Researchers at the University of Texas at Austin in the US and the University of Science and Technology of China have developed a new graphene analogue called vanadyl phosphate, VOPO4. The 2D nanosheet can be combined with graphene itself to make a novel electrochemical electrode for use in high-performance, flexible and ultrathin-film solid-state pseudocapacitors.
Indeed, a prototype capacitor made from the hybrid electrode has a capacitance of as high as 8.4 mF/cm2, which leads to energy and power densities of 1.7 mWh/cm2 and 5.2 mW/cm2. These are the highest values ever achieved for such a device says team leader Guihua Yu.
2D nanomaterials could be ideal for making flexible, ultrathin-film supercapacitors. The new α1-vanadyl phosphate nanosheet, developed Yu and colleagues is less than six atoms thick and has a high electrochemical working window of around 1.0 V in aqueous solution.
Yu’s team began by dispersing bulk VOPO4 in the solvent 2-propoanol. The researchers then mixed the two components together in an ultrasonic ice water bath for 15 minutes to form ultrathin VOPO4 nanosheets. After filtering, the nanosheets can easily be transferred onto a variety of substrates.
High specific capacitance and redox voltage, and a long cycle life
“We then made a flexible ultrathin pseudocapacitor on a flexible gold-coated PET substrate using the VOPO4/graphene hybrid as the working electrode,” explains Yu. “The capacitor that integrated VOPO4 in graphene sheets, in which electrons whizz through very fast, has a high specific capacitance of up to 8.4 mF/cm2, a high redox voltage of around 1 V and a long cycle life of more than 2000 cycles. The electrode is also very flexible and can withstand hundreds of folding/unfolding cycles and bending/unbending without any noticeable degradation in performance,” he told nanotechweb.org.
All these properties make for flexible ultrathin-film pseudocapacitors (a special type of electrochemical capacitor that is based on fast and reversible redox reactions at the surface of electroactive materials) with record energy and power densities of 1.7 mWh/cm2 and 5.2 mW/cm2.
The devices could be ideal for use in compact and efficient energy storage devices for portable electronics, as well as in integrated power sources for flexible and stretchable electronics, added Yu.
The team says that it is now busy developing other novel 2D materials with strong pseudocapacitive behaviour and trying to understand their charge storage properties.
The present work is detailed in Nature Communications doi:10.1038/ncomms3431.