Hydrogen charging curve

The galvanostatic and potentiodynamic charging curves of platinum electrodes shift approximately 60 mV in the negative direction when the solution pH is raised by 1 unit. This implies that when potentials which refer to the equilibrium potential of a hydrogen electrode in the same solution (RHE) are used, these curves remain practically at the same place within a wide range of solution pH. Hence, we shall use this scale while analyzing these curves. 

Pressure rise curve for X52 steel during high-pressure hydrogen permeation measurement. The measured pressure is normalized to the charging pressure on the upstream side. Here, the hydrogen charging pressure is 510 psi and the test temperature is 165°C. 

The best approach is normally an in situ determination based on voltammetry or charging curves, usually within the hydrogen adsorption region [96]. It is of course necessary to know the actual value of 0H for absolute determinations, but the method is practicable on a relative basis. The method becomes absolute only in a few cases, in particular for Pt electrodes [97] for which the catalytic activity per metal atom, which is the parameter really needed to evaluate electrocatalytic effects, can be calculated [98]. Sometimes, results are reported relative to the surface area measured on the basis of the limiting current for a redox reaction [99], but what is obtained is only the macroscopic surface in which asperities of a height higher than the diffusion layer thickness can only be accounted for. 

Such an equation can be applied for the determination of - pseudocapacitances (e.g., hydrogen capacitance of a platinum electrode) in the case of the so-called charging curve experiments. 

In contrast to the problem of oxygen adsorption, it has been clearly recognised since the work of Slygin and Frumkin [52] that a monolayer of hydrogen is adsorbed in the atomic form on Pt in the potential range from the equilibrium hydrogen potential to about 0.3 V more positive. They used equilibrium charging curves and later impedance measurements [53] but it... 

Fig. 15. A tjrpical anodic galvanostatio charging curve on platinum in sulfuric acid showing regions and extent of hydrogen ionization and of oxygen deposition.

Fio. 16. Typical galvanostatic charging curves in double charging method. Curves 1 and 2 are commenced at electrode potentials at which hydrogen coverage is required and at which no hydrogen is present, respectively (70). 

The change of potential due to the ohmic drop is instantaneous since if / = 0, then IR = 0. (This is the basis of the ohmic drop compensation by interruption techniques.) It is obvious that from the E vs. t function C can be determined. This is the principle of the determination of pseudocapacitance (e.g., chemisorbed hydrogen on platinum) by the method of charging curves. In this case, after the adsorption of hydrogen, a not too high anodic current (j < jo) is applied and E is followed as a function of time. 

Figure 3. A typical current-step charging curve of Pt in acid solution. Segments of the curve correspond to A, hydrogen desorption B, double layer charging C, oxygen adsorption D, end of oxygen monolayer buildup C, oxygen desorption.

Eq. 5 was used [13] to interpret the nearly linear relation between 0 and p observed on platinized platinum in hydrochloric acid and hydrobromic acid solutions when the adsorbed hydrogen is removed at constant current (anodic charging curve). 

The equilibrium of the Volmer reaction is practically established at sufficiently small sweep rates on platinum, iridium, and rhodium. Each of the i— U curves in Fig. 8 is nearly symmetrical to the U axis in the hydrogen region during the anodic and cathodic sweep. The condition i < io,v is fulfilled. Charging curves that were taken [14, 15] on electrolytically deposited ruthenium and osmium in 0.05 M H2SO4 indicate reversibility with respect to the Volmer reaction at much smaller current densities ( 11 < 0.1 p A/cm ). The exchange current densities of the Volmer reaction are considerably smaller for ruthenium and osmium than for the other four platinum metals. 

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