Proton-Conducting Ceramics for Electrochemical Ammonia Synthesis

Abstract

We report our progress in developing proton-conducting ceramics for flexible storage of intermittent renewable electricity in the form of ammonia. Various electrochemical and thermochemical synthesis strategies have been investigated in pursuit of “green ammonia”. Electrochemical ammonia synthesis can harness water-splitting to source hydrogen, displacing steam-methane reforming widely used in conventional ammonia production through the well-established Haber-Bosch process (HB).This electrochemical approach potentially provides an efficient solution to the energy-storage challenges presented by the expanding market penetration of solar and wind power. The intermittent nature of these renewables presents strain on the conventional power plants tasked with stable electricity production. Through creative application of novel proton-conducting electroceramics, intermittent renewable electricity can be used to drive the synthesis of green ammonia, effectively storing electricity in the form of an easily transported, carbon-neutral, energy-dense commodity chemical. Further, the process is reversible; NH3 can be electrochemically re-converted back into electricity as needed in meeting peak load demands.Displacing Haber-Bosch proves challenging, as it requires process solutions that can simultaneously achieve high NH3 conversion and high NH3-synthesis rate while minimizing energy costs. The Haber-Bosch process achieves both through high-pressure operation (up to 300 bar), where thermodynamics are more favorable. Electrochemical systems have yet to reach this stage of technological development. All previous work has been limited to atmospheric-pressure operation, where NH3-synthesis rates are orders-of-magnitude lower. Further, most reports on electrochemical ammonia synthesis have utilized molecular H2 as the source of protons for NH3 synthesis, rather than water electrolysis as described here.In this work, we present our efforts to apply proton-conducting ceramics to enable electrochemical ammonia synthesis from H2O and N2 feedstocks. Protonic ceramics are emerging materials finding applications in efficient electricity generation, energy storage, and fuels synthesis. As shown in the figure, renewable electricity is used to drive hydrogen production through high-temperature water splitting in the protonic-ceramic electrolyzer. The hydrogen mixes with nitrogen and is then converted to NH3 over a ruthenium catalyst. The process is executed at pressures up to 12 bar, bringing significant challenges in packaging of the protonic ceramic and the catalyst. In this presentation, we review our progress in establishing pressurized operation, and the benefits it brings to electrochemical synthesis of ammonia. Figure 1