Novel Iron-based catalysts for fuel cells – CAFERINNO

Fe(Co)/N/C catalysts will be synthesized through known as well as innovative approaches. The preparation of catalyst precursors leads to the dispersion of iron or cobalt coordinated to nitrogen-containing ligands and positioned in micropores (pore size 2 nm or less). Pyrolyzing the precursor at 900-1100C under controlled atmosphere (inert or reactive) results in a microporous carbon powder that includes Fe(Co)Nx active sites.
The various degradation mechanisms of the best performing catalysts will be separately studied and quantified with methodical and novel approaches. One of the bottleneck for NPMCs is their short lifetime in PEFC, today. Increasing their durability will require the identification of their major degradation mechanim(s) in order to be able to rationally act on the synthesis or post synthesis treatment.

The synthesis method of catalysts that will be used as reference catalysts for the degradation mechanisms has been implemented. It includes the preparation of a catalyst precursor (iron salt, N-containing ligand, metal-organic-framework = MOF). After a pyrolysis at 900-1100C in a controlled atmosphere (inert or reactive), a microporous carbon structure doped with nitrogen is obtained, with FeNx active sites in micropores.The precursor preparation step is important and the ligand as well as MOF choices are crucial. A detrimental reaction between some ligands and MOF has been found. This will enable us to rationally select ligand/MOF systems in order to avoid this reaction.
Another approach consists in the synthesis of ultramicroporous carbons of specific area 2000 m2g-1 or more, in order to prepare catalyst precursors by filling of these micropores with iron salt and N-ligand. Synthetic carbons with specific area of 1500 m2g-1 have already been obtained by template method.

The replacement of platinum by non precious metal catalysts in polymer electrolyte fuel cells would decrease by a factor of 2 the cost of PEFC stacks and would open their use for widespread application in portable electronics and cars. Replacing platinum by abundant metals would also solve the problem of platinum recycling if the latter must be used. Platinum reserves can barely match the amount of platinum needed if all vehicles were to be powered by PEFCs. Recycling Pt at 90% level or more would thus be crucial for this technology.

Two manuscripts are being written and a review article on the application of metal organic frameworks for electrochemical applications is being submitted.

Today, our energy needs are in large part satisfied by fossil fuels. In the future, renewable energies will play an ever increasing role. For transportation, a renewable fuel will be needed, except for battery vehicles with limited driving range. Hydrogen produced from water and renewable energies could be that fuel. In this respect, H2/air polymer electrolyte fuel cells (PEFCs) are interesting due to their twice higher energy efficiency compared to a H2-combustion engine.

A major drawback of today’s PEFCs is their dependence on platinum, a rare and expensive metal, for catalyzing the PEFC-reactions: hydrogen-oxidation and air-reduction. The latter reaction is by far the slowest and 90 % of the platinum in such a fuel cell is used at the air-reducing cathodes. Based on today’s price for platinum, studies have shown that 40-50% of the material’s cost of a PEFC-stack would be ascribed to the raw platinum metal. Therefore, eliminating platinum from the cathode would drastically reduce the cost of PEFCs and allow a massive utilization of this technology.

Recently, several breakthroughs have been reported in the field of non-precious-metal catalysts (NPMCs) made from iron (cobalt), nitrogen and carbon. Their activity for the oxygen reduction has been increased tremendously, making them suddenly interesting catalysts from a performance standpoint. Other less active NPMC catalysts have been reported to be stable for 700 h. In order to bring these NPMCs into real PEFC stacks, the highest activity reported for recent NPMCs will have to be combined with a stable behaviour for thousands of hours in an operating PEFC, as required for transportation application.

The aim of the present proposal is to investigate innovative approaches to obtain more durable NPMCs and simultaneously advance the science on the various degradation mechanisms that are specific to these catalysts in fuel-cell environment. Two novel approaches will be investigated. The first will consist in synthesizing new NPMCs by replacing the microporous carbon support by other microporous supports. The second will consist in modifying the surface of pre-existing NPMCs by various methods, in order to strengthen the resistance of Fe-based catalytic sites to demetallation, oxidative attack or anion adsorption. Simultaneously, an experimental methodology will be developed to quantify the importance and rate of each of these degradation mechanisms. Long fuel cell tests under controlled conditions will be coupled with advanced characterization techniques such as X-ray photoelectron spectroscopy, Mössbauer spectroscopy and on-line mass spectroscopy.