Reduced CoFe2O4/graphene composite with rich oxygen vacancies as a high efficient electrocatalyst for oxygen evolution reaction
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.
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)
- et al.
Durable and efficient hollow porous oxide spinel microspheres for oxygen reduction
Joule
(2018) - et al.
CoFe2O4 nanoparticles anchored on N/S co-doped mesoporous carbon spheres as efficient bifunctional electrocatalysts for oxygen catalytic reactions
Int J Hydrogen Energy
(2019) Electrocatalysts for hydrogen evolution reaction
Int J Hydrogen Energy
(2017)- et al.
One-pot synthesis of Ag-CoFe2O4/C as efficient catalyst for oxygen reduction in alkaline media
Int J Hydrogen Energy
(2016) - et al.
Mixed valence CoCuMnOx spinel nanoparticles by sacrificial template method with enhanced ORR performance
Appl Surf Sci
(2019) - et al.
Porous cobalt oxide nanoplates enriched with oxygen vacancies for oxygen evolution reaction
Nanomater Energy
(2018) - et al.
Facile loading mesoporous Co3O4 on nitrogen doped carbon matrix as an enhanced oxygen electrode catalyst
Mater Lett
(2019) - et al.
A partial sulfidation approach that significantly enhance the activity of FeCo layered double hydroxide for oxygen evolution reaction
Int J Hydrogen Energy
(2019) - et al.
A facile method for reduced CoFe2O4 nanosheets with rich oxygen vacancies for efficient oxygen evolution reaction
Int J Hydrogen Energy
(2017) - et al.
Activating CoFe2O4 electrocatalysts by trace Au for enhanced oxygen evolution activity
Appl Surf Sci
(2019)