Thermodynamics open cell voltage

An often very helpful strategy therefore is to modify existing materials. Compared to new compounds, such attempts would primarily not address thermodynamic quantities, such as open cell voltage. Instead, kinetic parameters can be optimized, and quantities such as practical cell voltage, electrode capacity, and power density can be drastically improved. 

There is evidence for a more complex discharge mechanism via lithium intercalation into (CF,) , yielding ternary compounds, (CLiyFJ 0,34.36 -pjjg interplanar distance in (CFj) increases during reduction and may reach 935 pm . Moreover, the open-cell voltage of the redox couple (CFj) /Li is only 3 V, whereas 4.57 V is calculated from thermodynamic data via Eq. (b) . ... 

At zero current, the fuel cell electrodes provide the thermodynamic open circuit voltage Vceii = Voc- Connection of a load induces current I in the system and reduces Vceii by V I). The current drawn from the fuel cell thus costs some potential the thermodynamic voltage Vgc is the capital at our disposal. The value of Voc is given by Nernst equation [1] in this chapter, Voc is assumed to be constant, and we will focus on 6 V f). 

We shall see in Chapter 6 that energy consumption during industrial synthesis increases in importance with the decrease of the unit price of a particular product. In this section we shall address the fundamental concepts of thermodynamics to the calculation of minimum energy requirements and open-cell voltages. We shall consider energy balances and heat transfer requirements, since both can be an essential part of practical reactor design. 

In engineering calculations one is rarely concerned with systems in their standard states and in consequence one has to use further thermodynamic calculations (see below) to determine the effect of say temperature on the open-cell voltage. The question is whether the required data are available in the literature. 

In the case of reversible systems the cell voltage may be derived from the thermodynamic data of the cell reactions according to Eq. (5). But often this equilibrium voltage cannot exactly be measured (even not at an open circuit), since the electrode process is not quite reversible, as in the case of the nickel electrode or since secondary reactions cause a slight deviation (cf. Fig. 1.18). Then the open cell voltage (OCV) actually is measured. 

At the cathode of a PEM fuel cell, a pure Pt surface is not easy to achieve because oxygen is present, leading to a mixed surface of Pt and PtO. Thus, the fuel cell thermodynamic open circuit voltage (OCV) at 25 °C is always... 

Open circuit voltage is the voltage across the terminals of a cell or battery when no external current flows. It is usually close to the thermodynamic voltage for the system. 

Equilibrium potentials can be calculated thermodynamically (for more details, see Chapter 3) when the corresponding electrode reaction is known precisely, even when they cannot be reached experimentally (i.e., when the electrode potential is nonequilibrium despite the fact that the current is practically zero). The open-circuit voltage of any galvanic cell where at least one of the two electrodes has an nonequilibrium open-circuit potential will also be nonequilibrium. Particularly in thermodynamic calculations, the term EMF is often used for measured or calculated equilibrium OCV values. 

The portion AQ = AH - AG = TAS of AH is transformed into heat. Ideal theoretical efficiencies % determined by the types and amounts of reactants and by the operating temperature. Fuel cells have an efficiency advantage over combustion engines because the latter are subdued to the Carnot limitation. High thermodynamic efficiencies are possible for typical fuel cell reactions (e.g., e,h = 0.83 (at 25°C) for H2 + I/2O2 -> H20(i)). The electrical potential difference between anode and cathode, = -AG/W(f, which is also called the electromotive force or open-circuit voltage, drives electrons through the external... 

At low discharge levels the concentration of reactants will be large and that of the products small, so that EccU will be relatively high. As the discharge continues, the thermodynamic-controlled open circuit voltage (i.e. the cell... 

The open circuit voltage (OCV) of a fuel cell is identical to the reversible cell potential, provided that the cell is in thermodynamic equilibrium. That is usually achieved, especially at high temperatures. There are some causes which... 

With its reactant and product connections isolated, but equilibrium concentrations remaining in the cell, the cell voltage is a maximum, namely V , at open-circuit, zero-flow equilibrium (Table A.1). Equilibrium entails the presence of a thermodynamically reversible, symmetrical or balanced exchange current, described by Marcus (1964 1982) as being an equilibrium exchange between electrode electrons and vibrating... 

Since it is impossible to measure the individual electric potential differences at the phase boundaries, we shall hereinafter speak only in terms of the difference in electric potential across the two terminals connected to the electrodes of the battery. When in a battery the current is not flowing or tends to zero, the measurable potential difference across the two terminals is called the open-circuit voltage (OCV), fJc, and it represents the battery s equilibrium potential (or voltage). Since it is related to the free energy of the cell reaction, the OCV is a measure of the tendency of the cell reaction to take place. Indeed, while the conversion of chemical into electric energy is regulated by thermodynamics, the behavior of a battery under current flow (the current is a measure of the electrochemical reaction rate) comes under electrochemical kinetics. 

Uniqueness of the Pt Catalyst The high overpotential at the PEFC cathode is the single most important source of loss in the PEFC, as it is in other low-temperature fuel cells. The sluggish nature of the ORR process is immediately reflected by the measured open-circuit voltage of around 1.0 V, to be compared with the thermodynamically expected value of... 

Of course, since AG and AH are used in the definition (3.16), the theoretical efficiency of a fuel cell depends on the redox reaction on which it is built. In any case the theoretical efficiency, calculated from thermodynamic quantities, corresponds to an operative condition of infinitesimal electronic flow (by definition of reversible process), which practically means no current drawn from the converter. As it is shown in the following sections, also at open-circuit (no current through the external circuit) the voltage of real fuel cells is slightly lower than °, and the main problem of the electrochemical energy conversion is to obtain potentials in practical conditions (when current is drawn) as near as possible the open-circuit voltage, in order to maximize the real efficiency of the device. 

These electrolyzers are operated at about 80°C, as opposed to the 95°C common in the industry. This lower temperature allows the use of vinyl ester resins and other plastics for cell construction. Moreover, the Dow diaphragm cell is optimized for low current density ( 0.5 kA/m2 ). It consumes less electrical energy per unit of production than others in the industry. The cell voltage at low current density is only 300—400 mV above the thermodynamic decomposition voltage. Dow does not license its diaphragm cell technology and operating data for these cells are not available in the open literature. 

For methanol as fuel, the cell voltage calculated from thermodynamic data is 1.215 V, but here in practice open circuit voltages of about 0.7 V are achieved and... 

The thermodynamic electromotive force of a polymer electrolyte membrane fuel cell at a temperature of 25°C is given by e = 1.229 V. The open-circuit voltage (OCV) of a hydrogen-oxygen polymer electrolyte membrane fuel cell has values between 0.95 and 1.02 V, depending on the temperature and gas pressures. 

For a system which initially begins in thermodynamic equilibrium, the magnitude of the perturbation to its equilibrium state is an important parameter for describing the phenomena it is the driving force behind the evolution. This is why the voltage between the terminals of an electrochemical cell with current flow is often compared to the open-circuit voltage . 

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