Water splitting by carbon under visible light

Abstract

Hydrogen production and storage remain one of the most challenging technologies for a clean and sustainable future energy scenario. If hydrogen is available in large amount and can be stored and transported safely, a hydrogen-based economy/society will be free of any greenhouse gas and the associated pollutant or wastes. Nowadays, hydrogen is mainly produced from fossil sources that result in the release of carbon and/or carbon dioxide. Water is abundant on the earth. Release of hydrogen from water with only oxygen left would be the best way for hydrogen production. We know from school experiment that water is formed by a strong oxidation reaction of H2 with O2 (explosive reaction). High energies are needed to split water to H2 and O2. This can be done by water electrolysis that accounts only for a small amount of current hydrogen production. However, current water electrolysis consumes too much energy and thus is uncompetitivewith production fromhydrocarbons. Production of H2 and O2 from water splitting using solar energy is a much elegant and promising means. Theoretically, only solar energy, water and a catalyst are needed. Water splitting occurs naturally in photosynthesis when photon energy is absorbed and converted into chemical energies through a complex biological pathway.However, the efficiency is very low and the system in which photosynthesis occurs (for instance, leaves) is very environment sensitive. Photocatalysts that could induce such process under light irradiation are semiconductor (the most famous example is TiO2), but most of them absorb mostly ultraviolet (UV) light. A host of inorganic and organic systems have been developed as photocatalysts for water splitting driven by visible light, however, they still suffer from low solar to hydrogen energy conversion efficiency and/or poor stability. There are twoways for water splitting: the four electronpathwaywith the formation of O2 and H2, and the two electron pathway with the formation of H2O2 and H2 [1].The former is thermodynamically more favorable, but the later has a much higher reaction rate when H2O2 is subsequently decomposed to H2 and O2 in a second two-electron reaction. For technical application, a high reaction rate is more important. For the stepwise twoelectron/two-electron water splitting to be viable and practical, the photocatalyst should be capable of promoting the generation as well as the subsequent decomposition of H2O2 with high efficiencies and less energy input allowing a considerable reduction in the energy cost for production of H2. It sounds somehow paradox as one catalyst should have the function to generate as well as to decompose the same molecule. This challenge was elegantly resolved by Kang et al. from Soochow University, Suzhou, China and Lifshitz from Technion, Haifa, Israel. They designed a new hybrid photocatalyst, a carbon nanodotcarbon nitride (Cdots-C3N4) nanocomposite, that can split water in two steps as schematically described in Fig. 1 [2].The task to split water and to split peroxide are done tandem by respective component: thefirst two-electron reaction is catalyzed by carbon nitride splittingwater to 2e 2e