Using rust to capture solar energy

Dr. Neil Canter, Contributing Editor | TLT Tech Beat January 2014

A process known as photoelectrochemical water splitting uses a semiconductor to split water into oxygen and hydrogen.

KEY CONCEPTS
• Photoelectrochemical water splitting involves using a semiconductor to use solar energy to split water into oxygen and hydrogen.
• Modifying alpha-iron (III) oxide (rust) or hermatite with a NiFeO_x overlayer results in the generation of a photovoltage that is closer to the required photovoltage needed to split water.
• Use of a dual-absorber composed of silicon nanowires reduces the gap needed to reach the required photovoltage to split water to 0.2 V.
AN ELUSIVE GOAL OF RESEARCHERS is to efficiently harness energy from the sun and convert it into electricity. The objective is to develop an artificial process comparable to photosynthesis undertaken by green plants.

One approach that has been covered periodically in this column is the preparation of more efficient solar cells. Most of the solar cells are based on inorganic silicon. In a previous TLT article, the development of an organic solar cell based on naturally derived cellulose nanocrystals is described (1). This cell exhibited the highest power conversion efficiency yet seen with a solar cell derived from renewable materials. An added benefit is that the organic solar cell can be recycled.

An alternative way to capture solar energy is to use a process known as photoelectrochemical (PEC) water splitting that involves the use of a semiconductor to use solar energy to split water into oxygen and hydrogen, which can be used as a cheap form of chemical energy. Dunwei Wang, associate professor of chemistry at Boston College in Chestnut Hill, Mass., describes the process as one in which energy, in the form of light, is the input and chemical energy is the output. He says, “The thermodynamic detail was worked out in the 1950s and is not different from what occurs in a solar cell.”

The water splitting occurs at a photoanode where the ideal sunlight wavelength used is 650 nanometers. An appealing semiconductor material to be used in this process is rust also known as hematite or alpha-iron (III) oxide. Wang says, “Hematite is an appealing material to work with because it is cheap and abundant. This material also has a relatively good efficiency because it can utilize up to 18 percent of the incident solar radiation when combined with photocathodes of suitable adsorption. One other important characteristic is that hematite is catalytically active up to a wavelength of 600 nanometers, which is close to the sweet spot required for using solar energy to split water.”

Hematite is an n-type where electrons are the majority carriers and holes are the minority carrier. Wang says, “One of the problems in using hematite is the limited distance that photoexcited holes can diffuse to the surface to be used in the water splitting process. Holes are trapped or recombined within two to four nanometers of the hematite.”

A second concern for hematite is common to all metal oxides tried as semiconductors in PEC. Wang says, “The electrochemical potential of metal oxides is too positive. Oxidation power is huge yet reduction power is not good enough.”

The photovoltage for hematite is typically less than 0.4 V, which is much too low. For successful water splitting, a photovoltage of 1.61 V or greater must be achieved. Progress towards generating the necessary photovoltage has now been achieved by surface modification of hematite.

NiFeO_x OVERLAYER
Wang and his fellow researchers modified hematite with a NiFeO_x overlayer, which led to the generation of a photovoltage of 0.61V bringing the research team closer to reaching the required photovoltage to actually split water. He says, “We selected NiFeO_x for several reasons. It is an abundant material that is easily produced in a process that does not require the use of water. NiFeO_x acts to change the thermodynamics of the system, which leads to a dramatic 0.4 V shift in the voltage needed to generate current which is known as the turn-on voltage.”

Figure 3 shows an image of the NiFeO_x layer on the hematite surface. Preparation of the overlayer consists of applying a 1:1 mixture of iron (III) 2-ethylhexanoate and nickel (II) 2-ethylhexanoate to a fluorine-doped, tin oxide-hematite electrode surface using a transfer liquid gun. The film was left in the air for five minutes followed by irradiation with ultraviolet light for three hours.


Figure 3. Application of a nickel iron oxide coating on hematite increases the photovoltage so that it is closer to the required photovoltage needed to split water in a process known as photoelectrochemical water splitting. (Courtesy of Boston College)

The result of this process is the formation of a relatively uniform NiFeO_x layer with a thickness of 100 nanometers. The NiFeO_x layer is amorphous, but transmission electron microscopy shows good contact with the crystalline hematite surface. NiFeO_x also absorbs light at the same wavelength as hematite but does not produce any photocurrent.

The effect of the NiFeO_x overlayer was evaluated by comparing the modified hematite with bare hematite. It was found that the benefit of the NiFeO_x was seen in experiments run in the dark as compared to when both surfaces were illuminated.

This shift in voltage can also be carried out through the use of a co-catalyst such as cobalt-phosphate. Wang says, “The co-catalyst acts in a similar fashion as any catalyst in a typical process by lowering the activation energy of the process and changing the kinetics of the system.”

The researchers did further testing by combining the hematite with the NiFeO_x overlayer with a dual-absorber composed of silicon nanowires. Wang says, “We obtained an even greater cathode shift of over 0.6 V, which further narrows the gap needed to reach the photovoltage for water splitting to 0.2 V.”

Future work involves finding a way to bridge the 0.2 V gap, and as Wang indicated even achieve a negative voltage. Additional information on this research can be found in a recent article (2) or by contacting Wang at dunwei.wang@bc.edu.

REFERENCES
1. Canter, N. (2013), “Recyclable Organic Solar Cells,” TLT, 69 (7), pp. 12-13.
2. Du, C., Yang, X., Mayer, M., Hoyt, H., Xie, J., McMahon, G., Bischoping, G. and Wang, D. (2013), “Hematite-Based Water Splitting with Low Turn-On Voltages,” Angewandte Chemie International Edition, 52 (48), pp. 12692-12695.

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