All-Iron Flow Battery Electrolyte With Cation Additives for pH Control
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Solution Overview
Problem
The iron salt battery experiences pH increase and hydrogen evolution reactions leading to inefficiencies and cell failure due to Fe(III) crossover and hydrogen gas formation, resulting in unbalanced charge states and insoluble iron hydroxide deposition.
Innovation Solution
Incorporation of specific cation additives such as Al3+, Na+, K+, Ca2+, Mg2+, and NH4+ into the electrolyte composition to control pH increase and enhance energy efficiency by reducing hydrogen evolution reactions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional electrolyte composition is used, then the battery operates with basic iron salt chemistry, but pH increases during cycling leading to hydrogen evolution reactions and cell failure
Solution Approach 1:
Aluminum ions (Al3+) are introduced as intermediary species that mediate between the Fe2+/Fe3+ redox reactions and the hydrogen evolution reaction. The aluminum ions preferentially interact with hydroxide ions produced during Fe3+ formation, forming aluminum hydroxide complexes that prevent pH buildup and suppress hydrogen evolution at the negative electrode.
Solution Approach 2:
The electrolyte composition is modified by adding cation additives (Al3+, Na+, K+, Ca2+, Mg2+, NH4+) to change the chemical parameters of the system. Specifically, the aluminum ion concentration is optimized at 0.03-0.3 M to control pH increase rate and suppress hydrogen evolution, while other cations at 0.1-3 M each provide additional benefits for overall battery performance.
2Reliability
If Fe(III) crossover occurs from positive to negative half-cell, then charge balance is disrupted, but adding separator membranes increases cell complexity
Solution Approach 1:
The electrolyte composition itself provides the protection against Fe(III) crossover through the added cation additives. The aluminum and other cations create a chemical environment that suppresses Fe(III) diffusion across the separator by competing for anionic sites and modifying the electrochemical potential gradient, eliminating the need for complex membrane structures.
3Loss of energy
If hydrogen evolution reaction occurs at negative half-cell, then energy efficiency decreases, but increasing acidity to suppress HER affects Fe2+ stability
Solution Approach 1:
The electrolyte pH is optimized to 0-4 through careful selection of cation additives and their concentrations. This pH range is sufficiently acidic to suppress hydrogen evolution reactions and improve energy efficiency, yet stable enough to prevent Fe2+ oxidation and precipitation. The aluminum ions play a key role in maintaining this optimal pH window by buffering against pH increases during cycling.
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 cation additives effectively limit pH rise, maintaining high energy efficiency and preventing cell degradation, thereby improving overall battery performance.
Implementation Method 1
During charge, hydrogen will evolve due to the hydrogen evolution reaction (HER). The acidic protons H+
Implementation Method 2
During the reaction, the charge within the electrolyte is balanced by migration of charged species through the separator.
Implementation Method 3
During charge, Fe(II) oxidizes to Fe(III) in the positive half-cell
Implementation Method 4
Energy storage is based on the electrochemical reaction of iron.
Implementation Method 5
in the negative half-cell Fe(II) is reduced to Fe(0). The latter reaction in the negative half-cell is also called the plating reaction, as Fe(0) is deposited on the negative electrode.
Implementation Method 6
During discharge, the plated Fe(0) is dissolved into the electrolyte forming Fe(II)
Implementation Method 7
while Fe(III) reduces to Fe(II) in the positive half-cell
Data Source
Figure 1
AI summary
The invention provides an electrolyte for use in an all-iron redox flow battery, comprising an aqueous solution of an Fe2+ salt; a first cation additive being aluminium Al3+; and a second cation additive selected from the group consisting of Na+, K+, Ca2+, Mg2+ or NH4+.