What is the name of the cell's energy currency molecule?

ATP, adenosine triphosphate, and currency is the right metaphor because it is spent rather than stored. Your body holds only about 250 grams of it at any moment and turns over something close to your entire body weight in it every day - each molecule recycled thousands of times.

The making of it is one of the strangest mechanisms in biology. Mitochondria pump protons across a membrane to build up a gradient, and those protons flow back through a protein that physically rotates - a molecular turbine, spinning at up to several hundred revolutions per second, with each turn producing three ATP. It is a rotary engine, discovered in the 1990s, and the demonstration that it actually spins won a Nobel Prize.

That gradient-and-turbine arrangement is shared by essentially all life, which is one of the arguments that it is very old - older than the split between bacteria and everything else. Some researchers argue the whole thing began at alkaline hydrothermal vents, where natural proton gradients across mineral membranes were available for free, and that early life did not invent the gradient so much as move in and take over one.