Enhanced Surface Charge Density of Nanogenerators by Small Molecules: A Review

Abstract

Electromagnetic generators, thermoelectric generators, solar cells, piezoelectric nanogenerators (PENGs), and triboelectric nanogenerators (TENGs) are examples of energy-conversion devices based on different operating principles. [1,2] Triboelectric devices link contact electricity with electrostatic induction, transferring inductive charges across electrodes to convert mechanical energy to electricity. [3,4] Mobile devices, Internet of things (IoT), smart wearable gadgets, and devices globally use thousands of sensors. [5] Solid-state embedded power sources require milliwatts or microwatts, yet their compactness allows for frequent charging or battery replacement. [6,7] Selfrecharging devices combine an energy harvester with a battery. [8,9] TENGs are promising for energy efficiency. These gadgets can capture energy from natural sources, mechanical power, and faint heartbeats, proving their compatibility with severe environments and the human body. [10][11][12] Surface polarization and Maxwell's displacement current affect triboelectric devices. TENG's performance has improved from the January 2012 report. Triboelectric systems can yield 1200 W m À2 area power density, 490 kW m À3 volume density, and 50-85% conversion efficiency. [13][14][15][16] Triboelectric charge density on contact surfaces affects TENG voltage and current output in all four modes. Charge density quadruples output power. Polymers and metals are triboelectric microscopically. This leads to small improvements. Chemists and materials scientists are working on theoretical and practical TENG initiatives. [17][18][19][20][21] TENG output can be enhanced by optimizing friction pairings based on electron affinity and surface modification via physical or chemical techniques. Complementary micro-or nanostructure reinforcements, such as nanotubes, polymer matrices, nanofillers, or small molecules, can be added to the triboelectric layers to enhance their performance. A "small molecule" refers to any organic substance with a molecular weight of less than 1500 Dalton. The 1500 Dalton cutoff limit was established at 900-1000 Daltons in the literature. Small molecules can perform a wide range of tasks and have a wide variety of functions, including energy harvesting, cell signaling, sensor applications, and many more. [22,23] Recently, in the 2020s, researchers are trying to use small molecules to be harnessed into TENGs to increase their charge density, as small molecules revolutionize the field of solar energy harvesting. Apart from this, very few articles are found on improving triboelectric performance using small molecules and expanding TENG applications. It might be due to the inability to choose small molecules and increase the charge density of the triboelectric surface with insufficient rudimentary physics. [24][25][26][27][28][29] This review introduces small molecules and possible selection mechanisms (such as electron denoting and withdrawing groups, including dielectric constant engineering, higher crystallinity, pK a value, and tailoring di...