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Development of a solid proton electrolyte direct methanol fuel cell


Development of electrolyte materials and membranes for a solid state direct methanol fuel cell, which are gas tight and chemically stable with methanol in the temperature range up to 120 C and which have a membrane resistance of 1-3 ohm s.q.m.
A wide range of new types of protonic conductor materials has been investigated as electrolyte for direct methanol fuel cell. The new protonic conductors are inorganic and organic types and their conductivities increase with increasing relative humidity. A couple of materials have emerged showing protonic conductivity higher than 10{-2}{-1}. They are in agreement with the target conductivity defined in the annex of the project. The group of inorganic proton conductors includes a clay (tin-laponite), a pillared layered material (kegging ion containing tin-phosphate) and a modified zeolite (tin-oxide containing mordenite). Their respective conductivities are 10{-2}, 3.10{-2} 10{-2}{-1} at temperature between 80-100 C. The organic protonic conductor is Ormolyte ((Polybenzylsulfonicacid)siloxane) (PBSS) which is an insoluble powder and has a conductivity of 3.10{-2}{-1} at 40 C. The second organic material is a protonic conducting membrane based on an organic backbone and a sulfonic acid group (AMPS). Membranes are prepared by radiation curing and showed conductivity of 8.10{-3}{-1} at 80 C.

Electrolyte membranes have been prepared from the protonic conductors and organic binders. Required properties for the new types of electrolyte membranes are low electrical resistance, low methanol diffusion and chemical stability. Organic binders are urethane acrylate or thermoplastic (polyvinylidene fluoride). Processes investigated for membrane preparation were radiation curing combined with serigraphy, or tape casting. Membrane thickness is between 100-200 um. Membrane shows reduced conductivity when compared to the pure materials. Conductivity is in a range 5.10{-4} - 6.10{-3}{-1} at 80 C. The best membranes based on the PBSS/PVDF showed resistance of 3{-2} which is very close to the targeted membrane resistance. Membrane resistance has been showed stable over a period of more than 800 hours at 60 C and 100% relative humidity. In general methanol absorption for the new type of membrane is half lower when compared to perfluorinated membrane (Nafion).

Assembly based on new membranes (PBSS/PVDF) and platinium electrode has been tested in methanol/O2 indicating current of 8{2} at 60 C.
Fuel cells predominantly oxidize hydrogen to produce electricity. If other fuels such as methane or methanol are used they first have to be converted into hydrogen with a reformer, which is expensive and bulky. This is a serious drawback for applications in transportation. Direct methanol fuel cells are therefore being developed which oxidize methanol directly and which don't require costly external reformers. The main problem is here the poisoning of the catalyst. In past EC R&D new ternary alloy catalysts have been developed which allow operation for 4000 hrs without poisoning of the catalyst. Ongoing work is now focused on increasing the current density and reducing the cost. This work is carried out in four coordinated contracts: JOUE-CT89-0011, JOUE-CT90-0026, JOUE-CT89-0007 and JOUE-CT90-0037.

The use of solid electrolytes in direct methanol fuel cells allows to increase the operation temperature of the fuel cell (80 C - 120 C) where we can expect an increased reaction rate and current density; this should lead to large cost reductions. The solid electrolyte also allows a very simple fuel cell concept which is suitable for cheap mass production. For this reason development of solid electrolyte direct methanol fuel cells is emphasized in the EC programme; it is the main topic of contract JOUE-CT90-0026.

The main effort of this project deals with development of solid proton conducting electrolyte materials. The following materials are being investigated: intercalates with a variety of guest species in host structures such as zeolites, transition metal phosphates and pillared clays (University of Odense); new proton conducting composite electrolytes, exchanged zeolites and clays and intercalated synthetic and natural minerals (University of Exeter); ormolytes and organic inorganic polymer electrolytes with good mechanical and thermal stability (University of Grenoble); acid M(III) sulphate hydrates and pillared high valency metal hydrogen phosphates and poly-phosphates (LAMMI). Acrylates with proton conducting acids and amines (INNOVISION). At INNOVISION electrolyte membranes are prepared from the electrolyte materials and suitable binders.

Until now, alternative proton conducting electrolyte materials have been identified including tin-oxide containing zeolites, pillared zirconium phosphates, acid ferric sulphate, ormolytes and polymer sulphonic acids have been developed, which show good conductivity for DMFC and a low diffusion of methonal.

In the extension these materials will be further developed and characterized. These electrolyte membranes will also be tested together with electrodes. The integration of the electrolyte and electrodes is a key task. The aim is to develop manufacturing methods which can lead to strong cost reductions.


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5260 Odense S

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