Sitti Rahmawati, Ira Sepriyani, Purnama Ningsih and Anang Wahid Muhammad Diah*
Phosphorylated nata de banana as polymer electrolyte membrane in fuel cells
https://doi.org/10.1515/ijmr-2021-8209
Received January 14, 2021; accepted November 9, 2021;
published online May 3, 2022
Abstract: The objective of this study was to prepare and characterize nata de banana (NDB) and phosphorylated nata de banana (NDBP) membranes for use in fuel cells as polymer electrolyte membranes. Banana peel juice was fermented with Acetobacter xylinum and subsequently molded into a nata de banana membrane in this study.
This membrane was phosphorylated by adding various quantities of phosphoric acid solution to it (1.5 M, 2 M, and 2.5 M). Several approaches were used to characterize these membranes, including functional group analysis using Fourier-transform infrared spectroscopy, ion exchange capacity (IEC), swelling degree (%SI), mechanical proper- ties, and morphological characterization using a scanning electron microscope-energy dispersive X-ray analyser. The results showed that NDBP has a higher ion exchange capacity and degree of swelling than NDB, and that the optimum membrane condition is 2 M phosphoric acid, with a 5.14 m Eq g−1ion exchange capacity and a 19.06 % swelling index. The cross-section of the NDB and NDBP membranes had a regular and good structure of the cellulose fiber pattern, according to morphological exami- nation. Using a scanning electron microscope and energy dispersive X-ray analyser, the phosphorus levels were identified in modest amounts (approximately 0.5 %).
This also suggests that the phosphorylation event on the membrane of the nata de banana was successful. It can be inferred that electrolyte membranes can be manufactured using environmentally benign natural resources. The phos- phorylated nata de banana membrane can be employed as a polymer electrolyte membrane in a fuel cell, according to these findings.
Keywords: Fuel cells; Membrane; Nata de banana; Phos- phorylated.
*Corresponding author: Anang Wahid Muhammad Diah, Discipline of Chemistry, Faculty of Teacher Training and Education, Tadulako University, Palu 94118, Indonesia,
E-mail: anangwmdiah@gmail.com. https://orcid.org/0000-0001- 6200-6991
Sitti Rahmawati, Ira Sepriyani and Purnama Ningsih, Discipline of Chemistry, Faculty of Teacher Training and Education, Tadulako University, Palu, Indonesia
1 Introduction
Indonesia’s energy demand has risen year after year in tandem with the country’s expanding economic growth and population. To date, the majority of energy sources have been derived from fossil fuels (fuel oil) such as petroleum, coal, and natural gas [1]. A fuel cell is a type of alternative energy for fuel that is safe, cost-effective, and ecologically beneficial, as well as having a high efficiency.
Fuel cells are energy sources that use two electrodes and work on electrochemical principles (anode and cathode).
The anode undergoes hydrogen oxidation, whereas the cathode undergoes oxygen reduction in the presence of a catalyst [2]. The type of electrolyte and fuel used in fuel cells can be categorized. The polymer electrolyte membrane fuel cell is one of the many varieties of this fuel cell (PEMFC).
An electrolytic polymer membrane serves as a conductor of protons from the anode to the cathode in a PEMFC [3].
Membranes can be seen of as fuel cell electrolytes in that they should have low electronic conductivity, low oxidant, be compatible with the component, be strong enough, be environmentally stable, and be inexpensive to make [4,5]. This is important when it comes to developing clean, efficient, and long-term energy technologies [6].
Nafion (C7HF13S.C2F4) is now the most extensively utilized polymer electrolyte membrane for PEMFC applica- tions. This membrane is comprised of sulfonated tetrafluo- roethylene, a synthetic polymer. Nafion has a strong proton conductivity and can work at temperatures up to 280◦C with thermal stability. Nafion, on the other hand, is very expensive and must be imported from abroad, as well as being unfriendly to the environment. As a result, this problem must be solved by replacing the Nafion membrane with a more cost-effective and simple-to-process electrolyte polymer membrane [7].
Previous investigations on bacterial cellulose, such as nata de coco from coconut water and nata de pina from pineapple, have been reported. Acetobacter xylinum bacteria were used to make both cellulose fibers [8,9].
Nata has various mechanical physical qualities that are advantageous, including high purity, tensile strength, crystallinity, water binding capacity, biocompatibility, and low gas permeability [10,11]. Nata de banana (NDB) is a bacterial cellulose similar to this that may be made from
banana peels utilizing A. xylinum bacteria and a fermen- tation method. When manufacturing bacterial cellulose, A. xylinum uses sugar (sucrose) as a food supply (nata).
The higher the sugar level as a primary component, the more bacterial cellulose will be produced. Because banana peel contains sugar, it has the potential to be employed as a substrate for the production of NDB (18.5 % carbohydrates).
The materials are simple to procure and manufacture, the cost is minimal, and the NDB product maintains thermal stability at temperatures as high as 260◦C [12].
Phosphorylated nata de banana (NDBP) membranes are nata de banana (NDB) membranes that have phosphor elements added to them after phosphorylation to improve their characteristics.
Because of its many free hydroxyl groups, NDB is hydrophilic. Hydrogen ions (H+) can be bound and trans- ported to additional hydroxyl groups in the presence of this hydroxyl group. As a result, NDB has a lot of potential as a polymer membrane to replace Nafion®in fuel cell applications. The electrolyte is currently the most important component in fuel cell systems [6]. Fuel cells can create energy indefinitely as long as hydrogen is available [13]. The hydrogen is converted back into electricity with fuel cells [2]. It is required to change the NDB membrane by phosphorylation process in order to improve the ability of H+ ion transport [14]. By exchanging hydroxyl groups with phosphate groups on the C-6 atom, this chemical alteration was carried out to improve the ability of a polymer membrane to transmit protons [15]. This study focuses on the phosphorylation reaction’s effect on the NDB membrane.
2 Experimental procedure
Banana peels were scrubbed and rinsed. The banana peels were then scraped and mixed with water at a ratio of 100 g–200 mL. After that, the juice was filtered, and the filtrate was collected. The filtrate was adjusted to pH 4 using glacial acetic acid. As much as 0.8 % of the total volume (2.4 g) of food-grade ZA fertilizer was added, while as much as 8 % of the total volume of glucose was added (24 g). After boiling and stirring for around 15 min, the material was placed into a mold or fermentation container.
At room temperature (27◦C), A. xylinum (starter) was added to the mixture. The mixture was wrapped in plastic, coated with paper, the gaps were then sealed to prevent air from entering the container, and allowed to stand for 12 days at a temperature of 26–27◦C [12].
The NDB gel was then rinsed in boiling water and immersed for 24 h in a 1 % (w/w) NaOH solution. This
treatment broke down microorganisms and removed con- taminants [10]. After soaking in a 1 % CH3COOH solution for 24 h, the process was completed by washing with water until the pH was neutral [16]. The thin NDB membrane was made by pressing the NDB gel between two plates at a pressure of 10 MPa for 5 min in a hydraulic press [8].
Phosphorylation of NDB membranes was accom- plished by soaking the membranes in a phosphoric acid (H3PO4) solution. For 10 h, the membranes were sub- merged in 100 mL of H3PO4 solutions of 1.5 M, 2 M, and 2.5 M concentrations. To eliminate excess phosphoric acid, each membrane was washed with distilled water and dried in air [8].
Physical and chemical features of NDB membranes were studied, including ion exchange capacity, swelling, surface shape, and functional group analysis. As described in the preceding approach, the ion exchange capacity (IEC) was measured usingEquation (1)[16].
IEC= (Vb−Vs) [Acid] . f p
m (1)
Where IEC stands for ion exchange capacity (m Eq g−1), Vb for the amount of H2SO4solution required to neutralize the blank solution (mL), Vs for the amount of H2SO4solution required to neutralize the membrane (mL), [Acid] for the concentration of H2SO4(M), fp for the dilution factor, and m for the sample mass (g).
Weighing the bulk yielded the swelling index (SI) membrane. Water was soaked into the membrane for 24 h. The membrane’s surface was dried with a tissue, and the bulk was weighed again.Equation (2)can be used to calculate the SI [8].
%SI= m−mo
mo
×100% (2)
Where m is mass of dry membrane (g), and mois mass of wet membrane (g).
Scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDX) was used to examine the mem- brane morphology (SEM Jeol JSM 6360 LA). The availability of phosphate in the membrane was determined using EDX. An Alpha Bruker Fourier transform infrared (FTIR) spectrophotometer was used to examine the functional groups on NDBP and NDB membranes [8].
3 Results
In this investigation, glacial acetic acid was used as a pH regulator in the synthesis of NDB. After neutralizing the membrane and treating it with a pressure of 10 MPa for
5 min until dry, a white gel of NDB with a thickness of 1 cm was obtained.
Equation (1)may be used to calculate the ion capacity.
The phosphorylation reaction of NDB improved the capa- bility of the polymer membrane matrix in exchanging ions (protons).Figure 1depicts the influence of phosphoric acid concentration on IEC.
The physical properties of membranes, particularly the diffusion parameter, are determined by their structure.
The SI of NDB and NDBP membranes after soaking in water for 24 h is shown inFigure 2. The influence of phosphoric acid concentration on the phosphorylation process of NDB was also investigated using SEM-EDX.Table 1shows the typical results of elements found in NDB and NDBP membranes that were examined using EDX.
4 Discussion
The addition of A. xylinum (starter) bacteria to the fer- mentation process can help to prevent rotten bacteria
Figure 1: Effect of H3PO4concentrations on membrane IEC values.
Figure 2: The swelling index (SI) value of NDB and NDBP membranes.
contamination and speed up the synthesis of nata. ZA fertilizer is a source of nitrogen and nutrients for A. xylinum bacteria [17,18]. Glucose is a carbon source, whereas ZA fertilizer is a source of nitrogen and nutrients for A. xylinum bacteria. The volume of the media, the area of the media, the pH, and the temperature all influence the preparation of NDB. The ideal pH for the synthesis of cellulose in A.
xylinum’s fermentation of NDB is between 4 and 6 [19].
After washing, neutralizing, and pressing the membrane, a white gel with a thickness of 1 cm resulted from the preparation for a 12-day incubation period.
NDB was then subjected to the phosphorylation pro- cess. The mechanism shows the process of NDB structural alteration via the substitution reaction of hydroxyl groups at C-6 with phosphate groups [15]. The higher phosphate concentration would result in a greater number of hydroxyl groups being changed. The IEC values of the membranes were used to examine the influence of the phosphorylation reaction at varied phosphate concentrations. The higher the IEC values in Figure 1. The higher the number, the more phosphate groups on the nata membrane [16].
Table 1:The percentage content of elements in NDB and NDBP membranes.
Sample Percentage of element (%)
C O P Na
NDB 61.67±0.19 35.67±0.44 – 2.40±0.00
NDBP 1.5 M 60.09±0.04 39.42±0.06 0.49±0.02 –
NDBP 2 M 59.84±0.13 39.59±0.15 0.58±0.01 –
NDBP 2.5 M 60.42±0.06 38.45±0.02 1.14±0.05 –
The IEC value of NDB membranes is improved by the phosphorylation reaction. Using 2 M phosphoric acid, the best IEC value of 5.14 m Eq g−1 was obtained. The lowest IEC value of 4.74 m Eq g−1was found in the NDB membrane without phosphorylation, indicating that the phosphorylation process happened on membranes and was expected to increase proton conductivity [20, 21].
Increasing the H3PO4 concentration to 2.5 M resulted in a lower IEC value. This means that the higher the H3PO4 concentration, the greater the crosslink between the NDBP chain membranes, as seen inFigure 3[16].
Figure 3: Cross-linked chain on NDBP membranes.
The influence of varying phosphoric acid concen- trations on the NDB can also be seen in the SI values of membranes. The addition of phosphate groups to membranes causes the spacing between polymer chains to increase. In comparison to NDBP membranes,Figure 2 reveals that the original NDB membrane has the lowest SI value of 2.92 %. When the concentration of phosphoric acid is increased to 1.5 M and 2 M H3PO4, the SI value increases.
After phosphorylation with 2 M H3PO4, this study produces the greatest SI value of 19.06 %. The SI value, on the other hand, lowers following the phosphorylation reaction with 2.5 M phosphate, indicating that the membrane absorbs less water due to the development of cross-links between the cellulose chains. The drop in SI value is caused by crosslinking cellulose chains on the NDBP 2.5 M [9,16,21].
The more water molecules absorbed by membranes, the further the polymer chains are separated. This will deter- mine the membrane’s ability to deliver protons. The data is pertinent to IEC values that are changed by phosphoric acid concentrations, as illustrated inFigure 1.
The FTIR spectra in Figure 4 show the success of the substitution reaction of hydroxyl groups at C-6 with a phosphate group on the phosphorylation reaction. The spectra of NDB and NDBP are near identical. Both spectra demonstrate the presence of C–H group vibrations in the 1429 cm−1region, O–H group vibrations in the 3415 cm−1 region, and the C=C group in the 1645 cm−1 region.
The phosphate group is successfully connected to the cellulose chain, as evidenced by a change in absorption at roughly 1055 cm−1, which is related to the vibration of the C–O–P group [1], and absorption at 1200 cm−1, which is attributable to the vibration of the P=O group [22,23].
Figure 4: The FTIR spectra of NDB and NDBP membranes.
Figure 5: Surface morphology images of (a) NDB, (b) NDBP 1.5 M, (c) NDBP 2 M, and (d) NDBP 2.5 M membranes.
Figures 5 and 6 show SEM images of the surface morphology and cross-section of NDB and NDBP mem- branes. The surface morphology of the NDB and NDBP membranes is clearly shown inFigure 5b–d. The cellulose chain fibers are clearly apparent in both surface pictures, although the membrane surface inFigure 5dappears to be more uniform, denser, and regular. SEM images of the cross-sectional morphology of NDB and NDBP membranes are shown inFigure 6. The cellulose that makes up the cross-sectional shape of the nata membrane is generated layer by layer [8]. The cellulose layer of the NDB membrane inFigure 6ais more ordered and neater than the multi- layered cellulose layer of all NDBP membranes shown in Figure 6b–d (sheets). Phosphorylation of cellulose modified the structure of membranes and influenced some physical properties [24]. The addition of acid can raise the membrane’s mechanical strength, which is one of the effects on physical qualities. The membrane’s mechanical strength increases as the acid concentration rises, but its elasticity diminishes due to the production of crosslinked
structures [8,9,17]. All of the membranes’ cross-sectional SEM pictures reveal a structure with many layers. Layers do not have the same thickness.
The effect of phosphoric acid concentration on the NDB membrane was further studied using SEM-EDX, as shown in Figure 7. Table 1 shows the average values of key elements and describes the elements found in NDB and NDBP membranes in detail. When the elements in NDB and NDBP membranes are compared, phosphor elements represent after the phosphorylation event, but sodium is not identified. The C, O, and Na contents in the NDB membrane have percentage values of 61.67 ± 0.19 %, 35.67±0.44 %, and 2.40 %, respectively. While C and O elements are present in similar amounts in NDBP membranes, the concentration of P increases as the quantity of phosphoric acid utilized during reactions increases, which is consistent with a prior work [8]. P is not identified in the NDB membrane, but after the phosphorylation event, the percentage values rise to 0.49
±0.02 %, 0.58±0.01 %, and 1.14±0.05 %.
Figure 6: Cross-sectional morphology images of (a) NDB, (b) NDBP 1.5 M, (c) NDBP 2 M, and (d) NDBP 2.5 M membranes.
Figure 7: Characterization using EDX on (a) NDB, (b) NDBP 1.5 M, (c) NDBP 2 M, and (d) NDBP 2.5 M.
5 Conclusions
The FTIR spectra in this study confirm a successful phosphorylation process of the nata de banana using phosphoric acid. The phosphorylation reaction raised the amount of phosphate in the cellulose membranes, which had an impact on the membranes’ physical and chemical properties. The best reaction was found at a concentration of 2 M H3PO4 with an IEC of 5.14 m Eq g−1 and a SI of 19.06 %. These findings show that the phosphatized nata de banana membrane can be employed in fuel cells as an electrolyte polymer membrane.
Author contribution: All the authors have accepted respon- sibility for the entire content of this submitted manuscript and approved submission.
Research funding: The authors would like to thank to The Laboratory of Chemistry, Faculty of Teacher Training and Education and Tadulako University Palu for the support in research and publication programs in 2020.
Conflict of interest statement: The authors declare no conflicts of interest regarding this article.
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