Making Fuel from Water

ISIS Report 01/07/09

An efficient and robust catalyst for oxidizing water brings us closer to converting sunlight into fuel Dr. Mae-Wan Ho

The holy grail of artificial photosynthesis is to mimic and improve on the green plant’s ability to turn sunlight directly into electrochemical energy that can be used as fuel [1] (Harvesting Energy from Sun with Artificial Photosynthesis, SiS 43). Research and development in this area within the OECD (Organisation for Economic Co-operation and Development) countries date back to the 1970s; and major efforts have been renewed by the United States Department of Energy (DoE) since 2007 [2].

These efforts are paying off. Important progress has been made by researchers Heinz Frei and Feng Jiao at DoE’s Lawrence Berkeley National Laboratory recently, bringing the dream of making fuel from water a closer to market. They’ve found that nano-sized crystals of cobalt oxide improves the status of the art by 1 550-fold

Effective photo-oxidation requires a catalyst that is both efficient in using solar photons and fast enough to keep up with the solar flux to avoid wasting those photons. Clusters of cobalt oxide nanocrystals are sufficiently efficient and fast, and also robust and abundant,” said Frei [3]. “They perfectly fit the bill.”

Efficient and robust catalysts required

The direct conversion of carbon dioxide and water to fuel depends on the availability of efficient and robust catalysts for the photochemical transformations [4] (see Splitting Water with Ease, SiS 43). Catalysts need to have high turnover frequency (TOF) and density to keep up with the solar flux at ground level (1 000 Wm-2) to avoid wasting incident solar photons. For example, a catalyst with a TOF of 100 s-1 requires a density of one catalytic site per square nanometre.

Catalysts with lower rates or taking up a larger space will require a high surface area nanostructure support that provides tens to hundreds of catalytic sites per square nanometre. Furthermore, catalysts need to work close to the thermodynamic potential of the redox reaction [1] so that a maximum fraction of the solar photon energy is converted to chemical energy. Stability considerations favour all- inorganic materials, as does the ability to withstand harsh reaction conditions of pH or temperature.

For the water oxidation half reaction, Jiang and Frei had found that iridium oxide fulfils these requirements in robustness, and has a reported TOF of 40 s-1 for IrO2 colloidal particles suspended in water. The catalyst was driven by a [Ru3+ (bpy)3] unit (bpy, 2,2-bipyridine), generated photochemically with visible light using the established [Ru2+(bpy)3]/persulphate (electron donor/acceptor) system and a modest overpotential of 0.37V. (The overpotential is the potential in excess of the theoretical electrochemical potential of 1.23V required [1] due to inefficiencies in the system.)

The researchers have previously demonstrated that the all- inorganic IrO2 nanoclusters (~ 2nm) directly coupled to a single centre chromium(VI) or a binuclear TiCrIII charge- transfer chromophore (a chemical group that gives colour to the molecule) [4] gave oxygen evolution under visible light with good quantum yield. While iridium oxide closely approaches the efficiency and stability required as catalyst for water oxidation, iridium is the least abundant metal on earth and is therefore not suitable for use on a very large scale. So Jiao and Frei explored more abundant metals, inspired by nature’s MnCa cluster of photosystem II; nature tends to use the most abundant materials [5] (Living with Oxygen, SiS 43). So they focussed on Co3O4 nanoclusters, and struck gold [6].

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