Hierarchical Edge-Rich Nickel Phosphide Nanosheet Arrays as

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Hierarchical Edge-Rich Nickel Phosphide Nanosheet Arrays as Efficient Electrocatalysts toward Hydrogen Evolution in both Alkaline and Acidic Conditions Qing Yan, Xi Chen, Tong Wei, Guiling Wang, Min Zhu, Yikuan Zhuo, Kui Cheng, Ke Ye, Kai Zhu, Jun Yan, Dianxue Cao, and Yiju Li ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b06861 • Publication Date (Web): 22 Mar 2019 Downloaded from http://pubs.acs.org on March 25, 2019

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ACS Sustainable Chemistry & Engineering

Hierarchical Edge-Rich Nickel Phosphide Nanosheet Arrays as Efficient Electrocatalysts toward Hydrogen Evolution in both Alkaline and Acidic Conditions

Qing Yan1, Xi Chen2, Tong Wei1, Guiling Wang1, *, Min Zhu1, Yikuan Zhuo1, Kui Cheng1, Ke Ye1, Kai Zhu1, Jun Yan1, Dianxue Cao1, Yiju Li1, *

1 Key

Laboratory of Superlight Materials and Surface Technology of Ministry of Education, College of Materials Science and Chemical Engineering, Harbin Engineering University, Harbin 150001, P. R. China 2 Department of Applied Physics, School of Physics and Optoelectronic Engineering, Xidian University, Xi’an 710071, P. R. China

* Corresponding authors: [email protected] (Guiling Wang); [email protected] (Yiju Li)

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ABSTRACT Searching highly-active, high stability, low-cost electrocatalysts toward hydrogen evolution reaction (HER) is imperative for electrochemical hydrogen production. We propose an effective synthetic method to obtain self-supported, hierarchical, and edge-rich nickel phosphide nanosheet arrays on nickel foam (Ni2P NSs-NF), which are successfully employed as a high-efficiency, three-dimensional (3D) binder-free electrode for HER. Benefiting from the 3D open nanostructure with abundant edges in the Ni2P NSs, the Ni2P NSs-NF electrode exhibits superior activity for HER both in alkaline and acidic conditions. It requires low overpotentials of 89 mV in the alkaline solution (1 M KOH) and 67 mV in the acidic solution (0.5 M H2SO4) to drive 10 mA cm-2 with low Tafel slopes of 82 and 57 mV dec-1, respectively. Besides, the hierarchical Ni2P NSs-NF electrode shows no apparent performance decay toward HER even after 10000 cycles. Theoretical calculation results prove that the free energy of hydrogen adsorption on the (211) crystal facet of Ni2P is closest to zero (−0.03 eV), which is supposed to be the most active site toward HER. The freestanding edge-rich Ni2P NSs-NF electrode is prospective for large-scale preparation of hydrogen.

KEYWORDS: Freestanding; Edge-Rich; Ni2P Nanosheets; Acid and Alkaline Solutions; Hydrogen Evolution Reaction

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INTRODUCTION With increasing fossil energy consumption and environmental hazards, searching renewable fossilfree pathways to produce clean fuels has triggered widespread concern.1-4 Hydrogen (H2) is a continuable and promisingclean fuel for its clean, natural abundance and high energy density.5,6 There are several methods to effectively obtain hydrogen, such as electrocatalytic water cracking, water gas conversion, methane reforming for hydrogen production and so on. Among the above methods, water electrolysis has been considered as a sustainable technology to produce high-purity hydrogen.7-10 Ptbased materials are acknowledged as the best electrocatalysts for HER, but they are scarce, highly expensive, and the stability of most Pt-based electrocatalysts is not good enough for industrial water splitting.11-13 Therefore, developing highly effective, resource-rich, and durable HER electrocatalysts is meaningful. With abundant source, bargain price, and favorable electrocatalytic activity, transition-metal phosphides (TMPs, including MoP, FeP, CoP, Ni5P4 etc.) are reported as promising alternatives to platinum-based electrocatalysts towards HER.14-19 Over the last several years, researchers have adopted many methods for improving the catalytic performance of TMPs, such as adjusting conductive substrates and coatings, regulating morphology and size, and manipulating dopants.20-22 Recently, nickel phosphides have raised much attention for the relatively high conductivity and favorable electrocatalytic performance towards HER. For example, Chen and partners reported that 3D porous multi-shelled Ni2P hollow microspheres need additional 98 mV to get 10 mA cm-2.23 Liu et al. reported the Ni2P–NiP2 hollow nanoparticle carrying abundant heterointerfaces shows a favorable catalytic performance toward the HER.24 However, these catalysts are in powder form, polymer binders are required to prepare the electrodes, which could impede the diffusion of the 3



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electrolyte ion and hydrogen bubble release, and inevitably reduce the exposed active sites of catalysts.25,26 Therefore, the construction of self-supported electrode architecture with open microstructure can well address this issue. For instance, Li et al. developed a carbon-covered hybrid Ni/Ni2P nanoparticles supported by macroporous carbon framework, which needs an overpotential of 92 mV towards HER to obtain 10 mA cm-2 in alkaline solutions.27 Liu et al. reported the nickel phosphide electrocatalysts fabricated by a direct phosphorization of Ni can achieve overpotentials of 98 mV in acidic and 117 mV in an alkaline solution for driving 10 mA cm-2, respectively.28 Note that, although much efforts have been made to improve HER performance, there is still much space for performance improvement for the transition-metal phosphides toward HER when comparing with Ptbased catalysts. Here we report a 3D self-supported edge-rich Ni2P NSs-NF electrode using a facile method for efficient hydrogen generation. Numerous in-plane nanopores were generated on the Ni2P NSs during the phosphatization process. The hierarchically porous mesh-like Ni2P nanosheet arrays with an open nanostructure can facilitate electrolyte permeation and hydrogen molecules release. Also, abundant active edges on the Ni2P NSs contribute to exposing larger amounts of active sites for HER. As a result, the freestanding Ni2P NSs-NF electrode demonstrates a high electrochemical activity towards HER. This Ni2P NSs-NF electrode shows low overpotentials of 89 mV in alkaline and 67 mV in acidic condition at 10 mA cm-2. Also, this Ni2P NSs-NF electrode displays extremely good durability even after 10,000 cycles.

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Scheme 1. Schematic diagram of the preparation method and structure of Ni2P NSs-NF electrode.

EXPERIMENTAL SECTION Synthesis of NiO NSs-NF The detailed contents of the chemical and materials are stated in the support information. We adopted the modified method according to our previous procedures.29 Firstly, 2.65 g NiSO4·6H2O and 0.4 g NH4NO3 were dissolved with 32 mL H2O, then 8 mL ammonia water was injected into the solution under continuous magnetic agitation to form a homogenous solution. Next, the pretreated nickel foam, which was washed by 6 M HCl, acetone and deionized water for 15 minutes successively, was immersed in the above solution and sealed in a petri dish with a cover for 12 h at 90 °C. The obtained nickel hydroxides precursor was then calcined at a muffle furnace, which was heated to 350 °C at a rate of 5 °C every minute and maintained for 2 h in an air atmosphere to obtain NiO NSs-NF. The mass loading of NiO on NF (1×1 cm2) is ~2.1 mg. Synthesis of Ni2P NSs-NF 5



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One piece of NiO NSs-NF loaded into a ceramic boat was at the middle of the tube. Then 1.0 g red P was at upstream in this tube. The tube furnace was heated to 500 °C at a rate of 5 °C every minute and maintained for 2 h in an Ar atmosphere, and then held at 250 °C for 3 h.24 The furnace was cooled naturally down to obtain the Ni2P NSs-NF electrode. The mass loading of Ni2P on NF (1×1 cm2) is 2.0 mg. Synthesis of Pt/C-NF Typically, 10 mg Pt/C powder, 0.5 mL ethanol, 0.5 mL H2O and 32 μL Nafion (5%) were mixed to prepare the slurry. Then Pt/C-NF was fabricated by drop-casting the slurry onto the NF (1×1 cm2) with a mass loading of ~2.0 mg. Electrochemical Measurements and Characterizations The electrochemical performance was tested in a standard three-electrode system. The catalysts serve as the working electrodes, a carbon rod serves as the counter electrode, and an Ag/AgCl (saturated KCl) electrode equipped with a salt bridge serves as the reference electrode.30 The detailed electrochemical test and physical characterizations methods can be seen in the support information. Free Energy Calculation of Hydrogen Evolution Reaction Density functional theory (DFT) calculations were performed by the Vienna ab initio simulation package (VASP) within the projector-augmented wave (PAW) method.31 Electron exchange and correlation were described with revised Perdew-Burke-Ernzerhof (RPBE) functional.32 A kinetic energy cutoff for plane wave expansion was taken as 400 eV. The total energy was converged to the difference of two iterated steps less than 10-5 eV. The Brillouin zone was sampled with 4×4×1 Monkhorst-Pack grid. All results were obtained by spin-polarized calculations. Ni2P (111), (021), (210), (002), (032), (211), (212), (300) and (321) facets were modeled by slabs cutting from primitive 6


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crystal with periodic boundary condition in the lateral directions. The slab thickness is not less than 6 Å. The DFT simulations were performed with the atoms below the second surface layer fixed. All the simulation models are shown in Figure S1. The free energy of hydrogen evolution reaction was calculated with the computational hydrogen electrode model.33 In hydrogen electrode, the reaction H+ + e- ↔ 1/2 H2 (g) is in equilibrium at 0 V, and thus, for proton-electron pair the free energy

is G (H+) + G (e-) = 1/2 G (H2). Within DFT

calculations, the hydrogen adsorption energy on a slab is obtained by ΔEH* = E (slab + H*) – E (slab) – 1/2 E (H2). The hydrogen adsorption free energy (ΔGH*) is then defined as ΔGH* = ΔEH* + ΔEZPE – TΔSH =ΔEH* + 0.24 eV. Here, T is the temperature. ΔEZPE is the difference of zero-point energy between the adsorbed state and gas phase. ΔSH is the entropy change. At T = 300 K, ΔEZPE – TΔSH = 0.24 eV is a reasonable and well-established approximation.34

RESULTS AND DISCUSSION The synthetic process of the Ni2P NSs-NF electrode was briefly illustrated in Scheme 1. The Ni(OH)2NF was first prepared through the wet chemical method under 90 °C for 12 h. The NF turns azury after loading Ni(OH)2 (Figure S2). The dense and interconnected nanosheets arrays were orderly grown on the NF (Figure S3), forming a 3D porous structure. The NiO NSs-NF was successfully obtained by a convenient one-step annealing at 350 °C, which was proved by X-ray diffraction (XRD) (Figure S4a). A 3D interconnected open framework of the NiO NSs-NF was remained after annealing (Figure S4b and c). Finally, the NiO NSs-NF was phosphatized under 500 °C for 2 h to obtain the Ni2P NSs-NF, whose morphology was revealed in Figure 1a and b. The thin Ni2P nanosheets connect to form 3D open nanostructures. Furtherly, transmission electron microscopy (TEM) was employed 7



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to investigate the morphology and crystallinity of Ni2P nanosheet. Figure 1c shows there are numerous nanopores generated on the ultrathin Ni2P nanosheet. Selected area electron diffraction (SAED) pattern (Figure S5) corresponds to the (211), (210), (021) and (111) crystal planes of the Ni2P, respectively, suggesting Ni2P is polycrystalline. The HRTEM image of the Ni2P nanosheet is shown in Figure 1d. The interplanar crystal spacings of 0.22 and 0.20 nm can be clearly distinguished, which is indexed into (111) and (021) planes of Ni2P, severally. In addition, rich edge sites and numerous defects can be clearly observed (Figure S6). Abundant edges and defects can offer more active sites for effective HER, thus enhancing the HER performance.35-37 Figure 1 e-h show the SEM and energy dispersive X-ray (EDX) elemental mapping images of Ni2P NSs-NF. The uniform elemental distributions of Ni and P indicate the homogenous growth of Ni2P NSs without obvious agglomeration. The corresponding EDX spectrum (Figure S7) shows the atomic ratio (Ni to P) is close to 2 ∶ 1.

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Figure 1. Morphology characterizations of Ni2P NSs-NF. (a, b) SEM images of the Ni2P under different magnifications. (c) TEM image for the Ni2P nanosheet. (d) HRTEM image for the Ni2P nanosheet. (e-h) SEM and corresponding elemental mapping images.

Figure 2a shows the XRD pattern of Ni2P NSs-NF. There are several obvious diffraction peaks located at 40.8, 44.6, 47.3, 54.2, 54.4, 55.0 and 74.9°, which are indexed into the (111), (021), (210), (300), (002), (211) and (212) planes of Ni2P, respectively (JCPDS # 65-3544). The surface composition and chemical valence state of Ni2P were further explored using the X-ray photoelectron spectroscopy (XPS). The binding energy of 852.7 eV is related to Ni-P, which is slightly blue-shifted compared with the metallic Ni (852.6 eV), suggesting the Ni is partially charged.38 The peaks at 857.8 and 875.1 eV correspond to the Ni 2p3/2 and 2p1/2 of the oxidized Ni species (Ni-O), respectively, accompanying with the satellite peaks at 863.3 and 881.1 eV.39 The P 2p XPS spectrum can be deconvoluted into three peaks at 128.9, 129.4 and 134.5 eV, which are assigned to the P 2p3/2, P 2p1/2 9



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as well as oxidized phosphorus species, severally (Figure 2c). The oxidized phosphorus species could be attributed to the superficial oxidation in air.17 The binding energy of 128.9 and 129.4 eV is lower than that of elemental P (130.0 eV), indicating that P is partially negatively charged.40,41 As a result, the negatively-charged P can facilely trap positively-charged protons under electrocatalytic process.40

The ΔGH* is an indicator for estimating the HER activity.42 To investigate the potential active sites, we performed quantum chemical calculations based on DFT for the hexagonal Ni2P. It is clear that (111), (021), (210), (002), (032), (211), (212), (300) and (321) are the main crystal planes of the hexagonal Ni2P according to the XRD result. Therefore, we adopted these three crystal planes of the Ni2P NSs-NF for the hydrogen adsorption energy calculation. The (111), (021), (210), (002), (032), (211), (212), (300) and (321) planes of the Ni2P, and (111) plane of the NiO were cleaved from the lattice data in Crystallography Open Database, with lattice constants a = 5.862 Å and c = 3.372 Å for hexagonal (P-62m) Ni2P bulk, and a = 4.177 Å for rock-salt (Fm-3m) NiO bulk. For the NiO (111) plane, the p(2×2) octopolar reconstructed model in the previous literature was employed.43 Slab models of eight atomic layers were constructed with the bottom six layers fixed. In the perpendicular direction, the replicas of the simulation system were separated by a vacuum layer of about 12 Å. Geometries were fully relaxed until the Hellmann-Feynman forces were below 0.01 eV/Å. On every surface, several different positions for hydrogen adsorption were tried to find the most stable configuration. Theoretically, the HER route can be expressed as a three-state diagram which starts from the initial state (H+ + e-), goes through an intermediate state (H*), and ends up in a final state (1/2 H2). 42 The values of ΔGH* for (111), (021), (210), (002), (032), (211), (212), (300) and (321) surfaces of the Ni2P, and (111) surface of the NiO are −0.10, +0.19, +0.21, +0.42, +0.13, −0.03, 10


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−0.16, +0.08, +0.36 and −1.16 eV, respectively, (Figure 2d). The results show the values of ΔGH* on the (211) crystal plane of Ni2P is closest to zero, which indicates the (211) plane of Ni2P could be the most active site for HER.

Figure 2. Structure characterizations of the Ni2P NSs-NF electrode. (a) XRD pattern of the Ni2P NSsNF electrode. (b) XPS of Ni 2p. (c) XPS of P 2p. (d) Calculated adsorption free energy of H (ΔGH*) of the (111), (021), (210), (002), (032), (211), (212), (300) and (321) crystal planes of Ni2P, and (111) crystal plane of NiO.

The HER performance of Ni2P NSs-NF electrode was explored. In contrast, the 20 wt% Pt/C catalysts loaded upon the nickel foam (Pt/C-NF), NiO NSs-NF and NF were also tested. Figure 3 shows the HER performance of above electrocatalysts in the alkaline condition. Figure 3a shows the LSV curves of these catalysts under 1 mV s-1. The Ni2P NSs-NF electrode has an obvious superiority compared 11



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with the NiO NSs-NF, NF and other reported transition metal based electrocatalysts (Table S1). The Ni2P NSs-NF electrode only requires the overpotential of 89 mV at 10 mA cm-2, much lower than those of the NiO NSs-NF (218 mV) and NF (266 mV) (Figure 3b). To get 20 and 100 mA cm-2, it needs 113 and 181 mV of overpotentials for the Ni2P NSs-NF, which are lower than those of the NiO NSs-NF (245 and 331 mV) and NF (301 and 413 mV). We also have investigated the corresponding current densities under an overpotential of 200 mV, and Figure 3c shows the results. The current densities of the Pt/C-NF and Ni2P NSs-NF are 244.9 and 144.6 mA cm-2, respectively. However, the NiO NSs-NF and NF are as low as 8.2 and 2.0 mA cm-2, respectively. Generally, Tafel slopes can be used to explore the reaction kinetics for HER. Figure 3d shows the Tafel figures of Pt/C-NF, Ni2P NSs-NF, NiO NSs-NF and NF. The Tafel slope of Ni2P NSs-NF is 82 mV dec-1, which is smaller to those of the NiO NSs-NF and NF. The value of the Tafel slope of the Ni2P NSs-NF electrode suggests the HER conforms to the Volmer-Heyrovsky mechanism, which can be described as following: 44 Volmer step: * + H2O + e - → * H + OH - (1) Heyrovsky step: H2O + e - + * H → H2 + OH - (2) The ∗ represents a site on the electrode surface. Furtherly, extrapolating the linear fit of the Tafel plot, the exchange current densities (J0) are received. Figure 3e shows the J0 of Ni2P NSs-NF is 0.082 mA cm-2, much larger than those of the NiO NSsNF and NF, which suggests the Ni2P NSs-NF possesses faster charge transfer reactions.45 Another important factor in evaluating the electrocatalyst is stability. Therefore, we adopted the accelerated degradation test (ADT) and chronopotentiometry (CP) to measure the stability of the Ni2P NSs-NF. The ADT was employed incessantly for 5000 and 10000 cycles from 0 to −0.6 V under 100 mV s-1. As exhibited in Figure 3f, even after 5000 and 10000 cycles, the curves are almost overlapped with 12


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the initial one, indicating excellent cycling stability. Benefitting from numerous in-plane nanopores, the electrolyte permeation and hydrogen molecules release can be accelerated, which can maintain the binding stability of nanosheets and ensure the sufficient contact between electrolyte and nanosheets. The stability of the Ni2P NSs-NF was also investigated for 20 h by chronopotentiometry (CP) at 10 mA cm-2 (Figure S8), it only increases by 7 mV of the potential during the whole process. Furtherly, the Ni2P NSs-NF was measured for 20 h under 50 mA cm-2 (Figure S9). The potential has no obvious change, suggesting excellent durability. The morphology and composition changes after HER were investigated by SEM and XPS. Figure S10 shows that the Ni2P NSs-NF basically maintains its 3D nanostructure after HER test in alkaline condition. In addition, Figure S11 shows the XPS spectra of the post-HER Ni2P NSs-NF. The binding energy around 852.9 eV is related to NiP, and the binding energy at 129.1 corresponds to the P 2p3/2 as well as 129.9 eV to P 2p1/2. These are similar with the fresh sample. The binding energy corresponding to Ni-O (856.1 eV for Ni 2p3/2 and 873.9 eV for Ni 2p1/2) and P-O (133.9 eV) shifts after HER, which is possibly due to the reduction of surface oxidation during long-time potential cycling.39

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Figure 3. HER performance in the alkaline condition. (a) Liner sweep voltammetry (LSV) curves. (b) Histograms of the overpotentials under different current densities. (c) Histograms of the current densities under an overpotential of 200 mV. (d) Tafel slopes. (e) Exchange current densities (J0). (f) LSV curves of Ni2P NSs-NF before and after 5000, 10000 cycles.

The HER performance of Ni2P NSs-NF electrode in acidic condition was also employed. For comparison, the Pt/C-NF and NF electrodes were tested as well. The LSV curves of these catalysts at 1 mV s-1 are shwn in Figure 4a. The Ni2P NSs-NF electrode has an obvious superiority compared with the NF and other reported transition metal based electrocatalysts (Table S2). The Ni2P NSs-NF merely needs the overpotential of 67 mV to drive 10 mA cm-2, much lower than those of the NF (338 mV) (Figure 4b). To drive the 20 and 100 mA cm-2, the Ni2P NSs-NF only requires the overpotentials of 86 and 154 mV, respectively. Under the overpotential of 200 mV, the Ni2P NSs-NF electrode displays a current density of 183.6 mA cm-2, much higher than the NF (0.99 mA cm-2) and close to the Pt/C-NF (237.2 mA cm-2). Tafel slopes are also used to investigate the reaction kinetics for HER in the acidic condition. Figure 4d exhibits the Tafel plots of the Pt/C-NF, Ni2P NSs-NF and NF. The Tafel slope of Ni2P NSs-NF is 57 mV dec-1, which approaches to that of the Pt/C-NF (36 mV dec-1) 14


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and is much superior to NF (119 mV dec-1). The slope value of the Ni2P NSs-NF is in the 40−120 mV dec-1, indicating the HER conforms to the Volmer-Heyrovsky mechanism.44 In addition, the J0 is obtained and exhibited in Figure 4e. The J0 of Ni2P NSs-NF is 0.668 mA cm-2, much larger than that of NF (0.014 mA cm-2). The stability of Ni2P NSs-NF is also investigated by ADT and CP. After 10000 cycles from 0 to −0.6 V at 100 mV s-1, there was a minimal change for the LSV curves (Figure 4f). The ability of the Ni2P NSs-NF was also confirmed by CP measurement for 20 h under 10 mA cm-2 (Figure S12). At the initial stage, the potential value decreases gradually, which indicates that the catalyst undergoes activation.46,47 After that, the potential keeps stable at the value about −0.067 V. Furtherly, the Ni2P NSs-NF was further measured for 20 h at 50 mA cm-2 (Figure S13). The potential also remains steady, suggesting excellent stability. Though the Ni2P NSs-NF electrode suffered from long-time HER test in acidic condition, the NF skeleton hasn’t been damaged obviously (Figure S14). Figure S15 shows the XPS spectra of the Ni2P NSs-NF electrode after HER in acidic condition. The binding energy located at 852.9 eV is related to the Ni-P, and the 129.1 and 129.9 eV correspond to the P 2p3/2 and P 2p1/2, respectively. The results are similar with the fresh sample. In addition, the binding energy corresponding to Ni-O (855.6 eV for Ni 2p3/2 and 872.7 eV for Ni 2p1/2) and P-O (133.9 eV) shifts and the intensity decreases, which could be possibly attributed to the reaction with the sulfuric acid.

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Figure 4. HER performance in the acidic solution. (a) LSV curves. (b) Histogram of the overpotentials at different current densities. (c) Histogram of the current densities at the overpotential of 200 mV. (d) Tafel plots. (e) The exchange current densities (J0). (f) LSV curves of Ni2P NSs-NF recorded before and after 5000, 10000 sweeps.

Electrochemical impedance spectroscopy (EIS) on these catalysts were performed to further explore the kinetics of these catalysts for HER in both alkaline and acidic conditions (Figure 5a and b). An overpotential of 200 mV was applied to obtain the EIS, and all impedance spectra have been fitted by the modified Randle equivalent circuit model, including Rs (the resistor between working and the reference electrode), R1 (the surface porosity at high frequencies) and Rct (the charge transfer at low frequencies) (Figure 4c). The Rct values are summarized and shown in Figure 5d. The Rct values of the Ni2P NSs-NF is 1.7 and 0.52 Ω in the alkaline and acidic conditions, respectively, which are a little bit higher than Pt/C-NF (0.55 and 0.32 Ω), and much smaller than those of the NF (17.8 and 100 Ω) and NiO NSs-NF (7.8 Ω in alkaline condition). The results indicate the Ni2P NSs-NF has faster electron transfer kinetics for HER in both alkaline and acidic conditions. Generally, the electrochemical active surface area (ECSA) value is proportional to the electrochemical double-layer 16


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capacitance (Cdl).41 The cyclic voltammograms in a non-Faradaic area were measured to get the Cdl. Figure S16 shows the capacitance of the Ni2P NSs-NF electrode is determined to be ca. 11.3 mF cm-2 in alkaline solution while the value is 19.4 mF cm-2 in acidic solution. It further proves that the distinct electrode/electrolyte feature plays a significant role in HER.48

Figure 5. EIS analysis of various electrodes. (a) Nyquist plots in the alkaline condition. Inset: the full-range Nyquist plots. (b) Nyquist plots in the acidic condition. Inset: the full-range Nyquist plots. (c) Modified Randle equivalent circuit models. (d) Histograms of Rct of the samples.

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CONCLUSIONS In summary, we designed freestanding hierarchical mesh-like Ni2P nanosheet arrays on nickel foam through a facile method for efficient and stable HER both in alkaline and acidic conditions. The 3D open nanostructures of the Ni2P nanosheet arrays with numerous in-plane nanopores can facilitate electrolyte ion diffusion and hydrogen molecules release. Additionally, abundant active edges on the Ni2P NSs can offer more active sites for HER. As a result, the Ni2P NSs-NF electrode requires low overpotentials of 89 and 181 mV at 10 and 100 mA cm-2, respectively, and possesses a low Tafel slope of 82 mV dec-1 in the alkaline condition. For the HER of the Ni2P NSs-NF electrode in the acidic condition, low overpotentials of 67 and 154 mV are needed to reach 10 and 100 mA cm-2, respectively, and the Tafel slope is only 57 mV dec-1. Moreover, the self-supported Ni2P NSs-NF electrode displays excellent cycling stability toward HER both in alkaline and acidic conditions even after 10000 cycles. The facile yet high-performance hierarchically porous Ni2P NSs-NF electrode holds great potential for the efficient large-scale hydrogen generation.

ASSOCIATED CONTENT Support Information Experimental sections containing chemicals and materials, electrochemical measurements and characterization; supplementary figures containing theoretical, morphological, structural, and electrochemical data for NF, Ni(OH)2-NF, NiO NSs-NF and Ni2P NSs-NF, and tables of the performance comparison with other published electrocatalysts.

AUTHOR INFORMATION Corresponding Authors 18


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*E-mail: [email protected]. *E-mail: [email protected]. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS We appreciate the financial support of this study by the National Natural Science Foundation of China (51572052 and 21403044).

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SYNOPSIS TOC The hierarchical edge-Rich Ni2P NSs-NF electrode shows efficient electro-catalytic performance for hydrogen evolution in both alkaline and acidic conditions.

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Hierarchical Edge-Rich Nickel Phosphide Nanosheet Arrays as

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