The study of activated carbon/CNT/MoO₃ electrodes for aqueous pseudo-capacitors

There is an emerging need to develop renewable energy, power sources and technologies for sustainable energy storage and conversion. Capacitive energy storage in the so-called supercapacitors is distinguished from other types of electrochemical energy stoage devices such as batteries by short charging times, the ability to deliver significantly higher power and to demonstrate very prolonged cycling (orders of magnitude better than rechargeable batteries). A key limitation to this technology is its low energy density and for that reason there is a considerable interest in exploring pseudo-capacitive electrodes' materials where faradaic mechanisms based on surface red-ox activity offer higher energy density than simple electrostatic interactions, yet with a very prolonged cycle life and high rate capability. Among the materials studied, one can find conductive polymers, redox functional groups and transition metals sulfides, nitrides, carbides and oxides. MoO₃ is a transition metal oxide which has several options for redox activities with protons and alkaline cations in aqueous solutions due to the multi-valence nature of molybdenum. In this work, we report on pseudo-capacitor electrodes, composed of monolithic, binder free, activated-carbon/carbon-nanotubes/molybdenumtrioxide (AC/CNT/MoO₃)matrices, demonstrating stable capacity up to 400 Fg^-1, within a potential window of 1V in acidic aqueous solutions, during at least 10,000 cycles. They also exhibit high and stable capacity (up to 250 Fg^-1) in alkaline solutions.