Electrochemical Performance of Porous Ceramic Supported Tubular Solid Oxide Electrolysis Cell
Solid oxide electrolysis cell (SOEC) is an efficient and clean energy conversion technology that can utilize electricity obtained from renewable resources, such as solar, wind, and geothermal energy to electrolyze water and produce hydrogen. The conversion of abundant intermittent energy to hydrogen energy would facilitate the efficient utilization of energy resources. SOEC is an all-ceramic electrochemical cell that operates in the intermediate to high temperature range of 500-750°C. Compared with traditional low temperature electrolysis technology (e.g., alkaline or proton exchange membrane cells operating at ~100°C), the high-temperature SOEC can increase the electrolysis efficiency from 80% to ~100%, providing a new way for energy saving. The SOEC single cells with the Ni-YSZ fuel electrode supported configuration have received most intensive research effort. This is due to the high catalytic activity and electronic conductivity of Ni, as well as good oxygen ionic conductivity of YSZ, promoting the electrochemical reduction of steam in fuel electrode. However, under the high steam partial pressures, the Ni in the electrode could be occasionally oxidized the NiO at the high operation temperature, leading to volume expansion of the supporting layer. This phenomenon would induce internal stress in cell functional layers, resulting cracking or even failure of the single cell. To address the above mentioned issues, we propose a porous yttira-stabilized zirconia (YSZ) supported tubular single cell with a configuration of porous YSZ support, Ni-YSZ fuel electrode current collector, Ni-YSZ fuel electrode electrochemical functional layer, YSZ/Ce0.8Sm0.2O1.9 bi-layer electrolyte, and La0.6Sr0.4Co0.2Fe0.8O3-δ air electrode. As the porous YSZ substrate exhibits high chemical and structural stability in a wide range of oxygen and steam partial pressures under the SOEC operating conditions, employing the porous YSZ as the single cell support is expected to improve mechanical stability of the whole single cell. In this work, the porous YSZ supported tubular electrolysis cell has been fabricated by extrusion and dip-coating technique. The porosity, pore size and mechanical property of the YSZ support are investigated with respect to the amount of polymethyl methacrylate (PMMA) pore former. At the PMMA amount of 25 wt.%, the porous YSZ support shows the optimum porosity of 40-45% and good bending strength of ~20 MPa. Electrochemical performance of the single cell for steam electrolysis has been characterized under the H2O-H2 co-feeding condition. At the operation temperature of 750 °C, the H2 production rate reaches 3 ml·min-1·cm-2 and the cell maintains 95% of its initial performance during 10 thermal cycles. The good results demonstrate the feasibility of the novel porous YSZ supported tubular cell design for solid oxide electrolysis cell.