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Evolution of Useless Iron Rust into Uniform α‑Fe2O3 Nanospheres: A Smart Way to Make Sustainable Anodes for Hybrid Ni−Fe Cell Devices Jianhui Zhu,*,†,∥ Linpo Li,‡,§,∥ Zuhong Xiong,† Yeqian Hu,† and Jian Jiang*,‡,§ †

School of Physical Science and Technology, Southwest University, No. 2 Tiansheng Road, BeiBei District, Chongqing 400715, P.R. China ‡ Institute for Clean Energy & Advanced Materials, Faculty of Materials and Energy, Southwest University, No. 2 Tiansheng Road, BeiBei District, Chongqing 400715, P.R. China § Chongqing Key Laboratory for Advanced Materials and Technologies of Clean Energies, No. 2 Tiansheng Road, BeiBei District, Chongqing 400715, P.R. China S Supporting Information *

ABSTRACT: The large amount of iron rust yielded in steel industries is undoubtedly a useless and undesired product since its substantial formation and recycle/smelting would give rise to enormous financial costs and environmental pollution issues. To best reuse such rusty wastes, we herein propose a smart and applicable method to convert them into uniform αFe2O3 nanospheres. Only after a simple and conventional hydrothermal treatment in HNO3 solution, nearly all of the iron rust can evolve into sphere-like α-Fe2O3 products with a typical size of ∼30 nm. When serving as actives for electrochemical energy storage, the in situ generated α-Fe2O3 nanospheres exhibit prominent anodic performance, with a maximum specific capacity of ∼269 mAh/g at ∼0.3 A/g, good rate capabilities (∼67.3 mAh/g still retains even at a high rate up to 12.3 A/g), and negligible capacity degradation among 500 cycles. Furthermore, by paring with activated carbons/Ni cathodes, a unique full hybrid Ni−Fe cell is constructed. The assembled full devices can be operated reversibly at a voltage as high as ∼1.8 V in aqueous electrolytes, capable of delivering both high specific energy and power densities with maximum values of ∼131.25 Wh/kg and ∼14 kW/kg, respectively. Our study offers a scalable and effective route to transform rusty wastes into useful α-Fe2O3 nanospheres, providing an economic way to make sustainable anodes for energy-storage applications and also a platform to develop advanced Fe-based nanomaterials for other wide potential applications. KEYWORDS: Smart evolution, Iron rust, α-Fe2O3 nanospheres, Sustainable anode, Hybrid Ni−Fe cell device

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INTRODUCTION Over the past centuries, the iron/steel has played a crucial role in shaping our materials and modern urban landscapes. The widespread use of steel products pushes forward continual development and progress in human civilization but also brings about economic losses that mainly stem from the inevitable corrosion when metals are exposed to moisture in air.1 In the USA alone, at least millions of tons of iron rust would be generated every day, typically in transportation, manufacturing industries, and infrastructure/utilities, etc. The total annual costs from corrosion, thereby, may add up to billions of US dollars.2,3 Despite the fact that iron rust is a natural product on earth and would not cause severe effects on our environment and human health, its substantial formation and existence may lead to formidable issues like mechanical structure failures or even building collapse.1−3 To reduce the vast rusty wastes, current handling approach is the recycling of such corrosives via high-temperature smelting methods.1,2 Though these treat© 2016 American Chemical Society

ments are rather direct and effective to reuse iron resources, their recovery is at the sacrifice of both high energy and expenditure, in the meantime leading to extra air/dust pollution and greenhouse gas emissions.1−5 Besides, the reused iron/steel would also have to suffer from the secondary corrosion problems, giving rise to additional financial losses. How humans process the large amount of rusty things into useful products via more rational, preferable, and cost-effective alternative routes is quite desirable but still needs our serious reconsidering. The energy storage has nowadays become a global concern because it possesses a wide range of critical applications (e.g., electronic consumables, modern electric vehicles and large-scale electrical grids, etc.) and plays a dominant role in the Received: July 3, 2016 Revised: October 2, 2016 Published: October 26, 2016 269

DOI: 10.1021/acssuschemeng.6b01527 ACS Sustainable Chem. Eng. 2017, 5, 269−276

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then dissolved into ∼40 mL of dilute HNO3 (concentration: ∼50%) at a temperature of 50 °C, aided by an ultrasonication treatment for 15 min. To avoid the air pollution led by HNO3, the overall dissolution process was performed in a sealed container. Afterward, the obtained solution was diluted by deionized water until its pH value increased to ∼1.82 (at this point, the total solution volume was recorded to be around 50 mL). The resulting mixture solution was then transferred into a 100 mL Teflon-lined stainless steel autoclave, which was sealed and placed in an electric oven at 220 °C for 2 h. When cooled down to room temperature naturally, the red powder samples were collected by centrifugation, washed with deionized water several times, and dried at 60 °C in the oven. For more environmental and sustainable concerns, the used acid solution can be reused to dissolve new rusty materials or totally recycled by alkali substances for other usage (e.g., making fertilizers etc.). Synthesis of ACs/Ni Hybrids. A mixture of 0.15 g of commercial AC powder (model, TF-B520, purchased from Shanghai Sino Tech Investment Management Co., Ltd.; specific surface area, ∼2000 m2/g), 0.7 g of hexamethylenetetramine (HMT), 0.3 g of Ni(NO3)2·6H2O, and distilled water (50 mL) was treated by ultrasonication for 15 min. The resulting suspension was then transferred into a sealed container (80 mL) and held at 95 °C for 6 h. Afterward, the intermediate of ACs@Ni(OH)2 samples was collected and washed with distilled water several times. The preparation of ACs/Ni hybrids was performed in a horizontal, quartz-tube furnace system. The intermediate samples placed in the center of the quartz tube were heated to 450 °C (heating rate: ∼ 10 °C/min) under constant N2 flow (50 sccm), kept for 15 min, and allowed to cool down to room temperature naturally. Characterization Techniques and Battery Testing. The morphology and the crystalline structure of as-made products were characterized with a JEOL JSM-7800F field emission scanning electron microscope (FE-SEM) with energy dispersive X-ray spectroscopy (EDS) and a JEM 2010F high-resolution transmission electron microscope (HR-TEM). X-ray powder diffraction (XRD) patterns were measured on a Bruker D8 Advance diffractometer using Cu Kα radiation. XPS (Thermo Electron, VG ESCALAB 250 spectrometer) was also used to characterize the products. The mass of electrode materials was measured on a microbalance with an accuracy of 0.01 mg (A&D Company N92, Japan). Electrochemical measurements were all performed using a CS310 electrochemical workstation. All working electrodes of Fe2O3 nanospheres were fabricated by the conventional slurry-coating method. Fe2O3 nanosphere powders, poly(vinylidene fluoride) (PVDF) binder, and acetylene black were mixed in a mass ratio of 80:10:10 and dispersed/homogenized in Nmethyl-2-pyrrolidone (NMP) to form slurries. The homogeneous slurry was then pasted onto a Ni foam (thickness: 1.5 mm) and dried at 100 °C for 10 h under vacuum. The mass loading on each current collector was controlled to be 2.5−4.0 mg/cm2. The fabrication of the ACs/Ni counter electrode is achieved by the same method except that the slurry mixture was pasted onto a stainless steel foil (thickness: 0.1 mm). For individual electrode testing, the performance was evaluated in a three-electrode system, with a Pt foil as counter electrode and an Ag/AgCl as reference electrode in 3 M KOH. Prior to testing, all electrodes were immersed in electrolyte for 15 min. Full HSCs devices were constructed with an activated Fe2O3 nanosphere anode and an ACs/Ni cathode in opposition to each other in 3 M KOH aqueous electrolyte. To balance the charge storage between electrodes, the cathode/anode mass ratio is eventually determined by referring to electrochemical behaviors of both electrodes. Note that before electrochemical measurements on rate performance, all electrodes were preactivated by continuous cyclic voltammetry (CV) scans (∼30−50 cycles at a rate of 10 mV s−1). Electrochemical impedance spectroscopy (EIS) measurements were performed by applying an AC voltage with 5 mV amplitude in a frequency range from 0.1 Hz to 100 kHz at open circuit potential. The specific capacities were calculated from galvanostatic charge/discharge curves by using

development of sustainable society.6,7 Research on rechargeable power sources using aqueous electrolytes (e.g., lithium salt/ alkaline solutions) has triggered great interest recently since water itself is the safest electrolyte and is moreover abundant/ friendly to our environment when compared to nonaqueous counterparts like ethylene carbonate, diethyl carbonate, and dimethyl carbonate, which are highly toxic, corrosive, and inflammable.8−10 Within diverse power-supply systems, the hybrid aqueous energy-storage devices made up of both powertype and energy-type electrodes can well inherit merits of supercapacitors and batteries, thereby showing great promise to serve as fast charging/discharging power sources in practical applications.11−13 To date, many types of aqueous cell constitutes have been proposed.14−16 Among them, such devices using ferruginous nanomaterials have drawn tremendous attention due to multielectron Faradaic reacting properties (based on Fe valence states variation) and great natural abundance of elemental Fe (the fourth richest element in earth’s crust), together with their beneficial nanoscale effects.17−20 Particularly, there is intensive research focusing on nanosize ferric oxide (Fe2O3) because it is readily available, environmentally benign, and more significantly has a threeelectron transfer redox behavior (Fe3+↔Fe0).19−22 However, their massive production and practical implementation are severely impeded by many aspects.23,24 The initial solution step, in any case, is to find a facile, high-yield, economical, and controllable way to produce Fe2O3 nanomaterials, which is the fundamental prerequisite for their future development in energy-storage devices. We herein put forward a scalable and smart strategy to convert useless iron rust into uniform α-Fe2O3 nanospheres for energy storage applications. The collected iron rust is initially ground into powder and dissolved in nitric acid (HNO3). Afterward, the as-formed homogeneous solution undergoes a conventional hydrothermal treatment, in which plenty of sphere-like α-Fe2O3 nanocrystals with a central size of ∼30 nm are in situ generated via a simple forced hydrolysis reaction. Note that the overall synthesis of α-Fe2O3 nanomaterials is reliable, effective and pretty low-cost; all of the Fe elements in final products comes from pristine rusty waste, and no other metallic salts or surfactants need to be involved. Such evolved Fe2O3 nanomaterials can directly function as electrochemical actives for the anode of Ni−Fe cell devices. During the half-cell testing, the α-Fe2O3 nanosphere electrode is able to exhibit outstanding anodic performances. A high specific capacity of 269 mAh/g can be obtained at a current density of 0.3 A/g, while good rate capability and cycling performance are also confirmed. As a proof-of-concept demonstration of functions of nanosize α-Fe2O3 in full cells, we purposely constructed a device of activated carbon (AC)/Ni (+)//α-Fe2O3 nanospheres (−) using aqueous KOH as the electrolyte. After being fully activated and optimized, this device shows both high specific energy and power densities, with maximum values reaching up to ∼131.25 Wh/kg and ∼14 kW/kg, respectively. These records definitely reflect that our integrated device well combines the superiorities of supercapacitors and thin-film batteries, showing great promise in the development of rechargeable power sources.

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EXPERIMENTAL SECTION

Synthesis of α-Fe2O3 Nanospheres. The iron rust scraped from corrosives was collected and employed as initiating materials. Typically, 0.2 g of bulky iron rust was first ground into powders and

Q spec = I × t /3.6m 270

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density of α-Fe2O3 anodes.25 Furthermore, it also facilitates the intrinsic electrode reaction kinetics and promises the utilization efficiency of α-Fe2O3 actives due to the shortened solid-state diffusion pathways for ions in aqueous electrolyte.25 On the other hand, only after a simple and controllable hydrolysis process, nearly all of the recycled rusty wastes can evolve into uniform α-Fe2O3 nanospheres successfully. Just like “killing two birds with one stone”, this evolution strategy, which is pretty low-cost and controllable, can not only avoid the high energy consumption and environmental pollution issues caused by conventional smelting treatments but also change rust wastes directly into useful nanomaterials with potential applications in the energy storage field. Scanning electron microscope (SEM) images (Figure 1b and c) show that the hydrothermally generated powder samples are made up of numerous nanospheres with a typical size around ∼30 nm. The small nanospheres have piled up and connected with each other, in the absence of any severe aggregations often occurring for nanomaterials with a diameter less than 100 nm. This loose architecture would be very beneficial for sufficient contact between the electrolyte and nanoactives, as well as deep/efficient OH− diffusions.26 Transmission electron microscope (TEM) observations (see the inset in Figure 1d) of αFe2O3 nanospheres further reveal that their size distribution is quite uniform, within a narrow range from 10 to 60 nm. As summarized in Figure 1d, the major nanoparticle size is centered at ∼30 nm (taking up a statistical ratio of more than 40% in total samples), which is in line with previous SEM characterizations. The crystalline nature of each nanosphere is evidenced by the high-resolution TEM (HRTEM) observation (Figure 1e). A well-defined lattice fringe with interspacing of 0.27 nm is presented, which accords well with the (104) facet of hematite Fe2O3.27 With respect to the formation mechanism, we believe that the forced hydrolysis reactions of iron salts in aqueous solutions may well account for the generation of such uniform α-Fe2O3 nanospheres. The overall chemical reactions can be expressed by the following formulas:

where I, t, and m represent the discharging current (A), the discharging time (s), and the mass of actives on electrode (g), respectively. The specific energy and power densities (E and P) based on the total mass of actives on electrodes were calculated according to E=

∫0

Δt

IV (t )dt

P = E /Δt wherein I is the discharging current (A), V is discharging voltage (V), dt is the time differential, and Δt is the discharging time (s).

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RESULTS AND DISCUSSION It is well-known that unlike the anticorrosive patina formed on copper, surface iron rust is very flaky and friable, offering no protection to the underlying iron. Therefore, given sufficient time, O2, and H2O, any iron substances would entirely convert into the rust that consists of hydrated iron oxides (Fe2O3· nH2O) and iron oxide-hydroxide species (FeOOH/Fe(OH)3).1,2 To better save/reuse these iron resources, we propose a smart, reliable, and efficient recycling strategy; that is the evolution of rust wastes into useful α-Fe2O3 nanosphere products, as schematically shown in Figure 1a. The overall

Δ

Fe3 + + 3H 2O → Fe(OH)3 + 3H+

(1)

2Fe(OH)3 → Fe2O3 + 3H 2O

(2)

The XRD pattern of as-prepared α-Fe2O3 nanospheres is shown in Figure 2a. Clearly, all diffraction peaks can be well indexed to hematite α-Fe2O3 (JCPDS Card No. 33-0664). No obvious signals from other impurities are detected. Energy dispersive X-ray spectroscopy (EDS) is further used to confirm the elemental composition of products. The EDS spectrum in Figure 2b demonstrates that except for the Cu signal coming from the loading substrate, only Fe and O elements are involved in the final products. X-ray photoelectron spectroscopy (XPS) measurements are further performed to determine the valence states of α-Fe2O3 nanospheres (see Figure 2c and d). The high-resolution Fe 2p spectrum shows that there are two distinct peaks appearing at binding energies of 710.8 and 724.2 eV, corresponding well with Fe 2P3/2 and Fe 2P1/3 spin-orbit peaks, respectively. After curve fitting, a satellite peak at 719.1 eV (marked by box) can be evidently distinguished, which is a representative fingerprint for Fe2O3.28,29 While in the O 1s spectrum, only a single strong peak is observed at 531.1 eV, which accords well with the results for metal oxide species.30 To investigate the porosity properties of α-Fe2O3 nanopowders, samples are then

Figure 1. (a) Schematic illustration of the evolution of rust waste into uniform ∼30 nm α-Fe2O3 nanospheres, (b,c) SEM images, (d) the diameter distribution histogram of α-Fe2O3 nanospheres from SEM analysis, and (e) HRTEM image of α-Fe2O3 nanosphere products. Inset in d is a typical TEM observation on α-Fe2O3 nanospheres.

synthetic flow is rather simple, and merely involves two major steps comprising (I) the ultrasonic dissolution of rust powders (scraped and collected from corrosives) into HNO3 and later (II) the forced hydrolysis treatments, leading to the formation of α-Fe2O3 nanospheres with an average diameter of ∼30 nm. This fabrication protocol is based on major aspects as follows. On the one hand, the evenly distributed particle size at ∼30 nm endows α-Fe2O3 products with notable nanoscale effects. Such a small size can bring about more active sites for electrochemical reactions, greatly enhancing the specific energy 271

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Figure 2. (a) XRD pattern and (b) EDS detecting of α-Fe2O3. (c,d) XPS spectra for Fe 2p and O 1s and (e) N2 adsorption−desorption isotherms of α-Fe2O3 nanospheres; the inset shows the pore-size distribution spectroscopy. (f) TEM observation of the mesopores.

Figure 3. (a) CV scan of α-Fe2O3 nanosphere electrode in 6 M KOH at a current rate of 2 mV/s. (b) CV plots and (c) galvanostatic discharge curves of α-Fe2O3 nanospheres at distinct operation rates. (d) Specific capacity as a function of scan rates, (e) cyclic test at a current rate of 4 A/g (the inset is the corresponding charge/discharge curve), and (f) EIS spectrum of the α-Fe2O3 nanosphere electrode.

examined by nitrogen (N2) adsorption−desorption measurement. The N2 adsorption−desorption isotherm (Figure 2e) with a type H1 hysteresis loop reveals that the as-obtained αFe2O3 nanomaterials possess a specific surface area of ∼39.8 m2/g. Also, it is interesting to note that there are mesopores centered at ∼2.8 nm and ∼6.2 nm dimensions, as distinguished from the pore-size distribution (PSD) plot (inset in Figure 2e). Such mesoporous structures are further verified by TEM observation. The sharp contrast in Figure 2f signifies that on the α-Fe2O3 nanosphere surface there are homogeneously distributed mesopores with typical diameters of ∼3.2 nm and ∼6.7 nm, which is highly consistent with the above PSD result. The existence of these mesoporous structures would correspondingly increase the redox reacting sites of α-Fe2O3

nanospheres and ensure the utilization efficiency of electrode actives during cell operations for electrochemical energy storage. To evaluate the electrochemical performances of α-Fe2O3 nanospheres, the as-prepared electrode is initially subjected to cyclic voltammetry (CV) measurement in a three-electrode system in 3 M KOH solution. As displayed in Figure 3a, there are multiple redox peaks present in the CV curve under a scanning rate of 2 mV/s. During the cathodic sweep, a strong reduction peak can be obviously observed at ∼ −1.14 V, corresponding to the electrochemical conversion reactions from Fe3+ to Fe2+.9,31−34 When scanned to a more negative potential (approaching the value of hydrogen evolution), the electrode shows another small peak at ∼ −1.26 V (marked by a black 272

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distribution of ample mesopores on the α-Fe2O3 nanosphere surface may also provide additional redox reaction places, helping to add to the large specific capacity of the α-Fe2O3 anode and strengthen the electrode kinetics.26,36 To obtain deep insights into the electrochemical reaction process, the electrochemical impedance spectroscopy (EIS) measurement has been conducted on the electrode of the α-Fe 2 O 3 nanospheres. The corresponding Nyquist plot is displayed in Figure 3f. On the basis of Randles equivalent circuit given in the inset of Figure 3f, it is obvious to find that at the high frequency region, the intersection (less than 1 Ω) at the real part indicates a very small equivalent series resistance (Rs) for the overall electrochemical system, and the semicircle with a diameter of ∼0.8 Ω reveals a low charge-transfer resistance (Rct) of the α-Fe2O3 nanosphere electrode.35 Meanwhile, in the low frequency range, the spike with a slope greater than 45° proves a low resistance for ions diffusion in the electrode system.22,35 A full-cell device is then assembled by coupling the α-Fe2O3 nanosphere anode with the ACs/Ni cathode using a 3 M KOH aqueous electrolyte. The choice of ACs/Ni as the counter electrode is due to its special hybrid functionalized configurations that involve both a non-Faradaic capacitive component as a fast power-output source and a battery-type Faradaic portion as a reliable energy-supply source. By referring to our previous work, the intermediate products of ACs@Ni(OH)2 can be obtained after a conventional hydrothermal process.36 SEM observations in Figure S1 clearly reveal that the AC blocks are entirely wrapped by Ni(OH)2 thin layers (the thickness for a single layer is only ∼7 nm).36 After a calcination reduction treatment, the final samples of AC/Ni can be evolved. The composition of AC/Ni hybrids is characterized by XRD, and the record is present in Figure S2a. Except peaks from ACs, the other pronounced peaks at 44.5° and 51.8° can be well indexed to Ni (JCPDS card no. 40-0850), corresponding to its facets of (111) and (200), respectively. EDS spectroscopy in Figure S2b is also used to verify the composition of AC/Ni, showing the presence of Ni and C elements (Cu signals come from the loading substrate). No other elements are detected. Representative SEM images of AC/Ni are displayed in Figure 4a,b. It is observed that the commercially available ACs intrinsically

box), which may originate from the conversion process of Fe2+ to Fe0.9,31−34 In the reverse anodic scan, the obvious oxidation peak emerging at ∼ −0.78 V should be attributed to the oxidation reaction from Fe2+ to Fe3+, and the weak one (indicated by a red box) lying at ∼ −1.12 V is due to the reversible phase variation from Fe0+ to Fe(OH)ads/Fe2+.9,31−34 The CV test distinctly confirms the realization of full valence state transformation of elemental Fe (Fe3+ ↔ Fe0+) within a potential window of −1.45 to −0.5 V (vs Ag/AgCl). CV curves of the α-Fe2O3 nanosphere electrode at varied scan rates from 2 to 20 mV/s are displayed in Figure 3b, signifying that the reversible Faradaic conversions between Fe3+ and Fe2+ play a dominant role in charge storage. Upon the increase of scan rates, the current response is also enhanced. However, there is no significant deformation on CV profiles, suggesting the good electrochemical stability and reaction kinetics of α-Fe2O3 nanoactives.22 Galvanostatic charge/discharge measurements are also performed at varied current densities toward α-Fe2O3 nanosphere electrodes. The corresponding discharge curves with current rates ranging from ∼3.2 to ∼12.3 A/g are shown in Figure 3c. Rather than a linear charge/discharge characteristic for electrical double-layer capacitors, the electrode of α-Fe2O3 nanospheres exhibits well-defined potential plateaus, indicative of a typical battery-type electrochemical behavior. The long discharge platform lying at around −0.8 V corresponds to the oxidation conversion of Fe2+ to Fe3+, highly in agreement with our former CV records. The stored capacity derived from the charge/discharge measurements have been calculated and plotted as a function of current rates, as shown in Figure 3d. At a current rate of 0.3 A/g, the α-Fe2O3 electrode can deliver a maximum capacity of ∼269 mAh/g, comparable to other promising alkaline rechargeable battery anodes.12,14 Along with the continual rise of current densities, the charge-storage capability declines gradually. However, even at a high current rate up to 12.3 A/g, the electrode is still capable of retaining a capacity as high as ∼63.7 mAh/g, which demonstrates the superior rate capability of α-Fe2O3 nanospheres. The cycling behavior of the α-Fe2O3 nanosphere electrode is then evaluated by constant charge/discharge measurements at a current density of 4 A/g (Figure 3e). The capacity value of α-Fe2O3 nanospheres initially keeps increasing until the capacity retention reaches 100% at the 20th cycle, which is mainly due to the electrolyte infiltration and the electrode activation process.35,36 In the following charge/discharge procedures, the delivered capacity maintains at ∼100% over 500 cycles. Afterward, though the capacity begins to decline due to electrode kinetics issues (e.g., negative volume expansions caused by repeated phase changes, electrode fatigue, etc.), there still remains a high retention of 80% after 600 cycles. The relative charge/discharge profile at 4 A/g is displayed in the inset of Figure 5e from which each different conversion stage can be found. The reason why the trace for Fe2+/Fe0 conversion is not quite distinct could be associated with electrochemical kinetics limitations of α-Fe2O3.34 Such a nearly symmetric characteristic implies the high Coulombic efficiency of α-Fe2O3 electrodes. We believe that the superior electrode performances of α-Fe2O3 nanospheres should be attributed to their nanosize effects. The uniform and well-dispersed nanospheres with a tiny diameter of ∼30 nm possess prominent nanoscale effects like more active reacting sites and shorter pathways for ionic diffusions, which are quite beneficial for charge storage as well as rate capabilities. Besides, the

Figure 4. (a,b) SEM observations of ACs/Ni hybrids. (c) CV tests at varied scan rates and (d) specific capacity as a function of scan rate for the ACs/Ni electrode. 273

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Figure 5. (a) Schematic illustration of the hybrid device of α-Fe2O3 (−)//ACs/Ni (+). (b) Comparative CV curves of the α-Fe2O3 anode and ACs/ Ni cathode at a current rate of 10 mV/s in three-electrode systems. (c) CV plots and (d) galvanostatic charge−discharge profiles of the hybrid device at varied scan rates. (e) The stored capacity as a function of scan rate, (f) the corresponding EIS spectrum, (g) Ragone plot, and (h) cyclic performance of the full device. (i−k) Optical images showing that two hybrid devices in series can light up three LED indicators.

balance the stored charges in Ni−Fe cell systems by referring to the electrochemical behaviors of each individual electrode (Figure 5b). Figure 5c shows CV curves of the optimized cell device over a potential range of 0−1.8 V at different scan rates. As clearly noted, the CV plots inherit the electrochemical characteristics of both anode and cathode. The quasirectangular geometrical shape in CV curves reveals the EDLC features arising from ACs, while the pairs of redox peaks demonstrate the battery-type contributions according to the following equation:

have plenty of hierarchical pore structures, while the evenly dispersed Ni nanoparticles are adhering tightly to the ACs outer surface. The large amount of pores inside AC backbones are capable of facilitating aqueous electrolyte penetration and making prominent capacitive contributions via rapid ion adsorption/desorption; while the electrochemically activated Ni surrounding ACs may supply sufficient faradic reaction sites to guarantee the specific energy density of an entire electrode.35,36 Figure 4c displays CV curves of AC/Ni electrodes at distinct scan rates. A pair of redox peaks appearing at around ∼0.2 and ∼0.31 V (vs. Ag/AgCl) corresponds well to the reversible conversion reactions of Ni2+/Ni3+.37 Additionally, unlike traditional cases for battery-type materials, the CV profile of AC/Ni hybrid expands greatly and exhibits a quasirectangular shape within the total potential range, which is attributed to EDLC contributions from ACs.38 According to constant charge/discharge tests, the ACs/Ni hybrid electrode demonstrates the specific capacities of 261.5 mAh/g (0.5 A/g), 248.4 mAh/g (1 A/g), 235.3 mAh/g (2.5 A/g), 206.5 mAh/g (5 A/g), 185 mAh/g (10 A/g), and 146.8 mAh/g (15 A/g) (Figure 4d). Even when the current rate is increased by 30 times (from 0.5 A/g to 15 A/g), the electrode still maintains 56% of the initial value, fully confirming its outstanding rate capabilities. The construction details for the full-cell device of α-Fe2O3 (−)//ACs/Ni (+) is schematically shown in Figure 5a. The mass ratio of α-Fe2O3 and Ni/ACs is carefully calculated to

Fe2O3 + 3Ni 2 + + 3OH− + 3e−2Fe + 3NiOOH

(3)

The cell performance is further evaluated by galvanostatic measurements, and the corresponding charge/discharge profiles at various current rates are present in Figure 5d. The distinct sloping plateaus resulting from faradic processes agree well with the former CV analysis. Note that the charge/ discharge curves with negligible IR drops are quite symmetric, further proving the good reversibility and reacting kinetics of our assembled device. The Coulombic efficiency always stays beyond ∼93.6%, indicative of outstanding electrochemical reversibility of such hybrid Ni−Fe cell devices. By referring to charge/discharge plots, the cell capacities have been calculated and plotted as a function of current rate (Figure 5e). At 0.3 A/g, a specific capacity of 93.3 mAh/g can be achieved. Even at a current density as high as 8 A/g (cells 274

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charged/discharged in few tens of seconds), the device can still deliver a capacity of 44.2 mAh/g, retaining ∼47.3% of the initial value and reflecting the superior rate capabilities of our hybrid device. The EIS spectrum of full devices is displayed in the inset of Figure 5e. In the high frequency range, the small values of Rct imply a fast charge-transfer property at the electrode interfacial regions, while at low frequency the line with a slope angle larger than 45° indicates smooth ionic diffusions between the electrode and electrolyte.22 To make clear the relationship between the energy and power densities, Ragone plots have been made (Figure 5g). This cell device delivers a maximum energy density of ∼131.25 Wh/kg at a power density of ∼0.35 kW/kg; more strikingly, even at the highest power density up to ∼14 kW/kg, a total energy density of ∼76.25 Wh/kg can still be retained. Such good energy-storage behaviors are superior to many iron-based full-cell devices in previous literature.39,40 The cyclic endurance toward the cell device is further examined at 4 A/g (Figure 5g). Initially, the discharge capacity of the hybrid device rises progressively due to the slow infiltrations of KOH electrolyte and electrochemical activation of electrode materials. After 100 cycles, a stable electrochemical equilibrium state is built. The capacity retention can always be preserved at a high level of ∼100% until the prime 400 cycles and still be kept at ∼90.2% even after the end of the cycling test. To demonstrate the potential in practical use, two device units (with a total effective area of 1 cm2) are connected in series to drive two different types of light emitting diodes (LEDs). Corresponding optical images are manifested in Figure 5 h,i. After 30 s of charging, such integrated devices are capable of powering three LED indicators (one red LED, ∼1.8 V, 20 mA; two yellow LEDs, ∼2.1 V, 20 mA). The above results reveal that the assembled devices hold great promise in practical applications.

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Research Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b01527. SEM images of ACs and ACs@Ni(OH)2 hybrid precursors; XRD pattern; and EDS spectrum of ACs/ Ni composite products (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*(J.Z.) Tel: +86-23-68254842. Fax: +86-23-68254969. E-mail: [email protected]. *(J.J.) E-mail: [email protected]. Author Contributions ∥

J.Z. and L.L. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the financial support from the Fundamental Research Funds for the Central Universities (SWU 115029 and XDJK2016C066) and Chongqing Natural Science Foundation (cstc2016jcyjA0477). J.J. thanks the Fundamental Research Funds for the Central Universities (SWU 115027 and XDJK2016C002) for support. Z.H.X. is thankful for the support from National Natural Science Foundation of China (11374242). This project is also supported by Program for Innovation Team Building at Institutions of Higher Educatio n in Chongqing (CXTDX201601011) and Graduate Student Research Innovation (CYS16048).

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REFERENCES

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CONCLUSIONS

In summary, a facile and scalable approach has been developed to convert the useless rusty wastes into uniform α-Fe2O3 nanospheres for hybrid Ni−Fe cell applications. Merely after a simple and controllable hydrothermal process conducted in HNO3 solution, almost all of the recycled rust can evolve into nanosized α-Fe2O3 products with a high surface-to-volume ratio and large amount of mesopores. When evaluated as electrodes for power-supply devices, the α-Fe2O3 nanospheres demonstrate high electrochemical activity and capacity (with a maximum value of ∼269 mAh/g), good cyclability (nearly no capacity decay after 500 cycles), and excellent rate capabilities. To check the functions of α-Fe2O3 nanospheres in practical application, a full-cell device of α-Fe2O3 nanospheres//ACs/Ni has been further developed. On account of its outstanding comprehensive performances, such an optimized two-electrode device can light up three LED indicators brightly, showing great promise as reliable power sources in our daily life. Our work not only provides a convenient, low-cost and effective way to convert rusty wastes into useful α-Fe2O3 nanospheres but also presents a sustainable and affordable platform to produce advanced Fe-based nanomaterials for other wide potential applications covering energy conversion devices and magnetic drug carriers, sensitive gas/electrochemical sensors, catalysts, wave absorption devices, etc. 275

DOI: 10.1021/acssuschemeng.6b01527 ACS Sustainable Chem. Eng. 2017, 5, 269−276

Research Article

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