Supporting Information. Efficient Energy Harvesting Using Processed Poly(vinylidene fluoride) Nanogenerator

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1 Supporting Information Efficient Energy Harvesting Using Processed Poly(vinylidene fluoride) Nanogenerator Anupama Gaur 1, Chandan Kumar 2, Shivam Tiwari 1, Pralay Maiti 1 * 1 School of Materials Science and Technology, Indian Institute of Technology, Banaras Hindu University, Varanasi-2215, India 2 School of Biomedical Engineering, Indian Institute of Technology, Banaras Hindu University, Varanasi-2215, India. *To whom correspondence should be addressed: pmaiti.mst@itbhu.ac.in (P. Maiti) 1. Experimental Section: 1.1. Materials: Commercial Poly(vinylidene fluoride) SOLEF 68, molecular weight is used for the study which is kindly supplied by Ausimont, Italy Sample preparation: Thin film samples were prepared using compression moulding machine (S. D. Instruments, India). Samples of mm 3 dimension were cut from the thin films for stretching purpose. Stretching of the sample is performed using universal testing machine at temperature of 9 o C. The details of PVDF sample abbreviations are as follows: P: unstretched S-1

2 P-S : half-stretched P-S: fully stretched P-S-5: stretched and poled at 5 kv/cm P-S-1: Stretched and poled at 1 kv/cm 2. Characterization: Tensile testing was performed by using Universal Testing Machine (Instron). Samples were stretched till the breaking. Strain rate of 5 mm/min was used for stretching. Fourier transform infrared spectroscopy (FTIR) measurement was performed using Nicolet 57 instrument in reflectance mode with resolution of 4 cm -1. X-ray diffraction (XRD) was done to analyze the structure of stretched and unstretched samples by Rigaku Miniflex 6 X-ray diffractometer using CuKα Radiation (λ =1.54 Å), operating at a voltage of 4 kv and current 15 ma with scan rate of 3 o /min at room temperature. Polarized optical microscopy (POM) was done to study the change in morphology before and after stretching of the samples using polarized optical microscope (Leitz, Biomed), after suitable crystallization. Scanning electron microscopy (SEM) was done to observe the change in surface morphology by using SEM (SUPRA 4, Zeiss). Before observation in SEM, thin film samples were gold coated using sputtering. Piezoelectric coefficient measurement: A wide range d 33 meter (Piezo Meter System PM2) was used to measure the piezoelectric coefficient of the samples. For this poled samples were used. Poling was done by applying a high electric field across the film samples at optimized high temperature (9 o C). S-2

3 Unimorph fabrication: The unimorph of pure PVDF stretched and unstretched samples are fabricated using poled samples. The unimorph has one non-active or structural layer and one active or piezoelectric layer. The thickness of the structural layer was taken such a way that the neutral axis of the unimorph lies in the structural layer. These layers were bonded with each other and the electrodes were attached for measurement using wires. Power measurement: To calculate the power, the device was made by coating conducting silver paste and then connecting the copper electrodes on the both side of the sample. The device is then encapsulated in the Poly(dimethyl sulfoxide) (PDMS) to keep it protected from external environment and any damages. The voltage output from the samples across varying resistance was measured using Digital Storage Oscilloscope Tektronix TBS-172B and the optimum resistance was obtained at which it gives the maximum power output. The current was measured using Agilent 3441A Digital Multimeter. S-3

4 Figure S1: Deconvolution of X-ray diffraction pattern of stretched sample. Figure S2: X-ray diffraction pattern for unpoled and poled samples. XRD pattern for unpoled and poled samples were obtained. There is no significant change after poling of the sample. This indicated that poling doesn t affect the structure. It only helps align the dipoles in the direction of applied electric field. S-4

5 Current / µa P-S-1 P-S P Time / s Figure S3: Experimentally measured current for the different device as indicated. The experimental current for the sample P, P-S and P-S-1 is.8,.21 and.95 µa respectively. The maximum current of.95 µa was observed for P-S-1 sample. We can see that the experimentally observed current (~.95 µa) is less than the theoretical current (~2.28 µa) at maximum power transfer condition. This difference is due to charge loss from power consumption by internal resistance in the measurement system. S-5

6 Voltage / V E7 1E8 1E9 Resistance / Ω Current / µa Figure S4: Variation in voltage and theoretical current with resistance for the samples. With the increment of resistance the output voltage increases due to ohmic losses of the device at constant mechanical load. As the electrical load increases the voltage increases and the voltage is maximum where the internal resistance of the material matches with the external resistance (Maximum power transfer theorem). After increasing resistance there is not significance rise in voltage and the current also follows the same in accordance with ohms law (V=IR). S-6

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8 Figure S5: (a) open circuit voltage and (b) corresponding power density at different modes of application of stress as mentioned. Open circuit voltage in different modes of stress application is shown. This shows that fabricated device is capable of producing voltage at different modes also and can be used for practical applications. S-8