Exchange current density hydrogen evolution

Table 21.13 Exchange current densities for the hydrogen evolution reaction...

FIGURE 28.2 Relation between the exchange current densities of hydrogen evolution and ionization at different metals and the electron work functions. 

Figure 9.4 Exchange current density for the hydrogen evolution reaction at various metals data taken from Trasatti [5].

Fig. 18. Dependence of the exchange current density of electrolytic hydrogen evolution on the electronic work function. [C

Since different surfaces will catalyze a given reaction to different degrees, it follows that io values are specific for a particular electrode surface. The exchange current densities for the evolution of hydrogen from 1 mol L-1 HC1, for example, range over some 11 powers of 10 (Table 15.1). 

Figure 15.7 Influence of exchange current density for different surfaces on the rate of metal dissolution by hydrogen evolution.

Fig. 7.114. A plot of exchange current density for hydrogen evolution reaction vs. M-H bonding strength. (Reprinted hom S. U. M. Khan, Some Fundamental Aspects of Electrode Processes," in Modern Aspects of Electrochemistry, Vol. 15, R. E. White, J. O M. Bockris, and B. E. Conway, eds., Plenum 1983, p. 339.)...

According to the different exchange current densities, i0, for hydrogen oxidation and hydrogen evolution on Ni and Pt, the catalytic activity of platinum is by a factor of several hundred to a thousand higher than that of nickel. Therefore, if the utilization of Raney-nickel particles below 10 jum size approaches 100%, it is clear that Pt-activated porous soot particles must be by a factor of from 10 to 30 smaller than Raney-nickel particles to achieve full utilization, that is, vanishing fuel starvation of the catalyst. This happens to be the case with soot agglomerates that are by their very nature of correct size (dv < 0.1 /im) (150, 151). 

Table 5.5 indicates die range of exchange current densities that have been observed for the hydrogen evolution reaction on various metals. Note that the... 

TABLE 5.5 The Exchange Current Density y0 for the Hydrogen Evolution Reaction in 1 M H2S04... 

Estimate the corrosion potential corr and the corrosion current density icorr of Zn in a deaerated HC1 solution of pH 1 at 298 K. In this solution Zn corrosion is accompanied by the hydrogen evolution reaction (h.c.r.). The parameters (standard electrode potential E°, exchange current density i0, Tafel slope b of Zn dissolution and the h.e.r. on Zn are... 

Metal Hydrogen overpotential, V Exchange current density for hydrogen evolution, A cm 2 Oxygen overpotential, V... 

Return to the Fe dissolution experiment discussed above, altering the solution to contain 5 pM M Fe2+. In addition, allow hydrogen evolution to occur on the iron surface with an exchange current density of 1(T5 A/cm2, whereas the exchange current density for the iron reaction is 1CT6 A/cm2. Assume that both reactions have Tafel slopes of 100 mV/decade. These conditions are illustrated graphically in the Evans diagram, named in honor of its creator, U. R. Evans, shown in Fig. 25. The lines represent the reaction kinetics of the two reactions considered. 

Figure 1.27 A mixed potential plot for the bimetallic couple of iron and zinc. The figure also explains the higher corrosion rate of iron than zinc in hydrochloric acid solution. Despite the more positive reduction potential of iron, the evolution of hydrogen on iron has a high exchange current density (Reproduced from Corrosion for Science and Engineering, Tretheway and Chamberlain, Copyright Pearson Education Ltd)...

Zn(OH)2 is soluble in the alkaline solution as [Zn(OH)3]- until the solution is saturated with K[Zn(OH)3]. In addition Zn(OH)2 can be dehydrated to ZnO. An enhanced power density (when compared with the - Leclanche cell) is accomplished by using particulate zinc (flakes) soaked with the alkaline electrolyte solution. This anode cannot be used as a cell vessel like in the Leclanche cell. Instead it is mounted in the core of the cell surrounded by the separator the manganese dioxide cathode is pressed on the inside of the nickel-plated steel can used as battery container. In order to limit self-discharge by corrosion of zinc in early cells mercury was added, which coated the zinc effectively and suppressed hydrogen evolution because of the extremely low exchange current density... 

---