Molybdenum Phosphide and Oxide Heterostructures

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Energy, Environmental, and Catalysis Applications

Cobalt/Molybdenum Phosphides and Oxides Heterostructures Encapsulated in N-doped Carbon Nanocomposite for Overall Water Splitting in Alkaline Media Lei Yu, Yun Xiao, Chenglong Luan, Juntao Yang, Hongyan Qiao, Yao Wang, Xin Zhang, Xiaoping Dai, Yang Yang, and Huihui Zhao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b15653 • Publication Date (Web): 17 Jan 2019 Downloaded from http://pubs.acs.org on January 17, 2019

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ACS Applied Materials & Interfaces

Cobalt/Molybdenum Phosphides and Oxides Heterostructures Encapsulated in N-doped Carbon Nanocomposite for Overall Water Splitting in Alkaline Media Lei Yu, [a] Yun Xiao, [a] Chenglong Luan, [a] Juntao Yang, [a] Hongyan Qiao, [a] Yao Wang, [a] Xin Zhang,*[a] Xiaoping Dai, [a] Yang Yang, [a] and Huihui Zhao[a]

[a]

State Key Laboratory of Heavy Oil Processing, College of Chemical Engineering, China University of Petroleum Beijing 102249 (China). E-mail: [email protected]

Keywords: metal phosphides, overall water splitting, N-doped Carbon, nanowires, synergistic effects

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Abstract: The development of designing and searching inexpensive electrocatalysts with highly activity for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is significant to enable water splitting as a future renewable energy source. Herein, we synthesis a new CoP(MoP)-CoMoO3 heterostructure coated by Ndoped carbon shell (CoP(MoP)-CoMoO3@CN) via thermal decomposing and phosphatizing the CoMoO4∙0.9H2O nanowires encapsulated in N-doped carbon. At 10 mA∙cm−2,

this

CoP(MoP)-CoMoO3@CN

nanocomposite

exhibits

superior

electrocatalytic activity of low overpotentials of 296 mV for OER and 198 mV for HER in alkaline media. More importantly, we achieve a current density of 10 mA∙cm−2 at 1.55 V by using this CoP(MoP)-CoMoO3@CN as both cathode and anode for overall water splitting. This promising performance could be due to the high activity of CoP(MoP)-CoMoO3 and the good conductivity of external mesoporous N-carbon shell, which makes the CoP(MoP)-CoMoO3@CN nanowires as a competitive alternative to noble metal based catalysts for water splitting.

Introduction Splitting water into hydrogen and oxygen has drawn tremendous research interest as an appealing solution to obtain sustainable and renewable energy.1-4 The reaction of water splitting electrolysis consists of the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER), both of which are crucial for the overall efficiency of water splitting.5 Currently, due to the superior catalytic activity, Ru-(Ir-) oxides and Pt have been considered as the benchmark catalysts


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for OER and HER.6-9 However, the widespread industrial deployment of these precious metal catalysts is largely limited by their cost and scarcity.10 To overcome these obstacles, many efforts have been focusing on the design of the electrocatalysts based on earth-abundant materials such as transition metal sulfides/phosphides/selenides/nitrides/borides/carbides for the HER11,

12and

transition metal oxides/hydroxides for the OER.13, 14 But different catalysts for separate OER and HER probably need different preparation processes which could increase the cost.15-18 In addition, because of the incompatibility of the activity and stability for the same catalyst system in a specific pH region, it is still difficult to pair the catalysts for OER and HER together in the same electrolyte.19 Thus, developing an electrocatalyst for both OER and HER is still a challenge.9, 20-22 To date, transition-metal phosphides have shown attractive activity for OER and HER due to the phosphorus in catalysts can moderate bonding to the reaction intermediates, which not only can create a surface with proton acceptor and hydride acceptor sites,23-27 but also promote the formation of oxides and peroxides.5, 23, 28 However, there are also some disadvantages of these phosphides such as uncontrolled agglomeration, poor conductivity, and easier oxidation.23 Meanwhile, we noticed that the bimetallic active sites exist synergistic cooperation effect and play a significant role in the electrochemical performance, such as Ni/MoxC,29 CoxMoy@NC,30 CuCo2S4,31 etc.. Our team have previously reported that there is synergistic effect in double active metal-Co and Ni which



exhibits good electrocatalytic activity compared to single active metal for HER and OER.32 Hence, we controllably integrated a CoP(MoP) and CoMoO3 heterojunction-like nanowires (NWs) embedded in N-doped carbon (CN) wall. The nanowires morphology of CoP(MoP)-CoMoO3@CN can afford a mass of uniformly distributed high active sites. The external nanosized mesoporosity Ncarbon wall can enhance the mechanical strength of the nanowires and allow fast and efficient electron transfer. Because of the remarkable structural characteristics, CoP(MoP)-CoMoO3@CN not only exhibits the excellent OER performance, but also demonstrates a high activity for cathodic HER counterpart with

robust

durability.

Additionally,

this

CoP(MoP)-CoMoO3@CN

nanocomposite enables a high activity and good stability for overall water splitting in alkaline media.

Results and Discussion As schematically shown in Scheme 1, we first developed a synthetic methodology to fabricate CoMoO4∙0.9H2O NWs by adding the cobaltous nitrate hexahydrate and sodium molybdate into deionized water/ethylene glycol solution (in the following discussion it is named as CoMoO4 for convenience). The resulting CoMoO4 NWs was then coated by polybenzoxazine (PB) and formed the CoMoO4@PB by the polymerization of the PB precursors (C2H8N2, HCHO, C6H6O2). After that, the obtained CoMoO4@PB was converted into CoP(MoP)-CoMoO3@CN NWs during a two-step calcination method: first,


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CoMo-CoMoO3@CN NWs were fabricated at 600 °C under a N2 flowing, affording partial reduction of CoMoO4@PB; subsequently, the CoMoCoMoO3@CN NWs were converted into ternary CoP(MoP)-CoMoO3@CN NWs via a simple solid state reaction with NaH2PO2 at 400 oC under the protection of a N2 flowing. The

X-ray

diffraction

(XRD)

of

CoMo-CoMoO3@CN,

CoP(MoP)-

CoMoO3@CN and CoMoO4 are shown in Figure S1. The XRD pattern of the asprepared CoMoO4 can be well indexed to the well-crystalline CoMoO4 (JCPDS No.14-0086). For CoMo-CoMoO3@CN composite, the peaks at 2-theta values of 35.7°, 36.6°, 52.1° and 62.3° can be assigned to (200), (004), (204) and (303) planes of CoMoO3 (JCPDS No. 21-0869), and the diffraction peaks at 40.5°, 44.2°, 43.0° and 40.5° in the XRD pattern of CoMo-CoMoO3@CN belong to the diffraction from (110) planes of Mo (JCPDS No. 42−1120), (111) planes of Co (JCPDS No. 15-0806), (111) and (021) planes of Co2C (JCPDS No. 50-1371), respectively. This result suggests the formation of metallic Mo/Co and Co2C during the pyrolysis process. According to the knowledge on pyrolysis of carbon,12, 32 it can be inferred that the Mo and Co/CoC nanocrystals originate from the reduction of CoMoO4 precursor by carbon during pyrolysis process. After the phosphating process, compared to the CoMo-CoMoO3@CN, five new peaks at 32.2°, 43.1°, 48.1°, 56.0° and 56.8° appear in the XRD pattern of CoP(MoP)-CoMoO3@CN, which can be assigned to (100), (101) planes of MoP (JCPDS No. 24-0771) and (211), (020), (301) planes of CoP (JCPDS No. 29-



0497). At the same time, the diffraction peaks of metallic Mo (40.5o) and Co (44.2 o) almost disappear, which means the metals transformed into metallic phosphides through a simple low-temperature phosphating process. The above results demonstrate that MoP and CoP coexisted in CoP(MoP)-CoMoO3@CN. The atomic ratio of Co, Mo and P in CoP(MoP)-CoMoO3@CN is 1.33:1:0.54, verified by inductively coupled plasma mass (ICP-MS, Table S1). The morphology and structure of the as-prepared catalysts were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Figure 1a shows the TEM images of CoMoO4 NWs. It is clearly observed that the product is smooth nanowires with the average diameter of 41.5±9.6 nm, and yields a high selectivity (≥95%), which is further demonstrated by SEM image (Figure 1b). After the calcination and phosphorization treatment, as shown in Figure 1c-f, the CoMo-CoMoO3@CN and CoP(MoP)-CoMoO3@CN still show the nanowires structure with the diameter of 44.3±10.8 nm and 43.7±7.3 nm respectively. It is noted that there are many tiny spherical particles located on the nanowires of CoMo-CoMoO3@CN and CoP(MoP)-CoMoO3@CN compared with pristine CoMoO4 NWs. Figure 2a exhibits a selected area electron diffraction (SAED) patterns with four rings, which may result from the crystal structures with different lattice fringe orientation (such as, CoP: (211), (111); MoP: (101); CoMoO3: (303)), in consistent with XRD results. As shown in Figure 2b the CoP(MoP)-CoMoO3@CN is surrounded by amorphous-like shells. The amorphous-like shells should be attributed to carbon, which is demonstrated


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by Raman thereafter. It can be inferred from the high-resolution transmission electron microscope (HRTEM, Figure 2c and d) that the lattice fringes in nanowires are about 1.91 Å and 2.75 Å, consistent with the CoP (211) and MoP (101) planes, respectively. Specifically, the fringes of Figure 1e should be Moiré fringes rather than crystal distortions. The Moiré fringes are essentially the interference between two or more periodic structures with similar frequency. 33 The Moiré fringes herein are possibly caused by the interference of crystallographic planes of MoP and CoMoO3. 34 Besides, the X-ray spectroscopy (EDS) mappings results (Figure 2fl) display that the Co, Mo, P, C, N and O elements are homogeneously distributed throughout the whole nanowires. As for the spherical particles located on the nanowires, it is observed that the lattice spacing is about 2.83 Å, which is attributed to (011) planes of CoP (Figure 2m and n), and the EDS mapping result shows that the Co, and P elements were homogeneously and overlapped distributed throughout the whole sphere (Figure 2o-u), revealing a formation of CoP component. Combined with the XRD result, it is concluded that the CoP, MoP and CoMoO3 component are successfully embedded in CN layers and the component of NWs composed of MoP, CoMoO3 and CoP, while the ingredient of sphere particles assembled on the NWs is attributed to CoP. Catalysts of CoMoO4, CoMo-CoMoO3@CN and CoP(MoP)-CoMoO3@CN were also investigated by the experiment of N2 adsorption-desorption isotherm (Figure 3a). It is evident that CoP(MoP)-CoMoO3@CN and CoMoCoMoO3@CN show type IV isotherm with an H1-type hysteresis, implying that



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the catalysts possess a mesoporous structures. For CoP(MoP)-CoMoO3@CN, CoMo-CoMoO3@CN, the BET surface areas are about 89 and 75 m2·g-1, pore volumes are about 0.13 and 0.15 cm3·g-1 respectively (Table S2 and Figure 3b). However, bimetal oxides CoMoO4 without carbon coat exhibit type III isotherm with the minimum surface area of 40 m2·g-1 and pore volume of 0.09 cm3·g-1, demonstrating that the CN materials pay an important role in fabricating uniform mesopores. This porous structure would accelerate the release of H2/O2 and allow fast and efficient electron transfer of the catalysts, consequently facilitating OER/HER.19, 35, 36 In addition, Raman spectroscopy were measured to get a deep insight into the local structure of carbons. As shown in Figure 3c, it can be seen that there are two bands centered at 1340 and 1585 cm-1 which related to the Dband and G-band, respectively,37 confirming the existence of carbon. The ID/IG of CoP(MoP)-CoMoO3@CN is 1.1, which is larger than the CoMoCoMoO3@CN (0.57) and CN (0.95), indicating the existence of partial graphitization and defects structure on the N-doped carbon shell, thus effectively enhancing the electroconductivity of the catalysts in OER and HER.38 To get an in-depth understanding of the catalytic performance of CoMoO4, CoMo-CoMoO3@CN

and

CoP(MoP)-CoMoO3@CN

catalysts,

X-ray

photoelectron spectroscopy (XPS) measurement was performed (Figure 4 and Figure S2). As shown in Figure 4a, the Co 2p peaks observed at 781.9/797.7 in CoP(MoP)-CoMoO3@CN, 780.1/795.4 eV in CoMo-CoMoO3@CN and 781.0/797.1 eV in CoMoO4 can be attributed to the oxidation state of Co


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species.39 For CoP(MoP)-CoMoO3@CN, the spectrum also comprises two pesks at 778.7 and 793.7 eV, which positively shifts from those of metallic Co0 2p3/2 and Co0 2p1/2 peak at 778.4 and 793.6 eV in CoMo-CoMoO3@CN, these two peaks in CoP(MoP)-CoMoO3@CN should be corresponding to the Co species in CoP.40 The high-resolution Mo 3d XPS (Figure 4b) shows peaks at 232.8/235.8 eV and 230.3/233.3 eV in CoP(MoP)-CoMoO3@CN, 231.74/234.7 eV and 229.85/233.0 eV in CoMo-CoMoO3@CN, which can be assigned to Mo6+ and Mo4+, respectively.25,

41

The rest of the XPS profile depicted in CoP(MoP)-

CoMoO3@CN is ascribed to MoP, represented by 229.4/232.4 eV doublets, which agrees well with previous reports.42-44 And the peaks at 229.2/232.8 eV in CoMo-CoMoO3@CN can be attributed to the metalic Mo in Figure 4b. Compared to CoMo-CoMoO3, the peak of Co-O (purple line) and Mo-O (dark yellow line) in CoP(MoP)-CoMoO3@CN were both shifted to higher binding energy. This probably because that Co/Mo-based species are doped with P atoms at the interface between CoP(MoP) and CoMoO3 during the phosphating process. The penetrating behaver of P atoms can modify the electronic structure of Co/Mo due to the charge transfer, resulting in a positive shifts of Co-O and Mo-O binding energy in CoMoO3.45 The P 2p XPS spectrum (Figure 4c) of CoP(MoP)-CoMoO3@CN exhibits two doublets with peak binding energies at 129.7 and 133.7 eV. The peak at 129.7 eV which is close to that of P0 (130.0 eV)39, 46 suggests the presence of metal phosphides (Pδ-, -1

Molybdenum Phosphide and Oxide Heterostructures

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