What Is the Second Law of Thermodynamics?

The electrolysis process itself is about 75% efficient. But the energy has changed form, and it's really important. Unfortunately, there's two types of energy. A kilowatthour is not a kilowatt hour. There's energy that we call work, and there's energy that We call heat. And when you do that, you have to convert that heat back into electricity again to use it. You can never get more than 50% efficiency out of it because of the second law of thermodynamics. So by doing the electrolysis yourself, you end up with something which is not nearly as useful,. It's very inefficient to convert that into the useful thing you need - electricity