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.”
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].
To form the Co3O4 nanoclusters, they used mesoporous silica (SBA-15) as the scaffold. The mesocopic structure of the silica consists of hollow channels connected by micropores. The CO3O4 clusters are formed exclusively inside the channels as parallel bundles of nanorods linked by short bridges, formed by CO3O4 growth in the micropores interconnecting the mesoscale channels. They loaded the silica at 4.2 and 8.6 percent by weight of CO3O4 in wet impregnation.
Transmission electron microscope images showed that the average spheroid-shaped bundle of CO3O4 has a short diameter of 35 nm and a long diameter of 65 nm for the sample prepared with 4.2 percent CO3O4; for the 8.6 percent sample, the short and long diameters were 65 and 170nm respectively. X-ray crystallographic analysis showed that the 4.2 percent samples were poorly crystallized, while the 8.6 percent sample corresponds to a 7.6 nm diameter rod structure. The 4.2 percent sample gave the highest rate of oxygen evolution when tested at pH 5.8 and 22 C (with an overpotential of 0.35V), about 40 percent higher than the 8.6 percent sample. The rate was linear for the first 30 minutes before gradually levelling off. When fresh Na2S2O8 electron acceptor was added and the pH value readjusted, oxygen evolution resumed at the initial rate. This finding confirmed that the slowdown was principally due to the consumption of the persulfate acceptor, and demonstrated that the activity of the CO3O4 nanoclusters did not degrade during photocatalysis in the several hours investigated.
In comparison, NiO nanocrystals in silica or micron sized CO3O4 particles were not effective. An estimated TOF of 1 140 s-1 per CO3O4 cluster was obtained in the 4.2 percent sample. The calculation is based on the geometry of the bundles of CO3O4 nanorods, bundle diameter 35 nm, rod diameter 7.6 nm, typically 14 rods per bundle, average rod length 50 nm. For the larger CO3O4 clusters (8.6 percent) the estimated TOF is 3450 s-1. The calculation assumed CO3O4 nanorod spheroid bundles of 48 per bundle, rod diameter 7.6 nm, average rod length 130 nm. The oxygen yield was 65 times smaller for the aqueous suspension of 200 mg of bare CO3O4 particles compared with the 4.2 percent nanocrystals impregnated in silica. When normalised to the same amount of CO3O4 the O2 yield for the silica impregnated nanocrystals at 4.2 percent exceeds that of the bare micron-sized particles by a factor of 1 550.
This was the first observation of efficient water oxidation, which is only half the artificial photosynthesis reaction. Nevertheless, the abundance of the metal oxide, the stability of the nanoclusters under use, the modest overpotential required, and the mild pH and temperature conditions make it a promising catalytic component for developing a viable integrated system for converting sunlight to fuel.
Article first published 01/07/09
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Dr.Amar Singh Azad Comment left 3rd July 2009 07:07:00
Excellent Article.