We have seen plenty of emergency smartphone chargers in the past, some of which let you charge your phone using a campfire. You could always build your own though. Joohansson has published a guide that shows you how to build your own compact, thermoelectric emergency generator. The goal is to build a portable charger with an adjustable voltage.
You are going to need a high temperature TEG module, an aluminum plate, a small heat sink, bolts, washers, and a few other parts to complete this project. All the instructions and list of parts are available here.