Can Sulfide Surfaces Catalyze Nitrogen Reduction to Ammonia Electrochemically for Energy Storage/Carrier Purposes?
Authors: Abghoui Younes , Skulason Egill 
Published: February 2020

Ammonia (NH3) is a significant energy storage intermediate, a renewable energy transportation medium, and a vital source of hydrogen1-3. It is also widely used in production of fertilizer and accordingly crucial for agriculture. Nitrogen fixation and production of ammonia is thus essential for feeding a growing world population as well as meeting its increasing energy demand. The dominant route to produce ammonia is the Haber-Botch process where H2 and N2 are highly heated and pressurized on a surface of transition metal(s) to form NH3. The Haber-Bosh’s sophisticated industrial setup results in high-energy consumption conditions where 2% of the world´s energy consumption is used for this process and thus hinders decentralization of the process. Another drawback of this process is high CO2 emission, which is around 1% of the global CO2 emission, originated from steam reforming of natural gases necessary to supply H2 for the process. Nonetheless, this industrial process is in stark contrast to the function of the enzyme nitrogenase in bacteria where NH3 is produced electrochemically from solvated protons, electrons and atmospheric nitrogen at ambient conditions. The active site of the enzyme is a metal sulfide cluster (MoFe7S9N). Nitrogen reduction reaction (NRR) to ammonia in an electrochemical manner and at ambient conditions is known as a promising alternative to the energy-intensive and CO2 emitting Haber-Bosch process. To mimic the enzymatic process and by taking inspiration from the nature, we have studied the possibility of catalyzing NRR on the surface of a range of transition metal sulfides (TMSs) that due to presence of the transition metal and sulfur atoms offer some similarity to the structure of the active site of the nitrogenase. However, there are other factors such as spin states and coordination geometries of the enzyme cofactor that may be important and these heterogeneous surfaces will not capture4,5.
With the use of density functional theory calculations, we explored the thermochemistry of the cathode reaction so as to construct the free energy profile and to predict the required onset potential for activation of nitrogen to ammonia. Since adsorption of N2 on the surface of the catalysts is usually an endergonic process, we have calculated and compared competition between adsorption of H and NNH on the surface of a range of TMSs and some were found favorable to bind NNH more strongly than H. For these surfaces, we calculated the energetics of the intermediates along the reaction path and construct free energy diagrams via the associative mechanism to estimate the onset potential for ammonia formation. We also investigated competition between adsorption of N and H when considering the dissociative mechanisms before exploring the free energy diagrams and possibility of ammonia formation dissociatively. We also found that the adsorption energies of the intermediates scale well with NNH and N free energies as descriptors when looking at the associative and dissociative mechanisms, respectively. With the use of these scaling relations we are able to construct the volcano plots where activity of these material is plotted as a function of adsorption energy of the descriptor and guiding experimentalists towards promising candidates worth testing for this reaction and possibility of their real-life applications. The outcome of our analyses show that it should be possible to reduce N2 to NH3 on these sulfide surfaces at overpotentials as small as 0.3-1.1 V vs. RHE.4 Although there are few TMSs that are predicted to be selective for NRR in aqueous environments, some of these candidates are anticipated to also show activity towards Hydrogen evolution reaction (HER) in aqueous media. Utilization of those surfaces in non-aqueous environments could help suppressing HER and increasing the yield of NH3, though.