Hybrid (liquid-solid) Redox Flow Batteries
July 2026
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Overview
Hybrid flow batteries are electrochemical energy storage systems in which one half-reaction deposits solid material onto an electrode while the other remains in solution. Unlike traditional redox flow batteries, where all active species stay dissolved and energy capacity can be scaled independently by increasing tank volume, hybrid designs introduce a solid-phase constraint that partially couples power and energy. For utilities, this means hybrid flow batteries behave more like a midpoint between conventional flow systems and fixed-electrode batteries, offering some flexibility in sizing but with practical limits on energy scaling.
The PNNL Energy Storage Grand Challenge Cost and Performance Assessment 2020 (ESGC 2020) notes that hybrid designs “[using] Earth-abundant zinc and iron electrodes for the anode in hybrid flow battery designs offer a pathway to lower cost”[1]. Even with this tradeoff, zinc-based hybrid flow batteries are highlighted as strong candidates for medium- and large-scale storage because of their safety characteristics, relatively low materials cost, high cell voltage, and higher energy density compared to many other aqueous systems. These attributes make hybrid flow batteries particularly well-suited for applications requiring multi-hour storage, predictable performance, and lower fire-risk profiles than lithium-ion. Hybrid flow batteries may also address several grid challenges such as reduced reliance on peaking generation, managing locally sourced energy variability, and improving resiliency in areas where long-duration storage adds value. However, these systems may require careful engineering to manage solid-electrode deposition, avoid uneven plating that can affect performance, and account for maintenance needs that differ from conventional flow systems. The partially coupled architecture can also affect dispatch flexibility, requiring thoughtful sizing and operational planning.
This technology page covers two hybrid flow battery chemistries: zinc-bromine (Zn-Br) and all-iron. Zn-Br has achieved commercial success and is already deployed at scale, while all-iron flow batteries reached early commercial deployment in 2025 with a 5-MW/50-MWh system installed at Salt River Project and broader commercialization beginning in 2026[1][2]. Both chemistries use earth-abundant, low-cost raw materials and position themselves as cost-competitive alternatives to vanadium. Their safety profiles differ significantly: Zn-Br involves bromine, which requires appropriate containment and safety protocols; all-iron systems use iron, salt, and water, offering non-toxic and non-flammable operation that may reduce permitting complexity and community concerns[3].
Benefits
Higher Energy Density Enables More Compact Multi-Hour Storage
Hybrid flow batteries, particularly zinc-bromine and all-iron, offer higher system-level energy density than traditional redox flow batteries[1][4]. This means more storage capacity within a smaller physical footprint, enabling siting in constrained substations, urban locations, or areas where land acquisition is costly. Higher energy density also improves the feasibility of 6-10 hour storage without dramatically expanding tank infrastructure[5].
Lower Raw Material Cost
Both chemistries use earth-abundant, industrially available materials. ARPA-E confirms ESS Tech’s all-iron battery uses “easy-to-source iron, salt, and water”[3]. The Sandia assessment of Zn-Br confirms zinc bromide is commercially available[6]. This reduces exposure to commodity volatility associated with vanadium or lithium and helps with planning around more predictable long-term procurement and replacement costs[1]. Lower materials cost also improves total cost of ownership, making multi-hour energy storage more financially viable for regulated and competitive utility environments.
Strong Safety Profile (Especially for All-Iron Systems)
All-iron flow batteries are non-toxic and non-flammable. ARPA-E confirms the technology is “environmentally friendly”[3]. The National Science Review comprehensive analysis (2025) confirms iron-based systems offer “excellent safety”[7]. While Zn-Br includes bromine, it remains safer than many alternative systems when properly contained, offering a lower-risk alternative for long-duration applications.
Modular, Siting-Flexible Infrastructure for Grid Expansion
Hybrid flow batteries share the geographic flexibility typical of flow systems: they can be installed in flat or varied terrain and do not require elevation or geological conditions. Their modular nature allows utilities to incrementally add capacity as system needs evolve, supporting phased deployment strategies and aligning capital investments with grid demand growth[8].
Technology Readiness Level (TRL)
- Final system completed
- Tested and validated under expected conditions
- Near operational readiness
- Design essentially finalized
Zn-Br and all-iron hybrid flow batteries are at TRL 8, reflecting commercial deployment and real-world validation in operational environments. Zn-Br systems are already commercially available and field tested, while all-iron systems entered early commercial deployment with multi-megawatt utility pilots beginning in 2025 with broader commercialization underway[9].
Adoption Readiness Level (ARL)
Value Proposition
Delivered Cost
Medium Risk
Both chemistries use earth-abundant materials at lower raw material cost than vanadium. The PNNL ESGC 2020 identifies zinc-based systems as cost-advantaged[1]. ARPA-E confirms all-iron technology is cost-effective due to its use of iron, salt, and water[3]. The Faraday Institution 2025 report confirms that hybrid systems are expected to achieve competitive system-level energy densities[5]. Sandia notes that Zn-Br economics are expected to be “much more favorable when large-volume runs become feasible”[6].
Functionality Performance
Medium Risk
Zn-Br systems supply power for 2–10 hours at energy efficiency of 70% or higher with energy density of 65–75 Wh/kg[4]. Sandia performance testing documents specific operational requirements including mandatory strip cycles every 1–4 charge/discharge cycles[10]. All-iron systems have demonstrated 12.2 hours of supply at rated power and 17.8 hours of supply at reduced power. The National Science Review (2025) notes remaining challenges including hydrogen evolution and reversibility of iron deposition, which active research is addressing[7].
Ease of Use/Complexity
Medium Risk
All-iron systems use non-hazardous materials that simplify siting and handling, while Zn-Br systems require trained personnel for bromine management and adherence to specific cycling protocols[3]. Although both chemistries have real-world deployment experience and maturing operational frameworks, it should be expected to experience some learning curve associated with fluid management, stack maintenance, and system controls – all of which contribute to moderate integration complexity[10].
Market Acceptance
Demand Maturity/Market Openness
Medium Risk
Zn-Br is included in the PNNL ARL Assessment’s near-term commercial viability analysis (2024–2027)[8]. All-iron has a validated utility pilot with Salt River Project[2], confirming utility market acceptance. Growing federal and state support for long-duration storage, including the $349 million in DOE funding and 12 state energy storage procurement targets, creates a receptive market environment, though adoption is still emerging[8].
Market Size
Low Risk
Flow batteries are expected to play a significant role in growth of long-duration storage market, driven by locally sourced energy generation integration[11]. ESS Tech’s Q3 2025 SEC filings report over 1.1 GWh in active proposals, indicating substantial near-term market demand. The DOE August 2024 report projects flow batteries can reach below $0.05/kWh levelized cost with targeted innovation[12].
Downstream Value Chain
Medium Risk
Zn-Br solutions benefit from an existing, though still maturing, downstream value chain supported by commercial deployments and independent testing[10]. All-iron systems have developing domestic manufacturing capacity and predominantly U.S.-sourced components, but scale remains early-stage. Relative to vanadium, supply chains for hybrid chemistries are less mature, so utilities should expect medium risk associated with vendor availability, component sourcing, and long-term service infrastructure[8].
Resource Maturity
Capital Flow
Medium Risk
Federal support for long-duration storage, including the DOE SI 2030 program, is helping advance hybrid flow battery development[8]. Both chemistries are benefitting from increased public and private investment, with all-iron technology showing early commercial traction through multi-megawatt contracts[2].
Project Development, Integration, and Management
Medium Risk
Hybrid flow batteries have demonstrated successful field deployments – Zn-Br in telecom applications and all-iron in recent multi-MWh utility projects – providing a foundation of real-world operational experience[10]. As deployment scale, utilities should expect the need for expanded engineering, construction, and commissioning support, and sector-wide workforce constraints could affect project timelines, contributing to moderate integration risk[8].
Infrastructure
Low Risk
Both chemistries are modular and site-flexible with no geographic constraints[8]. All-iron systems impose minimal special infrastructure requirements. Zn-Br requires bromine containment and handling systems, but these are well-defined and manageable[10].
Manufacturing and Supply Chain
Medium Risk
Zinc, bromine, and iron are commercially produced at industrial scale with established global supply chains. However, hybrid chemistries, particularly all-iron, are still scaling up from early commercial production. As demand grows toward GWh-scale manufacturing, supply chain expansion and vendor diversification will be critical, presenting moderate exposure to manufacturing bottlenecks[7].
Materials Sourcing
Low Risk
Both chemistries use earth-abundant, globally traded materials with no rare earth or critical mineral requirements. ARPA-E confirms all-iron systems use “easy-to-source” materials[3]. The National Science Review (2025) confirms iron-based systems derive cost-effectiveness from iron’s abundance[7]. The PNNL ESGC 2020 identifies zinc-based systems as materially cost-advantaged[1]. Material availability is unlikely to constrain deployment, giving this category a low-risk rating.
Workforce
Medium Risk
Deployment and operation of hybrid flow batteries require trained technicians, but workforce demands vary by chemistry. The PNNL ARL Assessment projects a need for 197,000–376,000 jobs in grid-connected battery storage by 2030[8]. All-iron systems’ non-hazardous electrolytes reduce specialist chemical training requirements compared to bromine-based systems. Zn-Br requires workers trained in bromine safety and Zn-Br-specific operational procedures documented in Sandia testing[10].
License to Operate
Regulatory Environment
Medium Risk
All-iron flow battery systems follow standard regulatory pathways typical for non-hazardous aqueous storage technologies[3]. Zn-Br involves bromine, a hazardous substance subject to EPA, OSHA, and DOT chemical storage and transport regulations. The broader flow battery sector is still seeing development of standards and codes of practice, contributing to moderate regulatory uncertainty for utilities[11].
Policy Environment
Low Risk
Hybrid flow batteries benefit from broad federal support for long-duration storage. Federal incentives under the Inflation Reduction Act and Bipartisan Infrastructure Law, combined with energy storage procurement targets across multiple states, create favorable conditions for utility adoption[8]. Domestic manufacturing initiatives, such as all-iron systems’ U.S.-based supply chain, further align with current policy trends[11][12].
Permitting & Siting
Medium Risk
All-iron systems have low permitting risk due to non-toxic, non-flammable electrolytes[3]. Zn-Br requires additional permitting from bromine’s hazardous substance classification including chemical storage permits, secondary containment verification, and fire safety review are standard requirements. These requirements are manageable but can add time to permitting processes, especially for urban or public-facing sites.
Environmental & Safety
Medium Risk
All-iron flow batteries are non-flammable and non-toxic. ARPA-E confirms the environmentally friendly profile[3], and the National Science Review (2025) confirms “excellent safety” for iron-based systems[7]. Zn-Br involves documented environmental risk: the PNNL ESGC 2020 confirms that Zn-Br systems “will precipitate bromine during use, which will pollute the environment and cause corrosion”[1], requiring active management and containment engineering.
Community Perception
Low Risk
All-iron flow batteries carry low community perception risk. Iron, salt, and water are universally recognized as benign materials, and ARPA-E confirms the technology is “environmentally friendly”[3]. Zn-Br is associated with commercial clean energy storage and has an established track record. While bromine handling may require community communication, the technology’s commercial deployments provide evidence of safe operation. Both chemistries are free of the thermal runaway risk associated with lithium-ion batteries.
Case Studies & Implementation
Salt River Project – Iron Flow Battery Pilot (Arizona, USA)
Salt River Project, a major Arizona utility, announced in October 2025 a storage purchase agreement with ESS Tech for a 5-MW/50-MWh iron flow battery system to be installed at SRP’s Copper Crossing Energy and Research Center in Florence, Arizona. The project resulted from an SRP request for proposals for long-duration energy storage pilots issued in 2024. ESS Tech’s iron flow battery uses iron, salt, and water as its electrolyte and is manufactured with over 98% domestically sourced components.
https://www.utilitydive.com/news/salt-river-project-ess-iron-flow-battery/802579/
ARPA-E All-Iron Flow Battery – ESS Tech
ESS Tech developed its all-iron hybrid flow battery technology with ARPA-E support, described by ARPA-E as “a cost-effective, reliable, and environmentally friendly all-iron hybrid flow battery.” ESS Tech is a U.S. developer of iron flow long-duration energy storage systems, featuring a 25-year design life and a domestic manufacturing model.
https://arpa-e.energy.gov/technologies/projects/iron-flow-battery
Sandia National Laboratories – Zinc-Bromine Field Testing for Remote Telecom Sites
Sandia National Laboratories, under the U.S. Department of Energy’s Energy Storage Program, conducted third-party performance testing of commercial zinc-bromine flow battery modules for remote telecommunication applications. The testing evaluated performance, lifecycle, auxiliary component durability, and failure mechanisms under real operating conditions, providing independently verified performance data for the Zn-Br chemistry.
https://www.sandia.gov/app/uploads/sites/163/2021/09/SAND2013-2818C.pdf
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.
- Walton, Robert. Salt River Project Taps ESS for 50-MWh Iron Flow Battery. UtilityDive. [Online] UtilityDive, October 10, 2025. [Cited: July 13, 2026.] https://www.utilitydive.com/news/salt-river-project-ess-iron-flow-battery/802579/.
- Song, Julia. Energy Storage Systems (ESS). ARPAE. [Online] ARPAE, August 30, 2017. [Cited: July 13, 2026.] https://arpa-e.energy.gov/programs-and-initiatives/search-all-projects/iron-flow-battery.
- Jimenez-Blasco, Uxua, et al. Enhanced Performance of Zn/Br Flow Battery Using N-Methyl-N-Propylmorpholinium Bromide as Complexing Agent. PubMed Central. [Online] PubMed Central, August 27, 2021. [Cited: July 13, 2026.] https://pmc.ncbi.nlm.nih.gov/articles/PMC8431348/. PMCID: PMC8431348.
- Roberts, Diarmid, et al. Market and Technology Assessment of Flow Batteries for Developing Economies. Didcot, United Kingdom : The Faraday Institution, 2025.
- Rose, David M. and Ferreira, Summer R. Initial Test Results from the RedFlow 5 kWh Zinc-Bromide Module, Phase 1. Albuquerque, NM : Sandia National Laboratories, 2012. SAND2012-1352.
- Aqueous iron-based redox flow batteries for large-scale energy storage. He, Cailing, et al. 7, s.l. : National Science Review, 2025, Vol. 12. https://doi.org/10.1093/nsr/nwaf218.
- Hollas, Aaron, et al. Adoption Readiness Level Assessment of Redox Flow Batteries. Richland, WA : Pacific Northwest National Laboratory, 2024. PNNL-36780.
- Hede, Karyn. New All-Liquid Iron Flow Battery for Grid Energy Storage. [Online] Pacific Northwest National Laboratory, March 25, 2024. [Cited: July 13, 2026.] https://www.pnnl.gov/news-media/new-all-liquid-iron-flow-battery-grid-energy-storage.
- Rose, David M. and Ferreira, Summer R. Performance Testing of Zinc-Bromine Flow Batteries for Remote Telecom Sites. Albuquerque, NM : Sandia National Laboratories.
- Sprenkle, Vince, et al. Technology Strategy Assessment. Washington, D.C. : U.S. Department of Energy, 2023. DOE/OE-0033 – Flow Batteries Technology Strategy Assessment.
- U.S. Department of Energy. Achieving the Promise of Low-Cost Long Duration Energy Storage. Washington, D.C. : U.S. Department of Energy, 2024.
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