Synthesis and Characterization of Composites-Based Bacterial Cellulose by Ex-Situ Method as Separator Battery

: Many studies have been conducted and developed on cellulose-based battery separator materials, including bacterial cellulose, which has characteristics like plant cellulose. This research aims to synthesize BC/Al2O3 composite and analyze its potential as a battery separator. The synthesis of the composite with the ex-situ method is to immerse BC from tofu liquid waste (fermentation time variation of 6, 7, and 8 days) into Al2O3 suspension. The characterization results showed that the immersion of Al2O3 in BC can increase porosity, electrolyte absorption, and conductivity, indicating that the composite has the potential to be used as a battery separator.


Introduction
The separator is a component part of the battery that functions as a separator between electrodes for the transfer of ions in the electrolyte and ensures that there is no short-circuit between the electrodes in the battery (Lee et al., 2018;Lu et al., 2018). Materials commonly used as commercial battery separators are based on polyolefins such as polypropylene (PP), polyethylene (PE), and tri-layer PP/PE/PP (Mun et al., 2021). However, there are some drawbacks, namely nondegradability, low porosity (36.31-44.00 %), electrolyte absorption, and low conductivity (Lee et al., 2018;Xu et al., 2017).
The search for separator materials has been extensive to address such weaknesses, and cellulose is one of the biopolymers that has been extensively studied as a separator battery because it has high porosity and affects ion conductivity (Ginting et al., 2023;Pan et al., 2019;Tanpichai et al., 2019;Wang et al., 2018;Xu et al., 2017;Zhu et al., 2021). Cellulose produced by bacteria, or bacterial cellulose (BC), has characteristics such as porosity, electrolyte absorption, conductivity, tension strength, and thickness that are not much different from the standard (Choi et al., 2022;Li et al., 2021;Muddasar et al., 2022;Xu et al., 2017), so it has the potential to be a separator battery (Ginting et al., 2023;Li et al., 2021;Qian et al., 2022).
The thickness and low tensile strength of the separator can affect battery safety (Hao et al., 2022). The selection of fermentation media used in the manufacture of BC greatly affects the thickness and tensile strength. The use of tofu and corn liquid waste media produces BC with a thickness 4-52 times greater than that of nonwaste and is accompanied by low tensile strength (> 98.06 MPa) (Costa et al., 2019;Yasa et al., 2020). Several studies have conducted the addition of ceramic oxidebased fillers (SiO2, Al2O3, TiO2, and others) to overcome this weakness (Huy et al., 2021;Wei et al., 2019;Xu et al., 2017;Yu et al., 2021). The study by Xu et al. (2017), added Al(NO3)3.9H2O to BC with a simple in situ thermal decomposition method and produced BC/Al2O3 composites that have porosity, electrolyte absorption, thickness, tensile strength, and conductivity that can meet commercial separator standards.
According to the explanation above, the chemical and physical properties of BC and BC/Al2O3 composite were investigated to establish its potential as a separator battery. BC synthesis has been performed using Gluconacetobacter xylinus bacteria based on tofu liquid waste media, and BC/Al2O3 composite synthesis have been carried out ex-situ.

Preparation of BC and BC/ Al2O3 composites
The production of BC in this study was carried out following the procedure used by Sarkono et al. (2014) with modifications. In this research, tofu liquid waste was used as a production medium, with nutrient compositions of 10.0, 0.5, and 0.5% (w/v) sugar, ammonium sulfate, and yeast extract in 100 mL production scale. The pH of the media was set to 5 and sterilized for 15 min in an autoclave at 121°C and 2 atm. The media were inoculated with coconut water-based starter culture (10% v/v) and static fermented for 6, 7, and 8 days at 30°C. BC hydrogels were harvested and cleaned with cold water to remove residual media before being boiled for approximately 15 min and soaked in 0.5 M NaOH for 24 h. The water-reduced BC sheets were dried in an oven (50℃, 10 h). The dry BC was immersed into γ-Al2O3 suspension (1 g γ-Al2O3 in 1 L 0.8 M NaOH and stirred for 16 h) and ultrasonicated for 4 h and decanted to obtain γ-Al2O3 suspension (33 %), and dried in an oven (50 ℃, 2 h).

Characterization
Chemical characterization of BC and BC/Al2O3 composites performed with FTIR-ATR and Raman (Bruker). Porosity was measured by immersing BC and BC/Al2O3 composites in n-butanol 80% (v/v) and calculated based on equation (1).
where Mk and Mb are dry and wet mass of the sample, ⍴B is density of n-butanol (g/cm 3 ), and Vk is dry volume of the sample (cm 3 ). Electrolyte absorption was measured by immersing the sample in NaOH (1 M) electrolyte solution for 1 h and calculated using equation (2).
where Ae is electrolyte absorption, M1 and M2 is sample mass before and after immersion. The conductivity of BC and BC/Al2O3 composites measured and calculated based on equation (3).
where ρ is resistivity. Tensile strength is measured based on ASTM D638 with Tensilon RTG-1310 at load cell capacity 5.0 kN with a sampling rate of 5 mm/min. Composite surface morphology analysis using Scanning Electron Microscopy (FEI, Inspect-S50). Figure 2 shows the FTIR-ATR spectra for BC and the BC/Al2O3 composite. Based on the absorption bands obtained, the spectra of BC and BC/Al2O3 composite have similarities with the difference in wavenumber magnitude as shown in Table 1. The difference in wavenumber for reference BC with the research results as shown in Table 1., may be due to the base BC media used, where Galdino et al. (2020) and Güzel et al. (2019), using corn waste media, and orange peel. show any difference in absorption at wavenumbers 881-558 cm -1 (confirms the existence of Al-O-Al vibrations) with the BC membrane (Atrak et al., 2018). The spectrum of the BC/Al2O3 composite displays a narrower peak compared to the spectrum from BC. This difference is made possible by the increased degree of crystallinity that occurs in the BC/Al2O3 composite. The increase in crystallinity causes molecular vibrations in the BC/Al2O3 composite more districts compared to BC. The increase in crystallinity was also caused by using ultrasonication in the Al2O3 dispersion process.  (2019) Spectroscopy Raman was performed on BC and BC/Al2O3 composites to reconfirm the possibility of some vibrations not being recorded by the FTIR-ATR, with a maximum laser power of 50 mW. The spectrum results for the two composites are shown in Figure 2.

Result and Discussion
Spectroscopy Raman BC and BC/Al2O3 composites in Figure 3 shows the absorption with wavenumbers that are not much different. The vibrational region shows a similarity to the spectrum Raman for BC from aple cider waste media, with higher and narrower spectrum readings indicated by BC/Al2O3 composites. Visualization of BC and BC/Al2O3 composites produced in this study gave a different appearance, namely in the form of transparent white sheets for BC and opaque white for BC/Al2O3 composites. This difference indicates the dispersed Al2O3 into BC at the time of fermentation for 6, 7, and 8 consecutive days at 14.00; 6.36; and 24.58% of 33.00% suspension-Al2O3.  Xu et al. (2017) Porosity and electrolyte absorption test results on BC and BC/Al2O3 composites can be observed in Table  2. The porosity of BC and BC/Al2O3 composite for 7 days of fermentation is about 2 times higher than the standard separator, as well as the electrolyte absorption is higher than the standard. The existence of Al2O3 which dispersed the most in BC fiber at 8 days of fermentation (24.58%) resulted in greater absorption of electrolytes than without Al2O3, this is in line with the increased porosity of the BC/Al2O3 composite. [1] Wang et al. (2019) This increase is also influenced by the hydrophilicity of Al2O3, and properties of Al2O3 having moisture, wettability, and excellent electrolyte absorption (Xu et al., 2017). The separator must absorb and retain large amounts of liquid electrolyte to achieve low internal resistance and high ionic conductivity. The high tensile strength is also influenced by sonication during the dispersion of Al2O3 into BC fibers. This is because the use of prolonged ultrasonication increases the possibility of single BC fibers reacting with the microbubbles generated in the sonication process (acoustic cavitation effect), which can loosen the fiber surface and cause bond breakage to destroy the microsized microcrystalline cellulose fibers into nanocrystalline cellulose. The presence of Al2O3 through the dispersion process in BC fibers, can increase the thickness of BC/Al2O3 composites compared to BC (Fauza et al., 2019). The surface morphology of BC and BC/Al2O3 composites gives a difference in fiber arrangement with an average BC fiber diameter of 65.59 nm. BC/Al2O3 Composite showed better crystallinity with Al2O3coated fibers so that it shows a different visual from BC, this can be observed in the SEM image ( Figure 4). BC/Al2O3 composite in the SEM image with cross sections shows cellulose fibers that are arranged in stacks but not as dense as the cellulose arrangement in BC, this is due to the presence of Al2O3 in BC fiber, as well as the sonication process during Al2O3 dispersion into the BC fiber which can loosen the BC fiber so that it provides a distance between the cellulose piles in the BC/Al2O3 composite. Morphological comparison of BC and BC/Al2O3 composites indicates that the dispersion of Al2O3 gives a change in the physical properties of BC.

Conclusion
BC/Al2O3 composite tofu liquid waste-based was successfully synthesized by ex-situ method through immersed Al2O3 in BC with variations in fermentation time. The crystallinity of the BC/Al2O3 composite has increased compared to BC, this can be observed in the FTIR-ATR and Raman spectra. The existence of Al2O3 in BC through the ex-situ method can increase the porosity, electrolyte absorption, and conductivity of BC/Al2O3 composite, so it can potentially be developed as a battery separator.