In situ deposited cobalt-magnesium selenates as an advanced electrode for electrochemical energy storage

Currently, bimetallic selenates have attracted much attention as a prominent electrode composite material for supercapacitors owing to their higher redox chemistry and superior electrical conductivity. Herein, we synthesized cobalt-magnesium selenates (CoSeO3−MgSeO4, CMS) via a facile hydrothermal process, followed by selenization. At first, cobalt-magnesium oxide (Co2.32Mg0.68O4, CMO) was in situ prepared by a one-pot hydrothermal method. An investigation on the morphological change was performed by synthesizing the same CMO samples at different growth times by keeping the temperature constant. The CMO electrode designed for 8 h of growth time (CMO-8 h) with an attractive morphology showed a higher areal capacity of 101.7 µAh cm−2 (at 3 mA cm−2) than the other CMO electrodes prepared for 6 and 10 h. Further exalted performance was achieved by the selenization of the CMO-8 h sample to form the CMS material. At 3 mA cm−2, the resulted CMS exhibited nearly three times higher capacity, i.e., 385.4 µAh cm−2, than the CMO-8 h electrode. Additionally, an asymmetric cell fabricated with CMS as a positive electrode also revealed good energy storage performance. Within the applied voltage between 0 and 1.5 V, the asymmetric cell demonstrated maximum energy density of 0.159 mWh cm−2 (18.6 Wh kg−1) and maximum power density of 18.47 mW cm−2 (1938 W kg−1), respectively. Thus, novel magnesium-based metal selenates can act as an efficient electrode for energy storage.