Morphology controlled synthesis, magnetic properties and electrochemical study of hematite nanocrystals

Abstract

Rational design of the non-noble, robust and low-cost functional oxygen evolution and oxygen reduction reaction (OER/ORR) electrocatalysts are greatly desired for fuel cells, rechargeable Zn-air batteries and water electrolyzers. Zinc-air battery is considered as an ideal device for energy storage and conversion because of its projected high energy density and the long driving range per charge. Herein by varying the synthetic parameters we observed an interesting phase and morphological transformations in iron oxide nanostructures with interfacial chemistry selectively tunable towards OER or ORR. For the synthesis of electrocatalysts, a facile one-step hydrothermal route was developed utilizing lysine as a stabilizing agent. This study reveals that hybrid β-FeOOH @ α-Fe2O3 mixed phase favored selective OER over selective ORR favored on α-Fe2O3 nanocubes. Rate of the electrochemical OER is amplified by 300 times in β-FeOOH @ α-Fe2O3 that in turn is found to be significantly higher than the additive rate of their decoupled counterparts, indicating possible synergistic effects in the mixed phase. Perhaps the β-FeOOH sites act as water adsorption sites with its oxidation selectively catalyzed by α-Fe2O3 under applied bias. Integration of these electrocatalysts with selective interfacial activity to the air electrode of Zn-air battery ultimately lead to a rechargeable metal-air battery in tri-electrode configurations with 500 mV reduction in the charging voltage. To the best of our knowledge, rechargeable metal air battery by tuning the interfacial chemistry of iron oxide nanostructures has not been studied earlier. When the OER electrocatalyst was absent, the air cathode required higher overpotentials indicating the role of OER catalyst in increasing the voltage and the overall energy efficiency. Magnetic hysteresis measurement on selected samples display ferromagnetism with exceptionally high coercivity in the range of 3.3 to 5.3 kOe at room temperature confirming induced ferromagnetism with exceptionally high anisotropy. Apart from the search for the materials of higher anisotropy, the present work demonstrates a strategy towards developing low-cost and abundant functional electrocatalyst for new generation metal-air battery.