In the bottom of the slow cooker, whisk together coconut milk, chicken stock, tomato sauce, curry powder, salt and cayenne. Add chicken breasts, onion, chickpeas and sweet potatoes. Using tongs, gently toss ingredients together to ensure evenly coated. Cook on Low for 8 hours or High for 4 hours. Stir lemon juice 5 minutes before serving. Serve over rice and with plenty of fresh cilantro.