Supercritical Defect Engineering and Surface Modulation of Mixed‐Phase β‐Ni(OH) ₂ /α‐CoAl(OH) _m /NF Layered Double Hydroxides for High‐Performance Bifunctional Electrocatalysis in Energy Storage and Conversion

The development of layered double hydroxide (LDH) heterostructures as advanced electrode materials holds immense promise for next-generation energy storage and conversion technologies due to their potential for high specific capacitance and excellent electrocatalytic efficiency. In this study, a supercritical growth strategy is employed to achieve controlled phase formation and morphology optimization, resulting in a novel β-Ni(OH)₂/α-CoAl(OH)_m 2D nanosheet heterostructure directly grown on nickel foam (NF). X-ray diffraction (XRD) confirms the coexistence of β-Ni(OH)₂ and α-CoAl(OH)_m phases, with the expanded interlayer spacing of the α-phase contributing to enhanced electrochemical performance. The heterostructure delivers a high specific capacity of 1181 C g^-1 (2952 F g^-1) at 1 A g^-1 and an aerial capacitance of 1417 mC cm^-2 (3543 mF cm^-2) at 1 A cm^-2, maintaining 82% of its initial capacity after 10000 cycles. Electrocatalytic evaluation reveals efficient hydrogen and oxygen evolution reactions (HER/OER), requiring overpotentials of only 255 and 320 mV to reach 10 and 20 mA cm^-2, respectively, with corresponding Tafel slopes of 111 mV dec^-1 (HER) and 118 mV dec^-1 (OER). A high turnover frequency of 69.94 s^-1 at 1.65 V versus RHE is observed nearly threefold higher than the β-NiCoAl(OH)_m/NF. Moreover, the catalyst demonstrates bifunctionality in a water electrolyzers, achieving 10 mA cm^-2 at a low cell voltage of 1.69 V. This work highlights a scalable, low-cost, and binder-free route for fabricating high-performance bifunctional electrodes using earth-abundant metals. Despite these advantages, the relatively moderate Tafel slopes and the absence of long-term evaluation under industrial-scale conditions remain limitations. Further in situ studies and compatibility with flexible or integrated energy systems are essential for advancing practical deployment.