Water Splitting | ChemTalk

Core Concepts

In this article we will learn about what water splitting is, the process of water splitting, and its applications in research and industry.

What is water splitting?

Water splitting is the process of breaking down water (H₂O) into its two components: hydrogen (H₂) and oxygen (O₂). This process is currently generating a lot of interest for its potential to reduce of use of fossil fuels. Fossil fuels are the non-renewable resources formed over millions of years via the build-up of dead plants and animals underneath rock. A majority of the world’s energy comes from fossil fuels. However, the price of obtaining these fuels is the health of the land, clean water, and clean air.

On the other hand, renewable energy is replenished at a much faster rate than fossil fuels and yields fewer negative impacts on the earth over time. Water splitting is of interest because it opens avenues to produce economical and clean hydrogen. There are a variety of approaches to water splitting, each one uniquely identified by the way it splits water. Let’s examine these approaches in more depth.

The Splitting Process

Method 1: Water Electrolysis

Method one for splitting water is electrolysis, a process utilizing basic oxidation and reduction techniques to produce hydrogen gas! Electrolytic cells are electrochemical cells that use electricity as an external source of energy to drive non-spontaneous reactions. They are made up of two half-cells, a reduction half and an oxidation half, with electrons flowing from one half to another.

In this method of water splitting, the electrodes are immersed in water along with a small amount of electrolytes, a substance that conducts electricity via the movement of ions. The electrolyte can be an acid, a base, or a soluble salt, allowing for a more even distribution of the H₂O molecule’s anions and cations. At the anode, oxygen gas and hydrogen ions are formed through oxidation. At the cathode, hydrogen gas and oxygen ions are formed through reduction. The electrolyte is not consumed during the reactions.

Method 2: Thermochemical Water Splitting

Method two is a thermochemical process: using heat to separate hydrogen and oxygen into gaseous form. This method requires extreme heat, a temperature of approximately 500°C – 2000°C (932°F – 3632°F). This heat can be obtained via a variety of methods, one being the sun (by reflecting sunlight using mirrors onto a reactor tower).

The reactor tower holds a receiver that holds the heat transfer fluid (HTF), a gas or fluid used to move heat energy from one place to another, often steam or molten salt. In steam models, the water is heated and evaporated, turning a turbine and creating electricity. By contrast, the salt model involves pushing molten salt up into the receiver, at which point it proceeds into the steam generator.

Method 3: Photobiological Water Splitting

Method three of water splitting is the photobiological approach. This clever approach uses microorganisms and sunlight to split the water. Certain microorganisms, such as algae and bacteria, use the sunlight as energy to break down organic matter and release hydrogen gas. There is promise in this method, which can make use of non-drinkable water or even waste water!

Method 4: Photocatalytic Water Splitting

Finally, method four is the photocatalytic approach. This modified photosynthetic process uses a photocatalyst, a light source, and water. A photocatalyst is a material that can absorb light energy to accelerate a light reaction without being consumed.

Typically, the catalyst is a metal oxide, metal sulfide, or metal nitride. When the catalyst is exposed to light, holes form, oxidizing the water into O₂ and a positively charged hydrogen ion. Then, electrons generated during the reaction reduce the hydrogen into H₂.

Challenges of Water Splitting

Despite the numerous approaches to water splitting, none of them are 100 percent perfect. Each method comes with its own issues and challenges.

Some of these challenges are financial. Electrolysis and the thermochemical approach are both deeply hindered by the amount of capital needed upfront to begin the process. Researchers are trying to improve both the durability of the electrolyzer and the efficiency of thermochemical reactants. Ultimately, the goal in both scenarios is to improve longevity.

Method three, photobiological water splitting, has comparatively low hydrogen production. This is because the secondary byproduct of the splitting, oxygen, inhibits hydrogen production in the microorganisms. Research is being done to allow for a longer time period, during which the microbes actively produce hydrogen. The microbes also have low solar to hydrogen conversion efficiency, which refers to the amount hydrogen produced via their natural pathway. There is potential in the future to modify this pathway in order to improve hydrogen production.

The quality of the catalysts place limitations on the photocatalytic process. In general, the better the catalyst is at being a semiconductor, the more effective it is at water splitting. When using this approach, it’s important to put a significant amount of thought into choosing a proper catalyst, based on the amount of hydrogen required.

Conclusion

Water splitting is a technique used to separate water into its core components of oxygen and hydrogen. Splitting allows for the release of pure hydrogen, which is an increasingly prominent source of renewable energy. There are electrical, thermochemical, and biological, and catalytic methods to choose from when splitting water. No method is flawless, but avenues to reduce costs and increase production are being researched with much promise.