Water splitting is the chemical process of breaking down a water molecule (H₂O) into its constituent elements: hydrogen gas (H₂) and oxygen gas (O₂). This reaction is the reverse of combustion and is highly endothermic, requiring a significant energy input. Because of this, water does not naturally decompose at room temperature. Efficiently separating hydrogen from water is crucial for creating a clean, sustainable hydrogen economy, as the resulting hydrogen serves as a zero-emission fuel source.
Splitting Water Using Electricity
The most established method for separating water is electrolysis, which uses an electrical current to drive the decomposition reaction. This process takes place within an electrolyzer, which contains two electrodes—an anode and a cathode—submerged in an electrolyte solution. Applying a direct current forces the non-spontaneous chemical reaction to occur.
At the negatively charged cathode, water molecules gain electrons in a reduction reaction to form hydrogen gas and hydroxide ions. Simultaneously, at the positively charged anode, water molecules lose electrons in an oxidation reaction, producing oxygen gas and hydrogen ions. The overall reaction separates water into hydrogen and oxygen, with the hydrogen volume produced being twice that of the oxygen.
Different technologies manage this process, with Alkaline Electrolyzers (AEL) and Proton Exchange Membrane (PEM) Electrolyzers being the most common commercial types. AELs typically use a liquid electrolyte like potassium hydroxide and are known for their long lifespan and lower initial cost. PEM electrolyzers utilize a solid polymer membrane to transport protons, allowing them to operate at higher current densities and respond quickly to fluctuating power inputs, such as those from renewable sources.
The efficiency of an electrolyzer is measured in two ways: Faraday efficiency and energy efficiency. Faraday efficiency refers to the percentage of electrical charge resulting in hydrogen production, often approaching 99% in modern systems. Energy efficiency measures how much electrical energy is converted into the chemical energy stored in the hydrogen, typically ranging from 50% to 80%. Energy loss occurs due to the electrical resistance of cell components and the need for an overpotential—an extra voltage beyond the theoretical minimum of 1.23 volts—to overcome activation barriers.
Splitting Water Using Extreme Heat
Water can also be split using only heat, a process known as direct thermolysis, but this approach demands exceptionally high temperatures. Temperatures exceeding 2500°C are necessary for meaningful water decomposition, presenting immense material science challenges for reactor construction. Containment vessels at these temperatures often suffer corrosion, and the immediate recombination of the separated hydrogen and oxygen gases is difficult to prevent.
To circumvent the impracticality of direct thermolysis, scientists developed thermochemical cycles. These cycles use a series of two or more chemical reactions to split water at lower, manageable temperatures. They employ intermediate chemical compounds, such as metal oxides or sulfur compounds, that are recycled within the system to produce hydrogen and oxygen separately.
One example is the Sulfur-Iodine cycle, a three-step process using sulfur dioxide and iodine. This cycle can split water effectively with operating temperatures of 800°C to 1000°C. These temperatures are achievable using massive, concentrated heat sources, such as advanced high-temperature nuclear reactors or large-scale concentrated solar power facilities. The advantage of thermochemical cycles is that they convert heat directly into chemical energy (hydrogen), bypassing the less efficient step of converting heat into electricity required for conventional electrolysis.
Splitting Water Using Light and Catalysts
A third pathway utilizes the energy from light, typically sunlight, in a process that mimics natural photosynthesis. This method is explored through two routes: photocatalysis and photoelectrochemical (PEC) water splitting. Both aim for a direct conversion of solar energy into chemical energy, creating hydrogen without an external electrical grid.
Photocatalysis involves suspending microscopic semiconductor catalyst particles in water. When these particles absorb photons from sunlight, they generate electron-hole pairs. The electrons and holes migrate to the particle surface, driving the reduction of water to hydrogen and the oxidation of water to oxygen. This approach simplifies the system by combining the light absorption and reaction sites into a single material.
The PEC approach is more structured, using specialized semiconductor electrodes immersed in an electrolyte, forming a solar-powered electrochemical cell. The semiconductor material acts as both the light absorber and the electrode, directly converting solar energy into the electrochemical potential needed to split the water. This setup is often called “artificial photosynthesis” because it directly converts solar energy into a chemical fuel. These methods offer the potential for a sustainable, decentralized method of hydrogen production using only water and sunlight.