Optimized coupling of ammonia decomposition and electrochemical oxidation in a tubular direct ammonia solid oxide fuel cell for high-efficiency power generation

Abstract

With high energy density both by weight and volume, ammonia (NH3) is a promising hydrogen carrier. Furthemore, NH3 has a mature industrial background, and in liquid form storage and transportation is not a problem. Adding the merit of zero CO2 emission, NH3-to-power by direct ammonia solid oxide fuel cells (DA-SOFCs) is an acceptable strategy to facilitate hydrogen usage. Nonetheless, to achieve efficacy, a high compatibility between operating temperature and catalytic materials for NH3 decomposition is needed. In this work, we developed a tubular DA-SOFC with an output power capability of > 3 W. By combining experimental measurements and multi-physics simulation, we comprehensively studies the related intrinsic processes. Based on experimental data, we developed a two-dimensional multi-scale electro-thermo model of tubular DA-SOFC. Separately we evaluated the effects of inlet fuel gas composition, inlet flow velocity, operating temperature, and operating voltage on the rate of NH3 catalytic decomposition and H2 electrochemical oxidation, as well as on NH3 conversion, H atom utilization, and electrical efficiency of the tubular DA-SOFC. The results suggest that high H atom utilization could be realized by matching the rate of NH3 decomposition with that of H2 electrochemical oxidation. It was observed that with the decrease of temperature, the rate of H2 oxidation decreases more rapidly than that of NH3 decomposition, suggesting that the flow velocity of NH3 should be appropriately lowered to optimize H atom utilization. Finally, we established a correlation between H atom utilization, operating voltage, and electrical efficiency for synergistic optimization of operating conditions. At 0.7 V and 800 ℃, the tubular DA-SOFC fueled with NH3 of 27 mL·min−1 is capable of offering 3.2 W, displaying an efficiency of 60%. Compared to that of a tubular H2-SOFC (only 51% efficiency), the efficiency is significantly higher on the basis of equal voltage and fuel utilization ratio. The outcome of the present study demonstrates the potential of tubular DA-SOFC as a device for high-efficiency power generation.