Cobalt hydroxide is a promising electrode material for supercapacitors due to the high capacitance and long cyclability. However, the energy storage/conversion mechanism of cobalt hydroxide is still vague at the atomic level. Here we shed light on how cobalt hydroxide functions as a supercapacitor electrode at operando conditions. We find that the high specific capacitance and long cycling life of cobalt hydroxide involve a complete modification of the electrode morphology, which is usually believed to be unfavourable but in fact has little influence on the performance. The conversion during the charge/discharge process is free of any massive structural evolution, but with some tiny shuffling or adjustments of atom/ion species. The results not only unravel that the potential of supercapacitors could heavily rely on the underlying structural similarities of switching phases but also pave the way for future material design for supercapacitors, batteries and hybrid devices.

Voltage-biased solid-state nanopores are well established in their ability to detect and characterize single polymers, such as DNA, in electrolytes. The addition of a pressure gradient across the nanopore yields a second molecular driving force that provides new freedom for studying molecules in nanopores. In this work, we show that opposing pressure and voltage bias enables nanopores to detect and resolve very short DNA molecules, as well as to detect near-neutral polymers.

fluorinated solvent and additives. [5] Assisted with solid electrolyte interface (SEI)-forming additives allows for a stable cycling performance with a high Coulombic efficiency, as the negative electrode is protected from co-intercalation reactions of relatively large pyrrolidinium cations. [6] Nevertheless, slow electrode kinetics relating to Li intercalation/de-intercalation and notorious safety issue have been becoming the very bottleneck in improving power densities of these dual-ion batteries while achieving high energy density.To address the problem of low power density for dual-ion batteries, the electrochemical intercalation must be replaced, at least for one electrode, with means of physical adsorption. Accordingly, the electrodes must be abundant with a high specific area and excellent mechanical properties, e.g., graphene and few-layer graphite. [7] Without considering the desolvation/ dendrite issues for Li ions during the fast charge/discharge, the cations in electrolyte absorbing/desorbing onto a graphene surface can supply a high power density. Herein, we propose a dual-ion battery by selecting an electrolyte of (EMIm) + (PF 6 ) − ionic liquid using graphite as positive and reduced graphene oxide (RGO) as negative electrodes. The power density yields as high as 1333 W kg −1 , by simultaneously maintaining a competitive energy density of 70 Wh kg −1 (see Figure 1). The manifested harvesting of optimum performance excels as integration of an intercalation/de-intercalation mechanism and a fast surface absorption/desorption mechanism in a cell. Figure 2A shows the cyclic voltammetry (CV) curve of the graphite electrode at a scan rate of 5 mV s −1 . Although metallic lithium is usually used as the reference electrode or symmetry system without reference electrode, [8] herein the influence of interaction between Li and working electrode was excluded, as well as the complex reactions between two working electrodes. [5] This configuration thereof guarantees liable electrochemical characterization. As indicated by the arrows, a broad peak (1.6-2.0 V) and a small peak (1.46-1.6 V) appear with the potential increasing, unraveling a PF 6 − insertion process. Whereas the potential shifts negatively, two broad and small peaks ranging from 2.0 to 0.8 V were observed, revealing the de-insertion of anions from the graphite electrode. The occurrence of these redox peaks for ion uptaking and releasing in the CV curve describes a stage formation mechanism, in which multiple coordination possibilities exist for the anion in the graphite. These staging effects are well accepted for electrochemical lithium insertion into graphite and for TFSI − into graphitic carbons. [9] It is noted that the peak potential range of the charge process is narrower than that of the discharge process. The main peak in the charge process is consistent with a large current, whereas the curve of the discharge process consists For an electrochemical energy storage cell, it is challenging to synergistically harvest high energy density and hig...

Pseudocapacitance is commonly associated to the reversible redox reactions from electrode materials, but the enhancement in pseudocapacitance that only relies on electrode materials is limited. Here, we explore the possibility of enhancing pseudocapacitance through both Co(OH)2/graphene nanosheets (GNS) electrode and K3Fe(CN)6 electrolyte. With a good conductivity and favoring electron transfer, GNS are hybridized with Co(OH)2 to improve the pseudocapacitance of Co(OH)2, including enhancing its rate capability and electrochemical stability. Adding K3Fe(CN)6 into KOH electrolyte further enhances the pseudocapacitance via both directly contributing pseudocapacitance to Co(OH)2/GNS and promoting the electron gain and loss of Co ions. This novel Co(OH)2/GNS-K3Fe(CN)6/KOH electrode system shows an ultrahigh specific capacitance of 7514 Fg−1 at 16 Ag−1 in mixed 1 M KOH and 0.08 M K3Fe(CN)6, more than 100% coulombic efficiency, and long-term cycling stability (the capacitance retention is 75% after 20000 continuous charge-discharge cycles in mixed 1 M KOH and 0.04 M K3Fe(CN)6).

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