Catalyst ink

The catalyst inks were prepared by dispersing the catalyst nanoparticles into an appropriate amoimt of Millipore water and 5wt% Nafion solution. Then, both the anode and cathode catalyst inks were directly painted using a direct painting technique onto either side of a Nafion 117 membrane. A carbon cloth diffusion layer was placed on to top of both the anode and cathode catalyst layers [3-5]. The active cell area was 2.25cm. ... 

In practice, the catal5Tic layers are prepared by brushing or spraying catalyst ink (a suspension of the catalyst particles in water and/or an organic solvent with addition of ionomer) either onto diffusion media (carbon paper or carbon cloth, also referred to as substrates), resulting in so-called catalyst-coated substrates (CCS), or directly onto... 

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]. 

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. 

Catalyst layer ink can be deposited on gas diffusion layers to form a CCGDL, as discussed in the previous section. Alternatively, the catalyst ink can be applied directly onto the proton exchange membrane to form a catalyst-coated membrane (CCM). The most obvious advantage of the CCM is better contact between the CL and the membrane, which can improve the ionic connection and produce a nonporous substrate, resulting in less isolated catalysts. The CCM can be classified simply as a conventional CCM or as a nanostructured thin-film CCM. 

Apply the catalyst ink uniformly onto the GDL surface using a metering rod. 

Add a bridge-builder and a peptization agent into this mixture, followed by 30 minutes of stirring to form the catalyst ink. 

Numerous efforfs have been made to improve existing fhin-film catalysts in order to prepare a CL with low Pt loading and high Pt utilization without sacrificing electiode performance. In fhin-film CL fabrication, fhe most common method is to prepare catalyst ink by mixing the Pt/C agglomerates with a solubilized polymer electrolyte such as Nation ionomer and then to apply this ink on a porous support or membrane using various methods. In this case, the CL always contains some inactive catalyst sites not available for fuel cell reactions because the electrochemical reaction is located only at the interface between the polymer electrolyte and the Pt catalyst where there is reactant access. 

Blend the formed mixture until the catalyst is uniformly disfribufed and the mixture has adequate viscosity for coating. This step produces a catalyst ink. 

Apply this catalyst ink onto one side of fhe membrane (Na+ fype). Two coats are typically required for adequate catalyst loading. 

Similar to screen printing, the spray coating method [95] is widely used for catalyst fabrication, especially in labs. The major difference between the two is that the viscosity of the ink for spray coating is much lower than that for screen printing. The application apparatus can be a manual spray gun or an auto-spraying system with programmed X-Y axes, movable robotic arm, an ink reservoir and supply loop, ink atomization, and a spray nozzle with adjustable flux and pressure. The catalyst ink can be coated on the gas diffusion layer or cast directly on the membrane. To prevent distortion and swelling of the membrane, either it is converted into Na+ form or a vacuum table is used to fix the membrane. The catalyst layer is dried in situ or put into an oven to remove the solvent. 

Other efforts have also been made to modify the CL microstructure by controlling the agglomerate size in the catalyst ink. Uchida et al. [145] proposed a colloidal ink fabrication procedure using low-dielectric-constant solvents to generate a good network and a uniformity of perfluorinated... 

Zhang, H., Wang, X., Zhang, J., and Zhang, J. Conventional catalyst ink, catalyst layer, and MEA preparation. In PEM fuel cell electrocatalysts and catalyst layers Fundamentals and applications, ed. J. Zhang. London Springer, 2008. 

Coarse-grained molecular d5mamics simulations in the presence of solvent provide insights into the effect of dispersion medium on microstructural properties of the catalyst layer. To explore the interaction of Nation and solvent in the catalyst ink mixture, simulations were performed in the presence of carbon/Pt particles, water, implicit polar solvent (with different dielectric constant e), and ionomer. Malek et al. developed the computational approach based on CGMD simulations in two steps. In the first step, groups of atoms of the distinct components were replaced by spherical beads with predefined subnanoscopic length scale. In the second step, parameters of renormalized interaction energies between the distinct beads were specified. 

Formation of a thin catalyst layer on the clean electrode surface through surface attachment, such as dropping catalyst ink onto the electrode surface. For either a monolayer or a multilayer of the catalyst on the electrode surface, the bare electrode can be inserted into the soaking solution containing the catalyst, and then taken out and rinsed with water. 

Figure 5.4. Nyquist plots obtained at a DC bias potential of 0.2 V versus Ag/AgCl for electrodes with various amounts of catalyst ink applied [2], (Reprinted from Electrochimica Acta, 50(12), Easton EB, Pickup PG. An electrochemical impedance spectroscopy study of fuel cell electrodes, 2469-7, 2005, with permission from Elsevier.)...

In Abaoud et al. s work [18], the details of using polyethylene oxide (PEO) as a suspension agent in the preparation of catalyst ink for PEMFC electrodes were... 

The second method for catalyzing the membranes is to cast the same type of ink (TBA" " form of the ionomer) directly onto the membrane [44]. This process may have an advantage over the decal process in the formation of a more intimate membrane/ electrode interface. It may also be more amenable to scale-up. Indeed, initial attempts at laboratory-scale automated application of thin-film Pt/C//ionomer catalyst layers to ionomeric membranes have been quite successful. In this work, a computer-controlled mechanism of an X-Y recorder was applied to paint catalyst ink by the controlled repetitive motion of the pen of the recorder onto each of the membrane major surfaces. In this way, 100 cm areas of catalyzed membranes were re-producibly generated, yielding performances per cm of a similar level to that achieved previously with catalyzed membrane of 5 cm active area [44]. The laboratory-scale automation equipment is shown in Fig. 22. 

Figure 24 shows a cross section of a Nafion membrane catalyzed by direct application of catalyst inks to its two major surfaces, as observed by SEM [52], The thin slice of MEA required for SEM imaging was generated by microtome from the MEA encapsulated in epoxy. This figure actually describes an MEA prepared for a DMFC, with PtRu black and Pt black catalyst layers of relatively high loading, resulting in catalyst layers 10 and 14 pm thick (Fig. 24). The SEM image well depicts two generic characteristics of CCMs prepared by direct, ink-based application of the catalysts to the ionomeric membrane the interface between the catalyst layer and the membrane is sharp on the SEM scale and the thickness of the catalyst layer measured from the...

Cyclic voltammetric studies indicated that the activity of the Pt-Ru films increased with operating temperature just as in conventional catalyst layers produced from unsupported catalyst inks. Membrane electrode assemblies were fabricated from Pt-Ru films of the most active compositions, and a power density of 800 mW/mg was realized for anodes that were deposited with about 0.1 mg/cm of Pt-Ru (see Figure 1). Applying the catalyst layers by sputter deposition on the electrode was found to yield better performance than applying them on the membrane. This was attributed to the enhanced electrical connectivity achieved when the catalyst layer is applied on the electrode. However, this is only true for very thin films. When thicker composite films are produced, such as those planned later in this project, good electrical connectivity may be achieved even with membrane deposition. 

Another work of the University (Ha et al.) of Illinois deals with metallic miniature FC structure [20]. They described the design and performance of a passive air breathing direct formic acid FC (DFAFC). The MEA was fabricated in house with catalyst inks directly painted on a Nafion -117 membrane. The current collectors were fabricated from Ti foils elec-trochemically coated with gold. The miniature cell at 8.8 M formic acid produced a maximum power density of 33 mW cm with pieces of gold mesh inserted between the current collector and the MEA on both sides of the MEA. With Pd black used as the catalyst at the anode side [21], they recently obtained a maximum power density of 177 mW cm" at 0.53 V for their passive DFAFC with 10 M formic acid. 

Catalyst A. A catalyst ink comprising 0.24 weight percent palladium and 0.2 weight percent polyvinyl alcohol was prepared by dissolving 80 g Pd(OAc), in a mixture of 1600 mL deionized water and 320 mL concentrated ammonium hydroxide. The palladium solution was added to a solution of polyvinyl alcohol (125,000 mw, 88 mol percent hydrolyzed) in deionized water to provide a catalyst Ink having a viscosity of about 20 cp. 

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