Structural Engineering Hyperbranched NiCoP Arrays with Superior

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Structural Engineering Hyperbranched NiCoP Arrays with Superior Electrocatalytic Activities Toward Highly Efficient Overall Water Splitting Jian-Gan Wang, Wei Hua, Mingyu Li, Huanyan Liu, Minhua Shao, and Bingqing Wei ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b11576 • Publication Date (Web): 06 Nov 2018 Downloaded from http://pubs.acs.org on November 7, 2018

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ACS Applied Materials & Interfaces

Structural Engineering Hyperbranched NiCoP Arrays with Superior Electrocatalytic Activities Toward Highly Efficient Overall Water Splitting Jian-Gan Wang,†§* Wei Hua,† Mingyu Li,† Huanyan Liu,† Minhua Shao,§* Bingqing Wei †‡* † State

Key Laboratory of Solidification Processing, Center for Nano Energy Materials, School of Materials

Science and Engineering, Northwestern Polytechnical University and Shaanxi Joint Lab of Graphene (NPU), Xi’an 710072, China §Department

of Chemical and Biological Engineering, Hong Kong University of Science and Technology, Clear

Water Bay, Kowloon, Hong Kong 999077, China ‡

Department of Mechanical Engineering, University of Delaware, Newark, Delaware 19716, United States

Corresponding Authors *E-mail: [email protected] (J.-G. Wang); [email protected] (M. Shao); [email protected] (B. Wei) KEYWORDS: water splitting; NiCoP; hyperbranched architecture; bifunctional catalyst; hydrogen evolution reaction; oxygen evolution reaction

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ABSTRACT Developing inexpensive transition-metal-based nanomaterials with high electrocatalytic activity is of significant necessity for electrochemical water splitting. In this study, we propose a controllable structural engineering strategy of constructing a hyperbranched architecture for high efficient hydrogen/oxygen evolution reactions (HER/OER). Hyperbranched NiCoP architecture organized by hierarchical nanorod-on-nanosheet arrays is rationally prepared as a demonstration via a facile solvothermal and phosphorization approach. A strong synergistic benefit from the multiscale building blocks is achieved to enable outstanding electrocatalytic properties in an alkaline electrolyzer, including low HER and OER overpotentials of 71 and 268 mV at 10 mA cm-2, respectively, which significantly outperforms the counterparts of individual nanorods and nanosheets. The bifunctional catalysts also show highly efficient and durable overall water electrocatalysis with a small voltage of 1.57 V to drive a current density of 10 mA cm-2. The present study will open a new window to engineering hyperbranched architectures with exceptional electrocatalytic activities toward overall water splitting.

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1. INTRODUCTION Energy and environment are two critical concerns that the human being have to face in the 21st century.1 As the increasing depletion of the exhaustible fossil fuels has aroused unprecedented burden on the energy issue and environmental problem, it is extremely necessary to develop green and renewable power sources, such as solar and wind energies.2 Accordingly, electrochemical energy conversion and/or storage systems are necessary to make an efficient utilization of these intermittent energies.1,

3-4

Among them, electrochemical water splitting has been recognized as an attractive

method that enables conversion of electric energy into clean chemical energy (i.e., H2).5 Half-reactions of oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) are the two typical processes of the water splitting. Generally, it requires electrocatalysts to diminish the dynamic overpotentials of OER/HER and thereof maximize the conversion efficiency. The current state-of-the-art HER and OER electrocatalysts include precious metal Pt and Ir/Ru-based oxides, respectively.6-8 Nevertheless, the high price and natural scarcity of the materials significantly restrict their scale-up practical implementations. Consequently, it is highly imperative to explore inexpensive and earth-abundant catalysts as alternative candidates toward high-efficient water splitting. Transition-metal-based electrocatalysts have received remarkable research activities in recent years owing to their low cost, natural abundance, good catalytic activity, and easy processing. A great number of studies have made impressive progress on different transition metal compounds, including phosphides, carbides, oxides, nitrides, and sulfides.5,

9-12

An ideal electrocatalyst should

meet the rigid requirements of: (i) low overpotential, (ii) fast reaction kinetics, and (iii) excellent catalytic stability.5,

13-15

In addition, a single catalyst with bifunctional electrocatalytic activities

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toward both HER and OER and binder-free nature could further streamline the electrode fabrication process.16-18 To this end, transition metal phosphides are of significant promise for overall water splitting, because they show good electrocatalytic activities, high stability, and almost 100% Faradaic efficiency toward dual HER/OER in alkaline solutions.11, 19-21 Of particular note, bimetallic NixCoyP nanostructures have been proved to show advantageous electrocatalytic performance to the monometallic counterparts and many other bimetallic alloying phosphides.22-24 To further enhance the NixCoyP performance, great endeavor has been dedicated to elaborately tailoring the electronic structure and the surface adsorption energy by tuning the Ni/Co ratio. Previous results revealed that an optimal Ni/Co ratio of 0.5 results in the best performance.25-26 Moreover, structural engineering provides another feasible way to improve the performance by creating porous architecture, modifying surface wettability, and/or constructing hetero-interfaces.11 In this regard, a variety of NiCoP nanostructures, such as nanosheet,23,

27

nanorod/nanowire,24,

28-30

nanocone,31 nanocube,32

nanotube,33 nanoparticles,26, 34-35 and the like, have been prepared via different synthesis methods and showed good catalytic properties. It is worth to note that all these engineering efforts merely focus on a single nanostructure and how the structure influences the performance is difficult to identify. Considering the catalytic reactions only occur on the surfaces of catalyst, the increase in the active sites for a single nanostructure could only be achieved by prolonging the nanostructure along one direction.36 This inspires us to utilize different nanoscale building blocks perpendicular to each other to construct a three-dimensional (3D) hyperbranched architecture, which would afford more exposed surface area or active sites per geometric area for better electrocatalytic performance. In this work, we for the first time investigate and identify the true influence of structural engineering on the electrocatalytic performance. A facile solvothermal/phosphorization approach is

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developed to prepare three kinds of NiCoP nanostructures on conductive Ni foams, namely, nanorod arrays, nanosheet arrays, and particularly, 3D hyperbranched architecture assembled by hierarchical nanorod-perpendicular-on-nanosheet arrays. Expectedly, the unique 3D hyperbranched electrode exhibits the best HER/OER properties (e.g., lowest overpotential and smallest Tafel slope) in alkaline KOH solution among the three NiCoP nanostructures. Electrochemical results indicates that the 3D hyperbranched material provides more accessible electroactive sites per geometric area as well as improved reaction kinetics than the control electrodes of the single nanorod or nanosheets. The hyperbranched NiCoP electrode requires a small voltage of 1.57 V to drive an overall water splitting current density of 10 mA cm-2, holding a promise as a superior bifunctional electrocatalyst. 2. EXPERIMENTAL SECTION Materials synthesis. All chemicals were received from Sinopharm. In a typical synthesis process of hyperbranched precursor, a homogeneous aqueous solution (50 ml) containing 2 mmol nickel nitrate (Ni(NO3)2), 2 mmol cobalt nitrate (Co(NO3)2), 20 mmol of urea, and 8 mmol ammonium fluoride (NH4F) was prepared under stirring. The solution was put into a Teflon container. Then nickel foam (20 × 40 mm2, treated by sonication in HCl solution to eliminate the possible surface oxides) was placed into the solution. The container was subsequently sealed in a stainless steel autoclave, followed by heating to 100 °C and keeping at this temperature for 10 h. For the control samples, the amount of NH4F was changed to 0 and 16 mmol for the preparation of nanorods and nanosheets, respectively. Finally, the NiCo-precursor grown Ni foams were obtained and rinsed with water. After drying, the as-received foams were placed into a ceramic boat, and 100 mg of the sodium hypophosphite (NaHPO2) powder was placed at the upstream of the foams. Then the boat was put into a tube furnace, which was treated at 350 °C for 2 h under the inert N2 flow for phosphorization. The loading of the NiCoP nanostructures in the three samples is ca. 5.0 mg cm-2.

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Materials characterization.

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The microstructure of the as-prepared materials was investigated by

FEI scanning electron microscopy (SEM, NanoSEM 450) and transmission electron microscopy (TEM, JEOL 2100). The crystal phase was confirmed by powder X-ray diffraction (XRD, Shimadzu XRD-7000). X-ray photoelectron spectra (XPS) were obtained on Shimadzu Kratos Axis Supra and calibrated by C 1s peak at 284.8 eV. Electrochemical evaluation. Electrocatalytic properties were tested on the CHI 660E electrochemical workstation using a typical three-electrode system. The as-fabricated NiCoP-Ni foams were tailored as 10 × 10 mm2 for direct use as binder-free working electrodes, while the Ag/AgCl electrode and the carbon rod were served as the reference and counter electrodes, respectively. The electrolyte is 1.0 M KOH aqueous solution (pH=14). The calibration of the potentials to RHE was based on the formula of ERHE= EAg/AgCl + 0.197 + 0.059 pH. All polarization curves in this study were compensated by IR-correction method by ECorrected = ERaw - IRs. Herein, Rs corresponds to the series resistance, which can be obtained from the electrochemical impedance spectrum (EIS) measurements. Polarization profiles were acquired by linear sweep voltammetry (LSV) method at a sweep rate of 2 mV s-1. A durability examination was conducted using chronopotentiometry method at 10 mA cm-2 for HER, OER, and overall water splitting. The cycling test was also measured using cyclic voltammetry (CV) at 50 mV s-1 for 2000 cycles. The potentials are -0.08 V, 1.51 V and 1.60 V vs. RHE (without IR-correction) for 20 h HER, OER and overall water splitting durability tests, respectively. Electrochemical impedance spectrum (EIS) was obtained at -0.1 V vs. RHE for the HER and 1.6 V vs. RHE for the OER in the frequency region of 0.1-105 Hz (AC amplitude: 5 mV). CV was conducted in a narrow potential window without Faradaic reactions at various scan rates to determine the electrocatalytic active surface area (ECSA). By linearly fitting the capacitively anodic and cathodic current difference (Δj= janodic - jcathodic) against the sweep rate, the double layer capacitance (Cdl) is half of the slope value. The as-fabricated 6


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samples were assembled in a two-electrode system to measure the overall water splitting properties in 1.0 M KOH. The Faradaic efficiency of the gas production was measured during the first 60 min water splitting process at a current density of 10 mA cm-2. The evolved gas amounts were obtained and analyzed by gas chromatography (GC ， Shimadzu GC-2014C). The theoretical gas yield amounts (y) were calculated by Faraday law: y(H2 or O2)=Q/(nF), here, Q is the total charge passed on the electrodes, F corresponds to the Faradaic constant of 96485 C mol-1, and n is the transferred electron number per mol of gas (n=2 (for 1 mol H2) and 4 (for 1 mol O2)). 3. RESULTS AND DISCUSSION Figure 1a schematically illustrates the typical synthetic procedure of the 3D hyperbranched NiCoP architecture. Porous Ni foam substrate is employed as the 3D supporting scaffolds for the uniform formation of NiCo-precursor by a simple low-temperature solvothermal method. It is well-established that the morphology of the NiCo-precursor can be easily controlled by the reaction temperature/time, concentrations of metal salts, and types of additives.31,

37-38

Based on the

previously-documented knowledge, the morphology is mainly dependent on the deposition rate of NiCo-precursor during the solvothermal reactions. Rapid deposition rate results in the growth of nanorods, while slow rate contributes to the formation of nanosheets. We managed to manipulate the structures of NiCo-precursors by adjusting the dosage of NH4F, which could reduce the deposition rate of NiCo-precursors through surface coordination. Figure S1 shows different morphologies of NiCo-precursors. The morphology evolves from nanorod arrays (no NH4F additive, Figure S1c) to nanosheet arrays (16 mmol NH4F, Figure S1d) with increase of the dosage of NH4F. An interesting hyperbranched architecture of nanorod-perpendicular-on-nanosheet arrays is formed when the NH4F is at a mediate amount (8 mmol). Finally, various NiCoP nanostructures could be obtained by a typical phosphorization treatment. Figure 1b-e and Figure S2 exhibit the SEM images of the different NiCoP nanostructures. It is

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obvious that the parent morphologies of the NiCo-precursors are well inherited after the thermal phosphorization. As shown in Figure 1d-e, the interlaced nanosheet arrays are uniformly and vertically aligned on the surface of the Ni foam, which serves as primary backbone structures for the growth of secondary building blocks of the nanorods. Notably, the antenna-like nanorod arrays are radially planted into the nanosheets, thus constructing a unique hyperbranched architecture. The 3D hierarchically porous architecture is expected to afford sufficient channels for rapid mass transport and a large electroactive surface area for efficient HER/OER.39-41 Figure 1f shows a typical piece of the nanorod-on-nanosheet structure. The enlarged nanorod and nanosheet structures are displayed in Figure 1g and 1i. The diameter and length of the nanorods are 40-60 nm and 300-500 nm, respectively. The corresponding HRTEM images (Figure 1h and 1j) show distinct interplanar spacing of 0.22 nm, which belongs to (111) facets of the NiCoP phase. In addition, the EDX mapping result reveals homogeneous distribution of elemental Ni, Co, and P (Figure 1k) . Moreover, it is noteworthy that the NiCoP nanocrystals are enwrapped by an amorphous layer (Figure S3), primarily due to the surface oxidation of phosphide into phosphates in air. The phenomenon has been usually reported in metal phosphides.42-45 The amorphous outer layer of phosphide is demonstrated to have the favorable capability of safeguarding inner crystalline phosphides from corrosion during the HER/OER processes and enhancing the electrocatalytic activity of NiCoP by boosting electron transport kinetics.

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Figure 1. (a) Schematic of the synthesis c processes of hyperbranched NiCoP architecture. SEM images of (b) nanorod array, (c) nanosheet array and (d, e) nanorod-on-nanosheet array. (f, g, i) TEM and (h, j) HRTEM images of the hyperbranched NiCoP sample. (k) STEM image and the EDX mapping of Co, Ni, and P elements.

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Intensity (a.u.)

■ ●

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■ Ni foam ● NiCoP

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Sat. Niδ+

885

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2+

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810

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δ+

790

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855

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P-O

Intensity (a.u.)

Sat.

865

P 2p

Co 2p3/2

Co 2p1/2

870

Niδ+

Binding energy (eV)

d

Co 2p

Ni2+

Ni2+

2θ (degree)

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Ni 2p3/2

Ni 2p1/2

■

●

Ni 2p

Intensity (a.u.)

a

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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775

P 2p1/2 P 2p3/2

138

136

134

132

130

Binding energy (eV)

128

Figure 2. (a) XRD pattern of the hyperbranched NiCoP sample. Core-level spectra of (b) Ni 2p, (c) Co 2p, and (d) P 2p elements. The crystallographic phase of the as-fabricated sample is identified by XRD. As exhibited in Figure 2a, apart from the three sharp diffraction peaks belonging to the Ni substrate, the XRD pattern manifests a set of relatively weak peaks, which can be well-assigned to the hexagonal phase of NiCoP with a space group of P62m (a=5.834 Å, JCPDS # 71-2336).23 XPS measurement is conducted to examine the surface chemistry of the materials. The full scan XPS spectrum (Figure S4) elucidates the coexistence of elemental Ni, Co, P, and O components. The presence of O component coincides with the TEM observation of phosphates. The core-level spectrum of Ni 2p3/2 (Figure 2b) is deconvolved using three subpeaks with binding energies at 861.2, 856.5, and 853 eV, representing the satellite, oxidized Ni-POx (Ni2+), and partially charged Ni species (Niδ+) caused by Ni-P bonds,

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respectively.22-23 The Ni 2p1/2 region also shows two peaks at 870.5 and 874.5 eV and one satellite peak at 880.1 eV. Likewise, the Co 2p3/2 core-level spectrum (Figure 2c) exhibits a satellite peak at 785.4 eV, an oxidized Co-POx peak (Co2+) at 781.8 eV, and a positively-charged Co-P species (Coδ+, 778.8 eV) due to the formation of Co-P bonds.22, 46 The peaks at 793, 798.2 and 802.8 eV belong to the Co 2p1/2, which are Co-P, oxidized Co species, and the satellite, respectively. Figure 2d shows the high-resolution P 2p spectrum, which is composed of a doublet of P 2p3/2 component at 129.0 eV and P 2p1/2 component at 129.8 eV from metal phosphides, whereas the subpeak centered at 133.5 eV corresponds to the phosphate species (P-O).36, 47-48 The results again validate the formation of NiCoP and its partially oxidized phase of phosphate. The electrocatalytic activities of the samples toward water splitting were measured in 1.0 M KOH electrolyte using a three-electrode configuration. Figure 3a compares the IR-corrected LSV curves of the NiCoP nanorod (NR), nanosheet (NS), and nanorod@nanosheet (NR@NS) electrodes for HER. The three electrodes show much lower onset overpotentials than the bare Ni foam, indicating that the superior HER activity comes from the NiCoP component. More notably among the NiCoP electrodes, the hyperbranched NiCoP NR@NS electrode requires the lowest overpotentials of 71 and 149 mV to drive current densities of 10 and 100 mA cm-2, respectively. As for the commercial Pt/C (20%), it needs a lower overpotential of 43 mV to achieve the current density of 10 mA cm-2, but a higher overpotential of 244 mV is required to attain the current density of 100 mA cm-2. Tafel plot is conducted to characterize the HER kinetics. As exhibited in Figure 3b, the hyperbranched electrode manifests a Tafel slope as small as 57 mV dec-1, outperforming the individual nanorod (76 mV dec-1) and nanosheet (70 mV dec-1) electrodes. The Tafel slope value illustrates that the HER process is on the basis of the Volmer-Herrovsky mechanism:11, 19

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Volmer reation:

H2O + M + e− → M-Had + OH−

(1)

Herrovsky reaction:

H2O + M-Had + e− → H2 + OH−

(2)

Here, M represent the active sites on the electrode surface, which correspond to the Ni and Co species in this work. Specifically, the hydrogen intermediates (Had) are generated by the discharge of water molecules on the catalyst surface in the alkaline electrolyte (Eq. (1)). Then the hydrogen gas evolves through the electrochemical desorption process (Eq. (2)). Additionally, the contrast comparison (the lowest overpotential and smallest Tafel slope) reveals the significant importance of the hyperbranched architecture to promote water dissociation kinetics for boosting the HER performance. To demonstrate the enhanced HER kinetics, EIS testing was conducted with a resulting Nyquist plots displayed in Figure S5. The diameter of the semicircle denotes to the charge-transfer resistance (Rct) at the electrode/electrolyte interface. Notably, the hyperbranched electrode shows a much lower Rct (1.5 Ω) than the control electrodes (2.3 and 2.6 Ω for NS and NR, respectively), which is of great benefit in facilitating the hydrogen evolution efficiency. The HER properties of the hyperbranched electrode not only outperform that of the control electrodes, but also are superior to most transition metal compounds and comparable to the best results in literature (see Table S1 for detailed comparisons). The ECSA is evaluated to attain an in-depth understanding of the intrinsic activity of the electrocatalysts.22 The effective ECSA is estimated by Cdl, which is estimated by CV at various sweep rates (Figure S6). As expected in Figure 3(c), the hyperbranched electrode delivers the biggest Cdl value of 78.8 mF cm-2, indicating a corresponding highest electrocatalytic surface area among the electrodes. When normalized to the ECSA (Figure S7), the three electrodes show the similar current densities, illustrating their identical intrinsic catalytic activity. The higher catalytic activity can be

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rationally attributed to the hierarchically hyperbranched architecture that provide more electrolyte accessible sites for electrocatalysis.

a

b 300

Potential (V vs. RHE)

Current density (mA cm-2)

0 -50

20% Pt/C NiCoP NR c NiCoP NR@NS /de V NiCoP NS 5m 13 Ni foam

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20% Pt/C NiCoP NR NiCoP NR@NS NiCoP NS Ni foam

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Figure 3. HER characterization: (a) IR-corrected LSV profiles and (b) Tafel plots of the as-prepared electrodes. (c) Different in anodic/cathodic current densities as a function of sweep rate to determine double-layer capacitances for ECSA. (d) Chronoamperometric curve of the hyperbranched electrode at a potential of -0.08 V vs. RHE (without IR-correction) and the inset is the CV curves of the first and 2000th cycles.

An electrocatalyst desires for durable HER operation toward practical implementation. We

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evaluated the stability of the hyperbranched electrode by both current density-time measurement and CV test at 50 mV s-1. As displayed in Figure 3(d), the electrode possesses long-term stability with only a little degradation at 10 mA cm-2 for 20 h. In addition, the CV curve after 2000 cycles almost overlaps the initial shape, again validating the outstanding durability. After 20h HER stability test, the electrocatalyst was further characterized by SEM, XRD, Raman and XPS. The hyperbranched architecture has good preservation of its original morphology, indicating its robust structural integrity (Figure S8). It is worth noting that a new broad Raman peak in between 500 and 600 cm-1 appears in the cycling sample (Figure S9), suggesting that a new amorphous layer of Ni-Co oxides/hydroxides may be formed on the electrode surface.23,49 This is confirmed by the XPS result. As shown Figure S10, the disappearance of Niδ+/Coδ+ species related to Ni/Co-P bonds (Figure S10(a-b)) and the lower

intensity of the P 2p component (Figure S10(c)) elucidate the formation of

Ni-Co oxides/hydroxides. Such a surface composition change lowers the XRD peak intensity of NiCoP, but the main NiCoP phase is still maintained after the HER stability test (Figure S11). It is well known that Ni-Co oxides/hydroxides are essentially poor catalytic materials for HER, leading to the initial attenuation of the current density.50 However, the inner active NiCoP could change the local surface electronic state of the Ni-Co oxides/hydroxides, similar to chainmail catalyst,51-52 thereby maintaining the high catalytic activity. OER is a rate-determining process of water electrolysis due to the complex four electron transfer steps.53-54 Here, the OER performance of the as-prepared electrodes is further studied in the same alkaline solution. Figure 4a exhibits the LSV polarization curves. In stark contrast to the bare Ni foam, the NiCoP materials exhibit significantly enhanced catalytic activity toward OER. More encouragingly, the hyperbranched NR@NS electrode can afford a current density of 10 mA cm-2 at

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the lowest overpotential of 268 mV. By contrast, larger overpotentials of 277, 296 and 316 mV are required to obtain the same current density for the control RuO2, NR and NS electrodes, respectively. In addition, the hyperbranched electrode can sustain a rapid OER, since a low overpotential of 350 mV is merely required to deliver a large current density of 100 mA cm-2. Notably, the OER properties are at the top level of the recently reported transition metal compounds, as summarized in the Table S2. The better OER performance of the hyperbranched electrode can be rationalized by the increase of the ECSA (Figure S12). As displayed in Figure 4c, the double-layer capacitance attains to 13.5 mF cm-2, which is 2-2.5 times higher than that of the control electrodes. When normalized to the ECSA (Figure S13), the three NiCoP electrodes exhibit similar current densities owing to the same intrinsic catalytic nature. Figure 4b exhibits the Tafel slopes of all electrodes. The Tafel slopes of the NiCoP-based electrodes are very similar close to 71-75 mV dec-1, indicating the kinetics of the OER is almost identical. Similar results are reported in previous NiCoP materials.23 EIS characterization shows similar Rct values (2.75-3.25 Ω, Figure S14), further confirming the identical OER catalytic kinetics. The results demonstrate that the morphology exerts less kinetic influence on the OER than that on the HER. The OER stability of the hyperbranched electrode is examined by current density-time test (Figure 4d). The electrode exhibits a slight degradation in the anodic current density over the 20 h test. Moreover, the stability is verified by another CV cycling test (inset in Figure 4d), where the polarization curve of the 2000th cycle shows a negligible derivation from the initial one. The oxidation peaks in the range of 1.25~1.45 V vs. RHE of the LSV curves are associated with the oxidation reactions related to the Ni and Co species. The small difference of the peak shape before and after 2000-cycling stability test can be attributed to the change of morphology and composition

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of the electrode, which will be confirmed by the post-OER characterizations in the following part.

b1.72

RuO2



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NiCoP NR NiCoP NR@NS NiCoP NS Ni foam

250 200

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82 m


J 0.85 V (mA cm-2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0

4

80 60 40 20 0 1.1 1.2 1.3 1.4 1.5 1.6 1.7


8

12

Time (h)

16

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Figure 4. OER characterization: (a) IR-corrected LSV curves and its corresponding (b) Tafel plots of the as-prepared electrodes. (c) Different in anodic/cathodic current densities as a function of scan rate to determine double-layer capacitances for ECSA. (d) Chronoamperometric curve of the hyperbranched electrode at a potential of 1.51 V vs. RHE (without IR-correction) and the inset shows the CV curves of the initial and 2000th cycles at 50 mV s-1. It is worth to mention that there is a huge difference in the ECSAs for HER and OER albeit with a similarly hyperbranched architecture. This suggests that the HER/OER intrinsically occur on different electrocatalytic sites. Recent studies demonstrated that the metal phosphides would evolve

16


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into their oxide/oxyhydroxide counterparts in a strong alkaline electrolyte during the OER process.22, 49

Consequently, the morphology, component and surface chemistry of the electrode after the OER

duration test are deserved to be investigated for probing the true active sites. It is observed that the morphology of nanorods is evolved to nanosheets (Figure S15).31 The crystalline NiCoP phase is disappeared after the 20-h OER stability test (Figure S16), revealing the amorphous state of the newly generated nanosheets. Raman spectroscopy was performed to study the component of the formed nanosheets. As shown in Figure S17, the new broad peaks between 440 and 650 cm-1 are arising from Ni-Co oxides/hydroxides.49 Figure S18 shows the XPS spectra of the Co, Ni, and P elements. Of note, all deconvolved components of Co and Ni spectra are attributable to the corresponding oxidized species and satellites, and the original metal-P bonds disappear, indicating the full oxidation of the phosphides. Meanwhile, the sole presence of the P-O bond and the notable reduction of P intensity are in close correlation with oxidation during the catalytic processes. These results elucidate that the true activity of the OER primarily comes from the Ni-Co oxide/oxyhydroxides generated on the electrode surface, agreeing well with other previous studies.22-23, 31 It is well-known that the OER process is a four-step reaction and the possible mechanism under alkaline condition has been proposed as follows: M + OH− → MOH + e−

(3)

MOH + OH− → MO + H2O + e−

(4)

MO + OH− → MOOH + e−

(5)

MOOH + OH− → M + O2 + H2O + e−

(6)

The first step involves the adsorbed OH- ions on the active sites according to Eq. (3). After

17



recombination of a second OH- ion (Eq. (4)), the resulting MO reacts with OH- to form the intermediate of MOOH(Eqs. (5)), and finally, oxygen gas is evolved (Eq. (6)). These steps are accompanied by the redox reactions of nickel and cobalt species (i.e., Co2+ ↔ Co3+ ↔ Co4+ and Ni2+ ↔ Ni3+). Current density (mA cm-2)

b 300 250 200 150 20% Pt/C ‖ RuO2

100

NiCoP NR ‖ NiCoP NR NiCoP NS ‖ NiCoP NR@NS NiCoP NS ‖ NiCoP NS

50 0 1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8

1.9

2.0


c Current density (mA cm-2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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15

10

5

0 0

4

8

Time (h)

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Figure 5. Overall water splitting performance of the hyperbranched NiCoP NR@NS electrode in 1.0 M KOH. (a) Schematic of the two-electrode setup based on the hyperbranched NiCoP NR@NS electrodes. (b) LSV curves of the cells assembled by the three different NiCoP nanomaterials. The inset shows a digital photo of the two-electrode setup. (c) Stability of the cell assembled by the hyperbranched NiCoP NR@NS electrodes.

Based on the above analysis, the hyperbranched NiCoP NR@NS architecture could enable outstanding electrocatalytic capability for HER/OER in the same alkaline electrolyzer (1.0 M KOH). 18


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It is therefore interesting and fascinating to construct a two-electrode setup based on the bifunctional NiCoP NR@NS sample for full water splitting (Figure 5a). For comparison, the control NiCoP NR and NS electrodes were also measured. Figure 5b exhibits the LSV profiles of the three cell setups. As expected, the bifunctional NiCoP NR@NS electrode exhibits the best performance. Specifically, the NiCoP NR@NS‖NiCoP NR@NS electrode pair requires a small voltage of 1.57 V to drive 10 mA cm-2, compared with 1.61 and 1.64 V for the NS‖NS and NR‖NR electrodes. Notably, the current density of the NiCoP NR@NS‖NiCoP NR@NS electrode exceeds that of the Pt/C‖RuO2 electrode pair when the potential is higher than 1.66 V. Additionally, the Faradaic efficiency of oxygen and hydrogen production was measured by GC analysis, both of which exhibit 98-100% efficiency yields (Figure S19). Table S3 summarizes the previously-reported bifunctional NiCoP-based materials for full water splitting. The present results stand at the top level among these materials. Furthermore, the hyperbranched electrode manifests good durability with a slight degradation in the current density after 20 h, demonstrating its practical implementation for water electrolysis. The excellent electrocatalytic activity of the NiCoP NR@NS toward HER/OER can be attributable to the unique hyperbranched structure and the intrinsic nature of the ternary phosphide. More specifically, the hyperbranched architecture could afford large exposed surface sites per geometric area to facilitate the electrocatalytic activity. Secondly, the hierarchically porous structure provides sufficient channels for fast ion transport and gas release during the HER/OER processes. Thirdly, as documented in many studies,23-24 the ternary NiCoP phase itself is of highly catalytic in nature compared to the binary counterparts because of the enhanced electric conductivity for rapid electron transfer and the optimized electronic structure for favorable hydrogen adsorption/desorption. Finally,

19



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the 3D Ni scaffold holds strong mechanical and electrical connections to the active NiCoP nanostructures, thereby ensuring the long-term stability and high OER/HER kinetics. 4. CONCLUSIONS In summary, 3D hyperbranched NiCoP NR@NS architecture has been successfully regulated for the first time by a facile solvothermal and phosphidation process and the importance of the structure-performance relationship is elucidated. The hierarchical structure poses a significant influence on the electrocatalytic activities toward the HER/OER processes. The hyperbranched architecture is of great benefit in enabling numerous accessible active sites and fast mass transport, and accordingly, the NiCoP NR@NS can be employed as a superior bifunctional electrocatalyst to render outstanding HER/OER performance in an alkaline electrolyte. A small cell voltage of 1.57 V is sufficient for producing a stable current density of 10 mA cm-2 over 20 h in the two-electrode configuration, which is among the best performance of the transition metal compounds. The present structural engineering strategy would open new opportunities for constructing hierarchically hyperbranched nanomaterials toward high efficient overall water splitting.

Corresponding Authors *E-mail: [email protected] (J.-G. Wang); [email protected] (M. Shao); [email protected] (B. Wei) ACKNOWLEDGMENTS The work is supported by the research funds of NSFC (51772249, 51472204, and 51521061), NSF of Shannxi Province (2018JM5092), Fundamental Research Funds for the Central Universities (G2017KY0308), Hong Kong Scholars Program (XJ2017012), and Guangdong Special Fund for

20


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Science and Technology Development (Hong Kong Technology Cooperation Funding Scheme (201604030012, 201704030019, and 201704030065). Supporting Information Details of structural characterization and electrocatalytic properties of various NiCoP materials. Tables compare the previously-reported HER, OER, and overall water splitting performance with the present study. References (1) Seh, Z. W.; Kibsgaard, J.; Dickens, C. F.; Chorkendorff, I.; Nørskov, J. K.; Jaramillo, T. F., Combining Theory and Experiment in Electrocatalysis: Insights into Materials Design. Science 2017, 355, eaad4998. (2) Green, M. A.; Bremner, S. P., Energy Conversion Approaches and Materials for High-Efficiency Photovoltaics. Nat. Mater. 2016, 16, 23-34. (3) Yang, Z.; Zhang, J.; Kintner-Meyer, M. C. W.; Lu, X.; Choi, D.; Lemmon, J. P.; Liu, J., Electrochemical Energy Storage for Green Grid. Chem. Rev. 2011, 111, 3577-3613. (4) Wang, J.-G.; Kang, F.; Wei, B., Engineering of MnO2-based Nanocomposites for High-Performance Supercapacitors. Prog. Mater. Sci. 2015, 74, 51-124. (5) Zou, X.; Zhang, Y., Noble Metal-Free Hydrogen Evolution Catalysts for Water Splitting. Chem. Soc. Rev. 2015, 44, 5148-5180. (6) Han, X.; Wu, X.; Deng, Y.; Liu, J.; Lu, J.; Zhong, C.; Hu, W., Ultrafine Pt Nanoparticle-Decorated Pyrite-Type CoS2 Nanosheet Arrays Coated on Carbon Cloth as a Bifunctional Electrode for Overall Water Splitting. Adv. Energy Mater. 2018, 8, 1800935. (7) Gupta, U.; Rao, C. N. R., Hydrogen Generation by Water Splitting Using MoS2 and Other

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Structural Engineering Hyperbranched NiCoP Arrays with Superior

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