New High Performance Photocatalyst and Hybrid Photocatalytic-Electrolysis System Could Significantly Reduce Voltage Required and Cost for Hydrogen Production

. The activity of the treated catalyst is more than ten times that of untreated catalysts. The quantum yield of the new photocatalyst is 19% under visible light of wavelength 420 nm and is approximately 50 times the previously reported values (0.4%).

In its study of hybrid photocatalytic-electrolysis systems to reduce the cost of hydrogen production, AIST noted that while certain candidate redox media are used for oxidation-reduction reactions, the technique for low-voltage hydrogen production using Fe²⁺ ions has already been established. The researchers decided then that the use of iron (Fe²⁺ and Fe³⁺ ion pairs) as the redox medium was, at present, the most practical technique for the hybrid system.

A major challenge that was faced in the realization of this hybrid system was the development of a high-performance photocatalyst that would reduce the redox medium (from Fe³⁺ to Fe²⁺) while generating oxygen from water.

Prior AIST work had shown that a WO₃ semiconductor photocatalyst can absorb visible light and that its performance in environmental cleanup processes is significantly better than that of conventional TiO₂-based photocatalysts when a copper promoter or a palladium promoter is supported on the surface of the photocatalyst.

In this new study, the conditions for the preparation and surface-treatment of the WO₃ photocatalyst powder were optimized to improve the activity of the photocatalyst for the reaction in which Fe₃₊ ions are reduced while oxygen is generated from water. The researchers found that the treatment of the photocatalyst with cesium salt significantly improved the photocatalytic activity.

Fe²⁺ ions were stoichiometrically generated in the reaction. It was confirmed that Cs compounds that did not dissolve in water were present on the surface of the treated photocatalyst. The surface area, particle configuration, light absorption, and internal structure of the WO₃ semiconductor photocatalyst particles did not change considerably after Cs treatment.

There are two methods of Cs surface treatment: one involves the addition of cesium salt to a solution used for hydrothermal treatment, and another involves the impregnation of the WO₃ particles with cesium carbonate and sintering the particles at approximately 500 °C. High activity could be achieved by both the methods.

When the Cs-treated WO₃ photocatalyst surface was washed with highly acidic water to decrease the Cs ions on the surface or washed with an iron sulfate (FeSO₄) solution, the activity of the treated photocatalyst improved further (196 µmol/h) and was about 10 times that of untreated WO₃ photocatalysts (18 µmol/h).

The AIST team then investigated a mechanism for improving the activity of the WO₃ photocatalyst surface-treated with Cs. The Cs atoms that were unevenly distributed on the surface of the WO₃ photocatalyst were partly removed by using highly acidic water, thereby generating ion exchange sites that do not exist on normal WO₃ surfaces. Protons (H⁺) and water molecules, in the form of H₃O⁺, were specifically absorbed at these ion-exchange sites, where oxygen was efficiently generated by the oxidation of water. At some of these sites, ion-exchange of Fe²⁺ ions also occurred and Fe³⁺ ions are rapidly reduced to Fe²⁺ ions at the sites.

The reaction for oxygen generation efficiently progressed until all Fe³⁺ ions added at the beginning of the experiment were reduced to Fe²⁺ ions. The reaction in both aqueous solutions of iron sulfate and that of iron chloride proceeded stoichiometrically. The photocatalyst exhibited higher activity (256 µmol/h) in the aqueous solution of iron chloride than in that of iron sulfate. The activity of the catalyst did not deteriorate during the repeated experiments.

The quantum yield of 19% obtained under visible light (420 nm) was 48 times the previously reported values (0.4% for 405 nm) for WO₃ photocatalysts used for the generation of oxygen using Fe₃₊ ions. The AIST team achieved an efficiency of 0.3% for the conversion of solar energy to chemical energy, i.e., the production of Fe²+ ions; this is greater than the highest efficiency value reported previously for solar energy conversion by water decomposition using photocatalyst powder.

In comparison to the efficiency of conversion of solar energy to hydrocarbons in photosynthesis, this value exceeds the efficiency for switchgrass (0.2%), which is a well-known plant as a prospective raw material of biofuel. Such a significant improvement in the activity is a great step toward the realization of artificial photosynthesis, the researchers said.

The relation between the voltage and current of electrolysis in a small electrolysis apparatus that produces hydrogen using Fe2+ ions generated in the photocatalytic reaction. (a) Ordinary electrolysis. (b) Electrolysis in the presence of Fe2+ ions generated in the photocatalytic reaction. Source: AIST. Click to enlarge.

The photocatalyst-electrolysis hybrid system can directly produce hydrogen via the low-voltage electrolysis of an aqueous solution of Fe²⁺ ions. An electrolysis current was observed at a low voltage of approximately 0.8 V, and hydrogen corresponding to the current was generated at a counter electrode.

Theoretically, hydrogen production by ordinary water electrolysis without using Fe²⁺ ions requires a minimum electrolysis voltage of 1.23 V; however, in practice, a voltage of 1.6 V or higher is required because of the large over voltage of oxygen. In the new hybrid system, the photocatalyst can accumulate solar energy in an aqueous solution of Fe²⁺ ions, thereby enabling a low electrolysis voltage. Various types of power sources including solar cells and night-time electricity can be used for the electrolysis.

The results of this study represent a major step toward the development of a system for low-cost hydrogen production utilizing solar energy, AIST said.

If the current quantum yield of the photocatalyst is further increased and any light with a wavelength less than 480 nm can be used for this reaction, the theoretical limit of solar energy conversion efficiency will be 2.4%. If a semiconductor that can make use of wavelengths longer than that used for WO₃ is developed and light with wavelengths up to 600 nm can be utilized, the theoretical limit will increase to 7.5%. The AIST team intends to continue the study to improve photocatalysts so as to increase the efficiency of solar energy conversion.