Electrolysis of water
How can water be converted to H2 and O2?
Electrochemistry
Water ionizer Electrochemistry Hydrogen purifier Coal High-pressure electrolysis
Water_ionizer-Ionization Electrochemistry-Electrolysis_of_water Hydrogen_purifier-Catalytic_recombination_or_deoxygenation_purifier Coal-Gasification High-pressure_electrolysis-Ultrahigh-pressure_electrolysis
Electrolysis separates water into oxygen (O2) and hydrogen gas (H2) due to an electric current. 
Water can be converted to its component elemental gasses, H2 and O2 through the application of an external voltage. Water doesn't decompose into hydrogen and oxygen spontaneously as the Gibbs free energy for the process at standard conditions is about 474.4 kJ. The decomposition of water into hydrogen and oxygen can be performed in an electrolytic cell. In it, a pair of inert electrodes usually made of platinum immersed in water act as anode and cathode in the electrolytic process. The electrolysis starts with the application of an external voltage between the electrodes. This process will not occur except at extremely high voltages without an electrolyte such as sodium chloride or sulfuric acid (most used 0.1 M). Bubbles from the gases will be seen near both electrodes. The following half reactions describe the process mentioned above: "Anode (oxidation)": 2 H2O(l) → O2(g) + 4 H+(aq) + 4 e– "Cathode (reduction)": 2 H2O(g) + 2 e– → H2(g) + 2 OH–(aq) "Overall reaction": 2 H2O(l) → 2 H2(g) + O2(g) Although strong acids may be used in the apparatus, the reaction will not net consume the acid. While this reaction will work at any conductive electrode at a sufficiently large potential, platinum catalyzes both hydrogen and oxygen formation, allowing for relatively mild voltages (~2 V depending on the pH).
Catalytic recombination or deoxygenation is used to remove oxygen (O2) impurities. The process is also known as a 'deoxo' process. The oxygen reacts with the hydrogen to form water vapor, which can then be removed by a dryer if necessary. The catalysts that are used are based on platinum group metals (PGM). A typical system could handle up to 3% O2 in H2 in the feed, and reduce the O2 content to less than 1ppm. 
Coal gasification can be used to produce syngas, a mixture of carbon monoxide (CO) and hydrogen (H2) gas. This syngas can then be converted into transportation fuels, such as gasoline and diesel, through the Fischer-Tropsch process. This technology is currently used by the Sasol chemical company of South Africa to make motor vehicle fuels from coal and natural gas. Alternatively, the hydrogen obtained from gasification can be used for various purposes, such as powering a hydrogen economy, making ammonia, or upgrading fossil fuels. During gasification, the coal is mixed with oxygen and steam while also being heated and pressurized. During the reaction, oxygen and water molecules oxidize the coal into carbon monoxide (CO), while also releasing hydrogen gas (H2). This process has been conducted in both underground coal mines and in the production of town gas. C ("as Coal") + O2 + H2O → H2 + CO If the refiner wants to produce gasoline, the syngas is collected at this state and routed into a Fischer-Tropsch reaction. If hydrogen is the desired end-product, however, the syngas is fed into the water gas shift reaction, where more hydrogen is liberated. CO + H2O → CO2 + H2 In the past, coal was converted to make coal gas (town gas), which was piped to customers to burn for illumination, heating, and cooking. 
Ultrahigh-pressure electrolysis is high-pressure electrolysis operating at . At ultra-high pressures the water solubility and cross-permeation across the membrane of H2 and O2 is affecting hydrogen purity, modified PEMs are used to reduce cross-permeation in combination with catalytic H2/O2 recombiners to maintain H2 levels in O2 and O2 levels in H2 at values compatible with hydrogen safety requirements. 