20 w charger and iPad Air 1

Yes, you can charge your iPad Air1 with the Apple 20W Power Adapter - although your iPad Air1 will charge no faster than using the 12W Power Adapter with which it was originally supplied.

To charge your iPad from the 20W Power Adapter you will require a USB-C to Lightning charging cable:

• USB-C to Lightning Cable (1m)

https://store.apple.com/uk/xc/product/MM0A3ZM/A

No. Using the 20W Power Adapter will have absolutely no negative impact upon your iPad. Your iPad regulates its own battery charging - and will only draw the power that it needs from the Power Adapter.

The only discernible difference that you might notice is that the 20W Power Adapter may run noticeably cooler than the original 12W Power Adapter when charging your iPad Air1.