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− 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 , which is different from the two.
2MnO4 þ Fe2O3 þ OH ¼ 2MnO2 þ 2FeO42 þ H2O ð3Þ
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.
(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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The SO4 ions will not coat on BaFeO4, but enter into the electrolyte. It is favorable for BaFeO4 and the electrolyte to contact with each other directly. Figure 7 shows the
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