Electrosynthesis and Electrochemical Evaluation of Ni₂(CO₃)(OH)₂ as a Novel Bifunctional Material for Advanced Energy Storage

A scalable, membrane-assisted electrochemical synthesis strategy is reported for the controlled production of nickel carbonate hydroxide (Ni₂(CO₃)(OH)₂) using a three-compartment cell with selective ion transport across cation- and anion-exchange membranes under potentiostatic operation at 1.0 V. The catholyte and anolyte are isolated from the precipitation compartment using cation- and anion-exchange membranes, enabling selective ion transport while suppressing uncontrolled mixing and parasitic precipitation. The resulting product is structurally validated as a poorly crystalline monoclinic Ni₂(CO₃)(OH)₂ phase, with XRD reflections consistent with low crystallinity. Raman spectroscopy further confirms carbonate incorporation via the intense ν₁(CO₃²⁻) band at ∼1060 cm⁻¹ and supports the predominance of a poorly crystalline carbonate-hydroxide framework. FTIR spectra show diagnostic hydroxyl features and carbonate modes, corroborating the targeted carbonate-hydroxide chemistry. Thermogravimetric analysis exhibits a multi-step dehydration/decomposition profile consistent with conversion of nickel carbonate hydroxide to NiO. SEM reveals hierarchical, cauliflower-like sub-micrometer agglomerates with rough, porous surfaces, while EDS supports the expected elemental composition. To validate electrochemical quality, Ni₂(CO₃)(OH)₂ was used as the positive electrode in a Ni₂(CO₃)(OH)₂//activated carbon asymmetric supercapacitor employing 1 M TEATFB in acetonitrile. The device operates over a wide 0-2.5 V window and shows distinct pseudocapacitive behavior with excellent durability, retaining >80% of its initial capacity after 10,000 cycles at 5 A g⁻¹.