Temperature-entropy and energy utilization diagrams for energy, exergy, and energy level analysis in solar water splitting reactions

Abstract

Solar hydrogen production by water splitting reaction is an important route to realize the storage of solar energy. The involvement of light and chemical energy can reduce the high reaction temperature of direct thermochemical water splitting reactions, but the pivotal issues such as energy, exergy, and energy level relationships behind the temperature reduction remain under-researched. A graphic analysis of water splitting reactions using the temperature-entropy and energy utilization diagrams is proposed to research the energy, exergy, and energy level relationships. The energy composition and conversion mechanisms with the light and chemical energy are presented in the diagrams by comparing with direct thermochemical water splitting reactions. Three cases, including the photocatalytic, photo-thermochemical, and methane steam reforming reactions, are studied to explain the effect of light, the synergistic effect of heat and light, and the effect of chemical energy in reducing the temperature of water splitting reactions. The energy levels of light and methane are higher than those of hydrogen, thus lowering the energy level of heat and reducing reaction temperatures. The discovery of the energy level required for hydrogen production will provide guidance for reducing its temperature.