Yeah but that's a function of self-replication. If you could engineer a tuber or vine which did nothing but try to build up a huge H+ ion gradient between it's

Yeah but that's a function of self-replication. If you could engineer a tuber or vine which did nothing but try to build up a huge H+ ion gradient between it's roots and it's tip, then we could just let that grow indefinitely and use it as a battery directly. Similarly most models of "grey goo" would functionally be this: machines autonomously covering the biosphere in more efficient light harvesters.