Ni phosphide amorphous shell

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A unique Ni crystalline core/Ni phosphide amorphous shell heterostructured electrocatalyst for hydrazine oxidation reaction of fuel cells Jin Zhang, Xinyue Cao, Min Guo, Haining Wang, Martin Saunders, Yan Xiang, San Ping Jiang, and Shanfu Lu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 07 May 2019 Downloaded from http://pubs.acs.org on May 7, 2019

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ACS Applied Materials & Interfaces

A

Unique

Ni

Crystalline

Core/Ni

Phosphide

Amorphous

Shell

Heterostructured Electrocatalyst for Hydrazine Oxidation Reaction of Fuel Cells Jin Zhang†,[a], Xinyue Cao†,[a], Min Guo[a], Haining Wang*,[a], Martin Saunders[b], Yan Xiang[a], San Ping Jiang*,[c], and Shanfu Lu*,[a] Beijing Key Laboratory of Bio-inspired Material and Devices & School of Space and Environment, Beihang University, Beijing, 100191, China a

bCenter

for Microscopy, Characterization and Analysis (CMCA), The University of Western Australia, Perth, WA6009, Australia Fuels and Energy Technology Institute & Western Australia School of Mines: Mineral, Energy and Chemical Engineering, Curtin University, Perth, WA6102, Australia c

ABSTRACT: It is highly attractive but challenging to develop transition metal electrocatalysts for direct hydrazine fuel cells (DHzFCs). In this work, a nickel crystalline core@nickel phosphide amorphous shell heterostructured electrocatalyst supported by active carbon (Ni@NiP/C) is developed. Ni@NiP/C with P:Ni molar ratio of 3:100, [email protected]/C exhibits outstanding catalytic activity for the hydrazine oxidation reaction (HzOR) in alkaline solution, achieving a much better catalytic activity (2675.1 A [email protected] V vs RHE) and high stability, as compared to Ni nanoparticles supported on carbon (Ni/C) and Pt/C catalysts. The results indicate that formation of NiP amorphous shell effectively inhibits the passivation of the Ni core active sites and enhances the adsorption of hydrazine on Ni by improving the adsorption energy, leading to the high electrochemical activity and stability of [email protected]/C catalysts for HzOR. The density functional theory calculation confirms the structural and electrocatalytic effect of the core/shell heterostructure on the stability and activity of Ni active sites for HzOR. The unique crystalline core/amorphous shell structured Ni@NiP/C demonstrates a promising potential as effective electrocatalysts for DHzFCs. KEYWORDS: hydrazine oxidation reaction, Ni@NiP/C core/shell heterostructured electrocatalysts, Ni phosphide, amorphous structure, direct hydrazine fuel cells.

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1. INTRODUCTION Direct hydrazine fuel cells (DHzFCs) are promising power devices because of easy storage and transportation of liquid hydrazine fuel, high theoretical cell voltage (1.56 V), absent of CO2 gas emission and CO poisoning.1 Noble metals including Pt and Pd are usually used as effective electrocatalysts for the hydrazine oxidation reaction (HzOR).2,3 However, the scarcity and high cost of precious metal catalysts limit the development and commercial viability of DHzFCs. Moreover, Pt preferentially catalyzes the hydrazine into H2 and N2 in alkaline solutions via a chemical decomposition process, reducing the efficiency of fuel cells. Compared to Pt, transition metal catalysts4-5 including Ni and their alloys can efficiently oxidize hydrazine to nitrogen and water in alkaline solution by a four-electron step process6. Nevertheless, Ni-based electrocatalysts suffer from oxidation after exposed to air and agglomeration during HzOR in alkaline conditions, leading to the deterioration of shelf life and electrocatalytic activity of catalysts over time.7-8 Doping with non-metal elements including phosphorous, nitrogen and boron etc. has been shown to improve the performance of Ni-based electrocatalysts.9 The doping of phosphorous on metal catalysts significantly improves their electrocatalytic activity by altering bond distance, coordination number and electronic structure of metal catalysts.10 This would lead to favourable adsorption and desorption of reactants for reactions such as water splitting and HzOR etc.11 Sun et al. revealed that a crystalline Ni2P nanoarray on nickel foam shows a high catalytic activity for the HzOR in alkaline media.12-13 Nevertheless, it has been revealed that the active form of crystalline Ni2P catalyst for oxygen evolution reaction (OER) is Ni2P/NiOx core-shell structure.14 The intrinsic effect between the core and the shell promotes electrocatalytic OER performance on the surface as well as in the bulk of the heterostructured

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catalysts.15-16 In addition, crystalline metal phosphide has been employed as shell to improve the electrochemical performance of the metal or metal oxide core, such as crystalline [email protected] The surface electronic states of crystalline phosphide can be effectively influenced by a core metal via core-shell structure to tune interaction between phosphide and reaction intermediates to enhance electro-catalytic activity.18 On the other hand, amorphous alloys with short-range ordered structure show lower Tafel slope and overpotential as well as higher stability compared to crystalline metal phosphide,19-20 due to their numerous lowcoordination sites (terraces, steps and corner atoms) and defects in their structure.21,22 Moreover, the amorphous material increases electron and ion conductivity by providing large specific surface area and pathways for ion diffusion23. Thus, heterostructured core-shell catalysts are desirable for efficient hydrazine oxidation reactions. Here we design a unique Ni crystalline core@Ni phosphide amorphous shell heterostructured electrocatalyst supported by active carbon (Ni@NiP/C) for the hydrazine oxidation reaction. The NiP shell effectively protects the Ni core from passivation in air, similar to the effect of segregated graphene layers on metallic Ni catalyst.24 Moreover, the intrinsic effect between the crystalline Ni core and amorphous NiP shell increases the hydrazine adsorption energy and thus the activity for HzOR25. The core-shell structured Ni@NiP/C catalysts demonstrate significantly enhanced electrocatalytic performance for HzOR as compared to Ni nanoparticles (NPs) supported on carbon (Ni/C), supported by the density functional theory (DFT) calculations. 2. EXPERIMENTAL SECTION 2.1. Materials and synthesis NiCl26H2O, NaH2PO2, ethylene glycol, hydrous hydrazine (85 wt%) and KOH were purchased from Beijing Chemical Factory, China. Carbon black (Vulcan XC-72) was obtained from Cabot, USA. Nafion ionomer (5.0 wt%) was purchased from Dupont, USA.

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NiCl26H2O (7.5 mmol) was dissolved in 50.0 mL ethylene glycol at room temperature. Meanwhile, 0.64 g carbon black was dispersed in 50.0 mL ethylene glycol by ultrasound treatment for 30 min at room temperature. The NiCl2 solution was poured to the carbon black dispersion in a 250 mL flask, and then the mixture was heated to 60 oC. After that, 16.0 mL hydrous hydrazine and 20.0 mL KOH solution (10.0 M) were added to the mixture in sequence. Then the mixture was centrifuged after stirring for 15 min. Black powder product was collected and washed by ethanol and deionized water for four times, denoted as Ni/C. The as-synthesized Ni/C catalyst was dispersed in 20.0 mL ethylene glycol by ultrasound treatment. NaH2PO2 was dissolved in 10.0 mL ethylene glycol and then the NaH2PO2 solution was added to the Ni/C dispersion with different P:Ni molar ratios. The mixture was sealed in a Teflon-lined stainless steel autoclave and heated at 180 oC for 3h. After that, the autoclave was cooled down to room temperature. After centrifugation, black powder was collected, wash by ethanol and water for four times and then dried in a vacuum oven at 50 oC for 1 h. The product was denoted as Ni@NiP/C. 2.2. Characterizations The crystallinity of the Ni/C and Ni@NiP/C samples was measured by powder X-Ray diffraction (XRD) at room temperature using a Bruker D8 Advance diffractometer with Cu Kα radiation. The morphology of the samples was measured by high angle annular dark field scanning transmission electron microscopy (HAADF-STEM, FEI Titan G2) with an accelerating voltage of 200 kV. The element compositions and valence states of the catalysts were probed by X-ray photoelectron spectra (XPS) using a Thermo Scientific Escalab 250Xi spectrometer with an Al Kα (1486.6 eV) radiator. 5.0 mg catalyst and Nafion ionomer were dispersed in 1.0 mL isopropanol to form the catalyst ink. The loading of Nafion in the catalyst was 20.0 wt%. Then 5.0 L of the catalyst ink was deposited on a glassy carbon electrode to be used as working electrode. A Pt wire was used as

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the counter electrode while a home-made reversible hydrogen electrode (RHE, pH=14) was used as the reference electrode. 1.0 M KOH solution with 0.1 M hydrazine was employed as the electrolyte. Before the electrochemical tests, the electrolyte was bubbled with nitrogen for 30 min. All electrochemical tests were performed by a CHI760E instrument (Shanghai Chen Hua Instrument Co. Ltd.) Linear sweep voltammetry (LSV) was carried out with a scan rate of 10 mV s-1. Cyclic voltammetry (CV) test was measured with a scan rate of 50 mV s-1. Chronoamperometry (CA) test and electrochemical impedance spectroscopy (EIS) were conducted at 0.4 V (vs. RHE). The electrochemically active surface area (ECSA) of the samples was measured by CV. Briefly, after bubbling the 1.0 M KOH solution for 30 min, the potential of the working electrode was maintained at -0.6 V (vs. RHE) for 30 min. Then CV was conducted between 0.1 to 0.8 V (vs. RHE) until the CV curves became the same. The charge associated with the formation of a monolayer of OH on Ni is known to be 0.514 μC cm−2. The ECSA of samples was calculated as follows: ECSA = △Q/(0.514*mNi)

(1)

where △Q is the charge difference between the first and last scan of CV curves. 2.3. DFT calculation The geometries and electronic property of Ni@NiP/C samples were studied by Density Functional Theory (DFT) calculations with the Vienna Ab Initio Simulation Package (VASP). 26-27

The Ni(111) surfaces was represented by 4-layer slab and 4×4 supercell constructed with

lattice bulk constants. The varied ratio of P was represented by different number of P atoms replacing Ni atoms at the top layer. A vacuum layer of ca. 15 Å was used to eliminate the interaction between periodic slabs. The projector-augmented wave (PAW) formalism implementing the generalized gradient approximation as parameterized by Perdew et al. (PBE) was used. The plan-wave kinetic energy cut-off is 400 eV, and the k-points meshes were set as

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4×4×1. All structures were fully relaxed until the convergence in energy and force reached 1.0×10-5 eV and 1.0×10-4 eV/Å. The convergence tests revealed that all these parameters were sufficient. The adsorptive energy (Eads) of hydrazine molecule on metal surfaces is obtained as in equation (2), Eads=Ehydrazine-metal-Emetal-Ehydrazine

(2)

where Ehydrazine-metal, Emetal, and Ehydrazine are the energies of hydrazine-metal system, metal and the isolated hydrazine molecule, respectively. The adsorption energies were corrected using D3 correction method.28 3. RESULTS AND DISCUSSION

Figure 1 The XRD patterns of Ni@NiP catalyst with different P:Ni ratios. Inset is the enlarged (111) reflection plane of the Ni@NiP samples. 3.1.

Microstructure

of

Ni@NiP

heterostructured

electrocatalysts.

Ni@NiP/C

electrocatalysts were fabricated by a sequential reaction process (Figure S1). First, Ni NPs were directly grown on active carbon by a solution-phase chemical reduction method. Bright field transmission electron microscopy shows that Ni NPs have an average diameter of 13 nm with well-resolved lattice fringes in an interplanar distance of 0.205 nm corresponding to (111) plane of Ni NPs (Figure S2). XRD patterns confirms the face-centred cubic crystalline structure of Ni with peaks at 44.4o, 51.9o and 76.5o, corresponding to (111), (200) and (220) planes of

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Ni NPs (JCPDF#04-0850), respectively (Figure 1). To form the core-shell structure, PH3 derived from dismutation of H2PO2- ions reacted with Ni NPs to form a NiP shell.29 The content of P in the NiP shell was controlled by varying the concentration of P precursor. The P:Ni molar ratio of the Ni@NiP catalysts was measured by a inductively coupled plasma optical emission spectrometry (ICP-OES) analysis, and the samples with P:Ni molar ratio of 2.0:100, 3:100, 4.5:100 and 5.9:100 were obtained (Table S1), which was denoted as [email protected]/C, [email protected]/C, [email protected]/C and [email protected]/C, respectively. XRD patterns of Ni@NiP/C with various Ni/P ratios were similar to that of Ni/C (Figure 1), indicating the similar structure as that of Ni/C.

Figure 2 (A) TEM and (B) HR-TEM images of [email protected]/C core-shell nanoparticles. Inset is the SAED image of the sample. (C) HAADF-STEM image and (D) elemental mapping images of [email protected]/C NPs. Inset in figure C is the FFT image of selected areas. The as-synthesized [email protected] particles on carbon black, [email protected]/C were selected to study the microstructure of Ni@NiP/C catalysts (see Figure 2). [email protected]/C is characterized by irregular morphology and the selected area electron diffraction (SAED) pattern shows the presence of crystallinity in the catalysts (Figure 2A and B). The high resolution transmission 7



electron microscopy (HR-TEM) analysis shows the formation of the Ni@NiP core-shell structure with the Ni core along [111] direction covered by an amorphous layer with average thickness of ca. 4.0 nm (Figure 2C). High angle annular dark field scanning TEM image and elemental mapping provided further evidence of the formation of NiP shell on the Ni core (Figure 2D). The formation of amorphous NiP shell is likely due to the balance between the outward diffusion of Ni ions and inward diffusion of phosphorous and vacancies18. The longrange ordered structure of Ni atoms may be interrupted by the doping of P, resulting in the formation of amorphous structure30-31. Oxygen was found in the EDX spectrum of the sample (Figure S3), indicating the presence of phosphorous-oxygen, carbon-oxygen and metal-oxygen bonds, which is common in the synthesis of metal phosphide catalysts32.

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Figure 3 (A) XPS spectra of Ni@NiP/C and Ni/C NPs. (B) P 2p orbital binding energy and (C) Ni 2p orbital binding energy regions of Ni@NiP/C and Ni/C catalysts. Figure 3 shows the XPS spectra of Ni@NiP/C and Ni/C electrocatalysts. In the case of Ni@NiP/C and Ni/C (Figure 3B), two peaks centred at 855.9 eV and 861.4 eV were observed and can be assigned to Ni oxide species (e.g. Ni2O3 or Ni(OH)2) and the satellite peak, respectively, indicating the surface oxidation of Ni NPs33. In the case of Ni@NiP/C, two new peaks centred at 853.0 eV and 858.0 eV were observed in addition to the peaks of NiOx, which can be assigned to Ni(0).34 The presence of crystalline Ni NPs is also supported by the XRD results as shown in Fig.1. This indicates that the presence of NiP amorphous shell inhibits the

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oxidation of Ni core. The protection effect is likely due to the fact that the doping of P atoms on the surface of Ni NPs decreases the oxophilicity in Ni due to the weakened 2 donation from O2 to the Ni atoms.35 With the increase in the P:Ni ratio, the content of Ni(0) on the catalyst surface increases from 6.5 at.% for [email protected] to 9.6 at.% for [email protected], while the binding energy (BE) of Ni(0) slightly shifts from 853.0 eV to 853.3 eV. With the increase in the P content, the BE of P2p (0) increases from 129.8 eV for [email protected] to 130.4 eV for [email protected], a change of 0.6 eV (Figure 3C). The results imply that the introduction of P element can effectively tune the electronic structure of Ni because of the covalent interactions between Ni 3d and P 3p electronic bands.36-37 Since Ni carries a partial positive charge while P carries a partial negative charge,38 the significant down shift of the BE of P indicates that the introduction of P increases the electron transfer from Ni to P.

Figure 4 (A) Linear sweep voltammetry curves of Ni@NiP/C catalysts in 1.0 M KOH + 1.0 M hydrazine solution purged by N2 at a scan rate of 10 mV s-1 and 1600 rpm. (B) Tafel plots of the Ni@NiP/C catalyst for HzOR. (C) Electrochemical impedance spectra of Ni@NiP catalyst at 0.25 V vs RHE for the HzOR. (D) Durability of the Ni@NiP catalysts with a constant potential of 0.25 V vs RHE at 25 oC.

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3.2. Electrocatalytic activity of Ni@NiP core-shell catalysts for HzOR. The electrochemical property of the Ni@NiP/C, Ni/C and Pt/C electrocatalysts was characterized by linear scan voltammetry in N2-saturated 1.0 M hydrazine + 1.0 M KOH solution (Figure 4). Ni/C showed a very poor activity for HzOR, exhibiting a rapid drop in the current at potential above 0.25 V vs RHE (Figure 4A). The rapid decrease in the current density is related to the Ni oxidation reaction around 0.25 V (Figure S4) due to the formation of higher valence of Ni surface oxygenates via the reaction of Ni surface with OH- in alkaline solution.39 Ni@NiP/C electrocatalysts show a much better electrocatalytic activity for HzOR and the current density for HzOR increases with the polarization potential (Figure 4A). In the case of Ni@NiP/C no oxidation peak was observed at 0.25 V as in the case of Ni/C (Figure S4). This indicates that the NiP amorphous shell efficiently inhibits the oxidation of Ni core, thus preventing the passivation of Ni core catalysts. This is consistent with the TEM and XPS results of the Ni@NiP/C electrocatalysts as shown in Figures 2 and 3. The electrocatalytic activity of Ni@NiP/C depends strongly on the P:Ni ratio and the best performance is obtained on [email protected]/C, achieving a current density of 2675.1 A gNi -1 at 0.25 V vs. RHE for HzOR. This is substantially higher than 875.4 A gNi-1 obtained on Ni/C and 2250.4 A gPt -1 on Pt/C electrocatalysts under identical conditions. [email protected]/C exhibited a smallest hydrazine oxidation onset potential of -0.10 V. The electrocatalytic activity of [email protected]/C is also significantly better than those of electrocatalysts recently reported for HzOR (see Table S2), including Ni-B/C,40 Ni0.6Co0.4,41 Ni0.4Fe0.6, 5 Ni0.5Mn0.5, 5 Ni0.4Zn0.6 5 and Ni0.87Zn0.1342. Tafel slopes of Ni@NiP/C electrocatalysts for HzOR are in the range of 48.7 – 57.3 mV dec-1 (Figure 4B), which is comparable with the Tafel slope obtained on crystalline Ni2P electrode on Ni foam for HzOR.12 The Tafel slope for the reaction also depends strongly on the P:Ni ratio in Ni@NiP/C with the lowest value of 48.7 mV dec-1 observed for [email protected]/C,

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lower than 55.8 mV dec-1 obtained on the Ni/C catalysts. The low Tafel slopes of Ni@NiP/C catalyst demonstrates the fast kinetics of the HzOR as compared to that on Ni/C catalyst. To further study the catalytic kinetics, electrochemical impedance spectroscopy (EIS) was measured at 0.25 V vs RHE (Figure 4C). The high frequency intercept in Nyquist plots corresponds to the ohmic resistance, while the differences between the low and high frequency intercepts can be assigned to the charge transfer resistance of the catalyst for HzOR.43 Consistent with the activity, [email protected]/C shows the lowest charge transfer resistance. Besides, [email protected]/C also exhibits a significantly better stability measured at 0.25 V (vs. RHE) among the electrocatalysts studied (Figure 4D).

Figure 5 The LSV curves of (A) Ni/C and (B) Ni@NiP catalyst in 1.0 M KOH and 1.0 M hydrazine solution with a scan rate of 10 mV s-1. (C) The Arrhenius plots of Ni/C and [email protected]/C at 0.2 V. 12


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Figure 5 shows the electrocatalytic activity of Ni/C and [email protected]/C for HzOR at different temperatures. With increasing the operating temperature of HzOR, the current density for HzOR increases while the onset potential decreases. More specifically, the current density of Ni/C at 0.2 V increases from 861 A gNi-1 at 40 oC to 1604 A gNi-1 at 80 oC (Figure 5A). The onset potential decreases from 0.013 V at 40 oC to -0.047 V at 80oC. These results indicate that increasing temperature tends to enhance the kinetics of HzOR. Compared to Ni/C, the current density of [email protected]/C at 0.2 V increases from 3049 A gNi-1 at 40 oC to 4653 A gNi-1 at 80 oC (Figure 5B), which is 2.54 and 1.84 times higher than that of the Ni/C at the same test condition, respectively. The onset potential decreases from -0.102 V at 40 oC to -0.146 V at 80 oC. We employed Arrhenius equation, k=Aexp(-Ea/RT), to calculate the activation energy of catalyst for HzOR, where k is rate constant, A is frequency factor, Ea is activation energy, R is gas constant and T is absolute temperature. According to the equation, the activation energy of [email protected]/C is 11.6 kJ mol-1, lower than 13.3 kJ mol-1 for the reaction on Ni/C catalyst (Figure 5C). The results indicate that the formation of NiP shell significantly increase the kinetics of HzOR for Ni.

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Figure 6 (A) DFT calculation model for the adsorption of hydrazine molecules on the surface of Ni@NiP catalyst. Adsorption energy of NiP shell for the (B) O atom and (C) hydrazine molecules. (D) Electro-catalytic activity of Ni@NiP/C catalysts in 1.0 M KOH + 1.0 M hydrazine solution purged by N2 at 0.25 V vs RHE with a scan rate of 10 mV s-1 and 1600 rpm. 3.3. Promotion mechanism of NiP shell for HzOR. To fundamentally understand the promotion mechanism of P doping or NiP shell, we employed DFT calculations to investigate the effects of P content in the core-shell catalyst on the adsorption of O on Ni (Figure S5A) and NiP (Figure S5B) surface. Different number of Ni(111) atoms of the top layer was replaced by P atoms to denote NiP layers with varied P ratio (Figure 6A). DFT calculation indicates that the O atom adsorption energy of Ni on NiP shows U shape dependence against the doping level of P with the lowest value of -0.87 eV for NiP at a doping level of 4/16, while the value is 1.08 eV for pristine Ni atoms, as shown in Figure 6B. When half of Ni atoms are replaced by P (i.e., doping level of 8/16), the adsorption energy of O in Ni atom could not be obtained as the O atoms attach on Ni to form NiO species. In addition, amorphous nickel phosphides films are immune to attack at grain boundaries when the content of P is low.44 Thus, the doping of P on Ni decreases adsorption of O on Ni and thus inhibits the passivation effect of Ni metal45. Moreover, the outer layer electron density of Ni increases from 0 eV to 0.12 eV when 6 of 16 Ni atoms were replaced by P atoms (Table S4). That is consistent with the higher peak shift of BE of Ni, as shown in Figure 3A. DFT calculations show that the adsorption energy of NiP surface for hydrazine molecules shows bell shape with increasing the content of P in the surface layer with an adsorption energy of -1.92 eV for Ni12P4 (Figure 6C). The trend is in consistent with the performance of these core-shell catalysts for HzOR (Figure 6D), indicating that the adsorption energy is the dominated factor for HzOR on Ni based catalysts. Agusta et al. revealed that the hydrazine molecules are firstly adsorbed on the surface of Ni-based catalyst, while the co-adsorption with OH from the alkaline solution promotes the dehydrogenation reaction of N2H446-47. In addition,

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Strasser et al. proposed that the existence of a hydrazine surface adsorbate serves as a reactive intermediate for the electrochemical HzOR, while the hydrazine adsorbates tend to absorb on the surface sites or defects on the Ni based catalysts.41 Thereby, the presence of defects in the amorphous NiP shell could aid the effective adsorption of hydrazine onto the catalytic surfaces,48 leading to efficient oxidation of N2H4 to N2 and H+ that is transformed to water by reacting with OH- in the alkaline solution. A 150

B 150 1st cycle 10th cycle

1st cycle 10th cycle

100

Current  A

100

Current / A

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


50

0

-50 0.0

50

0

0.2

0.4

0.6

-50 0.0

0.8

0.2

0.4

0.6

0.8

E / V vs RHE

E / V vs RHE

Figure 7 The 1st and 10th cyclic voltammetry curves of Ni and [email protected]/C during the CV test at a N2 purged 1.0 M KOH solution with a scan rate of 10 mV s-1. Before the CV test, the samples were reduced at -0.6 V vs RHE for 30 min in the solution. In addition, the electrochemical surface area (ECSA) of Ni/C calculated from CV curves is 7.61 m2 g-1, while the ECSA of [email protected]/C calculated from CV curves is 9.6 %, as shown in Fig.7. The ECSA of [email protected]/C is actually lower than that of Ni/C catalysts despite the significantly higher activity of the [email protected]/C catalyst for HzOR as compared to that of Ni/C (see Fig.4). This clearly indicates that the high electrocatalytic activity of Ni@NiP/C is not due to the high ECSA. Also, orthophosphate, which is associated with the peak around 133.0 eV (see Figure 3) might exist in the Ni@NiP/C catalysts, but the electrocatalytic activity of orthophosphate for the HzOR activity is negligible.49 Consequently, the higher electrocatalytic activity of Ni@NiP core-shell catalyst is likely due to the intrinsic effect between Ni and P atoms including electronic effect, geometric effect and bifunctional mechanism, etc.50 The

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amorphization could increase the activity through the influence on the atomic and electronic structures in crystalline materials.51-52 Sun et al. proposed that the good performance of a crystalline Co(ZnxNi2-x)O4/amorphous Ni heterostructured catalyst for OER is due to the unique architecture with synergistic effects of amorphous shell and atomic structure of crystalline core.25 In addition, the electron may transfer from the metal core to the shell structure (Fe@C, Co@C and FeNi@C etc.),53-55 resulting in the increase of the Femi level energy and consequent inducing promoted charge transfer to adsorbates and strength the chemical adsorption.56 Wang et al. concluded that the metal-phosphide electron interactions in the crystalline Fe@FeP core-shell structure influence the binding of hydrogen atoms on the phosphide surface and enhanced activity for hydrogen evolution reaction.18 Thereby, hydrazine molecules could adsorb more strongly on the Ni@NiP surface, consequently enhancing the HzOR electrocatalytic activity on the crystalline Ni core/amorphous NiP shell heterostructured catalysts. 4 Conclusions In summary, crystalline Ni core/amorphous NiP shell heterostructured catalysts were successfully fabricated and the catalytic performance for hydrazine oxidation reaction was investigated. Crystalline Ni and amorphous NiP core/shell catalysts exhibited excellent activity, low Tafel plot and low charge transfer resistance than Ni/C with the optimized structure of [email protected]/C catalysts. The intrinsic effect between metal and P atoms contribute to increase the electro-catalytic activity of Ni@NiP core-shell catalyst. Moreover, DFT calculation demonstrates that the doping of P on Ni surface decreases the electron density of Ni atoms, leading to the weak interaction of O atom and Ni atom. In addition, it also increases the adsorption of hydrazine molecule on Ni atoms, leading to higher electrochemical activity and stability of HzOR for [email protected]/C catalyst than Ni/C and even Pt/C. The current work shows

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the unique crystalline Ni@amorphous NiP heterostructure catalysts holds promising potential application as anode catalysts for direct hydrazine fuel cells. ASSOCIATE CONTENT Supporting Information Tables S1 lists the results of ICP-OES. Table S2 lists the performance of the samples in this work and data from reports. Tables S3 lists the ECSA and Table S4 shows the adsorption energy data by DFT. Figure S1 to S4 show the procedures for the synthesis of Ni/NiP samples, TEM images, EDS curve and electrocatalytic performance of samples in the KOH solution. Figure S5 shows the DFT calculation model for the Ni and Ni@NiP samples. AUTHOR INFORMATION Corresponding authors *Email: [email protected] (H.L.W.) *Email: [email protected] (S.P.J.) *Email: [email protected] (S.F.L.) Author Contributions All authors have contributed to the writing of this work and have given approval to the submission of this manuscript. J.Z. and X.Y.C. contribute equally to this work. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was financially supported by the Key Research and Development Program of Beijing (Z171100000917011), National Natural Science Foundation of China (No. 21503010, 21722601), the Fundamental Research Funds for the Central Universities and the Australian Research Council under Discovery Project scheme (project No. DP150102044, DP180100731 and DP180100568).

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For Table of Contents Use Only

The formation of NiP amorphous shell effectively inhibits the passivation of the Ni core active sites and enhances the adsorption of hydrazine on Ni by improving the adsorption energy.

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Ni phosphide amorphous shell

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