Fuel cell mass requirements

A comprehensive analysis of solid oxide fuel cells phenomena requires an effective multidisciplinary approach. Chemical reactions, electrical conduction, ionic conduction, gas phase mass transport, and heat transfer take place simultaneously and are tightly coupled. 

The ohmic loss is relatively easy to understand because the electrical resistance of the cell components behaves as a cause of voltage loss. However, determination of overpotential from the electrochemical reaction resistance at the electrodes has been an interesting research topic. The fuel cell electrodes require a large surface area to increase the reaction rate, and thus porous materials are employed. In addition, the electrode surface is covered by thin electrolyte film to provide the three-phase boundary of gas-liquid-solid where the electrochemical reaction occurs. Thus, the electrochemical resistance in MCFC is comprised of charge-transfer resistance on the electrode surface and mass transfer through the liquid film and gas channel as shown in Fig. 8.2. 

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 gas diffusion electrode components and electrode structures in HT-PEM fuel cells should differ from those in LT-PEM fuel cells. Because water flooding is not a problem in HT-PEM fuel cells, the requirements for mass transport of the fuel and oxidant within the bipolar plate channels and gas diffusion electrodes may not be as critical as for LT-PEM fuel cells. 

Fuel cell mass balance requires that the sum of all mass inputs must be equal to the sum of all mass outputs. The inputs are the flows of fuel and oxidant plus water vapor present in those gases. The outputs are the flows of unused fuel and oxidant, plus water vapor present in those gases, plus any liquid water present in either fuel or oxidant exhaust. 

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. 

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. 

In addition to high-profile fuel cell applications such as automotive propulsion and distributed power generation, the use of fuel cells as auxiliary power units (APUs) for vehicles has received considerable attention (see Figure 1-9). APU applications may be an attractive market because it offers a true mass-market opportunity that does not require the challenging performance and low cost required for propulsion systems for vehicles. In this section, a discussion of the technical performance requirements for such fuel cell APUs, as well as the current status of the technology and the implications for fuel cell system configuration and cost is given. 

The information in the previous sections can be used to determine a mass balance around a fuel cell and describe its electrical performance. System analysis requires an energy or heat balance to fully understand the system. The energy balance around the fuel cell is based on the energy absorbing/releasing processes (e.g., power produced, reactions, heat loss) that occur in the cell. As a result, the energy balance varies for the different types of cells because of the differences in reactions that occur according to cell type. 

Issues with mass transport resistance, especially at higher current densities, represent an important hurdle that fuel cells need to overcome to achieve the required efficiencies and power densifies that different applications require. Diffusion layers represenf one of fhe major fuel cell components that have a direct impact on these mass transport issues thus, optimization of the DLs is required through the use of differenf experimental and characterization techniques. 

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 most numerically solved models, a control-volume approach is used. This approach is based on dividing the modeling domain into a mesh. Between mesh points, there are finite elements or boxes. Using Taylor series expansions, the governing equations are cast in finite-difference form. Next, the equations for the two half-boxes on either side of a mesh point are set equal to each other hence, mass is rigorously conserved. This approach requires that all vectors be defined at half-mesh points, all scalars at full-mesh points, and all reaction rates at quarter-mesh points. The exact details of the numerical methods can be found elsewhere (for example, see ref 273) and are not the purview of this review article. The above approach is essentially the same as that used in CFD packages (e.g.. Fluent) or discussed in Appendix C of ref 139 and is related to other numerical methods applied to fuel-cell modeling. ... 

Recent fuel processor performance is summarized in Table 4. The fuel processors were operated at atmospheric pressure, and the water and methanol feed mixture was about 60 wt % methanol. The typical composition of the reformate stream was 72— 74% hydrogen, 24—26% carbon dioxide, and 0.5—1.5% carbon monoxide on a dry gas basis. The carbon monoxide levels were significantly below equilibrium (5.4% at 350 °C), but they still require additional cleanup for use in fuel cells. The fuel processor efficiency was calculated using eq 5 and was reported to be greater than 80%. It is interesting to note that increasing the power 5-fold, from 20 to 100 W, only resulted in a 50% increase in volume and a 33% increase in mass. 

In addition to mass transport from the bulk of the electrolyte phase, electroactive material may also be supplied at the electrode surface by homogeneous or heterogeneous chemical reaction. For example, hydrogen ions required in an electrode process may be generated by the dissociation of a weak acid. As this is an uncommon mechanism so far as practical batteries are concerned (but not so for fuel cells), the theory of reaction overvoltage will not be further developed here. However, it may be noted that Tafel-like behaviour and the formation of limiting currents are possible in reaction controlled electrode processes. 

Thus, the heat release is directly related to the amount of product water. The next consideration is the amount of heat needed to raise fuel cell temperature from, for example, -30 to 0°C (AT = 30 K). The thermal mass of the fuel cell components comes in large part from the bipolar plates (BPPs), neglecting the end plates. With graphite bipolar plates of 1 mm thickness each, and assuming an adiabatic system, the required heat is... 

Three dimensional electrode structures are used in several applications, where high current densities are required at relatively low electrode and cell polarisations, e g. water electrolysis and fuel cells. In these applications it is desirable to fully utilize all of the available electrode area in supporting high current densities at low polarisation. However conductivity limitations of three-dimensional electrodes generally cause current and overpotential to be non-uniform in the structure. In addition the reaction rate distribution may also be non-uniform due to the influence of mass transfer.1... 

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