Solar energy has been an intriguing topic of discussion of the past years, especially where renewable resources are concerned. Solar cells use various metallic nanowires and/or crystalline wafers to convert sunlight into useable energy. Another type of cell used to generate solar energy is the photoelectrochemical water splitting cell, which involves dividing water (H2O) into hydrogen (H2) and oxygen (O2). These water-splitting cells are convenient because they not only convert solar power into energy but they also store the energy in a clean chemical fuel, H2. Unfortunately, the most popular material used in these cells, silicon, is slower in converting sunlight and water into energy and fuel. The slow reaction time causes these silicon cells to be much less efficient than they could be. Recently, however, scientists at the China-Australia Joint Research Center for Functional Molecular Materials (CAJRC) have discovered a catalytic coating made of tungsten sulfides that promotes an increased production on hydrogen from the silicon nanowires in the water-splitting cells.
Tungsten is a heavy metal, found in the 6th row of periodic table among the transition metals. As one of the strongest and densest elements, tungsten is often used in hardware such as drills and circular saws because of its immense durability. Militaries will employ tungsten alloys in artillery, especially for high impact penetrating incendiaries and other weapons. Jewelers will also use tungsten compounds in wedding bands because of the long lasting properties and the increased sheen obtained in the metallic mixtures. It is also hypoallergenic in its metallic form, so tungsten based jewelry has become popular among many people with sensitive skin. Tungsten compounds are also often used as catalysts for various chemical reactions such as energy production and conversion. It is also used in high temperature situations, as either hardware or lubricant, because it has the highest melting point of any element on the periodic table. One of the more useful properties of tungsten is its ability as a conductor, leading many manufacturers to use it in the production of light bulbs. It is this conducting ability that helps allow tungsten compounds to act as catalysts for energy converting reactions such as those that occur in solar cells.
The CAJRC scientists, led by Zhipeng Huang and Chi Zhang from Jiangsu University in China in 2014, made and tested two different versions of a tungsten sulfide in order to measure the enhancement of the photoelectrochemical cells. The two types, WS2 (which is a crystalline—or ordered—solid) and WS3 (which is an amorphous—or disordered—solid), were prepared and tested using a set of electrodes to measure the voltage running through the cells. Since solar cells respond to specific types of light, the researchers used UV-vis spectroscopy—which is the analysis of a material’s interaction with light waves in the ultraviolet and visible light ranges—to determine whether the two tungsten sulfide compounds responded to different ranges of wavelengths. The results indicate that both WS2 and WS3 respond to the same wavelength ranges of light (300 nm to 1100 nm). These findings allowed the Huang and Zhang group to accurately and confidently compare the efficiencies of the two compounds in the same environment without worrying that variances in light wavelengths were causing discrepancies. By looking at the yield of hydrogen from the cells and maintenance of current over time of the cells, Huang and Zhang were able to conclude that both tungsten sulfide compounds improved the efficiency of the water-splitting solar cells. Both WS2 and WS3 work to catalyze the water-splitting reaction in the cell when activated by solar light and help maintain a mostly consistent initial photocurrent—due to the light source—over time. The scientists as CAJRC were also able to show that WS3 was a better enhancer than WS2 as its cell produced a greater hydrogen yield (roughly 100%) and maintained a higher percent of the initial photocurrent in the cell (90%).
Both sets of results indicate that tungsten sulfide compounds are promising materials for improving energy generation in the solar cell industry. Increasing the efficiency and production rate of photoelectrochemical water splitting solar cells without inhibiting any other functions of the cell is an important goal in the field of renewable energy science. These tungsten sulfide compounds look to be promising leads into better and perhaps more accessible solar cells for the public, which is never a bad thing for a society so reliant on the energy we generate. Tungsten is also favorable because it occurs naturally in biological systems, making it somewhat more benign with regards to the environment, which is very important when considering the potential waste hazards associated with any form of mass production and consumerism. It seems the future of solar power may lie in the hands of one of the world’s densest heavy metals.
1 Huang, Z.; Chen, Zho,; Chen, Zhi,; Han, R.; Meng, H.; Lv, C.; Wang, C.; Zhang, C. ACS Appl. Mater. Interfaces 2014, 6, 10408-10414.
2 Strigul, N. W.; Koutsospyros, A.; Arienti, P.; Braida, W.; Christodoulatos, C.; Dermatas, D. J. Chemosphere 2005, 61, 248-258.