Main Hydrogen Production Processes: An Overview

Hierarchically Structured Catalysts Toward Sustainable Hydrogen Economy: Electro- and Thermo-Chemical Pathways

Hydrogen, as an important clean energy source, plays a more and more crucial role in decarbonizing the planet and meeting the global climate challenge due to its high energy density and zero-emission. The demand for sustainable hydrogen is increasing drastically worldwide as driven by the global shift towards low-carbon energy solutions. Thermochemical catalysis process dominates hydrogen production at scale given its relatively mature technology and commercialization status, as well as the established manufacturing infrastructure. While due to its environmentally friendly nature and growing abundant sources of renewable electricity, the electrochemical path for hydrogen production is rising as a major alternative to the thermochemical means. Nevertheless, hierarchically structured catalysts and devices have gradually taken the center stage toward replacing the traditional counterparts, especially with the rapid advancement of the design and manufacture of such ordered nanostructure assemblies toward high activity, efficient mass transport, and superb stability. In this review, the latest progress of the hierarchically structured catalysts for hydrogen production have been surveyed on electro- and thermo- chemical pathways comparatively. It covers the structure designs of atomic dispersion, nanoscale surfaces and interfaces for achieving highly active and durable catalysts, components, and devices. Both electrochemical and thermochemical approaches are reviewed in terms of the vast design details, engineered benefits, and understandings of various Pt-group metal (PGM) and non-PGM based transition metal catalysts for hydrogen production. As the growing trend, brief discussions are also presented toward the high-level assembly and manufacture of complexly structured components and devices at scale in the electrochemical and thermochemical energy systems.

Advances in Catalysts for Hydrogen Production: A Comprehensive Review of Materials and Mechanisms

This review explores the recent advancements in catalyst technology for hydrogen production, emphasizing the role of catalysts in efficient and sustainable hydrogen generation. This involves a comprehensive analysis of various catalyst materials, including noble metals, transition metals, carbon-based nanomaterials, and metal-organic frameworks, along with their mechanisms and performance outcomes. Major findings reveal that while noble metal catalysts, such as platinum and iridium, exhibit exceptional activity, their high cost and scarcity necessitate the exploration of alternative materials. Transition metal catalysts and single-atom catalysts have emerged as promising substitutes, demonstrating their potential for enhancing catalytic efficiency and stability. These findings underscore the importance of interdisciplinary approaches to catalyst design, which can lead to scalable and economically viable hydrogen production systems. The review concludes that ongoing research should focus on addressing challenges related to catalyst stability, scalability, and the integration of renewable energy sources, paving the way for a sustainable hydrogen economy. By fostering innovation in catalyst development, this work aims to contribute to the transition towards cleaner energy solutions and a more resilient energy future.

First-Principles Calculations and Machine Learning of Hydrogen Evolution Reaction Activity of Nonmetallic Doped β-Mo2C Support Pt Single-Atom Catalysts

The most widely used catalyst for the hydrogen evolution reaction (HER) is Pt, but the high cost and low abundance of Pt need to be urgently addressed. Single-atom catalysts (SACs) have been an effective means of improving the utilization of Pt atoms. In this work, we used a nonmetal (NM = B, N, O, F, Si, P, S, Cl, As, Se, Br, Te, and I) doped β-Mo2C (100) C-termination surface as the support, with Pt atoms dispersed on the support surface to construct Pt@NM-Mo2C. Using density functional theory (DFT) calculations, we selected catalysts with excellent HER activity. Among 117 candidate catalysts, 49 catalysts exhibited ideal catalytic performance with Gibbs free energy of hydrogen intermediate (H*) adsorption (ΔGH*) values less than 0.2 eV. The ΔGH* values of 16 catalysts were even lower than that of Pt (ΔGH* ≈ 0.09 eV), with PtI@N2/4-a-Mo2C demonstrating the best performance (ΔGH* = -0.01 eV). Combined with electronic structure analysis, we could understand the impact of charge transfer between Pt and the underlying NM atoms on the strength of the Pt-H bond, thereby promoting HER activity. Using machine learning (ML), we identified that the primary influencing factors of the HER catalytic activity in the Pt@NM-Mo2C system were the Bader charge transfer of Pt (NePt), the d-band center of Pt (εdPt), and the atomic radius of NM (RNM), with NePt having the greatest impact on the HER catalytic activity.

Ligand-induced hollow binary metal-organic framework derived Fe-doped cobalt-carbon nanomaterials for oxygen evolution

There is significant anticipation for high-efficiency and cost-effective non-precious metal-based catalysts to advance the industrial application of the anodic oxygen evolution reaction (OER) for hydrogen production. This study introduces an efficient strategy that utilizes ligand-induced metal-organic framework (MOF) building blocks for the synthesis of hollow binary zeolitic imidazolate frameworks 67 (ZIF-67) and Prussian blue analogues (PBAs) (ZIF-67@PBA) heterostructures through a hybrid MOF-on-MOF approach. Manipulating the Co2+/Zn2+ ratio in the precursor ZIF-67 allows for the convenient synthesis of the final product, denoted as CoxFe-ZP, after pyrolysis, where the inclusion of Zn effectively modulates the distribution of Co in the catalyst. The resulting CoxFe-ZP catalysts exhibit a positive synergistic effect between hollow graphitic carbon nanomaterials and Fe-doped Co nanoparticles. The optimal Co0.3Fe-ZP catalyst demonstrates satisfactory OER performance, achieving an overpotential of 302 mV at 10 mA cm-2 and a small Tafel slope of 60.0 mV dec-1. Further analysis of the activation energy confirms that the enhanced OER activity of Co0.3Fe-ZP can be reasonably attributed to the combined influence of its morphology and composition. This study demonstrates a ligand-induced method for examining the morphology and electrochemical properties of grown binary MOF-on-MOF heterostructures for OER applications.

Valorization of dragon fruit waste to value-added bioproducts and formulations: A review

Owing to the increasing worldwide population explosion, managing waste generated from the food sector has become a cross-cutting issue globally, leading to environmental, economic, and social issues. Circular economy-inspired waste valorization approaches have been increasing steadily, generating new business opportunities developing valuable bioproducts using food waste, especially fruit wastes, that may have several applications in energy-food-pharma sectors. Dragon fruit waste is one such waste resource, which is rich in several value-added chemicals and oils, and can be a renewable resource to produce several value-added compounds of potential applications in different industries. Pretreatment and extraction processes in biorefineries are important strategies for recovering value-added biomolecules. There are different methods of valorization, including green extractions and biological conversion approaches. However, microbe-based conversion is one of the advanced technologies for valorizing dragon fruit waste into bioethanol, bioactive products, pharmaceuticals, and other valued products by reusing or recycling them. This state-of-the-art review briefly overviews the dragon fruit waste management strategies and advanced eco-friendly and cost-effective valorization technologies. Furthermore, various applications of different valuable bioactive components obtained from dragon fruit waste have been critically discussed concerning various industrial sectors. Several industrial sectors, such as food, pharmaceuticals, and biofuels, have been critically reviewed in detail.

Biohydrogen from waste feedstocks: An energy opportunity for decarbonization in developing countries

In developing economies, the decarbonization of energy sector has become a global priority for sustainable and cleaner energy system. Biohydrogen production from renewable sources of waste biomass is a good source of energy incentive that reduces the pollution. Biohydrogen has a high calorific value and emits no emissions, producing both energy security and environmental sustainability. Biohydrogen production technologies have become one of the main renewable sources of energy. The present paper entails the role of biohydrogen recovered from waste biomasses like agricultural waste (AW), organic fraction of municipal solid waste (OFMSW), food processing industrial waste (FPIW), and sewage sludge (SS) as a promising solution. The main sources of increasing yield percentage of biohydrogen generation from waste feedstock using different technologies, and process parameters are also emphasized in this review. The production paths for biohydrogen are presented in this review article, and because of advancements in R and D, biohydrogen has gained viability as a biofuel for the future and discusses potential applications in power generation, transportation, and industrial processes, emphasizing the versatility and potential for integration into existing energy infrastructure. The investigation of different biochemical technologies and methods for producing biohydrogen, including anaerobic digestion (AD), dark fermentation (DF), photo fermentation (PF), and integrated dark-photo fermentation (IDPF), has been overviewed. This analysis also discusses future research, investment, and sustainable energy options transitioning towards a low-carbon future, as well as potential problems, economic impediments, and policy-related issues with the deployment of biohydrogen in emerging nations.

A comprehensive review on unleashing the power of hydrogen: revolutionizing energy systems for a sustainable future

Population growth and environmental degradation are major concerns for sustainable development worldwide. Hydrogen is a clean and eco-friendly alternative to fossil fuels, with a heating value almost three times higher than other fossil fuels. It also has a clean production process, which helps to reduce the emission of hazardous pollutants and save the environment. Among the various production methodologies described in this review, biochemical production of hydrogen is considered more suitable as it uses waste organic matter instead of fossil fuels. This technology not only produces clean energy but also helps to manage waste more efficiently. However, the production of hydrogen obtained from this method is currently more expensive due to its early stage of development. Nevertheless, various research projects are underway to develop this method on a commercial scale.

Bipolar Membrane Seawater Splitting for Hydrogen Production: A Review

The growing demand for clean energy has spurred the quest for sustainable alternatives to fossil fuels. Hydrogen has emerged as a promising candidate with its exceptional heating value and zero emissions upon combustion. However, conventional hydrogen production methods contribute to CO2 emissions, necessitating environmentally friendly alternatives. With its vast potential, seawater has garnered attention as a valuable resource for hydrogen production, especially in arid coastal regions with surplus renewable energy. Direct seawater electrolysis presents a viable option, although it faces challenges such as corrosion, competing reactions, and the presence of various impurities. To enhance the seawater electrolysis efficiency and overcome these challenges, researchers have turned to bipolar membranes (BPMs). These membranes create two distinct pH environments and selectively facilitate water dissociation by allowing the passage of protons and hydroxide ions, while acting as a barrier to cations and anions. Moreover, the presence of catalysts at the BPM junction or interface can further accelerate water dissociation. Alongside the thermodynamic potential, the efficiency of the system is significantly influenced by the water dissociation potential of BPMs. By exploiting these unique properties, BPMs offer a promising solution to improve the overall efficiency of seawater electrolysis processes. This paper reviews BPM electrolysis, including the water dissociation mechanism, recent advancements in BPM synthesis, and the challenges encountered in seawater electrolysis. Furthermore, it explores promising strategies to optimize the water dissociation reaction in BPMs, paving the way for sustainable hydrogen production from seawater.

Spatial charge separated two-dimensional/two-dimensional Cu-In2S3/CdS heterojunction for boosting photocatalytic hydrogen production

Two-dimensional (2D) beta indium sulfide (β-In2S3) shows great potential in photocatalytic hydrogen production due to its broad-spectrum response, relatively negative conduction band edge, high carrier mobility and low toxicity. However, the high charge recombination rate limits the application of In2S3. Here, we in-situ grew 2D cadmium sulfide (CdS) on the surface of In2S3 doped with copper ions (Cu2+) to construct a heterojunction photocatalyst that suppresses charge recombination. The in-situ grown method and shared sulfur composition were conducive to forming the efficient interface contact between In2S3 and CdS, promoting charge transfer and showing the high spatial charge separation rate, resulting in a hydrogen production rate of 868 µmol g-1h-1. The induced Cu2+ extended the light absorption range and stabilized the photocatalyst. By creating stable 2D/2D heterojunction photocatalysts with high charge separation efficiency, this work opens new possibilities for applying In2S3 materials in photocatalytic hydrogen production.

Construction of ZnxCdyS with a 3D Hierarchical Structure for Enhanced Photocatalytic Hydrogen Production from Water Splitting

The ZnxCdyS has been proven to have unique photoelectric properties, but its synthesis method and photocatalytic water cracking performance need to be further improved. In this paper, Cd-MOF@ZIF-8 with a MOF-on-MOF (MOF = metal-organic framework) structure was prepared by a simple ion adsorption method. Then, a CdS/ZnxCdyS heterojunction with a 3D hierarchical structure was formed by solvothermal sulfidation. The prepared catalysts with different Zn/Cd ratios show an improved hydrogen production performance for photocatalytic water splitting, and the hydrogen evolution rate of Zn1Cd1S can reach up to 29.2 mmol·g-1·h-1. The excellent photocatalytic activity not only benefits from ZnxCdyS strong light conversion ability but also is closely related to the hierarchical structure and large specific surface area. A type II heterojunction also plays an important role in the spatial separation of photogenerated carriers. This paper provides a simple and feasible idea for the synthesis of a photocatalyst with a large specific surface area using a MOF-on-MOF synthesis strategy.

Chitosan hydrogel anchored phthalocyanine supported metal nanoparticles: Bifunctional catalysts for pollutants reduction and hydrogen production

Metal nanoparticles possess high catalytic activity in various organic transformation reactions. A catalyst must be recovered and re-used effectively and economically to lower the overall reaction cost. The recovery of a catalyst remains a challenge due to their extreme small size. In this research work, catalytic metal nanoparticles were synthesized on Zn-phthalocyanine (ZnPc) and chitosan hydrogel (CH) composite which acts as catalyst support. The ZnPc-CH support facilitate the easy recovery of the loaded metal nanoparticles. Metal nanoparticles (M⁰) based on Cu⁰, Ag⁰, Ni⁰, Co⁰ and Fe⁰ were decorated inside and on ZnPc-CH hydrogel surface. The developed M⁰@ZnPc-CH were utilized for the enhanced selective reduction of toxins and hydrogen production by methanolysis and hydrolysis of NaBH₄. Effective catalytic reduction and hydrogen generation was successfully achieved with Co⁰@ZnPc-CH and ZnPc-CH. Under optimized conditions, Co⁰@ZnPc-CH showed complete reduction of 4-nitrophenol (4-NP) in 8.0 min with the fast 4-NP reduction kinetics (K = 0.611 min^-1). Among the developed catalysts, ZnPc-CH showed fast H₂ generation with high H₂ generation rate (HGR = 4100 mLg^-1min^-1) under optimized conditions. Metal leaching from Co⁰@ZnPc-CH was negligible during recycling of the catalyst, suggesting that it could be implemented to 4-NP treatment from real water samples. Similarly, ZnPc-CH could produce same quantity of H₂ throughout 4 continuous cycles of durability testing without any deactivation and leaching and ZnPc-CH showed high stability, indicating the effectiveness of the catalyst to be applied for H₂ production on large scale.

Biological Hydrogen Production from Biowaste Using Dark Fermentation, Storage and Transportation

Hydrogen is widely considered as the fuel of the future. Due to the challenges present during hydrogen production using conventional processes and technologies, additional methods must be considered, like the use of microorganisms. One of the most promising technologies is dark fermentation, a process where microorganisms are utilized to produce hydrogen from biomass. The paper provides a comprehensive overview of the biological processes of hydrogen production, specifically emphasizing the dark fermentation process. This kind of fermentation involves bacteria, such as Clostridium and Enterobacterium, to produce hydrogen from organic waste. Synthetic microbial consortia are also discussed for hydrogen production from different types of biomasses, including lignocellulosic biomass, which includes all biomass composed of lignin and (hemi)cellulose, sugar-rich waste waters, and others. The use of genetic engineering to improve the fermentation properties of selected microorganisms is also considered. Finally, the paper covers the important aspect of hydrogen management, including storage, transport, and economics.

Green Energy by Hydrogen Production from Water Splitting, Water Oxidation Catalysis and Acceptorless Dehydrogenative Coupling

In this review, we want to explain how the burning of fossil fuels is pushing us towards green energy. Actually, for a long time, we have believed that everything is profitable, that resources are unlimited and there are no consequences. However, the reality is often disappointing. The use of non-renewable resources, the excessive waste production and the abandonment of the task of recycling has created a fragile thread that, once broken, may never restore itself. Metaphors aside, we are talking about our planet, the Earth, and its unique ability to host life, including ourselves. Our world has its balance; when the wind erodes a mountain, a beach appears, or when a fire devastates an area, eventually new life emerges from the ashes. However, humans have been distorting this balance for decades. Our evolving way of living has increased the number of resources that each person consumes, whether food, shelter, or energy; we have overworked everything to exhaustion. Scientists worldwide have already said actively and passively that we are facing one of the biggest problems ever: climate change. This is unsustainable and we must try to revert it, or, if we are too late, slow it down as much as possible. To make this happen, there are many possible methods. In this review, we investigate catalysts for using water as an energy source, or, instead of water, alcohols. On the other hand, the recycling of gases such as CO2 and N2O is also addressed, but we also observe non-catalytic means of generating energy through solar cell production.

Carbon neutrality: a comprehensive bibliometric analysis

In recent years, excessive emissions of carbon dioxide have further intensified global warming, which poses a great threat to human society. The best way to reverse this situation is to take action to equalize the "carbon emissions" with the "carbon absorption," i.e., carbon neutrality (CN). To better understand the evolution and display a broad panorama of CN research, this paper provides a comprehensive bibliometric analysis of publications in the field of CN from 2006 to 2021. We use the Web of Science Core Collection as the data source, and a total of 633 publications have been retrieved. In the paper, we measure the productivity and influence of publications through recognized bibliometric metrics, and visually analyze publications using VOS Viewer and CiteSpace. The analysis results show that China is the most productive country/region. The study illustrates that in order to reach the goal of CN, it is necessary to reduce carbon emissions such as full using renewable energy. Simultaneously, it is also essential to enhance the "negative emissions" of carbon such as taking advantage of carbon sink and carbon capture, utilization, and storage. This paper provides some references for scholars who are interested or research in this field.

A Review of the CFD Modeling of Hydrogen Production in Catalytic Steam Reforming Reactors

Global demand for alternative renewable energy sources is increasing due to the consumption of fossil fuels and the increase in greenhouse gas emissions. Hydrogen (H₂) from biomass gasification is a green energy segment among the alternative options, as it is environmentally friendly, renewable, and sustainable. Accordingly, researchers focus on conducting experiments and modeling the reforming reactions in conventional and membrane reactors. The construction of computational fluid dynamics (CFD) models is an essential tool used by researchers to study the performance of reforming and membrane reactors for hydrogen production and the effect of operating parameters on the methane stream, improving processes for reforming untreated biogas in a catalyst-fixed bed and membrane reactors. This review article aims to provide a good CFD model overview of recent progress in catalyzing hydrogen production through various reactors, sustainable steam reforming systems, and carbon dioxide utilization. This article discusses some of the issues, challenges, and conceivable arrangements to aid the efficient generation of hydrogen from steam reforming catalytic reactions and membrane reactors of bioproducts and fossil fuels.

A Review on Green Hydrogen Valorization by Heterogeneous Catalytic Hydrogenation of Captured CO2 into Value-Added Products

The catalytic hydrogenation of captured CO2 by different industrial processes allows obtaining liquid biofuels and some chemical products that not only present the interest of being obtained from a very low-cost raw material (CO2) that indeed constitutes an environmental pollution problem but also constitute an energy vector, which can facilitate the storage and transport of very diverse renewable energies. Thus, the combined use of green H2 and captured CO2 to obtain chemical products and biofuels has become attractive for different processes such as power-to-liquids (P2L) and power-to-gas (P2G), which use any renewable power to convert carbon dioxide and water into value-added, synthetic renewable E-fuels and renewable platform molecules, also contributing in an important way to CO2 mitigation. In this regard, there has been an extraordinary increase in the study of supported metal catalysts capable of converting CO2 into synthetic natural gas, according to the Sabatier reaction, or in dimethyl ether, as in power-to-gas processes, as well as in liquid hydrocarbons by the Fischer-Tropsch process, and especially in producing methanol by P2L processes. As a result, the current review aims to provide an overall picture of the most recent research, focusing on the last five years, when research in this field has increased dramatically.

Nanoengineering of Catalysts for Enhanced Hydrogen Production

Hydrogen (H2) has emerged as a sustainable energy carrier capable of replacing/complementing the global carbon-based energy matrix. Although studies in this area have often focused on the fundamental understanding of catalytic processes and the demonstration of their activities towards different strategies, much effort is still needed to develop high-performance technologies and advanced materials to accomplish widespread utilization. The main goal of this review is to discuss the recent contributions in the H2 production field by employing nanomaterials with well-defined and controllable physicochemical features. Nanoengineering approaches at the sub-nano or atomic scale are especially interesting, as they allow us to unravel how activity varies as a function of these parameters (shape, size, composition, structure, electronic, and support interaction) and obtain insights into structure–performance relationships in the field of H2 production, allowing not only the optimization of performances but also enabling the rational design of nanocatalysts with desired activities and selectivity for H2 production. Herein, we start with a brief description of preparing such materials, emphasizing the importance of accomplishing the physicochemical control of nanostructures. The review finally culminates in the leading technologies for H2 production, identifying the promising applications of controlled nanomaterials.

Hydrogen Production and Its Applications to Mobility

Hydrogen has been identified as one of the key elements to bolster longer-term climate neutrality and strategic autonomy for several major countries. Multiple road maps emphasize the need to accelerate deployment across its supply chain and utilization. Being one of the major contributors to global CO₂ emissions, the transportation sector finds in hydrogen an appealing alternative to reach sustainable development through either its direct use in fuel cells or further transformation to sustainable fuels. This review summarizes the latest developments in hydrogen use across the major energy-consuming transportation sectors. Rooted in a systems engineering perspective, we present an analysis of the entire hydrogen supply chain across its economic, environmental, and social dimensions. Providing an outlook on the sector, we discuss the challenges hydrogen faces in penetrating the different transportation markets. Expected final online publication date for the Annual Review of Chemical and Biomolecular Engineering, Volume 13 is October 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

The Route from Green H2 Production through Bioethanol Reforming to CO2 Catalytic Conversion: A Review

Currently, a progressively different approach to the generation of power and the production of fuels for the automotive sector as well as for domestic applications is being taken. As a result, research on the feasibility of applying renewable energy sources to the present energy scenario has been progressively growing, aiming to reduce greenhouse gas emissions. Following more than one approach, the integration of renewables mainly involves the utilization of biomass-derived raw material and the combination of power generated via clean sources with conventional power generation systems. The aim of this review article is to provide a satisfactory overview of the most recent progress in the catalysis of hydrogen production through sustainable reforming and CO2 utilization. In particular, attention is focused on the route that, starting from bioethanol reforming for H2 production, leads to the use of the produced CO2 for different purposes and by means of different catalytic processes, passing through the water–gas shift stage. The newest approaches reported in the literature are reviewed, showing that it is possible to successfully produce “green” and sustainable hydrogen, which can represent a power storage technology, and its utilization is a strategy for the integration of renewables into the power generation scenario. Moreover, this hydrogen may be used for CO2 catalytic conversion to hydrocarbons, thus giving CO2 added value.

Analysis of Control-System Strategy and Design of a Small Modular Reactor with Different Working Fluids for Electricity and Hydrogen Production as Part of a Decentralised Mini Grid

Hydrogen is increasingly being viewed as a significant fuel for future industrial processes as it offers pathways to zero emission. The UK sees hydrogen as one of a handful of low-carbon solutions for transition to net zero. Currently, most hydrogen production is from steam reforming of natural gas or coal gasification, both of which involve the release of carbon dioxide. Hydrogen production from mini decentralised grids via a thermochemical process, coupled with electricity production, could offer favourable economics for small modular reactors (SMRs), whereby demand or grid management as a solution would include redirecting the power for hydrogen production when electricity demand is low. It also offers a clean-energy alternative to the aforementioned means. SMRs could offer favourable economics due to their flexible power system as part of the dual-output function. This study objective is to investigate the critical performance parameters associated with the nuclear power plant (NPP), the cycle working fluids, and control-system design for switching between electricity and hydrogen demand to support delivery as part of a mini grid system for a reactor power delivering up to approximately 600 MWth power. The novelty of the work is in the holistic parametric analysis undertaken using a novel in-house tool, which analyses the NPP using different working fluids, with a control function bolt-on at the offtake for hydrogen production. The results indicate that the flow conditions at the offtake can be maintained. The choice of working fluids affects the pressure component. However, the recuperator and heat-exchanger effectiveness are considered as efficiency-limiting factors for hydrogen production and electricity generation. As such, the benefit of high-technology heat exchangers cannot be underestimated. This is also true when deciding on the thermochemical process to bolt onto the plant. The temperature of the gas at the end of the pipeline should also be considered to ensure that the minimum temperature-requirement status for hydrogen production is met.

Catalysts for Sustainable Hydrogen Production: Preparation, Applications and Process Integration

The earth is experiencing a series of epochal emergencies, directly related to the overexploitation of natural resources [...]

Hydrogen as a Clean and Sustainable Energy Vector for Global Transition from Fossil-Based to Zero-Carbon

Hydrogen is recognized as a promising and attractive energy carrier to decarbonize the sectors responsible for global warming, such as electricity production, industry, and transportation. However, although hydrogen releases only water as a result of its reaction with oxygen through a fuel cell, the hydrogen production pathway is currently a challenging issue since hydrogen is produced mainly from thermochemical processes (natural gas reforming, coal gasification). On the other hand, hydrogen production through water electrolysis has attracted a lot of attention as a means to reduce greenhouse gas emissions by using low-carbon sources such as renewable energy (solar, wind, hydro) and nuclear energy. In this context, by providing an environmentally-friendly fuel instead of the currently-used fuels (unleaded petrol, gasoline, kerosene), hydrogen can be used in various applications such as transportation (aircraft, boat, vehicle, and train), energy storage, industry, medicine, and power-to-gas. This article aims to provide an overview of the main hydrogen applications (including present and future) while examining funding and barriers to building a prosperous future for the nation by addressing all the critical challenges met in all energy sectors.

Recent Progress in the Steam Reforming of Bio-Oil for Hydrogen Production: A Review of Operating Parameters, Catalytic Systems and Technological Innovations

The present review focuses on the production of renewable hydrogen through the catalytic steam reforming of bio-oil, the liquid product of the fast pyrolysis of biomass. Although in theory the process is capable of producing high yields of hydrogen, in practice, certain technological issues require radical improvements before its commercialization. Herein, we illustrate the fundamental knowledge behind the technology of the steam reforming of bio-oil and critically discuss the major factors influencing the reforming process such as the feedstock composition, the reactor design, the reaction temperature and pressure, the steam to carbon ratio and the hour space velocity. We also emphasize the latest research for the best suited reforming catalysts among the specific groups of noble metal, transition metal, bimetallic and perovskite type catalysts. The effect of the catalyst preparation method and the technological obstacle of catalytic deactivation due to coke deposition, metal sintering, metal oxidation and sulfur poisoning are addressed. Finally, various novel modified steam reforming techniques which are under development are discussed, such as the in-line two-stage pyrolysis and steam reforming, the sorption enhanced steam reforming (SESR) and the chemical looping steam reforming (CLSR). Moreover, we argue that while the majority of research studies examine hydrogen generation using different model compounds, much work must be done to optimally treat the raw or aqueous bio-oil mixtures for efficient practical use. Moreover, further research is also required on the reaction mechanisms and kinetics of the process, as these have not yet been fully understood.

Construction of Chemically Bonded Interface of Organic/Inorganic g-C3N4/LDH Heterojunction for Z-Schematic Photocatalytic H2 Generation

The design and synthesis of a Z-schematic photocatalytic heterostructure with an intimate interface is of great significance for the migration and separation of photogenerated charge carriers, but still remains a challenge. Here, we developed an efficient Z-scheme organic/inorganic g-C3N4/LDH heterojunction by in situ growing of inorganic CoAl-LDH firmly on organic g-C3N4 nanosheet (NS). Benefiting from the two-dimensional (2D) morphology and the surface exposed pyridine-like nitrogen atoms, the g-C3N4 NS offers efficient trap sits to capture transition metal ions. As such, CoAl-LDH NS can be tightly attached onto the g-C3N4 NS, forming a strong interaction between CoAl-LDH and g-C3N4 via nitrogen-metal bonds. Moreover, the 2D/2D interface provides a high-speed channel for the interfacial charge transfer. As a result, the prepared heterojunction composite exhibits a greatly improved photocatalytic H2 evolution activity, as well as considerable stability. Under visible light irradiation of 4 h, the optimal H2 evolution rate reaches 1952.9 μmol g-1, which is 8.4 times of the bare g-C3N4 NS. The in situ construction of organic/inorganic heterojunction with a chemical-bonded interface may provide guidance for the designing of high-performance heterostructure photocatalysts.

Improved H-Storage Performance of Novel Mg-Based Nanocomposites Prepared by High-Energy Ball Milling: A Review

Hydrogen storage in magnesium-based composites has been an outstanding research area including a remarkable improvement of the H-sorption properties of this system in the last 5 years. Numerous additives of various morphologies have been applied with great success to accelerate the absorption/desorption reactions. Different combinations of catalysts and preparation conditions have also been explored to synthesize better hydrogen storing materials. At the same time, ball milling is still commonly and effectively applied for the fabrication of Mg-based alloys and composites in order to reduce the grain size to nanometric dimensions and to disperse the catalyst particles over the surface of the host material. In this review, we present the very recent progress, from 2016 to 2021, on catalyzing the hydrogen sorption of Mg-based materials by ball milling. The various catalyzing routes enhancing the hydrogenation performance, including in situ formation of catalysts and synergistic improvement achieved by using multiple additives, will also be summarized. At the end of this work, some thoughts on the prospects for future research will be highlighted.

Evaluation of Goethite as a Catalyst for the Thermal Stage of the Westinghouse Process for Hydrogen Production

This work focuses on the evaluation of goethite as a catalyst for the transformation of sulfuric acid into sulfur dioxide, a reaction with great interest for the hybrid electrochemical-thermoelectrochemical Westinghouse cycle for hydrogen production. A comparison of the performance of goethite with that of CuO, Fe2O3, and SiC has been carried out. Moreover, a mixture of those catalysts was evaluated. The results demonstrate that goethite can be used as a catalyst for the thermal decomposition of sulfuric acid in the Westinghouse cycle, with an activity higher than that of SiC but lower than that of Fe2O3 and CuO. However, it does not undergo sintering during its use, but just produces small particles in its surface, which remain after the treatment. Mixtures of Fe2O3 with SiC or goethite do not produce synergism, thus operating each catalyst in an independent way.

Microalgal Hydrogen Production in Relation to Other Biomass-Based Technologies—A Review

Hydrogen is an environmentally friendly biofuel which, if widely used, could reduce atmospheric carbon dioxide emissions. The main barrier to the widespread use of hydrogen for power generation is the lack of technologically feasible and—more importantly—cost-effective methods of production and storage. So far, hydrogen has been produced using thermochemical methods (such as gasification, pyrolysis or water electrolysis) and biological methods (most of which involve anaerobic digestion and photofermentation), with conventional fuels, waste or dedicated crop biomass used as a feedstock. Microalgae possess very high photosynthetic efficiency, can rapidly build biomass, and possess other beneficial properties, which is why they are considered to be one of the strongest contenders among biohydrogen production technologies. This review gives an account of present knowledge on microalgal hydrogen production and compares it with the other available biofuel production technologies.

Development Process of Energy Mix towards Neutral Carbon Future of the Slovak Republic: A Review

Global climate change is putting humanity under pressure, which in many areas poses an unprecedented threat to society as we know it. In an effort to mitigate its effects, it is necessary to reduce the overall production of greenhouse gases and thus, dependence on fossil fuels in all areas of human activities. The presented paper deals with an evaluation of energy mix of the Slovak Republic and four selected neighboring countries in the context of achieving their carbon neutral or carbon negative future. The development of the evaluated energy mixes as well as greenhouse gas emissions is presented from a long-term perspective, which makes it possible to evaluate and compare mutual trends and approaches to emission-free energy sectors.

Incorporation of Nonmetal Group Dopants into g-C3N4 Framework for Highly Improved Photocatalytic H2 Production

The incorporation of nonmetal group dopants into a graphitic carbon nitride (g-C₃N₄) framework is fabricated by adding a small amount of hexamethylenetetramine during the thermal polymerization process. The material shows an excellent visible-light photocatalytic H₂ production performance that is eight times higher than bulk g-C₃N₄. This outstanding performance is ascribed to the introducing of N-doped carbon, which not only enhances the light absorption and favorsa narrower band gap, but also upshifts the conductionband (CB) potential, resulting in a better reduction ability of electrons. This discovery has potential significancefor the designing of high performance, economic, and environmental friendly photocatalyst for solar energy conversion.