Electrochemical and Dynamic Characterization of a 6-Cell Solid Oxide Cells Stack in Steam Electrolysis and Steam and Carbon Dioxide Co-Electrolysis

Abstract

High temperature steam electrolysis and co-electrolysis of steam and carbon dioxide for hydrogen and syngas production is gaining attention for its potential to play a key role in the production of renewable fuels and for long-term energy storage. Unlocking a longer lifetime and lower costs for these systems is required if they are to be widely deployed. In this work, we consider a 6-cell Solid Oxide Cell (SOC) short-stack, manufactured by SOLIDpower S.p.a. (Italy), that has been installed and tested at UC Irvine. The short-stack is installed in a test bench that maintains the operating temperature in a controlled range of 650-850°C, via an electric furnace. The cells are characterized by an 8 mol% Y2O3 stabilized Zirconia (YSZ) electrolyte (8 ± 2 μm) supported on a conventional porous Ni/YSZ anode electrode (240 ± 20 μm). The cathode electrode is comprised of a composite of metallic perovskite Sr-doped LaMnO3 (LSM) and oxide-ion electrolyte YSZ (40 ± 10 μm). The active area of each cell is 80 cm2.The aim of the study is the experimental characterization of the short-stack via electrochemical and dynamic techniques to: i) characterize stack performance in steady-state; ii) investigate single cell degradation and interdependencies between adjacent cell operating conditions to determine stack degradation; and iii) determine characteristic time responses of thermal, fluid-dynamic and electrochemical phenomena. The experiments are conducted for both steam electrolysis and co-electrolysis of steam and carbon dioxide. The experimental electrochemical methods employed include chronopotentiometry, electrochemical impedance spectroscopy and current interrupt. Moreover, fluid-dynamic step response and thermal ramp dynamic response of the stack are evaluated to achieve a better understanding of the effect of an operating control system on local fuel starvation and local temperature hot spot regions, which are possible contributors to increased degradation.Preliminary results show unequal degradation rates for the six cells under thermal cycles (which put stack under continuous switching between exothermic and endothermic operating modes). The unequal degradation rates of the cells were investigated and are possibly related to unequal flow distribution within the stack. Degradation rates were below 1% for thirty consecutive thermal cycles for operation in steam electrolysis. Responses that included fluid-dynamic step step changes allowed determination of the fluid-dynamic and thermal time constants associated with preheating flows introduced to the stack of the order of 200 to 300 seconds, depending on the step size. The evaluation of the fluid-dynamic and thermal response allowed decoupling of the thermal capacitance of the stack and that associated with the furnace and piping, via an equivalent circuit model approach. This has led to an ability to estimate the amount of heat produced by the stack when operating in exothermic conditions and the absorbed heat from the furnace when operating in endothermic (sub-thermoneutral voltage) conditions. Figure 1