A Bimetallic Electrocatalyst Platform for Understanding the Roles of Surface Chemistry and Functionalization on Nitrogen Reduction to Ammonia

The Haber-Bosch industrial process for ammonia production is the cornerstone of modern commercial-fertilizer-based agriculture. Haber-Bosch ammonia fueled the global population growth of the 20th century, and approximately half of the nitrogen in human bodies today originates from ammonia-based fertilizer produced by the Haber-Bosch process. However, the Haber-Bosch process operates at high temperature and high pressure to achieve high conversion efficiencies, and the hydrogen input comes from steam reforming of coal or natural gas. In addition to the energy costs, the large production of carbon dioxide as a greenhouse gas and the large required economies of scale motivate research efforts to explore other possible options for ammonia production. One potential option is low temperature electrochemical synthesis of ammonia from nitrogen and water. An electrochemical process that directly synthesizes ammonia molecules from nitrogen gas and the hydrogen atoms of water molecules would eliminate the need for fossil-fuel-based hydrogen as a reactant and decrease CO2 emissions. Further, an electrochemical system based on already-developed technology in the fuel cell and electrolysis arenas would enable a modular, scalable, and energy efficient process that could be connected to renewables (i.e., wind or solar) as the energy input. While electrochemical ammonia synthesis is an enticing option in theory, in practice the process is severely limited by the lack of available catalysts that can produce ammonia at high Faradaic efficiencies. All heterogeneous catalyst surfaces that are active for the reduction of di-nitrogen to ammonia in the presence of water are also highly active for the reduction of water molecules to hydrogen gas. Both reactions occur within a similar potential range, and thus electrochemical systems will inherently battle against this reaction selectivity issue. Today, all reported heterogeneous catalysts remain limited to low efficiencies of less than 1% for ammonia production at low temperature because the majority of the applied electrical potential goes towards water splitting and the evolution of hydrogen. In our research, we are interested in understanding how catalyst surface chemistry and ligand functionalization can be used to direct and control reaction selectivity of the electrocatalyst. We synthesize non-precious metal-based catalysts comprised of iron and nickel, which, as a bimetallic material, are theoretically-predicted to potentially result in optimal surfaces for nitrogen reduction. However, these mixed oxide/hydroxide surfaces are also expected to still be dominated by hydrogen adsorption and evolution. Thus, to further address reaction selectivity and create a catalyst surface that is more selective for ammonia synthesis, we also design our catalysts such that the local surface environment of the catalyst is controlled through ligand functionalization. In this talk, our ongoing work to design, synthesize and characterize iron-nickel hydroxide bimetallic catalysts for efficient electrochemical ammonia synthesis will be discussed.