Fuel Crossover and Internal Currents

Althongh the electrolyte of a fuel cell would have been chosen for its ion conducting properties, it will always be able to support very small amounts of electron conduction. The sitnation is akin to minority carrier conduction in semicondnctors. Probably more important in a practical fuel cell is that some fuel will diffuse from the anode through the electrolyte to the cathode. Here, because of the catalyst, it will react directly with the oxygen, producing no current from the ceU. This small amount of wasted fuel that migrates through the electrolyte is known as fuel crossover. 

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. 

as in the last section, we suppose that we have a fuel cell that only has losses caused by the activation overvoltage on the cathode, then the voltage will be as in equation 3.4 

For the case in point, a PEM fuel cell using air, at normal pressure, at about 30°C, reasonable values for the constants in this equation are 

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Even if the polymeric membrane in a PEM fuel cell is designed to permit only the passage of hydrated hydrogen ion, some electric conductivity and gas permeation cannot be avoided. Then at open-circuit, when no current can be observed through the external circuit, two phenomena can occur at the anode side of a PEM fuel cells, called fuel crossover and internal current. They can be described as follows ... 

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... 

The total electrical current is the sum of external (useful) current and current losses due to fuel crossover and internal currents ... 

There is another voltage loss, which is caused by fuel erossover and internal currents. Basically, fuel crossover and internal currents have very marked effeets on the OCV, Eg y. For more information on this voltage loss, please see Seetion 1.2.4. 

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. 

The fuel crossover and internal current are obviously not easy to measure - an ammeter cannot be inserted in the circuit One way of measuring it is to measure the consumption... 

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. 

Using the Tafel equation (Equation 5.114), the overpotential caused by fuel crossover and internal currents is given as... 

The actual useful voltage V obtained from a fuel cell with the load is different from the theoretical/ideal voltage E from thermodynamics. This is due to losses associated with the operation, fuel cell materials used, and the design. These losses are ohmic ir, activation A ln(i/io), fuel crossover and internal current leakage A ln(i /io), and mass transport or concentration losses m exp (ni) (Larminie and Dicks, 2(X)3) ... 

Although hydrogen crossover and internal currents are equivalent, they physically have different effects in a fuel cell. The loss of electrons occurs after the electrochemical reaction has taken place and therefore the effect on both anode and cathode activation polarization would have the effect as depicted by Equation (III.33). Hydrogen that permeates through the membrane does not participate in the electrochemical reaction on the anode side, and in that case the total current resulting from the electrochemical reaction would be the same as the external current. However, hydrogen that permeates through the membrane to the cathode side may... 

The response of the fuel cell is determined by the electrochemical processes and associated kinetics at the electrode and electrode interface. The electrochemical processes depend on the mass and charge transfer between the bulk electrolyte solution and electrode surface. The rates at which these transfers occur are determined by the number of localized phenomena and largely depend on the materials involved. These processes are presented in this chapter and the relations between the fuel cell potential and current density are given in terms of BV and Tafel equations. The key losses in the fuel cell include the activation losses, ohmic losses, mass transport losses, and losses owing to reactant crossover and internal currents that are discussed in this chapter. The fuel cell polarization curve is presented and is discussed for low-temperature and high-temperature fuel cells such as PEMFC and SOFC, respectively. 

Although Figure 3-21 shows that fuel cell efficiency above 60% may be possible, albeit at very low current and power densities, in practice that is rarely the case. At very low current densities, hydrogen crossover and internal current losses, although very small, become important, and the efficiency-power curve flattens. For the particular case, a maximum efficiency of -55% is reached (Figure 3-22). 

Current density must be determined from the polarization curve. Because no hydrogen crossover and internal current losses and no limiting current are given, the fuel cell polarization curve may be calculated from ... 

A 100-cm hydrogen/oxygen fuel cell operates at 0.5A/cm and 0.7V. Hydrogen flow rate is kept proportional to current generated at stoichiometric ratio of 1.5. Hydrogen crossover and internal current loss account for 2mA/cm. ... 

The equivalence of the fuel crossover and the internal currents on the open circuits is an approximation, but is quite a fair one in the case of hydrogen fuel cells where the cathode activation overvoltage dominates. However, the term mixed potential is often used to describe the situation that arises with fuel crossover. 

So, in this case the losses correspond to a current / of 1.40 x 10 X 2 X 9.65 X 10 = 27 mA. The cell area is 10 cm, so this corresponds to a current density of 2.7mAcm . This current density gives the total of the current density equivalent of fuel lost because of fuel crossover and the actual internal current density. 

In the previous chapter we discussed the polarization curve and all of the losses associated with the generation of current that result in decreased operating efficiency and generation of heat. At a fundamental level, all of these polarizations are a result of transport limitations. The ohmic polarization is a result of ion and electron transport losses, and the concentration and activation polarization is a result of mass transport limitations of the reactant to the catalyst surface and charged particles across the double layer, respectively. Even the crossover and internal short current loss from the expected Nemst potential is a result of transport. Optimization of the fuel cell design therefore must include an optimization of the (desired) modes of transport and minimization of the undesired modes of transport. In this chapter, the modes of transport relevant to fuel cells are described in greater detail. 

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... 

E is the reversible OCV given by equations 2.1 and 3.1 in is the internal and fuel crossover equivalent current density described in Section 3.5... 

In Figure 5.25, a schematic of the various processes that occur is shown where both the fuel (A) and oxidant (B) are considered to diffuse to the other side of the electrolyte and some electrons transfer from the anode to the cathode through electrolyte. Even though there is no external current density (4xt = 0) under open-circuit conditions, there are internal short-circuiting currents because of (1) the small electronic conductivity of the electrolyte membrane, the electrical-short-circuit current, and (2) the permeating fuel (A) and oxidant (B) across the membrane that cause small local crossover currents at the cathode and the anode, respectively. This leads to a net potential loss even under open-circuit conditions. 

The concentration polarization values, and rj, . from Eq. (4.86), and the crossover losses can be included by adding either the fuel crossover current density to the cathode current (mass transfer) or an internal short resistor for the case of a mixed conductivity in the electrolyte. 

FIGURE 3-14. The effect of internal currents and/or hydrogen crossover on the fuel... 

FIGURE 3-22. Fuel cell efficiency vs power density curve solid line with and dashed line without internal current and/or hydrogen crossover losses. 

FIGURE 21.27 Hydrogen crossover current density under the long-term RH cycling comparison between Nafion cast membrane and reinforced membrane. (From Choudhury, B., Material challenges in proton exchange membrane fuel cells. International Symposium on Material Issues in a Hydrogen Economy, November 12-15, Richmond, VA, 2007.)... 

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