**1. Introduction**

Electrodeposition of metals or its oxides is one of oldest themes in electrochemical science [1-6]. The first studies on this topic are dated from early nineteenth century, using galvanic cells as a power source [3-6]. Despite the antiquity, electrodeposition remains a much studied topic. Themes as supercapacitors or electrochemical cells devices have raised considerable attention [7-17]. In this case, the electrodeposition technique is of great interest due to their unique principles and flexibility in the control of the structure and morphology of the oxide electrodes.

One of the most modern applications of oxides electrodeposition is solar cells. Investigation of the development of environmentally friendly low cost solar cells with cheaper semicon‐ ductor materials is extremely important for the development of green energy technology. The electrodeposition of Cu2O, TiO2 and many others oxides, is a very promissory research field due the low cost and high efficiency of this electrochemical method.

Moreover, two new topics that also deserves attention is the application of electrodeposition in solid oxide fuel cells (SOFC) and the metals recycling. In this field of study the electrode‐ position also contributes very significantly. For SOFC, the electrodeposition is used to formation of protective coating on electrical interconnects. By other hand, in the metal recycling, the electrodeposition is the cheaper and simple method by obtention of metallic elements.

Thus, in this chapter will be reported some theoretical aspects about electrodeposition of metals and oxides and their applications more modern and relevant. Among these applications will be treated with one application of electrodeposition in supercapacitors, solar cells, recycling of metals and electrical interconects for solid oxide fuel cells (SOFC).

© 2013 Garcia et al.; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2013 Garcia et al.; licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### **1.1. Theoretical foundation of electrodeposition**

In the early stages of electrodeposition, the limiting step corresponds to electrons transfer from work electrode for metallic ions in solution. In this case the relation between the current and the overpotential for electrodeposition is given by Eq. 1 [1-4]. In this equation, F is Faraday's constant, k is a constant, C is the concentration of metal ions in solution, α corresponds to a coefficient of symmetry (near 0.5), η corresponds to overpotential, R is the ideal gas constant and T the absolute temperature, in Kelvin. There is an exponential dependence between the current and applied overpotential. Obviously that, with increasing of overpotential, the ionic current that electrolyte can supply is limited by the other processes as such material transport or electrical conductivity [2]. Through Coulomb's law, we obtain the relation of thickness with the charge density (d = MMq/nFρ). Where, MM corresponds to the molecular weight, q is the charge density, n corresponds the charge of metal ions and ρ is the density.

$$\dot{q} = -FkC \exp\left(\frac{aF\eta}{RT}\right) \tag{1}$$

by Equation 2. In pH =6.00 the concentration of H+

6.00.

authors this effect appears due the H+

(instantaneous nucleation).

tration, thus, the cobalt electrodeposition is the principal reaction.

(a) (b)

**40**

**60**

**Charge Efficiency (**

**Figure 1.** a) Cobalt electrodeposition on a platinum electrode. The ionic concentration of Co+2 is 1.00 molL-1 and the scan rate was 1 0mVs-1. (b) Charge efficiency for cobalt electrodeposition in a range of potentials, and at pH 1.50 and

The morphological aspects are also influenced by parallels reactions1,2,3.The figure 2 shows the metallic cobalt electrodeposited on ferritic steel in pH = 1.50 and 6.00. It clearly appears that the deposit at pH = 1.50 is more compact than the deposit at pH = 6.00. According to many

of the nucleation models to the initial electrodeposition stages shows that at pH=6.00, the nuclei grow progressively (progressive nucleation). SEM showed a three-dimensional nucleus growth. With the decrease in pH to 1.50, the nucleation process becomes instantaneous

(a) (b)

**Figure 2.** Scanning Electron Microscopy images of cobalt electrodeposited in: (a) pH= 6.00 and (b) pH 1.50. The po‐

tential applied of −1.00 V and charge density was 10.0 C cm− 2 and the electrolyte was CoSO4 1 molL-1.

**)**

**80**

**100**

is very low compared with cobalt concen‐

Metallic and Oxide Electrodeposition http://dx.doi.org/10.5772/55684 103

**-1.8 -1.6 -1.4 -1.2 -1.0 -0.8**

**E (V)**

 **pH = 1.50 pH = 6.00**

reduction onto surface of growth islands. The application

In electrodeposition, is very common the use of potential versus current curves called vol‐ tammetry. The figure 1-a shows a typical cyclic voltammetry for cobalt electrodeposition on a steel electrode. The cyclic voltammetry is used to identify the potential where begins the electrodeposition and the electrodissolution. In cathodic scan, for potential more negative than -0.70 V occurs the Co+2 reductions for metallic cobalt onto steel electrode. In anodic scan, the cobalt dissolution begins in -0.3 V. The voltammogram shown in figure 1-a represents the cobalt electrodeposition, however, many others metallic electrodeposition follow the same pattern.

In the electrodeposition of Mn+ ions using the aqueous media (Eq. 2) is always observed the hydrogen evolution reaction (HDR) represented by equation 2. This results principally in reducing the loading efficiency of electrodeposition [1-5].

$$\rm M\_{(aq)}^{n+} + \rm n\bar{e}^- \to \rm M\_{(s)} \tag{2}$$

$$2H^{+}\_{\text{(aq)}} + 2e^{-} \to H\_{2(g)} \tag{3}$$

The charge efficiency is an important aspect in metal electrodeposition. In this case, the parallel reaction showed in the equations 2 has a great influence in the electrodeposition. The figure 1b shows the charge efficiency for cobalt electrodeposition in pH = 1.50 (high concentration of H+ ) and pH = 6.00 (low concentration of H+ ). In pH = 1.50 note that the maximum efficiency (about 68%), while in pH = 6.00 the charge efficiency value can easily reach 95%. This occurs because at high concentrations of H+ , part of charge is used for promotes the reaction shown by Equation 2. In pH =6.00 the concentration of H+ is very low compared with cobalt concen‐ tration, thus, the cobalt electrodeposition is the principal reaction.

**1.1. Theoretical foundation of electrodeposition**

102 Modern Surface Engineering Treatments

pattern.

H+

In the early stages of electrodeposition, the limiting step corresponds to electrons transfer from work electrode for metallic ions in solution. In this case the relation between the current and the overpotential for electrodeposition is given by Eq. 1 [1-4]. In this equation, F is Faraday's constant, k is a constant, C is the concentration of metal ions in solution, α corresponds to a coefficient of symmetry (near 0.5), η corresponds to overpotential, R is the ideal gas constant and T the absolute temperature, in Kelvin. There is an exponential dependence between the current and applied overpotential. Obviously that, with increasing of overpotential, the ionic current that electrolyte can supply is limited by the other processes as such material transport or electrical conductivity [2]. Through Coulomb's law, we obtain the relation of thickness with the charge density (d = MMq/nFρ). Where, MM corresponds to the molecular weight, q is the

charge density, n corresponds the charge of metal ions and ρ is the density.

exp *<sup>F</sup> i FkC*

= - ç ÷

*RT* æ ö a h

In electrodeposition, is very common the use of potential versus current curves called vol‐ tammetry. The figure 1-a shows a typical cyclic voltammetry for cobalt electrodeposition on a steel electrode. The cyclic voltammetry is used to identify the potential where begins the electrodeposition and the electrodissolution. In cathodic scan, for potential more negative than -0.70 V occurs the Co+2 reductions for metallic cobalt onto steel electrode. In anodic scan, the cobalt dissolution begins in -0.3 V. The voltammogram shown in figure 1-a represents the cobalt electrodeposition, however, many others metallic electrodeposition follow the same

In the electrodeposition of Mn+ ions using the aqueous media (Eq. 2) is always observed the hydrogen evolution reaction (HDR) represented by equation 2. This results principally in

> ( ) ( ) *<sup>n</sup> M ne M aq <sup>s</sup>*

( ) 2( ) 2 2 *H eH aq <sup>g</sup>*

The charge efficiency is an important aspect in metal electrodeposition. In this case, the parallel reaction showed in the equations 2 has a great influence in the electrodeposition. The figure 1b shows the charge efficiency for cobalt electrodeposition in pH = 1.50 (high concentration of

(about 68%), while in pH = 6.00 the charge efficiency value can easily reach 95%. This occurs

reducing the loading efficiency of electrodeposition [1-5].

) and pH = 6.00 (low concentration of H+

because at high concentrations of H+

è ø (1)

+ - + ® (2)

+ - + ® (3)

). In pH = 1.50 note that the maximum efficiency

, part of charge is used for promotes the reaction shown

**Figure 1.** a) Cobalt electrodeposition on a platinum electrode. The ionic concentration of Co+2 is 1.00 molL-1 and the scan rate was 1 0mVs-1. (b) Charge efficiency for cobalt electrodeposition in a range of potentials, and at pH 1.50 and 6.00.

The morphological aspects are also influenced by parallels reactions1,2,3.The figure 2 shows the metallic cobalt electrodeposited on ferritic steel in pH = 1.50 and 6.00. It clearly appears that the deposit at pH = 1.50 is more compact than the deposit at pH = 6.00. According to many authors this effect appears due the H+ reduction onto surface of growth islands. The application of the nucleation models to the initial electrodeposition stages shows that at pH=6.00, the nuclei grow progressively (progressive nucleation). SEM showed a three-dimensional nucleus growth. With the decrease in pH to 1.50, the nucleation process becomes instantaneous (instantaneous nucleation).

**Figure 2.** Scanning Electron Microscopy images of cobalt electrodeposited in: (a) pH= 6.00 and (b) pH 1.50. The po‐ tential applied of −1.00 V and charge density was 10.0 C cm− 2 and the electrolyte was CoSO4 1 molL-1.

#### **1.2. Oxide electrodeposition**

In the electrodeposition of oxides, more of a method can be adopted. From a metal layer previously electrodeposited can be performed a polarization in alkaline solution such as NaOH, KOH etc. This results can be expressed by equations 4 and 5. The first step is the metal dissolution that is an electrochemical process (Eq. 4). The second step (Eq. 5) is the chemical process due precipitation of hydroxide on the surface of substrate. This method produces films with high adherence and a reduced thickness. This probably occurs due to formation of an oxide layer on a metal layer with a high surface area [4-5].

$$M\_{(s)} \to M\_{(aq)}^{n+} + n e^- \tag{4}$$

Another method used for the formation of metal oxides is the anodic electrodeposition In this

Basically in the most papers, the electrodeposition can occur by passing a current fixed or by imposing a fixed potential. In both cases, the energy for the formation of nuclei for growth is given by Eq. 7 (the nucleation energy). In this equation, N\* is the number of atoms per nucleus

corresponds to the overpotential applied. The second term represents the increased surface tension caused by the addition of ad-atoms. Thus, the greater number of atoms larger the cluster size as shows in the Eq. 7. Optimizing the nucleation energy appropriately (dΔG/dη=0) we get the expression for maximum atoms number that each nuclei may contain (Eq. 8). where

> <sup>0</sup> \* || ( ) *G N ze N <sup>i</sup>* D =- + h j

> > <sup>2</sup> ( ) *crit i <sup>s</sup> <sup>N</sup> z e* pe

mAcm-2. In the higher current density, the grain size is much smaller.

**2. Modern applications of metallic electrodeposition**

is possible formation of oxide layers through electrochemical methods.

0

Thus, it is not difficult to observe that the higher overpotential, lower the number of atoms per growth nucleus and consequently the smaller grain size obtained. The electrodeposition created under low overpotential have smaller grains, while samples prepared under high overpotential are formed by pyramidal structures. In fact this observation can also be seen in Figure 2. This Figure shows the SEM of electrodeposited cobalt obtained in 100 and 200

Now we can see that, unlike other deposition techniques, in the electrodeposition, the characteristics of formed film are related with simple parameters as pH and overpotential applied. This makes with this method became versatile and inexpensive compared to other

With the electrodeposition is possible to achieve very thin layers of metal over the other metal or another electrical conductor material. Since that a metallic film is onto conductive substrate

2

h

is the charge of the metal ion, e0 is the elementary charge and | η |

<sup>¬</sup> (7)

Metallic and Oxide Electrodeposition http://dx.doi.org/10.5772/55684 105

(8)

<sup>=</sup> (9)

( ) 2 2 () ( ) <sup>2</sup> *<sup>n</sup> M mH O Mn O mH ne aq n s aq* <sup>+</sup> ® + - + ++

case the equation that represents this generic reaction is shows in Eq 6.

**1.3. Electrodeposition morphology**

ε is the surface energy and s is the area of each atom.

growth (cluster), zi

deposition methods.

$$\text{M}^{n+}\_{\text{(aq)}} + \text{OH}^{-}\_{\text{(aq)}} \to \text{M(OH)}\_{n\text{(ads)}}\tag{5}$$

Figure 3-a shows an electrodeposited layer of cobalt over a stainless steel. Figure 3-b represents the cobalt film after a potentiostatic polarization at 0.7 V (Ag / AgCl reference) during 200 s. The electrolyte used was NaOH 6 molL-1.

The cobalt film electrodeposited showed a high contact area. This can also be visualized in the oxide/ hydroxide formed after the anodic polarization in NaOH 6 molL-1.

**Figure 3.** *Scanning electron microscopy images of (a) Cobalt electrodeposition on a steel electrode.(b)* Cobalt film after a potentiostatic polarization at 0.7 V (Ag / AgCl reference) during 200 s in NaOH 6 molL-1.

Another method frequently used for electrodeposition of hydroxides on the conductive substrates is the use of nitrates as counter ions. In this case the hydroxyl ions are generated by reduction of nitrate ions in solution (Eq. 5).. The film of metallic hydroxide is then generated as shown by Eq 4.

$$\text{NO}\_{3(aq)}^{-} + 7H\_{2}O + 8e^{-} \rightarrow \text{NH}\_{4(aq)}^{+} + 10\text{OH}^{-}\_{(aq)}\tag{6}$$

Another method used for the formation of metal oxides is the anodic electrodeposition In this case the equation that represents this generic reaction is shows in Eq 6.

$$\text{Mn}^{n+}\_{\text{(aq)}} + m\text{H}\_2\text{O} \overset{\rightarrow}{\longleftrightarrow} \text{Mn}\_2\text{O}\_{n\text{(s)}} + 2m\text{H}^+\_{\text{(aq)}} + ne^- \tag{7}$$

#### **1.3. Electrodeposition morphology**

**1.2. Oxide electrodeposition**

104 Modern Surface Engineering Treatments

oxide layer on a metal layer with a high surface area [4-5].

The electrolyte used was NaOH 6 molL-1.

as shown by Eq 4.

In the electrodeposition of oxides, more of a method can be adopted. From a metal layer previously electrodeposited can be performed a polarization in alkaline solution such as NaOH, KOH etc. This results can be expressed by equations 4 and 5. The first step is the metal dissolution that is an electrochemical process (Eq. 4). The second step (Eq. 5) is the chemical process due precipitation of hydroxide on the surface of substrate. This method produces films with high adherence and a reduced thickness. This probably occurs due to formation of an

*<sup>n</sup> M M ne s aq* ® + + - (4)

+ - + ® (5)

() ( )

oxide/ hydroxide formed after the anodic polarization in NaOH 6 molL-1.

a potentiostatic polarization at 0.7 V (Ag / AgCl reference) during 200 s in NaOH 6 molL-1.

() () ( ) ( ) *<sup>n</sup> M OH M OH aq aq n ads*

Figure 3-a shows an electrodeposited layer of cobalt over a stainless steel. Figure 3-b represents the cobalt film after a potentiostatic polarization at 0.7 V (Ag / AgCl reference) during 200 s.

The cobalt film electrodeposited showed a high contact area. This can also be visualized in the

(a) (b)

**Figure 3.** *Scanning electron microscopy images of (a) Cobalt electrodeposition on a steel electrode.(b)* Cobalt film after

Another method frequently used for electrodeposition of hydroxides on the conductive substrates is the use of nitrates as counter ions. In this case the hydroxyl ions are generated by reduction of nitrate ions in solution (Eq. 5).. The film of metallic hydroxide is then generated


3( ) 2 4( ) ( ) 7 8 10 *NO H O e NH OH aq aq aq*

Basically in the most papers, the electrodeposition can occur by passing a current fixed or by imposing a fixed potential. In both cases, the energy for the formation of nuclei for growth is given by Eq. 7 (the nucleation energy). In this equation, N\* is the number of atoms per nucleus growth (cluster), zi is the charge of the metal ion, e0 is the elementary charge and | η | corresponds to the overpotential applied. The second term represents the increased surface tension caused by the addition of ad-atoms. Thus, the greater number of atoms larger the cluster size as shows in the Eq. 7. Optimizing the nucleation energy appropriately (dΔG/dη=0) we get the expression for maximum atoms number that each nuclei may contain (Eq. 8). where ε is the surface energy and s is the area of each atom.

$$
\Delta G = -N^\* z\_i e\_0 \mid \eta \mid +\varphi(N) \tag{8}
$$

$$N\_{crit} = \frac{\pi \mathbf{z}^2 \mathbf{s}}{\left(\mathbf{z}\_i \mathbf{e}\_{\phantom{0}} \eta\right)^2} \tag{9}$$

Thus, it is not difficult to observe that the higher overpotential, lower the number of atoms per growth nucleus and consequently the smaller grain size obtained. The electrodeposition created under low overpotential have smaller grains, while samples prepared under high overpotential are formed by pyramidal structures. In fact this observation can also be seen in Figure 2. This Figure shows the SEM of electrodeposited cobalt obtained in 100 and 200 mAcm-2. In the higher current density, the grain size is much smaller.

Now we can see that, unlike other deposition techniques, in the electrodeposition, the characteristics of formed film are related with simple parameters as pH and overpotential applied. This makes with this method became versatile and inexpensive compared to other deposition methods.

#### **2. Modern applications of metallic electrodeposition**

With the electrodeposition is possible to achieve very thin layers of metal over the other metal or another electrical conductor material. Since that a metallic film is onto conductive substrate is possible formation of oxide layers through electrochemical methods.

**Figure 4.** SEM images of cobalt electrodeposited from 0.5 molL-1 sulphate cobalt at 200mAcm-2 and (b) at 100 mAcm-2.

#### **2.1. Supercacitors**

The energy always played an important role in human being's life [4]. Thus, it is necessary to study about renewable energy sources to reduce the energy consumption. Because this, the capacitors are used in the transport systems as a mean to store energy and reuse it during short periodic intervals. Basically, the conventional capacitors consist of two conducting electrodes separated by an insulating dielectric material and The application of this electrical device is the storage of energy.

When a voltage is applied to a capacitor, opposite charges accumulate on the surfaces of each electrode. The differential capacitance is defined as C = dQ/dV where V is the difference of potential between the capacitor plates and Q is the accumulated charge in the active surface of capacitor material. The geometric relation of capacitance can be providing by equation 3 [6-7]. Where A and d are the geometric area and distance between capacitors plates respec‐ tively. The ε and ε<sup>o</sup> are the dielectric constant between the plates (no unit) and the vacuum permittivity (8.8 x 10-12 Fm-1) respectively.

$$\mathbf{C} = \frac{A\varepsilon\varepsilon^{0}}{d} \tag{10}$$

redox reaction are known as supercapacitors. The most used and commercial supercapaci‐ tors are basically made of an oxide semiconductor with a reversible redox couple. This oxide has to show a transition metal ion, which must be able to assume different valen‐

Metallic and Oxide Electrodeposition http://dx.doi.org/10.5772/55684 107

Among the oxide materials for application in supercapacitors, ruthenium and iridium oxides have achieved more attention. In many cases the capacitance depends on the preparation method and the deposition of oxides materials. Capacitances up to 500 F/g [5] or 720 F/g are reported for amorphous water-containing ruthenium oxides [6-9]. The great disadvantages of RuO2 is the high cost and its low porosity structure [9-10]. Thus in recent works great efforts were undertaken in order to find new and cheaper materials [4-6]. A cheaper and widely used alternative is the activated carbon. The specific capacitance of this material is about 800 F/g [6]. In this context the researchers have tried to develop a material having a low cost, high surface

Cobalt oxides are attractive in view of their layered structure and their reversible electro‐ chemical reactions. The possibility of enhanced performance through different preparative methods, also interests the scientists. The spinel cobalt oxide Co3O4, for example, can be obtained from hydroxide cobalt (II) previously deposited onto a conductive substrate (steel for example). Is enough a thermal oxidation in 400 degrees in air atmosphere, for formation of Co3O4. In the Co3O4 oxide, the reversibility of both redox process (Co+2/Co+3) (Eq. 4) and (Eq. 5) (Co+3/Co+4) in KOH 6 molL-1 is very high and promising for capacitive applications in

> 2 3 C OH [C OH ] e (net) (aq) (ads) *o o* ® + - + - ¬

3 4 (ads) (ads) (aq) (ads) 2 [C OH ] OH [C O ] e *o o H O*

¬

In figure 3 is shown a schematic of obtaining Co3O4. The previously electrodeposited cobalt can be subjected to a anodic polarization in solution of KOH 6molL-1. With a thermal treatment

leads to the formation of a layer of chemical composition of Co(OH)2 [8-9]. The Figure 5 represents a cyclic voltammetry of a steel electrode coated with metallic cobalt in KOH 6molL-1. Note that the first peak (around -0.6 V) is related with the electrodissolution of cobalt

2

® +- + -


+ -+ (11)

C the phase Co3O4 is formed onto substrate. This procedure

() ( ) <sup>2</sup> *Co Co e s aq* ® + + - (13)

area, high conductivity and high stability in alkaline medium.

ces and strong bonding power.

*2.1.1. Cobalt oxides*

electrochemical devices [7-14].

of 20 hours at a temperature of 400 o

(Eq. 13) that leads to the formation of Co(OH)2 (Eq. 14) [11-12].

In this form C is given in Faraday (F). Thus, for practical applications the capacitors plates must have high area and very small separation distance. The first capacitors of carbon materials can provide the specific capacitances of 15 – 50 μFcm-2 [8]. The materials with capacitance densities of one, two or more hundreds of Fcm-2 or Fg-1 have been denominat‐ ed as"supercapacitors" or "ultracapacitors"[4-8]. There are two principal types of superca‐ pacitors: (a) the double-layer capacitor, and (b) the redox pseudocapacitor, the latter being developed in this paper. The high performances of capacitors that work with reversible redox reaction are known as supercapacitors. The most used and commercial supercapaci‐ tors are basically made of an oxide semiconductor with a reversible redox couple. This oxide has to show a transition metal ion, which must be able to assume different valen‐ ces and strong bonding power.

Among the oxide materials for application in supercapacitors, ruthenium and iridium oxides have achieved more attention. In many cases the capacitance depends on the preparation method and the deposition of oxides materials. Capacitances up to 500 F/g [5] or 720 F/g are reported for amorphous water-containing ruthenium oxides [6-9]. The great disadvantages of RuO2 is the high cost and its low porosity structure [9-10]. Thus in recent works great efforts were undertaken in order to find new and cheaper materials [4-6]. A cheaper and widely used alternative is the activated carbon. The specific capacitance of this material is about 800 F/g [6]. In this context the researchers have tried to develop a material having a low cost, high surface area, high conductivity and high stability in alkaline medium.

#### *2.1.1. Cobalt oxides*

**2.1. Supercacitors**

106 Modern Surface Engineering Treatments

mAcm-2.

the storage of energy.

tively. The ε and ε<sup>o</sup>

permittivity (8.8 x 10-12 Fm-1) respectively.

The energy always played an important role in human being's life [4]. Thus, it is necessary to study about renewable energy sources to reduce the energy consumption. Because this, the capacitors are used in the transport systems as a mean to store energy and reuse it during short periodic intervals. Basically, the conventional capacitors consist of two conducting electrodes separated by an insulating dielectric material and The application of this electrical device is

(a) (b)

**Figure 4.** SEM images of cobalt electrodeposited from 0.5 molL-1 sulphate cobalt at 200mAcm-2 and (b) at 100

When a voltage is applied to a capacitor, opposite charges accumulate on the surfaces of each electrode. The differential capacitance is defined as C = dQ/dV where V is the difference of potential between the capacitor plates and Q is the accumulated charge in the active surface of capacitor material. The geometric relation of capacitance can be providing by equation 3 [6-7]. Where A and d are the geometric area and distance between capacitors plates respec‐

*<sup>C</sup>* <sup>=</sup> *<sup>A</sup>εε* <sup>0</sup>

In this form C is given in Faraday (F). Thus, for practical applications the capacitors plates must have high area and very small separation distance. The first capacitors of carbon materials can provide the specific capacitances of 15 – 50 μFcm-2 [8]. The materials with capacitance densities of one, two or more hundreds of Fcm-2 or Fg-1 have been denominat‐ ed as"supercapacitors" or "ultracapacitors"[4-8]. There are two principal types of superca‐ pacitors: (a) the double-layer capacitor, and (b) the redox pseudocapacitor, the latter being developed in this paper. The high performances of capacitors that work with reversible

are the dielectric constant between the plates (no unit) and the vacuum

*<sup>d</sup>* (10)

Cobalt oxides are attractive in view of their layered structure and their reversible electro‐ chemical reactions. The possibility of enhanced performance through different preparative methods, also interests the scientists. The spinel cobalt oxide Co3O4, for example, can be obtained from hydroxide cobalt (II) previously deposited onto a conductive substrate (steel for example). Is enough a thermal oxidation in 400 degrees in air atmosphere, for formation of Co3O4. In the Co3O4 oxide, the reversibility of both redox process (Co+2/Co+3) (Eq. 4) and (Eq. 5) (Co+3/Co+4) in KOH 6 molL-1 is very high and promising for capacitive applications in electrochemical devices [7-14].

$$\text{Co}^{\cdot 2} \overset{\text{\tiny \text{\textquotedblleft}}{\text{ (net)}}}{\text{\textquotedblleft}} \text{\textquotedblright} \overset{\text{\tiny \text{\textquotedblleft}}{\text{ (aq)}}}{\text{\textquotedblleft}} \text{\textquotedblright} \text{\textquotedblleft} \text{\textquotedblright} \text{\textquotedblleft} \text{\textquotedblright} \text{\textquotedblright} \text{\textquotedblleft} \text{\textquotedblright} \text{\textquotedblleft} \text{\textquotedblleft} \text{\textquotedblright} \text{\textquotedblleft} \text{\textquotedblleft} \text{\textquotedblleft} \text{\textquotedblright} \text{\textquotedblleft} \text{\textquotedblleft} \text{\textquotedblleft} \text{\textquotedblleft} \text{\textquotedblleft} \text{\textquotedblleft} \text{\textquotedblleft} \text{\textquotedblleft} \text{\textquotedblleft} \text{\textquotedblleft} \text{\textquotedblleft} \text{\textquotedblleft} \text{\textquotedblleft} \text{\textquotedblleft} \text{\textquotedblleft} \text{\textquotedblleft} \text{\textquotedblleft} \text{\textquotedblleft} \text{\textquotedblleft} \text{\textquotedblleft} \text{\textquotedblleft} \text{\textquotedblright}} \text{\textquotedblleft} \text{\textquotedblleft} \text{\textquotedblleft} \text{\textquotedblleft} \text{\textquotedblleft} \text{\textquotedblleft} \text{\textquotedblleft} \text{\textquotedblright}} \text{\textquotedblleft} \text{\textquotedblleft} \text{\textquotedblleft} \text{\textquotedblleft} \text{\textquotedblleft} \text{\textquotedblleft} \text{\textquotedblleft} \text{\textquotedblleft} \text{\textquotedblright}} \text{\textquotedblleft} \text{\textquotedblleft} \text{\textquotedblleft} \text{\textquoted$$

$$\left[\text{Co}-\text{OH}\_{\text{(ads)}}\right]^{+3}\text{(ads)} + \text{OH}^{-}\_{\text{(aq)}} \overset{\rightarrow}{\leftarrow} \left[\text{Co}-\text{O}\_{\text{(ads)}}\right]^{+4} + H\_2\text{O} + \text{e}^-\tag{12}$$

In figure 3 is shown a schematic of obtaining Co3O4. The previously electrodeposited cobalt can be subjected to a anodic polarization in solution of KOH 6molL-1. With a thermal treatment of 20 hours at a temperature of 400 o C the phase Co3O4 is formed onto substrate. This procedure leads to the formation of a layer of chemical composition of Co(OH)2 [8-9]. The Figure 5 represents a cyclic voltammetry of a steel electrode coated with metallic cobalt in KOH 6molL-1. Note that the first peak (around -0.6 V) is related with the electrodissolution of cobalt (Eq. 13) that leads to the formation of Co(OH)2 (Eq. 14) [11-12].

$$\text{Co}\_{(s)} \rightarrow \text{Co}\_{(aq)}^{2+} + 2e^- \tag{13}$$

$$\text{Co}^{2+}\_{\text{(aq)}} + \text{OH}^{-}\_{\text{(aq)}} \rightarrow \text{Co(OH)}\_{\text{2(ads)}} \tag{14}$$

with another the substrate, Zhou et al [14] obtained a specific capacitance of 2700Fg-1. In this case the substrate was Ni. Are shown in table 1 some values of specific capacitance for oxides

Potentiostatic Ti plates 1000 F g-1 Co(NO3)2 Ozone [13]

Potentiostatic Ni sheets 2700 F g−1 Co(NO3)2 - [14]

**Table 1.** Some values of specific capacitance for oxides and hydroxides of cobalt obtained by electrodeposition or

One promising material is hydrated amorphous or nanocrystalline manganese oxide, MnO2 nH2O this material has exhibited capacitances exceeding 200 F/g in solutions of several alkali salts, such as LiCl, NaCl, and KCl [16]. Chang and Tsai [17] have reported supercapacitance of 240 Fg−1 for hydrous MnO2 synthesized by potentiostatic method. By other hand Feng et al [16] reported supercapacitors of 521 Fg−1 for MnO2 multilayer nanosheets prepared galvanos‐ tatically. Both in galvanostatic or potentiostatic electrodeposition, the anodic electrodeposition

> () 2 2( ) ( ) 2 42 *Mn H O MnO H e aq s aq* <sup>+</sup> ® + - + ++

2( ) ( ) <sup>2</sup> ( ) *MnO Na e MnO Na s aq ads*

+ +

The model more relevant proposed to describe the charge and discharge cycles of MnO2 at electrolyte constituted by Na2SO4 is given in Eq. 8. This equation demonstrated that the capacitance of MnO2 is result of redox reaction that occurs in its surface. This redox reaction

+ - -+ ®

A fact which makes the MnO2 becomes most promising as supercapacitor, is the recycling Zn-MnO2 batteries. Few studies in the literature report the recycling of Zn-MnO2 batteries [18]. Thus, this line of research becomes extremely important to make supercapacitors based in

**Solution**

electrodeposit 310 Fg-1 LiOH 150 oC [15]

**Method for Co3O4**

<sup>¬</sup> (18)

ions [16-17].

<sup>¬</sup> (19)

**Reference**

109

Metallic and Oxide Electrodeposition http://dx.doi.org/10.5772/55684

and hydroxides of cobalt found most frequently in literature.

**Substrate Specific**

Co

for formation of MnO2 can be express by Eq. 7:

MnO2 more environmentally correct.

2

corresponds of electrochemical adsorption/desorption of Na+

**capacitance**

**Method for Co(OH)2**

Cyclic voltammetry

electrodeposition/calcination.

*2.1.2. Manganese oxides*

The second peak, around 0.0 V, is related to the formation of Co(OH)3 (Eq.15) [6-10]. The scheme showed in figure 5-b shows the obtention of Co3O4 from thermal oxidation.

$$\text{Co(OH)}\_{2(ads)} + \text{OH}^-\_{\text{(aq)}} \rightarrow \text{Co(OH)}\_{3(ads)} + 1e^- \tag{15}$$

**Figure 5.** a) A cyclic voltammetry of a steel electrode coated with metallic cobalt in KOH 6molL-1. (b) The obtention of Co3O4 from thermal oxidation.

There is also a method for the electrodeposition of a layer of Co(OH)2 without the need for an alkaline solution. The Co(OH)2 can be electrodeposited from a solution of cobalt nitrate. The many papers described this electrodeposition. The first step is the reduction of nitrate ions (Eq. 5). The reaction shown in the equation 5 promotes the alkalinization of electrical interface of metal "M". In a chemical step the Co(OH) is formed onto "M"(Eq. 6) [13-14].

$$\text{NO}\_{3(aq)}^{-} + \text{H}\_{2}\text{O} + 2e^{-} \rightarrow \text{NO}\_{2(g)} + \text{2OH}^{-}\_{\text{(aq)}}\tag{16}$$

$$\text{Ca}^{\text{+}2}\_{\text{(aq)}} + 2\text{OH}^{-}\_{\text{(aq)}} \xrightarrow{M} \text{M / Co(OH)}\_{\text{(s)}} \tag{17}$$

In the vast majority of papers about supercapacitors based on cobalt oxides at least one of the steps corresponds to electrodeposition. In an interesting case in the literature, Kung et al [13] performed the electrodeposition of Co(OH)2 on a Ti plate as shown by Eq.6. To obtain the Co3O4, the film of Co(OH)2 it was submitted to an atmosphere ozone. They obtained an excellent capacitance specific value around 1000Fg-1. Practically the same method however


with another the substrate, Zhou et al [14] obtained a specific capacitance of 2700Fg-1. In this case the substrate was Ni. Are shown in table 1 some values of specific capacitance for oxides and hydroxides of cobalt found most frequently in literature.

**Table 1.** Some values of specific capacitance for oxides and hydroxides of cobalt obtained by electrodeposition or electrodeposition/calcination.

#### *2.1.2. Manganese oxides*

2

() () 2( ) ( ) *Co OH Co OH aq aq ads*

scheme showed in figure 5-b shows the obtention of Co3O4 from thermal oxidation.

2( ) ( ) 3( ) ( ) () 1 *Co OH OH Co OH e ads aq ads*

(a) (b)

of metal "M". In a chemical step the Co(OH) is formed onto "M"(Eq. 6) [13-14].

2

3( ) 2 2( ) ( ) 2 2 *NO H O e NO OH aq <sup>g</sup> aq*

() () ( ) <sup>2</sup> /( ) *<sup>M</sup> Co OH M Co OH aq aq <sup>s</sup>*

In the vast majority of papers about supercapacitors based on cobalt oxides at least one of the steps corresponds to electrodeposition. In an interesting case in the literature, Kung et al [13] performed the electrodeposition of Co(OH)2 on a Ti plate as shown by Eq.6. To obtain the Co3O4, the film of Co(OH)2 it was submitted to an atmosphere ozone. They obtained an excellent capacitance specific value around 1000Fg-1. Practically the same method however


+ - + ¾¾® (17)

Co3O4 from thermal oxidation.

108 Modern Surface Engineering Treatments

**Figure 5.** a) A cyclic voltammetry of a steel electrode coated with metallic cobalt in KOH 6molL-1. (b) The obtention of

There is also a method for the electrodeposition of a layer of Co(OH)2 without the need for an alkaline solution. The Co(OH)2 can be electrodeposited from a solution of cobalt nitrate. The many papers described this electrodeposition. The first step is the reduction of nitrate ions (Eq. 5). The reaction shown in the equation 5 promotes the alkalinization of electrical interface

The second peak, around 0.0 V, is related to the formation of Co(OH)3 (Eq.15) [6-10]. The

+ - + ® (14)


One promising material is hydrated amorphous or nanocrystalline manganese oxide, MnO2 nH2O this material has exhibited capacitances exceeding 200 F/g in solutions of several alkali salts, such as LiCl, NaCl, and KCl [16]. Chang and Tsai [17] have reported supercapacitance of 240 Fg−1 for hydrous MnO2 synthesized by potentiostatic method. By other hand Feng et al [16] reported supercapacitors of 521 Fg−1 for MnO2 multilayer nanosheets prepared galvanos‐ tatically. Both in galvanostatic or potentiostatic electrodeposition, the anodic electrodeposition for formation of MnO2 can be express by Eq. 7:

$$\text{Mn}^{+2}\_{\text{(aq)}} + 2\text{H}\_2\text{O} \overset{\rightarrow}{\longleftrightarrow} \text{MnO}\_{\text{2(s)}} + 4\text{H}^+\_{\text{(aq)}} + 2e^- \tag{18}$$

The model more relevant proposed to describe the charge and discharge cycles of MnO2 at electrolyte constituted by Na2SO4 is given in Eq. 8. This equation demonstrated that the capacitance of MnO2 is result of redox reaction that occurs in its surface. This redox reaction corresponds of electrochemical adsorption/desorption of Na+ ions [16-17].

$$\text{MnO}\_{2(s)} + \text{Na}^+\_{(aq)} + e^- \overset{\rightarrow}{\leftarrow} \text{(MnO}\_2\text{"}\text{Na}^+\text{)}\_{ads} \tag{19}$$

A fact which makes the MnO2 becomes most promising as supercapacitor, is the recycling Zn-MnO2 batteries. Few studies in the literature report the recycling of Zn-MnO2 batteries [18]. Thus, this line of research becomes extremely important to make supercapacitors based in MnO2 more environmentally correct.

#### **2.3. Solar cells**

Photovoltaic (PV) cells are made up of two semi-conductor layers and one electrolyte. One layer containing a positive charge, the other a negative charge [19-22]. The n-type semicon‐ ductor receive radiation *hν,* thus valence electron is promoted to conduction band. One potential difference is established between the n-type semiconductor and the p-type semicon‐ ductor. This causes a photocurrent to flow through of the system. Basically, the electrolyte is used for circulation of ionic current which restores the nature of semiconductors. The figure 6 shows the scheme of photovoltaic cell. The researches are basically focuses on development of semiconductor electrodes and electrolytes cheaper and more efficient. Thus in this context the electrodeposition can contribute greatly. This is because the electrodeposition is a method simple, practical and inexpensive to produce both p-type as n-type [21,22].

Particularly, the obtention of *n-*type or *p*-type semiconductor can be controlled from pH of

Metallic and Oxide Electrodeposition http://dx.doi.org/10.5772/55684 111

**Figure 8.** The cyclic voltammetry of a Pt electrode in Na2SO4 and CuSO4 + Na2SO4 solution.

electrodeposition bath (Figure 8).

**Figure 7.** Representation of energy diagram for formation of Cu2O.

**Figure 6.** Representation of a photovoltaic cell using two semiconductor electrodes and an electrolyte.
