[news.cnet.com]

The concept of actual combustion taking place in my pocket causes me to hearken back to the days of catalytic 'hand warmers'. Which, when poorly maintained, would cause actual flames.

I tried a previous version from another firm (based on sodium borohydrate)

Many of the cells were defective, and even when it worked it barely managed to put out even 1 watt (not nearly enough for today's power-hungry smartphones).

As the article notes, lithium-based battery packs were the better alternative.