Mass Transport or Concentration Losses

Similarly, if the anode of a fuel cell is supplied with hydrogen, then there will be a slight drop in pressure if the hydrogen is consumed as a result of a current being drawn from the cell. This reduction in pressure results from the fact that there will be a flow of hydrogen down the supply ducts and tubes, and this flow will result in a pressure drop due to their fluid resistance. This reduction in pressure will depend on the electric current from the cell (and hence H2 consumption) and the physical characteristics of the hydrogen supply system. 

the change in pressure caused by the use of the fuel gas can be estimated as follows. We postulate a limiting current density i at which the fuel is used up at a rate equal to its maximum supply speed. The current density cannot rise above this value, because the fuel gas cannot be supplied at a greater rate. At this current density the pressure would have just reached zero. If P is the pressure when the current density is zero, and we assume that the pressure falls linearly down to zero at the current density i, then the pressure P2 at any current density i is given by the formula 

If we substitute this into equation 2.10 (given above), we obtain 

This gives us the voltage change due to the mass transport losses. We have to be careful with signs here equation 2.10 and 3.8 are written in terms of a voltage gain, and the term inside the brackets is always less than 1. So if we want an equation for voltage drop, we should write it as 

Now the term that in this case is RT/IF will be different for different reactants, as should be evident from equation 2.8. For example, for oxygen it will be RT/AF. In general, we may say that the concentration or mass transport losses are given by the equation 

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Mass transport or concentration losses. These result from the change in concentration of the reactants at the surface of the electrodes as the fuel is used. We have seen in Chapter 2 that concentration affects voltage, and so this type of irreversibility is sometimes called concentration loss. Because the reduction in concentration is the result of a failure to transport sufficient reactant to the electrode surface, this type of loss is also often called mass transport loss. This type of loss has a third name - Nemstian . This is because of its connections with concentration, and the effects of concentration are modelled by the Nemst equation. 

The concentration polarisation is predominant for high temperature fuel cells. When the hydrogen is supplied via reformer, then the rate of supply of hydrogen at the anode decreases as compared to its consumption rate then the concentration polarisation dominates in the fuel cell. The removal of water can also be a prominent cause for the mass transport or concentration losses. 

When fuel cell begins to operate and electrical power begins to output, the electrochemical reaction can lead to the depletion of reaction in catalyst layer. This depletion will affect the performance of fnel cell through mass transportation or concentration losses. The two major mass transportation effects considered in fuel cell modeling are (1) convective mass transfer, which occnrs in flow channels due... 

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) ... 

The limiting current increases, thus reducing the mass transport or concentration overvoltage losses. This is because of the absence of nitrogen gas, which is a major contributor to this type of loss at high current densities (see Section 3.7). 

As we have mentioned earlier, one of the fuel cell voltage losses is the mass transfer loss or concentration loss caused by lower reactant gas concentration distribution at the reaction sites. Mass transport establishes reactant gas concentration distributions in gas supply channels and in the electrodes of a fuel cell, and hence in the distribution of local current densities. The gas supply rates to the anode-membrane and cathode-membrane interface must be sufficient enough to meet the gas consumption rate given by the electrochemical reaction rates. Any insufficient supply of gas to reaction sites may cause sluggishness in the reactions and cause mass transfer loss and reduction in fuel cell output voltage. 

It is assumed that all electrons transfers from the particle conduction band and surface states to the electrode take place under conditions where the current is mass transport controlled. The first order rate constant kg describes electron promotion either by thermal or photonic processes, and the rate constant k describes the loss of the electrons from the conduction band or surface states by a process which is first order in electron concentration. The validity of this assumption will be discussed later. There will be an equation similar to equation (71) for each value of m. If each equation is multiplied by its value of m and the engendered set of equations summed, it is possible to obtain the simple result that ... 

There are different kinds of DAFC operation conditions depending of the way the fuel and the oxidant (oxygen/air) are fed into the cell. In complete active fuel cells the liquid fuel (neat alcohol or aqueous solution) is pumped and gas is compressed, using auxiliary pumps and blowers, in order to improve mass transport and reduce concentration polarization losses in the system. On the other hand, in complete passive DAFC the alcohol reaches the anode catalyst layer by natural convection and the cathode breathes oxygen directly from the air. A number of intermediate options have been also studied and tested. 

The rate of oxygen evolution is lower at high NaCl concentrations. However, recycling the electrolytic solution to the undivided cell is cumbersome, and so the utilization of NaCl decreases. Modem processes use relatively dilute NaCl solutions to improve the utilization of the salt. Many electrolyzers use platinized titanium or DSA electrodes. Operation at high current densities increase the yield because the rates of the loss reactions (24) and (25) are independent of current density. Finally, agitation of the solution promotes undesirable mass transport of OCl to the electrodes. 

In addition, it can be seen from Figure 3.19 and Figure 3.20 that the mass transport loss becomes significant when the fuel cell is operated at high current density. This is created by the concentration gradient due to the consumption of oxygen or fuel at the electrodes. 

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