Carbonate Formation during Nickel Hydroxide Precipitation Reduces Pseudocapacitive Performance

Nickel hydroxide (Ni(OH)2) is a promising pseudocapacitive material owing to its high theoretical capacitance and reversible Ni2+/Ni3+ redox activity. Here we demonstrate that carbonate incorporation during hydrothermal synthesis is the key structural factor limiting its electrochemical performance. Ni(OH)2 was prepared using hexamethylenetetramine (HMT) and urea at different synthesis temperatures, and carbonate incorporation was quantified by XRD, FTIR, Raman, TGA-MS, and CaCO3 precipitation. HMT-derived samples at a low temperature (80 °C) formed a turbostratic α-phase with interlayer water, delivering the highest specific capacitance (∼870 F g–1 at 1 A g–1) and excellent cycling stability (92–96% retention after 1000 cycles). In contrast, increasing the synthesis temperature promoted carbonate incorporation and crystallization into nickel carbonate hydroxide, reducing the interlayer spacing and surface area and increasing charge-transfer resistance. Urea-derived samples incorporated carbonate at all synthesis temperatures, yielding phases with capacitances an order of magnitude lower than those of HMT analogues. Electrochemical impedance spectroscopy confirmed that carbonate incorporation blocks redox-active sites and hinders ion/electron transport. These results provide a quantitative mechanistic understanding of how carbonate formation governs transition metal layered hydroxide performance, establishing guidelines for optimizing hydrothermal synthesis of pseudocapacitive electrodes.