Synthesis of Nanostructured Manganese (Hydro)Oxides in Microreactor with Intensive Counter-Current Swirling Flows for High-Performance Pseudocapacitive Electrodes

Efficient and scalable synthesis of nanostructured pseudocapacitive materials is essential for advancing high-performance energy storage systems. In this work, manganese (hydro)oxides were synthesized via a continuous-flow microreactor equipped with intensive counter-current swirling flows, providing enhanced micromixing and precise control over reaction kinetics. By varying the flow rates of reagents (1.5, 2.2, and 3.0 L/min), we tuned the phase composition, crystallinity, and porosity of the resulting materials. Powder X-ray diffraction, Raman spectroscopy, and EDX analysis revealed a transition from Mn(OH)2 and MnOOH phases at lower flow regimes to highly crystalline Mn3O4 spinel at 3.0 L/min. SEM and BET analysis showed the formation of layered mesoporous structures with surface areas up to 120 m2/g. Electrochemical characterization in 1 M Na2SO4 demonstrated a strong correlation between synthesis conditions and capacitive performance. The best-performing electrode (MR-3.0) exhibited a specific capacitance of 425 F/g at 5 A/g, low charge transfer resistance, and ideal capacitive behavior. These enhancements are attributed to optimized ion transport and accessible surface area resulting from flow-assisted synthesis. Our results highlight the potential of swirling-flow microreactors as a robust platform for producing advanced pseudocapacitive materials with tunable properties, suitable for next-generation supercapacitor electrodes in hybrid energy storage systems.