Renewable electrocatalytic synthesis of ammonia by nitrogen reduction reaction (NRR) at room temperature and ambient pressure has attracted numerous attentions in order to find a promising alternative to the centralized, energy-intensive, and CO₂ emitting Haber-Bosch process. Despite all the efforts that have been made in this regard, NRR has yet been plagued with low faradaic efficiencies (FE) and reaction rates under ambient conditions with hydrogen evolution reaction (HER) considered as an important competitive side reaction. Here with the use of density functional theory, we have investigated a wide range of transition metal nitride (TMN) surfaces for possibility of catalyzing the NRR at ambient conditions^1–6. 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. Some of these materials were found not suitable for catalyzing ammonia formation mainly due to either poisoning or decomposition in electrochemical environment. However, some of earlier TMNs are predicted to catalyze ammonia formation at small onset potentials (-0.5 to -0.75 V) with considerable selectivity, while some other favoring the HER. When comparing the activity and the onset potentials predicted on these TMNs, it is seen that the Mars van Krevelen mechanism should be more favourable mechanism for NRR compared to the conventional associative and dissociative mechanisms. We recently tested few of the DFT predicted TMNs in experiments, and preliminary results indicate proof of concept and interesting performance of these materials under operating conditions.

Reference:

(1) Abghoui, Y.; Garden, A. L.; Hlynsson, V. F.; Björgvinsdóttir, S.; Ólafsdóttir, H.; Skúlason, E. Enabling Electrochemical Reduction of Nitrogen to Ammonia at Ambient Conditions through Rational Catalyst Design. Phys. Chem. Chem. Phys. 2015, 17 (7), 4909–4918.

(2) Abghoui, Y.; Garden, A. L.; Howalt, J. G.; Vegge, T.; Skúlason, E. Electroreduction of N2 to Ammonia at Ambient Conditions on Mononitrides of Zr, Nb, Cr, and V: A DFT Guide for Experiments. ACS Catal. 2016, 6 (2), 635–646.

(3) Abghoui, Y.; Skúlason, E. Electrochemical Synthesis of Ammonia via Mars-van Krevelen Mechanism on the (111) Facets of Group III–VII Transition Metal Mononitrides. Catal. Today 2017, 286, 78-84.

(4) Abghoui, Y.; Skúlason, E. Onset Potentials for Different Reaction Mechanisms of Nitrogen Activation to Ammonia on Transition Metal Nitride Electro-Catalysts. Catal. Today 2017, 286, 69–77.

(5) Abghoui, Y.; Skulason, E. Computational Predictions of Catalytic Activity of Zincblende (110) Surfaces of Metal Nitrides for Electrochemical Ammonia Synthesis. J. Phys. Chem. C 2017, 121 (11), 6141–6151.

(6) Abghoui, Y.; Skúlason, E. Hydrogen Evolution Reaction Catalyzed by Transition-Metal Nitrides. J. Phys. Chem. C 2017, 121 (43), 24036–24045.