Your question is general and there are several fields under physical chemistry such as corrosion, energy, nanotechnology, etc. However, choosing a research topic depends a lot on your supervisor’s field and what resources you have. Once you know your focus like solar energy, for example you start by checking out recent papers and studies to see what's already been done. Then you look for a gap something missing or not explored yet and that’s where your research can begin.

You can also use AI tools to help you out. like Semantic Scholar or SciSpace can find papers and summarize papers.