The Ni-Mn hydroxide/carbon cryogel (20 to 80 wt%) composite electrodes were synthesized by freeze-drying to improve electrochemical properties of the Ni-Mn hydroxide. XRD results revealed that the composite electrode showed a distorted spinel-like structure similar to the Ni-Mn hydroxide plus the graphitized carbon. The crystallinity of carbon cryogel in composite electrode was improved dramatically with increasing the carbon cryogel content. The presence of the carbon cryogel played a significant role in inhibiting the crystal growth of Ni-Mn hydroxide on the surface of carbon cryogel. Electrochemical properties of the composite electrodes, such as capacitance, power density, and energy density, were observed to be always higher than those of the Ni-Mn electrodes. The highest specific capacitance of 221 F/g was observed for the electrodes containing 20 wt% of carbon cryogel at a scan rate of 5 mV/s and an electrode loading amount of 15 mg/cm² due to easy transport of the ions into pores. However, the highest energy density of 10.8 Wh/kg and power density of 927 W/kg were observed for the composite electrodes containing 80 wt% of carbon cryogel. Experimental results suggested that tunable electrochemical properties of the composite electrodes can be adjusted by varying the carbon cryogel content.

Keywords: Ni-Mn hydroxide, Carbon cryogel, Capacitance, Power density, Energy density, Supercapacitor