At various points in this chapter it has been asserted that ‘such and such an overvoltage is important in so and so conditions’. For example, it has been said that in SOFC the ohmic voltage drop is more important than activation losses. What is the evidence for these claims?
Some of the evidence is derived from experiments using specialised electrochemical test equipment such as half-cells, which is beyond the scope of this book to describe. (For such information, see, for example, Greef et al. (1985).) A method that is fairly straightforward to understand is that of electrical impedance spectroscopy. A variable frequency alternating current is driven through the cell, the voltage is measured, and the impedance calculated. At higher frequencies, the capacitors in the circuits will have less impedance. By plotting graphs of impedance against frequency, it is possible to find the values of the equivalent circuit of Figure 3.7. It is sometimes even possible to distinguish between the losses at the cathode and the anode, and certainly between mass transport and activation-type losses. Wagner (1998) gives a particularly good example of this type of experiment as applied to fuel cells. However, because the capacitances are large and the impedances are small, special signal generators and measurement systems are needed. Frequencies as low as 10 mHz may be used, so the experiments are often rather slow.
The current interrupt technique is an alternative that can be used to give accurate quantitative results (Lee, 1998), but is also used to give quick qualitative indications. It can be performed using standard low-cost electronic equipment. Suppose a cell is providing a current at which the concentration (or mass transport) overvoltage is negligible. The ohmic losses and the activation overvoltage will in this case cause the voltage drop. Suppose now that the current is suddenly cut off. The charge double layer will take some time to disperse, and so will the associated overvoltage. However, the ohmic losses will immediately reduce to zero. We would therefore expect the voltage to change as in Figure 3.8 if the load was suddenly disconnected from the cell.
The simple circuit needed to perform this current interrupt test is shown in Figure 3.9. The switch is closed and the load resistor adjusted until the desired test current is flowing. The storage oscilloscope is set to a suitable timebase, and the load current is then switched off. The oscilloscope triggering will need to be set so that the oscilloscope moves into ‘hold’ mode – though with some cells the system is so slow that this can be done by hand. The two voltages V r and V a are then read off the screen. Although the method is simple, when obtaining quantitative results, care must be taken, as it is possible to overestimate V r by missing the point where the vertical transition ends. The oscilloscope timebase setting
Immediate rise in voltage, V r Slow final rise
to OCV, Va
Time Time of current interrupt
Voltage
Figure 3.8 Sketch graph of voltage against time for a fuel cell after a current interrupt.
Fuel cell
Digital storage oscilloscope
A
Figure 3.9 Simple circuit for performing a current interrupt test.
needed will vary for different fuel cell types, depending on the capacitance, as is done in the three example interrupt tests overleaf. These issues are addressed, for example, by B¨uchi et al. (1995).2
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 irreversibilities, and both methods are widely used.
Typical results from three current interrupt tests are shown in Figures 3.10, 3.11, and 3.12. These three examples are shown because of the clear qualitative indication they give of the importance of the different types of voltage drop we have been describ- ing. Because oscilloscopes do not show vertical lines, the appearance is slightly different 2This paper also outlines an interesting variation on the current interrupt test, in which a pulse of current is applied to the cell.
Va
Vr
Figure 3.10 Current interrupt test for a low-temperature, ambient pressure, hydrogen fuel cell. The ohmic and activation voltage drops are similar. (Time scale 0.2 s/div−1, i= 100 mA cm−2.)
Vr
Va
Figure 3.11 Current interrupt test for a direct methanol fuel cell. There is a large activation overvoltage at both electrodes. As a result, the activation overvoltage is much greater than the ohmic, which is barely discernible. (Time scale 2 s/div. i= 10 mA cm−2.)
Va
Vr
Figure 3.12 Current interrupt test for a small solid oxide fuel cell working at about 700◦C. The large immediate rise in voltage shows that most of the voltage drop is caused by ohmic losses. (Time scale 0.02 s/div. I= 100 mA cm−2.)
from Figure 3.8, as there is no vertical line corresponding to V r. The tests were done on three different types of fuel cell, a PEM hydrogen fuel cell, a direct methanol fuel cell, and a solid oxide fuel cell. In each case the total voltage drop was about the same, though the current density certainly was not.
These three examples give a good summary of the causes of voltage losses in fuel cells. Concentration or mass transport losses are important only at higher currents, and in a well-designed system, with good fuel and oxygen supply, they should be very small at rated currents. In low-temperature hydrogen fuel cells, the activation overvoltage (at the cathode) is important, especially at low currents, but the ohmic losses play an important part too, and the activation and ohmic loses are similar (Figure 3.10). In fuel cells using fuels such as methanol, there is a considerable activation overvoltage at both the anode and cathode, and so the activation overvoltage dominates at all times (Figure 3.11). On the other hand, in higher-temperature cells the activation overvoltage becomes much less important and ohmic losses is the main problem (Figure 3.12).
We now have a sufficient understanding of the principles of fuel cell operation, and in the following chapters we look much more closely at the practical details of different types of fuel cell systems.
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