Redox Flow Battery
Promise of Redox Flow Batteries
The promise of Redox Flow Batteries (RFBs) utilizing soluble redox couples is becoming recognized for large-scale energy storage. Flow batteries offer a unique solution to grid-scale energy storage because of their electrolyte tanks which allow easy scaling of storage capacity. As energy-intensive storage systems can support integration of intermittent renewables, such as solar and wind, into the grid, the scalability of RFB systems presents an economic opportunity.
Experimental diagnostics and Material characterization
Experimental diagnostics and material characterization have be developed to improve energy density, operating temperature and electrolyte imbalances over time, including
Alternative Cell Design
Various configurations of redox flow batters have been proposed and evaluated (a) a conventional redox flow battery with two divided compartments containing dissolved active species, (b) a hybrid redox flow battery with gaseous reactant at one electrode, (c) a redox flow battery with membraneless structure using laminar flow and (d) a redox flow battery with solid electroactive materials as flowing media.
The design of flow fields (serpentine, interdigitate, column) and channel dimensions potentially affects the performance of a redox flow battery.
Advanced Electrode Design
Developing advanced electrodes for redox flow cell batteries has been a major focus. Different surface pre-treatments have demonstrated dramatically impact catalytic activity, including
Membrane Separator
An ideal membrane of a flow battery keeps the redox‐active couples completely apart but is permeable to the supporting electrolyte ions to enable charge balance. The selectivity of the membrane is crucial, as a cross‐contamination of catholyte and anolyte leads to reduced efficiencies and, in the case of asymmetric electrolytes, a long‐term decay of capacity. All-vanadium redox flow batteries have V 3+/V 2+ redox reactions on the negative side and VO2 +/VO 2+ on the positive side. Such battery uses the same metal ions on both sides. Crossover of metal ions through the membrane will then not cause contamination of the electrolyte. In contrast, for redox flow batteries with different metal ions such as Fe 3+/Fe 2+ and Cr 3+/Cr 2+ in an iron-chromium flow battery, the cross-contamination via ion penetration may cause irreversible performance loss.
Alternative Electrolyte
Conventional redox flow batteries have two divided electrolyte reservoirs. Catholyte and anolyte are separated by a membrane, which permits ions to pass through it. Related RFB research areas that seek to increase energy density and/or employ inexpensive electrolytes, such as nonaqueous systems, bromine, sulfur, and iron-based chemistries, generally focus on development of materials that overcome cell voltage and the cost of vanadium. The composition of the electrolyte impacts the hydraulic, kinetic, ohmic, and mass-transfer resistances, as well as the system stability.
Test Systems
Flow battery test systems have been developed and used for evaluation of the charge/discharge characteristics and alternating current (AC) impedance of a single-cell redox flow battery. By combining AC electrochemical impedance spectroscopy and equivalent circuit model, equivalent circuits and equivalent elements are investigated.
Functional flow battery hardware
Dual vs single-membrane design for measuring crossover of redox flow batteries provides simple methods to measure crossover in complete cells. Dual-membrane can be used to quantify the crossover, which is especially important when comparing different types of membranes.
The promise of Redox Flow Batteries (RFBs) utilizing soluble redox couples is becoming recognized for large-scale energy storage. Flow batteries offer a unique solution to grid-scale energy storage because of their electrolyte tanks which allow easy scaling of storage capacity. As energy-intensive storage systems can support integration of intermittent renewables, such as solar and wind, into the grid, the scalability of RFB systems presents an economic opportunity.
Experimental diagnostics and Material characterization
Experimental diagnostics and material characterization have be developed to improve energy density, operating temperature and electrolyte imbalances over time, including
- Effect of mesopore microstructure for carbon electrode
- Real-time measurements and experimental analysis of material
- Cell optimization using reference electrode and potentiostatic analysis
- Experimental studies of crossover measurement and hydrodynamics study
- Characteristics of charge/discharge and alternating current impedance in redox flow batteries
Alternative Cell Design
Various configurations of redox flow batters have been proposed and evaluated (a) a conventional redox flow battery with two divided compartments containing dissolved active species, (b) a hybrid redox flow battery with gaseous reactant at one electrode, (c) a redox flow battery with membraneless structure using laminar flow and (d) a redox flow battery with solid electroactive materials as flowing media.
The design of flow fields (serpentine, interdigitate, column) and channel dimensions potentially affects the performance of a redox flow battery.
Advanced Electrode Design
Developing advanced electrodes for redox flow cell batteries has been a major focus. Different surface pre-treatments have demonstrated dramatically impact catalytic activity, including
- Hydrophobic non-activated electrode vs hydrophilic activated electrodes optimizing transport properties
- Thermal oxidation for activity gradient electrode improving electrolyte utilization
- Cobalt oxide facile modification on graphite felt promoting the electrochemical activity and stability
- Surface etching and introduction of functional groups improving mass transfer and electrode process
- Micro-porous layer enhancing surface areas with highly kinetically-active surfaces
Membrane Separator
An ideal membrane of a flow battery keeps the redox‐active couples completely apart but is permeable to the supporting electrolyte ions to enable charge balance. The selectivity of the membrane is crucial, as a cross‐contamination of catholyte and anolyte leads to reduced efficiencies and, in the case of asymmetric electrolytes, a long‐term decay of capacity. All-vanadium redox flow batteries have V 3+/V 2+ redox reactions on the negative side and VO2 +/VO 2+ on the positive side. Such battery uses the same metal ions on both sides. Crossover of metal ions through the membrane will then not cause contamination of the electrolyte. In contrast, for redox flow batteries with different metal ions such as Fe 3+/Fe 2+ and Cr 3+/Cr 2+ in an iron-chromium flow battery, the cross-contamination via ion penetration may cause irreversible performance loss.
Alternative Electrolyte
Conventional redox flow batteries have two divided electrolyte reservoirs. Catholyte and anolyte are separated by a membrane, which permits ions to pass through it. Related RFB research areas that seek to increase energy density and/or employ inexpensive electrolytes, such as nonaqueous systems, bromine, sulfur, and iron-based chemistries, generally focus on development of materials that overcome cell voltage and the cost of vanadium. The composition of the electrolyte impacts the hydraulic, kinetic, ohmic, and mass-transfer resistances, as well as the system stability.
Test Systems
Flow battery test systems have been developed and used for evaluation of the charge/discharge characteristics and alternating current (AC) impedance of a single-cell redox flow battery. By combining AC electrochemical impedance spectroscopy and equivalent circuit model, equivalent circuits and equivalent elements are investigated.
Functional flow battery hardware
Dual vs single-membrane design for measuring crossover of redox flow batteries provides simple methods to measure crossover in complete cells. Dual-membrane can be used to quantify the crossover, which is especially important when comparing different types of membranes.