Impact of Thickness on Direct Ammonia Decomposition in Solid Oxide Fuel Cells, Numerical and Experimental Research

Abstract

The growing share of renewable energy sources (RES) in the energy mix and the desire to reduce greenhouse gas and pollutant emissions are driving widespread technological progress. One of the main areas is the development of hydrogen technologies, as manifested, for example, in the emerging hydrogen strategies of countries around the world. However, the widespread use of H2 is limited by the high energy input required for storage. This makes ammonia a promising carbon-free hydrogen carrier. Its application is being considered, e.g. in power systems for deep-sea transportation. Ammonia can be used like a fuel in energy systems, but often requires proper processing or decomposition into hydrogen and nitrogen in the presence of a catalyst. These activities increase financial or technological inputs which can complicate energy systems and reduce their efficiency or energy density. Power plants based on solid-oxide fuel cells (SOFCs) have the ability to process NH3 as fuel whether in external or internal cracking mode. However due to the nickel-based anode and operating temperatures above 600°C, SOFCs can directly use NH3 as a fuel - direct-ammonia SOFCs (DA-SOFCs). Lowering the operating temperature of the cells and reducing their thickness brings with it a wider opportunity for SOFC-based systems e.g. decrease energy density, and reduction of used materials. An additional advantage of direct ammonia conversion in SOFC cells is the endothermic nature of the reaction, allowing for a reduction in the amount of cooling air supplied to the system. In this study, the performance of cells with different anode thickness was compared under similar operating conditions for direct ammonia-fueled solid oxide fuel cells (SOFCs). The experimental results were reproduced in the open source computational fluid dynamics (CFD) software, Open Fuel Cell, in which the source term responsible for internal cracking was introduced. Then calculations were made to determine the distribution of ammonia and temperature in the functional part. It was determined that in the case of counter-current flow of fuel and oxidizer, thinner cells show a smaller temperature gradient. Lowering the operating temperature and cell thickness does not lead to incomplete conversion of ammonia in the cell. Post-mortem analysis of the cells used in the experiment showed no negative effect of ammonia on the functional layers of the cells.