PTFE-bonded carbon electrodes

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. - " ... 

In 1970, J. E. Wynn (in Koscher et al. 2003) described a fuel cell that contains three electrodes, an anode, a cathode and a grid electrode (Fig. 13). The anode consisted of an unsintered Ag/PdO catalyst in a carbon/ polytetrafluoroethylene (PTFE) bonded structure. The cathode consisted of a platinized carbon/PTFE bonded structure (15% Pt by wt.). A mixture of 9 M KOH and 6 M methanol was used as the anolyte and 9 M KOH solution was used as the catholyte. The anolyte was separated from the catholyte by an anion exchange membrane. The third electrode, a platinum catalyzed nickel grid, was placed in-between the membrane and the cathode. The grid electrode was electrically connected to thecatbode through a small resistance. The three-electrode cell assembly was used to reduce the unwanted methanol oxidation at cathode. Any methanol that... 

Phosphoric Acid Fuel Cell This type of fuel cell was developed in response to the industiy s desire to expand the natural-gas market. The electrolyte is 93 to 98 percent phosphoric acid contained in a matrix of silicon carbide. The electrodes consist of finely divided platinum or platinum alloys supported on carbon black and bonded with PTFE latex. The latter provides enough hydrophobicity to the electrodes to prevent flooding of the structure by the electrolyte. The carbon support of the air elec trode is specially formulated for oxidation resistance at 473 K (392°F) in air and positive potentials. 

Pt-alloy catalysts, 40 132-133 PTFE-bonded carbon electrodes, 40 133-134 Pt microcrystal particle size on soot, 40 131-132... 

Morphology and Structure of Complete PTFE-Bonded Carbon Electrodes... 

FIG. 21. Cross section through a commercial Prototech electrode fabricated from a carbon fabric impregnated with PTFE- bonded, Pt-catalyzed soot particles. The layer of light color below the electrode is the SiC matrix, determined to keep the electrolyte between the electrodes. 

The PTFE-bonded electrode was introduced by Neidrach and Alford [11]. It consists of metal blacks or carbon supported metal catalysts that are hydrophilic, blended with fine particles of hydrophobic PTFE, that flow and bind the structure as a result of heat treatment during fabrication. Thanks to its physical properties, PTFE flows and penetrates the pores, thus allowing a good interfacial contact between catalyst and carbon and providing hydrophobic gas pores for reactants. 

Nowadays, the main type of H2 electrodes used in AFCs is a PTFE-bonded electrode with a Pt load of about 0.3 mg cm [12-14]. PAFCs employ H2-difiusion PTFE-bonded electrodes with Pt supported on carbon as catalyst for low loadings of O.l-l.O mg cm [15]. In contrast, PEFCs utilize H2 electrodes in which the catalyst (Pt/C) and the ionomer (Nafion ) are... 

The evolution of cell components from 1965 to the present day for PAFCs is summarized in Table 5-1. In the mid-1960s, the conventional porous electrodes were polytetrafluoroethylene (PTFE) - bonded Pt black, and the loadings were about 9 mg Pt/cm. During the past two decades, Pt supported on carbon black has replaced Pt black in porous PTFE-bonded electrode structures as the electro-catalyst. A dramatic reduction in Pt loading has also occurred the loadings are currently about 0.10 mg Pt/cm in the anode and about 0.50 mg Pt/cm in the cathode. 

Many pilot-scale cells have utilized fuel-cell type electrodes (Chapter 11), in particular, porous, PTFE-bonded carbon surfaces. Here, a dispersed carbon surface provides high electroactive area and the reaction zone is limited to the electrolyte side of the electrode Such electrodes are, however, prone to NaH02 precipitation which blocks the micropores, hindering flow distribution, lowering the mass transport and blinding active cathode area. 

Like the PEM fuel cell, the PAFC uses gas diffusion electrodes. In the mid-1960s, the porous electrodes used in the PAFC were PTFE-bonded Pt black, and the loadings were about 9 mg Pt cm on each electrode. Since then, Pt supported on carbon has replaced Pt black as the electrocatalyst, as for the PEMFC, as shown in Figure 4.6. The carbon is bonded with PTFE (about 30-50 wt%) to form an electrode-support structure. The carbon has important functions ... 

Oxygen reduction studies and cyclic voltammetry were carried out at ambient temperature (22 2°C) in 1 cm gas diffusion electrodes as previously described (8). The catalyst, mixed with ca. 50 mass % PTFE as a binder (and Nafion solution in one case), was spread on carbon fibre paper (CFP Toray TGPH090), which was then hot-bond to a Nafion 117 membrane. In the cell, a controlled flow of gas (O2, or N2 for cyclic voltammetry) was passed over the CFP and the membrane was in contact with a 1 M H2S04(aq) solution containing counter and reference (SCE) electrodes. 

---