Synthesis of Copper Phosphide Nanotube Arrays as Electrodes for

Research Article pubs.acs.org/journal/ascecg

Synthesis of Copper Phosphide Nanotube Arrays as Electrodes for Asymmetric Supercapacitors Ying-Chu Chen,† Zhong-Bo Chen,‡ Yan-Gu Lin,§ and Yu-Kuei Hsu*,‡ †

Karlsruhe Institute of Technology (KIT), Institut für Anorganische Chemie, Engesserstraße 15, D-76131 Karlsruhe, Germany Department of Optoelectronic Engineering, National Dong Hwa University, Hualien 97401, Taiwan § National Synchrotron Radiation Research Center, Hsinchu 30076, Taiwan ‡

S Supporting Information *

ABSTRACT: A copper(I) phosphide (Cu3P) nanostructure with tubelike morphology has been directly synthesized on copper foil via a two-step process of electro-oxidation and phosphidation. The microstructure and morphology of the Cu3P nanotube (NT) arrays are systematically examined with the measurements of a SEM, a TEM, XRD, and XPS. Electrochemical measurements manifest that the Cu3P NT exhibits excellent properties of a negative electrode of a supercapacitor. An optimized asymmetric supercapacitor with a negative electrode of Cu3P NT and a positive electrode of CNT has been fabricated that can deliver energy density of 44.6 Wh kg−1, power density of 17 kW kg−1, and stability test with over 81.9% of specific capacitance remaining after 5000 cycles; these features are valuable for prospective applications of Cu3P NT in systems for energy storage and conversion. This work paves the way to explore phosphide-based materials in a new class for supercapacitor applications. KEYWORDS: Copper(I) phosphide, Nanotubes, Electrochemical, Asymmetric supercapacitor

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INTRODUCTION

practical negative electrode materials are hence highly required for the deployment of EC in commercial applications. Copper(I) phosphide, Cu3P, has been attracting attention because it exhibits an excellent prospect for electrochemical applications of many kinds.7−9 For example, Ni et al. reported a large specific energy density of Cu3P on Cu foam as an anode for Li-ion batteries.10 Tian et al. fabricated Cu3P nanowires (NW) as an integrated three-dimensional hydrogen-evolving cathode, showing a small onset, 62 mV, of overpotential and a Tafel slope 67 mV dec−1.11 Besides, the metal phosphides acting as efficient OER precatalysts recently was reported.12 Li et al. demonstrated the use of Cu3P NW as a new platform for cathodic analysis for the sensitive and selective electrochemical nonenzymatic detection of H2O2 with a detection limit 2 nM.13 These investigations throw light on the positive prospects of Cu3P as an effective electrochemical electrode material, but reports on Cu3P for a supercapacitor electrode are rare until

Because energy requirements due to rapid consumption of energy and increasing demand exceed the supply of electronic devices, the development of high-performance devices for energy storage is an urgent issue. Electrochemical capacitors (EC), also called supercapacitors, are the most promising reserve force alternative to secondary lithium ion batteries on the basis of their high power density, rapid kinetics of charge propagation, and longevity.1,2 To accelerate the energy and power density of supercapacitors, an effective approach is to assemble asymmetric supercapacitors incorporating an electrochemical double-layer capacitor (EDLC) electrode and a pseudocapacitor electrode to enlarge the potential window with two distinct electrode materials. Much effort has been devoted to develop active electrodes, such as carbonaceous materials, metal oxides or hydroxides, and conducting polymers; although positive electrode materials have been intensively investigated and used to realize EC with satisfactory performance, only a few reports of the development of negative electrode materials are found in the literature.3−6 Novel and © 2017 American Chemical Society

Received: December 9, 2016 Revised: February 16, 2017 Published: April 6, 2017 3863

DOI: 10.1021/acssuschemeng.6b03006 ACS Sustainable Chem. Eng. 2017, 5, 3863−3870

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ACS Sustainable Chemistry & Engineering Scheme 1. Schematic Diagram of Synthesis of Cu3P NT on Copper Foil

system, an asymmetric supercapacitor was conducted with Cu3P NT as the anode and CNT/CC as the cathode. EIS measurements were performed with this apparatus over a frequency range 100 kHz to 0.1 Hz at varied potential with AC amplitude 10 mV.

now. In this work, we demonstrate a simple and reliable procedure for synthesis of 1-D Cu3P nanotube (NT) arrays on copper foil without conducting additive and binder. The capacitive performance of directly grown Cu3P NT arrays in EC has been systematically evaluated. A hybrid supercapacitor was constructed using Cu3P NT as prepared as the negative electrode and carbon nanotube (CNT) on carbon cloth (CC) as the positive electrode of which the electrochemical characteristics were reported by our group.14 The experimental results show that the assembled Cu3P and CNT hybrid EC can be operated reversibly at cell voltage 1.5 V, delivering energy density of 44.6 Wh kg−1 and power density of 17045.7 W kg−1 and having 81.9% retention of capacitance after 5000 cycles.

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RESULTS AND DISCUSSION Structural and Compositional Analysis. Figure 1a shows the surface morphologies of Cu(OH)2 NT arrays fabricated with electro-oxidation in alkaline solution at 10 °C. The straight and dense NT structures cover uniformly a large surface area of the copper foil substrate. From a magnified FESEM (Figure 1b), each NT has a length over 10 μm and a diameter about 150 nm. The Cu(OH)2 NT samples as grown were then placed in a tube furnace with an amount of NaH2PO2 powder for phosphidation. The sample color gradually altered from indigo color to black with increasing duration of reaction; this color change implies that the phosphidation transformed the composition of the NT samples. Through the phosphidation treatment for 30 min, the morphology of the sample still showed a wirelike shape with almost the same length and diameter as shown in Figure 1c, but the surfaces of the NT became slightly deformed to a curled shape through the phosphidation treatment.11 To elucidate the microscopic structure of the Cu 3 P NT, we conducted a detailed investigation using a field-emission transmission electron microscope (FE-TEM). Figure 1d shows a TEM image of a Cu3P NT fragment at small magnification; the tubelike shape is confirmed. The HRTEM indicates that the wall structure of the Cu3P tubes as shown in Figure 1e is formed with a) large crystallites of size about 25 nm and b) poorly crystallized or even amorphous regions. The region within the green frame on the HRTEM image is, however, a single Cu3P crystal of hexagonal structure, as indicated by the agreement between its diffractogram and the calculated diffraction pattern of bulk hexagonal Cu3P (space group P63cm, space group number 185) in the [423]-zone axis. In the inset of Figure 1d, all reflections in the SAED pattern of tubes are assignable to hexagonal Cu3P (green) with a = 6.959 Å and c = 7.143 Å. No additional diffraction signals of copper oxide are observed in the SAED pattern, which indicates that the tube walls are not oxidized, in agreement with HRTEM. In addition, EDXS maps of (a) O− Kα1, (b) Cu−Kα1−O−Kα1, (c) P−Kα1−O−Kα1, and (d) Cu− Kα1−P−Kα1−O−Kα1 were shown in Figure 2, and this indicates that, first, tube walls are composed of Cu3P and no O inside the tube wall is observed on the EDXS map of O, according to HRTEM; second, an O containing material forms a thin external shell surrounding the tubes and fills up to a different degree the tube canals. This is in agreement with HRTEM, where a thin amorphous shell with an inhomogeneous thickness of up to t ∼ 5 nm can be observed on all images of tubes, which might have resulted from natural formation in ambient conditions.15 We undertook XRD analysis to analyze the structural crystalline of the NT-array electrodes after a phosphidation

EXPERIMENTAL SECTION

The Cu3P NT arrays on copper foil were synthesized through direct electro-oxidation and phosphidation, as shown in Scheme 1. Typically, Cu foil was first washed successively with ethanol, acetone, HCl ,and water several times to remove impurities from the surface, followed by electro-oxidation at 4 mA/cm2 in an alkaline solution (NaOH, 1 M, 20 min) to form Cu(OH)2 NT on Cu foil at a temperature of 10 °C. The resulting Cu(OH)2 NT was then washed with water several times and dried in air. To prepare Cu3P NT, we placed NaH2PO2 (72 mg) at the center of the tube furnace and Cu(OH)2 NT downstream at carefully adjusted locations to set the temperature; the distance between them was about 42 cm. After flushing with N2, the center of the furnace was elevated to 300 °C at a heating rate of 7.5 °C min−1 and was held at this temperature for 30 min. The Cu(OH)2 NT was kept at ∼150 °C. After natural cooling to near 23 °C, the loading for Cu3P on Cu foil was determined, with the use of a high precision microbalance, to be 0.43 mg cm−2 by measuring the weight before and after scratching Cu3P out of Cu foil. For the electrochemical measurements, the hierarchical electrode was prepared on bonding a copper wire onto the edge of approximately 1 × 1 cm2. The bonding was accomplished with silver paste and cured for 20 min at 80 °C. The bonding pad was covered with epoxide to expose only the Cu3P NT surface to the test solutions. The CNT/CC electrodes were prepared with a nickelcatalyst-assisted microwave-plasma-enhanced chemical-vapor-deposition technique; for the detailed process we refer the reader to ref 14. The morphology of hierarchical Cu3P NT arrays was examined with a scanning electron microscope (SEM, JEM-4000EX); the structure of the samples was analyzed with high-resolution scanning transmission electron microscopes (HR-STEM, JEM2010F, and JEM2200FS operated at 200 kV, JEOL) and a X-ray diffractometer (XRD, Bruker D8 Advance, Cu Kα radiation, λ = 0.1506 nm). The chemical states of the elements were determined by X-ray photoelectron spectra (XPS, PerkinElmer model PHI 1600). To evaluate the electrochemical performance of Cu3P NT arrays, we employed cyclic voltammetry (CV) and a galvanostatic charge/discharge method (CHI 6273D potentiostat/galvanostat). All electrochemical measurements were conducted in H2SO4 electrolyte solution (1 M). A conventional three-electrode system comprised the hierarchical Cu3P NT array as a working electrode, the square platinum sheet as an auxiliary electrode, and an Ag/AgCl reference electrode in KCl solution (3 M). All potentials reported in this article are relative to Ag/AgCl (3 M KCl, 0.207 V vs SHE). The loading amount of Cu3P NT on the copper foil electrode was determined with a microbalance (Sartorius BP 211D, Germany, accuracy 10 μg). In the two-electrode electrochemical 3864


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Figure 1. (a) and (b) FESEM images of Cu(OH)2 NT; (c) SEM images of Cu3P NT; (d) FE-TEM of Cu3P NT; and (e) HRTEM image within green frame and image of Fourier transformation.

treatment, as shown in Figure 3a. The XRD result of the indigo color sample, corresponding to the SEM picture in Figure 1a, shows that the material belongs to Cu(OH)2 of the orthorhombic phase (JCPDS card no. 72-0140) with the signals marked with triangles, except the copper substrate with an asterisk mark. After phosphidation for 30 min, the diffraction signals with circles mark indexing to hexagonal-phase Cu3P (JCPDS card no. 71-2261) appeared. No other signal beyond

the diffraction signals of Cu foil and Cu3P was observed, implying no existence of other phases or materials. The conversion of Cu(OH)2 to Cu3P by phosphidation was confirmed, consistent with the HRTEM results. To reveal the chemical composition and oxidation states of Cu and P elements of Cu3P NT, we recorded in XPS, the core-level spectra of Cu 2p and P 2p, as displayed in Figure 3b and 3c. After phosphidation, the Cu 2p lines at 932.8 and 952.3 eV 3865


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Figure 2. EDXS maps of (a) O−Kα1, (b) Cu−Kα1−O−Kα1, (c) P−Kα1−O−Kα1, and (d) Cu−Kα1−P−Kα1−O−Kα1.

934.2 and 954.3 eV correspond to oxidation states of Cu2+. The occurrence of a satellite characteristic that located at the highenergy side of the Cu 2p3/2 main feature at 943.2 eV indicates the existence of Cu(II), which might have resulted from natural formation in ambient conditions.15 The XPS P 2p of the Cu3P NT sample is presented in Figure 3c, which shows two signals at binding energies of 129.4 and 133.5 eV. The former signal, at an energy less than the binding energy of red P (130.0 eV), results from the binding energy of P in Cu3P; the signal at 133.5 eV belongs to oxidized P species owing to superficial oxidation of Cu3P when Cu3P contacted with air.16 The areas of the XPS Cu 2p and P 2p signals for the NT as prepared show a ratio of Cu to P = 3.05:1, indicating the formation of a Cu3P compound. Those findings support the structural results of XRD and HRTEM. Electrochemical Characteristics of Cu3P NT Array Electrodes. The electrochemical properties of the samples, as prepared, were investigated as electrode materials for electrochemical capacitors. Figure 4a shows cyclic voltammograms (CV) of the Cu3P NT array electrodes in a threeelectrode system in the electrolyte of H2SO4 (1 M) at a varied negative voltage cutoff. From the negative voltage scan the CV curves have completely the same trace, showing the highly reversible feature of Cu3P NT electrodes. Meanwhile, the currents from anodic process increase slightly as the CV of negative voltage cutoff reaches a further negative value of −0.5 V. This phenomenon can be contributed to redox reactions occurring at the Cu 3 P NT arrays; their counterpart simultaneously occurs during a positive voltage scan. At the range of operated voltage from 0.1 to −0.5 V, the two remarkable humps corresponding to cathodic and anodic maxima indicate that the Cu3P NT electrode possesses pseudocapacitive properties; the obvious redox reactions might be ascribed to the transition from metallic Cu(0) and Cu(I) species at potential approximately −0.3 V vs Ag/ AgCl.17,18 As the potential was further increased to a positive potential, the oxidations of Cu3P to Cu(II) species and water might occur at potential 0.1 V vs Ag/AgCl.19 So far, the investigation on copper phosphide electrodes for supercapacitors is rare, and a related reaction mechanism is not mentioned. Although the identification of signals for assignment requires further investigation, a possible reaction

Figure 3. (a) X-ray diffraction pattern of Cu3P NT; (b) Cu 2P; and (c) P 2P XPS of Cu3P NT.

(Figure 3a), corresponding to Cu 2p3/2 and 2p1/2, indicate an oxidation state of Cu1+. The signals from high-energy peaks at 3866


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Figure 4. (a) CV of a Cu3P NT electrode in a three-electrode cell at a sweep rate of 50 mV s−1. (b) CV of a Cu3P NT electrode with a varied sweep rate. (c) Galvanostatic charge/discharge curve of a Cu3P NT electrode with varied current density. (d) Variation in gravimetric capacitance of a Cu3P NT electrode with varied current density.

from the anodic charging process were generally symmetric with their corresponding cathodic discharging counterparts for the Cu3P NT array, indicating a typical capacitive material with the great reversibility. The specific capacitance (Cs) of the Cu3P NT electrode in an electrolyte of H2SO4 solution is evaluated by using the following equation21

mechanism is proposed, as described in the following equations based on the relevant literature19,20 Cu(I)3 P + x H+ + x e− ⇔ Hx{Cu(0)x Cu(I)3 − x }P

(1)

Cu(I)3 P + H 2O ⇔ Cu(II)y Oy Cu(I)3 − y P + 2y H+ + 2ye− (2)

Cs =

Figure 4b shows the CV of the Cu3P NT arrays recorded in H2SO4 solution (1 M) at varied scan rates. With increasing scan rate (ν), the maximum current enhances continuously, indicating that under the applied scan rates the rates of electronic and ionic transport are sufficiently fast. There is no remarkable change in the appearance of the CV, indicating the satisfactory reversibility of the Cu3P electrode in the rapid charge−discharge response. In addition, the cathodic peak current (ip) against ν1/2 (insert of Figure 4b) shows an almost linear behavior, indicating that the redox reaction is dominated by diffusion in the electrolyte of H2SO4. The curves of galvanostatic charge/discharge measurements in Cu3P NT arrays were also tested at varied current density, as shown in Figure 4c. The charge/discharge curves illustrate that most capacitance is produced in the potential region of 0 to −0.5 V, corresponding to the redox reaction of Cu(I) and Cu(II). Those results were similar to that in the CV. The segments

iΔt mΔV

(3)

in which i (mA) is the discharge current for applied duration Δt (s); ΔV (V) is the operated voltage window; and m is the mass of Cu3P NT. The specific capacitance values of Cu3P NT arrays are 300.9, 214.2, 177.1, 148.5, and 130.4 F g−1 at current densities 2.5, 3, 3.5, 4, and 4.5 mA cm−2, respectively, as shown in Figure 4d. With increasing current density the decrease in capacitance is possibly due to the increased potential drop from the resistance of the Cu3P electrode and insufficient Faradaic redox reaction at large current densities, or it might reflect the large resistance of Cu3P NT. Moreover, the excellent capacitive characteristics are comparable with other negative/positive battery-type materials (like MnO2, NiO/Ni(OH)2, Co(OH)2/ Co3O4, FeOOH, NiCo2O4, and others),22−27 suggesting that Cu3P NT electrodes are the promising electrode materials in the application of supercapacitor. 3867


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capacitance, of which the detailed electrochemical performance could be found in Figure S1 and ref 14. The comparative CV curves of the two electrodes were performed at a scan rate of 50 mV s−1, as shown in Figure 6a. The Cu3P NT electrode can be performed at a voltage window 0 to −0.5 V; the CNT/CC electrode can be operated within a potential window 0 to 1.0 V; the asymmetric supercapacitor device can operate at a voltage of 1.5 V. To achieve an optimum capacitance of the device and to fulfill the requirement of charge balance between, the optimal mass ratio of Cu3P NT and CNT was considered, based on their specific capacitances and potential windows. Therefore, an optimal CNT to Cu3P NT mass ratio of 0.81 enables charge flow balance between the positive and negative electrodes (Q+ = Q−) according to the equation m+/m− = C−ΔV−/C+ΔV+. Figure 6b shows CV curves of the asymmetric supercapacitor at varied scan rates; meanwhile, the areas of their CV curves increase when the scan rate increases, indicating that within the scan rates the charge/discharge processes are not kinetically limited. The shapes of the CV curves combine the characteristics of faradaic pseudocapacitance and EDLC and repeat reversibly, thus ensuring the stability of the aqueous electrolyte of the cell during charging and discharging. We performed galvanostatic charge−discharge measurements at varied current density with potential window 1.5 V (Figure 6c). The asymmetric supercapacitor (CNT// Cu3P) exhibits specific capacitance 142.8 F g−1 at current density of 0.75 mA cm−2. Even at current density of 10 mA cm−2, the specific capacitance still attained 97.8 F g−1, approximately 68.5% of the capacitance retained. Furthermore, in order to investigate the capacitive performance of asymmetric supercapacitor, the values of specific power and specific energy can be calculated by the following equations29

Electrical impedance spectra (EIS) of the Cu3P NT electrode were performed at an open-circuit potential with AC perturbation 10 mV in frequency range 100 kHz− 0.1 Hz. Figure 5 displays Nyquist plots of impedance spectra, which

Figure 5. Nyquist plots of impedance spectra at varied applied potential; the inset shows an equivalent circuit.

measured for Cu3P NT arrays in the electrolyte of H2SO4 (1 M) and analyzed with complex nonlinear least-squares (CNLS) fitting according to an equivalent circuit, as shown in the inset of Figure 5. RW is the diffusion resistance of counterion, Rs is the solution resistance of the electrolyte, and Rct is the chargetransfer resistance across the interface between Cu3P and the electrolyte, whereas the constant-phase elements of CPE1 and CPE2 represent the interfacial capacitance and the pseudocapacitance from the redox reaction of Cu3P, respectively. The fitted results of RW, Rct, and τd are listed in Table 1. As the Table 1. Best-Fitted Values of Equivalent Circuit Parameters −0.1 V −0.3 V −0.5 V a

Rct (Ω)

RW (Ω)

τd (s)

0.63 0.64 0.65

1.57 8.43 23.9

0.59 0.25 0.07

P (W kg −1) = [I(A) × ΔV ]/m (kg)

(4)

E (W h kg −1) = [I(A) × t × ΔV ]/m (kg)

(5)

where t is the discharge time in seconds corresponding to the potential difference in volt, I is the discharge current in ampere, and m is the Cu3P electrode mass in gram. The energy density that can be extracted from the charge and discharge is about 44.6 W h kg−1 at current density of 0.75 mA cm−2 and approximately 17045.7 W kg−1 at current density of 10 mA cm−2. The energy and power density values of the hybrid cell exceed those of EC, listed in Figure 7, which employ other phosphide-based materials, namely, AC//Ni2P/Co3V2O8,30 AC//CoP,31 and graphene//Co2P.32 The cyclability of the asymmetric supercapacitor was investigated with continuous measurements of charge and discharge over 5000 cycles (Figure 6d) at current density of 10 mA cm−2. The asymmetric supercapacitor illustrates approximately 81.9% retention of initial specific capacitance after 5000 cycles, which indicates superior cycling stability. All these results elaborate a fact that the Cu3P NT is a promising material that can be prospectively used for the development of an EC negative electrode.

a

τd: diffusion time coefficient

applied potential decreased, proton intercalation was expected, which resulted in increased values of Rct and RW, but the value of τd decreased at the same time. With decreasing applied potential, the gradually increased intercalation of protons and fewer available protons sites within Cu3P, which might hinder diffusion of subsequent protons, thus resulted in further increased values of Rct and RW.28 The gradually increased resistances also caused a decreased τd, indicating a shortened path for proton diffusion in Cu3P NT. The satisfactory capacitive characteristics are similar to those of phosphidebased supercapacitors and indicate that Cu3P NT arrays can be excellent and promising materials of a supercapacitor electrode. To explore further the application of the Cu3P NT products, as synthesized, in the field of electron storage, we fabricated an asymmetric supercapacitor device with a negative electrode of Cu3P NT and a positive electrode of CNT/CC. The electrochemical characteristics of CNT/CC were evaluated in a three-electrode system as shown in Figure 6a; the results imply its excellent performance as an electric double-layer

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CONCLUSION The tubelike nanostructures of Cu3P standing on copper foil were fabricated with a simple electro-oxidation process and phosphidation reaction to realize a negative electrode without binding paste for electrochemical capacitors. Due to highly reversible charge and discharge characteristics, the Cu3P NT electrode could achieve the specific capacitance of 300.9 F g−1. The experimental results demonstrate the ability of the CNT// 3868


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Figure 6. (a) CV curves of the Cu3P NT electrode and the CNT/C electrode at 50 mV/s measured in a three-electrode system; (b) CV curves of the asymmetric supercapacitor collected at a varied scan rate; (c) galvanostatic charge/discharge curves collected at varied current density for asymmetric supercapacitor devices in a voltage window 1.5 V; (d) cycling performance of an asymmetric supercapacitor at current density of 10 mA cm−2.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b03006. Electrochemical properties of the CNT/CC electrode in a three-electrode cell with 0.5 M H2SO4; specific energy of the asymmetric supercapacitor with the other positive/ negative mass ratios; and SEM and EDX data of Cu3P NT on Cu foil after scratching Cu3P NT (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Phone: +886-3-863-4196. Fax: +886-3-863-4180. E-mail: [email protected].

Figure 7. Ragone plot of asymmetric supercapacitor device.

ORCID

Yu-Kuei Hsu: 0000-0003-1963-5172 Notes

Cu3P asymmetric supercapacitor cell, as fabricated, to achieve an energy density of 44.6 W h kg−1 at current density of 0.75 mA cm−2 and a power density of 17045.7 W kg−1 at current density of 10 mA cm−2, with 81.9% retention of capacitance after 5000 cycles at current density of 10 mA cm−2. This work demonstrates that a Cu3P NT array is a prospective material for the development of a supercapacitor negative electrode.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS National Science Council and National Dong Hwa University supported this research under contracts MOST 105-2221-E259-024-MY3 and MOST 105-2221-E-259-026, respectively. 3869


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Synthesis of Copper Phosphide Nanotube Arrays as Electrodes for

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