Conventional Batteries vs. Flow Batteries

A flow battery, also known as redox flow battery, is a special type of electrochemical cells. They are batteries composed of two chemical components dissolved in appropriate solvents and stored in vessels separated using a membrane. The dissolved substances in the electrolytes (anolyte and catholyte) provide the chemical energy that is transformed to electrical energy. A pump is used to circulate the liquids in the system. Reactions occur at the current collectors sandwiching the ion-selective membranes. Ion exchange and electron transfer occur. The cell voltage of flow batteries typically range from 1 to 2.2 V. Redox flow batteries are also rechargeable. Once an external voltage is applied, the reactions are reversed, driving the regeneration of the fuels in the electrolytes. Aside from that, flow batteries can be instantly recharged simply by changing the liquid electrolyte.

Various chemistries and electrolytes have been explored for flow batteries, with an increasing trend among battery manufacturers to adopt vanadium as a fundamental material for both small-scale and large-scale applications. In the production of batteries, vanadium salts in sulfuric acid form the electrolyte. The utilization of vanadium in redox-flow batteries (VFBs) is gaining prominence as energy companies seek advancements in energy storage solutions.

The difference between conventional and flow batteries is that the chemical energy converted in flow batteries is stored as the electrolyte, whereas in conventional batteries, it is stored as the electrode material. 

What makes fuel cells and flow batteries different? Flow batteries work in a similar way to fuel cells. In fact, they have been commonly referred to as regenerative fuel cells. The key difference is that fuel cells directly consume a fuel, such as hydrogen, while flow batteries store and release electrical energy through the redox reactions of liquid electrolytes. In terms of components, fuel cells often use expensive catalysts like platinum, while flow batteries rely on redox-active materials for electrolytes.

How do Vanadium Redox Flow Batteries Work?

The key strength of vanadium RFBs lies in the multiple valence states of vanadium (+2, +3, +4, and +5). This unique feature enables the use of vanadium as redox couples in both negative and positive electrodes. Unlike other RFBs using different electrolytes on each side, vanadium RFBs notably minimize cross-contamination concerns. In a vanadium RFB, the active vanadium species of different valence states are stored in electrolyte tanks. The negative tank contains the V³⁺/V²⁺ couple, whereas the positive tank has the V⁵⁺/V⁴⁺ pair. In both tanks, the vanadium species (1–3 M) are dissolved in sulfuric acid (1–2 M). The electrolytes are non-flammable, non-explosive and no toxic products are generated through cycling.

During discharge, V³⁺ is oxidized to V²⁺ at the negative side. The generated electron flows through the current collector, the load, and finally to the current collector at the positive side. Ions from the electrolyte (either H⁺ or SO₄^2–) flow through the ion exchange membrane. Using the ions and generated electrons, VO₂⁺ is then reduced to VO²⁺ on the positive electrode.

During charging, the reverse processes occur. In simpler terms, throughout the charge-discharge cycle, the active vanadium species in the electrolyte undergo reduction and oxidation, effectively converting chemical energy into electrical energy. Maintaining electroneutrality is seamlessly achieved by transporting ions within the electrolyte through the membrane. The presence of dissolved redox species in the recirculated solution distinguishes RFBs from other batteries that store energy within electrode structures, like lead-acid and lithium-ion batteries. The unique 'non-participating electrode' structure of VRBs allows for the independent customization of power rating and storage capacity.

Flow Battery Components

Conventional batteries, exemplified by lithium-ion batteries, employ fixed anodes and cathodes made of materials like graphite or lithium-containing compounds. They utilize either liquid or solid electrolytes to facilitate ion movement between the anode and cathode, separated by a porous material. A Battery Management System (BMS) is crucial for monitoring and managing various parameters during the charging and discharging processes.

In contrast, redox flow batteries (VRFBs) feature an innovative design. Both the anode and cathode active materials in VRFBs consist of salts dissolved in a liquid electrolyte, eliminating the need for distinct materials. The electrolyte is stored in external tanks, offering scalable energy capacity. An ion exchange membrane (IEM) separates the two electrolyte tanks, allowing protons to pass through while preventing mixing of ions. Multiple cells are combined in a stack, and a pump and flow system circulate the electrolyte between the tanks and through the stack during charging and discharging. This unique design provides advantages in scalability and long cycle life, particularly in applications like grid energy storage.

In vanadium redox flow batteries (VRFBs), the electrodes commonly used are primarily carbon-based. This includes materials like graphite felt, carbon felt, and carbon paper. These carbon-based electrodes serve as crucial components, facilitating the electrochemical reactions and the flow of vanadium ions during the battery's charging and discharging processes. The use of carbon materials contributes to the electrodes' conductivity and durability, making them integral to the overall performance of VRFB systems.