Battery technologies that rely on flammable, toxic, unsustainable, and expensive energy sources contribute significantly to climate change. To address this issue and mitigate the impact of climate change, it is crucial to transition from fossil fuels to cleaner and environmentally friendly energy sources. One way to support this transition is by improving the efficiency of energy storage systems, making them safer, more stable, and sustainable while achieving high energy and power density.
Researchers have been focusing on molecular engineering approaches to develop aqueous-based redox-enhanced electrochemical capacitors (redox ECs). These advanced hybrid electric double-layer capacitors use redox-active molecules at the electrode-electrolyte interface to increase the energy density.
Redox ECs offer several advantages, such as cost effectiveness, the use of abundant elements, and structural tunability, thanks to the utilization of organic redox-active electrolytes. However, one major challenge in their development is the limited solubility of these species in aqueous systems, resulting in low energy density. Previous attempts to enhance solubility have been time-consuming and costly.
Researchers from Korea have recently made progress in addressing this challenge by using hydrotropic-supporting electrolyte (HSE) to enhance the solubility of organic redox-active species. The study, led by Assistant Professor Seung Joon Yoo and Professor Sukwon Hong from the Gwangju Institute of Science and Technology in Korea, was published in ACS Energy Letters.
The researchers employed the process of hydrotropy, which involves using a class of amphiphilic molecules. This unique solubilization phenomenon allows for a significant increase in the solubility of sparingly soluble solutes by increasing the volume of the surfactant relative to the hydrophobic component. As a model species, the researchers tested various quinones due to their utility as redox-active additives and acceptable electrochemical stability.
Through their experiments, the researchers discovered that using HSE (p-toluene sulfonic acid (p-TsOH), 2-naphthalenesulfonic acid (2-NpOH), and anthraquinone-2-sulfonic acid (AQS)) improved the solubility of hydroquinone (HQ) without requiring chemical functionalization. Importantly, they demonstrated that the increase in solubility was directly proportional to the concentration of the respective HSEs.
Additionally, the researchers designed a biredox salt called 2-[N,N,N-tris(2-hydroxyethyl)] anthracenemethanaminium-9,10-dione bromide (AQM-Br), which could participate in Faradaic reactions at both positive and negative electrodes. They tested this salt in the HSE system in a concentration-dependent manner. Dr. Yoo emphasized that “the solubility of HQ in HSE was increased sevenfold, and we synthesized a designer multifunctional dual-redox species (AQM-Br) with significantly enhanced solubility, from being barely soluble to >1 M, by optimizing the HSE.”
Furthermore, the researchers sought to understand the solubilization mechanisms for both the HQ and AQM-Br electrolytes. Through intermolecular nuclear Overhauser effect and dynamic light scattering analyses, they found that hydrotrope solubilization for HQ/HSE was achieved through the co-solubilizer mechanism, while for AQM-Br/HSE, it occurred due to the formation of quasi-micelle nanostructures.
In conclusion, the researchers believe that their simple approach can be readily applied to different classes of redox species and can be beneficial for various applications, including redox flow batteries. The study also provides a guideline for designing energy-dense redox-active electrolytes and selecting optimal HSE and red generation of electric vehicles (EVs) with longer driving ranges and faster charging capabilities are highly dependent on the advancement of battery technology. Currently, dominant battery technologies utilize flammable, toxic, unsustainable, and expensive energy sources, which contribute significantly to climate change. To curtail the impacts of climate change, it is crucial to transition from fossil fuels to cleaner and more environmentally friendly energy sources.
Researchers are actively working on improving the efficiency of energy storage systems to support this transition. One area of research focuses on the development of aqueous-based redox-enhanced electrochemical capacitors (redox ECs) using molecular engineering approaches. Redox ECs are a type of advanced hybrid electric double-layer capacitors that employ redox-active molecules at the electrode-electrolyte interface to increase the energy density.
Organic redox-active electrolytes used in redox ECs offer advantages such as cost merit, the use of earth-abundant elements, and structural tunability. However, a significant challenge in their development is the limited solubility of these species in aqueous systems, leading to low energy density. Previous attempts to improve solubility have been time-consuming and expensive.
A recent study conducted by researchers from Korea, led by Assistant Professor Seung Joon Yoo and Professor Sukwon Hong from the Gwangju Institute of Science and Technology, explores the use of hydrotropic-supporting electrolyte (HSE) to enhance the solubility of organic redox-active species. Their findings were published in ACS Energy Letters.
The researchers employed hydrotropy, a solubilization phenomenon that involves using a class of amphiphilic molecules. Hydrotropy allows for a significant increase in the solubility of sparingly soluble solutes by increasing the volume of the surfactant relative to the hydrophobic component. Quinones were chosen as a model species due to their utility as redox-active additives and acceptable electrochemical stability.
By utilizing HSE, which includes p-toluene sulfonic acid (p-TsOH), 2-naphthalenesulfonic acid (2-NpOH), and anthraquinone-2-sulfonic acid (AQS), the researchers successfully improved the solubility of hydroquinone (HQ) without the need for chemical functionalization. Importantly, they demonstrated that the solubility increase was directly proportional to the concentration of the HSEs used.
Additionally, the researchers synthesized a biredox salt called 2-[N,N,N-tris(2-hydroxyethyl)] anthracenemethanaminium-9,10-dione bromide (AQM-Br), which could participate in Faradaic reactions at both positive and negative electrodes. The solubility of AQM-Br was significantly enhanced from being barely soluble to >1 M by optimizing the HSE.
To gain insights into the solubilization mechanisms, the researchers employed intermolecular nuclear Overhauser effect and dynamic light scattering analyses. They discovered that the solubilization of HQ/HSE occurred through the co-solubilizer mechanism, while AQM-Br/HSE formed quasi-micelle nanostructures.
In conclusion, this study presents a simple and effective approach to enhance the solubility of organic redox-active species in aqueous systems. The findings have potential implications for the development of energy-dense redox-active electrolytes, not only for redox ECs but also for other applications such as redox flow batteries. This research provides a valuable guideline for the design of sustainable and efficient energy storage systems, promoting the transition to cleaner energy sources and mitigating the impacts of climate change.
Source: GIST (Gwangju Institute of Science and Technology)