Vacancies Revitalized Ni3ZnC0.7 Bimetallic Carbide Hybrid

Publication Date (Web): August 6, 2018 ... Moreover, Li-ion storage mechanism for the bimetallic counterparts has been substantiated mainly ... which ...
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Vacancies Revitalized Ni3ZnC0.7 Bimetallic Carbide Hybrid Electrodes with Multiplied Charge-Storage Capability for High-Capacity and Stable-Cyclability Lithium-Ion Storage Huawei Song, Jian Su, and Chengxin Wang* State Key Laboratory of Optoelectronic Materials and Technologies, School of Materials Science and Engineering, Sun Yat-Sen (Zhongshan) University, Guangzhou 510275, People’s Republic of China

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S Supporting Information *

ABSTRACT: Transition metal carbides (TMCs) exhibit good electron conductivity and structural stability; however, the limited active sites hindered the application in lithium-ion batteries. Unlike many alloying or conversion-reaction-type active materials, most of them store charges by capacitive processes. Moreover, the Li-ion storage mechanism for the bimetallic counterparts has been substantiated mainly through an adsorption or intercalation Faradaic pseudocapacitance effect. Taking vacancy-enriched Ni3ZnC0.7 nanohybrids as example, we demonstrated for the first time the occurrence of local phase segregation and abnormally efficient Li-ion storage from in situ formed active nanocrystals in organo-functionalized ultrasmall Ni3ZnC0.7 (∼3−5 nm) hybrid electrodes. Those hybrid electrodes not only delivered high reversible Li-ion storage capability of 1100 mAh g−1 at 50 mA g−1 but also exhibited an excellent rate performance of 10 A g−1 with fully reversible capacity recovery and ultralong cycling stability (nearly no capacity decay) at different rates (e.g., 0.1 A g−1 for 300 cycles, 0.2 A g−1 for more than 500 cycles, and 1 A g−1 for 1000 cycles with nearly 100% capacity retention). Physicochemical characterization of the postcycled electrodes revealed highly active Zn and Ni nanocrystals newly generated around the parent vacancy-enriched Ni3ZnC0.7 nanocrystals, which enriched the active sites for Li-ion storage and simultaneously elevated the capacitive and diffusion-controlled charge-storage capability, probably accounting for the enhanced electrochemical performance, besides the highly electron-/ion-conductive porous carbon matrix and TMCs themselves. KEYWORDS: bimetallic carbides, vacancy, Ni3ZnC0.7, phase segregation, nanocrystals, Li-ion storage

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preferable to develop new electrode materials with stable and indestructible structures during battery operation, simultaneously inserting Li ions through multiple types of mechanisms, unlike that of graphite. Benefiting from the high electron conductivity and good mechanical strength, TMCs have been intensively explored in energy storage and conversion. In particular, the TMC-based electrodes could afford diverse ion-storage mechanisms for application in rechargeable batteries and electrochemical capacitors. On the one hand, those TMC electrodes could utilize both non-Faradaic and Faradaic charge-storage processes, especially for the MXene nanosheets and those porous hybrid electrodes which exhibit excellent ion storage capability due to their large specific surface; on the other hand, the hardness of TMCs endow those electrodes with durable stability for very long-term tests. However, so far, TMCs in lithium-ion batteries are mostly inactive themselves, only

owadays, to meet the energy-intensive demand, the key to advance rechargeable batteries mainly lies in designing advanced electrodes of high energy and power performance.1 As in lithium-ion batteries, new-generation electrode materials based on multielectron transfer attract most of the attention. Among them, transition metal oxides, nitrides, sulfides, and fluorides have been explored because of their superior Li-ion storage capability compared with those commercial electrodes based on the Li+ intercalation mechanism such as graphite and LiCoO2.2−4 However, those electrodes based on multielectron transfer mostly suffer from huge volume expansion as a result of taking in more Li ions, unavoidably leading to many undesired side effects as pulverization and exfoliation of the active materials. Hence, various measures are adopted to address these issues. The main techniques that are welcomed are designing novel nanostructures or nanocomposites, which should not only afford enough void space suitable for the volume expansion effect but also be conductive enough for electrons and Li ions.5,6 For example, active electrode materials are engineered in various porous carbon bubbles, graphene and their derivatives, carbon nanotubes, ordered mesoporous carbon, and organic−metal frameworks, etc.7−9 It is actually © XXXX American Chemical Society

Received: June 19, 2018 Accepted: August 6, 2018 Published: August 6, 2018 A

DOI: 10.1021/acsaem.8b00992 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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ACS Applied Energy Materials

which Zn occupies the eight top points of simple cubic lattice, and Ni is found at the six face centers, while C takes up 70 percent of the octahedral interstices formed by Ni atoms. The widening peak indicated that Ni3ZnC0.7 was highly nanocrystallized, while the shift to the higher 2theta range probably occurred as a result of the loss of Zn and C in the lattice, considering the synthetic vacuum atmosphere. To substantiate the supposition, Rietveld structure refinement of series of Ni3ZnxCy (0 ≤ x ≤ 1, 0 ≤ y ≤ 0.7) calculated structures was performed. Among them, the consistent powder XRD patterns between as-prepared nanohybrids and the Ni3Zn0.89C0.7 or Ni3Zn0.85C0.35 verified ratios of zinc vacancies could reach up to at least 11% without consideration of carbon extraction and increased another 4% when half of the carbon has been extracted out of the nanocrystals (Figure 1a and Table S1). The speculation of the not rigidly stoichiometric Ni, Zn, or C would be further visually substantiated by the TEM images. The EDX spectrum (Figure 1b) implied only the existence of C, O, Zn, and Ni in the nanohybrid. The oxygen element probably derived from the O-containing organic functional groups (e.g., −C−OH, −COOH) in the carbon matrix according to the emerging C−O and OC−O peaks in the deconvoluted C 1s XPS spectrum and vibration absorbance bands of C−O−C (1119 cm−1), COO− (1469, 1623 cm−1), and − OH (3450 cm−1) in the FT-IR spectra (Figures 1(c) and S1).23 The sharp 2p3/2 peaks accompanying weak satellite peaks in the Ni 2p spectrum and marked peak pack of the loss feature in Zn 2p spectrum demonstrated the metal-like character of Zn and Ni, very consistent with the alloy-type Ni3ZnC0.7 nanocrystals in the nanohybrid.24 Moreover, the well-defined D and G bands (1314 and 1582 cm−1) in the Raman spectrum (Figure 1d) verified that the carbon matrix was locally graphitized, which probably resulted from the ordered extraction of carbon in Ni3ZnC0.7.25,26 The shoulder peak around 1169 cm−1 was contributed by the C−O−C vibration mode, while the peak pack at 506 cm−1 was probably derived from Zn−C and Ni−C vibration modes in the Ni3ZnC0.7 nanocrystals.27,28 In combination of the EDX, XPS, Raman, and FT-IR spectra, one readily concluded that those nonstoichiometric Ni3ZnC0.7 nanocrystals were dispersed in a graphitized carbon matrix fully of various oxygen-containing organic functional groups. The typical type IV N2 adsorption− desorption isotherms (Figures 1e,f and S2) verified that there were plenty of mesopores in the nanohybrids as shown in the BJH desorption pore distribution.29 The BET surface area and BJH pore volume of the nonstoichiometric Ni3ZnC0.7 appeared higher than those of stoichiometric ones, in good accordance with the synthetic atmospheres and the FT-IR results. The loss of excessive Zn would repeatedly etch the carbon matrix because of the relevant complicated processes of oxidation, reduction, and evacuation, leading to abundant pores. The porous microstructure of the as-prepared Ni3ZnC0.7 nanohybrids could also be verified in the STEM and TEM images (Figures 2 and S3). As shown in Figure 2a,b, those particles smaller than 5 nm were uniformly distributed in an irregular carbon matrix, and the gathering Zn and Ni distribution in the elemental mapping images implied that those particles consisted of Zn and Ni just conforming to the physicochemical characterization depicted above. The dispersive oxygen distribution indicated that those oxygen atoms might come from the carbon matrix, agreeing well with the many oxygen-containing organic groups included. Occasionally appearing large particles were further confirmed as agglomer-

serving as robust skeletons in composites electrodes. For example, TiC has been reported as a highly conductive, mechanically robust, and electrochemically inactive scaffold in fabricating Si/TiC/C, TiC/NiO, and Zn/TiC/C composite electrodes;10−12 Fe3C in composite electrodes has been reported inactive;13 even the so-called MXene materials including Ti3C2, Nb2C, V2C, etc. have also been reported to store Li ions through adsorption or intercalation pesudocapacitance among interlayer spaces of those using MC x polyhedron stacking.14−16 At least for now, the only exception is Mo2C, which has been substantiated to store Li ions through conversion or alloying reaction (Mo2C + xLi+ + xe− → 2Mo + LixC).17 Consequently, nanocomposites of MoxC have frequently been reported to exhibit excellent Li-ion storage performance. Similarly, bimetallic carbides, except for common merits of being good electron conductors and having high mechanical structural stability, also possess a wider dimension of choices for regulation of physicochemical properties. For example, the content of both metals could be altered to adjust them to be suitable for catalysts and energy storage. However, as far as we are concerned, very little relevant work has been reported. Only Co3ZnC nanohybrids have been reported to deliver a superior Li-ion storage performance;18 however, the Co3ZnC itself was verified to be inert without any chemical reaction or phase transformation during the lithiation/delithiation process. The major capacity in the nanocomposites was attributed to the enhanced capacitive charge-storage as a result of a large specific surface. Ni3ZnC0.7 in [email protected] and ZnO/ Ni3ZnC0.7/C hybrids have also been demonstrated to be inactive and served as a buffering layer with good conductivity for hindering the coarsening and exfoliation of ZnO.19,20 Recently, Koketsu et al. reported Ti vacancies highly availed to enhance Mg2+/Al3+ storage capability in anatase TiO2.21 Oxygen vacancies were also verified to improve capacitive charge-storage properties in MoO3−x.22 Hence, altering the content of components in the bimetallic carbides might help to generate many vacancies and improve the Li-ion storage performance, meanwhile making full use of their good conductivity and mechanical structural stability. Through a fast evaporation−solidification method, we have successfully fabricated Ni3ZnC0.7 nanohybrids with organofunctionalized ultrasmall nanocrystals (Scheme 1). To obtain Scheme 1. Illustrations Depict the Universal Procedure for Fabricating Ni3ZnC0.7 Nanohybrids with Ultrasmall Nanocrystals

the vacancy-enriched nonstoichiometric Ni3ZnC0.7, the annealing process for the uniform Zn−Ni organic precursor was performed under vacuum, while a flowing H2 atmosphere was used to obtain the stoichiometric nanohybrids. (For the synthetic procedure in detail, see the Methods Section in the Supporting Information). The powder XRD pattern of the asprepared nanohybrids in Figure 1a shows two wide peaks around 42.8 and 49.7° (2theta), very consistent with that of cubic Ni3ZnC0.7 (space group: Pm-3m, a = 0.366 nm), in B

DOI: 10.1021/acsaem.8b00992 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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Figure 1. Structures, composition, and bonds of the as-prepared nonstoichiometric Ni3ZnC0.7 nanohybrids. (a) Powder XRD pattern of the experimental pattern in comparison with the literature pattern as well as those of the Rietveld refinement Zn-and/or C-vacancy-enriched simulated patterns, (b) EDX spectrum, (c) deconvoluted XPS spectra for C 1s, Ni 2p, and Zn 2p peaks, (d) Raman spectrum, and (e,f) N2 adsorption− desorption isotherms and distribution of the BJH desorption pore.

simulations and TEM images, the missing rate of Zn was at least higher than 11%. Those vacancies would play a significant role in enhancing the related charge-storage properties. Charge-storage performance was tested by fabricating lithium batteries with nonstoichiometric Ni3ZnC0.7 nanohybrids (stoichiometric Ni3ZnC0.7 nanohybrids for comparison) as the cathodes and lithium plates as the anode, and 20 μL of 1 M LiPF6 in ethylene carbonate (EC) and diethyl carbonate (DEC) with a weight ratio of 1:1 was used as the electrolyte. The typical charge−discharge curves and cyclic voltammograms (Figures 3a,b and S6a−d)) clearly demonstrated that the nonstoichiometric Ni3ZnC0.7 nanohybrids exhibited multiplied Li-ion storage capability compared with the stoichiometric nanohybrids. In the first cathodic cycle, the former was characteristic of three different slopes, in which two around 1.25 and 1 V were probably attributed to Li insertion into Ni3ZnC0.7 and the formation of Ni3C, which resulted from phase segregation of the deformed nonstoichiometric Ni3ZnC0.7 structure, while the slope around 0.5 V was ascribed to the formation of Zn and its alloying reaction like that of reported ZnO composites.20 As to the stoichiometric Ni3ZnC0.7 hybrid electrode, there was only one steady slope

ates of several small nanocrystals with different orientations but all separately encapsulated by graphitized carbon (Figures 2d and S3). The elegant microstructure would be quite beneficial to maintain structural integrity in severe deformation brought by a considerable amount of Li-ion intercalation. As mentioned, nonstoichiometric Ni and Zn would favor the generation of many vacancies. The variation in intensity of atom columns in the atomic-resolution TEM images could allow us to directly observe vacancies.21 As shown in Figure 2e (the crystal orientation was defined in the lower-left, and the theoretical atomic connection mode equivalent to the observed image was also presented in the top-right) and 2f, the varying intensity and dark contrast were clearly observed in the 10 atom-column-marked rectangular zone, which was also presented in the corresponding colored image with a profile showing intensity variation (lower-left in Figure 2f), indicating the existence of vacancies. The intensity in variation of the 10 × 10 atom columns in a parallelogram zone (Figure S4) also demonstrated that there were many vacancies in the asprepared nonstoichiometric Ni3ZnC0.7 nanocrystals (atomicresolution TEM image of stoichiometric Ni3ZnC0.7 is also presented in Figure S5 for comparison). According to the XRD C

DOI: 10.1021/acsaem.8b00992 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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Figure 2. Microstructures of the as-prepared nonstoichiometric Ni3ZnC0.7 nanohybrids. (a,b) STEM images in different magnifications, (c) element mapping images for the marked zone in (b), (d) TEM image, and (e,f) atomic-resolution TEM image of a nanocrystal and its colored view with a profile (lower-left of (f)) showing varying intensity of the marked rectangular zone, indicating the existence of vacancies. (The crystal orientation was defined in the lower-left, and the theoretical atomic connection mode equivalent to the observed image was also presented in the top-right of (e).)

delivered excellent cycling performance (Figure 3c−e) with high capacity more than 1000 mAh g−1 at 0.1 A g−1 for nearly 300 cycles, more than 900 mAh g−1 at 0.2 A g−1 for approximately 600 cycles, and 635 mAh g−1 at 1A g−1 for more than 1000 cycles. The almost 100% capacity retention clearly verified that the advantage of vacancies improved charge-

around 0−1 V because of adsorption−desorption of Li from Ni3ZnC0.7 and partially graphitized carbon. After galvanostatic testing at a rate of 50 mA g−1, the former delivered a superior reversible capacity of ∼1100 mAhg−1, twice more than that of the latter (∼400 mAh g−1). Except for the multiple chargestorage capability, the vacancy-enriched nanohybrids also D

DOI: 10.1021/acsaem.8b00992 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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Figure 3. Electrochemical properties of the as-prepared nonstoichiometric Ni3ZnC0.7 nanohybrids in comparison with the stoichiometric nanohybrids. (a) Typical charge−discharge curves after initial five cycles at 50 mA g−1, (b) cyclic voltammograms after initial five scans at 0.2 mV s−1, (c−e) cycling performance at different rates of 0.1, 0.2, and 1 A g−1, and (f) rate performance at varying rate from 50 mA g−1 to 10 A g−1 and its capacity retentions in long-term cycling after varying rates.

porous microstructures and nonstoichiometric-related vacancies but also highly dependent on the conductive carbon matrix derived at vacuum atmosphere and the intrinsic electron conductors of the bimetallic carbides themselves (Figure S6f). To fully understand the effect of vacancies on the chargestorage properties of the nonstoichiometric Ni3ZnC0.7 nanohybrids, sweep voltammetry was performed at different rates in comparison with the stoichiometric nanohybrids (Figure 4). According to the equation (i = k1v + k2v0.5), the response current (i) is linearly dependent on sweep rate (v) in capacitive charge-storage processes and power sweep rate (v0.5) in diffusion-controlled charge-storage processes, respectively.30 Consequently, the charge-storage properties could be determined by slopes of lg(i) versus lg(v) plots at several given voltages. As shown in the cyclic voltammograms (Figure 4a,c), the nonstoichiometric Ni3ZnC0.7 provided three couples of cathodic and anodic peaks indicating more complicated Liion storage mechanisms than those of the stoichiometric

storage properties in bimetallic carbides. Moreover, a good rate performance (Figure 3f) was also achieved in the nonstoichiometric nanohybrids, specifically, 1105 mAh g−1 at 50 mA g−1, 934 mAh g−1 at 100 mA g−1, 845 mAh g−1 at 200 mAh g−1, 720 mAh g−1 at 500 mA g−1, 594 mAh g−1 at 1A g−1, 460 mAh g−1 at 2 A g−1, 339 mAh g−1 at 5 A g−1, and 190 mAhg−1 at 10 A g−1, respectively, quite superior to those of stoichiometric nanohybrids (Figure S6e). When the rate was returned to 100 mA g−1, the storage capability quickly recovered nearby and even fully recovered after being successively tested at 10 A g−1 for more than 1500 cycles. The reversible capability could remain for at least more than 3000 cycles with a high capacity retention of 94.5%. It was important to note that the slight rise in the capacity shown in the cycling tests was attributed to the vacancy-related phase segregation at the surface of the nonstoichiometric Ni3ZnC0.7 nanocrystals, which will be fully discussed below. As to the improved rate performance, it is not only contributed to by the E

DOI: 10.1021/acsaem.8b00992 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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Figure 4. Sweep voltammetry analyses of the charge-storage process in the Ni3ZnC0.7 nanohybrids. (a) Cyclic voltammograms of nonstoichiometric Ni3ZnC0.7 at various sweep rates. The main cathodic and anodic peaks are marked as c(n) and a(n), respectively, for analyzing the kinetics. (b) Lg(i) versus lg(v) plots of the current response at the marked peaks in (a). The slopes (s) of the plots provided lie in the range of 0.5−1, indicating a mixed charge-storage mechanism of a diffusion-controlled Li+ intercalating process and capacitive charge-storage process. (c) Cyclic voltammograms of stoichiometric Ni3ZnC0.7 at various sweep rates, and (d) corresponding lg(i) versus lg(v) plots. Charge-storage contribution analysis in the cyclic voltammogram at 0.5 mV s−1 of the (e) nonstoichiometric and (f) stoichiometric Ni3ZnC0.7 based on calculated k1 and k2 (i = k1v+k2v0.5).

Figure 5. (a) Ex situ XRD patterns and (b) Zn LMM Auger spectra for the postcycled electrodes of nonstoichiometric Ni3ZnC0.7 after different numbers of cycles, exhibiting marked phase segregation process. (All of the electrodes characterized were in the fully charged stage of 3 V.)

counterpart with only peaks for one couple. According to the corresponding lg(i) versus lg(v) plots (Figure 4b,d), the slopes for the cathodic peaks (c1, c2, and c3) and the anodic peaks (a1, a2, and a3) lie between 0.5 and 1. This implied that both capacitive and diffusion-controlled processes contributed to the charge-storage capability of the nanohybrids. However, the capacitive contribution at higher voltage is more prominent than that at lower voltages. It is quite different that almost no

high-voltage peaks existed for the stoichiometric Ni3ZnC0.7. Hence, the vacancies highly improved the capacitive chargestorage capability of Ni3ZnC0.7, which was very consistent with the previous reported phenomenon in MoO3−x and F-doped TiO2.21,22 Moreover, the diffusion-controlled charge-storage capability was also revitalized, as more large peaks and larger area were presented in the cyclic voltammograms, which could also be clearly distinguished by the colored area and the F

DOI: 10.1021/acsaem.8b00992 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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ACS Applied Energy Materials noncolored area in Figure 4e,f) achieved on the basis of calculated k1 and k2 at fixed voltages.30 It is already well known that vacancies could significantly improve the capacitive charge storage by affording a considerable amount of active sites for Li-ion intercalation with fast kinetics, while what causes the enhanced diffusioncontrolled Li-ion storage capability is unclear. To fully get the insights, powder XRD, Zn LMM Auger spectra, and highresolution TEM were carried out for the postcycled electrodes after different numbers of cycles. In the powder XRD patterns (Figure 5a), marked diffraction peaks appearing around 36.3 and 38.5° (2theta) at the fifth cycle indicated that Zn nanocrystals were formed, while the peak near 44.8° (2theta) implied the existence of Ni3C or Ni. It is to note that the main peaks for Ni3ZnC0.7 have not disappeared. This indicated the phase segregation of Ni3ZnC0.7 to Zn and/or Ni3C or Ni is local, and Ni3ZnC0.7 could not be fully decomposed. The partial phase segregation highly contributed to the capacity of the nanohybrid and also resulted into reduction peaks around 1 and 0.5 V in the first cathodic scan in the CVs, except for the formation of overlapping solid electrolyte interphase. Moreover, the phase segregation could be more clearly observed in the patterns after more cycles. The diffraction peaks around 31.8, 34.0, and 36.1° (2theta) ascribed to ZnO and 37.0° of NiO further substantiated the newly formed Zn and Ni nanocrystals that suffered from full lithiation/delithiation diffusion-controlled charge-storage processes (LiZn → Zn → ZnO and Ni → NiO in the charge process, where O probably comes from various O-containing groups in the carbon matrix or decomposition of SEI, which also accounts for the slight rise of storage capability after long-term testing25,31), which indirectly confirmed that the vacancies in the bimetallic carbides improved the capacitive charge storage capability, simultaneously revitalizing the diffusion-controlled Li-ion storage capability due to newly regenerated active Li-ion intercalating sites around by local phase segregation. The shoulder peak ascribed to metal-like Zn and its disappearance in the Zn LMM Auger spectra (Figure 5b), and clear Zn and Ni3C or Ni nanocrystals formed around the surface of Ni3ZnC0.7 nanocrystals accompanied with their further conversion into their oxide nanocrystals in the TEM images (Figure 6a−h) also agreed with that presented in the XRD patterns.24 The small active particles after long-term cycling with no marked crystal coarsening revealed that the vacancyinduced Li-ion storage capability improvements also avails to durably maintain the structural integrity of the electrodes. Interestingly, segregated graphene-like carbon (SG carbon) was also observed in the postcycled TEM and XRD results after 100 cycles.

Figure 6. TEM images for the postcycled electrodes of nonstoichiometric Ni3ZnC0.7 (a−d) at the 5th cycle and (e−h) at the 100th cycle, exhibiting a marked phase segregation process around the surface of Ni3ZnC0.7 nanocrystals. (All of the electrodes characterized were in the fully charged stage of 3 V.)

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CONCLUSIONS In summary, we have developed a universal fast evaporation− solidification method for fabricating multicomponent carbides with ultrasmall nanocrystals and abundant organic functional groups. Through controlling the annealing processes, nonstoichiometric vacancy-enriched Ni3ZnC0.7 nanohybrids have been synthesized. The as-prepared nonstoichiometric Ni3ZnC0.7 exhibited superior Li-ion storage capability as compared with the stoichiometric counterpart. Both capacitive and diffusion-controlled charge-storage properties of Ni3ZnC0.7 have been significantly improved by the introduction of vacancies. The former was due to increasing sites for Li-ion adsorption, while the latter was attributed to newly generating

active nanocrystals derived by vacancy-induced local phase segregation for Li-ion intercalation. The vacancy-induced local phase segregation and the vacancy-improved capacitive charge storage may be applicable to other carbides, and the defect regulated property evolution also opens up a new avenue to promote the application of bimetallic carbides or other compounds in energy storage and catalysis fields in future.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsaem.8b00992. G

DOI: 10.1021/acsaem.8b00992 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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Experimental section, additional XRD, FT-IR, TG, BET, SEM, and TEM characterization and electrochemical test, structure refinement results (PDF)

AUTHOR INFORMATION

Corresponding Author

*Tel & Fax: +86-020-84113901; E-mail: [email protected]. edu.cn. ORCID

Huawei Song: 0000-0001-9091-2297 Chengxin Wang: 0000-0001-8355-6431 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the grants from the National Natural Science Foundation of China (grant nos. 51602355 and U1401241) and the Postdoctoral Science Foundation of China (grant no. 2015M582467).

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DOI: 10.1021/acsaem.8b00992 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX