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Hybridizing amorphous nickel cobalt phosphate and nickel phosphide as an efficient bifunctional nanocatalyst towards overall water splitting
Journal Pre-proof Hybridizing amorphous nickel cobalt phosphate and nickel phosphide as an efficient bifunctional nanocatalyst towards overall water splitting Cong Li, Xuanhao Mei, Frank Leung-Yuk Lam, Xijun Hu
PII:
S0920-5861(20)30082-1
DOI:
https://doi.org/10.1016/j.cattod.2020.02.031
Reference:
CATTOD 12699
To appear in:
Catalysis Today
Received Date:
4 March 2019
Revised Date:
19 October 2019
Accepted Date:
19 February 2020
Please cite this article as: Li C, Mei X, Leung-Yuk Lam F, Hu X, Hybridizing amorphous nickel cobalt phosphate and nickel phosphide as an efficient bifunctional nanocatalyst towards overall water splitting, Catalysis Today (2020), doi: https://doi.org/10.1016/j.cattod.2020.02.031
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2020 Published by Elsevier.
Hybridizing amorphous nickel cobalt phosphate and nickel phosphide as an efficient bifunctional nanocatalyst towards overall water splitting Cong Li, Xuanhao Mei, Frank Leung-Yuk Lam* and Xijun Hu* Department of Chemical and Biological Engineering The Hong Kong University of Science and Technology Clear Water Bay, Kowloon, Hong Kong E-mail: [email protected]; [email protected]
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Graphic abstract
Highlights
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Amorphous Ni and Co phosphate is successfully synthesized on the Ni phosphide foam. The substrate enhancement effect of nickel phosphide is disclosed and verified. The catalyst shows really small overpotentials for HER and OER to reach 10 mA cm-2. A cell voltage of only 1.55 V at 10 mA cm-2 is verified for overall water splitting. The excellent properties of amorphous and crystalline materials are integrated.
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Abstract
Here a nanohybrid material consisting of amorphous nickel cobalt phosphate loaded on the nickel phosphide foam was designed and synthesized as an outstanding nanocatalyst to catalyze both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in alkaline conditions. The high activity of amorphous materials and the great stability as well as the good 1
conductivity of crystalline materials were successfully integrated. The nickel cobalt phosphate was recognized as the active component, while the nickel phosphide was of crucial importance and showed a substrate enhancement effect on the elevated catalytic activity, including the amplification of active surface area and constructing a channel between the electrode and the active sites. Notably, the nanohybrid system can drive the cathodic/anodic current density to be 10 mA cm-2 with the overpotentials of only 73 mV in HER test and 234 mV in OER test, which
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outperformed most of the cobalt-based nanocatalysts so far. Finally, a two-electrode configuration with the nanohybrid catalyst as both cathode and anode was established, which required a cell voltage of only 1.55 V to aff ord a water splitting current density of 10 mA cm−2
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in 1.0 M KOH.
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Keywords: transition metal phosphate/phosphide, nanohybrid, overall water splitting, substrate enhancement
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1. Introduction
The sustainability and efficiency of the worldwide economic development have always been
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heavily dependent on the energy consumption. However, it is said that the key to one's success is also one's undoing. In recent decades, the issue of the depletion of energy storage, mainly
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referring to fossil fuels, is more than ever and has severely threatened the global community [1, 2]. Scientists were hence motivated to propose plenty of strategies to address this problem, expecting to discover renewable energy sources with reasonable production efficiency and reliable capacity, including fuel cell [3-5], Li-ion battery [6-8] and hydrogen energy coming from water electrolysis [9, 10]. Among all these potential alternatives, hydrogen energy has 2
attracted numerous attention and research interest due to its high heat value and zero-emission of carbon after combustion. For various strategies for hydrogen production, electrochemical water splitting with only hydrogen and oxygen as products is considered as one of the most promising methods, since the predominant steam reforming approach unavoidably induces remarkable carbon pollution [10]. Nevertheless, one tough problem hindering the practical employment of hydrogen is the sluggish kinetics on the cathode part (HER) as well as the anode
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part (OER). It implies that a lot of extra potential is required to overcome the energy barriers of the two half reactions compared to the theoretical value, indicating a great necessity of designing and fabricating high-performance and durable catalytic systems.
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Substantial research effort has been devoted to developing the electrocatalysts for either HER
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or OER [11, 12], and as we know, some representative noble metals and metal oxides, such as Pt, RuO2 and IrO2, were recognized as the state-of-the-art catalysts in both acidic and basic
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electrolyte. However, on the one hand, the scarcity and prohibitive cost cause serious problems
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for their large-scale application. On the other hand, it can be easily observed that both RuO2 and IrO2 suffer from the unavoidable oxidation under high voltage in OER process, hence the
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degradation issue of the catalysts makes the long-term durability another problem to be tackled [13, 14]. Besides these, currently most developed catalysts generally exhibited activity in either
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HER or OER, with an apparent performance reduction when they were paired together owing to the possible incompatibility [15]. Also, in practical scenarios, the two half reactions must be carried out in the same electrolyte, consequently the overall water splitting is a more effective method from an industry-oriented view [16]. Thus, it is of great significance to explore costeffective and robust catalysts with excellent activity for both HER and OER. 3
Remarkable progress has been made with respect to the bifunctional catalysts, especially those transition-metal-based nanocatalysts with rational electronic configuration, high charge transfer efficiency and superior catalytic activity [17-19]. So far, various transition metal oxides/hydroxides [20, 21], sulfides [22-24], nitrides [25] and phosphides [26-28] have been intensively exploited. Among all those candidates, cobalt-based transition metal compounds have recently been highlighted with promising potential to replace the noble metal catalysts.
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The capability of forming in-situ catalytically active cobalt centers is very beneficial to the water electrolysis kinetics [29]. Besides the intrinsic activity, the density/number of active sites, the electron transfer efficiency and the mass diffusion rate are all crucial to the overall catalytic
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performance and have been investigated to further enhance the activity [30-32]. Recent studies
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have shown that the superior catalytic performance can be achieved by preparing 3D amorphous nanomaterials [33-35]. Their short-range ordered structure provides abundant active sites,
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making more active sites exposed preferably to reactive ions in the solution. Another attractive
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strategy is to grow nanocatalysts on conductive substrates [29, 36, 37]. The substrate, on the one hand, can greatly enlarge the surface area of the catalysts, increasing the odds of being
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accessible to the reactants. On the other hand, the good conductivity of the substrate is able to accelerate the electron transfer process from the electrode to the catalysts, minimizing the
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transmission loss at the interfaces. Moreover, the bubble effect usually becomes more and more prominent when the applied voltage is really high [38]. The formed H2 or O2 bubbles stick to the surface of catalysts, impeding the sufficient contact between the active sites and reactants. Therefore, the whole reaction kinetics is inevitably weakened. In this regard, an appropriate substrate is very important to the overall performance of the catalysts. Accordingly, it is 4
desirable to design a catalytic system consisting of a bifunctional nanocatalyst, which is capable of catalyzing the overall water splitting efficiently, supported on a rational substrate to achieve distinguished electrolytic performance towards large-scale production. Herein, we successfully synthesized amorphous cobalt phosphate (NiCoPi) on nanosheet-like nickel phosphide (Ni5P4/Ni2P) via a two-step method. The Ni foam after the phosphorization process in a chemical vapor deposition (CVD) furnace was directly used as the substrate in the
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following solvothermal reaction. The NiCoPi/Ni-P/NiCoPi achieved a current density of 10 mA cm−2 at an overpotential of 73 mV in basic media for HER with high stability. Significantly, in the OER process, the NiCoPi/Ni-P/NiCoPi electrode exhibited an overpotential of only 234 mV
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to drive the geometric current density to be 10 mA cm−2 with mechanical robustness. And it
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was found out that the Ni phosphide (Ni-P) as the substrate played an essential role of strengthening the charge transfer and enlarging the effective surface area. Furthermore, we
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constructed a two-electrode electrolysis configuration to test the performance of overall water splitting where the NiCoPi/Ni-P/NiCoPi served as both anode and cathode. Surprisingly, this
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cell required a cell voltage of only 1.55 V to reach a current density of 10 mA cm−2, suggestive
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of its promising feature for practical realization of water splitting.
2. Experimental
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2.1 Synthesis of catalysts
A piece of Ni foam with the size of 1 cm×5 cm was successively washed with acetone, diluted HCl (6 mol L-1) and deionized (DI) water several times to fully remove the organic impurities and oxidation layer. Then the Ni foam was put in the middle of a temperature-controlled furnace, with 0.2 g red phosphorus placed upstream. Then 100 sccm pure Ar gas was purged into the 5
furnace for one hour to remove all residue air and kept during the whole process. The temperature was increased with a ramping rate of 20 oC min-1 and kept for another one hour. Then the system was cooled down naturally to room temperature. In the following solvothermal synthesis, typically, 0.66 mmol of Co(NO3)2·6H2O was dissolved in the mixture of 3 mL oleylamine (OM) and 10 mL 1-octadecene (ODE) by stirring for 1 h. In the meantime, 2 mmol Na2HPO4 was dissolved in 10 mL DI water under ultrasonication for 1 h. Then they were mixed
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together by stirring for 30 min and transferred into a 100 mL Teflon-lined autoclave. One piece of Ni-P was put into the solution. After sealing, the autoclave was heated in an oven at 150 oC for 24 h. Then the sample was carefully washed with cyclohexane/ethanol mixed solution (4:1,
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v/v) and ethanol/DI water (1:1, v/v) mixed solution three times. Finally, the sample was put
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into a furnace again and heated at 200 oC in Ar gas to remove the residue organics. 2.2 Materials characterization
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Scanning electron microscope (SEM) measurements were performed on a JEOL-6700F Scanning Electron Microscope with an accelerating voltage of 10 kV. X-ray diff raction (XRD)
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patterns were obtained on a PW1830 (Philips) Powder X-ray diffractometer fitted with Cu Kα
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radiation. X-ray photoelectron spectroscopy (XPS) measurements were carried out on a Physical Electronics 5600 multi-technique system with an exciting source of Al with the
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working power of 150 W. Transmission electron microscope (TEM) measurements were performed on a JEM 2010F (JEOL) Transmission Electron Microscope with an acceleration voltage of 200 kV. The ion concentration was monitored by an inductively coupled plasma atomic emission spectrometer (ICP-AES, Optima 7300DV, PerkinElmer). 2.3 Electrochemical measurements 6
A CHI 660D electrochemical analyzer (CH Instruments, Inc. USA) was utilized for all the electrochemical measurements. In a typical three-electrode system, the NiCoPi/Ni-P/NiCoPi was directly utilized as the working electrode, with a carbon plate as the counter electrode and a saturated calomel electrode (SCE) as the reference electrode. Typically, the metal ratio of Ni: Co was determined by XPS to be 9:1 (Ni1.35Co0.15PO4). After confirming the Co loading amount by ICP-AES, the total NiCoPi/Ni-P/NiCoPi mass loading is 0.45 mg cm-2. Alternatively, the
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catalyst powder with the same metal ratio, which was collected from the reaction mixture, was first dispensed in 1 mL isopropanol/DI water (1:3) mixture solvent with 50 μL Nafion 117 solution (Sigma-Aldrich) added inside. Then the catalyst dispersion was dropped on the Ni-P
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foam with the same loading (denoting as NiCoPi@Ni-P@NiCoPi). All potentials applied were
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calibrated in 1 M KOH to reversible hydrogen electrode (RHE) following the equation: E(RHE) = E(SCE) + 1.068 V. All cyclic voltammetry (CV) and linear sweep voltammetry (LSV)
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polarization curves were recorded in a scan rate of 5 mV s-1. A 95% iR compensation was
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employed in all LSV tests for HER, while a 90% iR compensation was employed for OER tests. The durability test was conducted at a constant potential without iR compensation for over 20
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h to record the current. Electrochemical impedance spectroscopy (EIS) measurements were performed at a potential of -60 mV (vs RHE) with the frequency range from 105 to 10 Hz. The
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ZView software was used to fit the data and calculate the charge transfer resistance (Rct). The electrochemically active surface area measurements were conducted by a CV technique with the potential region where no Faradaic reaction occurred under various scan rates. By fitting the current density difference (Δj) against the scan rate, the double-layer capacitance (Cdl) can be calculated as half of the slope. For the overall water splitting measurements in 1 M KOH, a 7
two-electrode configuration was assembled with one piece of the as-prepared NiCoPi/NiP/NiCoPi acting as both the counter electrode and the reference electrode, and another piece as the working electrode.
3. Results and discussion The Ni foam has already been acknowledged to be very important as substrate in the electrochemical field, which possesses good conductivity and large specific surface area,
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leading to a high density of active sites. However, a further optimization of the substrate is still essential and can be expected to be beneficial to the upgradation of catalytic activity. Therefore, in this work, the Ni foam (Fig. 1a) first underwent a phosphorization process in a CVD furnace
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to synthesize Ni-P on the Ni foam skeleton. After the Ni is converted to Ni-P, it still has
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adequate conductivity serving as electrodes. As shown in Fig. 1b, the Ni-P exhibits a nanosheet morphology with a broad size distribution (a few microns to a dozen microns). It can be seen
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that some large Ni-P nanosheets with the size of a dozen microns embed in the Ni foam, while
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most of the Ni-P nanosheets are relatively small with a few microns of size on average, locating on the surface of Ni foam randomly but with full coverage. It can be clearly observed that,
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compared to the Ni foam with relatively flat surface in Fig. 1a, the Ni-P nanosheets are capable of providing higher surface area, and at the same time maintaining the conductive Ni skeleton.
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And the XRD patterns of Ni-P are presented in Fig. 1d. The specific diff raction peaks of Ni5P4 (PDF#18-0883) and Ni2P (PDF#74-1385) shown on Ni-P demonstrates the successful phosphorization, which is consistent with the SEM-Energy-dispersive X-ray spectroscopy (SEM-EDX) results in Fig. S1.
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Fig. 1. Materials characterization of the NiCoPi/Ni-P/NiCoPi. SEM images of (a) Ni foam, (b) Ni-P and (c) NiCoPi/Ni-P/NiCoPi. (d) XRD results of Ni-P and NiCoPi/Ni-P/NiCoPi. (e) SEM-
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EDX images of (c).
Then the Ni-P was put into an autoclave to synthesize amorphous NiCoPi through a
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solvothermal method. From Fig. 1c we can find out that the nanosheet morphology of Ni-P is maintained perfectly after the deposition process. Moreover, the edges of Ni-P nanosheets are
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not as sharp as before, demonstrating that the surface of all Ni-P nanosheets has been fully
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covered by the NiCoPi. The TEM images in Fig. S2 reveals the nanosheet morphology of the catalyst powder (CoPi) with tens of nanometers collected from the reaction mixture in the solvothermal process. The composition of CoPi can be verified by the TEM-EDX measurements. Also, the SEM-EDX images in Fig. 1e confirms the existence of various elements (Ni, Co, P and O) and their uniform distribution. It is important to note that, different 9
from the powder sample, the Ni species appears in the NiCoPi/Ni-P/NiCoPi (Fig. 1e). As the Ni-P has been fully covered by the as-prepared catalyst which will be discussed later, the Ni species should be considered in this case to come from the as-prepared NiCoPi, rather than the Ni species of the underneath Ni-P. Thus, it can be deduced that the Ni-P was partially converted into the NiCoPi. And it is notable that no other diff raction peak except those of Ni5P4 and Ni2P presents in the XRD results after solvothermal reaction, indicating that the substrate is still Ni-
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P but with lower content, as well as the amorphous nature of NiCoPi.
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Fig. 2. High-resolution XPS spectra of (a) Ni 2p, (b) Co 2p, (c) P 2p and (d) O 1s.
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To further confirm the chemical composition of NiCoPi/Ni-P/NiCoPi, XPS analysis was also
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conducted. After deconvolution, Fig. 2a shows that the two characteristic peaks for Ni 2p3/2 and 2p1/2 are located at binding energies of 856.14 eV and 873.98 eV and their satellite peaks locate at 861.21 eV and 879.59 eV, respectively. And in Fig. 2b the peaks at 781.63 eV and 797.41 eV can be assigned to Co 2p3/2 and 2p1/2. The corresponding satellite peaks locate at 786.65 eV and 801.87 eV, respectively. All the binding energy results are in good accord with the literature 10
[27]. Interestingly, in the Ni 2p spectrum, the peak at 853 eV is generally considered as the partially charged Ni species caused by Ni−P bonds [39, 40], which is not found here. This verifies our statement before that the surface has been fully covered by NiCoPi, which can be authenticated again by the inexistence of a doublet of P 2p3/2 peak at around 129.0 eV and P 2p1/2 peak at around 129.8 eV (Fig. 2c) coming from metal phosphides [41]. Furthermore, we could not observe any characteristic peak of Co-P bond locating at 778.8 eV [42], illustrating
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that the Co species only exists in NiCoPi form. Fig. 2c shows the spectrum of P 2p with a single peak at 133.64 eV, implying the existence of phosphate. The peak at 531.16 eV in the O 1s region belongs to the P-O bond, as shown in Fig. 2d.
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Now it can be confirmed that we have successfully synthesized NiCoPi on the Ni-P nanosheets
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with full coverage. Then the NiCoPi/Ni-P/NiCoPi was directly used as working electrode in a typical three-electrode configuration to test its HER catalytic activity in 1 M KOH. The iR
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compensation was applied automatically by the electrochemical workstation to eliminate its
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influence. Fig. 3a shows the polarization curves of Ni-P, NiCoPi/Ni/NiCoPi, NiCoPi/NiP/NiCoPi and NiCoPi@Ni-P@NiCoPi. To note, the Ni-P itself exhibits a rather inferior
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performance, with a large overpotential applied to initiate the reaction. After the decoration with NiCoPi, the NiCoPi/Ni-P/NiCoPi requires only 73 mV to drive the current density to be
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10 mA cm-2, which is even competitive to the noble Pt. Since the Ni-P is not accessible to the electrolyte here, its contribution to the HER should be considered negligible. Therefore, it can be concluded that the amorphous NiCoPi is very HER-active with a great potential to replace the state-of-the-art Pt in the hydrogen production. Furthermore, the sample with the NiCoPi depositing on the pristine Ni foam was tested. As a result, a larger overpotential (119 mV) is 11
needed to obtain 10 mA cm-2. Also, the NiCoPi@Ni-P@NiCoPi with ex-situ formation of catalysts (see Experimental section for details) shows a performance similar to Ni-P, which is not as good as NiCoPi/Ni-P/NiCoPi, even worse than the NiCoPi/Ni/NiCoPi. Besides that, its LSV curve drops very slowly comparing with the other counterparts, especially in the high
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potential region.
Fig. 3. HER performance measurements of various as-prepared catalysts. (a) HER polarization
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curves for various catalysts. (b) Corresponding Tafel fitting plots derived from the data in (a). (c) Nyquist plots of various samples. All the total resistance of the cell (Rs) were standardized
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to 2.42 Ω (the Rs of NiCoPi@Ni-P@NiCoPi) for the convenience of comparison of Rct from
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different catalysts. The actual values of Rs were summarized in Table S1. (d) The CV curves of NiCoPi/Ni-P/NiCoPi at different scan rates for the evaluation of ECSA in HER. (e) The corresponding ECSA fitting results for various samples. (f) Durability test recorded under a constant applied potential for over 20 h.
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To disclose the intrinsic factors causing the activity difference, the Tafel slopes extracted from their corresponding polarization curves were calculated and shown in Fig. 3b. This indicator is able to provide kinetic information on different electrodes. The NiCoPi/Ni-P/NiCoPi shows the lowest Tafel slope of 87.0 mV dec-1, while NiCoPi@Ni-P@NiCoPi, NiCoPi/Ni/NiCoPi and Ni-P demonstrate higher Tafel slopes of 123.4, 129.1 and 109.9 mV dec-1, respectively. One possible explanation is that the underneath P atoms could modulate the electronic structure of
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NiCoPi. The excess electrons of P atoms make the materials more electronegative, so that the hydrogen atoms can be more easily absorbed [43]. The highest Tafel slope value of NiCoPi/Ni/NiCoPi strongly support this. Another crucial parameter to evaluate the intrinsic
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activity of individual active site is the Rct in EIS results (Fig. 3c). After fitting, the Rct can be
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calculated to be 7.344Ω, 1.461 Ω, 7.187 Ω and 1.530 Ω for Ni-P, NiCoPi/Ni-P/NiCoPi, NiCoPi/Ni/NiCoPi and NiCoPi@Ni-P@NiCoPi, respectively. Except the Ni-P itself, the other
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two electrodes with Ni-P as substrate (NiCoPi@Ni-P@NiCoPi and NiCoPi/Ni-P/NiCoPi) both
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exhibit small Rct, no matter how the NiCoPi forms on the surface of Ni-P, while the sample with the Ni foam as substrate (NiCoPi/Ni/NiCoPi) possesses a rather high Rct. So, it is quite
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convincing that the P atoms can affect the electronic structure of catalysts, and at the same time offer a better channel for electron to pass through.
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Furthermore, the electrochemical surface area (ECSA) is a powerful tool to assess the HER performance. Although this method is supposed to compare the catalysts made by the same materials, it may provide some preliminary ideas about the surface areas of various catalysts here. And as an appropriate depiction of ECSA, the double-layer capacitance (Cdl) was measured and shown in Fig. 3d-e. The Cdl of NiCoPi/Ni-P/NiCoPi is 10.57 mF cm−2 which is 13
higher than those of the NiCoPi@Ni-P@NiCoPi (5.98 mF cm−2) and Ni-P (5.07 mF cm−2), especially higher than that of NiCoPi/Ni/NiCoPi (0.84 mF cm−2) by an order of magnitude, indicating the enlarged electrochemically active surface area of NiCoPi/Ni-P/NiCoPi. Also, it’s obvious that the nanosheet morphology of Ni-P could truly provide more active surface area compared the Ni foam (Fig. S3), which is beneficial to the HER. Then the stability of NiCoPi/Ni-P/NiCoPi was investigated with the chronoamperometry technique in Fig. 3f. Only
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a slight decrease of the current density can be detected for up to 20 hours’ continuous electrolysis, demonstrating the good stability of the NiCoPi/Ni-P/NiCoPi for HER. The robustness of this material could also be verified by the SEM image, XPS spectra and XRD
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result (Fig. S4a, S5-S6). The crystallinity of Ni-P may play a role in the view of stability, which
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provides a robust support to the amorphous catalyst to prevent the degradation from occurring. And at the same time the underneath Ni-P remains almost unchanged from the original state
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(Fig. 1d) after electrochemical tests.
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Fig. 4. OER performance measurements of various as-prepared catalysts. (a) OER polarization curves for various catalysts. (b) Corresponding Tafel fitting plots derived from the data in (a). (c) Multicurrent process of NiCoPi/Ni-P/NiCoPi. The current density started from 110 mA cm−2 to 230 mA cm−2, with an increment of 20 mA cm−2 per 500 s without iR correction. (d) The CV curves of NiCoPi/Ni-P/NiCoPi at different scan rates for the evaluation of ECSA in OER. (e) The corresponding ECSA fitting results for various samples. (f) Durability test recorded under
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a constant applied potential for over 20 h.
The OER activity of NiCoPi/Ni-P/NiCoPi was then assessed in alkaline solution (Fig. 4). From
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the polarization curves in Fig. 4a, an evident preoxidation reaction is observed for Ni-P prior to
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the OER process. This anodic peak in the potential region of 1.3−1.5 V (vs RHE) can be attributed to the formation of NiOOH [44]. No anodic peak as distinct as that of Ni-P is observed
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from the other three electrodes with amorphous catalyst coated onto them, indicating the
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oxidation is mainly related to the Ni-P species alone. To eliminate the interference of the oxidation current, the cathodic sweep curves in the corresponding CV tests of the catalysts were
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employed for providing quantitative information in Fig. S7. The NiCoPi/Ni-P/NiCoPi requires an overpotential of 234 mV to achieve a geometric current density of 10 mA cm−2, and this
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OER performance is even better than that of the conventional RuO2 catalysts. In contrast, 258 mV, 260 mV, 266 mV are needed for NiCoPi/Ni/NiCoPi, Ni-P and NiCoPi@Ni-P@NiCoPi to achieve the same current density, respectively. And the superior OER activity of NiCoPi/NiP/NiCoPi is particularly evident in the high current density region (Fig. 4a), showing a promising perspective of industrial deployment. The corresponding Tafel slope of NiCoPi/Ni15
P/NiCoPi is 69.9 mV dec−1 (Fig. 4b), verifying the OER-favorable kinetics comparing to the other counterparts. Figure 3c displays a multistep chronopotentiometric curve for NiCoPi/NiP/NiCoPi. The potential remains stable with constant demanding current density and immediately reaches a new level with respect to the new applied current density, implying the excellent mass transportation property of our NiCoPi/Ni-P/NiCoPi catalyst. Similarly, the NiCoPi/Ni-P/NiCoPi also exhibits the highest Cdl (Fig. 4d-e), further verifying the substrate
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enhancement in the view of the surface area. Also, the long-term stability of NiCoPi/NiP/NiCoPi was probed and shown in Fig. 4f, it can be identified that there is no obvious current density change for up to 20 hours’ OER test, implying the excellent durability of NiCoPi/Ni-
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P/NiCoPi for OER in strong alkaline solution (also see Fig. S4b, S5-S6).
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What’s more, it was demonstrated that the Fe impurities in the KOH electrolyte usually exhibited enhancement effect on the Ni-based OER catalysts [45, 46]. To unveil whether Fe
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impurities play a crucial role in our NiCoPi/Ni-P/NiCoPi catalyst for OER, 20 CV scans were recorded after each 5 min aging period (Fig. S8). Both oxidation and reduction peaks show
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merely 18 mV shifting, and the potentials at 10 mA cm-2 vary in the range of 232 mV to 234
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mV, with no obvious performance improvement. Therefore, the Fe impurities here are not a
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key factor in the superior OER activity.
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Fig. 5. Two-electrode cell measurements with NiCoPi/Ni-P/NiCoPi acting as both cathode and anode. (a) Polarization curves for overall water splitting of NiCoPi/Ni-P/NiCoPi. Inset: the corresponding polarization curves of HER and OER half reactions. (b) Durability test recorded under a constant applied potential for around 40 h.
The superior performance of NiCoPi/Ni-P/NiCoPi both in HER and OER inspires us the
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possibility of building a real two-electrode system, with the NiCoPi/Ni-P/NiCoPi serving as both cathode and anode. In Fig. 5a, the overall current density reaches 10 mA cm-2 with only 1.55 V applied. This value is consistent with the results of the two corresponding half reactions,
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where an overpotential of 73 mV in HER and an overpotential of 234 mV in OER (1.23
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V+0.073 V+0.234 V=1.537 V in total) are required to drive the cathodic/anodic current density to be 10 mA cm-2. The long-term electrochemical stability of the cell was examined in Fig. 5b.
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There is no obvious change in the current density up to 40 hours, suggesting the excellent
4. Conclusions
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durability of NiCoPi/Ni-P/NiCoPi for overall water splitting.
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In summary, the amorphous NiCoPi was successfully synthesized on the surface the Ni-P nanosheets by a two-step preparation method, which can be verified by various characterization
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techniques, including XPS, XRD, TEM and SEM-EDX. The as-prepared NiCoPi/Ni-P/NiCoPi endows great catalytic activity both for HER and OER in a strong alkaline solution. The requisite overpotentials to achieve 10 mA cm-2 are 73 mV for HER and 234 mV for OER, respectively. As the substrate, the Ni-P has been identified to be vital to the enhanced activity from the following aspects. Firstly, the P atoms could modulate the electronic structure of 17
NiCoPi, and so that offer a better channel for electron to flow. More importantly, the nanosheet morphology of Ni-P is catalysis-favorable and provides more active surface area than normal Ni foam substrate does. The overall electrochemical water splitting using NiCoPi/Ni-P/NiCoPi as both HER catalyst and OER catalyst in 1.0 M KOH further reveals its bright prospect for practical hydrogen production, with only 1.55 V necessary for realizing a current density of 10 mA cm-2. This work may provide an insightful understanding on the substrate enhancement
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effect and offer an effective strategy for the optimization of catalyst design in water electrolysis.
Acknowledgements
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This work is financially supported by the Energy Institute at The Hong Kong University of
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Science and Technology.
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References
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20
PII:
S0920-5861(20)30082-1
DOI:
https://doi.org/10.1016/j.cattod.2020.02.031
Reference:
CATTOD 12699
To appear in:
Catalysis Today
Received Date:
4 March 2019
Revised Date:
19 October 2019
Accepted Date:
19 February 2020
Please cite this article as: Li C, Mei X, Leung-Yuk Lam F, Hu X, Hybridizing amorphous nickel cobalt phosphate and nickel phosphide as an efficient bifunctional nanocatalyst towards overall water splitting, Catalysis Today (2020), doi: https://doi.org/10.1016/j.cattod.2020.02.031
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Hybridizing amorphous nickel cobalt phosphate and nickel phosphide as an efficient bifunctional nanocatalyst towards overall water splitting Cong Li, Xuanhao Mei, Frank Leung-Yuk Lam* and Xijun Hu* Department of Chemical and Biological Engineering The Hong Kong University of Science and Technology Clear Water Bay, Kowloon, Hong Kong E-mail: [email protected]; [email protected]
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Graphic abstract
Highlights
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Amorphous Ni and Co phosphate is successfully synthesized on the Ni phosphide foam. The substrate enhancement effect of nickel phosphide is disclosed and verified. The catalyst shows really small overpotentials for HER and OER to reach 10 mA cm-2. A cell voltage of only 1.55 V at 10 mA cm-2 is verified for overall water splitting. The excellent properties of amorphous and crystalline materials are integrated.
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Abstract
Here a nanohybrid material consisting of amorphous nickel cobalt phosphate loaded on the nickel phosphide foam was designed and synthesized as an outstanding nanocatalyst to catalyze both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in alkaline conditions. The high activity of amorphous materials and the great stability as well as the good 1
conductivity of crystalline materials were successfully integrated. The nickel cobalt phosphate was recognized as the active component, while the nickel phosphide was of crucial importance and showed a substrate enhancement effect on the elevated catalytic activity, including the amplification of active surface area and constructing a channel between the electrode and the active sites. Notably, the nanohybrid system can drive the cathodic/anodic current density to be 10 mA cm-2 with the overpotentials of only 73 mV in HER test and 234 mV in OER test, which
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outperformed most of the cobalt-based nanocatalysts so far. Finally, a two-electrode configuration with the nanohybrid catalyst as both cathode and anode was established, which required a cell voltage of only 1.55 V to aff ord a water splitting current density of 10 mA cm−2
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in 1.0 M KOH.
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Keywords: transition metal phosphate/phosphide, nanohybrid, overall water splitting, substrate enhancement
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1. Introduction
The sustainability and efficiency of the worldwide economic development have always been
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heavily dependent on the energy consumption. However, it is said that the key to one's success is also one's undoing. In recent decades, the issue of the depletion of energy storage, mainly
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referring to fossil fuels, is more than ever and has severely threatened the global community [1, 2]. Scientists were hence motivated to propose plenty of strategies to address this problem, expecting to discover renewable energy sources with reasonable production efficiency and reliable capacity, including fuel cell [3-5], Li-ion battery [6-8] and hydrogen energy coming from water electrolysis [9, 10]. Among all these potential alternatives, hydrogen energy has 2
attracted numerous attention and research interest due to its high heat value and zero-emission of carbon after combustion. For various strategies for hydrogen production, electrochemical water splitting with only hydrogen and oxygen as products is considered as one of the most promising methods, since the predominant steam reforming approach unavoidably induces remarkable carbon pollution [10]. Nevertheless, one tough problem hindering the practical employment of hydrogen is the sluggish kinetics on the cathode part (HER) as well as the anode
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part (OER). It implies that a lot of extra potential is required to overcome the energy barriers of the two half reactions compared to the theoretical value, indicating a great necessity of designing and fabricating high-performance and durable catalytic systems.
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Substantial research effort has been devoted to developing the electrocatalysts for either HER
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or OER [11, 12], and as we know, some representative noble metals and metal oxides, such as Pt, RuO2 and IrO2, were recognized as the state-of-the-art catalysts in both acidic and basic
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electrolyte. However, on the one hand, the scarcity and prohibitive cost cause serious problems
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for their large-scale application. On the other hand, it can be easily observed that both RuO2 and IrO2 suffer from the unavoidable oxidation under high voltage in OER process, hence the
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degradation issue of the catalysts makes the long-term durability another problem to be tackled [13, 14]. Besides these, currently most developed catalysts generally exhibited activity in either
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HER or OER, with an apparent performance reduction when they were paired together owing to the possible incompatibility [15]. Also, in practical scenarios, the two half reactions must be carried out in the same electrolyte, consequently the overall water splitting is a more effective method from an industry-oriented view [16]. Thus, it is of great significance to explore costeffective and robust catalysts with excellent activity for both HER and OER. 3
Remarkable progress has been made with respect to the bifunctional catalysts, especially those transition-metal-based nanocatalysts with rational electronic configuration, high charge transfer efficiency and superior catalytic activity [17-19]. So far, various transition metal oxides/hydroxides [20, 21], sulfides [22-24], nitrides [25] and phosphides [26-28] have been intensively exploited. Among all those candidates, cobalt-based transition metal compounds have recently been highlighted with promising potential to replace the noble metal catalysts.
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The capability of forming in-situ catalytically active cobalt centers is very beneficial to the water electrolysis kinetics [29]. Besides the intrinsic activity, the density/number of active sites, the electron transfer efficiency and the mass diffusion rate are all crucial to the overall catalytic
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performance and have been investigated to further enhance the activity [30-32]. Recent studies
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have shown that the superior catalytic performance can be achieved by preparing 3D amorphous nanomaterials [33-35]. Their short-range ordered structure provides abundant active sites,
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making more active sites exposed preferably to reactive ions in the solution. Another attractive
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strategy is to grow nanocatalysts on conductive substrates [29, 36, 37]. The substrate, on the one hand, can greatly enlarge the surface area of the catalysts, increasing the odds of being
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accessible to the reactants. On the other hand, the good conductivity of the substrate is able to accelerate the electron transfer process from the electrode to the catalysts, minimizing the
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transmission loss at the interfaces. Moreover, the bubble effect usually becomes more and more prominent when the applied voltage is really high [38]. The formed H2 or O2 bubbles stick to the surface of catalysts, impeding the sufficient contact between the active sites and reactants. Therefore, the whole reaction kinetics is inevitably weakened. In this regard, an appropriate substrate is very important to the overall performance of the catalysts. Accordingly, it is 4
desirable to design a catalytic system consisting of a bifunctional nanocatalyst, which is capable of catalyzing the overall water splitting efficiently, supported on a rational substrate to achieve distinguished electrolytic performance towards large-scale production. Herein, we successfully synthesized amorphous cobalt phosphate (NiCoPi) on nanosheet-like nickel phosphide (Ni5P4/Ni2P) via a two-step method. The Ni foam after the phosphorization process in a chemical vapor deposition (CVD) furnace was directly used as the substrate in the
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following solvothermal reaction. The NiCoPi/Ni-P/NiCoPi achieved a current density of 10 mA cm−2 at an overpotential of 73 mV in basic media for HER with high stability. Significantly, in the OER process, the NiCoPi/Ni-P/NiCoPi electrode exhibited an overpotential of only 234 mV
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to drive the geometric current density to be 10 mA cm−2 with mechanical robustness. And it
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was found out that the Ni phosphide (Ni-P) as the substrate played an essential role of strengthening the charge transfer and enlarging the effective surface area. Furthermore, we
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constructed a two-electrode electrolysis configuration to test the performance of overall water splitting where the NiCoPi/Ni-P/NiCoPi served as both anode and cathode. Surprisingly, this
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cell required a cell voltage of only 1.55 V to reach a current density of 10 mA cm−2, suggestive
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of its promising feature for practical realization of water splitting.
2. Experimental
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2.1 Synthesis of catalysts
A piece of Ni foam with the size of 1 cm×5 cm was successively washed with acetone, diluted HCl (6 mol L-1) and deionized (DI) water several times to fully remove the organic impurities and oxidation layer. Then the Ni foam was put in the middle of a temperature-controlled furnace, with 0.2 g red phosphorus placed upstream. Then 100 sccm pure Ar gas was purged into the 5
furnace for one hour to remove all residue air and kept during the whole process. The temperature was increased with a ramping rate of 20 oC min-1 and kept for another one hour. Then the system was cooled down naturally to room temperature. In the following solvothermal synthesis, typically, 0.66 mmol of Co(NO3)2·6H2O was dissolved in the mixture of 3 mL oleylamine (OM) and 10 mL 1-octadecene (ODE) by stirring for 1 h. In the meantime, 2 mmol Na2HPO4 was dissolved in 10 mL DI water under ultrasonication for 1 h. Then they were mixed
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together by stirring for 30 min and transferred into a 100 mL Teflon-lined autoclave. One piece of Ni-P was put into the solution. After sealing, the autoclave was heated in an oven at 150 oC for 24 h. Then the sample was carefully washed with cyclohexane/ethanol mixed solution (4:1,
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v/v) and ethanol/DI water (1:1, v/v) mixed solution three times. Finally, the sample was put
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into a furnace again and heated at 200 oC in Ar gas to remove the residue organics. 2.2 Materials characterization
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Scanning electron microscope (SEM) measurements were performed on a JEOL-6700F Scanning Electron Microscope with an accelerating voltage of 10 kV. X-ray diff raction (XRD)
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patterns were obtained on a PW1830 (Philips) Powder X-ray diffractometer fitted with Cu Kα
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radiation. X-ray photoelectron spectroscopy (XPS) measurements were carried out on a Physical Electronics 5600 multi-technique system with an exciting source of Al with the
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working power of 150 W. Transmission electron microscope (TEM) measurements were performed on a JEM 2010F (JEOL) Transmission Electron Microscope with an acceleration voltage of 200 kV. The ion concentration was monitored by an inductively coupled plasma atomic emission spectrometer (ICP-AES, Optima 7300DV, PerkinElmer). 2.3 Electrochemical measurements 6
A CHI 660D electrochemical analyzer (CH Instruments, Inc. USA) was utilized for all the electrochemical measurements. In a typical three-electrode system, the NiCoPi/Ni-P/NiCoPi was directly utilized as the working electrode, with a carbon plate as the counter electrode and a saturated calomel electrode (SCE) as the reference electrode. Typically, the metal ratio of Ni: Co was determined by XPS to be 9:1 (Ni1.35Co0.15PO4). After confirming the Co loading amount by ICP-AES, the total NiCoPi/Ni-P/NiCoPi mass loading is 0.45 mg cm-2. Alternatively, the
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catalyst powder with the same metal ratio, which was collected from the reaction mixture, was first dispensed in 1 mL isopropanol/DI water (1:3) mixture solvent with 50 μL Nafion 117 solution (Sigma-Aldrich) added inside. Then the catalyst dispersion was dropped on the Ni-P
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foam with the same loading (denoting as NiCoPi@Ni-P@NiCoPi). All potentials applied were
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calibrated in 1 M KOH to reversible hydrogen electrode (RHE) following the equation: E(RHE) = E(SCE) + 1.068 V. All cyclic voltammetry (CV) and linear sweep voltammetry (LSV)
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polarization curves were recorded in a scan rate of 5 mV s-1. A 95% iR compensation was
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employed in all LSV tests for HER, while a 90% iR compensation was employed for OER tests. The durability test was conducted at a constant potential without iR compensation for over 20
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h to record the current. Electrochemical impedance spectroscopy (EIS) measurements were performed at a potential of -60 mV (vs RHE) with the frequency range from 105 to 10 Hz. The
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ZView software was used to fit the data and calculate the charge transfer resistance (Rct). The electrochemically active surface area measurements were conducted by a CV technique with the potential region where no Faradaic reaction occurred under various scan rates. By fitting the current density difference (Δj) against the scan rate, the double-layer capacitance (Cdl) can be calculated as half of the slope. For the overall water splitting measurements in 1 M KOH, a 7
two-electrode configuration was assembled with one piece of the as-prepared NiCoPi/NiP/NiCoPi acting as both the counter electrode and the reference electrode, and another piece as the working electrode.
3. Results and discussion The Ni foam has already been acknowledged to be very important as substrate in the electrochemical field, which possesses good conductivity and large specific surface area,
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leading to a high density of active sites. However, a further optimization of the substrate is still essential and can be expected to be beneficial to the upgradation of catalytic activity. Therefore, in this work, the Ni foam (Fig. 1a) first underwent a phosphorization process in a CVD furnace
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to synthesize Ni-P on the Ni foam skeleton. After the Ni is converted to Ni-P, it still has
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adequate conductivity serving as electrodes. As shown in Fig. 1b, the Ni-P exhibits a nanosheet morphology with a broad size distribution (a few microns to a dozen microns). It can be seen
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that some large Ni-P nanosheets with the size of a dozen microns embed in the Ni foam, while
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most of the Ni-P nanosheets are relatively small with a few microns of size on average, locating on the surface of Ni foam randomly but with full coverage. It can be clearly observed that,
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compared to the Ni foam with relatively flat surface in Fig. 1a, the Ni-P nanosheets are capable of providing higher surface area, and at the same time maintaining the conductive Ni skeleton.
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And the XRD patterns of Ni-P are presented in Fig. 1d. The specific diff raction peaks of Ni5P4 (PDF#18-0883) and Ni2P (PDF#74-1385) shown on Ni-P demonstrates the successful phosphorization, which is consistent with the SEM-Energy-dispersive X-ray spectroscopy (SEM-EDX) results in Fig. S1.
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Fig. 1. Materials characterization of the NiCoPi/Ni-P/NiCoPi. SEM images of (a) Ni foam, (b) Ni-P and (c) NiCoPi/Ni-P/NiCoPi. (d) XRD results of Ni-P and NiCoPi/Ni-P/NiCoPi. (e) SEM-
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EDX images of (c).
Then the Ni-P was put into an autoclave to synthesize amorphous NiCoPi through a
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solvothermal method. From Fig. 1c we can find out that the nanosheet morphology of Ni-P is maintained perfectly after the deposition process. Moreover, the edges of Ni-P nanosheets are
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not as sharp as before, demonstrating that the surface of all Ni-P nanosheets has been fully
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covered by the NiCoPi. The TEM images in Fig. S2 reveals the nanosheet morphology of the catalyst powder (CoPi) with tens of nanometers collected from the reaction mixture in the solvothermal process. The composition of CoPi can be verified by the TEM-EDX measurements. Also, the SEM-EDX images in Fig. 1e confirms the existence of various elements (Ni, Co, P and O) and their uniform distribution. It is important to note that, different 9
from the powder sample, the Ni species appears in the NiCoPi/Ni-P/NiCoPi (Fig. 1e). As the Ni-P has been fully covered by the as-prepared catalyst which will be discussed later, the Ni species should be considered in this case to come from the as-prepared NiCoPi, rather than the Ni species of the underneath Ni-P. Thus, it can be deduced that the Ni-P was partially converted into the NiCoPi. And it is notable that no other diff raction peak except those of Ni5P4 and Ni2P presents in the XRD results after solvothermal reaction, indicating that the substrate is still Ni-
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P but with lower content, as well as the amorphous nature of NiCoPi.
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Fig. 2. High-resolution XPS spectra of (a) Ni 2p, (b) Co 2p, (c) P 2p and (d) O 1s.
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To further confirm the chemical composition of NiCoPi/Ni-P/NiCoPi, XPS analysis was also
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conducted. After deconvolution, Fig. 2a shows that the two characteristic peaks for Ni 2p3/2 and 2p1/2 are located at binding energies of 856.14 eV and 873.98 eV and their satellite peaks locate at 861.21 eV and 879.59 eV, respectively. And in Fig. 2b the peaks at 781.63 eV and 797.41 eV can be assigned to Co 2p3/2 and 2p1/2. The corresponding satellite peaks locate at 786.65 eV and 801.87 eV, respectively. All the binding energy results are in good accord with the literature 10
[27]. Interestingly, in the Ni 2p spectrum, the peak at 853 eV is generally considered as the partially charged Ni species caused by Ni−P bonds [39, 40], which is not found here. This verifies our statement before that the surface has been fully covered by NiCoPi, which can be authenticated again by the inexistence of a doublet of P 2p3/2 peak at around 129.0 eV and P 2p1/2 peak at around 129.8 eV (Fig. 2c) coming from metal phosphides [41]. Furthermore, we could not observe any characteristic peak of Co-P bond locating at 778.8 eV [42], illustrating
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that the Co species only exists in NiCoPi form. Fig. 2c shows the spectrum of P 2p with a single peak at 133.64 eV, implying the existence of phosphate. The peak at 531.16 eV in the O 1s region belongs to the P-O bond, as shown in Fig. 2d.
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Now it can be confirmed that we have successfully synthesized NiCoPi on the Ni-P nanosheets
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with full coverage. Then the NiCoPi/Ni-P/NiCoPi was directly used as working electrode in a typical three-electrode configuration to test its HER catalytic activity in 1 M KOH. The iR
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compensation was applied automatically by the electrochemical workstation to eliminate its
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influence. Fig. 3a shows the polarization curves of Ni-P, NiCoPi/Ni/NiCoPi, NiCoPi/NiP/NiCoPi and NiCoPi@Ni-P@NiCoPi. To note, the Ni-P itself exhibits a rather inferior
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performance, with a large overpotential applied to initiate the reaction. After the decoration with NiCoPi, the NiCoPi/Ni-P/NiCoPi requires only 73 mV to drive the current density to be
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10 mA cm-2, which is even competitive to the noble Pt. Since the Ni-P is not accessible to the electrolyte here, its contribution to the HER should be considered negligible. Therefore, it can be concluded that the amorphous NiCoPi is very HER-active with a great potential to replace the state-of-the-art Pt in the hydrogen production. Furthermore, the sample with the NiCoPi depositing on the pristine Ni foam was tested. As a result, a larger overpotential (119 mV) is 11
needed to obtain 10 mA cm-2. Also, the NiCoPi@Ni-P@NiCoPi with ex-situ formation of catalysts (see Experimental section for details) shows a performance similar to Ni-P, which is not as good as NiCoPi/Ni-P/NiCoPi, even worse than the NiCoPi/Ni/NiCoPi. Besides that, its LSV curve drops very slowly comparing with the other counterparts, especially in the high
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potential region.
Fig. 3. HER performance measurements of various as-prepared catalysts. (a) HER polarization
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curves for various catalysts. (b) Corresponding Tafel fitting plots derived from the data in (a). (c) Nyquist plots of various samples. All the total resistance of the cell (Rs) were standardized
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to 2.42 Ω (the Rs of NiCoPi@Ni-P@NiCoPi) for the convenience of comparison of Rct from
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different catalysts. The actual values of Rs were summarized in Table S1. (d) The CV curves of NiCoPi/Ni-P/NiCoPi at different scan rates for the evaluation of ECSA in HER. (e) The corresponding ECSA fitting results for various samples. (f) Durability test recorded under a constant applied potential for over 20 h.
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To disclose the intrinsic factors causing the activity difference, the Tafel slopes extracted from their corresponding polarization curves were calculated and shown in Fig. 3b. This indicator is able to provide kinetic information on different electrodes. The NiCoPi/Ni-P/NiCoPi shows the lowest Tafel slope of 87.0 mV dec-1, while NiCoPi@Ni-P@NiCoPi, NiCoPi/Ni/NiCoPi and Ni-P demonstrate higher Tafel slopes of 123.4, 129.1 and 109.9 mV dec-1, respectively. One possible explanation is that the underneath P atoms could modulate the electronic structure of
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NiCoPi. The excess electrons of P atoms make the materials more electronegative, so that the hydrogen atoms can be more easily absorbed [43]. The highest Tafel slope value of NiCoPi/Ni/NiCoPi strongly support this. Another crucial parameter to evaluate the intrinsic
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activity of individual active site is the Rct in EIS results (Fig. 3c). After fitting, the Rct can be
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calculated to be 7.344Ω, 1.461 Ω, 7.187 Ω and 1.530 Ω for Ni-P, NiCoPi/Ni-P/NiCoPi, NiCoPi/Ni/NiCoPi and NiCoPi@Ni-P@NiCoPi, respectively. Except the Ni-P itself, the other
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two electrodes with Ni-P as substrate (NiCoPi@Ni-P@NiCoPi and NiCoPi/Ni-P/NiCoPi) both
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exhibit small Rct, no matter how the NiCoPi forms on the surface of Ni-P, while the sample with the Ni foam as substrate (NiCoPi/Ni/NiCoPi) possesses a rather high Rct. So, it is quite
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convincing that the P atoms can affect the electronic structure of catalysts, and at the same time offer a better channel for electron to pass through.
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Furthermore, the electrochemical surface area (ECSA) is a powerful tool to assess the HER performance. Although this method is supposed to compare the catalysts made by the same materials, it may provide some preliminary ideas about the surface areas of various catalysts here. And as an appropriate depiction of ECSA, the double-layer capacitance (Cdl) was measured and shown in Fig. 3d-e. The Cdl of NiCoPi/Ni-P/NiCoPi is 10.57 mF cm−2 which is 13
higher than those of the NiCoPi@Ni-P@NiCoPi (5.98 mF cm−2) and Ni-P (5.07 mF cm−2), especially higher than that of NiCoPi/Ni/NiCoPi (0.84 mF cm−2) by an order of magnitude, indicating the enlarged electrochemically active surface area of NiCoPi/Ni-P/NiCoPi. Also, it’s obvious that the nanosheet morphology of Ni-P could truly provide more active surface area compared the Ni foam (Fig. S3), which is beneficial to the HER. Then the stability of NiCoPi/Ni-P/NiCoPi was investigated with the chronoamperometry technique in Fig. 3f. Only
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a slight decrease of the current density can be detected for up to 20 hours’ continuous electrolysis, demonstrating the good stability of the NiCoPi/Ni-P/NiCoPi for HER. The robustness of this material could also be verified by the SEM image, XPS spectra and XRD
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result (Fig. S4a, S5-S6). The crystallinity of Ni-P may play a role in the view of stability, which
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provides a robust support to the amorphous catalyst to prevent the degradation from occurring. And at the same time the underneath Ni-P remains almost unchanged from the original state
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(Fig. 1d) after electrochemical tests.
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Fig. 4. OER performance measurements of various as-prepared catalysts. (a) OER polarization curves for various catalysts. (b) Corresponding Tafel fitting plots derived from the data in (a). (c) Multicurrent process of NiCoPi/Ni-P/NiCoPi. The current density started from 110 mA cm−2 to 230 mA cm−2, with an increment of 20 mA cm−2 per 500 s without iR correction. (d) The CV curves of NiCoPi/Ni-P/NiCoPi at different scan rates for the evaluation of ECSA in OER. (e) The corresponding ECSA fitting results for various samples. (f) Durability test recorded under
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a constant applied potential for over 20 h.
The OER activity of NiCoPi/Ni-P/NiCoPi was then assessed in alkaline solution (Fig. 4). From
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the polarization curves in Fig. 4a, an evident preoxidation reaction is observed for Ni-P prior to
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the OER process. This anodic peak in the potential region of 1.3−1.5 V (vs RHE) can be attributed to the formation of NiOOH [44]. No anodic peak as distinct as that of Ni-P is observed
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from the other three electrodes with amorphous catalyst coated onto them, indicating the
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oxidation is mainly related to the Ni-P species alone. To eliminate the interference of the oxidation current, the cathodic sweep curves in the corresponding CV tests of the catalysts were
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employed for providing quantitative information in Fig. S7. The NiCoPi/Ni-P/NiCoPi requires an overpotential of 234 mV to achieve a geometric current density of 10 mA cm−2, and this
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OER performance is even better than that of the conventional RuO2 catalysts. In contrast, 258 mV, 260 mV, 266 mV are needed for NiCoPi/Ni/NiCoPi, Ni-P and NiCoPi@Ni-P@NiCoPi to achieve the same current density, respectively. And the superior OER activity of NiCoPi/NiP/NiCoPi is particularly evident in the high current density region (Fig. 4a), showing a promising perspective of industrial deployment. The corresponding Tafel slope of NiCoPi/Ni15
P/NiCoPi is 69.9 mV dec−1 (Fig. 4b), verifying the OER-favorable kinetics comparing to the other counterparts. Figure 3c displays a multistep chronopotentiometric curve for NiCoPi/NiP/NiCoPi. The potential remains stable with constant demanding current density and immediately reaches a new level with respect to the new applied current density, implying the excellent mass transportation property of our NiCoPi/Ni-P/NiCoPi catalyst. Similarly, the NiCoPi/Ni-P/NiCoPi also exhibits the highest Cdl (Fig. 4d-e), further verifying the substrate
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enhancement in the view of the surface area. Also, the long-term stability of NiCoPi/NiP/NiCoPi was probed and shown in Fig. 4f, it can be identified that there is no obvious current density change for up to 20 hours’ OER test, implying the excellent durability of NiCoPi/Ni-
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P/NiCoPi for OER in strong alkaline solution (also see Fig. S4b, S5-S6).
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What’s more, it was demonstrated that the Fe impurities in the KOH electrolyte usually exhibited enhancement effect on the Ni-based OER catalysts [45, 46]. To unveil whether Fe
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impurities play a crucial role in our NiCoPi/Ni-P/NiCoPi catalyst for OER, 20 CV scans were recorded after each 5 min aging period (Fig. S8). Both oxidation and reduction peaks show
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merely 18 mV shifting, and the potentials at 10 mA cm-2 vary in the range of 232 mV to 234
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mV, with no obvious performance improvement. Therefore, the Fe impurities here are not a
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key factor in the superior OER activity.
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Fig. 5. Two-electrode cell measurements with NiCoPi/Ni-P/NiCoPi acting as both cathode and anode. (a) Polarization curves for overall water splitting of NiCoPi/Ni-P/NiCoPi. Inset: the corresponding polarization curves of HER and OER half reactions. (b) Durability test recorded under a constant applied potential for around 40 h.
The superior performance of NiCoPi/Ni-P/NiCoPi both in HER and OER inspires us the
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possibility of building a real two-electrode system, with the NiCoPi/Ni-P/NiCoPi serving as both cathode and anode. In Fig. 5a, the overall current density reaches 10 mA cm-2 with only 1.55 V applied. This value is consistent with the results of the two corresponding half reactions,
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where an overpotential of 73 mV in HER and an overpotential of 234 mV in OER (1.23
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V+0.073 V+0.234 V=1.537 V in total) are required to drive the cathodic/anodic current density to be 10 mA cm-2. The long-term electrochemical stability of the cell was examined in Fig. 5b.
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There is no obvious change in the current density up to 40 hours, suggesting the excellent
4. Conclusions
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durability of NiCoPi/Ni-P/NiCoPi for overall water splitting.
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In summary, the amorphous NiCoPi was successfully synthesized on the surface the Ni-P nanosheets by a two-step preparation method, which can be verified by various characterization
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techniques, including XPS, XRD, TEM and SEM-EDX. The as-prepared NiCoPi/Ni-P/NiCoPi endows great catalytic activity both for HER and OER in a strong alkaline solution. The requisite overpotentials to achieve 10 mA cm-2 are 73 mV for HER and 234 mV for OER, respectively. As the substrate, the Ni-P has been identified to be vital to the enhanced activity from the following aspects. Firstly, the P atoms could modulate the electronic structure of 17
NiCoPi, and so that offer a better channel for electron to flow. More importantly, the nanosheet morphology of Ni-P is catalysis-favorable and provides more active surface area than normal Ni foam substrate does. The overall electrochemical water splitting using NiCoPi/Ni-P/NiCoPi as both HER catalyst and OER catalyst in 1.0 M KOH further reveals its bright prospect for practical hydrogen production, with only 1.55 V necessary for realizing a current density of 10 mA cm-2. This work may provide an insightful understanding on the substrate enhancement
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effect and offer an effective strategy for the optimization of catalyst design in water electrolysis.
Acknowledgements
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This work is financially supported by the Energy Institute at The Hong Kong University of
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Science and Technology.
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