Reversible Solid Oxide Cells: Durability of Fuel Electrodes Against Voltage Cycling

Abstract

Introduction Reversible Solid Oxide Cells (rSOCs, or solid oxide reversible cells) are an attractive electrochemical energy conversion technology that can act as both a Solid Oxide Fuel Cell (SOFC) for power generation, and a Solid Oxide Electrolysis Cell (SOEC) for steam electrolysis (1). In the near future, renewable energy sources, such as wind and solar, are set to dominate energy production, however, these suffer from intermittency issues based on weather conditions. Therefore, it is desirable to adjust the supply of fluctuating electricity production from renewable energy sources to match demand using rSOCs, for hydrogen storage in the SOEC mode and for power generation using stored hydrogen in the SOFC mode. This concept is schematically described in Figure 1. The purpose of this study is to examine the degradation of rSOCs using conventional fuel electrode materials, for comparison with alternative fuel electrode materials, which are under development. Experimental Reversible voltage cycle tests simulating rSOC operation were conducted using conventional cell materials: Cermet of Ni- with Sc2O3-doped ZrO2 (ScSZ) for the fuel electrode; dense 200 m thick ScSZ membrane for the electrolyte; and La0.6Sr¬0.4Co0.2Fe0.8O0.3 oxide (LSCF) air electrode with Gd-doped CeO2 (Gd0.1Ce0.9O2) acting as a buffer layer between the ScSZ electrolyte. A Pt-based reference electrode was deposited beside the air electrode, such that the fuel electrode potential was measured as the voltage between the fuel electrode and the reference electrode. The durability of the fuel electrodes were evaluated at an operating temperature of 800C. Air was supplied to the air electrode, while 50%-humidified hydrogen was supplied to the fuel electrode. Reversible operation was investigated by repeatedly changing between the SOFC and SOEC modes by switching the direction of current for up to 1,000 cycles, as shown in Figure 2. The current density in the SOFC and SOEC modes are expressed as positive and negative values, respectively. The current sweep rate was 1.5 mA cm-2 s-1. Current density was varied within ± 0.2 A cm-2, and the fuel electrode potential was measured at the peak current of ± 0.2 A cm-2 for each cycle. Here, the performance degradation of the Ni-cermet fuel electrode was examined by switching between the SOEC mode and the SOFC mode, simulating possible rSOC operation conditions. Results and discussion Figure 3 shows the fuel electrode potential measured at ± 0.2 A cm-2, every 50 cycles. Degradation within 1,000 cycles was calculated to be 30.6%, as the average value of voltage degradation between the SOFC and SOEC modes. Such degradation could be associated with microstructural changes, e.g. due to a redox reaction between Ni and NiO (2), during rSOC operation. Such degradation could be reduced by using redox-tolerant fuel electrode materials, in which redox-stable materials are used as the stable backbone in the porous electrode structure. Current-voltage characteristics, rSOC cycle durability, and long-term stability of various fuel electrodes will be presented, including alternative co-impregnated electrodes (3,4). Such results will be compared with those using the conventional Ni-cermet fuel electrode. Acknowledgements A part of this study was supported by “Research and Development Program for Promoting Innovative Clean Energy Technologies Through International Collaboration” of the New Energy and Industrial Technology Development Organization (NEDO). Collaborative support by Prof. H. L. Tuller, Prof. B. Yildiz, and Prof. J. L. M. Rupp at Massachusetts Institute of Technology (MIT) is gratefully acknowledged.