Hexagonal Co- and Ni-Based Hydroxide Nanoplates and Their Topotactic Conversion into Spinel Oxides for Nitrite Electroreduction

Transition-metal hydroxides and spinel nickel cobaltite (NiCo2O4) nanomaterials are promising for supercapacitors, batteries, fuel cells, electrochemical sensing, and electrocatalysis. Because performance and stability are strongly governed by size and morphology, numerous studies target nanostructure control. Ideally, synthesis should combine (i) operational simplicity, (ii) precise control of shape and size, and (iii) high performance. Here, we report a simple, surfactant-free hydrothermal route that yields highly crystalline, ultrafine hexagonal nanoplates of β-Co(OH)2, β-Ni(OH)2, and mixed (Ni,Co)(OH)2 using only alkaline conditions (no stabilizers or complex reagents). The Ni:Co ratio tunes morphology, size, and interplanar (d-)spacings, while mixed compositions exhibit the characteristic intermediate between the single-component hydroxides. Upon annealing, (Ni,Co)(OH)2 converts topotactically to NiCo2O4, and β-Co(OH)2 converts to Co3O4 while preserving the hexagonal nanoplate morphology and high crystallinity. All products display high morphological uniformity and narrow size distributions, demonstrating the robust control afforded by this minimalist synthesis. As a proof of concept, NiCo2O4 and Co3O4 nanoplates were evaluated as electrocatalysts for the nitrite reduction reaction, achieving Faradaic efficiencies of 10.0 and 92.5%, respectively, highlighting Co3O4 as a particularly effective catalyst for environmentally and energy-relevant applications.