Ever since its discovery in 2004, graphene has been revolutionizing the field of materials science and beyond. Graphene comprises two-dimensional sheets of carbon atoms, bonded into a thin hexagonal shape with a thickness of one atom layer. This gives it remarkable physical and chemical properties. Despite its thinness, graphene is incredibly strong, lightweight, flexible, and transparent. It also exhibits extraordinary electrical and thermal conductivity, high surface area, and impermeability to gasses. From high-speed transistors to biosensors, it boasts an unrivaled versatility in applications.
Nanocellular graphene (NCG) is a specialized form of graphene that achieves a large specific surface area by stacking multiple layers of graphene and controlling its internal structure with a nanoscale cellular morphology.
NCG is coveted for its potential to improve the performance of electronic devices, energy devices and sensors. But its development has been stymied by defects that occur during the manufacturing process. Cracks often appear when forming NCG, and scientists are looking for new processing technologies that can fabricate homogeneous, crack-free and seamless NCGs at appropriate scales.
“We discovered that carbon atoms rapidly self-assemble into crack-free NCG during liquid metal dealloying of an amorphous Mn-C precursor in a molten bismuth,” says Won-Young Park, a graduate student at Tohoku University.
Dealloying is a processing technique that exploits the varying miscibility of alloy components in a molten metal bath. This process selectively corrodes certain components of the alloy while preserving others.
Park and his colleagues demonstrated that NCGs developed by this method exhibited high tensile strength and high conductivity after graphitization. Moreover, they put the material to the test in a sodium-ion battery (SIB).
“We used the developed NCG as an active material and current collector in a SIB, where it demonstrated a high rate, long life and excellent deformation resistance. Ultimately, our method of making crack-free NCG will make it possible to raise the performance and flexibility of SIBs – an alternative technology to lithium-ion batteries for certain applications, particularly in large-scale energy storage and stationary power systems where cost, safety, and sustainability considerations are paramount.”
Working alongside Park was Soo-Hyun Joo from the Institute of Materials Research (currently based at Dankook University) along with Hidemi Kato from the same institute. Details of their research were published in the journal Advanced Materials (“Mechanically Robust Self-Organized Crack-Free Nanocellular Graphene with Outstanding Electrochemical Properties in Sodium Ion Battery”).
Their endeavors were made possible through collaboration with researchers from the Tohoku University’s Frontier Research Institute for Interdisciplinary Sciences and the Fracture and Reliability Research Institute; Pohang University of Science and Technology; and Johns Hopkins University.
The ability to convert carbon dioxide into useful chemicals and fuels could help address rising atmospheric CO2 levels while simultaneously producing valuable industrial feedstocks.
Read moreThe new method of making mixed halide-perovskites results in solar cells with improved stability and performance. The new method results in better control over perovskite crystallization rates. This means the crystal structure is more ordered, in part due to researchers understanding and taking advantage of the faster crystallization of bromide relative to iodide.
The result is a material with fewer defects and less halide migration and thus less segregation of the bromide and iodide. This in turn means uniform mixing of bromide and iodide across the material, which allows the material to absorb light evenly. The end result is that solar cells made using the new method will perform better under real-world conditions.
Typical halide perovskite solution deposition uses an anti-solvent drip procedure to initiate crystallization of the halide film. The standard anti-solvent method for producing bromide-iodide mixed halide perovskite films often leads to excessive defect formation (e.g., bromide vacancies) owing to the rapid crystallization of bromide vs. iodide-perovskite phases. Simulations show that halide migration is enhanced in the presence of a large population of halide vacancies. This limits the stability of bromide-iodide mixed halide perovskites under light and heat.
In comparison to the anti-solvent approach, the gentler gas-quench method better controls crystallization, first producing a bromide-rich surface layer that then induces top-down columnar growth to form a gradient structure with less bromide in the bulk than in the surface region. The anti-solvent method does not produce such a gradient structure.
In this study, researchers from the National Renewable Energy Laboratory, the University of Toledo, and the University of Colorado Boulder demonstrated that the gas-quench method also produces fewer bromide vacancies and results in materials with a higher quality opto-electronic performance. Solar cells made using the gas-quench method retain desirable light absorption properties and provide enhanced performance in the form of a higher charge carrier mobility, higher open circuit voltage, and enhanced stability.
