Associate Professor Robert Braun (ME) and collaborators developed a novel storage method combining recent advances in reversible solid oxide electrochemical cells with sub-surface storage of CO2 and CH4, thereby enabling large-scale electricity storage with a round-trip efficiency exceeding 70 percent and an estimated storage cost around 3 ¢ kW-1 h-1, i.e., comparable to pumped hydro and much better than previously proposed technologies. The results are published in the latest issue of “Energy & Environmental Science.”