Chemical Communications Blog

Written by Tianyu Liu, University of California, Santa Cruz

Batteries are indispensable components that are powering a diverse array of electronics used almost every day. In recent years, due to the mass production of rechargeable electronics such as cell phones and electric vehicles, the need for reliable and economically viable batteries is rapidly increasing. Lithium-ion batteries represent a dominated rechargeable battery category that has been commercialized since early 1990s. However, the uneven distribution and high cost of lithium pose concerns on the sustainability of lithium-ion batteries.

Since the last decade, a number of scientists have shifted their attention to metal-ion batteries with more abundant and inexpensive metals than lithium, such as sodium and potassium. Change of ions calls for the need of seeking electrode materials with suitable structures that are able to host sodium or potassium ions. Most recently, Naguib and coworkers from Oak Ridge National Laboratory in USA and Purdue University in USA have identified a new two-dimensional material belonging to the MXene family that exhibits promising performance as an electrode for potassium-ion batteries. Their works has been published in Chem. Commun.

MXenes are a group of two-dimensional transition metal carbides and carbonitrides (Figure a) with chemical formula Mn+1XnTz; where M, X and Tz stand for an early transition metal element (e.g., Ti, V, Cr), carbon and/or nitrogen, and termination element (usually O, OH or F), respectively. Based on previous theoretical studies, MXenes are predicted to be capable of hosting potassium ions. In this work, Naguib et al. first synthesized one of the MXenes, Ti3CNOF, and experimentally investigated its energy storage performance.

The researchers first synthesized the Ti3CNOF powder by a wet etching process of its precursor. The obtained powder was then blended with other additives (including carbon black powder and polymer binders) and cast onto a piece of copper foil to prepare the electrode. The Ti3CNOF electrode delivered a high capacity (a measure for amount of energy that can be stored) of 710 mAh/g in the first discharging process and retained 75 mAh/g after 100 charge and discharge cycles (Figure b). In addition, the researchers gauged the charge storage mechanism of the synthesized Ti3CNOF using X-ray diffraction and X-ray photoelectron spectroscopy. The key conclusion is that potassium ions are able to intercalate in between layers of Ti3CNOF without triggering any phase change (Figure c). This mechanism is similar with lithium-ion intercalation into graphite.

Though the capacity performance reported here is not as outstanding as other graphene-based electrodes, this work provides the encouraging potential of MXenes serving as potassium-ion battery electrodes. Exploring other MXenes and modifying Ti3CNOF demonstrated here are expected to further enhance the charge storage performance of MXene-based potassium-ion batteries.

To find out more please read:
Electrochemical Performance of MXenes as K-ion Battery Anodes
Michael Naguib, Ryan A. Adams, Yunpu Zhao, Dmitry Zemlyanov, Arvind Varma, Jagjit Nanda, Vilas G. Pol.
DOI: 10.1039/C7CC02026K

About the author:
Tianyu Liu is a Ph.D. in chemistry from University of California-Santa Cruz. He is passionate about scientific communication to introduce cutting-edge researches to both the general public and the scientists with diverse research expertise. He is a web writer for the Chem. Commun. and Chem. Sci. blog websites. More information about him can be found at http://liutianyuresearch.weebly.com/.

Written by Tianyu Liu, University of California, Santa Cruz

Chitosan is a polysaccharide derived from chitin, a second most abundant bio-polymer on earth that consists of the shells of crustaceans (such as shrimps and crabs). It is a commonly used bio-compatible material for bio-medical applications such as drug delivery.

Previous studies have shown that chitosan is pH sensitive in water: it forms a viscous solution at low pH values and becomes insoluble at high pH values. Preliminary investigations suggest that such pH responsiveness is associated with the protonation and de-protonation processes of the amine groups on the chitosan polymer chain (the red box in Figure a). However, there still lacks fundamental understanding on how the pH affects the behavior of chitosan.

Now writing in Chem. Commun., Xu and Matysiak from University of Maryland, USA provided us new insights on the chitosan’s pH responsiveness at the molecular level. They adopted a method called “coarse-grained molecular simulation” to illustrate the self-assembly behaviors of chitosan polymer chains at different pH values. Unlike atomistic molecular simulations that focus on individual atoms of a molecule, the coarse-grained molecular simulation treats a group of atoms as one ensemble and probes the collective behavior of each ensemble (Figure a and b). This simulation technique demands less time than the atomistic counterparts without significantly reducing the simulation accuracy. It is suitable for characterizing polymers composed of thousands of atoms, such as chitosan.

The key discovery of this work is that the chitosan polymer chains can adopt different configurations at different pH values. At high pH values, each chain tends to crosslink perpendicularly with adjacent chains. The crosslinking reaction propagates and eventually builds up a three-dimensional dense chain network (Figure c). At low pH values, the protonated amine groups favor parallel crosslinking. Thus, each chain aligns in parallel with each other, which leads to a loosely-packed structure (Figure d). The perpendicularly cross-linked configuration reduces the solubility of chitosan in water but renders robustness and elasticity of the chitosan networks. The parallel cross-linked morphology increases water solubility but decreases the elasticity of the chitosan assembly. These conclusions obtained by the simulation are consistent with experimental results.

To find out more please read:
Effect of pH on Chitosan Hydrogel Polymer Network Structure
Hongcheng Xu and Silvina Matysiak
DOI: 10.1039/C7CC01826F

About the author:
Tianyu Liu is a Ph.D. in chemistry from University of California-Santa Cruz. He is passionate about scientific communication to introduce cutting-edge researches to both the general public and the scientists with diverse research expertise. He is a web writer for the Chem. Commun. and Chem. Sci. blog websites. More information about him can be found at http://liutianyuresearch.weebly.com/