Oxygen redox reaction is the ubiquitous reaction involved in the metal-air batteries, regenerative low temperature fuel cells, fuel cells, chlor alkali and water electrolysis cells ¹. Among these metal-air and regenerative fuel cells require bifunctional catalysts. Developing cost effective, highly active and durable bi-functional electrocatalyst is one of the major challenges faced by the several energy storage and conversion technologies. Oxygen redox reaction proceeds with sluggish kinetics. To enhance the sluggish kinetics, noble metal, metal oxides and doped carbon composites have been studied. However, noble metals application are limited due to their low availability, stability and high cost ^{2, 3}.

In this scenario, transition metal oxides based on perovskites, pyrochlores, and spinels have been considered as promising candidates for the oxygen reduction/evolution activity, thanks to their structural robustness, cost, accessibility and stable performances in aqueous alkaline electrolytes. Among these different structures , spinel based oxides pose superior ORER performance, due to their mixed valence states and 2⁺ valence ion in 6 fold coordination⁴. Spinels (AB₂O₄) with earth abundant 3d transition metals (Fe, Co, Ni, Cu, and Mn) are considered to be alternative to the noble metals. By altering the B site ion or using different A and B combinations, surface redox sites and metal–oxygen bonds, subsequently leads to superior performance. Spinel catalyst performance greatly depends on composition, shape, size, and structure⁵. Ni and Fe based materials are attractive due to their abundance and performance ⁶.

In this work, we synthesized spinel NiFe₂O₄ through the modified co-precipitation method and the resulting samples were heat treated showing different particle sizes. The various sample phases and morphologies were characterized using XRD and TEM. Their structure to electrochemical performance relationship was studied in three electrode electrochemical setup. Their electrochemical ORR activities were characterized by linear sweep voltammetry by RDE in oxygen saturated 0.1 M KOH. Electrode activity was measured using the CV in the potential range of -0.9 to 0.7 V vs Hg/HgO and stable performances by chronoamperometry. Particle sizes to electrochemical activity of the catalysts were systematically investigated.

Acknowledgement

This work is funded by the Swedish Energy Agency.

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