Synthesis of manganese nitride doped with rare-earth elements and their oxygen reduction reaction activity

Abstract

Binary and ternary metal nitrides with d-block elements have been attracting attention as non-platinum oxygen reduction reaction (ORR) electrocatalysts. However, nitrides with f-block elements have not been explored as ORR catalysts, presumably because of their instability in an aqueous solution. By combining the stability of d-block metal nitrides in an alkaline solution and a high affinity toward OH− of the f-block elements, novel ORR nitride catalysts in an aqueous alkaline solution would emerge. Herein, we synthesized novel manganese nitrides with rare-earth elements (Y, Er, Tm, Yb) via the self-combustion reaction and evaluated their ORR activities in an aqueous alkaline solution. Ytterbium-doped manganese nitrides with different compositions were synthesized by the combustion reactions between MnCl2-YbCl3 mixtures and NaNH2 powder. The lattice parameter of the synthesized nitrides increased with an increase in the concentration of rare-earth elements, and EDX exhibited the distribution of Mn and Yb on the nanometer scale. A significant change in the magnetic properties by adding Yb supported the incorporation of Yb into an MnN framework. The enhancement of the ORR activity and high affinity of OH− on the surface were found in the nitride catalysts with ∼1% Yb/Mn ratio. In addition, manganese nitrides containing small amounts of various rare-earth elements (Y, Er, Gd, Tm) with enhanced catalytic activities were also synthesized. This work provides new strategies for synthesizing metal nitrides with rare-earth elements, and for improving the ORR catalytic activities of metal nitrides by doping rare-earth elements.