Researchers at the University of Delaware have demonstrated a direct ammonia fuel cell (DAFC) prototype with a peak power density of 135 mW cm−2. The DAFC employs an ammonia-tolerant precious-metal-free cathode catalyst and a high-temperature-stable hydroxide exchange membrane. Their paper is publishedin the journal Joule.
The researchers started by analyzing the economics of hydrogen-, nitrogen-, and carbon-based fuels made by carbon-neutral pathways in a post-fossil world in which only water, air, and renewable electricity are available for fuel synthesis. They found that ammonia has the lowest source-to-tank energy cost by a significant margin.
Source-to-tank cost comparison of carbon-neutral transportation fuels. Zhao et al.
As a nitrogen-based liquid fuel, ammonia is cheaper to store and distribute than hydrogen and avoids the carbon dioxide emissions of other liquid fuels, which are expensive to capture.—Brian Setzler, one of the lead authors and a postdoctoral associate at UD
The challenges, however, are that ammonia does not work in a proton exchange membrane fuel cell; and that ammonia is more difficult to oxidize than hydrogen, which causes ammonia fuel cells to produce less power than hydrogen fuel cells.
The team solved the first problem by using hydroxide exchange membrane fuel cells that have been studied for over a decade in the lab of Yushan Yan, a Distinguished Engineering Professor at UD.
Assisted by a $2.5-million grant from the REFUEL program of the Advanced Research Projects Agency-Energy (ARPA-E) in the US Department of Energy (DOE), the UD team engineered a fuel cell membrane that can operate at higher temperatures to speed up ammonia oxidation. They also identified catalysts that were not poisoned by ammonia.
With these improvements, we have demonstrated a new direct ammonia fuel cell prototype with a peak power density of 135 milliwatts per square centimeter, which closes much of the performance gap compared to hydrogen.—Yun Zhao, lead author
Zhao, Yun et al. (2019) “An Efficient Direct Ammonia Fuel Cell for Affordable Carbon-Neutral Transportation” Joule, doi: 10.1016/j.joule.2019.07.005