Magnetic field-enhanced crystalline-amorphous hybrid nickel-cobalt hydroxide nanotubes for high-energy and 20,000-cycle stability in supercapacitors: mechanistic insights and performance enhancement

The quest for high-performance supercapacitors with enhanced energy density and cycling stability poses a significant challenge in sustainable energy storage. In this study, we engineered a hybrid material combining amorphous domains to facilitate rapid ion diffusion with crystalline phases of CoNiO₂ and Ni(OH)₂ to enhance electronic conductivity. Remarkably, when exposed to magnetic fields, it demonstrated a 23.9 % capacity increase (from 709.3 to 879.1 C g^-1), attributed to magnetohydrodynamic effects that enhance OH^- ion transport and reduce charge recombination. At 15 A g^-1, the device retained 81.9 % of its capacity at 1 A g^-1. Magnetic fields were found to lower charge-transfer resistance (from 0.743 to 0.481 Ω at 100 mT) and promote diffusion-controlled contributions via Lorentz force-driven ion convection. In asymmetric supercapacitor configurations, the device achieved 41.00 Wh kg^-1 at 937.4 W kg^-1 without a magnetic field. At 200 mT, it delivered 44.38 Wh kg^-1 at 926.7 W kg^-1, with 96.2 % capacity retention after 20,000 cycles, demonstrating an enhanced energy storage performance. This work demonstrates a novel strategy for leveraging magnetic fields to address the conductivity-diffusivity trade-off in transition metal hydroxides, providing a universal strategy for developing high-energy storage systems in electromagnetically active environments.