Open circuit voltage temperature effect

The value of E - y is called the open-circuit voltage of the cell, which is related to the composition of the product. Note that the steam conversion ratio, X, depends on the open-circuit voltage, and is not affected by the pressure or flow rate of the reactant. Also, the open-circuit voltage decreases with increasing temperature because of the endothermic nature of the reaction. However, due to the temperature dependence of the logarithmic term in Equation 4.5, this effect decreases with the value of X. 

The mechanism and characteristics of thermal cutoff devices in several prismatic lithium-ion cells was studied by VenugopaF ° by monitoring the impedance at 1 kHz and the open circuit voltage (OCV) of the cells as a function of temperature. All the cells studied contained PE-based separators with a shutdown temperature between 130 and 135 °C. Within this narrow temperature range, the shutdown separators caused a sharp and irreversible rise in impedance of the cell. Single layer PE separators were effective up to around 145 °C, above which they... 

There are also two well-known thermoelectric effects resulting from the joining of dissimilar materials (forming a junction) the Seebech effect, on which thermocouples are based, and the Peltier effect, used for thermopiles. The Seebech effect results when the two junctions of the dissimilar materials are held at different temperatures. The Seebech coefficient, e, is defined as the open-circuit voltage generated per unit temperature differential of the two junctions ... 

Katz et al. investigated the performance of the polymer solar cells under elevated temperatures in the range of 25-60 °C, which represents real operating conditions due to heating under solar irradiation [122]. While the open circuit voltage ( oc) decreased linearly with temperature, the short circuit current (Jsc) and the fill factor (FF) increased up to about 50 °C, followed by a saturation region (Fig. 28). These effects overcompensated the dropping 7oc and thus the efficiency was maximal for a 50 °C cell temperature [122]. 

The effect of temperature on the open-circuit voltage can be determined by the application of the Gibbs-Helmholtz equation, written as ... 

Qi Z, Buelte S. Effect of open circuit voltage on performance and degradation of high temperature PBI-H3PO4 fuel cells. J Power Sources 2006 161 1126-32. 

Increased temperatures have a strong correlation to some of the key degradation mechanisms. Membrane degradation due to chemical mechanisms as indicated by fluoride release rate has been shown to follow an Arrhenius relationship with temperature, with an increase of 1.8 times for a 10 °C rise. Figure 6.4 shows the relationship between temperature and the fluoride release rate as measured in the cathode and anode effluent at open-circuit voltage operation for an MEA containing DuPont Nafion 112 membrane. This trend is consistent with data reported in the Uterature, e.g. Madden et al. (2009). Sethuraman et al. (2008) showed the effect of temperature on lifetime at open-circuit voltage and found a lifetime of -500 h at 80 °C was dramatically decreased to -60 h at 100 °C and only -30 h at 120 °C (Sethuraman et al., 2008). 

So, to sum up, the internal current and/or diffusion of hydrogen through the electrolyte of a fuel cell is not usually of great importance in terms of operating efficiency. However, in the case of low-temperature cells, it has a very marked effect on the open circuit voltage. 

The same effect holds tme for longer immersion times of a few days. In the following experiments, the samples were immersed for 63 h. Then, the samples were dried for 48 h at 85 °C. Moreover, it should be noted that the open-circuit voltage (OCV) decreased by 2.3 % on immersion in water. Since the OCV is directly related to the state of charge of the battery, it can be viewed as an indirect measurement of die delithiation rate of the battery. Indeed, this result is fully consistent with the 4 % delithiation rate deduced from the magnetization measurements, and the 1-3 % loss of Fe and P in the immersion process estimated from the physical and chemical analyses. It thus fully confirms that the delithiation process is located in the surface layer. The effect of H2O on the electrochemical properties was also evaluated by exposure of the sample to ambient air. This effect is illustrated for a hydrothermal sample in Fig. 15, which shows the change of the capacity as a function of time at different temperatures in dry atmosphere and in ambient air (55 % relative humidity) [63]. 

Fig. 2 Effects of humidification on FRR during an open-circuit voltage (OCV) durability test at 80°C, H /air 150cm min at atmospheric pressure. Humidifier temperature from 60 to 77.5°C corresponds to RH from 42 to 88% (Inaba et al. 2006)...

To study the ion diffusion within solid oxide electrolytes, Krishnamurthy et al. performed DFT calculations and KMC simulations of oxygen diffu-sivity in a YSZ electrolyte. Also, Modak and Lusk simulated the open-circuit voltage of a one-dimensional YSZ electrolyte model and then compared the predicted voltages and concentration profiles with an analytical Guoy-Chapman solution. Within the KMC model, many properties were predicted including the oxygen concentration distribution, the voltage profile, the local electric field, and the effects of the temperature and the relative permittivity. 

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