Batteries have long been recognized for their capacity to convert and store electrical energy efficiently. Compared with lithium-ion batteries, rechargeable metal-air batteries such as zinc-air batteries could be considered as idealized energy devices due to their high power density, economic viability and high safety.^1,2 Active and durable electro-catalysts on the cathode side are required to catalyze the slow oxygen reduction reaction (ORR) during discharge process for zinc-air batteries.^3,4 In this work, we have successfully synthesized mesoporous Co₃O₄@CNT@PQ7 through a sol-gel method⁵. Benefiting from the large special surface area and highly active sites, the obtained Co₃O₄@CNT@PQ7-200 hybrid catalyst shows excellent catalytic performance for ORR, which ensures an exceptionally high performance for zinc- air batteries.

First, 5 g of nano-silica with the particle size of 500 nm were mixed with 25 g of hydrochloric acid solution (1 M) and sonicated for 8 minutes. Then, 22.506 g PQ-7 solution was added into the pre-synthesized SiO₂ solution. 3.7 g of FeSO₄ 7H₂O was added into this mixture under stirring for 5 hours. Then the resulting solution was dried for 48 hours at 85 ^oC and then pyrolyzed at 800 ^oC for 1 hour under a nitrogen atmosphere with a temperature ramp rate of 20 ^oC min^-1. The SiO₂ was leached out using excess amount of sodium hydroxide (NaOH, 4 M) solution for 48 hours and the resulting powder was washed with deionized water for neutralization, and then dried overnight. After that the sample had to be acid-leached using 0.5 M H₂SO₄ at 85 ^oC for 8 hours, then re-pyrolyzed at 800 ^oC for 1 hour under the same conditions as those during the first heat treatment to obtain the catalyst sample (denoted as PQ7)⁶. Meanwhile, 0.01 mol Co(NO₃)₂ 4H₂O were dissolved in 10 mL of deionized water. Then 0.05 g CNTs were dispersed in the above prepared solution by ultrasonication for 30 minutes. At the same time, 0.01 mol C₆H₈O₇•H₂O was dissolved in 5 ml of deionized water. After that the solution has to be added into prepared solution drop by drop by stirring for 5 hours and then dried at 90 ^oC for 6 hours. The collected powder of Co₃O₄@CNT hybrid materials were calcined in air at 100 ^oC, 150 ^oC, 200 ^oC, 350 ^oC and 500 ^oC for 1 hour to obtain the product (denoted as Co₃O₄@CNT-x). 10% of PQ7 and Co₃O₄@CNT were dissolved in ethanol and deionized water (volume ratio 1:1) and sonicated for 1 hour. The above solution has to be dried at 80 ^oC for 6 hours. The obtained solid was ground to a fine powder in an agate mortar (denoted as Co₃O₄@CNT@PQ7-x, x=100, 150, 200, 350 and 500).

Co₃O₄@CNT@PQ7-x is also directly used as the working electrode catalyst for the measurements toward ORR by half-cell test⁶. The hybrid material displays good bifunctional ORR activity as is shown in Fig. 1. Higher onset potential is achieved for Co₃O₄@CNT@PQ7-200. The Co₃O₄@CNT@PQ7-200 exhibits a half-wave potential (E_1/2) of 0.75 V. In contrast, the E_1/2 for Co₃O₄@CNT@PQ7-100, Co₃O₄@CNT@PQ7-150, Co₃O₄@CNT@PQ7-350 and Co₃O₄@CNT@PQ7-500 is 0.62, 0.58, 0.64 and 0.69 V, respectively. The above results indicate that more ORR active sites could be obtained, which should be attributed to the assistance of PQ7.

Fig. 1 ORR polarization curves of Co3O4@CNT@PQ7-x

References

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