Carbon-supported membrane electrode blacks

Adjacent the ionomeric membrane on both sides are the catalyst layers (Fig. 1). As described above, these are platinum black/PTFE composites with high platinum loadings (typically 4 mg Pt/cm on each electrode) or composites of carbon-supported platinum and recast ionomer, with or without added PTFE, of much lower platinum loading (as low as 0.1 mg Pt/cm on each electrode). The electrochemical processes in the fuel cell take place at these electrocatalysts. In the hydrogen (or methanol reformate)/air fuel cell, the processes at the anode and cathode, respectively, are ... 

In this work, three membrane electrode assemblies (MEAs) with the same composition of the anode Pt/Ru black layer and different cathode layers were prepared. The cathode layer of the first MEA ( 1) was made using a Pt-black catalyst with 3.6 mg/cm Pt loading on the cathode side. The cathode layer of the second MEA ( 2) consisted of two layers the external Pt-black layer in contact with the gas diffusion layer (2.8 mg/cm ) and the internal Pt/C layer (0.3 mg/cm ) in contact with the membrane. The cathode layer of the third MEA ( 3) was made using carbon-supported Pt/C catalyst from Tanaka Inc. (1.3 mg/cm ). The anode layers were made in a similar way using Pt/Ru-black with 5.0 O.lmg/cm catalyst loading and 10 wt % of Nafion. 

The preparation of MEAs constimtes a vital part of fuel cell evaluation, with the performance of the fuel cell strongly dependent on the quality of the MEA prepared. The MEA consists of the PEM of a given thickness (usually between 25 and 200 pm), two electrodes made from Pt or Pt-Ru alloys (either as unsupported blacks or supported on carbon) combined with an ionomeric binder, and porous gas diffusion layers (GDLs) to facilitate reactant gas transport to the electrodes. The electrodes may be directly apphed on to the surface of the PEM, or may be applied on to the porous carbon gas diffusion layer and subsequently attached by hot-pressing on to the PEM. In the latter case, the combined electrode and GDL is termed a gas diffusion electrode. The presence of an ionomeric binder in the electrode is vital to ensure that proton transport from the reactive sites of the electrocatalyst to the membrane interface and vice versa proceeds with minimal resistive losses. In the interests of membrane electrode interfacial stability, it is advisable to use the same ionomeric material in the PEM and electrode. 

It is interesting to note that conventional carbon black supports promote the formation of peroxide, which then decomposes into radicals that attack the membrane. However, the role of graphitized carbon materials (such as CNTs) in peroxide formation is less clear. Smalley suggested that the curvy graphitic structure of CNTs deactivates free radicals by stabilizing them through enhanced delocalization. It would be worthwhile to determine whether the formation and fate of peroxide is any different between the carbon black and the CNT. At any rate, it is well known that the rate of formation of peroxide is greatly reduced by elimination of the carbon black support. Evidence of this is clear from the work we have done on carbonless electrodes (PTFE-bonded Pt black electrodes) and those with a hybrid structure. - " ... 

The PAFC works in a similar fashion to the PEM fuel cell described in Chapter 4. The PAFC uses a proton-conducting electrolyte, and the reactions occurring on the anode and cathode are those given in Figure 1.3. In the PAFC, the electrochemical reactions take place on highly dispersed electrocatalyst particles supported on carbon black. As with the PEM fuel cells, platinum (Pt) or Pt alloys are used as the catalyst at both electrodes. The electrolyte is an inorganic acid, concentrated phosphoric acid (100%) which, like the membranes in the PEM cells, will conduct protons. 

Electrodes used in PEM fuel cells typically employ an electrocatalyst layer with a porous, carbonaceous, electronically-conductive substrate that has been rendered hydrophobic. The catalyst layer usually comprises platinum, or a platinum-containing alloy, on a carbonaceous support (typically carbon black), dispersed ionomeric material similar in constituency to the electrolyte-membrane, and dispersed hydrophobic polymer such as polytetrafluoroethylene. The substrate, which serves as a reactant-gas diffusion layer, may be a carbon paper or a woven or non-woven cloth. The catalyst layer may be deposited directly onto the electrolyte-membrane or onto the substrate and later placed in contact with the membrane. 

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