(Fe, Co, Ni) as Bifunctional Oxygen Catalysts for ... - ACS Publications

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Design 3D hierarchical architectures of carbon and highly active transition-metals (Fe, Co, Ni) as bifunctional oxygen catalysts for hybrid lithium-air batteries Dongxiao Ji, Shengjie Peng, Dorsasadat Safanama, Haonan Yu, Linlin Li, Guorui Yang, Xiaohong Qin, Srinivasan Madhavi, Stefan Adams, and Seeram Ramakrishna Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b05056 • Publication Date (Web): 06 Feb 2017 Downloaded from http://pubs.acs.org on February 6, 2017

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Chemistry of Materials

Design 3D hierarchical architectures of carbon and highly active transition-metals (Fe, Co, Ni) as bifunctional oxygen catalysts for hybrid lithium-air batteries Dongxiao Ji, †‡ Shengjie Peng,* ‡§ Dorsasadat Safanama, ⊥ Haonan Yu, † Linlin Li,§∥ Guorui Yang, ‡ Xiaohong Qin,* † Madhavi Srinivasan, ∥ Stefan Adams, ⊥ and Seeram Ramakrishna‡ †

Key Laboratory of Textile Science & Technology, Ministry of Education, College of Textiles,

Donghua University, Shanghai 201620, P. R. China. ‡

Department of Mechanical Engineering, National University of Singapore, 117574, Singapore.

§

College of Material Science and Engineering, Nanjing University of Aeronautics and

Astronautics, Nanjing 210016, P. R. China. ⊥

Department of Materials Science and Engineering, National University of Singapore, 17579,

Singapore. ∥

School of Materials Science and Engineering, Nanyang Technological University, 639798,

Singapore.

ACS Paragon Plus Environment

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KEYWORDS: electrospinning; non-precious metals; free-standing catalyst; oxygen reduction reaction; hybrid Li-air battery

ABSTRACT: Flexible power sources and efficient energy storage devices with high energy density are highly desired to power a future sustainable community. Theoretically, rechargeable metal-air batteries are promising candidates for the next-generation power sources. The rational design of oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) catalysts with high catalytic activity is critical to the development of efficient and durable metal-air batteries. Herein, we propose a novel strategy to mass synthesize non-precious transition-metal-based nitrogen/oxygen co-doped carbon nanotubes grown on carbon-nanofiber films (MNO-CNTCNFFs, M= Fe, Co, Ni) via a facile free-surface electrospinning technique followed by in-situ growth carbonization. Combining the high catalytic activity of Fe-catalyzed CNTs and the efficient mass transport characteristics of 3D carbon fiber films, the resultant flexible and robust FeNO-CNT-CNFFs exhibit the highest bifunctional oxygen catalytic activities in terms of a positive half-wave potential (0.87 V) for ORR and low overpotential (430 mV @ 10 mA cm-2) for OER. As a proof-of-concept, newly designed hybrid Li-air batteries fabricated with FeNOCNT-CNFFs as air electrode present high voltage (~ 3.4 V), low overpotential (0.15 V) and long cycle life (over 120 hours) in practical open air tests, demonstrating the superiority of the freestanding catalysts and their promising potential for the applications in fuel cells and flexible energy storage devices.


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INTRODUCTION The demand for advanced energy storage devices today is unprecedented and growing, owing to the rapid development of electric vehicles, flexible electronics, and renewable energy conversion systems in recent years.1 Rechargeable lithium-air (Li-air) batteries are among the most promising technologies being studied in order to meet the surging energy needs of the modern world due to their high theoretical energy density.2-5 However, one major limitation hindering the adoption of traditional non-aqueous Li-air batteries in practical applications is the formation of insoluble discharge products which eventually block the air electrode.6-9 Although hybrid Liair batteries with the air electrodes immersed in an aqueous catholyte has the potential to overcome this limitation,10 they also suffer from a large overpotential derived from the sluggish ORR and OER kinetics - a measure of the catalytic performance of the catalysts.11 Generally, precious metal-based catalysts are considered the benchmark materials in addressing the issue of poor reaction kinetics of OER and ORR. Nevertheless, this excellent performance comes at a significant cost as the materials required are prohibitively expensive, and have poor durability, thereby limiting their industrial application. Comparatively, non-precious metal based bifunctional catalysts are inexpensive with promising catalytic activity and stability, making them ideal candidates for advanced hybrid Li-air batteries. In recent years, there have been investigations into various efficient, non-precious metal based ORR and OER catalysts, including heteroatom doped (N, P, S, etc.) carbons,12-14 metal-nitrogen-doped carbons (M-NC),15-17 and transition-metal oxides.18-21 However, it remains a significant challenge to optimize the practical activity of these nanocatalysts by engineering design due mainly to the adjunction of the insulating and inactive binders, as well as the easy detachment of the nanocatalysts from


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the porous air electrode.22,

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These highlight just how important combining a highly active

bifunctional non-precious metal catalyst with a conductive porous electrode in a suitable architecture is in improving the practical electrocatalytic activity for hybrid Li-air batteries. From the catalyst point of view, transition-metal-based N-doped carbon shows much promise in replacing Pt for catalyzing ORR due to their low cost and high catalytic activity generated from the M-Nx structure and the synergistic effects from the interactions between transition metal and the N-doped graphite layer.24-26 Recently, N/O co-doped carbon nanomaterials containing positively charged carbon atoms were found to be efficient catalysts for OER.27, 28 Theoretically, the combination of N/O co-doped carbon with ORR active transition-metal-based active sites could be a good pathway toward an efficient bifunctional oxygen electrocatalyst. Moreover, to the best of our knowledge, the use of transition metal-based N/O co-doped carbon catalysts in hybrid Li-air battery systems is still rare. From the electrode point of view, the electrospinning technique is attracting growing attention in assembling free-standing carbon fiber film electrodes for various batteries because it enables the high output production of micro/nano fiber networks with superior mechanical strength, highly porous structures, and large surface areas,29-33 which are desirable properties of high performance electrodes. Due to the direct growth nature of M-catalyzed (M: Fe, Co, Ni) CNTs, M-based N/O co-doped CNTs directly grown on electrospun carbon fiber can be realized through a facile design albeit with many challenges. Compared to typical nanocatalyst-on-support materials, the in-situ growth of active CNTs on electrospun carbon fibers provides strong and stable interconnectivity between the active materials and the electrode, which enhances the catalytic activity and more importantly, improves the stability of the air electrode. Moreover, the hierarchical and porous structure of the electrospun carbon fiber film is a boon to enable rapid oxygen and electrolyte


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diffusion, another key factor in improving the practical catalytic activity. Therefore, to take the full advantage of the catalyst, in situ growth of M-based CNTs on electrospinning carbon fiber films is highly desirable to form a fast oxygen and electrolyte diffusion 3D architecture with high practical catalytic activity for advanced hybrid Li-air batteries. Herein, we present a novel strategy to mass synthesize M-based N/O co-doped CNTs grown on carbon fiber films (MNO-CNT-CNFFs, M= Fe, Co, Ni) as superior freestanding, bifunctional catalysts for ORR and OER. The fabrication is based on a simple free-surface electrospinning technique followed by in-situ growth carbonization (Figure S1, Supporting Information). The obtained flexible and robust FeNO-CNT-CNFFs present unique nanoarchitectures with the highest bifunctional catalytic activity in the series, which is comparable to that of commercial Pt/C (20%) and Ir/C (20%). The hybrid Li-air batteries fabricated with FeNO-CNT-CNFF as the air electrode exhibit superior performance with a low overpotential of 0.15 V, high voltage (~ 3.4 V) and a long cycle life under open air conditions. Benefiting from its high electrical conductivity, stable micro/nano architecture, and abundant accessible active sites, the FeNOCNT-CNFF is one of the most active freestanding non-precious metal bifunctional oxygen catalysts with promising potential for the applications in metal-air batteries and flexible energy storage devices. Experimental Section Mass Preparation of Free-standing MNO-CNT-CNFFs. M-rich nanofiber film was fabricated by free-surface electrospinning. The precursor solution for electrospinning was first prepared by dissolving 6 g of Polyacrylonitrile (PAN, Mw = 150 000) and 6 g of cellulose acetate (CA, Mr ~ 29 000) in 100 mL N, N-dimethylformamide (DMF). The mixture was stirred at 60 °C for 4 hours to obtain a pale yellow solution. Then, 5 g of Iron(III) acetylacetonate (Fe (Acac)3) /


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Co(Acac)2 / Ni(Acac)2 was added into the solution, and stirred in oil bath at 60 °C overnight. A stepped pyramid-shape spinneret was used to electrospin the precursor solution, and a nonwoven cloth was used as the collector (Figure S1, Supporting Information). 85-90 kV voltage was applied to produce nanofibers. The working distance (the distance between the top surface of the spinneret and the grounded collector) was 25 cm to ensure good formation of the multiple electrospinning jets generated from the free surface. The as-prepared solution was loaded into a self-designed solution supply system at a flow rate of 60 mL h-1, resulting in a production yield of 4-10 g h-1 for the M-rich nanofiber films. The resultant M-rich nanofiber films (180 × 80 cm) were carefully cut out according to the requirement and stabilized at 280 °C in air for 2 h with the heating rate of 1 °C min-1. Thereafter, the prepared M-rich nanofiber film was placed into a porcelain boat with ~ 1 g melamine (The weight ratio of melamine and the sample is 5:1) paved under the film. The mixture was calcined at different temperatures (700, 800 and 900 °C) for 2 h with a heating rate of 5 °C min-1 under the protection of N2. The comparative Fe-CNFF was prepared through the same procedure only without the adding of melamine at 800 °C. Materials Characterization. The morphologies of the MNO-CNT-CNFFs was observed by field emission scanning electron microscopy (FE-SEM, JEOL-6700F) and Field emission transmission electron microscopy (FE-TEM, JEOL JEM-2010F). Elemental mapping was performed using a JEOL JEM-2010F transmission electron microscope operating at 200 kV. The phase compositions of the carbon films were analysed by X-ray diffraction (XRD) on a Bruker D8 Advance diffractometer using Cu Kα radiation (λ = 1.5406 Å) and scanning the 10° to 80° 2θ range at 0.02° s-1. N2 adsorption-desorption isotherms were measured by a Micromeritics ASAP 2020 Surface Area and Pore Size Analyser at liquid nitrogen temperature. The specific surface areas of the samples were calculated by the multipoint Brunauer-Emmett-Teller (BET) procedure.


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XPS analysis was carried out on a Kratos Axis Ultra DLD spectrometer, with binding energy correction by referencing the C 1s peak of adventitious carbon to 284.6 eV. Thermogravimetric analysis (TGA) measurements were carried out on a Shimadzu DSC-60 differential scanning calorimeter from room temperature to 900 °C using a heating rate of 5 °C min-1. Raman spectra were collected on LabRAM HR instrument with a 532 nm excitation laser. The Hydrophilic/hydrophobic nature of the FeNO-CNT-CNFFs was measured by water contact angle measurement using VCA Optima Surface Analysis System (AST products, Billerica, MA). The tensile properties of the FeNO-CNT-CNFFs were determined using a tabletop tensile tester (Instron 5943) at a load cell capacity of 10 N. Test specimens of dimensions 10 mm breadth and 20 mm length with a thickness of 110 µm. The crosshead speed was 5 mm min-1 and tested under ambient conditions. The conductivity of the free-standing catalyst is measured using a four-point probe method. The samples are cut into 1cm x 1cm squares and thickness of each sample is measured. Electrochemical Measurements. All electrochemical measurements were performed on a rotating electrode system (pine Inc.) with an Autolab electrochemical work station (Autolab Instrument) bipotentiostat at room temperature. A three-electrode cell configuration was employed with a working electrode of glassy carbon rotating disk electrode (RDE) 5 mm in diameter or rotating ring-disk electrode (RRDE) 5.61 mm in diameter. A Pt foil and a Ag/AgCl (3 M KCl) electrode were used as the counter electrode and reference electrode, respectively. All potentials were referenced to the reversible hydrogen electrode (RHE) scale according to the Nernst equation (ERHE = EAg/AgCl + 0.059 × pH + 0.21 V, at 25 °C). The electrolyte used were 0.1 M KOH and 1 M LiOH aqueous solution. The linear sweep voltammograms (LSVs) were collected at a scan rate of 5 mV s-1. The onset potential is noted as the potential which is able to


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yield 0.1 mA cm-2 current density during the ORR or OER process based on the tested steady state polarization plots. OER data were corrected for an ohmic drop ~ 44 Ω in 0.1M KOH and ~ 11 Ω in 1M LiOH. The catalyst ink was prepared by adding 5 mg of catalyst into 1 mL of 1:1 v/v water/ethanol mixed solvent that contained Nafion solution (5 wt%, Sigma-Aldrich), the ink was ultrasonically dispersed for at least 30 min. The same recipe was used to prepare the Pt/C (Premetek Co. 20 %) and Ir/C (Premetek Co. 20 %) catalysts ink. 16 µL of the catalyst ink was pipetted and dispensed onto the glassy carbon electrode (RDE) to reach a catalyst loading of approximately 0.4 mg cm-2, and for RRDE, the catalyst ink was added 20 µL to reach as the same catalyst loading as RDE. To further investigate the effect of mass loading to the catalytic activity, the mass loading of 0.2 mg cm-2 and 0.1 mg cm-2 were also performed on RDE. The mass loading of Pt/C and Ir/C onto the RDE and RRDE surface was 0.1 mg cm-2. For ORR experiment, O2 (99.999%) was bubbled for 30 min prior to the test and maintained in the headspace of the electrolyte throughout the testing process. The catalyst covered working electrode was cycled by cyclic voltammetry (CV) at a scan rate of 50 mV s-1 and 5 mV s-1 until the current became stable. RDE measurements were carried on by linear sweep voltammetry (LSV) at a scan rate of 5 mV s-1 with various rotating speeds from 400 to 1600 rpm. For RRDE measurement, the ring potential was fixed at 1.5 V versus RHE. The Koutecky-Levich equation can be used for determining the electron transfer number (n): 53, 54 1 1 1 1 1 = + = − ⁄ I I I nFAk 0.62nFAD v ⁄ w ⁄

where I is the measured current density, Ik is the kinetic current density, Id is the diffusionlimiting current density, ω is the angular velocity (ω = 2πN, N is the rotation speed), F is the


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Faraday constant (96485 C mol-1), A is the geometric electrode area (cm2), k is the rate constant of the reaction, C0 is the bulk concentration of O2 (1.2 × 10-6 mol cm-3), DO2 is the diffusion coefficient of O2 (1.9 × 10-6 cm2 s-1) and ν is the kinematic viscosity of the electrolyte (0.01 cm2 s-1). For RRDEs, the electron transfer number (n) and percentage of peroxide species (yperoxide) relative to the total products was determined by the following equations:

n=

4NId NId + Ir

y peroxide =

200I r NId + Ir

Where Id, Ir and N are the disk current, the ring current, and the current collection efficiency of RRDE, respectively. Here, N=0.38. Synthesis of NASICON-type Li1+xAlxGe2-x(PO4)3 (LAGP) Glass. The NASICON-type Li1+xAlxGe2-x(PO4)3 (LAGP) glass was prepared using the melt quenching method.48 In brief, stoichiometric amounts of Li2CO3 (Alfa Aesar, Ward Hill, MA), Al2O3 (Sigma Aldrich, Milwaukee, WI), GeO2 (Alfa Aesar, Ward Hill, MA) and NH4H2PO4 (Merck) were mixed and ballmilled (Fritsch Pulverisette 7) using zirconia bowls and balls, pre-heated to 700 °C, molten at 1450 °C and quenched using stainless steel plates (as the heat sink). Then, the NASICON-type Li1+xAlxGe2-x(PO4)3 (LAGP) glass was obtained. Battery Assembly and Test. The battery cell consists of a protected Li-metal anode and the FeNO-CNT-CNFFs cathode. Briefly, rechargeable hybrid LABs are assembled by sandwiching LAGP pellets (thickness of 1-2 mm) between lithium anode (0.5 mm thick, Honjo Metal Co.,


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Ltd) and 1 M LiOH (98%, Sigma Aldrich) catholyte. 1 M LiPF6 in EC/DMC electrolyte is used between lithium anode and the ceramic solid electrolyte to facilitate interfacial charge transfer. The oxygen reduction and formation is catalyzed by FeNO-CNT-CNFF-800 air electrode. The area of the FeNO-CNT-CNFF-800 cathode was cut into 0.6 cm2, which is also defined as the effective LAB area. For comparison, Pt/C + Ir/C air electrode were prepared. The catalyst ink was prepared as described in electrochemical experiments. The air electrodes were consisted of carbon fiber mat, with a gas diffusion layer (GDL) on the air-facing side and a catalyst layer (CL) on the electrolyte-facing side. The GDL was made by dispersing CNTs in ethanol, and dropped on the carbon fiber mat. The CL was made by loading catalyst ink onto the other side of the carbon fiber mat with a loading of 0.4 mg cm-2. Tests of the hybrid LAB were performed in an ambient environment at room temperature. A battery test station (Arbin, GT2000) connected to a computer installed with the Arbin Data Pro software was used in the charge-discharge data acquisition. The cell was discharged at a current density from 0.03 to 0.35 mA cm-2. For longterm discharge and charge experiments, a discharging current density of 0.03 mA cm-2 was used, and each cycle is limited to 6 hours without the adding of catholyte. RESULTS AND DISCUSSION The strategy for the fabrication of free-standing MNO-CNT-CNFF is schematically illustrated in Scheme 1a. Firstly, a solution containing polyacrylonitrile (PAN), cellulose acetate (CA), and iron

(III)

acetylacetonate

(Fe(Acac)3)/

Co(Acac)2

/

Ni(Acac)2

dissolved

in

N,N-

dimethylformamide was prepared and used as the spinning precursor to mass synthesize the metal-rich nanofiber film (M-NFF) (Scheme 1b). The obtained M-NFF was then stabilized at 280 °C (Scheme 1c), before a controlled carbonization treatment (@ 800 °C) applied to pyrolyze the stabilized M-NFF in the presence of melamine. During the carbonization process, melamine


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undergoes pyrolysis to form graphitic carbon nitride and deposited on the M-NFF.34 Meanwhile, CA decomposes with the release of oxygen-containing gases, leaving channels within the carbonized PAN fibers (Figure S2, Supporting Information).35 The formation of the channels is beneficial in exposing the transition metal compositions that act to catalyze the growth of CNTs, in addition to forming the MNO-CNT active materials (Figure S3, Supporting Information). A uniform and freestanding MNO-CNT-CNFF is obtained after cooling the carbonized product down to room temperature (Scheme 1d). This synthesis route is simple and results in a high yield, characteristics of a potentially upscaleable industrial process. Figure 1 and Figure S5 (Supporting Information) show the morphology of the MNO-CNTCNFFs, confirming the successful growth of Fe, Co, Ni-catalyzed CNTs. The electrospun carbon fibers with a diameters of ~ 600 nm were homogenously covered by the in-situ grown Fe-based CNTs which had peapod-like structures (Figure 1a-c). These nanoscale peapods possess diameters of approximately 10-30 nm, and lengths in the range of several hundred nanometers. The “peas” were well crystallized, 5-20 nm diameter Fe-based particles having a lattice distance of ~ 0.21 nm (Figure 1e), and were enclosed in “peapods” comprising 4-6 graphitic layers. The crystalline structure was identified as Fe3C (cementite, JCPDS No. 35-0772) and cubic Fe (JCPDS No. 06-0696) according to the X-ray diffraction (XRD) pattern (Figure 1f). Empty, hollow pods were also observed, which could increase the active surface area and further enhance the catalytic performance of the material (Figure 1d). The crystal structure of the CNTs are investigated and showed in Figure 1d and e, in which the lattice fringes with an inter-planar distance of ~ 0.34 nm corresponds to the C (002) plane. Intriguingly, we found the outer graphitic layer of the peapods were not perfectly parallel to the CNT axis, but instead protrude in random directions resulting in a large number of exposed edges on the surface (Figure 1d and e)


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contributing to the high electrocatalytic performance.38,39 The elemental mapping results (Figure 1g-k) indicate the homogeneous distribution of C, O, N, and Fe elements in the prepared materials. The homogeneity is believed to result from the optimized electrospinning of the nanofibers and the well controlled in-situ synthesis of Fe-based CNT. The morphology and the purity of the as-prepared NiNO-CNT-CNFFs and CoNO-CNT-CNFFs are shown in Figure S5 (Supporting Information). The resultant free-standing FeNO-CNT-CNFF are highly flexible (Figure 2a) with good mechanical strength up to 3.1 MPa (Figure 2b). The good mechanical performance could guarantee the stability and flexibility of free-standing catalysts, demonstrating the practical application potential of FeNO-CNT-CNFFs as flexible electrodes. 36, 58 Due to the porous and hierarchical nature of electrospun carbon fiber films, FeNO-CNT-CNFFs are believed to possess fast mass-transport kinetics for oxygen and electrolyte diffusion. To estimate the mass-transport property of FeNO-CNT-CNFFs, 0.5 mL KOH electrolyte (dyed with Methylene Blue) was adsorbed by 1 cm2 samples of FeNO-CNT-CNFFs, Pt/C coated Ni-foam, and Pt/C coated carbon cloth each. The samples with the adsorbed electrolyte were moved into 5 mL of KOH solution, and the color change of each solution was recorded (Figure 2c-e). Compared to Pt/C coated Nifoam and Pt/C coated carbon cloth, the dyed electrolyte diffuses out faster in FeNO-CNT-CNFF800, revealing the extent to which its unique nanoarchitecture benefits mass transport of the aqueous electrolyte.37 Furthermore, contact angle tests (Figure 2f-h), reveal that FeNO-CNTCNFFs are hydrophilic, with a small contact angle of ~ 0° due to the presence of the C=O bond (Figure S15, Supporting Information). Compared to Pt/C coated Ni-form (contact angle ~ 127°) and carbon cloth (contact angle ~ 132°), FeNO-CNT-CNFFs have greater wettability, making them suitable for aqueous metal-air batteries with high energy densities.55-57


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The ORR and OER bifunctional catalytic activity of the MNO-CNT-CNFFs were first investigated by linear sweep voltammetry (LSV) on a rotating disk electrode (RDE) in 0.1 M KOH. The bifunctional performance can be quantified by the potential difference between ORR and OER (noted as ∆E = Ej=10 - E1/2, where Ej=10 represents the potential @10 mA cm-2). FeNOCNT-CNFF exhibited the highest bifunctional catalytic performance (∆E = 0.786V) in the series (Figure S7, Supporting Information). Since the temperature at which pyrolysis occurs could significantly affect the structure and the performance of the obtained materials, several pyrolysis temperatures were examined for FeNO-CNT-CNFF in this work (700 °C, 800 °C, and 900 °C, denoted as FeNO-CNT-CNFF-700, 800, or 900). A control sample of Fe decorated carbon fiber film (Fe-CNFF) was also prepared and pyrolysed in the absence of melamine at 800 °C to elucidate the effect of in-situ CNT growth on the material’s performance. Notably, we did not find CNTs generated on FeNO-CNT-CNFF-700 (Figure S6a and b, Supporting Information) as the temperature may have been too low to form CNTs. The CNTs observed on FeNO-CNTCNFF-900 (Figure S6e and f, Supporting Information) were found to be much longer than those in FeNO-CNT-CNFF-800. The XRD patterns of FeNO-CNT-CNFF-700 and FeNO-CNT-CNFF900 display similar peak positions to that of FeNO-CNT-CNFF-800, which can be attributed to C (002), Fe3C, and cubic Fe (Figure S8, Supporting Information). The ORR catalytic performances of FeNO-CNT-CNFFs are shown in Figure 3. It was observed that FeNO-CNT-CNFF-800 exhibited the best ORR catalytic performance with the onset potential (Eonset) of 1.01 V and half-wave potential ~ 0.87 V (Figure 3a and Fig. S13, Supporting Information). In sharp contrast, the control Fe-CNFF performed much worse in terms of catalytic activity as compared to FeNO-CNT-CNFFs. This result indicates that melamine not only functions as the CNT precursor, but also plays an important role in enhancing the


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electrocatalytic activity, since even FeNO-CNT-CNFF-700 (on which CNT growth was absent) showed better catalytic activity for ORR than Fe-CNFF. The ORR activity of FeNO-CNTCNFF-800 was further investigated by cyclic voltammetry (CV) and rotating ring disk electrode (RRDE) techniques. As shown in Figure 3b, oxygen reduction peaks arise in an O2-saturated KOH electrolyte and are absent in a N2-saturated KOH solution. The well-defined cathodic reduction peak of FeNO-CNT-CNFF-800 appears at ~ 0.90 V, at a larger potential than the reduction peak of Pt/C at ~ 0.87 V. The electron transfer number (noted as n) and the H2O2 yield (%) obtained from RRDE are shown in Figure 3d. For FeNO-CNT-CNFF-800, the H2O2 yield is extremely low (< 3%) in the potential range between 0.2 V and 0.9 V, and since almost 4 electrons are transferred (n > 3.9), it can be deduced that oxygen was reduced to water through a four-electron pathway in the same potential range. Consequently, the n and the H2O2 yield is congruent to that of Pt/C. Figure 3c shows the corresponding Tafel plots of FeNO-CNT-CNFF800 and Pt/C. Comparable tafel slopes (56 mV dec-1 for FeNO-CNT-CNFF-800 and 64 mV dec-1 for Pt/C) are observed in the two materials, suggesting a similar ORR reaction mechanism occurs on both the catalyst surfaces. Different catalyst mass loadings of FeNO-CNT-CNFF-800 were also investigated (Fig. S14, Supporting Information), wherein the limiting currents and half-wave potentials were found to be enhanced significantly with increasing mass loading. Notably, the ORR activity of FeNO-CNT-CNFF-800 outperforms that of Pt/C at equivalent mass loadings. In evaluating the stability of FeNO-CNT-CNFF-800 at a constant voltage of 0.65 V， a meagre 8% decrease of the current density was observed after 55000 s, compared to a decrease of more than 20% observed for Pt/C under the same operating conditions (Fig. S16a, Supporting Information). Figure 4a demonstrates the performance of FeNO-CNT-CNFFs as OER catalysts. The onset of the sharp rise in anodic current occurs at relatively low Eonset of 1.431 V, 1.380 V, and 1.399


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V for FeNO-CNT-CNFF-700, 800, and 900 respectively, suggesting they are potential good OER catalysts. Furthermore, the good OER catalytic activities of FeNO-CNT-CNFFs are also reflected by their higher current densities and lower onset potentials when compared to Pt/C. Even when compared to the state-of-the-art Ir/C as the benchmark OER catalyst, FeNO-CNTCNFFs show a better onset potential, similar Tafel slops (63 mV dec-1 for FeNO-CNT-CNFF800 and 46 mV dec-1 for Ir/C, Figure 4b), as well as greater OER stability at a constant voltage of 1.65 V after 25000 s (Fig. S16b, Supporting Information). The exceptional ORR and OER bifunctional performance of FeNO-CNT-CNFF-800 is shown in Figure 4c and d (listed in table S3 and table S4, Supporting Information), which is better than that of precious metals in 0.1M KOH and 1M LiOH. To further investigate the active nature of the FeNO-CNT-CNFFs electrode, a nitrogen adsorption isotherm was performed (Figure S9, Supporting Information). FeNO-CNT-CNFF-800 shows the highest Brunauer-Emmett-Teller derived specific surface area of 220.24 m2 g-1 among the series, ascribed to the in-situ formation of the peapod-like CNTs. This result compares favorably against other reported freestanding air electrodes, including electrospun macroporous hollow carbon fibers (90.71 m2 g-1),37 Co3O4 nanowire grown on SS mesh (22 m2 g-1),40 2D freestanding platinum nanosheets (15.8 m2 g-1)41 and overlapped g-C3N4 and Ti3C2 nanosheets film (205 m2 g-1).42 The iron content of the as-prepared FeNO-CNT-CNFF-800 was evaluated by themogravimetric analysis (TGA) and calculated to be 15.18% (Figure S10, Supporting Information). The Raman spectra of FeNO-CNT-CNFFs revealed the characteristic G and D bands of carbon (Figure 5a) and the relatively high ID (1325 cm-1) / IG (1555 cm-1) ratios of FeNO-CNTCNFFs obtained from 700 °C, 800 °C and 900 °C were 1.137, 1.188 and 1.139 respectively, indicating the defect abundant nature of FeNO-CNT-CNFFs. The X-ray photoelectron


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spectroscopy (XPS) survey spectra of FeNO-CNT-CNFFs showed the presence of C, O, N, and Fe elements (Figure S11 and table S1, Supporting Information), confirming that the iron, nitrogen, and oxygen containing species were successfully doped into the carbon matrix. The content of N and O species were respectively determined to be 4.37 at% and 2.25 at% in FeNOCNT-CNFF-900, 4.99 at% and 4.80 at% in FeNO-CNT-CNFF-800, 3.42 at% and 2.39 at% in FeNO-CNT-CNFF-700. Detailed high-resolution N 1s and Fe 2p XPS spectra are shown in Figure 5b and c. Pure iron (II) phthalocyanine (FePc) is taken as a reference to investigate the binding state of N and Fe species. Five N binding states have been observed in the samples prepared from different pyrolysis temperatures (Figure 5b). The peaks at ~ 398.5 eV, 400.3 eV, 401.3 eV, and 402.0 eV are attributed to pyridinic N, pyrrolic N, graphitic N and oxidized pyridinic N, respectively.12 The peaks at ~ 399.63 eV in three samples correspond to the N binding state similar to nitrogen atoms coordinating to iron atom in FePc (noted as Fe-Nx and decorated with red bond in the insert Scheme in Figure 6b).43 Pyridinic N, which is believed to have a positive effect in promoting catalytic activity for ORR and OER,32, 44 is generated as the main N binding state in the three samples (Figure S12, Supporting Information). As the Fe-Nx moieties are considered to be active sites in traditional Fe-N-C ORR catalyst, the amount of Fe-Nx in the prepared samples are listed (Table S2, Supporting Information). FeNO-CNT-CNFF-800 contains the highest number of Fe-Nx moieties, approximately 11.2% in all N binding types, higher than FeNO-CNTCNFF-900 (9.0%) and FeNO-CNT-CNFF-700 (4.2%). The Fe 2p XPS spectrum shown in Figure 5c shows two peaks arising for FePc at ~ 709.9 eV and ~ 723.0 eV, corresponding to the Fe atom coordinating to N atoms (Fe-Nx), and the binding energy of 2p1/2 Fe2+ respectively.45,50 The signal at ~ 706.5 eV in FeNO-CNT-CNFF-700 and FeNO-CNT-CNFF-800 is attributed to


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metallic Fe or carbide Fe (noted as Fe-C).15 The peaks observed at ~ 710.0 in three samples can be assigned to iron atom in Fe-Nx configuration.26 The differences in the Fe 2p XPS spectra elucidates the effect of the different pyrolysis temperatures on the composition of the fabricated material. It is noteworthy that the peak at ~ 706.5 eV is not detected in FeNO-CNT-CNFF-900 although Fe/Fe3C nanocrystals have been found by XRD and TEM. The absence of this peak demonstrates that Fe-Nx configuration dominates the Fe binding state on the surface of FeNOCNT-CNFF-900. Whereas in FeNO-CNT-CNFF-700, Fe-C coexists with the Fe-Nx configuration but with significantly higher Fe-C content compared with Fe-Nx. However, in FeNO-CNT-CNFF-800, the surface Fe 2p3/2 binding state mainly consists of Fe-Nx and Fe-C. It is known that Fe-Nx and N-doped C activated by Fe-based particles (noted as Fe@C) may promote the catalytic activity of the Fe-N-C type catalyst for ORR by acting as active sites.25, 26, 46, 47

Taking into account the XPS spectra and the order of ORR activity for the three FeNO-

CNT-CNFF samples, FeNO-CNT-CNFF-800 with both Fe@C and the highest content of Fe-Nx, delivers the highest activity for ORR. Meanwhile, FeNO-CNT-CNFF-700 with a large proportion of Fe@C and a lower amount of surface Fe-Nx exhibits the lowest ORR activity. With regard to FeNO-CNT-CNFF-900 displaying moderate performance for catalyzing ORR, in light of the absence of the Fe-C binding state on material’s surface, it can be deduced that Fe-Nx may be the main active sites in FeNO-CNT-CNFF-900. These findings lead to the conclusion that FeNx configuration should be the ORR catalytically active sites in the as-prepared FeNO-CNTCNFFs and Fe-based particles can greatly promote the activity of the nearby Fe-Nx and N-doped C to catalyze ORR. Moreover, the doping pyridinic N atoms and oxygen atoms into the carbon framework (Figure S15, Supporting Information) can generate positively charged carbon atoms,


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favoring the absorption of OH- and H2O via electrostatic force, thus becoming the active sites for OER.27,28,32 Based on the excellent bifunctional electrocatalytic performance and the robust physical properties of the FeNO-CNT-CNFF-800, proof-of-concept rechargeable hybrid Li-air batteries (LABs) were assembled with self-standing FeNO-CNT-CNFF-800 as an air electrode. Figure 6a shows the schematic illustration of the rechargeable hybrid LAB composed of a lithium metal anode and NASICON-type Li1.5Al0.5Ge1.5(PO4)3 (LAGP)48 anode-protecting membrane. The anode chamber is filled with 1 M LiPF6 in EC-DMC to minimize interfacial resistance, and 1 M LiOH aqueous solution is used as the catholyte solution. The overall cell reaction of the asprepared battery can be described as follows: ° ' 4 + ! + 2" ! ↔ 4 !" $%& = 3.44 *

Signiﬁcantly, the hybrid LAB reaches very high voltage ~ 3.4 V, which is able to power green LEDs after charging, indicating the opportunity to be commercial application (Figure 6b). Figure 6c compares the first charge–discharge curves of the hybrid cell with different cathodes, where ∆V denotes the voltage difference between the charge and discharge voltages. The FeNO-CNTCNFF-800-based battery exhibits the best performance in the open air test with a ∆V of 0.15 V at the current density of 0.03 mA cm-2, much lower than that of Pt/C + Ir/C air electrode (∆V = 0.43 V). These results clearly confirm the superiority of FeNO-CNT-CNFF-800 as bifunctional air electrodes in the full-cell configuration. The excellent FeNO-CNT-CNFF-800 performance can be attributed to the combined advantages of the freestanding FeNO-CNT-CNFF-800, namely i) the superior bifunctional catalytic activity of FeNO-CNT-CNFF-800 in 1 M LiOH promoting the ORR/OER; ii) the hierarchically porous structure with hydrophilic surface allowing fast mass


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transport of oxygen and electrolyte; and iii) the good conductivity of CNTs and interconnected 1D carbon fiber without binder providing a “highway” for electron transfer. Figure 6d shows the effect of current density on the charge and discharge voltages, and confirms that the as-prepared hybrid LAB performs a stable 4e- reduction mechanism under varying current densities. The room temperature cycling performance of the cell in the open-air conditions with FeNO-CNTCNFF-800 is shown in Figure 6 e and f, where stable charge and discharge curves are achieved for over 21 cycles (~ 120 hours of cycling) with a specific capacity of 30 mAh/(g of active catalyst). As observed in Figure 6e, the overpotential of the cell gradually increases with cycling, reaching the value of 0.3 V after 21 cycles, with the increase likely stemming from the slow formation of Li2CO349, 51, 52 (Figure S17, Supporting Information) and the evaporation of the catholyte. Based on the results presented above, we submit that the synthesized FeNO-CNTCNFF-800 is a highly promising bifunctional air electrode for hybrid Li–air batteries with its low overpotential, high voltage and high charge–discharge stability in operational open-air conditions. CONCLUSION In conclusion, a general synthesis strategy combining a free-surface electrospinning technique with an in-situ growth approach has been developed to fabricate free-standing MNO-CNTCNFFs (M = Fe, Co, Ni) on a large scale. The resultant MNO-CNT-CNFFs possess hierarchical and porous structures with CNTs grown in-situ on the carbon fiber surface. The unique architecture imparts a high degree of flexibility, excellent mechanical properties, and outstanding stability. Due to the high catalytic activity of Fe-based CNTs and the efficient mass transport of CNFFs, the robust FeNO-CNT-CNFFs as novel bifunctional electrocatalysts demonstrate superior ORR and OER catalytic activity comparable to that of precious metals. Rechargeable


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hybrid Li-air batteries using FeNO-CNT-CNFFs as air electrodes exhibit excellent battery performance with a long cycle life (more than 120 h in open air), high voltage (~3.4 V) and a low charge-discharge voltage gap (with the lowest ∆V of 0.15 V). The developed fabrication strategy to tailor the structure and the performance of this nanoengineered material shows great potential to be extended to other systems for a wide range of applications, including supercapacitors, metal-air batteries, and fuel cells.

ASSOCIATED CONTENT Supporting Information Available: Photo of mass fabrication of Fe-NFF, SEM images of surface morphology of Fe-PAN/CA carbon nanofibers, the morphology and purity of CoNO-CNT-CNFF and NiNO-CNT-CNFF,SEM images of morphology FeNO-CNT-CNFF-700, 800 and 900, Bifunctional activities for MNO-CNT-CNFFs in 0.1M KOH at 1600 rpm, XRD patterns of FeNO-CNT-CNFF-700 and FeNO-CNT-CNFF-900, N2 adsorption-desorption isotherms and pore distribution of the samples, TGA curve of FeNO-CNT-CNFF-800, The XPS survey spectra and the proportion of different N bonding types of three samples, the LSVs curves of different catalysts for ORR at different rotating speeds, and corresponding K-L plots of different catalysts at different potentials, The LSVs of FeNO-CNT-CNFF-800 in different loading @ 1600 rpm, the XPS O1s spectra of the samples, the stability of FNO-CNT-CNFF-800 for ORR and OER in 0.1 M KOH. The XRD pattern of FeNO-CNT-CNFF-800 air electrode after 20 cycles

AUTHOR INFORMATION


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Corresponding Author

* Shengjie Peng. E-mail: [email protected]

* Xiaohong Qin. Email: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT We are thankful for the ﬁnancial support from the Singapore National Research Foundation (NRF-CRP10-2012-06), China Jiangsu Specially Appointed Professor, the Chang Jiang Youth Scholars Program of China (Grant no. 51373033 and 11172064), the Key Grant Project of Chinese Ministry of Education (Grant no. 113027A), “The Fundamental Research Funds for the Central Universities” and “DHU Distinguished Young Professor Program” to Xiaohong Qin and the China Scholarship Council (CSC).

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FIGURES AND CAPTIONS


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Scheme 1. (a) the schematic illustration of the fabrication process of MNO-CNT-CNFFs. (b) mass production of Fe-NFF through free-surface electrospinning technique in 15 min, and the size of the prepared Fe-NFF is 180×80 cm. (c) the stabilized M-NFFs. (d) the MNO-CNTCNFFs obtained after the carbonization of M-NFFs with melamine.


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Figure 1. (a-b) Typical SEM images of FeNO-CNT-CNFF. (c-e) TEM images of FeNO-CNTCNFF. (f) XRD patterns of FeNO-CNT-CNFF. (g-k) corresponding elemental mapping images of C, O, Fe and N. Scale bar: (a) 1 µm, (b) 100 nm, (c) 50 nm, (d) 10 nm, (e) 5 nm, (g)-(k) 100 nm.


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Figure 2. (a) a digital picture of double knotted FeNO-CNT-CNFF-800 showing excellent flexibility. (b) the stress-strain curve of FeNO-CNT-CNFFs. (c-e) mass-transport kinetics of the adsorbed electrolyte: (c) initial, (d) after 5 min, and (e) after 10 min. (f-g) The water contact angle of the samples.


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Figure 3. (a) LSV curves of the as-prepared catalysts for ORR at 1600 rpm in 0.1 M KOH. (b) CV curves of FeNO-CNT-CNFF-800 and Pt/C in N2-saturated and O2-saturated 0.1M KOH at 5 mV s-1. (c) ORR Tafel plots of FeNO-CNT-CNFF-800 and Pt/C. (d) The number of electron transfer and the H2O2 yield (%) of FeNO-CNT-CNFF-800 and Pt/C.


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Figure 4. (a) LSV curves of the catalysts for OER at 1600 rpm in 0.1 M KOH. (b) OER Tafel plots. (c) Bifunctional activities for different catalysts in 0.1M KOH at 1600 rpm. (d) The bifunctional activity of different catalysts in 1 M LiOH at 1600 rpm.


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Figure 5. (a) Raman spectra of FeNO-CNT-CNFFs. (b) The deconvoluted N 1s spectra of FePc and FeNO-CNT-CNFFs. (c) The Fe 2p narrow scan spectra of FePc and FeNO-CNT-CNFFs


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Figure 6. (a) Schematic illustration of rechargeable hybrid Li-air battery. (b) A digital photo shows that FeNO-CNT-CNFF-based hybrid Li-air battery lighted up a green LED (> 3 V). (c) Comparison of voltage gap between charge-discharge voltage plateaus of hybrid Li-air batteries with different catalysts. (d) Charge-discharge curves of FeNO-CNT-CNFF-based hybrid Li-air battery at different current densities. (e) The overpotential change of FeNO-CNT-CNFF-based hybrid Li-air battery at different cycle number. (f) The cycling performance of hybrid Li-air battery using FeNO-CNT-CNFF as the air electrode tested in the open-air condition.


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Graphical abstract：


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(Fe, Co, Ni) as Bifunctional Oxygen Catalysts for ... - ACS Publications

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