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Efficient production of H2 from water without the use of an extra bias or any sacrificial reagents is possible using a GaN photocatalyst with a NiO cocatalyst. The average energy conversion efficiency from light energy to H2 energy was approximately 1% for 500 h. The total amount of hydrogen was 184 mL/cm2 of the GaN surface. H2 production rate was as high as 0.37 mL/(cm2·h).

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This paper presents a new data-driven controller synthesis based on the Koopman operator and the extended H2 norm characterization of discrete-time linear systems. We model dynamical systems as polytope sets which are derived from multiple data-driven linear models obtained by the finite approximation of the Koopman operator and then used to design robust feedback controllers combined with the H2 norm characterization. The use of the H2 norm characterization is aimed to deal with the model uncertainty that arises due to the nature of the data-driven setting of the problem. The effectiveness of the proposed controller synthesis is investigated through numerical simulations.

We confirmed that GaN photocatalyst with NiO cocatalyst (GaN-NiO) continuously produced hydrogen from water for 500 hours without any extra bias. The GaN-NiO photocatalyst was hardly etched and 184-mL hydrogen gas was produced from the electric charge of 1612 coulombs, the Faradic efficiency of which was 89.2%. The conversion efficiency from incident light energy to hydrogen chemical energy was 0.98% in average for 500 h. The incident photon-to-current conversion efficiency (IPCE) was 50% at 300 nm and 35% at 350 nm after the experiment, which was much higher than those of other semiconductor-based photocatalysts.

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