Redox-Cycling – A Tool for Artificial Electrochemical Aging of Solid Oxide Cells

Abstract

Solid Oxide Cells (SOC) have an important role and hold great promise with their diverse and versatile portfolio in the energy application sector, ensuring opportunities for integration of renewable energy sources into the overall energy system. However, the spectrum of applications still meets some general barriers, summarized as durability and costs. The initial performance degrades over their lifetime due to the effect of use (electrochemical ageing), of time (calendar ageing), of different permanent and/or accidental stress conditions - thermal, current load, mechanical, poisoning etc. Considering the expected useful commercial maintenance-free lifetime of up to 80 000 hours for stationary applications, the performance of durability tests looks quite unrealistic, which opens the niche for introduction of accelerated stress tests.In respect to the fuel electrode, the main degradation mechanism during operation in both fuel cell and electrolysis mode is the reduction of the triple phase boundary points and of the Ni/gas specific surface area caused by microstructural changes in the Ni network due to Ni migration and coarsening. The migration over long distances can bring to Ni depletion near the electrolyte interface. Similar processes of Ni coarsening and migration can occur faster during oxidation of Ni caused by incidental change in the operation conditions (leakage, fuel starvation). Thus, artificial redox cycling can be applied for accelerated degradation testing. However, the oxidation/reduction cycles should mimic the normal aging.The most common experimental approach for evaluation of the redox cycling effect on the degradation is the oxidation in a furnace at high temperature for at least one hour, followed by post mortem analysis. However, this approach introduces severe oxidation conditions, even full oxidation of the Ni network, which does not represent the most common electrochemical and calendar aging.The aim of this work is to develop a procedure for accelerated stress tests by artificial aging of the anode via redox cycling. It is performed before the operation of the cell thus eliminating the influence of the degradation mechanisms coming from other components. The proposed procedure ensures reproducible and fine-tuned level of oxidation based on in situ impedance monitoring of the Ni network reduction/oxidation. A two-step algorithm is developed and approbated. Step 1. Performance of redox cycling on anode sample with in situ impedance monitoring of the Ni network reduction/oxidation. The anode sample (pellet with thickness 250 µm and diameter 20 mm) is placed between two Pt meshes. In the initial state (YSZ/NiO) the impedance of the anode has capacitive behavior with resistance about 20 kΩ which after reduction transforms into inductive one with resistance about 40 mΩ corresponding to the electronic conductivity of the produced Ni network (Fig. 1). In the same way the oxidation can be registered. Its level may vary from small oxidation depth (for instance two times increase of the resistance, i.e. from 40 mΩ to about 70-80 mΩ as it is presented in Fig. 1) to big oxidation (above 10 kΩ).For the performance of Step 2 small level of oxidation was chosen, as shown in Fig. 1, which prevents from potential irreversible damages. Thus, the redox cycling of single anode serves for definition of the experimental redox cycling conditions. The deepness of degradation is regulated by the number of cycles. Step 2. Performance of redox cycling on full cell. In this configuration the oxidation of the anode Ni network cannot be monitored, but the conditions are already defined in Step 1 which guarantees reproducible identical redox cycles. The impedance measurements ensure monitoring of the total cell behavior during redox cycling. The procedure is applied for fuel cell mode, but it is also valid for electrolyzer mode. The results are shown in Fig. 2 where the impedance diagrams after the initial reduction and after the 8th redox cycle are compared. Although there is a decrease of the ohmic resistance which is usually observed during the first 1000 hours of standard operation mainly due to improved contacts, the polarization resistance after the 8th cycle is about 14% higher. This behavior is similar to that observed for standard testing for at least 600 hours. The 8 cycles are performed for 16 working hours which confirms the accelerated aging produced by the developed algorithm. Acknowledgements: The research leading to these results received funding from the Fuel Cells and Hydrogen 2 Joint Undertaking under grant agreement No 825027. The experiments were performed with equipment supported by the Bulgarian Ministry of Education and Science under the National Roadmap for Research Infrastructure 2017-2023 “Energy storage and hydrogen energetics (ESHER)”, approved by DCM No 354/29.08.2017. Figure 1