Impressive! This came out really really well and would probably make a great shirt. I like how the bat signal connects inside and outside. Very effective use of the different values. The city appears to have some depth although all the buildings, the bats and the fog themselves are flat.
The only thing that could be changes is the border. I know it's the cape that surrounds the cityscape, but it seems too straight. Especially the borders to the left and right could use some wavey form to make it look more like and actual flowing cape. As of now, it still comes across but appears a bit stiff.
What I really like is how you added light color tones to the face and the lightning. It adds a lot of energy (both figuratively and literally) to an otherwise calm and grim image.
Overall, well done, of course.