Add the chopped onion, minced garlic, drained black beans, drained corn, diced tomatoes with juices, and vegetable broth to the slow cooker.
Sprinkle in the chili powder, cumin, cayenne pepper, salt, and black pepper. Stir until the spices are evenly distributed.
Cover and cook on low for 6 to 8 hours or on high for 3 to 4 hours, until the chili thickens and the flavors meld.
Stir once more, taste, and adjust salt or cayenne if needed before serving.