Water in catalyst layers

The MHM and the water balance model decouple when the liquid saturation is constant with this assumption, effective parameters of transport and reaction will be constant as well. This is the situation normally evaluated in CCL modeling. It will be considered next. Specific effects due to the complex coupling between porous morphology, liquid water formation, oxygen transport, and reaction rate distributions will be discussed in the section Water in Catalyst Layers The Watershed. ... 

When liquid water accumulates in cathode electrode, it can hinder the transportation of the reactant species by blocking the pores in the porous GDL. In addition, the active sites can be covered by liquid water in catalyst layer creating a barrier through which oxygen would have to diffuse. 

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. 

For typical catalyst layers impregnated with ionomer, sizes of hydrated ionomer domains that form during self-organization are of the order of 10 nm. The random distribution and tortuosity of ionomer domains and pores in catalyst layers require more complex approaches to account properly for bulk water transport and interfacial vaporization exchange. A useful approach for studying vaporization exchange in catalyst layers could be to exploit the analogy to electrical random resistor networks of... 

The use of hydrogen with heptane did not completely prevent the formation of a catalyst layer complex regardless of conditions, however, the complex which was obtained in its presence contained less hydrocarbon and was more active as indicated by the heat evolved by its reaction with water. The catalyst layer formed under nitrogen pressure was a brown tar. 

Cold start Shut-down strategy importance on freezing of process water on catalyst layer of MEA Results the degree of dryness in the stack significantly influences cold start-up ability, increasing dryness improves performance the optimal shut-down strategy allows start-up from -6 C without any performance loss, lower temperatures will see temporary performance loss Schiebwohl et ai, 2009... 

Cold start Product water absorbed in ionomer in catalyst layer, taken away as vapor in gas flow, and frozen into ice in catalyst layer pores Results increasing membrane thickness increases water capacity but decreases water absorption process, increasing ionomer volume fraction increases ionomer water capacity and enhances membrane water absorption cell start-up is better under potentiostatic condition than galvanostatic condition Jiao and Li, 2010... 

As described in this chapter, the physieal theory and molecular modeling of catalyst layers provide various tools for relating the global performance metrics to local distributions of physical parameters and to structural details of the complex composite media at the hierarchy of scales from nanoscale to macroscale. The subsisting challenges and recent advances in the major areas of theoretical catalyst layer research include (i) structure and reactivity of catalyst nanoparticles, (ii) selforganization phenomena in catalyst layers at the mesoscopic scale, (iii) effectiveness of current conversion in agglomerates of carbon/Pt, and (iv) interplay of porous structure, liquid water formation, and performance at the macroscopic scale. 

Jd — On Dt20 [134] Celtec -P 1000. Fictitious water diffusivity of 1 X 10 m s used. Effective water diffusivity in catalyst layer accounts for transport in the gas and membrane phase... 

This chapter provides a systematic account of the pertinent challenges and approaches in catalyst layer design. The hierarchy of structural effects and physical phenomena discussed includes materials design for high surface area and accessibility, statistical utilization of Pt evaluated on a per-atom basis, transport properties of charged species and neutral reactants in composite media with nano- to meso-porosity, local reaction conditions at internal interfaces in partially electrolyte-filled porous media, and global performance evaluated in terms of response functions for electrochemical performance and water handling. 

Coarse-grained molecular dynamics (CGMD) simulations have become a viable tool to unravel self-organization phenomena in complex materials and to analyze then-impact on physicochemical properties (Malek et al., 2007 Marrink et al., 2007 Peter and Kremer, 2009). Various MD simulations to study microstructure formation in catalyst layers will be discussed. The impact of structures obtained on pore surface wettability, water distribution, proton density distribution, and Pt effectiveness will be evaluated. 

Higher Pt loading causes a transition of the Pt/C surface from predominantly hydrophobic to predominantly hydrophilic. The change in surface wettability of Pt/C triggers a structural inversion in the ionomer film. This inversion of the orientation of the ionomer film has implications for catalyst utilization, density distribution and conductivity of protons, electrocatalytic activity, and water balance in catalyst layers. 

A viable approach to optimize the dispersion and structure of ionomer in catalyst layers is, thus, to make the surface of PPG agglomerates sufficiently hydrophilic, as to achieve the favorable orientation of the ionomer film, depicted on the left-hand side of Figure 3.41. Essentially, in this configuration, water exists where it is needed, at the interface with Pt. The ionomer structure is optimally utilized to provide high proton concentration in the interfacial water film. At the same time, the secondary pore space remains hydrophobic and thus water-free. 

The structural picture of ionomer in catalyst layers, unraveled in this section, suggests that extrapolation of bulk membrane properties in terms of water uptake, water binding, and proton transport skews specific properties of ionomer in CLs and is not generally feasible for the purpose of CL modeling. One needs to adapt mechanisms of water and proton transport to the thin-film ionomer morphology, where (i) proton transport is dominated by surface properties of ionomer and (ii) electrocatalytic properties are determined by the interfacial thin-film structure formed by Pt/C surface, ionomer film, and a thin intermediate water layer. 

The right-hand side of Equation 4.282 includes the dominant sources of heat in catalyst layers of low-temperature fuel cells. Note that part of the heat flux from the CCL is transported with liquid water produced in the ORR. Being not represented explicitly, this flux is taken into account in the equations of this section (see below). 

The extracted parameters are vital for rationalizing mechanisms and amounts of water fluxes in PEFCs. The model could be applied for the analysis of sorption data at varying PEM thickness and equilibrium water content. Experiments running at varying T would provide activation energies of the vaporization-exchange rate constant and bulk transport coefficients. Similar modeling tools can be developed for the study of water sorption and fluxes in catalyst layers. They can be extended, furthermore,... 

Local Water Balance Catalyst Layer Mass Balance In this section we discuss the local water balance, which can be (and usually is) highly nonhomogeneous throughout the fuel... 

Catalyst Layer Cracking and Delamination Catalyst layers are typically sprayed, deposited, or spread onto the electrolyte from a viscous mixture. This mixture is then baked at an elevated temperature, which drives off volatile compounds in the catalyst mixture used to control mixture viscosity and dispersion. As a result, small fissures, or mudcracks are common in catalyst layers, as shown in Figure 6.32, with widths much greater than the average pore size in a continuous portion of the catalyst layer. Over time, and as a result of the electrolyte expansion and contraction with water content variation, these cracks can grow and lead to delamination or catalyst layer degradation. 

In catalyst layers, the source term includes the heat released by the electrochemical reaction, heat generated due to ionic and electronic resistance, and heat of water evaporation (again, only if there is liquid water present, and the gas is not saturated) ... 

When catalyst layer is flooded, the active sites are covered by liquid water. Baschuk and Li employed R R-, R g to denote the resistance of oxygen in a partially flooded catalyst layer and derived the equivalent diffusion coefficient in catalyst layer as... 

The resistance of oxygen in catalyst layer R i R g varies with different degree of water flood, which indeed influence the diffusion coefficient. However, this effect on the cell voltage is relatively small. Due to the fact that the resistance caused by membrane fraction in the void region of the catalyst layer is so high, the added resistance caused by liquid water is nearly negligible. 

Khorasani, A.N. Molecular modeling of proton and water distribution in catalyst layer pores of polymer electrolyte fuel cells. Abstract MA2012-011059. 

Endo-exo product mixtures were isolated using the following procedure. A solution of cyclopentadiene (concentration 2-10" M in water and 0.4 M in oiganic solvents) and the dienophile (concentration 1-5 mM) in the appropriate solvent, eventually containing a 0.01 M concentration of catalyst, was stirred at 25 C until the UV-absorption of the dienophile had disappeared. The reaction mixture (diluted with water in the case of the organic solvents) was extracted with ether. The ether layer was washed with water and dried over sodium sulfate. After the evaporation of the ether the... 

In a typical procedure, a solution of 0.175 mmol of L- -amino acid and 0.175 mmol of NaOH in 1 ml of water was added to a solution of 0.100 mmol of Cu(N03)2in 100 ml of water in a 100 ml flask. Tire pH was adjusted to 6.0-6.5. The catalyst solution was cooled to 0 C and a solution of 1.0 mmol of 3.8c in a minimal amount of ethanol was added, together with 2.4 mmol of 3.9. The flask was sealed carefully. After 48 hours of stirring at 0 C the reaction mixture was extracted with ether, affording 3.10c in quantitative yield After evaporation of the ether from the water layer (rotary evaporator) the catalyst solution can be reused without a significant decrease in enantioselectivity. 

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