Simulating Complex Catalysts key to making Cheap, Powerful Fuel Cells

Modeling how methanol interacts with platinum catalysts inside fuel cells in realistic environments becomes even more complicated because distances between the atoms can change as molecules dance near the charged surface. Credit: Manos Mavrikakis

Using a unique combination of advanced computational methods, University of Wisconsin-Madison chemical engineers have demystified some of the complex catalytic chemistry in fuel cells – an advance that brings cost-effective fuel cells closer to reality. “Understanding reaction mechanisms is the first step toward eventually replacing expensive platinum in fuel cells with a cheaper material,” says Prof. Manos Mavrikakis.

Fuel cells generate electricity by combining electrons and protons — provided by a chemical fuel such as methanol – with oxygen from the air. To make the reaction that generates protons faster, fuel cells typically contain catalysts. With the right catalyst and enough fuel and air, fuel cells could provide power very efficiently. Someday, fuel cells could make laptop batteries obsolete. Mere tablespoons of methanol could potentially provide up to 20 hours of continuous power. But alternatives to the expensive platinum catalyst in today’s fuel cells haven’t emerged because scientists still don’t fully understand the complicated chemistry required to produce protons and electrons from fuels.

At first glance, the chemistry sounds straightforward: Methanol molecules awash in a watery milieu settle down on a platinum surface and give up 1 of 4 H atoms. The movement of those electrons from that H atom make an electric current. The water molecules are not wallflowers, sitting on the sidelines of the methanol molecules reacting with platinum; rather, they occasionally cut in to the chemical dance. And varying voltage on the electrified surface of the platinum catalyst tangles the reaction’s tempo even further.

They first used density functional theory to solve for quantum mechanical forces and energies between individual atoms, then built a scheme upon those results using molecular dynamics methods to simulate large ensembles of water and methanol molecules interacting among themselves and with the platinum surface. The detailed simulations revealed that the presence of water plays a huge role in dictating which hydrogen atom breaks free from methanol first – a result that simpler methods could never have captured. Electric charge also determined the order in which methanol breaks down, surprisingly switching the preferred first step at the positive electrode.

This type of information enables scientists to predict which byproducts might accumulate in a reaction mixture, and select better ingredients for future fuel cells. “Modeling enables you to come up with an informed materials design,” says Mavrikakis, whose work was supported by the Department of Energy and the National Science Foundation. “We plan to investigate alternative fuels, and a range of promising and cheaper catalytic materials.” The results represent the culmination of six years of effort across two continents. http://news.wisc.edu/simulating-complex-catalysts-key-to-making-cheap-powerful-fuel-cells/