(Invited) High Performance Alkaline Membrane Fuel Cells Powered By Direct Ammonia

Abstract

Alkaline exchange membrane fuel cells (AEMFCs) have gained significant attention in recent years owing to several advantages over the industry-standard proton exchange membrane fuel cells (PEMFCs) such as lower cost materials (e.g., non-PGM electro-catalysts, hardware, etc.) Less commonly cited are potential advantages of AEMFCs with respect to alternative fuels. Ammonia is a particularly attractive carbon-free fuel, being liquid at room temperature under ~7 atm, resulting in very high volumetric energy density and simplified distribution relative to hydrogen which is typically compressed to 200-700 atm. Furthermore, ammonia cannot be used as a fuel in PEMFCs since in the acidic environment ammonia exists primarily as ammonium which displaces the H+ ions that impart conductivity to the polyelectrolyte. One of the key challenges in direct ammonia fuel cells is to execute the sluggish ammonia oxidation reaction (AOR) effectively. Previous successful attempts at operating direct ammonia fuel cells, therefore, all utilized high-temperature approaches such as solid oxide fuel cells (SOFCs) operated at ~600-850 °C [1] or alternatively, alkaline fuel cells (AFCs) operated at ~140 °C with concentrated KOH solutions as the electrolyte [2]. Additionally, a few attempts have been made to operate at lower temperatures with ammonia as a fuel, yielding very low power densities (<20 mW/cm2) [3-4]. In this study, we report for the first time, a direct NH3/O2 AEMFCs operated completely free of liquid electrolyte – in both the membrane-electrode assembly and the fuel feed. Utilizing operating temperatures of 100-120 °C to minimize losses from the AOR, and optimizing operation conditions and MEA design, yielded a peak power density of 270 mW/cm2 with no added electrolyte, and 450 mW/cm2 with the addition of dilute (1M) KOH to the fuel feed. Optimization was also carried out to stabilize performance over several hours with minimal voltage loss, thereby demonstrating sustained ammonia oxidation in the absence of any hydroxide additives. This work thus demonstrates the applicability of a high-performance direct ammonia-powered AEMFCs. [1] Hagen A., "Use of alternative fuels in solid oxide fuel cells." Fuel 2.2H2 (2007): H2. [2] Simons E.L., Cairns E.J., Surd D.J., "The performance of direct ammonia fuel cells." Journal of The Electrochemical Society 116.5 (1969): 556-561. [3] Rong L., Tao S., "Direct ammonia alkaline anion-exchange membrane fuel cells." Electrochemical and Solid-State Letters 13.8 (2010): B83-B86. [4] Kuntke P., Sleutels T.H.J.A., Saakes M., Buisma C.J.N., "Direct ammonia fuel cell performance using PtIr/C as anode electrocatalysts." international journal of hydrogen energy 39.10 (2014): 5148-5152.