Phase engineering of cobalt hydroxides using magnetic fields for enhanced supercapacitor performance

We report on the selective synthesis of single crystal sheet-like α-Co(OH)2 and raft-like nanostructures of β-Co(OH)2 hydrothermally using hydrazine as a reducing agent. α-Co(OH)2 sheets of several micrometers in width and about 20 nm in thickness were reproducibly yielded in the presence of an applied strong magnetic field (2 tesla) at 180 °C. In the absence of a magnetic field, raft-like nanostructures were obtained, composed of rod-like β-Co(OH)2 with a length of about 1.5 μm. The applied magnetic field was believed to provide a phase transition driving force and promote the phase transition from β-Co(OH)2 to α-Co(OH)2. When evaluated as an electrode material for supercapacitors, the as-obtained sheet-like α-Co(OH)2 exhibited higher capacitance and better cycling stability than β-Co(OH)2 prepared in the absence of a magnetic field.