operation for more than 150 cycles. This approach also demonstrated the potential of CLH as a practical method for hydrogen storage and transport using solid iron as a safe and environmentally friendly carrier. Overall, the ACCEPTOR project established new material design principles and scalable reactor-compatible solutions that significantly improve the durability and efficiency of chemical looping hydrogen systems. The results provide a strong foundation for industrial implementation and contribute to the development of CO-neutral hydrogen technologies for future energy systems.