Originally on Earth, hydrogen existed mainly as water. Then plants split water into hydrogen and oxygen to form carbohydrates and protein. So huge amounts of hydrogen are combined with carbon as biomass and over time this became gas, oil, and coal.

To produce hydrogen we can:

• split it from water, 
• remove it from an organic compound,
• steam reforming - a combination of both

Sources of hydrogen

From water

• Electrolysis of water
• Heating water with a catalyst to split the water directly
• Sunlight on water with a catalyst
• Microorganisms can produce hydrogen
• Photobiological Water Splitting

From organic compounds

• Fermentation: Biomass is converted into sugar-rich feedstocks that can be fermented to produce hydrogen.
• partially burning methane or any hydrocarbon 
• Thermal decomposition of methane (pyrolysis)

From organic compound plus water - reforming

• Natural Gas Reforming:
• Liquid Reforming eg Ethanol + steam
• Gasification: Coal or biomass

Sources of global hydrogen production:

• 50% via steam reforming of natural gas,
• 30% from oil/naphtha reforming
• 18% from coal gasification,
• 3.9% from water electrolysis, and
• 0.1% from other sources. Source

Worldwide hydrogen production causes about 5% of the global CO2 emissions.  Source

One kilogram of hydrogen has the same energy as 3 KG of petrol / gasoline. 

Hydrogen is used mainly for:

• production of NH3 - ammonia for fertiliser,
• Reforming of oil to petrol
• Reduction of metals from ore

In future it will be needed for:

• Fuel cell cars
• Industrial heating
• Storage of energy
• Steelmaking

Electrolysis  

Passing electricity through water is one of the simplest ways of producing hydrogen. If the electricity is from renewable sources, then this process produces no greenhouse gases. One aim is to have excess electricity produce hydrogen that can then be used to produce electricity when required. Like a battery, it would iron out the peaks and lows.

Energy content of Hydrogen is 147 MJ/kg. At an efficiency of  80%, 50 kWh (180 MJ) of electricity produces 1 kilogram of hydrogen.  

1 KG of hydrogen has the same energy as 3 KG of petrol.

Electrolysis at Low temp.

Low temperature electrolysis is carried out at <100^oC,  with an electrolyte of an alkaline (hydroxyl ion conducting) solution, or a polymer membrane (proton conducting).

Low temperature electrolysis is expensive, but has a few advantages:

• on-demand (distributed) generation,
• high purity hydrogen, 
• unit modularity,
• fast start-up and shutdown,
• good load following capability that makes them suitable for integrating with intermittent renewable energy source

Polymer electrolyte membrane (PEM)-based electrolysis systems offer additional advantages over alkaline systems:

• higher current densities and production rate,
• solid state system requiring no alkaline solutions or electrolyte top-up, 
• higher purity hydrogen and
• hydrogen generation at higher pressures 

Companies selling Low Temp electrolysis are:

• Proton OnSite,
• Giner Electrochemical Systems,
• Hydrogenics,
• New Horizon,
• ITM Power)  

Efficiency is 50-55%.

Source

Electrolysis at high temp.

If the water, or steam, temperature is around 1000^oC, then the process is more efficient because up to one third of the energy can come from  a heat source, such as solar mirrors, or nuclear. The ceramic yttria stabilised zirconia electrolyte, can transfer either O^2- or H⁺ ions. It is the same as a solid oxide fuel cell.

- Sunfire

Sunfire's reversible electrolysis combines both fuel cell and electrolysis in one single device. The process electrolyses steam under high pressure (> 20 bar) and temperatures over 800 deg C. 

Their website claims various efficiencies of 70-90 %  (Energy in H₂ / electrical energy used.)  A Solid Oxide Power Core ) generates hydrogen. 

They use the hydrogen to produce liquid fuels (-CH₂-), such as diesel, or methane (CH₄) by combining H₂ with CO₂. This is a convenient way to store H₂. Sunfire

Electrolysis Very high temp nuclear hydrogen 

One plan is to use a very high temperature (850-1,000^oC) next generation nuclear reactor to generate hydrogen by high temperature electolysis. The plan is for the reactor to generate electricity by day, and hydrogen at night.

Electrolysis at high pressure

3% of the energy can be saved by electrolysing water at 120–200 bar (1740–2900 psi). Compressing water takes almost no energy, and the hydrogen is produced, already compressed.

Electrolysis with catalysts

In 2012 MIT discoved that a catalyst using cobalt and phosphate makes electrolysis more efficient.

A startup company called Sun Catalytics was formed to exploit this discovery. It was bought by Lockheed Martin and is working on flow batteries with no mention of the catalyst.  ref  ref2

Catalyst - Cobalt atoms on graphene 

The best catalyst for helping the splitting of water is platinum. But it is expensive. Scientists at Rice university have taken graphene, doped it with nitrogen, and added with cobalt atoms. It can split water as well as platinum, and a lot more cheaply. It uses very little cobalt as every atom is exposed to the water because it is on the surface of the graphen, not locked up inside a metal particle.

Water reacts at the anode to form oxygen and positively charged hydrogen ions (protons).

The electrons flow through an external circuit and the hydrogen ions selectively move across the PEM to the cathode.

At the cathode, hydrogen ions combine with electrons from the external circuit to form hydrogen gas.
Anode Reaction:    2H₂O → O₂ + 4H+ + 4e^-
Cathode Reaction: 4H⁺ + 4e- → 2H₂

ALKALINE ELECTROLYZERS

Alkaline electrolyzers operate via transport of hydroxide ions (OH-) through the electrolyte from the cathode to the anode with hydrogen being generated on the cathode side. Electrolyzers using a liquid alkaline solution of sodium or potassium hydroxide as the electrolyte have been commercially available for many years. Newer approaches using solid alkaline exchange membranes as the electrolyte are showing promise on the lab scale.

SOLID OXIDE ELECTROLYZERS

Solid oxide electrolyzers, which use a solid ceramic material as the electrolyte that selectively conducts negatively charged oxygen ions (O2-) at elevated temperatures, generate hydrogen in a slightly different way.

Water at the cathode combines with electrons from the external circuit to form hydrogen gas and negatively charged oxygen ions.

The oxygen ions pass through the solid ceramic membrane and react at the anode to form oxygen gas and generate electrons for the external circuit.

Solid oxide electrolyzers must operate at temperatures high enough for the solid oxide membranes to function properly (about 700°–800°C, compared to PEM electrolyzers, which operate at 70°–90°C, and commercial alkaline electrolyzers, which operate at 100°–150°C). The solid oxide electrolyzers can effectively use heat available at these elevated temperatures (from various sources, including nuclear energy) to decrease the amount of electrical energy needed to produce hydrogen from water.  USDOE

Graphene is an incredible single sheet of graphite.

A new catalyst just 15 microns thick has proven nearly as effective as platinum-based catalysts but at a much lower cost, according to scientists at Rice University. The catalyst is made of nitrogen-doped graphene with individual cobalt atoms that activate the process. (Credit: Tour Group/Rice University)

Photo-electrolysis

The photoelectrode is a semiconducting device absorbing solar energy and simultaneously creating the necessary voltage for the direct decomposition of water molecule into oxygen and hydrogen.

If the semiconductor photoelectrode is submerged in an aqueous electrolyte exposed to solar radiation, it will generate enough electrical energy to support the generated reactions of hydrogen and oxygen. The voltage necessary for electrolysis is about 1.35 V.

A catalyst on the surface improves efficiency.

Photocatalytic

There is a lot of research into making an artificial leaf. Sunlight hits a catalyst, which then splits water molecules producing hydrogen. There are many catalysts, most are quite complex.

Photosynthesis is about 1% efficient.  Wikipedia   Ref

Chemical - Olivine

Hydrogen is produced when water meets the mineral olivine under the high temperatures and pressure. In the process, olivine turns into the mineral serpentine and water splits into its components, hydrogen and oxygen. Serpentinite can be used to fix carbon dioxide. Source

Splitting water - heat

At 2200 °C about three percent of all H₂O molecules are dissociated into various combinations of hydrogen and oxygen atoms, H, H₂, O, O₂, and OH. At 3000 °C more than half of the water molecules split.

The main problem for commercial production is finding material to withstand the high temperature.

Thermochemical cycles

A way oif reducing thiis temperature is to use thermochemical cycles involves a series of chemical reactions driven by heat alone. Hydrogen is produced, and the chemicals recycled.

The heat would come from solar mirrors, or nuclear reactor.

There are over 300 processes whereby heat and chemicals can be used to produce hydrogen from water. The temperatures are around 1,000^oC.

E,G, Zinc oxide powder passes through a reactor heated by a solar concentrator operating at about 1,900°C. The zinc oxide dissociates to zinc and oxygen. The zinc reacts with water to form hydrogen gas and zinc oxide.  Source 

2ZnO + heat → 2Zn + O₂
2Zn + 2H₂O → 2ZnO + 2H₂

Thermochemical cycles

The US Dept of Energy USDOE is developing a Very High temperature Reactor VHTR for the production of hydrogen and electricity. This would be a gen. IV nuclear reactor. The plan is to build one of 50 MW by 2017. Then it would be used to produce hydrogen by the best method discovered to date.

In Europe, the high temperature gas cooled reactor was exploring ways of using the high temperatures produced. This work was followed up in the US by the The Gas research Institute (now the Gas Technology Institute). They looked at:

• 200 thermochemical cycles were considered
• 125 were considered feasible
• 80 were tested in the lab.

The best were:

• Hybrid sulfur,
• Sulfur-iodine,
• hybrid copper sulphate 

See end of this page for more details.

Splitting water - Photobiological

See page on BioHydrogen

Hydrogen from hydrocarbons

Hydrogen can be produced from hydrocarbon fuels through four basic technologies:

1. steam reforming (SR),
2. partial oxidation (POX),
3. autothermal reforming (ATR).  Source
4. Biological processes - see Biohydrogen page

Steam Reforming

Reforming is a process by which the molecular structure of a hydrocarbon is rearranged to alter its properties.

Steam methane reforming

Steam reacts with methane to yield carbon monoxide and hydrogen.   This is the most common method of producing hydrogen.

CH₄ + H₂O ⇌ CO + 3 H₂

It  is carried out at 700 – 1100 °C, with nickel based catalyst. This reaction consumes heat (endothermic, ΔHr= 206 kJ/mol)

Next, steam is reacted with CO which removes the oxygen to leave hydrogen. 

CO + H₂O ⇌ CO2 + H₂

This is called a water gas shift reaction. It gives off heat, (exothermic,  ΔHr= -41 kJ/mol).

Renewable Liquid Reforming: Renewable liquid fuels, such as methanol or ethanol, are reacted with high-temperature steam to produce hydrogen near the point of end use.

​Reforming of Methane - Solar gas.

CSIRO has been developing a method of using concentrating solar thermal heat to convert methane to H₂ and CO₂Solar gas

Plasma reforming

Plasmatrons can generate temperatures >2000°C. They supply the heat for the steam reforming reaction. They are compact, and can be shut Down quickly.

Hydrogen is generated mainly from methane and coal involving three major steps requiring separate reactors, all operating at temperatures in excess of 500°C:

1. Methane reforming or coal gasification to produce syngas (a mixture of hydrogen and carbon monoxide) at temperatures close to 800°C;
2. water gas shift reaction to convert carbon monoxide to hydrogen and carbon dioxide at around 500°C; and
3. H2/CO2 separation and gas cleaning.

Partial oxidation - Hydrocarbons, coal or biomass

Gasification of coal

Coal is partially burned to produce heat and CO.

This produces synthesis gas (syngas), a mixture of CO and H₂. It is how coal is gasified for use in gas turbines. For hydrogen production autothermal reforming is needed.

Gasification of biomass

Biomass is a bit more difficult. The feedstock is moist, and tars are produced. Small plant normally cannot afford the oxygen production equipment. If they use air, then the gas is diluted with nitrogen. The main costs are the transport of biomass, and the removal of the tars.

CH₄ + O₂  ⇌  CO + 2H₂

CH₄ +H₂O ⇌ CO + 3H₂

Autothermal reforming

The steam reforming process is endothermic, so needs a source of heat.  If the heat is not available, then the hydrocarbon can be partially burnt to supply the heat. Then steam is added for steam reforming to convert CO to H₂ and CO₂.

Pyrolysis of hydrocarbons

The process takes place in the absence of oxygen and air, and therefore the formation of dioxins can be almost ruled out. The products are hydrogen and carbon.  No CO₂ is produced.

Splitting of methane - Pyrolysis

Thermal decomposition of methane in a high-temperature bubble column reactor. The column is filled with liquid metal (Pb, Sn, etc) that is at 600-1000°C. Methane rises up to the surface and is decomposed as it rises. The process is experimental

Pyrolysis to hydrogen and carbon fibres

A pyrolysis process, developed by Eden with the University of Queensland produces hydrogen and carbon nanofibres or carbon nanotubes. The fibres ae being used to strengthen concrete. So far it in creaseing strength about 25%.

If successful on a commercial scale, the process could have important implications. Eden Energy

Methane is bubbled up a liquid-metal bubble column reactor filled with molten tin at 750-900^oC. The ascending methane bubbles are decomposed into hydrogen and carbon, Ref 

Hydrogen may be obtained from methane by pyrolysis in the temperature range 1000°−1200°C. 

 At present, the most widely used and cheapest method for hydrogen production is the steam reforming of methane (natural gas). This method includes about half of the world hydrogen production, and hydrogen price is about US$7/GJ. A comparable price for hydrogen is provided by partial oxidation of hydrocarbons. However, greenhouse gases generated by thermochemical processes must be captured and stored, and thus, an increase in the hydrogen price by 25–30% must be considered.

The further used thermochemical processes include gasification and pyrolysis of biomass. The price of hydrogen thus obtained is about three times greater than the price of hydrogen obtained by the steam reforming process. Therefore, these processes are generally not considered as cost competitive of steam reforming. The price of hydrogen from gasification of biomass ranges from US$10–14 /GJ and that from pyrolysis US$8.9–15.5 /GJ. It depends on the equipment, availability, and cost of feedstock.

Electrolysis of water is one of the simplest technologies for producing hydrogen without byproducts. Electrolytic processes can be classified as highly effective. On the other hand, the input electricity cost is relatively high and plays a key role in the price of hydrogen obtained. Source

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Where Will the Hydrogen Come From?

Source
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