Understanding the Current-Voltage Behavior of High Temperature Solid Oxide Fuel Cell Stacks

Abstract

High temperature solid oxide fuel cell (SOFC) stacks are highly efficient and environmentally friendly electrochemical systems, which convert the chemical energy of fuel gases with oxygen from air directly into electrical energy. During operation of SOFC stacks under system operating conditions pronounced temperature and fuel gas composition gradients along the cell area and along the height of the stack occur. Therefore, in contrast to SOFC cells, the electrochemical behavior of SOFC stacks is much more complex and has not sufficiently been studied. Specially, a shortcoming exists in terms of understanding the homogeneity, performance loss mechanisms, and various resistances and overvoltages within the stack repeat components. Therefore, this paper focuses on the improvement of the understanding and of the interpretation of different current-voltage curves of solid oxide fuel cell stack repeat units. Three different cases are discussed: repeat units with high power performance, with high cell contact resistance and with high fuel utilization. The stacks were investigated by current-voltage curves, electrochemical impedance spectroscopy and gas analysis. In order to understand the electrochemical behavior of these three cases both experimental and modeling results are presented, compared and discussed.

Microkinetic Modeling of Nickel Oxidation in Solid Oxide Cells: Prediction of Safe Operating Conditions

“…Indeed, experimental studies on segmented SOFCs by Willich show a similar hysteresis upon high fuel utilization, which was interpreted as nickel oxidation . In a follow‐up work, Lang et al conclude that a kink in the current‐voltage behavior at high fuel utilization is due to nickel oxidation . Thus, there is qualitative experimental evidence for the behavior predicted with the present model.…”

Electrochemical Quality Assurance of Solid Oxide Electrolyser (SOEC) Stacks

“…4, whereas in SOFC a continuous increase in temperature is observed. 35 In Figure 3 all temperature curves show local minima at current densities around -550 mA/cm 2 . Beyond this current density the stack temperature increases again, which can be explained by the higher dissipative heat generation within the stacks.…”

“…Due to the high steam content in the fuel gas the OCV is much lower compared to stacks operated in SOFC mode with H 2 + N 2 mixture. 14,35,36 Moreover, the OCV in SOEC is much less sensitive toward changes in the fuel gas composition compared to SOFC. Therefore, the stacks have also been operated in SOFC mode, which leads to reproducible stack OCVs of 6.0 V or 1.2 V per RU.…”

“…9 and 10). 36 No difference between the influence of the air inlet temperature and the fuel gas inlet temperature on the electrolysis voltage can be identified. For a reliable and reproducible jV-characterization of SOEC stacks it is obligatory to accurately monitor the gas inlet temperatures.…”

“…This is in good agreement with results obtained in SOFC mode at OCV and at the same temperature. 35,36 Especially in the high frequency range signal disturbances are often observed and reported in literature. 34 These artefacts were eliminated in the measured spectra by minimization of the inductive interferences by twisting both the current probes of the stack and the voltage probes of the RUs to each other (Figure 2).…”