Energy Harvesters Integrated with Energy Storage AND/OR End Users
In practice, it is common to connect a solar cell with a Li- ion battery for simultaneous energy harvesting and storage. However, a monolithic photo-battery has been invented to substitute this two-component solar cell-battery combination . Apart from their electrochemical properties, Li-ion intercalation materials are also mechanically active. When a stress is applied, they can exhibit a mechanical- electrochemical coupling that increases the voltage of the battery . Therefore, a study into using Li-ion batteries to simultaneously harvest energy has been carried out . The efficiency of this harvester-storage integration was only 0.012%, much smaller than that of conventional kinetic energy harvesters such as piezoelectric, electromagnetic, and triboelectric systems . The theoretical efficiency was predicted to be 2.9%. A supercapacitor has also been reported for the same purpose of simultaneously harvesting and storing energy . The electrolyte started to flow when subjected to pressure, causing the electrokinetic supercapacitor to operate in both harvesting and storage modes. The efficiency could reach 0.03%-0.1% under one bar pressure or regulated external load.
The harvesting, storage, and utilization of energy are becoming a worldwide issue due to the energy crisis, environmental pollution, and the fast development of electronics for our daily life [202-204]. Recent developments in hybrid devices are based on nanogenerators and energy storage systems through integration, hybridization, and all-in-one designs for self-charging energy systems for future electronics. Basically, the nanogenerator can be divided into representative two devices depending on piezoelectric and triboelectric properties, called the piezoelectric nanogenerator (PENG) and triboelectric nanogenerator (TENG), respectively. The first demonstrated nanogenerator was the PENG using a ZnO NW , which was operated by the piezoelectric property. The formation of a piezoelectric potential or piezo-potential was arisen from the breakage of central symmetry in the ZnO crystal structure by external force. The ZnO-based PENGs have been developed as various types [206, 207-209]. This basic principle and the model of power generation apply to other PENGs based on various piezoelectric materials, such as PZT [210-212]. Material selection and structural design are the key factors for the development of PENGs, which are based on the coupling of piezoelectric materials and flexible substrates. The all-polymer-based flexible TENG based on triboelectrification and electrostatic induction was invented in 2012 , which could convert mechanical energy into electricity. Interestingly, triboelectrification can be found everywhere in the surrounding environment and in most common materials used every day in our daily lives. The detailed mechanisms for PENG and TENG are described in the literature [213,214,215]. Both PENG and TENG generate AC pulse output. In order to provide stable power using nanogenerators, energy storage systems (ESSs) such as supercapacitors and rechargeable batteries are essential for future electronic devices. Thus, the selection of the materials, structural design, and circuit connections in designs for hybridizing ESSs and nanogenerators should be properly considered and designed because most nanogenerators have thin, lightweight, flexible substrates, and require bendable, and stretchable device formation .
In addition, wired and textile ESSs and other new functions are required according to the developments of advanced nanogenerators with higher output power and new application in future.
PENG technology is highly reliable, has stable operations, smaller device area, and diverse application fields. To date, extensive fabrication methods, growth of various one-dimensional (lD)/two-dimensional (2D) inorganic piezoelectric nanostructures (NSs) on plastic substrates , flexible piezoelectric polymer films, and device designs (planar, stretchable, cylindrical, or fiber) were developed to improve the PNG (literature uses PNG and PENG interchangeably) technology as a prominent energy-harnessing approach for creating the sustainable independent power source to drive the low-power consumed electronic devices/sensors. Moreover, the device compatibility, electrical output performance (nW/cm2 to pW/cm2) under various harsh environments, and flexibility issues were optimized to think about the realtime commercialized PNG or PENG (piezo electric nanogenerator) product. On the other side, few PNGs have dual functionality such as wearable/portable independent power source to drive the commercial electronic devices and can also work as a self- powered sensor (or a battery-free sensor) to measure/monitor the various physical, chemical, biological, and optical stimuli .
The study by Nagamalleswara et al.  suggests that PNG (or PENG) technology based on the type of materials is classified into three categories such as (a) inorganic NSs-based PNG, (b) polymer matrix-based PNG, and (c) composite (polymer + inorganic NPs) PNG as shown in Figure 9.4 . Following the development of PENG by ZnO nanowire mentioned above, many other researchers across the world extend the growth of other NSs and its device designs for inorganic PNG, but it has few ample drawbacks such as typical growth process of piezoelectric NSs, brittleness, lower force limits, failure instability, and leakage current issues . Further, PNG was implemented with the flexible PVDF and its copolymers due to its high flexibility, easy process to prepare flexible films, low electrical output performance, and accepting large mechanical force. However, it has one major disadvantage such
FIGURE 9.4 The schematic represents the overview of PNGs (or PENGs): classification, drawbacks, and advantages .
as low piezoelectric coefficient and relative permittivity at room temperature than the inorganic piezoelectric NSs. Nagamalleswara et al.  overcome the issues of the inorganic and polymer PNGs by developing the composite PNG technology. The major key factor in designing the efficient, flexible composite PNG device is the development of multifunctional hybrid or composite piezoelectric structures. It indirectly depends on the selection of individual high-performance inorganic and polymer materials and cost-effective fabrications processes. These kinds of composite PNGs are highly suitable to work as sustainable independent power sources as well as self-powered sensors to measure various physical parameters such as physical, optical, biological, and chemical stimuli. Zhao et al.  developed hybrid piezo-/ TENG for highly efficient and stable rotation energy harvesting.
The all-polymer-based flexible TENG based on triboelectrification and electrostatic induction was invented in 2012 , which could convert mechanical energy into electricity. Interestingly, triboelectrification can be found everywhere in the surrounding environment and in most common materials used every day in our daily lives. Both PENG and TENG generate AC pulse output. In order to provide stable power using nanogenerators, ESSs such as supercapacitors and rechargeable batteries are essential for future electronic devices. Thus, the selection of the materials, structural design, and circuit connections in designs for hybridizing ESSs and nanogenerators should be properly considered and designed because most nanogenerators have thin, lightweight, flexible substrates, and require bendable, and stretchable device formation . In addition, wired and textile ESSs and other new functions are required according to the developments of advanced nanogenerators with higher output power and new application in future.
To test the efficient charging of the battery for hybrid devices, recently, researchers have studied how to integrate the TENGs with lithium-ion batteries (LIBs) using a rectifier. In 2016, Pu et al. demonstrated efficient charging of LIBs by a rotating TENG with pulsed output current . Fast Li-ion extraction from the typical electrode materials LiFeP04 and Li4Ti5012 was achieved by the TENG at a rotation speed of 250 rpm. The estimated coulombic efficiency of the TENG charging and the following 0.5 C discharging can be higher than 90%, comparable with constant current charging. Interestingly, improvement of the power utilization efficiency (up to 72.4%) in transferring power from the TENG to the LiFeP04-Li4Ti5012 full battery was achieved by optimizing the coil ratio of a transformer. High efficiency was achieved when the impedance of the TENG was reduced to close to that of the battery cell. In addition, they showed that a 1 h charging of a commercial LIB by the rotating TENG (600 rpm, 36.7 transformer coil ratio) can exhibit a discharge capacity of 130 mAh. In fact, the energy conversion efficiency is very important when the generated output power is stored in ESSs. Nan et al. prepared the cathodic material Li,V2(P04)3/C and compared the storage efficiency with most popular cathodic materials: LiCoO,, LiFeP04, and LiMn204 . They showed that the selection of electrode materials is important for efficient charging in hybrid devices.
The term “all-in-one hybrid device” means that a single device has both an energy harvesting nanogenerator and an ESS in the device without complicated connections. The all-in-one hybrid device design has many advantages for compact, simple, and portable devices, but more efforts are needed to fabricate the devices because of the problem of matching device and material characteristics, such as flexibility, coatings, compositions, electrochemical properties, etc., between the nanogenerator and the ESS. Very recently, researchers have reported and suggested new concepts for all-in-one devices.
In 2016, Wang et al. reported a new nanoenergy cell (NEC) that uses high-density piezoelectric nanowires to harvest mechanical energy and has a large electrolyte (phosphoric acid/polyvinylalcohol (H,P04/PVA) gel electrolyte)-nanowire interface to store electricity in the all-in-one system consisting of a PENG and an electric double-layer supercapacitor (EDLC) . The device achieved a continuous output current for over 90s, and the mechanical-electric energy conversion efficiency of the NEC was over 10 times higher than that of the PENG without increasing the device volume or reducing the efficiency. Interestingly, Ramadoss et al. made an all-in- one device from a PENG and a pseudo-capacitor based on PVDF-ZnO and MnO,, respectively . The device exhibited self-charging capability under palm impact (aluminum-foil-based device to 110 mV over 300s; fabric-based device to 45 mV over 300 s). Most recently, Song et al. demonstrated an integrated sandwich-shaped, selfcharging power unit (SCPU) with a wrinkled poly(dimethyl siloxane)-based TENG and a carbon nanotube (CNT)/paper-based solid-state EDLC . During vibrations, the device can be utilized to simultaneously harvest and store the mechanical energy as electrochemical energy, and it could be charged to 900 mV in 3h under the compressive stress at 8 Hz. This study showed that their developed novel all-in- one device is a promising candidate for flexible electronics and wearable devices.
Guo et al. reported the concept of an all-in-one shape-adaptive self-charging power package based on a TENG and EDLC that has been simultaneously demonstrated for harvesting body motion energy to sustainably drive wearable/portable electronics. Subsequent research made advances on washability, flexibility, water proof characteristics, and stretchability of the all-in-one devices .