Mass Production of Metallic Fe@Carbon Nanoparticles with Plastic

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Mass Production of Metallic Fe@Carbon Nanoparticles with Plastic and Rusty Wastes for High-Capacity Anodes of Ni–Fe Batteries Han Zhang, Yani Liu, Ting Meng, Lai Ma, Jianhui Zhu, Maowen Xu, Chang Ming Li, Weiwei Zhou, and Jian Jiang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.9b02083 • Publication Date (Web): 23 May 2019 Downloaded from http://pubs.acs.org on May 27, 2019

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ACS Sustainable Chemistry & Engineering

Mass Production of Metallic Fe@Carbon Nanoparticles with Plastic and Rusty Wastes for High-Capacity Anodes of Ni–Fe Batteries Han Zhanga, Yani Liua, Ting Menga, Lai Maa, Jianhui Zhub, Maowen Xua, Chang Ming Lia*, Weiwei Zhou *, Jian Jianga*

c

aSchool

of Materials and Energy, and Chongqing Key Lab for Advanced Materials and Clean Energies of

Technologies, Southwest University, No.2 Tiansheng Road, BeiBei District, Chongqing 400715, P.R. China. bSchool

of Physical Science and Technology, Southwest University, No.2 Tiansheng Road, BeiBei District,

Chongqing 400715, P.R. China. c

School of Materials Science and Engineering, Harbin Institute of Technology at Weihai, No.2 Wenhua Road, Weihai

264209, P.R. China. To whom correspondence should be addressed: Tel: +86-23-68254842. *E-mail:

[email protected] (C.M. Li); [email protected] (W. W. Zhou); [email protected] (J. Jiang).

ABSTRACT: The recycle/reuse of wastes is significant for green and sustainable development of our society. Herein, we present the mass production of Fe@C nanoparticles (NPs) with plastic and rusty wastes for making high-capacity anodes of Ni–Fe batteries. The total conversion is achieved by a facile one-step chemical vapor deposition process, where plastics decomposition offers rich hydrocarbons and iron rust-derived Fe2O3 are reduced to Fe NPs, in-situ launching the catalytic growth of carbon (C) shells on their surfaces. All Fe NPs are thereby tightly sealed by C layers, whose thickness can be controlled by tuning the reaction time. Benefiting from superb reactivity/conductivity of Fe core and good stability/robustness of C shell, such unique Fe@C hybrid configurations exhibit a high delivered capacity (Max. value: ∼405.2 mA h g−1), excellent rate performance and superb cyclic stability (∼91.9% capacity retention after 4000 cycles). The further assembled full-cell (-)Fe@C//NiO@C(+) devices can deliver impressive specific energy density (∼138 W h kg−1) and rate capability (∼14.5 kW kg−1), verifying their great potential in real applications. This paradigm work may guide us to massive and smart evolution of disposable/useless wastes into useful materials for energy-related applications.

Keywords: mass production; Fe@C NPs; plastic/rusty wastes; high-capacity anode; Ni–Fe batteries

ACS Paragon Plus Environment

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Introduction Plastic products have become worldwide commodities and pervaded our daily life owing to their moldable, durable, lightweight and affordable features.1 Only in the past 2018, the global production of plastics has statistically reached up to 380 million tons.2 Among them, the polyethylene (PE), a typical category of plastics, has already taken up a giant market portion.3 One major and widespread PE application is to manufacture disposable packaging materials (e.g., plastic bags, films, geomembranes, containers, etc.), which often have short service lifespan and immediately enter into waste streams after use.4 While enjoying the convenience of these products, human beings have to worry how to handle the huge amount of yielded plastic wastes.6 For the moment, the common ways to process the plastic garbage are realized by either landfill or direct incineration.1,7 However, such simple and convenient treatments are inadvisable because they normally lead to severe soil contaminations and/or secondary air pollution, making little recycle/reuse of these resources.9 In this context, it is highly significant and desirable to come up with an eco-friendly, efficient and economical strategy of “converting the large amount of disposable plastics into useful products”, which is much favorable for green and sustainable development of our society. Apart from aforementioned environmental/renewable concerns, the sustainable development also needs to pay tremendous attention to energy-related issues, typically like green and scalable energy conversion/storage.11-12 Generally, highly efficient energy-storage systems could endow us with more options/flexibilities since they solve the problems for intermittent collections of solar and wind energy.13-14 Recently, aqueous rechargeable batteries (ARBs) have revived and attracted great interest as alternative power sources owing to their prominent power density, little environmental footprint, low cost and good safety.16,19,41 Especially, there has been a major thrust toward advanced Ni–Fe


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battery researches in that ferruginous anodes (FeOx31-33, FeSx34-36, Fe37, etc.) are earth-abundant, inexpensive, low-toxic, with a wide operation window (-1.4~-0.3 V vs. Ag/AgCl) in alkaline electrolytes.39-40 Among various Fe-based candidates, metallic Fe shows great prospect as the prominent anode by virtue of its outstanding physicochemical properties, remarkable theoretical capacity (~1441 mA h g-1), good electronic conductivity and superb electrochemical reactivity.38 Currently, Fe plates are used as negative electrodes for commercial Ni–Fe batteries. However, bulky Fe plates can merely offer small numbers of active sites on external surface regions and deep solidstate ion-diffusion pathways, resulting in poor electrochemical performances (like limited materials utilization ratio, low specific capacity, etc.).37 To break through above barriers, the efficient methodology is downsizing Fe bulks into the nanoscale.38 However, bare Fe nano-metals are seldom used for ARBs due to their highly active electrocatalysis and instable chemistry in alkaline conditions.38 In this regard, many efforts have been devoted to the surface modifications/engineering on Fe nanoparticles (NPs) for better utilization in ARBs.28 The robust carbon (C) encapsulation is an advisable and adoptable route, as it can promote the chemical stability of dispersive Fe NPs, restrain their electrocatalysis properties and preserve the intrinsic electrical conductivity.28,38 Nevertheless, current methodologies for preparing Fe@C NPs often require multiple steps and expensive Fecontained precursors. Meanwhile, the generated samples are usually imperfect due to the coexistence of other impurities/by-products (like FexC, etc.) in some cases.28 Thereby, it is urgently desired to seek a facile yet applicable strategy for the mass production of Fe@C hybrid NPs so as to construct superior Ni–Fe batteries. Based on above considerations, we herein develop a facile chemical vapor deposition (CVD) approach to produce metallic Fe@C NPs in a scalable way, by using the PE plastic and rusty wastes



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as the carbon feedstock and initiating materials, respectively. During this CVD process, the instant PE thermal decomposition can provide plenty of hydrocarbon gases; meanwhile, all iron rust-derived Fe2O3 NPs are reduced to Fe NPs, in-situ catalyzing the growth/encapsulation of C layers on their surfaces and thus resulting in the vast production of core-shell Fe@C NPs. We need to emphasize that there involve several highlights in this case study: i) Both Fe and C sources stem from our daily wastes, which can not only enable the smart recycle/reuse of discarded resources but also reduce the battery cost and environment pollutions, greatly fulfilling our demands in a long-term sustainable development. ii) The generation of Fe nano metals and C shells are simultaneously achieved via a conventional onestep CVD process, largely simplifying the overall synthetic procedures. iii) On account of ordinary fabrication techniques and easily available raw materials, our proposed strategy can be readily scaled up. When applied as anodes of ARBs, the resulting Fe@C NPs can exhibit high delivered specific capacities (Max. value: ∼405.2 mA h g−1), superb cyclic stability (nearly ∼91.9% capacity preservation after 4000 times of cycling) and rate capabilities (~114.8 mA h g-1 when running at 20 A g-1). This may chiefly benefit from the superb reactivity/conductivity of Fe inner core and good protective capability of C outer shells. To further testify their potential utilization in practical usage, (+)NiO@C//Fe@C(−) full-cell devices have been assembled and tested, which can output both high energy density (∼138 W h kg−1) and power density (∼14.5 kW kg−1). Our success in mass production of Fe@C NPs with living wastes may set up an economical platform, not only to make sustained highcapacity anodes for Ni–Fe ARBs but also to fabricate other advanced hybrids for distinct practical applications.

Experimental Data


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Preparation of Fe@C NPs. Fe2O3 nanospheres (average size: ~30 nm) were pre-made from iron rust by according to previous reports.13 Then, 10 g of Fe2O3 nanospheres were placed into a ceramic boat, transferred into a quartz tube furnace and heated at a ramping rate of 10 oC min−1 under N2 flow (50 sccm). When the furnace temperature reached 800 oC, 20 g PE waste (e.g., preservative film) was put into the heating zone instantly and kept for 1.5 h. After cooled down naturally to room temperature, Fe@C NPs were obtained without any further treatments. For comparison study on anodic behaviors, Fe3O4@C NPs were also fabricated by according to our previous study.32 Preparation of NiO@C nanowall arrays (NWAs). In details, 1.24 g nickel acetate tetrahydrate (C4H6NiO4·4H2O), 1.5 g urea (CO(NH2)2) and 0.37 g ammonium fluoride (NH4F) were dissolved into 50 mL deionized (DI) water. Then, the mixture solution (color: green) was transferred into a Teflonlined autoclave wherein a piece of chemically inert stainless steel (SS) substrate (2.5×4.0×0.03 cm-3) was placed beforehand, and heated/maintained at 130 oC for 5 h in an electric oven. Afterwards, the substrate covered with Ni2(OH)2CO3 NWAs precursors was soaked (soaking time: 6 h) into a prepared homogeneous liquid with 50 mg dopamine (DA) and 100 mL Tris-buffer solution (pH value: ~8.5). Later, the NWAs samples were rinsed by DI water for several times, dried at 60 oC and calcined at 550 oC

for 1 h in N2 atmosphere, leading to the generation of NiO@C NWAs products.

Characterization Techniques and Battery Testing. The basic samples identification was conducted by X-ray powder diffraction (XRD) diffractometer (Bruker, D8 Advance; Cu Kα source). Morphological and crystalline features of our samples were further analyzed by using transmission electron microscope (JEM 2010F), and field-emission scanning electron microscope (JEOL JSM7800F) coupled with high-resolution energy-dispersive X-ray spectroscopy (EDS). X-ray photoelectron spectroscopy (XPS) detection was regulated with the Al Kα radiation and fixed analyzer



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transmission mode (Thermo Electron, VG ESCALAB 250 spectrometer). The Raman spectrum was conducted on Raman spectroscope (Witech. CRM200; 532 nm) and the thermogravimetric analysis (TGA; SDT600) was performed under a heating rate of ∼5 K min−1 in air atmosphere. The nitrogen adsorption−desorption isotherm measurements were performed on a NoVa 1200e nitrogen adsorption apparatus. The mass of electrode samples were measured on a microbalance with a precision of 10-5 g (Ohaus, USA). The film electrodes were fabricated by the conventional slurry-coating method. The working electrodes were made by mixing Fe@C powders, poly(vinylidene fluoride) (PVDF) binder, carbon black with a mass ratio of 8:1:1 and moderate N-methyl-2-pyrrolidinone (NMP) to form a uniform slurry, which is then pasted onto a Ni foam (thickness: 1.8 mm) and dried at 120 oC for overnight under vacuum condition. The actives weight loaded on each electrode was controlled to be ~2.4−3.8 mg cm-2. NiO@C NWAs grown on SS substrate (area: ∼2 × 4 cm2) was straightly used as the cathode after washed by DI water for several times. For half-cell electrochemical testing, the electrode performance was examined by using a three-electrode system in 3 M KOH solution, with an Ag/AgCl as the reference electrode and a Pt wire as the counter electrode. To assemble the full cells, the Fe@C hybrid anode, NiO@C NWAs cathode and an electrolyte-soaked separator were packed by two PET plates with conventional heat-seal techniques. In order to balance the charge storage between cathode and anode, the mass ratio is finally determined by according to the single electrode behaviors. Note that before electrochemical tests, all working electrodes were pre-activated via continuous cyclic voltammetry (CV) scans (50 cycles at 10 mV s-1). The specific capacities were calculated from galvanostatic charge/discharge curves by using the formula of: Qspec. = I × t/3.6m

(1)

where I is the operation current (A), m is the actives mass on electrodes (g), and t is the discharge time


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(s). The energy (E) and power densities (P) of the full cell were calculated by the following formulas: ∆𝑡

𝐸 = ∫0 𝐼𝑉(𝑡)𝑑𝑡

(2)

𝑃 = 𝐸/∆𝑡

(3)

wherein I is the charge/discharge current (A), V is cell voltage (V), dt is the time differential, and Δt is the discharge time (s).

Results and Discussions Figure 1 presents the schematic for the entire fabrication flow. All starting materials of Fe2O3 NPs (Figure S1) are produced from the useless rusty wastes.13 In the course of CVD process at 800 oC, plenty of hydrocarbons (including methane, ethylene, acetylene, short-chain PE residuals, etc.) or even H2 molecules would be yielded due to the instant/deep pyrolysis of PE, with a chain of chemical reactions below: -[-CH2 - CH2 -]-n → CH2 = CH2 → C + H2

(4)

As a consequence, Fe2O3 NPs tend to react with these gases and fast reduced into metallic Fe NPs, which serve as catalysts for the in-situ formation of graphitic C layers on their outer surfaces. The related chemical equations mainly involve: 2Fe2O3 +3C→4Fe + 3CO2

(5)

Fe2O3 + 3H2→2Fe + 3H2O

(6)

After such a one-step calcination process, each of pristine Fe2O3 NPs can straightforwardly convert into core-shell Fe@C NPs. Particularly noteworthy is that all involved reactants in this work originate from useless plastic and rusty wastes, not only alleviating the environmental pollutions but also reducing the synthetic cost. Also note that the whole synthetic procedure is controllable and rather



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simple, which could be readily scaled up for industrial development. Moreover, the as-configured Fe@C NPs are quite preferable for ARBs application; the inside Fe NPs can be readily activated, and thereby participate in reversible redox reactions, while the tight C encapsulation can help to restrain the electrocatalysis of Fe NPs and promote their physicochemical stability. After battery use, these ferruginous NPs could be recycled by washing and heating treatments, and further collected as fresh Fe2O3 NPs for novel samples manufacturing.13 Figure 2a-b showcase the field emission scanning electron microscope (FE-SEM) images of Fe@C NPs. The over-view FE-SEM observation (see Figure 2a) reveals that Fe@C NPs exhibit a similar geometrical morphology to the pristine Fe2O3 precursors (Figure S1) but with a larger size of ∼100 nm. The obvious swell in particle size highly suggests the successful C layers formation. Note that even under SEM detection, the contrast between the inner core and outer shell can be clearly distinguished (Figure 2a and b). Moreover, almost each single Fe NP is covered with thick C layers; no evident PE-derived amorphous carbonaceous aggregates are observed. Figure 2c shows the Raman spectrum of Fe@C NPs. Two distinguishable peaks are evidently present at ~ 1452 and 1577 cm−1, which successively correspond to the D and G bands of graphitic constituents. The ID/IG intensity ratio is determined to be 1:1.3, implying relatively high graphitization degree of C layers. Figure 2d compares the samples’ X-ray diffraction (XRD) patterns before and after calcination treatments. The pristine precursors are well indexed to hematite α-Fe2O3 (blue pattern; JCPDS No. 33-0664). After CVD process, the main diffraction peaks (black pattern) appear at 44.8°, and 65.2°, corresponding to the (110), (200) facets of metallic Fe (JCPDS No. 87-0722), respectively. Whereas, the weak and broad peak at ∼25.3° should be correlated with the (002) facet of PE-evolved graphitic C (JCPDS No. 656212). Above XRD analysis reveals the successful transformation of Fe2O3 nanocrystals into core-


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shell Fe@C after calcination. By according to thermogravimetric (TGA) record (Figure S2), the total C content in Fe@C can be determined around 11.7%. In addition, the Fe 2p XPS detection (Figure 2e) shows there are two peaks fitted at binding energies of ~709.6 and ~723.5 eV, highly confirming the existence of Fe0 and consistent with our former XRD result.28,38 Figure 2f shows the XPS spectrum of C 1s, wherein the signals can be fitted into three individual component peaks at ~284.6, ~285.9 and ~288.1 eV. They are successively deconvoluted to C atoms in chemical conditions of C=C, C−O and C=O.32 The nitrogen adsorption−desorption isotherm with a type IV hysteresis loop shows the asformed Fe@C nanoparticles possess a specific surface area of ∼45.65 m2 g-1 (Figure S3a), and their major pore diameter is distributed around ∼4 nm (Figure S3b). The presence of such mesoporous structures may facilitate the ionic transport from the electrolytic phase into deep inner regions of Fe@C nanoparticles. The energy dispersive X-ray spectroscopy (EDS) with elemental mapping analysis and transmission electron microscopy (TEM) are further used to clarify the samples’ structure and composition. Typical TEM observations (Figure 3a-b) of Fe@C NPs definitely reveal the core-shell configurations; the mean thickness for C coating layer lies at ~30 nm, and the average size of Fe core increases to the level of ∼50 nm (larger than that of Fe2O3 precursors at ∼30 nm). The core diameter variation is possibly due to aggregations and/or fusions of adjacent Fe NPs during the thermal treatment. In the high-resolution TEM (HRTEM) observation toward Fe/C interface regions (Figure 3c), the crystalline lattice spacing is well-defined at ~0.20 and ~0.34 nm, highly consistent with the (110) plane of metallic Fe and (002) facet of graphitic C, respectively. Note that such PE-derived C shells are regularly arranged/packed (rather than in a disordered/amorphous state). The EDS pattern verifies the existence of Fe and C elements (see Figure 3d; the Cu signal stems from the background).



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In addition, the EDS mappings toward a selected region (see SEM image in Figure 3e) evidently exhibit the homogeneous distribution of elemental Fe and C in our samples (Figure 3f-g), in the absence of any other impurities. The delicate relationship between the key reaction parameters and geometric characteristics of as-built C layers deserve our deep investigation/clarification. Figure 3h displays the C layer thickness and total C content as a function of thermal reaction time, which is plotted by according to TG measurements and component calculations. As observed by TEM monitoring, Fe@C NPs can statistically possess a mean C layer thickness of ~5, ~10, ~25, ~28 and ~30 nm at the durable heating time of 0.25, 0.5, 1, 1.5 and 2 h, respectively. As the reaction time proceeds, the total C content still keeps growing owing to the ceaseless decomposition of PE wastes; extra disordered carbonaceous species formed at a high-temperature condition would continuously accumulate/deposit onto samples, giving rise to a moderate mass ratio increase. However, after a long reaction time over 2 h (2.5 and 3 h), the C thickness is still preserved at the level of ~30±2 nm, which is mainly attributed to the saturation of C catalytic formation on the outer surface of each Fe NP. The electrochemical performances of Fe@C NPs were firstly carried out in 3 M KOH electrolyte using a three-electrode testing system. To confirm unique advantages of Fe@C hybrids, samples of Fe3O4@C and bare Fe2O3 NPs are also tested for comparison. Figure 4a displays their respective cyclic voltammetry (CV) curves at a potential window of −1.4 to −0.35 V (vs. Ag/AgCl) at 5 mV s-1. Distinctly, they all exhibit a similar CV profile but different peak positions. In the Fe@C case, two reduction peaks located at −1.13 and −1.26 V in the cathodic sweep successively correspond to the conversion reactions of Fe3+ to Fe2+ and Fe2+ to Fe0. Whereas, oxidation peaks at −1.09 and −0.74 V in the anodic scan are highly related to stepwise phase variations from Fe0 to Fe2+ and Fe2+ to Fe3+. The peak separation for Fe3+ to Fe2+ redox pairs is measured to be ∼390 mV, much lower than that of


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Fe3O4@C (∼460 mV) and bare Fe2O3 (∼610 mV) counterparts. This strongly evidences the superior electrochemical kinetics of Fe@C hybrids to either Fe3O4@C or Fe2O3 electrode. Figure 4b reveals the CV curves of Fe@C electrodes at different scan rates from 10 to 60 mV s−1. In spite of the gradual rise in scan rates, there is no obvious deformation of CV shapes (the peak shift is quite small), evidencing their great stability and rapid electrochemical dynamics. Figure 4c shows galvanostatic discharge curves for Fe@C hybrids at various current densities from ~1 to ~20 A g−1. Under a current rate of ~1 A g-1, the discharge curve exhibits a long plateaus at ∼−0.85 V and a short one at ∼−1.05 V, revealing a representative battery-type property and well in line with our former CV results. Also notice that, upon the rise of current rates, those discharge plateaus nearly keep the same, once again testifying the excellent anodic reaction dynamics. Figure 4d shows the specific capacity of Fe@C NPs as a function of current rates. Specifically, when measured at current densities of 1, 2, 5, 10 and 15 A g-1, such a hybrid electrode can deliver high capacities of ~405.2, ~350.8, ~285.6, ~198.2 and ~150.9 mA h g-1, respectively; even underneath a current as large as 20 A g-1, an impressive specific capacity is still retained up to ~114.8 mA h g-1, demonstrating the excellent high-rate capability of Fe@C NPs. We need to stress that the corresponding specific capacity of Fe@C is much higher than the values of Fe3O4@C and Fe2O3 counterparts at all current densities (Figure 4d), which mainly results from superb reactivity/conductivity of Fe core and good stability/robustness of thick C shells. Electrochemical impedance spectroscopy (EIS) spectra also show that Fe@C hybrids own the smallest charge-transfer resistance (Figure 4e). Moreover, the long-lasting cyclic behavior has been tested for all anodes (Fe2O3, Fe3O4@C and Fe@C hybrids) at a potential range of −1.4 to −0.35 V under a rate of 5 A g-1 (see Figure 4f). Firstly, the capacity retention of the three electrodes rises stepwise owing to a retard permeation of KOH electrolyte and progressive activations of Fe-based actives. Then, a steady electrochemical



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equilibrium state is established for all electrode cases. Evidently, after hundreds of cycles activation period (for single-phased Fe2O3: ~600 cycles; for Fe3O4@C: ~450 cycles), Fe2O3 and Fe3O4@C electrodes deliver the Max. capacity retention but later suffer from the severe capacity decay. In sharp contrast, Fe@C hybrid exhibits far superior cyclic stability (capacity retention at ∼91.9% after 4000 cycles) to either pure Fe2O3 (capacity retention at ∼42.1% after 2000 cycles) or Fe3O4@C anodes (capacity retention at ∼78.6% after 3000 cycles). After 4000 cycles, the sharp diffraction peaks in XRD pattern (Figure S4) are consistent with ferroferric oxide (JCPDS No.75-0033), revealing a fact that the involved ferruginous species have already changed from the pristine Fe into Fe3O4. Additional SEM observations (See Figure S5) on cycled Fe@C electrodes illustrate that the core-shell hybrid nanoarchitecture is still preserved and distinguished (the outer shells almost keep intact in the absence of splits/breaks), though the particle size for inner ferruginous species indeed becomes expanded after long-term cycling. We believe the principal reason should be ascribed to the robust thick C package, which can indeed protect interior Fe nanocrystals (e.g., by preventing them from adverse NPs aggregations or growth into large bulks, and alleviating volume expansions of ferruginous actives). Besides, the irreversible conversion of Fe (within intact/thick C shells) into inactive γ-Fe2O3 is relatively retarded when compared to the case of FeOx.38,

41

Table 1 summarizes the cycling

performances of some previous C/Fe-based hybrids derived from various C and ferruginous precursors. Clearly, our Fe@C NPs made from rusty and PE wastes show comparable or far superior cyclic stability to previously reported examples using relatively expensive raw materials and tedious fabrication procedures. This demonstrates the great potential of our Fe@C NPs as affordable electrode materials as well as the effectiveness in recycling/reusing waste materials. To further evaluate the feasibility of Fe@C hybrids for practical applications, we have assembled


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full-cell devices by paring the Fe@C anode with the NiO@C NWAs cathode (denoted as (+)NiO@C//Fe@C(−); see the device schematic in Figure 5a). Figure 5b reveals the CV plot of (+)NiO@C//Fe@C(−) (scan rate: 5 mV s−1) within a voltage range of 0−1.7 V. Also, corresponding CV plots for Fe@C anode and NiO@C cathode are provided as well (see the inset of Figure 5b). Nearly all of them exhibit equivalent CV integral areas, implying that both anode and cathode are fully matched on charge storage. Clearly, there are a pair of symmetric redox peaks recorded in the CV curve of this device, indicative of highly reversible electrochemical reactions and typical battery characteristics in our systems. Specifically, the anodic peak P1 (at ∼1.43 V) represents to the yield of NiOOH and Fe0 in charging reactions, while the cathodic peak P2 (situated at ∼1.30 V) accords to the reverse discharging process. The small peak separation of ~130 mV indicates the superb reaction kinetics of the full-cell devices. The total electrochemical reaction is present below: Fe + 3NiOOH↔FeOOH + 3NiO + H2O

(7)

Figure 5c shows the galvanostatic discharge curves of (+)NiO@C//Fe@C(−) device at varied current rates from 1 to 16 A g−1 (one typical galvanostatic charge/discharge profile in full-cell testing is shown in Figure S6). In the discharge reaction, the cell voltage plateau is steadily kept at ∼1.2 V; no obvious IR drop is viewed along the augment of current rates. The capacity for the device is plotted as a function of current rate (Figure 5d). The device can deliver a great specific capacity (∼138.9 mA h g−1) at ~1 A g−1, exceeding the values for most Fe-based ARBs. In addition, the specific capacities of this device are measured to be around ~115, ~90 and ~74 mA h g−1 at 2, 4 and 8 A g−1, respectively. Even when operated at a current density of ∼16 A g−1 (charge/discharge process accomplishes in few seconds), our full-cell device is still able to output a capacity of ∼58 mA h g−1, further confirming its excellent rate capability in power-supply usage. Figure 5e reveals the cyclic endurance at a current density of 4



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A g−1. Clearly, the capacity retention also rises at first and then remains at ∼95.1% after 4000 cycles, verifying its excellent long-term cyclic stability. Moreover, two fully-charged (+)NiO@C//Fe@C(−) cells connected in series are able to drive a blue LED light (rated power: 3-5 W, operating voltage: 2.5 V) for over 5 min (see the optical images in Figure 5e), demonstrating their great promise in real applications. To manifest the superiority of our devices, Ragone curve of energy density vs. power density is plotted (Figure 5f; data in classic literatures are involved for comparison). As calculated, the assembled devices are capable to output a highest specific energy of ∼138 W h kg−1 at a power density of ∼0.61 kW kg−1. When evaluated under the Max. power density up to ∼14.5 kW kg−1, the (+)NiO@C//Fe@C(−) cell devices still maintain an extraordinary energy density of ∼75.7 W h kg−1, which is far higher than those of Ni–Fe cells built with FeOx anodes.

Conclusions In this work, we have proposed a sample and scalable strategy that can efficiently turn wasted plastic and iron rust into high-quality Fe@C NPs, which can serve as preferable anodes for Ni–Fe ARBs. In view of the fact that the involved reactants are from wasted materials and the subsequent conversion into useful materials is accomplished via a facile one-step calcination, our proposed strategy is appealing for not only the prevention of environmental pollution but also the sustainable development of our society. When tested as electrodes for power-supply applications, the Fe@C NPs can output high charge-storage capacity (with a Max. value of ∼405.2 mA h g−1), good rate capabilities (350.8, 285.6, 198.2, 150.9 and 114.8 mA h g-1 at 2, 5, 10, 15 and 20 A g-1), and remarkable cycling performance (91.9% capacity retention after 4000 cycles). The as-assembled (+)NiO@C//Fe@C(−) full-cell device is capable of outputting Max. energy/power densities up to ∼138 W h kg−1/∼14.5 kW


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kg−1. Such electrochemical properties are better than those of other previously reported Fe@C hybrids for Ni–Fe ARBs. This work may provide a feasible strategy toward constructing C-based composites using plastic wastes as carbon sources, which is meaningful from the economic and renewable perspectives.

Acknowledgements This project is supported by National Natural Science Foundation of China (11604267, 51802269 and 21773138), Chongqing Natural Science Foundation (cstc2016jcyjA0477 and cstc2018jcyjAX0624) and Fundamental Research Funds for the Central Universities (XDJK2018C005, SWU115027 and SWU115029). Authors also acknowledge financial supports from Venture & Innovation Support Program for Chongqing overseas returnees (cx2018027) and Program for Innovation Team Building at Institutions of Higher Education in Chongqing (CXTDX201601011, XDJK2017A002).

Supporting Information SEM images of Fe2O3 nanosphere precursors; TGA analysis, N2 adsorption−desorption isotherms and the pore-size distribution plot of Fe@C NPs; XRD pattern and SEM images of cycled Fe@C electrodes; a typical charge-discharge profile for (+)NiO@C//Fe@C(−).

References (1) Brooks, A. L.; Wang, S.; Jambeck, J. R. The Chinese Import Ban and Its Impact on Global Plastic Waste Trade. Sci. Adv. 2018, 4, eaat0131-eaat0137. (2) Schweitzer, J.; Gionfra, S.; Pantzar, M.; Mottershead, D.; Watkins, E.; Petsinaris, F.; ten Brink, P.; Ptak, E.; Lacey, C.; Janssens, C. Unwrapped: How Throwaway Plastic is Failing to Solve Europe’s



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Page 20 of 27

Figure Capations:

Figure 1 General schematic displaying the mass production of Fe@C Nanoparticles with plastic and rusty wastes.

Figure 2 (a-b) SEM observations, (c) Raman spectrum, (d) XRD pattern and (e-f) XPS results for Fe@C Nanoparticles.

Figure 3 (a-c) TEM observations, (d) EDS detection and (e-g) elemental mapping record of Fe@C nanoparticles. (h) The plot showing the involved C layer thickness (black) and total content (red) as a function of reaction time.

Figure 4 (a) Typical CV scan at 5 mV s-1 for distinct Fe-based anodes. (b) CV plots of Fe@C NPs at various scan rates, and (c) galvanostatic discharge curves of Fe@C NPs. (d) Specific capacity as a function of current densities, (e) EIS spectra and (f) long-term cyclic behaviors (black: Fe2O3 NPs; red: Fe3O4@C NPs; blue: Fe@C NPs).

Table 1 Electrochemical property comparisons of our Fe@C NPs anodes with typical C/Fe-based nanohybrids in previous literatures.

Figure 5 (a) The schematic showing the NiO@C//Fe@C device configuration. (b) CV scan of this full-cell device at 5 mV s-1 (Inset: CV scans for Fe@C anode and NiO@C cathode). (c) Galvanostatic discharge profiles at varied current densities, (d) the stored capacity as a function of current rate, (e) cyclic performance and (f) Ragone plot of full-cell device (Inset: optical images showing two ARBs devices linked in series can light up a high-power LED indicator).


Page 21 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41


Recycle & Reuse

+

-[-CH2 - CH2 -]-n Plastic Wastes

Alkaline Battery

CH2 = CH2 C

H2

Iron Rust

-

N2-Filled Furnace Plastic Wastes

CH2 = CH2

Fe2O3

Collection ~100 nm

~30 nm

Fe2O3 Nanoparticles

Self-catalytic Reactions Fe@C Nanoparticles

Massive Evolution

Figure 1


Application


a

c Intensity (a.u.)

b

100 nm

D band

Carbon

(110)

e

Fe

(002)

JCPDS No. 87-0722 (110) Fe2O3 (104) (214) (113) (024)(116) (300) (119) (012)

Fe 2p 1/2

Intensity (a.u.)

(200)

1400

1600

1800

Raman shift (cm-1)

f

Fe 2p

Fe 2p 3/2

1200

C 1s

C=C

Intensity (a.u.)

d

G band

50 nm 1000

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41

Page 22 of 27

C-O C=O

JCPDS No. 33-0664 10

20

30

40

50

2 Theta

60

70

80

700

710

720

730

Binding Energy (eV)

Figure 2 ACS Paragon Plus Environment

740 280

282

284

286

288

290

292

Binding Energy (eV)

294

Page 23 of 27

a

c

b

Fe (110) 0.20 nm

C 25 nm

10 nm

e

C Fe

45 40 35 30 25 20 15 10 5 0

f

2

4

6

Energy (eV)

8

2 nm

g

200 nm

Fe

0

(002) 0.34 nm

Fe

C

10

16

h

12

Saturation

8 4 0

0.0

0.5

1.0

1.5

Reaction Time (h) ACS Paragon Plus Environment

Figure 3

2.0

2.5

3.0

Total C Content (%)

Intensity (a.u.)

d Cu (Background)

C Layer Thickness (nm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41



b

Fe2+ to Fe3+

0.10

Fe3O4@C

Fe0 to Fe2+

Current (A)

Current (A)

390 mV 0.02

Fe2O3 0.00

-0.02

-0.04

Fe3+ to Fe2+ H2 Evolution -1.4

-1.2

-1.0

Scan Rate：5mV s-1 -0.8

0.05 0.00 -0.05

60 mV s-1

-0.10

Fe@C NPs

Fe2+ to Fe0

c

0.15

-0.6

-0.4

-1.6

-0.2

Potential (V vs. Ag/AgCl)

e

d

10 mV s-1

-0.15 -1.4

-1.2

-1.0

-0.8

-0.6

-0.4


2.0 1.5

15

-Z'' (ohm)

Fe3O4@C

-Z'' (ohm)

200

Fe@C NPs

Fe2O3

20

10

100

1.0 0.5

5

Fe2O3

0

0

5

0.0

10

15

Current Density (A g-1)

20

0

0

5

10

0

1

15

Z' (ohm)

Figure 4

2

Z' (ohm) 20


-0.8 -0.6

10 A g-1 5 A g-1 20 A g-1

-0.4

0

f

25

Fe@C NPs

-1.0

300

2 A g-1 600

1 A g-1

900

1200

1500

1800

Time (s)

Fe3O4@C

300

-1.2

-0.2

30

400


0.04

3

25

4

30

Capacity Retenition (%)

a

Capacity (mAh g-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41

Page 24 of 27

120

Fe@C NPs

100

Fe3O4@C 91.9%

80

78.6% 60

Fe2O3 40

42.1%

20 0

0

1000

2000

3000

Cycle Number

4000

Page 25 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41


Fe-based precursors

Carbon precursors

Cyclic performance

Ref.

Fe(CO)5

Glucose

93.5% (1000 cycles)

[21]

Fe(NO3)3•9H2O

Dextran (C6H10O5)n

88.9% (1500 cycles)

[20]

FeCl3•6H2O

Carbon cloth

96.1% (2000 cycles)

[25]

FeCl3•6H2O

Natural graphite flake

73.2% (3000 cycles)

[23]

FeCl3•6H2O

Polyacrylonitrile (PAN) 91% (1000 cycles)

FeCl3

Poly(acrylic acid) (PAA)

FeSO4•7H2O

[24]

90% (1000 cycles)

[28]

Graphite oxide

67.8% (1000 cycles)

[26]

Fe(NO3)3•9H2O

Graphene

91.1% (1000 cycles)

[29]

FeCl2•4H2O

Graphene/PANI

92% (5000 cycles)

[27]

FeCl3•6H2O

Polystyrene (PS)

82% (1000 cycles)

[30]

Iron rust

PE

91.9% (4000 cycles)

Table 1


This work

ACS Sustainable Chemistry & Engineering 1.8

Anode Current (A)

0.06

Ni(II) Ni(III)

Fe(0) Fe(III)

1.4 0.00

1.2

P1

-0.03 -1.5 -1.2 -0.9 -0.6 0.0

0.2

0.4


0.00

-0.03

0.0

Fe@C

0.3

0.6

0.9

Voltage (V)

1.2

1.5

114.9 90.3

90

70.1 50.3

60 30 0

Unit: mAh g-1 0

2

4

6

8

10

12

14

-1

Current Density (A g )

16

e

120

0.6 0.4

8 A g-1

100

95.1%

80 60 40 20 0

0

1000

2000

3000

Cycle Number

Figure 5 ACS Paragon Plus Environment

4000

4 A g-1

2 A g-1 1 A g-1

16 A g-1 0

100

200

300

Time (s)

400

1000

Energy Density (Wh/Kg)

-1

138.9

120

0.8

-0.2

1.8

180 150

1.0

0.0

P2

Capacity Retenition (%)

d

0.03

c

1.6

0.2

KOH Electrolyte

NiO@C

b

0.03

Voltage (V)

Cathode

0.09 Current (A)

a

Capacity (mAh g )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41

Page 26 of 27

500

f This work

100

10

Ref.22

Ref.17 Ref.5 Ref.8 Ref.18 Ref.10 i Supercapacitors Ref.15

1 0.01

0.1

1

10

Power Density (KW/Kg)

100

Page 27 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41


Table of Content (TOC) ~100 nm

-[-CH2 - CH2 -]-n Fe@C Nanoparticles Plastic Wastes Self-catalytic Reactions ~30 nm

Fe2O3 Nanoparticles

Massive Evolution

Cathode

Anode

Ni(II) Ni(III)

Fe(0) Fe(III)

KOH Electrolyte

NiO@C

Fe@C

A facile and scalable approach is developed that can efficiently turn wasted plastic and iron rust into high-quality Fe@C NPs as preferable anodes for Ni/Fe ARBs, rendering them with outstanding behaviors in both half-cell and full-cell systems. The proposed strategy is appealing for the prevention of environmental pollution and the sustainable development of our society. This work not only provide a new way for preparing renewable high-performance anode materials for Ni//Fe ARBs but also set up an economical platform for fabricating ACS Paragon Plus Environment advanced carbon-based composites by recycling waste materials for other practical usages.

Mass Production of Metallic Fe@Carbon Nanoparticles with Plastic

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