CoP Heterostructures for Highly Efficient Hydrogen

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Energy, Environmental, and Catalysis Applications

Robust NiCoP/CoP Heterostructures for Highly Efficient Hydrogen Evolution Electrocatalysis in Alkaline Solution Hui Liu, Xiao Ma, Han Hu, Yuanyuan Pan, Weinan Zhao, Jialiang Liu, Xinyu Zhao, Jialin Wang, Zhongxue Yang, Qingshan Zhao, Hui Ning, and Mingbo Wu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b00592 • Publication Date (Web): 05 Apr 2019 Downloaded from http://pubs.acs.org on April 5, 2019

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ACS Applied Materials & Interfaces

Robust NiCoP/CoP Heterostructures for Highly Efficient Hydrogen Evolution Electrocatalysis in Alkaline Solution Hui Liu#, Xiao Ma#, Han Hu*, Yuanyuan Pan, Weinan Zhao, Jialiang Liu, Xinyu Zhao, Jialin Wang, Zhongxue Yang, Qingshan Zhao, Hui Ning, Mingbo Wu*

State Key Laboratory of Heavy Oil Processing, Institute of New Energy, College of Chemical Engineering, China University of Petroleum (East China), Qingdao, 266580, (China)

#H.

L. and X. M. contributed equally to this work.

*Corresponding author. Email: [email protected] (M. W.), [email protected] (H. H.)

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ABSTRACT: Electrocatalytic hydrogen evolution reaction (HER), the cornerstone of the emerging hydrogen economy, can be essentially facilitated by robustly heterostructural electrocatalysts. Herein, we report a highly active and stably heterostructual electrocatalyst consisting of NiCoP nanowires decorated with CoP nanoparticles on a nickel foam (NiCoPCoP/NF) for effective hydrogen evolution. The CoP nanoparticles are strongly interfaced with NiCoP nanowires producing abundant electrocatalytically active sites. Combined with the integrated catalyst design, the NiCoP-CoP/NF affords remarkable hydrogen evolution performance in terms of high activity, enhanced kinetics, and outstanding durability in an alkaline electrolyte, superior to most of Co (or Ni)-phosphide-based catalysts reported previously. Density functional theory (DFT) calculations demonstrate that there is an interfacial effect between NiCoP and CoP, which allows a preferable hydrogen adsorption and thus contributes to significantly enhanced performance. Furthermore, an electrolyzer employing the NiCoP-CoP/NF as the cathode and RuO2/NF as the anode (NiCoP-CoP/NF||RuO2/NF) exhibits excellent water splitting activity and outstanding durability, which is comparable with the benchmark PtC/NF||RuO2/NF electrolyzer.

KEYWORDS: hydrogen evolution reaction; electrocatalysis; heterostructure; transition metal phosphides;

water

splitting.


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1. INTRODUCTION With the fast consumption of fossil fuels and growing concerns about environmental issues, the exploration and application of green and renewable energy are now urgently demanded.1 Hydrogen, a clean and sustainable energy with a calorific value of 282 KJ mol-1, is an emerging alternative to the widely employed oil, coal, and gas, whose utilization causes very serious pollution.2,3 Among all the available strategies for hydrogen production, electrochemical water splitting using renewable electrical energy represents the most promising means because of its reliability, stability, and highly pure product.4-6 To facilitate hydrogen evolution reaction (HER), some noble metals such as gold, palladium and platinum have been involved to catalyze the water splitting. Unfortunately, the exorbitant price and limited yields of those metals have hindered the popularization of the HER technology. As a result, it is imperative to design and construct highperformance catalysts utilizing the abundant elements on earth.7-9 In the last several years, numerous research studies have helped to engineer economically feasible catalysts with remarkable progress achieved. Related examples include heteroatomdoped carbon and transition metal compounds, for instance sulfides, selenides, nitrides, carbides, phosphides, etc.10-31 Despite these successes, the practical performance of these alternative catalysts is still in need of improvement to rival their noble metal-based counterparts.32 One feasible solution is to construct heterostructured interfaces between different components. The rational combination of them can regulate the electronic structures and rearrange atoms at the interfaces, thus modulating the binding energy, transformation, and transportation of surface



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species.33,34 For example, Li et al. found that the HER can be efficiently and robustly catalyzed at the interfaces of Ni and WC.35 By interfacing Ni with NiO, Dai and his colleagues demonstrated a significantly enhanced HER performance over the nanoscale Ni/NiO heterostructure, far superior to its counterparts only consisting of either Ni or NiO.36 To maximize the advantage of this kind of elecctrocatalysts, special attention should be paid to their durability and robustness. The turbulence caused by the continuous gas evolution can easily cause the deterioration of multi-component catalysts without strong binding force. Intrinsically, the high fusion of different components is likely to be realized between components with sufficiently low lattice mismatches. Besides, the in-situ production of such multi-component catalysts through single precursor mediated one-step synthesis is also an appealing technology. On the basis of these design rationales, we demonstrated a novel heterostructural electrocatalyst made of NiCoP nanowires decorated with CoP nanoparticles on a nickel foam (denoted as NiCoP-CoP/NF). The NiCo precursor is first synthesized through a facile hydrothermal process which is then transformed into the dual-component catalyst by subsequent phosphorization. The adjacency of Ni and Co in the periodic table secures the sufficiently low lattice mismatch between NiCoP and CoP and the direct transformation from one single NiCo precursor further contributes to the robust interfaces. Because of these structural merits, the overpotential of the NiCoPCoP/NF electrode can reach as low as 73 mV to deliver a HER current density of 10 mA cm-2 in KOH (1.0 M). Moreover, this catalyst delivers superior durability without any deterioration after 5000 catalytic cycles. Density functional theory (DFT) calculations display that the robust


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heterostructured interfaces between NiCoP and CoP possess an appropriate hydrogen adsorption free energy, thus attributing to the excellent performance. In addition, an overall water-splitting electrolyzer employing the NiCoP-CoP/NF as the cathode and RuO2/NF as the anode (NiCoPCoP/NF||RuO2/NF) achieves a current density of 31 mA cm-2 at a given potential of 1.60 V in 1.0 M KOH electrolyte, superior to the benchmark Pt-C/NF||RuO2/NF electrolyzer. 2. EXPERIMENTAL SECTION 2.1. Preparation of NiCo precursor Briefly, 0.582 g Ni(NO3)2∙6H2O, 1.164 g Co(NO3)2∙6H2O, 0.6 g urea and 0.185 g NH4F were added in deionized water (30 mL) under ultrasonication. Ni foam (cut into 1 cm × 4 cm) was sonicated in acetone, alcohol, and deionized water for 15 minutes, orderly. Then the precleaned Ni foam and the solution were transferred into a Teflon-lined stainless steel autoclave (60 mL), and heated at 120 °C for 5 h in a rotating oven to uniformly grow NiCo precursor. After the reaction, the NiCo precursor was thoroughly rinsed with deionized water, and then dried at room temperature for the following experiments. 2.2. Preparation of NiCoP-xCoP/NF, NiCoP/NF, NiP/NF and CoP/NF In a typical preparation of NiCoP-CoP/NF, the as-made NiCo precursor was located in the middle of a quartz tube with NaH2PO2 (1.0 g) at the upstream side near the NiCo precursor. The sample was subsequently heated to 350 °C with a rate of 2 °C min-1 and kept for 2 h in N2 atmosphere. After that, the as-prepared product of NiCoP-CoP/NF was collected for further characterization and test. The loading amount of NiCoP-CoP was about 1.5 mg cm-2, which was



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calculated by the weight variation of NF before and after reaction. For comparison, the NiCoP/NF and other NiCoP-xCoP/NF (denoted as NiCo-0.5CoP/NF and NiCo-2CoP/NF) samples were synthesized in a similar way by adjusting the amount of Co(NO3)2∙6H2O from 4 mmol to 2 mmol, 3 mmol and 6 mmol, respectively. In addition, the monometallic phosphides (denoted as NiP/NF and CoP/NF) were also achieved without adding Co(NO3)2·6H2O and Ni(NO3)2·6H2O, respectively, while keeping other conditions same. 2.3. Physical Characterizations The as-prepared products were characterized with an X-ray diffractometer (XRD, Cu Kα radiation). The morphology of NiCoP-CoP/NF nanowires arrays was studied by a scanning electron microscope (SEM). Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images were recorded on a JEM-2100F transmission electron microscope. Energy dispersive spectroscopy (EDS) analysis was taken on Tecnai TEM with EDS detector under scanning unit electron microscopy (STEM) mode. Element content was measured with inductively coupled plasma-atomic emission spectrometry (ICP-AES). X-ray photoelectron spectra (XPS) analysis was conducted by using an ESCALab 250XI electron spectrometer, in which a monochromatic Al Kα source (hυ = 1486.6 eV) was applied. 2.4. Electrochemical Measurements The HER performance of the as-obtained electrocatalysts was tested using a CHI 760E electrochemical workstation (Shanghai Chenhua) with a classic three-electrode system in N2saturated KOH aqueous solution (1.0 M) at room temperature. The working electrode was one of


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NiCoP-xCoP/NF, NiCoP/NF, NiP/NF, CoP/NF, NiCo precursor, Pt-C/NF and bare Ni foam, and carbon rod was selected as the counter electrode while Ag/AgCl electrode as the reference electrode. The polarization curves were obtained by linear sweep voltammetry (LSV) technique at 5 mV s-1 without iR correction. Electric impedance spectroscopy (EIS) were tested at an overpotential of 200 mV with frequency range of 10-1  105 Hz. The cycling stability was performed in a potential range (+0.1  -0.2 V vs RHE) for 5000 cycles. Time-dependent current density curves were recorded at a constant overpotential of 150 mV over 24 and 100 h, respectively. In addition, the calculation of current densities was based on the area of electrode (1 × 1 cm2) actually immersed into the electrolyte. All of the measured potentials were displayed vs reversible hydrogen electrode (RHE) following the equation: E(RHE) = E(Ag/AgCl) + 0.197 V+ 0.059 × pH. The NiCoP-CoP/NF electrode was behaved as a cathode for the HER and RuO2 supported on the Ni foam (RuO2/NF) was behaved as an anode for the OER in 1.0 M KOH solution in the two-electrode alkaline electrolyzer. For comparison, Pt/C supported on the Ni foam (Pt-C/NF) was also acted as the cathode with RuO2/NF as the anode (Pt-C/NF||RuO2/NF) to drive the overall water splitting process. The loading mass of noble-metal catalysts is approximately 1.5 mg cm-2. 2.5. DFT Computation The density functional theory (DFT) analysis were calculated by the Dmol3 module in the Materials Studio program of Accelrys.33 The generalized gradient approximation (GGA) method with Perdew-Burke-Ernzerhof (PBE) was applied for the exchange-correlation functional.34 The



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core treatment was treated using effective core potential (ECP) and the double-numericalpolarization (DNP) functions basis set was employed.35 The correction of Van der Waals interaction was included using the DFT-D method of Grimme.36 The structures were optimized with 1×10-5 Hartree for energy change, 0.002 Hartree/Å for max force and 0.005 Å for max displacement, respectively. For all the calculations of slab models, 3×3×1 Monkhorst-Pack grid k-points mesh was employed in the Brillouin zone. To avoid periodic interactions, a vacuum space of 15.0 Å was used along the normal direction to the catalyst surface. Correlative theoretical models were built to simulate NiCoP, CoP, and composite NiCoPCoP catalysts phases. Typically, the (001) facet with Co-termination was adopted to act as active surface for NiCoP, which was constructed as a slab with four layers. For CoP, the (011) facet was used in the creation of the slab model with three layers. NiCoP-CoP is composed of NiCoP (001) facet and CoP (011) facet and lattice parameter is 14.9399 Å, 14.8772 Å, 30.0002 Å, respectively. The hydrogen absorption free energy ΔGH* was calculated according to the following formulas: ΔGH* = ΔEH* + ΔEZPE – TΔS

(1)

ΔEH* = E(slab + H*) – (E(slab) + 1/2 E(H2))

(2)

where ΔE(H*), ΔEZPE, T, ΔS, E(slab + H*), E(slab), and E(H2) represent the binding energy, zero-point energy, temperature, the entropy change, the total energy of slab covered with a H, the energy of slab, and the energy of H2(g), respectively. The vibrational entropy of hydrogen in the adsorbed state is negligible, i.e. ΔS = SH* – 1/2 S(H2) ≈ –1/2 S(H2), where S(H2) is the entropy of H2(g) under


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standard conditions. Hence, the total corrections were taken as in37 ΔGH* =ΔEH* + 0.24 eV

(3)

3. RESULTS AND DISCUSSION The typical synthesis strategy of NiCoP-CoP/NF is schematically demonstrated in Figure 1a. The NF was selected as a substrate owing to its high porosity and excellent electrical conductivity (Figure 1b).42,43 Firstly, evenly aligned NiCo precursor nanowires were grown on the NF through the hydrothermal treatment of Ni2+ and Co2+ in the presence of ammonium fluoride and urea (Figure 1c).44 The X-ray diffraction (XRD) pattern reveals that the diffraction peaks of NiCo precursor can be assigned to NiCo2(CO3)1.5(OH)3 (Figure S1), which is similar to Co(CO3)0.5OH·0.11H2O (JCPDS: 48-0083), except they shift a slight to the low diffraction region. Previous reports have shown that the partial substitution of Co2+ by Ni2+ can only change a minor in lattice parameters, but not alter the crystal structure owing to the similarity of Co and Ni atoms,45-48 which may facilitate the formation of robust heterostructural interface. Then, NaH2PO2 was chosen as the P source at elevated temperature phosphating the NiCo precursor. Interestingly, their morphology can be well maintained without deterioration (Figure 1d and Figures S2a,b). Meanwhile, the digital images of NiCoP-CoP/NF (Figures S2c,d) manifest the flexibility of this architecture and the capability of serving as an integrated electrode directly.



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Figure 1. (a) Illustrated scheme of fabrication process for the NiCoP-CoP/NF. SEM images of (b) Ni foam, (c) NiCo precursor, and (d) NiCoP-CoP/NF. Figure 2a shows the XRD pattern of the NiCoP-CoP/NF. The diffraction peaks at 2 = 40.9, 45.0 and 47.4 are readily indexed to the (111), (201) and (210) planes of hexagonal NiCoP (JCPDS: 71-2336), respectively. The existence of CoP is confirmed from the peaks at 2 = 31.6, 36.3, 48.1 and 56.0, which correspond to the (011), (111), (211) and (301) planes of orthorhombic CoP (JCPDS: 29-0497), respectively. The scanning electron microscopy (SEM) image (Figure 2b) shows that vertically aligned NiCoP-CoP nanowires with lengths of several microns and diameters of 40-60 nm are uniformly anchored on the NF. The large active surface area produced by this unique hierarchical nanowire array structure could facilitate the transmission of electrolyte. As a result, significantly enhanced HER activity can be anticipated. For comparison, NiP/NF and CoP/NF were also fabricated in the same way and their structural information is provided in supporting information (Figure S3). To further illustrate the structural details of the NiCoP-CoP, an individual nanowire was investigated by transmission electron microscopy (TEM). In Figure 2c, the average diameter of the NiCoP-CoP is around 50 nm, in accordance with the SEM analysis (Figure 2b inset). The


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enlarged TEM images (Figure 2c inset and Figure S4a) reveal that plenty of nanoparticles of around 5-8 nm are uniformly decorated on the nanowire. As shown in Figure S4b, these nanoparticles have two sets of lattice fringes forming a 90° angle, and the interplanar spacing is around 0.279 and 0.254 nm, belonging to the (002) and (200) crystallographic planes of orthorhombic CoP phase (Figure S5), respectively. In addition, Figure 2d shows the characteristic spacing of 0.335 nm for the (001) plane of NiCoP, while the spacing of 0.283 and 0.279 nm with a 45o angle can be ascribed to (011) and (002) planes of CoP, respectively (Figure S5). The (001) facet of NiCoP and the neighboring (011) and (002) surfaces of CoP contributed to robust interfaces. The corresponding selected area electron diffraction (SAED) image (Figure 2d inset) displays several bright rings, which are composed of discrete spots and match well with the (001) plane of NiCoP and the (011) plane of CoP, respectively. To illustrate the element distribution of different elements, the energy dispersive X-ray spectroscopy (EDS) was carried out under TEM observation (Figure 2e). As shown, Ni is uniformly distributed among the region of the nanowire while obvious signal enhancement of Co and P is observed in the region where nanoparticles are located. Such an uneven element distribution reveals that the nanowire is made of NiCoP and the nanoparticles mainly contain CoP. The EDS (Figure 2f) further indicates that the atomic ratio of Ni: Co: P is estimated to 1: 2: 2.3, demonstrating the equal content NiCoP and CoP in terms of molar ratio. This is consistent with the Ni/Co ratio measured by inductively coupled plasmaatomic emission spectroscopy (ICP-AES) (Table S1). All the above observations easily reveal that the NiCoP nanowires are interfaced with CoP nanoparticles to produce the robust



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heterostructures.

Figure 2. Fundamental analysis of NiCoP-CoP/NF. (a) XRD pattern, (b) SEM image, (c) TEM image, (d) HRTEM image and the SEAD pattern (insert in (d)), (e) elemental mapping images and (f) EDS spectrum. In order to study the detailed formation mechanism, the ratio effect of Ni/Co precursor on the NiCoP-CoP heterostructures were explored. As shown in Figure S6a, when Ni2+ and Co2+ precursor were taken in 1: 1 ratio, only nanowire was observed, which can be identified as NiCoP (JCPDS: 71-2336) (Figure S6d). Further increasing the amount of Co precursor (Ni/Co to 1: 1.5, 1: 2, 1: 3) yielded the formation of nanoparticles decorated on nanowires heterostructures (Figures S6b,c and Figure 2c). Moreover, it is obvious that the density of the newly produced nanoparticles increases with increasing the ratio of Co to Ni in the nanowires. Furthermore, the XRD patterns of various NiCoP-xCoP display similar to NiCoP-CoP, except that the CoP signals are gradually increased with increasing the amount of Co precursor (Figure S6d and Figure 2a), which can be further proved by the ICP-AES results (Table S1). This result suggests that the NiCoP nanowires were fabricated with the stoichiometry of Ni/Co to 1: 1, while the remaining 12 ACS Paragon Plus Environment

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Co species could diffuse outward and would be captured by P species to form CoP nanoparticles in the nanowires immediately. Furthermore, X-ray photoelectron spectroscopy (XPS) was used to further characterize the electronic states of NiCoP-CoP heterostructures. Figure 3a demonstrates the high-resolution Ni 2p spectrum, in which the existence of Ni2+ species can be verified by a shoulder observed on the main peak at 856.7 eV. The lower binding energy (BE) at 853.1 eV is due to Niδ+ species (δ is close to 0).49,50 Similarly, Co2+ species are apparent from the peaks at 781.4 and 797.6 eV with two satellites. The binding energy at 778.6 eV in Co 2p3/2 spectra is attributed to Co-P bond, which is positively compared with metallic Co (778.2 eV) (Figure 3b).51 With regard to the P 2p spectrum (Figure 3c), the peak at 129.3 eV can be assigned to P bonded with Ni or Co (metal phosphide). A higher peak at 133.2 eV is recognized as the oxidized phosphate species (P-O), which are probably induced by the partial oxidation of metal phosphides in air.52 To better confirm the formation of P-O bond, the O 1s spectrum was further investigated as shown in Figure 3d. Two peaks are involved in the O 1s spectrum, of which one is ascribed to P-O bond at 533.2 eV and the other one suggests the presence of M-O species (M = Co, Ni) at 531.0 eV.53-56 All these observations demonstrate the successful synthesis of NiCoP-CoP heterostructures.



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Figure 3. High-resolution (a) Ni 2p, (b) Co 2p, (c) P 2p and (d) O 1s XPS spectra of NiCoP-CoP heterostructures. The HER activity of as-synthesized NiCoP-CoP/NF was then evaluated. For comparison, the catalytic activities of NiCoP/NF, NiP/NF, CoP/NF, NiCo precursor, Pt-C/NF and bare NF were also measured under the same conditions. It is noted that no binder or additional substrate was required owing to the unique self-supporting structure and conductive characteristic of the Ni foam. Figure 4a presents the HER polarization curves without iR correction. The Pt-C/NF catalyst, as expected, behaves excellent HER performance with a near-zero onset potential. To drive the cathodic current density (j) of 10 mA cm-2 and 100 mA cm-2, the NiCoP-CoP/NF requires overpotentials as small as 73 mV (10) and 183 mV (100), respectively. These overpotentials are much lower than those of NiCoP/NF (10 =80 mV, 100 =220 mV), NiP/NF 14 ACS Paragon Plus Environment

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(10 = 123 mV, 100 = 304 mV), CoP/NF (10 = 108 mV, 100 = 254 mV), NiCo precursor (10 = 239 mV, 100 = 440 mV), and bare NF (10 = 299 mV, 100 = 558 mV) (Figure 4b). Moreover, Figures S7a,b show that Ni/Co ratios also exert an effect on the HER performance of NiCoPxCoP/NF, where the overpotentials at 10 mA cm-2 (100 mA cm-2) for NiCoP-0.5CoP/NF and NiCoP-2CoP/NF were 80 and 72 mV ( 185 and 205 mV), respectively. Furthermore, Figure 4c shows the corresponding Tafel plots, which are related to HER kinetics. The NiCoP-CoP/NF yields a lower Tafel slope (91.3 mV dec-1) than the NiCoP/NF (94.7 mV dec-1), NiP/NF (183.4 mV dec-1), CoP/NF (102.2 mV dec-1), NiCo precursor (183.0 mV dec-1) and bare NF (198.8 mV dec-1), revealing a more efficient Volmer–Heyrovsky process over the resultant NiCoP-CoP/NF in the alkaline media. It is worthwhile to mention that the NiCoP-CoP/NF exhibits superior electrocatalytic performance to other Co (or Ni)-phosphide-based catalysts reported elsewhere, such as CoP/CoP2/Al2O3 spheres,57 Ni2P@NPCNFs,58 nest-like NiCoP/CC,53 Ni2P/NiCoP@Ndoped carbon nanocones,59 and so on (Table S2). Moreover, cyclic voltammograms (Figure S8) were conducted to calculate the the electrical double−layer capacitance (Cdl), which is positively related to the electrochemically active surface area (ECSA) of catalysts. In Figure 4d, the NiCoP-CoP/NF shows a larger Cdl (120.9 mF cm-2) than NiCoP/NF (56.9 mF cm-2), NiP/NF (39.7 mF cm-2), CoP/NF (29.0 mF cm-2), NiCo precursor (10.8 mF cm-2) and pure NF (2.2 mF cm-2), which implies that the NiCoP-CoP/NF contains the largest active sites and the HER performance can thus be promoted. Meanwhile, the electrochemical impedance spectra (EIS) results suggest the charge transfer resistance (Rct) of



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NiCoP-CoP/NF (5 Ω) is obviously lower than that of NiCoP/NF (9 Ω), NiP/NF (15 Ω), CoP/NF (12 Ω), NiCo precursor (25 Ω) and bare Ni foam (34 Ω) (Figure 4e), further suggesting a faster electron transport within the NiCoP-CoP/NF. Furthermore, NiCoP-CoP/NF also exhibits remarkable catalytic stability with an imperceptible current change after 5000 cycles and shows a maintained current density of 50 mA cm-2 through continuous electrolysis at an overpotential of 150 mV over 24 h (Figure 4f). Figure S9 demonstrates that NiCoP-CoP/NF can maintain its activity even after 100 h test. Combined with SEM image (Figure S10a), TEM image (Figure S10b) and XRD pattern (Figure S10c) of the NiCoP-CoP/NF after the HER test, the above results reveal that NiCoP-CoP/NF presents robust durability in the alkaline media.


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Figure 4. (a) Polarization curves of NiCoP-CoP/NF, NiCoP/NF, NiP/NF, CoP/NF, NiCo precursor, Pt-C/NF and bare NF at a sweep rate of 5 mV s-1 in 1.0 M KOH. (b) The required overpotential () at the current density of 10 and 100 mA cm-2. (c) The corresponding Tafel plots. (d) Cdl (Δj = (janode − jcathode)/2 at 0.15 V vs RHE; data obtained from the cyclic voltammograms in Figure S8) and (e) Electrochemical impedance spectra (EIS) of catalysts at  = 200 mV. (f) Polarization curves of NiCoP-CoP/NF initially and after 5000 cycles between +0.1 to -0.2 V (vs RHE). The inset in (f) shows time-dependent current density curve at  = 150 mV over 24 h. The role of NiCoP-CoP interface was deeply studied with the assistances of density 17 ACS Paragon Plus Environment


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functional theory (DFT) analysis. The adsorption free energy of H* (ΔGH*) has been proved to be a key indicator for the HER activity and the ideal value of ΔGH* should be closed to zero. In this context, we calculated ΔGH* on the (001) surface of NiCoP, the (011) surface of CoP, and the interface between NiCoP and CoP, respectively (Figure S11-S13). As shown in Figure 5a, ΔGH* value on the interface of NiCoP and CoP was calculated to be the smallest (−0.15 eV) compared to that of NiCoP (−0.23 eV) and CoP (−0.75 eV). It indicates that the former possesses more optimal hydrogen adsorption free energy, leading to a significantly improved HER activity. Through the above results and analysis, a possible mechanism on the NiCoP-CoP interface in alkaline electrolytes is proposed in Figure 5b. When an H2O molecule arrives at the surface of catalysts, an H atom and OH− can be produced by the dissociation of the water molecule. Previous researches have suggested that the desorption of OH− is more likely to take place on Ni site than Co site in alkaline solutions, while Co site helps in H2 generation and release.56,60 On the constructed NiCoP-CoP interface regions, the OH− tend to be adsorbed on Ni site, while the H atom transfers to an adjacent interface Co site and converts into an adsorbed H (H*). Thus the sufficient amounts of CoP in the NiCoP-CoP heterostructures can provide adequate sites to absorb the H timely and thus eventually improve the HER performance in alkaline media. In addition, if the NiCoP-CoP interfaces are limited on account of the insufficient amounts of CoP (that is, NiCoP-0.5CoP/NF), the intermediate H produced by H2O could not be absorbed efficiently, leading to limited improvement of HER. By contrast, if the CoP nanoparticles grow densely (that is, NiCoP-2CoP/NF), the NiCoP nanowires would be covered too tight to provide


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adequate NiCoP-CoP interfaces, which leads to moderate HER activity (Figure S7). Therefore, the appropriate constituent on the NiCoP-CoP interfaces is crucial to the HER performance.

Figure 5. (a) DFT calculated ΔGH* for NiCoP(001), CoP(011), and NiCoP(001)–CoP(011) systems. (b) The proposed mechanisms of water dissociation on the NiCoP-CoP heterostructures in alkaline solutions. Given that the obtained NiCoP-CoP/NF showed excellent electrocatalytic HER activity, a two-electrode cell was assembled by using NiCoP-CoP/NF as the cathode and RuO2/NF as the anode (NiCoP-CoP/NF||RuO2/NF) for overall water splitting. When a potential of 1.60 V was applied, the current density of this system can reach 31 mA cm-2, which is somewhat higher than that of the benchmark Pt-C/NF||RuO2/NF electrode (20 mA cm-2) (Figure 6a). Meanwhile, the NiCoP-CoP/NF||RuO2/NF couple demonstrated robust durability and a stable current output was observed for 24 h (Figure 6b). These excellent results demonstrate that the NiCoP-CoP/NF catalyst has a broad application prospect in substituting noble metal electrocatalysts for HER.



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Figure 6. (a) Polarization curves of the NiCoP-CoP/NF||RuO2/NF and Pt-C/NF||RuO2/NF electrodes for overall water splitting system. (b) Chronoamperometric technology under a static overpotential of 1.60 V of the NiCoP-CoP/NF||RuO2/NF two-electrode. The inset in (b) shows the generation of H2 and O2 bubbles on the NiCoP-CoP/NF||RuO2/NF two-electrode. 4. CONCLUSIONS In summary, we fabricated a heterostructure consisting of NiCoP nanowires decorated with CoP nanoparticles on Ni foam as a high-efficiency HER electrocatalyst. The robust interfaces between NiCoP and CoP are highly active, allowing enhanced and durable hydrogen evolution at a low overpotential. DFT calculations reveal that the interface effects allow a preferable ΔGH* value, thus allowing more effective hydrogen evolution. Moreover, an electrolyzer with NiCoPCoP/NF as the cathode and RuO2/NF as the anode delivered a current density of 31 mA cm-2 at an applied potential of 1.60 V, outperforming the benchmark electrolyzer consisting of one PtC/NF cathode and one RuO2/NF anode. This work may offer a new path to fabricate more active HER catalysts, and is also expected to motivate more studies on the modulation of the interfacial properties of diversified composites in heterogeneous catalysis, energy storage/conversion, and so on. ASSOCIATED CONTENT 20 ACS Paragon Plus Environment

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Supporting Information. The Supporting Information is available free of charge via the Internet at http://pubs.acs.org. XRD pattern for NiCo precursor (Figure S1), SEM images of NiCo precursor and NiCoP-CoP/NF (Figure S2), SEM images and XRD patterns of NiP/NF and CoP/NF (Figure S3), TEM image and HRTEM image of NiCoP-CoP nanowires (Figure S4), Schematic representation of the relative orientation of (002), (200) and (011) facets in orthorhombic CoP structure (Figure S5), TEM images and XRD patterns of NiCoP/NF, NiCoP-0.5CoP/NF and NiCoP-2CoP/NF (Figure S6), Polarization curves and the required overpotential to achieve current density of 10 and 100 mA cm-2 for NiCoP/NF, NiCoP-0.5CoP/NF, NiCoP-CoP/NF and NiCoP-2CoP/NF (Figure S7), Cyclic voltammetry curves of NiCoP-CoP/NF, NiP/NF, CoP/NF, NiCo precursor and Ni foam (Figure S8), Durability test of NiCoP-CoP/NF at  = 150 mV over 100 h (Figure S9), SEM image, TEM image and XRD pattern of the NiCoP-CoP/NF after continuous long-term test (Figure S10), Bulk structures of NiCoP and CoP (Figure S11), Structures of NiCoP(001) and CoP(011) surfaces (Figure S12), Structures of NiCoP(001)-CoP(011) (Figure S13), ICP-AES results of NiCoP/NF and NiCoP-xCoP/NF with different compositions (Table S1) and comparison of the HER performance for NiCoP-CoP/NF with that of some recently reported Co (or Ni)-phosphide-based catalysts (Table S2). (PDF) AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (M. W.), [email protected] (H. H.). 21 ACS Paragon Plus Environment


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ORCID Mingbo Wu: 0000-0003-0048-778X

Author Contributions #H.

L. and X. M. contributed equally to this work.

Notes The authors declare no conflict of interest.

ACKNOWLEDGMENTS We acknowledge financial support from the National Natural Science Foundation of China (Nos. 51572296, U1662113), the Fundamental Research Funds for the Central Universities (15CX08005A, 17CX06029), Scientific Research and Technology Development Project of Petrochina Co., LTD (2016B−2004(GF)), and the Financial Support from Taishan Scholar Project.


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Table of Contents Graphic


CoP Heterostructures for Highly Efficient Hydrogen

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