Vanadium Flow Battery

The rechargeable lead-acid battery, invented in 1859, is still the one of the most common methods of storing energy. However, even though there have been many improvements made to the original 1859 battery, the lead-acid battery is far from ideal.

Lead is poisonous. That’s not a problem while the lead is inside the battery, but lead-acid batteries need replacing every few years, and the old ones recycled. Lead levels around most lead-acid recycling facilities tend to be high, indicating that lead does get into the environment during the recycling process.
Lead is also very dense, which means lead-acid batteries are heavy. That’s not an issue if the batteries are on shelves at a wind farm, but it’s a major problem if you hope to run an electric car using lead-acid batteries.
During discharge, both the anode (Pb) and the cathode (PbO₂) form Pb²⁺ in the form of suspended particles of PbSO₄(s). In time, these tiny particles come together to form larger crystals of PbSO₄ which cake the electrodes and even fall off onto the floor of the cell. While the suspended particles are readily turned back into Pb and PbO₂ when the battery is recharged, the solid crystals of PbSO₄ do not decompose easily. This ‘sulfation’ process significantly reduces the performance of the battery. It is most likely to occur if the battery is left standing in a discharged state for some time. Fully-charged batteries also undergo sulfation, simply because a fully-charged battery slowly discharges even when there is no current being drawn.

Even since the lead acid battery was invented, chemists have been trying to make a better battery. While quite a wide variety of cells for small devices such as cell phones, hearing aids, cameras and computers have been developed; nothing has rivalled the lead-acid battery for large-scale energy storage. Until now, that is.

And the new...

Professor Maria Skyllas-Kazacos is an Australian chemist specialising in electrochemistry. In 1984 she started working on a new style of battery called a redox flow cell. In flow cells, the half-cells are separated by a membrane and the reagents for each half-cell are pumped through the stack of cells. Both oxidised and reduced forms in each half-cell must be in solution.

The first flow cell investigated by scientists at NASA (National Aeronautic and Space Administration) laboratories in the USA was an iron-chromium cell, but they soon discovered an fundamental problem with flow cells — there would always be some transfer of reagents across the membrane. Maria and her team realised that what was needed was a combination of half-cells where it wouldn’t matter if the reagents did mix. The answer was to use vanadium, which exists in four oxidation states — all soluble. On one half-cell vanadium (V) is reduced to vanadium (IV), while in the other half-cell vanadium (II) is oxidised to vanadium (III). Any ion that migrates across the membrane is quickly oxidised or reduced to the required state, so no long-term contamination occurs.

VO₂⁺(aq) + 2H⁺(aq) + e^–	VO²⁺(aq) + H₂O(l)
V²⁺(aq)	V³⁺(aq) + e^–

Each cell produces a voltage of 1.4–1.6 V (depending on the concentration of the solutions). The desired voltage can be produced by combining a large number of cells together in a cell stack — all connected filled by the same electrolyte tank and pump.

Vanadium is a relatively cheap and abundant metal and while the higher oxidation states of vanadium are toxic, vanadium flow batteries are very much safer than lead-acid batteries, because they do not need to be pulled apart and rebuilt every 2–3 years. The electrolyte solution can be charged and discharged indefinitely. The only part likely to require replacing are the cell membranes — about every 8–10 years.

Like lead-acid batteries, vanadium flow batteries are charged by reversing the current flow, however flow batteries can also be recharged simply be replacing the spent electrolyte solution with fresh solution. So, for example, an electric car could be powered by a vanadium flow battery. Under normal use, the owner drives the car during the day, then recharges the battery overnight using cheap night-rate electricity. However, in an emergency (perhaps the owner forgot to plug it in overnight, or they’ve driven further than usual), the electrolytes in the tanks can be replaced at a service station within minutes.

There are vanadium flow batteries in a number of locations around the world:

On King Island (between Australia and Tasmania) 70 000 litres of electrolyte solution stores the energy from a wind farm to supply energy to the residents when the wind does not blow. This significantly reduces the islander’s dependence on fossil fuels for electricity generation.
Many buildings in Japan, Canada and USA use vanadium flow batteries as back-up power supplies, to maintain current when the main electricity supply fails.
During peak periods, electricity demand may be 20% or 30% higher than for the rest of the day. These peaks put a strain on electricity generation systems (power stations), and also on the transmission lines that deliver the current. In Japan, vanadium flow batteries are used in several places to supply peak electricity where it is needed, avoiding the need to replace aging transmission lines or build new power stations. This solution would work in a number of places in New Zealand as well.

The disadvantage with the vanadium flow battery described above is that, although all the ions involved are soluble, they’re not very soluble. This low solubility means that the battery has a relatively low energy density (the amount of energy that can be stored per kg of battery). Maria is working on a second generation vanadium battery involving vanadium bromide, which is considerably more soluble than the current species. It is hoped that the new battery will have an energy density about twice that of the current version.