If we consider the dissociation of hydrogen:
the right hand side of the equation can supply electrons to another reaction. The tendency for this to occur is in part dependent on the pH of the solution. If we consider alkaline hydrothermal vents we have a pH of around 11, this drives the reaction to the right because the protons avidly combine with hydroxyl ions to give water:
Which means that there is a marked tendency to supply electrons for any electron-accepting reaction. The electrons can hop on to an FeS barrier (each changing the charge on an Fe from 3+ to 2+) which separates the vent fluid from CO2 rich, acidic oceanic water:
Deriving from fluid with a pH of 11 these electrons have a redox potential of -650mV, ie they are highly reducing.
If we now look at the situation on the oceanic side of the barrier we have:
and by adding on the factor of an acidic pH, with lots of protons driving the reaction to the right we have this:
Under these conditions electrons supplied at -650mV are very able to allow the reaction to proceed to the right yielding CO. Repeating the process yields CH2O and metabolism is on its way.
OK. Nick Lane makes these points in his paper:
1. There is no contact between the H2 in the vent fluid and the CO2 in the ocean fluid. The two Hs in the formaldehyde come from oceanic protons combining with vent H2 derived electrons.
2. I've shown the reaction occurring once to CO and again to CH2O. Why stop at twice? Given a supply of -650mV electrons why not keep generating CO and inserting it, along with e- and H+, in to whatever hydrocarbon you have already got in the vent fluid? Nick Lane has reaction sketches for generating almost all of the Krebs cycle components on this basis.
Theoretically, if you wanted to make an origin of life reactor to test whether you can generate a multitude of the hydrocarbons at the core of metabolism you don't actually need a supply of alkaline hydrogen rich fluid. This only supplies electrons at -650mV. An alternative supply would be a 1.5 volt battery with some sort of voltage reduction to get from -1500mV to -650mv and you're away.
A microporous FeS electrode in Perrier water, energised by an AA battery via a couple of resistors and you might just be set up. Getting the apparatus anoxic and detecting the products might be more of a challenge!
Edit Finally followed Nick Lane's final reference. These folks have reached pyruvate via an energised FeS electrode. It's a lot more complex than Perrier water but it works. End edit