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NF catalyst for efficient oxygen evolution
Journal of Alloys and Compounds 826 (2020) 154210
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Three-dimensional (3D) hierarchical coral-like Mn-doped Ni2PeNi5P4/ NF catalyst for efficient oxygen evolution Siran Xu a, Yeshuang Du a, Xian Liu a, Xin Yu a, Chunlin Teng a, Xiaohong Cheng b, Yunfeng Chen c, Qi Wu a, * a
College of Chemistry and Chemical Engineering, Hubei Normal University, Huangshi, 435002, China Hubei Key Laboratory of Low Dimensional Optoelectronic Materials and Devices, Hubei University of Arts and Science, Xiangyang, 441053, Hubei Province, PR China c School of Chemistry and Environmental Engineering, Wuhan Institute of Technology, Wuhan, 430073, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 31 October 2019 Received in revised form 3 February 2020 Accepted 5 February 2020 Available online 6 February 2020
Developing earth-abundant and high-efficient catalysts for the oxygen evolution reaction (OER) to enhance the efficiency of water splitting is highly desirable. Metal-doping and construction of heterogeneous structure are two start-of-the-art strategies to increase the OER activity of transition metal materials. In this work, we exhibit the design and construction of a unique coral-like 3D hierarchical Mndoped Ni2PeNi5P4/NF (MneNieP/NF) OER catalyst. The Mn-doped Ni2PeNi5P4/NF hybrid catalyst is synthesized on Ni foam by hydrothermal and followed in situ phosphidation, which exhibits superior OER catalytic performance with an overpotential of 230 mV (vs. RHE) at a current density of 10 mA/cm2 and 70 mV lower than Ni2PeNi5P4/NF (300 mV). Furthermore, it also exhibits good long-term stability for 20 h. This work provides a good thought named metal-doping transition metal phosphide complexes to improve the catalytic activity for OER. Such a superior OER performance is main attributed to the unique morphology, the doping of metal Mn regulating the metal phosphide Ni2PeNi5P4 nanosheet structure to be a 3D rough high-active-sites nanorod structure with the in situ formed oxidized Ni species on the surface and effective composite nanostructures. © 2020 Elsevier B.V. All rights reserved.
Keywords: Coral-like Hierarchical Mn-doping Ni2PeNi5P4/NF Catalytic activity Oxygen evolution
1. Introduction The energy crisis caused by the abuse of fossil fuels in the world is increasing [1]. Electrochemical water splitting not only has a jumbo progress in sustainable energy storage (wind and solar energy), but also this energy conversion approach has become a mature, environmentally-friendly and clean hydrogen production technology for large-scale applications [2e4]. Water splitting (2H2O / 2H2 þ O2) consists of two half-reactions: Oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) [5]. And the water splitting systems is able to generate high pure oxygen and hydrogen which is alternatives to fossil fuels [6]. However, the huge energy loss and low electrolysis efficiency caused by the high overpotential of HER and OER severely limit the large-scale practical application of water splitting [7]. Among them, the kinetically slow-four-electron-transfer steps of OER (2H2O/O2 þ 4Hþ þ 4e)
* Corresponding author. E-mail address: [email protected] (Q. Wu). https://doi.org/10.1016/j.jallcom.2020.154210 0925-8388/© 2020 Elsevier B.V. All rights reserved.
and rigid oxygen-oxygen bonds are the main limiting factors [8]. Therefoelectrocatre, alysts were indispensable to reduce the energy barrier and speed up the reaction kinetics. Recently, Ir, Ru-based oxides (h10:330 mV) [9], which are regarded as the most representative OER catalysts, but it cannot be put into practical production applications due to its high-cost and scarcity [10,11]. Therefore, it is very urgent to develop an efficient, low-cost and high-abundance OER catalyst. Currently, researchers have made stupendous progress in preparing high-activity electrocatalysts with earth-abundant elements, including transition metal phosphides (TMPs) [12], sulfides [13,14], selenides [15], nitrides [16], oxides [17] and double layer hydroxides (LDHs) [18]. Recently, TMPs have become a new favorite of catalyst materials due to its high activity, good electrical conductivity, and rapid oxidation to metal oxyhydroxide on the electrode surface during OER [19e21]. Some single-metal phosphides have been spanking developed in this regard, such as FeP [22,23], Ni2P [24], CoP [25,26] et al., especially nickel-based compounds. The low-price, earth-abundant, rich-valence states holding an
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excellent electrocatalytic activity of metal Ni have arouse wide concern. Porous nickel phosphide with good electrical conductivity and large surface area provides abundant active sites and increases effectiveness of facilitating charge and mass transport. However, corrosion resistance and activity of single metal phosphides are worse owing to uncontrolled agglomeration and larger series resistance, more susceptible oxidation [27,28]. So optimization of electrocatalytic performance of single metal phosphide active materials is essential. Currently, the assembly of single-metal twodimensional nanosheets into a three-dimensional hierarchical structure has been reported as a good strategy for improving the OER performance of two-dimensional nano-materials [29]. Two methods have been received widespread attention. One is metaldoping, which is recognized as a superb strategy to constitute bimetallic phosphides which introduce hetero-ion to provoke synergistic effect and therefore enrich redox sites, maximize exposure surface active edge sites and improve electronic conductivity [30]. Meanwhile, metal-doping availably optimizes the adsorption/desorption kinetics of intermediates by regulating surface morphology and can effectively improve the catalytic performance [31]. For example, Qin et al. [14] synthesized metal Zndoped Ni3S2 nanosheet arrays on NF as efficient OER catalyst with superior OER performance for overpotential of 330 mV at 100 mA/ cm2, 90 mV less than single-transition metal Ni3S2/NF. Feng et al. [20] synthesized 5% Mn-doped CoP3 nanowire arrays with the low OER overpotential of 280 mV while the overpotential of 340 mV of CoP3 NWs/CFP, which gap of overpotential demonstrated that Mndoping leads to better catalytic performance. Wen et al. [32] reported V-doped Ni2P/CC with better catalyst performance than single-metal phosphide Ni2P. Therefore, a large number of related literatures have proved that metal-doping is a practical strategy to optimize the electronic structure of the catalyst and improve the OER efficiency [29]. The other is to form heterogeneous interface from multi-phases. The multi-phase materials hold outstanding OER activity for two superiorities. Firstly, heterogeneous interface can effectively reduce the chemical adsorption free energy of interface ions(H* and OH*) and accelerate electronic recombination efficiency by tight connection of heterogeneous interface active materials to enhance interface transfer efficiency and therefore enhance OER performance. Secondly, multi-phases nanostructure surface with large active site can synergistically promote kinetics of mass and electron transmission to accelerate OER kinetics [33e36]. Some heterogeneous nanomaterials such as Ni2P/FeP [29], CoP/FeP [37], and NiMoP [38] are often used as high-efficiency OER catalysts. Simultaneously, Luo et al. [36] synthesized tree-like NiSeNi3S2/NF heterostructure with epitaxial growth strategy, which has 269 mV OER overpotential at 10 mA/cm2. Yan et al. [34] constructed hetero-interfaces of porous Ni2PeNi5P4 heterostructured arrays holding excellent water splitting catalytic performance with 1.69 V to reach 10 mA/cm2. Moreover, directly growing the active material on the conductive substrate not only improves the electrical conductivity of the material as a whole, but also the close contact between the active material and the substrate further enhances the electron transport efficiency, which has been proved as an effective method for improving OER activity [39]. In conclusion, regulating the materials structure (heterostructure) and controlling that morphology (metal-doping) to exposure more active sites can improve the performance of water splitting [34,35,39]. However, there are few reports in the literature that combine the two strategies to achieve "two in one". In this paper, we reported a high-efficiency 3D coral-like Mndoped Ni2P/Ni5P4 nanorods composite catalyst ("two in one") grown on NF. In the special structures, Ni2P/Ni5P4 ultrathin nanosheet has been transformed into a coral-like rough nanorod structure (Mn-doping Ni2PeNi5P4/NF) with abundant active sites,
high charge transfer efficiency and superior OER catalytic activity after Mn-doping. Such Mn-doped Ni2PeNi5P4/NF catalyst requires only an overpotential of 230 mV (vs. RHE) at a current density of 10 mA/cm2, which reduces the overpotential of 70 mV over Ni2PeNi5P4/NF under the same conditions. In addition, it exhibits long-term electrochemical stability at a current density of 10 mA/ cm2 for at least 20 h. 2. Experimental 2.1. Materials NiCl2∙6H2O is purchased from Kaitong Chemical Reagent Co., Ltd. (Tianjin, China). MnCl2∙4H2O is purchased from Shanpu Chemical Reagent Co., Ltd. (Shanghai, China). Red phosphorus is purchased from Damao Chemical Reagent Factory (Tianjin, China). NH4F, CH2N2O and KOH are all purchased from Sinopharm Chemical Reagent Co., Ltd. (Tianjin). RuCl3∙ xH2O is obtained in Aladdin and Nafion 117 solution (5%; Dupont) is purchased from Alfa Aesar (China) Chemicals Co. Ltd. . All medicines belonged to AR. Nickel foam (NF) is provided by Shenzhen Tianchenghe Technology Co., Ltd. All Nickel foam is clean before usage with ultrasonically using 3 M of HCl for 10 min to remove the oxides on the surface, then ethanol and water for 10 min, respectively [21]. The ultrapure water used throughout all experiments is produced from a Millipore system. All chemicals are used as received without further purification. 2.2. Preparation of NiMn-precursor/NF Generally, NiMn-precursor on Ni foam is fabricated with hydrothermal synthesis method. 0.25 mmol NiCl2∙6H2O, 1.5 mmol MnCl2∙4H2O, 0.1 g NH4F and 1 g CO(NH)2 are dissolved in 30 mL deionized water with magnetically stirring for 30min to get mixed precursor fluid. Then the solution is transferred to a 50 mL Teflonlined stainless autoclave with a piece of clean nickel foam (1 2 cm2). The autoclave is sealed and constant at 120 C for 12 h and naturally cool down to room temperature. After cooling, we remove the NF with tweezers and thoroughly wash it several times with deionized water to get a uniform NF surface color, then the catalyst is dried at 50 C for 6 h in an oven. Ni-hydroxide precursor is prepared under same conditions without MnCl2∙4H2O. Other NiMn-precursors are fabricated via the similar procedure except that the raw material ratios of Ni and Mn salts are set as 1:4, 1:5 and 1:7. 2.3. Preparation of MneNieP/NF nanorod array The MneNieP/NF catalyst is prepared with phosphidation treatment using typical chemical vapor deposition(CVD). The NiMn-precursor/NF is placed in a ceramic boat while other one boat is filled with 0.17 g P lying in upstream of the tube furnace. Then two ceramic boats are temperature programmed to 550 C and maintain that for 2 h under N2 flow with a heating speed of 5 C/ min, then naturally cool down to room temperature. Ni2P/NF is prepared by its hydroxide precursor without the presence of Mn salt. 2.4. Synthesis of RuO2 electrode To preparation RuO2 powder, 1.3 g of RuCl3$xH2O, 5 mL 1.0 M KOH and 50 mL distilled water are vigorous stirred for 45 min at 100 C. Then solution is centrifuged for 10 min and washed several times, dried at 80 C for 3 h to obtain RuO2 powder. To preparation RuO2 working electrode on Ni foam, 5 mg RuO2, 1 mL 1% mixed
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solution of Nafion and isopropanol are ultrasonicated for 30 min to turn into a homogeneous dispersed solution, then adding 1e2 mg XC-72R to enhance inherent conductivity. After ultrasonicating dispersion, RuO2 solution is dispersed onto NF and this NF is placed in the air overnight. The load of RuO2 electrode on NF is ~7 mg/cm2. 2.5. Characterization The X-ray diffraction (XRD) patterns are obtained from a LabX XRD-6100 X-ray diffractometer with Cu Ka radiation (40 kV, 30 mA) of wavelength 0.154 nm (SHIMADZU, Japan). Scanning electron microscope (SEM) measurements are performed on a XL30 ESEM FEG scanning electron microscope at an accelerating voltage of 20 kV. The structures of the samples are determined by Transmission electron microscopy (TEM) images on a JEOL JEM 2100 electron microscopy operated at 200 kV. X-ray photoelectron spectroscopy (XPS) data of the samples are collected on an ESCALABMK II x-ray photoelectron spectrometer using Mg as the exciting source. 2.6. Electrochemical measurement Electrochemical measurement is performed on a CHI 660E electrochemical workstation (Chenhua,Shanghai). A threeelectrode system is used throughout the experiment, which include the counter electrode (a graphite rod), the reference electrode (mercuric oxide electrode (Hg/HgO) and the work electrode (the resulting catalyst (MneNieP/NF). All the tests are carried out at room temperature in 1.0 M KOH solution. The potential in our work is all adjusted with reversible hydrogen electrode (RHE): E(RHE) ¼ E (Hg/HgO)þ0.924V [37]. The electrochemical impedance spectroscopy (EIS) measurements are performed under a bias of 0.20 V vs RHE in the frequency ranging from 100 kHz to 10 mHz with an amplitude of 5 mV. 3. Result and discussion Fig. 1 shows simple synthesis steps of hydrothermal method and high temperature chemical vapor deposition of MneNieP/NF catalyst. First, NiMn-precursor/NF nanorods are grown on bare NF by a hydrothermal method, and then tube furnace phosphidation (see ESI for details). The X-ray powder diffraction (XRD) pattern of NieMneP/NF (Fig. 2a) shows the main diffraction peaks of the resulting catalyst appeared at 40.6 , 44.8 , 47.4 , 54.2 and 55.0 , which are indexed to the (111), (201), (210), (300), (211) planes of pure Ni2P (JCPDS No. 74e1385), respectively. Moreover, there are
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also five distinct characteristic peaks at 36.1, 43.9 , 45.2 , 47.9 and 53.1 corresponding to the (104), (212), (204), (213) and (214) planes of Ni5P4 (JCPDS No. 18e0883). The two remaining characteristic peaks correspond to NF pattern (JCPDS No. 01e1258). Mn element is not clearly observed in Fig. 2a, which maybe exist in MneNieP/NF as doping state. Details of Fig. 2a reveals that the main diffraction peak of the product is slightly shifted to the right compared to Ni5P4 and NF owing to Mn metal doping [40], and the XRD of MneNieP/NF has no other impurity peaks. Moreover, we observe that the diffraction peaks does not show any deviation without Mn element in Fig. S1a. Further, the distinct weak diffraction peaks of Mn(OH)4 (JCPDS No. 15e0604) and NiMn mixed oxide (JCPDS No. 75e2089) are observed from the XRD pattern of NiMn-precursor/NF in Fig. S2a, suggesting that the Mn element does exist in the precursor. And Ni(0) (JCPDS No. 04e0850) and Ni(III) oxide (JCPDS No. 14e0481) have strong diffraction peaks in the precursor. The diffraction peaks of Mn element compounds are not directly observed in the XRD pattern of the final product catalyst. We suspect that it may be due to the fact that the Ni-based phosphide content accounts for most of the NieMneP/NF hybird catalyst under the 550 C high temperature [41]. As is shown in Fig. S3, the SEM pattern of pure NF shows smooth surface and overall three-dimensional macroporous structure. Fig. S2b and Fig. 2b show the structure of MnNi-precursor before and after phosphidation, respectively, Fig. 2b indicate that the structure of MneNieP/NF hybrid catalyst is no longer a simple nanoflake of Ni2P/Ni5P4 (Fig. S1b) or nanorods of NiMnprecursor(Fig. S2b), but a coral-like 3D nanorod array that is inlaid by a combination of two single structures. Fig. 2c shows the actual coral map similar to morphology of the resulting product. For clearer images and low-magnification SEM images of the threedimensional nanorods, see Fig. S4. It is obvious that the morphology after phosphidation, the surface becomes porous and rough, which facilitates the gas diffusion and thus beneficial to enhance catalytic activity. The TEM-EDS for the final product have been done (Fig. S5), the result indicates that the presence of Ni, Mn and P elements were confirmed, and the Ni/Mn atomic ratio was measured to be 1:5.05, which is very close to that of Ni/Mn feed ratio (1:6). Simultaneously, Fig. 2d shows the EDS elemental mapping of MneNieP/NF, which is evenly distributed. TEM(Transmission electron microscopy) image (Fig. 2e) shows that MneNieP/NF hybrid catalyst does exhibit nanorod morphology, we can observe that the surface is composed of nanosheets, but obvious nanorod arrays cannot be completely observed due to the close packing of nanorods. Fig. 2f shows the HRTEM pattern belonging to MneNieP/NF, there are obvious two-phase coupling
Fig. 1. Synthetic route map of MneNieP/NF nanorod array.
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Fig. 2. (a) XRD pattern of MneNieP/NF. (b) SEM image of the final catalyst of coral-like MneNieP/NF. (c)Image of actual coral. (d) SEM image and EDS elemental map of Ni, Mn, P of MneNieP/NF. (e,f,g)TEM and HRTEM images of MneNieP/NF nanorods.
interface with lattice spacing of 2.59 nm belonging to Ni5P4 and 2.81 nm belonging to Ni2P nanomaterial. As shown in Fig. 2g, highresolution TEM (HRTEM) image of MneNieP/NF also presents lattice stripe with space of 2.53 nm, corresponding to (200) crystal plane of Ni2P. Meanwhile, lattice distances of 2.84 nm and 3.11 nm
exactly corresponds to (201) and (103) crystal planes of Ni5P4 (Fig. 2h). SAED pattern also reveals other planes of Ni2P (Fig. S6), indicating the polycrystalline structure of MneNieP/NF. The above results fully demonstrate that MneNieP/NF hierarchical nanorod structure has been grown on the NF surface.
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The X-ray photoelectron spectroscopy (XPS) survey spectrum is used to comprehend surface chemical states of the final product. In Fig. 3a, the XPS spectrum of MneNieP/NF further confirm the existence of Mn, Ni and P elements. There are some differences in the MneNieP/NF survey spectrum before and after oxygen evolution for 20 h and it is worth noting that there is almost no P element in the XPS spectrum of the tested product. More carefully, Fig. S7 shows the P 2p XPS spectra, on which the signal intensity of P element on MneNieP/NF surface is significantly reduced after 20 h of oxygen evolution. It has been reported that the reduction of the phosphide on the catalyst surface means that the phosphide on the near-surface is converted into Ni-(oxy)hydroxide during OER progress. Meanwhile, The XPS spectrum of Ni 2p after long-trem OER progress showed two peaks at 855.9 and 873.7 eV which can be attributed to the Ni 2p3/2 and Ni 2p1/2 of Ni-(oxy)hydroxide, respectively [42,43]. For the detailed XPS spectrum of other elements after 20 h, please see Fig. S7. Fig. 3b, c and 3d present the XPS spectra of Mn、Ni、P of MneNieP/NF, respectively. Fig. 3b shows the Mn 2p high-definition spectrum, and the binding energy (BE) characteristic peak at 642.5 eV corresponded to Mn2þ of Mn 2p3/2 and the BE peaks at 654.7eV belonged to Mn 2p1/2 [44]. Ni 2p (Fig. 3c) has two BE peaks at 857.2 eV and 875.1 eV complied with Ni 2p3/2 and Ni 2p1/2, corresponding to Nidþ [36,45]. The remaining two BE peaks at 861.5 eV and 879.9 eV are considered as two satellite peaks. In Fig. 3d, the peak at 129.7eV is assigned to P 2p and the peak at 134.9 eV from P 2p high-resolution spectrum is ascribed to PeO peak, due to the inevitable exposure of the sample in the air [46]. A three-electrode system is used to measure the OER activity of MneNieP/NF catalyst in 1.0 M KOH. The prepared catalyst is used as work electrode directly. Meanwhile, Hg/HgO immersed in 1.0 M KOH and a graphite rod served as reference electrode and counter electrode, respectively. All evaluation data are based on reversible
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hydrogen electrodes (RHE) and iR compensation is used to ensure authentic data of OER performance [47]. In order to better judge the catalytic performance of the final catalyst, pure Ni2P/Ni5P4 and NF are tested under the same conditions for comparison. In order to eliminate the interference of Ni(II)/Ni(III) redox peak at about 1.4 V, the OER activity are evaluated according to the polarization curves swept from high potential to low potential. First, we explore the influence of catalyst composition on OER catalytic activity. The effect of the ratio of Ni:Mn on catalyst activity is studied to obtain the optimal ratio (Fig. S8), Fig. S8 performs the cyclic voltammetry (CV) curves of MneNieP/NF with different Ni/Mn ratios (Ni:Mn ¼ 1:5, 1:6, 1:7 and 1:8). The results showed that OER activity of assynthesized catalysts is in connection with metal ratio. Among them, Ni:Mn ¼ 1:6 is the optimal ratio which exhibits the best catalytic activity. Therefore, the mole ratio of Ni/Mn is fixed at 1:6 in the following research. Fig. 4 detailedly shows the OER performance of the resulting catalyst and control samples. In Fig. 4a, we observe the corresponding CV curves of MneNieP/NF, Ni2PeNi5P4/ NF,RuO2/NF and clean NF. For all samples, a certain peak corresponding to reduction reaction can be observed after the end of OER process. It is known that anodic and cathodic peaks wound appear in CV curves for Ni-based catalysts, which is caused by the reversible reaction of Ni(II)/Ni(III). The reduction peak of Ni(II)/ Ni(III) at about 1.2 V has little effect on the anodic current caused by oxygen evolution reaction. The enhanced OER catalytic activity is extraordinary obvious after phosphidation and Mn incorporation from polarization curves. As expected, NF exhibits poor OER activity with an overpotential of 380 mV at the current density of 10 mA/ cm2, and MneNieP/NF hybrid catalyst could effectively promote OER catalytic activity compared with Ni2PeNi5P4/NF, which demands only 230 mV overpotential at 10 mA/cm2, while the overpotential for Ni2PeNi5P4/NF is 300 mV at the same current density. Meanwhile, there are a rapid anodic current increase when the
Fig. 3. (a) XPS survey spectrum for MneNieP/NF before and after OER measure for 20 h. XPS spectra in the (b) Mn 2p, (c) Ni 2p, and (d) P 2p regions for MneNieP/NF.
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Fig. 4. (a) CV curves for MneNieP/NF, Ni2PeNi5P4/NF and NF with a scan rate of 5 mV/s for OER in 1.0 M KOH. (b) Tafel plots of NF, Ni2PeNi5P4/NF and MneNieP/NF.(c) CV curves for MneNieP/NF before and after 500 CV cycles.(d) Time dependence of the potential of MneNieP/NF under overpotential of 230 mV at 10 mA/cm2.
potential is scanned to more positive section. Moreover, the OER performance of the catalyst is superior to other non-noble-metal catalysts in literature (see Table S1 in detail). Additionally, the reaction kinetics of OER generally are evaluated by Tafel plots. The smaller the Tafel slope, the faster the OER kinetics. Tafel plots are derived from the polarization curves, revealing the connection of the potential and the log of the relevant current density [48,49]. As shown in Fig. 4b, the tafel slopes of MneNieP/NF, Ni2PeNi5P4/NF, RuO2/NF and pure NF are 64.3 mA dec1,126.1 mA dec1, 104.5 mA dec1 and 152.9 mA dec1, respectively. The MneNieP/NF exhibits the smallest Tafel slope, which suggests the faster OER kinetics of MneNieP/NF and implies better OER performance, corresponding to CV curves (Fig. 4a). Moreover, basic research indicates that the OER overpotential is related to the free energy of the reaction [50,51]. In our example, MneNieP/NF hybrid catalyst exhibits superior catalytic activity, which may be presumed to be due to the decrease in the free energy of the reaction, resulting from the introduction of biphasic phosphide, Mn-doping and a suitable composition, which provides an advantageous environment for catalyzing OER. Stability is also considered as a major evaluation criterion for the practicality of the catalyst [52]. The long-term electrochemical durability of MneNieP is further tested by cyclic voltammetry (CV) scanning and chronopotentiometric measurement. Fig. 4c demonstrates the polarization curves of the final sample before and after CV scanning of 500 cycles with negligible decay at 10 mA/cm2 in 1.0 M KOH, which indicates that the catalyst has good cycle test stability and high durability. Simultaneously, the catalyst could also maintains an overpotential of 230 mV at a current density of 10 mA/ cm2 for at least 20 h, which can further confirm good stability (Fig. 4d). Also, the NieMneP catalyst after stability is characterized by XPS to explore the states of different elements. It is obvious that
the content of oxidized Ni and oxidized Mn increased and the content of P decreased in Fig. S7. We deduced that the metal phosphide on the surface of catalyst has changed to metal (oxy) hydroxide during continuous oxygen evolution process, which is regarded as real active species for OER. The results were in consistence with phenomena recorded in other literature [42,43,53]. It is known that metal phosphides are thermodynamically less stable than metal oxides in solution under strongly oxidative environment, which might be oxidized to corresponding metal oxides/oxyhydroxides in high potential range during OER process [53]. According OER mechanism as follow [54], it is rational that the surface of MneNieP/NF is oxidized to Ni-oxyhydroxides. S þ OH / SeOH* þ e (S¼Surface) SeOH* þ OH / SeO* þ H2O þ e SeO* þ OH / SeOOH* þ e SeOOH* þ OH / SeO2* þ H2O þ e SeO2* / S þ O2 It has been reported that NieFe oxy/hydroxide active species are formed on the surface of FeP/Ni2P [29]. A review has introduced this phenomenon and summarized that the insitu formed oxides/ oxyhydroxides could possibly enhance the OER activity [43]. It maybe origin from synergistic electronic interactions between metal phosphides and oxy-hydroxides on the surface [53]. To further analyze the superior OER performance of MneNieP/ NF, we test the electric double-layer capacitance (Cdl) and electrochemical impedance spectroscopy (EIS). As shown in Fig. 5(a-c), CV
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Fig. 5. CV image within scan rate of 70e110 mV/s of (a) NF, (b) Ni2PeNi5P4/NF and (c) MneNieP/NF.(d) The capacitive current densities at 0.42 V as a function of scan rate for MneNieP/NF, Ni2PeNi5P4/NF and NF. (e) Nyquist plots of MneNieP/NF and Ni2PeNi5P4/NF.
scanning is performed at different scanning rates of 70e110 mV/s in the illegal pull zone (100 mV apart) of MneNieP/NF, Ni2PeNi5P4/ NF and NF. As shown in Fig. 5d, the Cdl value (2.16 mF/cm2) of MneNieP/NF is observed larger than that of Ni2PeNi5P4/NF (1.84 mF/cm2), which implies heterogeneous nanostructure and Mn-doping with more effective active sites are in favor of a better OER performance. The improved electrical conductivity could be demostrated by EIS. As in Fig. 5e, the semicircular radius of the MneNieP/NF is smaller than that of Ni2PeNi5P4/NF, revealing lower charge transfer resistance and the faster electron transfer efficiency in comparison with control samples [55], The above datas indicate that it has smaller resistance, greater electron transfer efficiency, faster OER kinetics and better activity. The results are consistent with the high-performance of MneNieP/NF nanohybrids. Therefore, the good OER activity and stability of the prepared MneNieP/NF may be attributed to the following factors: (1) unique coral-like structure with porous and rough surfaces facilitate the gas diffusion and thus beneficial to enhance catalytic activity. (2) Doped state of metal Mn and synergistic effect of two-phase coupling of Ni2P/Ni5P4 nanosheet heterostructure may play an important role in the enhancement of the OER performance. (3) The in situ formed oxidized Ni species on the surface is in favor of improving OER performance. (4) The favorable composition factor plays a role in enhancing catalytic activity. As a result, all of the above indicate that MneNieP/NF exhibits good OER performance. 4. Conclusions In summary, Mn-doping Ni2PeNi5P4/NF electrocatalyst with a unique coral-like hierarchical structure has been prepared by a two-step method, which results in a significant enhancement of the OER performance. At a current density of 10 mA/cm2, only a low overpotential of 230 mV is required, which is 70 mV less than the overpotential of Ni2PeNi5P4/NF. In addition, such catalyst exhibits good long-term electrochemical stability, which can be stabilized for at least 20 h. This certainly benefits from a unique morphology, metal Mn-doping and the synergistic effects of Ni5P4eNi2P, the in situ formed oxidized Ni species on the surface and effective
composite nanostructures.. This work not only offers an attractive and cost-effective catalyst for alkaline oxygen evolution, but also provides important strategy for designing and synthesizing Nibased phosphate catalysts with enhanced OER activity. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. CRediT authorship contribution statement Siran Xu: Writing - original draft, Writing - review & editing. Yeshuang Du: Writing - original draft, Writing - review & editing. Xian Liu: Data curation. Xin Yu: Data curation. Chunlin Teng: Data curation. Xiaohong Cheng: Resources. Yunfeng Chen: Resources. Qi Wu: Supervision, Writing - original draft, Writing - review & editing. Acknowledgements This work is supported by the National Natural Science Foundation of China (Grant No. 21601056), the Hubei Provincial Department of Education (no. Q20162505), the Natural Science Foundation of Hubei Province (Grant No. 2019CFB569) and the Science and Technology foundation for Creative Research Group of HBDE (Grant Nos. T201810). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jallcom.2020.154210. References [1] L. Yu, H. Zhou, J. Sun, F. Qin, D. Luo, L. Xie, F. Yu, J. Bao, Y. Li, Y. Yu, S. Chen, Z. Ren, Nano Energy 41 (2017) 327e336. [2] J. Sun, M. Ren, L. Yu, Z. Yang, L. Xie, F. Tian, Y. Yu, Z. Ren, S. Chen, H. Zhou, Small 15 (2019), e1804272.
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Three-dimensional (3D) hierarchical coral-like Mn-doped Ni2PeNi5P4/ NF catalyst for efficient oxygen evolution Siran Xu a, Yeshuang Du a, Xian Liu a, Xin Yu a, Chunlin Teng a, Xiaohong Cheng b, Yunfeng Chen c, Qi Wu a, * a
College of Chemistry and Chemical Engineering, Hubei Normal University, Huangshi, 435002, China Hubei Key Laboratory of Low Dimensional Optoelectronic Materials and Devices, Hubei University of Arts and Science, Xiangyang, 441053, Hubei Province, PR China c School of Chemistry and Environmental Engineering, Wuhan Institute of Technology, Wuhan, 430073, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 31 October 2019 Received in revised form 3 February 2020 Accepted 5 February 2020 Available online 6 February 2020
Developing earth-abundant and high-efficient catalysts for the oxygen evolution reaction (OER) to enhance the efficiency of water splitting is highly desirable. Metal-doping and construction of heterogeneous structure are two start-of-the-art strategies to increase the OER activity of transition metal materials. In this work, we exhibit the design and construction of a unique coral-like 3D hierarchical Mndoped Ni2PeNi5P4/NF (MneNieP/NF) OER catalyst. The Mn-doped Ni2PeNi5P4/NF hybrid catalyst is synthesized on Ni foam by hydrothermal and followed in situ phosphidation, which exhibits superior OER catalytic performance with an overpotential of 230 mV (vs. RHE) at a current density of 10 mA/cm2 and 70 mV lower than Ni2PeNi5P4/NF (300 mV). Furthermore, it also exhibits good long-term stability for 20 h. This work provides a good thought named metal-doping transition metal phosphide complexes to improve the catalytic activity for OER. Such a superior OER performance is main attributed to the unique morphology, the doping of metal Mn regulating the metal phosphide Ni2PeNi5P4 nanosheet structure to be a 3D rough high-active-sites nanorod structure with the in situ formed oxidized Ni species on the surface and effective composite nanostructures. © 2020 Elsevier B.V. All rights reserved.
Keywords: Coral-like Hierarchical Mn-doping Ni2PeNi5P4/NF Catalytic activity Oxygen evolution
1. Introduction The energy crisis caused by the abuse of fossil fuels in the world is increasing [1]. Electrochemical water splitting not only has a jumbo progress in sustainable energy storage (wind and solar energy), but also this energy conversion approach has become a mature, environmentally-friendly and clean hydrogen production technology for large-scale applications [2e4]. Water splitting (2H2O / 2H2 þ O2) consists of two half-reactions: Oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) [5]. And the water splitting systems is able to generate high pure oxygen and hydrogen which is alternatives to fossil fuels [6]. However, the huge energy loss and low electrolysis efficiency caused by the high overpotential of HER and OER severely limit the large-scale practical application of water splitting [7]. Among them, the kinetically slow-four-electron-transfer steps of OER (2H2O/O2 þ 4Hþ þ 4e)
* Corresponding author. E-mail address: [email protected] (Q. Wu). https://doi.org/10.1016/j.jallcom.2020.154210 0925-8388/© 2020 Elsevier B.V. All rights reserved.
and rigid oxygen-oxygen bonds are the main limiting factors [8]. Therefoelectrocatre, alysts were indispensable to reduce the energy barrier and speed up the reaction kinetics. Recently, Ir, Ru-based oxides (h10:330 mV) [9], which are regarded as the most representative OER catalysts, but it cannot be put into practical production applications due to its high-cost and scarcity [10,11]. Therefore, it is very urgent to develop an efficient, low-cost and high-abundance OER catalyst. Currently, researchers have made stupendous progress in preparing high-activity electrocatalysts with earth-abundant elements, including transition metal phosphides (TMPs) [12], sulfides [13,14], selenides [15], nitrides [16], oxides [17] and double layer hydroxides (LDHs) [18]. Recently, TMPs have become a new favorite of catalyst materials due to its high activity, good electrical conductivity, and rapid oxidation to metal oxyhydroxide on the electrode surface during OER [19e21]. Some single-metal phosphides have been spanking developed in this regard, such as FeP [22,23], Ni2P [24], CoP [25,26] et al., especially nickel-based compounds. The low-price, earth-abundant, rich-valence states holding an
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excellent electrocatalytic activity of metal Ni have arouse wide concern. Porous nickel phosphide with good electrical conductivity and large surface area provides abundant active sites and increases effectiveness of facilitating charge and mass transport. However, corrosion resistance and activity of single metal phosphides are worse owing to uncontrolled agglomeration and larger series resistance, more susceptible oxidation [27,28]. So optimization of electrocatalytic performance of single metal phosphide active materials is essential. Currently, the assembly of single-metal twodimensional nanosheets into a three-dimensional hierarchical structure has been reported as a good strategy for improving the OER performance of two-dimensional nano-materials [29]. Two methods have been received widespread attention. One is metaldoping, which is recognized as a superb strategy to constitute bimetallic phosphides which introduce hetero-ion to provoke synergistic effect and therefore enrich redox sites, maximize exposure surface active edge sites and improve electronic conductivity [30]. Meanwhile, metal-doping availably optimizes the adsorption/desorption kinetics of intermediates by regulating surface morphology and can effectively improve the catalytic performance [31]. For example, Qin et al. [14] synthesized metal Zndoped Ni3S2 nanosheet arrays on NF as efficient OER catalyst with superior OER performance for overpotential of 330 mV at 100 mA/ cm2, 90 mV less than single-transition metal Ni3S2/NF. Feng et al. [20] synthesized 5% Mn-doped CoP3 nanowire arrays with the low OER overpotential of 280 mV while the overpotential of 340 mV of CoP3 NWs/CFP, which gap of overpotential demonstrated that Mndoping leads to better catalytic performance. Wen et al. [32] reported V-doped Ni2P/CC with better catalyst performance than single-metal phosphide Ni2P. Therefore, a large number of related literatures have proved that metal-doping is a practical strategy to optimize the electronic structure of the catalyst and improve the OER efficiency [29]. The other is to form heterogeneous interface from multi-phases. The multi-phase materials hold outstanding OER activity for two superiorities. Firstly, heterogeneous interface can effectively reduce the chemical adsorption free energy of interface ions(H* and OH*) and accelerate electronic recombination efficiency by tight connection of heterogeneous interface active materials to enhance interface transfer efficiency and therefore enhance OER performance. Secondly, multi-phases nanostructure surface with large active site can synergistically promote kinetics of mass and electron transmission to accelerate OER kinetics [33e36]. Some heterogeneous nanomaterials such as Ni2P/FeP [29], CoP/FeP [37], and NiMoP [38] are often used as high-efficiency OER catalysts. Simultaneously, Luo et al. [36] synthesized tree-like NiSeNi3S2/NF heterostructure with epitaxial growth strategy, which has 269 mV OER overpotential at 10 mA/cm2. Yan et al. [34] constructed hetero-interfaces of porous Ni2PeNi5P4 heterostructured arrays holding excellent water splitting catalytic performance with 1.69 V to reach 10 mA/cm2. Moreover, directly growing the active material on the conductive substrate not only improves the electrical conductivity of the material as a whole, but also the close contact between the active material and the substrate further enhances the electron transport efficiency, which has been proved as an effective method for improving OER activity [39]. In conclusion, regulating the materials structure (heterostructure) and controlling that morphology (metal-doping) to exposure more active sites can improve the performance of water splitting [34,35,39]. However, there are few reports in the literature that combine the two strategies to achieve "two in one". In this paper, we reported a high-efficiency 3D coral-like Mndoped Ni2P/Ni5P4 nanorods composite catalyst ("two in one") grown on NF. In the special structures, Ni2P/Ni5P4 ultrathin nanosheet has been transformed into a coral-like rough nanorod structure (Mn-doping Ni2PeNi5P4/NF) with abundant active sites,
high charge transfer efficiency and superior OER catalytic activity after Mn-doping. Such Mn-doped Ni2PeNi5P4/NF catalyst requires only an overpotential of 230 mV (vs. RHE) at a current density of 10 mA/cm2, which reduces the overpotential of 70 mV over Ni2PeNi5P4/NF under the same conditions. In addition, it exhibits long-term electrochemical stability at a current density of 10 mA/ cm2 for at least 20 h. 2. Experimental 2.1. Materials NiCl2∙6H2O is purchased from Kaitong Chemical Reagent Co., Ltd. (Tianjin, China). MnCl2∙4H2O is purchased from Shanpu Chemical Reagent Co., Ltd. (Shanghai, China). Red phosphorus is purchased from Damao Chemical Reagent Factory (Tianjin, China). NH4F, CH2N2O and KOH are all purchased from Sinopharm Chemical Reagent Co., Ltd. (Tianjin). RuCl3∙ xH2O is obtained in Aladdin and Nafion 117 solution (5%; Dupont) is purchased from Alfa Aesar (China) Chemicals Co. Ltd. . All medicines belonged to AR. Nickel foam (NF) is provided by Shenzhen Tianchenghe Technology Co., Ltd. All Nickel foam is clean before usage with ultrasonically using 3 M of HCl for 10 min to remove the oxides on the surface, then ethanol and water for 10 min, respectively [21]. The ultrapure water used throughout all experiments is produced from a Millipore system. All chemicals are used as received without further purification. 2.2. Preparation of NiMn-precursor/NF Generally, NiMn-precursor on Ni foam is fabricated with hydrothermal synthesis method. 0.25 mmol NiCl2∙6H2O, 1.5 mmol MnCl2∙4H2O, 0.1 g NH4F and 1 g CO(NH)2 are dissolved in 30 mL deionized water with magnetically stirring for 30min to get mixed precursor fluid. Then the solution is transferred to a 50 mL Teflonlined stainless autoclave with a piece of clean nickel foam (1 2 cm2). The autoclave is sealed and constant at 120 C for 12 h and naturally cool down to room temperature. After cooling, we remove the NF with tweezers and thoroughly wash it several times with deionized water to get a uniform NF surface color, then the catalyst is dried at 50 C for 6 h in an oven. Ni-hydroxide precursor is prepared under same conditions without MnCl2∙4H2O. Other NiMn-precursors are fabricated via the similar procedure except that the raw material ratios of Ni and Mn salts are set as 1:4, 1:5 and 1:7. 2.3. Preparation of MneNieP/NF nanorod array The MneNieP/NF catalyst is prepared with phosphidation treatment using typical chemical vapor deposition(CVD). The NiMn-precursor/NF is placed in a ceramic boat while other one boat is filled with 0.17 g P lying in upstream of the tube furnace. Then two ceramic boats are temperature programmed to 550 C and maintain that for 2 h under N2 flow with a heating speed of 5 C/ min, then naturally cool down to room temperature. Ni2P/NF is prepared by its hydroxide precursor without the presence of Mn salt. 2.4. Synthesis of RuO2 electrode To preparation RuO2 powder, 1.3 g of RuCl3$xH2O, 5 mL 1.0 M KOH and 50 mL distilled water are vigorous stirred for 45 min at 100 C. Then solution is centrifuged for 10 min and washed several times, dried at 80 C for 3 h to obtain RuO2 powder. To preparation RuO2 working electrode on Ni foam, 5 mg RuO2, 1 mL 1% mixed
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solution of Nafion and isopropanol are ultrasonicated for 30 min to turn into a homogeneous dispersed solution, then adding 1e2 mg XC-72R to enhance inherent conductivity. After ultrasonicating dispersion, RuO2 solution is dispersed onto NF and this NF is placed in the air overnight. The load of RuO2 electrode on NF is ~7 mg/cm2. 2.5. Characterization The X-ray diffraction (XRD) patterns are obtained from a LabX XRD-6100 X-ray diffractometer with Cu Ka radiation (40 kV, 30 mA) of wavelength 0.154 nm (SHIMADZU, Japan). Scanning electron microscope (SEM) measurements are performed on a XL30 ESEM FEG scanning electron microscope at an accelerating voltage of 20 kV. The structures of the samples are determined by Transmission electron microscopy (TEM) images on a JEOL JEM 2100 electron microscopy operated at 200 kV. X-ray photoelectron spectroscopy (XPS) data of the samples are collected on an ESCALABMK II x-ray photoelectron spectrometer using Mg as the exciting source. 2.6. Electrochemical measurement Electrochemical measurement is performed on a CHI 660E electrochemical workstation (Chenhua,Shanghai). A threeelectrode system is used throughout the experiment, which include the counter electrode (a graphite rod), the reference electrode (mercuric oxide electrode (Hg/HgO) and the work electrode (the resulting catalyst (MneNieP/NF). All the tests are carried out at room temperature in 1.0 M KOH solution. The potential in our work is all adjusted with reversible hydrogen electrode (RHE): E(RHE) ¼ E (Hg/HgO)þ0.924V [37]. The electrochemical impedance spectroscopy (EIS) measurements are performed under a bias of 0.20 V vs RHE in the frequency ranging from 100 kHz to 10 mHz with an amplitude of 5 mV. 3. Result and discussion Fig. 1 shows simple synthesis steps of hydrothermal method and high temperature chemical vapor deposition of MneNieP/NF catalyst. First, NiMn-precursor/NF nanorods are grown on bare NF by a hydrothermal method, and then tube furnace phosphidation (see ESI for details). The X-ray powder diffraction (XRD) pattern of NieMneP/NF (Fig. 2a) shows the main diffraction peaks of the resulting catalyst appeared at 40.6 , 44.8 , 47.4 , 54.2 and 55.0 , which are indexed to the (111), (201), (210), (300), (211) planes of pure Ni2P (JCPDS No. 74e1385), respectively. Moreover, there are
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also five distinct characteristic peaks at 36.1, 43.9 , 45.2 , 47.9 and 53.1 corresponding to the (104), (212), (204), (213) and (214) planes of Ni5P4 (JCPDS No. 18e0883). The two remaining characteristic peaks correspond to NF pattern (JCPDS No. 01e1258). Mn element is not clearly observed in Fig. 2a, which maybe exist in MneNieP/NF as doping state. Details of Fig. 2a reveals that the main diffraction peak of the product is slightly shifted to the right compared to Ni5P4 and NF owing to Mn metal doping [40], and the XRD of MneNieP/NF has no other impurity peaks. Moreover, we observe that the diffraction peaks does not show any deviation without Mn element in Fig. S1a. Further, the distinct weak diffraction peaks of Mn(OH)4 (JCPDS No. 15e0604) and NiMn mixed oxide (JCPDS No. 75e2089) are observed from the XRD pattern of NiMn-precursor/NF in Fig. S2a, suggesting that the Mn element does exist in the precursor. And Ni(0) (JCPDS No. 04e0850) and Ni(III) oxide (JCPDS No. 14e0481) have strong diffraction peaks in the precursor. The diffraction peaks of Mn element compounds are not directly observed in the XRD pattern of the final product catalyst. We suspect that it may be due to the fact that the Ni-based phosphide content accounts for most of the NieMneP/NF hybird catalyst under the 550 C high temperature [41]. As is shown in Fig. S3, the SEM pattern of pure NF shows smooth surface and overall three-dimensional macroporous structure. Fig. S2b and Fig. 2b show the structure of MnNi-precursor before and after phosphidation, respectively, Fig. 2b indicate that the structure of MneNieP/NF hybrid catalyst is no longer a simple nanoflake of Ni2P/Ni5P4 (Fig. S1b) or nanorods of NiMnprecursor(Fig. S2b), but a coral-like 3D nanorod array that is inlaid by a combination of two single structures. Fig. 2c shows the actual coral map similar to morphology of the resulting product. For clearer images and low-magnification SEM images of the threedimensional nanorods, see Fig. S4. It is obvious that the morphology after phosphidation, the surface becomes porous and rough, which facilitates the gas diffusion and thus beneficial to enhance catalytic activity. The TEM-EDS for the final product have been done (Fig. S5), the result indicates that the presence of Ni, Mn and P elements were confirmed, and the Ni/Mn atomic ratio was measured to be 1:5.05, which is very close to that of Ni/Mn feed ratio (1:6). Simultaneously, Fig. 2d shows the EDS elemental mapping of MneNieP/NF, which is evenly distributed. TEM(Transmission electron microscopy) image (Fig. 2e) shows that MneNieP/NF hybrid catalyst does exhibit nanorod morphology, we can observe that the surface is composed of nanosheets, but obvious nanorod arrays cannot be completely observed due to the close packing of nanorods. Fig. 2f shows the HRTEM pattern belonging to MneNieP/NF, there are obvious two-phase coupling
Fig. 1. Synthetic route map of MneNieP/NF nanorod array.
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Fig. 2. (a) XRD pattern of MneNieP/NF. (b) SEM image of the final catalyst of coral-like MneNieP/NF. (c)Image of actual coral. (d) SEM image and EDS elemental map of Ni, Mn, P of MneNieP/NF. (e,f,g)TEM and HRTEM images of MneNieP/NF nanorods.
interface with lattice spacing of 2.59 nm belonging to Ni5P4 and 2.81 nm belonging to Ni2P nanomaterial. As shown in Fig. 2g, highresolution TEM (HRTEM) image of MneNieP/NF also presents lattice stripe with space of 2.53 nm, corresponding to (200) crystal plane of Ni2P. Meanwhile, lattice distances of 2.84 nm and 3.11 nm
exactly corresponds to (201) and (103) crystal planes of Ni5P4 (Fig. 2h). SAED pattern also reveals other planes of Ni2P (Fig. S6), indicating the polycrystalline structure of MneNieP/NF. The above results fully demonstrate that MneNieP/NF hierarchical nanorod structure has been grown on the NF surface.
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The X-ray photoelectron spectroscopy (XPS) survey spectrum is used to comprehend surface chemical states of the final product. In Fig. 3a, the XPS spectrum of MneNieP/NF further confirm the existence of Mn, Ni and P elements. There are some differences in the MneNieP/NF survey spectrum before and after oxygen evolution for 20 h and it is worth noting that there is almost no P element in the XPS spectrum of the tested product. More carefully, Fig. S7 shows the P 2p XPS spectra, on which the signal intensity of P element on MneNieP/NF surface is significantly reduced after 20 h of oxygen evolution. It has been reported that the reduction of the phosphide on the catalyst surface means that the phosphide on the near-surface is converted into Ni-(oxy)hydroxide during OER progress. Meanwhile, The XPS spectrum of Ni 2p after long-trem OER progress showed two peaks at 855.9 and 873.7 eV which can be attributed to the Ni 2p3/2 and Ni 2p1/2 of Ni-(oxy)hydroxide, respectively [42,43]. For the detailed XPS spectrum of other elements after 20 h, please see Fig. S7. Fig. 3b, c and 3d present the XPS spectra of Mn、Ni、P of MneNieP/NF, respectively. Fig. 3b shows the Mn 2p high-definition spectrum, and the binding energy (BE) characteristic peak at 642.5 eV corresponded to Mn2þ of Mn 2p3/2 and the BE peaks at 654.7eV belonged to Mn 2p1/2 [44]. Ni 2p (Fig. 3c) has two BE peaks at 857.2 eV and 875.1 eV complied with Ni 2p3/2 and Ni 2p1/2, corresponding to Nidþ [36,45]. The remaining two BE peaks at 861.5 eV and 879.9 eV are considered as two satellite peaks. In Fig. 3d, the peak at 129.7eV is assigned to P 2p and the peak at 134.9 eV from P 2p high-resolution spectrum is ascribed to PeO peak, due to the inevitable exposure of the sample in the air [46]. A three-electrode system is used to measure the OER activity of MneNieP/NF catalyst in 1.0 M KOH. The prepared catalyst is used as work electrode directly. Meanwhile, Hg/HgO immersed in 1.0 M KOH and a graphite rod served as reference electrode and counter electrode, respectively. All evaluation data are based on reversible
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hydrogen electrodes (RHE) and iR compensation is used to ensure authentic data of OER performance [47]. In order to better judge the catalytic performance of the final catalyst, pure Ni2P/Ni5P4 and NF are tested under the same conditions for comparison. In order to eliminate the interference of Ni(II)/Ni(III) redox peak at about 1.4 V, the OER activity are evaluated according to the polarization curves swept from high potential to low potential. First, we explore the influence of catalyst composition on OER catalytic activity. The effect of the ratio of Ni:Mn on catalyst activity is studied to obtain the optimal ratio (Fig. S8), Fig. S8 performs the cyclic voltammetry (CV) curves of MneNieP/NF with different Ni/Mn ratios (Ni:Mn ¼ 1:5, 1:6, 1:7 and 1:8). The results showed that OER activity of assynthesized catalysts is in connection with metal ratio. Among them, Ni:Mn ¼ 1:6 is the optimal ratio which exhibits the best catalytic activity. Therefore, the mole ratio of Ni/Mn is fixed at 1:6 in the following research. Fig. 4 detailedly shows the OER performance of the resulting catalyst and control samples. In Fig. 4a, we observe the corresponding CV curves of MneNieP/NF, Ni2PeNi5P4/ NF,RuO2/NF and clean NF. For all samples, a certain peak corresponding to reduction reaction can be observed after the end of OER process. It is known that anodic and cathodic peaks wound appear in CV curves for Ni-based catalysts, which is caused by the reversible reaction of Ni(II)/Ni(III). The reduction peak of Ni(II)/ Ni(III) at about 1.2 V has little effect on the anodic current caused by oxygen evolution reaction. The enhanced OER catalytic activity is extraordinary obvious after phosphidation and Mn incorporation from polarization curves. As expected, NF exhibits poor OER activity with an overpotential of 380 mV at the current density of 10 mA/ cm2, and MneNieP/NF hybrid catalyst could effectively promote OER catalytic activity compared with Ni2PeNi5P4/NF, which demands only 230 mV overpotential at 10 mA/cm2, while the overpotential for Ni2PeNi5P4/NF is 300 mV at the same current density. Meanwhile, there are a rapid anodic current increase when the
Fig. 3. (a) XPS survey spectrum for MneNieP/NF before and after OER measure for 20 h. XPS spectra in the (b) Mn 2p, (c) Ni 2p, and (d) P 2p regions for MneNieP/NF.
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Fig. 4. (a) CV curves for MneNieP/NF, Ni2PeNi5P4/NF and NF with a scan rate of 5 mV/s for OER in 1.0 M KOH. (b) Tafel plots of NF, Ni2PeNi5P4/NF and MneNieP/NF.(c) CV curves for MneNieP/NF before and after 500 CV cycles.(d) Time dependence of the potential of MneNieP/NF under overpotential of 230 mV at 10 mA/cm2.
potential is scanned to more positive section. Moreover, the OER performance of the catalyst is superior to other non-noble-metal catalysts in literature (see Table S1 in detail). Additionally, the reaction kinetics of OER generally are evaluated by Tafel plots. The smaller the Tafel slope, the faster the OER kinetics. Tafel plots are derived from the polarization curves, revealing the connection of the potential and the log of the relevant current density [48,49]. As shown in Fig. 4b, the tafel slopes of MneNieP/NF, Ni2PeNi5P4/NF, RuO2/NF and pure NF are 64.3 mA dec1,126.1 mA dec1, 104.5 mA dec1 and 152.9 mA dec1, respectively. The MneNieP/NF exhibits the smallest Tafel slope, which suggests the faster OER kinetics of MneNieP/NF and implies better OER performance, corresponding to CV curves (Fig. 4a). Moreover, basic research indicates that the OER overpotential is related to the free energy of the reaction [50,51]. In our example, MneNieP/NF hybrid catalyst exhibits superior catalytic activity, which may be presumed to be due to the decrease in the free energy of the reaction, resulting from the introduction of biphasic phosphide, Mn-doping and a suitable composition, which provides an advantageous environment for catalyzing OER. Stability is also considered as a major evaluation criterion for the practicality of the catalyst [52]. The long-term electrochemical durability of MneNieP is further tested by cyclic voltammetry (CV) scanning and chronopotentiometric measurement. Fig. 4c demonstrates the polarization curves of the final sample before and after CV scanning of 500 cycles with negligible decay at 10 mA/cm2 in 1.0 M KOH, which indicates that the catalyst has good cycle test stability and high durability. Simultaneously, the catalyst could also maintains an overpotential of 230 mV at a current density of 10 mA/ cm2 for at least 20 h, which can further confirm good stability (Fig. 4d). Also, the NieMneP catalyst after stability is characterized by XPS to explore the states of different elements. It is obvious that
the content of oxidized Ni and oxidized Mn increased and the content of P decreased in Fig. S7. We deduced that the metal phosphide on the surface of catalyst has changed to metal (oxy) hydroxide during continuous oxygen evolution process, which is regarded as real active species for OER. The results were in consistence with phenomena recorded in other literature [42,43,53]. It is known that metal phosphides are thermodynamically less stable than metal oxides in solution under strongly oxidative environment, which might be oxidized to corresponding metal oxides/oxyhydroxides in high potential range during OER process [53]. According OER mechanism as follow [54], it is rational that the surface of MneNieP/NF is oxidized to Ni-oxyhydroxides. S þ OH / SeOH* þ e (S¼Surface) SeOH* þ OH / SeO* þ H2O þ e SeO* þ OH / SeOOH* þ e SeOOH* þ OH / SeO2* þ H2O þ e SeO2* / S þ O2 It has been reported that NieFe oxy/hydroxide active species are formed on the surface of FeP/Ni2P [29]. A review has introduced this phenomenon and summarized that the insitu formed oxides/ oxyhydroxides could possibly enhance the OER activity [43]. It maybe origin from synergistic electronic interactions between metal phosphides and oxy-hydroxides on the surface [53]. To further analyze the superior OER performance of MneNieP/ NF, we test the electric double-layer capacitance (Cdl) and electrochemical impedance spectroscopy (EIS). As shown in Fig. 5(a-c), CV
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Fig. 5. CV image within scan rate of 70e110 mV/s of (a) NF, (b) Ni2PeNi5P4/NF and (c) MneNieP/NF.(d) The capacitive current densities at 0.42 V as a function of scan rate for MneNieP/NF, Ni2PeNi5P4/NF and NF. (e) Nyquist plots of MneNieP/NF and Ni2PeNi5P4/NF.
scanning is performed at different scanning rates of 70e110 mV/s in the illegal pull zone (100 mV apart) of MneNieP/NF, Ni2PeNi5P4/ NF and NF. As shown in Fig. 5d, the Cdl value (2.16 mF/cm2) of MneNieP/NF is observed larger than that of Ni2PeNi5P4/NF (1.84 mF/cm2), which implies heterogeneous nanostructure and Mn-doping with more effective active sites are in favor of a better OER performance. The improved electrical conductivity could be demostrated by EIS. As in Fig. 5e, the semicircular radius of the MneNieP/NF is smaller than that of Ni2PeNi5P4/NF, revealing lower charge transfer resistance and the faster electron transfer efficiency in comparison with control samples [55], The above datas indicate that it has smaller resistance, greater electron transfer efficiency, faster OER kinetics and better activity. The results are consistent with the high-performance of MneNieP/NF nanohybrids. Therefore, the good OER activity and stability of the prepared MneNieP/NF may be attributed to the following factors: (1) unique coral-like structure with porous and rough surfaces facilitate the gas diffusion and thus beneficial to enhance catalytic activity. (2) Doped state of metal Mn and synergistic effect of two-phase coupling of Ni2P/Ni5P4 nanosheet heterostructure may play an important role in the enhancement of the OER performance. (3) The in situ formed oxidized Ni species on the surface is in favor of improving OER performance. (4) The favorable composition factor plays a role in enhancing catalytic activity. As a result, all of the above indicate that MneNieP/NF exhibits good OER performance. 4. Conclusions In summary, Mn-doping Ni2PeNi5P4/NF electrocatalyst with a unique coral-like hierarchical structure has been prepared by a two-step method, which results in a significant enhancement of the OER performance. At a current density of 10 mA/cm2, only a low overpotential of 230 mV is required, which is 70 mV less than the overpotential of Ni2PeNi5P4/NF. In addition, such catalyst exhibits good long-term electrochemical stability, which can be stabilized for at least 20 h. This certainly benefits from a unique morphology, metal Mn-doping and the synergistic effects of Ni5P4eNi2P, the in situ formed oxidized Ni species on the surface and effective
composite nanostructures.. This work not only offers an attractive and cost-effective catalyst for alkaline oxygen evolution, but also provides important strategy for designing and synthesizing Nibased phosphate catalysts with enhanced OER activity. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. CRediT authorship contribution statement Siran Xu: Writing - original draft, Writing - review & editing. Yeshuang Du: Writing - original draft, Writing - review & editing. Xian Liu: Data curation. Xin Yu: Data curation. Chunlin Teng: Data curation. Xiaohong Cheng: Resources. Yunfeng Chen: Resources. Qi Wu: Supervision, Writing - original draft, Writing - review & editing. Acknowledgements This work is supported by the National Natural Science Foundation of China (Grant No. 21601056), the Hubei Provincial Department of Education (no. Q20162505), the Natural Science Foundation of Hubei Province (Grant No. 2019CFB569) and the Science and Technology foundation for Creative Research Group of HBDE (Grant Nos. T201810). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jallcom.2020.154210. References [1] L. Yu, H. Zhou, J. Sun, F. Qin, D. Luo, L. Xie, F. Yu, J. Bao, Y. Li, Y. Yu, S. Chen, Z. Ren, Nano Energy 41 (2017) 327e336. [2] J. Sun, M. Ren, L. Yu, Z. Yang, L. Xie, F. Tian, Y. Yu, Z. Ren, S. Chen, H. Zhou, Small 15 (2019), e1804272.
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