Boosting the Electrocatalytic Water Oxidation Performance of

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Boosting the Electrocatalytic Water Oxidation Performance of CoFe2O4 Nanoparticles by Surface Defect Engineering Gang Ou, Fengchi Wu, Kai Huang, Naveed Hussain, Di Zu, Hehe Wei, Binghui Ge, Huizhen Yao, Lai Liu, Henan Li, Yumeng Shi, and Hui Wu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b19265 • Publication Date (Web): 09 Jan 2019 Downloaded from http://pubs.acs.org on January 10, 2019

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ACS Applied Materials & Interfaces

Boosting the Electrocatalytic Water Oxidation Performance of CoFe2O4 Nanoparticles by Surface Defect Engineering Gang Ou,†,‡,‖ Fengchi Wu,†,‡ Kai Huang,‖ Naveed Hussain,‖ Di Zu,‖ Hehe Wei,‖ Binghui Ge,£,$ Huizhen Yao,† Lai Liu,† Henan Li,§ Yumeng Shi,†,‡,* and Hui Wu‖,* †

International Collaborative Laboratory of 2D Materials for Optoelectronics Science and

Technology of Ministry of Education, College of Optoelectronic Engineering, Shenzhen University, Shenzhen 518060, China. ‡ Engineering

Technology Research Center for 2D Material Information Function Devices and

Systems of Guangdong Province, College of Optoelectronic Engineering, Shenzhen University, Shenzhen 518060, China. §

College of Electronic Science and Technology, Shenzhen University, Shenzhen 518060, China.

‖ State

Key Laboratory of New Ceramics and Fine Processing, School of Materials Science and

Engineering, Tsinghua University, Beijing 100084, China. £ Beijing

National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese

Academy of Sciences, Beijing 100190, China. $

Institute of Physical Science and Information Technology, Anhui University, Hefei, Anhui

230601, China.

ACS Paragon Plus Environment

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ABSTRACT: Spinel oxides have attracted widespread interest for electrocatalytic applications owing to their unique crystal structure and properties. The surface structure of spinel oxides significantly influences the electrocatalytic performance of spinel oxides. Herein, we report a Li reduction strategy that can quickly tune the surface structure of CoFe2O4 (CFO) nanoparticles and optimize its electrocatalytic oxygen evolution reaction (OER) performance. Results show that a large number of defective domains have been successfully introduced at the surface of CFO nanopowders after Li reduction treatment. The defective CFO nanoparticles demonstrate significantly improved electrocatalytic OER activity. The OER potential observed a negative shift from 1.605 V to 1.513 V at 10 mA cm-2 while the Tafel slope is greatly decreased to 42.1 mV dec-1 after 4 wt% Li reduction treatment. This efficient Li reduction strategy can also be applied to engineer the surface defect structure of other material systems and broaden their applications.

KEYWORDS: spinel, CoFe2O4, surface engineering, defect, OER


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1. INTRODUCTION Hydrogen is a source of clean, highly effective and renewable energy.1,2 As far as we are concerned, electrocatalytic water splitting is a promising method for hydrogen production considering our Earth is a watery place. During the process of water electrolysis, oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) are occurring at the anode and cathode, respectively.3,4 However, the overall efficiency of this reaction is extremely limited on account of the sluggish kinetics of OER. Hence, the emergence of high-efficiency and environmental friendly OER electrocatalysts will benefit the wide use of hydrogen.5,6 Currently, researchers have developed a series of OER electrocatalysts, in which spinel oxides have become a research hotspot due to its unique crystal structure and excellent performance.7,8 To further improve the electrocatalytic performance of spinel oxides, a variety of strategies have been proposed, such as defect engineering9,10, constructing porous structure11,12, growth on carbon substrate13-15 and building heterogeneous structure16,17. As a heterogeneous electrocatalyst, the spinel oxides’ surface structure plays a key role in the electrocatalytic property. Implantation of defects upon the surface of electrocatalyst would increase the number of active sites.18-20 Therefore, engineering of the surface defect structure is a preferred strategy to enhance the electrocatalytic performance. So far, various surface structure engineering methods have been developed, such as high temperature reduction9,21,22, chemical reduction10,23,24 and plasma treatment25,26. These methods, however, often involve high temperature and long-time treatment, while influencing the phase structure and morphology of nanomaterials. Importantly, controlling the defect content in these nanomaterials is even more challenging. Hence, it is necessary to develop a method to engineer the spinel oxides’ surface defect structure with high efficiency and controllability. Here, we report a Li reduction mean to tune the surface defect structure of CFO


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nanoparticles at room temperature. Results show that certain amount of defective domains structure can be effectively implanted at the surface of CFO nanoparticles by Li reduction treatment. Furthermore, this work reports significantly improved electrocatalytic OER activity of CFO nanoparticles owing to the introduction of defects, in which the potential dropped from 1.605 V to 1.513 V at 10 mA cm-2 after reduction by 4 wt% Li. Meanwhile, the defective CFO nanoparticles also demonstrated excellent stability in OER. 2. EXPERIMENTAL METHOD 2.1. Materials preparation. CFO nanopowders used in the experiment were commercially available (99.5%, Aladdin). In order to engineering the surface defect structure of CFO nanoparticles, we applied a previously reported Li reduction strategy.27 Li powders (99.9%, Cellithium) and dimethyl carbonate (99%, Aladdin) were added in a mortar and then finely ground with CFO nanopowders for 1 h in an argon-filled glove box. The mixture was then washed 3 times with deionized water. In order to tune the defects content at the surface of CFO nanoparticles, 4 weight ratios of Li to CFO, 2 wt%, 4 wt%, 6 wt% and 8 wt%, were used in the experiments, the corresponding Li reduced samples were named as CFO-2Li, CFO-4Li, CFO6Li and CFO-8Li, respectively. 2.2. Physicochemical characterization. The X-ray diffraction (XRD) plots were recorded by a Rigaku X-ray diffractometer (D/max-2500). Scanning electron microscopy (SEM) images were taken by a Zeiss microscope (MERLIN VP Compact). Transmission electron microscopy (TEM) images were collected by a JEOL microscope (JEM-ARM200F). X-ray photoelectron spectra were obtained by a Thermo Fisher spectrometer (Escalab 250Xi). The specific surface areas (SSA) were tested by a Quantachrome surface area analyzer (QuadraSorb SI). The elements


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content were measured by a Perkin Elmer inductively coupled plasma mass spectrometry (ICPMS, ELAN DRC-e). 2.3. Electrocatalytic measurements. The electrocatalytic oxygen evolution performance was characterized by a three-electrode electrochemical system (PGSTAT204, Autolab) in 1 M KOH solution with Hg/HgO as reference electrode and Pt sheet as counter electrode. The ink solution was prepared by adding CFO nanoparticles and Nafion 117 (Aldrich) in mixed solution with equal volume ratio of deionized water and ethanol. The working electrode was prepared by dipping uniformly dispersed ink solution on carbon fiber paper (CFP, Toray), in which the loading was 0.2 mg cm-2. The commercial IrO2 (99.9%, Aladdin) and RuO2 (99.9%, Aladdin) oxides were also prepared as working electrodes with the same method and mass loading as CFO nanoparticles. The OER activity was characterized by linear sweep voltammetry (LSV) with scanning rate of 5 mV s−1. The resistance was measured by electrochemical impedance spectroscopy (EIS) with high frequency of 100K Hz and low frequency of 0.1 Hz at 0.55 V vs Hg/HgO. The electrochemically active surface area (ECSA) was fitted by cyclic voltammetry (CV) curves in potential of 0-0.1 V vs Hg/HgO at 10-50 mV s-1. For stability test, chronopotentiometry was applied for electrocatalytic water splitting with CFO-4Li as anode and commercial Pt sheet as cathode at current of 10 mA. Meanwhile, the generated H2 and O2 were collected with two inverted cylinder filled with electrolyte at the top of the two electrodes. The volume of the H2 and O2 were recorded every hour for the calculation of Faradic efficiency. The potential conversion between Hg/HgO and RHE was calculated according to: ERHE=EHg/HgO + 0.926 V. In order to eliminate the effect of solution resistance, the LSV curves were iR corrected by 95% of the solution resistance. All assays were run at room temperature. 3. RESULTS AND DISCUSSION


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Li metal possess strong reducibility, as a consequence, its contact with metal oxides will seize the surface oxygen to generate Li oxides at room temperature, thereby implant defects at the same time. As demonstrated in Figure 1, the surface of spinel oxide will lose partial oxygen to reveal the defect-rich surface structure due to the redox reaction with Li after Li reduction treatment. More importantly, the introduction of defects will strongly influence the surface electronic structure, microstructure and catalytic performance of the spinel oxides. To prove the effect of Li reduction on the surface structure and performance of spinel oxides, we chose commercial CFO nanopowders as model material. We first explored the crystal structure of CFO nanopowders by XRD. The pristine CFO nanopowders showed pure spinel phase structure (Figure 2a), which is consistent with the standard card (PDF#22-1086).13 In addition, CFO-2Li, CFO-4Li and CFO-6Li demonstrated the same diffraction peaks with pristine CFO, suggesting no phase change appeared in these samples after reduction by Li. However, miscellaneous peaks correspond to CoFe alloy were found in CFO-8Li (Figure S1), indicating small part of the oxides has been reduced to metal phase at this experimental condition. It is apparent that a suitable amount of Li reduction treatment would not influence the phase structure of CFO nanopowders. Here, the content of Li should be controlled less than 8 wt% without phase change from the above XRD results. The impact of Li reduction on the morphology of CFO nanopowders was tested as well. The SEM images of pristine and Li reduced CFO nanopowders were exhibited in Figure 2b-2e, in which no obvious difference can be observed, showing that the Li reduction treatment almost had no influence on the particle size of CFO nanopowders. To explore the impact of Li reduction on the surface defect structure of CFO nanopowders, we conducted XPS to characterize the elements’ binding energies. Three peaks named O1, O2 and O3 were shown in O 1s spectra (Figure 3a ). Obviously, O1 was associated with lattice oxygen.


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O2 was chemically adsorbed oxygen species, which means there are oxygen deficiencies at the corresponding sites. O3 was physically adsorbed surface oxygen species.9,21 The oxygen chemisorption will appear in place of oxygen vacancies, in which the corresponding oxygen binding energy is different from O1 and O3, where the amount of oxygen vacancies was found to be proportional to the peak intensity. The O2 peak increased comprehensively after reduction by 4 wt% Li, suggesting that the surface oxygen defect content was greatly improved after Li reduction. It is noted that the introduced oxygen defect sites were usually chemically adsorbed by H2O molecule and formed as MOH-H (M=Co, Fe).28 It can be seen from the XPS spectra of Fe and Co that the implantation of oxygen defect shifted its binding energy by about 0.3~0.5 eV and no peak corresponds to Co0 and Fe0 can be found (Figure S2), suggesting the surface phases of CFO-4Li nanoparticles are still in the form of oxides. Besides, no Li peak can be found in Li 1s XPS spectra, as shown in Figure S2, indicating all generated Li oxides during grinding have been removed by deionized water rinsing, in other words, no residue at the surface of CFO-4Li nanoparticles. After that, we characterized the surface microstructure of CFO nanoparticles by using TEM. The pristine CFO nanoparticles demonstrated integral surface structure, complete crystallization and almost no defect (Figure 3b and 3c), while the surface of CFO-4Li revealed obvious changes (Figure 3d and 3e), showing complete disruption of surface structure and the appearance of many disordered structures. From Figure 3e, it is clear that several dark regions with size of a few nanometers can be found at the surface of CFO-4Li nanoparticles in the high angle annular dark field (HAADF) image. We speculate it can be ascribed to the non-uniform distribution of the defects at the surface of CFO nanoparticles. A disorder region with certain thickness will appear with the implantation of oxygen defects, in which the ions were randomly distributed. Here, these randomly distributed ions will make less contribution to the brightness at


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the transmission direction compared with the ordered ions in the lattice, making this defective region darker than the nearby region with much less defects. This is the reason for the formation of dark region at the surface of CFO nanoparticles and we named this kind of region as defective domain. In addition, it can be also considered as an auxiliary evidence for the implantation of defects by comparison with pristine CFO nanoparticles. It has to be pointed out that the smooth surface of pristine CFO nanoparticles became rough after Li reduction treatment, as the introduction of defective domains had destroyed the surface completeness (Figure S3). Besides, we also measured the SSA of CFO nanoparticles and shown in Figure S4. It is clear that the SSA of Li reduced CFO is larger than that of CFO-Pristine, which can be ascribed to the rough surface of CFO nanoparticles by Li reduction treatment. Interestingly, the SSA of CFO nanoparticles enlarged with the increase of Li content, suggesting more defects can be implanted in CFO with the adding of lithium. For the purpose of assess the influence of Li reduction upon the metal elements of CFO nanoparticles, we further measured their contents by ICP-MS (Table S1). The ratios of Co:Fe in Li reduced CFO are very close to that of pristine CFO nanoparticles, suggesting the Li reduction treatment has little effect on the ratio of metal elements. Moreover, the Li content in the treated CFO samples are also so low that can be ignored, indicating the generated Li oxides have been totally washed. This result is consistent with the XPS data. Based on these results, we believe Li reduction treatment can successfully introduce surface defects on CFO nanoparticles. Next, the electrocatalytic OER property was investigated. Figure 4a presents the OER activity before and after Li reduction. Clearly, the OER activity of CFO nanoparticles has been raised dramatically after the treatment. At 10 mA cm-2, the potentials obtained for pristine CFO, CFO2Li, CFO-4Li and CFO-6Li were 1.605 V, 1.545 V, 1.513 V and 1.528 V, respectively. In


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addition, with the increase of Li content, the OER activity of CFO nanoparticles displayed the trend of an increase followed by a decrease, while peaked at 4 wt%. As a comparison, we measured the OER performances of commercial IrO2 and RuO2. They demonstrated a low onset potential at about 1.475 V (Figure 4a). But their current density revealed a relative low increase rate with the increase of potential maybe due to their large particle size (Figure S5). It is apparent that the OER activity of CFO-4Li and CFO-6Li are better than that of commercial IrO2 and RuO2 oxides. In addition, compared with that of previously reported spinel type OER electrocatalysts, we found out that our CFO nanoparticles reduced by 4 wt% Li demonstrated significant performance advantages (Table S2). Besides, the Tafel slope of CFO nanoparticles significantly decreased after Li reduction treatment (Figure 4b). In particular, the Tafel slope decreased from previous 73.3 mV dec-1 to 42.1 mV dec-1 for CFO-4Li, suggesting that the oxygen evolution dynamics has been significantly improved. Moreover, we can see that the Tafel slopes of Li reduced CFO nanoparticles are even lower than that value for commercial IrO2 and RuO2. In addition, we further tested the EIS of CFO nanoparticles at 0.55 V vs Hg/HgO. The pristine CFO nanoparticles possess a relatively high resistance (Figure 4c), but they turn out to possess a much smaller resistance after Li treatment, and demonstrate the optimal conductivity after 4 wt% Li reduction. Here, the Nyquist plots were fitted in terms of the equivalent circuit (inset of Figure 4c) as well as the corresponding resistance of solution (Rs) and CFO nanopowders (Rnp) were summarized in Table S3. It is apparent that the resistance of CFO-4Li is about one fifteenth that of pristine CFO, showing significantly reduced resistance after Li reduction treatment, which is helpful for uplifting the OER performance. Furthermore, we also measured the ECSA of pristine and Li reduced CFO nanoparticles by applying cyclic voltammetry at different scanning rates (Figure S6).29 It is noted that the capacitance contributed by the CFP substrate has been deducted


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before the calculation of ECSA of CFO nanoparticles (Figure S7). The ECSA of CFO nanoparticles grow with the increase of Li content, showing the same trend with SSA, suggesting Li reduction treatment is beneficial for the improvement of ECSA as well as active sites. The OER activity of CFO nanoparticles based on ECSA was also shown in Figure S8. However, the OER activity of CFO-6Li is not the best although its ECSA is the largest one, which may be due to the decrease of conductivity (Figure 4c). At last, the catalytic stability of CFO-4Li nanoparticles was examined by chronopotentiometry for electrocatalytic water splitting with CFO-4Li as anode and commercial Pt sheet as cathode. As shown in Figure 4d, it demonstrated superior catalytic stability, in which the voltage showed a small increase of about 20 mV at the first hour and kept almost unchanged in the next few hours. Here, we speculate that it is because surface reconstruction may appear at the surface of CFO-4Li nanoparticles in the first hour of the electrocatalytic process and then stable surface structure was formed. At the same time, the volume of the generated H2 and O2 were recorded every hour (Figure S9) and the corresponding Faradaic efficiency was calculated and shown in Figure 4d. The volume ratio of H2 and O2 is 2:1 and the calculated Faradaic efficiency is very close to 100% except the first hour, suggesting the superior efficiency of electricity to hydrogen energy. Furthermore, we also characterized the microstructure of CFO-4Li nanoparticles after electrochemical stability test (Figure S10). It demonstrated almost the same defective surface structure as that of fresh CFO-4Li nanoparticles, suggesting its excellent defect structural stability, which is also the basis for the superior electrochemical stability of CFO-4Li nanoparticles. The above results demonstrated that Li reduction treatment can effectively implant a certain amount of defective domains at the surface of CFO nanoparticles. Meanwhile, the Li reduction method also exhibits advance relative to all-room-temperature process, high controllability and


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high efficiency than the previous reported chemical reduction methods, such as H2, NaBH4 Mg and Al reduction. In addition, the introduced defects revealed a positive impact on the OER performance. We speculate that the reasons include the following three points: 1) the introduction of defects brings noticeable changes to the surface electronic structure of CFO nanoparticles and generates many high-energy metastable state structures, hence producing more defect active sites and ultimately promoting the electrocatalytic performance;25,30 2) the introduction of defects is helpful for improving the oxygen evolution dynamic performance, so significantly accelerating the electrocatalytic reaction speeds; 3) the introduction of defects improve the electronic conductivity of CFO nanoparticles, namely the charge transfer dynamic performance.28 In addition, we also observed the phenomenon that the OER performance of CFO nanoparticles displays the trend of an increase followed by a decrease and an optimal value exists with the increase of Li content. This is mainly due to the introduction of a small amount of defects that can help improve the catalytic performance. However, further introduction of defects will bring certain negative impact on the surface structure and the corresponding OER performance of CFO nanoparticles, such as excessive incomplete surface lattice and poor conductivity. Therefore, controlling the surface defect content of CFO nanoparticles to a reasonable extent can optimize it’s OER performance. Our experiment results show that the CFO nanoparticles can obtain the best OER performance after reducing by 4 wt% Li. 4. CONCLUSIONS In summary, we have successfully tuned the surface defect structure of CFO nanoparticles by a versatile and facile Li reduction strategy at room temperature. Results showed that the surface defect engineering had no obvious influence on the intrinsic crystal structure, but it embeds numbers of defective domains at the surface of CFO nanoparticles, which could significantly


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improve the electrocatalytic OER performance. The potential decreases from pristine 1.605 V to 1.513 V at 10 mA cm-2, while the corresponding Tafel slope demonstrated a significant decrease to 42.1 mV dec-1 after 4 wt% Li reduction treatment. It demonstrates that Li reduction method can effectively optimize the OER performance of spinel CFO nanoparticles. Apart from spinel oxides, other defect-rich material systems can also be prepared by this general Li reduction method and applied in the areas of catalysis, energy storage and environmental protection.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: XRD plot of CFO-8Li, XPS spectra of Co, Fe and Li, HAADF image of CFO nanoparticles, SSA of CFO nanoparticles, SEM images of IrO2 and RuO2 powders, ECSA of CFP and CFO nanoparticles, OER performance based on ECSA of CFO nanoparticles, Volume of the generated H2 and O2 by CFO-4Li, HAADF images CFO-4Li nanoparticles after OER test, Table S1 for ICP-MS of CFO nanoparticles, Table S2 for the comparison of OER performance with previously reported spinel oxide materials, Table S3 for resistance of CFO nanopowders under OER conditions. AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (Y.S.). *E-mail: [email protected] (H.W.). Notes The authors declare no competing financial interest. ACKNOWLEDGMENT


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This study was supported by the National Natural Science Foundations of China (Grant 51788104, 51706117 and U1564205), and National Basic Research of China (Grants 2015CB932500, 2016YFE0102200 and 2018YFB0104404). Y.S. acknowledges the support from the Thousand Young Talents Program of China, the National Natural Science Foundation of China (Grant No. 51602200 and 61874074), Science and Technology Project of Shenzhen (JCYJ20170817101100705, JCYJ20170817100111548) and the (Key) Project of Department of Education of Guangdong Province (Grant No. 2016KZDXM008). This project was supported by Shenzhen Peacock Plan (Grant No. KQTD2016053112042971).


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Figure 1. Schematic illustration for the fabrication of defective CFO nanoparticles.


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Figure 2. (a) XRD plots of pristine and reduced CFO nanopowders. (b-e) SEM images of pristine CFO, CFO-2Li, CFO-4Li and CFO-6Li, respectively.


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Figure 3. (a) XPS spectra (O 1s) of pristine CFO and CFO-4Li nanoparticles. HAADF images of pristine CFO (b, c) and CFO-4Li (d, e) nanoparticles.


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Figure 4. (a) LSV curves of CFO nanoparticles and commercial IrO2 and RuO2. (b) Tafel slope. (c) EIS of CFO nanoparticles with inserted equivalent circuit. (d) Chronopotentiometry plot and Faraday efficiency of CFO-4Li.


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REFERENCES (1) Mahmood, J.; Li, F.; Jung, S. M.; Okyay, M. S.; Ahmad, I.; Kim, S. J.; Park, N.; Jeong, H. Y.; Baek, J. B. An Efficient and pH-universal Ruthenium-based Catalyst for the Hydrogen Evolution Reaction. Nat. Nanotechnol. 2017, 12, 441-446. (2) Roger, I.; Shipman, M. A.; Symes, M. D. Earth-abundant Catalysts for Electrochemical and Photoelectrochemical Water Splitting. Nat. Rev. Chem. 2017, 1, 3. (3) Ou, G.; Fan, P. X.; Zhang, H. J.; Huang, K.; Yang, C.; Yu, W.; Wei, H. H.; Zhong, M. L.; Wu, H.; Li, Y. D. Large-scale Hierarchical Oxide Nanostructures for High-performance Electrocatalytic Water Splitting. Nano Energy 2017, 35, 207-214. (4) Liu, Y. Y.; Wu, J. J.; Hackenberg, K. P.; Zhang, J.; Wang, Y. M.; Yang, Y. C.; Keyshar, K.; Gu, J.; Ogitsu, T.; Vajtai, R.; Lou, J.; Ajayan, P. M.; Wood, B. C.; Yakobson, B. I. Selfoptimizing, Highly Surface-active Layered Metal Dichalcogenide Catalysts for Hydrogen Evolution. Nature Energy 2017, 2, 17127. (5) Zhang, B.; Zheng, X.; Voznyy, O.; Comin, R.; Bajdich, M.; Garcia-Melchor, M.; Han, L.; Xu, J.; Liu, M.; Zheng, L.; Garcia De Arquer, F. P.; Dinh, C. T.; Fan, F.; Yuan, M.; Yassitepe, E.; Chen, N.; Regier, T.; Liu, P.; Li, Y.; De Luna, P.; Janmohamed, A.; Xin, H. L.; Yang, H.; Vojvodic, A.; Sargent, E. H. Homogeneously Dispersed Multimetal Oxygen-evolving Catalysts. Science 2016, 352, 333-337. (6) Maayan, G.; Gluz, N.; Christou, G. A Bioinspired Soluble Manganese Cluster as a Water Oxidation Electrocatalyst with Low Overpotential. Nat. Catal. 2018, 1, 48-54. (7) Zhao, Q.; Yan, Z. H.; Chen, C. C.; Chen, J. Spinels: Controlled Preparation, Oxygen Reduction/Evolution Reaction Application, and Beyond. Chem. Rev. 2017, 117, 10121-10211. (8) Suen, N. T.; Hung, S. F.; Quan, Q.; Zhang, N.; Xu, Y. J.; Chen, H. M. Electrocatalysis for the


18

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


Oxygen Evolution Reaction: Recent Development and Future Perspectives. Chem. Soc. Rev. 2017, 46, 337-365. (9) Liu, D. L.; Zhang, C.; Yu, Y. F.; Shi, Y. M.; Yu, Y.; Niu, Z. Q.; Zhang, B. Hydrogen Evolution Activity Enhancement by Tuning the Oxygen Vacancies in Self-supported Mesoporous Spinel Oxide Nanowire Arrays. Nano Res. 2018, 11, 603-613. (10) Wang, Y. C.; Zhou, T.; Jiang, K.; Da, P. M.; Peng, Z.; Tang, J.; Kong, B.; Cai, W. B.; Yang, Z. Q.; Zheng, G. F. Reduced Mesoporous Co3O4 Nanowires as Efficient Water Oxidation Electrocatalysts and Supercapacitor Electrodes. Adv. Energy Mater. 2014, 4, 1400696. (11) Zhong, H. X.; Zhang, Y.; Zhang, X. B. Superior Oxygen Reduction Electrocatalyst: Hollow Porous Spinel Microsphere. Chem 2018, 4, 191-198. (12) Wang, W. H.; Kuai, L.; Cao, W.; Huttula, M.; Ollikkala, S.; Ahopelto, T.; Honkanen, A.; Huotari, S.; Yu, M. K.; Geng, B. Y. Mass-production of Mesoporous MnCo2O4 Spinels with Manganese(IV)- and Cobalt(II)-rich Surfaces for Superior Bifunctional Oxygen Electrocatalysis. Angew. Chem. Int. Edit. 2017, 56, 14977-14981. (13) Li, T. F.; Lv, Y. J.; Su, J. H.; Wang, Y.; Yang, Q.; Zhang, Y. W.; Zhou, J. C.; Xu, L.; Sun, D. M.; Tang, Y. W. Anchoring CoFe2O4 Nanoparticles on N-doped Carbon Nanofibers for Highperformance Oxygen Evolution Reaction. Adv. Sci. 2017, 1700226. (14) Zhao, X. J.; Pachfule, P.; Li, S.; Simke, J. R. J.; Schmidt, J.; Thomas, A. Bifunctional Electrocatalysts for Overall Water Splitting from an Iron/Nickel-based Bimetallic Metal-organic Framework/Dicyandiamide Composite. Angew. Chem. Int. Edit. 2018, 57, 8921-8926. (15) Wang, X. J.; Li, Y.; Jin, T.; Meng, J.; Jiao, L. F.; Zhu, M.; Chen, J. Electrospun Thin-walled CuCo2O4@C Nanotubes as Bifunctional Oxygen Electrocatalysts for Rechargeable Zn-air Batteries. Nano Lett. 2017, 17, 7989-7994.


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(16) Wu, X. J.; Niu, Y. L.; Feng, B. M.; Yu, Y. N.; Huang, X. Q.; Zhong, C. Y.; Hu, W. H.; Li, C. M. Mesoporous Hollow Nitrogen-doped Carbon Nanospheres with Embedded MnFe2O4/Fe Hybrid Nanoparticles as Efficient Bifunctional Oxygen Electrocatalysts in Alkaline Media. ACS Appl. Mater. Inter. 2018, 10, 20440-20447. (17) Huang, J. Z.; Han, J. C.; Wang, R.; Zhang, Y. Y.; Wang, X. J.; Zhang, X. H.; Zhang, Z. H.; Zhang, Y. M.; Song, B.; Jin, S. Improving Electrocatalysts for Oxygen Evolution Using NixFe3xO4/Ni

Hybrid Nanostructures Formed by Solvothermal Synthesis. ACS Energy Lett. 2018, 3,

1698-1707. (18) Wang, G. M.; Yang, Y.; Han, D. D.; Li, Y. Oxygen Defective Metal Oxides for Energy Conversion and Storage. Nano Today 2017, 13, 23-39. (19) Yan, D. F.; Li, Y. X.; Huo, J.; Chen, R.; Dai, L. M.; Wang, S. Y. Defect Chemistry of Nonprecious-metal Electrocatalysts for Oxygen Reactions. Adv. Mater. 2017, 29, 1606459. (20) Chen, D. J.; Chen, C.; Baiyee, Z. M.; Shao, Z. P.; Ciucci, F. Nonstoichiometric Oxides as Low-cost and Highly-efficient Oxygen Reduction/Evolution Catalysts for Low-temperature Electrochemical Devices. Chem. Rev. 2015, 115, 9869-9921. (21) Bao, J.; Zhang, X. D.; Fan, B.; Zhang, J. J.; Zhou, M.; Yang, W. L.; Hu, X.; Wang, H.; Pan, B. C.; Xie, Y. Ultrathin Spinel-structured Nanosheets Rich in Oxygen Deficiencies for Enhanced Electrocatalytic Water Oxidation. Angew. Chem. Int. Edit. 2015, 54, 7399-7404. (22) Vitucci, F. M.; Paolone, A.; Palumbo, O.; Greco, G.; Lombardo, L.; Köntje, M.; Latini, A.; Panero, S.; Brutti, S. High-temperature Structural Evolution of the Disordered LiMn1.5Ni0.5O4. J. Am. Ceram. Soc. 2016, 99, 1815-1822. (23) Zhu, C. Z.; Fu, S. F.; Du, D.; Lin, Y. H. Facilely Tuning Porous NiCo2O4 Nanosheets with Metal Valence-state Alteration and Abundant Oxygen Vacancies as Robust Electrocatalysts


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Towards Water Splitting. Chem.-Eur. J. 2016, 22, 4000-4007. (24) Tung, C. W.; Hsu, Y. Y.; Shen, Y. P.; Zheng, Y. X.; Chan, T. S.; Sheu, H. S.; Cheng, Y. C.; Chen, H. M. Reversible Adapting Layer Produces Robust Single-crystal Electrocatalyst for Oxygen Evolution. Nat. Commun. 2015, 6, 8106. (25) Xu, L.; Jiang, Q. Q.; Xiao, Z. H.; Li, X. Y.; Huo, J.; Wang, S. Y.; Dai, L. M. Plasmaengraved Co3O4 Nanosheets with Oxygen Vacancies and High Surface Area for the Oxygen Evolution Reaction. Angew. Chem. Int. Edit. 2016, 55, 5277-5281. (26) Liu, R.; Wang, Y. Y.; Liu, D. D.; Zou, Y. Q.; Wang, S. Y. Water-plasma-enabled Exfoliation of Ultrathin Layered Double Hydroxide Nanosheets with Multivacancies for Water Oxidation. Adv. Mater. 2017, 29, 1701546. (27) Ou, G.; Xu, Y. S.; Wen, B.; Lin, R.; Ge, B. H.; Tang, Y.; Liang, Y. W.; Yang, C.; Huang, K.; Zu, D.; Yu, R.; Chen, W. X.; Li, J.; Wu, H.; Liu, L. M.; Li, Y. D. Tuning Defects in Oxides at Room Temperature by Lithium Reduction. Nat. Commun. 2018, 9, 1302. (28) Yue, Q. D.; Liu, C. M.; Wan, Y. Y.; Wu, X. J.; Zhang, X. Y.; Du, P. W. Defect Engineering of Mesoporous Nickel Ferrite and its Application for Highly Enhanced Water Oxidation Catalysis. J. Catal. 2018, 358, 1-7. (29) McCrory, C. C. L.; Jung, S.; Peters, J. C.; Jaramillo, T. F. Benchmarking Heterogeneous Electrocatalysts for the Oxygen Evolution Reaction. J. Am. Chem. Soc. 2013, 135, 16977-16987. (30) Wang, H. Y.; Hung, S. F.; Chen, H. Y.; Chan, T. S.; Chen, H. M.; Liu, B. In Operando Identification of Geometrical-site-dependent Water Oxidation Activity of Spinel Co3O4. J. Am. Chem. Soc. 2016, 138, 36-39.


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Boosting the Electrocatalytic Water Oxidation Performance of

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