Fuel cell electrode fabrication

Fuel cell
There are several kinds of fuel cells, and each operates a bit differently. But in general terms, hydrogen atoms enter a fuel cell at the anode where a chemical reaction strips them of their electrons and electricity is produced through a chemical reaction with oxygen or another oxidizing agent.
Fuel cell consists of an anode (negative side), a cathode (positive side) and an electrolyte that allows charges to move between the two sides of the fuel cell. Electrons are drawn from the anode to the cathode through an external circuit, producing direct current electricity. The reactions that produce electricity take place at the electrodes. The first generation of polymer electrolyte membrane fuel cells (PEMFC) used PTFE-bound Pt black electro catalysts that exhibited excellent long-term performance.
There are two methods of preparing PTFE bonded fuel cell electrodes, namely dry and wet methods depending upon the form of PTFE used.
Wet Method
In this method, aqueous PTFE emulsion is used. The procedure used is as follows: The depyrophorised Raney-Ni powder is first mixed with promoters (e.g. Cu2O). This mixture is added to PTFE suspensions. Isopropanol is added to stabilize the rubber like resultant mixture. During this mixing the suspension of PTFE breaks and water is removed. This paste is heated in order to evaporate some water and isopropanol resulting into a plastisizable mass. Cold rolling of this mass is done to obtain a felt of 0.2 to 0.5 mm thickness. Finally this felt is rolled with a nickel net which also works as a current collector. The cold rolling results into more linkage of catalyst particles with PTFE strands.
D-1 PTFE suspension containing 60% of PTFE, mixed with the catalyst, is milled for one hour at 20±20C. The paste is calendar rolled into 0.1 to 0.2 mm thick sheet. The surfactants are removed by boiling the sheet in acetone. Thus obtained layer is used as catalyst layer. Gas side layers are prepared by using nickel black powder blended with PTFE dispersion. These two layers are rolled with a mesh of stainless steel to get the final electrodes. In later, use of dopants like chromium and titanium improves and stabilizes the polarization characteristics of Raney nickel electrodes.
Dry Method
Use of dry PTFE powder in making fuel cell electrodes is relatively new. Dry PTFE powder with Raney nickel catalyst is used to prepare fuel cell hydrogen electrodes. In this method 5-8 wt% of PTFE is added to catalyst. The blend is milled into a high speed machine with sharp blades. It formed a network of PTFE treads and lumps with catalyst grains in between. The high speed blade milling of the catalyst particles with PFE leads to PTFE coating on the catalyst grains. As a result of this process which is known as reactive mixing, a fluffy mass of the powder is produced. The next step is the rolling of this fluffy PTFE-catalyst mixture into a calendar to form a tape. This tape is further rolled on to a wire mesh of nickel.