Sector coupling using SOECs via co-electrolysis of CO2 and H2O

Abstract

The shift to renewable energy sources for power production in order to decrease the CO2 emissions has led to rise in the demand for research in the field of energy storage technologies. One of the interesting options for storing excess renewable energy obtained from sources such as solar and wind is via the production of hydrocarbons. Utilization of solid oxide electrolysis cells (SOECs) facilitates this technology, known as Power-to-Liquid (PtL) and Power-to-Gas (PtG). This thesis is titled Sector coupling using SOECs, since integration of renewable energy production and storage to meet the grid demands can be approached with the use of SOECs for chemical production. However, the commercialization of SOECs is accompanied by the challenge of degradation under long-term operation. This PhD thesis focuses on the production of syngas, which is a valuable precursor for downstream production of fuels such as methane, using co-electrolysis wherein excess renewable energy can be stored. The aim of this work is to form methane internally at high pressure conditions, for which SOEC process model has been designed in Chapter 7. The novelty also lies in the use of wind profile from the Danish island of Bornholm for running cell and stack for dynamic energy storage. State-of-the-art (SoA) cells consisting of a nickel-yttria stabilized zirconia (Ni-YSZ) fuel electrode, YSZ electrolyte and lanthanum strontium cobaltite ferrite-gadolinium doped ceria (LSCF-CGO) composite oxygen electrode, were initially tested under co-electrolysis (H2O+CO2) conditions. A comparison between galvanostatic and potentiostatic modes was performed. For the purpose of degradation analysis, electrochemical impedance spectroscopy (EIS) was used along with microstructural analysis. Area specific resistance (ASR) was used as an indicator for comparing the extent of degradation. Potentiostatic mode of operation helped protect the cell by lowering the current density during long-term operation thereby keeping the overpotential over the cell constant. Following this, improvement in the cell structure was suggested in the form of modification of electrode microstructure. For the oxygen electrode modification, infiltration of solution was performed on a porous backbone layer. The solution consisted of lanthanum strontium cobaltite (LSC) which was infiltrated on CGO backbone For the fuel electrode modification, change in the particle size and density of the Ni-YSZ was performed. The improved cell was further tested under dynamic load cycling conditions as a candidate for energy storage in the form of methane. However, this is a very challenging task which requires the optimization of electrodes, test protocols etc. obtained as a result of ambient pressure tests carried out in Chapter 2 and 3. Co-electrolysis operation was performed on cell and stack level for 1000 hours. In the context of co-electrolysis operation, the presence of leaks was investigated in detail. Electrochemical and microstructural analysis of the electrodes and electrolytes helped in the formation of the theory of diffusion of H2 and O2 through the pinholes along with flow dependency. The improved cell was also tested under pressurized conditions up to 10 bar. The high pressure test was performed by applying steam electrolysis. Impedance data modeling was performed to draw a correlation between pressure and the electrode performance. Ultimately, the long-term goal will be to enable direct methane production via high pressure SOE Coperation. In this perspective, process modeling was performed, using the data from the experimental analysis to validate the model.