Electrochemical polarization hydrogen overpotential

The overpotentials at the anode qAnode (oxygen overpotential) and cathode qcathode (hydrogen overpotential), also referred to as charge transfer overpotentials, result from the inhibition of electron transport in the separate electrochemical reactions (see Fig. 11.2). In order for current to flow through the electrolysis cell, the resistance polarization must also be overcome. It is caused by the ohmic resistance of the ceU (electrolytes, separator and electrodes). The ohmic voltage drop can be calculated from the current density i in A cm and the surface-specific resistance R of the ceU in Q cm. ... 

Thus, in the upper part of the polarization curve, the hydrogen overpotential is found to depend strongly on the solution composition. This can be explained on the basis of the theory of a slow ordinary discharge or electrochemical desorption. 

For isolating the overpotential of the working electrode, it is common practice to admit hydrogen to the counter-electrode (the anode in a PEMFC the cathode in a direct methanol fuel cell, DMFC) and create a so-called dynamic reference electrode. Furthermore, the overpotential comprises losses associated with sluggish electrochemical kinetics, as well as a concentration polarization related to hindered mass transport ... 

FIGURE 2.19 Anode overpotential versus current density in hydrogen with different concentration of H20. The plot at the bottom is the enlarged part for the polarization in region I. (From Jiang, S.P. and Badwal, S.P.S., J. Electrochem. Soc., 144 3777-3784, 1997. Reproduced by permission of ECS-The Electrochemical Society.)... 

The driving force for an electrochemical reaction to proceed is polarization of the electrode, i.e. the potential difference between the equilibrium potential of the reaction and the electrode potential. The rate of the electrochemical reaction depends on the hindrances that have to be overcome by the reacting particles for the reaction to proceed. The hydrogen reaction on lead proceeds with great hindrances, i.e. at high overpotential. Hence, the competing reaction of lead sulfate reduction to lead proceeds with high coulombic efficiency and kinetic stability. This, in turn, ensures stable performance of the lead—acid battery. 

As it has been introduced in the previous section, hydrogen electrosorption in carbon materials under negative polarization has a direct impact on the energy density of a supercapacitor operating in an aqueous electrolyte. Due to the overpotential of dihydrogen evolution, the electrochemical stability window can be extended to lower potential values. Moreover, the electro-desorption of hydrogen by anodic oxidation gives rise to a pseudo-faradic contribution in addition to the EDL capacitance of the material. 

This non-linear curve is divided into two parts. If > Eearr-, the upper curve represents an anodic polarization behavior for oxidation of the metal M On the contrary, if f < Ecorr the lower curve is a cathodic polarization for hydrogen reduction as molecular gas (hydrogen evolution). Both polarization cases deviate from the electrochemical equilibrium potential (Ecorr) due to the generation of anodic and cathodic overpotentials, which are arbitrarily shown in Figure 3.2 as and rtc, respectively. 

Boron-doped diamond electrodes have high electrochemical stability, are not deactivated during operation, exhibit high overpotentials for both oxygen and hydrogen evolution, and furthermore produce active hydroxyl radicals under polarization at high anodic potentials. These properties of BDD can open up new possibilities in industrial electrosynthesis. 

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