Review of the Stateoftheart of the Methanol Crossover in Direct Methanol Fuel Cells
Nanomaterials (Basel). 2020 Sep; 10(nine): 1641.
Composite Proton Exchange Membranes Based on Chitosan and Phosphotungstic Acid Immobilized One-Dimensional Attapulgite for Direct Methanol Fuel Cells
Received 2020 Jun 30; Accustomed 2020 Aug 19.
Abstract
In order to obtain biopolymer chitosan-based proton substitution membranes with excellent mechanical properties as well as high ionic conductivity at the same time, natural attapulgite (AT) with ane-dimensional (1D) construction was loaded with a stiff heteropolyacid and also a super proton conductor, phosphotungstic acid (PWA), using a facial method. The obtained PWA anchored attapulgite (WQAT) was and so doped into the chitosan matrix to prepare a series of Chitosan (CS)/WQAT composite membranes. The PWA blanket could improve the dispersion and interfacial bonding between the nano-additive and polymer matrix, thus increasing the mechanical strength. Moreover, the ultra-strong proton conduction ability of PWA together with the interaction between positively charged CS bondage and negatively charged PWA can construct effective proton ship channels with the help of 1D AT. The proton conductivity of the composite membrane (4 wt.% WQAT loading) reached 35.iii mS cm−one at lxxx °C, which was 31.8% higher than that of the pure CS membrane. Moreover, due to the decreased methanol permeability and increased conductivity, the blended membrane with iv% WQAT content exhibited a peak power density of 70.26 mW cm−2 fed at 2 Grand methanol, whereas the pure CS membrane displayed only xl.08 mW cm−2.
Keywords: chitosan, attapulgite, phosphotungstic acid, proton commutation membranes, fuel cells
one. Introduction
Directly methanol fuel cell (DMFC) is a highly efficient and eco-friendly energy conversion device that can straight convert chemic free energy into electric free energy by using liquid methanol every bit the anode fuel. Moreover, when compared with proton exchange membrane fuel cells (PEMFCs) using gaseous hydrogen every bit the fuel, DMFCs have a wider application prospect especially in the fields of portable devices and vehicles because they tin fully employ the existing infrastructure for gasoline [i,2,iii]. As a key component in a DMFC, the proton exchange membrane (PEM) plays an of import role of a separator of anode and cathode as well every bit a deliverer of protons [4,5]. PEMs must possess high proton conductivity, excellent methanol barrier power, and good thermal and mechanical backdrop. Upwards to now, perfluorosulfonic acid (PFSA) membranes (represented by Dupont'south Nafion) are known every bit currently commercial PEMs in PEMFCs for their relatively high proton electrical conductivity and adept chemical and electrochemical stabilities [6]. All the same, the high methanol permeability of PFSAs is a big obstacle when utilizing PFSAs as PEMs in DMFCs. In addition, the very high toll and limited operation temperature of PFSAs also bring adverse effects on their widespread application in DMFCs [7]. Therefore, numerous researchers accept turned to seek cost-effective and loftier-performance alternatives to PFSAs.
In contempo years, chitosan (CS), a natural polymer, has attracted wide attention as a PEM cloth for its abundance in nature, environmental friendliness and low cost [eight,9]. As the only degradable polycationic bio-polysaccharide in nature, CS is a deacetylated product of chitin and has been practical to various fields such as food chemistry, biomedicine, cosmetics, cloth, papermaking and so on. CS has a large number of amino and hydroxyl groups, and good ability of alcohol–h2o separation, which has preferential permeability to water under the condition of alcohol resistance. So, CS is as well a skilful candidate of PEMs to solve the trouble of methanol permeation [viii]. In the process of membrane preparation, the hydrophilic groups of CS generate strong intra molecular and intermolecular hydrogen-bonding interactions, which drive CS chains to form numerous crystalline regions and cantankerous-linked networks. On the one paw, the loftier crystallinity packs the CS chains and thus decreases the costless volume cavities (only 0.56 nm), which tin can effectively prevent methanol penetration through the CS membrane [10], but on the other hand highly crystallized CS has very low proton conductivity because the ion conduction in a PEM mainly occurs in the amorphous phase rather than crystalline phase [8,ix]. Additionally, the poor mechanical properties of CS are also a concern when using CS-based PEMs operation in fuel cells. To improve the mechanical properties and proton conductivity, many efforts have been taken to modify CS. For example, chemical modification [11,12] (e.g., sulfonation, phosphorylation, chemic cross-linking) and concrete modification [13,14,15,16] (eastward.g., blending with other polymers, organic-inorganic hybrids). Among these methods, training of organic-inorganic composites is considered equally a facile and efficient approach to solve the bug facing CS because this method can combine both their merits and sometimes generate the synergistic effect.
Many inorganic nano-additives such every bit 0-dimensional (0D) nanoparticles [17,18,nineteen] (e.thousand., silica, titania, zirconia, zeolites, silicon-aluminum oxides), one-dimensional (1D) nanotubes or nanorods [14,15,sixteen,20,21,22] (e.k., carbon nanotubes, halloysite tubes) and 2-dimensional (2D) nanoplates [23,24] (e.thou., montmorillonite, grapheme oxide) take been extensively applied to modify polymers to improve their thermal and mechanical stabilities while endowing them with some new properties due to the synergistic result. Amidst those inorganic nanomaterials, 1D nanotubes or nanorods stand up out for their unique anisotropic 1D shape because such nanomaterials tin contribute to construct better aqueduct-like ion transport pathways in the composites for proton conduction [25,26,27]. By and large, nanomaterials including 1D nano-additives demand to be surface modified to improve the compatibility between nanomaterials and polymer matrix, and thus, can fully play their functions. Wen and co-workers designed and prepared functionalized carbon nanotubes (CNTs) with different surface coating substances (such as chitosan, sulfate zirconia, SiO2, TiO2, organic long-chain ions) to change CS to fabricate a series of composite membranes [14,15,16,26,28]. These surface coating materials tin non only promote the dispersion of CNTs, and thus, fully play the reinforcement role of CNTs, but also improve the proton conductivity due to the newly formed proton conducting network along the surface of functionalized CNTs. Autonomously from 1D CNTs, 1D clay nanotubes or nanorods have also been used every bit an constructive additive in the field of PEMs. Wang et al. [21] synthesized halloysite nanotubes bearing sulfonated polyelectrolyte brushes (SHNTs) via distillation-precipitation polymerization and and so added into CS matrix to prepare nanohybrid membranes. The SHNTs is helpful to create continuous pathways along the nanotube, thus generating acid-base of operations pairs at SHNT-CS interface, which tin work every bit depression-barrier proton-hoping sites. Moreover, the wider pathways could form with the help of the long brushes on SHNTs in the CS matrix.
Recently, phosphotungstic acid (PWA), recognized as a potent heteropolyacid and a super proton conductor, was immobilized onto the surface of poly (vinylidene fluoride) (PVDF) electrospun nanofibers to obtain a new three-dimensional proton conducting network [29]. Afterward impregnating with CS, which can tightly anchor PWA to avoid its leaching out from PEMs in water or methanol, the resulting composite membrane demonstrated ultrahigh proton conductivity and satisfactory fuel cell functioning. Inspired by that, herein, we utilized PWA equally a surface modification material to glaze 1D natural clay attapulgite (AT) with a theoretical formula of SiviiiMg5O20(OH)2(H2O)four·4HtwoO [30]. Information technology should exist mentioned that AT is cheap and abundant in nature when compared with other carbon nanomaterials [23,25]. Then, the obtained PWA-decorated AT was added into the CS matrix to fabricate novel organic-inorganic composite membranes. Expectedly, the PWA coating could amend the dispersion and interfacial bonding betwixt the nano-additive and polymer matrix, thus increasing the mechanical strength. Moreover, the ultra-potent proton conduction power of PWA together with the interaction between positively charged CS chains and negatively charged PWA can construct effective proton send channels with the help of 1D AT. The effects of PWA-coated AT on the structure and properties of CS were evaluated. The unmarried directly methanol fuel cell performance was also tested.
two. Materials and Methods
2.i. Materials
Attapulgite clay was obtained from the Huaiyuan Mining Industry Co. (Huai'an, Jiangsu Province, Red china). Chitosan (M w = 500 kDa) with a degree of 90% deacetylation was purchased from Zhejiang Aoxing Biotechnology CO., Ltd. (Yuhuan, Zhejiang Province, China), DC5700 [(CHiiiO)3Si(CH2)3Northward+(CH3)ii(C18H37)Cl−] was bought from Green Chem. International Co., Ltd. (Shanghai, Red china). BaCl2 was purchased from Annaiji Chemical Agent CO., Ltd. (Beijing, China). Ethanol (A.R.), acetic acid (A.R.), PWA (A.R.), and ammonia (A.R.) were supplied past Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Sulfuric acrid (98%) was supplied by Kaifeng Shenma Group Co., Ltd. (Kaifeng, Communist china).
2.2. Synthesis of Phosphotungstic Acid Immobilized Attapulgite
Phosphotungstic acrid immobilized on attapulgite (WQAT) was prepared using the following main three steps: Firstly, AT was ultrasonically dispersed in a H2SO4 solution (2 M) at 60 °C for two h. The suspension was filtered, followed by rinsing with deionized water until SO4 ii − was undetectable past a BaCl2 solution. The obtained sample was stale and gained acrid-activated AT. Secondly, 2 g of acid-activated AT was dispersed into the mixture of ethanol and deionized water (1:one, volume ratio), and ultrasonically oscillated for 30 min. After adjusting the pH to ten–12 using NaOH solution, 15 mL of DC5700 was added to the mixture under ultrasonic dispersing at seventy °C for 120 min, then slowly stirred for 12 h. The mixture was filtered, repeatedly done by h2o and dried at 80 °C to obtain quaternized AT (QAT). Finally, WQAT was obtained past immersing QAT in a PWA solution (v mg mL−1) for 24 h and then washed by deionized water to remove the residual PWA, and dried in a vacuum oven at lx °C overnight.
2.iii. Grooming of CS-Based Composite Membranes
An corporeality of 0.7 yard of CS was dissolved in a xx mL of acerb acid aqueous solution (2% v/five) and stirred at room temperature. Simultaneously, a certain amount of nanoparticle was dispersed into a 10 mL of acetic acid solution with ultrasonic treatment for thirty min. And then, these ii solutions were mixed together and stirred vigorously for another 24 h. The resultant homogenous mixture was bandage onto a articulate glass plate and dried at 40 °C for 24 h to obtain blended membranes. To easily uncover the membranes and remove the remained acetic acid, the membrane was immersed in a NaOH solution for 2 h and and then rinsed with deionized h2o until the pH value was 7. Subsequently, the membrane was ionic cross-linked by immersion in a H2SOiv solution (2 M) for 24 h and and so extensively washed with water to remove the residual HiiSo4. Finally, the membrane was stale nether vacuum at 30 °C for 24 h. The obtained blended membranes are named as CS/WQAT-ten, where x is the mass percent of the WQAT in the CS matrix. The average thickness of the dry membranes falls in the range of 50–55 μm.
2.4. Characterization
Nicolet 380 Fourier transform infrared spectrometer (Thermo Electron Co., Waltham, MA, USA) and a X-ray photoelectron spectroscopy (XPS, ThermoFisher Scitific Co., Waltham, MA, United states) were used to character the chemical structure of AT and modified AT samples. The thermal backdrop of WQAT and composite membranes were conducted past thermo gravimetric analysis (TGA). TGA was carried out on a STA 499F instrument (NETZSCH Co., Deutschland) from 30 to 800 °C with a heating charge per unit of x °C min−one under nitrogen atmosphere after beingness kept for 5 min at 130 °C to remove the absorbed water. The thermal transition beliefs of the membranes was tested on a differential scanning calorimeter (DSC, NETZSCH Co., Selb, Germany). The samples were get-go preheated from room temperature to 130 °C with 10 °C min−ane nether nitrogen atmosphere, then cooled to ninety °C, and reheated to 260 °C. In lodge to verify the dispersion of nanorods in the blended membranes, Scanning Electron Microscope (JSM-6510, Electronics Co., Fukuoka, Japan) was used to discover the surface and cross-sections of the membranes at 5 kV. The membrane sample was freeze-fractured in liquid nitrogen previously and so sputtered with gold. The mechanical properties of the hybrid membranes (one.0 × iv.0 cm) were investigated by a universal tensile testing automobile (AG-IC 5KN, Shimadzu Co., Kyoto, Japan) using an elongation rate of ii mm min−1 at room temperature.
Water uptake and swelling ratio of the membranes were carried out through measuring the changes in the weight and area of the samples in dry out and wet conditions. After weighing the weight (Wdry) and recording the size (Adry), the dry out membrane was soaked in deionized h2o for 24 h at 80 °C and so recorded the weight (Wwet) and size (Amoisture) of the moisture sample. H2o uptake and swelling ratio of the membrane are calculated co-ordinate to the post-obit equations:
(one)
(ii)
Proton electrical conductivity was obtained by electrochemical impedance spectroscopy (EIS) on an electrochemical station (Autolab PGSTAT 302N, Netherland) with voltage amplitude of 5 mV in the frequency range of 1 Hz–one MHz. Initially, all the membranes were fully hydrated by immersing in deionized h2o for 24 h, and then put onto a dwelling-made ii-electrode mould with platinum wires every bit electrodes. Afterward that, the measurements were carried out in the temperature range of 20–80 °C. The calculation formula of proton conductivity (σ, Due south cm−1) is as follows:
where L (2.sixteen cm) is the distance between the two electrodes, and R and A are the resistance (Ω) and cantankerous-sectional area (cm2) of the membrane sample, respectively.
ii.five. DMFC Performance and Methanol Crossover Test
The DMFC single cell performance was evaluated at seventy °C with an agile area of iv cm2 (2 cm × 2 cm). The anode and cathode catalysts were Pt-Ru with a loading of 4.0 mg cm−2 and Pt of two.0 mg cm−2, respectively. The membrane was sandwiched betwixt the anode and cathode, and then hot pressed for 3 min at 125 °C and 2 MPa to obtain a membrane electrode assembly. Methanol solution (one mol L−ane H2And so4 solution in different ratio) was fed every bit a fuel into the anode at a menstruation rate of i mL min−1 past a peristaltic pump, and oxygen was fed into the cathode at a flow charge per unit of 90 mL min−1. The electric current density (I) and potential (V) of the fuel cells were recorded by an electrochemical workstation (Autolab PGSTAT 302N, Herisau, Switzerland) at a browse rate of five mA s−1.
Methanol crossover was tested using a voltammetric method. A methanol solution (1, two or 5 M) was fed at a flow rate of 1.0 mL min−i into the anode side of the MEA while the cathode side was kept in an inert N2 atmosphere at a period charge per unit of 90 mL min−ane. The whole examination setup was kept at a temperature of 70 °C. By applying a positive potential at the cathode side, the flux rate of permeating methanol was adamant by measuring the steady-country limiting current density resulting from the complete electro-oxidation at the membrane/Pt goad interface at the cathode side.
3. Results and Discussion
3.one. Synthesis and Characterization of WQAT
The synthesis process of WQAT could be divided into three steps as illustrated in Scheme 1: The showtime step is a conventional acid treatment, which can introduce some hydroxyl groups on the surface of AT. With the aid of these introduced -OH groups, DC5700, a long-chain silane coupling agent with quaternary ammonium groups, grafted onto the surface of AT to obtain QAT through typical hydrolysis and condensation reactions. In the post-obit step, with the AT nanorods and the surface grafted quaternary ammonium groups acting as a template and anchoring sites respectively, PWA, a heteropolyacid with strong acidity and have excellent proton ship ability, which can immobilize on AT through a strong acid-base of operations interaction between positively charged QAT and negatively charged PWA.
FTIR, TG, and XPS were used to characterize the synthesized WQAT powders. The chemical limerick of pristine AT and modified AT was confirmed past FTIR as shown in Figure onea. As for the FTIR spectra of AT, QAT, and WAQT, the three samples showed the characteristic absorption bands of attapulgite at 3400–3600 cm−ane, which correspond to the four hydroxyl structures of the hydroxyl stretching vibration of bonds to aluminum and/or magnesium, the hydroxyl stretching vibration of adsorbed water, and the angle vibration of zeolite water in the channels of AT [31]. The absorption peaks at 1030 and 1018 cm−i were characteristic peaks of symmetric and asymmetric stretching vibration of Si-O of Si-O-Si and Si-O-Al, respectively. Besides, the FTIR spectrum of QAT appeared ii new absorption peaks at 2924.1 and 2853.0 cm−ane, which corresponded to the C-H stretching vibration of CHthree and CH2 in the alkyl chain of DC5700, indicating that AT was successfully modified by DC5700. The feature IR bands of HPW in WQAT were somewhat unlike from those of bulk HPW. Amid the four feature bands of HPW, those at 1080 cm−1 (P-Oa) and 981.4 cm−ane (Due west-Od) were overlapped by the potent and broad Si-O-Si band and Si-O band of AT, while those at 890.3 cm−1 (Due west-Ob-W, corner-sharing) and 798.8 cm−ane (W-Oc -W, edge-sharing) shifted to 897.1 cm−1 and 817.0 cm−i, respectively. These shifts are probably assigned to the germination of the secondary construction of phosphotungstic acid with a potent chemic interaction between oxygen terminated of heteropolyacid polyanion and H5Oii + of QAT, which illustrated HPW has a adept stability on the surface of QAT [32].
(a) FTIR and (b) XPS full-scan spectra of AT and functionalized AT.
XPS was used to verify the elements (such as Al 2p, Si 2p, Si 2s, C 1s, O 1s, and others) presented on the surface of AT. From the XPS spectrum of QAT (Figure 1b), it tin be found that a new N1s acme appeared at 401.86 eV, which was assigned to the quaternary amine groups from the grafted DC5700 chains, indicating that DC5700 long bondage containing -NRthree + groups were successfully grafted onto the surface of AT through the reaction between the ethoxy group of DC5700 and the hydroxyl groups of AT. For WQAT, except for the peaks similar to QAT, a new superlative of W4f at a binding energy of 35.34 eV became remarkably visible. The upshot also proved that HPW was anchored onto the surface of DC5700 modified AT through the potent electrostatic interaction between -NR3 + cations of grafted DC5700 and Pw12O40 3− anions of PWA.
To make up one's mind the coating content of PWA in WQA, TGA was carried out in a nitrogen flow from 30 to 800 °C. The obtained TGA and derivative thermo-gravimetric (DTG, dW/dT) curves are shown in Figure 2a,b. As for the pristine AT, it showed a three weight loss thermal degradation beliefs: The get-go loss is due to the adsorbed h2o and zeolite water in the attapulgite; the second loss comes from the removal of structural water; the tertiary loss is because of the destruction of the internal hydroxyl groups of AT. As for QAT, the commencement weight loss (almost iv.06%) was lower than that of AT (9.86%), which was mainly attributed to the hydrophobic organic long chains covering the surface of AT. Apart from this, QAT had a weight loss of 12.24% in the range of 150–270 °C and 29.09% in the range of 280–600 °C, which should exist due to the introduction of a new bond of Si-O-Si betwixt organo-silane coupling agent and AT and the decomposition of DC5700 organic molecular chains. After PWA blanket, WQAT exhibited a similar decomposition procedure to that of QAT. From the difference of the final residue content between QAT and WQAT, the loading content of PWA could exist calculated to exist 11.48%, because PWA can keep thermally stable until 700 °C [33].
TGA (a) and DTG (b) curves of AT, QAT and WQAT.
3.2. Characterization of Pure CS and CS/WQAT Composite Membranes
three.2.1. Construction and Morphology
The dispersion of nanofillers in an organic-inorganic composite arrangement can bear upon the load transfer from the polymer matrix to stiff inorganic nanofiller. Too, the good distribution of 1D WQAT in the blended membranes could as well contribute to form new aqueduct-similar pathways for proton transport. The cross-sectional morphology of the blended membranes was probed using Scanning Electron Microscope (SEM) as shown in Figure 3a–f. The pure CS membrane (Figure 3a) showed a homogeneous and impact morphology without obvious cracks or micro-voids. With the introduction of WQAT, which are nanorods with a diameter of ~10 nm and length of several microns (as shown in Figure threeg), obvious white particle-like substances can be observed in the composites (as shown in Effigy 3b–f). These white particles are WQAT nanorods distributed in the CS matrix after liquid nitrogen quenching during the training of SEM cross-sectional samples. Conspicuously, these WQAT nanorods were uniformly dispersed in the polymer matrix as for the sample CS/WQAT-ii and CS/WQAT-four, which can exist verified past the Energy Dispersive Ten-Ray Spectroscopy (EDX) mapping images of Si (Figure 3h) and Westward (Figure iiii). Still, some WQAT agglomerations (marked with yellow circles in Figure threed–f) can be seen in the cross-sections of the composite membranes when the content of WQAT was more than 4%.
Cantankerous-sectional SEM images of the membranes: (a) CS, (b) CS/WQAT-2, (c) CS/WQAT-4, (d) CS/WQAT-8, (east) CS/WQAT-10, (f) CS/WQAT-15; (g) SEM epitome of WQAT; EDX mapping images of Si (h) and Due west (i) distribution in the CS/WQAT-4 membrane.
By and large, the proton conduction in PEMs mainly occurs in amorphous phase rather than crystalline phase [eight,ix]. Due to the being of intramolecular hydrogen bonds between oxygen atoms and intermolecular hydrogen bonds betwixt amino and hydroxyl groups in CS molecule, CS regularly accumulates into a semi-crystalline structure during the process of movie formation. The XRD patterns of the as-prepared membranes are depicted in Effigy 4. The pure CS membrane exhibited typical three characteristic diffraction bands at 2θ = 12.ii°, thirty.eight° and twoscore.five°. Obviously, as for the blended membranes, the intensity of the strong band of CS matrix at 12.21° decreased kickoff so increased with increasing WQAT contents. Among all the membranes, CS/WQAT-4 had the weakest diffraction intensity. The reason for this trend of intensity change is maybe because the ordered arrangement of CS molecular chains is partly destroyed by the added WQAT, and thus, reduced their crystalline domains. Nevertheless, when the content of WAQT was high (due east.one thousand., above 4%), the agglomerated inorganic particles (as illustrated in SEM images in Figure 3) had no obvious damage ability to the crystal areas. This effect of WQAT on the crystallization ability of CS matrix may besides bring some influences on the proton conductivity, which will be discussed in Section iii.2.4.
Comparing of XRD patterns of CS and CS/WQAT blended membranes.
three.2.2. Thermal and Mechanical Properties of the Composite Membranes
Every bit a primal component in PEMFCs or DMFCs, PEMs must have fantabulous thermal and mechanical stabilities for long-fourth dimension operation. The thermal properties of the equally-prepared membranes were investigated by TGA-DTG and DSC as shown in Figure 5. From Effigy va, the pure CS membrane exhibited two thermal weight loss stages: (i) The first phase may be related to the degradation of chitosan side groups around 200–250 °C; (two) the 2d phase in the temperature range of 250–350 °C is the decomposition of polymer backbone of CS, which is similar to the result in the literature [14]. After the incorporation of WQAT, the CS/WQAT composite membranes demonstrated similar degradation behavior to that of the pure CS membrane. By comparison the initial decomposition temperature of the first stage, the CS/WQAT composite membranes showed a slightly college degradation temperature than that of pure CS from the DTG curves. This may be due to the interaction between phosphotungstic acid and -NH2 of chitosan, which partly inhibits the movement of polymer segments and thus increases the thermal stability of the composites. To farther ostend the thermal behavior, DSC was as well conducted from 90 to 260 °C, and the obtained curves are shown in Effigy vb. Clearly, the decomposition temperatures (Td) increased from 217.iv °C (pure CS membrane) to 232.2 °C (CS/WQAT-15 membrane) with the increase in WQAT content, verifying the plenty thermal stability of our composite system for the application in PEMFCs and DMFCs.
(a) TGA-DTG and (b) DSC curves of CS/WQAT composite membranes.
PEMs should possess enough mechanical strength and adequate flexibility to come across the requirements of fuel cell assembly and operation. The tensile strength, elongation and modulus of the pure CS and CS/WQAT composite membranes are shown in Table 1. The pure CS membrane possessed a tensile force of 41.31 MPa, elongation of 17.01% and Immature's modulus of 618.9 MPa. As for the composite membranes, the tensile strength, elongation and Young'south modulus values increased with the increase in the content of WQAT up to iv wt.%. The comeback of tensile strength and Young's modulus might be attributed to the good dispersion of WQAT, which contributes to the germination of more acrid-base pairs, and thus, can partly prevent the slippage of CS bondage when the composites are subjected to stress. The increment of elongation, which is unlike other typical inorganic nanofiller reinforced composite systems, may be ascribed to the flexible DC5700 long chains grafted on AT. Amid all the composite membranes, the CS/WQAT-4 membrane exhibited the highest tensile strength, elongation and Young's modulus of 58.65 MPa, 23.21% and 1152.69 MPa, which improved by 42.0%, 36% and 86.25% respectively when compared to those of the pure CS membrane. However, with the further increment in WQAT content, the overall decrease of mechanical backdrop is due to the bunch of WQAT, which weakens its power of strengthening and toughening. Fortunately, the mechanical properties of the composite membranes were still improve than those of the pure CS membrane. In summary, the above upshot showed that the incorporation of WQAT tin can enhance the thermal and mechanical stabilities of CS matrix, making the composite membranes more than suitable for DMFCs awarding.
Table 1
Mechanical properties of the pure CS and CS/WQAT blended membranes.
| Sample | Tensile Strength (MPa) | Elongation (%) | Young'due south Modulus (MPa) |
|---|---|---|---|
| CS | 41.31 ± two.0 | 17.07 ± 4.eight | 618.xc ± x |
| CS/WQAT-2 | 50.xiv ± 2.vii | twenty.89 ± 3.one | 990.48 ± 11 |
| CS/WQAT-4 | 58.65 ± 2.5 | 23.21 ± 1.viii | 1152.69 ± 14 |
| CS/WQAT-8 | 52.37 ± ii.2 | nineteen.96 ± two.5 | 980.78 ± xvi |
| CS/WQAT-10 | 48.67 ± 1.eight | 18.36 ± 4.5 | 890.54 ± 12 |
| CS/WQAT-fifteen | 42.8 ± 3.viii | 16.36 ± 2.4 | 720.54 ± 14 |
3.two.3. Water Uptake and Area Swelling of the Membranes
Water molecules play an of import role for proton conduction in proton substitution membranes because H+ ions transport generally along the hydrogen-bonded network formed past water (diffusion mechanism) or through the ion exchange groups dissociated by water (hopping mechanism). All the same, too high h2o uptake by and large results in excessive swelling of PEMs and thus sharply decreases their mechanical stability. In addition, it should be mentioned that too big swelling ratio of a PEM also brings a severe methanol crossover from the anode to cathode through electro-osmosis, thus producing a mixed potential to subtract the open circuit voltage. Therefore, the water absorption and swelling of PEMs are important indicators for judging the quality of proton commutation membranes. The water uptake values of the CS/WQAT composite membranes with various weight fractions of WQAT at unlike temperatures are shown in Table ii. Compared with the pure membrane, the water absorption values of the composite membranes increased by 8.35% to 41.16% at 80 °C with unlike content WQAT. This increment in water uptake may exist because AT itself has many hydroxyls on its structure, which is helpful to absorbing h2o. Besides, the existence of heteropolyacid anions on the surface of AT could also contain water molecules through hydrogen bonds. As for the upshot of temperature on wet uptake, all the membranes showed an increasing trend of water content with the increase in temperature. Generally, the increased temperature can advance the movement of h2o molecules and polymer chains of chitosan, which makes H2O easily lengthened into the membranes.
Table 2
Water uptake (%) of the pure CS and CS/WQAT blended membranes.
| Sample | twenty °C | 40 °C | 60 °C | 80 °C |
|---|---|---|---|---|
| CS | 68.15 ± 5.24 | 72.03 ± 6.52 | 88.05 ± 7.38 | 114.11 ± 7.seven |
| CS/WQAT-2 | 71.21 ± iii.37 | 74.00 ± 5.61 | xc.11 ± 4.28 | 121.17 ± 8.v |
| CS/WQAT-4 | 72.09 ± iii.58 | 74.76 ± iv.53 | 92.69 ± three.52 | 161.08 ± seven.3 |
| CS/WQAT-8 | 70.05 ± iv.12 | 74.86 ± v.72 | 86.21 ± 6.92 | 150.93 ± 5.7 |
| CS/WQAT-10 | 68.44 ± four.87 | 75.86 ± three.57 | 89.27 ± 5.47 | 137.78 ± 8.iii |
| CS/WQAT-15 | 65.49 ± iii.73 | 75.98 ± 4.38 | 85.87 ± 3.39 | 123.64 ± 9.five |
Equally for the area swelling of the composite membranes (as shown in Tabular array 3), all the membranes brandish increased area swelling values with the testing temperatures, which is similar to the consequence of water uptake. However, unlike the alter trend of water uptake with the content of WQAT, the area swelling values follow a slight decrease trend, indicating the increased dimensional stability of the composites. The consequence is probably due to the strong interfacial interaction between CS and WQAT. In summary, the to a higher place results indicated that the blended membranes possessed higher h2o uptake ability and dimensional stability than those of the pure CS membrane.
Table 3
Area swelling ratio (%) of the composite membranes.
| Sample | 20 °C | 40 °C | sixty °C | 80 °C |
|---|---|---|---|---|
| CS | 5.17 ± 4.57 | 9.50 ± 4.36 | 17.81 ± ii.52 | 38.02 ± 5.33 |
| CS/WQAT-2 | 5.xvi ± three.97 | nine.32 ± 4.38 | fifteen.64 ± five.31 | 35.56 ± six.30 |
| CS/WQAT-4 | 4.20 ± 5.37 | 8.16 ± 5.35 | 14.92 ± 3.xxx | 30.02 ± four.31 |
| CS/WQAT-8 | four.52 ± 2.97 | nine.79 ± 5.07 | 17.0 ± 3.36 | 33.92 ± iii.67 |
| CS/WQAT-10 | 4.88 ± v.34 | 9.86 ± 4.27 | 16.60 ± half dozen.01 | 33.65 ± 5.47 |
| CS/WQAT-15 | v.68 ± 4.85 | 9.90 ± iv.31 | xvi.07 ± 4.77 | 39.22 ± four.36 |
3.2.4. Proton Conductivity, Methanol Permeability and Single Prison cell Operation
The proton conductivities of the every bit-prepared membranes were tested at 100% R.H. in the temperature range from 20 to 80 °C using a two-probe electrochemical impedance spectroscopy method, and the results are shown in Figure half-dozen. According to the curves, the electrical conductivity values of the composite membranes increased firstly and then decreased with the add-on of WQAT. Among all the membranes, the CS/WQAT-iv membrane showed a highest conductivity of 35.3 mS cm−i at eighty °C that was nigh 37% higher than that of the pure CS membrane (25.viii mS cm−1). To meliorate empathize the effect of WQAT on the proton conductivity, we also prepared the CS/AT composite membrane with 4% AT loading and tested its proton electrical conductivity under the same condition. The result showed the proton conductivity of the CS/AT (4% AT content) composite membrane was 28.8 mS cm−1 at 80 °C, which was much lower than that of the CS/WQAT blended membrane with the same inorganic nanofiller content, indicating the positive influence of the PWA surface medication on AT. This significantly improved proton conductivity and may be explained equally follows: (1) the reduced degree of crystallinity of chitosan matrix (every bit proven in Effigy five) is conducive to proton transport; (two) the super-acidic PWA coated on the surface of AT can act as new H+ transfer sites; (3) with the aid of 1D structure of AT, the homogeneously dispersed WQAT (equally shown in Figure 3c) could construct continuous proton ship nanopathways forth the interface between CS and WQAT considering of the acid-base interaction betwixt the two components [29]; (4) the increased water uptake power tin can also contribute to the fast ship of H+. However, information technology can too be noted that further increasing WQAT did not produce college proton conductivity. For case, the CS/WQAT-fifteen blended membrane exhibited a conductivity of 25.8 mS cm−ane (80 °C) that was 73% of that of the CS/WQAT-4 membrane. Such a trend is unsurprising because the aggregated inorganic nanofiller may partly block the fast proton transportation in the composite to some extent.
Proton electrical conductivity of the composite membranes at different temperatures.
In addition to proton electrical conductivity, the penetration of methanol is another serious problem deserving to be discussed because methanol crossover can not only partly waste the anode fuel, simply also touch the output voltage and fuel cell performance [34]. Considering the optimal properties of the CS/WQAT-4 composite membrane amongst all the composites, this sample was selected to be further evaluated for the methanol permeability and single-cell performance. The methanol crossover was tested using a voltammetric method [35], in which the limited current density that comes from consummate electro-oxidation of methanol permeation at the interface of membrane/catalyst is used to evaluate the methanol permeability. Generally, the higher the methanol crossover current density, the more serious the methanol permeability. Figure 7a compares the methanol crossover current density for the hybrid membrane (CS/WQAT-4) at lxx °C in different methanol solutions (1 One thousand, two M and v M); it tin exist seen that the crossover electric current density gradually increased with the methanol concentration. This is because methanol permeability is more serious when using a methanol solution with higher CH3OH concentration as the anode fuel, due to the bigger concentration gradient diffusion. At the same fourth dimension, we as well compared the methanol crossover current densities of the pure CS, CS/WQAT-4 and commercial Nafion 212 membranes in a ii M methanol solution. Every bit shown in Effigy 7b, Nafion 212 membrane exhibited the highest current density of 535.9 mA cm−2 amidst the three membrane samples, indicating its poor methanol barrier ability due to its highly hydrophobic/hydrophilic stage separated morphology [35]. As for the pure CS membrane, it as well showed a relatively high current density of 445 mA cm−2, while this value for the CS/WQAT-4 hybrid membrane was only 325 mA cm−ii. This result revealed that the hybrid membrane had excellent methanol bulwark power, which is attributed to the formation of tortuous pathways confronting penetrating the methanol molecules in the composites.
Comparison of the methanol crossover current densities of (a) CS/WQAT-4 at 70 °C in methanol solutions with dissimilar methanol concentrations; (b) CS, CS/WQAT-4 and Nafion 212 at seventy °C in a ii M methanol solution.
In order to farther evaluate the possibility of the applied awarding of our composite membranes, single DMFC was equipped and tested at 70 °C using methanol solution as the anode fuel and oxygen as the cathode gas. Figure 8a depicts the potential-electric current density (I-V) and the power density-current density curves of the CS/WQAT-4 composite membrane at 1 G, two M and five Thou methanol concentrations. From Figure 8a, the open up circuit voltages (OCVs) were 0.74 V, 0.67 Five and 0.64 V respectively when the methanol concentrations were 1 One thousand, ii K and five M, which showed a similar trend to that of the methanol crossover. Simultaneously, the summit power densities were 45.23, 70.26 and 59.ane mW cm−2 respectively at 1 Yard, 2 M and iii Chiliad methanol solutions. Information technology is obvious that the CS/WQAT-4 blended membrane exhibited a highest power density at 2 K methanol. So we further compared the single-cell performance of the CS/WQAT-iv composite membrane, pure CS and commercial Nafion 212 membranes using ii M methanol as the anode fuel at 70 °C. Figure viiib shows that the maximum ability density of Nafion 212 membrane was 79.87 mW cm−2, which was just slightly higher than that of the CS/WQAT-4 composite membrane (70.26 mW cm−2) under the same test condition. Equally a dissimilarity, the DMFC equipped with the pure CS membrane output the peak ability density of as low as 40.08 mW cm−two, which was only 57% of that of our composite membrane. The improved proton conductivity together with decreased methanol crossover may be responsible for the satisfactory fuel jail cell functioning of our designed composite membranes.
Polarization and power density curves of DMFC tests of (a) the CS/WQAT-4 composite membrane at unlike methanol concentrations, and (b) different PEMs at 70 °C with a 2 K methanol solution as the anode fuel.
4. Conclusions
In summary, super-strong proton conductor, PWA, anchored AT was prepared and direct used as a novel nanofiller employed in the chitosan matrix to fabricate blended proton exchange membranes. The mechanical strength of the composite membranes increased owing to the incorporation of uniformly-dispersed 1D WQAT, which can serve equally physical cross-linking points to prohibit the rupture of polymer chains. Moreover, the ultra-potent proton conduction ability of PWA together with the interaction between positively charged CS chains and negatively charged PWA could construct effective proton transport channels with the help of 1D AT. As a effect, the proton conductivity of the CS/WQAT-4% composite membrane increased by 31.8% when compared with that of the pure CS membrane at eighty °C. Besides, the methanol permeability of the composite membrane can as well be remarkably decreased. The increased ionic conductivity and decreased methanol permeability pb to the increase of maximum power density for the blended electrolyte, which exhibited the value of 70.26 mW cm−two at seventy °C (2 Grand methanol as the anode fuel), whereas the pristine membrane displayed simply 40.08 mW cm−2.
Synthesis process of WQAT and CS/WQAT-x composite membranes.
Funding
This enquiry received no external funding.
Conflicts of Involvement
The writer declares no conflict of interest.
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