Did you remember a plot where Heisenberg use a metal chemical process to produce electricity on Breaking Bad. However, Patreon like chemistry teacher Walter White instead of Heisenberg (heavy-duty drug trafficker) who make a Lemon battery to generate electricity. The results, of course, are usually abominable. How playful lemon generate electricity?
In fact, it worked! As long as metal on your side, you can generate electricity to create fire by acid substance. This is not a new fangled thing but it can save your life when you are in the fields.
ps: Copper and Zinc is best choice. Because they can form a good aside battery environment with citric acid.