Electrolysis

Electrolysis of water is the decomposition of water (H2O) into oxygen (O2) and hydrogen gas (H2) due to an electric current being passed through the water.

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At optimum efficiency, you can generate hydrogen with an energy use of:

39.4 kilowatt-hours per kilogram (142 MJ/kg), 12.749 Kilo-joules per liter (12.75 MJ/m3).

One liter of water will produce 1235 liters of hydrogen and 622 liters of oxygen (0.89kg).

1 litre of water weights 1 kilo and when electrolysed will produce hydrogen and oxygen as described by the following equation 2 H2O(l) → 2 H2(g) + O2(g) in atomic weight terms 36.0012kg of water with give 4.0032kg of hydrogen and 31.998kg of oxygen so a single kilo you will get 4.0032/36.0012 or 111.19gm of hydrogen and 31.998/36.0012 or 888.81gm of oxygen.

One liter of water has a mass of 1000 grams. Molar mass of water is 18.016 g/mol. One liter of water contains 55.5 moles of water molecules (55.5 moles of hydrogen and 27.75 moles of oxygen). Avogadro’s rule for molar volume states that one mole of any ideal gas occupies 22.4 liters of volume. 55.5 moles of hydrogen occupy 1243.2 liters of hydrogen gas. 27.75 moles of oxygen occupy 621.6 liters.

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This means that, at optimum efficiency, in order to generate the necessary 550 liters of oxygen that is used by a by a person in a day, you will need almost a liter of water, and 14 Mega-joules of energy???

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Practical electrolysis (using a rotating electrolyser at 15 bar pressure) may consume 50 kilowatt-hours per kilogram (180 MJ/kg), and a further 15 kilowatt-hours (54 MJ) if the hydrogen is compressed

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The reaction has a standard potential of −1.23 V, meaning it ideally requires a potential difference of 1.23 volts to split water.

This technique can be used to make hydrogen fuel (hydrogen gas) and breathable oxygen; though currently most industrial methods make hydrogen fuel from natural gas instead.

A DC electrical power source is connected to two electrodes, or two plates (typically made from some inert metal such as platinum, stainless steel or iridium) which are placed in the water. Hydrogen will appear at the cathode (where electrons enter the water), and oxygen will appear at the anode. Assuming ideal faradaic efficiency, the amount of hydrogen generated is twice the amount of oxygen, and both are proportional to the total electrical charge conducted by the solution. However, in many cells competing side reactions occur, resulting in different products and less than ideal faradaic efficiency.

The following cations have lower electrode potential than H+ and are therefore suitable for use as electrolyte cations: Li+, Rb+, K+, Cs+, Ba2+, Sr2+, Ca2+, Na+, and Mg2+. Sodium and lithium are frequently used, as they form inexpensive, soluble salts.

If an acid is used as the electrolyte, the cation is H+, and there is no competitor for the H+ created by disassociating water. The most commonly used anion is sulfate (SO2− 4), as it is very difficult to oxidize, with the standard potential for oxidation of this ion to the peroxydisulfate ion being −2.05 volts.

Strong acids such as sulfuric acid (H2SO4), and strong bases such as potassium hydroxide (KOH), and sodium hydroxide (NaOH) are frequently used as electrolytes due to their strong conducting abilities.

A solid polymer electrolyte can also be used such as Nafion and when applied with a special catalyst on each side of the membrane can efficiently split the water molecule with as little as 1.5 Volts.

High pressure High pressure electrolysis is the electrolysis of water with a compressed hydrogen output around 120–200 Bar (1740–2900 psi). By pressurising the hydrogen in the electrolyser, the need for an external hydrogen compressor is eliminated; the average energy consumption for internal compression is around 3%.

High-temperature High-temperature electrolysis (also HTE or steam electrolysis) is a method currently being investigated for water electrolysis with a heat engine. High temperature electrolysis may be preferable to traditional room-temperature electrolysis because some of the energy is supplied as heat, which is cheaper than electricity, and because the electrolysis reaction is more efficient at higher temperatures.

Nickel/iron In 2014, researchers announced an electrolysis system made of inexpensive, abundant nickel and iron rather than precious metal catalysts, such as platinum or iridium. The nickel-metal/nickel-oxide structure is more active than pure nickel metal or pure nickel oxide alone. The catalyst significantly lowers the required voltage.Also nickel–iron batteries are being investigated for use as combined batteries and electrolysis for hydrogen production. Those “battolysers” could be charged and discharged like conventional batteries, and would produce hydrogen when fully charged.

There are two main technologies available on the market, alkaline and proton exchange membrane (PEM) electrolysers. Alkaline electrolysers are cheaper in terms of investment (they generally use nickel catalysts), but less efficient; PEM electrolysers, conversely, are more expensive (they generally use expensive platinum-group metal catalysts) but are more efficient and can operate at higher current densities, and can therefore be possibly cheaper if the hydrogen production is large enough.

A 100%-efficient electrolyser would consume 39.4 kilowatt-hours per kilogram (142 MJ/kg) of hydrogen, 12,749 joules per litre (12.75 MJ/m3). Practical electrolysis (using a rotating electrolyser at 15 bar pressure) may consume 50 kilowatt-hours per kilogram (180 MJ/kg), and a further 15 kilowatt-hours (54 MJ) if the hydrogen is compressed.

Reported working efficiencies were for alkaline in 1996 lying in the 50–60% range for the smaller electrolysers and around 65–70% for the larger plants. Theoretical efficiency for PEM electrolysers are predicted up to 94% Ranges in 2014 were 43–67% for the alkaline and 40–67% for the PEM, they should progress in 2030 to 53–70% for the alkaline and 62–74% for the PEM.