Scientists from University of Picardy Jules Verne in France made an important step forward in developing miniature energy storage systems. They succeeded in creating electrodes from quantum dots of molybdenite (MoS₂), which demonstrated very high performance within micro-supercapacitors – small-size energy storage devices operating in IoT sensors, portable electronics and medical implants.
Until recently, relatively low energy density remained the main limitation for micro-supercapacitors (valued for their quick recharging and high cyclic resource); i.e., the volume of energy the device is capable of accumulating given the minimal mass. The researchers assumed: this problem could be solved in case of combining high electrical conductivity and very big active surface accessible for electrolyte ions in one material. They chose molybdenite for such material used not in its usual stratiform, but in the form of quantum dots – nano particles with just a few nanometers in size. Given such scale, the quantum-size effects are manifested radically changing and improving electrochemical behavior of the material.
Not only the material itself is critical, but the method of its receiving as well. The team applied the electrochemical deposition pulse method, when the voltage is supplied to the base in short pulses. Such mode allows for accurate separation of the stages of nanoparticles conception and growth and for forming a homogenous dense layer of quantum dots on the electrode surface. At the same time, the researchers deliberately controlled the internal structure of the material. They were able to achieve the co-existence of two MoS₂ crystalline phases within the same nano particles – metallic 1T-phase and semi-conductor 2H-phase. The first one provides for electric conductivity and reduces internal losses, and the second one – for structural stability and forming a big number of active centers, where the charge is accumulated. This combination was the key factor of high performance.
The tests showed: the optimized electrode based on such quantum dots reaches 741 farad per gram capacity in sulfuric acid electrolyte significantly exceeding the values of the majority of previously known MoS₂-based electrodes. At the same time, the material turned out to be strongly resistant to degradation: after 35 thousand charge/discharge cycles it maintains 89.6% of initial capacity evidencing high resource and reliability.
At the next stage, the scientists assembled a symmetrical micro-supercapacitor from two similar electrodes – a full-featured miniature device. Its parameters turned out to be just as impressive: 272 f/g capacitance, 112 W·h/kg energy density, and – 3,712 W/kg power density. Even after 20 thousand cycles, the appliance maintained 77% of initial capacity. From the standpoint of cumulative performance, this development is not only highly competitive to the best world standards, but is compatible or even exceeding the state-of-the-art systems based on MXens and graphene (reference materials for small-size energy storage devices), from the standpoint of certain characteristics.
