A 2012 pre-print, Self-Charged Graphene Battery Harvests Electricity from Thermal Energy of the Environment claims that a graphene device was used to generate a voltage of 0.35V by capturing some of the energy of ionic thermal motion in a solution of CuCl2.
They claim to have excluded the possibility that this voltage was driven by a regular chemical reaction.
This result has been widely positively cited on the web. However, it also had its detractors:
Nature had a blog article, Sparks fly over graphene energy device:
Few researchers are convinced.
It cites a few researchers pooh-poohing the result.
I haven't heard of any further developments regarding it.
Was a prototype device built that showed that power could be drawn from ionic thermal motion?