Beating the Efficiency of Photovoltaics-Powered Electrolysis with Tandem Cell Photoelectrolysis

Beating the Efficiency of Photovoltaics-Powered Electrolysis with Tandem Cell Photoelectrolysis S olar water splitting is a promising route for hydrogen production from renewable sources: water and sunlight.The hydrogen can be stored and converted to power on demand by means of fuel cells, used as a clean fuel to refuel electric vehicles, or serve as a feedstock for the production of valuable chemicals such as ammonia (via the Haber-Bosch process) or drop-in liquid fuels (by CO 2 hydrogenation).Solar water splitting can be readily achieved by combining two commercial technologies: photovoltaics (PV) and electrolysis.Such combinations have been demonstrated and tested in selfsufficient solar houses, 1 solar hydrogen pilot plants, 2 solar hydrogen refueling stations, 3 and PV solar power plants. 4nfortunately, PV-powered electrolysis (PV-electrolysis for short) is too expensive for large-scale solar hydrogen production. 5,6This motivates research on photoelectrochemical (PEC) cells that convert solar energy directly to hydrogen fuel by water photoelectrolysis, thereby combining the functions of light harvesting and electrolysis together.Unlike separate PVelectrolysis systems that have already been demonstrated and tested in several pilot plants, [1][2][3][4] PEC water splitting is still under development, and research in this area is academic, for the most part, dealing mostly with lab tests of small-size components and prototype devices.Therefore, it is too early to predict whether PEC water splitting will beat PV-electrolysis in the commercial playground.Recent technoeconomic analyses suggest that PEC water splitting might become costcompetitive if the efficiency and lifetime could be improved significantly compared to present reports on PEC cells. 5,6owever, some of the underlying assumptions are quite speculative, and unforeseen challenges might get in the way on scaling <1 cm 2 prototype devices to >10 000 m 2 solar fields.On the bright side, new advances in PEC materials, device architectures, and operation schemes, such as the power management and light management schemes presented in this Viewpoint, may render PEC water splitting more competitive than the current scenarios.PEC water splitting is an evolving research field, and therefore, it is difficult to make reliable predictions for large-scale solar water splitting plants based on current knowledge and experience.As a result, much uncertainty still remains as for the prospects of PEC water splitting to be cheaper than PV-electrolysis.While cost predictions are quite speculative, efficiency is a tangible parameter that has a profound impact on the economy of solar energy technologies.This Viewpoint addresses the efficiency of solar water splitting systems, proposing power management and light management schemes that enable boosting the efficiency of PV-PEC tandem cells with minimal changes in system components and therefore with minimal add-on costs.Thus, the proposed schemes are expected to