Recent Highlights of Solid Oxide Fuel Cell and Electrolysis Research at DTU Energy

Abstract

A successful Green Transition requires efficient, and cost competitive technologies for converting & storing energy from renewable sources and for providing energy to the stationary and the transport sector. Solid oxide fuel cells and electrolysis (SOFC, SOE - SOC) provide high efficiencies for electricity generation and production of storage media / fuels, respectively. In addition, they are flexible and versatile, which makes them attractive in many different application areas and energy scenarios.Research on solid oxide concepts has a long tradition at the Department of Energy Conversion and Storage (DTU Energy) of the Technical University of Denmark. It spans from fundamental topics such as materials & electrode development over methodology development for example in micro structural & electrochemical characterization, up to the applied level, where integration of SOC with up- and down-stream processes becomes important. Significant funding from National and European programs supported this research. The presentation will introduce highlights of recent achievements in the areas of (i) materials & electrode development to reach higher activity and better durability, (ii) cell & stack development for lowering costs and increasing robustness, (iii) power to X concepts with special emphasis on X= fuels, (iv) lifetime & diagnostics approaches for accelerating durability studies, and (v) modelling from materials to stack levels aiding lifetime evaluation and designing of stacks & systems.(i) Materials & electrode development: With the aim to reduce use of critical raw materials in SOCs, Co-free oxygen electrodes were developed based on lanthanum, strontium, and iron containing perovskite materials (LSF) applying a nano engineered hybrid catalyst coating via co-infiltration. The electrodes were integrated into cells and a high electro catalytic activity was achieved leading to power densities of ca. 1 W/cm2 in SOFC mode. Such cells with CGO infiltrated Ni/YSZ fuel electrode allowed for a stable electrolysis operation at 650 oC and -0.5 A/cm2 with a voltage close to 1.3 V.(ii) Cell & stack development: A novel, metal-based monolithic concept for SOFC stacks was developed. It is based on tape casting & lamination to build a monolithic structure thereby combining interconnect and gas distribution into one layer. Electro catalytic materials are infiltrated after sintering. This concept allows for an increase of the power density >5 kW/l, a decrease of materials costs by a factor of ca. 4-5 as compared to conventional stacks, and robustness for cycling operations.(iii) Power to X concepts: A novel and efficient route for producing green methanol was demonstrated by use of residue from the agriculture. A 6 kW SOEC stack unit produced hydrogen, which was combined with syngas from a thermal gasifier operating on straw in a down-stream methanol reactor. Methanol is considered a promising green fuel for the shipping sector. The same applies for ammonia. SOFC anodes catalyze the cracking of ammonia. This endothermal process can be integrated and tailored with the exothermal hydrogen oxidation in the electrode. The cracking of ammonia was studied on materials and stack level.(iv) Lifetime & diagnostics approaches: The long lifetimes –both required and achieved - of mature SOC generations make it necessary to develop new approaches for degradation studies and lifetime assessment. With the aim to harvest gained results from single cell durability testing over the last ca. 20 years at DTU Energy, a database was established allowing for a new evaluation of durability behavior. The database makes it possible to apply machine learning routines to reveal linear and non-linear correlations and single & combined effects of operating parameters on durability over a large range of data. The aim is to identify accelerating testing approaches and establish lifetime prediction.(v) Modelling: To further optimize the operation of an SOC and avoid degradation, a time dependent 3D stack model describing the degradation over the entire lifetime was developed. For this to be computational feasible, the model was built using the multi-scale, multi-physics homogenization concept, making it possible to simulate 40,000 hours of operation for a full 3D stack in one hour and 15 minutes. Same modelling concept was also utilized to describe local mechanical failures by use of a sub-model for the mechanical stress concentrations near assembly points inside the stack.