Power/Performance Bits: Oct. 10

Asphalt anode
Scientists at Rice University developed an anode for lithium metal batteries enabling them to charge 10 to 20 times faster than commercial lithium-ion batteries.

The anodes are a porous carbon made from asphalt mixed with conductive graphene nanoribbons and coated with composite with lithium metal through electrochemical deposition. The lab combined the anode with a sulfurized-carbon cathode to make full batteries for testing. The batteries showed a high-power density of 1,322 watts per kilogram and high-energy density of 943 watt-hours per kilogram.

It also showed exceptional stability after more than 500 charge-discharge cycles. A high-current density of 20 milliamps per square centimeter demonstrated the material’s promise for use in rapid charge and discharge devices that require high-power density.

“The capacity of these batteries is enormous, but what is equally remarkable is that we can bring them from zero charge to full charge in five minutes, rather than the typical two hours or more needed with other batteries,” said James Tour, professor of chemistry at Rice.

Scanning electron microscope images show an anode of asphalt, graphene nanoribbons and lithium at left and the same material without lithium at right. The material was developed at Rice University and shows promise for high-capacity lithium batteries that charge 20 times faster than commercial lithium-ion batteries. (Source: The Tour Group/Rice)

Testing revealed another benefit: The carbon mitigated the formation of lithium dendrites, metal whiskers that form in a battery’s electrode and potentially cause short circuits, fires, and explosions.

Tour noted the battery is simpler and cheaper to manufacture than the group’s previous attempts. “There is no chemical vapor deposition step, no e-beam deposition step and no need to grow nanotubes from graphene, so manufacturing is greatly simplified.”

More stable perovskites
Researchers at Ecole Polytechnique Fédérale de Lausanne (EPFL) improved the stability of perovskite solar cells, which show promise for providing high efficiency with low manufacturing costs but suffer from rapid degradation under normal conditions.

To achieve efficiencies exceeding 20%, perovskite solar cells use hole-transporting materials (HTMs, which selectively transport positive charges in a solar cell). By virtue of their ingredients, these hole-transporting materials adversely affect the long-term operational stability of the cell.

The team focused on cuprous thiocyanate (CuSCN), an inorganic HTM which stands out as a stable, efficient and cheap candidate ($0.5/gr versus $500/gr for the commonly used spiro-OMeTAD). But previous attempts to use CuSCN as a hole transporter in perovskite solar cells have yielded only moderately stabilized efficiencies and poor device stability, due to problems associated with depositing a high-quality CuSCN layer atop of the perovskite film, as wells as the chemical instability of the CuSCN layer when integrated into a perovskite solar cell.

Cross-sectional scanning electron microscopy micrograph of a complete CuSCN solar cell displaying various layers (Source: M. Grätzel/EPFL)

To address this, the researchers developed a simple dynamic solution-based method for depositing highly conformal, 60-nm thick CuSCN layers that allows the fabrication of perovskite solar cells with stabilized power-conversion efficiencies exceeding 20%. This is comparable to the efficiencies of the best performing, state-of-the-art spiro-OMeTAD-based perovskite solar cells.

Second, the scientists introduced a thin spacer layer of reduced graphene oxide between the CuSCN and a gold layer. This allowed the perovskite solar cells to achieve excellent operational stability, retaining over 95% of their initial efficiency while operating at a maximum power point for 1000 hours under full-sun illumination at 60 °C. This surpasses even the stability of organic HTM-based perovskite solar cells that are heavily researched and have recently dominated the field.

According to Michael Grätzel, professor of chemistry at EPFL, “this is a major breakthrough in perovskite solar-cell research and will pave the way for large-scale commercial deployment of this very promising new photovoltaic technology.”

Wave power
Researchers at the Okinawa Institute of Science and Technology are working on a project to harness wave energy using turbines placed close to the shoreline.

Previously, the team explored using submerged turbines anchored to the sea floor through mooring cables that convert the kinetic energy of sustained natural currents into usable electricity, which is then delivered by cables to the land. While the project was successful, the team wanted an ocean energy source that was cheaper and easier to maintain.

The group focused on areas of the coastline where concrete tetrapods and wave breakers are placed to weaken the force of incoming waves and protect the shore from erosion.

“Surprisingly, 30% of the seashore in mainland Japan is covered with tetrapods and wave breakers,” said Tsumoru Shintake, professor in the Quantum Wave Microscopy Unit at OIST. Placing the turbines near tetrapods and wave breakers or among coral reefs exposes them to ideal wave conditions, allowing them to generate renewable energy as well as help protect the coasts from erosion while being affordable for those with limited funding and infrastructure.

“Using just 1% of the seashore of mainland Japan can [generate] about 10 gigawatts [of energy], which is equivalent to 10 nuclear power plants,” said Shintake. “That’s huge.”

The blades of this five-blade turbine are made of a soft material and they rotate on their axis when influenced by ocean waves—the diameter of the turbine is about 0.7 meters. The axis is attached to a permanent magnet electric generator, which is the part of the turbine that transforms the ocean wave energy into usable electricity. The ceramic mechanical seal protects the electrical components inside of the body from any saltwater leakage. This design allows the turbine to function for ten years before it need replacing. (Source: OIST Quantum Wave Microscopy Unit)

The turbines themselves are built to withstand the forces thrust upon them during harsh wave conditions as well as extreme weather, such as a typhoon. The blades are flexible, and thus able to release stress rather than remain rigid and risk breakage. The supporting structure is also flexible. They are built to be safe for surrounding marine life–the blades rotate at a carefully calculated speed that allows creatures caught among them to escape.

The team’s first commercial demonstration will use half-scale models, with 0.35-meter diameter turbines powering LEDs.