Transition-Metal PhosphideâCarbon Nanosheet Composites Derived

Research Article Cite This: ACS Appl. Mater. Interfaces 2017, 9, 40171-40179

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Transition-Metal Phosphide−Carbon Nanosheet Composites Derived from Two-Dimensional Metal-Organic Frameworks for Highly Efficient Electrocatalytic Water-Splitting Mengke Zhai, Fei Wang, and Hongbin Du* State Key Laboratory of Coordination Chemistry, Collaborative Innovation Center of Chemistry for Life Sciences, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210023, China S Supporting Information *

ABSTRACT: The preparation of highly active, sustainable, nonprecious metal materials as hydrogen evolution and oxygen evolution reaction (HER and OER) catalysts that can relieve the environmental pollution and energy shortage problems present a great challenge to chemists. We herein report the fabrication of a highly active metal phosphide−carbon composite catalyst for HER and OER in acid and basic solution, respectively. The catalyst is derived through carbonization and subsequent phosphorization of two-dimensional (2D) cobalt porphyrinic metal-organic framework nanosheets. It consists of cobalt phosphide nanoparticles embedded in mesoporous N-doped graphitic carbon materials. The catalyst shows good electrocatalytic activities for HER in 0.5 M H2SO4 and OER in 1 M KOH with overpotentials of 98 and 370 mV at a current density of 10 mA cm−2 and the Tafel slopes of 74 and 79 mV dec−1, respectively. In addition, the catalyst also shows good durability. The method used in this study could be applied to prepare new, highly efficient water-splitting catalysts by using diverse 2D metal-organic frameworks as templates. KEYWORDS: two-dimensional metal-organic frameworks, hydrogen evolution reaction, oxygen evolution reaction, cobalt phosphide, nanoparticles been paid much attention.12,13 For example, transition-metal phosphides have been known as hydrodesulfurization and hydrodenitrification catalysts owing to their activation of H2.14,15 These materials have also been shown to be efficient catalysts for HER,16−21 among which Co-based phosphides exhibited excellent catalytic performance. Nowadays, two-dimensional (2D) materials, such as graphene, metal oxides, metal sulfides, and metal hydroxides, have attracted great interest because of their unique physical and chemical properties.22 They are widely studied in many fields, such as electronic, energy storage, gas separation, and water splitting due to their large lateral dimension and nanometer thickness.23,24 Metal-organic frameworks (MOFs) are porous crystalline materials that are orderly constructed by coordination of metal ions or clusters with polytopic organic ligands,25,26 possessing diverse structures, functions, and properties.27,28 These materials have shown promises as adsorbents and energy storage materials. Recently, MOFs have been transformed into metal oxides, porous carbon materials, metal carbides, and metal phosphides upon thermal treatment,29−33 showing excellent properties in water splitting.

1. INTRODUCTION The ever increasing awareness of the global energy crisis and the greenhouse effect prompts scientists to look for renewable and clean resources as replacements of fossil fuels.1 The hydrogen evolution reaction (HER) from water through converting energy from light or electricity into chemical energy storage provides a renewable, sustainable, and clean energy system.2−4 The obtained hydrogen (H2) is pollution-free and high-output energy without the release of the greenhouse gas carbon dioxide.5−7 At the same time, the catalytic water decomposition technology through the electrolysis of water allows efficient energy storage in the chemical bonds, thus solving the problem of the difficulty of storing H2. Designing the oxygen evolution reaction (OER) catalysts is more challenging because of the slow four-electron transfer process during water oxidation.8 Platinum is the most active catalyst in water splitting, especially for HER, whereas noble metal oxides (RuO2 and IrO2) are efficient catalysts for the OER.9,10 The noble metal catalysts can reach high current density at low overpotentials during the water-splitting process.11 But the high cost and rarity of noble metals limit their large-scale use in HER and OER. So it is highly desirable to develop more efficient and/or non-noble metal catalysts for water splitting. Much work so far has focused on exploiting low-cost and highly efficient alternatives. Transitional-metal-based compounds have © 2017 American Chemical Society

Received: July 20, 2017 Accepted: November 3, 2017 Published: November 3, 2017 40171

DOI: 10.1021/acsami.7b10680 ACS Appl. Mater. Interfaces 2017, 9, 40171−40179

Research Article

ACS Applied Materials & Interfaces

porphyrinic framework-derived electrocatalyst active for both HER and OER.

The 2D MOF nanosheets are a new member of the 2D material family.34 Compared with three-dimensional bulk MOFs, the 2D MOF nanosheets have larger surface area and more available active sites on the surfaces because of their specific 2D morphology.34,35 Therefore, it is likely these materials possess unique properties. Herein, we report the preparation of cobalt phosphide nanoparticles embedded in N-doped carbon materials (denoted as Co-P@NC) by using a Co-based porphyrin paddlewheel framework-3 (PPF-3) 2D MOF nanosheet35 as a sacrificial template through carbonization and phosphorization procedures (Scheme 1). The Co-P@NC material exhibited good

2. EXPERIMENTAL SECTION 2.1. Materials. 4,4′-Bipyridine (BPY) was purchased from Bide Pharmatech Ltd. Cobalt nitrate hexahydrate (Co(NO3)2·6H2O), N,Ndimethylformamide (DMF), poly(vinyl pyrrolidone) (PVP), ethanol, sodium hypophosphite monohydrate (NaH2PO2·H2O), and Nafion were purchased from Sinopharm. Commercial Pt/C (20% Pt) was purchased from Macklin. All of the materials were used as received without further purification. The porphyrinic ligand 4,4′,4″,4‴((4,4′,4″,4‴-(porphyrin-5,10,15,20-tetrayl)tetrakis(benzoyl))tetrakis(azanediyl))tetrabenzoic acid (TCPP) was synthesized according to the reported procedure.36 2.2. Synthesis of Porphyrin Paddlewheel Framework-3 (PPF3) Nanosheets. PPF-3 was synthesized according to the literature procedure.35 Typically, BPY (1.56 mg), Co(NO3)2·6H2O (4.4 mg), and PVP (10.0 mg) were added in 6 mL of a mixture of DMF and ethanol (v/v = 3:1) in a 10 mL vial. TCPP (4.0 mg, 0.005 mmol) dissolved in 2 mL of the mixture of DMF and ethanol was then poured into the above solution. The mixture was fully sonicated for 25 min and then capped and put into an oven at 80 °C for 24 h. The resulting product was washed with ethanol twice and collected by centrifuging. 2.3. Synthesis of Co@NC and Co-P@NC. First, PPF-3 powders were put into a tube furnace and carbonized under Ar atmosphere at 600, 700, 800, and 900 °C for 5 h, respectively, with a heating rate of 5 °C min−1. The obtained materials were Co-embedded N-doped carbon materials, denoted as Co@NC-600, Co@NC-700, Co@NC800, and Co@NC-900, respectively. Then, 50 mg of carbonized PPF-3 (i.e., Co@NC) was fully grinded with 600 mg of NaH2PO2·H2O (i.e., PPF-3/NaH2PO2·H2O weight ratio of 1:12, molar ratio 1:50). The mixture was thermally treated under Ar at 300 °C for 5 h for phosphorization, with a heating rate of 5 °C min−1. The obtained catalyst was washed three times with water and dried under vacuum at 100 °C for 8 h to obtain Co-P@NC (i.e., Co-P@NC-600, Co-P@NC700, Co-P@NC-800, and Co-P@NC-900). Co-P@NC with different PPF-3/NaH2PO2·H2O weight ratios of 1:7 and 1:17 was also synthesized by using the same procedure.

Scheme 1. Schematic Presentation for the Preparation of Co-P@NC

electrocatalytic properties with overpotentials of 98 and 370 mV at a current density of 10 mA cm−2 and Tafel slopes of 74 and 79 mV dec−1 for HER and OER in acidic and alkaline electrolytes, respectively, which are comparable to some of the best nonprecious metal water-splitting catalysts known so far. To the best of our knowledge, Co-P@NC is the first 2D metal-

Figure 1. SEM images of (a) PPF-3 nanosheets (inset shows enlarged nanosheets), (b) Co@NC-800, and (c) Co-P@NC-800. (d) Transmission electron microscopy (TEM) and (e) high-resolution TEM (HRTEM) images of Co-P@NC-800. (f) Selected-area electron diffraction (SAED) patterns of Co-P@NC-800, showing diffraction spots of CoP and Co2P. 40172


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ACS Applied Materials & Interfaces 2.4. Physical Methods. Powder X-ray diffraction (PXRD) data were obtained on a Bruker D8-Advance diffractometer with a Cusealed tube (λ = 1.54178 Å) at 40 kV and 40 mA in the 2θ range with 0.2 s per step. The scanning electron microscopy/energy dispersive spectrometry (SEM/EDS) images were collected on a Hitachi S-4800 field emission scanning electron microscope with an accelerating voltage of 5 and 20 kV, respectively. Scanning transmission electron microscopy (STEM) measurements were collected on a JEM-2100 (Japan). X-ray photoelectron spectroscopy (XPS) characterizations were performed on a PHI 5000 Versa Probe instrument. Nitrogen sorption measurements were carried out at 77 K (Micrometrics ASAP 2020 analyzer) after the sample was treated under vacuum at 150 °C for 8 h. Raman data were collected on an InVia-Reflex instrument (England). 2.5. Electrochemical Measurements. The electrochemical data were recorded on a CHI-660D electrochemical workstation (CH Instruments, Inc.) with a three-electrode system. The working electrode was a glassy carbon electrode (d = 3 mm, S = 0.07065 cm2) modified with the sample, the reference electrode was an Ag/ AgCl electrode, and the counter electrode was a graphite electrode. The glassy carbon electrode was polished by 0.3 μm alumina slurry and 0.05 μm alumina slurry to obtain a mirror-like surface before use. The homogenous inks of the catalysts, e.g., Co-P@NC-800, commercial 20 wt % Pt/C, and RuO2, were prepared by mixing 1 mg of catalyst powder, 132 μL of ethanol, 66 μL of deionized water, and 2 μL of 5% Nafion solution. The solution was sonicated for 30 min. Then, 4 μL of catalyst ink was drop-cast onto the polished glassy carbon to form the working electrode with a mass loading of 0.283 mg cm−2 after the ink was air-dried. Before the HER test, high-purity N2 was bubbling through the electrolyte for 30 min to expel dissolved O2. All of the potentials were referenced to a reversible hydrogen electrode (RHE) by adding a figure of (0.197 + 0.059pH) V. The HER and OER tests were carried out both in an N2-saturated 0.5 M H2SO4 solution and in a 1 M KOH solution. All of the tests were conducted at room temperature, and the electrochemical test data are presented with iR (internal resistance) compensation by using the CHI software.

Figure 2. PXRD patterns of (a) Co@NC-800 and (c) Co-P@NC-800, compared with those of (b) Co, (d) CoP, and (e) Co2P.

bands (ID/IG). The intensity ratios ID/IG of Co@NC-600, -700, -800, and -900 were calculated to be 2.26, 1.36, 1.33, and 1.30, respectively, suggesting the higher degree of graphitization of PPF-3 were achieved at a higher temperature. After phosphorization, the PXRD pattern of Co-P@NC-800 (Figure 2c) showed diffraction peaks owing to the (011), (111), (211), (103), and (301) planes of CoP (JCPDS 290497), (121) and (211) planes of Co2P (JCPDS 32-0306) and (002) plane of graphitic C, respectively. Further experiments showed that the phosphorized product mainly consisted of metal-rich Co2P when the PPF-3/NaH2PO2·H2O weight ratio of 1:7 (molar ratio ∼1:29) was used during the phosphorization. The diffraction peaks of Co could be clearly seen, which meant there remained parts of Co nanoparticles unphosphorized. The molar ratio of CoP/Co2P was estimated to be 1:1.5 based on the PXRD peak intensities. When more NaH2PO2· H2O was added during the phosphorization, the peak intensities of CoP increased and those of Co decreased. The molar ratios of CoP/Co2P were calculated to be 1:1.03 and 1:1.08 for Co-P@NC-800 prepared with the PPF-3/NaH2PO2· H2O weight ratios of 1:12 and 1:17, respectively (Figure S1b). The results showed that the molar ratio of CoP/Co2P in CoP@NC-800 increased with the increase in the PPF-3/ NaH2PO2·H2O weight ratio during the preparation, but leveled off at the PPF-3/NaH2PO2·H2O weight ratio of 1:12. The formation of Co2P and CoP during the phosphorization with sodium hypophosphite has been reported in the literature.38 Because CoP is known to exhibit better electrocatalytic activity than Co2P,39−41 the catalysts used in the following tests were prepared with the precursor/hypophosphite weight ratio of 1:12. Additionally, further PXRD analyses showed that the particle sizes of Co2P and CoP in Co-P@NC-800 (1:12) are ca. 13 and 12 nm, respectively, as determined from the PXRD data by using the Scherrer formula. In comparison, the particle sizes of Co2P and CoP in Co-P@NC-900 (1:12) were slightly larger (15 and 16 nm, respectively). SEM and TEM images were taken to investigate the morphology evolution of Co@NC-800 and Co-P@NC-800 (SEM images of Co@NC-700 and Co@NC-900 are shown in Figure S3). As shown in Figure 1b, SEM images showed that Co@NC-800 consisted of some broken and wrinkled particles after carbonization at 800 °C. After extensive grinding and a subsequent phosphorization process, the morphology of CoP@NC-800 showed little changes compared to that of Co@

3. RESULTS AND DISCUSSION 3.1. Material Preparation and Characterization. Scheme 1 shows the procedure for the preparation of the title catalyst. The Co-based porphyrin paddlewheel framework3 (PPF-3) was successfully synthesized according to the literature method.35 PXRD patterns showed that PPF-3 had high crystallinity with preferred crystal orientation, indicating a sheet-like crystal morphology (Figure S1a, Supporting Information). SEM images showed that the square-sheet crystals of PPF-3 were obtained with a lateral size of ca. 1.5 μm and a thickness of about 50−100 nm (Figure 1a). Carbonization and phosphorization of PPF-3 were carried at various temperatures in inert atmosphere. Because the electrocatalytic tests showed that the products carbonized at 800 °C gave the best performance, the characterization of the thus-obtained catalysts are given as follows. After carbonization of PPF-3 at 800 °C, the obtained Co@ NC-800 showed three PXRD diffraction peaks related to the (111), (200), and (220) planes of Co (JCPDS 15-0806) and one attributable to the (002) plane of graphitic C (Figure 2a). To confirm the graphitization, Raman spectra were taken, which are shown in Figure S2. Raman spectra of Co@NC-800 showed two bands typical of partially graphitic carbon materials. The D-band at about 1324 cm−1 was attributed to the vibration of carbon atoms with dangling bonds in the plane termination of disordered graphite, whereas the G-band at 1598 cm−1 was associated with the vibration of sp2 hybridized carbon atoms in a graphite layer.37 The disorder degree of the carbon was in proportion to the relative intensity ratio of the two 40173


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Figure 3. IR-corrected polarization curves of (a, c) HER and (b, d) OER for Co-P@NC materials prepared with different precursor/hypophosphite weight ratios (a, b) and carbonization temperatures (c, d).

g−1, respectively. The BET surface area showed a rising trend as the treating temperature went up, but the pore volume of CoP@NC-800 was the highest compared to that of Co@NC-700 and Co@NC-900. After phosphorization, however, the surface area of Co-P@NC-700, Co-P @NC-800, and Co-P@NC-900 decreased significantly, with the calculated BET surface area of 39.8, 1.9, and 0.1 m2 g−1, respectively. The decreased BET surface area after phosphorization could be attributed to particle sintering at high temperature and incorporation of P via phosphorization by hypophosphites, which resulted in the formation of CoP and Co2P nanoparticles. X-ray photoelectron spectra were recorded to further characterize the Co-P@NC-800. As shown in Figure S4, the X-ray photoelectron survey spectra of Co-P@NC-800 confirmed the presence of Co, P, C, and N, which was consistent with the SEM element mapping results. The high-resolution Xray photoelectron spectrum of the Co 2p region showed four peaks at 781.2, 785.8, 797.2, and 802.3 eV, respectively (Figure S6b). The peaks at 781.2 and 797.2 eV could be attributed to Co 2p3/2 and Co 2p1/2, respectively, whereas the peaks at 785.8 and 802.3 eV were due to the satellite peaks.43,44 The binding energy of Co 2p3/2 in Co-P@NC-800 was positively shifted from that of the metal Co (777.9 eV),45 comparable to those of cobalt phosphides reported in the literature.46 The highresolution X-ray photoelectron spectrum of the P 2p region showed three peaks located at 129.7, 130.9, and 133.3 eV, respectively (Figure S6c). The peaks at 129.7 and 130.9 eV were due to P 2p3/2 and P 2p1/2, respectively, assigned to CoP or Co2P.47 The peak at 133.3 eV was attributed to the

NC-800 (Figure 1c). The TEM image showed that Co-P@NC800 consisted of nanosheets decorated with nanoparticles of 10−20 nm sizes (Figure 1d) that coincided with those of metal phosphides obtained from the PXRD analyses. Moreover, the HRTEM image of Co-P@NC-800 showed clear crystal lattice fringes with a distance of 0.162 nm, corresponding to the (301) plane of CoP (Figure 1e). Selected-area electron diffraction (SAED) patterns of Co-P@NC-800 (Figure 1f) showed the diffraction spots belonging to CoP and Co2P nanoparticles. A series of rings and spots could be assigned to the (101), (201), (230), (400), (202), (210), (410), and (430) planes of CoP and to the (031), (130), (341), (220), and (251) planes of Co2P, respectively. EDS element mapping for Co-P@NC-800 were collected, which further confirmed the uniform dispersion of Co, C, N, and P elements within the squarelike nanosheets (Figure S4). N2 sorption analysis was used to further characterize the texture properties of the 2D MOF-derived materials. As shown in Figure S5a,b, all of the isotherms of Co@NC-700, Co@NC800, and Co-P@NC-900 were of type IV, with an obvious H3type hysteresis loop. The type-IV isotherms suggested that Co@NC-700, Co@NC-800, and Co-P@NC-900 possessed mesopores in the structure. The associated H3 hysteresis loops indicated that the pores in these materials were of slit-shape, likely formed by aggregates of platelike particles,42 which was in agreement with the SEM and TEM results. The calculated Brunauer−Emmett−Teller (BET) surface areas of Co@NC700, Co@NC-800, and Co@NC-900 were 206, 267, and 279 m2 g−1, with the pore volume of 0.190, 0.353, and 0.281 cm3 40174


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Figure 4. (a) IR-corrected HER polarization curves and (b) Tafel plots of 20% Pt/C, Co@NC-800, and Co-P@NC-800 in a N2-saturated 0.5 M H2SO4 solution at a scan rate of 5 mV s−1. (c) HER polarization curves of Co-P@NC-800 before and after the stability test at a current density of 10 mA cm−2 (d).

phosphate species.45 It is noted that from the XPS analysis of P 2p spectra based on the reported references, there was no P 2p peak contributable to the P−C bonding in Co-P@NC-800. Consistently, Raman spectra of Co-P@NC-800 (Figure S2b) gave no apparent peaks in the P−C bond stretching range of 670−780 cm −1. These results indicated that the obtained carbon materials were without or with negligible P doping, coinciding with the literature report.48 The high-resolution Xray photoelectron spectrum of Co-P@NC-800 in the N 1s region gave an asymmetric peak that could be deconvoluted into two peaks at 399.3 and 401.9 eV (Figure S6d). The two peaks were related to pyrrolelike and graphitic nitrogen,49 respectively. These nitrogen were originated from the porphyrinic ligand used in the precursor. The incorporation of these nitrogen in the carbon matrix could facilitate the electrocatalytic activities.50 The above XPS results indicated that Co-P@NC-800 consisted of cobalt phosphides embedded in an N-doped carbon matrix, in agreement with the PXRD and TEM results. 3.2. Electrocatalytic Performances of HER and OER. The obtained Co@NC and Co-P@NC were tested to electrocatalyze water splitting, including HER and OER. Initially, the linear sweep voltammetry (LSV) tests on Co-P@ NC were carried out in acidic and alkaline electrolytes solution at a scan rate of 5 mV s−1 to optimize the water-splitting catalysts prepared under different conditions. As shown in Figure 3a,b, the LSV measurements showed that the catalyst Co-P@NC-800 (1:7) prepared with a precursor/hypophosphite weight ratio of 1:7 was active for both HER and OER. The performance was much improved when the precursor/

hypophosphite weight ratio increased to 1:12. Further increase in amounts of hypophosphites (i.e., precursor/hypophosphite weight ratio of 1:17) during the phosphorization did not achieve better catalytic activities. In the meanwhile, as shown in Figure 3c,d, the optimized carbonization temperature for the preparation of the catalyst was 800 °C; lower or higher carbonization temperature led to deteriorating HER and OER performance of the catalyst. The better performance of the highly phosphorized Co-P@NC-800 catalyst could be related to its composition. A recent study confirmed that CoP showed higher HER catalytic activity than the morphologically equivalent Co2P.39 The Co-P@NC-800 (1:7) mainly consisted of less reactive Co2P, whereas in Co-P@NC-800 (1:12) and Co-P@NC-800 (1:17), Co2P was partially transformed into highly active CoP. On the other hand, the better performance of Co-P@NC-800 obtained via carbonization at 800 °C than that of Co-P@NC-700 and Co-P@NC-900 might be attributed to an optimal balance of accessible catalytic sites and electron conductivity.32 On the basis of the above results, the catalysts used in the following tests were prepared under the optimized conditions, where the carbonization temperature was set at 800 °C and the precursor/hypophosphite weight ratio was 1:12. The electrocatalytic performances of Co@NC-800 and CoP@NC-800 were studied in detail in both acid and alkaline solution. Figure 4 shows the HER performance of Co@NC-800 and Co-P@NC-800 in an N2-saturated 0.5 M H2SO4 solution. Both Co@NC-800 and Co-P@NC-800 exhibited good HER activities. The HER polarization curve of Co@NC-800 gave an overpotential of 270, 326, and 412 mV at a current density of 10, 20, and 50 mA cm−2, respectively (Figure 4a). After 40175


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Figure 5. (a) IR-corrected OER polarization curves and (b) Tafel plots of 20% Pt/C, Co@NC-800, and Co-P@/NC-800 in 1 M KOH electrolyte at a scan rate of 5 mV s−1. (c) OER polarization curves of Co-P@NC-800 before and after the stability test at a current density of 10 mA cm−2 (d).

transition-metal electrocatalysts (Table S2), but inferior to that of commercial RuO2 (290 mV). The Tafel slopes for RuO2, CoP@NC-800, and Co@NC-800 electrodes were 53, 79, and 105 mV dec−1, respectively (Figure 5b). Similarly, Co-P@NC-800 showed better kinetics than Co@NC-800, with the Tafel slope value close to that of the RuO2 electrode. Galvanostatic measurements at the current density of 10 mA cm−2 showed that Co-P@NC-800 possessed good stability for at least 6000 s despite the fact that ample O2 gas bubbles were generated and abruptly liberated from the electrode, which might result in the catalyst falling off from the electrode (Figure 5c,d). It is noted that the OER performance of Co-P@NC-800 deteriorated quickly after ca. 3 h, accompanied with the disintegration of nanosheets into nanoparticles (Figure S8). The deterioration of the OER performance of Co-P@NC-800 could be attributed to the oxidation of metal phosphides in an alkaline solution (1 M KOH), which has been well documented in the literature.12,51 In comparison, the OER performance of Co-P@NC-800 in alkaline solution was not as extraordinarily good as its excellent HER performance in acid solution. This is in accordance with the general observations of transition-metal phosphide-based OER catalysts, where partial oxidation of CoPx into CoOx in alkaline solution likely limited their catalytic OER performance.52 Finally, it is pointed out that Co-P@NC-800 exhibited moderate HER activities in 1 M KOH solution (Figure S7). It achieved a current density of 10 mA cm−2 at an overpotential of 339 mV with a Tafel slope of 154 mV dec−1, inferior to its HER performance in acid solution. The good HER and OER performance of Co-P@NC-800 in acid and alkaline solution, respectively, could be attributed to

phosphorization, Co-P@NC-800 showed much more improved HER activities than Co@NC-800, reaching the current densities of 10, 20, and 50 mA cm−2 at overpotentials of 98, 126, and 181 mV, respectively. The Tafel slopes were obtained from the linear fitting of the Tafel plots, giving rise to Tafel values of 57, 74, and 178 mV dec−1 for 20% Pt/C, Co-P@NC800, and Co@NC-800, respectively (Figure 4b). The Tafel slope of Co-P@NC-800 was much better than that of Co@NC800 and was close to that of 20% Pt/C, indicating its excellent HER catalytic kinetics.32 To test the stability of Co-P@NC-800, a galvanostatic method was used with the current density set at 10 mA cm−2. The measurements showed that Co-P@NC-800 possessed good electrocatalytic HER stability, performing for at least 12 h with little change in the overpotential (Figure 4c,d). Even though the HER performance of Co-P@NC-800 was not better than that of commercial 20% Pt/C, Co-P@NC-800 displayed excellent electrocatalytic HER behavior with little catalyst loading, superior to that of most previously reported transition-metal electrocatalysts (Table S1). Contrary to the HER tests, Co-P@NC-800 exhibited poor OER performance in 0.5 M H2SO4 solution (overpotential of 630 mV at a current density of 2 mA cm−2, Figure S7). Figure 5 shows the OER performance of Co@NC-800 and Co-P@NC-800 in 1 M KOH solution. Similarly, both Co@ NC-800 and Co-P@NC-800 exhibited good OER activities. Co-P@NC-800 had better OER performance than Co@NC800, achieving a current density of 10 mA cm−2 at an overpotential of 370 mV (c.f. 450 mV for Co@NC-800) (Figure 5a). The OER performance of Co-P@NC-800 in alkaline solution is better than that of many previously reported 40176



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both the unique structure and composition of the catalyst. First, the catalyst consists of cobalt phosphides, including CoP and Co2P, which are known as highly active water-splitting catalysts.53,54 Second, cobalt phosphides forms nanoparticles that are well dispersed in the N-doped carbon matrix as a result of carbonization of Co-porphyrin MOF PPF-3 and subsequent phosphorization. The resulted catalysts with large surface area and pores make the active CoPx nanoparticles easily accessible during the electrolysis. Last but not the least, the ultrathin 2D MOF nanosheets serve as templates during the preparation of the catalysts and are carbonized into graphitic carbon thin layers, which provides an efficient electron transport passway and facilitates the electrolysis of water. The unique structure of the porphyrinic ligand results in N-doped graphitic carbon, which could alter the electronic structure of carbon and thus benefit the electrocatalytic properties.35,55,56

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b10680. Additional PXRD, Raman, XPS, STEM, N2 sorption curves, X-ray photoelectron spectra and tables (PDF)

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REFERENCES

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4. CONCLUSIONS We used a simple method through the combination of carbonization and phosphorization of 2D cobalt porphyrinic MOF nanosheets to obtain a electrocatalytic Co-P@N-doped C material with good properties for HER and OER in acid and basic solution, respectively. The prepared catalyst Co-P@NC800 reached a current density of 10 mA cm−2 at low overpotentials of 98 and 370 mV for HER and OER with Tafel slopes of 74 and 79 mV dec−1, respectively. The HER performance is comparable to the best known non-noble transition-metal-based water-splitting catalysts. The excellent electrocatalytic properties of Co-P@NC-800 can be attributed to its unique structure, which consists of accessible, ultrafine CoP and Co2P nanoparticles well dispersed in a nitrogen-doped graphitic carbon matrix. This study demonstrates that it is a promising method to use 2D MOF nanosheets as templates to prepare multifarious non-noble metal−carbon composite materials for electrocatalysis. Because there are various 2D MOFs available so far that possess different organic and inorganic components and structures, there are great opportunities awaiting exploration and discovery of new catalysts with superior catalytic water-splitting properties by using this method.

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Research Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Hongbin Du: 0000-0002-5293-2323 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for the financial support from the National Natural Science Foundation of China (21471075 and 21673115). 40177


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