Chemistry and materials

Ceramic processing

Electrochemical flue gas purification relies on a porous ceramic backbone structure. The backbone structure needs to be mechanical strong, porous and it must be able to conduct electrons and ions. A lot effort has therefore been put in to optimizing such ceramic backbone structures. The ceramic backbone structure is fabricated using standard ceramic processing techniques like tape casting, lamination and sintering. After sintering the ceramic backbone structures are evaluated using scanning electron microscopy. Examples of micro-graphs of the ceramic backbone structure are shown below (only the electrode). As it can be seen the structure is highly porous. Porosities in the structure are made by adding pore formers like spherical PMMA particles or carbon fibres to the tape cast slurries. Here the large holes are formed when the PMMA particles are burned off and the small pores are formed when the carbon fibres are burned off during sintering. 

 

Chemistry

Electrochemical flue gas purification is an interdisciplinary scientific field, comprising solid state chemistry, catalysis and electrochemistry. At Risø DTU we are studying three different reactions: electrochemical reduction of nitric oxide, electrochemical assisted oxidation of propene, and electrochemical assisted reaction between nitric oxide and propene. Whereas we have been studying the electrochemical reduction of nitric oxide for several years, we have first recently started studying the two other reactions.

Electrochemical reduction of nitric oxide: The main problem with electrochemical reduction of nitric oxide under oxidizing conditions is the simultaneously reduction of oxygen at the cathode leading to a low current efficiency. At Risø DTU we are trying to solve the problem of low selectivity by adding alkaline- or alkaline earth-nitrates to the electrodes. It is known that the addition of BaO or K2O to the electrodes enhances the selectivity of the electrodes towards electrochemical NOX reduction in a net oxidizing atmosphere. However, further development is needed. Under oxidizing conditions the NOX can be stored as Ba(NO3)2 or KNO3 as given in the equations below (with Ba as an example):

(1) BaO + 2NO + 3/2O2 → Ba(NO3)2

(2) BaO + 2NO2 + 1/2O2 → Ba(NO3)2

These processes can also be electrochemically activated. The formed nitrates can then be reduced electrochemically as follows:

(3) Ba(NO3)2 + 10e- → BaO + N2 + 5O--,

thereby recycling the BaO or K2O.

Electrochemical characterisation

The ceramic backbone structure is evaluated using standard electrochemical techniques like cyclic voltammetry and electrochemical impedance spectroscopy. An example of voltammograms recorded on a ceramic backbone structure is shown below. Peaks are revealed in the voltammograms. The peaks are due to oxygen evolution.



To enhance the performance and selectivity of the backbone structures, the backbone structures are infiltrated with various metal-nitrates. Infiltration is a well known technique in the fabrication of catalytic converters, and it has also gained attention in the field of solid oxide fuel cell electrodes. A SEM micro-graph of a backbone structure infiltrated with a metal-nitrate is shown below. The infiltrated material is seen to be nano-sized.



Voltammograms recorded on a backbone structure infiltrated with another type of metal-nitrates are shown below. Pronounced peaks are revealed in the voltammograms, indicating formation of nitrates.

Page updated  by   25.01.2011


Kent Kammer Hansen
Senior Researcher
Fuel Cells and Solid State Chemistry (ABF)
Dir tel+45 46775835


Frederik Berg Nygaard
Business Development Engineer
Fuel Cells and Solid State Chemistry (ABF)
Dir tel+45 46775666