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Introduction
The current energy demand is mostly satisfied through the use of resources such as natural gas, coal and oil. This form of energy production is responsible for a considerable amount of greenhouse gas (GHG) emissions, representing 66,667% of global emissions, as reported by the United States Environmental Protection Agency (EPA) 1. During 2021, the release of carbon dioxide (CO2) linked to power generation will increase by 6%, reaching a record 36.3 billion tonnes 2,3. This increase is largely due to population growth and the continued exploitation of fossil fuels, which has resulted in a 68% increase in CO2 and anthropogenic carbon concentrations in the atmosphere 4.
For these reasons, efforts have been made to find other ways to generate energy with zero carbon emissions, an energy resource that has had some relevance in recent decades has been hydrogen, its importance is given by the ability of the element to store energy in relation to its mass, which is estimated to be around the 143MJ/Kg 5-7, this energy density is the highest among known fuels. Another important factor is the versatility of this element to generate clean energy, either by burning it directly 8 or by using it as a fuel or producing energy using fuel cells to produce electricity 9, his element is considered environmentally friendly when generating energy, however, its carbon neutrality is based on the way in which the hydrogen is produced 10.
For many years, hydrogen has played a crucial role in the industrial sector, especially for synthesising ammonia and in the manufacture of fertilisers 11, driving the development of specialised technologies for its separation from fossil resources, the development of these technologies being due to the fact that hydrogen is not isolated in nature 12. Over the years, different procedures have been explored to obtain hydrogen isolated from these compounds, based on the type of feedstock used: fossil resources such as oil, coal and natural gas, or renewable resources such as water and biomass 13,14.
In Latin America, there has been a growing interest in hydrogen production and storage at the global level. This is partly due to the fact that several countries in the region have a low cost of clean energy as a result of the energy transition towards a cleaner energy matrix, such as Brazil and Colombia, which generate a significant proportion of their energy through hydroelectric sources. 15-17. In addition, there has been interest in building green hydrogen production plants in the region, an industry that could generate multiple benefits, including job creation, increased affordability of energy services for sectors that do not yet have them, climate change mitigation and poverty reduction 18. Among the countries in the region, Chile has developed a greater emphasis with two projects under development, countries such as Colombia are in a state of analysis for the development of a roadmap and in Costa Rica an alliance between public and private companies has been consolidated with the aim of generating supply and demand for this resource 18-20.
The main objective of this research is to address the different hydrogen generation routes that have evolved up to the present day. On the one hand, the consolidated technologies for hydrogen production at industrial level will be reviewed. On the other hand, emerging technologies that allow hydrogen to be generated in a sustainable way, without producing environmental pollution, will be explored, with the main emphasis on understanding the operation of technologies to produce green hydrogen. In order to develop this objective, different research articles will be examined that present experimental results of the various technological alternatives for hydrogen production. In this way, the study seeks to provide a comprehensive overview of the current status and trends in the development of efficient and clean technologies for hydrogen generation, in order to contribute to the energy transition towards more sustainable sources.
Methodology
In this section we will detail the process we carried out to perform the literature search, which was crucial in identifying relevant studies for our state-of-the-art article. The bibliographic sources selected for data collection are the following: Scopus, Science Direct, Google Scholar, Web of Science, Springer Link, Taylor & Francis.
These platforms greatly facilitate the filtering of research articles, allowing us to limit the search to review and research articles, as well as by year. We developed search equations employing Boolean operators (AND, OR, NOT), using the following keywords: “Hydrogen production" "Fuel cells" "Green hydrogen" "Technologies for hydrogen production" "Electrolyzers" "Electrolytic cells" "Biohydrogen" "Pyrolysis" "Natural gas" "Gasification" "Autothermal reforming" "Thermochemical".
The use of these equations allowed us to significantly reduce the number of documents retrieved, limiting it to less than 50 per search equation. This strategy streamlines the review process and ensures a more accurate selection of relevant documents. Regarding the inclusion criteria, we prioritized papers from the last 10 years and in English language. Initially, we included a large number of review articles to obtain an overview of all production technologies. Subsequently, we focused our attention on individual investigations of each technology, with the objective of delving deeper into operational aspects, efficiency, advantages and disadvantages of each one, this approach allowed us to obtain a detailed and complete knowledge of each technology considered. Thus, the structure of the research was organized according to the different hydrogen production technologies, such as electrolysis, gasification and biological production, among others, allowing a systematic analysis of the various approaches and approaches used in each area, as well as the identification of emerging trends and promising areas of research.
Hydrogen generation technologies
Over the years, global demand for hydrogen has experienced a remarkable increase, driving a significant increase in hydrogen production. According to recent data, global hydrogen production will amount to 90 million tonnes (Mt) in 2020 21, while by 2021, this figure rose to 94 million tonnes 22. In addition, over the course of 2022, total hydrogen production will increase by a further 11.1 million tonnes per year. 23. Importantly, most of this growing hydrogen production continues to rely exclusively on fossil resources resulting in the release of approximately 900 Mt CO222,24.
In general, these technologies are classified using a colour coding system, in order to facilitate the distinction and description of the various production processes 25,26. Figure 1 shows the colour coding of hydrogen, with their respective production processes and the raw materials most commonly used in the generation of hydrogen, these are classified into different categories according to the production processes used, which results in a wide range of colours that reflect their origin and environmental impact, which are, grey hydrogen, blue hydrogen and green hydrogen are the main variants, each with their particularities 27,28.
Firstly, grey hydrogen is obtained by reforming hydrocarbons, especially methane, through thermochemical processes 29, being the most economical method, but with high CO2 emissions (9,5 KgCO2/KgH2). Blue hydrogen incorporates carbon capture and storage technologies to mitigate emissions, reducing them to between KgCO2/KgH2, although it still relies on non-renewable resources 30. In contrast, green hydrogen is produced by electrochemical processes, such as the electrolytic dissociation of water, using electricity from sustainable sources such as wind, solar or hydroelectric power, with low emissions 31,32 (8,2 KgCO2/KgH2 in Colombia) 33. This colour classification allows the identification of the origin and environmental footprint of each type of hydrogen, encouraging informed decisions and a more sustainable approach to energy management 34,35.
Production from fossil fuels
So far, hydrogen production has been mostly linked to the use of fossil fuels as the main raw material, which are mainly composed of hydrocarbons characterised by large amounts of hydrogen and carbon in their molecular structure, the physical state at room temperature of these fuels (coal, oil and natural gas) varies depending on the amount of carbon present 36. The procedures for obtaining hydrogen focus on the decomposition of the bonds between hydrogen and carbon in these hydrocarbons, representing one of the most widely used methods for the production of hydrogen from fossil fuels today 37.
Hydrocarbon reforming
Currently, the predominant technological route for hydrogen generation on a global scale is hydrocarbon reforming, mainly methane, accounting for about 48% of total production 38. In this field, three different approaches have been established: steam reforming 39, partial oxidation and autothermal reforming 40), (41.
Firstly, steam methane reforming (SMR) involves reacting methane with steam at temperatures in the range of 700 and 1000°C and moderate pressures between 3 and 25 bar 40-42, where this reaction takes place in the presence of a metal catalyst composed mainly of nickel. The endothermic reaction is presented in Equation 1, separating and recombining the methane molecules (CH4) and water (H2O), to generate carbon monoxide and hydrogen 43.
Subsequently, using the gas-water shifting technique, convert CO to CO2 and release more hydrogen, as shown in Equation 2 44, this reaction is considered exothermic because it has a standard enthalpy reaction of -41,17 kJ/mol, but this heat release is not adequate to maintain the temperature required in the reforming reaction, and some of the natural gas is used as fuel to meet the temperature demand.
Subsequently, the aim is to purify the hydrogen by separating the CO2 and other pollutant gases, finally obtaining high purity hydrogen 44. Although this process is considered the most economical option, the hydrogen produced is categorised as "grey hydrogen" because of its high direct CO2 emissions, producing 9,5 KgCO2/KgH237,39, it is feasible to implement carbon capture and storage (CCS) systems that enable the cost-effective and low-emission production of short-term hydrogen called "blue hydrogen" 45.
Another process used in hydrocarbon reforming is partial oxidation, this process consists of the incomplete oxidation of the hydrocarbon by oxidising the carbon and leaving the hydrogen free, as can be seen in equation 3, the process consists of injecting a stream of air or oxygen into a reactor containing the hydrocarbon 37, where only carbon is oxidised leaving CO and releasing H2.
The CO produced is subjected to water and gas displacement to obtain additional hydrogen, as in equation 2 46; in addition, the reaction rate exceeds that of steam reforming, and this makes it possible to use a reactor of smaller dimensions 47, a remarkable aspect is its ability to handle the presence of sulphur in the feedstock without the need for catalysts, although in cases of low sulphur concentration in the feedstock, catalysts can be used to reduce the reaction temperature and facilitate thermal control of the process 44.
Finally, autothermal reforming is presented, this process combines partial oxidation and SMR in a single reactor to optimise the thermal equilibrium, taking advantage of the energy generated by the partial oxidation to drive the SMR reactions 48. In this process, oxygen and water vapour are introduced into the reactor simultaneously, allowing precise temperature control by adjusting the air to fuel ratio, the energy efficiency of this process is relatively high as it demands less energy than steam reforming, but with a lower efficiency than SMR, although higher than partial oxidation 42,49,50.
Pyrolysis of hydrocarbons
The pyrolysis process is a thermochemical reaction involving the thermal decomposition of hydrocarbon molecules by the application of heat in an oxygen-depleted environment, yielding hydrogen and carbon as expressed in equation 4 45,51. The products derived from this transformation are affected by various elements, such as the type of fuel used, the pressure and temperature conditions during operation, as well as the time the material spends in the processing unit 42.
During this procedure, hydrocarbon molecules, such as methane (CH4), ethane (C2H6), propane (C3H8), among others, which are found in fossil fuels, are subjected to high temperatures, generally in the range of 700 to 900°C 52, as thermochemical separation occurs from heavy residual fractions with boiling points above 350 °C, it makes sense that hydrogen production is carried out in two steps, hydro-gasification as described in equation 5, where methane is subjected to hydrogen-rich gas and water vapour to convert it into syngas 53.
This leads to the breaking of the chemical bonds connecting carbon and hydrogen atoms, resulting in hydrogen and solid carbon in the form of coke as in equation 6 50.
The hydrogen produced can be separated and purified much like the last two phases of the SMR, on the other hand, the solid carbon or coke that is formed can be gasify with steam to produce more hydrogen, making use of the displacement reaction as in equation 7.
The pyrolysis process is notable for its ability to prevent the generation of carbon dioxide (CO2), making it a low-emission technology for the production of hydrogen from fossil fuels 49. Although this process requires higher temperatures in contrast to SMR, it has the advantage of dispensing with expensive catalysts 52.
Coal gasification
Gasification consists of converting a solid material into combustible gas by means of a thermochemical process that makes it possible to obtain synthesis gas from materials such as biomass or coal 54. Synthesis gas consists of carbon monoxide (CO), hydrogen (H2), carbon dioxide (CO2), methane (CH4), these gases can be used as fuel on their own, or can be separated to produce different chemical products 51.
Gasification is carried out in a special reactor, known as a gasifier, in which feed materials are heated to high temperatures ranging from 700 to 1500°C, usually in an oxygen deficient environment limited to 10 to 50% of stoichiometric 55. This process allows the feedstock to be thermally decomposed and converted into gas, instead of being burnt completely as in conventional combustion 56,57.
Gasification is presented as an alternative to conventional coal combustion, offering notable advantages such as higher efficiency for power generation and a significant reduction of GHGs 58. However, it should be noted that this process still involves considerable costs and requires specialised equipment, which restricts its application on a large scale 50.
Production from renewable resources
Conventional hydrogen production from fossil fuels, although efficient, generates high CO2 emissions. 59. However, cleaner technologies have been developed that make use of renewable energy resources such as sunlight, electricity and biochemical processes 60. These production routes, which include electrolysis of water, photolysis with direct solar energy and biomass conversion through biological and thermochemical processes, allow clean hydrogen to be obtained using raw materials such as water and biomass. 61,62. This approach not only reduces pollutant emissions, but also contributes to the transition towards a more sustainable and environmentally friendly energy matrix 63.
Water division
Water splitting emerges as an extremely important production route, as it has established itself in the production of hydrogen, based on breaking down water molecules through specialised water splitting processes such as electrolysis, thermolysis and photolysis 62,63.
Thermolysis
Linares and Moratilla define thermolysis as "the extraction of hydrogen from the molecule that contains it through the application of heat; we speak of thermolysis when the heat comes from an external source". 64) , these authors call thermolysis, "when the heat comes from an external source". For water splitting to occur, it is necessary to bring the Gibbs free energy (∆G) to zero, which is necessary for the decomposition of water to take place. However, the main limitation of this way of producing hydrogen lies in reaching the required level of Gibbs free energy, for which temperatures around 2500 K are necessary 65. The thermodynamic properties of water decomposition reactions under standard conditions hinder the efficient realisation of direct thermal synthesis at lower temperatures 66.
Electrolysis
Electrolysis is based on the splitting of a chemical compound into its most basic components by the use of electric current, the basic reaction of electrolysis is found in equation 7. Lladó et al. 67, explain how the combination of Gibbs free energy and thermal energy satisfies the theoretical energy demand, which is responsible for the dissociation of the water molecule. Also, the theoretical voltage that must be applied for water splitting to occur is 1.23V as explained by Fabregas et al. 65, this value is obtained by linking the free energy to the concept of useful work, applying the first law of thermodynamics 65,67,68.
It should be noted that this technology has been developed and used on a commercial scale 10,69, its constant development has allowed the introduction of four types of electrolysis, which are differentiated by their operating parameters and materials of construction, where (i) alkaline water electrolysis, (ii) anion exchange membrane (AEM) water electrolysis, (iii) proton exchange membrane (PEM) water electrolysis and (iv) sodium oxide water electrolysis, where the operating principles are the same for each 62; where the operating principles are the same for each of the cases 62, Table 1 lists the different existing technologies, their operating conditions and the advantages and disadvantages they present.
Table 1 Performance of different electrolysis technologies.
| Type of electrolysis | Electrolyte | Temperature (°C) | Voltage (V) | Current density (A/m2) | Catalyst | Efficiency | Ref. |
|---|---|---|---|---|---|---|---|
| Alkaline (AWE) | KOH, NaOH | 60 - 80 | 1,4 - 3 | 0,2 - 0,8 | Nickel, iron and cobalt | 50% - 71% | 69-71 |
| Anion Exchange (AEM) | DVB polymeric support with KOH/NaOH 1 M | 20 - 80 | 1,4 - 2 | 0,2 - 2 | Noble metals such as platinum | 57% - 59% | 62 |
| Proton Exchange (PEM) | Polymer solid electrolyte (PFSA) | 60 - 80 | 1,4 - 2,5 | 1,4 - 2,5 | Noble metals such as platinum | 50% - 83% | 72-74 |
| Solid Oxide (SOE) | Yttria-stabilised zirconia (YSZ) | 800 - 1000 | 1 - 1,5 | 1 - 1,5 | Noble metals | 89% (Laboratory) | 75,76 |
Current research on water electrolysis has focused on increasing the efficiency of the process, which is hampered by poor kinetics due to the transfer of four electrons in the oxygen evolution reaction (OER), which is slower than the hydrogen evolution reaction (HER), which needs only two electrons 77. Song et al., provides a comprehensive review of advances in the synthesis, catalytic mechanisms and applications of oxygen evolution catalysts (OER), highlighting the need for efficient and low-cost catalysts. Several types of promising catalysts are explored, including transition metal hydroxides and oxides, transition metal phosphates, complex metal composites and metal-organic materials (MOFs), while highlighting the importance of understanding and optimising the reactivity, stability and scalability of these catalysts through various characterisation techniques, continued research into the development of economical and efficient electrocatalysts is essential to achieve an affordable and sustainable water electrolysis process, as well as for the large-scale production of hydrogen and the efficient conversion of solar energy into chemical fuels 78.
For these reasons, the field of electrolysis has remained focused on the development of low-cost electrocatalytic materials, which allow for increased energy efficiency, safety, durability, operability, portability and high installation and operating costs 68. Angeles-Olvera et al. 79, carried out a review of nickel-based electrocatalysts as an abundant metal on earth, where they point out that it is key to elucidate the reaction mechanisms and the role of heteroatoms, defects, dopants and nanostructures, and it is also important to design synthesis and characterisation techniques that are accessible at an industrial level.
Chen et al., 80 extends this perspective by examining various renewable energy sources, such as urea, hydrazine and biomass, for the sustainable production of hydrogen through water electrolysis. The advantages of urea oxidation reactions in alkaline media, the advantageous use of hydrazine as an environmentally friendly and economical fuel, and the possibility of harnessing biomass with pre-treatment strategies are explored. Overall, the article highlights the importance of diversifying renewable energy sources, offering opportunities for more efficient and economical technologies that reduce dependence on fossil fuels and mitigate environmental impact.
On the other hand, the article written by Liu 78, addresses the need to develop affordable electrocatalysts for hydrogen production through proton exchange water electrolysis (PEMWE), presenting alternatives not based on platinum group metals (PGMs), platinum, palladium, rhodium, rhodium, ruthenium, iridium and gold, seeking to replace them with carbon and boron nitride-based compounds. This research highlights the importance of addressing challenges in the design of non-PGM based catalysts, proposing solutions such as atomic layer deposition and the injection moulding technique. Finally, the economic analysis suggests cost efficiency in the medium and long term with the implementation of these catalysts.
In the same way, Angeles-Olvera et al., 79 provide a detailed review on the use of nickel catalysts for water electrolysis, highlighting the importance of developing sustainable and affordable electrocatalysts. Specific types of nickel catalysts are discussed, such as nanotubes, aerosols, alloys and nanoparticles, which have demonstrated efficiency and stability. The article emphasises the need to determine the catalytic activity under standard conditions to compare and evaluate the efficiency of these materials.
Biomass production
Biomass has attracted a lot of attention in hydrogen production because it is a feedstock that can be obtained from biological resources and generated naturally, making biomass a renewable resource that can be produced in a sustainable way 81. A crucial aspect is the possibility of using thermochemical processes to extract hydrogen from fossil fuels; however, using biomass has the advantage of significantly lower environmental impact 61. Likewise, hydrogen production can also be carried out by biological processes, which offer a number of benefits. However, it is essential to optimise these processes and improve energy efficiency to maximise their potential 82.
In contrast to the previously described methods, biological approaches allow the use of various types of organic waste, thanks to the activity of different groups of microorganisms. Figure 2 shows all the production processes for producing hydrogen by means of microorganisms, this form of production makes it possible to reduce CO2 emissions, in addition to the efficient elimination of a large amount of waste and residual biomass.
Fermentation
In general, fermentation processes use microorganisms to produce alcohols, acetone, H2 and CO2 from organic substrates, depending on the type of microorganism, the type of fermentation, as there are groups of bacteria that carry out fermentation in the absence of light, this variety of bacteria gives rise to two types of fermentation, dark fermentation (DF) and photofermentation. Table 2 shows experimental research on the production of hydrogen by means of these fermentation processes 83.
Table 2 Experimental investigations on hydrogen production by fermentative processes with respect to different substrates, pH and microorganism.
| Microorganism | Process | Substrate | Substrate concentration | pH | Production | Ref. |
|---|---|---|---|---|---|---|
| Mixed saccharolytic crops | Dark Fermentation | Distillery Waste Water (DWW) | 40g/L | 6,5 | 0,8-1,6 L H2/L | 84 |
| Rhodobacter sphaeroides B-3059 | Photofermenta-tion | Distillery Waste Water (DWW) | 40g/L | 7 | 17,6 L H2/L | |
| Rhodospirillum rubrum, Rhodobacter capsulatus and Rhodopseudomonas palustris | Photofermenta-tion | Potato Starch | Not specified | 6 - 7 - 8 | 45mL/L*h | 85 |
| Photosynthetic bacteria HAU-M1 | Photofermenta-tion | Alfalfa | 31,23g/L | 6,95 | 12,5mL/h | 86 |
| Rhodobacter sphaeroides 158 DSM | Photofermenta-tion | Brewery Wastewater Pretreated With Banana Peel | 50% treated wastewater and 50% of the standard environment | Not speci-fied | 408,33 ml/h | 87 |
| Mixed bacterial cultures | Dark Fermentation | Food Waste | 10g/L | 7 ± 3 | 74,91ml/g | 88 |
| Mixed crops | Dark Fermentation | Hydrolysed Sugar Beet Pulp | Not specified | 5,5 | 36dm3/Kg | 89 |
| Bacterial strain TERI S7 | Dark Fermentation | ▪ Xylose ▪ Glucose ▪ Sucrose ▪ Sucrose ▪ Maize Syrup Solution ▪ Soluble Starch | 10g/L | 7 | ▪ 1899 ± 15 ml/L ▪1725 ± 37 ml/L ▪ 1651 ± 43 ml/L ▪ 1581 ± 63 ml/L ▪ 1416 ± 41 ml/L | 90 |
| Mixed crop obtained from a water treatment plant | Dark Fermentation | Hydrolysed Wheat Residue | 15g/L | 5,0 - 6,0 | Not specified | 91 |
Firstly, dark fractionation (DF) decomposes biomass-based substrates or wastewater from industrial processes using groups of anaerobic bacteria in a light-free environment, which use sugars as an energy source, however, this form of production presents several challenges., Ren et al. 92, explains that the main challenges are in the construction of the bioreactor, the selection of the glucose-rich substrate and the inhibition of the substrate, it is also necessary to control the growth rate of the bacteria, as Wheelock Gutiérrez explains}, as low production is related to a high proportion of acids and short-chain alcohols, causing pH reduction affecting the activity of the hydrogen-synthesising microorganism.
Photofermentation, on the other hand, employs photosynthetic bacteria that use light energy as a source of electrons under conditions of nitrogen and oxygen deficiency. In this process, these bacteria take the necessary carbon and electrons from the organic matter present in the substrate 49; an anaerobic medium is necessary, because H2 formation is due to the enzyme nitrogenase, fixing nitrogen and releasing hydrogen in the process, however, nitrogenase is inhibited in the presence of oxygen 93. Photofermentation presents a number of challenges, including bioreactor design, light availability, controlling variables such as substrate concentration, bioreactor pH, temperature and light penetration, all of which make photofermentation a complex process to operate with high hydrogen production costs 6,61,81,94.
Biophotolysis
Biophotolysis is a biological process that involves the use of light and organisms such as photosynthetic bacteria, microalgae and cyanobacteria, using the natural photosynthesis of these plants to break down the water molecule into hydrogen and oxygen by the action of two enzymes that are key to biophotolysis, hydrogenase and nitrogenase 82,93,95. On the one hand, hydrogenase is responsible for the production of hydrogen from water, while nitrogenase is responsible for facilitating the fixation of atmospheric nitrogen, which can improve the yield of the process by providing essential nutrients for the micro-organisms, but these enzymes act differently for direct and indirect biophotolysis 81,96,97.
Regarding the function of enzymes in each of the following, Nagarajan et al., 98, in his article explains, "direct biophotolysis occurs when photosynthetic electrons derived from water splitting are transferred via PS II, PS I and ferredoxin to hydrogenase under anaerobic conditions". As for indirect biophotolysis, Nagarajan et al. state that an external source of electrons required for hydrogenase action is obtained from "fermentative metabolism of stored carbohydrate reserves occurring under dark conditions” 98.
On the other hand, Kosourov et al., 99 explains that hydrogenase is involved in the reduction of electrons to hydrogen, harnessing the energy generated by photosynthesis and electron flow, making use of hydrogenase production to convert protons to hydrogen; for indirect biophotolysis, Kosourov et al. explain that the enzyme nitrogenase plays a crucial role in providing reducing agents derived from nitrogen fixation, which can be used as substrates in hydrogen generation.
In spite of the research that has been developed to understand the functioning of this process, there have also been several studies focused on the optimisation of the biophotolysis process, in Table 3, some of them concentrate their efforts on the design of photobioreactors, also in the area of cellular genetics and experimentation with different types of microorganisms.
Table 3 Research in different areas of biophotolysis
| Authors | Outline of the document | Reference |
|---|---|---|
| Schumann et al. | They focus on the use of hydrogenases to catalyse hydrogen production, emphasising hydrogenases as superior catalysts due to their higher catalytic rates and more efficient energy utilisation. | 95 |
| Kosourov er al. | It presents advances in the understanding of hydrogen metabolism and its impact on cellular bioenergetics, opening up possibilities for the development of hydrogen-producing cell factories with improved performance, as well as exploring the use of synthesised hydrogenase. | 99 |
| Kamshybayeva et al. | It studies the sensitivity of enzymes to oxygen and electron competition in different metabolic pathways, focusing on the limitations of hydrogen production at the industrial level, also using advances in genetic engineering and biotechnology, it explores solutions to enhance hydrogen production in cyanobacteria. | 100 |
| Kossalbayev et al. | Describes in detail the design features of photobioreactors and the conditions necessary to grow cyanobacteria optimally. | 101 |
| Bozieva et al. | Research on the possibility of hydrogen production by means of biophotolysis in different species of cyanobacteria. The strains Cyanobacterium sp., Dolichospermum sp. and Sodalinema gerasimenkoae IPPAS B-353 are studied, showing that the most efficient strains are Dolichospermum sp. under light anaerobic conditions and Sodalinema gerasimenkoae IPPAS B-353 in the dark. | 102 |
Microbial electrolysis cell
The microbial electrolysis cell (MEC) is related to the traditional electrolysis already described in previous sections, with the difference that it combines electrochemistry with bacterial metabolism 103, the organic material present in the anode chamber is degraded by the action of microorganisms, particularly an exoelectrogenic species, with the help of a small amount of electrostatic charge, the organic material is degraded producing electrons, CO2 and protons.(+H) 103, where protons pass through the proton exchange membrane (PEM) into the cathodic chamber by the action of "hydrogenotrophic methanogenic micro-organisms and a small potential difference" 104 the reduction reaction takes place by pairing protons (+H) with electrons resulting in the production of hydrogen.
Interest in this form of hydrogen production has led to various designs for the configuration of MCE reactors. Murugaiyan et al, 105, conducted research focused on the study of different reactor configurations, together with the materials required for their construction. Jensen et al., 106 conducted a complete review focused on hydrogen production using (MEC), where an important point of this document is also the configurations and design of the reactors, giving importance to the anode and cathode materials; in Table 4 there are different investigations making use of microbial electrolysis, testing different types of configurations, substrates, membranes, etc.
Table 4 Hydrogen production rate with respect to various substrates, materials and reactor type.
| Reactor type | Anode | Cathode | Membrane | Substrate | Inoculum | Production rate | Ref. |
|---|---|---|---|---|---|---|---|
| Dual chamber | Metal oxide and graphene nanocomposites | Nickel Foam (NF) | Nafion | Waste water from the sugar industry (SWW) | Not specified | 4,38 ± 0,11 mmol/L/D | 107 |
| Single chamber | Carbon fabric | Molybdenum Phosphide (MoP) | MEC without membrane | Acetate and glucose | Not specified | 39,8 ± 1,9 L/L/D | 108 |
| Double chamber | graphite felt 5 mm thick | Stainless Steel Mesh Electrodes | Cation exchange membrane | Pig slurry | digestate from a local sewage treatment plant | 0,2 LH2/L-day | 109 |
| Double chamber | Biochar | Grafito | Cation exchange membrane (CMI-7000) | synthetic water | Hoeflea sp. and Aquiflexum sp | 0,89 ± 0,10 m3 /day*m3 | 110 |
| Single chamber | carbon felt | Carbon Paper Cathode Nano-Mg(OH)2/Gr | Non-specific | phosphate buffer | bacteria solution MFC | 0,63 ± 0,11 m3/day*m3 | 111 |
Conclusions
The development and utilisation of hydrogen energy has become a relevant direction in the field of modern energy research, positioning itself as an energy matrix with great potential to replace fossil fuels in the future. Current research is focused on advancing various mechanisms for the production of hydrogen without environmental pollution.
On the one hand, electrolysis is at the forefront, a form of production that appears to be the main source of clean hydrogen when implemented with renewable energy sources. On the other hand, there are the various processes that encompass biohydrogen production. The latter is characterised by the use of renewable resources such as water and biomass, paving a way towards cleaner and more sustainable hydrogen production. Within the field of biohydrogen, different methods have been found to suit the microorganisms and the type of feedstock used. With the development of this research, we can analyse each of these different methods and what are the main challenges to be overcome in order to achieve competitive efficiency compared to traditional industrial production processes. Regarding fermentation processes, the challenges to be overcome focus on the design and construction of suitable reactors, the need for a substrate rich in glucose and the control of the bacterial growth rate. In biophotolysis, solutions must be explored in conjunction with genetic engineering to overcome the limitations of industrial production.
In addition, it is crucial to continue researching and developing more efficient and scalable technologies for hydrogen production from renewable sources in order to reduce dependence on fossil fuels and mitigate environmental impact. Furthermore, challenges such as hydrogen storage, transport and distribution, as well as the integration of these technologies into existing energy systems must be addressed for a sustainable energy transition.
