MATERIAL DEVELOPMENT TO ENHANCE REVERSIBLE SOLID OXIDE CELLS FOR HYDROGEN PRODUCTION AND POWER GENERATION

Abstract

In electrolysis mode, reversible solid oxide cells (RSOCs) can use electricity from renewable sources to produce green hydrogen and, in reverse, in fuel cell mode, use hydrogen to generate electricity.1 RSOCs present a viable opportunity to solve renewable energy intermittency and achieve on-demand hydrogen and electricity production. However, multiple requirements, including high catalytic activity, high ionic and electronic conductivity, and cell component stability, must be simultaneously satisfied for electrolytic to fuel cell mode switching to be effective in RSOCs. While the state-of-the-art materials for fabricating solid oxide cells cannot currently fulfil these multiple electrochemical requirements, an exsolution process can simultaneously improve such functionalities in these materials. An exsolution process entails the segregation of metallic cations to form catalytically active nanoparticles on the surface of a perovskite lattice (the support structure) – this results in highly active, anchored and therefore stable catalytic sites.2,3 Also, the growth of such nanoparticles to reside within the bulk of the perovskite lattice (bulk exsolution) has been recently shown to improve ionic conductivity.3 Therefore, this research seeks to develop new perovskite materials using surface and bulk exsolution to fulfil the multiple electrochemical requirements of RSOCs. An A-site deficient perovskite with a stoichiometric composition such as (Sr,Ca)1-α(Ti,Fe,Ni)O3, known for its ability to drive B-site exsolution in their tendency to revert to a stable ABO3 perovskite stoichiometry,2 is targeted in this research. Parameters related to the synthesis of the new perovskite have been examined at the current stage of this research. Also, five potential precursor materials, (Fe(NO3)3.9H2O, Ni(NO3)26H2O, SrCO3, CaCO3, and TiO2) have been studied to ascertain their suitability and develop a synthesis route for the new perovskite materials. The methodology adopted for the study involved the characterisation of the potential precursor materials using thermogravimetry analysis (TGA), scanning electron microscopy (SEM), and X-ray diffraction (XRD)analysis. The TGA results revealed Fe2O3, NiO, SrO and CaO as the decomposition products of Fe(NO3)3.9H2O, Ni(NO3)26H2O, SrCO3 and CaCO3, while no substantial decomposition occurred in TiO2. These decomposition products have indicated the suitability of the different materials as precursors for the desired A-site deficient perovskite material. Furthermore, the XRD analysis of Fe(NO3)3.9H2O, Ni(NO3)2.6H2O and CaCO3 have confirmed their respective crystalline composition and shall help in understanding the complex crystal changes the precursor materials will undergo to form the desired perovskite material. Considering the decomposition temperature ranges: 50 – 400 oC, 56 – 573 oC, 600 – 850 oC, and 620 – 900 oC, respectively, for Fe(NO3)3.9H2O, Ni(NO3)26H2O, CaCO3, and SrCO3, it is expected that all the precursors, except TiO2, shall fully decompose into reactive oxides before 950 oC. This indicates that the desired perovskite synthesis reaction will likely occur between the 600 – 1000 oC temperature range, following the decomposition time of CaCO3 and SrCO3.