On the Optimization of Nitrogen-Reduction Electrocatalysts: Breaking Scaling Relation or Catalytic Resonance Theory?

Ammonia is one of the most commonly used chemicals to produce fertilizers used in agriculture. With the rapid growth in the global population and subsequent growth in the demand for food, the global demand for ammonia is projected to skyrocket in the future. Presently, ammonia is mainly produced through the Haber-Bosch process, a thermal catalytic process which requires extreme reaction conditions due to pressures of several hundred bars. Unfortunately, this process accounts for more than one percent of the world’s energy consumption and CO₂ admission, which is considered unsuitable and unsustainable in the contemporary industrial setting. Thus, developing alternative methods for efficient and green production of ammonia is inevitable.

The electrochemical nitrogen reduction reaction (NRR) has recently drawn research attention as a promising alternative route for environmentally friendly large-scale ammonia production. Interestingly, this reaction can be sufficiently driven by energy/electricity obtained from renewable sources like wind and solar power energy. Nevertheless, the application of NRR is underdeveloped due to several challenges. They include low nitrogen solubility, the competition between NRR and the hydrogen evolution reaction under cathodic reaction conditions, and the sluggish NRR reaction kinetics attributed to the transfer of six proton-electron pairs. These limitations result in large overpotentials and low selectivity for the desired product ammonia.

NRR electrocatalyst optimization is a promising approach for overcoming the above issues. Specifically, the breaking scaling relation concept has been employed to improve the turnover of many electrocatalytic processes, including the oxygen evolution and reduction reactions in electrolyzers and fuel cells, respectively. However, this concept has been implemented with limited progress and has not been adequately discussed for developing NRR catalysts. Recently, the catalytic resonance theory concept has been proposed as an alternative for electrocatalyst optimization and to achieve reaction rates beyond the limiting Sabatier volcano. Nevertheless, determining the most effective concepts requires a detailed comparison of both methodologies which, hitherto, is still missing.

Herein, Professor Kai S. Exner from University Duisburg-Essen in Germany conducted a detailed and unbiased comparison of the two optimization strategies: catalytic resonance theory by programmable catalysis and breaking scaling relation. These two concepts were evaluated in a case study for the NRR over transition-metal oxides (TMOs), aiming to derive design principles to improve electrocatalyst toward selective nitrogen formation. His work is currently published in the research journal, ChemCatChem.

Volcano plots, which can be constructed by the analysis of scaling relations between the adsorption-free energies of the intermediate states, are a popular approach to differentiate between active and inactive catalyst, located at the apex or the legs of the volcano, respectively. The author demonstrated that enhancing the electrocatalytic activity of electrocatalysts at the volcano apex can be successfully achieved through fine tuning rather than the full breaking of nitrogen-containing scaling relations. Breaking scaling relation proved more advantageous for catalyst optimization for cases characterized by positive scaling correlations between NRR intermediates. These observations make breaking scaling relations a viable strategy for improving the material design. On the other hand, the impact of catalytic resonance theory on electrocatalyst optimization mainly depends on material location in the volcano plot for cases of positive scaling correlation whereas this approach has proven to be successful in the case of negative scaling correlations.

In summary, Professor Kai S. Exner provided thorough discussion of the electrocatalyst optimization strategies for NRR on the example of transition-metal oxides (TMOs). The resulting catalyst optimization relations in the NRR over TMOs could be generalized if the preconditions of the derived volcano plot and the assumed reaction mechanism hold true in other material classes. In a statement to Advances in Engineering, Professor Exner explained that the presented design principles contribute to the development of improved electrode materials for sustainable and green ammonia production.