Insight into the Superior Electrocatalytic Performance of Ternary Nickel

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Insight into the Superior Electrocatalytic Performance of Ternary Nickel Iron Poly-Phosphide Nanosheet Array: an X-ray Absorption Study Xiaodeng Wang, Mingyu Pi, Dingke Zhang, haiyun li, Jiajia Feng, Shijian Chen, and Jinhua Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b22114 • Publication Date (Web): 28 Mar 2019 Downloaded from http://pubs.acs.org on March 28, 2019

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ACS Applied Materials & Interfaces

Insight into the Superior Electrocatalytic Performance of Ternary Nickel Iron Poly-Phosphide Nanosheet Array: an X-ray Absorption Study Xiaodeng Wang a, Mingyu Pi b, Dingke Zhangb, Haiyun Lia, Jiajia Fenga, Shijian Chena,*, Jinhua Lic,* a Chongqing

Key Laboratory of Soft Condensed Matter Physics and Smart

Materials, College of Physics, Chongqing University, Chongqing, 401331, China b College

of Physics and Electronic Engineering, Chongqing Normal University, Chongqing, 401331, China

c

International Joint Research Center for Nanophotonics and Biophotonics, School of Science, Changchun University of Science and Technology, Changchun, Jilin Province, China, 130022

*Corresponding author: Prof. Shijian Chen, E-mail: [email protected], Tel: +86-2365678362; Prof. Jinhua Li, E-mail: [email protected].

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Abstract Though ternary components and doping with foreign atoms have been widely studied to enhance the electrocatalytic performance of transition metal phosphides, the underlying mechanism is not clear. Here, we fabricated a ternary Ni-Fe-P nanosheets on carbon fiber paper as efficient electrodes and studied the local atomic and electronic structure alteration through X-ray absorption spectroscopy. The optimized ternary NiFe-P nanosheets electrode exhibited superior hydrogen evolution activity and stability in 0.5 M H2SO4, with a low overpotential of 56 mV at 10 mA cm-2. X-ray absorption spectroscopy study revealed that with the Fe ions incorporation into the system, the NiP bonds elongated and less electrons transferred from Ni to P which resulted in a reduced oxidation state of Ni and reduced the interaction between hydrogen atom and the catalyst surface. Our work not only demonstrate the future potential of highperformance electrocatalyst based on ternary Ni-Fe-P but also offer a promising method to explore the unique synergistic effect in ternary compounds.

KEYWORDS: HER, Nickel Diphosphide, Nanosheets, Electrocatalysis, Absorption

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X-ray

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1. Introduction Nowadays, the exhausted fossil energy source, the continuously increasing of energy demands and the growing environmental crisis have prompted human beings to search for sustainable and renewable energy alternatives as future energy carriers. H2, as a promising clean energy carrier, has been considered to be an effective way to meet the growing demand for energies.1-5 Electrochemical water electrolysis is an extensive and efficient method to produce hydrogen fuel. Although rare metals (e.g., Pt, Ir and Ru) based electrocatalysts possess excellent hydrogen evolution reaction (HER) performance, the high cost and scarcity of noble metals are seriously detrimental to the large-scale commercial application of clean energy technology.5 Even more unfortunately, noble-metal-based electrocatalysts suffer from poor durability.3, 6-9 Thus, it is urgent that earth-abundant and environmentally friendly elements are used for developing high-efficient, low-cost, and durable HER catalysts. For HER, the traditional transition metal phosphides, selenides, and sulfides were used as promising alternative electrocatalysts, and much associated work had been done to improve the HER active.5, 10-14 Up till now, transition-metal phosphides, especially MoP2, FeP, CoP, Co2P, NiP2, CoP3, Fe2P, Fe3P and Ni2P,10, 15-25 possessed excellent stability and high electrochemistry performance within a wide pH range, which can be regarded as the possible alternative catalysts toward water splitting. Recently, to further enhance the water splitting performance, ternary transition metal phosphides have been explored in electrocatalytic research and much relevant work has been carried out. It is particularly noteworthy that electrocatalytic activities of ternary transition metal phosphides are highly depended on metal ratio, such as Co/Ni, Co/Fe, Ni/Fe, Mo/Ni, Fe/Mn, which benefits from the unique electrical conductivity, charge transfer, unique 3



electronic structure and synergistic coupling effect.26-33 Much work has been focused on optimizing metal ratios to enhance electrocatalytic active. For example, Sun et al. reported that CoP-Fe possessed lower adsorption energy barrier and exhibited excellent HER performance than CoP, which was attributed to the modified electronic structures by Fe-doped.34 Qu et al. reported that Co-doped Ni2P catalysts hybrided with reduced graphene exhibited outstanding electrocatalytic active for water splitting when the Ni/Co ratio is 1:1,27 which was profited from plenty of surface active sites and the excellent conductivity of hybrid electrodes. Though ternary components and doping with foreign atoms have been widely studied to enhance the electrocatalytic active of transition metal phosphides, the mechanism for the enhanced catalytic performance is not clear. Therefore, investigations on the local coordinate changes and electronic structure alteration of ternary metal phosphides may provide new insights into the catalytic mechanisms. In this work, we synthesized iron nickel poly-phosphide nanosheets on carbon paper (Ni-Fe-P) by one-step Ni-Fe nanosheets precursor phosphating and further probed the local coordinate and electronic structure by X-ray absorption spectroscopy. Benefiting from the controllable composition, the optimized ternary Ni-Fe-P nanosheets electrode possessed excellent electrocatalytic performance and outstanding durability. Only 56 mV of overpotential in 0.5 M H2SO4 is required for optimized ternary Ni-Fe-P nanosheets to reach 10 mA cm−2, which possesses much better electrocatalytic performance than pure NiP2 under identical conditions. The XANES, XPS and DFT calculations indicate that the local coordinate and electronic structure of NiP2 has been changed. The above analysis indicated that electronic structure engineering of the NiP2 catalyst by the strategy of Fe-doping has triggered the synergistic effect to enhance the catalytic kinetics, resulting in improved electrocatalytic performance. 4 ACS Paragon Plus Environment

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2. Experimental 2.1 Materials Carbon fiber paper (CP) was provided by Shanghai Dongli Corp. Iron nitrate nonahydrate (Fe(NO3)3·9H2O), Ammonium fluoride (NH4F), Urea (CON2H4), and Nickel nitrate hexahydrate (Ni(NO3)2·6H2O)were bought from Tianjin Fuchen Chemical Reagent Factory. Pt/C was bought from Alfa Aesar Chemicals Co. Ltd. Potassium hydroxide (KOH), Sulfuric Acid (H2SO4), Sodium phosphate dibasic dehydrate (Na2HPO4), Sodium dihydngen phosphate anhydrous (Na2HPO4·2H2O), and ethanol (C2H5OH) were bought from Beijing Chemical Corp. Nafion were bought from Sigma Aldrich Chemical Reagent Co., Ltd. All reagents were not further purified. 2.2 Preparation of Ternary Ni-Fe-P nanosheets and Pt/C In a typical procedure, CO(NH2)2 (50 mmol), Fe(NO3)2·9H2O (5mmol), NH4F (40 mmol) and Ni(NO3)2·6H2O (5 mmol) were dissolved in 180 mL distilled water with magnetic stirring over 1 h. In addition, CP (about 3 cm× 2 cm) was treated at 450 oC for 1 h in a muffle, and ultrasounded in dilute HNO3 solution, then cleaned with ethanol and deionized water for 0.5 h repeatedly. The transparent precursor solution and CP were poured into a 50 mL Teflon-lined autoclave and heated in a muffle at 100 °C for 8 h. Then, the CP grown with Ni-Fe hydroxide were rinsed by using deionized water repeatedly, and dried under air at 50 °C for one day. 25 mg red phosphorus and Ni-Fe hydroxide were sealed in a quartz tube at a vacuum of 10-4 Pa, which was heated at 550 oC

for 5 h. The products were denoted as NiP2-Fe-2. In addition, the initial ratio of Ni

and Fe source in the preparation of Fe-Ni hydroxide were also tuned as 1:0, 3:1 and 1:3, denoted as Ni(OH)2, Ni(OH)2-Fe-1 and Ni(OH)2-Fe-3. Then, the Ni-Fe hydroxide was heated at 550 °C for 5 h with red phosphorus in the sealed quartz tubes, and the final 5



products were denoted as NiP2, NiP2-Fe-1 and NiP2-Fe-3, respectively. 50 µL nafion solution and 20 mg Pt/ were dispersed in 1 mL water/ethanol solvent to prepare Pt/C, and then deposited on CP with a loading mass of 1.5 mg cm−2. 2.3 Characterization The phase structure of the composites was tested by using X-ray diffraction (XRD) patterns (Rigaku D/MAX2500PC). The microstructure and morphological of samples were measured by Field emission scanning electron microscopy (FE-SEM, TESCAN MIRA3) with an energy dispersive X-ray spectrometer (EDS) and transmission electron microscopy (TEM, FEI TECNAI G2 F20). X-ray photoelectron spectroscopy (XPS) was performed on a Thermo ESCALAB 250Xi. X-ray absorption spectroscopy was detected on the 1W1B beamline at the Beijing Synchrotron Radiation facility (BSRF) with 2.5 GeV energy and 250 mA storage current. The obtained EXAFS dates were treated by using ATHENA module in the IFEFFIT software packages. 2.4 Electrochemical measurements Electrochemical active of the catalysts were probed by using an electrochemical workstation. A conventional three-electrode cell was adopted, which included reference electrode, counter electrode and working electrode. A saturated calomel electrode was used as the reference and the graphite rob was used as a counter electrode. The Ni-FeP, Pt/C and bare CP were used as the working electrode. Linear sweep voltammetry (LSV) data was measured at the scan rate of 2 mV·s-1 and corrected to compensate for the iR loss and the background current. The durability testing was operated by using the continuous cyclic voltammetry and the chronopotentiometric electrolysis over 20 h. The Nyquist plots were probed in the same configuration at different potentiostatic modes from 0.1 to 105 Hz without iR compensation. 2.5 Theoretical calculation 6 ACS Paragon Plus Environment

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All the calculations based on density functional theory (DFT) were performed at the generalized gradient approximation (GGA) of the Perdew-Burke-Ernzerhof (PBE) form with the projector-augmented wave potentials implemented in the simulation package VASP 35. The structure model was relaxed by setting 0.02 eV/Å in force. A plane-wave energy cutoff is 450 eV because this cutoff was satisfactory for this type of calculation.

17, 36, 37

The surface energies of (111), (101) and (100) facets are 0.059,

0.069, 0.047 J/m2, respectively, and the (100) facet was thermodynamically stable. As calculation of all the possible facets is time-consuming, the (100) facet was selected to explore the HER mechanism. NiP2 (100) slab with a 15 Å thick vacuum space along caxis was used to evaluate the electronic properties and the performance of electrocatalytic active. For Fe-doped NiP2, Ni atoms were replaced by Fe atoms at different sites. The Gibb's free energy change (∆GH*) was counted to reflect the activity of HER and obtained based on the formula of ∆GH* = Etotal - Esurf - EH2/2 + ∆EZPE T∆S.17 EH2 and ∆EZPE are the energy of H2 gas and the change of zero-point energy, respectively. T∆S is the entropy change, which is estimated to be 0.2 eV at room temperature.

3. Results and discussion

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Scheme 1 Scheme of Ni-Fe-P NSs synthesis process on CP. The fabrication process was exhibited in Scheme 1, Ni-Fe-P nanosheets was synthesized via the following two processes: (1) Ni-Fe hydroxide nanosheets was directly grown on CP by hydrothermal method (Fig. S1); (2) Ni-Fe hydroxide nanosheets were annealed with red phosphorus in a vacuum sealed quartz tube to chemically transform to Ni-Fe-P nanosheets. The SEM images of bare CP shows a smooth surface in Fig. S2a. After hydrothermal reaction, the CP was entirely covered with well-defined Ni hydroxide nanosheets and Ni-Fe hydroxide nanosheets. With the incorporation of Fe ions, the Ni-Fe NSs precursor still maintained sheet-like morphology in Fig. S1 and beta-phase structure (JCPDS no. 14-0117) in Fig. S3a, the size of the nanosheet decreased with the increase of Fe content. Besides, after annealed with P, the primal nanosheet morphology was maintained on a large scale (Fig. 1a and Fig. S2). The related element mapping indicated that P, Ni, and Fe elements were uniformly distributed on CP (Fig. S4) and the Ni/Fe ratio of Ni-Fe-P was shown in Table S1. TEM image confirmed the nanosheet morphology (Fig. 1b). HRTEM possessed legible and recognizable lattice fringes with a plane spacing of 0.193 nm, which can correspond to the (220) plane (Fig. 1c). Fig. 1d shows XRD patterns for the NiP2-Fe-2, which contains the diffraction peaks characteristic of CP substrate, cubic structure of NiP2 (JCCDS no. 21–0590, space group: Pa3, a=b=c=5.453) and orthorhombic structure of FeP2 (JCPDS no. 89-2261, space group: Pnnm, a = 2.7293, b = 4.981, c = 5.6652), which were possessed in Fig. S3. In addition, the XRD peak of the Ni-Fe-P remained unchanged with the increase of Fe content, but the peak intensity weakened with the increase in Fe content in Fig. S3b, indicating that the crystallinity of NiP2 became worse, which were ascribed to the fact that the size of the nanosheet decreased with increasing Fe content. All these results indicated that the Ni-Fe-P 8 ACS Paragon Plus Environment

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nanosheets was successfully prepared. As shown in Fig. 2a, the XPS spectrum of the NiP2-Fe-2 suggests the existence of Fe, Ni, and P elements. The Ni 2p spectrum (Fig. S5) can be split into two peaks, the peaks at 874.5 and 856.6 eV assigned to oxidized Ni were corresponding to Ni 2p1/2 and Ni2p3/2, respectively. The peaks at 854.1 eV can correspond to Ni-P in NiP2. The P 2p signal consists of two peaks, which can be ascribed to the peak of P 2p3/2 and P 2p1/2, respectively. The peak at 134.4 eV of P region is corresponding to the oxidized phosphate, which meaning that the surface of NiP2 was oxidized in air. The P 2p spectrum can be indexed to Ni-P in NiP2 at 129.6 and 130.3 eV. The peaks of Fe 2p at 720.4 and 707.8 eV can be indexed to Fe-P in FeP2 and the peak at 712.5 eV is corresponding to the oxidized Fe. Compared with the peak position of pristine NiP2 (Fig. S5), XPS results indicated that both P and Ni peaks of the NiP2-Fe-2 exhibited a small deviation to their elemental valent states, which manifested that Fe doping modified the electronic structures of both Ni and P. The BE of P shifts to that of element P and the BE of Ni exhibits shift to that of Ni metal in the NiP2-Fe-2, indicating enriched electron density around the Ni atoms.

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Fig. 1. SEM (a), TEM (b) and HRTEM (c) images of the NiP2-Fe-2. (d) XRD patterns of NiP2-Fe-2. The HER active was measured in 0.5 M H2SO4 by using the prepared Ni-Fe-P as work electrode. Fig. 2a showed LSV curve for the Ni-Fe-P. For comparison, pristine NiP2 nanosheets, benchmark Pt/C and CP were also tested. As one might expect, the Pt/C electrode presented very high electrocatalytic activity, while CP possessed very poor HER activity. The polarization curves showed that the catalytic activity of the NiFe-P varied with the Fe content from 0 to an optimum value of 0.5, and the onset potential of the NiP2-Fe-2 electrode was 12 mV (Fig. S6), which was markedly lower than that of NiP2-Fe-3 (26 mV), NiP2-Fe-1 (21 mV) and NiP2 (48 mV), but still inferior to 20 wt% Pt/C (7 mV). The NiP2-Fe-2 electrode demanded overpotentials of 56 and 111 mV vs. RHE at 10 and 100 mA cm-2, respectively, which were comparable with the benchmark Pt/C electrode. Compared with pure FeP2 (153mV), NiP2 (107mV) in Fig. S7 and the reported catalysts in Table S2, our NiP2-Fe-2 showed higher HER active among the reported catalysts. Besides, the NiP2-Fe-2 demonstrated the smallest Tafel slope of 49.5 mV dec−1, in comparison with NiP2 (62.1 mV dec−1), NiP2-Fe-1 (58.8 mV dec−1) and NiP2-Fe-3 (60.8 mV dec−1).


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Fig. 2. (a) LSV curves of Ni-Fe-P, CP and Pt/C on CP in acid solution. (b) The corresponding Tafel plots of the Ni-Fe-P and Pt/C on CP. (c) Overpotentials (left) at 10 mA cm−2 and Tafel slopes (right) for the Ni-Fe-P and Pt/C on CP. (d) Cdl values of the Ni-Fe-P (Δj = ja − jc) vs scan rates. (e) The Nyquist plots of the Ni-Fe-P electrocatalysts and equivalent circuit (inset). (f) Durability tests of LSV polarization curve and continual electrolysis over 20 h at the fixed overpotential (inset) for the NiP2-Fe-2. The electrochemical double-layer capacitances of the Ni-Fe-P were measured to explore the high HER activity (Fig. S8). It can be seen that the NiP2-Fe-2 expressed a real capacitance of 55.2 mF cm−2, which was higher than NiP2 (10.3 mF cm−2), NiP211



Fe-1 (20.6 mF cm−2) and NiP2-Fe-3 (27.9 mF cm−2) (Fig. 2d), indicating that the NiP2Fe-2 had the largest specific surface area exposing more active sites for HER among all the tested catalysts. To eliminate the effect of ECSA for activity, we calculate the normalized exchange current density to evaluate the HER performance. The exchange current density (j0) was calculated by extrapolating the Tafel plots in Fig. S9 and we normalized the exchange current density by corresponding Cdl. The normalized exchange current density of the NiP2-Fe-1 (0.231 mA cm−2), NiP2-Fe-2 (0.181 mA cm−2) and NiP2-Fe-3 (0.177 mA cm−2) are higher than NiP2 (0.149 mA cm−2) in table S3, indicating that the higher HER performance of the ternary Ni-Fe-P resulted from enhanced intrinsic activity after Fe doping. In addition, the turnover frequency (TOF) of catalyst was designed to assess the intrinsic catalytic activity.38-40 As shown in Fig. S10, NiP2-Fe-1 (1.22 s-1), NiP2-Fe-2 (1.06 s-1), and NiP2-Fe-3 (0.66 s-1) possessed a higher TOF value than NiP2 (0.41 s-1) at 100 mV, indicating favorable kinetic process by Fe doping. Furthermore, the NiP2-Fe-2 exhibited a very low charge transfer resistance of 2.49 Ω at 100 mV, contrasting to NiP2 (4.81Ω), NiP2-Fe-3 (4.15Ω) and NiP2-Fe-1 (3.20Ω), which indicated a high coulombic efficiency in the HER process for the NiP2-Fe-2 (Fig. 2e and Fig. S11). We also found that the NiP2-Fe-2 exhibited high stability with little current loss for 2000 cycles CV and 20 hours tests without any structural and morphological changes (Fig. 2f and Fig. S12). Besides, we further investigated the HER active of Ni-Fe-P in 1.0 M PBS (pH 7) and 1.0 M KOH (Fig. S13). The optimized NiP2-Fe-2 electrode delivered the best HER performance with lower overpotential of 86 and 104 mV to drive 10 mA cm-2 and exhibited superior durability and high electrocatalytic active in alkaline and acidic solutions, respectively. In addition, our NiP2-Fe-2 showed higher electrocatalytic active in alkaline and acidic solutions for HER than the most reporting catalysts in Table S4 and Table S5. 12 ACS Paragon Plus Environment

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In order to probe the reason for enhanced electrocatalytic performance with Fe incorporation, the atomic structures and coordination modes of the NiP2-Fe-2 and NiP2 electrodes, as well as reference sample Ni foil, were further investigated by XANES and EXAFS (Fig. 3).41-42 The pre-edge peaks of Ni K-edge was corresponding to the electronic transitions from the 1 s orbital to an unoccupied 3d orbital and the white line originates from the 1s to the 4p orbital, respectively.43-44 The Ni K-edge XANES of the NiP2-Fe-2 in Fig. 3a shown that the absorption edge was negatively shifted, and the peak of the white line decreased comparing with the pure NiP2 nanosheets, indicating the lower oxidation state, which was corresponding to XPS results. We also found that the position of Ni absorption edge in the NiP2-Fe-2 was located between NiP2 and Nifoil based on the first derivative XANES, which also indicated that Ni exhibited a reduced oxidation state than that in pristine NiP2. The Fourier-transformed (FT) k3weighted Ni K-edge EXAFS spectra was carried out to further research the structure of NiP2 at atomic level, as exhibited in Fig. 3b. With the incorporation of Fe, the 1st shell Ni-P of NiP2-Fe-2 showed a higher peak intensity and a deviation. The EXAFS fitting was performed, which was exhibited in Table S6 and Fig. S14. It showed that 1st shell Ni-P of the NiP2-Fe-2 contained two different Ni-P scattering paths, but the additive mean coordination number was 6, staying the same with that of NiP2. The bond lengths of 1st shell Ni-P were 2.268 Å and 2.291 Å, respectively. Compared with pure NiP2 (2.276 Å), the bond length of 1st shell Ni-P was smaller. In addition, the bond length of 2nd shell of Ni-P in the NiP2-Fe-2 was 3.478 Å, which possessed a smaller deviation than that of NiP2 (3.476 Å). The Ni-Ni/Fe in the NiP2-Fe-2 possessed similar distance with Ni-Ni in the NiP2 within the error range. The above analysis implied that there was a lattice distortion in the NiP2 after Fe doping. Moreover, the Ni L-edge of the NiP213



Fe-2 showed about 0.2 eV negative shift, which also manifested a longer Ni-P bond and an enriched electron density around the Ni atoms, comparing with that of pristine NiP2 (Fig. S15). The NiP2 lattice would stretch as Ni ions were replaced bigger Fe ions. DFT calculations also obtained an increase of the lattice constant increased after Fe doping, as listed in table S7. Based on the above analysis, we found with the Fe ions incorporation into the system, the Ni-P bonds elongated and less electrons transferred from Ni to P, which enhance the metallicity of Ni to improve the HER performance of NiP2.45

Fig. 3. (a) The Ni K-edge XANES spectra of NiP2-Fe-2 (Inset: first derivatives of the pre-edge region). (b) The Ni K-edge FT-EXAFS of the NiP2-Fe-2. The alteration of local atomic and electronic structures induced by Fe ions incorporation into the system should have great effect on the HER performance of Fedoped NiP2. We also carried out DFT calculations to explore the effect of the atomic and electronic structure changes for the ΔGH* of ternary Ni-Fe-P. The HER process is normally described as three steps, including hydrogen attached to the surface (catalystH* state), the intermediate state (catalyst-H-H*) and the final state (catalyst-H2).35 Fig. 4a show the top view of NiP2-Fe6% (100) schematic model. Here, we calculated the ΔGH* on the (100) surface of the NiP2 and NiP2-Fe. It is well accepted that the smaller |ΔGH*| catalyst should possesses higher electrocatalytic active, which can be 14 ACS Paragon Plus Environment

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considered as a good candidate considered for HER.46 As the free energy diagram (Fig. 4b) has shown, the ΔGH* of NiP2-Fe6% is only 0.02 eV, smaller than those in NiP2Fe9% (-0.20eV), NiP2-Fe3% (-0.10eV) and NiP2 (-0.24 eV), which indicated that proper amount of Fe doping into NiP2 facilitated the H* adsorption kinetics during the HER process.

Fig. 4. (a) Top view of NiP2-Fe6% (100). Orange, Red, and Cyan balls represent P, Fe, and Ni atoms, respectively. (b) The ΔGH* for H adsorption on NiP2-Fe3%, NiP2-Fe6%, NiP2-Fe9% and pristine NiP2 (100).

4. Conclusions In summary, we have demonstrated that metal doping can intrinsically enhance the HER active of NiP2 nanosheets and gained atomic-level understanding of ternary NiFe-P nanosheets electrocatalysts. XPS, XANES, and DFT studies consistently reveal that Fe doping can urge the local coordinate changes and electronic structure alteration of NiP2, which leads to a reduced thermo-neutral ΔGH* and facilitated HER performance. Furthermore, our work shed light on the mechanism of ternary metal poly-phosphides with tunable metal ratio and possess a novel platform to develop lowcost and high-active HER electrocatalysts for clean energy strategy. 15



Associated Contents Supporting Information TOF calculation method; SEM and Optical photograph of Ni(OH)2-Fe; SEM images for CP and NiP2; Large-scale SEM images for NiP2 and the NiP2-Fe-2; EDX elemental mapping for NiP2-Fe-2; XPS spectra of Fe 2p, Ni 2p and P 2p regions for the NiP2-Fe-2 and NiP2; XRD, CVs, EIS, TOF and calculated exchange current density for Ni-Fe-P; LSV curves of the NiP2-Fe-2, NiP2 and FeP2 on CP; XRD pattern and SEM image for NiP2-Fe-2 after HER hydrolysis; LSV and Tafel plots of Ni-Fe-P in alkaline and acidic solutions; LSV of the NiP2-Fe-2 before and after 2000 CV cycles with a chronopotentiometric curve in alkaline and acidic solutions; R space with inverse FTEXAFS Fitting results of the Ni K-edge EXAFS spectra and Ni L-edge spectra for NiP2 and the NiP2-Fe-2; Table of EXAFS fitting parameters, calculated lattice parameters, J0,normalized and the HER performance of TMPs HER electrocatalysts.

Acknowledgements This work is supported by the National Natural Science Foundation of China (grants 51672031), Nanophotonics and Biophotonics Key Laboratory of Jilin Province (20140622009J). We also appreciate the project No. 2018CDJDWL0011 from the Fundamental Research Funds for the Central Universities and the sharing fund of largescale equipment of Chongqing University. We also appreciate the Beijing Synchrotron Radiation facility (1W2A and 1W1B) for characterizations.

References (1) Shi, Y.; Zhang, B. Recent Advances in Transition Metal Phosphide Nanomaterials: 16 ACS Paragon Plus Environment

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Insight into the Superior Electrocatalytic Performance of Ternary Nickel

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