Reduced CoFe2O4/graphene composite with rich oxygen vacancies as a high efficient electrocatalyst for oxygen evolution reaction

https://doi.org/10.1016/j.ijhydene.2020.02.045Get rights and content

Highlights

  • The reduced CoFe2O4/graphene composite was synthesized by simple, low-cost and versatile method.

  • The catalyst has abundant oxygen vacancies.

  • The catalyst exhibits superior OER catalytic activity to the commercial RuO2 catalyst.

Abstract

Oxygen evolution reaction (OER) is regarded as a limit-efficiency process in electrochemical water splitting generally, which needs to develop the effective and low-cost non-noble metal electrocatalysts. Oxygen vacancies have been verified to be beneficial to enhance the electrocatalytic performance of catalysts. Herein, we report the facile synthesis of reduced CoFe2O4/graphene (r-CFO/rGO) composite with rich oxygen vacancies by a citric acid assisted sol-gel method, heat treatment process and the sodium borohydride (NaBH4) reduction. The introduction of graphene and freezing dry technique prevents the restacking of GO and the aggregation of CFO nanoparticles (NPs) and increases the electronic conductivity of the catalyst. Fast heating rate and low anneal temperature favors to obtain low crystallinity and lattice defects for CFO. NaBH4 reduction treatment further creates the rich oxygen vacancies and electrocatalytic active sites. The obtained r-CFO/rGO with high specific surface area (108 m2 g−1), low crystallinity and rich oxygen vacancies demonstrates a superior electrocatalytic activity with the smaller Tafel slope (68 mV dec−1), lower overpotential (300 mV) at the current density of 10 mA cm−2, and higher durability compared with the commercial RuO2 catalyst. This green, low-cost method can be extended to fabricate similar composites with rich defects for wide applications.

Graphical abstract

A reduced CoFe2O4/graphene catalyst with rich oxygen vacancies prepared by simple, low-cost method shows superior electrocatalytic performance for oxygen evolution reaction compared with the commercial RuO2 catalyst.

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Introduction

Up to now, electrochemical water splitting has been developed drastically because it is very important for green energy storage and conversion [[1], [2], [3], [4], [5], [6], [7], [8], [9]]. But the anodic oxygen evolution reaction (OER) hinders the practical applications of water splitting due to the complex four-electron redox process and high energy barrier [[10], [11], [12], [13], [14]]. Thereby, it seems particularly significant to lower the overpotential, reduce the energy consumption and enhance the durability of electrocatalysts toward the OER. RuO2 and IrO2 is widely accepted as the good electrocatalysts tor the OER [[15], [16], [17]]. But its high cost, scarcity and instability hampers wider applications, which promotes us to explore cheap, high-efficiency non-noble metal electrocatalysts.

A large number of electrocatalysts based on transition metals display the unique and promising characteristics for OER, including metal oxides [[16], [17], [18], [19], [20], [21], [22], [23]], hydroxides [[24], [25], [26], [27], [28], [29]], spinel oxides [[30], [31], [32], [33], [34], [35], [36], [37], [38]], phosphides [[39], [40], [41], [42], [43]], nitrides [44,45], and so on. Among them, the spinel ferrites MFe2O4 (M = Co, Ni, Cu, etc.) have drawn great attention due to their specific geometric and electronic structures toward the OER, which is beneficial for the adsorption and activation of electroactive species on the surface redox active metal centers [46,47]. To further improve the electrocatalytic performance of spinel oxides, defect engineering including creation of oxygen vacancies is regarded as one of the effective strategies to increase the reactivity and number of active sites [2,5,13,[20], [21], [22], [23], [24], [25],[30], [31], [32],48]. Based on the density functional theory (DFT) calculation reported in the previous literatures [5,12,21,27], the oxygen vacancies can lower the adsorption energy of H2O and weaker metal-oxygen bonds to make the electrons exchange easily and boost the electrocatalytic performance effectively. Nowadays, vast efforts have been focused on generating oxygen vacancies by the facile methods including high temperature and chemical reduction [2,21,30,44,45], plasma measurement [49], where the sodium borohydride (NaBH4) reduction treatment is an easy approach to create the oxygen vacancies. Apart from oxygen vacancies, the electrical conductivity of the spinel ferrites also has an important influence on the electrocatalytic activity during the electrochemical process. To address this issue, it is a good route for the spinel ferrites growing or supporting on the carbon substrate, such as porous carbon [[50], [51], [52]], carbon nanotubes [[53], [54], [55]], graphene [[56], [57], [58], [59]]. Especially, graphene is a good candidate because of the outstanding electrical conductivity, high specific surface area and good mechanical stability. Some spinel ferrites/graphene composites have been developed [[56], [57], [58], [59]], but most of them show inferior electrocatalytic activity compared with the commercial RuO2 catalyst, which need to be further modified and improved.

Herein, we report a simple and economic route to synthesize the reduced CoFe2O4/graphene (r-CFO/rGO) composite with rich oxygen vacancies by a citric acid assisted sol-gel method, heat treatment process and NaBH4 reduction. Compared with the commercial RuO2 catalyst, the prepared product possesses smaller Tafel slope (68 mV dec−1), lower overpotential (300 mV) at the current density of 10 mA cm−2 and higher durability. The outstanding OER electrocatalytic performance and green, low-cost synthetic method of r-CFO/rGO is beneficial for promising practical applications.

Section snippets

Synthesis of CoFe2O4/graphene (CFO/rGO) composite

CoFe2O4/graphene composite was prepared by a citric acid assisted sol-gel and vacuum freeze-drying method and a subsequent calcination process. In a typical procedure, 15 mmol FeCl3 and 7.5 mmol CoCl2 were firstly dissolved in 50 mL distilled water under vigorous magnetic stirring for 30 min. Then, 20 mg graphene oxide (GO) synthesized by modified Hummers’ method was dispersed into citric acid solution (0.45 mol L−1, 50 mL). The above solution was slowly mixed with the former solution under

Characterization of the catalysts

The chemical composition and phase structure of the sample was analyzed by An X-ray diffraction (XRD) measurement. Fig. 1 is the wide-angle XRD patterns of r-CFO/rGO and CFO/rGO over the range 20° ≤ 2θ ≤ 80°. The observed diffraction peaks arising at 30.0°, 35.4°, 43.1°, 56.9°, 62.5° (2θ) correspond to (220), (311), (400), (511), (440) crystal planes of the spinel cubic structure CoFe2O4 (JCPDS: 22–1086). It should be noted that the intensity of diffraction peaks become weak for r-CFO/rGO,

Conclusions

We report a simple and economic route to synthesize the r-CFO/rGO composite with rich oxygen vacancies by a citric acid assisted sol-gel method, heat treatment process and NaBH4 reduction. As prepared r-CFO/rGO composite exhibits a low overpotential of 300 mV and a small Tafel slope of 68 mV dec-1, which overwhelms the commercial RuO2 catalyst and most spine-based catalysts. Characterizations confirmed the outstanding electrocatalytic performance, which is attributed to the rich oxygen

Author contributions

Y. Ma and H. Zhang contributed equally.

Prime novelty statement

This paper describes the synthesis of the reduced CoFe2O4/graphene composite with rich oxygen vacancies as an electrocatalyst for OER by a simple citric acid assisted sol-gel method, heat treatment process and the sodium borohydride (NaBH4) reduction. The approach is simple, green, low-cost and can be extended to synthesize other similar catalysts. The prepared catalyst shows unique and outstanding catalytic performance for oxygen evolution reaction compared with commercial the RuO2 catalyst.

Acknowledgments

The authors are grateful for National Natural Science Foundation of China (No.21671107), and Nanjing Xiaozhuang University Research Project (No. 2018NXY23).

References (60)

  • S. Liu et al.

    Bacterial-cellulose-derived carbon nanofiber-supported CoFe2O4 as efficient electrocatalyst for oxygen reduction and evolution reactions

    Int J Hydrogen Energy

    (2016)
  • A. Ashok et al.

    Highly active and stable bi-functional NiCoO2 catalyst for oxygen reduction and oxygen evolution reactions in alkaline medium

    Int J Hydrogen Energy

    (2019)
  • Z. Li et al.

    Spinel NiCo2O4 3-D nanoflowers supported on graphene nanosheets as efficient electrocatalyst for oxygen evolution reaction

    Int J Hydrogen Energy

    (2019)
  • X. Liu et al.

    ZIF-67 derived hierarchical hollow sphere-like CoNiFe phosphide for enhanced performances in oxygen evolution reaction and energy storage

    Electrochim Acta

    (2019)
  • W. Yan et al.

    An efficient Bi-functional electrocatalyst based on strongly coupled CoFe2O4/carbon nanotubes hybrid for oxygen reduction and oxygen evolution

    Electrochim Acta

    (2015)
  • W. Bian et al.

    A CoFe2O4/graphene nanohybrid as an efficient bi-functional electrocatalyst for oxygen reduction and oxygen evolution

    J Power Sources

    (2014)
  • H. Yang et al.

    Defect locating: one-step to monodispersed CoFe2O4/rGO nanoparticles for oxygen reduction and oxygen evolution

    Int J Hydrogen Energy

    (2017)
  • T. Zhang et al.

    Spinel CoFe2O4 supported by three dimensional graphene as high-performance bi-functional electrocatalysts for oxygen reduction and evolution reaction

    Int J Hydrogen Energy

    (2019)
  • R. Cao et al.

    Recent progress in non-precious catalysts for metal-air batteries

    Adv Energy Mater

    (2012)
  • S. Peng et al.

    Necklace-like multishelled hollow spinel oxides with oxygen vacancies for efficient water electrolysis

    J Am Chem Soc

    (2018)
  • K. Huang et al.

    Direct immobilization of an atomically dispersed Pt catalyst by suppressing heterogeneous nucleation at -40 oC

    J Mater Chem A

    (2019)
  • L. Lang et al.

    Hollow core-shell structured Ni-Sn@C nanoparticles: a novel electrocatalyst for the hydrogen evolution reaction

    ACS Appl Mater Interfaces

    (2015)
  • J. Suntivich et al.

    A perovskite oxide optimized for oxygen evolution catalysis from molecular orbital principles

    Science

    (2011)
  • D.G. Nocera et al.

    The nature of lithium battery materials under oxygen evolution reaction conditions

    J Am Chem Soc

    (2012)
  • J. Bao et al.

    Ultrathin spinel-structured nanosheets rich in oxygen deficiencies for enhanced electrocatalytic water oxidation

    Angew Chem

    (2015)
  • J. Sun et al.

    A facile strategy to construct amorphous spinel-based electrocatalysts with massive oxygen vacancies using ionic liquid dopant

    Adv Energy Mater

    (2018)
  • A. Bergmann et al.

    Reversible amorphization and the catalytically active state of crystalline Co3O4 during oxygen evolution

    Nat Commun

    (2015)
  • H.H. Duan et al.

    High-performance Rh2P electrocatalyst for efficient water splitting

    J Am Chem Soc

    (2017)
  • D. Kuo et al.

    Measurements of oxygen electroadsorption energies and oxygen evolution reaction on RuO2(110): a discussion of the sabatier principle and its role in electrocatalysis

    J Am Chem Soc

    (2018)
  • J. Kim et al.

    High-performance pyrochlore-type yttrium ruthenate electrocatalyst for oxygen evolution reaction in acidic media

    J Am Chem Soc

    (2017)
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