Development of perovskite-like structures for hydrogen production via two-step thermochemical water splitting
Creators: name: Wilson, George E.
Publisher: name: Imperial College London
Year: 2022

description: Hydrogen powered technologies are proposed to help mitigate climate change as low carbonemitting technologies. Devices such as fuel cells convert the chemical energy stored within hydrogen molecules via electrochemical redox processes to electrical energy for work. These technologies have the primary benefit of not emitting carbon dioxide – one of the main contributing pollutants towards the greenhouse effect. However, current commercial hydrogen production technologies require fossil fuel reactants and emit carbon dioxide as a product. Therefore, research into ways of producing hydrogen from sustainable non-polluting sources has been of keen interest within the scientific community. One such technique is high temperature thermochemical water splitting. This process uses renewable concentrated solar power to heat up and thermally reduce metal oxide compounds and induce an oxygen nonstoichiometry within the lattice. The oxygen deficiency is then removed upon reoxidising with steam and producing hydrogen gas. Numerous thermochemical redox cycles have been proposed within the literature with the main aim to lower the reduction temperatures and increase the hydrogen production volumes. This has turned the attention of the field to investigate the ABO3 perovskite structures due to their ability to support a larger oxygen deficiency at lower temperatures compared to the benchmark material, cerium oxide, CeO2. This thesis combines theoretical first principle approaches and a wide range of experimental techniques to understand and discuss three different families of perovskite and perovskite-like metal oxide structures. The main findings of this thesis can be summarised as the following: Effect of antimony incorporation on the redox kinetics of SrCoO3-d - Thermal analysis techniques observe large oxygen production volumes onset between 300 and 400 °C under an inert gas flow with increased antimony content lowering total production. - Density Functional Theory (DFT) confirms the low reduction enthalpy in the region of 0.5 eV/O atom. Increased Sb concentration and proximity to the dopant increases vacancy formation energy. 6 - Low reduction enthalpy of the material was not favourable to drive thermochemical water splitting, however isothermal redox cycling demonstrated good performance for the alternative application of thermochemical oxygen separation compared to literature materials. - Antimony donor ions are postulated to lower the cobalt crystal field splitting to support an intermediate spin electron configuration with more favourable orbital filling for fast redox kinetics (eg=1). Effect of iron incorporation in (La0.8Sr0.2)0.95Cr1-xFexO3-d perovskites for thermochemical water splitting - Thermal analysis used to observed increasing Fe content coincides with an increase the oxygen production volumes and rates - DFT used to confirm lower vacancy formation energy in positions neighbouring Fe cations. Further predicted to have favourable thermodynamic properties for thermochemical water splitting. - Thermochemical water splitting observed hydrogen production rates similar to literature materials, Ce0.75Zr0.25O2-d. - Surface analysis techniques novel to this research field revealed increased strontium segregation towards the surface that prevented cyclability of the compounds. - Strontium-enriched perovskite surfaces can undergo reconstruction to form derivative phases such as Ruddlesden-Popper oxides, An+1BnO3n+1. Computational screening of n=1 Ruddlesden-Popper oxides for thermochemical water splitting - Screening study uses a combination of well-known crystallographic principles and DFT simulations to narrow down the field of this underexplored metal oxide family for use in thermochemical water splitting. - From an initial 27,899 structures, this study outlines a potential 30 A2BO4 Ruddlesden- Popper structures that have favourable reduction thermodynamics and “synthesisable” under laboratory conditions. - A new simpler and better fitting descriptor based on the lattice enthalpy is proposed to assist future screening work of Ruddlesden-Popper oxides at significantly reduced computational expense. Investigating Ca2MnO4 Ruddlesden-Popper oxide for thermochemical water splitting - Outputted compound from the prior screening study is explored further due its abundant constituent elements and favourable reduction thermodynamics. - Thermal analysis techniques observe similar oxygen production behaviour to the (La0.8Sr0.2)0.95Cr1-xFexO3-d perovskites investigated in a previous chapter. - Hydrogen was successfully produced via thermochemical redox reactions cycling between 1000 and 800 °C, thus experimentally verifying the screening study. - Further improvements are suggested by including doping ions to alter the thermodynamics or investigating the effect of perovskite/Ruddlesden-Popper heterostructures that have previously been observed to accelerate oxidation reactions.