The working potential of symmetric supercapacitors is not so wide because one type of material used for the supercapacitor electrodes prefers either positive or negative charge to both charges. To address this problem, a novel asymmetrical supercapacitor (ASC) of battery-type MnCoO nanofibers (NFs)//N-doped reduced graphene oxide aerogel (N-rGO) was fabricated in this work. The MnCoO NFs at the positive electrode store the negative charges, i.e., solvated OH, while the N-rGO at the negative electrode stores the positive charges, i.e., solvated K. An as-fabricated aqueous-based MnCoO//N-rGO ASC device can provide a wide operating potential of 1.8 V and high energy density and power density at 54 W h kg and 9851 W kg, respectively, with 85.2% capacity retention over 3000 cycles. To understand the charge storage reaction mechanism of the MnCoO, the synchrotron-based X-ray absorption spectroscopy (XAS) technique was also used to determine the oxidation states of Co and Mn at the MnCoO electrode after being electrochemically tested. The oxidation number of Co is oxidized from +2.76 to +2.85 after charging and reduced back to +2.75 after discharging. On the other hand, the oxidation state of Mn is reduced from +3.62 to +3.44 after charging and oxidized to +3.58 after discharging. Understanding in the oxidation states of Co and Mn at the MnCoO electrode here leads to the awareness of the uncertain charge storage mechanism of the spinel-type oxide materials. High-performance ASC here in this work may be practically used in high-power applications.

No abstract

No abstract

Membrane-based applications such as osmotic power generation, desalination and molecular separation would benefit from decreasing water friction in nanoscale channels. However, mechanisms that allow fast water flows are not fully understood yet. Here we report angstrom-scale capillaries made from atomically flat crystals and study the effect of confining walls’ material on water friction. A massive difference is observed between channels made from isostructural graphite and hexagonal boron nitride, which is attributed to different electrostatic and chemical interactions at the solid-liquid interface. Using precision microgravimetry and ion streaming measurements, we evaluate the slip length, a measure of water friction, and investigate its possible links with electrical conductivity, wettability, surface charge and polarity of the confining walls. We also show that water friction can be controlled using hybrid capillaries with different slip lengths at opposing walls. The reported advances extend nanofluidics’ toolkit for designing smart membranes and mimicking manifold machinery of biological channels.

No abstract

Carbon materials are ubiquitous in energy storage; however, many of the fundamental electrochemical properties of carbons are still not fully understood. In this work, we studied the capacitance of highly ordered pyrolytic graphite (HOPG), with the aim of investigating specific ion effects seen in the capacitance of the basal plane and edge-oriented planes of the material. A series of alkali metal cations, from Li+, Na+, K+, Rb+, and Cs+ with chloride as the counterion, were used at a fixed electrolyte concentration. The basal plane capacitance at a fixed potential relative to the potential of zero charge was found to increase from 4.72 to 9.39 μF cm–2 proceeding down Group 1. In contrast, the edge-orientated samples display capacitance ca. 100 times higher than those of the basal plane, attributed to pseudocapacitance processes associated with the presence of oxygen groups and largely independent of cation identity. This work improves understanding of capacitive properties of carbonaceous materials, leading to their continued development for use in energy storage.

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 stability of water-in-salt electrolyte systems is investigated using highly concentrated solutions of KF(aq) with graphite as a model system.