A more efficient solid polymer fuel cell with a catalyst layer that includes a nanohorn aggregate as a catalyst carrier has been developed by inventors Sreekumar Kurungot (Aichi-ken, JP) and Hirokazu Ishimaru (Aichi-ken, JP).
A voltage higher than the open circuit voltage of the solid polymer fuel cell is applied to the catalyst layer prior to start up so as to increase triphasic interfaces, the region at which the reaction gas is reduced. It is where the catalyst layer, the catalyst metal, and the polymer electrolyte meet.
Surface groups are produced on the surface of the carbon nanohorn aggregate by treating the carbon nanohorn aggregate using an oxygenated water beforehand, and then the catalyst metal is dispersed on the surface of the carbon nanohorn aggregate, according to the inventors in U.S. Patent Application 20100015475.
In Kurungot's and Ishimaru's method, voltage higher than the open circuit voltage of the solid polymer fuel cell is applied to the catalyst layer before the fuel cell is started up. The phrase "before the solid polymer fuel cell is started up" encompasses the period from when the catalyst layer is formed to when the fuel cell is shipped out from the factory. That is, the activation voltage may be applied to the catalyst layer in the period from when the fuel cell is assembled to when it is shipped out from the factory.
U.S. Patent Application 20100015475, FIG. 2 illustrates how the state of contact between the polymer electrolytes and the catalyst on the catalyst carriers improves as the catalyst layer, in which carbon nanohorns are used as the catalyst carriers, is activated. As shown in the left side of FIG. 2, a deep contact between the catalyst and the polymer electrolytes is difficult due to their size mismatch. However, when voltage is applied, the network of the polymer electrolytes shrinks or structurally changes as shown in the right side of FIG. 2, whereby the contact between the catalyst and the polymer electrolytes improves.
More specifically, before applying voltage, the polymer electrolytes, due to their viscosity, are present only at some parts of the surface of each carbon nanohorn. However, when voltage is applied, the polymer electrolytes enter between carbon nanotubes constituting each carbon nanohorn, so that the polymer electrolytes sufficiently contact the catalyst supported deep inside of the carbon nanohorn and thus sufficient triphasic interfaces are formed therein.
FIG. 3 illustrates how the reaction gas permeability improves as the catalyst layer on each carbon nanohorn is activated through the voltage application according to the invention. When voltage is not applied to the catalyst layer, the reaction gas permeability between carbon nanohorns is low due to the presence of the network of the polymer electrolytes. However, when voltage is applied to the catalyst layers, the network of the polymer electrolyte breaks up, so that the reaction gas permeability increases.
FIG. 3 illustrates how the reaction gas permeability improves as the catalyst layer on each carbon nanohorn is activated through the voltage application according to the invention. When voltage is not applied to the catalyst layer, the reaction gas permeability between carbon nanohorns is low due to the presence of the network of the polymer electrolytes. However, when voltage is applied to the catalyst layers, the network of the polymer electrolyte breaks up, so that the reaction gas permeability increases.
