Since the tech boom began decades ago, we’ve seen a dramatic transformation of electronics. Today, some technological dreamers are talking about “smart environments” where electronics are seamlessly integrated into our environment, providing comfort and convenience.

For these dreams to be achieved, we need to get electronics—not just the chips—miniaturized to the point where sensors can be pervasive. This involves developing high-performance electrochemical storage devices to enable long-lived sensors and radio frequency identification (RFID) tags. But efficient miniaturized energy storage devices have proven to be challenging to create; it can be done, but it's hard to integrate the results with other electronics.

Supercapacitors

According to an article in Science, an international team of scientists has now reported some progress in this area—specifically with the design of micro-supercapacitors. Supercapacitors are a class of materials that can store energy through accumulation of charge at the surface of a high-surface-area carbon sheet. They typically have a good cycle life, moderate energy density (6 Wh/kg), and high power densities (> 10 kW/kg). Supercapacitors are a great replacement for batteries in applications that require high power delivery and uptake with a very long charge-discharge cycle life; micro-supercapacitors are the same kind of material but much, much smaller.

Though a variety of materials have been explored for development of micro-supercapacitors, the processing techniques involved are not fully compatible with semiconductor device manufacturing processes. This means that you need separate hardware for energy storage, complicating any devices.

Out with the old, in with the new

Scientists have overcome this obstacle through development of a new wafer-scale process that enables manufacturing of micro-supercapacitors right on the chip. In this new process, titanium carbide (TiC) coatings are deposited onto a silicon dioxide (SiO 2 ) coated silicon wafer, producing carbon films. Through alteration of the deposition parameters, the resistivity, thickness, and mechanical strength of the TiC could be controlled. The team fabricated films up to 20 µm thick with controlled roughness.

The films were then treated with chlorine at 450 degrees Celsius in a furnace. This chlorination step transformed the films into carbide-derived carbon layers while releasing titanium tetrachloride (TiCl 4 ). The process was fast—carbon layers grew at a rate of 1 µm/min. A cross-section of one of these structures revealed a 5µm-thick carbon layer covering a 1.3 µm-thick film of metallic, conducting titanium carbide.

This process is advantageous because this transformation of TiC into porous carbon was achieved without any of the materials detaching from their interfaces (TiC/SiO 2 or carbon/TiC). As a result, the TiC/SiO 2 interface remains identical after the chlorination step. The remaining TiC layer functions as an adhesive that is able to absorb mechanical stresses. It can also be used as a current collector.

Characterization

Imaging of the films showed the presence of a uniform, porous structure with evenly distributed pore sizes. The carbon/TiC interface was rugged, with TiC needles penetrating into the carbon layer. This penetration allows for excellent adhesion. The surface roughness and porosity was estimated to be less than 1nm.

The electrochemical properties of the TiC/carbon films on Si wafer were also explored. When the samples were characterized in a sulfuric acid electrolyte, the capacitance per volume (volumetric) and per area (areal) were 410 F/cm3 and 205 mF/cm2, respectively. The electrode capacitance was found to be stable over 10,000 cycles. During a 0.9 s discharge (1 V/s), more than 200 F/cm3 were delivered. This system outperforms current supercapacitors and micro-supercapacitors.

The system doesn't need to be bathed in acid to work, though. Si/TiC/carbon film was annealed at 600 degrees Celsius in hydrogen prior to testing the electrode in an acetonitrile electrolyte. (This annealing step helps improve the capacitance by opening the carbon pores and improving accessibility to large cations.) The volumetric capacitance dropped, but still reached 170 F/cm3.

Manufactured devices

The team manufactured two-electrode micro-supercapacitor devices. They also demonstrated that the process could be scaled-up by manufacturing a Si wafer containing 40 patterned micro-supercapacitors. When they characterized these devices, they found that the electrochemical properties were maintained during manufacturing. For a device with a 2.1 µm layer of patterned TiC and a 1.4 µm carbon layer, the electrode volumetric capacitance reached 350 F/cm3 when tested in 1 M H 2 SO 4 electrolyte.

These materials differ from other cracked or low-strength carbon materials, which often exhibit delamination during cycling. When they looked at the mechanical properties, they found that the new coatings were stronger, exhibiting a hardness of 1.6 GPa and a Young’s modulus of 14.5 GPa. After the mechanical tests were performed, the team analyzed the fracture patterns. They found that the fractures propagated the entire way through the carbon down into the TiC layer. This finding suggests that both layers have similar mechanical properties. It also confirms the excellent adhesion exhibited by the TiC/carbon interface.

Self-supported carbon films were also prepared by increasing the chlorination time until the carbon film separates from the Si wafers. For these films, a high capacitance was also obtained (180 mF/cm2). An advantage of the freestanding carbon films is that they can be transferred to flexible substrates.

The team checked the electrochemical performance of an 8 µm-thick carbon film that had been transferred onto a common flexible, plastic substrate of polyethylene terephthalate. The capacitance reached 240 mF/cm2 (300 F/cm3) in H 2 SO 4 electrolyte. Freestanding patterned carbon microelectrodes could also be fabricated through full chlorination of the patterned TiC. This strategy provides a promising avenue for further development of high-performance, stand-alone, flexible micro-supercapacitors.

Energy storage devices like these could help drive the next generation of flexible and wearable electronics, pushing us even further beyond the futuristic “smart environment” to a reality where we can live, breathe, and walk alongside our devices.

Science, 2016. DOI: 10.1126/science.aad3345 (About DOIs).