•A simple concept for generating hydrogen from water splitting is proposed.•The system is direct-connected an electrochemical cell and a concentrated photovoltaic cell.•The system operates stably and high repeatability.•The energy conversion efficiency from light to hydrogen is over 12%.•The conditions for stable and high efficient operation are discussed.Energy storage is a key technology for establishing a stand-alone renewable energy system. Current energy-storage technologies are, however, not suitable for such an energy system because the technologies are cost ineffective and achieve low energy-conversion efficiency. The most realistic and expected technology is hydrogen generation from water splitting by an electrochemical cell directly connected with photovoltaic cell. In this study, a simple concept is proposed for generating hydrogen from water splitting by using a direct-electrically-connected polymer electrolyte electrochemical cell and a separately-located concentrated photovoltaic cell, named a “concentrated photovoltaic electrochemical cell (CPEC)”. The CPEC operates stably and achieves relatively high-energy conversion efficiency from light to hydrogen of over 12%. The conditions are comparison with those of the electrochemical cell connected with a polycrystalline Si solar cell.

The primary targets of our project are to drastically improve the photovoltaic conversion efficiency and to develop new energy storage and delivery technologies. Our approach to obtain an efficiency over 40% starts from the improvement of III–V multi-junction solar cells by introducing a novel material for each cell realizing an ideal combination of bandgaps and lattice-matching. Further improvement incorporates quantum structures such as stacked quantum wells and quantum dots, which allow higher degree of freedom in the design of the bandgap and the lattice strain. Highly controlled arrangement of either quantum dots or quantum wells permits the coupling of the wavefunctions, and thus forms intermediate bands in the bandgap of a host material, which allows multiple photon absorption theoretically leading to a conversion efficiency exceeding 50%. In addition to such improvements, microfabrication technology for the integrated high-efficiency cells and the development of novel material systems that realizes high efficiency and low cost at the same time are investigated.