Characterisation of Anion Exchnage Membrane Fuel Cells

Alkaline membrane fuel cells (AMFC) offer an exciting route to reduce the material cost of low temperature fuel cells.  In contrast to traditional proton exchange membrane fuel cells (PEMFC) hydroxide ions rather than protons carry charge [1].  The resulting rise in pH increases the stability and possibly reactivity of some oxygen reduction reaction (ORR) catalysts such as Ag and Ni [2].  Critically, unlike alkaline fuel cells with a liquid electrolyte AMFC are resistant to metal carbonate precipitation and can purge themselves of carbonate anions during operation [3].

We will report techniques to characterise the electrochemical surface area of silver fuel cell electrodes and carry out in-situ measurements of the extent of membrane carbonation.  We believe these techniques to be widely applicable and essential in understanding AMFC.

As well as having exceptional conductivity, silver has been shown to have an ORR price activity (£/mA) similar to the archetypal PEM catalyst platinum when in contact with alkaline membranes [4].  It is an outstanding material for both transport layers and cathode catalysts and is used widely in AMFC [5,6].  Unlike platinum, it is not possible to accurately measure the specific surface area of a silver catalyst using hydrogen adsorption or oxide formation.  We will show specific surface area measurements on both full silver electrodes and silver nanoparticles down to 10nm in size, using lead under potential deposition (UPD) [7].  Though UPD is a well-known technique, its use on nanoparticles has recently been the subject of increased attention [8].  Our work confirms that lead UPD on silver nanoparticles is possible for select electrolytes.

Anion exchange membranes exhibit their highest conductivity when hydroxide ions are the mobile anion.  When membranes are exposed to air carbonate and bicarbonate ions form (carbonation) and conductivity is reduced.  In an operating cell the reverse process occurs [3].  Previously this phenomenon has only been studied by online mass spectrometry.  We will report a new electrode, which can be placed inside an operating cell, to monitor the extent of carbonation locally during both start-up and steady state operation. Using this technique we will show an optimised method for purging the membrane during start-up.

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