Metal–Organic Framework-Derived Nickel–Cobalt Sulfide on Ultrathin

Jun 11, 2018 - Water oxidation is the key process for many sustainable energy technologies containing artificial photosynthesis and metal–air batter...
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Cite This: ACS Appl. Mater. Interfaces 2018, 10, 22311−22319

Metal−Organic Framework-Derived Nickel−Cobalt Sulfide on Ultrathin Mxene Nanosheets for Electrocatalytic Oxygen Evolution Haiyuan Zou,§,† Bowen He,§,† Panyong Kuang,† Jiaguo Yu,*,†,‡ and Ke Fan*,† †

State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, P. R. China ‡ Department of Physics, Faculty of Science, King Abdulaziz University, Jeddah 21589, Saudi Arabia

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S Supporting Information *

ABSTRACT: Water oxidation is the key process for many sustainable energy technologies containing artificial photosynthesis and metal−air batteries. Engineering inexpensive yet active electrocatalysts for water oxidation is mandatory for the cost-effective generation of solar fuels. Herein, we propose a novel hierarchical porous Ni−Co-mixed metal sulfide (denoted as NiCoS) on Ti3C2Tx MXene via a metal−organic framework (MOF)-based approach. Benefiting from the unique structure and strong interfacial interaction between NiCoS and Ti3C2Tx sheets, the hybrid guarantees an enhanced active surface area with prominent charge-transfer conductivity and thus a superior activity toward oxygen evolution reactions (OERs). Impressively, the hierarchical NiCoS in the hybrid is converted to nickel/cobalt oxyhydroxide−NiCoS assembly (denoted as NiCoOOH−NiCoS) by OER measurement, where NiCoOOH on the surface is confirmed as the intrinsic active species for the consequent water oxidation. The hybrid material is further applied to an air cathode for a rechargeable zinc−air battery, which exhibits low charging/ discharging overpotential and long-term stability. Our work underscores the tuned structure and electrocatalytic OER performance of MOF derivatives by the versatility of MXenes and provides insight into the structure−activity relationship for noble metal-free catalysts. KEYWORDS: MOF derivative, MXenes, oxygen evolution reaction (OER), post-OER analysis, zinc−air battery

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INTRODUCTION Sustainable routes for renewable energy to mitigate the reliance on the combustion of traditional fossil fuels trigger extensive attention.1−5 Recent innovative strategies for electrochemical energy conversion via water splitting and metal−air batteries offer significant potentials for addressing this issue.6−10 An essential process in these energy conversion systems is greatly dependent on the rate-limiting step of anodic oxygen evolution reactions (OERs), in which a multipleelectron−proton process and large overpotential are required for their sluggish kinetics.11−15 Although precious Ru/Ir oxides are regarded as the benchmark catalysts toward OERs, widespread implementation is hampered by their scarcity and high cost. Within this context, increasing efforts have been devoted to developing highly efficient, stable, and inexpensive electrocatalysts for OERs. Currently, low-cost metal−organic frameworks (MOFs) that can serve as promising precursors or templates to engineer robust electrocatalysts have ignited significant interest.16−18 The good compatibility of MOFs with diverse metal ions facilitates to construct homogenous multimetallic materials.19,20 Moreover, MOFs not only can tune the inherent electronic and/or surface structures of the © 2018 American Chemical Society

yielded derivatives but also can endow the resultant products with hollow and/or porous nanostructure, which can contribute to better catalytic activity.21−25 Unfortunately, these derivatives are commonly prone to aggregate during electrocatalysis, which leads to the decreased active surface area, and their intrinsic poor electrical conductivity severely restricts the electrocatalytic activity, stimulating the urgent requirement for efficient synthetic strategies to improve the moderate performance of MOF derivatives for electrocatalysis. Combining the MOF derivative electrocatalysts with twodimensional (2D) materials, such as graphene, is an effective strategy to solve the aforementioned problems.26−29 As a newly emerging group in the 2D material family with unique structure and electronic properties, MXenes have shed new light on the energy-related applications.30−36 MXenes are generally prepared by selectively removing the A layers from the bulk MAX phase, where M is an early transition metal, A is an A-group element, and X is the C and/or N element.37−40 During etching, the A atoms between MAX layers are Received: April 18, 2018 Accepted: June 11, 2018 Published: June 11, 2018 22311

DOI: 10.1021/acsami.8b06272 ACS Appl. Mater. Interfaces 2018, 10, 22311−22319

Research Article

ACS Applied Materials & Interfaces substituted by terminated functional groups such as fluorine, hydroxyl, and oxygen groups, rendering the etched MXenes with high hydrophilic surface, tunable properties, and superior electrical conductivity, which provide the feasibility for engineering novel hybrid systems with wider flexibility.41−44 Additionally, comparing with the conventional 2D materials such as graphene, layered double hydroxides (LDHs), and transition-metal dichalcogenides, besides the high conductivity, MXene ultrathin nanosheets can give extra merits of large active surface area, fast charge-transfer kinetics, and strong interfacial coupling for the energy conversion process (e.g., artificial photosynthetic systems, electronic apparatus, the catalysis, and so on).45−47 On the basis of these premises, it is anticipated that coupling of MOF derivatives with conductive MXenes could boost the catalytic properties and circumvent weaknesses of the dilemma above. In this research, we report a MOF derivative/2D MXene hybrid by in situ nucleation and conversion of ZIF-67 (ZIF indicates zeolitic imidazolate framework) to hierarchical porous Ni−Co-mixed metal sulfides on exfoliated Ti3C2Tx [Tx represents surface terminal groups such as −(Fx) and −(OH)x] MXene sheets (denoted as NiCoS/Ti3C2Tx hereinafter). The unique hybrid structure and intimate interfacial interaction between the two components not only ensure favorable electrical conductance and enlarged surface area but also provide sufficient active sites and superior activity toward OERs. Specially, postcatalytic analysis reveals that the hybrid catalyst of NiCoS/Ti3C2Tx undergoes a chemical structure transformation during water oxidation, with the rise of valence states of metal ions and in situ formation of nickel cobalt oxyhydroxides/NiCoS (denoted as NiCoOOH−NiCoS) assembly on Ti3C2Tx sheets, where NiCoOOH on the surface serves as the intrinsic active species for OERs. Furthermore, as a proof-of-concept application, the rechargeable Zn−air battery fabricated by NiCoS/Ti3C2Tx exhibits a small charging/ discharging voltage gap and excellent operation durability. This work highlights the optimized structure and electrocatalytic performance of MOF derivatives by the versatility of MXenes and provides insight into the structure−activity relationship for noble metal-free catalysts for OERs.

Figure 1. (a) Scheme for the synthesis of Ti3C2Tx sheets. SEM images of (b) bulk Ti3AlC2, (c) accordion-like Ti3C2Tx, and (d) exfoliated Ti3C2Tx sheets. TEM (e), HRTEM (f), and AFM (g) images of Ti3C2Tx sheets.

nucleation and conversion of the ZIF-67 by MOF-based strategy. The zeta potential of the prepared Ti3C2Tx sheets is −19.9 mV (Figure S1), which is ascribed to the functional groups [e.g., −(Fx) and −(OH)x] on the surface. The negative charge on the surface of Ti3C2Tx sheets could absorb Co2+ ions by electrostatic interaction. Then, the surface-anchored Co2+ could coordinate with 2-methylimidazole molecules and form the ZIF-67/Ti3C2Tx hybrid. After reaction for 15 min, ZIF-67 crystals were uniformly coated on both sides of the Ti3C2Tx sheets (Figure 2b). When an appropriate amount of Ni(NO3)2 was reacted with the ZIF-67/Ti3C2Tx hybrid, Ni2+ ions will etch ZIF-67 and the Co2+/Co3+ ions would coprecipitate with Ni2+ ions to form Ni−Co-mixed LDH (Figure 2a), which was denoted as NiCo-LDH/Ti3C2Tx (the ratio of Ni and Co here does not indicate the stoichiometric ratio). A typical SEM image (Figure 2c) of the as-formed product presents hierarchical porous NiCo-LDH (inset Figure 2c) on Ti3C2Tx sheets, forming a unique hybrid structure. Energy-dispersive Xray elemental mapping images confirm that Ni, Co, Ti, C, O, and F are homogeneously distributed (Figure S2a−h), suggesting the successful synthesis of the hybrid. After the sulfuration, NiCo-LDH/Ti3C2Tx was converted to NiCoS/ Ti3C2Tx with good preservation of the hierarchical porous structure (Figure 2d and inset). The TEM image (Figure 2e) clearly demonstrates that NiCoS is strongly anchored on the Ti3C2Tx sheets and presents continuous macropores (inset of Figure 2e), which could greatly facilitate the electrolyte diffusion. The HRTEM image of NiCoS/Ti3C2Tx (Figure 2f) shows that NiCoS is assembled by NiCoS nanoparticles hierarchically, where the lattice constants of 0.34 and 0.24 nm can be corresponded to (220) and (400) planes of Ni3S4, respectively. A high-angle annular dark-field (HAADF) image (Figure 2g) and the associated energy-dispersive X-ray spectroscopy (EDXS) mapping images (Figure 2h−m) of NiCoS/Ti3C2Tx further confirm the spatial dispersion of the elements in the composite, where Ni, Co, and S are homogeneously coated on the Ti−C−O composite, suggesting the successful hybridization of Ti3C2Tx with NiCoS. In contrast, the etching and sulfuration treatments of pristine

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RESULTS AND DISCUSSION A two-step process was applied to prepare the few-layer MXene Ti3C2Tx sheets, as schematically illustrated in Figure 1a (see Experimental Section in the Supporting Information for details). The Al layers of the pristine bulk Ti3AlC2 were first selectively etched by HF to yield the accordion-like Ti3C2Tx, as displayed in the scanning electron microscopy (SEM) images in Figure 1b,c. Subsequently, the resulting Ti3C2Tx was further delaminated by dimethyl sulfoxide under a nitrogen atmosphere to give isolated Ti3C2Tx sheets (Figure 1d). The transmission electron microscopy (TEM) image of the asexfoliated Ti3C2Tx shows a lateral size of hundreds of nanometers with a very low contrast to the background, indicating its ultrathin thickness (Figure 1e). A high-resolution TEM (HRTEM) image taken on the edge of the exfoliated sheets presents few layers thick, revealing its ultrathin nature again (Figure 1f). In addition, the atomic force microscopy (AFM) image indicates that the exfoliated Ti3C2Tx sheet renders a height of 3.66 nm, which is only approximately four layers of Ti3C2Tx. To integrate with MOF derivatives, the obtained Ti3C2Tx sheets were used as a synthetic substrate for the in situ 22312

DOI: 10.1021/acsami.8b06272 ACS Appl. Mater. Interfaces 2018, 10, 22311−22319

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ACS Applied Materials & Interfaces

The X-ray diffraction (XRD) patterns were obtained for detailed structural characterization of the samples. The diffraction peak for the (104) planes of bulk Ti3AlC2 is absent in the XRD pattern of Ti3C2Tx sheets, suggesting the successful etching of the Al layers (Figure S4a). Moreover, the (002) diffraction peak of Ti3C2Tx sheets shifts to a lower angle in comparison with that of bulk Ti3AlC2, verifying the enlarged interlayer distance of the exfoliated MXene Ti3C2Tx sheets. Figure 3a shows the XRD patterns of ZIF-67/Ti3C2Tx and the derived products of NiCo-LDH/Ti3C2Tx and NiCoS/ Ti3C2Tx. Notably, the XRD pattern of ZIF-67/Ti3C2Tx presents a superimposition of the ZIF-67 phase and Ti3C2Tx phase (Figure S4a,b), indicating the effective combination of ZIF-67 and Ti3C2Tx. After being etched by Ni2+, the original peaks of ZIF-67 disappear and are replaced by the Ni(OH)2 phase, forming the NiCo-LDH/Ti3C2Tx hybrid. In the XRD pattern of NiCoS/Ti3C2Tx, the diffraction peaks for NiCoS can be indexed to the cubic structure of Ni3S4 (PDF no. 76-1813), while the shift of the diffraction peaks to a higher angle in comparison with that of Ni 3S4 verifies the successful substitution of partial Ni ions by Co ions.48,49 This is consistent with the result of the HRTEM image (Figure 2f), which shows clear (220) and (400) planes of Ni3S4. In Raman spectra (Figure S5), compared to the bulk Ti3AlC2, peaks I and II in the Raman spectrum of Ti3C2Tx sheets are vanished, whereas peak III is broadened and downshifted. Such downshifting in Raman spectra verifies the successful exfoliation of bulk to very thin sheets, which is in good agreement with the reported literature.50 Notably, the Raman spectra of NiCoS/Ti3C2Tx present peaks which originate from both Ti3C2Tx and NiCoS. Moreover, the peaks in the hybrid are shifted relative to those in the bare ones, suggesting the strong interaction of Ti3C2Tx and NiCoS (Figure 3b).32 Furthermore, the bare Ti3C2Tx, NiCoS, and NiCoS/Ti3C2Tx exhibit the same type IV nitrogen adsorption−desorption curves (Figure 3c) in nitrogen adsorption−desorption isotherms, indicating mesopores within them. Impressively,

Figure 2. (a) Scheme of the preparation process of NiCoS/Ti3C2Tx. SEM images of (b) ZIF-67/Ti3C2Tx, (c) NiCo-LDH/Ti3C2Tx, and (d) NiCoS/Ti3C2Tx. Inset: the high-magnification SEM images of the samples. (e) TEM (inset: high-magnification TEM image) and (f) HRTEM images of NiCoS/Ti3C2Tx. (g) HAADF and (h−m) EDXS images of NiCoS/Ti3C2Tx.

ZIF-67 crystals give agglomerated polyhedron particles, which is unfavorable for the accessibility of the electrolyte (Figure S3a−c). The TEM image confirms the hollow structure of the bare NiCoS with an even size of ∼200 nm, whose surface is constituted by nanosheets (Figure S3d). Therefore, above results indicate that Ti3C2Tx sheets can serve as a highly conducted matrix to homogeneously deposit and distribute hierarchical NiCoS.

Figure 3. (a) XRD patterns of ZIF-67/Ti3C2Tx and derived NiCo-LDH/Ti3C2Tx and NiCoS/Ti3C2Tx products. (b) Raman spectra and (c) N2 sorption isotherms (inset: pore size distribution) of Ti3C2Tx, NiCoS, and NiCoS/Ti3C2Tx. 22313

DOI: 10.1021/acsami.8b06272 ACS Appl. Mater. Interfaces 2018, 10, 22311−22319

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10 mA cm−2, which is even smaller than the commercial RuO2 (397 mV). Notably, the OER performance of the physical mixture of NiCoS and Ti3C2Tx (NiCoS + Ti3C2Tx) is much lower than that of NiCoS/Ti3C2Tx (Figure S6), implying the weak synergetic effect between physically mixed NiCoS and Ti3C2Tx. Furthermore, the Tafel slope of NiCoS/Ti3C2Tx is 58.2 mV dec−1 (Figure 4b), which is obviously smaller than that of NiCoS (73.7 mV dec−1), NiCo-LDH/Ti3C2Tx (153.1 mV dec−1), NiCo-LDH (85.0 mV dec−1), RuO2 (88.3 mV dec−1), and Ti3C2Tx (227.3 mV dec−1), revealing the higher OER rate and favorable kinetics of NiCoS/Ti3C2Tx. According to the inductively coupled plasma atomic emission spectrometry (ICP-AES, Figure S7), the content of Ni and Co in NiCoS/Ti3C2Tx is 44.59 and 21.22 wt %, respectively, showing that the molar ratio of Ni/Co ∼2. Thus, the turnover frequency (TOF) of NiCoS/Ti3C2Tx is calculated to be 0.009 and 0.036 s−1 at overpotentials of 350 and 400 mV, respectively, assuming that all the metal sites (Ni and Co) in the catalysts are active (Figure 4c). These values are much higher than those of the commercial RuO2 (0.007 and 0.017 s−1 at 350 and 400 mV overpotentials, respectively), further confirming the high electrocatalytic activity of NiCoS/Ti3C2Tx hybrid. Note that these TOF values in our case are conservatively estimated because not all the metal sites are electrochemically accessible. Furthermore, the durability of the catalysts was measured under a constant current density of 10 mA cm−2. As shown in the chronopotentiometry curves (Figure 4d), the required potential of NiCoS/Ti3C2Tx remains nearly constant under the continuous electrolysis reaction for ∼6 h, which is superior to NiCoS, NiCo-LDH/Ti3C2Tx, NiCoLDH, and commercial RuO2, confirming the excellent stability of NiCoS/Ti3C2Tx. Actually, the NiCoS/Ti3C2Tx hybrid can be comparable to or even better than the most Ni−Co-based OER catalysts (Table S1). To understand the high catalytic performance of the NiCoS/ Ti3C2Tx, electrochemically active surface area (ECSA) was

NiCoS/Ti3C2Tx gives a Brunauer−Emmett−Teller (BET) surface area of about 55 m2 g−1 with a pore volume of 0.08 m3 g−1 (Table 1), much higher than those of the bare Ti3C2Tx Table 1. Physical Properties of the As-Prepared Samples samples

SBET (m2 g−1)

Vpore (m3 g−1)

dpore (nm)

NiCoS Ti3C2Tx NiCoS/Ti3C2Tx

8.0 15 55

0.02 0.04 0.08

6.1 9.9 7.9

(15 m2 g−1, 0.04 m3 g−1) and NiCoS (8 m2 g−1, 0.02 m3 g−1) counterparts. Additionally, the pore size distribution curves confirm that NiCoS/Ti3C2Tx possesses hierarchical pores containing mesopores (pore size ca. 3 nm) and macropores (pore size ca. 70 nm) simultaneously (inset of Figure 3c). Such hierarchical porosity of NiCoS/Ti3C2Tx with high surface area and pore volume can provide abundant active sites on the surface and is favorable for the accessibility of the electrolyte, thus enhancing the catalytic performance. The electrochemical characterizations were conducted on a typical three-electrode cell in 1.0 M KOH (pH 13.6) electrolyte with 0.21 mg cm−2 catalysts loading on glassy carbon electrodes. Before the electrochemical test, the electrodes underwent an activation treatment by several linear sweep voltammetry (LSV) until a stable curve was achieved, unless noted otherwise. Figure 4a presents the iR-corrected LSV curves of all the samples at a scan rate of 5 mV s−1. Apparently, Ti3C2Tx shows no OER activity. While, compared with the bare NiCo-LDH and NiCoS, both NiCo-LDH/ Ti3C2Tx and NiCoS/Ti3C2Tx hybrids exhibit enhanced catalytic activities with larger current densities and lower onset potentials, suggesting the positive effects of the Ti3C2Tx matrix to OER activity. Among these five electrocatalysts, NiCoS/Ti3C2Tx provides the best catalytic activity with a small overpotential (η) of 365 mV to deliver the current density of

Figure 4. (a) LSV curves and (b) Tafel plots of NiCoS/Ti3C2Tx, NiCoS, NiCo-LDH/Ti3C2Tx, NiCo-LDH, and RuO2. (c) TOFs at η = 350 and 400 mV and (d) chronopotentiometry curves at 10 mA cm−2 of NiCoS/Ti3C2Tx, NiCoS, NiCo-LDH/Ti3C2Tx, NiCo-LDH, and RuO2. (e) ΔJ (=Ja − Jc) plotted scan rates and (f) Nyquist plots with the bias of η 350 mV for different catalysts. 22314

DOI: 10.1021/acsami.8b06272 ACS Appl. Mater. Interfaces 2018, 10, 22311−22319

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Figure 5. High-resolution XPS spectrum of (a) Ni 2p, (b) Co 2p, and (c) O 1s for NiCo-LDH/Ti3C2Tx, NiCoS/Ti3C2Tx, and NiCoS/Ti3C2Tx after the OER.

Figure 6. (a) SEM, (b) TEM, (c) HRTEM, and (d−j) HAADF and EDXS images of NiCoS/Ti3C2Tx after the OER.

of NiCoS/Ti3C2Tx before and after the OER were analyzed and compared (Figures S10a and 5). In Ni 2p XPS spectra for NiCoS/Ti3C2Tx before the OER, two peaks at 853.28 and 870.55 eV could be assigned to the Ni−S bond,52,53 signifying the formation of nickel sulfide. Impressively, the Ni 2p XPS spectrum of NiCoS/Ti3C2Tx after the OER test shows no Ni− S bond and the pair peaks of Ni 2p3/2 and Ni 2p1/2 shift to 855.5 and 873.1 eV, respectively, which is ascribed to the oxidized Ni species (Ni−O bond), demonstrating the surface oxidation of NiCoS/Ti3C2Tx.54 Moreover, the Ni 2p spectra of NiCoS/Ti3C2Tx after the OER apparently shift to higher binding energies compared with the as-prepared NiCo-LDH/ Ti3C2Tx before the OER, suggesting its higher valence state of Ni, which is highly valuable for enhancement of the electrocatalytic activity. Similarly, the Co 2p spectra show the formation of the Co−S bond in NiCoS/Ti3C2Tx before the OER and a higher valence state of Co is achieved after the OER (Figure 5b). Furthermore, the O 1s spectrum of NiCoLDH/Ti3C2Tx before the OER can be deconvoluted into two species (Figure 5c): O2− at 529.2 eV and OH− at 531.0 eV.55,56 While, the chemical state of O 1s in NiCoS/Ti3C2Tx is mainly attributed to the adsorbed H2O (H2Oads, 532 eV).57,58 After the OER, the S signal on the surface of NiCoS/Ti3C2Tx disappeared and is substituted by OH−, O2−, and H2Oads instead (Figures 5c and S10b), which reveals the formation of nickel−cobalt oxides/hydroxides(very likely nickel cobalt oxyhydroxide, denoted as NiCoOOH). Therefore, the XPS analysis manifests that the surface of NiCoS/Ti3C2Tx under-

estimated for the catalysts for OERs. Normally, ECSA shows a positive correlation with the electrochemical double-layer capacitance (Cdl). Therefore, to evaluate the ECSA, Cdl is computed by the cyclic voltammetry (CV) curves under different scan rates in a proper potential range (Figure S8a−f). NiCoS/Ti3C2Tx exhibits much higher Cdl (2.4 mF cm−2) than NiCoS (1.2 mF cm−2), NiCo-LDH/Ti3C2Tx (0.58 mF cm−2), NiCo-LDH (0.27 mF cm−2), RuO2 (1.8 mF cm−2), and Ti3C2Tx (0.21 mF cm−2) (Figure 4e), suggesting more active sites of NiCoS/Ti3C2Tx. Furthermore, electrochemical impedance spectroscopy measurement is performed to analyze the interfacial resistance for the electrocatalysts. Distinguished semicircles can be observed in Nyquist plots (Figure 4f) of the electrocatalyst, which should be ascribed to the charge-transfer resistance (Rct) at the interface of the catalyst/electrolyte.51 NiCoS/Ti3C2Tx presents the smallest Rct, suggesting an optimized charge-transfer capacity during the OER process. This result is in good agreement with the Bode modulus plots (Figure S9), in which NiCoS/Ti3C2Tx shows the lowest resistance in various frequencies among our four electrodes. High conductivity and fast charge transfer of NiCoS/Ti3C2Tx must be another important contributor to its enhanced water oxidation performance. The aforementioned results validate that the electrocatalytic activity of bare NiCo-LDH can be significantly tailored by the corporation with Ti3C2Tx and sulfuration treatment. To get insight into the activity−structure relationship of NiCoS/ Ti3C2Tx, the X-ray photoelectron spectroscopy (XPS) spectra 22315

DOI: 10.1021/acsami.8b06272 ACS Appl. Mater. Interfaces 2018, 10, 22311−22319

Research Article

ACS Applied Materials & Interfaces

Figure 7. (a) Scheme of a homemade rechargeable Zn−air battery. (b) Charging and discharging polarization curves of the rechargeable Zn−air batteries. (c) Cycling performance at the current density of 10 mA cm−2.

On the basis of the promising OER performance, a proof-ofconcept test was carried out to evaluate the feasibility of NiCoS/Ti3C2Tx in a rechargeable Zn−air battery. As shown in Figure 7a, a homemade Zn−air battery device was assembled with a zinc plate as the anode, NiCoS/Ti3C2Tx coupled with Pt/C (the mass ratio of NiCoS/Ti3C2Tx:Pt/C was 1:1) loaded on a nickel foam (mass loading: 2 mg cm−2) as the cathode, and 6 M KOH with 0.2 M zinc acetate as the electrolyte. For compassion, the Pt/C and RuO2 mixed catalyst with the same proportion was utilized as the rechargeable noble metal−air cathode. The battery fabricated from NiCoS/Ti3C2Tx + Pt/C can work with a high open circuit of 1.43 V (Figure S13a), even slightly superior to that of the RuO2 + Pt/C air cathode (1.4 V). Figure 7b shows the charging and discharging polarization curves. Impressively, when the current density is raised to 50 mA cm−2, the voltage gap of charging (1.85 V) and discharging (1.15 V) for NiCoS/Ti3C2Tx + Pt/C-based battery is 0.7 V, which was 0.14 V lower than that of the noble metal− air cathode-based one, demonstrating a low voltage even at higher current density for NiCoS/Ti3C2Tx + Pt/C. Furthermore, the cycle discharging/charging measurement is recorded at 10 mA cm−2 to assess the stability of the constructed Zn−air battery. After 8 h galvanostatic charge−discharge test, NiCoS/ Ti3C2Tx + Pt/C shows virtually negligible voltage fading (Figure 7c), demonstrating the excellent stability of the battery, identical to RuO2 + Pt/C. By merely charging the NiCoS/Ti3C2Tx + Pt/C-based Zn−air battery for 5 min, the device can power a red light-emitting diode array (Figure S13b), implying its highly competitive potential for renewable energy applications.

goes an irreversible chemical structure transformation process during the OER, and the resulting oxides/hydroxides act as the intrinsic catalytically active species to afford water oxidation. The SEM and TEM images of NiCoS/Ti3C2Tx after the OER show that the nanosheets of NiCoS are transformed into a number of nanoparticles anchored on the surface of Ti3C2Tx (Figure 6a,b). The corresponding interplanar spacing of 0.24 and 0.14 nm is referred to (101) and (110) facets of NiOOH (Figure 6c). Clearly, the HAADF-scanning transmission electron microscopy (STEM) and EDXS mapping images demonstrate the uniform spatial distribution of Ni, Co, Ti, C, O, and only a trace of S elements, revealing the transformation of NiCoS to nickel cobalt oxides/hydroxides (Figure 6d−j). This is another strong evidence for the in situ irreversibly chemical structure transformation of the catalyst during the OER process, consistent with other reports. On the other hand, the post-OER NiCoS still maintains the initial hollow structure when the nanosheets are converted into nanoparticles (Figure S11a−c). Overall, by integrating the analyses of XPS and electron microscopy above, it is reasonably concluded that the initial NiCoS in the hybrid is partially converted to NiCoOOH on Ti3C2Tx irreversibly, which is the true intrinsic active phase to afford OER measurement. Interestingly, a pioneer work has verified that such electrochemical transformation during water oxidation can take place not only in sulfide catalysts but also in the MOFs.59 Inspired by these discoveries, the OER performance of ZIF-67/Ti3C2Tx and ZIF-67 is preliminarily studied (Figure S12). The overpotentials to achieve 10 mA cm−2 for ZIF-67/Ti3C2Tx and ZIF-67 are 387 and 416 mV, respectively, again, confirming the merit of Ti3C2Tx for the OER. The obtained OER performance of ZIF-67 and ZIF-67/Ti3C2Tx is very likely derived from the water-oxidized Co hydroxide/oxyhydroxides, rather than ZIF-67 itself. The detailed transformation of ZIF-67 in OER is in progress.

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CONCLUSIONS In summary, hierarchical porous NiCoS strongly coupled with exfoliated Ti3C2Tx MXene sheets was realized via the MOFbased strategy. The excellent activity and stability of the NiCoS/Ti3C2Tx hybrid in both OERs and Zn−air batteries are 22316

DOI: 10.1021/acsami.8b06272 ACS Appl. Mater. Interfaces 2018, 10, 22311−22319

Research Article

ACS Applied Materials & Interfaces

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comparable to those of noble-metal-based catalysts. The outstanding activity of the NiCoS/Ti3C2Tx hybrid is mainly attributed to the strong interaction between NiCoS and Ti3C2Tx, giving high conductivity for the efficient electron transport and tailored composite structure for more active sites by the Ti3C2Tx matrix. Specifically, morphological and chemical conversions are confirmed by postcatalytic analysis, where NiCoS on Ti3C2Tx is transformed to NiCoOOH-active phase on the parent catalyst surface to afford water oxidation. This study highlights the tuned structure and electrocatalytic performance of MOF derivatives by the versatility of MXenes and provides insight into the structure−activity relationship for noble metal-free catalysts in a wide range of energy-related applications.

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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/acsami.8b06272. Experimental section; structure characterizations; electrochemical measurements; zeta potential; EDXS; SEM; XRD; Raman spectra; ICP-AES; CV curves; Bode modulus plots; open-circuit plots and photograph of the rechargeable Zn−air batteries; XPS spectra; TEM; and performance comparison table (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Phone: 0086-27-87871029. Fax: 0086-27-87879468 (J.Y.). *E-mail: [email protected] (K.F.). ORCID

Jiaguo Yu: 0000-0001-9308-2882 Ke Fan: 0000-0003-2269-4042 Author Contributions §

H.Z. and B.H. contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (51320105001, 21433007, 51772234, and U1705251), the Natural Science Foundation of Hubei Province (2015CFA001), the Innovative Research Funds of SKLWUT (2017-ZD-4), and the Fundamental Research Funds for the Central Universities (WUT: 2017 IVA 092).

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REFERENCES

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DOI: 10.1021/acsami.8b06272 ACS Appl. Mater. Interfaces 2018, 10, 22311−22319

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ACS Applied Materials & Interfaces

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DOI: 10.1021/acsami.8b06272 ACS Appl. Mater. Interfaces 2018, 10, 22311−22319