(Invited) The Role of Electrochemistry in Renewable Ammonia Production
Authors: Ayers Katherine E , Gellett Wayne L. , Wiles Luke 
Published: February 2020

Ammonia-based fertilizers have enabled increases in food production to sustain the world’s population. Currently the major source of ammonia is the Haber-Bosch process, which requires high temperature and pressure and has low conversion efficiency, such that very large plants are required for economical production. Ammonia is therefore one of the most energy and carbon intensive chemical processes worldwide, largely due to the steam methane reforming step to produce the required hydrogen. Because of the very large plant scale and resulting centralization of production, ammonia may also be transported long distances to point of use, adding additional energy and emissions. Finally, the scale of most current ammonia plants is prohibitively expensive for many regions, such that subsistence farmers do not have access to products that can enhance the productivity of their farms.

Distributed, sustainable ammonia production would therefore have a huge impact on global energy use and related carbon emissions, as well as food supply in struggling areas. Electrochemical solutions generally are well-suited to modularity and integration with renewable energy sources, and can enable a renewable ammonia pathway in different ways. First, hydrogen from electrolysis has grown to a scale relevant to the Haber-Bosch process, and renewable electricity prices have dropped to levels enabling economic viability for renewable hydrogen. Ongoing work in modular Haber Bosch reactors can also provide a better match between and can operate at much milder temperatures and pressures, but a catalyst is needed which is selective to ammonia generation vs competing reactions. In addition, direct electrochemical production of ammonia has become an area of growing interest and research, and could enable smaller scale installations at reasonable cost, with milder reaction conditions.

Proton OnSite, in collaboration with the University of Arkansas, Colorado School of Mines, and Case Western Reserve University have demonstrated early feasibility for improved ammonia selectivity in the latter approach through tailoring nanoparticle catalyst morphology. The approach leverages an anion exchange membrane configuration for a wider range of catalyst compositions, and multiple pathways for catalyst design including peptide modification and control over core shell and alloy structures. Proton’s expertise in electrode fabrication, cell design and water management, combined with the universities’ expertise in catalyst design and synthesis, were combined to fabricate single cell stacks and demonstrate ammonia production from nitrogen over argon controls. A key element of the research has been analysis of residual ammonia and decoupling ammonia impurities or breakdown of nitrogen-containing materials in the cell from true ammonia generation from air. Strict protocols have been developed to avoid misleading results from impurities or degradation of components. This paper will provide an overview of the pathways to renewable ammonia and assessment pathway maturities and potential.

Figure 1