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Iron-tuned super nickel phosphide microstructures with high activity for electrochemical overall water splitting
Author’s Accepted Manuscript Iron-tuned Super Nickel Phosphide Microstructures with High Activity for Electrochemical Overall Water Splitting Huawei Huang, Chang Yu, Changtai Zhao, Xiaotong Han, Juan Yang, Zhibin Liu, Shaofeng Li, Mengdi Zhang, Jieshan Qiu www.elsevier.com/locate/nanoenergy
PII: DOI: Reference:
S2211-2855(17)30147-7 http://dx.doi.org/10.1016/j.nanoen.2017.03.016 NANOEN1844
To appear in: Nano Energy Received date: 9 December 2016 Revised date: 1 March 2017 Accepted date: 6 March 2017 Cite this article as: Huawei Huang, Chang Yu, Changtai Zhao, Xiaotong Han, Juan Yang, Zhibin Liu, Shaofeng Li, Mengdi Zhang and Jieshan Qiu, Iron-tuned Super Nickel Phosphide Microstructures with High Activity for Electrochemical Overall Water Splitting, Nano Energy, http://dx.doi.org/10.1016/j.nanoen.2017.03.016 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Iron-tuned Super Nickel Phosphide Microstructures with High Activity for Electrochemical Overall Water Splitting Huawei Huang1, Chang Yu1, Changtai Zhao, Xiaotong Han, Juan Yang, Zhibin Liu, Shaofeng Li, Mengdi Zhang, Jieshan Qiu* State Key Lab of Fine Chemicals, School of Chemical Engineering, Liaoning Key Lab for Energy Materials and Chemical Engineering, Dalian University of Technology Dalian 116024， Liaoning，China *
Corresponding author. [email protected] (J. S. Qiu)
Abstract Large-scale hydrogen production by electrolytic splitting of water is mainly governed by high-efficient yet cheap electrocatalysts that could be capable of accelerating the sluggish hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Herein, we report Fe-tuned Ni2P electrocatalysts with controllable morphology and structure by regulating atomic ratio of Ni/Fe, and reveal the Fe species-modulated electronic state behaviors and boosted catalytic activity for water splitting. The electrocatalytic activity of Fe-tuned Ni2P nanosheets for both HER and OER can be further enhanced by assembling the nanosheets vertically on conductive 2D carbon fiber (CF) matrix to make hierarchical monolithic 3D electrode (Ni1.5Fe0.5P/CF), which features more accessible active sites and open structure that helps to speed up both the HER and OER. The improved electrocatalytic activity of Ni1.5Fe0.5P/CF is due to the combined synergistic effects of the high conductivity of CF matrix and the strong interaction between active species and the CF support, as evidenced by a low overpotential of 293 mV to achieve a high current density of 100 mA cm-2 with superiorly long-term stability for OER. When the monolithic 3D Ni1.5Fe0.5P/CF electrodes were used as both anode and cathode for overall water splitting, a current density of 10 mA cm-2 is
1
The first two authors contributed equally to this work
1
generated at a low potential of 1.589 V, while at 20 mA cm-2, the potential is only 1.635 V. It has been demonstrated that modulating metal catalysts (nanosized nickel phosphide) with iron atoms is powerful, and may open up avenues to the design and fabrication of highly efficient catalysts for energy storage and conversion.
Graphical Abstract A strategy for assembling iron-tuned nickel phosphides on 2D carbon fiber sheet to configure binder-free 3D Ni1.5Fe0.5P/CF as water splitting catalyst is presented, indicative of the iron species-triggered positive effects on structure and electrochemical activities. Interestingly, the as-made 3D Ni1.5Fe0.5P/CF can achieve up to 100 mA cm-2 only at overpotential of 293 mV for oxygen evolution, and can generate 10 mA cm-2 at a low potential of 1.589 V for overall water splitting.
Keywords: Water splitting, Nickel-iron phosphide, 2D carbon fiber sheet, Oxygen evolution reaction, Bifunctional electrocatalyst
1. Introduction Electrochemical water splitting into hydrogen and oxygen provides an attractive pathway for sustainable energy conversion and storage.[1, 2] The two half reactions: hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), are involved in this system. Lowering the required activation energy barriers is highly concerned to achieve fast kinetic processes of both HER and OER for the practical requirements. To solve these concerned issues, the electrocatalysts with high-efficiency and long-term durability are highly required.[3-5] Platinum and noble metal oxides (RuO2 and IrO2) are generally used and possess unique superiorities. Nevertheless, their rareness and high cost make them less attractive for large-scale and practical application in water splitting.[6-10] In this case, it is the research focus to develop highly active and economical electrocatalysts based on earthabundant elements for HER and OER.[11-18] In particular, designing bifunctional 2
electrocatalysts that are simultaneously capable of catalyzing both HER and OER, are extremely promising and attractive owing to simplified operating system and the reduced overall cost associated with equipment.[19-22] Up to now, a series of transition metal selenides, oxides, chalcogenides, borides, and phosphides are capable of acting as symmetric bifunctional catalysts for overall water splitting, such as NiSe,[19] CoMnO,[20] Ni3S2,[23] Co9S8@MoS2/carbon nanofibers (CNFs),[24] Co2B,[25] CoP,[6, 26, 27] and Ni2P.[11, 12] Of these non-precious metal catalysts, transition-metal phosphides (TMPs) have been proved to be superior catalysts for HER and OER by theoretical calculation and experimental results. [2, 28, 29] Nevertheless, much remains to be done to gain an insight into the trends and limitations in their performance and further improve their intrinsic activity. It has been also demonstrated that modulating electronic structure of transition metal-based catalysts by doping additional metal atoms is an efficient way to enhance the intrinsically catalytic performance in energy-related devices.[30-35] The transition metal is capable of promoting the charge-transfer between transition metal atoms and metal platinum, thus leading to the enhanced electrochemical activities.[36] This is also the case for electrochemical water splitting. For example, Hu et al. found that Ni, Fe, and Co transition metals have significantly positive effects for enhancing the HER catalytic performance of the amorphous molybdenum sulfide films, in which these additional metals interact with unsaturated sulfur atoms in MoS3 and promote the growth of the MoS3 film with large surface area.[37] Recently, Vojvodic et al. developed amorphous FeWCo gel by introducing W into FeCo oxy-hydroxides to optimize the adsorption energy for OER intermediates, indicative of a highly efficient catalytic activity.[38] Mukerjee et al. found that the charge-transfer effects of the Co element in the Ni-Fe-Co mixed-metal oxides facilitate the formation of conductive NiOOH, and activate the Fe sites at a lower overpotential.[39] Zheng et al. reported that the bimetallic Ni0.33Co0.67S2 presents a higher HER catalytic activity in comparison with that of 3
the monometallic chalcogenide catalysts (NiS2 and CoS2) due to its fast electron transport and large surface area.[40] With the doping of Co atoms, the energy barrier for H-H bond formation on the iron pyrite FeS2 nanosheets decorated on carbon nanotubes (Fe0.9Co0.1S2/CNT) are greatly decreased and the HER catalytic activities are improved to a great degree.[31] Yang’s group reported that Fe incorporated into nickel sulfide (iron-nickel sulfide) ultrathin nanosheets, achieving an enhanced HER catalytic performance and stability.[41] These doping strategies feature intriguing charms and unique superiorities for electrocatalysis, which would be also applicable for TMPs system, and the binary metal phosphides as symmetric bifunctional electrocatalysts have been rarely explored. With this information in mind, it would be a promising strategy to modulate the electronic structure of phosphides by configuring highly active binary metal phosphides electrocatalysts elaborately to enhance the electrocatalytic activities. Also, it is necessary and significant to gain a deep insight into intrinsic catalytic activities of the as-made materials with well-controlled internal structure for the design of high-performance electrocatalysts. In the present study, we develop a facile and scalable strategy to synthesize a series of NixFe2-xP (0
2
, respectively. To further intensify the electron and mass transport, the Ni1.5Fe0.5P nanosheets
were further vertically assembled on the carbon fiber (CF) conductive substrate, yielding a hierarchically-structured three-dimensional (3D) monolithic electrode with more accessible active sites derived from open frameworks, strong binding force between CF substrate and electrochemically active species, and fast electron transport channels from interconnected CF matrix. Taking advantage of these unique merits, the as-made Ni1.5Fe0.5P/CF deliver an overpotential as low as 293 mV for achieving current density of 100 mA cm-2 and an amazingly long-term operational stability when directly utilized as an anode electrocatalyst for OER in 1 M KOH electrolyte. For overall water splitting, the current densities of 10 and 20 mA cm-2 can be generated at 1.589 and 1.635 V (iR corrected), respectively, which is superior to most of non-precious bifunctional catalysts reported in literature so far. 2. Results and discussion Preparation and characterizations of the as-made NixFe2-xP The fabrication processes of NixFe2-xP phosphides are illustrated in Scheme 1. For a typical run, NiFe LDHs precursors were first synthesized via urea hydrolysis of mixed nickel nitrate and iron nitrate with different molar ratios in a sealed Teflon-lined stainless steel autoclave at 120 oC, where urea was decomposed into ammonia and carbon dioxide, and further reacted with metal ions during this procedure.[13] Afterwards, the as-prepared NiFe LDHs suffer from a topotactic conversion reaction through a low-temperature phosphorization process to produce the corresponding NixFe2-xP phosphides.[42] Furthermore, single-component Ni2P and Fe2P samples were also prepared following the same procedure.
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Scheme 1. Schematic process for the synthesis of NixFe2-xP.
By regulating and controlling the Ni/Fe molar ratio from 5:1, 4:1, 3:1, 2:1 to 1:1 in the precursors, a series of binary metal phosphides were synthesized and the actual Ni/Fe molar ratio in NixFe2-xP was accurately tuned, which was determined by inductively coupled plasma (ICP) mass spectrometry (Table S1). The morphologies and structures of the Ni2P, Fe2P, NixFe2-xP and their corresponding precursors were examined by the field emission scanning electron microscopy (FE-SEM), of which the typical images are shown in Figure 1 and Figure S1. It can be noted that the morphologies of as-made electrocatalysts vary and associate with the molar ratio of Ni/Fe in NixFe2-xP, nevertheless, well keep the features of their precursors after the phosphorization treatment. For unary metal phosphides, the pure Ni2P (Figure 1a) has a flat sheet structure, while the Fe2P (Figure 1b) features sphere-shaped structure with different sizes. Interestingly, after phosphorization, the Ni1.0Fe1.0P phosphides retain sphereshaped characteristics of Fe2P, of which some sphere-shaped structures are constituted by nanosheets (red dotted line in Figure 1c). It is also noted from energy dispersive X-ray (EDX) analysis (Figure S2) that the sphere zone made of nanosheets has a high Ni content (red dotted line in Figure S2), while the rest solid spheres have a high iron content. The corresponding 6
EDX spectrum analysis in Figure S3 reveals that the molar ratio of Ni and Fe is nearly 1:1, which is consistent with the theoretical value. When the molar ratio of Ni and Fe increased to 2:1 or higher, the as-made Ni1.33Fe0.67P, Ni1.5Fe0.5P, Ni1.6Fe0.4P, and Ni1.67Fe0.33P show a sphere-shaped structure constituted by sheets radially grown from the center (Figure 1d-f and Figure S1), indicative of a well-preserved unique structure close to their corresponding precursors (Figure S1). And these subunit sheets feature honeycomb and rough texture derived from dehydration of the LDHs precursors during the annealing process,[42] implying the porous characteristics of the as-made NixFe2-xP, which will be evidenced by the following TEM results.
Fig 1. The morphologies of as-made samples: FE-SEM images of a) pure Ni2P, b) pure Fe2P, c) Ni1.0Fe1.0P, d, e) Ni1.5Fe0.5P, and f) Ni1.6Fe0.4P.
The typical powder X-ray diffraction (XRD) patterns of the as-made Ni2P, Fe2P, and bimetallic NixFe2-xP samples are displayed in Figure 2a. It can be clearly seen that the typical diffraction peak patterns of the as-made NixFe2-xP catalysts are basically consistent with that 7
of the Ni2P (JCPDS Card no. 03-0953). Noteworthily, the peaks of bimetallic NixFe2-xP broaden and slightly shift to large-angle due to the incorporation of Fe component (Figure 2 a, b and Table S2). Moreover, a weak peak at 40.26 o (blue dots in Figure 2b), corresponding to (111) plane of Fe2P (JCPDS Card no. 33-0670), appears when the atomic ratio of Ni/Fe decreased to 1:1 (namely, Ni1.0Fe1.0P), which may relate to the solid sphere-shaped structure illustrated in Figure 1c and Figure S2. These phenomena reveal that the Fe atoms can be doped into the Ni2P when the atomic ratio of Ni/Fe is larger than 2:1, and the crystal grain size decreases with the incorporation of Fe species.[2, 41, 43] In order to get more surface information about chemical composition and the electronic states of as-made samples, the Ni1.5Fe0.5P was further analyzed by depth X-ray photoelectron spectroscopy (XPS), of which the detailed results are shown in Figure 2c-f. The high-resolution depth spectrum of Ni 2p (Figure 2c) shows that the peak at 856.6 eV, corresponding to nickel phosphate, decreases rapidly with an increase of Ar ion etching time, which indicates that phosphate species resulting from partly surface oxidation only exist on the surface of catalyst. After sixty seconds of etching, such surface nickel phosphate species have been removed to a great degree, simultaneously, the intensity of the peaks at binding energies of 852.6 and 870.1 eV corresponding to Ni 2p3/2 and Ni 2p1/2, respectively, greatly increases. Compared with pure Ni2P (853.1 eV), a negative shift of Ni 2p3/2 peak was observed for Ni1.5Fe0.5P, which is probably attributed to the incorporation of Fe atom into Ni2P.[44] In the case of Fe 2p (Figure 2d), the peak intensity for Fe 2p1/2 and Fe 2p3/2, corresponding to the binding energy of Fe-P, also increase during Ar ion etching, while the peak of iron phosphate remains strong, indicative of a thick oxidized layer, which is also reflected in the high-resolution depth spectra of P (Figure 4e).[2, 45] It is well known that metal phosphate species dominating the surface layer of metal phosphides is an inevitable phenomenon, because the top layer of phosphides
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are prone to be oxidized when they are exposed to air, nevertheless the interior is homogeneous phosphides for the as-made Ni1.5Fe0.5P.[2, 28]
Fig 2. The structure characteristics and chemical compositions of the as-made samples: a) XRD patterns and b) zoom-in regional patterns of 39.5-45 o of the as-made NixFe2-xP samples. Depth XPS spectra of c) Ni, d) Fe, e) P and f) survey regions for Ni1.5Fe0.5P after 0-, 30-, 60-, 90-, 120-, 180-, 240-, and 360-s Ar ion etching. To further get insight into morphology and structural changes of materials before and after the phosphorization treatment, the as-synthesized Ni1.5Fe0.5 LDHs and Ni1.5Fe0.5P were further characterized by transmission electron microcopy (TEM) and high-resolution TEM (HR9
TEM), of which the detailed results are shown in Figure 3a-c. As illustrated in Figure 3a-b, the Ni1.5Fe0.5P keeps the integrated nanosheet-shaped structure similar to the LDHs precursor. Meanwhile, the numerous pores on the sheet surface are also observed, thus leading to more defects in the Ni1.5Fe0.5P matrix, which provide more contactable and accessible active sites to the electrolyte ions. Furthermore, the HR-TEM images in Figure 3c reveal the lattice fringes of 0.34 and 0.22 nm, corresponding to (001) and (111) planes of Ni1.5Fe0.5P, respectively, which further confirms the formation of the well crystallized Ni1.5Fe0.5P. In order to further study the chemical composition and element distribution of the as-made Ni1.5Fe0.5P sample, high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) was used. As shown in Figure 3d, it can be noted that the Ni, Fe, and P elements are almost uniformly distributed within the as-made Ni1.5Fe0.5P.
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Fig 3. TEM images of the as-made a) Ni1.5Fe0.5 LDHs precursors and b) Ni1.5Fe0.5P; c) HRTEM images of the Ni1.5Fe0.5P; d) HAADF-STEM image of Ni1.5Fe0.5P and the corresponding EDX elemental mapping images of Ni, Fe and P elements. Electrocatalytic performance of as-made samples The catalytic OER performance of Ni2P, Fe2P, and bimetallic NixFe2-xP samples were examined and compared with noble metal oxide (RuO2) in a typical three-electrode system, of which the detailed results are illustrated in Figure 4. It can be clearly seen from Figure 4a that the single-component Ni2P features a higher intrinsic activity in comparison to that of Fe2P. It is interesting that the catalytic activity of the single-component Ni2P can be also greatly improved in the presence of iron species. And the electrochemical activities vary with the molar ratios of Ni/Fe in NixFe2-xP, indicative of a component-dependent electrochemical behavior. Among these as-made electrocatalysts, the Ni1.5Fe0.5P (Ni/Fe = 3/1) delivers the best catalytic activity and only requires the overpotentials as low as 264 and 280 mV to achieve current densities of 10 and 20 mA cm-2, respectively (273 mV@10 mA cm-2 and 298 mV@20 mA cm-2, without iR corrected), which is much lower than commercial RuO2 (366 mV@10 mA cm-2 and 402 mV@20 mA cm-2) and superior to most of state-of-the-art OER catalysts (Figure 4b and Table S3 in the Supporting Information). The catalytic kinetics of as-made samples were estimated by the corresponding Tafel plots derived from Tafel equation: η = a + b log j (b is the Tafel slope and j represents the current density).[46-49] As illustrated in Figure 4c, of the adopted samples, the Ni1.5Fe0.5P delivers the lowest Tafel slope of 55 mV dec-1, indicative of fast kinetics for the efficient mass and electron transfer. Besides, the polarization curves of the as-made Ni1.5Fe0.5P catalyst (Figure S4) were also further tested at different scan rates (2, 5, 10, 15 and 20 mV s-1), where no significant changes in catalytic performance are observed, further confirming a sufficiently fast mass transport for the Ni1.5Fe0.5P electrode in the present system.[50, 51] To evaluate the stability of the catalyst, a 11
long-term galvanostatic test was performed at 10 mA cm-2 for 12 h. As shown in Figure 4d, the as-made Ni1.5Fe0.5P delivers a much better operational durability than RuO2. We also measured and analyzed the polarization curves of Ni1.5Fe0.5P sample before and after 1000 cycles with potential cycling between 1.2 and 1.7 V (vs RHE) at a scan rate of 100 mV s-1, of which the detailed results are shown in Figure S5. It can be noted that current density almost unchanged after 1000 cycles, indicative of a highly electrochemical cycling stability. The highly catalytic performance of Fe doped Ni2P is mainly attributed to the following combined factors: 1) the incorporation of Fe species endows the Ni2P nanosheets with smaller size, regular structure, and the modulated surface property and electronic states, thus leading to the enhanced reaction activity; 2) the open structure of non-solid spheres derived from the nanosheets are responsible for more accessible active sites to electrolytes, fast mass transport, and gas diffusion. The electrocatalytic HER polarization curves of the as-made catalysts and commercial Pt/C (20 wt% Pt) were also evaluated and compared in 1 M KOH. As we can see from Figure 4e, the as-made Ni1.5Fe0.5P still produces the optimal HER catalytic activity, achieving a cathodic current density of 10 mA cm-2 at the overpotential of 282 mV and Tafel slope of 125 mV dec-1 (Figure 4e-f), being lower than that of the monometallic Fe- or Ni-based phosphide catalysts. These results further imply that the Ni1.5Fe0.5P as the bifunctional catalyst is of great potential for both HER and OER.
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Fig 4. Catalytic OER performance of the as-made catalysts in 1 M KOH: a) Polarization curves with iR corrected (except the red dotted line, namely, the Ni1.5Fe0.5P without iR corrected) and b) overpotentials required at 10 mA cm-2 over a series of electrocatalysts. c) The corresponding Tafel plots. d) Long-term stability test measured at 10 mA cm-2 without iR corrected. Catalytic HER performance of the as-made catalysts in 1 M KOH: e) Polarization curves with iR corrected and f) corresponding Tafel plots of as-made samples.
Construction of 3D electrodes for water splitting For practical applications, a strong-coupled 3D architecture with highly electronic conductivity and abundantly active sites is required to avoid using the polymeric binders (e.g., 13
Nafion), because the utilization of these insulative binders may block the accessible active sites and increase the series resistance, thus resulting in degenerately catalytic activity.[51-53] As a highly conductive support material, CF is an ideal substrate to construct the 3D electrode due to the merits of lightweight and superior corrosion resistance.[54] Given that the as-made Ni1.5Fe0.5P features the best OER and HER catalytic activities, we further vertically assemble the Ni1.5Fe0.5P nanosheets on pretreated CF surface to produce a freestanding 3D electrode (Ni1.5Fe0.5P/CF), which can be directly used for electrolysis of water. The functional groups on pretreated CF surface act as the nucleation sites for the NiFe LDHs growth, thus leading to strong bonding force between CF and NiFe LDHs precursors.[54, 55] After hydrothermal process, the color of the CF varies from silver gray to faint orange, then turns to black after phosphorization process (Figure S6). The typical FE-SEM image of the 3D Ni1.5Fe0.5P/CF electrode is shown in Figure S7a-b. Compare with the pristine CF (Figure S8), it can be clearly observed that the Ni1.5Fe0.5P nanosheets uniformly and vertically grow on the CF. Furthermore, the corresponding EDX element mapping images (Figure S7c) reveal that the Ni, Fe and P elements homogeneously distribute on the CF and the corresponding molar ratio of Ni/Fe is 3.3:1 (Figure S7d). The representative XRD pattern (Figure S9) further demonstrates the existent carbon and crystallized Ni1.5Fe0.5P in the Ni1.5Fe0.5P/CF. The electrocatalytic OER performances of the integrated Ni1.5Fe0.5P/CF electrode (Ni1.5Fe0.5P mass loading: 1.38 mg cm-2) were also tested in alkaline medium (1 M KOH), of which the detailed results are shown in Figure 5a-c. For comparison, the pristine CF and RuO2 coated on CF (RuO2-CF) were also tested and shown in Figure 5. As shown in Figure 5a, the as-made 3D Ni1.5Fe0.5P/CF electrodes deliver a superior catalytic activity for OER and only need a low potential of 1.523 V (overpotential of 293 mV) to achieve up to a current density of 100 mA cm-2, outperforming RuO2-CF and most of the state-of-the-art electrocatalysts reported in the literature (for detailed comparison, see Table S3 in the Supporting 14
Information). While, the negligible OER catalytic activity for CF is also observed. The longterm stability test measured by galvanostatic test at a high current density of 100 mA cm -2 (Figure 5b) shows that oxygen is continuously produced on the electrode surface (Figure 5b inset) and the overpotential only increases by 8 mV after continuous electrolysis for 50 h, indicative of an outstanding stability for OER under alkaline condition. In contrast, the overpotential for RuO2-CF adhered by Nafion increases by 212 mV (Figure S10), which may be caused by the shedding of the active catalysts. In order to assess the binding force of the RuO2-CF and self-standing Ni1.5Fe0.5P/CF, these two samples were cut into small slices and sonicated in water for 10 min. As shown in Figure S11, it can be obviously noted that the sample bottle containing RuO2-CF turns black after ultrasonication testing (Figure S11e), which mainly results from the caducous RuO2 (Figure S11a-d). While the Ni1.5Fe0.5P/CF is almost unchanged after ultrasonication (Figure S11f-g), demonstrating a strong binding force between the Ni1.5Fe0.5P nanosheets and CF, which is better than the polymer binder (Nafion).[56] Figure 5c further illustrates a multi-current OER process on Ni1.5Fe0.5P/CF, in which the current increases from 50 mA cm-2 to 500 mA cm-2. At the beginning stage of 50 mA cm-2, the potential rapidly responds and levels off at 1.48 V, keeping unchanged for the rest 500 s. This is also the case as an increase of current, indicative of fast mass transfer properties, excellent conductivity and mechanical robustness of the integrated Ni1.5Fe0.5P/CF electrode.[19, 51] The catalytic HER activity for the 3D Ni1.5Fe0.5P/CF electrode was further evaluated in 1 M KOH. As shown in Figure 5d, in the case of the as-made Ni1.5Fe0.5P/CF, the overpotentials of 158 and 319 mV are required to achieve current densities of 10 and 100 mA cm-2, respectively, being inferior to that of the commercial Pt/C coated on CF (Pt/C-CF, 51 mV@10 mA cm-2 and 186 mV@100 mA cm-2). Moreover, it can be clearly observed from Figure S12 that the potential remains almost unchanged during 50 h continuous operation, indicative of a high HER catalytic stability and capability of long-term operation. 15
Fig 5. Catalytic OER performance of as-made Ni1.5Fe0.5P/CF and CF in 1 M KOH: a) polarization curves; b) long-term stability measured at 100 mA cm-2 for 50 h. Inset: a photograph showing the O2 bubbles produced on the Ni1.5Fe0.5P/CF electrode during stability test; c) a multi-current OER process of the integrated Ni1.5Fe0.5P/CF electrode. The current density increases from 50 mA cm-2 to 500 mA cm-2, with a growth of 50 mA cm-2 every 500 s; d) catalytic HER polarization curves of Ni1.5Fe0.5P/CF, Pt/C-CF and CF.
On the basis of the results discussed above, the as-made 3D Ni1.5Fe0.5P/CF electrocatalyst could serve as both anode and cathode for electrochemical water splitting in strong alkaline medium. In this case, the Ni1.5Fe0.5P/CF as a bifunctional catalyst for overall water splitting in two-electrodes system (schematic diagram shown in Figure S13) was further investigated. For comparison, a counterpart based on the commercial Pt/C-CF and RuO2-CF (mass loading: 1.38 mg cm-2) was also tested for overall water splitting. As shown in Figure 6a, the as-made Ni1.5Fe0.5P/CF can afford the current densities of 10 and 20 mA cm-2 at the applied potential of 1.589 and 1.635 V, respectively, which are comparable to those of Pt/C-CF // RuO2-CF 16
(1.505 @10 mA cm-2, 1.542 V@20 mA cm-2), much lower than those of two Pt foil electrodes (1.792 V @10 mA cm-2, 1.862 V @20 mA cm-2), and favorably comparable to most of the reported bifunctional electrocatalysts (Table S4). To better understand the electrocatalytic reaction process of the as-made Ni1.5Fe0.5P/CF, the Ni1.5Fe0.5P/CF samples after continuous electrolysis at 10 mA cm-2 for 5 h were characterized and compared with the original Ni1.5Fe0.5P/CF samples, of which the corresponding FE-SEM images, EDX and XPS spectra are shown in Figure S14. It can be noted that the post-HER Ni1.5Fe0.5P/CF (Figure S14a) and post-OER Ni1.5Fe0.5P/CF (Figure S14c) still keep the nanosheets-shaped structure. Compared with the original Ni1.5Fe0.5P/CF (Figure S7d), the relatively weaker peak of P in EDX spectrum (Figure S14b) for post-HER Ni1.5Fe0.5P/CF is observed. For the post-OER Ni1.5Fe0.5P/CF samples (Figure S14d), the P peak intensity also dramatically decreases and the O peak greatly increases. These results indicate that P leaches out from the Ni1.5Fe0.5P/CF during HER and OER process.[11, 29] Moreover, the high-resolution depth XPS spectra of Ni and P (Figure S14e-f) further demonstrate that the post-HER Ni1.5Fe0.5P/CF still maintains the phosphide phase and an increased oxide layer (increased peaks of Ni-O and P-O), which may be caused by released oxygen at anode dissolving in the electrolyte. And the post-OER Ni1.5Fe0.5P/CF is continually oxidized into corresponding hydroxides/oxides during OER process, forming phosphide/oxides/hydroxides heterojunctions as the primary active component, which is consistent with the results in literature.[11, 12, 29, 57] In order to verify the potential for practical application, the galvanostatic test at a current density of 20 mA cm-2 was also carried out to assess the long-term stability (Figure 6b). As observed, the potential was almost stabilized around 1.723 V during the continuously electrolysis over 20 h, revealing a great potential for practical applications.
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Fig 6. a) Polarization curves (with iR corrected) of the commercial Pt/C-CF (cathode) // RuO2-CF (anode), as-made Ni1.5Fe0.5P/CF // Ni1.5Fe0.5P/CF electrode, Pt foil // Pt foil, and CF // CF for overall water splitting in 1 M KOH, respectively; b) long-term stability test of Ni1.5Fe0.5P/CF // Ni1.5Fe0.5P/CF for overall water splitting measured at 20 mA cm-2 over 20 h without iR corrected.
3. Conclusion In summary, we present a facile route to synthesize active nickel phosphide nanosheets with Fe species-tuned structure based on earth-abundant transition metals (Ni and Fe). The electrocatalytic activities can be greatly improved by incorporating of iron species into Ni 2P, indicative of the ultrasensitive iron species-triggered electrochemical behaviors. Of a series of the as-made catalysts, the Ni1.5Fe0.5P nanosheets with 3:1 molar ratio of Ni to Fe deliver the best catalytic activities for both OER and HER. And the as-made Ni1.5Fe0.5P nanosheets can be vertically assembled on the CF to construct a 3D interconnected binder-free Ni1.5Fe0.5P/CF electrode. Such a kind of super structure features strong binding force, more accessible active sites to electrolyte ions and open space for gas release. Benefiting from these unique characteristics, the Ni1.5Fe0.5P/CF electrode delivers an excellent OER performance (293 mV@100 mA cm-2) and prominent electrochemical stability at a large current density. 18
Especially, the Ni1.5Fe0.5P/CF serves as both cathode and anode in alkaline electrolyte for overall water splitting, generating 10 and 20 mA cm-2 at low potentials of 1.589 and 1.635 V, respectively. The present strategy provides a novel approach to design highly efficient transition metal based catalysts with tunable components and electronic structure for highefficiency energy storage/conversion devices.
Acknowledgements The first two authors contributed equally to this work. This work was partly supported by the NSFC (Nos. 21522601, U1508201) the Fundamental Research Funds for the Central Universities (DUT16ZD217), and the National Key Research Development Program of China (2016YFB0101201)
Supporting Information Supporting Information: Experimental section, additional FE-SEM images, EDX elemental mapping/spectrum, polarization curves, XRD pattern, and Table S1-4.
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Author biographies
Huawei Huang received his B.S. degree from the Department of Materials Science and Engineering, Northeast Forestry University. Currently, he is a Ph.D. candidate at School of Chemical Engineering in Dalian University of Technology under the supervision of Prof. Jieshan Qiu. His research focuses on the synthesis of carbon-based nanomaterials and related functional materials for the applications in water splitting.
Prof. Chang Yu received her Ph.D. degree from the School of Chemical Engineering at Dalian University of Technology (DUT) in 2008. She is currently a professor for School of Chemical Engineering at DUT. She was also a visiting professor at Rice University (USA) in 2015. Her research interests mainly focus on carbon coupled two-dimensional inorganic layered materials for energy storage and conversion applications.
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Changtai Zhao is currently a Ph.D. candidate in Prof. Jieshan Qiu’s group at Dalian University of Technology, China. He was also a visiting student in Prof. Xueliang (Andy) Sun’s Group at the University of Western Ontario, Canada, in 2016. He gained his Bachelor’s degree from Department of Chemical Engineering, Qingdao University, China. His research interests focus on nanocarbon and advanced functional materials as well as their applications in energy conversion and storage, especially for Na/Li-ion batteries, Li-S batteries and Li-O2 batteries.
Xiaotong Han is a Ph.D. student in Prof. Jieshan Qiu’s group at Dalian University of Technology (DUT), China. He received his B.S. degree in Chemical Engineering and Technology in 2013 at Nanjing Tech University, China. He joined Prof. Jieshan Qiu’s group in 2013 and his current research interests focus on the design and fabrication of two-
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dimension nanohybrids derived from nanocarbons and transition metal compounds for energy storage and conversion devices including Li-ion batteries, supercapacitors as well as electrocatalysis.
Juan Yang received his Bachelor’s degree from the School of Chemical Engineering at Shenyang University of Chemical Technology in 2011, and he was a visiting student at Lawrence Berkeley National Laboratory (LBNL), USA. He is currently a Ph.D candidate in research group of Prof. Jieshan Qiu at Dalian University of Technology (DUT). His research interests mainly focus on design and optimization of carbon-based hybrids for energy storage and conversion application.
Zhibin Liu received his B.S. degree from Nanjing Tech University in 2014 and now he is a postgraduate student in Prof. Jieshan Qiu’s group at Dalian University of Technology. His
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research interests focus on the design of high-performance electrodes composed of carbon materials and transition metal compounds for electrocatalysis and supercapacitor.
Shaofeng Li received his Bachelor’s degree from the School of Chemistry and Chemical Engineering at Anhui University of Technology in 2014. He is currently a 1st year Ph.D. candidate in research group of Prof. Jieshan Qiu at Dalian University of Technology. His research interests mainly focus on rational design and optimization of carbon-based nanohybrids for energy storage and conversion application.
Mengdi Zhang received her B.S. degree in Chemical Technology from Dalian University of Technology in 2012. Currently, she is a Ph.D. candidate at School of Chemical Engineering in Dalian University of Technology under the supervision of Prof. Jieshan Qiu. Her research 25
mainly focuses on the design and synthesis of carbon-based nanomaterials for the application in supercapacitor and lithium secondary batteries.
Prof. Jieshan Qiu obtained his Ph.D. degree in the School of Chemical Engineering at Dalian University of Technology (DUT) in 1990. He was also a visiting professor at Pennsylvania State University (USA), West Virginia University (USA), and the University of Reading (UK). He was appointed to a Cheung-Kong Distinguished Professor in 2009. He is a professor of School of Chemical Engineering and director of the Carbon Research Laboratory at DUT. His current research includes functional carbon nanotubes, graphene, carbon nanohybrids, and their applications (energy conversion and storage, capacitive deionization technique, etc.).
Highlights
A strategy for controllable regulation of Fe-doped Ni2P is developed.
Fe species can modulate the structure and boost catalytic activities of Ni2P.
The Ni1.5Fe0.5P nanosheets feature dual function for HER and OER.
Ni1.5Fe0.5P/carbon fiber paper 3D electrodes deliver superior activity and stability for overall water splitting.
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Graphical Abstract
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PII: DOI: Reference:
S2211-2855(17)30147-7 http://dx.doi.org/10.1016/j.nanoen.2017.03.016 NANOEN1844
To appear in: Nano Energy Received date: 9 December 2016 Revised date: 1 March 2017 Accepted date: 6 March 2017 Cite this article as: Huawei Huang, Chang Yu, Changtai Zhao, Xiaotong Han, Juan Yang, Zhibin Liu, Shaofeng Li, Mengdi Zhang and Jieshan Qiu, Iron-tuned Super Nickel Phosphide Microstructures with High Activity for Electrochemical Overall Water Splitting, Nano Energy, http://dx.doi.org/10.1016/j.nanoen.2017.03.016 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Iron-tuned Super Nickel Phosphide Microstructures with High Activity for Electrochemical Overall Water Splitting Huawei Huang1, Chang Yu1, Changtai Zhao, Xiaotong Han, Juan Yang, Zhibin Liu, Shaofeng Li, Mengdi Zhang, Jieshan Qiu* State Key Lab of Fine Chemicals, School of Chemical Engineering, Liaoning Key Lab for Energy Materials and Chemical Engineering, Dalian University of Technology Dalian 116024， Liaoning，China *
Corresponding author. [email protected] (J. S. Qiu)
Abstract Large-scale hydrogen production by electrolytic splitting of water is mainly governed by high-efficient yet cheap electrocatalysts that could be capable of accelerating the sluggish hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Herein, we report Fe-tuned Ni2P electrocatalysts with controllable morphology and structure by regulating atomic ratio of Ni/Fe, and reveal the Fe species-modulated electronic state behaviors and boosted catalytic activity for water splitting. The electrocatalytic activity of Fe-tuned Ni2P nanosheets for both HER and OER can be further enhanced by assembling the nanosheets vertically on conductive 2D carbon fiber (CF) matrix to make hierarchical monolithic 3D electrode (Ni1.5Fe0.5P/CF), which features more accessible active sites and open structure that helps to speed up both the HER and OER. The improved electrocatalytic activity of Ni1.5Fe0.5P/CF is due to the combined synergistic effects of the high conductivity of CF matrix and the strong interaction between active species and the CF support, as evidenced by a low overpotential of 293 mV to achieve a high current density of 100 mA cm-2 with superiorly long-term stability for OER. When the monolithic 3D Ni1.5Fe0.5P/CF electrodes were used as both anode and cathode for overall water splitting, a current density of 10 mA cm-2 is
1
The first two authors contributed equally to this work
1
generated at a low potential of 1.589 V, while at 20 mA cm-2, the potential is only 1.635 V. It has been demonstrated that modulating metal catalysts (nanosized nickel phosphide) with iron atoms is powerful, and may open up avenues to the design and fabrication of highly efficient catalysts for energy storage and conversion.
Graphical Abstract A strategy for assembling iron-tuned nickel phosphides on 2D carbon fiber sheet to configure binder-free 3D Ni1.5Fe0.5P/CF as water splitting catalyst is presented, indicative of the iron species-triggered positive effects on structure and electrochemical activities. Interestingly, the as-made 3D Ni1.5Fe0.5P/CF can achieve up to 100 mA cm-2 only at overpotential of 293 mV for oxygen evolution, and can generate 10 mA cm-2 at a low potential of 1.589 V for overall water splitting.
Keywords: Water splitting, Nickel-iron phosphide, 2D carbon fiber sheet, Oxygen evolution reaction, Bifunctional electrocatalyst
1. Introduction Electrochemical water splitting into hydrogen and oxygen provides an attractive pathway for sustainable energy conversion and storage.[1, 2] The two half reactions: hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), are involved in this system. Lowering the required activation energy barriers is highly concerned to achieve fast kinetic processes of both HER and OER for the practical requirements. To solve these concerned issues, the electrocatalysts with high-efficiency and long-term durability are highly required.[3-5] Platinum and noble metal oxides (RuO2 and IrO2) are generally used and possess unique superiorities. Nevertheless, their rareness and high cost make them less attractive for large-scale and practical application in water splitting.[6-10] In this case, it is the research focus to develop highly active and economical electrocatalysts based on earthabundant elements for HER and OER.[11-18] In particular, designing bifunctional 2
electrocatalysts that are simultaneously capable of catalyzing both HER and OER, are extremely promising and attractive owing to simplified operating system and the reduced overall cost associated with equipment.[19-22] Up to now, a series of transition metal selenides, oxides, chalcogenides, borides, and phosphides are capable of acting as symmetric bifunctional catalysts for overall water splitting, such as NiSe,[19] CoMnO,[20] Ni3S2,[23] Co9S8@MoS2/carbon nanofibers (CNFs),[24] Co2B,[25] CoP,[6, 26, 27] and Ni2P.[11, 12] Of these non-precious metal catalysts, transition-metal phosphides (TMPs) have been proved to be superior catalysts for HER and OER by theoretical calculation and experimental results. [2, 28, 29] Nevertheless, much remains to be done to gain an insight into the trends and limitations in their performance and further improve their intrinsic activity. It has been also demonstrated that modulating electronic structure of transition metal-based catalysts by doping additional metal atoms is an efficient way to enhance the intrinsically catalytic performance in energy-related devices.[30-35] The transition metal is capable of promoting the charge-transfer between transition metal atoms and metal platinum, thus leading to the enhanced electrochemical activities.[36] This is also the case for electrochemical water splitting. For example, Hu et al. found that Ni, Fe, and Co transition metals have significantly positive effects for enhancing the HER catalytic performance of the amorphous molybdenum sulfide films, in which these additional metals interact with unsaturated sulfur atoms in MoS3 and promote the growth of the MoS3 film with large surface area.[37] Recently, Vojvodic et al. developed amorphous FeWCo gel by introducing W into FeCo oxy-hydroxides to optimize the adsorption energy for OER intermediates, indicative of a highly efficient catalytic activity.[38] Mukerjee et al. found that the charge-transfer effects of the Co element in the Ni-Fe-Co mixed-metal oxides facilitate the formation of conductive NiOOH, and activate the Fe sites at a lower overpotential.[39] Zheng et al. reported that the bimetallic Ni0.33Co0.67S2 presents a higher HER catalytic activity in comparison with that of 3
the monometallic chalcogenide catalysts (NiS2 and CoS2) due to its fast electron transport and large surface area.[40] With the doping of Co atoms, the energy barrier for H-H bond formation on the iron pyrite FeS2 nanosheets decorated on carbon nanotubes (Fe0.9Co0.1S2/CNT) are greatly decreased and the HER catalytic activities are improved to a great degree.[31] Yang’s group reported that Fe incorporated into nickel sulfide (iron-nickel sulfide) ultrathin nanosheets, achieving an enhanced HER catalytic performance and stability.[41] These doping strategies feature intriguing charms and unique superiorities for electrocatalysis, which would be also applicable for TMPs system, and the binary metal phosphides as symmetric bifunctional electrocatalysts have been rarely explored. With this information in mind, it would be a promising strategy to modulate the electronic structure of phosphides by configuring highly active binary metal phosphides electrocatalysts elaborately to enhance the electrocatalytic activities. Also, it is necessary and significant to gain a deep insight into intrinsic catalytic activities of the as-made materials with well-controlled internal structure for the design of high-performance electrocatalysts. In the present study, we develop a facile and scalable strategy to synthesize a series of NixFe2-xP (0
2
, respectively. To further intensify the electron and mass transport, the Ni1.5Fe0.5P nanosheets
were further vertically assembled on the carbon fiber (CF) conductive substrate, yielding a hierarchically-structured three-dimensional (3D) monolithic electrode with more accessible active sites derived from open frameworks, strong binding force between CF substrate and electrochemically active species, and fast electron transport channels from interconnected CF matrix. Taking advantage of these unique merits, the as-made Ni1.5Fe0.5P/CF deliver an overpotential as low as 293 mV for achieving current density of 100 mA cm-2 and an amazingly long-term operational stability when directly utilized as an anode electrocatalyst for OER in 1 M KOH electrolyte. For overall water splitting, the current densities of 10 and 20 mA cm-2 can be generated at 1.589 and 1.635 V (iR corrected), respectively, which is superior to most of non-precious bifunctional catalysts reported in literature so far. 2. Results and discussion Preparation and characterizations of the as-made NixFe2-xP The fabrication processes of NixFe2-xP phosphides are illustrated in Scheme 1. For a typical run, NiFe LDHs precursors were first synthesized via urea hydrolysis of mixed nickel nitrate and iron nitrate with different molar ratios in a sealed Teflon-lined stainless steel autoclave at 120 oC, where urea was decomposed into ammonia and carbon dioxide, and further reacted with metal ions during this procedure.[13] Afterwards, the as-prepared NiFe LDHs suffer from a topotactic conversion reaction through a low-temperature phosphorization process to produce the corresponding NixFe2-xP phosphides.[42] Furthermore, single-component Ni2P and Fe2P samples were also prepared following the same procedure.
5
Scheme 1. Schematic process for the synthesis of NixFe2-xP.
By regulating and controlling the Ni/Fe molar ratio from 5:1, 4:1, 3:1, 2:1 to 1:1 in the precursors, a series of binary metal phosphides were synthesized and the actual Ni/Fe molar ratio in NixFe2-xP was accurately tuned, which was determined by inductively coupled plasma (ICP) mass spectrometry (Table S1). The morphologies and structures of the Ni2P, Fe2P, NixFe2-xP and their corresponding precursors were examined by the field emission scanning electron microscopy (FE-SEM), of which the typical images are shown in Figure 1 and Figure S1. It can be noted that the morphologies of as-made electrocatalysts vary and associate with the molar ratio of Ni/Fe in NixFe2-xP, nevertheless, well keep the features of their precursors after the phosphorization treatment. For unary metal phosphides, the pure Ni2P (Figure 1a) has a flat sheet structure, while the Fe2P (Figure 1b) features sphere-shaped structure with different sizes. Interestingly, after phosphorization, the Ni1.0Fe1.0P phosphides retain sphereshaped characteristics of Fe2P, of which some sphere-shaped structures are constituted by nanosheets (red dotted line in Figure 1c). It is also noted from energy dispersive X-ray (EDX) analysis (Figure S2) that the sphere zone made of nanosheets has a high Ni content (red dotted line in Figure S2), while the rest solid spheres have a high iron content. The corresponding 6
EDX spectrum analysis in Figure S3 reveals that the molar ratio of Ni and Fe is nearly 1:1, which is consistent with the theoretical value. When the molar ratio of Ni and Fe increased to 2:1 or higher, the as-made Ni1.33Fe0.67P, Ni1.5Fe0.5P, Ni1.6Fe0.4P, and Ni1.67Fe0.33P show a sphere-shaped structure constituted by sheets radially grown from the center (Figure 1d-f and Figure S1), indicative of a well-preserved unique structure close to their corresponding precursors (Figure S1). And these subunit sheets feature honeycomb and rough texture derived from dehydration of the LDHs precursors during the annealing process,[42] implying the porous characteristics of the as-made NixFe2-xP, which will be evidenced by the following TEM results.
Fig 1. The morphologies of as-made samples: FE-SEM images of a) pure Ni2P, b) pure Fe2P, c) Ni1.0Fe1.0P, d, e) Ni1.5Fe0.5P, and f) Ni1.6Fe0.4P.
The typical powder X-ray diffraction (XRD) patterns of the as-made Ni2P, Fe2P, and bimetallic NixFe2-xP samples are displayed in Figure 2a. It can be clearly seen that the typical diffraction peak patterns of the as-made NixFe2-xP catalysts are basically consistent with that 7
of the Ni2P (JCPDS Card no. 03-0953). Noteworthily, the peaks of bimetallic NixFe2-xP broaden and slightly shift to large-angle due to the incorporation of Fe component (Figure 2 a, b and Table S2). Moreover, a weak peak at 40.26 o (blue dots in Figure 2b), corresponding to (111) plane of Fe2P (JCPDS Card no. 33-0670), appears when the atomic ratio of Ni/Fe decreased to 1:1 (namely, Ni1.0Fe1.0P), which may relate to the solid sphere-shaped structure illustrated in Figure 1c and Figure S2. These phenomena reveal that the Fe atoms can be doped into the Ni2P when the atomic ratio of Ni/Fe is larger than 2:1, and the crystal grain size decreases with the incorporation of Fe species.[2, 41, 43] In order to get more surface information about chemical composition and the electronic states of as-made samples, the Ni1.5Fe0.5P was further analyzed by depth X-ray photoelectron spectroscopy (XPS), of which the detailed results are shown in Figure 2c-f. The high-resolution depth spectrum of Ni 2p (Figure 2c) shows that the peak at 856.6 eV, corresponding to nickel phosphate, decreases rapidly with an increase of Ar ion etching time, which indicates that phosphate species resulting from partly surface oxidation only exist on the surface of catalyst. After sixty seconds of etching, such surface nickel phosphate species have been removed to a great degree, simultaneously, the intensity of the peaks at binding energies of 852.6 and 870.1 eV corresponding to Ni 2p3/2 and Ni 2p1/2, respectively, greatly increases. Compared with pure Ni2P (853.1 eV), a negative shift of Ni 2p3/2 peak was observed for Ni1.5Fe0.5P, which is probably attributed to the incorporation of Fe atom into Ni2P.[44] In the case of Fe 2p (Figure 2d), the peak intensity for Fe 2p1/2 and Fe 2p3/2, corresponding to the binding energy of Fe-P, also increase during Ar ion etching, while the peak of iron phosphate remains strong, indicative of a thick oxidized layer, which is also reflected in the high-resolution depth spectra of P (Figure 4e).[2, 45] It is well known that metal phosphate species dominating the surface layer of metal phosphides is an inevitable phenomenon, because the top layer of phosphides
8
are prone to be oxidized when they are exposed to air, nevertheless the interior is homogeneous phosphides for the as-made Ni1.5Fe0.5P.[2, 28]
Fig 2. The structure characteristics and chemical compositions of the as-made samples: a) XRD patterns and b) zoom-in regional patterns of 39.5-45 o of the as-made NixFe2-xP samples. Depth XPS spectra of c) Ni, d) Fe, e) P and f) survey regions for Ni1.5Fe0.5P after 0-, 30-, 60-, 90-, 120-, 180-, 240-, and 360-s Ar ion etching. To further get insight into morphology and structural changes of materials before and after the phosphorization treatment, the as-synthesized Ni1.5Fe0.5 LDHs and Ni1.5Fe0.5P were further characterized by transmission electron microcopy (TEM) and high-resolution TEM (HR9
TEM), of which the detailed results are shown in Figure 3a-c. As illustrated in Figure 3a-b, the Ni1.5Fe0.5P keeps the integrated nanosheet-shaped structure similar to the LDHs precursor. Meanwhile, the numerous pores on the sheet surface are also observed, thus leading to more defects in the Ni1.5Fe0.5P matrix, which provide more contactable and accessible active sites to the electrolyte ions. Furthermore, the HR-TEM images in Figure 3c reveal the lattice fringes of 0.34 and 0.22 nm, corresponding to (001) and (111) planes of Ni1.5Fe0.5P, respectively, which further confirms the formation of the well crystallized Ni1.5Fe0.5P. In order to further study the chemical composition and element distribution of the as-made Ni1.5Fe0.5P sample, high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) was used. As shown in Figure 3d, it can be noted that the Ni, Fe, and P elements are almost uniformly distributed within the as-made Ni1.5Fe0.5P.
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Fig 3. TEM images of the as-made a) Ni1.5Fe0.5 LDHs precursors and b) Ni1.5Fe0.5P; c) HRTEM images of the Ni1.5Fe0.5P; d) HAADF-STEM image of Ni1.5Fe0.5P and the corresponding EDX elemental mapping images of Ni, Fe and P elements. Electrocatalytic performance of as-made samples The catalytic OER performance of Ni2P, Fe2P, and bimetallic NixFe2-xP samples were examined and compared with noble metal oxide (RuO2) in a typical three-electrode system, of which the detailed results are illustrated in Figure 4. It can be clearly seen from Figure 4a that the single-component Ni2P features a higher intrinsic activity in comparison to that of Fe2P. It is interesting that the catalytic activity of the single-component Ni2P can be also greatly improved in the presence of iron species. And the electrochemical activities vary with the molar ratios of Ni/Fe in NixFe2-xP, indicative of a component-dependent electrochemical behavior. Among these as-made electrocatalysts, the Ni1.5Fe0.5P (Ni/Fe = 3/1) delivers the best catalytic activity and only requires the overpotentials as low as 264 and 280 mV to achieve current densities of 10 and 20 mA cm-2, respectively (273 mV@10 mA cm-2 and 298 mV@20 mA cm-2, without iR corrected), which is much lower than commercial RuO2 (366 mV@10 mA cm-2 and 402 mV@20 mA cm-2) and superior to most of state-of-the-art OER catalysts (Figure 4b and Table S3 in the Supporting Information). The catalytic kinetics of as-made samples were estimated by the corresponding Tafel plots derived from Tafel equation: η = a + b log j (b is the Tafel slope and j represents the current density).[46-49] As illustrated in Figure 4c, of the adopted samples, the Ni1.5Fe0.5P delivers the lowest Tafel slope of 55 mV dec-1, indicative of fast kinetics for the efficient mass and electron transfer. Besides, the polarization curves of the as-made Ni1.5Fe0.5P catalyst (Figure S4) were also further tested at different scan rates (2, 5, 10, 15 and 20 mV s-1), where no significant changes in catalytic performance are observed, further confirming a sufficiently fast mass transport for the Ni1.5Fe0.5P electrode in the present system.[50, 51] To evaluate the stability of the catalyst, a 11
long-term galvanostatic test was performed at 10 mA cm-2 for 12 h. As shown in Figure 4d, the as-made Ni1.5Fe0.5P delivers a much better operational durability than RuO2. We also measured and analyzed the polarization curves of Ni1.5Fe0.5P sample before and after 1000 cycles with potential cycling between 1.2 and 1.7 V (vs RHE) at a scan rate of 100 mV s-1, of which the detailed results are shown in Figure S5. It can be noted that current density almost unchanged after 1000 cycles, indicative of a highly electrochemical cycling stability. The highly catalytic performance of Fe doped Ni2P is mainly attributed to the following combined factors: 1) the incorporation of Fe species endows the Ni2P nanosheets with smaller size, regular structure, and the modulated surface property and electronic states, thus leading to the enhanced reaction activity; 2) the open structure of non-solid spheres derived from the nanosheets are responsible for more accessible active sites to electrolytes, fast mass transport, and gas diffusion. The electrocatalytic HER polarization curves of the as-made catalysts and commercial Pt/C (20 wt% Pt) were also evaluated and compared in 1 M KOH. As we can see from Figure 4e, the as-made Ni1.5Fe0.5P still produces the optimal HER catalytic activity, achieving a cathodic current density of 10 mA cm-2 at the overpotential of 282 mV and Tafel slope of 125 mV dec-1 (Figure 4e-f), being lower than that of the monometallic Fe- or Ni-based phosphide catalysts. These results further imply that the Ni1.5Fe0.5P as the bifunctional catalyst is of great potential for both HER and OER.
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Fig 4. Catalytic OER performance of the as-made catalysts in 1 M KOH: a) Polarization curves with iR corrected (except the red dotted line, namely, the Ni1.5Fe0.5P without iR corrected) and b) overpotentials required at 10 mA cm-2 over a series of electrocatalysts. c) The corresponding Tafel plots. d) Long-term stability test measured at 10 mA cm-2 without iR corrected. Catalytic HER performance of the as-made catalysts in 1 M KOH: e) Polarization curves with iR corrected and f) corresponding Tafel plots of as-made samples.
Construction of 3D electrodes for water splitting For practical applications, a strong-coupled 3D architecture with highly electronic conductivity and abundantly active sites is required to avoid using the polymeric binders (e.g., 13
Nafion), because the utilization of these insulative binders may block the accessible active sites and increase the series resistance, thus resulting in degenerately catalytic activity.[51-53] As a highly conductive support material, CF is an ideal substrate to construct the 3D electrode due to the merits of lightweight and superior corrosion resistance.[54] Given that the as-made Ni1.5Fe0.5P features the best OER and HER catalytic activities, we further vertically assemble the Ni1.5Fe0.5P nanosheets on pretreated CF surface to produce a freestanding 3D electrode (Ni1.5Fe0.5P/CF), which can be directly used for electrolysis of water. The functional groups on pretreated CF surface act as the nucleation sites for the NiFe LDHs growth, thus leading to strong bonding force between CF and NiFe LDHs precursors.[54, 55] After hydrothermal process, the color of the CF varies from silver gray to faint orange, then turns to black after phosphorization process (Figure S6). The typical FE-SEM image of the 3D Ni1.5Fe0.5P/CF electrode is shown in Figure S7a-b. Compare with the pristine CF (Figure S8), it can be clearly observed that the Ni1.5Fe0.5P nanosheets uniformly and vertically grow on the CF. Furthermore, the corresponding EDX element mapping images (Figure S7c) reveal that the Ni, Fe and P elements homogeneously distribute on the CF and the corresponding molar ratio of Ni/Fe is 3.3:1 (Figure S7d). The representative XRD pattern (Figure S9) further demonstrates the existent carbon and crystallized Ni1.5Fe0.5P in the Ni1.5Fe0.5P/CF. The electrocatalytic OER performances of the integrated Ni1.5Fe0.5P/CF electrode (Ni1.5Fe0.5P mass loading: 1.38 mg cm-2) were also tested in alkaline medium (1 M KOH), of which the detailed results are shown in Figure 5a-c. For comparison, the pristine CF and RuO2 coated on CF (RuO2-CF) were also tested and shown in Figure 5. As shown in Figure 5a, the as-made 3D Ni1.5Fe0.5P/CF electrodes deliver a superior catalytic activity for OER and only need a low potential of 1.523 V (overpotential of 293 mV) to achieve up to a current density of 100 mA cm-2, outperforming RuO2-CF and most of the state-of-the-art electrocatalysts reported in the literature (for detailed comparison, see Table S3 in the Supporting 14
Information). While, the negligible OER catalytic activity for CF is also observed. The longterm stability test measured by galvanostatic test at a high current density of 100 mA cm -2 (Figure 5b) shows that oxygen is continuously produced on the electrode surface (Figure 5b inset) and the overpotential only increases by 8 mV after continuous electrolysis for 50 h, indicative of an outstanding stability for OER under alkaline condition. In contrast, the overpotential for RuO2-CF adhered by Nafion increases by 212 mV (Figure S10), which may be caused by the shedding of the active catalysts. In order to assess the binding force of the RuO2-CF and self-standing Ni1.5Fe0.5P/CF, these two samples were cut into small slices and sonicated in water for 10 min. As shown in Figure S11, it can be obviously noted that the sample bottle containing RuO2-CF turns black after ultrasonication testing (Figure S11e), which mainly results from the caducous RuO2 (Figure S11a-d). While the Ni1.5Fe0.5P/CF is almost unchanged after ultrasonication (Figure S11f-g), demonstrating a strong binding force between the Ni1.5Fe0.5P nanosheets and CF, which is better than the polymer binder (Nafion).[56] Figure 5c further illustrates a multi-current OER process on Ni1.5Fe0.5P/CF, in which the current increases from 50 mA cm-2 to 500 mA cm-2. At the beginning stage of 50 mA cm-2, the potential rapidly responds and levels off at 1.48 V, keeping unchanged for the rest 500 s. This is also the case as an increase of current, indicative of fast mass transfer properties, excellent conductivity and mechanical robustness of the integrated Ni1.5Fe0.5P/CF electrode.[19, 51] The catalytic HER activity for the 3D Ni1.5Fe0.5P/CF electrode was further evaluated in 1 M KOH. As shown in Figure 5d, in the case of the as-made Ni1.5Fe0.5P/CF, the overpotentials of 158 and 319 mV are required to achieve current densities of 10 and 100 mA cm-2, respectively, being inferior to that of the commercial Pt/C coated on CF (Pt/C-CF, 51 mV@10 mA cm-2 and 186 mV@100 mA cm-2). Moreover, it can be clearly observed from Figure S12 that the potential remains almost unchanged during 50 h continuous operation, indicative of a high HER catalytic stability and capability of long-term operation. 15
Fig 5. Catalytic OER performance of as-made Ni1.5Fe0.5P/CF and CF in 1 M KOH: a) polarization curves; b) long-term stability measured at 100 mA cm-2 for 50 h. Inset: a photograph showing the O2 bubbles produced on the Ni1.5Fe0.5P/CF electrode during stability test; c) a multi-current OER process of the integrated Ni1.5Fe0.5P/CF electrode. The current density increases from 50 mA cm-2 to 500 mA cm-2, with a growth of 50 mA cm-2 every 500 s; d) catalytic HER polarization curves of Ni1.5Fe0.5P/CF, Pt/C-CF and CF.
On the basis of the results discussed above, the as-made 3D Ni1.5Fe0.5P/CF electrocatalyst could serve as both anode and cathode for electrochemical water splitting in strong alkaline medium. In this case, the Ni1.5Fe0.5P/CF as a bifunctional catalyst for overall water splitting in two-electrodes system (schematic diagram shown in Figure S13) was further investigated. For comparison, a counterpart based on the commercial Pt/C-CF and RuO2-CF (mass loading: 1.38 mg cm-2) was also tested for overall water splitting. As shown in Figure 6a, the as-made Ni1.5Fe0.5P/CF can afford the current densities of 10 and 20 mA cm-2 at the applied potential of 1.589 and 1.635 V, respectively, which are comparable to those of Pt/C-CF // RuO2-CF 16
(1.505 @10 mA cm-2, 1.542 V@20 mA cm-2), much lower than those of two Pt foil electrodes (1.792 V @10 mA cm-2, 1.862 V @20 mA cm-2), and favorably comparable to most of the reported bifunctional electrocatalysts (Table S4). To better understand the electrocatalytic reaction process of the as-made Ni1.5Fe0.5P/CF, the Ni1.5Fe0.5P/CF samples after continuous electrolysis at 10 mA cm-2 for 5 h were characterized and compared with the original Ni1.5Fe0.5P/CF samples, of which the corresponding FE-SEM images, EDX and XPS spectra are shown in Figure S14. It can be noted that the post-HER Ni1.5Fe0.5P/CF (Figure S14a) and post-OER Ni1.5Fe0.5P/CF (Figure S14c) still keep the nanosheets-shaped structure. Compared with the original Ni1.5Fe0.5P/CF (Figure S7d), the relatively weaker peak of P in EDX spectrum (Figure S14b) for post-HER Ni1.5Fe0.5P/CF is observed. For the post-OER Ni1.5Fe0.5P/CF samples (Figure S14d), the P peak intensity also dramatically decreases and the O peak greatly increases. These results indicate that P leaches out from the Ni1.5Fe0.5P/CF during HER and OER process.[11, 29] Moreover, the high-resolution depth XPS spectra of Ni and P (Figure S14e-f) further demonstrate that the post-HER Ni1.5Fe0.5P/CF still maintains the phosphide phase and an increased oxide layer (increased peaks of Ni-O and P-O), which may be caused by released oxygen at anode dissolving in the electrolyte. And the post-OER Ni1.5Fe0.5P/CF is continually oxidized into corresponding hydroxides/oxides during OER process, forming phosphide/oxides/hydroxides heterojunctions as the primary active component, which is consistent with the results in literature.[11, 12, 29, 57] In order to verify the potential for practical application, the galvanostatic test at a current density of 20 mA cm-2 was also carried out to assess the long-term stability (Figure 6b). As observed, the potential was almost stabilized around 1.723 V during the continuously electrolysis over 20 h, revealing a great potential for practical applications.
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Fig 6. a) Polarization curves (with iR corrected) of the commercial Pt/C-CF (cathode) // RuO2-CF (anode), as-made Ni1.5Fe0.5P/CF // Ni1.5Fe0.5P/CF electrode, Pt foil // Pt foil, and CF // CF for overall water splitting in 1 M KOH, respectively; b) long-term stability test of Ni1.5Fe0.5P/CF // Ni1.5Fe0.5P/CF for overall water splitting measured at 20 mA cm-2 over 20 h without iR corrected.
3. Conclusion In summary, we present a facile route to synthesize active nickel phosphide nanosheets with Fe species-tuned structure based on earth-abundant transition metals (Ni and Fe). The electrocatalytic activities can be greatly improved by incorporating of iron species into Ni 2P, indicative of the ultrasensitive iron species-triggered electrochemical behaviors. Of a series of the as-made catalysts, the Ni1.5Fe0.5P nanosheets with 3:1 molar ratio of Ni to Fe deliver the best catalytic activities for both OER and HER. And the as-made Ni1.5Fe0.5P nanosheets can be vertically assembled on the CF to construct a 3D interconnected binder-free Ni1.5Fe0.5P/CF electrode. Such a kind of super structure features strong binding force, more accessible active sites to electrolyte ions and open space for gas release. Benefiting from these unique characteristics, the Ni1.5Fe0.5P/CF electrode delivers an excellent OER performance (293 mV@100 mA cm-2) and prominent electrochemical stability at a large current density. 18
Especially, the Ni1.5Fe0.5P/CF serves as both cathode and anode in alkaline electrolyte for overall water splitting, generating 10 and 20 mA cm-2 at low potentials of 1.589 and 1.635 V, respectively. The present strategy provides a novel approach to design highly efficient transition metal based catalysts with tunable components and electronic structure for highefficiency energy storage/conversion devices.
Acknowledgements The first two authors contributed equally to this work. This work was partly supported by the NSFC (Nos. 21522601, U1508201) the Fundamental Research Funds for the Central Universities (DUT16ZD217), and the National Key Research Development Program of China (2016YFB0101201)
Supporting Information Supporting Information: Experimental section, additional FE-SEM images, EDX elemental mapping/spectrum, polarization curves, XRD pattern, and Table S1-4.
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Author biographies
Huawei Huang received his B.S. degree from the Department of Materials Science and Engineering, Northeast Forestry University. Currently, he is a Ph.D. candidate at School of Chemical Engineering in Dalian University of Technology under the supervision of Prof. Jieshan Qiu. His research focuses on the synthesis of carbon-based nanomaterials and related functional materials for the applications in water splitting.
Prof. Chang Yu received her Ph.D. degree from the School of Chemical Engineering at Dalian University of Technology (DUT) in 2008. She is currently a professor for School of Chemical Engineering at DUT. She was also a visiting professor at Rice University (USA) in 2015. Her research interests mainly focus on carbon coupled two-dimensional inorganic layered materials for energy storage and conversion applications.
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Changtai Zhao is currently a Ph.D. candidate in Prof. Jieshan Qiu’s group at Dalian University of Technology, China. He was also a visiting student in Prof. Xueliang (Andy) Sun’s Group at the University of Western Ontario, Canada, in 2016. He gained his Bachelor’s degree from Department of Chemical Engineering, Qingdao University, China. His research interests focus on nanocarbon and advanced functional materials as well as their applications in energy conversion and storage, especially for Na/Li-ion batteries, Li-S batteries and Li-O2 batteries.
Xiaotong Han is a Ph.D. student in Prof. Jieshan Qiu’s group at Dalian University of Technology (DUT), China. He received his B.S. degree in Chemical Engineering and Technology in 2013 at Nanjing Tech University, China. He joined Prof. Jieshan Qiu’s group in 2013 and his current research interests focus on the design and fabrication of two-
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dimension nanohybrids derived from nanocarbons and transition metal compounds for energy storage and conversion devices including Li-ion batteries, supercapacitors as well as electrocatalysis.
Juan Yang received his Bachelor’s degree from the School of Chemical Engineering at Shenyang University of Chemical Technology in 2011, and he was a visiting student at Lawrence Berkeley National Laboratory (LBNL), USA. He is currently a Ph.D candidate in research group of Prof. Jieshan Qiu at Dalian University of Technology (DUT). His research interests mainly focus on design and optimization of carbon-based hybrids for energy storage and conversion application.
Zhibin Liu received his B.S. degree from Nanjing Tech University in 2014 and now he is a postgraduate student in Prof. Jieshan Qiu’s group at Dalian University of Technology. His
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research interests focus on the design of high-performance electrodes composed of carbon materials and transition metal compounds for electrocatalysis and supercapacitor.
Shaofeng Li received his Bachelor’s degree from the School of Chemistry and Chemical Engineering at Anhui University of Technology in 2014. He is currently a 1st year Ph.D. candidate in research group of Prof. Jieshan Qiu at Dalian University of Technology. His research interests mainly focus on rational design and optimization of carbon-based nanohybrids for energy storage and conversion application.
Mengdi Zhang received her B.S. degree in Chemical Technology from Dalian University of Technology in 2012. Currently, she is a Ph.D. candidate at School of Chemical Engineering in Dalian University of Technology under the supervision of Prof. Jieshan Qiu. Her research 25
mainly focuses on the design and synthesis of carbon-based nanomaterials for the application in supercapacitor and lithium secondary batteries.
Prof. Jieshan Qiu obtained his Ph.D. degree in the School of Chemical Engineering at Dalian University of Technology (DUT) in 1990. He was also a visiting professor at Pennsylvania State University (USA), West Virginia University (USA), and the University of Reading (UK). He was appointed to a Cheung-Kong Distinguished Professor in 2009. He is a professor of School of Chemical Engineering and director of the Carbon Research Laboratory at DUT. His current research includes functional carbon nanotubes, graphene, carbon nanohybrids, and their applications (energy conversion and storage, capacitive deionization technique, etc.).
Highlights
A strategy for controllable regulation of Fe-doped Ni2P is developed.
Fe species can modulate the structure and boost catalytic activities of Ni2P.
The Ni1.5Fe0.5P nanosheets feature dual function for HER and OER.
Ni1.5Fe0.5P/carbon fiber paper 3D electrodes deliver superior activity and stability for overall water splitting.
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Graphical Abstract
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