Voltage decay

Lifetime performance degradation is a key performance parameter in a fuel cell system, but the causes of this degradation are not fully understood. The sources of voltage decay are kinetic or activation loss, ohmic or resistive loss, loss of mass transport, or loss of reformate tolerance (17). 

Surface voltage decay was measured with a Trek Model 341 high voltage electrostatic voltmeter (ESVM). The ESVM drives the potential of a probe body to the same potential as the film surface voltage. This device measures the voltage independent of distance within 5 mm of the film surface without causing current flow, which would modify the data. After poling, the corona needle was removed and the probe of the ESVM was placed 3 mm from the film surface. 

Figure 2. Surface voltage decay as a function of time for corona poled PS (O) and PMMA ( ) films doped with 4 wt.% DANS. Films poled at -3000V (0.6 cm air gap) for 15 min at 25°C.

Recent kinetic studies indicate that carbon corrosion can be significant under normal transient operation.56,57,60-62 The rate of voltage change, common in the automotive application, enhances cathode carbon-support corrosion.16 Hence, further model improvement shall be focused on finding the carbon corrosion kinetics associated with voltage cycling. Currently, the relationship between fuel cell performance decay and accumulated carbon-support loss is only empirical.22 More effort has to be made to incorporate mechanisms that can accurately quantify voltage decay with carbon-support loss.31,32... 

If we now assume somewhat arbitrarily that, at time t = 0, the voltage at the grid points is a sinusoidal pattern V(x, y) = V sin(27rx/k), and substitute this into the preceding equation, we find that the voltage decays exponentially. 

Constant power Voltage decay acceleration and current increase [71]... 

MEA life-accelerated conditions (approximately 1500 h, and <30 p.V h ) (voltage decay rate)... 

There are numerous techniques to measure the recombination lifetime. Some of the better known are photoconductive decay (13). diode reverse recovery (14). diode open circuit voltage decay (15). surface photovoltage (JL ) and forward-biased pn junction I-V characteristic (17. I will describe one particular photoconductive decay method, because it is a relatively new, non-contact method that requires no junctions. This makes it very suitable for a large number of measurements as for a process sequence characterization tool. 

A number of characterization methods for wafers are used today. One group of experiments aims at the purely electronic parameters like DLTS and Photoluminiscence. While DLTS and related measurements give information on the trap energies, methods like surface photovoltage (SPV) or open circuit voltage decay (OCVD) give information on effective diffusion length. 

What is impUed by unequal generation and recombination lifetimes The lifetime measurement techniques and the resulting lifetimes measured with them must be clearly understood to avoid confusion. For example, the recombination lifetime is measured by such methods as photoconductive decay, open-circuit voltage decay, diode reverse-recovery, surface photo voltage, electron-beam induced current and others. 

The open-circuit voltage decay (OCVD) lifetime characterization technique was one of the early lifetime measurement methods. [76] It is simple and is among the preferred lifetime characterization techniques because it is easy to implement It requires only a simple measurement circuit. The interpretation of the experimental data is fairly straightforward and a commercial instrument is available. 

Fig.6 A schematic open circuit voltage decay measurement circuit.

The open-circuit voltage decay V(t)-t curve of Fig.7(a) shows several... 

Fig.7 A schematic (a) and experimental (b) open circuit voltage decay curve.

Additonal complications are encountered for diodes in which the diode capacitance is appreciable [81] and when there is a shunt resistance across the diode as is usually encountered in solar cells. The voltage decay can then give unreliable lifetime measurements. 

SAT 14] SATHIYA M., ABAKUMOV A.M., FOK D., et al, Origin of voltage decay in high-capacity layered oxide electrodes . Nature Materials, December 2014 (advanced online publication). 

In an idealized case, the voltage decays linearly at constant current. There is complete overlap or symmetry between the cathode and the anode in terms of oxidation and reduction behavior, so the polarity of the device is not important. 

FIGURE 9.9 Comparison of voltage decay characteristics for Type I, Type n, and Type III and IV supercapacitors. 

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