When it comes to fully subsonic propulsion, all outlets are converging nozzles (I did say fully subsonic so no converging diverging). From what I understand, they are flow regulating devices (changing the exit diameter allows to control the mass flow through the propulsion system, be it a turbofan or ducted electric fan).
Why don't we ever use diverging outlets, i.e. diffusers? Is it due to the back pressure? Wouldn't such a shape increase the pressure recovery, i.e. more thrust.