Hydrogen electrode overvoltage

Hydrogen Overpotential (Overvoltage) the displacement of the equilibrium (or steady-state) electrode potential of a cathode required for the discharge of hydrogen ions at a given rate per unit area of electrode. 

Fig. 10-26. Energy diagram for a cell of photoelectrolytic decomposition of water consisting of a platinum cathode and an n-type semiconductor anode of strontium titanate of which the Fermi level at the flat band potential is higher than the Fermi level of hydrogen redox reaction (snao > epM+zHj) ) he = electron energy level referred to the normal hydrogen electrode ri = anodic overvoltage (positive) of hole transfer across an n-type anode interface t = cathodic overvoltage (negative) of electron transfer across a metallic cathode interface.

Some recent work by E. Newbery1 has provided a considerable number of trustworthy overvoltage determinations which should prove valuable in choosing the proper electrode for any given reduction. By overvoltage is understood the excess back electromotive force above that of a hydrogen electrode in the same electrolyte, and this has been measured by direct comparison with a hydrogen... 

Notes Here, a is a constant in Tafel equation for hydrogen evolution overvoltage on the corresponding metal electrode (for alcaline media) [39], Ti02 was exposed to the hydrothermal treatment at 175 °C for 24 h and then calcinated at 500 °C for 4 h. The mass of the photocatalyst in the reactor was 0.05 g, reactor volume was 10 ml, the metal concentration was 4-101 M, the ethanohwater ratio wass 95 5, and the irradiation wavelength ranged within Aj, = 310- 390 nm. 

The hydrogen overvoltage is also influenced by the condition of the surface of the electrodes. The starter sheets must be smooth as smooth surfaces lead to higher hydrogen evolution overvoltages. For this... 

The quality of zinc deposit depends on the purity of the electrolyte. With a pure electrolyte, it is possible to use higher temperatures, and thereby lower electrolyte resistance and decrease electrode overvoltages. With an impure electrolyte, the temperature must be lowered to 30-35 °C to hinder hydrogen evolution caused by the impurities. Temperatures below 30 °C can cause formation of calcium sulfate temperatures above 40 °C can increase lead corrosion, and above 45 °C can increase sticking of the deposit. 

Since it is a stable molecule, one wants to supply additional energy to convert the linear structure to bent form, i.e., CO2 to C02-. Because of the high LUMO level of carbon dioxide, the transfer of one electron to free CO2 is thermodynamically unfavourable, requires a very negative redox potential up to the order of -1.9V vs Normal Hydrogen Electrode (NHE). In photocatalytic route, no semiconductor possesses the conduction band potential required for CO2 activation, there is a need to supply additional overvoltage which will lead to another energy dilemma. 

Consider a hydrogen electrode in equilibrium with H ion at a concentration c and hydrogen gas at a pressure p. The equilibrium potential of this electrode is denoted by 00 The equilibrium is Hj 2H + 2e . If the potential of the electrode is increased (made more positive), this equilibrium will be disturbed. The reaction from left to right will predominate, H2 will be oxidized, and a positive current will flow into the solution. If the potential of the electrode is lowered (made more negative), the equilibrium will be disturbed. The reaction from right to left will predominate, H2 will be liberated, and a positive current will fiow into the electrode or a negative current will flow into the solution. The current that fiows to the electrode, therefore, depends on the departure of the potential from the equilibrium value, 0 — 0o This difference between the applied potential 0 and the equilibrium potential 0 is the overpotential, or overvoltage,... 

The sum of the surface and concentration overpotentials is called the total overpotential or overvoltage r) of the electrode which can be measured by standard reference electrodes, as already shown in Figure 21.1. All electrode potentials are related to the standard hydrogen electrode (SHE), the potential of which is set by definition at zero at all temperatures. The saturated calomel electrode (SCE), Hg/Hg2Cl2/Cr, is the most widely used reference electrode for potential measurement. Other electrode systems used are Hg/Hg2S04/S04, Hg/HgO/OH , and Ag/AgCI/Cr. 

Overvoltage, 782 Salt bridge, 763 Standard hydrogen electrode (SHE), 765... 

It is possible to measure this overvoltage at each electrode, either using reference electrodes within a working fuel cell or using half-cells. Table 3.1 below gives the values of io for the hydrogen electrode at 25°C, for various metals. The measurements are for flat smooth electrodes. 

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