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CoP3/CoMoP Heterogeneous Nanosheet Arrays as Robust Electrocatalyst for pH-Universal Hydrogen Evolution Reaction Deli Jiang, Yan Xu, Rong Yang, Di Li, Suci Meng, and Min Chen ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.9b00357 • Publication Date (Web): 25 Apr 2019 Downloaded from http://pubs.acs.org on April 26, 2019

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ACS Sustainable Chemistry & Engineering

CoP3/CoMoP Heterogeneous Nanosheet Arrays as Robust Electrocatalyst for pH-Universal Hydrogen Evolution Reaction Deli Jianga,*, Yan Xua, Rong Yanga, Di Lib, Suci Menga, Min Chena,* a

School of Chemistry and Chemical Engineering, Jiangsu University, Xuefu Road 301, Zhenjiang 212013, China

b

Institute for Energy Research, Jiangsu University, Xuefu Road 301, Zhenjiang 212013, China Corresponding author: Deli Jiang, Min Chen E-mail address: [email protected] (D.L. Jiang); [email protected] (M. Chen)

ABSTRACT Design and fabrication of highly active and earth-abundant hydrogen evolution reaction (HER) electrocatalyst which can work in a wide range of pH are pivotal for sustainable hydrogen production. In this work, we developed a novel CoP3/CoMoP heterogeneous nanosheet array supported on nickel foam (CoP3/CoMoP/NF HNAs) synthesized from a one-pot phosphidation of CoMoO4 nanosheet arrays. Within this heterojunction, plenty of tiny CoP3 nanopartices were embedded in the CoMoP nanosheet, forming abundant heterostructure interface, which can provide more active sites and improve the electric conduction, consequently improving the catalytic efficiency. The resulting CoP3/CoMoP/NF HNAs exhibited excellent HER activities with a low overpotential of 125 mV at 10 mA cm-2 and good durability in the acidic media, which ranks in the most-efficient Co-based TMPs electrocatalyst. Furthermore, the CoP3/CoMoP/NF HNAs showed high electrocatalytic HER activities and durability in both neutral and alkaline media. This facile synthesis of 1

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CoP3/CoMoP/NF heterostructure provides a novel way for the fabrication of other heterojunction metal phosphides for hydrogen production from water splitting.

KEYWORDS metal phosphides, nanosheet array, heterostructure, hydrogen evolution reaction, synergistic effect

INTRODUCTION Hydrogen is regarded as a renewable green energy which can alleviate the shortage of traditional fossil energy. Electrochemical water splitting is deemed as a promising environment-friendly way to produce hydrogen energy.1,2 Active and robust hydrogen evolution reaction (HER) electrocatalysts are indispensable to trigger the water splitting reaction efficiently.3,4 Owing to the proper hydrogen absorption energy, conventional highly active catalysts for HER are limited in Pt-group-based metals.5,6 However, the high cost and scarcity greatly limit their large-scale commercialization. Therefore, the search for the highly active and nonprecious catalysts for HER process is of great important.7-10 Recently, transition metal phosphides (TMPs), such as Ni2P, Ni5P4, CoP, FeP, Cu3P, MoP, WP, have received great attentions as a promising candidate owing to its intrinsic high HER activities, excellent stability, and earth abundance.11-25 Among them, Co-based monometallic (such as CoP, Co2P, CoP3) and bimetallic (such as NiCoP, FeCoP, Mn-Co-P, CoMoP) TMPs have shown promising HER activities due to their good electrical conductivity and high redox properties of Co.26-43 However, the HER activities of large majority of the still suffer from relatively large overpotential because of the unproper binding energy of the reaction immediate and the poor 2


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electron transfer kinetics. Therefore, it is highly desirable to develop an effective strategy to enhance the HER activity of Co-based TMPs electrocatalysts. Generally, compositional engineering to modulate the electronic structure and geometric engineering to provide more accessible active sites are two effective approaches to improve the electrocatalytic efficiency.44,45 In particular, combination of Mo elements and 3d transition metals (i.e., Fe, Co, Ni, etc.) to form bimetallic eletrocatalysts has recently emerged as a promising way to improve the catalytic performance owing to the synergistic interplay of the different active metal in the enhancing the electronic conductivity, providing more active sites, and modulating the binding energy of hydrogen.46-50 However, compared with the introduction of 3d transition metals, construction of CoMo or NiMo-based bimetallic TMPs electrocatalysts were less reported, possibly owing to the large difference in the atomic radius between the 3d transition metals and Mo, which renders the synthesis rather difficulty. As limited examples, Huang et al. developed Co5Mo1.0P nanosheet arrays electrocatalyst grown on the nickel foam collector, and the electrocatalyst delivers a low HER overpotential (173 mV@10 mA cm−2) in acidic solution.34 In another work, Li et al. recently reported a CoMoP@C electrocatalyst synthesized by a pyrolysis method using polyoxometalate as the precursor, and the CoMoP@C exhibited higher pH-universal HER catalytic activity as compared to Pt/C.51 On the other hand, the integration of different active materials into a multicomponent heterojunction is quite attractive in the development of high-performance HER catalyst. However, as far as we know, the CoMo-TMPs-based heterojunction electrocatalyst has never been reported, and a facile synthesis of Co-based TMPs heterojunctions is therefore highly desired and undoubtedly remains a great challenge. 3



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Herein, we demonstrated a novel CoP3/CoMoP heterojunction nanosheet arrays (HNAs) electrocatalyst supported on nickel foam (CoP3/CoMoP/NF HNAs) synthesized from a one-pot phosphidation of CoMoO4 nanosheet arrays. In this heterostructure, plenty of small CoP3 nanopartices were decorated on the CoMoP nanosheet and generated abundant heterostructure interface, which can expose more active sites and improve the electric conduction,

consequently

improving

the

catalytic

efficiency.

As

a

result,

this

CoP3/CoMoP/NF HNAs exhibited excellent HER activities with a low overpotential of 125 mV at 10 mA cm-2 in acidic media, which ranks in the most-efficient Co-based TMPs electrocatalyst.

Furthermore,

this

CoP3/CoMoP/NF

HNAs

catalyst

showed

high

electrocatalytic ability and durability under both neutral and alkaline electrolytes. Our work provides new light on the synthesis of heterojunction TMPs electrocatalyst with abundant interfaces for efficient HER.

EXPERIMENTAL Materials. Na2MoO4·2H2O, Co(NO3)2·6H2O, NaH2PO2·H2O, KOH, Na2HPO4, KH2PO4, HCl and H2SO4 were purchased from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China). Pt/C was purchased from Aladdin (Shanghai, China). All chemicals were analytical grade and used without further purification. Synthesis of CoP3/CoMoP/NF HNAs. The CoMoO4/NF was firstly fabricated by a hydrothermal method based on previously reported methods.52,53 The as-prepared CoMoO4/NF (2×2 cm, 36 mg) was placed at the center of the tube furnace, and the NaH2PO2·H2O in a porcelain boat was placed at upstream side of tube furnace. The sample was calcined at 350 °C for 2 h with a ramp rate of 2 °C/min under N2 atmosphere.54 The mass 4


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ratio of CoMoO4 and NaH2PO2·H2O is 1:5, and the samples were donated as CoP3/CoMoP-5/NF NAs. The CoP3/CoMoP-5/NF NAs was obtained after the temperature was cooled down to room temperature. By changing the mass ratios of CoMoO4 and NaH2PO2·H2O (1:3 and 1:8), the CoP3/CoMoP-3/NF and CoP3/CoMoP-8/NF NAs samples were obtained. Structural characterization. The X-ray diffraction patterns (XRD) of the samples were recorded on a Bruker D8 Advance X-ray diffractometer. Scanning electron microscopy (SEM, S-4800ⅡFESEM), transmission electron microscopy (TEM, Tecnai 12), and high-resolution TEM (HRTEM, Tecnai G2 F30) were used to characterized the morphology of the samples. The surface of samples was analyzed by the X-ray photoelectron spectroscopy (XPS, ESCA PHI500 spectrometer) measurements with an Al Kα radiator. Electrochemical measurements. The electrochemical measurements were carried out in a three-electrode setup on a CHI-760E electrochemical workstation. The CoP3/CoMoP/NF, Pt wire, and Ag/AgCl electrode were used as the working electrode, counter electrode, and reference electrode, respectively. The 0.5 M H2SO4 (pH = 0), 1 M PBS (pH = 7) and 1 M KOH (pH = 13.6) aqueous solutions were used as the electrolytes, respectively. All potentials measured in this study were referred to reversible hydrogen electrode (RHE) calculated by the equation of ERHE = EAg/AgCl + 0.197 V + 0.059 × pH.

The electrocatalytic activities of the samples were evaluated by the linear sweep voltammograms (LSV) curves at a scan rate of 5 mV s-1 with 90% iR-correction. The electrochemical active surface area (ECSA) was evaluated from the double-layer capacitance (Cdl), obtained from CVs at different scan rates of 20, 40, 60, 80, 100, 120, 140, and 160 mV 5



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s-1 in a small potential range. Electrochemical impedance spectroscopy (EIS) measurements were carried out at a frequency range from 0.01 Hz to 104 Hz. The stability of CoP3/CoMoP-5/NF NAs was estimated by successive CV for 1500 cycles and constant voltage chronoamperometry. During the measurement, fresh electrolyte solution was added to compensate its consumption.

RESULTS AND DISCUSSION The synthetic procedure of the CoP3/CoMoP/NF HNAs is schematically displayed in Figure 1a. Firstly, the three-dimensional CoMoO4 nanosheet arrays (NAs) precursor was fabricated by a facile hydrothermal treatment of Co(NO3)2·6H2O and Na2MoO4·2H2O. This CoMoO4 precursor was further transferred into the CoP3/CoMoP/NF HNAs by a subsequent phosphidation reaction. From the SEM image shown in Figure 1b, one can observe that the CoMoO4 nanosheets was vertically growth on the NF substrate, forming a 3D interconnected porous network with a plenty of void spaces favorable for the mass transport and electrolyte penetrate. The high-magnification image further reveals the smooth surface and good crystalline degree of these CoMoO4 nanosheets (Figure S1). After phosphidation, this nanosheet structure of CoMoO4 was largely retained, but the surface of the nanosheets became rough with many appeared small particles, forming a heterostructure (Figure 1c). This heterojunction

nanosheet

with

abundant

interface is

expected to

provide more

electrochemically accessible sites and enable the fast charge transfer, contributing to the enhanced HER efficiency (see the schematic in Figure 1a).

The structural feature can be further revealed by the TEM images, where many tiny 6


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nanoparticles were decorated on the surface of CoMoP (Figure 2a,b). The HRTEM image shows distinct lattice fringes with an interplanar spacing of 0.229 nm, corresponding well to the (112) lattice plane of the CoMoP, whereas the lattice spacing of 0.187 nm can be ascribed to the (400) plane of the CoP3 phase (Figure 2c), confirming the formation of CoP3/CoMoP heterostructure. Notably, no distinct lattice fringes can be observed in some part of the CoMoP substrate, indicative of the corresponding amorphous nature induced by the phosphidation reaction. Our observation is somewhat similar with the previously reported Ni2P@FePOx hybrid catalyst, of which the crystalline Ni2P nanoparticles were embedded in the amorphous FePOx substrate. The HAADF-STEM and the corresponding EDS elemental mapping images indicate that the Co and P elements not only homogeneously distribute the whole nanoplate but also concentrate on the surface crystalline nanoparticles, suggesting that the small nanoparticles domain are mainly composed of CoP3 and CoMoP (Figure 2d), which is consistent with the HRTEM result. All the above observations indicate that the heterojunction nanosheet mainly comprising of crystalline CoP3 and CoMoP nanosheets was fabricated. The XRD pattern and XPS were used to characterize the phase and chemical compositional evolution of the samples. As shown in Figure 3a, the diffraction peaks located at 28.4°, 33.9°, 36.9°, 39.5°, 55.6°, 59.3°, 63.2°can be indexed to the (002), (-222), (400), (040), (-532), (024) and (260) planes of CoMoO4 (JCPDS No. 21-0868).37 After phosphidation, all the diffraction peaks can be well indexed to the CoMoP (JCPDS No. 71-0478) and CoP3 (JCPDS No. 29-0496) phases.51,27 The chemical state and surface composition of the CoP3/CoMoP-5/NF HNAs was analyzed by XPS. For the Co 2p 2p3/2 7



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(Figure 3b), the binding energies at 779.6, 782.3, and 787.5 eV are attributed to Co-P, oxidized Co species and satellite peak, respectively. Similarly, for the Co 2p1/2, the binding energies at 797.2, 798.7, 804 eV can be ascribed to Co−P, oxidized Co species and satellite peak, respectively.55 The two peaks at 227.5 and 230.8 eV of Mo 3d are assigned to Moδ+ species (0 < δ < 4) (Figure 3c). Other two doublets located at 232.3/235.0 eV and 228.8/231.9 eV are assigned to Mo6+ and Mo4+, respectively, indicating the surface oxidation of CoMoP.56,57 For the P 2p spectra (Figure 3d), the doublet at 128.8 and 130.1 eV can be attributed to Mo−P or Co−P in metal phosphide.45 These results indicate that Co and Mo have positive charges (δ+), while the P has negative charge (δ-), suggesting the existence of electron density transfer from Co and Mo to P. The peak located at 133.0 eV can be ascribed to oxidized phosphorus species PO43-, possibly resulted from oxidation of the sample expose in the air, which is a common phenomenon observed in the metal phosphides.34 The electrochemical HER performances of the CoP3/CoMoP/NF HNAs catalysts were first measured in 0.5 M H2SO4. As shown in Figure 4a, the 20% Pt/C electrocatalyst delivers high HER activity with the onset overpotential approach to 0 mV and an overpotential of 54 mV to reach10 mA cm-2, whereas the bare CoMoO4 catalyst exhibits the poor HER activity with an overpotential of 271 mV when the current density is 10 mA cm-2. After conversion into CoP3/CoMoP HNAs, the CoP3/CoMoP/NF HNAs catalysts show significantly enhanced HER activity as compared to the bare CoMoO4 catalyst. Among the CoP3/CoMoP HNAs, the CoP3/CoMoP-5/NF HNAs shows the highest HER activity with a low overpotential of 125 mV to reach 10 mA cm-2 and a high current density towards 300 mA cm-2. The Tafel slope derived from the polarization curve was determined to illustrate catalytic kinetics for HER 8


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(Figure 4b). The commercial Pt/C loaded onto NF shows a Tafel slope of 34.3 mV dec-1. Remarkably, CoP3/CoMoP-5/NF HNAs exhibits a Tafel slopes of 61.1 mV dec-1, which is significantly smaller than those of CoMoO4 (102.5 mV dec-1), indicating the more favorable HER kinetics following a Volmer–Heyrovsky mechanism.58 It is worth mentioning that the electrocatalytic HER activity of CoP3/CoMoP compares favorably to those of recently reported high-performance earth-abundant HER catalysts (Figure 4c and Table S1). To provide insight into the reasons for the high HER performance in acidic solution, the double-layer capacitances (Cdl) was measured by a simple CV method (Figure S2a,b), which is proportional to the electrocatalytic active surface areas (ECSA) of the electrodes. As shown in Figure 4d, the CoP3/CoMoP-5/NF HNAs exhibits a larger Cdl value (6.72 mF cm-2), almost 2.6 folds than that of CoMoO4 (2.65 mF cm-2). This enhanced Cdl value of CoP3/CoMoP heterostructure should be attributed to the 3D NAs heterostructure which could render more active sites and contribute to a larger ECSA. Further, we normalized the polarization curves with the ECSA (Figure 4e) and found that the CoP3/CoMoP-5/NF HNAs still shows substantially improved HER performance as compared to the CoMoO4, revealing that the synergistic effect of the CoP3 and CoMoP in the heterostructure possibly dominates the intrinsic high HER performance. Electrochemical impedance spectrum (EIS) measurements are then carried out to investigate the reaction kinetics of the electrode under HER process at the interface between the electrode and electrolyte. It is obvious that CoP3/CoMoP-5/NF HNAs shows an even smaller Rct (1.91 Ω) than that of CoMoO4 (10.8 Ω) indicating a lower charge transfer resistance on the surface of CoP3/CoMoP-5/NF HNAs electrocatalyst (Figure 4f). These observations indicate that the interplay of CoP3 and CoMoP and the typical 9



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heterostructure with more active sites account for the subsequently enhanced HER performance. Significantly, CoP3/CoMoP/NF HNAs also exhibited outstanding HER performance in alkaline and neutral medias. Figure 5a shows the polarization curves of bare NF, NF modified with Pt/C, CoMoO4/NF, and CoP3/CoMoP-5/NF catalysts measured in 1 M KOH. The CoP3/CoMoP-5/NF HNAs shows much higher activity, achieving an overpotential of only 110 mV to reach 10 mA cm-2, significantly superior to that of CoMoO4 (251 mV) and comparable to other previously reported Co-based TMPs electrocatalysts (Table S2). The corresponding Tafel slopes of the catalysts were also calculated to illustrate the electron transfer kinetics (Figure 5b). Remarkably, CoP3/CoMoP-5/NF exhibits a much lower Tafel slope of 64.1 mV dec-1, compared to the CoMoO4 (126.2 mV dec-1), indicating the fast electron transfer progress in alkaline solution. We also calculated the Cdl value of the as-prepared CoMoO4/NF and CoP3/CoMoP-5/NF electrocatalysts based on the EDLCs. The CoP3/CoMoP-5/NF exhibits a larger Cdl value (4.52 mF cm-2) than the bare CoMoO4 (1.06 mF cm-2), again confirms that the CoP3/CoMoP-5/NF has an enlarged catalytically active surface area (Figure S2c,d). The EIS spectra result demonstrates that the CoP3/CoMoP-5/NF HNAs possesses the smaller charge-transfer resistance than that of CoMoO4/NF, indicating that the formed heterojunction nanosheets contributes to the promoted charge transfer efficiency and HER catalytic activity. For the case in the neutral solution, the CoP3/CoMoP/NF HNAs also shows high HER activity. The polarization curve in Figure 6a shows an overpotential of 89 mV to reach 10 mA cm-2 for CoP3/CoMoP-5/NF HNAs sample in 1.0 M PBS solution, much smaller than that of 10


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CoMoO4 (294 mV). As for the Tafel plots, the CoP3/CoMoP-5/NF HNAs exhibits a Tafel slope of 96.5 mV dec-1, which is lower than that of bare CoMoO4 (142.6 mV dec-1), indicating its faster kinetics for HER in neutral solution (Figure 6b). This enhanced electrocatalytic activity of the CoP3/CoMoP/NF HNAs can be attributed to the enlarged active surface area and promoted charge transfer. As shown in Figure 6c (obtained from the CV shown in Figure S2e,f), the CoP3/CoMoP-5/NF HNAs shows the much larger Cdl value than the CoMoO4/NF. The improved charge transfer can be verified by the EIS measurements, as shown in Figure 6d, where the CoP3/CoMoP-5/NF HNAs shows the much smaller charge-transfer resistance than that of CoMoO4/NF. Furthermore, the BET surface areas of the CoMoO4 and CoP3/CoMoP-5/NF were also measured (Figure S3). The enhanced surface area of CoP3/CoMoP-5/NF provides more active sites, contributing to the improved electrocatalytic activity. We compared the electrocatalytic HER performance of recent reports about Co- or Mo-containing TMPs and found that the present CoP3/CoMoP/NF HNAs is among the most active HER catalysts in neutral environment (Table S3). Catalytic stability is important for practical applications of electrocatalytic water splitting. We further tested the durability of the CoP3/CoMoP-5/NF HNAs catalyst under electrocatalytic operation by using the chronoamperometric method in the acidic, alkaline, and neutral electrolytes, respectively. It is evident that the initial current density exhibited no obvious decrease at a constant overpotential of -110 mV over an extended period of 20 h (Figure 7a). Moreover, the durability of CoP3/CoMoP-5/NF HNAs was further evaluated by 1000 continuous cycles and no significant change was observed compared to the initial one, indicating the excellent stability of the CoP3/CoMoP-5/NF HNAs catalyst. The XRD of the 11



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CoP3/CoMoP-5/NF HNAs catalyst after the stability tests are shown in Figure S4. Corresponding SEM images after stability measurements shows that the CoP3/CoMoP-5/NF HNAs essentially maintains the original structure (Figure S5). Based on these results, it can be concluded that the CoP3/CoMoP/NF heterostructures can be used as an all-pH electrocatalyst with high HER performance and stability. Overall, the high HER activity and stability of CoP3/CoMoP heterostructure nanosheet electrocatalyst can be attributed to the following advantages: (i) The Co and Mo in CoP3/CoMoP could effectively tunes the electronic structures and lower the energy barriers during the electrocatalyst process, and the CoP3/CoMoP heterostructure nanosheet have abundant interface between CoP3 particles and CoMoP nanosheet can enable the improved electronic conductivity which was confirmed by the EIS spectra results and the optimized reaction immediate energetics, which results in accelerated HER kinetics. (ii) The formation of CoP3/CoMoP heterostructure nanosheet with interconnected 3D architecture benefits for exposing of more active sites, electrolyte penetration, and release of gas bubbles for the efficient HER. (iii) The unique heterostructure nanosheet arrays supported on the highly conductive NF is beneficial for the charge transfer, diffusion of ions and structural integration, thus boosting HER activity and stability.

CONCLUSIONS In summary, a novel CoP3/CoMoP heterostructure electrocatalyst was synthesized by the phosphidation of CoMoO4 nanosheet arrays. The resultant CoP3/CoMoP/NF magnified excellent HER activities with a low overpotential of 125 mV to reach 10 mA cm-2 and durability in the acidic media, which rank in the most-efficient Co-based TMPs electrocatalyst. 12


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Furthermore, the CoP3/CoMoP/NF showed remarkable electrocatalytic HER ability and durability in both neutral and alkaline media. We attributed this high pH-universal HER activities to the synergistic effect of the typical HNAs structure and interfacial effect of the heterojunction between the two components, which could provide more active sites and improve the electric conduction, consequently improving the catalytic efficiency. This facile but effective synthesis of CoP3/CoMoP/NF HNAs heterostructure provides a new approach to the development of other heterojunction metal phosphides for energy conversion.

ASSOCIATED CONTENT Supporting Information. A SEM image of bare CoMoO4, CV curves measured in different electrolytes, BET surface areas, XRD pattern and SEM images after the stability test, comparisons of HER activities of reported Co- or Mo-based HER electrocatalysts in different electrolytes.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; [email protected]

NOTES The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by National Nature Science Foundation of China (21606111 and 21878130), Natural Science Foundation of Jiangsu Province (BK20150482), China 13



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Postdoctoral Science Foundation (2018M642181 and 2017T110453).

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(45) 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/Nickel Sulfide Interface Nanowires for Synergistic Hydrogen

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Page 23 of 30 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


a

H+, H2O

H2

Phosphorization Interface

CoMoO4/NF

CoP3/CoMoP/NF

c

b

1 μm

CoMoP

CoP3

d

1 μm

200 nm

Figure 1 (a) Synthetic procedure of the CoP3/CoMoP/NF HNAs and schematic of the interface between the CoP3 and CoMoP; SEM images of CoMoO4 (b) and CoP3/CoMoP/NF HNAs (c,d).

23



Page 24 of 30

c

a

c2

c1 5 nm

b

c2

c1 CoP3

CoMoP

d400= 0.187 nm

d112 = 0.229 nm

1 nm

200 nm

2 nm

d

500 nm

Co

Mo

P

Figure 2 TEM images of (a) CoMoO4 and (b) CoP3/CoMoP-5/NF HNAs; (c) HRTEM and (d) HAADF-STEM image and the corresponding EDS elemental mapping images of Co, Mo and P of CoP3/CoMoP-5/NF HNAs.

24


Page 25 of 30

(a)

(b)

Co 2p

Intensity (a.u.)

Intensity (a.u.)

CoP3/CoMoP-5/NF

PDF #29-0496 PDF #71-0478

Co 2p3/2 Co 2p1/2 Sat. Sat. Co-P

Co-P

CoMoO4/NF

PDF #21-0868 10

20

30

(c)

40 50 2 Theta ()

60

70

80

770 775 780 785 790 795 800 805 810 Binding Energy (eV)

(d)

P 2p

Mo 3d

Mo6+ Moδ+ 3d5/2

Intensity (a.u.)

Intensity (a.u.)

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


Mo4+

Moδ+ 3d5/2

226

228

230 232 234 236 Binding Energy (eV)

238

240

126

PO43-

P 2p3/2 P 2p

1/2

128

130 132 134 Binding Energy (eV)

136

Figure 3 (a) XRD patterns of the as-prepared samples grown on NF. High-resolution XPS spectra of (b) Co 2p, (c) Mo 3d and (d) P 2p region for the CoP3/CoMoP-5/NF HNAs.

25



b0.4

0

NF CoMoO4/NF

-30

CoP3/CoMoP-3/NF CoP3/CoMoP-5/NF

-60

CoP3/CoMoP-8/NF Pt/C

-90

-120 -1.0

CoMoO4/NF CoP3/CoMoP-5/NF

Overpotential (V)

Current Density (mA cm-2)

a

Pt/C

102.5 mV/dec

0.3

0.2

61.1 mV/dec

0.1

34.3 mV/dec -0.8

-0.6

-0.4

-0.2

0.6

0.0

0.9

1.2

1.8 log j (mA cm-2)

1.5

2.1

2.4


1.2

Δj (mA cm-2)

CoP3/NiP2

NiMoP2

p-MoO3-x

CoP UPNS

CoMoP

Co2P@C/CC

d1.6 CoP/CC

100

Co-P

150

Co0.9S0.58P0.42

200

Co5Mo1.0P

250

CoP@CC

300

CoP3/CoMoP

Overpotential (mV)

c

CoMoP-600

Potential(V) vs RHE

6.72 mF/cm2

0.8

0.4

2.65 mF/cm2

50

0.0

0

0

Catalysts

e

0

f

CoMoO4/NF

-Z'' (ohm)

CoP3/CoMoP-5/NF

JECSA (mA cm-2)

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

Page 26 of 30

-10 -20

40

80 120 Scan Rate (mV/s)

160

7

CoMoO4/NF

6

CoP3/CoMoP-5/NF

5

Rs

4

CPE Rct

3 2

-30 -40 -0.5

1 0 -0.4 -0.3 -0.2 Potential(V) vs RHE

-0.1

0

2

4

6

8 10 Z' (ohm)

12

14

16

Figure 4 HER performance of pure NF, NF modified with Pt/C, CoMoO4/NF, and CoP3/CoMoP/NF HNAs catalysts measured in 0.5 M H2SO4: (a) Polarization curves; (b) Corresponding Tafel plots derived from (a); (c) Comparisons of the overpotential at the current density of 10 mA cm-2; (d) Cdl measurements of CoMoO4/NF and CoP3/CoMoP-5/NF HNAs; (e) Polarization curves from (a) normalized to the ECSA. (f) Nyquist plots of CoMoO4/NF and CoP3/CoMoP-5/NF HNAs with the fitting curves.

26


Page 27 of 30

b0.4

0


-30

Overpotential (V)


a

-60 -90 -120

NF CoMoO4/NF

-150

CoP3/CoMoP-5/NF

-180

Pt/C

126.2 mV/dec

0.3

0.2

64.1 mV/dec

0.1

37.2 mV/dec

Pt/C

-0.6

c1.5

-0.5

-0.4 -0.3 -0.2 -0.1 Potential(V) vs RHE

0.0

0.6

0.1

4.52 mF/cm2

0.9 0.6

1.06 mF/cm2

0.3

1.2 1.5 1.8 log j (mA cm-2)

CoMoO4/NF

2.5

-Z'' (ohm)

CoP3/CoMoP-5/NF

0.9

d3.0

CoMoO4/NF

1.2

Δj (mA cm-2)

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


2.1

2.4

CPE

CoP3/CoMoP-5/NF

2.0

Rs

Rct

1.5 1.0 0.5 0.0

0.0 0

40


160

1

2

3

4 5 Z' (ohm)

6

7

8

Figure 5 HER performance of pure NF, NF modified with Pt/C, CoMoO4/NF, and CoP3/CoMoP-5/NF HNAs measured in 1 M KOH: (a) Polarization curves; (b) Corresponding Tafel plots derived from (a); (c) Linear fitting of the capacitive currents versus CV scan rates for CoMoO4/NF and CoP3/CoMoP-5/NF HNAs; (d) Nyquist plots of CoMoO4/NF and CoP3/CoMoP-5/NF HNAs.

27



b

CoMoO4/NF

0.4

CoP3/CoMoP-5/NF

-30

Overpotential (V)


a0

-60 Ni foam CoMoO4/NF

-90

CoP3/CoMoP-5/NF

c1.6

-0.5

0.2

96.5 mV/dec

0.1

44.8 mV/dec

-0.4 -0.3 -0.2 -0.1 Potential(V) vs RHE

0.0

0.1

0.0 0.6

d

CoMoO4/NF

0.8

0.4

0.83 mF/cm2

-Z'' (ohm)

7.06 mF/cm2

0.9

5

1.2 1.5 1.8 log j (mA cm-2)

CoMoO4/NF

4

CoP3/CoMoP-5/NF

1.2

142.6 mV/dec

Pt/C

0.3

Pt/C

-120 -0.6

Δj (mA cm-2)

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

Page 28 of 30

2.1

2.4

CPE

CoP3/CoMoP-5/NF

Rs

3

Rct

2 1 0

0.0 0

40


160

2

4

6

8 10 Z' (ohm)

12

14

Figure 6 HER performance of bare NF, NF modified with Pt/C, CoMoO4/NF, and CoP3/CoMoP-5/NF HNAs measured in 1 M PBS: (a) Polarization curves; (b) Corresponding Tafel plots derived from (a); (c) Linear fitting of the capacitive currents versus CV scan rates for CoMoO4/NF and CoP3/CoMoP-5/NF HNAs; (d) Nyquist plots of CoMoO4/NF and CoP3/CoMoP-5/NF HNAs.

28


0 -10 0

b Current Density (mA cm-2)

a -2

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


Current Density (mA cm )

Page 29 of 30

1 M KOH

-10 0

1 M PBS

-10

0.5 M H2SO4

-20

0

5

10 Time (h)

15

20

0

Initial 1000th

Initial 1000th

Initial 1000th

-20 -40 -60 -80 -100

1M PBS

1M KOH

-120 -0.4

-0.2

0.5 M H2SO4

-0.2 0.0 0.0 Potential(V) vs RHE

-0.2

0.0

Figure 7 HER stabilities of (a) Chronoamperometric curves (I-t) of CoP3/CoMoP-5/NF HNAs at the constant overpotential of 110 mV (in 1 M KOH), 90 mV (in 1 M PBS) and 100 mV (in 0.5 M H2SO4); (b) Polarization curves of CoP3/CoMoP-5/NF HNAs before and after 1000 cycles for the durability tests.

29



Page 30 of 30

For Table of Contents Use Only

A CoP3/CoMoP heterogeneous electrocatalyst was synthesized from a one-pot phosphidation of CoMoO4 nanosheet array and exhibited robust electrocatalytic activity for pH-universal hydrogen evolution reaction.

30


CoMoP Heterogeneous Nanosheet Arrays ... - ACS Publications

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