Solar energy is the wave of the future – powering our homes, businesses and even our cars. This sustainable, renewable source of energy is being studied extensively at the University of Wisconsin-Madison, and in May 2022, the Wisconsin Alumni Research Foundation (WARF) awarded a $100,000 grant to four groups of research under the Accelerator Electrification Challenge Grant.
One of the labs selected for this award, the Song Jin Research Group, is working on a new solar battery that could change the way we power our world.
This project is led by Professor Song Jin alongside Assistant Professor Dawei Feng and graduate student Ethan Auleciems. The project focuses on high performance solar batteries for public and commercial use. These batteries are a step above the traditional batteries that we use in our daily lives.
The batteries used in your cars, phones and other gadgets use reduction-oxidation (redox) reactions to produce electricity. During a redox reaction, molecules dissolved in the electrolyte solution gain and lose electrons. During oxidation, neutral (uncharged) molecules lose electrons. This reaction creates a positively charged cation and free electrons. In contrast, reduction occurs when a cation gains electrons, becoming a neutral molecule again.
This chemical reaction occurs at the current collectors – the positive and negative sides of the battery. Neutral molecules go to one current collector while electrons go to the other. The positive and negative sides cannot contact each other, otherwise the battery will stop generating electricity. A barrier in the battery allows electrons and neutral molecules to move through the solution but keeps the positive and negative sides separate. The movement of these molecules allows us to power our homes, cars, buildings and more.
Flow batteries, however, take a different approach.
Auleciems said his lab “is moving away from [traditional batteries].”
“We’re trying to separate the battery components into a flow battery,” he continued.
Flow batteries are unique because they separate the electrolyte from the collectors. Rather than storing the electrolyte with collectors, flow batteries store it in separate storage tanks.
“It allows us to separate the storage and the chemistry from it,” Auleciems explained. “Normally, if you want to increase the capacity of a battery, you have to buy a brand new battery, a bigger battery. With a flow battery, to increase the capacity, you just have to buy a bigger tank.
This separation makes flow batteries more flexible and economical than traditional batteries. As power requirements increase, the electrolyte reservoir can be inexpensively expanded.
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“Going into a solar flux battery, you’re essentially adding a solar cell as the third electrode,” Auleciems explained.
This “third electrode” can serve as an electrode donor or as a neutral molecule donor depending on the type of solar panel used.
A solar cell array has three modes. During the first mode, the solar cell battery can be used in the same way as a normal solar panel, converting the sun into electricity. The second mode is used for storage, where sunlight is converted into a charged electrolyte, but not transformed into electricity. The third mode can be used at night or on days with very little sunlight, where the stored and charged electrolyte is then converted into electricity.
Ordinary solar panels store their electricity in lithium-ion batteries or other rechargeable batteries. Auleciems noted that these types of solar panels can be good for small applications, but have limited expansion capability due to high cost.
“To feed the Memorial Union, you don’t want [traditional batteries],” he explained. “For large applications, flow batteries are very attractive, but the research isn’t there for energy density.”
Energy density and energy efficiency are issues that Auleciems and the lab are working on. Energy density is the amount of energy that can be stored in a system, while energy efficiency aims to limit the amount of energy lost as molecules move through the system. Auleciems’ project focuses on a solar flow battery with two solar cells instead of one. This doubles the battery voltage, in turn doubling the energy density of the system.
Energy efficiency has been an issue in flow batteries since their introduction in the 1980s. Recent advances by the lab group have increased efficiency by 1.4-20%, an exponential increase in energy efficiency.
“The end goal is to get [energy efficiency] as high as possible, but the fact is, at what cost? Auleciems said.
Generally, more expensive solar panels have higher efficiency, but Auleciems’ goal is to make this technology as accessible and affordable as possible.
“The holy grail would be to have a silicon-based solar cell, which I’m working on right now,” Aluciems explained.
Silicone has the advantage of being cheap, easily accessible and very well studied. Combining these solar cells with a similar electrolyte could further increase energy efficiency while keeping costs reasonable.
Auleciems’ vision with this project is to provide small communities with a reliable power supply all year round, especially in remote areas of the world.
He explained that communities in the mountainous regions of southern Brazil, for example, need electricity but are unlikely to be connected to the main grid due to their location. Solar cell batteries are a solution that can grow and shrink with the size of the community and the energy needed, while being durable and relatively inexpensive.
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