We show that bio-inspired molecular clusters based on molybdenum and sulphur evolve hydrogen at rates comparable to that of platinum. The incomplete cubane-like clusters (Mo₃S₄) efficiently catalyze the evolution of hydrogen when coupled to a p-type Si semiconductor that harvests red photons in the solar spectrum. The current densities at the reversible potential match the requirement of a photo-electrochemical hydrogen production system with a solar-to-hydrogen efficiency in excess of 10%. The experimental observations are supported by density functional theory calculations of the Mo₃S₄ clusters adsorbed on the hydrogen-terminated Si(100) surface, providing insights into the nature of the active site.

—Hou et al.

Today, most hydrogen is produced via steam methane reforming (SMR), resulting in large emissions of CO₂. An alternative, clean method is to make hydrogen fuel from sunlight and water via a photo-electrochemical (PEC, or water-splitting) process. When sun hits the PEC cell, the solar energy is absorbed and used for splitting water molecules into its components, hydrogen and oxygen.

Progress has so far been limited in part by a lack of cheap catalysts that can speed up the generation of hydrogen and oxygen. A critical component of the American-Danish effort was combining theory and advanced computation with synthesis and testing to accelerate the process of identifying new catalysts. This is a new development in a field that has historically relied on trial and error.

The team first tackled the hydrogen half of the problem. The DTU researchers created a device to harvest the energy from part of the solar spectrum and used it to power the conversion of single hydrogen ions into hydrogen gas. However, the process requires a catalyst to facilitate the reaction. Platinum is already known as an efficient catalyst, but platinum is too rare and too expensive for widespread use. The collaborators turned to nature for inspiration.

They investigated hydrogen-producing enzymes, using a theoretical approach Nørskov’s group has been developing to describe catalyst behavior. These studies led them to related compounds, which eventually took them to molybdenum sulfide. Molybdenum is an inexpensive solution for catalyzing hydrogen production, Chorkendorff said.

The team also optimized parts of the device, introducing a “chemical solar cell” designed to capture as much solar energy as possible. The experimental researchers at DTU designed light absorbers that consist of silicon arranged in closely packed pillars, and dotted the pillars with tiny clusters of the molybdenum sulfide. When they exposed the pillars to light, hydrogen gas bubbled up—as quickly as if they’d used platinum.

The hydrogen gas-generating device is only half of a full photo-electrochemical cell. The other half of the PEC would generate oxygen gas from the water; though hydrogen gas is the goal, without the simultaneous generation of oxygen, the whole PEC cell shuts down.

Many groups—including Chorkendorff, Dahl and Nørskov and their colleagues—are working on finding catalysts and sunlight absorbers to do this well.

This is the most difficult half of the problem, and we are attacking this in the same way as we attacked the hydrogen side.

—Søren Dahl

Resources

• Yidong Hou, Billie L. Abrams, Peter C. K. Vesborg, Mårten E. Björketun, Konrad Herbst, Lone Bech, Alessandro M. Setti, Christian D. Damsgaard, Thomas Pedersen, Ole Hansen, Jan Rossmeisl, Søren Dahl, Jens K. Nørskov & Ib Chorkendorff (2011) Bioinspired molecular co-catalysts bonded to a silicon photocathode for solar hydrogen evolution. Nature Materials doi: 10.1038/nmat3008

• Supplementary movie