NiCo-loaded carbon nanofibers obtained by electrospinning: Bifunctional behavior as air electrodes

Accepted Manuscript NiCo-loaded carbon nanofibers obtained by electrospinning: Bifunctional behavior as air electrodes C. Alegre, E. Modica, A. Di Blasi, O. Di Blasi, C. Busacca, M. Ferraro, A.S. Aricò, V. Antonucci, V. Baglio PII:

S0960-1481(18)30233-7

DOI:

10.1016/j.renene.2018.02.089

Reference:

RENE 9820

To appear in:

Renewable Energy

Received Date: 6 November 2017 Revised Date:

7 February 2018

Accepted Date: 17 February 2018

Please cite this article as: Alegre C, Modica E, Di Blasi A, Di Blasi O, Busacca C, Ferraro M, Aricò AS, Antonucci V, Baglio V, NiCo-loaded carbon nanofibers obtained by electrospinning: Bifunctional behavior as air electrodes, Renewable Energy (2018), doi: 10.1016/j.renene.2018.02.089. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT 2

NiCo-loaded carbon nanofibers obtained by electrospinning: bifunctional behavior as air electrodes

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C. Alegre*1,2, E. Modica1, A. Di Blasi1, O. Di Blasi1, C. Busacca1, M. Ferraro1, A.S. Aricò1, V. Antonucci1, V. Baglio*1

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Istituto di Tecnologie Avanzate per l’Energia, Nicola Giordano, CNR-ITAE,

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Salita Santa Lucia sopra Contesse, 5, 98126, Messina (Italy) Laboratorio de Investigación en Fluidodinámica y Tecnologías de la Combustión, LIFTEC,

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CSIC-Univ. of Zaragoza, Maria de Luna 10, 50018 – Zaragoza (Spain)

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*corresponding authors: [email protected] ; [email protected]

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Abstract

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In this work, carbon nanofibers (CNF) synthesized by electrospinning are loaded with a

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combination of nickel and cobalt, both in the metallic and oxide forms (NiCo-loaded

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CNF), and studied as bifunctional air electrodes for metal-air batteries. The performance

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of the NiCo-loaded CNF sample is compared with a similarly prepared CoOCo/CNF

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catalyst both for oxygen reduction and evolution reactions. The combination of nickel

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and cobalt, both in metallic and oxide form, leads to a bifunctional catalyst with a half-

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wave potential for the ORR of 874 mV vs. RHE and a reversibility (∆EOER-ORR) of 764

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mV. The stability of the catalyst is assessed by means of a 24 h chronopotentiometric

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test at -80 mA cm-2 and charge-discharge cycles (30 minutes each) at ±20 mA cm-2.

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NiCo-loaded CNF shows a remarkable stability, maintaining a constant potential during

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both tests.

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Keywords: bifunctional catalysts; metal-air batteries; air electrodes; carbon nanofibers;

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transition metal oxides

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ACCEPTED MANUSCRIPT 1. Introduction

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In the last decades, the research in the field of energy storage has exponentially grown

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due to the need of depleting an economy based on fossil fuels in search of a more

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sustainable conversion and storage of energy [1–4]. Due to the limitations of Li-ion

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batteries [5,6], there has been a great interest in new types of energy storage devices,

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like metal-air batteries that exhibit excellent features, such as high theoretical energy

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output (high energy density), low cost, environmental friendliness and safety [7–12].

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These devices are based on a negative electrode composed of a light metal such as Fe,

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Zn, Al, Mg or Na and a positive electrode, which takes oxygen from the air. As one of

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the reactants, air, is not stored in the cell, the energy density of metal-air batteries is

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significantly higher in comparison to Li-ion devices [8,9,13]. Besides, the abundance

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and environmental friendliness of the metals employed, in particular Fe and Zn, make

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the design of these systems easier, lighter and safer [8,10,14].

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The main challenge of these devices is regarding the positive electrode (air

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electrode), being the one receiving a greater deal of attention in literature. Air electrodes

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in metal-air batteries are responsible for the oxygen reduction reaction (taking place

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during the discharge of the battery) and for the oxygen evolution reaction (during the

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charge). The electrochemistry of oxygen is particularly challenging due to the known

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slow kinetics of the oxygen reduction reaction (ORR) and to the high potentials (> 1.8

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V vs. RHE) reached during the oxygen evolution reaction (OER), directly affecting the

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stability of the system [7,15–18]. In the latest years, several electrocatalysts have been

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developed as bifunctional materials [19], including perovskites [20–25], layered metal

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oxides [26,27], heteroatom-doped carbons [28–30] with or without metals and transition

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metal-based catalysts (nitrides, sulfides and oxides) [8,31–37].

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ACCEPTED MANUSCRIPT Among the latest ones, catalysts prepared by electrospinning have been recently

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studied as a fast and efficient way of producing bifunctional materials [38]. This

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technique allows for the preparation of highly active metals supported or embedded

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within a graphitic carbon support, carbon nanofibers (CNFs), characterized by a high

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interaction with the metal, porosity and good resistance to corrosion, due to their

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graphitic character [38]. Recently, carbon nanofibers synthesized by electrospinning,

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modified with a combination of cobalt oxide and metallic cobalt (CoO-Co/CNF), were

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investigated in our laboratories as bifunctional air electrodes showing good reversibility

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and stability [39].

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In the present work, electrospun CNFs were loaded with a combination of nickel

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and cobalt (both in the metallic and oxide forms) and evaluated as air electrodes in

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comparison with CoOCo/CNF prepared under the same conditions. The novelty of this

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work relies on the use of a combination of nickel and cobalt both in metallic and oxide

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form, instead of a commonly used NiCo2O4 spinel type, in order to take advantage of

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the presence of the metals to increase the activity towards ORR [31,40–42].

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Furthermore, the utilization of carbon nanofibers, with the presence of nitrogen species,

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can give a synergistic activity in this sense. Special attention was also paid to the

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stability of this new system.

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ACCEPTED MANUSCRIPT 2. Experimental

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2.1. Materials and methods

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A total amount of 200 mg of Ni(OAc)2 and Co(OAc)2 (Sigma Aldrich, 99%) precursors

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were mixed in order to obtain a right proportions of Ni(OAc)2 : Co(OAc)2 (1:2) with a

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solution obtained by dissolving 800 mg of PAN in 10 g of DMF (Sigma Aldrich,

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99.8%). This well-defined amount of precursors allowed to obtain a clear solution with

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the appropriate characteristics, suitable for the electrospinning technique. The whole

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mixture was kept under stirring at 60 °C for 24 h. The obtained solution was inserted

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into a glass syringe of 20 mL capacity and equipped with a 21G stainless steel needle.

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An electric field of 17 kV was then applied between the needle and a graphite-based

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collector on which the fibers were deposited. The distance between the needle and the

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target was fixed at 12 cm, temperature and relative humidity were maintained at 21 °C

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and below 40% respectively. Subsequently, the nanofibers so-obtained were subjected

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first to a stabilization procedure: 30 min at 270 °C in air and secondly at 900 ° C for 1 h

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under He flow (80 ml min-1) with a ramp of 5 °C min-1.

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2.2. Physico-chemical characterization

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NiCo-loaded CNF sample was characterized by means of X-ray diffraction (XRD), X-

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ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM),

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thermogravimetry in air and N2-physisorption. The XRD analysis was performed with a

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Philips X-pert 3710 X-ray diffractometer with Cu Kα radiation operating at 40 kV and

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20 mA. The surface composition was investigated by XPS using a Physical Electronics

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(PHI) 5800-01 spectrometer with a monochromatic Al Kα X-ray source at 300 W. X-

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ray photoelectron spectra were obtained with a pass energy of 58.7 eV for elemental

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analysis and a pressure in the analysis chamber of 1·10–9 Torr. Quantitative analyses

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ACCEPTED MANUSCRIPT were carried out by dividing the integrated peak area by atomic sensitivity factors,

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which were calculated from the ionization cross-sections, the mean free electron escape

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depth and the measured transmission functions of the spectrometer. XPS data were

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interpreted by using the on-line library of oxidation states implemented in the PHI

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Multipak 6.1 software [43]. SEM and energy dispersive X-Ray (EDX) analyses were

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carried out using a FEI XL30 SFEG microscope, operated at 25 kV. The textural

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features were obtained from the nitrogen adsorption-desorption isotherms, measured at -

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196°C, using an ASAP 2020 M Micrometrics. Thermogravimetric analysis (TG/DSC

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STA 409C NETZSCH-Gerätebau GmbH) was carried out from room temperature to

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800 °C at a heating rate of 5 °C min-1 in air atmosphere.

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2.3. Electro-chemical characterization

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All the electrochemical characterizations were performed using an Autolab

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potentiostat/galvanostat with GPES and NOVA softwares. Two configurations of

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electrode were employed: (i) a rotating disc electrode with the aim of obtaining kinetic

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parameters such as Koutecky-Levich plots and (ii) a gas diffusion working electrode to

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mimic as closely as possible the conditions encountered in a real cell.

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2.3.1. Rotating disk electrode: 5 mg of the catalyst was dispersed in a mixture of

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isopropanol andwater (3:1 v/v) and Nafion® (5 wt.% solution) under sonication for 30

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minutes. 15 µL of this ink (equivalent to 50 µg cm-2 of the active phase, NiCo) was

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deposited onto a glassy carbon electrode (GC, 5 mm in diameter) used as the working

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electrode. The counter electrode was a platinum mesh and the reference electrode was

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an Hg|HgO electrode (AMEL, 112 mV vs. SHE) with a 1 M KOH solution as the

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electrolyte. Room temperature was employed for all the electrochemical experiments.

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The solution was deaerated with He prior to any experiment for 45 minutes, and the

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ACCEPTED MANUSCRIPT electrode was cycled from 0 to 1.2 V vs. RHE until a stable voltammogram was

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obtained. Linear sweep voltammetries were performed saturating the electrolyte

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solution with pure O2. Different rotation speeds (100, 200, 400, 1000, 1600 and 2500

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rpm) were evaluated for the study of the oxygen reduction reaction (ORR), from 1.2 to

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0.3 V vs. RHE. The oxygen evolution reaction (OER) was evaluated at 1600 rpm from

126

1.0 to 1.8 V vs. RHE.

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2.3.2. Gas diffusion working electrode: An ink with a 70 wt. % of catalyst and a 30 wt.

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% of Nafion® (5 wt.% solution) was prepared by dispersing the powder in a mixture of

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isopropanol and water under sonication for 30 minutes. The ink was then sprayed onto a

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hydrophobic backing layer (LT 1200 W ELAT, E-TEK) until reaching a loading of 0.5

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± 0.02 mg cm-2 of active phase. An electrode of 14 mm diameter was cut and placed on

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a Teflon holder acting as working electrode in a 3-electrode half-cell. As the reference

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electrode, we employed the same Hg|HgO electrode from previous measurements

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(AMEL, 112 mV vs. SHE) and the same Pt mesh as counter-electrode. In this case, in

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order to reproduce the conditions of a metal-air battery, a 6 M KOH solution was used

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as the electrolyte. Both ORR and OER were evaluated by means of linear sweep

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voltammetries in the same range of potentials previously described. Stability tests

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consisted on a chronopotentiometry at -80 mA cm-2 for 24 h and, subsequently, charge-

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discharge cycles for 1 h each at ±20 mA cm-2 for a total of 8 h.

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3. Results and discussion

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3.1 Physico-chemical characterization

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Surface area and cumulative desorption pore volumes were determined for both the bare

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electrospun CNF and the composite electrospun NiCo-loaded CNF by means of N2

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physisorption applying the Brunauer-Emmett-Teller (BET) equation. A surface area of

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ACCEPTED MANUSCRIPT about 80 m2 g-1 and 309 m2 g-1 as well as pore volumes of about 0.04 cm3 g-1 and 0.28

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cm3 g-1 were recorded for bare CNF and NiCo-loaded CNF samples, respectively. The

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nitrogen adsorption-desorption isotherms and pore size distribution curves on CNF and

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NiCo-loaded CNF samples are reported in Figures 1 and 2. Pristine CNF showed a type

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I isotherm (Figure 1a) characterized by a flat profile up to high partial pressure values,

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indicative of a macroporous structure, in addition to a narrow hysteresis loop, due to the

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presence of mesopores. From pore size distribution curves (Figure 1b), the relative

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percentage of mesoporosity and macroporosity were calculated, being around 22% and

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78% respectively.

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0.06

Differential Pore Volume (cm 3 / gr)

CNF

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Quanti ty Adsorbed (cm 3 / gr)

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77,9 %

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Relative Pressure (p/p°)

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Pore width (nm)

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Figure 1. (a) N2 adsorption-desorption isotherms and (b) Pore size distribution of electrospun carbon nanofibers. (Coloured areas indicate the type and percentage of pores: meso- and macro-pores).

NiCo-loaded CNF sample exhibited a type II isotherm (Figure 2a) showing a

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hysteresis loop indicative of a mesoporous structure. In fact, a higher percentage of

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mesopores was calculated from the pore size distribution plot, 31.5 % for NiCo-loaded

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CNF versus 22 % for the pristine CNF. The increase in mesoporosity for the NiCo-

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loaded CNF sample could be attributed to both Ni and Co salts employed as precursors,

159

which act as pore generators, enhancing the textural development [44].

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0.15

Desorption Adsorption

200

100

0 0.0

NiCo-CNF

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Macro Meso 0.05

31,5 %

68,5 %

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NiCo-CNF Differential Pore Volume (cm 3 / gr)

Quantity Adsorbed (cm 3 / gr)

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Pore wi dth (nm)

Relative Pressure (p/p°)

Figure 2. (a) N2 adsorption-desorption isotherms and (b) Pore size distribution of NiCo-loaded CNF. (Coloured areas indicate the type and percentage of pores: meso- and macro-pores).

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The surface composition of the NiCo-loaded CNF was investigated by means of

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XPS. Table 1 shows the atomic concentration of the different elements, C, O, N, Ni and

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Co.

Table 1. Chemical speciation of NiCo-loaded CNF catalyst and pristine CNF derived from XPS analysis survey spectra. Sample

N1s

O1s

Co2p

Ni2p

CNF

89.0

6.8

4.2

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NiCo-loaded CNF

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4.7

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C1s

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Bare CNFs are mainly composed of C, with almost a 7 % of N. When Co and Ni

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are incorporated into the carbon matrix, the concentration of both elements amounts 1.2

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and 0.3 %, respectively. The ratio of metallic precursors for the synthesis of NiCo-

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loaded CNF was 1 :2 (Ni : Co). This explains the lower content of Ni on the surface.

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The concentration of N in the NiCo-loaded CNF becomes 3%. This ratio (Ni:Co = 1:2)

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was chosen since a spinel type NiCo2O4, known as a good bifunctional catalyst in the

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literature [31,40–42], shows a similar composition.

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ACCEPTED MANUSCRIPT Figure 3 shows the XPS spectra for the different orbitals : C1s, O1s, N1s, Ni2p

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and Co2p. The C1s core level spectrum (Figure 3a) was deconvoluted into four peaks.

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The large peak centered at 284.7 eV is attributed to the graphitic carbon. The peak

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centered at 285.5 eV can be ascribed to amorphous carbon. The low intensity peaks

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centered at 286.6 eV and 291 eV show the presence of hydoxyl and carboxyl species on

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the material surface, respectively. Table 2 shows the atomic concentration of species

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obtained from the deconvolution of the XPS spectrum. Graphitic carbon (C=C) is the

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predominant species, 49 %, C-O and amorphous carbon present a concentration around

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20 %.

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Table 2. Atomic concentration determined by XPS for orbitals C1s and O1s on NiColoaded CNF. C=C sp2 284.7 eV 49.0 Ni-O; Co-O 529.5 eV 24.0

C-C sp3 285.5 eV 19.7 C=O Carbonylic 531 eV 18.1

C-OH 286.6 eV 21.2 C-OH aliphatic 532 eV 41.3

C-OOH 291 eV 10.1 C-OH phenolic 533 eV 16.6

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The deconvolution of O1s core level spectrum (Figure 3b) shows the presence of

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four peaks. The peak centered at 529.5 eV is typical of metal-oxygen bonds,

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representing a 24 % of the total concentration, as seen in Table 2. The peak at 531 eV is

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attributed to the carbonyl species (C=O) while the peaks at 532 eV and 533 eV can be

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ascribed to the presence of both aliphatic and phenolic hydroxyl species (C-OH) on the

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surface, respectively. The N1s orbital (Figure 3c) was deconvoluted into five peaks. The

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peaks centered at 398 eV and 401 eV can be attributed to the pyridinic (NP) and

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graphitic (NG) species, respectively. These contributions are predominant, with atomic

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concentrations of 19 and 48 % (see Table 3). The peak at 399 eV confirms the presence

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of Co-Nx species, with an atomic contribution of 13 %. The low-intensity peak at 400

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eV is ascribed to a small amount of pyrrolic species. Moreover, a low percentage of

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oxidized nitrogen (Nox) is observed at about 403 eV.

(b)

(a) 292 290 288 286 284 282 Binding Energy / (eV)

538

280

Ni°

Intensity / a.u.

NiO Ni2O3

(c)

Co-Nx

Ni2p3/2

Ni2p1/2

890 885 880 875 870 865 860 855 850 Binding Energy / (eV)

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408 406 404 402 400 398 396 394 392 Binding Energy / (eV)

CoO

534 532 530 528 Binding Energy / (eV)

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NP (Pyridinic) Co-Nx NPr (Pyrrolic) NG (Graphitic) Nox (N-oxides)

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Ni-O and Co-O C=O C-OH aliphatic C-OH phenolic

Intensity / a.u.

Intensity / a.u.

C=C sp2 C-C sp3 C-OH C-OOH

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Intensity / a.u.

Co2p1/2

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Figure 3. XPS spectra for the NiCo-loaded CNF catalyst: (a) orbital C1s ; (b) orbital O1s ; (c) orbital N1s ; (d) orbital Ni2p and (e) orbital Co2p. 198

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The Ni2p spectrum (Figure 3d) shows the prevalence of two kinds of Ni species

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with the peaks centered at about 854 eV and 871 eV ascribed to NiO and the peaks

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centered at about 856 eV and 873 eV ascribed to Ni2O3. The low-intensity peak

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centered at about 852.7 eV can be attributed to the presence of Ni in metallic form.

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ACCEPTED MANUSCRIPT Finally, the deconvolution of Co2p spectrum (Figure 3e) shows a large peak centered at

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780 eV and the associated satellites at a higher B.E., which can be attributed to the

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typical CoO paramagnetic species. The low-intensity peak centered at 778.3 eV

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suggests the presence of a little amount of cobalt in metallic form. Moreover, the peak

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at about 782 eV is indicative of the presence of Co-Nx species, as also corroborated by

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the N1s deconvolution. The presence of metallic Ni and Co species is explained by the

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fact that during the carbothermal reduction at 900 °C, the environment is strongly

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reducing, causing formation of metallic Ni and Co particles. During subsequent cooling

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and exposure to the air, according to a passivation procedure, the outermost layers of the

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Ni and Co particles transform into an oxide phase, being this thermodynamically more

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stable. Table 3 shows the atomic concentration determined by XPS for orbitals N1s,

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Ni2p and Co2p. Co-O and Ni-O are the predominant forms.

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Table 3. Atomic concentration determined by XPS for orbitals N1s, Ni2p and Co2p on NiCo-loaded CNF. 19.0 Co 9.1

Co-Nx 12.7 Co-O 62.7

Ni 16.8

NiO 54.2

NPr

NG

9.9 Co-Nx 28.2 Ni2O3

47.9

Nox 10.5

28.9

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NP

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N1s orbital % species Co2p orbital % species Ni2p orbital % species

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SEM images were taken on both pristine CNF and NiCo-loaded CNF as shown in

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Figure 4. Both materials present a uniform size of about 250-300 nm. NiCo-loaded CNF

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sample shows a higher roughness degree due to the presence of NiCo nanoparticles

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along the nanofibers that confirms a higher specific surface area, as revealed by BET

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analysis.

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Figure 4. SEM micrographs at low (a) and (c) and high magnification (b) and (d) for pristine CNF and NiCo-loaded CNF electrospun samples, respectively.

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A comparison of the XRD patterns of the pristine CNF and NiCo-loaded CNF is

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reported in Figure 5. The peak at 2θ = 25.8°, attributed to the (002) reflection of carbon,

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is sharper, indicating a more graphitic structure, for the NiCo-loaded CNF sample than

227

for the pristine CNF. The presence of both nickel and cobalt, known as graphitization

228

catalysts, leads to a higher graphitic carbon. By observing the NiCo-loaded CNF

229

pattern, a composite structure (composed of 2 phases) is evident. However, it is not

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possible to distinguish between metallic Co and Ni neither between their oxidized forms

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(NiO and CoO), as observed in Figure 5 and reported in the ICSD database. The

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formation of an alloy between the two species can not be discarded. Typical peaks of

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the cobalt or nickel in metallic form are present at 2θ=45° (111) and 2θ=52° (200) as

234

well as the CoO e NiO structures, characterized by lattice planes (110) at 2θ=37°, (200)

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at 2θ=43°and (111) at 2θ=62°. The presence of Co and Ni in their metallic form is

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ascribed to the carbothermal reduction of the respective oxides at 900°C, as previously

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explained, in inert atmosphere [45].

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CoO [00-001-1227] NiO [00-022-1189] Co [00-015-0806]

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Intensity / a.u.

Ni [00-001-1258]

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Figure 5. XRD patterns for both the bare CNF and for the NiCo-loaded CNF composite.

Figure 6 shows the results obtained from the thermogravimetric analysis on both

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the bare CNF and the NiCo-loaded CNF composite. TG and DSC profiles of CNF and

241

NiCo-loaded CNF samples show an initial weight loss of about 5% due to a dehydration

242

process. A weight loss of about 95% and a typical exothermic curve between 400-

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600°C due to carbon thermal decomposition was recorded for pristine CNF. TG curve

244

of NiCo-loaded CNF shows a larger weight loss of about 15% with a variation from

245

exothermic to endothermic curve in the range from 300-380°C probably due to the

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metallic forms oxidation of the electrospun sample. A further weight loss of about 60%

247

at 400-550°C could be ascribed to CNF decomposition [46]. As well known, the

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presence of metal oxides act as a catalyst, decreasing carbon oxidation temperature

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[47,48]. The percentage of NiCo loaded on CNF was around 20 wt. %.

(a)

CNF 16

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90

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200

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10

70

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12

80

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600

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NiCo-loaded CNF

100

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80

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60 50

4

40

2

30

DSC / mW mg

110

20 0 10 0 0

100

200

300

400

500

600

-2 700

Temperature / ºC Figure 6. Thermogravimetric analysis in air at 5 ºC min-1 of (a) electrospun carbon nanofibers and (b) NiCo-loaded carbon nanofibers. 14

ACCEPTED MANUSCRIPT 3.1 Bifunctional activity towards the oxygen reduction and the oxygen evolution

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reactions.

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RDE measurements were carried out to analyze the catalytic activity of NiCo-loaded

253

CNF, eliminating other effects such as mass transport [8]. Figure 7a shows the LSV

254

curves for the NiCo loaded CNF at different rotation speeds in an O2-saturated 1 M

255

KOH solution. Limiting current density increases as a result of the better oxygen

256

diffusion with increasing rotating speed. Koutecky-Levich plots (Figure 7b) were

257

employed to calculate the number of electrons transferred in the oxygen reduction

258

reaction, being 4 e-.

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(a)

Potential vs. RHE / V

0.2

0.3

0.0

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0.8

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200 rpm

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-4.0 1600 rpm -4.5 -5.0 2500 rpm

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Current density / mA cm

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-5.5

NiCo-loaded CNF

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Potential vs. Hg|HgO / V

15

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ACCEPTED MANUSCRIPT

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0.50 V vs RHE 0.55 V vs RHE 0.60 V vs RHE 0.65 V vs RHE 0.70 V vs RHE

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i / mA cm

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NiCo-loaded CNF -1/2

ω

-1/2

/ rpm

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Figure 7. (a) LSV curves at different rotation rates recorded in a 1 M KOH O2saturated solution. Scan rate: 5 mV s-1. (b) Koutecky-Levich plots at different potentials. 259

Given certain similarities of this compound with the one published in our previous

261

work, a graph in which NiCo-loaded CNF was compared with CoO-Co/CNF is shown

262

in Figure 8 [39]. Pt/C and IrO2 are also included in the figure as state-of-the-art materials for

263

the ORR and the OER, respectively.

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Although Pt/C presents the best onset for the ORR (as expected), the combination of Ni

265

and Co species provides a better performance towards the oxygen reduction in

266

comparison to both referenced catalysts, CoO-Co/CNF and Pt/C. This is particularly

267

evident in terms of limiting current density and half-wave potential, especially when

268

comparing to the CoO-Co/CNF catalyst: NiCo-loaded CNF sample presents a half-wave

269

potential positively shifted 15 mV whereas the limiting current density for the NiCo-

270

based composite is a 30 % higher. Thus, we can consider the NiCo-loaded CNF sample

271

as a suitable oxygen reduction catalyst.

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272

Transition metals are known for their activity towards the oxygen reduction

273

reaction. Our NiCo-loaded CNF catalyst presents a combination of both NiO and CoO 16

ACCEPTED MANUSCRIPT with a significant amount of N-groups, also known for their activity for the ORR. In

275

particular, our NiCo-C composite possesses several groups that have already been

276

proved to be highly active, i.e, Co-Nx, responsible for the adsorption and conversion to

277

intermediate reaction products of the O2 molecule; graphitic N and pyridinic N, both

278

known for improving onset potential, 4 e- mechanism and increasing the activity

279

(depending on the amount of graphitic N) [49], metallic Co and Ni particles also present

280

activity for the ORR as secondary active sites [39]. Although not detected through

281

XRD, an alloy between Ni and Co species, with a promoting effect on the catalytic

282

activity for the ORR, cannot be discarded [50].

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The oxygen evolution reaction was also evaluated by LSVs, as shown in Figure

284

8b. IrO2 presents the best onset for this reaction, as expected. However, once again, the

285

NiCo-based catalyst outperforms both IrO2 and CoO-Co/CNF in terms of current

286

density. Among non-noble metal catalysts, metallic nickel or nickel peroxide materials

287

are the most effective for obtaining high catalytic activities in alkaline solution. This is

288

due to the interaction of Ni atoms with water molecules that easily form Ni-O bonds,

289

accelerating the OER processes [51].

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(a)

Potential vs. RHE / V 0.3 0.0

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CoO-Co/CNF NiCo-loaded CNF Pt/C

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Current density / mA cm

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Potential vs. Hg|HgO / V

(b)

Potential vs. RHE / V 1.2

1.3

100

1.7

1.8

1.9

NiCo-loaded CNF

CoO-Co/CNF

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40

1.6

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IrO2

30 20

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Current density / mA cm

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80

60

1.5

CoO-Co/CNF NiCo-loaded CNF IrO2

90

70

1.4

10 0

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Potential vs. Hg|HgO / V

Figure 8. (a) LSVs recording oxygen reduction reaction and (b) LSV recording oxygen evolution reaction for the NiCo-loaded CNF in comparison to Pt/C, IrO2 and CoO-Co/CNF [39]. (LSVs were obtained at 1600 rpm of rotation speed in an O2-saturated 1M KOH solution for the ORR and in an N2-saturated 1M KOH solution for the OER). 290

18

ACCEPTED MANUSCRIPT An important parameter to take into account when using these materials as

292

bifunctional catalysts is their reversibility, i.e., the difference (in terms of potential)

293

between both reactions. The lower this value, the better reversibility (better

294

bifunctionality) [50,52–54]. There are several ways of measuring this ∆E. One takes

295

into account the difference between the half-wave potential for the ORR and the

296

potential at a current density of 10 mA cm-2 for the OER [55,56]. Other authors consider

297

the potential at -3 mAcm-2 for the ORR and the potential at 10 mA cm-2 for the OER

298

[54,57]. Another way considers the potential at -1 mAcm-2 for the ORR and the onset

299

potential for the OER. The present approach considers the first way (taking into account

300

the half-wave potential for the ORR), since it was used in a previous work [39]. Values

301

are reported in Table 4.

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Table 4. Reversibility for the ORR and OER measured as ∆E = Ehwp ORR – E10mAcm-2 OER EORR / mV @ half-wave potential

303

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Catalyst

EOER / mV @ 10 mAcm-2

∆E / mV (EORR EOER)

NiCo-loaded CNF

874

1638

764

CoO-Co/CNF

858

1667

809

NiCo-loaded CNF shows a ∆E of 764 mV (see Table 4), a low value in

305

comparison to the ones found in literature for similar materials. For instance, Fu et al.

306

[50] reached a ∆E of 860 mV with a NiCo-hydrogel calculated in the same way as

307

reported here. Besides, this material showed a lower current density towards both

308

reactions in comparison to the present work. Other authors calculated ∆E as the

309

difference between the potential for the ORR at -3 mA cm-2 and the potential for the

310

OER at 10 mA cm-2. In that case, the NiCo-loaded CNF would have a reversibility ∆E

311

of 781 mV. For example, Prabu et al. [54] obtained a ∆E of 840 mV with a NiCo-spinel

312

synthesized by electrospinning. Other systems, like the one presented by Singhal et al.,

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19

ACCEPTED MANUSCRIPT based on CoFe oxide presented reversibility values similar to NiCo-loaded CNF, 785

314

mV [57]. These authors [57] reported a table of ∆E of different studies in literature. The

315

NiCo-loaded CNF sample would be the fourth in their table, in which the most active

316

bifunctional systems are : (1) CO-N-CNTs by Wang et al. [58] with ∆E of 560 mV, (2)

317

Co3O4/N-rGO by Liang et al. with 715 mV of ∆E and (3) Fe/C/N by Zhao et al, with ∆E

318

760 mV. These results prove that there is still margin for improvement, by appropriately

319

tailoring the nature and properties of this type of oxides based on transition metals.

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A gas diffusion working electrode was employed to mimic the conditions of an

322

air-breathing cathode of a metal-air battery. Figure 9 shows the polarization curves

323

obtained in a 6 M KOH solution for the NiCo-loaded CNF catalyst in comparison to the

324

referenced one. The trend is similar to the one obtained by means of RDE, being the

325

performances more similar between the two catalysts under these more practical

326

conditions.

0.30

1.50 1.35 1.20

0.15

1.05

0.00

0.90

-0.15

0.75

-0.30

0.60

-0.45 -300 -250 -200 -150 -100

-50

0

50

100

Current density / mA cm

150

200

250

Potential vs. RHE / V

0.45

1.65

AC C

Potential vs. Hg|HgO / V

0.60

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0.75

1.80

CoO-Co/CNF NiCo-loaded CNF

EP

0.90

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320 321

300

-2

Figure 9. Polarization curves for the ORR and the OER for NiCo-loaded CNF in comparison to CoO-Co/CNF [39]. Conditions: O2 flow during the ORR and N2 during the OER; scan rate: 5 mV s-1; 6 M KOH as the electrolyte. 20

ACCEPTED MANUSCRIPT The stability of air cathodes is still an un-resolved challenge for metal-air

328

batteries. The NiCo-loaded CNF catalyst was tested under two different conditions to

329

assess its stability. First, a chronopotentiometric analysis for 24 h at -80 mA cm-2 was

330

carried out. As ascertained from the graph, NiCo-loaded CNF sample shows a stable

331

potential along the whole test.

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-0.32

NiCo-loaded CNF

0.64

SC

-0.28 -0.26

0.66

-0.24

0.68

-0.22 -0.20

-0.16 0

2

4

TE D

-0.18

6

8

10

12

0.70

Potential vs. RHE / V

0.62

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Potential vs. Hg|HgO / V

-0.30

0.72 0.74 0.76

14

16

18

20

22

24

Time / h

332

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Figure 10. Chronopotentiometric test at -80 mA cm-2 for NiCo-loaded CNF performed in a 6 M KOH solution flowing O2.

For further assessment of the durability of the material, the same electrode was

334

subjected to charge-discharge cycles, performed at ± 20 mA cm-2 with a duration of 60

335

min each cycle (30 minutes charge, 30 minutes discharge) in a 6 M KOH solution.

336

AC C

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337 338 339 340

21

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1.05

NiCo-loaded CNF

0.75

0.60

0.60

0.45

0.45

0.30

0.30

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0.75

Potential vs. RHE / V

0.90

0.15

0.15

0.00

0.00

-0.15

-0.15

-0.30 -0.45 1

2

3

4

5

6

7

-0.30 -0.45 8

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0

SC

Potential vs. Hg|HgO / V

0.90

Time / h

Figure 11. Charge/discharge cycles at ± 20 mA cm−2 for the NiCo-loaded CNF performed in a 6 M KOH solution flowing O2. 341

4. Conclusions

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NiCo-loaded carbon nanofibers were in-house prepared by electrospinning. The

344

obtained material showed a meso-macroporous structure with a 20 wt.% of metallic

345

phase (Ni-Co). XRD and XPS studies revealed the presence of Ni and Co, both in the

346

metallic and the oxide form, as well as N surface groups, mainly graphitic and pyridinic

347

types. Co-Nx species were also encountered on the surface of NiCo-loaded CNF. This

348

material was evaluated as air electrode for metal-air batteries. Accordingly, the oxygen

349

reduction reaction and the oxygen evolution reaction were studied in an alkaline

350

solution. NiCo-loaded CNF showed a remarkable reversibility, with a potential

351

difference, ∆E, of 764 mV. The activity was ascribed to both the combination of NiO

352

and CoO, along with their metallic forms, as well as to the presence of N-groups. This

353

catalyst also showed a significant stability, sustaining a constant potential when

354

subjected to chronopotentiometric tests and charge-discharge cycles.

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Acknowledgments

356

The research leading to these results has received funding from the “Accordo di

357

Programma CNR-MiSE, Gruppo tematico Sistema Elettrico Nazionale – Progetto:

358

Sistemi elettrochimici per l’accumulo di energia”.

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