Water Oxidation on Spinel NiCo2O4 Nanoneedles Anode

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Water Oxidation on Spinel NiCo2O4 Nanoneedles Anode: Microstructures, Specific Surface Character, and the Enhanced Electrocatalytic Performance Huijie Shi and Guohua Zhao* Department of Chemistry, Tongji University, Shanghai 200092, People’s Republic of China S Supporting Information *

ABSTRACT: Aiming to improve the electrochemical catalytic performance of the spinel NiCo2O4 as water oxidation catalyst, solvothermal method was employed in this work to fabricate NiCo2O4 directly on conductive substrate FTO as integrated anode. By simply altering the solvent in the precursor solution, NiCo2O4 with different morphology was obtained. The electrocatalytic water oxidation behavior of both NiCo2O4 nanoneedles (NNs) and NiCo2O4 nanosheets (NSs) were investigated in analytical scale, and the results showed that NiCo2O4 NNs exhibited enhanced catalytic performance with lower onset potential, larger current density, and faster kinetics in water oxidation process compared with NiCo2O4 NSs. Meanwhile, both the anodes presented excellent stability in the basic conditions which favored oxygen evolution. The reasons for the superior catalytic activity of NiCo2O4 NNs were also discussed in depth by investigating the surface elements composition and distribution, as well as the different chemical state of the surface adsorbed oxygen. It suggested that the NiCo2O4 NNs anode surface which was better hydroxylated and had more physic- and chemisorbed water was beneficial for enhanced water oxidation performance. It was believed that the present work may provide valuable experimental foundation and an exemplary method for improving the activity of water oxidation catalyst.

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INTRODUCTION With the increasing consumption of fossil fuels and the related environmental problems, it has attracted much attention to look for clean and renewable energy resources.1 Solar energy conversion is considered as a sustainable energy resource, which is expected to last billions of years.2 However, it is noted that the solar energy has the disadvantage of periodicity for a given location;3 and therefore, it is of great significance to store the harvested solar energy in an efficient way. Water splitting is believed as a promising strategy to collect the solar energy and store it in the form of chemical bonds.4 In this process, water oxidation is supposed to be the critical step, which provides protons with high energy that later can be reduced to hydrogen or react with CO2, etc. to produce energy materials.5,6 However, it is worth noting that in water oxidation, a total of four electrons and four protons should be removed from two water molecules to generate one oxygen molecule.7 Owing to the multielectron nature of water oxidation reaction, it has a sluggish oxygen evolution kinetics, and a large overpotential (η) is usually required to facilitate the reaction. Up to now, the efficiency of water oxidation is not satisfying yet. As a result, a search for efficient and stable electrocatalyst is urgently needed in order to lower the overpotential, improve the water oxidation efficiency, and accelerate the evolution kinetics. Another factor that hinders the fast commercial development of solar energy conversion is the application of expensive metals. It is the fact that in the present many of the most promising results in water oxidation were obtained with the © 2014 American Chemical Society

electrocatalysts based on the complexes or oxides of ruthenium and iridium.8,9 Therefore, in order to achieve large-scale application of water oxidation and solar energy conversion, electrocatalysts composed of less expensive and more abundant metals such as manganese (Mn),10,11 iron (Fe),12,13 and cobalt (Co)14,15 are highly desired. Mixed valence oxides involving transition metals such as Co, Ni, Zn, Mn, Fe, etc., typically in a spinel structure, have recently attracted increasing research interest benefiting from the remarkable electrochemical properties,16,17 especially the nanostructured Co-based electrocatalysts owing to the potentially high activity and relatively easy preparation, which have the prospect to become substitutes for the catalysts based on noble metals in the fields of energy conversion and storage. In particular, the spinel binary nickel cobaltite (NiCo2O4) has found extensive application in the fields of magnetic materials,18 lithium ion batteries,19,20 fuel cells,21 and electrochemical capacitors,22,23 etc. Meanwhile, it has also aroused much interest and been employed as water oxidation catalyst (WOC) attributed to its promising electrocatalytic activity and stability in alkaline condition, which favors water oxidation.24,25 Compared with the cobalt oxides such as Co3O4 that has been reported with good WOC activity, Ni doping makes NiCo2O4 possess a much better electronic conductivity, at least Received: September 4, 2014 Revised: October 15, 2014 Published: October 16, 2014 25939

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Figure 1. (A and C) SEM and TEM images of NiCo2O4 NNs and (B and D) NiCo2O4 NSs.

2 orders of magnitude higher.25,26 Although NiCo2O4 has been applied as water oxidation catalyst with promising performance in many reports,26,27 there have been very little research to reveal the relationship between the structure, composition, and property of the materials. To date, efforts for further enhancing the performance of NiCo2O4 mainly include tailoring its microstructures or using nanostructured supports. Chao Jin et al. synthesized a mesoporous NiCo2O4 spinel nanowire arrays with a template-free coprecipitation route.26 And the high specific surface area is supposed to be beneficial for the oxygen reduction and evolution reactions in which the mesopores can function as “highways” for oxygen diffusion. Sheng Chen et al. reported a hierarchically porous nitrogen-doped grapheneNiCo2O4 hybrid paper as an advanced electrocatalytic watersplitting material with favorable kinetics and strong durability, which was comparable to the reported noble metal catalyst (IrO2).27 However, in recent studies, NiCo2O4 exists more likely in the form of powder. It is not only inconvenient for electrochemical test, but also the extra contact resistance and the increased “dead surface” due to the introduction of a conductive agent and a polymer binder during the conventional electrode modification will have a negative effect on the stability and electrocatalytic activity.16,28 As a result, in this work, NiCo2O4 was fabricated directly on conductive substrate FTO as an integrated anode via a facial template-free solvothermal method in order to avoid the abovementioned disadvantage. Different morphologies of NiCo2O4 can be obtained by simply changing the solvent N,Ndimethylformamide (DMF)/water to ethanol/water in the precursor solution. The different water oxidation performance of both NiCo2O4 electrodes was investigated. Particularly, the reasons for the enhanced electrochemical performance of NiCo2O4 anode were discussed in detail with relation to different morphology and microstructure, electron transfer ability, surface elements distribution, and surface-adsorbed oxygen, etc. It is believed that the research will provide useful

information for tailoring the morphology and water oxidation activity of an electrocatalyst.

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EXPERIMENTAL SECTION

Materials and Apparatus. Cobalt(II) chloride (CoCl2· 6H2O), Nickel(II) chloride (NiCl2·6H2O), and hexamethylenetetramine (HMTA) were purchased from Sigma-Aldrich. FTO (fluorine-doped tin oxide) was obtained from NSG group (Japan) with a sheet resistance of 14 ohm/sq. All the solutions were prepared with deionized water unless stated. Field emission-scanning electron microscope (FE-SEM, Hitachi S-4800, Japan) and high-resolution transmission electron microscope (HR-TEM, JEOL, Japan) were used for characterizing the morphology and crystal structure of the NiCo2O4 anodes. X-ray diffraction (XRD) measurements were performed on a Bruker D8 Advance diffractometer (Germany) with Cu Kα1 radiation (λ = 1.5406 Å, 40 kV, 100 mA) to study the crystalline structure of NiCo2O4. Raman spectra were recorded on a Rainshaw invia reflex Raman spectrometer using an infrared excitation laser source at the wavelength of 785 nm. X-ray photoelectron spectroscopy (XPS, AXIS Ultra DLD, Japan) was used to characterize the elements and oxidation states distribution of the anodes with a Mg Kα source. Brunauer−Emmett−Teller (BET) specific surface areas were determined from nitrogen sorption isotherms that were performed on a Micrometrics TriStar 3000 system. Fabrication of NiCo2O4 NanoNeedles (NNs) Anode. Before modification, FTO was ultrasonically cleaned for 60 min in a mixed solution of deionized water, acetone, and 2-propanol with a volume ratio of 1:1:1. Then 1 mmol NiCl2·6H2O, 2 mmol CoCl2·6H2O and 6 mmol HMTA were dissolved in 60 mL mixed solution of deionized water and DMF (v/v: 2/1) to form the precursor solution. After that, the solution was transferred to an 80 mL Teflon autoclave, and a piece of cleaned FTO was placed at an angle against the wall of the Teflon-liner with the conductive side facing down. The 25940

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solvothermal reaction was carried out at 90 °C for 10 h and cooled to room temperature naturally. The as-prepared electrode was then taken out, and the loosely attached products were removed ultrasonically. Finally, the electrode was dried at 60 °C and annealed in air at 320 °C for 2 h with a heating rate of 2 °C min−1 in order to make it crystallized. The catalyst loading was determined to be approximately 0.53 mg/cm2. Fabrication of NiCo2O4 Nanosheets (NSs) Anode. The NiCo2O4 NSs electrode was constructed with the similar method of that of NiCo2O4 NNs, except that during the solvothermal reaction, DMF was exchanged to ethanol with the same volume. The catalyst loading was determined to be approximately 0.93 mg/cm2. Electrochemical Measurements. All the electrochemical measurements were performed in 1 M KOH at room temperature using a CHI 660C electrochemical workstation (CHI Inc.) unless other stated. A conventional three-electrode system was employed with NiCo2O4 NNs or NiCo2O4 NSs as working electrode, a saturated calomel electrode (SCE) as a reference electrode, and a silver wire as the counter-electrode. Linear scan voltammograms and cyclic voltammetry were conducted with a scan rate of 5 mV s−1. Tafel plots were obtained at the scan rate of 10 mV s−1. The controlled potential electrolysis (CPE) was performed using the amperometric i−t curve. ac Impedance spectroscopy was recorded in 0.1 M KCl containing 10 mM [Fe(CN)6]3−/4− under the following conditions: ac voltage amplitude 5 mV, frequency ranges from 105 to 1−2 Hz and open circuit. Please note that the current density was normalized to the geometrical area and the measured potential versus SCE could be converted to a reversible hydrogen electrode (RHE), according to the Nernst equation (ERHE = ESCE + 0.059 pH + 0.2412); the overpotential (η) was calculated according to the formula: η(V) = ERHE − 1.23 V.

a diameter of approximately 150 nm. However, by simply exchanging the solvent of DMF/H2O (1/2) to ethanol/H2O (1/2) and the other conditions kept the same, the morphology of NiCo2O4 changed from nanoneedles to nanosheets (NSs) with the thickness of about 100 nm (Figure 1B). It was speculated that in the process of solvothermal reaction, the solvent DMF and ethanol played an important role in reforming the morphology of NiCo2O4 as structure-directing agents. TEM images of NiCo2O4 NNs and NiCo2O4 NSs were also recorded (Figure 1, panels C and D). It was shown that both the NiCo2O4 NNs and NiCo2O4 NSs were composed of nanocrystals mostly in the range of 10−20 nm, and the loose internal structure demonstrated the presence of porous structure in the sample. The small particle sizes led to a relatively large BET surface area, which were determined to be 71.9 and 73.7 m2/g, respectively. The porous structure of NiCo2O4 anode would contribute a lot to the fast mass transport and oxygen diffusion.26,27 Besides, the inset HRTEM images of both NiCo2O4 NNs and NiCo2O4 NSs showed welldefined lattice fringes with spacing of 0.28 nm, which could be readily indexed to (220) crystal planes of spinel NiCo2O4, indicating high crystallinity of the samples. XRD measurements were then conducted on both NiCo2O4 NNs and NiCo2O4 NSs electrodes. The results (Figure 2) indicated that both of them represented spinel phase compared with the standard PDF card (PDF no. 20-0781). The well-defined diffraction peaks at 2θ values of 31.3°, 36.8°, 44.3°, and 65.1° could be found in both NiCo2O4 NNs and NiCo2O4 NSs which could be to assigned to the (220), (311), (400), (511), and (440) crystal planes.29,30 These results confirmed that spinel NiCo2O4 with different morphology was successfully fabricated. Following, the electrochemical water oxidation behavior of both NiCo2O4 NNs and NiCo2O4 NSs anodes was investigated in 1 M KOH solution, employing linear sweep voltammetry (LSV). As shown in Figure 3A, both NiCo2O4 NNs and NiCo2O4 NSs exhibited separated oxidation features from the water oxidation. The oxidation wave at about 0.4 V versus SCE could be assigned to the oxidation of the catalyst [i.e., the oxidation of Co (III) to Co (IV) according to the literature],26 whereas the following sharp increment of the current was the result of water oxidation. The obvious oxidation wave of Co (III) to Co (IV) indicated that the cation of Co (III) in the octahedral site of the spinel was the main active sites that were responsible for the catalysis of water oxidation.25 For the cation of M (II) (M: Co, Ni), because they were deep located in the tetrahedral sites of the spinel and more difficult to be oxidized, the peaks corresponding to Co(II)/Co(III) and Ni(II)/Ni(III) were not visible in the LSV plots, although they also played an important role in water oxidation.31 The onset potential of the electrocatalytic water oxidation on NiCo2O4 NNs and NiCo2O4 NSs was determined by the intercept of lines extrapolated from the exchange current and the catalysis current in the high overpotential (Tafel) region.32 The onset potential of NiCo2O4 NNs obtained by this method was 365 mV, which was smaller than that of NiCo2O4 NSs (415 mV) and even the recent hottest water oxidation catalyst CoPi (413 mV), which was prepared by electrochemical deposition on FTO as reported (see Text SI 1 of the Supporting Information). This onset potential of NiCo2O4 NNs was much lower than that (600 mV) of the NiCo2O4 spinel nanowire arrays in the literature26 but larger than that (310 mV) of hierarchically porous nitrogen-doped grapheme-NiCo 2O4 hybrid paper which may be beneficial from the 3D conductive

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RESULTS AND DISCUSSION Microstructures and the Enhanced Electrocatalytic Water Oxidation on NiCo2O4 Anodes. In this work, a facial

Figure 2. XRD patterns of NiCo2O4 NNs and NiCo2O4 NSs.

solvothermal method was employed for fabricating NiCo2O4 on the conductive substrate FTO directly followed by a calcination process in air. As shown in Figure 1A, when the precursor solution was composed of DMF/H2 O (1/2), NiCo 2O 4 nanoneedles (NNs) could be uniformly grown on FTO, with 25941

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Figure 3. (A) LSV, (B) controlled potential electrolysis, and (C and D) Tafel plots of NiCo2O4 NNs and NiCo2O4 NSs recorded in 1 M KOH solution at a scan rate of 5 mV/s with a three-electrode system applying SCE as the reference electrode.

network.27 Comparison with other reported electrocatalysts was also summarized in Table SI 11 of the Supporting Information. The higher activity of NiCo2O4 NNs was also demonstrated by the larger current density compared with other control electrodes. As shown in Figure 3A, the overpotential needed to produce 5 mA cm−2 and 10 mA cm−2 catalytic current for NiCo2O4 NNs was about 165 mV and 323 mV lower than that of NiCo2O4 NSs and CoPi, respectively. The mass activity of NiCo2O4 NNs and NiCo2O4 NSs at 0.75 V versus SCE (potential needed to afford a current density of 10 mA cm−2) was also calculated to be 18.9 and 4.5 A g−1, respectively. Since the loading amount of NiCo2O4 NNs was only approximately one-half that of NiCo2O4 NSs, it was speculated that the structure of nanoneedles may be more beneficial for the exposure of active sites, leading to enhanced water oxidation performance and enlarged mass activity. Furthermore, the catalytic kinetics of NiCo2O4 NNs and NiCo2O4 NSs was also investigated by Tafel pots (Figure 3, panels C and D). The enhanced kinetics of NiCo2O4 NNs was proved by the relatively lower Tafel slope of 292 mV dec−1, compared with that of NiCo2O4 NSs (393 mV dec−1) and CoPi (312 mV dec−1). The water oxidation performance of NiCo2O4 anodes was also compared with that of Co3O4 nanosheets (NSs) prepared with the same methods as that of NiCo2O4 anode, except that Ni source was excluded. It showed that more positive onset potential and decreased current density could be observed on Co3O4 NSs and another control electrode Co3O4 NRs (Figure SI 8 of the Supporting Information). The results further confirmed that Ni doping could efficiently lower the onset

potential of water oxidation and improved the catalytic current density compared with Co3O4.25 Furthermore, NiCo2O4 NNs and NiCo2O4 NSs in the form of powders prepared with the same methods as that of NiCo2O4 anodes were also loaded on FTO to fabricate paste electrodes. And the water oxidation performance was investigated in order to clarify the advantages of integrated electrodes (Figure SI 9 of the Supporting Information). It could be found that much more negative onset potential and enhanced current density were obtained on NiCo2O4 NNs integrated anode compared with NiCo2O4 powder (NiCo2O4 NNs-P), owing to the “inert surface” present in NiCo2O4 NNsP introduced by the organic binder used.16 However, the difference between NiCo2O4 NSs integrated anode and NiCo2O4 NSs powder (NiCo2O4 NSs-P) was relatively small. It may be because that in NiCo2O4 NSs integrated anode, the nanosheets stacked together so that lots of active sites were blocked. But for NiCo2O4 NSs-P, more active sites may be exposed after dispersion and loading on FTO. What was more important was that the integrated anode exhibited enhanced stability in water oxidation, whereas the paste electrodes were unstable, and the catalyst film peeled off after only a few cycling, probably owing to the continuous oxygen evolution. The high catalytic stability of catalyst is of great importance for the large scale application in the energy conversion system. Therefore, the controlled potential electrolysis (CPE) was also performed at a higher potential (1 V vs SCE) to compare the catalytic activity and long-term stability. As shown in Figure 3B, the current density of NiCo2O4 NNs was approximately 3 times that of NiCo2O4 NSs, indicating higher catalytic activity of NiCo2O4 NNs. Moreover, it could be observed that after 25942

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was discovered that the electrolysis current increased steadily at the first half an hour, and then became stable. It was also confirmed by the CV cycling in 1 M KOH (Figure SI 10 of the Supporting Information), in which the peak current increased first and then became stable too. This phenomenon was consistent with that reported by S. W. Boettcher.33 It was presumed that in the electrochemical process, activation and wettability occurred on the electrode surface and it became increasingly ion-permeable through conversion of NiCo2O4 to M(OH)2 or MOOH (M: Ni, Co) on the electrode surface,33,34 which was more active in the water oxidation catalysis. Correlation between Water Oxidation Performance and the Specific Surface Character. On the basis of the above investigation, we could conclude that NiCo2O4 NNs exhibited superior catalytic activity toward water oxidation as well as faster kinetics. The reasons for the different electrochemical water oxidation behavior for NiCo2O4 NNs and NiCo2O4 NSs were discussed in detail. It was presumed that the unique one-dimensional microstructures of nanoneedles may be one reason for the enhanced activity of NiCo2O4 NNs, which may facilitate the electron transfer compared with NiCo2O4 NSs. In order to further clarify the factors causing these differences in water oxidation catalysis, the electroactive surface area was first measured for both NiCo2O4 NNs and NiCo2O4 NSs using cyclic voltammetry (CV) with 10 mM Fe(CN)63‑/4‑ in 0.1 M KCl (shown in Figure 4A). The electroactive surface area can be estimated according to the Randles-Sevcik equation.35 Figure 4. A CV plots of NiCo2O4 NNs and NiCo2O4 NSs recorded in 0.1 M KCl containing 10 mM Fe(CN)63−/4− with a scan rate of 10 mV/s; B EIS plots of NiCo2O4 NNs and NiCo2O4 NSs recorded in 0.1 M KCl containing 10 mM Fe(CN)63−/4− under open circuit potential.

Ip = 2.99 × 105nACD1/2v1/2

Where ip, n, A, C, D, and ν are the peak current, the number of electrons involved in the reaction, the electroactive surface area, the concentration of the reactant, the diffusion coefficient of the reactant species, and the scan rate, respectively. We can see from the equation that the electroactive surface area is

electrolysis for more than 3 h, the current density remained nearly unchanged, demonstrating the excellent stability of both NiCo2O4 NNs and NiCo2O4 NSs. An interesting phenomenon

Figure 5. XPS spectra for Co 2p (A, C) and Ni 2p (B, D) in NiCo2O4 NNs and NiCo2O4 NSs, respectively. 25943

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Figure 6. XPS spectra for O 1s in (A) NiCo2O4 NNs and (B) NiCo2O4 NSs; (C) Percentage of O1, O2, O3, and O4 in the O 1s in NiCo2O4 NNs and NiCo2O4 NSs.

proportional to the peak current (Ip). From calculation, the electroactive surface area of NiCo2O4 NNs was 5% larger than that of NiCo2O4 NSs, which were nearly the same. This result further confirmed that the nanoneedles structure was in favor of the exposure of active sites compared with the stacking character of nanosheets by comparison to the catalysts loading amount. The result was also consistent with the calculated mass activity. Besides, the relatively narrow peak interval of NiCo2O4 NNs (Figure 4A) suggested better electrochemical activity and electrode reaction reversibility than NiCo2O4 NSs. And it was further confirmed by the electrochemical impedance spectra (EIS) in which lower electron transfer resistance of NiCo2O4 NNs could be found compared with that of NiCo2O4 NSs (Figure 4B). From calculation, the electron transfer resistance (Rct) of NiCo2O4 NSs was approximately 1.5 times that of NiCo2O4 NNs. However, we noticed that the different morphologies and electron transfer ability cannot be the only reasons for the enhanced electrocatalytic water oxidation performance. It was speculated that the specific surface character, such as different surface element composition and chemical state of the anode may be the main reason for the different electrochemical activity. As a result, XPS measurements were taken to

Figure 7. (A) SEM of NiCo2O4 NNs, (B) NiCo2O4 NSs, and (C) Raman spectra of both NiCo2O4 NNs and NiCo2O4 NSs after LSV and controlled potential electrolysis characterization.

investigate the surface properties of the prepared NiCo2O4 anodes. As shown in Figure 5, in both NiCo2O4 NNs and NiCo2O4 NSs, the Co 2p spectra can be well-fitted to two kinds of cobalt species, characteristic of Co2+ and Co3+, while the Ni 2p spectra can be fitted to two sets of nickel species containing Ni2+ and Ni3+, together with two shakeup type peaks at the high binding energy side of the Ni 2p1/2 and Ni 2p3/2 edge.22,36 In the Co 2p spectra of NiCo2O4 NNs, the binding energy at 778.88 eV was ascribed to Co3+, while 780.28 eV was ascribed to Co2+. In Ni 2p spectra of NiCo2O4 NNs, the fitting peak at 855.36 eV was indexed to Ni3+, and the peak at 853.78 eV was related to Ni2+. Meanwhile, similar results have also been found in the Co 2P and Ni 2p spectra of NiCo2O4 NSs; the fitting peaks at 779.28 and 781.02 eV were ascribed to Co3+ and Co2+, and the fitting peaks at 855.74 eV and 853.98 were indexed to Ni3+ and 25944

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Ni2+, respectively. These results showed that both trivalent and bivalent cobalt and nickel existed in NiCo2O4 NNs and NiCo2O4 NSs, which were consistent with the literature.22,29 The surface content of Co and Ni for NiCo2O4 NNs and NiCo2O4 NSs was also calculated, and it was surprisingly found that the atomic ratio of Co/Ni was approximately 2 on the surface of NiCo2O4 NNs, while that on NiCo2O4 was only about 0.5. It suggested that the choice of solvent in solvothermal process not only influenced the morphology of the product but also altered the surface element composition and distribution, which further affected the catalytic activity. As reported, as a typical transitional-metal oxide, NiCo2O4 was particularly active for catalyzing water oxidation by interacting with water molecules to form Ni(Co)−O bonding and consequently facilitated dissociation of water.27 The results suggested that the element Co was more active in catalyzing water oxidation compared with Ni. By increasing the surface content of the Co element in NiCo2O4, its electrochemical activity would be improved. Furthermore, the surface oxygen was also investigated because they also played an important role and always functioned as active sites in the water oxidation reaction.37 Figure 6, panels A and B, showed the O 1s spectra of both NiCo2O4 NNs and NiCo2O4 NSs. Both the O 1s spectra could be fitted into four sub-bands which could be denoted as O1, O2, O3, and O4, respectively. Specifically for NiCo2O4 NNs, the component O1 at 528.8 eV was typical of metal−oxygen bonds.22,36 The component O2 sitting at 529.4 eV could be ascribed to the oxygen in OH− groups, indicating that the surface of NiCo2O4 NNs was hydroxylated to some extent. The component O3 at 530.7 eV was characteristic of a higher amount of defect sites with low oxygen coordination. The component O4 at 531.7 eV could be attributed to multiplicity of physic- and chemisorbed water at or near the surface. All the corresponding components could also be found in NiCo2O4 NSs. The relative content of different component in O 1s was all summarized in Figure 6C. As we could observe, for NiCo2O4 NNs, both the content of O2 and O4 was much higher than that in NiCo2O4 NSs. The content of component O2 which was dedicated to oxygen in OH− groups was 31.49% on the surface of NiCo2O4 NNs while that on the surface of NiCo2O4 NSs was 20.19%. It suggested that the surface of NiCo2O4 NNs, which was better hydroxylated and easy to adsorb water molecules, was beneficial for the enhanced electrocatalytic activity in water oxidation. From another perspective, as discussed above, the hydroxylated surface of NiCo2O4 indicated the existence of M(OH)2 or MOOH (M: Co, Ni) which was more ion-permeable and favorable for water oxidation as reported in the literature.33 Postcatalytic Characterization of the NiCo2O4 Anodes. To investigate the morphology and structure changes associated with the electrocatalysis, SEM and Raman spectroscopy were recorded on the NiCo2O4 electrodes prior to application of an oxidizing bias and again after all the electrochemical characterization. As indicated in Figure 7 (panels A and B), after LSV measurement and CPE for more than 3 h, the morphology of the NiCo2O4 NNs and NiCo2O4 NSs remained unchanged. Only a few crackles could be observed on NiCo2O4 NSs. Raman spectra of NiCo2O4 NNs and NiCo2O4 NSs before and after characterization also suggested the excellent stability of NiCo2O4 NNs and NiCo2O4 NSs from which we could see that the structure and crystallinity stayed nearly the same, except for the peak

intensity, which became a little weaker after characterization. It may be caused by the hydroxylation of the surface in the electrochemical process.

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CONCLUSIONS In this work, enhanced electrocatalytic water oxidation performance was obtained on spinel NiCo2O4 NNs anode with lower onset potential, larger current density, and faster kinetics. By simply altering the solvent used in the solvothermal process, not only have different morphologies been fabricated but also resulted in discriminated surface Co/Ni ratios and chemical state of adsorbed oxygen. The results showed that NiCo2O4 NNs exhibit small electron transfer resistance compared with NiCo2O4 NSs. Meanwhile, the surface of NiCo2O4 NNs was rich with the Co element, better hydroxylated and ion-permeable, and easily to adsorb water molecules. All these characteristics were beneficial for the enhanced water oxidation catalytic activity. This work will provide valuable information for improving the electrochemical activity of an electrocatalyst in the water oxidation process.

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ASSOCIATED CONTENT

S Supporting Information *

Methods for fabricating CoPi, Co3O4 NSs, and Co3O4 NRs and NiCo2O4 NNs-P and NiCo2O4 NSs-P; the SEM image of Co3O4 NRs and Co3O4 NSs; XRD patterns of Co3O4 NSs and Co3O4 NRs; water oxidation performance comparison between NiCo2O4 anodes and Co3O4 NSs, Co3O4 NRs, NiCo2O4 NNsP, and NiCo2O4 NSs-P; CV cycling of NiCo2O4 NNs and NiCo2O4 NSs in 1.0 M aqueous KOH solution. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +86-21-65981180. Fax: +86-21-65982287. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundations of China (Grants 21307091 and 21277099). REFERENCES

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The Journal of Physical Chemistry C

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dx.doi.org/10.1021/jp508977j | J. Phys. Chem. C 2014, 118, 25939−25946