I am a gap year student so I have achieved A* on math but retaking physcis and chem which I have A and A* predicted.
To answer your questions, I think it really varies with everyone how they write their personal statment. However, for me I chose one specific topic like eg.is it possible to use nanomatieral to treat regenerative diesease? this shows that I have focused on the certain mateiral that interested me and tried to dig very deep about the potential of nanomatieral on the development of the treatment. I think there are many ways to solve enviornmental problem with materials. for example some of the recycled and biodegrdeble materials helped us to reuse materials again for certain use which reduced some of the waste and CO2 emission etc. If you find it hard you can go on and research what are some of matierals that contributed to solve environemtnal problem. I feel like imperial likes creative, critical thinking and very self driven person for their passion so think of the matieral that hasnt really been talked or developed in this world and how you can apply that matieral to certain places or applications to solve the environment problems. For example, lets say you chose nanomaterials and you research how you could make nanopartilce based paint that can be painted on the building which can caputure toxic pollutant in the atmosphere that could improve air polluiton. And to find more definitive evidence you could i read one of the professor article and led me to think blah blah or I went to one of this conference blah blah or you actually did a lab work with some help from your school teacher to prove blah blah. I advice you to research on varieties of materials. You could talk about robotics but you have to think how does the types of matireals is used in robots changed the way they behave and move and eventually do some of the work and could solve enviroenmtal problems. Please ask if you have any other questions!