Year: 2023

The 4-point Optimal Point Charge (OPC) and 3-point OPC (OPC3) models are highly accurate water models, used extensively in molecular simulations to reproduce the properties of bulk water. However, there are no reports on whether these models can accurately reproduce the viscosity of water. Recently, a researcher from Japan tested the performance of the OPC and OPC3 models by evaluated their shear viscosities and comparing them to experimental results.

Water is one of the most abundant substances on Earth and partakes in countless biological, chemical, and ecological processes. Thus, understanding its behavior and properties is essential in a wide variety of scientific and applied fields. To do so, researchers have developed various water models to reproduce the behavior of bulk water in molecular simulations. While these simulations can provide valuable insights into the specific properties of water, selecting an appropriate model for the system under study is crucial. Today, two water models have become very popular among biomolecular researchers: the 4-point Optimal Point Charge (OPC) and 3-point OPC (OPC3) models. These models are known for their ability to reproduce several properties of water with high accuracy, including density, heat of vaporization, and dielectric constant. However, there is limited information on whether OPC and OPC3 water models can accurately predict the shear viscosity of water.

The viscosity of water greatly affects how water molecules interact with other substances and surfaces, dictating critical phenomena such as diffusion and absorption. This affects the texture and taste of foods and beverages, as well as how oils and liquids interact with food during cooking. More importantly, the viscosity of water needs to be considered when designing and manufacturing pharmaceutical products, as well as many types of lubricants and polymeric materials. In addition, it influences how water and water-based solutions flow through small tubes, such as those in our circulatory system and in microfluidic devices.

Recently, Associate Professor Tadashi Ando from Tokyo University of Science conducted a study to test the performance of the OPC and OPC3 models, by evaluating their shear viscosities and comparing the values to the experimental calculations. These findings were published in Volume 159, Issue 10 of The Journal of Chemical Physics on September 14, 2023.

First, Dr. Ando set up molecular dynamics simulations of up to 2,000 water molecules using popular water models, including OPC, OPC3, and variants of the Transferable Intermolecular Potential 3-point (TIP3P) and 4-point (TIP4P) models. Next, he used an approach known as the Green-Kubo formalism—a commonly used method from statistical mechanics to study viscosity and heat conduction in various materials— to calculate the viscosity of the models.

The calculated viscosities for both OPC and OPC3 water models were very close to each other for temperatures ranging from 273 K to 373 K. Notably, for temperatures above 310 K, the viscosity predicted by these models was very close to that predicted by previous experimental findings. However, this was not the case at lower temperatures. Dr. Ando explains, “Compared to other water models, the performance of the OPC and OPC3 models in terms of predicting the shear viscosity was lower than that of TIP4P and TIP3P variants, but only for temperatures below 293 K.” Notably, at 273 K and 293 K, the shear viscosities of the two models were around 10% and 20% lower, respectively, as compared to those derived experimentally.

In addition to viscosity, Dr. Ando also assessed the performance of the OPC and OPC3 models for predicting other important water properties, such as surface tension and self-diffusion. The performance of OPC and OPC3 for these properties was remarkably accurate. “Based on the results of this study, along with those from previous reports, we can conclude that the OPC and OPC3 are among the best nonpolarizable water models at present, accounting for the various static and dynamic properties of water,” highlights Dr. Ando.

Overall, this study provides a thorough understanding of the advantages and limitations of water models. With any luck, this will help scientists polish these models to make them even more useful across various technological fields!

Aqueous potassium-ion batteries are a promising alternative to lithium-ion batteries owing to their safety and low cost. However, not much is known about the properties of the solid-electrolyte interphases (SEI) that form between the electrode and the aqueous electrolyte. To address this knowledge gap, researchers from Japan have now conducted a study using advanced scanning electrochemical microscopy and operando electrochemical mass spectrometry. Their findings provide a deeper understanding of SEI in next-generation batteries.

Lithium-ion batteries (LIBs) have become immensely popular as the go-to power source for a wide variety of electronic devices and vehicles over the past two decades. Although it is hard to overstate the transformative effects that LIBs have had on modern societies, this technology has a fair share of disadvantages that cannot be ignored any further. These include the limited availability of lithium as well as safety and environmental concerns. These drawbacks have motivated scientists around the world to look for alternative battery technologies, such as aqueous batteries. Potassium-ion batteries (KIBs) are a prominent example; these batteries are made from abundantly available materials and are much safer than LIBs. Moreover, KIBs can utilize a water-in-salt electrolyte (WISE), which makes them more stable thermally and chemically.

However, the prevention of hydrogen evolution at the negative electrode for its stabilization is a major challenge in high-voltage aqueous batteries. While solid-electrolyte interphases (SEI) that form between these electrodes and the electrolyte solution help stabilize the electrodes in LIBs (by preventing electrolyte decomposition and self-discharge of the batteries), they have been scarcely researched in the context of KIBs.

To address this major knowledge gap, a research team from Tokyo University of Science– (TUS), Japan, has recently conducted a pioneering study to gain insights into SEI formation and their properties in WISE-based KIBs. Their findings were published online in the journal Angewandte Chemie International Edition on August 18, 2023. The study, led by TUS Professor Shinichi Komaba, is co-authored by Junior Associate Professor Ryoichi Tatara, Dr. Zachary T. Gossage, and Ms. Nanako Ito, all from TUS.

The researchers mainly employed two advanced analytical techniques—scanning electrochemical microscopy (SECM) and operando electrochemical mass spectrometry (OEMS)—to observe how SEI forms and reacts in real time during the operation of a KIB with a 3,4,9,10-perylenetetracarboxylic diimide negative electrode and 55 mol/kg K(FSA)0.6(OTf)0.4∙1H2O, a WISE developed by the team in a previous study.

The experiments revealed that SEI forms a passivating layer in WISE akin to that seen in LIBs, with slow apparent electron transfer rates, helping suppress hydrogen evolution. This can ensure stable performance and higher durability of KIBs. However, the researchers observed that the coverage of the SEI layer was incomplete at higher operating voltages, leading to hydrogen evolution.

Taken together, the results reveal the need to explore potential avenues to enhance SEI formation in future aqueous batteries. “While our results reveal interesting details on the properties and stability of SEI found in one particular WISE, we should also focus on reinforcing the SEI network to achieve improved functionality,” comments Prof. Komaba. “SEI could perhaps be improved by the development of other electrolytes that produce unique SEIs, but also through the incorporation of electrolyte additives or electrode surface pretreatment.”

This study also highlights the power of SECM and OEMS for gaining a solid understanding of electrode–electrolyte interactions in next-generation batteries. “These techniques provide a powerful means for tracking the development, coverage, ion transfer, and stability of SEI and can easily be adapted for a variety of electrolytes and electrodes,” explains Prof. Komaba. “We hope that this work encourages other researchers to further explore SECM and OEMS as advanced characterization methods that can be incorporated with traditional battery measurements to gain deeper insights.”

The development of aqueous batteries such as KIBs will be instrumental for sustainable societies in the future, since they could replace the expensive and hazardous LIBs currently used in electric vehicles, smart grids, renewable energy systems, and marine applications. By making energy storage more accessible, aqueous batteries will aid the transition toward carbon-neutral energy generation, paving the way for a greener future.

With any luck, further studies will lead us to promising LIB alternatives soon!