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Electrochemical doping: researchers improve carbon nanotube transparent conductors

Posted By Graphene Council, Wednesday, July 29, 2020
Skoltech researchers and their colleagues from Aalto University have discovered that electrochemical doping with ionic liquid can significantly enhance the optical and electrical properties of transparent conductors made of single-walled carbon nanotube films. The results were published in the journal Carbon.

A single-walled carbon nanotube (SWCNT) is a seamless rolled sheet of graphene, a list of graphite that is one atom thick. Just as other new carbon allotropes, SWCNTs demonstrate unique properties which can be employed in novel electronic devices that we use in our everyday life. One of the most promising applications is transparent conductors, which can be useful in medicine, green energy, and other fields: here, SWCNT films can replace the industrial standard indium-tin oxide (ITO). They are highly conductive, flexible, stretchable and can be easy doped due to the fact that all atoms in the nanotube are located on its surface.

Doping of SWCNTs allows to significantly increase film conductivity by eliminating the Schottky barriers between the tubes with different nature and increase the concentration of charge carriers. Moreover, the doping process leads to an increase in the transmittance of the films due to supersession of optical transitions.

While adsorption doping remains one of the most promising techniques for SWCNT modification, this method lacks uniformity and reversibility. In the new study, researchers propose a new reversible method to fine-tune the Fermi level of SWCNTs, dramatically increasing the conductivity while the optical transitions are suppressed. For this, they used electrochemical doping with an ionic liquid with a large potential window, which facilitates a high level of doping.

“We placed the SWCNT thin film into electrochemical cell and used standard three electrode scheme to apply potential to the nanotubes. With applying the negative/positive potential to the SWCNT film, an electrical double layer is formed at the SWCNT/ionic liquid interface. The latter acts as parallel plate capacitor causing positive/negative charge injection to SWCNT film surface and consequently the Fermi level shift,” explains Daria Kopylova, the first author of the study and senior research scientist at Skoltech.

The scientists were able to show that their electrochemical method can help achieve extremely high doping levels, comparable to the best results for doped SWCNTs films recently published in the field.

“The process is fully reversible so that it can be used to fine-tune the electronic structure of the single-walled carbon nanotubes in real time. Operating with the gate voltage, you can drive both optical transmittance and electrical conductivity of the films. The results open new avenues for future electronics, electrochromic devices, and ionotronics,” says Albert Nasibulin, head of Laboratory of Nanomaterials at the Skoltech Center for Photonics and Quantum Materials.

Tags:  Aalto University  Albert Nasibulin  carbon nanotube  Daria Kopylova  Graphene  Skoltech 

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Sulfur Provides Promising 'Next-Gen' Battery Alternative

Posted By Graphene Council, Thursday, June 18, 2020
With the increasing demand for sustainable and affordable energy, the ongoing development of batteries with a high energy density is vital. Lithium-sulfur batteries have attracted the attention of academic researchers and industry professionals alike due to their high energy density, low cost, abundance, nontoxicity and sustainability. However, Li-sulfur batteries tend to have poor cycle life and low energy density due to the low conductivity of sulfur and dissolution of lithium polysulfide intermediates in the electrolytes, which are generated when pure sulfur reacts with Li-ions and electrons.  

To circumvent these challenges, a multi-institutional research team led by Chunsheng Wang at the University of Maryland has developed a new chemistry for a sulfur cathode, which offers increased stability and higher energy of Li-sulfur batteries. Chao Luo - an assistant professor of chemistry and biochemistry at George Mason University - served as first author on the study, published in the Proceedings of the National Academy of Sciences (PNAS) on June 15.

Numerous conductive materials such as graphene, carbon nanotube, porous carbon and expanded graphite were used to prevent the dissolution of polysulfides and increase the electrical conductivity of sulfur cathodes - the challenge here is encapsulating the nano-scale sulfur in a conductive carbon matrix with a high sulfur content to avoid the formation of polysulfides.

"We used the chemical bonding between sulfur and oxygen/carbon to stabilize the sulfur," Luo said. "This included a high temperature treatment to vaporize the 'pristine' sulfur and carbonize the oxygen-rich organic compound in a vacuum glass tube to form a dense oxygen-stabilized sulfur/carbon composite with a high sulfur content."

In addition, scanning electron microscope (SEM) and transmission electron microscopy (TEM) instruments, X-ray photoelectron spectroscopy (XPS) and pair distribution function (PDF) were used to illustrate the reaction mechanism of the electrodes.

"In the dense S/C composite materials, the stabilized sulfur is uniformly distributed in carbon at the molecular level with a 60% sulfur content," Wang said. "The formation of solid electrolyte interphase during the activation cycles completely seal the sulfur in a carbon matrix, offering superior electrochemical performance under lean electrolyte conditions."

Li-sulfur batteries have applications in household and handheld electronics, electric vehicles, large scale energy storage devices and beyond.

Tags:  Battery  carbon nanotube  Chao Luo  Chunsheng Wang  George Mason University  Graphene  Li-sulfur batteries  University of Maryland 

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New material developed could help clean energy revolution

Posted By Graphene Council, Friday, March 27, 2020
Researchers developed a promising graphene–carbon nanotube catalyst, giving them better control over hugely important chemical reactions for producing hydrogen fuel

Fuel cells and water electrolyzers that are cheap and efficient will form the cornerstone of a hydrogen fuel based economy, which is one of the most promising clean and sustainable alternatives to fossil fuels. These devices rely on materials called electrocatalysts to work, so the development of efficient and low-cost catalysts is essential to make hydrogen fuel a viable alternative.  Researchers at Aalto university have developed a new catalyst material to improve these technologies.

The oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are the most important electrochemical reactions that limit the efficiencies of hydrogen fuel cells (for powering vehicles and power generation), water electrolyzers (for clean hydrogen production), and high-capacity metal-air batteries. Physicists and chemists at Aalto collaborating with researchers at CNRS France, and Vienna in Austria have developed a new catalyst that drive these reactions more efficiently than other bifunctional catalysts currently available. The researchers also found that the electrocatalytic activity of their new catalyst can be significantly altered depending on choice of the material on which the catalyst was deposited.

“We want to replace traditional expesive and scarce catalysts based on precious metals like platinum and iridium with highly active and stable alternatives composed of cheap and earth-abundant elements such as transition metals, carbon and nitrogen.” says Dr Mohammad Tavakkoli, the researcher at Aalto who led the work and wrote the paper.

In collaboration with CNRS the team produced a highly porous graphene–carbon nanotube hybrid and doped it with single atoms of other elements known to make good catalysts. Graphene and carbon nanotube (CNT) are the one‐atom‐thick two- and one‐dimensional allotropes of carbon, respectively, which have attracted tremendous interest in both academia and industry due to their outstanding properties compared more traditional materials. They developed an easy and scalable method to grow these nanomaterials at the same time, combining their properties in a single product. “We are one of the leading teams in the world for the scalable synthesis of double-walled carbon nanotubes. The innovation here was to modify our fabrication process to prepare these unique samples,” said Dr Emmanuel Flahut, research director at CNRS.

In this one-step process, they could also dope the graphene with nitrogen and/or metallic (Cobalt and Molybdenum) single-atoms as a promising strategy to produce single-atom catalysts (SACs). In catalysis science, the new field of SACs with isolated metal atoms dispersed on solid supports has attracted wide research attention because of the maximum atom-utilization efficiency and the unique properties of SACs. Compared with rival strategies for making SACs, the method used by the Aalto & CNRS team provides an easy method which takes place in one step, keeping costs down.
Catalyst substrate can boost performance

Catalysts are usually deposited on an underlying substrate. The role this substrate plays on the final reactivity of the catalyst is usually neglected by researchers, however for this new catalyst, the researchers spotted the substrate played an important part in its efficiency. The team found porous structure of their material allows to access more active catalyst sites formed at its interface with the substrate, so they developed a new electrochemical microscopy analysis method to measure how this interface could contribute to catalyze the reaction and produce the most effective catalyst. They hope their study of substrate effects on the catalytic activity of porous materials establishes a basis for the rational design of high-performance electrodes for the electrochemical energy devices and provides guidelines for future studies.

Tags:  Aalto university  carbon nanotube  CNRS  Emmanuel Flahut  energy  Graphene  Mohammad Tavakkoli 

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