When to use a Sigmoid function and when to use ReLu in a Convolutional Neural Network
Relu solves the gradient vanishing problem and stops the inactive neurons. Using Sigmoid in output layer because its range is (0, 1) and it can represent the probability of binary class. When you are doing multi-classification, it is more appropriate to use Softmax function.

Maybe you need to read more materials, you can refer to CS231n. Thanks.