Electrocatalytic hydrogen evolution reaction

In this section we treat some electrochemical reactions at interfaces with solid electrolytes that have been chosen for both their technological relevance and their scientific relevance. The understanding of the pecularities of these reactions is needed for the technological development of fuel cells and other devices. Investigation of hydrogen or oxygen evolution reactions in some systems is very important to understand deeply complex electrocatalytic reactions, on the one hand, and to develop promising electrocatalysts, on the other. 

Historically, electrocatalytic science developed from investigations into cathodic hydrogen evolution, a reaction that can be reahzed at many metals. It was found in a number of studies toward the end of the nineteenth century that at a given potential, the rate of this reaction differs by severaf orders of magnitude between metals. In one of the first theories of hydrogen evofution, the recombination theory of hydrogen overvoltage, the rate of this reaction was finked directfy to the rate of the catalytic... 

Only two general reviews [38, 39] entirely devoted to the hydrogen evolution reaction have appeared after the start of the development of cathode activation [40]. In several other cases, hydrogen evolution has been discussed within the general frame of electrocatalysis [4, 41-47] or kinetics of electrode reactions [48, 49]. However, only one of the two reviews mentioned above discusses electrocatalytic aspects with literature coverage up to the late 70 s, when the field of cathode activation was at the beginning of its development. 

Hydrogen evolution is the only reaction for which a complete theory of electrocatalysis has been developed [33]. The reason is that the reaction proceeds through a limited number of steps with possibly only one type of intermediate. The theory predicts that the electrocatalytic activity depends on the heat of adsorption of the intermediate on the electrode surface in a way giving rise to the well known volcano curve. The prediction has been verified experimentally [54] (Fig. 2) and the volcano curve remains the main predictive basis on which the catalytic activity is discussed [41, 55],... 

Other results point to no electrocatalytic increment with amorphous metals. Heusler and Huerta [591] have investigated amorphous Co75B25 and Ni67B33 with respect to corrosion. For the reaction of hydrogen evolution, in the case of the Co alloy, Tafel slopes of 120 mV, along with lower exchange currents for the amorphous material have been reported. Thus, the mechanism is the same as for the crystalline metal. In the case of the Ni alloy, some decrease in the Tafel slope has been observed with heat treatment (which promotes crystallization). Similarly, the same Tafel slope of 120 mV and the same exchange current as for pure Fe have been measured with... 

The reaction is dependent on time. At a freshly anodized electrode acetone is preferentially reduced to propane, whereas later isopropanol is the main product. Eventually both electrocatalytic reactions are suppressed and hydrogen evolution becomes the main reaction. 

While it is expected that electrocatalytic reactions on Ru surfaces should be strongly structure-sensitive, the first report on structural effects on hydrogen oxidation and evolution reactions appeared only recently The structural effects in the hydrogen oxidation reaction (HOR) and the hydrogen evolution reaction (HER) may be factors affecting the performance of hydrogen fuel cell anodes. 

F — F), SO that the whole acting force of a photoelectrode is concentrated on the anodic partial reaction taking place with minority carriers (holes) being involved. It is therefore convenient to carry out the cathodic partial reaction (e.g., hydrogen evolution) on a metal electrode, which possesses good electrocatalytic properties for this reaction. 

The hydrogen evolution reaction is an example where its electrocatalytic character shows that it is necessary for both the description of the adsorption process and the knowledge of the kinetic parameters. Most analysis of the electrocatalytic properties involve correlations from the estimated exchange current densities with characteristic electric potentials, free energies of adsorption, enthalpy of sublimations for the metal electrode, etc. [60]. 

Copper-based amorphous alloys also proved to be active in the oxidation of formaldehyde (108,109). As it was reported earlier in connection with the hydrogen evolution reaction (62) (see Section III,A,1), HF treatment leads to the formation of a copper-rich porous surface layer. As a result, electrodes with very high electrocatalytic activity for anodic formaldehyde oxidation could be prepared. It was found that the rate-determining step is a one-electron transfer and the oxidation proceeds via the hydroxymethanolate ion HOCH2O". However, it is not clear whether the catalytically active copper species is Cu° or Cu+. It would be interesting if either Cu° or Cu+ could be stabilized in amorphous alloys. 

One of the main electrocatalytic reactions is hydrogen evolution and the oxidation of molecular hydrogen at noble metal electrodes, due to the great interest both from the fundamental point of view, and for industrial... 

Hydrogen evolution Irom the electroreduction of protons at different modified polymer electrodes was first investigated by Tourillon and Gamier, who studied the inclusion of bimetallic Ag-Pt particles into poly-3-methylthiophene(PMeT) and observed their electrocatalytic properties towards the proton reduction reaction [46], They demonstrated the positive effect of the Ag particles (from 15 /ig/cm ) on the reduction current due to an increase of the electrode conduction at low potentials, where PMeT is in its neutral undoped state, and put in evidence a minimum Pt loading (of about 10 tg/cm for a 170 nm thick film) for obtaining an enhanced catalytic activity compared to a platinized Ag-coated Au electrode without a polymeric film. A remarkable stability with time was observed under polarization at a constant potential ( — 0.4 V/SCE) without degradation of the modified electrode. 

Low carbon steel (in mesh or perforated sheet form) is presently used as a cathode in diaphragm and membrane cells because of its low cost, long life (>20 yr), and good electrocatalytic characteristics for the hydrogen evolution reaction. Thus, the hydrogen overvoltage on steel is typically about 400 mV at 20 A/dm2 in 2.5 N NaOH at 90°C, the exact value being dependent on the surface state of steel. 

The electrocatalytic activity of the immobilized enzyme was studied by the suspension electrode method for the hydrogen evolution reaction. Curve 1 in Figure 18 characterizes hydrogen evolution at Er - -0.25 V as compared to the pure polymer (curve 2). Since the system contains no mediator, a direct electron exchange between the matrix and the active center of the enzyme must take place. 

Parallelism in the specific inhibition of electrocatalytic and enzymatic activity. The specific inhibitors of a particular enzyme are observed also to suppress its electrocatalytic activity in the adsorbed state. Experimental data demonstrate that a,a -dipyridyl completely suppresses the reaction of hydrogen evolution by immobilized hydrogenase fluorine ions inactivate laccase in the reaction of oxygen electroreduction and diphenylhydrazine has the same effect on peroxidase in the reaction of hydrogen peroxide electroreduction. A complete parallelism is also observed in the inactivating effect of hydrogen peroxide on peroxidase in the electrochemical reaction and enzymatic oxidation of o-dianisidine. 

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