New process for storing renewable energy

Dr. Neil Canter, Contributing Editor | TLT Tech Beat January 2013

A battery technology was developed to store energy generated by renewable resources such as solar and wind. 

KEY CONCEPTS
• An aqueous, electrolyte battery has been developed that can be used to store energy generated by renewable sources such as solar and wind.
• The battery is prepared with a cathode that has a composition similar to Prussian Blue and a hybrid anode combining activated carbon with polypyrrole.
• This battery displays good efficiencies and demonstrates good durability after testing over 1,000 deep-discharge cycles.

THE MOVEMENT TO DEVELOP ALTERNATIVE ENERGY SOURCES such as solar and wind (see Figure 2) has been ongoing due to the rising cost of energy from petroleum sources. But reliance on them as primary sources for powering the electrical grid are challenging because their ability to generate power in a consistent manner is questionable.


Figure 2. A new battery technology has been developed that can be used to store energy generated from renewable sources such as wind. (Courtesy of Stanford University)

Think about solar for a minute. It requires the sun, and we all know that there will be days when the sun is obscured by clouds. Wind is a variable parameter and does not usually persist in a specific environment at a steady rate.

This leads to the conclusion that there is a need for development of an effective way to store energy produced by these renewable sources so that a more consistent approach can be taken for the electrical grid. Battery development has been ongoing, particularly to be used in automobiles and in electrical devices.

In a previous TLT article, a new anode prepared from amorphous titanium dioxide nanotubes was discussed (1). Researchers found it to be a good alternative to carbon because it exhibits superior power and energy densities and converts into a crystalline material that can be even more effective because it accommodates more ions. Initial testing was done with a lithium cathode. As part of this work, an alloxide sodium battery was also developed and tested for the first time.

Currently, there are a number of storage technologies that have been tried with renewable energy sources but suffer from disadvantages. Dr. Yi Cui, associate professor in the department of materials science and engineering at Stanford University in Palo Alto, Calif., says, “One of the electrical- storage technologies currently available on the electric grid is pump-hydro that involves using available energy to pump water from an available source (such as the ocean or a lake) up to a high elevation and then enabling gravity to move water back down to the ground so that it can turn turbines and convert the potential energy of water to electricity.”

To use this technology, a water source is necessary, plus consideration must be taken to account for evaporation. Other processes under development include using a flywheel and working with lead-acid batteries. Cui comments on both of these, “The flywheel is very effective to store energy by spinning for a short period of time (10 minutes), but friction causes the spinning to flow down, stealing energy that could be used in a more beneficial fashion. After 150 years, lead-acid batteries work well in automobiles but do not have the long-term performance to be effective in supplying the electric grid.”

The need exists for a more effective way to store energy produced by renewable sources. Such a technology has now been developed.

AQUEOUS-RECHARGEABLE POTASSIUM BATTERY
Cui and his fellow researchers have developed a new aqueous, electrolyte battery technology by combining new cathode and anode materials. The battery uses potassium ions to cycle charge.

Cui says, “This battery exhibits an excellent efficiency between 95% and 99% when cycled at a low rate of 5 C and 79% efficiency at a higher rate of 50 C. It has good durability with no capacity loss seen after undergoing 1,000 deep-discharge cycles. We consider this to be a really high-end battery.”

The cathode material is derived from nanoparticles of copper hexacyanoferrate, which is similar in composition to the well-known dye, Prussian Blue. Cui says, “From past work reported in the literature, Prussian Blue is used as an electrochromic window, which means that it can initiate a color change as a coating on glass when voltage is applied.” Cui also noted that Prussian Blue has a very open structure which is good for the ion flow needed in batteries.

For the anode, the researchers combined the excellent cycling ability of activated carbon with the low potential of polypyrrole in a hybrid anode. Cui explains the challenge in developing the right type of anode, “It is very hard to find an anode voltage that has a low enough potential not to split water into hydrogen and oxygen in an aqueous system. Activated carbon cycles well but has too high of a potential relative to hydrogen. With a potential of -0.2 relative to hydrogen, polypyrrole has the right properties to reduce the overall potential when combined with activated carbon to an acceptable level.”

The researchers still needed to determine the optimum ratio of polypyrrole to activated carbon to maximize battery performance. Cui says, “We evaluated polypyrrole at 5%, 10%, 15% and 20% treat rates relative to activated carbon. Our objective was to find the right concentration of polypyrrole to ensure a high cycling rate, yet have a potential that was close to hydrogen.”

The researchers found 15% to be the ideal treat rate for polypyrrole. Assembling a full cell with the anode and cathode is straightforward, according to Cui. Future work will involve running long-term testing for at least one year to evaluate the performance and durability of the battery.

Cui also indicates that the researchers will be scaling up the technology to assess its commercial viability. He adds, “We are looking for even more effective anodes and are evaluating a Prussian Blue-type material as the anode.”

Additional information can be found in a recent article (2) or by contacting Cui at yicui@stanford.edu.

REFERENCES
1. Canter, N. (2012), “Titanium Dioxide: New Anode Material for Batteries,” TLT, 68 (2), pp. 12-13.
2. Pasta, M., Wessells, C., Huggins, R. and Cui, Y. (2012), “A High Rate and Long Cycle Life Aqueous Electrolyte Battery for Grid-Scale Energy Storage,” Nature Communications, 3 (1149), DOI:10.1038/ncomms2139.


Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat items can be sent to him at neilcanter@comcast.net.