Overall electrode reactions hydrogen evolution reaction

The hydrogen evolution reaction is the most studied electrode process. In spite of these efforts, essential features are not understood indeed, it is sometimes suggested that focusing on hydrogen evolution has delayed the development of modern electrochemistry by years, if not decades. The overall reaction in acid media is ... 

Determination of the pathways may be a demanding affair where more complex reactions are concerned, particularly when there may be more than one pathway occurring at one time. To make this meaning of pathway clear, let the simplest electrode reaction involving a reaction intermediate be chosen the hydrogen evolution reaction. The overall reaction hardly needs determination. It is (in acid solution)... 

With the exception of some simple single-electron reactions, most electrode processes involve several electrochemical or chemical steps. For example, the hydrogen evolution reaction, described by the overall equation... 

Despite the good agreement, there are two reasons why this test was flawed. First, the anodic half-reaction (5) had of necessity to be carried out at a higher pH, and thus in a different medium from the overall reaction (7). Second, and even more important, the electrode surface in the cathodic half-reaction (6) was not the same as the amalgam surface in reaction (7). Hydrogen evolution could be affected by the presence of zinc sites on the... 

Hydrogen evolution at metal electrodes is one of the most important electrochemical processes. The mechanisms of the overall reaction depend on the nature of the electrode and solution. However, all of them involve the transfer of proton from a donor molecule in the solution to the adsorbed state on the electrode surface as the first step. The mechanism of the elementary act of proton transfer from the hydroxonium ion to the adsorbed state on the metal surface is discussed in this section. 

It is recognized, that the overall process consists of the catalysis of reaction 1, an intermediate step being the reduction of water. Reaction 4 can be treated as hydrogen evolution on a compact silver electrode at negative potential. Silver has a small overpotential for evolution Of the two steps of which reaction 4 is composed... 

The intermediate remains on the electrode until it is transformed into another particle during the consecutive steps that make up the overall reaction. The simplest example is (Section 7.6.2) the mechanism of hydrogen evolution, in which one possible step involves chemical recombination between adsorbed H s, put onto the electrode surface by means of the discharge of H20+ from acid or H20 from alkaline solutions. The adsorbed H is the intermediate radical. 

Figure 14.5 illustrates the component and overall polarization curves when iron is inunersed in acidic solutions with and without Cu " ". The anodic polarization curves in both solutions are almost the same (line A). The counter electrode reaction in pure solution is the hydrogen evolution reaction, whose polarization behavior is shown by line B. These curves cross each other at point P, which is the mixed potential m(H). The anodic and the cathodic overvoltages are jFe(H) and — 7h respectively. The current density corresponding to m(H) is the corrosion current for iron in acid solutions, Jcorr(H)-When iron is placed in a solution containing Cu + ions, copper deposits on the iron substrate at the expense of iron corrosion. Note that the reversible potential of the Fe/Fe + couple of —0.44 V is less noble than that of the reversible potential for Cu/Cu " ... 

Tfie current-coUecting electrode where a known half-reaction (oxygen reduction, hydrogen evolution, etc.) completes the overall reaction occurring in an electrochemical cell. 

Hydrogen electrode reactions, that is hydrogen oxidation reaction (HOR) and hydrogen evolution reaction (HER) present one of the most important gas electrode reactions. When concerned with energy conversion applications HOR is particularly important as it presents anode reaction in proton fuel cells, while HER is cathode reaction in the case of water electrolysis. With overall reaction being ... 

An electrochemical cell consists of two electrodes as shown in Figure 1.9. This set-up separates an overall chemical reaction into two partial electrochemical reactions one at each electrode with an electronic exchange via the outer circuit. Figure 1.9 shows the Fe/Fe + electrode in combination with the H2/H+ electrode as an example. At the iron surface, Fe dissolution as Fe " is one half reaction, at the platinum electrode hydrogen evolution by H+ reduction is the other half-reaction. Both partial reactions compensate each other electronically by a transfer of electrons in the outer circuit from the Fe/Fe + electrode to the H2/H+ electrode. Both electrodes need an electrolytic contact to form... 

A relative hydrogen electrode (RHE) reference provides electrode potentials relative to a 0 set by the equilibrium potential for the overall hydrogen oxi-dation/evolution reaction at a pH of interest. 

It was demonstrated by R. Parsons and H. Gerischer that the adsorption energy of the hydrogen atom determines not only the rate of the Volmer reaction (5.7.1) but also the relative rates of all three reactions (5.7.1) to (5.7.3). The relative rates of these three reactions decide over the mechanism of the overall process of evolution or ionization of hydrogen and decide between possible rate-determining steps at electrodes from different materials. 

Fujishima and Honda [16, 158] reported the photodecomposition of water using semiconductor photoelectrolysis cells (Figure 4.10). When the surface of the Ti02 electrode was irradiated with UV light, oxygen evolution was observed at the Ti02 electrode surface and hydrogen at the Pt black electrode. The overall water photodecomposition reaction ... 

The influence of Pb + ions on the kinetics of zinc electrodeposition on Zn electrode in acidic sulfate electrolyte was discussed [217] in terms of a reaction model involving hydrogen adsorption and evolution, a multistep mechanism for zinc deposition and the overall reaction for zinc dissolution. The strongly adsorbed Pbads inhibited all the reactions taking place on the zinc electrode. 

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