Letter www.acsami.org
Native Cellulose Microfiber-Based Hybrid Piezoelectric Generator for Mechanical Energy Harvesting Utility Md. Mehebub Alam and Dipankar Mandal* Organic Nano-Piezoelectric Device Laboratory, Department of Physics, Jadavpur University, Kolkata 700032, India S Supporting Information *
ABSTRACT: A flexible hybrid piezoelectric generator (HPG) based on native cellulose microfiber (NCMF) and polydimethylsiloxane (PDMS) with multi wall carbon nanotubes (MWCNTs) as conducting filler is presented where the further chemical treatment of the cellulose and traditional electrical poling steps for piezoelectric voltage generation is avoided. It delivers a high electrical throughput that is an open circuit voltage of ∼30 V and power density ∼9.0 μW/cm3 under repeated hand punching. We demonstrate to power up various portable electronic units by HPG. Because cellulose is a biocompatible material, suggesting that HPG may have greater potential in biomedical applications such as implantable power source in human body.
KEYWORDS: native cellulose microfibers, lead-free green piezoelectric, MWCNTs, PDMS, mechanical energy harvesting generator
T
generation. In this context, recently epoxy resin, viz., polydimethylsiloxane (PDMS) has been successfully used as a host polymer for fabricating sensors and actuators due to its low Young modulus, adequate biocompatibility, excellent transparency, cost-effectiveness, and good thermal and chemical stability.14,15 It has been found that different types of piezoelectric materials are reported so far, for example, as a semiconductor (ZnO, ZnS, GaN, etc.),4−6 ceramics (PZT, BaTiO3, NaNbO3, KNbO3, KNN, ZnSnO3, etc.),3,8−10,12,13 polymers (PVDF and its copolymers, polyamides, parylene-C, etc.)16,17 and natural materials (bones, hairs, collagen fibrils, peptide, cellulose, sugar cane, etc.).18−20 Among them, natural piezoelectric materials are possess several advantages over the other class of piezoelectric materials. For example, cellulose, a novel piezoelectric polymer is almost endlessly available polymeric raw material that exhibits fascinating natural structure and also treated as eco-friendly and biocompatible product. In addition, cellulose is also utilized for versatile sensing applications and easily obtained from various plants as microfibrils consist of glucose−glucose linkages with linear chains.21 It is constituted of crystallites with interspersed amorphous regions of low degree of order.21,22 Native cellulose, namely cellulose I, is the crystalline cellulose. The term, regenerated cellulose, is called cellulose II, which is also refer as cellulose that usually needs a chemical treatment to prepare. Furthermore, cellulose exhibits both the direct and conversed piezoelectricity.23,24 Recently,
he piezoelectric materials have the ability of generating electric potential when subjected to certain mechanical stimuli, such as stress, elongation, tension, vibration, etc. Since 2006, after observing piezoelectric power generation from zinc oxide nanowires arrays, an effective green pathway of energy harvesting associated with mechanical vibration is opened up.1 Thereafter, a surge progress has been made on the nanoscale piezoelectric area where nanomaterials are generally grown on a substrate to fabricate the piezoelectric generator.2−7 It has been also realized that the necessary to design a HPG that not only suited to produce high piezoelectric potential, rather flexibility is another major concern. Thus, the fabrication of a flexible piezoelectric generator is one of the prime focus of interest in mechanical energy harvesting area. Substrate grown piezoelectric generators are involved a complicated fabrication procedure that limits the flexibility, suitable integration into robotic applications, biomedical sensing, electronic skin designing etc. Hence, preparation of a flexible composite film through dispersion of piezoelectric material in a host polymer by mechanical agitation becomes the favorable way to fabricate piezoelectric generators.8−10 Compared to substrate grown methodologies, this process is a very simple and easy to scale up. Furthermore, additional conducting fillers can be easily incorporated to improve the output performance of the generators.11−13 It noteworthy that conducting fillers are mainly used to make a conducting network between each piezoelectric constituents for smooth energy transmission to the top and bottom electrodes by enhancing the conductivity of the generator and avoiding internal energy dissipation. Besides this, conducting fillers are also assist the piezoelectric constituents to make them well dispersed within the host polymer matrix, which is the key aspect for high output power © 2016 American Chemical Society
Received: September 2, 2015 Accepted: January 11, 2016 Published: January 11, 2016 1555
DOI: 10.1021/acsami.5b08168 ACS Appl. Mater. Interfaces 2016, 8, 1555−1558
Letter
ACS Applied Materials & Interfaces cellulose based electro-active paper (EAPap) has been reported as a smart material as the piezoelectric charge constant (27−28 pC/N) of cellulose is comparable to the synthetic piezoelectric polymers.25−28 The fabrication of ultrathin film from cellulose has also been demonstrated for piezoelectric energy harvesting application.29 To fabricate these, chemical modifications of cellulose with toxic organic solvents and application of external electrical field or stretching is employed that limits the production yield because of the higher rate of electrical breakdown failure and could add significant cost to commercial scale production. In this work, we emphasized not to perform the chemical treatment of cellulose, rather NCMF (Loba Chemie., India) and MWCNTs (average diameter of 9.5 nm and length 1.5 μm, Nanocyl S.A., Belgium) are mixed together with PDMS (Sylgard 184, Dow Corning Corp., USA) by simply mechanical agitation to fabricate a HPG, where the typical electric poling step is also completely ruled out. The details fabrication procedure is also described in Figure S1. A high degree of crystallinity (χc ≈ 75%, estimated by curve deconvolution analysis of X-ray diffraction pattern, shown in Figure 1a) with microfibers (average diameter ∼10 μm, inset of
Figure 2. Chemical network structure of cellulose, attributing the hydrogen bonding linkages (dashed lines). The inset illustrates the Fourier transform infrared spectra (measured in attenuated mode) of native cellulose.
Figure 1. X-ray diffraction pattern with curve deconvolution (where crystalline peaks, Acr and amorphous halos, Aamr are shown) and crystalline planes indexing (a) and scanning electron microscopy image (enlarge view in inset) (b) of the native cellulose (digital photograph is placed at inset). The degree of crystallinity is estimated ΣAcr × 100%, where ΣAcr and ΣAamr are from the equation, χc =
Figure 3. (a) Open-circuit output voltage, (b) voltage and instantaneous power output as a function of the load resistance (corresponding circuit diagram is shown at inset), (c) performance of capacitor charging from HPG by repeating human hand punching and releasing motion. Green LEDs (spell “JU”, abbreviation of our university name) and LCD screen (displayed at inset of c) are powered up directly and from charged capacitor, respectively.
ΣAcr + ΣA amr
the total integral area of crystalline peaks and amorphous halos, respectively.
Figure 1b) are evident from native cellulose (digital photograph is placed at inset of Figure 1a) attributed from Field emission scanning electron microscopy images (Figure 1b) The network structure of cellulose (Figure 2) exhibits the multiple OH groups on the glucose from one chain form hydrogen bonding with oxygen atoms with the same or a neighbor chains and hold the chains together side-by-side (verified from Fourier transform infrared spectroscopy result, shown in the inset of Figure 2 and corresponding representing vibrational bands are listed in Table S1) is responsible for the spontaneous electric dipole formations in cellulose microfibrils inside the crystal lattice. Thus, the piezoelectric effect is produced by the displacement or reorientation of the dipoles in crystal lattice under external stress.30 The voltage generation of the HPG are assessed by tapping the device vertically under the repeated human hand punching and releasing motion with imparting stress amplitude of 40 kPa (calculation is shown in the Supporting Information, Text S1). Figure 3a represents the significant enhanced open circuit output voltage from the HPG that reveal the repeatedly generation of ∼30 V open circuit voltage (recorded with a
digital oscilloscope, Agilent DSO3102A). An enlarged view of the output signal is also shown, where a large positive peak is generated due to the direct impact of the stress (punching) and a negative peak corresponds to a relaxed state after removal of the initial stress. It should be noted that the positive peaks are always greater than their negative counterparts. These variation arise because of the different rates of compression and expansion, and the fact that the HPG can be considered as a very leaky capacitor. Thus, when HPG is placed under stress, resilience does not occur with the same rate as under stress and release motion, particularly “punch” takes considerably a longer time to return to its undistorted state. This increased time would allow the separation of piezoelectric charge carriers are returned to its neutral state, guiding a lower effective voltage. The possible mechanism of electrical power generation associated with the HPG is as follows: The cellulose microfibrils are randomly oriented inside the PDMS matrix as shown in Figure S2a. It is expected that each cellulose 1556
DOI: 10.1021/acsami.5b08168 ACS Appl. Mater. Interfaces 2016, 8, 1555−1558
Letter
ACS Applied Materials & Interfaces
S7b, c) as the MWCNTs played dual roles as a dispersing and conducting agent.13 To determine the optimal performance of HPG to use as an effective energy harvester, we varied the mixture ratio of NCMF and MWCNTs in a fixed amount of PDMS matrix. As shown in Figure S8a, the output voltage from HPG rises with increasing amounts of MWCNTs up to 0.5 wt %, at the fixed amount of NCMF (5 wt %). Because the MWCNTs play the dual role as dispersing and conducting agent, when it reached the percolation limit, open circuit output voltage attempts to decline. It is the higher concentration of MWCNTs weakening the insulating properties of the composite that results in the leakage of charge flow. When the amount of NCMF is varied (Figure S8b), the generated output voltage increases initially with the amount of NCMF for a fixed amount of MWCNTs (0.5 wt %), i.e., it directly attributes to the increment of piezoelectric material content in the HPG. However, in the presence of an excessive amount of NCMF (above 5 wt %) in the composite the output voltage start to diminish. The main reason behind this trend is associated with the dielectric property of the composite film.11 The presence of a large amount of piezoelectric material in a composite may also cause the agglomeration that results a lower output voltage. For potential utilization of our energy harvester, we used the HPG to operate different commercial portable electronic units. For example, HPG was directly connected with the array of LEDs to power up. Inset of Figure 3c and corresponding Video S1 demonstrated that green LEDs in series are successfully illuminated to spell the letters “JU”. Furthermore, HPG is also used to power up a LCD screen. For this, an electrolytic low leakage type capacitor (1 μF/25 V) is charged up with the typical full wave rectifier (IC type bridge diode is used) circuit, and an electronic unit is connected across the capacitor to power up, as shown in the circuit diagram of Figure S9a. The capacitor charging performance is presented in Figure 3c, indicating successfully charged up to 5 V, which is sufficient enough to power up tiny portable electronic units like LCD screen, calculator and wrist watch, as shown in inset of Figure 3c and Figure S9b−d. The corresponding Video S2 demonstrated that HPG can be utilized as an alternative energy harvesting power source for tiny portable electronic devices. The good stability (as shown in Figure S9e) of our HPG is an additional feature that ensures its applicability as effective and robust energy harvester that might be very much suitable for energy harvesting application driven by irregular excitations present in our living environment. In summary, we report a flexible piezoelectric hybrid generator made of NCMF and PDMS with MWCNTs as supplement material. Under repeating human hand punching, the NCMF-based HPG shows an open circuit output voltage of ∼30 V and short circuit output current of ∼500 nA, corresponding to a power density ∼9.0 μW/cm3. It can power up several number of LEDs, different portable electronic units, i.e., commercial LCD screen, calculator and wrist watch. Based on these results and considering the several merits, we conclude that the HPG might be potential useful as piezoelectric energy harvesting power source. Thus, a costeffective and industrially viable batch production of HPG is feasible with endlessly available, nontoxic cellulose as raw materials. Significantly, it may also open a new era to fabricate a lead-free piezoelectric energy generator in large scale device fabrication. Furthermore, because of the nontoxicity of device the power generation from inexhaustible biological energy such
microfibrils acts as the source of electric dipole (originated from the OH group) component pointing along the thickness direction. Thus, upon applying a vertical compressive stress, the cellulose microfibrils are subjected to compressive strain and subsequently strain-induced electric polarization takes place. As a result, positive and negative charges are accumulated at the two opposite electrodes (top and bottom) and finally the piezoelectric potential leads to electric power generation. When the compressive strain is released, the piezoelectric potential is tried to diminish as the accumulated charges are returned to the opposite direction resulting the reverse polarity. Therefore, continuously applying and releasing the compressive stress results the alternating voltages. The short circuit current (measured by Keithley 4200-SCS Parameter Analyzer) is found to be ∼500 nA (Figure S2b), corresponds to power density of ∼9.0 μW/cm3 (calculation is shown in Supporting Information, Text S2). Usually, it has been observed that an adequate level of voltage generation is feasible from piezoelectric nanogenerator only when an effective electrical poling treatment is under taken as shown in Table S2. In contrast, our HPG exhibits much superior output voltage where traditional electrical poling treatment step is completely obsoleted. The homolytic (leading to radical) chain breaking (Si−O−Si groups) of PDMS upon mechanical input may play an important role on the output performance of the HPG. Thus, bond breaking mechanism of PDMS (Figure S3) upon mechanical input might be a possible mechanism to execuate the high output parformance of HPG. It is due to the more OH groups opening produced from bond breaking of PDMS that form hydrogen bonding with the oxygen atom of the glucose units resulting increasing the piezoelectric dipoles density (Figure S4). This bond breaking phenomena and the viscoelastic behavior of the hybrid composite film might be responsible for the variation of piezoelectric coefficient (d33) from 8 to 15 pC/N (Supporting Information, Text S3). The output voltage generation form the HPG across a load resistor (a circuit diagram is shown at inset of Figure 3b) gradually rises with increasing resistance (Figure 3b) and saturate at infinitely high resistance similar like the open circuit 2 voltage. The instantaneous power (P = V ) has been reached to RL
the maximum value, i.e., ∼ 20 μW at a 30 M Ω of resistance, as shown in Figure 3b. These results are consistent with the other piezoelectric materials based similar type of devices.7,11 The theoretical current (I =
P RL
) is estimated to 816 nA at the
maximum power transfer condition, which is relatively higher than the experimental observation. This discrepancy is mainly attributed to the power consumption of internal resistance presents in the measuring unit. The output voltage of the HPG under a reverse connection confirmed the piezoelectric reversibility as shown in Figure S6. The polarities of the measured output signals are inverted when the HPG at reverse connection.9,12 In addition, we prepared three control samples (MWCNTs in PDMS, NCMF in PDMS and NCMF with MWCNTs in PDMS) and test the performances individually under the similar input conditions. PDMS-MWCNTs based device shows no significant output response (refer to Figure S7a) because there is no piezoelectric material present in the device and the PDMS-NCMFMWCNT-based device shows superior output response compared to the PDMS-NCMF-based device (refer to Figure 1557
DOI: 10.1021/acsami.5b08168 ACS Appl. Mater. Interfaces 2016, 8, 1555−1558
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ACS Applied Materials & Interfaces
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as cardiac motion, blood circulation, sensation, etc., can also be possible by simply interfacing the HPG of proper dimension with human body parts in future.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b08168. Detailed fabrication procedure, field-emission scanning electron microscopy (FE-SEM) image of the composite film, short circuit current from the HPG, concentrationdependent (NCMF or MWCNTs) output voltage, piezoelectric reversibility test. photograph of the OFF and ON conditions of the LCD screen, calculator and wrist watch, stability test of the HPG (PDF) Video S1, power up the LED (AVI) Video S2, power up the LCD screen (MPG)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Tel.: +91-33-2414-6666, ×2880. Fax: +91-33-2413-8917. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the Science and Engineering Research Board (SERB/1759/2014-15), Govt. of India. M.M.A. is supported by the UGC-BSR fellowship (P-1/RS/191/14). The authors are also thankful to Mr. Samiran Garain for his fruitful suggestions on bond breaking mechanism of the composite.
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REFERENCES
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DOI: 10.1021/acsami.5b08168 ACS Appl. Mater. Interfaces 2016, 8, 1555−1558
