Electrospun MOF-based FeCo nanoparticles embedded in nitrogen

Subscriber access provided by Karolinska Institutet, University Library

Article

Electrospun MOF-based FeCo nanoparticles embedded in nitrogen-doped mesoporous carbon nanofibers as an efficient bifunctional catalyst for oxygen reduction and oxygen evolution reactions in zinc-air batteries Lijuan Yang, Shizhan Feng, Guancheng Xu, Bei Wei, and Li Zhang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b06624 • Publication Date (Web): 01 Feb 2019 Downloaded from http://pubs.acs.org on February 2, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 34 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

ACS Sustainable Chemistry & Engineering

Electrospun MOF-based FeCo nanoparticles embedded in nitrogen-doped mesoporous carbon nanofibers as an efficient bifunctional catalyst for oxygen reduction and oxygen evolution reactions in zinc-air batteries Lijuan Yang, Shizhan Feng, Guancheng Xu*, Bei Wei, Li Zhang

Key Laboratory of Energy Materials Chemistry, Ministry of Education; Key Laboratory of Advanced Functional Materials, Autonomous Region; Institute of Applied Chemistry, Xinjiang University, Shengli Road No. 666, Urumqi, 830046 Xinjiang, P. R. China.

* Corresponding author. E-mail: [email protected]. Tel./Fax: +86-991-8580586

1

ACS Paragon Plus Environment

ACS Sustainable Chemistry & Engineering 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

Abstract Low-price, high-performance and strong-stability electrocatalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are highly significant in the application of clean energy devices like rechargeable zinc-air batteries and renewable fuel cells. In this paper, a Prussian blue analogue Co3[Fe(CN)6]2·nH2O (Co-Fe PBA), as a well-known member in metal-organic framework (MOF) family, was electrospun into polyacrylonitrile (PAN) nanofibers to obtain composite Co-Fe PBA@PAN nanofibers. Nitrogen-doped carbon nanofibers encapsulated FeCo alloy nanoparticles (FeCo-NCNFs-Ts, T = 700, 800, 900°C) were synthesized by pyrolysising Co-Fe PBA@PAN precursor at different temperatures under an argon atmosphere. The effects of different calcination temperatures and mass ratios between Co-Fe PBA and PAN on ORR/OER catalytic activity were explored. Among FeCo-NCNFs-Ts, FeCo-NCNFs-800 had the highest bifunctional electrocatalytic performance with a lower reversible overvoltage of 0.869 V between ORR (E1/2) and OER (Ej =10 mA cm-2), excellent stability and methanol durability, which even exceeded those of Pt/C and RuO2. The superb bifunctional activity for FeCo-NCNFs-800 was comparable to that of non-noble electrocatalysts reported in recent literatures. Moreover, zinc-air battery based on FeCo-NCNFs-800 air-cathode catalyst had a high power density of 74 mW cm-2 and strong cycling stability (125 cycles for 42 h), which can be comparable to Pt/C-RuO2 zinc-air battery. The impressive bifunctional activity on ORR and OER for FeCo-NCNFs-800 catalyst in zinc-air battery can be attributed to the synergistic effects of one-dimensional fibrous structure, FeCo alloy nanoparticles, Co-N (pyridinic-N) active sites, and numerous mesopores. Keywords: Prussian blue analogue, Electrospinning, Carbon nanofibers, Oxygen reduction reaction, Oxygen evolution reaction, Zn-air battery. Introduction Large quantities of fossil fuels are used widely in the industrial field, resulting in environmental pollution; thus, clean and sustainable energy sources must be developed to substitute traditional fossil fuels and satisfy environmental demands.1 Until now, metal-air batteries (especially zinc-air batteries) and fuel cells have attached great attention as up-and-coming energy sources because they can produce electricity with no pollutants or toxic by-products.2-4 Particularly, Zn-air batteries have stable discharge voltage and long-term service life. Furthermore, oxygen reduction and 2


Page 2 of 34

Page 3 of 34 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


evolution reactions (ORR and OER ) are two crucial reaction processes in zinc-air batteries.5-7 Hence, exploring efficient ORR/OER bifunctional catalysts is significant to the development of zinc-air batteries. Currently, precious metallic Pt-based catalysts are recognized as an optimal ORR catalyst, whereas they have some drawbacks such as exorbitant price, poor operation stability, sensitive to methanol fuel, and CO poising. Moreover, Pt-based catalysts show inferior OER activity.8 IrO2/RuO2-based catalysts for OER have also been well studied and optimized, but they display poor ORR performance and low overall energy conversion efficiency.9,10 These factors have hindered the wide utilization of energy devices (zinc-air batteries, fuel cells) in the field of business. Therefore, developing a promising non-precious metal bifunctional electrocatalyst with low cost and high ORR/OER performance to substitute Pt-based and IrO2/RuO2-based catalysts is critical for the commercialization of clean energy devices. So far, among various non-noble metal catalysts, transition metal (e.g., Fe and Co) nitrogen co-doped carbon compositions (M-N-Cs) have been demonstrated as ideal electrocatalysts due to the synergetic effects between metal and nitrogen along with porous structures and graphitic carbons.11-13 Most researchers have contended that M-Nx are actively catalytic sites for M/N co-doped carbons; metal atoms on the carbon surface, even with a small amount, may facilitate ORR/OER activity. Simultaneously, the electronic structure of carbon can be changed by the metal in the carbon matrix, which is largely responsible for ORR/OER catalytic activity.14 M-N-C catalysts are typically synthesized via high-temperature heat treatment of precursors consisting of metal salts, carbon and nitrogen sources in a N2 or Ar atmosphere.10,15,16 Han et al. reported the synthesis of a dual-metallic FeCo alloy activated N-doped graphitic carbon layers by utilizing cobalt nitrate, iron sulfate and cyanamide as precursors. By adjusting different molar rations between cobalt and iron salts, the optimized NC/Fe8Co2 catalyst exhibited outstanding ORR activity, high stability and methanol durability.17 The strategy of preparing M-N-C electrocatalysts for ORR/OER by direct carbonization of separate transition metal salts, nitrogen and carbon precursors is flexible and economical, but there is still some issues including uncontrolled structural uniformity, untunable pore structures, and low-density active sites.14,18 Therefore, rationally designing the structure of precursors is critical to acquire M-N-C catalysts with uniform structure, high density and available active centers. Metal-organic frameworks (MOFs), built by metal ions and N/C-contained organic linkers, have 3



received extensive attention as suitable precursors to acquire M-N-C ORR/OER catalysts due to their porous three-dimensional structures, high stability, and wide variety.19-21 Prussian blue (PB) or Prussian blue analogues (PBA) have shown promise in obtaining M-N-C ORR/OER catalysts via a direct pyrolysis in inert gases.22-24 The PB/PBA precursor intrinsically contains high density of M-Nx (nitrogen-coordinated metal) sites, which is a key factor for deriving ORR/OER catalysts. Porous M-N-C materials with highly graphitic structure were prepared by directly pyrolysising PB/PBA. Zhou et al. reported a Fe, N co-doped graphitic carbon bulb obtained by a direct calcination of PB. As Fe moieties in PB facilitate the formation of graphitic structure and provide metal active sites (Fe-Nx) during pyrolysis, the catalyst exhibited striking ORR performance in alkaline electrolyte, which was accessible to that of commercial Pt/C (20 wt%).25 Cai et al. synthesized FeCo alloy nitrogen-doped carbon (FeCo@NC) through a simple calcination of Co-Fe PBA. Electrochemical tests showed the obtained FeCo@NC can serve as an ORR/OER bifunctional catalyst in zinc-air battery with desirable activity and stability in alkaline media.26 Based on these research results, great accomplishments have been realized in PB/PBA-derived M-N-C ORR/OER catalysts. However, PB/PBA contains fewer carbon and nitrogen elements, and the calcined products exhibit low nitrogen/carbon contents, leading to few exposed active sites. Thus, challenges persist in PB/PBA-derived M-N-C ORR/OER catalysts, warranting further investigation to overcome the drawbacks and maximize the performance of M-N-C ORR/OER electrocatalysts based on PB/PBA. Currently, different carbon materials (e.g., graphene, Vulcan XC-72, and carbon nanotubes) supporting PB/PBA have been studied extensively, and the resultant catalysts achieve enhanced catalytic performance.27-29 For instance, Xue et al. reported the synthesis of iron nitrogen-doped porous carbons (Fe-C/NG) by directly calcinating PB/graphene oxide composite; the Fe-C/NG catalyst displayed excellent ORR catalytic property with superior stability and methanol durability in acidic and alkaline electrolyte solutions.30 Therefore, the strategy of a carbon-supported composite with PB/PBA is a feasible and efficient avenue to maximize electrocatalytic performance of M-N-C catalysts. In recent years, carbon nanofibrous materials have been applied in electrochemical energy storage devices due to their unique merits, such as a one-dimensional nanostructure, high porosity, and excellent conductivity.31-33 Carbon nanofibrous materials are typically prepared by a facile and cost-efficient electrospinning method along with heat treatment. As to electrospinning carbon 4


Page 4 of 34

Page 5 of 34 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


fibers, polyacrylonitrile (PAN) is the most widely employed precursor material because of its commercial viability, high carbon yields, abundant nitrogen sources, and easily to obtain uniform carbon fibers.34-36 Based on these findings, we designed carbon fibers as PBA support to obtain PBA@PAN fibrous composites. The resultant catalyst should promote catalytic performance due to more exposed catalytic active sites, good conductivity, and abundant pore structures. In this article,

nitrogen-doped

carbon

nanofibers

encapsulated

FeCo

alloy

nanoparticles

(FeCo-NCNFs-Ts) were synthesized by electrospinning Co-Fe PBA into PAN fibers and further annealing at different high temperatures (700, 800, 900°C) under Ar atmosphere. Evaluated as a novel bifunctional catalyst, FeCo-NCNFs-800 exhibited more notable bifunctional catalytic performance and stability than commercial Pt/C and RuO2 toward ORR and OER. More attractively, FeCo-NCNFs-800 can be employed as a promising air-cathode catalyst in zinc-air battery with excellent performance and cycling stability, comparable to Pt/C-RuO2 zinc-air battery. One-dimensional fibrous structure, FeCo alloy nanoparticles, abundant mesoporous structures, and sufficient Co-N (pyridinic-N) catalytic active sites are responsible for bifunctional catalytic activity in zinc-air batteries. Experimental section Preparation of Co-Fe PBA@PAN nanofibers Co-Fe PBA@PAN nanofibers were synthesized by an electrospinning method. Typically, 1 g of PAN was dissolved in 10 mL of DMF under vigorous stirring to obtain a PAN solution. Then, 0.25 g of Co-Fe PBA powders were added into the PAN solution under stirring followed by sonication for at least 30 min. The precursor solution was stirred continuously until a homogeneously dispersed solution was formed at ambient temperature. Then, the formed solution was poured into a 10 mL plastic syringe outfitted with a stainless steel tip with 0.70 mm inner diameter, and was electrospun at a distance of 18 cm between the rolling aluminum foil collector and the tip of needle at room temperature. The voltage and flow rate were 20 kV and 0.1 mm min-1 in the electrospinning process, respectively. Finally, Co-Fe PBA and PAN nanofibrous composite (Co-Fe PBA@PAN) was obtained after electrospinning for approximately 7–8 h. For comparison, pure PAN nanofibers without added Co-Fe PBA and Co-Fe PBA@PAN nanofibers with different Co-Fe PBA powder contents (0.15 g, 0.35 g, and 0.5 g) were prepared using the same experimental process. 5



Preparation of FeCo-NCNFs-Ts (T denotes pyrolysis temperature) As-prepared Co-Fe PBA@PAN nanofibers were placed flat on a carbon paper and transferred into a furnace, where they were calcined from room temperature to 280°C at a heating rate of 2°C min-1 for 1 h. Next, the temperature was increased to the target temperature (700, 800, and 900°C) at a heating rate of 5°C min-1 and held for 2 h under Ar atmosphere. After cooling naturally to room temperature, the resultant N-doped carbon nanofibers encapsulated FeCo alloy nanoparticles were labeled as FeCo-NCNFs-700, FeCo-NCNFs-800, and FeCo-NCNFs-900, respectively. For comparison, pure PAN nanofibers or Co-Fe PBA powders were carbonized directly using the same process at 800°C. The obtained catalysts were labeled as NCNFs-800 and FeCo-NC-800, respectively. Results and discussion Catalytic materials synthesis and characterizations Co-Fe PBA cubes were prepared by a facile and low-cost solution method (in supporting information).26,37 The X-ray diffraction (XRD) peaks of Co-Fe PBA precursor are aligned with the results from Cai et al.,26 elaborating the successful preparation of pure-phase Co-Fe PBA and exhibiting good crystallinity (Fig. S1a). The SEM image of Co-Fe PBA is presented in Fig. S1b, the Co-Fe PBA precursor exhibits uniform cubic shapes with an average diameter of roughly 200 nm. Co-Fe PBA@PAN nanofibrous composites were synthesized by electrospinning as-prepared Co-Fe PBA into PAN nanofibers, and followed by calcination in an Ar atmosphere at different temperatures to obtain black FeCo-NCNFs-Ts. Carbonization temperature of precursor is an important index for synthesizing highly active electrocatalysts toward ORR and OER. The thermogravimetric analysis (TGA) curves of Co-Fe PBA, PAN and Co-Fe PBA@PAN nanofibers precursors were plotted to identify a favorable calcination temperature (Fig. S2). Co-Fe PBA@PAN nanofibers exhibited highly thermal stability. A weight loss of 5.54% below 150°C was assigned to the removal of water and DMF molecules. As the temperature increased, Co-Fe PBA@PAN nanofibers underwent a rapid decomposition from 260 to 310ºC, and further carbonization and graphitization from 310 to 900ºC along with a weight loss of 35.46%. Additionally, TGA curves of Co-Fe PBA and PAN fibers revealed that the high heat stability of Co-Fe PBA@PAN composites was derived from that of Co-Fe PBA. In accordance with the TGA results, Co-Fe PBA@PAN nanofibers were pyrolyzed at temperatures of 6


Page 6 of 34

Page 7 of 34 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


700, 800, and 900ºC, respectively, using a simple two-step calcination process under high-purity Ar flow to fabricate nitrogen-doped carbon nanofibers encapsulated FeCo alloy nanoparticles named FeCo-NCNFs-Ts (T denotes carbonized temperature). Moreover, the optimum added content of Co-Fe PBA powders and suitable calcined temperature were determined to be 0.25 g and 800ºC for Co-Fe PBA@PAN nanofibers, respectively (Fig. S11). Co-Fe PBA or pure PAN fibers were calcined under the identical pyrolysis conditions at 800ºC for comparison, labeled as FeCo-NC-800 and NCNFs-800, respectively.

Fig. 1 (a) XRD patterns and (b) Raman spectra of FeCo-NCNFs-Ts. XRD was conducted to examine the crystalline phase of FeCo-NCNFs-Ts, FeCo-NC-800, and NCNFs-800 (Fig. 1a and Fig. S5a). All samples exhibit the (002) characteristic peak of graphitic carbon at 2θ = 26.40° (PDF#41-1487). In addition to NCNFs-800, FeCo-NCNFs-Ts and FeCo-NC-800 appear CoFe characteristic peaks at 2θ = 45.2° and 65.7°, corresponding to the crystalline planes of (110) and (200) (PDF#50-0795). For FeCo-NCNFs-Ts, by increasing the calcination temperature, the CoFe and (002) carbon peaks are clearly enhanced. This result indicated that the extent of crystallinity and graphitization increased with the growth and catalysis of FeCo alloy particles at higher temperatures.22,38 Furthermore, FeCo-NCNFs-Ts (especially FeCo-NCNFs-800 and FeCo-NCNFs-900) display obvious CoN characteristic peaks centered at 2θ = 43.68°, 50.86°, and 74.99° in line with (111), (200), and (220) crystalline planes (PDF#41-0943). Raman spectra of FeCo-NCNFs-Ts, FeCo-NC-800, and NCNFs-800 were measured to detect the graphitization degree and defective structure (Fig. 1b and Fig. S5b). Two distinct peaks appear for these materials centered around 1344 cm-1 (D band: disordered carbon) and 1581 cm-1 (G band: ordered sp2-graphitic carbon), respectively, and the ID/IG value (intensity

7



ratio) reflects the graphitic extent and defect sites. For FeCo-NCNFs-Ts, their ID/IG values were 0.99 (FeCo-NCNFs-700), 0.91 (FeCo-NCNFs-800), and 0.79 (FeCo-NCNFs-900), respectively. The graphitization degree of catalysts increased gradually with rising carbonization temperature. By contrast, the ID/IG values for FeCo-NC-800 and NCNFs-800 were 0.82 and 1.04, respectively. Remarkably, the graphitization degree of FeCo-NCNFs-800 was higher than that of NCNFs-800. This result suggests that the FeCo alloy can catalyze the graphitization of carbon well in the high-temperature pyrolysis process, concurring with the XRD analysis results.

8


Page 8 of 34

Page 9 of 34 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


Fig. 2 SEM images of (a, b) Co-Fe PBA@PAN nanofibers; (c, d) FeCo-NCNFs-700; (e, f) FeCo-NCNFs-800; (g, h) FeCo-NCNFs-900. The SEM image of Co-Fe PBA@PAN precursor exhibits a one-dimensional fibrous structure (Fig. 2a). The enlarged SEM image shows Co-Fe PBA cubes are obviously encapsulated within the fibers (Fig. 2b). Moreover, Fig. S3b reveals that the pure PAN nanofibers have smooth surface and 9



average size of about 550 nm, Co-Fe PBA is a cube with an average diameter of approximately 200 nm (Fig. S1b). Apparently, the size of Co-Fe PBA is smaller than that of PAN nanofibers; hence, Co-Fe PBA was successfully electrospun into the PAN nanofibers, and the cubic shape of Co-Fe PBA was well maintained during the electrospinning process. Through a carbonization process under a high-purity Ar atmosphere, as-prepared FeCo-NCNFs-Ts (T = 700, 800, and 900ºC) retained a web-like fibrous morphology encapsulating cubic protuberances (Fig. 2c–h). The diameter sizes of FeCo-NCNFs-Ts decreased to about 250 nm due to the shrinkage of PAN fibers during high-temperature carbonization. Thereby, cubes obviously protruded in the FeCo-NCNFs-Ts nanofibers (especially FeCo-NCNFs-900). The FeCo-NC-800 was obtained via heat treatment of Co-Fe PBA at 800ºC without PAN fibers, which do not retain the cubic shape of Co-Fe PBA and become aggregated nanoparticles (Fig. S3a). As evidenced by the TGA results (Fig. S2), the pyrolysis of PAN occurred prior to that of Co-Fe PBA. PAN firstly decomposed to form carbonized shells and coated Co-Fe PBA cubes during the heating process, further avoiding the damage of Co-Fe PBA cube. According to the SEM analysis, one-dimensional carbon fibers can provide sufficient contact areas between the electrolyte and reactant, enlarge exposed active catalytic sites, and offer an electron transport pathway over long fibers.39-41 Energy dispersive spectroscopy elemental mapping was carried out to detect the element distribution of FeCo-NCNFs-Ts (Fig. S4); the results confirmed that C, N, O, Co, and Fe elements were homogeneously distributed in the FeCo-NCNFs-800 carbon fibers.

10


Page 10 of 34

Page 11 of 34 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


Fig. 3 TEM images of (a) FeCo-NCNFs-800; (b) edge of FeCo-NCNFs-800 (inset: magnification image of the edge); (c) FeCo alloy nanoparticles; (d) HRTEM and lattice images of FeCo alloy nanoparticles (inset: lattice of single FeCo alloy nanoparticle). The nanofibrous structure of FeCo-NCNFs-800 was studied by TEM; the corresponding image exhibits web-like nanofibers with encapsulated cubes (Fig. 3a), implying that the cubic shapes of Co-Fe PBA precursor were successfully maintained during the calcination process, echoing the SEM results. The black spots in the cubes are FeCo alloy nanoparticles (Fig. 3b and c), which shows a crystal lattice spacing of 0.21 nm (in accordance with the [110] planes of CoFe) (Fig. 3d),42,43 indicating that Co and Fe ions were successfully reduced to bimetallic FeCo alloy by carbon during high-temperature pyrolysis. The FeCo alloy nanoparticles were protected by Co-Fe PBA-derived N-doped carbon in a limited space from agglomeration. Moreover, graphitic carbon layers on the exterior of cubes were formed by the catalysis of FeCo alloy with a crystal lattice 11



spacing of 0.33 nm (corresponding to the carbon [002] plane) (inset of Fig. 3b). Cubes were protected and separated by graphitic carbon layers, avoiding oxidization of FeCo alloy nanoparticles and catalyst corrosion in the alkaline electrolyte, which reinforced the high stability of catalyst.

Fig. 4 (a) N2 adsorption-desorption isotherms; (b) pore diameter distribution determined by BJH method of FeCo-NCNFs-Ts. N2 adsorption-desorption isotherms and corresponding pore size distribution patterns were plotted to assess the pore properties of FeCo-NCNFs-Ts (Fig. 4). As reference, NCNFs-800 and FeCo-NC-800 were characterized under the same conditions (Fig. S6). Pore size distribution curves were obtained using the BJH method. A clear type IV plot with a hysteresis loop appears in all as-synthesized catalysts, indicating the formation of mesoporous structures (Fig. 4a and Fig. S6a). Mesoporous structures are a key factor of offering more active sites for fast electrolyte/O2 diffusion and mass transportation in the electrochemical reaction over ORR/OER.44,45 Pore properties (surface area, total pore volume, pore size) of all catalysts were displayed in Table S1; the surface areas were calculated using the Brunauer-Emmett-Teller (BET) method. The BET surface area (63.209 m2 g-1) and total pore volume (0.146 cm3 g-1) of FeCo-NCNFs-800 were higher than those of FeCo-NCNFs-700 (37.552, 0.106) and FeCo-NCNFs-900 (48.628, 0.103). Furthermore, at a higher temperature of 900ºC, BET surface area and total pore volume decreased dramatically due to fiber shrinkage and pore collapse. In the pore size distribution (Fig. 4b), FeCo-NCNFs-Ts display one obvious peaks at approximately 3.82 nm, whereas the peak intensities of FeCo-NCNFs-700 and FeCo-NCNFs-900 are weaker than that of FeCo-NCNFs-800. Based on these results, FeCo-NCNFs-800 exhibited more porosity and clearly surpassed that of

12


Page 12 of 34

Page 13 of 34 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


FeCo-NCNFs-700 and FeCo-NCNFs-900. Moreover, Co-Fe PBA precursor mainly contributed to the large surface area and mesoporous structures of FeCo-NCNFs-800 compared with PAN carbon nanofibers due to the surface area (66.326 m2 g-1) and total pore volume (0.208 cm3 g-1) of FeCo-NC-800 being higher than those of NCNFs-800 (42.887, 0.056). Therefore, the catalyst precursor and calcined temperature appeared to exert prominent effects on the surface area and mesoporous structure.

Fig. 5 (a) Full spectrum; (b) C 1s spectra; (c) N 1s spectra; (d) schematic illustration of various N types in nitrogen-doped carbon materials; (e) Co 2p; (f) Fe 2p spectra of FeCo-NCNFs-800. 13



Chemical element types, contents, and bonding structures on the surface of as-prepared FeCo-NCNFs-Ts, FeCo-NC-800, and NCNFs-800 catalysts were probed by a X-ray photoelectron spectroscopy (XPS). As presented, the overall spectrum of FeCo-NCNFs-Ts and FeCo-NC-800 display the coexistence of C, N, O, Fe, and Co elements (Fig. 5a and Fig. S7). NCNFs-800 contains C, N, and O elements in the overall scan (Fig. S7). The relative atomic contents of C, N, O, Fe, and Co for FeCo-NCNFs-Ts, FeCo-NC-800, and NCNFs-800 were summarized in Table S2. Notably, for FeCo-NCNFs-Ts, the carbonization temperature substantially affected C and N contents. As the pyrolysis temperature elevating, atomic percentage contents of N (11.21%, 7.67%, 3.06%) decreased dramatically from FeCo-NCNFs-700 to FeCo-NCNFs-900, whereas the relative percentages of C gradually increased (75.83%, 80.34%, 83.18%). The results suggest that high temperature facilitates the graphitization of C and further enhances the electron conductivity of catalysts. As a comparison, among FeCo-NC-800, NCNFs-800, and FeCo-NCNFs-800, FeCo-NC-800 had a higher percentage of C contents (81.02%), whereas N was relatively lower (0.68%). On the contrary, NCNFs-800 had higher N and lower C relative contents (10.88%, 79.62%). Thus FeCo-NCNFs-800 contained considerable N and C percentage contents (7.67%, 80.34%) by combining their respective merits. The peak of O1s around 532.2 eV was observed in the full spectra, likely because it was easy to physically adsorb water or oxygen molecules on the surface of carbon materials during testing. The C1s spectrum reveal four peak positions at binding energies around 284.7, 286.0, 287.2, and 289.0 eV, aligning with the characteristic peaks of C-C, C-O, C=O, and -COO, respectively (Fig. 5b and Fig. S8a, c, e, g). A peak at 283.8 eV appears in the C1s spectrum of FeCo-NC-800 (Fig. S8e), confirming that a metallic chemical bond (M-N-C) was formed due to a strong force between metal and nitrogen elements.26,46 The M-N-C structure exhibits excellent catalytic properties for oxygen electrode reactions. The spectra of Co2p reveal five peaks around 779.0, 781.4, 785.7, 794.2, and 796.1 eV (Fig. 5e and Fig. S9b, d, f), in line with the characteristic peaks of Co3+2p3/2, Co2+2p3/2, the satellite peak, Co3+2p1/2, and Co2+2p1/2, respectively.47,48 The high-resolution Fe2p spectra reflect five peaks at about 710.9, 713.9, 716.9, 722.5, and 725.5 eV (Fig. 5f and Fig. S9a, c, e), assigned to Fe2+2p3/2, Fe3+2p3/2, the satellite peak, Fe2+2p1/2, and Fe+32p1/2, respectively.49,50 In the N1s spectrum (Fig. 5c and Fig. S8b, d, f, h), all as-synthesized catalysts show four different types of nitrogen, consisting of pyridinic-N (398.1–398.2 eV), pyrrolic-N (399.7–400.2 eV), graphitic-N (401.0–402.6 eV), and 14


Page 14 of 34

Page 15 of 34 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


oxidized-N (403.2–407.1 eV). The details of N configurations are schematically presented in Fig. 5d. Noticeably, pyridinic-N is at the margins of graphitic structures and can bond easily to metallic Co to form Co-N moieties as catalytic active sites.51 The binding energy of pyridinic-N is close to that of Co-N; thus, only a peak around 398.1 eV is considered pyridinic-N (Co-N) in the N1s spectrum.52,53 Graphitic-N is situated in the graphite carbon bonds with sp2 carbon, it can augment the conductivity of porous carbon. The relative proportions of various N bonding types were calculated based on the peak areas in the XPS spectrum, as illustrated in Table S3. For FeCo-NCNFs-Ts (T = 700, 800, 900ºC), pyridinic-N (48.03%, 41.64%, 30.43%), and pyrrolic-N (31.44%, 24.68%, 22.02%) contents decreased gradually with the increase of pyrolysis temperature. Graphitic-N elevated rapidly (13.21%, 24.56%, 39.69%); due to the poor heat stability of pyridinic- and pyrrolic-N, they could be transformed into graphitic-N at higher pyrolysis temperature. The existence of oxidized-N resulted from mild oxidation of pyridinic-N in the characterization process. By contrasting FeCo-NC-800 (38.21%) and NCNFs-800 (34.53%), FeCo-NCNFs-800 had the highest pyridinic-N amounts (41.64%), pyridinic-N demonstrates the largest binding energy that can improve the capacitance, and be helpful to electrocatalytic behavior. Above all, FeCo-NCNFs-800 offered a considerable balance point between the contents of pyridinic-N (41.64%) and graphitic-N (24.56%). Pyridinic-N (Co-N) and graphitic-N have been accepted as efficient active sites to favor remarkable ORR/OER electrocatalytic activity for Fe/Co-based nitrogen-doped carbon catalysts, and they contributed to the onset potential and limiting current density along with the electrical conductivity.54,55

15



ORR testing results

Fig. 6 (a) LSV curves of FeCo-NCNFs-Ts, commercial Pt/C, and RuO2 electrocatalysts at a rotation speed of 1600 rpm in 0.1 M O2-saturated KOH electrolyte; (b) LSV curves of FeCo-NCNFs-800 at different rotation speeds; (c) corresponding K-L curves of FeCo-NCNFs-800 at various potentials; (d) Tafel plots of FeCo-NCNFs-Ts and Pt/C electrocatalysts derived from the LSV curves at 1600 rpm; (e) current-time (I-t) chronoamperometric response curves of FeCo-NCNFs-800 and Pt/C catalysts at 0.665 V vs. RHE at 1600 rpm; (f) methanol tolerance experiments of FeCo-NCNFs-800 and Pt/C catalysts in 0.1 M O2-saturated KOH at 0.665 V vs. RHE at 1600 rpm by adding methanol. To assess the ORR catalytic performance of FeCo-NCNFs-Ts catalysts, cyclic voltammetry (CV) 16


Page 16 of 34

Page 17 of 34 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


was performed in KOH solution (0.1 M) saturated by either N2 or O2 at a scan rate of 50 mV s-1 (-0.2–1.2 V vs. RHE) (Fig. S10). CV curves of these catalysts show obvious cathodic oxygen reduction peaks in KOH electrolyte saturated by O2, but similar peaks are not seen in the N2-saturated KOH solution; this finding confirms the ORR activity of FeCo-NCNFs-Ts. In particular, the CV curve of FeCo-NCNFs-800 demonstrates a higher peak potential (0.797 V vs. RHE) than those of FeCo-NCNFs-700 (0.776 V) and FeCo-NCNFs-900 (0.787 V), indicating that FeCo-NCNFs-800 displayed better ORR activity among these catalysts. The rotating-disk electrode (RDE) testing was further utilized to demonstrate the catalytic activity of FeCo-NCNFs-Ts. Commercial Pt/C (20 wt%) and RuO2 were tested with the same conditions as comparison. Fig. 6a shows the LSV curves of catalysts in O2-saturated KOH (0.1 M) at 1600 rpm with a sweep rotation of 5 mv s-1 (-0.035–1.0 V vs. RHE). The onset and half-wave potentials (0.907, 0.817 V vs. RHE) of FeCo-NCNFs-800 outperform those of Pt/C (0.905, 0.781 V), FeCo-NCNFs-900 (0.899, 0.806 V), and FeCo-NCNFs-700 (0.864, 0.77 V). The ORR catalytic activity of commercial RuO2 is greatly inferior to these catalysts. Moreover, FeCo-NCNFs-800 presents a higher current density of 5.37 mA cm-2 at -0.035 V vs. RHE compared to Pt/C (4.72 mA cm-2), FeCo-NCNFs-700 (3.89 mA cm-2), and FeCo-NCNFs-900 (4.34 mA cm-2) vs. RHE. Their ORR electrocatalytic activities were summarized in Table S4. FeCo-NCNFs-800 displayed higher catalytic performance among these catalysts, being consistent with the CV results. In addition, CV and LSV comparison curves were constructed for FeCo-NCNFs-800, FeCo-NC-800, and NCNFs-800 (Fig. S16a and b), revealing that FeCo-NCNFs-800 demonstrated superior behavior on ORR compared to FeCo-NC-800 and NCNFs-800. Although FeCo-NC-800 contained large amounts of FeCo alloy and a higher degree of graphization, fewer carbon and nitrogen species led to poor ORR catalytic performance. On the contrary, NCNFs-800 offered abundant C, N moieties, and a one-dimensional fibrous structure, but the lack of FeCo alloy and Co-N active moieties resulted in inferior ORR catalytic behavior. The above facts revealed that the FeCo alloy and nitrogen-doped carbon nanofibers play a crucial part in the provision of effective active sites (Co-N), pore structures, large surface area, and high conductivity, and further improving the ORR activity of catalyst. ORR kinetics of FeCo-NCNFs-Ts and Pt/C catalysts were further analyzed by Tafel plots derived from their LSV curves at 1600 rpm. As seen in Fig. 6d, the Tafel slopes of FeCo-NCNFs-700, FeCo-NCNFs-800, FeCo-NCNFs-900, and Pt/C were calculated to be 82, 60, 17



Page 18 of 34

65, and 94 mV dec-1, respectively. Excitingly, all Tafel slopes of FeCo-NCNFs-Ts were smaller than that of Pt/C catalyst, verifying that FeCo-NCNFs-Ts catalysts (especially FeCo-NCNFs-800) had faster first-electron transfer (rate-determining step) and a stable dynamic process during electrochemical ORR. To evaluate the ORR diffusion reaction dynamics of FeCo-NCNFs-800 in detail, RDE measurements were carried out at different rotation speeds (400 to 2025 rpm) (Fig. 6b, Fig. S12, S13a, c, e and S17). With the increment of rotation speed, the current density elevated gradually, which can be ascribed to a shorter diffusion distance and enhanced mass transport rate (diffusion of O2 and electrolyte) at the electrode surface with high speeds. Furthermore, the reduction currents of catalysts were determined by rotation rates, indicating that the current was primarily kinetically controlled. The Koutecky-Levich (K-L) curves stemmed from the LSV plots of FeCo-NCNFs-Ts and Pt/C at various scan rates were plotted at different potentials (vs. RHE) to calculate their number of electron transfers (n), where n is integral to evaluating ORR activity, it was decided by the slopes of K-L plots and the equation. All K-L curves (J-1 vs. ω-1/2) of FeCo-NCNFs-Ts and commercial Pt/C at different potentials show good linearity and similar slopes (Fig. 6c and Fig. S13b, d f), confirming that FeCo-NCNFs-Ts and Pt/C possessed a stable dynamic process and first-order reaction kinetics. The average value of n were approximately 3.67 (FeCo-NCNFs-700), 3.91 (FeCo-NCNFs-800), 3.82 (FeCo-NCNFs-900) and 3.82 (Pt/C) respectively, approaching a four-electron reaction pathway and verifying that water was the main product during the ORR catalytic process. Apart from high catalytic activity and 4-electron selectivity, the stability of FeCo-NCNFs-800 and Pt/C

catalysts

were

also

implemented

by

chronoamperometric

methods

(Fig.

6e).

FeCo-NCNFs-800 exhibits stronger stability than commercial Pt/C catalyst. After continuous testing around 20000 s, FeCo-NCNFs-800 displays relative current retention of 81.30%. Comparatively, Pt/C catalyst retained only 69.69% of the original current density. An unique one-dimensional fibrous structure and high graphitization degree led to better stability of FeCo-NCNFs-800. Furthermore, the tolerance of ORR catalysts against contaminant poisoning (e.g., methanol) play a critical role in electrocatalytic application. Hence, the methanol durability of FeCo-NCNFs-800 and Pt/C were assessed by chronoamperometric responses at 1600 rpm (Fig. 18


Page 19 of 34 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


6f). The FeCo-NCNFs-800 catalyst demonstrates no obvious change when methanol was injected into O2-saturated KOH solution (0.1 M ), whereas Pt/C is vulnerable to methanol poisoning and exhibits a rapid shift under the same conditions. The result verified that FeCo-NCNFs-800 was more tolerant to methanol poisoning and a higher selectivity for ORR compared to Pt/C catalyst.

Fig. 7 (a) RRDE curves of FeCo-NCNFs-800 in 0.1 M O2-saturated KOH solution at a rotation rate of 1600 rpm; (b) electron transfer number (n) and H2O2 yield of FeCo-NCNFs-800 calculated by RRDE curves at various potentials. The ORR catalytic pathway and mechanism were further investigated and analyzed using the RRDE technique. Fig. 7a shows the tested current densities on ring and disk electrodes in 0.1 M KOH electrolyte (scan rate: 5 mV s-1 at 1600 rpm). The collected current at the ring electrode corresponds to the yields of H2O2. According to the disk and ring current, the values of n and H2O2 yield (%) could be estimated directly. For FeCo-NCNFs-800, the H2O2 yield during ORR was quite low (< 2%), and the average value of n was close to 4.00 within the same potential range (Fig. 7b). This result indicates that oxygen was reduced to water primarily through an ideal 4-electron reduction pathway in line with the K-L analysis.

19



OER testing results

Fig. 8 (a) LSV curves of FeCo-NCNFs-Ts, Pt/C and RuO2 in 0.1 M O2-saturated KOH solution at 1600 rpm; (b) Tafel curves of FeCo-NCNFs-Ts and RuO2 electrocatalysts; (c) cyclic voltammograms of FeCo-NCNFs-800 at scan rates from 2 mV s-1 to 10 mV s-1; (d) current densities of FeCo-NCNFs-Ts vs. different scan rates curves at 1.145 V; (e) LSV curves of FeCo-NCNFs-800 before and after 1000 cycles; (f) Nyquist plots of FeCo-NCNFs-Ts at 1.7 V vs. RHE. Based on the ORR testing results of as-synthesized catalysts, FeCo-NCNFs-800 exhibited remarkable ORR catalytic performance. Surprisingly, in addition to high ORR performance, the 20


Page 20 of 34

Page 21 of 34 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


FeCo-NCNFs-800 catalyst also demonstrated good OER performance. LSV plots of FeCo-NCNFs-Ts, Pt/C and RuO2 catalysts were curved at a RDE in O2-saturated 0.1 M KOH solution from 0.1 to 1.2 V vs. Ag/AgCl (Fig. 8a). LSV polarization curves of all catalysts display the increment of current densities with increasing potentials during the OER catalytic process. In particular, the FeCo-NCNFs-800 catalyst exhibits a higher current density, and a lower operating voltage (1.686 V vs. RHE) at a current density of 10 mA cm-2 compared with FeCo-NCNFs-700 (1.741V), FeCo-NCNFs-900 (1.733 V), and Pt/C (1.824 V). According to the comparison results, FeCo-NCNFs-800 displayed better OER catalytic performance than that of Pt/C catalyst, but which was inferior to that of commercial RuO2 (1.523 V). The OER activity of FeCo-NCNFs-800 catalyst was comparable to reported non-noble electrocatalysts in recent literatures (Table S6). To clarify the remarkable electrocatalytic activity of FeCo-NCNFs-800, the OER performance of FeCo-NC-800 and NCNFs-800 were tested for reference under the same conditions (Fig. S16c). Their catalytic activities toward OER are far worse than that of FeCo-NCNFs-800, indicating that a synergistic effect of the FeCo alloy and nitrogen-doped carbon fibers was highly important in the catalytic reaction. In addition, Tafel plots based on LSV curves at 1600 rpm were used to examine the OER catalytic kinetics for FeCo-NCNFs-Ts, and RuO2 catalysts (Fig. 8b). The Tafel slope of FeCo-NCNFs-800 (105.48 mv dec-1) is lower compared to that of FeCo-NCNFs-700 (282.39 mv dec-1) and FeCo-NCNFs-900 (275.34 mv dec-1), approximating that of commercial RuO2 (100.8 mv dec-1). This finding suggests that the FeCo-NCNFs-800 catalyst was more favorable for OER reaction kinetics and delivered good catalysis performance of OER. Electrochemical double-layer capacitance (Cdl) is a key parameter of OER, used as an index to evaluate the active electrochemical surface areas of catalysts; the value of Cdl is large, demonstrating that catalysts could expose more active sites to promote electrocatalytic oxygen production. Thus, the CV curves of FeCo-NCNFs-Ts were recorded at various scan speeds (2, 4, 6, 8, and 10 mv s-1) with a narrow potential range (Fig. 8c and Fig. S14). Based on the CV plots, the current densities of catalysts vs. different scan speeds at 1.145 V vs. RHE were plotted (Fig. 8d); they exhibit a linear relationship, and the slopes of these lines respectively reflect the Cdl values of different catalysts. Cdl of FeCo-NCNFs-800 was 37 mF cm-2, higher than that of FeCo-NCNFs-700 (21 mF cm-2) and FeCo-NCNFs-900 (31 mF cm-2). Thus the FeCo-NCNFs-800 catalyst exposed more active sites to enhance its OER catalytic activity. 21



As an OER catalyst, long-term durability is another critical parameter for OER activity, which was examined using LSV measurements in O2-saturated KOH solution (Fig. 8e and Fig. S15). The FeCo-NCNFs-800 catalyst only shows a slight potential shift around 0.032 V (vs. RHE, Ej = 10 mA cm-2 ) after 1000 cycles. FeCo-NCNFs-700 and FeCo-NCNFs-900 shift approximately 0.077 V and 0.040 V at the same environment, confirming that the FeCo-NCNFs-800 possessed higher stability. The FeCo alloy in FeCo-NCNFs-Ts was protected by carbon in nitrogen-doped carbon fibers, which was vital to high stability in the as-prepared electrocatalysts. The charge transfer process of FeCo-NCNFs-Ts catalysts was analyzed via electrochemical impedance spectroscopy (EIS) measurements at 1.7 V vs. RHE (Fig. 8f). Impedance data were acquired using an equivalent circuit (inset of Fig. 8f). Rs, Cd1, and Rct represent the solution resistance, constant phase element, and inter-facial charge transfer resistance, respectively. Rct is determined by the diameter of semicircle in EIS Nyquist plots. The Rct value of FeCo-NCNFs-800 (23.78 Ω) is lower than that of FeCo-NCNFs-700 (57.25 Ω) and FeCo-NCNFs-900 (42.21 Ω), demonstrating the highest charge transfer capability and fastest electron transport kinetics for FeCo-NCNFs-800 catalyst during OER. Bifunctional ORR and OER catalytic activities

Fig. 9. LSV curves of different catalysts showing the bifunctional ORR/OER activities in 0.1 M KOH at 1600 rpm. The bifunctional catalytic activities of FeCo-NCNFs-Ts, Pt/C, and RuO2 catalysts were further studied based on the difference in potential between OER (Ej = 10 mA cm-2 represents the potential of current density at 10 mA cm-2) and ORR (E1/2 represents the half-wave potential) (Fig. 9), where ΔE = Ej – E1/2 termed reversible overvoltage. The ΔE values of these catalysts were 22


Page 22 of 34

Page 23 of 34 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


listed in Table S5. FeCo-NCNFs-800 exhibited a smaller ΔE value (0.869 V) compared with FeCo-NCNFs-700 (0.971 V), FeCo-NCNFs-900 (0.927 V), Pt/C (1.043 V), and RuO2 (1.051 V). Comparison results verified that the FeCo-NCNFs-800 catalyst had better bifunctional activity for ORR and OER. Noticeably, the superb bifunctional activity for FeCo-NCNFs-800 was comparable to that of catalytic materials reported in the literatures (Table S6). This fascinating ORR and OER bifunctional electrocatalytic performance may have resulted from Co-N moieties embedded in the graphitic carbon layer as a catalytic active site and porous structures to bolster efficient contact between the reactant and catalyst. Most importantly, a one-dimensional carbon nanofibrous framework provided a conductive channel to accelerate electron transport. The FeCo alloy can catalyze the graphitization of carbon to achieve high electric conductivity and charge transfer ability. The enriched mesopore structure in carbon nanofibers can offer a proper specific surface area to promote diffusion of O2 and electrolytes. Overall, these advantages collectively improved the catalytic activity of FeCo-NCNFs-800 on ORR and OER. Rechargeable zinc-air battery testing results

Scheme 1 Schematic illustration of Zn-air battery assembly. The FeCo-NCNFs-800 catalyst had outstanding bifunctional electrocatalytic performance toward ORR and OER, it was highly expected that FeCo-NCNFs-800 could be applied in rechargeable zinc-air batteries. The activity of FeCo-NCNFs-800 as an air-cathode catalyst in zinc-air battery was hence evaluated. As shown in Scheme 1, a homemade zinc-air battery was constructed with a polished zinc sheet as the anode, and FeCo-NCNFs-800 catalyst-loaded hydrophobic carbon cloth 23



as the air cathode along with 6 M KOH solution with 0.2 M zinc acetate as the electrolyte. Pt/C-RuO2 (mass ratio of Pt/C: RuO2 = 1:1) was operated under identical conditions as a reference.

Fig. 10. (a) Digital image of assembled zinc-air battery using FeCo-NCNFs-800 as catalyst to display open-circuit voltage of 1.48 V; (b) LEDs powered by two assembled zinc-air batteries in series; (c) charge and discharge polarization curves; (d) discharge polarization curves and corresponding power density of zinc-air batteries using Pt/C-RuO2 and FeCo-NCNFs-800 as catalysts. Fig. 10a shows that the zinc-air battery with FeCo-NCNFs-800 can afford a high open-circuit voltage of 1.48 V, which is quite close to the theoretical value of a zinc-air battery in an ideal state. Two batteries were jointed in series to successfully turn on a light-emitting diode (LED) light (Fig. 10b), indicating that FeCo-NCNFs-800 based zinc-air device has potential practical applications in clean energy storage and conversion. Fig. 10c displays the charge-discharge polarization curves of zinc-air batteries based on FeCo-NCNFs-800 and Pt/C-RuO2 catalysts, respectively. The over-potential (discharge-charge voltage gap) of zinc-air battery with FeCo-NCNFs-800 was 0.87 V (2.03-1.16=0.87 V), slightly larger than that of Pt/C-RuO2 zinc-air battery (1.99-1.22 = 0.77 V);

24


Page 24 of 34

Page 25 of 34 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


this result implied that the zinc-air battery made by FeCo-NCNFs-800 catalyst possessed outstanding ORR and OER bifunctional catalytic performance. Discharge polarization and corresponding power density plots of zinc-air batteries with FeCo-NCNFs-800 and Pt/C-RuO2 catalysts are depicted in Fig. 10d. The zinc-air battery of FeCo-NCNFs-800 catalyst shows high peak power density (74 mW cm-2) and large current density (148 mA cm-2) at 0.5 V, approaching those of the zinc-air battery with Pt/C-RuO2 (88 mW cm-2 , 148 mA cm-2).

Fig. 11. Long-term cycling stability at 10 mA cm-2 of rechargeable Zn-air batteries. Cycling durability and rechargeability are key indices to assess zinc-air batteries. Therefore, the long-term cycling stability (reaching a current density of 10 mA cm-2) of rechargeable zinc-air batteries with FeCo-NCNFs-800 and Pt/C-RuO2 catalysts were tested. Fig. 11 illustrates the cyclic voltage curves of 2500 min along with the amplified cyclic voltage curves at 50 min (left) and 2500 min (right) in zinc-air batteries (inset of Fig. 11). The initial over-potential of FeCo-NCNFs-800 zinc-air battery is 0.88 V (2.07-1.19 = 0.88 V), which increases to 0.99 V (2.21-1.22 = 0.99 V) after cycling for 2500 min. The over-potential elevated relatively by 11%. This superior activity surpassed that of Pt/C-RuO2 (14%), and many other reported zinc-air batteries with transition metal and/or carbon catalysts.56-58 Importantly, at before and after cycling for 2500 min, the current efficiency of zinc-air battery with the FeCo-NCNFs-800 catalyst was 57.4% and 55.2%, respectively, with only 2.2% attenuation, less than that of Pt/C-RuO2 zinc-air battery (58.1%-53.3% = 4.8%). These results confirm that the zinc-air battery using FeCo-NCNFs-800 catalyst had better long-term cycling stability in discharge–charge processes. Above all, the superior bifunctional electrocatalytic activity and stability of FeCo-NCNFs-800 catalyst in zinc-air battery can be attributed to several key points: (1) one-dimensional mesoporous 25



nanofiber can provide a channel to promote mass diffusion, charge transportation, and higher electrocatalytic capability. (2) The FeCo bimetallic alloy can catalyze carbon graphitization at high temperature to augment electron conductivity and mitigate catalyst corrosion in alkaline electrolyte. (3) Mesoporous structures with a considerable surface area, which can provide more channels, enlarge the contact area of catalyst and electrolyte to expose more catalytic sites, reduce transportation resistance between reactants and active sites, realize fast O2 diffusion and mass transport to boost the electrocatalytic performance. (4) For proper pyridinic-N and graphitic-N, pyridinic-N can promote the onset potential by reducing O2 absorbed energy barrier and accelerating the first electron-transferred step of rate limiting, and graphitic-N can determine the limiting current density. (5) Co-N moieties obtained by Co coordinated with pyridinic-N, as another catalytic active site, prominently affected ORR and OER electrocatalytic activity. Conclusion In summary, a novel bifunctional ORR and OER catalyst, nitrogen-doped carbon nanofibers encapsulated FeCo alloy nanoparticles (FeCo-NCNFs-Ts), was fabricated by electrospinning MOF (Co-Fe PBA) into PAN nanofibers followed by calcination under a high-purity Ar atmosphere at different temperatures (700, 800, 900°C). The adding contents of Co-Fe PBA and carbonization temperature greatly influenced the ORR/OER performance of resultant FeCo-NCNFs-Ts. FeCo-NCNFs-800 showed outstanding ORR and OER bifunctional electrocatalytic activity coupled with excellent stability in alkaline solution with a smaller overall oxygen electrode activity index of ΔE = 0.869 V vs. RHE among all as-synthesized catalysts, even exceeding that of commercial Pt/C and RuO2. Results were comparable to those of other reported catalysts. Most importantly, when applied as an air-cathode catalyst of zinc-air battery, FeCo-NCNFs-800-based zinc-air battery displayed striking charge-discharge properties and long-time stability, comparable to those of a zinc-air battery with Pt/C-RuO2; hence, FeCo-NCNFs-800 can be an ideal air-cathode catalyst for potential application and development in zinc-air batteries. Such notable ORR and OER bifunctional activity on FeCo-NCNFs-800 catalyst for zinc-air battery primarily resulted from the synergistic effect of excellent structural properties, such as one-dimensional nanofibrous structures, FeCo alloy, Co-N (pyridinic-N) catalytic active sites, and mesoporous structures. As electrospinning polymers and MOF precursors are highly diverse, this work is expected to guide the synthesis of other MOF-based metal N-doped carbon nanofibers and their successful 26


Page 26 of 34

Page 27 of 34 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


applications in clean energy storage and conversion devices. Supporting Information XRD, TG, SEM, EDS, N2 adsorption-desorption, XPS, CVs, LSVs and other supplementary files of samples are available free of charge. (PDF) Conflicts of interest There are no conflicts of interest to declare. Acknowledgements This work was financially supported by China National Natural Science Foundation (grant numbers 21661029). Notes and references (1) Chu, S.; Majumdar, A. Opportunities and challenges for a sustainable energy future. Nature 2012, 488, 294−303, DOI 10.1038/nature11475. (2) Li, Y. G.; Lu, J. Metal-air batteries: will they be the future electrochemical energy storage device of choice. ACS Energy Lett. 2017, 2, 1370−1377, DOI 10.1021/acsenergylett.7b00119. (3) Jiao, Y.; Zheng, Y.; Jaroniec, M.; Qiao, S. Z. Design of electrocatalysts for oxygen- and hydrogen-involving energy conversion reactions. Chem. Soc. Rev. 2015, 44, 2060−2086, DOI 10.1039/C4CS00470A. (4) Han, X. P.; Wu, X. Y.; Zhong, C.; Deng, Y. D.; Zhao N. Q.; Hu, W. B. NiCo2S4 nanocrystals anchored on nitrogen-doped carbon nanotubes as a highly efficient bifunctional electrocatalyst for rechargeable

zinc-air

batteries.

Nano

Energy

2017,

31,

541−550,

DOI

10.1016/j.nanoen.2016.12.008. (5) Liu, S. H.; Wang, Z. Y.; Zhou, S.; Yu, F. J.; Yu, M. Z.; Chiang, C. Y.; Zhou, W. Z.; Zhao, J. J.; Qiu, J. S. Metal-organic-framework-derived hybrid carbon nanocages as a bifunctional electrocatalyst for oxygen reduction and evolution. Adv. Mater. 2017, 29, 1700874, DOI 10.1002/adma.201700874. (6) Osmieri, L.; Videla, A. H. A. M.; Specchia, S. Activity of Co-N multi walled carbon nanotubes electrocatalysts for oxygen reduction reaction in acid conditions. J. Power Sources 2015, 278, 296−307, DOI 10.1016/j.jpowsour.2014.12.080. (7) Liu, Q.; Zhu, J. H.; Zhang, L. W.; Qiu, Y. J. Recent advances in energy materials by electrospinning. Renew. Sust. Energ. Rev. 2018, 81, 1825−1858, DOI 10.1016/j.rser.2017.05.281. 27



(8) Li, J. K.; Alsudairi, A.; Ma, Z. F.; Mukerjee, S.; Jia, Q. Y. Asymmetric volcano trend in oxygen reduction activity of Pt and Non-Pt catalysts: in situ identification of the site-blocking effect. J. Am. Chem. Soc. 2017, 139, 1384−1387, DOI 10.1021/jacs.6b11072. (9) Chen, P. Z.; Zhou, T. P.; Xing, L. L.; Xu, K.; Tong, Y.; Xie, H.; Zhang, L. D.; Yan, W. S.; Chu, W. S.; Wu, C. Z.; Xie, Y. Atomically dispersed iron-nitrogen species as electrocatalysts for bifunctional oxygen evolution and reduction reactions. Angew. Chem. Int. Ed. 2017, 56, 610–614, DOI 10.1002/anie.201610119. (10) You, S. J.; Gong, X. B.; Wang, W.; Qi, D. P.; Wang, X. H.; Chen, X. D.; Ren, N. Q. Enhanced cathodic oxygen reduction and power production of microbial fuel cell based on noble-metal-free electrocatalyst derived from metal-organic frameworks. Adv. Energy Mater. 2016, 6, 1501497, DOI 10.1002/aenm.201501497. (11) Guo, X.; Jia, X. P.; Song, P.; Liu, J.; Li, E. L.; Ruan M. B.; Xu, W. L. Ultrahigh pressure synthesis of highly efficient FeNx/C electrocatalysts for the oxygen reduction reaction. J. Mater. Chem. A 2017, 5, 17470−17475, DOI 10.1039/C7TA05334G. (12) Ban, J. J.; Xu, G. C.; Zhang, L.; Xu, G.; Yang, L. J.; Sun, Z. P.; Jia, D. Z. Efficient Co-N/PC@CNT bifunctional electrocatalytic materials for oxygen reduction and oxygen evolution reactions based on metal-organic frameworks. Nanoscale 2018, 10, 9077−9086, DOI 10.1039/C8NR01457D. (13) Liu, K. H.; Zhong, H. X.; Meng, F. L; Zhang, X. B.; Yan, J. M.; Jiang, Q. Recent advances in metal-nitrogen-carbon catalysts for electrochemical water splitting. Mater. Chem. Front. 2017, 1, 2155−2173, DOI 10.1039/C7QM00119C. (14) Fu, S. F.; Zhu, C. Z.; Song, J. H.; Du D.; Lin, Y. H. Metal-organic framework-derived non-precious metal nanocatalysts for oxygen reduction reaction. Adv. Energy Mater. 2017, 1700363, DOI 10.1002/aenm.201700363. (15) Ma, X. J.; Chai, H.; Cao, Y. L.; Xu, J. Y.; Wang, Y. C.; Dong, H.; Jia, D. Z.; Zhou, W. Y. An effective bifunctional electrocatalysts: Controlled growth of CoFe alloy nanoparticles supported on N-doped carbon nanotubes. J. Colloid Interface Sci. 2018, 514, 656−663, DOI 10.1016/j.jcis.2017.12.081. (16) Hu, E. L.; Ning, J. Q.; He, B.; Li, Z. P.; Zheng, C. C.; Zhong, Y. J.; Zhang, Z. Y.; Hu, Y. Unusual formation of tetragonal microstructures from nitrogen-doped carbon nanocapsules with 28


Page 28 of 34

Page 29 of 34 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


cobalt nanocores as a bi-functional oxygen electrocatalyst. J. Mater. Chem. A 2017, 5, 2271−2279, DOI 10.1039/C6TA09943B. (17) Han, C.; Bo, X. J.; Liu, J.; Li, M.; Zhou, M.; Guo, L. P. Fe, Co bimetal activated N-doped graphitic carbon layers as noble metal-free electrocatalysts for high-performance oxygen reduction reaction. J. Alloys Compounds 2017, 710, 57−65, DOI 10.1016/j.jallcom.2017.03.241. (18) Wu, N.; Lei, Y. P.; Wang, Q. C.; Wang, B.; Han, C.; Wang, Y. D. Facile synthesis of FeCo@NC core–shell nanospheres supported on graphene as an efficient bifunctional oxygen electrocatalyst. Nano Res. 2017, 10, 2332−2343, DOI 10.1007/s12274-017-1428-3. (19) Lai, Q. X.; Zhu, J. J.; Zhao, Y. X.; Liang, Y. Y.; He, J. P.; Chen, J. H. MOF-based metal-doping-induced synthesis of hierarchical porous Cu-N/C oxygen reduction electrocatalysts for Zn-air batteries. Small 2017, 13, 1700740, DOI 10.1002/smll.201700740. (20) Ma, X.; Zhao, X.; Huang, J. S.; Sun, L. T.; Li, Q.; Yang, X. R. Fine Co nanoparticles encapsulated in a N-doped porous carbon matrix with superficial N-doped porous carbon nanofibers for efficient oxygen reduction. ACS Appl. Mater. Interfaces 2017, 9, 21747−21755, DOI 10.1021/acsami.7b02490. (21) Ahn, S. H.; Klein M. J.; Manthiram, A. 1D Co- and N-doped hierarchically porous carbon nanotubes derived from bimetallic metal organic framework for efficient oxygen and tri-iodide reduction reactions. Adv. Energy Mater. 2017, 7, 1601979, DOI 10.1002/aenm.201601979. (22) Wang, X. J.; Zou, L. R.; Fu, H.; Xiong, Y. F.; Tao, Z. X.; Zheng, J.; Li, X. G. Noble metal-free oxygen reduction reaction catalysts derived from prussian blue nanocrystals dispersed in polyaniline. ACS Appl. Mater. Interfaces 2016, 8, 8436−8444, DOI 10.1021/acsami.5b12102. (23) Wei, C.; Feng, Z. X.; Scherer, G. G.; Barber, J.; Shao-Horn, Y.; Xu, Z. J. Cations in octahedral sites: A descriptor for oxygen electrocatalysis on transition-metal spinels. Adv. Mater. 2017, 29, 1606800, DOI 10.1002/adma.201606800. (24) Feng, Y.; Yu, X. Y.; Paik, U. Formation of Co3O4 microframes from MOFs with enhanced electrochemical performance for lithium storage and water oxidation. Chem. Commun. 2016, 52, 6269−6272, DOI 10.1039/C6CC02093C. (25) Zhou, R. F.; Qiao, S. Z. Fe/N co-doped graphitic carbon bulb for high-performance oxygen reduction reaction. Chem. Commun. 2015, 51, 7516−7519, DOI 10.1039/C5CC00995B. (26) Cai, P. W.; Ci, S. Q.; Zhang, E. H.; Shao, P.; Cao, C. S.; Wen, Z. H. FeCo alloy nanoparticles 29



Page 30 of 34

confined in carbon layers as high-activity and robust cathode catalyst for Zn-air battery. Electrochim. Acta 2016, 220, 354−362, DOI 10.1016/j.electacta.2016.10.070. (27) Fu, G. T.; Liu, Y.; Chen, Y. F.; Tang, Y. W.; Goodenough, J. B.; Lee, J. M. Robust N-doped carbon aerogels strongly coupled with iron-cobalt particles as efficient bifunctional catalysts for rechargeable Zn-air batteries. Nanoscale 2018, 10, 19937−19944, DOI 10.1039/C8NR05812A. (28) An, T.; Ge, X. M.; Tham, N. N.; Sumboja, A.; Liu, Z. L.; Zong, Y. Facile one-pot synthesis of CoFe alloy nanoparticles decorated N-doped carbon for high-performance rechargeable zinc-air battery

stacks.

ACS

Sustainable

Chem.

Eng.

2018,

6,

7743−7751,

DOI

10.1021/acssuschemeng.8b00657. (29) Liu, Y. Y.; Wang, H. T.; Lin, D. C.; Zhao, J.; Liu, C.; Xie, J.; Cui, Y. A Prussian blue route to nitrogen-doped graphene aerogels as efficient electrocatalysts for oxygen reduction with enhanced active site accessibility. Nano Res. 2017, 10, 1213−1222, DOI 10.1007/s12274-016-1300-x. (30) Xue, J. H.; Zhao, L.; Dou, Z. Y.; Yang, Y.; Guan, Y.; Zhu, Z.; Cui, L. L. Nitrogen-doped 3D porous carbons with iron carbide nanoparticles encapsulated in graphitic layers derived from functionalized MOF as an efficient noble-metal-free oxygen reduction electrocatalysts in both acidic and alkaline media. RSC Adv. 2016, 6, 110820−110830, DOI 10.1039/C6RA24299E. (31) Wei, Q. L.; Xiong, F. Y.; Tan, S. S.; Huang, L.; Lan, E. H.; Dunn, B.; Mai, L. Q. Porous one-dimensional nanomaterials: Design, fabrication and applications in electrochemical energy storage. Adv. Mater. 2017, 29, 1602300, DOI 10.1002/adma.201602300. (32) Li, C. L.; Wu, M. C; Liu, R. High-performance bifunctional oxygen electrocatalysts for zinc-air batteries over mesoporous Fe/Co-N-C nanofibers with embedding FeCo alloy nanoparticles. Appl. Catal. B-Environ. 2019, 244, 150−158, DOI 10.1016/j.apcatb.2018.11.039. (33) Peng, S. J.; Li, L. L.; Lee, J. K. Y.; Tian, L. L.; Srinivasan, M.; Adams, S.; Ramakrishna, S. Electrospun carbon nanofibers and their hybrid composites as advanced materials for energy conversion and storage. Nano Energy 2016, 22, 361−395, DOI 10.1016/j.nanoen.2016.02.001. (34) Zhang, W. M.; Yao, X. Y.; Zhou, S. N.; Li, X. W.; Li, L.; Yu, Z.; Gu, L. ZIF-8/ZIF-67-derived Co-Nx-embedded 1D porous carbon nanofibers with graphitic carbon-encased Co nanoparticles as an efficient bifunctional electrocatalyst. Small 2018, 14, 1800423, DOI 10.1002/smll.201800423. (35) Zhang, C. L.; Lu, B. R.; Cao, F. H.; Yu, Z. L.; Cong, H. P.; Yu, S. H. Hierarchically structured Co3O4@carbon porous fibers derived from electrospun ZIF-67/PAN nanofibers as anodes for 30


Page 31 of 34 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


lithium ion batteries. J. Mater. Chem. A 2018, 6, 12962−12968, DOI 10.1039/C8TA03397H. (36) Li, Y.; Chen, S. M.; Xi, D. W.; Bo, Y. N.; Long, R.; Wang, C. M.; Song, L.; Xiong, Y. L. Scalable fabrication of highly active and durable membrane electrodes toward water oxidation. Small 2018, 14, 1702109, DOI 10.1002/smll.201702109. (37) Zhang, J.; Lyu, Z. Y.; Zhang, F.; Wang, L. J.; Xiao, P.; Yuan, K. D.; Lai, M.; Chen, W. Facile synthesis of hierarchical porous Co3O4 nanoboxes as efficient cathode catalysts for Li-O2 batteries. J. Mater. Chem. A 2016, 4, 6350−6356, DOI 10.1039/C6TA01995A. (38) Wang, Z.; Zuo, P. J.; Fan, L. Q.; Han, J. N.; Xiong, Y. P.; Yin, G. P. Facile electrospinning preparation of phosphorus and nitrogen dual-doped cobalt-based carbon nanofibers as bifunctional electrocatalyst. J. Power Sources 2016, 311, 68−80, DOI 10.1016/j.jpowsour.2016.02.012. (39) Zhao, Y. X.; Lai, Q. X.; Zhu, J. J.; Zhong, J.; Tang, Z. M.; Luo, Y.; Liang, Y. Y. Controllable construction

of

core-shell

polymer@zeolitic

imidazolate

frameworks

fiber

derived

heteroatom-doped carbon nanofiber network for efficient oxygen electrocatalysis. Small 2018, 14, 1704207, DOI 10.1002/smll.201704207. (40) Tang, H. J.; Chen, W.; Wang, J. Y.; Dugger, T.; Cruz, L.; Kisailus, D. Electrocatalytic N-doped graphitic nanofiber-metal/metal oxide nanoparticle composites. Small 2018, 14, 1703459, DOI 10.1002/smll.201703459. (41) Surendran, S.; Shanmugapriya, S.; Sivanantham, A.; Shanmugam, S.; Selvan, R. K. Electrospun carbon nanofibers encapsulated with NiCoP: A multifunctional electrode for supercapattery and oxygen reduction, oxygen evolution, and hydrogen evolution reactions. Adv. Energy Mater. 2018, 1800555, DOI 10.1002/aenm.201800555. (42) Liu, C.; Wang, J.; Li, J. S.; Liu, J. Z.; Wang, C. H.; Sun, X. Y.; Shen, J. Y.; Han, W. Q.; Wang, L. J. Electrospun ZIF-based hierarchical carbon fiber as an efficient electrocatalyst for the oxygen reduction reaction. J. Mater. Chem. A 2017, 5, 1211–1220, DOI 10.1039/C6TA09193H. (43) Han, L.; Yu, T. W.; Lei, W.; Liu, W. W.; Feng, K.; Ding, Y. L.; Jiang, G. P.; Xu, P.; Chen, Z. W. Nitrogen-doped carbon nanocones encapsulating with nickel-cobalt mixed phosphides for enhanced hydrogen evolution reaction. J. Mater. Chem. A 2017, 5, 16568–16572, DOI 10.1039/C7TA05146H. (44) Niu, Q. J.; Guo, J. X.; Tang, Y. H.; Guo, X. D.; Nie, J.; Ma, G. P. Sandwich-type bimetal-organic frameworks/graphene oxide derived porous nanosheets doped Fe/Co-N active 31



sites

for

oxygen

reduction

reaction.

Electrochim.

Acta

Page 32 of 34

2017,

255,

72–82,

DOI

10.1016/j.electacta.2017.09.125. (45) Xue, H. R.; Wang, T.; Gong, H.; Guo, H.; Fan, X. L.; Song, L.; Xia, W.; Feng, Y. Y.; He, J. P. Co3O4 Nanoparticle-decorated N-doped mesoporous carbon nanofibers as an efficient catalyst for oxygen reduction reaction. Catalysts 2017, 7, 189, DOI 10.3390/catal7060189. (46) Kim, J. B.; Jang, B.; Lee, H.; Han, W. S.; Lee, D.; Lee, H.; Hong, T. E.; Kim, S. A controlled growth of WNx and WCx thin films prepared by atomic layer deposition. Mater. Lett. 2016, 168, 218–222, DOI 10.1016/j.matlet.2016.01.071. (47) Deng, Y. J.; Dong, Y. Y.; Wang, G. H.; Sun, K. L.; Shi, X. D.; Zheng, L.; Li, X. H.; Liao, S. J. Well-defined ZIF-derived Fe-N codoped carbon nanoframes as efficient oxygen reduction catalysts. ACS Appl. Mater. Interfaces 2017, 9, 9699−9709, DOI 10.1021/acsami.6b16851. (48) Ji, D. X.; Peng, S. J.; Lu, J.; Li, L. L.; Yang, S. Y.; Yang, G. R.; Qin, X. H.; Srinivasan, M.; Ramakrishna, S. Design and synthesis of porous channel-rich carbon nanofibers for self-standing oxygen reduction reaction and hydrogen evolution reaction bifunctional catalysts in alkaline medium. J. Mater. Chem. A 2017, 5, 7507–7515, DOI 10.1039/C7TA00828G. (49) Wang, H.; Yin, F. X.; Chen, B. H.; He, X. B.; Lv, P. L.; Ye, C. Y.; Liu, D. J. ZIF-67 incorporated with carbon derived from pomelo peels: A highly efficient bifunctional catalyst for oxygen reduction/evolution reactions. Appl. Catal. B-Environ. 2017, 205, 55–67, DOI 10.1016/j.apcatb.2016.12.016. (50) Su, C. Y.; Cheng, H.; Li, W.; Liu, Z. Q.; Li, N.; Hou, Z. F.; Bai, F. Q.; Zhang H. X.; Ma, T. Y. Atomic modulation of FeCo-nitrogen-carbon bifunctional oxygen electrodes for rechargeable and flexible all-solid-state zinc-air battery. Adv. Energy Mater. 2017, 7, 1602420, DOI 10.1002/aenm.201602420. (51) Yin, P. Q.; Yao, T.; Wu, Y.; Zheng, L. R.; Lin,Y.; Liu, W.; Ju, H. X.; Zhu, J. F.; Hong, X.; Deng, Z. X.; Zhou, G.; Wei, S. Q.; Li, Y. D. Single cobalt atoms with precise N-coordination as superior oxygen reduction reaction catalysts. Angew. Chem. Int. Ed. 2016, 55, 10800–10805, DOI 10.1002/anie.201604802. (52) Su, Y. H.; Zhu, Y. H.; Jiang, H. L.; Shen, J. H.; Yang, X. L.; Zou, W. J.; Chen, J. D.; Li, C. Z. Cobalt nanoparticles embedded in N-doped carbon as an efficient bifunctional electrocatalyst for oxygen

reduction

and

evolution

reactions.

Nanoscale

32


2014,

6,

15080–15089,

DOI

Page 33 of 34 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


10.1039/C4NR04357J. (53) Aijaz, A.; Masa, J.; Rösler, C.; Antoni, H.; Fischer, R. A.; Schuhmann, W.; Muhler, M. MOF-templated assembly approach for Fe3C nanoparticles encapsulated in bamboo-like N-doped CNTs: Highly efficient oxygen reduction under acidic and basic conditions. Chem. Eur. J. 2017, 23, 12125–12130, DOI 10.1002/chem.201701389. (54) Lai, L. F.; Potts, J. R.; Zhan, D.; Wang, L.; Poh, C. K.; Tang, C. H.; Gong, H.; Shen, Z. X.; Lin, J. Y.; Ruoff, R. S. Exploration of the active center structure of nitrogen-doped graphene-based catalysts for oxygen reduction reaction. Energy Environ. Sci. 2012, 5, 7936–7942, DOI 10.1039/C2EE21802J. (55) Wang, H. T.; Wang, W.; Xu, Y. Y.; Asif, M.; Liu H. F; Xia, B. Y. Ball-milling synthesis of Co2P nanoparticles encapsulated in nitrogen doped hollow carbon rods as efficient electrocatalysts. J. Mater. Chem. A 2017, 5, 17563–17569, DOI 10.1039/C7TA05510B. (56) Prabu, M.; Ketpang, K.; Shanmugam, S. Hierarchical nanostructured NiCo2O4 as an efficient bifunctional non-precious metal catalyst for rechargeable zinc-air batteries. Nanoscale 2014, 6, 3173–3181, DOI 10.1039/C3NR05835B. (57) Wang, Y. Q.; Tao, L.; Xiao, Z. H; Chen, R.; Jiang, Z. Q.; Wang, S. Y. 3D carbon electrocatalysts in situ constructed by defect-rich nanosheets and polyhedrons from NaCl-sealed zeolitic

imidazolate

frameworks.

Adv.

Funct.

Mater.

2018,

28,

1705356,

DOI

10.1002/adfm.201705356. (58) Zhu, J. W.; Li, W. Q.; Li, S. H.; Zhang, J.; Zhou, H.; Zhang, C. T.; Zhang, J. N.; Mu, S. C. Defective N/S-codoped 3D cheese-like porous carbon nanomaterial toward efficient oxygen reduction and Zn-air batteries. Small 2018, 14, 1800563, DOI 10.1002/smll.201800563.

33



Table of Contents Entry

Nitrogen-doped carbon nanofibers encapsulated FeCo alloy nanoparticles (FeCo-NCNFs-Ts) were synthesized by electrospinning of Co-Fe PBA@PAN and post-calcination treatment, FeCo-NCNFs-800 exhibited superior bifunctional catalytic activity and stability towards ORR and OER in Zn-air battery to that of commercial Pt/C-RuO2 catalyst.

34


Page 34 of 34

Electrospun MOF-based FeCo nanoparticles embedded in nitrogen

Recommend Documents