Electrochemical Reduction of H2O and CO2 Using Anode Supported SDC Cells

Abstract

Solid oxide electrolysis cells (SOECs) have emerged as an attractive approach for electrochemical reduction of CO2 and/or H2O to high energy molecules, such as CO and/or H2, and demonstrated high faradic efficiency (good selectivity), high energy efficiency (low cell voltage), and high operating current density (high productivity). Conventionally SOECs based on YSZ as electrolyte material operate approximately 800oC in order to achieve a practical cell performance and to mitigate carbon deposition. However, high operating temperature leads to high cost and fast degradation of the SOEC system. Meanwhile, it becomes impossible to produce chemical fuels, such as methane, from the co-electrolysis of CO2 and H2O through in situ methanation.In this contribution, we studied electrochemical reduction of H2O and CO2 at reduced temperatures using anode supported SDC cells, with Ni-SDC anode and SSC cathode of effective area of 2 cm2, and the SDC electrolyte about 10-15 um in thickness. We found that the SDC cell performance (Cell#1) under electrochemical reduction of H2O reaches 0.5 A/cm2 at 1.15 V at 500oC with inlet gas composition of 50% H2O + 50% H2, at a flow rate of 100 ml/min/cm2. We repeated electrochemical reduction of H2O test using another SDC cell (Cell#2), and also conducted electrochemical reduction of CO2 at reduced temperatures. It was noticed that the cell performance under electrochemical reduction of H2O showed slightly better than, but quite similar to, that under CO2 reduction. The SDC cell (Cell#2) performance under electrochemical reduction of CO2 showed 0.5 A/cm2 at 1.17 V at 500oC with inlet gas composition of 80% CO2 + 20% H2, at a flow rate of 10 ml/min. This cell also showed no degradation during a short term (72hr) stability test under the CO2 reduction condition.Interestingly, under the tested conditions, SDC cells showed much better performance under the electrolysis cell (EC) mode than under the fuel cell (FC) mode. For example, at 500 oC, with 100mlpm H2-100mlpm H2O, the cell OCV is about 0.844V, the cell only reached 0.25A/cm2 at 0.4V under the FC mode, while under the EC mode it reached 1.26V at 1 A/cm2, which is approximately 4 times higher with the same polarization value of the cell voltage in comparison to the FC mode. The cause for the difference are investigated.