Hierarchically Porous Carbon as Superior Bifunctional

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Energy, Environmental, and Catalysis Applications

Co3O4/MnO2/Hierarchically Porous Carbon as Superior Bifunctional Electrodes for Liquid and All-Solid-State Rechargeable Zinc-Air Batteries Xuemei Li, Fang Dong, Nengneng Xu, Tao Zhang, Kaixi Li, and Jinli Qiao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b18684 • Publication Date (Web): 04 Apr 2018 Downloaded from http://pubs.acs.org on April 4, 2018

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ACS Applied Materials & Interfaces

Co3O4/MnO2/Hierarchically Porous Carbon as Superior Bifunctional Electrodes for Liquid and AllSolid-State Rechargeable Zinc-Air Batteries Xuemei Lia, Fang Donga, Nengneng Xua, Tao Zhangb*, Kaixi Lic, Jinli Qiaoa,b* a

State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of

Environmental Science and Engineering, Donghua University, 2999 Ren’min North Road, Shanghai 201620, China b

The State Key Lab of High Performance Ceramics and Superfine microstructure, Shanghai

Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai, 200050, China c

Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan, Shanxi 030001, China

*Corresponding author. Tel: +86-21-67792379. Fax: +86-21-67792159. E-mail: [email protected]; [email protected]

KEYWORDS: cobalt and manganese oxides, porous carbon, bifunctional electrodes, liquid-type, flexible all-solid-state, zinc-air batteries.

ACS Paragon Plus Environment

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ABSTRACT: The design of efficient, durable and affordable catalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is very indispensable in liquid-type and flexible all-solid-state zinc-air batteries. Herein, we present a high-performance bifunctional catalyst with cobalt and manganese oxides supported on porous carbon (Co3O4/MnO2/PQ-7). The optimized Co3O4/MnO2/PQ-7 exhibited a comparable ORR performance with commercial Pt/C, and a more superior OER performance than all the other prepared catalysts including commercial Pt/C. When applied to the practical aqueous (6.0 M KOH) zinc-air batteries, the Co3O4/MnO2/porous carbon hybrid catalysts exhibited exceptional performance, such as a maximum discharge peak power density as high as 257 mW cm-2 and the most stable chargedischarge durability over 50 hours with negligible deactivation so far. More importantly, a series of flexible all-solid-state zinc-air batteries can be fabricated by the Co3O4/MnO2/porous carbon with a layer-by-layer method. The optimal catalyst (Co3O4/MnO2/PQ-7) exhibited an excellent peak power density of 45 mW cm-2. The discharge potentials almost remained unchanged for 6 hours at 5 mA cm-2 and possessed a long cycle life (2.5 h @ 5 mA cm-2). These results make the optimized Co3O4/MnO2/PQ-7 as a promising cathode candidate for both liquid-type and flexible all-solid-state zinc-air batteries.


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1. INTRODUCTION

As a clean power source device, metal-air batteries are considered as one of the most promising candidates on account of their unique half-opened systems that consume inexhaustible atmosphere oxygen at the air electrode, resulting in high energy density.1,2 In particular, zinc-air batteries have received increasing attention and intensive research due to their low-cost materials, environmental friendly and easy handling.3,4 However, the sluggish oxygen reduction and evolution reaction (ORR&OER) kinetics have impeded the practical development of zinc-air batteries, which can be comprehended based on the high O=O bond energy of oxygen (498 kJ mol-1), meaning that this double-bond is very difficult to be broken during discharge and charge processes.5-7 Therefore, Pt/Ir-based precious catalysts have been widely used to increase the ORR/OER, although they are very expensive, scarce and have the limited availability and stability.8-11 Currently, numerous non-noble metal oxides, typically the MnO2 and Co3O4,12-15 are commonly regarded as cost-effective and conductivity-outstanding ORR/OER catalysts in alkaline electrolyte for zinc-air batteries. Nonetheless, MnO2 possibly possesses excellent catalytic abilities for rechargeable zinc-air batteries while the major defect is its poor electronic conductivity, as for the Co3O4 structure, it usually exhibits poor ORR catalytic activity. To solve these problems, an effective way is to use carbon materials as the conductive agent to enhance the electronic conductivity. For instance, Liu and co-workers 16 reported LixCo3-xO4 solid solution nanocrystals supported on carbon black as a superior electrocatalyst for oxygen reduction reaction, Co3O4/α-MnO2 supported on reduced graphene oxide,17 and (Fe2O3/MnO2)3/4-(CNTs)1/4 hybrid nanomaterials18 have also been studied for promoting electrochemical performance as an efficient catalyst in alkaline media. However, owing to carbon oxidation, the catalytic activities of these air electrodes gradually degraded, and their activity are still not particularly satisfactory


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in terms of both activity and stability. Based on the above considerations, this work aims to find the suitable carbon materials mixed with Co3O4/MnO2 to form an effective electrocatalyst for zinc-air batteries. According to our previous report,6,19,20 Fe/N/S-composited hierarchically porous carbons possess optimized surface area, active sites and porous structure, which are playing an important role on improving ORR performance. Note that by doping with nitrogen, the functional role of the carbon support has been improved because the extra electron of N was brought into the conjugated π-system to modulate the electronic structure or conjure new electron states that can lower the activation energy.21 To date, few researches about transition metal oxides supported on porous carbon materials was studied. Herein, three types of carbon materials are chosen as the conductive substrates for Co3O4/MnO2, including PQ-2, PQ-46 and PQ-7 porous carbon.

In addition, it is well worth noting that electrolyte leakage is still a challenge for liquid electrolyte-based zinc-air batteries. Moreover, the side-reaction products of K2CO3 or KHCO3 induced by the CO2 in atmosphere could lead to carbon corrosion problem for aqueous (6.0 M KOH) zinc-air batteries. And also conventional zinc-air batteries are typically bulky and rigid, thus they cannot meet the requirements of portable devices. Fortunately, recent progress in zincair battery has been demonstrated with excellent mechanical properties, such as flexibility, handiness and stretchability. For example, a zinc-air battery with cable-type structure had been put forward to attain the flexibility,22 but it was not stretchable and wearable. Xu et al.23 and Liu et al.24 studied different flexible, stretchable zinc-air batteries. Their batteries achieved high flexible activity, but the conductivity and stability were not high enough. Comparing to the previous study, developing new air electrodes with thin layer, light weight, good conductivity and flexibility, is also strongly required to be applied for zinc-air battery system. Nevertheless, at


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present, there is no report about the possibility of using transition metal oxides-supported porous carbon as ORR and OER bi-functional electrocatalyst in flexible all-solid state zinc-air batteries.

Based on the above conceptions, we herein explore a series of new Co3O4/MnO2/porous carbon (PQ-2, PQ-46, PQ-7) bi-functional electrocatalysts for liquid-type and flexible all-solid state zinc-air batteries. At first, different porous carbons were synthesized using our improved SiO2 template methods, in order to form a large specific surface area, which is conducive to the high exposure of the active sites and the outstanding transport properties.6,19,20 In this work, the electrochemical results demonstrated that the best Co3O4/MnO2/PQ-7 catalyst exhibited a comparable ORR performance and remarkable activity for OER with the commercial 20% Pt/C. Moreover, the Co3O4/MnO2/PQ-7 electrocatalysts was assembled into liquid-type primary and rechargeable zinc-air batteries, which validated the significant activity and durability. Therefore, the intimate coupling between Co3O4 nanoparticles and MnO2 nanotubes supported on porous carbon was found to play an important role in the improved bi-functional electrocatalytic activities and stability. More importantly, a novel flexible all-solid state zinc-air with superior electrochemical performance is successfully designed. In details, flexible all-solid state zinc-air batteries with Co3O4/MnO2/porous carbon air electrode showed superiorly mechanical properties and good cycling durability with maximum power density (45 mW cm-2), as well as a long cycle life (2.5 h @ 5 mA cm-2).

2. EXPERIMENTAL SECTION

2.1 Catalyst synthesis. Potassium permanganate (KMnO4), nano-silica (~500 nm), ferrous sulfate (FeSO4·7H2O), cobaltous nitrate (Co(NO3)2·6H2O), hydrochloric acid (HCl), sulfuric acid (H2SO4), sodium hydroxide (NaOH) and ammonia (NH3·H2O, 1.3 mol L-1) were all


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reacting reagents and obtained from Sinopharm Chemical Reagent Co., Ltd. (China). Polyquaternium-7 solution (10 wt.%), polyquaternium-46 solution (20 wt.%), polyquaternium-2 solution (62 wt.%) were bought from Sigma-Aldrich(China) chemicals Co., Ltd. Deionized water (D.I. water) and ethanol was used to prepare aqueous solutions. The overall synthetic procedure was shown in Scheme 1. The synthesis process of Co3O4/MnO2/porous carbon was as follows: (i) The MnO2 nanotube was firstly synthesized through our recently reported literature.25 (ii) Hierarchically porous carbons were fabricated by a modified SiO2 template method.19,20 In detail, 5 g of nano-silica (~500 nm) was mixed with 25 g of 1.0 M HCl, and sonicated for 10 minutes to obtain a homogeneous SiO2 colloid solution. Then, 18.0 g PQ-7 was dissolved in the above mixture under continuous stirring, resulting in the spontaneous coating of adherent PQ-7 layers. Subsequently FeSO4·7H2O was introduced, and the reaction was set to run for more than 3 hours. After stirring, the resulting viscous solution was dried for 48 hours at 85oC. The achieved solid was ground to a fine powder in an agate mortar and then heated at 800oC for 1 hour with a ramping rate of 20oC min-1. The calcination process was carried out under nitrogen atmosphere. After removal of silica by NaOH (4 M) etching for 48 hours, the sample was washed by D.I. water and dried in an oven overnight afterward. In order to remove a redundant phase, mainly contain unreacted metallic iron and iron compounds, the sample was acid-leaching using 0.5 M H2SO4 at 85oC for 8 hours. Finally, porous carbon samples were heat-treated under a nitrogen atmosphere at 800oC for another 1 hour. (iii) The final samples were performed by a co-precipitation procedure combined with post-heat treatment. In a typical approach, adding 30 mL D.I. water into 0.5 g Co(NO3)2·6H2O, followed by the addition of 5 mL NH3·H2O, the solution was sonicated for over 30 minutes. Afterward, 0.125 g pre-synthesized MnO2 nanotubes and 0.094 g as-prepared porous carbon was


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introduced and the mixture was stirred for another 1 hour to form homogeneous solution. After stirring, the dark product was collected and washed with D.I. water and ethanol by high-speed centrifugation (9000 rpm, 3 min) to neutralize it, then dried in an oven at 60°C for 5 hours, and lastly calcined in air at 300oC for 1 hour. The obtained samples are labeled as Co3O4/MnO2/PQ-7. For comparison purposes, Co3O4/MnO2/PQ-2 and Co3O4/MnO2/PQ-46 was synthesized by the same steps. 2.2.

Materials

characterization.

Scanning

electron

microscopy (SEM) with

HITACHI/S-4800 system was performed to observe the morphologies of materials. Images of transmission electron microscopy (TEM) were performed with a high-resolution Hitachi JEM2100F system at operating voltage of 200 kV. The Brunauer-Emmet-Teller (BET) model was applied to the isotherms to determine the apparent surface area at a relative pressure (P/P0) range of 0.05-0.3. The pore size distribution curves were calculated by the Barrett-Joyner-Halenda (BJH) method. The crystallographic information of the prepared samples was obtained by power XRD (Philips PW3830 X-ray diffractometer using Cu Kα radiation (λ = 1.5406 Å). X-ray photoelectron spectroscopy (XPS) studies were demonstrated using a RBD-upgraded PHI-5000C ECSA system (PerkinElmer) with an Al K X-ray anode source. 2.3. Electrode preparation and electrochemical measurements. To test the catalytic activity of the Co3O4/MnO2/porous carbon (PQ-2, PQ-46, PQ-7) samples, the catalyst ink was prepared by ultrasonically mixing 5.0 mg catalyst with 1 mL ethanol and 8 µL 5 wt.% Nafion® for more than 30 minutes. Then, 8 µL of the catalyst ink was loaded onto the pre-cleaned and polished glassy carbon disk (5 mm diameter) and dried at room temperature to form a uniform catalyst layer with the overall loading of 0.2 mg cm-2. For comparison, a commercially available 20 wt.% platinum on Vulcan carbon black (Pt/C, Johnson Matthey) was used as the baseline, and


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its catalyst ink and electrode were made under the same conditions as that for the measured sample. The electrochemical measurements were performed using a CHI Electrochemical Station (760 D) with a platinum wire and a saturated calomel electrode (SCE) used as the counter electrode and the reference electrode, respectively. Before each measurement of linear sweep voltammetry (LSV), the electrolyte (0.1 M KOH) was bubbled with oxygen for at least 30 minutes and constantly maintained during the measurement to ensure oxygen saturation. In this paper, all potentials were converted to the reversible hydrogen electrode (RHE) scale. To further analyze number of the transferred electrons by Kouteck-Levich plots, the electrode was rotated at several different rotation speeds @300, 600, 900 and 1200 rpm, and the sweep rate was 5 mV s-1 with a voltage window of 0-1 V. Chronoamperometric responses of the samples at 0.70 V were tested at rotation rate of 300 rpm @5 mV s-1 in a O2-saturated KOH (0.1 M) solution. 2.4. Fabrication of liquid-type zinc-air batteries. For the sake of obtain a more realistic cell operation condition, a home-made zinc-air battery is used to confirm the practical catalytic activity and stability for Co3O4/MnO2/porous carbon (PQ-2, PQ-46, PQ-7) samples. The air electrode was prepared by loading prepared samples on carbon paper (Toray TGP-H-090) (dried at 60°C for 2 h, catalyst loading 1.0 mg cm-2), with a polished zinc plate (purity > 99.99%, thickness: 0.5 mm, Shengshida Metal Mater. Co. Ltd., China) as the anode and 6.0 M KOH aqueous solution as the electrolyte. The discharge polarization and power density plots were acquired by a galvanodynamic method scaling the current from 0 to 1000 mA. Battery testing and cycling experiments were operated at room temperature using the recurrent galvanic pulse method, where one cycle consisted of a discharge step at 10 mA for 5 min followed by a charging step of the same current and duration. For comparison, a Pt/C air cathode was also prepared and measured in the same manner.


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2.5. Flexible all-solid state zinc-air batteries measurements. To evaluate the utility of Co3O4/MnO2/porous carbon (PQ-2, PQ-46, PQ-7) as the bi-functional electroactive material in flexible all-solid state zinc-air batteries, we built rechargeable zinc-air battery via a layer-bylayer method by which the electrodes were placed face-to-face with the membrane (Tokuyama A901, OH- conductivity: 11.4 mS cm-1, ion exchange capacity: 1.7 meq g-1). A polished zinc foil (purity > 99.99%, thickness: 0.01 mm, Shengshida Metal Mater. Co. Ltd., China) was used as the anode, an air electrode was prepared by Co3O4/MnO2/porous carbon sprayed onto a cleaned nickel foam (Fuel Cell Technology). The catalyst ink consisted of 20 mg sample, 5 mL ethanol, and 40 µL of 5 wt.% Nafion® was mixed, and then sonicated for 1 hour, followed by sprayed onto nickel foam as the gas diffusion layer with a catalyst loading of 1.0 mg cm-2. After spraying, the air electrode was dried in an oven at 60°C for 1 hour. Finally, the assembled device was pressed at rate of 3 MPa min-1 by a sheeting presser to enhance the integrity of the laminated structure.

3. RESULTS AND DISCUSSIONS

3.1 Characterization of the Co3O4/MnO2/porous carbon catalysts. To investigate the morphology and structure of the synthesized catalysts, the Co3O4/MnO2/porous carbon catalysts were first examined by scanning electron microscope (SEM) and transmission electron microscopy (TEM). Figure 1a,b showed that Co3O4 nanoparticles and porous carbon layers were rare and no certain shape. However, by introducing pristine PQ-7 porous carbon into transition metal oxides, it could be seen from the SEM images of Figure 1c that MnO2 nanotubes homogeneously dense coated by numerous Co3O4 nanoparticles on the surface anchor uniformly on the PQ-7 porous carbon layer, revealing efficient combination between MnO2 and Co3O4,


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Co3O4/MnO2 and PQ-7 porous carbon during the calcined treatment. Simultaneously, the length of the MnO2 nanotubes were in the range of 1-3 µm. As shown in Figure 1d-f, the Co3O4/MnO2/porous carbon hybrid consist of transparent MnO2 nanotubes, Co3O4 nanoparticles and porous carbon thin-sheet. In detail, Figure 1d,e and Figure S1a,b showed that thicker PQ2/PQ-46 porous carbon sheet displayed larger blocked structure, moreover, scarce Co3O4 nanoparticles have been completely invisible from transparent MnO2 nanotubes. In stark contrast, with further adding pure PQ-7 porous carbon into Co3O4/MnO2 hybrid, well dispersed and plentiful of smaller Co3O4 nanoparticles (5-8 nm) were formed on MnO2 nanotubes and embedding of the suitable Co3O4 nanoparticles on PQ-7 porous carbon thin-sheet surface (Figure 1f and Figure S1c). According to the above observation, the reason for causing particles narrow and homogeneous is most likely the strong coupling among PQ-7 porous carbon, Co3O4 nanoparticle and MnO2 nanotube, and such structure could promote both activity and stability, which are indispensable for a serviceable bi-functional catalyst.26 Further, the calculation of the d-spacing by the fringes analyzed in the HR-TEM image of the Co3O4/MnO2/PQ-7 has resulted in 0.69 nm (Figure S1d), which matches very closely with the theoretical d-spacing for the (110) orientation of MnO2. The inter layer spacing was found to be 0.24 nm, corresponding to the (311) plane of typical Co3O4 nanoparticles. Except of distinctly well-arranged crystal lattice stripes, surface graphitic porous carbon layers can be easily identified around metal oxides, which would be favorable for the effortless access to reactants and the formation of oxygenelectrode-electrolyte triple-phase boundary that consequently improves electro-catalytic activity.27 Hopefully, the evident graphitization degree can contribute to enhance an improveddispersion of active nanomaterials in the end.6


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The surface property and pore structure of Co3O4/MnO2/porous carbon, pristine porous carbon and Co3O4/MnO2 hybrid were investigated by N2 adsorption-desorption isotherms. From Figure S2a, we could observe the surface areas of pristine porous carbons increase in the order of PQ-7 porous carbon (940 m2 g-1) > PQ-46 porous carbon (728 m2 g-1) > PQ-2 porous carbon (602 m2 g-1). Further research was made to show that the optimal PQ-7 porous possessed both micropores and mesoporous centered at 0.54 and 14 nm, respectively, as indicated by Figure S2b. It is generally known that a larger BET surface area and suitable porous structure system could promote ORR catalytic activity.1,6,19 Next, the metal oxide/pristine porous carbon doping helped to increase specific surface area from 47 m2 g-1 (Co3O4/MnO2) to 249 m2 g-1 (Co3O4/MnO2/PQ-7), 73 m2 g-1 (Co3O4/MnO2/PQ-46) and 61 m2 g-1 (Co3O4/MnO2/PQ-2) as shown in Figure S2a and Figure 1g-i. The final Co3O4/MnO2/porous carbon hybrid still exit more mesoporous (Figure 1g-i inset), serving as reactive molecule reservoirs to short the diffusion pathways of reactive molecules.20 Herein, the results indicated that the optimum PQ-7 porous carbon endowed the final composite products (Co3O4/MnO2/PQ-7) with a high specific surface area and appropriate mesoporous structure, which could improve electron and ion transport, resulting in better electrochemical capacity.28,29

To reveal the catalysts’ structural change with different porous carbons, X-ray diffraction (XRD) patterns of porous carbons showed characteristic peaks of graphite at 2θ =26.6o and 43.0o (JCPDS: 26-108J0) (Figure 2). The coated cobalt species were identified in three forms (Co3O4), the cobalt diffraction peak was distinct and well indexed compared to standard JCPDS No. 421467. The diffraction peaks at 12.8o, 18.1o, 49.8o, 60.0 o and 69.7 o can be indexed to (110), (200), (411), (521) and (541) planes of MnO2 (JCPDS: 44-0141). Although no obvious major peak shift and structural variation are found in the XRD patterns of three compositions, the


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majority of C peaks were much broader in the case of Co3O4/MnO2/PQ-7 (Figure 2 inset), indicating there were smaller size of porous carbon layers and lower degree of crystallinity.

X-ray photoelectron spectroscopy (XPS) was then employed to identify the chemical composition and metal oxidation states of the Co3O4/MnO2/porous carbon. Representative two elements (N, C) were identified from the XPS spectrum of the individual porous carbon (Figure S3a-f). As shown in Figure S3a,b, the high resolution N1s spectra of PQ-2/PQ-46 porous carbon were fitted with only one signal (graphitic-N), which was believed to participate in the active sites.1,20 For PQ-7 porous carbon, it displayed two kinds of N, pyridinic-N and graphitic-N (Figure S3c). As well known, both the pyridinic-N and graphitic-N groups on the carbon substrate’s surface could play an important role in improving ORR catalytic activities, which could be attributed to the changes of the valence band structure, including the raising of density of π states near the Fermi level and the reduction of work function on carbon materials by pyridinic-N.6,20,30 Besides, the half-peak width of C 1s spectrum became narrower (Figure S3d-f) for PQ-2/PQ-46/PQ-7 porous carbon, implying a gradually enhanced graphitic degree and improved the ORR catalytic activity.6 Likewise, representative XPS spectrum of the Co3O4/MnO2/porous carbon hybrid as shown in Figure 3. The Mn 2p spectra showed two major peaks at binding energy of 654.2 and 642.5 eV (Co3O4/MnO2/PQ-2), 654.0 and 642.3 eV (Co3O4/MnO2/PQ-46), 653.8 and 642.3 eV (Co3O4/MnO2/PQ-7), with a spin-energy separation of 11.7 eV, 11.7 eV, 11.5 eV (Figure 3a-c), which corresponded well to the Mn 2p1/2 and Mn 2p3/2 in MnO2, respectively.17,28,31 In addition, the range of Mn 2p3/2 could be divided into two peaks by fitting, and the two pairs of spin-orbit doublets were assigned to Mn4+ and Mn3+ cations.32 Similarly, Deconvolution of complex Co 2p spectrum (Figure 3d,e) indicated the presence of two chemically distinct species: Co2+, Co3+ and 1~2 satellite peaks. The signal was


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not stable enough due to the lower content of Co 2p1/2 and large amount of noise. However, two peaks centered at 795.1 and 780.1 eV could be referred to Co 2p1/2 and Co 2p3/2 (Figure 3f), suggesting that Co element comes from Co3O4 for Co3O4/MnO2/PQ-7.33 The O 1s spectrum (Figure 3g) had three obvious groups. Besides the peaks at 531.7 and 533.5 eV owing to oxygen atoms in the hydroxyl groups and absorbed water,28,31 respectively, the strong peak at 530.2 eV could be attributed to oxygen atoms in the metal oxides of MnO2 and Co3O4. As shown in Figure 3h,i, the O 1s spectra could be divided into the peaks at 531.4 eV and 530.0 eV, corresponding to the binding energy of O2- bond with Co3O4/MnO2 hybrid, and OH- functionalities.34 Herein, the results clearly indicated that the existence of the oxygen-containing components with higher polarity could make the material more hydrophilic, enhancing a closer affinity to the electrolyte and dissolved oxygen.17 Furthermore, the survey spectrum of a typical Co3O4/MnO2/PQ-7 sample displayed the presence of Co (15.17 at%), Mn (12.96 at%), O (48.45 at%), and C (23.27 at%) elements (Figure S3g). Note that the Co/Mn atomic ratio was approximate ∼1.17 in the Co3O4/MnO2/PQ-7 composite. Therefore, we could calculate the ratio (weight percentage) of Co3O4 and MnO2 in the hybrid catalyst was about ~1.08.

3.2 Electrochemical evaluation of eletrocatalysts for ORR and OER. The electrocatalytic activity towards the ORR and OER was then investigated on a RDE in 0.1 M O2-saturated KOH solution with a standard three-electrode system. Among all of the samples, Co3O4/MnO2/PQ-7 showed lower onset potentials and higher current density compared to Co3O4/MnO2/PQ-2 and Co3O4/MnO2/PQ-46 for ORR (Figure 4a and Table S1). Moreover, the best-performing sample (Co3O4/MnO2/PQ-7) also showed an onset potential at 0.95 V, which was 10 mV more positive than the commercial 20% Pt/C (0.94 V) with a diffusion-limited current density slightly smaller than Pt/C (Figure 4a and Table S1). Obviously, the best Co3O4/MnO2/PQ-7 catalyst exhibited a


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comparable ORR performance with Pt/C, and was superior to that of individual PQ-7 porous carbon and Co3O4/MnO2 hybrid. Further kinetic calculation indicated that the number of the transferred electrons per oxygen molecule was nearly 4 for Co3O4/MnO2/porous carbon, which was a satisfying four-electron reaction pathway in the ORR (Figure 4b and Figure S4). The derived K-L plots exhibited a good linearity between Jk-1 and ω-1/2, signifying that the number of transferred electrons remained constant (~ 3.83) at different electrode potentials. In the meantime, these values such as ORR onset potential and number of transferred electrons for Co3O4/MnO2/porous carbon were superior to that of the non-noble metal catalyst in alkaline media studied previously.26 This suggests that the co-doped pristine PQ-7 porous carbon may be helpful to increase the oxygen reduction performance, and can provide defects and vacancies to oxygen to get into, implying exit strong coupling between PQ-7 porous carbon and the metal oxides.35 The improvement in hierarchically porous carbon catalyst bonding in Co3O4/MnO2 can also be confirmed by its exceptional stability for ORR. There was little loss for the Co3O4/MnO2/PQ-7 catalyzed ORR current (Figure 4c) after 48 000 s of invariable polarization at 0.7 V. In detail, the ORR current density produced in the Co3O4/MnO2/PQ-7 hybrid decreased by only 18% (retained up to 82%) over 48 000 s of continuous operation, indicating the excellent stability of the Co3O4/MnO2/PQ-7 catalyst for ORR. On the contrary, more than 40% loss in the corresponding

reduction

currents

could

be

found

for

among

Co3O4/MnO2/PQ-2,

Co3O4/MnO2/PQ-46, and 20% Pt/C. As proposed previously, the strong mutual impact between MnO2 and Co3O4 would effectively increase the durability of catalyst nanoparticles and prevent them from agglomeration.25 And with the incorporation of high graphitized and large specific surface area PQ-7 porous carbon into metal oxides structure, the support could be much less


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prone to oxidation and can also afford the corrosion resistance, contributing to the good stability of Co3O4/MnO2/PQ-7.19

The

details

regarding

bi-functional

catalytic

performance

of

the

novel

Co3O4/MnO2/porous carbon catalysts were further studied by evaluating their OER activities. In parallel, Co3O4/MnO2/PQ-2, Co3O4/MnO2/PQ-46, Co3O4/MnO2/PQ-7, Co3O4/MnO2, PQ-7 porous carbon and 20% Pt/C were measured by using linear sweep voltammetry (LSV) in alkaline solution, and the resultant curves were displayed in Figure 4d and Table S1. Among the results, Co3O4/MnO2/PQ-7 possessed the best OER performance in terms of onset potential and kinetic current density. For example, the optimized Co3O4/MnO2/PQ-7 catalyst demonstrated a small onset potential of 1.60 V (vs. RHE), which was better than those of Co3O4/MnO2/PQ-2 (1.70 V), Co3O4/MnO2/PQ-46 (1.71 V), Co3O4/MnO2 (1.71 V), PQ-7 porous carbon (1.76 V) and Pt/C (1.78 V). At the potential of 2 V, the current density of Co3O4/MnO2/PQ-7 was up to 25 mA cm-2, which was about 2~4 folds greater than other samples, meaning that the effective catalytic surface area of the best sample coated electrode was larger than other catalysts.36 Consequently, the comprehensive advantages of high activity, desirable kinetics and considerable durability prove that the optimized Co3O4/MnO2/PQ-7 holds promising potential as a bi-functional catalyst for the ORR and OER in renewable energy systems.

3.3 Co3O4/MnO2/porous carbon as air catalyst in liquid-type zinc-air batteries. In order to study the potential of Co3O4/MnO2/porous carbon catalyst in practical application, we assembled and measured home-made liquid-type (6.0 M KOH) zinc-air battery, as schematically illustrated in Figure 5a. The typical discharge curves and the corresponding power density against the discharge current density were shown separately in Figure 5b and Figure S5a. These


15


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batteries obtained an open-circuit voltage of nearly 1.4 V, which was only about 0.25 V deviated from the theoretical value (1.65 V), reasonably could attributed to the great activation over potential of the ORR. In the light of Table S2, for Co3O4/MnO2/PQ-7, both the current density at 1 V and peak power density (257 mW cm-2) were significantly improved over Co3O4/MnO2/PQ2 and Co3O4/MnO2/PQ-46, even higher than 20% Pt/C catalyst for liquid-type zinc-air battery. Moreover, the above results were also better than the most recently reported literature, such as Nf/PbMnOx (38 mW cm-2),37 α-MnO2/XC-72 (102 mW cm-2),38 CuFe alloy (212 mW cm-2),39 MnOx/Ketjenblack carbon (~190 mW cm-2)40 and Fe, Co and N precursors pyrolyzed with carbon (232 mW cm-2).41 In addition, we evaluated the constant-current (10 mA cm-2) discharge curves of the air cathode by comparing Co3O4/MnO2/porous carbon catalysts. The Co3O4/MnO2/PQ-7 based liquid-type zinc-air battery achieved a discharging voltage plateau of 1.2 V without noticeable decay for 70 hours (Figure 5c). After ~78 hours the voltage began to decrease, but easily replenishing the zinc plate and electrolyte could regenerate this battery for continuing runs at the same behavior level with the same catalyst. In contrast, the discharging voltage of Co3O4/MnO2/PQ-2 and Co3O4/MnO2/PQ-46 obviously decreased after 22 and 45 hours. With regard to Co3O4/MnO2/porous carbon catalyst, the specific capacity normalized to the mass of consumed zinc were 515, 578 and 670 mAh g-1 respectively, as shown in Figure S5b. Similarly, for the performance of Co3O4/MnO2/PQ-7, it was apparently superior to Co3O4/MnO2/PQ-2 and Co3O4/MnO2/PQ-46. This result again suggests a higher stability of Co3O4/MnO2/PQ-7 eletrocatalyst toward ORR, and Co3O4/MnO2/PQ-7 is also ideally suitable for liquid-type zinc-air battery.

In addition, for a competitive air catalyst, high electrical energy efficiency and durability are indispensable for rechargeable liquid-type (6.0 M KOH) zinc-air batteries. The charge and


16

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discharge (C-D) voltage gap should be as small as possible. We further studied rechargeable zinc-air battery in charging and discharging performance by using Co3O4/MnO2/PQ-7 and 20% Pt/C as the air cathode catalyst in 6.0 M KOH (Figure 5d). We could clearly find that the batteries based on Co3O4/MnO2/PQ-7 catalyst exhibit a similar discharge voltage than that of 20% Pt/C with current densities ranging from 0-70 mA cm-2. While in charging process, the batteries based on our catalyst (Co3O4/MnO2/PQ-7) obtained lower charge voltage compared to 20% Pt/C. These C-D voltage gaps of batteries were about 1.45 V (Co3O4/MnO2/PQ-7) and 1.60 V (20% Pt/C) at 70 mA cm-2, respectively. Meanwhile, Figure S6a displayed the galvanodynamic performance of Co3O4/MnO2/PQ-2 and Co3O4/MnO2/PQ-46 during battery charging and discharging processes, identically demonstrating that they were also inferior to Co3O4/MnO2/PQ-7. Those results could be explained by strong coupling effect between the metal oxides and PQ-7 support, which were responsible for the enhanced charge transfer and subsequently improved catalytic activities. Additionally, our Co3O4/MnO2/porous carbon catalysts underwent a C-D cycle (10 minutes per cycle) at a current density of 10 mA cm-2 (Figure 5e, Figure S6b,c). In detail, the battery based on Co3O4/MnO2/PQ-7 air cathode exhibited cycling stability for more than 324 cycles (or 54 h equivalent) with a slow degradation (0.6 V). It can be found clearly that the battery based on Co3O4/MnO2/PQ-7 exhibited approximate discharge and charge voltage gap (1.3 V) at the end of the test process in atmosphere air instead of pure oxygen or zinc plate that was gradually consumed, and this result was much higher than CMO-NF/C and CCBC catalysts.42,43 It was clearly seen that the battery based on the Co3O4/MnO2/PQ-7 catalyst showed more stable cycling performance with smaller voltage gap compared to the Co3O4/MnO2/PQ-2 and Co3O4/MnO2/PQ-46. Therefore, the much better cycling


17


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durability performance further emphasized the great potential of the Co3O4/MnO2/PQ-7 for rechargeable zinc-air batteries commercialization.

3.4 Co3O4/MnO2/porous carbon as air catalyst in flexible all-solid-state zinc-air batteries. To demonstrate potential applications in flexible all-solid-state zinc-air batteries, we have further developed battery composed of a zinc foil anode, alkaline membrane (Tokuyama A901) electrolyte, Co3O4/MnO2/porous carbon loaded on nickel foam air cathode (Figure 6a). As shown in Figure 6b, this is further supported by the fact that the corresponding peak power density of the Co3O4/MnO2/PQ-7-based battery (45 mW cm-2) is better than the Co3O4/MnO2/PQ-2-based one (18 mW cm-2) and the Co3O4/MnO2/PQ-46 based one (30 mW cm2

). Subsequently, Galvanostatic discharge measurements were conducted at current densities of 5

mA cm-2 to study the stability. The discharge curves in Figure 6c indicated that the Co3O4/MnO2/PQ-7 cathode derived an open circuit voltage of ~1.2 V, similar to that of the Co3O4/MnO2/PQ-2 and Co3O4/MnO2/PQ-46-based batteries, and a standing time as long as 6 hours, but the other two catalysts show much shorter working time. The noticeable performance manifests good stability of Co3O4/MnO2/PQ-7 for ORR. Apart from the good durability of Co3O4/MnO2/PQ-7 for ORR, the electrode design offers additional advantages to possess cycling stability. As shown in Figure 6d, the all-solid-state battery exhibited steady charge (1.98 V) and discharge (1.16 V) potentials at the current density of 5 mA cm-2 for 2.5 hours even when the device was bended to a large angle. This reveals not only a robust adhesion between the electrolyte Tokuyama A901 membrane and the Co3O4/MnO2/PQ-7 air electrode but also the capability of the flexible structure to maintain its better cycling stability under bending conditions. Those results signify possibilities of the all-solid-state zinc-air batteries based on the Co3O4/MnO2/PQ-7 to be used in flexible optoelectronic devices.


18

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4. CONCLUSIONS

In summary, a series of functionalized Co3O4/MnO2/porous carbon hybrids with a special structure are developed as effective bi-functional air catalysts for liquid-type, flexible all-solidstate zinc-air batteries. In terms of bi-functional performance, Co3O4/MnO2/PQ-7 possesses a well-balanced catalytic activity and stability in both the ORR and OER, which outperforms the PQ-2, PQ-46 counterparts, Co3O4/MnO2, individual PQ-7 porous carbon and commercial 20% Pt/C in half-cell testing. According to SEM/TEM/BET/XPS/LSV analysis, the high performance of this novel catalyst is believed to be attributed to the coupling effect among MnO2 nanotubes, Co3O4 particles and PQ-7 porous carbon layer. Under alkaline electrolyte condition, the primary and rechargeable zinc-air batteries, which used the hybrid Co3O4/MnO2/PQ-7 bi-functional air electrode showed outstanding discharge and charge performance while maintaining good stability when operated over 300 cycles compared to Co3O4/MnO2/PQ-2, Co3O4/MnO2/PQ-46 and commercial 20% Pt/C. Furthermore, a suite of excellent output power density and static discharge time were derived for these flexible all-solid-state Co3O4/MnO2/porous carbon-based primary zinc-air batteries benefiting from the use of highly flexible air electrodes based on nickel foam and polymer electrolyte membrane. The rechargeable battery using the Co3O4/MnO2/PQ-7 exhibited reliable cycling stability under stress at different bending angles. The neoteric Co3O4/MnO2/PQ-7 hybrid opens up an opportunity for the development of liquid-type and flexible all-solid-state zinc-air batteries even many other electrochemical energy conversion and storage systems.


19


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Scheme 1. Proposed mechanism for the synthesis of Co3O4/MnO2/porous carbon electrocatalysts and the application of those electrocatalysts in zinc-air battery.


20

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140 18

70

24

30 36 42 Pore Diameter / nm

48

(g) 0 0.0

0.2 0.4 0.6 0.8 Relative pressure ( P / P0 )

1.0

100 18

50

24


48

(h) 0 0.0


1.0

400

dV/dlog (D)

150

Volume absorbed ( cm-3g-1 )

210

200 dV/dlog (D)

dV/dlog (D)

280


500


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


300 200 18

100

24


48

(i) 0 0.0


Figure 1. SEM images of (a) Co3O4/MnO2/PQ-2, (b) Co3O4/MnO2/PQ-46, (c) Co3O4/MnO2/PQ7; TEM images of (d) Co3O4/MnO2/PQ-2, (e) Co3O4/MnO2/PQ-46, (f) Co3O4/MnO2/PQ-7; N2 adsorption-desorption isotherms of (g) Co3O4/MnO2/PQ-2, (h) Co3O4/MnO2/PQ-46, (i) Co3O4/MnO2/PQ-7. Insets show the pore size distribution from the BJH method of corresponding materials.


21

1.0


Co3O4 Intensity / a.u

○

x MnO2

Intensity / a.u

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

Page 22 of 37

● Porous cabon (311) ○ (002) ● (110) (200) x

24

x

26

28

30

32

2-theta / deg

(100)(411) (521) x (422) x (440) ● x (541) ○ ○

c

b

a

15

30

45

60

75

90

2-theta / deg Figure 2. XRD patterns of typical samples (a: Co3O4/MnO2/PQ-2, b: Co3O4/MnO2/PQ-46 and c: Co3O4/MnO2/PQ-7 electrocatalyst, inset shows an enlarged half-peak width of C).


22

Page 23 of 37

+

655 650 645 Binding Energy (eV)

(b) Mn 2p1/2

808

Mn 2p3/2

(e)

（ 654.0）

4

3

660

+


640

（ 653.8）

4

3

+

+


640

+

sat.

Co 2p3/2

2

（ 795.8） + 2

+

+

3

+

sat.

800 795 790 785 780 Binding Energy (eV) Co 2p3/2

2

805

800

+

3

+

2

+

+ 3

795 790 785 780 Binding Energy (eV)

775540

2-

O

-


528

O 1s

(i)

Co 2p1/2

（ 795.1）

528

O 1s （ 530.0）

OH

775540

（ 780.1）

-


(h) 3

Co 2p1/2

OH

2-

O

H2O

776 540

（ 780.6）

(f)

（ 642.3）

Mn 2p1/2

+ 3

+

+


805

Mn 2p3/2

(c)

660

+

2

sat.

640

（ 642.3）

3 2

Intensity (a.u.)

660

+

Co 2p1/2

（ 795.9）

（ 530.2）

Intensity (a.u.)

3

+

O 1s

(g)

（ 780.9）

Intensity (a.u.)

4

Intensity (a.u.)

Intensity (a.u.)

（ 654.2）

Intensity (a.u.)

Mn 2p1/2

Co 2p3/2

(d)

（ 642.5）

（ 530.0）

Intensity (a.u.)

Mn 2p3/2

(a)

Intensity (a.u.)

3

Intensity (a.u.)

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


OH

2-

O

-


528

Figure 3. High-resolution Mn 2p XPS spectra of Co3O4/MnO2/PQ-2 (a), Co3O4/MnO2/PQ-46 (b) and Co3O4/MnO2/PQ-7 (c); High-resolution Co 2p XPS spectra of Co3O4/MnO2/PQ-2 (d), Co3O4/MnO2/PQ-46 (e) and Co3O4/MnO2/PQ-7 (f); High-resolution O 1s XPS spectra of Co3O4/MnO2/PQ-2 (g), Co3O4/MnO2/PQ-46 (h) and Co3O4/MnO2/PQ-7 (i).


23

(a)

Co3O4/MnO2/PQ-46

0

Co3O4/MnO2/PQ-7 Co3O4/MnO2 PQ-7 Pt/C

-2

0.42

4 2 0

J-1 / cm2 mA-1

Co3O4/MnO2/PQ-2

Current density / mA cm

Current density / mA cm-2

2

0.36 0.30

-4

Page 24 of 37

Potential / V 0.25 0.30 0.35 0.40

(b)

n=3.83

0.24 0.18 0.02

-2

0.03

ω

-1/2

0.04

0.05

/ rpm-1/2

0.4

0.6

0.8

1.0

0.2

Potential / V (vs RHE)

60 Co3O4/MnO2/PQ-2 Co3O4/MnO2/PQ-46 Co3O4/MnO2/PQ-7 Pt/C

0

0

12000

(c) 24000

Time / s

36000


80

20

0.6

0.8

1.0


100

40

0.4

rpm 300 rpm 600 rpm 900 rpm 1200 rpm

-4

-6 0.2

Percentage / %

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

-2


48000

25 20

Co3O4/MnO2/PQ-2

(d)

Co3O4/MnO2/PQ-46 Co3O4/MnO2/PQ-7 Co3O4/MnO2

15

PQ-7 Pt/C

10 5 0 1.0

1.2

1.4

1.6

1.8

2.0


Figure 4. (a) ORR polarization curves of Co3O4/MnO2/porous carbon electrocatalysts, Co3O4/MnO2, pristine PQ-7 porous carbon and the commercial 20% Pt/C; (b) LSVs for Co3O4/MnO2/PQ-7 catalyst recorded in O2-saturated 0.1 M KOH at various rotation rates with a scan rate of 5 mV s−1 (the insets show the corresponding K-L plots at different potentials); (c) Chronoamperometric responses of Co3O4/MnO2/porous carbon and the commercial 20% Pt/C electrodes at 0.70 V in an O2-saturated 0.1 M KOH solution; (d) OER polarization curves of Co3O4/MnO2/porous carbon, Co3O4/MnO2, pristine PQ-7 porous carbon and the commercial 20% Pt/C.


24

800

150 100

400

(b)

3.0

150

300

450

0

600

1.45 1.6 V V

1.5 1.0 0.5 discharging 0

0.6

15

(d) 30

45

60


75

Co3O4/MnO2/PQ-2 Co3O4/MnO2/PQ-46

0.3 0.0

Co3O4/MnO2/PQ-7

0

20

40

(c)

60

80

Time / h

3.0

Co3O4/MnO2/PQ-7 Pt/C

2.0

replenish

0.9


2.5 charging

0.0

50

-2

1.2

Voltage / V

200

0

j = 10 mA cm

250

1200

0

1.5

300

Co3O4/MnO2/PQ-7 Pt/C

100

324 cycles

2.5

Voltage / V

Voltage / mV

1600

Voltage / V

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


Power density / mW cm-2

Page 25 of 37

2.0 1.3 V

1.5 0.7 V

1.0 0.5 j = 10 mA cm-2 0.0

5 mins charge 5 mins discharge

0

1

2

3

4

50

52

(e) 54

Time / h

Figure 5. (a) A home-made zinc-air battery; (b) Polarization curve and corresponding power density plots of primary zinc-air battery using Co3O4/MnO2/PQ-7 and the commercial 20% Pt/C as air cathode electrocatalyst; (c) Long-time discharge curves of the primary battery using


25


Page 26 of 37

Co3O4/MnO2/porous carbon as cathode electrocatalysts at 10 mA cm-2; (d) Charge and discharge polarization (v-i) curves of the rechargeable zinc-air battery using Co3O4/MnO2/PQ-7 and the commercial 20% Pt/C as the cathode electrocatalysts; (e) Cycling data at 10 mA cm-2 in short cycle periods (10 minutes per cycle) of rechargeable zinc-air battery using Co3O4/MnO2/PQ-7 as cathode electrocatalyst.


26

Co3O4/MnO2/PQ-2 Co3O4/MnO2/PQ-46

Voltage / mV

1600

(b) 50

Co3O4/MnO2/PQ-7

40

1200

30 800

20

400 0 1.6

j = 5 mA cm-2

3.0

(c)

2.5

Voltage / V

1.2

Voltage / V

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


Power density / mW cm-2

Page 27 of 37

0.8 0.4 0.0 0.0

Co3O4/MnO2/PQ-2 Co3O4/MnO2/PQ-46 Co3O4/MnO2/PQ-7

1.5

3.0

Time / h

10 0

15

30

45

0

60

Current density / mA cm-2 j = 5 mA cm-2

(d)

5mins charge 5mins discharge

2.0 1.5 1.0 0.5

4.5

6.0

0.0 0.0

0.5

1.0

1.5

2.0

2.5

Time / h

Figure 6. (a) A schematic Illustration of flexible all-solid-state zinc-air battery; (b) Polarization curves and corresponding power density plots of the flexible all-solid-state zinc-air battery using Co3O4/MnO2/porous carbon as the cathode electrocatalysts; (c) Long-time discharge curve of Co3O4/MnO2/porous carbon electrocatalysts; (d) Cycling data at 5 mA cm-2 in short cycle periods (10 minutes per cycle) of Co3O4/MnO2/PQ-7 electrocatalysts, applying bending angle 45°and 120°.


27


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ASSOCIATED CONTENT Supporting Information. The following files are available free of charge. TEM and HRTEM image; BET and BJH theoretical calculations; High-resolution N 1s, C 1s XPS spectra and the survey spectrum; LSV curves; Polarization curve and corresponding power density plots of primary zinc-air batteries; Charge and discharge polarization curves and cycling data at 10 mA/cm2; Summary of LSV data; Peak power density of primary zinc-air batteries with several key parameters extracted from previous literatures (PDF)

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]. Phone: +86-21-67792379. Fax: +86-21-67792159 (J.Q.). * E-mail: [email protected] (T.Z.). Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work is financially supported by the National Natural Science Foundation of China (U1510120, U1510204, 91645110, 51172251), the College of Environmental Science and Engineering, State Environmental Protection Engineering Center for Pollution Treatment and


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TOC 54x35mm (300 x 300 DPI)


Hierarchically Porous Carbon as Superior Bifunctional

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