Electrochemical deposition is used for manufacturing various products of our modern life, as a process by which a film is deposited from a solution of ions onto an electrically conducting surface.
During the electrochemical process, the ions in a solution are driven toward a substrate or electrode by an applied electrical field and then coated on the electrode. The simplest structure of an electrochemical cell is of a two-electrode system that consists of a cathode and an anode. Such a cell works well under the equilibrium conditions. However, in the nonequilibrium conditions, measuring the potential between the two electrodes is inaccurate due to the redox potentials and overpotential. To tackle this problem, a third electrode needs to be used as the reference potential. In general, the normalized hydrogen electrode (NHE), which is defined to be zero potential, is widely used as the reference. In addition, silver chloride electrode is another common reference electrode for electrochemical measurements.
The electrochemical processes can be summarized to consist of following steps (Paunovic 2007):

• Ions migration: The hydrated metal ions in the solution migrate to the cathode under the influence of applied potential.
• Electron transfer: Near the cathode surface, a hydrated metal ion enters the diffused double layer where the water/solvent molecules of the hydrated ion are aligned. Subsequently the metal ion enters the Helmholtz double layer where it is deprived of its hydrate envelope.
• The dehydrated ion is neutralized and adsorbed on the cathode surface.
• The adsorbed atom then migrates or diffuses to the growth point on the cathode surface.

Metals

The electrochemical deposition process has been widely used for coating thin layers of different metals (Tsang et al. 2009; Magnussen et al. 1990; Ogata et al. 2006) in many traditional technologies. For example, copper plating is used to prevent case/surface hardening of steel on the specified parts of the components (McCullough and Reiff 1924), silver plating on tableware and electrical contacts along with engine bearings(Faust and Thomas 1939), gold platting on jewelry and watch cases, and zinc coating to prevent the corrosion of steel articles (Hinton and Wilson 1989; Marder 2000; Srivastava and Mukerjee 1976), while nickel and chromium plating are used on automobile wheel rims and household appliances. In the modern electronic industry, the electrochemical deposition also has many applications such as the printed circuit boards, through-hole plating, multilayer read/write heads and thin-film magnetic recording media, etc.

Principle

A substrate is immersed in a solution (the bath) containing the desired metal in the oxidized form either as an aquatic cation or as a complex ion. The anode is usually a bar of the metal being plated. During electrolysis, the metal is deposited onto the working electrode/substrate by following reactions as:

The amount of metal deposited on the substrate can be estimated by Faraday’s laws of electrolysis.
Structures: The structures of the as-deposited metal layers, e.g., morphology, crystallinity, thickness, etc., can be controlled by varying the process parameters such as voltage, deposition temperature, and adding agents (Wang et al. 2005). For example, the metal nanowires with high length/diameter ratio can be prepared by electrochemical deposition with the addition of porous aluminum oxide (PAO) template (Gelves et al. 2006). The self-organized morphology of metal nanowires can also be prepared by two-step template synthesis using electrodeposition in which the anodic alumina membrane (AAM) was used. In the first step, the metal nanowires embedded in the AAM were fabricated by pulsed electrodeposition. After the growth of nanowires embedded in the AAM, these nanowires array can act as nano-electrode array or more specifically as cathode for the following deposition process. With applied electrical field vertically to the metal (Bi)-nanoelectrode array, the nanowires may bend down, which cause their ends assembled or knitted together from different orientations, thus generating bunches and the nests of the bunch-like metal nanostructure (Zhang et al. 2009). The schematic diagram for the process is illustrated in Fig. 1.

The metal nanowires prepared by the electrochemical deposition are usually polycrystalline in nature. In a study carried by Tian et al., the simultaneous growth of single and polycrystalline metal nanowires, such as Ag, Au, Ni, Cu, Co, and Rh, was reported via the aforementioned electrochemical deposition route (Tian et al. 2003). Different growth models have been proposed to understand the growth mechanism of the noble metal nanowire. The single crystals of low melting point metallic nanowires, such as Au, Ag, and Cu, in the electrodeposition are grown with the 2D-like nucleation mechanism (Tian et al. 2003). Such finding revealed that the new grains grew when the size of an initial cluster exceeds the critical dimension Nc during the deposition. The larger Nc facilitates the single crystal to grow from a previously nucleated seed grain. The critical dimension Nc for a 2D-like nucleus can be expressed as Nc = bsε2/(Zeη)2, where s is the area occupied by one atom on the surface of the nucleus, ε the edge energy, Z the effective electron number, η the over potential, and b a constant, respectively.
The electrochemical deposition process has also been applied to prepare transition-metal nanowires (Pan et al. 2005). In a study, Pan et al. reported that [220] is the favorable growing orientation of Ni nanowire. It was found that the single crystal structure of the nanowires can be changed to polycrystalline structure at lower potential or higher processing temperature. The texture of thicker Ni layer is a result of competitive growth among adjacent grains at the stage of grains formation. The grains grow faster along the normal of the planes that have low facet index due to their lower surface energy, resulting in the increase of grain size and facilitation of the columnar grain formation. In addition, the confinement of the nanoporous structure in the anodic aluminum oxide (AAO) template can also promote the formation of columnar grains and the single-crystalline nanowires. Chu et al. (2002) demonstrated a novel approach for the fabrication of metal nanostructure arrays on the conducting glass substrate. During the growth process, a film of aluminum was firstly deposited by sputtering on tin-doped indium oxide (ITO) film. With oxidizing anodically, alumina template with 5- to 150-nm pores diameter were obtained, and then 0.5- to 2.6-μm-long Ni nanowires were grown within the alumina template pores. So the size and orientation of the nanowires could be controlled by the porous alumina films.
A number of morphologies, such as nanowires, decahedral and icosahedral shapes, porous nanowires, multipods, nanobrushes, and even snowflake-shaped structures, have been successfully prepared with the electrochemical deposition technique. Even noble metal-based cubic nanobox-like structure can also be prepared with the electrochemical deposition technique (Peng et al. 2010). In order to successfully obtain cubic nanobox-like structure, various electrodeposition parameters including potential and the concentration of solution with the supporting electrolyte and surfactants were optimized.
Figure 2 schematically illustrated that the Au nanotubes can be fabricated on the template of polycarbonate tract-etching (PCTE) membrane with electroless deposition technique. By slowing down the electrochemical deposition, Te was deposited on the surfaces of Au nanotubes to form Au-Te nanocables. The final product Au-Te nanocables in which Au acts as shell and Te as core was found to have radial consistent dimensions with starting templates.

Nanostructured copper (Cu) particles were prepared electrochemically on polypyrrole (PPy) film (Zhou et al. 2004). The PPy film was deposited on a sputter-coated thin gold film grown on a Si (100) substrate. The deposition conditions were optimized by altering several deposition parameters, such as film thicknesses of gold and PPy, potential and Cu concentration to tune the particle density, and crystal size of copper. It was also observed that the gold and PPy film thicknesses are critical to exclude gold–silicon interface and to control shape uniformity. It was found that the deposition potential was important to control the size and density of nanocrystals.
Cobalt nanoplate array on copper substrate using the electrochemical deposition without using any template was fabricated by Xu et al. (2012). The length and height of the vertically aligned nanoplates were found to be few micrometers and 350 nm respectively. The dimensions of nanoplates were controlled by manipulating the electroplating conditions.
From the above discussion, the electrochemical deposition had been proved an effective route to fabricate metal micro-/nanostructures. By tuning the reaction parameters or using the additional template, the desired morphology can be achieved.

Corrosion of Metals

Corrosion of metals is the spontaneous chemical destruction processing of materials in the corrosive environment. Most often it follows an electrochemical mechanism, where the anodic dissolution (oxidation) of the metal and cathodic reduction of an oxidizing agent occur simultaneously. It causes enormous losses in our economy. Rusting weakens the metal structures of buildings and bridges, the equipment of chemical and metallurgical plants, underground pipelines, river and sea vessels, and other structures.
Direct losses attributable to the corrosion include expenditures for the replacement of individual parts, units, entire lines, or plants and for various preventive and protective tasks (such as the application of coatings for corrosion protection). Indirect losses arise when the corroded equipment leads to the defective products that must be rejected; they also arise during downtime required for the preventive maintenance or repair of equipment. About 30 % of all steel and cast iron are lost due to the corrosion. A part of the corroded metals can be reprocessed as scrap, but about 10 % is irrevocably lost. In this case, corrosion protection has become very critical in recent years, for example, (1) the use of thin metallic support structures that are designed to reduce the overall metal content, (2) the use of technologically advanced equipment operated under extreme conditions such as nuclear reactors and jet and rocket engines, and (3) the mass-scale development of products having extremely thin metal films, such as printed circuit boards and integrated circuits.

Corrosion Protection

The electrochemical and non-electrochemical ways to protect metals against corrosion can be distinguished. The non-electrochemical processing includes dense protective films that isolate the metal from the medium by using paint, polymer, bitumen, enamel, etc. There is a general shortcoming of such a technique. When the coating is scratched mechanically, the protection loses and the local corrosion activity arises.
The electrochemical methods of protection rely on different perceptions: (1) electroplating of the corroding metal with a thin protective layer of a more corrosion-resistant metals, (2) the electrochemical oxidation of the surface, and (3) polarization control of the corroding metal (the position and shape of its polarization curves). The polarization of a corroding metal can be controlled by various additives to the solution, called corrosion inhibitors, which adsorb on the metal to lower the rates of the cathodic and/or anodic reaction. Inhibitors are used primarily for the acidic electrolyte solutions, sometimes also for neutral solutions. Various organic compounds with –OH, –SH, –NH2, –COOH, and so on as the functional groups are used as best-known inhibitors.
In summary, metal deposition from solution has attracted renewed interest in microelectronics and the related areas as a simple and no versatile technique. On the other hand, the metal deposition from solution remains a fascinating area of fundamental research, which studies the important aspects including electrocrystallization, solid–solid interactions, surface dynamics and diffusion, epitaxial growth, and atom to solid transitions.

Silicon

Silicon is a widely used semiconductor for electronic and photovoltaic devices because of its earth abundance, chemical stability, and tunable electrical properties by doping. The production of pure silicon films by simple and inexpensive methods has been the topic of many investigations. The demand for low-cost silicon-based photovoltaic devices has encouraged the quest for solar-grade silicon production through processes alternative to the currently used Czochralski process or other processes. The electrodeposition of silicon offers a clean, effective, and inexpensive alternative processing over the conventional silicon processing. The following sections will discuss the reaction solvents and crystallinity control of the products.
Solvents: The electrodeposition of silicon is usually processed in a nonaqueous medium due to its very large reduction potential and high reactivity to water. It has been studied more than 30 years, and a number of solution media, such as molten salts (LiF-KF-K2SiF6 and BaO-SiO2-BaF2), organic solvents (acetonitrile, tetrahydrofuran), and room-temperature ionic liquids, have been employed. The key factor of the silicon electrodeposition is the purity of the deposited silicon. The Si used for the photovoltaic devices is in the solar grade of >99.9999 % (6N), while the electronic grade of Si needs a purity of 11N for electronic devices. In most of the cases, the electrodeposited silicon does not meet these requirements without further purification.
Crystallinity: Silica or fluorosilicates have been used as the solutes to obtain either the amorphous silicon or metallic silicides. Gu et al. (2013) offer one possible means for the direct electrodeposition of crystalline Si. The initial stage involved electroreduction of an oxidized precursor (SiCl4) to the fully reduced state (Si) at the electrode–electrolyte interface. The electroreduction of SiCl4 at the solid electrodes has been investigated previously and found to produce purely amorphous Si. In the electrochemical liquid–liquid–solid (ec-LLS) scheme, the initially reduced Si could be partitioned into the liquid gallium phase. The solubility of Si in Ga between room temperature and 100o C as determined by extrapolations of the published metallurgical data for the Ga–Si system ranges from 10-8 to 10-6 at.%. Although it is very low, the solubility of Si in Ga at 100 C is comparable to the solubility of Ge in Hg at room temperature. The dissolved Si in Ga(l) could then reach saturation and supersaturation conditions if SiCl4 is continually reduced at the electrode–electrolyte interface. When a critical supersaturation condition is reached, a phase separation of Si(s) from Ga(l ) followed by crystal growth occurs, as shown in Fig. 3 (Gu et al. 2013).

Most recently, Mallet et al. (2013) demonstrated a promising technique for synthesizing silicon nanotubes via electrodeposition technique. In this study, polycarbonate (PC) membrane was used as an insulated nanoporous template in which electrodeposition was carried out by using room-temperature ionic liquids (RTILs). A fine adjustment of electrochemical parameters influencing ionic diffusion inside the nanopores of the template was used to grow the Si nanotubes at the expense of Si nanowires. This indicates that the electrodeposition process in the presence of RTILs is competitive to grow the nanostructures with a high surface to volume ratio with low cost in a large scale.
In addition, Salman et al. also reported the room-temperature template (polycarbonate membranes (PC))-assisted synthesis of germanium and silicon nanowires with the electrochemical deposition technique (Al-Salman et al. 2008). In the synthesis process, the air- and water-stable ionic liquid 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl) amide ([Py1,4]Tf2N) containing GeCl4 and SiCl4 as a Ge and Si source was used.
The electrochemical synthesis of amorphous silicon in two non-oxygenated organic solvents, acetonitrile and dichloromethane, under controlled atmosphere was reported by Bechelany et al. (2012). In both solvents, tetraethylammonium chloride had been used as a supporting electrolyte and silicon tetrachloride as a silicon precursor. By tuning solvent concentrations and experimental parameters, the formation from dense to highly porous Si deposits can be observed. A heat treatment under hydrogen was performed to enhance the stability of the deposits prior to air exposure.
Amorphous silicon has also been successfully electrodeposited on copper using a SiCl4-based organic electrolyte under galvanostatic conditions (Epur et al. 2012). The deposited films were tested for their possible application as anodes for Li-ion battery. The results indicate that this binder-free amorphous silicon anode exhibits a reversible capacity of ~1,300 mAh g-1 with a coulombic efficiency of >99.5 % up to 100 cycles. Impedance measurements at the end of each charge cycle show a non-variable charge transfer resistance which contributes to the excellent cyclability over 100 cycles observed for the films. This approach of developing thin amorphous silicon films directly on copper eliminates the use of binders and conducting additives, rendering the process simple, facile, and easily amenable for large-scale manufacturing (Fig. 4).
The electrochemical deposition of Si films on Ni substrate for lithium-ion battery anodes was reported by Zhao and co-workers (2012). Their findings revealed that the Li+ diffusion coefficient of the Si film was much higher than the pristine Si nanoparticles (Fig. 5).

It was proposed that the Si film supported on 3D conductive framework could accommodate the volume change in Li+ insertion/extraction effectively, which caused a high capacity and a stable cycle performance, and that Li+  ions exhibited fine diffusion kinetics in the Si film.
In summary, electrodeposition is a promising environmental friendly, low-temperature, and low-cost approach to deposit both amorphous and crystalline silicon for industry applications, such as anode materials for solar cells and thin film transistors, etc. It is prospected that in the near future, the electrodeposition of silicon from low-toxic, inexpensive solvents will have great potential for largescale production applications.

Metal Oxides

The electrochemical deposition of metal oxides is mainly performed in alkaline solutions containing metal precursors. The deposition of metal oxides can be carried out under oxidizing conditions as well as reducing conditions from the alkaline solutions. In both cases, the metal ion dissociates from the precursor and precipitates on the electrode as the oxide (Azaceta et al. 2012; Schrebler et al. 2006; Tench and Warren 1983; Stevenson et al. 1999; Wang et al. 2010, 2011; Qu et al. 2006; Anees et al. 2011; Fernandes et al. 2009). What controls the ability to deposit an oxide is the stability of oxide under the experimental conditions, i.e., the potential, temperature, and local pH values near the electrode surface (Dube´ et al. 1995; Kendig et al. 1994; Wu et al. 2010b).

Principle

Metal oxide can be obtained from the decomposition of metal hydroxide with thermal treatment produced by metal and oxygen precursor during the electrochemical deposition. Hydroxyl ions are used as the oxygen precursor and can be obtained by two methods. One is directly changing the solution pH value by adding alkali. But this method usually requires complex agent to stabilize the metal ion in high pH value solution. Once the metal ion gets electrons from substrate, the valence of the metal ion will be changed, and then the metal ion becomes unstable with the complex agent and comes out to react with hydroxyl ions to form metal hydroxide.
The other method is using water or other oxygen compounds, O2, H2O2, or NO3, to generate hydroxyl ions by getting electrons. The local pH increases and the metal ions become metal hydroxide precipitated on the substrate electrode.

Deposition of the binary oxides under the reductive conditions includes the reduction of metal ions that form stable oxides. On the other hand, the cathodic deposition of metal oxides relies on precipitations due to the electrochemically induced local pH alterations. The reduction of the dissolved oxygen or nitrate ions results in the local production of hydroxide ions, which, for example, enables the deposition of CdO and ZnO. However, the hydroxide ions are also produced upon the dissociation of complexes with ligands like lactic, tartaric, and citric acid containing hydroxyl groups. During the recent years, there have been tremendous reports in the nanostructural morphology control of metal oxide thin films with the electrochemical process because of the morphology-dependent physical and chemical properties of metal oxide thin films. Thus, the following sections will discuss recent progress in shape and size control of these oxide nanostructures.
One-dimensional nanostructures: One-dimensional (1D) metal oxide nanostructures, including nanowires, nanorods, nanotubes, and nanoribbons, have been intensively studied owing to their unique physical properties and fascinating potential applications in future nanoelectronic devices. Their superior physical properties including superconductivity, enhanced magnetic coercivity, and the unusual magnetic state of some 1D nanostructure have been theoretically predicted and experimentally demonstrated. Various nanodevices, such as sensors, lasers, field effect transistors, optical waveguides (Revie and Uhlig 2008), cantilever resonators, and generators, have been fabricated by using the 1D nanostructure as the building blocks.
Generally, there are two strategies to produce the 1D metal oxide nanostructures through the electrochemical process. One is the template-assisted electrodeposition, where the 1D anisotropic growth is realized by using various templates to confine the growth space of the electrodeposits. The template-assisted electrodeposition has been demonstrated to be a versatile approach for the preparation of the 1D nanostructure of numerous materials, including metals, semiconductors, and conductive polymers. Another strategy is the template-free electrodeposition, where the 1D anisotropic growth is achieved by utilizing the intrinsic anisotropic crystallographic structure of a targeted material.
Binary metal oxide nanowires can be fabricated via the electrodeposition by dissolving molecular ions such as Cl-, SO42-, and CH3COO- in solutions having different anions (Elias et al. 2008a). The nature of anions in electrolyte solution was found to be critical in tuning length and diameter of nanowires. The nanowires with higher aspect ratio can be obtained in acetate solution for preparing binary oxide nanowires. For example, the single-crystalline ZnO nanotubes with an external diameter ranged 200–500 nm and length 1–5 μm were prepared electrochemically and shown in Fig. 12 (Elias et al. 2008b). The whole process was completed in three steps with combination of electrochemical and chemical approaches. In the first step, the ZnO nanowires were prepared by the electrochemical deposition. Second, the selective etching of ZnO nanowires in KCl solution was carried out to transform them into nanotubular architecture. The KCl concentration and temperature with immersion time were critical during the whole etching process. The evolution of the etching (dissolution) starts from the tip of the nanowires toward their bottom. The dissolution affects the nanowire core preferentially, leaving the lateral faces, resulting in a tubular structure. Degradation of the lateral walls starts when the core is fully dissolved, inducing a decrease of the length of the nanotubes until the complete dissolution of the ZnO nanostructures.
The electrochemical synthesis of TiO2 nanotubes is one of the research focuses in the electrodeposition of metal oxides (Roy et al. 2011), which has been widely used in dye-sensitized solar cells. The first self-organized anodic oxides on titanium were reported for the anodization in chromic acid electrolytes containing hydrofluoric acid by Zwilling and co-workers in 1999 (Zwilling et al. 1999). The 500-nm-thick nanotube structure was not highly organized and the tubes showed considerable sidewall inhomogeneity. However, it was recognized that small additions of fluoride ions to an electrolyte are the key to form these self-organized oxide structures.
Kelly (1979) explored the influence of fluorides on the passivity of titanium and concluded that porous oxide layers formed for low fluoride concentrations. Unfortunately, the (likely) presence of self-organized TiO2 nanotube layers was not verified by performing high-resolution electron microscopy (Fig. 6).

Most crucial improvements to the geometry of the tubes were established by Macak et al. (2005a). It demonstrated that the pH plays a crucial role in improving the tube layer thickness, that is, at neutral pH values, much longer tubes could be grown (Macak et al. 2005b). In nonaqueous electrolytes, smooth tubes without sidewall inhomogeneity (ripples) can be grown to much higher aspect ratios and show a strongly improved ordering.
Wang et al. reported highly ordered TiO2 nanotube arrays via the electrochemical anodization of high-purity Ti foil and Ti thin film-coated indium tin oxide (ITO) glass in fluorine containing electrolytes (both aqueous and nonaqueous). The formation of the well-aligned TiO2 nanotube arrays was affected by the electrolyte temperature and the applied anodization potential. In the aqueous electrolyte, the anodization potential exerted significant influence on the formation of TiO2 nanotube arrays, while the limited effect from the electrolyte temperature was observed. In the nonaqueous electrolyte, the electrolyte temperature markedly affected the TiO2 nanotube dimensions while the anodization potential exhibited slight influence in this regard. As a consequence, TiO2 nanotube arrays with tube diameters ranging from 20 to 90 nm and film thicknesses ranging from several hundred nanometers to several micrometers were obtained.
Two-dimensional nanostructures: Two-dimensional (2D) nanostructures, especially nanosheets, which possess atomic or molecular thickness and infinite planar dimensions, are regarded as the thinnest functional nanomaterials. To date, the synthesis of 0D (e.g., nanoparticles and nanodots), 1D (e.g., nanotubes and nanowires), and 3D nanomaterials (e.g., nanocubes) has been well documented. In contrast, 2D nanomaterials have remained conspicuously absent until the recent emergence of processes for producing graphene and inorganic nanosheets. However, graphene is a conductor, and electronic technology also requires insulators, which are essential for many devices such as memories, supercapacitors, and gate dielectrics. One reason why 2D nanomaterials have been less thoroughly studied may be related to the special synthetic process, involving the delamination of layered compounds into single layers. There are many known layered compounds with strong in-plane bonds and weak coupling between layers.
Two-dimensional nanosheets are considered to be excellent candidates for future nanoelectronic applications. Such nanostructures and their electronic states play an important role in realizing the innovative electronic, optical, and magnetic functionalities. For example, the operation of almost all semiconducting devices relies on the application of two-dimensional interfaces. To date, various nanosheets have attracted increasingly fundamental research interest because of their potential to be used for different applications like electrochemical capacitors and supercapacitors For example, it has been reported that Co3O4 thin films with nanosheet structure were prepared with a facile electrochemical deposition method, as shown in Fig. 7.

Other oxides, e.g., manganese dioxide (MnO2) nanosheets on flexible carbon cloth (CC) via anodic electrodeposition technique, have also been reported. Petal-shaped MnO2, having sheet thickness of a few nm and typical width of 100 nm, with a strong adhesion on CC has been observed. Besides, freestanding ZnO nanosheets have been electrodeposited on an indium tin oxide substrate in zinc nitrate solution. It suggests that the crystallization of the ZnO nanosheet below room temperature slows down the whole process.
The electrochemical deposition of metal oxide nanosheets represents a facile approach to fabricate oxide nanomaterials with high surface area, especially suitable for energy conversion and storage applications. However, there are still several challenges: (1) the stability of as-prepared nanosheet structure. As the nanosheets are weak-bonded, the reliability, durability, and chemical and thermal stabilities are still a concern for practical applications. (2) Up to date, most reported nanosheets are in the category of binary metal oxides. This is because the complex oxide nanosheets, such as perovskite oxides that are unique in both structural diversity and electronic properties, are difficult to be fabricated for their potential applications in electronics. (3) As these nanosheets are easily agglomerated, it is crucial to obtain stable nanosheet suspension solution for thin film and device applications.
Three-dimensional nanostructures: The development of well-defined porous metal oxides with nanometer-scale periodicity is of intense interest for a number of applications including photonics, sensing and detection arrays, catalysis, separations, microfluidics, and low dielectric constant (k) thin films in the microelectronics. To date, many of these materials have been prepared by the electrodeposition approach with the aid of different templates. For instance, the single-crystalline ZnO nanorod/amorphous and MnO2 nanoporous shell composites were prepared by electrochemical deposition, as shown in Fig. 8 (He et al. 2011).

A graphene/MnO2 nanocomposite-based porous nanofibers in which graphene nanosheets were coated on the surface were also reported via electrodeposition route (Yu et al. 2011). These porous nanofibers facilitate both loading of active electrode and easy approach of electrolyte to the electrodes materials. The dip and dry method was evolved first to coat porous fibers on graphene nanosheets and then MnO2 was deposited electrochemically on afore prepared porous fibers on graphene nanosheets. It was also suggested that these architectures could be promising candidates for multifunctional designs.
In summary, nanostructured metal oxides with various morphologies from electrochemical process have been extensively studied during the last two decades due to their interesting electronic, magnetic, electrochemical, and optical properties and potential use as catalytic and electrode materials in various devices. In particular, the electrodeposition of metal oxide nanostructure with high surface area and high chemical activity may be widely used as electrodes for the electrochemical energy storage device applications.

Non-oxides

To date, there are only a few non-oxides, such as copper indium diselenide (Hu et al. 2012; Sovannary et al. 2007) (CuInSe2, CIS) and bismuth tellurides (Magri et al. 1996; Chen et al. 2010), reported that can be deposited by electrochemical processes. Copper indium diselenide (CuInSe2 or CIS) and related compounds are being used in low-cost photovoltaic applications. Conversion efficiencies of these solar cells are now approaching close to 20 %. CuInSe2 has large optical absorption coefficient, which results from a direct energy gap. CIS solar cells have already surpassed the conversion efficiency of 19.5 % based on a multistep process using evaporation. The evaporation technology is excellent for good quality film growth, but difficult to scale up. Currently, a great deal of effort is directed to a large-scale, high-quality, and low-cost technology for preparing CIS thin films. Electrodeposition is highly suitable to achieve this goal. The record efficiency of 11.3 % was reported for a cell using electrodeposition route.
In order to obtain high-quality CIS thin films, the optimization of electrodeposited precursor film composition and morphology becomes extremely important. Sovannary et al. (2007) in their report presented vertically aligned arrays of CuInSe2 (CIS) nanowires with controllable diameter and length by pulse cathodic electrodeposition from a novel acidic electrolyte solution into anodized alumina (AAO) templates, followed by annealing at 220o C in vacuum.
Bismuth telluride and its derivatives doped with antimony and selenium are the best-known materials for room-temperature thermoelectric applications. Electrodeposition has been recognized as an effective method for preparing these materials due to its unique features: simple, fast, low cost, and capable of selective deposition. The number of publications on electrodeposition of thermoelectric materials, particularly bismuth telluride-based materials, has increased by a factor of three in the last decade. Most of these studies dealt with the electrodeposition of bismuth telluride from acidic solutions using 1 M HNO3 that can dissolve only about 20 mM Te species, which in turn limits the Bi concentration in order to obtain the correct alloy composition (Bi2Te3). The low metal ion concentration causes composition inhomogeneities across the film thickness due to the unbalanced depletion of Bi and Te species in plating bath. The rather low mass transport in solution also limits the maximum deposition current and hence growth rate that can be attained. Magri et al. (1996) presented electrochemically prepared bismuth telluride alloy films of uniform thickness from a solution containing Bi3+ and HTeO2+ ions in 1 mol dm-3 nitric acid (pH=O) on stainless steel. The electrodeposited films were found to be monophasic, polycrystalline structure and their stoichiometries vary according to the solution composition.
In another report (Chen et al. 2010), the bismuth telluride film and nanowires array were fabricated by potentiostatic electrodeposition, as shown in Fig. 9. Both materials were found to exhibit slightly Te-rich, n-type Bi2Te3 with preferred orientation in rhombohedral structure, and they were found to show much better thermoelectric performance than their counterparts in bulk.
Nonaqueous electrolytes often provide advantages such as wide electrochemical window, good thermal stability, and high metal ion solubility. Tellurium and tellurium alloy films have been electrochemically prepared from dimethyl sulfoxide (DMSO) and dimethyl formamide (DMF). Bismuth antimony thermoelectric nanowires were prepared from DMSO (Paunovic 2007). Li and co-workers used DMSO for the electrodeposition of bismuth telluride and bismuth antimony telluride materials.
In summary, the electrochemical deposition is an effective way to fabricate non-oxides with the controlled morphologies for energy storage applications. In particular, the combination of electrochemical deposition and solvothermal can result in high-crystalline, high-quality non-oxide nanomaterials with improved performance.

Conducting Polymers

There are several unique properties of conducting polymers (CPs), which distinguish them from traditional organic coatings. Like metals, CPs are electronically conducting, so galvanic coupling with the metal is expected, with concomitant alteration of corrosion behavior. CPs are redox active materials, with potentials that are positive of iron and aluminum. Thus, as with chromate, interesting and potentially beneficial redox interactions of CPs with active metal alloys such as steel and aluminum are anticipated, again with concomitant alteration of their corrosion behavior. The ability of a CP to function as a reservoir for the release of dopant ions that can act as corrosion inhibitors may also be an important distinguishing feature (Tallman et al. 2004).
One factor that limits the application of CPs as corrosion control coatings is the difficulty in casting such polymers as films or coatings. Since most common conjugated conducting polymers (e.g., polyaniline, polypyrrole, or polythiophene) are insoluble in environmentally friendly solvents such as water or alcohol, processability is a key issue. Solvent-soluble polymers can be synthesized by polymerization of derivatized monomers containing either an ionic side chain (for water solubility) or an alkyl side chain (for organic solvent solubility). For example, a long-term corrosion protection of an aluminum alloy (Al 2024-T3) by a poly (3-octyl pyrrole) primer with a polyurethane topcoat was reported by Gelling et al. (2001), where the CP was cast from a carbon tetrachloride/dichloromethane/acetonitrile solvent mixture.
The direct electrodeposition of CPs onto active metals is appealing but is generally not feasible due to the positive potentials required for polymerization. At such potentials, the metal oxidizes (corrodes) and an adherent, continuous film is not deposited unless special surface preparation and electrolytes are used, and even then only at high monomer concentrations (Beck and Hűlser 1990).
Electron-conductive polymers were first reported in 1971 by Shirakawa and co-workers. They synthesized conducting polyacetylene and found that it had a considerably high conductivity relative to other organic compounds, 103 Scm-1. Since 1971, various conducting polymers and their synthesis mechanisms have been studied actively by many researchers. Basically, the conducting polymers have electrons that are delocalized in p-conjugated systems along the whole polymer chain; their electron conductivity is much higher than that of other polymers that do not have a p-conjugated system.
Furthermore, in 1979, Diaz and co-workers synthesized conductive polypyrrole film by electropolymerization. Electropolymerization is similar to electrodeposition of metal films. Its polymerization reactions occur electrochemically; that is, electrons go through the electrode–electrolyte interface when a potential is applied to an electrode. In the case of an insoluble polymer generated on the electrode, a film of conducting polymer is thus obtained easily. Essentially, regarding electron transfer, all electrochemical reactions are classifiable into oxidative or reductive reactions. Electrochemical polymerizations can be classified in a similar way. In almost all cases, electropolymerized polymers have been synthesized by electrooxidation. It seems, however, that films obtained by this method contain branching or cross-linking defects. On the other hand, electroreductive polymerization performed by using a zerovalent nickel complex is sensitive to the halogen atom in the monomer, and therefore it yields polymers with well-defined linkages among the monomer units.

Principle

Very recently, Armel et al. (2012) proposed two different methods to deposit metallic and semiconducting nanoparticles inside a conducting polymer (CP) by controlling the diffusion of an active ion within the film (Fig. 10). The pulsed electrodeposition was used as primary method, where the “off” intervals allow the ions to flow into the CPs as they reoxidize. The second method involved poly (ethylene glycol) (PEG) or ionic liquid (IL) to assist the transportation of the ion within the CP. Various nanoparticles such as Pt, Ni, Cu, and CdS were successfully deposited inside a conductive film of poly(3,4-ethylenedioxythiophene) (PEDOT) by adopting the aforementioned techniques. 

The morphological examination of prepared structures revealed that the nanoparticles were well dispersed throughout the films and formed the open and layer structures in the presence of PEG as shown in Fig. 11.

Using a phosphonium-based (triisobutylmethylphosphonium tosylate) IL and heat during electrodeposition results in the incorporation of CdS inside a layered polymer scaffold as shown in Fig. 12. The presence of the nanoparticles throughout the film was confirmed by EDAX measurements. This study provided facile routes to functionalize CP electrodes with either organic or inorganic catalysts by assuming the crucial interconnection between the catalysts and the polymer.

In another case study, Luo and Cui (2011) presented a conducting polymer -based smart coating with magnesium (Mg) implant to improve the corrosion resistance of Mg and also release drug in a controllable fashion. By considering the facts that the ionic liquids are highly conductive and stable solvent with a very wide electrochemical window, they directly electrodeposited conducting polymer coatings poly (3,4-ethylenedioxythiophene) (PEDOT) on the active metal Mg in ionic liquid at mild conditions, and Mg remained considerably stable during the electrodeposition. The coating remained uniform and significantly improved the corrosion resistance of Mg as shown in Fig. 13.

In addition, the PEDOT coatings can load the anti-inflammatory drug dexamethasone during the electrodeposition which can be subsequently released upon electrical stimulation, which is illustrated in Fig. 14.
From this study, it is expected that the proposed CPCs could be electrodeposited on other active metals and alloys besides pure Mg, and such CPCs with drug-releasing properties may find applications in Mg-based implantable devices.
Electrodes coated with conducting polymer films have attracted considerable interest in the last two decades (Tallman et al. 2004; Luo and Cui 2011; Murotani et al. 2008; Li and Albery 1992). A multitude of reviews and monographs have been written on the subject. Many electrochemical works were performed with aromatic or conjugated compounds yielding insulating polymer too. But few studies were made up to now with aliphatic molecules concerning their electropolymerization behavior. For example, poly(3,4-ethylenedioxythiophene) (PEDOT) is one of most attractive electrochromic materials. Cho et al. (2005) presented nanotube structure of PEDOT by using electrochemical deposition in the pores of the alumina template film. The detailed mechanism of nanotube formation was also presented in these studies shown in Fig. 15.
In summary, the conducting polymers can combine the electronic characteristics of metals with the engineering properties of polymers. In the past 40 years, considerable knowledge of such materials has been gained and this is reflected in a number of texts and early reviews. It is notable that novel electrolyte and ionic liquid have important applications for depositing conducting polymers. As the ionic liquid is a highly conductive and stable solvent with a very wide electrochemical window, the conducting polymer coatings can be directly electrodeposited on the substrates in ionic liquid under mild conditions.