Fat-bar training increases grip and overall strength. You can buy training tools to create a fatter/open grip or you can just use kettlebells, just not in the traditional manner. Instead of using the handles, grasp the cannonball part of the kettlebell and curl away. You can only perform palms-up curls with this method, but what a burn! Try them standing, seated, in an incline position, or on a preacher bench.
Always implement thick-grip work slowly and gradually. Guys often buy fat-grip attachments and then rush to the gym to try them out on anything they can get their hands on. And what happens only days later? Tendonitis, and it takes a while to heal. Don't be overzealous. Be a tortoise with these things and you'll win the race over time.