With the Internet of Things (IoT) becoming an increasingly large part of our day-to-day reality, there has been a massive increase in research into physical devices with network connectivity, ranging from wearable electronics to bio/chemical sensing.
Along with the increase in IoT technology comes the necessity to power these devices with an autonomous power source. Battery manufacturers have been struggling to produce a power source that not only functions at the micro-scale but has high power density and energy. Micro supercapacitors seem to be the new research flavor of the month when it comes to meeting these demands.
Electric batteries and capacitors do a relatively similar job: They store energy. However, they do it in completely different ways. Electric batteries make use of the chemical reactions between materials such as lithium and graphite, moving ions to generate power. In contrast, supercapacitors will use electrostatic energy between electrodes to do the same.
Supercapacitors have been a developing technology for decades now and are slowly approaching the typical values associated with batteries. At the same time, they are capable of reaching much greater power densities than batteries while being able to charge and discharge very quickly over the course of hundreds of thousands of cycles.
Comparison of energy storage techniques. Image via Wiki Commons.
Micro supercapacitors have been sought out for their potential in on-chip integration since 2010. The materials of interest have generally remained the same throughout this period, where research was mainly vested in carbon, TiC, and graphene.
Now researchers from the VTT Technical Research Centre of Finland have developed a new method for manufacturing supercapacitor energy inside a micro circuit, potentially realizing the independent power source battery manufacturers have been looking for.
Most supercapacitors we use today make use of carbon-based electrodes due to their substantial surface area and subsequent ability to store charge. Unfortunately, the fabrication of carbon-based electrodes typically includes a high-temperature process which convolutes its integration into silicon-based technologies; specifically those involving microfabrication. The solution the researchers from VTT had was to exploit the high surface-area-to-volume ratio of porous silicon and its compatibility with current microfabrication processes.
The problem that prior researchers faced with porous silicon was that the material has low chemical stability, narrow wettability, and large resistance values in its matrix which formed restraints on the power capabilities of the material. The VTT researchers were able to overcome these obstacles by coating the porous silicon matrix with titanium nitride using atomic layer deposition.
A series of images that shows the increase in wettability before and after the addition of TiN via ALD. Image courtesy of the VTT Technical Research Centre of Finland.
With the application of titanium nitride to the porous silicon matrix, the researchers were able to develop a new hybrid nano-electrode. The design boasts a paramount conductive surface-area-to-volume ratio, resulting in extremely efficient energy storage for a supercapacitor.
The resulting form factor also happened to be very small. The new design was able to further increase the energy and power density values to rival the current state-of-the-art carbon and graphene capacitors. The design reached an energy density of (1.3 mWhcm-3), power densities of (214 Wcm-3), and specific capacitance of (15 Fcm-3) compared to the current graphene on-chip supercapacitors which reach an energy density of (2 mWhcm-3) and a power density of (200 Wcm-3).
A Cross section of the test capacitor and circuitry
B Porous silicon layer cross section
C Porous silicon layer magnified, before and after TiN coating
D Illustration of how the TiN is added in cycles of Atomic Layer Deposition
The new design also happens to complement the current IoT technologies very well. The researchers were capable of mounting the supercapacitor on a 1mm silicon chip, while reserving enough space on the chip to also embed numerous devices and electronic components such as sensors and microcircuits.
VTT plans on continuing its research, particularly aimed at increasing the efficiency of the electrodes. The original research article, published in the journal Nano Energy, can be found here.