Strong Surface Hydrophilicity in Co-Based Electrocatalysts for Water

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Strong Surface Hydrophilicity in CoBased Electrocatalysts for Water Oxidation Fumin Tang, Weiren Cheng, Yuanyuan Huang, Hui Su, Tao Yao, Qinghua Liu, Jinkun Liu, Fengchun Hu, Yong Jiang, Zhihu Sun, and Shiqiang Wei ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b07088 • Publication Date (Web): 31 Jul 2017 Downloaded from http://pubs.acs.org on August 2, 2017

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ACS Applied Materials & Interfaces

Strong Surface Hydrophilicity in Co-Based Electrocatalysts for Water Oxidation Fumin Tang†, Weiren Cheng†, Yuanyuan Huang, Hui Su, Tao Yao, Qinghua Liu,* Jinkun Liu, Fengchun Hu, Yong Jiang, Zhihu Sun, and Shiqiang Wei* National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei 230029, Anhui, P. R. China †These authors contributed equally to this work. *E-mail: [email protected]; [email protected]

Abstract Developing efficient and durable oxygen evolution electrocatalyst is of paramount importance for the large-scale supply of renewable energy sources. Herein, we report a design of significant surface hydrophilicity based on the cobalt oxyhydroxide (CoOOH) nanosheets to greatly improve the surface hydroxyl species adsorption and reaction kinetics at the Helmholtz double layer for high-efficiency water oxidation activity. The as-designed CoOOH-graphene nanosheets achieve a small surface water contact angle of ~23° and a large double-layer capacitance (Cdl) of 8.44 mF/cm2, thus could evidently strengthen surface species adsorption and trigger electrochemical oxygen evolution reaction (OER) under a quite low onset potential of 200 mV with an excellent Tafel slope of 32 mV/dec. X-ray absorption spectroscopy and first-principles calculations demonstrate that the strong interface electron coupling between CoOOH and graphene extracts partial electrons from the active sties and increases the electron state density around the Fermi level, and effectively promotes the surface intermediates formation for efficient OER. Keywords: Surface hydrophilicity; Surface adsorption; 3d electron structure; Proton-electron transfer; Electrochemical water oxidation

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Introduction Electrochemical water splitting is a cheap, sustainable and efficient technology for energy conversion and storage in the form of carbon-neutral chemical energy.1-4 As is well known, the water electrolysis process is composed of two crucial half-reactions: oxygen evolution reaction (OER) on the anode and hydrogen evolution reaction (HER) on the cathode. Unfortunately, the overall efficiency of water splitting was greatly limited by the kinetically sluggish OER, which requires high activity and excellent durability anodic electrocatalysts.5-8 it is known that the OER involves multiple proton-coupled electron transfer process, which is restricted by the unmatched surface activation energy and dull surface species reaction kinetics.9-11 Moreover, the poor surface wettability and the absence of effective charge conductive corridor in electrocatalysts further deteriorate the OER activity at electrode/electrolyte interface.[12-14] Therefore, it is highly imperative to effectively improve the on-demand surface electronic structures to tune the species adsorption and reaction process in the surface double-layer of the electrocatalysts for the optimal OER efficiency. Up to date, numerous efforts have been devoted to synthesizing novel nanostructured materials or modifying the elemental composition to trigger the surface activity of the electrocatalysts. For instance, IrO2 and RuO2 nanoparticles and ultrathin γ-CoOOH nanosheets have been elaborately designed as efficient OER catalysts to modulate electronic structure of surface active metal sites, which significantly reduce the overpotential to ~300 mV at 10 mA/cm2.11,15 Moreover, perovskite oxides SrCoOx nanoparticles and CoOOH nanomaterials have been incorporated with rare and transition metal impurities to effectively tune the surface activation energy barrier, achieving low overpotential of ~350 mV and ~230 mV at 10 mA/cm2, respectively.12,16,17 Indeed, morphology modification and element doping are effective strategies to tune the electronic structure configuration of the surface active metal ions for accelerated four proton-coupled electron transfer process of OER.18 However, the element doping also leads to serious lattice damage at high doping 2


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concentration or sharp morphology change. Recently, two-dimensional (2D) surface coupling with distinct surfaces on the opposite faces of 2D materials has been demonstrated an effective way to maintain the original lattice structure and excellent properties of each component and remarkably enhance the asymmetric chemical reactivity and stability via interfacial electron field coupling.19,20 Toward this goal, the surface of the host electrocatalyst should be high-energetic and with an open structure. To maximize the interface electron coupling, it is necessary to seek a compatible two-dimensional substrate material with superior charge conductivity and mobility. Herein, via a strategy of two-dimensional surface coupling between the ultrathin CoOOH and graphene nanosheets, we effectively improve the surface hydrophilicity and proton-electron transfer kinetics at the Helmholtz double layer for high water oxidation efficiency (Figure 1a). The graphene with strong electron affinity could capture the 3d electrons of Co, and thus enhance the hydrophilic capacity of the surface active sites (Figure 1a). To commensurate with surface periodic OH open structure of CoOOH, the graphene has been elaborately functionalized with homo-oxo group to increase its surface free energy and simultaneously match the interface lattice to guarantee efficient in-situ interfacial oriented growth of CoOOH nanosheets atop graphene. The as-synthesized CoOOH-graphene nanosheets could effectively decrease the water contact angle to ~23°, and improve the surface adsorption dynamics for fast proton-electron transfer kinetics at the electrolyte/catalyst interface. Consequently, the CoOOH-graphene nanosheets successfully triggers OER under a quite low onset potential of 200 mV with an excellent Tafel slope of 32 mV/dec and quickly reaches 10 mA/cm2 at a small overpotential of 248 mV. The experimental results combined with theoretical analysis unravel that the redistribution of surface charge density induced by strong interfacial coupling synergistically promotes the surface species adsorption dynamics and proton-electron transfer kinetics, contributing to prominent OER performance for CoOOH-graphene nanosheets.

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Figure 1. Schematic of surface hydrophilic coupling and characterizations of the CoOOH-graphene structure. (a) Schematic of surface hydrophilic coupling, (b) HRTEM image and SAED image, (c) AFM image, and (d) EDS mapping image of the sample.

Results and discussion The formation of the interface coupling structure CoOOH-graphene structure can be verified by high-resolution transmission electron microscopy (HRTEM) and atomic force microscopy (AFM) image. In the HRTEM image of Figure 1b, some distinct and ordered lattice fringes surrounded by the disorder edge can be observed, which preliminary indicates that the CoOOH nanosheets are grown atop the graphene layer. Further, The AFM image (Figure 1c) and corresponding height profiles (Figure S1) of the sample display two typical platform features,11,19,20 confirming the growth of CoOOH nanosheet on the graphene. The measured step heights of the coupling structure are 0.6 and 1.4 nm, in consistence with the thickness of single-layer reduced graphene oxide (RGO) and half-unit-cell γ-CoOOH, respectively. The HRTEM image in Figure 1b shows distinct lattice spacing distance of 0.143 and 0.246 nm, attributed to the (110) and (100) planes of γ-CoOOH, respectively. The EDS mapping results confirm the random elemental distribution of C, O and Co in the CoOOH-graphene 4


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nanosheets (Figure 1d). These results strongly demonstrate the successful formation of CoOOH-graphene coupling structure nanosheets.

Figure 2. Characterizations of the CoOOH-graphene. (a) XANES spectra of C K-edge, (b) EPR spectra, (c) XANES spectra of O K-edge, and (d) water contact angle measurements.

To gain an in-depth insight into the d-orbital electronic structure of CoOOH-graphene nanosheets, the electron paramagnetic resonance spectroscopy (EPR) and X-ray absorption near edge structure spectroscopy (XANES), which are highly sensitive to outer-orbital electron distribution in materials,21,22 were performed. First, the C K-edge X-ray absorption near-edge structure (XANES) results in Figure 2a show that characteristic peaks of C 1s→π*(~285.5 eV) and C 1s→σ*(~292.5 eV) transitions are observed in the XANES spectra of RGO, consistent with the previously reported results.23,24 In comparison with RGO, the CoOOH-graphene nanosheet displays a remarkably enhanced intensity of the peak at ~288 eV. This result corresponds to the formation of interfacial Co–O–C bonds in the coupling 5



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structure,24-26 confirming the strong covalent bonding between CoOOH and RGO. As Figure 2b shows, there is no obvious vibrated EPR signal for pure CoOOH nanosheets and only a faint one at g=2.003 for RGO.27 Strikingly, a significantly intensified EPR signal at g=2.003 is observed for the CoOOH-graphene nanosheets, clearly indicating the introduction of large amount of unpaired d-orbital electrons in the CoOOH-graphene nanosheets. In the O K-edge XANES results of Figure 2c, the first, second, and third absorption peaks are associated with the transition of O 1s to hybrid unoccupied Co eg states and O 2p→Co 4s, 4p, respectively.28,29 For CoOOH-graphene nanosheet, the peak at ~532 eV is broadened and shifted to the lower-energy side with peak intensity significantly decreased by ~50% compared to pure CoOOH nanosheet. The peak intensity of O 1s→Co eg is proportional to the final unoccupied eg states, and thus the half decay of intensity means the nearly half occupancy eg state for CoOOH-graphene compared with pure CoOOH nanosheets. Taking the unchanged Co3+(d6) oxidation state and the increased intensity of EPR into account, the d electron structure of CoOOH-graphene is estimated to a near-unity occupancy of the eg-orbital. This optimized d-orbital configuration would effectively improve the surface property of CoOOH-graphene to strengthen the surface interaction between solution ions and active sites, facilitating the surface species adsorption dynamics at the electrolyte/catalyst interface. To further support this deduction, the static contact angle measurements were carried out to investigate the surface wettability of CoOOH-graphene. It is well known that the contact angle is highly sensitive to the interfacial ions/molecules interaction as well as the surface energy variation.30 As seen from Figure 2d, the static contact angles of CoOOH and RGO nanosheets are 70° and 105°, respectively, obviously indicating a hydrophilic surface of these materials. In sharp contrast, the static contact angle of CoOOH-graphene is markedly decreased to 23°, inferring a super-hydrophilic surface with strong surface ions/molecules interaction and high surface energy after the combination of CoOOH and RGO. The enhanced interfacial interaction and increased surface energy could effectively boost the surface species adsorption dynamics to form dense Helmholtz double layer at electrolyte/catalyst interface with high surface capacitance for a prominent surface reactivity.

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Figure 3. Electrochemical characterization of the RGO, pure CoOOH nanosheets, mixed sample (denoted as CoOOH+RGO), CoOOH-graphene nanosheets, and IrO2 for OER. (a) Polarization Curves, (b) Corresponding Tafel slopes, (c) The differences in current density variation at an overpotential of 20 mV plotted against scan rate for estimation of double-layer capacitance (Cdl), and (d) Electrochemical impedance spectroscopy (EIS) Nyquist plots.

To evaluate the OER activity of the CoOOH-graphene structure nanosheets, the water oxidation performances of the electrocatalysts were carefully investigated by steady-state electrochemistry measurements in 1 M KOH solution using a typical three-electrode setup (for details see in Supporting Information). The polarization curves of CoOOH-graphene nanosheets in Figure 3a demonstrates a notably small onset overpotential of 200 mV for the OER. After the onset potential, the anodic current quickly rises from 3 mA/cm2 at 230 mV to 30 mA/cm2 at 270 mV. More importantly, the anodic current of CoOOH-graphene nanosheets rapidly reaches to 10 mA/cm2 at quite low overpotential of only 248 mV, superior to most reported values for benchmarking IrO2 and RuO2 ( 330–400 mV at 10 mA/cm2) OER catalysts.31-33 In 7



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contrast, pure CoOOH nanosheets and the random mixture of CoOOH+RGO samples show an inferior OER activity with larger overpotential of ~300 mV and ~310 mV at 10 mA/cm2, respectively, and the RGO alone exhibits poor OER activity, illustrating that the prominent OER performance of the CoOOH-graphene nanosheets is attributed to the covalent-bonding structure. Meanwhile, a lower Tafel slope of 32 mV/dec as shown in Figure 3b presents the improved catalytic activity of CoOOH-graphene. This value is obviously lower than that of pure CoOOH nanosheets (~40 mV/decade) and IrO2 (~52 mV/decade), resulting in a much higher enhancement of the OER activity at η > 248 mV. Furthermore, the electrochemical double-layer capacitance (Cdl)34 results show that the CoOOH-graphene nanosheets possess the largest Cdl of 8.44 mF/cm2 (Figure 3c, S11 and Table S3), twice as much as that of pure CoOOH nanosheets (3.56 mF/cm2). This result indicates larger effective active sites of CoOOH-graphene nanosheets for superior OER performance. Figure 3d presents the electrochemical impedance spectroscopy (EIS) results for various electrodes under OER process. It can be seen from the Nyquist plots that the charge transfer resistance of CoOOH-graphene nanosheets at electrode-electrolyte interface is as low as 20 ohm and only one fifth of CoOOH nanosheets, which reveals an ultra-fast charge transfer and surface reaction kinetics at the electrode-electrolyte interface.35,36 More interestingly, the charge transport in CoOOH-graphene electrode exhibits characteristic transmission line behavior under low resistance of ~2 ohm. This suggests a convenient electron diffusion in internal CoOOH-graphene electrode and almost no charge accumulation at electrode-substrate interface,37,38 which means lower overpotential is required for driving the electrons from CoOOH-graphene to external circuit. The EIS fitting results in Table S1 show that the double layer capacitance of CoOOH-graphene is greatly increased by a factor of ~3 relative to pristine CoOOH and the corresponding charge transfer resistance is clearly reduced from 104 to 21 Ω with respect to pristine CoOOH, demonstrating the evidently improved surface hydrophilicity and charge transfer property for CoOOH-graphene. Moreover, to determine the Faradic efficiency of CoOOH-graphene, the gas evolution of sample is characterized with a rotating ring disk electrode (RRDE) in argon-saturated 1 M NaOH. The detailed results shown in Figure S10 reveal that the gas, produced from the surface of CoOOH-graphene electrode, is verified to be 8


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oxygen, and the corresponding Faradic efficiency is calculated to be ~96% based on four-electron oxygen evolution process. To verify the durability of the electrocatalyst, the chronopotentiometric curve in Figure S12 illustrates a stable current density of 10 mA/cm2 and the overpotential remains almost constant (248 mV) during 20 h operation. Furthermore, the accelerated degradation polarization curve after 1000 cycles almost overlaps the first cycle of J–V curve, reflecting a strong long-term OER durability of the CoOOH-graphene. The morphology and primary crystal lattice of CoOOH-graphene were maintained before and after OER operation, indicating high stability of CoOOH-graphene during OER operation as revealed by TEM and HRTEM results shown in Figure S2. Moreover, the valence states of Co ions were slightly increased after water oxidation (Figure S6c), inferring the surface Co ions of CoOOH-graphene may serve as OER active sites for OER performance. For CoOOH-graphene, homogeneous ultrathin CoOOH nanosheets with surface periodic OH open structure have been in-situ oriented growth atop the surface of GO. Consequently, there is presence of numerous C–O–Co bonds at the CoOOH-graphene interface, as confirmed by the XANES results in Figure 2a. These strong interfacial sp3 C–O–Co bond is helpful to prevent GO from high reduction, leading to a moderate and suitable GO reduction degree for CoOOH-graphene unraveled by XPS and Raman results shown in Figure S7. Hence, the CoOOH-graphene could both improve the surface hydrophilicity and the proton-electron transfer kinetics for high OER activity. Therefore, all above results reveal that the CoOOH-graphene structure possesses excellent electrocatalytic activity and prominent electrochemical stability as a promising OER electrocatalyst. To understand the origin of the high OER activity for the CoOOH-graphene structure, we performed atomic structure and electronic characterizations on the samples. The Co K-edge X-ray absorption fine structure (XAFS) spectra39-41 in Figure 4a show that the k3χ(k) oscillation displays a noticeable difference in the range of 2.4– 14 Å in comparison with pure CoOOH nanosheets, implying the different local atomic arrangements around Co ions.42,43 Moreover, the Fourier transform (FT) curve of the CoOOH-graphene nanosheets shows that the intensity of peak at 1.48 Å is obviously enhanced relative to pure CoOOH nanosheets, indicating an increased Co– 9



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O coordination. Quantitative EXAFS fitting results further reveal that the coordination number of Co–O bond is increased from 5.1 for pure CoOOH nanosheets to 5.5 for CoOOH-graphene. These results mean that the Co–O dangling bonds in the pristine CoOOH nanosheet are refilled by the hydroxyl at the GO surface, confirming the interface hybridized CoOOH-graphene nanosheets heterostructure. The covalent bonding between CoOOH and RGO would change the chemical environment and electronic structure of Co ions. The Co L2,3-edge XANES spectra in Figure S9 show that the characteristic peaks at 779.8 and 794.6 eV, arising from Co 2p→3d electron transitions, are significantly reduced in intensity after hybridizing with RGO. This indicates a new Co 3d electron occupation in CoOOH-graphene, primarily due to the increase of unpaired electrons introduced by RGO hybridization.44,45 From the first-principles band structure calculations in Figure 4c, it is readily seen that the density of states (DOSs) around the Fermi level are greatly enhanced for CoOOH-graphene nanosheets compared with that of CoOOH nanosheets (Figure S14), confirming the fact that more mobile electrons are introduced after RGO hybridization. The spin-up and spin-down states of CoOOH-graphene nanosheets deeply extend across the Fermi level and partially occupied antibonding eg orbitals along with the unoccupied t2g states are introduced, resulting in the half-occupancy eg configuration which is consistent with the above analysis of EPR and O K-edge XANES. This new d-electron structure of CoOOH-graphene nanosheets will extremely promote surface proton-electron transfer and greatly decrease the energy barriers of the OER intermediates. As the calculated free energy shown in Figure 4d, The CoOOH-graphene nanosheets can realize lower energy levels of intermediates and products than CoOOH nanosheets during the whole OER process. Especially, it can fast adsorb OH- to form *OH with quick electron transfer under near zero free energy change (0.08 eV) and significantly lower the energy level of *OOH formation by ~1 eV compared with CoOOH nanosheets.

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Figure 4. (a) Fourier transforms of the Co K-edge EXAFS k3χ(k) functions for CoOOH and CoOOH-graphene nanosheets. The inset displays the k3χ(k) functions, (b) Charge density waves for RGO and CoOOH-graphene nanosheets, (c) Calculated DOSs for CoOOH-graphene nanosheets, and (d) Calculated free energy profiles for CoOOH and CoOOH-graphene nanosheets where M refers to the catalyst.

In the OER process in alkaline solution, the formation of *OOH requires the highest activation energy and is the rate determining step (RDS) for the CoOOH-graphene electrocatalyst. The enhanced hydrophilic surface property of CoOOH-graphene could effectively boost the surface adsorption dynamics of hydroxyl and water molecule at the first stage of OER, and thus accelerate the OER process in the alkaline solutions. Furthermore, the density-functional theory calculations reveal that the binding energies of OH−, H2O, and O2 at the surface of pristine CoOOH are −0.25, −0.47, and −0.52 eV, respectively. While for CoOOH-graphene, the surface binding energies of OH−, H2O, and O2 are −1.18, −0.59, and −0.75 eV, respectively. It can be seen that the binding energy of OH− at CoOOH-graphene surface is significantly increased by 0.93 eV compared to CoOOH; 11



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whereas for H2O and O2, the increase amplitude of binding energy is not so evident. For the CoOOH material, its first step of the adsorption of OH− in the OER process is relatively weak, and thus improving the OH− adsorption is helpful to accelerate the water oxidation kinetics of CoOOH. The binding energy calculation results confirm the enhanced surface hydrophilicity of CoOOH electrocatalyst by the combination of graphene, which will show great improvement on the OER activity of the CoOOH-graphene electrocatalyst. The density-functional theory calculation results reveal that the binding energy of hydroxyl, water and oxygen on CoOOH-graphene surface has been increased by 0.93, 0.12 and 0.23 relative to CoOOH respectively, indicating the enhanced surface hydrophilicity has greater influence on hydroxyl adsorption than on water and oxygen release. For CoOOH-graphene, the improved surface super-hydrophilicity could effectively boost the surface adsorption dynamics of hydroxyl, and then promote the formation of *O and *OOH intermediates at the Helmholtz double layer, which is considered as the rate determining step during OER process. In spite of the possible presence of some negative effects on the oxygen release, the improved surface hydrophilicity of CoOOH-graphene has totally promoted OER kinetic at the Helmholtz double layer for higher OER activity. Summarizing all above results, the interface coupling structure nanosheets of CoOOH-graphene has made a great breakthrough as the promising OER electrocatalysts which trigger oxygen evolution under quite low onset overpotential of 200 mV and significantly reduce the OER overpotential to 248 mV at 10 mA/cm2, superior to those of CoOOH bulk (374 mV), IrO2 bulk (330 mV) and CoOOH nanosheet (300 mV). In this work, we present a design by combination of CoOOH and graphene to improve the surface hydrophilicity of the electrocatalyst. The enhanced surface property of CoOOH-graphene could effectively boost the surface adsorption dynamics of hydroxyl and water molecule at the first stage of OER, and then promote the formation of *O and *OOH intermediates during OER process. Therefore, the Tafel slope of CoOOH-graphene is significantly reduced to 32 mV/dec, superior to the most promising Co-based electrocatalysts reported by far (Table S4). As for the electronic structure of CoOOH-graphene, the 3d bands of Co is evidently rearranged with high electron density extending across the Fermi level, contributing to a new d-electron configuration with half-occupancy eg for Co ions. 12


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The new electronic feature plays an important role in promoting fast proton-electron transfer

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electrocatalysts. First, the unpaired electrons in the t2g and eg orbitals with strong electrophilicity can quickly couple with the aqueous hydroxyl and fast transfer its electrons to regenerate dynamic holes and form *OH at active cobalt sites. The successive holes will accelerate further hydroxyl chemisorption and proton release to facilitate the sequential formation of *OOH intermediates on the active sites and then decrease the oxygen evolution barrier.46 The improved OER kinetics can be confirmed by the reduced onset overpotential of 200 mV and the small Tafel slope of 32 mV/dec (Figure 3a,b). Second, the strong hybridization of Co d states and O/C p states as revealed by XANES, EPR, and DFT results is quite beneficial for electron transfer from surface active sites to the external circuit, maintaining the consecutive supply of vigorous holes to meet the fast oxidization rates of OER intermediates for continuous oxygen production at active cobalt sites. Especially, the σ-bonding eg orbital of Co has a strong overlap with inner O 2p orbital along c axis which can obviously reduce electron transfer resistance across CoOOH-graphene interface,10 exhibiting a typically quick electron transmission behavior as demonstrated by the EIS results (inset of Figure 3d).47 Finally, the significant enhancement of mobile electrons around the Fermi level activates the Janus structure nanosheet, effectively decreasing its Gibbs free-energy of the intermediates state. Indeed, as shown in Figure 4d, the activation energy barrier of CoOOH-graphene nanosheets is significantly decreased by 0.2 eV relative to that of pristine CoOOH nanosheets, corresponding to more favorable thermodynamic OER process induced by RGO.

Conclusion In summary, we present an interface coupling structure of CoOOH-graphene nanosheets with interfacial chemical contact as an efficient OER electrocatalyst. The strong covalent bonding between CoOOH and RGO evidently mediates the electronic structure of the CoOOH, and effectively transfer proton-electron across the electrolyte/catalyst interface and provide sustained surface holes for oxidizing the *OOH intermediate. The CoOOH-graphene nanosheets could effectively decrease the 13



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water contact angle to ~23°, and improves the surface adsorption dynamics for fast proton-electron transfer kinetics at the electrolyte/catalyst. Benefiting from the accelerated interface charge transfer, the CoOOH-graphene nanosheets realize impressive OER performance with small onset overpotential of 200 mV and quite low overpotential of 248 mV at 10 mA/cm2. The insights gained from structure characterization and first-principle calculations clearly reveal that the newly formed 3d-electron structure induced by RGO endows CoOOH-graphene with charge transmission line characteristic to promote interfacial proton-electron transfer and the formation of OER intermediates. These results may open up a new avenue into the design of efficient and cost-effective OER electrocatalysts.

Experimental section Synthesis of CoOOH-graphene and CoOOH nanosheets. Graphene oxide (GO) was synthesized from commercial graphite by the modified Hummer’s method and then was further decorated with surface functional hydroxyl groups via peroxide treatment. The heterostructure CoOOH-graphene nanosheets were synthesized by a two-step strategy with GO-template-assisted growth hydrothermal reaction in combination with a subsequent topochemical transformation process. Firstly, the initial α-Co(OH)2 nanosheet seeds nucleated at the surface of GO-template due to the functional groups coupling and surface electrostatic effect. Subsequently, the Co ions precursor chemisorbed on the nucleus and enabled an oriented growth along the surface of GO-templates at a high autoclaving temperature of 130 °C for 24 h. Then, the synthetic products were subjected to centrifugation, ultrasonication, and oxidation to obtain the CoOOH-graphene nanosheets. Typically, 0.75 mmol CoCl2·6H2O was dissolved in 40 mL mixed solution of ethylene glycol and deionized water containing 5 mg graphene oxide under vigorous stirring to form a homogeneous solution. After bubbled by N2 for 1 h, 100 µL ammonia was added, and then the solution was transferred into a 50 mL Teﬂon-lined stainless steel autoclave and maintained at 130 °C for 24 h. After that, the obtained products were collected by centrifugation, washed with ethanol three times, and then ultrasonic dispersed in deionized water again for future oxidation treatment. NaClO solution (5.2 wt%) was dropped into the 14


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dispersion under vigorous stirring for topochemical transformation. The final product was filtrated, collected, and thoroughly washed with deionized water several times, then dried at 60 °C under vacuum. For comparison, the CoOOH nanosheets were synthesized in the same process without GO-template. The reduced graphene oxide (RGO) was gained via hydrothermal treatment of graphene oxide. The IrO2 was commercial product obtained from Alfa Aesar. Structure characterization. The FE-SEM was performed by using a FEI Sirion-200 scanning electron microscope. TEM, HRTEM, EDS, and SAED patterns were conducted on a JEOL-2010 TEM with an acceleration voltage of 200 kV. AFM was acquired on Veeco DI Nano-scope MultiMode V system. XPS spectra were taken on an ESCALAB MKII with Mg Kα (hυ = 1253.6 eV) as the excitation source. The binding energies obtained in the XPS were corrected for specimen charging S3 by referencing C 1s to 284.5 eV. XRD patterns were recorded by using a Philips X’Pert Pro Super diffractometer with Cu Kα radiation (λ = 1.54178 Å). Raman spectra were recorded at ambient temperature with a NEXUS 670 FT-IR Raman spectrometer under a 532 nm excitation laser source. The XAFS data were collected at BL14W1 station in Shanghai Synchrotron Radiation Facility and 1W1B station in Beijing Synchrotron Radiation Facility. The C and O K-edge and Co L2,3-edge XANES were measured at BL12B-a beamline of National Synchrotron Radiation Laboratory in the total electron yield mode under a vacuum better than 5 × 10-6 Pa. The beam from the bending magnet was monochromatized utilizing a varied line-spacing plane grating and refocused by a toroidal mirror. Electrochemical measurement methods. Electrochemical measurements were performed using an electrochemical workstation (Model CHI760D, CH instruments, Inc., Austin, TX) with a three-electrode system, operated with the modified glassy carbon disk electrode as working electrode, platinum mesh as the counter electrode, and saturated Ag/AgCl (3M KCl) as reference electrode. All the electrochemical measurements were conducted in 1 M NaOH (aq) electrolytes continuously purged with 99.999% N2 (Praxair) and at a sweep rate of 10 mV s−1. The glassy carbon disk electrodes with a diameter of 3 mm covered by various catalyst films are used as the working electrodes. All polarization curves were corrected for iR losses unless otherwise noted. Tafel slopes were obtained from the extrapolation of the linear 15



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region of a plot of overpotential versus current density. Cyclic voltammetry curves were measured in the region of 1.10–1.40 V at various scan rates (20, 40, 60 mV S-1, etc.) for the calculation of the double-layer capacitance (Cdl). Electrochemical impedance spectroscopy (EIS) dates were recorded with frequency range of 0.1–100 kHz at overpotential of 250 mV vs RHE.

Supporting Information Characterization of CoOOH-graphene and CoOOH samples, details of XAFS and electrochemical measurements, and the DFT calculation details and results.

Acknowledgements This work was supported by the National Natural Science Foundation of China (Grants No. 21533007, 11621063, U1532265, 21603207, 11435012, 11305174, and 11422547), the Fundamental Research Funds for the Central Universities (WK2310000054), and the China Postdoctoral Science Foundation (2016M590581).

References (1) Wang, J.; Cui, W.; Liu, Q.; Xing, Z.; Asiri, A. M.; Sun, X. Recent Progress in Cobalt-Based Heterogeneous Catalysts for Electrochemical Water Splitting. Adv. Mater. 2016, 28, 215-230. (2) Cobo, S.; Heidkamp, J.; Jacques, P. A.; Fize, J.; Fourmond, V.; Guetaz, L.; Jousselme, B.; Ivanova, V.; Dau, H.; Palacin, S.; Fontecave, M.; Artero, V. A Janus Cobalt-Based Catalytic Material for Electro-Splitting of Water. Nat. Mater. 2012, 11, 802-807. (3) Thoi, V. S.; Sun, Y.; Long, J. R.; Chang, C. J. Complexes of Earth-Abundant Metals for Catalytic Electrochemical Hydrogen Generation Under Aqueous Conditions. Chem. Soc. Rev. 2013, 42, 2388-2400.

16


Page 16 of 22

Page 17 of 22

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(4) Ma, T. Y.; Dai, S.; Jaroniec, M.; Qiao, S. Z. Metal-Organic Framework Derived Hybrid Co3O4-Carbon Porous Nanowire Arrays as Reversible Oxygen Evolution Electrodes. J. Am. Chem. Soc. 2014, 136, 13925-13931. (5) Jin, H.; Wang, J.; Su, D.; Wei, Z.; Pang, Z.; Wang, Y. In Situ Cobalt-Cobalt Oxide/N-doped Carbon Hybrids as Superior Bifunctional Electrocatalysts for Hydrogen and Oxygen Evolution. J. Am. Chem. Soc. 2015, 137, 2688-2694. (6) Lee, J. G.; Hwang, J.; Hwang, H. J.; Jeon, O. S.; Jang, J.; Kwon, O.; Lee, Y.; Han, B.; Shul, Y. G. A New Family of Perovskite Catalysts for Oxygen-Evolution Reaction in Alkaline Media: BaNiO3 and BaNi0.83O2.5. J. Am. Chem. Soc. 2016, 138, 3541-3547. (7) Gorlin, M.; Chernev, P.; Ferreira de Araujo, J.; Reier, T.; Dresp, S.; Paul, B.; Krahnert, R.; Dau, H.; Strasser, P. Oxygen Evolution Reaction Dynamics, Faradaic Charge Efficiency, and the Active Metal Redox States of Ni-Fe Oxide Water Splitting Electrocatalysts. J. Am. Chem. Soc. 2016, 138, 5603-5614. (8) Lewis, N. S. Research Opportunities to Advance Solar Energy Utilization. Science 2016, 351, aad1920. (9) Lu,

Q.;

Yu,

Y.;

Ma,

Q.;

Chen,

B.;

Transition-Metal-Dichalcogenide-Nanosheet-Based

Zhang, Composites

H.

2D for

Photocatalytic and Electrocatalytic Hydrogen Evolution Reactions. Adv. Mater. 2016, 28, 1917-1933. (10) Suntivich, J.; May, K. J.; Gasteiger, H. A.; Goodenough, J. B.; Shao-Horn, Y. A Perovskite Oxide Optimized for Oxygen Evolution Catalysis from Molecular Orbital Principles. Science 2011, 334, 1383-1385. (11) Huang, J. H.; Chen, J. T.; Yao, T.; He, J. F.; Jiang, S.; Sun, Z. H.; Liu, Q. H.; Cheng, W. R.; Hu, F. C.; Jiang, Y.; Pan, Z. Y.; Wei, S. Q. CoOOH Nanosheets with High Mass Activity for Water Oxidation. Angew. Chem. Int. Ed. 2015, 54, 8722-8727. (12) Zhu, Y.; Zhou, W.; Chen, Z. G.; Chen, Y.; Su, C.; Tade, M. O.; Shao, Z. SrNb(0.1)Co(0.7)Fe(0.2)O(3-delta) Perovskite as a Next-Generation Electrocatalyst for Oxygen Evolution in Alkaline Solution. Angew. Chem. Int. Ed. 2015, 54, 3897-3901. (13) Busch, M.; Ahlberg, E.; Panas, I. Water Oxidation on MnOx and IrOx: Why Similar Performance? J. Phys. Chem. C 2013, 117, 288-292. 17



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 22

(14) Zhang, B.; Zheng, X.; Voznyy, O.; Comin, R.; Bajdich, M.; García-Melchor, M.; Han, L.; Xu, J.; Liu, M.; Zheng, L. Homogeneously Dispersed Multimetal Oxygen-Evolving Catalysts. Science 2016, 352, 333-337. (15) Lee, Y.; Suntivich, J.; May, K. J.; Perry, E. E.; Shao-Horn, Y. Synthesis and Activities of Rutile IrO2 and RuO2 Nanoparticles for Oxygen Evolution in Acid and Alkaline Solutions. J. Phys. Chem. Lett. 2012, 3, 399-404. (16) Petrie, J. R.; Jeen, H.; Barron, S. C.; Meyer, T. L.; Lee, H. N. Enhancing Perovskite Electrocatalysis through Strain Tuning of the Oxygen Deficiency. J. Am. Chem. Soc. 2016, 138, 7252-7255. (17) Zhang, B.; Zheng, X. L.; Voznyy, O.; Comin, R.; Bajdich, M.; Garcia-Melchor, M.; Han, L. L.; Xu, J. X.; Liu, M.; Zheng, L. R.; de Arquer, F. P. G.; Dinh, C. T.; Fan, F. J.; Yuan, M. J.; Yassitepe, E.; Chen, N.; Regier, T.; Liu, P. F.; Li, Y. H.; De Luna, P.; Janmohamed, A.; Xin, H. L. L.; Yang, H. G.; Vojvodic, A.; Sargent, E. H. Homogeneously Dispersed Multimetal Oxygen-Evolving Catalysts. Science 2016, 352, 333-337. (18) Bonaccorso, F.; Colombo, L.; Yu, G.; Stoller, M.; Tozzini, V.; Ferrari, A. C.; Ruoff, R. S.; Pellegrini, V. 2D materials. Graphene, Related Two-Dimensional Crystals, and Hybrid Systems for Energy Conversion and Storage. Science 2015, 347, 1246501. (19) Deng, D.; Novoselov, K. S.; Fu, Q.; Zheng, N.; Tian, Z.; Bao, X. Catalysis with Two-Dimensional Materials and Their Heterostructures. Nat. Nanotechnol. 2016, 11, 218-230. (20) Geim, A. K.; Novoselov, K. S. The Rise of Graphene. Nat. Mater. 2007, 6, 183-191. (21) McAlpin, J. G.; Stich, T. A.; Ohlin, C. A.; Surendranath, Y.; Nocera, D. G.; Casey,

W.

H.;

Britt,

R.

D.

Electronic Structure

Description

of a

[Co(III)3Co(IV)O4] Cluster: A Model for the Paramagnetic Intermediate in Cobalt-Catalyzed Water Oxidation. J. Am. Chem. Soc. 2011, 133, 15444-15452. (22) Sun, Z. H.; Liu, Q. H.; Yao, T.; Yan, W. S.; Wei, S. Q. X-Ray Absorption Fine Structure Spectroscopy in Nanomaterials. Sci. China Mater. 2015, 58, 313-341. (23) Sun, Z. H.; Yang, X. Y.; Wang, C.; Yao, T.; Cai, L.; Yan, W. S.; Jiang, Y.; Hu, F. C.; He, J. F.; Pan, Z. Y.; Liu, Q. H.; Wei, S. Q. Graphene Activating 18


Page 19 of 22

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Room-Temperature Ferromagnetic Exchange in Cobalt-Doped ZnO Dilute Magnetic Semiconductor Quantum Dots. ACS Nano 2014, 8, 10589-10596. (24) Liang, Y.; Li, Y.; Wang, H.; Zhou, J.; Wang, J.; Regier, T.; Dai, H. Co(3)O(4) Nanocrystals on Graphene as a Synergistic Catalyst for Oxygen Reduction Reaction. Nat. Mater. 2011, 10, 780-786. (25) Liang, Y.; Wang, H.; Zhou, J.; Li, Y.; Wang, J.; Regier, T.; Dai, H. Covalent Hybrid of Spinel Manganese–Cobalt Oxide and Graphene as Advanced Oxygen Reduction Electrocatalysts. J. Am. Chem. Soc. 2012, 134, 3517-3523. (26) Wang, J.; Zhou, J.; Hu, Y.; Regier, T. Chemical Interaction and Imaging of Single Co3O4/Graphene Sheets Studied by Scanning Transmission X-Ray Microscopy and X-Ray Absorption Spectroscopy. Energy Environ. Sci. 2013, 6, 926-934. (27) Yang, L.; Zhang, R.; Liu, B.; Wang, J.; Wang, S.; Han, M. Y.; Zhang, Z. Pi-Conjugated Carbon Radicals at Graphene Oxide to Initiate Ultrastrong Chemiluminescence. Angew. Chem. Int. Ed. 2014, 53, 10109-10113. (28) Zhou, S.; Miao, X.; Zhao, X.; Ma, C.; Qiu, Y.; Hu, Z.; Zhao, J.; Shi, L.; Zeng, J. Engineering Electrocatalytic Activity in Nanosized Perovskite Cobaltite through Surface Spin-State Transition. Nat. Commun. 2016, 7, 11510. (29) Rojas, T. C.; Sánchez-López, J. C.; Sayagués, M. J.; Reddy, E. P.; Caballero, A.; Fernández, A. Preparation, Characterization and Thermal Evolution of Oxygen Passivated Nanocrystalline Cobalt. J. Mater. Chem. 1999, 9, 1011-1017. (30) Raj, R.; Maroo, S. C.; Wang, E. N. Wettability of Graphene. Nano Lett. 2013, 13, 1509-1515. (31) McCrory, C. C.; Jung, S.; Peters, J. C.; Jaramillo, T. F. Benchmarking Heterogeneous Electrocatalysts for the Oxygen Evolution Reaction. J. Am. Chem. Soc. 2013, 135, 16977-16987. (32) Meng, Y.; Song, W.; Huang, H.; Ren, Z.; Chen, S. Y.; Suib, S. L. Structure-Property Relationship of Bifunctional MnO2 Nanostructures: Highly Efficient, Ultra-Stable Electrochemical Water Oxidation and Oxygen Reduction Reaction Catalysts Identified in Alkaline Media. J. Am. Chem. Soc. 2014, 136, 11452-11464.

19



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(33) Stoerzinger,

K.

A.;

Qiao,

L.;

Biegalski,

M.

Page 20 of 22

D.;

Shao-Horn,

Y.

Orientation-Dependent Oxygen Evolution Activities of Rutile IrO2 and RuO2. J. Phys. Chem. Lett. 2014, 5, 1636-1641. (34) Liu, Y.; Hua, X.; Xiao, C.; Zhou, T.; Huang, P.; Guo, Z.; Pan, B.; Xie, Y. Heterogeneous Spin States in Ultrathin Nanosheets Induce Subtle Lattice Distortion To Trigger Efficient Hydrogen Evolution. J. Am. Chem. Soc. 2016, 138, 5087-5092. (35) Klahr, B.; Gimenez, S.; Fabregat-Santiago, F.; Bisquert, J.; Hamann, T. W. Photoelectrochemical and Impedance Spectroscopic Investigation of Water Oxidation with "Co-Pi"-Coated Hematite Electrodes. J. Am. Chem. Soc. 2012, 134, 16693-16700. (36) Gao, M. R.; Chan, M. K.; Sun, Y. Edge-Terminated Molybdenum Disulfide with a 9.4-A Interlayer Spacing for Electrochemical Hydrogen Production. Nat. Commun. 2015, 6, 7493. (37) Cummings, C. Y.; Marken, F.; Peter, L. M.; Wijayantha, K. G.; Tahir, A. A. New Insights into Water Splitting at Mesoporous Alpha-Fe2O3 Films: a Study by Modulated Transmittance and Impedance Spectroscopies. J. Am. Chem. Soc. 2012, 134, 1228-1234. (38) Adachi, M.; Sakamoto, M.; Jiu, J.; Ogata, Y.; Isoda, S. Determination of Parameters of Electron Transport in Dye-sensitized Solar Cells Using Electrochemical Impedance Spectroscopy. J. Phys. Chem. B 2006, 110, 13872-13880. (39) Sun, Z. H.; Yan, W. S.; Yao, T.; Liu, Q. H.; Xie, Y.; Wei, S. Q. XAFS in Dilute Magnetic Semiconductors. Dalton Trans. 2013, 42, 13779-13801. (40) Yao, T.; Sun, Z. H.; Li, Y. Y.; Pan, Z. Y.; Wei, H.; Xie, Y.; Nomura, M.; Niwa, Y.; Yan, W. S.; Wu, Z. Y.; Jiang, Y.; H., L. Q.; Q., W. S. Insights into Initial Kinetic Nucleation of Gold Nanocrystals. J. Am. Chem. Soc. 2010, 132, 7696-7701. (41) Yao, T.; Zhang, X. D.; Sun, Z. H.; Liu, S. J.; Huang, Y. Y.; Xie, Y.; Wu, C.; Yuan, X.; Zhang, W. Q.; Wu, Z. Y.; Pan, G. Q.; Hu, F. C.; Wu, L. H.; Liu, Q. H.; Wei, S. Q. Understanding the Nature of the Kinetic Process in a VO2 Metal-Insulator Transition. Phys. Rev. Lett. 2010, 105, 226405. 20


Page 21 of 22

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(42) Huang, J. H.; Shang, Q. C.; Huang, Y. Y.; Tang, F. M.; Zhang, Q.; Liu, Q. H.; Jiang, S.; Hu, F. C.; Liu, W.; Luo, Y.; Yao, T.; Jiang, Y.; Pan, Z. Y.; Sun, Z. H.; Wei, S. Q. Oxyhydroxide Nanosheets with Highly Efficient Electron-Hole Pair Separation for Hydrogen Evolution. Angew. Chem. Int. Ed. 2016, 128, 2177-2181. (43) He, S.; Huang, Y. Y.; Huang, J. H.; Liu, W.; Yao, T.; Jiang, S.; Tang, F. M.; Liu, J. K.; Hu, F. C.; Pan, Z. Y.; Liu, Q. H. Ultrathin CoOOH Oxides Nanosheets Realizing Efficient Photocatalytic Hydrogen Evolution. J. Phys. Chem. C 2015, 119, 26362-26366. (44) Lee, J. S.; You, K. H.; Park, C. B. Highly Photoactive, Low Bandgap TiO2 Nanoparticles Wrapped by Graphene. Adv. Mater. 2012, 24, 1084-1088. (45) Cai, L.; He, J. F.; Liu, Q. H.; Yao, T.; Chen, L.; Yan, W. S.; Hu, F. C.; Jiang, Y.; Zhao, Y. D.; Hu, T. D.; Sun, Z. H.; Wei, S. Q. Vacancy-Induced Ferromagnetism of MoS2 Nanosheets. J. Am. Chem. Soc. 2015, 137, 2622-2627. (46) Lu, Z.; Wang, H.; Kong, D.; Yan, K.; Hsu, P. C.; Zheng, G.; Yao, H.; Liang, Z.; Sun, X.; Cui, Y. Electrochemical Tuning of Layered Lithium Transition Metal oxides for Improvement of Oxygen Evolution Reaction. Nat. Commun. 2014, 5, 4345. (47) Liu, Q. H.; He, J. F.; Yao, T.; Sun, Z. H.; Cheng, W. R.; He, S.; Xie, Y.; Peng, Y. H.; Cheng. H.; Sun, Y. F.; Jiang, Y.; Hu, F. C.; Xie, Z.; Yan, W. S.; Pan, Z. Y.; Wu, Z. Y.; Wei, S. Q. Aligned Fe2TiO5-Containing Nanotube Arrays with Low Onset Potential for Visible-Light Water Oxidation. Nat. Commun. 2014, 5, 5122.

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Strong Surface Hydrophilicity in Co-Based Electrocatalysts for Water

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