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Optimization of electro-catalysts for the acid methanol fuel cell

Objective

Continue the search for new catalysts. Prepare high surface anodes for methanol oxidation and oxygen diffusion cathodes for direct methanol fuel cells; both liquid and solid polymer electrolytes are investigated.
The inhibition of oxygen gas reduction on platinum (the most effective catalyst for the reaction) was attributed to commencement, at ca. 0.8 V(RHE), of surface oxidation which is assumed to block active sites which play a crucial role in the interfacial reaction. On the other hand the inhibition of the methanol oxidation reaction, which is required to proceed at a much lower potential (< ca. 0.4 V), seems to be due to the absence of a significant coverage of oxygen species on platinum at low potentials. This lack of oxygen permits the formation of COads blocking species which deactivate the electrocatalyst surface. The addition of ruthenium to platinum improves the electrocatalytic activity for methanol oxidation - the platinum activating the alcohol while the ruthenium enhances the oxygen activity of the surface. Pure ruthenium, and most other noble metals, eg Ir and Rh, are poor electrocatalysts for methanol oxidation as they have too strong an affinity for oxygen.

A new type of flexible carbon-cloth-bound-porous-carbon electrode has been developed that allows the use of extremely thin layers of supported catalyst with PTFE binder. Such electrodes have been found to give very high performance levels with Pt-Ru catalysts chemically deposited from anionic sulphito-complexes that have been heat treated and then dispersed, levels that promise considerable technological application.

Teranry alloys of Pt/Ru/M where M is Pb, Ga and Cr have been investigated as well as a large number of binary alloys of Pt. In general, five methods for catalyst improvement have been identified from the mechanistic studies: deposition of reproducible very finely dispersed particles of 1-4 nm size; electro-deposition of Pt alloys; modification of the underlying carbon functionality to control particle size and morphology; alloying of Pt with another metal that then dissolves to give an active surface; and use of composite electrodes allowing exceptionally high loadings of Pt/Ru in very thin layers.

Investigations on the catalytic activity towards the oxidation of methanol, the selective reduction of oxygen in the presence of methanol and on the corrosion of smooth and high surface area binary and ternary noble metal and transition metal-noble alloys have been performed.

High activities for methanol oxidation were obtained with PtFe, PtRu, PtSn PtPb and PtRuNi alloys. PtRu combines a good activity with high corrosion resistance. Noble metal alloys less active to methanol oxidation exhibit also low activities for oxygen reduction. Therefore, the design and the working parameters of the cell have to care for a sufficiently low methanol conversion at the cathode.

Modified carbon materials were investigated as electro-catalysts for oxygen reduction in methanol fuel cells. Purified P33 has been used as a substrate material. Two types of catalysts were investigated:

1. CoTMePP (Cobalttetramethylphenylporphyrin)

2. Transition metal chalcogennids (TMC): MoxRuySez

The experiments were carried out under pure oxygen and air, at the temperature 23 C and 60 C and the presence of methanol (without and 20%).

In agreement with previous results the catalysts of N4-chelate type are selective for oxygen reduction whereas for TMC catalysts an additional voltage drop in the presence of methanol was observed.

Both catalysts showed a lower activity for oxygen reduction in sulphuric acid is in comparison to Pt as a catalyst.

First experiments on TMC catalysts showed that at constant current density the voltage decreased continuously at a rate of about 1 mV/h. This might be related to the corrosion of the catalyst in sulphuric acid.

- Pt, Ru, Re, different PtRu alloys as well as codeposits of Pt, Ru and a third metal like Ni, Cr and Ga have been studied as methanol catalysts.

- Using in situ FTIR spectroscopy the main products of methanol oxidation (CO2 and formic acid) and the main poisons (CO and COH) can be controlled.

- The catalytic effect of PtRu alloys upon CO oxidation has been investigated via cyclic voltammetry and TDS in UHV.

- Chemical and electrochemical codeposition of metals on different carbons has been used with parameter variation in order to develop methanol anodes.

- The methanol effect on the oxygen cathode has been studied.

- Methanol permeation was followed via online MS for the Dow membrane, Nafion117 and polyoxyphenylene.

- Complete cells using gaseous methanol have been successfully tested.

- Demonstration cells were tested at room temperature.
Fuel cells predominantly oxidize hydrogen to produce electricity. If other fuel 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.

In project JOUE-CT89-0011 work is distributed in the following way. Siemens is carrying out a search for new catalysts for methanol oxidation and oxygen reduction; the corrosion stability of these catalysts will also be studied; the University of Newcastle will characterize the new catalysts. The optimization of catalyst material, the deposition of metal and other catalysts on a carbon substrate and the development of solid polymer electrolyte based electrodes for oxygen reduction and methanol oxidation are carried out by the universities of Bonn and Newcastle. The University of Cork is focusing on improvement of the oxygen electrode. Half cells are tested by the University of Bonn.

In the extension the research for non-Pt catalysts for the O2 electrode will be itensified (Cork, Newcastle, Ulm). In the field of catalysts of the fuel electrode work will be carried out on the stability of multimetal alloy catalysts. Research will in future be more directed towards DMFC with solid electrolytes using gaseous methanol.

Funding Scheme

CSC - Cost-sharing contracts

Coordinator

RHEINISCHE FRIEDRICH-WILHELMS-UNIVERSITAET BONN
Address
Regina Pacis Weg 3
Bonn
Germany

Participants (3)

SIEMENS AG
Germany
Address
Wittelsbacherplatz 2
Muenchen
UNIVERSITY COLLEGE CORK, NATIONAL UNIVERSITY OF IRELAND, CORK
Ireland
Address
Western Road, Presentation Buildings
30 Cork
UNIVERSITY OF NEWCASTLE UPON TYNE
United Kingdom
Address

NE1 7RU Newcastle Upon Tyne