Monolayer V4C3Tz is synthesized and its atomic structure is studied using high-end transmission electron microscopy. X-Ray diffraction reveals key 3D to 2D crystal transformations as a V4AlC3 crystal is transformed into V4C3Tz in synthesis processes. The charge storage properties of V4C3Tz film electrodes are investigated for supercapacitor applications in 3 M H2SO4. V4C3Tz film electrodes shows an excellent capacitance of up to 469.6 F g(-1) and 845.7 F cm(-3), rate performance up to 30 A g(-1) and cycling stability up to 10,000 cycles. A combination of electrochemical kinetics/mass transport models, staircase potentio-electrochemical impedance spectroscopy and in situ X-Ray absorption spectroscopy reveals, for the first time for this MXene, the underlying charge storage mechanisms, consisting of double layer capacitance, pseudocapacitance and a minor contribution from mass transport-controlled processes. The latter two implying a outstanding redox activity superior to Ti-based MXenes. The stability in standard environments, mechanical flexibility and the demonstrated excellent charge storage performance of V4C3Tz makes it one of the best candidates for supercapacitor applications, especially in miniaturized devices.