Electricity storage is needed on an unprecedented scale to sustain the ongoing transition of electricity generation from fossil fuels to intermittent renewable energy sources like wind and solar power. Today pumped hydro is the only commercially viable large-scale electricity storage technology, but unfortunately it is limited to mountainous regions and therefore difficult to expand. Emerging technologies like adiabatic compressed air energy storage (ACAES) or storage using conventional power-to-gas (P2G) technology combined with underground gas storage can be more widely deployed, but unfortunately for long-term to seasonal periods these technologies are either very expensive or provide a very low round-trip efficiency. Here we describe 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% and an estimated storage cost around 3 ¢ kW⁻¹ h⁻¹, i.e., comparable to pumped hydro and much better than previously proposed technologies.

Observing Sperm Factories: The blood-testis barrier creates a permissive microenvironment for germ cell differentiation and protects meiotic germ cells from autoimmunity. To become sperm, large cysts of germline progenitors must cross the blood-testis barrier without causing damage. Smith and Braun (p. 798 , published online 20 September) show that somatic Sertoli cells, the gate-keepers of the blood-testis barrier, manage this task by building a network of compartments bounded by old and new tight junctions. Intercellular bridges, which connect germ cells within cysts, span the transient compartments by passing through vertical strands at tricellular junctions. Only after the formation of a new blood-testis barrier are old tight junctions removed and germ cells released.