Thermodynamics PEMFC

However, lower-temperature operation would thermodynamically hinder CO formation via methanol decomposition and rWGS. A low CO content is desired for MSR-PEMFC combinations (see Section 5.3.7). To that end, the development of catalysts active at lower temperatures still remains the central goal of methanol catalysis research. 

While natural gas reforming is the primary process for the industrial production of H2, the reforming of other gaseous hydrocarbons such as ethane, propane, and n-butane have been explored for the production of H2 for fuel cells.52,97 The reforming of propane and n-butane received particular attention in recent years, because they are the primary constituents of liquefied petroleum gas (LPG), which is available commercially and can be easily transported and stored on-site. LPG could be an attractive fuel for solid oxide fuel cells (SOFCs) and PEMFCs for mobile applications.98 01 The chemistry, thermodynamics, catalysts, kinetics, and reaction mechanism involved in the reforming of C2-C4 hydrocarbons are briefly discussed in this section. 

The thermodynamic calculations on the basis of Equations 5.6-5.8 indicate that ZnO might be unable to reduce the sulfur in the reformate to the level for PEMFC applications when the fuel gas contains a large amount of H20, CO, and C02 due to the thermodynamic factor. ZnO is not efficient for removing COS. It needs to catch more close attention to the effects of the coexisting H20, CO, and C02 on the H2S removal from the reformate in the design of a post-desulfurization process in a hydrocarbon fuel processor, especially at high temperature. 

A PEMFC converts the chemical energy of reactants into electrical energy by electrochemical reactions, which involve the transfer of electrons across the interface. Thermodynamic analysis describes funda-... 

It is clear that the activity at the interface influences the potentials of both electrodes as well as terminal cell potential of PEMFC. Table 2 provides a list of standard potentials for electrode reactions and the overall reactions for the system of interest based on standard thermodynamic data.1 1... 

Table 2 Thermodynamic values of H2, O2, and some hydrocarbon fuels of PEMFC interest...

The application of the first law of thermodynamics to a PEMFC provides the relation between enthalpy change and heat added to the system, as well as the electric work done by the system, in the absence of kinetic and potential energy changes... 

If the PEMFC works ideally, the change of Gibbs free energy of the reaction can be completely converted to electrical energy. Thus, the ideal or thermodynamic efficiency of PEMFC can be given as... 

Carbon is thermodynamically unstable at operating conditions of a PEMFC cathode because the equihbrium potential of its electrooxidation to carbon dioxide is ES/C02 = 0-207 V versus RHE at 298 K [8,247], In 1970s-1980s, corrosion... 

Table 2.1 lists AG, AH, efficiency limits, and thermodynamic voltage E° at different temperatures. Water is in the gaseous form at all the temperatures for this table. Although there are only two temperature points below 100°C, the theoretical efficiency and the thermodynamic voltage are quite close to those at 127°C. So for PEMFCs that typically operate at temperatures lower than 80°C, the theoretical efficiency of 94.5% and the thermodynamic voltage of 1.18 V should not cause significant errors at temperatures from ambient to 80°C. The data at 227°C, 627°C, and 927°C can be used for phosphoric... 

For a PEMFC, the OCV is always smaller than the thermodynamic voltage for several reasons. One reason is that there are some side reactions occurring at the electrodes, especially at the cathode. The participants in the side reactions can be some impurities or a component of the catalyst layer. For example, when the cathode catalyst is Pt/C, besides Pt being oxidized to Pt Ox (E° = 0.98 V), the carbon catalyst support could be oxidized by the following reaction ... 

The standard reduction potential of this reaction is around 0.21 V, much lower than the standard reduction potential of oxygen (R. 2.6) therefore, a mixed potential between 0.21 and 1.18 V is produced. Since the oxygen reduction reaction and Pt oxidation reaction dominate at the cathode due to kinetic reasons, the mixed potential is near the higher end. The other reason is hydrogen crossover through the PEM from the anode to the cathode. This is like an internal current flow in the cell and thus leads the electrodes (mainly the cathode) away from the 0 current thermodynamic equilibrium conditions. Due to the slow kinetics of Reaction 2.6, this internal current flow significantly lowers the cathode potential. Detailed estimation is given later in this chapter. For these two reasons, the OCV of a PEMFC is typically between 0.95 and 1.0 V. 

The data found in the literature are not completely consistent Especially the temperature dependence of the polyphosphoric acid equilibria below 200 °C is not clear. In addition to the thermodynamic states of the equilibria, there is also no extensive information on the kinetics of these condensation reactions as a function of temperature. According to other studies in the literature, the kinetics at room temperature are presumably slow, that is, the half-life of the polyphosphoric acid species might be in the order of hours. At temperatures above 200 °C, the equilibria will be established within minutes. There is a lack of information on kinetic data for the temperature range from 150 to 180°C, which is important for HT-PEMFC operating regimes. 

Carbon is generally used as catalyst support material because of its high electric and thermal conductivity, chemical stability, and porous structure [11]. The catalytic activity of the catalyst layer increases with increasing carbon surface area due to better platinum dispersion. High surface area carbon blacks such as Ketjenblack and Vulcan are therefore preferred in PEMFC application. However, carbon is thermodynamically unstable at normal cathode potentials between 0.5 and 1V. As shown in Figure 20.1a, carbon is oxidized to carbon dioxide (CO2) or carbon monoxide (CO) at high electrode potentials whereas it is reduced to methane (CH4) at low electrode potentials. The following reactions are relevant for fuel-cell operation ... 

Platinum has a low but non-negligible solubility within the polysulfonic acid membranes that are usually used in PEMFCs. The equilibrium concentration increases with rising temperature and electrode potential, as shown in Figure 20.1. Carbon, which is usually used as catalyst support, is thermodynamically unstable under normal operating conditions of the cathode [apc > 0.208 V, see Eqs. (20.3) and (20.4)]. Nonetheless, it is one of the most stable catalyst support materials for... 

CEMs such as Nation remain state-of-the-art H" ion conductors in fuel cells to this day. However, much research is being done to improve CEMs in PEMFC and DMFC. AEMs as OH ion conductors are also being investigated for alkaline hydrogen and alcohol fuel cells (see entry Fuel Cells, Principles and Thermodynamics ). Such new membranes will likely be suitable for SPE electrosynthesis in the future. 

Franco has designed this model to coimect within a nonequilibrium thermodynamics framework atomistic phenomena (elementary kinetic processes) with macroscopic electrochemical observables (e.g., I-V curves, EIS, Uceii(t)) with reasonable computational efforts. The model is a transient, multiscale, and multiphysics single electrochemical cell model accounting for the coupling between physical mechanistic descriptions of the phenomena taking place in the different component and material scales. For the case of PEMFCs, the modeling approach can account for detailed descriptions of the electrochemical and transport mechanisms in the electrodes, the membrane, the gas diffusion layers and the channels H2, O2, N2, and vapor... 

Fig. 4.2 (a) Ideal thermodynamic efficitaicy of polymer electrolyte membrane fuel cells (PEMFCs) compared to that obtained in the Camot process, (b) Comparison of processes in a cogenerated heat engine with fuel cell performance (From [2])... 

The dynamic model of a PEMFC can be realized in MATLAB and Simulink software for implementation in power systems [10]. Beginning with hydrogen flow, the three significant factors are input, output, and reaction flows during operahon [11]. The thermodynamic potential of the chemical energy that can be converted into electrical energy is derived from Nernst s law and is dependent on the partial pressures of the reactants and temperature. For reaction kinetic considerahon, overpotentials at both anode and cathode essentially constitute the energy required to drive a reaction beyond the state of thermodynamic reversibility. 

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