Plasma sputtering deposition of platinum into porous fuel cell electrodes

Platinum is deposited into porous carbon materials relevant for fuel cell electrodes using plasma sputtering techniques. The resulting platinum concentration profile extends up to 2μm into the porous carbon and is well fitted by a generalized stretched Gaussian function, which displays the non-thermal nature of the penetration process. Platinum deposits are observed to grow as clusters. On the outermost carbon particles, platinum nano-cluster sizes of 3.5 nm have been measured. In tests using actual PEM fuel cells, current densities as high as 1000 mA cm−2 have been obtained at 400 mV with 25 cm2 plasma electrodes. This compares favourably with commercially available electrodes but the present electrodes have a platinum density 4.5 times lower and hence can be considered to be significantly more efficient. The proton exchange membrane fuel cell (PEMFC) is a promising power source due to its high energy conversion efficiency at relatively low operating temperatures (near 80 ̊C). Such power sources are being seriously considered for use on portable power sources for laptop computers and cellular phones, for transportation such as buses and cars, and for stationary applications like auxiliary power sources for domestic use in housing possibly in conjunction with gas turbines. One way of increasing fuel cell efficiency and reducing costs is to improve the electrode characteristics. In particular, a reduction of the amount of noble metal catalyst (typically platinum) and the way it is distributed in the gas diffusion electrodes would be of great value. Conventional electrodes are fabricated by coating a carbon cloth with an ink composed of carbon particles (carbon black), clusters of platinum catalyst and polytetrafluoroethylene (PTFE) particles. This coating is called the gas diffusion layer and is 50μm thick with a platinum catalyst content of 0.35 mg cm−2. Some attempts have been made to reduce the amount of catalyst [1] and one way is to simply expose the uncatalysed gas diffusion layer (figure 1) to a flux of platinum, which then diffuses into the gas diffusion layer to a sufficient depth so that adequate fuel cell performance is reached. A technique that meets these conditions is plasma sputtering, because plasma sputtering has the advantage of controlling the depth profile. Earlier experiments [2–8] have shown great promise for this technique, but a clear description of catalyst penetration into the uncatalysed diffusion electrodes and resulting concentration profiles remains to be established. In order to obtain more detailed information on the deposition process, a low pressure very high frequency (VHF) inductive plasma sputtering system has been constructed (APRIM VIDE) as displayed in figure 2. An argon plasma is created in the stainless steel deposition chamber of inner 0022-3727/04/243419+05$30.00 © 2004 IOP Publishing Ltd Printed in the UK 3419