I’m not sure about an app with just katakana words, but I know that Torii teaches you a 10k vocab deck, which has a large chunk of katakana words in it. So you learn katakana words and you’ll improve your grammar.
It also has a setting to remove all vocab presented in Wanikani, so that you won’t learn the same word twice.
I’m pretty sure Kitsun.io has multiple decks that are specifically katakana focused, but it is a paid service, so keep that in mind.