Electrochemical properties of Li+ ion insertion in
nanoporous TiO2 (anatase) electrodes were studied
by
voltammetry. Linear and cyclic potential scans were recorded as a
function of electrolyte concentration, film
thickness, and temperature. The currents were directly
proportional to the inner electrode area of the
electrodes.
The reduction of Ti4+ and oxidation of
Ti3+ are sluggish and follows irreversible kinetics.
The standard rate
constant was (3.5 ± 0.5) × 10-10 cm/s.
The transfer coefficient was close to 0.5, indicating that the
potential
drop appears mainly across the Helmholtz layer. The capacitive
currents govern largely the shape of the
i
−
v
curves, except within a region near the peak potential where
diffusion-limited insertion and extraction of
Li+
ions in the anatase lattice are dominating. The diffusion
coefficient at 25 °C in the nanoporous structure was
approximately 2 × 10-17 cm2/s
for insertion and 4 × 10-17
cm2/s for extraction. The activation energy
was
0.4 eV for insertion and 0.5 eV for extraction. The maximum
obtained mole fraction of Li+ in
Li
x
TiO2 was
x = 0.47.
Laser flash induced photocurrent transient measurements have been
used to investigate the electron transport
in nanostructured TiO2 (anatase) thin film electrodes in
contact with an electrolyte. The shape and the time
domain for the current transients were found to be dependent on film
thickness and electrolyte conductivity.
The experimental results are discussed using a diffusion model.
If the experimental results are interpreted as
diffusion, a chemical diffusion coefficient for the electrons in the
nanostructured system is determined to 1.5
× 10-5 cm2/s using 700 mM
LiClO4 in ethanol as electrolyte.
Photoelectrochemical properties of nanostructured ZnO thin film
electrodes have been investigated in the
UV and visible regions. For films consisting of 15 nm large
undoped crystallites a maximum monochromatic
current conversion efficiency of 58% was obtained in the visible using
a ruthenium-based dye as a sensitizer.
The overall solar energy conversion efficiency for this film was
2%. In comparison, sensitized films consisting
of 150 nm large Al-doped crystallites yield a monochromatic current
conversion efficiency of 31% and an
overall solar energy conversion efficiency of 0.5%. The study
also shows that nanostructured ZnO may give
high efficiencies in the UV region, approaching unity for the Al-doped
films.
Electrochemical insertion of lithium in nanoporous and CVD samples
of TiO2 (anatase) was studied by
chronoamperometry. The currents following cathodic and anodic
potential steps were monitored as a function
of film thickness, temperature, and electrolyte concentration. The
time dependence of the currents generally
exhibit the behavior of a diffusion-limited process. It is
demonstrated that the magnitude of the currents
scales directly with the inner area of the electrodes. The
potential dependence on the rate of insertion and
extraction indicates that the reduction of Ti4+ and
oxidation of Ti3+ is kinetically hindered. The
double-layer
capacitance and the adsorbate concentration at 0 V of the nanoporous
structure were determined to be 30−40
μF/cm2 and 1.7−2.4 mol/cm2, respectively.
The chemical diffusion coefficient at 25 °C for insertion
and
extraction in the nanoporous structure was 1 × 10-17
and 4 × 10-17 cm2/s, respectively. The
corresponding
values for the CVD samples, using the projected area, were 2 ×
10-15 cm2/s for insertion and 6 ×
10-15
cm2/s for extraction. The activation energy for
lithium insertion and extraction was 0.35 and 0.38 eV for
the
nanoporous films and 0.54 and 0.78 eV for the CVD samples.
The charge transport in dye-sensitized nanostructured TiO 2 was studied by laser pulse induced photocurrent transients. The experimental curves were compared to simulations using a diffusion model with an initial electron distribution of an exponential decay. The simulations were optimized with respect to the experimental curves giving an apparent diffusion coefficient of 6 × 10 -6 cm 2 /s for the electrons with an electrolyte of 0.1 M KI in propylene carbonate, the potential being +300 mV vs Ag/AgCl in ethanol. The charge transport was highly dependent on electrolyte composition and light intensity.
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