Composites of Manganese oxide/Carbon aerogel from Water Hyacinth for supercapacitor application

This study describes the synthesis of cellulose aerogel using water hyacinth, glutaraldehyde (GA), polyvinyl alcohol (PVA), and carboxymethyl cellulose (CMC). The carbonization process of cellulose aerogel into carbon aerogel was investigated under N2 atmosphere at temperatures of 600 °C, 700 °C, and 800 °C, using a heating rate of 10 °C/min and a fixed duration of 2 h. To enhance the properties of the carbon aerogel, Manganese oxide particles were attached to its surface through a metal wetting method utilizing Mn(NO3)2.4H2O at a concentration of 5 %. The results indicate that cellulose aerogel carbonized at 700 °C exhibits higher electrical storage capacity compared to the aerogels carbonized at the other two temperatures. The capacitance of the supercapacitor reaches its maximum value when the Mn content is 24.31 %. In galvanostatic charge/discharge (GCD) test, the maximum capacitance is achieved at a current density of 0.2 A/g.


Introduction
In the era of scientific and technological revolution to meet the increasing demands of human beings, fossil energy sources are becoming depleted.In addition to researching and developing new energy sources, energy storage devices play a role in providing continuous energy supply and have attracted attention [1].Two prominent devices in this field are batteries and supercapacitors.Batteries are devices with high energy density that can store large amounts of energy but have slow charging times.Conversely, supercapacitors can meet the energy storage capacity while ensuring fast charging [2,3].The research and development of materials for synthesizing supercapacitors is currently receiving much attention, especially carbon-based materials such as graphene and carbon nanotubes [4].The specific capacitance of those supercapacitors is relatively low, and the production cost is high, making them difficult to apply in practical applications.
Supercapacitor can be divided into electric double layer capacitor and pseudo-capacitor according to different energy storage mechanisms [5].Electric double layer capacitor mainly store electric energy by forming an electric double layer by electrostatic interaction between electrolyte ions and 1340 (2024) 012014 IOP Publishing doi:10.1088/1755-1315/1340/1/012014 2 electrode materials.In pseudoc-apacitors, charge is stored by rapid reversible redox reactions of the electroactive substances.The electric double layer capacitor has a lower specific capacitance than the pseudo-capacitor.Metal oxides have high specific capacitance, high energy density, good chemical stability and low cost [6].Moreover, they can perform rapid faraday redox reaction.Therefore, metal oxide/carbon aerogel composites have been demonstrated as promising pseudocapacitive electrode materials [7].
With rapid reproduction characteristics, water hyacinth has become a major cause of waterway congestion and a reduction in the quality of underwater living organisms.With a high carbon content, water hyacinth is a potential raw material for carbon-based materials production [8].
To enhance the cross-linking ability of the raw material with adhesive substances, preprocessing of the material is required before synthesis.The commonly used preprocessing methods for water hyacinth are biological and chemical methods.The biological method is considered environmentally friendly.This method is time-consuming and complex.On the other hand, the chemical preprocessing method with fast processing time and high efficiency in removing lignin and hemicellulose has become one of the most promising methods used to increase the cellulose content in the raw material and enhance the bonding ability of cellulose molecules with adhesive substances.
There are many methods for doping metals onto materials.Among the methods for metal doping on material surfaces, the wet impregnation method has the fastest speed and is the least disadvantageous.The wet impregnation method has the advantage of easy preparation of a catalyst layer on the surface without the need to calculate the porous volume of the material [9].In this study, we used a chemical method to obtain cellulose from water hyacinth and used the wet impregnation method to make composites of manganese oxide/carbon aerogel materials.

Characterization
Morphology of samples was investigated using a Hitachi S-4800 Scanning electron microscopy (SEM) operated at an acceleration voltage of 10 keV.The structure of the samples were characterized by X-ray diffraction (XRD).
Density of cellulose aerogel is calculated by the following (1.1) [10]: Where, m is the mass of the sample (kg) and V is the volume of the sample (m 3 ) Porosity is calculated by the following (1.2) [10]: Where s  is the density of the solid material.
The density of the solid material was calculated based on the solid density of component, by the following Eq(1.3)[10]: Where w cellulose , w PVA , w CMC , w GA are the weight percentages of cellulose, PVA, CMC and GA in the sample and

GA
 are the solid densities of cellulose, PVA, CMC and GA.Cyclic Voltammetry (CV) was used to study the electrochemical processes occurring between the electrode surface and the electrolyte.The experimental system consists of a three-electrode setup: a Titanium (Ti) coated with Platinum (Pt) counter electrode, a Standard Calomel Electrode (SCE) reference electrode, and a working electrode connected to a CHI660C electrochemical analyzer.The experiments were conducted in a 1 M H2SO4 environment.The potential scanning range was set from 0.7 V to 0 V, at different scan rates (5 mV/s, 10 mV/s, 25 mV/s, 100 mV/s, 200 mV/s).Galvanostatic charge/discharge (GCD) method was used for the estimation of the specific capacitance of the material.The experiments were performed in the voltage range of 0.7 V to 0 V, at current densities of 0.2 A/g, 0.5 A/g, 1 A/g, and 2 A/g.

Preprocessing of water hyacinth
The water hyacinth was collected from Chau Thanh district, Tien Giang province, Vietnam.Initially, the water hyacinth was trimmed by removing the roots and leaves, thoroughly washed, chopped into small pieces, and dried under sunlight for 3 to 4 days.After drying, the water hyacinth stems were finely ground into powder.Subsequently, the water hyacinth powder was treated with a 2 % NaOH solution (1:50 wt) at 65 ºC.Then, the powder was filtered and washed with distilled water until reaching pH 7. Finally, the mixture was dried at 60 ºC for 24 h.

Synthesis of carbon aerogel
0.5 g of preprocessed water hyacinth was dispersed in 20 mL of distilled water.Then, cross-linking agents (PVA and CMC) were added with a PVA to CMC ratio of 1:1, 1.5:1, and 2:1.The mixture was stirred at 70 °C for 1 h.Afterwards, a mixture of GA solution was added and stirred until gelation occurred.The resulting gel was then subjected to freeze-drying for 48 h to obtain cellulose aerogel material.The synthesized cellulose aerogel was subsequently carbonized at temperatures of 600 °C, 700 °C, and 800 °C in a nitrogen environment, with a heating rate of 10 °C/min for 2 h to obtain carbon aerogel.The samples obtained were denoted as CA600, CA700, and CA800.0.05 g of carbon aerogel powder was added to 0.25 mL, 0.43 mL, 0.62 mL, 0.95 mL, and 1.59 mL of 5 % Mn(NO3)2.4H2Osolution, respectively.The mixture was then sonicated for 1 h.The resulting mixture was dried at 70 °C for 12 h and subsequently annealed in ambient air at 250 °C for 5 h to 4 obtain Mn-doped CA.The samples corresponding to each volume of the 5 % Mn(NO3)2.4H2Osolution were designated as CA@Mn1, CA@Mn2, CA@Mn3, CA@Mn4, and CA@Mn5.

The influence of PVA and CMC
In the process of synthesizing cellulose aerogel material, PVA and CMC play an important role in the formation of crosslinking between polymer chains, resulting in the formation of porous structure within the material.Figure 3 illustrates the influence of the PVA:CMC ratio on the bulk density and porosity of the material.As the PVA:CMC ratio increases, the formation of crosslinking within the aerogel increases, leading to the preservation of the structural integrity of the material and reduced shrinkage after drying, increasing the volume and decreasing the bulk density while enhancing the material's porosity.

The effect of carbonization temperature
The CV curves of CA600, CA700, and CA800 samples are shown in Figure 4(a).The absence of oxidation-reduction peaks in the CV curves is a characteristic of supercapacitors.In the CA600 sample, the small area under the CV curve indicates poor conductivity.In the CA700 and CA800 samples, the CV curves exhibit wide current intensity oscillations and large integrated areas, indicating that an increase in carbonization temperature contributes to enhanced conductivity of the material.To compare the stability and cycling performance of the samples, the electrodes coated with CA600, CA700, and CA800 samples were subjected to GCD measurements at a current density of 0.2 A/g.The charge/discharge curves of the samples are shown in Figure 4(b), and the capacitance values obtained from the GCD results are presented in Table 1.The CA600 sample exhibits the fastest charge/discharge process, completing a cycle in only 6.3 s, while the CA700 and CA800 samples demonstrate better stability.Notably, the CA700 sample shows the longest charge/discharge cycle of up to 793 s.The triangular and symmetrical shape of the CA700 sample indicates its reversible behavior, which is desirable for the stability of supercapacitors.The capacitance calculated from the GCD results of the CA700 sample also demonstrates its superior energy storage capability compared to the other two samples.Based on the CV and GCD results of carbon aerogel samples carbonized at different temperatures, the CA700 sample, prepared using carbon aerogel carbonized at 700 °C, exhibits the best electrochemical properties.Both the specific capacitance calculated from CV and GCD are higher than those of the 600 °C and 800 °C samples.The fast charge and slow discharge curve of the CA700 sample indicates its stable nature.At the carbonization temperature of 700 °C, the carbon aerogel significantly improves the specific capacitance of the supercapacitor.The porous framework formed at 700 °C, along with a reasonable pore distribution, facilitates the diffusion of electrolyte ions across the electrode surface, thereby enhancing ion conduction.The optimal carbonization temperature chosen is 700 °C.

The effect of Mn(NO3)2.4H2O amount
The SEM images of the analyzed samples are presented in Figure 5.The Mn-doped CA materials exhibit three-dimensional sizes and observable porous structures.The high temperature during the carbonization process leads to the decomposition of organic carbohydrate compounds within the aerogel.As the temperature increases, these compounds are lost, creating voids within the material structure.The SEM results demonstrate the even distribution of Mn particles on the surface of the carbon structure, with an increasing amount of Mn distribution as the amount of 5 % Mn(NO3)2.4H2Oincreases.
The influence of the Mn(NO3)2.4H2Oamount was investigated using cyclic voltammetry with a fixed scan rate of 10 mV/s, as shown in Figure 6(a).From the plots, it can be observed that the CA Blank sample, which utilizes carbon aerogel as the active material for the electrode without the addition of Mn metal, exhibits a smaller range of current intensity oscillation compared to the Mndoped CA samples.On the other hand, the CA@Mn1 and CA@Mn2 samples show an increasing capacitance with higher Mn concentrations, reaching the highest capacitance at CA@Mn3, as indicated by the wide range of current intensity oscillation and larger area.To compare the effect of Mn amount on the stability and charge/discharge cycle of the samples, the samples were subjected to galvanostatic charge/discharge (GCD) measurements at a current density of 0.2 A/g.The charge/discharge curves of the samples are shown in Figure 5(b), and the calculated capacitance values from the GCD results are presented in Table 2.
From the SEM, CV, and GCD results at different amounts of Mn(NO3)2.4H2O, it can be observed that the CA@Mn3 sample exhibits superior electrochemical properties and charging/discharging capabilities of the supercapacitor, characterized by a high specific capacitance and a fast charging rate and slow discharging rate.The improved capacitance of the material can be attributed to the characteristics of manganese (III) oxide on the surface of the carbon aerogel structure, which significantly enhances the capacitance of the material.
As the Mn content in the Mn-doped CA sample increases, the capacitance is improved.If the Mn content is too high, it can lead to thermal degradation of the material due to the excessive heat generated from the oxidation reaction of manganese during the annealing stage.The SEM results of the CA@Mn5 sample also show that at excessively high concentrations, manganese (III) oxide tends to reform into larger clusters, reducing the capacitance as protons require more time to reach these larger-sized manganese (III) oxide clusters.The CA@Mn3 sample with a Mn content of 24.31 % exhibits superior characteristics due to the even distribution of manganese (III) oxide particles on the metal surface.

. Thermal conductivity
The effect of current density on the material capacitance in GCD test was investigated.From the results in Figure 7 and Table 3, it can be observed that the capacitance of the CA@Mn3 sample decreases as the current density increases.At a current density of 2 A/g, the charge/discharge curve reaches the shortest value, with a significantly decreased specific capacitance of 91.43 F/g.The maximum specific capacitance of 134.29 F/g is achieved at a current density of 0.2 A/g, indicating the highest energy storage capacity of the capacitor at this current density.This can be explained by the fact that as the current density increases, the charge transferred through the capacitor increases at a faster rate, resulting in higher internal resistance and decreased specific capacitance.

Conclusions
Composites of manganese oxide/carbon aerogel were successfully prepared from water hyacinth, GA (glutaraldehyde), PVA (polyvinyl alcohol), and CMC (carboxymethyl cellulose) and Mn(NO3)2.4H2Osolution.700 °C was found as the optimal carbonization temperature.The CA@Mn3 sample with a Mn content of 24.31 % exhibits superior capacitance.In GCD investigation, the material showed the highest capacitance at the current density of 0.2 A/g.The Mn-doped carbon aerogels are promising for supercapacitor applications.

Figure 1 .
Figure 1.The process of synthesizing the composite of manganese oxide/carbon aerogel.

5 Figure 3 .
Figure 3.The effect of the PVA:CMC ratio on the bulk density and porosity of cellulose aerogel.

Figure 4 .
Figure 4.The effect of carbonization temperature on the electrochemical properties of the material.(a) The cyclic voltammetry curves of samples and (b) The galvanostatic charge/discharge of samples.

Figure 6 .
Figure 6.The effect of the amount of Mn(NO3)2.4H2Oon the electrochemical properties of the material.(a) The cyclic voltammetry curves of samples and (b) The galvanostatic charge/discharge of samples.

Figure 7 .
Figure 7.The GCD curves of the CA@Mn3 sample at different current densities

Table 1 :
Capacitance of the capacitor at different pyrolysis temperatures Sample Carbonization temperature ( o C)Capacitance (F/g)

Table 3 :
The capacitance of the CA@Mn3 sample calculated from the GCD curves at different current densities.