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

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.