“Twistron the charges away from the CNT

“Twistron harvester”- which is the
given name for this carbon nanotube (CNT) yarn twist, has the ability of transforming mechanical energy into electrical energy via
twisting and stretching motions. In comparison to piezoelectric and ferroelectric
devices which are generally limited to low-strain deformations, this elastic
harvester utilizes mechanical energy that can be easily obtained from our
surrounding. Thus, it is useful and effective for various usages, namely
generating electricity from tidal waves, self-powered sensors, and health
monitoring devices. Consisting of highly coiled multi walled carbon nanotubes
(MWNTs) which resemble yarns, after normalizing them to the harvester’s overall
weight, they are able to produce peak electrical power of 250 Watts per
kilogram when cyclically stretched up to 30 Hertz. As for each mechanical
cycle, the generated electrical energy attained as high as 41.2 Joules per
kilogram. Unfortunately, only low short circuit current can be produced despite
large stress and external bias voltage were applied. Currently its potential
application is restricted to externally driven strain sensor. Several MWNTs
yarn twisting-insertion methods were investigated and cone-spinning yarns were
found to generate approximately four times the peak and average power compared
to dual Archimedean yarns. This is attributed to the quasi-uniform tension that
is maintained along the CNT array in cone-spinning configuration. The working
principles behind its power generating property is by gathering the electric
charges closely within the harvester. As the volume of CNT yarn reduces upon
twisting or stretching, this actually causes the electric charges on the yarns
to become closer and increase their internal energy. This will increase the
voltage due to the accumulation of charge densely stored around the yarn, hence
making electricity harvesting possible. The internal friction induced when
stretched will release the charges away from the CNT and hence generating
electricity. In addition, since
the Young’s modulus of
individual MWNT is very high, a huge stress generation can be induced by a
relatively small applied strain, subsequently causing massive work densities
per cycle.

 

By using MWNTs, electrolyte ions can
penetrate into their adequately large pores, allowing charge to be stored
within them when immersed in electrolyte solution. To charge these harvesters, firstly they need to be either
submerged in or coated with an ionically conducting material. They work well
with electrolytes of various concentration. They perform like a supercapacitor
as the yarns will self-charged when immersed in an electrolyte without the need
of any external bias voltage. This is caused by the difference in chemical
potential between the electrode and electrolyte. Ionic charge will be injected
and stored into the harvester when immersed while electricity will be released
when elongated or twisted. However, the yarns should achieve elongated
configuration with sufficient applied tension, or else the sinusoidal
stretching will not give sinusoidal variation in open-circuit voltage (OCV) or
short-circuit current (SCC). When tested in 0.1M HCl as electrolyte by fully
stretching it, voltage peaked most sharply while the peak power surpassed the
average power by a factor of 3.34. While the exact mechanism behind this is yet
to be fully understood, the hysteric nature of twist insertion and removal is
found to be the contributing factors which affect the overall performance. By
slightly untwisting the coiled yarn, compressive force and densification will
decrease, leading to increase in capacitance. Depending on the kinds
application, coil index can be adjusted to allow energy collection in various
strain range. The adjustment of harvester’s coil index varies its stiffness and
affecting the reversible coil deformation. For all tests, the spring index was
fixed at 0.43.

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For evaluating the equilibrium charge
state when immersed in an electrolyte, the measurement of potential of zero
charge (PZC) was done via piezoelectrochemical spectroscopy (PECS) method.
Among all electrolytes tested, 0.1M HCl produced the largest intrinsic bias
voltage (0.4V) and largest yarn potential increment when stretched (150mV for
30% strain). Besides, conditions similar to sea water (0.6M NaCl) were tested
and it was found to exhibit promising prospect in obtaining energy from ocean
waves. The results showed that it is possible to harvest energy from ocean as
the value of PZC was below ±7mV from 3°C to 60°C, which resembles the typical
temperature range for ocean. It is also found that charge introduced by the
electrolyte is hugely unaffected by strain as the value of PZC was below ±5mV
by stretching the harvester by 20%. By varying the pH of electrolyte, the
intrinsic bias voltage drops as pH increases, thus showing that low pH
electrolyte is hole contributor and vice versa. However, the major drawback is
that electrical impedance actually restricts its output power. This is because
the capacitor impedance will reduce as frequency increases and internal
resistance starts to dominate. Eventually it will lead to a stagnation in power
output against frequency. It is then found that impedance is actually affected
by the yarn resistance itself. By introducing Platinum (Pt) wire coiled among
the yarns, peak power was found to be improved and it did not affect the
elastic behavior of the harvester. Furthermore, in order to further increase
the voltage, these harvesters can be connected in series for diverse
applications. 

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