CNT composites as an efficient electrocatalyst for oxygen evolution reaction

Accepted Manuscript Title: Fe2 O3 hollow nanorods/CNT composites as an efficient electrocatalyst for oxygen evolution reaction Author: H.A. Bandal A.A. Chaugule A.R. Jadhav W-J. Chung H. Kim PII: DOI: Reference:

S0013-4686(16)32443-4 http://dx.doi.org/doi:10.1016/j.electacta.2016.11.107 EA 28397

To appear in:

Electrochimica Acta

Received date: Revised date: Accepted date:

10-7-2016 17-11-2016 18-11-2016

Please cite this article as: H.A.Bandal, A.A.Chaugule, A.R.Jadhav, W-J.Chung, H.Kim, Fe2O3 hollow nanorods/CNT composites as an efficient electrocatalyst for oxygen evolution reaction, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2016.11.107 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.

Fe2O3 hollow nanorods/CNT composites as an efficient electrocatalyst for oxygen evolution reaction

H.A. Bandal, A.A. Chaugule, A.R. Jadhav, W-J. Chung, H. Kim*

Department of Energy Science and Technology, Smart Living Innovation Technology Center, Myongji University, Yongin, Gyeonggi-do 17058, Republic of Korea

Corresponding author: Tel.: +82 31 330 6688; fax: +82 31 336 6336 Email address: [email protected] (H. Kim)

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Graphical abstract

Highlights  Fe2O3 hollow nanorods/ CNT composites were successfully prepared. 

Catalytic activity of Fe2O3/CNT composites was tested for water oxidation reaction

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Fe2O3/CNT composite showed higher electrochemically active surface area and reduced charge transfer resistance.

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The composite showed excellent activity under alkaline condition

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Abstract Oxygen evolution reaction is a key half reaction in overall water splitting process and metal air batteries. However, this reaction is kinetically unfavorable and requires a large overpotential when conducted in the absence of a precious metal catalyst. Substituting these precious metal catalysts with more abundant transition metal oxide such as Fe2O3 is a key challenge for widespread applications of water splitting. The catalytic activity of the Fe2O3 for water oxidation is mostly limited by the poor electric conductivity and low electrolyte permeability on catalyst surface. To overcome these limitations in this work, we have synthesized a composite between Fe2O3 nanorods and oxidized multi-walled carbon nanotubes via a simple urea assisted coprecipitation method. The Fe2O3 nanorods with hollow interior and low crystallinity provided improved electrochemically active surface area and larger number of transportation channels for diffusion of electrolyte. While, introduction of the carbon nanotube caused significant drop in resistance associated with faradic process. Consequently, the Fe2O3/carbon nanotube composite displayed excellent catalytic activity which is superior to Fe2O3, carbon nanotubes and their physical mixture. It is noted that the present work provides a simple strategy for the development of high performance oxygen evolution catalyst based on Fe2O3.

Keywords: Electrocatalyst; Water electrolysis; Oxygen evolution; Fe2O3/CNT.

1. Introduction Oxygen evolution reaction (OER) is a vital part of numerous energy generation and storage technologies such as water electrolysis [1], metal air batteries [2,3], and solar cells [4] etc. However,

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the unfavorable thermodynamics and sluggish kinetics of the OER process necessitates the use of a noble metal catalyst [5,6]. Substitution of these rare and expensive metal catalysts with cheap and readily available transition metal oxide will be curial for making these technologies commercially viable. Recently, as a part of ongoing research to develop comparatively cheaper OER catalysts, nanoparticles of transition metal oxides [7], some perovskite oxides [8], and their composites were evaluated as alternative catalysts for the OER [9]. However, these metal oxide catalysts are generally less active and durable than the state-of-the-art Ir-based catalysts. It has been observed that the OER can only occur at a potential higher than the redox potential of catalyst material [10–12]. Hence, an ideal OER catalyst must have the redox potential lower or similar to the thermodynamic potential of water electrolysis. Though this guideline holds true for the several conventional OER catalyst based on semiconducting oxides of Ni, Co and their composites, however, the oxides of iron represents an exceptional case. Iron oxides such as hematite or magnetite have significantly lower redox potentials than the theoretical potential of OER yet they shows very poor electrocatalytic activity for the OER [13]. In recent years however, several reports highlighting the vital contribution from the traces of Fe ions (present as impurities in the electrolyte solution) to the electrocatalytic activity of Ni-based electrodes are published [14]. It is now believed that the high catalytic activity of Ni oxide and hydroxides originates entirely from the traces of Fe ions present in the electrolyte solution [15]. Moreover, deliberate doping of the iron into the oxides, hydroxides or hydrated oxides of nickel and cobalt caused a significant improvement in their catalytic activity [15–18]. This enhancement in catalytic activity is mostly ascribed to the presence of catalytically active Fe in a conducting medium [19]. Indeed FeOOH thin films deposited on conducting substrates showed to a unique catalytic activity that is even comparable to IrO2 [20]. Apart from improving the electrical conductivity, doping of Fe in Ni or

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Co modifies the coordination environment of Fe thereby affecting the energetic of interactions between of Fe and the active intermediates formed during the OER [21]. In light of these results Fe is quite often used as a dopant to improve the catalytic activity of the other transition metal oxides. However, as the elemental abundance of Fe is several order of magnitude higher than any other transition metal hence, development of the OER catalyst having iron oxide as its key component is highly desirable. This has inspired several studies focused on improving the electrocatalytic activity of the Fe2O3[22,23]. However the low conductivity of iron oxide and higher overpotential required to drive OER on Fe2O3 is a major obstacle for practical applications[24]. The catalytic activity of a semiconducting oxide towards the OER is governed mainly by the adsorption energy of reaction intermediate on the surface of the catalyst and electrical conductivity of the catalyst. The optimum adsorption energies of reaction intermediates on the catalyst ensure easy adsorption and desorption of reactants and products [25]. Whereas, high electrical conductivity will reduce the overall loss of electrons due to resistance. Besides, increment in catalytic activity of metal oxide can also be achieved by carefully tailoring morphology of the catalyst to improve the surface area and diffusion processes on the catalyst. For instance by carefully varying the structure size and geometry of the iron oxide, its electrocatalytic activity can be increased by almost three orders of magnitude with respect to the planer iron oxide [26]. However, the catalytic activity of this nanostructure iron oxide was still considerably lower than the cobalt oxide. Alternatively catalytic efficiency of the metal oxide can also be increased by immobilizing it on a conducting substrate such as graphene [27], carbon fiber paper [28], or carbon nanotubes [29] etc. These carbon substrates augment the catalytic activity of the metal oxides by assuring the fast electron transport. Anchoring of the metal oxide on the carbon nanomaterials also

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improves stability of the electrocatalyst by averting the aggregation of nanoparticles [30]. Among these substrates CNTs have particularly high current carrying capacity (approximately 1000 times higher than Cu), large surface area, high mechanical strength and structural flexibility, which make them an excellent substrate for metal oxides [31]. Besides, surface of the carbon nanotubes can be easily oxidized so as to introduce hydrophilic functional groups. These functional groups not only enable easy dispersion of CNT in polar solvents but also provide the anchoring points for adsorption of the metal ions, which enables a uniform distribution of metal nanoparticles. As structure of the inner walls of carbon nanotubes remains intact during the oxidation process, CNTs retain their high electrical conductivity [32]. Deriving inspiration from these facts, in this work, we have synthesized the Fe2O3 hollow nanorods decorated on oxidized multi-walled carbon nanotubes (Fe2O3/CNTs) via a simple urea assisted co-precipitation method. When compared with other morphologies such as sphere nanoparticles with rod-like shape are often found to have larger surface to volume ratio [33,34] and hence are expected to possess higher number of electrocatalytically active sites. Thus the composite between a rods shaped nanoparticle and carbon nanotubes can simultaneously relax the barriers associated with mass transfer and charge transfer processes. In this regards the primary objective of this work is to study the influence of carbon nanotubes on catalytic activity and durability of the iron oxide for OER. 2. Experimental 2.1. Materials Multi-walled carbon nanotubes (CNT), were purchased from Sigma Aldrich, ferric chloride hexahydrate (FeCl3.6H2O) was acquired from Across Organics, urea (CON2H4) was procured from Samchung Chemicals, and KOH was obtained from Showa Chemicals. All these chemicals were

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of sufficient purity and used without any further purification. 2.2. Oxidation of CNT During a typical experiment 1g of MWCNT (P-CNT) was dispersed in mixture of concentrated H2SO4/HNO3 (3:1). This mixture was sonicated for 1 hrs and then refluxed at 80°C for 4 hrs, so as to graft hydroxy groups on the surface of CNTs. The product was isolated by centrifugation, washed with DI water to remove excess of acid, and finally dried at 80°C for 24 hrs. 2.3. Synthesis of Fe2O3/CNT composites Process for the synthesis of the Fe2O3/CNT is summarized in Fig. 1. In brief, for synthesis of Fe2O3/CNT composite with 30% loading of CNT, 0.1 g oxidized carbon nanotubes were dispersed in 100 mL DI water with the aid of ultrasconication. Afterward, FeCl3 (1.08g, 0.004 M), and urea (2.16 g, 0.036 M) were added to CNT solution and the mixture was heated at 100°C for 24 h with rapid stirring. The obtained black solid was washed successively with DI water and ethanol, dried overnight at 100oC. The final product that was obtained after calcinations at 350°C for 2 h was labelled as Fe2O3/CNT30 where the subscript denotes % of the CNT. For comparison, the composite with other loadings of carbon nanotubes were also prepared by adjusting the concentration of the carbon nanotubes in the reaction mixture. 2.4. Characterization of catalyst A Fourier transform infrared spectrophotometer (FT-IR, Varian 2000) was used to record Infrared absorption spectra of the sample. X-ray diffraction (XRD) spectra were used to examine phase purity of the prepared samples. The XRD spectra were obtained with help of powder X-ray difractometer (, ‘X’ pert MPD diffractometer) using CuKα radiation. X-ray difractometer was operated within Bragg angle range of 10 to 90°. Morphology of sample was examined using a field

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emission scanning electron microscope (SEM, Hitachi S-3500N) coupled with energy dispersive X-ray spectroscope, and The transmission electron microscopy (TEM, JEOL-3010, Japan) equipped with EDS operated at an accelerating voltage of 300 kV. The chemical state of Fe, C, and O in the composite was verified by X-ray photoelectron spectroscopy (XPS, Thermo Electron, U.K.) using Mg Kα radiation. 2.5. Electrochemical measurements Electrochemical measurements were conducted on ZIVE MP1 multichannel workstation potentiostat. All the tests were performed at room temperature using 1M KOH as electrolyte. A glassy carbon (GC) electrode (3 mm diameter), Ag/AgCl electrode and Pt wire electrode, were used as the working electrode, reference electrode and counter electrode respectively. Before electrochemical analysis GC electrode was carefully polished using alumina powder (0.01 μm) followed by successive washings with 0.1M HCl, water, and acetone. Deposition of catalyst on the working electrode was then accomplished by drop casting 10 μL of catalyst ink obtained by dispersing 2 mg of catalyst and 30 μL nafion solution (5wt% in IPA) in 1 mL N-methyl pyrolidone. The Stability of catalyst was evaluated by operating the system at a constant potential. The impedance spectra was recorded in a frequency range of 0.1 to 10000 hrz under a bias potential of 10 mV. 3. Result and discussion 3.1. Catalyst synthesis We have synthesized Fe2O3/CNT composites by hydrolysis of FeCl3 in the presence of the CNT. The CNTs were oxidized prior to their use to introduce hydrophilic functionalities on their surface. The FT-IR spectra (Fig. S1) of these oxidized CNT indicate that during oxidation surface of CNT was functionalized by hydroxyl, ether, and carbonyl functional groups. The electrostatic

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forces of repulsion among these newly introduced functional groups averts the aggregation of CNT in aqueous solutions and a stable colloidal dispersion of CNT in water was achieved. After addition of the FeCl3 some of the Fe3+ ions are adsorbed on the surface of carbon nanotubes thereby disturbing the forces of repulsion between the CNT hence aggregates of CNT settles down readily. The Fe3+ adsorption capacity of the carbon nanotube was determined using UV visible spectroscopy to be 73 mg g-1 of CNT (supporting info Fig. S2). The rest of the Fe3+ exists as a hexacoordinated aquo complex [Fe(OH)6]+3. Due to polarization of coordinated water molecule by Fe3+ this complex is a strong acid and easily undergoes hydroxylation reaction even at low pH [35]. The hydroxylated complexes produced in this process are not stable and undergoes polymerization reaction with elimination of water molecules [36]. The identity of the product of this reaction is determined by the experimental conditions. For instance, at highly acidic or alkaline pH Fe2O3 is produced [37], while at intermediate pH FeOOH is obtained as a major product [35]. In the present work Fe3+ was slowly hydrolyzed in presence of the CNT. Urea was added to reaction mixture so as to maintain the pH of solution close to 8. As reaction proceeds the hydrolysis of urea produces NH3 thereby increasing pH of solution gradually. Increase in pH of the solution promotes the hydroxylation of adsorbed Fe3+ into FeOOH embryos (heterogeneous nucleation). At the same time the excess Fe3+ ions present in the solution also undergoes hydroxylation to produce FeOOH embryos (homogenous nucleation). Subsequent condensation of these embryo leads to the formation of FeOOH/CNT composite which could be transformed to Fe 2O3 nanorods/CNT composite simply by thermal decomposition. 3.2. Crystal structure analysis The crystallographic structure of FeOOH/CNT composites is determined by X-ray differaction spectra (Fig. 2). Most of the peaks observed are assigned to FeOOH phase however

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few small peaks characteristic of Fe2O3.H2O phase were also observed at 2θ values of 24.17, 33.09, 40.75 and 49.19. The X ray diffraction spectra of Fe2O3/CNT30 composite obtained by calcining FeOOH/CNT at 350°C showed a broad diffraction peak at 26.7 (002) for graphitic carbon of CNT along with peaks at 2θ=24.09 (012), 33.28 (104), 35.74 (110), 40.99 (113), 49.49 (024), 54.29 (116), 56.09 (122) which corresponds to a rhombohedral unit cell (R-3c point group, JCPDF 00001-1053) of Fe2O3. The broad nature of diffraction peaks indicates the presence of small particles with low crystallinity. The particle size of Fe2O3/CNT composite calculated with help of Scherer equation was 20 nm. The XRD spectrum of Fe2O3 and Fe3O4 has lot of similarities therefore purity of iron oxide cannot be estimated solely based on XRD. Therefore, Raman spectroscopy was employed to verify the phase purity of iron oxide in Fe2O3/CNT30 composites. The Raman spectrum of Fe2O3/CNT30 composite (Fig. 3) shows the presence of peaks at 207 cm-1, 277 cm-1 and 387 cm-1. Here, later two peaks correspond to Eg mode of Fe2O3 while, the former is observed due to A1g mode. Apart from these peaks observed at 1580 cm-1 and 1347 cm-1 are outcome of inplane bond starching motion of adjacent carbon (G mode E2g symmetry) and disorder in sp2 bonded carbon clusters of graphitic structure (D band) respectively. Since the D band represents the disorder in the structure of carbon nanotubes the ratio D and G band is often used to measure the disorder density of carbon nanotubes. In the present case the D/G ratio of 1.27 suggests the presence of carbon nanotubes of smaller size and large degree of disorder. This observation is consistent with the results from the FT-IR (Supporting info Fig. S1) spectra were intensification of adsorption peaks at 2995 cm-1 (C-H stretching) and 658 cm-1 (C-H bending) was observed on oxidation of CNT. The D/G ratio in Fe2O3/CNT30 composites is slightly higher than that in oxidized CNT (1.21) this increment in D/G ratio could be ascribed to partial deterioration of CNT during calcination. Nonetheless large degree of disorder in the structure of carbon nanotubes should act

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in favor of the reaction by providing the new channels for the transportation of electrolyte. 3.3. Chemical structure analysis In order to verify the chemical composition and the corresponding chemical state of elements present in the Fe2O3/CNT30 composites we studied the XPS spectra of the composite material. The XPS characterization results of the Fe2O3/CNT30 composite are presented in Fig. 4a– d. The overall XPS spectrum of the Fe2O3/CNT30 composite (Fig. 4a) reveals the presence of Fe, C, O, and trace amount of N in the composite. The presence of N was unexpected, probably during synthesis of the composite reaction between nucleophilic ammonia and electrophilic carbonyl group of the CNT may have caused incorporation of nitrogen in the composite. Fig. 4b shows the Fe 2p core-level binding energy spectrum of the catalysts. The peaks observed at 710.8 eV and 724.6 eV could be assigned to Fe 2p3/2 and Fe 2p1/2, respectively and the separation of the 2p doublet was 13.8 eV. All of these features are typical of Fe2O3. Moreover, no obvious signals are detected for Fe (0) at 707 eV and Fe 2p3/2 of Fe (II) at 709.5 eV, suggesting the formation of pure ferric oxides without impurities. The C 1s XPS spectra (Fig. 4c) shows the carbon binding energies of 284.5 and 286.3 these binding energies corresponds to carbon bonding in C-C and C-O respectively. A satellite peak corresponding to π-π* transition is also observed at 293 eV. The peaks observed at 530.1 eV, 531.3 eV, and 532.6 eV in O 1s spectra of Fe2O3/CNT (Fig. 4d) are best ascribed to Fe-O bond in Fe2O3, oxygen bridges at the interface of the structure and remaining COH, COOH functionalities of CNT. These results in combination with results from XRD and Raman analysis confirm the successful formation of Fe2O3/CNT composite. 3.4. Morphology analysis The morphology of the Fe2O3/CNT30 composites was studied with the help of FE-SEM and TEM analysis. SEM (Fig. 5a, b) images reveal that the Fe2O3/CNT30 constitutes of small

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nanoparticles of iron oxide aggregated around each other to form larger clusters of no definite size or shape as a result, exact shape of these nanoparticles is difficult to access with help SEM analysis. The TEM images of Fe2O3/CNT30 composite are presented in Fig. 5c-f. The overview of Fe2O3/CNT30 composite shows presence iron oxide particles with carbon nanotubes embedded within them. The iron oxide particles are present as large cluster that have completely covered the surface of CNT and free CNTs are clearly visible at the edges of this cluster. The Fig. 5d shows the magnified TEM images of iron oxide particle in the composite. These iron oxide particles are capsule like rods that are approximately 35-45 nm in length and 5-10 nm in width. Moreover, these nanorods have hollow interior which could be helpful in reducing the mass transfer resistance and enhancing the catalytic activity of composite. Difference between the particle sizes calculated by Scherer equation and observed in TEM images suggest that Fe2O3 nanorods have low crystallinity and large number defects. The HR-TEM image displayed in Fig. 5e shows the lattice fringes with spacing of 0.269 nm and 0.182 nm which can be indexed as (104) and (113) planes of Fe2O3 respectively. The SADE pattern of Fe2O3/CNT30 composite (Fig. 5f) consists of small spots dispersed in rings indicating presence of the polycrystalline Fe2O3 particles. The ring pattern observed in SADE images was indexed to (110), (214), and (217) planes of Fe 2O3. These results from SADE and HR-TEM analysis are consistent with XRD spectra and indicate the formation of pure Fe2O3 phase with low crystallinity. Thus on the basis of these results we conclude that the composite of Fe2O3 nanorods with CNT has been successfully synthesized. The presence Fe 2O3 nanorods with large degree amorphization, hollow structure, and small particle size should reduce the mass transport resistance by providing the higher number of transportation channels. Furthermore, doping of carbon nanotube is expected to reduce the electrical resistance of composite thereby making it an ideal catalyst for OER reaction.

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3.5. Electrocatalytic experiment The electrocatalytic activity of a material for oxygen evolution reaction is primarily evaluated on the basis of onset potential and Tafel slope. Whereas, former represent the minimum potential required for commencement of OER the later describes the sensitivity of current to changes in applied potential. Besides, the potential required to achieve a current density of 10 mA cm-2 is also an important parameter as it corresponds to a benchmark solar fuel generation efficiency of 10%. In the present work we have evaluated electrocatalytic activity of the catalyst deposited on a glassy carbon electrode in 1M KOH solution. At first we have compared the electrocatalytic activity of Fe2O3/CNT30 composites with Fe2O3 and CNT. The Fig. 6a shows the polarization curve while the corresponding Tafel plots are plotted in Fig. 6b. As expected both Fe2O3 and CNT possesses negligible catalytic activity as evident from the high onset potentials, low current density and higher Tafel slope of 126.39 mV dec-1 and 247 mV dec-1 respectively. The OER onset potential of the Fe2O3/CNT30 composite was only 0.52 mV while current density of 10 mA cm-2 is reached at 0.58 mV which corresponds to an overpotential of 343 mV and 410 mV respectively. Moreover, the Tafel slope of Fe2O3/ CNT composite is only 62 mV dec-1 which is significantly more negative than Fe2O3 as well as CNT. To further ascertain the origin of superior catalytic activity we compared electrocatalytic activity of Fe2O3/CNT30 composite and Fe2O3+CNT mixture. The Fe2O3+CNT mixture was prepared by physically mixing appropriate quantities of suspensions of Fe2O3 and CNT in NMP. Though the Fe2O3+CNT easily outperformed CNT and Fe2O3 nanorods its catalytic activity is significantly lower than Fe2O3/CNT30 composite. This suggests that the superior catalytic activity of Fe2O3/CNT30 nanoparticles probably originates from the close association of high conductivity of carbon nanotube, and hollow Fe2O3 nanorods in the composite. Apart from the catalytic activity the stability of catalyst is also of vital importance

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thus the stability of the Fe2O3/CNT30 was investigated by performing the potentiostatic analysis at a constant potential of 600mV Vs Ag/AgCl (Fig. 6c). The Fe 2O3/CNT30 displayed good stability and maintain 80% of its initial activity up to 12 hrs of reaction. Beyond, this however a rapid decline in the catalytic activity was observed. To get better understanding of this result we deposited the Fe2O3/CNT30 on ITO substrate and recorded XRD spectra of this electrode before and after stability test (Supporting info Fig. S3). The XRD spectra of the Fe2O3/CNT30 obtained after stability test showed a significant enhancement in the intensity of peak at 2θ =26.7º corresponding to (002) reflection of graphitic carbon. This suggest that during OER slow dissolution of Fe2O3 occurs hence, the catalytic activity of the composite was found to decreases with time. These results highlight the superior catalytic activity of Fe 2O3/CNT30 composites over Fe2O3 and CNT. Generally such enhancement in the catalytic activity on doping of CNT is assigned to the improved electrical conductivity of the composite material. However, we will like to point out that the Fe2O3/CNT30 composite reaches current density of 10 mA cm-2 at applied potential of 0.70 V and 0.58 mV Vs Ag/AgCl electrode in 0.1M (Supporting info Fig. S5b) and 1 M KOH respectively which is comparable to CoFe2O4/CNT [30], FeCo2O4/hollow graphene spheres [38], Co3O4/RGO[39] sphere. This observation is even more significant as CoFe 2O4, and Co3O4 are known to be far better electrocatalyst than Fe2O3. Moreover, the catalytic activity of Fe2O3/CNT30 composite is even comparable with some of the best known OER catalyst like NiCo2O4 [40], Thus this improved catalytic activity cannot be only due to the heightened electrical conductivity instead the morphology of Fe2O3 also has a rather vital contribution to the activity of the catalyst. It has been demonstrated that a nanomaterial with rod like shape has higher surface to volume ratio than other morphologies [34] also, a hollow nanoparticle is also expected to show higher surface to

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mass ratio than a denser particle. Thus the Fe2O3 nanorods with hollow architecture will provide larger electrochemically accessible surface area which is also vital for oxygen evolution reaction. The combination of the higher electrochemically accessible surface are and enhanced electrical conductivity results in the extremely high catalytic activity of the Fe 2O3/CNT30 composite. Recently Tavakkoli et al. reported synthesis of γFe2O3-CNT (mag-CNT) composite by adopting CVD method followed by in situ conversion Fe nanoparticles into γFe2O3 [41]. Their catalyst has displayed better catalytic activity and stability than Fe2O3/CNT30 composite. The difference between the catalytic activities of the two catalyst can be ascribed to their crystal structure. In case Fe2O3/CNT30 the crystal structure resembles that of rhombohedral hematite phase. Whereas, iron oxide in mag-CNT was found to adopt cubic maghemite structure that shows close resemblance with spinal structure of magnetite except in case of maghemite the Fe2+ atom from tetrahedral sites are removed by cation vacancies [42]. Since vacancies in crystal structure can also play a pivotal role in determining the catalytic activity of the OER catalyst, [43] mag-CNT showed better catalytic activity than Fe2O3/CNT30 composite. However, the process used for synthesis of magCNT requires sophisticated instrument and significantly higher temperature (up to 1100oC) moreover, control over the morphology and composition of the catalyst is not possible. On contrary, our method is much simpler, requires lower temperature (350 oC max) and a precise control over morphology and structure is also possible. In order to determine the optimum composition of catalyst Fe2O3/CNT composites with different mass fractions CNTs were prepared and tested for their electrocatalytic activity. The results of these experiments are presented in Fig. 6d. Addition of the CNT results in enhancement in the catalytic activity and maximum catalytic activity is observed at Fe 2O3:CNT mass ratio of 1:1. This is expected as increasing the amount of carbon nanotubes will result in better electrical

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conductivity. However, increasing the CNT loading beyond this point has an adverse effect on the catalyst performance. This observation can be explained on the basis of TEM images of the composite with higher CNT loading. The SEM and TEM images of the composites with 50% CNT and 70% loading are shown in Fig. 7. The TEM images of the Fe2O3/CNT50 composite (Fig. 7b) showed that most of the surface of carbon nanotube is covered by Fe2O3 nanorods on the surface of CNT. The Fe2O3 nanoparticles in this composite exists in two forms the first group consists of hollow rods grown on the surface of carbon nanotubes while the second group comprise of the dense particles having no definite size or shape. If loading of CNT is further increased, formation of Fe2O3 rods does not take place instead dense particles with random size are produced. Consequently, even though increasing the concentration of CNT will improve the electrical conductance, a concurrent decrease in the electrochemically accessible surface area due to loss of hollow structure will result in the enhanced resistance towards mass transfer that reduces the catalytic activity of Fe2O3/CNT composite towards OER. To verify this we determined the double layer capacitance of the Fe2O3/CNT composite with different mass fraction of the carbon nanotubes (Supporting info Fig. S5). The double layer capacitance of an electrode is directly proportional to the electrochemically active surface area as well as the OER active catalytic sites on the electrode surface. The Fe2O3/CNT50 composite showed highest double layer capacitance while the least value was obtained for Fe2O3/CNT70 composite. These values of double layer capacitance are in good agreement with the catalytic activity of the catalyst. The influence of CNT on morphology of Fe2O3 is interesting and probably arises due to adsorption of Fe3+ ions on the surface of carbon nanotubes (Fig. 7). As mentioned earlier the nucleation of FeOOH embryos occurs in two ways namely heterogeneous nucleation of adsorbed Fe3+ on the surface of CNT and homogeneous nucleation from the solution. Among these embryos

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the FeOOH embryos formed on surface of CNT are immobilized due to the attractive forces between oxygen rich functionality of CNT and metal thus are unable to merge with each other to form larger particles. On the other hand FeOOH seeds formed due to homogeneous nucleation are free to move and hence they are capable of forming particles of larger size. As concentration of the CNT is increased fraction of Fe+3 adsorbed on CNT increases and heterogeneous nucleation becomes a preferred pathway for formation of FeOOH seeds. As a result, at higher concentration of CNT most of the seeds are immobilized on the surface of CNT this hinders the assembly of FeOOH particles to larger particle. Therefore, at higher concentration of CNT small irregularly shaped particles are produced. 3.6. Impedance analysis Electrochemical impedance spectroscopy (EIS) is often employed to study various factors that influence the activity of catalyst. The impedance of the transition metal oxides for oxygen evolution reaction is often explained with help of equivalent circuit (EC) shown in Fig. 8b while, Nyquist plot of Fe2O3/CNT composite with different concentration of CNT is displayed in Fig. 8a. As evident from the figure impedance spectra of Fe2O3/CNT composites is not in the form of an ideal semicircle rather it appears like ark of a semicircle thus capacitive elements in the EC circuits were replaced with a constant phase element (CPE) defined by following equation. 𝑍𝐶𝑃𝐸 = 𝑄

1

𝑄 𝑦 (𝐽𝜔) 𝑎

(eq. 1)

Were ZCPE is impedance of the CPE. If Qa=1 the CPE behaves as an ideal capacitor while, it will behaves as ideal resistor if Qa=0. Though the physical significance of CPE is not clear the value of Qa is often used to represent the structural homogeneity of the electrode. The Nyquist plot of Fe2O3 and Fe2O3/CNT composites (Fig. 9a) are significantly different especially in higher frequency region. In higher frequency region the impedance spectra of all

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Fe2O3/CNT composites shows a linear relationship while at lower frequency semicircle is observed. This behavior is typical of diffusion controlled process moreover, if cathodic and anodic peak current observed in CV of Fe2O3/CNT composite are plotted against square root of scan rate a straight line with R2 of 0.987 and 0.989 respectively is obtained (Fig S5). Suggesting that reaction is diffusion controlled. Interestingly the impedance spectra of Fe2O3 did not show diffusion controlled behavior, may be in absence of carbon nanotubes due to low conductivity of Fe 2O3 the progress of reaction is mostly limited by the charge transfer processes. Introduction of CNT reduces the resistance associated with faradic process thus increasing the dominance of diffusion process on OER. To account for this behavior an additional EC element 𝑂 (Nerst diffusion) was introduced. This modified EC is represented in the Fig.9c. Here, R-Ω denotes solution resistance and remains reasonably constant for all the catalyst studied in this work. The term Q -dl is related to the double-layer capacitance of the electrode and can be correlated with the number of active sites on the surface of the catalyst. Among the catalyst under current study the Q-dl was found to increase in following order Fe2O3/CNT50 >Fe2O3CNT30 > Fe2O3/CNT70 which is also the order in which catalytic activity increases. The other circuit components, Rs, Rp, and Qϕ, account for different steps within the overall reaction. For instance, Rp, and Rs are related with the charge-transfer resistances and the ease with which various surface intermediates are formed, respectively. The overall faradaic resistance can be obtained simply the adding of Rs and Rp (Rfara= Rp+Rs). In the case of Fe2O3/CNT composite the faradaic resistance was found to increase in the following order Fe 2O3/CNT70 > Fe2O3/CNT30 > Fe2O3/CNT50. These results are more or less consistent with the catalytic activity of the composites. The term Qϕ in equivalent circuit describes the change in charge of the surface species during OER and, the Rs Qϕ loop describes the charge relaxation process associated with the

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formation of surface intermediates. The doping of the CNT caused augmentation in the value of Qϕ by enabling the easy conduction of electrons which is beneficial during the charge relaxation of the surface intermediate. The final EC element O accounts for the Nernst diffusion, as expected the value of O was found to increase linearly with mass fraction of carbon nanotubes suggesting that resistance for diffusion process has increased with mass fraction of carbon nanotubes. Thus based on these results from impedance analysis of Fe2O3/CNT composite it is suffice to say that unique catalytic activity of the Fe2O3/CNT50 is outcome of the synergistic effect between CNT and Fe2O3 nanorods that helps to reduce both mass transfer and charge transfer restrictions. 4. Conclusion In summary, Fe2O3/CNT composite was successfully synthesized by hydrolyzing FeCl3 in presence of the CNT. The synergistic effect of highly conductive CNT on hollow iron oxide nanorods caused a negative shift in faradic resistance as well as mass transfer resistance. Consequently, the as-synthesized Fe2O3/CNT composite exhibited high electrocatalytic activity for OER along with good stability in basic media. The Fe2O3/CNT50 composite displayed maximum catalytic activity and achieved the current density of 1 mA cm-2 and 10 mA cm-2 at a low overpotential of 323 and 383 mV, respectively. Moreover, the Fe2O3/CNT50 composite displayed the Tafel slope of 68 mv dec-1 which is significantly lower than Tafel slope recorded for either Fe2O3, CNT, or their physical mixture. Acknowledgement This project was supported by National Research Foundation of Korea (NRF) – Grants funded by the Ministry of Science, ICT and Future Planning (2014R1A2A2A01004352) and the Ministry of Education (2009-0093816), Republic of Korea.

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Figure caption Fig.1.

Schematic representation of experimental procedure for preparation of the Fe 2O3/CNT composites.

Fig.2.

XRD spectra of a) oxidized CNT, (b) FeOOH/CNT30 Composite, (b) Fe2O3, and (d) Fe2O3/CNT30 Composite.

Fig.3.

Raman spectra of (a) oxidized CNT, and (b) Fe2O3 /CNT30 composite.

Fig.4.

XPS spectra of Fe2O3/CNT30 composite (a) XPS survey (b) Fe2p, (c) C1s, and (d) O1s spectra.

Fig.5.

(a), (b) FE-SEM images, (c), (d), TEM images, (e), HR-TEM images, and (f) SADE image of Fe2O3/CNT30 composite.

Fig.6.

Comparison of electrocfatalytic activity of Fe2O3 and Fe2O3/CNT30 composite (a) polarization curve of catalyst deposited on glassy carbon electrode, (b) curresponding tafel slope, (c) potentiostatic analysis at a constant potential of 600 mV Vs Ag/AgCl electrode, (d) effect of CNT loading on onset overpotential and overpotential at current density of 10 mA cm-2, inset of figure 5d shows the changes in tafel slope of catalyst as a function of CNT doping.

Fig.7.

SEM and TEM images of Fe2O3/CNT composite with (a), (b) 50% CNT doping and (c), (d) 70% CNT doping.

Fig.8.

Probable mechanism formation and growth of the FeOOH particles in presence of the CNT.

Fig.9.

Impedance analysis of Fe2O3/CNT composite with different loading of CNT; (a) nyquist plot, (b) typical equivalent circuit diagram for OER, (c) equivalent circuit diagram used in current work.

27

Figure 1

28

Figure 2

29

Figure 3

30

Figure 4

31

Figure 5

32

Figure 6

33

Figure 7

34

Figure 8

35

Figure 9

36

Table. 1. Best fitted values from impadance spectra % CNT

R-Ω (Ω)

Qy-dl Qa-dl n (μS s cm-2)

Rp (Ω) Rs (Ω)

Qy-dl Qaϕ n (μS s cm-2)

Oy

Ob (sec-1)

30

8.53

3.457

0.68

23.70

82.5

393

0.71

7.8

0.33

50

8.43

5.9

0.55

16.36

56.03

650

0.76

46

0.38

70

9.17

1.11

0.71

66.42

61.16

257

0.74

146

0.43

37