Electrochemical conversion of nitrogen to ammonia-experimental and theoretical studies

This University of Oxford led project will demonstrate the feasibility of producing ammonia, an energy vector with multiple applications, in a carbon free synthesis powered by renewable energy. This part of the study aims to simulate the electrochemical syntheses of ammonia from nitrogen and hydrogen/water.

Bulk models to represent the experimental systems in a realistic fashion At the first stage, we will focus on electrode materials with varying amounts of Samarium, namely Sm2-xSrxNiO4 (with x=0.4, 0.5, 0.8 and 1.2) and SmFe0.7Cu0.3- xNixO3 (with x=0, 0.1, 0.2 and 0.3), that have been reported in the literature. These systems have been characterized experimentally and shown a strong difference in catalytic activity towards electrochemical synthesis of ammonia. Aim of the computational simulations is to understand the difference in activity in order to help decision making on potential improved experimental systems and/or predict trends in catalytic activity.

To succeed in this ambitions, a broad skill set of computational modelling techniques is required. As this might become a re-appearing pattern for newly identified materials or potential catalysts, we'll establish a protocol that can be followed for any new material of interest for the electrodes This will form the basis for the work undertaken in the first part of the project.
- Generation of starting structure of bulk material based on available experimental evidence
- Monte Carlo (MC) simulations (using the in-house code DL_MONTE ) to the thermodynamically stable phase
- Validation of the model by theoretical reproduction of bulk properties, e.g. the ionic conductivity with Adaptive Kinetic Monte Carlo (AKMC) techniques (using the in-house code DL_AKMC )

Establishing a bulk structure is key for validation of most experimental values. The reactivity of the electrode, however, depends on the catalytically active surface. In the secnd part of the project, we'll therefore establishe a reliable and reproducible protocol to generate different crystal surfaces and determine the likelihood of their appearance in the experimental systems (using the in-house code DL_POLY , and/or CRYSTAL)To keep the computational efficiency as low as possible, we'll establish realistic, but still efficient representations of the systems of interest by using a cutting cluster approach.

In the third part of the study, we'll use these representations to study the reactivity at the interface, taking into account different competing reactions:We'll simulate the catalytic conversion of nitrogen to ammonia, which is the reaction we are actually interested in. In addition, we'll investigate potential competing reaction like the generation of hydrogen at the cathode (2H+ + 2e- -> H2), reducing the Faradaic efficiency of the MEAs. Namely, we will evaluate the influence of the electrolyte, and/or electromagnetic fields on the reaction energies, but also on the geometries (an effect that is normally neglected).

At the end of the project, we aim to be able to use computational simulations to understand and to be anle to make predictions on the difference in activity in a diverse group of materials in order to help decision making on potential improved experimental systems and/or predict trends in catalytic activity.

Ammonia is a commodity and the sole differentiators from the customers/market perspective are price and environmental impact. The business opportunity is to deliver a carbon-free production process for ammonia which can then be utilized a flexible asset for 4 sub-areas:
(i) Currently ammonia is used as a feedstock for the chemical industry. About 80% is used in the fertilizer industry with the remaining 20% being used in varied range of chemical pro-cesses and the plastics industry.
(ii) As an energy storage medium for heat and electricity, which can be turned into heat/electricity (via gas turbines or fuel cells) on demand. (short-medium opportunity)
(iii) A fuel for the transportation sector.
(iv) As a highly efficient hydrogen carrier, thereby supporting the implementation of the Hydrogen Economy.
There is currently no cost-competitive renewable based carbon-free synthetic route in the marketplace. If this technical feasibility study is successful, the impact for this new electrochemical synthesis method is
(a) Carbon-free,
(b) capable of integration with an intermittent energy source and
(c) achieves high efficiencies.
Thus, the key technology in such a synthesis and output from this work, will be the Membrane Electrode Assembly (MEA), which could be integrated with a source of
intermittent electricity generation.