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Hydrogen - a singular atom

The author, Daniel Hartley
Daniel Hartley
Reading time: 6 to 7 minutes

Hydrogen produced from renewable sources has the potential to be a source of green energy, and an alternative energy carrier to electricity.

Symbol H; atomic number 1; atomic mass 1.0080uNonmetal, naturally occurring as a gas H2 at room temperature.Derived from greek hydro, "water", and genes, "forming".

Hydrogen atoms were fleetingly formed during the Big Bang but it wasn't until the recombination 370,000 years later that conditions became favourable for electrons to remain in orbit around atomic nuclei.

Hydrogen was the first element in the universe and it is the most abundant. From it all other elements are formed.

Covalent and hydrogen bonds

Hydrogen readily shares covalent bonds (video) with other elements to form compounds, including water H2O, ammonia NH3, hydrogen sulfide H2S, and organic compounds, such as hydrocarbons CnH2n+2 and carbohydrates (CH2O)x.

The bonds in these compounds are polar. In a molecule of water, for example, the shared electrons spend more time near the (more electronegative) oxygen atom, which therefore takes on a partial negative charge. The hydrogen end of the molecule, where the electron spends less time, takes on a partial positive charge. Molecules of water position themselves in response to these partial charges; the hydrogen of one molecule being attracted to the oxygen of another (unlike charges attract), to form a hydrogen bond.

Hydrogen bonds explain cohesion (why water molecules 'bead'), and the relatively high boiling point of water - energy is needed to break apart the hydrogen bonds.

The polarity of water makes it a good solvent. When salt Na+Cl- is added to water, it dissolves; positive sodium ions - charged atoms are called ions - are attracted to the slightly negative hydrogen end of the water molecule, and positive chlorine ions are attracted to the oxygen end, thereby breaking apart, or dissolving, the bonds between the sodium and chlorine atoms.

Why are the sodium and chlorine atoms in salt charged? The sodium and chlorine atoms in salt are bound together by an ionic bond (video). Ionic bonds occur where an electron, or electrons, are transferred from one atom to another, in this case from a sodium atom to a chlorine atom. Since the two atoms now have opposite charges they attract. Na becomes Na+ and Cl becomes Cl-.

Hydrogen bonds can also form between molecules of different species, for example in DNA, where they hold together the two (polynucleotide) chains. The hydrogen bonds are strong enough to keep the double helix together, but not so strong as to prevent the chains separating when they need to be replicated.

Production

95% of hydrogen is produced by natural gas reforming in two ways.

The methane in natural gas reacts with steam under high pressure in the presence of a catalyst to produce hydrogen, carbon monoxide and carbon dioxide.

The carbon monoxide and steam are then reacted using a catalyst to produce carbon dioxide and more hydrogen in the "water-gas shift reaction".

In a final step, the "pressure-swing adsorption" reaction, carbon dioxide and other impurities are removed from the gas stream, to leave pure hydrogen.

Steam-methane reforming reactionCH4 + H2O (+heat) → CO + 3H2Water-gas shift reactionCO + H2OCO2 + H2 (+ small amount of heat)

The methane and other hydrocarbons in natural gas react with a limited amount of oxygen to partially oxidise the hydrocarbons. The carbon monoxide produced is once again reacted with water in the "water-gas shift reaction".

Partial oxidation of methane reactionCH4 + ½O2CO + 2H2 (+ heat)Water-gas shift reactionCO + H2OCO2 + H2 (+ small amount of heat)

There are several other methods of hydrogen production including photobiological, photoelectrochemical, solar thermochemical, biological processes, and electrolysis.

Electrolysis

Electrolysis is a process whereby electricity is used to split water into hydrogen and oxygen. The process varies depending on the electrolyte selected. Oxygen gas forms at the anode, hydrogen gas at the cathode.

Anode Reaction 2H2OO2 + 4H+ + 4e-Cathode Reaction 4H+ + 4e- → 2H2
An electrolyte is a "substance that produces an electrically conducting solution when dissolved in a polar solvent, such as water." Wikipedia

Electrolysis can produce hydrogen with no greenhouse gas emissions, if the original source of the energy used is renewable. The process is currently 70% to 80% efficient; for every 40kWh of energy produced 50kWh to 55kWh of electricity is required. 95% efficiency has been claimed but not proven in production.

Transporting hydrogen

Converting pipelines to carry a natural gas hydrogen mix is feasible. The cost of adapting existing lines for compressed hydrogen varies depending on whether you ask industry or their critics. At the heart of the debate is the feasibility of producing green hydrogen in large volumes soon. If the hydrogen passing through pipelines does not come from renewable sources, it will lock us in to further use of, and dependency on, fossil gas at the expense of investment in renewables.

Hydrogen gas has a low energy density by volume at ambient temperatures even compared to natural gas. It is therefore either compressed or liquified (molecular hydrogen has high energy density by mass) for transportation by road or rail.

Hydrogen fuel cells

Fuel cells can be used at a wide range of scales from a power station to a laptop. They work like batteries but do not run down, producing electricity as long as there is fuel. They are also efficient, converting chemical energy to electrical energy at around 60% efficiency; this compares well to an average of 16% efficiency for gas combustion engines.

How fuel cells work
A fuel, such as hydrogen, is fed to the anode, and air is fed to the cathode. In a hydrogen fuel cell, a catalyst at the anode separates hydrogen molecules into protons and electrons, which take different paths to the cathode. The electrons go through an external circuit, creating a flow of electricity.

Current production and use

A fraction of hydrogen is currently produced from low carbon energy; the latest figure for the EU is 0.1%. Global production of hydrogen from electrolysis is less than 0.1%.

Hydrogen use today is dominated by industry, namely: oil refining, ammonia production, methanol production and steel production.

Potential

Hydrogen as a fuel is attractive because at the point of use it emits no greenhouse gases, and it can be used for long term storage at potentially lower cost than electricity. There are, however, drawbacks.

Drawbacks

The majority of current hydrogen production involves greenhouse gas emissions. Production is often described using a colour spectrum (image). 'Grey' hydrogen is produced by burning coal without capturing CO2 emissions. The gas industry in particular is promoting 'blue' hydrogen whereby greenhouse gases emitted during the combustion of gas are sequestered. Carbon capture and storage (CCS) is not, however, established at the scale required despite a decade or more of trials, heavy investment, and government subsidies. In 2020 40 million tonnes of CO2 was captured from 26 operating facilities worldwide.

'Green' hydrogen, produced only using renewable energy, and transported through 'hydrogen ready' pipelines is a seductive idea but if the path to that goal is overly long or impossible, we will be saddled with the cost of maintaining CCS infrastructure, and safeguarding sequestered CO2 indefinitely, whilst still emitting greenhouse gases.

Greasing the wheels of failure

Talk of 'blue' hydrogen and carbon capture and storage means burning gas with no guarantee of a transition towards 'green' hydrogen.

Producing hydrogen from natural gas with carbon capture and storage, so-called blue hydrogen, could also be the key to keeping Norwegian gas valuable in a low carbon future [emphasis added].

This scenario is understandably attractive to fossil fuels companies who maintain their grip on the market, secure profits on existing investments, and receive subsidies in the name of reducing emissions. They are lobbying hard in the EU to protect their interests.

A mixed future

Hydrogen may have a successful future if it is unburdened by hype, it just might take time for sensible methods of production and suitable uses to be found. Produced close to sources of excess renewable energy it may be a good fit for larger forms of transportation (fuel cells) including shipping, for energy storage, and domestic and commercial heating (hydrogen boilers).

One positive example of combining wind power and hydrogen storage is the Surf 'n' Turf community project on the Orkney Islands north of Scotland where the abundance of energy makes conversion to hydrogen and back viable.

How much do your remember?

References

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