Artificial Photosynthesis Provides Practical Approach for Storing Solar Energy as Hydrogen Fuels

Photosynthesis is a process where algae, plants, and certain bacterial species absorb light from the sun to chemically convert carbon dioxide (CO2) and water into energy.

This energy is then stored for subsequent use. For many years, researchers have been attempting to mimic this natural process. A simulated model of photosynthesis can offer a clean and renewable source of energy that could help to meet the rising demands of the society.

Synthetic photosynthesis could provide a practical alternative to fossil fuels, and for this to happen, the rate and efficiency of the water oxidation process must be enhanced. Water oxidation is a reaction that converts water into electrons, oxygen gas, and hydrogen ions.

Now, a research team at the U.S. Department of Energy's Brookhaven National Laboratory, Adam Mickiewicz University, and Baruch College, City University of New York, have developed a couple of novel molecular catalysts for this water oxidation process. These catalysts are ruthenium complexes covered by binding molecules (ligands) comprising phosphonate groups. They expedite the development of the oxygen-oxygen bond, which is considered to be the slowest and most energy-intensive step of water oxidation.

Preliminary studies that were published in Angewandte Chemie International Edition showed that these complexes of ruthenium could provide a low-energy pathway towards faster water oxidation.

Storing solar energy as hydrogen fuel or carbon-based fuels like methanol requires catalysts that can oxidize water at fast rates, with high efficiency, and for long periods of time. Our ruthenium complexes catalyze the oxygen-oxygen bond formation faster than any other known catalysts, generating hundreds of oxygen molecules per molecule of catalyst per second. With these catalysts, the electrical potential required to start the reaction is approximately 10 times less than that of a AA battery.

Javier Concepcion, Chemist, Artificial Photosynthesis Group, Brookhaven Lab

Forming the oxygen-oxygen bond

During the process of water oxidation, four electrons and four protons, which are needed in the following reaction to change CO2 into usable energy, are taken off from two water molecules, and this eventually leads to the formation of an oxygen-oxygen bond. To promote water oxidation, it is important to break the bonds existing between oxygen and hydrogen atoms in the two water molecules. With regard to synthetic photosynthesis, this molecular breakup is activated by a chemical catalyst.

Water is a very stable molecule, so getting two water molecules to react with each other is very difficullt. Our ruthenium complexes provide the reactivity needed to break those bonds.

Yan Xie, Doctoral Candidate, Stony Brook University

The research paper outlines the details of the sequence of steps through which the reaction is initiated and completed by the catalyst. When one of the water molecules attaches to the complex of ruthenium, it loses protons during the process as the ruthenium complex is oxidized (loses electrons), leading to an electron-deficient ruthenium-oxo group, which is highly reactive. With the aid of a phosphonate group, the other water molecule reacts with the ruthenium-oxo group to produce molecular oxygen.

The phosphonate group accepts protons, or hydrogen ions, from water. It is positioned near the active site of the ruthenium complex where water oxidation occurs. Incorporating the phosphonate group and ruthenium in a single complex makes it easy for the water molecule to find that one site and react.

David Shaffer, Research Associate, Brookhaven Lab

The protons are ultimately moved from the phosphonate group to the surrounding solution.

Studying the electrochemistry of the ruthenium complexes

In order to find out the speed and efficiency of the water oxidation process with the ruthenium catalysts, the researchers explored the electrochemistry of individual oxidation state by applying different levels of currents and then determining the amount of current passing via the system at different pH values (protons’ concentration in the solution).

The voltage at which catalysis starts tells you about the energy efficiency of water oxidation, while the current tells you how quickly water oxidation is occurring. Our ruthenium complexes minimize the amount of energy lost as heat, both in terms of the voltage and the rate that would be required for the catalyst, if incorporated into a device, to make use of all incoming sunlight.

Javier Concepcion, Chemist, Artificial Photosynthesis Group, Brookhaven Lab

Computational modeling was also used for studying the activation parameters, which are the molecular and energy order, needed to make and break bonds during the critical reaction between the ruthenium-oxo group and the water molecule.

These computational studies eventually revealed why the phosphonate group led to faster catalysis.

Phosphonate is a good proton acceptor, so it energetically favors the reaction. Because it is part of the ligand, it is already positioned and ready to interact with water, removing the need for a more ordered arrangement of molecules.

Javier Concepcion, Chemist, Artificial Photosynthesis Group, Brookhaven Lab

From another set of studies, the team was able to discover that it was not the oxygen-oxygen bond formation step, but rather one of the oxidation steps that was restricting the speed of the catalysis. To improve this step, the researchers are now creating second-generation catalysts and hoping to develop similar reactive catalysts using cobalt and iron, which are more abundantly available and also less costly than that of ruthenium, but whose chemistries are quite challenging.

By incorporating these catalysts into systems capable of absorbing sunlight and combining them with catalysts that reduce carbon dioxide or water into fuels, artificial photosynthesis could become a practical approach for storing solar energy as fuels.

Javier Concepcion, Chemist, Artificial Photosynthesis Group, Brookhaven Lab

The DOE Office of Science supported the study.