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Zinc Bromine Flow Batteries

Zinc bromine flow batteries or Zinc bromine redux flow batteries (ZBFBs or ZBFRBs) are a type of rechargeable electrochemical energy storage system that relies on the redox reactions between zinc and bromine. Like all flow batteries, ZFBs are unique in that the electrolytes are not solid-state that store energy in metals. They store energy in electrolyte liquids held in two tanks one containing a positively-charged anode and the other with a negatively-charged cathode, separated by a membrane.


How They Work

ZBFBs incorporate two separate electrolyte solutions held in tanks, each containing zinc bromide dissolved in water or another suitable solvent and the other containing a solution of bromine. The two solutions are separated by a microporous and ion-exchange-capable membrane that allows ions to pass through but prevents the two solutions from mixing.


During charging, an electric current is passed reactor stack from one tank to the other. This causes zinc ions to move from the zinc bromide solution to the negative electrode, the anode, and bromine ions to move from the bromine solution to the positive electrode, the cathode. At the anode, the zinc ions are reduced to zinc metal, which is deposited on the electrode. At the cathode, the bromine ions are oxidized to bromine gas, which is dissolved in the electrolyte.


During discharging, the process is reversed. The zinc metal on the anode is oxidized back to zinc ions, which move into the zinc bromide solution. The bromine gas at the cathode is reduced back to bromide ions, which move into the bromine solution. The electric current flows through the external circuit as the ions move through the membrane.


In no-membrane zinc flow batteries (NMZFBs) or iterations of the ZBFB that does not use a membrane to separate the positive and negative electrolytes, the electrolytes are separated by a porous spacer that allows ions to pass through but prevents the two electrolytes from mixing. The porous spacer in a NMZFB is designed to allow ions to pass through but prevent the two electrolytes from mixing. This is achieved by using a material with very small pores. The pores are small enough to prevent the two electrolytes from mixing, but they are large enough to allow ions to pass through.


Primus Power's ZBFB. Warrantied for up to 20 years.
Primus Power's ZBFB. Warrantied for up to 20 years.

Advantages of Zinc Bromine Flow Batteries

Zinc bromine flow batteries offer several advantages that make them an appealing choice for energy storage:


Scalability

These flow batteries are highly scalable, allowing for adjustments in energy storage capacity by simply resizing the electrolyte tanks.


Long Cycle Life

ZBFBs are known for their extended cycle life, capable of enduring a high number of charge and discharge cycles without significant degradation. This reliability ensures longevity in energy storage applications.


Energy Efficiency

Some ZBFB systems can achieve energy efficiencies reaching up to 80%, which means a substantial portion of the energy input during charging can be efficiently recovered during discharge.


Chemical Stability

Zinc and bromine are stable elements, and the corresponding electrolytes are chemically stable, contributing to the overall safety and longevity of ZBFBs.


Recyclable

ZBFM are recyclable. This is an important part of the circular economy. It helps to reduce the amount of waste that goes to landfills and conserve natural resources.

Redflow’s ZBM3 battery. Redflow describes it is the world’s smallest commercially available zinc-bromine flow battery. Source: Redflow
Redflow’s ZBM3 battery. Redflow describes it is the world’s smallest commercially available ZBFB. Source: Redflow
How It Is Going

The roots of ZBFBs can be traced back to the exploration of redox flow battery (RFB) technology in the mid-20th century. Researchers were intrigued by the concept of using redox reactions to store and release electrical energy. During this period, the groundwork was laid for the development of flow battery systems, including ZBFBs.


As interest in energy storage technologies grew, companies like the Australian manufacturer, RedFlow (formerly known as ZBB Energy) began exploring the commercialization of ZBFBs. RedFlow, founded in 2005 and headquartered in Brisbane, Australia, played a pivotal role in advancing ZBFBs, developing and marketing ZBFB systems for stationary energy storage applications. Their efforts helped bring ZBFBs closer to commercial viability. Bearing in mind the bulky nature of ZBRBs, one of RedFlow's notable achievements was the development of the ZBM2, a 10 kWh ZBFB system designed for residential and small commercial use. This system offered 100% depth of discharge capability, ensuring that users could maximize the utilization of stored energy. It includes a smaller stack design and a bi-directional DC-DC converter.


In recent years, more companies, besides RedFlow and Primus Power and Gelion, and EOS Energy Enterprises that make non-flow ZBRB, have continued to develop and deploy ZBFB systems. These deployments include grid-scale installations and collaborations with renewable energy projects.

Redflow’s QuadPod. It incorporates switch gear, BMS, protection, communications and hybrid inverter. Source: RedFlow
Redflow’s QuadPod. It incorporates switch gear, a BMS unit, protection, a communications unit and a hybrid inverter. Source: RedFlow

The Disadvantages

While zinc bromine flow batteries offer a plethora of benefits, they do come with certain challenges. These include lower energy density compared to lithium-ion batteries, lower round-trip efficiency, and the need for periodic full discharges to prevent the formation of zinc dendrites, which could puncture the separator. In addition, ZBFBs require sequestering agents to prevent toxic bromine vapor emissions, adding to the overall cost of ownership.


Generally, due to their architecture and low energy densities, ZBFBs store less electrical energy than lithium-ion for the same volume or weight. They are thus often too bulky to be used in mobile applications like electric vehicles and in electronics unlike lithium-ion solid state batteries. They are more suited to stationary energy storage applications, such as grid support and renewable energy integration. They are often deployed in common containers that are immovable and take up larger space. Their complex and integrated construction with moving parts, such as pumps and tanks can make them more expensive to manufacture and maintain than other types of batteries.


They have a relatively low power density, meaning that they cannot deliver a lot of power quickly and can only be used for storage purposes. Also, while lithium-ion batteries can achieve efficiencies of 90% or more, ZBFBs often operate in the range of 70-80%. While zinc and bromine are relatively low-cost materials, ZBFBs require expensive sequestering agents to prevent toxic bromine vapor emissions. These agents add to the overall cost of the system and can complicate the handling of the battery's components.

Gelion's Endure battery is a zinc-bromide non-flow battery that uses a gel electrolyte
Gelion's Endure battery is a zinc-bromide non-flow battery that uses a gel electrolyte instead of two tanks.
Current Research

Recent progress has been made in improving the cost and performance of VRFBs. For example, researchers have developed new electrolyte compositions that can reduce the amount of vanadium required, and they have also developed new membrane and electrode materials that can improve the efficiency and power and current densities of VRFBs.


Researchers have also developed new electrolyte compositions that can reduce the amount of vanadium required, such as using multivalent vanadium ions or adding other redox couples to the electrolyte. New membrane materials have been developed that can improve the efficiency and power and current densities of VRFBs, such as anion exchange membranes (AEMs) and composite membranes. New electrode materials have been developed that can improve the efficiency and power and current densities of VRFBs, such as carbon-based electrodes and metal-organic framework (MOF) electrodes.


ZFBs represent a compelling solution for long-duration energy storage needs. Their scalability, cycle life, and energy efficiency as well as the interest of market movers make them a viable contender in the evolving landscape of energy storage technologies. As renewable energy sources continue to expand, these flow batteries are poised to play a crucial role in ensuring a stable and sustainable energy future as well as disrupting lithium's stranglehold on the industry.

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