Hydrogen is recognized as a powerful, clean energy carrier that produces only water vapor when consumed in a fuel cell. While it is the most abundant element in the universe, it rarely exists in its pure, diatomic form (\(\text{H}_2\)) on Earth. The vast majority of hydrogen is chemically bound, notably within water molecules (\(\text{H}_2\text{O}\)). Extracting it requires overcoming the strong chemical bonds, presenting a significant challenge in chemistry and engineering. The goal is to efficiently separate the hydrogen from the oxygen without using more energy or generating more carbon emissions than the resulting fuel can offset.
Splitting Water Using Electricity
The most commercially mature method for extracting hydrogen from water is electrolysis, which uses an electric current to drive the chemical reaction \(\text{2H}_2\text{O} \to \text{2H}_2 + \text{O}_2\). This process occurs within a device called an electrolyzer, which contains two electrodes, an anode and a cathode, separated by an electrolyte. Two primary types of electrolyzers are used: alkaline and proton exchange membrane (PEM).
Alkaline electrolyzers are a long-established technology that operates using a liquid electrolyte, typically a concentrated solution of potassium hydroxide (\(\text{KOH}\)) or sodium hydroxide (\(\text{NaOH}\)). Water molecules are split at the cathode, producing hydrogen gas and hydroxide ions (\(\text{OH}^-\)). These ions migrate through a porous diaphragm to the anode, where they release electrons to form oxygen gas and water. This method is known for its durability, lower material costs, and often uses nickel-based catalysts.
Proton Exchange Membrane (PEM) electrolyzers use a solid polymer membrane as the electrolyte. Water is fed to the anode, where it is oxidized, producing oxygen gas, electrons, and positively charged hydrogen ions (\(\text{H}^+\)), or protons. These protons pass through the solid membrane to the cathode. At the cathode, the protons recombine with electrons to form high-purity hydrogen gas. PEM systems are favored for their compact size, rapid response time to fluctuating power inputs, and ability to produce hydrogen at high pressure.
Using High Heat and Chemical Cycles
Directly splitting water using only heat, known as thermal decomposition, requires temperatures exceeding \(2500^\circ\text{C}\). This extreme heat requirement creates immense material challenges and makes the process impractical for industrial application. To circumvent this high energy barrier, researchers developed thermochemical cycles that use a sequence of two or more chemical reactions to achieve the same result at much lower temperatures, relying solely on heat energy rather than electrical input.
A prominent example is the Sulfur-Iodine (S-I) cycle, a three-step process that uses sulfur and iodine compounds as intermediate chemical agents. The cycle’s net reaction is the decomposition of water, with all intermediate chemicals recycled. The process begins with the Bunsen reaction, combining sulfur dioxide (\(\text{SO}_2\)), iodine (\(\text{I}_2\)), and water (\(\text{H}_2\text{O}\)) to produce two separate acid phases: sulfuric acid (\(\text{H}_2\text{SO}_4\)) and hydriodic acid (\(\text{HI}\)).
In the subsequent two steps, these acids are thermally decomposed to release hydrogen and oxygen and regenerate the intermediate chemicals. Hydriodic acid is broken down around \(450^\circ\text{C}\) to release hydrogen gas. The sulfuric acid decomposition step requires the highest temperature, typically between \(800^\circ\text{C}\) and \(850^\circ\text{C}\), to release oxygen gas and regenerate the sulfur dioxide. This high-temperature requirement necessitates a powerful heat source, such as a high-temperature nuclear reactor or concentrated solar thermal facility.
Direct Conversion Using Sunlight
A different approach uses sunlight directly, bypassing the need to first convert solar energy into electricity. This is achieved through photoelectrochemical (PEC) or photocatalytic water splitting, a one-step process driven by photons. The system uses specialized semiconductor materials immersed directly into the water or electrolyte solution, functioning as photoelectrodes that absorb solar energy.
When photons strike the semiconductor material, they excite electrons, generating electron-hole pairs. The material is engineered so the electrons and holes move to different locations on the surface. Electrons travel to the cathode-like site to reduce water into hydrogen, while the holes travel to the anode-like site to oxidize water into oxygen. This uses the photon’s energy to drive the chemical reaction.
Materials like titanium dioxide (\(\text{TiO}_2\)) or hematite (\(\text{Fe}_2\text{O}_3\)) are often studied as candidate semiconductors. The absorbed photon energy must be sufficient to overcome the minimum thermodynamic energy requirement for water splitting, which is \(1.23\) electron volts. By integrating light absorption and the chemical reaction into a single device, PEC water splitting offers a simplified, highly localized production pathway for hydrogen fuel.
Technical Energy Barriers and Costs
Regardless of the method employed, a significant energy barrier must be overcome to break the strong hydrogen-oxygen bond in the water molecule. While the theoretical minimum voltage for electrochemical water splitting is \(1.23\) volts, practical operation consistently demands a higher energy input, typically \(1.5\) to \(1.6\) volts. This difference is known as overpotential, and it represents wasted energy.
Research suggests that a portion of this extra energy is consumed by water molecules, which must “flip” their orientation at the electrode surface before releasing oxygen atoms. This molecular reorientation is energetically demanding and contributes to the overall inefficiency of the oxygen evolution reaction (OER) half-step. This need for additional energy directly translates into a higher cost for the resulting hydrogen.
Material costs also present a substantial hurdle for commercial viability. PEM electrolyzers rely on expensive platinum group metals (PGMs), such as platinum and iridium, to serve as highly efficient catalysts. Similarly, PEC and thermochemical systems require stable, high-performance, and inexpensive materials that can withstand highly corrosive environments and extreme temperatures, such as \(850^\circ\text{C}\) in the S-I cycle. These combined technical and material challenges keep the current cost of water-extracted hydrogen high compared to hydrogen produced from fossil fuels.