Low Temperature Synthesis of Mn- and Fe-substituted mesoporous Ni0.5Co2.5O4 spinels toward higher OER activity

The development of new low-cost water splitting electrocatalysts to replace the expensive and scarce established Pt and RuO2/IrO2 catalysts is a major challenge on the way to green hydrogen production. Ordered mesoporous transition metal oxides, especially Co3O4 based, were hence studied extensively owing to their good performance and stability in alkaline electrolytes for the oxygen evolution reaction (OER).[1] The preparation of bimetallic transition metal oxides (TMOs) like NiCo2O4, but also the addition of small amounts of transition metals – as dopants - led to improved electrocatalysts due to synergistic effects between the metal oxide species. [2, 3] In this work, we synthesized ordered mesoporous Co3O4 (CO) and Ni0.5Co2.5O4 (NCO) as model nanostructured materials and evaluated the effects of partial Ni-substitution by Mn (Mn0.2Ni0.3Co2.5O4, MNCO), Fe (Fe0.2Ni0.3Co2.5O4, FNCO), and both (Mn0.1Fe0.1Ni0.3Co2.5O4, MFNCO). The materials were obtained via one-step impregnation nanocasting using KIT-6 (aged at 40 °C) as a hard template to obtain highly ordered mesoporous electrocatalysts exhibiting bimodal pore size distribution and high surface area. By implementation of a low-temperature calcination approach, the temperature necessary to obtain spinel-type transition metal oxides could be significantly reduced from typically 300-500 °C to 200 °C. This was achieved by exploiting the ‘container effect’ using the covered calcination crucible.[4] This time, to highlight the benefits of this procedure for the synthesis of our multi-metal oxides, the complex Mn0.1Fe0.1Ni0.3Co2.5O4 composition was calcined with a closed crucible lid at 200 °C (MFNCO-200) and compared to the materials obtained via standard conditions (MFNCO-300, 300 °C for multi-metal oxides [5]).