INVESTIGATION OF ANION EXCHANGE MEMBRANE UREA ELECTROLYSIS

Abstract

Nitrogen oxides (NOx) are one of the most dangerous anthropogenic forms of pollution known. NOx can cause health hazards linked to respiratory and cardiovascular systems. NOx emissions can also contribute to the formation of tropospheric ozone, acid rain, and depletion of the ozone layer. In this work, an innovative H2-deNOx technology, alternative to the selective catalytic reaction (SRC) technology, is studied for NOx abatement in mobility applications. The H2-deNOx approach could help diesel engines to comply with the strict regulation for NOx emissions that have been implemented in the EU over the years. This approach enables high efficiencies of NOx removal from the engine exhaust gas even in cold conditions, where SCR tends to fail. Anion exchange membrane electrolysis, a promising and emerging technology for electrochemical water splitting, was adapted for the conversion of an aqueous solution of urea, AdBlue, into ammonia and hydrogen (eU2A). These gases are subsequently used for the reduction of NOx in the exhaust gas stream. The influence of several components that compose this electrochemical device are studied. The influence of operating conditions such as temperature and potassium hydroxide (KOH) concentrations are investigated. Three main experimental tests were used: a) rotating disk electrode (RDE) was performed to provide insights into the electrocatalytic activity in the anodic reaction (oxygen and ammonia production); b) submerged cells and c) closed electrolyzer cells were used for MEA characterization – in submerged cells different KOH concentrations and temperatures were used while in closed cells, catalyst coating techniques (like CCM and CCS) were the focal point. It was concluded that the current density of the electrochemical conversion of urea into ammonia by AEM electrolysis is low and needs to be increased. With pure AdBlue, current densities of 70 mA cm-2 were obtained for submerged cells at 2 V, while using the RDE anodic reaction this value was 3 mA cm-2, at 1.8 V. Adding 1 M KOH improves drastically the current density, reaching 300 mA cm-2 in submerged cells, 125 mA cm-2 in RDE and almost 500 mA cm-2 in closed electrolyzer cell; though much higher than without supporting electrolyte, this approach still underperforming for making the technology commercial. The relatively low current density was assigned to the poor electric conductivity and poor mass transport kinetics of the urea solution. Besides increasing the current density, for making the H2-deNOx technology commercial, it should also be addressed the stability of the anion exchange membranes used.