CoP Nanoparticles in Situ Grown in Three-Dimensional Hierarchical

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CoP Nanoparticles in Situ Grown in Three-Dimensional Hierarchical Nanoporous Carbons as Superior Electrocatalysts for Hydrogen Evolution Weiyong Yuan, Xiaoyan Wang, Xiaoling Zhong, and Chang Ming Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b05304 • Publication Date (Web): 28 Jul 2016 Downloaded from http://pubs.acs.org on August 1, 2016

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ACS Applied Materials & Interfaces

CoP Nanoparticles in Situ Grown in ThreeDimensional Hierarchical Nanoporous Carbons as Superior Electrocatalysts for Hydrogen Evolution Weiyong Yuan,*,†,‡ Xiaoyan Wang,†,‡ Xiaoling Zhong,†,‡ and Chang Ming Li†,‡ †

Institute for Clean energy & Advanced Materials, Faculty of Materials & Energy, Southwest

University, Chongqing 400715, China. ‡

Chongqing Key Laboratory for Advanced Materials and Technologies of Clean Energies,

Chongqing 400715, China. KEYWORDS: three-dimensional hierarchical nanoporous carbon, in-situ growth, transition metal phosphides, CoP nanoparticles, hydrogen evolution

ABSTRACT: The development of efficient and low-cost hydrogen evolution reaction (HER) catalysts is critical for storing energy in hydrogen via water splitting, but still presents great challenges. Herein, we report synthesis of three-dimensional (3-D) hierarchical nanoporous carbon (HNC) supported transition metal phosphides (TMPs) for the first time by in-situ growth of CoP nanoparticles (NPs) in CaCO3 NP-templated Cinnamomum platyphyllum leaf extractderived carbon. They were subsequently employed as a HER catalyst, showing an onset potential of 7 mV, an overpotential of 95.8 mV to achieve 10 mA cm-2, a Tafel plot of 33 mV dec-1, and

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an exchange current density of 0.1182 mA cm-2, of which the onset overpotential and the Tafel plot are the lowest reported for non-noble-metal HER catalysts, and the overpotential to achieve 10 mA cm-2 and the exchange current density also compare favorably to most reported HER catalysts. In addition, this catalyst exhibits excellent durability with negligible loss in current density after 2000 CV cycles ranging from +0.01 to -0.17 V vs. RHE at a scan rate of 100 mV s-1 or 22 h of chronoamperometric measurement at an overpotential of 96 mV and a high Faraday efficiency of close to 100%. This work not only creates a novel high-performance non-noblemetal HER electrocatalyst and demonstrates the great advantages of the in-situ grown 3-D HNC supported TMP NPs for the electrocatalysis of HER, but also offers scientific insight into the mechanism for the in-situ growth of TMP and their precursor NPs, in which an ultra-low reactant concentration and rich functional groups on the 3-D HNC support play critical roles.

1. INTRODUCTION Storage of renewable but intermittent energy such as sunlight and wind in the covalent bonds of hydrogen through electrochemical/photoelectrochemical water splitting is a highly promising strategy to solving today’s energy and environmental crises.1-5 A key step in this process is the hydrogen evolution reaction (HER), for which it is highly desirable to minimize the overpotential, while maximizing the hydrogen production rate.2-4 Pt group metals are the most efficient and widely used HER catalysts, but are quite expensive and scarce.2-4 Thus, development of earth-abundant, low-cost, and active catalysts is critical for the large-scale energy storage via water splitting. Among all the reported non-noble-metal HER catalysts, transition metal phosphides (TMPs) such as CoP, Ni2P, and FeP have attracted much interest for their potentially higher catalytic activity than that of conventionally reported ones and good acid-


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stability, both of which are favorable to polymer electrolyte membrane (PEM) based water splitting.6-9 However, there are still great challenges to further significantly increase the catalytic performance of these TMP-based catalysts to meet the requirements of practical applications due to lack of unique nanostructured supports with large accessible surface area, high conductivity, and rich nucleation sites, and lack of facile and economical controllable strategies with the capability of growing ultrasmall and well-dispersed nanocatalysts.10-13 Three-dimensional (3-D) hierarchical nanoporous carbons (HNCs) possessing a 3-D structure and multiscale pore sizes with at least one between 1 and 100 nm, are particularly promising as catalyst supports since the 3-D carbon structure provides an interconnected network for fast electron transport and avoids the aggregation frequently occurring between 0, 1, and 2-D nanostructures during and after loading of the catalysts,14-16 the larger pores can facilitate the diffusion of reactants and products to accelerate HER, the smaller pores are able to increase the specific surface area to improve the dispersion of catalysts,17-19 and the confinement of catalyst particles in the pores could enhance their durability toward HER under high overpotentials.11,20 Nevertheless, it is a great challenge to load TMP or their precursor nanoparticles (NPs) in the 3D HNCs using conventional approaches including gas-phase deposition, physical/chemical adsorption, impregnation: the gas-phase deposition approach requires sophisticated equipment and harsh conditions, and is not suitable for low-cost mass production of the catalysts;21,22 it is difficult for the pre-synthesized NPs to enter inside pores of the 3-D HNCs via physical/chemical adsorption, and organic linkers frequently used can easily block the nanopores;23-25 the NPs obtained via impregnation are usually large and aggregated due to the weak interaction between the metal ions and the carbon substrates, and the evaporation induced non-uniform distribution of these ions.26,27


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The direct in-situ growth could produce small and well-dispersed NPs with intimate contact between the NPs and supports for maximizing individual properties and introducing synergistic effects.10,28,29,30,31 However, it is rather difficult to realize in 3-D HNCs due to the long and tortuous reactant diffusion path, a large lattice mismatch between carbon and TMPs or their precursors, very few functional groups on the surface of conventionally synthesized HNCs, and high nucleation rate during hydrothermal or high-temperature colloidal synthesis most successful up to date for the creation of TMP or their precursor NPs,6,10,18,29,32 all of which will lead to nucleation and growth in solution, thus forming large particles with severe aggregation and poor contact with the supports. Furthermore, critical factors affecting the in-situ growth, which are important for understanding and further controlling the process, have not been well identified to date.33-35 In this study, we explored the possibility for the in-situ growth of TMP NPs in 3-D HNCs for the first time using CoP as the model TMP and our recently reported 3-D HNC (denoted as BMHNC), which were fabricated from Cinnamomum platyphyllum leaf extract via a NPtemplated biomass derivation approach, as the model 3-D HNC. BMHNC was used due to its well-defined hierarchical nanoporous structure, rich surface functional groups with functionalization degree adjustable via changing the carbon precursor, and their centimeter-scale size which could be claimed as truly 3-D (see the results part).19 The main factors to affect the synthesis were investigated to understand the possible mechanism. The electrocatalytic performance of BMHNC supported CoP NPs toward HER was further studied. 2. EXPERIMENTAL SECTION


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2.1. Materials. Sodium hypophosphite was purchased from Aladdin. Ammonium fluoride, Urea, Co(NO3)2·6H2O, and Nafion (5 wt %) were purchased from Sigma-Aldrich. MWCNT was purchased from Shenzhen Nanotech Port Co., Ltd. Commercial Pt/C catalyst (30 wt %) was bought from E-TEK. The deionized (DI) water used throughout all experiments was purified through a Milli-Q water purification system (Millipore). 2.2. Synthesis of BMHNC. BMHNC was prepared according to our previous work.19 Briefly, Cinnamomum platyphyllum leaves were rinsed with DI water, cut into small pieces, and mixed with a few CaCO3 nanoparticles. Then the mixture was ground thoroughly with the help of a mixed solvent of EtOH and H2O (EtOH/H2O=1:1, v/v), heated for 2 min in a microwave oven, and filtered by using Whatman filter paper with a pore size of 0.2 mm. The filtrate was dried at 70 °C, resulting in a viscous leaf extract. 3 g of the leaf extract was then mixed with 3 g of CaCO3 nanoparticles with the help of ethanol, dried in a blast oven at 60 °C for 2 h, and heated to 800 °C for 2 h under the protection of Ar. Finally, the product was washed with 1 M HCl and DI water several times and dried in an oven at 60 °C. 2.3. Synthesis of non-functionalized HNC (NFHNC). NFHNC was prepared according to the work of Zhao et al.18 The mass ratio of PF resin to nano-CaCO3 was 1:1. 2.4. Synthesis of BMHNC supported CoP NPs (CoP/BMHNC). 10 mg of the BMHNC was added into 30 ml of a mixed solution prepared by dissolving 0.83 mM Co(NO3)2•6H2O, 4.17 mM urea, and 2.08 mM NH4F in DI water, ultrasonicated for 20 min, and stirred overnight. The suspension was further ultrasonicated for 20 min and then transferred to a Teflon-lined stainless steel autoclave of 50 mL capacity. Subsequently, the autoclave was sealed and heated at 120 °C for 12 h, and then cooled to room temperature naturally. The product was filtered with large


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amount of DI water and then dried under vacuum for 5 h. To prepare CoP/BMHNC, 10 mg of the obtained product above and 0.2 g of NaH2PO2 were put at two separate positions in a porcelain boat with NaH2PO2 at the upstream side of the tube furnace. Subsequently, the samples were heated at 300 ℃ for 2 h in an Ar atmosphere, and then naturally cooled to ambient temperature. 2.5. Synthesis of CoP/BMHNC with doubled concentration of reactants (CoP/BMHNCD). The preparation method and conditions are the same as described above except that the concentrations of all the reactants including BMHNC are doubled. 2.6. Purification of pristine MWNTs without introducing functional groups. The purification of pristine MWNTs was carried out according to our reported procedures with minor modifications.36 Briefly, the MWNTs were dispersed in 60 ml of a 37% HCl solution, ultrasonicated for 1.5 h, and stirred for 12 h. They were then repeatedly centrifuged and washed with 37% HCl for three times, before being dispersed in 60 ml of the 37% HCl solution again. The above process was repeated for 3 times. Subsequently, the MWNTs were dispersed in 50 mL of DI water, ultrasonicated for 1.5 h, stirred for 10 h, and then filtered with large amount of DI water. The final product was dried at 70 °C under vacuum for 5 h. 2.7. Synthesis of CoP/MWNT. The preparation method and conditions were the same as that used for the synthesis of CoP/BMHNC except that purified pristine MWNTs were used as the support. 2.8. Synthesis of CoP/NFHNC. The preparation method and conditions were the same as that used for the synthesis of CoP/BMHNC except that NFHNC was used as the support.


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2.9. Synthesis of CoP/graphene oxide (GO). The preparation method and conditions were the same as that used for the synthesis of CoP/BMHNC except that GO was used as the support. 2.10. Characterization. Field emission scanning electron microscopy (FESEM) imaging was carried out on a JEOL JSM-7800F microscope. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images were taken on a JEOL JEM-2100 microscope. The Scanning TEM characterization was performed on a JEOL JEM-ARM200F microscope equipped with a cold field emission gun and probe Cs corrector. X-ray powder diffraction (XRD) patterns were obtained using a Shimadzu XRD-7000 diffractometer with Cu K α line. X-ray photoelectron spectra (XPS) were recorded on a Thermo Scientific ESCALAB 250Xi X-ray photoelectron spectrometer, using Al Kα radiation as the excitation source.

2.11. Electrochemical measurements. All electrochemical measurements were performed with a three-electrode system in 0.5 M H2SO4 on a CHI660E electrochemical workstation (CH Instruments, Inc., USA). A saturated calomel electrode (SCE) and a platinum foil were used as the reference electrode and counter electrode, respectively. For the preparation of working electrode, 60 µL of catalyst solutions with Nafion (mass ratio of catalyst to Nafion is 4) was dropped on the clean glassy carbon electrode and then dried under the infrared lamp. The loading of the catalysts on the electrode is 300 µg cm-2. Linear sweep voltammetry (LSV) measurements were conducted at a scan rate of 2 mV s-1. Cyclic voltammetry (CV) measurements were carried out between +0.01 V and -0.17 V at 100 mV s-1. Electrochemical impedance spectra (EIS) were obtained at an overpotential of 200 mV with a frequency range from 1 Hz to 1 MHz and an amplitude of 10 mV. All the electrochemical data are presented with the potentials/overpotentials iR corrected. The onset potentials of the HER are determined from the intersection of the x-axes


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and the tangents of the LSV curves. All the potentials reported in our work are versus the reversible hydrogen electrode (RHE). In 0.5 M H2SO4, E (RHE) =E (SCE) + 0.259 V, based on the RHE calibration experiment (see the detailed procedure in Supporting Information). 2.12. Hydrogen production measurement. The amount of hydrogen produced during electrochemical water splitting was measured via an online gas chromatograph (GC-2014, Shimadzu). The cell setup was the same as that used for electrochemical measurements. The cathode potential was maintained at -0.25 V. 3. RESULTS AND DISCUSSION 3.1. Scheme for fabrication of CoP/BMHNC. The process for the in-situ synthesis of 3-D BMHNC supported CoP NPs is schematically shown in Scheme 1. Firstly, BMHNC was synthesized from the extract of Cinnamomum platyphyllum leaves using the CaCO3 NPtemplated biomass derivation approach. Secondly, CoP precursor NPs were in situ grown in BMHNC via one-step hydrothermal synthesis. Finally, the CoP precursor NPs were in situ converted to CoP NPs via low-temperature phosphidation.


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Scheme 1. The process for the synthesis of 3-D BMHNC supported CoP NPs. 3.2. Synthesis and characterization of BMHNC. The representative photograph of BMHNC (Fig. 1A) shows that it is a 3-D carbon material. Its FESEM images at different magnifications (Fig. 1B, Fig. 1C, and Fig. 1C inset) reveal a large-area and highly uniform porous structure with pore sizes ranging from ~10 nm to ~100 nm. Moreover, it is composed of ultrathin nanosheets which have a thickness smaller than 10 nm and interconnect together (Fig. 1C and its inset). The TEM image further shows its nanoporous structure as well as 3-D networked and ultrathin nanosheet-assembled architecture (Fig. 1D). The N2 adsorption-desorption isotherm (Fig. 1E) exhibits a type IV characteristics: the steep increase in the adsorbed volume at low relative pressure, the desorption hysteresis at medium relative pressure, and the almost vertical tails at a relative pressure close to 1.0 indicate the presence of micropores, mesopores, and macropores, respectively.18,37 The pore size distribution obtained by the BJH method reveals the existence of large macro/mesopores within a broad size range from 12-70 nm as well as small mesopores within a size range from 3-5 nm (Fig. 1F). The former should be caused by the removal of CaO NPs, which are formed from pyrolysis of CaCO3 NPs,18,19 and the latter caused by activation of CO2 also produced from the CaCO3 pyrolysis.18,19


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Figure 1. Photograph (A), large-area FESEM (B), FESEM (C), TEM (D), N2 adsorptiondesorption isotherm (E) and pore size distribution (F) of BMHNC. The inset in (C) is the highmagnification FESEM image of BMHNC, and the inset in (F) is pore size distribution of BMHNC in the size range of 3-10 nm.


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The FTIR spectrum of BMHNC (Fig. 2A) shows several characteristic peaks: the one at 3438 cm-1 could be ascribed to –OH stretching vibration; 2914, 2854 and 1452 cm-1 are due to stretching and bending vibrations of C–H bands, respectively; 1468 and 1402 cm-1 are associated with C=C stretching; 1132 cm-1 is due to –CH2 vibrations typical for substituted aromatics; the ones at 1622 and 1573 cm-1 are attributed to δ(–CO–NH–) and δ(–NH–), respectively; the one at 725 and 800 cm-1 could be ascribed to aromatic –CH bending, and the one at 670 cm-1 could be due to some inorganic species.19,38-40 The NMR spectrum of BMHNC (Fig. 2B and C) exhibits a few peaks: the peaks at 2.65, 6.55, 6.98, 7.09 and 7.2 ppm (Fig. 2B inset and Fig. 2C) are due to the R-NH–Ar–SH; the ones at 1.36, 1.48, 1.66 and 2.38 ppm (Fig. 2B inset) correspond to R-Ar–CO–NH–R; those at 0.86, 1.25, 2.51 ppm are ascribed to CH3, CH2, and DMSO (solvent), respectively.19,41,42 The FTIR and NMR spectra clearly indicate the functionalization of the carbon materials with groups such as amide- and sulfur- containing ones. The high-resolution C 1s XPS spectrum of BMHNC in Fig. 2d shows C-C peak at 284.1 and 285.2 eV, C=C peak at 284.6 eV, C-O/C-N peak at 286.04 eV, N-C=O peak at 288.8 eV, and OC=O peak at 289.09 eV.43-45 The high-resolution S 2p XPS spectrum shows C-S-C peak at 164.85 eV and S-H peak at 163.77 eV, (-)SO3R peak at 168.9 eV, and RSO2 peak at 167.77 eV.46-48 The high-resolution N 1s spectrum shows Pyrridinic N at 399.1 eV, Pyrridonic N peaks at 400.4 eV, and R–NH–R peak at 399.5 eV.19,42 These data demonstrate the successful functionalization of BMHNC with groups of Pyrridonic N, –NH–, –SH, (-)RSO3, and RSO2.


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Figure 2. FTIR (A), NMR (B and C), and high-resolution C 1s (D), S 1s (E), and N 1s (F) XPS spectra. 3.3 In-situ growth and characterization of CoP in BMHNC. BMHNC was then used as a support for growth of CoP NPs. Fig. 3A shows the XRD patterns of the samples before and after the in-situ low-temperature phosphidation. For the sample before phosphidation, several characteristic peaks of orthorhombic Co(CO3)0.5(OH)•0.11H2O (JCPDS card no. 48-0083) are


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identified.49,50 However, after phosphidation, only the characteristic diffraction peaks of orthorhombic CoP (JCPDS card no. 29-0497) are observed (Fig. 3B is its crystal structure).7,9 This result indicates the complete conversion of cobalt-hydroxide-carbonate to CoP. The size of CoP NPs can be estimated according to Scherrer's equation:51

=

0.9

where L is the average particle size, is the X-ray wavelength, is the angle of the (011) face, and is the half-peak width of the (011) face. The calculated size of CoP NPs is 2.9 nm. Fig. 3C and Fig. 3D display the high-resolution XPS spectra of the sample after phosphidation. The high-resolution Co 2p3/2 spectrum (Fig. 3C) shows two peaks at 779.6 and 781.3 eV, while the P 2p spectrum (Fig. 3D) exhibits three peaks at 130.1 (P 2p3/2), 130.8 (P 2p1/2), and 133.9 eV. The peaks at 779.6 and 130.1 eV are close to the binding energies of Co and P in CoP,7,9 thus indicating the formation of CoP, and the peaks at 782.1 and 133.9 eV correspond to oxidized states of Co and P, which arise from surface oxidation of CoP by O2 in the air.7,9 It needs to be noted that the XPS were measured immediately after the phosphidation. The fast oxidation of CoP suggests that BMHNC could greatly facilitate O2 diffusion.


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Figure 3. (A) XRD patterns of BMHNC supported CoP precursor and CoP. (B) The crystal structure of orthorhombic CoP. (C) High-resolution Co 2p3/2 XPS spectra of CoP/BMHNC. (D) High-resolution P 2p XPS spectra of CoP/BMHNC. Fig. 4 shows FESEM images of CoP/BMHNC at different magnifications. CoP NPs cannot be seen at low magnifications (Fig. 4A and B), suggesting their very small size and good dispersion. The architecture of BMHNC remains the same as that before growth of CoP NPs, indicating its structure is robust enough to withstand harsh hydrothermal growth and phosphidation conditions. At high magnifications (Fig. 4C and D), ultra-small NPs are indeed observed with a very high surface density, and they are distributed quite uniformly on BMHNC. This result clearly demonstrates the successful growth of CoP NPs on the BMHNC.


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Figure 4. FESEM images of CoP/BMHNC at different magnifications. Fig. 5A shows the low-magnification TEM image of CoP/BMHNC. No NPs can be clearly observed as well. This further suggests that CoP NPs are very small and highly dispersed without severe aggregations. The formation of these NPs is confirmed by the high-magnification TEM images (Fig. 5B and C), from which small, uniform, and well-dispersed NPs are clearly observed. No NPs are observed in other places than BMHNC, suggesting that the CoP NPs are preferentially grown on BMHNC rather than in solution. It is found that the NPs are a little fuzzy at high magnifications even under the optimized imaging parameters (Fig. 5C). This should be due to their entrapment in the 3-D carbon matrix. The NP size is ~2.6 nm, as evaluated from diameters of ~50 CoP NPs in TEM image (Fig. S2). This value is slightly smaller than that estimated from the XRD data (Fig. 3A). Lattice spacings of ~0.28 nm are observed from the HRTEM image (Fig. 5D), corresponding to the (011) planes of orthorhombic CoP (JCPDS card


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no. 29-0497).9,11 It is noteworthy that reported carbon supported CoP NPs are usually much larger than the ones synthesized here.11,52 One study shows the synthesis of 2~3 nm CoP NPs, but their precursors are obtained via two reaction steps in long reaction time (over 20 h) and the NPs are aggregated together.7 The STEM images were obtained to further evaluate the distribution of CoP (Fig. 6). The CoP NPs are distributed uniformly all over BMHNC. This result is also supported by the high-magnification FESEM images (Fig. 4C and D). The synthesis of CoP precursors reported herein is simple with only one step and accomplished in shorter time, and the obtained CoP NPs are the smallest up to date that are still kept well separated without aggregation.

Figure 5. Representative TEM (A, B, and C) and HRTEM (D) images of CoP/BMHNC at different magnifications.


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Figure 6. STEM images of CoP/BMHNC at different magnifications. 3.4. Growth mechanism of CoP/BMHNC. The mechanism for the synthesis of 3-D BMHNC supported CoP NPs was then investigated. No morphological difference can be observed from TEM for the samples before and after phosphidation (data not shown). Thus, the NP formation is fully dependent on the hydrothermal reaction process. We here argue that the formation of small and well-dispersed Co(CO3)0.5(OH)•0.11H2O NPs in BMHNC is ascribed to the in-situ nucleation and growth during the hydrothermal synthesis. In our system, the concentrations of reactants are one to two orders of magnitude lower than what are conventionally adopted.49,53,54 If the concentrations of BMHNC and all reactants were deliberately doubled (see Experimental part for the synthesis of CoP/BMHNC-D), much larger NPs (>100 nm) were formed, which were clearly nucleated and grown in solution (Fig. 7A), and no small NPs could be found in the carbon matrix (Fig. 7B). Under this condition, the nucleation rate on BMHNC is essentially


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unchanged due to the same ratio of reactants to BMHNC, but that in solution increases due to the doubled concentration of reactants. Thus, it is very likely that the nucleation rate in solution has exceeded that on BMHNC, leading to the preferential growth in solution. This result shows that the Co(CO3)0.5(OH)•0.11H2O NPs are formed by in-situ nucleation and growth. Moreover, it indicates that the concentrations of reactants play a critical role in the in-situ seed nucleation, and for the 3-D HNC supports which possess a long and tortuous diffusion path, they must be kept very low. To further reveal the importance of in-situ synthesis of Co(CO3)0.5(OH)•0.11H2O NPs in BMHNC, NFHNC, which had the same architecture as BMHNC as our previous report showed,19 was adopted as the support (also see its TEM images in Fig. S3). Very large NPs (>100 nm) from solution but no NP in the carbon matrix were observed (Fig. 7C and D), obviously due to the nucleation of Co(CO3)0.5(OH)•0.11H2O in solution rather than on the support. This result also indicates that the functional groups on BMHNC can significantly promote the nucleation on its surface for in-situ growth of Co(CO3)0.5(OH)•0.11H2O NPs. Pristine MWNTs were also used (see Experimental part) since they are benchmark catalyst supports.55,56 Quite large and severely aggregated particles were obtained (Fig. 7E) and no NPs were attached on the nanotube walls (Fig. 7F), due to their inert surface greatly reducing the nucleation rate on them. This further demonstrates the importance of functional groups in the insitu synthesis. GO has a sheet-like structure and rich functional groups similar to BMHNC, and therefore was further employed as the support. However, the obtained NPs are much larger than that on BMHNC (Fig. 7G and H). GO nanosheets have no pores, and easily restack to form dense multilayer structure during the synthesis (Fig. 7G and H). This result suggests that the 3-D porous structure of BMHNC is beneficial for the formation of small NPs in our in-situ growth approach.


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Figure 7. TEM image (A) and high-magnification TEM image (B) of CoP/BMHNC-D. TEM image (C) and high-magnification TEM image (D) of CoP/NFHNC. TEM image (E) and highmagnification TEM image (F) of CoP/MWNT. TEM image (G) and high-magnification TEM image (H) of CoP/GO. The in-situ conversion of Co(CO3)0.5(OH)•0.11H2O to CoP via the low-temperature phosphidation is likely to occur according to the following reaction steps: firstly, PH3 is generated

from

the

in-situ

thermal

decomposition

of

NaH2PO2

to

reduce


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Co(CO3)0.5(OH)•0.11H2O to Co;57 then, the Co catalyzes a further decomposition of PH3 to elemental P;58 finally, the Co and P react to form CoP.59 It needs to be noted that the in-situ synthesis of TMP or its precursor NPs in 3-D HNC is a great challenge. In this study, we realize the in-situ growth of CoP using an ultra-low reactant concentration and BMHNC with rich functional groups. To our knowledge, the in-situ growth of CoP in 3-D HNC has never been studied before. In addition, the effects of concentration and surface functionalization on the in-situ growth have not been investigated. 3.5. Electrocatalytic performance of CoP/BMHNC. The HER catalytic activity of CoP/BMHNC was subsequently investigated. Fig. 8A shows the LSV curve of CoP/BMHNC. To reveal the effect of in-situ synthesis on the activity, CoP/BMHNC-D with the same support as CoP/BMHNC, but different growth mechanism was used as a comparison. To reveal the effect of porosity of the carbon support on the activity, CoP/GO was also used for comparison. In addition, the curves of CoP/MWNT and commercial Pt/C are displayed to demonstrate the great potential of using BMHNC as the support. As expected, the commercial Pt/C catalyst shows excellent HER activity with an overpotential close to zero. Interestingly, CoP/BMHNC exhibits very high catalytic activity as well. Its onset overpotential is only 7 mV, which is the lowest reported for non-noble-metal HER catalysts, and it only requires an overpotential of 95.8 mV to achieve a current density of 10 mA cm-2, which is much lower than reported CoP supported on homogeneous nanoporous carbon,11 and among the lowest reported for non-noble-metal HER catalysts (see Table 1 for detailed comparison of reported most active non-noble-metal HER catalysts). The onset overpotentials of CoP/BMHNC-D, CoP/MWNT, and CoP/GO are 70 mV, 79 mV, and 127 mV, respectively, all of which are much larger than that of CoP/BMHNC, and the overpotentials to achieve 10 mA cm-2 are 163 mV, 165 mV, and 299 mV, also significantly


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larger than that of CoP/BMHNC. This result demonstrates the unique advantages of in-situ growth of CoP NPs in BMHNC and high porosity of the BMHNC for greatly promoting the catalytic activity toward HER. Fig. 8B displays the Tafel plots of CoP/BMHNC, CoP/BMHNC-D, CoP/MWNT, CoP/GO, and commercial Pt/C. The Tafel slope of Pt/C is 28.1 mV dec-1, close to the reported values.7,8 CoP/BMHNC has a slightly higher Tafel slope of 33 mV dec-1, which is the lowest value reported for the non-noble-metal HER catalysts. Although the Tafel slope does not match the expected values of 29, 38, and 116 mV dec-1 corresponding to different rate-determining steps in the HER, this result clearly shows that the kinetics of HER on CoP/BMHNC is very fast.3,60 In comparison, the Tafel slopes for CoP/MWNT, CoP/BMHNC-D, and CoP/GO are 38.6 mV dec-1, 62.1 mV dec-1, and 93.5 mV dec-1, respectively, indicating their slower HER rate. The exchange current density of CoP/BMHNC achieves 0.1182 mA cm-2, which is at the same magnitude as that of Pt/C (0.7939 mA cm-2). This exchange current density is among the highest reported for non-noble-metal based HER catalysts, including CoP supported on homogeneous nanoporous carbon (Table 1).11 However, the exchange current densities of CoP/MWNT, CoP/BMHNC-D, and CoP/GO are only 0.00042 mA cm-2, 0.02561 mA cm-2, and 0.00836 mA cm-2, respectively. Therefore, their intrinsic catalytic activities are much poorer than that of CoP/BMHNC. The superior HER catalytic activity of CoP/BMHNC could be ascribed to several reasons: firstly, the 3-D interconnected carbon nanostructure provides a conducting network for fast charge transport;15,16,66 secondly, the hierarchical nanoporous structure of BMHNC facilitates the ion and mass diffusion, and improves the dispersion of CoP NPs for more exposed active sites toward HER;17-19 thirdly, the in-situ growth of CoP NPs not only significantly reduces their size


21


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Page 22 of 48

and improves their dispersion, but also ensures their good contact with BMHNC for reduced charge transfer resistance.10,28,29 Electrochemical impedance spectroscopy was carried out to investigate the interfacial charge transfer during HER. Indeed, CoP/BMHNC has a much lower charge transfer resistance than CoP/MWNT, CoP/BMHNC-D, and CoP/GO, indicating that the direct in-situ growth of CoP in BMHNC can greatly facilitate the charge transfer resistance (Fig. 8C). Since a smaller charge transfer resistance corresponds to a faster reaction kinetics, this result also further confirms much more favorable HER kinetics of CoP/BMHNC than those of CoP/BMHNC-D, CoP/MWNT, and CoP/GO. The durability of an electrocatalyst is very critical for its practical applications. The LSV curves of CoP/BMHNC that were measured before and after 2000 CV cycles ranging from +0.01 to -0.17 V at a scan rate of 100 mV s-1 are shown in Fig. 8D. The LSV curve after the CV experiment exhibits negligible loss in current density compared to the initial one. The inset in Fig. 8D shows the time dependence of the current density at an overpotential of 96 mV, revealing that CoP/BMHNC can maintain its catalytic activity for at least 22 hours.


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Figure 8. （ A ） LSV curves of CoP/BMHNC, CoP/BMHNC-D, BMHNC, CoP/MWNT, CoP/GO, and commercial Pt/C. (B) Tafel plots of CoP/BMHNC, CoP/BMHNC-D, CoP/MWNT, CoP/GO, and commercial Pt/C. (C) Nyquist plots of CoP/BMHNC, CoP/BMHNC-D, CoP/MWNT, and CoP/GO measured at an overpotential of 200 mV in a frequency range from 106-1 Hz. (D) LSV curves of CoP/BMHNC at a scan rate of 2 mV s-1 before and after 2000 CV cycles at a scan rate of 100 mV s-1 between +0.01 to -0.17 V. Inset: time dependence of the current density of CoP/BMHNC at an overpotential of 96 mV. Table 1. Detailed comparison of the performance of CoP/BMHNC with those of representative non-noble-metal HER catalysts.


23


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Catalyst CoP/CNT CoP/CC CoP nanotubes CoP/Ti CoP/RGO CoP/macroporous carbon CoP–mesoporous carbon vesicles CoP–ordered mesoporous carbon CoP nanowires MoS2/RGO Hollow Ni2P nanoparticles Ni5P4-Ni2P nanosheet array Nanoporous FeP nanosheets FeP/graphene nanosheets CoP/BMHNC

Loading (mg cm-2)

Onset potential (V)

Overpotential at 10 mA cm-2

Tafel plot (mV dec-1)

0.285 0.92 0.2 2.0 0.285

-0.04 -0.038 -0.04 -0.1189

122 67 129 90 156.89

54 51 60 43 70.22

0.285

-0.1053

141.73

0.285

-0.0956

0.285 0.35 1

Page 24 of 48

Exchange current density (mA cm-2) 0.13 0.288

Reference

0.057

7 9 13 61 11

69.83

0.074

11

134.34

63.10

0.091

11

-0.0777

112.18

56.67

0.161

11

-0.04 -0.1

110

54 41

0.16

62 63

116

46

0.033

6

-0.054

120

79.1

0.116

64

0.28

-0.1

240

67

0.28

-0.03

123

50

0.12

65

0.3

-0.007

95.8

33

0.1182

This work

1

8

3.6. Hydrogen production measurement. The amount of hydrogen produced through electrochemical water splitting was measured quantitatively using gas chromatography (GC). The Faradaic efficiency (FE) of the HER process can be obtained by dividing the measured amount of hydrogen with calculated one (assuming 100% FE). The excellent agreement of the two sets of values (Fig. 9) indicates that the FE is close to 100%.


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Figure 9. The calculated (solid line) and measured (red dot) amount of hydrogen at different times for CoP/BMHNC at -0.25 V for 120 min in 0.5 M H2SO4. 4. CONCLUSION 3-D HNC supported TMPs was synthesized for the first time by in-situ growth of CoP NPs in CaCO3 NP-templated Cinnamomum platyphyllum leaf extract-derived carbon. This 3-D hybrid catalyst shows an onset potential of 7 mV, an overpotential of 95.8 mV to achieve 10 mA cm-2, a Tafel plot of 33 mV dec-1, and an exchange current density of 0.1182 mA cm-2, of which the onset overpotential and the Tafel plot are the lowest reported for non-noble-metal HER catalysts, and the overpotential to achieve 10 mA cm-2 and the exchange current density also compare favorably to most reported HER catalysts, including homogeneous nanoporous carbon supported CoP. In addition, it exhibits excellent durability with negligible loss in current density after 2000


25


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CV cycles ranging from +0.01 to -0.17 V vs. RHE at a scan rate of 100 mV s-1 or 22 h of chronoamperometric measurement at an overpotential of 96 mV and a high Faraday efficiency of close to 100%. This work not only offers a novel high-performance non-noble-metal HER electrocatalysts and reveals great advantages of in-situ grown 3-D HNC supported TMP NPs toward the electrocatalysis of HER, but provides scientific insight into the mechanism for the insitu growth of TMP and their precursor NPs, in which an ultra-low reactant concentration and rich functional groups on the 3-D HNC support play critical roles. ASSOCIATED CONTENT Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org. Cyclic voltammogram of Pt foil in H2-saturated 0.5 M H2SO4 with a scan rate of 1 mV/s, size distribution of CoP NPs in CoP/BMHNC, TEM images of FNHNC with different magnifications. AUTHOR INFORMATION Corresponding Author *Email: [email protected]. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT


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This work was financially supported by Scientific Research Foundation for the Returned Overseas Chinese Scholars, Ministry of Education of China, Chongqing Natural Science Foundation (No. cstc2015jcyjA50029), the Fundamental Research Funds for the Central Universities (Grant No. XDJK2015C026), and Start-up grant from Southwest University, China (Grant No. SWU114090). REFERENCES (1)

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(55) Huang, H.; Sun, D.; Wang, X. Low-Defect MWNT–Pt Nanocomposite as a High Performance Electrocatalyst for Direct Methanol Fuel Cells. J. Phys. Chem. C 2011, 115, 1940519412. (56) Zykwinska, A.; Radji-Taleb, S.; Cuenot, S. Layer-by-Layer Functionalization of Carbon Nanotubes with Synthetic and Natural Polyelectrolytes. Langmuir 2010, 26, 2779-2784. (57) Guan, Q.; Li, W. A Novel Synthetic Approach to Synthesizing Bulk and Supported Metal Phosphides. J. Catal. 2010, 271, 413-415. (58) Xue-jiao, T.; Zong-ming, X.; Chang-xiu, H.; Bao-gui, Z. In Sustainable Nanotechnology and the Environment: Advances and Achievements; Shamim, N., Sharma, V. K., Eds.; American Chemical Society: 2013; Chapter 11, pp 181-199. (59) Henkes, A. E.; Vasquez, Y.; Schaak, R. E. Converting Metals into Phosphides: A General Strategy for the Synthesis of Metal Phosphide Nanocrystals. J. Am. Chem. Soc. 2007, 129, 1896-1897. (60) Duan, J.; Chen, S.; Chambers, B. A.; Andersson, G. G.; Qiao, S. Z. 3D WS2 Nanolayers@Heteroatom-Doped Graphene Films as Hydrogen Evolution Catalyst Electrodes. Adv. Mater. 2015, 27, 4234-4241. (61) Pu, Z.; Liu, Q.; Jiang, P.; Asiri, A. M.; Obaid, A. Y.; Sun, X. CoP Nanosheet Arrays Supported on a Ti Plate: An Efficient Cathode for Electrochemical Hydrogen Evolution. Chem. Mater. 2014, 26, 4326-4329.


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(62) Jiang, P.; Liu, Q.; Ge, C.; Cui, W.; Pu, Z.; Asiri, A. M.; Sun, X. CoP Nanostructures with Different Morphologies: Synthesis, Characterization and a Study of Their Electrocatalytic Performance toward the Hydrogen Evolution Reaction. J. Mater. Chem. A 2014, 2, 1463414640. (63) Li, Y.; Wang, H.; Xie, L.; Liang, Y.; Hong, G.; Dai, H. MoS2 Nanoparticles Grown on Graphene: An Advanced Catalyst for the Hydrogen Evolution Reaction. J. Am. Chem. Soc. 2011, 133, 7296-7299. (64) Wang, X.; Kolen'ko, Y. V.; Bao, X.-Q.; Kovnir, K.; Liu, L. One-Step Synthesis of SelfSupported Nickel Phosphide Nanosheet Array Cathodes for Efficient Electrocatalytic Hydrogen Generation. Angew. Chem., Int. Ed. 2015, 54, 8188-8192. (65) Zhang, Z.; Lu, B.; Hao, J.; Yang, W.; Tang, J. FeP Nanoparticles Grown on Graphene Sheets as Highly Active Non-Precious-Metal Electrocatalysts for Hydrogen Evolution Reaction. Chem. Commun. 2014, 50, 11554-11557. (66) Li, Z.; Zhang, L.; Amirkhiz, B. S.; Tan, X.; Xu, Z.; Wang, H.; Olsen, B. C.; Holt, C. M. B.; Mitlin, D. Carbonized Chicken Eggshell Membranes with 3D Architectures as HighPerformance Electrode Materials for Supercapacitors. Adv. Energy Mater. 2012, 2, 431-437.


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Scheme 1. The process for the synthesis of 3-D BMHNC supported CoP NPs. Scheme 1 22x9mm (300 x 300 DPI)


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Figure 1. Photograph (A), large-area FESEM (B), FESEM (C), TEM (D), N2 adsorption-desorption isotherm (E) and pore size distribution (F) of BMHNC. The inset in (C) is the high-magnification FESEM image of BMHNC, and the inset in (F) is pore size distribution of BMHNC in the size range of 3-10 nm. Fig. 1 65x85mm (300 x 300 DPI)



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Figure 2. FTIR (A), NMR (B and C), and high-resolution C 1s (D), S 1s (E), and N 1s (F) XPS spectra. Fig. 2 58x68mm (300 x 300 DPI)


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Figure 3. (A) XRD patterns of BMHNC supported CoP precursor and CoP. (B) The crystal structure of orthorhombic CoP. (C) High-resolution Co 2p3/2 XPS spectra of CoP/BMHNC. (D) High-resolution P 2p XPS spectra of CoP/BMHNC. Fig. 3 41x34mm (300 x 300 DPI)



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Figure 4. FESEM images of CoP/BMHNC at different magnifications. Fig. 4 38x28mm (300 x 300 DPI)


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Figure 5. Representative TEM (A, B, and C) and HRTEM (D) images of CoP/BMHNC at different magnifications. Fig. 5 67x44mm (300 x 300 DPI)



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Figure 6. STEM images of CoP/BMHNC at different magnifications. Fig. 6 50x49mm (300 x 300 DPI)


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Figure 7. TEM image (A) and high-magnification TEM image (B) of CoP/BMHNC-D. TEM image (C) and highmagnification TEM image (D) of CoP/NFHNC. TEM image (E) and high-magnification TEM image (F) of CoP/MWNT. TEM image (G) and high-magnification TEM image (H) of CoP/GO. Fig. 7 76x114mm (300 x 300 DPI)



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Figure 8.（A）LSV curves of CoP/BMHNC, CoP/BMHNC-D, BMHNC, CoP/MWNT, CoP/GO, and commercial Pt/C. (B) Tafel plots of CoP/BMHNC, CoP/BMHNC-D, CoP/MWNT, CoP/GO, and commercial Pt/C. (C) Nyquist plots of CoP/BMHNC, CoP/BMHNC-D, CoP/MWNT, and CoP/GO measured at an overpotential of 200 mV in a frequency range from 106-1 Hz. (D) LSV curves of CoP/BMHNC at a scan rate of 2 mV s-1 before and after 2000 CV cycles at a scan rate of 100 mV s-1 between +0.01 to -0.17 V. Inset: time dependence of the current density of CoP/BMHNC at an overpotential of 96 mV. Fig. 8 37x27mm (300 x 300 DPI)


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Figure 9. The calculated (solid line) and measured (red dot) amount of hydrogen at different times for CoP/BMHNC at -0.25 V for 120 min in 0.5 M H2SO4. Fig. 9 37x27mm (300 x 300 DPI)



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For Table of Contents Use Only 35x15mm (300 x 300 DPI)


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CoP Nanoparticles in Situ Grown in Three-Dimensional Hierarchical

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