Energy-harvesting And Pressure-sensing Devices Based On Carbon Nanotubes

Carbon nanotubes(CNT)are the one-dimensional nano-material,which have a configuration of hexatomic ring.Their nano-scale diameter and millimeter-scale length(even up to centimeter)endow them with large aspect ratio and high specific surface area.Due to their carbon-carbon covalent bond,CNTs have excellent mechanical,electrical,thermal properties.Carbon materials comprised of CNTs have excellent intrinsic properties of CNTs and some special properties that depended on their geometrical configurations,which allow them to be potentially used in energy harvesting and intelligent sensing fields.Our strategy is to focus on the fabrication of CNT sheets with specific structure and the study of pressure-induced electrochemical double-layer capacitance change of CNT sheets.According to the pressure-induced capacitance changes,CNT sheets were used to harvest the compressive mechanical energy underwater,monitor the underwater pressure changes,and detect the pressure changes in air.The main research contents and results are listed as follows:(1)Non-aligned CNT forests were synthesized using a combination of reactive ion etching(RIE)and water-assisted chemical vapor deposition(CVD),and then were drawn to fabricate CNT sheets by adopting mechanical draw.The mechanical strength,structure stability,and capacitance behavior of CNT sheets were studied.It shows that CNT sheets made using a 200 k Pa draw stress had nano-gap structure,high electrical conductivity of 1100 S/m(axial direction),high density of 520 mg/cm3(areal density of 4.1 mg/cm2),high degree of alignment of 0.68(Herman's orientation factor,HOF).Moreover,they provided exceptional resistance to shrinkage in various aqueous electrolytes,and had high tensile strength of ~700 k Pa.More important,the structure parameters(density,degree of alignment)and surface modification(non-ionic surfactant,Triton X-100)dependence of capacitance of CNT sheets were explored to obtain CNT sheets with excellent capacitance properties.(2)CNT energy harvesters were designed and assembled to electrochemically convert the compressive mechanical energy into electric energy.Here,mechanical-drawn CNT sheets were used as the working electrode.Then,the working mechanism(pressure-induced electrochemical potential variation),output power and energy,and energy conversion efficiency of the CNT energy harvester were discussed in detail.The results show the output power of CNT energy harvesters in 0.6 M Na Cl aqueous solution(similar to the salinity of seawater)reach up to 1.28 m W/m2(31.5 m W/kg)at low frequency range.Moreover,the energy per cycle(at 0.02 Hz)and energy conversion efficiency was 34.3 m J/m2(0.85 J/kg)and 4.7%,respectively.More important,these CNT energy harvesters could harvest more mechanical energy and generate large voltage and current when they were operated in other electrolytes.For example,a CNT energy harvester in 0.1 M HCl aqueous solution generated a peak-to-peak open-circuit voltage(OCV)of 7 m V,peak short-circuit current(SCC)of 0.8 A/m2,and peak power of 3.98 m W/m2(119 m W/kg).(3)The CNT energy harvesters also could be used to monitor the underwater pressure changes due to their pressure-dependent electrical signals.These electrochemical sensors,which could use seawater as the electrolyte,generated pressure-change-response electrical current.The results show that these CNT pressure sensors had wide pressure detection range of 10 Pa-300 k Pa(1 mm-30 m water level change),high pressure sensitivity of 0.008 A/(m2?k Pa),which covers most of the wave height monitoring in ocean.They were used to detect and distinguish the moving objects,tides,whirlpools and underwater navigation.Moreover,their ability to operate at wide temperatures(4-60 °C),frequencies(0.01-5 Hz),and salinities(0.1-5 M)enabled them excellent environmental adaptability.Besides,they could also used in corrosive chemical environments(0.1 M HCl and 0.1 M KOH)to monitor the pressure changes.(4)According to the electrochemical working mechanism(pressure-induced electrochemical potential variation),CNT sheets were used for the wearable self-powered pressure sensors.The results show that these electrochemical sensors could electrochemically generate electrical signals in response to pressure change without requiring an external power supply.The high sensitivity covered wide medium-high pressure range of 10-330 k Pa for human motion monitoring.They were used for breathing,pulse rate,sound vibrations,air-flow disturbances monitoring.Moreover,the output power(at 0.5 Hz,sinusoidal pressure of 330 k Pa)and energy reached up to 54 ?W/g and 52 ?J/g,respectively,which could be used for driving a low-power-consumption Bluetooth or Wi-Fi wireless device.Besides,several wearable CNT pressure sensors were integrated into a sensing array for imaging pressure-load shape and pressure distribution.