New class of catalyst for fuel cells beats pure
platinum by a mile.
Hydrogen fuel cells will power the automobiles of the future; however,
they have so far suffered from being insufficiently competitive. At the
University of Houston, Texas, USA, a team led by Peter Strasser has now
developed a new class of electrocatalyst that could help to improve the
capacity of fuel cells. The active phase of the catalyst consists of
nanoparticles with a platinum-rich shell and a core made of an alloy of
copper, cobalt, and platinum. This catalyst demonstrates the highest
activity yet observed for the reduction of oxygen.
Hydrogen fuel cells are a tamed version of the explosive reaction that
occurs between oxygen and hydrogen gases to form water. To allow the
reaction to proceed gently and the energy released to be tapped in the
form of an electrical current, the reactants are separated within the
fuel cell, and each half-reaction occurs in its own chamber. In one
half-cell, oxygen takes up electrons from an electrode (reduction); in
the other, hydrogen gas gives up electrons (oxidation). The cells are
linked by a polymer electrolyte membrane, across which exchange occurs.
To get the reaction to proceed, the electrodes must be catalytic. For
decades, the material of choice for the electrode in the oxygen
half-reaction has been the precious metal platinum. Now, Strasser and
his team have developed a new material, an alloy of platinum, copper,
and cobalt that is deposited onto carbon supports in the form of
nanoparticles. The active catalytic phase is formed in situ: when a
cyclic alternating current is applied to the electrode, the less
precious metals, especially the copper, on the surface of the
nanoparticles separate from the alloy. This process results in
nanoparticles with a core made of the original copper-rich alloy and a
shell containing almost exclusively platinum.
�The oxygen-reducing activity of our new electrocatalytic material is
unsurpassed - it is four to five times higher than that of pure platinum.
In addition, we have demonstrated how to incorporate and activate this
material in situ in a fuel cell,� says Strasser. The observed increase
in surface area of the nanoparticles is not enough to explain the
increased activity. Strasser suspects that special altered structural
characteristics of the surface play a role. Although the surface
consists mostly of platinum, the distances between the platinum atoms
on the particle surface seem to be shorter than those in pure platinum.
This compression can be stabilized by the alloy core, which shows even
shorter Pt-Pt distances because of the presence of copper and cobalt.
In addition, the copper-rich core seems to influence the electronic
properties of the platinum shell. Theoretical calculations have
suggested that the oxygen can thus bind optimally to the particle
surface, allowing it to be more easily reduced.