Integrated Photo-Electrocatalytic (PEC) Systems for Water Splitting to Hydrogen and Oxygen under Concentrated Sunlight: Effect of Internal Parameters on Performance

The development of stable integrated photo-electrocatalytic (PEC) systems for hydrogen production from water is poised to reduce global CO2 emission. While some PEC systems have reached efficiencies as high as 18%, most demonstrated limited stability, and a non-negligible fraction of these are based on either H-2 production or current measurements, without evidence of O-2 production shedding doubts on the overall requirement of a catalytic cycle. In this work, we focus on a system that works under concentrated sunlight composed of a triple junction, a 3 J InGaP/GaAs/Ge photovoltaic cell with its front side covered with a poly(methyl methacrylate) sheet and its backside protected by metallic Ni foil. The hydroxylated Ni metal, in the alkaline environment used, acts as the oxygen evolution catalyst, in addition to protection from corrosion. It does also act as a heat-dissipating material, which is crucial for cell stability under concentrated sunlight. The complete system is stable, producing stoichiometric amounts of H-2 and O-2, and the reaction is therefore catalytic. Under concentrated light, we have studied the effect of changing the area of the Ni foil (effectively the amount of oxygen evolution catalyst) while keeping the light-receiving front-cell area constant. This dimensionless ratio is found to determine the reaction rate, below a ratio (Ni area to cell area) of about 1. Meanwhile, above this number, the reaction rate showed marginal changes; it is desired to have this ratio as large as possible in order to decrease the current density and therefore increase the lifetime of the complete system. A compilation of the most stable and most active similar systems is also given and discussed, in particular, in view of the very small number of available work reporting both H-2 and O-2 measurements. The solar-to-hydrogen efficiencies in this work are found to be 14% at 1-15 suns and 12% at 39 suns.