Development and characterization of high temperature and -pressure alkaline electrolysis cells (HTP-AECs)

Abstract

During the last three decades the Intergovernmental Panel on Climate Change (IPCC) has published several reports. Each new report has made it increasingly clear that climate change needs to be handled now - sooner rather than later. There is strong evidence that climate change is caused predominantly by human activities, grounded in the unprecedented increase in greenhouse gas concentrations, including carbon dioxide, since the beginning of the industrial era. The fact that the worlds energy supply has been relying mainly on fossil fuels during the entire industrial era and until now elucidates the need to implement sustainable energy technologies that carry a significantly smaller carbon footprint compared to fossil fuel based technologies. A major challenge in the shift from a fossil based energy system to sustainable energy system, based on renewable energy, is the storage of large amounts of electricity and the conversion of electricity to other useful forms of energy. We know that hydrogen can serve as an energy carrier between various forms of energy.Currently, electrolysis is the most mature approach to produce hydrogen from renewable sources which means the electricity has to come from renewable sources. Hydrogen can be stored as chemical energy for long periods of time and production is feasible on the kW-GW scale. In addition, the hydrogen can be used as a precursor to make synthetic fuels, e.g., for parts of the transport sector that is hard to electrify. As mentioned hydrogen can be produced using electrolysis which is the electrochemical splitting of water into its constituents, oxygen and hydrogen gas, using electricity as the driving force. Alkaline electrolysis is the most proven electrolysis technology and the most utilized technology on a commercial level. There is, however, room for improvement of the technology, in terms of improving the efficiency and increasing the hydrogen production rate per unit area of electrolysis cell.The work in this thesis is focused on improving the alkaline electrolysis technology utilizing a cell concept that allows for operation at high temperatures (150-250°C) and high pressure (20-40bar), as this can significantly improve the cell efficiency and hydrogen production rate per unit area of cells. Ultimately this can lead to lower production prices of electrolyzed hydrogen. These electrolysis cells are termed high temperature and pressure alkaline electrolysis cells (HTP-AECs).The main objective in this project has been to develop, process and test porous oxygen electrodes. The starting point of the work carried out has been to identify electrocatalyst materials that are suitable to catalyse the evolution of oxygen; having in mind that the materials have to be stable during operation at HTP conditions. Ceramic oxides (mixed metal oxides) have been investigated as candidates and among these some La, Ni and Fe based oxides with the perovskite (or related) structure have been tested. The overpotential of the materials towards the oxygen evolution reaction (OER) in 1M KOH is in the range 0.38-0.45V at 10mAcm−2. The most active electrocatalysts were a multiphase-LaNiO3 and a La2Ni0.9Fe0.1O4. None of the candidates were chemically stable after exposure to concentrated KOH at 220°C for one week; the main secondary phases identified were La2O3/La(OH)3 and NiO. The perovskite LaNi0.6Fe0.4O3 (LNF) is stable at 100°C and appear to be the most stable among the candidates. It is also sufficiently active towards the OER, thus it has been used for the further work.The processing of porous LNF electrodes was done by screen printing optimized inks stabilized with polyvinylpyrrolidone (PVP) as dispersant and graphite and poly(methyl methacrylate) (PMMA) as pore formers. Larger pores were generated after burn-out of Graphite and PMMA with different sizes and shapes and thus used to generate different microstructures. A partial sintering of the electrode layers left finer inter-particle pores. The resulting structures have 58-72% porosity and the d10 % d90 pore sizes are 0.19-0.28µm and 1.5-3.5µm respectively. Electrodes were tested, at conditions relevant for conventional alkaline electrolysis cells, in 8M KOH at 65°C. The electrodes were tested as flooded electrodes, used in conventional alkaline electrolysis cells and gas diffusion electrodes, used in HTP-AECs, and showed similar performance in both cases. The overpotential of the most suitable cell was 0.42V and 0.46V at 0.2Acm−2 in the flooded and gas diffusion mode respectively. The electrodes are sufficiently stable at these conditions. A screening of different electrode microstructures at room temperature in 1 M KOH showed that the sintering temperature was the most important parameter. A lower sintering temperature, resulted in more porous bodies, large inter-particle pore sizes and better electrode performance. The electrodes sintered at lower temperatures turned out to be too mechanically too weak. Aqueous LNF suspensions containing rice starch as a pore former were also investigated as a possible way to fabricate porous electrodes with similar microstructures as mentioned above. The main advantage being that water based suspensions are more environmentally friendly than the organic solvents usually used in ceramic processing. Starch consolidation casting was used to process thin electrode layers but crack formation and delamination from the substrate could not be avoided after deposition and sintering. Quantification of the particle size and zeta-potential in the suspensions along with the sedimentation- and rheological behaviour, revealed that LNF is stabilized well electrostatically in aqueous suspensions at their intrinsic pH or sterically using the polymer PVP. In relation to the HTP-AEC technology, the LNF electrodes developed in this work, though chemically unstable at HTP conditions, are relevant to test at HTP-AEC conditions. The reason being that valuable information, about performance of the microstructures and about the long-term chemical and mechanical stability, can be retrieved. This is valuable information for comparison with previous work and it can serve as a guide for future development work.