Spray slow cooker with a non stick cooking spray. Place chicken, ( frozen or thawed ) inside slow cooker.
In a medium bowl mix together brown sugar, vinegar, soda, garlic, soy sauce, and black pepper together. Pour sauce over chicken. Cook on high 4 - 5 hours or on low 6 - 8 hours.
Take chicken carefully out of slow cooker. ( if it falls apart that's ok.)Put the chicken in any serving dish that's at least 2 inches deep. Pour the remaining sauce left in the slow cooker into a saucepan. Place saucepan on high heat. Mix together water and cornstarch and pour into the saucepan. Mix well. Let sauce come to a boil and continue to boil for 3 minutes or until it starts to thicken and turns into a glaze.
Remove from the heat and let it sit 2 minutes. It will get thick as it cools. Pour the sauce/glaze over the chicken. Sprinkle with red pepper flakes.