Tailoring of the synthesis gas composition during high-temperature co-electrolysis

High-temperature co-electrolysis is the simultaneous electrochemical conversion of H2O and CO2 in the temperature range of about 700 – 900 °C. In this thesis solid oxide cells with ceramic electrolyte are used. The cells consist of a cathode made from a cermet (ceramic metal) of nickel and yttria-stabilized zirconia (YSZ), a YSZ electrolyte and an anode made of lanthanum strontium cobalt ferrite (LSCF) or lanthanum strontium cobaltite (LSC) as well as a barrier layer made of gadolinium-doped ceria (GDC) between electrolyte and anode. In the thesis, the performance and underlying processes were analyzed by direct and alternating current measurements in CO2 and co-electrolysis as well as the boundary in between. The aim was the identification of the processes especially with regard of the role of electrochemical conversion of CO2 in order to optimize the co-electrolysis process in future applications. It was shown that the performance increases with increasing H2O partial pressure between 0 and 30 %. In this range H2O and CO2 are electrolyzed simultaneously. Above 30% the performance stayed more or less constant and equals that of pure steam electrolysis. Pure H2O electrolysis is dominating and CO2 is converted in the reverse water gas shift reaction (RWGS). The impedance spectroscopy measurements showed, that the largest losses were caused by diffusion processes in the cathode. These losses increase with decreasing H2O:CO2 ratio, that is with predominating CO2 electrolysis. This could be explained by the significantly larger diffusion coefficient of CO2 compared to H2O. The unique selling point of high-temperature co-electrolysis is the ability to tailor the syngas ratio in one single process step. The tailoring was investigated by varying temperature, gas composition and flow rate. The temperature only showed a small influence on the H2:CO ratio, which is caused by the water-gas shift equilibrium. The ratio of H2:CO in the output gas is especially dependent on the H2O:CO2 ratio of the input gas. The relation is about linear, causing the input H2O:CO2 ratio to react into H2:CO of about the same value. The decrease of flow rate results in an increased gas utilization, however, it is also responsible for a decreased current density at the same voltage. Thus, the space-time conversion is reduced. Although the percental conversion is increased (gas utilization), the current density determines the absolute conversion. High-temperature co-electrolysis is a very versatile method to utilize and store energy from renewable sources, while at the same time CO2 is recycled and valorized