Air carrier Captain reported taking possession of a passenger's electronic device in a thermal containment bag for safe keeping until destination arrival. The electronic device; which had previously experienced a thermal runaway; was turned over to ground personnel for proper handling.

Narrative

I'm filing this because I understand the company is tracking TCB (Thermal Containment Bag) use. I don't have much of substance as regards the actual onboard event to add to my report. The bag was really used out of an abundance of caution in anticipation of a potential issue. The device was not hot to the touch; but it was swollen and cracking as if it had been quite hot at some time. The passenger knew we had those bags and notified one of our Flight Attendants (FAs) who properly placed the device in the bag just in case.A more interesting question had to do with what do we do with a used bag upon landing. Nobody really knew. Dispatch suggested I contact operations and that the fire and rescue folks would want to meet the plane to take possession of the bag; which is ultimately what happened. Dispatch ran interference for us and advised operations at ZZZ that we would be handing them a TCB. I called operations in 'range' and verified that the fire department had been notified; which they had. ZZZ Approach Control had also been advised; which I guess is a good thing; just in case we needed to hurry up at any time. They asked if we needed any assistance; which we did not. Landing and taxi in were uneventful. The fire chief met us on the jet bridge and was waiting when we opened the door. Oddly; he asked us what he was doing there; that is; why was he called out? Apparently no one had advised him of our exact situation and what was expected of him. Once I explained it to him; he jumped right into action and took the bag outside to evaluate. Once he determined it was safe; he turned it over to a supervisor who was going to have it disposed of 'at the hanger.' I didn't get any additional details from him as to what that meant.