  The current state of the art in direct water splitting in
photo-electrochemical cells (PECs) is presented together with: (i) a case study
of water splitting using a simple solar cell with the most efficient water
splitting electrodes and (ii) a detailed mechanism analysis. Detailed analysis
of the energy balance and efficiency of solar hydrogen production are
presented. The role of hydrogen peroxide formation as an intermediate in oxygen
evolution reaction is newly revealed and explains why an oxygen evolution is
not taking place at the thermodynamically expected 1.23 V potential.
  Solar hydrogen production with electrical-to-hydrogen conversion efficiency
of 52% is demonstrated using a simple ~0.7%-efficient n-Si/Ni Schottky solar
cell connected to a water electrolysis cell. This case study shows that
separation of the processes of solar harvesting and electrolysis avoids
photo-electrode corrosion and utilizes optimal electrodes for hydrogen and
oxygen evolution reactions and achieves ~10% efficiency in light-to-hydrogen
conversion with a standard 18% efficient household roof Si-solar cells.
