A small filter unit in a test installation of a 0.5-liter diesel engine at Dinex Emission Technology A/S. Click to enlarge.

Research on the electrochemical reduction of pollutants has been underway since 1975, when Pancharatnam et al. used a zirconia-based oxide ion conducting electrolyte to reduce NO to elemental nitrogen and oxide anions on a Pt-electrode. The main obstacle with this technique, according to Risø, is that the presence of oxygen lowers the activity of this type of system towards the reduction of NO and thereby increases the current consumption.

Dinex Filter Technology A/S, a participant in Risø project, has demonstrated the electrochemical oxidation of soot. Dinex captures the soot in a porous structure consisting of an oxide ionic conducting electrolyte and a pair of electronic conducting ceramic electrodes. Reactive oxide anions are pumped to the anode where they react with the soot particles forming CO₂.

The electrochemical oxidation of hydrocarbons in an all solid state electrochemical cell has been studied by several authors. The main problem is that not all the hydrocarbons are converted to CO₂, but that CO and partially oxidized hydrocarbons also are formed.

In principle, says Risø, it is possible to combine all the processes in one porous filter built with alternating layers of electrodes and electrolyte. The driving force for the reactions (see diagram at top) is an external potential difference imposed between the top and the bottom of the filter.

Such electrochemical exhaust gas treatment has a number of advantages over existing filters making it attractive to target this research at the car industry, Risø says. Purification of carbon particles, nitrogen oxides (NO_x) and unburned hydrocarbons from the exhaust can all happen in the same filter unit.

Existing diesel exhaust aftertreatment solutions generally require the installation of particulate filters and either an SCR catalyst (Selective Catalytic Reduction), a NO_x absorber, or use recirculation of the exhaust gas. This leads to additional cost.

Another advantage of using electrochemical methods, according to Risø, is that it is not necessary to add other substances to the fuel. Conventional SCR technology typically uses nitrogen-containing urea as a reducing agent to remove NO_x from the exhaust. Also, the filter can be produced without the use of precious metals.

The treatment of exhaust gas is conducted independently of the engine operation, potentially leading to fuel savings compared with the leading alternative technologies. The technology could also be applied in the purification of flue gas from power plants, and possibly in the shipping industry.

The project intends to develop the technology into a prototype for use under realistic conditions in a diesel engine.

The project is led by Kent Kammer Hansen, Senior Scientist in the Fuel Cells and Solid State Chemistry Division at Risø National Laboratory for Sustainable Energy, the Technical University of Denmark. Also participating in the project are the Department of Mechanical Engineering at DTU and the company Dinex Emission Technology A/S.