A Simple Electrochemical Route to Access Amorphous Mixed-Metal Hydroxides for Supercapacitor Electrode Materials

Supercapacitors or electrochemical capacitors, as energy storage devices, require very stable positive electrode materials for useful applications. Although most positive electrodes are based on crystalline mixed-metal hydroxides, their pseudocapacitors usually perform poorly or have a short cycle life. High activities can be achieved with amorphous phases. Methods to produce amorphous materials are also not typically amenable towards mixed-metal compositions. It is demonstrated that electrochemistry in an ambient environment can be used to produce a series of amorphous mixed-metal hydroxides with a homogeneous distribution of metals for use as positive electrode materials in a supercapacitor. The integrated performance of the amorphous ternary mixed-metal hydroxide pseudocapacitor is superior to that of crystalline materials. The amorphous Ni-Co-Fe hydroxide supercapacitor is characterized by a long-term cycling stability that retained 94% of its capacity after 20 000 cycles. This is much higher than the cycle life of crystalline devices. These results show the broad applicability of this methodology towards new electrode materials for high-performance supercapacitors, especially amorphous mixed-metal hydroxides, as advanced electrode materials.