Effects of additives and surface treatment on electrochemical performance for bafeo4 electrode/ Comparison of different additives effect, страница 2

Characterization The products were characterized by X-ray diffraction (XRD), using a D/max-γA diffractometer with Cu Kα radiation operated at 40 kV and 100 mA. Data were collected in the 2θ ranges from 10 °C to 70 °C (1= 0.15418 nm). The scanning electron microscope (SEM, Hitachi S-450, Japan) was used to observe the grain morphology and particle size. Infrared radiation (IR) results of the samples were determined by Perkin Elmer Spectrum one FTIR.

Electrode preparation and electrochemical performance test The cathode was prepared by mixing 80 wt% BaFeO₄ (or BaFeO₄&additives or TiO₂-coated) with 15 wt% acetylene black and 5 wt% polyvinylidene fluoride, using N-methyl-2pyrrolidine as solvent. The additives are KMnO₄, SrTiO₃, and K₂S₂O₈, respectively. After the electrode surface was treated by TiO₂ sol, the electrode was dried under vacuum for 12 h. The mixed slurry was besmeared onto a foam nickel patch. The anode was commercial zinc and the electrolyte was 14 mol/L KOH. The mass of active materials was obtained by chemical method.

All the galvanostatic discharge tests were performed by using LandCT2001A Test System with current density of 0.5 mA/cm² in the range of 0.8–2.0 V at room temperature.

Results and discussion

XRD analysis

The XRD pattern is shown in Fig. 2. All diffraction lines could be indexed as the orthorhombic system with a space group Td, which is consistent with the literatures reports [13, 14].

IR analysis

The IR spectrum of BaFeO₄ is illustrated in Fig. 3, in which

−¹ the peaks at 868, 814 and 779 cm are assigned to FeO₄ tetrahedron stretching vibration peak, from which it can be seen that there is no triple degenerate at the fundamental frequency band but three detached peaks due to departure of

Fig. 2 XRD pattern of BaFeO₄

87 Fig. 3 IR spectrum of BaFeO4

BaFeO₄ to Td that agrees with what is reported [13, 14]. The

−¹ peaks at 3,463 and 1,629 cm are assigned to stretching vibration absorption peak and bending vibration absorption

−¹ peak of H₂O. The peak at 1,384 cm is attributed to C-O stretching vibration absorption peak, this may due to the absorption of H₂O and CO₂ in air when tested.

SEM analysis

Figure 4 shows SEM image of BaFeO₄. As can be seen from Fig. 4, the sample exhibits spherical morphology with average particle size of 50~100 nm.

Electrochemical performance

Additive K₂S₂O₈

Figure 5 shows the discharge curves within the potential range 0.8–2.0 V at the current density of 0.5 mA/cm². The discharge curve of BaFeO₄ is shown in Fig. 5. The curve a is the discharging curve of BaFeO₄ and the curves b, c, and d are the discharging curves of BaFeO₄ with additive K₂S₂O₈ of 15%, 10%, and 5%, respectively. From a comparison of a, b, c, and d in Fig. 5, some features were worthy to be mentioned. Firstly, the discharge capacity and the discharge platform of d are increased by 40 mAh/g and 0.3 V, respectively. Secondly, the discharge platform of c

descend and the smoothness is lower than a. Thirdly, the discharge capacity of b is improved unconspicuously and the platform also descend. The above results indicate that the electrochemical performance has been improved with adding K₂S₂O₈. When 5% K₂S₂O₈ is added into BaFeO₄, both the discharge capacity and the discharge platform exhibit the best.

Comparison of different additives effect

Figure 6 shows the effect of different additives on discharge performance of BaFeO₄ electrode. The samples contain 5% of additives except for curve a is pure BaFeO₄. As can be

Fig. 6. Different additives effect on electrochemical performance of BaFeO₄ electrode

seen from Fig. 6, the discharge capacity is the highest and the value is 240 mAh/g after adding 5% K₂S₂O₈ (curve d) under the same condition. When KMnO₄ and SrTiO₃ are added, the discharge capacity is 220 mAh/g and 230 mAh/g, respectively. From the above results, the BaFeO₄ with 5% K₂S₂O₈ provides a flattest discharge profile and highest overall energy capacity.