Self-Supported Cedarlike Semimetallic Cu3P Nanoarrays as a 3D

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Self-Supported Cedarlike Semimetallic Cu3P Nanoarrays as a 3D High-Performance Janus Electrode for Both Oxygen and Hydrogen Evolution under Basic Conditions Chun-Chao Hou,† Qian-Qian Chen,† Chuan-Jun Wang,† Fei Liang,‡ Zheshuai Lin,‡ Wen-Fu Fu,†,§ and Yong Chen*,† †

Key Laboratory of Photochemical Conversion and Optoelectronic Materials and HKU-CAS Joint Laboratory on New Materials, and Key Laboratory of Functional Crystals and Laser Technology, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, P.R. China § College of Chemistry and Chemical Engineering, Yunnan Normal University, Kunming 650092, P.R. China ‡

S Supporting Information *

ABSTRACT: There has been strong and growing interest in the development of cost-effective and highly active oxygen evolution reaction (OER) electrocatalysts for alternative fuels utilization and conversion devices. We report herein that semimetallic Cu3P nanoarrays directly grown on 3D copper foam (CF) substrate can function as effective electrocatalysts for water oxidation. Specifically, the surface oxidationactivated Cu3P only required a relatively low overpotential of 412 mV to achieve a current density of 50 mA cm−2 and displayed a small Tafel slope of 63 mV dec−1 in 0.1 M KOH solution, on account of the collaborative effect of large roughness factor (RF) and semimetallic character. Following that, investigations into the mechanism revealed the formation of a unique active phase during the water oxidation process in which conductive Cu3P was the core covered with a thin copper oxide/ hydroxide layer. Moreover, this Cu3P 3D electrode was also applied to the hydrogen evolution reaction (HER) and showed good catalytic performance and stability under the same basic conditions. KEYWORDS: copper phosphide, electrocatalysis, oxygen evolution, hydrogen evolution, water splitting Because of the rich redox properties and relatively low price,8 Cu-based materials have been widely used in various fields including oxidizing organic compounds,9 cross-coupling, and gas-phase reactions benefiting from the practical and straightforward multiple ways of preparation.10 Moreover, Cubased catalysts are also found to show good catalytic ability toward electrochemical decomposition of CO2 in a simple twocompartment electrochemical cell which involves water splitting and reduction of CO2 to CO (or other oxygenates) to finally achieve conversion of natural solar energy to the “solar fuels”.11 Among these above reactions, it is essential to overcome the challenge of the oxygen evolution half reaction due to its multielectron and multiproton nature. Also, it is highly favorable to use the same one catalyst to fulfill the multitarget reactions, as the matching of reactivity is required.12 From this point of view, the development of Cu-based materials has an irreplaceably significant role compared with other nonprevious metals. However, at the present stage, the catalytic performance of Cu-based catalysts is yet to be improved. Their

1. INTRODUCTION Increasingly serious environmental pollution and the global energy crisis resulting from the depletion of fossil fuels has prompted tremendous efforts in the exploration of various emerging technologies based on catalysis for the storage and conversion of renewable and clean energies.1 In particular, water electrolysis has been developed to serve as a prospective and attractive means to achieve this purpose.2,3 Due to the existence of surface polarization at practical electrodes, it would require additional voltage to overcome overpotentials other than the theoretical value of 1.23 V in order to drive water splitting.4 Accordingly, it is imperative to explore robust water splitting catalysts to achieve respectable performance subject to the two-electron transport process for the hydrogen evolution reaction (HER) and the sluggish four-electron transfer kinetics for the oxygen evolution reaction (OER).5 Currently, state-ofthe-art electrocatalysts used for water splitting are mainly Ruand Ir-based compounds for the OER and platinum for the HER, but they suffer from unfavorable cost and scarcity, severely limiting their scale-up practical applications.6 It is thus highly favorable to explore less expensive and efficient catalysts especially made from earth-abundant elements for both the HER and OER.7 © XXXX American Chemical Society

Received: May 25, 2016 Accepted: August 12, 2016

A

DOI: 10.1021/acsami.6b06251 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Scheme 1. Preparation of Cedarlike Cu3P/CF as a 3D Cathode for Water Reduction and Cu−P@Cu3P/CF Nanohybrid by in Situ Surface Oxidation as a 3D Anode for Water Oxidation, Respectively

Therefore, it is quite conceivable to apply this strategy to Cubased OER catalytic systems to improve the intrinsic activity. The currently prevalent strategies using completely different OER and HER catalysts in one electrolyte often suffer from mismatched activity of two kinds of electrocatalysts and thus mediocre overall water splitting performance.28,29 Therefore, the parallel development of a homologous bifunctional OER and HER catalyst, which originated from the same source with similar structures and exhibited matching reactivity when applied as complementary electrocatalyst pairs for water splitting, would potentiate the kinetics of electron transfer and represent an intriguing approach.30,31 However, most of the existing OER catalysts, including Cu-based ones, show negligible catalytic effects when applied as catalysts for the HER. Consequently, it is still a challenging task to find a Cubased catalyst that has not only a large roughness factor, more active sites, and metallic properties, factors that would bring high electrocatalytic external and intrinsic activity for OER, but also acts as a Janus catalyst toward both the HER and OER.32−34 Herein we first report on cedarlike semimetallic Cu3P nanoarrays directly grown on 3D copper foam (CF) substrate as highly effective OER electrocatalysts. The large RF of Cu3P nanoarrays resulted in high electrochemical active surface area, while the semimetallic Cu3P core character facilitated charge transfer (Scheme 1). Consequently, the surface oxidationactivated Cu3P reached a current density of 50 mA cm−2 at an overpotential of 412 mV and showed a low Tafel slope of 63 mV dec−1 in basic solution, representing one of the best Cubased water oxidation electrocatalysts reported to date. Moreover, this Cu3P 3D electrode was also applied to the HER and showed an intriguing catalytic performance and stability under the same basic conditions.

use as non-noble electrocatalysts for water splitting reactions is still in the primary stage. Over the past years, a handful of homogeneous copper(II) complexes have been pioneered to function as efficient molecular catalysts for water splitting.13−21 Their tunability in electronic and molecular structure is facile for deep insight into the catalytic mechanism. However, these molecular copper catalysts generally need high overpotential to realize water splitting, and the relatively low activity and stability for the OER compared with the Cu-based heterogeneous nanomaterials make them far from industrial requirements. Meanwhile, several groups have found that copper(II) complexes can act as precursors of catalytically active heterogeneous copper(II) oxide films.22−24 For example, we have devised a heterogeneous CuO-based electrocatalytic system for water oxidation by electrodepositing a water-soluble catalyst precursor [Cu(TEOA)(H2O)2][SO4].23 Unfortunately, the electrodeposited CuO-based catalysts often show a deteriorative activity (∼1.5−2 mA cm−2 at 1.3 V vs NHE) and moderate stability for the OER owing to the large size of microparticles and densely packed morphology. It is reasonable to suppose that a 3D open Cu-based electrode with a large roughness factor (RF) and high electrochemical surface area (ECSA) will provide more active sites for catalysis and favor the diffusion of solution to improve the external performance. On the other hand, we have noticed that transition-metal phosphides (TMPs), e.g., Ni2P25 and Ni5P4,26 which possess distinct metallic character, have been reported recently as component catalysts for OER in basic medium. In previous work, we also observed the same phenomenon that the surface of CoP is readily oxidized to high-valence Co−P species to form unique core−shell structure, possessing indisputable advantages over Co oxide for OER.27 The oxidized species on the shallow surface could serve as active sites for catalysis while the electrically conductive CoP core facilitates the charge transfer and thus markedly accelerates the OER kinetics. B


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Figure 1. (a) The crystal packing diagram of Cu3P. (b) The magnified view of Cu3P surface structure along the c-axis and (c) a three-dimensional portion of Cu3P structure. (d) Calculated density of states (DOS) and (e) corresponding electric band structure of Cu3P. (f) Typical I−V curves and (g) temperature-dependent electrical resistance of Cu3P/CF film sample. in the air by steel wire upstream of a quartz boat of the tube furnace. Then NaH2PO2 (∼1.0 g) was placed at the downstream side of the quartz boat to make sure that the distance between them was about 0.5 cm. Subsequently, this tube furnace was heated to 300 °C at a rate of 2 °C min−1 under Ar atmosphere and held at this temperature for 120 min. The Ar flow is slightly fast (∼3 bubbles per second) to ensure that the Cu foam and NaH2PO2 were mixed uniformly during the heating process. After this reaction, the color of the CF became dark. Finally, the film was rinsed with 2 M HCl solution and deionized water a couple of times to remove residual reactant and was dried under vacuum overnight. The CuOx/CF film was prepared by simple oxidation of CF. The CF was put into a quartz boat and then sealed in a muffle furnace. Next, the muffle furnace was heated to 400 °C at a rate of 2 °C min−1 for 120 min. After cooling to room temperature, the CuOx/CF film was obtained.

2. EXPERIMENTAL SECTION 2.1. Chemicals and Materials. All solvents and chemicals were of analytical grade and used as received without further purification. Sodium hypophosphite (NaH2PO2, AR, 99.0%) was purchased from Aladdin Ltd. (Shanghai, China). Copper foam (100 mm × 100 mm) was purchased from Suzhou Jia Shi De Metal Foam Co., Ltd. Platinum foil was bought from Tianjin AIDA Hengsheng Science-Technology Development Co., Ltd. RuCl3·xH2O (48.2% Ru) was obtained from Alfa Aesar. Nafion (5 wt %) was bought from Sigma-Aldrich. H2SO4, NaNO3, NaOH, HCl, graphite rods, and all solvents were obtained from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). 2.2. Synthesis of Cu3P/CF and CuOx/CF Films. In a typical procedure, copper foam (CF) was first sonicated in dilute HCl solution for 15 min and then washed with water several times to remove the surface oxides or pollutants. Following this, the clean CF was carefully cut into 0.5 × 1.0 cm2 pieces, and four pieces were hung C


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Figure 2. (a) XRD pattern of Cu3P/CF. (b) SEM image of bare CF. (c−f) SEM images of Cu3P/CF film. (g) TEM (inset) and HRTEM images of Cu3P sample. (h) Line-scan EDX and (i) corresponding EDX elemental mapping of P and Cu elements. Scale bar: 100 um. 2.3. Methods for Characterization. X-ray diffraction patterns (XRD) were recorded on a Bruker AXSD8 X-ray diffractometer (Cu Kα, λ = 1.5406 Å, 100 mA, and 40 kV). For X-ray photoelectron spectroscopy (XPS) analysis, the samples were first deoxygenated in Ar for at least 1 h, and then the measurement was carried out on a PHI 5300 ESCA system equipped with an Al Kα X-ray source. The binding energies were calibrated with reference to that of C1s (285.0 eV). Transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM), and selected-area electron diffraction (SAED) were conducted by using a transmission electron microscope (JEM 2100F) with a 200 kV accelerating voltage. Scanning electron microscopy (SEM) images and corresponding energydispersion X-ray spectra were obtained on a field emission scanning electron microscope (Hitachi S-4800). For all TEM, HRTEM, SAED, and corresponding EDX analyses, the samples were first dispersed in alcohol and sonicated for at least 0.5 h, followed by dropping on an ultrathin carbon film and leaving it to dry naturally at room temperature in air prior to these measurements. Fourier-transformed infrared data were recorded on a Bruker ALPHA FTIR spectrometer from 4000 to 450 cm−1 at room temperature. Raman spectroscopy measurements were performed on an inVia-Reflex confocal laser micro-Raman spectrometer using Ar+ laser excitation (λex = 532 nm). The electronic transportation properties were measured on a Keithley 4200-SCS semiconductor characterization system with a two-point probe method on pressed pellets of the Cu3P/CF film sample that was prepared by phosphorization of CF.35 2.4. Electrochemical Measurements. Electrochemical experiments were carried out using a computer-controlled electrochemical workstation (CHI660E) and conducted in a conventional threeelectrode setup in a one-compartment cell with the 3D CF as working electrode, saturated Ag/AgCl as reference electrode, and Pt foil as auxiliary electrode for the OER (using a graphite rod for the HER). All

experiments were performed in an aqueous solution of 0.1 M KOH, and the potentials reported here were calibrated with respect to the reversible hydrogen electrode (RHE): E(RHE) = E(Ag/AgCl) + 0.059 × pH + 0.197. Linear sweep voltammetry (LSV) measurements were conducted in O2-saturated 0.1 M KOH aqueous solution for OER (Arbubbled solution for the HER) with a sweep rate of 5 mV s−1. 2.5. Theoretical Methods. The first-principles calculations for Cu3P crystal were performed using CASTEP software package by the plane-wave pseudopotential method.36 The kinetic cutoff energy was set as 320 eV, and the Brillouin zone was (4 × 4 × 4) with a separation of 20 − − close to 100% − − 0.264 0.095 0.234 580 72 120 145 18 30 63 128 75 412 707 570 4.65 9.40 5.37 Cu3P/CF CuOx/CF RuO2/CFe

373 420 340

η@J = 50 mA cm−2 [mV] onset overpotential [mV]c Rs [Ω cm2]b sample

Table 1. OER Activity Data of Different Electrocatalystsa

Tafel slope [mV dec−1]

ECSA [cm2]

RF (roughness factor)

SA (specific activity) @η = 490 mV [mA cm−2]d

Faradaic efficiency [%]

stability [h]


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Figure 4. (a) Multicurrent process of the as-prepared Cu3P/CF film. The current density was obtained under different potentials from 1.66 to 1.96 V, with an increment of 100 mV without correction of iR. (b) Current density trace of as-prepared Cu3P/CF electrode obtained at an applied potential of 1.76 V vs RHE in 0.1 M KOH.

identified as the real catalytically active species for OER by Yeo and co-workers through in situ Raman spectroscopy.60 In the present work, the presence of hydroxo or aqua groups is also important for these Cu-based heterogeneous catalysts as was found for the well-studied homogeneous single site polypyridyl Ru or Cu complexes [eqs 4 and 5)]. These groups probably assist the surface oxidation of Cu(II) to Cu-oxo intermediates through a proton-coupled electron transfer (PCET) process for catalyzing OER.61−63 −e−, −H+

[Ru II−OH 2]2 + ⎯⎯⎯⎯⎯⎯⎯⎯⎯→ [Ru III−OH]2 + −e−, −H+

−e −

⎯⎯⎯⎯⎯⎯⎯⎯⎯→ [Ru IV =O]2 + ⎯⎯⎯→ [Ru V =O]3 + Figure 5. High-resolution Cu(2p) XPS (left) and AES (right) spectra of as-prepared Cu3P film, post-OER Cu3P film (CPE, 10 h), and XPS 15 nm depth profile of post-OER Cu3P film.

(4)

−e−, −H+

[Cu II−OH 2]2 + ⎯⎯⎯⎯⎯⎯⎯⎯⎯→ [Cu III−OH]2 + −e−, −H+

⎯⎯⎯⎯⎯⎯⎯⎯⎯→ [Cu IV =O]2 + ([Cu III−O•]2 + ])

often observed for the transition metal phosphides such as Ni2P,25,59 CoP,27,57 and Ni5P4.26 In our case, a broad oxidation peak appeared at ∼1.05 V vs RHE before the OER catalytic current for Cu3P/CF could be ascribed to Cu(II) to Cu(III) (Figures 3c and S16).18 Recently, Cu(III) species were

(5)

Then the unstable high-valence Cu-oxo intermediates can easily decompose into Cu oxides/hydroxide covering the original surface of Cu3P to form this unique Cu−P@Cu3P core−shell structure.

Figure 6. (a)The XRD patterns and (b) Raman spectra of the as-prepared Cu3P/CF and post-OER Cu3P/CF films. (c) EDX and line-scan EDX of the post-OER Cu3P/CF sample. The TEM and HRTEM images of (d) the as-prepared Cu3P/CF and (e) post-OER Cu3P/CF films (inset: SEM image). (f) EDX elemental mapping of post-OER Cu3P. H


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Figure 7. (a) Polarization curves of Cu3P/CF, CF, and commercial Pt HER cathode. (b) The corresponding Tafel plots of Cu3P/CF, CF, and commercial Pt. (c) Comparison of overpotentials at a current density of 10 mA cm−2 and Tafel slopes for different samples. (d) Current density trace of Cu3P/CF electrode at a static overpotential of 360 mV. All the curves were obtained in 0.1 M KOH aqueous solution with iR correction.

3.3. Hydrogen Evolution Reaction. It is very intriguing to explore bifunctional electrodes for both HER and OER in water splitting devices, in which the installation would be simplified by employing the identical electrode initiated on anodic oxygen evolution for catalyzing the corresponding opposite hydrogen evolution process as well. In this way, we also put the 3D Cu3P/CF as cathode for the first time in alkaline (0.1 M KOH) solution for the HER (Figure 7a and 7b). For comparison, Pt and CF were also studied. From the LSV curves (Figure 7a), it is not unexpected that Pt shows the highest activity with negligible onset overpotential. Cedarlike 3D Cu3P/CF also displays good HER performance with an onset potential of ∼100 mV, comparable to that for Cu3P reported in acidic medium.35 The Cu3P/CF, CF, and Pt electrodes need overpotentials of 222, 541, and 57 mV to achieve a current density of 10 mA cm−2 (Figure 7c), respectively. To afford the same current density, CoP-CNT,64 FeP NAs,65 and MoPCA266 in acid medium (0.5 H2SO4) only require overpotentials of 122, 55, and 125 mV, respectively. Furthermore, the Tafel plots were recorded to reveal the HER kinetics (Figure 7b). A Tafel slope of 148 mV dec−1 for Cu3P/CF is close to the reported value for CoP on carbon cloth under basic conditions67 and lower than that of CF (184 mV dec−1). It is well-known that the HER in alkaline solution usually proceeds through the Volmer−Heyrovsky or Volmer−Tafel mechanisms [eqs 6 and 7)]:68,69

atoms (Hads), after which adsorbed H intermediates would subsequently yield H2 and the OH− is then detached to refresh the original catalyst surface. The striking feature for metal phosphides is the partial charge transfer from metal center to P element. For Cu3P, the positively charged Cu(δ+) centers favor the adsorption of nearby OH− originating from water decomposition, while the negatively charged P(δ−) sites prefer to combine with H atoms.35 It is believed that both factors are beneficial to the enhancement of HER performance (Figure S17). Figure 7d shows the chronoamperometric curve for Cu3P/ CF at a static overpotential of ∼360 mV. It is found that the current density slightly decreased during the first 2 h and then was almost constant for the following 8 h. This finding indicates excellent durability of the Cu3P/CF electrode for the HER under such harsh conditions (0.1 M KOH). Finally, a device consisting of two pieces of Cu3P/CF as both anode and cathode electrodes was constructed to probe the catalytic performance of this bifunctional Cu3P catalyst for overall water splitting (Figure S18). From the LSV of water electrolysis on Cu3P/CF electrodes in 0.1 M KOH, we could easily observe that Cu3P/CF shows an indisputable advantage over the bare copper foam system under the same conditions (Figure S18a). The controlled potential electrolysis experiment in 0.1 M KOH reveals that the overall water splitting performance of Cu3P/CF is stable for at least 20 h (Figure S18b), which is further confirmed by comparing the LSV curves of Cu3P before and after 20 h of electrolysis (Figure S18b inset).

H 2O + e− → Hads + OH−(Volmer) and H 2O + Hads + e− → H 2 ↑ +OH−(Heyrovsky)

(6)

4. CONCLUSION Cedarlike copper phosphide nanoarrays are directly grown on copper foam through a topotactic phosphorization method and are used to function as a reliable bifunctional metallic electrode for both OER and HER in alkaline solution for the first time. The very electrochemically active surface area and unique 3D open electrode configuration of Cu3P/CF are responsible for

H 2O + e− → Hads + OH−(Volmer) and Hads + Hads + e− → H 2 ↑ (Tafel)

(7)

wherein both of the above pathways involve similar process: the adsorption of H2O molecules and the following electrochemical dissociation of captured H2O into OH− species and adsorbed H I


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the considerable OER catalytic performance, which is comparable to the state-of-the-art noble-metal catalyst systems (Table S1). The catalytic mechanism of Cu3P nanoarrays is elucidated to involve the in situ formation of active species that are semimetallic Cu3P in the core covered with a thin layer of copper oxide/hydroxide originating from surface oxidation of Cu3P during the OER process. Furthermore, a device consisting of two pieces of Cu3P/CF as both anode and cathode electrodes is successfully constructed for overall water splitting. This finding not only enriches the Janus catalyst family in an “all in one” system but also stimulates further interest in the in situ formation of nonprevious Cu-based electrocatalysts for emerging renewable energy utilization and conversion.

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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/acsami.6b06251. Experimental procedures, synthesis of RuO2, details of electrochemical measurements and preparation of working electrode, and XRD, SEM, XPS, and EDX spectra for the as-prepared materials, etc. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We appreciate the financial support from the National Key Basic Research Program of China (973 Program 2013CB834800 and 2013CB632403) and the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB17030200). This work was partially supported by the Natural Science Foundation of China (21371175) and CASCroucher Funding Scheme for Joint Laboratories. Y.C. acknowledges support from the Chinese Academy of Sciences (100 Talents Program).

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REFERENCES

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