Performance of Fuel Cells

For low-temperature fuel cells, electrocatalysts are required to achieve desirable reaction rates at the anode and cathode, whereas the high-temperature fuel cells do not require any electrocatalyst. The most common type of electrode reactions and their corresponding Nemst potential are as follows  

There is a simple way to obtain the Nemst equations. For example, consider a reaction 

The Gibbs free energy gets modified and can be written as AGf = G°f-RT In 

Equation (2.66) infers the increase in the potential with increase in the activity of reactants. Equation (2.66) is also referred to as a Nernst equation and can also be written in terms of E.M.F. Let us demonstrate it with the help of the following reaction inside fuel cell (operating at high temperature). 

If P02 andpHjO are unchanged and the partial pressure of hydrogen increases from Pi to then the change in voltage will be given by 

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Fig. 1. Effect of catalyst types on the performance of fuel cell at 25 °C (9M HCOOH, Air).

The performance of fuel cells is affected by operating variables (e.g., temperature, pressure, gas composition, reactant utilizations, current density) and other factors (impurities, cell life) that influence the ideal cell potential and the magnitude of the voltage losses described above. Any number of operating points can be selected for application of a fuel cell in a practical system, as illustrated by Figure 2-4. 

In order to improve the performance of fuel cells, Wilkinson and St-Pierre [92] and Johnson et al. [93] compared typical CFP cathode DLs with modified DLs (from CFP or CC) that improved the mass transport at high current densities. Figure 4.14 shows the different DLs that were used to improve the cell s performance. Similar strategies can be implemented in other types of DLs, such as metallic or engineered. 

The recent study by Lakshmi et al. [63] is just one example of the very extensive research efforts devoted to the improvement of performance of fuel cells by increasing the dispersion of the electrocatalyst on the carbon support by virtue of carbon surface functionalization [64], Without acknowledging their familiarity with the most relevant prior studies, they did note that the point of neutral charge evaluation helps in identifying the platinum complex to be used for electrocatalyst synthesis based on their charge, in the sense that, for example, if the carbon surface is positively charged an anionic platinum complex is needed (see Section 5.2.1). 

The performance of fuel cell systems can be severely influenced by operating conditions such as temperature, fuel and oxidant flow rates, pressure, and fuel humidity. Performance can drop significantly if the cell is not properly operated. The following sections describe how AC impedance can change under different operating conditions. 

Wang, S.Z. and Tatsumi, 1., Improvement of the performance of fuel cells anodes with Sm doped Ce02, Acta Physico-Chim. Sin., 19, 844-848 (2003). 

Based upon the results of detailed models of these two types, overall models for the performance of fuel cells may be formulated in terms of simple equivalent electric circuit models that parametrise the loss terms and allow calculations of overall efficiencies as a function of such parameters. 

Characterize and subsequently optimize the performance of fuel cells that contain POEMs. 

The performance of fuel cells with PSSA-PVDF membranes was observed to have a strong dependence upon the flow rate of oxygen supplied to the cathode, as illustrated in Fig. 1.97. The best performance observed with PSSA-PVDF membranes was under conditions of low oxygen/air flow rates. As shown in Fig. 1.97, at 60 °C the best electrical performance was obtained with a flow rate of 0.1 L/min. oxygen (20 psig pressure). 

In order to understand the performance of fuel cells under conditions deviating from ideal thermodynamic behavior it is necessary to discuss the relationship of ceU voltage to the electrochemical potentials of the electrodes. The equUibrium ceU voltage is the difference between the equUibrium electrode potentials of the cathode (positive electrode) Uo,c and the anode (negative electrode) Uo.a, which are determined by the electrochemical reactions at the respective electrode ... 

Cha, S.W., et al.. Geometric scale effect of flow channels on performance of fuel cells. Journal of the Electrochemical Society, 2004. 151(11) pp. A1856-A1864. 

Cha SW, O Hayre R, Prinz FB (2004) The influence of size scale on the performance of fuel cells. J PowCT Sources 175 789-795... 

There are only a few reported studies that examine mechanical degradation and review the effect of compression of gas-diffusion layers on the performance of fuel cells [133-135]. Lee and Merida [133] characterized some GDL properties after 300 h. of compression at constant temperature in an ex situ test and found that the dry gas-phase permeabilities remained roughly constant. 

H. Zhu and R. J. Kee. A general mathematical model for analyzing the performance of fuel-cell membrane-electrode assemblies. J. Power Sources 117, (2003) 61-74. 

Y.Z. Wang, K.S. Ho, S.D. Wu, K.H. Hsieh, C.H. Lee, Influence of sulfonationity of epoxy-based semi-interpenetrating polymer networks of sulfonated polyimides as proton exchange membranes on the performance of fuel cell application, J. Fuel Cell Sci. Technol. 7 (2) (2010) 021014(1-7). 

Al-Baghdadi [37] developed a semi-empirical model to provide a tool for the design and analysis of fuel cell total systems. The model take into account the process variations, such as the gas pressure, temperature, humidity, and used to cover operating processes, which are important factors in determining the real performance of fuel cell. 

In recent decades, various electrode materials have been investigated to improve the performance of fuel cells [299]. A conventional low-temperature fuel cell electrode is composed of polytetrafluoroethylene, a high-surface-area carbon black loaded with a precious metal catalyst, and a current collector, as well as other minor components. The most challenging issue for electrode performance is the electrocatalyst [327]. Carbon has been established as the best catalyst support because of its good electrical conductivity, high surface area, surface hydro-phobicity, and stability [328-331]. In the past few years, template-synthesized carbons with various structures have been tried as components of fuel cells. 

Progress made in the development of fuel cell drives can be shown by referring to performance data for passenger cars (Figure 1.6). The figure shows that in an early phase of development, the stack performances were comparatively low, certainly due to considerably lower power densities or specific performances of fuel-cell systems. This allowed only a limited driving performance. The range was... 

The surface properties of the individual cell components have a major influence on the water management and, hence, the performance of fuel cells. The influence of GDL wettabihty on the cell performance was studied in situ by SchrSder et al. 

For instance, Thiedmann et al. [76] made a structural comparison between pore phases of paper- and non-woven GDLs. Despite the obvious differences of the fiber systems, the pore phases possess some similarities. Note that this is in accordance with physical experiments showing that the performances of fuel cells under the same operating conditions, but with different GDL types, vary [77]. 

Table 2 provides performances of fuel cells based on a PS GDL. 

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