Can I use an sdxc card for Time Machine backups

You are welcome to try, but it is not considered one of Apple's supported backup devices: Backup disks you can use with Time Machine - Apple Support.

You are likely to find its performance unacceptably slow. I tried once out of morbid curiosity and gave up.

See if the following support article answers your question:

• Backup disks you can use with Time Machine - Apple Support

Addendum

I agree with John Galt on this. Some things you need to consider:

• You would need a card that can hold 2-3 times the capacity of your Mac.
• The card would need to be formatted for APFS.
• The card would need to be recognized by Time Machine as being an available backup destination.
• The "wear & tear" caused by the number of backups that will be written to this card.
• The fact that these cards are not considered reliable for storing data long term.