Metal deposits hydrogen evolution

The reaction kinetics of hydrogen evolution is particularly sensitive to trace amounts of noble metal impurities in the solution such as Cu and Au, which tend to deposit on the silicon surface [24, 25]. Figure 9 shows the effect of different metal deposits on the photocurrent of p-Si in a H2SO4 solution [26]. There appears to be a correlation between the effect of these metals on photocurrent and the exchange current density of hydrogen evolution on the metals. In addition to metal deposits, hydrogen evolution can also be catalyzed through deposition of a layer of metal... 

This definition of overpotential is phenomenological and is always valid, irrespective of the reasons for the deviation of the potential from its reversible value. The overpotential is always defined with respect to a specific reaction, for which the reversible potential is known. When more than one reaction can occur simultaneously on the same electrode, there is a different overpotential with respect to each reaction, for any value of the measured potential. This situation is encountered most commonly during the corrosion of metals. When iron corrodes, for example, in a neutral solution, the overpotential may be + 0.4 V with respect to metal dissolution and -0.8 V with respect to oxygen reduction. During metal deposition, hydrogen evolution often occurs as a side reaction. At any given potential the overpotential with... 

Figure 6.3 shows the effect of different metal deposits (as islands equivalent to a few monolayers with about 90% surface coverage) on the photocurrent ofThe shift of the i-V curve from that of bare material is due to the catalytic effect of the metal on hydrogen evolution. For a metal deposit the photocurrent is parallel to the exchange current density for the dark evolution of H2 Pt, as a catalyst, has the highest exchange current whereas Pb, as an inhibitor, has a very low exchange current. 

These metal structures can be formed in both potentiostatic and galvanostatic regimes of electrolysis and their formation are always accompanied by strong hydrogen co-deposition. Hydrogen evolution is the second reaction which occurs at the cathode during electrodeposition processes from aqueous solutions in some cases it can be... 

Thus, hydrogen is not evolved as copper is deposited. (Hydrogen evolution should be avoided because it makes the copper deposit spongy and of poor quality.) Also, metals more difficult to reduce, such as nickel, cannot deposit along with the copper because of the excess nitric acid. 

The low current efficiency of this process results from the evolution of hydrogen at the cathode. This occurs because the hydrogen deposition overvoltage on chromium is significantly more positive than that at which chromous ion deposition would be expected to commence. Hydrogen evolution at the cathode surface also increases the pH of the catholyte beyond 4, which may result in the precipitation of Cr(OH)2 and Cr(OH)2, causing a partial passivation of the cathode and a reduction in current efficiency. The latter is also inherently low, as six electrons are required to reduce hexavalent ions to chromium metal. 

In contrast to metal ion discharge, hydrogen evolution according to reaction (15.4) causes a pH increase of the solution layer next to the cathode. At a certain value of pH in this layer, hydroxides or basic salts of the metal start to precipitate, which affects the mechanism of further metaf deposition and also the structure and properties of the deposit produced. 

More recently, Ikeda et a/.108 have examined C02 reduction in aqueous and nonaqueous solvents using metal-deposited p-GaP and p-InP electrodes under illumination. Metal coatings on these semiconductor electrodes gave much improved faradaic efficiencies for C02 reduction. In an aqueous solution, the products obtained were formic acid and CO with hydrogen evolution at Pb-, Zn-, and In-coated electrodes, while in a nonaqueous PC solution, CO was obtained with faradaic efficiencies of ca. 90% at In-, Zn-, and Au-coated p-GaP and p-InP, and a Pb coating on a p-GaP electrode gave oxalate as the main product with a faradaic efficiency of ca. 50% at -1.2 V versus Ag/AgCl. 

The composition of the codeposition bath is defined not only by the concentration and type of electrolyte used for depositing the matrix metal, but also by the particle loading in suspension, the pH, the temperature, and the additives used. A variety of electrolytes have been used for the electrocodeposition process including simple metal sulfate or acidic metal sulfate baths to form a metal matrix of copper, iron, nickel, cobalt, or chromium, or their alloys. Deposition of a nickel matrix has also been conducted using a Watts bath which consists of nickel sulfate, nickel chloride and boric acid, and electrolyte baths based on nickel fluoborate or nickel sulfamate. Although many of the bath chemistries used provide high current efficiency, the effect of hydrogen evolution on electrocodeposition is not discussed in the literature. 

The coevolution of H2 gas in electroless deposition processes is a phenomenon that needs to be understood not only to elucidate the mechanism of deposition, but also since it impacts the properties of deposits by H inclusion. Van den Meerakker [51] first proposed a correlation between simultaneous hydrogen evolution in electroless deposition and the heat of adsorption of hydrogen. In this useful endeavor, however, he has been criticized for erroneously calculating the heats of adsorption of H at Cu by Gottesfeld et al. [52], and Group I (or SP type) metals in general by Bindra and Tweedie [53]. 

Metal deposition is an example of a more general class of electrochemical reactions, ion transfer reactions. In these an ion, e.g. a proton or a chloride ion, is transferred from the solution to the electrode surface, where it is subsequently discharged. Many ion-transfer reactions involve two steps. The hydrogen-evolution reaction, for example, sometimes proceeds in the following way ... 

Another factor that may prevent the occurrence of excessive local potentials (and hence excessive deposition rates) on the metal surface is incursion of a different electrochemical reaction, such as hydrogen evolution, at such places as an alternative to the metal deposition that predominates at lower potentials. 

Saturating the electrolyte with iron(lll) hydroxide (e.g., by addition of aqueous solutions of ferric nitrate) and simultaneously adding cobaltous salts leads to in situ formation of a mixed Fe(llI)/Co(ll)/Co(IIl) deposit, which exhibits catalytic activity comparable to that of Fe304 shown by the current voltage curve in Fig. 11. Such mixed oxidic catalyst coatings are composed of very small oxide crystals, which evidently are dissolved upon current interruption due to dissociative oxide dissolution. The transfer of dissolved metal ions to the cathode followed by cathodic deposition of the metal, however, can be completely prohibited, if the potential of the cathode due to optimal electrocatalysis of cathodic hydrogen evolution proceeds with an over-... 

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