Effects of additives and surface treatment on electrochemical performance for bafeo4 electrode/ Comparison of different additives effect, страница 3

It can be known from Figs. 5 and 6 that the sample without any additives compared with the samples added with KMnO₄ and SrTiO₃ has a better electrochemical performance of BaFeO₄, the reason of which can be explained by the following equations. The FeO₄^[1]− ions arise the discharge reaction in the electrolyte. At the beginning, there is a little Fe₂O₃ is adhered on the BaFeO₄ surface. With the proceeding of the reaction, the products amount increases gradually. Owing to the insulating ability of Fe₂O₃, when the attaching amount increase, the proceeding of Eq. (1) will be baffled and the discharge performance gets down. Under the existence of a little K₂S₂O₈, Fe₂O₃ can be again oxidated to become FeO₄²⁻

Fig. 8 The discharge performance curve of the samples

ions. This process is depicted by Eq. (2). It makes the electrochemical reactions go on wheels and enhances the electrode discharge performance.

2FeO²₄ þ 5H₂O þ 6e ¼ Fe₂O₃ þ 10OH                                          ð1Þ

3S2O28þFe2O3 þ 10OH ¼ 6SO24 þ FeO42 þ 5H2O ð²Þ

Fig. 7 Schematic representation of the mechanism for additives

The Eq. (3) denotes the mechanism about adding a bit KMnO₄, which is similar to K₂S₂O₈. As can be seen from Eq. (3), the products MnO₂ also adhere on the electrode surface. It is similar to Fe₂O₃ and has a disadvantage influence on BaFeO₄ discharge reactions. The Eq. (2) displays that the products are SO₄²⁻ after adding K₂S₂O₈. mechanism for the two additives. The mechanism can explain why the sample with additive K₂S₂O₈ has an obvious effect on improving BaFeO₄ electrochemical performance, and the performance is better than that KMnO₄. The mechanism of additive SrTiO₃ had been discussed by our previous study [12], which is different from the two.

2MnO₄ þ Fe₂O₃ þ OH ¼ 2MnO₂ þ 2FeO₄² þ H₂O ð³Þ

Surface treating of electrode

In order to improve BaFeO₄ discharge performance, in this work, surface treating of BaFeO₄ electrode containing additive K₂S₂O₈ was processed with TiO₂ sol. Figure 8 shows the discharge curve of the samples. In Fig. 8, the discharge curve of BaFeO₄ (curve a) is as comparison, BaFeO₄ with 5% K₂S₂O₈ is shorted for BK (curve b) BaFeO₄ electrode containing 5% K₂S₂O₈ that is surface treating with TiO₂ sol shorts for BKT (curve c). As can be seen from Fig. 8, c has the best platform and the highest capacity. The capacity of curve c is increased by 65 mAh/g and 25 mAh/g from a comparison of a and b, respectively. The electrochemical performance has been much more obviously enhanced by combining additives with surface treating. The reason is that a thin protecting film appears on the electrode surface after BK electrode is surface treated with trace TiO₂ (1%) sol. It can protect additive and ensure the action of additive under the discharge process. The result is that the discharge performance of BaFeO₄ electrode is further enhanced.

89 Conclusions

(1)  The results show that the discharge performance of BaFeO₄ is obviously enhanced, the discharge platform is increased by 0.3 V, and the capacity reached up 240 mAh/g and increased by 40 mAh/g after 5% of K₂S₂O₈ was added. Compare with the adding of KMnO₄ and SrTiO₃, the effect is obvious.

(2)  The discharge capacity is increased by 65 mAh/g via surface treating the electrode with 1% TiO₂ sol, reached up 265 mAh/g, and the discharge platform rises. The result indicates that electrochemical performance can be obviously improved by combining additives with surface treating.

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