A solar flow battery was developed that is made of silicon solar cells combined with advanced solar materials integrated with optimally designed chemical components. While solar flow batteries are years away from commercialization, they offer the potential to provide reliable electricity generation and storage for lighting, cellphones, or other fundamental uses for homes in remote areas. They combine the advantages of photovoltaic cells that convert sunlight into electricity with the advantages of flow batteries, which use tanks of chemicals that can react to produce electricity and be recharged by the solar cells.

Since the Sun doesn’t always shine, storage is key for practical solar electricity, especially in remote and rural regions with a lot of sunlight such as in the sunbelt regions of the US, Australia, Saudi Arabia, and Africa. Many solar home systems use lead-acid or lithium-ion batteries for electricity storage. Flow batteries could be less expensive at a larger scale and are an ideal storage choice for merging with solar cells.

The researchers turned to an increasingly popular material for photovoltaic cells: halide perovskites. The solar conversion efficiency of these special materials has dramatically increased. The materials can also increase the efficiency of traditional silicon solar cells by capturing more energy from the Sun. Yet silicon remains key for making a stable device that can withstand the chemicals in a flow battery.

The integrated solar flow battery contains both a solar cell and chemicals to generate, store, and redeliver renewable electricity from the Sun in one device. (Photo: Wenjie Li)

The team fabricated the perovskite-silicon solar cells with an additional protection layer on the silicon surface. To predict the ideal voltage at which the flow batteries should run, the team developed a new theoretical modeling method that allowed them to select a pair of chemicals in the flow battery that would operate at the ideal voltage based on the characteristics of the solar cell, maximizing efficiency. The chemicals are organic compounds, not expensive metals as in traditional flow batteries, and are dissolved in a benign water solution of table salt rather than strong acids.

The resulting device maintained a high efficiency over hundreds of hours and hundreds of charge-discharge cycles while retaining most of its capacity. That lifespan was several times longer than earlier devices.

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