Flow-induced Voltage Generation Research Articles

Voltage generation induced by thermo-driven ion solution flow in CNTs for low-grade thermal energy harvesting

The flow induced voltage generation has been regarded as an advanced energy harvesting technology at nanoscale with broad application prospects. The ionic liquid flow inside carbon nanotubes (CNTs) can be potential to harvest the low-grade thermal energy. In this work, the flow of aqueous solution of sodium chloride (NaCl) driven by the temperature gradient and the performance of voltage generation in CNT are investigated with the non-equilibrium molecular dynamics (MD) simulations. The accumulated displacement and the number density distribution of ions and water molecules are recorded in the simulations. The average flow velocities of ions are obtained and the flow-induced voltage is then predicted. The results show the thermal gradient along the axial direction of CNT drives the NaCl solution to flow from its hot end to cold one, which can drag the free charge carriers of CNT with a directional shift and generate a considerable voltage output. Moreover, the effects of temperature difference, CNT diameter and solution concentration on the ions flow and the flow-induced voltage are also studied. It is found the influence of the average flow velocity difference between Na+ and Cl− plays a dominant role on the flow-induced voltage, the different influencing factors show different rules in the variation of voltage generation. The findings in this work can contribute to the development of new energy-harvesting nano-devices for the low-grade thermal energy.

A study on the effects of the number of layers of graphene for flow-induced power generation on graphene/polyetrafluoroethylene membranes

Abstract We report generation of flow-induced electrical voltage and current associated with changing numbers of graphene layers in graphene surfaces prepared on PTFE (polytetrafluoroethylene) substrates by moving an ionic droplet of NaCl to investigate the effects of pseudocapacitance phenomena between each graphene surface and the ionic solution. Each sample exhibited different surface energies and electrical ionic double layers on the graphene surface. For each sample, these properties were evaluated by water contact angle measurements and Raman spectra, respectively. Additionally, we demonstrated changes in flow-induced voltage and current generation using various ionic concentrations of NaCl (0.2, 0.4, and 0.6 M). Based on triboelectrification and pseudocapacitance phenomena, the generated voltage and power for monolayer graphene were 8.1 and 15.9 times larger, respectively, than the values obtained for 3 layers.

Highly Increased Flow-Induced Voltage Generation on Hybrid Membranes of Monolayer Graphene and Single-Walled Carbon Nanotubes

We investigate the hybrid structure composed of single-walled carbon nanotubes (SWCNTs) and monolayer graphene to highly increase flow-induced voltage generation by an ionic droplet on these hybrid carbon membranes. These properties were characterized by Raman spectra, a field-emission-scanning probe, and optical microscope. We demonstrated flow-induced voltage generation on the hybrid structure at various ion concentrations of NaCl. The generated voltage for the membrane of SWCNTs/graphene/SWCNTs was 8.636 and 4.92 times larger than for the SWCNTs, and graphene/SWCNTs membranes, respectively, based on the highly increased electron dragging mechanism.

Highly sensitive and reliable octyltrichlorosilane coated silicon sensors for nitrogen gas flow detection

In this article, we present highly sensitive and reliable nitrogen gas flow sensors based on surface modified n-type (n-Si) and p-type (p-Si) silicon wafers. The surfaces of n-Si and p-Si wafers were coated with octyltrichlorosilane (OTS) molecules. The surface modified n-Si and p-Si wafers were used as sensing elements. The sensing elements were kept on the 45° angle inclined to the experimental mount and nitrogen gas was flown over the inclined surface at subsonic velocities. The flow induced voltage generation is caused by the interplay between the mechanisms of Bernoulli's principle and Seebeck effect. The experimental results show that the half coated and full coated n-Si and p-Si wafers generate more voltage than that of the uncoated at a given velocity. In the case of n-Si wafers, the half coated surface generated voltage was ≈4.80 times than that of the uncoated surface. In the case of p-Si wafers, the half coated surface generated voltage was ≈3.83 times than that of the full coated. The surface analysis of the half and full coated surfaces show that the OTS SAM molecules on n-Si surfaces were homogeneous and well ordered, however on p-Si surfaces, the OTS SAM molecules were heterogeneous and disordered. The enhanced voltage generations and high sensitivities are caused by an effective change in the gradient of Fermi energy (EF) due to OTS SAM coating. The experimental observations and theoretical analysis of enhanced voltage generation show that the OTS surface modification causes synergism of different mechanisms, contributing to the cumulative increase of flow induced voltage. The effect of aging process on the reliability of the half coated OTS/n-Si and OTS/p-Si sensors were tested for 24 h, 30 days and 90 days, the experimental results confirm that the flow induced voltages are consistent and reproducible and both the OTS/n-Si and OTS/p-Si sensors are very reliable. The investigation of the effects of the coverage of the OTS SAM and the angle of inclination on the flow induced voltage shows that the 50% coverage of the OTS/n-Si, OTS/p-Si and the 45° angle of inclination produced a maximum flow induced voltage at any given velocity.

Nitrogen doping effect on flow-induced voltage generation from graphene-water interface

Liquid-flow-induced generation of electricity using nanocarbons, particularly graphene-water interface, has received attention for energy harvesting. Here, we have obtained voltage generation from a single water droplet motion on graphene. We have investigated the effect of the graphene surface condition on flow-induced voltage generation, which is controlled by heteroatom doping. Nitrogen-doped graphene shows three times higher voltage generation compared to pristine graphene due to the doping-induced surface charge of graphene. Graphene surface potential tuning by doping is shown to play an important role in voltage generation.

Flow-induced voltage generation by driving imidazolium-based ionic liquids over a graphene nano-channel

In this study, the flow-induced voltage is investigated by driving the pure bulk room temperature ionic liquid (RTIL) 1-ethyl-3-methylimidazolium tetrafluoroborate ([Emim][BF4]) flowing over a graphene nano-channel consisting of two parallel single-layered graphene sheets using molecular dynamics simulation for the first time.

Effects of perfluoroalkylsilane molecular assembly on flow induced voltage generated by doped silicon wafers

We report the effects of surface modifications on (i) Seebeck coefficients and (ii) flow induced voltage generations of the n-type (n-Si) and p-type (p-Si) silicon wafers. The surfaces of n-Si and p-Si wafers were coated with 1H,1H,2H,2H- perfluorooctyltrichlorosilane (FOTS) molecules. The surface modified n-Si and p-Si of size 12 mm×4 mm were mounted on the π/4 angle inclined experimental mount, and nitrogen gas was flown over the inclined surface at the subsonic velocities, 5.3, 10.61, 15.91, 21.22, and 26.52 ms−1, and the voltage difference between the lead and rear ends of pristine and surface modified n-Si and p-Si was measured. The experimental results and theoretical relations are presented. The flow induced voltage generation is caused by the interplay between the Bernoulli flow and Seebeck effect. The flow-voltage response results show that the half coated and full coated n-Si and p-Si wafers generate more voltage than that of the uncoated at a given velocity. The band theory reveals that the flow of nitrogen gas accumulates charge carriers at the FOTS self assembled monolayer (SAM)—silicon interfaces, which resulted in the more voltage generation by full and half coated ni-Si and p-Si surfaces than that of the pristine surface. The enhanced voltage generations and high sensitivities are caused by an effective increase of the gradient of Fermi Energy (EF) (Seebeck coefficient) due to FOTS SAM coatings. Because of that the FOTS SAM modified n-Si and p-Si are become highly sensitive to nitrogen gas flow.

Highly Increased Flow-Induced Power Generation on Plasmonically Carbonized Single-Walled Carbon Nanotube .

We generate networks and carbonization between individualized single-walled carbon nanotubes (SWCNTs) by an optimized plasmonic heating process using a halogen lamp to improve electrical properties for flow-induced energy harvesting. These properties were characterized by Raman spectra, a field-emission-scanning probe, transmission electron microscopy, atomic force microscopy and thermographic camera. The electrical sheet resistance of carbonized SWCNTs was decreased to 2.71 kΩ/□, 2.5 times smaller than normal-SWCNTs. We demonstrated flow-induced voltage generation on SWCNTs at various ion concentrations of NaCl. The generated voltage and current for the carbonized-SWCNTs were 9.5 and 23.5 times larger than for the normal-SWCNTs, respectively, based on the electron dragging mechanism.

Flow-induced voltage generation by moving a nano-sized ionic liquids droplet over a graphene sheet: Molecular dynamics simulation.

In this work, the phenomenon of the voltage generation is explored by using the molecular dynamics simulations, which is performed by driving a nano-sized droplet of room temperature ionic liquids moving along the monolayer graphene sheet for the first time. The studies show that the cations and anions of the droplet will move with velocity nonlinearly increasing to saturation arising by the force balance. The traditional equation for calculating the induced voltage is developed by taking the charge density into consideration, and larger induced voltages in μV-scale are obtained from the nano-size simulation systems based on the ionic liquids (ILs) for its enhanced ionic drifting velocities. It is also derived that the viscosity acts as a reduction for the induced voltage by comparing systems composed of two types of ILs with different viscosity and temperature.

Galvanism of continuous ionic liquid flow over graphene grids

Flow-induced voltage generation on graphene has attracted great attention, but harvesting voltage by ionic liquid continuously flowing along graphene at macro-scale is still a challenge. In this work, we design a network structure of graphene grids (GG) woven by crisscrossed graphene micron-ribbons. The structure is effective in splitting the continuous fluid into “droplets” to generate consistent voltage using the mechanism of electrochemical energy generation. Key parameters such as flow rate, mesh number of GG, and slope angle are optimized to obtain maximum voltage in energy generation. The results suggest great potential of this graphene-based generator for future applications in energy harvesting.

Flow-induced voltage generation in graphene network

We report a voltage generator based on a graphene network (GN). In response to the movement of a droplet of ionic solution over a GN strip, a voltage of several hundred millivolts is observed under ambient conditions. In the voltage-generation process, the unique structure of GN plays an important role in improving the rate of electron transfer. Given their excellent mechanical properties, GNs may find applications for harvesting vibrational energy in various places such as raincoats, umbrellas, windows, and other surfaces that are exposed to rain.

Flow-induced voltage generation over monolayer graphene in the presence of herringbone grooves.

While flow-induced voltage over a graphene layer has been reported, its origin remains unclear. In our previous study, we suggested different mechanisms for different experimental configurations: phonon dragging effect for the parallel alignment and an enhanced out-of-plane phonon mode for the perpendicular alignment (Appl. Phys. Lett. 102:063116, 2011). In order to further examine the origin of flow-induced voltage, we introduced a transverse flow component by integrating staggered herringbone grooves in the microchannel. We found that the flow-induced voltage decreased significantly in the presence of herringbone grooves in both parallel and perpendicular alignments. These results support our previous interpretation.

Flow-induced voltage generation in non-ionic liquids over monolayer graphene

To clarify the origin of the flow-induced voltage generation in graphene, we prepared a new experimental device whose electrodes were aligned perpendicular to the flow with a non-ionic liquid. We found that significant voltage in our device was generated with increasing flow velocity, thereby confirming that voltage was due to an intrinsic interaction between graphene and the flowing liquid. To understand the mechanism of the observed flow-induced voltage generation, we systematically varied several important experimental parameters: flow velocity, electrode alignment, liquid polarity, and liquid viscosity. Based on these measurements, we suggest that polarity of the fluid is a significant factor in determining the extent of the voltage generated, and the major mechanism can be attributed to instantaneous potential differences induced in the graphene due to an interaction with polar liquids and to the momentum transferred from the flowing liquid to the graphene.

Flow-induced voltage generation in high-purity metallic and semiconducting carbon nanotubes

We investigated the flow-induced voltage generation of single-walled carbon nanotubes (SWCNTs), comparing metallic and semiconducting types, flow velocity, and different ionic concentration solutions. The induced fluid-flow voltage was measured using a microfluidics chip that we fabricated with a SWCNT film embedded between its metal electrodes. We found that the voltage generated for semiconducting nanotubes was three times greater than that for metallic nanotubes and that both types of SWCNTs showed an unexpected reversal in signal sign, likely due to the switching of the major carrier between holes and electrons. These generated voltages increased proportionally for both types of SWCNTs as functions of flow velocity and ionic concentration.

Multiwalled carbon nanotubes for flow-induced voltage generation

Recently it has been reported that voltage can be generated by passing fluids over single-walled carbon nanotube (SWCNT) arrays with potential application to flow sensors with a large dynamic range. The present work investigates voltage generation properties of multiwalled carbon nanotubes (MWCNTs) as a function of the relative orientation of the nanotube array with respect to the flow direction, flow velocity, and solution ionic strength. It was found that the flow-induced voltage can be significantly enhanced by aligning the nanotubes along the flow direction, increasing the flow velocity and/or the ionic strength of the flowing liquid. A flow-induced voltage of ∼30mV has been generated from our perpendicularly-aligned MWCNT in an aqueous solution of 1M NaCl at a relatively low flow velocity of 0.0005m∕s, which is 15 times higher than the highest voltage reported for single-walled carbon nanotubes. The results are generally consistent with the pulsating asymmetric ratcheting mechanism proposed for SWCNT arrays, in which an asymmetrical spatial distributed strain forms from interactions with the polar and ionic species at the tube surface and is driven along the tube by the fluid flow.

Flow-induced voltage and current generation in carbon nanotubes

New experimental results, and a plausible theoretical understanding thereof, are presented for the flow-induced currents and voltages observed in single-walled carbon nanotube samples. In our experiments, the electrical response was found to be strongly sublinear -- nearly logarithmic -- in the flow speed over a wide range, and its direction could be controlled by an electrochemical biasing of the nanotubes. These experimental findings are inconsistent with the conventional idea of a streaming potential as the efficient cause. Here we present a new, physically appealing, Langevin-equation based treatment of the nanotube charge carriers, assumed to be moving under coulombic forcing by the correlated ionic fluctuations, advected by the liquid in flow. The resulting 'Doppler-shifted' force-force correlation, as seen by the charge carriers drifting in the nanotube, is shown to give a strongly sublinear response, broadly in agreement with experiments.