Ultrathin Nitrogen-Doped Carbon Coated with CoP for Efficient

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An Ultrathin Nitrogen-Doped Carbon Coated with CoP for Efficient Hydrogen Evolution Fulin Yang, Yongting Chen, Gongzhen Cheng, Shengli Chen, and Wei Luo ACS Catal., Just Accepted Manuscript • Publication Date (Web): 24 Apr 2017 Downloaded from http://pubs.acs.org on April 24, 2017

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An Ultrathin Nitrogen-Doped Carbon Coated with CoP for Efficient Hydrogen Evolution Fulin Yang‡, Yongting Chen‡, Gongzhen Cheng, Shengli Chen*, and Wei Luo* College of chemistry and molecular sciences, Wuhan University, Wuhan, Hubei 430072, China

ABSTRACT

Searching for non-noble metal based electrocatalysts with high efficiency and durability toward hydrogen evolution reaction (HER) is vitally necessary for the upcoming clean and renewable energy systems. Here we report the synthesis of CoP nanoparticles encapsulated in ultrathin nitrogen-doped porous carbon (CoP@NC) through a metal-organic framework (MOF) route. This hybrid exhibits remarkable electrocatalytic activity toward HER in both acidic and alkaline media, with good stability. The experiment and theoretical calculation reveal that the carbon atoms adjacent to N dopants on the shells of CoP@NC are active sites for hydrogen evolution, and CoP and N dopants synergistically optimize the binding free energy of H* on the active sites, which results in a higher electrocatalytic activity than its counterparts without nitrogen doping and/or CoP-encapsulation.

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KEYWORDS: hydrogen evolution; cobalt phosphide; catalytically active site; ultrathin nitrogen-doped carbon layer; DFT

Hydrogen has been considered as one of the most environmental-friendly and renewable energy carrier, which can be a potential candidate for future energy strategies due to the depletion of fossil fuel and environmental pollution.1,2 Electrochemical water-splitting is one of the most efficient ways to produce highly pure hydrogen.3-5 Currently, Pt and Ru/Ir-based noble metal materials have been regarded as the most effective catalysts to reduce the overpotential of hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), the two half-reactions of the overall water-splitting, respectively.6-8 However, the scarcity and high-cost hinder their large-scale application. To this end, a large number of efforts have been devoted to searching for highly active, stable, non-noble metal based HER/OER electrocatalysts, such as transition metal carbides,9-11 nitrides,12-15 chalcogenides,16-20 oxides,21,22 alloys,23,24 and phosphides.25-30 Transition metal phosphides (TMPs) showing promising performance for HER, probably due to their hydrogenase-like catalytic mechanism, have been widely studied.31-36 Shao-Horn et al. reported that surface P could act as the active site for HER on CoP.37 Qu and Sun reported that introduction of Ni and Fe to form the ternary NiCoP and FeCoP could enhance the HER activity of CoP through modifying the free energy of hydrogen adsorption on the catalyst surface, respectively.38,39 Other studies have shown that CoP coupled with nanocarbon substrates, such as graphene,40 carbon nanotubes,32 and porous carbon derived from metal-organic frameworks (MOFs) with high specific surface area and porous structures, exhibit further increased HER performance.41-43 In these works, the active sites have been considered to be presented on the


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ACS Catalysis

surface of CoP, while the nanocarbons are regarded to increase the electric conductivity and prevent the corrosion of CoP.44 Metal nanoparticles (NPs) encapsulated in carbon layers are considered as promising nanostructures with high stability compared to the naked ones, probably due to the protection from encapsulating carbon shells. Thus, few carbon layers encapsulated metal NPs have been proved to be efficient oxygen electrode reaction and HER catalysts.9, 10, 45-49 However, to the best of our knowledge, TMPs encapsulated in carbon shell with enhanced HER activity, and the effect of dopants and carbon shells, have not been experimentally and theoretically studied. Herein, we report a facile two-step metal-organic framework (MOF)-derived route to prepare CoP nanoparticles encapsulated in ultrathin nitrogen-doped porous carbon shell (CoP@NC) with ZIF-9 as the precursor. The as-synthesized CoP@NC can serve as an outstanding catalyst for HER in both acidic and alkaline media, with extraordinary electrocatalytic activity and long-term durability. Interestingly, as the result of the presence of few N-doped carbon layers surrounding CoP NPs, the direct active sites may not be on the surface of CoP NPs as mentioned in the previous literatures.31,32 In order to discern this problem, the experimental methods have been carried out by synthesizing and comparing the catalytic performance of CoP NPs with/without N-doped carbon shells. Furthermore, density functional theory (DFT) calculations indicate that the carbon atoms adjacent to N dopants on the shells of CoP@NC are the active sites for hydrogen evolution. RESULTS AND DISCUSSION Morphology and Structure of Samples. The synthetic process of CoP@NC is illustrated in Figure 1a. Firstly, well-defined nanostructured ZIF-9 was constructed via a simple solvothermal method in accordance with the preceding literature (Figure S1a).50 This Co-based MOF was then


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pyrolyzed in a high temperature (750 °C) under N2 atmosphere, with an isothermal process at 500 °C for 2 h during the heating, to obtain Co NPs encapsulated in few N-doped carbon layers (Co@NC). As shown in the powder X-ray diffraction (XRD) pattern (Figure S2), two strong diffraction peaks appeared at 44.3°and 51.6°are identical with the (111) and (200) planes of metallic Co (JCPDS-15-0806). The morphology of the hybrids was characterized by transmission electron microscope (TEM) (Figure 1b). As observed, Co NPs are encapsulated by few layers carbon shells, and uniformly embed in the carbon matrix derived from the organic ligands of ZIF-9. High-resolution TEM (HRTEM) images in Figure 1c and inset reveal that the lattice spacings on the nanopaticle and graphitized carbon layer are about 0.20 nm and 0.34 nm, corresponding to the (111) plane of Co and the (002) plane of graphitized carbon, respectively. It should be noted that the 500 °C was chosen for the crucial isothermal process to make the MOFs break down and carbonize preliminarily, and to avoid the formation of Co NPs embedded in thick carbon layers (Co@NC-T) under a higher temperature simultaneously (Figure S1b and S3).51


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Figure 1. (a) Schematic illustration of the synthetic process of CoP@NC; TEM images of (b) Co@NC and (d) CoP@NC; HRTEM images of (c and inset) Co@NC and (e and inset) CoP@NC.


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Subsequently, a low-temperature phosphidation strategy was adopted to prepare CoP@NC with Co@NC as the Co-based precursor and NaH2PO2 as the phosphorus source. The product was etched in 0.5 M H2SO4 for 8 h to remove the unstable species. XRD pattern of CoP@NC indicates a successful transition from metallic Co to CoP without other changes of the phases (Figure S2). As observed, the obvious diffraction peaks at 31.6°, 36.4°, 46.2°, 48.2°, 52.3°and 56.8°are corresponded to the (011), (111), (112), (211), (103) and (301) planes of CoP phase (JCPDS-29-0497), respectively.32 In addition, the extra diffraction peaks at 26.0° of the two samples indicate the (002) reflection of graphitized carbon. As shown in Figure 1d, the novel core-shell structure maintains well after the phosphidation process. Simultaneously, some hollow carbon shells without CoP NP cores can be observed, which may form after removing the unstable Co-cased species by acid etching. HRTEM images (Figure 1e and inset) of CoP NPs further corroborate that CoP NPs are completely wrapped in 2-4 layers of carbon shells. The observed lattice spacing on CoP is about 0.28 nm, corresponding to the (011) plane of CoP. Besides, the lattice spacing on carbon layers is 0.34 nm, corresponding to the (002) plane of graphitic carbon. Figure S4a exhibits the scanning TEM (STEM) and the corresponding Energy dispersive X-ray spectroscopy (EDX) line scanning image, which further confirms that the nanoparticles are constituted by Co and P, and wrapped in N-doped carbon layers. EDX analysis from Figiure S4b indicates that the contents of the elements in CoP@NC are approximate to 3.8%, 3.8%, 82.8%, 4.0% and 5.6% for Co, P, C, N and O, respectively. The porous structure of this hybrid derived from MOFs was further characterized by N2 adsorption-desorption isotherms and shown in Figure S5. The Brunauer-Emmett-Teller (BET) surface area and total pore volume of CoP@NC were measured to be 367.8 m2 g-1 and 0.389 cm3 g-1, respectively.


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X-ray photoelectron spectroscopy (XPS) measurement was performed to elucidate the chemical composition and the valence state of CoP@NC. Figure 2a displays the XPS survey spectrum of CoP@NC, and indicates the existance of Co, P, C, N and O in the sample. The contents of these elements are presented in Table S1. As shown in high resolution XPS spectra of Co 2p (Figure 2b), two main peaks at 778.6 eV and 793.5 eV are assigned to Co 2p3/2 and Co 2p1/2, respectively, which are attributed to Co0. And the peak at 780.4 eV in Co 2p3/2 region and 797.2 eV in Co 2p1/2 region are assigned to oxidized Co species (Co2+/3+). The remaining two peaks deconvoluted at 785.7 eV and 802.1 eV are satellite peaks. Moreover, the high resolution XPS spectra of P 2p core levels can also be deconvoluted into three peaks which locate at 129.3 eV for P 2p3/2, 130.2 eV for P 2p1/2 and 133.2 eV for oxidized P species, respectively (Figure 2c). It is suggested that the inevitable contact between the CoP surface and oxygen leads to the emergence of the peaks at 780.4 eV and 133.2 eV. This can be also demonstrated by the high resolution XPS spectra of O 1s, which can be deconvoluted into three peaks of P-O (533.2 eV), C-O (531.9 eV) and Co-O (531.3 eV) (Figure S6a). Most importantly, compared with Co 2p3/2 in metallic Co (778.1 eV) and P 2p3/2 in elemental P (130.2 eV), the binding energy of Co 2p3/2 (778.6 eV) and P 2p3/2 (129.3 eV) in CoP@NC shift positively and negatively, respectively, similar to other reports.31,32,41 As observed from Figure 2d, the high resolution XPS spectra of N 1s is deconvoluted into three peaks as Pydinic-N (398.5 eV), Pyrrolic-N (400.1 eV) and Graphitic-N (401.5 eV). Meanwhile, an asymmetric signal of the high resolution C 1s spectrum at 284.3 eV with a tail lying in the higher binding energy region further confirms that N atoms are doped in carbon (Figure S6b).


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Figure 2. XPS of CoP@NC: survey scan (a) and high resolution XPS spectra of (b) Co 2p, (c) P 2p and (d) N 1s. For comparison, CoP NPs embedded in pristine graphitized carbon layers (denoted as CoP@C), and metal-free N-doped porous carbon (NC) were prepared in a similar way with the ZIF-9 replaced by Co-HKUST and ZIF-7, respectively. Meanwhile, if an oxidation process of Co@NC was executed in air at 250 °C for 12 h before phosphidation, naked CoP NPs supported on carbon matrix without N-doped carbon layers surrounding (denoted as CoP) were obtained. Furthermore, CoP@NC-T was synthesized with the same low-temperature phosphidation strategy when the Co@NC-T used as the precursor. XRD patterns and (HR)TEM images of these samples were displayed in Figure S7-S11. Electrocatalytic Performance. HER activities of these Co-based catalysts under acidic media (0.5 M H2SO4) were evaluated in a three-electrode system, and 20 % Pt/C was used for comparison. Due to the huge improvement of CoP@NC came from the acid pickling progress


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before electrochemical tests (Figure S12), all of our catalysts were used after washing with 0.5 M H2SO4 for 8 h. As shown in Figure 3a, Pt/C exhibits excellent HER activity with the onset overpotential (which is denoted as the overpotential at the current density of 1 mA/cm-2 in our work) close to zero as expected, whereas the catalytic activity of NC is poor with a large onset overpotential of 165 mV. Moreover, CoP@NC shows an excellent performance with the onset overpotential of about 21 mV and an overpotential of 78 mV at a current density of 10 mA cm-2 (ƞ10), which is much lower than those of CoP@C (ƞ10 = 130 mV) and CoP (ƞ10 = 128 mV) as observed. Meanwhile, when CoP is wrapped by thick N-doped carbon layers, as CoP@NC-T, the HER performance reduces (ƞ10 = 124 mV). Considering the same catalyst loading, the ƞ10 value of CoP@NC should be among the lowest ones in the reported values on transition metal phosphides and other non-noble metal based HER catalysts (Table S2). Tafel slope fitted from the polarization curve by Tafel equation (ƞ = b log j + a, where a and b are constants) is an inherent property of the electrocatalyst, which is contributed to elucidate the HER mechanism. As observed in Figure 3c, Pt/C shows the lowest Tafel slope of 29 mV dec-1, which is in agreement with the previous literatures.52 The Tafel slope of CoP@NC is 49 mV dec-1, which is lower than those of CoP@NC-T (59 mV dec-1), CoP@C (64 mV dec-1), CoP (56 mV dec-1) and NC (228 mV dec-1), and indicates that the HER on the CoP@NC probably follows the VolmerHeyrovsky mechanism.53,54 Furthermore, under the large overpotential, the Tafel slope of CoP@NC is 52 mV dec-1, lower than that of Pt/C (78 mV dec-1) as shown in Figure S13, indicating the superior HER performance of CoP@NC at large electric current condition.


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Figure 3. HER polarization curves in 0.5 M H2SO4 (a) and 1.0 M KOH (b), and the Tafel slopes in 0.5 M H2SO4 (c) and 1.0 M KOH (d) of CoP@NC, CoP@NC-T, CoP@C, CoP, NC and Pt/C, with scan rate of 5 mV s−1. HER polarization curves of CoP@NC before and after 1000 CV cycles from 0.05 V to -0.20 V in 0.5 M H2SO4 (e) and 1.0 M KOH (f). Chronoamperometry i-t curve of CoP@NC at the overpotential of 100 mV in 0.5 M H2SO4 (inset of e) and 150 mV in 1.0 M KOH (inset of f). To further understand the intrinsic activities of the four CoP-based catalysts, charge transfer resistances (Rct) have been tested by electrochemical impedance spectra (EIS). From Figure S14,


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the smaller Rct value is realized on CoP@NC, which corresponds to a faster reaction rate.55 In addition, we normalized the currents from the HER polarization curves in 0.5 M H2SO4 by the electrochemically active surface area (ECSA), which were calculated from the double-layer capacitances (Cdl) measured in Ar-saturated 0.5 M H2SO4 by CVs with the scanning rates from 50 mV s-1 to 10 mV s-1 around the open circuit potential (OCP).56 The ECSA of a catalyst could be obtained by dividing Cdl by the specific capacitance (Cs) of the sample. However, there is few related report about the value of Cs in the similar HER system with CoP/carbon-based composite material as the catalyst. Thus, we use the general specific capacitances of Cs = 0.035 mF cm-2 to calculate the ECSA.57-69 As shown in Figure S15, CoP@NC possesses the largest active area, with the best specific activity, which further confirm that the peculiar core-shell structure is benefit for HER, and the intrinsic activities of its active sites are higher than the other counterparts. To explore the durability of CoP@NC, long-term cyclic voltammetry (CV) and amperometric i-t curve were carried out and shown in Figure 3e and the inset. After continuous CV scanning for 1000 cycles with the potential from 0.05 V to -0.20 V vs RHE, only 2 mV negative displacements of ƞ10 is observed. Meanwhile, negligible loss of current density at the overpotential of 0.10 V is observed after continuous hydrogen evolution for 24 h. In addition, CoP@NC also exhibits a remarkably electrocatalytic performance in alkaline media. As shown in Figure 3b, only a small overpotential of 129 mV is needed to afford a current density of 10 mA cm-2 for CoP@NC, which is superior to those of CoP@NC-T (169 mV), CoP@C (ƞ10 = 170 mV), CoP (ƞ10 = 163 mV), NC (ƞ10 = 503 mV), and lower than most of the reported TMP-based catalysts as shown in Table S3. The Tafel slope of CoP@NC is 58 mV dec-1, smaller than those of CoP@NC-T (79 mV dec-1), CoP@C (83 mV dec-1), CoP (77 mV dec-


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), and NC (170 mV dec-1) (Figure 3d). Furthermore, long-term CV scanning and i-t curve were

also studied to test the stability. As shown in Figure 3f and the inset, only insignificant increase of the overpotential at a current density of 10 mA cm-2 was observed after 1000 CV circles, as well as a slight decrease of the current density after a long period of 24 h of amperometric i-t curve operation. These results indicate that CoP@NC could be an efficient alternative for HER in a wide pH condition. Besides that, OER could be catalyzed by CoP@NC in alkaline media with a small ƞ10 of about 300 mV and Tafel slope of 70 mV dec-1, with a favorable stability (Figure S16), indicating CoP@NC could be used as a bi-functional electrocatalyst for both HER and OER in alkaline condition. In addition, the approximately 100 percent of faradaic efficiencies of CoP@NC for HER and OER are shown in Figure S17. Interpretation of the Active Sites. From the above electrochemical studies, CoP@NC possesses the best catalytic activity in both acidic and alkaline media among the catalysts tested. In consideration of the core-shell structure of CoP@NC, we speculate that the direct active sites for HER locate on the N-doped carbon shells rather than on Co or P atoms due to the isolation between CoP and electrolyte by N-doped carbon layers. In order to prove that, the influence of CoP@NC and CoP by thiocyanate ions (SCN−) on the HER was investigated, because the metal centered catalytic active sites would be deactivated by SCN− in acid.47 As expected, after adding SCN-, the ƞ10 of about 22 mV increases for CoP and only 5 mV for CoP@NC (Figure 4). The trivial change in ƞ10 of CoP@NC is probably due to the masking effect by SCN− for CoP surface where locates at the defects of the carbon shells, which could be further explained by Raman spectrum (Figure S18). The characteristic at about 1350 and 1580 cm−1 are attributed to D band and G band, respectively, while the G band is highly dependent on the degree of graphitization and the D band represents the defect.60-62 These results indicate that the HER active sites of


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CoP@NC should be predominately located on the carbon-based shells. Combining with the polarization curves of CoP@NC, CoP@NC-T, CoP@C, CoP, and NC, it is suggested that the electronic-transfer process between CoP, C, and N stemming from synergistic effect between CoP and N dopants is the key for boosting the HER activity of CoP@NC.

Figure 4. HER polarization curves for CoP@NC (a) and CoP (b) with and without the addition of KSCN in 0.5 M H2SO4. DFT Calculation. To further seek insight of the outstanding HER performance of the CoP@NC, density functional theory (DFT) calculations were performed (see supporting information for model and computation details). Theoretically, the HER pathway could be described as a three-state diagram which starts from the initial state (H+ + e-), passes by an intermediate state (H*), and ends up in a final state (1/2 H2). As is well known, the adsorption free energy of H (ΔG(H*)) is an appropriate parameter for evaluating the HER activity of a catalytic material, and the catalyst with ΔG(H*) close to 0 is considered as a promising candidate for HER, which could facilitate the charge transfer processes for both H* intermediate and H2 formation.9, 10, 47 For CoP@NC, the value of ΔG(H*) with N atoms as the H adsorption site is much more positive than the adjacent C atoms, indicating that the N dopants should not be the HER active sites, which could however, activate the adjacent C atoms (Figure S19 and Table S4).45-47 As described in Figure 5, CoP@NC exhibits the most optimized ΔG(H*) value among


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various materials, which indicates that this nanocomposite material should possess the highest catalytic activity for HER, agreeing well with the electrochemical testing results. In general, different from the previous reports which proposed that the Co (δ+) with the neighboring pendant base P (δ−) acted as the active sites,31 the present calculation results indicate that for the CoP@NC core-shell structure, the C atoms above CoP (011) and adjacent to the N dopants, are the most effective active sites.

Figure 5. The calculated adsorption free energy of H (ΔG(H*)) for CoP@NC, CoP@C, and CoP. CONCLUSIONS In summary, CoP nanoparticles encapsulated in ultrathin N-doped porous carbon (CoP@NC) have been synthesized through a MOF-derived route and further applied in both acidic and alkaline media for water electrolysis. The N-doped graphitic carbon layers coated on CoP NPs play a key role in boosting the HER performance. DFT theoretical calculation and a series of experiments demonstrate that the synergistic effect between CoP cores and N-doped graphitic carbon layer shells lead to an outstanding electrocatalytic HER activity, and the direct active


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sites are located on the carbon atoms adjacent to the N dopants in carbon shells. This work introduces a new catalyst for efficient and stable HER, and may open new avenues for designing other encapsulated transition metal oxides, carbides, nitrides, sulfides, and selenides for more applications.

EXPERIMENTAL SECTION Chemicals. Cobalt nitrate hexahydrate (Co(NO3)2•6H2O), zinc nitrate hexahydrate (Zn(NO3)2•6H2O), benzimidazole (C7H6N2), benzenetricarboxylic acid (C9H6O6), N, Ndimethylformamide (C3H7NO), ethanol (C2H5OH), sodium hypophosphite (NaH2PO2), sulfuric acid (H2SO4), potassium chloride (KCl), potassium hydroxide (KOH), isopropyl alcohol (C3H8O), Nafion solution (5 wt.%), and Pt/C (20 wt.%) were utilized directly as received without any further purification. Ultrapure water was used throughout the whole experiment. Characterization. Powder X-ray diffraction (XRD) patterns were performed on a Bruker D8Advance X-ray diffractometer with Cu Kα radiation source (λ = 0.154178 nm). The morphologies and sizes of the samples were observed by using a Zeiss Sigma scanning electron microscope (SEM) and a Tecnai G20 U-Twin transmission electron microscope (TEM) equipped with an energy dispersive X-ray detector (EDX) at an acceleration voltage of 200 kV. Raman spectroscopy was measured with a laser micro-Raman spectrometer (Renishaw in Via, 532 nm excitation wavelength). X-ray photoelectron spectroscopy (XPS) measurement was carried out with a Thermo Fischer ESCALAB 250Xi spectrophotometer. The surface area was analyzed from N2 adsorption–desorption isotherms at liquid nitrogen temperature (77 K) after dehydration under vacuum at 120 °C for 12 h by using a Quantachrome NOVA 4200e.


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Synthesis of MOFs. Micro-scale ZIF-9 was synthesized via a solvothermal process according to the previous literature.15a In a typical synthesis, 0.71 g Co(NO3)2•6H2O and 0.57 g benzimidazole were dissolved in 60 mL N, N-dimethylformamid. After dissolved, the solution was put into a Teflon-lined stainless autoclave. The sealed autoclave was then put into an electric oven and maintained at 140 °C for 24 h with a heating rate of 5 °C min-1. After cooling to ambient temperature with a colding rate of 0.4 °C min-1, ZIF-9 crystals were gained. The product were washed 3 times with ethanol and then dried at 60 °C for 8 h. ZIF-7 was synthesized via the same process with Co(NO3)2•6H2O substituted by Zn(NO3)2•6H2O. Co-HKUST was prepared according to the literature.63 In detail, 0.82 g Co(OAc)2•4H2O and 0.4 g H3btc with 30 mL H2O was placed into a Teflon-lined stainless autoclave and maintained at 140 °C for 24 h with a heating rate of 5 °C min-1, then cooled down to 120 °C with the cooling rate of 0.1 °C/min and held for 5 h, followed by further cooling down to 100 °C with the same rate, and held for another 5 h. Finally the red crystals were centrifuged, washed with H2O and EtOH after cooling to room temperature. Synthesis of CoP@NC. ZIF-9 crystals were pyrolyzed into a tube furnace at 750 °C for 2 h with a heating rate of 5 °C min-1 under Ar atmosphere with an isothermal process at 500 °C for 2 h during the heating. 50 mg pyrolysis products (Co@NC) were further phosphidated at 300 °C for 2 h with a heating rate of 2 °C min-1 in N2 to prepare CoP@NC with 1.0 g NaH2PO2 locating at the upstream. The product was washed by 0.5 M H2SO4 for 8 h, and then centrifuged and wash by ultrapure water and EtOH. For comparison, CoP@C and NC were prepared in the same way of CoP@NC but ZIF-9 replaced by Co-HKUST and ZIF-7, respectively. When an oxidation step of Co@NC was


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performed in air at 250 °C for 12 h before phosphidation without other change, the final product was CoP. CoP@NC-T were prepared in the same way of CoP@NC without an isothermal process at 500 °C for 2 h during the pyrolysis of ZIF-9. Preparation of working electrodes. To make the working electrode, 5 mg catalysts was dispersed in the solution of 0.98 mL isopropyl alcohol mixed with 0.02 mL 5 wt. % Nafion solution and then sonicated for 1 h at least. The glassy carbon electrode (GCE, 5 mm in diameter) was polished with 1.0, 0.5 and 0.05 μm alumina powder successively to obtain a mirror-like surface, and then wash with ultrapure water and ethanol by sonication. After GCE dried in air, 12 μL of the inks was drop-casted on the surface of the GCE intermittently (loading: 0.306 mg cm-1) and dried under air before any electrochemical measurements. Electrochemical measurements. The electrochemical properties of all samples were measured by using a CHI 760E electrochemical analyzer (CH Instruments, Chenhua Co., Shanghai, China) in a three-electrode system at room temperature. The modified GCE with various catalyst samples, a saturated calomel electrode (in acid, or Hg/HgO in base), and a platinum foil (CV, LSV)/graphite rod (stability test) were used as a working electrode, reference electrode and a counter electrode, respectively. Cyclic voltammetry (CV), linear sweep voltammetry (LSV), and amperometric i–t curve were carried out to study the electrochemical properties of the working electrode. Before the test, the actual value of the potential vs. reversible hydrogen electrode (RHE) of the reference electrode was calibrated by using hydrogen electrode reaction with the 20 wt. % Pt/C as working electrode (0.153 mg cm-2). 0.5 M H2SO4 and 1.0 M KOH were used as the electrolyte solution to provide the electrochemical environment with different pH (~ 0 and 14), respectively. Faraday efficiencies were testified in 1.0 M KOH with the CoP@NC modificated carbon cloth as the working electrode (loading: ~ 0.3 mg cm-2)


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via a drainage gas-collecting method. All potentials in this work were conducted with iRcompensation.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website. Additional XRD, TEM images, DTA, N2 adsorption-desorption isotherms, electrochemical tests, and DFT calculation details. AUTHOR INFORMATION Corresponding Author *[email protected]; [email protected] Author Contributions ‡

These authors contributed equally.

Notes The authors declare no competing financial interests. ACKNOWLEDGMENT This work is financially supported by the National natural Science Foundation of China (21571145, 21633008), the Creative Research Groups of Hubei Province (2014CFA007), and the Large-scale Instrument and Equipment Sharing Foundation of Wuhan University.


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Table of Contents.


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Ultrathin Nitrogen-Doped Carbon Coated with CoP for Efficient

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