11.4.5.2: Methods of Green Hydrogen Production

Electrolytic hydrogen. The only truly zero emission method of obtain ing hydrogen is the electrolysis of water – i.e., the decomposition of H₂O molecules into molecules of the constituent elements, H₂ and O₂. This occurs when electric current flows through water . Pure water, however, is a very bad conductor of electricity. To make the current flow, dissolve some acid, alkali or salt in the water to form an electrolyte. The molecules of such substances undergo dissociation in water that is, they split into positive and negative ions (“cations” and “anions”, respectively):

\[\mathrm{H}_{2} \mathrm{SO}_{4} \leftrightarrow 2 \mathrm{H}^{+}+\mathrm{SO}_{4}^{2-} \notag\]
\[\mathrm{NaOH} \leftrightarrow 2 \mathrm{Na}^{+}+\mathrm{OH}^{-} \notag\]
\[\mathrm{Na}_{2} \mathrm{SO}_{4} \leftrightarrow 2 \mathrm{Na}^{+}+\mathrm{SO}_{4}^{2-} \notag\]

When current runs through the electrolyte between two immersed electrodes, the positive cations (H⁺, Na⁺) flow towards the “cathode”, i.e., the negative electrode, and the negative anions (SO²⁻, OH⁻) flow towards the “anode”, i.e., the positive electrode. At the electrodes additional chemical reaction may occur (more details are given in the YouTube video linked above), resulting in liberating hydrogen gas H₂ at the cathode, and liberating oxygen gas O₂ at the anode.

The water electrolysis device is called an electrolyzer. Large plants with units of a total capacity of the order of 10 MW are already being built, and facilities with a capacity as large as 100 MW are expected to emerge in the near future in countries actively implementing the “hydrogen economy”.

Figure \(\PageIndex{1}\): Electrolysis of water – i.e., decomposing H₂O molecules to H₂ and O₂ by sending electric current through a water solution of an acid (e.g., H₂SO₄) or a base (e.g., NaOH) – from Wikimedia Commons.

At an industrial scale, three types of electrolyzers are currently used. We will give only their names and their performance, but technical details concerning the design can be found on the web pages linked. So these three types are: alkaline electrolyzers, Proton Exchange Membrane (PEM) electrolyzers, and cells for high temperature electrolysis (HTE) usually referred to as solid oxide electrolytic cells (SOEC).

It is instructive to examine the energy r elations in water electrolysis. They are presented, e.g., in this Hyperphysics website. As can be seen in this page, an electric energy input of 237.13 KJ is needed brake down one mole of H₂O into hydrogen and oxygen. In the same illustration one can also see a value of ∆H = 285.83 kJ. What’s the meaning of this symbol? It’s simply the heat of combustion of hydrogen, i.e. the thermal energy released when one mole of hydrogen is burned in the process of combining it back with oxygen. So, does it mean that from the latter reaction one gets 285.83 kJ 237.13 kJ MJ = 48.7 kJ of thermal energy more than the electric energy that was put in? At first glance, this may seem like a violation of energy conservation! But no, everything is fine: as can be seen again on the Hyperphysics Website, it’s the “environment” which provides this missing energy. As is indicated at the Web page, the process considered by Hyperphysics is carried out at a temperature of 298 K – meaning that there is a “thermal energy component” present in the water subjected to electrolysis. In the course of the process, part of this thermal energy contained in water is taken away from it to augments the electrolysis – which would result in lowering the water temperature, if not for the new heat still flowing in from outside.

In view of the above, we may conclude that the “practical efficiency” (also called the “voltage cell efficiency”) of water electrolysis – it is, the ratio of the energy contained in the released hydrogen to the electric energy put in is (285.83 kJ/ 237.13 kJ) 100% = 120.5%. Well, it would indeed be so at ideal conditions. In real situations, there are many additional factors that lower the efficiency. It depends on the type of the electrolyzer, and it is also correlated with the price one has to pay for it:

• Alkaline electolyzers are the least expensive ones. They use water solution of NaOH or KOH as the electrolyte, work at temperature 60-80^◦C, at pressure up to 30 bar; and their efficiency may may be up to 70%. The design of the electrolyzer most closely resembles the diagram in Figure 11.16, the only major difference is a membrane between the anode and cathode, which is transparent for ions but prevents gas mixing.
• PEM electrolyzers work at similar temperatures and pressures as the alkaline ones, but the arrangement of the electrodes is quite different than in a “classical” electrolysis experiment. It uses pure water that is suplied only to the anode, where it is split by a catalyzer to O₂ molecules and H⁺ ions. The latter, which are “bare” protons, migrate through the PEM membrane to the cathode, where they recombine with incoming electrons and form H₂ molecules (the functioning of a PEM electrolyzer is clearly explained by an animated diagram in this Wikipedia article). The voltage cell efficiency of PEM electrolyzers reportedly reaches 80% and this is not expected to be the upper limit because intensive R&D efforts are still conducted on this technology. However, the electrolyzers are more expensive than the alkaline ones.
• The HTE electrolyzers, also known as “Solid Oxide Electrolytic Cells”, are even more expensive than the PEM onesa. They work at temper-atures as high as 500-850^◦. They use not liquid water but steam which is fed fed to cathode and there it is split to H₂ molecules and O⁻ ions. The latter migrate across the solid oxide layer to the anode, where give away their electrons and form O₂ molecules. Due to the high temperature, the amount ∆H of thermal energy augmenting the reaction is much higher than in the other two methods, so that the “cell voltage efficiency” in an HTE electolyzer may exceed 100%. However, the “environment” is not able to supply the thermal energy needed, it must be provided by an additional heat source. Therefore, good conditions for the deployment of HTE technology exist where therer is no need to pay extra for heat for example, close to installations that produce a lot of waste heat, or in places where there is access to geothermal heat.

In industrial practice, the amount of hydrogen is not measured in moles, but in cubic meters which the gas occupies under standard conditions. One such cubic meter contains 44.6 moles of hydrogen. Therefore, to produce it under ideal conditions, 237.13 kJ / mol 44.6 mol = 10.59 MJ = 2.94 kWh of electric energy is needed. With the efficiency offered by the technologies considered, electricity consumption will be then from 3 to 5 kWh/m³. Another measure is 1 kg of hydrogen, which occupies 11.2 cubic meters – so that the electricity consumption is 34 56 kWh/kg depending on the method used. The average price per 1 kWh paid by industrial consumers in the US is about 7 cents/kWh. Hence, the cost of 1 kg of electrolytic hydrogen produced at industrial scale is between $2.40 and $3.90. And this is only the price for electricity; the cost of operating the equipment and amortization of the costs invested in its purchase will certainly add something to this price. This does not compare favorably with the price of hydrogen obtained by steam reforming of natural gas, which – as can be found in various sources – may be as low as $1.25/kg.

To sum up as long as the price is a relevant factor, “green” electrolytic hydrogen loses in competition with that produced by steam reforming of natural gas. But it wouldn’t lose if electricity were for free ... And this is not just a hypothetical situation, but quite real. In countries that are actively implementing a zero-carbon economy and are increasingly replacing fossil fuel power plants with wind and solar farms, this is already happening. Thanks to the whims of intermittent weather! Every once in a while there is too much wind and sun. The renewable sources suddenly start generating too much power, the consumers are not able to use the surplus. What is being done then? Energy production gets curtailed, some windmills and PV panels are simply shut down. Which is an obvious waste, this surplus power can be sent, after all, to electrolysis plants. At electricity cost reduced to zero or nearly zero, the hydrogen obtained in such situations will certainly be cheaper than that obtained from methane. There will be more and more of this cheap electrolytic hydrogen as the potential of the renewable energy sector increases. And not only will it be obtained from excess power generated by strong winds or too much sunshine but, with time, dedicated wind turbine and solar farms will be built with the main purpose of supplying electricity to large water electrolysis plants. These are plans for the future in Australia, Germany, UK, Belgium, the Netherlands... – and in California. Interesting analyzes regarding the perspectives of the developing “hydrogen economy” in the context of prices and other economic aspects can be found, for example, in this British publication, or in this report compiled by Akin, Gump, Strauss, Hauer & Feld, a renowned Washington, D.C. “think tank”.

Carbon-neutral Hydrogen derived from methane. The world today is literally “flooded” with cheap methane. When the production of cheap electrolytic hydrogen develops to such an extent that it begins to threaten the dominance of methane, its producers certainly will not want to “give up the field” without a fight. If stringent restrictions against CO₂ emissions are implemented, they may fail. But there is still a way to stay on the market in this situation namely, the conversion of methane into hydrogen without releasing CO₂ into the atmosphere. Is it possible? Yes! A reaction known as methane pyrolysis can be used for this purpose. At temperatures as high as 800-1000^◦C in the presence of metallic catalysts, the following reaction takes place:

\[\mathrm{CH}_{4} \cdot-2 \mathrm{H}_{2} \cdot+\cdot \mathrm{C}_{\text {solid }} \notag\]

The catalyst can be copper alloy with bismuth, it can also be lead. At high temperatures, these metals are molten. They are placed in a high vertical column, Methane under high pressure is blown into the column from below. Bubbles of gas travel through the molten metal, decomposing into hydrogen and carbon in solid form along the way. Hydrogen is discharged from the upper end, and carbon in a sooth-like form accumulates above the metal from where it can be removed. For every four kilograms of methane processed, one kilogram of hydrogen and three kilograms of solid carbon are obtained. Blowing methane through molten metal is not the only possible method of carrying out the pyrolysis. A German chemical giant BASF is experimenting with a whole spectrum of other methods, as described in this presentation.

But let’s do the numbers. The heat of combustion of hydrogen is 141.6 MJ/kg and of methane it’s 55.6 MJ/kg. So from pyrolysis conversion on gets a fuel that will release only 141.6/(4x55.6) = 63.7% of the energy that would be released by burning the methane used as the input. Yet, say, if California imposed a total ban on methane fuel, natural gas suppliers might prefer to convert their stocks to zero-emission fuel rather than to lose the market entirely.

A question arises, what to do with the solid carbon byproduct? It’s not an acute problem, though, because solid carbon can be utilized in many ways. For instance, if mixed with top soil, it may act as sort of fertilizer, facilitating circulation of water and air.

Nearly carbon-neutral hydrogen from biomass. Biomass is a term that covers a whole variety of things mainly of plant origin: agricultural residues such as stems and leaves; residues from logging and the wood industry such as branches and sawdust; municipal waste, such as leaves removed in autumn; garbage left after metal, glass and plastic have been removed; and many other residues.

Figure \(\PageIndex{2}\): Different types of gasifiers used for processing biomass (from commons.wikimedia).

The main components of biomass are cellulose, hemicellulose and lignin.

In Chapter 10, we discussed one way to use biomass, namely, obtaining bioethanol, for which only one ingredient can be used – the cellulose. But we have not mentioned yet another technology, called biomass gasification, in which all its components are used. The biomass in it is subjected to high temperature (> 700^◦C) in the presence of steam and oxygen in a device called “gasifier”.

Cellulose is a polymer consisting of chained “links” of glucose molecules, C₆H₁₂O₆, and in a gasifier it yields hydrogen, carbon dioxide oxide and carbon monoxide – and the latter is then oxidized:

\[\mathrm{C}_{6} \mathrm{H}_{12} \mathrm{O}_{6} \cdot+\cdot 2 \mathrm{H}_{2} \mathrm{O} \cdot+\mathrm{O}_{2} \cdot \rightarrow 8 \mathrm{H}_{2} \cdot+\cdot 4 \mathrm{CO}_{2} \cdot+\cdot 2 \mathrm{CO}\notag \]
\[2 \mathrm{CO} \cdot+\cdot 2 \mathrm{H}_{2} \mathrm{O} \cdot \rightarrow 2 \mathrm{H}_{2} \cdot+\cdot 2 \mathrm{CO}_{2} \uparrow\notag \]

The two equations can be combined into a single overall one:

\[\mathrm{C}_{6} \mathrm{H}_{12} \mathrm{O}_{6} \cdot+\cdot 4 \mathrm{H}_{2} \mathrm{O} \cdot+\cdot \mathrm{O}_{2} \cdot \rightarrow 1 \mathrm{OH}_{2} \cdot+\cdot 6 \mathrm{CO}_{2}\notag\]

The building blocks for hemicellulose are five-carbon (pentose) sugars, their common chemical formula is C₅H₁₀O₅. The overall equation for the pentose sugars is:

\[\mathrm{C}_{5} \mathrm{H}_{10} \mathrm{O}_{5} \cdot+\cdot \mathrm{H}_{2} \mathrm{O} \cdot+\cdot \cdot 2 \mathrm{O}_{2} \cdot 6 \mathrm{H}_{2} \cdot+\cdot 5 \mathrm{CO}_{2}\notag \]

As can be readily calculated, the mass of hydrogen obtained from the gasification of cellulose and hemicellulose is about 11%, and 8%, respectively, of the mass of the “feedstock” put into the gasifier.

Lignin has a very complex atomic structure. The proportion of carbon, oxygen and hydrogen atoms in lignin from different plants may vary, so that it is not possible to formulate a single gasification equation. The process has to be studied experimentally, as described, e.g., in this research report. The relative mass of hydrogen obtained from lignin gasification is lower than than that obtained from the other two biomass components. But the relative percent of lignin in different kinds of biomass is usually lower than 20%, so that it is reasonable to assume that the mass of hydrogen obtained from gasification is about 10% of the mass of the “feedstock” used.

The 10% or so of hydrogen in the output gas fron the gasifier has to be separated from the other components – there are several methods of doing this, e.g., by using porous ceramics or a technique known as Pressure Swing Adsorption (PSA). The latter is widely used to separate hydrogen from CO₂ in gas obtained from steam reforming og natural gas. Note that the gases produced by biomass gasification and natural gas reforming contain similar fractions of hydrogen (˜10% and ˜15% by weigh, respectively), while the remainder is mainly CO₂.

Just a moment, just a moment! At the beginning of the story, it was clearly stated that it would be about “nearly carbon neutral” method – and now it turns out that about 90% of the exhaust from the gasifier is CO₂. It makes no sense!

Well, it does make. Note that the CO₂ produced by biomass gasification is not “a new CO₂” added to the atmosphere. In order to create biomass, plants absorbed CO₂ from the air – and now this CO₂ is only given back to it. It’s the same story as with all biofuels. The release of CO₂ into the atmosphere when burning biofuels should be regarded as a ”loan return”, and not as “littering”. CO₂ released to the atmosphere is not “evil in its own right” – it’s evil only if it’s produced by burning fossil fuels. If we do not collect biomass to produce hydrogen, but instead we leave it where it has grown then we will not release CO₂ to the atmosphere, but, eventually, it will get there anyway after a somewhat longer period, due to the decaying processes caused by fungi, molds, bacteria and a range of other microorganisms.