A primary atomic-scale effect accompanying Li-ion insertion into rechargeable battery electrodes is a significant intercalation-induced change of the unit cell volume of the crystalline material. This generates a variety of secondary multiscale dimensional changes and causes a deterioration in the energy storage performance stability. Although traditional in situ height-sensing techniques (atomic force microscopy or electrochemical dilatometry) are able to sense electrode thickness changes at a nanometre scale, they are much less informative concerning intercalation-induced changes of the porous electrode structure at a mesoscopic scale. Based on a electrochemical quartz-crystal microbalance with dissipation monitoring on multiple overtone orders, herein we introduce an in situ hydrodynamic spectroscopic method for porous electrode structure characterization. This new method will enable future developments and applications in the fields of battery and supercapacitor research, especially for diagnostics of viscoelastic properties of binders for composite electrodes and probing the micromechanical stability of their internal electrode porous structure and interfaces.
Employing single
molecules as electronic circuit building blocks
is one promising approach to electronic device miniaturization. We
report single-molecule junction formation where the orientation of
molecules can be controlled externally by the working electrode potential.
The scanning tunneling microscopy break junction (STM-BJ) method is
used to bridge tetrafluoroterephthalic acid (TFTPA) and terephthalic
acid (TPA) molecules between the Au(111) electrode and the STM tip
to measure the single-molecule conductance through the junction. When
the Au(111) electrode is at negative potentials (with respect to the
zero-charge potential), a highly ordered and flat-oriented superstructure
forms, allowing for direct contact between the π system of the
benzene ring of the molecules and the Au(111) electrode, leading to
junction formation with no anchoring group involvement. Our first-principles
nonequilibrium Green’s function (NEGF) computation shows a
flat configuration yields a conductance that is 3 orders of magnitude
larger than for a molecule vertically connected to the electrodes
via anchoring groups. Conductances of 0.24 ± 0.04 and 0.22 ±
0.02 G0 are experimentally measured with the flat configurations
of TFTPA and TPA, respectively. These values are at least 2 orders
of magnitude higher than the experimental values previously reported
for the conductance of TPA bridged through carboxylic acid anchoring
groups (3.8 × 10–4–3.2 × 10–3 G0). In contrast, a positively charged
surface triggers an order–disorder transition eliminating the
high-conductance states, most likely because the formation of the
flat-oriented junction is prevented. The dependence of TFTPA conductance
on the electrode potential (electrode Fermi level) suggests a LUMO
mediated transport mechanism. Calculation confirms the lack of an
effect of the addition of an electron-withdrawing group are investigated.
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