Effect of reduced graphene oxide (rGO) in chitosan/Pahae natural zeolite-based polymer electrolyte membranes for direct methanol fuel cell (DMFC) applications
Yuan Alfinsyah Sihombing
⇑, Susilawati, Siti Utari Rahayu, Masnita Desy Situmeang
Department of Physics, Faculty of Mathematics and Natural Science, Universitas Sumatera Utara, Jl. Bioteknologi No.1, Medan 20155, Indonesia
a r t i c l e i n f o
Article history:
Received 28 November 2022 Revised 11 January 2023 Accepted 13 January 2023 Available online 18 January 2023
Keywords:
Chitosan
Pahae Natural Zeolite reduced Graphene Oxide Proton conductivity Direct Methanol Fuel Cells
a b s t r a c t
The use of electrolyte membranes for fuel cell applications has grown rapidly, and one of the materials often developed is chitosan. In this study, polymer electrolyte membranes were successfully produced using chitosan and Pahae natural zeolite with the addition of various reduced Graphene Oxide (rGO) con- centrations. Furthermore, water and methanol uptake values, methanol permeability, ion exchange capacity, and proton conductivity were the basic characteristics of Direct Methanol Fuel Cells (DMFC) applications. The optimum water uptake value was observed in the CS/PNZ/rGO 2.0 % membrane at 294.5 %. This value increased with the addition of rGO concentration and is in line with the increasing ion exchange capacity. The CS/PNZ/rGO 2.0 % membrane has the optimum ion exchange capacity with a value of 0.8121 mmol/g. Meanwhile, the membrane permeability value tends to decrease with increas- ing rGO composition at each variation of methanol concentration. The proton conductivity value also increased along with rGO concentration, and the highest value was found in CS/PNZ/rGO 2.0 % at 6.777106S/cm. Based on the results, high ion exchange capacity, low permeability, and high proton conductivity indicate that CS/PNZ/rGO-based polymer electrolyte membranes can be used in Direct Methanol Fuel Cells (DMFC) applications.
Ó2023 The Authors. Publishing services by Elsevier B.V. on behalf of KeAi Communications Co. Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
1. Introduction
Daily human activities, such as cooking, driving a vehicle, watching television, and turning on a device, cannot be separated from energy. Furthermore, there has been a continuous increase in energy consumption at a fairly high rate. The average growth in demand in Indonesia was approximately 5.3 % annually. The lar- gest household energy requirement was fuel oil, namely 40.1 %, fol- lowed by electricity, natural gas, coal, and biomass[1].
Indonesia is currently facing a major problem related to the energy crisis due to the continuous increase in household demand along with significant population growth. The availability of fossil fuel reserves in 2016 was approximately 7.25 billion barrels, but it decreased by 0.74 % compared to the previous year. Previous stud- ies also showed there was a 5.04 % decrease in natural gas. Accord- ing to the Ministry of Energy and Mineral Resources, 92.1 % of the total energy reserves have been processed into oil[2]. Meanwhile, one-third of natural gas has been provided from the total reserves.
Based on these data, the oil resources are expected to run out in 9 years time and natural gas is likely to take 42 years.
This indicates that it is necessary to find alternatives to provide new energy sources due to the increase in environmental demands. In selecting this alternative, several factors need to be considered, including ease of handling, sustainable availability, economical cost, and smart technology. One of the environmen- tally friendly energy sources is fuel cells, which are essentially open thermodynamic systems. They operate through electrochem- ical reactions and consume reactants from external sources[3]. The fuel cells consist of an anode and a cathode plate, as well as a mem- brane inserted between the plates. The membranes that are cur- rently been developed are derived from modified polymeric materials, and they are also known as Proton Exchange Membrane (PEM). They are the main component of Polymer Electrolyte Mem- brane Fuel Cells (PEMFC), which serves as an electrolyte in good proton conduction to become a barrier in crossing the fuel cell sys- tem[4,5].
Chitosan is a polymer material that is widely used in the man- ufacture of these products. It also has good properties, such as biodegradability, chemical inertness, and biocompatibility, hydrophilic, relatively inexpensive, and can be made into films
https://doi.org/10.1016/j.mset.2023.01.002
2589-2991/Ó2023 The Authors. Publishing services by Elsevier B.V. on behalf of KeAi Communications Co. Ltd.
This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
⇑Corresponding author.
E-mail address:[email protected](Y.A. Sihombing).
CHINESE ROOTS GLOBAL IMPACT
Contents lists available atScienceDirect
Materials Science for Energy Technologies
j o u r n a l h o m e p a g e : w w w . k e a i p u b l i s h i n g . c o m / e n / j o u r n a l s / m a t e r i a l s - s c i e n c e - f o r - e n e r g y - t e c h n o l o g i e s
[6,7]. Furthermore, chitosan has chemical and thermal stability, mechanical properties, and low conductivity due to the lack of hydrogen ions moving in the membrane[8].
Chitosan-based fuel cells are modified with the addition of other materials. There are several methods to modify chitosan to improve the chemical and physical properties of PEM, such as the addition of organic and inorganic materials as filler, blending, cross-linking, quaternization, phosphorylation, and sulfonation [8–11]. The most effective method for developing composites with excellent properties is the complex technique. In the complex method, inorganic materials with good thermal stability and mechanical features were mixed into organic materials, which have good flexibility, chemical reactivity, and process properties [12]. Several recent studies have combined organic and inorganic materials into chitosan-based PEM to increase proton conductivity, ion exchange capacity, mechanical properties, oxidation stability such as carbon nanotubes (CNTs), Polyaniline, Polyaniline/nanosil- ica (PAni/SiO2), Quaternary poly (ether ether ketone), Sulfonation of Graphene Oxide, etc [13–17]. They were all found to have improved polymer electrolyte membrane performance.
Furthermore, zeolites consist of aluminosilicate compounds, where the chemical structure was tetrahedral and contains bonds with oxygen. This natural material is environmentally friendly and can be obtained from abundant mountainous mineral rocks in Indonesia. One of the sources of zeolites in North Sumatra is the Pahae Natural Zeolite (PNZ), Tarutung, North Tapanuli. PNZ has been widely used as a filter, water vapor filter for hydrogen, and ethanol purification[18–23]. Zeolites can also be used for fuel cell applications due to their ion-exchange properties as well as good properties, such as mechanical features and thermal stability. Pol- tarzewski et al successfully prepared this material in a poly(tetraflu- oroethylene) matrix with high ionic conductivity [24]. Zeolite Y with Cu + and Ag + was used to adsorb sulfur from commercial fuels [25]. Zeolite was also prepared into Nafion, which was then applied in direct methanol fuel cell (DMFC) to increase power density, inter- face bonding, proton conductivity, as well as reduce methanol per- meability with high-temperature applications[26–31]. They can also be used as inorganic fillers in the chitosan matrix. Wu et al revealed that the membrane conductivity was increased up to 25.8 mS/cm after its usage [32]. This material can also improve the thermal and mechanical stability of hybrid products due to the presence of hydrogen bonds between zeolite and chitosan [33]. Furthermore, the addition of sorbitol as a plasticizer on the chi- tosan/zeolite membrane reduces methanol permeability[34].
Carbon nanomaterials have received much attention for facili- tating proton conduction in PEM because of their unique properties and selection of multiple structures [35]. One-dimensional (1D) carbon nanotubes (CNT) and two-dimensional (2D) graphene oxide (GO) have received the most attention. GO has a continuous 2D structure, many functional groups, and a large surface area [36,37]. Because of its high conductivity, graphene oxide (GO) is widely used in the production of fuel cell membranes [38]. PEM composites were successfully prepared using GO mixed with zeo- lite, sulfonated poly (ether sulfone) addition in GO, chitosan mixed with phosphorylated GO [39–41], and so on. They all show increased proton conductivity due to functional groups on the sur- face and unique structures. Reduced Graphene Oxide (rGO) is a reduced GO in which the oxide’s functional group has been reduced.
rGO refers to nanomaterials with nanoscale particle diameters that have mechanical, optoelectronic, or conductivity properties similar to graphene. rGO is a candidate material for membrane fabrication due to its excellent thermal, mechanical, and electronic properties and its very large surface area and resistance to chemical reactions [42]. Because of the exceptional property of phosphoric acid in pro- ton conduction, GO is functionalized and modified with phosphoric acid groups (-PO3H2)[43,44]. Moreover, it can easily form hydro-
gen bonds with water for proton transport, benefiting from its large hydration and polarization energies[45]. Several studies have also succeeded in functionalizing GO to improve the performance of membrane electrolyte polymers such as amino acids, sulfonated GO/Chitosan, PGO/Nafion, etc.[46–48].
In this study, Chitosan (CS) was selected as an organic material, while PNZ served as an inorganic filler. CS-PNZ was used as a com- posite membrane and a polymer electrolyte for various fuel cell applications with various reduction of graphene oxide (rGO). The CS/PNZ/rGO membrane was tested for water and methanol uptake properties, swelling ratio, methanol permeability, ion exchange capacity, and proton conductivity. This product is expected to be used for Direct Methanol Fuel Cells (DMFC) applications as an envi- ronmentally friendly alternative energy source.
2. Materials and method 2.1. Materials
The main material in this study was chitosan (CS) with medium molecular weight from Sigma-Aldrich. Fillers, such as natural zeo- lite (PNZ) were obtained from Pahae, Tarutung, North Tapanuli, North Sumatra, Indonesia, while reduced Graphene Oxide (rGO) was purchased from Sigma-Aldrich. Furthermore, acetic acid, sul- furic acid, aquadest, methanol, sodium chloride, and sodium hydroxide were obtained locally.
2.2. Sample preparation
Sample preparation was carried out based on the procedures in previous studies[49,50]. The schematic of the membrane manu- facturing process is presented inFig. 1.
2.3. Characterization
The zeolite powder was tested to determine the chemical com- position and type using X-ray Fluorescence (XRF). All membrane samples, namely CS/PNZ, CS/PNZ/rGO 0.5 %, CS/PNZ/rGO 1.0 %, CS/PNZ/rGO 1.5 %, and CS/PNZ/rGO 2.0 % were tested for water and methanol uptake by measuring the difference in weight before and after immersion for 24 h. For the measurement of methanol absorption, various concentrations were used, namely 1 M, 2 M, 3 M, 4 M, and 5 M. The calculation of water and methanol uptake was carried out using Eq.(1):
%Water=Methanol Uptake¼WwetWdry
Wwet 100% ð1Þ
WhereWwetis the weight of the membrane after immersed in pure water for 24 h andWdry is the weight of the membrane before immersed.
The Swelling ratio was tested based on the increase in the mem- brane area after immersion in water and methanol. This calculation can be seen in Eq.(2):
Swelling ratio¼AwetAdry
Adry 100% ð2Þ
WhereAwetis the surface area of the membrane when wet andAdry
is the surface area of the membrane when dry.
The methanol permeability was obtained using the cell diffu- sion method at room temperature. The membrane was placed between two containers. Furthermore, container A was filled with methanol of various concentrations, namely 1 M, 2 M, 3 M, 4 M, and 5 M, while B contains distilled water. They were then stirred at a speed of 350 rpm continuously and the methanol permeability was determined using the equation:
P¼CBð ÞVt Bl
tACA ð3Þ
Where P is permeability (cm2/s),CBis methanol concentration in container B at t,CA is methanol concentration in container A,VB
solution volume in container B (cm3), timet(s),Ais membrane area (cm2) andlmembrane thickness (cm).
Ion Exchange Capacity (IEC) was tested by the acid-base titra- tion method using sodium chloride as the medium. The membrane in the form of ion Hþwas converted to Naþafter immersion in 1 M NaCl solution for 24 h at room temperature. Furthermore, Ion Hþ was then titrated with 0.01 M NaOH solution using a phenolph- thalein indicator [51,52]. IEC (mmol/gram) was calculated using the equation:
IEC¼ðCNaOHVNaOHÞ
Wdry ð4Þ
WhereCNaOH(mol/L) andVNaOH(mL) are the concentration and vol- ume of NaOH solution required to neutralize the solution andWdry
the dry mass of the membrane.
All membranes were then tested for proton conductivity. They were immersed in demineralized water (aqua DM) for 24 h at room temperature, and their resistance value was measured using Elec- trochemical Impedance Spectroscopy (EIS). The EIS was set to Potentiostatic with a frequency of 10 mHz to 100 kHz with a DC voltage of 1 mV and an AC voltage of 10 mVrmsat room tempera- ture and a stabilization time is about 10 to 15 min. All samples have an area of 12:5cm2, and the proton conductivity was deter- mined using the equation:
r
¼ LRA ð5Þ
WhereLis membrane thickness (cm),Ais membrane area (2:5cm2), andRis equivalent resistance of the membrane (X).
3. Results and discussion 3.1. X-ray Fluorescence (XRF) test
The chemical content of Pahae Natural Zeolite (PNZ) was deter- mined through the X-ray Fluorescence (XRF) test. The results of the PNZ composition spectrum are presented inFig. 2.
Based on Fig. 2, PNZ content was dominated by Si and Al.
There were also other minerals, such as potassium and calcium.
The percentages of the constituent elements of Pahae’s natural zeolite are presented in Table 1. The value of the Si/Al ratio determines the type of natural zeolite. The ratio value was 4.896, and the mordenite zeolite type has a range of 2–5 [53].
Furthermore, these results indicated that the purity of the Silica-Alumina content was 77.6 %. The purity of the zeolite plays a role in the number of pores formed. The Si/Al ratio can be used to determine the hydrophilic/hydrophobic properties. A low ratio causes the zeolite surface to be hydrophilic, while a high ratio leads to hydrophobic nature[54], which indicates that PNZ has hydrophobic properties.
3.2. Water and methanol uptake
Water uptake determines the amount of water that can be absorbed, which serves as a proton transport medium. However, excess uptake can cause the membrane to become brittle [55].
The value obtained for all samples is presented inFig. 3.
Fig. 3shows that the 2.0 % CS/PNZ/rGO membrane has the opti- mum water uptake of 294.5 %. The higher the composition of the rGO, the higher the percentage obtained. This is because the zeolite structure is porous, hence, it can absorb water. The higher the uptake, the higher the proton conductivity. This is because the amount of water molecules in the membrane that can become pro- ton transfer media also increased[56].
Methanol uptake test measured the membrane’s ability to absorb this solvent. Therefore, the value of the methanol perme- ability can be predicted, which involves the passage of methanol into the product. The higher the volume that passes through the membrane, the higher the decrease in the performance of DMFC.
This is because the presence of an excess amount of the solvent can flood the cathode and damage the electrode assembly [57,58]. The value of membrane uptake for various methanol con- centrations is illustrated inFig. 4.
Fig. 4 shows that the highest percentage of methanol uptake was obtained at 0.5 % CS/PNZ/rGO membrane with 1 M methanol concentration of 254.4 %. Furthermore, the value obtained decreased with increasing solvent concentration and percentage of rGO. The lowest percentage was obtained in 2.0 % CS/PNZ/rGO membrane at a 5 M methanol concentration of 78.2 %. The decrease in methanol uptake indicates that the membrane has low perme- ability. This is due to the concentrated methanol. hence, the mem- brane cannot accommodate a large amount of the solvent before being saturated.
Fig. 1.Schematic of the CS/PNZ/rGO membrane manufacturing process.
3.3. Swelling ratio
The membrane water absorption capacity is similar to the swel- ling. Furthermore, water absorption gives an idea of the weight of water that can be absorbed on the membrane, while swelling is the expansion of its volume due to trapping. The swelling ratio was
determined by measuring the membrane area in dry and wet con- ditions after being immersed in water and methanol for 24 h, as shown inFigs. 5 and 6.
Fig. 5 shows that the Swelling ratio in water increases along with the rGO concentration in the CS/PNZ/rGO membrane. This is in line with increasing ionic groups (–OH and –SO3H) in the chain on product, hence, high water absorption causes an increase in swelling. The highest ratio value was obtained on the CS/PNZ/
Fig. 2.PNZ test results spectra using XRF.
Table 1
PNZ Composition Elements.
Element Al Si K Ca Ti V Mn Fe Rb Sr Eu
Percentage (%) 13.162 64.437 10.898 5.694 0.867 0.014 0.051 2.913 0.078 0.085 0.028
Fig. 3. Water uptake of CS/PNZ/rGO membrane with variations in rGO concentration.
Fig. 4.Methanol uptakeof CS/PNZ/rGO membrane with variations in rGO concen- tration to the methanol concentration.
rGO 2.0 % with a percentage of 194.7 %, where the membrane expe- rienced an increase of about 30 % compared to others without rGO.
Subsequently, the swelling ratio of the membrane in methanol is presented inFig. 6. The highest ratio was obtained in CS/PNZ/rGO 0.5 % with a 1 M methanol concentration of 161 %. The percentage obtained decreased along with the increasing methanol and rGO concentrations. The swelling ratio has a relationship with a decrease in methanol uptake. The lesser the amount of the solvent, the smaller the increase in area of the product.
3.4. Methanol permeability
Methanol permeability is a process of measuring the mecha- nism of methanol and water solution movement through a diffu- sion mechanism, and this depends on the pore volume. There are two types of pores in polymer membranes, namely pore networks or ionic clusters and aggregate. Proton transport occurs through
both types, while mass transport (methanol, water, and gas) is expressed by the aggregate type[12,50]. The membrane is placed between two vessels, which helps to measure the methanol con- centration that diffuses from vessels A to B. In this study, the methanol permeability value was determined through the density method using a pycnometer[59]. The values obtained with varia- tions in rGO to methanol concentration are presented inFig. 7.
The decrease in the methanol permeability can be caused by the size and volume of the pores, which are estimated to be smaller than the size of the solvent’s molecules, hence, it cannot cross the membrane. InFig. 7, the permeability tends to decrease with increasing rGO composition because its addition can cover the pores. The highest value was obtained in CS/PNZ with a 1 M methanol concentration of 31:5105cm2=s.
Fig. 5.Swelling ratioof CS/PNZ/rGO membrane in water with rGO variations.
Fig. 6.Swelling ratioof CS/PNZ/rGO membrane inmethanolwith rGO variation to methanol concentration.
Fig. 7.Methanol permeability of CS/PNZ/rGO membrane with rGO variation to methanol concentration.
Fig. 8.Ion exchange capacity (IEC) of CS/PNZ/rGO membrane with rGO variations.
3.5. Ion exchange capacity (IEC)
Ion exchange capacity indicates the number of ionic groups in the polymer matrix, which is indirectly related to the proton con- ductivity of a polymer. The value of the membrane ion IEC is pre- sented inFig. 8.
InFig. 8, the CS/PNZ/rGO 2.0 % membrane has the optimum ion exchange capacity value of 0.8121 mmol/g. These results indicate that the higher the composition of rGO, the higher the IEC. Further- more, rGO has acidic properties, which leads to an increased ability to transfer ions in the membrane. It also has a high conductivity value, and this indicates that it can easily conduct ions[60]. The surface area was high and can improve the charge transfer process [61].
3.6. Proton conductivity
The proton conductivity test on the membrane was carried out to determine the proton exchange capacity. The greater the con- ductivity, the higher the ability of the membrane to exchange pro- tons, and the better the performance of the fuel cell. The imaginary and real resistance values of each membrane were plotted using NyQuist, and the results are presented inFig. 9.
The impedance diagram is approximately semicircle in the high frequency region and straight line in the low frequency region (Fig. 9). The equivalent resistance (R) is the point where the straight line intersects the X-axis[62]. The higher the real resis- tance value, the lower the frequency obtained. At low frequencies, the ability of the membrane to transfer proton reduces and the NyQuist curve also becomes more volatile. However, the curve gets
better with the addition of rGO, especially at low frequencies.
Increasing the rGO concentration on the membrane shows the resistance curve of real and imaginary resistance fluctuations are decreasing. Consequently, the semicircle curve becomes better.
This is because rGO had good electrical conductivity and can con- duct electricity even at low frequencies of 10 mHz. Its addition to CS can increase the conductivity of the membrane because the functional group can bind oxygen. The hydrogen bond network between the –OH group of CS and the –COOH group of rGO facili- tates continuous ion transfer[63].
The proton conductivity values for each membrane are pre- sented inTable 2.
The highest proton conductivity value was observed in CS/PNZ/
rGO 2.0 % w/w at 6.777106S=cm. The addition of rGO led to an increase in the conductivity value due to several possibilities. The hydrophilic nature of rGO in oxygenated functional groups, such as carboxylate, hydroxyl, and epoxy oxygen groups attached to the sp3 carbon surface attracts protons, which move through hydrogen bonds along the water molecule membrane and transfer them using the Grotthuss mechanism.[64,65]. This was also caused by the electrostatic interactions between chitosan, Pahae’s natural zeolite, rGO, and sulfuric acid as crosslinkers. The membrane pro- duced has a lot of hydroxyl ions OH–, H+, –NH3+, hence, it can trans- fer protons along ionic and hydrogen bonds by jumping from one functional group to another[66].
4. Conclusion
Polymer electrolyte membranes have been successfully pro- duced using chitosan and Pahae natural zeolite with the addition of various rGO concentrations. The optimum water uptake was observed in CS/PNZ/rGO 2.0 % at 294.5 %. Meanwhile, the largest methanol uptake was obtained in CS/PNZ/rGO 0.5 % membrane with a 1 M methanol concentration of 254.4 %. The absorption of water increases with the addition of the rGO. This was supported by the increased ion exchange capacity, where the more water the membrane can absorb, the more ions can be exchanged. The methanol uptake value decreases with increasing rGO and metha- nol concentrations. This was also supported by the decreasing methanol permeability. The proton conductivity value also increased along with rGO. The largest proton conductivity value Fig. 9.Plot of Nyquist Membrane CS/PNZ/rGO with variations in rGO concentration and equivalent circuit withRsis resistance of solution,Rctis charge transfer resistance and Cdlis dual layer capacitance.RsþRctis equivalent resistance.[62].
Table 2
Proton conductivity value of CS/PNZ/rGO membrane variations in rGO concentration.
Sample Average
thickness (cm)
Resistance (X) Conductivity (S=cmÞ
CS/PNZ 0.0188 2274 3.307106
CS/PNZ/rGO 0.5 % 0.0226 2523 3.583106
CS/PNZ/rGO 1.0 % 0.0200 2103 3.804106
CS/PNZ/rGO 1.5 % 0.0220 1989 4.424106
CS/PNZ/rGO 2.0 % 0.0286 1688 6.777106
was obtained in CS/PNZ/rGO 2.0 % membrane at 6.777106S/cm.
Based on the results, CS/PNZ/rGO-based polymer electrolyte mem- branes have high ion exchange capacity, low permeability, and good conductivity, which indicates that they can be used in Direct Methanol Fuel Cells (DMFC) applications.
CRediT authorship contribution statement
Yuan Alfinsyah Sihombing: Conceptualization, Methodology, Validation, Supervision. Susilawati: Data curation, Investigation.
Siti Utari Rahayu:Visualization, Writing – review & editing.Mas- nita Desy Situmeang:Investigation, Writing – original draft.
Declaration of Competing Interest
The authors declare that they have no known competing finan- cial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The authors are grateful to the Directorate of Research and Community Service Ministry of Education, Culture, Research, and Technology for funding this study on the College Excellence Basic Research scheme with contract number 39/UN5.2.3.1/PPM/KP-DR TPM/L/2022 on March 18, 2022.
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