Mixed-MetalâOrganic Framework Self-Template Synthesis of Porous

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Mixed-Metal-Organic Framework Self-Template Synthesis of Porous Hybrid Oxyphosphide for Efficient Oxygen Evolution Reaction Dickson D. Babu, Yiyin Huang, Ganesan Anandhababu, Muhammad Arsalan Ghausi, and Yaobing Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b13359 • Publication Date (Web): 13 Oct 2017 Downloaded from http://pubs.acs.org on October 13, 2017

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ACS Applied Materials & Interfaces

Mixed-Metal-Organic

Framework

Self-Template

Synthesis of Porous Hybrid Oxyphosphide for Efficient Oxygen Evolution Reaction Dickson D. Babu ‡, Yiyin Huang ‡, Ganesan Anandhababu, Muhammad Arsalan Ghausi and Yaobing Wang* Key Laboratory of Design and Assembly of Functional Nanostructures, Fujian Provincial Key Laboratory of Nanomaterials, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, YangQiao West Road 155#, Fuzhou, Fujian 350002, P. R. China.

ABSTRACT: Developing efficient, stable yet cost-effective electrocatalyst is the key link along the path to hydrogen fuel produced by water splitting. The current bottleneck in water electrolysis technology is the sluggish oxygen-evolving reaction (OER) which is also central to the rechargeable metal–air batteries. Herein, we report a promising mixed-metal–organic framework (MMOF) self-template strategy to synthesize CoFe hybrid oxyphosphide with highly porous morphology. Aided by the porous hybrid bulk structure beneficial to fast ion diffusion to abundant highly active sites, the as-synthesized Co3FePxO exhibited excellent electrocatalytic activity towards OER, with an overpotential of 291 mV at 10 mA cm-2 and a low Tafel slope of 85 mV dec-1. With the underpinnings of MMOF endowing the structural rigidity and stability, the material also showed long life for OER without detectable activity decay.

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KEYWORDS:

Mixed-metal-organic

framework,

Self-template,

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Porous

structure,

Oxyphosphide, Oxygen Evolution INTRODUCTION The ever-increasing energy demand to satisfy the development of modern society and the growing concerns about environmental pollution as well as climate change have rendered it imperative to seek for alternatives of fossil-fuels.1-4 A typical sustainable and clean fuel is hydrogen, usually generated by two electron-transfer from electrochemical water splitting.5-7 However, the other half-reaction of water splitting is oxygen evolution reaction (OER) with four electron-transfer, which is kinetically sluggish and generally requires a large overpotential to overcome the kinetic barrier.8-12 Therefore, water splitting reaction is typically performed in either strongly acidic or alkaline solution to minimize the kinetic barrier. As a result of the fact that acidic OER is plagued by the limitation of using acid-insoluble high-cost noble metal catalysts such as RuO2 and IrO2,13-17 alkaline OER based on non-noble metal materials have emerged as a credible alternative. Many transition metal materials and their hydroxides, oxides, borides and chalcogenides have been exploited over the past years towards efficient OER.18,19 Among these transition metals, cobalt- and iron-based OER catalysts stand out, thanks to their earth-abundance, environmental benignity, variable valence states and high theoretical catalytic activity,20-23 whereas the critical issues related to their low intrinsic conductivity and relatively easy self-agglomeration still remain unresolved, leading to the high overpotential and decreased reaction rate during OER.24 Numerous works reported lately suggest that, the key to enhance electron mobility of materials is by doping with other elements, including both metal


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and nonmetal.25 For example, either amalgamation of Co and Fe materials in a hybrid,26 or oxyphosphorization of metal materials,25 could ameliorate OER activity. The origin of improved electrocatalytic performance was revealed to be the improved charge transfer, together with modification of electronic structures to lower the kinetic energy barriers for favourably bonding to the adsorbates. Such being the case, it is fair to infer that mixed metal oxyphosphides could be more efficient for catalyzing OER as compared to their counterparts of monometal oxyphosphide.27-29 Another aforesaid issue of self-agglomeration can also be mitigated by fabrication of bulk materials with porous structure. An ideal template for synthesizing such structure is metal– organic frameworks (MOFs), which are composed of metal ions or clusters linked by organic ligands to aﬀord crystalline structures with features of diversified pores, highly ordered structures and unprecedented surface areas.30-33 Note that different metals can simultaneously combine with one kind of ligand to generate mixed-metal–organic frameworks (MMOFs), which provide the basis of fabrication of integral porous mixed metal oxyphosphide. The route from MMOFs to integral porous hybrid oxyphosphide thus brings about two merits: one is integral porous sturcture in favour of expediting the diffusion of oxygen gas bubbles whereas keeping structural rigidity and stability; the other is multicomponent incorporation for inducing strong electron coupling to enhance electron transfer and electro-catalytic reactions. Both factors are expected to expedite OER. Herein, we report a promising strategy to integrate the advantages of multi-atom doping and porous rigid integral structure. An open framework consisting of CoFe mixed-metal–organic framework (Co3Fe MMOF) with nanodiamond morphology was synthesized from a mixed solution of Co2+, Fe3+ and p-benzene dicarboxylic acid (BDC). Later, the nanodiamond MMOFs


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was subjected to phosphorylation by treating with NaH2PO2 and annealing in air to give CoFe oxyphosphide (Co3FePxO). It required an overpotential of only 291 mV for reaching a current density of 10 mA cm−2, with a Tafel slope as low as 85 mV dec-1 for OER. The porous hybrid bulk structure beneficial to fast ion diffusion to abundant highly active sites with probably favorable O* adsorption energy was revealed. Based on this, the MMOF self-template synthesis towards hybrid oxyphosphide brings up a general strategy for exploring efficient electrocatalysts towards extensive application in renewable energy technologies

RESULTS AND DISCUSSION Figure 1 depicts the schematic illustration of the synthesis of metal-oxyphosphides. The highly porous Co3FePxO was prepared by following a three-step protocol involving the hydrothermal synthesis of Co3Fe MMOF using p-Benzenedicarboxylic acid as ligand, followed by the phosphorylation of the as obtained MMOF with NaH2PO2 at 350 oC for 4 hours to obtained Co3FePx, which was further subjected to annealing at 550 oC for 5 hours to obtain the mixed metal oxyphosphides


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Figure 1. Schematic illustration of the synthesis of metal-oxyphosphides Figure 2a depicts the X-ray diffraction (XRD) patterns of Co3Fe MMOF and Co3FePx. The XRD pattern of Co3FePx shows a series of diffraction peaks at 24.0ᵒ and 36.0ᵒ, 38.6ᵒ, 47.1ᵒ, 51.6ᵒ, 53.7ᵒ and 54.9ᵒ which were indexed to the (-121), (310), (150), (400), (-171), (-114) and (124) planes of CoP and FeP4 phases with JCPDS 20-0336 and JCPDS 27-1171, respectively. Further, the relatively weak and broad diffraction peaks centred at 24.5ᵒ also provides an evidence for ultrafine particle size. All the aforesaid observations confirm the successful conversion of Co3Fe MMOF into Co3FePx. The metal phosphide-related peaks turn unconspicuous after annealing in air, suggesting elimination of part of metal phosphides by oxygen incorporation. In addition, the XRD plots of all the remaining samples are incorporated in the SI (Figure S1).


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. Figure 2. (a) PXRD patterns of Pristine Co3Fe MOF, Co3FePx and Co3FePxO, (b) TEM images of Co3Fe MOF nanodiamonds. Morphology of the as prepared Co3Fe MMOF as well as the phosphatized materials (Co3FePx) and the corresponding oxyphosphides was obtained using a JEOL-6700F scan electron microscopy (SEM). The typical SEM images of the as-formed Co3Fe MMOF depict the nanodiamond morphology (Figure S2). Further, Figure 2b depicts the morphology of Co3Fe MOF. After phosphorylation and the subsequent annealing, the nanodiamond morphology of the MMOF precursor is transformed into a highly integral porous framework, while, a distinct phosphorous coating can be evidenced from the Figure 3c. Furthermore, Figure S3 and S4 illustrate the porous nature of Co3FePx and Co3FePxO. In addition, the existence of Co, Fe, C, P, and O elements on the as-prepared porous framework was also vindicated by the EDS mapping as well as energy dispersive X-ray (EDX) spectroscopy analysis (Figure S5 and S6). The signal for O can be attributed to the incorporation of oxygen in the sample during annealing in air, while, surface phosphate species on metal phosphides may


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Figure. 3 (a) TEM images of Co3FePx, (inset shows Co3Fe MOF nanodiamonds), (b) TEM of Co3FePxO, (c) TEM of Co3FePx (red dotted line indicates phosphorous coating layer over Co3Fe MOF), (d-e) HRTEM images of different lattice spacing of Co3FePxO, (f) SAED pattern for Co3FePxO (g) HAADF of Co3FePxO (h-k) EDS mapping of Co, Fe, O and P of Co3FePxO.


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also be formed. Similar morphologies were obtained for all the remaining MOFs and their corresponding phosphatized materials (Figure S7 and S8). Interestingly, Co MOF revealed stacked nano-needle morphology, while pertaining CoPxO displayed highly porous structure (Figure S9). The transmission electron microscopy (TEM) images of the Co3Fe MMOF nanodiamonds and Co3FePx are depicted in Figure S10 and Figure 3a; the morphology of the pertaining oxyphosphides is presented in Figure 3b. From Figure 3c, it is clearly evident that the treatment with NaH2PO2 results in a uniform phosphorous coating on the material. Additionally, HRTEM images of Co3FePxO illustrate the lattice fringes with d spacing of 0.24 nm which corresponds to the (310) plane of the CoP and with d spacing of 0.17 nm which can be ascribed to the (-114) plane of FeP4, respectively (Figure 3d-e).34 Furthermore, the corresponding selective area electron diffraction (SAED) pattern of the sample can be indexed to the (310), (400) and (420) planes of CoP (Figure 3f). The high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image (Figure 3g) of Co3FePxO confirms its distinct feature of porous architecture. The EDS elemental mapping images of Co3FePxO (Figure 3h-k) confirm the even distribution of Co, Fe, P and O elements throughout the whole material. X-ray photoelectron spectroscopy (XPS) measurements were undertaken to probe the chemical nature as well as the composition of the surface (Figure 4). The full survey spectra for Co3FePxO, Fe3CoPxO, CoPx and FePx demonstrate the presence of Co, Fe, P, C and O elements as shown in Figure S11. The deconvolution of high resolution Co 2p spectrum exhibits predominant peaks at 781.8 eV and 798.1 eV in Co3FePxO hybrid.35 The spectrum in the Fe 2p region exhibits two spin orbit doublet (712.1, 725.5 eV) and (716.3, 729.7 eV ). The first doublet


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is assigned to the binding energy of Fe in Co3FePxO while the second doublet at higher binding energies is attributed to Fe in Fe-O species caused by the surface oxidation of Co3FePx.36 It is well-known that the binding energies of phosphorus 2p3/2 and metal satellite features are exceedingly sensitive to the variations in charge of a given atom and local environment, including the adjacent atoms. The deconvoluted P 2p spectrum shows a doublet located around 129 eV, attributed to binding energy of metal phosphide in Co3FePxO.

Figure 4. High-magnification XPS spectra (a) Co2p region of Co3FePxO, (b) Fe 2p region of Co3FePxO, (c) P 2p region of Co3FePxO and (d) O 1s region of Co3FePxO.


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An additional peaks observed in P region at about 133 eV represent oxidized P species due to surface oxidation of Co3FePx.37 The incorporation of oxygen in the material was also confirmed by Figure 4d, which probably strengthen the O* adsorption and thus enhance the activity on active sites.38 Besides, Co 2p and Fe 2p peaks positively shifted and P 2p peak negatively shifted. These shifts are mainly due to the effect of partial electron transfer. The negative shift of P 2p3/2 binding energy reveals the increased electron occupation, resulting in enhanced electron-donating ability. The positive shifts of Co and Fe 2p3/2 indicate enhanced electron transfer capability.39 Further, during the electrocatalytic processes Co and Fe centres with positive shifts and P centres with negative shift function as hydride-acceptor and protonacceptor sites, respectively. Thus the electrons are diffused from metallic Co and Fe centres to P, beneficial to the adsorption and desorption processes of reactant and product molecules, respectively.40 Additionally, the enlarged difference could lead to increased local electric dipole, thus reducing the energy barrier of the electrocatalytic process, and thereby subsidising the overall catalytic reaction. All these results substantiate the formation of Co3FePxO by oxyphosphorization reaction between Co3Fe MMOF and PH3 gas derived from the thermal decomposition of NaH2PO2, followed by annealing in air. The electrocatalytic activity of the as-synthesized metal oxyphosphides (Co3FePxO, CoPxO, FePxO and Fe3CoPxO) for OER in 1M KOH is investigated using a typical threeelectrode setup. Figure 5a shows the corresponding polarization curves with IR correction. It can be seen that the Co3FePxO displays the lowest potential of 291 mV at 10 mA cm-2 with highest current density in the test potential range for OER, hence placing it among the top tier of OER catalysts. In contrast, CoPxO and Fe3CoPxO require overpotentials of 350 and 430 mV, while FePxO displays an overpotential of 501 mV to drive the same current density.


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Figure 5. (a) LSV curves for Co3FePxO, CoPxO, FePxO, Fe3CoPxO and commercial Ir/C in 1 M KOH solution with IR corrections. (b) Corresponding Tafel plots in 1 M KOH solution. (c) Electrochemical double-layer capacitance measurements (difference in current (∆j = (ja − jc)) plotted as a function of scan rate), (d) Overpotential comparison between Co3FePxO and different metal phosphide based catalysts, (e) Rotating ring disk electrode voltammogram obtained for Co3FePxO in 1 M KOH, (f) Charge potential curves of Zn-air batteries at a current density of 10 mA cm-2 with different cathode catalysts. (Inset- A fabricated Zn-ar battery).


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Furthermore, the aforesaid overpotential value stacks up favourably to many other recently reported metal-phosphide-based high-performance OER catalysts (Table S1 and Figure 5d). Besides, we also compared the OER performance of the aforementioned metal oxyphosphides with their corresponding phosphides. As evidenced from Figure S12, the oxyphosphides outperform the analogous phosphates by 40-70 mV, thereby vindicating the employment of annealing of metal phosphides to give metal oxyphosphides. The highly porous features of these oxyphosphides warrant fast mass transport along the 2D basal planes and large number of highly exposed active sites, ergo, ameliorating the catalytic efficient. Tafel plots can give a deeper insight into the kinetics of electrocatalytic OER, 41-43 ergo, the polarization curves were then fitted to Tafel equation (η= blogj + a, where b is the Tafel slope) and the pertaining plots are presented in Figure 5b. From the classical Butler–Volmer relationship, a Tafel slope close to 60 mV dec-1 attributes that, the first electron transfer is the rate-limiting step, whereas, a Tafel slope of around 40 mV dec-1 implies that the second electron transfer is the rate-determining step. As evidenced from the figure, Co3FePxO displays the lowest Tafel slope of 85 mV dec−1 compared to CoPxO (137 mV dec−1) and Fe3CoPxO (143 mV dec-1), whereas FePxO (165 mV dec−1) exhibits the highest slope value. Interestingly, the Tafel slope for the standard Ir/C (81 mV dec-1) is very close to the slope of Co3FePxO (85 mV dec-1), implying that the OER kinetics in the two materials are quite comparable. From all the aforementioned values it is evident that not only Co3FePxO sample has a favourable OER kinetics but also there are distinct mechanistic differences between the samples. The Brunauer-Emmett-Teller (BET) specific surface area of the Co3FePxO was characterized by employing the nitrogen adsorption-desorption isotherm techniques (Figure S13). Additionally, the nitrogen adsorption–desorption isotherms of the Co3Fe oxyphosphide


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electrocatalyst exhibited a hysteresis loop in the medium pressure region, (P/Po = 0.9) which is a typical characteristic of mesopores and/or the neck sections of macropores. The shape of the isotherms depicts small hysteresis, which is consistent with a wide distribution of pore size, spanning the microporous (

Mixed-MetalâOrganic Framework Self-Template Synthesis of Porous

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