Solar water splitting: Efficiency discussion

Abstract 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 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. Mechanism of cascading oxidation of water molecules via the peroxide stage is proposed and conforms with experimental observation. 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. Strategy to increase efficiency above 15% for a single junction cell is outlined.