- Email: [email protected]
Porous coordination polymer-derived ultrasmall CoP encapsulated in nitrogen-doped carbon for efficient hydrogen evolution in both acidic and basic media
international journal of hydrogen energy xxx (xxxx) xxx
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Porous coordination polymer-derived ultrasmall CoP encapsulated in nitrogen-doped carbon for efficient hydrogen evolution in both acidic and basic media Shuai Liu a,b,1, Anning Jiang b,1, Zegao Wang c, Mengxia Lin a, Dan Xia a,*, Qiang Li b,**, Mingdong Dong c,*** a School of Materials Science and Engineering, Hebei University of Technology, And Tianjin Key Laboratory of Materials Laminating Fabrication and Interface Control Technology, Tianjin, 300130, China b Key Laboratory of Colloid and Interface Chemistry of Ministry of Education, And School of Chemistry and Chemical Engineering, Shandong University, Jinan, 250100, Shandong, China c Interdisciplinary Nanoscience Center (iNANO), Aarhus University, Aarhus C, DK, 8000, Denmark
highlights u-CoP@NC were directly synthesized through phosphorization of a PCPs precursor. Remarkable electrocatalytic activity of u-CoP@NC towards HER was achieved. The synergistic effect between u-CoP and NC could improve the HER performance. The confinement by NC layers enhanced the stability of the u-CoP@NC catalysts.
article info
abstract
Article history:
Exploiting high-efficient and stable non-precious metal-based electrocatalysts toward
Received 16 September 2019
hydrogen evolution reaction (HER) is of enormous significance to address the shortage of
Received in revised form
global power source, but there remain major challenges. Here we present a facile and
4 November 2019
controllable strategy to synthesize a strongly coupled ultrasmall-cobalt phosphide/nitro-
Accepted 6 November 2019
gen-doped graphitic carbon (u-CoP@NC) hybrid structure via phosphorization from a
Available online xxx
porous coordination polymer (PCP) precursor. The PCP-derived u-CoP@NC exhibits remarkable activity and stability for HER, achieving a current density of 10 mA cm2 with a
Keywords:
low overpotential of 131 mV in acidic media and 111 mV in basic media. The corresponding
Cobalt phosphide
Tafel slopes present in acidic and basic media are 62.7 and 70.3 mV dec1, respectively.
Nitrogen-doped graphitic carbon
Results reveal that the enhanced electrocatalytic performance of u-CoP@NC originates
layer
from the strongly coupled u-CoP nanoparticles and graphitic carbon layer, and the perfect
Hydrogen evolution reaction
dispersity of the active sites. This research opens up new avenues for designing earth-
Porous coordination polymer
abundant metal-based electrocatalysts with high capability for water splitting applications.
Electrocatalyst
© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
* Corresponding author. ** Corresponding author. *** Corresponding author. E-mail addresses: [email protected] (D. Xia), [email protected] (Q. Li), [email protected] (M. Dong). 1 These authors contributed equally to this work. https://doi.org/10.1016/j.ijhydene.2019.11.063 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Liu S et al., Porous coordination polymer-derived ultrasmall CoP encapsulated in nitrogen-doped carbon for efficient hydrogen evolution in both acidic and basic media, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.11.063
2
international journal of hydrogen energy xxx (xxxx) xxx
Introduction As a renewable and clean source of energy, hydrogen has been regarded as a promising candidate to replace the traditional fossil fuels [1]. Compared to most current technologies for hydrogen production from fossil fuels, which inevitably consume much high energy, meanwhile, emit greenhouse gas [2], electrochemical water splitting method primarily involving hydrogen evolution reaction (HER) is inexpensive, reliable and clean to generate industrial-levels hydrogen [3,4]. However, robust and stable electrocatalysts are an essential prerequisite to improve the energy conversation efficiency and reaction kinetics of water splitting [2,5]. Up till now, noble-metal-based compounds, such as Pt/C and Ru/Ir/C, are still the predominated electrocatalysts [6]. However, their high price and limited storage in Earth hinder their large-scale practical applications, which drives the research on searching for earth-abundant metal-based electrocatalysts with high performance [7e17], including oxide catalysts [13], carbide catalysts [14], nitride catalysts [15] and phosphide catalysts [18]. Among these newly developed electrocatalysts, cobalt phosphide (CoP) has attracted wide attention owing to the low-cost, abundance and relatively high HER activity [18e36]. It is widely accepted that P atoms in CoP crystal lattice could grab electron from Co atoms and serving as proton carriers, which plays key roles for the HER activity [8,29]. The activity of CoP could be further improved by confining CoP with carbon layers [18,20,22,23,27,34,37,38], as the electrons will transport from the CoP to their neighboring carbon atoms, thus results in the increased density of states to the Fermi level and the optimized free energy of H* binding to the active sites during the HER catalytic processes [7]. Furthermore, the outer carbon layers could protect the CoP from corrosion and dissolution in the electrolyte during electrocatalysis, which will further improve the HER catalytic performance. However, the confined CoP nanoparticles are always easy to form large and non-uniform aggregates during the pyrolysis process, thereby decreases the HER catalytic activity. Moreover, too thick or less porous outer carbon layers will in turn impede the mass transfer and result in the decreasing of HER catalytic activity [39]. Therefore, it is still challenging for finding a facile method for synthesizing uniform ultrasmall-CoP nanoparticles confined in carbon layers as HER electrocatalyst with high performance. Recently, metal-organic compounds such as metal-organic frameworks (MOFs) have been utilized as exceptional good precursors to synthesize well-defined nanohybrids [40e44]. Through appropriate pyrolysis, the precursors could be converted into conductive carbons containing confined metal nanoparticles and doping elements for many catalytic applications (e.g. HER, oxygen evolution and oxygen reduction reaction) [32,40]. However, these kind of nanohybrids are usually composed of some low degree of graphitization carbons and weak interactions between the metal nanoparticles and the derived carbons [45]. In order to increase the charge transfer and mass diffusion path towards HER, we apply this concept through employing porous coordination polymers (PCPs) [46,47] instead of conventional MOFs to synthesize ultrasmall-CoP confined into carbon layers, as the strong host-
guest interactions and the confinement effect of the Co-based PCPs might effectively prevent agglomeration of nanoparticles during the pyrolysis treatment and offer opportunities for the formation of nanocatalysts down to nanometer size. Herein, we synthesized an ultrasmall-CoP/N-doped graphitic carbon (u-CoP@NC) heterostructure as efficient HER electrocatalysts from PCPs of [Co2(BDC)2(BPY)]n (BDC ¼ 1,4-benzenedicarboxylate, BPY ¼ 4,40 -bipyridine) precursors [48]. The in situ and confined formation of CoP nanoparticles results in ultrasmall nanoparticle diameter and strong coupling between CoP and the outer carbon layer. The as-synthesized u-CoP@NC electrocatalysts show high catalytic activity for HER with low overpotentials (h10 ¼ 131 mV in acidic and 111 mV in basic media), and the corresponding Tafel slopes are as small as 62.7 and 70.3 mV dec1, respectively. Various characterization techniques revealed that the synergistic effect of high conductivity of graphitic carbon layer, the good dispersity of the active sites, and the confinement effect by N-doped graphitic carbon layers result in the enhanced activity and stability of catalysts.
Experimental Chemicals Cobalt (II) acetate tetrahydrate (Co(OAC)24H2O, 99.5%) and acetic acid (99.5%) were purchased from Sinopharm. 4,40 bipyridine (BPY, 98%) and 1,4-benzenedicarboxylate (BDC, 99%) were purchased from Aladdin. Polyvinylpyrrolidone (PVP, MW ¼ 40,000) and sodium hypophosphite monohydrate (NaH2PO2, 99%) were purchased from Sigma-Aldrich. Cobalt phosphide (CoP, 98%) were purchased from Alfa Aesar. Pt/C (20 wt %) were obtained from Macklin. All of the chemicals and materials were used without further treatment.
Synthesis of CoP@NC nanobelts The CoP@NC nanobelts were fabricated through a two-step process. Firstly, 49.8 mg of Co(OAC)24H2O and 10 mg PVP were dissolved into 12 ml methanol and 575 ml acetic acid (Solvent A). Then, 33.6 mg BDC dissolved into 32 ml methanol and 10 ml DMF (Solvent B) was added into solvent A slowly. After stranding for 24 h, the mixture was transferred to 10 ml methanol containing 15.6 mg BPY and then stand for 2 h. By sonicating for 30 min, the mixture was incubated for another 5 h. The precipitate was collected by centrifuging. Then the samples were washed by methanol and dried under vacuum atmosphere for obtaining the cobalt-based nanobelt precursors ([Co2(BDC)2(BPY)]n). Secondly, the as-prepared nanobelt precursors were pretreated through annealing at 400 C for 6 h, and then phosphorized in a tube furnace. The precursors of 20 mg and NaH2PO2 of 200 mg were loaded in a quartz tube (200 mm in length and 10 mm of diameter) placed in the tube furnace. The temperature was program controlled and raised to 700, 800 or 900 C from room temperature with a heating rate of 5 C/min, and maintained at the preset temperature for 2 h. After the reaction completed, the furnace was cooled to room temperature. The black products were washed
Please cite this article as: Liu S et al., Porous coordination polymer-derived ultrasmall CoP encapsulated in nitrogen-doped carbon for efficient hydrogen evolution in both acidic and basic media, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.11.063
international journal of hydrogen energy xxx (xxxx) xxx
3
with water and ethanol, and then dried in vacuum. The final products were labeled as u-CoP@NC-T (T ¼ 700, 800, 900 C).
Materials characterization X-ray diffraction (XRD) was applied to characterize the phase of the as-synthesized CoP@NC-T nanobelts. Scanning electron microscopy (SEM, 200 kV) was employed to investigate the morphology of the sample surface. High-resolution transmission electron microscopy (HR-TEM, FEI Talos F200X, 200 kV) was operated for imaging and elemental mapping. The X-ray photoelectron spectroscopy (XPS, ESCALAB 250) was carried out for the valence states of elements analysis. The Raman spectroscopy measurements were recorded using a Renishaw Raman microscope with 514 nm laser excitation.
Electrochemical (EC) measurements The EC measurements were conducted on a conventional three-electrode configuration (CHI760E, CH Instruments), in which a saturated calomel electrode (SCE) is used as reference electrode whereas a graphite rod and a glass carbon electrode (GCE, 3 mm in diameter) coated with electrocatalyst are employed as counter electrode and working electrode, respectively. The as-synthesized electrocatalysts were dispersed in Nafion solution with concentration of 5 mg/mL by 30 min sonication forming an ink solution. The ink solution (5 mL) was then loaded onto the GCE surface and dried in vacuum. The prepared samples then underwent a series of EC measurements, including (a) linear sweep voltammetry (LSV) measurements, which were implemented in both acidic (0.5 M H2SO4) and basic solutions (1 M KOH) with a scan rate of 5 mV s1, (b) cyclic voltammetry (CV) measurement, which characterizes the electrochemically active surface areas (ECSAs) at continuous scan rate starting from 10 mV s1 and ending by 100 mV s1 with an interval of 10 mV s1, (c) continues CV cycles and i-t tests, which measures the stability of the electrocatalysts for 50 h in both acidic and basic electrolytes, (d) the EC impedance spectroscopy (EIS) measurements, which were tested in 0.5 M H2SO4 and 1 M KOH solution with alternating voltage of 0.18 V and 0.17 V, respectively, within a frequency range between 0.1 and 100 kHz. All the recorded potentials were iR-corrected and converted to a reversible hydrogen electrode (RHE) in accordance with the following equation, ERHE ¼ ESCE þ 0.059 pH þ E0SCE where E0SCE is the standard potential of SCE. For comparison, the commercial Pt/C electrocatalysts (20 wt %) were employed as control to perform measurements in the same way.
Results and discussions The synthetic route of the u-CoP@NC is presented in Scheme 1. Briefly, the [Co2(BDC)2(BPY)]n PCPs were first synthesized by ultrasonic assisted assembly of cobalt ions and BDC, BPY ligands. The morphologies of the as-synthesized
Scheme 1 e Schematic route for the u-CoP@NC electrocatalysts synthesis.
[Co2(BDC)2(BPY)]n exhibit a well-defined nanobelt shape with length of several micrometers and width of 124.4 ± 5.9 nm (Fig. 1a). The [Co2(BDC)2(BPY)]n nanobelts were then pyrolyzed at 400 C for harvesting ultrasmall cobalt nanoparticles confined in N-doped carbon layers and further transferred into u-CoP@NC through a high temperature phosphorization with PH3 generated by NaH2PO2. To evaluate the temperature effect on final catalytic performance, a series of phosphorization temperatures (T ¼ 700, 800 and 900 C) were employed. Low-magnification TEM images (Fig. 1b and Fig. S1 supporting information) reveal that the [Co2(BDC)2(BPY)]n nanobelts after phosphorization still keep the nanobelt shape morphology. The zoom-in TEM image (Fig. 1c) shows that untrasmall CoP nanoparticles with average diameter of ~8 nm disperse uniformly on the N-doped carbon substrate. The HR-TEM image (Fig. 1d) clearly shows the confined CoP nanoparticles dispersed in the graphitic carbon layers. The lattice spacing of 0.205 nm and 0.342 nm could be indexed to the (112) CoP crystal plane and the (002) graphitic carbon plane, respectively. The element distribution could be revealed by the highangle annular dark-field scanning TEM (HAADF-STEM) and energy-dispersive X-ray (EDX) characterization, which shows the uniformly distribution of C and N elements whereas concentrated distribution of Co and P elements within certain parts of u-CoP@NC-900 (Fig. 1eei). The crystallinity and surface chemical state of the uCoP@NC-T samples were investigated by XRD, XPS and Raman spectroscopy measurements. The XRD diffraction patterns of the u-CoP@NC-T samples (Fig. 2a) match well with the standard diffraction spectrum of CoP (JCPDS Card No. 29e0497) [25], which indicates the PCP precursors are converted into crystal CoP successfully after pyrolysis and phosphorization treatment. The XPS survey spectrum (Fig. S2a supporting information) reveals that the u-CoP@NC is composed of Co, P, C, N, and O elements. The high-resolution XPS spectrum of Co 2p (Fig. 2b) shows two main peeks of Co 2p3/2 and Co 2p1/2 locate at 778.5 and 793.38 eV, which attributes to Co0. Meanwhile, the other two peaks located at 781.18 eV and 797.18 eV attribute to the oxidation state of Co species Co2þ/3þ, indicating the Co 23/2
Please cite this article as: Liu S et al., Porous coordination polymer-derived ultrasmall CoP encapsulated in nitrogen-doped carbon for efficient hydrogen evolution in both acidic and basic media, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.11.063
4
international journal of hydrogen energy xxx (xxxx) xxx
Fig. 1 e (a) SEM image of [Co2(BDC)2(BPY)]n nanobelts. (b, c) TEM, (d) HR-TEM images and (eei) HAADF-STEM image and the corresponding EDX elemental mapping of the u-CoP@NC-900 sample.
and Co 2p1/2 regions, respectively. Whereas the two peaks remaining are satellite peaks situated around 785.08 and 802.08 eV. In addition, the XPS spectrum of P 2p are divided to three peaks at 129.28 eV, 130.08 eV and 133.88 eV, which allocates to P 2p3/2, P 2p1/2, and PeO, respectively (Fig. 2c). From Fig. 2bec, one can observe that the two peaks at 781.18 and 133.88 eV are attributed to the unavoidable contact of the CoP surface and ambient oxygen [31,33]. Moreover, the peak position of Co 2p3/2 (778.5 eV) is positively shifted while P 2p3/2 (129.28 eV) is negatively shifted in u-CoP@NC compared with that in metallic Co (778.1 eV) and in elemental P (130.2 eV), respectively, which is in agreement with the previously reported results [25]. The N 1s peaks investigated by XPS
spectrum could also be divided into three components residents at 397.68 eV, 399.78 eV and 400.48 eV (Fig. S2b, supporting information), attributing to pyridinic-N, pyrrolic-N, and graphitic-N, respectively. Meanwhile, the XPS spectrum of C 1s shows asymmetric signal with one peak located at 284.48 eV while a tail lying at 285.18 eV, which further confirms the successful doping of N atoms into carbon layers (Fig. S2c, supporting information) [49,50]. The Raman spectroscopy measurements of u-CoP@NC-T samples are shown in Fig. 2d, which shows the intensity ratio of the D band to G band (ID/IG) drops from 0.931 to 0.804 with the increasing phosphorization temperatures. It indicates that the higher phosphorization temperature results in higher degree of
Please cite this article as: Liu S et al., Porous coordination polymer-derived ultrasmall CoP encapsulated in nitrogen-doped carbon for efficient hydrogen evolution in both acidic and basic media, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.11.063
international journal of hydrogen energy xxx (xxxx) xxx
5
Fig. 2 e (a) XRD patterns of u-CoP@NC-T samples. (b, c) HR-XPS spectra of Co 2p and P 2p of the u-CoP@NC-900 sample. (d) Raman spectra of u-CoP@NC-T samples.
graphitization of carbon layer, which leads to higher electrical conductivity and enhanced catalytic activity [49,50]. The catalytic activities of u-CoP@NC-T catalysts toward HER were firstly assessed in 0.5 M H2SO4. Fig. 3a presents the polarization curves of the as-synthesized u-CoP@NC-T samples recorded at a scan rate of 5 mV s1. It is clearly revealed that the phosphorization temperature greatly influences the HER performance. The u-CoP@NC-900 transfers a current density of 10 mA cm2 (j10) at an overpotential of 131 mV lower than those of u-CoP@NC-800 (156 mV) and uCoP@NC-700 (174 mV) as investigated. The linear sweep polarization curves of NC, CoP and u-CoP@NC-900 are compared as shown in Fig. S3 (supporting information), indicating that ultrasmall-CoP nanoparticles confined into N-doped graphitic carbon layer could effectively enhance the catalytic activity of CoP. The corresponding Tafel slopes of uCoP@NC are calculated to elucidate the rate controlling step of HER process. Consistent with the reported study [25], Pt/C presents the lowest Tafel slope (32.8 mV dec1) shown in Fig. 3b. Besides, the u-CoP@NC-900 presents a Tafel slope of 62.7 mV dec1 smaller than those of u-CoP@NC-800 of 65.1 mV dec1 and u-CoP@NC-700 of 88.1 mV dec1, which illustrates a controlled EC desorption rate process. As a result, HER of the u-CoP@NC-900 may obey the VolmerHeyrovsky mechanism [51]. To further evaluate the intrinsic activity of the fabricated u-CoP@NC toward HER, the double-layer capacitance (Cdl) was measured in nonFaradaic potential region and the ECSAs of the u-CoP@NC samples are analyzed [52]. The CV curves of u-CoP@NC catalysts (Fig. S4a-c supporting information) were adopted for obtaining the straight lines slops as a function of current
densities versus scan rates (Fig. 3c). The calculated Cdl values for u-CoP@NC-900, u-CoP@NC-800 and u-CoP@NC-700 are 31.7, 11.4, and 8.7 mF cm2, respectively. The largest Cdl value of u-CoP@NC-900 indicates the rougher surface of uCoP@NC-900 leads to more active sites for HER [53,54]. The charge transfer between the catalysts and electrolyte was investigated by EIS. The Nyquist plots (Fig. 3d) suggest that the u-CoP@NC exhibits similar electrolyte resistance (Rs) whereas dissimilar charge transfer resistance (Rct). The uCoP@NC-900 presents the lowest Rct, indicating the boosted reaction kinetics at the interface between the catalyst and electrolyte [55]. The in situ formed CoP nanoparticles strengthen the formation of graphitic carbon and, hereafter, a higher graphitization degree in u-CoP@NC-900 sample, thus enhance its electrical conductivity. In addition, the u-CoP@NC catalysts also present outstanding electrocatalytic activity in 1.0 M KOH solution. The linear sweep polarization curves of the as-synthesized uCoP@NC-T samples in 1.0 M KOH are shown in Fig. 3e. The uCoP@NC-900 delivers j10 at an overpotential of 111 mV preceding to those of u-CoP@NC-800 (146 mV) and u-CoP@NC-700 (202 mV). The corresponding Tafel slope of u-CoP@NC-900 is 70.3 mV dec1, which is lower than the values of u-CoP@NC800 (87.8 mV dec1) and u-CoP@NC-700 (109.6 mV dec1) as shown in Fig. 3f. Furthermore, the CV curves of u-CoP@NC-T scanning with different rates are illustrated in Fig. S4d-f (supporting information). The tendency of calculated Cdl values for the u-CoP@NC-T samples is similar to the HER activities showing in 0.5 M H2SO4, where u-CoP@NC-900 presents the optimum value of 23.8 mF cm2 (Fig. 3g), which contributes to the high electrocatalytic activity of u-CoP@NC-
Please cite this article as: Liu S et al., Porous coordination polymer-derived ultrasmall CoP encapsulated in nitrogen-doped carbon for efficient hydrogen evolution in both acidic and basic media, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.11.063
6
international journal of hydrogen energy xxx (xxxx) xxx
Fig. 3 e (a, e) Linear sweep polarization curves, (b, f) the corresponding Tafel plots, (c, g) capacitive currents at (c) 0.3 V and (g) 0.2 V as a function of scan rate, and (d, h) EIS Nyquist plots of u-CoP@NC samples toward HER measured (aed) in 0.5 M H2SO4 solution and (eef) in 1 M KOH solution.
900. Moreover, the charge transfer at the interface between uCoP@NC-900 and electrolyte is fast, which can also be indicated by the smallest semicircle of the Nyquist plots evaluated in the high-frequency zone (Fig. 3h).
In order to evaluate the stability of u-CoP@NC catalysts, continuous CV scanning were performed and chronoamperometric curves were plotted. It is apparent that uCoP@NC-900 in acidic media shows negligible displacement
Please cite this article as: Liu S et al., Porous coordination polymer-derived ultrasmall CoP encapsulated in nitrogen-doped carbon for efficient hydrogen evolution in both acidic and basic media, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.11.063
international journal of hydrogen energy xxx (xxxx) xxx
7
Fig. 4 e (a, b) HER polarization curves before and after 2000 CV cycles and (c, d) chronoamperometry i-t curves at high current density of 50 mA cm¡2 of u-CoP@NC-900 (a,c) in 0.5 M H2SO4 solution and (b,d) in 1 M KOH solution.
of overpotentials at j10 after 2000 cycles of continuous CV scanning (Fig. 4a). In basic media, a slight voltage (20 mV) decreases after 2000 CV cycles with potentials between 0 and -0.2 V versus RHE (Fig. 4b). The HER electrolysis performance at a high current density (50 mA cm2) is shown in Fig. 4ced. It is apparent that no obvious current density decreases were observed after continuous HER for ~50 h in both acidic and basic solutions. The fluctuation of the current density is attributed to the H2 bubbles produced on the electrodes. These results confirm the stability of u-CoP@NC-900 catalysts, which indicates the promising practical applications. The morphology of the catalysts undergone the long-time HER tests was characterized by TEM (Fig. S5 supporting information). It is shown that the nanobelt morphology and ultra-small particles of u-CoP@NC-900 are generally maintained, without obvious collapse and aggregation. The above results indicate the good stability of the u-CoP@NC catalysts in both acidic and basic solutions, which attributes to the confinement effect by the graphitic carbon layers [49].
Conclusions In conclusion, ultrasmall-CoP nanoparticles strongly coupled with nitrogen doped graphitic carbon layers have been designed and fabricated through a PCP-derived route. The assynthesized u-CoP/NC-900 exhibited remarkable HER electrocatalytic activity, achieving a current density of 10 mA cm2 with small overpotentials of 131 mV and 111 mV in acidic and basic media, respectively, and the corresponding Tafel slopes are as low as 62.7 and 70.3 mV dec1. The desired
electrocatalytic activity of u-CoP@NC originates from the strongly coupled u-CoP nanoparticles and graphitic carbon layer and the perfect dispersed active sites. The strong interaction between u-CoP nanoparticles and the outer graphitic carbon layer not only improve the charge transport rate but also prevent the corrosion of CoP during electrocatalysis in the electrolyte, thereby increasing the catalytic activity and the stability. This work opens up new avenues for synthesizing non-precious metal-based highly efficient electrocatalysts towards water splitting applications.
Acknowledgments The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (No. 21703119 and 31600753), the Natural Science Foundation of Tianjin (No. 16JCYBJC43400), the National Science Foundation of Hebei Province (No. C2017202206), the Scientific Research Foundation for the Returned Overseas Chinese Scholars of Hebei Province (No. CL201711), the Natural Science Foundation of Shandong Province (No. ZR2017MB036) and the Grant for Taishan Scholar Advantage Characteristic Discipline, the Danish National Research Foundation, AUFF NOVA-project, and EU H2020 RISE (MNR4SCell 734174 project).
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.11.063.
Please cite this article as: Liu S et al., Porous coordination polymer-derived ultrasmall CoP encapsulated in nitrogen-doped carbon for efficient hydrogen evolution in both acidic and basic media, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.11.063
8
international journal of hydrogen energy xxx (xxxx) xxx
references
[1] Turner JA. Sustainable hydrogen production. Science 2004;305:972. [2] Roger I, Shipman MA, Symes MD. Earth-abundant catalysts for electrochemical and photoelectrochemical water splitting. Nat Rev Chem 2017;1:0003. [3] Xu X, Zhong Y, Shao Z. Double perovskites in catalysis, electrocatalysis, and photo (electro) catalysis. Trends in Chemistry 2019;1:410e24. [4] Jiang P, Liu Q, Liang Y, Tian J, Asiri AM, Sun X. A costeffective 3D hydrogen evolution cathode with high catalytic activity: FeP nanowire array as the active phase. Angew Chem Int Ed 2014;53:12855e9. [5] Zou X, Zhang Y. Noble metal-free hydrogen evolution catalysts for water splitting. Chem Soc Rev 2015;44:5148e80. [6] Walter MG, Warren EL, McKone JR, Boettcher SW, Mi Q, Santori EA, et al. Solar water splitting cells. Chem Rev 2010;110:6446e73. [7] Wang Y, Kong B, Zhao D, Wang H, Selomulya C. Strategies for developing transition metal phosphides as heterogeneous electrocatalysts for water splitting. Nano Today 2017;15:26e55. [8] Shi Y, Zhang B. Recent advances in transition metal phosphide nanomaterials: synthesis and applications in hydrogen evolution reaction. Chem Soc Rev 2016;45:1529e41. [9] Wang Z, Li Q, Xu H, Dahl-Petersen C, Yang Q, Cheng D, et al. Controllable etching of MoS2 basal planes for enhanced hydrogen evolution through the formation of active edge sites. Nano Energy 2018;49:634e43. [10] Xu H, Cheng D, Cao D, Zeng XC. A universal principle for a rational design of single-atom electrocatalysts. Nat Catal 2018;1:339. [11] Yang Q, Wang Z, Dong L, Zhao W, Jin Y, Fang L, et al. Activating MoS2 with super-high nitrogen-doping concentration as efficient catalyst for hydrogen evolution reaction. J Phys Chem C 2019;123:10917e25. [12] Jiang H, Wang Z, Yang Q, Tan L, Dong L, Dong M. Ultrathin Ti3C2TX(MXene) nanosheet-wrapped NiSe2 octahedral crystal for enhanced supercapacitor performance and synergetic electrocatalytic water splitting. Nano-Micro Lett 2019;11:31. [13] Xu X, Chen Y, Zhou W, Zhu Z, Su C, Liu M, et al. A perovskite electrocatalyst for efficient hydrogen evolution reaction. Adv Mater 2016;28:6442e8. [14] Zheng Y, Jiao Y, Zhu Y, Li LH, Han Y, Chen Y, et al. Hydrogen evolution by a metal-free electrocatalyst. Nat Commun 2014;5:3783. [15] Zhu Y, Chen G, Xu X, Yang G, Liu M, Shao Z. Enhancing electrocatalytic activity for hydrogen evolution by strongly coupled molybdenum nitride@nitrogen-doped carbon porous nano-octahedrons. ACS Catal 2017;7:3540e7. [16] Fang X, Wang Z, Wang Y, Kang S, Jiang Z, Dong M. Tuning quaternary hybride CoeNieSeSe composition as a bifunctional electrocatalyst for hydrogen and oxygen evolution reactions. Int J Hydrogen Energy 2019;44:27685e94. [17] Wang Z, Wu H-H, Li Q, Besenbacher F, Li Y, Zeng XC, et al. Reversing interfacial catalysis of ambipolar WSe2 single crystal. Adv Sci 2019. https://doi.org/10.1002/advs.201901382. [18] Zhou Q, Shen Z, Zhu C, Li J, Ding Z, Wang P, et al. Nitrogendoped CoP electrocatalysts for coupled hydrogen evolution and sulfur generation with low energy consumption. Adv Mater 2018;30:1800140. [19] Zeng Y, Wang Y, Huang G, Chen C, Huang L, Chen R, et al. Porous CoP nanosheets converted from layered double hydroxides with superior electrochemical activity for hydrogen evolution reactions at wide pH ranges. Chem Commun 2018;54:1465e8.
[20] Yan L, Cao L, Dai P, Gu X, Liu D, Li L, et al. Metal-organic frameworks derived nanotube of nickelecobalt bimetal phosphides as highly efficient electrocatalysts for overall water splitting. Adv Funct Mater 2017;27:1703455. [21] Tang C, Zhang R, Lu W, He L, Jiang X, Asiri AM, et al. Fedoped CoP nanoarray: a monolithic multifunctional catalyst for highly efficient hydrogen generation. Adv Mater 2017;29:1602441. [22] Tabassum H, Guo W, Meng W, Mahmood A, Zhao R, Wang Q, et al. Metaleorganic frameworks derived cobalt phosphide architecture encapsulated into B/N Co-doped graphene nanotubes for all pH value electrochemical hydrogen evolution. Adv Energy Mater 2017;7:1601671. [23] Wang H, Min S, Wang Q, Li D, Casillas G, Ma C, et al. Nitrogen-doped nanoporous carbon membranes with Co/ CoP Janus-type nanocrystals as hydrogen evolution eectrode in both acidic and alkaline environments. ACS Nano 2017;11:4358e64. [24] Yang H, Zhang Y, Hu F, Wang Q. Urchin-like CoP nanocrystals as hydrogen evolution reaction and oxygen reduction reaction dual-electrocatalyst with superior stability. Nano Lett 2015;15:7616e20. [25] Tian J, Liu Q, Asiri AM, Sun X. Self-supported nanoporous cobalt phosphide nanowire arrays: an efficient 3D hydrogenevolving cathode over the wide range of pH 0e14. J Am Chem Soc 2014;136:7587e90. [26] Sharma AK, Joshi H, Ojha K, Singh AK. Graphene oxide supported cobalt phosphide nanorods designed from a molecular complex for efficient hydrogen evolution at low overpotential. Chem Commun 2019;55:2186e9. [27] Lu Y, Hou W, Yang D, Chen Y. CoP nanosheets in-situ grown on N-doped graphene as an efficient and stable bifunctional electrocatalyst for hydrogen and oxygen evolution reactions. Electrochim Acta 2019;307:543e52. [28] Wu T, Pi M, Wang X, Guo W, Zhang D, Chen S. Hierarchical cobalt poly-phosphide hollow spheres as highly active and stable electrocatalysts for hydrogen evolution over a wide pH range. Appl Surf Sci 2018;427:800e6. [29] Liu T, Xie L, Yang J, Kong R, Du G, Asiri AM, et al. Selfstanding CoP nanosheets array: a three-dimensional bifunctional catalyst electrode for overall water splitting in both neutral and alkaline media. ChemElectroChem 2017;4:1840e5. [30] Liu T, Wang K, Du G, Asiri AM, Sun X. Self-supported CoP nanosheet arrays: a non-precious metal catalyst for efficient hydrogen generation from alkaline NaBH4 solution. J Mater Chem 2016;4:13053e7. [31] Tang C, Qu F, Asiri AM, Luo Y, Sun X. CoP nanoarray: a robust non-noble-metal hydrogen-generating catalyst toward effective hydrolysis of ammonia borane. Inorg Chem Front 2017;4:659e62. [32] Xie L, Zhang R, Cui L, Liu D, Hao S, Ma Y, et al. Highperformance electrolytic oxygen evolution in neutral media catalyzed by a cobalt phosphate nanoarray. Angew Chem Int Ed 2017;56:1064e8. [33] Zhang R, Ren X, Hao S, Ge R, Liu Z, Asiri AM, et al. Selective phosphidation: an effective strategy toward CoP/CeO2 interface engineering for superior alkaline hydrogen evolution electrocatalysis. J Mater Chem 2018;6:1985e90. [34] Chen Z, Jia H, Yuan J, Liu X, Fang C, Fan Y, et al. P-co-doped carbon coupled with CoP as superior electrocatalysts for hydrogen evolution reaction and overall water splitting. Int J Hydrogen Energy 2019;44:24342e52. [35] Jin J, Zhu Y, Liu Y, Li Y, Peng W, Zhang G, et al. CoP nanoparticles combined with WS2 nanosheets as efficient electrocatalytic hydrogen evolution reaction catalyst. Int J Hydrogen Energy 2017;42:3947e54.
Please cite this article as: Liu S et al., Porous coordination polymer-derived ultrasmall CoP encapsulated in nitrogen-doped carbon for efficient hydrogen evolution in both acidic and basic media, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.11.063
international journal of hydrogen energy xxx (xxxx) xxx
[36] Zhang M, Ci S, Li H, Cai P, Xu H, Wen Z. Highly defective porous CoP nanowire as electrocatalyst for full water splitting. Int J Hydrogen Energy 2017;42:29080e90. [37] Yang F, Chen Y, Cheng G, Chen S, Luo W. Ultrathin nitrogendoped carbon coated with CoP for efficient hydrogen evolution. ACS Catal 2017;7:3824e31. [38] Yang S, Chen L, Wei W, Lv X, Xie J. CoP nanoparticles encapsulated in three-dimensional N-doped porous carbon for efficient hydrogen evolution reaction in a broad pH range. Appl Surf Sci 2019;476:749e56. [39] Jin H, Wang J, Su D, Wei Z, Pang Z, Wang Y. In situ cobaltecobalt oxide/N-doped carbon hybrids as superior bifunctional electrocatalysts for hydrogen and oxygen evolution. J Am Chem Soc 2015;137:2688e94. [40] Zhang H, Liu X, Wu Y, Guan C, Cheetham AK, Wang J. MOFderived nanohybrids for electrocatalysis and energy storage: current status and perspectives. Chem Commun 2018;54:5268e88. [41] Dang S, Zhu Q-L, Xu Q. Nanomaterials derived from metaleorganic frameworks. Nat Rev Mater 2017;3:17075. [42] Cao X, Tan C, Sindoro M, Zhang H. Hybrid micro-/nanostructures derived from metaleorganic frameworks: preparation and applications in energy storage and conversion. Chem Soc Rev 2017;46:2660e77. [43] Wang W, Xu X, Zhou W, Shao Z. Recent progress in metalorganic frameworks for applications in electrocatalytic and photocatalytic water splitting. Adv Sci 2017;4:1600371. [44] Gao L, Chen S, Zhang H, Zou Y, She X, Yang D, et al. Porous CoP nanostructure electrocatalyst derived from DUT-58 for hydrogen evolution reaction. Int J Hydrogen Energy 2018;43:13904e10. [45] Li Y, Ben T, Zhang B, Fu Y, Qiu S. Ultrahigh gas storage both at low and high pressures in KOH-activated carbonized porous aromatic frameworks. Sci Rep 2013;3:2420. [46] Kitagawa S, Kitaura R, Noro S-i. Functional porous coordination polymers. Angew Chem Int Ed 2004;43:2334e75.
9
[47] Chen C, Zhang W, Hong Y, Le Z, Li Q, Li W, et al. Synthesis of coordination polymer thin films with conductanceresponse to mechanical stimulation. Chem Commun 2019;55:2545e8. [48] Sakata Y, Furukawa S, Kondo M, Hirai K, Horike N, Takashima Y, et al. Shape-memory nanopores induced in coordination frameworks by crystal downsizing. Science 2013;339:193e6. [49] Liu Y, Zhu Y, Shen J, Huang J, Yang X, Li C. CoP nanoparticles anchored on N, P-dual-doped graphene-like carbon as a catalyst for water splitting in non-acidic media. Nanoscale 2018;10:2603e12. [50] Su J, Yang Y, Xia G, Chen J, Jiang P, Chen Q. Ruthenium-cobalt nanoalloys encapsulated in nitrogen-doped graphene as active electrocatalysts for producing hydrogen in alkaline media. Nat Commun 2017;8:14969. [51] Pentland N, Bockris JO, Sheldon E. Hydrogen evolution reaction on copper, gold, molybdenum, palladium, rhodium, and iron. J Electrochem Soc 1957;104:182e94. [52] Lukowski MA, Daniel AS, Meng F, Forticaux A, Li L, Jin S. Enhanced hydrogen evolution catalysis from chemically exfoliated metallic MoS2 nanosheets. J Am Chem Soc 2013;135:10274e7. [53] Pan Y, Lin Y, Chen Y, Liu Y, Liu C. Cobalt phosphide-based electrocatalysts: synthesis and phase catalytic activity comparison for hydrogen evolution. J Mater Chem 2016;4:4745e54. [54] Li X, Jiang L, Liu J, Hua Q, Wang E, Xie G. Size-dependent catalytic activity of cobalt phosphides for hydrogen evolution reaction. J Energy Chem 2020;43:121e8. [55] Chang J, Ouyang Y, Ge J, Wang J, Liu C, Xing W. Cobalt phosphosulfide in the tetragonal phase: a highly active and durable catalyst for the hydrogen evolution reaction. J Mater Chem A 2018;6:12353e60.
Please cite this article as: Liu S et al., Porous coordination polymer-derived ultrasmall CoP encapsulated in nitrogen-doped carbon for efficient hydrogen evolution in both acidic and basic media, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.11.063
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Porous coordination polymer-derived ultrasmall CoP encapsulated in nitrogen-doped carbon for efficient hydrogen evolution in both acidic and basic media Shuai Liu a,b,1, Anning Jiang b,1, Zegao Wang c, Mengxia Lin a, Dan Xia a,*, Qiang Li b,**, Mingdong Dong c,*** a School of Materials Science and Engineering, Hebei University of Technology, And Tianjin Key Laboratory of Materials Laminating Fabrication and Interface Control Technology, Tianjin, 300130, China b Key Laboratory of Colloid and Interface Chemistry of Ministry of Education, And School of Chemistry and Chemical Engineering, Shandong University, Jinan, 250100, Shandong, China c Interdisciplinary Nanoscience Center (iNANO), Aarhus University, Aarhus C, DK, 8000, Denmark
highlights u-CoP@NC were directly synthesized through phosphorization of a PCPs precursor. Remarkable electrocatalytic activity of u-CoP@NC towards HER was achieved. The synergistic effect between u-CoP and NC could improve the HER performance. The confinement by NC layers enhanced the stability of the u-CoP@NC catalysts.
article info
abstract
Article history:
Exploiting high-efficient and stable non-precious metal-based electrocatalysts toward
Received 16 September 2019
hydrogen evolution reaction (HER) is of enormous significance to address the shortage of
Received in revised form
global power source, but there remain major challenges. Here we present a facile and
4 November 2019
controllable strategy to synthesize a strongly coupled ultrasmall-cobalt phosphide/nitro-
Accepted 6 November 2019
gen-doped graphitic carbon (u-CoP@NC) hybrid structure via phosphorization from a
Available online xxx
porous coordination polymer (PCP) precursor. The PCP-derived u-CoP@NC exhibits remarkable activity and stability for HER, achieving a current density of 10 mA cm2 with a
Keywords:
low overpotential of 131 mV in acidic media and 111 mV in basic media. The corresponding
Cobalt phosphide
Tafel slopes present in acidic and basic media are 62.7 and 70.3 mV dec1, respectively.
Nitrogen-doped graphitic carbon
Results reveal that the enhanced electrocatalytic performance of u-CoP@NC originates
layer
from the strongly coupled u-CoP nanoparticles and graphitic carbon layer, and the perfect
Hydrogen evolution reaction
dispersity of the active sites. This research opens up new avenues for designing earth-
Porous coordination polymer
abundant metal-based electrocatalysts with high capability for water splitting applications.
Electrocatalyst
© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
* Corresponding author. ** Corresponding author. *** Corresponding author. E-mail addresses: [email protected] (D. Xia), [email protected] (Q. Li), [email protected] (M. Dong). 1 These authors contributed equally to this work. https://doi.org/10.1016/j.ijhydene.2019.11.063 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Liu S et al., Porous coordination polymer-derived ultrasmall CoP encapsulated in nitrogen-doped carbon for efficient hydrogen evolution in both acidic and basic media, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.11.063
2
international journal of hydrogen energy xxx (xxxx) xxx
Introduction As a renewable and clean source of energy, hydrogen has been regarded as a promising candidate to replace the traditional fossil fuels [1]. Compared to most current technologies for hydrogen production from fossil fuels, which inevitably consume much high energy, meanwhile, emit greenhouse gas [2], electrochemical water splitting method primarily involving hydrogen evolution reaction (HER) is inexpensive, reliable and clean to generate industrial-levels hydrogen [3,4]. However, robust and stable electrocatalysts are an essential prerequisite to improve the energy conversation efficiency and reaction kinetics of water splitting [2,5]. Up till now, noble-metal-based compounds, such as Pt/C and Ru/Ir/C, are still the predominated electrocatalysts [6]. However, their high price and limited storage in Earth hinder their large-scale practical applications, which drives the research on searching for earth-abundant metal-based electrocatalysts with high performance [7e17], including oxide catalysts [13], carbide catalysts [14], nitride catalysts [15] and phosphide catalysts [18]. Among these newly developed electrocatalysts, cobalt phosphide (CoP) has attracted wide attention owing to the low-cost, abundance and relatively high HER activity [18e36]. It is widely accepted that P atoms in CoP crystal lattice could grab electron from Co atoms and serving as proton carriers, which plays key roles for the HER activity [8,29]. The activity of CoP could be further improved by confining CoP with carbon layers [18,20,22,23,27,34,37,38], as the electrons will transport from the CoP to their neighboring carbon atoms, thus results in the increased density of states to the Fermi level and the optimized free energy of H* binding to the active sites during the HER catalytic processes [7]. Furthermore, the outer carbon layers could protect the CoP from corrosion and dissolution in the electrolyte during electrocatalysis, which will further improve the HER catalytic performance. However, the confined CoP nanoparticles are always easy to form large and non-uniform aggregates during the pyrolysis process, thereby decreases the HER catalytic activity. Moreover, too thick or less porous outer carbon layers will in turn impede the mass transfer and result in the decreasing of HER catalytic activity [39]. Therefore, it is still challenging for finding a facile method for synthesizing uniform ultrasmall-CoP nanoparticles confined in carbon layers as HER electrocatalyst with high performance. Recently, metal-organic compounds such as metal-organic frameworks (MOFs) have been utilized as exceptional good precursors to synthesize well-defined nanohybrids [40e44]. Through appropriate pyrolysis, the precursors could be converted into conductive carbons containing confined metal nanoparticles and doping elements for many catalytic applications (e.g. HER, oxygen evolution and oxygen reduction reaction) [32,40]. However, these kind of nanohybrids are usually composed of some low degree of graphitization carbons and weak interactions between the metal nanoparticles and the derived carbons [45]. In order to increase the charge transfer and mass diffusion path towards HER, we apply this concept through employing porous coordination polymers (PCPs) [46,47] instead of conventional MOFs to synthesize ultrasmall-CoP confined into carbon layers, as the strong host-
guest interactions and the confinement effect of the Co-based PCPs might effectively prevent agglomeration of nanoparticles during the pyrolysis treatment and offer opportunities for the formation of nanocatalysts down to nanometer size. Herein, we synthesized an ultrasmall-CoP/N-doped graphitic carbon (u-CoP@NC) heterostructure as efficient HER electrocatalysts from PCPs of [Co2(BDC)2(BPY)]n (BDC ¼ 1,4-benzenedicarboxylate, BPY ¼ 4,40 -bipyridine) precursors [48]. The in situ and confined formation of CoP nanoparticles results in ultrasmall nanoparticle diameter and strong coupling between CoP and the outer carbon layer. The as-synthesized u-CoP@NC electrocatalysts show high catalytic activity for HER with low overpotentials (h10 ¼ 131 mV in acidic and 111 mV in basic media), and the corresponding Tafel slopes are as small as 62.7 and 70.3 mV dec1, respectively. Various characterization techniques revealed that the synergistic effect of high conductivity of graphitic carbon layer, the good dispersity of the active sites, and the confinement effect by N-doped graphitic carbon layers result in the enhanced activity and stability of catalysts.
Experimental Chemicals Cobalt (II) acetate tetrahydrate (Co(OAC)24H2O, 99.5%) and acetic acid (99.5%) were purchased from Sinopharm. 4,40 bipyridine (BPY, 98%) and 1,4-benzenedicarboxylate (BDC, 99%) were purchased from Aladdin. Polyvinylpyrrolidone (PVP, MW ¼ 40,000) and sodium hypophosphite monohydrate (NaH2PO2, 99%) were purchased from Sigma-Aldrich. Cobalt phosphide (CoP, 98%) were purchased from Alfa Aesar. Pt/C (20 wt %) were obtained from Macklin. All of the chemicals and materials were used without further treatment.
Synthesis of CoP@NC nanobelts The CoP@NC nanobelts were fabricated through a two-step process. Firstly, 49.8 mg of Co(OAC)24H2O and 10 mg PVP were dissolved into 12 ml methanol and 575 ml acetic acid (Solvent A). Then, 33.6 mg BDC dissolved into 32 ml methanol and 10 ml DMF (Solvent B) was added into solvent A slowly. After stranding for 24 h, the mixture was transferred to 10 ml methanol containing 15.6 mg BPY and then stand for 2 h. By sonicating for 30 min, the mixture was incubated for another 5 h. The precipitate was collected by centrifuging. Then the samples were washed by methanol and dried under vacuum atmosphere for obtaining the cobalt-based nanobelt precursors ([Co2(BDC)2(BPY)]n). Secondly, the as-prepared nanobelt precursors were pretreated through annealing at 400 C for 6 h, and then phosphorized in a tube furnace. The precursors of 20 mg and NaH2PO2 of 200 mg were loaded in a quartz tube (200 mm in length and 10 mm of diameter) placed in the tube furnace. The temperature was program controlled and raised to 700, 800 or 900 C from room temperature with a heating rate of 5 C/min, and maintained at the preset temperature for 2 h. After the reaction completed, the furnace was cooled to room temperature. The black products were washed
Please cite this article as: Liu S et al., Porous coordination polymer-derived ultrasmall CoP encapsulated in nitrogen-doped carbon for efficient hydrogen evolution in both acidic and basic media, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.11.063
international journal of hydrogen energy xxx (xxxx) xxx
3
with water and ethanol, and then dried in vacuum. The final products were labeled as u-CoP@NC-T (T ¼ 700, 800, 900 C).
Materials characterization X-ray diffraction (XRD) was applied to characterize the phase of the as-synthesized CoP@NC-T nanobelts. Scanning electron microscopy (SEM, 200 kV) was employed to investigate the morphology of the sample surface. High-resolution transmission electron microscopy (HR-TEM, FEI Talos F200X, 200 kV) was operated for imaging and elemental mapping. The X-ray photoelectron spectroscopy (XPS, ESCALAB 250) was carried out for the valence states of elements analysis. The Raman spectroscopy measurements were recorded using a Renishaw Raman microscope with 514 nm laser excitation.
Electrochemical (EC) measurements The EC measurements were conducted on a conventional three-electrode configuration (CHI760E, CH Instruments), in which a saturated calomel electrode (SCE) is used as reference electrode whereas a graphite rod and a glass carbon electrode (GCE, 3 mm in diameter) coated with electrocatalyst are employed as counter electrode and working electrode, respectively. The as-synthesized electrocatalysts were dispersed in Nafion solution with concentration of 5 mg/mL by 30 min sonication forming an ink solution. The ink solution (5 mL) was then loaded onto the GCE surface and dried in vacuum. The prepared samples then underwent a series of EC measurements, including (a) linear sweep voltammetry (LSV) measurements, which were implemented in both acidic (0.5 M H2SO4) and basic solutions (1 M KOH) with a scan rate of 5 mV s1, (b) cyclic voltammetry (CV) measurement, which characterizes the electrochemically active surface areas (ECSAs) at continuous scan rate starting from 10 mV s1 and ending by 100 mV s1 with an interval of 10 mV s1, (c) continues CV cycles and i-t tests, which measures the stability of the electrocatalysts for 50 h in both acidic and basic electrolytes, (d) the EC impedance spectroscopy (EIS) measurements, which were tested in 0.5 M H2SO4 and 1 M KOH solution with alternating voltage of 0.18 V and 0.17 V, respectively, within a frequency range between 0.1 and 100 kHz. All the recorded potentials were iR-corrected and converted to a reversible hydrogen electrode (RHE) in accordance with the following equation, ERHE ¼ ESCE þ 0.059 pH þ E0SCE where E0SCE is the standard potential of SCE. For comparison, the commercial Pt/C electrocatalysts (20 wt %) were employed as control to perform measurements in the same way.
Results and discussions The synthetic route of the u-CoP@NC is presented in Scheme 1. Briefly, the [Co2(BDC)2(BPY)]n PCPs were first synthesized by ultrasonic assisted assembly of cobalt ions and BDC, BPY ligands. The morphologies of the as-synthesized
Scheme 1 e Schematic route for the u-CoP@NC electrocatalysts synthesis.
[Co2(BDC)2(BPY)]n exhibit a well-defined nanobelt shape with length of several micrometers and width of 124.4 ± 5.9 nm (Fig. 1a). The [Co2(BDC)2(BPY)]n nanobelts were then pyrolyzed at 400 C for harvesting ultrasmall cobalt nanoparticles confined in N-doped carbon layers and further transferred into u-CoP@NC through a high temperature phosphorization with PH3 generated by NaH2PO2. To evaluate the temperature effect on final catalytic performance, a series of phosphorization temperatures (T ¼ 700, 800 and 900 C) were employed. Low-magnification TEM images (Fig. 1b and Fig. S1 supporting information) reveal that the [Co2(BDC)2(BPY)]n nanobelts after phosphorization still keep the nanobelt shape morphology. The zoom-in TEM image (Fig. 1c) shows that untrasmall CoP nanoparticles with average diameter of ~8 nm disperse uniformly on the N-doped carbon substrate. The HR-TEM image (Fig. 1d) clearly shows the confined CoP nanoparticles dispersed in the graphitic carbon layers. The lattice spacing of 0.205 nm and 0.342 nm could be indexed to the (112) CoP crystal plane and the (002) graphitic carbon plane, respectively. The element distribution could be revealed by the highangle annular dark-field scanning TEM (HAADF-STEM) and energy-dispersive X-ray (EDX) characterization, which shows the uniformly distribution of C and N elements whereas concentrated distribution of Co and P elements within certain parts of u-CoP@NC-900 (Fig. 1eei). The crystallinity and surface chemical state of the uCoP@NC-T samples were investigated by XRD, XPS and Raman spectroscopy measurements. The XRD diffraction patterns of the u-CoP@NC-T samples (Fig. 2a) match well with the standard diffraction spectrum of CoP (JCPDS Card No. 29e0497) [25], which indicates the PCP precursors are converted into crystal CoP successfully after pyrolysis and phosphorization treatment. The XPS survey spectrum (Fig. S2a supporting information) reveals that the u-CoP@NC is composed of Co, P, C, N, and O elements. The high-resolution XPS spectrum of Co 2p (Fig. 2b) shows two main peeks of Co 2p3/2 and Co 2p1/2 locate at 778.5 and 793.38 eV, which attributes to Co0. Meanwhile, the other two peaks located at 781.18 eV and 797.18 eV attribute to the oxidation state of Co species Co2þ/3þ, indicating the Co 23/2
Please cite this article as: Liu S et al., Porous coordination polymer-derived ultrasmall CoP encapsulated in nitrogen-doped carbon for efficient hydrogen evolution in both acidic and basic media, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.11.063
4
international journal of hydrogen energy xxx (xxxx) xxx
Fig. 1 e (a) SEM image of [Co2(BDC)2(BPY)]n nanobelts. (b, c) TEM, (d) HR-TEM images and (eei) HAADF-STEM image and the corresponding EDX elemental mapping of the u-CoP@NC-900 sample.
and Co 2p1/2 regions, respectively. Whereas the two peaks remaining are satellite peaks situated around 785.08 and 802.08 eV. In addition, the XPS spectrum of P 2p are divided to three peaks at 129.28 eV, 130.08 eV and 133.88 eV, which allocates to P 2p3/2, P 2p1/2, and PeO, respectively (Fig. 2c). From Fig. 2bec, one can observe that the two peaks at 781.18 and 133.88 eV are attributed to the unavoidable contact of the CoP surface and ambient oxygen [31,33]. Moreover, the peak position of Co 2p3/2 (778.5 eV) is positively shifted while P 2p3/2 (129.28 eV) is negatively shifted in u-CoP@NC compared with that in metallic Co (778.1 eV) and in elemental P (130.2 eV), respectively, which is in agreement with the previously reported results [25]. The N 1s peaks investigated by XPS
spectrum could also be divided into three components residents at 397.68 eV, 399.78 eV and 400.48 eV (Fig. S2b, supporting information), attributing to pyridinic-N, pyrrolic-N, and graphitic-N, respectively. Meanwhile, the XPS spectrum of C 1s shows asymmetric signal with one peak located at 284.48 eV while a tail lying at 285.18 eV, which further confirms the successful doping of N atoms into carbon layers (Fig. S2c, supporting information) [49,50]. The Raman spectroscopy measurements of u-CoP@NC-T samples are shown in Fig. 2d, which shows the intensity ratio of the D band to G band (ID/IG) drops from 0.931 to 0.804 with the increasing phosphorization temperatures. It indicates that the higher phosphorization temperature results in higher degree of
Please cite this article as: Liu S et al., Porous coordination polymer-derived ultrasmall CoP encapsulated in nitrogen-doped carbon for efficient hydrogen evolution in both acidic and basic media, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.11.063
international journal of hydrogen energy xxx (xxxx) xxx
5
Fig. 2 e (a) XRD patterns of u-CoP@NC-T samples. (b, c) HR-XPS spectra of Co 2p and P 2p of the u-CoP@NC-900 sample. (d) Raman spectra of u-CoP@NC-T samples.
graphitization of carbon layer, which leads to higher electrical conductivity and enhanced catalytic activity [49,50]. The catalytic activities of u-CoP@NC-T catalysts toward HER were firstly assessed in 0.5 M H2SO4. Fig. 3a presents the polarization curves of the as-synthesized u-CoP@NC-T samples recorded at a scan rate of 5 mV s1. It is clearly revealed that the phosphorization temperature greatly influences the HER performance. The u-CoP@NC-900 transfers a current density of 10 mA cm2 (j10) at an overpotential of 131 mV lower than those of u-CoP@NC-800 (156 mV) and uCoP@NC-700 (174 mV) as investigated. The linear sweep polarization curves of NC, CoP and u-CoP@NC-900 are compared as shown in Fig. S3 (supporting information), indicating that ultrasmall-CoP nanoparticles confined into N-doped graphitic carbon layer could effectively enhance the catalytic activity of CoP. The corresponding Tafel slopes of uCoP@NC are calculated to elucidate the rate controlling step of HER process. Consistent with the reported study [25], Pt/C presents the lowest Tafel slope (32.8 mV dec1) shown in Fig. 3b. Besides, the u-CoP@NC-900 presents a Tafel slope of 62.7 mV dec1 smaller than those of u-CoP@NC-800 of 65.1 mV dec1 and u-CoP@NC-700 of 88.1 mV dec1, which illustrates a controlled EC desorption rate process. As a result, HER of the u-CoP@NC-900 may obey the VolmerHeyrovsky mechanism [51]. To further evaluate the intrinsic activity of the fabricated u-CoP@NC toward HER, the double-layer capacitance (Cdl) was measured in nonFaradaic potential region and the ECSAs of the u-CoP@NC samples are analyzed [52]. The CV curves of u-CoP@NC catalysts (Fig. S4a-c supporting information) were adopted for obtaining the straight lines slops as a function of current
densities versus scan rates (Fig. 3c). The calculated Cdl values for u-CoP@NC-900, u-CoP@NC-800 and u-CoP@NC-700 are 31.7, 11.4, and 8.7 mF cm2, respectively. The largest Cdl value of u-CoP@NC-900 indicates the rougher surface of uCoP@NC-900 leads to more active sites for HER [53,54]. The charge transfer between the catalysts and electrolyte was investigated by EIS. The Nyquist plots (Fig. 3d) suggest that the u-CoP@NC exhibits similar electrolyte resistance (Rs) whereas dissimilar charge transfer resistance (Rct). The uCoP@NC-900 presents the lowest Rct, indicating the boosted reaction kinetics at the interface between the catalyst and electrolyte [55]. The in situ formed CoP nanoparticles strengthen the formation of graphitic carbon and, hereafter, a higher graphitization degree in u-CoP@NC-900 sample, thus enhance its electrical conductivity. In addition, the u-CoP@NC catalysts also present outstanding electrocatalytic activity in 1.0 M KOH solution. The linear sweep polarization curves of the as-synthesized uCoP@NC-T samples in 1.0 M KOH are shown in Fig. 3e. The uCoP@NC-900 delivers j10 at an overpotential of 111 mV preceding to those of u-CoP@NC-800 (146 mV) and u-CoP@NC-700 (202 mV). The corresponding Tafel slope of u-CoP@NC-900 is 70.3 mV dec1, which is lower than the values of u-CoP@NC800 (87.8 mV dec1) and u-CoP@NC-700 (109.6 mV dec1) as shown in Fig. 3f. Furthermore, the CV curves of u-CoP@NC-T scanning with different rates are illustrated in Fig. S4d-f (supporting information). The tendency of calculated Cdl values for the u-CoP@NC-T samples is similar to the HER activities showing in 0.5 M H2SO4, where u-CoP@NC-900 presents the optimum value of 23.8 mF cm2 (Fig. 3g), which contributes to the high electrocatalytic activity of u-CoP@NC-
Please cite this article as: Liu S et al., Porous coordination polymer-derived ultrasmall CoP encapsulated in nitrogen-doped carbon for efficient hydrogen evolution in both acidic and basic media, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.11.063
6
international journal of hydrogen energy xxx (xxxx) xxx
Fig. 3 e (a, e) Linear sweep polarization curves, (b, f) the corresponding Tafel plots, (c, g) capacitive currents at (c) 0.3 V and (g) 0.2 V as a function of scan rate, and (d, h) EIS Nyquist plots of u-CoP@NC samples toward HER measured (aed) in 0.5 M H2SO4 solution and (eef) in 1 M KOH solution.
900. Moreover, the charge transfer at the interface between uCoP@NC-900 and electrolyte is fast, which can also be indicated by the smallest semicircle of the Nyquist plots evaluated in the high-frequency zone (Fig. 3h).
In order to evaluate the stability of u-CoP@NC catalysts, continuous CV scanning were performed and chronoamperometric curves were plotted. It is apparent that uCoP@NC-900 in acidic media shows negligible displacement
Please cite this article as: Liu S et al., Porous coordination polymer-derived ultrasmall CoP encapsulated in nitrogen-doped carbon for efficient hydrogen evolution in both acidic and basic media, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.11.063
international journal of hydrogen energy xxx (xxxx) xxx
7
Fig. 4 e (a, b) HER polarization curves before and after 2000 CV cycles and (c, d) chronoamperometry i-t curves at high current density of 50 mA cm¡2 of u-CoP@NC-900 (a,c) in 0.5 M H2SO4 solution and (b,d) in 1 M KOH solution.
of overpotentials at j10 after 2000 cycles of continuous CV scanning (Fig. 4a). In basic media, a slight voltage (20 mV) decreases after 2000 CV cycles with potentials between 0 and -0.2 V versus RHE (Fig. 4b). The HER electrolysis performance at a high current density (50 mA cm2) is shown in Fig. 4ced. It is apparent that no obvious current density decreases were observed after continuous HER for ~50 h in both acidic and basic solutions. The fluctuation of the current density is attributed to the H2 bubbles produced on the electrodes. These results confirm the stability of u-CoP@NC-900 catalysts, which indicates the promising practical applications. The morphology of the catalysts undergone the long-time HER tests was characterized by TEM (Fig. S5 supporting information). It is shown that the nanobelt morphology and ultra-small particles of u-CoP@NC-900 are generally maintained, without obvious collapse and aggregation. The above results indicate the good stability of the u-CoP@NC catalysts in both acidic and basic solutions, which attributes to the confinement effect by the graphitic carbon layers [49].
Conclusions In conclusion, ultrasmall-CoP nanoparticles strongly coupled with nitrogen doped graphitic carbon layers have been designed and fabricated through a PCP-derived route. The assynthesized u-CoP/NC-900 exhibited remarkable HER electrocatalytic activity, achieving a current density of 10 mA cm2 with small overpotentials of 131 mV and 111 mV in acidic and basic media, respectively, and the corresponding Tafel slopes are as low as 62.7 and 70.3 mV dec1. The desired
electrocatalytic activity of u-CoP@NC originates from the strongly coupled u-CoP nanoparticles and graphitic carbon layer and the perfect dispersed active sites. The strong interaction between u-CoP nanoparticles and the outer graphitic carbon layer not only improve the charge transport rate but also prevent the corrosion of CoP during electrocatalysis in the electrolyte, thereby increasing the catalytic activity and the stability. This work opens up new avenues for synthesizing non-precious metal-based highly efficient electrocatalysts towards water splitting applications.
Acknowledgments The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (No. 21703119 and 31600753), the Natural Science Foundation of Tianjin (No. 16JCYBJC43400), the National Science Foundation of Hebei Province (No. C2017202206), the Scientific Research Foundation for the Returned Overseas Chinese Scholars of Hebei Province (No. CL201711), the Natural Science Foundation of Shandong Province (No. ZR2017MB036) and the Grant for Taishan Scholar Advantage Characteristic Discipline, the Danish National Research Foundation, AUFF NOVA-project, and EU H2020 RISE (MNR4SCell 734174 project).
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.11.063.
Please cite this article as: Liu S et al., Porous coordination polymer-derived ultrasmall CoP encapsulated in nitrogen-doped carbon for efficient hydrogen evolution in both acidic and basic media, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.11.063
8
international journal of hydrogen energy xxx (xxxx) xxx
references
[1] Turner JA. Sustainable hydrogen production. Science 2004;305:972. [2] Roger I, Shipman MA, Symes MD. Earth-abundant catalysts for electrochemical and photoelectrochemical water splitting. Nat Rev Chem 2017;1:0003. [3] Xu X, Zhong Y, Shao Z. Double perovskites in catalysis, electrocatalysis, and photo (electro) catalysis. Trends in Chemistry 2019;1:410e24. [4] Jiang P, Liu Q, Liang Y, Tian J, Asiri AM, Sun X. A costeffective 3D hydrogen evolution cathode with high catalytic activity: FeP nanowire array as the active phase. Angew Chem Int Ed 2014;53:12855e9. [5] Zou X, Zhang Y. Noble metal-free hydrogen evolution catalysts for water splitting. Chem Soc Rev 2015;44:5148e80. [6] Walter MG, Warren EL, McKone JR, Boettcher SW, Mi Q, Santori EA, et al. Solar water splitting cells. Chem Rev 2010;110:6446e73. [7] Wang Y, Kong B, Zhao D, Wang H, Selomulya C. Strategies for developing transition metal phosphides as heterogeneous electrocatalysts for water splitting. Nano Today 2017;15:26e55. [8] Shi Y, Zhang B. Recent advances in transition metal phosphide nanomaterials: synthesis and applications in hydrogen evolution reaction. Chem Soc Rev 2016;45:1529e41. [9] Wang Z, Li Q, Xu H, Dahl-Petersen C, Yang Q, Cheng D, et al. Controllable etching of MoS2 basal planes for enhanced hydrogen evolution through the formation of active edge sites. Nano Energy 2018;49:634e43. [10] Xu H, Cheng D, Cao D, Zeng XC. A universal principle for a rational design of single-atom electrocatalysts. Nat Catal 2018;1:339. [11] Yang Q, Wang Z, Dong L, Zhao W, Jin Y, Fang L, et al. Activating MoS2 with super-high nitrogen-doping concentration as efficient catalyst for hydrogen evolution reaction. J Phys Chem C 2019;123:10917e25. [12] Jiang H, Wang Z, Yang Q, Tan L, Dong L, Dong M. Ultrathin Ti3C2TX(MXene) nanosheet-wrapped NiSe2 octahedral crystal for enhanced supercapacitor performance and synergetic electrocatalytic water splitting. Nano-Micro Lett 2019;11:31. [13] Xu X, Chen Y, Zhou W, Zhu Z, Su C, Liu M, et al. A perovskite electrocatalyst for efficient hydrogen evolution reaction. Adv Mater 2016;28:6442e8. [14] Zheng Y, Jiao Y, Zhu Y, Li LH, Han Y, Chen Y, et al. Hydrogen evolution by a metal-free electrocatalyst. Nat Commun 2014;5:3783. [15] Zhu Y, Chen G, Xu X, Yang G, Liu M, Shao Z. Enhancing electrocatalytic activity for hydrogen evolution by strongly coupled molybdenum nitride@nitrogen-doped carbon porous nano-octahedrons. ACS Catal 2017;7:3540e7. [16] Fang X, Wang Z, Wang Y, Kang S, Jiang Z, Dong M. Tuning quaternary hybride CoeNieSeSe composition as a bifunctional electrocatalyst for hydrogen and oxygen evolution reactions. Int J Hydrogen Energy 2019;44:27685e94. [17] Wang Z, Wu H-H, Li Q, Besenbacher F, Li Y, Zeng XC, et al. Reversing interfacial catalysis of ambipolar WSe2 single crystal. Adv Sci 2019. https://doi.org/10.1002/advs.201901382. [18] Zhou Q, Shen Z, Zhu C, Li J, Ding Z, Wang P, et al. Nitrogendoped CoP electrocatalysts for coupled hydrogen evolution and sulfur generation with low energy consumption. Adv Mater 2018;30:1800140. [19] Zeng Y, Wang Y, Huang G, Chen C, Huang L, Chen R, et al. Porous CoP nanosheets converted from layered double hydroxides with superior electrochemical activity for hydrogen evolution reactions at wide pH ranges. Chem Commun 2018;54:1465e8.
[20] Yan L, Cao L, Dai P, Gu X, Liu D, Li L, et al. Metal-organic frameworks derived nanotube of nickelecobalt bimetal phosphides as highly efficient electrocatalysts for overall water splitting. Adv Funct Mater 2017;27:1703455. [21] Tang C, Zhang R, Lu W, He L, Jiang X, Asiri AM, et al. Fedoped CoP nanoarray: a monolithic multifunctional catalyst for highly efficient hydrogen generation. Adv Mater 2017;29:1602441. [22] Tabassum H, Guo W, Meng W, Mahmood A, Zhao R, Wang Q, et al. Metaleorganic frameworks derived cobalt phosphide architecture encapsulated into B/N Co-doped graphene nanotubes for all pH value electrochemical hydrogen evolution. Adv Energy Mater 2017;7:1601671. [23] Wang H, Min S, Wang Q, Li D, Casillas G, Ma C, et al. Nitrogen-doped nanoporous carbon membranes with Co/ CoP Janus-type nanocrystals as hydrogen evolution eectrode in both acidic and alkaline environments. ACS Nano 2017;11:4358e64. [24] Yang H, Zhang Y, Hu F, Wang Q. Urchin-like CoP nanocrystals as hydrogen evolution reaction and oxygen reduction reaction dual-electrocatalyst with superior stability. Nano Lett 2015;15:7616e20. [25] Tian J, Liu Q, Asiri AM, Sun X. Self-supported nanoporous cobalt phosphide nanowire arrays: an efficient 3D hydrogenevolving cathode over the wide range of pH 0e14. J Am Chem Soc 2014;136:7587e90. [26] Sharma AK, Joshi H, Ojha K, Singh AK. Graphene oxide supported cobalt phosphide nanorods designed from a molecular complex for efficient hydrogen evolution at low overpotential. Chem Commun 2019;55:2186e9. [27] Lu Y, Hou W, Yang D, Chen Y. CoP nanosheets in-situ grown on N-doped graphene as an efficient and stable bifunctional electrocatalyst for hydrogen and oxygen evolution reactions. Electrochim Acta 2019;307:543e52. [28] Wu T, Pi M, Wang X, Guo W, Zhang D, Chen S. Hierarchical cobalt poly-phosphide hollow spheres as highly active and stable electrocatalysts for hydrogen evolution over a wide pH range. Appl Surf Sci 2018;427:800e6. [29] Liu T, Xie L, Yang J, Kong R, Du G, Asiri AM, et al. Selfstanding CoP nanosheets array: a three-dimensional bifunctional catalyst electrode for overall water splitting in both neutral and alkaline media. ChemElectroChem 2017;4:1840e5. [30] Liu T, Wang K, Du G, Asiri AM, Sun X. Self-supported CoP nanosheet arrays: a non-precious metal catalyst for efficient hydrogen generation from alkaline NaBH4 solution. J Mater Chem 2016;4:13053e7. [31] Tang C, Qu F, Asiri AM, Luo Y, Sun X. CoP nanoarray: a robust non-noble-metal hydrogen-generating catalyst toward effective hydrolysis of ammonia borane. Inorg Chem Front 2017;4:659e62. [32] Xie L, Zhang R, Cui L, Liu D, Hao S, Ma Y, et al. Highperformance electrolytic oxygen evolution in neutral media catalyzed by a cobalt phosphate nanoarray. Angew Chem Int Ed 2017;56:1064e8. [33] Zhang R, Ren X, Hao S, Ge R, Liu Z, Asiri AM, et al. Selective phosphidation: an effective strategy toward CoP/CeO2 interface engineering for superior alkaline hydrogen evolution electrocatalysis. J Mater Chem 2018;6:1985e90. [34] Chen Z, Jia H, Yuan J, Liu X, Fang C, Fan Y, et al. P-co-doped carbon coupled with CoP as superior electrocatalysts for hydrogen evolution reaction and overall water splitting. Int J Hydrogen Energy 2019;44:24342e52. [35] Jin J, Zhu Y, Liu Y, Li Y, Peng W, Zhang G, et al. CoP nanoparticles combined with WS2 nanosheets as efficient electrocatalytic hydrogen evolution reaction catalyst. Int J Hydrogen Energy 2017;42:3947e54.
Please cite this article as: Liu S et al., Porous coordination polymer-derived ultrasmall CoP encapsulated in nitrogen-doped carbon for efficient hydrogen evolution in both acidic and basic media, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.11.063
international journal of hydrogen energy xxx (xxxx) xxx
[36] Zhang M, Ci S, Li H, Cai P, Xu H, Wen Z. Highly defective porous CoP nanowire as electrocatalyst for full water splitting. Int J Hydrogen Energy 2017;42:29080e90. [37] Yang F, Chen Y, Cheng G, Chen S, Luo W. Ultrathin nitrogendoped carbon coated with CoP for efficient hydrogen evolution. ACS Catal 2017;7:3824e31. [38] Yang S, Chen L, Wei W, Lv X, Xie J. CoP nanoparticles encapsulated in three-dimensional N-doped porous carbon for efficient hydrogen evolution reaction in a broad pH range. Appl Surf Sci 2019;476:749e56. [39] Jin H, Wang J, Su D, Wei Z, Pang Z, Wang Y. In situ cobaltecobalt oxide/N-doped carbon hybrids as superior bifunctional electrocatalysts for hydrogen and oxygen evolution. J Am Chem Soc 2015;137:2688e94. [40] Zhang H, Liu X, Wu Y, Guan C, Cheetham AK, Wang J. MOFderived nanohybrids for electrocatalysis and energy storage: current status and perspectives. Chem Commun 2018;54:5268e88. [41] Dang S, Zhu Q-L, Xu Q. Nanomaterials derived from metaleorganic frameworks. Nat Rev Mater 2017;3:17075. [42] Cao X, Tan C, Sindoro M, Zhang H. Hybrid micro-/nanostructures derived from metaleorganic frameworks: preparation and applications in energy storage and conversion. Chem Soc Rev 2017;46:2660e77. [43] Wang W, Xu X, Zhou W, Shao Z. Recent progress in metalorganic frameworks for applications in electrocatalytic and photocatalytic water splitting. Adv Sci 2017;4:1600371. [44] Gao L, Chen S, Zhang H, Zou Y, She X, Yang D, et al. Porous CoP nanostructure electrocatalyst derived from DUT-58 for hydrogen evolution reaction. Int J Hydrogen Energy 2018;43:13904e10. [45] Li Y, Ben T, Zhang B, Fu Y, Qiu S. Ultrahigh gas storage both at low and high pressures in KOH-activated carbonized porous aromatic frameworks. Sci Rep 2013;3:2420. [46] Kitagawa S, Kitaura R, Noro S-i. Functional porous coordination polymers. Angew Chem Int Ed 2004;43:2334e75.
9
[47] Chen C, Zhang W, Hong Y, Le Z, Li Q, Li W, et al. Synthesis of coordination polymer thin films with conductanceresponse to mechanical stimulation. Chem Commun 2019;55:2545e8. [48] Sakata Y, Furukawa S, Kondo M, Hirai K, Horike N, Takashima Y, et al. Shape-memory nanopores induced in coordination frameworks by crystal downsizing. Science 2013;339:193e6. [49] Liu Y, Zhu Y, Shen J, Huang J, Yang X, Li C. CoP nanoparticles anchored on N, P-dual-doped graphene-like carbon as a catalyst for water splitting in non-acidic media. Nanoscale 2018;10:2603e12. [50] Su J, Yang Y, Xia G, Chen J, Jiang P, Chen Q. Ruthenium-cobalt nanoalloys encapsulated in nitrogen-doped graphene as active electrocatalysts for producing hydrogen in alkaline media. Nat Commun 2017;8:14969. [51] Pentland N, Bockris JO, Sheldon E. Hydrogen evolution reaction on copper, gold, molybdenum, palladium, rhodium, and iron. J Electrochem Soc 1957;104:182e94. [52] Lukowski MA, Daniel AS, Meng F, Forticaux A, Li L, Jin S. Enhanced hydrogen evolution catalysis from chemically exfoliated metallic MoS2 nanosheets. J Am Chem Soc 2013;135:10274e7. [53] Pan Y, Lin Y, Chen Y, Liu Y, Liu C. Cobalt phosphide-based electrocatalysts: synthesis and phase catalytic activity comparison for hydrogen evolution. J Mater Chem 2016;4:4745e54. [54] Li X, Jiang L, Liu J, Hua Q, Wang E, Xie G. Size-dependent catalytic activity of cobalt phosphides for hydrogen evolution reaction. J Energy Chem 2020;43:121e8. [55] Chang J, Ouyang Y, Ge J, Wang J, Liu C, Xing W. Cobalt phosphosulfide in the tetragonal phase: a highly active and durable catalyst for the hydrogen evolution reaction. J Mater Chem A 2018;6:12353e60.
Please cite this article as: Liu S et al., Porous coordination polymer-derived ultrasmall CoP encapsulated in nitrogen-doped carbon for efficient hydrogen evolution in both acidic and basic media, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.11.063