Fuel cell mass transport

Polymer Electrolyte Fuel Cells, Mass Transport... 

Polymer Electrolyte Fuel Cells, Mass Transport, Flg.l Accumulation of water in the flow channels can disturb the distribution of gas on the cell level channel to channel, see top) and on the stack level cell to cell, see bottom)... 

Polymer Electrolyte Fuel Cells, Mass Transport, Fig. 2 Mass transport through the cathode GDL, interconnecting the channel of the bipolar plate and the catalyst layer... 

Polymer Electrolyte Fuel Cells, Mass Transport, Fig. 4 Modeled catalyst layer cutout with a side length of 100 nm. The shown random structure includes 64 carbon particles with a diameter of 28 nm colored in blue. The ionomer coating has a thickness of 10 nm and is shown in colors from green to red... 

Charge transport in the electrolyte and mass transport in the gas phase are the dominant transport phenomena in fuel cells. Mass transport in gas mixtures is generally described by the Stefan-Maxwell equations ... 

As mentioned above, the operation of fuel cells strongly depends on processes that take place in very small regions with specific features, in the so-called catalyst layer (CL). This CL, a few tens of a micron thick, requires several features to facilitate and promote the main processes for the operation of fuel cells mass transport, electrons (negative electrical current) transfer and transport, ion transport, and even heat transport, not to mention other complex physical phenomena such as adsorption of reactants on catalyst surfaces, secondary reactions, etc. Mass transport is required as CLs are the place that both reactants and products of the electrochemical reactions in the fuel cell should reach or leave respectively. Anodic CLs need an adequate structure to allow the access of hydrogen gas (in the case of a PEMFC) and sometimes the exit of liquid water. The water comes from the cathode side due to its diffusion through the membrane or from... 

The choice of immobilization strategy obviously depends on the enzyme, electrode surface, and fuel properties, and on whether a mediator is required, and a wide range of strategies have been employed. Some general examples are represented in Fig. 17.4. Key goals are to stabilize the enzyme under fuel cell operating conditions and to optimize both electron transfer and the efficiency of fuel/oxidant mass transport. Here, we highlight a few approaches that have been particularly useful in electrocatalysis directed towards fuel cell applications. 

As can be seen from Table 1.57, the efficiency of fuel cells is about double that of gas-burning internal combustion (IC) engines. Therefore, if fuel cells can be made at the same cost as IC engines, hydrogen fuel cell-based transportation is already cost competitive with gasoline-based transportation. Unfortunately, at this point in time, fuel cells are much more expensive, but that is likely to change as soon as mass production starts. 

The HFP focuses on the acceleration and development for cost reduction necessary to competitively market fuel cells for transportation, stationary, and portable power applications. Their Hyways Project Roadmap aims for a mass market rollout by 2020 for European class vehicles with efficiencies of at least 40% on the New European Drive Cycle (NEDC) at a cost of 100 Euros kW and lifetimes of 5000 h for automobiles and 10,000 h for buses. The goals for stationary power systems mainly target residential power systems which the HFP hopes to see tangible market penetration by 2020. By 2009-2012, these systems are expected to maintain 34-40% electrical efficiency, a total fuel efficiency of 80%, >12,000 h of operation and cost less than 6000 Euro per system stack (Borup et al., 2007). 

Due to the considerable differences in reactant chemistries and testing conditions between the cells presented in Table 6.1, it is difficult to draw final conclusions about which specific device architecture may be optimal. Factors such as reactant species, reactant and electrolyte concentration, flow rate, temperature, separators, patterned electrodes, and the presence of catalysts can all significantly affect the power output of a device. Power density comparisons would ideally be made with devices benchmarked at standardized conditions for example, a liquid/liquid cell could be benchmarked at room temperature with standardized vanadium electrolytes (e.g., 1 M vanadium in 1 M sulfuric acid) and widely available porous carbon electrodes (e.g., Toray carbon paper). Conducting polarization curve and impedance measurements at a range of flow rates would enable full characterization of fuel utilization, mass transport, and ohmic losses which are inherent to the cell structure, and the peak volumetric power density measurements would then enable a direct comparison with other devices. 

Fuel cells are the subject of vast amounts of research and most experts now predict that by about 2020 they will be widely used for mass transportation. There are four major potential benefits to using fuel cell technology compared to more conventional sources of energy ... 

The transient response of DMFC is inherently slower and consequently the performance is worse than that of the hydrogen fuel cell, since the electrochemical oxidation kinetics of methanol are inherently slower due to intermediates formed during methanol oxidation [3]. Since the methanol solution should penetrate a diffusion layer toward the anode catalyst layer for oxidation, it is inevitable for the DMFC to experience the hi mass transport resistance. The carbon dioxide produced as the result of the oxidation reaction of methanol could also partly block the narrow flow path to be more difScult for the methanol to diflhise toward the catalyst. All these resistances and limitations can alter the cell characteristics and the power output when the cell is operated under variable load conditions. Especially when the DMFC stack is considered, the fluid dynamics inside the fuel cell stack is more complicated and so the transient stack performance could be more dependent of the variable load conditions. 

For isolating the overpotential of the working electrode, it is common practice to admit hydrogen to the counter-electrode (the anode in a PEMFC the cathode in a direct methanol fuel cell, DMFC) and create a so-called dynamic reference electrode. Furthermore, the overpotential comprises losses associated with sluggish electrochemical kinetics, as well as a concentration polarization related to hindered mass transport ... 

Summing up this section, we would like to note that understanding size effects in electrocatalysis requires the application of appropriate model systems that on the one hand represent the intrinsic properties of supported metal nanoparticles, such as small size and interaction with their support, and on the other allow straightforward separation between kinetic, ohmic, and mass transport (internal and external) losses and control of readsorption effects. This requirement is met, for example, by metal particles and nanoparticle arrays on flat nonporous supports. Their investigation allows unambiguous access to reaction kinetics and control of catalyst structure. However, in order to understand how catalysts will behave in the fuel cell environment, these studies must be complemented with GDE and MEA tests to account for the presence of aqueous electrolyte in model experiments. 

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