Voltage thermodynamic

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. 

The cell voltage Vop is largely decided by the operating temperature and pressure. The thermodynamic voltage for water splitting (Vrev) reduces with an increase in operating temperature as given by the relation [23,24] for °K. 

Sensor. The control of the exhaust composition was essential to maintain the air-to-fuel ratio close to stoichiometric for simultaneous conversion of all three pollutants. This control came about with the invention of the 02 sensor.21,22 The sensor head of this device was installed in the exhaust immediately at the inlet to the catalyst and was able to measure the 02 content instantly and precisely. It generates a voltage consistent with the Nemst equation in which the partial pressure of 02 (P02)exhaust in the exhaust develops a voltage (E) relative to a reference. The exhaust electrode was Pt deposited on a solid oxygen ion conductor of yttrium-stabilized zirconia (Zr02). The reference electrode, also Pt, was deposited on the opposite side of the electrolyte but was physically mounted outside the exhaust and sensed the partial pressure (P02)ref in the atmosphere. E0 is the standard state or thermodynamic voltage. R is the universal gas constant, T the absolute temperature, n the number of electrons transferred in the process, and F the Faraday constant. 

As shown in Figure 3.13, the voltage losses can be analyzed from the polarization curve. Generally, the voltage losses consist of four parts (1) loss due to gas crossover this is represented by the open circuit voltage (OCV), which is lower than the thermodynamic voltage (2) loss due to activation resistance (3) loss due to ohmic resistance and (4) loss due to mass transport limitation. 

Metal Electrolyte Tem- perature [°q Thermodynamical voltage [V] Cell voltage M Current density [Am 2] Current efficiency m Energy consumption [kWh kg-1]... 

The energy consumption in electrowinning depends on the thermodynamical voltage, electrode polarization, and electrolyte resistivity. The conductivity of nickel electrowinning electrolytes (catholyte) at 60 °C is typically 120mScm 1 and for nickel electrorefining electrolytes at 60 °C it is about 200 mS cm-1 [47]. The sulfate electrolyte conductivity can be estimated using Eq. (26)... 

Here V is the so called thermodynamic voltage and from Equation 4 is found to be... 

There are the many open circuit emf studies that have been carried out to search for new solid electrolyte materials. The specimen is placed in contact with electrodes which establish different but known partial pressures Px2 Px2 measured. The ratio of the measured emf E to the thermodynamic voltage calculated from Equation 7 gives a quick estimate of the materials ionic transference number, as implied by Equation 6 cibove. This is an estimate to be sure, because the ratio actually averages the value of tj over the scale. But the method very quickly reveals which materials might best be ruled out for further studies. Indeed, those which persist in registering a zero measured emf are predominantly electronic conductors and probably not much can be done (e.g., by way of doping etc.) to make them into ionic conductors, i.e., solid electrolytes. 

The quantity is of fundamental importance and will be referred to hereafter as the thermodynamic voltage for species i—it represents the chemical driving force or affinity which acts to drive the i-type ions through the medivon. Like Vj, has units of volts. 

In contrast to the ionic species, the thermodynamic voltage for electrons always remains at zero. Their state of charge is... 

Figure 7. Ternary schematic of the overall thermodynamic voltage of an electrochemical cell...

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). 

A single cell in a DMFC has a maximum thermodynamic voltage of 1.18V at 25°C. The reacdons that take place in a DMFC are given below ... 

Therefore the thermodynamic voltage of this cell is mainly a function of the mean activity of HCI, whereas the activities of H" or Cl do not play any part when taken separately. Here, it must be underlined that because the solution contains three types of ions (H IC and CT), one therefore needs to include the different H" and CT concentrations when calculating the mean activity coefficient of HCI. 

E = thermodynamic voltage (Volt) associated with Reaction [9.3]. 

The thermodynamic voltage required to decompose water into hydrogen and oxygen is therefore E° = AG (H2O) / 2f 1.23 V, a value close to the voltage required to evolve chlorine from NaCl (1.36 V). However, the electric potential of the anode and the cathode varies markedly with the pH of the electrolyte. Let us consider the water splitting reaction in acidic media ... 

Therefore, under the same operating conditions, the cell voltage E = E - E = 1.23V (Fig. 9.1). The thermodynamic voltage required to split water molecules into hydrogen and oxygen is the same, whatever the pH. The only difference is that the potential of each electrode is shifted along the potential axis, as a function of electrolyte pH. The consequences are mostly to do with electrode material stability. 

At pH = 0 (Fig. 9.10), the thermodynamic potential of the chlorine-evolving anode is close to -t-1.36 V whereas the thermodynamic voltage of oxygen evolution from water in acidic media is approximately -t-1.23 V. The difference is not very large but the kinetics of the OER are slow and require elevated operation potentials of ca. -1-I.6 V to proceed at a significant rate. Therefore, pure chlorine is evolved at the surface of the DSA. 

On the other hand, when the cell voltage V exceeds the thermodynamic voltage Vq, the direction of the cell current (i.e., that of electrons and ions) and that of the electrodes and the OR (Eqn (15.1)) is reversed. In other words, for the case of Figure 15.4, electrolysis of water (Choi, Bessarabov, Datta, 2004) occurs to produce the H2 and O2. Then the power density, P = V i < 0, is negative and must be supplied externally from a power source, as per the first law of thermodynamics. 

---