There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

Flow Batteries: Current Status and Trends

The development of improved rechargeable batteries represents a major technological challenge for this new century, as batteries constitute the limiting components in the shift from petrol (gasoline) powered to electric vehicles, while also enabling the use of more renewable energy on the grid. To minimize the ecological implications associated with their wider use, we must integrate sustainability of battery materials into our research endeavours, choosing chemistries that have a minimum footprint in nature and that are more readily recycled or integrated into a full circular economy. Sustainability and cost concerns require that we greatly increase the battery lifetime and consider second lives for batteries. As part of this, we must monitor the state of health of batteries continuously during operation to minimize their degradation. It is thus important to push the frontiers of operando techniques to monitor increasingly complex processes. In this Review, we will describe key advances in both more sustainable chemistries and operando techniques, along with some of the remaining challenges and possible solutions, as we personally perceive them.

Sustainability and in situ monitoring in battery development

Electrical energy storage is expected to be important for decarbonizing personal transport and enabling highly renewable electricity systems. This study analyses data on 11 storage technologies, constructing experience curves to project future prices, and explores feasibl…

The future cost of electrical energy storage based on experience rates

Storage of photovoltaic and wind electricity in batteries could solve the mismatch problem between the intermittent supply of these renewable resources and variable demand. Flow batteries permit more economical long-duration discharge than solid-electrode batteries by using liquid electrolytes stored outside of the battery. We report an alkaline flow battery based on redox-active organic molecules that are composed entirely of Earth-abundant elements and are nontoxic, nonflammable, and safe for use in residential and commercial environments. The battery operates efficiently with high power density near room temperature. These results demonstrate the stability and performance of redox-active organic molecules in alkaline flow batteries, potentially enabling cost-effective stationary storage of renewable energy.

http://science.sciencemag.org/content/sci/349/6255/1529.full.pdf

Alkaline quinone flow battery

The development of new high capacity redox active materials is key to realizing the potential of non-aqueous redox flow batteries (RFBs). In this paper, a series of substituted 1,4-dimethoxybenzene based redox active molecules have been developed via a subtractive design approach. Five molecules have been proposed and developed by removing or reducing the bulky substituent groups of DBBB (2,5-di-tert-butyl-1,4-bis(2-methoxyethoxy)benzene), a successful overcharge protection material for lithium-ion batteries. Of these derivatives, 2,3-dimethyl-1,4-dimethoxybenzene (23DDB) and 2,5-dimethyl-1,4-dimethoxybenzene (25DDB) are particularly promising as they demonstrate favorable electrochemical characteristics at gravimetric capacities (161 mA h g−1) that approach the stability limit of chemically reversible dimethoxybenzene based structures. Diffusivity, solubility, and galvanostatic cycling results indicate that both 23DDB and 25DDB molecules have promise for non-aqueous RFBs.

A subtractive approach to molecular engineering of dimethoxybenzene-based redox materials for non-aqueous flow batteries

This study aims to advance our understanding of the physical and electrochemical behavior of nonaqueous redox electrolytes at elevated concentrations and to develop experimentally informed structur...

https://par.nsf.gov/servlets/purl/10100666

Tailoring Two-Electron-Donating Phenothiazines To Enable High-Concentration Redox Electrolytes for Use in Nonaqueous Redox Flow Batteries

Electrolyte Development for Non-Aqueous Redox Flow Batteries Using a High-Throughput Screening Platform

Towards Low Resistance Nonaqueous Redox Flow Batteries

Stationary energy storage systems are needed to facilitate the widespread integration of intermittent renewable electricity generators, such as solar photovoltaic and wind turbines, and to improve the energy efficiency of the electrical grid. While no single technology can meet all needs, redox flow batteries (RFBs) have shown a favorable balance of cost, safety, and performance for many high-value applications. RFBs are rechargeable electrochemical devices that utilize the reversible redox reactions of two soluble electroactive species for energy storage. A compelling feature of the RFB configuration is the independent scaling of power and energy which enables cost-effective implementation of electrochemical couples with low energy density. Aqueous RFBs have been the subject of the vast majority of research efforts to date, which have yielded industry-level demonstrations. By comparison, non-aqueous RFBs are still in their infancy but have the potential for high energy density due to the extended stability window of non-aqueous electrolytes and the enriched selection of redox materials due to the broad variety of organic solvents. This chapter aims to introduce emerging, potentially transformative, strategies for enhancing RFB technologies through molecular design, electrolyte development, and cell-level engineering. Detailed discussions focus on recent developments in redox active materials (inorganic – aqueous, organic – aqueous, inorganic – non-aqueous, and organic – non-aqueous) and in system design (interdigitated flow fields, semi-solid flow cells, and hybrid flow cells). Future research directions and key challenges for RFB technologies are also highlighted.

Jeffrey A. Kowalski

Papers

A subtractive approach to molecular engineering of dimethoxybenzene-based redox materials for non-aqueous flow batteries

Tailoring Two-Electron-Donating Phenothiazines To Enable High-Concentration Redox Electrolytes for Use in Nonaqueous Redox Flow Batteries

Electrolyte Development for Non-Aqueous Redox Flow Batteries Using a High-Throughput Screening Platform

Towards Low Resistance Nonaqueous Redox Flow Batteries

Recent Developments and Trends in Redox Flow Batteries