Mass transport of fuel

Comparison of current densities, and the potenhal for onset and for maximum current, is hampered when attempts are made, as will be undertaken in the following sechons, to report on results from diverse sources, as there is as yet poor adherence to normalization of such data to standard or accepted conditions. The variability in experimental approaches includes differences, often unspecified, in specific surface areas of the elechodes used, the control of mass transport of fuel/ oxygen to the surface, the coverage and activity of the enzyme on the surface, the stabihty of the system, the operational (and ophmal) condihons (pH, ionic strength. 

The steady state temperature of the catalyst surface under mass-transport-limited conditions can exceed the adiabatic flame temperature if the rate of mass transport of fuel to the surface is faster than the rate of heat transport from the surfaee. The ratio of mass diffusivity to heat dilTusivity in a gas is known as the Lewis number. Reactor models [9] show that for gases with a Lewis number close to unity, such as carbon monoxide and methane, the catalyst surface temperature jumps to the adiabatic flame temperature of the fuel/air mixture on ignition. However, for gases with a Lewis number significantly larger than unity the rate of mass transport to the surface is much faster than the rate of heat transport from the surface, and so the wall temperature can exceed the adiabatic gas temperature. The extreme case is... 

The complexity of the bio-electronic interface necessitates that material selection and electrode fabrication be understood to obtain an optimal result. In the design of an anode or a cathode, the materials should enhance stability of the immobilized biocatalyst and foster electron transfer (ET) between the biocatalyst and the conductive material. As a result, various materials and methodologies have been described in the literature to coordinate redox biocatalysts and electrodes [24,25]. In addition, several factors should be considered when selecting materials, including the electro-chemically accessible surface area (EASA), mechanical stability, and conductivity. The electrode architecture can be designed to enhance mass transport of fuel to the biocatalyst, and, in some instances, to include mediators to shuttle electrons between... 

The proper design and organization of hierarchically structured electrodes optimize the bioelectronic performance and enhance mass transport of fuel to the biocatalysts. By using nanostructured materials at the interface, the relative surface area for catalyst loading is increased compared with bulk materials and the distance between catalyst and the conductive surface may be decreased to enhance DET. Nanostructured modification or functionalization of 2D surfaces may enhance power output however, the essentially planar surface can limit scalability. 

The apphcation of porous, conductive scaffolds as components of anode architectures can enhance power production by three processes (1) transporting fuel from the bulk solution to the surface-confined biocatalysts, (2) increasing the amount of 3D surface area suitable for enzyme immobilization or biofilm formation, and (3) enhancing conductivity and, in turn, ET between the biocatalyst and the electrode. When considering electrode design, the support should be biocompatible and highly conductive at the same time. Additionally, the scaffold should have enough porosity to enhance mass transport of fuel. 

The field of enzyme immobilization, although mature, stiU falls short of a universal protocol for every enzyme. For BFC applications, this is particularly true. In the last 5 years, however, the toolbox of usable methodologies has certainly expanded and a number of reliable technologies now exist for BFC fabrication. Encapsulation of biocatalysts in conductive polymer, for example, would benefit from attempts to optimize mass transport of fuel. Stability of enzyme electrodes in nonphysiological environments will also become a cmx of EFC research, as advancements in fuel cell fabrication result in prototypes that can undergo reafistic operational field testing. Only then can the tme operational stabUity and the effects of temperature, salinity, pressure, pH, humidity, and so on be determined. Enzyme immobilization as a factor... 

While the mass production of fuel cell cars is some time away, if cost-competitive fuel cell stacks are available soon, it can change the competitive mix of transportation options. 

The experimental optimization of Nafion ionomer loading within a catalyst layer has attracted widespread attention in the fuel cell community, mainly due to its critical role in dictating the reaction sites and mass transport of reactants and products [15,128-134]. Nafion ionomer is a key component in the CL, helping to increase the three-phase reaction sites and platinum utilization to retain moisture, as well as to prevent membrane dehydration, especially at low current densities. Optimal Nafion content in the electrode is necessary to achieve high performance. 

For this type of fuel cell, a number of reports studying anode MPLs have been published. Neergat and Shukla [124] used a hydrophobic MPL on the cathode (carbon black and PTFE) and a hydrophilic MPL on the anode (carbon black and Nafion) (see Section 4.3.2). Different types of carbon particles were used (Vulcan XC-72, acetylene black, and Ketjenblack) and it was concluded that Ketjenblack was the carbon that showed the best performance when it was used on both the anode and cathode MPLs with 10 wt% Nafion and 10 wt% PTFE, respectively. A similar design was also used by Ren et al. [173] in a passive DMFC. Improvement of the DMFC performance by using a hydrophilic MPL, as discussed previously, was also demonstrated by Lindermeir et al. [125]. They compared both hydrophilic and hydrophobic MPLs for the anode DL, and it was observed that the former improves the mass transport of the MEA. 

One of the main parameters that would improve the overall performance of a fuel cell is better mass transport of reactants through the diffusion layer toward the active catalyst zones. In order to quantify and characterize how well the gas mass transport is in a specific DL material and design, it is important to measure the in-plane and through-plane permeabilities. Most of the published permeability results report the viscous permeability... 

Mass Transport at Very Low Concentrations. Reactor Circuits. Early in the development of water-cooled reactors, it became apparent that at temperatures of 250-300 C with a non-isothermal circuit, corrosion of carbon steel could lead to significant mass transport of iron if the chemistry of the system were not properly controlled. The resulting buildup of large deposits of crud" on fuel surfaces caused fuel failure. However, the large cost differential between carbon steel and stainless steel provided an incentive to identify chemistry conditions for the successful use of carbon steel. 

It is possible to make nonstoichiometric solids that have ionic conductivities as high as 0.1-1000 S m-1 (essentially the same as for liquid electrolytes) yet negligible electronic conductances. Such solid electrolytes are needed for high energy density electrical cells, fuel cells, and advanced batteries (Chapter 15), in which mass transport of ions between electrodes is necessary but internal leakage of electrons intended for the external circuit... 

Mass transport within the electrodes is of particular importance in determining the reflection of the porous media structure on the fuel cell performance. In fact, the main results of mass transport limitation is that the reactant concentrations (H2 and CO for the anode, and O2 for the cathode) at the reaction zone are lower than in the gas channel. When applying Equations (3.40) and (3.42), the result is that the lower the concentration of the reactants, the lower the calculated cell performance. The loss of voltage due to the mass transport of the gas within the electrodes is also referred to as concentration overpotential. Simplified approaches for determining concentration overpotential include the calculation of a limiting current, i.e. the maximum current obtainable due to mass transport limitation (cf. Appendix A3.2). 

Regardless of the specific type of fuel cell, gaseous fuels (usually hydrogen) and oxidants (usually ambient air) are continuously fed to the anode and the cathode, respectively. The gas streams of the reactants do not mix, since they are separated by the electrolyte. The electrochemical combustion of hydrogen, and the electrochemical reduction of oxygen, takes place at the surface of the electrodes, the porosities of which provide an extensive area for these reactions to be catalysed, as well as to facilitate the mass transport of the reactants/products to/from the electrolyte from/to the gas phase. 

Electrocatalysts One of the positive features of the supported electrocatalyst is that stable particle sizes in PAFCs and PEMFCs of the order of 2-3 nm can be achieved. These particles are in contact with the electrolyte, and since mass transport of the reactants occurs by spherical diffusion of low concentrations of the fuel-cell reactants (hydrogen and oxygen) through the electrolyte to the ultrafine electrocatalyst particles, the problems connected with diffusional limiting currents are minimized. There has to be good contact between the electrocatalyst particles and the carbon support to minimize ohmic losses and between the supported electrocatalysts and the electrolyte for the proton transport to the electrocatalyst particles and for the subsequent oxygen reduction reaction. This electrolyte network, in contact with the supported electrocatalyst in the active layer of the electrodes, has to be continuous up to the interface of the active layer with the electrolyte layer to minimize ohmic losses. 

Nanocasting can be considered as the best method to obtain a narrow pore size distribution. This is a very important feature because carbon supports with tunable properties allow the modeling of the system, in order to optimize the processes of methanol oxidation (adsorption and re-dissolution) and mass transport in fuel cells. 

Ordered mesoporous carbons (OMCs) offer most of all controllable pore sizes in a well-defined cubic or hexagonal arrangement and large pore volumes, which are advantageous in fuel cell operation, as they improve the mass transport of products and reactants [8]. They are obtained by using ordered mesoporous silica, such as MCM-41 and SBA-15, as hard templates. After their infiltration with a suitable carbon precursor and its subsequent carbonization, the template is removed and the templated carbon powder recovered (Figure 7.3). The pore structure of the carbon thus depends on the 3D porous structure of the template, with, for instance, CMK-3 carbon being an exact replica of SBA-15 silica [10]. 

A controlled porosity is of significant importance in fuel cell applications, since mass transport of the reactants to and the products away from the catalytically active sites plays an essential role in the device performance. It has been observed that the structural collapse of the electrode during durability tests impacts the performance of the fuel cell even more than the loss of active platinum particles [11], In addition to the tailored properties, which can be achieved by rational design of the template, another advantage is that the synthesis route is comparatively simple and not too expensive to be developed to industrial scale. 

The pores are for the transport of fuel cell reactants and product(s). Optimal porosity and pore size distribution can facilitate the mass transport process to minimize the fuel cell performance loss due to concentration overpotential. If some pores are more hydrophobic than others, what is the relative distribution Is the distribution of pore sizes and hydrophobicity within the allowable range ... 

Decay heating will raise the temperature of the fuel at rates that are usually less than 1 K/s. Eventually, a temperature is reached at which the exoergie reaction of steam with zireonium alloy cladding is limited only by the mass transport of steam to the elad. Fuel then experiences a temperature exeursion at rates of 20 K/s or more. Diffusion of radionuelides through solid and even liquid fuel produces a pronounced increase in the radionuelide release at these elevated temperatures. As diseussed further in Chapter III, only a fraction of the radionuclides released from the fuel successfully negotiates... 

Electrodes consisting of supported metal catalysts are used in electrosynthesis and electrochemical energy conversion devices (e.g., fuel cells). Nanometer-sized metal catalyst particles are typically impregnated into the porous structure of an sp -bonded carbon-support material. Typical carbon supports include chemically or physically activated carbon, carbon black, and graphitized carbons [186]. The primary role of the support is to provide a high surface area over which small metallic particles can be dispersed and stabilized. The porous support should also allow facile mass transport of reactants and products to and from the active sites [187]. Several properties of the support are critical porosity, pore size distribution, crush strength, surface chemistry, and microstructural and morphological stability [186]. 

Many statements and descriptions are plain and brief, but they are the crystallization of many years of my experience and research. You ll explore fundamental questions Will the catalyst particles be able to participate in the electrochemical reaction if they are fully covered by a thin ionomer film What can the limiting current density be based on the mass transport of air How severe will the voltage loss be if a thin layer of liquid water forms How can you quickly assess catalytic activity difference based on voltage difference in V-I curves How can a direct methanol fuel cell work using neat methanol How high can H2and O2 gases be pressurized within a PEM electrolyzer ... 

A combination of synergistic improvements in the catalyst, support, gas diffusion layers, membrane, and essentially the entire porous electrode structure in conjunction with bipolar plates/flow fields is expected to improve the mass-transport of reactant gases, protons, and water management. Thus, an increase in the peak current density (A/cm ) and peak power density (W/cm ) will result this in turn will lower the stack volume, the amount of catalyst, and membrane material used and raise the kW/L, kW/kg, and lower the /kW stack metrics. It should be noted that the rated or peak power for automotive stacks is based in part on maintaining an electrical efficiency of >50% this dictates that the cell voltage has to be maintained above 0.60 V. At this time, volumetric power densities of practical stacks in fuel ceU vehicles have been reported to be as high as 2 kW/L... 

In addition to the effect of support on the catalyst dispersion, the interaction with the ionomer (Nafion ) is crucial for the electrochemical performance of the catalyst layer in the fuel cell. Catalyst nanoparticles that are isolated from the ionomer network are electrochemically inactive. Furthermore, the distribution of the ionomer will affect the ohmic resistance and the mass transport of the reactants and/or products in the catalyst layer. Hence, the interface between the catalyst/support/ionomer will influence the overall polarization behavior of the anode. 

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