Lessons Learned from Operating a Solid Oxide Electrolysis Cell at 1.25 a/cm2 for One Year

Abstract

Solid oxide electrolysis cells (SOECs) convert electrical energy to chemical energy stored in H2 and/or CO and has a great potential to become a key enabling technology in the transition towards renewables. The key challenges for the successful commercialization of SOEC are the limited long-term durability and cost. To ensure high production rate and hence reduce the investment cost, operation at high current densities needs to be considered. Conventional Ni/yttria stabilized zirconia (YSZ) fuel electrode supported SOEC cells demonstrate acceptable long-term durability when operated at below 1 A/cm2, with a number of tests running for years. These cells suffer however severe degradation when operated at above 1 A/cm2, where the major degradation phenomena are reported to be delamination of the oxygen electrode, crack formation in the YSZ electrolyte, depletion of Ni in the active Ni/YSZ electrode, deterioration of the Ni-YSZ interface, and formation of ZrO2 nano-particles on Ni surface. A large part of the cell resistance increase is associated with the Ni/YSZ electrode. By introducing Ce0.8Gd0.2O2- d (CGO) nanoparticles into the Ni/YSZ electrode via solution infiltration, we were able to reduce the cell degradation (evaluated based on cell voltages of 0-1000h) from 699 mV/1000h to 66 mV/1000h when operating the cells at 800 oC and 1.25 A/cm2. The CGO nano-particles facilitate the steam splitting reaction at the Ni/YSZ triple phase boundaries (TPB), reduce the electrode polarization and effectively mitigate the microstructural deterioration typically seen in the non-modified Ni/YSZ electrodes. The remaining 66 mV/1000h degradation is mainly associated with formation of a poorly conducting CeAlO3 phase, which is a reaction product between Al2O3 as the sintering aid and the CGO particles on the Ni grains. An effort to break down the CeAlO3 phase was explored, though not successful. Testing the cell with modified Ni/YSZ electrode was extended to one year. A steam supply failure at 4000 h caused certain damage to the cell and resulted in a higher degradation rate in the second half of the one-year test. A counter-balance strategy was successfully tried out by increasing the cell operating temperature to 830 oC eventually. The test was successfully terminated after reaching one year, with cell voltages of 1124 and 1468 mV at the beginning and end of the test, corresponding to an average degradation rate of 40 mV/kh. The current work shows high potential for modifying conventional SOEC cells for use in high current density electrolysis applications.