Solar battery runs on light and air

Scientists from The Ohio State University (OSU) have succeeded in combining a battery and a solar cell into one hybrid device, described as the world’s first solar battery. Their study has been published in the journal Nature Communications.

Professor Yiying Wu and doctoral student Xiaodi Ren recently invented a high-efficiency, air-powered battery that discharges by chemically reacting potassium with oxygen - a design which won the $100,000 clean energy prize from the US Department of Energy in 2014. Wu described the invention as a “breathing battery”, saying, “It breathes in air when it discharges and breathes out when it charges.”

The researchers sought to combine a solar panel with a battery similar to their award-winning effort. The problem was that solar cells are normally made of solid semiconductor panels, which would block air from entering the battery. So doctoral student Mingzhe Yu designed a permeable mesh solar panel from titanium gauze - a flexible fabric on which he grew vertical rods of titanium dioxide. Air passes freely through the gauze while the rods capture sunlight.

Normally, connecting a solar cell to a battery would require the use of four electrodes, but the hybrid design uses only three. Beneath the mesh solar panel (the first electrode), the researchers placed a thin sheet of porous carbon (the second electrode) and a lithium plate (the third electrode). Between the electrodes, they sandwiched layers of electrolyte to carry electrons back and forth.

During charging, light hits the mesh solar panel and creates electrons. Inside the battery, electrons are involved in the chemical decomposition of lithium peroxide into lithium ions and oxygen. The oxygen is released into the air and the lithium ions are stored in the battery as lithium metal after capturing the electrons. When the battery discharges, it chemically consumes oxygen from the air to re-form the lithium peroxide. An iodide additive in the electrolyte acts as a ‘shuttle’ that transports electrons between the battery electrode and the mesh solar panel.

Scanning electron microscope images show the researchers’ solution: nanometre-sized rods of titanium dioxide (larger image) which cover the surface of a piece of titanium gauze (inset). The holes in the gauze are approximately 200 micrometres across, allowing air to enter the battery while the rods gather light. Image courtesy of Yiying Wu, The Ohio State University.

The researchers used a red dye to tune the wavelength of light captured by the mesh, thus creating a dye-sensitised solar cell. They originally used the chemical ruthenium as the dye, but this was consumed in the light capture, causing the battery to run out of dye after eight hours of charging and discharging. They turned to a dark red semiconductor that wouldn’t be consumed - iron oxide, or rust - which enabled the battery to charge from sunlight while retaining its red colour.

Wu and his students believe their device will not only bring down renewable energy costs by 25%, but it will also improve solar energy efficiency. Typically, only 80% of electrons emerging from a solar cell make it into a battery, due to the loss of electricity that normally occurs when electrons travel between a solar cell and an external battery. With the new design, light is converted to electrons inside the battery, so nearly 100% of the electrons are saved.

The team believes their solar battery’s lifetime will be comparable to rechargeable batteries already on the market. They hope to explore ways to enhance its performance with new materials and plan to license the battery to industry.

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