Mixed-Counterion Iron Hexacyanide Electrolytes for Higher Solubility
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Solution Overview
Problem
The low solubility of ferrocyanide/ferricyanide salts limits their use in energy storage systems, restricting energy density and requiring workarounds like elevated temperatures or elaborate crystallization processes.
Innovation Solution
Stable aqueous solutions of iron hexacyanides are created by combining sodium and potassium ions with iron hexacyanide salts, exceeding the concentration limits of individual salts, allowing for higher energy storage densities.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Quantity of substance
If single counterion salts (Na4Fe(CN)6 or K4Fe(CN)6) are used, then the solution is simple to prepare, but the concentration of Fe(CN)64− is limited to below 0.8 M due to solubility constraints
Solution Approach 1:
The patent combines multiple counterion salts (sodium and potassium ferrocyanide) in a mixed electrolyte solution. This merging of different salt sources allows the solution to achieve Fe(CN)64− concentrations exceeding 0.8 M, overcoming the solubility limitations of individual single-counterion salts while maintaining solution stability.
2Quantity of substance
If alkaline earth metal salts (e.g., Ca2Fe(CN)6) are used to increase concentration, then the Fe(CN)64− concentration can reach 3 M, but metal hydroxide precipitation (e.g., Ca(OH)2) occurs in alkaline systems
Solution Approach 1:
The patent changes the counterion composition parameters by using a mixed sodium-potassium counterion system instead of alkaline earth metal counterions. This parameter change allows achieving high Fe(CN)64− concentrations (above 0.8 M) without the harmful side effect of metal hydroxide precipitation that occurs with alkaline earth metals in alkaline conditions.
3Quantity of substance
If elaborate flow-through crystallizers are used to increase energy density, then insoluble crystallites are separated out, but the system complexity and operational difficulty increase significantly
Solution Approach 1:
The patent extracts and eliminates the need for complex flow-through crystallization systems by formulating a mixed-counterion electrolyte that naturally achieves high Fe(CN)64− concentrations in solution. This approach removes the disturbing complex equipment while achieving the desired high energy density through improved solubility characteristics of the mixed salt system.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The solutions achieve significantly higher energy storage densities, enabling theoretical charge/discharge densities of at least 20 A-h/L, surpassing previous limitations and enhancing the performance of energy storage systems.
Implementation Method 1
Stable aqueous solutions of iron hexacyanides are created by combining sodium and potassium ions with iron hexacyanide salts, exceeding the concentration limits of individual salts
Implementation Method 2
The ferrocyanide/ferricyanide redox couple, Fe(CN)63−/4−, is well understood and is frequently used in energy storage applications
Data Source
AI summary
Stable solutions comprising high concentrations of charged coordination complexes, including iron hexacyanides are described, as are methods of preparing and using same in chemical energy storage systems, including flow battery systems. The use of these compositions allows energy storage densities at levels unavailable by other iron hexacyanide systems.