Synthesis Of Transition Metals Doped Manganese Dioxides And The Oxygen Reduction Reaction Activities

Due to its bright application prospects and good prospects for development, fuel cell is considered as one of the most clean and energy efficient devices. The traditional fuel cell uses precious metal as catalyst of oxygen reduction reaction on its negative electrode. But it can’t be producted and applicated commercially in a large scale because of its high cost and instability. In recent years, many researchers focus on non precious metals and their oxides because of its lower cost. Among them, manganese dioxide has attracted the widest attention due to its resources and environment friendly etc. Manganese dioxide plays an excellent performance on ORR with its good structural and electronic properties. However, the catalytic activity is still unsatisfied compared with the Pt catalyst. Therefore, to prepare a new kind manganese dioxide that modified to be more efficient and has high catalytic activity has a great significance on the researches and development of the high-performance fuel cell. Therefore, according to the mechanism of ORR and considering the request of the cathode material, this paper carry out the following work to modify the manganese dioxide using as the ORR catalyst:1. Use reflux method to prepare V-KOM-S-2 by one step, without adding any surface active agents or template materials and only use MnSO4·H2O、KMnO4 and NaVO3 as reactants. By controlling the proportion and concentration of the reactants, K-OMS-2、1% V-K-OMS-2、2.5% V-K-OMS-2 and 5% V-K-OMS-2 four samples with different vanadium contents were obtained. The characterization results show that, doping vanadium doesn’t change the crystal form of K-OMS-2, but its morphology becomes shorter while its BET changes not so much. The catalytic activities of the four samples were investigated by electrochemical workstation. We figure out that the initial potential, half wave potential and the limiting current density of the four samples were increased with the increase of the vanadium doping amount. By calculating from K-L equation, we got the number of transferred electrons on ORR catalyzing by K-OMS-2、1% V-K-OMS-2、2.5% V-K-OMS-2 and 5% V-K-OMS-2 that were 4.19、4.29、4.31 and 4.33 respectively, which suggested that the ORR mechanisms catalyzed by the four samples were closed to "direct four electron process", When the vanadium doping content was below 5%, the catalytic activity of the four catalysts on ORR become better and better with the increase of vanadium doping content. According to the result of ICP, the average oxidation states of Mn in the K-OMS-2、 1% V-K-OMS-2、2.5% V-K-OMS-2 and 5% V-K-OMS-2 were 3.91.3.89.3.87 and 3.85 respectively, suggesting that the difference of the catalytic effect between the four samples may due to the different vanadium contents. Because the radius of V5+ was close to that of Mn4+, so it is easy for the V5+ to replace the Mn4+in the lattice, which will latter increase the ratio of Mn3+/Mn4+. As a result, the O2 adsorption capacity of the sample increases as there are more oxygen vacancies exist in the crystal, which finally lead to the improvement of the ORR catalytic activity of the V-K-OMS-2 samples.2. Use reflux method to prepare M-KOM-S-2(M includes Co、Ni and Cu) by one step, without adding any surface active agents or template materials and only use MnSO4·H2O, KMnO4, Ni(HNO3)2 · 6H2O, Cu(HNO3)2·3 H2O and Co(HNO3)2·6H2O as reactants. By controlling the proportion and concentration of reactants, several M-KOM-S-2 samples with different metal contents were obtained. For the convenience of comparing the catalytic activity of different kind of doping metals, we select 2.5% Co-K-OMS-2、2.5% Ni-K-OMS-2 and 2.5% Cu-K-OMS-2 that have the same doping content to analyze compare with K-OMS-2. The characterization results show that doping 2.5% M2+(Co2+、Ni2+、Cu2+) doesn’t change the crystal form and morphology of K-OMS-2, but its BET increase a lot. Among them, the BET of 2.5% Co-K-OMS-2 and 2.5% Ni-K-OMS-2 reach 99.20m2/g and 93.07m2/g. The catalytic activities of the four samples were investigated by electrochemical workstation. We figure out that the initial potential, half wave potential and the limiting current density of the samples were increased after doping Co、Ni and Cu and follow the order 2.5% Co-K-OMS-2> 2.5% Ni-K-OMS-2> 2.5% Cu-K-OMS-2> K-OMS-2. By calculating from K-L equation, we got the number of transferred electrons on ORR catalyzing by K-OMS-2,2.5% Co-K-OMS-2、2.5% Ni-K-OMS-2 and 2.5% Cu-K-OMS-2 that were 4.19、4.32.4.27 and 4.23 respectively, which suggested that the ORR mechanisms catalyzed by the four samples were closed to "direct four electron process". According to BET data and position of Co、Ni、Cu in the periodic table of elements, the improved catalytic activity of the 2.5% Co-K-OMS-2、2.5% Ni-K-OMS-2 and 2.5% Cu-K-OMS-2 on ORR may due to the BET rules and the electro negativities of Co2+、Ni2+ and Cu2+.