Co3O4âÎ´ Quantum Dots As a Highly Efficient Oxygen Evolution

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Co3O4-# Quantum Dots as a Highly Efficient Oxygen Evolution Reaction Catalyst for Water Splitting Guangxing Zhang, Jie Yang, Han Wang, Haibiao Chen, Jinlong Yang, and Feng Pan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 27 Apr 2017 Downloaded from http://pubs.acs.org on April 27, 2017

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ACS Applied Materials & Interfaces

Co3O4-δ Quantum Dots as a Highly Efficient Oxygen Evolution Reaction Catalyst for Water Splitting

Guangxing Zhang, Jie Yang, Han Wang, Haibiao Chen, Jinlong Yang⃰, and Feng Pan⃰

School of Advanced Materials, Shenzhen Graduate School, Peking University, Shenzhen 518055, People’s Republic of China.

KEYWORDS: Lithiation/delithiation, Co3O4-δ quantum dots, oxygen vacancies, electrocatalyst, oxygen evolution reaction.

ABSTRACT Co3O4-δ quantum dots (Co3O4-δ-QDs) with a crystallite size of approximately 2 nm

and

oxygen

vacancies

were

fabricated

through

multicycle

lithiation/delithiation of mesoporous Co3O4 nanosheets. Used as an oxygen evolution reaction (OER) electrocatalyst for water splitting, the catalytic performance (an overpotential of 270 mV@10 mA cm-2 and no decay within 30 h) of Co3O4-δ-QDs is superior to that of previously reported Co-based catalysts and the state-of-the-art IrO2. Compared with the Co3O4 nanosheets, the enhanced OER activity of Co3O4-δ-QDs is attributed to two factors: one is the increased quantity of the Faradaic active sites, including the total active sites (q*Total), the most accessible active sites (q*Outer) and their ratio (q*Outer/q*Total); the other is the enhanced intrinsic electroactivity per active site evaluated by the turnover frequency (TOF) and the current density normalized by the most accessible active sites (j/q*Outer) related to OER. This multicycle lithiation/delithiation method can be applied to other transition metal oxides as well, offering a general approach to develop high performance electrocatalysts for water splitting.

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Introduction As excessive utilization of unsustainable fossil fuels will escalate the greenhouse effect, establishing renewable and hybrid energy systems is becoming more and more urgent.1, 2 Water electrolysis is a promising approach for producing, storing and using clean energy, especially for making hydrogen.3, 4 Therefore, water electrolysis has attracted significant attention. However, electrochemical water splitting is mainly hindered by the oxygen evolution reaction (OER) that involves a stepwise four-electron transfer process.5, 6 The O-H bond breaking and the subsequent O-O bond formation have to overcome high energy barriers, which makes OER the rate-determining step. Currently, noble metal oxides such as RuO2 and IrO2 are still considered as the most efficient OER catalysts, but their practical applications are limited by scarcity and high cost.7 Recently, oxides of earth-abundant and inexpensive transition metals (Fe, Co, Ni, and Mn) have been widely researched as important alternative OER catalysts.3, 8-11 However, bulk transition metal oxides are normally less active for OER owing to their low surface area and poor conductivity.12 Previous studies generally used the following three approaches to optimize the performance of OER catalysts. The first approach is to create a larger surface area through the design of nanostructures, such as nanoparticles13, nanowires14, nanosheets15 and mesopores16,

17

. For example,

Tüysüz et al. fabricated ordered mesoporous Co3O4 through a template-directed approach.16 Grewe et al. prepared mesoporous CuCo2O4 by using KIT-6 as a hard template.18 Sun et al. synthesized atomically-thin non-layered cobalt oxide to enhance the catalytic activity in OER.15 The second approach is to fabricate carbon-transition metal nanohybrids or to anchor catalysts onto electrically conductive supports to improve the conductivity.15,

19, 53-55

For instance, Ma et al. designed hybrid

Co3O4-carbon nanowires on Cu foil by carbonization of metal-organic framework to enhance their OER performance.20 The third approach is to tune the oxidation state by doping21-25 and facet control26. For example, Fominykh et al. reported that Fe-doped NiO nanoparticles showed higher OER activity than pure NiO nanoparticles.27 Among transition metal oxides, cobalt-based oxide is a stable and highly 2


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competitive OER catalyst. The high OER catalytic activity of cobalt-based oxide is owing to the Co4O4 cubane subunits and pinning of the Co3+/4+.28 Usually, Co3O4 comprises cobalt ions of two different oxidation states: Co2+ locating at the tetrahedral sites and Co3+ occupying the octahedral sites. The relative population of Co2+ and Co3+ has been demonstrated as the key to influence the catalytic performance.14, 29 The tetrahedral Co2+ site favors the formation of cobalt oxyhydroxide (CoOOH) which is active site for water oxidation.30,

31

In addition, the OER catalytic activity of

cobalt-based oxide can be changed greatly by generating oxygen vacancies in the surface.56 For example, NaBH4 treatment14 and plasma-engraving12, 32 were reported as effective methods for the modulation of oxygen vacancies to enhance the OER catalytic activity of Co3O4. It is well-known that several stable transition metal oxides, such as Fe2O333, Co3O434, MnO235 and NiO36 can be used as anode materials for lithium-ion batteries (LIBs). During the lithiation (discharge) process, the transition metal oxides are reduced to metal, and then the metal is oxidized during delithiation (charge). It is worth noting that significant volume expansion will happen when Li-ions insert into the transition metal oxides during lithiation, causing the oxides to break into a mass of small crystallites, which can expose more active sites for water oxidation.37 In addition, the cycled crystallite oxides in LIBs are usually low-valence oxides with oxygen vacancies, different from the initial oxides before cycling. The crystallites with oxygen vacancies are beneficial to H2O/OH- adsorption in the KOH solution during the OER process.38 Therefore, after multicycle lithiation/delithiation, it is expected that Co3O4 evolves into Co3O4-δ crystallites with low-valence and oxygen vacancies, which may deliver high OER activity. Herein, Co3O4 quantum dots (Co3O4-δ-QDs) with a crystallite size of approximately 2 nm were fabricated through multiple lithiation/delithiation cycles (named as Li-multiple-cycle method) of mesoporous Co3O4 nanosheets. Besides, this method could tune the concentration of oxygen vacancies and increase the population of Co2+ over Co3+. Used as an oxygen evolution electrocatalyst for water splitting, Co3O4-δ-QDs prepared with 20 lithiation/delithiation cycles (designated as 3



Co3O4-δ-QDs-20th cycle) required an overpotential of ∼270 mV to reach a current density of 10 mA cm-2, and it was superior to many previously reported Co-based catalysts and the state-of-the-art IrO2. Electrochemical analysis showed that Co3O4-δ-QDs-20th cycle possessed a low charge transfer resistance (Rct) and a large quantity of Faradaic active sites, as well as a higher TOF and a higher j/q*Outer related to the intrinsic electroactivity per active site to support the superior oxygen evolution activity. More importantly, the QDs had excellent stability to maintain rapid oxygen evolution at a high current density and demonstrated a long lifetime. This Li-multiple-cycle method can be applied to other transition metal oxides as well, providing a general approach to develop promising electrocatalysts for water splitting. Experimental sections Synthesis of mesoporous Co3O4 nanosheets: Cobalt acetate (8 mmol) and urea (16 mmol) were dissolved in 80 mL deionized water, followed by ultrasonic agitation for 30 min. Then the mixture was poured into a 100 mL Teflon-lined autoclave, sealed and heated at 100 °C for 12 h. After being cooled naturally to room temperature, the obtained pink powders were centrifuged and washed with water and ethanol many times, and then dried at 70 °C overnight. The as-prepared powders were then treated in a box furnace at 400 °C for 30 min to obtain Co3O4 nanosheets. Synthesis Co3O4-δ quantum dots: Firstly, 5 mg of Co3O4 nanosheets and 50 µL of Nafion solution and 0.95 mL of ethanol were sonicated for 30 min to form a homogeneous ink. Then, 50 µL of the ink was dropped onto a carbon fiber paper substrate (1×1 cm), and dried under lamp. The carbon fiber paper containing Co3O4 nanosheets was sealed in a 2032 coin cell in a glove box filled with pure argon. Lithium foil was used as the anode. The electrolyte was 1 M solution of LiPF6 in an ethylene carbonate (EC)/dimethyl carbonate (DMC) mixture (1/1 in volume). The galvanostatic charge/discharge test was performed between 0.4 and 3 V versus Li+/Li, and the current was set at 178mA/g. For the sake of thorough delithiation, the coin cell was discharged to 4.3 V in the final pass. After 20 cycles, the cobalt oxide 4


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quantum dots were obtained by washing the fiber paper with DMC many times in a glove box filled with pure argon. Materials characterization: The crystal structures of the samples were characterized via X-ray diffraction (Bruker D8 Advance diffractometer with Cu Kα). The micro-structures of the samples were investigated via scanning electron microscopy (FE-SEM, ZEISS Supra 55) and transmission electron microscopy (TEM, Tecnai G2 F20 S-TWIN, 200KV). The surface area and the pore size distribution were determined by Brunauer-Emmett-Teller nitrogen adsorption/desorption (BET, Micromeritics ASAP 2020 HD88). XPS analyses were performed using an ESCALAB 250XL. The galvanostatic discharging-charging tests were conducted on a NEWARE battery cycler at room temperature. The electrochemical impedance spectra was recorded by a CHI 660E from 100,000 Hz to 0.01 Hz, the amplitude of the used perturbation was 10 mV, and the initial potential was 0.5 V vs. Ag/AgCl. Electrochemical measurements: The electrochemical measurements were carried out with a CHI 660E at room temperature. The electrolyte was a 1 M KOH solution. In the three electrode system, a saturated Ag/AgCl electrode and platinum were used as the reference electrode and the counter electrode, respectively. The working electrode was a layer of Nafion impregnated catalyst casted onto carbon fiber paper as detailed in the following: 5 mg of Co3O4 nanosheets and 50 µL of Nafion solution (5 wt%, DuPont) and 0.95 mL of ethanol were sonicated for 30 min to form a homogeneous ink. Then, 50 µL of the ink was dropped onto a carbon fiber paper substrate (1×1 cm), and dried under lamp. Results and discussion The synthesis process of Co3O4-δ-QDs is schematically illustrated in Figure 1a. In brief, the precursor Co(CO3)0.5(OH) was synthesized via a hydrothermal method, then, the Co3O4 nanosheets were prepared by calcining the precursor in the box furnace at 400 °C for 30 min. Finally, the Co3O4 nanosheets were drop-casted onto carbon fiber paper substrate and assembled into a 2032 coin cell. Co3O4-δ-QDs were obtained after 5



galvanostatic lithiation (discharging) and delithiation (charging) for a different number of cycles (Experimental Section). Figure 1b shows the characteristic X-ray diffraction (XRD) patterns of Co(CO3)0.5(OH) precursor, Co3O4 nanosheets and Co3O4-δ-QDs. The major peaks of Co3O4 nanosheets can be indexed to spinel Co3O4 (JCPDS, No.43-1003). The average crystallite sizes obtained using the Scherrer equation are 14.42 nm (based on the (311) plane) and 14.31 nm (based on the (220) plane). However, we cannot assign any peaks to Co3O4-δ-QDs in which the crystallite size might be too small to detect. Both broad peaks near 30° and 40° can be assigned to the carbon fiber paper. The morphology of the Co(CO3)0.5(OH) precursor was characterized with SEM, and the micro-structure of the Co3O4 nanosheets and the Co3O4-δ-QDs was characterized by TEM. As shown in Figure 1c, Co(CO3)0.5(OH) consists of nanosheets which are about 15 nm thick. After calcination in air, the resulted Co3O4 retains the nanosheet morphology but with porosity (Figure 1d and Supporting Information Figure S1a). The BET surface area of the Co3O4 nanosheets is 100.42 m2 g-1, and the average pore size is about 5 nm (Supporting Information Figure S1). Figure 1e shows the morphology of the sample after going through Li-multiple-cycle. It can be observed that the size of the crystallites has reduced to about 2 nm and the material can be classified as quantum dots. Figure 2a shows galvanostatic charging/discharging profile of the initial 20 cycles of the Co3O4 nanosheets. According to the design, the Co3O4 nanosheets electrode, similar to most reported pure transition metal oxide anodes in LIBs, underwent an unstable charging and discharging process, because the structure was gradually destroyed to small crystallite in the initial several cycles, as well as gradually increased side reaction on the crystallites in the subsequent cycles.39 X-ray photoelectron spectroscopy (XPS) was performed on samples to trace the elements as well as their binding energy (B.E.) during Li-multiple-cycles. Full XPS spectra (Supporting Information Figure S2a) shows that the elements in the samples include Co, O, F, C and Li. The ubiquitous C (B.E. of C1s: 284.6 eV) and F (B.E. of C-F bond49: 688 eV) elements in all samples result from the carbon fiber substrate and the Nafion binder. Meanwhile, it was found that very little Li (Supporting Information 6


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Figure S2b) appeared in the samples after Li-multiple-cycle, and the Li-atoms almost completely leaved the crystal structure when charging to the high voltage of 4.3 V. Importantly, the B.E. (Supporting Information Figure S2c) of Co 2p3/2 and Co 2p1/2 located at 780 and 795 eV40 shift by ~1 eV to higher B.E., and the satellites at 786 and 802.5 eV appear in the samples after Li-multiple-cycles, which corresponds to feature peaks of the reduced Co2+.14, 47 The peak fitting of Co 2p in Figure 2b shows that the ratio (~1.8) of Co2+/Co3+ in Co3O4-δ QDs-20th cycle is larger than that (~1.2) in pristine Co3O4. In addition, The O 1s spectrum of the samples after Li-multiple-cycle (Supporting Information Figure S2d) shows double peaks, including one peak related to the Co-O bond41 at 529.8 eV and the other at ~531 eV. The peak fitting of O 1s in Figure 2c shows that a large number of oxygen vacancies (B.E. at 532.4 eV)12, 48 is created in the Co3O4-δ QDs-20th cycle. Therefore, we asserted that the high ratio of Co2+ favors the formation of the OER active site and large numbers of oxygen vacancies facilitate the OH- adsorption in the KOH solution, causing that the Co3O4-δ QDs processed high OER activity. The structural change of Co3O4 nanosheets after multiple lithiation/delithiation cycles can be also proven by TEM. Figure2d shows the HRTEM image and corresponding fast Fourier transform (FFT) images of the Co3O4 nanosheets, suggesting the monocrystalline nature of the Co3O4 nanoparticles in sheet structures. Figure 2e shows that the crystal size reduces to ~2 nm after 20 lithiation/delithiation cycles, resulting in the increment of the electrochemical double-layer capacitance (EDLC) (Supporting Information Figure S3). In addition, within the QDs, the lattice spacing is 0.21 nm, which may be expanded from the 0.202 nm of the (400) planes during the lithiation/delithiation process. The region outside of the QDs without lattice fringe contains created defects, which could be attributed to the formation of oxygen vacancies during the lithiation/delithiation. Therefore, the formation process of Co3O4-δ-QDs with oxygen vacancies can be summarized using the following chemical equation (Eq. 1): Co3O4 + 8Li+ → 3Co + 4Li2O ↔ Co3O4-δ-QDs + 8Li+ 7


(1)


The OER performance of Co3O4 nanosheets, Co3O4-δ-QDs, benchmark IrO2 and commercial Co3O4 catalysts was probed in alkaline electrolyte (1 M KOH) by using a three electrode system. As can be seen in Figure 3a, the OER profiles show that IrO2, commercial Co3O4, and Co3O4 nanosheets need overpotentials of ∼340 mV, ∼420 mV, and∼330 mV to reach 10 mA cm-2, respectively, which is higher than the overpotential of the Co3O4-δ QDs-20th cycle (∼270 mV). The activity comparison demonstrates the excellent electrocatalytic performance of Co3O4-δ-QDs for OER. In Figure 3b, the Tafel slope (38.8 mV dec-1) of the Co3O4-δ QDs-20th cycle is closed to the minimum among all of the OER catalyst. Chronoamperometry (j-t) curves (Figure 3c) of all OER catalyst loaded onto carbon fiber paper confirm that the catalytic activity and stability (no decay within 30 h) of the Co3O4-δ-QDs are better than those of IrO2. The Faraday efficiency was measured by water drainage method45, 46, and it was nearly 100% (Supporting Information Figure S4). TEM characterizations of the Co3O4-δ-QDs after OER (Supporting Information Figure S5) shows that the nanosheets also consists of quantum dots, and the crystallite size and the lattice spacing are still 2 nm and 0.21 nm, respectively, indicating the excellent stability of Co3O4-δ-QDs. As shown in Figure 3d and Table S2 (Supporting Information), the Co3O4-δ QDs-20th cycle is in the top list in all Co-based OER catalysts developed in recent years. The enhanced activity should be owed to the ultra-small crystalline size with more active sites and unique surface properties with oxygen vacancies in Co3O4-δ-QDs. Especially, the oxygen vacancies can improve the conductivity of Co3O4.12,

14

Thus the oxygen vacancies of Co3O4-δ-QDs formed by the

lithiation/delithiation process may enhance the electrocatalytic activity of single active site. In Figure 4a, the OER activity of Co3O4-δ-QDs prepared using different number of lithiation/delithiation cycles (2nd, 6th, 10th, 40th-cycle) was explored. Among these cycled Co3O4-δ-QDs, the Co3O4-δ-QDs-20th cycle have the lowest overpotential. In Figure 4b, the Tafel slope of the Co3O4-δ-QDs-20th cycle is the lowest among the Co3O4-δ-QDs with different number of cycles, which agrees with the semicircles in the Nyquist plots of the Co3O4-δ-QDs (Supporting Information Figure S3). However, the specific capacitance of Co3O4-δ-QDs increases with the 8


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cycle number (Figure 4c, d), which is not related to the OER catalytic performance and the charge transfer resistance of Co3O4-δ-QDs (Supporting Information Figure S7). Currently, the specific capacitance is widely used to evaluate the OER activity of catalysts.42 While we find that the same Co3O4 nanosheets loaded to different substrates (CNF and Ni foam) perform significantly different in OER tests, although they show similar specific capacitance (Figure 5). The reason can be attributed to that the specific capacitance only reflects the adsorption capacity of OH* or H2O*, while the OER activity should be the statistical contribution of the active sites. Before the water oxidation begins, the overall current, as shown in Figure 6a, can be attributed to two kinds of electrochemical contributions: i) Faradaic contribution corresponding to the redox process of Co2+/Co3+ and Co3+/Co4+, ii) capacitive.43 The faradaic contribution is usually resulted from the reconstruction of catalytic interfaces, which serves as the active sites for water oxidation. Hence, we use Faradaic active sites to evaluate the OER activity of our Co3O4 series catalysts. The Faradaic active sites can be calculated from the voltammetric charge (q*) estimated according to Eq. 244: q* =

1 E2 i(E)dE vms ∫E1

(2)

Here, v (V s−1) is the scan rate, S (cm2) is the surface area of the working electrode, m is the mass of the catalyst, E1 and E2 are the range of the scanning voltage of voltammogram (from 0.0 V to 1.55 V vs. RHE). According to previous study, the OH- ions can access all active sites throughout the whole catalyst layer when the scan rate is slow. However, only the most accessible active sites can be reached by the OH- ions when the scan rate is high. The total and the most accessible active sites can be evaluated by the total charge (q*Total) and the most accessible charge (q*Outer) values, which can be extrapolated according to the following Eq. 3 and 443:

9



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q* = q*Outer + C1 v-1/2

(3)

1/q* = 1/q*Total + C2 v1/2

(4)

Here, v is the scan rate. C1 and C2 are constants, and the q* values were calculated using Eq. 2. The q*Total and q*Outer of Co3O4 can be deduced from Figure 6b and Figure 6c, respectively. The results are shown in Table 1. It shows that the q*Total

of

the

Co3O4-δ QDs-20th

cycle

increases

rapidly,

indicating

that

lithiation/delithiation process can create multiple active sites. The q*Outer/q*Total ratio (Table 1) can be used to evaluate accessibility of the active sites. The highest accessibility is also found for Co3O4-δ-QDs-20th cycle. We further compare the q*Total and q*Outer with Co3O4-δ-QDs prepared with different number of cycles (Supporting Information Figure S10 and Table S1) and find that the Co3O4-δ-QDs-20th cycle possesses the maximal q*Total, q*Outer and q*Outer/q*Total, indicating the largest number and the highest accessibility of the active sites for OER. The OER charge transfer resistance was evaluated at 1.5 V vs. RHE via electrochemical impedance spectroscopy (EIS) method to investigate the kinetics of electrode reactions. The semicircles in the Nyquist plots in Figure 6d correspond to the charge transfer resistance (Rct) which is connected with the kinetics for water splitting. The Rct of pristine Co3O4 nanosheets is 19.6 Ω, Rct of the Co3O4-δ QDs-20th cycle is 5.1 Ω, which is the smallest among Co3O4-δ-QDs prepared with different number of cycles (Supporting Information Figure S7 and Table S1), indicating that the Co3O4-δ QDs-20th cycle has relatively lower charge transfer resistance for water splitting. To get further insights into the intrinsic electroactivity per active site, turnover frequency (TOF) and the current density normalized by q*Outer (j/q*Outer) were calculated, in which TOF was normalized with every surface metal atom.50, 51 As shown in Figure 7a, the TOF of the Co3O4-δ-QDs-20th cycle is 0.024 s-1 at η = 300 mV, which is 8 times of that of Co3O4 nanosheets (0.003 s-1 at η = 300 mV). Meanwhile, the current density was also normalized with the most accessible charges (j/q*Outer).52 In Figure 7b, j/q*Outer represents the per site activity of the surface 10


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accessible metal atom, which can be considered as the electrocatalytic activity per active site. The j/q*Outer of the Co3O4-δ-QDs-20th cycle at η = 300 mV is 0.253 A C-1, larger than that (0.178 A C-1) of Co3O4 nanosheets. Therefore, the larger TOF and j/q*Outer suggest that the Co3O4-δ-QDs process higher intrinsic electroactivity per active site, which can be attributed to the increased ratio of Co2+/Co3+ and oxygen vacancies beneficial to OH- adsorption in the KOH solution during OER process.38 Conclusion In summary, we have demonstrated an efficient Li-multiple-cycle method to synthesise Co3O4-δ-QDs with oxygen vacancies. Used as an oxygen evolution electrocatalyst for water splitting, Co3O4-δ-QDs-20th cycle requires an overpotential of only ∼270 mV at 10 mA cm-2, which is superior to many previously reported Co-based catalysts and the state-of-the-art IrO2. Compared with the Co3O4 nanosheets, the enhanced OER activity of Co3O4-δ-QDs is attributed to two factors: one is the increased quantity of the Faradaic active sites, including the total active sites (q*Total), the most accessible active sites (q*Outer) and their ratio (q*Outer/q*Total); the other is the enhanced intrinsic electroactivity per active site evaluated by TOF and j/q*Outer related to OER. More importantly, the QDs have excellent stability to maintain rapid oxygen evolution at a high current density and demonstrate a long lifetime. This Li-multiple-cycle method can be applied to other transition metals oxides as well, providing a general approach to develop promising electrocatalyst for water splitting. ASSOCIATED CONTENT Supporting Information Additional

figures

about

detailed

materials

characterization

(BET,

XPS),

electrochemical measurements (EDLC curves, LSV curves of Co3O4 nanosheets@Ni foam and q*Total- q*Outer measurement for different galvanostatic cycle of Co3O4-δ). AUTHOR INFORMATION Corresponding Authors 11



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School of Advanced Materials, Peking University Shenzhen Graduate School, Shenzhen

518055,

China.

E-mail:

[email protected]

(JL

Yang),

[email protected] (F Pan); Tel: 86-755-26033200 (F Pan). ACKNOWLEDGMENT The work was financially supported by National Science Foundation of China (No. 51602009) and Postdoctoral Science Foundation (2016M600008) and National Materials Genome Project (2016YFB0700600). References (1) Dresselhaus, M. S.; Thomas, I. L. Alternative Energy Technologies. Nature 2001, 414, 332-337. (2) Gray, H. B. Powering the Planet with Solar Fuel. Nat. Chem. 2009, 1, 7-7. (3) Liang, Y.; Li, Y.; Wang, H.; Zhou, J.; Wang, J.; Regier, T.; Dai, H. Co3O4 Nanocrystals on Graphene as a Synergistic Catalyst for Oxygen Reduction Reaction. Nat. Mater. 2011, 10, 780-786. (4) Jiao, F.; Frei, H. Nanostructured Cobalt and Manganese Oxide Clusters as Efficient Water Oxidation Catalysts. Energy Environ. Sci. 2010, 3, 1018-1027. (5) Gao, M. R.; Xu, Y. F.; Jiang, J.; Zheng, Y. R.; Yu, S. H. Water Oxidation Electrocatalyzed by an Efficient Mn3O4/CoSe2 Nanocomposite. J. Am. Chem. Soc. 2012, 134, 2930-2933. (6) Zhao, Y.; Nakamura, R.; Kamiya, K.; Nakanishi, S.; Hashimoto, K. Nitrogen-Doped Carbon Nanomaterials as Non-Metal Electrocatalysts for Water Oxidation. Nat. Commun. 2013, 4, 2390. (7) 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. 12


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Table and Figure captions Figure 1. (a) Scheme for the synthesis of Co3O4-δ-QDs; (b) XRD profiles of the Co(CO3)0.5(OH) precursor, Co3O4 nanosheets and Co3O4-δ-QDs; (c) SEM image of Co(CO3)0.5(OH); TEM images of (d) Co3O4 nanosheets and (e) Co3O4-δ-QDs. Figure 2. (a) Galvanostatic cycling profile of Co3O4 nanosheets; (b) Fitting binding energies of Co 2p; (c) O 1s spectra of initial Co3O4 nanosheets and Co3O4-δ-QDs-20th cycle; HRTEM images of (d) initial Co3O4 nanosheets and (e) Co3O4-δ-QDs-20th cycle. Figure 3. Oxygen evolution performance of Co3O4 series catalysts. (a) LSV curves at 1 mV s-1 and (b) Tafel plots of Co3O4 nanosheets, Co3O4-δ-QDs-20th cycle, benchmark IrO2 and commercial Co3O4 catalysts in O2-saturated 1 M KOH

solution;

(c)

Chronoamperometry

(j-t)

of

Co3O4

nanosheets,

Co3O4-δ-QDs-20th cycle and IrO2; (d) OER activity comparison graph showing η = 10 mA cm-2 and Tafel slopes for Co3O4 catalysts reported in the last three years. Figure 4. Oxygen evolution performance of different galvanostatic cycle Co3O4-δ. LSV curves (a) and Tafel plots (b) of different galvanostatic cycle Co3O4-δ, in O2-saturated 1 M KOH solution (scan rate: 1 mV s-1); (c) EDLC curves of Co3O4-δ-QDs with 20th cycle with different scan rates; (d) Plots of current densities at 0.25 V versus scan rates of different galvanostatic cycle Co3O4-δ respectively. Figure 5. Plots of current densities at 250 mV versus scan rates of Co3O4 nanosheets, Co3O4 nanosheets@Ni foam and Co3O4-δ-QDs-20th cycle respectively (EDLC curves are shown in Figure S3). Inset is the LSV curves of Co3O4 nanosheets and Co3O4 nanosheets@Ni foam at 1 mV s-1 in O2-saturated 1 M KOH solution. Detail of LSV curves for Co3O4 nanosheets@Ni foam is shown in Figure S8. Figure

6.

(a)

Cyclic

voltammograms

of

Co3O4

nanosheets,

and

Co3O4-δ-QDs-20th cycle before OER reaction at 3 mV s-1. Dependence of voltammetric charges on the scan rate: extrapolation of (b) q*Total and (c) q*Outer. Cyclic voltammetry curves with scan rate from 3 to 40 mV s-1 are 20


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shown in Figure S9. (d) Nyquist plots for the Co3O4 serials catalysts measured at 1.5 V vs. RHE. Figure 7. (a) Turnover frequency (TOF) and (b) the current density normalized by the most accessible charges (j/q*Outer) for Co3O4 nanosheets, and Co3O4-δ-QDs-20th cycle. Table 1. Electrochemical parameters obtained for Co3O4 nanosheets, Co3O4-δ-QDs.

21



Figures Figure 1

Figure 1. (a) Scheme for the synthesis of Co3O4-δ-QDs; (b) XRD profiles of the Co(CO3)0.5(OH) precursor, Co3O4 nanosheets and Co3O4-δ-QDs; (c) SEM image of Co(CO3)0.5(OH); TEM images of (d) Co3O4 nanosheets and (e) Co3O4-δ-QDs.

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Figure 2

Figure 2. (a) Galvanostatic cycling profile of Co3O4 nanosheets; (b) Fitting binding energies of Co 2p; (c) O 1s of initial Co3O4 nanosheets and Co3O4-δ-QDs-20th cycle; HRTEM images of (d) initial Co3O4 nanosheets and (e) Co3O4-δ-QDs-20th cycle.

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Figure 3

Figure 3. Oxygen evolution performances of Co3O4 series catalysts. (a) LSV curves at 1 mV s-1 and (b) Tafel plots of Co3O4 nanosheets, Co3O4-δ-QDs-20th cycle, benchmark IrO2 and commercial Co3O4 catalysts in O2-saturated 1 M KOH

solution;

(c)

Chronoamperometry

(j-t)

of

Co3O4

nanosheets,

Co3O4-δ-QDs-20th cycle and IrO2; (d) OER activity comparison graph showing η = 10 mA cm-2 and Tafel slopes for Co3O4 catalysts reported in the last three years. 24


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Figure 4

Figure 4. Oxygen evolution performances of different galvanostatic cycle Co3O4-δ. LSV curves (a) and Tafel plots (b) of different galvanostatic cycle Co3O4-δ, in O2-saturated 1 M KOH solution (scan rate: 1 mV s-1); (c) EDLC curves of Co3O4-δ-QDs-20th cycle with different scan rates; (d) Plots of current densities at 0.25 V versus scan rates of different galvanostatic cycle Co3O4-δ respectively.

25



Figure 5

Figure 5. Plots of current densities at 250 mV versus scan rates of Co3O4 nanosheets, Co3O4 nanosheets@Ni foam and Co3O4-δ-QDs-20th cycle respectively (EDLC curves are shown in Figure S3). Inset is the LSV curves of Co3O4 nanosheets and Co3O4 nanosheets@Ni foam at 1 mV s-1 in O2-saturated 1 M KOH solution. Detail of LSV curves for Co3O4 nanosheets@Ni foam is shown in Figure S8.

26


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Figure 6

Figure

6.

(a)

Cyclic

voltammograms

of

Co3O4

nanosheets,

and

Co3O4-δ-QDs-20th cycle before OER reaction at 3 mV s-1. Dependence of voltammetric charges on the scan rate: (b) extrapolation of q*Total and (c) q*Outer. Cyclic voltammetry curves with scan rate from 3 to 40 mV s-1 are shown in Figure S9. (d) Nyquist plots for the Co3O4 serials catalysts measured at 1.5 V vs. RHE.

27



Figure 7

Figure 7. (a) Turnover frequency (TOF) and (b) the current density normalized by q*Outer (j/q*Outer) for Co3O4 nanosheets, and Co3O4-δ-QDs-20th cycle.

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Table 1 Table 1. Electrochemical parameters obtained for Co3O4 nanosheets, Co3O4-δ-QDs.

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Co3O4âÎ´ Quantum Dots As a Highly Efficient Oxygen Evolution

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Co3O4âÎ´ Quantum Dots As a Highly Efficient Oxygen Evolution