Overpotential deposited hydrogen

It has been established that H d is the reactive intermediate in the HOR, and therefore the kinetics of the HOR is mainly determined by the interaction between H d and the Pt surface atoms. There are two different possible states of adsorbed hydrogen. One is the Hupd (the underpotentially deposited hydrogen), which is the strongly adsorbed stated formed on the surface at potentials more positive than the Nernst potential, and the other is the H pd (the overpotentially deposited hydrogen), which is the weakly adsorbed state formed close to or negative with respect to the Nernst potential. ... 

It is evident that decreases with a decrease in the electrode potential in the entire range. At the lowest potentials there is an influence of the overpotentially deposited hydrogen (HOPD), which is related to the classical hydrogen evolution, and dissolved hydrogen formation at the solution around the electrode. The kinetics is very fast and was measured without a potentiostat [242]. Because the adsorption isotherm is rather complex [248] no rate constants were determined. It should be added that in earlier studies, at monocrystaUine Pt, it was found that R i was potential independent [249], which may be connected with equipment artifacts. 

Conway BE, Bai L (1985) Determination of the adsorption behaviour of overpotential-deposited hydrogen-atom species in the cathodic hydrogen-evolution reaction by analysis of potential-relaxation transients. J Chem Soc Faraday Trans 81 1841-1862... 

A mercury cathode finds widespread application for separations by constant current electrolysis. The most important use is the separation of the alkali and alkaline-earth metals, Al, Be, Mg, Ta, V, Zr, W, U, and the lanthanides from such elements as Fe, Cr, Ni, Co, Zn, Mo, Cd, Cu, Sn, Bi, Ag, Ge, Pd, Pt, Au, Rh, Ir, and Tl, which can, under suitable conditions, be deposited on a mercury cathode. The method is therefore of particular value for the determination of Al, etc., in steels and alloys it is also applied in the separation of iron from such elements as titanium, vanadium, and uranium. In an uncontrolled constant-current electrolysis in an acid medium the cathode potential is limited by the potential at which hydrogen ion is reduced the overpotential of hydrogen on mercury is high (about 0.8 volt), and consequently more metals are deposited from an acid solution at a mercury cathode than with a platinum cathode.10... 

For reductions, hanging mercury drop electrodes or mercuryfilm electrodes are frequently used owing to their microscopic smoothness and because of the large overpotential for hydrogen evolution characteristic for this electrode material. Mercury film electrodes are conveniently prepared by the electrochemical deposition of mercury on a platinum electrode from an acidic solution of an Hg2+ salt, e.g. the nitrate. However, the oxidation of mercury metal to mercury salts or organomercurials at a low potential, 0.3-0.4 V versus the saturated calomel electrode (SCE), prevents the use of these electrodes for oxidations. 

In Section 8.2 the basics of pulsed dectrodeposition (PED) will be described for the case of aqueous electrolytes which allow the deposition of comparatively noble metals like Cu, Ni, Pd, or Au less noble metals like Fe or Zn can still be electrodeposited from aqueous electrolytes because they exhibit a comparatively large overpotential for hydrogen evolution. However, the main limitation of aqueous dectrolytes, of course, is their narrow electrochemical window which adversely affects the electrodeposition of metals like A1 or Ta. Therefore, recently, the PED technique has been extended to ionic liquids as electrolytes. General electrochemical aspects of ionic liquids can be found in Ref. [44] here, in Section 8.3, we will only address the technical aspects with respect to PE D. Examples of nanometals and nanoalloys electrodeposited from chloroaluminate-based ionic liquids are given in... 

It is known that a number of metals which do not amalgamate with mercury are capable of lowering the overpotential of hydrogen production at mercury electrodes and indeed catalytic hydrogen waves attributable to metal deposition can be observed in polarograms of simple salts of Rh, jj. 394,395 pj 394,395 Qy which amalgamate with... 

Kvokova and Lainer electrodeposited pure Re and Re-Cr alloy on Mo substrate. For the deposition of Re itself, two baths were used one containing perhhenic acid, and the other containing potassium perrhenate. In the first bath, the discharge of hydrogen ions was enhanced. The authors attributed the low overpotential of hydrogen on Re to its lattice parameter (a = 2.758 A). However, a justification to this theory has not been proposed. For both deposition of Re and Re-Cr alloy, the concentration polarization was foimd to be insignificant compared to the activation polarization. 

In most cases, additional metal ions are present, mainly those with a positive standard potential, which are deposited preferentially at the cathode and have a low overpotential for hydrogen evolution. On the other hand, zinc sponge is deposited even at relatively low current densities during electrolysis of ZnSO solutions in the presence of Cu, As or Sb ions these are nobler than zinc and are deposited at the maximum rate (i.e. at their limiting current) in a powdery form, and the zinc deposited on them "copies" their powdery structure. Powdered metals (Fe, Cu, Mi) are usually deposited at the cathode at current densities higher than the limiting one with simultaneous evolution of hydrogen. 

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