The reason this does not work is because it takes more energy to break the Hydrogen bond than is able to be captured by recombining the Hydrogen and Oxygen to produce power. While not everyone can be expected to do the math that proves this, the basic idea is simple and easy to understand.

Every process is inefficient to some degree, and each operation loses some of the energy potential to it due to inefficiency. The theoretical power to be gained from combining Hydrogen and Oxygen is just less than the amount of power necessary to break the bond in water. This alone means that using water as fuel requires additional energy than water can produce. Now, add the rate of efficiency loss to that loss, and it should be logically obvious that breaking down water using electricity from a battery to power an internal combustion engine (that also loses the power in the mechanisms that translate the power from burning into mechanical energy, and then each time the energy is transferred mechanically) is not going to produce more power than is in the battery to begin with.

Indeed, just driving an electric motor with the battery is orders of magnitude more efficient, as fewer transformations are necessary to induce motion in a car.

Using solar power to produce Hydrogen and Oxygen from water to propel a vehicle requires huge amounts of solar power, which is not available from the surface area of a vehicle. 50 watts will not propel a car very fast. The total amount of power available from the solar panel is greatly reduced from it's stated capacity, starting with the fact that the panel is rated at 50 watts, but in real life conditions produces less, as it is impossible to perfectly position the panel in only perfectly sunny conditions, and losses in power proceed from there throughout the system.

Thanks!