Natural Biomass-Derived Porous Carbon from Water Hyacinth Used as Composite Cathode for Lithium Sulfur Batteries

Abstract

We have successfully prepared porous carbon from water hyacinth plants using several steps, i.e., carbonization, activation, and calcination processes. Water hyacinth porous carbon is an alternative as a carbon material due to the ease and low cost of the manufacturing process, abundant raw materials in nature, and its contribution to solving environmental problems. Utilization of water hyacinth weed plants as raw materials for porous carbon will provide added value to water hyacinth. In this research, porous carbon is used as the host material of sulfur in the electrodes of lithium sulfur batteries. The N₂ adsorption desorption characterization showed a porous carbon surface area of around 642 m²/g and a total pore volume of 0.713 cm³/g. The sulfur content of the composite electrode of C/S 1:2.5 (%w/w) was 60.6%. The four-line probe (FLP) testing showed electrical conductivity of porous carbon of around 3.93 × 10⁻² S/cm and the electrical conductivity of the composite electrode was around 5.4 × 10⁻⁴ S/cm. Furthermore, the composite electrodes were applied as cathodes of lithium sulfur batteries, which have thicknesses around 200 µm and sulfur loading of 3.57 mg/cm². The highest discharge capacity of the battery was 312 mAh/g and the Coulombic efficiency was around 70%.

1. Introduction

Carbon nanomaterials have shown wide potential application in many fields, such as catalysis, biotechnology, energy storage, and conversion, due to their unique chemical physical and electronic properties within the limited dimensions of sp² hybridized carbon [1]. Graphene is a carbon material representing carbon nanomaterials with excellent flexibility, high electrical conductivity, and large surface area. However, the practical application of graphene is still constrained by its high cost and scarcity of supply. Instead, porous carbon is a prime candidate for carbon nanomaterials because of its high intrinsic conductivity, large surface area, and engineerable pore structure [2,3].

Biomass has enormous potential to become a porous carbon raw material as it is renewable, economical, and abundantly available in nature and has an easy process. Porous carbon from biomass can be practically applied as an electrode, especially in energy storage systems [1]. In the application as an electrode of supercapacitors or batteries, a porous carbon electrode was demonstrated to work with fast charge discharge rates and long cycles and to produce high power and energy density [4].

Various types of biomasses have been made as porous carbon materials for carbon electrodes, including coconut shells [5], leaves [6], apricot shells [7], poplar catkin [8], bamboo [9], olive stones [10], tea leaf [11], banana peel [12], corn [13], rapeseed shell [14], peony shell [15], shrimp shell [16], reed flower [17] and full pouch shell [18]. These biomasses are used because they are easy to obtain, renewable, cheap, and environmentally friendly. Carbon electrodes from biomass can show good performance. However, most of the biomass comes from agricultural products, such as coconut shells, bamboo, leaves, bananas, and corn, with limited availability for sustainable fabrication. On the other hand, there are sources of biomass that grow at an uncontrolled growth rate and generally have a negative impact on the environment as weeds.

Water hyacinth (Eichhornia crassipes) is the most problematic plant in the world due to its uncontrolled abundant growth in open ponds, irrigation, and other water reservoirs. Water hyacinth is able to grow quickly to very high densities (more than 60 kg/m²); this means spreading across the surface of the water can occur, which has unfavorable effects on the environment, human health, and economic development [19]. Water hyacinth has biochemical components consisting of 20.8% hemicellulose, 30.5% cellulose, and 21.3% lignin [20]. Water hyacinth naturally contains alkaline or alkaline earth elements such as Na⁺, K⁺, Ca²⁺, Mg²⁺, and about 30% protein [21]. Water hyacinth porous carbon (WHPC) has been prepared and applied to lithium/sodium ion batteries and supercapacitors [21,22]. WHPC has the potential to be applied to Li-S batteries.

The lithium-sulfur battery (LSB) is an environmentally friendly battery that is currently being researched and developed. Theoretically, lithium metal has a specific capacity of 3861 mAh/g, and sulfur has a specific capacity of 1675 mAh/g. The sulfur electrode has a theoretical specific energy density of 2600 Wh/kg, which is 3–5 times greater than conventional cathode materials such as LiCoO₂ and LiFePO₄ [23]. In addition, replacing conventional cathodes with sulfur has many advantages, including a lower operating voltage (2.15 V vs. Li/Li⁺), which results in increased safety [24]. Sulfur is also a promising material because of its large reserves, low production costs, and environmental friendliness compared to other toxic transition metal compounds [25]. However, there are some significant problems with the cathode of LSBs, namely rapid capacity loss, poor performance, and low Coulombic efficiency. The disadvantages are caused by several problems; first, the low sulfur conductivity of 5 × 10⁻³⁰ S/cm at 25 °C. Second, the large volumetric expansion of sulfur during the sulfur conversion process to Li₂S₂/Li₂S is due to the difference in density between sulfur and lithium sulfide (2.03 and 1.66 g/cm³) and the fast sulfur shrinkage, respectively. Sulfur has a greater rate of volume expansion when fully lithium sulfide, which can cause cracking and electrode damage [26]. Third, long-chain polysulfide (PS) (Li₂S_x, 3 < x < 8) formed at the cathode moves to the anode through the separator and dissolves in the electrolyte, which inhibits charge mobility during the charging and discharging process [27].

The general strategy to solve the problem of LSBs is using the method of encapsulation and impregnation of sulfur in a porous carbon matrix [27]. The high sulfur content of the sulfur porous carbon composite cathode determines the energy density and discharge capacity of the battery. In previous studies, porous carbon has been made from water hyacinth and its composite material with high sulfur content [28]. In this study, WHPC/S composites were studied for application as the cathode of LSBs.

2. Materials and Methods

2.1. Synthesis of Porous Carbon

Water hyacinth (WH) was collected from swamps in several cities in West Java, Indonesia. Water hyacinth porous carbon (WHPC) was synthesized by simply carbonizing the water hyacinth as the precursor for carbon. After cleaning, the WH stems and leaves were taken and dried in the sun for 5–6 days, followed by drying in an oven at 100 °C for 2 h. The dried WH was carbonized at 600 °C for 2 h at a heating rate of 10 °C/min in air and then cooled to room temperature naturally, resulting in a black powder. The carbon was then pulverized to a size of 200 mesh, then mixed with ZnCl₂ 30% (Sigma Aldrich, CAS Number: 7704-34-9) in the weight ratio of 1:3; then, it was soaked for 24 h at room temperature and dried at 110 °C for 2 h and heated at 800 °C for 1 h with a heating rate of 10 °C/min under argon (Ar) gas flow.

2.2. Composite Preparation of Water Hyacinth Porous Carbon_Sulfur

WHPC and the sulfur (Sigma Aldrich 99%, St. Louis, MO, USA) were mixed with the weight ratio of 1:2.5 in a quartz mortar. The mixture was heated at 155 °C for 12 h to achieve the lowest viscosity of sulfur so that sulfur could easily enter the pores of the WHPC matrix. Then, the composite was heated again at 300 °C under Ar gas flow for 30 min to remove the sulfur present on the surface.

2.3. Characterization

Characterization of the WHPC and WHPC/S composites was carried out by scanning electron microscopy energy-dispersive X-ray spectroscopy (SEM-EDS, S-4800, Hitachi Limited, Tokyo, Japan), X-ray diffraction (XRD, Rigaku-TTRIII), Brunauer–Emmett–Teller (BET, Quantachrome Nova 4200e, Anton Paar, Graz, Austria), and thermogravimetric analysis (TGA, Perkin Elmer TGA 4000, Waltham, MA, USA). XRD patterns were determined by using Cu Kα (wavelengths = 0.15418 nm) in the range of 10° ≤ 2θ ≤ 80° (scan speed 5°/min and scan step 0.02°) at room temperature. Morphologies and microstructures of the samples were observed and analyzed by using the SEM operating at 10 kV. The sulfur content was detected using TGA measurement under an air atmosphere, and the temperature range was from room temperature to 700 °C with a heating rate of 10 °C/min. Nitrogen adsorption isotherms at 77 K were measured using the BET method.

2.4. Electrochemical Measurement

The cathode of the battery was made by mixing WHPC/S composite materials, black acetylene (Super P, 10 wt%), and polyvinylidene fluoride (PVDF, 10 wt%) with a composition of 8:1:1 with N-methyl pyrrolidone (NMP) solvent. The mixture was then stirred for 3 h with a magnetic stirrer and then coated with a doctor blade on aluminum foil which was dried at 60 °C for 12 h in a vacuum oven. Furthermore, the film was cut into a circle with a diameter of 14–15 mm according to the diameter of a coin. The 2025 type coin cell was assembled by making WHPC/S composite as a cathode, Li as an anode, and Celgard 2400 as a separator, and glued in a glove box with Ar atmosphere. The electrolyte was prepared from a mixture of 1.0 M lithium bis(trifluoro-methanesulfonyl)imide (LiTFSi) solution dissolved in 1,3-dioxolane (DOL) and 1,2-dimethoxyethane (DME) (v/v, 1:1) containing 1 wt% LiNO₃ as additive [29]. Galvanostatic testing was carried out at a voltage range of 1.6–3.4 V to test the capacity of the battery and the cyclability of the battery at the rate of variation (1C = 1675 mAh/g).

3. Results and Discussion

The results of the N₂ absorption desorption test on WHPC samples obtained a surface area of 642.39 m²/g, a total pore volume of 0.713 cm³/g, and a pore radius of 2.22 nm with a mesoporous structure [28].

The composite XRD patterns of WHPC and WHPC/S are shown in Figure 1. In Figure 1a, the diffraction peaks of WHPC are very noisy, which may be caused by the presence of elements derived from the raw material during the carbonization process and elements from the activator that cannot be removed during the calcination process. This is supported by the results of the EDS data showing the elements contained in the WHPC material.

Table 1. Elements of WHPC.

Elements	Average
	Weight (%)	Atomic (%)
C	66.33	80.32
O	9.48	8.61
S	1.56	0.71
Cl	8.67	3.56
K	2.41	0.90
Ca	4.04	1.47
Zn	4.29	0.94
Na	3.22	2.09
Total	100.00	100.00

shows the elements contained in WHPC with the average %weight and %atomic obtained from EDS measurements at eight measurement points. It is observed that the weight % of the C element in the WHPC sample is about 66.33%. Other elements were also observed in a considerable amount of about 33.67%.

The results of the EDS mapping confirmed the presence of elemental impurities in WHPC originating from activators and from raw materials.

In Figure 1b, the diffraction pattern of the WHPC/S composite shows the crystal lattice peak at a d-spacing of 0.385 nm, which corresponds to the hkl peak of (222) of sulfur element at 2θ = 23.06° with reference data COD 96-9011363/1364. The presence of a peak (222) in the WHPC/S composite indicates that sulfur is the bulk content in the WHPC composite [28].

The SEM images of the WHPC and WHPC/S composite are shown in Figure 2. The SEM images in Figure 2a clearly show the well-developed pores on the surface of the WHPC [28]. Figure 2b–f are EDS mapping of WHPC with C element still dominant. In addition to the C element, there are other elements derived from raw materials and activators, such as Zn and Cl. Figure 2g is SEM images of WHPC/S composite and Figure 2h–j are an EDS mapping for WHPC/S composite and C and sulfur (S) elements in the WHPC/S composite. The elemental maps of C and S indicate that the elemental sulfur is almost homogeneously distributed in the structure of the WHPC/S sample.

Figure 3a shows the N₂ adsorption desorption tests of WHPC and WHPC/S composites with precursor sulfur ratios of 1:2.5. The specific surface area and total pore volume for composites with a ratio of 1:2.5 are 46.6 m²/g and 0.042 cm³/g, these results indicated that sulfur had entered the pores of the porous carbon [28]. Figure 3b shows a sharp decrease in the weight loss of pure sulfur from 100 wt% to close to 0 wt% in the range of temperatures of 200 °C to 320 °C. As for the WHPC/S (1:2.5) composite, the weight decreased from 90% to 30% at temperatures of 200 °C to 290 °C.

The temperature relatively stabilized when the composite was heated to temperatures above 300 °C due to the evaporation of sulfur from the mesoporous WHPC [7]. The sulfur loading is around 60.6 wt% calculated from the thermogravimetric (TG) curve. Weight loss above 450 °C is due to carbon oxidation [7]. From the TG curve, it can also be seen that the temperature of the composite lower than pure sulfur indicates a weak bond between sulfur and carbon atoms [25].

The performances of the battery are shown in Figure 4. Figure 4a shows the process of charging and discharging battery capacity at a scan rate of 1C. The initial discharge capacity is 312 mAh/g. In the 10th cycle, the discharge capacity is 300 mAh/g with a retention capacity of about 96%. The specific discharge capacity decreases due to the small amount of polysulfide solubility and Li₂S aggregation during the electrochemical reduction process [30]. Figure 4a also describes the battery operating voltage from 1.8–2.4 V. The peak voltage of 2.25 V is the oxidation voltage of Li₂S₂ and Li₂S to long-chain PS (Li₂S_x, 4 < x < 8), the peak voltages of 2.35 V and 2.05 V are the cyclic reduction of S₈ open ring soluble long-chain polysulfide and polysulfide reduction to Li₂S₂ and Li₂S, respectively [7].

Figure 4b shows the value of the charge and discharge capacity of the battery for 50 cycles. The battery capacity value decreases gradually until the 30th cycle and decreases drastically until the 50th cycle. This phenomenon is due to the solubility of lithium polysulfide and the gradual accumulation of Li₂S on the cathode surface during the electrochemical reduction process in organic electrolytes [7]. The Coulomb efficiency value in Figure 4b shows an increasing trend to close to 90% at the end of the cycle. The average Coulomb efficiency is about 70%.

The rate performance of the cells at various C-rates from 0.1C to 1C in the voltage range 1.8–2.4 V is evaluated and displayed in Figure 5.

The rate capability of the WHPC/S composite electrode is explored and illustrated in Figure 5. After effectively activating the WHPC/S electrode, the WHPC/S cell can provide a stable specific capacity of about 400, 370, 325, and 312 mAh/g at different rates of 0.1C, 0.2C, 0.5C, and 1C, respectively. When the charging process is returned to a rate of 0.1C, a specific capacity value of about 380 mAh/g is obtained.

If the results obtained in this research are compared with lithium sulfur batteries with a porous carbon matrix from other types of biomasses, such as apricot [7] or commercially conductive carbon black [30], the battery performance of this work is still low, which is indicated by the low value of initial discharge capacity and Coulombic efficiency. This is influenced by several factors, one of which is the level of purity of the porous carbon. The purity of the porous carbon can be improved by optimizing several steps during the synthesis process. Several researchers have also reported studies using porous carbon from water hyacinth as a battery cathode matrix in lithium-ion batteries [21]. With several optimization steps during the synthesis and fabrication of the samples, performance improvements for water hyacinth-based lithium sulfur batteries can be achieved in the near future.

4. Conclusions

Porous carbon from water hyacinth (WHPC) for application as a cathode matrix in lithium sulfur batteries has been successfully prepared. The resulting porous carbon has a surface area of 642.39 m²/g, a total pore volume of 0.713 cm³/g, and a mesoporous structure that is suitable for use as a cathode matrix. Porous carbon from water hyacinth biomass was made as a composite with sulfur (WHPC/S) with a sulfur content of 60.6 wt% by weight with a weight ratio of 1:2.5 WHPC/S. The lithium sulfur battery cathode that has been made has a sulfur content of 3.57 mg/cm². This sulfur content value met the requirements as an active battery ingredient. The resulting battery cells have an initial discharge capacity of 312 mAh/g and an average Coulomb efficiency of 70%. The resulting performances of these batteries are still lower compared to lithium sulfur batteries with porous carbon from other biomasses or commercial conductive black carbon. As a prototype of a water hyacinth-based lithium sulfur battery, this result has shown a good first step for further development.