Controlled Conversion of Metal–Organic Frameworks to Ultrathin Phosphite Nanosheet Arrays for High-Performance Supercapacitors

Transition metal phosphites are attracting increased attention in the field of electrochemistry due to their open-framework structure, superior conductivity, and high stability. In this work, Ni₁₁(HPO₃)₈(OH)₆ (Ni-POH) nanosheet arrays were prepared from a hexamethylenetetramine solution containing varying amounts of Ni(NO₃)₂·6H₂O, employing plate-like phosphonate-based metal–organic frameworks (Ni-PMOF) as both the template and the phosphorus source. Also, bimetallic phosphites Ni/Co-POH-x could be obtained by replacing Ni(NO₃)₂·6H₂O with a mixture of Ni(NO₃)₂·6H₂O and Co(NO₃)₂·6H₂O in certain molar ratios, where x is the molar ratio of the added Ni/Co. Experiments revealed that among as-prepared phosphites, Ni/Co-POH-2 exhibited the most uniform nanosheet morphology, the biggest specific surface area, the highest structural stability, and the optimum electrochemical performance as a supercapacitor electrode material. At a current density of 1 A g^–1, the specific capacitance of Ni/Co-POH-2 reached 1905.60 F g^–1. Notably, the asymmetric supercapacitors assembled with activated carbon (Ni/Co-POH-2//AC) exhibited a high energy density of 36.59 Wh kg^–1 at 0.85 kW kg^–1. After 5000 cycles, the capacitance retention was 67.98%, showing excellent cycle stability. The present work provides a useful attempt for synthesizing MOF-derived phosphite electrode materials with a high electrochemical storage performance.