Ann: SOR Powers Bluetooth Sensor Device with Moisture, page-71

https://www.nature.com/articles/s41467-018-06633-z
Here is another good article. Seems that there are two elextrode plates between the graphene substance. This substance has the ability to disassociate water molecules, producing protons (charged particles). The graphene substance also has a gradient which keeps those free protons separated, creating a potential voltage between the electrodes.

The tricky part seems to be getting the graphene substance right, by modification with other molecules so that it maintains its gradient and efficiency.

Then, I suppose it's about engineering the product (where we are up to), so that it produces a reliable and usable current. But also, that it can be cheaply mass produced, small enough and unlikely to degrade easily. If these are achieved, then it's all on. I for one would like to see a pair of smart sneakers. Perfect place to house sensors and a battery pack.