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. 2022 Nov 27;7(48):43647–43656. doi: 10.1021/acsomega.2c04553

Tin Oxide (SnO2)-Decorated Reduced Graphene Oxide (rGO)-Based Hydroelectric Cells to Generate Large Current

Aarti †, Anurag Gaur †,‡,*, Prakash Chand †, Jyoti Shah §, Ravinder Kumar Kotnala §
PMCID: PMC9730460  PMID: 36506139

Abstract

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Nonphotocatalytic water splitting through oxygen-deficient, mesoporous metal oxide design-based hydroelectric cells (HECs) is a well-known phenomenon. To exploit more power from HECs, a metal oxide with more oxygen deficiency is desirable. In this study, oxygen-deficient mesoporous SnO2 via a sol–gel method and its composites with reduced graphene oxide (rGO) have been presented. Raman spectra of SnO2-rGO nanocomposites revealed an increase in the oxygen vacancies, while the X-ray diffraction (XRD) pattern confirmed the strain formation in the nanocomposite lattice owing to defect formation. The X-ray photoemission spectroscopy (XPS) results also indicated the presence of oxygen vacancies on the surface of SnO2, whereas Brunauer–Emmett–Teller (BET) measurements revealed that adding rGO into SnO2 increased the surface area from 44.54 to 84.00 m2 g–1. The water molecules are chemidissociated on the oxygen-deficient mesoporous surface of the pellet followed by physiodissociation at the mesopores. The redox reaction of the dissociated ions at the Zn anode and the Ag inert cathode produces current in the outer circuit. Interestingly, adding few drops of water into a SnO2-rGO-based HEC resulted in a short-circuit current of 148 mA with an open-cell voltage of 1.0 V. The maximum power delivered by the SnO2-rGO-based HEC is 148 mW. The addition of rGO into SnO2 boosts the peak current remarkably from 68 to 148 mA, which is the highest reported current generated by a hydroelectric cell.

Introduction

Oxide materials have gained popularity in recent years due to their high chemical stability and ease of processing.1−3 Porous ceramic materials are widely used for a variety of applications in both research and industry. Because of their extensive properties such as high structural integrity, good thermal capacity, and harsh environmental stability, oxide-based porous ceramic humidity sensors have been known to provide a better response than polymer-based sensors.4 The important feature of a porous oxide ceramic material is that water molecules can easily pass through the pores, and vacancies result in a more effective surface area.5 Oxygen vacancies act as trapping centers for electrons and holes that define the physicochemical properties of oxide materials. Recently, defects like oxygen vacancies and porosity in a variety of metal oxides have been used to generate green electricity via water molecule dissociation in a hydroelectric cell (HEC) without the use of any electrolyte/light/alkali/temperature.6,7 In comparison with other metal oxides, tin oxide (SnO2) has been studied extensively for applications such as catalytic breakdown, gas sensing, and organic molecule detection. These applications take advantage of the intrinsically defective chemical structure of tin oxide.8 Its use in industry is encouraged by its relative abundance, cheap cost availability, environmental friendliness, faster response time, and improved chemical stability. Tin oxide has been successfully employed in a range of modern applications due to its intrinsic defects, low electrical resistance, great optical transparency, and tunable valency of the tin cation.9−13 The use of oxygen-deficient SnO2 in the form of green power generation using a hydroelectric cell (HEC) technology was recently reported.7 At ambient temperature, the device adsorbs water molecules on defective surfaces and spontaneously chemidissociates them into H+ and OH– ions. Nanopores in the cell operate as trapping sites for hopping protons, generating a high electric field inside the nanopore and allowing physisorbed water molecules to split for a long time.6,14 Surface defect states including oxygen vacancies, interstitials, and unsaturated surface bonds have been found to increase active sites for water molecule attraction and chemidissociation reactions. Metal ion doping has been discovered to be particularly successful in increasing defect concentration, which has improved the current generation of SnO2-based HEC. Apart from this, several carbon-based nanomaterials are also used to enhance the electrical properties of SnO2.15

The importance of graphene-based metal oxide nanocomposites in science and technology is immense, and it holds enormous promise for energy conversion devices. Mixing SnO2 with rGO improves adsorption by introducing grain boundaries, interfaces, and surface imperfections. Several articles on the production and use of SnO2-rGO nanocomposites have been published to date. According to Gui, the presence of the oxygen vacancy enhances gas sensing performance toward NO2 in the SnO2/rGO heterostructure at room temperature.16 Using a simple hydrothermal process, Van Tuan et al. demonstrated that SnO2/rGO nanocomposites have significantly improved photocatalytic capabilities.17

In the present study, SnO2-rGO nanocomposites have been studied first time in the context of hydroelectric cells. We have synthesized SnO2 and SnO2-rGO nanocomposites using a simple and convenient sol–gel approach. To increase the conductivity as well as the electric power output of SnO2, rGO is added. This research suggests SnO2-rGO nanocomposite as a potential candidate for green energy production through a hydroelectric cell.

Experimental Section

Material Used

Graphite fine powder (98%), stannous chloride dihydrate (SnCl2·2H2O), ethylene glycol (C2H6O2), citric acid (C6H8O7), sulfuric acid, dilute hydrogen chloride, hydrogen peroxide (extra pure, 30% w/v, H2O2), potassium permanganate, hydrazine monohydrate, absolute ethanol (99.9%, CH3CH2OH), and sodium hydroxide were used for the synthesis. All of the compounds were of analytical grade and were utilized exactly as received. In the synthesis, deionized (DI) water was employed.

Synthesis of GO

Graphene oxide was made using a modified Hummer's technique. Natural graphite powder (2 g) was mixed with concentrated H2SO4 (90 mL) in a beaker, which was agitated and chilled to 5 °C in an ice–water bath for 2 h. KMnO4 (9 g) was progressively added to the graphitic solution at a temperature below 20 °C, then heated at 35 °C for 4 h while stirring. The mixture was stirred for 30 min at 98 °C after being diluted with DI water (200 mL). The reaction was stopped by adding a 30% H2O2 solution (10 mL) after the dilution, resulting in a yellowish-brown precipitate. Centrifugation was used to separate the obtained product, which was then washed multiple times with 10% dilute HCl, ethanol, and DI water. The obtained graphene oxide precipitate was dried for 3 days at 40 °C.

Preparation of SnO2 Nanoparticles

A simple sol–gel method was used to make SnO2 nanoparticles, as shown in Figure 1a. In a typical SnO2 synthesis, 50 mM SnCl2·2H2O was dissolved in 200 mM ethylene glycol at ambient temperature. After that, 50 mM citric acid was added to the above-mentioned solution at 50 °C under constant stirring. The solution was then stirred for 3 h at 70–80 °C (to obtain a clear solution), and then held at 110 °C for a few hours until the formation of a thick white gel occurred. The gel was dried for 2 days at 150 °C in an oven. To make the final product, the dried gel was calcined at 350 °C for 3 h in a muffle furnace.

Figure 1.

Figure 1

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Schematic representation of the synthesis procedure of (a) SnO2, (b) reduced graphene oxide (rGO), and (c) SnO2-rGO nanocomposite.

Preparation of SnO2-rGO Nanocomposites

The sol–gel approach was used to bind SnO2 nanoparticles on graphene sheets. The whole synthesis procedure of rGO, SnO2, and SnO2-rGO nanocomposites is depicted in Figure 1a–c as a schematic picture. To obtain a homogeneous suspension, an appropriate quantity of GO powder (0.01 g) was added to DI water (100 mL) in a beaker and ultrasonicated for 2 h. Next, 50 mM SnCl2·2H2O and 200 mM ethylene glycol were separately prepared and added to the suspension and swirled on a magnetic stirrer continuously for 1 h.

The 50 mM citric acid was added to the above solution at 50 °C. The solution was then stirred for 3 h at 70–80 °C (to obtain a clear solution). A freshly prepared NaBH4 solution was added to the aforesaid solution and agitated for 1 h to reduce GO to rGO. The solution was then stirred for a few hours at 110 °C until the formation of a thick white gel occurred. The gel was dried for 2 days at 150 °C in an oven. To make the final product, the dried gel was calcined at 350 °C in a muffle furnace.

Characterizations

Crystallinity, phase, and crystallite size were analyzed through X-ray diffraction using a Bruker D8 ADVANCE ECO P-XRD system having Cu Kα (λ = 1.54 Å) radiation with a scan speed of 2° min–1 and a 0.02° step size. Raman spectra from a laser Raman spectrometer (Wi Tec alpha 300 RA) at a laser excitation wavelength of 532 nm were used to confirm the purity of the samples. X-ray photoelectron spectroscopy (XPS) at room temperature was used to analyze the chemical states of the elements and oxygen vacancies in synthesized samples. Surface morphology, elemental mapping, and particle size measurements were completed using field emission scanning electron microscopy (FESEM JEOL), scanning electron microscopy (SEM JSM6390LV), and transmission electron microscopy (JEM-1400 TEM), respectively. The nitrogen adsorption–desorption isotherm was recorded via the Brunauer–Emmett–Teller (BET) technique (Quantachrome) used to assess the pore size distribution, surface area, and pore volume. A hydraulic press machine was used to compress the produced powders into 2.0 × 2.0 cm2 pellets (6 ton pressure). One side of the pellet was painted in a comb pattern with conducting silver paste, and the other side was covered with a 0.3 mm thick anode zinc plate. By adjusting external loads, a pulse source meter (Keithley-2400) was utilized to depict V–I polarization properties of the fabricated cell. The Nyquist plot of wet cells was accomplished using a CHI760E Electrochemical workstation within a frequency range of 0.1 to 105 Hz.

Results and Discussion

Structural Characterization

The components and crystalline phase of as-prepared SnO2 and SnO2-rGO nanocomposites are investigated using XRD. The obtained diffraction pattern has been found in good agreement with JCPDS file no. 41-1445 for the rutile phase of tetragonal SnO2 structure with space group P42/mnm.18 No distinctive peaks of additional contaminants were identified in the XRD pattern of SnO2-rGO nanoparticles. FULLPROF suite software was used to perform Rietveld refinement of the X-ray data of the SnO2 sample. Figure 2a delineates the Rietveld refinement of X-ray diffraction at room temperature for the SnO2 sample.19,20

Figure 2.

Figure 2

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(a) Rietveld analysis of SnO2 nanoparticles; (b) room-temperature X-ray diffraction patterns of SnO2, GO, rGO, and SnO2-rGO nanocomposites; and (c–e) structural model of SnO2.

The lattice parameters of the unit cell a = b = 4.756 Å, c = 3.194 Å, and the volume of the unit cell V = 72.256 Å3 for pure SnO2 were found to be in good agreement with the literature using Rietveld refinement. Six atoms make up the unit cell. As illustrated in Figure 2c–e, each tin Sn4+ ion is in the center of an extremely regular octahedron created by six oxygen ions, while each O2– is surrounded by three Sn4+ positioned at the vertices of an isosceles triangle.21−23 The XRD patterns in Figure 2b of as-prepared SnO2 and SnO2-rGO nanocomposites are observed to be similar, with diffraction peak intensities decreasing, showing strain in the SnO2 lattice caused by electrostatic interaction between the rGO sheet and SnO2 nanoparticles.24,25

According to Debye Scherrer’s formula,26 the average crystallite size (D) of our compound was calculated from the full-width at half maximum (FWHM) of the most prominent peak (110).

Debye Scherrer’s formula is given as

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where λ is the X-ray wavelength (λ = 0.154 nm), β is the full-width at half maximum (FWHM) intensity, and θ is the diffraction angle.

For lattice strain27

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where ε is the strain, βhkl is the FWHM of the most intense (110) peak, and θ is the Bragg angle.

It can be seen that the peaks of pure SnO2 are narrower than that of SnO2 nanocrystals in the SnO2-rGO composite, indicating the high crystallinity and larger crystal size of pristine SnO2. It has been proved that the presence of rGO can modify the growth of SnO2 grains.28 Due to the confinement effect of the graphene sheet, the particle size of SnO2 in the SnO2-rGO composite is much smaller than that of pure SnO2.29,30 The Sn–O bond length fluctuates due to lattice strain produced by the development of oxygen vacancies31,32 (Table 1).

Table 1. Structural Properties of SnO2 and SnO2-rGO Using the (110) Diffraction Plane.

samples FWHM (β) (degree) diffraction angle (2θ) (degree) crystallite size (nm) lattice strain
SnO2 0.51 26.47 16 0.009
SnO2-rGO 1.13 26.56 8 0.021
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Raman Analysis

The microstructure and defect characteristics of SnO2-rGO nanocomposites were studied using Raman spectroscopy. The Raman spectra of GO, rGO, and SnO2-rGO nanocomposites at room temperature are shown in Figure 3. The D band and the G band are two significant distinctive bands that are assigned to graphene. A one-phonon defect-assisted process of vibrations of k-point phonons of A1g symmetry gives rise to the diamondoid (D) band, demonstrating the presence of disorder and defects introduced by functional groups in the graphene planes. The E2g phonon of sp2-hybridized carbon atoms of isolated double bonds in the graphene plane gives birth to the graphite (G) band.33 It is observed that there is a shift in Raman peak positions, which confirms the bond formation between SnO2 nanoparticles and graphene layers.34,35 The degree of defects in synthesized samples is determined by the ratio of D and G band intensity (ID/IG). ID/IG ratios for SnO2-rGO (1.07) are higher than graphene oxide (0.83) and rGO (1.01), indicating the reduction in the sp2 carbon domain and a greater number of defects caused by the growth of grain boundaries, vacancies, amorphous carbon species, and the attachment of SnO2 nanoparticles on the rGO plane.36−38

Figure 3.

Figure 3

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Raman spectra of synthesized GO, rGO, and SnO2-rGO nanocomposites.

XPS Analysis

The chemical state and surface compositions on the surface of prepared SnO2 and SnO2-rGO nanocomposites were studied using X-ray photoelectron spectroscopy (XPS). Figure 4a–d delineates the XPS spectra of the synthesized samples. The survey spectra (Figure 4a,b) demonstrate the primary constituents of SnO2 and SnO2-rGO samples, which are Sn, O, and C.

Figure 4.

Figure 4

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X-ray photoelectron spectroscopy. (a, b) Survey scan of SnO2 and SnO2-rGO. High-resolution XPS spectra of (c) O 1s and (d) C 1s for SnO2-rGO.

Three peaks at 529.4, 531.6, and 533.1 eV are observed in the high-resolution O 1s XPS spectra (Figure 4c). The 529.4 eV peak is attributed to lattice oxygen (Sn–O), the 531.6 eV peak is attributed to the oxygen-deficient region, and the third peak is attributed to chemisorbed oxygen species and −OH groups on the surface.39,40 The C 1s spectrum is shown in Figure 4d. The broad band may be separated into three XPS peaks centered at 284.9, 286.8, and 287.9 eV, which correspond to C–C, C–O, and C=O groups of rGO, respectively.41−43 We can conclude that the SnO2-rGO sample obtained via a sol–gel process consists of rGO and SnO2 species with more oxygen vacancies and OH groups.

Morphological Analysis

The surface morphology of the prepared samples was investigated using the FESEM technique. Figure 5a–c displays the FESEM images of as-prepared SnO2 nanoparticles, SnO2-rGO nanocomposites, and rGO nanosheets. Figure 5a,b shows the FESEM images of the surface morphology of SnO2 NPs and SnO2-rGO nanocomposites. The spherical-shaped SnO2 nanoparticles with approximately uniform size are shown in this FESEM picture.44 In SnO2-rGO nanocomposites, it is shown that the rGO sheets are immersed in clusters of SnO2 nanoparticles. The structure of rGO nanosheets can be observed as a carbon layer, as shown in Figure 5c.45 SnO2 nanoparticles scattered on the surface of nanosheet-like graphene layers in a SnO2-rGO nanocomposite were also exhibited in the low-magnification TEM image in Figure 5d.46 Further, the particle size distribution histogram for the SnO2-rGO nanocomposite obtained from the TEM image, as shown in inset Figure 5e, is in the range of 7–8 nm, which is very close to the value calculated through XRD data. The signals of Sn, O, and C in the SnO2-rGO nanocomposite are observed in the energy dispersive X-ray (EDX) spectrum in Figure 5f. To confirm the distribution of Sn (blue color), O (green color), and C (red color) onto the surface, elemental mapping of the area was carried out, as shown in Figure 5g–j.

Figure 5.

Figure 5

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(a–c) FESEM of SnO2, SnO2-rGO, and rGO; (d) TEM image of the SnO2-rGO nanocomposite; (e) particle size distribution histogram of the SnO2-rGO nanocomposite; and (f–j) EDX spectra and mapping of the SnO2-rGO nanocomposite.

BET Analysis

The Brunauer–Emmett–Teller (BET) and Barrett–Joyner–Halenda (BJH) methods were used to measure the porosity and specific surface area of SnO2 and SnO2-rGO samples. Figure 6a–d shows the nitrogen adsorption/desorption isotherms and the BJH pore size distribution plot of pure SnO2 and SnO2-rGO nanocomposites. According to IUPAC standards, the resulting isotherm curves are type IV isotherms following an H3-type hysteresis loop, which is compatible with the De Boer classification, indicating that the materials are mesoporous.47 The pore volume, surface area, and pore size of pure SnO2 and SnO2-rGO nanocomposites are listed in Table 2. The addition of rGO in SnO2 produces more oxygen vacancies, which is also confirmed by XPS. A large surface area produces a large number of surface dangling bonds with associated oxygen vacancies. This increased surface area not only offers additional active sites but also provides effective transport channels for reactant molecules.48−50

Figure 6.

Figure 6

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(a–d) N2 adsorption–desorption isotherm and the BJH pore size distribution plot of SnO2 and SnO2-rGO nanocomposites.

Table 2. Data of the Average Surface Area, Pore Volume, and Pore Radius of SnO2 and SnO2-rGO Nanocomposites.

samples surface area (m2 g–1) pore volume (cm3 g–1) pore radius (Å)
SnO2 44.536 0.084 17.078
SnO2-rGO 84.008 0.209 19.103
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The BJH plots for the determination of the pore volume and the radius are delineated in Figure 6c,d. The average pore radius for pure SnO2 and SnO2-rGO was obtained nearly equal to 17.078 and 19.103 Å, respectively. The total pore volume increases from 0.084 cm3 g–1 (SnO2) to 0.209 cm3 g–1 (SnO2-rGO), suggesting the formation of additional pores.

The overall pore volume increases from 0.084 cm3 g–1 for SnO2 to 0.209 cm3 g–1 for SnO2-rGO, indicating the development of extra pores caused by the reduction of GO to rGO caused by the removal of various oxygen functional groups from the nanocomposites.30 The observed results suggest that synthesized samples to fabricate HEC cells are highly mesoporous in nature because of oxygen vacancies

Performance of SnO2 and SnO2-rGO Nanocomposites

As illustrated in Figure 7a–d, the performance of SnO2-based hydroelectric cells was assessed using voltage–current polarization (V–I) plots. With distinct working regions of HEC, the voltage drop across the external variable load is given as a function of current. The short-circuit current (Isc) and open-cell voltage Voc of SnO2-based HEC showed a 68 mA current and 1.1 V open-cell voltage and SnO2-rGO nanocomposite cells showed 148 mA current and 1.0 V open-cell voltage, respectively, as shown in Table 3. The maximum offload current is reported at 93 mA by a Sb–SnO2-based HEC among other metal oxides like ZnO, SnO2, MgO, Al2O3, SiO2, and TiO2.7,51

Figure 7.

Figure 7

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(a–d) Polarization curves and power generated by pure SnO2 and SnO2-rGO-based hydroelectric cells.

Table 3. Obtained Parameters of Fabricated Hydroelectric Cells.

samples Isc (mA) Voc (volts) ideal power (mW) Pmax = Voc × Isc
SnO2 68 1.1 74
SnO2-rGO 148 1.0 148
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Table 4 presents a comparison of current and voltage generated by different HECs. It is observed that the metal oxide’s surface has unsaturated metal cations Mδ+ and oxygen vacancies Vδ− on the surface.7 This Mδ+–Vδ− pair is known as active defect pair sites for adsorption as well as for chemidissociation of water molecules.52 In SnO2-rGO, the unsaturated metal cations and oxygen vacancies (as confirmed by the XPS, BET, and Raman spectroscopy) attract oxygen from a water molecule. These oxygen vacancies trap the electrons and attract hydroxyl ions of water molecules. This water molecule dissociates into hydronium and hydroxide ions when it approaches the defect pairs of electronegative surface cations and oxygen vacancies. Hydroxide ions create chemical bonds with unsaturated cations on the surface, and a chemisorbed layer of OH– ions physisorbed the water molecules with the help of hydrogen bonding. Hydronium ion hopping occurs inside the open nanopores on the surface due to hydrogen bonding traps. The physisorbed water molecules in hydronium and hydroxide ions are further dissociated by the concentrated trapped ions, which provide enough electric potential. As a result of the Grotthuss chain reaction, hydronium ions migrate toward the cathode. The following are the electrochemical half-reactions that should occur on the electrode and the SnO2-rGO surface.

At the metal-oxide surface,
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At the Zn surface,
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At the Ag surface,
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SnO2-rGO HEC generates a higher offload current in comparison to SnO2 HEC. It is due to more surface energy generated by the unsaturated surface cations and oxygen vacancies formation in SnO2-rGO, which is validated by XPS and BET results. Thus, SnO2-rGO requires less activation energy for water molecule dissociation, which results in a higher current value.

Table 4. Comparison of Different HECs.

HECs current (mA) voltage (volts) refs
SnO2 22.2 0.75 (7)
Mg–SnO2 41.69 0.78 (14)
Co–SnO2 77.52 0.45 (14)
Li–SnO2 70 0.98 (53)
Sb–SnO2 93 1.43 (54)
CeO2-rGO 21.3 0.83 (30)
TiO2 2.3 0.90 (7)
SiO2 1.1 0.96 (7)
ZnO 5.0 0.90 (7)
MgO 1.5 0.94 (7)
Al2O3 6.5 0.93 (7)
SnO2-rGO 148 1.0 present work
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Typical polarization zones are shown in the V–I polarization curves for SnO2 and SnO2-rGO HECs at various load levels in Figure 7. The breakdown of water (H2O) molecules into H3O+ and OH ions, as well as redox processes at electrodes, generate current and voltage in HECs.55 The V–I curves are divided into three portions, AB, BC, and CD (as shown in Figure 7), which explain the different types of polarization losses used to characterize the cell’s operational zones.56,57 The open-circuit voltage of the cell at an infinite load was denoted by point A. The activation loss, which is prevalent in the lower current density regions, is given by the region AB. The activation loss is directly related to the rate of the electrochemical reaction. This loss is responsible for the delay in the initiation of the water dissociation process on the mesoporous surface and in the collection of ions by the respective electrodes. Pure SnO2 showed that a high activation loss may be due to low defect density dissociating a small number of water molecules, which, in turn, require more energy for the charge transfer reaction to proceed. The Ohmic region (BC) represented the internal resistance of the hydroelectric cell material to ion flow along with electron flow resistance offered by cell electrodes. It is observed that a large linear Ohmic region is useful for HEC operation observed in the SnO2-rGO cell, which signifies the low internal resistance of the cell material. This may be because of increased defect centers, which promote the ion transfer via H3O+ hopping and migration of OH– via oxygen vacancies.

The mass transport loss/concentration loss, which is present in the higher current density zones, is represented by CD. The higher concentration loss is observed for SnO2-rGO, which accounts for a higher concentration of dissociated ions and a limited amount of charge transport at highly active electrodes. The active linear region and the high current value obtained for SnO2-rGO HEC are beneficial for facile, portable, and low-cost electricity generation by dissociation of the water molecule.

In the present study, cells’ maximum powers were calculated to be 74 and 148 mW for SnO2 and SnO2-rGO-based HECs, respectively. The maximum power of 148 mW obtained for a SnO2-rGO HEC cell with an area of 4.0 cm2 is higher among all reported HECs, as shown in Table 4. The addition of GO to SnO2 enhances the current magnitude from 68 mA for SnO2 to 148 mA for SnO2-rGO. The improved oxygen vacancies and the surface area increase the water dissociation, resulting in an increase in the current. Because of the facile charge transfer between rGO and SnO2, the barrier to OH– diffusion via grain boundaries is reduced, resulting in a lower Ohmic loss. The current HEC’s potential is higher than previously reported HEC cells, indicating that the redox reaction and the water dissociation rates are balanced.

The charge transport mechanism within hydroelectric cells has been observed by the Nyquist impedance response of fabricated HECs. Figure 8a,b shows the Nyquist plots of wet SnO2 and SnO2-rGO HEC cells, respectively. In the given frequency range, both graphs in Figure 8a,b exhibit a single semicircular curve. It is observed that the addition of rGO in SnO2 decreased the series resistance by approximately 10 times, revealing high OH– and H3O+ conduction in the SnO2-rGO cell because of which the current magnitude increased. In the case of a pure SnO2 cell, a semicircular arc at a high frequency illustrated the cell’s internal resistance of approximately 13 Ω, as obtained by the fitting curve in Figure 8a. The hydronium ions transfer toward the respective electrodes via a hopping mechanism, and hydroxide ions move via defects, which decreases the internal resistance of the cell. The highly reduced internal resistance is observed for SnO2-rGO (∼8 Ω) due to the high concentration of defects, which provide a conduction path for the dissociated ions.

Figure 8.

Figure 8

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Nyquist equivalent circuit fit. (a) Wet SnO2 and (b) wet SnO2-rGO hydroelectric cells, with an analogous circuit for simulating their Nyquist spectra.

In the wet condition, SnO2 cells have the highest impedance. The value of impedance was reduced for the SnO2-rGO cell when GO was added to SnO2. This decrease in impedance suggests that electrons move from the surface of SnO2 nanoparticles to defects sites on the rGO surface generated during GO reduction in nanocomposites.31,58,59 The impedance of water-soaked cells dramatically drops to ∼30 Ω for the SnO2-rGO cell as compared to ∼100 Ω for the SnO2 cell. The plots indicate diminished semicircular loops in the high and intermediate frequencies. The water dissociation in nanocomposite HECs led to a dramatic drop in resistance, allowing ionic flow. Because of the high proton conductivity in GO, the conductivity of nanocomposite-based HEC increased after wetting, as seen in the Nyquist plot. Adding GO content into SnO2 lowers grain boundary resistance, allowing facile hydroxyl ion migration through grain boundaries when compared to the pure SnO2 cell.60

Conclusions

In summary, the oxygen-deficient mesoporous SnO2 and SnO2-rGO nanocomposites were synthesized via a sol–gel method. The addition of rGO into a SnO2 lattice induced oxygen deficiency as evidenced by XPS and Raman spectroscopy studies. The amount of water molecule dissociation/splitting in SnO2-rGO nanocomposites was boosted by an increase in oxygen vacancies, Sn3+ surface ions, and increased surface area. The addition of rGO into SnO2 increased the conductivity of hydroxyl ions via migration. The SnO2-rGO-based HEC delivers the highest reported current of 148 mA and a voltage of 1.0 V. The ability to build vast amounts of oxygen defects in SnO2 and achieve a high current density with such a simple approach has enormous potential to replace solar cells and fuel cells.

Acknowledgments

One of the authors, A.G., is thankful to the Haryana State Council of Science & Innovation Technology (HSCSIT) for providing funding through research project no: HSCSIT/R&D/2022/158. The authors are also thankful to MRC, MNIT, Jaipur, for the XPS, and JNU Delhi for the RAMAN facility.

Author Contributions

Aarti contributed to the methodology, data curation, and writing—original draft; P.C. contributed to supervision; A.G. contributed to conceptualization, editing, validation, and supervision; J.S. contributed to editing and data analysis; and R.K.K. contributed to conceptualization, results, and discussion.

The authors declare no competing financial interest.

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