SYNTHESIS, FABRICATION, AND CHARACTERIZATION OF 2D LAYERED NANOMATERIALS AND POLYMER BASED FLEXIBLE PIEZOELECTRIC AND TRIBOELECTRIC NANOGENERATORS FOR ENERGY HARVESTING APPLICATIONS

Abstract

With increasing demand of energy, the emission of carbon is increasing in our environment and also the limited resources of fossil fuels have brought the green and renewable energy generation on high demand. Most of the electronic gadgets use batteries that also requires to be recharged after some time. The advances in low powered electronics, wireless sensors and micro electromechanical systems have tremendously raised the power requirement in the existing digital world. In the present scenario, we are completely surrounded with several electronics devices, such as, cell phones, tablets, I-pods, smart watches etc., which collectively require huge amount of power for their operation. A green energy generating device, which can harvest the energy from ambient sources present in our surroundings to fulfill the energy needs of future technologies without polluting our surrounding is becoming more in demand. The development of nanogenerators offers the best hope for the current global energy crisis rises due to social globalization. Piezoelectric nanogenerators (PENGs) and triboelectric nanogenerators (TENGs) are currently gaining a lot of attention for harvesting mechanical energy from the ambient environment to serve as the power source for powering electronic devices. When a piezoelectric material is used as one of the two components in the triboelectric nanogenerator, these two effects can be coupled. It is possible to further improve the output efficiency of the nanogenerator by integrating these two effects to form a hybrid nanogenerator. The present thesis involves the synthesis of nanocomposite films based on PVDF and 2D layered nanomaterials and the subsequent investigation of their applications within energy harvesting devices, primarily through the utilization of piezoelectric and triboelectric effects. With this aim, PVDF films with different nanocomposites have been synthesized, where the systematic effect of the chalcogen atom of the Transition metal dichalcogenides (TMDCs) (MoS2, MoSSe, MoSe2) and polyvinylidene fluoride (PVDF) based composites on the piezoelectric performance of the fabricated piezoelectric nanogenerator has been investigated. Raman and FTIR spectroscopy revealed the enhancement of piezoelectric behaviour due to increase in the polar β-phase content of PVDF after addition of TMDCs in PVDF. Out of all the fabricated piezonanogenerator, the PVDF/MoSSe based nanogenerator showed the maximum peak to peak open circuit voltage of 31.2 V and short circuit current of 1.26 μA. This enhancement was achieved just by adding TMDCs without any further treatment. The enhanced piezoelectric performance is attributed to the synergistic contributions from inherent piezoelectricity of synthesized TMDCs and intensified β-phase for the PVDF/TMDCs vi composites. The highest output voltage for the PVDF/MoSSe device is ascribed to the lack of reflection symmetry in MoSSe structure. The generated voltage was also used for lightning a commercial LED and for charging of a capacitor of 4.7 uF. After studying the effect of different TMDCs on the piezoelectric properties of PVDF, we have checked the effect of different percentages of MoS2 as a nanofiller in the PVDF matrix. The concentration of nanofiller in the PVDF also contributes to the device output performance. To check the ideal concentration of nanofiller, we have fabricated the different piezoelectric energy harvesters based on PVDF-MoS2 materials by varying the weight percentage (0 %, 2%, 5 % and 7 wt %) of MoS2. The piezoelectric output of the PVDF was found to be increased due to the incorporation of MoS2. In comparison to bare PVDF film, the piezoelectric nanogenerator based on 7 wt% of MoS2 as filler in PVDF shows nearly 2-fold increase in peak-to-peak output voltage from 9.4 V to 18.0 V. After that, we have also investigated the impact of MoS2 weight percentage on the triboelectric performance of PDMS/PVDF-MoS2 based nanogenerator. The results show that 7 wt% of MoS2 is the optimum amount of doping for triboelectric energy harvesting similar to the case of piezoelectric energy harvesting; above this level, the energy harvesting performance of the nanogenerator abruptly decreased due to MoS2 agglomeration in the PVDF matrix. Therefore, the triboelectric nanogenerator with 7 wt% of MoS2 exhibits maximum output voltage of 189 V and a short circuit current of 1.61 µA. The TENG with 7 wt% MoS2 produced a power density of 104.5 µWcm 2 which is ~2.7 times of the power density produced by the bare PVDF based TENG. Further, a piezo-tribo based hybrid nanogenerator (HNG) is fabricated by integrating the MoS2-PVDF film having 7 wt% of MoS2 with PDMS thin film as two layers required for the HNG device. Our study reveals that inclusion of MoS2 in the PVDF matrix stimulates the generation of β-phase and also the intrinsic piezoelectric properties which contributes in the overall enhancement of the piezoelectric output of PVDF. In addition to enhanced β-phase, the distribution of MoS2 increase the surface roughness which increases the contact area and also boosts the surface potential and dielectric constant thereby increasing the triboelectric output of the device. Each of these factors combine to significantly increase the performance of the hybrid nanogenerator. The fabricated TENG demonstrated the practical application, by powering electronic stopwatch and scientific calculator. After the analysis of the effect of filler concentration, to further improve the efficiency of the PENG and TENG, the PVDF-MoSe2 nanofibers were synthesized via electrospinning technique with different wt% of MoSe2 (0 wt%, 3 wt%, 5 wt%, 7 wt% and 10 wt%). In this work, hybrid triboelectric nanogenerator coupling both the piezoelectric and triboelectric effects have been vii fabricated to drive the environment friendly approach of splitting the water for hydrogen production. Here, electrospun PVDF-MoSe2 fiber deposited over aluminium foil, and Nylon fiber based triboelectric negative and positive material, respectively have been used for fabricating the hybrid nanogenerator. The PVDF-MoSe2/Nylon based TENG exhibits remarkable peak to peak open circuit voltage of 113.6 V and 26.5 µA of short-circuit current. Moreover, it exhibits a high power density of 230.4 µWcm-2 under piezo-tribo coupling, which is even superior than the majority of previously fabricated similar type of devices. This TENG, which has great output performance, can also capture energy from a variety of mechanical and biological motions, and the device versatility has also been shown by its use in hydrogen evolution. It has been observed that material selection is critical for TENGs since the triboelectric properties of the materials have significant effect on the performance of TENGs. Therefore, after fabricating a high performance PVDF-MoSe2 nanofiber based TENG, in the last part, we have studied the effect of triboelectric layer material on the output performance of the TENG device. In this study, the PVDF-MoSe2 nanofiber film has been fixed as one of the triboelectric layers, whereas the second layer material has been varied including PTFE, PDMS, PET, Paper and Nylon. The TENG consisting of Nylon as second triboelectric material demonstrates the maximum power density of ~231 µWcm-2 . The effect of tapping frequency and force on the triboelectric output of PVDF-MoSe2/Nylon was also studied and an increasing trend of output voltage and current is observed with increasing force and frequency thereby producing a maximum voltage of ~206 V. The increase in output performance of the TENG with increase in frequency is attributed to faster charge transfers process, when the contact frequency is higher. With the rise in contact force, the effective area and capacitance of the TENG also increases which further results in the enhancement of triboelectric output of the nanogenerator. This study proposes an effective approach for enhancing the performance of triboelectric nanogenerator just by selecting the suitable material.