Thin-film catalyst layers

Wilson, M. S. and Gottesfeld, S., Thin film catalyst layers for polymer electrolyte fuel cell electrodes, J. Appl. Electrochem., 22, 1, 1992. 

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. 

To overcome these disadvantages, a thin-film CL technique was invented, which remains the most commonly used method in PEM fuel cells. Thin-film catalyst layers were initially used in the early 1990s by Los Alamos National Laboratory [6], Ballard, and Johnson-Matthey [7,8]. A thin-film catalyst layer is prepared from catalyst ink, consisting of uniformly distributed ionomer and catalyst. In these thin-film catalyst layers, the binding material is not PTFE but rather hydrophilic Nafion ionomer, which also provides proton conductive paths for the electrochemical reactions. It has been found that the presence of hydrophobic PTFE in thin catalyst layers was not beneficial to fuel cell performance [9]. 

Because thin-film catalyst layers are the most commonly used in today s PEM fuel cell technology, we will mainly focus in this sechon on the properties of the thin-film catalyst layer as well as its effect on fuel cell performance. 

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. 

There are two main types of thin-film catalyst layers catalyst-coated gas diffusion electrode (CCGDL), in which the CL is directly coated on a gas diffusion layer or microporous layer, and catalyst-coated membrane, in which the CL is directly coated on the proton exchange membrane. In the following sections, these catalyst layers will be further classified according to their composition and structure. 

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... 

Thin-Film Catalyst Layer Fabrication 2.4.2. t Ink-Based Catalyst Layer Fabrication... 

Fig. 25. Air cathode catalyst utilization for different types of catalyst layers in contact with ionomeric membranes. , Platinum black/PTFE (4 mg/cm ) ionomer-impregnated gas-diffusion electrodes (0.45 mg Pt/cm ) A, thin film of Pt/C//ionomer composite (0.13 mg Pt/cm ). The advantage of thin-film catalyst layers increases particularly at high current density (lower cell voltage) because transport limitations within the catalyst layer are minimized.

The thin-film catalyst layers are typically cast from inks consisting of the supported Pt catalyst and solubilized ionomer [10]. Because the ionomer must bind the thin film structure together, special treatments of the recast films are necessary during fabrication to impart robustness to... 

As seen in Fig. 22, ME As based on such thin-film catalyst layers can be constructed using a decal process, in which the ink is cast onto Teflon blanks for transfer to the membrane by hot-pressing. A second approach is to cast the same type of ink (TBA+ form of the ionomer) directly onto the membrane [12]. The latter process has an advantage over the decal process in... 

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].

Preparation of thin film catalyst layers from a catalyst dispersion in polymer electrolyte solution and other additives [26]. 

A concentration of the platinum loading close to the electrolyte interface via Pt-covered polymer nanofibers has been proposed by 3 M [31]. The concentration of the platinum catalyst coated onto nanofibers within approximately 300 nm distances from the electrolyte membrane surface, however, is leading to specific operation characteristics. While high power densities can be achieved under comparatively dry operating conditions even at elevated temperatures, the thin catalyst layers show a tendency for flooding under wet conditions at low temperatures. A summary of the behavior of this type of catalyst layers under specific operating conditions is given in [56]. The low temperature behavior has been improved by addition of a conventional catalyst layer on to this thin film catalyst layer [57]. 

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. 

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