Interview: Fuel cells a hot topic
02 October 2007
Stephen Paddison talks to James Mitchell Crow about fuel cell science, and his search for the ideal electrolyte
|Stephen J Paddison is an associate professor of chemical engineering at the University of Tennessee in Knoxville, US. His research, in the general area of computational materials chemistry, focuses on modelling proton exchange membranes used in low temperature fuel cells.|
What's the focus of your research?
For about the past 11 years we've been in the business of polymer electrolyte membrane (PEM) fuel cells. Over the last seven years it's been pretty much exclusively theoretical and computation work, although I still have very close ties with experimental groups. My own interest has been in the electrolyte, the central component of the fuel cell. These PEM fuel cells are limited by current electrolytes to 90°C, and there's quite a push to elevate that to 110 or 120°C.
Both the oxidation at the anode and reduction at the cathode are at present catalysed by noble metals, typically platinum and ruthenium. And unless you elevate the temperature, the catalysts get poisoned by carbon monoxide, which has several sources including as a fuel impurity. The CO coats the catalyst, so your efficiency goes to the proverbial hell in a handbasket - but if you raise the temperature, the CO doesn't stick.
How will modelling help us find higher temperature electrolytes?
My work has been to understand how existing electrolytes conduct protons, and propose new materials that offer higher performance. The current electrolytes all require water to conduct protons well, they have to be pretty much fully hydrated. Moving to temperatures above 100°C - beyond the boiling point of water - will cause problems. So there's a push to find materials that will either retain the water, or require no or very little water to function.
These materials are very difficult to understand because in the presence of water they exhibit very fine phase separation - the electrolyte polymer backbone is very hydrophobic, and the end groups of the polymer side chains are very hydrophilic. So with this very fine, nano separation, you have a real puppy's breakfast, it's a very complex system and it's very difficult to understand the transport phenomena over those scales. The science is unique in that we're forced to work in domains in which our understanding of macroscopic phenomena doesn't really carry, yet it's larger than molecules. It lends itself to examining the system at many different levels to get the real picture.
Of the various types of fuel cell, is one likely to emerge as the best?
They have very different applications. The PEM fuel cells have a range of applications, from portable power supplies for electronic devices, to vehicular power, to stationary power. The solid oxide-type fuel cell, where the electrolyte is a solid ceramic, pretty much only has applications in stationary devices, because they operate at very high temperatures [typically 700-1000°C] so wouldn't be safe or feasible for vehicles. They have the opposite problem to us; they would really like to come down in temperature. The difficulty is, can we find a good electrolyte for this intermediate, 110-150°C temperature regime?
How far away are we from mass market fuel cells?
I think seeing these cells in a mass-produced vehicle that's affordable is still quite a few years away. And then there is the question of hydrogen distribution. I think in some respects we're closer for portable power supplies for small devices, and perhaps also for stationary power supplies, to provide back-up power in remote areas, for example. With vehicular power, we've had decades of developed car technology, so people have clear expectations of their car, which the technology must meet.
What other hurdles do fuel cells need to get over?
Generation and distribution of the hydrogen, which isn't quite as convenient as a bed of coal or a pool of oil. Producing it and storing it remain open questions, but other people are working hard on that. And there's the catalyst - it would be wonderful if they could get away entirely from noble metals. Can we use something more abundant and less pricey, and still get the performance? We've come a long way since Nasa's early fuel cells, which cost thousands of dollars, but we're still not quite there.
Funding agencies still have money to put towards this, and companies depend on this funding because it's high risk research and the payoff may not be for five or 10 years. But I really think we have some momentum. I've seen a big increase in people involved in this field, and that's exactly what it needs.