Environment Friendly Mesoporous Magnetite Nanoparticles Based

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Environment Friendly Mesoporous Magnetite Nanoparticles Based Hydroelectric Cell Shipra Jain, Jyoti Shah, Sanjay R Dhakate, Govind Gupta, Chhemendra Sharma, and Ravinder Kumar Kotnala J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b12561 • Publication Date (Web): 20 Feb 2018 Downloaded from http://pubs.acs.org on February 23, 2018

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The Journal of Physical Chemistry

Environment Friendly Mesoporous Magnetite Nanoparticles Based Hydroelectric Cell Shipra Jainab, Jyoti Shaha, S. R. Dhakatea, Govind Guptaa, C. Sharmaa and R. K. Kotnalaa* a

b

CSIR-National Physical Laboratory,

AcSIR- National Physical Laboratory

Dr. K. S. Krishnan Road, New Delhi-110012, India *Corresponding Author

Abstract Recently invented hydroelectric cell (HEC) is emerging as a better alternative to green electrical energy device. In this direction oxygen deficient mesoporous magnetite nanoparticles have been synthesized by chemical method to fabricate magnetite based hydroelectric cell. Water molecule chemidissociates on the surface Fe cations and oxygen vacancies followed by physisorbed water molecule dissociation due to charges trapped inside mesopores of magnetite. Mesoporosity and oxygen vacancies in magnetite nanoparticles have been confirmed by BET and X-ray photoelectron spectroscopy. Dissociated H3O+ and OHions migrate towards attached silver and zinc electrodes respectively in magnetite hydroelectric cell via capillary and surface diffusion. Ionic diffusion of dissociated ions has been confirmed by Nyquist plot for dry and wet magnetite HEC. Ohmic loss in magnetite has been found less due to Fe2+/ Fe3+ hopping process which results in increased cell current. A typical magnetite hydroelectric cell of 4.8 cm2 area delivers 50 mA peak current with maximum output power of 38.5 mW. An emf of 0.77 V is generated due to redox reaction at respective electrodes in the cell. Reduction in cell emf is attributed to the oxidation of Fe2+ to Fe3+ ion forming internal cells in magnetite, it ultimately impedes OH- ion diffusion towards

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Zn electrode. Byproducts of hydroelectric cell, zinc hydroxide and H2 gas are not harmful for the environment. Electricity generation by magnetite hydroelectric cell is safe, environment friendly and facile technique. Introduction Electric current and voltage generation using deionized water at room temperature by hydroelectric cell has unfolded a new field of research1. Room temperature water molecule dissociation capability of nanoporous and oxygen deficient Li- substituted magnesium ferrite has been transformed into electrical energy device invention2,3. Spinel structure of ferrite is extremely stable, exists in the form AB2O4 which consists of 64 tetrahedral and 32 octahedral sites. In a spinel unit cell 24 metal ion fill almost 1/4th octahedral (B) and tetrahedral sites (A), remaining 3/4th oxygen sublattices4. There is a wide spectrum of spinel compound which can be explored for hydroelectric cell application. Among spinel family, magnetite has attracted a lot of interest in the field of electrochemistry5, catalysis6, spintronics7 due to its half metallic conductivity by the presence of both Fe2+ and Fe3+ ions. Ferrimagnetic magnetite has a cubic inverse spinel structure8, the chemical formula Fe3O4 denoted as [Fe3+]t[Fe3+,Fe2+]oO4, where ferric ions occupy tetrahedral sites and equal number of ferrous and ferric ions occupy octahedral sites. Use of Fe3O4 as a catalyst in environmental redox reactions9,10, water–gas shift reaction11, synthesis of quinoxalines in water12, photo-catalytic splitting of water13,14 over magnetite electrodes reveal interaction of water with magnetite surface. Water molecule adsorption explored on thin films of magnetite showed that Fe3O4 is active towards water15. Water molecule adsorption on Fe3O4 could be molecular or dissociative. Adsorption occurs on oxygen vacancy and surface defects significantly16-18. The study of water interaction with Fe3O4 has been reported only on thin films under ultra high vaccum conditions at water pressure < 10-3 mbar19,20. Several imaging techniques have been used to identify the dissociative adsorption of water on magnetite thin films. No such 2 ACS Paragon Plus Environment

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experimental studies have been reported on bulk Fe3O4 based on active sites responsible for water molecule dissociation. In present work, we have synthesized oxygen deficient mesoporous magnetite nanoparticles for fabrication of hydroelectric cell. Oxygen deficient magnetite nanoparticles have been synthesized by chemical coprecipitation technique. Detailed structural, microstructural, electrical and ionic distribution studies of magnetite nanoparticles have been investigated. Electric current and voltage generation by magnetite hydroelectric cell has been explored indepth. Possible mechanism of water molecule dissociation by mesoporous magnetite nanoparticles has been reported in this study. Magnetite nanoparticles pellet used in recently invented hydroelectric cell has shown significant increase in output of the cell. Experimental Section Magnetite nanoparticles have been synthesized by chemical co-precipitation technique. All the chemicals used were of analytical grade. High purity precursors Ferrous chloride (FeCl2.2H2O, 99% pure), Anhydrous ferric chloride (FeCl3, 99% pure) and Ammonia solution (25% NH3 by weight) were used. Millipore deionized water was used for synthesis. Anhydrous FeCl3 32.44g and FeCl2.2H2O 16.22g were taken and mixed in 200 ml de-ionized water. Solution was magnetically stirred at 60°C for 1 hour and it was precipitated out in the form of hydroxides by adding aqueous ammonia. The black precipitate was magnetically stirred for 1 hour. Filtered precipitate was washed several times with deionized water and acetone to neutralize its pH. Black powder was further vacuum dried at 40ºC for 5 hours. A detailed description of synthesis mechanism of magnetite has been provided in Text S1. Dried powder was pressed into 1 inch square pellet by a hydraulic press. Silver comb pattern was screen printed on one face and Zn sheet was pasted on opposite face of the square pellet. Electrical contacts were made from the silver and zinc electrodes respectively to test the cell performance as a hydroelectric cell. 3 ACS Paragon Plus Environment


Phase confirmation and crystallite size of the black powder was analysed by taking X-ray diffraction pattern from Bruker AXS D8 advance XRD with Cu-Kα radiation of wavelength λ= 1.542 Å operating at 40 kV/30 mA. Chemical composition and crystal structure symmetry were identified by Raman spectroscopy technique using Renishaw in-Via Raman spectrometer at laser excitation wavelength of 514.5 nm. Infra-red vibrational spectroscopy was used to identify the vibrational Fe-O bonds of black powder using PerkinElmer 1750 FTIR spectrophotometer. The nanoparticle shape, size distribution and lattice spacing were measured by high resolution transmission electron microscope Tecnai F30 HRTEM system. Electron spin resonance measurement was carried out at room temperature at X- band of fixed frequency 9.54 GHz using Model-A-300, Bruker biospin to determine spin density of Fe3O4 nanoparticles with reference to known spin density of 2,2-diphenyl-1-picrylhydrazyl (DPPH). V-I polarization characteristics of hydroelectric cell was plotted using different external loads. The ionic diffusion of dissociated ions of water molecule in magnetite cell was observed using a Wayne-Kerr Impedance Analyzer 6500B (UK). A rectangular 1cm X 0.5cm black pellet with silver electrical contacts on both the ends were used to obtain the Nyquist plot of pellet in dry and wet state by applying 10 mV AC voltage in frequency range 20 Hz-120 MHz. Estimate of specific surface area, pore size distribution, pore volume were determined by Nitrogen adsorption-desorption isotherm using BET technique by Quanta Chrome Instrument (NOVA 2000e USA). Redox reaction in Fe3O4 has been analyzed by cyclic voltammetry using potentiostat/galvanostat of MetroOhm Autolab, PGSTAT30 with a conventional three electrode set up. Porous microstructure of the cell pellet was measured by field emission scanning electron microscope Carl-Zeiss SUPRA V40 at an accelerating voltage of 10 kV. Elemental identification of Fe3O4 was determined by energy dispersive Xray spectroscopy (Oxford ISIS 300 EDS) as an attachment to FESEM. Ionic state of surface ions has been examined by high resolution X-ray photoelectron spectroscopy under ultrahigh


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vacuum based multiprobe surface analysis system (Scienta Omicron) at a base pressure of 5×10-11 Torr using monochromatic Al Kα radiation source of energy 1486.7 eV. Grains on Fe3O4 pellet were determined by Bruker multimode atomic force microscopy using a conducting tip to capture the surface microstructure, particle size and pore structure. Results and Discussion 1.1 Structural Analysis by XRD, Raman Spectra, FTIR and HRTEM: Crystalline structure of Fe3O4 has been identified by X-ray diffraction pattern as shown in fig. 1(a). Diffraction peaks at 30.4°, 35.43°, 37.09, 43.08°, 53.43°, 56.94°, 62.57° correspond to (220), (311), (222), (400), (422), (511), (440) lattice plane of spinel structure respectively in accordance with Fe3O4 standard JCPDS Card No.(79-0419). No additional peak appeared which indicates high purity and good crystallinity of synthesized magnetite. Broadening of XRD peaks indicated nanoparticle formation. The interplanar spacing for the most intense (311) peak of Fe3O4 using Bragg’s diffraction formula was calculated 2.53 Å with lattice parameter 8.38 Å close to the reported value of 8.39 Å in JCPDS Card No. (79-0419). The crystallite size has been calculated by Scherrer equation 1: =

⅄

………..(1)

where d is the crystallite size, β is the full width at half-maximum of the most intense (311) peak, and λ is the X-ray wavelength. The crystallite size calculated is 11.88 nm. In order to differentiate among Fe3O4 , γ-Fe2O3 and α-Fe2O3 iron oxide phases, Raman spectra of synthesized powder has been performed and is shown in fig. 1(b). Five Raman active bands A1g, Eg , 3T2g have been predicted by group theory for spinel magnetite21. Raman peaks observed at 200 (Eg), 317 (T2g), 524 (T2g), 533 (T2g), 668 (A1g) cm-1 validate the formation of magnetite nanoparticles22,23. Since oxidation of magnetite to maghemite is highly sensitive to laser power, the presence of maghemite peaks at 352 (T2g), 709 (A1g) cm-1



might be due to oxidation of magnetite owing to laser power exposure24,25. Raman spectral peaks of magnetite have been shifted by 3-7 cm-1 towards higher frequency, the shift in peaks attributed to quantum confinement effects26. Functional group presence in the synthesized nanoparticles has been further characterized by FT-IR spectroscopy as shown in fig. 1c. Two distinct bands at 570 and 446 cm-1 have been observed due to the intrinsic stretching vibration mode associated with Fe-O absorption bond of respective tetrahedral and octahedral sites in Fe3O4 spinel structure27,28. IR spectroscopy thereby confirms the sub-lattices in spinel phase of Fe3O4. High resolution TEM micrograph of Fe3O4 nanoparticles depicted the morphology of nanoparticles as shown in fig. 1(d). Particles are spherical in shape with variable particle size distribution shown in the inset of fig. 1(d). Agglomeration of particles could be seen along with distribution of nanopores among the particles. The average particle size of magnetite is 11.02 nm measured by the scale provided in HRTEM image. Surface lattice fringe pattern has been measured by a high resolution image with inter planar spacing of 0.21 nm, 0.25 nm and 0.16 nm corresponding to (400), (311) and (220) lattice planes of Fe3O4 respectively (JCPDS Card No. 79-0419). Vacancy defects in lattice fringe pattern have been observed, marked by arrows as shown in fig. 1(d). Uneven d spacing in lattice fringes as shown in the encircled area validates the existence of stacking faults, the defects are more pronounced near grain boundaries. Vacancy defects and planar defects might be due to presence of oxygen vacancies. 1.2 Morphology and Microstructural Analysis by FESEM, BET, AFM Surface morphology of magnetite nanoparticles has been captured by FESEM images shown in fig. 2(a, b). Aggregation of nanoparticles leads to wide pores explicitly visible from the images. Scanned images of Fe3O4 nanoparticles revealed uniform distribution of the


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nanoparticles along with aggregation of smaller grains. Elemental identification has been obtained by EDX spectra as shown in inset of fig. 2(a). Presence of Fe and O peaks with 71 ; 29 wt%, and 39 ; 61 atomic% respectively confirms magnetite composition. A high magnification image fig. 2(b) has been used to estimate the average grain size and pore size distribution. Grain size varies from 9-20 nm with an average grain size of 11 nm. Meso pores with size less than 50 nm along with some macro-pores (size >50 nm) can be seen over the entire surface. Distribution of nanopores with an average pore size of 10.12 nm has been observed. Surface morphology of magnetite nanoparticles has been further evaluated by AFM technique as shown in fig. 2(c). Agglomerated spherical nanograins with fine mesopores and pore size varying from 2 to 18 nm with an average pore size of 8.7 nm are observed in AFM image. Surface microstructure analysed by all imaging techniques clearly shows very fine spherical nanoparticles less than 10 nm distributed throughout the sample. Pore size and specific surface area of magnetite nanoparticles have been determined by BET Nitrogen adsorption-desorption isotherms shown in fig. 2(d). Magnetite adsorptiondesorption Nitrogen isothermal curve exhibited type III isotherm with hysteresis according to the IUPAC conventions29. Specific surface area of Fe3O4 has been found 89.78 m²/g calculated by multipoint BET method. The specific surface area, pore volume and pore size distribution results are summarized in Table S1. Fe3O4 nanoparticles exhibited a wide mesoporous size distribution30,31 with an average pore diameter of 9.81 nm shown in inset of fig. 2(d). The cumulative pore volume with pore diameter less than 190 nm was 0.335 cc/g. The total porosity of Fe3O4 nanoparticles calculated using volumetric ratio analysis has been estimated 46%. Highly mesoporous spherical magnetite nanoparticles have been synthesized for fabricating hydroelectric cell. 1.3 Iron ionic state analysis by XPS: Insight into Oxygen Vacancy



Surface composition and valance states of Fe and O have been investigated by X-ray photoelectron spectroscopy as shown in fig. 3(a, b). XPS spectra for survey peaks upto 1000 eV and C 1s spectra have been depicted in fig. S1 elucidating the presence of surface ion valance state in magnetite. Fe 2p (fig. 3(a)) and Fe 3p (fig. S1(c)) XPS spectral peaks have been taken to investigate the ionic state of Fe in magnetite. Fe2p peak splitted into Fe 2p1/2 peak at 710.5 eV and Fe 2p 3/2 peak at 724.1 eV confirmed the presence of both Fe2+ and Fe3+ ions leading to the formation of Fe3O4 23. Deconvolution of Fe 2p1/2 peak and Fe 2p3/2 peak affirmed the presence of both Fe2+ peak at 709.5 eV and 723.5 eV and Fe3+ peak at 711.6 eV, 725 eV respectively32. A satellite peak appeared at 718.5 eV, attributed to Fe3+ ion, indicating partial oxidation of magnetite to γ-Fe2O3 33. Partial formation of γ-Fe2O3 might be possible, as Fe2+ is subjected to aired oxidation to Fe3+ state during synthesis. Deconvolution of Fe 3p peak (fig. S1) at 55.5 eV resulted to Fe2+ and Fe3+ peaks at 54.1 eV and 55.8 eV respectively32. The mean relative area under the curve of two Fe 2p peaks attributed to Fe2+ and Fe3+ ionic state respectively has been used to calculate the stoichiometric ratio of Fe2+ and Fe3+ ions. Ratio of Fe2+/ Fe3+ ions determined as 31.5: 68.5 is close to 33:67 stoichiometric ratio of Fe2+/ Fe3+ in Fe3O4. Slight deviation in stoichiometric ratio might be due to the uncertainity involved in calculations and/or partial oxidation of Fe3O4. Existence of oxygen vacancy in Fe3O4 nanoparticles has been investigated using O 1s XPS spectra. Fine structure, broad (~ 6 eV) core level spectra as shown in fig. 3(b) indicated multiple oxygen environment. Three different chemical shifts has been identified by Gaussian peak fitting in the curve, peaks centred around 529.7, 531, 532.3 eV with FWHM < 1.3 eV. Major peak around 529.7 eV with integrated peak area 77% has been attributed to lattice oxygen of Fe3O4 33. Dominant band around 531 eV with 16% integrated peak area is due to O atoms in vicinity of an oxygen vacancy, this band has been related to oxygen defects in the 8 ACS Paragon Plus Environment

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matrix of metal oxides34-37. Higher binding energy band around 532.3 eV with peak area 7% can be attributed to surface hydroxyl groups, Fe-OH species37. Surface oxygen defects in the form of oxygen vacancies and hydroxyl species exist in high concentration in Fe3O4 nanoparticles, Oxygen vacancies act as donor centres for water molecule dissociation17. 1.4 Unsaturated spins analysis by EPR Electron spin resonance of magnetite has been carried out to calculate the dangling bonds in Fe3O4 nanoparticles shown in fig. 4. The resonance field, line-width and g value has been summarized in text S2. The intensity of EPR spectra is directly proportional to the number of unpaired spins in magnetite, unpaired spins have been estimated by spin density. It has been calculated by comparing area under the curve of Fe3O4 with the standard sample DPPH based on known spin concentration of 1.52 × 1018. Spin density of Fe3O4 has been determined 8.37×1024 spin/g which is a measure of unsaturated/dangling bonds present in the composition38. Dangling bonds are due to unpaired electrons39. High value of spin density indicates the presence of large number of unpaired electrons in Fe3O4 acting as active sites for water molecule dissociation40. Adsorbed water molecule can be chemidissociated on the active Fe3O4 surface into OH- and H3O+ ions owing to the high spin density41. 2

Water Molecule Dissociation by Magnetite

Water molecule dissociation mechanism on magnetite (111) plane has been explained in schematic diagram fig. 5. Chemi-dissociation of water molecule occurs on the surface of Fe3O4 due to attraction between oxygen lone pair electron of H2O molecule and octahedrally coorndinated unsaturated Fe2+ and Fe3+ surface cations leading to chemidissociation19. Decrease in –OH bond dissociation energy of water molecule due to strong binding of oxygen lone pair electron of water molecule with highly electronegative Fe2+ and Fe3+ ions in magnetite leads to the heterolytic dissociation of H2O molecule42. After the formation of FeOH bond, H+ ion binds to the nearby surface oxygen atom to form another –OH surface 9 ACS Paragon Plus Environment


hydroxyl group43,44. Water molecule dissociation has also been observed on oxygen vacancies as reported in literature16. Chemidissociation of water molecule occurs on surface Fe2+/ Fe3+ ions and on oxygen vacancies leading to surface hydroxylation. Dissociated surface –OH ions physisorb water molecules by hydrogen bonding, traping H+ ions to form H3O+ ions. These surface H3O+ ions get trapped inside the mesopores of Fe3O4 and generate high electric field sufficient to dissociate further physisorbed water molecule spontaneously. Further electrolytic conduction by H+/H3O+ proton hopping occurs due to Grotthus chain reaction45. Thus, dissociation of water molecule on magnetite surface produces hydronium and hydroxide ions. 2.1 Analysis of ionic conduction behaviour in magnetite by Nyquist plot: Nyquist plot of dry magnetite pellet (1cm × 0.5cm × 0.2 cm) with silver contacts on both faces has been shown in fig. 6. A characteristic semicircular loop of an equivalent parallel RC circuit of dry magnetite pellet has been obtained with charge transfer resistance Rc = 4.6×105Ω. High resistance could be due to high porosity 46% as measured by volumetric method of synthesized magnetite nanoparticles46. When millipore de-ionized water was poured on magnetite pellet, a significant reduction in the semicircular loop with long low frequency tail has been observed in Nyquist plot shown in the inset of fig. 6. Resistance of the pellet decreased to 9.8KΩ. Decrease in the resistance of the pellet attributed to water molecule dissociation by magnetite. Initially, chemi-dissociation of water takes place on the unsaturated surface cations followed by physisorption of water molecule. Protonic conduction occurs on physisorbed H3O+/H2O layer increasing ionic diffusion current47-49. Warburg diffusion tail appeared at low frequency has been attributed to ionic diffusion at magnetite and electrode interface. Zeta potential of magnetite nanoparticles dispersed in water measured 22.4 mV (fig. S2) is indicative of strong electrostatic attraction of the surface Fe2+/ Fe3+ ions with the negatively charged hydroxyl group of water molecule. To study electrochemical reactions occurring on


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the surface of Fe3O4, Cyclic voltammetry has been performed using platinum as an auxiliary electrode, saturated Ag/AgCl as reference electrode, black Fe3O4 pellet as working electrode, measurement taken in 0.1 M KOH solution. The potential scan range from -0.75 V to +0.75 V at scan rate of 4 mV/s. CV measurement at room temperature has been depicted in fig. 7. Oxidation of iron ions in magnetite has been observed with a large cathodic peak current of 1.5 mA and anodic peak current of 2 mA respectively revealing the role of magnetite as an anode. An irreversible reaction mechanism occurs on Fe3O4 surface owing to the existence of anodic peak at ~ 0.4V and a cathodic peak at ~ -0.18V with peak to peak seperation of 0.58 V.

+ 4O → 3FeOOH + 2 O + ------(2)

Chemidissociated surface –OH group interacts with the octahedrally coordinated unsaturated Fe2+ ions of magnetite oxidizing it electrochemically to Fe3+ state. Oxidation of Fe2+ to Fe3+ ions may lead to the formation of iron oxy hydroxide/ hydrated ferric oxide (eq. 2) on the magnetite anodic surface50. Role of magnetite as an anode has been reported in literature51. Cyclic voltammetric results of magnetite anodic oxidation are consistent with the reported CV results of Fe2+/Fe3+oxidation in magnetite52. 2.2 Magnetite Hydroelectric Cell: V-I Polarization Characteristics The magnetite HEC comprises a magnetite square pellet of 4.8 cm2 area painted with silver comb electrodes at one face and Zn plate pasted on opposite face as shown in fig. 8. Magnetite HEC partially dipped in deionized water generates 50 mA short circuit current and 0.77 V open cell voltage. The electrical power response of magnetite HEC has been evaluated by performing V-I polarization curve as shown in fig. 9. V-I polarization plot of magnetite HEC with Zn and Ag electrodes in water at room temperature exhibited typical polarization



region53,54. Current and voltage obtained by magnetite hydroelectric cell reveals the dissociation of water molecule into hydronium and hydroxide ions. Decrease in voltage with increase in current is attributed to the kinetics of the electrochemical cell with the existence of three prominent loss regions55. The open circuit voltage 0.77 V of magnetite cell, point A is lower than the redox potential of Ag and Zn electrodes used in the cell. Decrease in voltage may be attributed to internal anodic behaviour of magnetite owing to octahedral Fe2+ - Fe3+ ion oxidation within Fe3O4 as confirmed by CV measurements (fig. 7). Activation loss (region AB) is dominant at low current density, loss exhibits the delay in initiation of the process of water dissociation on the mesoporous Fe3O4 surface and in collection of dissociated ions by the electrodes due to the existence of an activation energy. Activation energy is the energy barrier which is required to overcome the electrochemical reaction occurring on mesoporous magnetite surface and electrode. The voltage drop in intermediate current density region BC is linear due to ohmic losses provides resistance to the flow of ions through porous structure of Fe3O4. Sharp voltage drop is observed in high current density region CD due to crowding of the electrode surface with high concentration of ions, dissociated ions are taken away to respective electrodes to form current. This concentration loss is also referred as mass transport loss due to concentration polarization observed at high current56. Peak power obtained by polarization power curve is 6.4 mW. However, the initial maximum power generated by magnetite hydroelectric cell is 38.5 mW. Unsaturated Fe2+, Fe3+ surface cations and oxygen vacancies in magnetite facilitate chemidissociation of water molecule. High electron spin density creates more dangling bonds with trapped electrons as active sites for water molecule dissociation in Fe3O4 confirmed by EPR results44. Dissociated hydronium ions get trapped inside mesopores of Fe3O4 and thereby generate high electric field. Electric field inside mesopore of 10 nm diameter and length 50 nm has been calculated ~105 V/cm1. This electric field is high enough to dissociate


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further physisorbed water molecules spontaneously (eq. 3). Zinc acts as anode due to its high oxidation potential and silver acts as inert cathode. Diffusion of hydroxide ions toward Zn electrode and hydronium ions towards Ag electrode through Fe3O4 occurs via capillary and surface diffusion. Zinc gets oxidized to produce Zn(OH)2 (eq. 4) and H3O+ ions reduces at Ag cathode with the evolution of H2 gas (eq. 5) by capturing electrons from Zn anode that generates voltage in the cell shown in fig. 10. At magnetite surface: 4H O → 2 O + 2O

…..(3)

At anode: Zn + 2 O → Zn(OH) + 2e …..(4) At cathode: 2 O + 2 → (g) + 2H O …..(5) In the process of electric current generation Zn(OH)2 gets deposited at zinc anode that has been confirmed by X-ray diffraction measurement. Hydrogen gas evolution at silver anode has been detected using MQ-8 H2 gas sensor57. After a long operation of cell, an orangebrownish color deposition appeared on black magnetite pellet as a result of oxidation of Fe2+ ions (eq. 2). Identification of α-FeOOH, goethite formation has been confirmed by Raman spectra (fig. S3). Magnetite HEC generates 50 mA short circuit current which is quite high compared to Limagnesium ferrite HEC1. Higher short circuit current may be attributed to the contribution of both unsaturated Fe2+, Fe3+ octahedral cations with higher concentration of dangling bonds present on the surface dissociating more water molecules. Synthesized magnetite nanoparticles mesopores are within size distribution of 8-20 nm. Due to Fe2+/ Fe3+ hopping ohmic losses of magnetite has been found lesser compared to Li-magnesium ferrite. Magnetite hydroelectric cell open circuit voltage recorded has been found lower than reported 0.98V open cell voltage of hydroelectric cell1. The reduction in potential may be due to occurrence of an additional oxidation reaction within magnetite as an internal anode in the



cell, it has been confimed by CV measurement. Interaction of Fe2+ ions of magnetite with diffused surface hydroxyl ions facilitates its oxidation hence reducing the reaction rate of zinc to zinc hydroxide. Thus, tiny internal electrochemical cells are formed within magnetite58,59. Hydroelectric cell with magnetite forms tiny cells, obstructs further OH- ion diffusion towards external zinc anode resulting in decrease in emf of the cell due to such local action . Over passage of time reduction in cell current is observed which might be due to passivation of interface between zinc anode and magnetite surface due to zinc hydroxide formation. Nyquist plot of magnetite HEC with Zn and Ag electrodes is shown in fig. 11(a). The resistance of dry magnetite cell has been found 105Ω observed by extrapolating the incomplete semicircle in the frequency range 20 Hz to 120 MHz. After dipping the cell into deionized water, resistance of the cell decreased to approximately 80 Ω as shown in fig. 11(b). Wet magnetite HEC Nyquist plot represented a superposition of capacitive tail, high frequency grain contributed partial semicircle, grain boundary contributed semicircle and partial existence of ionic diffusion tail at low frequency60. Sharp semicircle with a large time constant in the frequency region (20 Hz-50 KHz) has been observed due to contribution of grain boundaries in nanoparticles61. A small partial semicircle loop in middle frequency region (50 KHz -0.2 MHz) is also recorded due to the grain contribution having a lower time constant. High frequency capacitive tail (0.2 MHz -120 MHz) appeared due to the reaction of OH- ions with Zn electrode to form a Zn(OH)2 passive film62. The low frequency diffusion tail appeared below 30 Hz due to diffusion of H3O+ ions. A complete Warburg impedance tail due to protonic diffusion via H3O+ ions from magnetite to electrode surface appeared at lower frequency region shown in inset fig. 11(b).

Conclusions


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Magnetite based hydroelectric cell has been fabricated on the similar lines as Li-magnesium ferrite HEC. Mesoporous oxygen deficient magnetite nanoparticles have been synthesized by chemical route. Crystalline phase of spinel Fe3O4 has been analysed by different spectroscopic techniques. Uniform particle shape and size distribution has been confirmed by scanning electron microscopy. Mesopores distribution of average pore size 9.8 nm with 46% porosity has been obtained in magnetite by BET technique. Oxygen vacancy and cationic composition distribution in magnetite has been confirmed by XPS study. High mesoporosity with lesser resistivity of magnetite than Li-magnesium ferrite HEC generated 50 mA current with maximum power 38.5 mW. Dissociation of water molecules at surface Fe sites and oxygen vacancies provides surface hydroxylation. On hydroxylated surface, water molecules get physisorbed and trap hydronium ions inside mesopores generates high electric field which further splits water molecule. This phenomenon resulted into increased ionic current in the cell. Oxidation of Fe2+/ Fe3+ ions within Fe3O4 reduced emf of the cell. Magnetite hydroelectric cell performance is stable, repetitive and durable. Magnetite HEC is a clean source of energy and environment friendly. Fabrication technique of magnetite hydroelectric cell is facile and economic. It has the potential to replace existing fuel cell and solar cell clean energy technology.

Figure Captions: Figure 1: (a) X-ray diffraction pattern of synthesized powder for 2Θ range 20° - 70°, (b) Raman spectra of magnetite powder with wavenumber from 100 to 1000 cm-1, (c) FT-IR spectroscopy of magnetite with wavenumber versus transmittance % showing Fe-O bonds, 15 ACS Paragon Plus Environment


(d) High-resolution transmission electron microscopy image of magnetite nanoparticles [showing particle size and shape distribution in inset of (d)], High magnification image of spinel lattice fringes showing defects in the lattice marked by arrows, encircled area showing grain boundary of crystal lattice with large number of defects along with irregular lattice pattern. Figure 2: (a) Field Emission scanning electron microscopy image of magnetite showing the porous microstructure, EDX spectra of magnetite nanoparticles is shown in inset, (b) High magnification image taken by FESEM at a scale of 100 nm showing particle size and pore size distribution with marked line spacing, (c) 2D Topographical image of magnetite nanoparticles using AFM, (d) Nitrogen adsorption-desorption isotherm of magnetite nanoparticles, BJH pore size distribution and pore volume distribution plot of mesoporous magnetite nanoparticles in inset of (d). Figure 3: XPS spectra of Fe3O4: (a) Fe 2p peak spliting, (b) Core level O 1s spectra. Figure 4: First Derivative Electron Spin Resonance (ESR) spectra of Fe3O4 sample.The inset: ESR spectra of DPPH reference sample. Figure 5: Adsorption of water molecule and its dissociation on Fe2+, Fe3+ ions and oxygen vacancy in (111) plane of Fe3O4. Figure 6: Nyquist plot of magnetite pellet in dry state, (inset) shows the nyquist plot of magnetite pellet in wet conditions. Figure 7: Cyclic voltammagram of magnetite electrode in 0.1 M KOH solution. Figure 8: Magnetite hydroelectric cell with silver comb pattern electrode on front face and Zinc electrode pasted on rear face of pellet. Figure 9: Polarization curve and energy generated by Magnetite Hydroelectric cell dipped in deionized water.


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Figure 10: Schematic diagram of magnetite hydroelectric cell showing water molecule dissociation, diffusion of hydronium and hydroxide ions towards Ag cathode and Zn anode respectively generating cell voltage, physisorbed water molecule dissociation inside mesopore of Fe3O4 due to trappped hydronium ions into hydronium and hydroxide ions. Figure 11: Nyquist plot: (a) Magnetite hydroelectric cell in dry condition, (b) in wet conditions, low frequency region of magnetite cell in wet conditions showing the diffusion tail (inset of b).



Figure 1


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Figure 11

Acknowledgements The authors are grateful to the Director of NPL for providing facilities, motivation and support to conduct this study. Shipra Jain is thankful to University Grants Commission (UGC) ref no. 20/12/2015 (II) EU-V for providing fellowship.

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Supplementary file Information Text S1: Synthesis mechanism of Fe3O4 nanoparticles. Table S1: Surface Area, pore size and pore volume summary of magnetite nanoparticles. Figure S1: XPS spectra: broad survey scans for identification of all elements, their ionic state and bonding. Text S2: Resonance line width calculation. Figure S2: Zeta potential Measurement. Figure: S3- Raman spectra of orange deposition on the magnetite surface. AUTHOR INFORMATION

Corresponding Author *Dr. R. K. Kotnala, Chief Scientist Head (former), Environmental Sciences and Biomedical Metrology Division CSIR-National Physical Laboratory Dr. K.S. Krishnan Road New Delhi –110012, India Fax: 91-11-45609310; Phone: 91-11-45608599 E-mail: [email protected]

Author Contributions SJ synthesized, characterized the material, JS performed some characterization and written the manuscript, SRD performed the BET measurements, GG carried out the XPS measurement, CS helped in writing and RKK analysed and supervised the work and writing the manuscript.



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Table of Content 82x44mm (150 x 150 DPI)

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Environment Friendly Mesoporous Magnetite Nanoparticles Based

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