Ammonia as an efficient CO_X-free hydrogen carrier: Fundamentals and feasibility analyses for fuel cell applications

A CO_X-free 1 kW-class hydrogen power pack fueled by liquid ammonia is presented. For applications in a practical-scale hydrogen production system in conjunction with a polymer electrolyte membrane fuel cell, Ru catalysts supported on La-doped alumina (Ru/La(x)-Al₂O₃) were pelletized by varying the lanthanum doping content (x mol%) to control catalytic activities. An optimized Ru(1.06 wt%)/La(20)-Al₂O₃ pellet catalyst presents a >99.7% conversion efficiency at 500 °C under a gas hourly space velocity of 5000 mL g_cat ⁻¹ h⁻¹. Various materials were screened to remove residual ammonia from the product stream, and the X zeolite was chosen as a highly capable adsorbent. Based on the synthesized catalyst and screened adsorbent, a power pack consisting of a dehydrogenation reactor, an adsorbent tower, and a 1 kW-class polymer electrolyte membrane fuel cell was designed and manufactured. The as-integrated system can convert 9 L min⁻¹ of ammonia into 13.4 L min⁻¹ of hydrogen, powering a 1 kW-class fuel-cell continuously for >2 h without any performance degradation. To achieve autothermal and CO_X-free operations, heat required for ammonia dehydrogenation was provided by unutilized hydrogen from the fuel cell, drastically increasing the overall efficiency of the system to >49% while removing the external heat source, isobutane. Finally, a drone tethered to the system was operated, demonstrating the feasibility of an elongated flight time of >4 h, much longer than 14 min with Li-polymer battery loaded on the drone. The system is expected to meet the United States Department of Energy's 2020 gravimetric and volumetric hydrogen storage targets of 4.5 wt% and 30 gH₂ L⁻¹ at system weights of 43 kg and 50 kg, respectively.