The Solid Oxide Fuel Cell (SOFC) is characterized by high operating temperatures (600-900°C). Within an SOFC electrode, fundamental physicochemical processes involve heterogeneous catalytic chemistry and electrochemistry, which are coupled to transport processes in the porous electrode structures. The complex interaction between these processes requires models with detailed kinetic mechanisms and transport on a microscopic level. A number of crucial issues concerning the influence of catalyst structure and composition, reforming chemistry and direct oxidation, carbon deposition, nickel oxide formation, cell aging etc. are addressed.
An example for the research activities on the surface and electrode level is the investigation of electrocatalysis at SOFC anode and cathode (Fig. 1). Two main materials are considered for SOFC anode, i.e. Ni/YSZ system and anodes based on perovskite compounds (LaSrTiO3). For the SOFC cathodes LSCF, LSM or LSC electrodes are modeled. In the case of Ni/YSZ based anodes investigation is focused on electrocatalysis at the three-phase boundary. This is the region where the gas phase and the two solid phases of electrode (Nickel, Ni) and electrolyte (yttria-stabilized zirconia, YSZ) meet. Here, effects such as surface reaction, surface diffusion and surface phase transitions due to spillover between the two solid phases are simulated. Proposed reaction mechanisms vary not only in the degree of simplification, but also in the fundamental pathway. In contrast to the Ni/YSZ, in which nickel degradation and poisoning are potential problems, perovskite oxides anodes are stable promising alternatives. For such mixed ionic/electronic conductors a bulk path is the most appropriate scenario of charge-transfer and a TPB interface is not strictly necessary for the reactions to occur. Similar situation is observed for LSCF based SOFC cathode where oxygen reduction taking place at the LSCF surface with further interfacial transport throughout LSCF/electrolyte phase. The goal of the work in this field is the elucidation of the elementary electrochemical reaction mechanism at the SOFC anode and cathode. An additional goal of our work is to assess degradation processes at SOFC electrodes. This includes mechanical (stresses) and chemical (solid carbon, sulfur and nickel oxide formation etc.) phenomena at electrode surface and bulk phases.
Activities on the cell level include the investigation of the coupling of detailed kinetics with macroscopic transport processes, needed for the prediction of spatially resolved chemical and electrochemical properties (Fig. 2). Depending on the boundary conditions (fixed or periodic), situations in either a single cell or a stack repeat element can be modeled.
In last years hydrogen became a prominent candidate as energy carrier due to its good clean, storable and transportable features. However, the production of hydrogen is based upon the use of fossil fuels such as steam reforming, partial oxidation of heavy hydrocarbons or gasification of coal, which significantly increases pollutions concentration. Among the new techniques one of the most cleanest and efficient method of hydrogen production is high temperature water electrolysis using Solid Oxide Electrolysis Cell (SOEC). In this respect, we work under development of elementary kinetic mechanism of fuels reduction/oxidation at different electrodes taking into account complex heat management within the cell.
(a)Reformate operated SOFC anodes with solid carbon and nickel oxide (NiO) formation
(b)Elementary kinetic mechanism of oxygen reduction at LSCF based SOFC cathode
(c)Elementary kinetic investigation of sulfur formation at Ni/YSZ based SOFC anode
Fig. 1: Elementary kinetic modeling of catalytic and electrochemical processes in SOFC electrodes taking into account different degradation phenomena
Fig. 2: Typical planar SOFC geometry used in our modeling study (upper picture) and simulated spatial distribution of gas species along channel and porous phases of the planar SOFC including surface, charge-transfer and degradation processes