Fast charger

The wattage of the power supply doesn’t matter. You could charge with a gigaWatt power supply if there was such a thing. As long as the power supply outputs a USB standard 5 volts, the iphone itself will regulated the current draw. The power supply needs to be a minimum of 20W to support fast charging, but any Apple USB power supply of 20 or more watts will work just fine.

iPhones, iPads and Mac laptops all self-regulate their own charging. They will never draw more current then they are designed to use, regardless of how much excess current a power supply could provide.

Yes you can use it and no it will not harm the battery. An iPhone regulates its own charging and will not take any higher current than it is designed to use.