Normal hydrogen half cell

Three kinds of equilibrium potentials are distinguishable. A metal-ion potential exists if a metal and its ions are present in balanced phases, e.g., zinc and zinc ions at the anode of the Daniell element. A redox potential can be found if both phases exchange electrons and the electron exchange is in equilibrium for example, the normal hydrogen half-cell with an electron transfer between hydrogen and protons at the platinum electrode. In the case where a couple of different ions are present, of which only one can cross the phase boundary — a situation which may exist at a semiperme-able membrane — one obtains a so called membrane potential. Well-known examples are the sodium/potassium ion pumps in human cells. 

A redox potential exists if both phases exchange electrons and the electron exchange is in equilibrium for example, the normal hydrogen half cell with an electron transfer between hydrogen and protons at the platinum electrode. 

The potentials of the metals in their 1 mol U salt solution are all related to the standard or normal hydrogen electrode (NHE). For the measurement, the hydrogen half-cell is combined with another half-cell to form a galvanic cell. The measured voltage is called the normal potential or standard electrode potential, E° of the metal. If the metals are ranked according to their normal potentials, the resulting order is called the electrochemi-... 

As indicated previously, it is desirable to consider the individual electrode reactions independently. One might suppose that this could be achieved by characterizing the individual electrodes as described in Section 3.1.3. However, for reasons of sound thermodynamics, another method has been established. It was decided to relate all electrode reactions to one common reference electrode. Electrochemists have chosen the H+/H2 reaction under standard conditions (ct 1+ = 1M p 12 = 1 bar) as such a general reference electrode. It is termed the normal hydrogen electrode or the standard hydrogen electrode (SHE). Thus, whenever E and E° values are presented for individual electrode reactions (half cells), it is understood that these values pertain to a complete cell in which the SHE constitutes the second electrode. 

Under theoretical cell voltage conditions, for both half-cell reactions (HOR and ORR) there is no net reaction. In other words, both half-electrochemical reactions are in equilibrium, and no net current passes through the external circuit. The cell voltage can be considered the OCV. At 25 °C, if the pressures of both H2 and 02 are 1 atm, the OCV should be 1.23 V. However, in reality the OCV is normally lower and an OCV of 1.23 V is never observed. This is due to the mixed potential at the cathode side, and hydrogen crossover from the anode side to the cathode side [22, 23], At 1.23 V, Pt is not stable so oxidation of Pt occurs ... 

There are two other common versions of this half-cell the normal and tenth normal csAo-mel electrodes, in which the KCl concentration is either 1.0 or 0.1 N. The saturated electrode is the easiest to prepare and the most convenient to use but has the largest temperature coefficient. The half-cell potential for each of the calomel electrodes has a different value relative to the standard hydrogen electrode these emf valnes are given in Table 1. Calomel electrodes can be easily prepared in the laboratory and are also available commercially. Two typical calomel electrode designs are shown in Fig. 7. 

The potential calculated from Equation 13.3 is the potential relative to the normal hydrogen electrode (NHE—see Section 13.3). The potential becomes increasingly positive with increasing Ag" (the case for any electrode measuring a cation). That is, in a ceU measurement using the NHE as the second half-cell, the voltage is... 

To develop a useful list of relative half-cell or electrode potentials, we must have a carefully delined reference electrode that is adopted by the entire chemical community. The standard hydrogen electrode, or the normal hydrogen electrode, is such a half-cell,... 

Single-electrode potentials, corresponding to half-cell reactions, are often listed. These are actually potentials of the given electrodes relative to the normal hydrogen electrode as a standard. For example, in order to determine the electrode potential for the half-cell reaction... 

Here rev is the reversible potential of the half cell measured with respect to an arbitrary reference. Normally, we measure the reversible potential of half cells by using the standard hydrogen electrode as the point of reference. Equation (2.158) then becomes ... 

The electromotive series is a list of the elements in accordance with their electrode potentials. The measurement of what is commonly known as the "single electrode potential", the "half-reaction potential" or the "half-cell electromotive force" by means of a potentiometer requires a second electrode, a reference electrode, to complete the circuit. If the potential of the reference electrode is taken as zero, the measured E.M.P. will be equal to the potential of the unknown electrode on this scale. W. Ostwald prepared the first table of electrode potentials in 1887 with the dropping mercury electrode as a reference electrode. W. Nernst selected in 1889 the Normal Hydrogen Electrode as a reference electrode. G.N. Lewis and M. Randall published in 1923 their table of single electrode potentials with the Standard Hydrogen Electrode (SHE) as the reference electrode. The Commission of Electrochemistry of the I.U.P.A.C. meeting at Stockholm in 1953 defined the "electrode potential" of a half-cell with the SHE as the reference electrode. 

Eo = the specific standard half-cell potential for the reaction cited at unity activity for dissolved species and 1 atmosphere fugacity for gaseous substances at 25°C. The standard half cell potential for the reaction 2H + 2e" = H2 is defined as 0.0 V versus the normal hydrogen electrode (NHE) at 25°C. 

Each half-cell reaction has a specific standard potential reported as the potential of the reduction reaction vs. the normal hydrogen electrode (NHE). In an elecdochemical cell, there is a half-cell corresponding to the working electrode (WE), where the reactions under study take place, and a reference half-cell. Experimentally the cell potential is measured as the difference between the potentials of the WE half-cell and the reference electrode/ref-erence half-cell (see Chapter 4). The archetypal reference electrode is the NHE, also known as the standard hydrogen electrode (SHE) and is defined, by convention, as 0.000 V for any temperature. Although the NHE is not typically encountered due to difficulty of operation, all conventional electrodes are in turn referenced to this standard to define their absolute potential (i.e., the Ag/AgCl, 3 M KCl reference has a potential of 203 mV vs. the NHE). In practice, experimental results are either stated as being obtained vs. a specific reference electrode, or converted to potentials vs. NHE. 

The internal element appears as a grayish-white cylindrical pack with shiny mercury at the top of the element, if it is a calomel internal. This mercury-mercurous chloride half cell provides a potential of 244 mV versus the normal hydrogen electrode at 25°C if it is surrounded by saturated potassium chloride filling solution. It is important that this element be kept wet and uncontaminated in order to provide a stable and reproducible potential. With use, the pack may show some separation within the element tube, but this usually does not cause error or deviation of its potential. 

The standard hydrogen electrode (SHE), also referred to as normal hydrogen electrode, is the universal reference for reporting relative half-cell potentials. The SHE could be used as either an anode or a cathode depending upon the nature of the half-cell it is used with. SHE is based on the redox half-cell ... 

The half-cell electrode that is normally chosen to have a potential of zero is the standard hydrogen electrode (SHE). This half-cell consists of an inert platinum electrode immersed in 1 M HCl with hydrogen gas at 1 atm bubbling through the solution, as shown in Figure 18.6 t. When the SHE acts as the cathode, the following half-reaction occurs ... 

Among the papers dealing with potential measurements at isothermal half-cells at normal pressure, such of industrial interest are remarkable, e.g. of the chlorine electrode [41]. Very precise studies at thermocells with the hydrogen electrode, the silver-silver chloride electrode and the silver electrode [42] provided single ion entropies and activation entropies. 

These electrodes are electrochemical half cells providing a constant potential they are basically nonpolarizable, which means that their potential is independent from a current flow through the cell, or at least they follow Ohm s law. They contain a redox couple where both partners are present in well-defined concentrations (in fact, activities). Basically it is possible to realize a normal hydrogen electrode (NHE) as a potential reference with 0.000 V (per definition) but the practical efforts are enormous (proton activity 1.000 mol/L, hydrogen gas pressure 1.000 atm using a specially activated platinum electrode) additionally the potential is not very robust because small current flows or temperature changes may cause significant deviations already. 

Reference electrode An electrochemical half-cell with a stable and known potential. The most common is the saturated calomel electrode (SCE,-1-0.242 V vs NHE). The primary standard reference electrode is the normal hydrogen electrode (NHE, 0.0000 V by definition). 

Tildesley, 1990). Next, structural relaxation is performed based on MC trial moves of all the atoms in the supercell and acceptance or rejection of the generated configurations according to the Metropolis criterion. This hydrogenation of the a-Si layers results in a layered a-Si H/c-Si superstructure with structurally stable interfaces. The simulation cell is then cut in half and the periodic boundary conditions (PBCs) are removed in the direction normal to the plane of the a-Si H/c-Si interface. Additional c-Si atomic layers are placed at the appropriate interlayer distance at the bottom of the resulting simulation cell and are held fixed to simulate contact with an infinite rigid substrate. MC equilibration is used to distribute the hydrogen in the presence of the free surface. Finally, the surface is relaxed further by MD at the temperature of interest. 

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