(22b) The Solar Flow Battery: A Hybrid Device for Base-Load Solar Electricity

Solar energy is by far the most abundant renewable energy resource; indeed the energy in sunlight that strikes the troposphere exceeds global primary consumption by a factor of ~10,000. After many decades of development, photovoltaic technologies are approaching cost parity with fossil-derived electricity. However, the temporal intermittency of sunlight prevents photovoltaics from scaling indefinitely while maintaining a stable electrical grid. Thus there is a critical need for efficient and low-cost energy storage to enable dispatchable solar electricity. To this end, significant advances have been made in the academic and commercial sectors in solar-driven fuel synthesis as well as large-scale battery technologies. Among the most attractive approaches for grid-scale electricity storage is the redox flow battery (RFB), a flowable galvanic cell that, unlike solid-state batteries, decouples energy and power density and can be optimized for efficiency, cost, and robust operation at a large scale. Although several RFB technologies have achieved limited commercial success, significant advances are needed to prove that RFB technologies can be cost-effective when used with intermittent renewables.

Herein I will present a promising hybrid technology that combines aspects of solar photovoltaics with redox flow batteries, which we call the solar flow battery (SFB). The SFB concept is similar to solar-driven water splitting devices, in that it uses semiconductor light absorbers directly interfaced with chemical species that can undergo uphill redox reactions for net solar energy storage. Unlike solar fuels devices, however, the SFB makes use of electrolytes that remain in the liquid phase and exhibit facile electron-transfer kinetics. I will discuss first the operating principles of the SFB, emphasizing several straightforward design rules and the proposed benefits over alternative energy storage approaches. Then I will describe laboratory-scale demonstration devices that we have built based on a unique earth-abundant semiconductor material, WSe₂, that is attractive for use in SFBs due to its favorable electronic properties and high stability under electrochemical conditions.