Metal Oxide Based Hydroelectric Cell for Electricity Generation by

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C: Energy Conversion and Storage; Energy and Charge Transport

Metal Oxide Based Hydroelectric Cell for Electricity Generation by Water Molecule Dissociation without Electrolyte/Acid Ravinder Kumar Kotnala, Rekha Gupta, Abha Shukla, Shipra Jain, Anurag Gaur, and Jyoti Shah J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b04999 • Publication Date (Web): 24 Jul 2018 Downloaded from http://pubs.acs.org on July 25, 2018

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The Journal of Physical Chemistry

Metal Oxide Based Hydroelectric Cell for Electricity Generation by Water Molecule Dissociation without Electrolyte/Acid R. K. Kotnala1*, Rekha Gupta1, Abha Shukla1, Shipra Jain1, Anurag Gaur2, Jyoti Shah1 1

CSIR-National Physical Laboratory, New Delhi 110012, India

2

National Institute of Technology, Kurukshetra 136119, India

*Corresponding Author Fax: 91-11-45609310; Phone: 91-11-45608599/8266 E-mail: [email protected]

Abstract Never before electricity has been generated out of metal oxides without using any light (UV/IR), acid or alkali but it has been achieved by adding few drops of water on nano porous metal oxide based Hydroelectric cell (HEC) at room temperature. Electricity generation has been validated and unified for six different metal oxides based on the principle of water dissociation at oxygen deficient nonporous pellet. Presence of oxygen vacancies at the surface of all metal oxide samples has been confirmed by Raman and Photoluminescence spectroscopy. Oxide based HEC delivers maximum power ~16.6 mW in 4.48 cm2 area of SnO2 cell with peak current 22.2 mA, approximately 2.075 times higher than reported 8 mA current in ferrite based HEC. Water chemidissociation at metal oxide surface was found to be reinforced predominantly by electronegativity of metal cations and defect structures at nanoporous surface. Divergent peak current values ranging from 22.2 mA to 1.1 mA were obtained depending on internal resistance, grain boundary nature, water molecule dissociation capability and nanopores connectivity of different oxides. Slow diffusion of ions in certain metal oxides pertaining to highly impeding

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grain boundaries have reduced current that confirmed by dielectric and impedance spectroscopy. Metal oxide HEC provides an ecofriendly, cost effective and portable green energy source with almost no running cost. Introduction The quest for searching new green energy sources has been focus of intensive research for last fifty years due to dwindling conventional energy resources. Recently invented green energy device Hydroelectric Cell (HEC) produces electricity by dissociation of water molecules at octahedrally coordinated unsaturated cations and oxygen vacancies on Mg0.8Li0.2Fe2O4

1-3

. In this device, initially water molecules are chemidissociated on oxygen

deficient porous surface of Mg0.8Li0.2Fe2O4 followed by physidissociation of water molecules at nanopores. Hydroelectric cell based on ferrite material is one of the best alternative for green energy harvesting without disturbing ecological balance of earth and it is attracting a lot of attention of masses globally

1-9

. In present work, different metal oxide Hydroelectric Cell

configurations have been put forward, by replacing ferrite, for their low-cost availability, versatility and to obtain more electrical power output. Metal oxides are a versatile class of material that find vital technological utility in new generation solid state sensors, antibacterial agents, electrochemical reaction modulations and solar power generation

10-14

. By the virtue of

their intriguing physical and chemical properties and low-cost, metal oxides offer their promising role in the development of new version of Hydroelectric Cells to produce green electricity. The most common point defects observed at oxide surface are oxygen vacancies that have been long established to increase surface reactivity of metal oxides. Dissociation of water molecules has been most studied as a surface reaction which occurs at oxygen vacancies or defect sites 15-16. For last few decades, this phenomenon is being utilized for hydrogen production, gas phase shift 2 ACS Paragon Plus Environment

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reactions, photocatalytic activities and other gas sensing applications but not for direct current generation in external circuit 17-18. In present work, we have taken SnO2, Al2O3, ZnO, TiO2, MgO and SiO2 for Hydroelectric cell fabrication to validate water dissociation mechanism at room temperature without use of acid/alkali and light. The oxides have been explored exhaustively theoretically as well as experimentally for interaction of water molecules with oxygen vacancies and exploited for collection of dissociated ions based on Hydroelectric cell principle to generate electrical power as green energy for the first time. Experimental The metal oxide test samples for hydroelectric cell were prepared by solid state sintering method. Analytical grade precursor powders of SnO2, Al2O3, ZnO, TiO2, MgO and SiO2 were grinded separately in a pestle mortar for 30 minutes and presintered at 750O C for 2hrs. Presintered powders were again grinded for 30 minutes and pressed into 2.5 × 2.5 × 0.1 cm3 pellets. These pellets were finally sintered for 2hrs at different temperature ranges viz. 900O C 1200O C for different oxides to obtain nanoporous structure. Different processing temperatures were opted for different oxides to produce oxygen deficient metal oxide surface for optimum dissociation of water molecule. Comb patterned silver cathode of 1µm thickness was screen printed on one face of all pellets while opposite face was covered with a 0.3-mm thick zinc plate as anode. Raman Spectroscopy was performed by Renishaw Raman spectrometer at laser excitation wavelength of 514.5 nm. Photoluminescence measurements of all samples were observed using a photoluminescence spectrometer (Edinburgh Instruments, model FLSP-900) with a fixed excitation wavelength of 375 nm diode laser source. The current-voltage polarization was measured by varying load on metal oxide HECs using a Keithley source meter. BET surface analysis was performed by BET Quanta Chrome Instrument (NOVA 2000e USA). 3 ACS Paragon Plus Environment


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Surface morphology was analyzed using a scanning electron microscope (Zeiss: Evo MA-10). Impedance measurements of the cell pellets in dry and wet state were performed by Wayne-Kerr 6500B (UK) Impedance analyzer operated at 10 mV applied ac voltage in 20 Hz –120 MHz frequency range. Results & Discussion: Phase purity of all the samples has been confirmed by their characteristic Raman spectra indexed in Fig.1. SnO2 has been detected in Rutile phase and most intense Raman peak A1g at 634 cm-1 in SnO2 is assigned to antisymmetric vibrations of oxygen atoms in O-Sn-O bond

19

. The other

characteristic peaks of SnO2 are identified at 143 cm-1 (B1g), 475 cm-1 (Eg) and 776 cm-1 (B2g antisymmetric Sn-O stretching)

20.

An unspecified Raman peak appearing at 542 cm-1 is closely

associated to surface disorder activated Raman mode found in the region of 475 cm-1 to 775cm-1 by Dieguez et al. and Liu et al.

21, 22

. Broad peaks obtained in Raman spectra of Al2O3 can be

assigned to high defect density and disordered grain formation confirmed from SEM images. The characteristic modes of α-Al2O3 in crystalline phase have been observed at 378 cm-1 (Eg), 418 cm-1 (A1g), 578 cm-1 (Eg) and 749 cm-1(Eg)

23

. It is well known that SnO2 and Al2O3 are more

prone to form oxygen vacancies due to low defect formation energy, the surface disorder peaks observed in Raman spectra of these oxides may be assigned to high defect density or oxygen vacancy concentration present at their surface

24, 25

. The prominent peak observed at 438 cm-1

(E2g) in ZnO is assigned to displacement of lighter oxygen atoms in O-Zn-O bond 26. The other Raman modes are observed at 583 cm-1 (E1g) and 380 cm-1 (A1g) while the peaks at 143 cm-1, 332 cm-1 and 651 cm-1 are related to multi phonon process due to second order Raman spectra arising from zone boundary phonons 27, 28. The anatase phase of TiO2 has been confirmed from different Raman peaks at 147 cm-1 (Eg),197 cm-1 (Eg), 639 cm-1 (Eg) arising due to symmetric stretching


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vibrations of oxygen atoms in O-Ti-O bond 29. The other modes at 396 cm-1 (B1g) and 639 cm-1 (B1g) are assigned to antisymmetric bending vibrations of O-Ti-O bond in anatase phase. A little shift in prominent peak at 147 cm-1 may be due to nonstoichiometry or oxygen vacancies present at TiO2 surface 30, 31. Raman modes representing α-SiO2 are observed at 128 cm-1 (E1g), 208 cm-1 (A1g), 265 cm-1 (E2g), 354 cm-1 (A1g), 399 cm-1 (E1g), 465 cm-1 (A1g) and 699 cm-1 (E1g) respectively 32, 33. Single observed Raman peak at 1092 cm-1 in MgO is assigned to second order Raman scattering due to longitudinal optical phonon 34. Presence of oxygen vacancies on the surface of metal oxide HECs is one of the basic requirement of HEC working principle. Structural nonstoichiometry in the form of oxygen/cation vacancies and interstitial defects in metal oxides create F-centers with visible region luminescence

35

. Oxygen vacancies on the surface of all metal oxide samples were further

confirmed from Photoluminescence (PL) spectroscopy. Fig.2 shows PL spectra of metal oxide samples excited at wavelength of 375 nm. As the excitation energy for obtaining PL spectra for all metal oxide samples was kept lower than the band gap of samples under investigation, only surface defects were examined. Broad PL emission observed for all oxide samples in blue and blue green region (450 nm - 570 nm) of visible spectrum were ascribed to presence of in-plane and bridging oxygen vacancies respectively as depicted in Fig.2 35. The PL emission observed at 446 nm was assigned to presence of in-plane oxygen vacancies at SnO2 surface while a low shoulder at 471 nm represented bridging oxygen vacancies 36-38. PL emission observed at 501 nm with a hump at 474 nm in Al2O3 has been assigned to presence of neutral oxygen vacancies 39-40. Similar PL emission at 504 nm for ZnO and 501 nm for TiO2 indicated presence of singly ionized oxygen vacancy (F+ center)

41-44

. MgO and SiO2 samples also represented oxygen

vacancy related signature emission peaks at 498 nm and 502 nm respectively 45-46.



The grain size distribution as well as mesopore formation has been estimated by SEM images as depicted in Fig.3. Nanopores in the range of 3 nm - 6 nm at the surface of all metal oxides have been confiremed by BET analysis and porosity extending from minimum 6.22 % in SnO2 to maximum 38.4 % in TiO2 has been calculated by Archimedes method. The results are summarized in Table 1. The extent of chemidissociation of H2O molecules at metal oxide surface has been found to be strongly dependent upon electronegativity (Lewis acid strength), number of undercoordinated metal cations, defect density, surface orientation and oxygen vacancies 47,48. In order to identify degree of dissociated ions, a qualitative analysis of initial chemidissociated ions in metal oxides HECs has been carried out by inceptive surface current measurement by introducing few drops of water at their respective surface. Maximum current density observed ~ 0.79 µA/cm2 in SnO2 reduced to 0.62 µA/cm2 for Al2O3, 0.587 µA/cm2 for ZnO, 0.168 µA/cm2 for TiO2, 0.142 µA/cm2 for MgO and 0.119 µA/cm2 for SiO2 respectively (see supplementary Table S1). The strength of water molecule dissociation by different oxides has also been examined by indirect method of pH change of deionized water (see supplementary Table S2). Fig.4 represents voltage-current (V-I) polarization curves of different oxide hydroelectric cells. The peak power densities of different cells have been obtained as 16.6 mW, 6.4mW, 4.5mW, 2.07mW, 1.41mW, 1.05mW for SnO2, Al2O3, ZnO, TiO2, MgO and SiO2 respectively. Potential difference values of different cells has been measured as 0.75V, 0.93V, 0.90V, 0.90V, 0.94V, 0.96V for SnO2, Al2O3, TiO2, ZnO, MgO and SiO2 respectively. The voltage - current and other vital parameter values for six cells are depicted in Table 1. Various operating regions of hydroelectric cells have been explained by different polarization regions shown in V-I curves. Region AB represents activation polarization loss, where point A indicates actual open cell voltage at infinite load condition and directly measures chemical reactions prevailing at both


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electrodes. With decreasing load for SnO2, initial sharp drop of ~ 0.4V in AB region may be assigned to the high activation energy barrier for charge transfer reactions occurring at SnO2/electrode interface. It may be accounted to large number of water molecule dissociation spontaneously by SnO2 HEC which in turn raises the pH of SnO2 (Table S2). Increase in pH further increases hydrogen bonding energy (HBE) of silver metal resulting in to slow hydrogen evolution rate at silver cathode

49

. Reduced hydrogen evolution further lowers electron intake

from Zn anode to Ag cathode, which in turn increases barrier for OH- ion diffusion towards Zn anode. It ultimately leads to raised activation energy barrier and steep fall (~ 0.4V) in activation region. Similar high activation energy barrier ~ 0.25V has been observed in Al2O3 due to large number of dissociated ions. On the other hand, reduced activation polarization barrier ~ 0.15V0.1V has been observed for ZnO, TiO2, MgO and SiO2 respectively with reduced number of chemidissociated ions due to lower Lewis acid strength of metal cations or lesser oxygen vacancies available at the surface confirmed from Raman Spectra analysis. Region BC is attributed to ohmic polarization loss wherein voltage decreased almost linearly with increasing current. In this region current is limited by internal resistance of the cell material for H+ / OHion flow. Moreover, reducing external load CD region signifies concentration polarization loss where current attained the limiting values due to mass transfer of ions restricting further reaction at electrodes and material interface. Lower ohmic loss region in SnO2 (Fig. 4a) may be accounted for less resistance offered to the ion flow in pellet of SnO2 HEC due to low grain boundary impedance and similar situation prevails for Al2O3 HEC. Moreover, maximum ionic current also results in crowding of ions at electrode interface which in turn increases concentration polarization losses as shown in fig. 4(a-b). A nonohmic loss in ZnO HEC may be due to high impedance offered to ionic current flow across the cell resulting from highly resistive grain



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boundaries or lack of nanopores connectivity. Lower value of current obtained in TiO2, MgO and SiO2 HECs in ohmic region may be accounted for higher ohmic loss owing to high internal resistance created by highly resistive grain boundaries of these materials. Concentration losses in these cells fig.4(c-f) are restrained by smaller values of current obtained which further limits crowding of ions at electrode surface. Experimentally obtained polarization curves have also

been validated by modeling V-I data by semi empirical equation = − [ − −

ln(1 − )], where is reversible cell potential, A is Tafel slope, is exchange current

density, R is ohmic resistance for ion flow, m is constant and is limiting current density 50. The calculated parameters were observed in close approximation to experimental values as depicted in Table S3 (See supplementary Table S3) and fitted plots are shown in Fig.3. These results were further upheld by dielectric and impedance spectroscopy. Fig.5 represents frequency dependent dielectric loss of different oxide cells under wet condition. A characteristic low frequency relaxation peak of interfacial charge has been observed in all wet samples while no such peak was found in dry Hydroelectric cells (see supplementary Fig.S1). To negate the dipolar relaxation from reorientation motion of water molecule, dielectric relaxation of water has also been measured as shown in Fig.5 (blue filled circles). It is explicitly evident from Fig.5 that none of the cells showed dipolar relaxation as observed in case of water which indicates that water is present in dissociated form on the surface of cells. Loss peak observed at minimum frequency region in SiO2 and MgO indicates that maximum ions remain at the surface of these oxides only, either in dissociated or bound physiosorbed state. The shift of loss peak towards higher frequency side in ZnO, TiO2, Al2O3 and SnO2 respectively could be due to diffusion of ions inside the oxide surface, wherein Al2O3 and SnO2 clearly show dielectric relaxation in grain boundary region (~ 10 kHz) indicating presence of charge/ions at grain 8 ACS Paragon Plus Environment

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boundary of the material. An additional hump at frequency > 1MHz has also been detected in Fig.5 for SnO2, TiO2 and Al2O3 cells in wet state only which is assigned to the relaxation of charge carriers inside the grain

51

. Fig.6 represents complex impedance (Zʹʹ) vs frequency plots

of metal oxide cells in dry as well as in wet state. It is explicit that in wet state characteristic peaks of high impedance grain boundaries are visible (~10 kHz) for the constrained movement of ions, while in dry state (inset Fig.6) no such peaks are observed. Minimum grain boundary impedance has been observed for SnO2 while the maximum impedance recorded for ZnO followed by SiO2 and MgO in wet state also evident from V-I ohmic loss region. It is also confirmed by experiments conducted that maximum current and minimum bulk resistance for easy movement of ions has been observed in case of SnO2 cell. Moreover, maximum chemidissociated ions are produced at the surface of SnO2 evidenced from inceptive surface current measurement. Rutile structure of SnO2 readily forms defects due to its very low defect formation energy

52

and nonstoichiometry in SnO2 drops coordination of Sn atoms from

five to four with an apparent change in oxidation number from Sn4+ to Sn2+

53-55

. High electron

affinity (1.96) and reduced coordination of surface Sn atoms enhances their catalytic activity and makes them more receptive towards H2O molecule. Water interaction with SnO2 surface first comprises dissociative adsorption of H2O near defect site at Sn4+ metal atom resulting in to SnOH (terminal) and bridging O-H bonds formation. Presence of nanosize (3.4 nm) pores at SnO2 surface as examined by BET analysis, further dissociates the physiosorbed water molecules due to high electric field produced by trapped H3O+ ions inside the nanopores 1. A large amount of physidissociated H+ and OH- ions accelerates the diffusion of H+ ion towards Ag electrode through stabilized physiosorbed layer of water akin to Grotthuss diffusion. Further low grain



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boundary impedance and defective lattice promotes the diffusion of OH- ions towards Zn anode via capillary diffusion resulting into high current obtained in SnO2 HEC. Similarly, high current and low value of resistance for ion diffusion occurs in α-Al2O3 HEC. Alumina stabilizes in trigonal corundum structure consisting octahedrally coordinated Al ions 56. In bulk Al2O3, one third of the possible octahedrally coordinated cation sites are vacant and such vacancies reduce the nearest neighbour ligand coordination of Al ion by smallest amount. In addition to high electron affinity (1.61), the coordinative unsaturated surface Al ions behave as strong Lewis acid sites for H2O adsorption. It has also been found that α-Alumina readily adsorbs water at ambient conditions due to negative heat of formation of aluminum hydroxide

57

. Adsorbed H2O chemidissociates into H+ and OH- ions at defect site via proton

transfer to the nearby oxygen vacancy 48. A large number of chemidissoiciated ions has also been observed by surface current measurements (Table S1). Hydroxylated Al2O3 surface physiosorbs more water molecules to get physidissociated due to electric field developed inside the nanopores by trapped charges. Similar to SnO2, dissociated H+/OH- ions move towards respective electrodes via hopping and capillary diffusion promoted by low impedance grain boundaries and defective lattice of Al2O3. It has also been evidenced from Fig.4 that SnO2, Al2O3 and TiO2 confirmed the presence of ions inside the lattice of the oxide indicating diffusion of ions via lattice in these materials. Reduced value of current obtained even after large number of chemidissociated ions in TiO2 may be due to large number of grain boundaries present at the surface of TiO2 as depicted in SEM images. Abundance of grain boundaries in the oxide reduce H+ ions hopping towards Ag electrode at surface and limits the current generation

58

. Low

current obtained in ZnO may be assigned to its high grain boundary impedance for diffusion of physidissociated OH- ions towards Zn anode leading to limited current. Less amount of


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chemidissociated ions have been recorded for MgO and SiO2 respectively (Table S1). Low value of current in MgO may be accounted for the presence of maximum number of grain boundaries at MgO surface which reduces hopping of H+ ions towards Ag electrode. On the other hand, high impedance of grain boundary disrupts migration of OH- ions towards Zn electrode in SiO2 and MgO HECs giving rise to low current as confirmed from V- I plots. For further analyzing overall charge transfer process in HEC, Nyquist plots were obtained from electrochemical impedance spectroscopy (EIS) of metal oxide cells in dry and wet state as represented in Fig.7. A large drop in reactance ~ 105 ohm can be clearly seen in all plots by introducing a few drops of deionized water on the cell surface. A large single resistance arc has been obtained in overall frequency range in case of dry cells representing high resistance of oxide materials, while two depressed semicircles followed by a sloping line have been obtained in wet cells indicating actual charge transfer process. In order to identify individual resistance contribution from electrode/material/interface to the charge flow in HEC, equivalent circuit analysis has been performed by using EIS analyzer under wet condition of cells. The details of equivalent circuit are depicted in supplementary file Fig. S2. First semicircle in high frequency region reflects bulk resistance of the oxide materials for the ion current flow getting associated as ionic resistance due to large open pores or high grain boundary resistance, while second semicircle in middle frequency region is attributed to charge transfer resistance at anode surface 59-61. The sloping tail at low frequency is attributed to diffusion of ions at the electrode interface 62, 63. Fig. 7(e) & (f) exhibits high frequency semicircle dominates the Nyquist plots of SiO2 and MgO where high values of bulk resistance Rb have been obtained. Maximum bulk resistance value derived from Nyquist plots in SiO2 and MgO reflects slowest movement of dissociated ions in these materials which has been confirmed from 11 ACS Paragon Plus Environment


minimum value of current visible in V-I curves. It can also be interpreted from Table 2 that maximum Rct has been observed in SiO2. A low surface current is also confirmed in SiO2 and MgO due to less number of chemidissociated ions. It is evident that both intrinsic as well as extrinsic characteristics are responsible for this behavior. Theoretical and experimental investigations reveal that interaction of water with SiO2 forms Silanol (Si-OH) group and further breaking of Silanol bonds to form Si(OH)4 is a very slow reaction

64, 65

. While in case of MgO, water molecules are more associatively adsorbed on

the surface as compared to the dissociated one

66

. This may lead to a low surface current

recorded in SiO2 and MgO HECs. High grain boundary impedance in these metal oxides further lead to slow diffusion of ions and maximum bulk resistance as shown in Table 2. On the other hand, high Rct observed by Nyquist plots can be accounted for the clustering of grains and open macropores on SiO2 surface and maximum number of grain boundaries at MgO surface. High Rct further hinders the diffusion of ions at anode leading to overall low value of current in SiO2 and MgO HECs. Charge transfer resistance dominates in other metal oxides SnO2, Al2O3, ZnO and TiO2 inferred by highly resistive middle frequency semicircle obtained (Fig.7 a-d). Higher number of dissociation as well as faster diffusion of ions in these materials enhances the concentration of hydroxide ions on anode surface which is confirmed from V-I and dielectric measurements. It further results into high resistance of OH- ion interdiffusion at Zinc anode leading to increased Rct. Low value of bulk resistance determined from Nyquist plots in SnO2, Al2O3 and TiO2 may be due to their low grain boundary impedance and defective lattice leading to higher current in SnO2, Al2O3 and TiO2 HECs. While high grain boundary impedance as evident from Fig.6 may result in to high bulk resistance and low current in ZnO HEC. The


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calculated values of all parameters obtained from equivalent circuit fitting of Nyquist plots are depicted in Table 2. Conclusions: Electric current generation from metal oxide HECs by water dissociation has been thoroughly investigated. Current generation due to water dissociation at surface has been confirmed as distinctive property of all metal oxide Hydroelectric cells. In the group of six metal oxides investigated, SnO2 seems to be the best choice for metal oxide Hydroelectric Cell fabrication followed by Al2O3, ZnO, TiO2, MgO and SiO2 respectively. High defect density and low bulk resistance as well as grain boundary resistance of SnO2 makes it more susceptible for ionic current flow. However, all these oxides can be further explored for optimal current generation. This work confirms that working principle of HEC for electricity generation can be extended to any other material possessing nanoporous oxygen deficient structure along with cathode and anode. Table captions: Table 1: Current -Voltage and surface parameters of metal oxide HECs Table 2: Equivalent circuit fitting parameters of metal oxide HECs Figure Captions: Fig.1 Room temperature Raman spectra of as synthesized metal oxide samples. Fig.2 Room temperature Photoluminescence spectra of metal oxide samples in blue and green region of visible spectrum.



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Fig.3 SEM images of metal oxide HEC pellets (a) SnO2 (b) Al2O3 (c) ZnO (d) TiO2 (e) MgO (f) SiO2 respectively. Fig.4 Room temperature voltage-current polarization curves of metal oxide HECs representing different polarization regions. Red line indicated fitted plot to experimental data. Fig.5 Frequency dependence of dielectric loss (D) in wet metal oxide HECs. Fig.6 Reactance ( ) v/s frequency (f) plots of dry metal oxide HECs. Inset represents / f plots of wet metal oxide HEC’s where highly resistive grain boundary peaks of ZnO, SiO2 and MgO are clearly observable. Fig.7 Complex impedance (- v/s ) plot of dry metal oxides HECs. Inset represents fitted complex impedance spectra of wet metal oxide HECs. Fig.8 Dissociation of water molecules at metal oxide surface and migration of H+/OH- ions towards respective electrodes via Grotthouss and capillary diffusion Table 1: Current -Voltage and surface parameters of metal oxide HEC’s Sample I (mA)

V (Volts)

Peak Power (mW)

Grain Size(µm)

Pore Size (µm) Open surface (SEM)

% Porosity

BET

Surface Pore area Diameter (m2/g) (nm)

SnO2

22.2

0.75

16.65

0.200

0.150

6.22

4.978

3.459

Al2O3

6.5

0.93

6.04

0.400

0.326

15.78

10.968

6.6.03

ZnO

5

0.90

4.50

0.340

0.150

30.9

1.699

3.442


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TiO2

2.3

0.90

2.07

0.135

0.125

38.4

11.182

3.073

MgO

1.5

0.94

1.41

0.065

0.040

20.5

37.450

3.107

SiO2

1.1

0.96

1.05

0.500

0.280

10.3

5.670

3.279

Table 2: Equivalent circuit fitting parameters of metal oxide HEC’s Sample R1

Rct

Rb

Cct

Cb

n1

n2

Aw

L

SnO2

8.4E-17 22.02 28.65 2.256E-4

2.1845E-4 0.4427 0.7113 78.90 8.2E-08

Al2O3

1.0496

7.306E-10 0.6543 0.9355 173.3 1.7E-07

TiO2

1.3E-15 22.00 52.86 6.537E-10 4.906E-4

1

ZnO

21.397

0.7746 0.2947 20.39 3.0E-08

MgO

4.9E-24 69.85 306.4 2.573E-5

SiO2

3.4E-19 98.05 966.7 1.895E-05 1.850E-10 0.5736 0.9210 3222. 6.5E-08

12.42 33.53 5.0E-4

55.83 149.8 1.271E-4

5.0E-4

0.5227 985.4 6.6E-08

3.661E-11 0.7317 1


575.6 2.5E-07


Fig. 1

Fig.2


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Fig.3



Fig.4

Fig.5

Fig.6


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Fig.7

Fig. 8



Associated content Supplementary information S1. SEM analysis, S2. table of inceptive current measurements, S3. Table of change of pH of DI water S3. table of V/I fitting parameters, S4. BET surface area results and S5. dielectric spectroscopy of dry Hydroelectric cell test samples and S6. Impedance equivalent circuit analysis details. Acknowledgement

Authors are thankful to the Director, “CSIR-National Physical Laboratory" New Delhi for providing constant encouragement, motivation and support to carry out this work. References: (1)

Kotnala, R. K.; Shah, J. Green Hydroelectric Energy Source Based on Water Dissociation

by Nanoporous Ferrite. Int. J. Energy Res. 2016, 40, 1652-1661. (2)

Kotnala, R. K.; Shah, J. Lithium Substituted Magnesium Ferrite Material Based

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