Synthesis of nanostructured manganese (hydro)oxides in microreactor with intensive counter-current swirling flows for high-performance pseudocapacitive electrodes

Abstract Efficient and scalable synthesis of nanostructured pseudocapacitive materials is crucial for the development of high-performance energy storage systems. In this work, manganese (hydro)oxides were synthesized using a continuous-flow microreactor equipped with intensive counter-current swirling flows, ensuring enhanced micromixing and precise control over reaction kinetics. By varying the reagent flow rates (1.5, 2.2, and 3.0 L/min), the phase composition, crystallinity, and porosity of the resulting materials were effectively tuned. Powder X-ray diffraction, Raman spectroscopy, and EDX analysis revealed a phase evolution from Mn(OH) 2 and MnOOH at lower flow regimes to highly crystalline Mn 3 O 4 spinel at 3.0 L/min. SEM and BET analysis confirmed the formation of layered mesoporous structures with surface areas up to 120 m 2 /g. Electrochemical characterization in 1 M Na 2 SO 4 demonstrated a strong correlation between synthesis conditions and capacitive performance. The best-performing electrode (MR-3.0) exhibited a specific capacitance of 200 F/g at 5 A/g, low charge-transfer resistance, and ideal capacitive behavior. These enhancements are attributed to optimized ion transport and enhanced accessible surface area resulting from flow-assisted synthesis. Overall, the 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.