Methylene Blue in a High-Performance Hydrogen-Organic Rechargeable Fuel Cell.

A hydrogen-organic hybrid flow battery (FB) has been developed using methylene blue (MB) in an aqueous acid electrolyte with a theoretical positive electrolyte energy storage capacity of 65.4 A h L-1. MB paired with the versatile H2/H+ redox couple at the negative electrode forms the H2-MB rechargeable fuel cell, with no loss in capacity (5 sig. figures) over 30 100% discharge cycles of galvanostatic cycling at 50 mA cm-2, which shows excellent stability. A peak power density of 238 mW cm-2 has also been demonstrated by utilizing 1.0 M MB electrolyte. This represents a type of scalable electrochemical energy storage system with favorable properties in terms of material cost, stability, crossover management, and energy and power density, overcoming many typical limitations of organic-based redox FBs.

Aqueous Redox Flow Batteries: Small Organic Molecules for the Positive Electrolyte Species.

There are a number of critical requirements for electrolytes in aqueous redox flow batteries. This paper reviews organic molecules that have been used as the redox-active electrolyte for the positive cell reaction in aqueous redox flow batteries. These organic compounds are centred around different organic redox-active moieties such as the aminoxyl radical (TEMPO and N-hydroxyphthalimide), carbonyl (quinones and biphenols), amine (e. g., indigo carmine), ether and thioether (e. g., thianthrene) groups. We consider the key metrics that can be used to assess their performance: redox potential, operating pH, solubility, redox kinetics, diffusivity, stability, and cost. We develop a new figure of merit - the theoretical intrinsic power density - which combines the first four of the aforementioned metrics to allow ranking of different redox couples on just one side of the battery. The organic electrolytes show theoretical intrinsic power densities which are 2-100 times larger than that of the VO2+ /VO2 + couple, with TEMPO-derivatives showing the highest performance. Finally, we survey organic positive electrolytes in the literature on the basis of their redox-active moieties and the aforementioned figure of merit.