Iridium catalyst for splitting water

Dr. Neil Canter, Contributing Editor | TLT Tech Beat June 2015

A new heterogeneous catalyst is effective at producing oxygen under acidic conditions.

KEY CONCEPTS
• The single biggest obstacle to producing energy through artificial photosynthesis is water oxidation.
• A new iridium catalyst can bind to nearly any metal oxide forming a heterogeneous catalyst that splits water effectively.
• This catalyst functions well in an acidic environment and might also be useful as a corrosion inhibitor.

FINDING AN EFFICIENT PROCESS FOR SIMULATING PHOTOSYNTHESIS TO USE SOLAR ENERGY to split water into hydrogen and oxygen remains an elusive goal. Researchers have been looking to develop and refine catalysts to meet this objective.

In a previous TLT article, atomic layer deposition is used to place a thin layer of titanium dioxide on the nanostructured core of a dye-sensitized photoelectrosynthesis cell (1). The impact in adding the 3.6-nanometer thick layer is to improve the rate at which electrons are transferred to the cathode, leading to improved hydrogen generation and also facilitating the oxidation of water to form oxygen at a higher pH.

Water oxidation to generate oxygen remains the single biggest obstacle to producing energy through artificial photosynthesis. Staff Sheehan, doctoral student in the chemistry department at Yale University in New Haven, Conn., says, “Oxygen production is very difficult to achieve efficiently in an electrochemical cell because a much higher overpotential is required as compared with hydrogen production.”

A water oxidation catalyst of interest to Sheehan is iridium oxide. He says, “Iridium-based species such as iridium oxide are able to catalyze the oxidation of water at a low overpotential particularly under acidic conditions. Iridium oxide is also very stable in acidic solutions, and for these reasons it is considered the state-of-the-art water oxidation catalyst in polymer exchange membrane electrolysis.”

Unfortunately, there are several problems in working with iridium oxide. Sheehan says, “Iridium is one of the rarest elements in the earth’s crust and when an iridium oxide anode is prepared, only the very small fraction of iridium that is on the electrode’s surface and in direct contact with water is electrically active.”

Work done in the past by researchers determined that iridium can form molecular complexes that include an organic ligand known as Cp* (pentamethylcyclopentadienyl). These iridium-based species were predicted to be good homogeneous water oxidation catalysts, effectively, molecular analogues to iridium oxide. Researchers later discovered that this was not the case since the Cp* ligand degrades under oxidative conditions. In most cases, this leads to the formation of less active iridium oxide nanoparticles or films.

If a new, non-degradable, molecular catalyst could be developed for use in an anode for water oxidation that contains only active iridium, then there is potential for better utilizing iridium’s capabilities for water splitting. Such an approach has now been developed.

IRIDIUM SPECIES BOUND TO A METAL OXIDE SURFACE
Sheehan and his associates have developed an iridium catalyst that utilizes a bidentate ligand, 2-(2’pyridyl)-2-propanolate (known as pyalc), that is highly resistant to oxidative stress. This complex is formed from an iridium-containing precursor molecule that contains pyalc as well as an organic placeholder ligand. After removing the placeholder ligand from the precursor under strong oxidative conditions, the activated catalyst is formed. This activated catalyst has been found to effectively bind to metal oxides and oxidize water at a low potential with a high reaction rate and good durability.

Sheehan says, “This iridium-based catalyst demonstrates an outstanding and unique ability to bind to many different surfaces, from metal oxides to carbon-based materials. We initially saw this characteristic when attempts to isolate this species using chromatography failed because the catalyst would stick to the column and could not be removed. It can bind to nearly any metal oxide, including non-conductive species such as titanium dioxide and silica. What is more intriguing is that the catalyst chemically bonds to the surface of these metal oxides without any external bias, forming a conformal monolayer. It all happens in air, at room temperature, and all you have to do is immerse the substrate in a solution of the catalyst for a few hours.”

Initially, the researchers bound the iridium species to a mesoporous transparent conductive electrode film of tin-doped indium oxide nanoparticles to form a heterogeneous catalyst that they could study spectroscopically. Cyclic voltammetry was simultaneously used to evaluate the performance of the iridium catalyst in an oxygen saturated solution at pH 2.6 containing 0.1 molar potassium nitrate as an electrolyte.

The ability of the iridium catalyst to perform well in this sort of acidic environment may also have some additional benefits pertinent to lubrication. Sheehan says, “The iridium catalyst is very effective in protecting surfaces from corrosion under acidic conditions. This feature may prove to be of additional benefit in real-world applications.”

Figure 3 shows an image of the iridium catalyst splitting water at a potential of 1.6 volts. The bubbles appearing on the platinum wire in the foreground are hydrogen while the iridium catalyst is shown producing oxygen.


Figure 3. A new, more effective iridium catalyst used to split water is shown producing oxygen. The bubbles appearing in the foreground on the platinum wire are hydrogen. (Figure courtesy of Yale University.)

X-ray photoelectron spectroscopy was used to make sure the iridium species is still present after the water splitting reaction. Ultraviolet-visible measurements coupled with electrochemical measurements showed that greater than 90% of the iridium on the transparent conductive anode is active, which meets a key objective of the research.

The main problem faced with this particular system is the instability of the metal oxide substrate that the catalyst is bound to. Sheehan says, “The transparent conductive oxide we used in this initial study, tin-doped indium oxide, is not very acid-stable, so our ongoing work focuses on coupling the catalyst with a more stable substrate to improve overall anode stability. Currently, the iridium catalyst bound to a different metal oxide has been under evaluation for three months with no drop-off in performance.”

Additional information can be found in a recent article (2) or by contacting Sheehan at stafford.sheehan@yale.edu. 

REFERENCES
1. Canter, N. (2014), “Water splitting and stabilizing catalyst surfaces,” TLT, 70 (3), pp. 14-15.
2. Sheehan, S., Thomsen, J., Hintermair, U., Crabtree, R., Burdvig, G. and Schmuttenmaer, C. (2015), “A molecular catalyst for water oxidation that binds to metal oxide surfaces,” Nature Communications, 6, 6649, DOI: 10:1038/ncomms7469.


Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat can be submitted to him at neilcanter@comcast.net.