Voltage of an individual fuel cell

Design Principles An individual fuel cell will generate an electrical potential of about 1 V or less, as discussed above, and a current that is proportional to the external load demand. For practical apph-cations, the voltage of an individual fuel cell is obviously too small, and cells are therefore stacked up as shown in Fig. 27-61. Anode/ electrolyte/cathode assemblies are electrically connected in series by inserting a bipolar plate between the cathode of one cell and the anode of the next. The bipolar plate must be impervious to the fuel... 

Figure 2.56 shows a variety of stacked cell designs employed by SOFCs. Since an individual fuel cell produces a low voltage (typically < 1V), a number of cells are connected in series forming a fuel cell stack. An interconnect comprising a high-density material is used between the repeating anode-electrolyte-cathode units of... 

At moderately high values of the current, the voltage of an individual hydrogen-oxygen fuel cell, U, is about 0.7 V. 

The electrodes in Bacon s battery measured 370 cm, their thickness was 1.8 mm. At current densities from 200 to 400 mA/cm, the voltage of an individual cell in the battery was 0.90-0.95 V, which is considerably higher than that in phosphoric acid fuel cell (PAFC) and in modern proton exchange membrane fuel cell (PEMFC). A variety of corrosion problems were responsible for the total lifetime of Bacon s battery not reaching more than a few hundred hours. 

To conduct proton conductivity measurements, Buchi et al. [3] designed a current interruption device that used an auxiliary current pulse method and an instrument for generating fast current pulses (i.e. currents > 10 A), and determined the time resolution for the appropriate required voltage acquisition by considering the relaxation processes in the membrane of a PEM fuel cell [3]. They estimated that the dielectric relaxation time, or the time constant for the spontaneous discharge of the double-layer capacitor, t, is about 1.4 x 10 ° s. They found that the potential of a dielectric relaxation process decreased to <1% of the initial value after 4.6r (6.4 x 10 s) and that the ohmic losses almost vanished about half a nanosecond after the current changes. Because there is presently no theory about the fastest electrochemical relaxation processes in PEM fuel cells, the authors assumed a conservative limit of 10 s, based on observations of water electrolysis membranes. They concluded that the time window for accurate current interruption measurements on a membrane is between 0.5 and 10 ns. Another typical application of the current interruption method was demonstrated by Mennola et al. [1], who used a PEM fuel cell stack and identified a poorly performing individual cell in the stack. 

Another interesting attempt worth noting is the combination of porphyrin sensitized solar cell with a fuel cell made by Moore and Gust. The hybrid cell can realize an open circuit voltage of 1.2 V. The energy conversion efficiency of this photoelectrochemical biofuel cell can, in principle, produce more power than either a photoelectrochemical cell or a biofuel cell working individually [83],... 

Most electronic equipment shares the television set s need for a number of differing voltages for the operation of individual components. This alone may be sufficient justification for the inclusion of a direct current to alternating current converter in fuel cell power systems. In addition, alternating current electric motors are more suitable in most applications. They tend to operate at a rotational speed controlled by the frequency of the current. If completely unloaded they speed up to this fixed velocity and accelerate no further. Many types of direct current motors, if operated unloaded, will continue to accelerate until they fail. A belt driven fan operated by an alternating current motor is undamaged by the failure of the belt. A direct current motor will require a special safety circuit to shut it down in case of belt failure. If the belt and the safety circuit both fail, the motor will speed up until it destroys itself. 

The behavior of fuel cells in terms of individual and overall voltage as function of current density represents the basic performance from which an overall efficiency evaluation has to start. The impact of the parasitic consumption of auxiliary components involved in the system management strategies on the net power produced by the fuel cell stack represents the second important step to be carefully evaluated. 

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