Hydrophobic gas layer

The air gas-diffusion electrode developed in this laboratory [2] is a double-layer tablet comprising a porous hydrophobic gas layer and a catalytic layer in which the catalyst is placed. The gas layer of the electrode is prepared from hydrophobic material carbon black modified with PTFE by a special technology [3], The catalytic layer contains a porous catalyst. 

The hydrophobic gas layer of the air electrode [4] possesses high porosity (ca. 0,9 cm2/g), such that an effective oxygen supply through this layer is obtained. From the experimental porogrames measured by both mercury and 7 N KOH-porometiy the contact angle 0en of the hydrophobic material with water electrolytes is obtained (0eff =116° 118°). Because of... 

The investigation of the pore size distribution (Fig. 1) shows that nano-size pores (radius ca. 20 nm) predominate in the gas layer from this hydrophobic material. 

It is well known that Nafion ionomer contains both hydrophobic and hydrophilic domains. The former domain can facilitate gas transport through permeation, and the latter can facilitate proton transfer in the CL. In this new design, the catalyst loading can be further reduced to 0.04 mg/cm in an MEA [10,11]. However, an extra hydrophobic support layer is required. This thin, microporous GDL facilitates gas transport to the CL and prevents catalyst ink bleed into the GDL during applications. It contains both carbon and PTFE and functions as an electron conductor, a heat exchanger, a water removal wick, and a CL support. 

To summarize, to properly model liquid water transport and ensuing flooding effect on cell performance, one must consider four submodels (1) a model of catalytic surface coverage by liquid water inside the catalyst layer, (2) a model of liquid water transport through hydrophobic microporous layer and GDL, (3) an interfacial droplet model at the GDL surface, and last (4) a two-phase flow model in the gas channel. Both experimental and theoretical works, in academia and industry alike, are ongoing to build models for the four key steps of water generation, transport, and removal from a PEFC. 

Painted-on electrodes likewise need no binder. This is the type best suited for comparing various catalysts when working with gaseous reactants. The thin layer of the material and the direct contact between the catalyst particle and the gas phase at the hydrophobic covering layer ensures maximum utilization... 

Fig. 16. Schematic presentation of the morphological features of gas diffusion electrodes for fuel cells of (A) PTFE-bonded and Pt-activatcd Hi anodes and O2 cathodes used for Oi reduction in acidic and alkaline fuel cells (a) support, (b) hydrophobic gas diffusion layer, (c) hydrophilic electrode layer, (d) electrolyte, (e) magnified schematic of PTFE-bonded soot electrode, (f) adjacent hydrophobic layer, (g) microporous soot particles, (h) gas channels (mesopores), (k) PTFE particles, (I) flooded micro- and mesopores, (B) Schematic presentation of the morphology of PTFE-bonded Raney-nickel anodes used in alkaline fuel cells ol the Siemens technology.

A special kind of porous electrodes is gas diffusion electrodes which mostly have been used in fuel cells. A gas diffusion anode for hydrogen [68-73] may consist of three layers a metal current collector, a hydrophobic gas-porous layer, and a catalyst layer. The hydrophobic layer is often based on polytetrafluorethylene, which is made electric con-... 

A variation on simple solution casting is dip coating. In this technique, the sensor is dipped into a solution of the polymer for a length of time and removed. The same solution parameters hold for dip coating as for solution casting. Dip coating is typically used to coat an entire sensor with an outer protective layer. For example, hydrophobic gas-permeable membranes or interference barriers can be coated onto transducers using this technique [20]. 

To capture the dynamics of PEM fuel cell startup it is essential to include the dynamic mass balance for the water content in the membrane, TV . If water accumulation in the hydrophobic gas diffusion layer is neglected, then the water accumulation in the membrane is the difference between the water produced and the net water removed by convection from the anode and cathode, as given by Eq. (3.4). We measure all the quantities on the right-hand side of Eq. (3.4), so we can determine the accumulation rate of water. 

---