11-08-31 A Review of Advances in DMFC Catalyst Technology

A Review of Advances in DMFC Catalyst Technology

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31 Aug 2011PDF (369 kb)

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Earlier this month Dan considered the question of platinum catalysts and whether they were 'friend or foe' to fuel cells in terms of cost. The article garnered quite a response, indicating how much interest there is in this issue. Dan's conclusion was that platinum is a friend due to its superiority as a catalytic metal, but that the friendship needs work in order to succeed - specifically, by optimising the catalyst to allow thrifting of the high-value metal. This comment may sound merely optimistic, but is made with the full expectation that recent advances will deliver the required reduction of platinum loadings that are widely acknowledged as critical to the commercialisation of fuel cells.

A recent article published in the RSC journal Energy and Environmental Science provides a neat illustration of the form many of these efforts are taking, reviewing advances in understanding that are paving the way for more cost-effective and efficient catalysts for direct methanol fuel cells (DMFC). I'd like to pull out a few highlights from this paper.

The authors (Zhao et al.) first address the methanol oxidation reaction (MOR), taking place on the anode and typically catalysed by platinum-ruthenium. Fundamental insights by various researchers into the mechanisms and thermodynamics of the reaction have allowed several effects to be identified, over and above the composition of the alloy. It has been shown how and why MOR activity depends on the size and shape of the catalyst nanoparticles and how other factors also come into play, such as which state the active sites are in (metallic ruthenium or hydrous ruthenium oxide, for instance) and what the structure of the catalytic material is (whether it is a ruthenium-decorated platinum surface, or a core of one metal covered by the shell of another, or what crystallographic orientation it is in, and so on). Hence, not all platinum-ruthenium is created equal and significant improvements in performance can be obtained just by tweaking these parameters.

Of course, the search for alternative alloys continues: the article reviews the use of low-cost alloying metals to improve parameters such as the tolerance of platinum catalyst to carbon monoxide with some insight into the mechanistic effects of these additions.

On the cathode side, the challenge is primarily to improve the oxygen reduction reaction rate without pushing up platinum loadings. Here again size, shape, structure and state of the catalytic material all have their part to play, but optimisation is not always straightforward. For example, the authors note that at a certain point specific activity decreases with a decrease in platinum particle size or increase in specific area, which is somewhat unexpected. But once again, a better understanding of the mechanisms and thermodynamics of the catalysed reaction has suggested reasons for this and ways to structure the catalyst to counteract it.

Degradation of platinum catalysts is now also well understood, with a number of remedies under investigation. These include modifying the platinum with gold or base metals, optimising crystallographic structure, strengthening of the support-catalyst interaction and carbon 'riveting' of the platinum.

The inspection of such detailed effects has been made possible by ever more powerful analytic methods, and the fundamental insight into reaction mechanisms that has been gained is now allowing catalysts to be designed almost down to the atom level. Intelligent and finely controlled synthesis methods are allowing these novel designs to be realised. Hence, this is no longer an alchemist's quest to turn lead into gold: step-changes in catalyst activity per unit cost will be delivered by an accumulation of incremental adjustments.

This in itself is nothing new, as incremental advances have been core to catalyst development in many fields - fuel cells being no exception. Therefore our expectation that this apparent obstacle to wider commercialisation can be overcome is based on fundamental advancements like these taking place at research institutions across the world.

Marge Ryan     Market Analyst

margeryan@fuelcelltoday.com

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Marge Ryan
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