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

Abstract

Oxidation of the nickel electrode is a severe aging mechanism of solid oxide fuel cells (SOFC) and solid oxide electrolyzer cells (SOEC). This work presents a modeling study of safe operating conditions with respect to nickel oxide formation. Microkinetic reaction mechanisms for thermochemical and electrochemical nickel oxidation are integrated into a 2D multiphase model of an anode-supported solid oxide cell. Local oxidation propensity can be separated into four regimes. Simulations show that the thermochemical pathway generally dominates the electrochemical pathway. As a consequence, as long as fuel utilization is low, cell operation considerably below electrochemical oxidation limit of 0.704 V is possible without the risk of reoxidation.

Syngas Production from CO2 and H2O via Solid-Oxide Electrolyzer Cells: Fundamentals, Materials, Degradation, Operating Conditions, and Applications

“…294 When Ni is oxidized to NiO, the catalyst's redox reactions increase the electrode's volume by 69.9%, which generates mechanical stress that extends the electrode and both cracks and delaminates the electrolyte. 295 Hauch et al 296 observed Ni repositioning at high water-vapor pressures and current densities (e.g., at p Hd 2 O = 9.12 × 10 4 Pa, 2 A cm −2 , and 1223 K). The repositioned Ni densely covered the electrolyte, which may have prevented oxygen transfer from the fuel electrode to the electrolyte and permanently reduced the TPB length.…”

“…Yang and Irvine 297 showed that Ni−YSZ cathodes were rapidly oxidized in 3% H 2 /Ar mixtures. Neidhardt et al 295 modeled safe operating conditions to prevent the formation of nickel oxide. Simulations showed that because the thermochemical pathway usually dominated the electrochemical pathway, the cell could operate below the electrochemical oxidation limit of 0.704 V without any risk of reoxidation if the fuel utilization was low.…”

“…Nickel exhaustion is often accompanied by grain/particle coarsening . When Ni is oxidized to NiO, the catalyst’s redox reactions increase the electrode’s volume by 69.9%, which generates mechanical stress that extends the electrode and both cracks and delaminates the electrolyte . Hauch et al observed Ni repositioning at high water-vapor pressures and current densities (e.g., at p H 2 O = 9.12 × 10 4 Pa, 2 A cm –2 , and 1223 K).…”

Holistic Approach to Design, Test, and Optimize Stand-Alone SOFC-Reformer Systems

“…The likelihood of degradation due to carbon deposition and nickel oxidation can be estimated by the C-H-O (Carbon-Hydrogen-Oxygen) ratio of the fuel gas composition as shown in [32][33][34]. However, locally different fuel gas compositions and operating currents have a significant impact on the degradation process and change of the nickel grains as shown in [24,[35][36][37]. Nickel agglomeration is mainly caused by grain growth due to high temperatures as explained in [38].…”

Steady-State and Transient Operation of Solid Oxide Fuel Cell Systems with Anode Off-Gas Recirculation within a Highly Constrained Operating Range

“…Fuel utilization FU stack : Low hydrogen-to-steam ratios, which typically occur at anode outlets at high fuel utilizations, favor the thermochemical oxidation of the Ni catalyst, which represents a significant degradation mechanism [36]. Fuel utilization is, thus, constrained to a conservative steady-state value of FU stack,max = 75 %, allowing for small overshoots to up to 80 % during transient operation.…”