Triboelectric Nanogenerator Driven Self-Powered Solar Water Splitting Based On Hematite Photoanodes

Hydrogen energy is a kind of clean,green and environmentally friendly energy with high combustion value.It is considered as one of the most ideal alternative energy sources to handle energy and environmental problems.Photoelectrochemical(PEC)water splitting system based on semiconducting nanomaterials has become a fantastic and alternative method to obtain hydrogen energy due to its unique advantages.Among all materials,hematite is one of the most promising photocatalytic materials,which has been widely studied in recent years.However,the conduction band of hematite is too low to reduce water directly for hydrogen.External bias is necessarily required to release both oxygen and hydrogen in hematite-based PEC water splitting system,which greatly limits its application in real life.Triboelectric nanogenerator(TENG)can collect all kinds of mechanical energy in daily life and output electric energy.Through transformation and rectification,TENG can easily generate the desired electricity for hematite to produce hydrogen in PEC water splitting system.Considering those,we combined TENG with hematite-based PEC water spitting system in this paper,aiming at realizing hydrogen production without external power sources.The main contents are as follows(1)We successfully developed a rotatory disc-shaped triboelectric nanogenerator(RD-TENG)driven self-powered PEC water splitting system based on Ti-Fe2O3 photoanode to produce hydrogen energy.The external bias provided by RD-TENG enables hematite to overcome the disadvantage of unsuitable conduction band position for fully solar water splitting.The generated electricity is primarily transformed with a step-down transformer,followed by the full-wave rectification to output direct current as the necessary bias.At a relatively low rotation speed of 65 rpm,the peak current significantly increases to 0.12 mA under illumination compared to that in the dark with almost zero.With the rotation speed increasing to higher than 120 rpm,the peak current in the dark is nearly equal to that under illumination,indicating the direct electrolysis of water.The rate of hydrogen evolution is up to?6.67 ?L min-1 under illuminating at 140 rpm.This fantastic strategy would be an alternative and efficient route to collect solar power and mechanical energy simultaneously in the form of hydrogen energy.(2)We prepared a Lithium-ion battery based on Co3O4 material and introduced it into the self-powered PEC water splitting system based on Fe2O3 photoanode driven by TENG.Through light-operated switch,the external bias provided by TENG enables hematite to overcome the disadvantage of unsuitable conduction band position for fully solar water splitting under sufficient illumination.The peak current under illumination observed at a relatively low rotation speed significantly increased when compared to that in the dark,while no obvious change could be observed at a high rotation speed.When the illumination is insufficient or in the dark,the TENG turns to charge the lithium-ion battery.At the rotation speed of 100 rpm,the battery is charged to?1.8 V in about 80 minutes.With the increase of rotation speed,the charging time is shortened.The as-prepared hybrid system not only successfully collects solar and mechanical energy at the same time in the form of hydrogen energy,but also stores mechanical energy into lithium-ion batteries when the illumination is insufficient,which improves the efficiency of energy conversion.(3)We synthesized Molybdenum-modified hematite(Mo-Fe2O3)through a two-step hydrothermal method,and then treated it in hydrogen atmosphere to obtain H2-treated Mo-Fe2O3 photoanode.Compared with the pristine Fe2O3,the carrier concentration of Mo-Fe2O3 increases slightly,while that of H2-treated Mo-Fe2O3 increases significantly.The electrochemical impedance spectroscopy(EIS)also shows that H2-treated Mo-Fe2O3 photoanode has much smaller diameter of semicircle with lower resistance for the interfacial charge transfer and resistance for trapping holes by the surface states The significantly decreased values of both resistances indicate much faster charge transfer kinetics at the electrode interface and surface.At the same time,hydrogen treatment also reduces the onset potential of hematite photocathode,up to?100 mV.Both the samples with hydrogen treatment and Mo modification have higher bulk and surface separation efficiency.The photocurrent density of H2-treated Mo-Fe2O3 reaches 2.47 mA cm-2 at 1.23 V RHE,which is about 0.7 mA cm-2 higher than that of Mo-Fe2O3.Loaded with a layer of FeNiOOH co-catalyst,the photocurrent density of the final sample reaches 3.08 mA cm-2 at 1.23 V vs.RHE,about 3.3 times as much as that of the pristine Fe2O3.Furthermore,the performance of hematite can be enhanced through immersing in NaH2PO2 solution and heat treatment.In a word,decorated or modified hematite photoanodes exhibit good photocatalytic performance,and can be combined with the improved high-power triboelectric nanogenerator to develop self-powered PEC water splitting systems for hydrogen production in the future.