摘要/Abstract

氨(NH₃)是一种无碳氢载体, 比氢更易储运, 且体积能量密度更高, 因此直接氨燃料电池(DAFC)的研究具有重要的理论意义和实际价值. 本工作研究PtIr/C阳极催化剂在不同工作温度下电催化活性及其对DAFC性能的影响, 并探究了所用阴离子交换膜的渗氨量与DAFC性能的相关性. 结果表明, 从25~80 ℃, 基于PtIr/C阳极催化剂的DAFC在80 ℃下获得最优性能, 其开路电压(OCV) 0.50 V, 峰值功率密度3.2 mW•cm^-2, 可归功于Pt-Ir合金的协同作用和升温提高了催化活性. 不同温度下在DAFC阴极尾气中均检测到氨, 且氨含量随温度升高而上升, 致使阴极Pt/C催化剂毒化, 从而使DAFC的开路电压和功率密度下降.
关键词: 氨燃料, 催化剂, 渗透率, 燃料电池, 氢载体
Ammonia (NH₃), a carbon-free hydrogen energy carrier, is a key green energy for decarbonization, because of its easier storage and transport properties compared with hydrogen, as well as its higher volumetric energy density, which makes it suitable as alternative fuel and renewable energy carrier. To study the performance of PtIr/C anode electrocatalyst as direct ammonia (gas) fuel cells (DAFC), a laboratory test device was used to investigate the performance of DAFC employing anion exchange membrane (AEM) at different operating temperatures. Electrocatalytic activity of the PtIr/C electrocatalyst and performance of DAFC were examined by electrochemical workstation, respectively. Furthermore, the effect of NH₃-KOH concentration in anode fuel on ammonia oxidation reaction (AOR) activity was investigated. An on-line Fourier infrared (FTIR) gas analyzer was used to measure the ammonia concentration and spectra of cathode exhausted gas in DAFC. The results showed that the highest open circuit voltage (OCV) of 0.50 V and the peak power density of 3.2 mW•cm^-2 were obtained in DAFC at 80 ℃. The increased OCV might be attributed to the synergistic benefits (electronic effects) between Pt and Ir in the Pt-Ir transition alloys, which resulted in a shift to the lower over-potential of AOR. The rise in peak power density was mainly due to the increase of temperature, which improved the desorption of intermediate adsorption species (N_ads) from the electrocatalyst, as well as it enhanced the kinetics of AOR. The suitable NH₃-KOH concentration could reduce the onset potential of AOR and improve the catalytic activity. Both gas content and FTIR spectrum of the DAFC cathode exhausted gas confirmed the presence of ammonia concentration in exhausted gas at different temperatures, which demonstrated that ammonia fuel crossed the membrane electrode assembly (MEA) to the cathode site. It was further observed that ammonia cross-over increased with temperature, which led to a degradation of the cathode Pt/C electrocatalyst and resulted in the decrease of the OCV and power density of the DAFC. In this paper, the theoretical basis of DAFC agreed well with the experimental data. To further improve the performance of DAFC, it suggested that a high quality AEM allowing hydroxyl ion passing through but prohibiting ammonia cross-over, should be developed.
Key words: ammonia fuel, catalyst, permeability, fuel cell, hydrogen carrier


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