Electrolysis overvoltage

At CEA, the studies on this process have started more recently. The two critical components of the process components are the high-temperature decomposition reactor and the SDE. Beside the European project mentioned above on the process heat reactor for S03 decomposition, CEA studies therefore focus on the electrolysis section, with a pilot now in operation in Marcoule (see Figure 7). Indeed, a major challenge for the HyS process is the development of an efficient, cost-effective SDE. Prevention of S02 migration through the separation membrane of the electrolyser, which leads to undesired sulphur deposits, remains a major technical hurdle to overcome. Like for all electrolytic processes, economic competitiveness of the cycle will also depend on the minimisation of electrolysis overvoltage and on components lifetime (membrane, interconnectors). 

The electrolytic processes for commercial production of hydrogen peroxide are based on (/) the oxidation of sulfuric acid or sulfates to peroxydisulfuric acid [13445-49-3] (peroxydisulfates) with the formation of hydrogen and (2) the double hydrolysis of the peroxydisulfuric acid (peroxydisulfates) to Caro s acid and then hydrogen peroxide. To avoid electrolysis of water, smooth platinum electrodes are used because of the high oxygen overvoltage. The overall reaction is... 

In the past, elevated voltages in electrolysis cells (a cell overvoltage) had been attributed mainly to polarization of the hydrogen evolution reaction. Hence the term hydrogen overvoltage became common for this kind of polarization. 

In some cell types, especially those in which electrolysis generates gas at an electrode, the phenomenon of overvoltage may occur, which means that the voltage to be imposed must be higher than the emf plus an overvoltage the term overpotential must be strictly used for the single electrode. 

In an earlier note (p. 9) we mentioned the occurrence of overvoltage in an electrolytic cell (and overpotentials at single electrodes), which means that often the breakthrough of current requires an Uappl = Eiecomp r] V higher than Ehack calculated by the Nernst equation as this phenomenon is connected with activation energy and/or sluggishness of diffusion we shall treat the subject under the kinetic treatment of the theory of electrolysis (Section 3.2). 

Figure 2. Partial currents for C02 reduction at various electrodes during constant-current electrolysis at 16 mA/cm2 in a 0.5 M NaHC03 solution as a function of hydrogen overvoltage of the metal used.20 Values21 of hydrogen overvoltage of the metals were those obtained in acidic solutions at 1 mA/cm2.

The potential required to split water into and O, i.e., (E - E is equal to 1.229 V. Though the theoretical potential is 1.23 V for water electrolysis, in practice the actual water decomposition will occur only above 1.7 V. The extra potential, which is essential for the water decomposition, is called overpotential. Overvoltages are composed of activation or charge transfer overvoltage, concentration or diffusion or mass transfer overvoltage and resistance overvoltage. Overvoltage is evaluated mainly as a function of current and temperature (Viswanathan, 2006). 

The direct electrolysis of a number of organic substrates requires a considerable overvoltage in order to proceed at a reasonable rate. The rate of an electron transfer in solution is high when the standard potentials of the reacting systems have suitable values. 

Standard lead-acid batteries lose a small amount of water by evaporation, but the major mechanism for water loss is by electrolysis to form hydrogen and oxygen, as described by eqs (5.5) and (5.6). The presence of small quantities of foreign elements lowers the overvoltages for these processes and leads to an accelerated water loss it has been shown that the elements antimony, arsenic, cobalt, manganese, nickel, platinum and tellurium all have a deleterious effect, even at low levels. Two general approaches have been taken in the development of MF batteries ... 

Figure 19.17. Overvoltage and distribution of voltage drops in cells (Jtiine, 1985). (a) Overvoltage of hydrogen on some metals, (b) Voltage distribution in two kinds of cells for electrolysis of brine, (c) Variation of voltage distribution with current density in the electrolysis of HC1. (d) Schematic of voltage profile in a bipolar cell with five pairs of electrodes.

A photochemical process could use the energy of sunlight to split water into H2 and 02, the former then being used as a fuel which can be stored indefinitely. The splitting of water requires in principle a photoactivated catalyst dissolved or dispersed in water. The energy requirement for the overall reaction H20— H2 + 02 is 1.23 eV (per electron). In conditions of electrolysis it would however proceed extremely slowly at this potential difference which pertains to a thermodynamic equilibrium, and it is well known that a substantial overvoltage of the order of 0.5 V is required in practice to drive this reaction. 

Sodium ion acts as a spectator ion and is not involved in the electrode reactions. Thus, the sodium chloride solution is converted to a sodium hydroxide solution as the electrolysis proceeds. The minimum potential required to force this nonspontaneous reaction to occur under standard-state conditions is 2.19 V plus the overvoltage. 

For an electrolyser, instead of an isentropic coefficient, we take into account the overvoltage necessary for driving the electrolysis (ohmic resistance, anodic-cathodic overvoltage) here the exergy loss is simply connected to this overvoltage by Faraday s law ... 

Typical values are between 0.2 (optimistic goals for industrial high temperature electrolysis) to 0.6-0.8 V (effective overvoltage of alkaline electrolysis) or even more for some acid electrolysis. 

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