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Contemporary models are shown to significantly underestimate the attainable efficiency of solar energy conversion to water splitting, and experimentally a cell containing illuminated AlGaAs/Si RuO 2 /Pt black is demonstrated to evolve H 2 and O 2 at record solar driven water electrolysis efficiency. Under illumination, bipolar configured Al 0.15 Ga 0.85 As ( E g = 1.6 eV) and Si ( E g = 1.1 eV) semiconductors generate open circuit and maximum power photopotentials of 1.30 and 1.57 V, well suited to the water electrolysis thermodynamic potential: H 2 O → H 2 + 1 / 2 O 2; E ° H 2 O = E O 2 − E H 2; E ° H 2 O (25 °C) = 1.229 V. The E ° H 2 O /photopotential matched semiconductors are combined with effective water electrolysis O 2 or H 2 electrocatalysts, RuO 2 or Pt black . The resultant solar photoelectrolysis cell drives sustained water splitting at 18.3% conversion efficiencies. Alternate dual band gap systems are calculated to be capable of attaining over 30% solar photoelectrolysis conversion efficiency.
Licht et al. (2000) studied this question.