Improving Performance and Durability of Low Temperature Direct Ammonia Fuel Cells: Effect of Backpressure and Oxygen Reduction Catalysts

Abstract

Low temperature direct ammonia fuel cells (DAFCs) are attractive for transportation applications. The primary obstacle to their commercial use is their low performance and poor durability. In the present work, we focus on improving DAFCs performance and durability by examining the effect of operating backpressure and oxygen reduction reaction (ORR) catalysts such as Acta 4020, Pd/C and Pt/C. DAFCs with Acta 4020 cathode can reach a peak power density of 390 mW cm−2 which is among the best reported performance, but they can be operated for a period of 11 h at 300 mA cm−2. DAFCs with Pd/C cathode offer a moderate performance with a peak power density of 304 mW cm−2, but has a much improved durability - a continuous operation for up to 36 h with a slow decay rate of ∼1 mV h−1 at 300 mA cm−2. In addition, the degradation pathways for DAFCs with Pd/C cathode are probed by characterizing the initial and final electrodes by XPS, suggesting that cathodic Pd dissolution occurs during the durability test.

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“…Additionally, a continuous DAFC operation for 63 hours is observed at 300 mA cm -2 , operating at 100 °C. Although there is still degradation and voltage fluctuation issues, this DAFC durability has been better than the DAFC that has Acta 4020 cathode, which shows a dramatic degradation caused by its specific micropores structure (most active sites are located inside of micropores and thus are easier to be blocked by water) in previous report 27,45 . Based on these results, the Co2MnO4 catalyst is selected for further investigation.…”

“…To examine their performance as catalysts in DAFCs, Co2FeO4, Co2MnO4, and Co3O4 catalyst were sprayed on the cathode side of HEM to create catalyst coated membrane (CCM) with hydroxide exchange ionomer (HEI) as described in previous reports 6,27 . DAFCs with the abovementioned cathodes and PtIr/C anodes, were assembled and tested by feeding the anode with a 7 M NH3 in 1.25 M KOH solution.…”

“…The performance of a cathode in a polymer electrolyte fuel cell is determined jointly by the kinetics of the ORR and the oxygen transport resistance of the cathode catalyst layer (CL), where the latter could have a significant effect on the performance at higher current densities 27 . Previous experimental results suggested that uptake of liquid water by a more hydrophilic cathode CL, can result in oxygen transport limitations 46,47 .…”

“…The latter directive stems from the recognition that some crossover of aqueous ammonia into the cathode is existing in DAFCs used to date. Coupling with ammonia crossover, our previous work further shows that the cathode of DAFCs is subjected to severe flooding due to high water concentration gradient from anode to cathode and high water permeability of HEMs 27 . As such, an ideal cathode is a cathode with an ORR catalyst that is highly tolerant to ammonia and highly resistant to flooding, allowing for the best combination of cell performance and durability.…”

“…The optimum temperature for the operation of the DAFCs is ~100 °C --a compromise between (i) the need of higher cell temperature for enhancing the rate of the sluggish ammonia oxidation reaction (AOR) and, (ii) the upper limit of cell temperature defined by the demands of thermal stability for all cell components. As shown by us before 6,12,13,16,27,28 , the operation of the DAFC near 100 °C addresses to significant degree the power density demand, while leaving the door open for the use of ionic membranes --the electrolyte of choice for low temperature fuel cells over the last 60 years. Indeed, the significant advances reported in recent years in performance and longevity of the DAFCs, were enabled by the progress in the development of hydroxide exchange membranes (HEMs) of high conductivity, and improved chemical and mechanical stability 29 .…”