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Direct Methanol Fuel Cell component and concept evaluation

Objective

Methanol is an alternative fuel to hydrogen in Fuel Cells. To avoid an expensive and bulky reformer, Direct Methanol Fuel Cell (DMFC) oxidizes methanol directly and does not require an external reformer.
This project will evaluate Direct Methanol Fuel Cells operating above 100
components of a 100W-stack with large sized electrodes (100 - 250 cm2) will be designed and tested.

The expected results of operation with gaseous methanol at elevated temperatures and pressures are higher cell voltage, higher current densities at low catalyst loadings and thus a more efficient system.
Different methods in producing methanol vapour are discussed and the performance of the cell under these conditions was tested. The vapour production by complete vaporisation of a methanol/water mixture proved to be a successful method in operating the cell at 130 C. Thus, better cell characteristics (300 mA/cm{2} at 550 mV) than at 80 C could be obtained with liquid fuel supply (eg 100 mA/cm{2} at 460 mV).

These characteristics were reproduced and their stability observed in long-term endurance tests at higher temperatures. However, the values deteriorate with time to let the construction of a battery at this stage. The cathode causes a problem because of a high purging rate and an elevated pressure of oxygen. Under improved conditions (methanol conc. adopted) an endurance test lasting 250 h was carried out. The cell voltage dropped only by 0.4 mV/h.

To reduce the purging rate and pressure of oxygen, more fundamental investigations have been carried out. The experience of these measurements will lead to derive more suitable conditions of the cell operation. In parallel investigations, the noble metal load of the ME assembly was reduced to 0.25 mg/cm{2} and the U-I-characteristics were attained without a pronounced loss in cell voltage. The results demonstrate the possible cost reduction potential of the DMFC.

The goal of Innovision is to develop a process and to fabricate electrode with low catalyst content for a direct methanol fuel cell. In order to lower the catalyst content in the electrode, it was decided to create a thin active layer close to the protonic conducting membrane. In order to fabricate such a layer, a technique such as screen printing was selected and experimental verified.

The various steps of the electrode fabrication were investigated in detail and include, ink fabrication, ink printing and assembly treatment. Two methods were used for the fabrication of the electrode. One route was the printing of thin layer on the membrane followed by hot pressed procedure. The second route was based on a printing technique followed by a heat treatment of the assembly. The ink was developed and better properties in term of processability and reproducibility and thin layers with catalyst content of 0.15-0.25 mg/cm{2} were obtained.

Characterisation of the assembly was first performed using H2/O2 cell. Influence of process, assembly pre-treatment, components assembly and catalyst content on electrical performance were investigated. both U I-characteristics and impedance measurement were performed. The best results in term of catalyst utilisation were obtained for the thinner electrode. However, further improvement of the process and of the component content are needed in order to improve the electrical performance of the cell.

One of the processes used for preparing H2O2 cell was transferred for fabrication of methanol assembly. Initial tests of a cell based on 3.5 mg/cm{2} of Pt/Ru at the anode and 0.6 mg/cm{2} of platinum at the cathode were under progress.
In a first phase basic experiments with small sized electrodes are conducted. They include the influence of temperature, pressure and methanol supply on the characteristic of a direct methanol fuel cell. Furthermore, a control and management concept for a complete system will be evaluated and tested. It implies the fuel supply, CO2 separation, water- and temperature management.

The component work will include the development and optimization on alternative proton selective membranes (especially those with udapted ion exchange capacity), electrode structures, the assembly of
membrane-electrode units (MEs) and a modular cell design for large sized electrodes.

The operation and long term test of a rnulticelled stack of approx. 100 W shall verify the concept for components and operation rnanagement and outline the necessity of further investigations.

The membrane development and adaptation is a task of Innovision, Denmark. They will conduct membrane tests in small cells and - in parallel to Siemens - perform ME assembly. Selected large size membranes and assemblies will be delivered to Siemens.

Results of previous project :

In the Joule II project JOUE-CT91-0074 a cell concept with immobile electrolyte was tested. With 250 cm2 cells, 450 mV at 100 mA/cm2 could be obtained. Main problem remains the diffusion of H2O and methanol through the membrane. Membranes with a lower methanol leakage are under development (Innovision, Odense DK), but not yet available.

Expected results :

The expected achievements for an operation with gaseous methanol at elevated temperatures and pressures are higher cell voltage, higher current densities at low catalyst loadings. That means higher efficiency for the DMFC system. A rough estimation of its technical and economical acceptance should be possible.

Funding Scheme

CSC - Cost-sharing contracts

Coordinator

Siemens AG
Address
Hammerbacherstraße 12-14
91050 Erlangen
Germany

Participants (1)

Innovision A/S
Denmark
Address
75,Lindvedvej
5260 Odense S