*2.3.1. Semiconductive Cu2O*

Among various transition metal oxides, cuprous oxide (Cu2O) has attracted much attention. This because the Cu2O it is a non toxic and inexpensive semiconductor material. Cu2O is a direct band gap (1.80 eV) (Figure 7) semiconductor material and has a high absorption in the visible region of the solar spectrum [19,22,23].

The Cu2O may be a p-type semiconductor well as the n-type. In Cu2O-p there are vacancies of cuprous ions (Cu+ ) and in Cu2O-n, there are oxygen vacancies. Many papers on the Cu2O-p show that this material has a lower efficiency than 2% attributed to heterojunction represented by metallic cooper (Cu/Cu2O-p) [22]. Thus, many studies have been focused on Cu2O-n.

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

**2.3. Solar cells**

110 Modern Surface Engineering Treatments

*2.3.1. Semiconductive Cu2O*

cuprous ions (Cu+

visible region of the solar spectrum [19,22,23].

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].

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

Among various transition metal oxides, cuprous oxide (Cu2O) has attracted much attention. This because the Cu2O it is a non toxic and inexpensive semiconductor material. Cu2O is a direct band gap (1.80 eV) (Figure 7) semiconductor material and has a high absorption in the

The Cu2O may be a p-type semiconductor well as the n-type. In Cu2O-p there are vacancies of

show that this material has a lower efficiency than 2% attributed to heterojunction represented by metallic cooper (Cu/Cu2O-p) [22]. Thus, many studies have been focused on Cu2O-n.

) and in Cu2O-n, there are oxygen vacancies. Many papers on the Cu2O-p

Particularly, the obtention of *n-*type or *p*-type semiconductor can be controlled from pH of electrodeposition bath (Figure 8).

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

In copper electrodeposition several mechanisms are proposed. Considering the direct electro‐ deposition, the copper II ions are electrodeposited on one step as shown in Eq. 20. Siripala et al. [22] discusses the formation of an intermediate in the electrodeposition of copper. In this case the copper electrodeposition occurs through Cu+ forming the CuOH adsorbed (Eq. 21 and 22). Chemical occurs in one step the formation of Cu2O (Eq. 23).

$$\text{Cu}^{\ast 2}\_{\text{(aq)}} + 2e^- \to \text{Cu}\_{\text{(s)}} \tag{20}$$

$$\text{Cu}^{\ast 2}\_{\text{(aq)}} + \text{1e}^- \rightarrow \text{Cu}^+\_{\text{(aq)}} \tag{21}$$

$$\text{Cu}^{+}\_{\text{(aq)}} + \text{OH}^{-}\_{\text{(aq)}} \rightarrow \text{CuOH}\_{\text{(ads)}} \tag{22}$$

**Figure 9.** Representation of TiO2 particles electrodeposition on F TO.

**2.4. Electrical interconnects of Solid Oxide Fuel Cells (SOFCs)**

form H2O (1.3). Figure 10 depicts the scheme of operation of a SOFC.

The-Vinh et al. [25] showed that the TiO2/SiO2 nanocomposite electrode facilitated the increase of ca. 20% of photocurrent density and ca. 30% of photovoltaic efficiency in comparison with the bare TiO2 electrode. In accord to authors this electrodeposition method can be applied as an advanced microscopic way to control the electrochemical properties of the electrodes.

Metallic and Oxide Electrodeposition http://dx.doi.org/10.5772/55684 113

In the current scenario where the researches are focused on cleaner energy sources and efficient, the fuel cells are gaining more space. Among the most promising fuel cells for generation of large quantity of power, are the solid oxide fuel cells (SOFCs) [26-29]. Solid oxide fuel cells (SOFCs) are solid-state devices that produce electricity by electrochemically com‐ bining fuel and air across an ionically conducting electrolyte. During operation of solid oxide fuel cell (SOFC) the anode is fed of hydrogen and cathode is supply with O2 or air. At the operating temperature (600 to 800 °C), hydrogen is oxidized at the anode to H+ ions. The freed electrons are conducted to the external circuit to the cathode, enabling the reduction of the oxygen ions O-2(Eq.1.2) which are transported from the cathode to the anode through the electrolyte (ionic conductor). At the interface anode / electrolyte anions O-2 react with H+ to

In order to obtain high voltage and power density, a number of individual cells consisting of a porous anode, a dense thin-film electrolyte, and a porous cathode are electrically connected

$$\text{CuOH}\_{\text{(ads)}} + \text{CuOH}\_{\text{(ads)}} \rightarrow \text{Cu}\_2\text{O} + \text{H}\_2\text{O} \tag{23}$$

The reason for formation of Cu2O is due to oxygen reduction forming peroxide. Among other electrochemical steps, one of the possible reactions in this case is shows in Eq. 24. The presence of Cu+ certainly favor the formation of Cu2O.

$$2H\_2O\_{2(aq)} + Cu^{+2}\_{(aq)} \to 2Cu^{+}\_{(aq)} + 2H^{+}\_{(aq)} + O\_{2(g)}\tag{24}$$

According to Siripala et al. the type semiconductor (p or n) can be determined by ph of solution. If the copper electrodeposition occurs in pH below of 6.00 is formed preferably the Cu2O-n. For pH above 6.00, electrodeposition of Cu2O-p occurs preferably [22,23].

#### *2.3.1. Semiconductive TiO2*

Many papers have discussed the dye-sensitized solar cell (DSC) due its promising efficiency in very low cost [19-25]. In the literature is found cathodic electrodeposition of TiO2 nanopar‐ ticles on the optically transparent fluorine doped tin oxide-coated (FTO) glass [25]. This type of the oxide electrodeposition is already prepared. The deposition is made due to current of electrophoresis. In electrodeposition in pH higher than the zero charge point (PCZ) the particles TiO2 are negatively charged. Thus one cathodic electrodeposition is unfavorable due to electrostatic repulsion. In low pH there is a tendency for formation of bubbles of hydrogen gas due to reduction of H + ions. This causes a partial occupation of the electrode surface by decreasing the efficiency of the process. The electrodeposition of TiO2 on the FTO is pictured in the figure 9.

**Figure 9.** Representation of TiO2 particles electrodeposition on F TO.

In copper electrodeposition several mechanisms are proposed. Considering the direct electro‐ deposition, the copper II ions are electrodeposited on one step as shown in Eq. 20. Siripala et al. [22] discusses the formation of an intermediate in the electrodeposition of copper. In this case the copper electrodeposition occurs through Cu+ forming the CuOH adsorbed (Eq. 21

and 22). Chemical occurs in one step the formation of Cu2O (Eq. 23).

2

2

2

For pH above 6.00, electrodeposition of Cu2O-p occurs preferably [22,23].

2 2( ) ( ) ( ) ( ) 2( ) 2 2 *H O Cu Cu H O aq aq aq aq g*

certainly favor the formation of Cu2O.

of Cu+

*2.3.1. Semiconductive TiO2*

112 Modern Surface Engineering Treatments

in the figure 9.

( ) ( ) 2 *Cu e Cu aq <sup>s</sup>*

( ) ( ) 1 *Cu e Cu aq aq*

*Cu OH CuOH* () () *aq aq* ( ) *ads*

The reason for formation of Cu2O is due to oxygen reduction forming peroxide. Among other electrochemical steps, one of the possible reactions in this case is shows in Eq. 24. The presence

According to Siripala et al. the type semiconductor (p or n) can be determined by ph of solution. If the copper electrodeposition occurs in pH below of 6.00 is formed preferably the Cu2O-n.

Many papers have discussed the dye-sensitized solar cell (DSC) due its promising efficiency in very low cost [19-25]. In the literature is found cathodic electrodeposition of TiO2 nanopar‐ ticles on the optically transparent fluorine doped tin oxide-coated (FTO) glass [25]. This type of the oxide electrodeposition is already prepared. The deposition is made due to current of electrophoresis. In electrodeposition in pH higher than the zero charge point (PCZ) the particles TiO2 are negatively charged. Thus one cathodic electrodeposition is unfavorable due to electrostatic repulsion. In low pH there is a tendency for formation of bubbles of hydrogen gas due to reduction of H + ions. This causes a partial occupation of the electrode surface by decreasing the efficiency of the process. The electrodeposition of TiO2 on the FTO is pictured

+ - + ® (20)

+-+ + ® (21)

+ - + ® (22)

+ ++ +® + + (24)

*CuOH CuOH Cu O H O* () () 2 2 *ads ads* + ®+ (23)

The-Vinh et al. [25] showed that the TiO2/SiO2 nanocomposite electrode facilitated the increase of ca. 20% of photocurrent density and ca. 30% of photovoltaic efficiency in comparison with the bare TiO2 electrode. In accord to authors this electrodeposition method can be applied as an advanced microscopic way to control the electrochemical properties of the electrodes.

#### **2.4. Electrical interconnects of Solid Oxide Fuel Cells (SOFCs)**

In the current scenario where the researches are focused on cleaner energy sources and efficient, the fuel cells are gaining more space. Among the most promising fuel cells for generation of large quantity of power, are the solid oxide fuel cells (SOFCs) [26-29]. Solid oxide fuel cells (SOFCs) are solid-state devices that produce electricity by electrochemically com‐ bining fuel and air across an ionically conducting electrolyte. During operation of solid oxide fuel cell (SOFC) the anode is fed of hydrogen and cathode is supply with O2 or air. At the operating temperature (600 to 800 °C), hydrogen is oxidized at the anode to H+ ions. The freed electrons are conducted to the external circuit to the cathode, enabling the reduction of the oxygen ions O-2(Eq.1.2) which are transported from the cathode to the anode through the electrolyte (ionic conductor). At the interface anode / electrolyte anions O-2 react with H+ to form H2O (1.3). Figure 10 depicts the scheme of operation of a SOFC.

In order to obtain high voltage and power density, a number of individual cells consisting of a porous anode, a dense thin-film electrolyte, and a porous cathode are electrically connected

To improve the surface electrical properties and reduces the amount of chromium in the oxide film a coating of the stainless steel with semiconductor oxides has been proposed. Cobalt oxide Co3O4 is a promising candidate because of its interesting conductivity of 6.70 Scm-1 and an adequate linear thermal expansion coefficient [32-33]. A good strategy to obtain the Co3O4 layer over stainless steel is cobalt electrodeposition with subsequent oxidation in air at high temperatures (SOFC cathode conditions) (Figure 11). The cobalt electrodeposition can be a low cost technique. This methodology initially a cobalt layer is electroplated on steel. Under the conditions cathode (air and ~ 800) occurs the formation of Co3O4. In the figure 12, is shown the

Metallic and Oxide Electrodeposition http://dx.doi.org/10.5772/55684 115

MEV of a cobalt layer electrodeposited on a stainless steel plate.

**Figure 11.** Scheme of Co3O4 obtention over stainless steel using the electrodeposition method.

very regular and without preença chromium in its surface.

for achieving highly conductive films at high temperatures.

The figure 13 shows that the steel without coating of cobalt subjected a strongly oxidizing conditions of cathode forms an oxide film very irregular. Also is observed that the steel surface shows the presence of chromium. Moreover the sample of coating steel with cobalt shows up

In accord to Deng et al. [33] the stainless steel with electrodeposited cobalt tends to improve in high temperature and to reduce the chromium evaporation. The cobalt was oxidized to Co3O4. The coating not only maintains electrical contact, but it offers oxidation protection in ferritic stainless steels at lower chromium content and it is capable of significantly retarding chromium evaporation which reduces chromium poisoning of fuel cell. in accord to Wu et al [32], a uniform and smooth Mn–Co alloy can be successfully deposited on stainless steel substrate by electrodeposition. Compounds as Mn1.5Co1.5O4 can reach an electrical conductivity of 90 Scm−1 to 800 degrees [33]. This shows that the electrodeposition is an excellent alternative

**Figure 10.** Simple scheme of operation and gases flow of a SOFC.

by interconnects to form a "stack". These interconnects are in contact with both electrodes (cathode and anode) and must meet a number of requirements [27-29] :


Metallic interconnects have attracted a great attention due to their high electronic and thermal conductivity and a low cost and good manufacturability compared to traditional ceramic interconnects [30]. In recent years, many works have been focused on ferritic stainless steel due to its low cost and adequate linear thermal expansion coefficient (11-12 x 10-6 K-1) [28-30]. However, under the cathode working conditions (typically 800 o C in air) CrO3 evaporate from the Cr2O3 oxide film (Eq. 25) causing severe cell degradation [28-31].

$$
\frac{1}{2}\text{Cr}\_2\text{O}\_{3(s)} + \frac{3}{4}\text{O}\_{2(g)} \to \text{CrO}\_{3(g)}\tag{25}
$$

To improve the surface electrical properties and reduces the amount of chromium in the oxide film a coating of the stainless steel with semiconductor oxides has been proposed. Cobalt oxide Co3O4 is a promising candidate because of its interesting conductivity of 6.70 Scm-1 and an adequate linear thermal expansion coefficient [32-33]. A good strategy to obtain the Co3O4 layer over stainless steel is cobalt electrodeposition with subsequent oxidation in air at high temperatures (SOFC cathode conditions) (Figure 11). The cobalt electrodeposition can be a low cost technique. This methodology initially a cobalt layer is electroplated on steel. Under the conditions cathode (air and ~ 800) occurs the formation of Co3O4. In the figure 12, is shown the MEV of a cobalt layer electrodeposited on a stainless steel plate.

**Figure 11.** Scheme of Co3O4 obtention over stainless steel using the electrodeposition method.

by interconnects to form a "stack". These interconnects are in contact with both electrodes

**•** Low area specific resistance (ASR). An acceptable value, after 40,000 working hours, is 0.10

**•** Chemical stability in both atmospheres (reducing and oxidant) at high temperatures

**•** Linear thermal expansion coefficient, LTEC, compatible with the other components of the

Metallic interconnects have attracted a great attention due to their high electronic and thermal conductivity and a low cost and good manufacturability compared to traditional ceramic interconnects [30]. In recent years, many works have been focused on ferritic stainless steel due to its low cost and adequate linear thermal expansion coefficient (11-12 x 10-6 K-1) [28-30].

2 3( ) 2( ) 3( )

*Cr O O CrO sg g* + ® (25)

C in air) CrO3 evaporate from

(cathode and anode) and must meet a number of requirements [27-29] :

**Figure 10.** Simple scheme of operation and gases flow of a SOFC.

However, under the cathode working conditions (typically 800 o

the Cr2O3 oxide film (Eq. 25) causing severe cell degradation [28-31].

1 3 2 4

Ohm cm-2.

(between 600 and 1000 ºC).

114 Modern Surface Engineering Treatments

**•** Impermeability to O2 and H2.

cell (value close to 12.5 x 10-6 K-1).

The figure 13 shows that the steel without coating of cobalt subjected a strongly oxidizing conditions of cathode forms an oxide film very irregular. Also is observed that the steel surface shows the presence of chromium. Moreover the sample of coating steel with cobalt shows up very regular and without preença chromium in its surface.

In accord to Deng et al. [33] the stainless steel with electrodeposited cobalt tends to improve in high temperature and to reduce the chromium evaporation. The cobalt was oxidized to Co3O4. The coating not only maintains electrical contact, but it offers oxidation protection in ferritic stainless steels at lower chromium content and it is capable of significantly retarding chromium evaporation which reduces chromium poisoning of fuel cell. in accord to Wu et al [32], a uniform and smooth Mn–Co alloy can be successfully deposited on stainless steel substrate by electrodeposition. Compounds as Mn1.5Co1.5O4 can reach an electrical conductivity of 90 Scm−1 to 800 degrees [33]. This shows that the electrodeposition is an excellent alternative for achieving highly conductive films at high temperatures.

**Figure 12.** Scanning electron microscopy images of cobalt electrodeposited on stainless steel.

#### **2.5. Ion-Li batteries recycling**

The best way to achieve sustainable development is through recycling materials more precisely batteries. In this context, the electrodeposition takes shape more environmentally friendly. It is very interesting to note that metallic electrodeposition corresponds to a very interesting route for metal recycling. The principal metallic sources for recycling can be electronics circuits or spent batteries [6,34,35,36,37]. The Li-ion battery is a most attractive energy source for portable electronic products, such as cellular phones and laptop computers. Spinel structure LiCoO2 is most used as the cathode material for Li-ion batteries due to its good performance in terms of high specific energy density and durability [1]. Thus, the Li-ion batteries are a valuable source of cobalt. The global reaction for charge and discharge of a li-ion battery with LiCoO2 cathode can be represented b y Eq. 26. The LiCoO2 is initially over the Al current collector. This is represented by figure 14.

$$\text{LiCoO}\_2 + \text{6C} \overset{\rightarrow}{\text{C}} \text{Li}\_{\text{(1-x)}} \text{CoO} + \text{C}\_6 \text{Li}\_x \tag{26}$$

the increase of the acid concentration and temperature [6,34]. The addition of H2O2 is necessary

**Figure 13.** Scanning electron microscopy images and dispersive energy of X-ray for sample for steel coated with co‐

Metallic and Oxide Electrodeposition http://dx.doi.org/10.5772/55684 117

to increase the efficiency of cathode dissolution. H2O2 reduces cobalt from oxidation state +III,

insoluble in aqueous system, to +II, soluble in aqueous system. Considering that the active

(a) (b)

**Figure 14.** Photograph of the cathode of Li-ion batteries.

balt (low) and without cobalt (up) after 1000 hours at 850 degrees.

The first step in obtaining of cobalt from Li-ion batteries is removal of LiCoO2 of current collector. This is accomplished by performing a heat treatment of cathode at 400 degrees for approximately 24 hours. The powder of LiCoO2 obtained is dissolved in acidic solution under constant magnetic agitation at 80 ◦C for 2 h. The cathode dissolution efficiency increases with

**Figure 13.** Scanning electron microscopy images and dispersive energy of X-ray for sample for steel coated with co‐ balt (low) and without cobalt (up) after 1000 hours at 850 degrees.

the increase of the acid concentration and temperature [6,34]. The addition of H2O2 is necessary to increase the efficiency of cathode dissolution. H2O2 reduces cobalt from oxidation state +III, insoluble in aqueous system, to +II, soluble in aqueous system. Considering that the active

**Figure 14.** Photograph of the cathode of Li-ion batteries.

**2.5. Ion-Li batteries recycling**

116 Modern Surface Engineering Treatments

collector. This is represented by figure 14.

The best way to achieve sustainable development is through recycling materials more precisely batteries. In this context, the electrodeposition takes shape more environmentally friendly. It is very interesting to note that metallic electrodeposition corresponds to a very interesting route for metal recycling. The principal metallic sources for recycling can be electronics circuits or spent batteries [6,34,35,36,37]. The Li-ion battery is a most attractive energy source for portable electronic products, such as cellular phones and laptop computers. Spinel structure LiCoO2 is most used as the cathode material for Li-ion batteries due to its good performance in terms of high specific energy density and durability [1]. Thus, the Li-ion batteries are a valuable source of cobalt. The global reaction for charge and discharge of a li-ion battery with LiCoO2 cathode can be represented b y Eq. 26. The LiCoO2 is initially over the Al current

**Figure 12.** Scanning electron microscopy images of cobalt electrodeposited on stainless steel.

<sup>2</sup> (1 ) 6 <sup>6</sup> *x x LiCoO C Li CoO C Li* ®

The first step in obtaining of cobalt from Li-ion batteries is removal of LiCoO2 of current collector. This is accomplished by performing a heat treatment of cathode at 400 degrees for approximately 24 hours. The powder of LiCoO2 obtained is dissolved in acidic solution under constant magnetic agitation at 80 ◦C for 2 h. The cathode dissolution efficiency increases with

¬ - + + (26)

material is LiCoO2, the cathode dissolution reaction is represented by Eq. (2) [6]. Figure 15 shows a simplified diagram of li-ion batteries recycling using the electrodeposition.

the application of recycled cobalt in interconnects for SOFC [37]. In this work the metallic cobalt was electrodeposited on 430 steel in order to obtain a low electrical resistance film made to

Co3O4 phase. On the other hand, a sample without cobalt showed the usual Cr2O3 and

The electrodeposition remains a very important topic for technology development. Through changes in operating parameters, one can obtain metallic or oxide films with different characteristics. The electrodeposited Co3O4, have an excellent supercapacitive behavior with specific capacitive value around 2700 Fg-1. The MnO2 synthesized by electrodeposition method also have very good values of supercapacitance varying between 240 and 521 Fg−1. In solar cells, the electrodeposition is a very promising method for electrodes fabrication. This is because the electrodeposition is a method simple, practical and inexpensive to produce both p-type as n-type. Moreover, with the advancement of Solid Oxide Fuel Cells development, the electrodeposition is once again an important method to be considered. In this case the electrodeposition is an excellent method for improved of electrical interconnects. Finally, there is also an environmental aspect of electrodeposition. This because the metals recycling of

metals presents in spent batteries is made principally by electrodeposition method.

and Tulio Matencio2

2 Federal University of Minas Gerais, Minas Gerais, Belo Horizonte, Brazil

[2] Conway, В. Е. Electrochemical Data. Elsevier, Amsterdam: 1952.

1 Federal University of São João Del Rei- unit of Sete Lagoas, Minas Gerais, Sete Lagoas, Brazil

[1] Conway, В. Е.Theory and Principles of Electrode Processes. Ronald, New York: 1965.

[4] В J. O'M. Bockris and A. K. N. Reddy. Modern Electrochemistry. Vol. 2, Plenum,

[3] Parsons, R. Handbook of Electrochemical Data. Butterworths, London: 1959.

C for 1000 h in air, the cobalt layer was transformed into the

Metallic and Oxide Electrodeposition http://dx.doi.org/10.5772/55684 119

Co3O4. After oxidation at 850 o

FeCr2O4 phases.

**3. Conclusions**

**Author details**

, Vanessa F.C. Lins2

Eric M. Garcia1

**References**

New York:1970

**Figure 15.** Simplified diagram of li-ion batteries recycling using the electrodeposition.

Although many groups working with Li-ion batteries recycling, the group most advanced in recycling via electrodeposition seems to be the group's researcher Eric M. Garcia. Garcia and other researchers perform a study of cathode Li-ion batteries recycling using the electrodepo‐ sition technique. Among several conclusions was shown that the largest charge efficiency found was 96.90% at pH 5.40. Furthermore, this research group conducted a detailed study of the mechanism of electrodeposition using electrochemistry quartz crystal microbalance technique (EQCM) [34]. In this case, it was assessed that at pH below 5, the electrodeposition of cobalt follows the direct mechanism (Eq. 13). For pH less than 2.70, cobalt electrodeposition occurs simultaneously with the reduction of protons to hydrogen [34]. In other work of Garcia and colleagues of research, it was explained the morphology of material electrodeposited with relation to pH. Although other research papers also focused on the cobalt electrodeposition as way of recycling battery Li-ion, Garcia's group pioneered the application of recycled cobalt. Garcia and other researchers associated the recycling of Li-ion battery to production of supercapacitor based on composite formed by cobalt oxides and hydroxides. The specific capacitances calculated from cyclic voltammetry and electrochemical impedance spectroscopy show a good agreement with the value of 625 Fg-1 [36]. Moreover Garcia et al. also proposed the application of recycled cobalt in interconnects for SOFC [37]. In this work the metallic cobalt was electrodeposited on 430 steel in order to obtain a low electrical resistance film made to Co3O4. After oxidation at 850 o C for 1000 h in air, the cobalt layer was transformed into the Co3O4 phase. On the other hand, a sample without cobalt showed the usual Cr2O3 and FeCr2O4 phases.
