Ultrafine CoP Nanoparticles Supported on Carbon Nanotubes as

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Ultrafine CoP Nanoparticles Supported on Carbon Nanotubes as Highly Active Electrocatalyst for Both Oxygen and Hydrogen Evolution in Basic Media Chun-Chao Hou,† Shuang Cao,† Wen-Fu Fu,†,‡ and Yong Chen*,† †

Key Laboratory of Photochemical Conversion and Optoelectronic Materials and HKU-CAS Joint Laboratory on New Materials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China ‡ College of Chemistry and Chemical Engineering, Yunnan Normal University, Kunming 650092, People’s Republic of China S Supporting Information *

ABSTRACT: The development of low-cost and highly active electrocatalysts for two half reactions: H2 and O2 evolution reactions (HER and OER), is still a huge challenge to realize water splitting. Herein, we report that CoP nanoparticles (NPs) can act as a bifunctional catalyst for both HER and OER. Particularly, ultrafine CoP NPs decorated on Ndoped multiwalled carbon nanotube (MWCNT) exhibit remarkable catalytic performance for OER in 0.1 M NaOH aqueous solution, with a low onset overpotential of 290 mV, a Tafel slope of 50 mV dec−1, an overpotential (η) of 330 mV at 10 mA cm−2, and approximately 100% Faradaic efficiency, paralleling the performance of state-of-the-art Cobased OER catalysts including Co3O4, CoSe2, and Co−Pi. The hybrid catalyst is capable of maintaining a high catalytic current density for at least 10 h without any loss of catalytic activity. Meanwhile, the noblemetal-free catalyst also shows good activity and duarability for HER under the same basic condition. KEYWORDS: cobalt phosphide, electrocatalysis, hydrogen evolution, oxygen evolution, water splitting

1. INTRODUCTION Water splitting into H2 and O2 is considered as one of the most promising and attractive means to convert renewable energy resources, and soothe the human concerns about depletion of fossil fuels and increased environmental pollution.1−3 To this end, it is essential to develop highly efficient and competent water splitting catalysts because of the sluggish four-electron transfer kinetics for OER and two-electron for HER.4−7 Although precious metal Pt and metal oxides (i.e., RuO2 and IrO2) have been explored as excellent catalysts for HER and OER, respectively, the high cost considerably limits their largescale applications.8−11 Therefore, it is still demanding to seek novel, highly active, and inexpensive catalysts for both HER and OER. Recently, transition-metal-based sulfides,12−14 borides,15 carbides,15,16 and selenides17,18 have been discovered to be excellent HER catalysts in strongly acidic electrolyte, and on the other hand, Fe, Co, Ni, Cu, and Mn-based oxides or hydroxides have emerged as promising noble-metal-free materials for their catalytic power toward OER under neutral or strongly basic condition.19−23 Most interestingly, some noble-metal-free materials such as CoOx,24 Ni3N,25 CoSe,26 NiCo2S4,27 Ni2P, Ni5P4, etc.28,29 display high performance for both HER and OER, which are of vital importance for the development of “all in one” electrocatalytic water splitting systems.24−33 © XXXX American Chemical Society

Transition-metal phosphides (TMPs) which are intrinsically metallic have been intensively studied as catalysts for the hydrodesulfurization (HDS) reaction.34 Recently, Schaak’s, Sun’s, and our groups exploited their intriguing applications for electro- and photocatalytic hydrogen evolution from water.35−38 For instance, self-supported nanoporous cobalt phosphide (CoP) nanowires were found to be robust HER catalysts over the wide range of pH 0−14.39 During these pioneering works, it has been noticed that metal phosphides have good electrical conductivity with metallic behavior40 and that the metal on the surface could be readily oxidized to high valent oxo/hydroxo species.41,42 We thus expect that TMPs may function as a bifunctional catalyst for both hydrogen and oxygen evolution in the same electrolyze. Multiwalled carbon nanotube, a class of carbon-based nanomaterial with high electrical conductivity, high mechanical strength, and large surface area, has been widely used as support to improve the dispersion of electrocatalysts and thus to enhance their catalytic activities.43 Herein, we report on the ultrafine CoP NPs supported on CNT as highly active and robust bifunctional electrocatalyst for both oxygen and Received: September 29, 2015 Accepted: December 7, 2015

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DOI: 10.1021/acsami.5b09207 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 1. Characterization of the CoP−CNT nanohybrid. (a) SEM, (b and d) TEM, (c) distribution of the particle size, and (e and f) HR-TEM images. (g) Cs-corrected HAADF-STEM image, (h) bright field image, and (i) corresponding EDX elemental mapping of Co, P, C, and N of the CoP−CNT composite. the solution was stirred for at least 30 min. The solution was then diluted cautiously with water (about 50 mL). After the flask was cooled to room temperature, 140 mL of water and 10 mL of 30% H2O2 were added to end this reaction. After stirring for 5 min at room temperature, the precipitate was then repeatedly centrifuged and washed with 5% HCl solution twice, copious amounts of water, followed by drying under vacuum overnight. 2.3. Synthesis of Co3O4−CNT Nanohybrid. The Co3O4−CNT nanohybrid was prepared following the reported literature.45 First, the ox-CNT (90 mg) was dispersed uniformly in 120 mL of ethanol and 0.5 mL of water and sonicated for 30 min. Then, 3 mL of 0.6 M Co(Ac)2 aqueous solution was allowed to dissolve in the solution, followed by the addition of 2.5 mL of NH3·H2O at room temperature. After keeping at 80 °C with stirring for 20 h, the mixture was transferred to a Teflon-lined autoclave with a stainless steel shell at 150 °C for 3 h. Finally, the resulting product was collected by centrifugation, washed with ethanol and water and dried under vacuum for 24 h. The resulting Co3O4−CNT hybrid product was ∼223 mg. 2.4. Synthesis of CoP−CNT Nanohybrid. The CoP−CNT nanohybrid was synthesized using Co3O4−CNT as precursor according to our previous work and literatures.44,46 Typically, 20 mg of NaH2PO2 was placed at the upstream side of a quartz boat, and 100 mg of Co3O4−CNT was placed at the downstream side of the same quartz boat with a distance of 4 cm between them, followed by carefully placing at the center of the tube furnace. After being flushed with Ar, the tube furnace was elevated to 300 °C, with a heating rate of 2 °C min−1 and kept at this temperature for about 2 h under Ar gas flow (one bubble per second). After cooling to room temperature, the obtained CoP−CNT nanohybrid was washed repeatedly by 10% HCl solution and copious amounts of water, followed by drying under vacuum. 2.5. Characterizations and Analyses. Powder X-ray diffraction patterns (XRD) were carried out using a Bruker AXSD8 X-ray diffractometer with Cu Kα radiation (λ = 1.5406 Å). X-ray photoelectron spectroscopy (XPS) samples were deoxygenated under Ar for 1 h and investigated on a PHI 5300 ESCA system. An

hydrogen evolution reactions in identified basic media. The asprepared CoP−CNT nanohybrid as anode shows a low onset overpotential of 290 mV and a small Tafel slope of 50 mV dec−1, achieves a current density of 10 mA cm−2 at an overpotential of 330 mV and almost 100% Faradaic efficiency for OER in 0.1 M NaOH, which are favorably comparable and even superior to the performance of RuO2 and other Co-based OER catalysts. When used as a cathode, the CoP−CNT composite also displays good catalytic activity and duarability for HER under the same alkaline condition.

2. EXPERIMENTAL SECTION 2.1. Synthesis of CoP Nanoparticles. CoP nanoparticles were prepared according to literature methods.35−38,44 In a typical procedure, 200 mg of Co(NO3)2·6H2O and 50 mg of sodium citrate were added to 100 mL of water and stirred for 15 min; then, 3 mL of 1.0 M NaOH was added dropwise to form a Co(OH)2 precipitate. After that, the suspension was dried at 60 °C for 3 h and used for the next procedure. CoP nanoparticles were synthesized by a thermal reaction. 50 mg Co(OH)2 and 250 mg NaH2PO2 solid were mixed by grinding and heated in a quartz boat of the tube furnace at 300 °C for 1 h in a flowing 30 mL min−1 Ar. After cooling to room temperature, the obtained black solid was washed subsequently by water and ethanol three times and dried at vacuum oven. 2.2. Synthesis of Mildly Oxidized Carbon Nanotubes (oxCNT). The mildly oxidized CNTs (ox-CNT) were prepared by a reported method.45 Briefly, MWCNT was first purified by calcining it at 500 °C for 2 h. Then, the powder was washed several times with diluted hydrochloric acid (10%) and distilled water and dried under vacuum. The purified MWCNT (1.0 g) was put into a 250 mL roundbottom flask, followed by addition of 23 mL of concentrated H2SO4 and stirring at room temperature overnight. After that, NaNO3 (200 mg) was added to the flask, which was heated at 40 °C, and allowed to dissolve for about 5 min. Next, the reaction temperature was kept below 45 °C, KMnO4 (1.0 g) was slowly added to the suspension, and B


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Figure 2. (a) Polarization curves for HER on modified GC electrodes comprising Pt, CoP, CNT, CoP−CNT composite catalysts. Scan rate: 5 mV s−1. (b) Tafel plots of Pt, CoP, CoP−CNT composite catalysts. (c) Nyquist plots of electrochemical impedance spectra of the as-prepared CoP, CoP−CNT, Pt (inset) at η = 340 mV. (d) Polarization data for the CoP−CNT sample before and after 300 and 600 CV sweeps between −0.14 and −0.44 V vs RHE. (e) Time dependence of the current density for CoP−CNT at a static overpotential of 340 mV. (f) Theoretical and experimental hydrogen evolution from water by bulk electrolysis at η = 340 mV using CoP−CNT catalyst. All the measurements were performed in 0.1 M NaOH aqueous solution. The iR loss from the solution resistance was corrected. Al Kα X-ray source with a power of 250 W was used. The charge effect was calibrated using the binding energy of C(1s) (285.0 eV). Samples for transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), selected-area electron diffraction (SAED) and energy-dispersion X-ray (EDX) spectroscopy measurements were analyzed by using a transmission electron microscope (JEM 2100F) with an accelerating voltage of 200 kV. Scanning electron microscope (SEM) images and corresponding energy-dispersion X-ray spectroscopy measurements were conducted on a Hitachi S-4800 field emission scanning electron microscope. Double spherical aberration-corrected high-angle annular dark-field scanning TEM (HAADF-STEM) and their corresponding energy dispersive X-ray mapping were recorded on a JEM ARM200F atomic resolution analytical microscope with an operating voltage of 200 kV with high-resolution 0.078 nm. Bright field STEM images were recorded by using a collection semiangle of 11 mrad. Electron dispersive X-ray spectroscopy spectra were obtained using a probe size of 0.13 nm with the probe current 86 pA. For all TEM, HRTEM, SEM, SAED, Cs-corrected HAADF-STEM and corresponding EDX measurements, the samples were first dispersed in ethanol and sonicated for at least 30 min, then dropped on an ultrathin carbon film copper mesh and allowed to dry in air at room temperature prior to these measurements. Fourier-transformed (FTIR) infrared data were collected at ambient temperature on a Bruker ALPHA FTIR spectrometer from 4000 to 450 cm−1. The experimental backgrounds were collected using the OPUS software package. Raman spectroscopy was carried out on an inVia-Reflex confocal laser micro-Raman spectrometer using Ar+ laser excitation with a wavelength of 532 nm.

observed in the case of CoP−CNT. After phosphidation of Co3O4−CNT at 300 °C for 2 h, only the diffraction peaks at 31.7, 36.5, 46.4, 48.4, 52.1, and 56.6°, indexed to the (011), (111), (112), (211), (103), and (301) planes of orthorhombic CoP phase (JCPDS No. 29-0497), are perceived. It should be pointed out here that the relatively weak and broad diffraction peaks are associated with the quantum effect of CoP NPs on account of their ultrafine size of 1.5−2 nm. No other impurities such as metallic Co, Co3O4, and Co2P are detected in this sample, indicative of a successful chemical conversion of Co3O4 into CoP. Observations by scanning electron microscopy (SEM), transmission electron microscopy (TEM), high revolution TEM (HR-TEM) and STEM (Figure 1 and Figures S2−S5) reveal that the CoP NPs with size in about 1.5−2 nm are well distributed on the CNT (diameter 20−40 nm). The HR-TEM image taken for one single nanocrystal of CoP in this composite shows the lattice fringes with a d-spacing of 0.189 nm, attributable to the reflection of the (211) plane. The corresponding selected area electron diffraction (SAED) pattern shows discrete and sharp spots, which can be indexed to the (011), (111), and (112) planes of orthorhombic CoP nanocrystals (Figure S6). The energy-dispersive X-ray (EDX) analysis reveals that the CoP−CNT nanohybrid is mainly comprised of C, N, Co, and P elements with the molar ratio of Co/P close to 1:1 (Figure S7), in good agreement with the value of 50.43:49.57 detected by X-ray photoelectron spectroscopy (XPS) spectrum. Figure S8 illustrates the XPS spectrum of the CoP−CNT nanohybrid in the Co(2p) and P(2p) regions. Two apparent peaks located at 779.3 and 782.9 eV originate from the Co 2p3/2 species, and two additional peaks at 794.1 and 799.2 eV corespond to the Co 2p1/2 species. The high-resolution image of the P(2p) region also shows two broad peaks at ∼130.0 and 135.0 eV. The peaks at 779.3 eV (Co 2p3/2), 794.1 eV (Co 2p1/2) and 130.0 eV (P 2p) are very close to the binding energies (BE) of Co(2p) and P(2p) in CoP,47−49 and the peaks at 782.9 eV (Co 2p3/2), 799.2 eV (Co 2p1/2), and 135.0 eV (P 2p) are presumably assigned to the oxidized cobalt and

3. RESULTS AND DISCUSSION 3.1. Materials Characterization. The X-ray diffraction (XRD) patterns of CNT, Co3O4−CNT, and CoP−CNT are depicted in Figure S1. The CNT shows two major peaks located at about 26.2 and 42.9°, corresponding to the (002) and (101) reflections of hexagonal graphite, respectively. Five additional diffraction peaks for Co3O4−CNT at 2θ = 31, 37, 45, 59, and 65° can be observed, which correspond to the (220), (311), (400), (511), and (440) planes of Co3O4 (JCPDF No. 42-1467), respectively. The strong peak (002) for CNT was greatly suppressed in Co3O4−CNT due to the loading of Co3O4 with high density. Similar phenomenon was also C


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Figure 3. (a) Polarization curves for OER on bare GC electrode and modified GC electrodes comprising CNT, Co3O4, CoP, CoP−CNT composite and RuO2 catalysts. Scan rate: 10 mV s−1. (b) Tafel plots. (c) Comparison of Tafel slopes, onset potentials, and potentials required to reach j = 10 mA cm−2 for CoP−CNT composite, pure CoP NPs, RuO2, and Co3O4 catalysts. (d) OER polarization curves of CoP−CNT composite, CoP, and RuO2 catalysts before and after potential sweeps (0.6−0.8 V vs Ag/AgCl). (e) Nyquist plots of electrochemical impedance spectra of the as-prepared CoP, CNT, CoP−CNT, Co3O4, RuO2 at η = 430 mV. (f) Current density trace obtained by controlled-potential 1.8 V vs RHE electrolysis of ITO electrode (1 cm2). All the measurements were performed in 0.1 M NaOH aqueous solution. The iR loss from the solution resistance was corrected.

phosphorus species arising from superficial oxidation of CoP in the CoP−CNT sample upon exposure to air.42 The presence of oxygen in this sample was further confirmed by the XPS survey spectrum, which furnished indirect evidence of the oxidation of CoP. Structural information on the Co3O4−CNT and CoP−CNT samples were further investigated by FTIR and Raman. As presented in Figures S9 and S10, the IR spectrum of the Co3O4−CNT sample displays two distinct bands at 564 and 662 cm−1 that originate from the vibrational absorption modes of the cobalt−oxygen bonds.50−52 The signals at ∼669, 513, and 470 cm−1, assigned to the A1g, Eg, and F2g−active modes of Co3O4, are also discernible in the Raman spectrum.53,54 After phosphidation, all peaks of Raman and FTIR spectra indexed to the Co3O4 are not observed in the CoP−CNT nanohybrid, which further confirms the complete conversion of Co3O4− CNT into CoP−CNT composite. The Raman spectra of oxCNT, Co3O4−CNT, and CoP−CNT samples all present three peaks at around 1344, 1578, and 2687 cm−1, corresponding to D (arises from the sp3 defect sites), G (arises from sp2-bonded pairs), and a 2D carbon band, respectively.55 The similar ID/IG ≈ 1 ratio of these three samples reveals that the walls of CNT retain intact even after deposition of Co3O4 and CoP NPs.55,56 It is worth noting that the oxidization pretreatment for CNT is crucial to the successful generation of small CoP NPs.45 When untreated CNT is used as support to fabricate the CoP−CNT nanohybrid by following the same procedure, the resulting CoP NPs with relatively large size of 8−15 nm are randomly decorated on the surface of CNT (Figure S11), and shows poor OER catalytic activity (Figure S12). For comparison, other nanomaterials including unsupported CoP, Co3O4, RuO2 were also synthesized and characterized by XRD, TEM, HRTEM, SAED, EDX, and XPS (Figures S13−S16). 3.2. Hydrogen Evolution Reaction. The catalytic activities of the unsupported CoP NPs and CoP−CNT composite for HER in strongly alkaline electrolyte (0.1 M NaOH) were first examined using a typical three-electrode system. Figure 2a shows the polarization curves of CoP NPs and CoP−CNT together with that of commercial noble Pt for comparison. It is evident that CoP NPs can function as an active HER catalyst in alkaline solution with an onset

overpotential of about 220 mV. The value is dramatically reduced to about 80−90 mV when CoP−CNT is used as cathode, similar to the results reported by Sun et al.39 and lower than other non-noble metal HER catalysts in basic media. Under similar experimental conditions, Pt exhibits a negligible onset potential; CNT and the blank GC electrode show no obvious voltammetric response. The CoP−CNT composite displays a Tafel slope of 56 mV dec−1, comparable to that of Pt (45 mV dec−1) (Figure 2b). The Tafel slopes for both CoP and CoP−CNT are in the range of 40−120 mV dec−1, implying that the HER should proceed via a Volmer-Heyrovsky HER mechanism.57 The Faradaic yield for hydrogen evolution of CoP−CNT was assessed at a cathodic current density of about 9.5 mA cm−2 for 10000 s. The experimentally measured amount of hydrogen agreed closely with the theoretically calculated value based on Faraday’s law, indicating a close to 100% Faradaic efficiency (FE). Also, durability experiments were performed through the long-term CV sweeps (Figure 2d) and controlled potential electrolysis (CPE) method (Figure 2e). After 600 cyclic voltammetry (CV) cycles between −0.14 and −0.44 V, the polarization curve shows negligible difference in comparison with the initial one. This finding coupled with a 10000 s longterm electrochemical process manifests that the CoP−CNT composite is durable in such an alkaline system. 3.3. Oxygen Evolution Reaction. Next, we evaluated the OER activity of the CoP NPs and CoP−CNT composite in the same strongly alkaline electrolyte (Figure 3). As a reference point, the state-of-the-art RuO2, Co3O4, and CNT catalysts were also examined under similar conditions. The samples were deposited with the same loading of about 0.285 mg cm−2 on glassy carbon electrodes (Figure S17). The bare glassy carbon does not show any OER catalytic activity before 1.8 V vs RHE (reversible hydrogen electrode). The ohmic potential drop (iR) losses arising from the solution resistance were all corrected. It is exciting that the CoP−CNT nanohybrid exhibits excellent OER activity with an OER onset overpotential of 290 mV. This value is much smaller than those of pure CNT, CoP, and Co3O4 nanoparticles and only slightly larger than that of RuO2, suggesting that the CoP−CNT nanohybrid is a very active OER catalyst. Remarkably, the CoP−CNT composite can D


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ACS Applied Materials & Interfaces Table 1. Comparison of OER Activity Data for Different Catalysts catalyst CNT Co3O4 CoP CoP− CNT RuO2

onset potential (V vs RHE)

η@ J = 10 mA cm−2 (mV)

mass activity @ η = 370 mV [A g−1]

Tafel slope (mV dec−1)

TOF @ η = 370 mV (s−1)a

Cdl (mF cm−2)

Faradaic efficiency (%)

stability (h)b

1.60 1.64 1.58 1.52

470 570 400 330

3.53 0.89 14.7 78.3

97 85 80 50

0.0001c 0.0002 0.0034 0.0287

0.70 0.23 9.01

close to 100% close to 100%

> 10 h > 10 h

1.48

437

29.8

72

0.0101

9.10

a

The values of TOF were calculated by assuming that every metal atom are involved in the catalysis (see Experimental Section for the calculated method). bThe stability was tested on ITO electrode in 0.1 M NaOH solution. cBased on the amount of carbon.

afford a current density of 10 mA cm−2 at a small overpotential of ∼330 mV, superior to other catalysts. The OER kinetics of the above electrocatalysts was also probed by corresponding Tafel plots and then fitted to the Tafel equation (η = a + b logj, where j is the current density), yielding Tafel slopes of approximate 50, 72, 85, 80, 97 mV dec−1 for CoP−CNT, RuO2, Co3O4, CoP, and CNT, respectively. Note here that the CoP−CNT composite exhibits the smallest Tafel slope and is therefore the most efficient one among the investigated Co-based noble-metal-free OER catalysts. It is very interesting to point out that the catalytic performance of CoP−CNT nanohybrid is very close to that of the reported CoSe2 NPs supported on nitrogen-doped graphene nanohybrid under identical basic conditions, suggesting the outstanding intrinsic OER kinetics of this kind of Co-based material.58 To further assess the catalytic activity of the CoP NPs and CoP−CNT composite for OER, the mass activity and turnover frequency (TOF) at an overpotential of 370 mV were also provided (Table 1). The calculated mass activity for CoP− CNT nanohybrid is 78.3 A g−1, outperforming other catalysts, even RuO2. Noticeably, the constructed CoP−CNT material was found to exhibit the highest TOF of 0.0287 s−1 on the postulation that every metal atom is catalytically active. Besides the OER activity, the long-term durability is another critical parameter that determines the practicability and is of great significance for energy conversion and storage. To evaluate this, we also performed continuous potential cycling between 0.6 and 0.8 V vs Ag/AgCl for CoP NPs, CoP−CNT, and RuO2. As shown in Figure 3d, after 1000 and 2000 cycles, the CoP−CNT catalyst merely needs a 14 and 26 mV increase in η to reach the current density of 8 mA cm−2, and CoP NPs needs a 21 mV η increase after 1000 cycles, whereas RuO2 needs a 63 mV η increase after 2000 continuous potential cycles at an accelerated scanning rate of 100 mV s−1, demonstrating the excellent stability of the CoP NPs and CoP−CNT composite OER catalysts. The CPE experiments further reveal that they can maintain good catalytic activity for at least 10 h (Figures 3f and S18). Interestingly, the catalytic OER current density of CoP NPs is almost independent of the scan rate ranging from 30 to 150 mV s−1, indicating the highly efficient transport and favorable catalytic kinetics within electrodes (Figure S19).59 The generated oxygen in the headspace was confirmed by gas chromatography and measured quantitatively by an Ocean Optics optical probe to monitor the percentage of oxygen. The FEs of the CoP NPs and CoP−CNT composite during the electrocatalytic oxygen evolution process were calculated by comparing the theoretically calculated and experimentally determined amounts of oxygen. The two above values for

both CoP and CoP−CNT show a good agreement, suggesting a close to 100% FE, as shown in Figures S20 and S21. 3.4. Mechanism Aspect. To get insight into the OER mechanism, we carefully analyzed and compared the XPS spectra of unsupported CoP NPs before and after electrolysis. Note that the CoP NPs investigated here are in the form of agglomeration (Figure S16). Figure 4 shows high-resolution

Figure 4. High-resolution (a) Co(2p) and (b) P(2p) XPS spectra of the as-prepared CoP, post-OER CoP (after several CVs from 1.0 to 2.0 V vs RHE until a stable and reproducible LSV was obtained) and XPS 10 nm depth profile of the post-OER CoP sample.

Co(2p) and P(2p) XPS spectra of the as-prepared and postOER CoP NPs. The as-prepared sample demonstrates apparent Co(2p) peaks for both CoP at 778.5 (2p3/2) and 793.4 eV (2p1/2) and oxidized cobalt at 781.5 (2p3/2) and 797.2 eV (2p1/2), which is consistent with the observation in the case of CoP−CNT. In sharp contrast, only two broad peaks corresponding to the oxidized cobalt species were observed for the post-OER sample, and the characteristic signals for CoP almost completely disappeared with P element in the similar situation, implying the occurrence of oxidization during the OER process. Furthermore, XPS 10 nm depth profile collected for the post-OER sample shows two typical peaks for CoP in the Co(2p) region. These findings reveal that the oxidation process takes place merely on the surface of bulk CoP and the CoP core remains intact after OER. However, powder XRD analysis on the post-ORE sample failed to detect any new peaks for oxidized cobalt species, probably due to its poor crystallinity (Figure S22). Similar surficial oxidation phenomena were also observed in the Ni5P4 and Ni2P OER systems.28,29 Further evidence for superficial oxidation were furnished by the following experimental observations. First, the post-HER CoP NPs, in which the oxidized species are presumably reduced, shows similar OER catalytic performance with the asprepared CoP NPs. This finding precludes the feasibility that only the oxidized Co−P species on the surface of the asprepared sample contribute to the OER. Therefore, the surficial oxidation of CoP should occur during electrolysis and could be responsible for OER. Second, the post-OER CoP NPs basically lose the catalytic activity for HER (Figure S23), due to the E


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catalytic reaction. To confirm this, the electrochemical doublelayer capacitance (Cdl) was measured to determine the electrochemical surface area (ECSA) of electrocatalysts because the Cdl is proportional to the active surface area (Table 1).65−68 For OER, the Cdl of post-OER CoP−CNT (9.0 mF cm−2) exhibited a 39.1 and 12.8-times increase than those of postOER CoP (0.23 mF cm−2) and Co3O4 (0.7 mF cm−2), respectively, and is comparable to that of RuO2 (9.1 mF cm−2) under the present condition (Figure S28). The results demonstrated that ultrafine CoP directly grown on CNT has higher catalytically active surface area with respect to bulk CoP and Co3O4 nanoparticles. For HER, the Cdl of the as-prepared CoP and CoP−CNT is 0.43 and 1.42 mF cm−2, respectively. Second, CNT is an excellent conductive material, which will favor the fast charge transfer in this composite. This has been confirmed by electrochemical impedance spectroscopy (EIS) measurements. For both HER and OER (Figures 2c and 3e), the CoP−CNT composite has a much lower impedance and thus a rapider charge-transfer kinetics relative to CoP alone. Additionally, for the CoP−CNT composite, in situ oxidation of the whole CoP NPs other than the superficial CoP into active Co−P species is favorable for sufficient usage of CoP for OER.

complete conversion of surficial CoP into oxidized Co−P species, which cover on the surface of CoP and block the HER activity sites. Although it is not quite clear about the exact components of the oxidized species generated during OER at this stage, the existence of Co3O4 can be completely ruled out because it has been verified to be active toward HER.24 The molar ratio of Co to P elements for the oxidized species is determined by XPS to be about 4:1, which exceeds the value for Co3(PO4)2·xH2O (3:2). In view of the fact that Co(2p) and P(2p) binding energies of cobalt hydroxides and oxides are in the same range as those of cobalt phosphate, we tentatively describe the oxidized species as a combination of cobalt(II) phosphate with a cobalt oxo/hydroxo species CoxOy(OH)z, probably in the Co(II) state. Very recently, Sun and co-workers reported that metallic Co NPs can serve as a promising noble-metal-free catalyst for efficient OER in alkaline media.60 It was found that the surficial metallic Co atoms were oxidized during electrochemical oxidation process, as indicated by the anodic peaks observed at ∼1.10 V for oxidation of CoII to CoIII. In our case, the peak observed at ∼1.20 V for CoP NPs and that at ∼1.13 V for CoP−CNT are probably responsible for such an oxidation process (Figure 3a). The Co oxidation potential was also found to be pH dependent under alkaline condition (Figures S24 and S25), implying the involvement of a deprotonated intermediate. Taken together, we propose the following mechanism for OER using CoP NPs as catalyst. During the catalytic process, the surface of CoP is readily oxidized to high-valence Co-based species. Further polarization would induce the formation of surface Co(IV) sites, as key intermediates in the pathway for O2 evolution.61−63 We note that a similar mechanism was also proposed for metallic Ni3N nanosheets as an efficient OER electrocatalyst by Xie et al.64 The unique core−shell structure in situ generated during oxidation process possesses undisputable advantages for OER. On the one hand, the oxidized species on the shallow surface can act as active site for catalytic reactions and, on the other hand, the metallic CoP with high electrical conductivity as core facilitates charge transfer and thus markedly accelerates the OER kinetics. On the basis of the observations and discussion above for CoP NPs in the bulk state, we reasonably infer that CoP NPs supported on CNT are entirely converted into oxidized species during OER, due to their ultrafine size (1.5−2 nm) and homogeneous distribution on the support. Indeed, the XPS spectra in the Co(2p) and P(2p) regions for the post-OER CoP−CNT sample only demonstrated the peaks of Co(2p) at 783.4 eV and P(2p) at 135.0 eV, assigned to the oxidized Co−P species, whereas the Co(2p) of CoP at 779.3 eV and P(2p) at 130.0 eV disappeared (Figure S26). In consideration of the fact that the size of CoP NPs loaded on CNT is smaller than the detection range of XPS, we did not perform XPS depth analysis (>5 nm) on the CoP−CNT sample. The double spherical aberration-corrected STEM image and elemental mapping analysis were used to directly detect the change of element of CoP−CNT before and after OER. As shown in Figure S27, the existence of oxygen element was unambiguously observed for the post-OER CoP, while it is hardly discernible for the asprepared sample. The improved catalytic performance of CoP−CNT composite over CoP NPs for both HER and OER in the same electrolyte may receive contributions from two aspects as follows. First, the CNT-supported CoP NPs with smaller size and higher surface area can provide more active sites for

4. CONCLUSION In conclusion, we have demonstrated that CoP NPs and CoP− CNT hybrid can act as bifunctional catalysts for OER and HER (Figure S30). The as-prepared CoP−CNT composite exhibits remarkable OER catalytic performance, with a low onset overpotential of 290 mV, a Tafel slope of 50 mV dec−1, a η of 330 mV at 10 mA cm−2 and almost 100% Faradaic efficiency. They also reveal good HER activity and durability in highly corrosive alkaline solution. The excellent activity at relatively low overpotential and the simple synthesis of CoP provide obvious advantages over the mostly reported Co-based systems (Tables S1−S2). Most importantly, the present study will stimulate further research efforts about other metal phosphides such as FeP, Cu3P, and so on as bifunctional catalysts for hydrogen and oxygen evolution reactions. It could be anticipated that metal phosphides will open new opportunities for overall water splitting in the future.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b09207. Experimental procedures, synthesis of RuO2 and Co3O4, details on electrochemical measurements, preparation of working electrode and calculation methods, XRD, TEM, HR-TEM, XPS, EDX spectra for the as-prepared materials, etc. (PDF)

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AUTHOR INFORMATION

Corresponding Author

* E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Key Basic Research Program of China (973 Program 2013CB834800 and 2013CB632403) and by the Ministry of Science and Technology of China (2012DFH40090). We thank the Natural F


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Science Foundation of China (21371175), Beijing Natural Science Foundation (2142033), and CAS-Croucher Funding Scheme for Joint Laboratories. Y.C. acknowledges the support of the Chinese Academy of Sciences (100 Talents Program and KGZD-EW-T05).

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