Catalyst layer electrodes

A fuel cell consists of an ion-conducting membrane (electrolyte) and two porous catalyst layers (electrodes) in contact with the membrane on either side. The hydrogen oxidation reaction at the anode of the fuel cell yields electrons, which are transported through an external circuit to reach the cathode. At the cathode, electrons are consumed in the oxygen reduction reaction. The circuit is completed by permeation of ions through the membrane. 

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],... 

MEAs used in this study were prepared in the following procedure [5]. The diffusion backing layers for anode and cathode were a Teflon-treated (20 wt. %) carbon paper (Toray 090, E-Tek) of 0.29 mm thickness. A thin diffusion layer was formed on top of the backing layer by spreading Vulcan XC-72 (85 wt. %) with PTFE (15 wt. %) for both anode and cathode. After the diffusion layers were sintered at a temperature of 360 C for 15 min., the catalyst layer was then formed with Pl/Ru (4 mg/cm ) and Nafion (1 mg/cm ) for anode and with Pt (4 mg/cm ) and Nafion (1 mg/cm ) for cathode. The prepared electrodes were placed either side of a pretreated Nafion 115 membrane and the assembly was hot-pressed at 85 kg/cm for 3 min. at 135 C. 

Catalyst layer architecture As a consequence of the diminishing remrns from ever higher dispersion, the effort to increase the active catalyst surface area per unit mass of Pt has centered in recent years primarily on optimization of catalyst layer properties, aiming to maximize catalyst utilization in fuel cell electrodes based on Pt catalyst particle sizes of 2-5 nm. High catalyst utilization is conditioned on access to the largest possible percentage of the total catalyst surface area embedded in a catalyst... 

The most recent improvements in Pt catalyst utilization U by optimization of catalyst layer composition and stmcture have led to catalyst utilizations as high as 80%, or more, determined as the ratio between measured ORR current per geometric square centimeter of electrode area and the current expected from the total measured Pt surface area per geometric square centimeter of the electrode, i.e.,... 

Yasuda K, Taniguchi A, Akita T, loroi T, Siroma Z. 2006a. Characteristics of a platinum black catalyst layer with regard to platinum dissolution phenomena in a membrane electrode assembly. J Electrochem Soc 153 A1599-A1603. 

Figure 13.9 Reaction scheme for Ci molecule oxidation on a Pt/C catalyst electrode, including reversible diffusion from the bulk electrolyte into the catalyst layer, (reversible) adsorption/ desorption of the reactants/products, and the actual surface reactions. The different original reactants (educts) and products are circled. For removal/addition of H, we do not distinguish between species adsorbed on the Pt surface and species transferred directly to neighboring water molecule (H d, H ) therefore, no charges are included (H, e ). For a description of the individual reaction steps, see the text.

Thin catalyst layers on a GC rotating disk electrode (RDE) or a rotating ring-disk electrode (RRDE) serve for studies of ORR kinetics. In order to separate the kinetic current from the measured current j, Schmidt and co-workers [Schmidt et al., 1998b] corrected the latter for the influence of oxygen diffusion in the aqueous electrolyte and in the polymer film using the foUowing equation ... 

Figure 15.3 Simulated effectiveness factor for porous carbon electrode as a function of the exchange current density jo and DCo for Ip] = 0.4 V for a 10wt% Pt/C catalyst layer with 7= 10, A = 140m g p = 2gcm, Nafion volume fraction 0.6, thickness p,m, and ionic conductivity 0.05 Scm See the text for details. (Reproduced from Gloaguen et al. [1994], with kind permission from Springer Science and Business Media.)...

Our recent our works show that even higher activity and stability can be demonstrated by the three-layer electrodes with nickel layer, active in the oxygen evolution, middle layer with catalyst, active in the oxygen reduction (Mn02, pyropolymer or a perovskite), and a diffusion (waterproof) layer,... 

The main components of a PEM fuel cell are the flow channels, gas diffusion layers, catalyst layers, and the electrolyte membrane. The respective electrodes are attached on opposing sides of the electrolyte membrane. Both electrodes are covered with diffusion layers, and the flow channels/current collectors. The flow channels collect current from the electrodes while providing the fuel or oxidant with access to the electrodes. The gas diffusion layer allows gases to diffuse to the electro-catalysts and provides electrical contact throughout the catalyst layers. Within the anode catalyst layer, the fuel (typically H2) is oxidized to produce electrons and protons. The electrons travel through an external circuit to produce electricity, while the protons pass through the proton conducting electrolyte membrane. Within the cathode catalyst layer, the electrons and protons recombine with the oxidant (usually 02) to produce water. 

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

The function of the electrolyte membrane is to facilitate transport of protons from anode to cathode and to serve as an effective barrier to reactant crossover. The electrodes host the electrochemical reactions within the catalyst layer and provide electronic conductivity, and pathways for reactant supply to the catalyst and removal of products from the catalyst [96], The GDL is a carbon paper of 0.2 0.5 mm thickness that provides rigidity and support to the membrane electrode assembly (MEA). It incorporates hydrophobic material that facilitates the product water drainage and prevents... 

Another approach has been developed to fabricate electrodes with loading as low as 0.1 mg Pt/cm (32). The electrode structure was improved by increasing the contact area between the electrolyte and the platinum clusters. The advantages of this approach are that a thinner catalyst layer of 2 to 3 microns and a uniform mix of catalyst and ionomer are produced. For example, a cell with a Pt loading of 0.17 to 0.13 mg/cm has been fabricated. The cell generated 3 A/cm at > 0.4V on pressurized O2 and 0.65 V at 1 A/cm on pressurized air (32,... 

There is considerable interest in extending PEFC technology to the direct methanol and formaldehyde electro-oxidation (34, 35). This requires Pt-based bi-metallic catalysts. Tests have been conducted with gas diffusion type Vulcan XC-72/Toray support electrodes with Pt/Sn (0.5 mg/cm, 8% Sn) and Pt/Ru (0.5 mg/cm, 50% Ru). The electrodes have Teflon content of 20% in the catalyst layer. 

The evaluation of catalysts typically uses two techniques. The first is evaluation as a thin layer on a bulk electrode (e.g., glassy carbon) in dilute liquid electrolyte (e.g., H2 4) either as a static electrode or an RDE. In the study of oxygen reduction, there has been much discussion as to the most appropriate electrolyte to use. In general, dilute perchloric acid (HCIOJ is preferred because of its noncoordinating nature, it is thus closest to the environment foxmd within a FEM catalyst layer with perfluorosulfonic acid ionomer. A possible alternative is trifluoromethylsulfonic acid (CF3SO3H), which mimics perfluorosulfonic acids closely, but there are relatively few studies with this acid. Rotating... 

Figure 2.1 shows a schematic structure of the fuel cell membrane electrode assembly (MEA), including both anode and cathode sides. Each side includes a catalyst layer and a gas diffusion layer. Between the two sides is a proton exchange membrane (PEM) conducting protons from the anode to the cathode. 

Schematic structure of a fuel ceU membrane electrode assembly (MEA), including both anode and cathode catalyst layers. (Based on Lister. S. and McLean, G. Journal of Power Sources 2004 130 61-76. With permission from Elsevier.)...

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.)... 

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. 

Cell voltage versus current density of the electrode with a Nation content of 1.0 mg/cm on the surface and various Nation contents inside the catalyst layer. (Reproduced from Lee, D. and Huang, S. International Journal of Hydrogen Energy 2008 33 2790-2794. With permission from the International Association of Hydrogen Energy.)... 

Brush 5 wt% Nafion solution onto the catalyst layer of this electrode. 

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