there are those that might suppose pyrophosphite (with an i) as the first energy source (and we pretty much use the addition and subtraction of phosphate as out currency today) with something like iron as the terminal electron acceptor.
power comes from things, harnessing it involves moving electrons, and one needs a sink.
We use oxygen. There wasn't any then.
As far as energy source, the assumptions are that the first forms were heterotrophic, so you want something reasonably abundant and reasonably renewable.
Some of these extremophiles are worth a peak:
http://www.geobacter.org/Life-Extreme
The greater problem in some ways is coming up with compartments in the first place. Folks can spit out all sorts of organic molecules, and important ones, from primordial soup type of best-guess replication experiments ... but to actually harness anything, one needs to put things in compartments ... membranes, the mighty lipid bilayer (and friends). It is so much easier to get to making and maintaining DNA or RNA or even proteins than it is to get to making these massive fatty acid membranes. But, if the soup somehow was cranking out the end salted alkanes/alkenes, then early life is like a hermit crab ... the lipid providing the opportunity for segregation (electron transport, proton gradients, the stuff the the energy really is), but not a product of life ... yet, until something stumbled into the benefits of making its own fats.
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