THE ARTIFICIAL LEAF DETAILED

The Artificial Leaf

Daniel G. Nocera, April 4, 2012 (Accounts of Chemical Research)

“To convert the energy of sunlight into chemical energy, the leaf splits water via the photosynthetic process to produce molecular oxygen and hydrogen, which is in a form of separated protons and electrons. The primary steps of natural photosynthesis involve the absorption of sunlight and its conversion into spatially separated electron–hole pairs…For a synthetic material to realize the solar energy conversion function of the leaf, the light-absorbing material must capture a solar photon to generate a wireless current that is harnessed by catalysts, which drive the four electron/hole fuel-forming water-splitting reaction under benign conditions and under 1 sun (100 mW/cm2) illumination.

“This Account describes the construction of an artificial leaf comprising earth-abundant elements by interfacing a triple junction, amorphous silicon photovoltaic with hydrogen- and oxygen-evolving catalysts made from a ternary alloy (NiMoZn) and a cobalt–phosphate cluster (Co-OEC), respectively…X-ray absorption spectroscopy studies have established that the Co-OEC is a structural relative of Mn3CaO4–Mn cubane of the PSII-OEC, where Co replaces Mn and the cubane is extended in a corner-sharing, head-to-tail dimer…”

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The structure of the leaf

“…The artificial leaf achieves direct solar-to-fuels conversion at 1 sun (AM 1.5, 100 mA cm–2) under benign conditions, using earth-abundant materials, and without the use of wires. The artificial leaf mimics nature inasmuch as it stores solar energy in water splitting by conforming to the functional elements of a leaf. We note that the actual leaf does not use hydrogen but stores it as a solid fuel, for example, carbohydrate. In the case of the artificial leaf, the hydrogen is available for combination with CO2 as new catalysts for this process are discovered.

“…The artificial leaf described here, as a simple, stand-alone device composed of earth-abundant materials, provides a first step down a path aligned with the low-cost systems engineering and manufacturing that is required for inexpensive solar-to-fuels systems…In doing so, solar energy can become a viable energy supply to those in the nonlegacy world…Considering that it is the 6 billion nonlegacy users that are driving the enormous increase in energy demand by midcentury…a research target of delivering solar energy to the poor with discovery such as the artificial leaf provides global society its most direct path to a sustainable energy future.”

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