Nest-like NiCoP for Highly Efficient Overall Water Splitting - ACS

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Nest-like NiCoP for Highly Efficient Overall Water Splitting Cheng Du, Lan Yang, Fulin Yang, Gongzhen Cheng, and Wei Luo ACS Catal., Just Accepted Manuscript • Publication Date (Web): 10 May 2017 Downloaded from http://pubs.acs.org on May 10, 2017

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Nest-like NiCoP for Highly Efficient Overall Water Splitting Cheng Du,a Lan Yang,a Fulin Yang,a Gongzhen Chenga and Wei Luoa,b*

a

College of Chemistry and Molecular Sciences, Wuhan University, Wuhan, Hubei

430072, P. R. China, Tel.: +86-27-68752366 *Corresponding author. E-mail addresses: [email protected]. b

Key laboratory of Advanced Energy Materials Chemistry (Ministry of Education),

Nankai University, Tianjin 300071, P. R. China Abstract The investigation of high-efficiency non-precious electrocatalysts for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is of great significance for renewable energy technologies. Here, we provide successive hydrothermal, oxidation and phosphidation method to fabricate 3D nest-like ternary NiCoP/CC electrocatalyst with superior catalytic activity and stability toward HER/OER. Nest-like NiCoP/CC requires overpotentials of 44 and 62 mV to reach the current density of 10 mA cm-2 in acidic and alkaline media toward HER, respectively. For OER, the NiCoP/CC exhibits high active and durable performance with an overpotential of 242 mV at current density of 10 mA cm-2 in alkaline solutions. Furthermore, the practical application of NiCoP/CC as a bifunctional catalyst for overall water splitting reaction yields current densities of 10 and 100 mA cm-2 at 1.52 and 1.77 V, respectively. Keywords: TMP, nest-like, NiCoP, HER, OER, water splitting 1

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1. Introduction One of the challenges of the 21st century is to develop and sustain a carbon-neutral economy in place of the currently prevalent petroleum economy.1,2 Hydrogen, has been considered as one of the most potential fossil fuel alternatives due to its high energy density and environmental benignity.3 The electrolysis of water to generate hydrogen has been considered an attractive and carbon-free strategy.4 However, both the hydrogen evolution reaction (HER) at the cathode and oxygen evolution reaction (OER) at the anode need satisfied catalysts to obtain high catalytic performance and superior stability.5,6 To date, Pt and RuO2 or IrO2 are considered as the most efficient catalysts toward HER and OER, however, scarcity and high cost hinder their widespread applications.7,8 To this end, a great deal of efforts have been made to design and fabricate efficient non-precious metal based catalysts for HER and OER, especially the bifunctional overall water splitting.9 Accordingly, earth-abundant 3d transition metals (Co, Ni, Fe, Mn, V, etc.) alloys,10-14 and their chalcogenides,15-17 carbides,18-20 nitrides,21,22 and phosphides23-26 have been widely explored to replace the noble metal based catalysts. Among them, transition metal phosphides (TMPs) with unique charged natures (positive and negative charges in metal and phosphorus, respectively), have attracted special attention due to the high catalytic activity derived from their hydrogenase-like catalytic mechanism.27 In addition, it is known that the performance of the TMPs could be further increased by additional metal doping.28 Thus the bimetallic phosphides usually exhibit superior activity to the monometallic phosphides, probably due to the synergistic effect.29 2


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It is known that the performance of material strongly depends on its size, shape, dimensionality and morphology.30 Owing to large surface area, highly active sites exposure, as well as the fast rate of mass transfer, 3D nanocatalysts have attracted considerable attention.31 Electrocatalysts with the morphologies of 0D nanoparticle,32 1D nanowire/nanorods33 have received more attention in either HER or OER. Moreover, these electrocatalysts are usually suffered from low-speed electron transmission and slow diffusion of electrolyte and evolved gas bubbling. Adding insult to injury, they are easy to agglomerate, resulting in poor long-term stability.34 Therefore, developing 3D TMPs toward overall water splitting with higher activity and superior stability is highly desirable. Herein, we reported a controllable fabrication of 3D nest-like NiCoP architecture directly grown on carbon cloth (CC). Taking advantage of the unique 3D structure, the as-synthesized NiCoP/CC exhibited excellent electrocatalytic performance toward HER with exceptional overpotentials of 44 mV and 62 mV at a current density of 10 mA cm-2, small Tafel slopes of 39 and 66.5 mV dec-1 in acidic and alkaline solution, respectively. In addition, NiCoP/CC showed high active and durable OER performance with an overpotential of 242 mV at 10 mA cm-2 in alkaline solutions. Furthermore, when used as both anode and cathode for water splitting in basic solution, the current densities of 10 and 100 mA cm-2 were achieved at only cell voltages of 1.52 and 1.77 V, respectively. 2. Material and methods 2.1 Chemicals and materials 3


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Cobalt (II) nitrate hexahydrate, nickel (II) nitrate hexahydrate, urea and ammonium fluoride were purchased from Sinopharm Chemical Reagent Co. Ltd. (China). Carbon cloth was purchased from CeTech Co. Ltd. (China). NaH2PO2 was purchased from Aladdin. 5 wt. % Nafion solution was purchased from Sigma-Aldrich. Pt/C (Johnson Matthey Hispec 3600) was purchased from Shanghai Hesen Electric Co. Ltd. (China). 2.2 Preparation of nest-like NiCo2O4/CC 1.5 mmol Cobalt (II) nitrate hexahydrate, 1.5 mmol nickel (II) nitrate hexahydrate, 15 mmol urea and 7.5 mmol NH4F were dissolved in 15 mL deionized water by sonication. A piece of carbon cloth (CC) (1 cm × 2 cm) was first washed with acetone, deionized water, and alcohol by ultrasonication for about 30 min, respectively. Then the aqueous solution and CC were transferred into a Teflon-lined stainless autoclave (20 mL volume) and maintained at 120 ºC for 6 h. After washed with deionized water and dried at 80 ºC under vacuum, followed by calcined at 450 ºC for 2 h, the final nest-like NiCo2O4/CC product was obtained. 2.3 Preparation of nest-like NiCoP/CC NiCo2O4/CC and 2 g of NaH2PO2 were placed separately in a porcelain boat before heated into a tube furnace at 300 ºC for 2 h under N2 atmosphere. The heating speed was kept as 2 ºC min-1. After washed by deionized water, the as-prepared NiCo2O4/CC was immersed into 0.5 M H2SO4 for a while, followed by washed by deionized water and ethanol and dried at 80 ºC under vacuum. For comparison, CoP/CC, Ni2P/CC and NiCoP/CC nanoarray were prepared in a similar way without adding nickel (II) nitrate hexahydrate, Cobalt (II) nitrate hexahydrate, or urea, 4


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respectively. Ni33Co67P/CC and Ni67Co33P/CC were fabricated by the same way using different initial molar rations of Cobalt (II) nitrate hexahydrate to Cobalt (II) nitrate hexahydrate as 1:2 and 2:1, respectively. 2.4 Physical characterizations TEM was carried on FEI Tecnai G20 TEM instrument operating at 200 kV. SEM was studied on Zeiss Sigma scanning electron microscope. XRD were measured on Bruker D8 Advance X-ray diffractometer using CuKα radiation with a velocity of 10º min-1. XPS was carried on Thermo Fischer ESCALAB 250Xi spectrophotometer. The N2 adsorption-desorption experiment was measured on Quantachrome NOVA 4200e. 2.5 Electrochemical measurements The electrocatalytic activities of all samples was measured on CHI 760E electrochemical analyzer in a three-electrode at room temperature. The electrolyte solution was 0.5 M H2SO4 or 1.0 M KOH, with NiCoP/CC as working electrode. The loading amount of NiCoP on CC was ~2 mg cm-2, with Pt foil as counter electrode and saturated calomel electrode (SCE) as the reference electrode. The mass loading for Ni2P/CC and CoP/CC was also controlled to be about 2 mg cm-2. Bulk NiCoP and commercially available Pt/C catalysts electrodes were prepared for comparison. The NiCoP and Pt/C were respectively dispersed in isopropanol solvent (0.1% Nafion), the catalytic ink (5 mg mL-1) was deposited onto the glassy carbon rotating disk electrode (ϕ = 5 mm) with an overall catalyst loading of 2 and 0.15 mg cm-2, respectively. The actual value of the potential vs. reversible hydrogen electrode (RHE) of the SCE was calibrated by using the 20 wt% Pt/C deposited onto the glassy carbon as working 5


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electrode to obtain the hydrogen electrode reactions’ polarization curve at 5 mV s−1 under H2 atmosphere. Before electrochemical test, working electrode was activated at 500 mV s-1 for several cycles. Linear sweep voltammetry (LSV) was carried out at 5 mV/s.

Linear

sweep

voltammetry

(LSV),

and

amperometric

i-t

and

chronopotentiometric v-t curve were carried out to explore the electrocatalytic performance of working electrode. Furthermore, chronopotentiometry tests were implemented at 100 mA cm-2. The hydrogen and oxygen generated from cathode and anode could be measured by a water-gas displacing method in one hour of durability test. The theoretical O2 yields calculated as follows: n(O2)=Q/ n*F, where n(O2) is the number of moles of oxygen produced, Q is the charge passed through the electrodes, F is the Faradaic constant (96485 C mol−1), and n is the number of electrons transferred during water splitting (four moles of electrons per mole of O2); the theoretical H2 yields calculated as the same as O2 except four moles of electrons per 2 mole of H2. 3. Results and Discussion The nest-like NiCoP/CC was prepared by hydrothermal process, oxidized procedure, followed by a solid-state phosphorization using NaH2PO2 as P source, as shown in Scheme S1. The scanning electron microscopy (SEM) in Figure S1 shows the 3D nest-like NiCo2O4 catalysts synthesized by the hydrothermal method with an average size of 15 µm are uniformly grown on carbon cloth. After the phosphorization, the samples retained its morphology well, as shown from SEM images in Figures 1a, 1b.

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Figure 1. (a)-(d) Morphology of nest-like NiCoP/CC. (a)-(b) SEM images of nest-like NiCoP/CC, (c)-(d) TEM image and high-resolution TEM image of nest-like NiCoP/CC.

Furthermore, EDX elemental mapping images of NiCoP/CC (Figure S2) indicate Co, Ni and P elements are uniformly distributed throughout the whole nest. The XRD patterns (Figure S3) indicate the complete conversion of NiCo2O4 (JCPDS card No. 73-1702) to hexagonal NiCoP (JCPDS card No.71-2336). As determined by N2 sorption measurement, the nest-like NiCoP/CC shows a Brunauer-Emmett-Teller (BET) surface area of 30.3 m2 g−1 (Figure S4a). The SEM and transmission electron microscopy (TEM) images show nest-like NiCoP is constructed by NiCoP nanorods. A high-resolution TEM image confirms the NiCoP has an interplanar spacing of 0.220 nm, corresponding to the (111) plane of the hexagonal NiCoP (Figure 1d). The EDX 7


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spectrum (Figure S5) further implies the coexistence of Co, Ni and P elements, with the molar ratio of Co, Ni and P atomic ratio ~ 1:1:1. To further investigate the detail formation mechanism, effect of various experimental parameters on the unique morphology were studied. We found that the addition of urea, CC, and co-existence of Ni, Co precursors are the key factors for this unique 3D nest-like morphology. As shown in Figure 2a and Figure 2e, NiCoP nanoarray grown on CC were observed without the addition of urea during the synthesis. Without CC, only aggregated NiCoP microspheres were obtained (Figure 2b and Figure 2f). For comparison, binary CoP and Ni2P were prepared by the same method. However, CoP nanoarray and Ni2P nano-flower were observed, respectively (Figures 2c, 2g, 2d, and 2h). The Ni2P/CC and CoP/CC show BET surface area of 9.7 and 6.0 m2 g−1, respectively (Figures S4b, S4c). Further experiments indicates that high-quality 3D nest-like NiCoP architecture could only be obtained by optimizing the specific amount of urea. For example, irregular block NiCoP/CC with smooth surface was obtained when 5 mmol urea was used (Figures S6a, S6d), and hexagonal NiCoP/CC nanosheet with small amount of nanowire growing at the edge of the NiCoP/CC nanosheet was observed when 10 mmol urea was used (Figure S6b). Further increasing the amount of urea (20 mmol) yielded the formation of carambola-like NiCoP/CC (Figures S6c, S6f). The effect of the molar ratio of Ni to Co precursor on the morphology of the NiCoP/CC was also studied. Figure S7 indicates Ni0.33Co0.67P/CC nanoarray and Ni0.67Co0.33P/CC nano-flower are obtained, respectively. From Figure S7 and Figure 2c-d, 2g-h, it can be seen that the more Co 8


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inclined to form nanoarray morphology, while the more Ni inclined to form nano-flower morphology. The XRD patterns of Ni0.33Co0.67P/CC nanoarray and Ni0.67Co0.33P/CC nano-flower were quite similar to that of NiCoP/CC, as shown in Figure S8.

Figure 2. SEM images for (a, e) NiCoP/CC (nanoarray), (b, f) bulk NiCoP, (c, g) CoP /CC, (d, h) Ni2P /CC.

XPS was performed to further analyze the surface structure of the as-prepared nest-like NiCoP/CC. As shown by the XPS survey spectra in Figure S9a, Ni, Co, P, C and O are coexist in the as-prepared nest-like NiCoP/CC. Figure S9b-d displays the high resolution spectra of the Co 2p, Ni 2p and P 2p core levels. In Co 2p spectra, peaks at 778.4 and 781.2 eV are attributed to Co 2p3/2, while the peak at 785.8 eV is belonged to the satellite peak of Co 2p3/2. The Co 2p1/2 region also exhibits two main peaks at 793.2 eV and 797.8 eV and one satellite peak at 802.8 eV. For Co 2p3/2, the peak located at 778.4 eV is assigned to Co-P. The peak at 781.2 eV could be ascribed to a Co oxidized state.35 The peaks at 853.1, 856.4 and 860.5 eV are attributed to Niδ+ 9


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in NiCoP, oxidized Ni species and the satellite of Ni 2p3/2 peak, respectively, as shown in Figure S9c. The other peaks at 870.3, 874.3 and 879.8 eV are corresponded to Ni 2p1/2 energy level, which are Niδ+ in NiCoP, oxidized Ni species and the satellite of the Ni 2p1/2 peak, respectively.36 Figure S9d shows P 2p spectra, the doublet situated at 129.0 and 129.8 eV correspond to P 2p3/2 and P 2p1/2 from NiCoP, respectively, and the peak at 133.5 eV could be assigned to PO43- or P2O5.37 These results confirm the presence of NiCoP and some oxidized NiCoP. To further reveal the oxidized Co and oxidized Ni, the O1s binding energy for the NiCoP/CC was shown in Figure S9e. The peak at 531.4 eV suggests the presence of either MOx or M(OH)x (M = Co, Ni).35 Additionally, the peak at 533.4 eV is attributed to oxidized phosphate species. The peaks assigned to oxidized Co, Ni and P species might be caused by the surface oxidation of NiCoP due to contact with air.36 Figure S9f shows the XPS of the C 1s, the peaks at 284.4, 286.1, 286.8 and 289.1 eV stand for sp2C, -C-O, -COO and -C=O, respectively.38 Electrocatalytic activity of the nest-like NiCoP/CC catalysts toward HER were evaluated under strongly acidic (0.5 M H2SO4) and alkaline (1.0 M KOH) conditions

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Figure 3. Hydrogen evolution reaction electrocatalysis in 0.5 M H2SO4 (a-c) and 1.0 M KOH solution (d-f). (a), (d) LSV curves of HER and (b), (e) Tafel plots of nest-like NiCoP/CC, along with Pt/C, NiCoP/CC nanarray, Ni33Co67P/CC, Ni67Co33P/CC, Ni2P/CC, CoP/CC, NiCoP and CC for comparison; (c), (f)Polarization curves of nest-like NiCoP/CC initially and after 5000 CV scans (c) or 2000 CV (f) scans; (c), (f) Inset: Time-dependent catalytic current density curve for NiCoP/CC in 0.5 M H2SO4 at η = 50 mV (c) and in 1.0 M KOH at η = 100 mV (f).

using a typical three-electrode setup at 5 mV s-1. Figure 3 shows the LSV curves of NiCoP/CC catalyst for HER in 0.5 M H2SO4 and 1.0 M KOH with current density normalized by geometric surface area (GSA) (1×1 cm2) of these electrodes. For comparison, NiCoP/CC nanoarray, Ni33Co67P/CC, Ni67Co33P/CC, bare CC, bulk NiCoP, Ni2P/CC, CoP/CC, and commercial Pt/C (20 wt%) were also studied. Under acidic condition in 0.5 M H2SO4, Pt/C exhibits excellent HER activity with the onset overpotential close to zero as expected, while bare CC exhibits almost no HER 11


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activity. As limited by the intrinsic activity, binary CoP/CC exhibits inferior HER activity with an overpotential of 118 mV to afford 10 mA cm−2. Ni2P/CC electrodes performed better than CoP/CC (ƞ10 = 72 mV and ƞ100 = 268 mV). As expected, NiCoP/CC nanoarray exhibits Pt-like HER activity with overpotential as low as 48 mV and 137 mV at 10 mA cm-2 and 100 mA cm-2. However, bulk NiCoP requires 381 mV to reach a current density of 10 mA cm-2. Remarkably, the activity of nest-like NiCoP/CC is much better than NiCoP/CC nanoarray, while the overpotential at 10 mA cm-2 and 100 mA cm-2 is 44 mV and 101 mV, respectively. Moreover, it is interesting to note that the HER performance of NiCoP/CC was also depends on Ni/Co ratios, while the overpotential at 10 mA cm-2 for Ni33Co67P/CC and Ni67Co33P/CC were 67 and 61 mV. Figure 3b shows the corresponding Tafel slopes. 3D nest-like NiCoP/CC shows a Tafel slop of 38.5 mV dec-1, close to that of Pt/C (30.4 mV dec-1), much smaller than those for NiCoP/CC nanoarray (46.5 mV dec-1), Ni2P/CC (54.1 mV dec-1), Ni33Co67P/CC (58.5 mV dec-1), Ni67Co33P/CC (58.4 mV dec-1), CoP/CC (66.7 mV dec-1), and bulk NiCoP (135.6 mV dec-1). The lowest overpotential and smallest Tafel slope indicate the superior HER activity of the 3D nest-like NiCoP/CC, which is among the highest reported non-precious metal HER catalysts as shown in Table S1. Moreover, after 1000, or even 5000 cycles of the CV measurement, only a slight increase of the overpotential was observed compared with the initial CV of the nest-like NiCoP/CC (Figure 3c). Meanwhile, amperometry was performed at the overpotential of 50 mV. As shown in Figure 3c, the current density of NiCoP/CC was almost retained during the continuous hydrogen-release for 24 12


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hours. These results confirmed that the unique 3D nest-like structure of NiCoP provided a faster charge transfer rate and facile release of the evolved H2 to improve the HER activity, as well as excellent stability. Figure 3d shows the electrocatalytic HER activity for the different TMP electrodes in alkaline media. Under alkaline conditions, the overpotentials of nest-like NiCoP/CC at 10 and 100 mA cm-2 are 62 and 158 mV, respectively, much lower than those of the NiCoP/CC nanoarray (ƞ10 = 84 mV and ƞ100 = 190 mV), CoP/CC (ƞ10 = 290 mV), Ni2P/CC electrodes (ƞ10 = 155 mV and ƞ100 = 248 mV) Ni33Co67P/CC (ƞ10 = 120 mV and ƞ100 = 238.5 mV), Ni67Co33P/CC (ƞ10 = 100.3 mV and ƞ100 = 216.5 mV) and bulk NiCoP (ƞ10 = 320 mV). Moreover, the Tafel slope of nest-like NiCoP/CC is measured to be 68.2 mV dec-1, which is smaller than those of NiCoP/CC nanoarray, Ni2P/CC, CoP/CC, Ni33Co67P/CC, Ni67Co33P/CC and bulk NiCoP, with 88.3, 93.5, 115.9, 99.6, 102.3 and 108.6 mV dec-1, respectively (Figure 3e). Even after 2000 cycles of the CV measurement, only 2 mV decrease compared with the initial CV of the nest-like NiCoP/CC is observed at 10 mA cm-2 (Figure 3f). It is worth noting that the electrocatalytic performance of 3D nest-like NiCoP/CC is better than most of other TMPs based catalysts in alkaline condition as shown in Table S2. Their OER performance were also evaluated in 1.0 M KOH (Figure 4). 3D nest-like NiCoP/CC exhibits an OER activity with low overpotentials of 242 and 330 mV to obtain the current density of 10 and 100 mA cm-2, which are lower than those of NiCoP/CC nanoarray (ƞ10 = 246 mV and ƞ100 = 352 mV), Ni2P/CC (ƞ10 = 318 mV and ƞ100 = 418 mV), CoP/CC (ƞ10 = 302 mV and ƞ100 = 450 mV), Ni33Co67P/CC (ƞ10 = 13


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281 mV and ƞ100 = 384 mV), Ni67Co33P/CC (ƞ10 = 254 mV and ƞ100 = 366 mV), bulk NiCoP (ƞ10 = 460 mV), and most of the reported values in the literatures (Table S2). The Tafel slope for nest-like NiCoP/CC is measured to be 64.2 mV dec-1, which is smaller than those of NiCoP/CC nanoarray (76.5 mV dec-1), Ni2P/CC (83.2 mV dec-1), CoP/CC (111.2 mV dec-1), NiCoP (109.8 mV dec-1), Ni33Co67P/CC (78.2 mV dec-1), Ni67Co33P/CC (75.3 mV dec-1) and CC (261.5 mV dec-1), indicating the HER proceed Volmer-Heyrovsky mechanism.24 We further test the stability of the 3D nest-like NiCoP/CC by a chronoamperometric method. The NiCoP catalyst exhibits superior stability toward OER at ≈100 mA cm−2 for 40000 s as shown in Figure 4c. The nest-like morphology of NiCoP/CC is maintained well as indicated in Figure 4d, further demonstrates the high stability of the as-synthesized NiCoP/CC. In addition, to facilitate the comparison of the specific activity of the nest-like NiCoP/CC, Ni2P/CC

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Figure 4. (a) Oxygen evolution reaction LSV curves and (b) Tafel plots of nest-like NiCoP/CC, NiCoP/CC nanoarray, Ni2P/CC, CoP/CC, NiCoP and CC in 1.0 M KOH. (c) v-t curve of NiCoP/CC at 100 mA cm-2. (d) SEM of NiCoP/CC after stability test.

and CoP/CC for both HER and OER in 1 M KOH, geometric activity and specific activity per BET surface area are shown in Figure S10. Noted that the nest-like NiCoP/CC operated with highest specific current densities per geometric surface area and BET surface area for HER and OER, further suggesting the superior catalytic activities of the nest-like NiCoP/CC. In addition, we reduced the mass loading of the nest-like NiCoP/CC and NiCoP/CC nanoarray to 1.3 mg cm-2 to make a much more clear comparison for their catalytic activities toward HER and OER. As shown in Figure S11, 3D nest-like NiCoP/CC shows a higher activity for HER in acidic 15


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solution with low overpotentials of 64 and 182 mV to obtain10 and 100 mA cm-2, respectively, which are lower than those of NiCoP/CC nanoarray (ƞ10 = 83 mV and ƞ100 = 216 mV). In addition, 3D nest-like NiCoP/CC also exhibits excellent performance for OER with the overpotential of 254 and 367 mV at 10 and 100 mA cm-2, respectively. However, NiCoP/CC nanoarray requires 270 and 380 mV to reach the current density of 10 and 100 mA cm-2. These results indicate the intrinsic superior HER and OER activity of 3D nest-like NiCoP/CC. To further investigate the superior OER activity, the NiCoP catalyst after OER is characterized by XPS (Figure S12). Figure S12a shows two main peaks around 795.4 and 802.6 eV for the Co 2p1/2 energy level, corresponding to oxidized Co species and the satellite of Co 2p1/2, respectively. The peaks at 781.3, 779.3 and 785.5 eV for the Co 2p3/2, are assigned to oxidized Co and satellite Co 2p3/2, respectively. The Ni 2p spectrum (Figure S12b) displays two peaks at 856.2 and 874 eV, which are assigned to the 2p3/2 and 2p1/2 peaks of oxidized Ni. The total amount of P signal is significantly lower and only the broad the phosphorus oxidation component at 133.5 eV is observed (Figure S12c). These results indicate the oxidation of the surface of NiCoP. Furthermore, the O 1s is deconvoluted to three components at 533.2, 531.9 and 531.1 eV, which attributed to phosphorus-oxygen, carbon-oxygen and metal-oxygen, respectively, as shown in Figure S12d. Therefore, the catalytic activity of the as-synthesized 3D nest-like electrocatalysts could be originated from the formation of Ni-Co oxo/hydroxide on the surface of catalysts during the OER process under a strong alkaline media, consistent to the previous reports. 39, 40 16


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The superior electrocatalytic activity of nest-like NiCoP/CC over CoP/CC and Ni2P/CC was further examined by electrochemical double layer capacitance (Cdl) and electrochemical impedance spectroscopy (EIS) analysis. NiCoP/CC exhibits the lowest Rct value (Figure S13), implying the smallest electron and charge transfer resistance, and thus fast electrode kinetics.41 To gain insights into the intrinsic activity of the catalysts, we measured the double-layer capacitances (Cdl), which are associated to electrocatalytic active surface areas (ECSAs).42 3D nest-like NiCoP/CC shows the highest Cdl in acidic and basic media (Figure S14, S15), together with the BET results, indicating the largest catalytically relevant surface area. These results indicate that the enhanced electrocatalytic performance on NiCoP/CC could also arise from the high ECSA. Encouraged by the high activity of 3D nest-like for both OER and HER, we built an alkaline electrolyzer using two identical NiCoP/CC as both cathode and anode. The activity of water electrolysis was evaluated by linear scan voltammetry. From Figure 5a, we can see that NiCoP/CC exhibits superior activity with a cell voltage of 1.52 V at 10 mA cm-2. For comparison, bifunctional Ni2P/CC and CoP/CC catalysts were also examined (Figure 5a). As expected, NiCoP/CC exhibits superior activity in comparison to Ni2P/CC and CoP/CC catalysts, even those of most of the previously reported bifunctional electrocatalysts in 1.0 M KOH media for water splitting as shown in Table S2. Due to the importance of the durability of the electrode for water splitting

in

view

of

practical

use,

NiCoP/CC

17


was

continued

to

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Figure

5.

(a)

LSV

curves

using

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nest-like

NiCoP/CC||NiCoP/CC,

Ni2P/CC||Ni2P/CC, CoP/CC||CoP/CC as two electrodes in 1 M KOH, (b) Chronopotentiometric response curve of NiCoP/CC at 100 mA cm-2. release hydrogen and oxygen for 40000 s at 100 mA·cm-2 (Figure 5b), the voltage increase only increased 80 mV after 40000 s. The current-time slopes of chronoamperometric tests for HER and voltage-time slopes of chronopotentiometric test for OER and water splitting were also shown in Figure S16. These slopes indicate the excellent durability in long-term electrochemical process. In addition, the faraday efficiency of O2 and H2 production was measured, and the obtained number of moles varying with time are shown in Figure S17 at 100 mA, which fitted well with the theoretical yields with molar ratio of H2 to O2 close to 2. The enhanced performance of HER and OER might be attributed to the unique 3D morphology and ternary composition of the nest-like NiCoP. Specifically, highly open hierarchical nest-like structure with high exposure of active sites should be beneficial for close contact with electrolyte. Also, the porous structure may also facilitate facile release of the evolved gas during HER and OER catalysis.43 In addition, it has been reported that by DFT calculation, doping Co may alter the 18


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ACS Catalysis

electronic structure of Ni2P, and further optimize atomic hydrogen trapping and desorption.44 Moreover, the use of carbon cloth as support could further improve the conductivity and charge transfer capability.45 4. Conclusions In summary, the 3D nest-like NiCoP have been successfully prepared on CC by a successive hydrothermal, oxidation and phosphidation process for the first time. The nest-like structure of NiCoP greatly increased the active sites and fast facile transport of electrons throughout the entire electrode, as well as the facile release of the evolved gas during HER and OER catalysis. The as-synthesized nest-like NiCoP/CC exhibits excellent HER catalytic performance and durability at both acidic and alkaline conditions. Furthermore, it also shows superior OER performance in alkaline solution, enables as-synthesized NiCoP/CC as bifunctional electrocatalysts for overall water splitting. It affords current density of 10 mA cm-2 at 1.52 V, 100 mA cm−2 at about 1.77 V, with only 80 mV increased over 40000 s. The use of NiCoP/CC as both cathode and anode will significantly simplify the electrode fabrication, facilitate their widespread application. Also the facile synthetic method for carbon cloth supported TMPs can be expanded to obtain other 3D nanostructures on carbon cloth or Ni foam for more applications (e.g., batteries, supercapacitors, catalysis).

Acknowledgements This work was financially supported by the National Natural Science Foundation of China (21571145, 21633008), the Creative Research Groups of Hubei Province 19


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(2014CFA007), the Large-scale Instrument and Equipment Sharing Foundation of Wuhan University.

Appendices A. Supporting information

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Nest-like NiCoP for Highly Efficient Overall Water Splitting - ACS

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