Control of Manganese Oxide Hybrid Structure through Electrodeposition and SILAR Techniques for Supercapacitor Electrode Applications

Abstract

Manganese oxide has been studied as a promising supercapacitor electrode due to its high theoretical capacitance, low cost, and environmental friendliness. Supercapacitor performance such as specific capacitance, resistance, and cycle life greatly depends on the morphology and crystal structure of manganese oxide. In this study, a Mn₃O₄ hybrid structure was successfully synthesized using electrodeposition and successive ionic layer adsorption and reaction (SILAR) techniques which are simple, cost-effective, and low-temperature wet chemical processes. It was found that Mn₃O₄ morphology is different depending on manganese precursors and synthesis techniques. Sea-grape-like and bird nest-like morphologies were obtained via the electrodeposition technique, while flower-like and nanoparticle morphologies were formed via the SILAR technique using manganese acetate and manganese sulfate as precursors, respectively. The hybrid structure of the nanoparticle-decorated bird nest-like heterostructure was prepared using manganese sulfate electrodeposition and subsequent SILAR deposition of manganese acetate. X-ray photoelectron spectroscopy confirmed the Mn₃O₄ formation. Electrochemical properties of manganese oxide hybrid structure were systematically studied with cyclic voltammetry and galvanostatic charge–discharge, showing the highest areal capacitance of 390 mF cm⁻² at 0.1 mA cm⁻² with series and charge transfer resistances down to 4.55 and 4.91 Ω in 1 M sodium sulfate electrolyte.

1. Introduction

Supercapacitor is an energy storage device bridging between battery and conventional capacitor. It has the advantages of long life-cycle [1], fast charge–discharge rate [2], cost effective and environmental friendliness [3,4,5] which has potential applications for hybrid electric vehicles [6,7], handheld electronic devices [8], device for monitoring human motion and deaf-mute voicing [9], and support component for energy storage system [10]. The charge storage mechanisms of supercapacitor can be divided into two categories: electrical double-layer capacitance (EDLC) and pseudocapacitance. The first one utilizes electrolyte ions which electrostatically attach to the surface of electrodes with opposite charges to store energy. This type of charge storage mechanism benefits from the high surface area of the electrode, so the material of choice usually is carbon materials (e.g., activated carbon [11], carbon nanotube [12], graphene [13]). For pseudocapacitance, it utilizes faradaic reaction between the electrolyte ion and electrode. Pseudocapacitive materials include transition metal oxide (e.g., RuO_x [14], FeO_x [15], and MnO_x) and conductive polymers [16].

Manganese oxide is one of the most promising transition metal oxides due to its abundance in nature, high theoretical capacitance, and multiple oxidation numbers which leads to a variety of crystal structures [17,18]. These crystal structures play an important role in determining rate capability, cyclic stability [19], and ultimately, energy density and power density of supercapacitor [20]. The reduced KMnO₄ was calcined in air at various temperatures to obtain α-MnO₂ with different morphological structures [21]. These structures changed from granule flake-like shape into thin nanowires with higher degree of crystallinity, which decreased specific capacitance from 177 F g⁻¹ to 110 F g⁻¹ but showed higher pseudocapacitive behavior.

For the film deposition method, electrodeposition has been a dominant technique for many years due to its simple setup, easy to control parameters, and time efficiency [17,22,23]. The structural and electrochemical properties of MnO₂ electrodeposited in potentiostatic and galvanostatic mode were compared at nearly identical current density with 0.5 M KMnO₄ as a precursor [24]. The obtained MnO₂ had crystal structure in the ramdellite phase and formed nanosheet structures which exhibited specific capacitance of 196 F g⁻¹ and 128 F g⁻¹ for galvanostatic and potentiostatic deposition mode, respectively. Many works utilized high surface area substrate to improve the specific capacitance of the manganese oxide film, but to study the stability of manganese oxide in various crystal structures, smooth surface substrate should be used to eliminate the effect taking advantage of the substrate roughness [25].

Another interesting deposition technique is the successive ionic layer adsorption and reaction (SILAR) method which is an emerging technique that recently has gained more attention due to its easily controlled deposition rate and film thickness, affording relatively uniform film on any substrates, cost efficiency, and low-temperature operation. SILAR is mainly based on the adsorption and reaction of the ion from solution and rinsing between every immersion with deionized water to avoid precipitation in solution [26]. Mn₃O₄ nanoparticles deposited by SILAR method achieved the maximum specific capacitance of 194 F g⁻¹ [27]. A study on the effect of different Mn precursors on electrochemical properties of manganese oxide film deposited through the SILAR method revealed that manganese acetate (Mn(CH₃COO)₂), manganese sulfate (MnSO₄), and manganese chloride (MnCl₂) resulted in granular type of morphology with different sizes [28,29]. In addition, the hybrid structure of electrodeposited manganese oxide film from KMnO₄ and MnSO₄ precursors could improve the electrochemical properties with the highest specific capacitance of 279 F g⁻¹ [30]. The hybridization of manganese oxide could improve the specific capacitance of film by provided additional active sites and higher surface area. However, the synthesis of manganese oxide hybrid film through electrodepostion has suffered the high electric field for the second layer deposition due to the low conductivity of the first manganese oxide layer.

In this work, we report the synthesis of Mn₃O₄ hybrid structure utilizing electrodeposition and SILAR techniques. The effect of manganese precursors (Mn(CH₃COO)₂) and MnSO₄) and deposition techniques on morphologies and electrochemical properties was investigated. The hybrid structure of nanoparticle-decorated bird nest-like heterostructure was prepared through manganese sulfate electrodeposition and subsequent SILAR deposition of manganese acetate, resulting in an enhancement of an areal capacitance without hindering the stability. The electrochemical properties of Mn₃O₄ hybrid structure were systematically studied with cyclic voltammetry and galvanostatic charge–discharge, showing the highest areal capacitance of 390 mF cm⁻² at 0.1 mA cm⁻² with series and charge transfer resistances down to 4.55 and 4.91 Ω in 1 M sodium sulfate electrolyte.

2. Materials and Methods

2.1. Chemical Reagents

All chemical reagents in this study were used without further purification. Manganese(II) sulfate monohydrate (MnSO₄·H₂O), sodium sulfate (Na₂SO₄) anhydrous crystals, and sodium hydroxide (NaOH) were purchased from CARLO ERBA Reagents, Val-de-Reuil, France. Manganese(II) acetate tetrahydrate (Mn(CH₃COO)₂·4H₂O) was purchased from Acros Organics (Geel, Belgium).

2.2. Mn₃O₄ Deposition

2.2.1. Mn₃O₄ Deposition Using the Electrodeposition Method

Firstly, 1 cm × 1 cm stainless steel 304 (SS304) was used as a substrate for cathodic electrodeposition using a graphite rod with a diameter of 6 mm as a positive electrode. The SS304 substrates were cleaned with acetone, ethanol, and deionized (DI) water, respectively, for 10 min in ultrasonic bath. Mn₃O₄ film was deposited on SS304 substrate in galvanostatic mode at a current density of 1 mA cm⁻² for 10 min, then rinsed with DI water and heated at 80 °C overnight. The Mn₃O₄ precursors were 0.1 M MnSO₄ and 0.1 M Mn(CH₃COO)₂ (hereinafter referred to as EPD-S and EPD-A, respectively).

2.2.2. Mn₃O₄ Deposition Using the Successive Ionic Layer Adsorption and Reaction (SILAR) Method

The cleaned SS304 substrate was first soaked in manganese precursor for 1 min and immersed in DI water. After that, the SS304 substrate was soaked in 1 M NaOH for 1 min and immersed in DI water. These steps were repeated for 20 cycles. The deposited film was kept at 80 °C overnight. The Mn₃O₄ precursors were 0.1 M MnSO₄ and 0.1 M Mn(CH₃COO)₂ (hereinafter referred to as SIL-S and SIL-A, respectively).

2.2.3. Mn₃O₄ Hybrid Structure Using the Electrodeposition and Successive Ionic Layer Adsorption and Reaction (SILAR) Methods

Hybrid structure of Mn₃O₄ was done by combining two deposition techniques, electrodeposition and SILAR methods. In the first step, the electrodeposition method was chosen for the deposition of the first layer to ensure a good contact between Mn₃O₄ and SS304 substrate. The electrodeposition process was carried out in the same procedure to that of a single-layer deposition, as mentioned earlier. For the second layer, to prevent damaging the electrodeposited structure, the SILAR method was employed. The SILAR deposition processes were identical to the single layer deposition as described above. The resulting hybrid film, known as HYB-A, was created using MnSO₄ precursor for the first layer and Mn(CH₃COO)₂ precursor for the second layer. The hybrid film, however, is referred to as HYB-S when Mn(CH₃COO)₂ precursor was employed for the first layer and MnSO₄ precursor for the second layer.

2.3. Morphological Characterization

Field emission scanning electron microscopy (FE-SEM) was performed with Schottky FE-SEM SU5000 Hitachi with an accelerating voltage of 5 kV at various magnifications. X-ray photoelectron spectroscopy (XPS) was carried out on Kratos Axis ultra DLD spectrometer. The peak fitting and analysis of XPS data were performed with CasaXPS. The oxidation number of manganese oxides was determined by binding energy difference between Mn 3s multiplet [31].

2.4. Electrochemical Characterization

Electrochemical characterization was performed in 1 M Na₂SO₄ at a potential window of −0.2–0.8 V with Ag/AgCl and Pt sheet as reference and counter electrodes, respectively. Cyclic voltammetry (CV) was measured at a scan rate range from 5 to 100 mV s⁻¹. Galvanostatic charge–discharge (GCD) was measured at various current densities from 0.1 mA cm⁻² to 1 mA cm⁻². Electrochemical impedance spectroscopy was measured in frequency range from 0.1 to 10⁵ Hz at 0.01 V_rms. The stability test of Mn₃O₄ electrodes was performed with GCD at a current density of 1 mA cm⁻² for 1500 cycles.

3. Results

3.1. SILAR Film Formation Mechanism

Mn₃O₄ deposited by SILAR method was exposed to cationic and anionic (NaOH 1 M) precursor by soaking in solution bath and then being dumped in DI water to remove weakly attached ion on the surface. This method deposited film ion layer-by-layer, the thickness of which could be controlled by the number of dipping cycles and soaking time. Mn₃O₄ from manganese acetate was formed following Equations (1) and (2) [28]. Mn₃O₄ from manganese sulfate was formed following Equations (3) and (4).

Mn(CH₃COO)₂ + 2H₂O → Mn(OH)₂ + 2CH₃COOH

(1)

3Mn(OH)₂ + 2(OH)⁻ → Mn₃O₄ + 4H₂O + 2e⁻

(2)

MnSO₄ + 2H₂O → Mn(OH)₂ + H₂SO₄

(3)

3Mn(OH)₂ + 2(OH)⁻ → Mn₃O₄ + 4H₂O + 2e⁻

(4)

3.2. Morphological Structure of Mn₃O₄

The morphological structures of Mn₃O₄ deposited on SS304 substrates were captured by FE-SEM at various magnifications (Figure 1). The Mn₃O₄ film electrodeposited by the Mn(CH₃COO)₂ precursor showed the sea-grape-like morphology as seen in Figure 1a. Nanoparticles with a diameter range from 70 to 80 nm attached on a long branch. The Mn₃O₄ film electrodeposited by MnSO₄ precursor showed the interconnected bird nest-like morphological structure as seen in Figure 1b, which could act as a high surface layer for hybrid film to enhance areal capacitance. For the SILAR deposition technique, the obtained Mn₃O₄ films from both precursors are shown in Figure 1c,d. Both films exhibited a structure of uniformly dispersed Mn₃O₄ nanoparticles, with sporadically grown spiky flowers observed on the surface of the electrodes (as shown in the inset images). When comparing between two manganese precursors, the flowers from Mn(CH₃COO)₂ were larger in diameter. This was caused by higher deposition rate when the more stable Mn(III) species, compared to MnSO₄, were oxidized to Mn(IV). The hybrid Mn₃O₄ films were the combination of two deposition techniques, electrodeposition and SILAR. The initial layer was electrodeposited with one of the manganese precursors, and the second layer was deposited using the SILAR technique and a different precursor. Figure 1e depicts the interconnected bird nest-like structure of the Mn₃O₄ film electrodeposited by MnSO₄ precursors, which served as a high surface area substrate for the second layer of Mn₃O₄ film deposited with Mn(CH₃COO)₂ precursors employing the SILAR technique (so-called HYB-A). For the HYB-S film, the precursors for electrodeposition and SILAR were changed to Mn(CH₃COO)₂ and MnSO₄ for the first and second layers, respectively. The hybrid Mn₃O₄ film structure retained the characteristics of both precursors, resulting in an aggregated nanoparticle structure, as shown in Figure 1f. Low magnification SEM images of all samples are shown in Figure S1.

The X-ray photoelectron spectroscopy (XPS) results of the Mn₃O₄ films deposited using the electrodeposition and SILAR methods are shown in Figure 2. The oxidation number, determined by the multiplet energy difference, of the films ranged from 2 to 3, indicating that the films were composed of Mn₃O₄, which could undergo redox reactions changing between Mn(II) and Mn(IV) [31]. The Mn 2p spectra exhibited distinct Mn 2p^1/2 and Mn 2p^3/2 peaks at 641 eV and 653 eV, respectively. The XPS spectra of the HYB-A film are shown in Figure 3. The oxidation number calculated from the O 1s spectra was identical to the one determined by the Mn 1s spectra. Furthermore, Figure 3c displays a prominent hydration peak, indicating the presence of molecular water residue within the film structure, potentially facilitating the diffusion of electrolyte ions into the internal film structure.

3.3. Electrochemical Properties

Figure 4a shown cyclic voltammetry of Mn₃O₄ films deposited with electrodeposition and SILAR technique at scan rate 5 mV s⁻¹ in the potential range from −0.2 to 0.8 V. The electrodeposited Mn₃O₄ showed a significantly larger cyclic loop, indicating the superior charge storage capability. At 0.55 V, a couple of redox peaks, responsible for reversible redox process of Mn(II) ↔ Mn(III), were observed only for electrodeposited film. The lack of redox peak in SILAR-deposited film was possibly caused by lower active sites on electrode surface and led to a decrease in pseudocapacitive behavior and lower charge storage capability. The rapid change in current at −0.2 V and 0.8 V of Mn₃O₄ electrodeposited with MnSO₄ indicated the low electrical resistance which could provide efficient electron transfer between active mass and stainless steel substrate. When combined with the significantly larger cyclic loop caused by the higher surface area of the interconnected structure, EPD-S was clearly more superior than EPD-A. The Mn₃O₄ deposited with the SILAR method, SIL-A and SIL-S, showed identical cyclic loop shape, indicating that both precursors could provide charge storage capability with almost no difference in performance.

As shown in Figure 4b, HYB-A has a significantly larger cyclic loop than HYB-S and EPD-S (which has the largest loop in the single layer group), indicating the superior charge storage capability compared to other films. The higher surface area of the film can be attributed to the preservation of the electrodeposited interconnected structure during SILAR deposition, allowing for the formation of aggregated nanoparticles on top without damaging the electrodeposited structure. The slightly slower current change at potential −0.2 and 0.8 V was due to the increased film thickness.

Figure 4c,d displays the CV curves of the EPD-S and HYB-A electrodes at various scan rates. Even at the higher scan rate of up to 100 mV s⁻¹, both electrodes maintained their semi-rectangular CV curves, indicating the stability of the electrodes. The redox peaks were observed to be more shifted in the EPD-S electrode than in the HYB-A electrode, suggesting that the redox reaction on the surface of the EPD-S required longer time for ion diffusion to facilitate the redox reaction [32]. The CV curves of each electrode studied in this work are shown in Figure S2.

The galvanostatic charge–discharge was carried out at current density of 0.1 mA cm⁻² in the potential range from −0.2 to 0.8 V (Figure 5a). In general, the charge–discharge curves closely resembled a symmetrical triangle. The small peak at potential 0.5–0.6 V was the reversible redox reaction, the same as the one observed in CV curves which did not show up in films deposited using the SILAR method.

The areal capacitance of EPD-A, EPD-S, SIL-A, SIL-S, HYB-A, and HYB-S at 0.1 mA cm⁻² is 76, 146, 123, 123, 390, and 216 mF cm⁻², respectively. In Figure 5b, the HYB-A has the highest areal capacitance, followed by HYB-S. Hybrid films may have higher mass compared to single layer film, but when considering the two-time increase in areal capacitance compared to individual film, it is indicated that there is a synergistic effect between electrodeposition and the SILAR method. However, when utilizing higher current density, the hybrid film cannot exhibit its performance as in low current density. This can be explained through the lower accessibility of electrolyte ions. When charged and discharged at low current density, electrolyte ions would have enough time to access deeper layers of active mass and give higher areal capacitance.

Figure 5c,d displayed the GCD curves of the EPD-S and HYB-A electrodes at current densities ranging from 0.1 mA cm⁻² to 1.0 mA cm⁻². An obvious redox peak was observed in both electrodes at the low current density. The semi-symmetrical triangular shape of the GCD curves was still retained even at the higher current density of 1 mA cm⁻², indicating the good stability of the electrode performance. The GCD curves of each electrodes studied in this work at various current densities are shown in Figure S3.

The cycling performance test of HYB-A electrode was tested at a current density of 1 mA cm⁻² for 1500 cycles and compared with the EPD-S electrode. As shown in Figure 6a, at the first 50 cycles, the areal capacitance of HYB-A was slightly increased, possibly due to penetration of the electrolyte ion into the internal structure of film. After 150 cycles, the areal capacitance rapidly decreased to 70% of its initial value, and it was subsequently maintained at approximately 56% of its initial value for up to 1500 cycles. In contrast to the hybrid film, the areal capacitance of EPD-S electrode gradually dropped to 75% of its initial value after 300 cycles and still retained at 65% for up to 1500 cycles. This indicated that the electrodeposited layer was robust for both cycling test and high current density. The slightly lower cycling stability of the HYB-A electrode could be attributed to its higher mass loading, which, in turn, resulted in the collapse of the interconnected structure during an extended cycling test. Figure 6b shows electrochemical impedance spectroscopy (EIS) of HYB-A films which had low series resistance (R_s) and charge transfer resistance (R_ct) of 4.55 and 4.91 Ω, respectively.

4. Conclusions

In this work, we provided the morphological structure and electrochemical performance of Mn₃O₄ deposited using electrodeposition and the SILAR method and the hybrid film with SILAR on top of electrodeposited film to enhance the areal capacitance without hindering the stability. The obtained films were characterized by FE-SEM. Electrodeposited films show the structure of nanoparticles and interconnected network when deposited with Mn(CH₃COO)₂ and MnSO₄, respectively. Mainly, nanoparticles were shown up when deposited Mn₃O₄ using the SILAR method with the spiky flower sporadically dispersed on the surface. The hybrid film structures were dominated by the electrodeposited layer with some characteristics of the SILAR layer. At a low scan rate of 0.1 mA cm⁻², HYB-A achieved the highest areal capacitance of 390 mF cm⁻² with a maximum areal energy density of 15.92 μWh cm⁻² and power density of 1.24 mW cm⁻² and retained low electrical resistance, indicating the superior electrochemical performance.