Hydrogen over-voltage

It has been established that the hydrogen over-voltage on Pb electrodes immersed in 0.5 M H2SO4 follows the Tafel equation, as illustrated in Eig. 2.13 [27]. It has been found experimentally that = 0.13 V per decade, but its value depends greatly on the purity of the solution. Impurities at concentrations of 10 mol E cause an increase in the Tafel slope up to 0.24 V per decade. 

The hydrogen over-voltage on lead at a given current density depends on the time of polarization and the nature of the anions in the solution [28]. Figure 2.14 shows the 77 vs Ig I c relationship at two different polarization rates. It can be seen that the Tafel curve displays hysteresis phenomena which depend on the speed with which the polarization is changed [28]. The observed hysteresis has been attributed (a) to adsorption of anions on the Pb surface which alter both the distribution of the surface charges and the surface H concentration [28,29] or (b) to dissolution of a certain amount of hydrogen in the Pb [30,31]. 

E. Pressure Hydrogen over-voltage increases rapidly with decreasing pressure. 

In addition to the 3-phase, the most potent cathodes in Mg-Al alloys were thought to be the iron-rich phases the iron-aluminium intermetallic phase FeAl is one of the most detrimental cathodic phases in Mg-Al alloys on the basis of its potential and low hydrogen over-voltage [22]. AlMn [17,18] is also detrimental, and Mg2Si [18] seems to have no influence, while Mg2Pb facilitates localised corrosion and leads to a negative difference effect. 

CCEs have also been used in conjunction with enzymes. Glucose oxidase was encapsulated within the methyl silicate (or silica)-graphite composite [216, 220]. Reduced oxygen from the enzymatic reaction results in the production of H2O2, which is oxidized at the electrode. Palladium and rhodium catalysts have been added to the electrode to help lower the over voltage required for hydrogen peroxide oxidation [219, 221]. Glucose oxidase and horseradish peroxidase have been simultaneously entrapped within a CCE for a biosensor that does not require a... 

In order to minimise, or even eliminate, the need for maintenance, the battery should operate with minimum water loss. This is achieved when the over-voltage of hydrogen and oxygen evolution on Ph and Pb02 surfaces, respectively, is very high. This is of utmost importance for VRLA and maintenance-free batteries. Hence, it is essential to reduce the rate of hydrogen evolution at the negative plates and to find appropriate methods to oxidize the evolved... 

By assuming that the pressure loss of one of the reactants, say hydrogen, determines the concentration over-voltage and that the pressure drop between the bulk and the TPB region is linear, a simple expression for the concentration over-voltage can be derived as follows. If the system is at steady state, then the absolute rate of diffusion of the gas for a unit of geometric area must be equal to the current. Therefore... 

It is assumed that the pressure loss of hydrogen alone is significant and is responsible for the concentration over-voltage. Imposing this assumption on (10.44) results in... 

Therefore, the enzyme peroxidase acts as an electrocatalyst (bio-electrocatalyst) eliminating the over-voltage for hydrogen peroxide reduction at the electrode. As result a significant increase of the electrode potential (AE) - anodic shift towards the equilibrium potential of the redox couple H2O2/H2O occurs. The rate of this increase (AE/At) is proportional to the hydrogen peroxide production rate. 

Horiuti J, Polanyi M (1933) Catalyzed reactirai of hydrogen with water, and the nature of over-voltage. Nature (London) 132 931... 

---