Microporous layer properties

Ilford is calling the microporous layer "nanoporous" since the particle and pore size in the layer they produce is weU below the micron level, with typical 20 nanometer particles while, practically, no particles are larger then 70 nanometers. The mineral oxides used in the porous products are surface treated in a proprietary process to balance physical properties such as brittleness and gloss with imaging properties such as color brilliance, layer transparency, and permanence. 

Double-layer properties of porous carbon materials have been widely investigated in relation to the development of the electrochemical capacitors. For detailed information the reader should consult specialized literature. For porous carbons materials, the double-layer capacitance depends on their specific snrface area [82,83], pore stmcture (notably, the pore size distribntion) [84-87], and their crystalline stmctnre and snrface chemistry [83,88,89], Shi [84] measnred the dc capacitance of varions carbons in a KOH electrolyte and noticed that the overall capacitance may reasonably be described as a sum of the capacitance of micro- and mesopores. Assuming that the electrical double layer propagates into micropores accessible for N2 adsorption, the author estimated the differential donble-layer capacitance per unit of micropore surface area as 15 to 20 p,F/cm. Lower values were reported by Vilinskaya... 

Although the physical and mathematical models as well as the various property-value expressions are taken from experimental results and basic physics, there is still a need to validate the approach taken. This has been done both qualitatively [70] and quantitatively [71], The qualitative validation involved comparing trends in terms of properties, and the quantitative validation involved using the membrane model in a simple pseudo 2-D fuel-cell model to explain and agree with experimental water-balance data. The model has also been validated in a more complicated fuel-cell model, such as the ones used to examine the effects of microporous layers in fuel cells [40]. However, such comparisons serve to validate the entire model and not necessarily just the one for the membrane. 

In order to improve the properties of the aforementioned GDLs, they can be further modified by different coatings. These are mainly treatment with polytetrafluour-ethylene (PTFE) and/or the deposition of a microporous layer (MPL) on the face at which the catalyst will be added [102]. 

CB, and in particular Vulcan XC-72, is the standard support material in fuel cell research. Therefore, it is hardly surprising that most recent publications do no longer focus on ways to describe, analyze, and optimize the support, but rather report on strategies to, for example, increase the dispersion of the catalytically active metal nanoparticles on the carbon surface by various treatments [43-47]. Apart from that, also the investigation of CB composites, such as CB blended with CNT [48] and CB/reduced graphene oxide [49], came into the focus of recent fuel cell research, as well as its application in the microporous layer (MPL) of the GDLs, which are not subject of this chapter [50, 51]. In addition, the comparison of low-surface-area Vulcan XC-72 with high-surface-area black pearls or Ketjen-black with respect to their electrochemical properties, capacitive behavior [52], and durability [53] has been a frequent subject of recent publications. 

Carbon is used extensively in fuel cells due to the favorable properties of high electrical and thermal conductivity, the relatively low cost, the absence of damaging degradation products when compared to metals, and the ability to form various structures from high surface area particles for the carbon support and in the microporous layer, to carbon or graphitic fibers for the gas diffusion substrate, to graphite or carbon-polymer composite flow field plates. Dicks provides an overview of the role of carbon in fuel cells [45]. 

In general, the phosphoric acid electrolyte is required to conduct protons in the membrane and a certain limited amount of the doping acid is needed to provide ionic conductivity in the catalyst layers. In the GDL (microporous layer and bulk) as well as in the bipolar plate, phosphoric acid is not required and even highly undesired because it compromises the gas transport properties in the GDL and may catalyze... 

Abstract The polymer electrolyte fuel cell (PEFC) consists of disparate porous media microstructures, e.g. catalyst layer, microporous layer, gas diffusion layer, as the key components for achieving the desired performance attributes. The microstmcture-transport interactions are of paramount importance to the performance and durability of the PEFC. In this chapter, a systematic description of the stochastic micro structure reconstmction techniques along with the numerical methods to estimate effective transport properties and to study the influence of the porous structures on the underlying transport behavior is presented. 

Akey performance limitation in the polymer electrolyte fuel cell (PEFC) originates from the multiple, coupled and competing, transport interactions in the constituent porous components. The suboptimal transport behavior resulting from the underlying complex and multifunctional microstmctures in the catalyst layer (CL), gas diffusion layer (GDL) and microporous layer (MPL) leads to water and thermal management issues and undesirable performance loss. Therefore, it is imperative to understand the profoimd influence of the disparate porous microstmctures on the transport characteristics. In this chapter, we highhght the stochastic microstmcture reconstmction technique and direct transport simulation in the CL, GDL and MPL porous stmctmes in order to estimate the effective transport properties and imderstand the microstmctural impact on the imderlying transport behavior in the PEFC. 

Cellulose acetate Loeb-Sourirajan reverse osmosis membranes were introduced commercially in the 1960s. Since then, many other polymers have been made into asymmetric membranes in attempts to improve membrane properties. In the reverse osmosis area, these attempts have had limited success, the only significant example being Du Font s polyamide membrane. For gas separation and ultrafUtration, a number of membranes with useful properties have been made. However, the early work on asymmetric membranes has spawned numerous other techniques in which a microporous membrane is used as a support to carry another thin, dense separating layer. 

The stmcture of activated carbon is best described as a twisted network of defective carbon layer planes, cross-linked by aHphatic bridging groups (6). X-ray diffraction patterns of activated carbon reveal that it is nongraphitic, remaining amorphous because the randomly cross-linked network inhibits reordering of the stmcture even when heated to 3000°C (7). This property of activated carbon contributes to its most unique feature, namely, the highly developed and accessible internal pore stmcture. The surface area, dimensions, and distribution of the pores depend on the precursor and on the conditions of carbonization and activation. Pore sizes are classified (8) by the International Union of Pure and AppHed Chemistry (lUPAC) as micropores (pore width <2 nm), mesopores (pore width 2—50 nm), and macropores (pore width >50 nm) (see Adsorption). 

Ghosh [548] used cellulose nitrate microporous filters (500 pm thick) as scaffold material to deposit octanol into the pores and then under controlled pressure conditions, displace some of the oil in the pores with water, creating a membrane with parallel oil and water pathways. This was thought to serve as a possible model for some of the properties of the outermost layer of skin, the stratum comeum. The relative proportions of the two types of channel could be controlled, and the properties of 5-10% water pore content were studied. Ibuprofen (lipophilic) and antipyr-ine (hydrophilic) were model drugs used. When the filter was filled entirely with water, the measured permeability of antipyrine was 69 (in 10 6 cm/s) when 90% of the pores were filled with octanol, the permeability decreased to 33 95% octanol content further decreased permeability to 23, and fully octanol-filled filters indicated 0.9 as the permeability. 

Aravamudhan, Rahman, and Bhansali. [70] developed a micro direct ethanol fuel cell with silicon diffusion layers. Each silicon substrate had a number of straight micropores or holes that were formed using microelec-tromechanical system (MEMS) fabrication techniques. The pores acted both as microcapillaries/wicking structures and as built-in fuel reservoirs. The capillary action of the microperforations pumps the fuel toward the reaction sites located at the CL. Again, the size and pattern of these perforations could be modified depending on the desired properties or parameters. Lee and Chuang [71] also used a silicon substrate and machined microperforations and microchannels on it in order to use it as the cathode diffusion layer and FF channel plate in a micro-PEMFC. 

The typical properties of some commercial microporous membranes are summarized in Table 4. Celgard 2730 and Celgard 2400 are single layer PE and PP separators, respectively, while Celgard 2320 and 2325 are trilayer separators of 20 and 25 fim thickness. Asahi and Tonen separators are single layer PE separators made by the wet process. Basic properties, such as thickness, gurley, porosity, melt temperature, and ionic resistivity are reported in Table 4. These properties are defined in section 6.1.3. 

To overcome the poor mechanical properties of polymer and gel polymer type electrolytes, microporous membranes impregnated with gel polymer electrolytes, such as PVdF. PVdF—HFP. and other gelling agents, have been developed as an electrolyte material for lithium batteries.Gel coated and/ or gel-filled separators have some characteristics that may be harder to achieve in the separator-free gel electrolytes. For example, they can offer much better protection against internal shorts when compared to gel electrolytes and can therefore help in reducing the overall thickness of the electrolyte layer. In addition the ability of some separators to shutdown... 

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