β-Ni(OH)₂ as the active phase for high rectification electrochemical ion diodes

Electrochemical ion diodes (CAPodes) integrate charge storage with directional ion transport, but the rectifying phase in nickel foam-based CAPodes remains unresolved. Here we show that rectification arises predominantly from the reversible OH^--coupled β-Ni(OH)₂/β-NiOOH redox transition rather than from metallic Ni itself. In situ Raman and ex situ XPS analyses reveal potential-dependent hydroxide/oxyhydroxide conversion on nickel foam, and the same rectification function can be reconstructed on an inert Ti substrate by electrodepositing β-Ni(OH)₂. The optimized electrode delivers a rectification ratio I (RRI) of 303.6 and a rectification ratio II (RRII) of 97.1%, outperforming commercial nickel foam, while the corresponding CAPode provides 123.7 F g^-1 at 1 A g^-1 and supports stable OR/AND logic outputs. The rectification behavior is further reproduced in β-Ni(OH)₂ powders prepared via co-precipitation, confirming that the rectification originates from the active phase rather than the substrate or synthesis route. These findings establish β-Ni(OH)₂ as the key electroactive phase in nickel-based CAPodes and provide a mechanism-guided strategy for designing redox-active ion diodes.