Using something like cytochrome c reductase to turn hydrocarbons (bio- or geo-) into electricity and then into motion would be a big efficiency improvement over

Using something like cytochrome c reductase to turn hydrocarbons (bio- or geo-) into electricity and then into motion would be a big efficiency improvement over our current heat engines. Still, it's only half the problem. We also want an efficient and compact way to turn electricity into energy-dense chemicals. AFAIK, that doesn't exist in nature; we'd have to start from scratch.