Power/Performance Bits: July 7

Direct solar energy storage; more developments in wooden chips; leaves inspire micro-supercapacitors.


Direct solar energy storage

Storing solar energy as hydrogen is a promising way for developing comprehensive renewable energy systems. To accomplish this, traditional solar panels can be used to generate an electrical current that splits water molecules into oxygen and hydrogen, the latter being considered a form of solar fuel. However, the cost of producing efficient solar panels makes water-splitting technologies too expensive to commercialize. EPFL scientists developed an unconventional method to fabricate high-quality, efficient solar panels for direct solar hydrogen production with low cost.

The researchers focused on one of the best 2-D materials for solar water splitting, tungsten diselenide. Past studies have shown that this material has a great efficiency for converting solar energy directly into hydrogen fuel while also being highly stable.

To make the thin films, the researchers injected tungsten diselenide ink at the boundary between two liquids that do not mix. Exploiting this oil-and-water effect, they used the interface of the two liquids as a “rolling pin” to form an even and high-quality thin film with minimal clumping and restacking. The liquids were then carefully removed and the thin film was transferred to a flexible plastic support, which is much less expensive than a traditional solar panel.

A photograph of a single-flake-layer WSe2 thin film deposited on flexible Sn:In2O3 (ITO)-coated PET plastic. (Source: Kevin Sivula/EPFL)

A photograph of a single-flake-layer WSe2 thin film deposited on flexible Sn:In2O3 (ITO)-coated PET plastic. (Source: Kevin Sivula/EPFL)

The thin film was tested and found to be superior in efficiency to films made with the same material but using other comparable methods. At this proof-of-concept stage, the direct solar-to-hydrogen conversion efficiency was around 1%, an improvement over thin films prepared by other methods, and with potential for higher efficiencies in the future.

Plus, this liquid-liquid method can be scaled up on a commercial level. “It is suitable for rapid and large-area roll-to-roll processing,” says Kevin Sivula of EPFL. “Considering the stability of these materials and the comparative ease of our deposition method, this represents an important advance towards economical solar-to-fuel energy conversion.”

More developments in wooden chips

Joining the search for compostable chips, researchers from the University of Wisconsin-Madison demonstrated the feasibility of making microwave biodegradable thin-film transistors from cellulose nanofibrillated fiber, a transparent, flexible biodegradable substrate made from wood.

“We found that cellulose nanofibrillated fiber based transistors exhibit superior performance as that of conventional silicon-based transistors,” said Zhenqiang Ma, professor of electrical and computer engineering at UW-Madison. “And the bio-based transistors are so safe that you can put them in the forest, and fungus will quickly degrade them. They become as safe as fertilizer.”

An array of microwave silicon transistors sitting on a wood-derived CNF substrate. (Source: Jung-Hun Seo, Shaoqin Gong and Zhenqiang Ma/University of Wisconsin-Madison)

An array of microwave silicon transistors sitting on a wood-derived CNF substrate. (Source: Jung-Hun Seo, Shaoqin Gong and Zhenqiang Ma/University of Wisconsin-Madison)

To create high-performance devices, Ma’s team employed silicon nanomembranes as the active material in the transistor – pieces of ultra-thin films peeled from the bulk crystal and then transferred and glued onto the cellulose nanofibrill substrate to create a flexible, biodegradable and transparent silicon transistor.

But to make portable electronics, the biodegradable transistor needed to be able to operate at microwave frequencies. The researchers conducted a series of experiments to study the device’s functional performance, which showed the biodegradable transistor has superior microwave-frequency operation capabilities comparable to existing semiconductor transistors.

Next, the team plans to develop more complicated circuit system based on the biodegradable transistors.

Leaves inspire micro-supercapacitors

Researchers from the Institute for Basic Science and Sungkyunkwan University in South Korea devised a new technique for creating a solid-state micro-supercapacitor (MCS) modeled on natural vein-textured leaves in order to take advantage of the natural transport pathways which enable efficient ion diffusion parallel to the graphene planes found within them.

To create this final, efficient shape, the team layered a graphene-hybrid film with copper hydroxide nanowires. After many alternating layers they achieved the desired thickness, and added an acid solution to dissolve the nanowires so that a thin film with nano-impressions was all that remained.

To fabricate the MSCs the film was applied to a plastic layer with thin, ~5μm long parallel gold strips placed on top. Everything not covered by the gold strips was chemically etched away so that only the gold strips on top of a layer of film were left. Gold contact pads perpendicular to the gold strips were added and a conductive gel filled in the remaining spaces and was allowed to solidify. Once peeled from the plastic layer, the finished MSCs resemble clear tape with gold electrical leads on opposite sides.

The team produced stunning test results. In addition to its superior energy density, the film is highly flexible and increases capacitance after initial use. The volumetric energy density was 10 times higher than currently available commercial supercapacitors. The MSCs are displaying electrical properties about five orders of magnitude higher than similar lithium batteries and are comparable to existing, larger supercapacitors. According to Young Hee Lee, the team’s leader, “To our knowledge, the volumetric energy density and the maximum volumetric power density in our work are the highest values among all carbon-based solid-state MSCs reported to date.”

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