Is it safe to store my passwords in an encrypted .7z/.zip by 7-zip?

7-zip uses AES-256, which is rated acceptable for TOP SECRET documents by the NSA.

Assuming you use a strong pass-phrase that should be more than enough to persuade the attacker not to bother with trying to crack the file, but to move on immediately to beating it out of you with a wrench.

I'd personally use KeePass.

Not only KeePass has a portable version (that you can run straight off of a UFD), it's a full-featured password database, with an 'auto-type' feature so no one need to see what your password is.

If AES-256 encryption is safe enough, then 7-Zip will do that. It also provides an additional option to encrypt the filenames. If you're encrypting your data, you should probably encrypt the filenames, too.