Green Energy Applications of Hematite (α-Fe₂O₃), Magnetite (Fe₃O₄), and Maghemite (γ-Fe₂O₃) Nanoparticles Based Hydroelectric Cell

Abstract

Recently invented hydroelectric cell (HEC) is emerging as a better alternative for green electrical energy devices. HEC is fabricated as to generate electricity via splitting of water into H3O+ and OH− ions without releasing any toxic product. In iron oxides, Hematite (α-Fe2O3), magnetite (Fe3O4) and maghemite (γ-Fe2O3) nanoparticles HEC are recently reported for their remarkable electrical response by splitting water molecules. Fe3O4 HEC 4.8 cm2 surface size has delivered 50 mA short circuits current. Li ions into Fe3O4 stabilize electrical cell response to 44.91 mA with open-circuit voltage 0.68 V. Maghemite based HEC delivered a maximum short circuit current 19 mA with emf 0.85 V using water 200 μL. Maximum off-load output power 27.6 mW has been delivered by 4.84 cm2 area hematite-HEC which is 3.52 times higher with 7.84 mW power as generated by Li-Mg ferrite HEC. Maximum electrical power 16.15 mW delivered by maghemite HEC is 0.58, 0.42 times lower than respective magnetite, hematite HECs. In more applicability of iron oxides, the multiferroic nanocomposites of BaTiO3 with 85% CoFe2O4 has been shown maximum short circuit current 7.93 mA and 0.7 V emf by sprinkling few drops of water on HEC surface. Li0.3Ni0.4Fe2.3O4 and Mg0.8Li0.2Fe2O4 HECs also have some remarkable results for green energy generation.

1. Introduction

World is fast becoming a global village due to the increasing daily requirement of energy by all population across the world while the earth in its form cannot change. Energy demand necessities for all societies to be services of energy to meet basic human needs like health, lighting, cooking, space comfort, mobility and communication, and serve as generative processes [1]. World’s growing energy need, alongside increasing population led to the continual use of fossil fuel-based energy sources (coal, oil, and gas) which became problematic by creating several challenges. It includes depletion of fossil fuel reserves, greenhouse gas emissions and other environmental concerns, geopolitical and military conflicts, and the continual fuel price fluctuations. Recently, renewable energy sources are the most outstanding alternative and the only solution to the growing challenges of energy. In 2012, renewable energy sources supplied 22% total world energy generation which was not possible a decade ago. Renewable energy is an abundant source of energy and environment friendly as well [2]. Solar and wind energy for instance, are currently doing so well but their intermittency requires that energy storage or converting device system become more efficient and cost effective. Fuel cells are emerging energy converting devices which has low or no environmental effect but with the actual energy efficiency and energy density are lower than theoretical ones. Similarly dye synthesized solar cells are attractive energy conversion device have some limitations due to their stability. Renewables remained fastest growing source of energy in buildings, increasing 4.1% annually on average between 2009 and 2019. Use of renewable electricity for heat, that is, electric heat pumps, provided the second largest renewable energy contribution in recent years. Meantime, more than 256 gigawatts (GW) of renewable power capacity were added globally during recent year, surpassing the previous record by nearly 30.4% [1, 3, 4]. Table 1 has shown present status of power capacity. Global population without access to electricity continued to shrink, although 771 million people (10% of the world’s population) still lacked electricity access in 2019, nearly 75% of them in sub-Saharan Africa.

1.1 Global energy transition, 2050

Figure 1(a) provides a geographical breakdown of the renewable power generation capacity [5]. China accounts for over one third, followed by the United States, India, and the European Union. Around 85% renewables in the power sectors with a large share from recurrent solar PV and wind is not possible without some strong combination of flexible dispatchable power, transmission interconnection, storage, smart grids, and demand-side management. Innovative technologies, operational practices, market designs, and business models are needed. Digital technologies open up new opportunities that yield new forms of flexibility such as aggregators that bundle services from small systems into marketable packages or consumer real-time price signals. In 2017, 50 Hz grid operator in eastern Germany recorded an annual average 53.4% variable renewable energy. Growth of direct use of renewables in end use sectors (buildings, industry, and transport) would contribute 0.3% points annual renewables share growth, around a quarter of the total. Biomass alone would account for two-thirds of direct use of renewable energy in 2050. In primary energy terms annual bioenergy supply would roughly double from present levels to around 116 EJ in 2050.

Figure 1.

(a) Geographical breakdown of renewable power generation capacity (2018–2050) [5]. (b) Global electricity production by source and share of renewables, 2010–2020 [3].

1.2 Global electricity productions

In early 2020, global electricity demand dropped sharply in the wake of the COVID-19 pandemic. However, demand rebounded by year’s end, resulting overall in a slight decline of around 2%, the first annual decline since the global economic crisis of 2008/2009. Production of electricity from renewables was favored under these low-demand circumstances due to its inherent low operating costs, as well as dispatch rules in many countries that prioritize renewable electricity. Renewables generated an estimated 29.0% of global electricity in 2020, up from 27.3% in 2019 (Figure 1(b)). Progress in renewable energy, and the decline in fossil fuels (coal), has been especially pronounced in certain countries and regions. Wind power, hydropower, solar power, and bioenergy became the EU-27’s main source of electricity in 2020, growing from 30% of generation in 2015 to 38%. Electricity generation from these renewable sources grew 23% as production from coal power fell by half over this period. Similarly, in the United Kingdom, renewables grew to a 42% share of generation which become main source of electricity in 2020, beating out fossil gas and coal at 41%. In United States, renewable energy reached nearly 20% of net electricity generation, with solar and wind energy accounting for more than half of this in 2020. More than 19% of Australia’s electricity came from wind and solar energy in 2020. In China, electricity from hydropower, solar energy, and wind energy provided more than 27% of production, up from around 26% in 2019. While variable renewables contributed more than 9% of global electricity in 2020, in some countries they met much higher shares of production, including in Denmark (63%), Uruguay (43%), Ireland (38%), Germany (33%), Greece (32%), Spain (28%), the United Kingdom (28%), Portugal (27%), and Australia (20%).

1.3 Renewable energy status in India

At present India is sixth largest country in the world in electricity generation with aggregate capacity 149 GWs out of which 25% hydro, 64% thermal, 3% nuclear, and about 8% renewable energy (small hydro, wind, cogeneration and biomass-based power generation, and solar) [6]. India’s energy consumption has been increasing at a relatively fast rate because of economic development which grow at 8–9% per annum. Due to this India needs to attains a target of having 70% renewable energy use by 2050. Initiating with a very low base of renewable in 2000, the installed capacity of grid-connected renewable has reached 27.5 GW in 2013. Figure 2 shows only 2.1% of renewable energy share to the Indian electricity grid. Government funded solar energy in India only accounted ~6.4 MW/year power as of 2005, 25.1 MW was added in 2010 and 468.3 MW in 2011. In view of these discussion about renewable energy sources, there is a need to develop more growth system for green energy from other sources like Hydroelectric cell (HEC) that recently investigated by Kotnala et al. on iron oxide nanoparticles [7, 8, 9, 10, 11, 12].

Figure 2.

Electricity grid-renewable energy share in India [6].

1.4 Hydroelectric cell as novel renewable energy source

Hydroelectric cell is newly invented as green energy source which offers many advantages over other renewable energy sources without using any solar/UV irradiation/electrolyte, while it uses little water for energy generation [7, 8, 9, 10, 11, 12, 13]. Most important thing towards green energy generation due to HEC is that the residues are non-toxic and its low-cost component raw materials. Electricity generation of HEC was first invented on LiMgFe₂O₄ due to water molecule dissociates on octahedrally under coordinated cations and oxygen defects [14]. Electrochemical redox reaction at respective Ag/Zn electrode with dissociated H₃O⁺ and OH⁻ ions develop an emf 0.98 V. HEC is environmentally benign, low cost, easy manufacturing, facile in electricity generation with significantly useful by-products, and it is a potential candidate to replace existing portable green energy sources. HEC not required any other toxic chemicals unlike solar cell it can work in day or night and can run small scale devices like LED and fan. Two byproducts of HEC are hydrogen gas (highly pure 99.98% H₂ gas) and Zn(OH)₂ nanoparticles which has higher value into industrial commercial products and environment friendly.

1.5 Applications of hydroelectric cell

Green electricity production by hydroelectric cell has applications in geographically tough regions like rural areas, farms, forests, and mountains. It utilized for domestic and residential purposes in decentralized mode. Hydroelectric cell also be used as energy source in automotive industry. Using facile process, HEC can produce high-quality H₂ gas which can be stored for further use as a clean fuel. HEC acts as portable power generator for charging mobile phone, torch, video camera, laptop, etc. Furthermore, HECs can be used as power panels for stationary power generation. HEC produced high purity nanoparticles of zinc hydroxide and after thermally dissociation, ZnO nanoparticles are formed as the HEC byproduct for industrial applications.

1.6 Iron oxide based hydroelectric cell

Iron oxides have considerable potential into split water molecule and generate electricity through it. Defects in terms of oxygen vacancies of iron oxide nanoparticles make it suitable materials for HEC fabrication. Advantage of iron oxide nanoparticles is that the defects are easily formed during synthesis and suitable heating conditions that easily produce both Fe²⁺ and Fe³⁺ ions via oxygen vacancies [15, 16, 17, 18, 19, 20]. Naturally occurring iron oxide like magnetite, maghemite, and hematite that easily splits water due to humid atmosphere [11]. Iron oxide nanoparticles have an amplified active surface suitable their reactivity towards polar water molecules. Water molecule interaction with iron oxide nanoparticles is highly dependent on coordination number of Fe ions along with surface composition. While dissociative adsorption of water is prominent on defective surface with coexisting Fe and O ions.

1.7 Iron oxides

Physical properties of iron oxides are given in Table 2. Iron oxides exist in eight forms in nature, with magnetite (Fe₃O₄), maghemite (γ-Fe₂O₃), and hematite (α-Fe₂O₃) which are probably most common due to their polymorphism involving temperature-induced phase transition [21]. Hematite is oldest known iron oxide that widespread in rocks and soils. Hematite has blood-red color that become black or gray in crystalline form. It extremely stable at ambient conditions, and often is the end product of the transformation of other iron oxides. Magnetite is also known as black iron oxide, ferrous ferrite, or Hercules stone and exhibits strongest magnetism. Maghemite occurs in soils as a weathering product of magnetite, or a product due to heating of other iron oxides.

Energy sources	Energy conversion and usage options	Power capacity 2020 (in GW)
Renewable power capacity (not including hydropower)	Power generation	1668
Renewable hydropower	Power generation	1170
Solar	Photovoltaic, thermal power generation, and water heaters	760
Wind power	Power generation, wind generators, windmills, and water pump	743
Bio-power	Heat and power generation, pyrolysis, gasification, and digestion	145
Geothermal power	Urban heating, power generation, hydrothermal, and hot dry rock	14.1
Solar thermal power	Solar home systems, solar dryers, and solar cookers	6.2
Ocean power	Numerous design, barrage, and tidal stream	0.5

Table 1.

Renewable energy sources, their use, and power capacity (in 2020) [1, 3].

Property	Fe oxides
	Hematite	Magnetite	Maghemite
Molecular formula	α-Fe2O3	Fe3O4	γ-Fe2O3
Density	5.26 g cm^−3	5.18 g cm^−3	4.87 g cm^−3
Melting point	1350°C	1583–1597°C	—
Hardness	6.5	5.5	5
Magnetism	Weakly ferromagnetic or antiferromagnetic	Ferromagnetic	Ferrimagnetic
Curie temperature	956 K	850 K	820–986 K
MS at 300 K	0.3 Am^2 kg^−1	92–100 Am^2 kg^−1	60–80 Am^2 kg^−1
Crystallographic system	Rhombohedral, hexagonal	Cubic	Cubic or tetrahedral
Structural type	Corundum	Inverse spinel	Defect spinel
Space group	R3c (hexagonal)	Fd3m	P4332 (cubic); P41212 (tetragonal)
Lattice parameter (nm)	a = 0.5034, c = 1.375 (hexagonal), aRh = 0.5427, α = 55.3° (rhombohedral)	a = 0.8396	a = 0.83474 (cubic), a = 0.8347, c = 2.501 (tetragonal)
Band gap (Eg)	n-type; Eg ~ 2.3 eV	n- and p-type; Eg ~ 0.1 eV	n-type; Eg ~ 2.0 eV
Applications	Catalysts, pigments, gas sensors, and HEC	Solar energy conversion, context of imaging, drug delivery, environmental catalysis, recording devices, and HEC	Magnetic recording, high-frequency switch modes, electromagnetic absorbers, biomedical, and HEC

Table 2.

Physical and magnetic properties of iron oxides [8, 9, 11, 18, 19].

1.7.1 Hematite (α-Fe₂O₃)

It is a most stable iron oxide under ambient conditions and can be used as a starting material for synthesis of magnetite and maghemite. Crystalline structure of hematite is shown in Figure 3(a), where Fe³⁺ ions occupy two-thirds octahedral sites that confined by nearly ideal hexagonal close-packed O lattice. Oxygen ions are in a hexagonal close-packed arrangement, with Fe(III) ions occupying octahedral sites. Iron atom has a strong magnetic moment due to four unpaired electrons in 3d orbitals. In crystalline state, hematite is paramagnetic at temperatures above its Curie temperature 956 K. At room temperature, it is weakly ferromagnetic with phase transition 260 K (the Morin temperature, T_M) to an antiferromagnetic state [22]. Hematite Morin temperature decreases with decreasing particle size and tends to vanish below 8–20 nm. Recently investigated hematite HEC has remarkable results due to heterolytic dissociation of water into two surface hydroxyl groups is energetically more favorable [9].

Figure 3.

Crystalline structure: (a) hematite, (b) magnetite, and (c) maghemite [19].

1.7.2 Magnetite (Fe₃O₄)

As shown in Figure 3(b), Fe₃O₄ has face centered cubic spinel structure (32 O²⁻ ions and close-packed along [111] direction) where all Fe²⁺ ions occupy half of the octahedral sites and Fe³⁺ are split evenly across the remaining octahedral and tetrahedral sites. Fe₃O₄ contains iron oxides into both divalent and trivalent states. Stoichiometric magnetite Fe^II/Fe^III = 1/2, and the divalent iron may partly or fully replaced by other divalent (Co, Mn, Zn, etc.) ions. Fe₃O₄ has lowest resistivity among iron oxides due to small bandgap 0.1 eV. Magnetite is ferrimagnetic at 300 K. Magnetite particles smaller than 6 nm are superparamagnetic at room temperature with coercivities ranging from 2.4 to 20 kA m⁻¹ [23]. Dissociation of H₂O molecules at surface Fe sites and oxygen vacancies provides surface hydroxylation. On the hydroxylated surface, H₂O molecules get physisorbed and trap H₃O⁺ ions inside mesopores to generate a high electric field, which further splits water molecules. This process results into increased ionic current in the cell. Li was doped into Fe³⁺ in Fe(II, III) oxide to create more oxygen vacancies to trap more electrons within oxygen vacancies [10].

1.7.3 Maghemite (γ-Fe₂O₃)

As shown in Figure 3(c), maghemite has a spinel structure that is similar with magnetite irrespective to cation sublattice vacancies. Two-thirds sites are filled with Fe(III) ions arranged regularly, with two filled sites being followed by one vacant site. Oxygen anions give rise a cubic close-packed array while ferric ions distributed over tetrahedral sites (eight Fe ions per unit cell) and octahedral sites (remaining Fe ions and vacancies). Maghemite is ferrimagnetic at room temperature, unstable at high temperatures, and loses its susceptibility with time. Maghemite magnetic behavior is stabilized by doping of transition ions. Maghemite particles smaller than 10 nm are superparamagnetic at 300 K. Fe²⁺ ions oxidized into Fe³⁺ state in an ambient air by creating octahedral surface iron vacancies, V^Fe_s for maintaining maghemite charge neutrality [11]. Non-stoichiometry in maghemite lattice is enhanced by creating oxygen vacancies with existing V^Fe_s increases surface reactivity significantly.

1.8 Surface oxygen vacancies formations in iron oxides

Iron oxides include hematite, magnetite, and wustite (FeO) forms [24]. Magnetite Fe₂O₃ exhibits various polymorphs such as hematite α-Fe₂O₃ (rhombohedral), maghemite γ-Fe₂O₃ (cubic), β-Fe₂O₃ (cubic), and ε-Fe₂O₃ (orthorhombic), among which α-Fe₂O₃ is most thermodynamically stable phase. Both γ-Fe₂O₃ (space group: P4₁32, a = b = c = 0.8347 nm) and Fe₃O₄ (space group: Fd3m, a = b = c = 0.8394 nm) share cubic structure with close-packed O atoms along <111> direction with vary Fe oxidation states. Applications for iron oxides intimately depend on their ability to redox (reduction and oxidation) cycle between +2 and + 3 oxidation states. Based on temperature-programmed reduction studies, two mechanisms have been proposed: a three-step mechanism, Fe₂O₃ → Fe₃O₄ → FeO → Fe; and a two-step mechanism, Fe₂O₃ → Fe₃O₄ → Fe [24]. In general, reduction of Fe₂O₃ is the hematite, does not occur directly metallic iron Fe. If the reduction temperature is lower than 570°C, reduction to Fe occurs stepwise from Fe₂O₃ to Fe₃O₄, called magnetite, and continues to Fe. Intermediate oxide, wustite Fe_1 − xO, is not stable at temperatures lower than 570°C. For this, reduction occurs from Fe₂O₃ via Fe₃O₄ to Fe_1 − xO and continues afterward to Fe [25].

Even without addition from dopants or extrinsic defects, the properties of an oxide can be altered considerably if the material is made non-stoichiometric. Resulting, so-called intrinsic defects [vacancies at either O (V_O) or metal (V_M) site or interstitials of both types, O_i or M_i], alter number of electronic charge carriers. Defects formation change both geometric and electronic structure. When Fe₃O₄ is reduced, the non-stoichiometry is accommodated through Fe interstitials, and when it is oxidized, Fe vacancies created [26]. For hematite α-Fe₂O₃, Fe, and O vacancies are said to mediated water splitting process through localization of optically-derived charges. Adsorbed species and doping on the hematite surface, intrinsic vacancies can lead to formation of charge sites with consequent bending of the electronic valence and conduction bands. This increases the abundance of acceptor Fe vacancies [27]. Negreiros et al. [28] suggested hematite (α-Fe₂O₃) is a potential candidate for photo-electrochemical water splitting. Under oxygen rich conditions, the hematite, Fe₂O₃ (0001) terminated surface more stable [29]. On this surface termination, the isolated water molecule forms a heterolytically dissociated structure with OH⁻ group attached with surface Fe³⁺ ion and proton to surface O²⁻ ion. In contrast, in corundum, the vacancy site is filled with two electrons that repel OH⁻ ions. Here, the proton resulting from dissociated water forms a hydride ion (H⁻). In the present chapter we have given a short review description on renewable energy sources in the form of hydroelectric cell based on iron oxides.

2. Synthesis of iron oxides hydroelectric cells

2.1 Co-precipitation technique

2.1.1 Magnetite HEC

Anhydrous FeCl₃ (32.44 g) and FeCl₂·2H₂O (16.22 g) (in molar ratio 1:2) were mixed in deionized water and the solution magnetically stirred at 60°C/1 h to get hydroxide precipitates by the addition of aqueous ammonia. Filtered precipitate was washed with deionized water and acetone to neutralize its pH. Black powder further vacuum-dried at 40°C/5 h. Dried powder pressed into 4.8 cm² pellet by a hydraulic press. Ag paste and Zn sheet also used as electrodes. Electrical contacts were made from the Ag and Zn electrodes to test the HEC performance [8, 9, 11].

2.1.2 Hematite HEC

Magnetite nanoparticles were synthesized by co-precipitation technique as discussed above. Filtered and washed precipitates with neutral pH are annealed at 500°C/2 h to obtain hematite nanoparticles. Crystalline hematite powder grinded and pressed into 2.5 × 2.5 × 0.1 cm³ pellet using hydraulic press. Pellet was further annealed at 600°C/2 h. Ag electrode in comb pattern was screen printed on one face of pellet and its second face was pasted with Zn anodic sheet of 0.3 mm thickness (Figure 4(b) and (c)) [30]. Measurement setup for HEC performance is shown in Figure 4(a).

Figure 4.

(a) Experimental setup for V-I process in HEC using Keithley source meter (multiferroic BaTiO₃/CoFe₂O₄) [7, 30]. (b) Magnetite HEC with Ag comb pattern and Zn electrodes on the faces of pellet [8]. (c) Fabricated HEC in Li-doped Fe₃O₄ [10].

2.1.3 Maghemite HEC

Maghemite nanoparticles were synthesized by the oxidation of magnetite nanoparticles using a co-precipitation technique as discussed above. Filtered and washed precipitates annealed at 300°C/2 h. Oxidized brown powder was grinded and pressed into a 2.48 × 2.48 × 0.1 cm³ pellet using hydraulic press and further annealed at 350°C/2 h Ag paste and Zn sheet used as electrodes.

2.2 Mg_0.8Li_0.2Fe₂O₄ HEC using solid state reaction method

Mg_0.8Li_0.2Fe₂O₄ pellet was synthesized by solid state reaction method. High-purity precursors MgCO₃, Li₂CO₃, and Fe₂O₃ were taken in 0.8:0.1:1 molar ratio. Precursor powders mixed and grinded for 2 h in pestle and mortar and annealed at 850°C/10 h. Crystalline powder was grinded for 30 min and pressed into a 2.2 × 2.2 × 0.1 cm³ pellet and 4.6 cm diameter circular pellet of 0.1 cm thickness. Pellets were subjected to sintering 1050°C/6 h. Comb-patterned Ag of 0.1 μm thickness and Zn sheet used as electrode of the pellet [12, 31].

2.3 BaTiO₃/CoFe₂O₄ HEC using sol-gel method

Ethanol and acetic acid are mixed in 75:25 ratio and C₁₆H₃₆O₄Ti is added to it. A required amount of Ba(CH₃COO)₂, CoCl₂·6H₂O, FeCl₃ are mixed in distilled water and then added to the precursor of C₁₆H₃₆O₄Ti to get solution M. Solution M and PVA are mixed in 5:2 ratio and dried at 700°C/7 h. Pellet samples heated at 800°C/2 h. To fabricate this multiferroic HEC, the BTO-CFO crystalline powder was mixed with 3–5 drops of PVA solution (as binder) and then pressed into 4.5 cm² circular pellet of thickness 1 mm using hydraulic press with 5 bar pressure. BTO-CFO pellets finally sintered at 800°C/2 h. In order to increase porosity in pellet nanocomposite, more quantity of binder PVA was added and sintering temperature may reduce.

3. Results and discussion

3.1 Hematite HEC for generating electricity by water splitting

Hematite HEC delivered highest 30 mA current with an emf 0.92 V using ~500 μL deionized water. Rhombohedral structured hematite is confirmed with XRD pattern [9]. Values of lattice constant, a = 5.034 Å and c = 13.757 Å is matched well with corundum structure. Hematite average particle size is 20.76 nm as calculated with Scherrer relation. FESEM confirm large number of evenly distributed mesopores which might be due to coalescence and strong agglomeration of hematite nanoparticles with value of average pore size is 17 nm. Existence of characteristic A_1gbands at 221 and 491 cm⁻¹ and E_g bands at 239, 287, 401, and 605 cm⁻¹, respectively, ascribed with hematite structure using Raman spectroscopy. E_g bands at 401 and 605 cm⁻¹ exists due to symmetric mode of O atoms related with cations in a plane perpendicular to crystallographic c-axis and Fe-O stretching vibrations, respectively [32]. FTIR spectra have investigated two IR absorption bands at 536 and 464 cm⁻¹ are the characteristic of Fe-O stretching vibrations of crystalline hematite [33]. The stretching and bending vibrations of adsorbed water molecules with bands located around 3270 and 1624 cm⁻¹ are observed [34]. Decreased intensity and slight Fe-O band shifting towards lower wavenumbers in wet hematite indicates change in Fe-O bond distance due to significant water adsorption. In XPS study, the high-resolution Fe 2p core-level spectra consisted of Fe 2p peak splitting into Fe 2p_3/2 peak at 711.2 eV and Fe 2p_1/2 peak at 724.6 eV is observed. Associated satellite peaks of Fe 2p_3/2 and Fe 2p_1/2 around 719.3 and 733.1 eV, respectively at ~8.1 and 8.5 eV high energy indicates presence of Fe³⁺ ions. O 1 s spectra leading to three components O_I, O_II, and O_III centered at 530.1, 531.3, and 532.6 eV, respectively is observed. Binding energy O_II peak around 531.3 eV is attributed with oxygen defects/vacancies. Broad PL emission band centered at ~564 nm (2.2 eV) corresponds to band edge transition. Non-radiative peaks at 656 nm (1.89 eV), 682 nm (1.82 eV), and 753 nm (1.65 eV) appeared from the transitions of trapped electrons in different defect states of oxygen vacancies on the surface of nanostructured α-Fe₂O₃.

3.1.1 TEM analysis

High-resolution TEM micrograph as given in Figure 5(a) depicted disoriented irregular hexagonal shaped nanoparticles with bright-dark fringes. Value of average grain size is 18.8 nm. Polycrystalline surface lattice fringes with 0.25 and 0.22 nm interplanar spacing correspond to (110) and (113) planes of hematite, respectively detected (Figure 5(a′)). Moire fringe pattern is observed in Figure 5(a″) which might be due to overlapping of strained crystallite fringes. Strain is relaxed due to structural defects, and the morphology of these defects is strongly dependent on growth conditions [35]. Planar surface defects and vacancy defects results into Moire fringes [36].

Figure 5.

(a) HRTEM image showing disoriented hexagonal hematite nanoparticle, (a′) surface lattice fringe pattern (inset), unequal d spacing (white arrows). (a″) Surface defect Moire fringe pattern, (b) BJH cumulative pore volume variation with pore size (inset is the active surface area of mesoporous), (c) V-I polarization curve of wet hematite HEC (inset is the low current activation polarization region), (d) respective electric power loss curve (schematic hematite HEC). Nyquist plot of (e) dry and (f) wet hematite HEC [9].

3.1.2 Microstructural analysis

Specific surface area 72.2 m² g⁻¹ has been calculated by adsorption isotherm using multipoint BET method (Figure 5(b)). Average pore width 16.8 nm is obtained from pore size distribution curve using Barrett-Joyner-Halenda (BJH) method. Cumulative pore volume 0.289 cc g⁻¹ is obtained with pore diameter less than 39 nm indicating mesoporous structure. The value of porosity is 27.4% which has been calculated by volumetric ratio method.

3.1.3 Voltage-current conduction process

Figure 5(c) is the voltage-current (V-I) polarization that shows 30 mA short circuit current and an open cell potential 0.92 V of wet hematite HEC of size 4.84 cm². The highly electronegative surface Fe³⁺ cations act as Lewis acid attracting water molecules strongly due to interaction of oxygen lone pair electron with octahedrally coordinated surface Fe ions having empty 3d orbitals [37]. This is the process of heterolytic dissociation of water molecule occurs with OH⁻ ion bonded on surface Fe³⁺ ion and dissociated H⁺ ion bonded nearby surface O²⁻ ion [38, 39]. Two OH⁻ groups formed due to initial chemidissociation of single water molecule. The physisorption of water molecule occurs via hydrogen bonding on chemidissociated surface OH group layer and therefore, the physisorbed layers form H₃O⁺ ions by H⁺/H₂O ion due to hopping mechanism [40]. Surface defects due to oxygen vacancies significantly enhance the reactivity of hematite surface. In Figure 5(c), the internal losses in HEC lead reduction of open cell voltage (E_oc) into 0.92 V. Since there is formation of different irreversible polarization losses (PQ, QR, and RS region) in HEC including activation polarization, ohmic polarization, and mass transport polarization which induces voltage drop with increment in operating current [41]. Therefore, the output voltage of HEC may describe by following Eq. (1):

$$V_{\mathit{out}}=E_{\mathit{open}\mathit{cell}}-V_{\mathit{activation}}-V_{\mathit{ohmic}}-V_{\mathit{mass}\mathit{concentration}}$$E1

where V_out is the output overvoltage and E_oc open cell voltage. V_act, V_oh, and V_conc are activation, ohmic, and mass transport overvoltage loss, respectively. To overcome the energy barrier of electrochemical reaction occurring at Ag/Zn electrodes due to initial surface chemidissociated ions, the activation overvoltage (region PQ) might be required which is 0.11 V at low current density [9]. Due to larger number of chemidissociated surface hydroxyl groups occurring at hematite surface added physisorb water molecules via hydrogen bonding. Concurrently high electric potential is generated in mesopores of hematite due to trapped H₃O⁺ ions which responsible into spontaneous dissociation of physisorbed water molecules into H₃O⁺ and OH⁻ ions. In the intermediate current region BC, the linear voltage drop with increment in operating current is observed. This might be due to decrement in hematite cell resistance by spontaneous water molecule dissociation and charge/ion transport through crystalline boundaries. However, the voltage drop occurs due to overcoming loss due to flow of dissociated H₃O⁺/OH⁻ ions through mesoporous electrode surface. Sudden voltage degradation observed at high current density region (CD) which is the effect of concentrated polarization overvoltage. In this case, the charge/ions transported in high concentration at electrode surface are quickly taken away by active Zn/Ag electrodes to flow high current. Maximum hematite HEC current 30 mA recorded which result by polarization losses fallow lower rate of charge/ion transport phenomenon. Therefore, the output current might be affected with mass transport overvoltage loss [42]. Figure 5(d) showed electrical power (P_out = V × I) due to presence of polarization losses which is 4.5 mW with maximum off-load output power (P_max = E_oc × I_sc) is 33.94 mW.

3.1.4 Ionic conduction by impedance spectroscopy

Dynamics of charge/ion transport from hematite surface-electrode interface in wet HEC is determined by Nyquist plot equivalent circuit as given in Figure 5(e) and (f) [inset of Figure 5(f) is the equivalent circuit]. Values of impedances in dry and wet state hematite HEC is given in Table 3. A single semicircular resistive arc in dry hematite HEC is observed with impedance in the order ~10⁷ Ω which might be the effect of large number of defects and high porosity in the pellet sample. With few microliter of Millipore deionized water onto dry hematite HEC, the overall impedance is decreased 100 Ω. Equivalent circuit consists of R_c (resistance of crystallite); R_cb (resistance of crystallite boundary); R_ct (resistance by charge transport at the interface between electrode-hematite surface); W (Warburg element contribution by diffusion of ions at electrode surface); and C stand respective capacitance [43]. Fitted parameters from the equivalent circuit modeling for wet HEC are R_c (Ω) = 43, R_sc (Ω) = 23.7, R_el (Ω) = 19, C_c (F) = 5 × 10⁻⁹, n₁ = 0.84, C_sc (F) = 4.5 × 10⁻⁷, n₂ = 0.68, C_el (F) = 0.66, and A_w = 32. The higher frequency depressed semicircle is the bulk crystallite contribution where the water molecules chemidissociate at unsaturated surface cations. The middle frequency depressed semicircle is the effect of nanocrystallites boundaries contribution which has comparatively higher defect concentration to dissociate more water molecules. The single semicircle observed at lower frequency region corresponds to charge/ion transport phenomenon by ionic diffusion at crystallite surface/electrode interface.

x	Synthesis method	HEC area size (cm^2)	Crystalline phase	D (nm)	Pore size/p (%)	Isc (mA)	Voc (V)	RD (Ω)	RW (Ω)
Fe3O4 [8]	Co-precipitation	4.8	Cubic spinel	11	10 nm/46	50	0.77	1 × 10^5	80
Li:Fe3O4 [10]	Co-precipitation	4.08	Cubic spinel	400	4 nm	44.91	0.68	1 × 10^3	71
γ-Fe2O3 [11]	Co-precipitation	4.84	Cubic spinel	9–17	10.78 nm	19	0.85	1 × 10^6	1000
α-Fe2O3 [9]	Co-precipitation	4.84	Rhombohedral	18	39 nm/27.4	30	0.92	1 × 10^7	100
Mg0.8Li0.2Fe2O4 [12]	Solid state reaction	4.8	Cubic spinel	400	30	8	0.98	1 × 10^8	100
Li0.3Ni0.4Fe2.3O4 [30]	Solid state reaction	4	Cubic spinel	47	2.5 nm	15.3	0.90	5.3 × 10^6	15.5
Mg0.7Li0.3Fe2O4 [31]	Solid state reaction	4.9	Inverse spinel	1000	—	17	0.859	—	—
BaTiO3/CoFe2O4 [7]	Sol-gel using PVA	4.5	Tetragonal/Cubic spinel	70	2.3–10.8	1.2–7.9	0.7–0.97	0.5 × 10^6	5

Table 3.

Iron oxides HECs: Composition (x), synthesis method, cell area size (cm²), crystalline phase, particles size (D), pore size/porosity (p), short circuit current (I_sc), open circuit voltage (V_oc), dry cell resistance, R_D (bulk), and wet cell resistance, R_W (bulk).

3.2 Magnetite nanoparticles HEC

Typical magnetite HEC of 4.8 cm² area delivers 50 mA peak current with a maximum output power 38.5 mW. An electromotive force (emf) of 0.77 V is generated due to a redox reaction at respective electrodes. Figure 6 shows the HEC results of magnetite [8] where the water molecule chemidissociates on Fe surface cations and oxygen vacancies due to physisorbed water molecule dissociation results into charges trapped inside mesopores. Magnetite crystalline structure by XRD pattern gives diffraction peaks at 30.4°, 35.43°, 37.09, 43.08°, 53.43°, 56.94°, and 62.57° correspond to (220), (311), (222), (400), (422), (511), and (440) lattice planes of spinel phase, respectively. Interplanar spacing of most intense Fe₃O₄ (311) peak is calculated to be 2.53 Å with the value of lattice constant 8.38 Å. Crystallite size using the Scherrer relation ~12 nm.

Figure 6.

(a) Raman spectra of magnetite Fe₃O₄ nanoparticles measured at room temperature. (b) FTIR spectroscopy of magnetite showing Fe-O bonds. (c) FESEM image showing porous grains (inset is the EDX spectrum). XPS spectra of Fe₃O₄: (d) Fe 2p and (d′) corelevel O 1 s. (e) V-I characteristics and output power in wet deionized water condition [8].

3.2.1 Defects/vacancies study by Raman analysis

As Raman spectra shown in Figure 6(a), the five active bands at 200 (E_g), 317 (T_2g), 524 (T_2g), 533 (T_2g), and 668 (A_1g) cm⁻¹ confirm the formation of spinel magnetite [44, 45]. Since oxidation of magnetite to maghemite is highly sensitive with laser power, therefore the maghemite peaks at 352 (T_2g) and 709 (A_1g) cm⁻¹ may due to oxidation of magnetite. Raman spectral peaks of magnetite are shifted by 3–7 cm⁻¹ towards higher frequency which may attribute by quantum confinement effect.

3.2.2 Structural and defects study by FTIR analysis

Figure 6(b) is the functional group study by FTIR spectroscopy which shown two distinct bands at 570 and 446 cm⁻¹ due to intrinsic stretching vibration mode associated with Fe-O absorption bonds, respectively tetrahedral and octahedral sites of Fe₃O₄ spinel structure [46].

3.2.3 Nanopores and particles size by FESEM pattern

FESEM image of magnetite nanoparticles given in Figure 6(c) shows aggregation type product with nanopores formation. Elemental identification by EDX spectra is also given in the inset of Figure 6(c). Presence of Fe and O peaks with 71/29 wt% and 39/61 atomic% confirms the magnetite stoichiometry. Fe₃O₄ average grain size is 11 nm with mesopores size <50 nm, along with some macropores of size >50 nm is detected. Nanopore size and specific surface area of present magnetite nanoparticles is determined with BET nitrogen adsorption-desorption isotherms [8]. Specific surface area of Fe₃O₄ is found to be 89.78 m²g⁻¹ and the average pore diameter is 9.81 nm. Cumulative pore volume with pore diameter < 190 nm is 0.335 cm³g⁻¹. Using volumetric ratio analysis, the total porosity of Fe₃O₄ nanoparticles is estimated to be 46%.

3.2.4 Iron and oxygen vacancies confirmation with XPS analysis

Magnetite valence states of Fe and O are investigated with X-ray photoelectron spectroscopy as given in Figure 6(d) and (d′). Satellite peak at 718.5 eV is attributed with Fe³⁺ ion indicates partial oxidation of magnetite into γ-Fe₂O₃ which might be possible when Fe²⁺ is subjected to air oxidation into Fe³⁺ state [47]. Mean relative area under the curve of two Fe 2p peaks confirms attributed to Fe²⁺ and Fe³⁺ ionic states which used to calculate the stoichiometric ratio of Fe²⁺/Fe³⁺ ions, that is, 31.5:68.5 close to 33:67 (as standard stoichiometric ratio of Fe²⁺/Fe³⁺ in Fe₃O₄). Figure 6(d′) is the O 1 s XPS spectra which clearly shown peaks at 529.7, 531, and 532.3 eV, respectively of Fe-O, oxygen vacancies and OH ions. A major peak around 529.7 eV with an integrated peak area of 77% is attributed with lattice oxygen of Fe₃O₄ [47]. The observed surface oxygen defects in terms of oxygen vacancies and hydroxyl species act as donor centers for water molecule dissociation.

3.2.5 Magnetite HEC V-I performance

Figure 6(e) shows the electrical results of magnetite HEC that partially dipped in deionized water and generates 50 mA short-circuit current and 0.77 V open-cell voltage. This observed electrical response has a typical polarization region [48]. Decrease in voltage with increasing current is attributed with kinetics of electrochemical cell having three prominent loss regions. At point A, the open-circuit voltage of 0.77 V is lower than the redox potential of Ag and Zn electrodes. Decrease in voltage may attribute due to internal anodic behavior of magnetite owing to octahedral Fe²⁺-Fe³⁺ ionic oxidation. Activation loss (region AB) is dominant at lower current which results from delay in initiation of water dissociation process in mesoporous Fe₃O₄ surface and collection of dissociated ions by the electrodes. This activation energy is the energy barrier which is required to overcome electrochemical reaction that happening on mesoporous magnetite surface and electrode. Voltage drop in intermediate current density (region BC) is linear which is ohmic losses provides resistance to flow of ions through porous structure. Current density region CD has a sharp voltage drop which is observed due to crowding of electrode surface with high concentration of ions (mass transport loss) [42]. Output power has maximum value 38.5 mW is observed.

3.2.6 Mechanism for current conduction in magnetite HEC

Since the unsaturated Fe²⁺ and Fe³⁺ surface cations and oxygen vacancies in magnetite help to chemidissociation of water molecules. The high electron spin density makes more dangling bonds with trapped electrons as active sites for water molecule dissociation. The dissociated H₃O⁺ ions get trapped inside Fe₃O₄ mesopores to generate high electric field. The trapped electrons pull nearer H₂O molecule and consequently, the unsaturated surface metal cations immediately take OH⁻ ion out of it. When the OH⁻ ion is taken away, the H₃O⁺ ion remains, and these H₃O⁺ ions are trapped within nanopores of HEC. It leads to generate very high electric field inside pores. Hydronium ions, H₃O⁺ hops towards Ag electrode and hydroxide OH⁻ ions diffuse towards Zn electrode through surface vacancies. Redox reaction occurs between both electrodes leads to voltage and current generation in HEC due to Grotthuss process. Due to presence of nanosized pores on Fe₃O₄ surface, the H₂O molecules dissociates with high electrostatic potential by trapped H₃O⁺ ions inside nanopores. Zn gets oxidized to produce Zn(OH)₂, and H₃O⁺ ions are reduced at Ag cathode to the evolution of H₂ gas by capturing electrons from the Zn anode, generating emf in the cell due to reactions:

$$At\mathit{magnetite}\mathit{surface}:4H_{2}O\to 2H_3{O}^{+}+2O{H}^{-}$$E2

$$At\mathit{Anode}:Zn+2O{H}^{-}\to Zn{\left(OH\right)}_2+2e^{-}$$E3

$$At\mathit{Cathode}:2H_3{O}^{+}+2e^{-}\to H_2\left(g\right)\uparrow +2H_{2}O$$E4

In this process of electric current generation, Zn(OH)₂ deposited at Zn anode. Hydrogen H₂ gas evolution at the Ag anode. Resistance of dry magnetite cell is found to be 10⁵ Ω, which is reduced to 80 Ω after dipping the cell with deionized water.

3.3 Green energy enhancement with Li ions in magnetite HEC

The 4.08 cm² size HEC of Li-doped Fe₃O₄ (Li_0.4Fe_2.6O₄) delivers short circuit current, emf, and off-load output power is 44.91 mA, 0.68 V, 30.80 mW, respectively [10]. XRD pattern revealed spinel cubic structure with lattice constant 8.35 Å is measured. Using Debye-Scherrer relation, the value of crystallite size from most intense (311) peak is 8 nm. Porous microstructure using BET technique in FESEM analyzed the specific surface area to be 45 m² g⁻¹, cumulative pore volume 0.04683 cm³ g⁻¹ and pore size 4 nm at a relative pressure 0.99663 (P/Po) are measured. Li ions in Fe₃O₄ have created more oxygen vacancies in the spinel lattice to enhance the capability for water dissociation. XPS analysis indicates the existence of Fe into +2 and + 3 oxidation states of Fe₃O₄ and monovalent Li¹⁺ at the divalent Fe²⁺ site because to the concentration oxygen vacancy is enhanced. Oxygen vacancies accelerate the process of water splitting to produce larger current being generated by Li doping as compared with pure Fe₃O₄ HEC.

3.3.1 HEC V-I response

In Figure 7(a) the observed V-I mechanism is described with four types of different control segments [49]. Segment OP is the internal loss region where voltage 0.98 V as theoretical maximum exists due to internal losses in the cells. Segment PQ at low current density region is the activation loss where the voltage needed to overcome electrochemical reactions between Li:Fe₃O₄ surface cations and electrodes. Intermediate segment QR is the voltage reduction by cell ohmic losses which is mostly responsible to provide hindrance to the flow of ions via porous network. At high current density segment RS, a sudden decline in voltage is found is attributed due to highly reactive electrodes that do not get enough ions to react. This is also called concentration loss or mass transport loss [50]. On-load peak power shown by polarization plot is 5.39 mW.

Figure 7.

(a) V-I characteristics in Li-doped Fe₃O₄ (Li_0.4Fe_2.6O₄) HEC in deionized water under wet condition [10]. (b) Corresponding Nyquist plot in both dry and wet states.

3.3.2 Impedance measurement and Nyquist plots

The electrostatic impedance spectroscopy is an important tool to determine the dynamic behavior of a cell/battery [51]. A Nyquist Cole-Cole for Li-doped Fe₃O₄ HEC is shown in Figure 7(b) for both wet and dry states. It helps us to study the diffusion of ions due to water molecules splitting. From the equivalent circuit [inset of Figure 7(b)], the resistance R₁ corresponds to the first semicircle in the high frequency region and resistance R₂ from second semicircle in the middle frequency. Constant phase element Q is used for a capacitor to compensate non-ideal behavior of the electrode due to surface roughness or porosity and the Warburg element, W identify the diffusion of ions. Li ions into Fe₃O₄ HEC exhibits impedance in its dry state in the order of 10³ Ω which reduces into 71 Ω in its wet state. The HEC in moist state displays a tail at a low frequency which confirms the ionic diffusion of H₃O⁺.

3.4 HEC performance in maghemite

Due to non-stoichiometric behavior of maghemite, the water dissociation is highly performed to increase cell capacity [11]. The surface active sites oxygen vacancies, V^O_s in magnetite and hematite HECs stimulated water dissociation to generate 50 and 30 mA current, respectively which is higher with maghemite HEC 19 mA. XRD pattern results into cubic spinel maghemite with lattice parameter, a = 8.35 Å. Vibrational Raman broad band are measured around 360 (T_2g), 500 (E_g), and 700 cm⁻¹ (A_1g) which showed characteristic peaks of maghemite [11, 45]. The broad A_1g mode attributed due to vibration of O atoms along Fe-O bonds. The V^Fe_s related with local surroundings in lattice resulting into different Fe-O distance near vacancy defects [52]. Band at 667 cm⁻¹ is assigned to FeO₄ tetrahedra vibration of non-defective spinel while the band at 719 cm⁻¹ exists due to Fe-O vibrations adjacent to V^Fe_s. From FTIR study, the strong IR band at ~570 cm⁻¹ exists due to Fe-O vibrations which eventually broadens and splits into newer bands due to oxidation process. A strong IR band at 638 cm⁻¹ along with a shoulder near 538 cm⁻¹ is attributed with shifting and splitting of Fe-O stretching vibrational mode at (ν₁) ~570 cm⁻¹ [53]. However, a weak absorption at ~440 cm⁻¹ is assigned to shifting of octahedral Fe-O vibrational mode ν₂ [45]. The HRTEM study has revealed average grain size 13.1 nm nanopore size 11.6 nm. Using BET method, the value of effective surface area 90.37 m² g⁻¹, and an average pore size 10.78 nm, pore volume 0.33 cc g⁻¹ and maximum pore size 120 nm have been analyzed. XPS peaks obtained ~710.7 eV, 717.7 eV and 724.3 eV, 733 eV corresponded to respective Fe 2p_3/2 and Fe 2p_1/2 peaks, respectively, of maghemite [11, 54].

3.4.1 Lattice defects formation with photoluminescence

Figure 8(a) shows the photoluminescence (PL) emission peaks observed from nanostructured maghemite. The PL peak at ~550 nm (2.25 eV) is assigned with radiative recombination of electrons due to crystal field splitting energy levels in maghemite octahedral sites [55]. PL peak at 615 nm confirm the transition from tetrahedral crystal field splitting energy respect to O (2p). A hump at ~655 nm and strong PL peak at ~830 nm is attributed with radiative recombination of electron trap levels, respectively with tetrahedral and octahedral sites with O (2p) band which resulted by V^O_s. V^Fe_s corresponds with octahedral sites acts as acceptor type surface defects may responsible with an additional peak at ~740 nm.

Figure 8.

(a) Photoluminescence spectra of maghemite γ-Fe₂O₃. (b) Variation of capacitance with frequency in its wet state (inset is the maghemite pellet with corresponding capacitive plot in its dry state) [11].

3.4.2 Charge transport kinetics with dielectric measurement

Since low conductivity of crystallite boundaries in maghemite arises due to presence of V^Fe_s disrupting Fe²⁺/Fe³⁺ hopping path breaking its electronic conduction [11]. As shown in Figure 8(b), the magnetite pellet is found to be more conducting than maghemite even in its wet state. A sharp increment in capacitance is observed from dry to wet state (varying capacitance from 10 pF to 3 μF in maghemite compared with 100 nF to 2.8 μF in magnetite) along the electrode region. Comparatively higher capacitance (10⁶ order) in maghemite inferred to the formation of larger doubly and triply hydrogen bounded physisorbed water molecules at V^Fe_s than singly bound water molecules in magnetite. The V-I characteristics of maghemite HEC are given [11]. With the addition of 200 μL deionized water on 4.84 cm² area maghemite HEC generates 19 mA short circuit current with an emf 0.85 V (Table 3). In comparison to hematite and magnetite HECs, the less output electricity generated by maghemite HEC may attributed by presence of acceptor type V^Fe_s with reduced V^O concentration. The molecular H₂O adsorption on V^Fe sites is responsible to reduced chemi-dissociation rate of water in maghemite [56].

3.5 Progress on Li substituted MgFe₂O₄ HEC

The Mg_0.8Li_0.2Fe₂O₄ HEC cell of size 17 cm² is able to generate short circuit current 82 mA and 920 mV emf with maximum output power 74 mW [12]. This current conduction process occurs due to dissociated H₃O⁺ and OH⁻ ions are transport through surface and capillary diffusion in porous ferrite towards Zn and Ag electrodes. Figure 9(a) is the mechanism for current conduction process in Mg_0.8Li_0.2Fe₂O₄ HEC [12]. Li¹⁺ ions in MgFe₂O₄ create oxygen vacancy which acts as dangling/unsaturated bond to produce trapped electrons. Electric field developed inside the pore is physisorbed water molecule spontaneously. Further, voltage is generated by oxidation reaction occurring at Zn electrode and reduction of H₃O⁺ occurring at Ag electrode due to Eqs. (2)–(4). Generated voltage helps to transport H₃O⁺ and OH⁻ ions towards respective electrodes. Surface lattice fringe widths 0.25 and 0.16 nm correspond to (311) and (511) lattice planes, respectively for Mg_0.8Li_0.2Fe₂O₄ measured by HRTEM analysis [12]. Using BET method, the specific surface area of Mg_0.8Li_0.2Fe₂O₄ pellet is determined to be 165 m² g⁻¹. Total pore volume for pores smaller than 455 nm diameter is obtained to be 0.74 cc g⁻¹ along with 30% total porosity. Mg_0.8Li_0.2Fe₂O₄ HEC exhibits high reactance in its dry state (order of 10⁸ Ω). When HEC is partially dipped in deionized water, the reactance of cell pellet is decreased to ~100 Ω.

Figure 9.

(a) HEC working (schematic) due to water molecules dissociation and conduction of H₃O⁺ and OH⁻ ions (zoom picture shows nanopores on pellet surface and further proton hopping inside nanopore to generates enough electric field) [12]. Voltage-current responses in (b) Li_0.3Ni_0.4Fe_2.3O₄ [30], (c) (1 − x) BaTiO₃ − xCoFe₂O₄ [7], HECs under soak water condition.

3.6 Progress on Li_0.3Ni_0.4Fe_2.3O₄ HEC

The Ni substituted lithium ferrite, Li_0.3Ni_0.4Fe_2.3O₄ (LNFO) HEC generate green electricity has been reported [30]. The Ni substituted at octahedral site of Li ferrite (Li_0.5Fe_2.5O₄) because the reasoning behind Ni⁺² (0.72 Å) substitution at Li⁺¹ (0.76 Å) and Fe^+3/2 (0.63/0.77 Å) site is the ionic radius and lower/higher valances. Due to occurrence of valance and ionic radii mismatch, the resulting lattice strain produces oxygen defects [7]. XRD pattern revealed cubic spinel phase with the value of lattice constant 8.319 Å and average particles size 47.8 nm is measured. From FESEM, the average grain size from pellet sample is 475 nm. From BET analysis, the value of pore size is 2.5 nm. FTIR has revealed the higher frequency absorption band ν₁ at 606 and 605 cm⁻¹ is attributed due to stretching vibrations of tetrahedral sites. Lower frequency absorption band ν₃ found at 412 and 411 cm⁻¹ is the stretching vibrations of octahedral sites [57, 58]. Middle frequency absorption band ν₂ at 479 cm⁻¹ confirm the availability of Li-O complexes at octahedral sites. Absorption band at 3425 cm⁻¹ corresponds to stretching vibrations of the surface adsorbed H₂O molecules. PL emission observed peak at 472 nm corresponds to blue emission and 613 nm (orange emission) confirms to the presence of defects states within forbidden energy gap. PL emission peak at lower wavelength [around ~611 nm (2.02 eV)] might be attributed with lattice defects (oxygen vacancies, interstitials, etc.) which acts as unsaturated or dangling bonds in ionic oxides. Due to differences in ionic radii of Fe³⁺, Li⁺, and Ni²⁺ ions, the strain is induced in the spinel Li_0.3Ni_0.4Fe_2.3O₄ lattice [59]. This lattice strain might to create defect vacancies (XPS analysis). Voltage-current polarization characteristics of LNFO based HEC are obtained in Figure 9(b) [30]. Value of maximum offload output power is 13.77 mW, maximum offload current 15.3 mA and open cell voltage 0.9 V measured. The highly electronegative Ni²⁺ and Fe³⁺ cations on LNFO surface attract a lone pair of electrons of oxygen present in water molecules. This strong attraction at octahedrally unsaturated surface cations leads to chemidissociation of water molecules into H₃O⁺ and OH⁻. At low current density, activation loss PQ is the potential required to overcome the energy barrier of electrochemical reaction. Ohmic loss QR is the resistance faced by ions during migration through the porous network. Concentration polarization loss RS signifies that the insufficient ions available at electrodes in the hyper-reactive state. The values of impedances of LNFO HEC in both dry and wet condition are given in Table 3.

3.7 Progress on BaTiO₃-CoFe₂O₄ multiferroic HEC

Lattice strain formation in multiferroic nanocomposites is highly induced defects associated vacancies [60]. Two constituent phases ferroelectric and ferromagnetic of multiferroic causes accumulation of charge carriers at grain boundaries and acts as active center for water molecule adsorption. Figure 9(c) shows the V-I results of multiferroic (1 − x) BaTiO₃ − xCoFe₂O₄ [x = 0.0 (BTO), 1.0 (CFO) 0.85 (BTO85), 0.75 (BTO75), 0.65 (BTO65), 0.55 (BTO55)] HEC synthesized by sol-gel process [7]. XRD pattern revealed the tetragonal BTO and spinel cubic CFO phases. For pure BTO, BTO85, BTO75, BTO65, and BTO55, the values of lattice constant for BTO are a = 0.399 nm, c = 0.403 nm; a = 0.399 nm, c = 0.401 nm; a = 0.400 nm, c = 0.402 nm; a = 0.400 nm, c = 0.403 nm; and a = 0.400 nm, c = 0.404 nm, respectively. Value of lattice constant for CFO phase a = 0.839 nm, 0.837 nm, 0.837 nm, 0.838 nm and 0.839 nm for pure CFO, BTO85, BTO75, BTO65, and BTO55 nanocomposite, respectively. Using method of XRD density, experimental (apparent) density, apparent density using Archimedes principle, the value of porosity is 2.304, 7.59, 10.8, 7.15, 6.18, and 5.54%, respectively measured for BTO, CFO, BTO85, BTO75, BTO65, and BTO55. From FESEM pattern, the average particle size, D = 80, 66, 70, 83, 277 and 300 nm, measured for BTO, CFO, BTO85, BTO75, BTO65, and BTO55, respectively. Maximum defects/vacancies in BTO85 nanocomposites are formed as confirm with PL emission. In Figure 9(c), the pure BTO and CFO based HEC generated low current while nanocomposite cells exhibited current in mA. The observed variation in short circuit current, emf and output cell power is explained due to capacity of water dissociation that depends upon oxygen vacancies concentration, surface unsaturated bonds and nanoporosity. Oxygen vacancies provide dangling bonds in cell surface to attract more polar water molecules due to chemidissociation process by OH⁻ and H⁺ ions. Pure BTO and CFO HECs generated short circuit current 2 and 1.2 mA, and emf 0.97 and 0.95 V, respectively. The BTO85, BTO75, BTO65, and BTO55 nanocomposite HEC generated short circuit current and emf 9.4 mA, 0.7 V; 4.2 mA, 0.90 V; 6.4 mA, 1 V; and 6.1 mA, 0.94 V, respectively.

3.8 Usability of iron oxide as HEC for future directions

Two major needs for good electric generation process in HEC of any metal-oxide material is that the nanomaterial to be highly defect states such as oxygen vacancies and porous formation or high porosity in pellet specimen. Iron oxides act as candidate material for HEC as discussed in the present chapter. Actually, iron oxides such as hematite, magnetite and maghemite are good source for Fe from +2 and + 3 valence states formation within oxygen vacancies. Therefore, the formation of oxygen vacancies in iron oxide based HEC depends upon the concentrations of Fe²⁺ and Fe³⁺ ions. The doping from transition metal and rare earth ions into hematite, magnetite, and maghemite nanomaterials, the surface oxygen vacancies might be increased. The concept of surface oxygen vacancies in iron oxides is already discussed in the introduction part. Non-stoichiometric nature of iron oxides also plays an important role for their usability in HEC. In case, the occurring of oxygen vacancies and valence fluctuations of Fe³⁺ and Fe²⁺ ions at magnetite surface attract water molecules towards its surface and chemidissociates it into H⁺ and OH⁻ ions. Surface of Fe₃O₄ allows chemidissociation of water molecules due to attraction between the octahedrally coordinated unsaturated Fe²⁺ and Fe³⁺ cations and lone pair electron of oxygen in the H₂O molecule. Heterolytic splitting of the H₂O molecule takes place on Fe₃O₄ surface because relatively stable bondage of the highly electronegative Fe³⁺ and Fe²⁺ ions with lone pair electron of oxygen in H₂O. After heterolytic splitting, a Fe-OH bond is formed which lets the H⁺ ion binds the neighboring oxygen atoms present on the surface to create another -OH surface group. Electrons trapped in oxygen vacancies act as dangling/unsaturated bonds attract the polar water molecule and unsaturated ferrous and ferric cations present on the surface of Fe₃O₄ lattice pull out OH⁻ ion from H₂O molecule and chemidissociates water into H₃O⁺ and OH⁻ ions. Trapped H₃O⁺ ions into nanopore develop a very high electric potential inside the nanopores due to Grotthuss chain reaction [10]. Spin density of Fe₃O₄ is determined to be 8.37 × 10²⁴ spins/g, which is a measure of unsaturated/dangling (unpaired electrons) bonds present in the composition [8]. Water molecule interaction with hematite is highly dependent on the coordination number of Fe³⁺ ions along with surface composition, while dissociative adsorption of water is prominent on defective surface with coexisting Fe and O ions [9]. Non-stoichiometry in maghemite lattice can be enhanced by creating oxygen vacancies with existing surface iron vacancies increases surface reactivity significantly. In another case, surface of BTO-CFO nanocomposite unsaturated ions Fe²⁺, Ti⁺³, and oxygen vacancies attract polar water molecules to its closest approach followed by electron transfer and dissociated into hydronium and hydroxide ions [7].

4. Conclusions

The electricity generation by iron oxide based hydroelectric cell is a non-polluting and facile technique for green energy devices. Iron oxide nanoparticles in the form of hematite, magnetite, maghemite, ferrites, and ferrite multiferroic composite are prepared by solid state reaction, co-precipitation, and sol-gel methods. Hematite HEC delivered 30 mA current with an emf 0.92 V in wet deionized water condition. The maximum off-load power generated by magnetite and hematite is much higher than maghemite and ferrites based HEC. This is due to the enhancement capability of water molecule dissociation at surface Fe sites by highest number of oxygen vacancies formation to fallow surface hydroxylation process. The HEC of 4.08 cm² size of Li-doped Fe₃O₄ delivers I_sc, emf and P_out = 44.91 mA, 0.68 V, 30.80 mW, respectively. A 4.84 cm² size maghemite HEC generated stable and repetitive 19 mA I_sc with emf 0.85 V in de-ionized water. Mg_0.8Li_0.2Fe₂O₄ HEC of sizes 4.8 and 17 cm² in deionized water generated 8 and 82 mA I_sc, respectively. The Li_0.3Ni_0.4Fe_2.3O₄ HEC delivered output current density of 3.8 mA cm⁻² which is two times higher with Mg_0.8Li_0.2Fe₂O₄ HEC (1.7 mA/cm²).

Acknowledgments

The author K.C. Verma thankfully acknowledges the financial support by UGC of Dr. DS Kothari Post Doctorate Fellowship [No. F4-2/2006(BSR)/PH/16-17/0066] and CSIR-HRDG for SRA (Pool Scientist) fellowship Grant No. B-12287 [SRA (Pool No): 9048-A].