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High Spin State Promotes Water Oxidation Catalysis at Neutral pH in Spinel Cobalt Oxide Lin Ma, Sung-Fu Hung, Liping Zhang, Weizheng Cai, Hong bin Yang, Hao Ming Chen, and Bin Liu Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b04812 • Publication Date (Web): 12 Jan 2018 Downloaded from http://pubs.acs.org on January 12, 2018

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Industrial & Engineering Chemistry Research

High Spin State Promotes Water Oxidation Catalysis at Neutral pH in Spinel Cobalt Oxide Lin Ma,† Sung-Fu Hung,# Liping Zhang,† Weizheng Cai,† Hong Bin Yang,† Hao Ming Chen,# and Bin Liu*,† †

School of Chemical and Biomedical Engineering, Nanyang Technological University, 62 Nanyang Drive, Singapore 637459, Singapore # Department of Chemistry, National Taiwan University, No. 1, Sec. 4, Roosevelt Road Taipei 106, Taiwan Email: [email protected] Abstract: In this work, we present Co3O4 quantum dots (QDs) as a highly efficient and stable oxygen evolution reaction (OER) catalyst at neutral pH. The Co3O4 QDs with a mean size of 5 nm were synthesized by reacting cobalt acetate with benzyl alcohol in the presence of ammonia under reflux conditions. The as-synthesized Co3O4 QDs show extraordinary water oxidation activity with onset overpotential as low as 398 mV and mass activity as high as 567 A/g (at 1.75 V vs RHE) in a 0.2 M phosphate buffer electrolyte (pH ~ 7), which are among the most efficient earth-abundant OER catalysts at neutral pH reported in the literature, reaching a stable current density of 10 mA/cm2 at an overpotential of ~ 490 mV with a Tafel slope of 80 mV/decade. Through in-depth investigations by X-ray photoelectron spectroscopy and X-ray absorption spectroscopy, the high spin Co2+ and Co3+ cations on the surface of Co3O4 QDs were found to be important to promote the OER kinetics at neutral pH. Keywords: neutral pH; oxygen evolution reaction; spin state; spinel; cobalt oxide. Electrochemical water splitting has been regarded as a promising strategy to provide clean and sustainable energy to address the global energy crisis and the environmental deterioration. One of the key challenges to make the electricity-driven water splitting feasible is to develop efficient and earth-abundant electrocatalyst for the oxygen evolution reaction (OER), which involves four electron transfer and is thermodynamically up-hill, limiting the overall efficiency of water electrolysis. Over the past few decades, extensive efforts have been made 1 ACS Paragon Plus Environment

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to develop earth-abundant electrocatalysts for OER. Examples include first raw transition metal oxides and metal hydr(oxy)oxides.1-16 However, while most of the aforementioned electrocatalysts are active in the alkaline solution, only few can be operated at neutral pH. Neutral pH water electrolysis is preferable in practical applications in terms of minimizing the adverse environmental impacts and reducing the operational costs (e.g., catalyst destabilization and reactor corrosion). Till now, the development of a stable, earth-abundant electrocatalyst that can be operated at neutral pH with low overpotential for water oxidation remains a fundamental challenge. In this work, we present Co3O4 quantum dots (QDs) as a highly efficient electrocatalyst for water oxidation under neutral pH conditions. The Co3O4 quantum dots were synthesized through reacting cobalt acetate with benzyl alcohol in the presence of ammonia under reflux condition. Figure 1a shows the transmission electron microscope (TEM) image of assynthesized Co3O4 QDs. The Co3O4 QDs are well separated from each other and are uniform in size with a mean diameter of ~ 5 nm (Figure S1a). The X-ray diffraction (XRD) pattern as shown in Figure S1b reveals that the as-synthesized Co3O4 QDs are phase-pure cubic spinel. The high-resolution TEM (HRTEM) image (inset in Figure 1a) shows a clear-defined lattice spacing, corresponding to the distance between the (400) planes of spinel Co3O4. Different from the color of bulk Co3O4, which is black, the color of Co3O4 QDs is yellowish brown, which may be caused by the quantum-confinement effect due to the ultra-small size. X-ray photoelectron spectroscopy (XPS) was performed to study the chemical states of cobalt on the surface of Co3O4 QDs. Figure 1b shows the high-resolution Co2p XPS spectrum, which could be deconvoluted into Co3+, Co2+ and cobalt satellite peaks. The Co3+ to Co2+ ratio on the surface of Co3O4 QDs was determined to be around 2.1, which agrees with the chemical composition of spinel Co3O4. The electrochemical activity of as-prepared Co3O4 QDs was evaluated in a 0.2 M phosphate buffer solution (pH ~ 7). For comparison, spinel Co3O4 nanocubes (~ 40 nm) were 2 ACS Paragon Plus Environment

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also synthesized via an oxidative reflux method as reference. The crystal structure and morphology of the resultant Co3O4 nanocubes were well characterized and are displayed in Figure S2 and S3. Figure 2 shows the cyclic voltammetry (CV) curves for both Co3O4 QDs and nanocubes. Two sets of redox peaks were observed in the CV measurements. The first redox peak at 1.37 V vs. RHE is due to the Co2+ to Co3+ transition and the second redox peak at 1.57 V vs. RHE can be assigned to the redox couple of Co3+/Co4+. The Co3O4 QDs show much larger redox current densities, which can be partially attributed to the larger external surface area of the Co3O4 QDs, arising from the smaller size. Figure 3a compares the linear sweep voltammetry curves of Co3O4 QDs and nanocubes. It can be clearly observed that the Co3O4 QDs exhibit much higher OER activity across the entire potential window by showing much larger current densities. Video S1 shows the evolution of O2 bubbles on Co3O4 QDs coated FTO glass. The OER onset overpotential for Co3O4 QDs is 398 mV. The overpotentials required to driving anodic current densities of 10, 20, 50 mA/cm2 are 490, 530 and 610 mV, respectively (Figure 3a), which are much smaller as compared to the Co3O4 nanocubes and among the best for earth-abundant OER catalysts reported in the literature at neutral pH (Table S1). The Tafel slope obtained from the low overpotential region for Co3O4 QDs is 80 mV/decade, which is similar to that for IrO2 nanoparticles (81 mV/decade) (Figure S4), but much smaller than that for Co3O4 nanocubes (115 mV/decade) (Figure 3b), suggesting better OER catalytic kinetics for Co3O4 QDs as compared with that for Co3O4 nanocubes. Furthermore, the mass activity reaches as high as 340 A/g for Co3O4 QDs at 1.75 V vs. RHE, which is about 17 times larger than that for Co3O4 nanocubes and ~ 50% of that for IrO2 nanoparticles (the most efficient OER catalyst in neutral electrolyte) (Figure S5, S6, S7 and S8). The mass activity could be further increased to 567 A/g by optimizing the catalyst loading amount (Figure S6). Furthermore, by taking into account the size effect (the OER activities of Co3O4 samples were normalized to their electrochemically active surface area (ECSA). Cyclic voltammetry with varying scan rates (Figure S9) was performed to estimate 3 ACS Paragon Plus Environment


the ECSA based on the method discussed by Trasatti and Petrii)17, the Co3O4 QDs still display more than 3 times enhancement in the intrinsic catalytic activity as compared to the Co3O4 nanocubes (Figure 3c). Additionally, as compared with IrO2 nanoparticles and Co3O4 nanocubes, Co3O4 QDs show much improved OER stability (Figure S10 and Figure 3d) with close to 100% Faraday efficiency (Figure S11). Here, it has to be mentioned that the stability tests were carried out on conductive FTO glass substrates instead of glassy carbon electrodes, as a result, the FTO electrodes required higher overpotentials to reach the same current densities as those on glassy carbon electrodes. To probe the rationale of much enhanced OER activity for the Co3O4 QDs, cobalt L-edge X-ray absorption (XAS) spectra were collected (Figure 4 and Figure S12), which can be further deconvoluted into high spin Co3+, low spin Co3+, and high spin Co2+ cations.18 The content of the high spin Co2+ and Co3+ cations are much higher in Co3O4 QDs as compared to that in Co3O4 nanocubes. Besides the high spin Co2+ that was found to be active to induce the µ-OOH moieties,4 the high spin Co3+ has partially occupied eg and t2g orbitals, which offer stronger overlaps with oxygen-related species than the empty eg and fully occupied t2g orbitals offered by low spin Co3+.19, 20 As a result, the high spin Co2+ and Co3+ on the surface of Co3O4 QDs during OER would facilitate the adsorption of water oxidation intermediates, thus improving the OER activity. Besides, the formation of CoPi as an active catalyst on Co3O4 in the phosphate buffer solution was ruled out for several reasons based on experimental observations. First of all, the Co3O4 QDs exhibit all expected features of a cubic spinel material, including TEM and XRD characterizations before and after water oxidation catalysis. Secondly, current densities did not increase during repeated cycles of water electrolysis, which discards the in situ hypothetical formation of a more active catalyst. Thirdly, the Tafel slope for Co3O4 QDs is clearly different from that reported for CoPi, suggesting a different catalytic mechanism. Fourthly, the calculated OER mass activity of Co3O4 QDs is significantly larger than that 4 ACS Paragon Plus Environment

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reported for CoPi, indicating a more efficient water oxidation catalyst of Co3O4 QDs. Finally, after bulk water electrolysis, EDX and XPS analyses (Figure S13) did not detect any traces of phosphate, which is different from the fact that CoPi requires the incorporation of phosphate from solution during water oxidation in a self-repairing process.21, 22 In summary, we have successfully synthesized Co3O4 QDs based on a facile reflux method. The as-synthesized Co3O4 QDs are uniform in size and show excellent water oxidation catalytic activities at neutral pH. The OER mass activity of Co3O4 QDs reaches 567 A/g at 1.75 V vs. RHE in a phosphate buffer solution (pH ~ 7), which is the highest value reported so far among water oxidation catalysts made of earth-abundant materials. The calculated turn over frequency of Co3O4 QDs (based on 50 µg/cm2 catalyst loading) reaches as high as 0.07 s1

at neutral pH under an overpotential of 520 mV (Supporting Information). X-ray

photoelectron spectroscopy and X-ray absorption spectroscopy analyses revealed that the high spin Co2+ and Co3+ on the surface of Co3O4 QDs is important in the OER process at neutral pH, which played critical roles for enhancing OER performance.

Experimental Section Catalysts synthesis. All chemicals including Co(CH3COO)2·4H2O, Co(NO3)2·6H2O, benzyl alcohol, NH3·H2O (25%), NaOH, NaNO3, HCl, K2IrCl6, HNO3, were purchased from Sigma Aldrich and used without further purification. In a typical synthesis of Co3O4 quantum dots (QDs), 160 mg of Co(CH3COO)2·4H2O was added into 7 ml of benzyl alcohol in a 100 ml three neck flask and stirred for 2 hours. Subsequently, 7 ml of NH3·H2O (25%) was added into the mixture drop by drop. The flask was then transferred into an oil bath with temperature kept at 165 °C. The reaction was continued under vigorous stirring for 2 hours. After reaction, the solution was cooled down naturally to room temperature and the product was thoroughly washed with ethanol and collected by centrifugation. In a typical synthesis of Co3O4 nanocubes,23 100 ml of 0.3 M NaOH solution and 35 g of NaNO3 were mixed in a 250 ml 5 ACS Paragon Plus Environment


three neck flask. The flask was then transferred into an oil bath, which was kept at 120 ºC. Thereafter, purified air with a flow rate of 50 ml/min was bubbled into the flask under vigorously stirring following by adding 20 ml of 1.0 M Co(NO3)2·6H2O aqueous solution. The reaction was conducted for 12 hours. After cooling down to room temperature, the product was harvested by centrifugation and thoroughly washed with HCl (2.0 M) and deionized water. In a typical synthesis of IrO2 nanoparticles,24 50 ml of 2.0 mM aqueous K2IrCl6 solution was prepared, and the pH of the solution was adjusted to 13 using 10 wt% NaOH. The solution was heated to 90 ºC and kept at this temperature for 20 minutes. Afterwards, the solution was quickly transferred to an ice bath. Then 3 M HNO3 was used to adjust the pH of the solution to 1. Finally, the product was washed with deionized water and collected by centrifugation. Characterization. X-ray diffraction patterns (XRD) were recorded on a Bruker AXS D8 Advance with Cu Kα radiation (λ = 1.5406 Å). Morphological and lattice structural information were examined by field-emission scanning electron microscopy (FESEM, JEOL, JSM6700F) and transmission electron microscopy (TEM, FEI Tecnai G2 30). X-ray photoelectron spectroscopy (XPS) measurements were performed on an ESCALAB 250 photoelectron spectrometer (Thermo Fisher Scientific) at 2.4 × 10-10 mbar using a monochromatic Al Kα X-ray beam (1486.60 eV). Soft X-ray absorption spectra of Co L-edge were measured in total electron yield mode at room temperature using BL-20A at National Synchrotron Radiation Research Center (NSRRC, Hsinchu, Taiwan), in which the electron storage ring was operated at 1.5 GeV with a beam current of 300 mA. Electrochemical measurements. All electrochemical properties were studied on a CHI 760D electrochemical workstation in a three-electrode configuration using Pt plate as the counter electrode and Ag/AgCl electrode as the reference electrode. The oxygen evolution reaction (OER) activities were evaluated with rotating disk electrode (RDE) measurements. To prepare 6 ACS Paragon Plus Environment

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the working electrode, catalysts were drop-cased on glassy carbon electrode with catalyst loading fixed at 50 µg/cm2. All potentials were calculated with respect to reversible hydrogen electrode (RHE) using the following equation: E(RHE) = E(Ag/AgCl) + 0.059 pH + 0.197 V. Linear sweep voltammetry was recorded at a scan rate of 10 mV/s. Tafel plots were recorded at a scan rate of 1 mV/s. Double-layer capacitance was derived from the slope of capacitive current versus scan rate plot. Cyclic voltammetry (CV) measurements were performed in the potential region without redox processes (1.25 – 1.30 V vs. RHE). Faraday efficiency was estimated using the volumetric method. To determine the composition and purity of the gas product, the collected gas was analyzed using gas chromatograph (Agilent GC 490). The electrolyte was 1 M KOH aqueous solution (pH ~ 13.6) or 0.2 M phosphate buffer solution (pH ~ 7). All data were corrected for a small Ohmic drop (~ 6 ohm in 1 M KOH, ~ 13 ohm in 0.2 M phosphate buffer). Figure S14 compares the polarization curves obtained with and without IR correction. Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Turnover frequency calculation and additional experimental results of the characterization and performance of catalysts, including Figures S1-S14 and Tables S1 (PDF). Notes The authors declare no competing financial interests. Acknowledgements This work was supported by Singapore Ministry of Education Academic Research Fund (AcRF) Tier 1: RG111/15 and RG10/16, Tier 2: MOE2016-T2-2-004, Public Sector Funding from Agency for Science, Technology and Research of Singapore (A*Star): M4070232.120,

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and the National Research Foundation (NRF), Prime Minister’s Office, Singapore under its Campus for Research Excellence and Technological Enterprise (CREATE) program.

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Liu, S.; Zhao, H. J.; Tang, Z. Y. Ultrathin Metal-Organic Framework Nanosheets for Electrocatalytic Oxygen Evolution. Nat. Energy 2016, 1, 16184. (13) Song, F.; Hu, X. Ultrathin Cobalt-Manganese Layered Double Hydroxide Is an Efficient Oxygen Evolution Catalyst. J. Am. Chem. Soc. 2014, 136, 16481. (14) Subbaraman, R.; Tripkovic, D.; Chang, K. C.; Strmcnik, D.; Paulikas, A. P.; Hirunsit, P.; Chan, M.; Greeley, J.; Stamenkovic, V.; Markovic, N. M. Trends in Activity for the Water Electrolyser Reactions on 3d M(Ni,Co,Fe,Mn) hydr(oxy)oxide Catalysts. Nat. Mater. 2012, 11, 550. (15) Li, H. Y.; Chen, S. M.; Jia, X. F.; Xu, B.; Lin, H. F.; Yang, H. Z.; Song, L.; Wang, X. Amorphous Nickel-Cobalt Complexes Hybridized with 1T-Phase Molybdenum Disulfide via Hydrazine-Induced Phase Transformation for Water Splitting. Nat. Commun. 2017, 8, 15377. (16) (a) Zhao, Y. F.; Jia, X. D.; Chen, G. B.; Shang, L.; Waterhouse, G. I. N.; Wu, L. –Z.; Tung, C. –H.; O’Hare, D.; Zhang, T. R. Ultrafine NiO Nanosheets Stabilized by TiO2 from Monolayer NiTi-LDH Precursors: An Active Water Oxidation Electrocatalyst. J. Am. Chem. Soc. 2016, 138, 6517. (b) Zhao, Y. X.; Chang, C.; Teng, F.; Zhao, Y. F.; Chen G. B.; Shi, R.; Waterhouse, G. I. N.; Huang, W. F.; Zhang, T. R. Defect-Engineered Ultrathin δ-MnO2 Nanosheet Arrays as Bifunctional Electrodes for Efficient Overall Water Splitting. Adv. Energy. Mater. 2017, 7, 1700005. (17) Trasatti, S.; Petrii, O. A. Real Surface Area Measurements in Electrochemistry. Pure & Appl. Chem. 1991, 63, 711. (18) Guillou, F.; Zhang, Q.; Hu, Z.; Kuo, C. Y.; Chin, Y. Y.; Lin, H. J. Chen, C. T.; Tanaka, A.; Tjeng, L. H.; Hardy, V. Coupled Valence and Spin State Transition in (Pr0.7Sm0.3)0.7Ca0.3CoO3. Phys. Rev. B 2013, 87, 115114. (19) Jin, S.; Kevin, J. M.; Hubert, A. G.; John, B. G.; Shao-Horn, Y. A Perovskite Oxide Optimized for Oxygen Evolution Catalysis from Molecular Orbital Principles. Science 2011, 334, 1383. (20) Huang, J.; Chen, J.; Yao, T. He, J.; Jiang, S.; Sun, Z.; Liu, Q.; Cheng, W.; Hu, F.; Jiang, Y.; Pan, Z.; Wei, S. CoOOH Nanosheets with High Mass Activity for Water Oxidation. Angew. Chem. Int. Ed. 2015, 54, 8722. (21) Kanan, M. W.; Nocera, D. G. In Situ Formation of an Oxygen-Evolving Catalyst in Neutral Water Containing Phosphate and Co2+. Science 2008, 321, 1072. (22) Yogesh, S.; Matthew, W. K.; Daniel, G. N. Mechanistic Studies of the Oxygen Evolution Reaction by a Cobalt-Phosphate Catalyst at Neutral pH. J. Am. Chem. Soc. 2010, 132, 16501. (23) Chen, H. M.; Liu, R. S.; Li, H.; Zeng, H. C. Generating Isotropic Superparamagnetic Interconnectivity for the Two-Dimensional Organization of Nanostructured Building Blocks. Angew. Chem. Int. Ed. 2006, 45, 2713. (24) Zhao, Y.; Hernandez-Pagan, E. A.; Vargas-Barbosa, N. M.; Dysart, J. L.; Mallouk, T. E. A High Yield Synthesis of Ligand-Free Iridium Oxide Nanoparticles with High Electrocatalytic Activity. J. Phys. Chem. Lett. 2011, 2, 402.



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(a)

! (b)

Figure 1. (a) TEM image of as-synthesized Co3O4 QDs. Inset shows the HRTEM image of a single Co3O4 QDs. (b) High resolution Co 2P XPS spectrum of as-synthesized Co3O4 QDs.


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(a)

(b)

Figure 2. Cyclic voltammograms of (a) Co3O4 QDs and (b) Co3O4 nanocubes at a scanning rate of 10 mV/s in 0.2 M phosphate buffer.



Figure 3. (a) Polarization curves recorded at a scan rate of 10 mV/s on a rotating disk electrode at a 1600 rpm rotating speed in 0.2 M phosphate buffer (pH ~ 7). (b) Corresponding Tafel plots. (c) Normalized polarization curves. (d) Chronopotentiometric curves of Co3O4 QDs and Co3O4 nanocubes (catalyst loading: 50 µg/cm2 on FTO) under constant current density of 8 mA/cm2 in 0.2 M phosphate buffer.


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Figure 4. Co-L3 XAS spectra for (a) Co3O4 QDs and (b) Co3O4 nanocubes. Co3+ (Oh) HS stands for the Co3+ cation in the octahedral site with high electronic spin state, Co3+ (Oh) LS stands for the Co3+ cation in the octahedral site with high electronic spin state, and Co2+ (Td) HS stands for the Co2+ cation in the tetrahedral site with high electronic spin state.



For Table of Contents Only

2

Current density (mA / cm )

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50

At neutral pH

40 30 20 10

Co3O4 QDs Glassy carbon

0 1.2 1.4 1.6 1.8 Potential (V vs RHE)


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High Spin State Promotes Water Oxidation ... - ACS Publications

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