Biomedical Engineering Reference
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process. Self organized TiO
nanotube arrays with a controlled pore
size and tube length are feasible by an electrochemical anodization
method. The superior energy-storage performance of NiO on TiO
2
2
nanotubes has been mostly attributed to its highly accessible redox
reaction sites [57]. Brezesinski et al. have revealed the importance
of the combination of nanocrystallinity and mesoporosity to attain
greater power densities and energy densities in noncomposited
TiO
films. According to these authors polymer-directed assembly
of preformed nanocrystals into three-dimensional frameworks is an
alternative route to overcome the loss of periodicity and porosity
upon crystallization [58].
Recently, Telcordia Technologies has proposed nonaqueous
asymmetric hybrid supercapacitor as a new concept to provide high
energy densities to pseudocapacitors [US Patent no. 6,252,762.].
This device utilizes a nonaqueous electrolyte coupled with a Faradaic
lithium-intercalation anode. The stored charge usually comes from
the reversible adsorption/desorption reaction of anions on the
surface of the carbonaceous cathode material and the reversible
lithium insertion/extraction reaction in the anode material [59]. The
first asymmetric supercapacitors were based on Li
2
anodes
though others titanium oxides have been further researched as
TiO
Ti
O
4
5
12
-B nanowires [60] and TiO
nanotubes [61].
Anode in Li-ion batteries:
2
2
the interest on titanium oxides
based materials comes from the early nineties when several
reports
revealed
the
lithium
insertion
properties of spinel
related structures as LiTi
2
[62]. However, the
specific capacity is limited to 175 mAh g
O
4
and Li
Ti
O
12
4
5
is recently
regaining interest as anode for Li-ion batteries due to its higher
theoretical capacity (335 mAh g
-
1
. TiO
2
-
1
) when 1 Li is inserted per TiO
2
unit. Additional advantageous features are its high mechanical
stability upon continuous battery cycling, low cost and lack of
toxicity. Its potential versus the Li
/Li redox couple is about
1.75 V. It limits undesirable side reaction with the electrolyte and
avoids Li electroplating, inherent of graphite anodes and origin
of safety concerns. Nevertheless, the poor ionic and electronic of
TiO
+
polymorphs limit the electrochemical insertion of lithium.
Previous electrochemical studies have shown that the different
modifications of TiO
2
accommodate lithium in different ratios
2
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