Co-Fe-P nanotubes electrocatalysts derived from metal-organic frameworks for efficient hydrogen evolution reaction under wide pH range

Author’s Accepted Manuscript Co-Fe-P Nanotubes Electrocatalysts Derived from Metal-Organic Frameworks for Efficient Hydrogen Evolution Reaction under Wide pH Range Jiahui Chen, Jianwen Liu, Jin-Qi Xie, Huangqing Ye, Xian-Zhu Fu, Rong Sun, Ching-Ping Wong www.elsevier.com/locate/nanoenergy

PII: DOI: Reference:

S2211-2855(18)30864-4 https://doi.org/10.1016/j.nanoen.2018.11.051 NANOEN3207

To appear in: Nano Energy Received date: 17 October 2018 Revised date: 17 November 2018 Accepted date: 18 November 2018 Cite this article as: Jiahui Chen, Jianwen Liu, Jin-Qi Xie, Huangqing Ye, XianZhu Fu, Rong Sun and Ching-Ping Wong, Co-Fe-P Nanotubes Electrocatalysts Derived from Metal-Organic Frameworks for Efficient Hydrogen Evolution Reaction under Wide pH Range, Nano Energy, https://doi.org/10.1016/j.nanoen.2018.11.051 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Co-Fe-P Nanotubes Electrocatalysts Derived from Metal-Organic Frameworks for Efficient Hydrogen Evolution Reaction under Wide pH Range

Jiahui Chena,b,1, Jianwen Liuc,d,1, Jin-Qi Xiea,b, Huangqing Yea,b, Xian-Zhu Fua,c,*, Rong Suna,*, Ching-Ping Wonge

a

Shenzhen Institutes of Advanced Technology, Chinese Academy of Sciences, Shenzhen 518055,

China. b

Shenzhen College of Advanced Technology, University of Chinese Academy of Sciences,

Shenzhen 518055, China. c

College of Materials Science and Engineering, Shenzhen University, Shenzhen 518055, China.

d

National Supercomputing Center in Shenzhen, Shenzhen, 510855, China.

e

School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia

30332, United States. E-mail address: [email protected] [email protected].

*Corresponding authors. Abstract The development of high-performance and cost-effective electrocatalysts is of great significance for hydrogen production by water splitting but remains challenge. Herein, a metal organic frameworks (MOFs) templating approach is proposed to synthesize Co incorporating FeP nanotubes (Co-Fe-P nanotubes) for efficient for hydrogen evolution reaction (HER). The MOFs-derived tubular structure with in situ Co substitution lead to abundant catalytic sites, fast mass and charge transport pathways, and desirable electronic configuration. These beneficial effects enable Co-Fe-P nanotubes efficient HER catalytic activity in a wide pH range, achieving small overpotentials of 86, 138, and 66 mV at a current density of 10 mA cm-2 in 1 M KOH, 1 M phosphate buffer solution (PBS), and 0.5 M H2SO4, respectively. The catalytic activity of Co-Fe-P

1

Authors contributed equally. 1

nanotubes outperforms most of the reported FeP-based electrocatalysts. Density functional theory calculations further reveal that Co substitution results in increased density of states near Fermi level, boosting the intrinsic electrocatalytic activity of Co-Fe-P nanotubes. This work affords a feasible way to the synthesis of cheap and efficient FeP-based electrocatalysts for HER. Graphical Abstract:

Keywords Electrocatalysis, Hydrogen evolution reaction, Metal phosphide, Hollow structure, Nanotube

1. Introduction Electrochemical water splitting driven by intermittent wind and solar energy sources is a desirable technology for the generation of clean and sustainable hydrogen that could replace traditional fossil fuels and thus alleviate energy crisis [1-3]. As an essential half-reaction of water splitting, hydrogen evolution reaction (HER) is kinetically sluggish that needs high efficient electrocatalysts to reduce the overpotential and improve energy conversion efficiency. To date, Pt-group noble metal-based materials are the state-of-the-art electrocatalysts for HER over a wide pH range (0-14) owing to their optimal hydrogen adsorption free energies (GH) on catalyst surface [4, 5]. Unfortunately, these noble metals due to their scarcity and high cost, which strongly limit their wide spread applications. Therefore, substantial efforts have been devoted to the development of high-performance and cost-effective electrocatalysts based on earth-abundant 3d transition metals [6, 7], such as metal alloys [8], phosphides [9], sulfides [10], and selenides [11]. Among the various non-precious materials, iron phosphide (FeP) is of particular interest for large scale hydrogen generation because iron is more abundant and much cheaper than other transition metal [12-14]. 2

Recently, FeP has been considered as a promising electrocatalyst for HER, but its catalytic performance is still unsatisfying in terms of large overpotential, undesirable stability, and poor compatibility in different pH solutions [15-18]. Generally, the HER performance of FeP could be improved by two strategies: one is to tune its composition by nano carbon hybridization or metallic heteroatom doping, the other is to construct nanostructures with specific morphology including solid or hollow nanoparticles, nanowires, nanosheets, and nanotubes. It is well established that encapsulation of metal phosphide nanoparticles by carbon nanotube, graphitic carbon, and graphene can not only protect the nanoparticles from aggregation, but also play an important role in modulating the electronic configuration of nanoparticles and provide additional active sites for HER [19-21]. For example, the carbon-shell-coated FeP nanoparticles showed impressive HER performance in 0.5 M H2SO4 with low overpotential of 71 mV at 10 mA cm-2 and remarkable long-term durability [13]. Furthermore, incorporation of one or more foreign metal atoms is a more favorable approach to directly alter the electronic configuration of catalyst and optimize the GH on catalyst surface, which in turn improve its intrinsic activity via the intriguing synergetic effect [22-24]. Co0.59Fe0.41P binary transition metal phosphide nanocubes synthesized by using this strategy could achieve low potentials of 72 and 92 mV at 10 mA cm-2 in acidic and alkaline solutions, respectively [25]. Moreover, the electrocatalytic performance greatly depends on the morphology and nanostructure. Different from the varieties of nanostructures, nanotubes have attracted increasing attention because of their unique functional properties [26-28]. The one dimensional structure with internal void space bring many benefits for electrocatalysis in virtue of high surface area, low density, as well as short mass and charge transport pathways. Despite continuous progress of FeP-based catalyst for HER, most of them could function only in acidic solution or require large overpotential to reach a current density of 10 mA cm-2. Therefore, it is highly desired to explore high-performance FeP-based catalyst that work under wide pH range. Metal organic frameworks (MOFs) are a class of coordination network compound, in which metal ions or clusters are periodically linked by organic ligands. The easy control of composition, size, and morphology endow MOFs with multiple functions and broad applications [29-31]. In recent years, MOFs are employed as versatile platforms to synthesize metal oxides [32], carbon materials [33], carbon coated metals [34], and their derivations [35-37] for electrocatalysis. In this context, MOFs-derived hollow nanostructures are of great importance, which offer huge 3

possibilities to improve catalytic performances [38, 39]. Typically, Co4Ni1P nanotubes derived from Ni-Co bimetallic MOF-74 could afford 10 mA cm-2 at overpotentials around 130 mV in acidic, neutral, and alkaline solutions [40]. CoP nanoparticle-embedded N-doped carbon nanotube hollow polyhedrons derived from core-shell MOFs exhibited small overpotentials of 140 and 115 mV at 10 mA cm-2 in 0.5 M H2SO4 and 1 M KOH, respectively [41]. Nevertheless, the MOFs derived FeP-based hollow one-dimensional structure has been rarely reported. Inspired by the above considerations, we report a MIL-88B MOFs (MIL=Materials of Institute Lavoisier) templating approach to synthesize Co-incorporating FeP nanotubes (named Co-Fe-P nanotubes), aiming at improving the HER catalytic performance of FeP. The synthetic route of the desired nanotubes is shown in Fig. 1. Firstly, two metal salts and 1,4-benzenedicarboxylic acid (1,4-BDC) linkers are used to assemble CoFe MIL-88B nanorods with the assist of solvothermal process. The CoFe MIL-88B nanorods are then converted to semi-hollow mixed metal oxides (CoFeOx) through a calcination process in air. After phosphidation, the final Co-Fe-P nanotubes with completely hollow interior and uniform dispersity of Co, Fe, and P atoms are obtained. The substitution of Co dramatically enhances the HER performances of FeP in wide pH value solution, featuring low overpotentials of 86, 138, and 66 mV at a current density of 10 mA cm-2 in 1 M KOH, 1 M phosphate buffer solution (PBS), and 0.5 M H2SO4, respectively. The high surface area of the tubular structure and the synergetic effect induced by Co substitution endow Co-Fe-P catalyst with abundant catalytic sites, short mass diffusion pathways, low charge transport resistance, and desirable electronic configuration for hydrogen evolution at low overpotentials. In addition, the robust Co-Fe-P nanotubes with high structural stability shows excellent long-term stability and durability.

Fig. 1. Schematic illustration of the synthesis of Co-Fe-P nanotubes. 4

2. Experimental section 2.1. Materials and chemicals Iron chloride hexahydrate (FeCl3·6H2O, 99%), cobalt nitrate hexahydrate (Co(NO3)2·6H2O, 99%), iron nitrate nonahydrate (Fe(NO3)3·9H2O, 98%), 1,4-benzenedicarboxylic acid (1,4-BDC, 99%), acetonitrile (CH3CN, 99%), N,N-dimethylformamide (DMF, 99.5%), sodium hydroxide (NaOH, 98%) and sodium hypophosphite (NaH2PO2, 99%) were purchased from Aladdin Reagent. 20% Pt/C and Nafion (5 wt%) were purchased from Alfa Aesar. All the reagents were used as received. The deionized water used in this work was supplied by a Millipore system. 2.2. Synthesis of CoFe MIL-88B CoFe MIL-88B was synthesized by a solvothermal method. For the synthesis of CoFe MIL-88B with Co/Fe feeding molar ratio of 1:2, 0.4984 g of 1,4-BDC, 0.5406 g of FeCl3·6H2O and 0.2911 g of Co(NO3)2·6H2O were dissolved in 30 mL of DMF. The mixture was stirred for 15 min before 6 mL of 0.4 mol L-1 NaOH was added. After stirred for another 15 min, the reaction mixture was transferred into a Teflon-lined autoclave and heated at 100 C for 15 h. After cooling down naturally, the product was collected by centrifugation and washed with DMF and methanol several times and vacuum dried at 60 °C for 24 h. For comparison, Co MOFs (without the addition of FeCl3·6H2O) and CoFe MIL-88B (with Co/Fe molar ratios of 1:1 and 2:1) were also synthesized by using the same procedure. 2.3. Synthesis of Fe MIL-88B Specifically, 0.2243 g of 1,4-BDC and 0.6060 g of Fe(NO3)3·9H2O were dissolved in 32.5 mL of DMF to form a clear solution, and 32.5 mL of CH3CN was poured into the above solution under stirring for 15 min. The resulting mixture was then transferred into a Teflon-lined autoclave and heated at 120 C for 1 h. After being cooled naturally to room temperature, the solid product was collected by centrifugation and washed with DMF and methanol several times and dried in vacuum at 60 °C for 24 h. 2.4. Synthesis of Co-Fe-P nanotubes, FeP nanotubes, and CoP nanoparticles The as-prepared CoFe MIL-88B was placed into a tube furnace, heated to 500 C with a ramp rate of 2 C min-1, and held for 6 h in flowing air to obtain the metal oxide (CoFeOx). To prepare the phosphides, 20 mg of CoFeOx and 600 mg NaH2PO2 were put at two separate position 5

in a ceramic boat inside of a tube furnace with NaH2PO2 at the upstream side of the Ar gas flow. Then the sample was heat to 300 C at a ramp rate of 2 C min-1 and kept for 2 h. After phosphidation, the black powders were washed with 0.5 M H2SO4 and deionized water to remove any unstable species, and finally dried in vacuum at 60 °C, yielding Co-Fe-P nanotubes. The synthesis of FeP nanotubes and CoP nanoparticles are almost identical with that of Co-Fe-P nanotubes, except that Fe MIL-88B and Co MOFs were used as precursor and the phosphidation temperature was fixed at 300 C, respectively. 2.5. Characterizations Scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) measurements were conducted using a FEI Nova NanoSEM 450. Transmission electron microscopy (TEM), high-resolution TEM (HRTEM) images, line scan profiles, and elemental mapping results were taken by a FEI Tecnai G2 F20 S-Twin microscope at 200 kV. Fourier transform infrared (FTIR) spectra were collected on a Bruker Vertex 70 FTIR spectrometer using KBr pellet. Thermal gravimetric analysis (TGA) measurements were performed on a TA Q600 under air atmosphere with a heating rate of 10 C min-1 from room temperature to 800 C. Elemental analysis was performed on a Perkin Elmer 7000DV inductively coupled plasma atomic emission spectrometer (ICP-AES). N2 adsorption-desorption measurements were carried out on a Micromeritics ASAP 2020 instrument at -196 C. X-ray diffraction (XRD) patterns were recorded on a Bruker D8 Advance diffractometer using Cu K radiation at a scan rate of 5  min-1. X-ray photoelectron spectroscopy (XPS) spectra were obtained on a Thermal Fischer ESCALAB 250 Xi instrument using a focused monochromatic Al K radiation under ultra-high vacuum. All XPS spectra were calibrated by hydrocarbon peak at 284.8 eV. 2.6. Electrochemical measurements All the electrochemical measurements were carried out on a CHI 760e electrochemical workstation in a typical three-electrode system using a glassy carbon electrode (GCE, 5 mm in diameter), graphite rod and Ag/AgCl electrode as working electrode, counter electrode and reference electrode, respectively. The HER performances were elevated in N2-saturated 0.5 M H2SO4, 1 M PBS and 1 M KOH, respectively. 4 mg catalyst powder was dispersed in 1 mL dispersant of ethanol/water/Nafion (v/v/v=85/10/5) and sonicated for 15 min to prepare catalyst ink. Then 14 L of the ink was dropped onto a carefully polished GCE with loading amount of 6

0.285 mg cm-2 and fully dried. Polarization curves were recorded by linear sweep voltammetry (LSV) tests at a scan rate of 5 mV s-1. The stability measurements were carried out by cyclic voltammetry (CV) at a scan rate of 100 mV s-1 and chronoamperometry tests at different potentials. The recorded potentials were corrected against IR drop automatically and converted to reversible hydrogen electrode (RHE) according to ERHE = EAg/AgCl + 0.197 V + 0.059pH. Electrochemical impedance spectroscopy (EIS) was measured at overpotential of 100 mV in 1 M KOH and 0.5 M H2SO4, and 200 mV in 1 M PBS from 105 Hz to 0.1 Hz with an amplitude of 5 mV. Electrochemical active surface area (ECSA) was estimated from the double-layer capacitance (Cdl) normalized by a specific capacitance for a flat surface according to the equation: ECSA =

𝐶𝑑𝑙 0.04 𝑚𝐹 𝑐𝑚−2

The Cdl was measured by CV with multiple scan rates in non-faradaic potential region. Turn over frequency (TOF) values were calculated using the following equation, while assuming all metal atoms is involved in the HER: TOF =

𝑗𝑆 2𝑛𝐹

Where j is the measured current density (mA cm-2), S is the geometric area of GCE (0.196 cm2), F is Faraday’s constant (96485 C mol-1), n is the moles of the metal atom determined based on ICP-AES results. Faraday efficiency (FE) was determined by a drainage gas collecting method. The volume of generated H2 was measured by a graduated tube, and converted to mole by ideal gas law. The theoretical amount of H2 was then calculated based on the Faraday’s law assuming that all charges that passed through the working electrode were 2e-. Then FE was calculated by the following equation: FE =

2𝑛𝐹 𝑄

Where n is the amount of hydrogen generated (mol), F is the faraday constant (96485 C mol-1) and Q is the total amount of charge passed through the cell (C). 2.7. Density functional theory (DFT) Calculations The spin-polarized DFT calculations were performed for FeP and Co-Fe-P using the Vienna Ab-initio Simulation Package (VASP) package [42, 43]. The generalized gradient approximation (GGA) with the Perdew-Burke-Ernzerhof (PBE) functional were used to describe the electronic 7

exchange and correlation effects [44]. The FeP was optimized using as-obtained lattice parameters (a=5.193, b=5.792, c=3.099; α=β=γ=90°). One Fe atom was substituted by one Co atom to model the Co doped FeP. A 444 mesh was employed to generate 32 K-points automatically using the Monkhorst-Pack scheme. The criteria for the geometry optimization and ionic steps were set as 0.02 eV/Å and 10–5 eV for the force and energy, respectively.

3. Results and Discussion Single-metallic and bimetallic MIL-88B MOFs are first synthesized by a solvothermal method according to the previous literatures [45, 46]. SEM and TEM images reveal that Fe MIL-88B crystals have a uniform spindle-like morphology with average size of ~600 nm in length and ~80 nm in diameter (Fig. S1). The morphology of the Co/Fe bimetallic MOFs is strongly related to the feeding molar ratio of the metal salts in this synthetic case. Similar morphology with the Fe MIL-88B MOFs can be obtained only in a specific Co/Fe feeding molar ratio of 1:2, otherwise nanorods tend to coexist with stacked flakes in the products, which might attribute to the heterogeneous coordination process, in which higher Co2+ concentration favors the growth of two dimensional flakes (Fig. S1 and S2). Notably, Co MOFs obtained in pure Co2+ feeding condition show irregular flake shape (Fig. S2). Therefore, Co/Fe bimetallic MOFs with the uniform morphology, which denotes as CoFe MIL-88B, is used in the following investigations. The real Co/Fe molar ratio in CoFe MIL-88B determined by ICP-AES is 1:5.46. XRD patterns indicate that the as-prepared Fe MIL-88B and CoFe MIL-88B exhibit almost identical diffraction peaks with the simulated results from references (Fig. S3) [47, 48]. As revealed by FT-IR analysis (Fig. S4), the two MOFs show four distinct vibrational bands at 544, 750, 1391 and 1588 cm-1 characteristics of Co/Fe-O stretching, C-H stretching, C=O symmetric and asymmetric stretching, respectively. The band at 1655 cm-1 corresponds to the C=O stretching vibration of dimethyl formamide that trapped in the MOFs [49]. The above results imply the successful synthesis of MIL-88B MOFs with high purity and well-defined structure. Fig. S5 presents the TGA curves of MIL-88B MOFs calcined in air. The first weight loss below 300 C ascribes to the removal of adsorbed solvent molecules, and the second one at 300-400 C relates to the decomposition of the frameworks, which in turn provides a calcination temperature, i.e. 450 C, to produce metal oxides. 8

After calcination, the as-synthesized CoFeOx has the same Co/Fe molar ratio as the MOFs precursor. The SEM images of CoFeOx (Fig. 2a, b) indicate a curving and shrunken morphology, preserving the spindle-like structure of the CoFe MIL-88B. As further revealed by TEM image (Fig. 2c), CoFeOx has a semi-hollow interior, and is composed of small nanocrystals. During the pyrolysis process, metal ions diffused outward with the produced CO2 and H2O, facilitating the formation of hollow structure.[50] In HRTEM image (Fig. 2d), two lattice fringes with interplanar distances of 0.485 and 0.252 nm are observed and assigned to (111) plane of CoFe2O4 and (111) plane of Fe2O3, respectively. Selected area electron diffraction (SAED) pattern (Fig. 2e) confirms the polycrystalline nature of the CoFeOx and coexistence of CoFe2O4 and Fe2O3. Elemental mapping images of CoFeOx in Fig. 2f illustrate that Co, Fe and O elements are evenly distributed throughout the entire structure. Consistent with the HRTEM analysis, the XRD peaks of CoFeOx (Fig. S6) can be indexed to CoFe2O4 (PDF No. 22-1086) and Fe2O3 (PDF No.33-0664). In addition, the semi-hollow CoFeOx nanotubes possesses a BET surface area of 34.3 m2 g-1 with pore size around 11 nm (Fig. S7b and S8b). For comparison, Fe2O3 is also prepared in a similar way by using Fe MIL-88B as precursor, relative characterizations are shown in Fig. S5-S10.

Fig. 2. (a, b) SEM, (c) TEM, (d) HRTEM, (e) SAED, and (f) Elemental mapping images of

9

CoFeOx nanostructure.

Phosphidation of CoFeOx with NaH2PO2 is employed to obtain Co-Fe-P nanostructure. SEM images (Fig. 3a, b) verify that Co-Fe-P exhibits similar morphology feature to CoFeOx except for some broken ends. From TEM image shown in Fig. 3c, complete hollow Co-Fe-P nanostructure with a shell thickness of around 7 nm is clearly observed. The interior spacing of the nanotube is beneficial for the exposure of active sites, the mass and charge transport, and the increased accessibility of electrolyte solution. HRTEM images given in Fig. 3d and e reveal fringe spacing of 0.188, 0.253, 0.259, and 0.273 nm, which are associated with the (211), (102), (200), and (011) crystal planes [51]. The angle between (211) and (200) is 43.5, confirming the existence of these two crystal planes. The SAED pattern (Fig. S11) of Co-Fe-P presents three concentric circles composed by tiny dim spots, implying its poorer crystallinity than the former CoFeOx. TEM line scan analysis (Fig. 3f) provides direct evidence for the hollow nature of Co-Fe-P. Moreover, the elemental mapping images (Fig. 3g) of a Co-Fe-P nanotube demonstrate the homogenously distributed Co, Fe and P are mainly concentrated at the edge of the nanotube, which is accordance with the above results. The crystalline structures of phosphide nanotubes are confirmed by XRD shown in Fig. S12. As for FeP nanotubes, the dominated peaks at 32.7, 35.5, 37.2, 46.9, 48.3, 56.1, and 59.6 are attributed to the (011), (102), (111), (202), (211), (212), and (020) planes of orthorhombic FeP (PDF No. 78-1443). With incorporation of Co atoms, the diffraction peaks negatively shift and locate between the reference peaks of FeP and CoP (PDF No. 29-0497), indicating the formation of a kind of Co-Fe-P ternary solid solution that Co atoms are uniformly doped into the lattice of FeP [40, 51, 52]. According to the ICP-AES result, the atomic ratio of Co:Fe:P in Co-Fe-P nanotubes is measured to be 0.22: 0.66: 1, while Fe: P in FeP nanotubes is 0.89: 1, showing a lower metal content than the stoichiometric ratio due to the acid washing.

10

Fig. 3. (a, b) SEM, (c) TEM, and (d, e) HRTEM images of Co-Fe-P nanotubes. (f) Line scan profile across a Co-Fe-P nanotube. (g) Elemental mapping images for Co, Fe, and P of a length of Co-Fe-P nanotube.

N2 sorption measurement curves of Co-Fe-P and FeP nanotubes exhibit hysteresis loops, suggesting their mesoporous structure features (Fig. S7c, d). After phosphidation, the BET surface areas of Co-Fe-P and FeP nanotubes are dramatically increased, more than two-fold over those of CoFeOx and Fe2O3, to 84.6 m2 g-1 and 75.7 m2 g-1. Likewise, the pore sizes of these two phosphides are larger than the metal oxide precursors (Fig. S8c, d). The high surface area and well-developed porosity could accelerate the access of HER reactants, leading to fast reaction kinetics. XPS tests are carried out to further investigate the surface chemical composition of Co-Fe-P and FeP nanotubes. The full-scan spectra of phosphide nanotubes (Fig. S13) clearly show the signals of Co, Fe, P, O and C elements, wherein O and C originate from the superficial oxidation and contamination of the samples. A series peaks indicative of oxidized species could be observed in the detailed XPS profiles. The high-resolution Fe 2p spectra of Co-Fe-P nanotubes has two intense peaks at 707.4 (2P3/2) and 720.3 eV (2P1/2), corresponding to Fe-P bonding in Co-Fe-P (Fig. 11

4a). The other two peaks at 711.4 and 724.7 eV with a satellite peak at 716.2 eV belong to Fe3+ arose from the superficial oxidation [24]. The binding energy of Fe-P in Co-Fe-P nanotubes has slightly and positively shift compared to that of FeP nanotubes, which could be explained by the electronegativity difference between Fe (1.83) and Co (1.88). In the P 2p region of Co-Fe-P (Fig. 4b), P 2P3/2 and P 2p1/2 located at 129.3 and 130.1 eV are characteristic peaks of metal phosphide, showing slightly negative shift compared to FeP. These results indicate that the addition of Co gives rise to the charge redistribution in Co-Fe-P catalyst, which generally promotes the water adsorption and dissociation process in HER [53]. The broad peaks in P 2p spectra around 133.4 eV is ascribed to oxidized P species, such as phosphates, which is confirmed by the dominated peaks in O 1s spectra (Fig. 4c) [54]. Additionally, the small peak at 533.0 eV is related to the surface-adsorbed H2O molecule [55]. The Co 2p spectra is deconvolved into two sets of spin-orbit doublets, corresponding to the binding energies of Co 2P3/2 and Co 2p1/2 with shakeup satellite peaks at 785.1 and 803.2 eV, respectively (Fig. 4d). The peaks at 778.7 and 793.7 eV are assigned to Co-P in Co-Fe-P, while the peaks at 781.6 and 797.5 eV are attributed to oxidized Co species [56].

Fig. 4. High-resolution XPS spectra of (a) Fe, (b) P, (c) O, and (d) Co for the as-prepared Co-Fe-P nanotubes. 12

The electrocatalytic HER activities of the as-prepared metal phosphides and commercial 20 wt% Pt/C are evluated by using of a three-electrode configuration in 1 M KOH, 1 M PBS, and 0.5 M H2SO4. The catalysts are coated on a GCE with a fixed mass loading of 0.285 mg cm-2. Fig. 5a-c show the IR-corrected LSV curves at a scan rate of 5 mV s-1 in the above three solutions. In expectation, the Pt/C catalyst exhibits the highest electrocatalytic HER activities with overpotentials of 59, 54, and 39 mV at a current density of 10 mA cm-2 (j10) in 1 M KOH, 1 M PBS, and 0.5 M H2SO4, respectively. On the contrary, the GCE shows negligible activity in the measurement range. As shown in Fig. 5a, the overpotential for Co-Fe-P to achieve j10 is 86 mV in alkaline electrolyte, over two times lower than that of FeP (178 mV). In addition, the Co-Fe-P exhibits larger overpotential of 138 mV at j10 in neutral PBS as compared to that in alkaline electrolyte (Fig. 5b). In spite of this, it is still superior to FeP (198 mV). More impressively, the Co-Fe-P also reveals better catalytic performance than FeP in acidic electrolyte, only requiring overpotential of 66 mV to reach j10 (Fig. 5c). These results suggest that Co-Fe-P is highly active toward HER in a wide pH range, prevailing against most of recently reported MOFs-derived and FeP-based catalysts (Table S1 and S2). Notably, Co-Fe-P nanotubes show much higher activity than the reported Co-Fe-P film with similar composition [57], highlighting the superiority of tubular structure that possesses large surface area and consequently provides more available active sites for catalysis. For better comparison, CoP nanoparticles are synthesized by using Co MOFs as precursor (Fig. S14 shows further details). The catalytic performances of CoP nanoparticles are better than Co-Fe-P nanotubes in 1 M KOH and 1 M PBS, but worse in 0.5 M H2SO4, showing overpotentials of 72, 130, and 80 mV at j10. The similar performances of CoP and Co-Fe-P imply that constructing tubular structure and tuning composition are effective strategies to gain desirable catalytic performances with relative lower amount of Co. Comparisons of Tafel slopes derived from polarization curves are demonstrated in Fig. 5d-f. Typically, generation of H2 involves two different reaction routes, namely, Volmer-Heyrovsky and Volmer-Tafel mechanisms as indicated by the variation of Tafel slopes [58]. Pt/C, Co-Fe-P, and FeP manifest Tafel slopes of 59, 66, and 111 mV dec-1 in 1 M KOH, respectively, suggesting a Volmer-Heyrovsky mechanisms in which the Volmer reaction for FeP and the Heyrovsky reaction for Pt/C and Co-Fe-P becomes the rate-determining step (RDS). Similar trends and results of the 13

three catalysts can be also seen in 0.5 M H2SO4. However, large Tafel slope of 138 mV dec-1 for Co-Fe-P is observed in 1 M PBS, demostrating the Volmer reaction is the dominated RDS. Although the Co-Fe-P catalyst shares different RDSs in different electrolytes, it exhibits much smaller Tafel slope than that of FeP. The smaller Tafel slope of Co-Fe-P highlights the positive effect of Co dopant, leading to fast reaction kinetics and high catalytic activity toward HER.

Fig. 5. HER catalytic performances of the developed catalysts in (a, d, g) 1 M KOH, (b, e, h) 1 M PBS, and (c, f, i) 0.5 M H2SO4. (a-c) LSV curves of Pt/C, Co-Fe-P, and FeP collected at a scan rate of 5 mV s-1 with IR compensation. (d-f) The corresponding Tafel slopes. (g-i) LSV curves of Co-Fe-P nanotubes before and after 1000 CV cycles. Insets in (g-i) show the chronoamperometry curves at overpotential of 100 mV in 1 M KOH and 0.5 M H2SO4, and 200 mV in 1 M PBS.

The ECSA is estimated from Cdl obtained by CV curves in non-Faradic potential region (Fig. S15). As shown in Fig. S16, the Cdl values of Co-Fe-P are calculated to be 40.73, 11.01, and 18.5 mF cm-2 with ECSAs of 1018, 462.5, and 275.3 in 1 M KOH, 1 M PBS, and 0.5 M H2SO4, respectively, which are significantly larger than those of FeP. This result suggests that the Co 14

heteroatoms help to expose more active sites, promoting the catalytic reaction. The unique hollow structure of Co-Fe-P with high BET surface area is also responsible for the enlarged ECSAs. EIS measurements of Pt/C, Co-Fe-P, and FeP are performed at specific overpotentials to reveal the charge transfer properties of the catalysts. Fig. S17 in Supporting Information presents the Nyquist plots wherein a semicircle appears in the middle frequency region. The semicircular diameter of Co-Fe-P is greatly reduced under all pH conditions as compared to FeP, indicating its enhanced charge transfer ability that favors the combination of electron and reactants at the catalyst-electrolyte interphase. The TOF at overpotential of 100 mV is calculated to further elucidate the intrinsic electrocatalytic activity of the catalyst and shown in Table S3. Co-Fe-P exhibits TOF values of 4.610-3, 1.310-3, and 7.810-3 s-1 in 1 M KOH, 1 M PBS, and 0.5 M H2SO4, respectively, which are 6, 2, and 11 times higher than those of FeP. The remarkable TOF values confirm the excellent HER catalytic performance that produce considerable H2 in a given time. Furthermore, faradaic efficiency is evaluated by collecting H2 gas in a water displacement setup. The faradaic efficiencies of Co-Fe-P in all pH conditions are maintained above 95.8% over the entire 2 hours electrolysis (Fig. S18), reflecting the high HER catalytic efficiency of Co-Fe-P nanotubes and promising potential for practical implementation. Accordingly, the durability and long-term stability of the catalyst in alkaline, neutral, and acidic electrolytes are investigated by using CV scanning and chronoamperometry tests. As presented in Fig. 5g and i, the Co-Fe-P catalyst possesses good stable HER performance in alkaline and acidic electrolytes, showing slightly negative shift in LSV curves after 1000 CV cycles and slight decrease of catalytic current density over 20 h operation. Unexpectedly, the Co-Fe-P nanotubes suffers from a loss of catalytic activity at high current density after 1000 CV cycles in neutral electrolyte (Fig. 5h). Chronoamperometry result (Fig. 5h insert) also shows a decrement of around 10% in the current density after 20 h. The activity deterioration of Co-Fe-P in neutral media could be ascribed to the following two points: firstly, the large resistance of PBS electrolyte causes large Ohmic loss as indicated by EIS result in Fig. S17b; secondly, large water absorption and activation energy barriers should be overcome due to the low proton concentration of PBS electrolyte [58]. Although the catalytic performance in 1 M PBS is not as stable as those in 1 M KOH and 0.5 M H2SO4, it is still comparable with or even better than some of the recently reported catalysts [17, 59, 60]. 15

The morphologies, structures, and surface compositions of Co-Fe-P catalyst after chronoamperometry measurements in multiple pH electrolytes are further characterized by SEM, TEM, XRD, and XPS. From the SEM and TEM images in Fig. S19, the tubular morphologies are well-preserved with lattice fringes indexed to Co-Fe-P. As evidenced by XRD patterns (Fig. S20), diffraction peaks at 32.7, 35.5, 48.3, and 56.1 for Co-Fe-P after electrocatalysis are identical with the as-prepared one, indicating its high structural stability. Meanwhile, XPS is employed to analyze the surface chemistry of Co-Fe-P since it can reveal more detailed information than the panoramic morphology and structure characterizations. Fig. S21 shows the high-resolution XPS spectra of post-HER Co-Fe-P. The characteristic peaks of Co-Fe-P after reactions in alkaline and neutral media are basically retained except the peak intensity. The intensities of Fe-P and Co-P peaks decrease as electrolyte pH value raised, while the intensive peaks of oxidized species are observed. This result suggests the partial oxidation/hydroxylation of the catalyst after long-term HER tests, which agrees well with the previous reports [40, 61, 62]. The intensity variations of the oxidized P species (phosphates) and metal phosphides peaks further verify the oxidation of Co-Fe-P in 1 M KOH and 1 M PBS. In 0.5 M H2SO4, examination of post-catalytic Co-Fe-P shows the unchanged Fe 2p, Co 2p, and P 2p peaks, indicating the high surface stability of Co-Fe-P in strong acidic environment. In addition, the new peak around 535.5 eV in O 1s spectra is characteristic of Nafion residues and is further proved by the distinct F 1s peak in survey spectrum (Fig. S22). DFT calculations are performed to gain insight into the doping effect of Co on HER catalytic activities. Two models, i.e. the FeP and Co substituted FeP (Co-Fe-P) models with orthorhombic structures have been built (Fig. 6a). With the doping of Co, the total density of states (DOS) near the Fermi level increase significantly as shown Fig. 6b. For the FeP model, the most dense DOS peak is around -1.5 eV for the spin up states. However, for the Co-Fe-P model, the most dense DOS peaks shift to ~-0.5 eV by increasing both the spin up and spin down DOS. Based on the detailed analysis of the dx2, dxy, dxz, dyz, and dz2 contributions for Fe and Co atoms (Fig. S23-S25), it is shown that the main contribution for the most dense DOS (the highest peak at ~ -1.5 eV) arises from the Fe (dxz) orbitals. With the doping of Co, the Fe (dxz) contribution decreases significantly at ~-1.5 eV for the Co-Fe-P model. In addition, the Co (dz2) increases the DOS at ~-0.5 eV. As a result, the DOS near the Fermi level increases significantly for Co-Fe-P, indicating 16

higher activity as compared to FeP. The increased DOS of Co-Fe-P near Fermi level also leads to fast charge transfer kinetics since the intensity of DOS is related to the electrical conductivity. Fig. 6c presents the charge density images of FeP and Co-Fe-P at the same surface of (0, 0, 0.5). As compare to the change of DOS, the charge density change is little, which is probably due to the lower amount of Co. On the basis of DFT calculations, it shows that Co substitution has a positive effect on the DOS near Fermi level, which could boost the catalytic activity of FeP.

Fig. 6. (a) Crystal structure models of FeP and Co-Fe-P. (b) The calculated density of states for FeP and Co-Fe-P. (c) Charge density distribution image of FeP (top) and Co-Fe-P (bottom).

Based on the above analysis, the excellent catalytic performances of Co-Fe-P nanotubes could be originated from the following factors: (i) The tubular nanostructure of Co-Fe-P possesses high surface area and well developed porosity, benefiting the exposure of active sites and the access of HER relevant species. (ii) The one dimensional structure helps to enhance the structural stability of Co-Fe-P, showing excellent durability and long-term stability. (iii) The Co-Fe-P affords good electrical conductivity pathways and fast charge transfer kinetics as indicated by EIS result. (iv) Co incorporation significantly increases the DOS for d-orbital near Fermi level, resulting in higher elctrocatalytic activity. By taking these advantages, Co-Fe-P nanotubes electrocatalyst demonstrates dramatically improved catalytic performances as compared to FeP nanotubes in a wide pH range.

4. Conclusions A MOFs templating approach is presented to improve the HER catalytic performances of FeP,

17

which involves hollow nanostructure formation and simultaneously Co substitution. Owing to the synergetic effect of desired morphology and atomic level electronic structure engineering, the as-synthesized Co-Fe-P nanotubes show excellent catalytic performances, achieving a current density of 10 mA cm-2 at small overpotentials of 86, 138, and 66 mV in 1 M KOH, 1 M PBS, and 0.5 M H2SO4, respectively. The Co-Fe-P nanotubes also have high structural stability during the long-term measurements. Theoretical calculations reveal that the Co incorporation increases the DOS near Fermi level, improving the catalytic activity eventually. This work demonstrates a promising route to achieve high-performance and low-cost FeP-based electrocatalysts for hydrogen evolution in wide pH conditions.

Acknowledgements This work was financially supported by the National Key R&D Project from Minister of Science and

Technology

of

(JCYJ20160229195455154),

China

(2016YFA0202702),

Guangdong

Department

Shenzhen of

Science

research and

plan

Technology

(2017A050501052), and Guangdong Province Industrial-Academic-Research Cooperation Program (No. 2014 B090901017).

Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at

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Lett. 16 (2016) 7718-7725.

Jiahui Chen is currently a Ph.D. candidate under the supervision of Prof. Rong Sun at Shenzhen Institutes of Advanced Technology, Chinese Academy of Sciences. He received his B.S. and M.S. degrees at Shenzhen University. His research interests mainly focus on the synthesis of non-noble metal-based nanomaterials and their applications for electrocatalysis.

Jianwen Liu is a research Professor in the College of Materials Science and Engineering, Shenzhen University. He received a Ph.D. degree from the Chinese University of Hong Kong. His current research interests focus on theoretical studies of energy materials and their catalytic/electrocatalytic properties using first principles calculations in combination with molecular dynamics.

22

Jin-Qi Xie is currently a Ph.D. candidate under the supervision of Prof. Rong Sun at Shenzhen Institutes of Advanced Technology, Chinese Academy of Sciences. His research concentrates in materials and devices for energy storage and thermoelectrics.

Huangqing Ye is currently a Ph.D. candidate under the supervision of Prof. Rong Sun at Shenzhen Institutes of Advanced Technology, Chinese Academy of Sciences. He received his B.S. degree from Huazhong University of Science & Technology, and M.S. degree from South China University of Technology. His research focuses on the synthesis and applications of carbon/metal based nanomaterials.

Xian-Zhu Fu received his Ph.D. degree in Chemistry from Xiamen University in 2007. After postdoctoral stay at University of Alberta in Canada, he joined the Shenzhen Institutes of Advanced Technology, Chinese Academy of Sciences. He is currently a Professor in the College of Materials Science and Engineering, Shenzhen University. His research interests focus on 23

electrochemistry and energy materials.

Rong Sun is a Professor at Shenzhen Institutes of Advanced Technology (SIAT), Chinese Academy of Sciences. She is also the director of Center for Advanced Materials and the deputy director of Institute of Advanced Integration Technology at SIAT. She received her Ph.D. degree from Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences. She was authorized for special government allowance granted by the State Council of China, and also elected as member of Shenzhen Double hundred Plan, which is an exceptional honor for those who have great contribution to Shenzhen development. Her research interests include electronic packaging materials and energy storage materials.

Ching-Ping Wong received his B.S. degree from Purdue University and Ph.D. degree from the Pennsylvania State University. He was awarded a postdoctoral fellowship under Nobel laureate Prof. Henry Taube at Stanford University. He is a Regents’ Professor and the Charles Smithgall Institute Endowed Chair at the School of Materials Science and Engineering, Georgia Institute of Technology, the Chief Scientist at Shenzhen Institutes of Advanced Technology, Chinese Academy of Sciences. Professor Wong is also a member of the US National Academy of Engineering and a foreign member of the Chinese Academy of Engineering.

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Highlights 1. A MOFs templating approach is proposed to synthesize Co-Fe-P nanotubes as HER electrocatalysts. 2. The as-synthesized electrocatalysts show excellent HER catalytic activity in wide pH range. 3. The tubular nanostructure of Co-Fe-P with high surface area and well developed porosity contributes to the superior HER performances. 4. The electronic states modulation caused by Co incorporation also boosts the elctrocatalytic activity of Co-Fe-P nanotubes.

25