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Advanced Batteries & Energy Storage Research
Posted on November 9, 2023 by  & 

Promising Redox Flow Battery Chemistries for Stationary Energy Storage

Engineer control energy storage system or battery container unit in factory
Redox flow battery (RFB) installations are expected to increase over the next decade as demand for stationary energy storage technologies is expected to increase. This is due to potential Li-ion material supply shortages coming toward the end of the decade and increasing variable renewable energy (VRE) sources penetrating electricity grids, creating greater variability and uncertainty in energy supply. Increasing VRE penetration will, therefore, create demand for technologies that can dispatch energy over longer timeframes to accommodate for periods when energy from these intermittent energy sources is unavailable.
Redox flow batteries could be a technology well suited to such applications. Depending on the chemistry, their power output and energy capacity can be decoupled, providing cost reduction benefits (on a $/kWh basis) versus Li-ion at longer durations of storage. However, the most widely deployed RFB, the vanadium RFB (VRFB), suffers from expensive electrolyte, reducing its potential for future cost reductions. Therefore, as identified in IDTechEx's recent report "Redox Flow Batteries Market 2024-2034: Forecasts, Technologies, Markets", several players are developing and commercializing alternative RFB chemistries, which use cheaper and more widely abundant materials than vanadium.
While the increasing growth of the RFB market will see it valued at US$2.8B in 2034, some RFB chemistries are likely to be deployed in greater volumes than others due to varying player activity and technical and economic limitations.
Vanadium RFBs and alternatives
The vanadium redox flow battery (VRFB) is the most commercially deployed and well understood RFB technology. VRFBs are the RFB chemistry being developed by the most companies, with IDTechEx having identified 19 players.
Proportion of companies developing various redox flow battery chemistries. Source: IDTechEx
One of the advantages of VRFBs is their easily recyclable or regeneratable electrolyte, which can typically be performed at the end of the system's lifetime. This improves the overall sustainability of the system, as well as providing residual system value. However, cross-mixing of catholyte and anolyte across membranes used in VRFBs has been a known issue that could require more frequent electrolyte regeneration, increasing operation and maintenance costs. Although, some VRFB developers stated in interview with IDTechEx that much of their R&D efforts have been focused on engineering proprietary materials to reduce cross-mixing, and they now believe this is no longer a major issue in their VRFBs.
Despite this, VRFBs suffer from high CAPEX, and this is due to expensive vanadium electrolyte. This is the key disadvantage of VRFBs and will limit future system cost reductions, influencing how competitive these systems can be with existing energy storage solutions such as Li-ion batteries. Several alternative RFB chemistries are being developed by some key players, which use more widely available and cheaper materials for use in the electrolyte. These include zinc-bromine, zinc-iron, all-iron, hydrogen-bromine, hydrogen-manganese, and organic RFBs (ORFBs). The active material/electrolyte costs in these RFB systems are estimated by IDTechEx to be approximately an order of magnitude less than vanadium electrolytes, presenting a key advantage.
Zinc-based RFBs
Zinc-bromine RFBs are being developed by Redflow and, across 2022 - H2 2023, were the only non-vanadium RFBs to be deployed commercially. Zinc-iron batteries are being developed by WeView, who started a strategic alliance with and took a minority stake in ViZn Energy in 2019. The two parties formed a China licensing agreement that will see a mutual commitment to using ViZn Energy's technology for utility-scale storage across China. It is unclear whether ViZn Energy is still an active or independent player after the strategic alliance with WeView. Across 2022/23, WeViewraised US$143.7M to commercialize its zinc-iron RFB. This is some of the most substantial funding for any one RFB developer in recent years.
While promising, zinc-based RFBs suffer from zinc-plating and dendrite formation on the anode. This can reduce the lifetime of the RFB, as well as create difficulties with scaling RFB power output and energy capacity independently. Research in the literature has looked at using more highly engineered membranes with greater mechanical strength to resist dendrite penetration, though it is unknown whether WeView are adopting such materials. Ultimately, while these systems have shown some commercial promise, there may be technical limitations that reduce their widespread commercial viability.
All-iron RFBs
All-iron RFBs are only being developed by ESS Inc. and VoltStorage. ESS Inc. plans to install a large 10-hour 500 MWh all-iron RFB in Germany by 2027 with LEAG. ESS Inc. has also announced various other pilot projects and plans to expand its distribution of all-iron RFBs to Australia, New Zealand, and Oceania. The company has a semi-automated 800 MWh production facility in Oregon, US. However, all-iron RFBs can also suffer from iron-plating and dendrite formation, but this can be mitigated by using organic ligands to form coordination compounds with iron ions. Ultimately, all-iron RFBs could look to be a contending and cheaper RFB technology in future.
Other RFB chemistries in development
Hydrogen-bromine and hydrogen-manganese RFBs are being developed by Elestor and RFC Power, respectively. Elestor is starting to enter the pilot/demonstration phase, while RFC Power is still in early R&D phase. Organic RFBs are being developed by five players identified by IDTechEx, making these the RFB chemistry with the second-greatest number of players developing these systems, behind VRFBs. These players are predominantly in R&D phase, but some have deployed small-scale demonstration projects.
Organic redox flow batteries make use of organic species as the redox active electrolyte species. As such, there are a wide range of molecules and molecule derivatives that can be utilized in the ORFB anolyte and catholyte. However, these systems typically have lower power capability and higher levelized cost of storage (LCOS) versus VRFBs given their lower expected cycle life. Time will tell whether these players will be successful in commercializing their ORFBs.
VRFBs will remain the key RFB technology at least over the next few years, due to established supply chains already in place between vanadium/component suppliers and RFB developers and more substantial production capacity being planned. Increasing installations of pilot projects and demonstrations are likely to be seen from all-iron, zinc-bromine, and zinc-iron systems. However, the number of players involved in these latter three chemistries are far fewer than those manufacturing VRFBs.
If these lower-cost systems can be demonstrated to be effective in providing long durations of storage, VRFB companies could lose RFB market share, but likely only in the longer term.
For more information on RFB technologies, players, value chains, materials, economics, and granular 10-year RFB market forecasts, please refer to IDTechEx's report "Redox Flow Batteries Market 2024-2034: Forecasts, Technologies, Markets".
For more information on this report, including downloadable sample pages, please visit
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Authored By:

Senior Technology Analyst

Posted on: November 9, 2023

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