Carbon-free sustainable energy technology : direct ammonia fuel cells

Abstract

Ammonia, an indirect hydrogen storage media containing a high content of hydrogen (17.8 wt. %), could be an ideal carbon-free fuel for fuel cells. The DAFCs employed alkaline anion exchange membranes (AEMs), referring to the low temperature AEM-DAFCs, not only have merits of the high energy efficiency, but are compatible with non-precious catalysts without ammonia decomposition process, which means a lower cost compared to proton exchange membrane fuel cells. Unlike high-performance of direct ammonia solid oxide fuel cells (high temperature SO-DAFCs), the low catalytic activity of the electro-catalysts and the difficulty of ammonia oxidation at low temperatures lead to far worse performance of low temperature AEM-DAFCs. Therefore, this article is trying to offer some incentives and indicate a direction for the future development of DAFCs. First, this review emphasizes previous development tracks and current progress on low temperature AEM-DAFCs and high temperature SO-DAFCs. For the low temperature AEM-DAFCs, the current progress of platinum-based and non-platinum-based electro-catalysts, high conductivity membranes, the low catalytic activity and membrane degradation issues will be summarized. The performance comparison of high temperature SO-DAFCs with various electrode and electrolyte materials and long-term stability issues will be discussed in the later section.