What substance can add water to produce electricity (i.e. it can be used instead of dry batteries) U

There are many things that can produce electricity by adding water. Current liquid batteries use metals and other active oxides as two electrodes, but I think the next one is more interesting. You can take a look at the "water watch". priced at 1980 yen. This is a green watch that can generate electricity after adding water without using dry batteries. This product is a green digital watch that does not require dry batteries or external power. After pouring water into the water supply tank, it can generate electricity and the liquid crystal part will display the time. When the water runs out, the time display also stops, so it is necessary to replenish the water in time when the water is insufficient. A water injection can last 2 to 3 months. The metal and activated carbon inside the green watch will react with water and play thesame role as a dry battery. The metal part and activated carbon serve as the electrode materials, the negative electrode and the positive electrode respectively, and water becomes the electrolyte to generate current between the electrodes.