Bimetallic Fe-Mo Films Deposited via Dynamic Hydrogen Bubble Templating (DHBT) for Electrocatalytic Nitrogen Reduction to Ammonia
Authors: Lobo Carlos M. S. , Kammal Hannah , Kaulbersch Julian Merlin , Marschall Roland , Roth Christina 
Published: December 2024

Ammonia is a very valuable chemical, not only due to its role as a fertilizer precursor in the agricultural sector, but also as an energy storage medium which could potentially substitute oil as the fuel of a society powered by renewable energy[1]. However, it is currently synthesized at the industrial scale by the Haber-Bosch process, which is responsible for around 3% of the global CO2 emissions[2-3]. This fact, coupled with the current global climate crisis caused by the excess emissions of greenhouse gases (GHG) demands that alternative industrial processes be found to produce ammonia that are, at a minimum, GHG neutral. With this perspective in mind, the electrochemical reduction reaction of N2 (NRR) to ammonia becomes an attractive way to make use of the renewable energy to produce ammonia without emitting CO2 in the process.

Currently, the electrochemical production of ammonia at an industrial level is unfeasible due to the very low production rates which are observed at the laboratory scale. These lie in the nanomole per second per square centimeter range, and arise from the high stability of the nitrogen molecule due to its triple bond that in turn leads to more competition occurring from the parasitic hydrogen evolution reaction. In our research we attempt to increase the ammonia production rate by using nanostructured and hierarchically porous Gas Diffusion Electrodes (GDEs) produced by the Dynamic Hydrogen Bubble Templating method. The use of GDEs should allow industrially relevant current densities to be reached and the use of Fe and Mo as the catalysts of choice is driven by the fact that these are well-known catalysts for the NRR in the Haber-Bosch process. Additionally, they are also incorporated in the active center of nitrogenases, enzymes that catalyse the conversion of nitrogen at ambient conditions.

To take full advantage of the porous nature of the DHBT-deposited metallic foams, it is first necessary to understand how the pore size changes over deposition time. Thus, the first step was to analyse the influence of deposition time and current density on the DHBT deposition of Fe foams, which showed that the average pore diameter on the surface of the Fe foams can reach the order of 20 micrometers within 5 minutes of deposition time.

Since pure Fe cannot be used as a NRR catalyst due to its dissolution tendency under typical NRR reaction conditions, the next step was to include Mo in the mix. It was found that the initial electrolyte solution needed only the addition of the Mo salt (its concentration being one tenth of the Fe concentration) and that Fe and Mo co-deposit as expected at a current density of 1.25 A·cm-2. In contrast to the pure Fe foams, the average pore diameter in the presence of Mo decreases to around half of the original value. While the newly deposited foam is more resistant to oxidation in air than the pure Fe foams (pure Fe foams suffer from extensive oxidation after a couple of hours exposed to air), it is still not stable enough in the NRR electrolyte, as it dissolves into the solution already at open circuit potential (OCV) within 30 minutes. To address this stability issue, annealing procedures were implemented in the synthesis routine, with the first tests performed by holding the samples at 400 °C for 1 or 3 hours with a heating ramp of 10 °C·min-1. While this post-treatment reduces the pore sizes even further, initial electrochemical results already reveal that foams annealed for 3h become much more stable in the NRR electrolyte compared to the untreated samples, as evidenced by the more positive OCV that can be observed after 2 hours of immersion in the electrolyte, and the fact that the film did not appear to be damaged. These results are promising to obtain a stable GDE catalyst that can withstand the NRR conditions. Investigations are still pending regarding the effect of annealing at higher temperatures and longer times to allow a fully functional GDE for NRR to be fabricated.

References:

[1] MacFarlane, D. R. et al., A Roadmap to the Ammonia Economy, Joule, 2020, 4, 1186-1205.

[2] Wang, S. et al., Nitrogen Fixation Reaction Derived from Nanostructured Catalytic Materials, Adv. Funct. Mater., 2018, 28(50), 1803309.

[3] Green Ammonia Synthesis. Nat. Synth., 2023, 2, 581-582.