Abstract
In addition to being an essential chemical for fertilizers, ammonia can also be used as a hydrogen carrier or directly as a fuel. Compared to hydrogen, the main advantages are that ammonia can be liquefied in mild conditions and the technology for ammonia shipping and pipelines already exists. Compared to carbon-based synthetic fuels, no carbon dioxide is produced by the oxidation, only water and nitrogen. The theoretical voltage of a direct ammonia fuel cell (DAFC) is 1.17 V, nearly equal to the 1.23 V of hydrogen fuel cells (HFC) but a central drawback is the slow kinetics of the ammonia oxidation reaction (AOR). The AOR is even slower than the oxygen reduction reaction (ORR) at the cathode, which in turn is the main voltage loss component in HFCs. Consequently, due to the AOR kinetics, DAFCs typically require significantly higher platinum group metal (PGM) catalyst loadings than HFCs for the same power output.
To improve the performance of our aqueous-feed DAFC, especially using non-PGM catalysts, we carried out comparisons of catalysts and anion exchange membranes (AEM), and supplemented the comparison of power, voltage, and current parameters with electrochemical impedance spectroscopy (EIS). Additionally, we also compared measurements in two different test cells to try to identify the cause of measurement repeatability issues that we had when using one of the cells.
Initially, we assumed that the reproducibility of catalyst deposition (or lack of it) and AEM or catalyst degradation in the ammonia-containing electrolyte had caused our repeatability problems. However, following a preliminary analysis of measurement statistics, we consider problems with the cell itself a more likely cause. The maximum power of the cell depended almost inversely on the series resistance (RS, i.e., the high-frequency resistance/real-axis intercept of the EIS spectrum), and the current at given voltage varied more than the differences in the resistance alone could explain when most measurement parameters were not varied (e.g., ammonia and hydroxide concentrations, liquid and gas flow rates). The general inverse dependence on RS remained also when measuring the same electrodes in different cells, and the performance did not necessarily degrade over time so, clearly, degradation and differences between electrodes could not explain everything. When testing different screw torques and gaskets in our AEM electrolyser, we had noticed that the voltage differences were often high compared to the RS differences, and it is known that clamping torque can affect the operation of fuel cells and electrolysers significantly. Nevertheless, more detailed analysis is ongoing to determine possible other contributing factors, to optimize the use of the cell(s) in measurements, and to guide future work.
This research was done under the TELEGRAM project. This project has received funding from the European Union's Horizon 2020 Research and Innovation programme under grant agreement No 101006941. The project started on the 1st of November 2020 with a duration of 42 months. The authors acknowledge support from the Federal Ministry of Education and Research in the framework of the project Catlab (03EW0015A).
Figure 1