Andri Sokka

Marek Mooste

Maike Käärik

Viktoria Gudkova

Jekaterina Kozlova

Arvo Kikas

Vambola Kisand

Alexey Treshchalov

Aile Tamm

Päärn Paiste

Jaan Aruväli

Jaan Leis

Andres Krumme

Steven Holdcroft

Sara Cavaliere

Frédéric Jaouen

Kaido Tammeveski
email: kaido.tammeveski@ut.ee

Iron and cobalt containing electrospun carbon nanofibre-based cathode catalysts for anion exchange membrane fuel cell

Keywords: Carbon nanofibres, Electrospinning, Anion exchange membrane fuel cell, Nonprecious metal catalyst, Oxygen reduction, Electrocatalysis

Introduction

Carbon nanofibres (CNFs) as novel multifunctional materials have a large number of applications due to their unique structure, properties and functions. As an example, CNFs have been employed in the fields of nanotechnology, materials science, energy conversion and storage, environmental science and medicine. To prepare polymer nanofibres, electrospinning is an effective, facile, and easy-to-operate and scale-up technique that leans on static electricity [START_REF] Zhou | Production, structural design, functional control, and broad applications of carbon nanofiber-based nanomaterials: A comprehensive review[END_REF][START_REF] Barhoum | Nanofibers as newgeneration materials: From spinning and nano-spinning fabrication techniques to emerging applications[END_REF]. Doped CNFs derived from electrospun polymer precursors have been widely studied for application in energy storage and conversion devices (e.g. supercapacitors, batteries, fuel cells and solar cells) [START_REF] Lei | Electrospun Inorganic Nanofibers for Oxygen Electrocatalysis: Design, Fabrication, and Progress[END_REF][START_REF] Liu | Recent advances in energy materials by electrospinning[END_REF][START_REF] Wang | Metal-organic frameworkderived one-dimensional porous or hollow carbon-based nanofibers for energy storage and conversion[END_REF]. The oxygen reduction reaction (ORR) is one of the key reactions in the fuel cells and metal-air batteries. Non precious metal catalyst (NPMC) materials based on electrospun CNFs are very promising for ORR in terms of adjustable morphology, high electrical conductivity, tuneable porosity and excellent structural stability [START_REF] Xu | Noble-metal-free electrospun nanomaterials as electrocatalysts for oxygen reduction reaction[END_REF][START_REF] Rauf | Non-precious nanostructured materials by electrospinning and their applications for oxygen reduction in polymer electrolyte membrane fuel cells[END_REF][START_REF] Cavaliere | Electrospinning: designed architectures for energy conversion and storage devices[END_REF].

The ORR can proceed via 4e -or 2e -pathway giving water or hydrogen peroxide, respectively, as a final product [START_REF] Sarapuu | Electrocatalysis of oxygen reduction on heteroatom-doped nanocarbons and transition metal-nitrogen-carbon catalysts for alkaline membrane fuel cells[END_REF]. Pt-based electrocatalysts are regarded currently as the most suitable for the ORR due to their unique characteristics, such as direct 4e -pathway, low overpotential and high current density in polymer electrolyte fuel cells and metal-air batteries [START_REF] Rauf | Non-precious nanostructured materials by electrospinning and their applications for oxygen reduction in polymer electrolyte membrane fuel cells[END_REF][START_REF] Sarapuu | Electrocatalysis of oxygen reduction on heteroatom-doped nanocarbons and transition metal-nitrogen-carbon catalysts for alkaline membrane fuel cells[END_REF]. In addition to ORR, the electrocatalysis of the oxygen evolution reaction (OER) is also essential for rechargeable metal-air batteries. Therefore, the bifunctionality of catalysts for both reactions is an important field of research for electrospun CNF-based NPMCs [START_REF] Lei | Electrospun Inorganic Nanofibers for Oxygen Electrocatalysis: Design, Fabrication, and Progress[END_REF][START_REF] Alegre | NiCo-loaded carbon nanofibers obtained by electrospinning: Bifunctional behavior as air electrodes[END_REF][START_REF] Li | High-performance bifunctional oxygen electrocatalysts for zincair batteries over mesoporous Fe/Co-N-C nanofibers with embedding FeCo alloy nanoparticles[END_REF][START_REF] Alegre | Toward more efficient and stable bifunctional electrocatalysts for oxygen electrodes using FeCo2O4/carbon nanofiber prepared by electrospinning[END_REF][START_REF] Cao | Electrospinning synthesis of transition metal alloy nanoparticles encapsulated in nitrogen-doped carbon layers as an advanced bifunctional oxygen electrode[END_REF].

According to the literature, the preparation of pyrolysed nanostructures composed of transition metal (TM) and nitrogen-doped electrospun CNFs is a favourable way to develop efficient and stable NPMCs for ORR [START_REF] Xu | Noble-metal-free electrospun nanomaterials as electrocatalysts for oxygen reduction reaction[END_REF][START_REF] Rauf | Non-precious nanostructured materials by electrospinning and their applications for oxygen reduction in polymer electrolyte membrane fuel cells[END_REF]. In this case, single transition metal atoms coordinated to nitrogen (M-Nx) are embedded into the CNF and are active sites for the ORR. Also, the metallic particles wrapped in N-C are beneficial for high ORR activity in alkaline media [START_REF] Cao | Electrospinning synthesis of transition metal alloy nanoparticles encapsulated in nitrogen-doped carbon layers as an advanced bifunctional oxygen electrode[END_REF][START_REF] Feng | Bamboo-like nitrogen-doped porous carbon nanofibers encapsulated nickel-cobalt alloy nanoparticles composite material derived from the electrospun fiber of a bimetal-organic framework as efficient bifunctional oxygen electrocatalysts[END_REF]. Furthermore, it has been found that when the CNF contains two or more TM dopants, its catalytic performance can be boosted [START_REF] Xu | Noble-metal-free electrospun nanomaterials as electrocatalysts for oxygen reduction reaction[END_REF]. Therefore, different research groups have prepared and studied Zn/Co [START_REF] Liu | Electrospun ZIF-based hierarchical carbon fiber as an efficient electrocatalyst for the oxygen reduction reaction[END_REF][START_REF] Zang | Cobalt/zinc dual-sites coordinated with nitrogen in nanofibers enabling efficient and durable oxygen reduction reaction in acidic fuel cells[END_REF][START_REF] He | Single Cobalt Sites Dispersed in Hierarchically Porous Nanofiber Networks for Durable and High-Power PGM-Free Cathodes in Fuel Cells[END_REF], Ni/Co [START_REF] Alegre | NiCo-loaded carbon nanofibers obtained by electrospinning: Bifunctional behavior as air electrodes[END_REF][START_REF] Feng | Bamboo-like nitrogen-doped porous carbon nanofibers encapsulated nickel-cobalt alloy nanoparticles composite material derived from the electrospun fiber of a bimetal-organic framework as efficient bifunctional oxygen electrocatalysts[END_REF][START_REF] Deng | NiCo-doped C-N nano-composites for cathodic catalysts of Zn-air batteries in neutral media[END_REF] co-doped and even trimetallic [START_REF] Cao | Electrospinning synthesis of transition metal alloy nanoparticles encapsulated in nitrogen-doped carbon layers as an advanced bifunctional oxygen electrode[END_REF][START_REF] Liu | Trimetallic FeCoNi-N/C nanofibers with high electrocatalytic activity for oxygen reduction reaction in sulfuric acid solution[END_REF] electrospun CNF-based catalysts. As the single Fe or Co doped carbon nanomaterials show good ORR electrocatalytic performance [START_REF] Xu | Noble-metal-free electrospun nanomaterials as electrocatalysts for oxygen reduction reaction[END_REF], Co-Fe co-doped CNFs witness an increased interest. In more specific, Uhm et al. and Alegre et al. have proposed the use of Co and Fe codoped CNF catalysts in alkaline ethanol fuel cells [START_REF] Uhm | A facile route for preparation of non-noble CNF cathode catalysts in alkaline ethanol fuel cells[END_REF] and rechargeable alkaline metal-air batteries [START_REF] Alegre | Toward more efficient and stable bifunctional electrocatalysts for oxygen electrodes using FeCo2O4/carbon nanofiber prepared by electrospinning[END_REF], respectively. Li et al. have additionally tested their Co and Fe co-doped CNF in zinc-air battery (ZAB) and reported comparable results to Pt/C [START_REF] Li | High-performance bifunctional oxygen electrocatalysts for zincair batteries over mesoporous Fe/Co-N-C nanofibers with embedding FeCo alloy nanoparticles[END_REF]. In addition, CNF-based NPMCs co-doped with more than one TM have already shown considerable results in proton exchange membrane fuel cell (PEMFC) [START_REF] Zang | Cobalt/zinc dual-sites coordinated with nitrogen in nanofibers enabling efficient and durable oxygen reduction reaction in acidic fuel cells[END_REF][START_REF] He | Single Cobalt Sites Dispersed in Hierarchically Porous Nanofiber Networks for Durable and High-Power PGM-Free Cathodes in Fuel Cells[END_REF]. Until now however, there is no information available about Co and Fe co-doped electrospun CNF-based catalyst application at the cathode of anion exchange membrane fuel cells (AEMFCs). This research is appealing since cobalt ferrite nanoparticles supported on Vulcan carbon XC-72 cathode catalyst was recently shown to reach a remarkable peak power density (Pmax) of 1350 mW cm -2 in AEMFC [START_REF] Peng | High-Performing PGM-Free AEMFC Cathodes from Carbon-Supported Cobalt Ferrite Nanoparticles[END_REF].

The versatility of electrospinning can be exploited by using porogens and additives in the precursor solution to further tune the inner porosity and structure of the obtained fibres. In our previous study on CNFs containing only one TM (Fe or Co) we investigated the influence of ionic liquids (ILs) as porogens in the electrospinning solution and demonstrated that the highest Pmax was obtained for the catalyst prepared with the addition of IL [START_REF] Mooste | Electrospun Polyacrylonitrile-Derived Co or Fe Containing Nanofibre Catalysts for Oxygen Reduction Reaction at the Alkaline Membrane Fuel Cell Cathode[END_REF].

In the present work, Co and Fe co-doped CNFs were prepared by electrospinning polyacrylonitrile (PAN) with acetate salts as TM precursors with and without the IL. The electrospun CNFs were further pyrolysed in N2 atmosphere and studied as ORR and OER electrocatalysts in alkaline and acidic media by rotating disc electrode method, and as a cathode catalyst in the AEMFC.

Material and methods

Preparation of catalyst materials

Two types of carbon nanofibre materials were prepared using a single needle electrospinning setup and the rotating drum configuration similarly to our previous work [START_REF] Mooste | Oxygen Reduction on Catalysts Prepared by Pyrolysis of Electrospun Styrene-Acrylonitrile Copolymer and Multi-walled Carbon Nanotube Composite Fibres[END_REF]. The chemicals used were 1 butyl 3 methylimidazolium acetate ([BMIm]OAc, IoLiTec), cobalt(II) acetate (Co(OAc)2, 98%, Alfa Aesar™), iron(II) acetate (Fe(OAc)2, 95%, Aldrich), polyacrylonitrile (PAN, MW=150,000, Sigma Aldrich) and N,N-dimethylformamide (DMF, 99.8%, Sigma-Aldrich). The electrospinning mixture in DMF for the first material (Fe/Co/IL-CNF) contained [BMIm]OAc:PAN:Co(OAc)2:Fe(OAc)2 (weight ratio of 1:7:1:1). In the case of the second material (Fe/Co-CNF), the mixture of PAN:Co(OAc)2:Fe(OAc)2 (weight ratio of 8:1:1) in DMF was used. Both electrospinning solutions were prepared under N2 atmosphere via mechanical stirring of 10 wt% PAN in DMF at 40 for 72 h. Then the other components were added and the mechanical stirring continued for additional 24-48 h. The electrospinning conditions for Fe/Co-CNF were as follows: voltage 12 kV, distance to the collector (rotating drum) 10-10.5 cm and the feed rate of the solution 1.5 mL h -1 . For the Fe/Co/IL-CNF, the voltage 12-14 kV, distance to the collector (rotating drum) 12-14 cm and the feed rate of the solution 0.6-1.0 mL h -1 were used. For both materials, the inner diameter of the needle was 0.1 mm and the speed of the drum (7.5 cm diameter) was 4400 rpm.

After electrospinning, the materials were pyrolysed at 800, 900 or 1000 under N2

atmosphere according to the previously published procedure [START_REF] Mooste | Electrospun Polyacrylonitrile-Derived Co or Fe Containing Nanofibre Catalysts for Oxygen Reduction Reaction at the Alkaline Membrane Fuel Cell Cathode[END_REF]. The designation of the catalyst consists of the material followed by carbonisation temperature. Selected NPMCs were also leached in aqueous solution containing 0.5 M H2SO4 and 0.5 M HNO3 followed by a second pyrolysis in the same conditions as the first carbonisation. For acid treatment, two different sets of conditions were used. The additional letter a or b in the material designation stands for 8 h at 50 °C (procedure 1) or 12 h at 80 °C (procedure 2), respectively.

Physical characterisation

For physical characterisation, the NPMC materials were used in the form of powder or coated on a substrate material using catalyst inks without Nafion ® . The scanning electron microscopy (SEM) experiments were performed with a high resolution scanning electron microscope (Helios NanoLab 600, FEI Company). For assessing the elemental composition of the samples and performing the elemental mapping, the INCA Energy 350 EDS spectrometer (Oxford Instruments) connected to the SEM was used. X ray photoelectron spectroscopy (XPS) studies were carried out at a pressure below 10 -9 Torr using a SCIENTA SES 100 spectrometer with an electron take off angle 90°, 300 W non monochromatic Mg Kα X ray source and pass energy 200 eV. The obtained data were processed with the Casa XPS software (version 2.3.17). The processing included the removal of X ray satellites, peak fitting using the Gauss-Lorentz hybrid function, combination of linear and Shirley backgrounds and correction for electron spectrometer transmission.

Raman spectroscopy measurements in back-scattering geometry were carried out using a Renishaw inVia spectrometer with 514.5 nm excitation wavelength from the Ar ion laser and confocal microscope. To prevent overheating due to irradiation, the laser power on the sample was limited to 3 mW. The 4-band fitting was used with the peaks of G (~1580 cm -1 ) -ideal graphitic lattice; D1 (~1350 cm -1 ) -disordered graphitic lattice, graphene layer edge; D3 (~1500 cm -1 ) -amorphous carbon and D4 (~1200 cm -1 ) -disordered graphitic lattice [START_REF] Sadezky | Raman micro spectroscopy of soot and related carbonaceous materials: Spectral analysis and structural information[END_REF]. A baseline correction was used to compensate for the small fluorescence background. The spectra were normalised to the intensity of the G band and the intensities of ID1 and IG (integrated areas under the bands) as well as FWHM of D1 and G bands were extracted from the fitting of the curves by Voigt functions.

The composition of the catalysts crystalline phases was studied using the powder X-ray diffraction (XRD) analysis with a Bruker D8 Advance diffractometer employing Ni-filtered Cu Kα radiation. The XRD patterns were registered using a step of 0.013°2θ from 5 to 85°2θ and the collecting time of 525 s per step.

For the microwave plasma atomic emission spectroscopy (MP AES) analysis, the samples were dissolved with Anton Par Multiwave PRO microwave digestion system using NXF100 digestion vessels (PTFE TFM liner) in 8 N rotor prior to analysis. 10 mg of sample was weighed into PTFE vessels into which 4 ml of 69 % HNO3 (Carl Roth ROTIPURAN Supra) and 2 ml of H2O2 (Sigma-Aldrich TraceSelect) were sequentially added. After the initial reaction had subsided the vessels were capped and digested in the microwave unit at 240 °C and 45-50 bar. After digestion the samples were diluted using 2 % HNO3 to a final concentration of around 4 mg L -1 and analysed using Agilent MP-AES 4210. Concentrations

were determined using analytical wavelengths Co 350.631 nm, Fe 371.993 nm.

The porosity parameters of the materials were found using low-temperature N2 physisorption with NOVAtouch LX2 (Quantachrome Instruments). Prior to measurements, the samples were degassed under vacuum at 300 °C for 12 h. The BET surface area (SBET) [START_REF] Brunauer | Adsorption of Gases in Multimolecular Layers[END_REF] of samples was calculated in the P/P0 interval of 0.02-0.2. The total pore volume (Vtot) was calculated at P/P0 0.97. Specific surface area (Sdft), volume of micropores (Vμ) and pore size distribution (PSD) were calculated from N2 adsorption/desorption isotherms using a quenched solid density functional theory (QSDFT) equilibria model for slit type pores.

Electrochemical experiments

The electrochemical experiments were performed using cyclic voltammetry (CV) and rotating disc electrode (RDE) methods in Ar-saturated (99.999 %, Linde) or O2-saturated (99.999 %, Linde) 0.1 M KOH (p.a. quality, Merck) or 0.5 M H2SO4 (Suprapur, Merck) solutions. The catalyst inks with Nafion ® were prepared in 2-propanol (99.8 %, Honeywell Riedel-de Haën)

similarly as in our previous study [START_REF] Mooste | Transition metal phthalocyanine-modified shungite-based cathode catalysts for alkaline membrane fuel cell[END_REF]. After drop-casting the ink onto the polished and cleaned [START_REF] Mooste | Electrocatalysts for oxygen reduction reaction based on electrospun polyacrylonitrile, styrene-acrylonitrile copolymer and carbon nanotube composite fibres[END_REF] glassy carbon (GC, A = 0.196 cm 2 ) electrodes (GC-20SS, Tokai Carbon, Japan), the NPMC loading of 0.2 and 0.8 mg cm -2 was used for the experiments in alkaline and acidic media, respectively. In the case of the commercial Pt/C (46.1 wt% Pt, Tanaka Kikinzoku Kogyo K.K., Japan) catalyst, a 20 µgPt cm -2 loading was used [START_REF] Mooste | Electrospun Polyacrylonitrile-Derived Co or Fe Containing Nanofibre Catalysts for Oxygen Reduction Reaction at the Alkaline Membrane Fuel Cell Cathode[END_REF]. The electrochemical experiments were conducted in a 3-electrode glass cell using Autolab potentiostat/galvanostat PGSTAT30 (Metrohm Autolab, The Netherlands), Nova 2.1 software, EDI101 rotator and CTV101 (Radiometer) speed control unit. Pt-wire served as a counter electrode and saturated calomel electrode (SCE) as the reference electrode. All the reported potentials are referred with respect to the SCE. The polarisation curves for oxygen reduction and evolution reaction were recorded in the cathodic and anodic direction, respectively. The polarisation curves were iR compensated using the ohmic resistance values obtained with electrochemical impedance spectroscopy [START_REF] Ping | Fused Hybrid Linkers for Metal-Organic Framework-Derived Bifunctional Oxygen Electrocatalysts[END_REF].

AEMFC testing

The membrane-electrode assembly (MEA) was prepared using poly[2,2′-(2,2′′,4,4′′,6,6′′hexamethyl-p-terphenyl-3,3′′-diyl)-5,5′-bibenzimidazole] (HMT-PMBI) anion exchange membrane (AEM) with benzimidazolium cationic groups [START_REF] Wright | Hexamethyl-pterphenyl poly(benzimidazolium): a universal hydroxide-conducting polymer for energy conversion devices[END_REF], gas-diffusion layer (GDL, Sigracet 28BC), NPMC or Pt/C at the cathode and Pt-Ru/C (50:25:25 by weight, Alfa Aesar) at the anode. According to the previously published procedure [START_REF] Mooste | Electrospun Polyacrylonitrile-Derived Co or Fe Containing Nanofibre Catalysts for Oxygen Reduction Reaction at the Alkaline Membrane Fuel Cell Cathode[END_REF][START_REF] Mooste | Transition metal phthalocyanine-modified shungite-based cathode catalysts for alkaline membrane fuel cell[END_REF], the catalyst loadings of 2 mgNPMC cm -2 (or 0.4 mgPt cm -2 ) and 0.6 mgPt-Ru cm -2 at the gas diffusion electrodes were used for cathode and anode, respectively . The catalyst-coated GDL and the AEM were nm, respectively. The variance in diameter of the electrospun fibre material is a well-known feature [START_REF] Li | High-performance bifunctional oxygen electrocatalysts for zincair batteries over mesoporous Fe/Co-N-C nanofibers with embedding FeCo alloy nanoparticles[END_REF][START_REF] Liu | Trimetallic FeCoNi-N/C nanofibers with high electrocatalytic activity for oxygen reduction reaction in sulfuric acid solution[END_REF][START_REF] Uhm | A facile route for preparation of non-noble CNF cathode catalysts in alkaline ethanol fuel cells[END_REF]. Similar fibre diameters to the present work have been recently obtained with Ni/Co co-doped [START_REF] Feng | Bamboo-like nitrogen-doped porous carbon nanofibers encapsulated nickel-cobalt alloy nanoparticles composite material derived from the electrospun fiber of a bimetal-organic framework as efficient bifunctional oxygen electrocatalysts[END_REF] and Co-doped [START_REF] Mooste | Electrospun Polyacrylonitrile-Derived Co or Fe Containing Nanofibre Catalysts for Oxygen Reduction Reaction at the Alkaline Membrane Fuel Cell Cathode[END_REF] CNF catalysts. The SEM images with higher magnification are presented in Fig. 1. In the case of Fe/Co/IL-CNF-800-based materials, the peculiar formations are seen on the CNF surface compared to the Fe/Co-CNF-900-based nanofibres. Similar formations have been observed for Fe/Co co-doped CNFs prepared with different TM precursor salts and other electrospinning solution components [START_REF] Li | High-performance bifunctional oxygen electrocatalysts for zincair batteries over mesoporous Fe/Co-N-C nanofibers with embedding FeCo alloy nanoparticles[END_REF][START_REF] Uhm | A facile route for preparation of non-noble CNF cathode catalysts in alkaline ethanol fuel cells[END_REF]. In the study by Li et al., these formations were found to be related with the formed TM nanoparticles according to the elemental mapping [START_REF] Li | High-performance bifunctional oxygen electrocatalysts for zincair batteries over mesoporous Fe/Co-N-C nanofibers with embedding FeCo alloy nanoparticles[END_REF]. Herein, these peculiar nanostructures could also correspond to the metal nanoparticles formed on the surface of CNFs. Locally the elongated protrusions can be seen on the surface of the fibres (especially prominent in Fig. 1c). These structures could be formed due to the presence of metal nanoparticles, which during pyrolysis can initiate a formation of nanotube-like structures. The SEM-EDX elemental mapping of the Fe/Co/IL-CNF-800-based catalysts in Fig. 2 showed a uniform distribution of Fe and Co throughout the material, indicating that the formations observed herein have a rather similar composition with the fibre itself. This is also supported by the presence of formations after the acid treatment procedures (Figs. 1b andc).

The bulk elemental composition of the NPMCs was investigated using the EDX analysis and the TM content was also verified via MP-AES (Table 1). For both, Fe/Co/IL-CNF-800 and Fe/Co-CNF-900, the acid treatment by procedure 1 (8 h at 50 °C) has no effect on the TM content, while procedure 2 with higher temperature and longer treatment time (12 h at 80 °C) has leached out a considerable amount of both Fe and Co and introduced a low amount of S.

Similarly, the presence of S has been previously detected in carbon nanotubes after their purification in the HNO3 + H2SO4 mixture [START_REF] Kruusenberg | Effect of purification of carbon nanotubes on their electrocatalytic properties for oxygen reduction in acid solution[END_REF]. Raman spectroscopy was employed to study the carbon structure in the NPMCs. As typical to electrospun polymer-based CNF [START_REF] Cao | Electrospinning synthesis of transition metal alloy nanoparticles encapsulated in nitrogen-doped carbon layers as an advanced bifunctional oxygen electrode[END_REF][START_REF] Liu | Electrospun ZIF-based hierarchical carbon fiber as an efficient electrocatalyst for the oxygen reduction reaction[END_REF][START_REF] He | Single Cobalt Sites Dispersed in Hierarchically Porous Nanofiber Networks for Durable and High-Power PGM-Free Cathodes in Fuel Cells[END_REF][START_REF] Wang | Metal-containing Ionic Liquid/Polyacrylonitrile-derived Carbon Nanofibers for Oxygen Reduction Reaction and Flexible Zn-Air Battery[END_REF], two broad peaks with similar height were observed at ca. 1350 and 1600 cm -1 for all the samples (Fig. 3). After the deconvolution of the spectra using 4-band fitting, the ID1/IG ratios were calculated for all the catalysts to be higher than 2. This indicates that the ID1/IG ratio is insensitive to the degree of structural disordering of these very defective carbon materials. Although, according to the comparison of D1 and G band width (full width at half maximum, FWHM), the structural disorder (amorphous carbon with appreciable sp 3 bonding) in the Fe/Co/IL-CNF-800b material is slightly higher than in Fe/Co-CNF-900b. Similarly, ID1/IG ratios higher than 2 have been previously reported by Liu et al. for trimetallic Co/Fe/Ni-doped CNF catalysts [START_REF] Liu | Trimetallic FeCoNi-N/C nanofibers with high electrocatalytic activity for oxygen reduction reaction in sulfuric acid solution[END_REF]. To obtain information about the surface composition of Fe/Co/IL-CNF-800 and Fe/Co-CNF-900 catalysts and their acid-leached counterparts (treated at 80 °C for 12 h), XPS analysis was performed (Table 2, Figs. 4,S2). The surface of the catalyst materials contains the same elements as the bulk material (Table 1). Although, S was not detected via XPS analysis compared to the EDX data, most likely due to the low amount of this element. Similarly to other studies on TM-doped CNF catalysts [START_REF] Mooste | Electrospun Polyacrylonitrile-Derived Co or Fe Containing Nanofibre Catalysts for Oxygen Reduction Reaction at the Alkaline Membrane Fuel Cell Cathode[END_REF][START_REF] Wang | Metal-containing Ionic Liquid/Polyacrylonitrile-derived Carbon Nanofibers for Oxygen Reduction Reaction and Flexible Zn-Air Battery[END_REF], the content of specific elements on the surface is different from the bulk. The detection of Co and Fe on the catalyst surface provides a possibility for the presence of M-Nx sites (insets to Figs. 4,S2). The M-Nx species were also possibly detected in the deconvoluted N1s spectra (Table 2, Figs. 4,S2). In addition to the high ORR activity, the M-Nx sites are also known to be resistant to the acid leaching [START_REF] Buan | Active sites for the oxygen reduction reaction in nitrogen-doped carbon nanofibers[END_REF]. The main components in the N1s spectra of all the studied catalysts are pyridinic-N, pyrrolic-N and graphitic-N. While the relative content of pyrrolic-N is around 30 % for all the catalysts, the amount of pyridinic-N is considerably higher in Fe/Co/IL-CNF-800 compared to Fe/Co-CNF-900. The latter also applies to their acid-leached counterparts.

Table 2 Elemental composition (at%) of different catalyst materials and relative content (%) of various N species (N1 -imine, N2 -pyridinic-N, N3 -metal-Nx, N4 -pyrrolic-N, N5graphitic-N, N6 -bulk N-H, N7 -pyridine-N-oxide) as determined by XPS (Figs. 4 andS2). The crystalline structure of the prepared NPMCs was evaluated by XRD analysis. The XRD patterns of all catalyst materials exhibited a distinctive peak at ca. 25° as observed in Fig. 5a for Fe/Co/IL-CNF-800b and Fe/Co-CNF-900b. The latter peak and the smaller signal at 44° correspond to the (002) and (101) crystal faces of graphitic carbon, respectively, which is known to be present in the XRD pattern of pyrolysed CNFs [START_REF] Feng | Bamboo-like nitrogen-doped porous carbon nanofibers encapsulated nickel-cobalt alloy nanoparticles composite material derived from the electrospun fiber of a bimetal-organic framework as efficient bifunctional oxygen electrocatalysts[END_REF][START_REF] Zang | Cobalt/zinc dual-sites coordinated with nitrogen in nanofibers enabling efficient and durable oxygen reduction reaction in acidic fuel cells[END_REF][START_REF] He | Single Cobalt Sites Dispersed in Hierarchically Porous Nanofiber Networks for Durable and High-Power PGM-Free Cathodes in Fuel Cells[END_REF][START_REF] Deng | NiCo-doped C-N nano-composites for cathodic catalysts of Zn-air batteries in neutral media[END_REF][START_REF] Liu | Trimetallic FeCoNi-N/C nanofibers with high electrocatalytic activity for oxygen reduction reaction in sulfuric acid solution[END_REF][START_REF] Wang | Metal-containing Ionic Liquid/Polyacrylonitrile-derived Carbon Nanofibers for Oxygen Reduction Reaction and Flexible Zn-Air Battery[END_REF]. The high graphitic carbon content is supported by the presence of Co and Fe, regarded as the graphitisation catalysts [START_REF] Alegre | NiCo-loaded carbon nanofibers obtained by electrospinning: Bifunctional behavior as air electrodes[END_REF]. Another set of XRD peaks at 45°, 65° and 83°, which were present in the diffraction patterns of all catalyst materials, are attributed to the (110), ( 200) and (211) crystal facets of FeCo (1:1) alloy, respectively [START_REF] Li | High-performance bifunctional oxygen electrocatalysts for zincair batteries over mesoporous Fe/Co-N-C nanofibers with embedding FeCo alloy nanoparticles[END_REF]. In the XRD pattern for Fe/Co/IL-CNF-800 and Fe/Co-CNF-900 catalysts no peaks for Fe or Co oxides were detected (data not shown). After the acid treatment and second pyrolysis, the XRD peaks for Fe2O3 and Co3O4 were present, indicating the acid leaching of metallic particles as metal cations but also partial reprecipitation or adsorption of cations on CNF as oxides during the drying step (Fig. 5a). Additionally, the total TM content in both materials (with or without IL additive) has decreased at least by 30 % after the acid treatment at 80 °C for 12 h (procedure 2) according to the comparison of the obtained XRD data. The TM content did not decrease during the acid treatment by procedure 1 (8 h at 50 °C), which is also supported by the EDX data (Table 1).

To assess the materials specific surface area (SSA) before and after the acid treatment by procedure 2 (12 h at 80 °C), low temperature N2 physisorption studies were carried out. The SSA values using the Brunauer Emmett Teller method (SBET) were calculated to be 272 and 165 m 2 g -1 for the Fe/Co/IL-CNF-800 and Fe/Co-CNF-900, respectively. The difference indicates that the IL acts as a porogen and can provide higher surface area and higher porosity [START_REF] Wang | Metal-containing Ionic Liquid/Polyacrylonitrile-derived Carbon Nanofibers for Oxygen Reduction Reaction and Flexible Zn-Air Battery[END_REF]. The latter can be supported by the higher content of TMs on the material surface in case of Fe/Co/IL-CNF-800 compared to the Fe/Co-CNF-900 (Table 2). After the acid treatment and additional pyrolysis, considerably higher SBET values were obtained, i.e. 351 and 452 m 2 g -1 for Fe/Co/IL-CNF-800b and Fe/Co-CNF-900b, respectively. For comparison, several studies regarding Co/Fe co-doped CNF-based catalysts have reported SBET values similar to Fe/Co/IL-CNF-800b around 350 m 2 g -1 [START_REF] Li | High-performance bifunctional oxygen electrocatalysts for zincair batteries over mesoporous Fe/Co-N-C nanofibers with embedding FeCo alloy nanoparticles[END_REF][START_REF] Alegre | Toward more efficient and stable bifunctional electrocatalysts for oxygen electrodes using FeCo2O4/carbon nanofiber prepared by electrospinning[END_REF][START_REF] Uhm | A facile route for preparation of non-noble CNF cathode catalysts in alkaline ethanol fuel cells[END_REF]. The shape of the N2 isotherms on Fe/Co/IL-CNF-800b and Fe/Co-CNF-900b (inset to Fig. 5b) corresponds to type II with H4 hysteresis according to IUPAC, which is characteristic of micro-mesoporous materials [START_REF] Thommes | Physisorption of gases, with special reference to the evaluation of surface area and pore size distribution (IUPAC Technical Report)[END_REF].

The pore size distribution (PSD) plots (Fig. 5b) show the presence of three pore size maxima at about 0.8, 1.5 and 3.5 nm, with higher mesopore volumes in case of Fe/Co-CNF-900b. In more specific, the amount of micropores according to the Vμ in respect to Vtot was calculated to be 44 % and 36 % for Fe/Co/IL-CNF-800b and Fe/Co-CNF-900b, respectively. The inset to b shows N2 adsorption/desorption isotherms that were used for PSD calculation.

Oxygen reduction studies

Firstly, both electrospun precursors were pyrolysed at 800, 900 or 1000 °C, and studied as NPMCs for ORR in 0.1 M KOH (Fig. S3). The most positive onset potential (Eonset) and halfwave potential (E1/2) were obtained in the case of Fe/Co/IL-CNF-800 and Fe/Co-CNF-900 for the materials with and without IL additive, respectively. Therefore, these two materials were chosen for further acid-treatment procedures (Fig. 6a, Table 3). According to the E1/2 value, the procedure 2 (12 h at 80 °C) provided a higher ORR activity than acid treatment by catalysts similar [START_REF] Li | High-performance bifunctional oxygen electrocatalysts for zincair batteries over mesoporous Fe/Co-N-C nanofibers with embedding FeCo alloy nanoparticles[END_REF] or more negative [START_REF] Cao | Electrospinning synthesis of transition metal alloy nanoparticles encapsulated in nitrogen-doped carbon layers as an advanced bifunctional oxygen electrode[END_REF][START_REF] Feng | Bamboo-like nitrogen-doped porous carbon nanofibers encapsulated nickel-cobalt alloy nanoparticles composite material derived from the electrospun fiber of a bimetal-organic framework as efficient bifunctional oxygen electrocatalysts[END_REF][START_REF] Liu | Electrospun ZIF-based hierarchical carbon fiber as an efficient electrocatalyst for the oxygen reduction reaction[END_REF] E1/2 values than Fe/Co/IL-CNF-800b have been reported. In contrast, more positive E1/2 of -0.13 V has been observed by Alegre et al.

for Ni/Co co-doped CNFs [START_REF] Alegre | NiCo-loaded carbon nanofibers obtained by electrospinning: Bifunctional behavior as air electrodes[END_REF]. In a recent work with excellent AEMFC performance (peak power density (Pmax) of 1350 mW cm -2 ) using cobalt ferrite nanoparticles supported on Vulcan XC-72, a relatively low current density at 0.8 V in AEMFC was observed that is in turn consistent with the reported low E1/2 of -0.30 V in 0.1 M KOH [START_REF] Peng | High-Performing PGM-Free AEMFC Cathodes from Carbon-Supported Cobalt Ferrite Nanoparticles[END_REF]. As multiple TM containing CNFs have been studied as cathode catalysts in PEMFCs [START_REF] Zang | Cobalt/zinc dual-sites coordinated with nitrogen in nanofibers enabling efficient and durable oxygen reduction reaction in acidic fuel cells[END_REF][START_REF] He | Single Cobalt Sites Dispersed in Hierarchically Porous Nanofiber Networks for Durable and High-Power PGM-Free Cathodes in Fuel Cells[END_REF],

then the ORR performance of the prepared NPMCs in 0.5 M H2SO4 was also explored (Fig. 6b, Table 3). Similarly to that observed in alkaline media, the E1/2 value closest to the one of Pt/C (E1/2 = 0.56 V) was observed for Fe/Co/IL-CNF-800b (E1/2 = 0.46 V) in 0.5 M H2SO4.

For comparison, in several studies with different TM co-doped CNFs, more positive E1/2 values have been reported, which are comparable to the one obtained for Pt/C herein [START_REF] He | Single Cobalt Sites Dispersed in Hierarchically Porous Nanofiber Networks for Durable and High-Power PGM-Free Cathodes in Fuel Cells[END_REF][START_REF] Liu | Trimetallic FeCoNi-N/C nanofibers with high electrocatalytic activity for oxygen reduction reaction in sulfuric acid solution[END_REF]. The ORR activity of Fe/Co/IL-CNF-800b and Fe/Co-CNF-900b catalysts was also studied in more detail. The RDE voltammetry curves for ORR obtained in 0.1 M KOH at different rotation rates and the Koutecky-Levich (K-L) plots derived from these polarisation curves are presented in Fig. 7. According to a comparison of the K-L plots, the diffusion-limited current is obtained at more positive potential values in the case of Fe/Co/IL-CNF-800b catalyst as the extrapolated K-L lines yield zero intercept at higher potential values compared to the ones of Fe/Co-CNF-900b. The electron transfer number (n) was calculated from the slopes of the K-L lines using the K-L equation and parameters taken from our previous work [START_REF] Mooste | Electrocatalysts for oxygen reduction reaction based on electrospun polyacrylonitrile, styrene-acrylonitrile copolymer and carbon nanotube composite fibres[END_REF]. The obtained n values (insets to Figs 7b,d) demonstrated that the ORR proceeds via 4e -pathway for both the studied catalysts, which is promising for the targeted application in AEMFC cathodes.

The RDE voltammetry curves at different rotation rates, the corresponding K-L plots and n values recorded in 0.5 M H2SO4 are presented in Fig. S4. The parameters for n calculation in acid solution were taken from our previous study [START_REF] De Almeida E Silva | Polymer-derived Co/Ni-SiOC(N) ceramic electrocatalysts for oxygen reduction reaction in fuel cells[END_REF]. In the acidic medium, the diffusionlimited current plateau is reached only for Fe/Co-CNF-900b catalyst from -0.3 V vs. SCE (Fig. S4d). On the other hand, the n value is closer to 4 at more positive potentials in the case of Fe/Co/IL-CNF-800b (inset to Fig. S4b). As the bifunctionality of the NPMCs to catalyse both OER and ORR reactions is very often evaluated for the development of rechargeable metal-air batteries [START_REF] Alegre | Toward more efficient and stable bifunctional electrocatalysts for oxygen electrodes using FeCo2O4/carbon nanofiber prepared by electrospinning[END_REF][START_REF] Cao | Electrospinning synthesis of transition metal alloy nanoparticles encapsulated in nitrogen-doped carbon layers as an advanced bifunctional oxygen electrode[END_REF][START_REF] Kisand | Bifunctional multi-metallic nitrogen-doped nanocarbon catalysts derived from 5-methylresorcinol[END_REF][START_REF] Kazimova | Shungitederived graphene as a carbon support for bifunctional oxygen electrocatalysts[END_REF], the OER performance in 0.1 M KOH was also studied (Fig. S5). The values of potential to achieve the OER current of 2 mA (E2mA), which is equal to 10 mA cm -2 are presented in Table 3. The lowest E2mA values were determined as 0.65 and 0.63 V vs. SCE for Fe/Co/IL-CNF-800b and Fe/Co-CNF-900b, respectively. For comparison, a very similar E2mA value to Fe/Co-CNF-900b has been previously reported for Ni/Co co-doped CNFs [START_REF] Alegre | NiCo-loaded carbon nanofibers obtained by electrospinning: Bifunctional behavior as air electrodes[END_REF]. For the evaluation of the catalyst bifunctionality, the difference of potential (E) between E1/2 (ORR) and E2mA (OER) [START_REF] Feng | Bamboo-like nitrogen-doped porous carbon nanofibers encapsulated nickel-cobalt alloy nanoparticles composite material derived from the electrospun fiber of a bimetal-organic framework as efficient bifunctional oxygen electrocatalysts[END_REF] should be calculated (Table 3). Herein, the lowest E value of 0.80 V was observed for Fe/Co/IL-CNF-800b and Fe/Co-CNF-900b electrocatalysts. For comparison, rather close E values (0.76-0.79 V) have been reported for several Ni/Co co-doped CNFs [START_REF] Alegre | NiCo-loaded carbon nanofibers obtained by electrospinning: Bifunctional behavior as air electrodes[END_REF][START_REF] Feng | Bamboo-like nitrogen-doped porous carbon nanofibers encapsulated nickel-cobalt alloy nanoparticles composite material derived from the electrospun fiber of a bimetal-organic framework as efficient bifunctional oxygen electrocatalysts[END_REF]. While, lower E values (0.62-0.69 V) have been obtained for Co/Fe/Ni [START_REF] Cao | Electrospinning synthesis of transition metal alloy nanoparticles encapsulated in nitrogen-doped carbon layers as an advanced bifunctional oxygen electrode[END_REF] and Fe/Co [START_REF] Li | High-performance bifunctional oxygen electrocatalysts for zincair batteries over mesoporous Fe/Co-N-C nanofibers with embedding FeCo alloy nanoparticles[END_REF] codoped CNF-based NPMCs in 0.1 M KOH. As an exception, in the remarkably more concentrated alkaline solution of 6.0 M KOH, the E value as low as 0.48 V has been reported for iron-cobaltite spinel supported CNFs [START_REF] Alegre | Toward more efficient and stable bifunctional electrocatalysts for oxygen electrodes using FeCo2O4/carbon nanofiber prepared by electrospinning[END_REF]. Regarding the latter study, the use of 6

M KOH is more suitable environment when the ZAB is the intended application.

AEMFC testing

According to the ORR studies in 0.1 M KOH, the Fe/Co/IL-CNF-800b and Fe/Co-CNF-900b catalyst materials were chosen for testing as the NPMCs at the AEMFC cathode (Fig. 8, Table 4). For comparison, the data for commercial Pt/C catalyst are also presented. The Pmax values show that the IL-containing nanofibre-based Fe/Co/IL-CNF-800b outperforms the IL-free Fe/Co-CNF-900b. Moreover, the Pmax of Fe/Co/IL-CNF-800b itself makes up ca 78 % of the Pmax obtained with commercial Pt/C. This shows the superiority of Pt/C in an AEMFC in agreement with the RDE results obtained in 0.1 M KOH (Fig. 6a). Although, the Pmax is often reached at the cell voltages at which the practical use is hindered due to the heat rejection.

Therefore, a comparison at a higher cell voltage (e.g. 0.65 V, P0.65V in Table 4) has been proposed for possible real-life applications [START_REF] Ramaswamy | Alkaline Anion-Exchange Membrane Fuel Cells: Challenges in Electrocatalysis and Interfacial Charge Transfer[END_REF]. Using the P0.65V values for comparison, a difference between Fe/Co/IL-CNF-800b and Pt/C is reduced from 22 % (Pmax) to 11 % (P0.65V).

It is complicated to compare the obtained results with the data from the literature as very different operating conditions and materials have been used for AEMFC testing [START_REF] Ramaswamy | Alkaline Anion-Exchange Membrane Fuel Cells: Challenges in Electrocatalysis and Interfacial Charge Transfer[END_REF][START_REF] Mustain | Understanding how high-performance anion exchange membrane fuel cells were achieved: Component, interfacial, and cell-level factors[END_REF].

Furthermore, there is no prior information available about the AEMFC performance with similar catalysts based on multiple TM co-doped electrospun CNFs. Therefore, the selection of results for comparison from other studies has been extended to include several TM codoped carbon-based catalyst materials of different origin (Table 4). As an exception, only Fedoped commercial NPMC material from our previous work (Fe-N-C) has been included as the AEMFC testing has been carried out in similar conditions [START_REF] Lilloja | Mesoporous iron-nitrogen co-doped carbon material as cathode catalyst for the anion exchange membrane fuel cell[END_REF]. The Fe-N-C catalyst, which was prepared using VariPore™ method by Pajarito Powder, LLC, outperformed both NPMCs prepared herein according to both, Pmax and P0.65V, values [START_REF] Lilloja | Mesoporous iron-nitrogen co-doped carbon material as cathode catalyst for the anion exchange membrane fuel cell[END_REF]. Among the selection of several TM co-doped materials presented in Table 4, the Fe/Co/IL-CNF-800b outperforms several catalyst materials that have been studied in AEMFC conditions with cell testing performed using the commercial Fumatech or Tokuyama AEMs with only Pt/C at the anode.

In addition to the ORR activity, at least as important role in the AEMFC performance is played by the effective integration of NPMC with the specific GDLs and membrane [START_REF] Li | Designing the 3D Architecture of PGM-Free Cathodes for H2/Air Proton Exchange Membrane Fuel Cells[END_REF][START_REF] Gottesfeld | Anion exchange membrane fuel cells: Current status and remaining challenges[END_REF].

Also, the high thickness of the NPMC catalyst layer at the cathode and water management issues (e.g. flooding or drying) have a great influence on the overall fuel cell performance [START_REF] Mustain | Catalysts for Polymer Membrane Fuel Cells[END_REF][START_REF] Omasta | Beyond catalysis and membranes: visualizing and solving the challenge of electrode water accumulation and flooding in AEMFCs[END_REF]. Considering these aspects together with different GDL material and gas flow rates can provide a possible explanation for the high Pmax of 415 mW cm -2 obtained with FeCoNC-at catalyst in an earlier investigation, where otherwise similar conditions to the present study were used [START_REF] Kisand | Transition metalcontaining nitrogen-doped nanocarbon catalysts derived from 5-methylresorcinol for anion exchange membrane fuel cell application[END_REF]. Also, more than two times higher Pmax of FeCoNC-at compared to the Fe/Co/IL-CNF-800b could not be solely caused by the catalysts ORR activity as in the kinetically controlled region remarkably closer current density values at 0.85 V (J0.85V) around 60 mA cm -2 were observed for both catalysts. Furthermore, more negative E1/2 value was observed in the case of FeCoNC-at (-0.18 V) compared to Fe/Co/IL-CNF-800b (-0.15 V) during the RDE experiments in 0.1 M KOH [START_REF] Kisand | Transition metalcontaining nitrogen-doped nanocarbon catalysts derived from 5-methylresorcinol for anion exchange membrane fuel cell application[END_REF]. In Table 4, the NPMCs showing higher Pmax values than FeCoNC-at have all been obtained with laboratory AEMs that are not commercially available. Among the selected catalysts, the highest AEMFC performance with Pmax of 1350 mW cm -2 has been obtained with cobalt ferrite nanoparticles supported on Vulcan carbon XC-72 (CF-VC), although, its E1/2 value measured in RDE experiments in 0.1 M KOH was moderate, at -0.30 V [START_REF] Peng | High-Performing PGM-Free AEMFC Cathodes from Carbon-Supported Cobalt Ferrite Nanoparticles[END_REF]. This observation confirms the critical importance of other aspects in addition to the ORR kinetic activity of a cathode, in order to achieve a high AEMFC performance. The observed ORR activity of the NPMC materials prepared in this work should be attributed to their specific composition and architecture. Firstly, the unique fibrous CNF structure with high SSA can provide a large amount of ORR-active sites [START_REF] Xu | Noble-metal-free electrospun nanomaterials as electrocatalysts for oxygen reduction reaction[END_REF][START_REF] Mooste | Electrocatalysts for oxygen reduction reaction based on electrospun polyacrylonitrile, styrene-acrylonitrile copolymer and carbon nanotube composite fibres[END_REF]. The studied NPMCs present a hierarchical porosity, with both micro-and mesopores (Fig. 5b). The micropores are considered necessary to reach high density of M-Nx sites [START_REF] Jaouen | Heat-treated Fe/N/C catalysts for O-2 electroreduction: Are active sites hosted in micropores?[END_REF][START_REF] Ratso | Non-precious metal cathodes for anion exchange membrane fuel cells from ball-milled iron and nitrogen doped carbide-derived carbons[END_REF], while the mesopores can promote faster mass transfer [START_REF] Xu | Noble-metal-free electrospun nanomaterials as electrocatalysts for oxygen reduction reaction[END_REF][START_REF] Zang | Cobalt/zinc dual-sites coordinated with nitrogen in nanofibers enabling efficient and durable oxygen reduction reaction in acidic fuel cells[END_REF]. The presence of high number of ORR-active sites was also supported by Raman spectroscopy, which showed a very defective carbon structure indicated by the bonding of different species through sp 3 carbon. The XRD patterns showed the formation of FeCo alloy, which could indicate to the co-existence of FeCo alloy and Fe/Co-Nx active sites that can promote the electrocatalytic activity towards the ORR [START_REF] Li | High-performance bifunctional oxygen electrocatalysts for zincair batteries over mesoporous Fe/Co-N-C nanofibers with embedding FeCo alloy nanoparticles[END_REF]. The presence of M-Nx sites was supported by XPS analyses in the N1s, Co2p and Fe2p regions (Table 2). In addition to the M-Nx sites, the pyridinic-N, pyrrolic-N and graphitic-N species were also observed in the N1s spectra. Both, pyridinic-N and graphitic-N are generally recognised to contribute to the high ORR activity of CNFs [START_REF] Xu | Noble-metal-free electrospun nanomaterials as electrocatalysts for oxygen reduction reaction[END_REF][START_REF] Alegre | Toward more efficient and stable bifunctional electrocatalysts for oxygen electrodes using FeCo2O4/carbon nanofiber prepared by electrospinning[END_REF][START_REF] Meng | ZIF67@MFC-Derived Co/N-C@CNFs Interconnected Frameworks with Graphitic Carbon-Encapsulated Co Nanoparticles as Highly Stable and Efficient Electrocatalysts for Oxygen Reduction Reactions[END_REF][START_REF] Lu | Electrospun nitrogen-doped carbon nano fibers for electrocatalysis[END_REF][START_REF] Peng | ZIF-67-derived Co nanoparticles anchored in N doped hollow carbon nanofibers as bifunctional oxygen electrocatalysts[END_REF]. In more specific, the pyridinic-N has been found to improve the ORR onset potential [START_REF] Zang | Cobalt/zinc dual-sites coordinated with nitrogen in nanofibers enabling efficient and durable oxygen reduction reaction in acidic fuel cells[END_REF], while the graphitic-N provides higher ORR current density through increased electrical conductivity [START_REF] Feng | Bamboo-like nitrogen-doped porous carbon nanofibers encapsulated nickel-cobalt alloy nanoparticles composite material derived from the electrospun fiber of a bimetal-organic framework as efficient bifunctional oxygen electrocatalysts[END_REF][START_REF] Wang | Metal-containing Ionic Liquid/Polyacrylonitrile-derived Carbon Nanofibers for Oxygen Reduction Reaction and Flexible Zn-Air Battery[END_REF]. In several studies [START_REF] Xu | Noble-metal-free electrospun nanomaterials as electrocatalysts for oxygen reduction reaction[END_REF][START_REF] Mooste | Electrocatalysts for oxygen reduction reaction based on electrospun polyacrylonitrile, styrene-acrylonitrile copolymer and carbon nanotube composite fibres[END_REF][START_REF] Shang | Electrospun Nitrogen-Doped Carbon Nanofibers Encapsulating Cobalt Nanoparticles as Efficient Oxygen Reduction Reaction Catalysts[END_REF], the pyrrolic-N has been also regarded beneficial for ORR as it could provide higher charge mobility and better donor-acceptor properties than other doped N species [START_REF] Liu | Free-standing nitrogen-doped carbon nanofiber films as highly efficient electrocatalysts for oxygen reduction[END_REF]. *The value is determined from the polarisation curve.

The higher AEMFC performance obtained with Fe/Co/IL-CNF-800b compared to Fe/Co-CNF-900b could be also explained in terms of the porous structure and site distribution in the fibre-based electrodes. Firstly, the IL was found to cause the appearance of peculiar formations on the nanofibre surface (Fig. 1) providing an easily accessible surface area compared to the IL-free fibres. Despite the SBET being higher in the case of Fe/Co-CNF-900b catalyst, the amount of surface pores and active sites in Fe/Co/IL-CNF-800b could be higher due to these structures. The pores on the nanofibre surface are more easily accessible to the O2 molecules compared to the inner porosity of CNF. Additionally, the physical characterisation showed a higher amount of micropores (Fig. 5b) and structural disorder (Fig. 3) in the case of Fe/Co/IL-CNF-800b catalyst. The more positive ORR Eonset value obtained for Fe/Co/IL-CNF-800b compared to the Fe/Co-CNF-900b in 0.1 M KOH (Table 3) could be due to the higher relative content of pyridinic-N (Table 2) [START_REF] Zang | Cobalt/zinc dual-sites coordinated with nitrogen in nanofibers enabling efficient and durable oxygen reduction reaction in acidic fuel cells[END_REF].

In

immersed overnight in 3 M

 3 KOH prior to assembling the MEA and sandwiching it into a 5 cm 2 cell (Fuel Cell Technologies Inc., USA) with silicone gaskets and a torque of 7 Nm. The performance of the prepared MEA was studied in a Greenlight Fuel Cell Test Station (G50 Fuel Cell System, Hydrogenics, Canada). During the testing, a flow rate of 0.05 NLPM for humidified (80 % RH) H2 and O2 gases, backpressure of 200 kPa, cell temperature of 60 °C and a scan rate of 10 mV s -1 were used.

3. Results and discussion 3 . 1

 31 Physical characterisation of catalystsThe physical characterisation was carried out on the catalyst materials with optimised pyrolysis temperature (i.e., 800°C for Fe/Co/IL-CNF and 900°C for Fe/Co-CNF), according to the ORR activity in 0.1 M KOH (as shown later in Section 3.2). SEM was employed to study the surface morphology of the prepared catalysts. The SEM micrographs for fibre diameter determination are shown in Fig.S1(Supplementary material). The diameter of Fe/Co/IL-CNF-800-based and Fe/Co-CNF-900-based fibres was about 450-650 and 550-750

Fig. 1 .

 1 Fig. 1. SEM images of (a) Fe/Co/IL-CNF-800, (b) Fe/Co/IL-CNF-800a, (c) Fe/Co/IL-CNF-800b, (d) Fe/Co-CNF-900, (e) Fe/Co-CNF-900a and (f) Fe/Co-CNF-900b catalyst materials.

Fig. 2 .

 2 Fig. 2. SEM EDX element mapping of the (a) Fe/Co/IL-CNF-800, (b) Fe/Co/IL-CNF-800a and (c) Fe/Co/IL-CNF-800b catalyst materials.

Fig. 3 .

 3 Fig.3. Raman spectra of Fe/Co/IL-CNF-800 and Fe/Co-CNF-900 catalysts and their acidleached counterparts. In the material designation a and b stand for acid treatment at 50 °C for 8 h (procedure 1) and 12 h at 80 °C (procedure 2), respectively, followed by second pyrolysis.

Fig. 4 .

 4 Fig. 4. (a, c) XPS survey spectra and (b, d) deconvoluted high resolution N1s spectra of (a, b) Fe/Co/IL-CNF-800, (c, d) Fe/Co/IL-CNF-800b catalyst materials. The insets to a and c show high resolution spectra in the Fe2p, Co2p regions deconvoluted to peaks characteristic to different Co and Fe oxidation states.

Fig. 5 .

 5 Fig. 5. (a) XRD patterns and (b) PSD for Fe/Co/IL-CNF-800b and Fe/Co-CNF-900b catalyst. The inset to b shows N2 adsorption/desorption isotherms that were used for PSD calculation.

procedure 1 (

 1 8 h at 50 °C). In more specific, the IL-containing CNF-based catalyst Fe/Co/IL-CNF-800b showed among the NPMCs studied the most positive Eonset and E1/2 values of -0.04 and -0.15 V vs. SCE, respectively. Although, these are still outperformed by Pt/C. On the other hand, compared to single TM (Fe or Co) containing CNFs in our previous work, the most positive E1/2 was reported as -0.19 V for Co-PAN-A1000 catalyst [22]. This indicates the advantage of Fe and Co co-doping compared to the use of single TM to achieve a higher ORR activity. In other studies with two or three TMs (Fe, Co, Ni, Zn) co-doped CNF-based

Fig. 6 .

 6 Fig. 6. RDE voltammetry curves for oxygen reduction on Fe/Co/IL-CNF-800-and Fe/Co-CNF-900-based catalysts in O2-saturated (a) 0.1 M KOH and (b) 0.5 M H2SO4 at 1900 rpm,  = 10 mV s -1 . NPMC loadings of (a) 0.2 and (b) 0.8 mg cm -2 were used. For comparison, the ORR polarisation curve for a commercial Pt/C (20 μgPt cm -2 ) catalyst is also presented.

Fig. 7 .

 7 Fig. 7. (a, c) RDE voltammetry curves and (b, d) Koutecky-Levich plots for O2 reduction recorded with (a, b) Fe/Co/IL-CNF-800b and (c, d) Fe/Co-CNF-900b coated GC electrodes in O2 saturated 0.1 M KOH at (1) 360, (2) 610, (3) 960, (4) 1900, (5) 3100 and (6) 4600 rpm at 10 mV s -1 . NPMC loading: 0.2 mg cm -2 . The insets to (b, d) show the electron transfer number (n) as a function of potential.

Fig. 8 .

 8 Fig. 8. Power density and polarisation curves for H2/O2 single cell AEMFC with the HMT PMBI anion exchange membrane and Fe/Co/IL-CNF-800b, Fe/Co-CNF-900b or Pt/C (0.4 mgPt cm -2 ) as a cathode catalyst. NPMC loading: 2 mg cm -2 . Pt-Ru/C (0.6 mgPt Ru cm -2 ) was used as the anode catalyst. T=60 °C.

  summary, we propose for the first time Co and Fe co-doped electrospun CNF-based cathode catalysts for AEMFC application. The power density obtained with Fe/Co/IL-CNF-800b catalyst at -0.65 V was 89 % of the power density obtained with commercial Pt/C. 4. Conclusions In the present work, the NPMC materials were prepared via pyrolysis of Co and Fe co-doped electrospun CNFs with and without an IL additive as porogen. The low temperature N2 physisorption studies indicated the formation of micro-mesoporous nanofibre structure and the XPS experiments showed the presence of different N and M-Nx species, which are all known to be beneficial for high ORR activity. Raman spectroscopy showed very defective carbon structure and the XRD patterns indicated the presence of a FeCo alloy, which can also boost the ORR activity. The IL-containing nanofibre-based Fe/Co/IL-CNF-800b material possess the highest activity towards the ORR as demonstrated by RDE studies in both acidic and alkaline media. The OER studies revealed that the ORR/OER reversibility (E) of such catalyst was 0.80 V. Very similar E value was obtained for the most active IL-free fibrebased NPMC (Fe/Co-CNF-900b). The Fe/Co/IL-CNF-800b and Fe/Co-CNF-900b were studied as a cathode catalyst in an AEMFC and the Pmax values of 168 and 195 mW cm -2 , respectively, were obtained. The Pmax value of the Fe/Co/IL-CNF-800b was 78 % of that obtained for Pt/C catalyst.

Table 1

 1 Elemental composition of different catalyst materials (wt%) as determined by EDX analysis. The transition metal content is also determined by MP-AES analysis*.

	Catalyst material	C	N	O	S	Fe	Co
	Fe/Co/IL-CNF-800 72.8	5.3 3.9 0	8.3 (7.82 +/-0.05)*	9.7 (8.13 +/-0.04)*
	Fe/Co/IL-CNF-800a 72.3	4.5 5.3 0	8.7 (8.33 +/-0.07)*	9.3 (9.43+/-0.08)*
	Fe/Co/IL-CNF-800b 75.2	6.6 4.7 0.5	6.4 (6.39 +/-0.12)*	6.7 (7.19 +/-0.07)*
	Fe/Co-CNF-900	72.8	3.5 5.0 0	8.1 (7.58 +/-0.16)*	10.6 (9.36 +/-0.14)*
	Fe/Co-CNF-900a	72.9	5.2 3.5 0	8.2 (8.75 +/-0.05)*	10.2 (10.27 +/-0.10)*
	Fe/Co-CNF-900b	80.4	4.2 3.2 0.

Table 3

 3 ORR onset potential (Eonset), ORR half-wave potential (E1/2), potential at OER current density at 10 mA cm -2 (E2mA) and the ORR/OER reversibility (E) values for different catalyst material coated electrodes. Test solutions: O2-saturated 0.1 M KOH (a) and 0.5 M H2SO4 (b). All the potential values have been obtained from the iR-drop corrected I-E curves.

	Electrode	Eonset (V) a	E1/2 (V) a E2mA(V) a E (V) a Eonset (V) b	E1/2 (V) b
	Fe/Co/IL-CNF-800	-0.07	-0.19	0.71	0.90	0.57	0.36
	Fe/Co/IL-CNF-800a	-0.06	-0.17	0.67	0.84	0.55	0.43
	Fe/Co/IL-CNF-800b	-0.04	-0.15	0.65	0.80	0.57	0.46
	Fe/Co-CNF-900	-0.07	-0.20	0.72	0.92	0.59	0.34
	Fe/Co-CNF-900a	-0.06	-0.19	0.68	0.87	0.59	0.41
	Fe/Co-CNF-900b	-0.07	-0.17	0.63	0.80	0.57	0.40
	Pt/C	-0.02	-0.13	-	-	0.68	0.56

Table 4 .

 4 Comparison of AEMFC performances using different TM co-doped electrocatalysts in the literature. As an exception, the Fe-N-C is only doped with Fe.

	Cathode catalyst	Pmax (mW cm -2 )	P0.65V (mW cm -2 )	J0.85V (mA cm -2 )	Cathode loading (mg cm -2 )	Anode cm -2 ) loading (mg	T (°C)	Membrane	Ref.
	Fe/Co/IL-CNF-800b	195	179	56	2	0.6 Pt-Ru 60	HMT-PMBI	This work
	Fe/Co-CNF-900b	168	168	53	2	0.6 Pt-Ru 60	HMT-PMBI	This work
	Pt/C	250	201	87	0.4 Pt 0.6 Pt-Ru 60	HMT-PMBI	This work
	Fe-N-C	220	217	133	2	0.6 Pt-Ru 60	HMT-PMBI	[39]
	NiCo/NCNTs 65	43*	14*	4	0.12 Pt 50	Fumatech FAA-3-50	[52]
	Co-Fe3O4/C 114	69*	0	0.8	0.8 Pt 60 Fumatech FAA-3 [53]
	CoFeN/C-HLH	177 146*	22*	4	0.4 Pt 50 Tokuyama A201 [54]
	FeCoNC-at 415	350	63	2	0.6 Pt-Ru 60	HMT-PMBI	[44]
	Fe-Co-N-C 420 181*	-	1	0.7 Pt-Ru	60	LDPE-based AEM	[55]
	α-
	Mn2O3/Fe0.5-	980 629*	154*	1.5	0.9 Pt-Ru 60 ETFE-based AEM [56]
	NH3
	CoFe-N-CDC/CNT	1120 815	24	0.01 metal 0.7 Pt-Ru 60 ETFE-based AEM [57]
	MnCo2O4/C 1200 775*	71*	0.8 metal oxides	0.4 Pt-Ru 80	QAPPT	[58]
	CF-VC	1350 651*	28*	2.4	0.7 Pt-Ru 70	polyethylene-based AEM	[21]

Acknowledgements

The present study was financially supported by the Estonian Research Council (grants PRG723 and PRG4) and the M-ERA.Net project "C-MOF.cell" (SLTKT20445). This research was also supported by the EU through the European Regional Development Fund (TK141 "Advanced materials and high-technology devices for energy recuperation systems", TK134 "Emerging orders in quantum and nanomaterials" and TK143 "Molecular cell engineering"). SC acknowledges IUF for financial support.