CL, proton conductivity

Table 23.1 lists the typical parameters and critical current densities for the three electrodes. As can be seen, for the polymer electrolyte membrane fuel cell (PEMFC) cathode, fait is large and under a typical current density of about 1 A cm the cathode works in the Tafel regime. However, if the CL proton conductivity were to decrease by a factor of 3, jcnt would be three times lower and the CL would enter the transition region, in which the polarization voltage increases. 

Following Eikerling and Kornyshev (1998) and Wang et al. (2004), the CL proton conductivity is taken to be proportional to the square of the ionomer content therefore, Equation 4.267 contains the factor on the right-hand side. It can be assumed that the exchange current density is proportional to the catalyst surface, so that g(x) represents the normalized shape of the catalyst loading through the electrode depth. Thus, the functions g and p must obey the normalization condition... 

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]. 

Typically, Nation ionomer is the predominant additive in the catalyst layer. However, other types of CLs with various hygroscopic or proton conductor additives have also been developed for fuel cells operafed xmder low relative humidity (RH) and/or at elevated temperatures. Many studies have reported the use of hygroscopic y-Al203 [52] and silica [53,54] in the CE to improve the water retention capacity and make such CEs viable for operation af lower relative humidity and/or elevated temperature. Alternatively, proton conducting materials such as ZrP [55] or heteropoly acid HEA [56] have also been added... 

The fabrication of catalyst layers for PEM fuel cells involves maintaining a delicate balance between gas and water transport, and electron and proton conduction. The process of CL fabrication should be guided by both fuel cell performance and cost reduction. 

An effective catalyst layer must serve multiple functions simultaneously electron and proton conduction, oxygen or hydrogen supply, and water management. The composition and structure of a CL can affecf all fhese functions... 

The catalyst layer is composed of multiple components, primarily Nafion ion-omer and carbon-supported catalyst particles. The composition governs the macro- and mesostructures of the CL, which in turn have a significant influence on the effective properties of the CL and consequently the overall fuel cell performance. There is a trade-off between ionomer and catalyst loadings for optimum performance. For example, increased Nafion ionomer confenf can improve proton conduction, but the porous channels for reactanf gas fransfer and water removal are reduced. On the other hand, increased Pt loading can enhance the electrochemical reaction rate, and also increase the catalyst layer thickness. 

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. 

The catalyst layer usually consists of carbon-supported catalyst or carbon black mixed with PIPE and/or proton-conducting ionomer (e.g.. Nation iono-mer). Because the sizes of the pores in a t) ical DL are in the range of 1-100 pm and the average pore size of the CL is just a few hundred nanometers, the risk of having low electrical contact between both layers is high [129]. Thus, the MPL is also used to block the catalyst particles and does not let them clog the pores within the diffusion layer [57,90,132,133]. 

A poorly balanced water distribution in the fuel cell can severely impair its performance and cause long-term effects due to structural degradation. If PEMs or CLs are too dry, proton conductivity will be poor, potentially leading to excessive joule heating, which could affect the structural integrity of the cell. Too much water in diffusion media (CLs and GDLs) blocks the gaseous supply of reactants. As these examples show, all processes in PEECs are linked to water distribution and the balance of water fluxes. 

Impregnating these layers with PFSA ionomer for enhanced proton conduction or hydrophobizing agents like Teflon for sufficient gas porosity is optional. However, ionomer impregnation is indispensable in CLs with thicknesses of > 1 ftm. Ultrathin CLs with - 100-200 nm, on the other hand, can operate well without these additional components, based on sufficiently high rates of transport of dissolved reactant molecules and protons in liquid water, which could ensure uniform reaction rate distributions over the entire thickness of the layer. 

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. 

Water content affects many processes within a fuel cell and must be properly managed. Proton conductivity within the polymer electrolyte typically decreases dramatically with decreasing water content (especially for perfhiorinated membranes such as Nation ), while excessive liquid water in the catalyst layers (CLs) and gas diffusion layers (GDLs) results in flooding, which inhibits reactant access to the catalyst sites. Water management is complicated by several types of water transport, such as production of water from the cathode reaction, evaporation, and condensation at each electrode, osmotic drag of water molecules from anode to cathode by... 

Proper water management in proton exchange membrane fuel cells (PEMFCs) is critical to PEMFC performance and durability. PEMFC performance is impaired if the membrane has insufficient water for proton conduction or if the open pore space of the gas diffusion layer (GDL) and catalyst layer (CL) or the gas flow channels becomes saturated with liquid water, there is a reduction in reactant flow to the active catalyst sites. PEMFC durability is reduced if water is left in the CL during freeze/thaw cycling which can result in CL or GDL separation from the membrane,1 and excess water in contact with the membrane can result in accelerated membrane thinning.2... 

A typical PEFC, shown schematically in Fig. 1, consists of the anode and cathode compartments, separated by a proton conducting polymeric membrane. The anode and cathode sides each comprises of gas channel, gas diffusion layer (GDL) and catalyst layer (CL). Despite tremendous recent progress in enhancing the overall cell performance, a pivotal performance/durability limitation in PEFCs centers on liquid water transport and resulting flooding in the constituent components.1,2 Liquid water blocks the porous pathways in the CL and GDL thus causing hindered oxygen transport to the... 

S0l., So2 and SHlo refer to the respective source terms owing to the ORR, e is the electrolyte phase potential, cGl is the oxygen concentration and cHlo is the water vapor concentration, Ke is the proton conductivity duly modified w.r.t. to the actual electrolyte volume fraction, Dsa is the oxygen diffusivity and is the vapor diffusivity. The details about the DNS model for pore-scale description of species and charge transport in the CL microstructure along with its capability of discerning the compositional influence on the CL performance as well as local overpotential and reaction current distributions are furnished in our work.25 27,67... 

MEA performance is mainly limited by ORR kinetics, as well as oxygen transport to the cathode catalyst. Another major loss is due to proton conduction, in both the membrane and the cathode catalyst layer (CL). Characterization of the ionic resistance of fuel cell electrodes helps provide important information on electrode structure optimization, and quantification of the ionomer degradation in the electrodes [23],... 

Fig. 1.2. The idealized bacteriorhodopsin liposome containing a light-driven proton pump in a membrane with some proton, K, Cl, HCl conductance and allowing some exchange.

Be compatible with proton-conducting polymers so that composites containing a catalyst and a proton conductor provide efficient proton transport through the CLs... 

It shonld be noted that high utilization factors measnred with cyclic voltammetry by no means warrant the assnmption that nnder dynamic conditions of fnel cell operation the CLs deliver the same cnrrent as they wonld without mass transport and ohmic constraints. To acconnt for the latter, Gloagnen et al. [185] employed the effectiveness factor the ratio of the actnal reaction rate to the rate expected in the absence of mass and ionic transport limitations. The effectiveness factor is a fnnction of the total catalyst area, the exchange cnrrent density, the overpotential, the diffusion coefficient D, the concentration of electroactive species Co, the thickness of the CL, and the proton conductivity of the electrolyte, and drops sharply below 100% with increased exchange current density and decreased the product DCq. 

Fuel cell CLs are the key components in the entire fuel cell device because the reactions such as hydrogen—oxidation reaction (HOR) at anode and the ORR at cathode occur inside the CLs. Particularly, in order to carry out the ORR, the catalyst particles inside the cathode CL must be in contact with each other for electrical conductivity and also in contact with protonic conducting (in acidic PEM fuel cells), or hydroxide conducting (in alkaline PEM fuel cells) ionomer for ionic conductivity. In addition, there must be some channels within the CL for transporting the reactants and the products. In other words, the catalyst particles must be in close contact with each other, with the electrolyte, and also with the adjacent diffusion medium (DM). Moreover, the reactants gas O2) and the produced water travel mainly through the voids, so the CL must be porous enough to allow gas to diffuse to the reaction sites and liquid water to wick out. 

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