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A new approach for the synthesis of electrocatalytically active CoFe2O4 catalyst for oxygen reduction reaction
Journal of Electroanalytical Chemistry 847 (2019) 113183
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Journal of Electroanalytical Chemistry journal homepage: www.elsevier.com/locate/jelechem
A new approach for the synthesis of electrocatalytically active CoFe2O4 catalyst for oxygen reduction reaction
T
Arpan Samanta, C. Retna Raj
⁎
Functional Materials and Electrochemistry Laboratory, Department of Chemistry, Indian Institute of Technology, Kharagpur 721302, West Bengal, India
ARTICLE INFO
ABSTRACT
Keywords: Multi-metal complex CoFe2O4, rGO Oxygen reduction reaction Electrocatalysis
Synthesis of non-precious electrocatalyst for the reduction of oxygen is of significant interest in the development of electrochemical energy conversion devices. Herein, we demonstrate a new, low-cost, and facile non-hydrothermal approach for the synthesis of electrocatalytically active spinel CoFe2O4 by thermal annealing of the multimetal complex, potassium cobalt hexacyanoferrate [K2CoFe(CN)6]. The as-synthesized CoFe2O4 is characterized by spectroscopic, electron microscopic and electrochemical techniques. The polyhedral-shaped crystalline CoFe2O4 particles have size ranging from 200 to 300 nm. The CoFe2O4 catalyst is integrated with reduced graphene oxide (rGO) and its electrocatalytic activity toward oxygen reduction reaction is evaluated. The integrated hybrid catalyst rGO/CoFe2O4 favors the 4-electron reduction of oxygen to water at low overpotential in alkaline pH with Tafel slope of 63 mV dec−1. The hybrid catalyst (rGO/CoFe2O4) has high durability and methanol tolerance. The catalytic performance is superior to that of the as-synthesized CoFe2O4 and rGO. The enhanced electrocatalytic performance is ascribed to the synergistic effect of CoFe2O4 and rGO, large surface area and high electronic conductivity. The ideal integration of CoFe2O4 with electronically conducting rGO favor the facile diffusion and promote the electron transfer for the reduction of oxygen.
1. Introduction The development of alternative environment-friendly energy sources is of utmost importance in order to decrease the dependence on the fossil fuel to meet the global energy demand. The electrochemical devices such as metal-air batteries, capacitors, and fuel cells are among the most promising choices as these are sustainable and environment friendly. One of the major challenges in the fabrication of efficient fuel cells and other energy conversion devices is to overcome the sluggish electron transfer kinetics associated with oxygen reduction reaction (ORR) at cathode [1–3]. Traditionally, the state-of-the-art Pt and Ptbased alloys have been used as cathode catalyst [4–6]. However, the high cost, lack of durability and severe poisoning by anode fuel limits the use Pt catalyst in the widespread commercialization of fuel cells [7–9]. Synthesis of efficient low-cost non-precious electrocatalyst for the cathodic reduction of oxygen in polymer electrolyte membrane fuel cell is the need of this hour [10,11]. Transition metal-based mixed valence oxides including spinels have received immense importance among the scientific community owing to their interesting physiochemical and electronic properties [12–17]. Among them, ferrite-based spinel oxide with the molecular formula MFe2O4 (M = Ni2+, Co2+, Cu2+, etc.) has widely been explored in ⁎
various fields especially in the magnetic device applications due to their high magnetic anisotropy, high coercivity, mechanical hardness and chemical stability [18–23]. The cobalt ferrite (CoFe2O4)-based materials received considerable attention for various electrochemical applications including energy conversion and storage [24–31]. Recently, several attempts have been made to utilize ferrite-based materials for electrocatalytic oxygen reduction and oxygen evolution reactions [32–37]. The limited electrical conductivity [38] of ferrites is one of the main challenges in effectively using them for the electrocatalytic application. One approach to enhance the electrical conductivity and hence the electrocatalytic activity is to support the ferrites on suitable conducting supports like carbon. The nanostructured carbon such as carbon nanotubes [39], graphene [27,35], and biocarbon [40] has been used to overcome the poor electrical conductivity of ferrite spinel-based catalyst. Among these carbon materials, graphene has attracted much interest in electrocatalysis, owing to its unique physical and chemical properties such as high electrical conductivity, chemical stability and large surface area [41]. To the best of our knowledge, only a handful of studies report the catalytic activity of cobalt ferrite toward ORR [35,39,40,42]. Traditionally, ferrites have been synthesized by sol-gel, hydrothermal, co-precipitation, sonochemical, aerosol, ball milling,
Corresponding author. E-mail address: [email protected] (C.R. Raj).
https://doi.org/10.1016/j.jelechem.2019.05.065 Received 25 January 2019; Received in revised form 29 April 2019; Accepted 27 May 2019 Available online 28 May 2019 1572-6657/ © 2019 Elsevier B.V. All rights reserved.
Journal of Electroanalytical Chemistry 847 (2019) 113183
A. Samanta and C.R. Raj
microemulsion [43–51], etc. approaches. The hydrothermal methods have been extensively used for the shape-controlled synthesis of ferrites. The use of organic solvents and shape-regulating organic additives such as surfactants and polymer has the adverse effects on the surface properties of these materials due to the adsorption of shape directing reagents. The surface adsorbed species can impair the electrocatalytic properties as the electrochemical properties of nanostructured materials largely depends on the surface morphology, structure, etc. Suitable pre-treatment procedures are required to remove the surface adsorbed surfactants/polymers in order to study the electrocatalytic properties. It is desirable to develop a simple approach devoid of such shape regulating templates/reagents. Our group is interested in the development of non-Pt electrocatalyst for ORR and OER [10,13,52,53]. In continuation of our earlier efforts, herein, we demonstrate a non-hydrothermal facile approach for the synthesis of non-precious metal catalyst CoFe2O4 by the thermal annealing of multi-metal complex potassium cobalt hexacyanoferrate [K2CoFe(CN)6] and its electrocatalytic performance toward ORR. The as-synthesized CoFe2O4 is integrated with the catalyst support rGO and the electrocatalytic activity is evaluated in terms of catalytic current density, onset potential, Tafel slope, durability, etc. in alkaline medium.
and ethanol to remove the unreacted reactants, if any. The product was then dried in vacuum at 80 °C for 12 h. The dried product was subjected to thermal annealing at 900 °C for 1 h in air at a heating rate of 5 °C min−1. The integrated hybrid material rGO/CoFe2O4 was obtained by mixing an aqueous dispersion of 1:2 weight ratios of rGO and CoFe2O4 under constant stirring for overnight. 2.4. Material characterization
Graphite powder, Nafion® and Pt/C (20%) were purchased from Sigma-Aldrich. All other chemicals used in this investigation were obtained from Merck, India without further purification. All the solutions were made of Millipore water (Milli Q system).
The infrared spectrum was recorded on Perkin-Elmer FTIR spectrophotometer RX1. X-ray diffraction (XRD) analysis was performed with a Bruker D8 advance unit. Transmission electron microscopy (TEM) images were captured with an FEI-TECNAI G2 20S TWIN electron microscope operating at a voltage of 200 kV. Field emission scanning electron microscopy (FESEM) analysis was carried out using an FEI NOVA NANOSEM 450, and mapping analysis were performed with a BRUKER EDS microanalyzer attached to the FESEM instrument. The X-ray photoelectron spectrum (XPS) was analyzed with PHI 5000 Versa Probe II scanning XPS microprobe (ULVACPHI) =1486.6 eV). All the electrochemical responses were recorded by using Autolab potentiostat-galvanostat (302 N) with computer controlled NOVA software in 0.1 M KOH aqueous solution as the electrolyte. All the experiments have been reproduced at least three times by using the rotating disk (glassy carbon) (RDE) and rotating ring (Pt) disk (glassy carbon) (RRDE) as working, Pt wire as auxiliary, and Hg/HgO as reference electrodes. The durability test was performed using a graphite rod counter electrode and the working electrode was held at the potential of 0.4 V. All the potentials are referred against the reversible hydrogen electrode (RHE) using the thermodynamic potential according to the equation E (RHE) = E (Hg/HgO) + 0.908 V.
2.2. Synthesis of rGO
2.5. Electrode preparation
Graphene oxide (GO) was synthesized by the exfoliation of graphite according to the modified Hummers method [54] and characterized with XRD and FTIR measurements (Fig. S1). rGO was obtained by thermal annealing of GO at 900 °C in an argon atmosphere in a tubular furnace for 1 h. The black colored rGO was then washed well with copious amount of water and dried in vacuum.
The catalyst ink was prepared by dispersing 1.5 mg of the integrated hybrid electrocatalyst with 50 μL of Nafion® solution and 250 μL of isopropanol for 30 min using an ultrasonicator. All working electrodes were modified by drop casting required amount (10–15 μL) of the homogeneous catalyst ink and was allowed to dry at room temperature for 6 h.
2.3. Synthesis of CoFe2O4 nanoparticles and rGO/CoFe2O4
3. Results and discussion
CoFe2O4 was synthesized using a single-source precursor potassium cobalt hexacyanoferrate [K2CoFe(CN)6], a multi-metal complex, which was synthesized by mixing an aqueous solution of 1:1 M ratio of K3[Fe (CN)6] and CoCl2.6H2O under constant stirring at room temperature (Scheme 1). The pink color solution of CoCl2.6H2O gradually turned to deep brown during the addition of K3[Fe(CN)6] confirming the formation of multi-metal complex K2CoFe(CN)6. The precipitate was centrifuged after 4 h and the residue was washed thoroughly with water
The multi-metal complex K2[CoFe(CN)6] was characterized with FTIR and XRD analyses (Fig. S2). The FTIR spectral profile of K2[CoFe (CN)6] shows the characteristic signature in the region of 4000–400 cm−1 (Fig. S2a). The peaks at ~ 2162 cm−1 and 2117 cm−1 correspond to the CeN stretching in FeIII-CN-CoII and FeII-CN-CoIII environments, respectively [55]. The peaks at 541 cm−1 and 591 cm−1 are attributed to the formation of Fe-CN-Co [56]. The four major diffractions at 2θ values of 17.2°, 24.5°, 34.8°, 39.1° correspond to the
2. Materials and methods 2.1. Reagents and materials
Scheme 1. Scheme illustrating the syntheses of CoFe2O4 and rGO/CoFe2O4. 2
Journal of Electroanalytical Chemistry 847 (2019) 113183
A. Samanta and C.R. Raj
The surface survey scan XPS profile shows the presence of oxygen, iron, and cobalt (Fig. 2a). The high-resolution decovoluted Fe 2p spectra show two peaks at 710.5 and 724.08 eV corresponding to Fe 2p3/2 and Fe 2p1/2, respectively accompanied by a satellite peak at 717.8 (Fig. 2b), confirming the presence of Fe in +3 oxidation state [26,59]. The peak at 712.8 eV suggests the presence of Fe+3 in more than one coordination environment in the partially inverse spinel structure [60]. This is possibly due to the high affinity of Co+2 in octahedral sites of inverse spinel structure. In the case of Co 2p, the peaks appear at the binding energy of 796.04 and 780.7 eV corresponds to Co 2p1/2 and 2p3/2, respectively. The satellite peaks at 785.1 and 803.3 eV confirm that Co exists in +2 oxidation state [61,62] (Fig. 2c). The O 1s spectra originate from the lattice oxygen and have the binding energy of 529.2 eV [62] (Fig. 2d). The FESEM and TEM images (Fig. 3a–c) reveal that the CoFe2O4 particles have polyhedral shape with a size distribution ranging from 200 to 300 nm. In the high-resolution TEM image, the fringe spacing of 0.25 nm corresponding to the (311) plane of CoFe2O4 (Fig. 3d) was observed. The selected area diffraction (SAED) shows a spotty ring pattern (inset of Fig. 3c) corresponding to the (200), (311), (400), and (511) planes, further supports the crystalline nature of CoFe2O4. Elemental mapping analysis on a single particle of CoFe2O4 confirms the presence of Co, Fe, and O (Fig. S3). The FESEM and TEM images of rGO/CoFe2O4 evidence the presence of both CoFe2O4 nanoparticles and rGO sheets; the particles are randomly distributed over the rGO sheets (Fig. 3e and f). rGO/CoFe2O4 and CoFe2O4 catalysts were first electrochemically characterized by evaluating the specific capacitance (Csv) with cyclic voltammetry (Fig. S4) following the known procedures [63]. Interestingly, integrated hybrid catalyst rGO/CoFe2O4 has significantly large Csv (141 μF g−1) compared to the as-synthesized CoFe2O4 (0.5 μF g−1)
Fig. 1. XRD profile of CoFe2O4 and rGO/CoFe2O4.
(200), (220), (400) and (420) planes of face-centred cubic cobalt hexacyanoferrate [57] (Fig. S2b). FESEM image (Fig. S2c) reveals that the as-synthesized K2[CoFe(CN)6] has agglomerated morphology. Fig. 1 shows the XRD profile of as-synthesized CoFe2O4, and the hybrid material rGO/CoFe2O4. Both the catalysts show the characteristic peaks corresponding to the cubic spinel structure of CoFe2O4 (JCPDS #221086). The sharp diffraction peaks suggest the crystalline nature of CoFe2O4 particles. The crystallite size of CoFe2O4 particles was calculated from X-ray line broadening analysis using Scherrer formula [58] for both the hybrid and CoFe2O4. The average calculated crystallite size is found to be ~22 nm in both the catalysts. In case of rGO/CoFe2O4, the broad diffraction ~25.1° is indexed to the (002) plane of carbon.
Fig. 2. XPS surface survey scan (a) and high resolution Fe 2p (b), Co 2p (c) and O 1s (d) spectral profiles of CoFe2O4. 3
Journal of Electroanalytical Chemistry 847 (2019) 113183
A. Samanta and C.R. Raj
Fig. 3. FESEM (a, b, e), TEM (c, f) and HRTEM (d) images of CoFe2O4 (a, b, c, d) and rGO/CoFe2O4 (e, f). Inset in (c) is the SAED pattern of CoFe2O4.
and rGO (14 μF g−1), suggesting that the hybrid material has high electronic conductivity and is easily accessible to the supporting electrolyte [64]. The very low specific capacitance of CoFe2O4 implies its poor electronic conductivity. The integration of poorly conductive CoFe2O4 with conducting rGO enhances the overall electronic conductivity and makes the active catalyst accessible. The electrocatalytic activity of rGO/CoFe2O4 toward ORR was evaluated with RDE and RRDE in O2 saturated 0.1 M KOH solution. Fig. 4a illustrates the electrocatalytic performance of rGO/CoFe2O4 and as-synthesized CoFe2O4 and rGO toward ORR. The onset potential for ORR on rGO/CoFe2O4 is 50–170 mV more positive than the free rGO and CoFe2O4, respectively. The well-defined polarization curves obtained with rGO/CoFe2O4 hybrid suggests facile electron transfer kinetics. The limiting current obtained with rGO/CoFe2O4 is two times higher than those of the other catalyst examined in this investigation, further highlighting the high activity of the hybrid. Linear increments in the limiting current density with increasing the rotation speed supports the diffusion controlled electron transfer kinetics (Fig. 4b). It should be noted here that the ORR performance of as-synthesized CoFe2O4 and rGO is rather poor in terms of onset potential and current density (Figs. 4a, S5). As can be seen in Fig. 4 that the CoFe2O4-based electrode does not yield limiting current and we observed a peak around 0.45 V and broad hump at more negative potential presumably due to the non-homogeneous electron transfer across the electrode-solution interface due to its poor electronic conductivity. More importantly, the geometric surface area normalized current density for ORR on rGO and CoFe2O4 catalyst is 2 times lower than the hybrid catalyst. The slope of the Koutecky-Levich plot for the hybrid catalyst remains almost same in the potential range of 0.6 to 0.45 V, supporting the involvement of the same number of electrons during ORR in the entire potential range (Fig. 4c). It is interesting to note that the mass-specific activity (mass normalized kinetic current density) of rGO/CoFe2O4 hybrid (14.4 A g−1) is ~2 times higher than
the rGO (7.6 A g−1) and CoFe2O4 (7.2 A g−1) catalysts with identical catalyst loading (0.25 mg cm−2). The ideal integration of electronically conducting large surface area rGO with poorly conducting CoFe2O4 enhances the overall performance presumably by enhancing the electrical conductivity of CoFe2O4. The rGO support offers a large surface area, prevents the aggregation of CoFe2O4 and favors the facile diffusion of the electrolyte. It is worth noting here that all the kinetic analysis has been carried out with the limiting current obtained in the cathodic sweep at the scan rate of 5 mV/s though the true steady state was not achieved (Fig. S6). In order to understand the reaction pathway for ORR and the % of HO2¯ yield, RRDE experiment was carried out with the ring potential held at 1.4 V vs RHE while sweeping the disk potential. The number of electrons (n) and the peroxide yield was obtained from the ratio of ring and disk currents (IR/ID) at 1600 rpm (Fig. S7), using the following equations where N is the current collection efficiency of RRDE (0.37).
n = 4NID/(NID + IR )
(1)
%HO2 ¯ = 200IR /(NID + IR )
(2)
As shown in Fig. 4d, the average number of electrons transferred (n) over the potential range from 0.2 to 0.6 V vs RHE is 3.9. The number of electrons transferred in the entire potential range is shown in Fig. S7b. The HO2¯ yield was only < 5% confirming that the hybrid catalyst favors 4-electron pathway for ORR. In the case of rGO and CoFe2O4 generation of > 40–60% HO2¯ with n value of 2.8–3.2 is observed under the same potential range. The rGO of the hybrid material favors the 2-electron pathway with more positive onset potential whereas the CoFe2O4 tend to favors the 4-electron pathway with less positive onset potential. The synergistic effect between CoFe2O4 and rGO improves the overall performance of the integrated hybrid catalyst [65] in terms of more positive onset potential and high current density. The 4
Journal of Electroanalytical Chemistry 847 (2019) 113183
A. Samanta and C.R. Raj
Fig. 4. (a) Polarization curves illustrating the ORR activity of different catalysts. Electrolyte: 0.1 M KOH. Rotation: 1600 rpm. (b) Polarization curves at different rotations. (c) Corresponding K-L plot. Sweep rate: 5 mV/ s. (d) Plot showing the average % of HO2¯ and number of electron transferred (n) during ORR with different catalysts under the same potential range 0.2 V to 0.6 V vs RHE. (e) Tafel plot for rGO/CoFe2O4, CoFe2O4 and 20% Pt/C catalysts. (f) Plot illustrating the durability of rGO/CoFe2O4 and CoFe2O4 catalysts.
synergistic effect can be explained by considering a 2 + 2-electron transfer mechanism proposed by Hermann et al. [66]. First O2 is reduced to HO2– preferably at the rGO active site through a 2-electron pathway followed by the 2-electron HO2– reduction to OH– at the adjacent CoFe2O4 sites, resulting in an overall 4-electron pathway for ORR. The kinetics of ORR was further studied with Tafel analysis and Tafel slope of 63 mV dec−1 was obtained with the hybrid catalyst which is very close to that of the traditional Pt catalyst (67 mV dec−1), suggesting the facile electron transfer kinetics (Fig. 4e). The Tafel slope approaches the theoretical value of 59 mV dec−1 (2.303RT/F) at room temperature (25 °C), implying high intrinsic ORR activity of the integrated hybrid catalyst. The ORR performance of rGO/CoFe2O4 integrated hybrid catalyst is comparable to the other CoFe2O4-based catalysts reported in the literature [35,39,40,42,67] (Table S1). For instance, CNT/CNF/BCCoFe2O4 based catalysts show an onset potential of 0.82–0.87 V vs RHE
with n value of ~3.8–3.9 [39,40,67]. The rGO/NG-CoFe2O4 based catalysts have the onset potential between 0.83 and 0.9 V with n value of ~3.85 [35,42]. In our case, we could achieve an onset potential of 0.85 V vs RHE with n values of 3.9 and Tafel slope of 62 mV dec−1, which is close to the state-of-the-art Pt catalyst, highlighting the enhanced activity of our hybrid catalyst. The durability of the traditional Pt-based catalyst is one of the major concerns. The durability of both the rGO/CoFe2O4 hybrid and CoFe2O4 catalysts was examined by holding the potential of 0.4 V vs RHE, where high current density is observed, for 5 h and rotating the electrode at 1600 rpm (Fig. S8). A significant drop in current density (~58%) for the CoFe2O4 catalyst was observed whereas in case of rGO/CoFe2O4 hybrid only a ~6% decrease in current density was observed after 5 h of continuous operation (Fig. 4f), implying the high durability of the hybrid catalyst. Traditional Pt-based catalysts do not have tolerance toward methanol and are known to catalyse the oxidation of methanol 5
Journal of Electroanalytical Chemistry 847 (2019) 113183
A. Samanta and C.R. Raj
and reduction of oxygen. Our rGO/CoFe2O4 hybrid and as-synthesized CoFe2O4 catalysts has high tolerance toward methanol; the onset potential and the limiting current density for ORR remain unchanged even after the addition of 1 M MeOH (Fig. S9).
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4. Conclusions We have demonstrated a new facile solid-state approach for the synthesis of CoFe2O4 using a single-source multi-metal complex precursor. The hybrid catalyst derived from rGO and CoFe2O4 has good catalytic activity toward ORR. It promotes the direct 4-electron pathway for the reduction of oxygen at low overpotential. The onset potential, mass specific activity and durability is higher than the assynthesized rGO and CoFe2O4. Superior ORR activity of rGO/CoFe2O4 hybrid is due to the ideal integration of electronically conductive large surface area rGO with poorly conductive CoFe2O4 and the synergistic effect. Low Tafel slope suggests that the hybrid catalyst has high intrinsic ORR activity. Our study demonstrates that the rational hybridization of conductive rGO with poorly conductive metal oxides is a promising approach to achieve high catalytic activity. Acknowledgments This work was financially supported by Science and Engineering Research Board (SERB), India (Grant No. EMR/2016/002271). Samanta is thankful to DST and IIT Kharagpur, India for the financial support. We are thankful to the Department of Physics, IIT Kharagpur for providing the DST-FIST funded XPS facility. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.jelechem.2019.05.065. References [1] R. Bashyam, P. Zelenay, A class of non-precious metal composite catalysts for fuel cells, Nature 443 (2006) 63–66. [2] C. Song, J. Zhang, Electrocatalytic oxygen reduction reaction, in: J. Zhang (Ed.), PEM Fuel Cell Electrocatalysts and Catalyst Layers: Fundamentals and Applications, Springer, 2008, pp. 89–134. [3] M.K. Debe, Electrocatalyst approaches and challenges for automotive fuel cells, Nature 486 (2012) 43–51. [4] N.M. Markovic, T.J. Schmidt, V. Stamenkovic, P.N. Ross, Oxygen reduction reaction on Pt and Pt bimetallic surfaces: a selective review, Fuel Cells (2) (2001) 105–116. [5] N. Jung, D.Y. Chung, J. Ryu, S.J. Yoo, Y.-E. Sung, Pt-based nanoarchitecture and catalyst design for fuel cell applications, Nano Today 9 (2014) 433–456. [6] S. Ghosh, R.K. Sahu, C.R. Raj, Shape-regulated high yield synthesis of electrocatalytically active branched Pt nanostructures for oxygen reduction and methanol oxidation reactions, J. Mater. Chem. 21 (2011) 11973–11980. [7] A. Morozan, B. Jousselme, S. Palacin, Low-platinum and platinum-free catalysts for the oxygen reduction reaction at fuel cell cathodes, Energy Environ. Sci. 4 (2011) 1238–1254. [8] M.S. El-Deab, F. Kitamura, T. Ohsaka, Poisoning effect of selected hydrocarbon impurities on the catalytic performance of Pt/C catalysts towards the oxygen reduction reaction, J. Electrochem. Soc. 160 (2013) F651–F658. [9] C. Su, T. Yang, W. Zhou, W. Wang, X. Xu, Z. Shao, Pt/C–LiCoO2 composites with ultralow Pt loadings as synergistic bifunctional electrocatalysts for oxygen reduction and evolution reactions, J. Mater. Chem. A 4 (2016) 4516–4524. [10] C.R. Raj, A. Samanta, S.H. Noh, S. Mondal, T. Okajima, T. Ohsaka, Emerging new generation electrocatalysts for the oxygen reduction reaction, J. Mater. Chem. A 4 (2016) 11156–11178. [11] M. Shao, Q. Chang, J.-P. Dodelet, R. Chenitz, Recent advances in electrocatalysts for oxygen reduction reaction, Chem. Rev. 116 (2016) 3594–3657. [12] J. Xu, P. Gao, T. Zhao, Non-precious Co3O4 nano-rod electrocatalyst for oxygen reduction reaction in anion-exchange membrane fuel cells, Energy Environ. Sci. 5 (2012) 5333–5339. [13] S. Bag, K. Roy, C.S. Gopinath, C.R. Raj, Facile single-step synthesis of nitrogendoped reduced graphene oxide-Mn3O4 hybrid functional material for the electrocatalytic reduction of oxygen, ACS Appl. Mater. Interfaces 6 (2014) 2692–2699. [14] W. Zhou, L. Ge, Z.-G. Chen, F. Liang, H.-Y. Xu, J. Motuzas, A. Julbe, Z. Zhu, Amorphous iron oxide decorated 3d heterostructured electrode for highly efficient oxygen reduction, Chem. Mater. 23 (2011) 4193–4198. [15] Z.-S. Wu, S. Yang, Y. Sun, K. Parvez, X. Feng, K. Müllen, 3D nitrogen-doped graphene aerogel-supported Fe3O4 nanoparticles as efficient electrocatalysts for the
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A. Samanta and C.R. Raj [45] X.-M. Liu, S.-Y. Fu, L.-P. Zhu, High-yield synthesis and characterization of monodisperse sub-microsized CoFe2O4 octahedra, J. Solid State Chem. 180 (2007) 461–466. [46] L. Zheng, L. Guan, G. Yang, S. Chen, H. Zheng, One-pot synthesis of CoFe2O4/rGO hybrid hydrogels with 3D networks for high capacity electrochemical energy storage devices, RSC Adv. 8 (2018) 8607–8614. [47] Y.I. Kim, D. Kim, C.S. Lee, Synthesis and characterization of CoFe2O4 magnetic nanoparticles prepared by temperature-controlled coprecipitation method, Physica B 337 (2003) 42–51. [48] K.V.P.M. Shafi, A. Gedanken, R. Prozorov, J. Balogh, Sonochemical preparation and size-dependent properties of nanostructured CoFe2O4 particles, Chem. Mater. 10 (1998) 3445–3450. [49] J. Fu, C.J. DeSantis, R.G. Weiner, S.E. Skrabalak, Aerosol-assisted synthesis of shape-controlled CoFe2O4: topotactic versus direct melt crystallization, Chem. Mater. 27 (2015) 1863–1868. [50] E. Manova, B. Kunev, D. Paneva, I. Mitov, L. Petrov, Mechano-synthesis, characterization, and magnetic properties of nanoparticles of cobalt ferrite, CoFe2O4, Chem. Mater. 16 (2004) 5689–5696. [51] F. Foroughia, S. A. H.-Tabrizib, J. Amighian, Microemulsion synthesis and magnetic properties of hydroxyapatite-encapsulated nano CoFe2O4, J. Magn. Magn. Mater., 382 (2015), 182–187. [52] A. Samanata, C.R. Raj, Catalyst support in oxygen electrocatalysis: a case study with CoFe alloy electrocatalyst, J. Phys. Chem. C 122 (2018) 15843–15852. [53] S. Mondal, C.R. Raj, Copper nitride nanostructure for the electrocatalytic reduction of oxygen: kinetics and reaction pathway, J. Phys. Chem. C 122 (2018) 18468–18475. [54] W. S. Hummers, R. E. Offeman, Preparation of graphitic oxide, J. Am. Chem. Soc., 80 (1958), 1339–1339. [55] R.O. Lezna, R. Romagnoli, N.R. de Tacconi, K. Rajeshwar, Cobalt hexacyanoferrate: compound stoichiometry, infrared spectroelectrochemistry, and photoinduced electron transfer, J. Phys. Chem. B 106 (2002) 3612–3621. [56] M. Berrettoni, M. Giorgetti, S. Zamponi, P. Conti, D. Ranganathan, A. Zanotto, M.A. Saladino, E. Caponetti, Synthesis and characterization of nanostructured
cobalt hexacyanoferrate, J. Phys. Chem. C 114 (2010) 6401–6407. [57] L.-H. Shi, T. Wu, M.-J. Wang, D. Li, Y.-J. Zhang, J.-H. Li, Molecule-based cobalt hexacyanoferrate nanoparticle: synthesis, characterization, and its electrochemical properties, Chin. J. Chem. 23 (2005) 149–154. [58] H.P. Klug, L.E. Alexander, X-ray Diffraction Procedures for Polycrystalline and Amorphous Materials, 2nd ed., Wiley, New York, 1974. [59] G.C. Allen, S.J. Harris, J.A. Jutson, J.M. Dyke, A study of a number of mixed transition metal oxide spinels using X-ray photoelectron spectroscopy, Appl. Surf. Sci. 37 (1989) 111–134. [60] Z.J. Gu, X. Xiang, G.L. Fan, F. Li, Facile synthesis and characterization of cobalt ferrite nanocrystals via a simple reduction-oxidation route, J. Phys. Chem. C 112 (2008) 18459–18466. [61] H.L. Yuan, Q. Wang, S.M. Zhou, L.S. Liu, X.L. Chen, S.Y. Lou, R.J. Yuan, Y.M. Hao, N. Li, Low-temperature preparation of superparamagnetic CoFe2O4 microspheres with high saturation magnetization, Nanoscale Res. Lett. 5 (2010) 1817–1821. [62] S. Pylypenko, S. Mukherjee, T.S. Olson, P. Atanassov, Non-platinum oxygen reduction electrocatalysts based on pyrolyzed transition metal macrocycles, Electrochim. Acta 53 (2008) 7875–7883. [63] J.-S.M. Lee, M.E. Briggs, C.C. Hu, A.I. Cooper, Controlling electric double-layer capacitance and pseudocapacitance in heteroatom-doped carbons derived from hypercrosslinked microporous polymers, Nano Energy 46 (2018) 277–289. [64] Z.-H. Huang, T.-Y. Liu, Y. Song, Y. Li, X.-X. Liu, Balancing the electrical double layer capacitance and pseudocapacitance of hetero-atom doped carbon, Nanoscale 9 (2017) 13119–13127. [65] X. Li, W. Qu, J. Zhang, H. Wang, Electrocatalytic activities of La0.6Ca0.4CoO3 and La0.6Ca0.4CoO3-carbon composites toward the oxygen reduction reaction in concentrated alkaline electrolytes, J. Electrochem. Soc. 158 (2011) A597–A604. [66] V. Hermann, D. Dutriat, S. Müller, Ch. Comninellis, Mechanistic studies of oxygen reduction at La0.6Ca0.4CoO3-activated carbon electrodes in a channel flow cell, Electrochim. Acta 46 (2000) 365–372. [67] S. Liu, W. Yan, X. Cao, Z. Zhou, R. Yang, Bacterial-cellulose-derived carbon nanofiber-supported CoFe2O4 as efficient electrocatalyst for oxygen reduction and evolution reactions, Int. J. Hydrog. Energy 41 (2016) 5351–5360.
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Journal of Electroanalytical Chemistry journal homepage: www.elsevier.com/locate/jelechem
A new approach for the synthesis of electrocatalytically active CoFe2O4 catalyst for oxygen reduction reaction
T
Arpan Samanta, C. Retna Raj
⁎
Functional Materials and Electrochemistry Laboratory, Department of Chemistry, Indian Institute of Technology, Kharagpur 721302, West Bengal, India
ARTICLE INFO
ABSTRACT
Keywords: Multi-metal complex CoFe2O4, rGO Oxygen reduction reaction Electrocatalysis
Synthesis of non-precious electrocatalyst for the reduction of oxygen is of significant interest in the development of electrochemical energy conversion devices. Herein, we demonstrate a new, low-cost, and facile non-hydrothermal approach for the synthesis of electrocatalytically active spinel CoFe2O4 by thermal annealing of the multimetal complex, potassium cobalt hexacyanoferrate [K2CoFe(CN)6]. The as-synthesized CoFe2O4 is characterized by spectroscopic, electron microscopic and electrochemical techniques. The polyhedral-shaped crystalline CoFe2O4 particles have size ranging from 200 to 300 nm. The CoFe2O4 catalyst is integrated with reduced graphene oxide (rGO) and its electrocatalytic activity toward oxygen reduction reaction is evaluated. The integrated hybrid catalyst rGO/CoFe2O4 favors the 4-electron reduction of oxygen to water at low overpotential in alkaline pH with Tafel slope of 63 mV dec−1. The hybrid catalyst (rGO/CoFe2O4) has high durability and methanol tolerance. The catalytic performance is superior to that of the as-synthesized CoFe2O4 and rGO. The enhanced electrocatalytic performance is ascribed to the synergistic effect of CoFe2O4 and rGO, large surface area and high electronic conductivity. The ideal integration of CoFe2O4 with electronically conducting rGO favor the facile diffusion and promote the electron transfer for the reduction of oxygen.
1. Introduction The development of alternative environment-friendly energy sources is of utmost importance in order to decrease the dependence on the fossil fuel to meet the global energy demand. The electrochemical devices such as metal-air batteries, capacitors, and fuel cells are among the most promising choices as these are sustainable and environment friendly. One of the major challenges in the fabrication of efficient fuel cells and other energy conversion devices is to overcome the sluggish electron transfer kinetics associated with oxygen reduction reaction (ORR) at cathode [1–3]. Traditionally, the state-of-the-art Pt and Ptbased alloys have been used as cathode catalyst [4–6]. However, the high cost, lack of durability and severe poisoning by anode fuel limits the use Pt catalyst in the widespread commercialization of fuel cells [7–9]. Synthesis of efficient low-cost non-precious electrocatalyst for the cathodic reduction of oxygen in polymer electrolyte membrane fuel cell is the need of this hour [10,11]. Transition metal-based mixed valence oxides including spinels have received immense importance among the scientific community owing to their interesting physiochemical and electronic properties [12–17]. Among them, ferrite-based spinel oxide with the molecular formula MFe2O4 (M = Ni2+, Co2+, Cu2+, etc.) has widely been explored in ⁎
various fields especially in the magnetic device applications due to their high magnetic anisotropy, high coercivity, mechanical hardness and chemical stability [18–23]. The cobalt ferrite (CoFe2O4)-based materials received considerable attention for various electrochemical applications including energy conversion and storage [24–31]. Recently, several attempts have been made to utilize ferrite-based materials for electrocatalytic oxygen reduction and oxygen evolution reactions [32–37]. The limited electrical conductivity [38] of ferrites is one of the main challenges in effectively using them for the electrocatalytic application. One approach to enhance the electrical conductivity and hence the electrocatalytic activity is to support the ferrites on suitable conducting supports like carbon. The nanostructured carbon such as carbon nanotubes [39], graphene [27,35], and biocarbon [40] has been used to overcome the poor electrical conductivity of ferrite spinel-based catalyst. Among these carbon materials, graphene has attracted much interest in electrocatalysis, owing to its unique physical and chemical properties such as high electrical conductivity, chemical stability and large surface area [41]. To the best of our knowledge, only a handful of studies report the catalytic activity of cobalt ferrite toward ORR [35,39,40,42]. Traditionally, ferrites have been synthesized by sol-gel, hydrothermal, co-precipitation, sonochemical, aerosol, ball milling,
Corresponding author. E-mail address: [email protected] (C.R. Raj).
https://doi.org/10.1016/j.jelechem.2019.05.065 Received 25 January 2019; Received in revised form 29 April 2019; Accepted 27 May 2019 Available online 28 May 2019 1572-6657/ © 2019 Elsevier B.V. All rights reserved.
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microemulsion [43–51], etc. approaches. The hydrothermal methods have been extensively used for the shape-controlled synthesis of ferrites. The use of organic solvents and shape-regulating organic additives such as surfactants and polymer has the adverse effects on the surface properties of these materials due to the adsorption of shape directing reagents. The surface adsorbed species can impair the electrocatalytic properties as the electrochemical properties of nanostructured materials largely depends on the surface morphology, structure, etc. Suitable pre-treatment procedures are required to remove the surface adsorbed surfactants/polymers in order to study the electrocatalytic properties. It is desirable to develop a simple approach devoid of such shape regulating templates/reagents. Our group is interested in the development of non-Pt electrocatalyst for ORR and OER [10,13,52,53]. In continuation of our earlier efforts, herein, we demonstrate a non-hydrothermal facile approach for the synthesis of non-precious metal catalyst CoFe2O4 by the thermal annealing of multi-metal complex potassium cobalt hexacyanoferrate [K2CoFe(CN)6] and its electrocatalytic performance toward ORR. The as-synthesized CoFe2O4 is integrated with the catalyst support rGO and the electrocatalytic activity is evaluated in terms of catalytic current density, onset potential, Tafel slope, durability, etc. in alkaline medium.
and ethanol to remove the unreacted reactants, if any. The product was then dried in vacuum at 80 °C for 12 h. The dried product was subjected to thermal annealing at 900 °C for 1 h in air at a heating rate of 5 °C min−1. The integrated hybrid material rGO/CoFe2O4 was obtained by mixing an aqueous dispersion of 1:2 weight ratios of rGO and CoFe2O4 under constant stirring for overnight. 2.4. Material characterization
Graphite powder, Nafion® and Pt/C (20%) were purchased from Sigma-Aldrich. All other chemicals used in this investigation were obtained from Merck, India without further purification. All the solutions were made of Millipore water (Milli Q system).
The infrared spectrum was recorded on Perkin-Elmer FTIR spectrophotometer RX1. X-ray diffraction (XRD) analysis was performed with a Bruker D8 advance unit. Transmission electron microscopy (TEM) images were captured with an FEI-TECNAI G2 20S TWIN electron microscope operating at a voltage of 200 kV. Field emission scanning electron microscopy (FESEM) analysis was carried out using an FEI NOVA NANOSEM 450, and mapping analysis were performed with a BRUKER EDS microanalyzer attached to the FESEM instrument. The X-ray photoelectron spectrum (XPS) was analyzed with PHI 5000 Versa Probe II scanning XPS microprobe (ULVACPHI) =1486.6 eV). All the electrochemical responses were recorded by using Autolab potentiostat-galvanostat (302 N) with computer controlled NOVA software in 0.1 M KOH aqueous solution as the electrolyte. All the experiments have been reproduced at least three times by using the rotating disk (glassy carbon) (RDE) and rotating ring (Pt) disk (glassy carbon) (RRDE) as working, Pt wire as auxiliary, and Hg/HgO as reference electrodes. The durability test was performed using a graphite rod counter electrode and the working electrode was held at the potential of 0.4 V. All the potentials are referred against the reversible hydrogen electrode (RHE) using the thermodynamic potential according to the equation E (RHE) = E (Hg/HgO) + 0.908 V.
2.2. Synthesis of rGO
2.5. Electrode preparation
Graphene oxide (GO) was synthesized by the exfoliation of graphite according to the modified Hummers method [54] and characterized with XRD and FTIR measurements (Fig. S1). rGO was obtained by thermal annealing of GO at 900 °C in an argon atmosphere in a tubular furnace for 1 h. The black colored rGO was then washed well with copious amount of water and dried in vacuum.
The catalyst ink was prepared by dispersing 1.5 mg of the integrated hybrid electrocatalyst with 50 μL of Nafion® solution and 250 μL of isopropanol for 30 min using an ultrasonicator. All working electrodes were modified by drop casting required amount (10–15 μL) of the homogeneous catalyst ink and was allowed to dry at room temperature for 6 h.
2.3. Synthesis of CoFe2O4 nanoparticles and rGO/CoFe2O4
3. Results and discussion
CoFe2O4 was synthesized using a single-source precursor potassium cobalt hexacyanoferrate [K2CoFe(CN)6], a multi-metal complex, which was synthesized by mixing an aqueous solution of 1:1 M ratio of K3[Fe (CN)6] and CoCl2.6H2O under constant stirring at room temperature (Scheme 1). The pink color solution of CoCl2.6H2O gradually turned to deep brown during the addition of K3[Fe(CN)6] confirming the formation of multi-metal complex K2CoFe(CN)6. The precipitate was centrifuged after 4 h and the residue was washed thoroughly with water
The multi-metal complex K2[CoFe(CN)6] was characterized with FTIR and XRD analyses (Fig. S2). The FTIR spectral profile of K2[CoFe (CN)6] shows the characteristic signature in the region of 4000–400 cm−1 (Fig. S2a). The peaks at ~ 2162 cm−1 and 2117 cm−1 correspond to the CeN stretching in FeIII-CN-CoII and FeII-CN-CoIII environments, respectively [55]. The peaks at 541 cm−1 and 591 cm−1 are attributed to the formation of Fe-CN-Co [56]. The four major diffractions at 2θ values of 17.2°, 24.5°, 34.8°, 39.1° correspond to the
2. Materials and methods 2.1. Reagents and materials
Scheme 1. Scheme illustrating the syntheses of CoFe2O4 and rGO/CoFe2O4. 2
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The surface survey scan XPS profile shows the presence of oxygen, iron, and cobalt (Fig. 2a). The high-resolution decovoluted Fe 2p spectra show two peaks at 710.5 and 724.08 eV corresponding to Fe 2p3/2 and Fe 2p1/2, respectively accompanied by a satellite peak at 717.8 (Fig. 2b), confirming the presence of Fe in +3 oxidation state [26,59]. The peak at 712.8 eV suggests the presence of Fe+3 in more than one coordination environment in the partially inverse spinel structure [60]. This is possibly due to the high affinity of Co+2 in octahedral sites of inverse spinel structure. In the case of Co 2p, the peaks appear at the binding energy of 796.04 and 780.7 eV corresponds to Co 2p1/2 and 2p3/2, respectively. The satellite peaks at 785.1 and 803.3 eV confirm that Co exists in +2 oxidation state [61,62] (Fig. 2c). The O 1s spectra originate from the lattice oxygen and have the binding energy of 529.2 eV [62] (Fig. 2d). The FESEM and TEM images (Fig. 3a–c) reveal that the CoFe2O4 particles have polyhedral shape with a size distribution ranging from 200 to 300 nm. In the high-resolution TEM image, the fringe spacing of 0.25 nm corresponding to the (311) plane of CoFe2O4 (Fig. 3d) was observed. The selected area diffraction (SAED) shows a spotty ring pattern (inset of Fig. 3c) corresponding to the (200), (311), (400), and (511) planes, further supports the crystalline nature of CoFe2O4. Elemental mapping analysis on a single particle of CoFe2O4 confirms the presence of Co, Fe, and O (Fig. S3). The FESEM and TEM images of rGO/CoFe2O4 evidence the presence of both CoFe2O4 nanoparticles and rGO sheets; the particles are randomly distributed over the rGO sheets (Fig. 3e and f). rGO/CoFe2O4 and CoFe2O4 catalysts were first electrochemically characterized by evaluating the specific capacitance (Csv) with cyclic voltammetry (Fig. S4) following the known procedures [63]. Interestingly, integrated hybrid catalyst rGO/CoFe2O4 has significantly large Csv (141 μF g−1) compared to the as-synthesized CoFe2O4 (0.5 μF g−1)
Fig. 1. XRD profile of CoFe2O4 and rGO/CoFe2O4.
(200), (220), (400) and (420) planes of face-centred cubic cobalt hexacyanoferrate [57] (Fig. S2b). FESEM image (Fig. S2c) reveals that the as-synthesized K2[CoFe(CN)6] has agglomerated morphology. Fig. 1 shows the XRD profile of as-synthesized CoFe2O4, and the hybrid material rGO/CoFe2O4. Both the catalysts show the characteristic peaks corresponding to the cubic spinel structure of CoFe2O4 (JCPDS #221086). The sharp diffraction peaks suggest the crystalline nature of CoFe2O4 particles. The crystallite size of CoFe2O4 particles was calculated from X-ray line broadening analysis using Scherrer formula [58] for both the hybrid and CoFe2O4. The average calculated crystallite size is found to be ~22 nm in both the catalysts. In case of rGO/CoFe2O4, the broad diffraction ~25.1° is indexed to the (002) plane of carbon.
Fig. 2. XPS surface survey scan (a) and high resolution Fe 2p (b), Co 2p (c) and O 1s (d) spectral profiles of CoFe2O4. 3
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Fig. 3. FESEM (a, b, e), TEM (c, f) and HRTEM (d) images of CoFe2O4 (a, b, c, d) and rGO/CoFe2O4 (e, f). Inset in (c) is the SAED pattern of CoFe2O4.
and rGO (14 μF g−1), suggesting that the hybrid material has high electronic conductivity and is easily accessible to the supporting electrolyte [64]. The very low specific capacitance of CoFe2O4 implies its poor electronic conductivity. The integration of poorly conductive CoFe2O4 with conducting rGO enhances the overall electronic conductivity and makes the active catalyst accessible. The electrocatalytic activity of rGO/CoFe2O4 toward ORR was evaluated with RDE and RRDE in O2 saturated 0.1 M KOH solution. Fig. 4a illustrates the electrocatalytic performance of rGO/CoFe2O4 and as-synthesized CoFe2O4 and rGO toward ORR. The onset potential for ORR on rGO/CoFe2O4 is 50–170 mV more positive than the free rGO and CoFe2O4, respectively. The well-defined polarization curves obtained with rGO/CoFe2O4 hybrid suggests facile electron transfer kinetics. The limiting current obtained with rGO/CoFe2O4 is two times higher than those of the other catalyst examined in this investigation, further highlighting the high activity of the hybrid. Linear increments in the limiting current density with increasing the rotation speed supports the diffusion controlled electron transfer kinetics (Fig. 4b). It should be noted here that the ORR performance of as-synthesized CoFe2O4 and rGO is rather poor in terms of onset potential and current density (Figs. 4a, S5). As can be seen in Fig. 4 that the CoFe2O4-based electrode does not yield limiting current and we observed a peak around 0.45 V and broad hump at more negative potential presumably due to the non-homogeneous electron transfer across the electrode-solution interface due to its poor electronic conductivity. More importantly, the geometric surface area normalized current density for ORR on rGO and CoFe2O4 catalyst is 2 times lower than the hybrid catalyst. The slope of the Koutecky-Levich plot for the hybrid catalyst remains almost same in the potential range of 0.6 to 0.45 V, supporting the involvement of the same number of electrons during ORR in the entire potential range (Fig. 4c). It is interesting to note that the mass-specific activity (mass normalized kinetic current density) of rGO/CoFe2O4 hybrid (14.4 A g−1) is ~2 times higher than
the rGO (7.6 A g−1) and CoFe2O4 (7.2 A g−1) catalysts with identical catalyst loading (0.25 mg cm−2). The ideal integration of electronically conducting large surface area rGO with poorly conducting CoFe2O4 enhances the overall performance presumably by enhancing the electrical conductivity of CoFe2O4. The rGO support offers a large surface area, prevents the aggregation of CoFe2O4 and favors the facile diffusion of the electrolyte. It is worth noting here that all the kinetic analysis has been carried out with the limiting current obtained in the cathodic sweep at the scan rate of 5 mV/s though the true steady state was not achieved (Fig. S6). In order to understand the reaction pathway for ORR and the % of HO2¯ yield, RRDE experiment was carried out with the ring potential held at 1.4 V vs RHE while sweeping the disk potential. The number of electrons (n) and the peroxide yield was obtained from the ratio of ring and disk currents (IR/ID) at 1600 rpm (Fig. S7), using the following equations where N is the current collection efficiency of RRDE (0.37).
n = 4NID/(NID + IR )
(1)
%HO2 ¯ = 200IR /(NID + IR )
(2)
As shown in Fig. 4d, the average number of electrons transferred (n) over the potential range from 0.2 to 0.6 V vs RHE is 3.9. The number of electrons transferred in the entire potential range is shown in Fig. S7b. The HO2¯ yield was only < 5% confirming that the hybrid catalyst favors 4-electron pathway for ORR. In the case of rGO and CoFe2O4 generation of > 40–60% HO2¯ with n value of 2.8–3.2 is observed under the same potential range. The rGO of the hybrid material favors the 2-electron pathway with more positive onset potential whereas the CoFe2O4 tend to favors the 4-electron pathway with less positive onset potential. The synergistic effect between CoFe2O4 and rGO improves the overall performance of the integrated hybrid catalyst [65] in terms of more positive onset potential and high current density. The 4
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Fig. 4. (a) Polarization curves illustrating the ORR activity of different catalysts. Electrolyte: 0.1 M KOH. Rotation: 1600 rpm. (b) Polarization curves at different rotations. (c) Corresponding K-L plot. Sweep rate: 5 mV/ s. (d) Plot showing the average % of HO2¯ and number of electron transferred (n) during ORR with different catalysts under the same potential range 0.2 V to 0.6 V vs RHE. (e) Tafel plot for rGO/CoFe2O4, CoFe2O4 and 20% Pt/C catalysts. (f) Plot illustrating the durability of rGO/CoFe2O4 and CoFe2O4 catalysts.
synergistic effect can be explained by considering a 2 + 2-electron transfer mechanism proposed by Hermann et al. [66]. First O2 is reduced to HO2– preferably at the rGO active site through a 2-electron pathway followed by the 2-electron HO2– reduction to OH– at the adjacent CoFe2O4 sites, resulting in an overall 4-electron pathway for ORR. The kinetics of ORR was further studied with Tafel analysis and Tafel slope of 63 mV dec−1 was obtained with the hybrid catalyst which is very close to that of the traditional Pt catalyst (67 mV dec−1), suggesting the facile electron transfer kinetics (Fig. 4e). The Tafel slope approaches the theoretical value of 59 mV dec−1 (2.303RT/F) at room temperature (25 °C), implying high intrinsic ORR activity of the integrated hybrid catalyst. The ORR performance of rGO/CoFe2O4 integrated hybrid catalyst is comparable to the other CoFe2O4-based catalysts reported in the literature [35,39,40,42,67] (Table S1). For instance, CNT/CNF/BCCoFe2O4 based catalysts show an onset potential of 0.82–0.87 V vs RHE
with n value of ~3.8–3.9 [39,40,67]. The rGO/NG-CoFe2O4 based catalysts have the onset potential between 0.83 and 0.9 V with n value of ~3.85 [35,42]. In our case, we could achieve an onset potential of 0.85 V vs RHE with n values of 3.9 and Tafel slope of 62 mV dec−1, which is close to the state-of-the-art Pt catalyst, highlighting the enhanced activity of our hybrid catalyst. The durability of the traditional Pt-based catalyst is one of the major concerns. The durability of both the rGO/CoFe2O4 hybrid and CoFe2O4 catalysts was examined by holding the potential of 0.4 V vs RHE, where high current density is observed, for 5 h and rotating the electrode at 1600 rpm (Fig. S8). A significant drop in current density (~58%) for the CoFe2O4 catalyst was observed whereas in case of rGO/CoFe2O4 hybrid only a ~6% decrease in current density was observed after 5 h of continuous operation (Fig. 4f), implying the high durability of the hybrid catalyst. Traditional Pt-based catalysts do not have tolerance toward methanol and are known to catalyse the oxidation of methanol 5
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and reduction of oxygen. Our rGO/CoFe2O4 hybrid and as-synthesized CoFe2O4 catalysts has high tolerance toward methanol; the onset potential and the limiting current density for ORR remain unchanged even after the addition of 1 M MeOH (Fig. S9).
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4. Conclusions We have demonstrated a new facile solid-state approach for the synthesis of CoFe2O4 using a single-source multi-metal complex precursor. The hybrid catalyst derived from rGO and CoFe2O4 has good catalytic activity toward ORR. It promotes the direct 4-electron pathway for the reduction of oxygen at low overpotential. The onset potential, mass specific activity and durability is higher than the assynthesized rGO and CoFe2O4. Superior ORR activity of rGO/CoFe2O4 hybrid is due to the ideal integration of electronically conductive large surface area rGO with poorly conductive CoFe2O4 and the synergistic effect. Low Tafel slope suggests that the hybrid catalyst has high intrinsic ORR activity. Our study demonstrates that the rational hybridization of conductive rGO with poorly conductive metal oxides is a promising approach to achieve high catalytic activity. Acknowledgments This work was financially supported by Science and Engineering Research Board (SERB), India (Grant No. EMR/2016/002271). Samanta is thankful to DST and IIT Kharagpur, India for the financial support. We are thankful to the Department of Physics, IIT Kharagpur for providing the DST-FIST funded XPS facility. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.jelechem.2019.05.065. References [1] R. Bashyam, P. Zelenay, A class of non-precious metal composite catalysts for fuel cells, Nature 443 (2006) 63–66. [2] C. Song, J. Zhang, Electrocatalytic oxygen reduction reaction, in: J. Zhang (Ed.), PEM Fuel Cell Electrocatalysts and Catalyst Layers: Fundamentals and Applications, Springer, 2008, pp. 89–134. [3] M.K. Debe, Electrocatalyst approaches and challenges for automotive fuel cells, Nature 486 (2012) 43–51. [4] N.M. Markovic, T.J. Schmidt, V. Stamenkovic, P.N. Ross, Oxygen reduction reaction on Pt and Pt bimetallic surfaces: a selective review, Fuel Cells (2) (2001) 105–116. [5] N. Jung, D.Y. Chung, J. Ryu, S.J. Yoo, Y.-E. Sung, Pt-based nanoarchitecture and catalyst design for fuel cell applications, Nano Today 9 (2014) 433–456. [6] S. Ghosh, R.K. Sahu, C.R. Raj, Shape-regulated high yield synthesis of electrocatalytically active branched Pt nanostructures for oxygen reduction and methanol oxidation reactions, J. Mater. Chem. 21 (2011) 11973–11980. [7] A. Morozan, B. Jousselme, S. Palacin, Low-platinum and platinum-free catalysts for the oxygen reduction reaction at fuel cell cathodes, Energy Environ. Sci. 4 (2011) 1238–1254. [8] M.S. El-Deab, F. Kitamura, T. Ohsaka, Poisoning effect of selected hydrocarbon impurities on the catalytic performance of Pt/C catalysts towards the oxygen reduction reaction, J. Electrochem. Soc. 160 (2013) F651–F658. [9] C. Su, T. Yang, W. Zhou, W. Wang, X. Xu, Z. Shao, Pt/C–LiCoO2 composites with ultralow Pt loadings as synergistic bifunctional electrocatalysts for oxygen reduction and evolution reactions, J. Mater. Chem. A 4 (2016) 4516–4524. [10] C.R. Raj, A. Samanta, S.H. Noh, S. Mondal, T. Okajima, T. Ohsaka, Emerging new generation electrocatalysts for the oxygen reduction reaction, J. Mater. Chem. A 4 (2016) 11156–11178. [11] M. Shao, Q. Chang, J.-P. Dodelet, R. Chenitz, Recent advances in electrocatalysts for oxygen reduction reaction, Chem. Rev. 116 (2016) 3594–3657. [12] J. Xu, P. Gao, T. Zhao, Non-precious Co3O4 nano-rod electrocatalyst for oxygen reduction reaction in anion-exchange membrane fuel cells, Energy Environ. Sci. 5 (2012) 5333–5339. [13] S. Bag, K. Roy, C.S. Gopinath, C.R. Raj, Facile single-step synthesis of nitrogendoped reduced graphene oxide-Mn3O4 hybrid functional material for the electrocatalytic reduction of oxygen, ACS Appl. Mater. Interfaces 6 (2014) 2692–2699. [14] W. Zhou, L. Ge, Z.-G. Chen, F. Liang, H.-Y. Xu, J. Motuzas, A. Julbe, Z. Zhu, Amorphous iron oxide decorated 3d heterostructured electrode for highly efficient oxygen reduction, Chem. Mater. 23 (2011) 4193–4198. [15] Z.-S. Wu, S. Yang, Y. Sun, K. Parvez, X. Feng, K. Müllen, 3D nitrogen-doped graphene aerogel-supported Fe3O4 nanoparticles as efficient electrocatalysts for the
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