Energy from the Nanofluidic Transport of Water through Nanochannels

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Energy from the Nanofluidic Transport of Water through Nanochannels between Packed Silica Spheres Kundan Saha, Jumi Deka, and Kalyan Raidongia ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.9b01299 • Publication Date (Web): 16 Aug 2019 Downloaded from pubs.acs.org on August 23, 2019

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ACS Applied Nano Materials

Energy from the Nanofluidic Transport of Water through Nanochannels between Packed Silica Spheres Kundan Saha, Jumi Deka and Kalyan Raidongia* Department of Chemistry, Indian Institute of Technology Guwahati, Guwahati, 781039, Assam, India Email: [email protected]

ABSTRACT: Efficient harvesting of electrokinetic-streaming-potential requires a trade-off between high flow-rate and nanofluidic confinement. To attain the best out of these parameters, we have developed here a periodic network of tetrahedral and octahedral voids interconnected through fine biconical nanofluidic channels by close-packing nearly monodisperse silica spheres of diameters 285, 620, 1000, 1750 and 2900 nm. The interstices of close-packed silica spheres (diameter 285 to 1750 nm) simultaneously exhibit surface-charge-governed ionic conductivity and fast flow of water. The power density harvested from streaming water was found to be increasing with increased diameter of the close-packed spheres up to 1750 nm (151 mWm-2), and to be decreasing with further rise in the sphere diameter. The power density was found to be dependent on the mass loading of the silica spheres, contact area of the electrodes and pH of the streaming water. Pre-treatment of the silica spheres with concentrated nitric acid further enhanced the efficiency of the energy harvesting through streaming potential. Harvesting of streaming potential from packed silica spheres was found to be a convenient way of obtaining energy from water flowing through the household water taps, as they can be connected in a series to add up energy generated in multiple devices.

KEYWORDS Nanofluidic

crystals;

Streaming

potential;

Water-Energy;

Electrokinetics 1 ACS Paragon Plus Environment

Close-packed

spheres;

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INTRODUCTION The dependence of modern society on centralized power sources can be minimized by creating various portable devices that can harvest energy from resources in household environments. Numerous research efforts directed toward the development of sustainable energy harvesting systems have unveiled several innovative ways of generating power from unconventional sources like wind, water, wave, heat, and humidity. For example, electrical power has been produced from concentration gradient across the perm-selective membranes via a nanofluidic phenomenon called reverse electrodialysis.1-4 Nanogenerators were designed to harvest electrical power from the environmental humidity gradients.5-8 Numerous possibilities of converting the mechanical energy into electrical energy (or vice-versa) through triboelectricity (or electrochemical actuators) have also been demonstrated.9-13 Similarly, the chemical energy hidden in the interface of carbon-based materials and calm water was also transformed into electrical energy.14-15 Unfortunately, a majority of these demonstrations either fail to perform in the practical scenarios or they are not found to be commercially viable. The mechanical energy of flowing water has been a major source of green electricity for more than hundred years. However, the physical requirements of large dams and infrastructures limit their applicability as a decentralized energy source. On the other hand, the possibility of harvesting electrokinetic energy from fluids streaming through ultrafine charged nanochannels was indicated as well as theoretical verified more than half a century ago.16 However, due to its humble efficiency, electrokinetic-streaming-potential had failed to receive necessary attention until the recent advancements in the field of nanoscience and nanotechnology. In the wake of the demonstration of Sood and co-workers on the development of potential difference along the flow direction of water through bundles of single-walled carbon nanotube,17 several other graphitic nanomaterials have been explored for energy harvesting from flowing water.1821

Among them the most notable ones are the two-dimensional sheets of few-layer graphene 2 ACS Paragon Plus Environment

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and graphene-based hydrogels. Similarly, Dekkar and co-workers have exploited silica-based rectangular nanochannels to unveil various fundamental aspects of electrokinetic-streamingpotential.22-24 Even though channels of well-defined geometry are ideal as far as the fundamental understanding is concerned, they need to overcome many hurdles to meet the requirements for technological applications. For example, with channels of regular geometry it is challenging to tune and/or improve the energy efficiency as the basic requirements of streaming potential, namely nanofluidic confinement, surface-charge density and flow-rate are mutually exclusive. Therefore, in this article, we explored the complex nanofluidic network formed by the interconnected interstices of packed silica spheres with a view to harvesting streaming potential. The interstices of the theoretically favoured fcc close-packed structure can also be considered as a nanofluidic network of interconnected tetrahedral and octahedral pores arranged in a periodic manner, where each tetrahedron pore is connected to four octahedron pores, and each octahedron pore is connected to eight other tetrahedron pores through fine biconical nanofluidic channels.25-26 Interestingly, the size of the biconical fluidic channels that interconnect the octahedron and tetrahedron pores sharply drops from hundreds of nanometers to a few angstroms. Through a combination of analytical modelling and finite-element calculations, Gravelle et al. demonstrated that in biconical pores the conical entrance and suitable opening angle significantly improve the liquid permeability.27 This periodically arranged network of large voids and ultra-fine biconical fluidic channels provides a unique combination of fast flow and tight nanofluidic confinement, making it an ideal system for harvesting electrokinetic-streaming-potential. This nanofluidic network of silica interstices is found to be a convenient way of harvesting energy from water flowing through household water taps. The nanofluidic confinement of nanoparticle crystals has been exploited earlier to generate potential from concentration gradient via reverse electrodialysis.28 However, the present work exploits both high fluid-throughput and nanofluidic confinement of

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interconnected-biconical-nanochannels with a view to harvesting potential from pressuredriven flow of water. RESULTS AND DISCUSSION Due to its extraordinary properties and abundance, silica has been one of the most widely employed materials in a variety of energy harvesting applications. In this article, negative surface charges of silica spheres are exploited to break the electroneutrality of liquids that flow through its interstices with a view to generating potential gradient along the flow direction. At first, silica spheres of different average diameters (285, 620, 1000, 1750 and 2900 nm) were prepared through hydrolysis of tetraethyl orthosilicate (TEOS) in the presence of ammonia, following a procedure similar to that of Stӧber method.29-30 The field emission scanning electron microscopic (FESEM) images and XRD patterns of the as obtained spheres of different mean diameters are shown in Figure 1a and Figure S1 respectively. Upon vacuum filtration through a cellulose nitrate membrane (pore size 100 nm), the aqueous dispersions of these nearly monodispersed spheres yielded close-packed structures. In the close-packed structures, the silica spheres do not attach to each other with strong attractive forces and they can be easily re-dispersed in water under a mild stirring condition. The tap density of the 285, 620, 1000, 1750 and 2900 nm sized spheres was found to be 0.74, 0.56, 0.44, 0.22 and 0.16 gcm-3 respectively. A representative FESEM image revealing tiny interstices between the densely packed spheres is shown in Figure 1b. These interconnected interstices are exploited here as fluidic channels for the fast nanofluidic transport of the liquids. In order to understand the nanofluidic characteristics of the interstices, silica spheres were packed in a cylindrical column (diameter 1 cm) equipped with two Ag/AgCl electrodes at a distance of 1 cm apart. A schematic representation of the same is shown in Figure 1c. The columns packed with silica spheres were soaked with aqueous KCl solutions of different concentrations. Ionic conductivity between the electrodes was measured by recording I-V curves with a source meter instrument (Keithley 4 ACS Paragon Plus Environment

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2450). The slope of the I-V curve was normalized with the column dimensions in order to calculate the ionic conductivity, and was plotted as a function of concentration (shown in Figure 1d). The conductivity of the silica interstices formed by spheres of diameter 1750 nm and below displayed the typical characteristics of nanofluidic devices with two distinct conductivity regimes, known as the surface-charge-governed ionic conductivity. While, at the low concentration regime, conductivity was unaffected by the changing electrolyte concentration, at higher concentration regime, it increased with increasing salt concentration. Interestingly, the transition concentration from bulk-like conductivity to surface-chargegoverned conductivity was found to be dependent on the diameter of the spheres signifying a decreasing trend in nanofluidic confinement with increasing dimension of the spheres (Figure 1d). The conductivity of the silica interstices of sphere diameter 2900 nm displayed almost a bulk-like behaviour. Similar surface-charge-governed ionic conductivity was also reported by Chen et al. working with the nanochannel-network of close-packed nanoparticle crystals.26 Analogous to nanoparticle crystals, the characteristics of silica interstices can be expressed in terms of effective cross-sectional area (S) and cross-sectional perimeter (P), equation (1) 𝑆

𝑃

𝐺 = 10 ―3(µ + + µ ― )c𝑁𝐴𝑒𝐿 + µ + σ 𝐿

--------------------- (1)

The µ, NA, and e represent ionic mobility, Avogadro constant, and elementary charge respectively and L is the distance between the electrodes. P is defined as the total exposed surface of the spheres at per unit length. Considering the space occupancy for the ideal closepacked structure to be 74.05%, P was calculated as, 𝑃 =

4𝜋𝑟2 × 0.74 𝑆𝑚𝐿/ 𝐿

4𝜋𝑟3 3

, where, r represents

the radius of the sphere. S was calculated as [(1-74.05)×Sm×L/L], where, Sm = πR2, and R is the radius of the cylinder. The first term on equation (1) accounts for the bulk conductance, and proportional to S and the electrolyte concentration. The second term is proportional to the surface charge density (σ) and P, accounts for the electrical double layer formation. P for the 5 ACS Paragon Plus Environment


close-packed silica devices of different average diameters 285, 620, 1000, 1750 and 2900 nm were calculated to be 495, 227, 141, 81 and 48.6 m respectively. σ for silica spheres of different diameters were determined by fitting the conductance vs concentration data with equation (1). A minute increase in the σ values was observed with decreasing diameter of the spheres, shown in the bar diagrams of Figure 1e. The decreasing diameter of the spheres increases the curvature of the surface leading to a larger number of under-coordinated surface atoms. This slight variation in σ values is attributed to the increasing number of under-coordinated atoms in the surface. For the determination of σ, the mobilities of the ions were assumed to be unaffected by the flow of the medium (water) through the complex route of silica interstices. The goodness of the fitting can be seen in the Figure S2. In order to gain a rough idea about the overall confinement of the silica interstices, the conductivities of the silica interstices are also compared with that of an imaginary square cylinder of equivalent surface charge density at the transition point between surface-charge-governed ionic conductivity to bulk-like conductivity, by employing equation (2).31-32

heff =

𝜎 × 10 ―3 --------- (2) 𝑐𝑡𝑁𝐴𝑒

Here heff represents the dimension (height/width) of the squares. From the comparison, the nanofluidic confinement of the interstices formed by close-packing of the silica spheres of different diameters 285, 620, 1000, 1750 and 2900 nm was found to be equivalent to that of the imaginary square cylinders of size 220, 250, 270, 520 and 540 nm respectively. One of the most important characteristics of close-packed nanofluidic system is that irrespective of the dimensions of the spheres it has regions of very high nanofluidic confinement near the contact areas. A schematic illustration of the same is presented in the Figure 2a. In order to experimentally verify the existence of nanoconfined regions, 15 ml of aqueous methylene blue (MB) dye solution (3 ppm) was flowed through 250 mg of silica 6 ACS Paragon Plus Environment

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spheres packed in a cylindrical column of length 2 cm and diameter 1 cm. Figure S3a shows the UV-Vis spectra of the MB solution permeated through silica packed column of different sphere diameters. From the UV-Vis spectroscopic data, the amount of MB adsorbed by the column packed with silica spheres of diameter 285, 620, 1000, 1750 and 2900 nm was calculated to be 2.98, 2.95, 2.90, 2.80 and 2.76 ppm respectively. Similarly, the dye adsorption capacity of the silica spheres in the unconfined dispersed state was determined by stirring spheres of different diameter (250 mg each) with 15 ml of aqueous MB solution (3 ppm) for 30 minutes (Figure S3b). The difference in the adsorption capacity of the spheres in the closepacked state to unconfined bulk-state is attributed to the ultra-confined regions originating from close packing of the spheres. The bar diagrams in Figure S3c show that, with decreasing sphere dimension the ratio of dye adsorption by the spheres in packed and unpacked state was enhanced from 4.7 to 117 times. The interstices of close-packed silica spheres not only exhibit nanofluidic confinement but also support a fast flow of water. Under a vacuum pressure of 550 mmHg, 50 ml of water took just 27 min to flow through a cylindrical column (diameter 1 cm) packed with silica spheres (1000 nm) up to 1 cm height, yielding a flow-rate of 1.8 ml/ min. As described by Chen et al.,26 in a fcc structure, each tetrahedron void is connected to four octahedron voids, and each octahedron void is connected to eight tetrahedron voids through biconical channels. This highly interconnected network of voids and biconical nanochannels can account for the high flow-rate through the sphere-packed cylinder. The flow-rate of water was found to be increasing with increasing diameters of the packed silica spheres as shown in Figure 2b. This improvement in flow-rate is attributed to larger interstices formed by the larger spheres, and is concomitant with the results obtained from surface-charge-governed ionic conductivity. As spheres were supported on a membrane of pore size ~ 100 nm (the diameters of the spheres vary between 285 and 2900 nm), the close-packed structure was undisturbed by the water flow. 7 ACS Paragon Plus Environment


The nanofluidic confinement and high flow-rate of water through the complex nanofluidic network of silica interstices were explored for harvesting streaming potential. A cylindrical column of 1 cm diameter equipped with two Ag/AgCl electrodes at a distance of 1 cm apart was packed with silica spheres (1000 nm), and DI water was passed through its interstices. A vacuum pressure of 550 mmHg was applied to attaining a flow-rate of 1.8 ml/min. The opencircuit potential and zero-volt current were simultaneously monitored as a function of time by employing source meter instruments. Figure 2c shows that as soon as DI water started flowing through the interstices an open-circuit potential started appearing and got saturated at around 1.7 V, which was also accompanied by a current of 2.6×10-6 Amp (Figure 2d), yielding a stable power output of 69 mWm-2. The kinetic energy of the water flowing through the interstices was calculated to be 110 ×10-6 J, and the power density generated by per unit of water kinetic energy (in Joules) was found to be 627 Wm-2. The power generated from water flowing through the silica interstices is attributed to electrokinetic-streaming-potential. In short, upon contact with water the surface hydroxyl groups of silica spheres undergo hydrolysis yielding negative surface charges that are balanced by ions of opposite polarity forming an electrical double layer (EDL). The pressure-driven streaming of water drags the mobile ions of EDL, developing a potential difference along the flow direction, which is known as electrokinetic-streamingpotential. A two-dimensional schematic illustration of the streaming-potential through silica interstices is shown in Figure 2e. Under the constant flow-rate of water (0.05 ml/min), the potential value was found to be increasing with decreasing sphere diameter, and this result is attributed to the interstices of stronger nanofluidic confinement (shown in Figure S4). On the other hand, under identical vacuum pressure, the power density was found to be increasing with increasing diameter of the silica spheres (shown in Figure 2f), and this result is attributed to the enhanced flow-rate through the interstices of larger silica spheres. However, with silica interstices beyond the 8 ACS Paragon Plus Environment

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surface-charge-governed regime (for sphere diameter ~ 2900 nm), the enhanced flow-rate has negligible effect on the power output values. From simple geometrical considerations of fcc structure, the ratio of tetrahedron (rtetra) and octahedron void (rocta) to particle radius (Rsphere) are calculated to be 0.225 and 0.414 respectively (Figure S5). Therefore, in close-packed structure of silica spheres with diameter 1750 nm or less, most parts of the void (rtetra = 196 nm, and rocta = 363 nm) are covered with Debye length (~ 300 nm for electrolyte of concentration 10-6 M). However, in the system with silica particles of larger diameters (>1750 nm), regions with non-overlapping Debye length dominate. It is also reflected in their lower power output values. In order to highlight the importance of complex nanofluidic network of packed silica spheres, the experiment was repeated with reconstructed layered membranes of exfoliated vermiculite clay and silica aerogels. With a similar surface chemistry, the channels of vermiculite membrane exhibit strong nanofluidic confinement all through the channel,33 but offer tremendous resistance to water flow (typical flow-rates are in the range of 0.008 ml/min for a membrane of 1 cm thickness). Due to its minute flow-rate, the vermiculite membrane yielded only 0.4 mWm-2 power from the streaming potential (details are explained in the Supporting Figure S6). On the contrary, in the case of silica aerogels, very high flow-rate (30 ml/min) was accompanied by a humble nanofluidic confinement (Figure S7). Here again, irrespective of the high flow-rate, the power output was not found to be very significant (1.9 mWm-2). In comparison, the power density harvested from streaming of water through close-packed spheres (1750 nm) reaches up to 151 mWm-2. The efficiency of silica interstices in harvesting streaming potential is also compared with the existing literature (Table S1). However, due to the differences in operation conditions, device dimensions, and device architectures, a fair comparison between the systems was not possible.



In order to further confirm the role of surface-charges, aqueous solutions of HCl at different concentrations were flowed through the interstices silica spheres. At the low concentration regime, the open-circuit voltage remained constant up to threefold rise in the HCl concentrations, as shown in Figure 3a. However, at the high concentration regime, the opencircuit potential was found to be decreasing with increasing ionic concentration, and this result is attributed to the neutralization of the surface charges by the excessive protons present in the acidic medium.34-35 This experiment was repeated with KOH solutions of different concentrations. In the presence of base, the Si-OH groups get deprotonated and consequently improve the voltage output values, as shown in Figure 3b. However, at the higher concentration regime, the potential values decrease sharply with increasing concentration, for both HCl and KOH solutions. The diminishing potential values at higher concentration is attributed to a combined effect of decreasing Debye length and decomposition of Si-OH groups. Similarly, the dependence of power output values on the distance between the electrodes was also studied at a fixed flow-rate. As shown in the inset of Figure 3c, a cylinder of diameter 1.5 cm was packed with silica spheres (1000 nm) up to 8 cm of its length. Five Ag/AgCl electrodes were fixed along the length of the cylinder at a distance of 2 cm apart. Under a vacuum pressure of 550 mmHg, DI water was flowed (at a flow-rate of 0.6 mL/min) through this cylinder of packed silica spheres. Open-circuit potentials were measured by keeping one of the terminals of the source meter instrument fixed at the bottom-most electrode, and the connection of second terminal was successively varied with other four electrodes. In Figure 3c, the output power densities obtained as such are plotted as a function of the separation between the electrodes. The power output was found to be increasing with increasing distance between the electrodes, reaching a maximum potential value of 1.6 V (power density 8.4 mWm-2). The same has been attributed to the higher charge separation made by a large number of packed silica spheres present between the electrodes. However, further increase to the distance between the 10 ACS Paragon Plus Environment

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electrodes by increasing the mass loading was not possible as it decreases the flow-rate to a negligible value. The efficiency of the streaming-potential-harvesting was also found to be dependent upon the contact area of Ag/AgCl electrodes that come in contact with the flowing DI water. The Ag/AgCl electrodes of different lengths were coiled into circles so that they fitted inside the cylinder (diameter 1 cm), and the experiment was repeated with 500 mg of packed silica spheres of diameter 1000 nm. As the electrodes with higher contact areas can simultaneously interact with more number of charged ions continuously going out of the silica interstices along with the flowing water, the output current was found to be increasing from 0.6, to 3.7 μA when electrode area was increased from 16 to 94 mm2 respectively. However, the potential difference ( 1.6 V) remain constant with changing electrode area. As a result, energy harvesting efficiency was found to be increasing with increasing electrode area (Figure 3d). However, it was not possible to fit electrodes with area beyond 94 mm2 into the column of 1 cm diameter. In order to further improve the efficiency of the streaming potential, the silica particles (1000 nm) were pre-treated with concentrated nitric acid at 80 °C for 2 hours and subsequently washed with DI water. The streaming-potential-harvesting device fabricated with nitric acid treated silica particles (Ag/AgCl electrodes placed at 1 cm apart) has shown a streaming voltage of 2.3 V with 2.6 µA current under a constant flow of DI water (1.8 ml/min). It yielded a power density of 94 mWm-2. The plots for voltage and current generated upon streaming of DI water through nitric acid treated silica packing are shown in Figure 3e and 3f respectively. The enhancement in the surface charge density originating from etching of Si-O bonds of the silica particles by the concentrated nitric acid solution account for the improved values of the streaming potential. Because of its simplicity, the packed silica columns can also be used to harvest energy from water flowing through normal household taps, opening up new possibilities of harvesting energy from enormous amounts of water used for day to day domestic purposes. For that an 11 ACS Paragon Plus Environment


aspirator pump was attached to a water tap to create vacuum pressure in the Erlenmeyer flask through the Venturi effect. When water was allowed to flow (~ 7 litre/min) through the aspirator pump, it created enough pressure to pull DI water (0.4 ml/min) through the cylindrical column (diameter 1 cm) packed with nitric acid-treated silica spheres (1000 nm diameter), which was equipped with two Ag/AgCl electrodes (contact area 64 mm2) at a distance of 1 cm apart. The experimental set-up shown in Figure 4a conveniently generated a power density of 48 mWm-2 from the moderate flow of water through a household water tap (Supplementary video 1). It is noteworthy that the silica containing streaming-potential-harvesting devices can be connected in series to add up the power generated by each device. As shown in Figure S8, the total voltage generated from three devices (4.8 V) was found to be nearly three times of the voltage generated from an individual device (1.7 V) (spheres of diameter 1000 nm, electrodes placed at 1 cm apart). When the Ag/AgCl electrodes of three columns packed with silica spheres were connected in series, it produced sufficient power to light a LED of 3V (Figure 4b and Supplementary video 2). For each column, DI water was passed by applying a vacuum pressure of 550 mmHg, attaining a flow-rate of 1.7 ml/min. Unfortunately, without the external pressure, water flow through the packed silica column was found to be negligible. CONCLUSIONS In conclusion, we have demonstrated the possibility of harvesting sustainable energy from flowing water in portable devices by attaining the basic requirements of electrokineticstreaming-potential in the interstices of close-packed silica spheres. While the high flow-rate through the interstices of spheres is attributed to the network of tetrahedron and octahedron voids interconnected through biconical nanochannels, the surface hydroxyl groups of silica spheres have been credited for disrupting the electroneutrality of the flowing fluids. A significant enhancement in the power density is required to harvest maximum energy from streaming potential in real-world applications, and therefore multiple parameters that can be 12 ACS Paragon Plus Environment

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tuned to improve the energy efficiency have been identified and demonstrated. The key factors include the diameter of the close-packed spheres, number of the spheres, contact area of the electrodes, and pH of the streaming water. The current results also indicate that among the systems with an effective nanofluidic confinement, the one that allows the highest flow-rate of fluid would be ideal for the power generation through electrokinetic-streaming-potential. In view of the considerable amount of water that daily flows through household taps, energy harvesting through streaming potential could provide a valid contribution to our daily need for energy in a sustainable and decentralized manner. EXPERIMENTAL SECTION Materials: Tetraethyl orthosilicate (TEOS) was purchased from Alfa Aesar. Ammonium hydroxide (30 %), isopropanol, hydrochloric acid, sodium hydroxide were purchased from Merck. Raw vermiculite crystals, graphite powder and potassium chloride were purchased from Sigma-Aldrich. Methods: Synthesis of silica nanoparticles: Samples containing silica spheres of different average diameters were prepared by following the well-known Stӧber process.29 Typically, Tetraethyl orthosilicate (TEOS) was added dropwise to a mixture of ammonia and water in 600 ml isopropanol, the ratio of [NH3]:[TEOS] and [H2O]:[TEOS] was maintained as 0.81, and 6.25 respectively. For the synthesis of spheres of diameter 285, 620 and 1000 nm, the concentration of TEOS used were 0.45, 0.67 and 1.24 M respectively. Additionally, for the synthesis of spheres of diameter 1000 nm, the temperature of the reaction mixture was cooled down to 5 °C. The sample with large silica spheres was synthesised by adding a solution of 3.95 gram TEOS (for 1750 nm spheres) or 6.04 g TEOS (for 2900 nm spheres) in 33.3 ml ethanol with the help of a microfeeding pump at the rate of 13 ACS Paragon Plus Environment


0.2 ml/min to an aqueous mixture (65 ml water) of 6.75 ml ethanol, 9 ml ammonia and 0.017 g KCl at 30 °C.30 Exfoliation of raw vermiculite crystals: 100 mg of thermally expanded vermiculite crystals soaked in 100 ml of saturated aqueous solution sodium chloride (NaCl) was refluxed for 24 hours, followed by repeated washing with deionized water and ethanol via centrifugation technique. The as-obtained sodium exchanged vermiculite was further soaked in 2 M lithium chloride (LiCl) solution and refluxed for another 24 hours, followed by thorough washing with deionized water and ethanol. The as-obtained slurry was then re-dispersed in DI water (5 mg/ml) and was stacked via vacuum filtration in a cylinder equipped with two Ag/AgCl electrodes at a distance of 1 cm apart. Streaming potential with varying pH of the solutions: Silica spheres of diameter 1000 nm were packed in a cylindrical column (diameter 1 cm) equipped with two Ag/AgCl electrodes at a distance of 1 cm apart. HCl solution of varying concentrations (1 - 10-7 M) was allowed to flow through the interstices of the silica packing and open-circuit voltage was measured between the two Ag/AgCl electrodes. A similar column was packed with silica spheres of 1000 nm size for performing the experiment with a basic solution. Distance between electrodes: Silica spheres (1000 nm) was packed in a cylinder of diameter 1.5 cm equipped with five Ag/AgCl electrodes fixed along the length of the cylinder at a distance of 2 cm apart. Upon flowing DI water, the open-circuit potentials were measured by keeping one of the terminals of the source meter instrument fixed at the bottom-most electrode and varying the connection of the second terminal with other four electrodes, successively. Varying electrode area: The electrode area is varied by using Ag/AgCl electrode coiled into circles and varying the number of turns. The coiled electrodes are fixed inside a cylinder at a distance of 1 cm and were subsequently packed with silica sphere of diameter 1000 nm. Open14 ACS Paragon Plus Environment

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circuit voltage was measured between these electrodes during the flow of water through the packed column. Streaming potential from water flowing through household tap: An aspirator pump was attached to a water tap to create a vacuum pressure in the Erlenmeyer flask (Venturi effect). When water was flowed through the aspirator pump, the vacuum was created in the Erlenmeyer flask which helped to pull out DI water through the packed silica column mounted on the top (neck) of the Erlenmeyer flask. The open-circuit voltage was measured between the two Ag/AgCl electrodes placed at a distance of 4 cm encapsulating silica spheres of diameter 1000 nm. Characterisation: All electrokinetic and nanofluidic measurements were done using source meter instrument (Keithley 2450). The morphology of the samples was characterized by field emission scanning electron microscope (FESEM) (Zeiss, Model: Sigma), and field emission transmission electron microscope (FETEM) (JEOL, JEM 2100F). X-ray diffraction patterns were recorded by using Bruker D-205505 Cu-kα radiation (λ = 1.5406 Å) instrument. UV-Vis analysis of MB solution was done with Systronics UV-VIS Spectrometer 117. ASSOCIATED CONTENT Supplementary Information pXRD of silica spheres; calculation of σ; methylene blue adsorption; power generation using identical flow-rate; calculation of radius of tetrahedral and octahedral voids; microscopic images of vermiculite nanosheets; streaming potential through packed vermiculite nanosheets; streaming potential through silica aerogel; streaming potential through HNO3 treated silica spheres. ACKNOWLEDGEMENT



K.R. acknowledges the Ramanujan Research Grant (SB/S2/RJN-141/2014) and Nano Mission

Research Grant (DST/NM/NS/2018/141) of the Science and Engineering Research Board (SERB), India for financial support. All the authors thank the Central Instrumental Facility (CIF), IIT Guwahati for their help with sample characterizations, and Prof. Arup Kumar Goswami for proofreading the manuscript. K. S and J. D are grateful to IITG for PhD fellowships.

AUTHOR INFORMATION Corresponding author Email: [email protected] Kundan Saha (Email: [email protected]), Jumi Deka (Email: [email protected]) Notes The authors declare no competing financial interests.

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Figure 1. FESEM images of (a) silica spheres of different average diameter, and (b) closepacked structure of the spheres (diameter 285 nm) obtained through vacuum filtration of its aqueous dispersion. (c) Schematic representation of the experimental set-up used for the study of electrokinetic properties of the silica interstices. (d) Ionic conductivity as a function of KCl concentration through the interstices of close-packed silica spheres of different diameters. (e) Surface-charge-density (σ) of the silica spheres as a function of diameter.


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Figure 2. (a) Schematic illustration of the overlapping of EDLs in close-packed of silica spheres. (b) Flow-rate of water through the interstices of close-packed silica spheres of different average diameters. (c) Open-circuit potential, and (d) current generated upon flowing water through the interstices of close-packed silica spheres of diameter 1000 nm. (e) Twodimensional schematic representation of the streaming potential generated through silica interstices. (f) Power density harvested through streaming potential as a function of sphere diameter.



Figure 3. Open-circuit voltage generated as a function of the concentration of (a) HCl, and (b) KOH present in the aqueous solution flowing through close-packed silica spheres of diameter 1000 nm. (c) Power density harvested through streaming potential as a function of distance between the electrodes by employing silica spheres of diameter 1000 nm. The inset shows a schematic representation of the device fabricated for varying the distance between the electrodes. (d) Power density harvested through streaming potential as a function of contact area of the electrode encapsulated inside the silica packed column. (e) Voltage and (f) current generated upon flowing DI water through 1 cm packing of HNO3 treated silica spheres (1000 nm).


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Figure 4. (a) Optical image showing harvesting of streaming potential from water flowing through a household water tap and (b) photo showing functioning of a 3V LED by the streaming potential harvested from DI water through series connection of three column packed with silica spheres of 1000 nm diameter.



Image for the Graphical Abstract


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Energy from the Nanofluidic Transport of Water through Nanochannels

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