Electrochemical capacitors, commonly
known as supercapacitors,
are important energy storage devices with high power capabilities
and long cycle lives. Here we report the development and application
of in situ nuclear magnetic resonance (NMR) methodologies to study
changes at the electrode–electrolyte interface in working devices
as they charge and discharge. For a supercapacitor comprising activated
carbon electrodes and an organic electrolyte, NMR experiments carried
out at different charge states allow quantification of the number
of charge storing species and show that there are at least two distinct
charge storage regimes. At cell voltages below 0.75 V, electrolyte
anions are increasingly desorbed from the carbon micropores at the
negative electrode, while at the positive electrode there is little
change in the number of anions that are adsorbed as the voltage is
increased. However, above a cell voltage of 0.75 V, dramatic increases
in the amount of adsorbed anions in the positive electrode are observed
while anions continue to be desorbed at the negative electrode. NMR
experiments with simultaneous cyclic voltammetry show that supercapacitor
charging causes marked changes to the local environments of charge
storing species, with periodic changes of their chemical shift observed.
NMR calculations on a model carbon fragment show that the addition
and removal of electrons from a delocalized system should lead to
considerable increases in the nucleus-independent chemical shift of
nearby species, in agreement with our experimental observations.
Metal oxides with a tunnelled structure are attractive as charge storage materials for rechargeable batteries and supercapacitors, since the tunnels enable fast reversible insertion/extraction of charge carriers (for example, lithium ions). Common synthesis methods can introduce large cations such as potassium, barium and ammonium ions into the tunnels, but how these cations affect charge storage performance is not fully understood. Here, we report the role of tunnel cations in governing the electrochemical properties of electrode materials by focusing on potassium ions in α-MnO2. We show that the presence of cations inside 2 × 2 tunnels of manganese dioxide increases the electronic conductivity, and improves lithium ion diffusivity. In addition, transmission electron microscopy analysis indicates that the tunnels remain intact whether cations are present in the tunnels or not. Our systematic study shows that cation addition to α-MnO2 has a strong beneficial effect on the electrochemical performance of this material.
MXene for supercapacitor application which shows outstanding proton-induced pseudocapacitance in acidic aqueous electrolytes. [14,15] High electronic conductivity (up to 15 000 S cm −1), high packing density (up to 4 g cm −3), along with high pseudocapacitance endow Ti 3 C 2 T x ultrahigh volumetric capacitance (≈1500 F cm −3), which gives Ti 3 C 2 T x incomparable advantages over other electrode materials for supercapacitors. [16] However, Ti 3 C 2 T x electrodes suffer from long ion transport pathways due to the stacking nature of 2D materials, leading to ultra-low rate performance in a thick electrode. When used as a power supply for electronic devices where high areal energy densities at high rates are required, MXene electrodes need to be thick enough to ensure high charge storage capability. In this context, the ion transport issue becomes more critical because the low rate performance will deteriorate with the increase of film thickness. [17] Numerous efforts have been made to alleviate the restacking issue of Ti 3 C 2 T x film electrodes. Typical strategies include interlayer insertion of graphene, carbon nanotube or other nanomaterials, pillared structure design, template sacrifice method, vertical alignment, and etching holes. [17-27] However, by most of the reported approaches, the rate performances are increased at the expense of volumetric capacitance because of the introduction of inactive materials, excess spacing, or active materials with lower volumetric capacitance. For example, ≈15% decrease
Detailed investigation of the influence of surface modification using a typical oxide (TiO2) on the electrochemical cycling performance of LiNi0.5Mn1.5O4 at room temperature (25 °C) and elevated temperature (55 °C) is reported.
Summary
Metal‐organic frameworks (MOFs), as new class of porous materials, are constructed by inorganic metal centers and bridging organic links. Recently, MOFs have been proved to be effective templates for preparing metal oxides with large surface areas and controlled shape by directly annealing in air. There are lots of reports about metal‐organic framework‐derived metal oxides as electrode materials for supercapacitors. Metal‐organic framework‐derived metal oxides can offer higher capacitances compared with that prepared by other synthetic methods, likely attribute to high surface areas and optimal pore sizes. However, at present, the specific capacitances of MOF‐derived metal oxides received are far lower than theoretical values, and the cycle numbers could not meet practical demands. Accordingly, much effort has been made to improve the performance by further modifying MOFs. Thus, this paper focused on the advances in performance optimization of MOF‐derived metal oxide as electrode materials for supercapacitors as follows:
Dual metal MOF‐derived binary metal oxides. Metal oxides with 2 metal cations possess better electrical conductivity and richer redox active sites than single metal oxides.
Metal‐organic framework‐derived carbon/metal oxide composites (MO@C) or graphene/MOF‐derived graphene/metal oxide composites. Doping carbon not only facilitate transportation of electrodes but also contribution to extra double‐layer capacitance.
Hybrid MOF‐derived metal oxide composites (MO@MO). Metal oxide composites can produce some synergistic effects that the individuals cannot provide.
Metal‐organic framework‐derived metal oxides with a hollow structure. The Hollow structure could shorten ion diffusion distance and adapt to volume expansion generated during the ion intercalated/extracted process.
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