Modeling High-Power Density Ceria-Based Direct Ammonia Fueled SOFC Stacks for Mobile Applications

Abstract

The transport sector currently accounts for 23% of the global energy-related CO2 emissions, with an average annual growth of +1.8% since 2010, which is faster than for any other end-use sector [1]. Decarbonization of the mobility sector is particularly challenging due to the highly strenuous space, weight and efficiency demands for a power generation system applied for vehicle propulsion. Batteries have established themselves as a solution for the electric propulsion of passenger cars. However, their low energy densities and high recharging durations hinders their deployment in vehicles with large gross weight including trucks, ships, trains and airplanes [2, 3]. High-temperature solid oxide fuel cells (SOFCs) possess many advantageous characteristics compared to polymer electrolyte membrane (PEM) fuel cells that can be effectively exploited in a vehicle, such as high efficiency, fuel flexibility and impurity tolerance. SOFCs can be directly run with a great variety of fuels, such as NH3, which is currently regarded to be a very promising energy and hydrogen carrier, mostly due to its carbon neutrality, favorable volumetric energy density (12.7 MJ/L) and transportability [4].In this contribution, we assess the performance of intermediate temperature direct NH3-fueled SOFC stacks as a highly efficient power generation source for mobile applications by means of a detailed multi-physics modeling approach, comprising investigations on electrode, button cell, and stack scales. The modeled SOFC stack is intended to be integrated in a hybrid system concept by coupling with a gas turbine.Firstly, a 1D model of cell containing a gadolinium-doped ceria (GDC) electrolyte is developed based on an established multi-scale computational framework [2]. The electrochemical sub-model is subsequently parametrized and validated through reproducing measurements collected on a high-power density Ni-GDC/GDC/SSC-GDC cell. In order to physically account for the electronic leakage current due to mixed ionic-electronic conduction (MIEC) properties of the ceria-based electrolyte, the model contains the implementation of a distributed charge-transfer model that solves individual electronic and ionic conduction pathways across all constituents of the membrane-electrode assembly (MEA) [5]. The decomposition of NH3 on the surface of the Ni particles in the fuel electrode is modeled based on a thermodynamically consistent 12-step elementary kinetic mechanism [6]. The thermo-catalytic and electrochemical sub-frameworks are then integrated into a 3D stack model, which is built upon a lightweight design.The numerical simulations indicate that the performance of the SOFC stack based on the anode-supported GDC-electrolyte cell design is not only highly sensitive to the temperature, but also to the selection of the absolute pressure level. Results suggest the stack to reach a very promising performance in the intermediate temperature range with a predicted ASR of ~0.7 Ω cm2 at 550 °C, pure NH3 feed and 50% anode off-gas recirculation. By predicting species, temperature and current density distributions across the stack, the developed model proves itself to be highly instrumental for the identification of design points that provide a feasible trade-off between performance and safety metrics.