Highly efficient oxygen evolution electrocatalysts prepared by using reduction-engraved ferrites on graphene oxide

Jing-Bo Tana, Pathik Sahooa, Jia-Wei Wanga, Yu-Wen Hua, Zhi-Ming Zhang*b and Tong-Bu Lu*ab
aMOE Key Laboratory of Bioinorganic and Synthetic Chemistry, School of Chemistry, Sun Yat-Sen University, Guangzhou 510275, China. E-mail: lutongbu@mail.sysu.edu.cn; Fax: +86-20-8411-2921
bInstitute for New Energy Materials and Low Carbon Technologies, School of Materials Science and Engineering, Tianjin University of Technology, Tianjin 300384, China. E-mail: zmzhang@email.tjut.edu.cn

Received 3rd November 2017 , Accepted 28th November 2017

First published on 8th December 2017

Rational design and synthesis of efficient, stable and low-cost electrocatalysts for oxygen evolution reaction (OER) is critical for renewable energy conversion and storage. Herein, the reduction-engraved strategy was adopted to treat crystalline ferrite nanoparticles, which are highly dispersed on graphene oxide (GO) nanosheets. This reduction treatment generated abundant oxygen vacancies on the surface of nano-scale ferrites and dramatically enhanced their surface area, ensuring that the ferrite nanoparticles possess more accessible active sites for OER, and improve their electronic conductivity. Reduced cobalt/nickel ferrite (Co0.5Ni0.5Fe2O4, r-CNF), cobalt ferrite (CoFe2O4, r-CF) and nickel ferrite (NiFe2O4, r-NF) nanoparticles anchoring on the ultrathin GO nanosheets can act as highly active, stable and low-cost OER electrocatalysts in 1.0 M KOH solution. The r-CNF (Co[thin space (1/6-em)]:[thin space (1/6-em)]Ni = 1[thin space (1/6-em)]:[thin space (1/6-em)]1) on GO (r-CNFg) shows the best OER performance among the ferrite-based OER electrocatalysts, with an overpotential of 210 mV at 10 mA cm−2 in 1.0 M KOH solution, much more efficient than that of a commercial benchmark catalyst IrO2 (230 mV). The catalytic current density of r-CNFg at 1.49 V vs. RHE is about 50 times higher than that of CNF and CNFg. Also, it exhibits prominent electrochemical stability over 500 h in 1.0 M KOH.


Electrocatalytic water splitting includes two half reactions, oxygen evolution reaction (OER) and hydrogen evolution reaction (HER); it represents an attractive method to store spasmodic energy and supply sustainable and renewable clean energy.1–5 As is well known, OER, involving a stepwise four electron and four proton transfer process with a high overpotential, is considered a critical step of water splitting.1 In this field, noble metal-oxides, such as IrO2 and RuO2, are still considered as the most active electrocatalysts for OER.3–5 However, the scarcity and high-cost hinder them for large-scale applications. The search for non-precious electrocatalysts for dramatically reducing the overpotential and accelerating multistep proton-coupled electron transfer in OER has become a key goal for producing clean energy.6–15

A number of OER electrocatalysts, such as Co3O4,16–20 nickel oxides/oxyhydroxides,21–24 and transition-metal nitrides,25–29 have been explored and used as active electrocatalysts for OER. Also, various methods have been explored to improve their catalytic activity, such as design and synthesis of ultrathin or hollow nanoparticles, anchoring on carbon supports and the synthesis of mixed metal hydroxides/oxides.30–41 Among them, the preparation of binary/ternary metal hydroxides/oxides has been regarded as an efficient strategy that could significantly enhance the electrocatalytic activity of OER electrocatalysts, because of their altered electronic structure, optimized local coordination environment and/or enhanced charge transfer ability with doping elements.36–41 In this field, layered double hydroxide (LDH)-based nanomaterials, with binary metal centers evenly mixed at a molecular level, have been widely utilized as OER catalysts. Through persistent effort, a series of catalysts with efficient electrocatalytic activity for OER, contributed by small crystalline size, conductive support and doping elements, were explored by using iron, cobalt and nickel-based LDH as precursors.23,32,36–38,42–46

Spinel ferrites, denoted as MFe2O4 (M = Mn, Co, Ni, Zn, Cu, etc.), a kind of binary/ternary metal oxide, are greatly attractive as they are low-cost and earth-abundant, and possess many remarkable properties, such as magnetism, catalysis, and electricity.47–50 They are composed of Fe, Co and Ni elements, which are usually used to construct highly efficient OER catalysts.13,16–20,51–56 However, only a few studies have been conducted to explore spinel ferrite-based active OER electrocatalysts. For example, Guo et al. reported a series of electrospun MFe2O4 (M = Co, Ni, Cu, and Mn) spinel nanofibers for OER; among them, the lowest overpotential of 408 mV at a current density of 5 mA cm−2 was achieved with CoFe2O4 spinel nanofibers in 0.1 M KOH.47 Furthermore, the spinel CoFe2O4 nanoparticles were dispersed on the polyaniline-functionalized carbon nanotubes and carbon fiber paper to reduce their overpotentials to 314 mV and 378 mV, respectively.57,58 Also, NiO–NiFe2O4 composite nanoparticles were anchored on rGO to afford a small overpotential of 296 mV at a current density of 10 mA cm−2 in 1.0 M KOH.59 In addition, the porous CoFe2O4/C composite, which was obtained by pyrolysis of a bimetal metal–organic framework under a nitrogen atmosphere, exhibited an excellent OER performance with a low potential of 240 mV at 10 mA cm−2 in 1.0 M KOH, representing the best OER performance among the ferrite-based OER electrocatalysts.60 These results demonstrate that it is promising to explore excellent OER catalysts by employing ferrites as the precursors. However, the bulk spinel ferrites are normally less active for OER due to their intrinsic high electrical resistance and low surface area. Also, the magnetic ferrite nanoparticles usually suffer from severe aggregation. These problems dramatically limit the OER activity of the powder spinel ferrites.43,49 So, the design and synthesis of highly efficient ferrite-based OER catalysts with higher surface area, more exposed active sites, and higher electronic conductivity still represents a great challenge.

In this study, the reduction-engraved strategy was adopted to synthesize small-sized crystalline ferrite nanoparticles with abundant oxygen vacancies. It was confirmed that the reduction treatment strongly affected the conductivity, active sites and surface area of ferrites. Furthermore, small-sized ferrite nanoparticles are anchored on the GO sheets to prevent them from aggregating in the synthetic and electrochemical processes. The reduced nickel ferrite cobalt/nickel ferrite (Co0.5Ni0.5Fe2O4, r-CNF), cobalt ferrite (CoFe2O4, r-CF) and (NiFe2O4, r-NF) anchoring on the ultrathin GO nanosheets can act as highly active, stable and low-cost OER electrocatalysts (r-NFg, r-CFg, and r-CNFg, respectively). An appropriate control of the Co2+/Ni2+ ratio in mixed-metal ferrites can also modify their electronic and thus catalytic properties. The lowest overpotential of 210 mV at 10 mA cm−2 among the ferrite-based OER electrocatalysts was achieved with r-CNF on GO (r-CNFg), affording a small Tafel slope of 35 mV dec−1, and prominent electrochemical stability over 500 h in 1.0 M KOH.

Experimental section

Materials and characterization

All chemicals were obtained from commercial suppliers and used without further purification (Aladdin Co. Ltd, and Praxair Co. Ltd). GO was synthesized by a modified Hummers method based on ref. 19. The Fourier-transform infrared spectra (FT-IR) of the samples were recorded by using a Fourier transformation infrared spectrometer EQUINOX 55. Raman spectra were characterized by using a Laser Micro-Raman Spectrometer Renishaw inVia with 514 nm light. Element analysis of Fe, Co and Ni was conducted by inductively coupled plasma-atomic emission spectrometry (ICP-AES) using a TJA IRIS (HR). Thermogravimetric analysis (TGA) was conducted using a Netzsch TG-209 instrument under an air atmosphere. X-ray powder diffraction (XRD) measurements were performed with a Bruker D8 X-ray diffractometer with CuK radiation at a scanning rate of 1° min−1. Scanning electron microscopy (SEM) measurements were performed with a Hitachi SU-8010 scanning electron micro-analyzer with an accelerating voltage of 10 kV. Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) measurements were conducted by using an FEI Tecnai G2 F30 field-emission TEM. For TEM measurements, the samples were prepared by dispersing the products in ethanol and dropping the suspension on a holey carbon net supported on copper grids. N2 adsorption–desorption isotherms were obtained at 77 K on a BELmax sorption. X-ray photoelectron spectroscopy (XPS) measurements were conducted on an ESCAlab250.


Syntheses of CNF, CF and NF. In a typical synthesis process, 0.25 mmol CoCl2·6H2O (59.5 mg), 0.25 mmol NiCl2·6H2O (59.4 mg) and 1.0 mmol FeCl3 (162.1 mg) were mixed in 20 mL deionized (DI) water. After 30 min of sonication, the pH value of the mixture was adjusted to 10.5 using aqueous ammonia. Then the slurry was placed in a 50 mL stainless-steel autoclave and maintained at 180 °C for 6 hours. The resulting precipitate was centrifuged and washed thoroughly with DI water several times and then dried in an oven at 80 °C. CF and NF were synthesized using a similar process to that of CNF except that 0.5 mmol CoCl2·6H2O (119.0 mg) or 0.5 mmol NiCl2·6H2O (118.8 mg) was used instead of the mixed Co/Ni salts.
Syntheses of CNFg, CFg and NFg. CNFg, CFg and NFg were synthesized by similar methods to those of CNF, CF and NF, respectively, except that 51.0 mg GO was introduced into the reaction systems. The final products were dried by lyophilisation.
Syntheses of r-CNFg, r-CFg and r-NFg. 50.0 mg CNFg was dispersed in 50 mL of 0.5 M NaBH4 aqueous solution in an ice bath for 10 h. The resulting precipitate was centrifuged and washed thoroughly with DI water several times. Finally, it was dried in an oven at 60 °C. r-CFg and r-NFg were synthesized using a similar process to that of r-CNFg except that CFg and NFg were used instead of CNFg, respectively.
Preparation of electrodes. Typically, 1.5 mg CNFg, 10 μL 5% Nafion and 500 μL ethanol were sonicated for at least 30 min to form the ink. Then, the ink was dropped onto 1 × 1 cm nickel foam (NF) and dried in air. 3D nickel foam electrodes decorated by CNFg, CFg and NFg were all prepared by a similar method. All nickel foam electrode substrates were etched in a 1.0 M HCl solution for 30 min to remove the native nickel oxide on the surface and then washed with DI water several times prior to pasting. The obtained electrodes were immersed in 0.5 M aqueous NaBH4 solution at 0 °C for 10 h for preparing reduced composites.
Electrochemical measurements. Electrocatalytic activity of these samples for OER was studied in a 1.0 M KOH (pH = 13.8) solution and recorded on a CHI760 electrochemical instrument (Shanghai, ChenHua Co. Ltd) at ambient temperature. All the tests were performed in a three-electrode electrochemical cell with a Ag/AgCl reference electrode (3 M KCl) and a Pt foil counter electrode. The prepared electrodes were used as the working electrodes to investigate their electrocatalytic activities toward OER, each of which was cycled 30 times at a scan rate of 50 mV s−1 between 0 and 0.9 V vs. Ag/AgCl before data collection. The electrolyte was bubbled with oxygen for 30 min prior to OER measurements. The electrolyte was stirred at a constant rate of 1000 rpm to enhance mass transport and remove generated gas bubbles on the surface of the electrode for chronopotentiometry. The potentials were calibrated with respect to the reversible hydrogen electrode (RHE) using the following equation: E(RHE) = E(Ag/AgCl) + 0.197 + pH × 0.0592. The linear sweep voltammogram (LSV) curves were recorded at a scan rate of 1 mV s−1 for OER in the 1.0 M KOH solution and are shown with iR compensation. Electrochemical impedance spectroscopy (EIS) was performed in potentiostatic mode from 200 kHz to 50 mHz. To evaluate the effective electrochemical active surface area (ECSA), cyclic voltammograms (CVs) were carried out to probe the electrochemical double layer capacitance (Cdl) of various samples in the non-Faradaic region, identified from CV in quiescent solution. By plotting the current densities at 1.13 V vs. RHE against the scan rates, a linear trend was observed. The linear slope, equivalent to twice of the double-layer capacitance Cdl, and Cdl were used to represent the ECSA.

Results and discussion

Synthesis of ferrite-based catalysts and structural characterization

The synthetic strategy of CNFg and r-CNFg is illustrated in Scheme 1. In a typical hydrothermal synthesis, transition metal chlorides with different ratios were used to synthesize small-sized ferrite nanoparticles. Furthermore, all the composites (NFg, CFg, and CNFg) were reduced in NaBH4 aqueous solution, and the electrocatalytic performances of the reduced composites r-NFg, r-CFg, and r-CNFg were all tremendously improved, as the reduction-engraved treatment could introduce oxygen vacancies on the surface of the metal oxides, which could efficiently increase their conductivity and catalytic active sites.18,61,62 GO was obtained by mild oxidation of commercially available graphene, which was further used to support the CNF nanoparticles to prevent their aggregation during the synthetic process. The metallic stoichiometry in CNFg and r-CNFg was determined by ICP-AES, revealing that the ratio of Fe[thin space (1/6-em)]:[thin space (1/6-em)]Co[thin space (1/6-em)]:[thin space (1/6-em)]Ni in the spinel nanoparticles remained unchanged after reduction, and ∼70% spinel ferrite exists in the CNFg composite materials as shown by TGA analysis (Table S1 and Fig. S1).
image file: c7qi00681k-s1.tif
Scheme 1 The procedure for preparation of CNFg and r-CNFg.

TEM and HRTEM images demonstrate that small-sized ferrite nanoparticles (7–23 nm) are synthesized and anchored on the GO surface in a dispersed manner (Fig. 1a and S2), which is also confirmed by the SEM study (Fig. S3). During the hydrothermal synthesis, small-sized ferrite nanoparticles were obtained and immobilized by functional groups on the surface of GO. The distinct lattice spacings (0.252 nm, 0.251 nm and 0.253 nm) of CNF, NF and CF are identified carefully from HRTEM images and can be indexed to the (311) crystal facet of the corresponding spinel ferrites (Fig. 1b and S4–S8). Furthermore, the surface of pristine CNF becomes amorphous, and even in some regions cubic CNF particles turn to nanoflakes after the reduction-engraved treatment by NaBH4 (Fig. 1c). The HAADF-STEM image (Fig. 1f), SAED images (inset images in Fig. 1b and d), and the elemental mapping images (Fig. 1g–k) reveal that Co, Ni, Fe, and O are homogeneously distributed all over the r-CNF nanoparticles.

image file: c7qi00681k-f1.tif
Fig. 1 TEM and HRTEM images of (a, b) CNFg and (c, d) r-CNFg. The inset images corresponded to the SAED pattern of CNFg and r-CNFg. (e) Line scan of the HRTEM image of CNFg with a distinct lattice spacing of 0.252 nm, corresponding to the (311) crystal facet. (f) HAADF-STEM image of r-CNFg, (g–k) elemental mappings of r-CNFg, revealing the elemental distributions of C, O, Co, Ni and Fe.

Powder X-ray diffraction (PXRD) measurements were conducted to determine the crystalline phase and purity of all synthesized materials. In Fig. 2a, the diffraction peaks appearing at 13.1° and 26.5° are assigned to the (100) and (002) crystal facets of the GO, indicating that the GO is mildly oxidized and partially retains the network structure of graphene. In all the composites, the absence of the diffraction peak at 13.1° corresponding to the (001) plane of the GO demonstrates that spinel ferrites are efficiently immobilized on the GO.63,64 The PXRD patterns of NFg and r-NFg match well with cubic NiFe2O4 (JCPDS 10-0325), while the PXRD patterns of CFg and r-CFg also match well with cubic CoFe2O4 (JCPDS 22-1086). Moreover, the PXRD measurements of CNFg, r-CNFg and other spinel ferrite-based composites with different Co2+/Ni2+ stoichiometric ratios (Fig. S9) all exhibit similar PXRD patterns, because of the close ionic radii of Co2+ and Ni2+ and isostructural crystal packing in these ferrite nanoparticles. All these results indicate that all mixed metal oxides possess an inverse spinel structure and crystallize in the Fd[3 with combining macron]m space group, where half of the Fe3+ ions occupy tetrahedral (A) sites and the remaining Fe3+ ions along with M2+ distribute over the octahedral (B) sites.47,48 For the reduced composites, an obvious decrease in diffraction intensities of PXRD also confirmed the more amorphous phase of the reduced ferrites, which can be associated with the HRTEM result of r-CNFg.

image file: c7qi00681k-f2.tif
Fig. 2 (a) PXRD patterns of GO, spinel ferrite composites and their reduced composites. (b) Raman spectra of GO, spinel ferrite composites and their reduced composites.

Raman spectra were recorded to analyze the disorder on the surface of GO and characterize the spinel structure in the composites (Fig. 2b). The Raman spectra of the composites show both the D (disordered) band and G (graphitic) band at around 1349 cm−1 and 1575 cm−1, respectively, confirming the existence of GO in r-CNFg. For the ferrites, five Raman peaks were detected at 678, 560, 472, 311 and 197 cm−1, respectively. These peaks are attributed to three Raman active modes (A1g + Eg + 3T2g). These results are in agreement with that of the group theory calculation, where the spinel structure (M2+Fe3+)B(Fe3+)AO4 crystallizing in the space group Fd[3 with combining macron]m can give five Raman peaks (A1g + Eg + 3F2g).60 Also, FT-IR spectra also confirmed the existence of the GO and the ferrites in the composites (Fig. S10). In the FT-IR spectra of CNFg, CFg, and NFg, the intense stretching vibrations at 586 cm−1 (CNF), 576 cm−1 (CF) and 588 cm−1 (NF) were detected respectively, which can be attributed to the lattice absorption of Fe–O, confirming the presence of ferrites in the composites.47 Also, the C[double bond, length as m-dash]O stretching vibration peak at 1714 cm−1, the vibration and deformation peaks of the O–H groups at 3438 cm−1 and 1419 cm−1, the C–O (epoxy) stretching vibration peak at 1224 cm−1, and the C–O (alkoxy) stretching peak at 1049 cm−1 were observed in the FT-IR spectra, which confirmed the presence of GO in the composites. Compared to the FT-IR spectra of isolated GO, the characteristic peak of carboxylic groups at 1714 cm−1 in the FT-IR spectra of GO disappeared entirely in all of the composites, confirming that the carboxylic groups in GO coordinated with the metal-centers in the spinel ferrites.65,66

OER performance of the ferrite-based catalysts

Electrocatalytic OER performance of the composite materials and their reduced counterparts was studied in 1.0 M KOH aqueous solution with a three-electrode electrolytic cell. The catalyst on 3D nickel foam was used as a working electrode, and the electrocatalytic properties of the corresponding initial materials (GO, r-GO, CNF and r-CNF) were all investigated under the same conditions. As shown in Fig. 3, the overpotential reached 290 mV at a current density of 10 mA cm−2, when CNFg on the NF was used as the working electrode. After the reduction treatment, r-CNFg readily affords a much lower overpotential of 210 mV at the current density of 10 mA cm−2. Based on the control experiments, it can be observed that all the initial materials exhibit a much lower catalytic activity than that of r-CNFg (Fig. S11). In particular, the catalytic activity of r-CNFg is much better than that of r-CNF, revealing the existence of synergetic coupling effects between r-GO and r-CNF during the electrocatalytic process.67,68 Meanwhile, as shown in Fig. 3a, the OER catalytic activities of all the reduced composites (r-NFg: 220 mV, r-CFg: 220 mV) are much improved compared to those of the pristine composites (NFg: 300 mV, CFg: 290 mV). Also, these reduced composites (r-CNFg, r-NFg and r-CFg) all exhibit much more efficient performance for OER than that of the commercial benchmark catalyst IrO2 (230 mV). On the other hand, the onset potential of r-CNFg (≈1.39 V vs. RHE) is lower than those of the other reduced composites (r-NFg ≈ 1.41 V vs. RHE, r-CFg ≈ 1.42 V vs. RHE) and the noble metal oxide IrO2 (≈1.42 V vs. RHE). As shown in Fig. 3b, the Tafel slopes for r-CNFg, r-NFg and r-CFg are 35, 31, and 37 mV dec−1, respectively, much lower than those of CNFg, NFg, CFg and IrO2 (65 mV dec−1). These results demonstrate a much more efficient charge transfer at the interface of the spinel composites and electrolyte in OER.59,69 As shown in Fig. 3c, the catalytic current densities of different catalysts at 1.49 V vs. RHE show that the current density of r-CNFg is about 50 times higher than those of CNF and CNFg, and dramatically surpasses the commercial benchmark catalyst IrO2. Accordingly, reduction-engraved treatment represents a powerful strategy to enhance the OER catalytic ability of spinel composites, which can generate abundant oxygen vacancies on the surface of spinel ferrites.61
image file: c7qi00681k-f3.tif
Fig. 3 (a) LSVs of spinel composites (CNFg, NFg, and CFg), their reduced forms (r-CNFg, r-CFg, and r-NFg) and IrO2 on the nickel foam measured in 1.0 M KOH with a scan rate of 1.0 mV s−1. (b) Tafel plots. (c) The current densities of CNF, r-CNF, CNFg, r-CNFg and IrO2 at 1.49 V vs. RHE. (d) EIS plots of CNF, r-CNF, CNFg and r-CNFg on NF in 1.0 M KOH with a potential of 1.51 V (vs. RHE); the inset is the magnified view of the EIS plot.

To further study the influence of the Co2+/Ni2+ ratio on the OER activity of r-CNFg, a series of reduced spinel composites with different Co2+/Ni2+ stoichiometric ratios during the hydrothermal process were designed and synthesized. An electrochemical study demonstrates that r-CNFg with Co[thin space (1/6-em)]:[thin space (1/6-em)]Ni = 1[thin space (1/6-em)]:[thin space (1/6-em)]1 shows the best OER catalytic activity among them. As shown in Fig. S12, the lowest onset potential and the highest current density at the same potential were observed on the LSV of r-CNFg, indicating that the rational alteration of the Co2+/Ni2+ ratio in inverse spinel ferrite composites can enhance the OER performance. The loading amount of the catalyst was also optimized by chronopotentiometry (Fig. S13), showing that the best loading amount of r-CNFg is ∼1.5 mg cm−2. Moreover, the catalytic durability of spinel composites, their reduced forms and IrO2 is investigated by chronopotentiometry (Fig. S14a), and it is apparently performed over continuous 500 h at a current density of 20 mA cm−2 with negligible potential degradation for r-CNFg, showing the desired long-term stability for the OER process (Fig. S14b). The durability of r-CNFg was also proved by XPS; there is no obvious change in the XPS spectra of r-CNFg before and after electrolysis (Fig. S15). Additionally, no CO2/CO could be detected during the OER process, and the agreement of the theoretical and experimental amounts of O2 during the OER process suggests that the Faradaic efficiency for r-CNFg is almost 100% (Fig. S16 and S17).

All the above results reveal that the reduction treatment presents an efficient strategy that could significantly enhance electrocatalytic activity of the spinel oxides. The reduced composites (r-CNFg, r-NFg and r-CFg) all exhibit much better performance for OER than that of the as-synthesized composites CNFg, NFg, CFg and the commercial benchmark catalyst IrO2.

Understanding the enhanced OER activity of the ferrite-based catalysts

To elucidate these transformations, a series of characterization studies were conducted on pristine and the reduced composites. As shown in Fig. 3d, electrochemical impedance spectra (EIS) measurements reveal that r-CNFg (0.8 Ω) and r-CNF (1.2 Ω) both exhibit much smaller charge transfer impedance than that of the pristine CNFg (7.0 Ω) and CNF (24.9 Ω). It can be concluded that the reduced ferrite oxides can serve as highly conductive catalysts to provide a fast electron transfer pathway. This improvement can be attributed to the generation of oxygen vacancies,18,70 and the high electron conductivity of GO which also helps lower the resistance of the composite materials.

The effective electrochemical surface area (ECSA) was evaluated by the electrochemical double-layer capacitance (Cdl) measurement, which has been regarded as an efficient method to confirm active sites in the metal oxide electrocatalysts. Cdl measurement reveals that the ECSA of the reduced spinel ferrite composites is much higher than that of pristine ferrite@GO composites. In particular, the Cdl value of r-CNFg is 2.72 mF cm−2, ca. 2.4 times higher than that of CNFg (Fig. 4 and S18). These results demonstrate that more exposed active sites and higher surface area for electrocatalytic OER are generated during the NaBH4 treatment process. After reduction-engraved treatment, the TEM study also shows that the surface of crystalline CNF becomes amorphous (Fig. 1c and d), which is further confirmed by PXRD with the decrease of intensities in diffraction peaks of the reduced composites. Meanwhile, N2 absorption experiments are performed at 77 K and 1 atm to determine the specific surface area of GO, CNFg and r-CNFg (Fig. S19). The r-CNFg possesses a higher Brunauer–Emmett–Teller surface area of 67.53 m2 g−1 than that of CNFg (60.26 m2 g−1), although part of the r-CNFg nanoparticles fall off the GO nanosheets. These results demonstrate that r-CNF nanoparticles possess a higher surface area, because of the introduction of abundant oxygen vacancies in the spinel lattice. Moreover, the hydrophilicity of r-CNF was studied by water vapor absorption isotherm measurement at 298 K and 1 atm (Fig. S20). Compared to that of CNF, the high hysteresis of the desorption curve of r-CNF shows a strong affinity for water molecules in the reduced spinel structure, indicating the presence of more vacant sites at the surface of r-CNF. It was also confirmed by TGA analysis (Fig. S21), where r-CNF lose more water molecules than that of CNF.

image file: c7qi00681k-f4.tif
Fig. 4 CVs of (a) CNFg; (b) r-CNFg in 1.0 M KOH solution at varying scan rates, with the plots of the capacitive current density as a function of scan rate (c).

X-ray photoelectron spectroscopy (XPS) measurements are conducted to further elucidate the effects of reduction-engraved treatment on the surface properties of CNFg. As shown in Fig. 5a, the appearance of the bonding energies of Co 2p3/2 (781.4 eV), 2p1/2 (796.8 eV) and satellite peaks (786.9 eV and 803.0 eV) indicates the presence of Co2+ with a high-spin state in CNFg.41 The shift of the Co 2p3/2 peak of r-CNFg from 781.4 eV to 781.0 eV after NaBH4 treatment demonstrates the low oxygen coordination of Co2+. The peaks of Ni 2p3/2 and 2p1/2 at 856.1 eV and 873.8 eV, and satellite peaks at 862.4 eV and 880.1 eV in CNFg are in good agreement with those of Ni2+ in the oxides (Fig. 5b).71 Also, a slight shift of the peak of Ni 2p3/2 in CNFg towards a lower bonding energy at 855.8 eV in r-CNFg was also observed. These bonding energy shifts of Ni and Co atoms demonstrate electron gain (reduction) of metal centers from the surrounding oxygen atoms or the removal of lattice oxygen atoms from the oxides.72 This result implies the dramatic distortion of the crystal structure on the surface of spinel ferrites, resulting in an amorphous shell on CNF as observed by HRTEM.73–75 The XPS pattern also displays the bonding energies of Fe 2p3/2 peaks located at 711.3 eV and 713.3 eV, respectively (Fig. 5c), indicating that iron is present in a Fe3+ state in CNFg, while little partial Fe2+ (709.7 eV) exists.48,76 Meanwhile, the presence of the peak at 711.3 eV implies that the Fe3+ ions exist in more than one coordination environment. This can be ascribed to the high affinity of Co2+ and Ni2+ ions in the octahedral sites of inverse spinel structure, indicating the existence of Co0.5Ni0.5Fe2O4.57,60,77 However, there was no significant change in the bonding energies of Fe 2p after reduction. These shifts of the Ni2+ and Co2+ indicate the generation of surface oxygen vacancies in the reduction treatment of CNFg, which was further confirmed by the O 1s XPS spectra (Fig. 5d). In the O 1s XPS spectra, three peaks of O 1s of CNFg at 530.6 eV, 531.7 eV and 532.7 eV are attributed to metal–oxygen bonds (OI),78 oxygen defect species (OII),70,79–81 hydroxyl species from water on the spinel surface and the C–O (hydroxyl) groups of GO,82,83 respectively. Intriguingly, the area ratio of OII/OI in r-CNFg (1.35) is about four times higher than that in CNFg (0.36), indicating that oxygen vacancies increased rapidly on the amorphous surface of spinel ferrite after reduction-engraved treatment.84,85 To avoid the effect of GO on oxygen defects analysis, the O 1s spectra of CNF and r-CNF were also investigated. As shown in Fig. S22, the peak of O 1s at 531.6 eV was also detected, which shows the increase of oxygen vacancies after reduction treatment. The reduction treatment makes them more amorphous, which was also observed in the HRTEM and PXRD. Moreover, the TGA of CNF and r-CNF under an air atmosphere shows an increasing weight at 450 °C for r-CNF compared to CNF (Fig. S21), apparently revealing the generation of abundant oxygen vacancies after the reduction-engraved strategy.

image file: c7qi00681k-f5.tif
Fig. 5 XPS spectra of CNFg and r-CNFg: (a) Co 2p, (b) Ni 2p, (c) Fe 2p, and (d) O 1s.


In conclusion, we demonstrate a facile hydrothermal synthetic strategy to prepare a series of highly dispersed Co/NiFe2O4@GO composites by anchoring small ferrite nanoparticles on a conductive GO support. The reduction-engraved strategy was adopted to treat the crystalline ferrite nanoparticles which helps promote the OER efficiency of these ferrites. ECSAs, N2 absorption experiments, water vapor absorption isotherm measurements, XPS and TGA studies reveal that the reduction-engraved treatment made them possess higher conductivity and more exposed active sites on the nano-scale ferrites, which contributed to much enhanced electrocatalytic activity for OER of the ferrite catalysts. After the reduction treatment, the catalytic current density of r-CNFg at 1.49 V vs. RHE is about 50 times higher than that of the CNF and CNFg. With an appropriate control of the Co2+/Ni2+ ratio in mixed-metal ferrites, r-CNFg with Co[thin space (1/6-em)]:[thin space (1/6-em)]Ni = 1[thin space (1/6-em)]:[thin space (1/6-em)]1 exhibits the most excellent oxygen evolution activity and superior long-term stability as a promising alternative to noble metal OER electrocatalysts. This work provides a new strategy for the preparation of efficient mixed metal OER electrocatalysts by the generation of abundant oxygen vacancies on the surface of nano-scale ferrites supported by the conductive GO support, which ensures the enhanced surface area and improvement in their electronic conductivity.

Conflicts of interest

There are no conflicts to declare.


This work was supported by the 973 program of China (2014CB845602), the NSFC (21331007/21671032/21722104), and the NSF of Guangdong Province (S2012030006240).


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Electronic supplementary information (ESI) available. See DOI: 10.1039/c7qi00681k

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