Hierarchical Hollow Spheres Assembled with ultrathin CoMn Double

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Hierarchical Hollow Spheres Assembled with ultrathin CoMn Double Hydroxide Nanosheets as Trifunctional Electrocatalyst for Overall Water Splitting and Zn-Air Battery Kaiyue Li, Dong Guo, Jianyu Kang, Bo Wei, Xitian Zhang, and Yujin Chen ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b03232 • Publication Date (Web): 29 Sep 2018 Downloaded from http://pubs.acs.org on September 30, 2018

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ACS Sustainable Chemistry & Engineering

Hierarchical Hollow Spheres Assembled with ultrathin CoMn Double Hydroxide Nanosheets as Trifunctional Electrocatalyst for Overall Water Splitting and Zn-Air Battery Kaiyue Li, †,# Dong Guo, † Jianyu Kang, † Bo Wei, *,‡ Xitian Zhang # and Yujin Chen *,†

† Key Laboratory of In-Fiber Integrated Optics, Ministry of Education, and College of Science, Harbin Engineering University, Harbin 150001, China

# Key Laboratory for Photonic and Electronic Bandgap Materials, Ministry of Education, and School of Physics and Electronic Engineering, Harbin Normal University, Harbin 150025, China.

‡ Department of Physics, Harbin Institute of Technology, Harbin 150001, China. E-mail: [email protected]

KEYWORDS: layered double hydroxides, trifunctional catalysts,

water splitting,

Zn-air battery.

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ABSTRACT: Larger-scale usage of the clean energies requires advanced energy storage and conversion techniques. Overall water splitting and zinc-air systems are promising energy conversion and storage means, but need high-performance, low-cost and highly stable bifunctional electrocatalysts to hydrogen evolution/oxygen evolution reactions (HER/OER) and OER/oxygen reduction reactions (ORR), respectively. Herein we develop a facile method to fabricate CoMn double hydroxide (DH) hollow spheres as high-performance trifunctional electrocatalysts for overall water splitting and zinc-air battery. The outmost surfaces of the CoMn DH hollow spheres are composed of ultrathin nanosheets with a thickness of approximately 2.5 nm. Due to the hollow and ultrathin features, CoMn DH hollow spheres exhibit excellent activities and robust stabilities toward HER, OER, and ORR. The overall water electrolyser assembled with the CoMn DH hollow spheres can drive a current density of 10 mA cm-2 at an overpotential of merely 420 mV in 1.0 M KOH, among the best reported metal oxides and hydroxides. Remarkably, each all-solid-state Zn-air battery with the optimized CoMn DH hollow spheres air-cathode exhibited good cycling stability. Furthermore, two all-solid-state Zn-air batteries in series can power 55 red LED and the two-electrode electrolyzer catalyzed by the optimized CoMn DH hollow spheres with excellent operation stabilities.


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INTRODUCTION To resolve the issues relevant to the increasing environmental pollution and excessive consumption of petrochemical energy, clean and renewable energy sources have been explored widely. The large-scale utilization clean and renewable energy sources are involved in the development of energy storage and conversion systems such as electrochemical water splitting devices and metal-air batteries.1-10 To improve the electrochemical properties, highly active electrocatalysts for catalyzing hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) are required for the electrochemical water splitting, while highly active electrocatalysts for catalyzing oxygen reduction reaction (ORR) and OER are needed for metal-air batteries. Pt-based materials are the most active electrocatalysts for HER and ORR in acidic media,4, 6, 11 while RuO2 and IrO2-based material are the most active electrocatalysts for OER.12 However, the large-scale usage of the precious metal-based electrocatalysts is limited due to their high cost and scarce storage on Earth. Therefore, it is necessary to develop low-cost electrocatalysts with efficient and multifunctional electrohemical performance for water splitting and metal-air batteries. Recently, non-precious transition-metal based nanomaterials have been developed as electrocatalysts for catalyzing HER, OER and/or ORR.13-29 Among these non-precious electrocatalysts, transition-metal layered double hydroxides have attracted wide attention due to their facile and environmentally friendly fabrication methods.4,

15-29

Preliminary work mainly focused on the development of OER

electrocatalysts, including various NiFe, NiCo, NiMn, CoCo, NiCo, CoFe, CoAl, and 3 ACS Paragon Plus Environment


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CoMn hydroxide nanostructures.15-26, 30 Recently, some layered double hydroxides have shown bifunctional electrohemical activities through reducing their size and/or constructing hybrids. For example, through coupling with N-doped graphene oxide and graphene, respectively, NiCoFe layered double hydroxide nanosheets and NiFe layered double hydroxide nanoplates could be used as high-performance bifunctional electrocatalysts for OER and ORR;27, 28 carbon fiber cloth supported Co-Ni-based nanotubes/nanosheets could catalyze OER and HER for overall water splitting.29 However, to the best of our knowledge, trifunctional electrocatalysts based on non-precious transition-metal hydroxides for HER, OER and ORR have never been reported so far. To date, two strategies were common adopted to improve the catalytic activities of the electrocatalysts. One strategy is decreasing the size of the electrocatalysts to produce more exposed active site.15,

24, 26, 31-33

For example, Song et al. prepared

ultrathin CoMn hydroxide nanoplates with an average thickness of 3.6 nm through a simple coprecipitation method and found that the CoMn hydroxide on a glassy carbon (GC) electrode for OER only required 324 mV vs. reversible hydrogen electrode (RHE) overpotential to drive a current density of 10 mA cm-2 in 1.0 M KOH.26 Gong et al. grew NiFe layered double hydroxide nanoplates with a thickness of approximately 5 nm on carbon nanotubes (CNTs) and found that the onset potential of the NiFe nanoplates on GC electrode for OER was merely 1.50 V vs. RHE in 0.1 M KOH.15 Ping et al. assembled single-layer CoAl layered double hydroxide nanosheets on three-dimensional (3D) grapahene network, which showed significantly improved 4 ACS Paragon Plus Environment

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OER catalytic activity in comparison to bulk counterparts.24 Another strategy for the enhancement of

the catalytic

activity is

constructing porous

or hollow

electrocatalysts.29, 34-42 The porous or hollow structures not only allowed the active sites to contact with the electrolyte sufficiently, but also increased the electrochemically active area. As a consequence, the porous or hollow electrocatalysts showed significantly enhanced catalytic activities compared to the counterparts without porous or hollow features. For example, a alkaline water splitting system based on porous NiCoFe layered triple hydroxide nanosheets exhibited a low cell voltage of approximately 1.55 V at a current density of 10 mA cm-2 in 1.0 M KOH, compared

to

that

of

IrO2/Pt-based

water

splitting

device.27

Mo2C

nanoparticles/N-doped CNTs exhibited excellent HER performance with a current density of 10 mA cm-2 at an overpotential of 147 mV in 0.5 M H2SO4.31 FeOOH/Co/FeOOH nanotube arrays supported by NF only required an overpotential of about 250 mV to drive a current density of 20 mA cm-2 in 1.0 M NaOH.42 Recently, we assembled alkaline electrolyzers with hierarchical nickel-cobalt phosphide yolk-shelled spheres and hollow CoP nanopaticle/N-doped graphene hybrids, showing a current density of 10 mA cm-2 at cell voltages of 1.59 and 1.58 V, respectively.36, 37 Inspired by the pioneering results mentioned above, herein we develop a facile method to fabricate hierarchical CoMn double hydroxide (DH) hollow spheres for multifunctional electrocatalysts for HER, OER and ORR. The shell of the hollow spheres is composed of hierarchical CoMn DH nanosheets with an average thickness 5 ACS Paragon Plus Environment


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of 2~3 nm. To drive a current density of 10 mA cm-2, the overall water splitting system assembled with the hierarchical CoMn DH hollow spheres needs a voltage of 1.65 V, favourably comparable to those of the reported metal oxide and metal hydroxide based electrocatalysts. Furthermore, a primary Zn-air battery with the hierarchical CoMn DH hollow spheres as air cathode exhibits a high specific capacity of 684 mAh g-1 at a current density of 10 mA cm-2, outperforming that of a primary Zn-air battery with the Pt/C air-cathode (603 mAh g-1). Notably, a rechargeable Zn-air with the hierarchical CoMn DH hollow spheres as air-cathode shows higher round-trip efficiency than that of a rechargeable Zn-air with the Pt/C air-cathode. In addition, the water splitting device with the hollow spheres as bifunctional electrocatalysts can be self-powered by all-solid-state Zn-air battery assembled by the hollow spheres. EXPERIMETNAL SECTION Synthesis of the CoMn catalysts. Five CoMn catalysts, i.e., Co1Mn0, Co2Mn1, Co4Mn1, Co1Mn3 and Co0Mn1 spheres were prepared. The Co2Mn1 hollow spheres were synthesized through a two-step process. First, 190 mg (0.757 mmol) of Mn(NO3)2·4H2O and 218 mg (0.749 mmol) of Co(NO3)2·6H2O were dissolved into the mixed solvent containing 6 mL of glycerol, 18 mL of isproponal and 2 mL of water. Then the mixture above was transferred Teflon container, and hydrothermally reacted at 180 oC for 6 h. After being cooled to room temperature, the precipitate was collected by centrifugation, then washed with ultrapure water and ethanol for several times and dried at 40 oC in oven. The 100 mg of the precursor was distributed into 30 6 ACS Paragon Plus Environment

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mL of H2O and then transferred Teflon container with hydrothermal treatment at 150 o

C for 3 h. The precipitate was washed with ultrapure water and ethanol for several

times and dried at 40 oC in oven. Co4Mn1 was prepared through similar method to that of Co2Mn1 except that the water was not added in the first step. Co1Mn3 was prepared through similar method to that of Co4Mn1 but the added masses of Mn(NO3)2·4H2O and Co(NO3)2·6H2O were changed to 380 (1.514 mmol) and 218 mg (0.749 mmol), respectively. Co1Mn0 was prepared through the similar method to that of Co4Mn1 except that the added mass of Co(NO3)2·6H2O was changed to 436 mg and Mn(NO3)2·4H2O was not added into reaction system. Co0Mn1 was prepared through the similar method to that of Co1Mn3 except that Co(NO3)2·6H2O was not added into reaction system. Electrolytic water electrode preparation and measurements. Electrochemical measurements were performed in a three-electrode system at an electrochemical station (CHI660E, CH Instrument, USA). Three electrodes consisting of carbon paper-loaded CoMn catalysts, graphite rod and a Ag/AgCl (KCl saturated) were used as the working electrode, counter and reference electrodes, respectively. The loading mass of the CoMn catalysts was approximately 3.5 mg cm–2 for all the working electrodes. Electrochemical measurements of the catalysts were measured in 1 M KOH solution after purging the electrolyte with N2 gas for 30 min. Polarization curves were obtained using Linear sweep voltammetry (LSV) with scan rate of 2 mV s-1. The long-term stability test was carried out using chronopotentiometric measurements. All



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potentials measured were calibrated to RHE using the following Equation: E(RHE) = E(Ag/AgCl) + 0.21 V + 0.059 × pH.

Identification of produced gas and determination of Faraday efficiency (FE). Gas chromatography (GC) measurements were conducted on GC–2000C (Shimadzu Co.) with thermal conductivity detector and nitrogen carrier gas. Pressure data during electrolysis were recorded using a CEM DT-8890 Differential Air Pressure Gauge Manometer Data Logger Meter Tester with a sampling interval of 1 point per second. ORR electrode preparation and measurements. The ORR measurements were performed in a three-electrode system (CHI 760D, CH Instrument, USA), using a graphitic rod as a counter electrode and Ag/AgCl (KCl saturated) as a reference electrode. The preparation of working electrode was carried out as follows: 5.0 mg catalyst was firstly dispersed 450 µL ethanol, then 50 µL Nafion solution (5.0 wt%) was added, followed by 2.0 h sonication to form a relatively homogeneous suspension. 10.0 µL of the suspension was transferred onto the glassy carbon disk electrode or a rotating disk electrode (RED), which was mechanically polished and ultrasonically washed in advance. After solvent evaporation for 30 min in air, the working electrode was prepared for electrochemical measurements. The scan rate was 10.0 mV s-1 for LSV measurements. The HO2– yield for the Co2Mn1 DH hollow spheres was performed on the roating-ring-disk electrode (RRDE) in O2-saturated 0.1 M KOH solution at a rotation speed of 1600 rpm with a scan rate of 10.0 mV s-1. The hydrogen


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peroxide yield (HO2-1 %) and the electron transfer number (n) are determined by the following equations: HO2-1 (%) = 200 × Ir N-1/ ( Id + IrN-1) n= 4 × Id / ( Id+IrN-1) In the equations, Id is the disk current, Ir is the ring current, and N = 0.4 is the current collection efficiency of the Pt ring. OER electrodes of the electrocatalysts for evaluating overall oxygen reactions (OER and ORR) were prepared by the same method as that of ORR electrodes, and the OER properties were measured in 0.1 M KOH solution at a scan rate of 10 mV s-1. Assembly of liquid Zn-air battery. A home-made liquid zinc-air battery was employed. The air electrode was made by transferring a certain volume of catalyst ink onto a carbon paper with a gas diffusion layer with a catalyst loading of 0.50 mg cm-2 via a drop casting method. The catalyst ink was fabricated through the same method as that for ORR electrodes. A polished zinc foil (thickness: 0.25 mm) was used as the anode. The electrolyte was 6.0 M KOH for primary Zn-air battery and 6.0 M KOH with 0.2 M Zn(CH3COO)2 for rechargeable Zn-air battery. Measurements were carried out at 25oC with an electrochemical workstation (CHI 660E, CH Instrument, USA). The specific capacity and power density were calculated from the galvanostatical discharge results, normalized to the mass of consumed Zn and geometric area of the catalysts, respectively. The mass of consumed Zn was determined by the mass difference between the fresh Zn and resultant anode after discharging. 9 ACS Paragon Plus Environment


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Assembly of solid-state Zn-air battery. A polished zinc foil (thickness: 0.25 mm) was used as anode. The air electrode was made by dropping a certain volume of catalyst ink onto a cleaned carbon cloth substrate (1.5 × 3 cm2) with a catalyst loading of 1.50 mg cm-2. The gel polymer electrolyte was prepared as follows: 5.0 g of PVA-1788 powder (Aladdin) was dissolved in 50.0 mL deionized water at 90 oC under magnetic stirring for 2.0 h. Then 5.0 mL of 18.0 M KOH filled with 0.02 M Zn(CH3COO)2 was added and the electrolyte solution was kept stirring at 90 oC for 30 min. Then the solution was freezed at –10oC over 12 h, and then aged at room temperature. Then the flexible solid-state Zn-air battery was assembled with air electrode and zinc foil placed on the two sides of PVA gel (1.5 cm×2 cm×0.5 cm), and a piece of pressed Ni foam was used as current collector next to the air electrolyte. Characterization. Scanning electron microscope (SEM) images were taken using a HITACHI SU8000 operating at 15 keV. Transmission electron microscope (TEM) images were carried out on a FEI Tecnai-F20 transmission electron microscope equipped with a Gatan imaging filter (GIF). X-ray photoelectron spectroscopy (XPS) was performed on a Thermo ESCALAB250 X-ray Photoelectron Spectrometer. X-ray powder diffraction (XRD) was performed on a X’Pert Pro diffractometer with Cu Kα radiation (λ=1.5418Å). The metal content in the CoMn spheres were determined by energy dispersive spectrometer (EDS) and inductively coupled plasma optical emission spectrometer (ICP-OES).


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Figure 1. a) XRD pattern, b,c) TEM images, d) HRTEM image, e) EDX elemental mappings of the Co2Mn1 DH hollow spheres. RESULTS AND DISCUSSION The atomic ratios of Co/Mn determined by ICP-OES measurements are 2.2: 1, 3.8: 1 and 1: 3.2 for Co2Mn1 DH, Co4Mn1 and Co1Mn3 DH hollow spheres, respectively. Figure 1a shows the XRD pattern of the as-synthesized Co2Mn1 DH hollow spheres. The diffraction peaks can be indexed to (001), (100), (101), (002), (102), (110), (003) and (111) lattice planes of hexagonal Co(OH)2 (JCPDS card No. 30-0443), and (001), (101), (002) and (111) planes of hexagonal Mn(OH)2 (JCPDS card no. 18-0787), rsepectively. By careful comparison of the peak positions, we found that the diffraction peaks from (001) and (002) lattice planes of Co(OH)2 are overlapped with those from (001) and (002) lattice planes of Mn(OH)2, respectively due to their 11 ACS Paragon Plus Environment


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similar crystalline structures. Based on the XRD analysis, the Co2Mn1 DH hollow spheres are composed of crystalline CoMn double hydroxides. As shown in Figure S1, Co1Mn3 and Co4Mn1 DH hollow spheres have similar diffraction peaks, indicating that they have similar crystalline structures to that of Co2Mn1 DH hollow spheres. However, the diffraction peaks of the pure Co catalysts (Co1Mn0 spheres) can be indexed to Co(OH)2 and Co3O4 (JCPDS card No. 43-1003), while the diffraction peaks of the pure Mn catalysts (Co0Mn1 spheres) originate from Mn(OH)2 and Mn3O4 (JCPDS card No. 75-1560), respectively (Figure S1). The diffraction peak of (100) lattice plane of Co(OH)2 in the Co2Mn1 DH hollow spheres shifts toward higher diffraction angle by 0.30o compared to that in Co1Mn0 spheres, while the diffraction peak of (101) lattice plane of Mn(OH)2 in the Co2Mn1 DH hollow spheres shifts toward lower diffraction angle by about 0.45o compared to that in Co0Mn1 sphere (Figure S2), indicating that the slight change in the crystalline structures of Co(OH)2 and Mn(OH)2 in the Co2Mn1 DH hollow spheres occurs. SEM images (Figure S3) indicate that the Co0Mn1 and Co1Mn0 spheres have the largest and smallest sizes among our as-synthesized CoMn catalysts. TEM images (Figure 1b,c and Figure S4) indicates that the Co2Mn1, Co4Mn1, Co1Mn3 and Co1Mn0 spheres have hollow interior voids, whereas the inside parts of the Co0Mn1 spheres are solid. Furthermore, the outmost surfaces of the Co2Mn1, Co4Mn1, and Co1Mn0 spheres are composed of numerous thin nanosheets, while the outmost surfaces of the Co1Mn3 and Co0Mn1 spheres are composed of large nanoparticles. Thus, the incorporation of Mn not only has an important effect on the size, but also on the morphology of our CoMn 12 ACS Paragon Plus Environment

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trifunctional electrocatalysts. The thickness of the nanosheets in the outmost surfaces of the Co2Mn1 DH hollow spheres determined by AFM measurement is approximately 2.5 nm, suggesting their ulthratin features (Figure S5). High-resolution TEM (HRTEM) image in Figure 1d shows that the alignments of the visible lattice fringes of the nanosheets in the outmost surfaces of the Co2Mn1 DH hollow spheres are disordered and the multiple crystal domains are apparent. The TEM observations demonstrate numerous defects existed in the basal plane of the nanosheets. The ultrathin feature and the numerous defects allow more active sites to exposure to electrolyte and thereby facilitate the improvement of the catalytic activity of the DH hollow spheres. The labeled lattice fringes with interplanar distance of 0.280 nm correspond to the (100) lattice plane of Co(OH)2 or Mn(OH)2 nanosheets respectively (Figure 1d). The energy dispersive X-ray spectrometry (EDX) elemental mappings (Figure 1e) indicate that the Co2Mn1 DH hollow spheres contain Co, Mn, and O elements, and the distributions of these elements confirm the hollow feature of the DH spheres. As previously reported, the formations of Co1Mn0 hollow spheres (Co(OH)2 hollow spheres) could be achieved through the follow steps. Cobalt alkoxide solid spheres were first formed after the cobalt nitrate reacted with glycerol and isopropanol. Then, cobalt alkoxide solid spheres were reacted with water, and the Co(OH)2 hollow spheres were gradually formed through a so-call ‘self-template’ process during which the cobalt alkoxide solid spheres used as both templates and precursors for the formation of the nanosheet shell.43 In this work, we found that it was difficult to produce the hollow spheres for pure Mn-based catalysts (Co0Mn1), 13 ACS Paragon Plus Environment


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which may be due to the large amount of Mn3O4 particles in the Co0Mn1 sphere. Therefore, the hollow feature of our CoMn catalysts changed unapparent with the increase of Mn content (Figure S4).

Figure 2. XPS spectra of the Co2Mn1 DH hollow spheres. a) Co 2p, b) Mn 2p, and c) O 1s.

XPS measurements were carried out to analyze the surface compositions and the valence states of the elements. Figure 2a shows the Co 2p XPS spectra, in which the 2p3/2 peak at 781.2 eV and the 2p1/2 peak at 796.4 eV as well as the satellite peaks at 788.2 and 805.2 eV can be assigned to Co2+ species, while the 2p3/2 peak at 778.0 eV and the 2p1/2 peak at 795.2 eV correspond to Co3+ species.21, 44 The Co3+ species may be due to the oxidation of Co2+ upon air exposure. In the Mn 2p XPS spectra (Figure 2b), the 2p3/2 peaks at 641.5 and 642.6 eV and the 2p1/2 peaks at 653.5 and 654.7 eV can be observed, revealing the coexistence of Mn2+, Mn3+ and Mn4+ species in the nanosheets.45, 46 In comparison with pure cobalt and manganese hollow spheres, the binding energy of Co in Co2Mn1 DH shifts toward lower binding energy by approximately 0.8 eV; in contrast, the binding energy of Mn in Co2Mn1 DH has the opposite shift by 0.9 eV (Figure S6), suggesting the strong electron interaction 14 ACS Paragon Plus Environment

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between Co and Mn in the Co2Mn1 DH. The peaks at 529.7, 531.7 and 534.0 eV in the O 1s XPS spectra of Co2Mn1 DH spheres can be assigned to metal oxygen, hydroxyl species, and surface adsorbed water molecules, respectively (Figure 2c).21, 47, 48

The O 1s XPS spectra of the Co1Mn0 and Co0Mn1 spheres shows that they have

similar surface compositions to that of Co2Mn1 DH spheres (Figure S7). The HER activities of our CoMn electrocatalysts were first investigated using a standard three-electrode setup in 1 M KOH. The carbon paper-supported CoMn electrocatalysts with a mass loading of 2.0 mg cm-2, a graphitic rod and Ag/AgCl electrode were served as working, counter and reference electrodes, respectively. The HER activities of the Pt/C and carbon paper were also tested for comparison. Figure 3a shows the LSVs of the carbon paper, CoMn DHs and the Pt/C. As expected, the Pt/C exhibits excellent HER activity with a small overpotential of 25 mV at a catholic current density of 10 mA cm-2, while the carbon paper exhibits negligible HER activity over the tested potential range. Among the tested catalysts, the pure Mn (Co0Mn1) and pure Co (Co1Mn0) spheres exhibit poor HER activities with a cathodic current density of 10 mA cm-2 at overpotentials of 507 and 477 mV, respectively (Figure 3a,b). Upon incorporation of Mn to Co, the HER activities were increased significantly. For example, to deliver a cathodic current density of 10 mA cm-2 the Co2Mn1, Co4Mn1 and Co1Mn3 DH hollow spheres require overpotentials of 187, 272 and 337 mV, respectively. Therefore, the HER activity of our CoMn catalysts follows the order: Co2Mn1 > Co4Mn1 > Co1Mn3 > Co0Mn1 > Co1Mn0. The overpotential of Co2Mn1 DH hollow spheres at 10 mA cm-2 is smaller than that of the recently 15 ACS Paragon Plus Environment


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reported HER catalysts such as NiFe/NPC (260 mV at 10 mA cm-2), CoNi LDH (210 mV at 10 mA cm-2), FeCo NPC (340 mV at 10 mA cm-2), and NiCo2O4 NA/CC (456 mV at 10 mA cm-2) (Table S2),29, 44, 49, 50suggesting high HER activity of the DH hollow spheres. In addition, the LSV curve of Co2Mn1 hollow spheres without iR-compensation requires overpotential of 198 mV at 10 mA cm-2 (Figure S8a), merely 11 mV higher than that obtained from LSV curve with iR-compensation. This result further reveals high HER activity of Co2Mn1 hollow spheres. The outstanding catalytic activity of the Co2Mn1 DH hollow spheres was also reflected by Tafel slope,

which can be derived from the Tafel equation, η = a + b log | j |

(1)

where η represents overpotential, a is a constant and b is the Tafel slope, and j represents the catalytic current density. Figure 3b shows the Tafel plots of the CoMn spheres and Pt/C The Tafel slope of Pt/C is as small as about 50 mV dec-1, further suggesting the excellent HER performance. The Tafel slope of the Co2Mn1 DH hollow spheres is 60 mV dec-1, significantly smaller than that of Co1Mn0 (298 mV dec-1), Co0Mn1 (160 mV dec-1), Co4Mn1 (125 mV dec-1), and Co0Mn1(106 mV dec-1). The smallest Tafel slope suggests the fastest HER kinetics of the Co2Mn1 DH hollow spheres among these CoMn electrocatalysts, consistent with their large current densities at low HER overpotentials. Electrochemical impedance spectroscopy (EIS) measurements were carried out to analyze the fast HER kinetics. Nyquist plots (Figure 3c) indicate the Rct of the Co2Mn1 DH hollow spheres at an overpotential of


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250 mV is 1.21 Ω cm-2, lower than that of Co1Mn0 (2.08 Ω cm-2), Co0Mn1 (2.98 Ω cm-2), Co4Mn1 (1.28 Ω cm-2), and Co0Mn1 (1.43 Ω cm-2).

Figure 3. a) LSV curves of Co2Mn1, Co1Mn3, Co4Mn1, Co1Mn0, Co0Mn1 spheres and Pt/C toward HER, b) Comparisons of the overpotentials at 10 mA cm-2 and Tafel slopes among Co2Mn1, Co1Mn3, Co4Mn1, Co1Mn0, Co0Mn1 spheres and Pt/C. c) The Nyquist plots of Co2Mn1, Co1Mn3, Co4Mn1, Co1Mn0, Co0Mn1 spheres. d) The stability of the Co2Mn1 DH hollow spheres and Pt/C toward HER.

Besides the activity, the catalytic stability is a very important factor for the practical applications of the electrocatalysts. The stability of the Co2Mn1 DH hollow spheres toward HER was assessed by the chronoamperometry at an given overpotential of 190 mV. As shown in Figure 3f, at initial stage the current density is 10.2 mA cm-2, and 99.1% of the initial current density is retained after the 23 h continuous HER process, 17 ACS Paragon Plus Environment


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suggesting the robust stability of the Co2Mn1 DH hollow spheres toward HER. In contrast, the current density derived by the Pt/C catalyst at a given overpotential of 20 mV decreases to 99.1% of the initial current density after the 23 h continuous HER process. The HER-post Co2Mn1 DH hollow spheres were characterized by SEM, TEM, XRD and XPS. As shown in Figure S9c,d and Figure S10b, the HER-post CoMn catalysts still have hollow feature, and the ultratin nanosheets are still remained in the outmost surfaces. By comparing the Co and Mn XPS spectra before and after HER, we find that the valence states of the Co and Mn species after the HER have little difference from those of the initial CoMn catalysts. The XRD pattern indicates that the crystal compositions of the CoMn catalysts after the HER have also little change in comparison to the pristine CoMn catalysts (Figure S11). The SEM, TEM, XRD and XPS results demonstrate robust structural stability of the DH hollow spheres toward HER in the alkaline media.

Figure 4. a) LSV curves of Co2Mn1, Co1Mn3, Co4Mn1, Co1Mn0, Co0Mn1 spheres and IrO2 toward OER in 1.0 M KOH. b) Comparisons of the overpotentials at 10 mA cm-2 18 ACS Paragon Plus Environment

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and Tafel slopes among Co2Mn1, Co1Mn3, Co4Mn1, Co1Mn0, Co0Mn1 spheres and IrO2. c) The CV curves of Co2Mn1, Co1Mn3, Co4Mn1, Co1Mn0, Co0Mn1 spheres at a scan rate of 50 mV s-1. d) Nyquist plots of Co2Mn1, Co1Mn3, Co4Mn1, Co1Mn0, and Co0Mn1 spheres. e) The electrochemical double-layer capacitances of Co2Mn1, Co1Mn3, Co4Mn1, Co1Mn0, and Co0Mn1 spheres. f) Time-dependent current density curve for Co2Mn1 DH at an overpotential of 290 mV and IrO2 at overpotential of 340 mV toward OER.

We next investigate the OER activities of our CoMn electrocatalysts using a standard three-electrode setup in 1 M KOH. The carbon paper-supported CoMn electrocatalysts with a mass loading of 2.0 mg cm-2, a graphitic rod and Ag/AgCl electrode were served as working, counter and reference electrodes, respectively. The OER activities of the carbon paper and the commercial IrO2 on carbon paper were also tested for comparison. The electrocatalytic activities of the catalysts were evaluated by LSV measurements between 1.35 – 1.65 V vs. RHE. Figure 4a shows the iR-corrected polarization curves obtained from the LSVs at a scan rate of 2.0 mV s-1. The carbon paper exhibits negligible OER activity in the potential ranging from 1.35 to 1.65 V vs. RHE. Among the CoMn spheres, the Co2Mn1 DH hollow spheres exhibit the best OER activity. For example, to drive a current density of 10 mA cm-2, the Co2Mn1 DH hollow spheres require an overpotential of 233 mV, 55, 67, 87, and 97 mV lower than that of Co1Mn3, Co4Mn1, Co0Mn1, and Co1Mn0 spheres, respectively (Figure 4a). At the same time, the current density driven by the Co2Mn1 DH hollow spheres at 317 mV reaches to 188.1 mA cm-2, also larger than that of 19 ACS Paragon Plus Environment


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Co1Mn3 (21.10 mA cm-2), Co4Mn1 (16.60 mA cm-2), Co0Mn1 (8.70 mA cm-2), and Co1Mn0 (8.40 mA cm-2) spheres (Figure 4a). Therefore, the OER activity of our CoMn catalysts follows the order: Co2Mn1 > Co1Mn3 > Co4Mn1 > Co0Mn1 ≈ Co1Mn0. The OER performance of the Co2Mn1 DH hollow spheres outperform other reported hydroxide nanostructures such as CoCo-LDH ultrathin nanosheets (10 mA cm-2 at 350 mV), CoNi-LDH (10 mA cm-2 at 334 mV), α-Co4Fe(OH)x (10 mA cm-2 at 295 mV), NoCo2.7(OH)x (10 mA cm-2 at 350 mV), and NiFe LDH (10 mA cm-2 at 240 mV), also summarized in Table S1.5, 22, 23,24 Remarkably, the OER activity of the Co2Mn1 DH hollow spheres is superior to that of the state-of-the-art IrO2 catalyst. For example, to deliver a current density of 10 mA cm-2, IrO2 catalyst needs an overpotential of 272 mV, 39 mV higher than that of the Co2Mn1 DH hollow spheres (Figure 4b). The LSV curve without iR-correction (Figure S8b) indicates that the Co2Mn1 DH hollow spheres need an overpotential of 249 mV to drive a current density of 10 mA cm-2, merely 16 mV higher than that obtained from LSV curve with iR-compensation (Figure 4a). This result further confirms highly intrinsic OER activity of Co2Mn1 hollow spheres. The OER kinetics of our CoMn catalysts was evaluated by their Tafel slopes. As shown in Figure 4b, the Tafel slope of the Co2Mn1 DH hollow spheres is merely 57 mV dec-1, distinctly smaller than that of Co1Mn0 (182 mV dec-1), Co0Mn1 (274 mV dec-1), Co4Mn1 (59 mV dec-1), Co3Mn1 (88 mV dec-1), and IrO2 (73 mV dec-1). The smallest Tafel slope suggests the fastest OER kinetics of the Co2Mn1 DH hollow spheres, consistent with their large current densities at low overpotentials. 20 ACS Paragon Plus Environment

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To explain the reasons for the enhancements of the Co2Mn1 DH hollow spheres, cyclic voltammograms (CV) for the CoMn electrocatalsts were measured at a scan rate of 50 mV s-1. As shown in Figure 4c, there are no obvious redox peaks for Co0Mn1 spheres, indicating little change of Mn species in the valence state during OER. As for the Co1Mn0 spheres, the anodic peak before oxygen evolution is located at 1.29 V vs. RHE, resulting from the oxidation of the Co species. As for the Co2Mn1 DH hollow spheres, the anodic peak shifts negatively to 1.14 V vs. RHE, which may result from the strong electron interaction between Co and Mn. This shift of the anodic peak suggests Co species in the Co2Mn1 DH hollow spheres are more easily oxidized than those in the Co1Mn0 spheres, which facilitates the enhanced OER activity.29 As a result, compared to the Co1Mn0 spheres, the Co2Mn1 DH hollow spheres show enhanced OER activity including lower onset potential and faster increase rate of the OER current density (Figure 4a,b). Besides the electron interaction between Co and Mn, the charge-transfer resistance (Rct) is directly relevant to the OER performance. EIS measurements were performed to compare the Rct values of our electrocatalysts-based anodes. The Nyquist plots and corresponding equivalent circuit model in Figure 4d indicate that the Rct value of the Co2Mn1 DH hollow spheres is 1.37 Ω cm-2 at an overpotential of 350 mV, lower than that of Co1Mn3 (1.72 Ω cm-2), Co4Mn1 (1.50 Ω cm-2), Co0Mn1 (3.25 Ω cm-2), and Co1Mn0 (2.31 Ω cm-2) spheres. Small Rct value facilitates the improvement of increment rate of OER current density with increase of the overpotential. In addition, the electrochemical active surface area has an important effect on the activity of the electrocatalyst. The 21 ACS Paragon Plus Environment


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electrochemical active surface area (ECSA) is proportional to the double-layer capacitance (Cdl) of the electrocatalysts, which can be estimated by measuring CV curves at different scan rates (Figure S13). As shown in Figure 4e, the Cdl vaule for the Co2Mn1 DH hollow spheres is 83.5 mF cm-2, much larger than that of Co1Mn3 (52.30 mF cm-2), Co4Mn1 (51.61 mF cm-2), Co0Mn1 (11.56 mF cm-2), and Co1Mn0 (18.77 mF cm-2) spheres. In addition, the oxygen vacancy facilitates the adsorption of the intermediate products of the OER processes. Based on the discussion above, the enhanced OER activity of the Co2Mn1 DH hollow spheres can be explained by the electron interaction between Co and Mn, the larger ECSA and lower charge-transfer resistance. The stability of the Co2Mn1 DH hollow spheres was assessed by the chronoamperometry at an given overpotential of 290 mV. As shown in Figure 4f, at initial stage the current density is 50.2 mA cm-2, and 98.0% of the initial current density is retained after the 14 h continuous OER process, much better than that of IrO2, suggesting the robust stability of the Co2Mn1 DH hollow spheres toward OER. The structures of OER-post Co2Mn1 DH hollow spheres were characterized by SEM, TEM, XRD and XPS. As shown in Figure S9a,b and Figure S10a, the OER-post catalysts still exhibit hollow feature. Furthermore, the outmost surfaces are composed of the ultratin nanosheets, similar to those of the initial spheres. XRD pattern shows that the crystal compositions of the CoMn catalysts after the OER have also little change in comparison to the pristine CoMn catalysts (Figure S11). Thus, the structures of the Co2Mn1 DH hollow spheres are highly stable toward OER in the 22 ACS Paragon Plus Environment

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alkaline media. Figure S12a shows the Co 2p XPS spectra of the OER-post Co2Mn1 DH hollow spheres. Compared to the spectra of initial Co2Mn1 DH hollow spheres, the peaks of the Co 2p3/2 at 786.6 eV and the Co 2p1/2 at 803.0 eV appear, which can be assigned to CoOOH species.51 In contrast, the valence states of Mn species have little change before and after the OER process (Figure S12b), consistent with the CV measurements (Figure 4c). The XPS results suggest that the CoOOH at the surface is real active phase toward OER, consistent with the previous reports.22, 24 In terms of the experimental results above, the Co2Mn1 DH hollow spheres exhibit high activities and robust stabilities toward both HER and OER in 1.0 M KOH, we assembled a water electrolyzer using the Co2Mn1 DH hollow spheres as bifunctional electrocatalysts. For comparison, a water electrolyzer with Pt foil and IrO2 loaded on carbon paper as cathode and anode, respectively, was also assembled. The LSVs were tested in 1.0 M KOH solution at voltage ranging from 1.2 V to 2.0 V. As shown in Figure 5a, to drive overall water splitting current density of 10 mA cm-2, the water electrolyzer based on the Co2Mn1 DH hollow spheres requires an overpotential of 0.42 V, while the IrO2|Pt couple needs 0.34 V. Although the electrochemical property of the electrolyzers based Co2Mn1 DH hollow spheres is inferior to that of the IrO2|Pt couple, favorably comparable to that of the water electrolyzers assembled with recently reported bifunctional electrocatalysts such as Ni0.69Co0.31–P (10 mA cm-2 at overpotential of 0.51 V) and Ni0.6Co2.4O4 (10 mA cm-2 at overpotential of 0.53 V) (Table S3).36, 52 Meanwhile, the long-term stability of water splitting was tested at a voltage of 1.65 V in 1.0 M KOH solution with the current density of 10 mA cm-2. As 23 ACS Paragon Plus Environment


shown in Figure 5b, at initial stage the current density is 10.2 mA cm-2, and 96.3% of the initial current density is retained after the 5 h continuous HER process, suggesting the robust stability of the Co2Mn1 DH hollow spheres as bifunctioanl catalysts for overall water splitting. According to the experimentally measured and the theoretically calculated amounts of the hydrogen and oxygen at 20 mA cm-2, the Faradaic efficiencies for HER and OER are ∼100% (Figure 5c).

Figure 5. a) The LSV curves of Co2Mn1|Co2Mn1 and IrO2|Pt couples toward overall water splitting. b) Time-dependent current density curves for the water electrolyzer based the Co2Mn1 DH hollow spheres at a voltage of 1.65 V. c) Faraday efficiencies of Co2Mn1|Co2Mn1 couple at a given current density of 20 mA cm-2. Besides the HER and OER activities, the CoMn spheres also exhibit good ORR properties. The ORR performance of the CoMn spheres was evaluated using three-electrode system, in which the CoMn spheres coated onto RDE, graphitic rod, and Ag/AgCl electrode were served as working, counter and reference electrodes, respectively. For comparison, the ORR performance of Pt/C (20 wt.% Pt) coated onto a glassy carbon electrode was also measured under the same conditions. The LSVs of CoMn spheres were measured at a rotation speed of 1600 rpm in 0.1 M O2-saturated KOH solution to further assess the ORR properties of the CoMn spheres and Pt/C 24 ACS Paragon Plus Environment

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catalyst. As shown in Figure S14, Pt/C catalyst exhibits good ORR activity with the onset potential (E0) and the half wave potential (E1/2) of 0.91 and 0.81 V, respectively. Among the CoMn spheres, the Co0Mn1 spheres have more positive E0 (0.92 V) and E1/2 (0.83 V) than those of Co1Mn0 (E0 = 0.84V, E1/2 = 0.76 V), Co4Mn1 (E0 = 0.85 V, E1/2 = 0.76 V), Co2Mn1 (E0 = 0.87 V, E1/2 = 0.78 V), and Co1Mn3 (E0 = 0.94 V, E1/2 = 0.82 V). The above results indicate that the increase of the Mn content in the CoMn spheres facilitate the improvement of ORR property. The overall efficiency of the oxygen reactions is dependent on both ORR and OER activities of the oxygen catalysts. Thus, the OER properties of CoMn spheres loaded onto RDE electrodes were tested in O2-saturated 0.1 M KOH. As shown in Figure S15, the current density driven by the Co0Mn1 spheres is about 2.3 mA cm-2 at a high overpotential of 470 mV, suggesting their very poor OER activity. In contrast, the Co2Mn1 DH hollow spheres can deliver 10 mA cm-2 at merely 340 mV, lower than that of other CoMn spheres. The overall activity of the oxygen catalysts can be evaluated by ∆E calculated using the following equation, ∆E = Ej=10 – E1/2

(2)

where, E1/2 is the half wave potential for the catalyst catalysing ORR and Ej=10 is the potential at the current density of 10 mA cm-2 for the catalyst catalysing OER. The ∆E for the Co2Mn1 DH hollow spheres is 0.794 V, smaller than other CoMn catalysts and recently reported ORR catalysts (Table S4,5),34,57,58,59,60 suggesting that the Co2Mn1 DH hollow spheres have superior overall efficiency of oxygen electrocatalyst. In addition, the Tafel slope of Co2Mn1 DH hollow spheres for ORR is 46.7 mV dec-1, 25 ACS Paragon Plus Environment


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amongst the values of CoMn DH electrocatalysts (Figure S14b). To further investigate the ORR mechanism of the Co2Mn1 DH hollow spheres, LSV curves were measured in O2-saturated 0.1 M KOH solution at various rotation speeds from 400 to 2025 rpm. As shown in Figure 6a, the limited diffusion current density is increased with the increase of the rotation speeds, implying a smaller diffusion distance at a higher rotation speeds. The HO2– yield for the Co2Mn1 DH hollow spheres is lowered to 9.7 % over the potential range of 0.2 – 0.6 V through the roating-ring-disk electrode measurement in O2-saturated 0.1 M KOH solution at a rotation speed of 1600 rpm and the number of transferred electron is about 3.75, indicating the ORR on the CoMn DH hollow spheres occurs through nearly four-electron oxygen reduction process (Figure 6b). The stability of Co2Mn1 DH hollow sphere toward ORR was measured, as shown in Figure S17. It can be found that after 10000s continuous ORR process the ORR current density is increased to about 105% of the initial value, indicating the good stability of Co2Mn1 DH hollow sphere toward ORR. After the ORR test, the structure of Co2Mn1 DH hollow sphere was characterized by XRD, SEM, TEM and XPS. SEM and TEM images (Figure S9e,f and Figure S10c) indicate that the sphere-like morphologies and the hollow features of the Co2Mn1 DH catalysts are remained well, suggesting their structural stability toward ORR. XRD pattern (Figure S11) shows that there is little change of the ORR-post Co2Mn1 DH catalysts in crystal compositions in comparison to that of the Co2Mn1 DH catalysts. In Figure S12, the Co 2p XPS spectra show that the intensities of peaks at 778.0 and 796.2 eV


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corresponding to Co3+ are decreased obviously. That may be due to partial reduction of Co3+ to Co2+ during ORR process.

Figure 6. a) LSV curves of the Co2Mn1 DH hollow spheres toward ORR in O2-saturated 0.1 M KOH solution at 400– 2025 rpm. b) Electron transfer number (blue curve) and HO2- yield (black curve). c) The galvanostatic discharge-charge cycling curves of the rechargeable Zn-air batteries based on the Co2Mn1 DH and the commercial Pt/C air-cathodes at 10 mA cm-2, and the insets show the curves at initial and final stages. d) Photograph of 55 red LED powered by two all-solid-state Zn-air batteries with the Co2Mn1 DH air-cathode in series, and the inset shows open-circuit voltage of one all-solid-state Zn-air battery.

The results above demonstrate that the CoMn DH hollow spheres exhibit good ORR and OER activities in 0.1 M KOH. Thus we assembled Zn-air battery using the 27 ACS Paragon Plus Environment


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carbon paper-supported Co2Mn1 DH hollow spheres and Zn plate as air-cathode and anode, respectively. The electrochemical performance of the primary Zn-air battery was first assessed in 6.0 M KOH. Figure S17 indicates that the specific capacity of the primary Zn-air battery based on Co2Mn1 DH hollow spheres is 684 mAh g-1, larger than that of Pt/C based Zn-air battery (603 mAh g-1) at a current density of 10 mA cm-2. Even at a current density of 20 mA cm-2, the specific capacity of the primary Zn-air battery based on Co2Mn1 DH hollow spheres is up to 620 mAh g-1. The specific capacity of the Zn-air battery based on the Co2Mn1 DH hollow spheres is also higher than that of recently reported Zn-air batteries based on CoO, CoxMn1-xO, NiFe, MnOx and FeCo catalysts (Table S6).34,

35, 53-56

Inspired by the excellent

electrochemical performance of the primary Zn-air battery based on the Co2Mn1 DH hollow spheres, we assembled rechargeable Zn-air battery with 6 M KOH containing 0.2 M zinc acetate as electrolyte. The open-circuit voltage (OCV) of the rechargeable Zn-air battery based on the Co2Mn1 air-cathode is 1.413 V, comparable to that of Pt/C based Zn-air battery (1.452 V), as shown in Figure S18. Figure 5c shows the galvanostatic discharge-charge cycling curves of the rechargeable Zn-air batteries based on Co2Mn1 DH hollow spheres and the Pt/C as air-cathodes at a current density of 10 mA cm-2. The initial voltage gap between charge and discharge processes for the Co2Mn1-based Zn-air battery is 1.140 V, smaller than that of the Pt/C-based Zn-air battery (1.613 V). The voltage gap for the Co2Mn1-based Zn-air battery is almost kept unchanged after 14 h cycling measurements (Figure 6c and the inset), while it is increased to 2.1 V for the Pt/C-based Zn-air battery after only 4 h cycling 28 ACS Paragon Plus Environment

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measurements. The results above indicate good activity and cycling stability of the Co2Mn1 DH hollow spheres for rechargeable Zn-air battery. To further demonstrate the possibility of the practical applications, we made all-solid-state Zn-air batteries using Zn plate, the Co2Mn1 DH hollow spheres loaded carbon fibre cloth, and poly(vinyl alcohol)/KOH gel as anode, cathode, and solid-state electrolyte, respectively. Each all-solid-state Zn-air battery exhibits a high OCV of around 1.356 V (the inset in Figure 6d). Figure S19 shows the stability of charge-discharge cycling stability of the solid-state battery at a current density of 5 mA cm-2. The charge and discharge voltage are kept at average values of 0.835 V and 1.764 V, respectively, and have small fluctuation over 12 h cycling. Two all-solid-state batteries connected in series can lighten 55 red light-emitting diodes (LEDs) without obvious change in brightness over 7 h, as shown in Figure 6d. Furthermore, the water eletrolyzer based on the Co2Mn1 DH hollow sphere as HER and OER catalysts can be powered by two all-solid-state Zn-air batteries integrated in series with obvious gas bubbles produced on the electrode surfaces, as shown in Figure S20. In terms of the results above, the Co2Mn1 hollow spheres exhibit excellent activities and stabilities toward HER, OER and ORR in the alkaline media. The follow factors can be attributed to their trifunctional catalytic performance. i) The shell of the CoMn DH hollow spheres are composed of the ultrathin nanosheets with a thickness of approximately 2.5 nm. Furthermore, there are many defects in the basal plane of the nanosheets. The ultrathin thickness of the nanosheets and the defects in the basal planes allow more active sites to exposure to electrolyte and thereby facilitate the 29 ACS Paragon Plus Environment


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improvement of the catalytic activity of the DH hollow spheres. ii) The hollow feature of the DH spheres facilitates quick release of the formed gas bubbles generated from the electrode surface during the electrocatalysis even at a high current density. iii) Electrochemical measurements indicate the existence of the strong electron interaction between Co and Mn during the catalysis, which facilitates the improvement of the catalytic performance of the CoMn catalysts. Therefore, our strategy for the CoMn DH hollow spheres paves a novel way to development of high-performance, low-cost and highly stable multifunctional electrocatalysts for practical energy conversion and storage systems. CONCLUSIONS We develop a facile method for fabrication of the CoMn DH hollow spheres. The non-precious metal-based electrocatalysts exhibit high activities and robust stabilities in alkaline media toward HER, OER, and ORR. The optimized CoMn DH hollow spheres can drive 10 mA cm-2 at 187 mV for OER and at 233 mV for OER in 1.0 M KOH. The water electrolyzer assembled with the optimized CoMn DH hollow spheres requires an overpotential of 0.42 V to deliver a current density of 10 mA cm-2, outperforming some reported bifunctional electrocatalysts based on oxides and hydroxides. Moreover, the CoMn DH hollow spheres also have good ORR activity in 0.1 M KOH. The rechargeable Zn-air battery with the CoMn DH hollow sphere air-cathode show excellent cycling stability, superior to the battery with Pt/C air-cathode. Furthermore, two all-solid-state batteries integrated in series can power 55 red LEDs and full water splitting system using the CoMn DH hollow spheres as 30 ACS Paragon Plus Environment

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trifunctional catalysts. Our strategy for the CoMn DH hollow spheres paves a novel way to development of high-performance, low-cost and highly stable multifunctional electrocatalysts for practical energy conversion and storage systems.5, 16, 33, 35, 36, 49-62 ASSOCIATED CONTENT

Supporting Information Available: Figure S1-Figure S20 and Table S1-Table S6. XRD patterns of the CoMn spheres with different Co/Mn ratio. SEM and TEM images of the CoMn spheres with different Co/Mn ratios. AFM image of the nanosheets in the outmost surfaces in the Co2Mn1 DH. High-resolution XPS of Co in Co2Mn1 DH, Co1Mn0 hollow spheres and Mn in Co2Mn1 DH and Co0Mn1 hollow spheres. High-resolution XPS of O in Co1Mn0 and Co0Mn1 hollow spheres. CVs curves for CoMn catalysts the region of 0.83-0.93 V vs. RHE for Co2Mn1 DH spheres, Co4Mn1 DH spheres, Co1Mn3 DH spheres, Co1Mn0 spheres and Co0Mn1 spheres. SEM, TEM and XRD of Co2Mn1 DH hollow spheres after HER, OER and ORR. Co 2p, and Mn 2p XPS spectra of OER-post and HER-post Co2Mn1 DH hollow sphere. LSV curves of CoMn spheres and Pt/C catalysts toward ORR and OER in O2-saturated 0.1 M KOH solution at 1600 rpm.

Discharge polarization and power density curves of and

primary Zn-air batteries with the Co2Mn1 DH and the commercial Pt/C air-cathodes. Long-time discharge curves of primary Zn-air batteries with the Co2Mn1 DH hollow spheres air cathode. Open-circuit voltage of the rechargeable Zn-air batteries with the Co2Mn1 DH hollow and Pt/C air-cathodes. Galvanostatic discharge-charge cycling curves of all-solid-state battery. Photograph of the two-electrode electrolyser based on the Co2Mn1 DH hollow spheres powered by two all-solid-state Zn-air batteries with 31 ACS Paragon Plus Environment


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the Co2Mn1 DH air-cathode in series. AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]

*E-mail: [email protected]

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENT This work is supported by the National Natural Science Foundation of China (Grant No. 51572051), the Natural Science Foundation of Heilongjiang Province (E2016023), the Fundamental Research Funds for the Central Universities (HEUCF201708), and also the Open Project Program (PEBM 201703 and PEBM201704) of Key Laboratory for Photonic and Electric Bandgap Materials, Ministry of Education, Harbin Normal University, and also the 111 project (B13015) of Ministry Education of China to the Harbin Engineering University.

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Table of Contents Graphic and Synopsis

CoMn hydroxide hollow spheres exhibited multifunctional activities, which can be used as electrode materials for overall water splitting and zinc-air battery.


Hierarchical Hollow Spheres Assembled with ultrathin CoMn Double

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