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BY 4.0 license Open Access Published by De Gruyter Open Access December 31, 2019

Electrocatalytic Properties of Ni-Doped BaFe12O19 for Oxygen Evolution in Alkaline Solution

  • Mohammad Khotib , Bambang Soegijono EMAIL logo , Zainal Alim Mas’ud and Komar Sutriah
From the journal Open Chemistry


Transition metal oxide (TMO) continues to be studied and developed as an oxygen evolution reaction (OER) electrocatalyst due to its abundance and low price. The aim of this experiment was to evaluate Ni2+-doped BaFe12O19 (BHF) as an OER electrocatalyst in an alkaline medium. BHF and Ni-doped BHF was synthesized through a low temperature coprecipitation technique followed by a calcination process at 750oC for 4 h. Diffractograms of the BHF and the Ni-doped BHF indicated a single phase on the synthesized BHF and formed Fe2O3 impurities on the Ni-doped BHF. SEM images showed a homogenous plate-shaped particles in the BHF, while the Ni-doped BHF had larger inhomogeneous particles. Ni dopant increased OER electrocatalytic activity of BHF based on overpotential on specific current density. The Ni-doped BHF had comparable activity to some metallic oxides based on their overpotential values at specific current density value.

1 Introduction

Oxygen evolution reactions (OER)and oxygen reduction reactions (ORR) are the basic anodic reaction processes and are essential for applications in energy storage and conversion. Oxygen evolution mechanisms require a large overpotential to drive the reaction at practical rates, so that some extra energy is dissipated. Noble metals and their oxides (Pt, Ru, and Ir) are known as the best and most stable electrocatalysts for oxygen evolution. The limited sources and high cost of these noble catalysts have been the major obstacle to adoption for mass markets. Therefore, it is highly desirable to develop sustainable and low-cost OER and ORR catalysts for applications in energy storage and conversion.

Transition metal oxides (TMO) with spinel-structure [1, 2, 3, 4] and perovskite-structure [5, 6, 7] have been widely investigated recently and characterized for OER and ORR. NiFe2O4 phase in mixed oxide Ni-Fe has been shown to have a significant role in improving the oxygen evolution activity in 1M NaOH solution [1]. Co1.5Fe1.5O4 was found to be an active electrocatalyst for oxygen evolution reaction in 1 M KOH with Tafel slope values of 44 m Vdecade-1. Other ferrite types such as barium hex ferrites (BaFe12O19) are commonly used as magnetic materials and have excellent chemical stability and corrosion resistivity. Zn2+- doped BaFe12O19 nanoplate (2.7 mole% Zn2+, thickness ca. 50 nm) demonstrated a reversible capacity of 665.5 mA h g-1 after 250 cycles at current density of 100 mAg-1 as anode materials for lithium-ion batteries (LIBs) [8]. Zn2+-doping is beneficial to increase the electronic conductivity of BaFe12O19, and improving the cycling performance. Other Ni-Fe-based electrocatalysts have been under intense study, especially Ni-Fe oxy-hydroxide forms, because they are the most active catalysts measured in basic media [9, 10, 11].OER activity of first-row transition metal oxyhydroxidethin films areNi(Fe)OxHy > Co(Fe)OxHy >FeOxHy-AuOx >FeOxHy >CoOxHy >NiOxHy > MnOxHy.9The phase of Ni0.9Fe0.1OOH has an OER turnover frequency (TOF) more than 10-fold higher than that of IrOx. Incorporation of Fe impurities in Ni(OH)2/NiOOHis responsible to increased activity of aged Ni(OH)2/NiOOH [10]. In addition, precipitation of Fe in Ni(OH)2/NiOOH will form Ni1−xFex(OH)2/Ni1−xFexOOH phase which has a higher conductivity than Ni(OH)2/NiOOH, up to 30-fold [10].

In this work, we studied Ni2+-doped BaFe12O19 that was synthesized through a low temperature coprecipitation technique followed by a calcination process, as a catalyst for OER in an alkaline medium. The motivation of this work is to increase electronic conductivity of BaFe12O19 and form complex Ni-Fe oxyhydroxides on the catalyst surface, to improve the catalytic performance.

2 Experimental

2.1 Materials

Barium chloride dihydrate (BaCl2×2H2O), ion chloride hexahydrate (FeCl3×6H2O), nickel nitrate hexahydrate (Ni(NO3)2×6H2O), sodium hydroxide (NaOH), and hydrochloric acid (HCl) were all purchased from Merck Ltd. All chemicals were of analytical grade and were used as received without further purification.

2.2 Electrocatalyst Preparation

The BaFe12O19 was synthesized by a low temperature copresipitation technique that is modification of previous method [12]. The mixed solution of 2.1977g BaCl2·2H2O (9 mmoL) and 29.1866g FeCl3·6H2O (108 mmoL) in 40 mL of 0.6 M HCl was slowly introduced dropwise into 150 mL of 1.5 M NaOH solution. During the addition of the mixed solution, the temperature of reaction was constantly maintained within 3-4 °C with a circulating thermostatic bath and the solution was vigorously stirred. The dark brown precipitate immediately appeared as the mixed solution was added. The precipitate was centrifuged at 3500 rpm for 15 min. The precipitate was dried at 150 °C for 4 h, calcined at 750°C for 4 h and washed with deionized water until free of chloride ion to obtain crystalline BaFe 12O 19. The preparation procedure of Ni2+-doped BaFe12O19 was the same as that of BaFe12O19 with precursor compositions of 1:11.6:0.4 (BHFNi0.4) for BaCl2·2H2O:FeCl3·6H2O:Ni(NO3)2.6 H2O and 1:8:4 (BHFNi4) for BaCl2·2H2O:FeCl3·6H2O:Ni(NO3) 2.6H2O, respectively.

2.3 Characterizations of Electrocatalysts

The barium hexaferrite structure was determined by XRD GBC EMMA software Traces 28 mA, 35 kV with Cu Kq= 1.54056 A in the range 2q of 10-90 , step size of 0.02 , and scan rate of 2 /min. Functional groups of products was determined by using a Prestige-21 Shimadzu FTIR spectrometer. The morphologies of the samples were examined by using a JOEL scanning electron microscope (SEM) JS-IT100. The thermal properties of the precipitate were measured using DTA/TG under air purge with a temperature raising rate of 5°C/min.

2.4 ElectrocatalyticActivity

The catalyst slurries were prepared using polyvinyl acetate (PVAC) as a binder, and isopropanol as the solvent. The slurries were prepared by mixing of 50 mg material catalyst and 10 μL of 5% PVAC solutions in water-isopropanol (1:1), stirred to homogenous, and then dripped on the surface of SPCE DS-110. All the electrochemical analyses, such as cyclic voltammograms (CV) and Tafel polarization, were performed in a conventional three-electrode single compartment Pyrex glass cell using a computerized potentiostat (DY 2300 Digi-ivy). Polarization potential was measured in 1M NaOH solution with Ag/AgCl electrode as a reference electrode, platinum wire as an auxiliary electrode, and BHF electrocatalyst as a working electrode. Ethical approval: The conducted research is not related to either human or animal use.

3 Results and Discussion

3.1 Characteristics of BaFe12O19 and Ni-doped BaFe 12O19

XRD patterns of BHF synthesized by co-precipitation method at low temperatures with and without washing before calcination process are presented in Figure 1. The results showed that single phase of BaFe12O19 was obtained from the precipitates calcined at 750°C for 4 h and then washed with deionized water until free from chloride ion. This result is in line with previous research using the co-precipitation technique at 0±1°C using FeCl3 and BaCl2 as precursors [12]. In this method, formation of BHF at this condition was assisted by the presence of NaCl in the precipitate which served as a flux in the formation of BaFe12O19 crystals. The function of NaCl as a flux is evidenced by the absence of BHF in product calcinated at 750°C for 4 h that was prepared through washing of precipitate before calcination (Figure 1 c). It took a longer time at the calcination temperature of 750 °C or needed higher temperature to form BHF crystals. Other evidence that NaCl served as the flux can also be seen in Figure

Figure 1 Diffractograms of the washing effect before calcination on the formation of BHF.
Figure 1

Diffractograms of the washing effect before calcination on the formation of BHF.

1 b which exhibits presence of BHF peak in the product that prepared from precipitate without washing that was calcinated at 750°C for 2 h.The effect of NaCl flux to accelerate crystal formation BHF has been previously studied [13,14]. The salt is used since the metal oxide is more soluble in molten metal chloride of groups IA and IIA, compared to other forms. The metal oxides in the flux undergo rearrangement and diffuse to form the expected phase [13,14].

X-Ray diffraction pattern of BaFe12-xNixO19 (x=0, x=0.4, x=4) shows M-type hexaferrite crystal and a single-phase hexagonal structure as depicted in Figure 2. Meanwhile, the visualization of the BHF structure from Rietvield process is shown in Figure 3 using VESTA software. Distance and angle of inter atom O-Fe in BHF is difference depend on its atomic geometry and position. The unit cell parameters and cell volume of all samples are listed in Table 1. It was found that the lattice constant and cell volume decrease with the presence of Ni dopant. It is related to the replacement of the higher ionic radius of Fe3+ (0.078 nm, octahedral, high spin) by the lower ionic radius of Ni3+ (0.074 nm, octahedral, high spin). Substitution of Ni element caused contraction of the crystal structure of the barium hexaferrite, resulted in a decreasing size of the crystallite. It is indicated by the presence of peak broadening and intensities decreasing in the XRD pattern of the Ni-doped BaFe12O19.

Figure 2 XRD pattern of BHF (Top), BaNi0.4 Fe11.6O19 (middle) andBaNi4 Fe8O19 (bottom).
Figure 2

XRD pattern of BHF (Top), BaNi0.4 Fe11.6O19 (middle) andBaNi4 Fe8O19 (bottom).

Figure 3 Visualization of refined barium hexaferrite using Vesta software.
Figure 3

Visualization of refined barium hexaferrite using Vesta software.

Table 1

Lattice parameters of the synthesized materials.

PhaseCell unitCrystallite size (nm)
a=b (Å)c(Å)V (Å3)WHLangford
BaFe12O195.8938123.20737698.1494116.9(4.9)143.6 (2.9)
BaNi0.4 Fe11.6O195.8945423.19318697.896133.1(0.7)36.0(0.6)
BaNi4 Fe8O195.8933823.19432697.654846.6(0.6)50.9(0.5)

Infrared spectra for the precipitates and the powder calcinated at 750°C are shown in Figure 4. The dried precipitate spectrum illustrates the presence of absorption bands at 3331 cm-1 attributed to the O-H stretching of water and of the metal hydroxide. The distinctive OH-bending doublet associated with goethite (a-FeOOH) at 771 and 941 cm-1 werw observed along with well-defined peaks corresponding to Fe-O lattice vibrations at lower frequencies (607 and 476-463 cm-1) [15].The absorption bands at 590 cm-1, 543 cm-1, and 437 cm-1 belong to the metal–oxygen stretching vibrations of the BaFe12O19and of the Ni-doped BaFe12O19.

Figure 4 FTIR spectra of the dried precipitate (purple), BaFe12O19 (red), BaNi0.4Fe11.6O19 (blue) and BaNi4Fe8O19 (black).
Figure 4

FTIR spectra of the dried precipitate (purple), BaFe12O19 (red), BaNi0.4Fe11.6O19 (blue) and BaNi4Fe8O19 (black).

BHF formation from the precipitate precursor was studied through DTA/TGA thermal characterization. Intermediate phases of BHF synthesis using co-precipitation method are not clearly known. Ba(OH)2, Fe(OH)3, NaCl and its hydrated hydroxide are precipitates that resulted from coprecipitation process using BaCl2 and FeCl3 as the precursors and the NaOH as the precipitating agent. Thermal properties of the BHF dried precipitates showed endothermic peaks at 76.4oC, 638oC, and 805oC, and an exothermic peak at735oC (Figure 5a). At temperature ranges of 200-800°C, the weight loss of approximately 10%indicatesthe phase transition from the precipitates to barium hexaferrite. This DTA/TGA plot is in line with the thermogram of the dried precipitates resulting from Ba(NO3)2 and Fe(NO3)3 as the precursors and a NaOH-Na2CO3 system as the precipitating agent [16]. Strong endothermic peak at 805°C can be attributed to the melting point of NaCl in the precipitates. The onset temperature, which reflects the melting point, matches well with the value of 801°C in the previous study [17]. The DTA/TGA curves of Ni-BHF have a similar pattern to the DTA/TGA curves of the BHF, i.e. there are endothermic peaks at 76.4oC, 638oC, and 805oC, and an exothermic peak at 735oC (Fig 5b,c).

Figure 5 DTA/TG curves of the dried precursors for (a) BHF, (b) BHFNi0.4, (c) BHFNi4.
Figure 5

DTA/TG curves of the dried precursors for (a) BHF, (b) BHFNi0.4, (c) BHFNi4.

The morphology of the BHF and Ni-doped BHF was examined by scanning electron microscope (SEM). Figure 6a shows that co-precipitation technique followed by calcination at 750°C resulted in some nanoplates of BHF ca. 70-140 nm in thickness and ca. 400-600 nm in diameter. The plate shape in this result agrees with the findings of others [12,18,19]. Homogenous grain size is shown in the SEM image of the BHF, meanwhile the Ni-doped BHF tends to agglomerate, be inhomogeneous and larger in size than that of the BHF (Figure 6b). It indicates that the particle size distribution of the BHF was influenced by the Ni doping. Increasing grain size also occurs in Zn-Nb substituted BHF synthesized via sol-gel technique [18].

Figure 6 SEM image of BHF (a) and Ni-doped BHF (b) synthesized by coprecipitation technique.
Figure 6

SEM image of BHF (a) and Ni-doped BHF (b) synthesized by coprecipitation technique.

3.2 Electrochemical Study

The electrocatalyst stability of the BHF and the N- doped BHF were measured in the potential range of -1 - 1 (V vs Ag / AgCl) with a scan rate of 0.05 V/sec and sensitivity of 0.001 A/V in 1M NaOH medium. The electrocatalysts appeared to be stable during electrochemical measurements using the cyclic voltammetry of 20-cycles, so that the measurement could be continued to determine the catalytic activity of the OER (supplementary 1). SPCE DS-110 and PVAC binder were used to improve sensitivity and stability of electrocatalytic measurements from catalysts. PAVC has been studied as a binder in electrochemical applications, such as binders in the fabrication of LiFePO4 composite cathodes for lithium batteries [20] and binders in WO3/BiVO4 photoanodes for solar water splitting [21]. PVAC is categorized as an eco-friendly binder and could replace fluorinated BHFNi4. polymers such as PVDF and Nafion. In Figure 7c, cyclic voltammograms of BHF and Ni-doped BHF showed microelectrode that co-precipitation pattern, which oxidation and reduction peaks were not observed. This is in line with cyclic voltammograms of La0.9Ca0.1Co1−xFexO3 cathod [22] and CuxCo3-xO4 for oxygen evolution reaction [23]. Electrochemical measurement showed changes in Tafel slope (b), exchange current density (J0), overpotential (h), and charge transfer coefficient (a) after the SPCE was modified using the BHF and the Ni-doped BHF. The Tafel slope (b) for the BHF was still large up to > 500 mV/dec and decreased, reaching 179 mV/dec with the presence of Ni dopant (Figure 7 and Table 2). On the other hand, Ni dopant also decreased the BHF overpotential at 10 mA/cm2 from 393 mV to 180 mV in 1 M NaOH medium. The BHF doped with Ni0.4 and Ni4 had comparable activity to some metallic oxides based on their overpotential values at specific current density values. Overpotentials of IrO2 and RuO2 at 10 mA/cm2 current densities in 0.5 M KOH are 358 mV and 411 mV, respectively. Several spinel ferrites have been studied for the catalytic activity [24] and the tendencies are CoFe2O4>CuFe2O4> NiFe2O4>MnFe2O4.

Figure 7 Electrochemical parameters of BHF and Ni-doped BHF in 1M NaOH.
Figure 7

Electrochemical parameters of BHF and Ni-doped BHF in 1M NaOH.

Table 2

Measurement and kinetic parameters for BHF, Ni-doped BHF, and other electrocatalysts for OER.

MaterialElectrolyteSubstrateOverpotential (mV) at J0=10 mA/cmTafel slope (mV dec-1)charge transfer coefficient (a)Remarks
BHF1 M NaOHCarbon3935030.117This study
BHFNi0.41 M NaOHCarbon1741800.328This study
BHFNi41 M NaOHCarbon1801790.33This study
RuO20.5 M KOHFTO35855[24]
IrO20.5 M KOHFTO41191[24]
NiFe2O40.1 M KOHGCE440-[24]
NiFe2O41 M KOHNi-42[24]
CoCr2O41 M KOHGCE42263.3[24]
CoCr2O4/CNT1 M KOHGCE32651[24]

In addition, NiFe oxides in layer form such as oxyhydroxide have a high OER activity. Fe-doped nanocrystallized nickel oxide (FexNi1-xO) was synthesized an OER electrocatalyst in an alkaline medium [25]. The Fe0.1Ni0.9O has the highest electrocatalytic value for water oxidation with turn-over frequency (TOF) of 1.9 s-1 at 300 mV overpotential. The 10 mAcm-2 current density is achieved at 297 mV overpotential and has a Tafel slope of 37 mV dec-1. The performance of the iron oxide nickel and the evidence of this study indicates that the electrocatalytic performance of the Ni-doped BHF is due to the presence of redox complex systems of Fe2+/Fe3+; Fe3+/Fe4+ and Ni2+/Ni3+.

The OER electrocatalytic capability of the BHF may be due to the non-stoichiometric (defect) of Fe atoms, the occurrence of oxygen vacancy, and the geometry of Fe atoms. The defect of Fe atoms may cause destabilization of the crystal structures, resulting in M-O-M angular changes. The presence of oxygen vacancy can facilitate the formation of OH absorption in Fe atoms. The different geometry of Fe (octahedral, tetragonal and trigonal bipyramidal) affects the composition and position of electrons in different d (eg and t2g) orbitals, as well as the strength and length of the M-O bonds. Based on VESTA visualization, bond of Fe-O in octahedral and trigonal bipyramidal are longer than tetrahedral geometry (Figure 3). Fe-O bond has different length on octahedral at D position, tetrahedral, and trigonal pyramidal, so these are predicted as active site of electrocatalyst.

The Fe3+ orbital profiles in octahedral, tetrahedral, and trigonal bipyramidal geometries are presented in Figure 8. The oxidation state and the spin type of the transition metal complex indicate the tendency of the chemical sorption force of the perovskite compound, and the octahedral geometry of the Fe3+ ion is more dominant in the BHF structure. This may cause the electrocatalytic activity of the BHF and the Ni-doped BHF. In addition, Fe ions undergo deformation to oxy-hydroxide (FeOOH) due to the adsorption of OH ions in the alkaline medium, which are in line with the following mechanisms of oxygen evolution [24,26]:

Figure8 Splittingof Fe3+orbital based on crystal field theory.

Splittingof Fe3+orbital based on crystal field theory.

(i) Bockris Mechanism:

M + -OH → M-OH + e- (Tafel slope=120 mV/dec)

M-OH + -OH → M-O + H2O + e- (Tafel slope=40 mV/dec)

2M-O → 2M + O2(Tafel slope=15 mV/dec)

(ii) Bockris&Otagawa’s:

M + -OH → M-OH + e-

M-OH + -OH → M...H2O2 (physisorption) + e-

M...H2O2 + -OH → M…HO2 (physisorption) + H2O

M...H2O2 + M... HO2- → 2M + -OH + O2

(iii) Krasil’shchikov’s:

M + -OH → M-OH + e- (Tafel Slope=120 mV/dec)

M-OH + -OH → MO- + H2O (Tafel Slope=60 mV/dec)

M-O- → M-O+ e- (Tafel Slope=40 (low) & 120 mV/dec (high))

2M-O → 2M + O2(Tafel Slope=15 mV/dec)

The Ni-doped BHF will result in a Fe3+/Fe4+ and Ni2+/Ni3+ redox system which is assumed to be an active site on the electrocatalyst, whereas on the BHF there is only one active site on the electrocatalyst. Introduction of Fe into Ni oxy-hydroxide increases the Ni2+/Ni3+ potential and decreases the potential onset for the catalyst [27]. Another review of the catalytic activity of Fe and Ni can be seen from the strength of the M-OH bond based on the enthalpy of its formation. The higher enthalpy of M-OH formation the lower the current density by the tendency of Ni> Co> Fe>Mn> Cr> V [28].

4 Conclusion

BHF and Ni-doped BHF have been successfully synthesized using coprecipitation techniques with BaCl2 and FeCl3 precursors without washing/neutralizing process before sintering at 750°C. NaCl in precipitate of BHF assisted in formation of BHF crystal and act as flux. The Ni-doped BHF and the BHF on SPCE surface had OER electrocatalytic activity in NaOH medium with good performance based on the overpotential value at specific current density.

  1. Conflict of interest: Authors declare no conflict of interest.


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Received: 2018-11-30
Accepted: 2019-04-08
Published Online: 2019-12-31

© 2019 Mohammad Khotib et al., published by De Gruyter

This work is licensed under the Creative Commons Attribution 4.0 Public License.

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