Addressing Charge Losses in Triboelectric Mechanical Energy Harvester


Student thesis: Doctoral Thesis

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Award date22 Aug 2022


Anthropogenic climate change related to conventional concentrated and distributed energy production and consumption is a key challenge to our technological progress and humanity. For this aspect, mechanical energy with substantial reserves and zero pollution holds a potential to meet the energy demands via nanogenerator device architecture from a few volts to giga volts with power density upto 10000 W m-3. Triboelectric nanogenerators (TENG), based on the coupling of contact electrification and electrostatic induction effects, have attracted worldwide attention owing to their lightweight, simple design, low cost, and diverse material selection. However, the low-frequency harvesting technologies still face the dilemma of low charge density, low current output, and hence limited power density, which greatly limits their practical application. In this thesis, the low surface charge accumulation and further discrepancies between surface charge density and the corresponding current density are elaborated in terms of interfacial, bulk charge losses, and tribocharges delocalization.

First, this thesis summarizes the fundamentals of TENG and highlights the figure of merit issues thereof, starting from the origin of surface charges, and energy conversion, to practical application, which lays the foundation for designing this study.

Second, we report a symmetric TENG device design for quantitative measurements of the intra-layer and interlayers interfacial charge recombination centers and a new approach to eliminate this recombination by an efficient screening of charges from contacting interfaces to deep down charge blocking centers of the bulk matrix. Unlike conventional charge blocking approaches, where the electrons are blocked from the contacting interface, we introduce hole blocking material in the tribopositive PVDF matrix which captured positive charge from the interface and delocalizes it to the bulk. This way interfacial recombination and trapped charge-induced short-circuiting are simultaneously prevented, and the charge retention duration of the device increases to from 15 min to 5.5 h representing a 32-fold increase in current density to 4.4 mAm-2. This interfacial design increases the device output exponentially when stacked in a parallel interlayered structure, demonstrating the high potential to power high-demand electronics.

Third, the interfacial recombination is further eliminated by fabricating tribopositive layer from encapsulating magneto-sensitive magnetite into PVDF matrix via electrospinning. The magnetite accommodates the central part and embeds downward in the matrix developing charge transporting channels in the bulk and the positive groups floating outwards providing all positive surface. Owing to their magneto-sensitive nature, the embedded centers form an oblong structure in response to the magnetic field, which acts as a charge transporting channel and thus overcoming bulk losses to the dielectric matrix, enhancing the power density to 1.66 Wm-2.

Fourth, we introduce a few second fast autonomous self-healing property in solids without solvents by coupling ionic and/or covalent interactions with diffusion-less electrostatic interactions. A newly developed self-healing hydrogel matrix is enriched with intermolecular, intramolecular hydrogen bonding sites, non-bonding electron pairs, and dynamic covalent bonding sites crosslinked via secondary ionic bonding. A highly transparent hydrogel comprising PVA graft copolymerized with a hydrogen reservoir GA and crosslinked via ferric ions is synthesized via free radical polymerization. The hydrogel displays fast autonomous self-healing characteristics that are retained in dry, wet, and frozen states via an electrostatically driven diffusion-less self-healing mechanism. The self-healable, stretchable, transparent hydrogel acts as an induction electrode in single electrode TENG showing a high peak power density of 2.55 Wm-2, and its characteristic output remains intact after 18-day storage in a desiccator. The output remains invariant to the frequency of dynamics owing to the more charge delocalization centers, which develop a stable polarization interface between the electrification and induction layers. The power density is further enhanced to 8 Wm-2 with hand tapping, where 80% of this output is retained after device storage in aqueous and frozen (-30 ºC) environment for 24 h. The design strategy of combining chemical bonding, electrostatic interaction-driven self-healing, and electrical properties holds potential for application in prosthetics, robotics, cryogenics, and sportswear-based wearable electronics.

In summary, this thesis systematically investigated charge losses at various interfaces of TENG and proposed approaches to eliminate charge recombination at contacting interfaces, in the bulk, and on induction electrodes via electron and hole blocking mechanisms and embed charge transporting channels in the dielectric matrix and thus prolonging charge delocalization in the induction matrix. The concept of investigating amphoteric regions in electrification layer is introduced and new type of stacking device units is illustrated compared with the conventional parallel and series stack design. We believe our work on addressing charge losses provide important insights to the fundamental understanding of charge transfer through interface, bulk and induction to electrodes for a wide range of applications.