Catalyst layers cathode

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. ... 

One of the critical issues with regard to low temperamre fuel cells is the gradual loss of performance due to the degradation of the cathode catalyst layer under the harsh operating conditions, which mainly consist of two aspects electrochemical surface area (ECA) loss of the carbon-supported Pt nanoparticles and corrosion of the carbon support itself. Extensive studies of cathode catalyst layer degradation in phosphoric acid fuel cells (PAECs) have shown that ECA loss is mainly caused by three mechanisms ... 

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. 

In a PEM fuel cell, the CDLs are where the electrochemical reactions occur for electric power generation. For example, for H2/air (O2) PEM fuel cells, the reactions occurring at the anode and cathode catalyst layers are as follows ... 

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.)...

Efficient transport of protons from the anode catalyst layer to the cathode catalyst layer ... 

Xie, Z., Navessin, T, Shi, K., Chow, R., Wang, Q., Song, D., Andreaus, B., Eikerling, M., Liu, Z., and Holdcroft, S. Eunctionally graded cathode catalyst layers for polymer electrolyte fuel cells. Journal of the Electrochemical Society 2005 152 A1171-A1179. 

Kamarajugadda, S., and Mazumder, S. Numerical investigation of the effect of cathode catalyst layer structure and composition on polymer electrolyte membrane fuel cell performance. Journal of Power Sources 2008 183 629-642. Krishnan, L., Morris, E. A., and Eisman, G. A. Pt black polymer electrolyte-based membrane-based electrode revisited. Journal of the Electrochemical Society 2008 155 B869-B876. 

Kamarajugadda, S., and Mazumder, S. Numerical investigation of the effect of cathode catalyst layer structure and composition on polymer electrolyte membrane fuel cell performance. Journal of Power Sources 2008 183 629-642. 

Wang, G., Mukherjee, P P, and Wang, C. Y. Optimization of polymer electrolyte fuel cell cathode catalyst layers via direct numerical simulation modeling. Electrochimica Acta 2007 52 6367-6377. 

Figure 4.1 shows a schematic of a typical polymer electrolyte membrane fuel cell (PEMFC). A typical membrane electrode assembly (MEA) consists of a proton exchange membrane that is in contact with a cathode catalyst layer (CL) on one side and an anode CL on the other side they are sandwiched together between two diffusion layers (DLs). These layers are usually treated (coated) with a hydrophobic agent such as polytetrafluoroethylene (PTFE) in order to improve the water removal within the DL and the fuel cell. It is also common to have a catalyst-backing layer or microporous layer (MPL) between the CL and DL. Usually, bipolar plates with flow field (FF) channels are located on each side of the MFA in order to transport reactants to the... 

Influence of PTFE content in the anode DL of a DMFC. Operating conditions 90°C cell temperature anode at ambient pressure cathode at 2 bar pressure methanol concentration of 2 mol dm methanol flow rate of 0.84 cm min. The air flow rate was not specified there was a parallel flow field for both sides. The anode catalyst layer had 13 wt% PTFE, Pt 20 wt%, Ru 10 wt% on Vulcan XC-73R carbon TGP-H-090 with 10 wt% PTFE as cathode DL. The cathode catalyst layer had 13 wt% PTFE, Pt 10 wt% on carbon catalyst with a loading 1 mg cm Pt black with 10 wt% Nafion. The membrane was a Nafion 117. (Reprinted from K. Scott et al. Journal of Applied Electrochemistry 28 (1998) 1389-1397. With permission from Springer.)... 

One contradictory point regarding how the MPL works is related to the water saturation in the CL of the cathode. Nam and Kaviany [150] stated that using an MPL near the CL means that the water condensed in the DL carmot enter the CL, thus reducing the overall saturation of the active catalyst zones. This idea was also presented by Pasaogullari and Wang [151], who concluded that in the presence of an MPL, the liquid saturation in the CL is reduced substantially. These concepts contradict those presented earlier because it is not clear whether the liquid saturation does in fact increase in the cathode catalyst layer. This may depend directly on the rate at which the water goes back (or is forced) to the anode. 

W. Sun, B. A. Peppley, and K. Karan. Modeling the influence of GDL and flow-field plate parameters on the reaction distribution in the PEMFC cathode catalyst layer. Journal of Power Sources 144 (2005) 42-53. 

Concentrating on the operation of the so-called membrane electrode assembly (MEA), E includes irreversible voltage losses due to proton conduction in the PEM and voltage losses due to transport and activation of electrocatalytic processes involved in the oxygen reduction reaction (ORR) in the cathode catalyst layer (CCL) ... 

The catalyst layers (the cathode catalyst layer in particular) are the powerhouses of the cell. They are responsible for the electrocatalytic conversion of reactant fluxes into separate fluxes of electrons and protons (anode) and the recombination of these species with oxygen to form water (cathode). Catalyst layers include all species and all components that are relevant for fuel cell operation. They constitute the most competitive space in a PEFC. Fuel cell reactions are surface processes. A primary requirement is to provide a large, accessible surface area of the active catalyst, the so-called electrochemically active surface area (ECSA), with a minimal mass of the catalyst loaded into the structure. 

Similar to the above model, that of Ridge et al. ° examines the microstructure of the cathode catalyst layer in more detail. Their analysis is thorough and... 

Diffusion medium properties for the PEFC system were most recently reviewed by Mathias et al. The primary purpose of a diffusion medium or gas diffusion layer (GDL) is to provide lateral current collection from the catalyst layer to the current collecting lands as well as uniform gas distribution to the catalyst layer through diffusion. It must also facilitate the transport of water out of the catalyst layer. The latter function is usually fulfilled by adding a coating of hydrophobic polymer such as poly(tet-rafluoroethylene) (PTFE) to the GDL. The hydrophobic polymer allows the excess water in the cathode catalyst layer to be expelled from the cell by gas flow in the channels, thereby alleviating flooding. It is known that the electric conductivity of GDL is... 

In the PEFC system, the mean pore radii of catalyst layers are of the order of 0.1 pm. The Knudsen diffusion coefficients at 80 °C for O2 and H2O through the catalyst layer are thus estimated to be 0.32 and 0.43 cm /s, respectively. These values are comparable to the respective ordinary diffusion coefficients, indicating that Knudsen diffusion further restricts the rates of oxygen and water transport through the cathode catalyst layer in PEFCs and should be taken into account. 

The charge-transport equation includes the electrochemical kinetics for both anode and cathode catalyst layers. If we assume an infinitely large electric conductivity of the electronic phase, the electrode becomes an equipotential line, such that... 

The source term in the charge equation is used to describe the transfer current between the electronic and electrolyte phases inside of each anode and cathode catalyst layer. The transfer current densities are expressed as follows... 

These kinetic expressions represent the hydrogen oxidation reaction (HOR) in the anode catalyst layer and oxygen reduction reaction (ORR) in the cathode catalyst layer, respectively. These are simplified from the general Butler-Volmer kinetics, eq 5. The HOR... 

Specifically, Figure 16 shows that the current density in a cell with dry cathode gas feed drops nearly instantaneously once the cell voltage is relaxed from 0.6 to 0.7 V due to the fact that the electrochemical double-layer effect has a negligibly small time constant. Further, there exists undershoot in the current density as the oxygen concentration inside the cathode catalyst layer still remains low due to the larger consumption rate under 0.6 V. As the... 

As shown in Figure 23, the fully humidified case shows a maximum liquid saturation around 10% near the inlet and decreases in the flow direction due to decreasing reaction rate in the cathode catalyst layer. The 10% level of liquid saturation results from using a realistic GDL permeability on the order of 10 m2 67 Higher liquid saturation values reported in the literature were obtained only by using unrealistically small permeability, i.e., 95% maximum liquid saturation with GDL permeability of 7.3 x 10 m. °... 

Springer and Gottesfeld, Perry et al., and Eikerling and Kornyshev presented several analytical and numerical solutions for the cathode catalyst layer under various conditions. Perry et al. studied the effects of mass-transport limitations on the polarization characteristics of a reaction obeying... 

Figure 2.1 Schematic diagram of a DMFC, its electrode reactions and material transport involved, where (b) is the anode backing, (f) the cathode backing, (c) the Pt-Ru anode catalyst layer, (d) the Nafion 117 membrane and (e) the Pt cathode catalyst layer.

Equations (18-20) are discretized by the control volume method53 and solved numerically to obtain distributions of species (H2, 02, and N2) concentration, phase potential (solid and electrolyte), and the current resulting from each electrode reaction, in particular, carbon corrosion and oxygen evolution currents at the cathode catalyst layer, with the following initial and boundary conditions ... 

Knowing that 1 mg/cm2 of product water is a threshold, how much water can be stored at maximum within each component of the fuel cell, and how much can be removed to the outside For the cathode catalyst layer (CCL) with typical thickness of 10 p,m and 50% pore volume fraction, the CCL water storage capacity is approximately 0.5 mg/cm2. A 30- un-thick membrane can store 1.5 mg/cm2 of water, but its actual water storage capacity depends on the initial water content, A., and therefore is proportional to (ks.where A.sa, denotes the water content of a fully hydrated membrane. The escape of water into the GDL is unlikely due to the very low vapor pressure at cold-start temperatures (Pv>sa, = 40 Pa at —30°C). For reference, the GDL with 300 pm thickness and 50% porosity would store about 15 mg/cm2 of water, if it could be fully utilized. This capacity is too large to be used for cold start. From this simple estimation we can conclude that the CCL water storage capacity alone is not sufficient for successful cold start and that a successful strategy is to store water in the membrane. 

Ge and Wang also visualized the fuel cell cathode during cold start.6 Using a silver mesh as cathode GDL, they observed that when product water was less than 0.56 mg/cm2 water was not seen on the catalyst layer surface and that when it reached 1.12 mg/cm2 the liquid water emerged from the catalyst layer surface. This is consistent with the roughly estimated value of cathode catalyst layer water storage capacity of M).5 mg/cm2. They also estimated from their experiment that the freezing-point depression of water in the catalyst layer was at most 2°C. Then, they made the fuel cell optical... 

Wang15 investigated heat and mass transport and electrochemical kinetics in the cathode catalyst layer during cold start, and identified the key parameters characterizing cold-start performance. He found that the spatial variation of temperature was small under low current density cold start, and thereby developed the lumped thermal model. A dimensionless parameter, defined as the ratio of the time constant of cell warm-up to that of ice... 

Recently Meng17 developed a transient, multiphase, multidimensional PEFC model to elucidate the fundamental physics of cold start. The results showed the importance of water vapor concentration in the gas channels, which implies that large gas flow rates benefit cold-start performance. They also found that ice growth in the cathode catalyst layer during cold start was faster under the land than under the gas channels, and accumulated more at the interface between the cathode catalyst layer and GDL. 

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