Researchers at MIT Develop New Water-Splitting Catalyst That Works Under Benign Conditions; a “Giant Leap”
31 July 2008
Researchers at MIT—Prof. Daniel Nocera and Dr. Matthew Kanan—have developed a new water-splitting catalyst that is easily prepared from earth-abundant materials (cobalt and phosphorous) and operates in benign conditions: pH neutral water at room temperature and 1 atm pressure. A report on their discovery was published online 31 July 2008 in the journal Science.
The cobalt-phosphorous catalyst targets the generation of oxygen gas from water—the more complex of the two water-splitting half-cell reactions required (H2O/O2 and H2O/H2). Another catalyst generates the hydrogen. Although the new catalyst requires further work, it opens a very promising pathway for the development of systems that use artificial photosynthesis to store solar energy on a large scale in the form of O2 and H2 for subsequent use in a fuel cell.
Of the two reactions, the H2O/O2 reaction is considerably more complex. This reaction requires a four-electron oxidation of two water molecules coupled to the removal of four protons to form a relatively weak oxygen-oxygen bond. In addition to controlling this proton-coupled electron transfer (PCET), a catalyst must tolerate prolonged exposure to oxidizing conditions. Even at the thermodynamic limit, water oxidation requires an oxidizing power that causes most chemical functional groups to degrade. Accordingly, the generation of oxygen from water presents a significant challenge toward realizing artificial photosynthesis.—Nocera and Kanan (2008)
Other water oxidation catalysts exist, including first-row spinel and perovskite metal oxides; and precious metals and precious metal oxides. The first requires concentrated basic solutions (pH>13) and moderate overpotentials (<400 mV); the second operate under acidic conditions (pH<1).
However, few catalysts operate under the conditions of photosynthesis, i.e. in neutral water under ambient conditions. Neutral water is oxidized at Pt electrodes and some precious metal oxides have been reported to operate electrocatalytically in neutral or weakly acidic solutions. The development of an earth-abundant, first-row catalyst that operates at pH 7 at low overpotential remains a fundamental chemical challenge. Here we report an oxygen-evolving catalyst that forms in situ upon anodic polarization of an inert electrode in neutral aqueous phosphate solutions containing Co2+. Oxygen generation occurs under benign conditions: pH = 7, 1 atm and room temperature.—Nocera and Kanan (2008)
The new catalyst consists of cobalt metal, phosphate and an electrode, placed in water. When electricity—whether from a photovoltaic cell, a wind turbine or any other source—runs through the electrode, the cobalt and phosphate form a thin film on the electrode, and oxygen gas is produced. Combined with another catalyst, such as platinum, that can produce hydrogen gas from water, the system can duplicate the water splitting reaction that occurs during photosynthesis.
James Barber, a leading researcher in the study of photosynthesis who was not involved in this research, called the discovery by Nocera and Kanan a “giant leap” toward generating clean, carbon-free energy on a massive scale.
This is a major discovery with enormous implications for the future prosperity of humankind. The importance of their discovery cannot be overstated since it opens up the door for developing new technologies for energy production thus reducing our dependence for fossil fuels and addressing the global climate change problem—James Barber, the Ernst Chain Professor of Biochemistry at Imperial College London
Currently available electrolyzers, which split water with electricity and are often used industrially, are not suited for artificial photosynthesis because they are very expensive and require a highly basic (non-benign) environment that has little to do with the conditions under which photosynthesis operates.
If artificial photosynthesis is to enable the storage of solar energy commensurate with global demand, water-splitting chemistry will need to be performed at a daunting scale. Storing the equivalent of the current energy demand would require splitting greater than 1015 mol/yr of water, which is roughly 100 times the scale of nitrogen fixation by the Haber Bosch process. [The Haber Bosch process allows the mass synthesis of ammonia from nitrogen and hydrogen.]
The conditions under which water splitting is performed will determine how solar energy is deployed. The catalyst reported here has many elements of natural photosynthesis including its formation from earth abundant metal ions in aqueous solution, a plausible pathway for self-repair, a carrier for protons in neutral water and the generation of O2 at low overpotential, neutral pH, 1 atm and room temperature.—Nocera and Kanan (2008)
More engineering work needs to be done to integrate the new scientific discovery into existing photovoltaic systems, but Nocera said he is confident that such systems will become a reality. Nocera is the principal investigator for the Solar Revolution Project funded by the Chesonis Family Foundation and co-Director of the Eni-MIT Solar Frontiers Center.
This is just the beginning. The scientific community is really going to run with this.—Daniel Nocera
Nocera hopes that within 10 years, homeowners will be able to power their homes in daylight through photovoltaic cells, while using excess solar energy to produce hydrogen and oxygen to power their own household fuel cell. Electricity-by-wire from a central source could be a thing of the past.
The project is part of the MIT Energy Initiative, a program designed to help transform the global energy system to meet the needs of the future and to help build a bridge to that future by improving today’s energy systems.
This project was funded by the National Science Foundation and by the Chesonis Family Foundation, which gave MIT $10 million this spring to launch the Solar Revolution Project, with a goal to make the large scale deployment of solar energy within 10 years.
M W Kanan and D G Nocera (2008) In Situ Formation of an Oxygen-Evolving Catalyst in Neutral Water Containing Phosphate and Co2+. Science, doi: 10.1126/science.1162018
Robert F. Service (2008) New Catalyst Marks Major Step in the March Toward Hydrogen Fuel. Science 1 August 2008: Vol. 321. no. 5889, p. 62010.1126/science.321.5889.620
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