摘要
Spinel-type manganese-cobalt oxides have been regarded as important class of electrocatalysts for oxygen reduction reaction(ORR).However,they are usually synthesized through oxidation-precipitation under aqueous ammonia and then crystallization at high temperature(150–180℃),which not only increases the energy consumption but also induces the growth of particles that is unfavorable for ORR.Herein,through a facile precipitation-dehydration method,ultrasmall spinel manganese-cobalt oxide nanoparticles(~5 nm)homogeneously dispersed on conductive carbon black(MnxCo3-xO4/C)were fabricated at low temperature(60℃).And the bimetallic composite oxide(Mn1.5Co1.5O4/C)with cubic spinel structure and high Mn content exhibits remarkable enhancement of ORR activity and stability compared with single metal oxide(both Mn3O4/C and Co3O4/C).The essential reason for the enhancement of activity can be attributed to the presence of the mixed Mn^3+ and Mn^4+ cations in Mn1.5Co1.5O4/C.Moreover,the ORR activity of Mn1.5Co1.5O4/C is comparable to that of commercial 20 wt% Pt/C,and the relative current density only decreases 1.4% after 12 h test,exceeding that of Pt/C and most reported manganese-cobalt oxide electrocatalysts.
尖晶石型锰钴氧化物(MnxCo3-xO4)被认为是一类重要的氧还原反应(ORR)电催化剂.除了传统的高温固态合成方法,MnxCo3-xO4通常是通过前驱体在氨水中氧化-沉淀,再在高温(150-180 ℃)下晶化而合成的.高温合成方法不仅增加了能源消耗,还会导致氧化物纳米颗粒的生长.与较大颗粒的金属氧化物相比,金属氧化物超细纳米颗粒不仅可以降低扩散阻力,而且具有更大的比表面积,从而暴露出更多的活性位点,更有利于ORR的进行.因此,开发一种用于制备尖晶石型锰钴氧化物超细纳米颗粒的简便、低温合成方法是非常必要的.本文通过一种简单的低温沉淀-脱水方法,成功地合成了导电炭黑负载的尖晶石锰钴氧化物超细纳米颗粒(Mn1.5Co1.5O4/C).采用X射线衍射(XRD)、拉曼光谱(Raman)、透射电子显微镜(TEM)、X射线光电子能谱(XPS)等表征手段研究了所制备尖晶石氧化物的结构和形貌,通过电催化反应及表征测试了氧化物的ORR催化性能,并探究了双金属催化剂Mn1.5Co1.5O4/C活性提升的本质原因.TEM图显示MnxCo3-xO4氧化物纳米颗粒均匀分布在导电炭黑载体上,并且颗粒尺寸(~5 nm)较小.高分辨透射电子显微镜(HRTEM)图中有明显的晶格条纹和多晶衍射环,表明氧化物纳米颗粒具有较好的结晶性.XRD表征结果表明,双金属氧化物Mn1.5Co1.5O4/C具有立方尖晶石结构;并且与纯Co3O4相比,Mn1.5Co1.5O4/C的衍射峰略向小角度偏移,表明较大尺寸的Mn阳离子掺入立方Co3O4晶格中.分别采用TEM-EDS(能谱)和XPS测得的Mn/Co原子比都接近1,与投料比及文献中Mn1.5Co1.5O4金属原子比一致.以上结果说明通过这种简便的一步法可以成功制得组分可控的尖晶石锰钴氧化物.电催化测试表明,相比于单金属Mn3O4/C和Co3O4/C,复合双金属催化剂Mn1.5Co1.5O4/C的ORR活性和稳定性均显著增强.此外,Mn1.5Co1.5O4/C的ORR活性与商业化的20 wt%Pt/C相当,并且12 h后其相对电流密度仅降低1.4%,稳定性超过了Pt/C和大多数已报道的锰钴氧化物电催化剂.此外,Mn1.5Co1.5O4/C作为阴极催化剂在锌空气电池中表现出优异的放电性能.我们还通过XPS研究了Mn3O4/C和Mn1.5Co1.5O4/C中Mn电子态的差异,并推断锰钴双金属氧化物ORR活性和稳定性的增强可归因于高价态的Mn阳离子,特别是Mn^3+和Mn^4+混合态阳离子的存在.
基金
国家自然科学基金(21676193,51661145026).
