I'm a student starting a research project on sustainable aircraft technologies. I've been particularly interested by distributed electric propulsion (DEP), which is currently being tested by NASA with the X-57 Maxwell (I'm thinking the added efficiency of DEP combined with progress in battery technology could eventually make electric flight a more feasible option). My supervisor has also pointed me to ion drives. Are there any other technologies in development that I should be aware of? Information on progress, potential, who is developing the technology, as well as links to scholarly articles are all appreciated.