Catalyst layer microstructure

Figure 3. High resolution TEM image of a PEFC catalyst layer microstructure. Reproduced from Gostick et al.21 with permission from Karren L. More, Oak Ridge National Laboratory, USA.

Figure 4. Reconstructed catalyst layer microstructure along with pore and electrolyte phase volume fractions distribution.

Yang SC (2000) Modeling and simulation of steady-state polarization and impedance response of phosphoric acid fuel-cell cathodes with catalyst-layer microstructure consideration. J Electrochem Soc 147 71-77... 

Dobson, R, Lei, C., Navessin, T., and Secanell, M. 2012. Characterization of the PEM fuel cell catalyst layer microstructure by nonlinear least-squares parameter estimation. J. Electrochim. Soc... 159, B514-B523. 

Wang, Mukherjee, and Wang [124] investigated the effects of catalyst layer electrolyte and void phase fractions on fuel cell performance using a random microstructure. The model predicted volume fractions of 0.4 and 0.26 for void and electrolyte phases, respectively, as the optimal CL compositions. 

The microstructure of a catalyst layer is mainly determined by its composition and the fabrication method. Many attempts have been made to optimize pore size, pore distribution, and pore structure for better mass transport. Liu and Wang [141] found that a CL structure with a higher porosity near the GDL was beneficial for O2 transport and water removal. A CL with a stepwise porosity distribution, a higher porosity near the GDL, and a lower porosity near the membrane could perform better than one with a uniform porosity distribution. This pore structure led to better O2 distribution in the GL and extended the reaction zone toward the GDL side. The position of macropores also played an important role in proton conduction and oxygen transport within the CL, due to favorable proton and oxygen concentration conduction profiles. 

In general, we expect valuable insights for the advanced design of catalyst layers from understanding the microstructure of interconnected phases of... 

Microstructures of CLs vary depending on applicable solvenf, particle sizes of primary carbon powders, ionomer cluster size, temperafure, wetting properties of carbon materials, and composition of the CL ink. These factors determine the complex interactions between Pt/carbon particles, ionomer molecules, and solvent molecules, which control the catalyst layer formation process. The choice of a dispersion medium determines whefher fhe ionomer is to be found in solubilized, colloidal, or precipitated forms. This influences fhe microsfrucfure and fhe pore size disfribution of the CL. i It is vital to understand the conditions under which the ionomer is able to penetrate into primary pores inside agglomerates. Another challenge is to characterize the structure of the ionomer phase in the secondary void spaces between agglomerates and obtain the effective proton conductivity of the layer. 

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. 

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... 

The earliest models of fuel-cell catalyst layers are microscopic, single-pore models, because these models are amenable to analytic solutions. The original models were done for phosphoric-acid fuel cells. In these systems, the catalyst layer contains Teflon-coated pores for gas diffusion, with the rest of the electrode being flooded with the liquid electrolyte. The single-pore models, like all microscopic models, require a somewhat detailed microstructure of the layers. Hence, effective values for such parameters as diffusivity and conductivity are not used, since they involve averaging over the microstructure. 

Figure 36. Cathode voltage loss as predicted by direct numerical simulation of proton, oxygen, and water transport in a catalyst layer at the pore level (left), and three-dimensional oxygen concentration contours in a random microstructure of the catalyst layer (right).

Uchida, M. et ah. Effects of microstructure of carbon support in the catalyst layer on the performance of polymer-electrolyte fuel cells, J. Electrochem. Soc., 143, 2245, 1996. 

Fig. 45 Calculated 3-D contours of the oxygen concentration in a 10-pm thick cathode catalyst layer of a PEFC at 0.5 A cm-2 (a) and 1.5 A cm-2 (b) [94], The calculation was done using numerical reconstruction by a statistical method of the layer s microstructure.

To ensure good performance of a microstructured wall reactor, the thickness of the catalyst layer <5cat/inax must not exceed the value specified by Equation (25). 

Advances in the technology of microstructured catalytic reactors depend crucially on the ability to generate appropriate catalyst layers. The activity of the catalyst determines the thickness of the layer that needs to be deposited on the structured support or the walls of the MSR. Relatively thick layers of up to several hundred micrometers are necessary for moderate reaction rates to achieve good reactor performance, whereas thin layers are desirable for very fast catalytic reactions to avoid internal mass transfer limitations (Section 3.2.3). 

Aligned multiwall CNT arrays were synthesized as a basis for a microstructured catalyst, which was then tested in the Fischer-Tropsch reaction in a microchannel reactor [269]. Fabrication of such a structured catalyst first involved MOCVD of a thin but dense A1203 film on a FeCrAlY foam to enhance the adhesion between the catalyst and the metal substrate. Then, multiwall CNTs were deposited uniformly on the substrate by controlled catalytic decomposition of ethene. Coating the outer surfaces of the nanotube bundles with an active catalyst layer results in a unique hierarchical structure with small interstitial spaces between the carbon bundles. The microstructured catalyst was characterized by the excellent thermal conductivity inherent to CNTs, and heat could be efficiently removed from the catalytically active sites during the exothermic Fischer-Tropsch synthesis. 

The Jet Propulsion Laboratory (JPL) has researched the stated objectives by investigating sputter-deposition (SD) of designed anode and cathode nanostructures of Pt-alloys, and electronic structures and microstructures of sputter-deposited catalyst layers. JPL has used the information derived from these investigations to develop novel catalysts and membrane electrode assemblies (MEAs) that... 

Depending on the current density a rather non uniform utilization of the catalyst layer can be expected. While at low current density negligible effects are caused by the ionic resistance in the catalyst layer and almost all platinum particles can be used, the reaction concentrates close to the membrane interface at high current densities causing underutilization of the platinum present in the electrode [54, 55]. Optimization of electrode performance can be expected from microstructural optimization for example by designing catalyst layers having gradients in noble metal concentration and porosity. 

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