Enhanced Proton Conductivity and Methanol Permeability Reduction via Sodium Alginate Electrolyte-Sulfonated Graphene Oxide Bio-membrane

Enhanced Proton Conductivity and Methanol Permeability Reduction via Sodium Alginate... The high methanol crossover and high cost of Nafion® membrane are the major challenges for direct methanol fuel cell application. With the aim of solving these problems, a non-Nafion polymer electrolyte membrane with low methanol permeability and high proton conductivity based on the sodium alginate (SA) polymer as the matrix and sulfonated graphene oxide (SGO) as an inorganic filler (0.02-0.2 wt%) was prepared by a simple solution casting technique. The strong electrostatic attraction between -SO H of SGO and the sodium alginate polymer increased the mechanical stability, optimized the water absorption and thus inhibited the methanol crossover in the membrane. The optimum properties and performances were presented by the SA/SGO membrane with a loading of 0.2 wt% SGO, which gave a proton −3 −1 −7 2 −1 conductivity of 13.2 × 10 Scm , and the methanol permeability was 1.535 × 10 cm s at 25 °C, far below that of −7 2 −1 Nafion (25.1 × 10 cm s ) at 25 °C. The mechanical properties of the sodium alginate polymer in terms of tensile strength and elongation at break were improved by the addition of SGO. Keywords: Bio-membrane, Sodium alginate, Sulfonated graphene oxide, DMFC Background current electrocatalysts (palladium and ruthenium) [3]. The The simple conversion of chemical energy from a fuel proton electrolyte membrane is the most vital component through a chemical reaction into electricity can only be in DMFC because it functions as a fuel and oxidant separ- done by a fuel cell device. Regarding this capability, the dir- ator,aswell asa path forconductingprotons;consequently, ect methanol fuel cell (DMFC) has received great attention it can have a substantial effect on the overall system effi- because it can operate using only 17% methanol as the fuel ciency. Among the required membrane characteristics, the to produce electricity with reduced pollutant emissions membrane should have high proton conductivity and the compared with other methods and is also safe to use while ability to effectively block the methanol from crossing the flying [1]. DMFC has wide capabilities in many applications, membrane to avoid cathode side poisoning [4]. In addition, such as medical tools, hearing aids, and portable tools. Un- it is important to ensure the use of non-hazardous, inex- fortunately, its application has been hindered due to its lack pensive raw materials for the membrane. The current com- of commercialization, which is attributed to issues such as mercial membrane (Nafion) does not meet these major −2 the high cost of production (approximately 1000 USD m ) requirements; therefore, it is not a good membrane for [2], high methanol permeability of commercialized mem- DMFC applications due to its high methanol permeability, branes (Nafion) and low reactivity and low durability of the high cost, and use of hazardous materials. In addition, its proton conductivity is affected by these problems, conse- quently limiting its effectiveness in DMFC applications. * Correspondence: ctie@ukm.edu.my Currently, biomaterials are receiving attention because they Fuel Cell Institute, Universiti Kebangsaan Malaysia (UKM), 43600 Bangi, are safe and environmentally friendly, classifying them as Selangor, Malaysia Department of Chemical and Process Engineering, Faculty Of Engineering green technology materials. As a new and excellent bioma- and Built Environment, Universiti Kebangsaan Malaysia (UKM), 43600 Bangi, terial, alginates have intrigued many researchers from Selangor, Malaysia © The Author(s). 2018 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Shaari et al. Nanoscale Research Letters (2018) 13:82 Page 2 of 16 various areas for applications including tissue engineering, much lower cost, which makes it the most suitable can- biomedicine, delivery vehicles for drugs, food packaging, didate for membranes in DMFC applications [29]. Previ- and DMFC [5]. Alginate is a prominent water-soluble ous studies showed that GO strengthened natural polysaccharide found in brown seaweed, and it consists of polymers such as chitosan films and chitosan-gelatin (1-4)-linked β-D-mannuronic acid (M) and α-L-guluronic porous monoliths [19, 30]. Bayer et al. [31]prepared a acid (G) units. It has very high water absorption and can GO paper, which showed hydrogen permeability three absorb 200–300 times its own weight in water [6]. The times lower than Nafion and proton conductivity of 49.9 −1 proton conduction ability of pristine alginate is low due to mScm using an in-plane technique. The direct liquid the absence of continuous transfer pathways and the weak fuel cell (DLFC) performance was excellent when Lue et conducting ability of the polymer [6–9]. Previous studies al. [32] introduced GO into Nafion. However, the showed that the most effective method to enhance the performance of GO as a proton conductor is limited mechanical properties and specialize the other properties of because it lacks functional groups that can be proton car- this polymeric material is to introduce an inorganic riers in the membrane, which adversely affects proton material and a polymer backbone [7]. Composite materials conductivity and decreases fuel cell performance [19]. can extend or provide novel capabilities that are difficult to Karim et al. [33] reported that the conductivity of the GO −1 obtain by using each component individually. For instance, nanosheet in their study was 15 mS cm and the GO the mechanical strength of alginate has been successfully conductivities reported by Hatakeyama et al. [34]and −1 −1 enhanced by introducing carbon nanotube and graphene Bayer et al. [35] were 0.4 mScm and 0.55 mScm , oxide into the alginate polymer matrix [3, 10, 11]. Previous respectively. Based on these weaknesses, sulfonated GO is studies on the development of biopolymer-based considered as a better option than GO for this application membranes have shown good potential when combined because sulfonated GO has shown increased proton con- with other materials such as inorganic or synthetic ductivity, and it facilitates the formation of a homoge- −6 2 −1 polymers, e.g., double layer-chitosan (1.67 × 10 cm s ) neous membrane due to the high compatibility between −6 2 −1 [12], chitosan-PVA/Nafion (2.2 × 10 cm s )[13], the GOS and SO H[19]. Keith et al. [36]presented a −2 −1 chitosan-SHNT (0.76 × 10 Scm )[14], chitosan-zeolite SGO paper that showed a high maximum power density −2 −1 −2 −2 (2.58 × 10 Scm )[15], chitosan-PMA (1.5 × 10 Scm of 113 mWcm at 0.39 V for polymer electrolyte −1 −2 −1 )[16], chitosan-sodium alginate (4.2 × 10 Scm )[17], membrane fuel cell (PEMFC). The advantages of –SO H −2 −1 alginate-carrageenan (3.16 × 10 Scm )[18], sulfonated incorporation are as follows: (i) the acid groups can offer −2 −1 chitosan-SGO (72 × 10 Scm )[19], PVA-sodium algin- supplementary hopping sites for proton movement, and −2 −1 ate (9.1 × 10 Scm )[20], biocellulose-Nafion (7.1 × 10 (ii) the electrostatic attractions will improve the thermal −2 −1 −2 −1 Scm )[21], chitosan-SPSF (4.6 × 10 Scm )[22], and mechanical stabilities by interfering with the alginate −2 −1 chitosan-silica/carbon nanotube (CNT) (2.5 × 10 Scm ), chain mobility and packing. Based on our research, no −2 −1 chitosan-PVP (2.4 × 10 Scm )[23], nanocellulose/poly- nanocomposite alginate/SGO material has been produced −2 pyrrole (1.6 mW cm ) for enzymatic fuel cell [24], cellu- yet using this method. The use of biomaterials in the ap- −3 −1 lose nanofibres (CNFs) (0.05 × 10 Scm ) and cellulose plication of electrical devices will lead to interdisciplinary −3 −1 nanocrystals (CNCs) (4.6 × 10 Scm )[25], bacterial research between the biological sciences and sustainable cellulose (BC)/poly (4-styrene sulfonic acid) (PSSA) energy technologies. Therefore, this research will combine −1 (0.2 S cm )[26], and imidazole-doped nanocrystalline the advantages of alginate and SGO to form a novel bio- −2 −1 cellulose (2.79 × 10 Scm )[27]. However, the number of membrane with high durability, good proton conductivity, biopolymer-based membranes developed is too small and methanol permeability with the goal that it will compared to the studies involving synthetic polymers in perform better than Nafion or other commercial proton many areas including fuel cells. Additionally, it is undeni- exchange membranes (PEMs) as well as being much able that chitosan has received more attention than the cheaper to produce than Nafion. other carbohydrate polymers. Graphene oxide is a promising carbon-based material Methods with high potential in many applications, including elec- Materials tronics, nanocomposites, biomedicine, and fuel cells. TIMREX PG25 natural graphite was purchased from Graphene oxide has excellent properties, such as a high TIMCAL Ltd. Concentrated sulfuric acid (H SO , 95%), 2 4 aspect ratio, high conductivity, high mechanical strength, methanol (CH OH, 99.7%), potassium permanganate, unique graphitized plane structure, and electrical insu- hydrochloric acid, hydrogen peroxide aqueous solution lating properties [28]. As an additive material in a hydro- (H O , 35%), calcium chloride, ethanol, sulfanilic acid, 2 2 philic polymer matrix, it provides high resilience to sodium nitrite solution, and glycerol were obtained from resist swelling caused by moisture. Furthermore, gra- Sigma Aldrich. These chemicals were used as received phene oxide would be preferable to CNT due to its without further purification. Deionized (DI) water Shaari et al. Nanoscale Research Letters (2018) 13:82 Page 3 of 16 through a Millipore system (Milli-Q) was used in all times with water until reaching pH 7. The SGO particles experiments. were characterized by X-ray photoelectron spectroscopy (XPS). Sodium alginate was dissolved in 1% (w/v) Membrane Preparation double-distilled water to obtain a solution of alginate. Hummer’s method was modified and applied to provide The SGO content added to the alginate solution varied, a GOS from natural graphite [10, 37]. First, 2 g of graph- with values of 0.02, 0.05, 0.09, 0.13, 0.17, and 0.2 wt% to ite was mixed with 150 ml of H SO (95%) in a 500-ml produce a composite film. The mixture was stirred con- 2 4 flask. The mixture was stirred for 30 min in an ice bath. tinuously for 60 min with a magnetic stirrer. The hetero- Under continuous and vigorous stirring, 15 g of potas- geneous solution was transferred to a glass substrate and sium permanganate was added to the mixture. The was left at 60 °C for 72 h to allow for the thin film for- addition rate was carefully controlled to maintain the re- mation process. The dried alginate/sulfonated graphene action temperature at 20 °C. The mixture was then oxide membrane was then crosslinked using a calcium stirred and left overnight at room temperature, followed chloride/glycerol solution to increase the mechanical by the addition of 180 ml of water under vigorous stir- strength and to reduce the hydrophilic properties of al- ring and reflux at 98 °C for 24 h; this caused the solution ginate. The membrane was immersed for 30 min in to turn a yellow color. Eighty milliliters of 35% H O 100 ml of crosslinking solution whose cation concentra- 2 2 was added to the reaction mixture, which was allowed to tion was maintained at 1.5% w/v. Finally, any free cations cool to room temperature in order to quench the reac- were removed from the membrane surface by washing tion with KMnO . The resulting GO was washed by with DI water, and the membrane was dried at 25 °C. rinsing with 5% HCl followed by centrifugation. Finally, The preparation method is summarized in Scheme 1. the product was rinsed with DI water several times, fil- tered and dried under vacuum conditions. Membrane Characterization Fifty milliliters of graphene oxide was added to 8 ml of The Fourier transform infrared (FTIR PERKIN ELMER) a 0.06 M sulfanilic acid solution at 70 °C. With continu- spectra of graphene oxide, sulfonated graphene oxide, and ous stirring, 2 ml of sodium nitrite solution was added the membrane were analyzed. The FTIR wavelength was −1 dropwise to the mixture and allowed to stand for 12 h at in the range of 4000–500 cm . The microstructure of the a constant temperature of 70 °C. After the reaction was film membranes was examined using a field emission complete, the mixture was washed and collected by cen- scanning electron microscope (FEI QUANTA 400 trifugation. The collected SGO was washed several more FESEM) with an operating voltage of 5 kV as a precaution Scheme 1 Sulfonated graphene oxide (SGO) filler and SA/SGO biomembrane preparation method Shaari et al. Nanoscale Research Letters (2018) 13:82 Page 4 of 16 for the bio-material-based sample. The high-resolution thickness, and R is the resistance of the membrane, transmission electron microscopy (HRTEM) analysis was similar to the method in previous works [38, 39]. carried out using o Digital TEM HT7700 operated at an Two tank liquid permeability cells with 20 v/v% accelerating potential of 300 kV. methanol were used to determine the methanol perme- Samples were prepared on grids with lacey carbon ability of the membrane. The differences in the concen- support film. XPS was used to determine the chemical tration of methanol result in methanol crossover composition of the sample surface using an Axis Ultra through the membrane, and methanol permeability can DLD. The mechanical strength of the SA/SGO be determined. Equation 3 is used to calculate the per- membrane was tested with a Universal Testing Machine, meability of methanol: including tensile strength, Young’s modulus, and elong- ation at break. The load used was 3 kN at room 1 ΔCbðÞ t LVb P ¼ ð4Þ temperature. Changes in the weight and length (or thick- Ca Δt A ness) of wet and dry membranes can determine the rate of water absorption and the swelling ratio of the where P is the membrane diffusion permeability for 2 −1 membrane. Typically, the membrane was soaked in methanol (cm s ), C is the methanol concentration in −1 water for 2 days at 30 °C. For the wet membrane, the the feed chamber, i.e., cell A (mol L ), ΔCb(t)/Δt is the weight and length were recorded, and then, the water in methanol molar concentration variation in cell B as a −1 the membrane and the liquid droplets on the surface of function of time (mol L s), V is the volume of each the membrane were removed. In addition, the moist diffusion reservoir (cm ), A is the membrane area, and L membrane was dried under vacuum pressure and is the membrane thickness (cm). temperature of 120 °C for at least 24 h. The weight and The membrane characteristics can be determined by length of the membrane in the dry state were also re- calculating the selectivity of the membrane, which can corded. Using Eqs. 1 and 2, water intake (WU%) and be achieved by high proton conductivity and low metha- swelling ratio (SW%) can be determined, where L rep- wet nol permeability. The formula used for calculating the resents the wet mass and L represents the dry mass dry selectivity is as follows: obtained from the length of wet and dry membranes, respectively. σ φ ¼ ð5Þ mass −mass wet dry WU% ¼  100 ð1Þ mass dry where φ represents selectivity, σ represents ionic conductivity, and P represents methanol permeability. L −L wet dry SW% ¼  100 ð2Þ dry Results and Discussion Characterization of Sulfonated Graphene Oxide (SGO) and The methanol uptake calculation is the same as the SA/SGO Biomembrane water uptake calculation, except that the solution for The FTIR spectra in Fig. 1a, b show the difference immersion is changed to methanol rather than DI water. between GO and SGO, which can be clearly observed. The proton conductivity of the prepared membrane Figure 1b is the magnification of Fig. 1a to obtain a was calculated using a four-electrode conductivity cell clearer view of the peaks in the SGO spectra. The connected to a potentiostat/galvanostat (WonATech) −1 spectrum of SGO shows a new band at 1244 cm , operating over a frequency range of 1 MHz down to which is the typical absorbance of a sulfonic acid group 50 Hz. The membranes (1 cm × 4 cm in size) must be (-SO H), whereas the GO spectrum does not contain soaked in water for 24 h for the conductivity readings this band [40]. In addition, the spectrum shows new under the fully hydrated state. The potentiostat was run −1 peaks at the wavelengths of 1012, 1036, and 1125 cm , to obtain the graph of voltage versus current. The which are considered to be the symmetric and asymmet- gradient of the straight line is the membrane resistance. ric stretching vibrations of SO H groups. This new Scheme 1 presents the cell of the proton conductivity spectrum reveals that the graphene oxide solution was test. The proton conductivity can be calculated using the successfully modified into sulfonated graphene oxide following formula: using the simple method described above. At the same time, the sulfonation modification still kept the σ ¼ ð3Þ functional groups in GO such as the hydroxyl group at RWT −1 −1 3319 cm and the carboxyl group at 1636 cm . Further where L is the distance between the two electrodes, W is confirmation of the presence of SO H groups can be the width of the membrane, T is the membrane determined by XPS analysis. Shaari et al. Nanoscale Research Letters (2018) 13:82 Page 5 of 16 Fig. 1 a, b FTIR spectra for graphene oxide (GO) and sulfonated graphene oxide (SGO) Figure 2 shows the XPS spectra of the GO and SGO membranes in which the scanning spectra are in the range of 0–800 eV to recognize the surface of the existing elements via a measurable analysis. It can be observed that the C1s and O1s signals appeared at 286 and 531 eV, respectively, in both the GO and SGO spectra. It is also noticed that after the sulfonic acid groups were introduced into GO, a new S2p peak appeared at 168 eV. Sulfonic groups in SGO contributed to a slightly increased intensity in the O1s spectra compared with that of GO. The high- resolution spectrum of C1s, which is referred to as Gaussian spectral deconvolution, confirmed that GO was successfully customized via chemical modification [41]. The figure inside Fig. 2bisthe S2pspectra for functionalized GO at a larger magnification. The binding energy of the sulfonic groups contributed to the appearance of the S2p peak at 168 eV, and this peak confirmed that sulfonic acid groups were successfully attached to the GO nanosheet backbone [41, 42]. The successful production of GO via the Hummer’s method was confirmed by the sheet-shaped GO morphology as shown in the FESEM image (Fig. 3a). Bai et al. [43] also generated GO with Hummer’s method. The results of their studies showed that the morpholo- gies of both GO and RGO appeared to be slightly folded and formed some wrinkles, which resemble the GO morphology in this study. The FESEM image of SGO in Fig. 3b, c has a crumpled and rougher surface compared with the surface of GO, which is most likely due to the effects of the sulfonation process, confirming that the modification method was also successfully applied [41, 44]. This correlates with the existence of a new peak in the FTIR Fig. 2 XPS of a, b wide spectra GO and SGO and c S2p spectra transmittance spectra, which belongs to the sulfonic of SGO group. Moreover, the presence of sulfonic groups was Shaari et al. Nanoscale Research Letters (2018) 13:82 Page 6 of 16 ac bd Fig. 3 a FESEM image of GO. b, c FESEM images of SGO with various magnification and d EDX of SGO also confirmed in the GO sheet via the XPS analysis. is similar to the previous study reported by Marrella et SGO was different from GO, which had a multi-layered al. [46]. structure without any aggregation. The applied modifica- The presence of hydrogen bonding interactions be- tion method leads to the formation of a layered and tween SGO and the alginate polymer matrix is shown by restacked structured; thus, SGO demonstrated its flexi- the FTIR analysis. The FTIR results for the alginate and bility. The energy dispersive X-ray (EDX) result presents SGO alginate membranes are shown in Fig. 6. A slight that 1.76 wt% of sulfur element exists in the SGO sheets shift seems to occur for the hydrogen bonding site (Fig. 3d). spectra according to the hydrogen bond interactions. The surface image and cross section of the SA and The O-H group bands in the alginate membrane −1 SA/SGO bio membranes are shown in Fig. 4. Figure 4a– appeared at 1413 and 3440 cm ; however, the bands −1 c is a surface image, and Fig. 4d–f is a cross-sectional were shifted to 1406 and 3404 cm in the SA/SGO image of membranes with different SGO contents. Both membrane due to the hydrogen bonding among the low and high enlargements show that the SGO sheet is polar groups in SGO and the O–H groups in alginate completely dispersed homogeneously in the overall poly- [45]. The C=O group bands in the alginate membrane −1 mer matrix and is guided by intermolecular interactions; also shifted to 1046 from 1082 cm . The location of it is recognized that hydrogen bonds occur between the thesulfonicgroup (–SO H) bands in the alginate −1 sulfonic acid groups in SGO and polar groups (-O-, C = membrane also changed from 1284 to 1277 cm . O) in the SA/SGO membrane [45]. SGO is placed in the Thus, the results show that there is hydrogen bonding polymer matrix to function as a barrier to methanol between the SGO and alginate [47]. A complete dis- molecules. The image for SA/SGO6 looks better with persion of the SGO particles throughout the polymer the full spread to the entire sodium alginate polymer matrix can facilitate the proton conduction path in all matrix. Figure 5 is a TEM image for the composite directions of the membrane. As a result, the proper- formed in which the SGO nanosheets are well distrib- ties of SA/SGO membranes were assumed superior to uted in the sodium alginate polymer matrix. Sodium al- those of the pristine alginate membranes according to ginate exists in the nanosphere particle structure, which theSEMinterior structureand theFTIR spectra. Shaari et al. Nanoscale Research Letters (2018) 13:82 Page 7 of 16 a d b e c f Fig. 4 FESEM images of surface morphology and cross-section for a, d sodium alginate, b, e SA/SGO4, and c, f SA/SGO6 biomembranes Thermal Stability and Mechanical Properties Figure 7 shows the comparison of TGA analysis for all SA/SGO biomembranes with different contents of SGO. Losses at the first stage occurred below 200 °C due to the release of water molecules, which is known as the evaporation process. Generally, thermal decomposition of GO is at a temperature of approximately 200 °C due to the decomposition of the oxygen labile group, while for alginate polymers, heat decomposition at the first stage is at 178 °C [48, 49]. The SA/SGO biomembrane shows a heavy loss at a higher temperature of 198 °C. This increased temperature indicates that there is an interaction between sodium alginate and SGO, which in- creases the heat resistance for SA/SGO biomembrane. This shows that the presence of SGO has increased the thermal stability of the biomembrane due to favorable interfacial interactions, such as hydrogen-bonding or electrostatic interactions between the sodium alginate matrix and sulfonated graphene oxide nanosheets, thus making this membrane fit for DMFC application. The Fig. 5 TEM image of SGO nanosheets distributed in sodium alginate second stage of weight losses occurs at a temperature of polymer matrix 250 °C due to the decomposition of the sodium alginate Shaari et al. Nanoscale Research Letters (2018) 13:82 Page 8 of 16 Fig. 6 FTIR spectra of SA and SA/SGO membrane side-chain. The third stage (> 400 °C) involves the The Nafion membrane has a higher tensile stress process of decomposition of the polymer backbone [50]. compared to the SA/SGO6 biomembrane. However, it is Figure 8 presents the tensile stress and elongation at comparable between biomembrane categories. The gra- break of the membrane as the wt% of SGO varied. From phene oxide itself has very good mechanical properties, 0.02 to 0.13 wt% of SGO, the tensile stress increased and with an elastic modulus of 1100 GPa and an intrinsic then slightly decreased at 0.17 wt%. This might be attrib- strength of 125 GPa; this is the primary reason why uted to the restacking of graphene oxide sheets, which SGO can increase the mechanical properties of the al- can be related to the van der Waals forces in the GO ginate membrane [45]. nanosheets. The bulk of graphene oxide nanosheets Moreover, the formation of hydrogen bonds between leads to sliding and reduces the effect of graphene oxide SGO and the pure alginate matrix polymer can also re- in improving the mechanical properties of the mem- sult in good mechanical properties. A greater formation brane. The tensile stresses of Nafion and other biomem- of hydrogen bonding results in a stronger interfacial ad- branes in previous studies are listed in Table 1 [51–55]. hesion, consequently improving the mechanical strength Fig. 7 TGA curve for SA/SGO biomembranes with various SGO wt% Shaari et al. Nanoscale Research Letters (2018) 13:82 Page 9 of 16 Fig. 8 Tensile stress and elongation at break of biomembrane with various SGO wt% of the membrane. The elongation at break pattern is in results of water uptake and methanol uptake of the SA/ contrast to the tensile stress pattern. A lower tensile SGO membrane with varying SGO wt% values. As pre- stress results in a higher elongation at break percentage. sented, the SA/SGO membrane has a lower water Elongation at break indicates to what extent the uptake capacity with different contents of sulfonated GO membrane film can be stretched until the maximum (lowest WU - 57.9% by SA/SGO6) in the membrane point, which is also known as flexibility. Table 1 com- compared with pure alginate. An increasing amount of pares several membranes from previous studies with the SGO reduces the water uptake due to its blocking ability membrane of the current study in terms of elongation at as a filler in the membrane [5]. The addition of SGO break [51–56]. The different patterns between tensile facilitates the contraction of ionic pathways, thus hinder- stress and elongation at break are logical. As mentioned ing the movement of water and methanol. A higher above, the presence of SGO in the membrane increases SGO content results in a stronger barrier for the water the interfacial linkage due to the hydrogen bonding, thus absorption of the membrane. The hydrogen bonding be- reducing the flexibility of the membrane. tween the SGO filler and the sodium alginate polymer strengthens the interfacial adhesion of the membrane Liquid Uptake and Swelling Ratio of Membrane composite, thus reducing the water uptake capacity [19]. It is acknowledged that water is the prominent compo- The hydrogen bonding formation in the SA/SGO mem- nent in the proton exchange membrane because it acts brane involves the –OH groups in GO, the –O- and as a proton conductor in which the adsorbed water C=O groups on the SA chains, and contributions by facilitates proton transport [39]. Figure 9 presents the sulfonate groups (–SO H) [3, 19]. Similar to the pattern Table 1 Thickness, IEC, proton conductivity, methanol permeability, and membrane selectivity of SA/SGO composite biomembrane with different SGO content Sample Thickness IEC Proton conductivity Methanol permeability Selectivity Reference −1 −1 −7 2 −1 4 −3 (μm) (meq g ) (σ,mScm ) (P, × 10 cm s ) (SP × 10 Ss cm ) SA 198 ± 1 0.25 ± 0.03 6.36 ± 0.1 1.687 ± 0.22 3.7678 Current study SA/SGO1 200 ± 1 0.31 ± 0.05 6.63 ± 0.4 2.657 ± 0.39 2.495 Current study SA/SGO2 201 ± 2 0.34 ± 0.03 7.18 ± 0.5 2.453 ± 0.27 2.927 Current study SA/SGO3 203 ± 1 0.38 ± 0.02 7.75 ± 0.2 2.351 ± 0.23 3.296 Current study SA/SGO4 200 ± 1 0.45 ± 0.05 9.30 ± 0.1 2.045 ± 0.25 4.547 Current study SA/SGO5 201 ± 3 0.48 ± 0.03 10.6 ± 0.1 1.738 ± 0.23 6.098 Current study SA/SGO6 205 ± 2 0.56 ± 0.05 13.2 ± 0.1 1.535 ± 0.24 8.555 Current study Nafion 117 – 0.86 0.098 12.3 7.967 [28] Nafion 117 –– 0.081 20 4.05 [29] Nafion 117 –– 0.1056 25 4.22 [30] Shaari et al. Nanoscale Research Letters (2018) 13:82 Page 10 of 16 Fig. 9 Liquid uptake and swelling ratio of SA/SGO membrane with wt.% of SGO of the water uptake result, the methanol uptake of the Arrhenius behavior, the ion transport activation energy SA/SGO membrane also decreased with increasing SGO E of the SA/SGO membranes can be obtained wt% in the membrane. The presence of the same trend according to the Arrhenius equation: shows that there was good networking and bonding between SGO and the alginate polymer, which impeded E ¼ −bxR fuel crossing. From the experimental result, the presence of graphene oxide-based materials lowered the water where b is the slope of the line regression of ln σ (S/cm) −1 uptake capacity of the SA membrane and maintained its vs. 1000/T (K ) plots, and R is the gas constant −1 −1 mechanical strength. The swelling ratio decreased from (8.314472 JK mol ). The ion transport activation 106% to 61.12% with increasing SGO wt% in the alginate energy of the SA/SGO6 composite membrane is −1 polymer matrix (Fig. 9) due to the blocking effect [10]. 8.17 kJ mol , which is slightly greater than the E of −1 The strong hydrogen bonding also diminished the Nafion® 115 (6.00 kJ mol )[57] and lower than that of −1 pathways for absorbance of the ionic group into the Nafion 117 (12 kJ mol )[58]. This can be attributed to polymer [32]. the hydrophilic properties of the sodium alginate matrix, which provide high water content, and the introduction IEC, Proton Conductivity, Methanol Permeability, and of SGO still allows this property to remain due to the Selectivity hydrophilic properties of oxygenated functional groups. Ion exchange capacity (IEC) calculation is important The abundant water forms a continuous transferring since it is responsible for measuring the number of channel and makes the movement of ion easy. milliequivalents ions in 1 g of the prepared membranes Figure 13a presents the suggested proton mobility and is an indicator for proton conductivity in DMFCs. mechanism in SA/SGO plasticized with glycerol in Table 2 shows the IEC values of the membranes. A which high synchronization exists between H and elec- higher IEC value is achieved by the SA/SGO membrane tron lone pairs belonging to the oxygen atoms carrier in containing a higher wt% of SGO. This is due to the glycerol and the hydrophilic sulfonic acid groups in function of sulfonic acid groups in the SGO nanosheets. SGO nanosheets. We believe that the proton transport An increment in the IEC value increases the proton applies both Grotthus and vehicle mechanisms, conductivity value of the SA/SGO biomembrane. The strengthened by the SGO particles. proton conductivities of the SA/SGO membrane versus The SA/SGO biomembranes show very low methanol temperature are presented in Fig. 10. Increasing the permeability, and the lowest was achieved by SA/SGO6 −7 2 −1 temperature leads to the enhancement of proton (1.535 × 10 cm s ), as listed in Table 2. The low conductivity. The SA/SGO membrane features a consist- methanol permeability can be explained in terms of the ently increasing pattern in proton conductivity as the membrane microstructure between sodium alginate, SGO particle amount increases, with a maximal value of SGO, and glycerol plasticizer. The introduced SGO −1 13.2 mS cm at 0.2 wt% of SGO loading at temperature particles serving as fillers in the SA polymer create of 30 °C. The ln σ vs. 1000/T plot is also shown in substantial obstacles to the linked hydrophilic passages. Fig. 11. Assuming that the conductivity follows an The SGO filler blocks the migration of methanol passing Shaari et al. Nanoscale Research Letters (2018) 13:82 Page 11 of 16 Table 2 Comparison of condition in single-cell performance test with power density result for previous work and current study Membrane Anode catalyst loading Cathode catalyst loading Methanol feed concentration Temperature Pmax Mode Reference −2 −2 −3 −2 (mg cm ) (mg cm ) (mol dm ) (°C) (mW cm ) SA Pt-Ru:8 Pt:8 4 RT 2.8 Passive Current study SA/SGO6 Pt-Ru:8 Pt:8 4 RT 5.9 Passive Current study Nafion 117 Pt-Ru:8 Pt:8 4 RT 6.6 Passive Current study Nafion 117 Pt-Ru:8 Pt:8 2 RT 7.2 Passive [28] Alginate- Pt-Ru:5 Pt:5 2 50 10.4 Active [31] carrageenan through the membrane, and this is known as the It was noticed that a higher selectivity value resulted blocking effect, which reduces the methanol permeabil- in a higher DMFC capability. The selectivity values of ity. The methanol permeability also decreases because of the SA/SGO can be observed in Table 2, which the interfacial interaction between the SGO and SA compares the selectivity among SA and SA/SGO biopolymer [41]. The methanol permeability of the SA/ biomembranes as well as Nafion 117 membranes from SGO6 bio membrane at four different temperature previous work. The presence of SGO enhanced the conditions is shown in Fig. 12. As seen, the methanol selectivity of the SA/SGO polymer membrane (8.555 × 4 −3 permeability increases at a higher temperature, which 10 Sscm for 0.2 wt% SGO loading), which is higher 4 −3 can be related to the structure changes of the bio than that of SA (3.7678 × 10 Sscm ) and fortunately 4 −3 membrane. The higher temperature provides more heat, also higher than that of Nafion 117 (7.99 × 10 Sscm ) 4 −3 4 −3 which can shake the membrane chains and molecules, [38], 4.05 × 10 Sscm [61], and 4.22 × 10 Sscm thus leading to more free volume, which consequently [62], in which the low methanol permeability is the main reduces the methanol blocking effect. Less resistance factor to be considered. causes easier movement of methanol diffusion [59]. Mu et al. [60] reported the decrease in methanol crossover Single Cell in the presence of Au nanoparticles self-assembled on a Single-Cell Performance Evaluation Nafion membrane, which consequently improved the Figure 14 indicates the cell polarization result for pure overall performance. alginate, SA/SGO6 composite biomembrane and Nafion The interfacial interaction between SGO filler, glycerol, 117 under ambient temperature, 4 M methanol concen- and SA polymer confines the hydrophilic passage tration and passive mode condition. The SA/SGO6 com- formation in the membrane, and this wide hydrophilic posite biomembrane was applied due to the high passage is a significant factor in methanol migration selectivity factor and obviously had a higher open-circuit [19]. Thus, the presence of SGO facilitates methanol voltage (0.63 V), which can be related to the low permeability reduction [6]. The proposed mechanism of methanol permeability equaling to that the sodium methanol rejection is presented in Fig. 13b. alginate biomembrane. The OCV of Nafion 117 (0.52 V) Fig. 10 Proton conductivity of SA/SGO biomembranes with various content of SGO at different temperature Shaari et al. Nanoscale Research Letters (2018) 13:82 Page 12 of 16 Fig. 11 ln σ vs. 1000/T plot for the cross-linked QAPVA membranes, the lines indicate the linear regression in the current study is lower than SA/SGO and sodium Nafion achieves a better performance in DMFC applica- alginate, which might be due its higher methanol perme- tion due to the excellent proton conduction. However, ability. The crossing of methanol through the membrane the power density performance between Nafion 117 and leads to the reduction in the OCV value. The higher SA/SGO biomembranes does not show a big difference OCV of SA/SGO and alginate membrane is the big indi- quantitatively. Hence, SA/SGO can be an alternative cator that synthesized membrane has lower methanol membrane for DMFC in the future. However, the prop- permeability compared to Nafion, which the main erties of the membrane still need to be enhanced, and objective of this study is successfully achieved. The higher wt% of SGO filler can probably be used to obtain improvement in the power density of SA/SGO6 is due a higher power density. To the best of our knowledge, to the sulfonic acid group that functions as a proton there is only one previous work by Pasini Cabello et al. transferral pathway as well as a methanol inhibitor, thus that has examined the single-cell performance in DMFC −2 achieving 5.9 mW cm compared to the sodium application using an alginate biopolymer-based −2 alginate, which achieved only 2.83 mW cm . However, membrane [18]. They tested an alginate/carrageenan Nafion 117 achieved a higher power density, which was membrane at temperatures of 50, 70, and 90 °C in 2 M −2 6.62 mW cm . Thiam et al. [38] reported the perform- methanol concentration in the active mode, which ance of Nafion 117 membrane under the same condition achieved maximum power densities of 10.4, 13.9, and −2 2 with a power density of 7.95 mW cm . No doubt, 17.3 mW/m , respectively. The active mode has an Fig. 12 Methanol permeability of membrane SA/SGO6 vs. temperature Shaari et al. Nanoscale Research Letters (2018) 13:82 Page 13 of 16 Fig. 13 Suggested mechanism of a proton mobility and b methanol rejection advantage due to the continuous flow of the methanol Conclusions feed into the cell that allows the reaction to occur In conclusion, a membrane with low methanol continuously and thus is capable of achieving a higher permeability, high proton conductivity, and high select- power density. The higher power density could be ivity was successfully prepared through the simple achieved at a higher temperature due to the higher technique known as the blending method. The presence number of activated protons. Nevertheless, this work is of sulfonated graphene oxide enhanced the properties of an indicator that biopolymer-based membrane has a big the alginate-based polymer membrane in terms of potential that can be explored and applied in DMFC proton conductivity and methanol permeability. The systems. sulfonate groups facilitated the networking between the Shaari et al. Nanoscale Research Letters (2018) 13:82 Page 14 of 16 Fig. 14 Single-cell performance test for sodium alginate, SA/SGO6, and Nafion 117 (4 M methanol and 25 °C temperature, passive mode) alginate polymer and the graphene oxide filler. The Authors’ Contributions SKK had contributed the idea of the project. NS had performed the experimental blocking effect of SGO also reduced the methanol cross- work and wrote-tp this manuscript. SKK proof read and submiited this manuscript over in the membrane. The primary weaknesses of the for publication as corresponding author. SB, LKS, SM, ND co-supervised NS and alginate polymer, which are its mechanical properties of helped in analysis data. All the authors read and approved the final manusript. tensile strength and elongation at break, were also Authors’ Information improved by the addition of SGO into the polymer Mrs. Norazuwana Shaari—Student of Fuel Cell Institute, Universiti matrix. The presence of SGO improved the SA/SGO Kebangsaan Malaysia, Malaysia. membrane to a high level comparable to commercial Prof Ir. Dr. Siti Kartom Kamarudin—Director Of Fuel cell Institute, Universiti Kebangsaan Malaysia, Malaysia. membranes. Dr. Sahriah Basri—Fellow of Fuel Cell Institute, Universiti Kebangsaan Malaysia, Malaysia. Abbreviations Dr. Loh Kee Shyuan—Fellow of Fuel Cell Institute, Universiti Kebangsaan BC: Bacterial cellulose; CNC: Cellulose nanocrystal; CNFs: Cellulose nanofibers; Malaysia, Malaysia. CNT: Carbon nanotube; DI: Deionized; DLFC: Direct liquid fuel cell; Dr. Shahbudin Masdar—Associate Fellow of Fuel Cell Institute, Universiti DMFC: Direct methanol fuel cell; EDX: Energy dispersive X-ray; FESEM: Field Kebangsaan Malaysia, Malaysia. emission scanning electron microscope; FTIR: Fourier transform infrared; Dr. Darman Nordin—Associate Fellow of Fuel Cell Institute, Universiti GO: Graphene oxide; GOS: Graphene oxide sheet; HRTEM: High-resolution Kebangsaan Malaysia, Malaysia. transmission electron microscopy; IEC: Ion exchange capacity; L: Distance between the two electrodes; OCV: Open circuit voltage; P: Membrane Competing Interests diffusion permeability for methanol; PEMFC: Polymer electrolyte membrane We confirm that the work described has not been published before; it is not fuel cell; PEMs: Proton exchange membrane; PMA: Phospho molybdic acid; under consideration for publication anywhere else; and publication has been PSSA: Poly-styrene sulfonic acid; PVA: Poly vinyl alcohol; PVP: Poly (vinyl approved by all co-authors and the responsible authorities at the institute(s) pyrrolidone); R: Resistance of the membrane; RGO: Reduced graphene oxide; where the work has been carried out. The authors declare that they have no SA: Sodium alginate; SA/SGO: Sodium alginate/sulfonated graphene oxide competing interests. membrane; SGO: Sulfonated graphene oxide; SHNT: Sulfonated halloysite nanotube; SPSF: Sulfonated polysulfone; SW%: Swelling ratio percentage; T: Membrane thickness; TGA: Thermal gravimetric analysis; W: Width of the Publisher’sNote membrane; WU%: Water uptake percentage; XPS: X-ray photoelectron Springer Nature remains neutral with regard to jurisdictional claims in spectroscopy published maps and institutional affiliations. Acknowledgements Received: 5 October 2017 Accepted: 2 March 2018 The authors gratefully acknowledge the financial support given for this work by the Ministry of Education (MOE)-MALAYSIA under GSP/1/2015/TK01/UKM/ 01/1 and Universiti Kebangsaan Malaysia under DIP-2017-021. References 1. 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Electrochim Acta 56(24):8446–8456 http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Nanoscale Research Letters Springer Journals

Enhanced Proton Conductivity and Methanol Permeability Reduction via Sodium Alginate Electrolyte-Sulfonated Graphene Oxide Bio-membrane

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Materials Science; Nanotechnology; Nanotechnology and Microengineering; Nanoscale Science and Technology; Nanochemistry; Molecular Medicine
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1931-7573
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10.1186/s11671-018-2493-6
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Abstract

The high methanol crossover and high cost of Nafion® membrane are the major challenges for direct methanol fuel cell application. With the aim of solving these problems, a non-Nafion polymer electrolyte membrane with low methanol permeability and high proton conductivity based on the sodium alginate (SA) polymer as the matrix and sulfonated graphene oxide (SGO) as an inorganic filler (0.02-0.2 wt%) was prepared by a simple solution casting technique. The strong electrostatic attraction between -SO H of SGO and the sodium alginate polymer increased the mechanical stability, optimized the water absorption and thus inhibited the methanol crossover in the membrane. The optimum properties and performances were presented by the SA/SGO membrane with a loading of 0.2 wt% SGO, which gave a proton −3 −1 −7 2 −1 conductivity of 13.2 × 10 Scm , and the methanol permeability was 1.535 × 10 cm s at 25 °C, far below that of −7 2 −1 Nafion (25.1 × 10 cm s ) at 25 °C. The mechanical properties of the sodium alginate polymer in terms of tensile strength and elongation at break were improved by the addition of SGO. Keywords: Bio-membrane, Sodium alginate, Sulfonated graphene oxide, DMFC Background current electrocatalysts (palladium and ruthenium) [3]. The The simple conversion of chemical energy from a fuel proton electrolyte membrane is the most vital component through a chemical reaction into electricity can only be in DMFC because it functions as a fuel and oxidant separ- done by a fuel cell device. Regarding this capability, the dir- ator,aswell asa path forconductingprotons;consequently, ect methanol fuel cell (DMFC) has received great attention it can have a substantial effect on the overall system effi- because it can operate using only 17% methanol as the fuel ciency. Among the required membrane characteristics, the to produce electricity with reduced pollutant emissions membrane should have high proton conductivity and the compared with other methods and is also safe to use while ability to effectively block the methanol from crossing the flying [1]. DMFC has wide capabilities in many applications, membrane to avoid cathode side poisoning [4]. In addition, such as medical tools, hearing aids, and portable tools. Un- it is important to ensure the use of non-hazardous, inex- fortunately, its application has been hindered due to its lack pensive raw materials for the membrane. The current com- of commercialization, which is attributed to issues such as mercial membrane (Nafion) does not meet these major −2 the high cost of production (approximately 1000 USD m ) requirements; therefore, it is not a good membrane for [2], high methanol permeability of commercialized mem- DMFC applications due to its high methanol permeability, branes (Nafion) and low reactivity and low durability of the high cost, and use of hazardous materials. In addition, its proton conductivity is affected by these problems, conse- quently limiting its effectiveness in DMFC applications. * Correspondence: ctie@ukm.edu.my Currently, biomaterials are receiving attention because they Fuel Cell Institute, Universiti Kebangsaan Malaysia (UKM), 43600 Bangi, are safe and environmentally friendly, classifying them as Selangor, Malaysia Department of Chemical and Process Engineering, Faculty Of Engineering green technology materials. As a new and excellent bioma- and Built Environment, Universiti Kebangsaan Malaysia (UKM), 43600 Bangi, terial, alginates have intrigued many researchers from Selangor, Malaysia © The Author(s). 2018 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Shaari et al. Nanoscale Research Letters (2018) 13:82 Page 2 of 16 various areas for applications including tissue engineering, much lower cost, which makes it the most suitable can- biomedicine, delivery vehicles for drugs, food packaging, didate for membranes in DMFC applications [29]. Previ- and DMFC [5]. Alginate is a prominent water-soluble ous studies showed that GO strengthened natural polysaccharide found in brown seaweed, and it consists of polymers such as chitosan films and chitosan-gelatin (1-4)-linked β-D-mannuronic acid (M) and α-L-guluronic porous monoliths [19, 30]. Bayer et al. [31]prepared a acid (G) units. It has very high water absorption and can GO paper, which showed hydrogen permeability three absorb 200–300 times its own weight in water [6]. The times lower than Nafion and proton conductivity of 49.9 −1 proton conduction ability of pristine alginate is low due to mScm using an in-plane technique. The direct liquid the absence of continuous transfer pathways and the weak fuel cell (DLFC) performance was excellent when Lue et conducting ability of the polymer [6–9]. Previous studies al. [32] introduced GO into Nafion. However, the showed that the most effective method to enhance the performance of GO as a proton conductor is limited mechanical properties and specialize the other properties of because it lacks functional groups that can be proton car- this polymeric material is to introduce an inorganic riers in the membrane, which adversely affects proton material and a polymer backbone [7]. Composite materials conductivity and decreases fuel cell performance [19]. can extend or provide novel capabilities that are difficult to Karim et al. [33] reported that the conductivity of the GO −1 obtain by using each component individually. For instance, nanosheet in their study was 15 mS cm and the GO the mechanical strength of alginate has been successfully conductivities reported by Hatakeyama et al. [34]and −1 −1 enhanced by introducing carbon nanotube and graphene Bayer et al. [35] were 0.4 mScm and 0.55 mScm , oxide into the alginate polymer matrix [3, 10, 11]. Previous respectively. Based on these weaknesses, sulfonated GO is studies on the development of biopolymer-based considered as a better option than GO for this application membranes have shown good potential when combined because sulfonated GO has shown increased proton con- with other materials such as inorganic or synthetic ductivity, and it facilitates the formation of a homoge- −6 2 −1 polymers, e.g., double layer-chitosan (1.67 × 10 cm s ) neous membrane due to the high compatibility between −6 2 −1 [12], chitosan-PVA/Nafion (2.2 × 10 cm s )[13], the GOS and SO H[19]. Keith et al. [36]presented a −2 −1 chitosan-SHNT (0.76 × 10 Scm )[14], chitosan-zeolite SGO paper that showed a high maximum power density −2 −1 −2 −2 (2.58 × 10 Scm )[15], chitosan-PMA (1.5 × 10 Scm of 113 mWcm at 0.39 V for polymer electrolyte −1 −2 −1 )[16], chitosan-sodium alginate (4.2 × 10 Scm )[17], membrane fuel cell (PEMFC). The advantages of –SO H −2 −1 alginate-carrageenan (3.16 × 10 Scm )[18], sulfonated incorporation are as follows: (i) the acid groups can offer −2 −1 chitosan-SGO (72 × 10 Scm )[19], PVA-sodium algin- supplementary hopping sites for proton movement, and −2 −1 ate (9.1 × 10 Scm )[20], biocellulose-Nafion (7.1 × 10 (ii) the electrostatic attractions will improve the thermal −2 −1 −2 −1 Scm )[21], chitosan-SPSF (4.6 × 10 Scm )[22], and mechanical stabilities by interfering with the alginate −2 −1 chitosan-silica/carbon nanotube (CNT) (2.5 × 10 Scm ), chain mobility and packing. Based on our research, no −2 −1 chitosan-PVP (2.4 × 10 Scm )[23], nanocellulose/poly- nanocomposite alginate/SGO material has been produced −2 pyrrole (1.6 mW cm ) for enzymatic fuel cell [24], cellu- yet using this method. The use of biomaterials in the ap- −3 −1 lose nanofibres (CNFs) (0.05 × 10 Scm ) and cellulose plication of electrical devices will lead to interdisciplinary −3 −1 nanocrystals (CNCs) (4.6 × 10 Scm )[25], bacterial research between the biological sciences and sustainable cellulose (BC)/poly (4-styrene sulfonic acid) (PSSA) energy technologies. Therefore, this research will combine −1 (0.2 S cm )[26], and imidazole-doped nanocrystalline the advantages of alginate and SGO to form a novel bio- −2 −1 cellulose (2.79 × 10 Scm )[27]. However, the number of membrane with high durability, good proton conductivity, biopolymer-based membranes developed is too small and methanol permeability with the goal that it will compared to the studies involving synthetic polymers in perform better than Nafion or other commercial proton many areas including fuel cells. Additionally, it is undeni- exchange membranes (PEMs) as well as being much able that chitosan has received more attention than the cheaper to produce than Nafion. other carbohydrate polymers. Graphene oxide is a promising carbon-based material Methods with high potential in many applications, including elec- Materials tronics, nanocomposites, biomedicine, and fuel cells. TIMREX PG25 natural graphite was purchased from Graphene oxide has excellent properties, such as a high TIMCAL Ltd. Concentrated sulfuric acid (H SO , 95%), 2 4 aspect ratio, high conductivity, high mechanical strength, methanol (CH OH, 99.7%), potassium permanganate, unique graphitized plane structure, and electrical insu- hydrochloric acid, hydrogen peroxide aqueous solution lating properties [28]. As an additive material in a hydro- (H O , 35%), calcium chloride, ethanol, sulfanilic acid, 2 2 philic polymer matrix, it provides high resilience to sodium nitrite solution, and glycerol were obtained from resist swelling caused by moisture. Furthermore, gra- Sigma Aldrich. These chemicals were used as received phene oxide would be preferable to CNT due to its without further purification. Deionized (DI) water Shaari et al. Nanoscale Research Letters (2018) 13:82 Page 3 of 16 through a Millipore system (Milli-Q) was used in all times with water until reaching pH 7. The SGO particles experiments. were characterized by X-ray photoelectron spectroscopy (XPS). Sodium alginate was dissolved in 1% (w/v) Membrane Preparation double-distilled water to obtain a solution of alginate. Hummer’s method was modified and applied to provide The SGO content added to the alginate solution varied, a GOS from natural graphite [10, 37]. First, 2 g of graph- with values of 0.02, 0.05, 0.09, 0.13, 0.17, and 0.2 wt% to ite was mixed with 150 ml of H SO (95%) in a 500-ml produce a composite film. The mixture was stirred con- 2 4 flask. The mixture was stirred for 30 min in an ice bath. tinuously for 60 min with a magnetic stirrer. The hetero- Under continuous and vigorous stirring, 15 g of potas- geneous solution was transferred to a glass substrate and sium permanganate was added to the mixture. The was left at 60 °C for 72 h to allow for the thin film for- addition rate was carefully controlled to maintain the re- mation process. The dried alginate/sulfonated graphene action temperature at 20 °C. The mixture was then oxide membrane was then crosslinked using a calcium stirred and left overnight at room temperature, followed chloride/glycerol solution to increase the mechanical by the addition of 180 ml of water under vigorous stir- strength and to reduce the hydrophilic properties of al- ring and reflux at 98 °C for 24 h; this caused the solution ginate. The membrane was immersed for 30 min in to turn a yellow color. Eighty milliliters of 35% H O 100 ml of crosslinking solution whose cation concentra- 2 2 was added to the reaction mixture, which was allowed to tion was maintained at 1.5% w/v. Finally, any free cations cool to room temperature in order to quench the reac- were removed from the membrane surface by washing tion with KMnO . The resulting GO was washed by with DI water, and the membrane was dried at 25 °C. rinsing with 5% HCl followed by centrifugation. Finally, The preparation method is summarized in Scheme 1. the product was rinsed with DI water several times, fil- tered and dried under vacuum conditions. Membrane Characterization Fifty milliliters of graphene oxide was added to 8 ml of The Fourier transform infrared (FTIR PERKIN ELMER) a 0.06 M sulfanilic acid solution at 70 °C. With continu- spectra of graphene oxide, sulfonated graphene oxide, and ous stirring, 2 ml of sodium nitrite solution was added the membrane were analyzed. The FTIR wavelength was −1 dropwise to the mixture and allowed to stand for 12 h at in the range of 4000–500 cm . The microstructure of the a constant temperature of 70 °C. After the reaction was film membranes was examined using a field emission complete, the mixture was washed and collected by cen- scanning electron microscope (FEI QUANTA 400 trifugation. The collected SGO was washed several more FESEM) with an operating voltage of 5 kV as a precaution Scheme 1 Sulfonated graphene oxide (SGO) filler and SA/SGO biomembrane preparation method Shaari et al. Nanoscale Research Letters (2018) 13:82 Page 4 of 16 for the bio-material-based sample. The high-resolution thickness, and R is the resistance of the membrane, transmission electron microscopy (HRTEM) analysis was similar to the method in previous works [38, 39]. carried out using o Digital TEM HT7700 operated at an Two tank liquid permeability cells with 20 v/v% accelerating potential of 300 kV. methanol were used to determine the methanol perme- Samples were prepared on grids with lacey carbon ability of the membrane. The differences in the concen- support film. XPS was used to determine the chemical tration of methanol result in methanol crossover composition of the sample surface using an Axis Ultra through the membrane, and methanol permeability can DLD. The mechanical strength of the SA/SGO be determined. Equation 3 is used to calculate the per- membrane was tested with a Universal Testing Machine, meability of methanol: including tensile strength, Young’s modulus, and elong- ation at break. The load used was 3 kN at room 1 ΔCbðÞ t LVb P ¼ ð4Þ temperature. Changes in the weight and length (or thick- Ca Δt A ness) of wet and dry membranes can determine the rate of water absorption and the swelling ratio of the where P is the membrane diffusion permeability for 2 −1 membrane. Typically, the membrane was soaked in methanol (cm s ), C is the methanol concentration in −1 water for 2 days at 30 °C. For the wet membrane, the the feed chamber, i.e., cell A (mol L ), ΔCb(t)/Δt is the weight and length were recorded, and then, the water in methanol molar concentration variation in cell B as a −1 the membrane and the liquid droplets on the surface of function of time (mol L s), V is the volume of each the membrane were removed. In addition, the moist diffusion reservoir (cm ), A is the membrane area, and L membrane was dried under vacuum pressure and is the membrane thickness (cm). temperature of 120 °C for at least 24 h. The weight and The membrane characteristics can be determined by length of the membrane in the dry state were also re- calculating the selectivity of the membrane, which can corded. Using Eqs. 1 and 2, water intake (WU%) and be achieved by high proton conductivity and low metha- swelling ratio (SW%) can be determined, where L rep- wet nol permeability. The formula used for calculating the resents the wet mass and L represents the dry mass dry selectivity is as follows: obtained from the length of wet and dry membranes, respectively. σ φ ¼ ð5Þ mass −mass wet dry WU% ¼  100 ð1Þ mass dry where φ represents selectivity, σ represents ionic conductivity, and P represents methanol permeability. L −L wet dry SW% ¼  100 ð2Þ dry Results and Discussion Characterization of Sulfonated Graphene Oxide (SGO) and The methanol uptake calculation is the same as the SA/SGO Biomembrane water uptake calculation, except that the solution for The FTIR spectra in Fig. 1a, b show the difference immersion is changed to methanol rather than DI water. between GO and SGO, which can be clearly observed. The proton conductivity of the prepared membrane Figure 1b is the magnification of Fig. 1a to obtain a was calculated using a four-electrode conductivity cell clearer view of the peaks in the SGO spectra. The connected to a potentiostat/galvanostat (WonATech) −1 spectrum of SGO shows a new band at 1244 cm , operating over a frequency range of 1 MHz down to which is the typical absorbance of a sulfonic acid group 50 Hz. The membranes (1 cm × 4 cm in size) must be (-SO H), whereas the GO spectrum does not contain soaked in water for 24 h for the conductivity readings this band [40]. In addition, the spectrum shows new under the fully hydrated state. The potentiostat was run −1 peaks at the wavelengths of 1012, 1036, and 1125 cm , to obtain the graph of voltage versus current. The which are considered to be the symmetric and asymmet- gradient of the straight line is the membrane resistance. ric stretching vibrations of SO H groups. This new Scheme 1 presents the cell of the proton conductivity spectrum reveals that the graphene oxide solution was test. The proton conductivity can be calculated using the successfully modified into sulfonated graphene oxide following formula: using the simple method described above. At the same time, the sulfonation modification still kept the σ ¼ ð3Þ functional groups in GO such as the hydroxyl group at RWT −1 −1 3319 cm and the carboxyl group at 1636 cm . Further where L is the distance between the two electrodes, W is confirmation of the presence of SO H groups can be the width of the membrane, T is the membrane determined by XPS analysis. Shaari et al. Nanoscale Research Letters (2018) 13:82 Page 5 of 16 Fig. 1 a, b FTIR spectra for graphene oxide (GO) and sulfonated graphene oxide (SGO) Figure 2 shows the XPS spectra of the GO and SGO membranes in which the scanning spectra are in the range of 0–800 eV to recognize the surface of the existing elements via a measurable analysis. It can be observed that the C1s and O1s signals appeared at 286 and 531 eV, respectively, in both the GO and SGO spectra. It is also noticed that after the sulfonic acid groups were introduced into GO, a new S2p peak appeared at 168 eV. Sulfonic groups in SGO contributed to a slightly increased intensity in the O1s spectra compared with that of GO. The high- resolution spectrum of C1s, which is referred to as Gaussian spectral deconvolution, confirmed that GO was successfully customized via chemical modification [41]. The figure inside Fig. 2bisthe S2pspectra for functionalized GO at a larger magnification. The binding energy of the sulfonic groups contributed to the appearance of the S2p peak at 168 eV, and this peak confirmed that sulfonic acid groups were successfully attached to the GO nanosheet backbone [41, 42]. The successful production of GO via the Hummer’s method was confirmed by the sheet-shaped GO morphology as shown in the FESEM image (Fig. 3a). Bai et al. [43] also generated GO with Hummer’s method. The results of their studies showed that the morpholo- gies of both GO and RGO appeared to be slightly folded and formed some wrinkles, which resemble the GO morphology in this study. The FESEM image of SGO in Fig. 3b, c has a crumpled and rougher surface compared with the surface of GO, which is most likely due to the effects of the sulfonation process, confirming that the modification method was also successfully applied [41, 44]. This correlates with the existence of a new peak in the FTIR Fig. 2 XPS of a, b wide spectra GO and SGO and c S2p spectra transmittance spectra, which belongs to the sulfonic of SGO group. Moreover, the presence of sulfonic groups was Shaari et al. Nanoscale Research Letters (2018) 13:82 Page 6 of 16 ac bd Fig. 3 a FESEM image of GO. b, c FESEM images of SGO with various magnification and d EDX of SGO also confirmed in the GO sheet via the XPS analysis. is similar to the previous study reported by Marrella et SGO was different from GO, which had a multi-layered al. [46]. structure without any aggregation. The applied modifica- The presence of hydrogen bonding interactions be- tion method leads to the formation of a layered and tween SGO and the alginate polymer matrix is shown by restacked structured; thus, SGO demonstrated its flexi- the FTIR analysis. The FTIR results for the alginate and bility. The energy dispersive X-ray (EDX) result presents SGO alginate membranes are shown in Fig. 6. A slight that 1.76 wt% of sulfur element exists in the SGO sheets shift seems to occur for the hydrogen bonding site (Fig. 3d). spectra according to the hydrogen bond interactions. The surface image and cross section of the SA and The O-H group bands in the alginate membrane −1 SA/SGO bio membranes are shown in Fig. 4. Figure 4a– appeared at 1413 and 3440 cm ; however, the bands −1 c is a surface image, and Fig. 4d–f is a cross-sectional were shifted to 1406 and 3404 cm in the SA/SGO image of membranes with different SGO contents. Both membrane due to the hydrogen bonding among the low and high enlargements show that the SGO sheet is polar groups in SGO and the O–H groups in alginate completely dispersed homogeneously in the overall poly- [45]. The C=O group bands in the alginate membrane −1 mer matrix and is guided by intermolecular interactions; also shifted to 1046 from 1082 cm . The location of it is recognized that hydrogen bonds occur between the thesulfonicgroup (–SO H) bands in the alginate −1 sulfonic acid groups in SGO and polar groups (-O-, C = membrane also changed from 1284 to 1277 cm . O) in the SA/SGO membrane [45]. SGO is placed in the Thus, the results show that there is hydrogen bonding polymer matrix to function as a barrier to methanol between the SGO and alginate [47]. A complete dis- molecules. The image for SA/SGO6 looks better with persion of the SGO particles throughout the polymer the full spread to the entire sodium alginate polymer matrix can facilitate the proton conduction path in all matrix. Figure 5 is a TEM image for the composite directions of the membrane. As a result, the proper- formed in which the SGO nanosheets are well distrib- ties of SA/SGO membranes were assumed superior to uted in the sodium alginate polymer matrix. Sodium al- those of the pristine alginate membranes according to ginate exists in the nanosphere particle structure, which theSEMinterior structureand theFTIR spectra. Shaari et al. Nanoscale Research Letters (2018) 13:82 Page 7 of 16 a d b e c f Fig. 4 FESEM images of surface morphology and cross-section for a, d sodium alginate, b, e SA/SGO4, and c, f SA/SGO6 biomembranes Thermal Stability and Mechanical Properties Figure 7 shows the comparison of TGA analysis for all SA/SGO biomembranes with different contents of SGO. Losses at the first stage occurred below 200 °C due to the release of water molecules, which is known as the evaporation process. Generally, thermal decomposition of GO is at a temperature of approximately 200 °C due to the decomposition of the oxygen labile group, while for alginate polymers, heat decomposition at the first stage is at 178 °C [48, 49]. The SA/SGO biomembrane shows a heavy loss at a higher temperature of 198 °C. This increased temperature indicates that there is an interaction between sodium alginate and SGO, which in- creases the heat resistance for SA/SGO biomembrane. This shows that the presence of SGO has increased the thermal stability of the biomembrane due to favorable interfacial interactions, such as hydrogen-bonding or electrostatic interactions between the sodium alginate matrix and sulfonated graphene oxide nanosheets, thus making this membrane fit for DMFC application. The Fig. 5 TEM image of SGO nanosheets distributed in sodium alginate second stage of weight losses occurs at a temperature of polymer matrix 250 °C due to the decomposition of the sodium alginate Shaari et al. Nanoscale Research Letters (2018) 13:82 Page 8 of 16 Fig. 6 FTIR spectra of SA and SA/SGO membrane side-chain. The third stage (> 400 °C) involves the The Nafion membrane has a higher tensile stress process of decomposition of the polymer backbone [50]. compared to the SA/SGO6 biomembrane. However, it is Figure 8 presents the tensile stress and elongation at comparable between biomembrane categories. The gra- break of the membrane as the wt% of SGO varied. From phene oxide itself has very good mechanical properties, 0.02 to 0.13 wt% of SGO, the tensile stress increased and with an elastic modulus of 1100 GPa and an intrinsic then slightly decreased at 0.17 wt%. This might be attrib- strength of 125 GPa; this is the primary reason why uted to the restacking of graphene oxide sheets, which SGO can increase the mechanical properties of the al- can be related to the van der Waals forces in the GO ginate membrane [45]. nanosheets. The bulk of graphene oxide nanosheets Moreover, the formation of hydrogen bonds between leads to sliding and reduces the effect of graphene oxide SGO and the pure alginate matrix polymer can also re- in improving the mechanical properties of the mem- sult in good mechanical properties. A greater formation brane. The tensile stresses of Nafion and other biomem- of hydrogen bonding results in a stronger interfacial ad- branes in previous studies are listed in Table 1 [51–55]. hesion, consequently improving the mechanical strength Fig. 7 TGA curve for SA/SGO biomembranes with various SGO wt% Shaari et al. Nanoscale Research Letters (2018) 13:82 Page 9 of 16 Fig. 8 Tensile stress and elongation at break of biomembrane with various SGO wt% of the membrane. The elongation at break pattern is in results of water uptake and methanol uptake of the SA/ contrast to the tensile stress pattern. A lower tensile SGO membrane with varying SGO wt% values. As pre- stress results in a higher elongation at break percentage. sented, the SA/SGO membrane has a lower water Elongation at break indicates to what extent the uptake capacity with different contents of sulfonated GO membrane film can be stretched until the maximum (lowest WU - 57.9% by SA/SGO6) in the membrane point, which is also known as flexibility. Table 1 com- compared with pure alginate. An increasing amount of pares several membranes from previous studies with the SGO reduces the water uptake due to its blocking ability membrane of the current study in terms of elongation at as a filler in the membrane [5]. The addition of SGO break [51–56]. The different patterns between tensile facilitates the contraction of ionic pathways, thus hinder- stress and elongation at break are logical. As mentioned ing the movement of water and methanol. A higher above, the presence of SGO in the membrane increases SGO content results in a stronger barrier for the water the interfacial linkage due to the hydrogen bonding, thus absorption of the membrane. The hydrogen bonding be- reducing the flexibility of the membrane. tween the SGO filler and the sodium alginate polymer strengthens the interfacial adhesion of the membrane Liquid Uptake and Swelling Ratio of Membrane composite, thus reducing the water uptake capacity [19]. It is acknowledged that water is the prominent compo- The hydrogen bonding formation in the SA/SGO mem- nent in the proton exchange membrane because it acts brane involves the –OH groups in GO, the –O- and as a proton conductor in which the adsorbed water C=O groups on the SA chains, and contributions by facilitates proton transport [39]. Figure 9 presents the sulfonate groups (–SO H) [3, 19]. Similar to the pattern Table 1 Thickness, IEC, proton conductivity, methanol permeability, and membrane selectivity of SA/SGO composite biomembrane with different SGO content Sample Thickness IEC Proton conductivity Methanol permeability Selectivity Reference −1 −1 −7 2 −1 4 −3 (μm) (meq g ) (σ,mScm ) (P, × 10 cm s ) (SP × 10 Ss cm ) SA 198 ± 1 0.25 ± 0.03 6.36 ± 0.1 1.687 ± 0.22 3.7678 Current study SA/SGO1 200 ± 1 0.31 ± 0.05 6.63 ± 0.4 2.657 ± 0.39 2.495 Current study SA/SGO2 201 ± 2 0.34 ± 0.03 7.18 ± 0.5 2.453 ± 0.27 2.927 Current study SA/SGO3 203 ± 1 0.38 ± 0.02 7.75 ± 0.2 2.351 ± 0.23 3.296 Current study SA/SGO4 200 ± 1 0.45 ± 0.05 9.30 ± 0.1 2.045 ± 0.25 4.547 Current study SA/SGO5 201 ± 3 0.48 ± 0.03 10.6 ± 0.1 1.738 ± 0.23 6.098 Current study SA/SGO6 205 ± 2 0.56 ± 0.05 13.2 ± 0.1 1.535 ± 0.24 8.555 Current study Nafion 117 – 0.86 0.098 12.3 7.967 [28] Nafion 117 –– 0.081 20 4.05 [29] Nafion 117 –– 0.1056 25 4.22 [30] Shaari et al. Nanoscale Research Letters (2018) 13:82 Page 10 of 16 Fig. 9 Liquid uptake and swelling ratio of SA/SGO membrane with wt.% of SGO of the water uptake result, the methanol uptake of the Arrhenius behavior, the ion transport activation energy SA/SGO membrane also decreased with increasing SGO E of the SA/SGO membranes can be obtained wt% in the membrane. The presence of the same trend according to the Arrhenius equation: shows that there was good networking and bonding between SGO and the alginate polymer, which impeded E ¼ −bxR fuel crossing. From the experimental result, the presence of graphene oxide-based materials lowered the water where b is the slope of the line regression of ln σ (S/cm) −1 uptake capacity of the SA membrane and maintained its vs. 1000/T (K ) plots, and R is the gas constant −1 −1 mechanical strength. The swelling ratio decreased from (8.314472 JK mol ). The ion transport activation 106% to 61.12% with increasing SGO wt% in the alginate energy of the SA/SGO6 composite membrane is −1 polymer matrix (Fig. 9) due to the blocking effect [10]. 8.17 kJ mol , which is slightly greater than the E of −1 The strong hydrogen bonding also diminished the Nafion® 115 (6.00 kJ mol )[57] and lower than that of −1 pathways for absorbance of the ionic group into the Nafion 117 (12 kJ mol )[58]. This can be attributed to polymer [32]. the hydrophilic properties of the sodium alginate matrix, which provide high water content, and the introduction IEC, Proton Conductivity, Methanol Permeability, and of SGO still allows this property to remain due to the Selectivity hydrophilic properties of oxygenated functional groups. Ion exchange capacity (IEC) calculation is important The abundant water forms a continuous transferring since it is responsible for measuring the number of channel and makes the movement of ion easy. milliequivalents ions in 1 g of the prepared membranes Figure 13a presents the suggested proton mobility and is an indicator for proton conductivity in DMFCs. mechanism in SA/SGO plasticized with glycerol in Table 2 shows the IEC values of the membranes. A which high synchronization exists between H and elec- higher IEC value is achieved by the SA/SGO membrane tron lone pairs belonging to the oxygen atoms carrier in containing a higher wt% of SGO. This is due to the glycerol and the hydrophilic sulfonic acid groups in function of sulfonic acid groups in the SGO nanosheets. SGO nanosheets. We believe that the proton transport An increment in the IEC value increases the proton applies both Grotthus and vehicle mechanisms, conductivity value of the SA/SGO biomembrane. The strengthened by the SGO particles. proton conductivities of the SA/SGO membrane versus The SA/SGO biomembranes show very low methanol temperature are presented in Fig. 10. Increasing the permeability, and the lowest was achieved by SA/SGO6 −7 2 −1 temperature leads to the enhancement of proton (1.535 × 10 cm s ), as listed in Table 2. The low conductivity. The SA/SGO membrane features a consist- methanol permeability can be explained in terms of the ently increasing pattern in proton conductivity as the membrane microstructure between sodium alginate, SGO particle amount increases, with a maximal value of SGO, and glycerol plasticizer. The introduced SGO −1 13.2 mS cm at 0.2 wt% of SGO loading at temperature particles serving as fillers in the SA polymer create of 30 °C. The ln σ vs. 1000/T plot is also shown in substantial obstacles to the linked hydrophilic passages. Fig. 11. Assuming that the conductivity follows an The SGO filler blocks the migration of methanol passing Shaari et al. Nanoscale Research Letters (2018) 13:82 Page 11 of 16 Table 2 Comparison of condition in single-cell performance test with power density result for previous work and current study Membrane Anode catalyst loading Cathode catalyst loading Methanol feed concentration Temperature Pmax Mode Reference −2 −2 −3 −2 (mg cm ) (mg cm ) (mol dm ) (°C) (mW cm ) SA Pt-Ru:8 Pt:8 4 RT 2.8 Passive Current study SA/SGO6 Pt-Ru:8 Pt:8 4 RT 5.9 Passive Current study Nafion 117 Pt-Ru:8 Pt:8 4 RT 6.6 Passive Current study Nafion 117 Pt-Ru:8 Pt:8 2 RT 7.2 Passive [28] Alginate- Pt-Ru:5 Pt:5 2 50 10.4 Active [31] carrageenan through the membrane, and this is known as the It was noticed that a higher selectivity value resulted blocking effect, which reduces the methanol permeabil- in a higher DMFC capability. The selectivity values of ity. The methanol permeability also decreases because of the SA/SGO can be observed in Table 2, which the interfacial interaction between the SGO and SA compares the selectivity among SA and SA/SGO biopolymer [41]. The methanol permeability of the SA/ biomembranes as well as Nafion 117 membranes from SGO6 bio membrane at four different temperature previous work. The presence of SGO enhanced the conditions is shown in Fig. 12. As seen, the methanol selectivity of the SA/SGO polymer membrane (8.555 × 4 −3 permeability increases at a higher temperature, which 10 Sscm for 0.2 wt% SGO loading), which is higher 4 −3 can be related to the structure changes of the bio than that of SA (3.7678 × 10 Sscm ) and fortunately 4 −3 membrane. The higher temperature provides more heat, also higher than that of Nafion 117 (7.99 × 10 Sscm ) 4 −3 4 −3 which can shake the membrane chains and molecules, [38], 4.05 × 10 Sscm [61], and 4.22 × 10 Sscm thus leading to more free volume, which consequently [62], in which the low methanol permeability is the main reduces the methanol blocking effect. Less resistance factor to be considered. causes easier movement of methanol diffusion [59]. Mu et al. [60] reported the decrease in methanol crossover Single Cell in the presence of Au nanoparticles self-assembled on a Single-Cell Performance Evaluation Nafion membrane, which consequently improved the Figure 14 indicates the cell polarization result for pure overall performance. alginate, SA/SGO6 composite biomembrane and Nafion The interfacial interaction between SGO filler, glycerol, 117 under ambient temperature, 4 M methanol concen- and SA polymer confines the hydrophilic passage tration and passive mode condition. The SA/SGO6 com- formation in the membrane, and this wide hydrophilic posite biomembrane was applied due to the high passage is a significant factor in methanol migration selectivity factor and obviously had a higher open-circuit [19]. Thus, the presence of SGO facilitates methanol voltage (0.63 V), which can be related to the low permeability reduction [6]. The proposed mechanism of methanol permeability equaling to that the sodium methanol rejection is presented in Fig. 13b. alginate biomembrane. The OCV of Nafion 117 (0.52 V) Fig. 10 Proton conductivity of SA/SGO biomembranes with various content of SGO at different temperature Shaari et al. Nanoscale Research Letters (2018) 13:82 Page 12 of 16 Fig. 11 ln σ vs. 1000/T plot for the cross-linked QAPVA membranes, the lines indicate the linear regression in the current study is lower than SA/SGO and sodium Nafion achieves a better performance in DMFC applica- alginate, which might be due its higher methanol perme- tion due to the excellent proton conduction. However, ability. The crossing of methanol through the membrane the power density performance between Nafion 117 and leads to the reduction in the OCV value. The higher SA/SGO biomembranes does not show a big difference OCV of SA/SGO and alginate membrane is the big indi- quantitatively. Hence, SA/SGO can be an alternative cator that synthesized membrane has lower methanol membrane for DMFC in the future. However, the prop- permeability compared to Nafion, which the main erties of the membrane still need to be enhanced, and objective of this study is successfully achieved. The higher wt% of SGO filler can probably be used to obtain improvement in the power density of SA/SGO6 is due a higher power density. To the best of our knowledge, to the sulfonic acid group that functions as a proton there is only one previous work by Pasini Cabello et al. transferral pathway as well as a methanol inhibitor, thus that has examined the single-cell performance in DMFC −2 achieving 5.9 mW cm compared to the sodium application using an alginate biopolymer-based −2 alginate, which achieved only 2.83 mW cm . However, membrane [18]. They tested an alginate/carrageenan Nafion 117 achieved a higher power density, which was membrane at temperatures of 50, 70, and 90 °C in 2 M −2 6.62 mW cm . Thiam et al. [38] reported the perform- methanol concentration in the active mode, which ance of Nafion 117 membrane under the same condition achieved maximum power densities of 10.4, 13.9, and −2 2 with a power density of 7.95 mW cm . No doubt, 17.3 mW/m , respectively. The active mode has an Fig. 12 Methanol permeability of membrane SA/SGO6 vs. temperature Shaari et al. Nanoscale Research Letters (2018) 13:82 Page 13 of 16 Fig. 13 Suggested mechanism of a proton mobility and b methanol rejection advantage due to the continuous flow of the methanol Conclusions feed into the cell that allows the reaction to occur In conclusion, a membrane with low methanol continuously and thus is capable of achieving a higher permeability, high proton conductivity, and high select- power density. The higher power density could be ivity was successfully prepared through the simple achieved at a higher temperature due to the higher technique known as the blending method. The presence number of activated protons. Nevertheless, this work is of sulfonated graphene oxide enhanced the properties of an indicator that biopolymer-based membrane has a big the alginate-based polymer membrane in terms of potential that can be explored and applied in DMFC proton conductivity and methanol permeability. The systems. sulfonate groups facilitated the networking between the Shaari et al. Nanoscale Research Letters (2018) 13:82 Page 14 of 16 Fig. 14 Single-cell performance test for sodium alginate, SA/SGO6, and Nafion 117 (4 M methanol and 25 °C temperature, passive mode) alginate polymer and the graphene oxide filler. The Authors’ Contributions SKK had contributed the idea of the project. NS had performed the experimental blocking effect of SGO also reduced the methanol cross- work and wrote-tp this manuscript. SKK proof read and submiited this manuscript over in the membrane. The primary weaknesses of the for publication as corresponding author. SB, LKS, SM, ND co-supervised NS and alginate polymer, which are its mechanical properties of helped in analysis data. All the authors read and approved the final manusript. tensile strength and elongation at break, were also Authors’ Information improved by the addition of SGO into the polymer Mrs. Norazuwana Shaari—Student of Fuel Cell Institute, Universiti matrix. The presence of SGO improved the SA/SGO Kebangsaan Malaysia, Malaysia. membrane to a high level comparable to commercial Prof Ir. Dr. Siti Kartom Kamarudin—Director Of Fuel cell Institute, Universiti Kebangsaan Malaysia, Malaysia. membranes. Dr. Sahriah Basri—Fellow of Fuel Cell Institute, Universiti Kebangsaan Malaysia, Malaysia. Abbreviations Dr. Loh Kee Shyuan—Fellow of Fuel Cell Institute, Universiti Kebangsaan BC: Bacterial cellulose; CNC: Cellulose nanocrystal; CNFs: Cellulose nanofibers; Malaysia, Malaysia. CNT: Carbon nanotube; DI: Deionized; DLFC: Direct liquid fuel cell; Dr. Shahbudin Masdar—Associate Fellow of Fuel Cell Institute, Universiti DMFC: Direct methanol fuel cell; EDX: Energy dispersive X-ray; FESEM: Field Kebangsaan Malaysia, Malaysia. emission scanning electron microscope; FTIR: Fourier transform infrared; Dr. Darman Nordin—Associate Fellow of Fuel Cell Institute, Universiti GO: Graphene oxide; GOS: Graphene oxide sheet; HRTEM: High-resolution Kebangsaan Malaysia, Malaysia. transmission electron microscopy; IEC: Ion exchange capacity; L: Distance between the two electrodes; OCV: Open circuit voltage; P: Membrane Competing Interests diffusion permeability for methanol; PEMFC: Polymer electrolyte membrane We confirm that the work described has not been published before; it is not fuel cell; PEMs: Proton exchange membrane; PMA: Phospho molybdic acid; under consideration for publication anywhere else; and publication has been PSSA: Poly-styrene sulfonic acid; PVA: Poly vinyl alcohol; PVP: Poly (vinyl approved by all co-authors and the responsible authorities at the institute(s) pyrrolidone); R: Resistance of the membrane; RGO: Reduced graphene oxide; where the work has been carried out. The authors declare that they have no SA: Sodium alginate; SA/SGO: Sodium alginate/sulfonated graphene oxide competing interests. membrane; SGO: Sulfonated graphene oxide; SHNT: Sulfonated halloysite nanotube; SPSF: Sulfonated polysulfone; SW%: Swelling ratio percentage; T: Membrane thickness; TGA: Thermal gravimetric analysis; W: Width of the Publisher’sNote membrane; WU%: Water uptake percentage; XPS: X-ray photoelectron Springer Nature remains neutral with regard to jurisdictional claims in spectroscopy published maps and institutional affiliations. Acknowledgements Received: 5 October 2017 Accepted: 2 March 2018 The authors gratefully acknowledge the financial support given for this work by the Ministry of Education (MOE)-MALAYSIA under GSP/1/2015/TK01/UKM/ 01/1 and Universiti Kebangsaan Malaysia under DIP-2017-021. References 1. 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Nanoscale Research LettersSpringer Journals

Published: Mar 13, 2018

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