Electrochemical Synthesis of Ammonia Using Proton and Hydroxide Exchange Membrane Cells

Abstract

With the development of the Haber process and the subsequent work done by Bosch, ammonia production become an industrially and economically viable way to fix nitrogen. Ammonia used as a fertilizer has helped increase the global food production and estimates put it at about 40% of the global population’s food needs the ammonia fertilizer from the Haber process[1]. However, the Haber-Bosch process is an energy intensive process requiring high pressure (15-30 MPa) and relatively high temperature (430 °C – 480 °C) and is highly centralized with only about 13 companies producing at about 29 plants[2, 3]. Renewable energy resources offer a possible alternative way to fix nitrogen at low temperature and lower pressure to produce ammonia in a decentralized way. High temperature solid proton conductors have been used to produce ammonia selectively, but the high temperature can degrade the ammonia[5]. Low temperature polymer electrolyte membranes can be used which might reduce the overall energy input. Proton exchange membrane (PEM) suffers from conductivity loss due to the ammonia converting to ammonium[6], which reduces the cation conductivity. The other issue is the competing hydrogen evolution reaction (HER) which is especially fast in the acidic environment[7]. By moving to an alkaline environment, ammonia will not affect the conductivity because hydroxide exchange membranes (HEMs) use anions as the charge carriers. HEMs also allow the use of non-precious metal catalysts which could reduce the overall cost of these electrochemical devices. In this presentation, we will discuss the activity of a variety of catalyst, PGMs, metal nitrides, and non-precious metals in both HEM and PEM electrolyzers for their ammonia production. We utilized the hydrogen oxidation reaction (HOR) as the counter electrode which can also be utilized as a reference. This study allows for the direct comparison of PEM and HEM for electrochemical nitrogen reduction.