Activated carbon composites

Journal Pre-proof Enhanced oxygen evolution performance of spinel Fe0.1Ni0.9Co2O4/Activated carbon composites Yi-Ting Lu, Jianyuan Wu, Zhi-Xiu Lin, Ting-Hsuan You, Sheng-Chi Lin, Hsin-Yi Tiffany Chen, Laurence J. Hardwick, Chi-Chang Hu PII:

S0013-4686(19)31857-2

DOI:

https://doi.org/10.1016/j.electacta.2019.134986

Reference:

EA 134986

To appear in:

Electrochimica Acta

Received Date: 8 August 2019 Revised Date:

18 September 2019

Accepted Date: 30 September 2019

Please cite this article as: Y.-T. Lu, J. Wu, Z.-X. Lin, T.-H. You, S.-C. Lin, H.-Y. Tiffany Chen, L.J. Hardwick, C.-C. Hu, Enhanced oxygen evolution performance of spinel Fe0.1Ni0.9Co2O4/Activated carbon composites, Electrochimica Acta (2019), doi: https://doi.org/10.1016/j.electacta.2019.134986. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2019 Published by Elsevier Ltd.

The OER activity of Fe0.1Ni0.9Co2O4 is enhanced by the introduction of activated carbon (AC).

Enhanced

Oxygen

Evolution

Performance

of

Spinel

Fe0.1Ni0.9Co2O4/Activated Carbon Composites Yi-Ting Lua,b, Jianyuan Wuc, Zhi-Xiu Lina, Ting-Hsuan Youa, Sheng-Chi Lina, Hsin-Yi Tiffany Chend, Laurence J. Hardwick*b, Chi-Chang Hu*a a

Department of Chemical Engineering, National Tsing Hua University, HsinChu, 30013 Taiwan.

E-mail: [email protected] b

Stephenson Institute for Renewable Energy, Department of Chemistry, University of Liverpool,

Liverpool L69 7ZD, UK. E-mail: [email protected] c

Department of Materials Science and Engineering, National Tsing Hua University, HsinChu 30013

Taiwan. d

Department of Engineering and System Science, National Tsing Hua University, HsinChu 30013

----------------------------------------------------------------------------------------------------------------------------

1

Abstract: A series of iron-doped nickel cobalt oxide/activated carbon (Fe0.1Ni0.9Co2O4/AC) composites with various ratios of activated carbon were synthesised via a microwave-assisted hydrothermal method. The introduction of activated carbon significantly promotes the catalytic activity of Fe0.1Ni0.9Co2O4 for the oxygen evolution reaction (OER); meanwhile the complete coverage of nanoparticulate Fe0.1Ni0.9Co2O4 onto activated carbon particles prevents the carbon oxidation, leading to improved cycling durability of a zinc-air cell. Scanning electron microscopy images shows a good protective coverage of the nanoparticulate Fe0.1Ni0.9Co2O4 onto the

activated

carbon

particles.

Rotating

disk

electrode

voltammetry

and

electrochemical impedance spectroscopic results reveal that the OER overpotential and charge-transfer resistance of the Fe0.1Ni0.9Co2O4 electrocatalyst are substantially lowered by the introduction of activated carbon. With respect to the cycling stability, Fe0.1Ni0.9Co2O4/1.0wt%AC and Fe0.1Ni0.9Co2O4/1.5wt%AC composites maintain excellent activity for 80 h in a rechargeable zinc-air battery, due to the uniformly protective coverage of metal oxide on activated carbon. Furthermore, the Fe0.1Ni0.9Co2O4/3.7wt%AC composite was examined in a prolonged rechargeable zinc-air battery test for 120 h and the charge-discharge voltage gap was negligibly enlarged (0.02V increment after 120 h), which showed their potential as 2

electrocatalysts for long-term energy storage systems.

Keywords: iron-doped nickel cobalt oxide, activated carbon, oxygen evolution reaction, rechargeable zinc-air battery, cycling stability.

3

1. Introduction The oxygen evolution reaction (OER) requires a large overpotential to proceed at a sufficient rate [1-3]. Therefore, substantial research efforts have been paid on the development of efficient and durable electrocatalysts. Some precious metals and their oxides, including Ru and Ir, are frequently studied since they have been reported to be the most active catalysts [4, 5]. However, these noble-metal-based electrocatalysts suffer from disadvantages including scarcity and high cost, which obstructs the practical application in the OER catalysis [6]. By contrast, NiCo2O4-based oxides have drawn much attention because of their abundance, low cost, environmental benignity, good corrosion resistance and high electrocatalytic activity [7-10]. For example, in previous work, Fe0.1Ni0.9Co2O4 has been found to possess an enhanced electrocatalytic activity toward the OER [11]. Nevertheless, due to the naturally low conductivity and/or their tendency to agglomerate [12, 13], metal-oxide-based catalysts can be further improved by introducing conducting substrates such as carbon materials [14]. From the literature, numerous metal oxide-carbon composites have been reported to possess good catalytic properties in energy storage/conversion applications. However, many of these proposed composites suffer from performance decay after continuous utilisation in devices such as rechargeable metal-air batteries. For example, Xu et al. reported an 4

FeCo-based nanoparticles@heteroatom-doped carbon microsphere (FeCo@C MS) where the catalytic performance declined at 10 mA cm-2 after one hour [15]. Alegre et al. proposed La0.6Sr0.4Fe0.8Co0.2O3 mixed with carbon black (LSFCo/KB) catalyst which underwent a decrease in electrochemical performance at 80 mA cm-2 after 1500 seconds [16]. In addition, Li et al. synthesised carbon-encapsulated Co3O4-doped Co/CoFe nanostructured electrocatalyst that exhibited an apparently larger charge-discharge voltage gap at 5mA cm-2 [17]. Consequently, the carbon-supported composite should overcome the issue about long-term durability in order to be employed in practical energy-storage systems. One of the most common way to fabricate the transition metal oxide with uniform

composition

is

the

hydrothermal

method

[18-21].

Recently

the

microwave-assisted hydrothermal method has frequently been utilised since it provides many advantages; for instance, the homogeneous heating within the whole bulk solution, low energy consumption and short time of synthesis [22-26]. From the literature, carbon material is well known as a good microwave absorber [27], which suggests that the heating can be enhanced around the microwave-absorbing carbon materials [22]. Such selective heating favours the synthesis of carbon-supported composites. Furthermore, activated carbon has been reported to be the optimum microwave-absorbing carbon material, which can create many local hotspots that will 5

turn into active centres for the crystallite formation [27, 28]. Accordingly, the metallic ion (Ni2+, Co2+ and Fe3+) precursors can be selectively heated on the surface of activated carbon to facilitate the formation of metal oxide-coated activated carbon composites with strong metal oxide-carbon coupling [29-31]. However, activated carbon is rarely used as the supporting material in the OER application due to its vulnerability in alkaline electrolytes or in the highly positive potential region. Meanwhile, activated carbon has been reported to possess acceptable but relatively low conductivity compared to other carbon materials [32-34], which should be attributed to the large contact resistance of activated carbon particles and medium degree of graphitisation [35-37]. Despite these considerations, activated carbon is selected as the supporting material of the composite in this study due to the following reasons. Firstly, among carbon in various forms, activated carbon is frequently employed in carbon-supported catalysts due to its low cost and mass availability compared to other carbon materials [38-41]. Secondly, the large activated carbon particles can be well covered by small metal oxide nanoparticles (NPs) to form the core-shell-like structure via the microwave-assisted hydrothermal method, which creates another oxide-carbon electron transport pathway in addition to the original oxide-oxide electron transfer pathway [29, 42, 43]. Such a new pathway is thought to facilitate the electron 6

transport in the whole catalytic process [44, 45]. Thirdly, activated carbon naturally provides abundant surface functional groups, resulting in unique advantages in comparison with the other carbon materials. For example, the functional groups provide good hydrophilicity that leads to a more straightforward preparation of composite catalysts in aqueous solution [38, 46, 47]. In addition, the functional groups usually work as the anchoring sites for metallic ions during the preparation, so copious oxygen-containing functional groups can make both uniform dispersion and intimate interaction of metal oxide NPs upon the surface of activated carbon particles [38, 43, 48]. This uniform dispersion of metal oxides is believed to circumvent the possible carbon corrosion in the alkaline electrolyte or in the highly positive potential region, leading to better durability of the resultant composite electrocatalysts. When it comes to other carbon materials with high conductivity including carbon nanotube and graphene, they need additional chemical functionalisation processes to improve their poor hydrophilicity and chemical inertness, which inevitably reduces the electrical conductivity and structure stability instead [43, 49]. In this study, a two-step microwave-assisted hydrothermal method was applied to synthesise the Fe0.1Ni0.9Co2O4/AC composite and follows a similar procedure. previously reported in the preparation of nano-crystallite/graphene sheet composites [50]. A series of analyses, including scanning electron microscopy (SEM), X-ray 7

diffraction (XRD), and N2 adsorption/desorption isotherm, were utilised to investigate how the nanocrystals were supported by activated carbon in the as-prepared composites. The intrinsic catalytic activity of the OER was studied by the voltammetric tests in rotating disk electrode (RDE) tests, while the charge transfer resistance of the OER was investigated by electrochemical impedance spectroscopy (EIS). The practical use of the composite catalysts was examined in rechargeable zinc-air batteries since it underwent the OER during charging processes as well. The alkaline and high-voltage environment is considered the severe tests to prove the superior cycle stability and good performance of the Fe0.1Ni0.9Co2O4/AC electrocatalyst, which is practically applicable in long-term operation.

8

2. Experimental 2.1. Synthesis of Fe0.1Ni0.9Co2O4/AC electrocatalysts The electrocatalysts studied in this work are composites composed of metal oxides as catalytic material and activated carbon as support. The iron-doped nickel cobalt oxide, Fe0.1Ni0.9Co2O4, with the spinel structure was synthesised by a microwave-assisted hydrothermal method, followed by a 400oC calcination process. The precursors of CoCl2⋅6H2O, NiCl2⋅6H2O and FeCl3, in the molar ratio of 2 : 0.9 : 0.1, were dissolved in the ethanol-water mixture, with the addition of various amount (5, 10, 20 wt% of the transition metal precursors) of activated carbon powder (ACS-679, China Steel Chemical Corporation, Taiwan), and then stirred for 24 h to ensure the homogeneity. The solutions were subsequently titrated to pH 10 via slow addition of 1 M NaOH. The above solutions were transferred into glass containers and heated by a microwave synthesiser (Discover SP, CEM), with the heating power of 250W at 160oC for 30 min. After the hydrothermal process, the precipitates were centrifuged multiple times to remove the impurities and unreacted species, and then dried overnight in an 80oC vacuum oven. The as-prepared dried precipitates were further annealed at 400oC for 2 h in order to obtain the ternary spinel metal oxide/AC composites. The composites prepared from the precursor solutions containing 5, 10, and

20

wt%

activated

carbon

are

named

as

Fe0.1Ni0.9Co2O4/1.0wt%AC, 9

Fe0.1Ni0.9Co2O4/1.5wt%AC, Fe0.1Ni0.9Co2O4/3.7wt%AC, respectively (where the final carbon content in each sample was estimated from the TGA curves shown in Fig. S1). 2.2. Material Characterisation The inductively coupled plasma-mass spectrometry (ICP-MS, Agilent 7500ce, Agilent Technologies) was conducted to gain the atomic ratio of transition metal oxides. The thermogravimetric analysis from 105 to 800oC (TGA, SDT Q600, V20.9 Build 20, thermal analyser, USA) was used to understand the carbon contents in the composite catalysts. All catalysts were heated at 105oC for 10 min to remove the moisture. The as-prepared products were probed preliminarily by a scanning electron microscope (Hitachi SU8010 microscope), which showed the surface morphology of the composites. The X-ray diffraction (XRD) patterns were measured at the scan rate of (2θ) 0.6o min-1 by an X-ray diffractometer (D2 Phaser, Bruker) with Cu-Kα radiation (λ = 1.5406 Å). The specific surface area was analysed via the Brunauer-Emmett-Teller (BET) method from the N2 adsorption-desorption isotherm (ASAP 2020 Physisorption Analyser, Micromeritics Instrument Co.). Elemental analysis (EA) (or CHN analysis) was performed to confirm the absence of carbon leaching in the electrolytes (vario EL cube, elementar, Germany) in a long-term stability test.

10

2.3. Electrochemical characterisation Cyclic voltammetry (CV) and linear sweep voltammetry (LSV) were performed on the rotating disk electrode (RDE) to examine the electrocatalytic activities of the composites for the OER. These electrochemical tests were conducted by the electrochemical analyser (CH Instruments 724D). First, 0.5 mg of each catalyst was uniformly dispersed in the solution which consisted of 190 µL ethanol and 10 µL Nafion (E.I. du Pont de Nemours & Co.) in ultrasonic bath. Then, 20 µL of the solution was drop-casted on a glassy carbon disk electrode, which was then polished by an Al2O3 slurry. The electrode was dried in air for 24 h before the experiment began. The loading amount on the disk electrode was approximately 0.2 mg cm-2, since the disk area is 0.247 cm2. To investigate the OER catalysis on the catalyst-coated electrode, a three-electrode system was applied, where the counter electrode was a platinum gauze and the reference electrode was an Ag/AgCl electrode (Argenthal, 3 M KCl, 0.207 V vs. NHE at 25°C) in a Luggin capillary. The 0.1 M KOH electrolyte used here was kept at 25oC by a thermostatic water bath. The potential on the working electrode is expressed against the reversible hydrogen electrode (RHE) according to the relation: E (RHE) = E (Ag/AgCl) + 0.975 V. To assess the catalytic properties of OER, a positive LSV scan was conducted to obtain the polarisation curves, with a potential window ranging from the potential of 11

oxidation peak of the electrocatalyst, to 0.72V because the OER would occur at a higher potential than the oxidation of the redox couples, such as Ni2+/Ni3+ and Co2+/Co3+. The electrochemical impedance spectroscopy (EIS) was used to evaluate the inter-particle charge transport resistance and OER charge transfer resistance, which was conducted by an electrochemical analyser (CH Instruments 6211E). The electrocatalysts were coated on the carbon paper to make air electrodes, as described in the next section. All EIS data were recorded at 1.48 V (vs. RHE) where the OER occurs, from 0.01 Hz to 100000 Hz in 6 M KOH electrolyte. The applied AC amplitude was 10 mV. 2.4. Preparation of the air electrode The

air

cathode

was

fabricated

by

coating

each

catalyst

(Fe0.1Ni0.9Co2O4/1.0wt%AC, Fe0.1Ni0.9Co2O4/1.5wt%AC, Fe0.1Ni0.9Co2O4/3.7wt%AC) onto one piece of GDL240 carbon paper (CeTech Co., Ltd., Taiwan). First, a paste containing homogeneous mix of 0.1 g catalyst powder, 0.1 g ethylene glycol and 0.02 g 5% Nafion was prepared. Then it was evenly coated on the carbon paper without any metallic current collector, followed by an overnight 80oC drying treatment. The paste coated on the carbon paper was on a 1.0 cm2 area, serving as the catalytic layer, while the carbon paper provided the gas diffusion pathway. 2.5. Rechargeable zinc-air full-cell tests 12

The as-prepared air electrode was fixed in an acrylic cell, with the gas diffusion layer (GDL) exposed to the ambient air. The cell configuration has been previously reported, although in this work the Ti mesh was not used [51]. The negative electrode adopted here was a commercial zinc foil and the electrolyte was 6 M KOH + 0.2 M Zn(CH3COO)2. The long-term galvanostatic charge-discharge (GCD) cycling tests at a constant current density of 10 mA cm-2 and the GCD cycling tests at various current densities (2, 5, 10, 20, 50, 100 mA cm-2) were conducted by the CH Instruments 1128C.

13

3. Results and Discussion 3.1 Material characterisation ICP-MS was used to determine the Fe content and composition of all transition metal oxides (see Table S1 in Supporting Information). This table reveals similar composition for all metal oxides precipitated onto the activated carbon, which is comparable to pure Fe0.1Ni0.9Co2O4 and close to the nominal value of Fe0.1Ni0.9Co2O4 (within an error margin of ca. 5%). This result excludes the possibility that the enhanced catalytic activity of Fe0.1Ni0.9Co2O4/AC comes from the composition variation of metal oxide NPs caused by the introduction of activated carbon. TGA was applied to probe the amounts of carbon in the Fe0.1Ni0.9Co2O4/AC composites. From these data, the percentage of carbon rises as the amount of activated carbon powder in the precursors increases. The morphologies of pristine activated carbon and Fe0.1Ni0.9Co2O4 with/without activated carbon in Fig. 1 were probed by SEM. The pure activated carbon powders show obvious boundaries of each carbon particle in Fig. 1(a). In contrast, pure Fe0.1Ni0.9Co2O4 without activated carbon does not exhibit obvious edges. When the activated carbon is applied as the support, the small Fe0.1Ni0.9Co2O4 NPs are homogeneously grown on the large activated carbon particles, as shown by Fig. 1(c) - 1(e). The SEM images with various magnification are shown in Fig. S2 to further examine the uniformity of the Fe0.1Ni0.9Co2O4 on the 14

activated carbon. It is apparent that the activated carbon substrate is uniformly coated with a layer of Fe0.1Ni0.9Co2O4 NPs, so it is reasonable to propose that the activated carbon substrate in this core-shell-like structure is not significantly oxidised or decomposed in the alkaline, high-voltage process of the OER. The crystalline structure of Fe0.1Ni0.9Co2O4/AC composites was characterised by X-ray diffractometer (D2 Phaser, Bruker), as shown in Fig. 2. Since iron only takes up a low percentage (about 3.3 at% in the cation precursors), the crystal structure is similar to the original NiCo2O4. The XRD patterns show typical diffraction peaks corresponding to facets including (111), (220), (311), (222), (400), (422), (511), and (440), with the slight peak shift upward or downward resulting from crystal contraction or expansion. The presence of these peaks prove the main structure to be the same as the ICSD card shows (PDF#20-0781). Note that there are two apparent broad peaks corresponding to (002) and (100) of pure activated carbon, which range from 15° to 30° and 40° to 45° respectively and represent the certain degree of graphitisation [32, 35, 52-55]. Nevertheless, they almost disappear on those XRD patterns for composites, suggesting that the carbon support is completely covered by the metal oxide NPs [56, 57]. Here, all XRD data are fitted by the Rietveld method on the TOPAS software and the lattice parameters are 8.115, 8.113, 8.112 and 8.110 Å, for Fe0.1Ni0.9Co2O4, Fe0.1Ni0.9Co2O4/1.0wt%AC, Fe0.1Ni0.9Co2O4/1.5wt%AC, and 15

Fe0.1Ni0.9Co2O4/3.7wt%AC respectively (see Table 1). The crystal size of each composite, ranging from 42 nm to 46 nm, is also fitted and given in Table 1. There is no obvious change in the crystal size caused by the introduction of activated carbon. Fig. S3 shows the N2 adsorption-desorption isotherms of all electrocatalysts. Pure activated carbon exhibits the type I isotherm without a hysteresis loop, which is attributable to the chemical activation (e.g., KOH-activated carbon) process of activated carbon [58]. By contrast, Fe0.1Ni0.9Co2O4 shows the type V isotherm with a hysteresis loop of type H3 in the high-pressure region [59, 60]. Interestingly, all Fe0.1Ni0.9Co2O4/AC composites display typical isotherms similar to that of pure Fe0.1Ni0.9Co2O4, probably due to the tight agglomeration of Fe0.1Ni0.9Co2O4 NPs on the activated carbon surface (see Fig. 1 and S1). On the other hand, the specific surface areas of pure activated carbon, Fe0.1Ni0.9Co2O4, Fe0.1Ni0.9Co2O4/1.0wt%AC, Fe0.1Ni0.9Co2O4/1.5wt%AC, Fe0.1Ni0.9Co2O4/3.7wt%AC, calculated by the BET model, are respectively equal to 1868, 33, 28, 20, 22 m2 g-1. It is noticeable that all composites have slightly smaller specific surface area than Fe0.1Ni0.9Co2O4. Normally, the specific surface areas of composites should be between those of the original oxide NPs and activated carbon particles if the activated carbon is partially covered with Fe0.1Ni0.9Co2O4 NPs or if the activated carbon and Fe0.1Ni0.9Co2O4 coexist as a mixture [57, 61]. In contrast, Fe0.1Ni0.9Co2O4/AC composites even show smaller 16

specific surface areas than pure Fe0.1Ni0.9Co2O4. This phenomenon suggests that nearly all the pores of activated carbon particles are blocked by the metal oxide NPs, so that the activated carbon particles within the composites cannot significantly contribute to the resultant specific surface area. In addition, the outer metal oxide layer is probably a compact structure formed on the surface of activated carbon, consistent with Fig. 1 where pure Fe0.1Ni0.9Co2O4 seems to loosely agglomerate but Fe0.1Ni0.9Co2O4 NPs tightly grow on the activated carbon. Based on the results of N2 adsorption-desorption isotherms and BET calculations, the activated carbon powders are expected to be uniformly protected by a relatively compact Fe0.1Ni0.9Co2O4 NP layer. This interesting microstructure is beneficial not only to enhance the activity of Fe0.1Ni0.9Co2O4/AC catalysts but also to overcome the carbon corrosion issue in order to prolong the cycle life of the electrocatalysts in the OER catalysis and rechargeable Zn-air battery.

3.2 Electrochemical Characterisation in a three-electrode system The cyclic voltammograms (CVs) were measured in 0.1 M KOH at 25 mV s-1 for 75 cycles, to first activate the slightly hydrophobic catalysts, and then to estimate the number of active sites, i.e., electrochemically active surface area (ECSA) for the OER [62, 63]. The 1st, 25th, 50th, 75th cycles of CVs are given in the Fig. S4, where all 17

composites show stable i-E responses after 75 cycles. This process with repeated CV cycles is known as electrochemical activation and aims to obtain a stable i-E curves [64-66]. The cathodic voltammetric charges of all composites were integrated from the negative sweeps of such stabilised CV curves shown in Fig. 3 since the cathodic currents on the negative sweeps excluded the OER currents in order not to overestimate the ECSA. The voltammetric charges are 2.80, 3.01, 5.81, 6.04 mC cm-2, which reveals the ECSA is increased after activated carbon is employed as the substrate. The above result indicates that the composites display superior catalytic activities toward the OER, compared to the pure Fe0.1Ni0.9Co2O4. From the above BET results, the specific surface area decreases after the introduction of activated carbon support. Accordingly, the improved OER performance is not the result of an enlargement of the exposed surface area, but instead the higher catalytic activities. The OER electrocatalytic performance was further examined by LSV on the RDE. The rotating speed remained 1600 rpm in order to remove the evolving O2 bubbles during the OER process. Fig. 4(a) exhibits the polarisation curves for all electrocatalysts, where an enhanced electrocatalytic activity can be observed as activated carbon is used as the support. In addition, Fe0.1Ni0.9Co2O4/1.5wt%AC and Fe0.1Ni0.9Co2O4/3.7wt%AC show virtually no difference in the catalytic activity, which is in agreement with the result from voltammetric charges of CVs. By contrast, 18

Fe0.1Ni0.9Co2O4/1.0wt% exhibits a small improvement after supported by activated carbon, which means that the activated carbon content is too low to result in effective enhancement. The ECSA-normalised OER current densities are shown in Fig. S5, in order to evaluate the intrinsic activity of catalysts on the OER performance. Note that the order of catalysts with respect to decreasing the OER specific activity is: Fe0.1Ni0.9Co2O4/1.0wt%AC

>

Fe0.1Ni0.9Co2O4/1.5wt%AC

>

Fe0.1Ni0.9Co2O4/3.7wt%AC > Fe0.1Ni0.9Co2O4, which is different from the order with respect to the OER activity without normalisation: Fe0.1Ni0.9Co2O4/1.5wt%AC > Fe0.1Ni0.9Co2O4/3.7wt%AC

>

Fe0.1Ni0.9Co2O4/1.0wt%AC

>

Fe0.1Ni0.9Co2O4.

Nonetheless, the resultant current densities of all carbon-supported composites are higher than that of Fe0.1Ni0.9Co2O4. The CV, areal current densities in LSV, and ECSA-normalised current densities in LSV indicate that the enhanced OER behaviour of Fe0.1Ni0.9Co2O4/AC is due to not only a larger ECSA, but also a higher intrinsic activity. The corresponding Tafel plot is given in Fig. 4(b), which indicates similar mechanisms of the OER catalysis on all composites, reasonably owing to the same catalytic materials (i.e., Fe0.1Ni0.9Co2O4). According to the Tafel plot, the exchange current densities of the OER for all composites are higher than that for pure Fe0.1Ni0.9Co2O4, which means the OER catalytic activity is improved when activated carbon is introduced in Fe0.1Ni0.9Co2O4. In the composites, activated carbon powders 19

with relatively good electronic conductivity can provide other electron transport pathways that favour the OER. To support this claim, the EIS measurements were conducted to examine the inter-particle charge transport resistance and the OER charge transfer resistance for all electrocatalysts. The results are shown in Fig. 5(a), (b). The EIS spectra can be generally divided into three regions. At the high-frequency region end (see Fig. 5(b)), the equivalent series resistances (ESR) of all air electrodes are similar, which is attributed to the fact that the ESR is mainly dependent upon the distance between the open end of the Luggin capillary and the working electrode. The obvious high-frequency semicircle may correspond to the inter-particle impedance among oxide NPs and oxide/activated carbon powders in the coated paste, in parallel with the inter-particle capacitance [67]. However, the inter-particle resistance is significantly affected by the catalyst/carbon paper contact impedance and is due to variations in the contact impedance at the catalyst/carbon paper interface from electrode paste coatings. This effect probably causes the overlapping of the inter-particle impedance and the catalyst/carbon paper contact impedance (e.g., see the EIS data of Fe0.1Ni0.9Co2O4 and Fe0.1Ni0.9Co2O4/3.7wt%AC). In the middle-frequency region, a semicircle is observed for the air electrode containing Fe0.1Ni0.9Co2O4/1.0wt%AC. The semicircle indicates that the introduction of activated carbon into Fe0.1Ni0.9Co2O4 reduces the inter-particle 20

impedance. The same semicircle is visible for the air electrode containing Fe0.1Ni0.9Co2O4/1.5wt%AC. In the low-frequency region (see Fig. 5(a)), the larger semicircle corresponds to the OER at the bias potential, which can be modelled by a charge transfer resistance (Rct) of the OER in parallel with the double-layer capacitance [68]. Since the charge-transfer resistances of Fe0.1Ni0.9Co2O4/1.0wt%AC (11.6 ohm), Fe0.1Ni0.9Co2O4/1.5wt%AC (10.8 ohm), and Fe0.1Ni0.9Co2O4/3.7wt%AC (7.8 ohm) for the OER are smaller than the resistance (12.3 ohm) of the unsupported Fe0.1Ni0.9Co2O4, the introduction of activated carbon promotes electron transport to the Fe0.1Ni0.9Co2O4, leading to an enhancement of the observed OER catalysis in the composite electrode. In order to evaluate any possible carbon leaching out/corrosion of the activated carbon from the composite during the OER, EA was conducted to detect the carbon content

of

the

electrolytes

after

tests.

In

this

measurement

the

Fe0.1Ni0.9Co2O4/3.7wt%AC was selected as the representative sample because of its highest carbon content. The catalyst was coated on the RDE and polarised at 1700 mV (vs. RHE) in 6 M KOH for 20 h. Before and during the test, a purified N2 flow was used to purge the electrolyte in order to avoid any CO2 absorption in KOH from the air. After the 20-h test, the electrolytes were collected for subsequent EA measurements and the results are shown in Table S2 in the Supporting Information. 21

With a detection limit of 0.001 wt%, all samples did not show any carbon in the used electrolytes, suggesting the good protection of activated carbon by the tightly agglomerated Fe0.1Ni0.9Co2O4 NPs.

3.3. Evaluation of the OER performance and durability in a rechargeable Zn-air battery In order to investigate the OER catalytic performance and long-term stability of all Fe0.1Ni0.9Co2O4 composites in the alkaline electrolyte at highly positive potentials, the catalysts were tested as the bifunctional catalysts in a rechargeable zinc-air battery for which Fe0.1Ni0.9Co2O4 were originally designed and used [11]. The charge-discharge current density is 10 mA cm-2 and the duration of both charge and discharge steps is 20 min. There are many reasons why Fe0.1Ni0.9Co2O4/ACs may be intuitively considered inappropriate for the application of zinc-air batteries. Firstly, activated carbon is too vulnerable in extremely alkaline media. Secondly, as the oxygen evolution occurs, the evolving O2 bubbles would detach the metal oxide NPs from the activated carbon support. Thirdly, according to the literature, a couple of parasitic reactions, also called ageing process on activated carbon, have been reported in alkaline electrolytes and at highly positive potentials [69]. For example, the decomposition of surface functional groups has been recorded to occur as the 22

electrode potentials are higher than 0.25 V(vs. SHE) in aqueous electrolytes [69-71]. In such cases, the surface functional groups are believed to be continuously decomposed and reformed. Also, carbon corrosion has been reported to happen in neutral and alkaline environments, in which the surface of activated carbon is oxidised either indirectly to functional groups or directly to CO2 [69, 71-73]. All the above-mentioned side reactions would generate CO2, which is the reactant in another undesirable reaction, carbonate precipitation [74, 75]. Potassium carbonate tends to be deposited on the air electrode, which is detrimental to the electrocatalytic performance owing to the clogging of the channels [29, 74, 76]. As soon as the activated carbon support is oxidised to release CO2, the carbonate (K2CO3) will form in situ on the surface of the composites as well as within the gas diffusion layer on the air electrode, leading to a deteriorated performance. Despite these drawbacks of activated carbon, in this study the oxide-covered activated carbon powders were not exposed to the alkaline electrolyte, which suggested the above parasitic reactions were prohibited. In Fig. 6(a), the optimum OER electrocatalyst in the RDE test, namely Fe0.1Ni0.9Co2O4/1.5wt%AC, was utilised on the air electrode of a rechargeable zinc-air battery for 80 h. The Fe0.1Ni0.9Co2O4/1.0wt%AC was also tested for a comparison purpose.

The

GCD

cycles

of

Fe0.1Ni0.9Co2O4/1.0wt%AC

and

Fe0.1Ni0.9Co2O4/1.5wt%AC in selected time intervals were magnified in Fig. 6(b) to 23

provide more information on the electrochemical charge-discharge steps. The detailed charge-discharge voltages and gaps of all composites are listed in Table 2. From Fig. 6, the cell utilizing Fe0.1Ni0.9Co2O4/1.5wt%AC exhibits a lower charging cell voltage, i.e., a higher OER activity, than Fe0.1Ni0.9Co2O4/1.0wt%AC, in good agreement with the RDE results. On the other hand, both electrocatalysts remain good electrocatalytic activities for both ORR and OER, showing excellent cycling stability in the concentrated alkaline electrolyte (6 M KOH). It is noticeable that the applied cell voltages are up to 2 V, where all above-mentioned parasitic reactions can easily occur; however, no side reactions are observed in these tests. The cell using Fe0.1Ni0.9Co2O4/3.7wt%AC, which contains the highest activated carbon content among all composites, is further examined in a more prolonged 120-h test

(see

Fig.

7).

With

the

highest

amount

of

activated

carbon,

Fe0.1Ni0.9Co2O4/3.7wt%AC should be the most vulnerable in this high-voltage, concentrated alkaline environment. In Fig. 7(a) and (b), the charging voltage of the cell containing Fe0.1Ni0.9Co2O4/3.7wt%AC is slightly higher than those using Fe0.1Ni0.9Co2O4/1.0wt%AC and Fe0.1Ni0.9Co2O4/1.5wt%AC, which is attributed to the highest carbon content. However, the charging cell voltage in the 120th h is only 2.02V,

which

exhibits

negligible

decay

in

the

catalytic

activities

of

Fe0.1Ni0.9Co2O4/3.7wt%AC after the 120-h prolonged GCD test, compared to the 24

other Fe0.1Ni0.9Co2O4/AC composites. As a result, the proposed metal oxide/activated carbon composites are resistant to both the detachment of oxides and those common side reactions in the OER process. The durability in such a harsh environment can be explained by the results obtained from SEM, XRD and BET; that is, the robust metal oxide NPs uniformly protects the activated carbon supports and the coupling between each other is strong enough to overcome the 120-hour charge-discharge test. For a comparison purpose, pure activated carbon is also examined in the same condition for 80 h and the result is given in Fig. S6. In that case, the ORR and OER performances decline continuously, indicating the poor resistance to the high-voltage and concentrated alkaline electrolyte in practical applications. Here various current densities were also applied to investigate the charge-discharge

cell

voltage

gaps

of

a

full

cell

employing

the

Fe0.1Ni0.9Co2O4/3.7wt%AC composite. Each current density was tested for 5 cycles in order to examine the reversibility. In addition, a mixture consisting of 80 wt.% Fe0.1Ni0.9Co2O4 and 20 wt.% activated carbon was also tested under the same condition, in order to confirm the effective protection of activated carbon by Fe0.1Ni0.9Co2O4 NPs synthesised in this work. The discharge-charge cycles (20 min per cycle) of Fe0.1Ni0.9Co2O4/3.7wt%AC and Fe0.1Ni0.9Co2O4 are compared in Fig. 8, while the cell voltage gaps, discharge power density, and energy efficiencies of the 25

cell employing Fe0.1Ni0.9Co2O4/3.7wt%AC composite are listed in Table 3. From Fig. 8, the performance and stability of the Zn-air cell using the oxide/activated carbon mixture are much worse than the one utilizing the Fe0.1Ni0.9Co2O4/3.7wt%AC composite, especially at high current densities. These results indicate the degradation of unprotected activated carbon powders in the oxide/activated carbon mixture at high OER

rates,

which

support

the

statement

that

activated

carbon

within

Fe0.1Ni0.9Co2O4/3.7wt%AC is perfectly protected by the surrounded Fe0.1Ni0.9Co2O4 NPs with a compact microstructure. The energy efficiency for the cell using Fe0.1Ni0.9Co2O4/3.7wt%AC decreases from 64% to 41% as the applied current density is increased from 2 to 100 mA cm-2 since the required overpotentials of both ORR and OER are soaring when the current density becomes high. Moreover, the zinc-air battery can provide a large power density of 99 mW/cm-2 at 100 mA/cm-2, with a cell voltage of 0.99 V. The GCD test in Fig. 8 reveals that the cell is able to generate a constant electric power for a period of 10 min and can be recharged at high current densities.

26

4. Conclusions The introduction of activated carbon significantly enhances the OER activity of Fe0.1Ni0.9Co2O4 electrocatalysts and their long-term stability for the OER and ORR in the

concentrated

alkaline

electrolyte

(6

M

KOH).

A

simple

two-step

microwave-assisted hydrothermal method fabricated Fe0.1Ni0.9Co2O4/AC composites with activated carbon protected by the surrounding nanoparticulate metal oxides. This lead to the utilisation of activated carbon as a practical supporting material during the OER. For the OER, Fe0.1Ni0.9Co2O4/1.0wt%AC shows the highest specific activity but Fe0.1Ni0.9Co2O4/1.5wt%AC demonstrates the highest overall activity among all Fe0.1Ni0.9Co2O4/AC composites. The EIS data supported the finding that introducing activated carbon reduces the charge-transfer resistance of the OER for all Fe0.1Ni0.9Co2O4/AC composites comparing with Fe0.1Ni0.9Co2O4. The absence of significant carbon leaching during the 20-h OER test was confirmed, indicating the protection of activated carbon by the tightly agglomerated nanoparticulate Fe0.1Ni0.9Co2O4. In particular, the air electrode containing Fe0.1Ni0.9Co2O4/3.7wt%AC exhibited a durability up to 120 h without visible carbon corrosion and carbonate precipitation, demonstrating the promising application of Fe0.1Ni0.9Co2O4/AC electrocatalysts for the OER and the long-term rechargeable zinc-air battery.

27

ACKNOWLEDGMENTS This research was financially supported by the Ministry of Science and Technology, Taiwan under contract no. MOST 106-2923-E-007-005, MOST 106-2221-E-007-089–MY3, MOST 106-3113-1-239-001, which are gratefully acknowledged.

28

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Captions Fig. 1. SEM images of (a) pure activated carbon, (b) Fe0.1Ni0.9Co2O4, (c) Fe0.1Ni0.9Co2O4/1.0wt%AC,

(d)

Fe0.1Ni0.9Co2O4/1.5wt%AC,

and

(e)

Fe0.1Ni0.9Co2O4/3.7wt%AC. Fig. 2. XRD patterns of activated carbon, Fe0.1Ni0.9Co2O4, Fe0.1Ni0.9Co2O4/1.0wt%AC, Fe0.1Ni0.9Co2O4/1.5wt%AC, Fe0.1Ni0.9Co2O4/3.7wt%AC. A PDF card of NiCo2O4 is given as the reference. Fig.

3.

The

75th

cycles

of

Fe0.1Ni0.9Co2O4,

Fe0.1Ni0.9Co2O4/1.0wt%AC,

Fe0.1Ni0.9Co2O4/1.5wt%AC, Fe0.1Ni0.9Co2O4/3.7wt%AC in 0.1 M KOH at 25 mV s-1. Fig. 4. (a) LSV polarisation curves and (b) corresponding Tafel plots for (1) Fe0.1Ni0.9Co2O4, (2) Fe0.1Ni0.9Co2O4/1.0wt%AC, (3) Fe0.1Ni0.9Co2O4/1.5wt%AC, (4) Fe0.1Ni0.9Co2O4/3.7wt%AC in 0.1 M KOH at 2 mV s-1. Fig. 5. (a) Full EIS spectra and (b) magnified EIS in the high and middle-frequency regions

at

1.48V

(vs.

Fe0.1Ni0.9Co2O4/1.0wt%AC,

RHE)

in

6

M

KOH

for

Fe0.1Ni0.9Co2O4/1.5wt%AC,

Fe0.1Ni0.9Co2O4, and

Fe0.1Ni0.9Co2O4/3.7wt%AC, with the frequency from 100000 to 0.001 Hz and an ac amplitude of 10 mV. 35

Fig.

6.

Long-term

GCD

cycles

of

Fe0.1Ni0.9Co2O4/1.0wt%AC

and

Fe0.1Ni0.9Co2O4/1.5wt%AC for (a) 80 h and (b) the corresponding magnified cycles for selected time intervals under the ambient condition in a rechargeable zinc-air battery with 6 M KOH + 0.2 M zinc acetate, where the charge-discharge current density is 10 mA cm-2. Fig. 7. (a) A prolonged GCD test and (b) magnified cycles for selected time intervals under

the

ambient

condition

for

a

rechargeable

zinc-air

battery

using

Fe0.1Ni0.9Co2O4/3.7wt%AC in 6 M KOH + 0.2 M zinc acetate at 10 mA cm-2 for 120 h. Fig. 8. Comparison of charge-discharge cell voltages for 80 wt% Fe0.1Ni0.9Co2O4 physically

mixed

with

20

wt%

activated

carbon

(blue

line),

and

for

Fe0.1Ni0.9Co2O4/3.7wt%AC (red line) at the current densities of 2, 5, 10, 20, 50, 100 mA cm-2, with each cycle composed of 600-second discharge and 600-second charge steps.

36

Tables and Figures Table 1. The fitted lattice parameters and crystal size of Fe0.1Ni0.9Co2O4, Fe0.1Ni0.9Co2O4/1.0wt%AC, Fe0.1Ni0.9Co2O4/1.5wt%AC, Fe0.1Ni0.9Co2O4/3.7wt%AC, determined by the Rietveld method on the TOPAS software. Sample

Lattice parameter (Å)

Crystal size (nm)

Fe0.1Ni0.9Co2O4

8.115

41.9

Fe0.1Ni0.9Co2O4/1.0wt%AC

8.113

42.9

Fe0.1Ni0.9Co2O4/1.5wt%AC

8.112

46.1

Fe0.1Ni0.9Co2O4/3.7wt%AC

8.110

43.6

Table 2. The detailed cell voltages and the corresponding charge-discharge voltage gaps of zinc-air batteries using various Fe0.1Ni0.9Co2O4/AC composites and pure activated carbon. Cycle D/C in 10th hour

D/C in 80th hour

D/C in 120th hour

Fe0.1Ni0.9Co2O4/1.0wt%AC

1.19V/1.99V

1.21V/1.99V

X

Fe0.1Ni0.9Co2O4/1.5wt%AC

1.15V/1.93V

1.19V/1.96V

X

Fe0.1Ni0.9Co2O4/3.7wt%AC

1.19V/1.98V

1.21V/2.01V

1.21V/2.02V

Sample

37

Activated carbon

1.18V/2.16V

1.13V/2.29V

X

Table 3. The detailed cell voltages and corresponding charge-discharge gaps of Fe0.1Ni0.9Co2O4/3.7wt%AC at different current densities (2, 5, 10, 20, 50, 100 mAcm-2), where the energy efficiencies of the 5th cycle are listed here. D/C gap in the 5th

Discharge

cycle

Power density

2 mA cm-2

1.23V/1.93V

2.46 mW/cm-2

63.7%

5 mA cm-2

1.21V/1.96V

6.05 mW/cm-2

61.7%

10 mA cm-2

1.19V/1.99V

11.9 mW/cm-2

59.8%

20 mA cm-2

1.16V/2.04V

23.2 mW/cm-2

56.9%

50 mA cm-2

1.09V/2.16V

54.5 mW/cm-2

50.5%

100 mA cm-2

0.99V/2.40V

99 mW/cm-2

41.3%

Current density

Energy efficiency

38

Fig. 1

39

Fig. 2

40

Fig. 3

41

Fig. 4(a)

Fig. 4(b)

42

Fig. 5(a), (b)

43

Fig. 6(a), (b)

44

Fig. 7(a)

Fig. 7(b)

45

Fig. 8

46

Highlights 1. Activated carbon (AC) enhances the O2 evolution reaction activity of Fe0.1Ni0.9Co2O4. 2. Fe0.1Ni0.9Co2O4 particles cover the AC and prevent carbon corrosion. 3. The composite shows stable cycling in the 120-hour test. 4. The zinc-air cell shows a power density of 99 mW⋅cm-2 at 100 mA⋅cm-2.