polymers Article
Improve the Performance of Mechanoelectrical Transduction of Ionic Polymer-Metal Composites Based on Ordered Nafion Nanofibres by Electrospinning Yang Zhao 1 ID , Jiazheng Sheng 1 , Di Xu 1 , Minzhong Gao 2 , Qinglong Meng 1 , Dezhi Wu 1 , Lingyun Wang 1 , Wenlong Lv 1,3 , Qinnan Chen 1 , Jingjing Xiao 1, * and Daoheng Sun 1 1
2 3
*
Department of Mechanical and Electrical Engineering, Xiamen University, Xiamen 361102, China;
[email protected] (Y.Z.);
[email protected] (J.S.);
[email protected] (D.X.);
[email protected] (Q.M.);
[email protected] (D.W.);
[email protected] (L.W.);
[email protected] (Q.C.);
[email protected] (D.S.) School of Aerospace Engineering, Beijing Institute of Technology, Beijing 100081, China;
[email protected] Pen-Tung Sah Institute of Micro-Nano Science and Technology, Xiamen University, Xiamen 361005, China;
[email protected] Correspondence:
[email protected]; Tel.: +86-592-2185927
Received: 19 June 2018; Accepted: 20 July 2018; Published: 21 July 2018
Abstract: An ionic polymer–metal composite (IPMC) is a kind of soft material. The applications of IPMC in actuators, environmental sensing, and energy harvesting are currently increasing rapidly. In this study, an ordered Nafion nanofibre mat prepared by electrospinning was used to investigate the characteristics of the mechanoelectrical transduction of IPMC. The morphologies of the Nafion nanofibre mat were characterized. The proton conductivity, ion exchange capacities, and water uptake potential of the Nafion nanofibre mat were compared to traditional IPMC, respectively. A novel mechanism of Nafion nanofibre IPMC was designed and the open circuit voltage and short circuit current were measured. The maximum voltage value reached 100 mv. The output power was 3.63 nw and the power density was up to 42.4 µW/Kg under the load resistance. The Nafion nanofibre mat demonstrates excellent mechanoelectrcical transduction behavior compared to traditional IPMC and could be used for the development of self-powered devices in the future. Keywords: Nafion; ionic electrospinning; nanofibre
polymer–metal
composite;
mechanoelectrical
transduction;
1. Introduction Ionic polymer–metal composites (IPMCs) are a new kind of smart material. They consist of one proton exchange mat layer and two metal electrodes on both sides. The structure of IPMC is similar to a “sandwich”, as shown in Figure 1a. When a DC voltage difference is applied on both ends of the electrodes, IPMC generates mechanical bending in the direction of the anode [1]. On the contrary, if a mechanical deformation occurs in IPMC, a voltage is generated on both sides of the electrodes. Therefore, IPMC is a promising class of soft electroactive polymers that can be utilized as actuators [2,3], sensors [4,5], or energy harvesters [6,7] due to its mechanoelectrical transduction. Although IPMCs used as energy harvesters allow for energy conversion in a broad frequency range, especially in a low frequency, the electrical output is small [8–11]. It greatly limits the application of IPMC energy harvesting in practical use.
Polymers 2018, 10, 803; doi:10.3390/polym10070803
www.mdpi.com/journal/polymers
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Figure 1. 1. Schematic diagram diagram of of IPMC IPMC (a) (a) “Sandwich” “Sandwich” Structure Structure (b) (b) Molecular Molecular Formula Formulaof ofNafion. Nafion. Figure
Recently,most most work on ionic polymers hasto tried theimprove behavior, improve the Recently, work on ionic polymers has tried modelto themodel behavior, the performance, performance, and apply them in different fields. Wang et al. proposed a double-chamber valve-less and apply them in different fields. Wang et al. proposed a double-chamber valve-less pump pump actuated by a cantilever ionic polymer-metal composite, which validates the suitability use actuated by a cantilever ionic polymer-metal composite, which validates the suitability for use infor drug in drug applications delivery applications [12]. Horiuchi et al. ademonstrated a new voltage-controlled delivery [12]. Horiuchi et al. demonstrated new voltage-controlled accommodating accommodating IOL consisting of an IPMC actuator to change the lens’ focus in the eyeet [13]. IOL consisting of an IPMC actuator to change the lens’ focus applied in theapplied eye [13]. Hong al. Hong et al. investigated electrochemical and morphological characteristics of IPMCs by varying the investigated electrochemical and morphological characteristics of IPMCs by varying the morphology morphology their metal composite component et al.the investigated the sensing of their metalof composite component [14]. Dominik [14]. et al.Dominik investigated sensing capabilities of capabilities of IPMC as a displacement sensor [15]. Two mechanoelectrical transduction principles IPMC as a displacement sensor [15]. Two mechanoelectrical transduction principles have been have beenion reported: ionin transport in the constitutive mat [16–19] and an electrostatic effect [16,20] reported: transport the constitutive mat [16–19] and an electrostatic effect [16,20] resulting resulting from anionforming locationa forming a cluster. The researchers generally with the from cation andcation anionand location cluster. The researchers generally agree withagree the theory of theory of the “ion cluster network model” in Nafion [21,22]. Ma et al. adjusted the nano-scale the “ion cluster network model” in Nafion [21,22]. Ma et al. adjusted the nano-scale structure structure of the self-assembled proton conductive by enlarging the nano-scale pore size of the self-assembled proton conductive channels channels by enlarging the nano-scale pore size within within the macromolecules. The hybrid hyperbranched polyamide proton exchange mat (PEM) the macromolecules. The hybrid hyperbranched polyamide proton exchange mat (PEM) exhibits exhibits promising application [23]. Xia et al. performed nanoindentation teststhe to influence study the promising application potential potential [23]. Xia et al. performed nanoindentation tests to study influence of the temperature on the mechanical properties of theExchange Proton Exchange Mat. The of the temperature conditioncondition on the mechanical properties of the Proton Mat. The results results indicate that both hardness and elastic modulus show non-monotonic transition with the indicate that both hardness and elastic modulus show non-monotonic transition with the increase increase of temperature [24]. Tiwari et al. presented a physics-based mechanoelectric model that takes of temperature [24]. Tiwari et al. presented a physics-based mechanoelectric model that takes into into account material properties, electrostatic phenomenon, ion transport in IPMC the IPMC to study account material properties, electrostatic phenomenon, and and ion transport in the to study the the Mechanoelectric transduction in an ionic polymer-metal composite [25]. Tiwari et al. studied Mechanoelectric transduction in an ionic polymer-metal composite [25]. Tiwari et al. studied different different parameters in the mechanical and domain the electrical domain the to applicability investigate the parameters in the mechanical domain and domain the electrical to investigate of applicability of IPMC as an energy harvester in lower frequency regions [26]. Morohoshi et to al. IPMC as an energy harvester in lower frequency regions [26]. Morohoshi et al. established methods established methods to evaluate key properties such as water diffusion, gas permeability, and evaluate key properties such as water diffusion, gas permeability, and mechanical strength based on mechanical strength based on optimal coarse-graining and structure an optimal design of the polymer coarse-graining models, and an design ofmodels, the polymer could be established by the structure could be established by the method [27]. Bing Guo et al. synthesised a hydrophilic polymer method [27]. Bing Guo et al. synthesised a hydrophilic polymer shell on a silica microsphere template shellthe onsubsequent a silica microsphere andby the subsequent removal the template by etching to and removal of template the template etching to obtain the tinyofhydrophilic hollow chamber, obtain the tiny hydrophilic hollow chamber, it is critical retaining moisture and mats facilitating because it is critical to retaining moisture andbecause facilitating proton to transfer in the composite [28]. proton transfer in the composite mats [28]. The electrical output performance of IPMC is determined by characteristics of mechanoelectrical The electrical output performance of the IPMC is determined byof characteristics transduction of Nafion. Figure 1b shows molecular formula Nafion. Dueof tomechanoelectrical the hydrophobic transduction of Nafion. Figure 1b shows the molecular formula of Nafion. Due to the hydrophobic nature of the perfluorinated backbone and the hydrophilicity of the side chain containing sulfonic acid nature in of the the Nafion perfluorinated and the as hydrophilicity of the side forming chain containing sulfonic groups mat, the backbone Nafion mat exists microphase separation, a hydrophilic ion acid groups in the Nafion mat, the Nafion mat exists as microphase separation, forming a hydrophilic channel. The sulfonic group is immobilized on the side chain, and only hydrated cations can move ion channel. sulfonicdeformation group is immobilized on the side chain, and only hydrated cations move freely. When The mechanical of IPMC occurs, the volume of the compression side iscan reduced freely. When mechanical deformation of IPMC occurs, the volume of the compression side is reduced and that of the stretch side is increased. This leads to the movement of the hydrated cations in IPMC. and that the stretch side is increased. Thisinleads to the movement of the hydrated in IPMC. Owing toof changes in the local concentration the Nafion mat, an electrical potential cations difference forms Owing to changes in the local concentration in the Nafion mat, an electrical potential difference forms on both sides of the IPMC. However, the ion channel is random, irregular, and without a specific on both sides of Nafion the IPMC. the ion channel is random, irregular, andtransfer without a specific orientation in the mat.However, When mechanical deformation of IPMC occurs, the of hydrated orientation in the Nafion mat. When mechanical deformation of IPMC occurs, the transfer of cations requires a long transmission path and overcomes a large resistance from one side to the other. hydrated cations requires a long transmission path and overcomes a large resistance from one side Up to now, fewer researchers have concentrated on the structure to improve the electrical output to the other. Up now, fewer researchers haveMuchao concentrated structure improveconductivity the electrical by studying iontotransfer in the Nafion mat. Qu eton al.the studied thetoelectrical output by studying ion transfer in the Nafion mat. Muchao Qu et al. studied the electrical conductivity of melt spun composites consisting of PMMA containing both aligned carbon fibers and
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of melt spun composites consisting of PMMA containing both aligned carbon fibers and carbon black, which showed a decreased tensile strength and an increased E-modulus with the addition of fillers [29]. Qiang Chen et al. modified bulk metallic surfaces by the electrophoretic deposition of short phosphate glass fibers to improve bone-to-implant bonding [30]. Yahong Li et al. presented a combined electrospinning, ultrasonication adsorbing, and bobbin winder technique by electrospun thermoplastic polyurethane fiber yarns successively decorated with multi-walled carbon nanotubes and single-walled carbon nanotubes to obtain highly conductive and stretchable yarns [31]. This work aims to discuss the oriented transfer of ions by using an ordered Nafion nanofibre mat which has not been considered previously. In this paper, an ordered Nafion nanofibre mat is proposed to improve the mechanoelectrical transduction by electrospinning. The ordered Nafion nanofibre was collected by a rotated roller. The mechanism of IPMC based on the Nafion nanofibre mat was fabricated to test the characteristics of mechanoelectrical transduction. The surface morphologies of the Nafion nanofibre were characterized. The water uptake potential, ion exchange capacity, and proton conductivity were compared with traditional IPMC. The open circuit voltage and short circuit current of IPMC based on the Nafion nanofibre mat were measured under different thicknesses and different loads. The electrical output of IPMC based on the Nafion nanofibre mat under different load resistances was also tested. 2. Experimental 2.1. Materials DuPontTM Nafion PFSA polymer dispersions were made from chemically stabilized perfluorosulfonic acid/PTFE copolymer in the acid (H+ ) form that were used to electrospin the ordered nanofibre mat. The Nafion dispersion of DE2020 (the polymer content is 20–22%) was supplied by DuPont Co., (Wilmington, DE, USA), the Poly ethylene oxide (PEO: Molecular weights of 5,000,000) was purchased from Changchun Huayu Fine Chemical Co. Ltd., (Changchun, China), and absolute ethyl alcohol was obtained from Shantou Dahao Fine Chemical Co. Ltd., (Shantou, China). The PET film with a thickness of 0.1 mm was purchased from Suzhou Aiqiu Electronics Co., Ltd., (Suzhou, China). The silicone elastomer (PDMS) was obtained from Dow Corning Co. Ltd., (Midland, MI, USA). The PDMS was added with a curing agent (ratio 10:1) and was fully mixed with manual stirring for 3 min, and then the mixture was vacuumed for 10 min to remove the bubbles. It was prepared for future use. 2.2. Fabrication of Ordered Nafion Nanofibre Mat The mat of the ordered Nafion nanofibre was fabricated by the electrospinning method. Because the Nafion dispersion could not be electrospun, the PEO was added to Nafion dispersion to improve the electrospinning ability. The weight of nafion was 2 g. The weight ratio of nafion dispersion and PEO was 99:1. The proportion of solvent alcohol and deionized water was 1:1. The mixed solution was stirred by a magnetic stirrer for 4 h. Since PEO has no contribution to the proton conductivity of nanofibres, it was added to Nafion solution only to improve the solution viscosity and molecular entanglement force. At the same time, PEO was a useless impurity for Nafion nanofibres, so high purity Nafion solution was chosen for electrospinning. However, little PEO content could result in the poor quality of nanofibres, such as bead-on-nanofibres, and even make Nafion unable to be electrospun. Finally, 1% content of PEO was selected and high quality nanofibres were obtained. The electrospinning apparatus was schematically shown in Figure 2. The precision syringe pump provided a steady and continuous supply of solution for the syringe. The high-voltage power supply was applied to the nozzle to supply an electric field force for electrospinning. A rotating roller was used to collect nanofibres and the roller was mounted on the mobile platform moving laterally, so that the collected nanofibre mat had a certain width and uniform thickness. The related parameters of electrospinning are summarized in Table 1.
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Figure 2. Schematic diagram of the rotating roller electrospinning.
Figure 2. Schematic diagram of the rotating roller electrospinning. Table 1. The parameters of Nafion electrospinning.
Table 1. The parameters of Nafion electrospinning. Related Parameters Numerical Size Electrospinning voltage 3.0–4.0 KV Related Parameters Numerical Size Polymer solution flow rate 160 μL/h 3.0–4.0 KV InsideElectrospinning diameter of thevoltage syringe needle 200 μm Polymer solution flow rate 160 µL/h Spinneret-to-collector distance 4 cm Inside diameter of the syringe needle 200 µm Platform moving speed 60 mm/min Spinneret-to-collector distance 4 cm Roller rotation r/min Platform moving speed speed 60200 mm/min Diameter of roller 80 mm Roller rotation speed 200 r/min Diameter of roller 80 mm 2.3. Measurements and Characterizations of Nafion Nanofibre Mat Water uptake potential: The WUP was determined the difference between the weight of the 2.3. Measurements and Characterizations of Nafion Nanofibreby Mat vacuum-dried mat and the fully water-equilibrated mat. The mat was placed in deionized water for
Water uptake potential: WUP determined byremove the difference between thethe weight of the 24 h. The mat was quicklyThe wiped withwas absorbent paper to the surface water, and weight vacuum-dried mat and the fully water-equilibrated mat. The mat was placed in deionized water of the sample was recorded. Then, the sample was placed in a vacuum dried box to remove residual for 24 h. The matorwas quickly wiped paper remove the surface water, andmat thewas weight solvent water and the weightwith wasabsorbent recorded for 12 h.to The water uptake potential of the by the following equation: of thecalculated sample was recorded. Then, the sample was placed in a vacuum dried box to remove residual solvent or water and the weight was recorded for−12 h. The water uptake potential of the mat was 𝑊𝑤𝑒𝑡 𝑊𝑑𝑟𝑦 WUP = × 100% (1) calculated by the following equation: 𝑊𝑑𝑟𝑦 where 𝑊𝑤𝑒𝑡 and 𝑊𝑑𝑟𝑦 denote the weight W of wet the − wet dried mats, respectively. Wand dry WUP =the capacity of ion × 100% Ion exchange capacity: The IEC was exchange in the mat. First, the mat was (1) Wdry dried in a vacuum dryer for 12 h and then immersed in 10 mL 0.1 mol/L NaOH solution for 12 h. was taken out and the phenolphthalein solution was dropped in NaOH solution. whereThen Wwetthe andmat Wdry denote the weight of the wet and dried mats, respectively. Finally, 0.1 mol/L HCl solution was added to the extracted solution until it became neutral. The IEC Ion exchange capacity: The IEC was the capacity of ion exchange in the mat. First, the mat was value was calculated with the amount of the additive HCl solution. The IEC value was determined dried with in a the vacuum dryer for 12 h and then immersed in 10 mL 0.1 mol/L NaOH solution for 12 h. following equation:
Then the mat was taken out and the phenolphthalein solution was dropped in NaOH solution. Finally, 𝑉𝑁𝑎𝑂𝐻 × 𝑁𝑁𝑎𝑂𝐻 − 𝑉𝐻𝐶𝑙 × 𝑁𝐻𝐶𝑙 = extracted solution until it became neutral. The IEC 0.1 mol/L HCl solution was addedIEC to the (2)value 𝑤𝑒𝑖𝑔ℎ𝑡 𝑜𝑓 𝑑𝑟𝑦 𝑚𝑒𝑚𝑏𝑟𝑎𝑛𝑒 was calculated with the amount of the additive HCl solution. The IEC value was determined with the where V denotes the volume of the solution and N denotes normality. following equation: Proton conductivity: Proton conductivity of the between two copper V × NNaOH − mat VHClwas × Nmeasured HCl IEC = NaOH electrodes at both ends by connecting it weight to an impedance analyzer. The input frequency was 100–110 (2) o f dry membrane MHz and the amplitude was set at 5 mV. The equation used for calculating conductivity is given
wherebelow. V denotes the volume of the solution and N denotes normality. Proton conductivity: Proton conductivity of the mat was measured between two copper electrodes at both ends by connecting it to an impedance analyzer. The input frequency was 100–110 MHz and the amplitude was set at 5 mV. The equation used for calculating conductivity is given below.
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l𝑙 δ δ== 𝑅 × 𝐴 Rb𝑏× A
(3) (3)
where δδisisthe theproton protonconductivity, conductivity,l lisisthe thelength lengthof of the the film film between the electrodes, 𝑅𝑏b isisthe the bulk bulk where electrodes, R resistance of of the the mat, mat, and and A Aisisthe thecross crosssectional sectionalarea areaof ofthe themat. mat. resistance Scanningelectron electron microscopy: Scanning electron microscopy (SEM) imagesmorphology of surface Scanning microscopy: Scanning electron microscopy (SEM) images of surface morphology takenordered to observe ordered Nafionmats. nanofibre SEM images of allwere samples were were taken towere observe Nafion nanofibre SEM mats. images of all samples obtained obtained by SU-70 using an SU-70field thermal field emission microscope Inc., Japan). Tokyo, by using an thermal emission scanningscanning electron electron microscope (Hitachi(Hitachi Inc., Tokyo, Japan). The surfaces of thenanofibre Nafion nanofibre and other structures were observed. The surfaces of the Nafion mats andmats other structures were observed.
2.4. 2.4. Mechanoelectrical Mechanoelectrical Transduction TransductionMechanism Mechanismofofthe theNafion NafionNanofibre NanofibreIPMC IPMC Generally, Generally, ionic ionic polymer-metal polymer-metal composites composites were were fabricated fabricated by by electroless electroless plating plating Pt Pt electrodes electrodes on on both both sides sides of of the the Nafion-117 Nafion-117 film, film, just just like like aa “sandwich” “sandwich” structure, structure, but but this this structure structure was was not not suitable experimental mechanism schematic which waswas used to test the suitablefor forNafion Nafionnanofibre nanofibremats. mats.An An experimental mechanism schematic which used to test characteristics of mechanoelectrical transduction based on ordered Nafion nanofibre mats is shown the characteristics of mechanoelectrical transduction based on ordered Nafion nanofibre mats in is Figure Nafion nanofibres mat was fabricated electrospinning. The substrate a PET film. shown3.inThe Figure 3. The Nafion nanofibres mat was by fabricated by electrospinning. Thewas substrate was The electrodes evenlywere coated on the substrate bysubstrate silver paste. The size of The the silver a PET film. Thewere electrodes evenly coated on the by silver paste. size ofelectrodes the silver ◦ was 25 mmwas × 5 25 mm and× the spacing between them was 2them mm. was Then, it wasThen, heated at 60heated C forat 3060 min electrodes mm 5 mm and the spacing between 2 mm. it was °C to Nafionit.nanofibre matnanofibre was put onmat thewas electrodes the direction perpendicular to forsolidify 30 minit.toThe solidify The Nafion put onalong the electrodes along the direction the electrodes. The PDMS was directly poured onto the surface of the nanofibre mat and placed for perpendicular to the electrodes. The PDMS was directly poured onto the surface of the nanofibre mat 30 min to fully the to nanofibers Then, thePDMS. sampleThen, was placed in a drying oven and and placed for fuse 30 min fully fusewith the PDMS. nanofibers with the sample was placed in a heated three at for 70 ◦three C, which completely solidified the PDMS. Finally,the thePDMS. mechanoelectrical drying for oven andhours heated hours at 70 °C, which completely solidified Finally, the transduction mechanism was soaked in the saturated solution LiCl for 12 h to make the nanofibres mechanoelectrical transduction mechanism was soaked in theofsaturated solution of LiCl for 12 h to + + fully the cations Li . makeexchange the nanofibres fully of exchange the cations of Li .
Figure 3. 3. Schematic Schematic of of the the experimental experimental mechanism mechanism of of the the ordered ordered Nafion Nafion nanofibre nanofibremat. mat. Figure
The schematics of mechanoelectrical transduction based on the ordered Nafion nanofires mat The schematics of mechanoelectrical transduction based on the ordered Nafion nanofires mat are are illustrated in Figure 4. Under the state of no load, the hydration cations were evenly distributed illustrated in Figure 4. Under the state of no load, the hydration cations were evenly distributed in in Nafion nanofibres due to the effect of static electricity. When applying the pressure in the middle Nafion nanofibres due to the effect of static electricity. When applying the pressure in the middle of the of the Nafion nanofibre IPMC, the PDMS layer exhibited plastic deformation. Meanwhile, the Nafion nanofibre IPMC, the PDMS layer exhibited plastic deformation. Meanwhile, the deformation deformation was transferred to the Nafion nanofibres and resulted in the move of hydration cations was transferred to the Nafion nanofibres and resulted in the move of hydration cations to both sides to both sides along the direction of the nanofibres. A voltage difference was produced between the along the direction of the nanofibres. A voltage difference was produced between the intermediate intermediate electrode and the electrodes of both sides due to fixed anions and moving hydrated electrode and the electrodes of both sides due to fixed anions and moving hydrated cations. When the cations. When the electrodes were connected to the wire, the voltage and current signal could be electrodes were connected to the wire, the voltage and current signal could be measured. measured.
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Figure 4. Schematic of the mechanoelectrical transduction process. Figure 4. 4. Schematic Schematic of of the the mechanoelectrical mechanoelectrical transduction transduction process. process. Figure
3. Results and Discussion 3. Results Results and and Discussion Discussion 3. 3.1. Morphology of Ordered Nanofibre Mat 3.1. Morphology Morphology of of Ordered Ordered Nanofibre Nanofibre Mat Mat 3.1. Ordered Nafion Nanofibres were produced by electrospinning under high voltage. The OrderedNafion NafionNanofibres Nanofibres were produced by electrospinning under high The voltage. The Ordered were produced by electrospinning under high voltage. nanofibre mat was collected by using a rotated roller. It took 36 h to obtain a nanofibre matnanofibre with the nanofibre mat wasbycollected by usingroller. a rotated roller. It took 36 ahnanofibre to obtain mat a nanofibre mat with the mat was collected using a rotated It took 36 h to obtain with the dimensions of 300 μm in thickness and 20 mm in width. The surface morphology of thedimensions nanofibre dimensions 300 μm in thickness and 20 mmsurface in width. The surface morphology of the of 300 in of thickness 20can mmbeinseen width. of the nanofibre mat is nanofibre shown in mat is µm shown in Figureand 5a. It thatThe the orderedmorphology Nafion nanofibres have good directionality mat is shown in be Figure 5a. Itthe canordered be seen Nafion that thenanofibres ordered Nafion nanofibres have good directionality Figure 5a. It can seen that have good directionality and are arranged and are arranged closely under the collection of the rollers. The diameter of Nafion nanofibres was and areunder arranged closely under therollers. collection of the rollers. The diameter of Nafion nanofibres was closely the collection of the The diameter of Nafion nanofibres was between 600 between 600 nm and 800 nm and the nanofibres presented good homogeneity, as shown in Figure nm 5b. between 600 nm and 800 nm and the nanofibres presented good homogeneity, as shown in Figure 5b. and 800 nm and the nanofibres presented the good homogeneity, asin shown in compared Figure 5b. with Considering Considering the PDMS was hydrophobic, quality of the mat air was the mat Considering thehydrophobic, PDMS was hydrophobic, quality of in air was compared with the mat the PDMS was theThe quality ofthe the airthe wasmat compared with the mat immersed in immersed in deionized water. quality of mat the in mat was increased after immersing it in the immersed in deionized water. The quality of the mat was increased after immersing it in the deionized water. water. ItThe quality of the matthat wasPDMS increased it in the deionized of water. deionized could be concluded doesafter not immersing affect the water absorbability the deionized water. It could be concluded that PDMS does absorbability not affect theofwater absorbability of the It could be concluded that PDMS does not affect the water the nanofibres. nanofibres. nanofibres.
(a) (b) (a) (b) Figure 5. (a) SEM image of the ordered nanofibre mat and (b) the diameter of of Nafion nanofibres. Figure SEM image the ordered nanofibremat matand and(b) (b)the thediameter diameter Nafion nanofibres. Figure 5. 5. (a)(a) SEM image ofof the ordered nanofibre of Nafion nanofibres.
3.2. WUP, IEC and Proton Conductivity 3.2. 3.2. WUP, WUP,IEC IECand andProton ProtonConductivity Conductivity The properties of the sample mat are summarized in Table 2. The size of the sample was 15 mm The Table 2. 2. The size of of thethe sample was 15 15 mmmm in The properties propertiesof ofthe thesample samplemat matare aresummarized summarizedinin Table The size sample was in length and 10 mm in width. The data in Table 2 was the average value of five experiments. The length and 10 mm in width. The data in Table 2 was the average value of five experiments. The WUP in length and 10 mm in width. The data in Table 2 was the average value of five experiments. The WUP of the Nafion nanofibre mat was greater than the commercial film of Nafion-117. As shown in of the Nafion nanofibre mat wasmat greater than thethan commercial film of Nafion-117. As shownAs inshown Figure in 6, WUP of the Nafion nanofibre was greater the commercial film of Nafion-117. Figure 6, the WUP of the Nafion nanofibre was up to 52.92%, due to the large porosity between the WUP theWUP Nafion was up to 52.92%, due the large porosity nanofibres that Figure 6, of the of nanofibre the Nafion nanofibre was up toto52.92%, due to the between large porosity between nanofibres that can preserve the water molecules well. In addition, the nanofibre had a large specific can preserve thecan water molecules well.molecules In addition, the hadnanofibre a large specific surface area, nanofibres that preserve the water well. Innanofibre addition, the had a large specific surface area, which ensured that the hydrophilic ion cluster made full contact with water in the which that the hydrophilic ion cluster madeion full cluster contact made with water in the nanofibre. The surfaceensured area, which ensured that the hydrophilic full contact with water inIEC the nanofibre. The IEC of the Nafion nanofibre mat was higher than Nafion-117. As shown in Figure 7, nanofibre. The IEC of the Nafion nanofibre mat was higher than Nafion-117. As shown in Figure 7, the IEC of the Nafion nanofibre mat was up to 2.04, which indicates that the amount of hydrophilic the IEC of the Nafion nanofibre mat was up to 2.04, which indicates that the amount of hydrophilic
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of the Nafion nanofibre mat was higher than Nafion-117. As shown in Figure 7, the IEC of the Nafion nanofibre mat was up to 2.04, which indicates that the amount of hydrophilic ionic cluster in the Nafion nanofibre mat was more than that of Nafion-117, and the cation in the Nafion nanofibre mat Polymers 2018, 10, x FOR PEER REVIEW 7 of 13 was more than Polymers 2018,that 10, x of FORNafion-117, PEER REVIEWmeaning that it would generate more electrical output. The 7 of proton 13 −3 S/cm of conductivity of the Nafion nanofibre mat was 0.998 S/cm, compared with 1.24 × 10 ionic cluster in the Nafion nanofibre mat was more than that of Nafion-117, and the cation in the ionic cluster themat Nafion mat was more than that of Nafion-117, and the cation in the Nafion nanofibre was nanofibre more than that of Nafion-117, meaning would generate more Nafion-117. The in proton conductivity of the Nafion nanofibre mat that was itmuch greater than that of Nafion nanofibre mat was more than that of Nafion-117, meaning that it would generate more electricalThis output. The proton conductivity of the Nafion nanofibre mat was compared Nafion-117. indicates that the ion channel of the Nafion nanofibre mat0.998 has aS/cm, certain orientation, electrical Theofproton conductivity of the Nafion nanofibre mat was 0.998 S/cm, compared with ×output. 10−3 S/cm Nafion-117. Theeasily proton conductivity of the Nafion nanofibre mat was much so that the1.24 hydrated cations can transfer along the direction of the nanofibre. Moreover, due to −3 S/cm with 1.24 × 10 of Nafion-117. The proton conductivity of the Nafion nanofibre mat was much greater than that of Nafion-117. This indicates that the ion channel of the Nafion nanofibre mat hasthe a ion the constraints of the nanofibre boundary, it was difficult for the hydrated cations to leave greater than that of Nafion-117. indicates that the channeleasily of the along Nafionthe nanofibre matofhas certain orientation, so that theThis hydrated cations canion transfer direction thea channel in the nanofibreso and most of them could only transfer along thealong length direction ofof nanofibre, certain orientation, that cations transfer easily the direction the nanofibre. Moreover, due tothe thehydrated constraints of thecan nanofibre boundary, it was difficult for the so thehydrated proton conductivity was significantly improved. nanofibre. Moreover, due to the constraints of the nanofibre boundary, it was difficult for the cations to leave the ion channel in the nanofibre and most of them could only transfer along hydrated to leave the ion channel in the nanofibre andwas most of them could only transfer along the lengthcations direction of nanofibre, so the proton conductivity significantly improved. Table 2. Properties of Nafion-117 and the Nafion nanofibre mat. the length direction of nanofibre, so the proton conductivity was significantly improved. Table 2. Properties of Nafion-117 and the Nafion nanofibre mat. Table 2. Properties and the NafionProton nanofibre mat. WUP (%)of Nafion-117 IEC (mmol/g) Conductivity (S/cm)
Nafion-117 Nafion-117 Nafion nanofibre Nafion-117 Nafion nanofibre Nafion nanofibre
WUP (%) 11.77 WUP (%) 11.77 52.92 11.77 52.92 52.92
IEC (mmol/g) 1.69 IEC (mmol/g) 1.69 2.04 1.69 2.04 2.04
Proton Conductivity (S/cm) −3 Proton Conductivity 1.24 × 10(S/cm) 1.24 × 10−3 −3 0.998 1.24 × 10 0.998 0.998
Figure 6. WUP of Nafion-117 and the nanofibre mat.
Figure 6. 6. WUP andthe thenanofibre nanofibre mat. Figure WUPofofNafion-117 Nafion-117 and mat.
Figure 7. IEC of Nafion-117 and the nanofibre mat. Figure 7. IEC of Nafion-117 and the nanofibre mat.
Figure 7. IEC of Nafion-117 and the nanofibre mat. 3.3. Characteristics of Mechanoelectrical Transduction 3.3. Characteristics of Mechanoelectrical Transduction When an of external force is applied on the PDMS layer of Nafion nanofibre IPMC, the Nafion 3.3. Characteristics Mechanoelectrical Transduction Whenmat an external force is applied on theThis PDMS layer Nafiontonanofibre IPMC, Nafion nanofibre is compressed and deformed. causes theofcations transfer to both the sides with nanofibre mat is compressed and deformed. This causes the cations to transfer to both sides with When an external force is applied on the PDMS layer of Nafion nanofibre IPMC, the Nafion water molecules inside the nanofibre. A sample of Nafion nanofibre IPMC is illustrated in Figure 8. water molecules inside the nanofibre. A sample of Nafion nanofibre IPMC is illustrated in Figure 8. with nanofibre mat is compressed and deformed. This causes the cations to transfer to both sides Because of the movement of hydrated cations, the concentration of the cations was nonuniform. This Because of the movement of hydrated cations, the concentration of the cations was nonuniform. This watergenerates molecules inside the nanofibre. A sample of Nafion nanofibre is illustrated in Figure 8. a potential difference between the center electrode and theIPMC electrodes on both sides of generates a potential difference between the center electrode and the electrodes on both sides of
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Because of the movement of hydrated cations, the concentration of the cations was nonuniform. Polymers 2018,a10, x FOR PEER REVIEW 8 ofsides 13 This generates potential difference between the center electrode and the electrodes on both of Nafion nanofibre The generated differenceprompts prompts movement of external the external Polymers 2018, 10, IPMC. x FOR PEER REVIEW 8 of 13 Nafion nanofibre IPMC. The generatedpotential potential difference thethe movement of the circuit electrons to form an electrical signal. circuit electrons to form an electrical signal. Nafion nanofibre IPMC. The generated potential difference prompts the movement of the external circuit electrons to form an electrical signal.
Figure 8. The test sample of Nafion nanofibre IPMC.
Figure 8. The test sample of Nafion nanofibre IPMC. Figure 8. The test sample of Nafion nanofibre IPMC.
Two Nafion nanofibre IPMCs were fabricated to test the electrical output. The thickness of the Nafion nanofibre mat was 203 μm and 310 μm, respectively. Figures 9 and 10 show the electricalof the Two Nafion nanofibre IPMCs were to test testthe theelectrical electrical output. thickness Two Nafion nanofibre IPMCs werefabricated fabricated to output. TheThe thickness of the output under the reciprocating load, respectively. The load was 100 N and the frequency of the load Nafion nanofibre mat mat was was 203 µm 310 µm, respectively. Figures 9 and 10 show thethe electrical output Nafion nanofibre 203 and μm and 310 μm, respectively. Figures 9 and 10 show electrical was Hz. When measuring the output The of the sample, the openand circuit voltage and short circuit under the0.125 reciprocating load, respectively. load thethe frequency output under the reciprocating load, respectively. The was load 100 was N 100 N and frequencyofofthe the load load was current could be obtained. The graphs showed that the open circuit voltage and short circuit current Hz. When measuring the output of the sample, the open circuit voltage short circuit 0.125was Hz.0.125 When measuring the output of the sample, the open circuit voltage andand short circuit current were in pace with the periodic changes of the load. The open circuit voltage of the 203 μm thick could beThe obtained. Theshowed graphs showed the circuit open circuit voltage short circuitcurrent current were couldcurrent be obtained. graphs that thethat open voltage andand short circuit nanofibre mat was near 55 mv, and that which was 310 μm thick was near 100 mv. The short circuit were in pace with the changes periodic of changes of the load. Thecircuit open voltage circuit voltage of the 203 μm nanofibre thick in pace with the thick load. The open of the thick current ofthe theperiodic sample with the 203 μm nanofibre mat was near 25 nA, and203 thatµm which was 310 nanofibre mat was near 55 mv, and that which was 310 μm thick was near 100 mv. The short circuit mat was nearwas 55 mv, thatIt which was that 310 the µmthicker thick was near 100mat mv.is,The current of μm thick nearand 60 nA. can be seen the nanofibre the short larger circuit the electrical current of the sample with the 203 μm thick nanofibre mat was near 25 nA, and that which was 310 the sample with thethickness 203 µm thick matincreases, was near nA, and that which was 310More µm thick output is. As the of the nanofibre nanofiber mat the25 number of cations also increases. μm thick was near 60 nA. It can be seen that the thicker the nanofibre mat is, the larger the electrical was near 60 movement nA. It can forms be seen that thepotential thicker the nanofibre mat is, larger the electrical output is. cations a greater difference between thethe electrodes under mechanical output is. As the thickness of the nanofiber mat increases, the number of cations also increases. More deformation. As the thickness of theforms nanofiber matpotential increases, the number of cations also increases. More cations cations movement a greater difference between the electrodes under mechanical movement forms a greater potential difference between the electrodes under mechanical deformation. deformation.
Figure 9. Electrical output of Nafion nanofibre IPMC (Nafion nanofibre mat 203 μm). Figure 9. Electrical outputofofNafion Nafion nanofibre nanofibre IPMC nanofibre matmat 203 203 μm). Figure 9.toElectrical output IPMC(Nafion (Nafion nanofibre Compared traditional IPMC, the open circuit voltage of Nafion nanofibre IPMCµm). was greatly improved. In general, the open circuit voltage of traditional IPMC is 0.1–30 mV. The voltage of Nafion Compared to traditional IPMC, the open circuit voltage of Nafion nanofibre IPMC was greatly nanofibre IPMC could reachIPMC, 100 mVthe by comparison. But, compared to the short circuitIPMC current, it did Compared traditional openof circuit voltage of isNafion nanofibre greatly improved. Into general, the open circuit voltage traditional IPMC 0.1–30 mV. The voltage of was Nafion not increase as expected. In traditional IPMC, the Pt electrode was fabricated by electroless plating improved. In general, the reach open 100 circuit of traditional IPMC isto0.1–30 mV. The current, voltageitofdid Nafion nanofibre IPMC could mV voltage by comparison. But, compared the short circuit on the surface of the Nafion-117 film. The electrode of Nafion nanofibre IPMC was fabricated by not increase as expected. traditional IPMC, the Pt But, electrode was fabricated by electroless platingit did nanofibre IPMC could reach In 100 mV by comparison. compared to the short circuit current, coating. Obviously, the contact resistance of electrodes of Nafion nanofibre IPMC was larger than on the surface of the Nafion-117 film.IPMC, The electrode of Nafion nanofibre IPMC was fabricated by not increase as expected. In traditional the Pt electrode was fabricated by electroless plating coating. Obviously, the contact resistance of electrodes of Nafion nanofibre IPMC was larger than on the surface of the Nafion-117 film. The electrode of Nafion nanofibre IPMC was fabricated by
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Polymers 10, x FOR PEER REVIEW coating.2018, Obviously, the contact
of 13 resistance of electrodes of Nafion nanofibre IPMC was larger9 than 9 of 13 that of of traditional traditional IPMC. IPMC. Therefore, Therefore, the the short short circuit circuit current current of of the the Nafion Nafion nanofibre nanofibre mat mat did did not not that improved remarkably. that ofremarkably. traditional IPMC. Therefore, the short circuit current of the Nafion nanofibre mat did not improved Polymers 2018, 10, x FOR PEER REVIEW
improved remarkably.
Figure 10. Electrical output of Nafion nanofibre IPMC (Nafion nanofibre mat 310 μm).
Figure 10. Electrical output of Nafion nanofibre IPMC (Nafion nanofibre mat 310 µm). Figure 10. Electrical output of Nafion nanofibre IPMC (Nafion nanofibre mat 310 μm). The electrical output of Nafion nanofibre IPMC which was not soaked in water was also tested. The electricalmat output of Nafion Nafion nanofibre nanofibre IPMC which wasnot not soaked inwater water wasbe also tested. The nanofibre was of completely dry. FigureIPMC 11 shows the was results ofsoaked the experiment. Itwas can seen The electrical output which in also tested. that there was almost no output of Nafion nanofibre IPMC in the dry state. The nanofibre mat was completely dry. Figure 11 shows the results of the experiment. It can be seen The nanofibre mat was completely dry. Figure 11 shows the results of the experiment. It can be seen
that there there was was almost almost no no output output of of Nafion Nafion nanofibre nanofibre IPMC IPMC in that in the the dry dry state. state.
Figure 11. The open circuit voltage and short circuit current of Nafion nanofibre IPMC in a dry state.
The electrical output under different external loadscurrent was measured. The open circuit voltage and Figure The open circuit voltage and short circuit Nafion nanofibre IPMC a dry Figure 11.11. The open circuit voltage and short circuit current of of Nafion nanofibre IPMC in aindry state. short circuit current are shown in Figure 12, respectively. It can be seen that the open circuit voltage state. and short circuit current increased with the increase of the external load. But the increasing extent of The electrical output under different external loads was measured. The open circuit that voltage and openelectrical circuit voltage andunder short different circuit current was not very high. Therefore,The it can be inferred the and The output external loads was measured. open circuit voltage shortchange circuitofcurrent are shown in Figure respectively. can beions seen the nanofibre open circuit voltage external load has little effect 12, on the transfer of It internal ofthat Nafion IPMC. short circuit current are shown in Figure 12, respectively. It can be seen that the open circuit voltage Figurecircuit 13 shows the relationship theincrease applied of force the peak output It is a nonand short current increasedbetween with the theand external load. Butvoltage. the increasing extent and short circuit current increased with the increase of the external load. But the increasing extent of linear relationship. of open circuit voltage and short circuit current was not very high. Therefore, it can be inferred that open circuit voltage and short current was not very high.IPMC, Therefore, it can be inferred the In of order to measure thecircuit output power Nafion nanofibre a load was putthat inIPMC. the change external load has little effect onofthe transfer of internal ions ofresistance Nafion nanofibre change of external hasthelittle effect on output the transfer internal ionsload of Nafion nanofibre IPMC. series. Figure 14load shows results of the voltageofunder different resistances. It can be Figure 13 shows the relationship between the applied force and the peak output voltage. It is a Figure 13 that shows thethe relationship between the applied and theincreases. peak output voltage. It is of a nonseen with increase of the load resistance, the force voltage also The output power non-linear relationship. nanofibre IPMC could be calculated by: linearNafion relationship.
In order to measure the output power of Nafion nanofibre IPMC, a load resistance was put in 2 nanofibre IPMC, a load resistance was put in In order to measure the output power of Nafion 𝑉𝑜𝑢𝑡 series. Figure 14 shows the results of the output𝑃voltage under different load resistances. It can(4)be seen =voltage series. Figure 14 shows the results of the output under different load resistances. It can be 𝑅 that with the increase of the load resistance, the voltage also increases. The output power of Nafion seen where that with thepower increaseNafion of thenanofibre load resistance, the voltage also increases. The output power of P is the nanofibre IPMC could beofcalculated by: IPMC, Vout is the output voltage of Nafion nanofibre IPMC, Nafion nanofibre IPMC could be calculated by: and R is the load resistance. V2 P = 𝑉 2out (4) 𝑜𝑢𝑡 R (4) 𝑃= 𝑅 where P is the power of Nafion nanofibre IPMC, Vout is the output voltage of Nafion nanofibre IPMC, and R is the load resistance.
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where P is the power of Nafion nanofibre IPMC, Vout is the output voltage of Nafion nanofibre IPMC, and RPolymers is the2018, load10,resistance. x FOR PEER REVIEW 10 of 13 Polymers 2018, 10, x FOR PEER REVIEW
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Figure 12. The open circuit voltageand andshort short circuit different external loads. Figure 12. The open circuit voltage circuitcurrent currentunder under different external loads. Figure 12. The open circuit voltage and short circuit current under different external loads. Figure 12. The open circuit voltage and short circuit current under different external loads.
Figure 13. The peak output voltage under different external loads. Figure 13. The peak output voltage under different external loads. Figure 13. The peak output voltage under different external loads. Figure 13. The peak output voltage under different external loads.
Figure 14. Comparison of the output voltage under different load resistances. Figure 14. Comparison of the output voltage under different load resistances. Comparison the output voltage under load resistances. Figure Figure 15 Figure shows the output power with different loaddifferent resistances. When the external load 14. 14. Comparison ofofthe output voltage under different load resistances.
Figureis 15 shows theinternal outputresistance power with different load resistances. When power the external resistance equal to the of Nafion nanofibre IPMC, the output of the load load Figureis 15 shows the outputresistance power with different load resistances. When power the external resistance equal to the internal of Nafion nanofibre IPMC, the output of the load resistance is the maximum. The maximum value of output power was 3.63 nW, when the resistance equal to the internal resistancevalue of Nafion nanofibre IPMC, the output power the load resistance is the maximum. The maximum of output power was 3.63 nW, when theof resistance resistance is the maximum. The maximum value of output power was 3.63 nW, when the resistance
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Figure 15 shows the output power with different load resistances. When the external load resistance is equal to the internal resistance of Nafion nanofibre IPMC, the output power of the load resistance is2018, the 10, maximum. The maximum value of output power was 3.63 nW, when the11resistance Polymers x FOR PEER REVIEW of 13 was 300 KΩ. It illustrates that the internal resistance of the device is near 300 KΩ. The optimal was 300 KΩ. It illustrates that the internal resistance of the device is near 300 KΩ. The optimal electrical output would be gained under this external load resistance. Compared to traditional IPMC, electrical output would be gained under this external load resistance. Compared to traditional IPMC, the output power of 3.63 nW was effectively enhanced. Considering the mass of the Nafion nanofibre the output power of 3.63 nW was effectively enhanced. Considering the mass of the Nafion nanofibre mat (the of the nanofibre mat was powerdensity density Nafion nanofibre IPMC mat mass (the mass of the nanofibre mat was0.0856 0.0856 g), g), the the power ofof thethe Nafion nanofibre IPMC would be upbetoup42.4 µW/Kg. would to 42.4 μW/Kg.
Figure 15. The output power under different load resistances.
Figure 15. The output power under different load resistances. 4. Conclusions
4. Conclusions
In this paper, a novel structure of IPMC using an ordered Nafion nanofibre mat was presented.
In this paper,nanofibre a novel structure of used IPMCtousing an ordered Nafion nanofibre mat was presented. The Nafion IPMC was investigate the characteristics of mechanoelectrical transduction. The Nafion nanofibre mat was by electrospinning and collected by a rotated The Nafion nanofibre IPMC was used to fabricated investigate the characteristics of mechanoelectrical roller. Compared withnanofibre the Nafionmat commercial mat, theby nanofibre mat has the of high transduction. The Nafion was fabricated electrospinning andadvantages collected by a rotated water absorption and proton conductivity. Two samples of Nafion nanofibre IPMC were fabricated roller. Compared with the Nafion commercial mat, the nanofibre mat has the advantages of high water by using an ordered nanofibre mat with thicknesses of 203 μm and 310 μm, respectively. The absorption and proton conductivity. Two samples of Nafion nanofibre IPMC were fabricated by using electrical output of the open circuit voltage and short circuit current was compared. The maximum an ordered nanofibre mat with thicknesses of 203 µm and 310 µm, respectively. The electrical output of value of open circuit voltage was over 100 mv, which was generated by the Nafion nanofibre mat the open circuit voltage and short circuit current was compared. The maximum value of open circuit with a thickness of 310 μm. The results showed that the thicker the nanofibre mat is, the higher the voltage was overvoltage 100 mv,is.which was the Nafion nanofibre matoutput. with a Through thicknessa series of 310 µm. open circuit The mat ingenerated a dry state by exhibited almost no electrical The results showed that the thicker the nanofibre mat is, the higher the open circuit voltage is. The mat of load resistors, the output power and power density of the Nafion nanofibre IPMC were studied. in a dry statethe exhibited almostofno electrical output. Through a series load the output power Under load resistance 300 KΩ, the maximum of output power of and the resistors, power density was 3.63 nw anddensity 42.4 μW/Kg, The output power power Under densitythe of Nafion nanofibre of IPMC and power of therespectively. Nafion nanofibre IPMC wereand studied. load resistance 300 KΩ, was higherof than traditional IPMC. the maximum output power and the power density was 3.63 nw and 42.4 µW/Kg, respectively. In future study, how to reduce the contact nanofibre resistance of the electrodes andthan internal resistanceIPMC. to The output power and power density of Nafion IPMC was higher traditional achieve greater output power will be concentrated on. Also, there is a need to study the characteristics In future study, how to reduce the contact resistance of the electrodes and internal resistance to of microcosmic movement on ion transfer inside the Nafion nanofibre IPMC. achieve greater output power will be concentrated on. Also, there is a need to study the characteristics of microcosmic movement oncuration, ion transfer inside the Nafion nanofibre IPMC. Author Contributions: Data J.S., D.X., M.G., Q.M., and W.L.; Formal analysis, Q.C.; Investigation, L.W.; Methodology, D.W.; Supervision, D.S.; Writing–original draft, Y.Z.; Writing–review & editing, J.X.
Author Contributions: Data curation, J.S., D.X., M.G., Q.M., and W.L.; Formal analysis, Q.C.; Investigation, L.W.; Funding: D.W.; This research received no external funding. draft, Y.Z.; Writing–review & editing, J.X. Methodology, Supervision, D.S.; Writing–original Acknowledgments: This work financially supported by the National Natural Science Foundation of China Funding: This research received noisexternal funding. (No. 51505401, No. U1505243), Youth Key Project of Fujian Province College Natural Science Foundation (No.
Acknowledgments: This work isJoint financially by theProvince National Natural Science and Foundation of China JZ160493), Health-Education Research supported Projects of Fujian (No. WKJ2016-2-21), Fundamental (No. 51505401, No. U1505243), Youth Key Project of Fujian Province College Natural Science Foundation Research Funds for the Central Universities of Xiamen University (No. 20720150082). (No. JZ160493), Health-Education Joint Research Projects of Fujian Province (No. WKJ2016-2-21), and Fundamental Research Fundsoffor the Central Universities Conflicts Interest: There are no conflictsoftoXiamen declare. University (No. 20720150082). Conflicts of Interest: There are no conflicts to declare.
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