For years, I've been occasionally distracted with a fascination about robots that eat, either plants or meat or whatever. I have not investigated research in artificial chemical reactions of this sort for producing energy, but I suspect some work has been done (such as this example).
I think a carnivorous robot could also make a great just-barely-sci-fi story. Probably also been done. There's nothing new under the sun. I just haven't run across it myself yet, nor have I searched such a thing out much. I'm not much a fan of horror, so I wouldn't take it there myself, but I don't mind getting a bit edgy and uncomfortable. I think it could be well done.
In any case, just got reminded of that notion by this blog post. I haven't actually watched the mentioned ads yet, but I'll likely get around to it.
And, of course, solar powered robots are a lot more palatable (pun considered) to our sensibilities and are clearly practical for some situations.