I have been wondering about the switch charger today. I want a second one so I dont have to unplug it from the dock. I was thinking about three options, a normal 2.1 amp usb charger plus the USB C cable that comes with the Pro Controller, or the USB C charger for my phone (5V 3A), or the official charger.
I needed to know the specs of the official charger, so I tweeted Vooks (Aus Nintendo site). I got a reply with this

So it uses 5V and 15V. I dont know all the details of the new USB C power delivery standard, but I have heard about it using low and higher voltages. Getting the official one seems like the best idea.
i just hope it works with my portable charger, but I dont see why it wouldnt.