Standard Reversible Hydrogen Electrode

To model the water-splitting reaction and any electrochemical reaction, it is necessary to include the potential or bias. As long as only reaction energies are considered, it is possible to avoid explicit modeling of the electrochemical interface. With this approach, barriers for charge transfer reactions cannot be treated. In this section, the reference for the potential will be introduced. 

The work function of the metal electrode does not influence the bias. Imagine two electrodes in contact with each other and with the electrolyte. As all three elements are conducting, they will have a common Fermi level. At both sides, we let the equilibrium of the reversible hydrogen electrode evolve  

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E(she) is the electrode potential referred to the standard reversible hydrogen electrode AGHads(aq) is the same as in Equation 2.10b... 

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... 

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... 

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.

This is a thermodynamically reversible process that often serves as a standard reference electrode, known as the reversible hydrogen electrode (RHE), for all other electrochemical processes. 

Although the reversible hydrogen electrode is the primary standard, there are several other reference electrodes that are used in practice. Some of these secondary reference electrodes are described below (see also Section 4.1.7). 

This is a thermodynamically reversible reaction, and with platinum as the catalyst, is a standard reference known as the reversible hydrogen electrode (RHE), for which the potential is universally chosen as zero volts. The oxygen reduction reaction (ORR) at the cathode, on the other hand, is thermodynamically irreversible, and is generally expressed in terms of the dominant four electron reaction ... 

Oxidation of carbon to CO2 at standard conditions is thermodynamically possible at potentials greater than 0.207 V vs. the reversible hydrogen electrode (RHE) by Equation 3.1 (Tang et al., 2006). 

In contrast the SHE is an ideal device and cannot be rigorously realized experimentally. For the practical realization the standard potential of the hydrogen electrode is not measured at standard conditions but recalculated to at unit activity and 10 Pa. These electrodes are often called reversible hydrogen electrodes (RHE). The acronym RHE is also used for relative hydrogen electrode. In this special case both electrodes, the hydrogen reference electrode and the electrode to be studied, are immersed in the same electrolyte. In this way the junction potential between the reference and the sensing electrode is reduced [6]. 

The process of reversible hydrogen electroadsorption occurs at the smface of metal electrodes of the platinum group at potentials anodic with respect to the reversible hydrogen electrode at the given pH and at a pressure of 1 atm (half-standard reversible H+/H2 electrode at 1 atm. 

Since it is impossible to determine experimentally the metal/solution potential difference at a single interface, it has been necessary to measure all such values against a commonly accepted reference. The reversible hydrogen electrode, operating under standard conditions (pH = 0 Phi = f tm) was chosen for this purpose, and its potential was assigned an arbitrary value of zero. The potential quoted for any redox couple on the SHE scale is then the actual potential measured in a cell made up of the desired redox couple in one half-cell (imder standard conditions of concentration and pressure) and the SHE in the other half-cell. Taking the values of Cu/Cu and Fe/Fe as -H 0.340Vand -0.440 V vs. SHE, respectively, implies that these values will be measured vs. a SHE. 

Two things to notice are that fc, will depend on the reference electrode chosen and secondly E° is the equilibrium potential not for the standard hydrogen electrode but rather for the electrochemical equilibrium between H+ and H ds. This can be seen by explicitly writing out the equation corresponding to the reverse reaction ... 

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