O- functionalized oxygen-deficient Co3O4 nanorods as high performance supercapacitor electrodes and electrocatalysts towards water splitting

Author’s Accepted Manuscript O22-/O- Functionalized Oxygen-deficient Co3O4 Nanorods as High Performance Supercapacitor Electrodes and Electrocatalysts towards Water Splitting Guanhua Cheng, Tianyi Kou, Jie Zhang, Conghui Si, Hui Gao, Zhonghua Zhang

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S2211-2855(17)30322-1 http://dx.doi.org/10.1016/j.nanoen.2017.05.043 NANOEN1984

To appear in: Nano Energy Received date: 8 April 2017 Revised date: 17 May 2017 Accepted date: 21 May 2017 Cite this article as: Guanhua Cheng, Tianyi Kou, Jie Zhang, Conghui Si, Hui Gao and Zhonghua Zhang, O22-/O- Functionalized Oxygen-deficient Co3O4 Nanorods as High Performance Supercapacitor Electrodes and Electrocatalysts towards Water Splitting, Nano Energy, http://dx.doi.org/10.1016/j.nanoen.2017.05.043 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

O22-/O- Functionalized Oxygen-deficient Co3O4 Nanorods as High Performance Supercapacitor Electrodes and Electrocatalysts towards Water Splitting Guanhua Chenga,c, Tianyi Koub,c, Jie Zhanga, Conghui Sia, Hui Gaoa, Zhonghua Zhanga* a

Key Laboratory for Liquid-Solid Structural Evolution and Processing of Materials (Ministry

of Education), School of Materials Science and Engineering, Shandong University, Jingshi Road 17923, Jinan, 250061, P.R. China b

Department of Chemistry and Biochemistry, University of California, Santa Cruz, California

95064, United States c

The authors contribute equally to this work.

*Corresponding author. Email: [email protected] (Z. Zhang)

Abstract Owing to high theoretical specific capacitance of 3560 F g-1 and intrinsic activity towards oxygen evolution reaction (OER), inexpensive Co3O4 is drawing much attention as either a promising pseudocapacitive electrode or OER catalyst. However, restricted to poor conductivity and lack of active sites, Co3O4 usually exhibits limited experimental capacitance and OER activity, barely satisfying high energy density delivering of supercapacitors and low energy input of water-splitting systems. Herein, we report O22-/O- functionalized oxygendeficient Co3O4 nanorods for supercapacitor and water splitting dual applications. The CoC2O4∙2H2O converted oxygen-deficient Co3O4 nanorods show enhanced electrical conductivity as confirmed by the increased donor density. The increased number of Co2+ sites (oxygen vacancies) and CoOOH are believed to contribute to the improvement in faradaic reactions and OER activity. Additionally, surface functionalization by O22-/O- is realized in oxygen-deficient Co3O4 nanorods. On the basis of these merits, the as-synthesized Co3O4 1

nanorods demonstrate a significantly high specific capacitance of 739 F g-1 and an ultralow overpotential of 275 mV at 10 mA cm-2 for OER with ultralong stability of over 300 h (@ 100 mA cm-2). Specifically, an electrolyzer for overall water splitting can be driven by asymmetric supercapacitors with the optimized cobalt oxide as both electrocatalyst and electrode material. Graphical abstract

Keywords: cobalt oxide, supercapacitors, oxygen evolution reaction, water splitting, oxygen-deficiency

Introduction Increasing energy demands and environment pollution have stimulated intense research on developing renewable, sustainable and clean energy sources, as well as new technologies associated energy storage and conversion systems. Hydrogen, with high mass-specific energy density, is considered as one of the most promising substitutes for fossil fuels [1, 2]. The most efficient way to generate hydrogen is water splitting driven by electricity or solar energy [3, 4]. Oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) are the two half reactions of water splitting, both of which are vital for the overall efficiency of water splitting 2

[5]. Especially, OER is the kinetically determining step and requires high overpotentials to drive the four-electron oxidation process [6]. Although the catalysts based on noble metals, such as Ru and Pt, demonstrate favorable water splitting performances, the limited resources and high cost hinder their practical applications [7, 8]. Therefore, the discovery of effective and low-cost electrocatalysts for OER and HER lies at the heart of the development of advanced energy technologies, not only for water splitting, but also for fuel cells and metal-air batteries actually. Lithium-ion batteries and supercapacitors are promising candidates for energy storage devices, and both rely on electrochemical processes [9]. At this stage, they also face some challenges, such as limited lifespan, low energy density or power density. Similarly, their performances intimately depend on the properties of electrode materials. Overall, innovative materials hold the key to new generations of energy technologies. Recently, electrode materials based on transition metals have attracted significant attention, due to their abundant resources, multiple oxidation states and ability to myriad of electrochemical reactions [10-13]. However, the large intrinsic electrical resistance impedes their practical applications greatly. Numerous efforts have been employed to improve their electrochemical performances. For example, introducing oxygen vacancies [14] or heteroatoms [15-17] in transition metal oxides have been reported to enhance the electrical conductivity and therefore boost the capacitive performances. The development of nanostructured materials with desirable morphologies has boosted the electrode properties effectively, in term of the enlarged electroactive surface area, shortened diffusion pathway of ion/electron and capability of balancing stresses induced by cyclic volumetric variation [18]. Nanostructured materials with well-defined crystal plane structures show enhanced electrochemical properties, especially the rate capability [19]. This is because crystal planes with higher energy could reduce the oxidation-reduction gaps and then accelerate reaction rates. In addition, adjusting grain sizes and phases properly could contribute to enhanced performances [20]. However, complex processing methods and sophisticated techniques are 3

limited factors. Thus, to develop simple and universal strategies is still urgent for comprehensively enhancing the electrochemical performances. Among various transition metal oxides, Co3O4 has attracted extensive attention due to its high energy storage capacity (3560 F g-1), low-cost, environmental friendliness, multiple valence sites as well as high activity in water oxidation [21, 22]. Nonetheless, Co3O4 shares the common drawbacks of low electrical conductivity, which highly restricts its performances in energy storage and conversion. Also, the OER activity of Co3O4 should be further promoted for high efficiency water-splitting. It has been reported that calcination temperature can tune the surface components, such as the valance ratio of different metals in CoxNi1-xOy reported by Roginskaya et al. [23], and oxygen vacancies in Ce-Fe mixed oxides performed by Li et al. [24]. Herein, we report that the surface components and defects of electrode materials can be tuned by means of facilely controlling the calcination temperature, and thus enhance the electrochemical performances. Cobalt oxide acts as a model material to illustrate how this strategy effectively improves supercapacitive performances and electrocatalytic performances for water splitting. Specifically, we harvested highly active Co3O4 porous nanorods for electrochemical energy storage and catalysis through controllable CoC2O4∙2H2O conversion. Oxygen vacancies have been successfully introduced into Co3O4 nanorods which have largely increased the donor density for higher electrical conductivity. In addition, electrophilic O22-/Ospecies is functionalized on the surface and is believed to boost the OER activity [25]. The cobalt oxide calcined at 250 oC shows high specific capacitance (739 F g-1 at 5 mV s-1), long durability (no degradation of the overall capacitance after 50000 cycles), high OER activity (overpotential of 275 mV at 10 mA cm-2), good stability for 300 h as well as good HER activity. Moreover, an integrated electrolyzer for overall water splitting can be driven by asymmetric supercapacitors with the optimized cobalt oxide as both electrocatalysts and anodic electrode materials of the supercapacitors. 4

Experimental Preparation of cobalt oxide electrode materials The cobalt oxides were fabricated by means of a two-step approach. A two-electrode electrochemical cell with a DC stabilized power supply (Wenhua, China) was employed to prepare the precursors. Degreased and sealed commercial cobalt foils were used as the anode substrates and a smaller cobalt foil acted as the cathode. Anodization was performed in a 0.5 M oxalic acid aqueous solution at a constant voltage of 40 V for 15 min at room temperature. The anodized samples were rinsed with deionized water for several times to remove the residual acid, followed by a calcination step at different temperatures (250, 300, 350 and 400 o

C) for 30 min in air. Thus, the nanostructured Co3O4 decorated on cobalt foils was obtained.

The calcined samples are denoted as Co3O4-T. The mass loading of the cobalt oxide was 2.2 mg cm-2 by measuring changes in the weight of the samples before and after calcination according to the involved phase transformation (from CoC2O4∙2H2O to Co3O4) during the calcination process (specific details in acquiring the mass loading are described in the Electronic Supplementary Information). Characterization The phase composition of the as-prepared samples was characterized by X-ray diffraction (XD-3 diffractometer, Beijing Purkinje General Instrument Co., Ltd, China) equipped with Cu Kα radiation. Scanning electron microscopy (SEM, LEO 1530 VP) and transmission electron microscopy (TEM, FEI Tecnai G2) were employed to observe the microstructure and morphology of the cobalt oxides. Surface elemental information was detected by X-ray photoelectron spectroscopy (XPS, ESCALAB 250). Raman spectrum was recorded through Renishaw Raman spectrometer. Thermogravimetric (TG, Mettler-Toledo) measurement of the cobalt oxalate dihydrate was carried out in air at a heating rate of 5 oC min-1. Electrochemical measurements 5

Pseudocapacitive performances of the cobalt oxides were characterized through a threeelectrode system (Pt as the counter electrode and SCE as the reference electrode) using the electrochemical workstation (CHI660E, Shanghai, Chenhua). Cyclic voltammetry (CV), galvanostatic charge/discharge and electrochemical impedance spectroscopy (EIS) were measured, and the stability of the electrode was evaluated at a scan rate of 100 mV s -1. Electrochemical impedance information was obtained using a potentiostat (ZAHNER, Zennium) in a frequency range from 0.01 Hz to 100 kHz with a 5 mV amplitude at open circuit potential. An asymmetric supercapacitor was assembled using the Co3O4-250 as the positive electrode and an activated carbon (AC) electrode as the negative electrode. The preparation process of the AC electrode and the assembling method of the supercapacitor were described in our previous work [26]. Electrochemical measurements of the asymmetric supercapacitor were conducted using the CHI 660E potentiostat, including CV and galvanostatic charge/discharge in 2 M aqueous KOH. Electrocatalytic activity of the cobalt oxide electrodes toward OER was tested through a potentiostat (ZAHNER, Zennium) in O2-saturated 1 M KOH aqueous solution. CV was measured at 5 mV s-1, EIS was tested at a potential of 0.3 V (vs. SCE) with the frequency ranging from 0.1 Hz to 10 kHz and the durability was measured by chronopotentiometry at 100 mA cm-2. The measured potentials were normalized to RHE with the equation of E(RHE)=E(SCE)+0.244+0.059pH, and the pH value of the used KOH solution was measured as 13.5. HER performance of the cobalt oxide electrodes was evaluated by linear scan voltammetry (LSV) in N2-saturated 1 M KOH aqueous solution (scan rate 5 mV s-1). Overall water splitting was investigated by LSV using Co3O4-250 as both HER and OER catalysts in 1 M KOH. Stability test of the electrolyzer was carried out by chronopotentiometry at 10 mA cm-2. Mott-Schottky plots were measured using the potentiostat (CHI660E, Shanghai, Chenhua) in the 1 M KOH with 5 mV amplitude and frequency of 10 kHz. In addition, the electrochemically active specific surface area (EASSA) of the electrodes was estimated from 6

double-layer capacitance using the CV technique, varying the scan rate from 5 to 100 mV s-1 in the 1 M KOH. DFT calculation DFT calculations were performed by a Materials Studio CASTEP module, using the generalized gradient approximation (GGA) of the Perdew-Burke-Ernzerhof (PBE) functional for exchange-correlation and ultrasoft pseudopotentials. 300 eV was chosen for the cut-off energy of the plane wave basis set in all of the cases and the calculations were carried out with spin polarization. The k integrations over the Brillouin zone were performed up to 3×4×1 Monkhorst-Pack mesh. The self-consistent calculations were considered to be converged when the total energy of the system was stable within 10-6 eV atom-1. In the density of states (DOS) calculation, models of Co3O4 with 0, 1 and 2 oxygen vacancies were constructed.

Results and Discussion Anodization was employed to synthesize the precursor with cobalt oxalate dihydrate directly growing on cobalt foils. Subsequently, cobalt oxides were obtained by thermal decomposition of the as-prepared precursor at different calcination temperatures (Figure S1). In Figure S2, the SEM images demonstrate the nanostructured anodized product anchored on cobalt foil, and is confirmed to be cobalt oxalate dihydrate (CoC2O4∙2H2O) by the XRD (Figure S3) [27, 28]. After calcination, the two samples, Co3O4-250 and Co3O4-400 (denote the samples obtained by calcination at 250 and 400 oC respectively) have a similar nanorod structure, while the Co3O4-400 sample shows larger grains and nanopores (Figure 1a and b). Figure S4 presents the SEM images of the Co3O4-250 and Co3O4-400 samples at low magnification, showing nanostructured Co3O4 gathered together into clusters. Figure 1c demonstrates the TEM image of Co3O4-250. It is clear that there are mesopores between the grains. As expected, a higher surface area of Co3O4 can be achieved at a lower calcination temperature. Figure 1d presents the high-resolution TEM (HRTEM) image of the Co3O4-250. The grain 7

size roughly calculated from several HRTEM images is about 5.6 nm. The selected-area electron diffraction (SAED, selected area: ~ 200 nm in diameter) pattern exhibits polycrystalline rings (inset of Figure 1d), and the diffraction rings can be indexed as (111), (220), (311), (422), (511) and (440) crystal planes of the Co3O4 phase. In order to determine the active material mass as well as the decomposition temperature, TG of the precursor was measured. According to the TG curve (Figure S5), the cobalt oxide can be obtained by calcination of the as-anodized samples at different temperatures (250, 300, 350 and 400 oC). Figure 2a presents the XRD patterns of different Co3O4 samples. The diffraction peaks at 31.3, 36.8, 38.5, 55.7, 59.4 and 65.2 can be indexed to (220), (311), (222), (422), (511) and (440) planes of Co3O4 (PDF # 42-1647), while 44.57 for (002) and 47.57 for (101) (PDF # 05-0727) are ascribed to be Co. Additionally, it is apparent that the intensity of the Co3O4 diffraction peaks is enhanced and simultaneously, the peaks narrow with the increasing calcination temperature, which indicates the coarsening of the grains and enhanced crystallinity as well as fewer defects. Raman spectra of the different samples were measured to provide fingerprints to identify molecules (Figure 2b). Five Raman bands located at 188, 466, 509, 602 and 668 cm-1 are detected, corresponding to 3 F2g, 1 Eg and 1 A1g Raman active modes of the Co3O4 nanocrystals. The observed Raman peaks shift to lower wavenumbers compared to bulk Co3O4 which can be ascribed to the small grain sizes [29]. The phonon symmetries of Raman peaks are based on lattice vibrations of the spinel structure, in which Co2+ and Co3+ cations are situated at tetrahedral and octahedral sites in cubic lattice, confirming the presence of Co3O4. The peaks shift slightly to higher wave numbers with increasing calcination temperature, illustrating the larger grain size, which is consistent with the XRD results. To further investigate the electronic state of surface elements as well as their chemical environment, XPS spectra of the four Co3O4 electrodes were collected. Figure 2c and d 8

demonstrate the Co 2p2/3 spectra of Co3O4-250 and Co3O4-400, respectively. Co 2p2/3 spectra of Co3O4-300 and Co3O4-350 are shown in Figure S6. The fitting results show that Co 2p2/3 signal could be decomposed to two components at binding energy (BE) =779.5~779.6 and 780.9~781 eV ascribed to Co3+ (octahedral) and Co2+ (tetrahedral), respectively [30, 31]. The shake-up satellites with a low intensity at 788.7 eV are characteristic of pure Co3O4 [32]. The Co3+/Co2+ molar ratio of Co3O4-250 (1.21) is lower than that of Co3O4-300 (1.27), Co3O4-350 (1.33) and Co3O4-400 (1.43). Stoichiometric Co3O4 has a molar ratio of Co3+ to Co2+ equivalent to 2:1. After calcination, CoC2O4·2H2O was converted to oxygen vacancies enriched Co3O4. The loss of oxygen in the lattice leads to the increased donor (electron) density and the number of Co2+ in the vicinity area therefore is increased, with a lower molar ratio than 2 for Co3+ to Co2+ [33]. Nonstoichiometric Co3O4 converted at the low temperature (250 oC) possesses rich oxygen vacancies (Co2+ site). Nonetheless, higher temperature treatments (such as 400 oC) could re-oxidize some Co2+ back to Co3+, accompanied by the decreased density of oxygen vacancies (Co2+ site) and increased molar ratio of Co3+ to Co2+ [34]. As for the O 1s XPS spectra of Co3O4-250 and Co3O4-400 (Figure 2e and f), the signals for each catalyst can be decomposed into four components, lattice oxygen species (~529.6 eV for O2-), highly oxidative oxygen species (~530.4 eV for O22-/O-), hydroxyl groups or the surface-adsorbed oxygen (~531.5 eV for OH or O2) and adsorbed molecular water (~532.5 eV for H2O) [35]. The ratio of various oxygen species estimated from the relative area of the fitted subpeaks is listed in Table S1. It is clear that the O22- /O- content in Co3O4-250 (28.5%) is higher than that in Co3O4-400 (20.2%). Pseudocapacitive performances of the Co3O4 calcined at different temperatures were measured in the three-electrode system with 2 M KOH as the electrolyte solution. Figure 3a demonstrates the typical CV curves measured in a potential range from 0 to 0.55 V (vs. SCE) at 5 mV s-1. Redox peaks induced by faradaic reactions in the CV plot represent pseudocapacitive characteristics. In addition, it is clear that the anodic peaks shift slightly 9

from 0.392 to 0.378 V with the increasing calcination temperature. This is because the Co3O4 electrodes obtained at higher temperatures possess much bigger pores (Figure 1a-b). These pores work as ion buffer reservoirs, minimizing the influence of current polarization on electrode performances [36]. CV curves of different samples measured at different scan rates are shown in Figure 4a and Figure S7, and the decreasing peak current with the increasing calcination temperature indicates the reduction of capacitance. Figure 3b shows the specific capacitance of the Co3O4 electrodes as a function of scan rates. At the same scan rate, Co3O4 obtained at lower temperature possesses the higher specific capacitance. Figure 3c displays the cycling stability of the different samples measured by CV at 100 mV s-1 in 2 M KOH. All the four samples show excellent cycling performances. The slight increase of the specific capacitance at the beginning could be ascribed to the improvement of electrode surface wetting by the electrolyte during extended cycling [37]. Representative CV curves measured during the stability tests are shown in Figure S8 and Figure S9, further indicating the excellent stability. EIS analysis was employed to understand the impact of calcination temperature on the conductivity of different samples obtained at different calcination temperatures. Figure 3d demonstrates the typical Nyquist plots, which were acquired in the frequency range from 0.01 Hz to 100 kHz at open circuit with a 5 mV amplitude. The Nyquist plots are similar except for the diameter of the semicircles, which relates to the charge transfer. The semicircle diameter of Co3O4 obtained at the lower temperature is smaller. This indicates that the charge transfer resistance (Rct) is strongly influenced by the calcination temperature. There is still a little difference in the intersection value between the plots and the real axis, which represents the solution resistance and intrinsic resistance of the active material (Rs). It is apparent that the value increases with the increasing calcination temperature, showing the bigger intrinsic resistance of Co3O4 obtained at the higher calcination temperature. Based on Rs, Rct, double layer capacitance (Cdl), pseudocapacitance (Cps) and interfacial diffusive resistance (W), the 10

complex nonlinear least-square (CNLS) fitting method was used to quantify the impedance values. The fitted results are shown in Table S2. With increasing calcination temperature from 250 to 400 oC, Rs increases from 0.49 to 1.19 Ω, while Rct increases from 0.05 to 50.8 Ω. The capacitive performances of the Co3O4 obtained at 250 oC were measured and evaluated comprehensively. Figure 4a demonstrates the CV curves measured in a potential range between 0 and 0.55 V (vs. SCE) at various scan rates from 5 to 100 mV s-1. Figure 4b shows the specific capacitance as a function of scan rates. The calculated specific capacitance reaches to 739 F g-1 (1.63 F cm-2) at 5 mV s-1, and decreases to 388 F g-1 (0.85 F cm-2) at 100 mV s-1. This phenomenon is due to the presence of inner active sites that cannot contribute to the redox reactions at higher scan rate [38]. Galvanostatic charge/discharge profiles were also measured at various current densities ranging from 9 mA cm-2 to 60 mA cm-2 in the potential window of 0-0.45 V (vs. SCE), as shown in Figure 4c. At different current densities, the charge/discharge curves display symmetric charge and discharge processes, indicating excellent reversibility of the electrode materials. Figure 4d demonstrates the specific capacitance versus the current density calculated according to the charge/discharge profiles. The specific capacitance of the electrode based on the loading mass is 717, 712, 703, 685, 671 and 618 F g-1, and the areal specific capacitance is 1.58, 1.57, 1.55, 1.51, 1.48 and 1.36 F cm-2, corresponding to the current density of 9, 15, 20, 30, 40 and 60 mA cm-2 respectively. The cycling stability was explored by CV measurements at 100 mV s-1, which is necessary for practical applications. The specific capacitance undergoes a slight increase during the stability measurement, resulting in an overall increase of 3.1% after 50000 cycles (Figure 4e). The high specific capacitance and the excellent durability of Co3O4-250 are overwhelming compared to other Co-based electrode materials (Table S3) [39-45]. EIS was conducted before and after the cycling stability test, as shown in Figure 4f. No significant changes can be observed in the spectra. The spectra were analyzed by the equivalent circuit (inset in 11

Figure 3d) and related fitting results are shown in Table S2. In order to investigate the contribution of the cobalt substrate to the capacitance of Co3O4250, electrochemical tests of the cobalt foil calcined at 250 oC were carried out (Figure S10). The CV curve measured at 100 mV s-1 shows that the capacitance of the cobalt foil calcined at 250 oC can be neglected compared to the Co3O4-250 (Figure S10a). The specific capacitance increases initially, but levels off to 6.95 mF cm-2 after 20000 cycles, as shown in Figure S10b. The capacitance contribution (~ 1%) to the Co3O4-250 is negligible. Moreover, most of the substrate is covered by Co3O4 rather than being exposed in the solution. An asymmetric supercapacitor Co3O4-250//AC was assembled to evaluate further practical applications of the Co3O4-250 electrode. The charge between the two electrodes of the supercapacitor has been balanced based on the charge balance principle q+=q- in order to obtain an optimum electrochemical performance [46]. Figure 5a presents the CV curves of the Co3O4-250//AC supercapacitor at various scan rates from 5 to 100 mV s-1 with a potential window of 1.7 V. The current-potential response is dependent on the potential, as opposed to the potential-independent current response of an electrochemical capacitor based on a nonfaradaic process [47]. Figure 5b illustrates the galvanostatic charge/discharge profiles at different current densities. The curves show the typical symmetric shape, indicating the charge balance between the positive and negative electrodes. From the corresponding discharge profiles, the specific capacitance was calculated to be 60.3, 48.6, 44.3, 42.7 and 41.4 F g-1 at the current density of 0.68, 1.15, 1.83, 2.75 and 3.67 A g-1 respectively, based on the total mass of active materials on both electrodes (Figure 5c). Figure 5d presents the Ragone plot of the Co3O4-250//AC asymmetric supercapacitor. Our Co3O4-250//AC supercapacitor displays an excellent energy storage performance. The energy density reaches as high as 24.2 Wh kg-1 at a power density of 0.6 kW kg-1. Even at a higher power density of 2.9 kW kg-1, the supercapacitor could still have a high energy density of 14.2 Wh kg-1 (Figure 12

5d). The high energy density and power density of the present supercapacitor are superior compared to some other electrode materials, such as AC//nano-Co3O4 (20 Wh kg-1 at 209 W kg-1) [48], NiCo2O4-rGO//AC (23.3 Wh kg−1 at 324.9 W kg−1) [49] and CoMoO4//graphene (21.1 Wh kg−1 at 300 W kg−1) [50]. In addition, two supercapacitors in series can power a blue LED for hours (inset in Figure 5d), which demonstrates the potential of Co3O4-250 for practical application. Except for capacitive performances, OER and HER properties of the as-prepared Co3O4 were also investigated. CV curves of the different samples were measured at 5 mV s-1 to minimize the capacitive current in O2-saturated 1 M KOH (Figure 6a). These curves were corrected for average solution resistance, which was obtained from the Nyquist plots in Figure S12 (Figure S11 shows the CV curves before IR compensation). Co3O4-250 demonstrates the lowest onset potential of OER among these four catalysts, revealing the highest activity. Table S4 summarizes the overpotential of the four catalysts at certain current densities of 10, 100 and 500 mA cm-2, which demonstrates that the overpotential increases with the calcination temperature. When the current density is 10 mA cm-2, the overpotential of Co3O4-250, Co3O4-300, Co3O4-350 and Co3O4-400 is 275, 268, 276 and 283 mV, respectively, much lower than that of bare Co foil (430 mV). It should be noted that the overpotential is only 360 mV at 100 mA cm-2 and 440 mV at 500 mA cm-2 for the Co3O4-250 electrode. As shown in Table S5, the lower overpotential of 275 mV at 10 mA cm-2 demonstrates that Co3O4-250 is among the top tier of water oxidation catalysts, compared to the reported stateof-the-art Co-based OER catalysts, such as Mn3O4/CoSe2 (450 mV), N-CG-CoO (340 mV), CG-CoO (~340 mV), Co3O4@CoO (430 mV) at the same current density, and so forth [51-56]. The electrochemical stability of Co3O4-250 was operated by means of chronopotentiometry at 100 mA cm-2 (Figure 6b). The overpotential at this current density only increases about 9 mV after a significant long term of 300 h (much longer than the usually used 10 h), indicating the excellent durability of Co3O4-250. The O2 and H2 bubbles can be clearly observed on the 13

Co3O4-250 and Pt electrodes, as shown in the inset of Figure 6b and Movie S1. The catalytic ability of Co3O4 towards HER was also investigated in N2-saturated 1 M KOH. As shown in Figure 6c, all the four Co3O4 electrodes exhibit the same overpotential of ~260 mV at 10 mA cm-2, which is superior to the electrocatalytic performance of some other HER catalysts, such as Co-NRCNTs, MnNi, Ni/MWCNT, MoO3-MoS2 and Co3O4 NCs (Table S6) [57-61]. When the current density is over 60 mA cm-2, Co3O4 obtained at the lower calcination temperature shows the lower operating overpotential at the same current density, indicating the higher HER activity. It should be noted that Co3O4-250 only requires a 419 mV overpotential to achieve a current density of 100 mA cm-2. It is evidenced that the oxygen vacancy and O22-/O- functionalization might have little influence on the HER activity at the lower current density (below 60 mA cm-2), while at the higher current density, Co3O4 obtained at the lower calcination temperature demonstrates the higher HER activity due to more oxygen vacancies and larger amount of O22-/O-. XPS spectra of Co3O4-300 after electrochemical measurements (Figure S13a and c) have been obtained to explore the change in oxygen vacancy and O22-/O- functionalization after electrochemical reactions. Comparing the Co 2p2/3 XPS spectra of Co3O4-300 before (Figure S6a) and after (Figure S13a) electrochemical measurements, the Co3+/Co2+ molar ratio increases from 1.27 to 1.36, indicating the oxidation of Co2+ to Co3+ during electrochemical processes. According to the O 1s XPS spectra (Figure S13b and c), the content of O22-/O- increases from 22.6% to 26%. Based on the above results, it can be concluded that Co3O4-250 acts as good OER and HER electrocatalysts. Accordingly, an electrolyzer employing Co3O4-250 as both water reduction and oxidation catalysts has been constructed. In Figure 6d, the voltage for overall water splitting is 1.72 V at 10 mA cm-2, which is consistent with the HER and OER measurements. Durability as a vital criterion was also explored at 10 mA cm-2 as shown in inset of Figure 6d. The electrolyzer even exhibits considerable stability in 20 h. Inspiringly, two Co3O4-250//AC asymmetric supercapacitors in series could drive the electrolyzer in the 1 14

M KOH aqueous solution (Figure 6e and Movie S2). Obvious O2 and H2 bubbles can be observed in the enlarged photograph of Figure 6e. As noted above, Co3O4-250 possesses better capacitive performances and electrocatalytic activity for water splitting. Therefore, the enhanced electrochemical performances may be ascribed to the following factors. First, Co3O4-250 possesses higher electronic conductivity. Mott-Schottky plots were generated based on capacitance that was derived from the electrochemical impedance obtained at each potential with the frequency of 10 kHz in 1 M KOH aqueous solution. As shown in Figure 7a, all the four Co3O4 electrodes demonstrate negative slop in the Mott-Schottky plots, indicating the typical p-type semiconductor characteristic [62]. Carrier densities can be calculated according to Mott-Schottky equation (Equation (1)),

𝑁𝑑 =

−1 1 𝑑( 2 ) 𝐶 − (𝑒 𝜀𝜀 ) [ 𝑑𝑉 ] (1) 0 0 2

where Nd is the donor density, e0 is the electron charge, ε is the dielectric constant of Co3O4, ε0 is the permittivity of vacuum, and V is the applied potential. It is clear that the absolute value of the slop of the Mott-Schottky plots increases with the increasing calcination temperature (Figure 7a), which is inversely proportional to the carrier density. That is to say, the carrier density in Co3O4 reduces with the increasing calcination temperature and the Co3O4-250 has the highest donor density. Figure 7b shows the calculated density of states (DOS) of cobalt oxide without and in presence of one and two oxygen vacancies, and corresponding models are shown in Figure 7c-e. There are more oxygen vacancies (Co2+ sites) in Co3O4-250, as confirmed by XPS characterization. It is apparent that the density of states increases with the increasing oxygen vacancies, accompanied by the increase in the density of charge carriers. Specifically, oxygen vacancies (Co2+ sites) could introduce p-type defect band, and increase donor density to boost 15

the electrical conductivity of Co3O4, which is desirable either for rapid charge storage or fast electron transport in electrocatalysis [63]. In agreement with the theoretical calculation, the increased electrical conductivity of Co3O4-250 is confirmed by the EIS (Figure 3d), in which a value of 0.49 Ω for Rs is obtained in this sample, more than two times lower than that of Co3O4-400. The semicircle diameter in the EIS results represents the charge transfer resistance (Rct), and the Co3O4-250 exhibits a negligible Rct of 0.05 Ω, much smaller than that of Co3O4-400 (50.8 Ω), indicating good charge transfer property in electrochemical reactions (Table S2) due to the defect enriched surface nature [64, 65]. Moreover, the increased Co2+ sites were found to be active in the OER process and their promotion effect in forming both faradaic reaction and OER active cobalt oxyhydroxide (CoOOH) is also critical to enhance the electrochemical performances of Co3O4-250 [66]. O22-/O- species on the surface of Co3O4-250 with a higher content is important electrophilic reactant for water oxidation and is responsible for enhancing the OER activity (Table S1) [64, 67, 68]. In addition, the electrochemically active specific surface area (EASSA) of Co3O4-250 is higher than that of the other three electrodes (Figure S14), indicating more active sites for electrochemical reactions. Thus, the combined structure merits considerably promote the overall electrochemical performances of these defects enriched Co3O4 nanorods.

Conclusions Nanostructured Co3O4 obtained at lower temperatures demonstrates superior supercapacitive properties (high specific capacitance and ultrahigh durability) and electrocatalytic performances towards OER and HER (lower overpotentials, ultralong stability). The enhanced electrochemical performances can mainly be ascribed to the following factors: higher electronic conductivity, better charge transfer ability, higher content of O22-/O- and lower Co3+/Co2+ ratio. Therefore, tuning the surface defects and components of Co3O4 as well 16

as other transition metal oxides through the facile control of calcination temperature could produce highly competitive electrode materials for energy storage and conversion devices, such as supercapacitors and electrocatalysts for water splitting. This work also paves the way to drive electrolyzer for water splitting by supercapacitors.

Acknowledgements The authors gratefully acknowledge financial support by National Natural Science Foundation of China (51371106, 51671115), and Young Tip-top Talent Support Project (the Organization Department of the Central Committee of the CPC).

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24

Figures

200 nm

c

200 nm

d

（ 440） （ 422） （ 311） （ 111） （ 220） 20 nm

5 nm

（ 511）

Figure 1. (a, b) SEM images of the (a) Co3O4-250 and (b) Co3O4-400 electrodes. (c) TEM and (d) HRTEM images of the Co3O4-250 electrode. Inset in (a): a photograph of the Co3O4250 electrode. Inset in (d): SAED pattern corresponding to (c).

25

A1g

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O 1s

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Figure 2. (a) XRD patterns and (b) Raman spectra of the Co3O4-250, Co3O4-300, Co3O4-350 and Co3O4-400 electrodes. (c, d) Co 2p2/3 XPS spectra of the Co3O4-250, and Co3O4-400 electrodes. (e, f) O 1s XPS spectra of the Co3O4-250 and Co3O4-400 electrodes.

26

b

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Figure 3. Electrochemical measurements of the Co3O4-250, Co3O4-300, Co3O4-350 and Co3O4-400 electrodes in 2 M KOH aqueous solution. (a) CV curves at 5 mV s-1. (b)The specific capacitance as a function of the scan rate. (c) Cycling stability measured through CV at 100 mV s-1. (d) Nyquist diagrams obtained at open circuit potential with the frequency ranging from 0.01 Hz to 100 kHz. Inset in (d): equivalent circuit.

27

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Figure 4. Electrochemical measurements of the Co3O4-250 electrode in 2 M KOH aqueous solution. (a) CV curves at different scan rates ranging from 5 to 100 mV s -1. (b) The specific capacitance as a function of scan rate. (c) Galvanostatic charge/discharge profiles at different current densities. (d) The specific capacitance versus the current density. (e) Cycling performance measured by CV at 100 mV s-1. (f) Nyquist diagrams of the Co3O4-250 electrode before and after cycling stability measurement.

28

b

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0.68 A g-1 1.15 A g-1 1.83 A g-1 2.75 A g-1 3.67 A g-1

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Figure 5. Electrochemical characterization of the Co3O4-250//AC asymmetric supercapacitor (2 M KOH). (a) CV curves at various scan rates. (b) Galvanostatic charge/discharge curves at different current densities. (c) The specific capacitance calculated from the corresponding discharge curves for each current density based on the total mass of the active materials on both electrodes. (d) Ragone plot related to energy and power densities of the asymmetric supercapacitor. Inset in (d): a photograph of the blue LED powered by two supercapacitors in series.

29

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300

Time (h)

d 0.10 Current density (A cm-2)

Overall water splitting 2.1

0.08

-0.1

0.06

Co3O4-250 Co3O4-300

-0.2

1.5 1.2 0.9

0.04

Co3O4-350 Co3O4-400 -0.3

10 mA cm-2

0.6 0.3 0

2

4

0.00

-0.8

-0.6

-0.4

-0.2

Potential (V vs. RHE)

0.0

0.2

6

8

10 12 14 16 18 20

Time (h)

0.02

5 mV s-1 -0.4

1.8

Potential (V)

Current density (A cm-2)

H2 bubbles

-1

1.0

0.0

O2 bubbles

0.4

5 mV s-1 1.5

1.6

1.7

1.8

1.9

2.0

Potential (V)

Figure 6. (a) CV curves of the Co3O4-250, Co3O4-300, Co3O4-350, Co3O4-400 and bare Co foil electrodes in O2-saturated 1 M KOH aqueous solution after IR compensation (scan rate 5 mV s-1). (b) Cycling stability of the Co3O4-250 electrode measured by chronopotentiometry at a current density of 100 mA cm-2. Inset in (b): a photograph showing oxygen evolution at 100 mA cm-2. (c) LSV curves of the Co3O4-250, Co3O4-300, Co3O4-350, Co3O4-400 electrodes in N2-saturated 1 M KOH aqueous solution (scan rate 5 mV s-1). (d) LSV curve of water electrolysis using Co3O4-250 as both HER and OER catalysts in 1 M KOH aqueous solution, with the inset showing the stability test of the electrolyzer at 10 mA cm-2. (e) A photograph 30

showing that two Co3O4-250//AC asymmetric supercapacitors in series can drive overall water splitting with Co3O4-250 as both HER and OER catalysts in 1 M KOH aqueous solution.

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b 140

300

1/C2 (106 F-2 cm4)

250

DOS (states/eV atom)

a

Co3O4-250 Co3O4-300

200

Co3O4-350 150

Co3O4-400

100 50

no oxygen vacancy one oxygen vacancy two oxygen vacancies

120 100 80 60 40 20 0

0 0.1

0.2

0.3

0.4

0.5

0.6

-20

-20

-15

-10

c

0

5

10

15

e

d

Oxygen vacancy

-5

E-Ef (eV)

Potential (V vs. SCE)

Cobalt atom

Oxygen atom

Figure 7. (a) Mott-Schottky plots of the Co3O4-250, Co3O4-300, Co3O4-350 and Co3O4-400 electrodes measured in 1 M KOH aqueous solution with 5 mV amplitude and frequency of 10 kHz. (b) DFT calculation of the density of states (DOS) of pristine Co3O4 with different oxygen vacancies. (c-e) Models of pristine Co3O4: (c) without oxygen vacancy; (d) with one oxygen vacancy and (e) with two oxygen vacancies.

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Vitae

Guanhua Cheng received her Bachelor's degree of Materials Physics from Shandong University in 2013. Then she focused on the fabrication of transition metal oxides as well as their application in energy conversion and storage under the supervision of Prof. Zhonghua Zhang in Shandong University during her Mater study. She is pursuing her Ph.D. in Chemistry department at Technical University of Munich. Her research interests include the upgrading of bio-oils to fuels through electrochemical catalysis.

Tianyi Kou received his B.S. in composite materials and engineering from University of Jinan in 2011. Then he did M.S. degree research in Shandong University under the supervision of Prof. Zhonghua Zhang on the preparation of nanoporous metals for heterogeneous catalysis and energy storage. Currently he is a PhD student in Prof. Yat Li group at the Department of Chemistry and Biochemistry at University of California, Santa Cruz. His research interests include the design and synthesis of advanced nanomaterials for energy storage and conversion, such as supercapacitors and electrochemical water splitting.

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Jie Zhang is a master student at School of Material Science and Engineering, Shandong University. She received her Bachelor’s degree from Shandong University in 2014. She is currently doing her research under the supervision of Prof. Zhonghua Zhang. Her current research mainly focuses on hydrogen evolution reaction and oxygen evolution reaction.

Conghui Si received her Bachelor's degree in 2012 from Shandong University of Technology. She is currently pursuing her Ph.D. under the supervision of Prof. Zhonghua Zhang at School of Material Science and Engineering, Shandong University. Her research mainly focuses on high-efficient electrocatalyst for oxygen reduction reaction in fuel cells.

Hui Gao is a master student at School of Material Science and Engineering, Shandong University. He received his Bachelor's degree from Jilin University. He is at present doing research under the supervision of Prof. Zhonghua Zhang. His research focuses mainly on the 34

performance and mechanism of the electrode material used in sodium-ion and lithium-ion batteries.

Prof. Dr. Zhonghua Zhang received his Ph. D degree in 2003 from School of Materials Science and Engineering, Shandong University, Jinan, China. In 2004-2005, he worked as an Alexander von Humboldt postdoctoral research fellow at Institute of Materials, Ruhr University Bochum, Germany. At the end of 2005, he got a full professor position in Shandong University. At present, his research interests mainly focus upon dealloying mechanisms, dealloying-driven nanoporous metals, metal/metal oxide nanocomposites, transition metal chalcogenides, and their applications in energy and environment-related fields.

Highlights    

O22-/O- functionalized oxygen-deficient Co3O4 nanorods were facilely synthesized. Co3O4 shows specific capacitance of 739 F g-1 with superior stability of 50000 cycles. Co3O4 shows overpotential of 275 mV for OER with stability of 300 h @ 100 mA cm-2. The electrolyzer for overall water splitting can be driven by supercapacitors.

35