暖等离子体合成过渡金属掺杂氧化锰析氧电催化剂

Transition Metal-doped Manganese Oxide:Synthesis by Warm Plasma and Electrocatalytic Performance for Oxygen Evolution Reaction

可再生能源发电与质子交换膜水电解结合产生“绿色氢”对能源安全具有战略意义,其速控步骤是析氧反应。从稳定性、活性和成本角度考虑,本研究采用滑动弧暖等离子体一步合成了氧化锰(MnO_(x))及过渡金属掺杂(Fe-MnO_(x),Co-MnO_(x),Ni-MnO_(x)的析氧电催化剂,并对其晶体结构、形貌尺寸、元素组成和表面价态进行了表征。氧化锰主要由晶相Mn_(2)O_(3)和无定形Mn_(3)O_(4)组成。与之相比,掺杂的氧化锰尽管晶相组成基本不变,但其粒径明显变小、比表面积增大;掺杂Co促使氧化锰的表面电子增多。氧化锰基催化剂在酸性电解液的循环伏安测试中表现出独特的电流阶跃现象(低电势Ⅰ-Ⅱ区:1.4~1.8~2.4 V;高电势Ⅲ区:2.4~2.7 V)。该电流阶跃过程与Bulter-Volmer简化方程的电极动力学参照曲线相吻合,属于多价态锰参与的电催化反应。低电势区Fe-MnO_(x)的电化学活性最优,而高电势区Co-MnO_(x)表现最优。Co-MnO_(x)的起始电位比MnO_(x)低160 mV,且在恒电位电解中其末端电流密度是MnO_(x)的3倍。与其活性趋势一致,Fe-MnO_(x)、Co-MnO_(x)分别在低电势区、高电势区更具稳定性。本研究通过掺杂过渡金属优化氧化锰的颗粒尺寸、比表面积和电子结构,显著提高了催化剂析氧反应活性及稳定性。

By using renewable electricity proton exchange membrane(PEM),water electrolysis can produce“green hydrogen”of which the rate-determining step is oxygen evolution reaction.Considering the problems of stability,activity and cost,manganese oxides(MnO_(x))doped with transition metals as a kind of electrocatalysts(Fe-MnO_(x),Co-MnO_(x),and Ni-MnO_(x))were directly synthesized in one-step by gliding arc warm plasma.Complementary characterizations of the crystal structure,morphology,element composition,and surface valence state on this kind of synthetic catalysts were conducted.The results show that MnO_(x)mostly consists of crystalline Mn_(2)O_(3)and amorphous Mn_(3)O_(4).Although the transition metal doped catalysts possess the very approximate crystalline compositions to MnO_(x),they showed a remarkable decrease in particle size and an increase in specific surface area are achieved.Introducing Co to form Co-MnO_(x)can increase surface density of electrons as compared to pure MnOx.We further reveal a unique current step of MnO_(x)-based catalysts in acidic medium which appears in three consecutive potential regions(low I:1.4-1.8 V,low II:1.8-2.4 V,and high III:2.4-2.7 V)under cyclic voltammetry(CV)measurements.This current step process is quite consistent with the electrode dynamics curve(as reference)which is reduced by a simplified version of Bulter-Volmer equation,during which the electrocatalytic behavior involves manganese at multi-valence states.At low potential regions Fe-MnO_(x)achieves the highest activity among all catalysts,while at high potential region only Co-MnO_(x)achieves.Furthermore,Co-MnO_(x)onset potential is 160 mV lower than that of pure MnO_(x),and three times of the ending current density of pure MnO_(x) during bulk electrolysis under potentiostat.Consistent with the trend in activity,at low potential regions Fe-MnO_(x)is the most stable,while at high potential region Co-MnO_(x)is.Consequently,the significant increase in oxygen evolution reaction activity and stability of manganese oxides doped with transition metals is attributed to transition metal doping,which obviously optimizes the particle size,specific surface area,and electronic structure of MnO_(x).