All-liquid (All-soluble) Redox Flow Batteries
July 2026
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Overview
True redox flow batteries (RFBs) are long-duration electrochemical energy storage systems in which all electroactive species remain dissolved in liquid electrolytes stored in external tanks and circulated through a cell stack during charge and discharge. Unlike hybrid flow batteries, no solid-phase deposition occurs at either electrode. This means energy capacity is determined exclusively by electrolyte volume, while power output is set by cell stack size (making power and energy fully decoupled and independently scalable). This architectural separation enables flexible rightsizing of duration without redesigning power hardware, a key advantage for meeting evolving operational needs such as multi-hour peak support, locally sourced energy generation integration, and contingency reserves.
This technology page covers three chemistries that share the true RFB architecture: Vanadium Redox Flow Batteries (VRFB), Iron-Chromium Redox Flow Batteries (Fe-Cr), and Organic (Quinone-based) Redox Flow Batteries. According to the PNNL Energy Storage Grand Challenge Cost and Performance Assessment 2020 (ESGC 2020), vanadium systems have achieved the most commercial success, while classifying Fe-Cr among those that have “been demonstrated but falling short of commercialization”[1]. Organic quinone systems are currently advancing from TRL 6 toward TRL 8 through a DOE-funded demonstration[2]. Utilities evaluating their near-term deployability and supply chain risks should be aware of these distinctions as they translate directly into procurement confidence and project execution certainty.
All three chemistries share the architectural advantages of geographic flexibility, modular scalability, and long electrolyte cycle life. The PNNL Adoption Readiness Level Assessment of RFBs notes that inorganic electrolyte systems can achieve effectively limitless cycling at the electrolyte level, supporting asset lifetimes that can exceed those of conventional batteries[3]. Despite common design strengths, the chemistries differ significantly in cost drivers, supply chain concentration, chemical handling considerations, and commercial maturity.
Benefits
Independently Scalable Power and Energy for Flexible Grid Support
Electrolyte Longevity, Reducing Lifecycle Risk
Purely inorganic redox systems, as documented by PNNL, provide effectively limitless electrolyte cycling[3]. Vanadium systems have demonstrated lifecycles above 10,000 cycles[1]. This greatly reduces degradation-related uncertainty, supports multi-decade asset life, and enables predictable long-term performance in circuits with frequent cycling.
Siting Flexibility and Modular Deployment
True RFBs avoid the geographic limitations of technologies like pumped hydro and can be deployed on urban, industrial, and rural feeders without specialized terrain or geological requirements[3]. Modular sizing supports staged installation that aligns with feeder loading growth, distribution upgrade deferral strategies, and local-specific reliability needs.
Safety Profiles that Support Siting Near Load
True RFB systems span a range of safety characteristics including vanadium and Fe-Cr use aqueous electrolytes with known industrial handling practices, while emerging organic quinone chemistries use non-toxic, non-flammable, water-based electrolytes suitable for installations close to customer load centers[4]. This safety flexibility helps utilities expand storage into distribution substations or community-scale sites where risk mitigations and permitting are sensitive.
Pathway to Lower-Cost Storage
Iron-based and organic chemistries offer potential capital cost reductions relative to vanadium, aligning with DOE targets near $100/kWh[1]. Lower costs enable utilities to economically deploy multi-hour storage for load shifting, resilience support, and non-wires alternatives, especially in feeders experiencing rapid DER growth and shifting peak profiles.
Technology Readiness Level (TRL)
- Final system completed
- Tested and validated under expected conditions
- Near operational readiness
- Design essentially finalized
True RFBs collectively fall within TRL 8, reflecting a technology class that is deployable but with chemistry-dependent maturity. True RFBs, including commercially viable options today, complemented by emerging chemistries approaching readiness, making them a maturing long-duration storage solution for utility planning horizons.
Adoption Readiness Level (ARL)
Value Proposition
Delivered Cost
Medium Risk
Across the True RFB category, delivered costs remain strongly dependent on chemistry. Vanadium systems carry a relatively high electrolyte cost that sets a structural floor, while Fe-Cr and organic chemistries offer credible pathways to lower long-duration storage costs as they mature[1]. Although full commercial cost structures for Fe-Cr and organic systems are still emerging, the underlying materials are low-cost and widely available, suggesting favorable long-term cost potential, though full-scale delivered cost is improving but not yet fully predictable across all chemistries, placing the category in a medium-risk cost position[2][5].
Functionality Performance
Medium Risk
Ease of Use/Complexity
Medium Risk
Vanadium systems are the most operationally established, with commercial deployments providing operational precedents, though the PNNL ARL Assessment notes US customer hesitancy from limited domestic scale examples[3]. Fe-Cr systems are advancing in electrode and electrolyte engineering but do not yet have standardized operational frameworks[5]. Organic systems at TRL 6 are designed to use existing RFB hardware and infrastructure, reducing future adoption barriers as the technology matures[2].
Market Acceptance
Demand Maturity/Market Openness
Medium Risk
Long-duration storage is a rapidly expanding market segment, driven by state procurement targets and substantial U.S. DOE funding for 10- and 24-hour technologies[3]. Within this environment, vanadium flow batteries benefit from existing commercial channels and early utility deployments, while Fe-Cr and organic chemistries are positioned for future market entry as the broader LDES market develops under the DOE SI 2030 program[5]. Although demand signals are strong, market openness varies by chemistry, and emerging systems still require field validation, supporting a medium-risk assessment.
Market Size
Low Risk
Federal and state policy momentum indicates robust long-term market growth. The DOE SI 2030 Flow Batteries TSA identifies long-duration storage driven by locally sourced energy generation integration as a large and growing market[5]. Multiple states have established energy-storage procurement targets, and DOE funding continues to accelerate technology development[7].
Downstream Value Chain
Medium Risk
Vanadium systems benefit from the most developed supply chain, with established vendors, integration partners, and service ecosystems that support utility procurement[8]. In contrast, Fe-Cr and organic systems have emerging value chains that remain less mature but are expected to leverage shared flow-battery infrastructure as deployment scales. While supply-chain readiness is improving, chemistry-dependent variability leads to a medium-risk rating for downstream market maturity. Vanadium has a more developed downstream value chain than emerging alternatives, though the PNNL ARL Assessment identifies that non-vanadium chemistries still face limited supply chains[3]. The National Laboratory of the Rockies (NLR)/USAID Grid-Scale Energy Storage Technologies report confirms vanadium RFBs have been deployed across a range of applications, providing an established service and integration ecosystem[8].
Resource Maturity
Capital Flow
Medium Risk
The DOE SI 2030 program committed $349 million in LDES funding as of 2022, supporting all three chemistries[3]. The DOE August 2024 report identifies flow batteries as one of the LDES technologies capable of reaching below $0.05/kWh (Levelized cost of storage) with innovation investment[7]. Vanadium systems attract increasing private capital and early commercial financing signals, while Fe-Cr and organic chemistries rely more heavily on federal grants and emerging institutional interest[1].
Project Development, Integration, and Management
Medium Risk
Vanadium flow batteries benefit from international commercial-scale deployments that provide transferrable project development experience for utilities. In contrast, Fe-Cr and organic systems are still building field-validated operational frameworks and depend on broader RFB project development expertise as they mature. However, Fe-Cr systems still face technical barriers that limit commercial readiness, particularly the slow kinetics of the chromium redox reactions and parasitic hydrogen evolution at the negative electrode, both of which reduce efficiency and complicate system management[5]. The DOE Storage Innovations 2030 (SI 2030) Technology Strategy Assessment for Flow Batteries (TSA) (July 2023) positions Fe-Cr as a developing chemistry with cost and material advantages that warrant continued investment, provided these performance limitations can be further mitigated[5].
Infrastructure
Low Risk
True RFBs are modular, site-flexible, and require no specialized national infrastructure such as pipelines or geographic features. Their compatibility with standard construction practices and existing RFB manufacturing approaches, particularly for organic systems, reduces siting constraints and deployment barriers.
Manufacturing and Supply Chain
Medium Risk
Manufacturing maturity varies by chemistry. Vanadium systems have established suppliers but are subject to commodity price volatility due to geographically concentrated vanadium production[1][3]. Fe-Cr uses earth-abundant materials with widely available industrial supply chains[1]. Organic quinone materials are synthesized from low-cost, abundant feedstocks, with a domestic US pilot production line established in 2024 capable of producing flow battery reactants at costs comparable to vanadium[2].
Materials Sourcing
Low Risk
Fe-Cr and organic systems rely on abundant and broadly available materials with minimal geopolitical risk, offering strong long-term sourcing stability[1][4]. While vanadium presents higher sourcing risk due to commodity price volatility, noted in the PNNL ESGC 2020 at approximately $160/kWh for electrolyte based on V2O5 pricing[1]. Overall, the category benefits from the availability of lower-risk alternative chemistries.
Workforce
Medium Risk
The expected expansion of grid-connected storage will create workforce shortages across construction, manufacturing, and operations. Vanadium systems benefit from the most established operator base, while Fe-Cr and organic chemistries can leverage the broader flow-battery workforce pipeline as deployment scales. Workforce capability is growing, but not yet fully developed for all chemistries.
License to Operate
Regulatory Environment
Medium Risk
The DOE SI 2030 Flow Batteries TSA identifies regulatory and standards development as a key area for flow battery commercialization[5]. Vanadium and Fe-Cr systems require chemical handling permits under EPA and OSHA frameworks. Organic systems, using non-toxic water-based electrolytes, face a simpler regulatory path and are confirmed as safe for residential and commercial environments[4].
Policy Environment
Low Risk
True RFBs benefit from strong federal and state policy alignment. Current incentives under the Inflation Reduction Act, the Bipartisan Infrastructure Law, and state-level procurement targets directly support adoption of long-duration storage technologies[3]. The DOE SI 2030 program seeks 90% cost reductions for 10+ hour storage technologies, directly supporting all true RFB chemistries[5][7].
Permitting & Siting
Low Risk
True RFBs are modular with no geographic siting constraints[3]. Organic systems with non-toxic, non-flammable electrolytes have the simplest permitting profile. Vanadium requires standard acid containment review. Fe-Cr involves additional chromium handling requirements[5]. Overall, the category does not face the land use, geographic, or community opposition barriers associated with large conventional energy infrastructure.
Environmental & Safety
Medium Risk
Vanadium systems require corrosive acid management. Fe-Cr systems involve chromium, which peer-reviewed literature confirms is “known to be toxic in certain forms” with “environmental and health concerns associated with handling and disposal”[5]. Organic quinone systems are confirmed as “nontoxic, nonflammable, and safe for use in residential and commercial environments”[4], and the PNNL ESGC 2020 confirms water-soluble organic systems provide “inherent fire safety”[1].
Community Perception
Low Risk
True RFBs are broadly associated with clean, safe energy storage and do not carry the thermal runaway risk associated with lithium-ion batteries. Organic systems are confirmed safe for residential and commercial environments[4]. Vanadium systems have an established commercial track record. The PNNL ARL Assessment acknowledges some US customer hesitancy driven by limited domestic demonstration scale rather than fundamental safety concerns[3].
Case Studies & Implementation
San Diego Gas & Electric (SDG&E) Pilot (April 2022)
Sumitomo Electric’s California demonstration showed that VRFBs can reliably serve as the primary power source for a real distribution-level microgrid, supporting 66 customers during both normal market operations and emergency islanding. This project validated RFB’s ability to stabilize voltage and frequency, perform seamless transitions between grid-connected and islanded modes, operate without degradation across aggressive cycling, and maintain high system availability.
https://sumitomoelectric.com/press/2022/04/prs012
GridStar Flow Battery Installation at Fort Carson
The U.S. Army Engineer Research and Development Center and Fort Carson installed a 1 MW/10 MWh GridStar Flow RFB to provide long-duration energy storage, mission-critical backup power, and peak-demand management for the installation. The system can store solar energy, reduce grid strain during high-load periods, and supply reliable power during outages, offering over six hours of duration. This project demonstrates how True RFBs can deliver resilience, operational flexibility, and stable long-duration capacity for complex distribution environments.
References
- Mongird, Kendall, et al. 2020 Grid Energy Storage Technology Cost and Performance Assessment. Richland, WA : Pacific Northwest National Laboratory, 2020. Publication No. DOE/PA-0204.
- U.S. Department of Energy Office of NEPA Policy and Compliance. CX-034312: Demonstration of Low-Cost, Organic Quinone Flow Battery. Office of NEPA Policy and Compliance. [Online] U.S. Department of Energy, July 22, 2025. [Cited: July 13, 2026.] https://www.energy.gov/nepa/articles/cx-034312-demonstration-low-cost-organic-quinone-flow-battery.
- Hollas, Aaron, et al. Adoption Readiness Level Assessment of Redox Flow Batteries. Richland, WA : Pacific Northwest National Laboratory, 2024. PNNL-36780.
- Alkaline Quinone Flow Battery. Lin, Kaixiang, et al. 6255, s.l. : National Library of Medicine, 2015, Vol. 349. https://doi.org/10.1126/science.aab3033.
- Sprenkle, Vince, et al. Technology Strategy Assessment. Washington, D.C. : U.S. Department of Energy, 2023. DOE/OE-0033 – Flow Batteries Technology Strategy Assessment.
- Molecular engineering of dihydroxyanthraquinone-based electrolytes for high-capacity aqueous organic redox flow batteries. Huang, Shiqiang, et al. 4746, s.l. : Nature Communications, 2022, Vol. 13. https://doi.org/10.1038/s41467-022-32424-8.
- U.S. Department of Energy. Achieving the Promise of Low-Cost Long Duration Energy Storage. Washington, D.C. : U.S. Department of Energy, 2024.
- Bowen, Thomas, et al. USAID GRID-SCALE ENERGY STORAGE TECHNOLOGIES PRIMER. Golden, CO : National Laboratory of the Rockies, 2021. https://docs.nlr.gov/docs/fy21osti/76097.pdf.
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