Cell voltage uniformity

In Fig. 6.23a, b, the coefficient of variation is reported as function of time for R40 and R47 driving cycles, respectively. Fast and transient excursions outside the optimal region of Fig. 6.20 occur at fuel cell system minimum power of both cycles and for the highest power peak of R40 (170 s in Fig. 6.22a). However, for R40 cycle these excursions do not involve significant losses of cell voltage uniformity, as evidenced by values of coefficient of variation always below 2% (Fig. 6.23a), indicating that the short durations of shifts from the optimal region of... 

The results shown in previous sections suggest the crucial role of the oxidant supply system, in particular the effect of the stoichiometric ratio (air flow rate) on FCS efficiency and dynamic performance. In order to give further information about this issue, in this paragraph different air management strategies are closely examined, with particular reference to their influence on cell voltage uniformity and air compressor parasitic losses. 

The performance of each cell in a stack is typically monitored individually. The 1-V curves of all the cells are collected at the same time. Since the cells are connected in series, each cell is generating exactly the same amount of current at any given moment even if some cells are under reactant starvation. The difference in performance among cells is shown by the cell voltage. Uniform performance among cells is preferred. 

Fig. 5.10 Cell voltage variation within a 20 cell stack due to non-uniform fuel inflow normalized with the highest cell voltage of 0.71 V for an average current density of 667 mA/cm2.

The model equations are solved at every time step based on the given instantaneous boundary conditions. The solution starts by solving all nodal electrochemical reaction rates (current densities) given the specified cell voltage (or total current). If a quasi-steady activation loss is assumed (i.e., no double-layer dynamics to be solved), an iterative approach is used to determine the cell current densities at each node - node current at each time step is iterated so as to ensure a uniform cell voltage (e.g., to within 4 microvolt). The balance equation to be iterated at each node is Voeii = EN(I) - J2... 

Finite electrolyte conductivities and ionic current flow lead to ohmic voltage components in electrochemical cells. It is constructive at this point to review the effects of ohmic voltage contributions to driven and driving cells in the case of uniform current distributions. It will be shown that for each type of cell, the ohmic resistance lowers the true overpotential at the electrode interface for a fixed cell voltage even in the case of a uniform current distribution at all points on the electrode. 

The theory was also used to explore novel design ideas. It was predicted that functionally graded layers with enhanced ionomer content near the membrane interface and correspondingly reduced ionomer content near the GDL side would result in better performances compared to standard CCLs with uniform composition [122]. This prediction was recently verified in experiment [123]. Correspondingly, fabricated catalyst layers with a three-sublayer structure result in enhancements of fuel cell voltage by 5-10%. 

Then, the system based on the bubbler does not appear as the most convenient solution. A system based on electronic injection of water into the cathode inlet manifold, whose action depends on the uniformity of the individual cell voltages, could be the most effective technique for automotive applications. This solution would involve the use of an injection pump, which, however, would not affect significantly the FCS electric efficiency shown in Fig. 7.12. Another not predominant contribution to the energy losses due to the FCS auxiliary components can be identified in the electric consumption of the fan/radiator, which is useful to control the cooling water temperature on board of a vehicle [3]. 

This experimental analysis can be carried out on the FCS connected to a variable resistive load by means of the DC-DC converter, which is controlled to simulate the instantaneous current requests of electric drive. The tests described in the following are performed varying the slope of the acceleration phases, between 5 and 50 A s on the base of the stack current requirements. For each working condition, the uniformity of individual cell voltage during the transient step is assumed as indicator of stack operation reliability. 

Voltage uniformity does not exist in VRLA batteries unless all cells are fully saturated — and even then, this will change as water is lost and electrolyte is re-distributed from the separator into the plates. As noted previously, the ease of... 

An estimation of the effect of different parameters on the current density distribution can be made from Fig. 3.2, which shows the dependencies of the current densities at the closer, / > and further, if, part of the cathode from the anode on the cell voltage, (/, for different solution resistivity. As can be seen, the increase of the conductivity of the electrolyte leads to the more uniform current density distribution of deposits at the electrode surface. A similar but less pronounced effect of the increase of the cathodic Tafel slope can be seen, while the change of /q does not affect the current density distribution. It is necessary to note that a soluble anode is considered in this case and, hence, the anodic and cathodic exchange current densities are the same. 

The ability of an electrode to distribute uniformly current density on a whole cathode can be easily estimated by comparing the cathodic polarization curve with the cathodic current density-cell voltage dependence. The lower is the difference between them, the better distribution of the current density should be expected. 

Figure 3.14 shows that there is a large difference between the deposition overpotential and the cell voltage (tip overpotential) at low overpotentials which becomes negligible at high overpotentials, indicating a uniform current density distribution due to the additive adsorption, as illustrated in Fig. 3.15 [14]. 

As illustrated in Equation (17.20) for parallel electrodes and a uniform current distribution, the ohmic drop decreases with decrease in the inter-electrode gap and with increase in the electrolyte conductivity. In microstructured reactors, the small interelectrode gap together with the conductivity increase due to the coupling of the electrode processes leads to a substantial reduction in the ohmic perudty [7, 8j. Hence micro-structured designs permit one to minimize the cell voltage [Equation (17.19)], the specific energy consumption of the electrochemical cell [Equation (17.18)] and the heat generation terms [Equation (17.21)]. 

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