The lemon is acedic acid and most fruits and vegtables should work, however; a stronger acid generally benefits battery efficiency.
In reality though, the strength of a battery is a function of the anodic and the cathodic half cell. Moreover, rust is a battery and the first batteries used very different metals on the electronotive scale and a relatively neutral solution media like seawater as an electrolyte.
Look at
homemade batteries. for a goof basic discussion of batteries and electochemistry.
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