32.09: Biohydrogen - An Introduction

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H₂ as a fuel

Hydrogen gas would be ideal if it could be produced at scale, easily transported and stored, or produced at local sites on demand.   The reaction for the "burning" of hydrogen shows that the only greenhouse gas emitted is H₂O.

\begin{equation}
2 \mathrm{H}_2(\mathrm{~g})+\mathrm{O}_2(\mathrm{~g}) \rightarrow 2 \mathrm{H}_2 \mathrm{O}(\mathrm{g})
\end{equation}

H₂O comes and goes in our atmosphere in short timescales and does not continually build up, as does CO₂ from burning fossil fuels. The standard heat of combustion (in kJ/g or kcal/kg) for H₂ is far higher than any other fuel, as shown in Table  \(\PageIndex{1}\) below, making it an ideal fuel.

Name	Formula	State	-ΔHc° kJ/mol	-ΔHc° kJ/g or MJ/kg	-ΔHc°  kcal/kg
Ammonia	NH3	gas	383	22.48	5369
Butane	C4H10	gas	2878	49.50	11823
Carbon (graphite)	C	cry	394	32.81	7836
Ethanol	C2H6O	liq	1367	29.67	7086
Hydrogen	H2	gas	286	141.58	33817
Methane	CH4	gas	891	55.51	13259
methyl stearate (biodiesel)	(CH3(CH2)16(CO)CH3	liq	1764	40	9560
Naphthalene	C10H8	cry	5157	40.23	9609
Octane	C8H18	liq	5470	47.87	11434
Propane	C3H8	gas	2220	50.33	12021
wood (red oak)	-	solid	-	14.8	3540
coal (lignite)	-	solid	-	15	3590
coal (anthracite)	-	solid		27	4060

Table  \(\PageIndex{1}\):  Energy values for various fuels. Data source:  https://www.engineeringtoolbox.com/s...nt-d_1987.html

We won't discuss large-scale H₂ storage or transport, two fundamental engineering problems. Instead, we will focus on the production of "biohydrogen." Of course, the prefix "bio" can mean many things, including the production of H₂ in syngas using cellulose as a feedstock, the electrolysis of water powered by solar/wind energy, and its production by hydrogenases, enzymes found in some microbes.  

The fuel industry uses different colors as descriptors of hydrogen based on how it is produced. They are shown in Table  \(\PageIndex{2}\).

Color	Method of production
Green	electrolysis of H2O using solar/wind to generate electricity (expensive at present)
Blue	steam reforming of natural gas (CH4) with the other product, CO2 captured and stored (CCS)
Grey	steam reforming of natural gas (CH4) without CO2 capture and storage
Black (coal/oil)	gasification to form syngas
Pink (purple/red)	electrolysis powered by nuclear energy, which does not emit CO2; heat emitted produces steam for blue/gray H2 production
Turquoise	methane pyrolysis (heat in the absence of O2) to form H2 and C
Yellow	electrolysis using solar power without conversion to electricity as the power source.
White	underground H2 released through fracking

Table  \(\PageIndex{2}\):  Different "colors" of hydrogen based on production methods

Of course, H₂ in syngas can be produced from biomass, as described in Chapter 32.8, but it is unclear if a hydrogen color has been assigned to it.

At present, the important feedstocks for H₂ production around the world are natural gas (48%), oil (30%), coal (18%), and electrolysis (4%) - mostly all fossil fuels.

Methods of Production

H₂ production is also classified based on the chemical processes used to produce it. These processes include

1. Biological (use of live bacteria and algae cells)
2. Thermochemical:  (gas and liquid fuel reforming, coal and biomass gasification),
3. Electrochemical (electrolytic):  (photothermal, photoelectrolytic, and photobiological)

We will organize this chapter section using these three processes. We will start with Biological (1), followed by Electrochemical/Electrolytic (3), and end with Thermochemical (2). They are summarized in Figure \(\PageIndex{1}\). 

Figure \(\PageIndex{1}\):  The main pathways for H₂ production based on biomass. M.G. Eloffy et al., Chemical Engineering Journal Advances, 12 (2022). https://doi.org/10.1016/j.ceja.2022.100410.  Creative Commons license

Biomass can be used as the feedstock for all of these methods, so the resulting product can be called biohydrogen. Of course, nonbiological sources of feedstocks are the predominant ones used in thermochemical and electrochemical methods as well, as we discussed in the previous chapter section.  

2H⁺ ↔  H₂: An Overview

We will mostly discuss the production of H₂ as a society energy source. For industry use, it can be used in fuel cells to power spacecraft and cars, as shown in the reaction below.

\begin{equation}
\begin{aligned}
& \mathrm{O}_2+4 \mathrm{H}^{+}+4 \mathrm{e}^{-} \longrightarrow 2 \mathrm{H}_2 \mathrm{O} \\
& \mathrm{H}_2 \longrightarrow 4 \mathrm{H}^{+}+4 \mathrm{e}^{-}
\end{aligned}
\end{equation}

In the next chapter section, we will discuss in great detail the hydrogenases that produce and use H₂ in microbes so that this chapter will treat them very generally. However, we need to review the topic.

Use of H₂ as a source of electrons for reduction reactions.  

Each hydrogen in H₂ has an oxidation number of 0. Each hydrogen can be oxidized to H⁺ (oxidation number +1) with the 2 electrons passed on to a substrate/cofactor or a sequential series of substrates with higher and higher standard reduction potentials (better oxidizing agents), leading to the formation of reduced products.  

H₂ + (substrate)_OX  →  2H⁺  +  (product)_RED

This general reaction is analogous to the mitochondrial electron transport chain, in which electrons are passed from a source (NADH) to oxidized forms of acceptors. The general reaction below shows each redox pair in the electron transport chain.

NADH/NAD⁺ →  FAD/FADH₂ →  UQ/UQH2  →  Cyto C_OX/Cyto C_RED  →   O₂/H₂O 

Some organisms have evolved to produce energy by the oxidation of H₂. This reaction is analogous to those used by photosynthetic organisms to obtain energy through the oxidation of water. In photosystem II,  oxygen in H₂O (oxidation number -2) gets oxidized by the oxygen-evolving complex to produce O₂ (oxidation number 0). Some redox pairs, starting with H₂O/O₂, are shown below for photosystem II.  

H₂O/O₂ →  P680/P680* →  (Plastoquione)_OX/(Plastoquione)_RED 

The first reaction is endergonic and requires an energy source photons.

Use of H⁺ as a sink for electrons for oxidation reactions that produce H₂.  

H⁺ has an oxidation number of +1. Hence it can be reduced to H₂ (oxidation number of 0) as it gains electrons from substrates/cofactors, which get oxidized. This general reaction is shown below.  

2H⁺ + (substrate/cofactor)RED  →  H₂  + (substrate/cofactor)OX 

Many microorganisms can produce H₂ through variants of photosynthesis or through fermentation, both of which provide the two electrons needed. E. Coi has four hydrogenases (Hyd 1, 2, 3, and 4). It forms H₂ through two reactions catalyzed by:

• formate (HCO₂^-) dehydrogenase (FDH):  2HCO₂^- ⇌ 2CO₂ + 2H⁺ + 2e^- 
• hydrogenase (H₂ase):  2H⁺ + 2e^- → H₂ 

The C in formate has an oxidation number of +2 and is oxidized to CO_2, in which the C has an oxidation number of +4. 

Nothing is simple: H₂ is an indirect greenhouse gas.

H₂ itself is not a greenhouse gas as it doesn't have any bond vibrations that produce transient dipoles and hence does not absorb in the infrared region of the spectrum. Yet by affecting atmospheric levels of methane, a very potent greenhouse gas, as well as levels of ozone, it can lead to warming. It's not emission from the combustion of H₂ but rather the leakage into the atmosphere of transported and stored H₂ gas that is problematic.

Most of the H₂ that finds its way into the atmosphere diffuses into the soil and is taken up by bacteria. The rest reacts with hydroxy radicals (^.OH) in the atmosphere, as shown in the reaction below. 

H₂  +  ^.OH →  H₂O  +  H^. (atomic hydrogen)

The reaction of ^.OH with H₂ decreases the hydroxy radical's availability to react with the very potent greenhouse gas methane, CH₄. That reaction is shown below.

CH₄ + ^.OH →  ^.CH₃ + H₂O

The methyl radical ^.CH₃ reacts rapidly with oxygen to form the methylperoxy radical (CH₃O₂^.). This eventually forms formaldehyde, a water-soluble molecule that is removed from the atmosphere on precipitation.   Hence the reaction of H₂ with ^.OH increases the half-live of CH₄ in the atmosphere. 

^.OH is a key molecule in the troposphere and is considered a methane "sink"  that leads to the drawdown of methane.   We discussed the extreme reactivity of ^.OH in Chapter section's 12.3 and 12.4. It's so reactive that its half-life is in the order of seconds. It is also at very low concentrations of less than 1 part per trillion.

^.OH is produced from ozone, O₃, by the following reactions:

O₃  + hν (UV) →  O₂ + ^.O

 ^.O + H₂O  →  2 ^.OH

The first reaction is a photolysis, and experiments during a solar eclipse have shown the production of ^.OH in the atmosphere shuts down!  

Dr. Paul Crutzen, Nobel Prize winner in Chemistry, described ^.OH as the "detergent of the atmosphere" since it can react with and oxidize many deleterious trace gases in the troposphere, making them more water-soluble, leading to their elimination from the atmosphere. A main reaction of ^.OH is carbon monoxide (CO). It also reacts with volatile organic compounds (VOCs) and NOx (NO + NO₂), which are precursors of tropospheric ozone, a health hazard. Even though dioxygen, which comprises 20% of the atmosphere, is also an excellent oxidizing agent, it is kinetically slow to react. 

Very few gases are not oxidized by ^.OH. One set includes the refrigerant gases chlorofluorocarbons, which without oxidation by ^.OH enter the stratosphere, where they react with stratospheric ozone and reduce its protective effect against dangerous UV light. It does react with hydrochlorofluorocarbons (HCFCs).

Figure \(\PageIndex{2}\) below summarizes the adverse climatic effects of the oxidation of H₂ in the atmosphere.

Figure \(\PageIndex{2}\):  Effects of hydrogen oxidation on atmospheric greenhouse gas concentrations and warming. I. Ocko and Steven P. HamburgAtmos. Chem. Phys., 22, 9349–9368, 2022. https://doi.org/10.5194/acp-22-9349-2022.  Creative Commons Attribution 4.0 License.

Note in the central panel that H^. (atomic hydrogen) can start a free radical change reaction to produce tropospheric ozone, O_3, a pollutant that is not only a greenhouse gas but which also causes serious health consequences.

The message is this:  Care has to be taken to minimize methane and H₂ leakage during their production and use as fuels. 

Biohydrogen from Microalgae 

We will focus most of our attention on the Biological (1) and Electrolytic (3) processes for producing biohydrogen from microalgae. The Biological processes (1) require hydrogenases for H₂ production within cells. The Electrolytic (3) processes use microalgae as a feedstock to provide substrates that other microbes can ferment. These can be combined to increase production. Figure \(\PageIndex{3}\) below summarizes the Biological (1) and Electrolytic (3)  metabolic processes that can be used for microalgae H₂ production.

Figure \(\PageIndex{3}\): Metabolic pathways of biohydrogen production by micro-algal biomass. modified from Ahmed SF et al. Front. Energy Res. 9:753878. doi: 10.3389/fenrg.2021.753878.   Creative Commons Attribution License (CC BY).

These are mainly classified into three categories: i) the photobiological process through which biohydrogen is produced via direct and indirect photolysis in the microalgae, ii) fermentation, and iii) the electrochemical process that comprises photoelectrochemical and electrolytic.

 

BIOLOGICAL (1) - Biophotolysis (photosynthesis)

This consists of two processes, Direct and Indirect Photolysis (photosynthesis). Both use light to drive the ultimate reduction of 2H⁺ to H₂ using hydrogenase or nitrogenase. We will explore the details in the next chapter section. The biophotolysis process is divided into indirect (using electrons from substrates) and direct (using electrons from water). These processes are simplified in Figure \(\PageIndex{4}\).

Figure \(\PageIndex{4}\): Schematic diagram for biological (biophotolysis) process.   M.G. Eloffy et al.

Direct Biophotolysis (photosynthesis)

In direct biophotolysis (photosynthesis), water molecules are oxidized in Photosystem II, which contains the Oxygen Evolving Complex (OEC). This endergonic process is driven by light. The electrons lost from water are passed through Cytochrome b₆f and Photosystem I to ferredoxin then NADP⁺, which gets reduced to NADPH (as discussed in Chapter 20). These reactions are illustrated in Figure \(\PageIndex{5}\). 

Figure \(\PageIndex{5}\): Light reaction of photosynthesis and associated standard reduction potentials

In direct photolysis, electrons are passed directly from reduced ferredoxin to 2H⁺ in a reaction catalyzed by a hydrogenase, as shown in Figure \(\PageIndex{6}\) below.

Figure \(\PageIndex{6}\): Metabolic hydrogen production pathways used by Chlamydomonas reinhartii.FDX: ferredoxin; H₂ase: hydrogenase; NPQR: NADPH−plastoquinone oxidoreductase; PFR: pyruvate:ferredoxin oxidoreductase; PSI: photosystem I; PSII: photosystem II. Touloupakis, E.; Faraloni, C.; Silva Benavides, A.M.; Torzillo, G. Recent Achievements in Microalgal Photobiological Hydrogen Production. Energies 2021, 14, 7170. https://doi.org/10.3390/en14217170.  Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).

 The overall reaction is simplified in the equation below.

\begin{equation}
2 \mathrm{H}_2 \mathrm{O}+\text { Light } \rightarrow 2 \mathrm{H}_2+\mathrm{O}_2
\end{equation}

One problem with direct photolysis is that O2 can damage hydrogenases. Again we will discuss the biochemistry of hydrogenases in great detail in the next chapter section.

Indirect Biophotolysis

This process bypasses the damaging effects of O₂ on hydrogenase by being carried out in the absence of O₂ using fermentation to provide electrons for the hydrogenase reduction of 2H⁺ to H₂. Photosynthesis is required to make the carbohydrates necessary for fermentation. Glucose can then be oxidized anaerobically (in the dark to avoid O₂ formation from photosynthesis) to form pyruvate through the glycolytic pathway. Pyruvate can then be oxidatively decarboxylated through the pyruvate:ferredoxin oxioreductase (PFR) as ferredoxin gets reduced. It then passes its electrons on through hydrogenase to produce H₂. The pathway is illustrated in the top/right parts of the above figure and the reaction diagram in Figure \(\PageIndex{7}\) below.

Figure \(\PageIndex{7}\): Model of fermentative pathways involved in dark anaerobic H₂ production in C. reinhardtii. Proteins are shown as ovals. Photosynthetic ferredoxin (PETF).   Jens Noth et al., Journal of Biological Chemistry, 288 (2013). https://doi.org/10.1074/jbc.M112.429985.  Creative Commons license.

Glucose and some amino acids can be converted into pyruvate, a substrate for PFR1 in the single-cell algae C. reinhardtii. PFR1 converts pyruvate to acetyl-CoA and CO₂ with the electrons used to reduce ferredoxin. The reduced FDX2 passes electrons through hydrogenase (HYDA1) to form H₂. 

Another enzyme used to continue fermentation, pyruvate:formate lyase (PFL1), converts pyruvate to formate and acetyl-CoA, which can be metabolized further to acetate and ethanol. A shift to pyruvate oxidation to PFR1 occurs if PFL1 is mutated or long term anoxic conditions.

The key enzyme, pyruvate:ferredoxin oxioreductase (PFR), uses thiamine pyrophosphate (TPP) as a cofactor for the oxidative decarboxylation of the α-keto acid pyruvate, as expected. Figure \(\PageIndex{8}\) shows an interactive iCn3D model of the pyruvate ferredoxin oxidoreductase (PFOR) from Desulfocurvibacter africanus in anaerobic conditions (7PLM).

Figure \(\PageIndex{8}\): Pyruvate ferredoxin oxidoreductase (PFOR) from Desulfocurvibacter africanus in anaerobic conditions (7PLM). (Copyright; author via source). Click the image for a popup or use this external link: https://structure.ncbi.nlm.nih.gov/i...XRFMoVhaoRbW86

PFOR, abbreviated here, is a 267 kDa homodimer containing three [Fe₄S₄] clusters (spacefill) per monomer. Only one monomer is shown, and TPP is shown in sticks.

Again indirect photolysis occurs in the absence of O₂. Light illumination leads to only transient H₂ synthesis. If sulfur is limited in the growth media of the algae, more sustained H₂ production occurs, as the lack of sulfur reduces PSII activity. Hence H₂ production can be maximized by depleting sulfur and minimizing O₂ even in the presence of light. In the absence of O₂, hydrogenase gene expression increases. Nutrient depletion also leads to the production of formate and acetyl-CoA through the enzyme pyruvate:formate lyase (PFL1). This is predominant in Chlamydomonas cells in the dark.

The green microalgae C. reinhardtii makes most of its H₂ (approximately 90%) using direct photolysis.   Commercially, the production of H₂ in indirect photolysis is carried out in a separate sealed bioreactor to avoid  O₂. Indirect photolysis is shown in the above figures.

The reactions to this process are as follows :

\begin{equation}
\begin{gathered}
12 \mathrm{H}_2 \mathrm{O}+6 \mathrm{CO}_2+\text { hν } \rightarrow \mathrm{C}_6 \mathrm{H}_{12} \mathrm{O}_6+6 \mathrm{O}_2 \\
\mathrm{C}_6 \mathrm{H}_{12} \mathrm{O}_6+12 \mathrm{H}_2 \mathrm{O}+\text { hν } \rightarrow 12 \mathrm{H}_2+6 \mathrm{CO}_2
\end{gathered}
\end{equation}

 

BIOLOGICAL (1) - Fermentation

We have just discussed fermentation processes within living microalgae cells. Now let's consider fermentation processes using nonliving biomass feedstocks supplied to microbes to produce H₂. This offers a significant way to make biohydrogen. A schematic diagram for Biological (1) fermentation is shown below in Figure \(\PageIndex{9}\):

Figure \(\PageIndex{9}\): Schematic diagram for biological (fermentation) process. M.G. Eloffy et al.

Fermentation involves the decomposition of organic biomass to produce CO₂ and H₂.   The fermentation process can be separated into photofermentation (light fermentation) and dark fermentation.

Photofermentation

Some photosynthetic bacteria and microalgae use Photofermentation to produce H₂ from organic acids like acetic, butyric, lactic, and succinic acids. Oxidation of the acids produces CO₂ as well as H⁺s and e^- for H₂ production. Electrons are transferred through photosystem I and eventually, believe it or not, nitrogenase. It is a fermentation process as the process is anoxic. 

Some photosynthetic bacteria, like the purple nonsulfur bacteria, a facultative anoxygenic phototroph, and some microalgae, can produce H₂ using a simplified system that has only one photosystem and uses the enzyme nitrogenase to produce H₂. The photosystem can not generate an oxidizing agent strong enough to oxidize H₂O, but under anaerobic conditions, they can oxidize organic acids and even H₂S to provide electrons from H₂ production. These reactions are shown below in Figure \(\PageIndex{10}\).

Figure \(\PageIndex{10}\): Photofermentative hydrogen production in PNSB. 

Deo, D., Ozgur, E., Eroglu, I., Gunduz, U., & Yucel, M. (2012). Photofermentative Hydrogen Production in Outdoor Conditions. Hydrogen Energy - Challenges and Perspectives. doi: 10.5772/50390. Creative Commons Attribution 3.0 License, 

We studied nitrogenase in Chapter x.xx. The net reaction for the fixation of nitrogen is shown below.

\begin{equation}
\mathrm{N}_2+8 \mathrm{H}^{+}+8 \mathrm{e}^{-}+16 \mathrm{ATP} \rightarrow 2 \mathrm{NH}_3+\mathrm{H}_2+16 \mathrm{ADP}+16 \mathrm{Pi}
\end{equation}

In this reaction, N₂ is reduced as the N atoms go from a 0 oxidation state to +3 in NH₃. The needed electrons are made from organic acids and fed into the system and eventually go to ferredoxin, which transfers them to protons. The ratio of N₂ to H₂ produced is 1:1, at the expense of 16ATPs per H₂ produced.

The ATP produced by the collapse of the produced proton gradient through FoF1ATPase powers the reaction.

In the absence of N_2, the net reaction becomes

\begin{equation}
2 \mathrm{H}^{+}+2 \mathrm{e}^{-}+4 \mathrm{ATP} \rightarrow \mathrm{H}_2+4 \mathrm{ADP}+4 \mathrm{Pi}
\end{equation}

The electrons are still fed into nitrogenase, but in the absence of the substrate N_2, they are used to reduce 2H⁺ to H₂. Note that only 4 ATPs are required per each H₂ produced, a significant energy gain.

ATP produced during photosynthesis would be used for anabolic biosynthesis contributing to biomass, so extra ATP is needed to support H₂ synthesis past that needed for growth. As anabolism is a reductive process (compared to oxidative catabolism), adequate sources of electrons for reduction are required. Multiple pathways need electrons, including CO₂ fixation, N₂ fixation (with associated H₂ production, and organic acids like polyhydroxbutyrate.  The bacteria use photosynthesis and the Calvin cycle under photoautotrophic conditions to fix CO₂. When external energy supplies from organic acids are present, the bacteria can become photoheterotropic. Under these conditions, the Calvin cycle is used to maintain redox balance.  

Dark Fermentation

We studied this indirectly above section in our discussion of hydrogenases in microalgae. Hydrogenases are induced in dark conditions, and this pathway involved heterotrophic fermentation (anaerobic) in some bacteria and microalgae. Many microbial species are used. Industrial wastewater enriched in organic material can be used as a feedstock. 

Feedstock materials are hydrolyzed and subjected to fermentation, during which H₂ can be produced. For example, pyruvate produced by glycolytic fermentation can be oxidatively decarboxylated to acetyl-CoA and CO₂ by pyruvate:ferredoxin oxidoreductase with electrons passed on to ferredoxin and even through hydrogenase to form H₂ (as we described above). Addition H₂-produced steps after fermentation include acetogenesis and methanogenesis.    These processes are illustrated in Figure \(\PageIndex{11}\) below.

Figure \(\PageIndex{11}\): The steps involved in anaerobic digestion [9]. Rosa, P. R. F., & Silva, E. L. (2017). Review of Continuous Fermentative Hydrogen-Producing Bioreactors from Complex Wastewater. Frontiers in Bioenergy and Biofuels. doi: 10.5772/65548. Creative Commons Attribution 3.0 License

Examples of acidogenic (formation of short carboxylic/fatty acid), acetogenic (formation of acetic acid), and methanogenic (formation of methane) reactions that produce (and a few that consume) H₂ are shown in Table \(\PageIndex{3}\) below.

Acidogenic reactions

C6H12O6 + 2H2O → 2CH3COOH + 2CO2 + 4H2

C6H12O6 + 2H2O→ CH3CH2CH2COOH + 2CO2 + 2H2

Acetogenic reactions

CO2+ 4H2→ CH3COOH+ 2 H2O

CH3CHOHCOOH + H2O → CH3COOH + CO2 + 2H2

CH3CH2OH + H2O →CH3COOH + 2H2

CH3CH2COOH + 2 H2O → CH3COOH + CO2 + 3 H2

CH3(CH2)2COOH + 2 H2O → 2 CH3COOH + 2H2

Methanogenic reactions

4 H2 + CO2→ CH4 + 2 H2O

CH3COOH → CH4 + CO2

2CH3(CH2)2COOH + 2H2O + CO2→ 4CH3COOH + CH4

Table \(\PageIndex{3}\):  Example of acidogenic, acetogenic, and methanogenic reactions in dark fermentation. Adapted from Rosa, P. R. F., & Silva, E. L., ibid.

We have described a few of the enzymes involved in acidogenic reactions above. Figure \(\PageIndex{12}\) shows a summary of the steps in acidogenesis.

Figure \(\PageIndex{12}\): An overview of the metabolic pathways of acidogenesis.  Dzulkarnain, E.L.N., Audu, J.O., Wan Dagang, W.R.Z. et al. Microbiomes of biohydrogen production from dark fermentation of industrial wastes: current trends, advanced tools and future outlook. Bioresour. Bioprocess. 9, 16 (2022). https://doi.org/10.1186/s40643-022-00504-8.  http://creativecommons.org/licenses/by/4.0/.

A more complex list and summary of dark fermentation reactions are shown in Figure \(\PageIndex{13}\) below.

Figure \(\PageIndex{13}\): Key enzymes and dominant microbial taxa involved during anaerobic digestion of organic matter.  Dzulkarnain, E.L.N.et al. Ibid.

 

ELECTROCHEMICAL/ELECTROLYTIC (3)

Two primary electrochemical/electrolytic methods for H₂ production are photoelectrochemical and electrolytic, as shown in Figure \(\PageIndex{14}\) below.

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Figure \(\PageIndex{14}\):  Schematic diagram for the electrochemical process. M.G. Eloffy et al.

 

Electrolysis

In a microbial electrolytic cell (MEC), microalgae/cyanobacteria use industrially- and metabolically-processed feedstocks to oxidize organic substrates (for example, acetic acid) to CO₂. The released electrons move from the anode (where the oxidation occurs) to the cathode for H⁺ reduction to H₂. An external voltage is applied to increase electron flow to the cathode to facilitate the process. This increases the production of H₂ over and above that of just fermentation by microbes in the electrolytic cell. Cyanobacteria and a mix of green microalgae are used, as well as bacteria, which can use dark fermentation (i.e., combining the processes described above).  

 

Photoelectrochemical

Microbial photoelectrochemical cells (PEC) use light-sensitive semiconductor electrodes for water electrolysis. A membrane separates the two electrodes so the protons can be reduced to form H₂.  for the separated by a membrane, 

 

2. THERMOCHEMICAL from Biomass

We have already explored thermochemical methods to produce syngas  (H₂ and CO) and further use in the Fishcer-Tropsch reaction to make small and large molecules for chemical feedstocks and fuels. We also discussed electrochemical methods to produce syngas and other small organic molecules like formate and ethanol from CO₂. Figure \(\PageIndex{15}\) shows a schematic diagram for thermochemical (gasification) processes to produce H₂.  

Figure \(\PageIndex{15}\):  Schematic diagram for thermochemical (aqueous phase reforming) process. M.G. Eloffy et al.

 

Key Points - Beta version from Chat.openai

1. Biohydrogen is a form of biofuel that is produced from biomass through a process known as biological hydrogen production.
2. Biological hydrogen production involves the use of microorganisms, such as bacteria and algae, to convert organic matter into hydrogen gas.
3. Biohydrogen has the potential to be a clean, renewable, and sustainable source of energy, as it produces only water when burned and does not produce greenhouse gases.
4. Photosynthetic microorganisms such as algae and cyanobacteria are the most promising organisms for biohydrogen production, they can convert water and CO2 into hydrogen and oxygen through the process of photosynthesis.
5. Fermentative microorganisms such as bacteria and fungi can also be used to produce biohydrogen, they can convert organic materials such as sugars and starches into hydrogen through the process of fermentation.
6. Biohydrogen can be produced through different processes, including dark fermentation, light-driven fermentation, and photo-biological hydrogen production.
7. Dark fermentation is the process of using microorganisms to ferment organic matter in the absence of light to produce hydrogen gas.
8. Light-driven fermentation is the process of using microorganisms to ferment organic matter in the presence of light to produce hydrogen gas.
9. Photo-biological hydrogen production is the process of using algae or photosynthetic bacteria to produce hydrogen gas through photosynthesis.
10. Biohydrogen production is still in the early stages of development and research is ongoing to improve the efficiency and cost-effectiveness of the process.
11. Biohydrogen production from algae is considered more sustainable and environmentally friendly than traditional hydrogen production methods, which are often based on fossil fuels.
12. Biohydrogen has the potential to significantly reduce carbon emissions and to help decarbonize various sectors, including transportation and energy.