Although manganese oxide- and graphene-based supercapacitors have been widely studied, their charge storage mechanisms are not yet fully investigated. In this work, we have studied the charge storage mechanisms of K-birnassite MnO2 nanosheets and N-doped reduced graphene oxide aerogel (N-rGOae) using an in situ X-ray absorption spectroscopy (XAS) and an electrochemical quart crystal microbalance (EQCM). The oxidation number of Mn at the MnO2 electrode is +3.01 at 0 V vs. SCE for the charging process and gets oxidized to +3.12 at +0.8 V vs. SCE and then reduced back to +3.01 at 0 V vs. SCE for the discharging process. The mass change of solvated ions, inserted to the layers of MnO2 during the charging process is 7.4 μg cm−2. Whilst, the mass change of the solvated ions at the N-rGOae electrode is 8.4 μg cm−2. An asymmetric supercapacitor of MnO2//N-rGOae (CR2016) provides a maximum specific capacitance of ca. 467 F g−1 at 1 A g−1, a maximum specific power of 39 kW kg−1 and a specific energy of 40 Wh kg−1 with a wide working potential of 1.6 V and 93.2% capacity retention after 7,500 cycles. The MnO2//N-rGOae supercapacitor may be practically used in high power and energy applications.

The progression from quasi zero-dimensional (Q0D) nanoclusters to quasi one-dimensional (Q1D) nanorods, and, with increasing length, to nanowires, represents the most conceptually fundamental transition from the nanoscale to bulk-like length scales. This dimensionality crossover is particularly interesting, both scientifically and technologically, for inorganic semiconducting (ISC) materials, where striking concomitant changes in optoelectronic properties occur.(1,2) Such effects are most pronounced for ultra-thin(3) ISC nanorods/nanowires, where the confining and defective nature of the atomic structure become key. Although experiments on ISC materials in this size regime have revealed especially stable (or "magic") non-bulk-like Q0D nanoclusters,(4,5) all ISC Q1D nanostructures have been reported as having structures corresponding to bulk crystalline phases. For two important ISC materials (CdS and CdSe) we track the Q0D-to-Q1D transition employing state-of-the-art electronic structure calculations demonstrating an unexpected persistence of magic cluster stability over the bulk-like structure in ultra-thin nanorods up to >10 nm in length. The transition between the magic-cluster-based and wurtzite nanorods is found to be accompanied by a large change in aspect ratio thus potentially providing a route to nano-mechanical transducer applications.

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