Hydrogen PEFCs catalyst layer

Optimization of the catalyst layer composition and thickness in PEFCs for maximum catalyst utilization in operation on air and on impure hydrogen feed streams [Wilson, 1993 Springer et al., 1993]. 

Polymer electrolyte fuel cell (PEFC) is considered as one of the most promising power sources for futurist s hydrogen economy. As shown in Fig. 1, operation of a Nation-based PEFC is dictated by transport processes and electrochemical reactions at cat-alyst/polymer electrolyte interfaces and transport processes in the polymer electrolyte membrane (PEM), in the catalyst layers consisting of precious metal (Pt or Ru) catalysts on porous carbon support and polymer electrolyte clusters, in gas diffusion layers (GDLs), and in flow channels. Specifically, oxidants, fuel, and reaction products flow in channels of millimeter scale and diffuse in GDL with a structure of micrometer scale. Nation, a sulfonic acid tetrafluorethy-lene copolymer and the most commonly used polymer electrolyte, consists of nanoscale hydrophobic domains and proton conducting hydrophilic domains with a scale of 2-5 nm. The diffusivities of the reactants (02, H2, and methanol) and reaction products (water and C02) in Nation and proton conductivity of Nation strongly depend on the nanostructures and their responses to the presence of water. Polymer electrolyte clusters in the catalyst layers also play a critical... 

The high theoretical efficiency of a fuel cell is substantially reduced by the finite rate of dynamic processes at various locations in the cell. Substantial efficiency losses at typical operating temperatures occur already in the anodic and cathodic catalyst layers due to the low intrinsic reaction rates of the oxygen reduction and, in the case of the DMFC, of the methanol oxidation reaction. (The catalytic oxidation of hydrogen with platinum catalysts is very fast and thus does not limit PEFC performance.) In addition, at low temperatures, turnover may be limited by noble metal catalyst poisoning due to sulfur... 

Consider the cathode channel (similar arguments are applicable to the anode channel of DMFC or hydrogen PEFC). For simplicity we assume that (1) the catalyst layer is thin enough, so that there are no voltage losses associated with proton transport across the layer and (2) the diffusion losses of oxygen in the backing and catalyst layers are negligible. 

Fig. 20 Distributions at current density of 1 A cm 2, of electrode potential (top), reactant concentration (middle), and current generation (bottom) in a PEFC anode catalyst layer 5 pm thick, as result of limited transport rate of the hydrogen gas reactant and/or the limited transport rate of protons. Two cases of reactant concentration, 100% hydrogen and 10% hydrogen in the dry gas and two cases of effective protonic conductivity in the catalyst layer, 0.1 and 0.01 S cm-1, are considered in these calculations. A value of 2 x 10-4 cm2 sec-1 was used as estimate for effective Dh2 in the catalyst layer.

Fig. 23 Air cathode catalyst mass utilization (A mg-1 Pt) for different types of catalyst layers as developed chronologically for hydrogen/air PEFC. Squares PTFE-bonded Pt black at 4 mg Pt/cm2 circles ionomer-impregnated, PA- type electrodes (0.45 mg Pt/cm2) triangles thin-film Pt/C//ionomer composite (0.13 mg Pt/cm2). The relative advantage of thin-film catalyst layers is seen to increase with cell current density, as expected from the lower transport limitations involved (see Sect. 8.3.7.2.3) [10,11].

Fig. 27 Hydrogen/air PEFC performance at ambient pressure of both fuel and air, achieved at total Pt loading of only 0.12 mg Pt/cm2 with an MEA based on an ultrathin catalyst layer of the type described in Figs 24 and 25 [53b].

The influence of CO poisoning at the anode of an HT-PEFC was investigated by Bergmann et ul. [28]. The dynamic, nonisothermal model takes the catalyst layer as a two-dimensional plane between the membrane and gas diffusion layer into account. The effects of CO and hydrogen adsorption with respect to temperature and time are discussed in detail. The CO poisoning is analyzed with polarization curves for different CO concentrations and dynamic CO pulses. The analysis of fuel-cell performance under the influence of CO shows a nonlinear behavior. The presence of water at the anode is explicitly considered to take part in the electrooxidation of CO. The investigation of the current response to a CO pulse of 1.31% at the anode inlet showed a reversible recovery time of 20 min. 

One of the main challenges for the commercialization of PEFCs is the long-term stability. As was explained above, under fuel cell operation, hydrogen is oxidized at the anode while oxygen is reduced at cathode catalyst layer. 

In PEFCs the cathode side makes the largest contribution to voltage loss. This explains the great interest in CCL performance in these cells. We begin the analysis of CL performance with the cathode catalyst layer of a low-temperature hydrogen fuel cell. However, it should be emphasized that the performance of other catalyst layers of the cells considered in this book can be described by similar equations. The features of particular layers are taken into account by the expression for the rate of the electrochemical reaction. 

So far in this section, the system (5.85) through (5.87) has been discussed in the context of the CCL. However, exactly the same system of equations describes the impedance of the anode catalyst layer. The case of e 1 is characteristic of the PEFC anode, while e > 1 is typical of the PEFC cathode. Small e is typical for the PEFC anode because of the high exchange current density. Thus, the solutions above allow estimating the differential resistivity of the hydrogen anode CL, Raci-... 

FIGURE 5.33 The three fnel cells in a PEFC with the oxygen in the second half of the anode channel (cf. Figure 5.30). HOR, ORR, and CCR stand for the hydrogen oxidation, oxygen reduction, and carbon corrosion reactions, while the CL abbreviates the catalyst layer. Arrows indicate trajectories of proton (fdled circles) transport. The dashed line is the shape of the membrane phase potential. 

COMMENTS Typical PEFCs have a range of about 1-10 mA/cm hydrogen crossover. Also, there is some additional resistance from the ionomer in the catalyst layers which was not accounted for here. We could account for this by assuming an equivalent ionomer thickness in the catalyst layer based on the ionomer content. 

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