Hierarchically Structured Cu-Based Electrocatalysts with Nanowires

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Hierarchically Structured Cu-Based Electrocatalysts With Nanowires Array for Water Splitting Cui Lu, Jianying Wang, Steffen Czioska, Huan Dong, and Zuofeng Chen J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b08365 • Publication Date (Web): 31 Oct 2017 Downloaded from http://pubs.acs.org on October 31, 2017

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The Journal of Physical Chemistry

Hierarchically Structured Cu-based Electrocatalysts with Nanowires Array for Water Splitting Cui Lu, Jianying Wang, Steffen Czioska, Huan Dong, and Zuofeng Chen*

Shanghai Key Lab of Chemical Assessment and Sustainability, School of Chemical Science and Engineering, Tongji University, Shanghai 200092, China

ABSTRACT We report here the fabrication of CuO nanowires and their use as efficient electrocatalyst for the oxygen evolution reaction (OER) or as precursor for preparation of Cu3P nanowires for the hydrogen evolution reaction (HER). The surface-bound Cu(OH)2 nanowires are in situ grown on a three-dimensional copper foam (CF) by anodic treatment, which are then converted to CuO nanowires by calcination in air. The direct growth of nanowires from the underlying conductive substrate can eliminate the use of any conductive agents and binders, which ensures good electrical contact between the electrocatalyst and the conductive substrate. The hierarchically nanostructured Cu-based electrode exhibits excellent catalytic performance toward OER in 1 M KOH solution. Phosphorization of the CuO/CF electrode generates the Cu3P/CF electrode, which can act as an excellent electrocatalyst for HER in 1 M KOH. An alkaline electrolyzer is constructed using CuO and Cu3P nanowires coated copper foams as anode and cathode, which can realize overall water splitting with a current density of 102 mA/cm2 at an applied cell voltage of 2.2 V.

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INTRODUCTION The shortage of fossil fuel reserves coupled with the increasing environmental polution has called for development of renewable and more environmentally friendly energy sources. In view of this situation, the abundance of solar radiation reaching the earth surface has prompted tremendous efforts to utilize this energy. One promising approach is using solar energy to produce hydrogen gas via water splitting.1-3 Water splitting process contains two half-reactions, i.e., the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER).4 The main hindrance to realize effective water splitting is the sluggish OER due to its demanded four-proton and four-electon transfer proceeding to form the O-O double bond from two water molecules.5,6 To overcome this obstacle, researchers are making every effort to develop efficient and robust catalysts for OER under relatively benign conditions.7 On the other side, although precious metals, such as Pt and its alloys display remarkable HER efficiency, their low natural abundance and high cost have largely obstructed their widespread applications. Over the past decade, significant advances have been achieved in developing efficient and robust OER catalysts based on the first-row transition metals such as Fe8-10, Co11-13 and Ni14-18. Recently, their phosphide compounds have also been intensively investigated as new earthabundant electrocatalysts that can potentially replace Pt to catalyze HER.19-27 As an earthabundant and biologically relevant transition metal, Cu-based materials were relatively much


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less explored for both OER and HER.28-40 To boost the performance of Cu-based OER or HER electrocatalysts for practical applications, it is desirable to construct hierarchically nanostructured materials with large specific surface areas by facile synthesis routes. Herein, we report the three-dimensional (3D) Cu-based electrocatalysts for both OER and HER, which can simplify the fabrication procedure of electrolyzers, substantially lower the production cost and improve the catalytic performance. The fabrication process and the utilization of the electrode materials for the overall water splitting are illustrated in Scheme 1. The catalyst electrode benefits from the hierarchical structure of nanowires and the direct growth of nanowires from the underlying conductive substrate. The direct growth strategy can eliminate the use of any conductive binders and agents, which ensures good electrical contact between the electrocatalyst and the conductive substrate. The CuO/CF electrode exhibits impressive OER performance with a Tafel slope of 44 mV·dec–1 in 1 M KOH and an overpotential (η) of 430 mV to reach a current density of ~ 100 mA·cm–2. On the basis of the hierarchically structured CuO/CF electrode, we further developed a convenient and straightforward method to fabricate the copper phosphide (i.e. Cu3P/CF) electrode for HER, which was realized by a simple one-step phosphorization from the CuO/CF electrode. The electrode material maintains the nanowire morphology after phosphorization and exhibits excellent catalytic performance toward the HER in alkaline solution. With both CuO/CF and Cu3P/CF electrodes available, an alkaline electrolyzer was assembled for overall


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water splitting which could deliver a current density of 102 mA/cm2 at an applied cell voltage of only 2.2 V.

Scheme 1. Illustration of the fabrication of Cu-based nanowires on the copper foam (CF) surface for OER and HER.

EXPERIMENTAL Chemicals. Copper foam (thickness ~ 1.6 mm) was purchased from Suzhou Taili company. Copper foil was purchased from Alfa Aesar (42189, 99.999% purity). Potassium hydroxide (KOH, AR) was obtained from Macklin. Sodium hypophosphite (NaH2PO2, AR) was purchased from Aladdin Ltd. (Shanghai, China). All other chemical reagents were of analytical grade and used as received without further purification. All electrolyte solutions were prepared with deionized water (18 MΩ·cm) unless stated otherwise.


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Apparatus. Scanning electron microscope (SEM) images and energy dispersive X-ray analysis (EDX) data were obtained at Hitachi S-4800 (Hitachi, Japan) equipped with a Horiba EDX system (X-max, silicon drift X-Ray detector). SEM images were obtained with an acceleration voltage of 3 kV and EDX spectra were obtained with an acceleration voltage of 15 kV. Transmission electron microscopy (TEM) images, high resolution TEM (HRTEM) images and selected area electron diffraction (SAED) image were obtained using Tecnai G2 F20 STwin. The Cu3P/CF electrode was rinsed gently with deionized water and dried in air. The Cu3P nanowires were removed from the Cu3P/CF electrode substrate by sonication in the absolute ethanol, and a drop of the mixture was dried on a the micro grid copper network for analysis. Powder X-ray diffraction (XRD) was measured by Bruker Foucs D8 via ceramic monochromatized Cu Kα radiation of 1.54178 Å, operating at 40 kV and 40 mA. The scanning rate was 5° per min in 2θ and the scanning range was from 10 - 80°. X-ray photoelectron spectroscopy (XPS) for elemental analysis was conducted on a Kratos Axis Ultra DLD X-ray Photoelectron Spectrometer using 60 W monochromated Mg Kα radiation as the X-ray source for excitation. The carbon 1s peak (284.6 eV) was used for internal calibration. The peak resolution and fitting were processed with the XPS Peak 41 software. Electrochemical measurements were performed on a CHI 660E electrochemical workstation (Chenhua Corp., Shanghai, China). The three-electrode system consisted of a working electrode, a platinum wire counter electrode, and a saturated calomel reference electrode (SCE, 0.244 vs. NHE).


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Procedures. Prior to electrochemical measurements, the copper foam was first sonicated in dilute HCl solution for 15 min and then washed with water for several times to remove the surface oxides or pollutants. The electrodes were covered by scotch tape to define a projected geometric surface area of 0.4 cm2 for electrochemical measurements. Unless stated otherwise, all potentials in linear sweep voltammetry (LSV) and controlled potential electrolysis (CPE) were reported vs. RHE without iR compensation. To construct a Tafel plot, the current-potential data were obtained at working electrode by LSV at a very slow scan rate (0.1 mV/s) in 1 M KOH. The solution resistance measured prior to the data collection (using iR test function) was used to correct the Tafel plot for iR drop. All experiments were performed at 20 ± 2 °C. Preparation of the Cu(OH)2 nanowires and CuO nanowires electrodes. The Cu(OH)2 nanowires array on the copper foam (Cu(OH)2/CF) were prepared through anodization of CF according to an earlier report.41 In short, the anodization was performed by applying a constant current density of 20 mA/cm2 on CF for 20 min in 2 M KOH solution, during which a faintblue film was formed on the CF surface. The anodized CF was then washed with distilled water and the Cu(OH)2/CF electrode was obtained. The CuO nanowires array on the copper foam (CuO/CF) was prepared by annealing Cu(OH)2 nanowires. The Cu(OH)2/CF electrode was placed in a furnace oven and heat-treated under air atmosphere at 150 °C for 3 h and 200 °C for 3 h with a ramping rate of 5 °C/min. After being


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naturally cooled down to room temperature, the color of the electrode was changed to black and the CuO/CF electrode was obtained. Preparation of the Cu3P nanowires electrode. The Cu3P nanowires (Cu3P/CF) electrode was prepared according to the following procedure39,40: NaH2PO2 (0.5 g) was placed at the center of the tube furnace and the as-prepared CuO/CF electrode was hung at the downstream side of the furnace. After being flushed with Ar, the temperature of the furnace was elevated to 300 °C with a ramping rate of 2 °C/min and held at this temperature for 60 min. Finally, after being naturally cooled down to room temperature under Ar stream, the Cu3P/CF electrode was obtained. Calculation of ECSA. The electrochemically active surface area (ECSA) and roughness factor (RF) of the electrodes were estimated from the electrochemical double-layer capacitance (CDL) in 1 M KOH solution according to the literature.42 Briefly, a potential range where no apparent Faradaic process occurred was determined firstly using cyclic voltammetry (CV). The charging current (ic) in this potential range was then measured from CVs at different scan rates (ν). The relation between ic, CDL and ν was governed by eq 1, and CDL can be obtained from the slope of the plot of ic versus ν. The ECSA, which is directly proportional to the capacitance, can be evaluated by dividing CDL by CS (eq 2), where CS (= 0.04 mF·cm−2) is the specific capacitance of the same material with an atomically smooth planar surface per unit area under identical electrolyte conditions.43 The value of RF was calculated by dividing the estimated ECSA by the geometric area of the electrode (S) (eq 3).


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ic = ν CDL

(1)

ECSA = CDL / CS

(2)

RF = ECSA / S

(3)

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RESULTS AND DISCUSSION The Cu(OH)2 nanowires array on the copper foam (Cu(OH)2/CF) was prepared through anodization of CF.41 During this process, Cu was electro-oxidized to Cu2+, which combines with OH‒ in the electrolyte solution to form Cu(OH)2 NWs. The corresponding CuO nanowires electrode (CuO/CF) was obtained by further annealing Cu(OH)2/CF in air. The SEM images clearly show that the smooth surface of CF substrate is uniformly covered by a dense layer of nanowires by anodization (Figure 1A-C). The Cu(OH)2/CF and CuO/CF electrodes show no difference in SEM images, while the color of the electrode changes from blue to black after annealing (Figure 1D). The EDX analysis (Figure S1) shows that CF consists of only Cu element, and Cu(OH)2/CF and CuO/CF are both composed of Cu and O elements. The XRD pattern (Figure 1E) of CF shows peaks at 43.3°, 50.4° and 74.1°, corresponding to the (111), (200) and (220) reflections of the metallic copper (JCPDS No. 04-0836). In the XRD pattern of Cu(OH)2/CF, the peaks at approximately 16.7°, 23.8°, 34.1°, 39.8° and 53.2° are distinguishable from the strong background diffraction of the copper substrate, corresponding to the (020), (021), (002), (130) and (150) reflections of the orthorhombic Cu(OH)2 (JCPDS No. 13-0420), respectively.44,45 No other impurity peaks for CuO and Cu2O are detected in this


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sample. By annealing, the XRD pattern of CuO/CF displays two reflection peaks at 35.6° and 38.8°, which can be indexed to the (-111) and (111) planes of the monoclinic CuO (JCPDS No. 72-0629).45 No impurity peaks of Cu(OH)2 and Cu2O were observed, indicating the complete conversion of Cu(OH)2 nanowires to CuO nanowires. XPS was also recorded to further characterize the compositional changes during the fabrication process. The elements detected by survey XPS are consistent with those by EDX (Figures S2-S4). The high-resolution XPS of Cu(OH)2/CF shows peaks at 934.2 eV and 953.9 eV due to Cu 2p3/2 and Cu 2p1/2 core levels, respectively, which are accompanied with obvious shakeup satellite peaks (Figure 1F).46,47 The Cu 2p XPS of CuO/CF is similar with exception of peaks shifted toward lower binding energies by 0.8 - 0.9 eV.48 In addition, the Cu 2p XPS of CuO/CF after long-term water oxidation electrolysis (see below) is nearly unchanged.


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Figure 1. SEM images of (A) CF, (B) Cu(OH)2/CF, and (C) CuO/CF; (D) Photographs of CF (left), Cu(OH)2/CF (middle), and CuO/CF (right); (E) XRD patterns of CF, Cu(OH)2/CF, and CuO/CF; (F) The high-resolution Cu 2p XPS spectra of Cu(OH)2/CF, CuO/CF, and CuO/CF after CPE for 10 h.


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The catalytic performance of the electrodes toward OER was investigated in 1 M KOH solution at room temperature. The current density is normalized based on the projected geometric area of the electrode. From the LSV curves (Figure 2A) and the extracted values (Table 1), the catalytic activity follows the order CuO/CF > Cu(OH)2/CF > CF > copper foil under the identical experimental conditions. As the optimal electrode material, CuO/CF reaches a current density of 100 mA·cm–2 at an overpotential of only 430 mV. For evaluation of long-term performance and stability of the electrodes, CPE experiments were conducted in 1 M KOH at 1.8 V (η ≈ 570 mV) for 10 h (Figure 2B). In contrast to the other three electrodes, CuO/CF shows a considerably higher steady-state current density of ∼ 180 mA/cm2, which was sustained for at least 10 h. The SEM image (Figure S5) of CuO/CF after CPE reveals nearly no change in the structure and morphology. It is noted that the electrolysis with Cu(OH)2/CF suffers from continuous decrease in current density during the initial 2 h of CPE, which is finally stabilized at a current density of only 50 mA/cm2. The SEM images taken after electrolysis for 0.5, 1, 2 and 10 h (Figures S6-S8) revealed that Cu(OH)2 nanowires on the CF substrate peeled off gradually during electrolysis. After 2 h CPE, the Cu(OH)2 nanowires peeled off completely from the CF substrate. In consistency with the detachment of the O-rich Cu(OH)2 nanowires, the EDX spectra (Figure S9) show that the content of O element is decreased greatly during electrolysis. These results indicate that the annealing step is essential for preparation of the Cu-based nanowires electrode for OER, which


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not only converts the nanowires to catalytically more active species (evidenced by transient CV test), but also strengthens the stability of the nanowires coating (evidenced by CPE test). To further investigate the catalytic kinetics of OER, the Tafel plot was constructed by LSV at a very slow scan rate of 0.1 mV/s (Figure 2C). In the Tafel plot, the steady-state current density (j) was evaluated as a function of overpotential (η), η = Vappl – iR – EpH, where Vappl is the applied potential, i is the steady-state current, R is the uncompensated resistance, and EpH is the thermodynamic potential for water oxidation at this pH (EpH = 1.23 – 0.059 × pH). The Tafel slopes were fitted to be approximately 44, 89, 159 and 125 mV/dec for CuO/CF, Cu(OH)2/CF, CF and copper foil, respectively (Table 1). A small Tafel slope is representative of wellbalanced kinetics over all steps of the entire conversion chain. These results imply that the CuO/CF electrode possesses a much faster reaction kinetics compared to Cu(OH)2/CF, CF and copper foil. The small Tafel slope combining the low overpotentials, makes CuO/CF among the best Cu-based electrocatalysts for OER (Table S1). The high catalytic performance could be first attributed to the significantly increased ECSA of the 3D porous CF substrate. The ECSA of the samples were approximately evaluated by measuring the electrochemical double-layer capacitance (Figure 2D, S10-S12), assuming that the current measured in the non-Faradaic potential region is totally due to the double-layer charging. The ECSA and RF of the investigated electrodes were calculated by eqs 1-3 above and the values are listed in Table 1. Obviously, CuO/CF has significantly larger ECSA and RF in comparison with the other three electrodes. In addition, the direct growth of nanowires from


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the underlying conductive substrate can eliminate the use of any conductive agents and binders, which ensures good electrical contact between the electrocatalyst and the conductive substrate. Finally, the 1D nanowires grown on CF substrate could provide straight paths for electron transfer, which is also beneficial for the catalytic kinetics. All these factors are advantageous to the improvement of catalyst performance.

Figure 2. (A) LSV plots at 20 mV/s, (B) CPE curves at 1.8 V, and (C) Tafel plots of copper foil (pink), CF (black), Cu(OH)2/CF (blue), and CuO/CF (red) electrodes. (D) The anodic charging currents of the CuO/CF electrode measured at 0.73 V plotted as a function of scan rates. All electrolyte solutions are 1 M KOH.

Table 1. Comparison of the OER activity of different Cu-based electrodes.


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η @ 100 mA·cm–2 (mV)

Electrodes

ηonset (mV)

Copper foil

550

750

CF

470

Cu(OH)2/CF CuO/CF

Tafel slope (mV·dec–1)

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ECSA (cm2)

RF

125

0.49

2.45

660

159

2.5

12.5

400

568

89

23.3

116.5

320

430

44

31

155

Recently, the transition metal (such as Fe, Co and Ni) phosphides were found to be active for HER. In this study, the copper phosphide coated on the surface of copper foam was facilely fabricated via a direct phosphorization process of CuO/CF. In Figure 3A, the SEM image shows that the morphology of the electrode after phosphorization appears nearly unchanged with the exception that the surface of nanowires becomes rougher. Figure 3B shows TEM image of the material scraped from Cu3P/CF. In consistency with SEM image, the Cu3P nanowire consists of subunits of interconnected nanoparticles, which constitute a rough morphology. The nanowires array of rough surface can supply sufficient electrocatalytic active sites, which are crucial to an improved HER performance. The SAED pattern in Figure 3B inset shows several bright rings that consist of discrete spots, indicating the polycrystalline nature of this material. The HRTEM image of Cu3P in Figure 3C shows the well-resolved lattice fringes with an interplanar distance of 0.20 nm, corresponding to the (300) plane of Cu3P.[39]


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The EDX analysis in Figure 3D shows that Cu3P/CF is composed of Cu, O and P elements at a stoichiometric ratio of ∼3:1 Cu:P. In Figure 3E, the XRD pattern of Cu3P/CF shows diffraction peaks characteristic of Cu and Cu3P. The strong peaks at 43.3°, 50.4° and 74.1° originate from the the CF substrate as discussed above. Other peaks at around 36.0°, 39.1°, 41.6°, 45.1°, 46.2°, 47.3°, 53.5°, 59.0°, 66.5°, 73.4° and 78.3°, correspond to the (112), (202), (211), (300), (113), (212), (104), (222), (214), (322) and (314) reflections of Cu3P (JCPDS, No. 71-2261).49 The survey XPS spectrum shows additional P signal after phosphorization (Figure S13). In Figure 3F, the high-resolution P 2p XPS spectrum displays two broad peaks at 129.5 eV and 133.3 eV. The binding energy of 129.5 eV (P 2p) is typical of the Cu3P and the other peak at 133.3 eV (P 2p) is assigned to the oxidized phosphorus species (P-O).50 In Figure 3G, the binding energy of Cu 2p3/2 located at 932.8 eV and Cu 2p1/2 at 952.5 eV are assigned to Cu3P. Both effective phosphorization of the Cu(OH)2 precursor and preservation of the structure of the nanowires array are key for the high HER activity. It is known that phosphorization at temperatures lower than the optimized one will result in unsuccessful or incomplete conversion of CuO to Cu3P.39 On the contrary, at high temperatures Cu3P nanowires start to undergo coalescence or even change into porous film of microparticles.39 All above characterization data furnish the evidence of complete formation of Cu3P with the structure and morphology of nanowires wellretained at the processing temperature.


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Figure 3. (A) SEM images, (B) TEM image, (C) HRTEM image, (D) EDX spectrum, (E) XRD pattern, and (F, G) the high-resolution P 2p and Cu 2p XPS spectra of Cu3P/CF. The inset in (B) is the SAED pattern. The electrocatalytic HER activity of Cu3P/CF was also investigated in 1 M KOH solution at room temperature (Figure 4A). The LSV of a blank CF shows a poor HER


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activity with an overpotential (η) of 447 mV to reach a current density of 20 mA/cm2 at 2 mV/s. In obvious contrast, Cu3P /CF exhibits an overpotential as low as 210 mV to achieve the same current density. Tafel slopes of CF and Cu3P/CF fitted from slow-scanrate LSV are approximately 199 and 127 mV/dec, respectively (Figure S14). At a static overpotential of 300 mV, CPE of Cu3P/CF in 1 M KOH solution reached a steady-state current density of ~ 46 mA/cm2, which was sustained for at least 10 h (Figure 4B). This finding indicates excellent durability of the Cu3P/CF electrode toward HER in alkaline solution. (Table S2)

Figure 4. (A) LSV plots at 2 mV/s and (B) CPE curves at –0.3 V of the CF (black) and Cu3P/CF (red) electrodes. All electrolyte solutions are 1 M KOH.

Finally, an electrochemical cell consisting of CuO/CF as anode and Cu3P/CF as cathode electrode (CuO/CF-Cu3P/CF) was constructed to probe the catalytic performance of these catalysts for overall water splitting. From the LSV of the CuO/CFCu3P/CF electrodes in 1 M KOH, we could easily observe that CuO/CF-Cu3P/CF shows


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an indisputable advantage over the bare CF system (CF-CF) under the identical conditions (Figure 5A). We can see the obvious bubbles forming on the CuO/CFCu3P/CF electrodes (Figure 5A inset). The CPE experiment in 1 M KOH solution at a controlled potential of 2.2 V reveals that the overall water splitting of CuO/CF-Cu3P/CF is sustained with a current density of 102 mA/cm2 (Figure 5B).

Figure 5. (A) LSV (5 mV s–1) of overall water splitting using the as-prepared CuO/CF as anode and Cu3P/CF as cathode. For comparison, LSV with CF as both anode and cathode is also present. Inset is the photo of the two-electrode electrolyzer. (B) CPE of the two-electrode electrolyzer at an applied potential of 2.2 V. All electrolyte solutions are 1 M KOH.

In summary, we demonstrate the fabrication of hierarchically structured nanowires electrocatalysts based on copper foam and their use for efficient electrocatalytic water splitting. Cu(OH)2 nanowires were in situ grown on copper foam by anodic treatment, which were readily converted into CuO nanowires by a calcination procedure in air. Oxygen evolution


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initiates at a low overpotential, and reaches a current density of 100 mA cm−2 at an overpotential of 430 mV. The enormous electrochemically active surface area, the unique hierarchical porous structure, and the high conductivity of electrocatalyst due to the in situ growth strategy are the main reasons for the impressive OER performance. The CuO/CF could be furhter conveted to Cu3P/CF by phosphorization, which showed high performance for HER in alkaline solution. An alkaline electrolyzer combining both electrode materials was thus constructed to realize efficient overall water splitting. This work opens up a new avenue to fabricate Cu-based nanomaterial electrocatalysts for water splitting, which not only enriches the list of Cu-based nanomaterial, but also provides a new strategy to simplify the fabrication procedure of electrolyzers for water splitting.

ASSOCIATED CONTENT Supporting Information. Additional information as noted in the text.

AUTHOR INFORMATION Corresponding Author [email protected] (Z.-F. C.)

Notes The authors declare no competing financial interest.


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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21573160, 21405114), The Recruitment Program of Global Youth Experts by China, and Science & Technology Commission of Shanghai Municipality (14DZ2261100).

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TOC Graphic


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Hierarchically Structured Cu-Based Electrocatalysts with Nanowires

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