Spray the crockpot insert with non-stick cooking spray. Place the 24 oz of chicken breasts in a single layer at the bottom.
In a medium bowl, whisk together the 3 cups of chicken broth, 3 tbsp lemon juice, 1 tbsp lemon zest, 1 tbsp minced garlic, 1 tsp oregano, and 1 tsp thyme. Pour this mixture evenly over the poultry.
Cover and cook on LOW for 3 to 4 hours, or until the meat reaches an internal temperature of 165F.
Carefully remove the meat from the crockpot and set it aside on a cutting board to rest, loosely tented with foil.
Stir the 1.5 cups of rinsed white rice and 2 tbsp of butter into the remaining hot broth in the slow cooker. Cover and cook on HIGH for 45 to 60 minutes, or until the liquid is absorbed and grains are tender.
Slice or shred the rested meat, return it to the pot, and stir gently to combine before serving.