Redox dual-flow battery for combined electricity storage and hydrogen production

The energy transition towards a carbon-neutral and sustainable economy is one of the greatest challenges of the 21st century to combat global warming and pollution. The decarbonization process is affecting every sector of the economy (electricity, transportation, industrial
). For power generation, renewable energy resources are realistic alternatives to replace fossil resources such as gas and coal. However, the intermittent nature of wind and solar power limits their penetration into the conventional grid and increases the need for energy storage (battery, hydrogen storage
) for smoothing out fluctuations between electric supply and demand. In this context, the concept of the redox dual-flow battery was introduced to propose a hybrid storage solution that can store electrical energy both electrochemically and in the form of hydrogen fuel. The system differs from a traditional redox flow battery by including a secondary energy platform, in which the electrolytes can be discharged chemically in external catalytic reactors through redox-mediated water electrolysis to produce clean hydrogen. The dual storage feature improves the flexibility and enhances significantly the capacity of the system by storing energy beyond the capacity of the electrolytes in the form of hydrogen. Additionally, the redox-mediated water electrolysis offers several advantages over conventional electrolysis in terms of safety, durability and purity. In this thesis, the proof-of-concept of a dual-flow circuit using a vanadium-manganese redox flow battery for combined electricity storage and hydrogen production is demonstrated. The redox flow battery employs vanadium sulfate (V3+/V2+) and manganese sulfate (Mn3+/Mn2+) dissolved in concentrated sulfuric aqueous solution as negative and positive electrolyte, respectively. The galvanic cell achieves energy efficiency of 68% at a current density of 50 mA·cm–2 (cell voltage =1.92 V) and a relative battery energy density 45% higher than the conventional all-vanadium RFB in the same conditions. Once charged, the electrolytes can be spontaneously discharged through redox-mediated oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) in RuO2 and Mo2C catalytic reactors, resulting in an energy consumption for hydrogen production of ca. 50 kWh/kg H2. The system provides a competitive alternative for large-scale energy storage in renewable energy and transport applications.