Evaluation of Biohydrogen Production Depending on the Substrate Used—Examples for the Development of Green Energy

Abstract

Biohydrogen production is a promising alternative to replace fossil fuels in an environmentally friendly way. In addition to the many available renewable energy sources, the production of “colored” hydrogen and biohydrogen occupies an irreplaceable position due to the undeniable availability of biomass and the need to manage food waste (FW). This article presents the current state of biohydrogen production technology—examples on continents (America, Africa, Asia, Australia and Oceania) and in Europe in terms of the efficiency of dark methane fermentation (CH₄). Biophotolysis processes leading to the production of biohydrogen are indicated: directly and indirectly. The mechanism of the fermentation process of obtaining hydrogen and two-stage hydrogen fermentation are presented. The novelty of this article is the development of innovative trends in the development of the biohydrogen industry in Europe. Various models of the biohydrogen process are presented for different raw materials and proportions of substrates used in co-fermenters. Researchers from China are the undisputed pioneers in the use of renewable energy sources. However, improved energy self-sufficiency and environmental impacts are reflected in the growing number of pilot installations operating in European countries. This also gives hope for rapid progress towards full animal and FW management also in Poland.

1. Introduction

The progressing energy crisis, closely related to the deepening of the climate catastrophe, is currently the greatest obstacle to the development of civilization [1]. Rapidly decreasing reserves of fossil fuels pose a threat to global energy security, generating numerous social and economic problems [2]. The gradual shift away from fossil fuels to renewables is the starting point for slowing down the expansion of these crises.

Hydrogen (H₂) is considered to be the leading renewable energy source in terms of its benefits. Apart from the lack of generation of environmentally negative combustion products, it is characterized by a significant energy density [3]. Recognition of the benefits of using H₂ is reflected in a stable upward trend of interest in the implementation of technology for its production and use [4,5,6]. On the other hand, the factors hindering the dissemination of H₂ technologies are those resulting from its production (energy-intensive classical electrolytic methods) and storage (safety), but also the impossibility of an immediate departure from fossil fuels due to technical reasons.

H₂ is a clean energy carrier with great potential as an alternative fuel. Its potential lies primarily in the fact that it is a clean fuel, its use does not cause negative emissions to the environment [7].

Continuous development, urban growth and an intense lifestyle have enabled a model based on consumerism. Although society’s awareness is increasing, there is still a long way to go before non-ecological habits are replaced by the concept of reduction, reuse, and recycling. Meanwhile, the demand for energy is increasing year by year due to the continuous growth of the population and the improvement of the standard of living.

The prospects for the development of the energy market until 2050 assume transformations on a scale comparable to the next industrial revolution. According to the sources of the International Energy Agency, in agreement with government agendas and the “zero-emission” scenario, the use of fossil fuels will undergo a marginal reduction by 2030/2050 [8]. At the same time, we will observe a shift in the H₂ economy from emission-producing to emission-free production, i.e., from the so-called “grey” resource—obtained from non-renewable fossil fuels through the chemical industry—towards “green” H₂—classified as ecological, which is a new source of fuel and raw material for the sector of new technologies [9]. The current and projected distribution of H₂ sources from the acquisition and applications for each type is presented in Figure 1.

The figure presented in Figure 1 highlights the particularly important period of 2030–2035, during which the demand–supply market for H₂ sources will undergo a transformation. From around 2030, the demand for H₂ used in existing industries will begin to decline, while new markets will emerge in the transportation and modern manufacturing and energy sectors.

Currently, fossil fuels satisfy 84% of human energy needs, which is likely to increase in the coming decades [11,12]. The main sources of greenhouse gas emissions are transport, electricity production, and industry [13]. Petroleum-based fuels are the primary source of energy for transport purposes. The ease of travel and the need for motorization in everyday life mean that the growing demand for energy in this sector is unlikely to change in the future. It is predicted that at the current rate of consumption, non-renewable fuel reserves could be depleted by 2150 [14,15]. Transport generates more than 20% of anthropogenic carbon dioxide (CO₂) emissions, contributing to the rapidly escalating global warming effect [16]. Fossil fuels are limited resources whose source is restricted to specific regions of the world, which also raises economic and political concerns.

Finding alternative energy sources to fossil fuels is necessary to stop the deteriorating state of the natural environment. H₂, characterized by a high energy density as a substance that undergoes oxidation without creating harmful products, is considered one of the most promising energy carriers. Unfortunately, currently, a common source of H₂ remains to be fossil fuels, the extraction and processing of which is energy- and cost-intensive, or water electrolysis, a process requiring energy. Therefore, the use of renewable raw materials for the production of H₂ is an alternative option that shows high economic and environmental benefits for the development of sustainable raw materials and the circular economy.

Biohydrogen production opens up another niche for the management of sludge from municipal sewage treatment plants. Sewage sludge causes significant environmental pollution and also affects human health in many ways. Sewage sludge is used to produce many renewable resources to maintain environmental quality, reduce many risk factors, produce sustainable energy and serve as a reliable source of energy production.

This review aims to describe various methods of producing biohydrogen. It focuses on comparing their efficiency in terms of H₂ production and highlighting their main advantages and disadvantages. This article also discusses the current state of knowledge on biohydrogen production technology and identifies promising future development paths and presents the current state of the technologies used in the production of biohydrogen in Europe, with particular emphasis on Poland, in terms of efficiency and actual energy production. The novelty of this article is to describe innovative trends that have great potential to play an important role in this field in the near future.

1.1. Aspects of the Production of “Colored” Hydrogen and Biohydrogen

Due to the ongoing climate crisis caused by greenhouse gas emissions, this article pays particular attention to so-called green, emission-free methods of H₂ production, which are the least harmful to the environment and have the most promising potential in the current energy transformation. “Green” H₂ is produced in processes such as water (H₂O) electrolysis, using renewable energy sources. It is called green because there is no CO₂ emissions during the production process. Among the emission-free methods of H₂ production, we can mention [17]:

• Large-scale H₂O electrolysis powered by electricity from various renewable sources.
• Photovoltaic electrolysis [17,18], where sunlight acts on a membrane system (e.g., polymer) and the flow of electrons through a set of modules causes the breakdown of H₂O.
• Photoelectrochemical cells (PEC) [19] based on a chain of transformations initiated by sunlight in an aqueous environment. PEC uses photoactive semiconductors that create an electrode and absorb light (energy) to enable H₂O catalysis using sunlight and the production of gaseous H₂ and oxygen (O₂).
• Photofermentation [20], which uses bacteria for anaerobic fermentation to produce H₂ from organic acids under the influence of light (natural or artificial).
• Artificial photosynthesis, a process that imitates native plant photosynthesis, involves illuminating nanoparticles of titanium dioxide [21], which act as a catalyst for the breakdown of H₂O into H₂ [17,22].
• Photocatalysis based on the use of an activated and light-powered catalyst, which creates electron-hole pairs, allowing H₂O electrolysis to begin, similar to PEC. This process is subject to numerous limitations (e.g., efficiency or speed), and research into its better understanding is ongoing [17].

Despite the fact that the end product, H₂, is a zero-emission fuel that does not generate greenhouse gases or pollution to the environment during combustion, its production can be a more or less polluting process depending on the input materials and sources of energy used in the production process. Therefore, despite the emission-free advantages of using H₂ fuel, it is necessary to classify it based on the method of sourcing the raw material. Currently, we distinguish types of H₂ (Figure 2), such as gray, blue, brown, black, and turquoise—obtained from fossil sources, and green H₂, derived from renewable sources. Other production methods for H₂ are of lesser importance, including yellow H₂—obtained by electrolysis powered by solar energy; red, purple, and pink—produced by electrolysis based on nuclear energy; and white H₂—derived from geological natural sources [17,23,24].

The aforementioned model of dividing H₂ according to the sources of production, however, does not take into account the overall environmental burden associated with the environmental costs of producing the fuel. Dawood et al. [24] introduce a more holistic index called the HCI (Hydrogen Cleanness Index), based on a multilevel analysis of four basic tests that scale the actual “cleanliness” of the obtained H₂.

The HCI model predicts four areas (called “scopes”) considered in terms of the emission intensity of H₂ production (Figure 3).

These areas are scope 1, which looks at how much CO₂ is generated directly during H₂ production; scope 2, which focuses on CO₂ generated from ancillary and support processes; scope 3, which takes into account the life cycle of by-products; and scope 4, which analyzes the effectiveness of capturing the CO₂ generated. Each of these four areas is divided into three complementary clusters that determine the share of a particular type of energy in the process: gray (production from fossil fuels with uncontrolled CO₂ emissions into the environment), blue (using fossil fuel energy, but with carbon capture and storage—CCS) and green (from renewable sources). Clusters are scalable from 0% to 100%, which allows for a clear specification of whether the final H₂ obtained is “gray”, “blue” or “green”. Therefore, the HCI model describes H₂ according to the previously used color designations, such as gray or green, but with an expansion of the percentage level of the share of a particular type of energy powering the production process and an indication of the stage at which factors influencing the non-zero emission of greenhouse gases occurred in the process of obtaining H₂. For example, HCI (75 Green-2) indicates that H₂ is obtained from 75% renewable energy, where the “2” designation indicates non-direct (ancillary and support) greenhouse gas emissions during the production process of the discussed H₂ (this is precisely what the 2 scale in the HCI model refers to) [24].

When considering the issues of the emission and ecological impact of H₂ production, an important factor is also the selection of raw materials and sources of energy from which it is produced. The advantage is given to those that are easily and commonly available (such as H₂O, sunlight) or those that require the utilization of by-products from other production processes that would otherwise become hazardous waste—such as biomass (including manure, straw and other agricultural waste) from the widespread, mass production of meat [18].

One of the developing areas of interest in energy issues is a return to technologies related to agricultural sources. This involves using plant energy sources. One of the significant advantages of plants as an energy source is their growth, associated with the use of CO₂ for growth and the release of O₂. This means that plants can not only be a source of energy materials, but also absorb the CO₂ emitted during energy production. The problem to be solved is the identification of plants characterized by the rapid production of biomass, which in subsequent stages of processing is used for energy production.

Biomass is today the oldest and most widely used renewable energy source, which includes all organic matter on Earth, all substances of plant or animal origin that are biodegradable. Biomass also includes agricultural residues, forest residues and industrial and municipal waste.

Compared to the conventional cultivation of crops for other purposes, plants dedicated to the production of biomass have greater possibilities of locating fields. There are two main reasons why they can be grown in areas where the soil is severely degraded. Firstly, because they are not grown for consumption, so any accumulation of harmful substances in their tissues poses no risk to humans and animals (potential consumers), and secondly, they show faster biomass growth and have low soil requirements.

The combination of the raw material used and the technology used also affects the purity of the obtained H₂ fuel, which may require additional purification processes, generating additional emissions, costs, and energy requirements, or limit the possible applications of the obtained H₂. Another aspect is the distribution of the produced H₂, where even a cheaper, mass-produced, but less logistically dispersed method (and location) of production translates into higher (also in terms of emissions) costs of delivering H₂ fuel to the end consumer. Here, the “green” advantage will be gained by methods of producing H₂ that will enable its local, decentralized production, that is, located as close as possible to the consumer. Distributed systems have many advantages, such as a shorter distribution chain and lower transport costs. Biomass, associated with local agricultural production, is often physically distant from large factories and has significant potential in terms of logistical proximity to the end consumer. Biomass gasification effectively transforms it into a fuel suitable for dispersed (decentralized) energy systems. Therefore, biomass gasification can play a crucial role in future H₂ energy systems. Figure 4 illustrates the structure of H₂ synthesis in the industry, while biological methods of obtaining it are presented in Figure 5.

Figure 4 shows how the production and paths of obtaining H₂ currently look depending on the adopted production technology strategy. Green indicates paths based on biological methods, which rely on the selection, modification and research of microorganisms and simple organisms that produce H₂ as a byproduct of their metabolism. Red indicates a path based on subjecting H₂O to a high-temperature catalyst in the presence of pressure equal to or greater than the atmospheric pressure. Blue paths are associated with the interaction of an electric current or sunlight with H₂O in a cell, leading to the breakdown of the molecule and the release of H₂. Paths marked in purple are traditional methods of extraction based on breaking apart hydrocarbon chain molecules.

Figure 5 shows, in detail, the branch of biological synthesis. It divides biological methods into a process based on photosynthesis—the action of sunlight on a bed covered with a biological film. The film, under the influence of sunlight, initiates photosynthesis-based processes with the production of H₂ as a microbial waste product. The second branch is based on fermentation processes. Under anaerobic conditions, microorganisms process matter with or without access to light, producing H₂.

Fossil fuels are limited resources whose source is limited to certain regions of the world, which additionally generates economic and political concerns. This fact, confirmed by constantly rising fuel prices, along with broadly understood environmental factors, is the driving force behind the development of biofuels. These fuels are non-toxic, biodegradable, and free of harmful sulfur compounds, resulting in significantly lower emissions compared to conventional gasoline and diesel [26,27,28,29]. By definition, they are liquid or gaseous fuels used for transportation, most of which are produced from biomass [30]. Due to economic, political and environmental factors, alternative energy sources in transportation, such as bioethanol, biodiesel and H₂, will soon play a crucial role in the world’s future. The most promising and desirable renewable energy source is H₂. It is produced in the production process because it does not occur naturally in a free form.

Most biohydrogen-producing organisms are microorganisms and their ability to use or produce H₂ as a metabolite is due to the expression of metalloenzymes known as hydrorases [31].

Enzymes in this widely diverse family are usually divided into three different types depending on the metal content of the active site: [FeFe]-hydrogenases (iron-iron), [NiFe]-hydrogenases, (nickel–iron)-hydrogenases and [Fe]-hydrogenases (iron only) [32]. Notable examples are members of the genera Clostridium, Desulfovibrio and Ralstonia or the pathogen Helicobacter, most of which are strictly anaerobic or facultative microorganisms. Other microorganisms, such as green algae, also express highly active hydrogenases, as is the case with members of the genus Chlamydomonas.

Due to the enormous diversity of hydrogen enzymes, current efforts are focused on screening for new enzymes with improved properties [33,34], as well as on engineering already characterized hydrogenates to give them more desirable properties [35].

The biological production of H₂ from algae is a method of the photobiological decomposition of water (H₂O), which takes place in a closed photobioreactor, based on the production of hydrogen as a solar fuel by algae [36]. Algae produce H₂ under certain conditions. In 2000, it was discovered that if C. reinhardtii algae are deprived of sulfur, they will switch from producing oxygen, as in normal photosynthesis, to producing H₂ [37].

Photosynthesis in cyanobacteria and green algae breaks down water into H₂ ions and electrons. Electrons are transported by ferredoxins [38]. Fe-Fe hydrogenases (enzymes) combine them into H₂ gas. In Chlamydomonas reinhardtii, Photosystem II produces, as a result of the direct conversion of sunlight, 80% of the electrons that go to hydrogen gas [39].

Biological hydrogen production is also observed in N₂-fixing cyanobacteria. These microorganisms can grow to form fibers. Under nitrogen-limited conditions, some cells can specialize and form heterocysts, which provides an anaerobic intracellular space to facilitate N₂ fixation by the enzyme nitrogenase, also expressed internally.

Under N₂ fixation conditions, the nitrogenase enzyme accepts electrons and uses ATP (adenosiro-5′ triphophorate) to break the dinitrogen triple bond and reduce it to ammonia [40]. Molecular H₂ is also produced during the catalytic cycle of the nitrogenase enzyme. Biohydrogen belongs to third-generation fuels, produced using microorganisms [41] that convert H₂ into biohydrogen through photolysis (Figure 6) and fermentation (Figure 7).

Figure 6 shows the main methods of biohydrogen production using photosynthetic microorganisms, based on H₂O as the only electron donor. Biohydrogen is produced by microorganisms (cyanobacteria and algae) that can perform photosynthesis.

A simple system seems to be the so-called direct biophotolysis (Figure 6a), in which several simple, key reactions can be distinguished in the following order:

-

H₂O photolysis;

-

Transfer of electrons from photosystems to a protein called ferrhodoxin, which is then a direct donor of electrons transferred to protons;

-

Biohydrogen synthesis is catalyzed by hydrogenase.

Hydrogenases, as well as nitrogenases, are enzymes that, under precisely defined conditions, enable the synthesis of biohydrogen from electrons and protons. Reactions of this type may occur naturally under certain conditions, including as a kind of “safety valve”, enabling the removal of excess absorbed light energy, but their intensity is minimal.

Figure 6b,c shows an alternative method of producing biohydrogen from H₂O, the so-called indirect biophotolysis, which takes place in two stages. In the first stage, light energy is used to accumulate reduced compounds that take over electrons from photolysis (these compounds—NADPH (reduced form of NADP+—a dinucleotide phosphate ester)— are identical to those produced in the natural process of photosynthesis). Organic compounds are synthesized on the basis of NADPH. In the second stage, electrons from sugars are released again via NADPH and transferred via ferrodoxin to hydrogenases (Figure 6b) or nitrogenases (Figure 6c). The biohydrogen synthesis reaction itself is supported by the photosystem, which provides high-energy ATP molecules (adenosine-5′-triphosphate—the main energy carrier in cells) and these are used by energy-consuming enzymes that catalyze the formation of biohydrogen. Figure 6c is basically a modification of Figure 6b; the main difference is the spatial separation of both stages. The first stage, in which, in addition to generating a free pool of electrons, oxygen is also released (as a by-product), occurs in typical vegetative cells of cyanobacteria. The second stage occurs in special cells called heterocysts. The essence of this chapter is to create anaerobic conditions in place of the action of hydrogenases or nitrogenases, which are extremely sensitive to oxygen and in its presence are inactivated, in some cases, irreversible. Anaerobic conditions, created both locally and temporarily, i.e., only in certain areas of the cells where and when hydrogenases or nitrogenases act, are also necessary in the production of biohydrogen, as shown in Figure 6a,b, but in the case of Figure 6c, anaerobic conditions are ensured throughout the heterocyst area. Biohydrogen production takes place in specialized cells (so-called heterocysts) produced by cyanobacteria occurring in the form of filamentous colonies. For simplicity, the role of NADP+ in the electron transfer between photosystems and ferrodoxin was omitted [42].

In the fermentation process, glucose, which is the primary source of carbon, is converted to pyruvate during the glycolytic pathway—Figure 7.

Then, pyruvate is oxidized to acetyl coenzyme A in the presence of coenzyme A and the oxidized form of ferredoxin. Ferredoxin in its oxidized form is produced during the reduction of protons to molecular hydrogen with the participation of hydrogenase [44].

Unlike anaerobic hydrogen production processes using solar energy, the reactions of the decomposition of organic compounds in similar conditions, but without the use of photon energy, are called dark fermentation.

Dark fermentation contributes to the production of biohydrogen, i.e., a fuel of biological origin, specifically from biomass. The combination of processing biomass waste to produce energy is an advantage. It is known and common to process biomass by fermentation in order to produce biogas. The use of dark fermentation to produce biohydrogen is currently a small percentage of industrial installations [45,46]. Dark fermentation consists of the decomposition of biomass rich in carbohydrates by anaerobic bacteria into, among others, H₂. Microorganisms produce H₂ in dark conditions and at a temperature of 30–80 °C and a pH of 5–6. The products of dark fermentation, apart from H₂, are also CO₂, CH₄ and H₂S. It is generally assumed that a maximum of 4 moles of H₂ can be obtained from one mole of glucose [47,48,49]. In the latest studies, this gas is of great interest as an efficient energy carrier that can solve the biggest problem of the energy crisis in today’s world [50,51,52]. This gas has a very high energy efficiency, which can be even higher than that of hydrocarbon fuels [53]. Moreover, it can be stored in various carriers, such as metal hydrides, carbon nanostructures, borohydrides and others [54,55,56].

The photobiological [43] production of hydrogen using purple, sulfur-free bacteria has been known for over 60 years [57]. This process, commonly called photofermentation, takes place in anaerobic conditions, in the presence of light and organic substrates.

Fermentation may also be one of the stages of hydrogen production using a mixed (hybrid) method. In this case, the fermentation process involves a complex mixture of organic substances, most often waste products or technological sewage. Then, the obtained products constitute a medium for microorganisms during hydrogen bioproduction using visible radiation energy (photofermentation) [58].

Photofermentation uses brown bacteria, which are strict anaerobes. They adsorb electromagnetic radiation in the wavelength range 400 ÷ 950 nm. They are characterized by the presence of the enzyme nitrogenase, which is a catalyst for the reaction of hydrogen formation, especially in conditions of a lack of nitrogen. The advantage of photofermentation is the production of relatively pure hydrogen (with an admixture of 10 ÷ 20% CO₂), which, after relatively easy processing, can be directly used in a fuel cell. Since the substrates for photofermentation are the products of “dark” hydrogen fermentation, i.e., organic acids or alcohols, these two processes can be associated with each other according to the scheme presented in Figure 8. In relation to the hydrogen efficiency achievable in “dark” fermentation, the efficiency in the two-stage process can be up to three times higher [59].

“Dark” fermentation is already quite well-known, but there are still gaps in the knowledge about hydrogen photofermentation that require further research. From a technical point of view, it is very difficult to design a photobioreactor in such a way as to provide the appropriate amount of energy in the form of light necessary to achieve satisfactory process efficiency [59].

To sum up, biohydrogen can be produced using thermochemical, photocatalytic and biological methods. Thermochemical methods include gasification and pyrolysis. In this process, as a result of a high external temperature and the lack of O₂, the biomass decomposes into gases, including H₂. Catalytic and biological processes include dark fermentation and photofermentation. The advantages of dark fermentation are the production of biohydrogen without a light source, a high degree of H₂ production, and the flexibility of the method in relation to various substrates. During both dark fermentation and photofermentation, certain conditions must be met, such as process parameters including the temperature, reaction environment, mixing, and aerobic or anaerobic conditions [60].

Another type of biohydrogen production that is on the border between biological and chemical methods of obtaining H₂ is the steam reforming of biomethane. In this way, the so-called green H₂ is obtained [61].

1.2. Biohydrogen Status on Continents (America, Africa, Asia, Australia and Oceania)

There are different approaches to the topic of biohydrogen production depending on the region. The reason is differences in the number of plant species used for biomass production and municipal waste generated in given conditions on different continents.

The North American H₂ market reached approximately 55.6 million tonnes in 2022 and is expected to grow at a CAGR of 3.58% through to 2032. H₂ finds applications across a variety of industries, playing a key role in shaping the future of clean energy and sustainable practices. One of its main applications is energy production, where hydrogen fuel cells generate electricity using steam as the only by-product, providing a clean and efficient alternative. This also applies to transport, where H₂ powers vehicles and even planes, demonstrating its potential to revolutionize the automotive and aerospace sectors. On an industrial scale, H₂ participates significantly in processes such as hydrogenation, which is necessary in the production of chemicals such as methanol and ammonia. In the electronics industry, H₂ plays a key role in semiconductor production and metal reduction processes [62].

In South America, Becerra-Quiroz et al. [63] considered dark fermentation as the main strategy for biohydrogen production. Their research and technology development focused on defining operational parameters based on organic matter loading and the substrate-to-inoculum ratio. The research results were related to the mass of the expected municipal waste generated by Bogota (Colombia) in 2042. According to the authors, it will be possible to generate power exceeding 2.3 GWh per day.

Scientists from Brazil [64] examined the prospects for producing biohydrogen from palm oil and by-products generated during its processing. Particular attention was paid to POME, i.e., sewage waste generated during the production of palm oil. The work analyzed various types of raw material pre-treatment before dark fermentation and their impact on biohydrogen production. A significant increase in biohydrogen production was observed when acidic and alkaline hydrolysis and enzymatic pretreatment were used. At the same time, the authors note that low hydrogen production at the level of 1 to 3 millimoles of H₂/mol of glucose may result in a lack of implementation in the industrial extraction of H₂ from POME.

In recent years, green hydrogen has emerged as a possible solution to the global energy crisis. In Latin America, the release of Chile’s 2020 National Green H₂ Strategy has led to an acceleration of regional interest in the hydrogen industry. While the newness of the industry creates a number of challenges and uncertainties, it also offers opportunities for innovation and a chance for Latin American countries to capitalize on the industry. Latin America has great potential for green H₂, but realizing this potential requires navigating a complex market where, among many challenges, supply and demand will need to evolve simultaneously. The issue of supply and demand resonates with players in the organic H₂ market; potential suppliers are unsure of the market for their products and potential consumers are unsure of the availability of hydrogen that will meet their needs [65].

El-kebeer et al. [66] investigated the possibility of using sewage sludge to produce biohydrogen through dark fermentation. The tests were carried out on a pilot-scale installation equipped with a fermenter with a capacity of 100 dm³. The study showed a more beneficial effect of alkaline pre-treatment than acid pre-treatment. In the case of alkaline treatment, 1.04 dm³ H₂/g VS was obtained and, in the case of acid treatment, 0.74 dm³ H₂/g VS. The lack of pretreatment resulted in the production of only minimal amounts of H₂ (0.03 dm³ H₂/g VS). The highest biohydrogen yield was observed using alkali shock treatment (1.73 dm³ H₂/g VS).

Among the technological solutions for biohydrogen production being developed in China, the most important is dark fermentation. An interesting solution is also the biological conversion of lignocellulosic biomass based on the biocatalytic hydrolysis of lignin and cellulose taking place in the gastrointestinal tract of termites [67]. The maximum biohydrogen production achieved was 4.08 mmol/mL. However, this solution requires implementation in a full-scale biorefinery. The organisms that enable the conversion of biomass into biohydrogen include microorganisms of the genera Reticulitermes speratus, Nasusitermes spp., Pterotermes occidentis and others [67].

In China, biohydrogen production technologies were researched in terms of both photosynthetic production and fermentation processes. In the case of biohydrogen production by fermentation, the types of fermentation and their engineering control possibilities, the use of pure bacteria and the development of a two-phase anaerobic process for biohydrogen production were analyzed. In the 2000s, the first pilot plant producing hydrogen with a capacity exceeding 1200 m³ per day was established in Harbin, China [68].

Due to China’s desire to meet carbon peak and carbon neutrality goals, the important role of biohydrogen is noticeable. China’s national hydrogen economy system is being developed for this purpose. The plan is to develop a clean, low-emission, safe and efficient energy system based on H₂. The share of biomass in the production of biohydrogen is also expected. Xiangyu Meng et al. [69] predict that the potential of biomass for energy production in China is 2952 × 10⁴ kW. However, the role of biomass in the production of biohydrogen in this solution concerns only the function of the fuel to produce electricity, which would then be used to produce biohydrogen. According to the authors, the hydrogen development plan should be based on the principle “blue hydrogen takes the initiative, green hydrogen keeps pace, and gray hydrogen leaves”. At the same time, it is necessary to use biohydrogen more intensively in the industrial sector [69].

The authors of the work [70] point to the need for further industrial development and stimulating enterprises and scientific research institutions to jointly promote every aspect of H₂ production, storage and transport in the industrial chain and coordination with the process of building a new power system. This involves increasing investment in research and technology development. Basic hydrogen energy research in China is relatively weak and uninnovative and key technologies and critical materials remain at risk. It is important to fully leverage the leadership role of key energy giants such as China Petroleum and China Petrochemical in the energy sector. Meeting these requirements will, in turn, facilitate the promotion of the use of hydrogen energy, fully using the existing pipeline infrastructure and promoting infrastructure improvement (H₂ refueling stations). Hydrogen energy should play a greater role in areas of deep decarbonization in the industrial sector, distributed energy storage, aviation and shipping, with H₂-powered aircrafts and ships, as new energy transportation equipment represent major opportunities for China in terms of technology and the market and efforts should be made to realize the construction of a hydrogen energy society.

China, the world’s largest producer and consumer of H₂, in its medium- and long-term H₂ development plan for 2021–2035, indicates its intention to produce 50,000 hydrogen-fuel-cell vehicles by 2025 and to build a number of H₂ refueling stations. The plan assumes that by 2025, the production of green H₂ using renewable raw material resources will reach 100,000–200,000 tons per year. In addition to transport, the plan envisages the use of clean H₂ in other sectors: energy storage, electricity generation and industry [71].

On 24 February 2023, the Energy and Climate Change Ministerial Council (ECMC) agreed to review the 2019 National H₂ Strategy to ensure Australia is placed on a path to becoming a global leader in H₂ by 2030, both in terms of exports and in terms of decarbonizing Australian industry. H₂ has the potential to make a significant contribution to the transition to net zero through use in areas such as industry, transport, network reinforcement, chemicals and metals production. The investment potential for H₂ is AUD 300 billion, with projects focusing on domestic use as well as large export projects.

Although Australia was the third country to publish a hydrogen strategy in 2019, hydrogen opportunities have emerged from the relative margins to generate international excitement, with over 30 countries now publishing hydrogen strategies. The Australian Government is conducting a review of the National Hydrogen Strategy in partnership with states and territories [72].

1.3. Status of Biohydrogen in Europe

Referring to the latest IAE [26] Electricity Market Report 2023, the electricity market is undergoing a profound transformation. According to the data, trends in flattening global CO₂ emissions intersect, from 12,306 Mt recorded in 2020 to a projected 13,043 Mt in 2025. In Europe, the expected reduction will be a decrease from 950 Mt in 2020 to 763 Mt in 2025, representing a decrease of nearly 84.5%.

Analyzing the trends and forecasts for Europe—understood as the countries of the European Union, the European Free Trade Association (EFTA) and the UK (United Kingdom)—the number of planned projects to be completed by 2030 is expected to be 628, with a total estimated capacity of 138,554 MW. The year 2030 is again identified as a turning point in energy transformation for the region [73,74]. Compared to the Clean Hydrogen Monitor assumptions published in 2020 [75], i.e., 9101 MW, the forecasts have been revised to 138,554 MW by 2030 [76].

By trying to examine the issue from the perspective of investments in electricity generation (or overall energy potential) currently being made and in the future, we can indicate a growing trend, but encounter a problem with the precise forecasting of the scale of the phenomenon. There is a discrepancy in the publicly available sources regarding the data in particular years—

Table 1. Investment costs and performance levels for blue and green hydrogen production technologies over the course of a year [own work, based on [76,77,78,79,80,81,82,83]].

Technology	Year	Investment Cost	Efficiency	Ref.
		EUR * Million/MWh	%
Green—Alkaline electrolyzers (ALK)	2020	0.628–1.955	63–70	[77]
	2020	0.444–0.947	63–68	[78]
		1.395	51	[79]
		1.158–2.837	49–69	[80]
	2030	0.496–1.151	65–71	[77]
		0.361–0.740	68–69	[81]
		0.700	65	[79]
		0.736–1.531	52–73	[80]
	2050	0.220–0.880	70–80	[77]
		0.289	69	[81]
Green—Membrane cells with polymer electrolyte (PEM)	2020	1.613–2.828	56–60	[77]
		1.997	57	[79]
		1.474–3.402	55–63	[82]
		1.266–3.596	52–63	[80]
	2030	0.841–2.095	63–68	[77]
		1.037	64	[79]
		0.998–2.257	59–68	[82]
		0.772–2.739	52–69	[80]
Green—Oxide Electrolyzers (SOEC)	2020	3.041–6.658	74–81	[77]
		1.066	76	[82]
		2.132–3.664	80	[80]
	2030	0.838–3.199	77–84	[77]
		2.132–3.664	80	[82]
		0.799–3.331	80	[80]
	2050	0.489–1.143	77–90	[77]
		0.388	80	[82]
Blue—CCS for existing Steam Methane Reforming plant (SMR)	2020	0.701	-	[83]
Blue—New for steam methane reforming (SMR) and CCS	2020	1.650	-	[83]
	2020	0.963	-	[76]
		1.594	69	[77]
		0.792–1.408	-	[77]
		0.963	-	[76]
	2030	0.909	-	[76]
		1.290	69	[77]
	2050	0.856	-	[76]
		1.214	69	[77]
Blue—CCS for existing autothermal reforming (ATR)	2020	0.688	-	[83]
Blue—New installation autothermal reforming (ATR) and CCS	2020	1.498	-	[83]
		0.952	-	[78]

* 2019.

—depending on the analyzed source [76]. Table 1 compares the currently incurred costs and forecasts for the different methods of H₂ production.

An analysis of the numerical data indicates that by 2050, H₂ will only be obtained using green methods and methods of H₂ production from other sources will not be developed. As for the cost-effectiveness relationship, the efficiency currently achieved by technological means oscillates, on average, within 50–70%, with the maximum efficiency of green methods predicted to be around 80%, while reducing costs per unit of capacity.

In Europe, there are two large organizations that gather entities interested in H₂-related issues. The first and largest one is Hydrogen Europe, which has 250 members, including 9 Polish ones—for comparison, Germany has 37 and France 33. The second organization is the European Clean Hydrogen Alliance, founded by 30 EU members, which currently includes PGNiG, Orlen and Polenergia.

Referring to the Web of Science for the period of 2017–2020, there were about 1000 papers related to “hydrogen”, of which 83 were related to its use as fuel. As for patent applications, 79 were filed by 2020 (6th place in EU countries). In terms of expenditures, Poland spends 3 ppm of the national GDP on the research and development of H₂ technologies. Among the leading European countries are Denmark—35 ppm; Switzerland—37; and Estonia—32 [84].

Yagüe et al. [85] investigated the production of biohydrogen using the steam reforming of biomethane with the simultaneous capture and storage of CO₂. In this way, it is possible to achieve negative CO₂ emissions. This solution is called the production of so-called golden H₂. However, this technology also has its weaknesses. Although technologies for capturing CO₂ and storing it are already very advanced and ready for implementation, there is currently no effective infrastructure for transporting CO₂ to geological storage sites.

Susmozas et al. [86] studied technologies for the production of biohydrogen from by-products such as glycerol as a by-product from biodiesel production and bio-oil from the fast pyrolysis of poplar biomass. As a result of the process simulation carried out using Aspen Plus^® software V14 and the LCA life cycle analysis, the results were obtained that, from the point of view of efficiency and environmental aspects, bio-oil from the pyrolysis of poplar biomass is a more suitable raw material for steam reforming than bio-oil from rapeseed, which is waste from biodiesel production.

Panagiotopoulos and others [87] analyzed the possibility of producing biohydrogen under EU conditions using products from sugar beet cultivation and processing. The analyzed raw materials were sugar beet juice and beet pulp. The efficiency of biohydrogen production from these substrates was tested in relation to dark fermentation technology. Sugar beet has a relatively large potential for biohydrogen production due to its large production and availability in the EU combined with the decreasing demand for the sugar production process. Research has shown that no pre-treatment is required for the production of biohydrogen from sugar beets and their products. The estimated potential of the annual production from sugar beets in the EU was estimated by the authors as 300,106 kg of biohydrogen.

According to the JRC report [88] on the state of the development of hydrogen technology, the market for renewable H₂ and H₂O electrolysis continued to grow in 2022 compared to 2021, both in Europe and around the world. Estimates show that in Europe—including the EU, EFTA countries and the UK—the total installed electrolysis capacity increased from 85 MW in 2019 to 162 MW (expressed in electricity consumption) in August 2022. Short-term estimates indicate that by the end of 2023, the capacity will reach at least 191 MW and optimistically 500 MW. By the end of 2025, a 1371 MW electrolysis capacity is planned to be launched in Europe. However, despite clear signs of growth, which is expected to accelerate in 2023, according to the JRC report, the volume of renewable hydrogen currently produced compared to hydrogen obtained from fossil fuels is still negligible (0.2%). Outside Europe, estimates of the globally installed electrolysis capacity were in the range of 600–700 MW at the end of 2022. The latest available information indicates that the global capacity will reach 2 GW by the end of 2023. Cumulative deployments in Europe are accelerating and deployment plans are increasing year on year. The EU relies strongly on a regulatory framework for financing and financial support schemes, and European companies have a strong position as international patent holders. When it comes to high-value inventions, the EU continues to lead (31% of the total share) alongside Japan. Europe actively participates in research and development activities across the continent and, together with China and the United States, has a world-leading record in scientific publications. Among the anticipated applications, renewable H₂ could play a significant role in the decarbonization of industrial processes such as oil refining, steel and cement production, ammonia and fertilizers, or could be used as a fuel for heavy and long-distance transport (including e-fuel solutions in aviation and maritime transport). Finally, it can be used to support energy storage systems, especially in seasonal applications.

The EU targets set out in the RePower Plan [89] and hydrogen strategy [90] envisage the EU domestic production of 10 Mt of renewable H₂ and the import of the same number of Mt (of which 4 Mt will be in the form of ammonia). If 10 Mt of renewable H₂ was to be produced solely by water electrolysis, the European hydrogen industry estimates that 140 GW of the installed electrolysis capacity would be needed by 2030. Europe is heavily dependent on imported raw materials, but its global share of processed raw materials and components is gradually increasing, reaching a significant fraction when the final products are taken into account. The production of electrolyzers requires over 40 raw materials and 60 processed materials. The main suppliers of raw materials for electrolyzers are China (37%), South Africa (11%) and Russia (7%), while the EU’s share is only 2%. The other main perceived challenges will be contracting and securing the significant demand for renewable H₂ and ensuring an adequate supply of renewable electricity, which could be a key bottleneck to achieving Europe’s strategic renewable hydrogen production targets.

1.4. The Status of Biohydrogen in Poland

Transferring these considerations to the situation in Poland, the first fact worth noting is the high concentration of the energy sector around state-owned companies (PGNiG, Lotos, Orlen) and, to a lesser extent, in private initiatives. Investments in energy and innovation require significant financial resources, which seems to be a significant obstacle for smaller and medium-sized private initiatives [84]. According to the guidelines formulated by the European Commission, domestic production based on electrolysis by 2030 would amount to 40 GW of energy [91]. In 2020, the Polish authorities considered it appropriate to place greater emphasis on this sector of the economy [92].

Among the significant domestic entities with experience in H₂ technologies, we can distinguish [84] the following institutions, along with their areas of participation in this segment:

-

Jastrzębska Spółka Węglowa (JSW)—the separation of H₂ from coke oven gas;

-

Lotos, Polskie Sieci Elektroenergetyczne—Polish Power Grids (PSE)—the use of electrolyzers with electricity from renewable energy sources (RES);

-

Sescom sales—electrolyzers powered by PV;

-

Grup Azoty—scaling its own production of “gray” H₂ for sale;

-

Polenergia—production and use of “green” H₂—cogeneration converted to burn H₂;

-

RB Consulting—the distribution of electrolyzers;

-

Pątnów Adamów Konin Power Plant Complex (ZE PAK)—the use of electrolyzers with electricity from biomass;

-

Orlen—the use of electrolyzers with electricity from RES;

-

Tauron Wytwarzanie—the production of SNG (synthetic natural gas): H₂ from electrolysis with electricity from RES and CO₂ from emission installations;

-

Wałbrzyskie Zakłady Koksownicze “Victoria”—the separation of H₂ from coke oven gas;

-

Stalprodukt—steam CH₄ reforming.

In Poland, three groups of entities show interest in H₂ technology research. The first group is the sector of large and/or currently effective use of H₂ in their activities (mentioned above). The second group consists of research institutions, including the Institute of Energy, the Polish Academy of Sciences with its subordinate units, the State Research Institute, the Central Mining Institute, the Institute of Chemical Processing of Coal and the Institute of Oil and Gas. In 2019, the National Development Fund was established, consisting of the following elements: the Polish Development Fund (PFR) (strategic center), the Industrial Development Agency, Bank Gospodarstwa Krajowego (BGK), the Export Credit Insurance Corporation, the Polish Investment and Trade Agency, the Polish Agency for Enterprise Development and entities subordinate to the aforementioned consortium. The aim of this consortium is to use H₂ fuel in the field of heating and transport. Other agencies interested in H₂ topics are: the National Center for Research and Development, the National Science Center and the National Fund for Environmental Protection and Water Management.

The third identified stakeholder group consists of organizations with a mixed structure of shareholders, often referred to as “networks”. Such a network brings together local or national entities interested in the development of H₂ fuel procurement, storage and distribution. Networks with a local scope of activity include, for example, the Gdansk Hydrogen Cluster or the Wieluń Energy Cluster.

In addition to the above-mentioned environment, it is worth mentioning the impact of legislation and recommendations from the European Union, its strategy and policies outlined by the community. The guiding principle of EU policy is, as mentioned earlier, decarbonization. This model assumes a focus on the production of “green” H₂, using gray H₂ as an intermediate stage. H₂ is to be a component of the electricity grid, as a convenient energy storage unit and as a fuel for transportation in a broad sense [85].

H₂ has become one of the determinants of the new climate policy, the “New Green Deal” [93], and emerging aspirations to use only green energy. H₂ actually solves the basic problems of the energy sector as it is:

(1)

Potentially an excellent energy carrier (Power to Gas—P2G);

(2)

A practical energy carrier in the circular economy (CE).

The assumptions of H₂ production are in line with, i.a., strategies such as:

(1)

The Energy Policy of Poland until 2040 (specifically, objective 4. Development of energy markets (development of electromobility and alternative fuels));

(2)

The Polish Hydrogen Strategy to 2030 with a perspective to 2040 (development of 32 H₂ refueling stations and the use of RES for H₂ production based on electrolysis (the capacity of electrolyzers will reach 2 GW in 2030));

(3)

The Plan for the Development of Electromobility in Poland “Energy for the Future” (improvement of energy security, improvement of air quality and creation of conditions for the development of the electromobility of Poles);

(4)

The UN Sustainable Development Goals (Goal 13 Climate Action (reducing CO₂ emissions and reducing global warming));

(5)

The direction of the Łukasiewicz Research Network: A sustainable economy and energy. The subjects of research projects include the clean and efficient manufacturing, transmission and storage of energy and the effective use of surplus energy from RES, energy from waste and alternative fuels (in particular, H₂ technologies).

Poland is one of the leading producers of H₂ in Europe. Polish H₂ production accounts for about 10% of this gas consumption in Europe. Currently used technological processes are mainly based on CH₄ steam reforming. H₂ consumption is mainly related to ammonia production, oil refining processes and methanol production. It is expected that the value of the H₂ technology market in the world may amount to PLN 600 billion by the end of 2022, which is a 35% increase compared to 2015. For the H₂ R&D sector, since 2002, funds have already been allocated in the amount of over PLN 37 billion. The United States is investing the most in this direction, followed by Japan and France [94]. The most important sectors in which H₂ technologies would find potential application are steel processing, so-called natural gas blending and public and private transport. Currently, the steelmaking process contributes to significant CO₂ emissions due to the use of alkaline O₂ and shale furnaces. In this case, the use of HDR (Hydrogen Direct Reduction) technology can significantly reduce CO₂ emissions into the atmosphere. Particular attention should be paid to the fact that the use of H₂ in the economy involves the use of costly methods of its storage and transport. The solution may be the use of the existing gas infrastructure, in which natural gas can be combined with H₂ (blending). This process not only means the use of H₂ mixed with natural gas, but also enables its recovery at distribution points through, for example, membrane separation. According to the Polish Hydrogen Strategy, by 2030, H₂ will gradually become one of the key energy carriers used in the European Union. The implementation of H₂ technologies in the energy sector involves the launch of a P2G installation based on Polish technologies and the start of using H₂ as an energy storage. In addition, it is planned to use H₂ as an alternative fuel in transport. For this purpose, it is proposed, among other ideas, to allow 500 H₂-powered buses to operate by 2025, to develop 32 H₂ refueling stations and use RES for H₂ production based on electrolysis (the capacity of electrolyzers will reach 2 GW in 2030).

Work on the use of H₂ as an energy source is currently the most intensive in the automotive industry. Initially, the focus was on the construction of engines that would burn H₂, i.e., it would replace the H₂ fuels used so far.

H₂ is seen as the fuel of the future. Interest in H₂ as an energy source has grown rapidly over the last two decades and the main reason for this is the desire to reduce greenhouse gas emissions. H₂ fuel is defined as emission-free because during combustion, practically the only by-product is H₂O (water vapor) and, in trace amounts, nitrogen oxides, the amounts of which are much smaller than in the case of the combustion of conventional fuels. Therefore, H₂, due to its chemical and physical properties, can be an alternative to fossil fuels. In the light of the EU policy and legal regulations being introduced, the gradual elimination of high-emission fuels, including gas, will continue and force the introduction of low-emission fuels and eventually, where possible, zero-emission fuels, such as H₂. This is assumed in the development strategies until 2050, adopted in the EU, including Poland. The European Green Deal is the EU’s new economic development strategy. Its goal is a deep pro-ecological reconstruction of the EU economy, which is to become the first climate-neutral area within three decades.

Comparing the position of the Polish environment in relation to the European surroundings, one should focus on the amount of investment in research and the development of infrastructure and production for commercial and government entities, as well as the amount of research contributions to the overall state of knowledge and know-how, in the form of patent applications, publications, implementation projects, citation rates and the overall volume of information publicly released by research and development entities.

2. Results and Discussion

The main reactions responsible for the processes of H₂ formation include the oxidation of substrates, which results in the release of electrons. These, in turn, reduce the free (formally, actually hydrated) protons present in the reaction environments, creating molecular H₂. These reactions are catalyzed by enzymes from the family of enzymes known as hydrogenases [95,96].

They are organometallic compounds. They are usually classified into three groups (Figure 9), depending on the specificity of the active site: [FeFe] (iron–iron), [NiFe] (nickel–iron) and [Fe] (iron) [95,96,97,98,99,100].

The schematic structure of hydrogenase [FeFe] is shown in Figure 9a. Figure 9b shows the schematic structure of hydrogenase [NiFe]. Figure 9c shows a simplified structure of hydrogenase [Fe]. Many hydrogenases belonging to the [FeFe] group exist as a single subunit and contain a catalytically active metal center, although some enzymes consist of several subunits (up to heterotetramers). The model structure of [FeFe] consists of a metal cluster center of diiron with a disulfide bridgehead and the metals are bridge-coordinated by a carbonyl ligand (CO) [95,96,99]. In real crystal structures, these diatomic groups are adjacent to amino acid residues, stabilizing spatial structures through H₂ bonds [95,97,100,101].

A single [NiFe] cluster, which is an enzyme subunit, is considered to be the catalytic (active) center of the enzyme (Figure 9b).

This structure is very similar in practically all enzymes of the discussed group. Four sulfur atoms of cysteine moieties are coordinated with Ni, while two form bridges with the Fe ion. The other two cysteine moieties are terminally bound to nickel. The low-spin Fe(II) ion binds with three diatomic ligands and two cysteine thiolate bridges to form an octahedral complex. The presence of CO and CN⁻ moieties was first demonstrated by spectroscopic measurements in the infrared range (FT-IR). Cyanides are strong σ-donor ligands and can form H₂ bonds with amino acids of active sites of hydrogenases [NiFe]. The ligand CO does not show a similar tendency to form strong H₂ bonds. The third coordination bridge is formed between Ni and Fe atoms and its presence is determined by the redox potential of the enzyme. During activation, inactivation or inhibition and during the course of the catalytic process, the enzyme goes through several intermediate states. The study of the geometry of complex biochemical systems is an ambitious scientific challenge involving advanced methods of computational chemistry [95,96,97,98,99,100,101].

The potential of the biohydrogen produced is the higher carbohydrate content in the biomass. The efficiency of the process also depends on the microorganisms used and their type determines the temperature of the process. The microorganisms used can be mesophilic (25–40 °C), thermophilic (40–65 °C), extremely thermophilic (65–80 °C) or even hyperthermophilic (>80 °C). The choice of a particular culture or mixture of microorganisms depends on the specific requirements of the particular installation. There is a wide range of microorganisms that can be used for dark fermentation. For example, these can be Clostridia, methylotrophs, rumen bacteria, E. coli, Enterobacter, Citrobacter, Alcaligenes or Bacillus [102,103,104].

Obtaining H₂ by biotechnological methods is a promising alternative to electrochemical methods. The use of algae for this purpose, compared to other biological methods, brings many advantages: a high growth rate and low environmental requirements [3,4] and burdens [3,4,5,6,105]. In turn, the use of cyanobacteria [3,105,106,107] and photosynthetic bacteria [3,106,107] simultaneously contributes to a decrease in the CO₂ content in the atmosphere and also allows the use of waste H₂O as a substrate [3,4,107].

Table 2. Biohydrogen production technologies [3,4,5,6,105,106,107].

Technology	Advantages	Disadvantages
photofermentation	Use of a wide spectral range Use of different substrates Supporting wastewater treatment based on dark fermentation Bioremediation potential	Low conversion yield (quantum efficiency) Presence of organic by-products
dark fermentation	Numerous intermediates (metabolites) can act as secondary substrates Use of different substrates No need to supply light energy Bioremediation potential	Low therodynamic efficiency of hydrogen production Oxygen generation
direct biophotolysis	Use of solar energy and water Simplicity of implementation Impact on reducing the carbon dioxide content	High-intensity light and oxygen reduce conversion efficiency Inability to separate streams of hydrogen and oxygen of sufficiently high purity
indirect biophotolysis	Use of algae Separation of hydrogen and oxygen streams Use of atmospheric nitrogen Metabolite conversion	Carbon dioxide formation Relatively low efficiency of hydrogen generation
microbial electrolysis	Pollution-free technology Bioremediation potential Potential to reduce chemical oxygen demand Supporting wastewater treatment based on dark fermentation High hydrogen generation efficiency High efficiency of substrate processing	Use of expensive photovoltaic sets Obtaining hydrogen of moderate purity Sensitivity of the installation to scale effects and power supply stability

contains a summary of the advantages and disadvantages of the basic H₂ generation technologies.

From the point of view of profitability, it is important to obtain H₂ from H₂O containing dissolved components (e.g., sea water) which are not necessary to remove before decomposition. This paper [108] presents the results of the research on the use of amorphous carbon nitride as a photocatalyst. As a result of model tests, the operating conditions of the analyzed material were known, including the permissible salinity and the concentration of substances protecting the photocatalyst. The H₂ release rate resulting from artificial seawater splitting was well predicted by second-order polynomial regression (Box–Behnken method). Catalyst loading increases the release rate by providing a larger contact surface area, but can also lead to light shielding when excess light is present. A higher concentration of triethanolamine (sacrificial reagent) improves the reaction rate by increasing the rate of its adsorption onto carbon nitride. This effect may also adversely affect the process rate due to the corresponding lower rate of water adsorption onto the photocatalyst. Moreover, sea salt ions could beneficially improve the hydrogen evolution efficiency ratio, promoting the adsorption of triethanolamine and facilitating charge separation. At the same time, there is a risk that an excess of sea salt ions may also adversely affect the reaction rate due to the reduction of surface-active sites resulting from the undesirable precipitation of insoluble hydroxosol. Carbon nitride shows high photostability (75.1% within 18 h).

Research on the use of titanium dioxide (TiO₂) deposited on graphene in the photocatalysis process [109] has shown that the phenomenon of the recombination of photogenerated electron holes can be easily minimized, which translates into the improved photocatalytic efficiency of the synthesized composites. The band structure of the composite was analyzed by valence band XPS, revealing the reason for the high catalytic performance of the composite under visible light. The results showed that the synthesized TiO₂/graphene composites have attractive potential for applications in environmental and energy issues.

The work [110] was devoted to the use of oxide photocatalysts in a broader sense. Metal oxide materials can play a significant role in the photocatalytic production of H₂ from H₂O and sunlight, which is a promising direction in the transition to renewable and emission-free energy sources. Due to features such as abundance, a large band gap, stability in aqueous media, low cost, ability to change optoelectronic properties over a wide range and ease of obtaining them, oxide semiconductors are considered promising materials for photocatalytic water splitting and good candidates for long-term production H₂.

However, an optimal oxide photocatalyst with the properties necessary for effective water decomposition can only be obtained by modifying the catalyst. Combining various modification routes allows the formulation of possible ways to synthesize a semiconductor material with the properties required for the most effective light absorption in a broad spectrum, an efficient charge transfer and a reduced possibility of charge carrier recombination, high corrosion resistance and photochemical stability, as well as low cost.

The defect of most oxide semiconductors resulting from the improper level of the valence and conduction band edges can be solved by doping with metal or non-metal ions or combining with another semiconductor to obtain an optimal band structure and expand the light absorption area. The most optimal photoelectrode material, characterized by the high photocatalytic water splitting efficiency, can be found using complex ternary and ternary metal oxide semiconductor materials. The formation of a solid solution consisting of wide- and narrow-band-gap oxide and non-oxide semiconductors is a less suitable method for optimizing the photocatalyst band structure compared to the formation of heterojunctions that combine the properties of different components to increase the overall H₂O splitting efficiency.

It is necessary to find a compromise between increasing the efficiency of photon absorption and the not-too-high recombination of photoexcited charge carriers. The performance limitations of oxide-semiconductor photocatalytic materials due to the rapid recombination of electrons and holes can be solved by modifying the crystal structure and morphology of the photocatalyst. The recombination of donor–acceptor pairs can be significantly reduced by a combination of annealing and H₂ treatment. The morphology and crystallinity of oxide materials can be monitored and adjusted, most importantly, during their formation. The greater the defectiveness of the material, the higher the efficiency of the H₂O splitting reaction.

Nanostructuring of an oxide semiconductor allows the creation of a material with a reduced recombination rate and a relatively high redox cleavage reaction rate. The negative impact of accelerated charge recombination due to the developed transport network in nanoparticles, nanowires, nanotubes, nanorods, nanosheets or hierarchical nanostructures can be offset by heterojunctions due to their synergistic effect on visible light irradiation. The prospects of nanostructuring metal oxide materials to improve photocatalytic water decomposition were determined by comparing them in terms of performance; it is necessary to analyze the combined effect of several different modification methods and identify the multiplicative effect.

Visible absorption can be improved by sensitizing metal oxides with quantum dots, noticeably increasing the photocurrent density. Sequential chemical/electrochemical deposition at the quantum dot/metal oxide interface is a more promising way to expand the light absorption spectrum and improve carrier charge separation.

Various metals and other materials containing metals with variable valency states can be used as cocatalysts, ensuring the creation of active sites for hydrogen evolution and preventing the recombination of the photoexcited charge carrier. The main problem is to find more efficient cocatalysts to oxidize water molecules, effectively lowering the overvoltage of photoelectrochemical water splitting and increasing the photocurrent density, because this reaction dominates in terms of activation energy in the entire H₂O splitting reaction. An additional possibility of increasing the efficiency of oxide materials in H₂ production involves the appropriate selection of a sacrificial agent. To maximize the efficiency of photocatalytic H₂O decomposition, a combination of known strategies is recommended.

Table 3. Biotechnological methods for obtaining hydrogen [3,4,5,6,105,106,107].

Microorganism	Strain	Benefits	Limitations
green algae	Chlamydomonas reinhardii Chlamydomonas moewusii Chlorella vulgaris	Many times greater efficiency of photosynthesis compared to trees Efficient removal of carbon dioxide compared to traditional agricultural crops	Inhibiting effect of oxygen on hydrogen generation
cyanobacteria	Anabaena variabilis Cyanothece sp. Synechocystis PCC6803 Anabaena sp. PCC7120	Use of solar energy and water Impact on reducing the carbon dioxide content	Inhibiting effect of oxygen on hydrogen generation Use of hydrogen as a secondary substrate
photosynthetic bacteria	Rhodobacter capsulatus Rhodobacter sulidophilus Thiocapsa roseopersicina	Use of different substrates Use of a wide spectral range	Use of light Water pollution
fermentative bacteria	Enterobacter aerogenes Clostridium butyricum Magashaera elsdenii	Hydrogen generation under anaerobic conditions A wide range of cellulosic raw materials Obtaining valuable by-products as metabolites	The need to use carbon and nitrogen sources Water pollution Carbon dioxide generation

lists the characteristics of biotechnological methods for obtaining H₂.

Progress in research and development shows that biohydrogen produced from algae can be used as a clean energy in the future. On the other hand, it will involve significant economic complications, even over decades. It is predicted that the turning point may be reached through genetic testing [3].

Fermentation processes are more environmentally friendly, but are characterized by low sunlight conversion efficiency and biohydrogen efficiency. Biophotolysis methods also have low biohydrogen yields (<15%) and pose several challenges in terms of oxygen sensitivity and sunlight availability. Nevertheless, direct biophotolysis provides yields exceeding 80%. Electrochemical processes were found to be significantly energy-intensive and involve high requirements for pressure, temperature, power and advanced equipment, but produce 90% biohydrogen. The current cost of biohydrogen production is quite high, ranging from USD 10 to USD 20 per GJ, depending on the microalgae cultivation system, reactor design and metabolic pathway used. To compete with gasoline, this cost must be less than USD 0.33 per GJ. On the other hand, most economic evaluations are quite optimistic, but do not take into account important cost factors such as storage, handling and transportation [4].

Existing technologies offer potential for practical applications, but if biohydrogen systems are to become commercially competitive, they must be able to synthesize H₂ at a rate sufficient to power fuel cells large enough to perform practical work. Further research and development is necessary to increase the rate of synthesis and the final yield of H₂ [5,6,106].

Biomass conversion is a promising technology that has great potential to replace the use of fossil fuels. Various parameters such as feedstock type, operating conditions (pressure and temperature), residence time, bed material, etc., required for biofuel production using Fischer–Tropsch synthesis are discussed. The quality of biomass synthesis gas is strongly dependent on these factors; however, under some of the conditions discussed, biomass gasification cannot overcome the high costs of energy, capital expenditure and total product costs. Therefore, the scalability of biomass energy to large areas remains a challenging task in the field of renewable energy. SCWG is an excellent technology that can be cost-effective compared to various other gas generators, but is characterized by non-clogging, efficiency, corrosion and high hydrogen production costs.

Future research, including process optimization, reactor selection, low-cost catalyst preparation and the better selection of the gasification medium and technology, can exponentially advance the biomass era. Nickel-based catalysts have been extensively studied in recent years due to their economic viability and activity. However, they are easily poisoned and require improvement. Then come the transition metals and rare earth metals, which are doped in nickel-based catalysts to improve the performance of conventional catalysts. Therefore, new catalysts that increase the selectivity, activity and productivity of biomass gasification have not yet been discovered [105].

The results of a new biomass gasification simulation, which was confirmed by experimental data, for two sensitivity analyses for five different gasifying agents, such as air, O₂, steam, hydrogen peroxide (H₂O₂) and CO₂, show that:

(1)

The mass percentage of H₂, the H₂/CO molar ratio and the hydrogen yield were the highest in the steam gasification process. The yield of CH₄, LHV and CO was the highest in the CO₂ gasification process. The percentage of CO₂ was the highest in oxygen gasification and the degree of gas production was the highest in air gasification.

(2)

As the modified equivalence ratio (MER) increased, the hydrogen mass percentage, carbon dioxide mass percentage, H₂/CO molar ratio, hydrogen yield, and gas production increased. However, the mass percentage yield of CH₄, LHV and CO (except CO₂ gasification) decreased.

(3)

For all gasifiers, the mass percentage of H₂ and the mass percentage of CO increased with the increasing free space temperature, but the mass percentage of CO₂ and the mass percentage of CH₄ decreased.

(4)

As the temperature increased, the LHV of the synthesis gas decreased, but the H₂ yield, CO yield and gas production rate increased for all gasifying agents. As the free space temperature increased, the H₂/CO molar ratio decreased in the case of steam and H₂O₂ gasification, but increased in the case of O₂, air and CO₂ gasification [107].

Taking into account the selected studies on biowaste-derived biohydrogen production,

Table 4. Comparison of selected studies on biowaste hydrogen production [own study].

Source	Biogas Production Potential	Hydrogen Production Potential	Technology	Ref.
Food waste	Biogas 2.446 Nm3/d	H2 1.0 Nm3/d	A two-stage fermentation process for hydrogen/methane production	[111]
Mixture of food waste, cattle manure, potato pulp and pig manure		H2 21.0 mL/g VS	Multi-component system, laboratory scale	[112]
Pig manure (pm), coffee mucilage (cfm) and cocoa mucilage	91.85 mL H2/g VS,	4.367 mL H2	The pilot plant was operated under mesophilic conditions	[113]
Coffee mucilage Cocoa waste Pig manure	Coffee 2.80 Cocoa waste 4.88 Pig manure 3.30 L/Lsolution	Coffee 2.12 L/Lsolution Cocoa 0.07 L/Lsolution Pig manure 0.48 L/Lsolution		[114]
Coffee drink manufacturing wastewater	1.29 mol H2/mol hexose added	0.07 L H2//L/H	Up-flow anaerobic sludge blanket reactor	[115]
Food waste	H2 and 0.31 m3/kg· VSadded CH4 and 0.21 m3/kg· VSadded	H2 3.63 m3/m3·day CH4 1.75 m3/m3·day		[116]
Fruit–vegetable waste with swine manure ratio of 35/65	126 mL H2 g−1VS-added	3.27 L H2 L−1 d−1	Anaerobic fermentation	[117]
Organic waste: fruits (F), vegetables (V), meat–fish–cheese (MFC), bread–pasta (BP) and rejected materials	142 mL CH4/gVS/d	232 mLH2/gVS/d using only carbohydrates	Anaerobic digestion plant: Batch (35 °C, pH 5.5)	[118]
Organic waste: meat–fish–cheese (MFC), fruits (F), vegetable (V), bread–pasta (BP). The fraction MFC was composed of raw chicken breast, tuna chunks in brine and butter; the fraction F was composed of apple–banana mousse; the fraction V was composed of lyophilized minestrone soup; the fraction BP was composed of bread crumbs and raw pasta	244 mL/gVS	129 mL H2/gVS	Dark fermentation batch tests were carried out in 1-litre batch reactors under mesophilic conditions (35 ± 1 °C).	[119]
Rice Potato Lettuce		134 H2 mL/g-VS 106 H2 mL/g-VS 50 H2 mL/g-VS	Batch: (37 °C, pH 5.5)	[120]
Food waste; Pre-treatment: 90 °C, 20 min		148.7 H2 mL/g-VS	Batch (35 °C, pH 7.0)	[121]
Food waste (Cafeteria); pre-treatment acid: 12 h, pH 2		158 H2 mL/g-VS	Batch (37 °C, pH 8.0)	[122]
Food waste (cafeteria); pre-treatment alkaline: 6 h, pH 12		162 H2 mL/g-VS	Batch 37 °C, pH 6.0	[123]
Food waste; pre-treatment: ultrasonic, 30 min		140 H2 mL/g-VS	Batch (30 °C)	[124]
Food waste; pre-treatment: Autoclaving (121 °C, 15 min)		38.6 H2 mL/g-VS	Batch (35 °C)	[125]
Food waste (Cafeteria)	Biogas production rate 62.5 L/day with OLR (125.4 kg-COD/m3/day)	111.11 H2 mL/gVS added	Membrane bioreactor MBR, working volume 5 L 55 °C, pH 5.5	[126]
Food waste		80.9 H2 mL/gVS added	Anaerobic sequencing batch reactors (ASBR), working volume 4.5 L, 35 °C	[127]
Food waste		0.54 mol H2/mol hextose	Anaerobic sequencing batch reactors (ASBR), working volume 150 L, HRT 36 h, pH 5.3, 35 °C	[128]
Food waste		66.7 H2 L/kgVS	Stirred tank reactors (CSTR) with a working volume of 0.2 m3. pH 4.7, 55 °C, HRT 3.3 d	[129]
Fruit and vegetable unsold stock		240 L H2 containing H2 (49%)	Dark anaerobic fermentation in a pilot-scale reactor (V: 35 L)	[130]
Food waste		0.065 H2 m3/kgVS	Rotating drum: pH 5.2–5.8, 40 °C, SRT 160 h	[131]
Food waste; Heat pretreatment at 70 °C for 60 min	ASBR, working volume 12 L Produced CH4: 0.92 m3/m3 d	H2: 1.76 m3/m3 d	ASBR, working volume 3.6 L. 35 °C	[132]
Food waste	CSTR, working volume 40 L, HRT 5 d, 35 °C, pH 7.5 Produced CH4: 464 mL/gVS added	Produced H2: 205 mL/gVS added	CSTR, working volume 10 L, HRT 1.3 d, 55 °C, pH 5.5	[133]

, particular attention should be paid to the potential amount of “green H₂”.

Human economy leads to the creation of a series of intermediate and direct waste. Currently, research is being conducted on the management of waste from animal production, food processing and the HoReCa sector (Hotels Restaurants Catering). The publications compiled in Table 4 indicate a large range of biogas and H₂ yield depending on the analyzed substrate and the adopted processing method. Without deciding which study or substrate and strategy is the most promising in the future, we should refer to biogas itself. Biogas is a mixture of chain hydrocarbons and the production of biogas would be based on the fermentation or potentially photo-fermentation of organic matter into biogas and then the detachment of an H₂ molecule from the hydrocarbon chain or the direct biological conversion of organic matter into H₂ gas.

Among the substrates used, we can distinguish all kinds of biomass, both by-products from crops, biomass from energy crops and lignocellulosic waste and waste H₂O [102]. Among the plants grown for energy purposes, a significant production of biohydrogen can be obtained from such substrates as: sorghum stalks (3.15 mol H₂/mol glucose) [134], giant miscanthus after treatment with NaOH and Ca(OH)₂ (3.4 mol H₂/mol glucose) [135]. In the case of lignocellulosic waste, the literature data are available for such substrates as: wood fibers, corn cores, corn juice, wheat straw, corn leaves and pomace. In the case of these types of substrates, pre-treatment in the form of mechanical grinding was used and, in the case of pomace, alkali–thermal treatment with NaOH. A high yield of biohydrogen was obtained from corn juice and 69.4 dm³ H₂/kg of dry matter [136]. Pomace was also characterized by a relatively high yield of 13.39 moles of H₂/kg of dry matter [137]. In the case of other substrates, the amount of biohydrogen obtained was as follows: for wood fibers, 1.47 mol H₂/mol glucose [138], for corn cores, 3 mol H₂/mol glucose [139], for wheat straw, 3.8 mol H₂/mol glucose [140] and for corn leaves, 3.6 mol H₂/mol glucose [140].

Considering the above, green H₂ can be obtained during the H₂ fermentation process used for agricultural and food biomass, but in order to achieve this, previous research on polydisperse substrates occurring in the agricultural industry should be taken into account [141]. A commonly used technology for slurry utilization [142,143] (polydisperse substrate) is to subject it to the CH₄ fermentation process carried out in agricultural biogas plants. This technology is associated with high investment costs [144]. Additionally, the large volume of this substrate and the low content of organic matter make it difficult to raise the temperature of this process [145]. Carrying out intensive animal production on a small area favors the formation of excessive amounts of slurry. Slurry is commonly used as a fertilizer due to its low investment costs. Improper use leads to soil and H₂O contamination. Odor and greenhouse gases are also emitted [146]. CH₄ fermentation is facilitated by many properties of the leaven, such as the content of basic macro- and microelements supporting the development of bacterial microflora or the presence of anaerobic microorganisms. Based on the literature research, it was found that a big problem in agriculture is the management of pig manure so that it does not interfere with the natural environment.

In Poland, there was a rapid development of bedding-free industrial cattle and pig farms [147]. Large industrial farms (large-industrial, large-scale) are defined as installations requiring an integrated permit and the basic criterion determining the size of a farm is their staffing. A large factory farm is considered to be a farm with a stock of over 40,000 individual animals and for pigs it is two thousand pigs (fattening pigs) weighing more than 30 kg and/or 750 sows [148]. In 2008, the Helsinki Commission HELCOM [149] recognized large-scale farms as point sources of agricultural pollution. At the same time, cattle farms with a stocking capacity corresponding to 400 AU Animal Units were also considered industrial farms. The most unfavorable, from the point of view of environmental protection, is breeding in a bedding-free system, which is associated with the formation of huge amounts of liquid manure [150]. Comparing manure (generated during barn farming) and slurry (leachate from manure), slurry poses a number of problems related primarily to its storage, transport and further use. The main environmental threats resulting from large-scale animal husbandry and the related production of slurry include [147]:

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H₂O pollution, excessive soil fertilization and runoff of H₂O from fields to the ground and surface;

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Eutrophication, excessive fertilization of inland and marine H₂O (algal blooms, reduction of biodiversity and modification of aquatic ecosystems, loss of bottom fauna);

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Microbiological contamination where pathogenic microorganisms contained in slurry pose a serious sanitary threat.

Referring to the above discussed aspects of assessing environmental harm, energy consumption and resource usage, the assessment of the environmental impact of the analyzed methods can pose certain difficulties with classification. While the production method of biogas and H₂ itself can be evaluated as low-emission and environmentally neutral, the assessment of the raw materials may not be unambiguous, for example, obtaining biogas and biowater from by-products in animal husbandry can be evaluated as low-emission and environmentally friendly, and when summarizing the impact on the natural environment of animal husbandry itself, it can reduce the overall assessment of the proposed methods.

The phenomena described in the above analysis indicate a strong tendency towards changes in the energy sector. The International Energy Agency, in its latest published report, estimated the global demand for H₂ in 2021 at 94 million tons, mostly obtained in industry and petrochemistry, and a return to the upward trend before the outbreak of the pandemic in 2019, when the demand was about 91 Mt. The agency predicts a demand of 115 Mt for 2030. At the same time, it indicates that to meet the climate commitments made by governments of individual countries, H₂ production should reach 130 Mt. However, to meet the net zero emissions targets by 2050, it should be close to 200 Mt [8,151].

Based on Web of Science data, the number of publications for H₂ in the “Energy Fuels” section changed from 163 in 1990 to 7625 in 2019. This shows a drastic change in scientific trends and a redirection of research from mining and energy disciplines to H₂ as a source of energy worth considering (and investing in) again. H₂ is indicated as an intermediate medium in renewable energy as a method of storing energy produced by this industry. It is also considered as an alternative to petroleum in mass and individual transportation. These directions converge with the policy of combating global warming, reducing CO₂ emissions and phasing out fossil hydrocarbons.

Conducting biohydrogen production processes under catalytic conditions, especially using solutions in the field of photochemistry and organometallic chemistry, leads to an increase in their efficiency.

Hydrogenases are widely distributed in microorganisms and are involved in various metabolic pathways, such as the CH₄ formation pathway, N₂ fixation in the co-regulation of nitrogenase and hydrogenase, the removal of toxic heavy metals and the virulence of pathogenic bacteria and parasites [152]. Unlike hydrolases involved in many different metabolic pathways, nitrogenases are responsible for converting dinitrogen into ammonia through the biological N₂ fixation process, providing a source of N₂ for microorganisms [153].

Molecular O₂ is an important factor regulating anaerobic hydrogen production. Both nitrogenases and hydrogenases are sensitive to it. Nitrogenases must be processed under anaerobic conditions and their oxygen inactivation mechanisms probably involve oxidative damage to metal clusters [154,155]. There are several O₂-tolerant hydrogens in nature. Natural O₂-tolerant [FeFe] hydrogenase was discovered in Clostridium bjerinckii SM10 (CbA5H) [156]. Hydrogenases from the [NiFe] group were obtained from Ralstonia eutropha, but they are characterized by low enzymatic activity compared to O₂-sensitive enzymes [157]. Other [NiFe]-type hydrogenases have also been found in Aquifex aeolicus, Escherichia coli and Desulfovibrio fructosovorans [158,159]. Hydrogenase [NiFe] obtained from Klebsiella oxytoca HP1 showed remarkable tolerance to O₂ and showed significant H₂ release activity under significant atmospheric O₂ saturation [160].

Another tactic to solve current challenges involves implementing immobilization technologies. Hydrogenases are characterized by a high catalytic conversion rate and low overpotential under mild conditions, which has potential applications in replacing Pt as an electrocatalyst in the development of hydrogen biofuel cells [161]. Immobilization technology can help make the discussed enzymes reusable, maintain its stability and catalytic activity on the electrode surface and improve the electron transfer efficiency [162].

Designing novel nanostructured electrodes to attach enzymes can facilitate the direct electron transfer between enzymes and solid supports, thereby reducing the need for enzymes as electronic supports and simplifying biotechnological applications such as biofuel cells and biosensors [163]. The covalent immobilization of [NiFe] hydrolases on SAM-modified gold surfaces makes the enzyme electrodes relatively stable, the electron transfer rate increases and redox mediators are not needed [164].

Nanomaterials can be used to increase the efficiency of the electron transfer. The study of the immobilization of Fd-HydA1 on black TiO₂ nanotubes (bTNT) showed that a direct electron transfer took place between black TiO₂ and Fd-HydA1 [165]. The effect of the molecular weight on the catalytic and electrochemical properties of hydrogen was studied by fixing truncated enzymes (Pf αδ and Pf α, containing only the αδ subunit and the α subunit, respectively) derived from the four-subunit (αβγδ) [NiFe] Hydrogenase Pf SHI into multi-walled carbon nanotubes (MWCNTs) and the results showed that Pf αδ with a shorter distance between the electrode and enzymes showed a higher electron transfer rate than Pf SHI [166].

An equally promising direction, as discussed above, is the research on synthetic enzymes. Research on the molecular structures and catalytic mechanisms of various enzymes involved in the production of biohydrogen inspired researchers to develop new catalysts in the form of artificial hydrogenases and to construct more stable and efficient catalytic systems for the production of gaseous H₂ [167].

Metal center substitution was used to produce the first artificial nickel-substituted rubredoxin (NiRd) hydrolase, with the structure of a mononuclear Ni ion coordinated by four cysteine residues [168]. In heme-binding proteins, the native iron cofactor protoporphyrin IX has been replaced by cobalt protoporphyrin IX (CoP). Moreover, the CoP-myoglobin system showed a strong tolerance to O₂ [169]. Cobaloxime catalysts and electron-transfer proteins with photoabsorbing properties, such as ferredoxins and apoflavodoxins, can effectively self-assemble and provide a photocatalytic ability for proton reduction.

Although the simple combustion of an H₂ molecule seems to be the ideal solution to the aforementioned problems, attention should be paid to the technology for producing the fuel itself. Currently, the dominant method is to obtain it from fossil sources for industrial purposes. To meet the hopes placed in H₂ as a “fuel” and “storage”, new technologies are needed, including renewable energy sources, biotechnology and agriculture. When considering the issue, it is essential to change the previous model of dividing H₂ production technologies based on colors. It turns out that, besides focusing on the raw material, a broader, holistic approach to technology is crucial, i.e., analyzing the total environmental costs associated with producing one unit of energy.

Regarding the Republic of Poland, the environment has a different specificity than Western partners. Large state-owned companies are a strong and influential entity dominating the environment. They have widely understood resources: infrastructure, know-how in a related industry, human resources, financing and patronage from public authorities. Further, in the commercial market space, there is an empty space in the segment of this inverted pyramid. There are potential small private initiatives, conceptual projects, startups, etc. Small and medium-sized entities try to reduce the distance in the race by organizing larger organizations such as clusters, industry associations and associations. They try to involve the scientific community, local government and potentially other entities showing good will to cooperaters (including large market players).

There is a discrepancy between the commercial and scientific research sectors. The research sector achieves productivity in grants, publications, patents, etc., similar to neighboring countries. State-level expenditures do not rank at the forefront in relation to the gross domestic product. The lack of cooperation between the scientific and commercial sides leads to the distancing of both. Due to weak funding, the scientific community is heading towards concepts which are not ready for implementation in business. The number of experimental, test and semi-industrial initiatives is limited. The business environment is cautious in financing experimental ideas and seeks to improve already developed methods, such as purifying H₂ obtained from “old” paths and investing in storage and transmission infrastructure, such as the idea of the “Hydrogen Valley”, rather than innovation and revolution.

3. Conclusions

By 2050, there will be a need to redesign global energy strategies. The economy based on fossil fuels is facing another resource revolution due to economic and ecological reasons. Responding to the predictions of international institutions, government and intergovernmental agencies have established a path to phase out fossil fuels by 2050. H₂, currently commercially used in the chemical and petrochemical industry, is mostly derived from fossil fuels. According to forecasts, from around 2030, the industry will start to move away from current physico-chemical methods of H₂ synthesis and the demand for it will decrease in outdated industrial solutions. An example is direct biophotolysis, in which several simple, key reactions can be distinguished, in the following order:

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H₂O photolysis;

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The transfer of electrons from photosystems to a protein called ferrodoxin, which is then a direct donor of electrons transferred to protons;

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Biohydrogen synthesis which is catalyzed by hydrogenase.

At the same time, the search for new energy sources will force the global economy to incorporate H₂ into the newly shaping energy mix. H₂ is seen as the next fuel for mass and individual transportation, a product from the utilization of agricultural waste and flexible energy storage for renewable sources. In parallel with the growing interest in H₂, new technologies and methods for producing H₂ with a low environmental impact will emerge.

The main suppliers of raw materials for electrolyzers are China (37%), South Africa (11%) and Russia (7%), while the EU’s share is only 2%. The other main perceived challenges will be contracting and securing the significant demand for renewable H₂ and ensuring an adequate supply of renewable electricity, which could be a key bottleneck to achieving Europe’s strategic renewable hydrogen production targets.

Currently, the most promising concept is the tandem of renewable energy and H₂ fuel cells, as well as the biological utilization of biological waste to biogas and further to H₂. With new challenges, the resources for H₂ fuel production and methods of assessing their impact on the natural environment will change. The type of resource and how it was created, the energy efficiency of the method and the by-products it will generate will all play a role in modern method evaluation.

Investments, the number of research projects, patents and concepts will increase exponentially year after year. The situation will become extremely dynamic and it is not excluded that the current predictions may accelerate or undergo changes whose character cannot be assessed at the moment. The H₂ sector will grow due to the requirements set by global economic policy and further analysis will require continuous monitoring and updating.