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Graphene Dots Embedded Phosphide Nanosheet-Assembled Tubular Arrays for Efficient and Stable Overall Water Splitting Jungang Hou, Yiqing Sun, Shuyan Cao, Yunzhen Wu, Hong Chen, and Licheng Sun ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b06231 • Publication Date (Web): 03 Jul 2017 Downloaded from http://pubs.acs.org on July 4, 2017

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ACS Applied Materials & Interfaces

Graphene Dots Embedded Phosphide Nanosheet-Assembled Tubular Arrays for Efficient and Stable Overall Water Splitting Jungang Hou,*a Yiqing Sun,a Shuyan Cao,a Yunzhen Wu,a Hong Chen,b Licheng Sun*ab

a

Institute of Artificial Photosynthesis, State Key Laboratory of Fine Chemicals, DUT-KTH

Joint Education and Research Center on Molecular Devices, Dalian University of Technology (DUT), Dalian 116024, China. b

Department of Chemistry, KTH Royal Institute of Technology, 110044 Stockholm, Sweden.

ABSTRACT: Bifunctional electrocatalysts are highly desired for overall water splitting. Herein, the design and fabrication of three-dimensional (3D) hierarchical earth abundant transition bimetallic phosphide arrays constructed by one-dimensional tubular array that was derived from assembling two-dimensional nanosheet framework, has been reported by tailoring the Co/Ni ratio and tunable morphologies and zero-dimensional (0D) graphene dots were embedded on Co-Ni phosphide matrix to construct 0D/2D tubular array as a highly efficient electrode in the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Based on advanced merits, such as the high surface active sites, well-dispersed graphene dots and the enhanced electron transfer capacity as well as the confinement effect of the graphene dots on the nanosheets, the integrated GDs/Co0.8Ni0.2P tubular arrays as anode and cathode, exhibit the excellent OER and HER performance. By use of GDs/Co0.8Ni0.2P arrays in two electrode setup of the device, a remarkable electrocatalytic performance for full water spitting has been achieved with a high current density of 10 mA cm-2 at 1.54 V and the outstanding long-term operation stability in alkaline environment, indicating a promising system based on

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nonprecious-metal electrocatalysts towards potential practical devices of overall water splitting.

Keywords: Graphene dots, Transition bimetallic phosphide, Oxygen evolution reaction, Hydrogen evolution reaction, Overall water splitting

1. Introduction The increasing energy demands and environmental concerns have motivated worldwide intense research on developing sustainable alternative energy sources along with environmental benignity.1,2 The scalable and sustainable production of hydrogen from water splitting is an appealing approach yet requires a highly efficient, long-term stable electrocatalysts.3 It is well-known that Ir oxides, Ru oxides and Pt are among those active HER and OER catalysts, hindering the large-scale clean energy production due to the high cost and scarcity.4 Thus, it is of great importance to develop highly active nonprecious electrocatalysts for OER and HER. During the past years, most efforts have mainly focused on transition-metal based electrocatalysts with various components and nanostructures.6 For example, transition-metal sulfides, carbides, selenides and phosphides, including MoS2, Mo2C, CoSe2, CoPx and NiPx catalysts, have shown promising HER performance.7-11 Meanwhile, oxides, nitrides, phosphides, such as NiFeOx, Co3O4, NiNx, and Co-P catalysts were reported to be active for the OER activities.12-15 Notwithstanding significant progress, there are few earth abundant and efficient electrocatalysts to implement both OER and HER activities in the same electrolyte.715

To create high surface area and active site density, hierarchical three-dimensional materials

have been rationally produced for the electrochemical applications. Especially, the onedimensional (1D) nanostructures or the assembly of two-dimensional (2D) superstructures into three-dimensional (3D) hierarchical architectures, have substantially stimulated the 2 ACS Paragon Plus Environment

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catalytic performance in water splitting.16-18 Most Co or Ni-based catalysts grown on Ni foam, Ti or Cu foils and carbon cloth have received extensive research interest.7-15 To our knowledge, however, hierarchical Co-Ni-based phosphide nanosheet-assembled tubular array with high surface area and more active sites have not been reported to date. In attempts to improve the OER and HER performance, it is a challenge to maximize the surface area of 3D architectures integrated by additional building units. For examples, graphene-supported electrocatalysts have been used to increase the rate of the reactions.19-23 It is fascinating to endow layered materials with novel properties by tunable size and dimension engineering. Especially, zero-dimensional graphene dots (GDs) associated with quantumconfinement and edge effects have been developed for extensive applications.19-23 However, it is hard to integrate the HER and OER catalysts in the single electrolyte and hence mediocre overall performance.24-30 Enlightened by these points, it is easy to come up with the idea of improving the overall water splitting performance over the hierarchical graphene quantum dots grafted Co-Ni-based arrays. Herein, we developed the graphene dots (GDs) anchored Co–Ni phosphide nanosheetassembled tubular array for overall water splitting. Firstly, the Co–Ni phosphide electrodes assembled of 1D tubular arrays that can be visualized as the construction of 2D ultrathin nanosheets by tailoring the Co/Ni ratio and tunable morphologies have been fabricated, possessing the high surface active sites exposure yield and building the pathway for efficient charge transport. Secondly, the integration of the GDs/Co0.8Ni0.2P array demonstrates robust individually catalytic OER and HER activities. Finally, the high current density of 10 mA cm2

at 1.54 V in alkaline environment was achieved by the GDs/Co0.8Ni0.2P system as the two-

electrode device with for practical applications for full water splitting

2. Experimental Section 2.1 Preparation of GDs/Co-Ni based phosphide arrays 3 ACS Paragon Plus Environment


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Cu2O nanowire arrays as template and graphene dots were prepared by the previous works.31,32 In a typical process, CoCl2·6H2O (99.99%) and NiCl2·6H2O (99.9%) with the tunable Co/Ni ratio (2.0 mg the total amount) were added to 100 mL of the ethanol/water mixed solvent in the presence of 0.3 g PVP (Mw=30000). After stirring, the Na2S2O3·5H2O (1 M, 99.99%) was added into the suspension. After the reaction at room temperature, the Co(OH)2, CoxNi1-x(OH)y and Ni(OH)2 arrays were obtained. In a typical process, NaH2PO2 (0.06 g, 99%) was placed at the center of the tube furnace and the Co(OH)2, CoxNi1-x(OH)y and Ni(OH)2 textiles were placed at the downstream side of the furnace. After the reaction at 300 oC for 1 h, the CoP, Co0.8Ni0.2P, Co0.6Ni0.4P, Co0.4Ni0.6P, Co0.2Ni0.8P and NiP arrays with a wide range of Co/Ni ratios were obtained. Then Co0.8Ni0.2P array was placed in a 50 mL Teflon-lined autoclave reactor with the tunable graphene dots (GDs) solution (0, 5, 15, 25 and 50 mL). After the hydrothermal reaction at 110 oC, the GDs/Co-Ni based phosphide array was obtained. In comparison, IrO2 and Pt/C (20wt% Pt) as well as Nafion (5 wt%) were purchased from Sigma-Aldrich. 2.2 Structural Characterization XRD patterns were recorded at room temperature using Cu Kα radiation (λ=1.5418 nm) by X−ray diffractometer (Bruker AXS, Karlsruhe, Germany). SEM was performed by a fieldemission scanning electron microscope. TEM images were measured on the transmission electron microscopy at an acceleration voltage of 200 kV. The chemical states of the sample were determined by X-ray photoelectron spectroscopy in a VG Multilab 2009 system. Raman spectra were obtained on a Raman spectrometer with 633 nm wavelength incident laser. 2.3 Electrochemical Measurements The electrochemical measurements were performed with a CHI 660D electrochemical analyzer (CH Instruments Inc., Shanghai). The as-prepared electrode, a platinum foil and an Ag/AgCl electrode were used as the working electrode, the counter electrode and the reference electrodes, respectively. All polarization curves were corrected for the iR 4 ACS Paragon Plus Environment

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compensation in the three-electrode system. All tests were measured in 1 M KOH, and the potentials were displayed versus reversible hydrogen electrode (RHE) by: E(RHE) = E(Ag/AgCl) + 0.197 + 0.0591 × pH. The solutions were purged with N2 for 30 min prior to the HER or overall water splitting test or with O2 prior to the OER test. Polarization curves were obtained by linear sweep voltammetry (LSV) with a scan rate of 2 mV s-1 for HER and OER, respectively. The long time durability test was conducted by using controlled-potential electrolysis method. The electrochemical impedance spectra (EIS) of the electrodes under the following frequency ranges 106 to 1 Hz were obtained by AC impedance spectroscopy. The stability test was performed using the controlled potential electrolysis method. Cyclic voltammetry (CV) was carried out in 1 M KOH to probe the electrochemical double layer capacitance (Cdl) of various samples to estimate the effective electrode surface areas. For overall water splitting (both HER and OER) tests, the GDs/Co0.8Ni0.2P arrays in two-electrode water splitting system were used as both the anode and cathode electrodes and the potential scan range was from 1.4 to 1.75 V. All data for the two electrode electrolyzer were recorded without iR compensation.

3. Results and Discussion The design and synthesis strategies of GDs/phosphides arrays as 3D integrated electrocatalysts are illustrated in Fig. 1. The Cu2O/Cu nanowire array from Cu(OH)2/Cu was produced by thermal annealing after anodic oxidation according to our previous work.31 Through the solution-phase cation exchange by a coordinating etching and precipitating approach, a Co–Ni hydroxide (CoNi(OH)x) arrays grown on Cu woven gauze was obtained due to the continuous dissolution of the raw Cu2O nanowire, the formation a soluble complex [Cu2(S2O3)x]2-2x and the releasing of OH- ions from the coordinating etching procedure.33 Co– Ni hydroxides are precipitated by the reaction of metal ions and the released OH- ions. After the low-temperature phosphidation, the Co–Ni based phosphides arrays have been obtained 5 ACS Paragon Plus Environment


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from the as-prepared CoxNi1-x(OH)y, Co(OH)2 and Ni(OH)2 nanostructures. After the decoration of graphene dots, the bifunctional GDs embedded transition bimetallic phosphide nanosheet-assembled tubular array has been achieved.

Fig. 1 Illustrated scheme of synthesis process for GDs/Co-Ni phosphide nanosheet-assembled tubular array. Each step: (i) thermal annealing after anodic oxidation, (ii) cation exchange, (iii) phosphidation and (iv) hydrothermal deposition.

The intrinsic morphological and structural properties of the Ni-Co-based textiles are characterized by electron microscope analysis. At the beginning, the Cu substrate was uniformly covered with a dense layer of the Cu2O nanowires (Fig. S1/S2). After etching the Cu2O nanowire and subsequent phosphiding, SEM images of different CoP, NiP and GDs/Co0.8Ni0.2P arrays were shown (Fig. 2). Especially, the individual Co0.8Ni0.2(OH)y and Co0.8Ni0.2P electrodes were composed of 1D tubular array that was evidently formed from the nanosheets (Fig. S3~S5). For the GDs/Co0.8Ni0.2P arrays, the element mapping and energy 6 ACS Paragon Plus Environment

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Fig. 2 SEM images of (abc) CoP nanosheets-assembled tubular array, (def) NiP nanoparticles-assembled tubular array, and (ghi) GDs/Co0.8Ni0.2P nanosheets-assembled tubular array as well as (jkl) element mapping of Co, Ni and P.

dispersive spectra retained uniform spatial distribution of Co, Ni, P and C elements in the GDs/Co0.8Ni0.2P arrays (Fig. S6). The XRD patterns (Fig. S7) clearly suggest that the Co(OH)2, Ni(OH)2 and Co0.8Ni0.2(OH)y precursors were converted into individual CoP, NiP and Co0.8Ni0.2P nanostructures. Strikingly, the original 1D nanotubes with the average diameter of around 400 nm are distinctly converted into the hierarchical tubular feature with a 7 ACS Paragon Plus Environment


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large number of nanosheets on the tubular surface in different angles to each other. Whilist, the particle-assembled 1D tubular textiles were observed for the NiP textiles. It is worth to point out the almost identical morphologies between hydroxide and phosphide tubular textiles. After the decoration of graphene, there is no apparent change between Co0.8Ni0.2P and GDs/Co0.8Ni0.2P arrays (Fig. S8 and Fig. 2). More importantly, the tubular arrays derived from 2D nanosheets could provide substantial active sites for anchoring graphene, resulting into an enhancement of the electrocatalytic activities of GDs/Co–Ni-based array.

Fig. 3 TEM images of (a) Co0.8Ni0.2(OH)y, (b) NiP, (c) CoP and (d) GDs/Co0.8Ni0.2P and HRTEM images of (ef) Co0.8Ni0.2P, (gh) GDs and (ij) GDs/Co0.8Ni0.2P.


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To explore the details of the nanostructures, the typical TEM images clearly elucidate the tubular nanostructures in Fig. 3. The whole view of the Co0.8Ni0.2(OH)y, CoP and Co0.8Ni0.2P tubular nanostructures show that the 3D configuration is assembled by ultrathin nanosheets, while the NiP tubular nanostructure is composed of nanoparticles that are stacked along one dimension direction. Moreover, a closer look by high-resolution TEM image presents a wellresolved lattice fringe space of 0.189 nm indexed to the (211) plane of Co0.8Ni0.2P. After the introduction of GDs, HRTEM image showed fairly uniform GDs with the average diameters of ca. 2~5 nm and the d-spacing of the adjacent lattice planes for the GDs of ca. 0.210 nm, assigned to the (100) planes of graphene. A HRTEM image of a GDs/Co0.8Ni0.2P crystallite showed the interplanar spacings of 0.210 nm for GDs and 0.189 nm for Co0.8Ni0.2P nanosheets. Thus, the GDs/Ni-Co-based array as the 0D/2D nanostructures can achieve the integrated network to enhance the electrocatalytic performance. The chemical compositions of the GDs/Co0.8Ni0.2P were characterized by X-ray photoelectron spectroscopy (XPS). The XPS spectra (Fig. 4) reveals the presence of Co, Ni and P in the Co-Ni phosphide array. The high-resolution scan of the Co 2p electrons generated four peaks at 781.2 eV (Co 2p3/2) and796.9 eV (Co 2p3/2), followed by two satellite peaks at 785.2 eV and 802.7 eV.34 The Ni 2p2/3 core level spectrum involves that the broad satellite peak (centered at 861 eV) above the Ni2+ species is the shakeup peak of divalent Ni2+ species.35 With regard to P 2p spectra, a weak doublet lied at 129.3 and 130.2 eV can be indexed to P 2p3/2 and P 2p1/2 lines.36 The peaks at 778.2 eV for Co 2p3/2, 853.1 eV for Ni 2p3/2 and 129.3 eV for P 2p3/2 are associated with the typical binding energies for Co 2p, Ni 2p and P 2p contributions in phosphides, respectively. After the introduction of GDs, the main peak at 284.6 eV can be assigned to the C–C bond with a sp2 orbital.19-23 The peak at 285.9 and 288.4 eV can be indexed to the C–O and O–C=O bonds, respectively, indicating the existence of GDs. As shown in Raman spectra, the ordered G band at 1582 cm-1 is stronger than the disordered D band at 1372 cm-1 (Fig. S9).31 Moreover, the strong vibrations at 1270, 9 ACS Paragon Plus Environment


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1590 and 3400 cm-1 are ascribed to the C=C, C-OH and O-H bonds, facilitating the intimate contact of the GDs/Ni-Co-based phosphide arrays. Thus, all results demonstrate that the GDs could effectively couple the tubular walls of Ni-Co-based array.

Fig. 4 XPS spectra of (A) Co 2p, (B) Ni 2p, (C) P 2p and (D) C 1s of GDs/Co0.8Ni0.2P.

The water electrolysis activities of the GDs/Co-Ni-based phosphide arrays are evaluated by OER and HER in alkaline media by a typical three-electrode apparatus. Fig. 5a shows the polarization curves of tunable CoxNi1-xP arrays with different Co/Ni molar ratio. Because the nanostructures of nanosheets- or nanoparticles-assembled tubular arrays were controlled by the by tailoring the Co/Ni ratio, the as-prepared Co0.8Ni0.2P nanosheets-assembled tubular arrays with threedimensional hierarchical architectures yield the higher OER activity. Compared to the precious

IrO2 catalyst, the NiP, CoP and Co0.8Ni0.2P arrays, the GDs/Co0.8Ni0.2P electrode displays the highest OER activity (Fig. 5a/Fig. S10), corresponding the lowest onset potential of around 1.46 V (Fig. 5b). Compared to the overpotentials for the NiP, CoP, and Co0.8Ni0.2P catalysts as 10 ACS Paragon Plus Environment

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Fig. 5 (a) OER polarization curves of the CoP, Co0.8Ni0.2P, Co0.6Ni0.4P, Co0.4Ni0.6P, Co0.2Ni0.8P and NiP arrays. (b) OER polarization curves, (c) Tafel slopes and (d) Nyquist plots of the NiP, CoP, and Co0.8Ni0.2P and GDs/Co0.8Ni0.2P arrays in 1 M KOH solution at an applied potential of 1.55 V. (e) Polarization curves of the GDs/Co0.8Ni0.2P electrodes recorded before and after 5000 continuous CV cycles in the range of 1.45 to 1.75 V vs. RHE at 100 mV s-1. (f) Time dependence of current density under a constant potential of 1.55 V vs RHE of GDs/Co0.8Ni0.2P array.



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well as the reported OER catalysts (Table S1), the overpotential at the current density of around 15 mA cm−2 are as low as 287 mV for the GDs/Co0.8Ni0.2P electrode. To evaluate the kinetics for the OER, the linear fitting of the Tafel plot derived from the polarization curves result into a smaller Tafel slope (≈50 mV dec−1) of the GDs/Co0.8Ni0.2P electrode than that of NiP (≈90 mV dec−1), CoP (≈75 mV dec−1) and Co0.8Ni0.2P (≈60 mV dec−1) electrodes (Fig. 5c), indicating a favorable OER reaction kinetics for the GDs/Co0.8Ni0.2P electrode. Moreover, the electrochemical impedance spectra (EIS) analysis for the NiP, CoP, and Co0.8Ni0.2P arrays, the GDs/Co0.8Ni0.2P electrodes were conducted (Fig. 5d). The Nyquist plots reveal that the charge transfer resistances of the GDs/Co0.8Ni0.2P electrodes are obviously lower than that of the NiP, CoP, and Co0.8Ni0.2P electrodes. Thus, the 3D framework with high surface area (Fig. S11) could provide the loose nanostructures, facilitating the release of gas bubbles and the fast diffusion of electrolyte in the integrated electrode. The critical criterion to evaluate the catalyst in high concentration alkaline solutions is the long-term electrochemical stability in practical applications. An accelerated degradation test was conducted by cyclic voltammetry (CV) in the potential range of 1.45 to 1.75 V vs. RHE. The CV scanning for 5000 cycles illustrates an ignorable current loss (Fig. 5e), indicating the superior operation ability of the GDs/Co0.8Ni0.2P array. Combined with SEM image (Fig. S12) and XPS spectra (Fig. S13) of the GDs/Co0.8Ni0.2P array after 50 h OER test, the abovementioned results demonstrate that the GDs/Co0.8Ni0.2P array present the outstanding longterm stability. Thus, the synergistic effects of the hierarchical architectures and the incorporation of the GDs over the GDs/Co0.8Ni0.2P array make contribution to the improved oxygen evolution performance. The electrochemically catalyzed HER on the tunable CoxNi1-xP arrays with different Co/Ni molar ratio, and the GDs/Co0.8Ni0.2P array were assessed. Among the different CoxNi1-xP arrays and Pt/C, the significantly improved HER performance was achieved over Co0.8Ni0.2P array (Fig. 6a/Fig. S10). After the incorporation of GDs, the GDs/Co0.8Ni0.2P array presents 12 ACS Paragon Plus Environment

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the lowest onset potential in comparison of the NiP, Co0.8Ni0.2P, and CoP catalysts (Fig. 6b) as well as the reported HER catalysts (Table S2). In sharp contrast, the current recorded on the GDs/Co0.8Ni0.2P electrode shows a small onset potential with the tremendously enhanced current after further sweeping to negative applied potential, corresponding the substantial release of hydrogen gas bubbles from the electrode surface, indicating that the successful formation of the GDs/Co0.8Ni0.2P components can significantly enhance the HER activity. Compared to the overpotential of 102, 124 and 182 mV for the Co0.8Ni0.2P, NiP and CoP arrays, the measured overpotential at the current density of 15 mA cm−2 is as low as 93 mV for the GDs/Co0.8Ni0.2P array with the further increasing current density of 100 mA cm−2 at 166 mV (Fig. 6b), indicating that the HER activity of the GDs/Co0.8Ni0.2P array is superior over those earth-abundant HER materials.7-11 Moreover, the excellent electrocatalytic activities of the CoP, NiP, Co0.8Ni0.2P and GDs/Co0.8Ni0.2P electrodes were further confirmed by the Tafel plots derived from the polarization curves. After the linear fitting of the Tafel curves, as shown in Fig. 6c, a small Tafel slope of 51 mV dec−1 was obtained for the GDs/CoP array, lower than that of the CoP, NiP and Co0.8Ni0.2P benchmarks (67, 64 and 58 mV dec−1), which was superior to that many HER catalysts, such as Ni-P nanoparticles (87 mV dec−1), bulk MoS2 (94 mV dec−1), Mo2C nanocrystals (110-235 mV dec−1), carbon paper/carbon tubes/cobalt-sulfide (131 mV dec−1) and MoSe2/rGO

(101 mV dec−1),24-45

indicating a more rapid reaction kinetics of the GDs/Co0.8Ni0.2P array. The significant performance difference of Ni1-xCoxP hybrids might derive primarily from the synergistic effect of Co and Ni.26 With regard to HER protons reduction, the P sites collected protons, while the metal centers served as electron collectors. The more negative P species and more positive Ni and Co species in the GDs/Co0.8Ni0.2P arrays are extremely beneficial for collection of protons and electrons, respectively, in which it is more convenient and effective for protons to capture electrons, thus facilitating the generation of hydrogen.7-11 Especially, the introduction of GDs could further facilitate the electrochemical reduction of the absorbed 13 ACS Paragon Plus Environment


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Fig. 6 (a) HER polarization curves of the CoP, Co0.8Ni0.2P, Co0.6Ni0.4P, Co0.4Ni0.6P, Co0.2Ni0.8P and NiP arrays. (b) HER polarization curves, (c) Tafel slopes and (d) Nyquist plots of the NiP, Co0.8Ni0.2P, CoP and GDs/Co0.8Ni0.2P electrodes in 1 M KOH solution at an applied potential of -0.2 V. (e) Time dependence of current density under a constant potential of -0.15 V vs RHE of GDs/Co0.8Ni0.2P array. (f) The charging current density differences plotted against scan rates of the NiP, CoP and GDs/Co0.8Ni0.2P array. The linear slope is equivalent to twice of electrochemical double-layer capacitance (Cdl).


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H2O.44 In a word, the OH− generated by H2O splitting might preferentially attach to the surface active sites near the GDs/Co0.8Ni0.2P interface, while the GDs would also facilitate H adsorption and electron transfer, imparting synergistic HER activity to the GDs/Co0.8Ni0.2P. From the Nyquist plots, the observed resistance of the GDs/Co0.8Ni0.2P array is significantly less than that of the NiP, Co0.8Ni0.2P and CoP arrays and, thus, remarkably increased HER kinetics, demonstrating more fast charge transfer within the GDs/Co0.8Ni0.2P array. Moreover, the durability test was then performed at a fixed current density of ~80 mA cm-2 at an applied potential of –0.15 V vs. RHE for 50 h (Fig. 6e), indicating the excellent long-term durability of the GDs/Co0.8Ni0.2P array in alkaline medium. In addition, there is no evident structural failure and performance decline after being bent five times (Fig. S14), suggesting the flexible and recoverable ability. Based on the electrical double-layer capacitance tests, the electrochemically active surface area of the different arrays was evaluated (Fig. 6f/Fig. S15). After the calculation, the capacitance values of the GDs/Co0.8Ni0.2P, Co0.8Ni0.2P, CoP and NiP arrays of 179.4, 143.8, 102.8 and 47.7 mF cm–2, respectively. Compared with the other catalysts, the highest capacitance of the GDs/Co0.8Ni0.2P array suggested the highest electrochemical surface area and great active sites.46-55 Thus, these results of the GDs/Co0.8Ni0.2P array emerge the promise for diverse applications. To explore the details, the GDs/Co0.8Ni0.2P array with different concentration of GDs solution has been fabricated on the Co0.8Ni0.2P array. Especially, the assembly of component units, that is, 0D dots, 2D nanosheets and 1D tubular nanostructure into the 3D architectures has been produced. It is worth noting that the electrocatalytic performance of our 3D hierarchical architectures is highly dependent on the contents of the GDs. The electrocatalytic characteristics of the GDs/Co0.8Ni0.2P array as a function of GDs contents were presented (Fig. S16). Obviously, compared to the device without GDs decoration, the current densities of the GDs/Co0.8Ni0.2P array increase immediately upon decoration of GDs on the Co0.8Ni0.2P array, yielding an apparent enhancement for the OER and HER performance, which is more than 15 ACS Paragon Plus Environment


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twice that for the device without GDs. As the layer number increased, the current densities increased correspondingly. Nevertheless, on the contrary, the current density decreases due to the increased resistance in the circuit as the GDs content continues to increase. Reaching 25 mL of the GDs, the individual OER GDs/Co0.8Ni0.2P array as anode and HER GDs/Co0.8Ni0.2P array as cathode achieved the maximum values. Thus, the GDs/Co0.8Ni0.2P two-electrode system offers the high current density at the low applied voltage.

Fig. 7 (a) Linear sweep voltammetry and (b) time dependence of the current density under a static overpotential of 1.54 and 1.60 V vs RHE of GDs/Co0.8Ni0.2P||GDs/Co0.8Ni0.2P array in two-electrode water splitting system.

Based on the individual OER and HER results, the GDs/Co0.8Ni0.2P arrays can serve as the anode and the cathode, exhibiting the full water splitting to get one step closer to the real industrial application. The bifunctional GDs/Co0.8Ni0.2P array achieves 10, 40 and 60 mA cm−2 at applied biases of 1.54, 1.65 and 1.70 V (Fig. 7a), while the Co0.8Ni0.2P||Co0.8Ni0.2P 16 ACS Paragon Plus Environment

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arrays achieve 10 and 20 mA cm−2 at 1.60 and 1.65 V (Fig. S17). It is worthwhile mentioning that the GDs/Co0.8Ni0.2P arrays exhibit the superior electrocatalytic performance, dramatically higher than that of the reported catalysts (Table S3), such as CoP films, Ni5P4 films, NiSe array, Ni3S2/Ni||Ni3S2/Ni, NiFe hydroxide, NixCo3-xO4||NiCo/NiCoOx, NiCo2O4||Ni0.33Co0.67S2 and CoNi(OH)x||NiNx catalysts.6-15,46-58 Moreover, the GDs/Co0.8Ni0.2P arrays with excellent long-term durability for up to 50 h offer the higher current density of around 23 mA cm-2 at a low applied voltage of 1.60 V in alkaline solution. Especially, the GDs/Co0.8Ni0.2P arrays with a nominal voltage of 1.54 V can drive overall water splitting, noticeably, the generated hydrogen or oxygen gas with the illustration of bifunctional GDs/Co0.8Ni0.2P||GDs/Co0.8Ni0.2P nanosheet-assembled tubular arrays can be instantly and efficiently released from the surface of the electrodes as evolved gas bubbles (Movie S1). Thus, this demonstration presents nonprecious-metal electrocatalysts towards potential practical devices of overall water splitting.

3. Conclusion Herein, the design and fabrication of three-dimensional (3D) hierarchical earth abundant transition phosphide tubular array derived from two-dimensional (2D) nanosheets has been reported by use of the solution-phase cation exchange and subsequent low-temperature phosphidation approaches and zero-dimensional (0D) graphene dots were embedded on CoNi phosphide matrix to construct 0D/2D tubular array as a highly efficient electrode in the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Hierarchical graphene dots embedded Co-Ni phosphide tubular arrays as anode and cathode, exhibit the excellent OER and HER performance due to the high surface active sites, well-dispersed graphene dots and the enhanced electron transfer capacity as well as the confinement effect of the graphene dots on the nanosheets. The as-assembled GDs/Co0.8Ni0.2P system exhibited the remarkable electrocatalytic performance for both OER and HER with the high current 17 ACS Paragon Plus Environment


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densities of around 10 mA cm-2 at the low applied voltages of 1.54 V in alkaline solution. The excellent catalytic activity and outstanding long-term operational stability as well as the facile and scale-up fabrication process of this architecture offers promising features for potentially used in noble-metal-free water-splitting devices.

Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Experimental details, XRD patterns, SEM images, schematic illustrations, Raman spectra and FT-IR spectra, XPS spectra and electrochemical performance.

Corresponding Author *E-mail: [email protected] or [email protected]

Notes The authors declare no competing financial interest

Acknowledgements This work was supported by National Science Foundation of China (No. 51472027, 51672034, 21110102036), National Basic Research Program of China (973 program, 2014CB239402), the Swedish Energy Agency, and the K & A Wallenberg Foundation.

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