New, cubane-based co-catalysts for photocatalytic hydrogen production

Project introduction

The objective of the research is the development of a new class of co-catalysts for the hydrogen evolution reaction (HER) on photo cathodes without any scarce elements.
The largest available, permanent source of energy is sunlight. In order to provide the World with energy for the centuries to come we must find ways to harvest sunlight and effectively convert the energy into useful forms. The most direct approach is to make electricity using photovoltaics, but we must also find ways to efficiently and economically store energy in the form of chemicals. One of the most direct approaches is photocatalytic water splitting in which hydrogen and oxygen are produced from water when sunlight is absorbed by the photocatalytic material. One promising strategy for achieving overall water splitting while utilizing a large part of the solar spectrum is to divide the problem into two sub-reactions and design an optimized system for each:
  1. Absorb a photon in the semiconducting photo cathode promoting an electron to the conduction band of the photo cathode. Electrons from the conduction band may then reduce protons to hydrogen over a HER co-catalyst on the surface of the semiconductor:
    4 H+ + 4 e- ® 2 H2
  2. Absorb another photon in the semiconducting photo anode creating an energetic hole in the valence band of the photo anode. Holes may then oxidise water to oxygen and protons over an OER co-catalyst:
    2 H2O + 4 h+ ® 4 H+ + O2
With a well designed junction between the photo cathode and photo anode the leftover charges (holes in the photo cathode and electrons in the photo anode) annihilate and the overall process is: 2 H2O + 8hn ® 2 H2 + O2. A schematic representation of the entire process is shown in Fig. 1.
Two-photon energy levels
Fig. 1 Energy level diagram of a 2-photon water-splitting device under illumination. Electrons reduce protons to hydrogen at the photo cathode (right) while holes oxidize water to protons and oxygen on the photo anode (left).
Clearly, many things must work together smoothly for this whole reaction to work, but this project is focused on one small, but crucial part, namely the HER co-catalyst.

Cubanes as HER catalysts?

The best known catalyst for HER is platinum. Platinum works very well - it has a low overpotential, high current density, high stability in nearly every electrolyte - but platinum is also very scarce (and therefore also expensive). There is simply not enough platinum available to make photocatalytic hydrogen evolution feasible on the required scale. This means that alternative catalysts made entirely from earth abundant elements must be developed for this process to become practical. Inspired by the natural enzymes hydrogenase and nitrogenase Hinneman et al. have shown MoS2 to be a promising material for HER catalysis and this was experimentally confirmed to be the case by Jaramillo et al.. In our group the work on compounds based on molybdenum and sulphur for HER has carried on and cubanes (compounds with a core structure like a cube) have been found to be active for electrocemical HER when supported on carbon as published in 2008 by Jaramillo et al..
Recently, our group has used a new kind of cubane which is insoluble in water and which has a core of three molybdenum atoms and four sulphur atoms (see figure) and shown that it not only catalyses HER on carbon support (electrocatalysis), but that it is highly active when supported on p-type silicon (semiconductor). The p-silicon-Mo3S4 cubane system constitutes a candidate system for a rare-element free photocathode for water splitting (right hand side of Fig. 1).
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Acknowledgement

The research is sponsored by the Danish Council for Independent Research - FTP. Project number: 10-080861.

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On 24 Nov 2011, 08:48.