Nanoscale system reaches perfect efficiency for solar fuel production step

A major goal in renewable energy research is to harvest the energy of the sun to convert water into hydrogen gas, a storable fuel. Now, with a nanoparticle-based system, researchers have set a record for part of the process, reporting 100% efficiency for the half-reaction that evolves hydrogen (Nano Lett. 2016, DOI: 10.1021/acs.nanolett.5b04813).

To make such water-splitting systems, researchers must find the right materials to absorb light and catalyze the splitting of water into hydrogen and oxygen. The two half-reactions in this process—the reduction of protons to hydrogen, and the oxidation of water to oxygen—must be isolated from each other so their products don’t react and explode. “Completing the cycle in an efficient, stable, safe fashion with earth-abundant elements is an ongoing challenge,” says chemist Nathan S. Lewis of Caltech, who was not involved in this study.

Until recently, the efficiency of the reduction step had maxed out at 60%. One challenge is that electrons and positive charges formed in the light absorption process can rapidly recombine, reducing the number of electrons available for the hydrogen production reaction. To overcome this problem, several years ago, Lilac Amirav of Technion–Israel Institute of Technology and her colleagues designed a nanoparticle-based system (J. Phys. Chem. Lett. 2010, DOI: 10.1021/jz100075c) that would physically separate the charges formed during photocatalysis.

The team’s system comprises a light-harvesting cadmium sulfide quantum rod with a quantum dot made of cadmium selenide embedded near one end. A platinum tip on the opposite end catalyzes hydrogen production. When placed in water in a gas-tight reaction cell and exposed to visible light, the quantum rod absorbs photons, releasing electrons. The rod then transfers electrons to the platinum tip, reducing protons to form hydrogen, and leaving behind positively charged vacancies called holes at the cadmium selenide dot.

When the team initially tested the system in water, however, it had only 20% efficiency for hydrogen production. To improve performance, the researchers tested a variety of reaction conditions. In the new study, they achieved success by increasing the pH and adding isopropyl alcohol. Under these conditions, the holes oxidize hydroxide anions, which are more abundant at the high pH, to yield hydroxyl radicals. The alcohol then donates electrons to the radicals, preventing charge recombination and keeping more electrons available to evolve hydrogen.

When the team measured hydrogen production with this updated setup, they achieved up to 100% efficiency. “The results shatter the previous benchmarks for all systems,” Amirav says.

Lewis says the team’s achievement is “a step toward a stable, efficient system for solar fuels production.” He notes, however, that this work addresses only one of the necessary half-reactions.

Adapting the work to a full water splitting system may be challenging, says Amirav, because cadmium sulfide corrodes under long exposure to light. However, she has recently shown that adding another catalyst, such as iridium oxide or ruthenium, could improve its photochemical stability (J. Mater. Chem. A 2015, DOI: 10.1039/C4TA06164K; Angew. Chem., Int. Ed. 2015, DOI: 10.1002/anie.201411461).