Electrodeposition of Metals with Hydrogen Evolution

The cauliflower-like agglomerates of copper grains were formed at an overpotential of 550 mV, where there was no hydrogen evolution (Fig. 5.1a). The very branchy 3D (three dimensional) dendrites were formed at an overpotential of 650 mV where hydrogen evolution was very small and corresponds to of 

850 mV (//i,av(H2) = 30.7 %) [2], This type of deposits is denoted as the 3D (three dimensional) foam [3] or the honeycomb-like stmctures [4, 5], The uniform distribution of morphological forms clearly points out the strong effect of evolved hydrogen on formation of this deposit type. The concept of effective overpotential is proposed to explain formation of this structure type [4, 5]. 

2 Mechanism of Formation of the Honeycomh-Like Structure The Concept of Effective Overpotential  

The hydrogen evolution influences the hydrodynamic conditions inside electrochemical cell [6-8]. The increase in hydrogen evolution rate leads to the decrease of the diffusion layer thickness and hence to the increase of limiting diffusion current density of electrode processes. It was shown [6] that if the rate of gas evolutimi at the electrode is larger than 100 cm /cm min ( 5 A cm ), the diffusion layer becomes only a few micrometers thick. A coverage of an electrode surface with gas bubbles can be about 30 % [6]. If the thickness of the diffusion layer in conditions of natural convection is 5 10 cm and in strongly stirred electrolyte 5 10 cm [9], it is clear that gas evolution is the most effective way of the decrease of mass transport limitations for electrochemical processes in mixed activation-diffusion control. 

For electrochemical process in mixed activation-diffusion control, the overpotential t] and the current density i are related by Eq. (1.31) [10]  

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Over the past two decades, ionic liquids (ILs) have attracted considerable interest as media for a wide range of applications. For electrochemical applications they exhibit several advantages over the conventional molecular solvents and high temperature molten salts they show good electrical conductivity, wide electrochemical windows of up to 6 V, low vapor pressure, non-flammability in most cases, and thermal windows of 300-400 °C (see Chapter 4). Moreover, ionic liquids are, in most cases, aprotic so that the complications associated with hydrogen evolution that occur in aqueous baths are eliminated. Thus ILs are suitable for the electrodeposition of metals and alloys, especially those that are difficult to prepare in an aqueous bath. Several reviews on the electrodeposition of metals and alloys in ILs have already been published [1-4], A selection of published examples of the electrodeposition of alloys from ionic liquids is listed in Table 5.1 [5-40]. Ionic liquids can be classified into water/air sensitive and water/air stable ones (see Chapter 3). Historically, the water-sensitive chloroaluminate first generation ILs are the most intensively studied. However, in future the focus will rather be on air- and water-stable ionic liquids due to their variety and the less strict conditions under which... 

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... 

This technique is applied to mixtures of metal ions in an acidic solution for the purpose of electroseparation only the metal ions with a standard reduction potential above that of hydrogen are reduced to the free metal with deposition on the cathode, and the end of the reduction appears from the continued evolution of hydrogen as long as the solution remains acidic. Considering the choice of the cathode material and the nature of its surface, it must be realized that the method is disturbed if a hydrogen overpotential occurs in that event no hydrogen is evolved and as a consequence metal ions with a standard reduction potential below that of hydrogen will still be reduced a classic example is the electrodeposition of Zn at an Hg electrode in an acidic solution. 

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