Phase transformation of NiCo hydroxides derived from carbonate anion and its effect on electrochemical pseudocapacitor performance

Abstract Pseudocapacitors can exhibit higher capacity compared to traditional electrostatic or electric double–layer capacitors while they can provide faster power delivery and longer cycling ability than lithium–ion batteries. In this study, we demonstrate the hierarchical structures of NiCo hydroxides, which are grown on Ni foam via facile hydrothermal method and show enhanced pseudocapacitor performance. Moreover, we suggest the intriguing phase transformation of NiCo–layered double hydroxide (LDH) to NiCo–carbonate hydroxide (CH) and elucidate the morphological evolution mechanism from nanosheet to nanowire structures by increasing the concentration of carbonate anions during the synthesis procedure. The experimental results confirm that an increase of carbonate anions causes the phase and morphology of NiCo–LDH to transform into NiCo–CH. Consequently, the asymmetric device using NCCH16 electrode achieves excellent electrochemical performance, with an energy density of 31.1 Wh·kg-1 at a power density of 424.3 W·kg-1 due to their structural benefits, which provide effective electrolyte transport, charge transfer, and active site accessibility.