Cu2OâCu Hybrid Foams as High-Performance

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Cu2O-Cu Hybrid Foams as High-Performance Electrocatalysts for Oxygen Evolution Reaction in Alkaline Media Han Xu, Jin-Xian Feng, Yexiang Tong, and Gao-Ren Li ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b02911 • Publication Date (Web): 07 Dec 2016 Downloaded from http://pubs.acs.org on December 7, 2016

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Cu2O-Cu Hybrid Foams as High-Performance Electrocatalysts for Oxygen Evolution Reaction in Alkaline Media Han Xu, Jin-Xian Feng, Ye-Xiang Tong, and Gao-Ren Li* MOE Laboratory of Bioinorganic and Synthetic Chemistry, The Key Lab of Low-carbon Chemistry & Energy Conservation of Guangdong Province, School of Chemistry, Sun Yat-sen University, Guangzhou 510275, China

E-mail: [email protected]

Here 3D Cu2O-Cu hybrid foams are developed as high-performance electrocatalysts for OER in alkaline solution. The hybrid foams are composed of Cu2O-Cu dendrites with high surface area and high-speed electronic transmission networks, and they can provide fast transportation and short diffusion path for electrolyte and evolved O2 bubbles. As the special surface and structure effects, the Cu2O-Cu hybrid foams exhibit low onset overpotential of ~250 mV, small Tafel slope of 67.52 mV dec-1, and high durability over 50 h at a current density of 10 mA cm-2 for OER in alkaline solution. The results of this study may be particularly beneficial for the development of a type of hybrid porous foam electrocatalysts for the electrochemical process in which at least one gas-phase is involved, such as H2 or O2 evolution reaction and O2 or CO2 electroreduction reaction.

Keywords: Electrocatalyst, hybrid foam, Cu2O-Cu dendrite, core-shell structure, oxygen evolution reaction

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Electrochemical water splitting has been well known to be a high-efficient route for the sustainable molecular hydrogen generation, which has been proposed as a renewable energy production, storage and usage in the future.1-5 However, the production of hydrogen in the process of water splitting is severely restricted by the sluggish kinetics of oxygen evolution reaction (OER).6-10 To make the process of water splitting more efficient, the use of powerful electrocatalysts with high catalytic activity and durability for OER is essential.11-15 So far, it is well-known that RuO2 and IrO2 are the best OER electrocatalysts.16-19 However, the high-cost and scarcity of noble metal oxides limit their mass production for energy devices to become widespread around the world. For above reasons, there have motivated intensive efforts to develop the inexpensive and earth-abundant electrocatalysts with high catalytic activity and long-term stability for OER in alkaline media.20-23 Over the last few years, various economic metal oxides, oxyhydroxides and their derivatives, such as MnOx,24 CoxMn3–xO4,25 Co3O4,26 NiCo2O4,27 Ni-Fe hydroxide,28 Co1−xFex(OOH),29 LiNi0.8Al0.2O,30 FeP,31 and Ni3S2,32 have been widely investigated as promising electrocatalysts for OER. However, because of the intrinsic low electrical conductivity, the individual metal oxides, phosphides or sulfides as electrocatalysts have shown the nonideal electron transportation and slow kinetics during OER. Currently, the general approach to improve the electrical conductivity of electrocatalysts is to mix them with high conductive materials such as metal,33 conducting polymer,34 carbon,35,36 or graphene37,38 for catalytic performance enhancement. However, this kind of methods usually lead to out-of-order microstructures and low surface areas, making low availability of electrocatalysts and low-efficient transportation of active species. At present, the design and fabrication of low-cost metal oxide electrocatalysts with high catalytic activity and durability still remain a huge challenge for OER in alkaline media. As we all know, the metal/metal oxide core-shell structure can solve the poor conductivity of metal oxide.39,40 3D porous architectures can provide high specific surface area and accordingly can maximize the availability of catalysts and provide fast transportation of active species, and they have attracted great interest in heterogeneous catalysis in the past few years.41-43 By combining the advantages of core-shell and 3D porous structures, the design of 3D metal/metal oxide hybrid foams with core-shell structures will 2


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be particularly attractive for catalytic applications because of the multiple favorable structures and commpositions to accomplish exceptional properties of electrocatalysts. Cu-based oxides can undergo a variety of reactions due to the wide range of accessible oxidation states of Cu (Cu0, Cu+, Cu2+ and Cu3+), which are endowed with many potential applications, including electrocatalysts,44-46 photocatalysts,47 and lithium-ion battery anode materials.48 Previous experimental studies have shown that Cu-based oxides (e.g., Cu2O) own high electrocatalytic activity for OER.49,50 However, the extensive utilization of Cu-based oxides as electrocatalysts for OER is restricted by its inherent low electrical conductivity and instability. At present, the design and fabrication of Cu-based oxides as electrocatalysts with high catalytic activity and durability still remain a huge challenge for OER. Hybrid foams consisted of Cu2O-Cu dendrites were fabricated in solution of 0.75 M H2SO4 +0.1 M CuSO4 by hydrogen bubble soft template-assisted electrodeposition, and the formation process is shown in Scheme 1. During the electrodeposition, there was vigorous hydrogen gas evolution on the surface of electrode and these hydrogen bubbles played a crucial role as soft-template for the formation of foam structures consisted of dendrites (the vigorous hydrogen evolution on the surface of electrode can be clearly seen in Figure S1). The hydrogen-bubbles, deriving from the cathodic reaction on the surface of copper wire, build a continuous paths from substrate to electrolyte-air interface during the electrodeposition. Where there are hydrogen-bubbles, there will be no Cu ions available. Thus, the Cu was only deposited among the gaps of hydrogen bubbles. At the beginning of electrodeposition, a series of hydrogen-bubbles evolved at different locations on the surface of substrate and Cu foams began to form on the surface of substrate. Then, the size of Cu foams became larger and larger with electrodeposition time increasing, and the pore sizes in Cu foams increased with the distance away from the bottom because of the coalescence of hydrogen bubbles as shown in Figure S2 and Scheme 1. Finally, the outer layers of Cu dendrites were spontaneously converted to Cu2O because of the present of aqueous electrolyte during the electrodeposition, resulting in the formation of hybrid foams consisted of Cu2O-Cu dendrites.51,52 The hybrid Cu2O-Cu foams will bring high surface area and allow fast transportation of active species and easy release of the evolved O2 bubbles for OER. 3


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Figure 1a-c shows the typical SEM images of hybrid Cu2O-Cu foams deposited for 120 s with different magnifications. The uniform porous structure is clearly seen in the low-magnification SEM image as shown in Figure 1a, which shows the sizes of pores are about 20~60 µm. The thickness of Cu2O-Cu foams is ~200 µm as shown in Figure S3. The walls of Cu2O-Cu foams consist of large amounts of dendrites as shown in Figure 1b. The size of dendrite is ~5 µm as shown in Figure 1c. The hybrid Cu2O-Cu foams consisted of abundant pores and dendrites can significantly increase the specific surface area and provide fast transportation of active species. XRD pattern of the Cu2O-Cu foams is shown in Figure 1d. The diffraction peaks at 43º,50º,and 74ºcorrespond to (111), (200), and (220) phases of Cu (JCPDS No. 659026), respectively. The peaks at 36ºand 42ºdemonstrate the existence of Cu2O (JCPDS No. 65-3288). Cu2O-Cu foams were also analyzed by XPS and the results are shown in Figure S4a. The peaks at 932.2 and 952.0 eV correspond to the binding energy of Cu 2p3/2 and Cu 2p1/2, respectively.53 Due to the similar binding energy values of Cu 2p3/2 and Cu 2p1/2, Cu and Cu2O are very difficult to distinguish by XPS spectra of Cu 2p only. X-ray generated Auger Cu LMM spectrum has been utilized for a more accurate assignment for Cu and Cu2O, and Figure S4b shows Cu LMM spectrum of Cu2O-Cu electrode before water oxidation. Here Cu LMM spectrum of Cu2O-Cu electrode shows two peaks at 916.3 and 918.0 eV, which can be assigned to Cu2O and Cu, respectively, indicating the presence of Cu2O and Cu in Cu2O-Cu electrode.54 Composition quantitative analysis of Cu2O-Cu foams was performed on multiphase patterns by the RIR method (The details were described in the experimental section in the Supporting information), and the results showed that the Cu2O-Cu foams were comprised of 95.02 wt% Cu and 4.98 wt% Cu2O. In addition, the content of Cu2O in the Cu2O-Cu foams can be regulate with different electrodepositon time as shown in Table S1 and Figure S5. A typical transmission electron microscopy (TEM) image of Cu2O-Cu dendrite is shown in Figure 2a, which clearly shows that the dendritic structure consists of nanorods with the diameters of ~125 nm and lengths of ~500 nm. Figure 2b shows a typical TEM image with high magnification of the marked area with a red circle in Figure 2a, and it shows a uniform core-shell structure with an ultrathin Cu2O shell of about 5~8 nm in Cu2O-Cu dendrite. The selected area electron diffraction (SAED) pattern was also 4


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measured in the marked area with a red circle in Figure 2a and it was inserted in Figure 2b. A typical ring structure characteristic for polycrystalline materials is clearly seen. The diffraction patterns in SAED could be attributed to (111) plane of Cu and (111) plane of Cu2O, respectively, which are well consistent with XRD results. The high-resolution TEM (HRTEM) image of the marked area with a green circle in Figure 2b is shown in Figure 2c, which displays that the interplanar distances of core part are 2.08 Å and 1.80 Å, corresponding to (111) and (200) planes of Cu, respectively. The interplanar distances of 2.42 Å and 2.10 Å of shell part can be indexed to (111) and (200) planes of Cu2O according to PDF standard card database, but they became smaller than those in the standard card database (2.46 Å and 2.13 Å for (111) and (200) planes of Cu2O, respectively, JCPDS No. 65-3288), indicating that there is a lattice strain in the shell layer. The lattice strain in the Cu2O layer can result in the synergistic effects between the Cu and Cu2O in the Cu2O-Cu foams, this will benefit improvement the electrocatalytic performance of Cu2O-Cu foams for OER.55 The lattice strain existing in Cu2O-Cu foams can be further demonstrated by XRD results as shown in Figure 2d and S6. The positions of Cu2O (111) and (200) diffraction peaks of Cu2O-Cu foams both shift to higher 2θ compared with those of pure Cu2O foams, indicating the compressed lattices of Cu2O in Cu2O-Cu foams. This result is very good agreement with the results of HRTEM in Figure 2c. The synthesized Cu2O-Cu foams allow for convenient investigation of the electrocatalytic activity for OER without additional binders and extra substrates. The electrocatalytic activity of Cu2O-Cu foams is firstly evaluated by liner sweep voltammetry (LSV) using a standard three-electrode device in O2-saturated 1.0 M KOH solution at a scan rate of 5 mV s-1. For comparative study, the similar measurements of Cu foams, Cu2O foams and pristine Cu wire were also performed. Figure 3a shows LSV curves of various samples after iR-compensation. Noticeably, the pristine Cu wire shows very limited OER activity within the potential range of 1.35~1.75 V (vs RHE). The onset potential of oxygen evolution of Cu2O-Cu foams is about 1.48 V with a low overpotential (η0) of only 250 mV, which is much lower than those of Cu foams (356 mV) and Cu2O foams (330 mV) and is very close to that of previously reported IrO2 (~230 mV),56,57 indicating high electrocatalytic activity of Cu2O-Cu foams for OER in alkaline media. In addition, Cu2O-Cu foams only requires an overpotential (η10) of 350 mV to reach a current density of 10 mA cm-2, which is 5


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approximately the current density expected for 10% efficient solar-to-fuel conversion efficiency at the anode.1 The value of η10 at 10 mA cm-2 of Cu2O-Cu foams is 60 and 37 mV less than those of Cu foams and Cu2O foams, respectively. Such excellent OER activity of the Cu2O-Cu foams is superior to those of previous reported Cu-based materials and most earth-abundant transition-metal oxides or their derivative OER electrocatalysts (please see Table S2 in the Supporting information for details). In addition, we also studied the effect of electrodeposition time of Cu2O-Cu foams on electrocatalytic activity of OER. The LSVs and Tafel plots of Cu2O-Cu foams with different electrodeposition time are shown in Figure S8, and the summaries of η0 for onset potential, η10 at 10 mA cm-2 and Tafel plots are shown in Table S3. The results show that the Cu2O-Cu foams with electrodeposition time of 120 s exhibit the highest activity and owns the lowest η0, lowest η10 and smallest Tafel slope than those of Cu2O-Cu foams with electrodeposition time of 60 s, 90 s and 150 s, respectively. Long-term stability of the electrocatalyst is another critical index in the practical application. The chronopotentiometry was utilized to evaluate the electrocatalytic durability of Cu2O-Cu foams for OER in the solution of O2-saturated 1.0 M KOH. Cu foams and Cu2O foams were also tested under the same conditions as the references. As shown in Figure 3b, the overpotential of Cu2O-Cu foams almost remains unchanged at the current density of 10 mA cm-2 for 50 h, indicating that the Cu2O-Cu foams own excellent stability for OER in alkaline media, whereas the overpotentials of Cu foams and Cu 2O foams increase distinctly all the way. Therefore, the Cu2O-Cu foams exhibit a significantly enhanced stability compared with Cu foams and Cu2O foams. After the electrochemical testing at 10 mA cm-2 for 50 h, the structure of Cu2O-Cu foams was analyzed by SEM and XRD (Figure S9a-b). The surface morphology of Cu2O-Cu foams was maintained very well even after constant reaction over 50 h as shown in Figure S9a, indicating high structural stability of Cu2O-Cu foams. In addition, a small quantity of CuO (about 2.28 wt%) was identified in the Cu2O-Cu foams as shown in Figure S9b. The formation of CuO may be due to the surface oxidation of Cu2O-Cu foams during the testing process. XPS spectrum of Cu 2p of Cu2O-Cu electrode after the electrochemical testing is shown in Figure S10a. Compared to that of Cu2O-Cu electrode before the electrochemical testing, there is a broad Cu 2p3/2 peak and a satellite peak at ~943.0 eV for Cu2O-Cu 6


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electrode after the electrochemical testing, indicating the generation of CuO on the surface of Cu2O-Cu electrode after the electrochemical testing.53 In addition, the broad Cu 2p3/2 peak shown in Figure S10a could be deconvoluted into two peaks. The peak at 933.8 eV corresponds to CuO on the surface of Cu 2OCu electrode, and the peak at 932.7 eV can be assigned to Cu2O and Cu of Cu2O-Cu electrode. In addition, the existences of CuO, Cu2O and Cu in Cu2O-Cu electrode after the electrochemical testing can be verified by Cu LMM spectrum of Cu2O-Cu electrode as shown in Figure S10b. The peak at 917.6 eV can be assigned to CuO, and the peaks at 916.3 and 918.0 eV can be assigned to Cu2O and Cu, respectively.54 To eliminate effect of generated CuO on the catalytic property for OER, the electrocatalytic activity of single CuO foams is slao evaluated by LSV (Figure S9c-d), the single CuO foams exhibit higher overpotential to reach a current density of 10 mA cm-2 and bigger Tafel slope compared to those of Cu2OCu foams, indicating very limited OER activity of single CuO foam. To exclude the further oxidation of Cu2O-Cu foams during the OER, we have analyzed the cyclic voltammograms (CVs) of Cu2O-Cu foams for 20 cycles in 1.0 M KOH between -0.4 and 1.0 V (vs SCE) at a scan rate of 50 mV s-1, as shown in Figure S11. For the first cycle, a anodic peak was seen at -0.07 V (vs SCE) in the forward scan, which can be attributed to the oxidation of Cu+ to Cu2+,58 indicating the formation of CuO on the surface of Cu2OCu foams. However, the anodic peak in the forward scan disappear with increasing cycle number, indicating that the generation of CuO on the surface of Cu2O-Cu foams can effectively suppress further oxidation of Cu2O-Cu foams during the OER. The high OER electrocatalytic performance can be ascribed to the special surface effects and synergistic effects of Cu2O-Cu foams. Firstly, the 3D foam configuration provides more electrocatalytic sites and results in larger active surface area, which can be confirmed by the corresponding double-layer capacitance (Cdl) that represents electrochemically active surface area (ECSA).59-61 CVs of Cu2O-Cu foams at the different scan rates from 2~80 mV s-1 in the potential rang of 1.315~1.415 V (vs RHE) and the corresponding capacitive currents at 1.365 V (vs RHE) as a function of scan rate are shown in Figure 4a and 4b, respectively. The Cdl of Cu2O-Cu foams is caculated to be 193.9 mF cm-2, which is much larger than those of Cu foams (120.7 mF cm-2) and Cu2O foams (169.4 mF cm-2) (shown in Figure S12) and those reported 7


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for Co3O4 nanosheets(94.97 mFcm-2),26 Mo2C NPs@carbon composites (22.30 mF cm-2),62 g-C3N4/Ti3C2 nanosheets (29.70 mF cm-2),63 and CoS2 nanowires (21.50 mF cm-2).64 Therefore, the Cu2O-Cu foams own much enhanced ECSA compared with the above various electrocatalysts. Secondly, 3D Cu2O-Cu foams can provide short diffussion route and fast transportation for active species and also enable facile release of the evolved O2 gas bubbles. So the hybrid foams consisted of Cu2O-Cu dendrites can further improve the reaction kinetics for OER. Here the kinetics of electrocatalytic reaction of OER is evaluated by the corresponding Tafel plots in Figure 4c (η=a+blogj, where b is Tafel slope, j is current density). The Tafel plot of Cu2O-Cu foams is 67.52 mV dec-1, which is smaller than those of Cu2O foams (77.25 mV dec-1) and Cu foams (80.84 mV dec-1) and is even smaller than that of the reported IrO2 (71 mV dec-1).56,57 The smaller Tafel slope of Cu2O-Cu foams indicates more favorable reaction kinetics for OER. Thirdly, the Cu2O-Cu foams with Cu cores as electrocatalysts own high conductivity, which will provide the high-speed paths for electron transportation. The high conductivity can be demonstrated by the electrochemical impedance spectroscopy (EIS) as shown in Figure 4d and Figure S13, which show the plots of various samples all are composed of two semicircles. An equivalent circuit composed of 2RC elements as shown in Figure S14 was utilized to explain the acquired results. According to the previous reports in literature, the high-frequency response can be assigned to the intrinsic resistance of catalysts (Rc), and the lowfrequency response can be assigned to charge-transfer process of oxygen evolution at the catalyst interface (Rct).65,66 The Rct of Cu2O-Cu foams only is ~2.06 Ω, which is much smaller than those of Cu foams (~7.46 Ω) and Cu2O foams (~19.98 Ω), and the low-resistance will be favorable for OER kinetics on Cu2O-Cu foams. So the combination of the above different advantages of hybrid Cu2O-Cu foams obviously lowered the overpotential and Tafel slope and markedly enhanced electrocatalytic activity and durability as shown in Figure 3. In summary,we have developed the 3D hybrid foams consisted of Cu2O-Cu dendrites with high surface area for OER in alkaline solution. The hybrid Cu2O-Cu foams exhibited superior electrocatalytic activity, robust stability and favorable reaction kinetics for OER in alkaline media. The superior electrocatalytic performance could be attributed to unique 3D porous foams that could provide fast transportation and 8


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short diffusion paths for electrolyte and easy release of the evolved O2 bubbles and the core-shell dendrites could provide high speed electronic transmission networks and synergistic effects for electrocatalytic reactions. The above results not only indicate the potential of low-cost metal oxide as high catalytic activity, high durable and eco-friendly electrocatalysts for OER in alkaline media, but also will pave the way to the development of even more effective OER electrocatalysts through the design of other hybrid foams composed of core-shell dendrites.

Supporting Information The supporting Information is available free of charge on the ACS Publications website. Experimental section, Optical photographs, SEM images, XPS spectra, XRD patterns, Cyclic voltammograms, EIS Nyquist plots, and Table S1-S3.

Acknowledgements This work was supported by National Basic Research Program of China (2016YFA0202603 and 2015CB932304), Natural Science Foundation of Guangdong Province (S2013020012833 and 2016A010104004), and Fundamental Research Fund for the Central Universities (16lgjc67).

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Scheme 1. Schematic illustration for the fabrication of hybrid Cu2O-Cu foams.

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Figure 1. (a-c) SEM images of hybrid Cu2O-Cu foams with different magnifications; (d) XRD pattern of hybrid Cu2O-Cu foams.

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Figure 2. (a) TEM image of a typical Cu2O-Cu dendrite; (b) HRTEM image and SAED pattern (inset) of the area marked in red circle in (a); (c) HRTEM image with higher magnification of the area marked in green circle in (b); (d) XRD patterns of Cu2O-Cu foams and Cu2O foams.

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Figure 3. (a) LSV curves of Cu2O-Cu foams, Cu foams, Cu2O foams and Cu wire in deaerated 1.0 M KOH solution at a scan rate of 5 mV s-1; (b) Chronopotentiometric measurements of long-term stability of Cu2O-Cu foams, Cu foams and Cu2O foams at 10 mA cm-2.

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Figure 4. (a) CVs of hybrid Cu2O-Cu foams at the different scan rates from 2~200 mV s-1 in the potential rang of 1.30-1.40 V vs RHE; (b) Capacitive current at 1.35 V vs RHE as a function of scan rate for Cu2O-Cu foams; (c) Tafel plots of Cu2O-Cu foams, Cu foams and Cu2O foams; (d) EIS Nyquist plots of Cu2O-Cu foams, Cu foams and Cu2O foams at the open circuit voltage (the inset in (d) corresponds to the Nyquist plots at highfrequency.

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Cu2OâCu Hybrid Foams as High-Performance

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Cu2OâCu Hybrid Foams as High-Performance