Mesoporous Co3O4 Nanobundle Electrocatalysts

CHEMISTRY AN ASIAN JOURNAL www.chemasianj.org Accepted Article Title: Mesoporous Co3O4 Nanobundle Electrocatalysts Authors: Alam Venugopal Narendra Kumar, Yinghao Li, Shuli Yin, Chunjie Li, Hairong Xue, You Xu, Xiaonian Li, Hongjing Wang, and Liang Wang This manuscript has been accepted after peer review and appears as an Accepted Article online prior to editing, proofing, and formal publication of the final Version of Record (VoR). This work is currently citable by using the Digital Object Identifier (DOI) given below. The VoR will be published online in Early View as soon as possible and may be different to this Accepted Article as a result of editing. Readers should obtain the VoR from the journal website shown below when it is published to ensure accuracy of information. The authors are responsible for the content of this Accepted Article. To be cited as: Chem. Asian J. 10.1002/asia.201800651 Link to VoR: http://dx.doi.org/10.1002/asia.201800651 A Journal of A sister journal of Angewandte Chemie and Chemistry – A European Journal 10.1002/asia.201800651 Chemistry - An Asian Journal FULL PAPER Mesoporous Co3O4 Nanobundle Electrocatalysts Abstract: Tailoring metal oxide nanostructures with mesoporous architecture is highly important to improve their electrocatalytic performance. Here, we report the synthesis of two dimensional (2D) mesoporous Co3O4 (meso-Co3O4) nanobundles with uniform shape and size by employing hard template method. For this study, we have chosen incipient wetness impregnation technique for loading metal precursor into the silica hard template (SBA-15). The results reveal saturated precursor solution concentration plays a vital role in mesostructured ordering, size and shape of final meso-Co3O4 product. The optimized precursor concentration enables us to synthesize ordered meso-Co3O4 consisting 4-7 nanowires in the individual particle. The meso-Co3O4 structure exhibited excellent electrocatalytic activity for both glucose and water oxidation reactions. Introduction In the past few decades, extensive research has been carried out on the synthesis of transition metal oxides to show their physical and chemical properties changes with their structure. [1] Among several metal oxide species, the spinel-type metal oxides have received considerable attention owing to their multifunctional characteristics.[2] In particular, the spinel Co3O4 structure is widely used in biosensing, gas sensing, batteries, supercapacitor, electrocatalysis and magnetic applications. [3–7] The improved performance from this spinel structure were obtained by fine-tuning their electronic properties at nanoscale level by adopting different synthetic techniques. Currently, several synthetic strategies were established for the preparation of spinel Co3O4 particles with different size and shape. [8–11] For electrochemical application, surface area is considered as an important parameter that directly influences the performance of metal oxides. Recently, many reports have showed that the Co3O4 with mesoporous structure have improved electrocatalytic activity compared to simple Co3O4 (prepared without using SBA15 template). Hence, synthesis of metal oxides with mesoporous architecture has gained special attention. [12–14] SBA-15 and KIT-6 are the most extensively used hard templates in metal oxides preparation. [15] The final metal oxide nanostructure can be controlled by choosing the appropriate silica hard template. SBA-15 with p6mm symmetry offers a 2D structured mesoporous materials, whereas on using KIT-6 Dr. A.V. Narendra Kumar, Y. Li, S. Lin, Y. Li, Dr. H. Xue, Dr. Y. Xu, Prof. X. Li, Dr. H. Wang, Prof. L. Wang. College of Chemical Engineering Zhejiang University of Technology Hangzhou 310014, Zhejiang, P. R. China. E-mail: [email protected] and [email protected] Supporting information for this article is given via a link at the end of the document.((Please delete this text if not appropriate)) template the Ia-3d symmetry offers a 3D mesoporous structure.[16] In the hard template route, impregnation of metal precursor seems to be more crucial step, which affects the quality of final metal oxide product. Several impregnation methods have been established such as, i) wetness impregnation (excess solvent), ii) solid-liquid method, iii) twosolvent method and iv) incipient wetness impregnation. Recently, Deng et al. covered the merits and demerits of these impregnation methods in the preparation of mesoporous metal oxides.[15] In particular, wetness impregnation (excess solvent) is the most extensively studied method in hard template assisted metal oxide preparation. In this method the impregnation step is carried out twice to acquire high ordered mesoporous structure. For solid-liquid impregnation method, simple mechanical grinding of metal precursor with silica template is sufficient, and the impregnation process becomes simple and also there is no solvent evaporation step. However, slow heating rates were recommended during calcination process because, it allows sufficient amount of time for the molten metal precursor to diffuse into the silica pores before decomposing into corresponding metal oxides. The “two-solvents” impregnation method is highly time-consuming, in which the metal salt solution (aqueous) and silica containing hexane solution were stirred overnight to attain high precursor loading amount. Studies also show that the metal oxide formed via “two solvents” impregnation method has high homogeneity in the structure. [17] These three impregnation methods have widely been used in the preparation of mesoporous metal oxides. However, the incipient wetness impregnation is less frequently studied in hard template assisted metal oxide preparation, because of its difficulty with the scale up. The advantages of this “incipient wetness impregnation” method in controlling the mesostructure is described by Tiemann.[18] Although these impregnation methods produce a mesoporous structure in metal oxides. The degree of mesostructure ordering and particle size of the final metal oxide product depends on calcination temperature, silica pore size and precursor loading amount. Therefore, producing ordered Co3O4 material with high size uniformity is greatly challenging in hard template method. Previously, by adopting wetness impregnation technique in KIT-6 silica hard template, we have demonstrated the effect of calcination temperature on mesostructure ordering in Co3O4.[19] In this study, we demonstrate the synthesis of high surface area 2D meso-Co3O4 nanobundles by adopting “incipient wetness impregnation” technique. Instead of organic solvents, we use water in the preparation of saturated precursor solution. SEM and TEM analysis revealed the synthesized meso-Co3O4 nanobundles exhibit excellent size and shape regularity, while the control experiments indicate that the particle size and shape were controlled by the precursor loading amount. The technique used here allows us to synthesize meso-Co3O4 nanobundles with 4-7 nanowires. When compared with reported mesoporous Co3O4 structure, the meso-Co3O4 nanobundles exhibits high For internal use, please do not delete. Submitted_Manuscript This article is protected by copyright. All rights reserved. Accepted Manuscript Alam Venugopal Narendra Kumar, Yinghao Li, Shuli Yin, Chunjie Li, Hairong Xue, You Xu, Xiaonian Li, Hongjing Wang,* Liang Wang* 10.1002/asia.201800651 Chemistry - An Asian Journal specific surface area from BET analysis. Our electrochemical study also manifests that the synthesized meso-Co3O4 nanobundles has good catalytic ability to oxidize glucose and water. Results and Discussion Figure 1. Schematic representation for the preparation of mesoporous-Co3O4 nanobundles using SBA-15 hard template. Steps involved in the preparation of meso-Co3O4 nanobundles are outlined in Figure 1. Initially, the impregnation of Co(NO3)2 metal precursor into the SBA-15 hard template is carried out by simple physical mixing (step 1). After drying, the impregnated silica power is transferred into the furnace and heated (step 2). Finally, the silica template is etched using NaOH solution (step 3). The employed method is simple and it has widely been used in the preparation of Co3O4 and other mesostructured materials.[20,21] However, there is no studies conducted on controlling meso-Co3O4 particle morphology, size and structure uniformity by following these three synthetic steps. to load Co(NO3)2 precursor into SBA-15 hard template. In our typical synthesis the impregnation was carried out only once. This type of impregnation procedure has been previously demonstrated by us in the preparation of mesoporous Pt metallic nanostructures.[22] To the best of our knowledge, no attempts were made on the synthesis of mesoporous Co3O4 particles with high structure uniformity by adopting incipient wetness impregnation technique. Since the impregnation process adopted here differs slightly from the methods reported previously. Optical images were captured at different stages of the meso-Co3O4 nonbundles preparation (Figure 2). The impregnation process was carried out in the watch glass at open air atmosphere (Figure 2a). On adding the appropriate amount of Co2+ precursor solution to SBA-15 (see experimental details), initially the silica composite appears dry (Figure 2b). However, when the composite is continuously mixed with the spatula the precursor and silica form a thick paste (Figure 2c). This was then transferred to the air oven for drying. The impregnated silica powder was then grinded (Figure 2d) and taken for calcination. The obtained calcinated powder (Figure 2e) was transferred to Teflon liner with NaOH and heated to remove the silica hard template. The solution was washed several times with water to remove excess NaOH and dried (Figure 2f). Figure 2. Photographs captured at different stages during meso-Co3O4 synthesis. (a) SBA-15, (b) after the injection of Co(NO3)2 solution, (c) after complete mixing to form a paste, (d) dried Co(NO3)2 impregnated silica, (e) after the formation of Co3O4 with SBA-15 template and (f) meso-Co3O4 after removing the template. In this study, our experimental efforts made it possible by making simple modifications in the impregnation procedure and precursor loading amount. In contrast to the widely studied wetness impregnation procedures, Here, we adopt incipient wetness impregnate method (using minimum volume of water) SEM images of as synthesized meso-Co3O4 nanobundles were shown in Figure 3a and Figure 3b. For comparison other Figure 3. Low- and high-magnification SEM image of Co3O4 synthesized using SBA-15 hard template. (a and b) one-time impregnation of 0.25g Co(NO3)2, (c and d) two times impregnation of 0.125g Co(NO3)2 and (e and f) one-time impregnation of 0.50g Co(NO3)2. For internal use, please do not delete. Submitted_Manuscript This article is protected by copyright. All rights reserved. Accepted Manuscript FULL PAPER 10.1002/asia.201800651 Chemistry - An Asian Journal Figure 4. (a) XRD pattern for meso-Co3O4 sample. N2 adsorption-desorption isotherm profiles of SBA-15 (b), SBA-15/Co3O4 (c) and meso-Co3O4 nanobundles (d), respectively. The inset in (b, c and d) show the corresponding pore size distribution curves. The morphology of typical meso-Co3O4 particle, synthesized by one-time impregnation of 0.25 mg Co(NO3)2 precursor showed high particle size uniformity. Also, the high magnification image of same sample reveals the particles are made of individual Co3O4 nanowires (Figure 3b). To further understand the formation of meso-Co3O4 nanobundle structure, series of experiments were conducted by varying the precursor loading amount and number of impregnation steps. As shown in Figure 3c and 3d, when the impregnation process is conducted twice by keeping the total Co precursor loading amount (0.25 mg) constant. Co3O4 particles with different size and shape were formed. Again, on limiting the number of impregnation time to one and increase the Co precursor loading amount to 0.5 mg. The mesoporous Co3O4 particles show more shape irregularity and also large particle size, this is mainly because of high Co precursor loading into the silica channels. The influence of precursor loading amount with nanowires interconnectivity and mesostructured ordering has been previously described.[23] All of these observations clearly suggest that the Co precursor loading amount (0.25 mg) and one-time impregnation seems vital in manipulating meso-Co3O4 nanobundles with uniform particles through SBA-15 hard template route. From SEM analysis we found that the diameter of meso-Co3O4 nanobundles was less than 100 nm and length of roughly about 300 nm. Energy Dispersive X-ray (EDX) analysis were conducted to quantify the amount of silica present in the final meso-Co3O4. The EDX spectral data shows that the weight percentage of silica is less than 1% in the typical sample (Figure S1). Thus, the SEM results clearly demonstrate that we have successfully synthesized meso-Co3O4 nanobundles with high structural uniformity. The crystal structure of meso-Co3O4 particles were confirmed using powder X-ray diffraction (XRD) technique. As shown in Figure 4a, the meso-Co3O4 displayed diffraction peaks For internal use, please do not delete. Submitted_Manuscript This article is protected by copyright. All rights reserved. Accepted Manuscript FULL PAPER 10.1002/asia.201800651 Chemistry - An Asian Journal FULL PAPER (220) and (311) crystal planes of spinel Co3O4 structure, respectively. The redox characteristics of meso-Co3O4 nanobundles were tested using cyclic voltammetry (CV) in 0.1 M NaOH solution. In Figure 6a the CV curve of meso-Co3O4 sample show a strong redox peak with formal potential of 0.55 V corresponds to reversible CoOOH to CoO2 conversion. The peak current of this redox reaction increases linearly with the scan speed, implying the surface confined redox process of meso-Co3O4. Figure 5. (a, b and c) TEM images of the typical meso-Co3O4 sample at different magnification. (d) HRTEM image of meso-Co3O4. The inset in (c) shows the SAED pattern. Detection of glucose by electrochemical means is important to monitor blood sugar levels and control diabetes. [25] Glucose detection by regular blood extraction requires more sophisticated instrumentation and also time consuming. However, the electrochemical measurements offer some special advantages such as portability and short analysis time. Though a wide range of enzyme based electrochemical glucose sensors have already been appeared.[26] Since enzymes are sensitive to temperature and other chemical environments, acquiring reproducible results and electrode handling makes the overall process further complicated. Thus, developing a non-enzymatic glucose sensors electrode is vital for solving the later issues. Mesoporous material exhibits high electrocatalytic performance over simple nanostructured materials due to easy reactant accessibility to the surface. It has previously been showed that Co3O4 has an ability to electrochemically oxidize glucose. Taking advantage of the meso-Co3O4 structure with unique nanobundle morphology, which could provide promising activity for glucose oxidation reaction. So, the electrooxidation of glucose at mesoCo3O4 modified electrode is examined using CV in the potential range 0.00 V and 0.60 V vs Ag/AgCl. In the CV curves, the redox peaks appeared between 0.45 V and 0.55 V (CoO2/CoOOH) for meso-Co3O4, which is found to be the most active redox pair that mediates the conversion of glucose to gluconolactone.[27] As shown in Figure 6b, the oxidation peak current of CoO2/CoOOH redox couple increases with the For internal use, please do not delete. Submitted_Manuscript This article is protected by copyright. All rights reserved. Accepted Manuscript at 2θ value 19.00, 31.27, 36.85, 38.54, 44.81, 56.66, 59.36 and 65.24°. On indexing, the peaks 2θ value and their intensities were closely matching with the crystal planes of spinel Co 3O4 structure (JCPDS 42.1467). The oxidation state of Co in the spinel meso-Co3O4 also were investigated by X-ray photoelectron spectroscopy (XPS). The Co2p spectrum shows two intense peaks at 780.14 eV and 795.28 eV, these binding energy values correspond to 2p3/2 and 2p1/2 of Co3O4, respectively (Figure S2). A small peak splitting observed in 2p1/2 clearly reveals that Co atom exist in both Co 3+ and Co2+ in the meso-Co3O4. To determine the specific surface area of our meso-Co3O4 nanobundles, Brunauer–Emmett–Teller (BET) experiments were conducted. N2 adsorption-desorption isotherm profiles were collected at three different stages of synthesis (bare SBA-15, Co3O4 before silica etching and meso-Co3O4 after silica etching) Figure 4(b, c and d). The pore size distribution curves of these three samples were obtained from BJH measurement and depicted in the corresponding figures inset. The isotherms of all three sample exhibits a Type-IV pattern with a hysteresis loop between P/P0 = 0.4 and 0.8. The preceding BET features are similar to the previously formed mesopores metal oxides.[24] The SBA-15 used in this study shows a specific surface area of 759 m2 g-1, with average pore diameter 5.3 nm (Figure 4b). After the formation of Co3O4, the specific surface area is dropped to 339 m2 g-1, which is due to coverage of crystalline Co3O4 in silica mesopores. Upon silica template removal the meso-Co3O4 nanobundles displayed a specific surface of 222 m2 g-1, with the pore size of 4 nm. The measured surface area of meso-Co3O4 nanobundles in this study is much higher compared to previously reported mesoporous Co 3O4 synthesized via hard template route, which is easily understood by considering smaller particle size in which the mesopores are separated by the finite nanowire assembly. [23] Additional structural details of the meso-Co3O4 nanobundles were acquired from transmission electron microscope (TEM) images captured under bright-field mode. Figure 5a and 5b shows the low and high magnification images of meso-Co3O4, respectively. The captured image shows the formed meso-Co3O4 particles are well distributed and has almost uniform particle size throughout the sample (Figure 5a). The difference in image contrast within a single particle (Figure 5b) clearly indicates that the meso-Co3O4 nanobundles are composed of parallelly interconnected Co3O4 nanowires. Also, each particle comprises with 4 to 7 nanowires with various length and makes a mesoporous nanobundles like arrangement. The maximum wall thickness of these individual nanowires was found to be ~9 nm and the distance between the nanowires is ~3 nm (Figure 5c). The pore diameter (4 nm) measured through BJH for the meso-Co3O4 is very close to TEM based measurements. The inset in Figure 5c shows the selected area electron diffraction (SAED) pattern captured from single nanowire of meso-Co3O4 particle. Bright spots were identified in the circular pattern, which indicates high degree of crystallization in the meso-Co3O4 sample. Also, the high resolution TEM (HRTEM) image recorded by focusing individual nanowires displayed a clear lattice fringes. The lattice spacing of 0.286 nm and 0.224 nm measured at different location were matched with 10.1002/asia.201800651 Chemistry - An Asian Journal addition of 2 mM glucose. The oxidation peak current corresponding to CoO2/CoOOH redox couple rises significantly with increasing glucose concentration from 2 to 10 mM in the electrolyte solution, suggesting concentration dependant oxidation process on meso-Co3O4 modified electrode. We also extended our study using chronoamperometry technique to evaluate the glucose sensing ability of meso-Co3O4 nanobundle sample in terms of detection limit. For this experiment, the working electrode potential was kept constant at 0.58 V vs Ag/AgCl, and the glucose solution is added at multiple intervals. As seen from Figure 6c, the current spikes upon each addition of analyte indicating the oxidation process of glucose. A plot of current response vs glucose concentration is shown in inset of Figure 6c with the error bars. The calibration plot displayed two linear regions from 0 to 160 µM glucose and 160 µM to 400 µM glucose, respectively. From the calibration curve, the measured sensitivity of meso-Co3O4 catalyst was found to be 88 µA mM-1 cm-2 with lowest detection limit of 0.6 µM (S/N=3). The later obtained value seems reasonable when comparing with other non-enzymatic glucose sensors reported in the literature. [27–29] Species such as ascorbic acid (AA) and uric acid (UA) are commonly known interfering species during glucose sensing. Thus, we tested the current response of the meso-Co3O4 in the presence of these analyte molecules. For this study we choose 4, 0.125 and 0.33 mM, of glucose, AA and UA respectively. The preceding analyte concentrations were fixed based on the Hropovic et al. report.[30] In Figure 6d, we showed the oxidation currents obtained in the presence and absence of these interfering species. Figure 6. CV response of meso-Co3O4 sample at different scan rates (a) in presence of 0, 2, 4, 6, 8 and 10 mM glucose in the electrolyte at 20 mV.s-1 (b). (c) Amperometric response of meso-Co3O4 with successive addition of glucose at an applied potential of 0.58 V. The inset in (c) shows the corresponding calibration plot with error bar. (d) Comparison of meso-Co3O4 glucose oxidation current in the presence of common interfering species such as ascorbic acid and uric acid. For internal use, please do not delete. Submitted_Manuscript This article is protected by copyright. All rights reserved. Accepted Manuscript FULL PAPER 10.1002/asia.201800651 Chemistry - An Asian Journal FULL PAPER current. The variations observed for glucose oxidation current in the presence of AA and UA seem to be less significant. So, the meso-Co3O4 catalyst show good selectivity towards glucose in the presence of other possible interfering compounds. Figure 7. (a) LSV curves of the meso-Co3O4, Co3O4, RuO2 and GC electrode for OER in a 1 M KOH solution at the scan rate of 5 mV s-1. (b) The corresponding Tafel plots. (c) Nyquist plots of meso-Co3O4, Co3O4 and RuO2 samples. Previous studies have shown that mesoporous Co3O4 has very high catalytic activity for OER.[31–33] Motivated by the unique nanobundle like morphology of meso-Co3O4, we investigate its OER electrocatalytic activity by linear sweep voltammetry (LSV) measurements in 1 M KOH solution. As shown in Figure 7a, LSV curve of the meso-Co3O4 exhibits low OER onset potential (1.52 V vs RHE) compared to simple Co3O4 and GC electrode. When comparing with commercially available RuO2 catalyst under similar experimental conditions, the OER onset potential on RuO2 catalyst is ~10 mV lower than that of meso-Co3O4 sample. However, the benchmark current density of 10 mA cm -2 is achieved at the expense of 370 mV overpotential for mesoCo3O4, which is 60 mV smaller than that of commercially available RuO2 catalyst. The measured OER overpotential (370 mV) for meso-Co3O4 is also smaller than other mono and bimetal oxide based OER electrocatalyst.[23,34–36] The low overpotential exhibited by meso-Co3O4 when compared with simple Co3O4 itself is possibly due to optimum adsorption and desorption energy of reactant, intermediates and product molecules on meso-Co3O4 surface.[37] To obtain further insights on this complex proton-coupled electron transfer process at different catalytic surface Tafel plots have been constructed (Figure 7b). Tafel slope is a useful parameter to determine the rate determining step (RDS) in the OER. If the measured Tafel slopes is >120 mV dec-1, the OER is controlled by water adsorption step. Whereas, if the Tafel slope is 40 mV dec-1, O-O bond formation is the RDS.[38] For simple Co3O4, the Tafel slope was found to be 90 mV dec-1. Whereas, a relatively smaller Tafel slope of 58 mV dec-1 were obtained for meso-Co3O4 nanobundle catalyst, indicating both water adsorption and O-O formation step controls the OER rate.[39] The smaller Tafel slope measured for meso-Co3O4 nanobundle to that of simple Co3O4 catalyst manifests the faster OER kinetics on meso-structure. Although we observed a lower onset potential for commercial RuO2 catalyst, again the Tafel slope data clearly reveals that the O2 formation seems more favourable on meso-Co3O4 structure. Electrochemical impedance spectroscopy (EIS) measurements were conducted to evaluate the charge transfer resistance (Rct) of catalyst modified electrodes. The Nyquist plots of meso-Co3O4, simple Co3O4 and RuO2 were depicted in Figure 7c. The obtained Rct values are normalized with the working electrode geometric area. Based on the diameter of the semicircle the measured Rct value of meso-Co3O4 was found to be 8 Ω cm2. This value is smaller than simple Co3O4 (16.2 Ω cm2) and commercial RuO2 (38 Ω cm2) samples. On considering the key aspects of the meso-Co3O4 nanobundle structure, the high surface area and mesopores constructed by the individual nanowires would greatly improve the modified electrodes interfacial contact and mass transport, respectively. Therefore, EIS result clearly reveals that the meso-Co3O4 electrode has faster charge transfer ability, which contributes for greater OER activity. The stability of meso-Co3O4 was investigated by acceleration degradation test carried out in 1 M KOH solution. For this, the meso-Co3O4 modified GC electrode was allowed for 5000 potential scans between 1.50 V and 1.60 V vs RHE at the scan rate of 100 mV s-1. The same electrode was tested for OER by LSV to see the variations in current potential curves after continuous potential cycles. Interestingly, Figure 8a. shows no significant difference in the OER current potential profile before and after potential cycling. The later result clearly indicates the meso-Co3O4 structure possess excellent durability for OER. Figure 8. (a) Acceleration degradation tests of the meso-Co3O4 sample before and after 5000 continuous cycles. (b) Chronopotentiometry curves of the meso-Co3O4 and RuO2 at an applied current density of 10 mA cm-2. Also, the long-term stability of the typical meso-Co3O4 and commercial RuO2 catalyst were evaluated by chronopotentiometry technique at constant current density of 10 mA cm-2 for 10 h. As shown in Figure 8b, during the 10 h time scale the overpotential of meso-Co3O4 electrode remains steady. For internal use, please do not delete. Submitted_Manuscript This article is protected by copyright. All rights reserved. Accepted Manuscript The interference study results show that the glucose oxidation current is dropped by 9 % in the presence of AA (0.125 mM). Similarly, in the presence of UA (0.33 mM) the meso-Co3O4 modified electrode show 2 % current drop in glucose oxidation 10.1002/asia.201800651 Chemistry - An Asian Journal FULL PAPER Conclusions We have sucessfully demonstrated the synthesis of mesoCo3O4 nanobundles containing 4-7 nanowires by using a hard template strategy. The approach used here allows us to control the particle size and shape regularity of meso-Co3O4 nanobundles. We found the concentration of precursor solution plays a viral role in the the formation of meso-Co3O4 nanobundle and particle size. The choice of water and incipient wetness impregnation technique are the key advantages in the fabrication of the meso-Co3O4 nanobundle structure. Due to the unique morphology, the meso-Co3O4 nanobundles exhibited high BET surface area. The meso-Co3O4 shows better catalytic activity towards the oxidation of glucose and water to that of simple Co3O4 and commercial RuO2 catalyst, respectively. Experimental Section Chemicals Cobalt nitrate anhydrate (Co(NO3)2), Tetraethyl orthosilicate (TEOS), Pluronic P123 (PEO20-PPO70-PEO20), hydrochloric acid (HCl), potassium hydroxide (KOH), sodium hydroxide (NaOH), ethanol, glucose, ascorbic acid (AA) and uric acid (UA) were purchased from alladdin Co (Shanghai). Thereafter, the temperature was increased to 500 °C in 2 h and maintained for 6 h to obtain highly crystalline Co3O4. The entire heating process was carried out in the air atmosphere. Finally, the silica hard template was removed using hot 2 M NaOH. The solid material was transferred to the Teflon liner (50 ml) containing 10 ml of 2 M NaOH. The Teflon liner was placed in the stainless-steel autoclave and heated at 70 °C for 12 h. Again, the later procedure was repeated with fresh 2 M NaOH and this time at 90 °C for 12 h. The product was washed with water and ethanol several times and kept it for drying. As obtained product was taken for further characterization. The same procedure is adopted for the preparation of other composition except Co(NO3)2 precursor loading amount. Material characterization Scanning electron microscope (SEM) images of meso-Co3O4 samples were captured using HITACHI S-4700. Nitrogen sorption studies were carried out in Micromeritics Instrument Model No: ASAP2460. Transmission electron microscope (TEM) analysis was carried out in JEOL-2100F, with operating voltage of 200 kV. The wide-angle powder X-ray diffraction (XRD) patterns were collected from PANalytical X’Pert Powder with X-ray Source Cu Kα λ=0.154056 nm, voltage:40 kV, current 40 mA at the scan rate of 1˚ min-1 from10˚‒70˚ at the scan rate of 5˚. min1. All our electrochemical studies have been carried out in CHI 760E electrochemical workstation. For the electrode modification, meso-Co3O4 (2.5 mg) was mixed with water (625 µl) and 5 % Nafion solution (125 µl). This mixture was sonicated for 15 minutes to get a complete dispersion. Soon after, 3 µl of this catalyst ink was coated on to the glassy carbon (GC) electrode surface. A three-electrode configuration was employed in which GC was used as a working electrode, graphite rod and Ag/AgCl electrodes were used as counter and reference electrode respectively. For glucose oxidation reaction, GC electrode having 3 mm diameter was used as a working electrode. In the case of OER studies GC rotating disc electrode (2 mm diameter) was used. During the LSV measurement for OER, the working electrode was rotated at a constant speed of 1600 rpm. For stability study carbon paper was chosen as a working electrode. The obtained current values were normalized by the geometric area of working electrode used. Acknowledgements Synthesis of mesoporous silica (SBA-15) The mesoporous silica (SBA-15) hard template was prepared according to our previous paper.[22] In a typical synthesis, Pluronic P123 (4.0g) was added into distilled water (86.0 g) and conc. HCl (24.6 g 35%) solution and stirred it for 3 h to get complete dissolution of P123. To this solution, TEOS (8.50 g) was added and further continued the stirring for 24 h at 35 °C. Then, the solution was transferred into Teflon liner and sealed into a stainless-steel autoclave. After the hydrothermal treatment (100 °C for 24 h). The white solid product formed at the bottom of the container was collected after several washings with water and ethanol. This is then dried at 80 °C, followed by calcination at 550 °C for 6 h. As obtained silica powder was used as the hard template for synthesizing meso-Co3O4. The work was financially supported by the National Natural Science Foundation of China (No. 21601154, 21776255, 21701141), and Natural Science Foundation of Zhejiang Province (No. LQ18B010005). Keywords: Cobalt oxide • OER • Hard template • SBA-15 • Glucose sensing. References [1] X. Xia, Y. Zhang, D. Chao, C. Guan, Y. Zhang, L. Li, X. Ge, I. M. Bacho, J. Tu, H. J. Fan, Nanoscale 2014, 6, 5008– 5048. [2] M.-S. Park, J. Kim, K. J. Kim, J.-W. Lee, J. H. Kim, Y. Yamauchi, Phys. Chem. Chem. Phys. 2015, 17, 30963– 30977. [3] X. Liu, G. Qiu, X. Li, Nanotechnology 2005, 16, 3035–3040. [4] P. Dutta, M. S. Seehra, S. Thota, J. Kumar, J. Phys. Condens. Matter 2008, 20, 15218. [5] L. Bao, T. Li, S. Chen, C. Peng, L. Li, Q. Xu, Y. Chen, E. Ou, W. Xu, Small 2017, 13, 1602077. Synthesis of meso-Co3O4 nanobundles About 150 mg of SBA-15 was taken in the watch glass and 500 µL of Co(NO3)2 solution (0.5 mg/µL) was injected to it. Then, the silica power and the Co(NO3)2 solution was thoroughly mixed until we achieve a pink colour paste. (Note. Initially the water content in the silica seems insufficient and dry however upon continuous mixing the silica and Co(NO3)2 solution form a paste.) This paste was kept at 40 °C for completely evaporation of water. The dried powder was taken out and grinded well with the mortar. Then, the powder was transferred to the alumina boat and kept it in the tubular furnace. Initially, the furnace temperature was increased to 200 °C in 30 minutes and held for 4 h. For internal use, please do not delete. Submitted_Manuscript This article is protected by copyright. All rights reserved. Accepted Manuscript Whereas, the RuO2 modified electrode shows an overpotential increase and decrease over time indicating electrode instability. Therefore, the meso-Co3O4 nanobundle catalyst delivers high OER activity and stability to that of RuO2 catalyst. 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The prepared material shows decent electrocatalytic activity towards the oxidation of glucose and water. Mesoporous Co3O4 Nanobundle Electrocatalysts For internal use, please do not delete. Submitted_Manuscript This article is protected by copyright. All rights reserved. Accepted Manuscript Page No. – Page No.