Citric acid enables morphological optimization of Ni3(PO4)2 next-generation high-performance hybrid supercapacitors

Abstract

Nickel phosphate (Ni₃(PO₄)₂) exhibiting a cabbage-like hierarchical architecture was effectively synthesized by a hydrothermal process that was carefully treated with citric acid as a surface-regulating agent. The unblemished nickel phosphate (NP) electrode demonstrated a specific capacitance of 1090.1 F g⁻¹ in a three-electrode setup. Citric acid processing resulted in a significant improvement in electrochemical efficiency, yielding specific capacitances of approximately 791.4 and 2034.8 F g⁻¹ for NP treated with 0.3 g of citric acid (NPCA0.3) and 0.5 g of citric acid (NPCA0.5), respectively. This notable enhancement is ascribed to citric acid-induced surface modification, which augmented electronic mobility, improved electrolyte accessibility, and increased electrochemically active sites. Compared to prepared samples, NPCA0.5 exhibited exceptional charge storage capabilities and was hence chosen for device manufacture. The constructed asymmetric supercapacitor device demonstrated a remarkable specific capacitance of around 228.1 F g⁻¹, a substantial energy density of 81.1 Wh kg⁻¹ at a power density of 1146.1 W kg⁻¹, and exceptional cycling stability with 92.3% capacitance retention after 6 K cycles. The results underscore citric acid surface modification as an effective and scalable approach to markedly improve the electrochemical performance of phosphate-based electrode materials for high-efficiency advanced energy storage applications.