Principles of Energy Harvesting Devices for Self-Powered and Flexible Mechanical Sensors—Case Studies

The pursuit for developing self-powered touch sensor systems with flexible configuration has motivated the exploration of flexible and wearable electronic device applications, which potentially has profound importance in humanoid robotics and epidermal bioelectronics. It has also drawn attention from diverse medical engineers. The next-generation mechanical sensors are required to be highly sensitive to subtle stimuli, and to be deformable as well as self-powered. In this presentation, three different principles are introduced to fabricate the self-powered flexible and mechanical sensors based on the presenter’s research. First, the triboelectric energy harvesting technology using two-dimensional (2D) nanosheets is discussed.[1] My group demonstrates a fast, non-vacuum, wafer-scale, and patternable synthesis method for 2D MoS₂ using pulsed laser-directed thermolysis. The laser-based synthesis technique that we have developed can apply internal stress to MoS₂ crystal by adjusting its morphological structure, so that a surface-crumpled MoS₂ triboelectric energy harvesting device generates ~40% more power than a flat MoS₂ one. The direct-synthesized MoS₂ patterns are utilized to fabricate a self-powered flexible haptic sensor array. Additionally, some mechanisms of triboelectrifications are introduced.[2] My group firstly suggested the ‘Backflow and Stuck Charge Theory’ using the first-principles calculation as well as experimental data. The theory shows that the potential barrier between two triboelectric surfaces affect the backflow electron flows just after certain mechanical contact, rather than the forward electron flows at the mechanical contact. Secondly, the piezoelectric energy harvesting approach using simple nanoparticles is introduced.[3] In particular, my group studied that the concentration gradient based core-shell structure of ferroelectric nanoparticles can induce notable flexoelectric effects which coupled and enhanced its piezoelectric properties.[4] Lastly, the ionic diode-based energy harvesting method using ionic migration phenomena is presented as one of the energy harvesting technology.[5] Using two parts of agarose hydrogels with different ionic polymers, the soft ionic diode was developed. As confirmed by the finite element analysis, the underlying mechanism of the hydrogel ionic diode involves the formation of the depletion region by mobile cations and anions and the subsequent increase of the built-in potential across the depletion region in response to mechanical pressure. The applications of the self-powered hydrogel ionic diode to tactile sensing, pressure imaging, and touchpads are demonstrated, with sensing limitation is as low as 0.01 kPa. The versatility of the energy harvesting principles using advanced materials makes it highly promising for next-generation self-powered and wearable mechanical sensor systems.<br/>[1] “Laser-directed synthesis of strain-induced crumpled MoS₂ structure for enhanced triboelectrification toward haptic sensors”, <i>Nano Energy</i>, Vol. 78, pp. 105266, 2020.<br/>[2] “Triboelectrification: Backflow and Stuck Charges Are Key”, <i>ACS Energy Lett.</i>, Vol. 6, No. 8, pp. 2792–2799, 2021.<br/>[3] “Kinetic motion sensors based on flexible and lead-free hybrid piezoelectric composite energy harvesters with nanowires-embedded electrodes for detecting articular movements”, <i>Compos. B. Eng.</i>, Vol. 212, pp. 108705, 2021.<br/>[4] “Flexoelectric-Boosted Piezoelectricity of BaTiO₃@SrTiO₃ Core-Shell Nanostructure Determined by Multiscale Simulations for Flexible Energy Harvesters” <i>Nano Energy</i>, Vol. 89, pp. 106469, 2021.<br/>[5] “Hydrogel Ionic Diodes toward Harvesting Ultralow-Frequency Mechanical Energy”, <i>Adv. Mater., </i>Vol. 33, No. 36, 106469, 2021.