Fuel crossover effect

As evident from the i-E response, the 1.0 M solution of methanol delivered better performance at low current densities compared with all concentrations of TMM studied at 90 C. However, at very high current densities (>750 mA/cm ) the 0.5 and 1.0 M solutions of TMM shows improved performance with respect to methanol. This type of behavior was observed at a number of different ceil operating temperatures. When the effect of TMM concentration upon cell performance was investigated, it was observed that at low current densities the solutions of low fuel concentration showed less polarization, whereas at higher current densities solutions of higher concentrations showed better performance. This trend in performance is due to fuel crossover effects which dominate at low... 

Vertically layered streaming, T shape, channel length = 48 mm, channel width = 3.3 mm, Pt/Ru and Pt nanoparticles in Naflon-based ink solution brushed on carbon paper as electrodes, graphite plates as current collectors, implementing of nano-porous separator at the fuel-electrolyte interfece, running at 80 °C with O2 supply of 50 seem, hot-pressed thin film of Nation over cathode to alleviate fuel crossover effects... 

As illustrated in Figs. 1.107 and 1.108, high fuel efficiencies in operating 2 X 2 -size fuel cells equipped with PSSA-PVDF membranes are obtainable, especially at lower temperatures, due to the decreased methanol crossover obsen/ed. The fuel efficiency was determined for all samples at a number of different operating temperatures and ail displayed decreasing fuel efficiency with an increase in temperature, as shown in Fig. 1.107. This effect can be attributed solely to the amount of fuel crossover occurring as a function of... 

Problematic crossover effects can be minimized by strategic cell design, provided that inter-diffusion is restricted to a small interfacial width at the center channel, from which anode and cathode are adequately separated. However, the location and dimensions of the electrodes may also influence fuel utilization and overall cell resistance. 

Fuel crossover and internal currents. Although the type of electrolyte is chosen on the base of necessary ion transmission, the electrolyte is always able to let several electrons (electron conductivity) and fuel molecules to go through. If electron will reach the cathode by going through electrolyte, it will recombine and go out from the fuel cell without any effective work... 

V. A. Paganin, E. Sitta, T. Iwasita, and W. Vielstich. Methanol crossover effect on the cathode potential of a direct PEM fuel cell. J. Appl. Electrochem., 35 1239-1243, 2005. 

The theoretical OCV has the same value as the reversible eell potential. However, even when no current is drawn from a fuel cell, there is irreversible voltage loss, which means that the actual values of the OCV are always lower than the theoretically expected values. To date, a quantitative explanation for such OCV behavior has not been clear in the literature. One explanation attributes this behavior to H2 crossover and/or internal current, as described in the fuel cell book written by Larminie and Dicks [26]. A mixed potential [121-124] has also been widely used to interpret the lower OCV. The combined effects of fuel crossover, internal short, and parasitic oxidation reactions occurring at the cathode are the source of the difference between the measured open circuit cell voltage and the theoretical cell potential. Therefore, the actual OCV is expressed as... 

Fuel crossover occurs to some degree in all low-temperature fuel cells, particularly in DMFCs. For a DMFC, methanol crossover not only results in additional fuel consumption, but also reduces the cell voltage by the effect of mixed potential, and... 

Chippar P, Ju H (2013) Numerical modeling and investigation of gas crossover effects in high temperature proton exchange membrane (PEM) fuel cells. Int J Hydrogen Energy 38 7704—7714... 

Fuel crossover and internal currents. This energy loss results from the waste of fuel passing through the electrolyte, and, to a lesser extent, from electron conduction through the electrolyte. The electrolyte should only transport ions through the ceU, as in Figures 1.3 and 1.4. However, a certain amount of fuel diffusion and electton flow will always be possible. Except in the case of direct methanol cells the fuel loss and current is small, and its effect is usually not very important. However, it does have a marked effect on the OCV of low-temperatuie cells, as we shall see in Section 3.5. 

These effects - fuel crossover and internal currents - are essentially equivalent. The crossing over of one hydrogen molecule from anode to cathode where it reacts, wasting... 

Although internal currents and fuel crossover are essentially equivalent, and the fuel crossover is probably more important, the effect of these two phenomena on the cell voltage is easier to understand if we just consider the internal current. We, as it were, assign the fuel crossover as equivalent to an internal current. This is done in the explanation that follows. 

It should be noted that the performance of DMFCs depends very strongly on issues such as temperature and pressure. Temperature is very important, as it has a particularly strong impact on DMFCs. A rise in temperature greatly improves the anode reactions, as well as those at the cathode. The improved anode performance reduces the problem of fuel crossover, as it reacts properly on the anode and is not available for crossover. This not only has a further improving effect on the cathode but also improves efficiency. It has been shown by Dohle et al. (2002) that raising the temperature from 22°C to 77°C increases the power available by a factor of 4. Increasing the pressure can make further improvements, but whether the power required to compress the air makes this... 

Another important source of power loss performance in direct alcohol fuel cells is so-called crossover effect, which is the passage of the fuel from the anode to the cathode through the membrane. This crossover effect is responsible for a large part of power losses of DAFCs, especially in the open circuit region. Besides, ethanol can be oxidized at the cathode electrode resulting in mixed potentials that also contribute to the decreasing of the efficiency of the fuel cell. As a rule of thumb the crossover effect increases with the concentration of ethanol and with the increasing temperature and decreases with the thickness of the membrane thus... 

Theoretically, the exchange current density Jq can be obtained by measuring i versus q for a low range of q. Unfortunately, this measurement is not practical because of large experimental errors introduced by other fuel cell losses arising from ohmic resistances, mass transport effects, and reactant and product crossover effects. These losses are discussed in the next section. 

The fuels crossover and internal currents are equivalent that is, they both contribute voltage loss owing to a small equivalent cell current. However, fuel crossover and the internal cmrents have a different physical effect on fuel cell. In the internal current, the oxidation reaction has already taken place and the electrons are short-circuited through electrolyte. In case of fuel crossover such as hydrogen permeation from the anode to the cathode, first the fuel crosses over from the anode to the cathode and then oxidation and reduction reactions occur near the cathode. With reactant crossover and internal currents, a small amount of current is lost. In both cases, the current losses are similar to activation losses, and hence as an approximation, the current and potential behavior can be represented by the Tafel law. 

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