Neural Network Inspired Design of Highly Active and Durable N

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Neural Network Inspired Design of Highly Active and Durable N-doped Carbon Interconnected Molybdenum Phosphide for Hydrogen Evolution Reaction Zhiyan Guo, Ping Liu, Jing Liu, Fanglin Du, and Luhua Jiang ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b01026 • Publication Date (Web): 12 Sep 2018 Downloaded from http://pubs.acs.org on September 13, 2018

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ACS Applied Energy Materials

Neural Network Inspired Design of Highly Active and Durable N-doped Carbon Interconnected Molybdenum Phosphide for Hydrogen Evolution Reaction Zhiyan Guo,† Ping Liu,† Jing Liu, Fanglin Du, Luhua Jiang* Nanomaterial & Electrocatalysis Laboratory, College of Materials Science & Engineering, Qingdao University of Science & Technology, Qingdao, 266042, P.R. China †

These authors contribute equally.

ABSTRACT: Molybdenum phosphide, as a promising electrocatalyst for the hydrogen evolution reaction (HER), its activity suffers from two main limitations: low active surface area and unsatisfactory electrical conductivity. Herein, inspired by the neural network, we construct a novel artificial neural network-like molybdenum phosphide composite (denoted as MoP@NC-MF), using three dimensional (3D) melamine resin foams as the scaffold. The neural network-like MoP@NC-MF composite consists of carbon-wrapped MoP nanoparticles of abundant active sites as the "neurons" and N-doped carbon as the "axons". The MoP@NC-MF composite exhibits high electrocatalytic activity and stability for HER with a low overpotential of 125 mV required to achieve a current density of 10 mA cm-2 and a small Tafel slope of 53.0 mV·dec-1 under acidic conditions, which is one of the best HER electrocatalyst among the reported single molybdenum phosphide materials. Such a superior performance of the MoP@NC-MF composite is attributed to the unique neural network structure with both abundant MoP active sites to speed the surface reactions and the interconnected carbon "axons" to promptly transfer electrons. Additionally, the 3D network structure facilitates the liquid reactants/gaseous products transporting to/escaping from the reaction centers. Our findings demonstrate that elaborated design of the morphology and the structure can achieve highly efficient electrocatalysts. KEYWORDS: molybdenum phosphide, N-doped carbon, neural network-like, electrocatalysis, hydrogen evolution reaction.

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1. INTRODUCTION The fast depletion of fossil fuels and the severe deterioration of ecology have stimulated extensive research on alternative clean and sustainable energy. Hydrogen has been viewed as one of the most potential energy carriers and the most ideal substitutes for the traditional fossil fuels in the future owing to its high energy density, earth-abundant, convenient transportation and storage, environmental friendly and renewable features. 1, 2 Compared to the traditional hydrogen production technologies, the electrochemical water splitting to produce hydrogen has drawn more and more attention because of conforming to the economic and environmental sustainable development strategy.

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So far, Pt-group noble metals are considered as the most

efficient catalysts for HER, but the scarcity and high cost greatly restrict the practical application of hydrogen production on a large scale.

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Therefore, searching for

inexpensive and earth-abundant non-precious metal electrocatalysts for efficient hydrogen evolution has been a hotspot in recent years. In the past few decades, a large number of non-noble metal catalysts, such as transition-metal sulfides, 4, 8-12 carbides, 13-18 phosphide, 19-25 selenides, 26-29 nitride 30-33 and metal alloys34, 35 have been investigated as the potential electrocatalysts for HER. Among of them, molybdenum phosphides is one of the most extensively reported HER catalysts, due to the platinum-like electronic structures, electro-conductivity,

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acceptable

low cost, efficient catalytic activity and remarkable durability

over all pH range. However, molybdenum phosphide catalysts suffer from two main limitations: low active surface area and unsatisfactory electrical conductivity. To increase the active surface area, nanostructured molybdenum phosphide with varied morphologies have been designed to expose more active sites. For example, a closely interconnected network of MoP with high specific surface area was successfully 2


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prepared via the citric acid-assisted synthetic method, which showed high activity for the HER.

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It was reported that MoS2-MoP heterostructured nanosheets on

polymer-derived carbon with relatively-high specific surface area (162 m2·g-1) was successfully synthesized, which also exhibited high activity for the HER.

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Cation-doping is another strategy to improve the activity via tuning the electronic structure of phosphides. Recently, the MoP NPs encapsulated in N,P-codoped hollow nanospheres (MoP@NPC-H) was prepared through the phosphorization reaction by employing inorganic-organic Mo-P/polyaniline-pyrrole as a precursor for the first time, which showed high catalytic activity for HER because the heteroatom doping (N, P) in the MoP@NPC-H catalyst optimized the electronic configurations.40 Nevertheless, there are still large space to improve the catalytic activity of molybdenum phosphides by rational designing the micro-macrostructure. Another important issue have to be mentioned is that, for electrocatalysts, excellent electro-conductivity is the precondition for possessing high electrocatalytic activity by offering fast electron transfer during the reactions. The electronic conductivity of molybdenum phosphide is far from satisfactory, thus conductive substrates like carbon-based materials are usually introduced to improve the conductivity. However, the physically mixing of active components and substrates is hard to ensure a close contact interface and unfavorable for electron transfer during the HER. An ideal electrocatalysts is expected to possess both abundant active sites and fast mass transport of reactants/products. To this end, precursors with porous structures, such as molecular sieves, 41 MOFs,

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and recently melamine foams 45-50

have been employed as the skeletons of electrocatalysts and expected to form porous structures after thermal treatment to facilitate the mass transport in reactions. Particularly, for the Mo-based HER catalysts, a multi-functional hierarchical structure 3



is expected to construct, being able to speed the transportation of hydrated protons from electrolyte and electrons from the electrode towards the active sites, and simultaneously, the produced gaseous hydrogen is expected to move away fast from the active sites. Inspired by human nerves, as shown in Figure 1a, which can rapidly transmit electrical signals relying on the unique interconnected network architecture consisting of neurons and axons, we design a bionic structure as shown in Figure 1b, in which the MoP particles with small size of abundant active sites are contained in the "neurons", and the "neurons" are connected by the carbon "axons" to form a 3D network to ensure one side fast surface reaction on active MoP and the other side prompt electron transfer, meanwhile, the macropores are expected to facilitate the gaseous product escaping from the reaction center. With such a multi-functional bionic structure, it is expected to obtain a high catalytic activity. To realize such a bionic structure, melamine resin foams (MF) with 3D networks are adopted as the skeletons of the “artificial nerve”. Molybdenum and phosphorous sources are then deposited on the MF, after thermal treatment to form MoP "neurons" which are interconnected by carbon "axons". The unique neural network structured MoP@NC-MF displays as far as know one of the highest activity and stability for the HER among the single MoP catalysts.

Figure 1. (a) Diagrammatic sketch of the neural network and (b) FE-SEM image of the 4


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MoP@NC-MF. The scale bar in (b) is 10 µm.

2. EXPERIMENTAL SECTION 2.1. Chemicals and Materials Hexaammonium

heptamolybdate

tetrahydrate

[(NH4)6Mo7O24·4H2O)],

ammonium dihydrogen phophate (NH4H2PO4) and urea [CO(NH2)2] were purchased from Sinopharm Chemical Reagent Corp. Melamine resin foams (MF) were purchased from Clean World Corp. Anhydrous ethanol (CH3CH2OH), sulfuric acid (H2SO4) and potassium hydroxide (KOH) were purchased from Tianjin Chemical Reagent Corp. Pt/C (20 wt%) and Nafion solution (5 wt%) were purchased from Sigma-Aldrich. All the chemicals were analytical reagents and used without further purification. The highly purified water used in all experiments was produced through an Ulupure system.

2.2. Preparation of Electrocatalysts Preparation of MoP@NC-MF. The synthesis of the MoP@NC-MF involves an impregnation-calcination procedure, as illustrated in Figure S1. First, 0.2 mmol of (NH4)6Mo7O24·4H2O, 1.4 mmol of NH4H2PO4 and 0.033 mol of CO(NH2)2 were dissolved in 80 mL deionized water under magnetic stirring. Then the above solution was aged in an oil bath at 80ºC for 5 h under magnetic stirring. Afterwards, the cleaned melamine resin foams (MF, 30 mm × 10 mm × 10 mm) were put into the solution. Subsequently, MF were kept at 80 ºC for another 5 h to evacuate the bubbles and fully absorb the solution. After that, MF were dried in an oven at 80 ºC overnight. Second, the dried MF containing precursors were put into a tube furnace and calcined in N2 from room temperature to 900 ºC with a temperature ramp of 5 ºC/min and kept at 900 ºC for 2 h, followed by cooling down naturally to room temperature. The obtained MoP samples was denoted as MoP@NC-MF. For comparison, MoP@NC 5



sample was synthesized under the same conditions, just without adding MF.

Preparation of Bulk MoP. (NH4)6Mo7O24·4H2O of 0.1 mmol and 0.7 mmol of NH4H2PO4 were ground up to form a homogeneous white powders in an agate mortar for 30 min. Then the solid mixtures were sintered at 500ºC for 10 h in a muffle furnace to obtain the dark blue precursors. Subsequently, the precursors were put in a tube furnace and reduced in H2/Ar (5 wt%) from room temperature to 850℃ with a temperature ramp of 5 oC/min and kept at 850 ºC for 2 h, followed by cooling down naturally to room temperature. The prepared sample was denoted as Bulk MoP.

Preparation of NC. The N-doped C (NC) was prepared by direct pyrolyzing MF at 900 ºC for 2 h under N2 atmosphere.

Preparation of Bulk MoP-NC. Bulk MoP-NC was prepared by physically mixing 5 mg of Bulk MoP powder with 1 mg of NC.

2.3. Characterizations The crystalline structures of the samples were analyzed on an X-ray diffraction (XRD, Rigaku D/MAX 2550) equipped with Cu Kα radiation (λ = 1.54178 Å) by scanning from 2θ = 10º to 80º. The surface morphology of samples was characterized by a field emission scanning electron microscopy (FE-SEM, JEOL JSM-6700F). The TEM images were collected by a transmission electron microscopy (TEM, Tecnai G2 F20). The chemical components were analyzed by an X-ray photoelectron spectroscopy (XPS, Perkin-Elmer PHI 550) with Al Kα (h ν = 1486.6 eV) as a monochromatic X-ray source. The Brunauer-Emmett-Teller (BET) specific surface was determined by the N2 adsorption/desorption measurements on an automated Physisorption and Chemisorption Analyzer (Micromeritics ASAP 2020). Thermogravimetric analysis (TGA) was carried out in flowing air with a heating rate of 10 ºC /min from room 6


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temperature to 800 ºC using NETZSCH STA 449C. The TGA curve of MoP@NC-MF is shown in Figure S2. The weight loss prior to 300ºC is about 3%, which is attributed to the oxidation of carbon, and the significant weight gain after 300 ºC is to the oxidation of MoP. Thus, the loading of MoP in MoP@NC-MF is deduced to about 97 wt.%. H2-Temperature-Programmed Desorption (H2-TPD) analysis was carried out using the AutoChem II 2920. A 50 mg portion of sample was reduced in H2/Ar (10 wt%) flow from room temperature to 400 ºC with a heating rate of 5 ºC/min, followed by cooling down to room temperature. After an adsorption of H2/Ar (10 wt%) for 30 min at room temperature, the sample was swept in a Ar flow at a heating rate of 10 ºC per min until the TCD signal was stable. Meanwhile, the signial was recorded.

2.4. Electrochemical Measurements All electrochemical experiments were measured on a CHI 760D electrochemical station with a standard three-electrode system at room temperature. An L-shaped supporting glassy carbon electrode (L-GCE, 5mm in diameter) and a graphite rod were used as the working electrode and the counter electrode, respectively. A saturated calomel electrode (SCE) was used as the reference electrode in 0.5 M H2SO4 and a Ag/AgCl (3.0 M KCl) electrode was used as the reference electrode in 1 M KOH. The potentials in this work have been converted to refer to the reversible hydrogen electrode (RHE). The potential was calibrated to referring to RHE according to the reported method.51 Linear sweep voltammetry (LSV) measurements were tested with a scan rate of 5 mV·s-1.The Tafel slopes were determined by fitting the linear regions of Tafel plots according to the Tafel equation (η = a + b log j). Electrochemical impedance spectroscopy (EIS) measurements were carried out in the frequency range of 100 kHz-0.01 Hz with an amplitude of 10 mV in a potentiostatic 7



mode. To calculate the capacitance of samples representing the electrochemically active surface area (ECSA), the cyclic voltammograms (CV) were recorded at different scanning rates from 20 to 200 mV·s-1 in the potential range of 0.18-0.38 V vs.RHE. The electrochemical stability tests were conducted via cycling for 1000 cycles at the potential range from 0 to -0.2 V vs.RHE with a scanning rate of 100 mV·s-1. The electrolyte solution was saturated by N2 to eliminate the dissolved oxygen for all experiments.

Preparation of the working electrode. Five milligrams of catalysts was dispersed in the solution containing 660 µL of water, 380 µL of ethanol and 20 µL of Nafion solution (5 wt.%). Then the solution was sonicated for 30 min to form homogeneous ink. Subsequently, 14 µL of ink was dropped onto the L-GCE and naturally dried at room temperature (catalyst loading ca. 0.36 mg·cm-2).

3. RESULTS AND DISCUSSION Figure 2 shows the XRD pattern of MoP@NC-MF and the peak position and strength of the standard XRD of MoP (JCPDS No. 24-0771). The broad diffraction peak ranging at 20-30º indicates that amorphous carbon exists in the MoP@NC-MF sample. The characteristic peaks located at 27.9°, 32.3°, 43.1°, 57.4°, 57.9°, 64.9°, 67.0°, 67.8° and 74.3° can be assigned to the (001), (100), (101), (110), (002), (111), (200), (102) and (201) facets of MoP, respectively. The XRD patterns of Bulk MoP and MoP@NC, displayed in Figure S3, show clearly the same MoP crystalline structure as MoP@NC-MF. For Bulk MoP, MoP@NC and MoP@NC-MF, the average crystallite sizes of MoP calculated from the XRD diffraction peaks with Scherrer equation are around 35.5, 28.2 and 33.2 nm, respectively.

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Figure 2. XRD pattern of MoP@NC-MF. The SEM images of the as-prepared samples are displayed in Figure 3. The initial MF is of 3D ordered network structure (Figure 3a). After calcinated, the obtained NC inherited the 3D ordered network structure of MF as observed from Figure 3b. Interestingly, the interconnected “axons” of the network turned to be hollow after carbonization, as shown in circles of Figure 3b, which is consistent with the previous report.52 The hollow structure is a comprehensive result of shrinking effect and escaping of gaseous products during the calcination. For the MoP@NC-MF, the SEM images in Figure 3c-3e display that it is of neural network structure, possessing "neurons" interconnected by "axons". A further enlarged SEM image of the "neurons" (Figure 3d) shows clearly a loose and porous structure with pore sizes in the range of 15-300 nm (Figure 3e). In contrast, MoP@NC and Bulk MoP contain disordered and aggregated nanoparticles without obvious porous structures due to the aggregation of particles during the high-temperature calcination (Figure S4). To recognize the elemental distribution, the elemental mapping images of the MoP@NC-MF are analyzed in Figure 3g-3k. From Figure 3h and 3i, the distribution of Mo and P is almost overlapped and corresponds to the "neurons" of the "nerve", suggesting that the "neurons" is composed of molybdenum phosphide. The distribution of C and N is almost overlapped as shown in Figure 3j and 3k and 9



corresponds to the "axons" and the shell of the "neurons", indicating the MoP domains are enveloped and interconnected by porous carbon. The energy dispersive X-ray spectrum (EDS) also reveals that the atomic ratio of Mo to P for MoP@NC-MF is identified to be close to 1:1 (Figure S5), confirming again the existence of MoP crystalline phase in the sample. From the TEM image (Figure S6), MoP@NC-MF consists of small particles of about 10-50 nm, which is in accordance with the calculated result from the XRD patterns. Furthermore, the high-resolution TEM (HRTEM) image of MoP@NC-MF displays that MoP particle size shows clear lattice fringes with an interplanar distance of 0.276 nm, which corresponds to the (100) plane of MoP in Figure 3f. The formation of the unique structure of N-doped Carbon "axons" and MoP "neurons" could be traced from the preparation process. After impregnated in aqueous (NH4)6Mo7O24 and NH4H2PO4 solution, the melamine resin was dried. During the drying process, the liquid Mo and P precursors tend to aggregate at the junctions of the network, rather than at the "axons", with the action of the molecular surface tension to resulting a less surface energy. After calcination, the aggregated Mo and P precursors at the junctions form MoP with loose and porous structures due to the comprehensive result of shrinking effect and escaping of gaseous products during the calcination. Such a structure is as expected and is promising for fast electron/proton transfer and mass transport of gaseous products.

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Figure 3. FE-SEM images of (a) initial MF, (b) NC, (c-e) MoP@NC-MF with different magnifications, (f) HRTEM image of MoP@NC-MF, (g) FE-SEM image of MoP@NC-MF and corresponding EDX element mapping images of (h) Mo, (i) P, (j) C and (k) N of MoP@NC-MF, respectively. Scale bar: (a) 100 µm, (b) 10 µm and (c) 10 µm, (d) 1 µm, (e) 100 nm and (f) 10 nm.

In order to further acquire the information on the surface elemental compositions and valent states of MoP@NC-MF, X-ray photoelectron spectroscopy (XPS) was performed. The XPS spectra illustrate the presence of Mo, P, C, N, and O elements on the surface of the MoP@NC-MF sample (Figure S7). The Mo 3d XPS spectrum in Figure 4a reveals that Mo exists in the form of Moδ+ (0

Neural Network Inspired Design of Highly Active and Durable N

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