Since no one else is taking this up
Seriously...
You don't get more energy out of burning the O2 than you put into the cracking process. In fact, you get less, since these processes are by no means 100% effecient.
This means that fine, if you want to use photo energy, you can. But there are more effecient ways to get the electricity than photo. Cheapest way right now is burning something. So you burn natural gas, or diesel, or kerosene, or coal to generate electricity. And then use the electricity to crack the water.