robust nicop/cop heterostructures for highly efficient...

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

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

*S Supporting Information

ABSTRACT: Electrocatalytic hydrogen evolution reaction, the cornerstone of the emerginghydrogen economy, can be essentially facilitated by robustly heterostructural electrocatalysts.Herein, we report a highly active and stably heterostructural electrocatalyst consisting ofNiCoP nanowires decorated with CoP nanoparticles on a nickel foam (NiCoP−CoP/NF) foreffective hydrogen evolution. The CoP nanoparticles are strongly interfaced with NiCoPnanowires producing abundant electrocatalytically active sites. Combined with the integratedcatalyst design, NiCoP−CoP/NF affords a remarkable hydrogen evolution performance interms of high activity, enhanced kinetics, and outstanding durability in an alkaline electrolyte,superior to most of the Co (or Ni)-phosphide-based catalysts reported previously. Densityfunctional theory calculations demonstrate that there is an interfacial effect between NiCoP and CoP, which allows a preferablehydrogen adsorption and thus contributes to the significantly enhanced performance. Furthermore, an electrolyzer employingNiCoP−CoP/NF as the cathode and RuO2/NF as the anode (NiCoP−CoP/NF||RuO2/NF) exhibits excellent water-splittingactivity and outstanding durability, which is comparable to that of the benchmark Pt−C/NF||RuO2/NF electrolyzer.KEYWORDS: hydrogen evolution reaction, electrocatalysis, heterostructure, transition metal phosphides, water splitting

1. INTRODUCTION

With the fast consumption of fossil fuels and growing concernsabout environmental issues, the exploration and application ofgreen and renewable energy are now urgently demanded.1

Hydrogen, a clean and sustainable energy with a calorific valueof 282 kJ mol−1, is an emerging alternative to the widelyemployed oil, coal, and gas, whose utilization causes veryserious pollution.2,3 Among all of the available strategies forhydrogen production, electrochemical water splitting usingrenewable electrical energy represents the most promisingmeans because of its reliability, stability, and highly pureproduct.4−6 To facilitate the hydrogen evolution reaction(HER), some noble metals such as gold, palladium, andplatinum have been involved to catalyze the water splitting.Unfortunately, the exorbitant price and limited yields of thesemetals have hindered the popularization of the HERtechnology. As a result, it is imperative to design and constructhigh-performance catalysts utilizing the abundant elements onearth.7−9

In the last several years, numerous research studies havehelped engineer economically feasible catalysts, with remark-able progress achieved. Related examples include heteroatom-doped carbon and transition metal compounds, for instancesulfides, selenides, nitrides, carbides, phosphides, etc.10−31

Despite these successes, the practical performance of thesealternative catalysts is still in need of improvement to rival theirnoble metal-based counterparts.32 One feasible solution is toconstruct heterostructured interfaces between different com-ponents. The rational combination of these components can

regulate the electronic structures and rearrange atoms at theinterfaces, thus modulating the binding energy, transformation,and transportation of surface species.33,34 For example, Li et al.found that HER can be efficiently and robustly catalyzed at theinterfaces of Ni and WC.35 By interfacing Ni with NiO, Daiand colleagues demonstrated a significantly enhanced HERperformance over the nanoscale Ni/NiO heterostructure, farsuperior to its counterparts only consisting of either Ni orNiO.36

To maximize the advantage of this kind of electrocatalysts,special attention should be paid to their durability androbustness. The turbulence caused by the continuous gasevolution can easily cause the deterioration of multicomponentcatalysts without a strong binding force. Intrinsically, the highfusion of different components is likely to be realized betweencomponents with sufficiently low lattice mismatches. Besides,the in situ production of such multicomponent catalyststhrough single-precursor-mediated one-step synthesis is also anappealing technology. On the basis of these design rationales,we demonstrated a novel heterostructural electrocatalyst madeof NiCoP nanowires decorated with CoP nanoparticles on anickel foam (denoted as NiCoP−CoP/NF). The NiCoprecursor is first synthesized through a facile hydrothermalprocess, which is then transformed into the dual-componentcatalyst by subsequent phosphorization. The adjacency of Ni

Received: January 10, 2019Accepted: April 5, 2019Published: April 5, 2019

Research Article

www.acsami.orgCite This: ACS Appl. Mater. Interfaces 2019, 11, 15528−15536

© 2019 American Chemical Society 15528 DOI: 10.1021/acsami.9b00592ACS Appl. Mater. Interfaces 2019, 11, 15528−15536

Dow

nloa

ded

via

CH

INA

UN

IV O

F PE

TR

OL

EU

M o

n M

ay 6

, 201

9 at

10:

32:2

4 (U

TC

).

See

http

s://p

ubs.

acs.

org/

shar

ingg

uide

lines

for

opt

ions

on

how

to le

gitim

atel

y sh

are

publ

ishe

d ar

ticle

s.

www.acsami.orghttp://pubs.acs.org/action/showCitFormats?doi=10.1021/acsami.9b00592http://dx.doi.org/10.1021/acsami.9b00592

and Co in the periodic table secures the sufficiently low latticemismatch between NiCoP and CoP, and the direct trans-formation from a single NiCo precursor further contributes tothe robust interfaces. Because of these structural merits, theoverpotential of the NiCoP−CoP/NF electrode can reach aslow as 73 mV to deliver an HER current density of 10 mAcm−2 in KOH (1.0 M). Moreover, this catalyst deliverssuperior durability without any deterioration after 5000catalytic cycles. Density functional theory (DFT) calculationsdisplay that the robust heterostructured interfaces betweenNiCoP and CoP possess the appropriate hydrogen adsorptionfree energy, thus attributing to the excellent performance. Inaddition, an overall water-splitting electrolyzer employingNiCoP−CoP/NF as the cathode and RuO2/NF as the anode(NiCoP−CoP/NF||RuO2/NF) achieves a current density of31 mA cm−2 at a given potential of 1.60 V in 1.0 M KOHelectrolyte, superior to the benchmark Pt−C/NF||RuO2/NFelectrolyzer.

2. EXPERIMENTAL SECTION2.1. Preparation of NiCo Precursor. Briefly, 0.582 g of

Ni(NO3)2·6H2O, 1.164 g of Co(NO3)2·6H2O, 0.6 g of urea, and0.185 g of NH4F were added in deionized water (30 mL) underultrasonication. Ni foam (cut into 1 × 4 cm2) was sonicated inacetone, alcohol, and deionized water for 15 min, sequentially. Thenthe precleaned Ni foam and the solution were transferred into aTeflon-lined stainless steel autoclave (60 mL) and heated at 120 °Cfor 5 h in a rotating oven to uniformly grow the NiCo precursor. Afterthe reaction, the NiCo precursor was thoroughly rinsed withdeionized water and then dried at room temperature for the followingexperiments.2.2. Preparation of NiCoP−xCoP/NF, NiCoP/NF, NiP/NF, and

CoP/NF. In a typical preparation of NiCoP−CoP/NF, the as-madeNiCo precursor was located in the middle of a quartz tube withNaH2PO2 (1.0 g) at the upstream side near the NiCo precursor. Thesample was subsequently heated to 350 °C with a rate of 2 °C min−1

and kept for 2 h under a N2 atmosphere. After that, the as-preparedproduct of NiCoP−CoP/NF was collected for further character-ization and test. The loading amount of NiCoP−CoP was about 1.5mg cm−2, which was calculated by the weight variation of NF beforeand after reaction. For a comparison, the NiCoP/NF and otherNiCoP−xCoP/NF (denoted as NiCo−0.5CoP/NF and NiCo−2CoP/NF) samples were synthesized in a similar way by adjustingthe amount of Co(NO3)2·6H2O from 4 to 2, 3, and 6 mmol,respectively. In addition, 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 the otherconditions 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 nanowire arrays was studied by

a scanning electron microscope (SEM). Transmission electronmicroscopy (TEM) and high-resolution TEM (HRTEM) imageswere recorded on a JEM-2100F transmission electron microscope.Energy-dispersive spectroscopy (EDS) analysis was performed on aTecnai TEM with an EDS detector in the scanning unit electronmicroscopy mode. Element contents were measured with inductivelycoupled plasma-atomic emission spectrometry (ICP-AES). X-rayphotoelectron spectra (XPS) analysis was conducted by using anESCALab 250XI electron spectrometer, in which a monochromatic AlKα source (hν = 1486.6 eV) was applied.

2.4. Electrochemical Measurements. The HER performance ofthe as-obtained electrocatalysts was tested using a CHI 760Eelectrochemical workstation (Shanghai Chenhua) with a classicthree-electrode system in a N2-saturated KOH aqueous solution(1.0 M) at room temperature. The working electrode was one ofNiCoP−xCoP/NF, NiCoP/NF, NiP/NF, CoP/NF, NiCo precursor,Pt−C/NF, or Ni foam, and carbon rod was selected as the counter-electrode, whereas Ag/AgCl electrode was used as the referenceelectrode. The polarization curves were obtained by the linear sweepvoltammetry technique at 5 mV s−1 without iR correction. Electricimpedance spectroscopy (EIS) was performed at an overpotential of200 mV with a frequency range of 10−1−105 Hz. The cycling stabilitywas tested in a potential range (+0.1 to −0.2 V vs reversible hydrogenelectrode (RHE)) for 5000 cycles. Time-dependent current densitycurves were recorded at a constant overpotential of 150 mV over 24and 100 h, respectively. In addition, the calculation of currentdensities was based on the area of the electrode (1 × 1 cm2) actuallyimmersed into the electrolyte. All of the measured potentials weredisplayed vs the reversible hydrogen electrode (RHE) following theequation, E(RHE) = E(Ag/AgCl) + 0.197 V + 0.059 × pH. TheNiCoP−CoP/NF electrode served as a cathode for the HER andRuO2 supported on the Ni foam (RuO2/NF) served as an anode forthe OER in 1.0 M KOH solution in the two-electrode alkalineelectrolyzer. For a comparison, Pt/C supported on the Ni foam (Pt−C/NF) also acted as the cathode with RuO2/NF as the anode (Pt−C/NF||RuO2/NF) to drive the overall water-splitting process. Theloading mass of noble-metal catalysts is approximately 1.5 mg cm−2.

2.5. DFT Computation. Density functional theory (DFT)analyses were performed by the Dmol3 module in the MaterialsStudio program of Accelrys.37 The generalized gradient approxima-tion method with Perdew−Burke−Ernzerhof was applied for theexchange−correlation functional.38 The core was treated using theeffective core potential, and the double-numerical-polarizationfunctions basis set was employed.39 The correction of van derWaals interaction was included using the DFT-D method ofGrimme.40 The structures were optimized with 1 × 10−5 Hartreefor energy change, 0.002 Hartree Å−1 for maximum force, and 0.005 Åfor maximum displacement, respectively. For all of the calculations ofslab models, a 3 × 3 × 1 Monkhorst−Pack grid k-point mesh wasemployed in the Brillouin zone. To avoid periodic interactions, avacuum space of 15.0 Å was used along the normal direction to thecatalyst surface.

Correlative theoretical models were built to simulate NiCoP, CoP,and composite NiCoP−CoP catalyst phases. Typically, the (001)

Figure 1. (a) Illustrated scheme of the fabrication process of NiCoP−CoP/NF. SEM images of (b) Ni foam, (c) NiCo precursor, and (d) NiCoP−CoP/NF.

ACS Applied Materials & Interfaces Research Article

DOI: 10.1021/acsami.9b00592ACS Appl. Mater. Interfaces 2019, 11, 15528−15536

15529

http://dx.doi.org/10.1021/acsami.9b00592

facet with cotermination was adopted to act as the active surface forNiCoP, which was constructed as a slab with four layers. For CoP, the(011) facet was used in the creation of the slab model with threelayers. NiCoP−CoP is composed of the NiCoP(001) facet and theCoP(011) facet, and the lattice parameters are 14.9399, 14.8772, and30.0002 Å, respectively.The hydrogen absorption free energy ΔGH* was calculated

according to the following formulas

G E E STH H ZPEΔ = Δ + Δ − Δ* * (1)

E E E E( 1/2 )H (slab H ) (slab) (H )2Δ = − +* + * (2)

where ΔE(H*), ΔEZPE, T, ΔS, E(slab+H*), E(slab), and E(H2) represent thebinding energy, zero-point energy, temperature, entropy change, totalenergy of the slab covered with a H, energy of the slab, and energy ofH2(g), respectively. The vibrational entropy of hydrogen in theadsorbed state is negligible; that is, ΔS = SH* − 1/2S(H2) ≈ −1/2S(H2),where S(H2) is the entropy of H2(g) under standard conditions. Hence,the total corrections were taken as in the following equation41

G E 0.24eVH HΔ = Δ +* * (3)

3. RESULTS AND DISCUSSIONThe typical synthesis strategy of NiCoP−CoP/NF is schemati-cally demonstrated in Figure 1a. NF was selected as a substrateowing to its high porosity and excellent electrical conductivity(Figure 1b).42,43 First, evenly aligned NiCo precursor nano-wires were grown on NF through the hydrothermal treatmentof Ni2+ and Co2+ in the presence of ammonium fluoride andurea (Figure 1c).44 The X-ray diffraction (XRD) patternreveals that the diffraction peaks of NiCo precursor can beassigned to NiCo2(CO3)1.5(OH)3 (Figure S1), which is similarto those of Co(CO3)0.5OH·0.11H2O (JCPDS: 48-0083),except that they shift slightly to the low-diffraction region.Previous reports have shown that the partial substitution ofCo2+ by Ni2+ causes only a minor change in lattice parameters,but does not alter the crystal structure owing to the similarityof Co and Ni atoms,45−48 which may facilitate the formation ofa robust heterostructural interface. Then, NaH2PO2 waschosen as the P source at elevated temperature for phosphatingthe NiCo precursor. Interestingly, their morphology can bewell maintained without deterioration (Figures 1d and S2a,b).Meanwhile, the digital images of NiCoP−CoP/NF (Figure

S2c,d) manifest the flexibility of this architecture and thecapability of serving as an integrated electrode directly.Figure 2a shows the XRD pattern of NiCoP−CoP/NF. The

diffraction peaks at 2θ = 40.9, 45.0, and 47.4° are readilyindexed to the (111), (201), and (210) planes of hexagonalNiCoP (JCPDS: 71-2336), respectively. The existence of CoPis confirmed from the peaks at 2θ = 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 withlengths of several microns and diameters of 40−60 nm areuniformly anchored on the NF. The large active surface areaproduced by this unique hierarchical nanowire array structurecould facilitate the transmission of the electrolyte. As a result,significantly enhanced HER activity can be anticipated. For acomparison, NiP/NF and CoP/NF were also fabricated in thesame way and their structural information is provided in theSupporting Information (Figure S3).To further illustrate the structural details of NiCoP−CoP, an

individual nanowire was investigated by transmission electronmicroscopy (TEM). In Figure 2c, the average diameter ofNiCoP−CoP is around 50 nm, in accordance with the SEManalysis (Figure 2b inset). The enlarged TEM images (Figure2c inset and Figure S4a) reveal that plenty of nanoparticles ofaround 5−8 nm are uniformly decorated on the nanowire. Asshown in Figure S4b, these nanoparticles have two sets oflattice fringes forming a 90° angle, and the interplanar spacingsare around 0.279 and 0.254 nm, belonging to the (002) and(200) crystallographic planes of the orthorhombic CoP phase(Figure S5), respectively. In addition, Figure 2d shows thecharacteristic spacing of 0.335 nm for the (001) plane ofNiCoP, whereas spacings of 0.283 and 0.279 nm with a 45°angle can be ascribed to (011) and (002) planes of CoP,respectively (Figure S5). The (001) facet of NiCoP and theneighboring (011) and (002) surfaces of CoP contributed torobust interfaces. The corresponding selected area electrondiffraction (SAED) image (Figure 2d inset) displays severalbright rings, which are composed of discrete spots and matchwell with the (001) plane of NiCoP and the (011) plane ofCoP, respectively. To illustrate the element distribution ofdifferent elements, energy-dispersive X-ray spectroscopy

Figure 2. Fundamental analysis of NiCoP−CoP/NF: (a) XRD pattern, (b) SEM image, (c) TEM image, (d) HRTEM image and the selected areaelectron diffraction (SAED) pattern (inset in (d)), (e) elemental mapping images, and (f) EDS spectrum.



15530

http://pubs.acs.org/doi/suppl/10.1021/acsami.9b00592/suppl_file/am9b00592_si_001.pdfhttp://pubs.acs.org/doi/suppl/10.1021/acsami.9b00592/suppl_file/am9b00592_si_001.pdfhttp://pubs.acs.org/doi/suppl/10.1021/acsami.9b00592/suppl_file/am9b00592_si_001.pdfhttp://pubs.acs.org/doi/suppl/10.1021/acsami.9b00592/suppl_file/am9b00592_si_001.pdfhttp://pubs.acs.org/doi/suppl/10.1021/acsami.9b00592/suppl_file/am9b00592_si_001.pdfhttp://pubs.acs.org/doi/suppl/10.1021/acsami.9b00592/suppl_file/am9b00592_si_001.pdfhttp://pubs.acs.org/doi/suppl/10.1021/acsami.9b00592/suppl_file/am9b00592_si_001.pdfhttp://pubs.acs.org/doi/suppl/10.1021/acsami.9b00592/suppl_file/am9b00592_si_001.pdfhttp://pubs.acs.org/doi/suppl/10.1021/acsami.9b00592/suppl_file/am9b00592_si_001.pdfhttp://dx.doi.org/10.1021/acsami.9b00592

Figure 3. High-resolution (a) Ni 2p, (b) Co 2p, (c) P 2p, and (d) O 1s XPS spectra of NiCoP−CoP heterostructures.

Figure 4. (a) Polarization curves of NiCoP−CoP/NF, NiCoP/NF, NiP/NF, CoP/NF, the NiCo precursor, Pt−C/NF, and bare NF at a sweeprate of 5 mV s−1 in 1.0 M KOH. (b) Required overpotential (η) at current densities of 10 and 100 mA cm−2. (c) Corresponding Tafel plots. (d) Cdl(Δj = (janode − jcathode)/2 at 0.15 V vs RHE; data obtained from the cyclic voltammograms in Figure S8). (e) Electrochemical impedance spectra(EIS) of catalysts at η = 200 mV. (f) Polarization curves of NiCoP−CoP/NF initially and after 5000 cycles between +0.1 and −0.2 V (vs RHE).Inset in (f) shows the time-dependent current density curve at η = 150 mV over 24 h.



15531

http://pubs.acs.org/doi/suppl/10.1021/acsami.9b00592/suppl_file/am9b00592_si_001.pdfhttp://dx.doi.org/10.1021/acsami.9b00592

(EDS) was carried out under TEM observation (Figure 2e). Asshown, Ni is uniformly distributed in the region of thenanowire, whereas obvious signal enhancement of Co and P isobserved in the region where nanoparticles are located. Suchan uneven element distribution reveals that the nanowire ismade of NiCoP and the nanoparticles mainly contain CoP.The EDS (Figure 2f) further indicates that the atomic ratio ofNi/Co/P is estimated to be 1:2:2.3, demonstrating the equalcontent of NiCoP and CoP in terms of molar ratio. This isconsistent with the Ni/Co ratio measured by inductivelycoupled plasma-atomic emission spectroscopy (ICP-AES)(Table S1). All of the above observations easily reveal thatthe NiCoP nanowires are interfaced with CoP nanoparticles toproduce robust heterostructures.To study the detailed formation mechanism, the ratio effect

of the Ni/Co precursor on the NiCoP−CoP heterostructureswas explored. As shown in Figure S6a, when Ni2+ and Co2+

precursors were taken in a 1:1 ratio, only nanowires wereobserved, which can be identified as NiCoP (JCPDS: 71-2336)(Figure S6d). Further increasing the amount of the Coprecursor (Ni/Co to 1:1.5, 1:2, 1:3) yielded nanoparticlesdecorated on nanowire heterostructures (Figures S6b,c and2c). Moreover, it is obvious that the density of the newlyproduced nanoparticles increases with increasing ratio of Co toNi in the nanowires. Furthermore, the XRD patterns of variousNiCoP−xCoP’s are similar to that of NiCoP−CoP, except thatthe CoP signals gradually increase with increasing amounts ofthe Co precursor (Figures S6d and 2a), which can be furtherproved by the ICP-AES results (Table S1). This result suggeststhat the NiCoP nanowires were fabricated with a stoichiometryof Ni/Co of 1:1, whereas the remaining Co species coulddiffuse outward and would be captured by P species to formCoP 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 mainpeak at 856.7 eV. The lower binding energy (BE) at 853.1 eVis 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 withtwo satellites. The binding energy at 778.6 eV in Co 2p3/2spectra is attributed to the Co−P bond, which is positivelycompared with metallic Co (778.2 eV) (Figure 3b).51 Withregard to the P 2p spectrum (Figure 3c), the peak at 129.3 eVcan be assigned to P bonded with Ni or Co (metal phosphide).A higher peak at 133.2 eV is recognized as the oxidizedphosphate species (P−O), which are probably induced by the

partial oxidation of metal phosphides in air.52 To betterconfirm the formation of the P−O bond, the O 1s spectrumwas further investigated as shown in Figure 3d. Two peaks areinvolved in the O 1s spectrum, of which one is ascribed to theP−O bond at 533.2 eV and the other one suggests thepresence of an M−O species (M = Co, Ni) at 531.0 eV.53−56All of these observations demonstrate the successful synthesisof NiCoP−CoP heterostructures.The HER activity of as-synthesized NiCoP−CoP/NF was

then evaluated. For a comparison, the catalytic activities ofNiCoP/NF, NiP/NF, CoP/NF, NiCo precursor, Pt−C/NF,and bare NF were also measured under the same conditions. Itis noted that no binder or additional substrate was requiredowing to the unique self-supporting structure and conductivecharacteristics of the Ni foam. Figure 4a presents the HERpolarization curves without iR correction. The Pt−C/NFcatalyst, as expected, shows excellent HER performance with anear-zero onset potential. To derive cathodic current densities(j) of 10 mA cm−2 and 100 mA cm−2, NiCoP−CoP/NFrequires overpotentials as small as 73 mV (η10) and 183 mV(η100), respectively. These overpotentials are much lower thanthose of NiCoP/NF (η10 = 80 mV, η100 = 220 mV), NiP/NF(η10 = 123 mV, η100 = 304 mV), CoP/NF (η10 = 108 mV, η100= 254 mV), the NiCo precursor (η10 = 239 mV, η100 = 440mV), and bare NF (η10 = 299 mV, η100 = 558 mV) (Figure4b). Moreover, Figure S7a,b show that Ni/Co ratios also exertan effect on the HER performance of NiCoP−xCoP/NF,where the overpotentials at 10 mA cm−2 (100 mA cm−2) forNiCoP−0.5CoP/NF and NiCoP−2CoP/NF were 80 and 72mV (185 and 205 mV), respectively. Furthermore, Figure 4cshows the corresponding Tafel plots, which are related to HERkinetics. NiCoP−CoP/NF yields a lower Tafel slope (91.3 mVdec−1) than NiCoP/NF (94.7 mV dec−1), NiP/NF (183.4 mVdec−1), CoP/NF (102.2 mV dec−1), the NiCo precursor(183.0 mV dec−1), and bare NF (198.8 mV dec−1), revealing amore efficient Volmer−Heyrovsky process over the resultantNiCoP−CoP/NF in the alkaline media. It is worthwhile tomention that NiCoP−CoP/NF exhibits superior electro-catalytic performance compared with 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@N-doped carbon nanocones,59 and so on

(Table S2).Moreover, cyclic voltammograms (Figure S8) were con-

ducted to calculate the electrical double-layer capacitance(Cdl), which is positively related to the electrochemically activesurface area of catalysts. In Figure 4d, NiCoP−CoP/NF showsa larger Cdl (120.9 mF cm

−2) than NiCoP/NF (56.9 mF

Figure 5. (a) DFT-calculated ΔGH* for NiCoP(001), CoP(011), and NiCoP(001)−CoP(011) systems. (b) Proposed mechanisms of waterdissociation on the NiCoP−CoP heterostructures in alkaline solutions.



15532

http://pubs.acs.org/doi/suppl/10.1021/acsami.9b00592/suppl_file/am9b00592_si_001.pdfhttp://pubs.acs.org/doi/suppl/10.1021/acsami.9b00592/suppl_file/am9b00592_si_001.pdfhttp://pubs.acs.org/doi/suppl/10.1021/acsami.9b00592/suppl_file/am9b00592_si_001.pdfhttp://pubs.acs.org/doi/suppl/10.1021/acsami.9b00592/suppl_file/am9b00592_si_001.pdfhttp://pubs.acs.org/doi/suppl/10.1021/acsami.9b00592/suppl_file/am9b00592_si_001.pdfhttp://pubs.acs.org/doi/suppl/10.1021/acsami.9b00592/suppl_file/am9b00592_si_001.pdfhttp://pubs.acs.org/doi/suppl/10.1021/acsami.9b00592/suppl_file/am9b00592_si_001.pdfhttp://pubs.acs.org/doi/suppl/10.1021/acsami.9b00592/suppl_file/am9b00592_si_001.pdfhttp://pubs.acs.org/doi/suppl/10.1021/acsami.9b00592/suppl_file/am9b00592_si_001.pdfhttp://dx.doi.org/10.1021/acsami.9b00592

cm−2), NiP/NF (39.7 mF cm−2), CoP/NF (29.0 mF cm−2),the NiCo precursor (10.8 mF cm−2), and pure NF (2.2 mFcm−2), which implies that NiCoP−CoP/NF contains thelargest active sites, and the HER performance can thus bepromoted. Meanwhile, the electrochemical impedance spectra(EIS) results suggest that the charge transfer resistance (Rct) ofNiCoP−CoP/NF (5 Ω) is obviously lower than those ofNiCoP/NF (9 Ω), NiP/NF (15 Ω), CoP/NF (12 Ω), theNiCo precursor (25 Ω), and bare Ni foam (34 Ω) (Figure 4e),further suggesting a faster electron transport within NiCoP−CoP/NF. Furthermore, NiCoP−CoP/NF also exhibits re-markable catalytic stability with an imperceptible currentchange after 5000 cycles and shows a maintained currentdensity of ∼50 mA cm−2 through continuous electrolysis at anoverpotential of 150 mV over 24 h (Figure 4f). Figure S9demonstrates that NiCoP−CoP/NF can maintain its activityeven after 100 h of testing. Combined with the SEM image(Figure S10a), TEM image (Figure S10b), and XRD pattern(Figure S10c) of NiCoP−CoP/NF after the HER test, theabove results reveal that NiCoP−CoP/NF presents robustdurability in alkaline media.The role of the NiCoP−CoP interface was deeply studied

with the help of density functional theory (DFT) analysis. Theadsorption free energy of H* (ΔGH*) has been proved to be akey indicator for the HER activity, and the ideal value of ΔGH*should be close to zero. In this context, we calculated ΔGH* onthe (001) surface of NiCoP, the (011) surface of CoP, and theinterface between NiCoP and CoP, respectively (Figures S11−S13). As shown in Figure 5a, the ΔGH* value on the interfaceof NiCoP and CoP was calculated to be the smallest (−0.15eV) compared with that of NiCoP (−0.23 eV) and CoP(−0.75 eV). It indicates that the former possesses moreoptimal hydrogen adsorption free energy, leading to asignificantly improved HER activity.Through the above results and analyses, a possible

mechanism on the NiCoP−CoP interface in alkaline electro-lytes is proposed in Figure 5b. When an H2O molecule arrivesat the surface of catalysts, an H atom and OH− can beproduced by the dissociation of the water molecule. Previousresearches have suggested that the desorption of OH− is morelikely to take place on the Ni site than on the Co site inalkaline solutions, whereas the Co site helps in H2 generationand release.56,60 On the constructed NiCoP−CoP interfaceregions, the OH− tends to be adsorbed on the Ni site, whereasthe H atom transfers to an adjacent interface Co site andconverts into an adsorbed H (H*). Thus, sufficient amounts ofCoP in the NiCoP−CoP heterostructures can provideadequate sites for the timely absorption of the H and thus

eventually improve the HER performance in alkaline media. Inaddition, if the NiCoP−CoP interfaces are limited on accountof insufficient amounts of CoP (i.e., NiCoP−0.5CoP/NF), theintermediate H produced by H2O could not be absorbedefficiently, leading to limited improvement of HER. Bycontrast, if the CoP nanoparticles grow densely (i.e.,NiCoP−2CoP/NF), the NiCoP nanowires would be coveredvery tightly to provide adequate NiCoP−CoP interfaces, whichleads to moderate HER activity (Figure S7). Therefore, theappropriate composition on the NiCoP−CoP interfaces iscrucial to the HER performance.Given that the obtained NiCoP−CoP/NF showed excellent

electrocatalytic HER activity, a two-electrode cell wasassembled by using NiCoP−CoP/NF as the cathode andRuO2/NF as the anode (NiCoP−CoP/NF||RuO2/NF) foroverall 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). Mean-while, the NiCoP−CoP/NF||RuO2/NF couple demonstratedrobust durability and a stable current output was observed for24 h (Figure 6b). These excellent results demonstrate that theNiCoP−CoP/NF catalyst has a broad application prospect insubstituting noble-metal electrocatalysts for HER.

4. CONCLUSIONS

In summary, we fabricated a heterostructure consisting ofNiCoP nanowires decorated with CoP nanoparticles on Nifoam as a high-efficiency HER electrocatalyst. The robustinterfaces between NiCoP and CoP are highly active, allowingenhanced and durable hydrogen evolution at a low over-potential. DFT calculations reveal that the interface effectsallow a preferable ΔGH* value, thus allowing more effectivehydrogen evolution. Moreover, an electrolyzer with NiCoP−CoP/NF as the cathode and RuO2/NF as the anode delivereda current density of 31 mA cm−2 at an applied potential of 1.60V, outperforming the benchmark electrolyzer consisting of onePt−C/NF cathode and one RuO2/NF anode. This work mayoffer a new path to fabricate more active HER catalysts and isalso expected to motivate more studies on the modulation ofthe interfacial properties of diversified composites inheterogeneous catalysis, energy storage/conversion, and so on.

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acsami.9b00592.

Figure 6. (a) Polarization curves of the NiCoP−CoP/NF||RuO2/NF and Pt−C/NF||RuO2/NF electrodes for the overall water-splitting system.(b) Chronoamperometric technology under a static overpotential of 1.60 V of the NiCoP−CoP/NF||RuO2/NF two-electrode cell. Inset in (b)shows the generation of H2 and O2 bubbles on the NiCoP−CoP/NF||RuO2/NF two-electrode cell.



15533

http://pubs.acs.org/doi/suppl/10.1021/acsami.9b00592/suppl_file/am9b00592_si_001.pdfhttp://pubs.acs.org/doi/suppl/10.1021/acsami.9b00592/suppl_file/am9b00592_si_001.pdfhttp://pubs.acs.org/doi/suppl/10.1021/acsami.9b00592/suppl_file/am9b00592_si_001.pdfhttp://pubs.acs.org/doi/suppl/10.1021/acsami.9b00592/suppl_file/am9b00592_si_001.pdfhttp://pubs.acs.org/doi/suppl/10.1021/acsami.9b00592/suppl_file/am9b00592_si_001.pdfhttp://pubs.acs.org/doi/suppl/10.1021/acsami.9b00592/suppl_file/am9b00592_si_001.pdfhttp://pubs.acs.org/doi/suppl/10.1021/acsami.9b00592/suppl_file/am9b00592_si_001.pdfhttp://pubs.acs.orghttp://pubs.acs.org/doi/abs/10.1021/acsami.9b00592http://dx.doi.org/10.1021/acsami.9b00592

XRD pattern of the NiCo precursor (Figure S1), SEMimages of the NiCo precursor and NiCoP−CoP/NF(Figure S2), SEM images and XRD patterns of NiP/NFand CoP/NF (Figure S3), TEM image and HRTEMimage of NiCoP−CoP nanowires (Figure S4), schematicrepresentation of the relative orientations of (002),(200), and (011) facets in the orthorhombic CoPstructure (Figure S5), TEM images and XRD patterns ofNiCoP/NF, NiCoP−0.5CoP/NF, and NiCoP−2CoP/NF (Figure S6), polarization curves and the requiredoverpotential to achieve current densities of 10 and 100mA 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, the NiCo precursor, and Ni foam (FigureS8), durability test of NiCoP−CoP/NF at η = 150 mVover 100 h (Figure S9), SEM image, TEM image, andXRD pattern of NiCoP−CoP/NF after continuous long-term test (Figure S10), bulk structures of NiCoP andCoP (Figure S11), structures of NiCoP(001) andCoP(011) surfaces (Figure S12), structures ofNiCoP(001)−CoP(011) (Figure S13), ICP-AES resultsof NiCoP/NF and NiCoP−xCoP/NF with differentcompositions (Table S1), and comparison of the HERperformance for NiCoP−CoP/NF with those of somerecently reported Co (or Ni)-phosphide-based catalysts(Table S2) (PDF)

■ AUTHOR INFORMATIONCorresponding Authors*E-mail: [email protected] (H.H.)*E-mail: [email protected] (M.W.)ORCIDHui Liu: 0000-0001-7815-4200Han Hu: 0000-0002-3755-7342Mingbo Wu: 0000-0003-0048-778XAuthor Contributions†H.L. and X.M. contributed equally to this work.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSWe acknowledge financial support from the National NaturalScience Foundation of China (Nos 51572296, U1662113), theFundamental Research Funds for the Central Universities(15CX08005A, 17CX06029), Scientific Research and Tech-nology Development Project of Petrochina Co., LTD (2016B-2004(GF)), and the Financial Support from Taishan ScholarProject.

■ REFERENCES(1) Dresselhaus, M. S.; Thomas, I. L. Alternative EnergyTechnologies. Nature 2001, 414, 332−337.(2) Turner, J. A. Sustainable Hydrogen Production. Science 2004,305, 972−974.(3) Chen, J.; Xia, G. L.; Guo, Z. P.; Huang, Z. G.; Liu, H. K.; Yu, X.B. Porous Ni Nanofibers with Enhanced Catalytic Effect on theHydrogen Storage Performance of MgH2. J. Mater. Chem. A 2015, 3,15843−15848.(4) You, B.; Sun, Y. J. Innovative Strategies for ElectrocatalyticWater Splitting. Acc. Chem. Res. 2018, 51, 1571−1580.

(5) He, P.; Yu, X. Y.; Lou, X. W. Carbon-Incorporated Nickel-Cobalt Mixed Metal Phosphide Nanoboxes with Enhanced Electro-catalytic Activity for Oxygen Evolution. Angew. Chem., Int. Ed. 2017,56, 3897−3900.(6) Li, D.; Baydoun, H.; Verani, C. N.; Brock, S. L. Efficient WaterOxidation Using CoMnP Nanoparticles. J. Am. Chem. Soc. 2016, 138,4006−4009.(7) Wang, H.; Lee, H. W.; Deng, Y.; Lu, Z.; Hsu, P. C.; Liu, Y.; Lin,D.; Cui, Y. Bifunctional Non-Noble Metal Oxide NanoparticleElectrocatalysts through Lithium-Induced Conversion for OverallWater Splitting. Nat. Commun. 2015, 6, No. 7261.(8) Zhu, B. J.; Zou, R. Q.; Xu, Q. Metal-Organic Framework BasedCatalysts for Hydrogen Evolution. Adv. Energy Mater. 2018, 8,No. 1801193.(9) Wang, P. T.; Zhang, X.; Zhang, J.; Wan, S.; Guo, S. J.; Lu, G.;Yao, J. L.; Huang, X. Q. Precise Tuning in Platinum-Nickel/NickelSulfide Interface Nanowires for Synergistic Hydrogen EvolutionCatalysis. Nat. Commun. 2017, 8, No. 14580.(10) Zhang, J. T.; Dai, L. M. Nitrogen, Phosphorus, and FluorineTri-doped Graphene as a Multifunctional Catalyst for Self-PoweredElectrochemical Water Splitting. Angew. Chem., Int. Ed. 2016, 55,13296−13300.(11) Sheng, G. Q.; Chen, J. H.; Li, Y. M.; Ye, H. Q.; Hu, Z. X.; Fu,X. Z.; Sun, R.; Huang, W. X.; Wong, C. P. Flowerlike NiCo2S4 HollowSub-Microspheres with Mesoporous Nanoshells Support Pd Nano-particles for Enhanced Hydrogen Evolution Reaction Electrocatalysisin Both Acidic and Alkaline Conditions. ACS Appl. Mater. Interfaces2018, 10, 22248−22256.(12) Liu, H.; Ma, X.; Rao, Y.; Liu, Y.; Liu, J. L.; Wang, L. Y.; Wu, M.B. Heteromorphic NiCo2S4/Ni3S2/Ni Foam as a Self-StandingElectrode for Hydrogen Evolution Reaction in Alkaline Solution.ACS Appl. Mater. Interfaces 2018, 10, 10890−10897.(13) Zhang, J.; Wang, T.; Pohl, D.; Rellinghaus, B.; Dong, R. H.; Liu,S. H.; Zhuang, X. D.; Feng, X. L. Interface Engineering of MoS2/Ni3S2Heterostructures for Highly Enhanced Electrochemical Overall-Water-Splitting Activity. Angew. Chem., Int. Ed. 2016, 55, 6702−6707.(14) Yan, Y.; Xia, B. Y.; Xu, Z. C.; Wang, X. Recent Development ofMolybdenum Sulfides as Advanced Electrocatalysts for HydrogenEvolution Reaction. ACS Catal. 2014, 4, 1693−1705.(15) Tang, C.; Zhong, L.; Zhang, B.; Wang, H. F.; Zhang, Q. 3DMesoporous van der Waals Heterostructures for Trifunctional EnergyElectrocatalysis. Adv. Mater. 2018, 30, No. 1705110.(16) Zhu, H.; Gao, G. H.; Du, M. L.; Zhou, J. H.; Wang, K.; Wu, W.B.; Chen, X.; Li, Y.; Ma, P. M.; Dong, W. F.; Duan, F.; Chen, M. Q.;Wu, G. M.; Wu, J. D.; Yang, H. T.; Guo, S. J. Atomic-Scale Core/ShellStructure Engineering Induces Precise Tensile Strain to BoostHydrogen Evolution Catalysis. Adv. Mater. 2018, 30, No. 1707301.(17) Feng, J. X.; Wu, J. Q.; Tong, Y. X.; Li, G. R. Efficient HydrogenEvolution on Cu Nanodots-Decorated Ni3S2 Nanotubes byOptimizing Atomic Hydrogen Adsorption and Desorption. J. Am.Chem. Soc. 2018, 140, 610−617.(18) Qu, Y. D.; Medina, H.; Wang, S. W.; Wang, Y. C.; Chen, C. W.;Su, T. Y.; Manikandan, A.; Wang, K.; Shih, Y. C.; Chang, J. W.; Kuo,H. C.; Lee, C. Y.; Lu, S. Y.; Shen, G.; Wang, Z. M.; Chueh, Y. L.Wafer Scale Phase-Engineered 1T-and 2H-MoSe2/Mo Core-Shell 3D-Hierarchical Nanostructures toward Efficient Electrocatalytic Hydro-gen Evolution Reaction. Adv. Mater. 2016, 28, 9831−9838.(19) Zheng, Y. R.; Wu, P.; Gao, M. R.; Zhang, X. L.; Gao, F. Y.; Ju,H. X.; Wu, R.; Gao, Q.; You, R.; Huang, W. X.; Liu, S. J.; Hu, S. W.;Zhu, J.; Li, Z.; Yu, S. H. Doping-Induced Structural Phase Transitionin Cobalt Diselenide Enables Enhanced Hydrogen EvolutionCatalysis. Nat. Commun. 2018, 9, No. 2533.(20) Zhang, J. Y.; Wang, H.; Tian, Y.; Yan, Y.; Xue, Q.; He, T.; Liu,H.; Wang, C.; Chen, Y.; Xia, B. Y. Anodic Hydrazine OxidationAssists Energy-Efficient Hydrogen Evolution over a BifunctionalCobalt Perselenide Nanosheet Electrode. Angew. Chem., Int. Ed. 2018,57, 7649−7653.(21) Han, L.; Xu, M.; Han, Y. J.; Yu, Y.; Dong, S. J. Core-Shell-Structured Tungsten Carbide Encapsulated within Nitrogen-Doped



15534

http://pubs.acs.org/doi/suppl/10.1021/acsami.9b00592/suppl_file/am9b00592_si_001.pdfmailto:[email protected]:[email protected]://orcid.org/0000-0001-7815-4200http://orcid.org/0000-0002-3755-7342http://orcid.org/0000-0003-0048-778Xhttp://dx.doi.org/10.1021/acsami.9b00592

Carbon Spheres for Enhanced Hydrogen Evolution. ChemSusChem2016, 9, 2784−2787.(22) Han, N. N.; Yang, K. R.; Lu, Z. Y.; Li, Y. J.; Xu, W. W.; Gao, T.F.; Cai, Z.; Zhang, Y.; Batista, V. S.; Liu, W.; Sun, X. M. Nitrogen-Doped Tungsten Carbide Nanoarray as an Efficient BifunctionalElectrocatalyst for Water Splitting in Acid. Nat. Commun. 2018, 9,No. 924.(23) Yan, H. J.; Xie, Y.; Jiao, Y. Q.; Wu, A. P.; Tian, C. G.; Zhang, X.M.; Wang, L.; Fu, H. G. Holey Reduced Graphene Oxide Coupledwith an Mo2N-Mo2C Heterojunction for Efficient HydrogenEvolution. Adv. Mater. 2018, 30, No. 1704156.(24) Wang, Y. H.; Chen, L.; Yu, X. M.; Wang, Y. G.; Zheng, G. F.Superb Alkaline Hydrogen Evolution and Simultaneous ElectricityGeneration by Pt-Decorated Ni3N Nanosheets. Adv. Energy Mater.2017, 7, No. 1601390.(25) Liu, Z. H.; Tan, H.; Xin, J. P.; Duan, J. Z.; Su, X. W.; Hao, P.;Xie, J. F.; Zhan, J.; Zhang, J.; Wang, J. J.; Liu, H. Metallic IntermediatePhase Inducing Morphological Transformation in Thermal Nitrida-tion: Ni3FeN-Based Three-Dimensional Hierarchical Electrocatalystfor Water Splitting. ACS Appl. Mater. Interfaces 2018, 10, 3699−3706.(26) Xu, K.; Ding, H.; Zhang, M. X.; Chen, M.; Hao, Z. K.; Zhang,L. D.; Wu, C. Z.; Xie, Y. Regulating Water-Reduction Kinetics inCobalt Phosphide for Enhancing HER Catalytic Activity in AlkalineSolution. Adv. Mater. 2017, 29, No. 1606980.(27) Zhang, C.; Huang, Y.; Yu, Y. F.; Zhang, J. F.; Zhuo, S. F.;Zhang, B. Sub-1.1 nm Ultrathin Porous CoP Nanosheets withDominant Reactive {200} Facets: A High Mass Activity and EfficientElectrocatalyst for the Hydrogen Evolution Reaction. Chem. Sci. 2017,8, 2769−2775.(28) Hu, E. L.; Feng, Y. F.; Nai, J. W.; Zhao, D.; Hu, Y.; Lou, X. W.Construction of Hierarchical Ni-Co-P Hollow Nanobricks withOriented Nanosheets for Efficient Overall Water Splitting. EnergyEnviron. Sci. 2018, 11, 872−880.(29) You, B.; Zhang, Y. D.; Yin, P. Q.; Jiang, D. E.; Sun, Y. J.Universal Molecular-Confined Synthesis of Interconnected PorousMetal Oxides-N-C Frameworks for Electrocatalytic Water Splitting.Nano Energy 2018, 48, 600−606.(30) Wang, J. G.; Hua, W.; Li, M. Y.; Liu, H. Y.; Shao, M. H.; Wei,B. Q. Structurally Engineered Hyperbranched NiCoP Arrays withSuperior Electrocatalytic Activities toward Highly Efficient OverallWater Splitting. ACS Appl. Mater. Interfaces 2018, 10, 41237−41245.(31) Li, Y. J.; Zhang, H. C.; Jiang, M.; Kuang, Y.; Sun, X. M.; Duan,X. Ternary NiCoP Nanosheet Arrays: An Excellent BifunctionalCatalyst for Alkaline Overall Water Splitting. Nano Res. 2016, 9,2251−2259.(32) Zheng, Y.; Jiao, Y.; Vasileff, A.; Qiao, S. Z. The HydrogenEvolution Reaction in Alkaline Solution: From Theory, Single CrystalModels, to Practical Electrocatalysts. Angew. Chem., Int. Ed. 2018, 57,7568−7579.(33) Shao, Q.; Wang, P. T.; Huang, X. Q. Opportunities andChallenges of Interface Engineering in Bimetallic Nanostructure forEnhanced Electrocatalysis. Adv. Funct. Mater. 2019, 29, No. 1806419.(34) Kwon, T.; Jun, M. K.; Joo, J. W.; Lee, K. Nanoscale Hetero-interfaces between Metals and Metal Compounds for ElectrocatalyticApplications. J. Mater. Chem. A 2019, 7, 5090−5110.(35) Ma, Y. Y.; Lang, Z. L.; Yan, L. K.; Wang, Y. H.; Tan, H. Q.;Feng, K.; Xia, Y. J.; Zhong, J.; Liu, Y.; Kang, Z. H.; Li, Y. G. HighlyEfficient Hydrogen Evolution Triggered by a Multi-Interfacial Ni/WCHybrid Electrocatalyst. Energy Environ. Sci. 2018, 11, 2114−2123.(36) Gong, M.; Zhou, W.; Tsai, M. C.; Zhou, J.; Guan, M.; Lin, M.C.; Zhang, B.; Hu, Y.; Wang, D. Y.; Yang, J.; Pennycook, S. J.; Hwang,B. J.; Dai, H. J. Nanoscale Nickel Oxide/Nickel Heterostructures forActive Hydrogen Evolution Electrocatalysis. Nat. Commun. 2014, 5,No. 4695.(37) Delley, B. From Molecules to Solids with the DMol3 Approach.J. Chem. Phys. 2000, 113, 7756−7764.(38) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized GradientApproximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865−1868.

(39) Bergner, A.; Dolg, M.; Küchle, W.; Stoll, H.; Preuß, H. AbInitio Energy-Adjusted Pseudopotentials for Elements of Groups 13−17. Mol. Phys. 1993, 80, 1431−1441.(40) Grimme, S. Semiempirical GGA-Type Density FunctionalConstructed with a Long-Range Dispersion Correction. J. Comput.Chem. 2006, 27, 1787−1799.(41) Nørskov, J. K.; Bligaard, T.; Logadottir, A.; Kitchin, J. R.; Chen,J. G.; Pandelov, S.; Stimming, U. Trends in the Exchange Current forHydrogen Evolution. J. Electrochem. Soc. 2005, 152, 23−26.(42) Feng, L. L.; Yu, G. T.; Wu, Y. Y.; Li, G. D.; Li, H.; Sun, Y. H.;Asefa, T.; Chen, W.; Zou, X. X. High-Index Faceted Ni3S2 NanosheetArrays as Highly Active and Ultrastable Electrocatalysts for WaterSplitting. J. Am. Chem. Soc. 2015, 137, 14023−14026.(43) Lu, X. Y.; Zhao, C. Electrodeposition of HierarchicallyStructured Three-Dimensional Nickel-Iron Electrodes for EfficientOxygen Evolution at High Current Densities. Nat. Commun. 2015, 6,No. 6616.(44) Li, J. Z.; Wei, G. D.; Zhu, Y. K.; Xi, Y. L.; Pan, X. X.; Ji, Y.;Zatovsky, I. V.; Han, W. Hierarchical NiCoP Nanocone ArraysSupported on Ni Foam as an Efficient and Stable BifunctionalElectrocatalyst for Overall Water Splitting. J. Mater. Chem. A 2017, 5,14828−14837.(45) Sivanantham, A.; Ganesan, P.; Shanmugam, S. HierarchicalNiCo2S4 Nanowire Arrays Supported on Ni Foam: An Efficient andDurable Bifunctional Electrocatalyst for Oxygen and HydrogenEvolution Reactions. Adv. Funct. Mater. 2016, 26, 4661−4672.(46) Yang, J.; Yu, C.; Fan, X. M.; Zhao, C. T.; Qiu, J. S. UltrafastSelf-Assembly of Graphene Oxide-Induced Monolithic NiCo-Carbonate Hydroxide Nanowire Architectures with a SuperiorVolumetric Capacitance for Supercapacitors. Adv. Funct. Mater.2015, 25, 2109−2116.(47) Chen, H. C.; Jiang, J. J.; Zhang, L.; Wan, H. Z.; Qi, T.; Xia, D.D. Highly Conductive NiCo2S4 Urchin-like Nanostructures for High-rate Pseudocapacitors. Nanoscale 2013, 5, 8879−8883.(48) Yang, B.; Yu, L.; Yan, H. J.; Sun, Y. B.; Liu, Q.; Liu, J. Y.; Song,D. L.; Hu, S. X.; Yuan, Y.; Liu, L. H.; Wang, J. Fabrication of Urchin-like NiCo2(CO3)1.5(OH)3@NiCo2S4 on Ni Foam by an Ion-exchangeRoute and Application to Asymmetrical Supercapacitors. J. Mater.Chem. A 2015, 3, 13308−13316.(49) Tian, J. Q.; Liu, Q.; Asiri, A. M.; Sun, X. P. Self-SupportedNanoporous Cobalt Phosphide Nanowire Arrays: An Efficient 3DHydrogen-Evolving Cathode over the Wide Range of pH 0−14. J. Am.Chem. Soc. 2014, 136, 7587−7590.(50) Fang, Z. W.; Peng, L. L.; Qian, Y. M.; Zhang, X.; Xie, Y. J.; Cha,J. J.; Yu, G. H. Dual Tuning of Ni-Co-A (A = P, Se, O) Nanosheets byAnion Substitution and Holey Engineering for Efficient HydrogenEvolution. J. Am. Chem. Soc. 2018, 140, 5241−5247.(51) Tang, C.; Gan, L. F.; Zhang, R.; Lu, W. B.; Jiang, X.; Asir, A.M.; Sun, X. P.; Wang, J.; Chen, L. Ternary FexCo1‑xP Nanowire Arrayas a Robust Hydrogen Evolution Reaction Electrocatalyst with Pt-likeActivity: Experimental and Theoretical Insight. Nano Lett. 2016, 16,6617−6621.(52) Zhang, F. S.; Wang, J. W.; Luo, J.; Liu, R. R.; Zhang, Z. M.; He,C. T.; Lu, T. B. Extraction of Nickel from NiFe-LDH into Ni2P@NiFe Hydroxide as a Bifunctional Electrocatalyst for Efficient OverallWater Splitting. Chem. Sci. 2018, 9, 1375−1384.(53) Du, C.; Yang, L.; Yang, F. L.; Cheng, G. Z.; Luo, W. Nest-likeNiCoP for Highly Efficient Overall Water Splitting. ACS Catal. 2017,7, 4131−4137.(54) Yang, X. L.; Lu, A. Y.; Zhu, Y. H.; Hedhili, M. N.; Min, S. X.;Huang, K. W.; Han, Y.; Li, L. J. CoP Nanosheet Assembly Grown onCarbon Cloth: A Highly Efficient Electrocatalyst for HydrogenGeneration. Nano Energy 2015, 15, 634−641.(55) Feng, Y.; Yu, X. Y.; Paik, U. Nickel Cobalt Phosphides Quasi-Hollow Nanocubes as an Efficient Electrocatalyst for HydrogenEvolution in Alkaline Solution. Chem. Commun. 2016, 52, 1633−1636.(56) Zhang, R.; Wang, X. X.; Yu, S. J.; Wen, T.; Zhu, X. W.; Yang, F.X.; Sun, X. N.; Wang, X. K.; Hu, W. P. Ternary NiCo2Px Nanowires as



15535


pH-Universal Electrocatalysts for Highly Efficient HydrogenEvolution Reaction. Adv. Mater. 2017, 29, No. 1605502.(57) Li, W.; Zhang, S. L.; Fan, Q. N.; Zhang, F. Z.; Xu, S. L.Hierarchically Scaffolded CoP/CoP2 Nanoparticles: ControllableSynthesis and Their Application as a Well-Matched BifunctionalElectrocatalyst for Overall Water Splitting. Nanoscale 2017, 9, 5677−5685.(58) Wang, M. Q.; Ye, C.; Bao, S. J.; Chen, Z. Y.; Liu, H.; Xu, M. W.Ternary NixCo3‑xS4 with a Fine Hollow Nanostructure as a RobustElectrocatalyst for Hydrogen Evolution. ChemCatChem 2017, 9,4169−4174.(59) Han, L.; Yu, T. W.; Lei, W.; Liu, W. W.; Feng, K.; Ding, Y. L.;Jiang, G. P.; Xu, P.; Chen, Z. W. Nitrogen-Doped Carbon NanoconesEncapsulating with Nickel-Cobalt Mixed Phosphides for EnhancedHydrogen Evolution Reaction. J. Mater. Chem. A 2017, 5, 16568−16572.(60) Staszak-Jirkovsky,́ J.; Malliakas, C. D.; Lopes, P. P.; Danilovic,N.; Kota, S. S.; Chang, K. C.; Genorio, B.; Strmcnik, D.; Stamenkovic,V. R.; Kanatzidis, M. G.; Markovic, N. M. Design of Active and StableCo-Mo-Sx Chalcogels as pH-Universal Catalysts for the HydrogenEvolution Reaction. Nat. Mater. 2016, 15, 197−203.



15536

robust nicop/cop heterostructures for highly efficient...

Documents