Reversible hydrogen electrodes

Fig. 20.6 Electrocapillary curves for HCI at various concentrations determined using a reversible hydrogen electrode (R.H.E.) immersed in the same concentration of HCI as that used for the determination (after Bockris and Reddy )...

Metals in practice are usually coated with an oxide film that affects the potential, and metals such as Sb, Bi, As, W and Te behave as reversible A//A/,Oy/OH electrodes whose potentials are pH dependent electrodes of this type may be used to determine the solution s pH in the same way as the reversible hydrogen electrode. According to Ives and Janz these electrodes may be regarded as a particular case of electrodes of the second kind, since the oxygen in the metal oxide participates in the self-ionisation of water. 

According to Sato et al.,6,9 the barrier-layer thickness is about 1.5 to 1.8 nm V-1, and increases to 3 nm around the oxygen-evolution potential. In Fig. 5, the scale of the electrode potential, Vrhe, is that of the reversible hydrogen electrode (RHE) in the same solution. The electrode potentials extrapolated from the linear plots of the potentials against the film thickness suggested that the potential corresponding to the barrier thickness equal to zero is almost equal to 0.0 V on the RHE scale, independent of the pH of the solution, and approximately agrees with the equilibrium potential for the oxide film formation of Fe or Fe. Therefore it is concluded that the anodic overpotential AE applied from the equilibrium potential to form the oxide film is almost entirely loaded with the barrier portion. 

For many electrodes it is fonnd that one H+ or OH ion is involved in the reaction per electron hence, the electrode potential becomes 0.059 V more negative when the pH is raised by 1 unit this is the same potential shift as found for the hydrogen electrode. For such electrodes a special scale of electrode potentials is occasionally employed These potentials, designated as E refer to the potential of a reversible hydrogen electrode (RHE) in the same solution (i.e., at the given pH). For the electrodes of the type considered, potentials in this scale are independent of solution pH. 

The standard electrode potential of reaction (15.20) calculated thermodynamically is 1.229 V (SHE) at 25°C. For reachons (15.21) and (15.22), these values are 0.682 and 1.776 V, respechvely. The equihbrium potenhals of all these reactions have the same pH dependence as the potential of the reversible hydrogen electrode therefore, on the scale of potentials (against the RHE), these equilibrium potenhals are... 

Some metals are thermodynamically unstable in aqueous solutions because their equilibrium potential is more negative than the potential of the reversible hydrogen electrode in the same solution. At such electrodes, anodic metal dissolution and cathodic hydrogen evolution can occur as coupled reactions, and their open-circuit potential (OCP) will be more positive than the equilibrium potential (see Section 13.7). 

Since, by this reaction, two electrons are transferred from the reference electrode (which, for comparison with the experimental CV curves, we assume to be a reversible hydrogen electrode, giving f = 0) to the electrode, the term No(2eA) appears in (5.28). 

OHads formation has a clear voltammetric signature on a number of surfaces, including the (lll)-oriented surfaces of platinum group metals, Pt(lll) in alkaline and acid electrolytes of non-adsorbing anions [Markovic and Ross, 2002], and Au(lll), Au(lOO), and Ag(lll) in neutral and alkaline electrolytes [Savinova et al., 2002]. On these surfaces, the reaction has a reversible character. Anderson and co-workers calculated the reversible potential of Reaction (9.1) on Pt to be 0.62 V with respect to a reversible hydrogen electrode (RHE) [Anderson, 2002]. The Pt(lll)-OH bond energy has been estimated to be about 1.4 eV in an alkaline electrolyte [Markovic and Ross, 2002]. 

A standard rotating disk electrode (RDE) setup with a gas-tight Pyrex cell was used for the experiment on CO adsorption and the HOR. A Pt wire was used as counterelectrode. A reversible hydrogen electrode, RHE(t), kept at the same temperature as that of the cell (t, in °C), was used as the reference. All the electrode potentials in this chapter will be referenced to RHE(f). The electrolyte solution of 0.1 M HCIO4... 

An important eonelusion was that the best catalyst is not the alloyed one as expected, nor the mixture of Pt/XC 72 and Ru/XC 72 powders, but one eonsisting of a dispersion of Pt colloid and Ru colloid on the same carbon support, i.e., the Pt + Ru/XC 72 eatalyst. The latter leads to higher current densities for the eleetro-oxidation of methanol than the other catalysts with the same atomic ratio for potentials lower than 0.5 V versus a reversible hydrogen electrode (RHE) (Fig. 11.3). This result... 

The reversible hydrogen electrode (RHE) consists of Pt in contact with hydrogen at 1 atmosphere in the same solution us that employed in the electrochemical cell. On the RHE scale, therefore, hydrogen evolution always occurs at 0 V. 

Fig. 4. Catalytic activities of metals (as potentials measured at 10-4 A.cm-2) for anodic oxidation of different reductants. Er thermodynamic oxidation-reduction potentials of reductants. H2 reversible hydrogen electrode potential in solution used to study oxidation of each reductant. Adapted from ref. 38.

Figure 6.19. Experimental cyclic voltammograms of carbon-supported high surface area nanoparticle electrocatalysts in deaerated perchloric acid electrolyte. Solid curve pure Pt dashed curve Pt5oCo5o alloy electrocatalyst. Inset blow up of the peak potential region of Pt—OH and Pt— formation. Scan rate 100 mV/s. Potentials are referenced with respect to the reversible hydrogen electrode potential (RHE).

Figure 15.4 Tafel plot for the two half-reactions of the reversible hydrogen electrode.

Reversible, definition, 1419 Reversible electrode, definition, 834, 1113 Reversible hydrogen electrode. 815. 1207 Reversible reaction, 1251 Reversible region, 1255 Resistance, 1172 faradaic, 1175 ohmic, 1175... 

Fig. 6.57. Electrocapillary curves from solutions of different electrolyte (HG) concentrations. The symbol RHE stands for a reversible hydrogen electrode immersed, not in a standard solution, but in the same electrolyte as the electrode under study.

Figure 25 shows the evolution of cell voltage with time of Raney-nickel anodes that are deliberately operated at too high current densities so that the effectively applied overpotential was above the threshhold for nickel oxidation, which amounts to +80 mV vs the reversible hydrogen electrode. Evidently at a current density of 400 mA/cm2 and at 80°C the oxidation of Raney nickel proceeds within hours and at 300 mA/cm2 still within a week. 

Recalculated with reference to the reversible hydrogen electrode potential from D. Dobos, Electrochemical Data, Elsevier. Amsterdam. 1975. 

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