Small fuel cells performance

Figure 14.16 the shows fuel cell stack performance of a 1 kWe atmospheric PEMFC stack using PtRu anodes, operating on various gas compositions. As can be clearly seen, already small concentrations of CO lead to a large decrease of fuel cell performance. An air-bleed of 1.5% air in hydrogen is able to mitigate this ef-... 

Finally, there are some miscellaneous polymer-electrolyte fuel cell models that should be mentioned. The models of Okada and co-workers - have examined how impurities in the water affect fuel-cell performance. They have focused mainly on ionic species such as chlorine and sodium and show that even a small concentration, especially next to the membrane at the cathode, impacts the overall fuelcell performance significantly. There are also some models that examine having free convection for gas transfer into the fuel cell. These models are also for very miniaturized fuel cells, so that free convection can provide enough oxygen. The models are basically the same as the ones above, but because the cell area is much smaller, the results and effects can be different. For example, free convection is used for both heat transfer and mass transfer, and the small... 

In addition to the activity, other important requirements for the catalyst are the capability to start the reaction rapidly without the necessity for previous reduction with hydrogen and to perform effectively with intermittent operation these are essential properties for the catalyst in reformers, especially for portable and small-scale stationary fuel cell applications. In this respect, Dias and Assaf [61] focused on the potential of Pd, Pt and Ir to promote fast and intermittent ignition of methane ATR in Ni/y-Al203. They concluded that the three metals are very good promoters of the reduction of the nickel catalyst with methane, but the lower cost of palladium makes this metal more suitable than Pt and Ir for small fuel cells. 

In order to study cathode flooding in small fuel cells for portable applications operated at ambient conditions, Tuber et al.81 designed a transparent cell that was only operated at low current densities and at room temperature. The experimental data was then used to confirm a mathematical model of a similar cell. Fig. 4 describes the schematic top and side view of this transparent fuel cell. The setup was placed between a base and a transparent cover plate. While the anodic base plate was fabricated of stainless steel, the cover plate was made up of plexiglass. A rib of stainless steel was inserted into a slot in the cover plate to obtain the necessary electrical connection. It was observed that clogging of flow channels by liquid water was a major cause for low cell performance. When the fuel cell operated at room temperature during startup and outdoor operation, a hydrophilic carbon paper turned out to be more effective compared with a hydrophobic one.81... 

The analysis of the conditions within a gas channel can also be assumed to be onedimensional given that the changes in properties in the direction transverse to the streamwise direction are relatively small in comparison to the changes in the stream-wise direction. In this section, we examine the transport in a fixed cross-sectional area gas channel. The principle conserved quantities needed in fuel cell performance modeling are energy and mass. A dynamic equation for the conservation of momentum is not often of interest given the relatively low pressure drops seen in fuel cell operation, and the relatively slow fluid dynamics employed. Hence, momentum, if of interest, is normally given by a quasi-steady model,... 

The cation conductivity is very small which on one hand leads to the high preparation temperatures but on the other to the favorable defect chemical stability as well as to negligible kinetic de-mixing under fuel cell performance.94... 

Designing alloy electrocatalysts by the so-called ad-atom method, and by alloy sputtering for oxidation of CH3OH and CO, and for CO tolerance in H2 oxidation, respectively, as well as for O2 reduction are discussed. Many years of experience are summarized and collaborations with other groups are highlighted. The particle size effect in electrocatalysis by small particle electrodes, and the effect of corrosion of carbon-black supported nanoparticles on the electrocatalytic activity are also discussed. All these factors, as well as catalyst lifetimes, are very important in fuel cell performance and in the final cost estimates for the practical fuel cell applications. 

Figure 2.12. Capillary pressure as a function of position in the CCL for different fuel cell current densities as indicated on the graphs. For small jo, Tc z) indicating operation in the optimal wetting state. For jo >parts of the CCL are completely flooded, i.e. r z)>ryi, and the fuel cell performance is critically impaired [25],...

These contaminants are produced in the gasification process as a result of the presence of small amounts of sulphur in the biomass feed. The most important of these is H2S, followed by COS. H2S is chemisorbed on catalyst surfaces, thereby blocking active sites in the catalytic gas conditioning systems and limiting fuel cell performance in power generation applications. The loss of catalytic activity resulting from sulphur contamination is usually reversible in the systems dealt with in this chapter removal of H2S from the fuel gas results in the restoration of catalytic activity to the original level. 

The driving force for small nanoparticle catalysts is reduced cost by minimizing inactive non-surface atoms, which is the basis of most low Pt approaches. Yu and Pickup investigated the coverage dependence of Pb and Sb on commercial 40 wt% Pt supported on carbon in situ in a formic acid/02 fuel cell [29]. They found optimal coverages of 0.7 for both types of adatoms. The performance of both PtSb/C and PtPb/C far exceeded that of Pt/C. After nearly a 2 h hold at 0.6 V under fuel cell operation, the performance increase over Pt/C was 15- and 12.8-fold, respectively. Figure 4.2 is a comparison of fuel cell performance at 0.6 V as a function of adatom... 

However, as was mentioned above that even small traces of CO can cause significant decrease in fuel cell performance. It was reported that only 5 ppm CO in the hydrogen stream leads to a drop of the maximum power density to less than half the value obtained for pure hydrogen [31]. The preparation of an efficient electrocatalyst with high activity for the HOR with good CO tolerance is related to the understanding of reaction mechanisms on its surface. 

The state-of-the-art gas diffusion media are hydrophobized to such an extent that they allow transport of liquid water, an important mechanism at near-saturated conditions, as well as of water vapor and reactant gases. An important role is played by the micro porous layer (MPL). Because of the presence of small hydrophobic pores, a substantial hquid water capillary pressure can be bruit up, enabling a good gradient in the chemical potential of water to drier sections [10]. The optimization of gas diffusion media and the application of the MPL have led to significant improvement of the fuel cell performance at saturated conditions, showing their critical role. 

Hydrogen peroxide decomposition catalysts can be added to ionomer membranes in small amounts to slow down the decomposition of the ionomer during fuel cell operation. Additions of cerium and manganese, in both oxide and ionic forms, have been shown to increase the oxidative stability of membranes by orders of magnitude, and fuel cells prepared with such membranes have shown substantial increases in hfetime under aggressive hot and dry operation [60-62]. Unfortunately, these metal ions and oxides can consume ion exchange capacity and negatively impact fuel cell performance. 

Another key feature of fuel cells is that their performance and cost are less dependent on scale than other power technologies. Small fuel cell plants operate nearly as efficiently as large ones, with equally low emissions, and comparable cost. This opens up applications for fuel cells where conventional power technologies are not practical. In addition, fuel cell systems can be relatively quiet generators. 

Mitsubishi Electric Corporation investigated alloyed catalysts, processes to produce thinner electrolytes, and increased utilization of the catalyst layer (20). These improvements resulted in an initial atmospheric performance of 0.65 mV at 300 mA/cm or 0.195 W/cm, which was higher than the UTC Fuel Cells performance mentioned above (presented in Table 5-2 for comparison). Note that this performance was obtained using small 100 cm cells and may not yet have been... 

Small amounts of NOx can cause fuel cell performance drops. Knights et al. [21] found that a NO concentration of even 115 ppb could cause a cell performance drop of more than 25 mV at 0.175 A/cm. They also found that as NO concentration increased, cell voltage drop also increased accordingly [21]. Mohtadi et al. [35] found that the performance decay was increased as NO concentration was increased from 2.5 to 5 ppm, and Yang et al. [49] reported that when NO concentration was increased from 10 to 1480 ppm, the performance dropped significantly, as shown in Figure 6.7. 

The effect of CO contamination on fuel cell performance is shown in Figure 2.1. Evidently, even a small amount of CO (and only a trace amount of CO) in the fuel cell feed stream causes a dramatic decrease in performance [9,25], and a higher CO concentration can cause an even larger drop. In order to mitigate the effect of CO contamination on fuel cell performance, it is important to understand the contamination mechanism. 

Hydrogen is produced via reformation of hydrocarbons (Dicks, 1996 Hohlein et al., 1996 Schmidt et al., 1994), partial oxidation of small organics (Parsons, and VanderNoot, 1988), hydrolysis of sodium borohydride (Wee et al., 2006)) arrd water electrolysis (Onda et al., 2004 Smolinka, 2009). However, reformation techniques do not produce pitre hydrogen since low concentrations of impurities such as sitlfur (H2S), carbon oxides (CO2 and CO) and ammonia (NHj) exist in the reformate gas, resulting in fuel cell performance losses. 

In papers concerning the different prototypes of small fuel cell plants, unfortunately, the data reported are often not sufficiently detailed for an unambiguous estimate of the contributions of the various components to the specific performance indicators (the values of the constant coefficients a and P). 

The current interrupt test is particularly easy to perform with single cells and small fuel cell stacks. With larger cells the switching of the higher currents can be problematic. Current interrupts and electrical impedance spectroscopy give us two powerful methods of finding the causes of fuel cell irreversibihties, and both methods are widely used. 

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