In this talk, we present our experimental efforts to increase selectivity in electrochemical ammonia synthesis with electrolyte engineering. Ammonia synthesis is a vital chemical reaction for the modern world. The Haber-Bosh process is routinely credited with supporting somewhere around half of our current population, at a cost of about 1% of the worlds energy consumption. It also requires hydrogen, typically provided through hydrocarbon reforming and is concomitant with centralized infrastructure and distribution. Clearly, this presents an opportunity to develop a process that is not reliant on large capital infrastructure or hydrocarbon derived hydrogen.

One sought after option is a direct electrochemical process. This would enable ammonia to be produced directly using water and electricity from renewable resources. Unfortunately, faradaic efficiencies for ammonia synthesis in aqueous electrolytes are typically observed to be prohibitively low due to the competing hydrogen evolution reaction. Density functional calculations predict that ammonia synthesis will only proceed at potentials more negative than 1 volt, a potential at which most metal surfaces evolve hydrogen rapidly. While, in principle, it may be possible to find a material that activates nitrogen and not protons, we instead focus on changing the electrolyte to reduce HER. According to our simple kinetic model, reducing the availability of protons at the surface will allow nitrogen binding, the presumed rate limiting step, to compete with proton reduction.[1] While there are a number of possible electrolyte systems that may achieve this, we use aprotic, organic electrolytes with alcohols added as proton donors. By using alcohols as a proton donor, we hope to access an additional degree of freedom when engineering the reactivity of the interface. In preliminary measurements, we see faradaic efficiencies approaching 20%. We use both argon and nitrogen-15 labelled control experiments. We see no ammonia produced under argon purge and only labelled ammonia produced when using nitrogen-15 labelled N₂.

[1] Singh, A. R., Rohr, B. A., Schwalbe, J. A., Cargnello, M., Chan, K., Jaramillo, T. F., Nørskov, J. K. (2016). ACS Catalysis, 706–709. doi: 10.1021/acscatal.6b03035