Understanding why the vanadium redox battery, or VRB, is such an important development in battery technology requires a basic understanding of how batteries function. This article discusses very basic battery technology and explains the general principles behind flow batteries with an emphasis on VRB chemistry.

Batteries are broadly defined as primary or secondary batteries. These terms simply indicate whether or not the battery is rechargeable, and there are many different battery technologies within each group. While any battery can theoretically be recharged, primary batteries are designed to be discarded after the energy stored in the battery is depleted.

The very first battery was invented by Count Alessandro Giuseppe Antonio Anastasio Gerolamo Umberto Volta, an Italian physicist, in the year 1800. This primitive battery, which was called a Voltaic Pile, was made by stacking alternating discs of copper and zinc on top of each other, separated by a layer of cloth soaked in salt water. The outermost zinc disc served as the anode, and the outermost copper disc served as the cathode. Electrons were provided by the zinc, which was oxidized in the reaction, and flowed through a completed circuit to the copper, which was reduced. Volta did not understand the chemistry behind his own invention and failed to recognize the corrosion of the zinc discs as the source of the energy supplied by the Voltaic Pile.

From the Voltaic Pile, battery technology was improved by John F. Daniell, an English chemist, with the introduction of the Daniell Cell. His invention used solutions of differing densities to separate electrodes and circumvented the corrosion problems of the Voltaic Pile. His batteries were not portable, but this technology was used for many years for stationary applications. Before electrical utilities became widespread, Daniell Cells were commonly used to power doorbells and telegraph systems.In the late 1860s, Georges Leclanché, a French chemist, invented what is now called a wet cell battery. The Leclanché Cell used a reservoir of ammonium chlorite as the electrolyte solution. Zinc was used as the anode, and carbon was used as the cathode. This same chemistry was later developed into the dry cell battery by the Germanchemist Carl Gassner.In 1896, the National Carbon Companybegan producing paper-lined, 1.5 volt cylindrical dry cells that were called Columbia batteries. These were the first commercially available batteries in the US. National Carbon Company eventually became the Eveready Battery Company.All batteries convert chemical energy into electrical energy. The primary batteries discussed so far use up a component during the reaction and thus cannot easily be regenerated, but they do hold a charge for long periods of time when not in use.

Secondary batteries are specifically designed with reactants that will undergo a reversible chemical change. The reaction produces a flow of electrons to provide power. When electrical current is applied to the battery, the reaction is reversed, and the original chemicals are reformed.

The positive terminal of a primary battery is always known as the cathode, and the negative terminal is always known as the anode. In a secondary battery, this terminology is the same during discharge but reversed during recharge cycles. For this reason, terminals are usually described as positive or negative in secondary batteries, and the terms anode and cathode are not usually used.

Lead-acid batteries, familiar as standard automobile batteries, are a common secondary battery technology. They are the oldest type of secondary battery. Each cell of the lead-acid battery contains a lead negative electrode and a lead(IV) oxide positive electrode. A solution of sulfuric acid, usually 33 percent by volume, serves as the electrolytic conductor.

When a lead-acid battery is discharging, the sulfate ions from the sulfuric acid solution combine with the lead of the negative electrode to produce lead sulfate and free electrons. The electrons flow through the circuit and ultimately combine with the lead(IV) oxide and sulfate ions at the positive electrode to also produce lead sulfate. During the recharging cycle, electrons are stripped from the negative electrode and forced to the positive electrode. This changes the lead sulfate at both electrodes back to lead and lead(IV) oxide, respectively.

As all automobile owners know, lead-acid batteries eventually fail. Internal corrosion and loss of electrolyte activity eventually leave a build-up of lead sulfate on the internal plates and make recharging impossible.

Nickel-cadmium batteries were very common at one time, and they tolerated many more charge-discharge cycles than a typical lead-acid battery, but they were banned by the European Union in 2004 because of the toxicity of cadmium and have now been almost completely replaced by nickel-metal hydride, or NiMH, technology.

NiMH batteries use a hydrogen-absorbing alloy as the negative electrode and nickel oxyhydroxide as the positive electrode. They provide double or triple the storage capacity of nickel-cadmium batteries, but they lose charge at a fairly fast rate. They cannot be stored in a charged state for long periods of time and hold their charge.

Lithium-ion batteries are now widely used in consumer electronics in lieu of NiMH batteries. Lithium metal is extremely lightweight, so the batteries have a high energy density. Energy density is the term used to describe the energy provided per unit weight of the battery. They also exhibit a very slow loss of charge when not in use, so can be stored for long periods of time in a charged state.

The anode is typically carbon, and the cathode is a metal oxide. It is typically a layered oxide, such as lithium cobalt oxide, but it may also be constructed of lithium manganese oxide or a lithium iron phosphate. The electrolyte is a proprietary mixture of organic carbonates and non-aqueous, non-coordinating lithium ion complexes. The electrolytes are separated by a polymer membrane.During the discharge cycle, lithium ions carry charge through the polymer membrane from the anode to the cathode. When a higher voltage is applied, lithium ions move in the reverse direction, from the cathode to the anode.

Vanadium Redox Flow Batteries Or Fuel Cells

VRB Flow Batteries

Flow batteries differ from conventional rechargeable batteries in two very important ways. First, the electrochemical reaction occurs between two electrolyte solutions, not between an electrolyte and an electrode. This means that there is no deposition of material onto the electrode surface, and no electrolytes are lost as the battery is repeatedly discharged and charged.

Cell Stack “electrolyte solution”

The second difference is that the electrolytes are stored in external tanks. The electrolyte solutions are mechanically circulated through the portion of the battery where chemical energy is converted to electricity, which is called the cell stack, and the storage capacity of a flow battery is limited only by the physical size of the electrolyte storage reservoirs.

Typical Use For Flow Batteries

Flow batteries are the ideal technology for power storage at renewable source generators. Once a conventional rechargeable battery has been fully recharged, excess electricity that a wind turbine or solar photovoltaic panel might produce has nowhere to go and is wasted. Flow batteries, because they are limited in capacity only by the size of the electrolyte storage tanks, can be designed large enough to take advantage of all of the excess electricity generated.

What Does Redox Mean?

Because the reversible electrochemical reactions in these batteries are oxidation-reduction reactions, they are often called redox batteries. Redox is the term chemists use to describe reversible oxidation-reduction reactions.

What Determines The Power Output Of A Redox Battery?

The power of a flow battery is determined by the number and size of the cell stacks. The capacity of the battery is determined by the volume of electrolyte stored in the tanks. These factors are independent of each other, so flow batteries can be optimized for either energy storage, power delivery, or both.

Why is vanadium used in VRB’s “Vanadium Redox Batteries”?

Many electrolyte systems have been investigated for flow batteries, but vanadium redox batteries are currently the most popular. The primary reason for this popularity is the simplicity of the electrolyte system. Vanadium has four stable oxidation states, and so the electrolyte system can consist of a single active element.

Parts To A Vanadium Redox Battery VRB

In a VRB, the electrodes are constructed of carbon felts or porous, high surface area graphite plates. The solution in the cell with the negative electrode is called the anolyte, and the solution in the cell with the positive electrode is called the catholyte. VRBs use a mixture of VO2+ and VO2+ ions in the anolyte and a mixture of V3+ and V2+ ions in the catholyte. Both solutions contain sulfuric acid and are strongly acidic. The two solutions are separated by a proton-exchange membrane that selectively allows charge-carriers to cross but prevents diffusion of vanadium ions.

Chemical process that go on inside a VRB

When a VRB is discharging, electrons are removed from the negative terminal and V2+ ions are oxidized to V3+ ions. Electrons are added to the positive electrode and VO2+ ions are reduced to VO2+ ions in the positive half-cell. When the VRB is being recharged, these reactions are reversed.

Recent positive modifications to the electrolyte solution

In March of 2011, the Department of Energy’s Pacific Northwest National Laboratory announced that a modification to the electrolyte solution resulted in a 70 percent increase in the energy storage capacity and significantly expanded the operational temperature range. The modified VRBs use a hydrochloric-sulfuric acid mixture in the electrolyte solutions.

Vanadium Market VS Vanadium Demand

Supply of vanadium is a potential limiting factor in the continued development of VRB technology. Vanadium is primarily used in metallurgical applications, but the growth of VRB applications is placing a greater demand on a limited supply of vanadium materials. Continental Precious Minerals Inc. is a mineral exploration company headquartered in Toronto, Ontario. The prime focus of Continental is the exploration and development of their Sweedish vanadium properties. The company intends to produce vanadium especially for vanadium redox batteries.

VN:F [1.9.22_1171]

VN:F [1.9.22_1171]