Power/Performance Bits: April 30

Printed supercapacitors
Researchers at Drexel University and Trinity College created ink for an inkjet printer from MXene, a highly conductive two-dimensional material, which could be used to print flexible energy storage components, such as supercapacitors, in any size or shape.

The material shows promise as an ink thanks to its high conductivity and ability to apply easily to surfaces using a one-step process. A key attribute of MXenes is the ability to mix with liquids, like water and other organic solvents, while retaining their conductive properties.

“So far only limited success has been achieved with conductive inks in both fine-resolution printing and high charge storage devices,” said Yury Gogotsi, professor in Drexel’s Department of Materials Science and Engineering. “But our findings show that all-MXene printed micro-supercapacitors, made with an advanced inkjet printer, are an order of magnitude greater than existing energy storage devices made from other conductive inks.”

“For most other nano inks, an additive is required to hold the particles together and allow for high-quality printing. Because of this, after printing, an additional step is required – usually a thermal or chemical treatment – to remove that additive,” said Babak Anasori, a research assistant professor in Drexel’s Department of Materials Science and Engineering. “For MXene printing, we only use MXene in water or MXene in an organic solution to make the ink. This means it can dry without any additional steps.”

The development of highly conductive MXene ink allows for inkjet printing of energy storage devices like micro-supercapacitors. (Source: Drexel University)

Additionally, the solvent and MXene concentration in the ink can be adjusted to suit different kinds of printers. “If we really want to take advantage of any technology at a large scale and have it ready for public use, it has to become very simple and done in one step,” Anasori said. “An inkjet printer can be found in just about every house, so we knew if we could make the proper ink, it would be feasible that anyone could make future electronics and devices.”

In a series of tests, the MXene ink was used to print a simple circuit, a mirco-supercapacitor, and some text, on substrates ranging from paper to plastic to glass. The team was able to print lines of consistent thickness and found that the ink’s ability to pass an electric current varied with its thickness.

The researchers say that such advancements in printing with different materials will enable a large number of opportunities for new technologies.

Recycling cathodes
Researchers at the University of California San Diego improved the process for recycling degraded cathodes in used lithium-ion batteries, making it safer and less energy intensive.

“Due to the rapid growth of electric vehicle markets, the worldwide manufacturing capacity of lithium-ion batteries is expected to reach hundreds of gigawatt hours per year in the next five years,” said Zheng Chen, a professor of nanoengineering at UC San Diego. “This work presents a solution to reclaim the values of end-of-life lithium-ion batteries after 5 to 10 years of operation.”

The new recycling method involves collecting cathode particles from spent lithium ion batteries and then mixing them with a eutectic lithium salt solution, a mixture of two or more salts that melts at temperatures much lower than either of its components. The mixture is then heat treated in two steps: it is first heated to 300 C, then it goes through a short annealing process in which it is heated to 850 C for several hours and then cooled naturally.

Illustration of the process to restore lithium ions to degraded NMC cathodes using eutectic molten salts at ambient pressure. (Image courtesy of Advanced Energy Materials/Chen lab)

The process can be performed at ambient pressure, unlike the team’s previous method, which required pressurizing the cathode-salts mixture to around 10 atmospheres, a step that was more expensive and required extra safety precautions and specialized equipment.

Researchers used the method to regenerate NMC (LiNi0.5Mn0.3Co0.2), a popular cathode containing nickel, manganese and cobalt, which is used in many of today’s electric vehicles.

“We made new cathodes from the regenerated particles and then tested them in batteries built in the lab. The regenerated cathodes showed the same capacity and cycle performance as the originals,” said Yang Shi, a postdoctoral researcher at UC San Diego.

The team is tuning this process with the goal of making it a universal recycle method for any type of cathode materials used in lithium-ion and sodium-ion batteries and are also working on a way to recycle degraded anodes. The team has filed a provisional patent on this work.

Wood aids desalination
Researchers at the University of Maryland developed a wood-based component for water purification that could potentially be part of a cheaper, more portable method of desalination for remote or cost-sensitive areas.

The solar evaporator uses a technique known as interfacial evaporation, “which shows great potential in response to global water scarcity because of its high solar-to-vapor efficiency, low environmental impact, and portable device design with low cost,” said Liangbing Hu, associate professor of materials science and engineering at UMD. “These features make it suitable for off-grid water generation and purification, especially for low-income countries.”

Interfacial evaporators are made of thin materials that float on saline water, in this case basswood. Absorbing solar heat on top, the evaporators continuously pull up the saline water from below and convert it to steam on their top surface, leaving behind the salt. One downside of these, however, is salt buildup on the evaporative surface, which degrades performance until removed.

A key aim was to reduce the need for regular maintenance to remove this buildup.

Basswood naturally contains micron-wide channels that carry nutrients and water up the tree. The researchers drilled an additional array of millimeter-wide channels through a thin cross-section. The top surface is then carbonized by brief exposure to high heat, which aids solar absorption.

As the device absorbs solar energy, salt water is drawn up through the wood’s natural channels. Salt is spontaneously exchanged from these tiny channels through natural openings along their sides to the wider drilled channels, and then easily dissolves back into the water below.

“In the lab, we have successfully demonstrated excellent anti-fouling in a wide range of salt concentrations, with stable steam generation with about 75% efficiency,” said Yudi Kuang, a visiting scholar at UMD.

The researchers now are optimizing their system for higher efficiency, lower capital cost, and integration with a steam condenser to complete the desalination cycle.