Barrierless discharge

Thfel slopes different from the usual ones may also result if the r. d. s. proceeds barrierless (i.e., with a = 1) (or quasi-barrierless [201]). Thus, hydrogen evolution with barrierless discharge - step (3) - or barrierless electrochemical desorption - step (4) - as the r. d. s., is expected to occur with a Tafel slope of 60 and 30 mV, respectively [202], This behavior has been reportedly observed with Hg [203], Bi [204], Ag [205], and Au [206]. However, such an experimental observation takes place only under very special conditions and cannot have any relevance to practical electrolysis. 

Finally, we note here that the representation of the potential dependence of P in the diagram in Despic and Bockris s paper (their Fig. 14) implies ultimately that activationless discharge will set in, as treated by Krishtalik. Then, of course, the process becomes entirely diffusion limited. Correspondingly, barrierless discharge would arise at the other extreme. 

The actual situation involving barrierless discharge is not altogether clear when finite temperatures obtain, for then transitions to some excited vibrational states in the product species could be barrierless, while transition to the ground-state, zero-point, energy level may not be. 

A similar situation occurs when we consider the effect of the if/i potential. A shift of the potential affects the reaction rate both because of the variation of the surface ion concentration and because of the changing potential drop in the dense part of the double layer (see Section 4.1). In the case of ordinary discharge, the former factor is predominant since the latter one Involves a f/i potential with a = 1/2. In the case of barrierless discharge, a = 1, and both effects may balance out each other. This compensation of if/i effects is possible only with electron reduction of an n-charged cation (or a corresponding anion oxidation). In other cases, there is no compensation of the effect of the 1 1 potential on surface concentration and potential drop. 

As can be inferred from the above, in the case of barrierless discharge of hydrogen ions, for example, the overpotential is practically independent of the solution composition, which, along with a = 1, i.e., b = 59 mV, is a most typical feature of slow barrierless discharge. 

In order to experimentally detect barrierless discharge, the reaction of hydrogen evolution on mercury was chosen in the first place, since the hydrogen adsorption energy on this metal is low hence, one could expect a sufficient endothermicity of the discharge act. 

Barrierless discharge of ions was established with a sufficiently... 

For = 1, i.e., in the case of a barrierless discharge, the slope of the anodic curve corresponds to 0.030 V, while with j3 = 1/2 (ordinary discharge), itcorrespondsto0.040V. Near the equilibrium potential, j3 = 1, hence, a =0. In fact, in the cathodic region, a limiting current is observed, just as would be expected in an activationless process. 

The nature of intermediate has been rarely disputed. The suggestion [14] that reaction (2) might involve (H2 )ad, has not received any experimental confirmation. The possibility that the various steps proceed at comparable rates [15], rather than with a single rds, has also been suggested and theoretical calculated values of Tafel slopes for these cases are also presented in Table 1. Hydrogen evolutiOTi with barrierless discharge - step (1) or barrierless electrochemical desorption step - (3) as the rds [16] is expected to occur with a Tafel slope of —60 and —30 mV, respectively. This behavior has been observed with Au [17] and Ag. It has been also observed that hydrogen evolution could occur at Ni... 

Fig. 1.6. Potential energy curves for ordinary, activationless, and barrierless processes. The initial state for the ordinary discharge is shown by curves 1 and 2, while curves 3 and 4 show the activationless and barrierless discharge respectively. Curve 5 corresponds to the final state.

The position of the curve in Figure 1.7 with respect to the equilibrium potential primarily decides whether such a form of the potential dependence of current can be observed. Thus, if the region of transition from a = 1 to a = lies below the equilibrium potential (this corresponds to case (a) in Figure 1.7), the barrierless discharge cannot be observed, since below the equilibrium potential the reverse reaction, i.e. ionization, dominates. Apparently, this is the case encountered most frequently. If, on the other hand, the inflection region lies above the equilibrium potential (Figure 1.7(b), the barrierless discharge can be observed experimentally. 

Let us now consider the kinetic laws describing the barrierless discharge. 

It should also be emphasized that the overpotential independence of Ch30 and the double layer structure in the case of barrierless discharge does not contradict the idea that it is the slow discharge that determines the reaction rate. Indeed, if we compare the rates of evolution of hydrogen at a fixed potential, we shall find that it is proportional to the concentration of HaO ions. On the other hand, if we compare overpotentials for different solutions with the same surface concentration of hydrogen ions, the effect of the i i-potential would become clear. 

The lower the polarization curve for an ordinary discharge, the higher the current density and the overpotential at which it intersects the branch corresponding to a barrierless discharge, i.e. the easier the experimental observation of the phenomenon (this is shown schematically in Figure 1.11). Therefore, we studied hydrogen overpotential at mercury in concentrated solutions of salts of strongly... 

Fig. 1.11. Schematic form of polarization curves for ordinary and barrierless discharge in solutions of different compositions. Curves 1 to 3 correspond to different solution compositions.

In the previous chapter, we have shown that barrierless discharge may in principle exist at cathodes with a high hydrogen overpotential. In order to verify this statement, we conducted an experimental study of hydrogen overpotential at a mercury cathode in acidified iodide and bromide solutions, and in concentrated hydrochloric acid at low current densities[99,100]. ... 

It was also observed in 8 solutions of LiBr with concentration 6.5-11.9M, containing 0.02-0.9M HCl, as well as in two solutions of NaBr with concentration 6.6-7M, containing 0.85M HCl. In all these cases, the coefficient b was found to be close to 60 mV for the lower branch of the curve (the lowest value observed was 51 mV, while the highest observed value was 63 mV). In 23 experiments out of 31, b varied between 55 and 60 mV. The average value of b for all the experiments was found to be equal to 58 mV (the mean-square error is 3 mV), which is practically the same as the theoretical value of 59 mV for the case of a barrierless discharge. The polarization curves have the same form at other temperatures between 10 and 60 C. The coefficient b is also found to be close to the theoretical values (see Table 2). 

The value b = 58 mV and the independence of the overpotential from the solution composition are completely in accord with the predictions of the theory, and confirm that a slow barrierless discharge of HaO" " ions takes place at mercury cathodes in acidified salt solutions for low current densities. 

An important property of the barrierless discharge of hydrogen ions is that its rate is independent of the HaO concentration and the i( i-potential. By virtue of this property, this effect can be used for solving some general problems whose solution by other means is quite hard to obtain. 

---