Solar water splitting: fuelling the power and energy sectors towards a sustainable future

Researchers have always been aware that hydrogen, one of the lightest elements, is a promising green fuel and a potential energy store. Most hydrogen production plants require fossil fuels (in most cases, natural gas) to generate electricity, which is then used to electrolyse water in order to produce hydrogen. These fossil fuels include coal, petroleum, natural gas and even nuclear energy. Since water splitting is an endothermic process, a large amount of energy is used.

Although solar energy is widely available, using it to split water poses various challenges, including low energy conversion efficiency and the high costs involved, in particular the electrolyser unit.

Patent landscape
Recent IP filings show that 43.65% of the total number of patents for water splitting were filed between 2012 and 2017, demonstrating an increasing scientific interest in this field. The R&D is expected to grow at a compound annual rate of nearly 23% between 2017 and 2022. The hydrogen production market is estimated to be worth approximately $150 billion by 2022. The majority of research has been carried out in China, the United States, Japan and Germany, with China leading the way.

In China, 36.9% of the total patents in this area have been filed by universities and research institutes, including:

• Bohai University;
• East China University of Science and Technology;
• Xi'an University of Architecture and Technology;
• the Technical Institute of Physics and Chemistry;
• Dalian Institute of Chemical Physics; and
• the University of Electronic Science and Technology of China.

Researchers from these universities have begun to focus on materials such as perovskite-structured transition metal oxynitrides (eg, those based on gallium and tellurium) as a new type of photocatalyst with highly visible light content. In 2015 researchers from Bohai University filed patents focusing on photocatalytic materials that can be activated with visible light, as well as ultraviolet light such as CATON, CMTON and CSTON. These materials have shown high efficiency, stability and efficiency in water splitting.

Major industry players in this field include:

• Sunpower Technologies;
• Wuxi Thinkpower New Energy Technology Co Ltd;
• Sabic Global Technologies;
• Equos Research Co Ltd; and
• Everphoton Energy Corp.

Overall, the patent filings in this domain are predominantly focused on:

• reducing greenhouse gas emissions;
• increasing the efficiency of energy conversion;
• reducing the manufacturing cost of producing hydrogen;     
• increasing the efficiency of a photosynthetic system; and    
• improving photoactive materials.

Recent trends
Initial inventions were based around conducting catalytic reactions using different photocatalysts and then electrolysing water, effectively converting solar energy into hydrogen energy. Patents were then filed to overcome the problem of the costly processes involved in the direct conversion of solar energy into electricity, which is also less efficient. These problems were overcome by using thermochemical water dissociation by solar energy to produce hydrogen.

Due to the dynamic nature of this technology, the research shifted focus towards improving photoactive materials to:

• increase the efficiency of higher energy conversions;
• improve the limited lifetime and efficiency of hydrogen production devices; and
• reduce waste.

However, organisations such as Hyper Solar then began to introduce next-generation semiconductor materials, including silicide materials (eg, titanium silicide, nickel silicide, iron silicides, thallium silicide, boron silicide and cobalt silicide) which absorb solar or artificial radiation energy to split water and produce hydrogen and oxygen at relatively low cost. From thereon, the focus of the inventions has been on improving the properties of photoactive materials and devices that can be used for improving the photocatalytic reaction rate leading to higher energy conversion efficiency.

Latest advancements
Govinder Singh Pawar, a researcher at the University of Exeter, has taken a large step towards a pollution-free world by creating a photoelectrode that can store solar energy more efficiently than any other method and use it to generate enough voltage to extract hydrogen from water.

Further, scientists from Kyushu University have synthesised a compound that absorbs near-infrared light to produce hydrogen from water. The compounds consist of three ruthenium atoms connected by an organic molecule. This is the first successful use of infrared light for the production of hydrogen by water splitting.

Comment
Fulfilling the growing demand for sustainable energy is a major challenge for the scientific community. With the depletion of fossil fuels, the world will soon rely on renewable energy sources such as solar energy and fuels such as hydrogen fuel. Until now, the use of hydrogen as a fuel has been limited, but advances in fields such as solar water splitting are at an all-time high. In order to fully leverage the potential of this technology, researchers must improve the efficiency of energy conversion by utilising different combinations of photocatalytic materials and photoactive materials.