The electrochemical performance of deposited manganese oxide-based film.
The transition metal oxide has been recognized as one of the promising electrode materials for electrochemical capacitor application. Due to the participation of charge transfer reactions, the capacitance offered by transition metal oxide can be higher compared to double layer capacitance. The investigation on hydrous ruthenium oxide has revealed the surface redox reactions that contributed to the wide potential window shown on cyclic voltammetry curve. Although the performance of ruthenium oxide is impressive, its toxicity has limited itself from commercial application. Manganese oxide is a pseudocapacitive material behaves similar to ruthenium oxide. It consists of various oxidation states which allow the occurrence of redox reactions. It is also environmental friendly, low cost, and natural abundant. The charge storage of manganese oxide film takes into account of the redox reactions between Mn3+ and Mn4+ and can be accounted to two mechanisms. The first one involves the intercalation/deintercalation of electrolyte ions and/or protons upon reduction/oxidation processes. The second contributor for the charge storage is due to the surface adsorption of electrolyte ions on the electrode surface.
- energy storage devices
- metal oxides
- thin films
- electrode materials
The electronics technologies we are granted nowadays are the results of many years’ researches. From Benjamin Franklin, Alessandro Volta, Michael Faraday, and Nikola Tesla, the continuous efforts have developed the electrical knowledge which has been practicalized for the sake of living standards. The better quality of life and the transformation from agricultural-based economy into information-based economy indicating the globalization has taken place. Although the fruits of globalization are attractive, the globalization also causes unavoidable negative consequence such as global warming. The increased concentration of greenhouse gases in the atmosphere is referred as the impact of human activities such as the burning of fossil fuel for transports and power electricity plants. It brings into our concern that an alternative energy source is required, at the same time solving the energy demand issue that caused by globalization is needed. It is therefore, necessary to improve the energy management. Energy storage system, as part of the energy management, receives tremendous attentions for this purpose.
Energy storage system is a big family comprising various energy storage devices, for example, solar cell, battery, electrochemical capacitor, and fuel cell, which can be used for specific applications, for instance, the employment of fuel cell-based system over gasoline engine in transportation . By reducing the rated fuel cell power and controlling the energy loss, the fuel efficiency can be enhanced . Another technique of energy management is by distributing the power onto different energy sources. For example, a hybrid system based on a polymer electrolyte membrane fuel cell and nickel-metal hydride battery has been evaluated for tramway in Spain . Tsukahara and Kondo have inspected the prospective hybridizations of fuel cell with Li-ion battery and electric double layer capacitor (EDLC) to power the railway vehicles .
Apart from transportation, many portable electronic devices also supported by the battery, that is also an energy storage device. From nickel-cadmium battery, nickel-metal hydride battery, to Li-ion battery, the studies on batteries have never stopped. To date, there are more than 35,000 published papers regarding to lithium battery according to Web of Science. Nowadays, Li-ion battery is the most promising battery. A diagram describing charging-discharging mechanism of Li-ion battery is displayed in Figure 1.
As a rechargeable battery, Li-ion battery encourages a rapid growth in the mobile devices technology. Abraham has outlined the limitation and future outlook of Li-ion battery . Li-O2 or Li-air battery will be the battery that can achieve the utmost energy density. However, a down-to-earth Li-air battery is still well on the way to be practical. Recently, an all-solid-state rechargeable battery based on a solid glass electrolyte promises a safe, low cost, and longer cycle life properties for battery . The performances of batteries are summarized in Ragone plot , Figure 2.
Nowadays, the Ragone plot, which is an indication of possible development for certain energy storage devices, has included some storage devices other than batteries. For example, fuel cell, capacitor, and supercapacitor. However, it does not show all the other important properties for instance cycle stability, temperature range of operation, and energy efficiency. Therefore, the Ragone plot cannot be the only reference for evaluating the performance of an energy storage device. Nevertheless, it can be used as a source of information which is continually updated and improved .
An electrochemical capacitor, also known as supercapacitor, is an energy storage device whereby its electrochemical performance lies in between a conventional capacitor and a battery. It can store larger amount of energy compared to a capacitor, but lesser when compared to a battery. Nonetheless, an electrochemical capacitor has higher power density than a battery. The electrochemical capacitor is categorized into two types: electric double layer capacitor (EDLC) and pseudocapacitor. The working principle behind an EDLC is based on the electrostatic interaction. The EDLC charges/discharges are according to the adsorption/de-adsorption processes. The schematic diagram of an EDLC is depicted in Figure 3.
As shown, the positive and negative charges are attracted to the electrode surfaces with opposite charges respectively. The charges are thus stored by means of the electric double layer formed at the electrode surface. Therefore, material with high surface area and conductivity such as carbon materials are suitable to be employed as electrode for EDLC. Conversely, a pseudocapacitor makes use of the charge transfer reactions for charge storage purpose. The corresponding capacitance is named as pseudocapacitance.
Although the charge storage mechanisms are different, the electrochemical signatures of EDLC and pseudocapacitor are similar when they are examined using cyclic voltammetry test . As the charge stored is linearly dependent with the potential, the resultant voltammetry curve exhibits a rectangular and symmetry shape. The redox materials such as conducting polymers and transition metal oxides are usually studied as the potential electrode material for pseudocapacitor. The hydrous ruthenium oxide is found to exhibit a high specific capacitance of 720 F g−1, making it a promising electrode material for electrochemical capacitor . Nevertheless, ruthenium oxide is expensive and toxic. This has prevented hydrous ruthenium oxide from commercial use. Besides, the usage of strong acidic electrolyte also forbids it from application. Other materials such as SnO2, MnO2, TiO2, VO2, and MoO3 have been studied as the alternative electrode material and they are showing different potentials for the practical applications [11, 12, 13, 14, 15]. Among the materials, manganese oxide has been widely studied due to the fact that the manganese oxide can form a variety of composites with different materials using various synthesis routes [16, 17, 18]. In the following section, we will discuss about the properties of manganese oxide.
2. Manganese oxide
Manganese is one of the most abundant elements on Earth which is widely distributed across the crust. In general, manganese deposit forms from seawater. Thus, its redox sensitivity toward oxidation state of ocean enables us to explore the ancient environmental conditions . In human body, the manganese participates in carbohydrate metabolism as well as formation of bone and connective tissues. Manganese oxide has been used since ancient time. It was employed for elucidation of glass and acted as black pigment. Throughout the years, the researches on manganese oxide have deepen the understanding on the chemistry of this compound [20, 21, 22, 23, 24, 25, 26]. At the same time, the utilization of manganese oxide has been exploited and diversified.
The advantage of manganese oxide lies in the feasible formation of various structures based on different arrangements of basic building structure of manganese oxide, which is MnO6 octahedral. This is a structure where O2− ions are octahedrically coordinated to the central of Mn4+ ion, as shown in Figure 4.
The octahedral structure can be arranged through edge- and/or corner-sharing. In common, there are two types of structure: (a) tunnel or chain structure and (b) layer structure. Chain structure is made up of corner-sharing arrangement while tunnel structure is resulted from the combination of single, double, or triple chains of MnO6 octahedral. Layer structure is constructed by the sheets or layers stacking of MnO6 octahedral. Both kinds of structure can hold proton and electrolyte cation.
As a transition metal, manganese exists in various valence states which in turns form a variety of mineral with distinct physical and chemical properties. When different phases of manganese oxide are mixed, they can intergrow to form a new structure. The manganese oxides mineral is inclusive of MnO2, Mn2O3, Mn3O4, and MnOOH. The different forms of minerals are named as the polymorphs. For instance, MnO2 has three polymorphs: pyrolusite, ramsdellite, and nautile while MnOOH has polymorphs of manganite, groutite, and feiknechtite. The polymorphic form is dependent on the linkage between MnO6 octahedrals, which is determined by the preparation process of manganese oxide . Over the years, the variation in structural forms and properties has made manganese oxide a suitable candidate for different applications such as energy storage system, biosensor, coating, environmental, and nuclear science [28, 29, 30, 31, 32, 33, 34]. According to Pourbaix diagram, Figure 5, the electrochemical performances of manganese oxides in aqueous media are arisen from its various oxidation states (Mn2+, Mn3+, and Mn4+) that are emerged from different oxide phases (Mn3O4, Mn2O3, and MnO2) which are thermodynamically stable in alkaline region .
In general, the charge storage of manganese oxide relies on two mechanisms: (a) surface adsorption of electrolyte ions and (b) intercalation/deintercalation of electrolyte ions and/or proton upon reduction/oxidation . Although higher surface area can lead to higher value of specific capacitance, further increasing in surface area is found to not contribute to the specific capacitance [12, 37]. The structure of the manganese oxide can determine its electrochemical performance. A structure with more rooms for the insertion of electrolyte ions will offer higher charge storage capacity, and subsequently higher specific capacitance. To date, the researchers are still working best to figure out the charge storage mechanism of manganese oxide in order to better explain its electrochemical behavior [38, 39, 40]. This knowledge will inspire us and provide us a way on how to fully utilize the potential capacitance of manganese oxide.
Manganese oxide-based film can be prepared via a variety of methods. A material will exhibit different properties such as particle sizes and types of defects, depending on the fabrication routes. In addition to this, the change in experimental parameters also produces materials with different electrochemical properties. The common preparation methods are the hydrothermal, chemical bath deposition, polyol synthesis, sol–gel, electrodeposition, solvothermal, and co-precipitation [20, 36, 39, 41, 42, 43, 44, 45]. The specific capacitances obtained ranges from 121.5 to 698 F g−1, which is still lower than the theoretical specific capacitance of manganese oxide (1380 F g−1). To better utilize the electrochemical active sites, one should understand the relationship between the experimental method and the corresponding structure formed. This chapter will focus on the green and relatively simple method of electrodeposition. Thereafter, we will discuss about the electrochemical performance of manganese oxide-based film fabricated using electrodeposition technique.
3. General principle of electrodeposition
Electrodeposition refers to an electrical process such as electrolytic and electrophoretic deposition, which allows the accumulated mass of a metal ions, or deposit, coated onto an electrode. A common configuration of electrodeposition is displayed in Figure 6.
It is usually made up of a working electrode, counter electrode, and reference electrode. The working electrode is the substrate where the deposition reaction takes place. The reference electrode is used to maintain the voltage stability for the working electrode while the counter electrode or auxiliary electrode is utilized to complete the current flow. There is various reference electrodes served for different electrolyte solutions. A standard hydrogen electrode (SHE) consists of 1.0 M H+ (aq.) solution and is used to compare with other reference electrodes since the standard electrode potential of hydrogen is 0 V. Saturated calomel electrode (SCE) is a reference electrode composes of KCl solution and establishes based on the reaction between elemental mercury and mercury chloride. However, the dangerous nature of mercury content has prohibited the SCE reference electrode from widely use. Instead, the silver chloride (Ag/AgCl) electrode is employed. Ag/AgCl reference electrode is mostly utilized for electrochemical tests and industrial application due its simplicity for fabrication, stability, and non-toxicity. Nonetheless, the usage of Ag/AgCl in basic solution must be heedful because a long immersion of electrode allows the diffusion of OH− ions into the internal filling solution, which can subsequently increase the pH and shift the reaction potential. The measurement of potential using various reference electrodes can be referred to SHE and the corresponding values are recorded in Figure 7.
As electrolytic deposition makes use of solution containing metal ions for deposition purpose, electrophoretic deposition utilizes a mixture consists of suspended colloidal particles. Electrophoretic deposition was revealed when a Russian scientist observed the movement of clay particles in water induced by electric field. It occurs when the non-conductive electrically charged particles migrate to the electrode surface under an applied electric field. The charged particles suspend in the electrolyte due to the mutual electrostatic repulsion . Inside the suspension, the surface charges attract the electrolyte ions with opposite charge. As a result, the counter ions adsorb onto the surface charges, forming a diffuse cloud of counter ions. The process is governed by electrostatic interaction. At the same time, the adsorbed ions repulse from each other. When they are close enough to overcome the electrostatic force, van der Waals attraction will be predominant and adhesion can occur. To achieve electrophoretic deposition, a stable suspension containing well-dispersed particles with desired electrophoretic mobility must be prepared firstly. The usage of additives such as dopamine and triethanolamine can help to stabilize the suspension [47, 48]. In addition, it offers a more uniform and adherent deposit. The parameters that play the role on electrophoretic deposition are composition of dispersion medium, pH of electrolyte, and concentrations of particles and electrolyte. The principles behind electrolytic deposition and electrophoretic deposition have been studied [49, 50, 51]. An understanding about these two processes is required in order to fabricate desired electrode material. The oxidation kinetics of manganese oxide from Mn2+ is reliant on the deposition methods. Depends on the particle charge, electrodeposition can be divided into two types: anodic deposition and cathodic deposition. The anodic deposition is resulted from the oxidation of negative ions on anode (positively charged electrode). In contrast, the reduction of positive ions on cathode results in a cathodic deposition. For manganese oxide film, the anodic deposition involves the oxidation of cationic Mn2+ precursors while cathodic deposition is achieved by the reduction of anionic Mn7+ from MnO4−. Manganese (II) sulfate (MnSO4), manganese (II) nitrate (Mn(NO3)2), manganese (II) acetate (Mn(CH3COO)2), and manganese (II) chloride (MnCl2) can be used as precursors for Mn2+. As the deposition mechanism is independent of precursors, it does not affect significantly on the capacitive behaviors of manganese oxides formed . Due to this reason, the mass loading plays an important role in determining the capacity of manganese oxide formed. Among these precursors, Mn(CH3COO)2 decomposes at lower potential and offers higher deposition rate which make it a favorable precursor. In addition, the Mn3+ is more stable with acetate compared to sulfate which makes it a thermodynamic favor for oxidation kinetics . On the other hand, KMnO4 is usually the Mn7+ precursor for cathodic deposition.
As the oxidation process can lead to the dissolution of metal substrate, the cathodic reduction is usually preferred over anodic oxidation as various metals can be employed as the substrates . The anodic oxidation of Mn2+ involves other oxidation state of manganese ions. Initially the Mn2+ will diffuse and adsorb onto the electrode surface to form Mnads2+. The adsorbed ions oxidize to Mn3+ which subsequently forms intermediate with water. Under appropriate heating temperature, the MnOOH intermediate can be transformed to MnO2. The detailed oxidation reaction of Mn2+ to MnO2 in the medium with different acidity is described in Section 4 [54, 55]. According to Pourbaix diagram, the reduction process of Mn7+ occurs in a neutral aqueous solution is as follow, Reaction 1 :
The electrodeposition can be carried out either by a constant potential or current technique. For constant potential technique, the resulted current-time transient can be determined by following factors: (1) the potential and time of the oxide formation, (2) the potential and time of oxide reduction, and (3) the maintenance time of electrode at reversible potential before reduction takes place . The chronopotentiometry is an electrodeposition technique performed with a constant current. The constant current is applied between working and auxiliary electrodes while the potential of working electrode is measured against a reference electrode. The potential reveals the reaction takes place on the electrode during the electrodeposition process. Before the electrodeposition starts, there is no concentration gradient of oxidants in the solution regardless of the distance from electrode surface. This initial potential is also called as open circuit potential. However, a concentration gradient of oxidants is formed when the reduction initiates by the application of current. The reduction is a resultant process of oxidants responds to the applied current. As a result, the oxidants near to the electrode surface are consumed, causing the oxidants diffuse from bulk solution toward electrode surface in order to accommodate the reduction process. The potential is comparable to the redox potential of certain electron transfer reaction. Since the concentration changes with time, this potential alters correspondingly. Once the current cannot be further sustained by this redox potential, which is due to the concentration of oxidants turns to zero, this potential will adjust to another redox potential in order to maintain the fixed current. The changing potential during galvanostatic deposition can lead to the formation of various morphologies. Different from chronoamperometry, which is a deposition method employing a constant voltage, a desired morphology can be formed accordingly. Knowledge of the interactions between electrode and electrodeposition parameters allows us to construct and improve the electrode film performance. Herein, the manganese oxide deposited using various deposition techniques are discussed and evaluated.
4. Electrochemical performance of deposited manganese oxide-based film
4.1. Effect of electrolyte composition
For anodic deposition, manganese acetate (Mn(CH3COO)2) and manganese sulfate (MnSO4) are always chosen as the Mn2+ precursors. They offer different kinds of morphologies although the electrodeposition is carried out under the same conditions. For example, the morphology produced from 0.01 M Mn(CH3COO)2 at constant current density of 30 mA cm−2 exhibits interconnected but non-continuous nanorods structure, Figure 8(a) . On the other hand, 0.01 M MnSO4 leads to a continuous and homogenous nanorods structure (Figure 8(b)).
With the addition of H2SO4, a discrete crystallite of manganese oxide is formed (). H2SO4 acts as a supporting electrolyte and enhances the stability of the soluble Mn3+ intermediate before further reactions . This allows more manganese ions to be deposited and form a film on the substrate after certain electrodeposition time. The electrodeposition mechanism of manganese oxide with the presence of H2SO4 is shown as following :
Depending on the concentration of acid, there are two proposed pathways for the formation of manganese oxide: disproportionation and hydrolysis. Reaction 2 shows the oxidation of Mn2+ become soluble Mn3+ intermediate. If the supporting acid, which is H2SO4 in this case, has higher concentration, Mn3+ achieves relatively higher stability allowing it to experience disproportionation which subsequently forms Mn2+ and Mn4+ (Reaction 3). Mn4+ is then hydrolyzed to form MnO2 on the surface of substrate (Reaction 4). On the other hand, at lower acidity condition, Mn3+ is less stable and thus can hydrolyze easily to form MnOOH (Reaction 5). MnOOH is then converted to MnO2 under suitable annealing temperature (Reaction 6). The manganese oxide electrodeposited from the electrolyte containing H2SO4 can obtain specific capacitance as high as 5600 F g−1 . A less acidic medium also can be achieved by adding sodium sulfate (Na2SO4). It can act as the supporting electrolyte for Mn2+ and Mn7+ precursors [61, 62].
4.2. Effect of deposition potential
In an electrolyte of Mn(CH3COO)2, 0.5 V (vs. SCE) of anodic potential forms a manganese oxide consists of two oxidation states: trivalent (Mn3+) and tetravalent states (Mn4+), while divalent ions Mn2+ and Mn3+ are observed for manganese oxide formed at anodic potential lower than 0.5 V [63, 64]. The manganese oxide resulted from deposition potential of 0.2 V (vs. SCE) for 1 hour consists of γ-Mn2O3 and/or Mn3O4 while the phase transforms from tetragonal to hexagonal system when the deposition potential increased to 0.4 V and higher potentials. At anodic potential higher than 0.5 V, Mn4+ state becomes dominant and hydrous MnO2 can be formed .
The anodic potential is found to be correlated to the adsorbed water content in the manganese oxide structure. The increase in anodic deposition potential leads to the formation of higher oxidation state of manganese ions, at the same time results in the reduction in adsorbed water content [64, 66]. As the rate of nucleus formation increases more rapidly than the rate of crystal growth with the increase of anodic potential, the morphology preferably grows horizontally, which subsequently forms a compact and layered structure. The formation of this structure is resulted from the local fluctuation of reactants. As the reactant consumption rate increases with higher deposition potential, the reactions take place at the vicinity of electrode also occur rapidly. The release of adsorbed water molecules further resulted in the formation of an uneven surface. The highest specific capacitance obtained is 240 F g−1 at 5 mV s−1. A similar specific capacitance value is achieved when the technique applied is cathodic potentiostatic electrodeposition. The MnO2 prepared through cathodic reduction process at a constant potential of 0.55 V (vs. SCE) has achieved around 233 F g−1 . However, instead of a smooth surface which is produced at 0.5 V (vs. SCE) of anodic potential, a rough surface is formed for this cathodic potentiostatic electrodeposition. A further decrease in deposition potential (−1.8 V) can result in a porous structure with nanowall architecture .
For cathodic deposition, the increase in deposition potential leads to the uniform distribution of manganese ions onto the substrate. At relatively low cathodic deposition potential, the morphology formed is a cluster-like structure. The increase of cathodic deposition potential from −0.4 V to −0.1 V motivates the formation of flaky-like structure . Further increase in cathodic deposition potential forms a more homogenous and flat structure. The formation of various structures with the cathodic deposition potential arises from the different deposition mechanisms with different potential values. The first mechanism takes place in the cathodic potential range of 0.1−0.4 V whereby the MnO42− produced will form H2MnO4− that subsequently reduce to form manganese oxide, as shown in Reaction 7. Another mechanism occurs at cathodic potential less than 0.2 V. It takes into account of the dissolution of manganese oxide that has formed initially. This brings about the formation of Mn2+ which is then reacted with MnO4− to produce manganese oxide, as described in Reactions 8 and 9.
Depending on the potential range, a potentiodynamic deposition can produce a nanorod-like structure with various diameters, given that a same scan rate is employed, as shown in Figure 9. Smaller potential range (0.1−0.4 V vs. SCE) leads to the formation of nanorods structure with higher diameter while wider potential range (0.1−0.6 V vs. SCE) tends to form a nanowire structure, which has relatively smaller diameter compared to nanorods . Other studies that applying a small potential range (0.3−0.6 V vs. SCE) also led to the formation of nanorod structure with similar diameter, that is around 30 nm in average . This suggests the tendency to form nanorod structured manganese oxide at 0.3 V of potential range in electrolyte consisting manganese acetate and sodium sulfate. The specific capacitances estimated from charge–discharge at 0.1 mA cm−2 for nanorods formed in the potential ranges of (0.1−0.4) V and (0.3−0.6) V are in the range of 200 to 250 F g−1. The extension of deposition potential range from 0.3 to 0.5 V not only improves the surface area, at the same time it also enhances the specific capacitance for 44%.
Although the integration with other materials shown enhancement in charge storage, the incorporation of carbon nanotubes (CNT) does not always work in this way. As the sp2 carbon basal plane has low chemical and electrochemical reactivity, the nucleation sites on the surface of CNT-manganese oxide composite film are limited. In addition, the manganese oxide particles tend to grow at the CNT’s junction . A flower-like morphology built up by nanosheets that are originated radially from a central is thus formed. Nevertheless, this composite indeed acquires better cycle stability due to CNT acting as a conductive backbone that reduces the dissolution of particles. There is a variety of substrates can be used for deposition. Tantalum (Ta) foil has high melting point and provides good corrosion resistance, strength, and ductility. Cotton sheet supplies flexibility and textile structure that eases a uniform coating of material. Nickel (Ni) foil contributes a good electrical conductivity while the stainless steel is inert and owns a stable passivity. As a metal substrate, Ta foil and Ni foil usually offer a good cycle stability and specific capacitance of around 413 F g−1 in average for CNT-manganese oxide film [72, 73]. In spite of the fact that stainless steel has poorer electrical conductivity compared to other metals, the CNT-manganese oxide film composite film formed on stainless steel was found to achieve higher specific capacitance, that is 869 F g−1 . Meanwhile, it also has good cycle stability. Given a similar morphology, which is nanowires-structured manganese oxide coated on CNT, formed on these three substrates, the distinct specific capacitance value achieved suggests the hidden advantages of stainless steel as a substrate. Direct deposition of manganese oxide on CNT paper using potentiodynamic method gives rise to around 168 F g−1 . It is therefore important to choose an appropriate substrate for optimal electrochemical performance. Without the CNT, the manganese oxide tends to appear in nanorod-like structure and sphere-like structure at oxidation condition and reduction condition, respectively . For the manganese oxide deposited within the same potential range (0.1−0.4 V), a desired structure can be determined by applying different annealing temperatures. For example, 300°C leads to the formation of nanotubes structure while 100°C forms nanorods structure . The manganese oxide-based films prepared at different scan rates during potentiodynamic deposition possess distinct morphologies. At scan rate lower than 100 mV s−1, the morphology forms are irregular and dense. It starts to evolve and becomes more porous when the scan rate of deposition is increased. A nanoflake structure can be formed at scan rate of 200 mV s−1 . This structure is beneficial for charge storage purpose at which it has offered specific capacitance of 410 F g−1. The compact and non-porous structure produced at relatively scan rate achieved around 150 F g−1.
4.3. Effect of deposition current density
Potentiostatic deposition tends to form a more compact structure compared to the galvanostatic deposition. This is due to the consistent deposition rate during potentiostatic deposition. As the potential is maintained throughout the deposition process, the deposition rate is sustained. In contrast, the potential varies during galvanostatic deposition in order to sustain the current supply. The structure formed is thus less compact and higher in surface area . The difference in compactness contributes to 50% increment in specific capacitance value and 15% higher capacitance retention upon 5000 cycles for galvanostatic deposited manganese oxide film. During the galvanostatic deposition, the structure changes from irregular to regular and uniform structure with the increased in current density [79, 80]. When the current density exceeds the optimal value to deposit Mn4+, a clustered structure which consists of soluble Mn6+ and/or Mn7+ may forms. Other than oxidation states, the morphology also changes with current density. The evolution of morphology is easily observed using field emission scanning electron microscopy (FESEM). As the nucleation rate is directly related to the current density, a lower current density produces lower nucleation rate. As a result, there is not many nuclei formed on the substrate surface and a continuous deposit layer is hard to be constructed . In addition, the deposit usually possesses rough surface. Higher current density can lead to higher nucleation and growth rate. The deposit accumulates on the structure formed ahead and filling up the space or crack, which leads to the formation of a uniform coating, Figure 10.
At optimal current density, various structures such as nanowires, nanoflakes, nanosheets, and nanorods can be formed [57, 82, 83]. For example, manganese oxide formed from MnSO4 precursor at 4 mA cm−2 presents as agglomerated clusters. A small decreased in current density, 3.7 mA cm−2, brings about a grain-like structure constructed by nanowires . 2 mA cm−2 of galvanostatic deposition in Mn(NO3)2 produces a flower-like structure made up of nanowires . This manganese oxide film is capable to maintain 84% of specific capacitance after 1000 cycles.
4.4. Electrophoretic deposition
As one kind of the electrodeposition techniques, the oxidation and dissolution of the metal substrate during the anodic electrophoretic deposition is a concern. Thus, cathodic electrophoretic deposition is more favorable compared to its counterpart. Different from the electrodeposition techniques mentioned above, the electrophoretic deposition does not form the oxide during or after the process. Instead, the oxide is fabricated beforehand. The desired nanostructure can be manipulated using various preparation methods such as hydrothermal, chemical reduction, chemical precipitation, spray pyrolysis, and wet-chemical processes. The nanostructured oxide powder formed is then suspended in a dispersant electrolyte for further action, which is the electrophoretic deposition in this case. The structure of manganese oxide particles will be retained even after deposition, as depicted in Figure 11.
It can be seen that the stability of manganese oxide suspension is important to ensure the success of electrophoretic deposition. The ethanol can be used as the liquid medium for suspension. However, the deposit resulted from the manganese oxide suspension in ethanol is shown to be irregular and tends to form agglomerates. The addition of phosphate ester has been shown to enhance the stability of the manganese oxide suspension and increase the mass load . This improves the electrochemical performance of the manganese oxide film. Without the phosphate ester, the manganese oxide film only exhibits 236 F g−1 of specific capacitance and drops 15% of the initial specific capacitance after 25 cycles . Meanwhile, the phosphate ester has offered around 60% of increment in specific capacitance . Sodium alginate is a good dispersant as well. It has been proposed that the sodium alginate provides the electrostatic and steric stabilization for the manganese oxide suspension. Additionally, it supplies electric charge for the suspension particles which is beneficial for the deposition process . The manganese oxide film deposited from the dispersant electrolyte of sodium alginate has obtained specific capacitance (412 F g−1) a little higher than the one prepared from the ethanol with phosphate ester dispersant electrolyte (377 F g−1). The integration of carbon nanotubes or reduced graphene oxide with the manganese oxide does not alter the nanostructure of the oxide itself . The nanostructured manganese oxide particles attach on the carbon nanotube surface and deposit together onto the substrate .
We have discussed about the impact of few deposition parameters on the deposited manganese oxide-based film. The as-deposited manganese oxide-based film is dominantly amorphous with inherent cation deficiency . The defect is most likely to form at relatively low deposition temperatures (80−200°C). Except the one prepared using electrophoretic deposition, at which the manganese oxide particles are firstly fabricated before deposition, the as-deposited manganese oxide-based film requires certain post-treatments to improve the crystalline structure. Annealing is one of the common post-treatment. When the manganese oxide-based film undergoes post-heating process, different crystalline structure forms based on the annealing temperature. Most of the water content at the surface layer of structure desorbs gradually at 120−350°C . The manganese oxide presents as γ-manganese oxide at annealing temperature lower than 350°C. The transformation of crystalline structure is initiated at around 300°C, which allows the γ-phase changes to β-phase of MnO2. When the manganese oxide undergoes further heating, α-Mn2O3 phase starts to form . Any desired crystalline structure is thus can be prepared. Other than crystalline structure, the morphology of the manganese oxide-based film can also be modified by varying the annealing temperatures. At annealing temperatures lower than 200°C, a fibrous and granular structure is formed. The entanglement of fibers causes the morphology to evolve to a cluster-like structure when the temperature reaches 300°C. Further increasing in temperature can lead to the formation of flaky-like (500°C) and rod-like structures (600°C) , Figure 12.
There are still many studies carried out to investigate other possible factors affecting the deposited manganese oxide-based film. For example, by studying the porosity of the manganese oxide-based film, one can also inspect more details about the structure and charge transport properties . More and more deposition techniques have been developed and studied to prepare the manganese oxide-based film. For instance, a redox deposition that is took place when a substrate is immersed substrate into the Mn ions precursors [95, 96]. From chemical bath deposition, chemical vapor deposition to spray pyrolysis deposition, all deposition techniques are intended to grow the deposit with a good quality along with good physical and chemical properties in a large scale.
Manganese oxide-based film is shown to be prepared in various electrodeposition conditions by varying electrodeposition potentials, current densities, additives, and electrolytes. The potential application of manganese oxide-based film as the electrode material for electrochemical capacitor is thus discussed. As deposited manganese oxide film is amorphous in nature , the amorphousness can be transformed to crystalline phase by employing appropriate annealing temperature. The crystallinity start to arise when the annealing temperature increases to 300°C. MnO2 is the first crystal structure detected at 300°C, further increasing the temperature leads to the formation of Mn3O4 and Mn2O3 . Since proton participates in the charge storage mechanism of manganese oxide, it is believed that this oxide will perform better in hydrous form. Previous study about the RuO2 has shown the significant improvement on the electrochemical performance with the presence of hydrous phase . Not long after that, the crystalline manganese oxide has exhibited its potential application as electrode material as well . To date, the charge storage mechanism is found to be dependent on the crystalline structure, water content, and surface area. It turns out that, the electrochemical performance of manganese oxide-based film is not totally relied on any of these factors. In contrast, it is resulted from the combination of all the factors that have been found and studied. For this reason, the researchers are still making effort to understand this complication.
The addition of secondary or ternary materials can enhance the electrochemical performance of manganese oxide film. However, these are not shown in this chapter. The core idea is to gather the advantages of various materials then compensate shortcomings of each other. The common materials combination involves the carbon material, transition metal oxide, and conducting polymer. The carbon material offers conductivity while transition metal oxide and conducting polymer provides more electroactive sites for charge storage purpose. Electrodeposition is a widely used technique to protect and strengthen the function of parts used in various industries. The wide application of electrodeposition technology can be attributed to its simplicity, manufacturability, and scalability. The electrodeposition method also requires a relatively low fabrication cost for energy storage device compared to other methods. This technique allows a direct formation of film on the substrate desired and the film properties are governable by varying the deposition parameters. The traditional method of electrode fabrication for electrochemical capacitor involves the pressing of electrode material onto the substrate. This process can increase the contact resistance and reduce the porous surface area which brings about damage to the electrode materials formed. In overall, manganese oxide-based film prepared using electrodeposition is prospective and practical. There are numerous related studies carrying out every year, although the reported specific capacitance of manganese oxide-based film are yet far from the expected performances (Table 1), we are convinced that there is still a big room for improvement.
|Electrode materials||Deposition mode||Deposition electrolyte||Specific capacitance, F g−1||Reference|
|MnOx||Potentiostatic||0.2 V vs. SCE||0.001 M KMnO4 + 1 M Na2SO4||368.00|||
|MnOx||Potentiostatic||0.50 V vs. SCE||0.25 M Mn(CH3COO)2||240.00|||
|MnO2||Potentiostatic||0.55 V vs. SCE||KMnO4||232.94|||
|a-MnO2⋅nH2O||Potentiostatic||0.75 V vs. SCE||0.25 M MnSO4⋅5H2O||285.00|||
|MnOx||Potentiostatic||1.0 V vs. Ag/AgCl||2 mM MnSO4 + 50 mM KCl||163.40|||
|MnOx||Potentiostatic||1.1 V vs. Ag/AgCl||0.1 M MnSO4 + 0.01 M TTAB||343.00|||
|MnOx||0.1 M MnSO4||294.00|
|MnO2||Potentiostatic||10 V vs. Ag/AgCl||0.5 M KMnO4||128.00|||
|Mn3O4||Potentiostatic||−1.3 V vs. SCE||0.25 M MnNO3||416.00|||
|MnO2||Potentiostatic||0.60 vs. SCE||0.1 M Mn(CH3COO)2 + 0.1 M Na2SO4||240.00|||
|MnO2-CNT||Cyclic voltammetry||0.30–0.60 V vs. SCE||167.50|||
|Mn-Ni mixed oxide||Cyclic voltammetry||0.4 − 1.2 V vs. Ag/AgCl||0.05 M Mn(CH3COO)2 + 0.1 M Ni(CH3COO)2 + 0.2 M CH3COONa||169.00|||
|Mn-Mo mixed oxide||Cyclic voltammetry||0.00 − 1.00 V vs. Ag/AgCl||2 mM MnSO4 + 20 mM Na2MoO4||190.90|||
|MnOx||Cyclic voltammetry||1.10 − 1.50 V vs. SCE||0.5 M MnSO4 + 0.5 M Na2SO4 + 100 mM SLS||310.00|||
|MnO2||Galvanostatic||2 mA cm−2||5 mM Mn(NO3)2||246.00|||
|MnO2||Galvanostatic||3 mA cm−2||0.02 M KMnO4||188.00|||
|MnO2-PPy||Galvanostatic||4 mA cm−2||0.2 M MnSO4 + PPy||620.00|||
|Fe-doped MnOx||Galvanostatic||5 mA cm−2||0.1 M MnSO4 + 0.1 M citric acid||218.00|||
|MnOx||Galvanostatic||5 mA cm−2||0.01 M Mn(CH3COO)2||185.00|||
|MnO2||Galvanostatic||10.5 mA cm−2||0.02 M Mn(CH3COO)2||201.00|||
|Co-doped MnOx||Galvanostatic||50 mA cm−2||MnSO4, cobalt sulfate, EDTA||186.20|||
|Fe-doped MnOx||Galvanostatic||MnSO4, iron sulfate, EDTA||298.40|
|MnO2||Galvanostatic||165 mA cm−2||0.5 M KMnO4||196.00|||
|MnO2-RGO||Electrophoretic deposition||0.3 V vs. SCE||MnO2 + RGO||392.00|||
|MnOX||Electrophoretic deposition||5 − 10 V vs. SCE||Manganese oxide + sodium alginate||412.00|||
|MnOx||Electrophoretic deposition||10 − 100 V vs. SCE||Manganese oxide + ethanol + phosphate ester||377.00|||
|MnOx-CNT||Electrophoretic deposition||15 V vs. SCE||Manganese oxide + sodium alginate + carbon nanotubes||Around 210.00|||
|MnOx||Electrophoretic deposition||100 V vs. Ag/AgCl||Manganese oxide + ethanol||236.00|||
|MnOx||Electrophoretic deposition||100 V vs. Ag/AgCl||Manganese oxide + ethylene alcohol + H2SO4||275.00|||
The authors acknowledge the University of Malaya for providing financial support through the projects BKS030-2017 and FG034-17AFR.