PTFE-bound catalyst layer

PTFE-Bound Catalyst Layer Fabrication Using... 

Two main types of catalyst layers are used in PEM fuel cells polyfefrafluo-roethylene (PTFE)-bound catalyst layers and thin-film catalyst layers [3]. The PTFE-bound CL is the earlier version, used mainly before 1990. If confains two components hydrophobic PTFE and Pt black catalyst or carbon-supported Pt catalyst. The PTFE acts as a binder holding the catalyst together to form a hydrophobic and structured porous matrix catalyst layer. This porous structure can simultaneously provide passages for reacfanf gas fransport to the catalyst surface and for wafer removal from fhe cafalysf layer. In fhe CL, the catalyst acts as both the reaction site and a medium for electron conduction. In the case of carbon-supported Pt catalysts, both carbon support and catalyst can act as electron conductors, but only Pt acts as the reaction site. 

Polarization curves for Hj/Oj fuel cells at 50°C, 1 atm pressure. Curve A Nation impregnated (brush coated) PTFE-bound electrode (0.35 mg/cm Pt loading) curve B PTFE-bound catalyst layer (Pt loading 4 mg/cm ) curve C PTFE-bound electrode (Pt loading 0.35 mg/cm. (Based on Ticianelli, E. A. et al. Journal of the Electrochemical Society 1988 135 2209-2214. By permission of The Electrochemical Society.)... 

Using a carbon-supported Pt catalyst to replace Pt black can reduce the platinum loading by a factor of 10—from 4 to 0.4 mg/cm [74]. However, the platinum utilization in this PTFE-bound catalyst layer still remains low in the vicinity of 20% [75,76]. 

The most common electrode design currently employed is the thin-film design, characterized by the thin Nafion film that binds carbon-supported catalyst particles. The thin Nafion layer provides the necessary proton transport in the catalyst layer. However, this is a significant improvement over the PTFE-bound catalyst layer, which requires the less effective impregnation of Nafion . Sputter deposited catalyst layers have been shown to provide some of the lowest catalyst loadings, as well as the thinnest layers. The short conduction distance of the thin sputtered layer dissipates the requirement of a proton-conducting medium, which can simplify production. The performance of the state of the art sputtered layer is only slightly lower than that of the present thin-film convention [125]. 

In these catalyst layers, the catalyst particles are bound by a hydrophobic PTFE structure commonly cast to the diffusion layer. This method was able to reduce the platinum loading of prior PEM fuel cells by a factor of 10 from 4 to 0.4 mg cm [126]. In order to provide ionic transport to the catalyst site, the PTFE-bound catalyst layers are typically impregnated with Nafion by brushing or spraying. Even though the platinum utiUzation in PTFE-bound catalyst layers was approximately... 

TicianeUi and co-workers [126] fabricated the low-platinum loading PEM fuel cells featuring PI PE-bound catalyst layers. Cheng and co-workers [128] developed conventional PI PE-bound catalyst layer electrodes for direct comparison with the current thin-film method. The typical process employed for forming the PTFE-bound catalyst layer MEA in their study is detailed in the following [125],... 

In his 1995 patent, Wilson and co-workers desalbed the thin-film technique for fabricating catalyst layers for PEM fuel cells with catalyst loadings <0.35 mg cm. In this method the hydrophobic PTFE employed to bind the catalyst layer is replaced with hydrophilic perfluorosulfonate ionomer (Nafion ). As a result, the binding material in the catalyst layer is composed of the same material as the membrane. Thin-film catalyst layers have been found to operate at almost twice the power density of PTFE-bound catalyst layers. This corresponds to an inaease in active area utilization from 22% to 45.4% when a Nafion -impregnated and PTFE-bound catalyst layer is replaced with a thin-fihn catalyst layer [128]. Moreover, thin-fihn MEA manufacturing techniques are more established and apphcable to stack fabrication [130]. However, utilization of 45% suggests that there is still significant potential for improvement. 

In PTFE-bound catalyst layers, the catalyst particles were bound by a hydrophobic PTFE structure commonly cast to the diffusion layer. In order to provide ionic transport to the catalyst site, the PTFE-... 

As discussed in Section 2.2, there are two main types of catalyst layers PTFE-bound CLs and thin-film catalyst layers. Because the latter are almost always used in current work, we will focus only on different types of thin-film CLs in the following sections. 

Thin-film catalyst layers are usually hydrophilic, with no hydrophobic ingredients added inside the CL. Although PTFE is generally unnecessary for thin-film catalyst layers, sometimes hydrophobicity maybe required for better transport in the CL. Zhang et al. [11] designed a dual-bound composite CL that contained... 

Zhang and Shi [36] found that the dual-bound composite catalyst layer exhibited higher performance than either a PTFE-bound CL or a thin-film CL, as shown in Figure 2.9. Optimization of the dual-bound CL showed that impregnation of Nation between the two layers could lead to decreased cell performance [37]. Thus, the optimal structure for a dual-bound CL was a separate hydrophilic layer on top of a hydrophobic layer. 

There are two widely employed electrode designs the PTFE-bound and thin-film electrodes. Emerging methods include those featuring catalyst layers formed with electrodeposition and vacuum deposition (sputtering) [125]. 

---