Hydrogen

Liquid organic hydrogen carriers (LOHC) have emerged as a promising technology for the safe, efficient, and reversible storage and transportation of hydrogen, offering a viable pathway toward the development of a hydrogen-based economy. Despite the rapid expansion of this research field, a comprehensive understanding of its scientific evolution, intellectual structure, and emerging research directions remains limited. This study presents a comprehensive bibliometric and thematic analysis of LOHC research based on 1140 publications indexed in the Scopus database between 2007 and 2025. Bibliometric indicators, scientific mapping, Bradford’s and Lotka’s laws, thematic evolution, and strategic thematic mapping were performed using VOSviewer and Bibliometrix/Biblioshiny. The results reveal an exponential increase in scientific production after 2016, reflecting the growing global interest in LOHC technologies. Bradford’s law identified a highly concentrated publication landscape dominated by a small core of specialized journals, whereas Lotka’s law demonstrated that scientific production is driven by a limited group of highly productive authors. Keyword network and thematic evolution analyses show a clear transition from fundamental hydrogen storage concepts toward advanced catalyst development, molecular design, quantum chemical calculations, machine learning, techno-economic assessment, and hydrogen transport applications. Strategic thematic mapping further indicates that computational methodologies and sustainability-oriented analyses are becoming emerging drivers of future research. Overall, this study provides a comprehensive overview of the conceptual, intellectual, and thematic evolution of LOHC research while identifying the principal scientific trends and technological opportunities expected to shape the next generation of hydrogen storage systems. Full article

The rapid expansion of hydrogen supply chains introduces complex governance, financial, and operational risks that challenge sustainable transition pathways, particularly within urban and peri-urban industrial ecosystems. This study aims to identify the structural drivers of these risks and develop contradiction-based strategies to enhance systemic robustness. An integrated DEMATEL–ISM–TRIZ framework is employed to map causal interdependencies, construct hierarchical risk structures, and translate systemic tensions into innovation-oriented design principles. DEMATEL identifies cause-and-effect relationships, ISM establishes multilevel structural positioning, and TRIZ resolves critical contradictions through inventive principles. The results reveal that institutional coordination, regulatory adaptability, and strategic investment logic function as foundational drivers of cascading vulnerabilities. Hierarchical modeling confirms that many operational and social risks, including community acceptance and environmental conflicts, are dependent outcomes rather than root causes, with significant implications for urban governance and social legitimacy. Mapped to governance levels, the foundational drivers (R6, R9) operate mainly at the national and supply-chain-wide levels, the regulatory transmission risks (R10–R12) link national rule-setting with regional and municipal implementation, and the dependent social risks (R13–R15) materialize primarily at the urban/local level. TRIZ analysis indicates that robustness is achieved through adaptive governance, phased investment, modular infrastructure design, and socially embedded implementation sequencing. The TRIZ-derived inventive principles are subsequently contextualized as a strategic governance framework for city and regional policymakers, providing a roadmap for orchestrating a just and resilient hydrogen transition within urban and peri-urban industrial ecosystems. The integrated framework demonstrates that hydrogen supply chain sustainability emerges from structural redesign rather than incremental risk mitigation. This study contributes a coherent methodological architecture that links structural diagnosis with contradiction-driven governance innovation for sustainable energy transitions in cities. Full article

This study investigates how hydrogen refuelling station (HRS) operating parameters influence the representativeness of hydrogen fuel samples collected during parallel sampling events. Parallel sampling methodology is regularly used in Europe because it can be performed during normal vehicle refuelling with minimal disruption, but uncertainty remains regarding the effects of refuelling protocol, flow profile, or storage-bank usage. Experiments were conducted at the ZBT test HRS using the HySaM sampling system and at operating European HRS using the NPL ParSAM. Controlled comparisons were performed between several flow-rate profiles, while comprehensive impurity analysis was performed against ISO 14687:2025 requirements. Under controlled conditions, the refuelling protocol (excluding temperature and storage bank) and flow-rate profile had limited influence on most contaminants, whereas water showed the greatest variability and requires further investigation. However, repetitive sampling at normally operating HRS revealed larger differences for several species, including nitrogen, oxygen, argon, methane, carbon dioxide and sulphur, indicating that factors beyond protocol parameters tested in this study and nominal flow rate, such as storage-bank switching, flow path, and station hardware, may affect measured composition. Modelling indicates that the filling rates of the sampling cylinder and FCEV may diverge depending on the sampling-system strategy, highlighting the need for further improvements. The results show that parallel sampling may represent the fuel delivered during a specific refuelling event (if the filling rate of the cylinder and FCEV are sufficiently similar). However, parallel sampling may not fully represent the overall HRS and all operating conditions. Further research involving a broader range of HRS technologies and operating conditions may lead to validation and standardisation of the finding and possibly dedicated sampling protocols. Full article

Iron-based catalysts are widely investigated for CO₂ hydrogenation to light olefins due to their low cost, abundance, and ability to form oxidic and carbidic phases involved in the RWGS-FTS pathway. However, supports and promoters can obscure the intrinsic role of iron phase evolution. Here, unsupported Fe nanoparticles with distinct morphological, compositional and structural characteristics were synthesized, CO-pretreated, and evaluated for CO₂ hydrogenation toward C₂-C₄⁼. Their textural, structural, redox, and surface properties were examined to relate synthesis-induced differences to phase evolution and catalytic performance. CO pretreatment formed iron carbide phases in all samples, yet marked differences in catalytic behavior showed that the active state cannot be described solely by bulk phase composition. Iron oxide nanopolyhedra (Fe-NP) achieved 41.6% CO₂ conversion, the highest C₂-C₄⁼ selectivity of 27.7%, and the highest light-olefin yield of 11.5%. Surface analysis revealed notable differences in the relative contributions of carbidic and oxidic Fe species, suggesting that performance depends on the surface phase distribution established during activation and reaction. The coexistence of Fe₃O₄ and Fe_xC_y species appears to facilitate RWGS and subsequent Fischer-Tropsch pathways, as proposed in the literature. CO₂-TPD further showed that maximum CO₂ uptake did not correspond to superior performance, highlighting the combined influence of synthesis-induced structure, surface composition, and reduction-carburization behavior. Full article

It has been established that hydrogen diffusion can significantly influence the mechanical response of metallic materials under various loading conditions. This phenomenon is particularly evident in reducing the ability for macroscopic plastic deformation. The formation and collapse of bubbles inside a fluid, i.e., cavitation, near a solid surface, generates one of the most complex loading scenarios. The characteristics of localized, high-velocity, and cyclic loading inherent to this process may lead to material removal from the surface, a phenomenon known as cavitation erosion. The mechanical properties of the metal surface, affected by hydrogen, may alter the resistance against erosion wear under cyclic loading conditions during cavitation. In this research, 316L stainless steel samples were electrochemically charged in H₂SO₄ to assess the effect of diffused hydrogen on mechanical behavior, as determined by tensile and micro-indentation testing. The cavitation erosion experiments were conducted on hydrogen-charged and uncharged samples in an ultrasonic cavitation test rig. The findings showed that hydrogen diffusion resulted in a significant reduction of up to 80% in cavitation erosion damage after 5 h of testing compared to the uncharged sample. After seven days of storage, however, the cavitation erosion damage of the hydrogen-charged specimens returned to a level comparable to that of the uncharged specimens, indicating that the beneficial effect of hydrogen on cavitation erosion resistance is reversible. Following short-term cavitation exposure, notable plastic deformation was observed on the eroded surfaces of the uncharged specimens, particularly as raised grain boundaries and slip bands. In contrast, the surfaces of the hydrogen-charged specimens predominantly retained their integrity, suggesting an increased resistance to plastic deformation following hydrogen charging. Full article

Dark fermentation of market waste is repeatedly proposed as a decentralized biohydrogen pathway for Latin American cities, and its climate benefit is almost invariably credited using the IPCC Tier-1 term for avoided methane applied to the full diverted mass. This study evaluates how much hydrogen this waste stream can actually deliver and whether such a credit survives a stoichiometric balance. A seven-day gravimetric survey of 10 stalls at El Pino Market (Carabayllo, Lima, Peru) yielded 754.6 kg week⁻¹, distributed as 41.04% fruit, 22.82% vegetables, and 36.14% cooked food.Four mixtures reproducing the recorded generation ratios of fruit (F), vegetables (V), and cooked food (A) were fermented in batch reactors in quintuplicate for 72 h: (F + V, V + A, F + A, and F + V + A). were fermented in batch reactors, with cumulative gas recorded every 6 h, yielding 240 observations. The modified Gompertz model fitted all curves ($R^2=0.982$–0.994). Specific yields were 26.45 mL g⁻¹ VS for F + A, 18.89 for F + V, 18.41 for F + V + A, and only 0.63 for the fruit-free V + A mixture (Welch’s ANOVA $F_{3,7.32}=8.65\times {10}^4$, $p<0.001$; Games–Howell separated all pairs except F + V versus F + V + A, $p=0.713$). A linear Scheffé mixture model formulated on volatile-solids fractions and calibrated on the three fruit-containing mixtures predicted 5.48 mL g⁻¹ VS for V + A, whereas the observed value was 11.6% of that prediction (95% bootstrap CI 9.8–13.7%), an 88.4% suppression of total fermentative gas that persists under every admissible additive null (2.5–13.7%) and cannot be explained by substrate concentration. Because the gas phase was not speciated, hydrogen production is bounded between an upper bound in which all gas is H₂ and a reference case was measured at 45% v/v, yielding 11.90–26.45 mL H₂ g⁻¹ VS for the best-performing mixture. Even at the upper bound, stoichiometric closure relative to the acetate pathway reaches only 9.71%, and energy recovery amounts to 0.76–1.68% of the substrate chemical energy, compared with 73.8–84.3% for a methanogenic reference. A carbon and energy balance shows that more than 98% of the substrate energy and more than 95% of the carbohydrate carbon leave the process in the effluent. Acidogenesis therefore leaves most of the degradable organic carbon in the effluent, and the uncorrected Tier-1 credit of 29.44 t CO₂eq yr⁻¹ overestimates the attainable benefit by a factor of 10 to 23 when scaled by the H₂-based stoichiometric proxy (1.29–2.86 t CO₂eq yr⁻¹), and by a factor of 15 to 69 when scaled by the pathway-consistent carbon-based factor (0.43–1.90 t CO₂eq yr⁻¹). The biohydrogen potential of market waste is therefore bounded by its fruit fraction; fruit-free mixtures are not merely poorer but fermentatively suppressed; and avoided-methane credits must be constrained by measured process conversion and by the fate of residual carbon rather than by diverted waste mass alone. Full article

The decarbonization of hospitals located in remote regions with water stress requires a power source that is both reliable and cost-effective. The use of a green hydrogen energy system is proposed for use as the power source for the medical sector located in Ouarzazate, Morocco. The system utilizes photovoltaic panels to power proton exchange membrane (PEM) electrolyzers, which generate hydrogen fuel that is stored in a pipeline to the hospital site where it can be utilized in a fuel cell. A model was created in MATLAB/Simulink R2023a that considered the backward-propagation algorithm to size each of the components of the hydrogen energy system, which was optimized using the particle swarm optimization algorithm. The sizing results of the model indicated that a 924 kW electrolyzer, a 217 kW fuel cell, a 21.5 kW compressor, a 30 mm diameter pipeline, and a 7400 m² area for the photovoltaic panels are required to supply 161 kg of hydrogen per day to the hospital. The hydrogen fuel system will meet the demand of the hospital for 159 kg of hydrogen per day with a zero loss of load at the deterministic design point, in both the representative day and five-day cloudy-period stress test horizons; a full-physics Monte Carlo uncertainty analysis (N = 10,000 draws) further shows that this reliability outcome is not robust to combined ±20% uncertainty in electrolyzer efficiency and component unit costs, with zero loss of load maintained in 62.8% of draws. The installation cost of the hydrogen fuel system is approximately 8.21 M EUR. Furthermore, because the hydrogen fuel is stored upstream from the hospital, the flow rate of hydrogen fuel that passes through the pipeline is less than if it were stored downstream from the hospital. Finally, the levelized cost of hydrogen fuel of the system is approximately 13.67 EUR/kg, which shows limited sensitivity to the considered variations in solar irradiance. Thus, this hydrogen fuel system methodology can be applied to other types of critical loads, especially those critical loads within hospitals, in regions with high solar potential. Full article

A self-supported porous FeCoNiCuMo high-entropy alloy phosphide (HEAP) film electrode was prepared by brush coating followed by CVD phosphidation. XRD analysis identified the crystalline phases of the film electrodes. XPS was used to analyze the surface chemical states. Their surface features were observed by SEM, while EDS mapping resolved the spatial distribution of the constituent elements. Hydrogen evolution measurements on the HEAP electrodes were conducted in a three-electrode configuration. Four electrolytes were used: 1 M KOH, 0.5 M H₂SO₄, 1 M KOH + 1 M Na₂S, and 1 M KOH + 1 M NaCl. Phosphidation temperature had a pronounced effect on catalytic activity. Among the electrodes examined, HEAP-550 showed the best overall performance. It required overpotentials of 63, 59, 60, and 57 mV to deliver 10 mA·cm⁻² in the four electrolytes, respectively. Electrochemical impedance spectroscopy distinguished the electrodes in terms of charge-transfer resistance. The C_dl values derived from cyclic voltammetry reflected differences in electrochemically accessible surface area across the three phosphidation temperatures. HEAP-550 exhibited relatively low charge-transfer resistance together with a large ECSA. These characteristics are consistent with its superior HER activity. Furthermore, the HEAP-550 electrode demonstrated excellent long-term stability, maintaining stable operation at a current density of 100 mA cm⁻² for 24 h in all four electrolytes. This work provides a novel strategy for designing self-supported porous high-entropy phosphide film electrodes by integrating multicomponent alloy design with CVD phosphidation, offering new insights into the regulation of synergistic catalytic sites for efficient HER. The developed self-supported HEAP electrode holds great potential for practical applications in efficient water electrolysis and sustainable hydrogen production. Full article

Recently, efficient and robust electrocatalysts for alkaline HER and OER are essential for water splitting applications. In the present work, the combination of nickel (Ni) and cobalt (Co) bimetallic nanocomposites embedded on rGO were prepared by the solid-state sintering process. X-ray diffraction analysis shows a single face–centered–cubic metallic phase together with the (002) diffraction of reduced graphene oxide (rGO), consistent with the NiCo phase being supported on rGO. The field emission scanning electron microscopy analysis shows sub-micrometer metallic particles anchored on the layered rGO sheets in the NiCo/rGO bimetallic nanocomposite. The core-level spectrum of Ni and Co metals confirmed the existence of more oxidation on the surface of rGO. The NiCo/rGO bimetallic nanocomposites show an OER overpotential at 10 mA/cm² of 358 mV vs. RHE and a Tafel slope of 92 mV/dec. The HER overpotential is observed at 10 mA/cm² of −136 mV vs. RHE with a Tafel slope of 95 mV/dec. Both HER and OER stability analyses were performed for 25 h. Finally, full cell analysis of NiCo/rGO revealed that it requires 1.9 V to reach 10 mA/cm² and shows stable operation for 10 h. The present work paves the way for the formation of NiCo/rGO bimetallic nanocomposites for bifunctional electrocatalytic applications. Full article

Hydrogen will play a key role in decarbonising hard-to-abate sectors, yet the current supply is mostly fossil-based and carbon-intensive. Biohydrogen, produced from biomass and organic residues, offers a low-carbon alternative and can achieve negative emissions when integrated with carbon capture. However, its deployment is constrained by both feedstock availability and technology maturity. This review examines the global availability of different types of bio-feedstocks and their compatibility with biohydrogen production routes such as thermochemical and biological methods. Following PRISMA guidelines, it assesses the development status of the main thermochemical and biological routes, maps sustainable feedstocks to compatible technologies, and evaluates regional deployment potential. Thermochemical routes are at higher technology readiness levels (TRLs) and are better suited for large-scale centralised facilities, being compatible with dry lignocellulosic feedstocks. Conversely, biological methods are at lower maturity but are uniquely positioned for small-scale deployment and integration with wastewater and agro-industrial systems. Feedstock type critically influences performance, suggesting region-specific strategies. Future research should focus on technology demonstration in operational environments, regenerable catalysts, oxygen-tolerant enzymes, improved purification, and regional life-cycle assessments. Policy support on carbon pricing, hydrogen mandates, and streamlined permitting is vital to positioning biohydrogen as a cost-competitive complement to electrolysis-based hydrogen powered by renewable energy. Full article

Oxygen carriers (OCs) are essential materials in chemical looping technologies for low-carbon hydrogen production; however, the environmental implications of their synthesis remain insufficiently explored. This study presents a gate-to-gate life cycle assessment (LCA) of a chamotte–nickel OC produced via wet impregnation, following ISO 14040/44 guidelines. The functional unit was defined as 1 kg of oxygen carrier (OC), with input material requirements including chamotte, nickel nitrate, and water, as part of the synthesis process. Environmental impacts were modelled using ReCiPe 2016 Midpoint (H). Environmental hotspots varied across impact categories, with wet impregnation and energy-intensive thermal processes showing substantial contributions, while transportation-related burdens were relevant in selected toxicity and resource-related categories. Wet impregnation contributed 38.93% to climate change, approximately 55.6% to ozone depletion, and approximately 95.6% to terrestrial acidification in the modelled inventory. Thermal treatments also contributed substantially to several impact categories, primarily because of their high electricity requirements. Hotspot analysis identified precursor selection, energy supply, and logistics as key drivers of environmental burdens. Qualitative mitigation options include precursor substitution, renewable-energy integration, energy-efficiency improvements, and industrial symbiosis; however, these alternatives were not quantitatively evaluated in the present LCA. These results offer actionable guidance for sustainable OC design and highlight the importance of integrating LCA into early-stage oxygen-carrier design to reduce the environmental burdens associated with material synthesis. Further assessment incorporating OC performance, lifetime, regeneration, and hydrogen yield would be required to determine the implications of these material-level improvements for the environmental performance of hydrogen production. Full article

The growing global demand for electrical energy and the necessity for decarbonization highlight the importance of assessing the rational use of renewable resources and energy. Thus, this study examined the energy and exergy performance of two offshore wind energy destinations and conversions in Brazil: (i) storage in chemical bonds of hydrogen and ammonia for export and (ii) direct use of electrical energy in the Brazilian National Interconnected System (SIN). The method was based on an integrated exergy analysis, complemented by indicators such as Energy Return on Energy Investment (EROI), considering the entire production chain: seawater desalination, electrolysis, cryogenic air separation, ammonia synthesis and transport, and the electrical energy pathway through the SIN grid. The results demonstrated that the conversion and transport of ammonia lead to lower efficiencies, greater exergy losses, and lower EROI, whereas the direct use of electrical energy through the integrated system proved to be more efficient, with higher EROI and lower exergy losses. From a public policy perspective, the findings suggest that prioritizing hydrogen and ammonia production for export may result in the externalization of the main exergy and decarbonization benefits associated with their final use. The results therefore highlight the importance of aligning emerging hydrogen strategies with domestic decarbonization priorities, particularly by prioritizing the use of hydrogen and its derivatives in hard-to-abate sectors of the Brazilian economy. Full article

The global shift toward cleaner energy has positioned hydrogen-enriched methane (CH₄/H₂) as a practical bridge fuel. While it burns more efficiently and produces fewer carbon emissions than traditional hydrocarbons, it introduces operational and safety challenges. Hydrogen’s high reactivity and rapid burning velocity increase risks such as flashback and premature ignition. This study employs Computational Fluid Dynamics to examine the combustion behavior of methane−hydrogen blends in a 2D axisymmetric chamber based on RANS equations. Using ANSYS Fluent 19.1, the research utilizes a validated equilibrium mixture-fraction/PDF framework to ensure accuracy against physical experiments. The simulation framework successfully captures the complexity of non-premixed turbulent combustion by combining a probability density function approach with a realizable k-ε turbulence model. Moreover, this research explores how varying hydrogen concentrations and air mass flow rates, covering the full spectrum from lean to fuel-rich conditions, affect fluid dynamics, turbulence, and the development of recirculation zones. The data show that adding hydrogen fundamentally reshapes velocity fields and thermal profiles, which in turn dictate combustion efficiency and pollutant formation. It has been demonstrated that the optimal blend for combustion performance is the case containing 30% hydrogen. Furthermore, evaluations involving higher-fraction blends (approaching the 70% enrichment range) suggest that configurations exceeding this level necessitate a redesign of the injector near field to mitigate localized heat release and accelerated NO_x emissions. By identifying the operational limits for CH₄/H₂ blends in industrial settings such as steam boilers, this study offers a technical roadmap for engineering more stable, high-performance, and low-carbon energy infrastructure. Full article

The global interest in utilizing waste from agro–industrial processes through the implementation of clean technologies is rapidly increasing. This study evaluates the technical, economic, and environmental feasibility of valorizing regional agro–industrial residues in Santander, Colombia, specifically coffee mucilage, cocoa mucilage, and pig manure, using integrated biorefinery scenarios. Modernizing these processes is essential given a theoretical energy potential of 75,000 TJ/year, which could generate up to 20,833 GWh/year. Three biorefinery scenarios were designed and simulated using Aspen Plus^® to produce bio-hydrogen, methane, and methanol. The total plant capacity was established at 31.7 t/day, distributed according to regional biomass availability: 4.62 t/day of cocoa mucilage, 1.83 t/day of coffee mucilage, and 25.25 t/day of pig manure. The technical results for Scenario 1 indicated a reformed H₂ production of 0.15 t/day with an associated preheating energy requirement of 193,365 kJ/h. Environmental impact assessment using the ReCiPe 2015 Midpoint methodology identified Scenario 3 as having the lowest direct CO₂ mass flow (9.28 t/day) due to carbon consumption during methanol synthesis. However, Scenario 1 reported a CO₂ equivalent (CO₂e) of 194.37 t/day, reflecting indirect emissions from thermal and energy requirements. Economic analysis, validated through engineering cost references, demonstrated that while Scenario 3 offers a diverse product portfolio, it is currently not cost-effective as the production costs exceed the total expected income. Consequently, Scenario 2 is identified as the most balanced alternative for the Santander context, offering a sustainable compromise between technical efficiency, environmental mitigation, and economic viability. Full article

The World Bank’s Lighthouse Strategy identifies Morocco as a first mover exporter of green hydrogen and its derivatives to Europe; however, the engineering feasibility of the associated transport and storage network has not been quantitatively demonstrated. This study addresses that gap through an integrated spatial and multiperiod optimization framework that couples a spatially explicit Mixed Integer Linear Programming (MILP) model with a FlexSim/FloWorks digital twin for discrete event and hydraulic simulation. The MILP simultaneously optimizes electrolysis deployment, hydrogen storage technologies, and multimodal transport across a four node Moroccan export corridor (TanTan, Mohammedia, Jorf Lasfar, and Tanger Med) for the 2030, 2040, and 2050 planning horizons under a net present value objective. The optimal configuration combines a dedicated hydrogen backbone pipeline for the high volume production corridor with shortsea cabotage for the distribution branches, achieving a full chain levelized cost of ammonia (LCOA) of 1176 USD/t, consistent with the World Bank benchmark and reducing costs by 57 USD/t compared with an all cabotage configuration. The optimal network remains robust over a wide range of capital cost and financing assumptions, while the digital twin confirms the hydraulic and operational feasibility of the integrated pipeline–shipping system without critical port congestion. These findings demonstrate that combining optimization with digital twin validation provides a robust engineering basis for planning Morocco’s green hydrogen export infrastructure and supports investment decisions aligned with future CBAM compliant hydrogen and ammonia supply chains. Full article

Green hydrogen production through water electrolysis is a key pathway to the decarbonization of future energy systems. However, part of the electrical input is transformed into waste heat. In this study, alkaline, proton-exchange membrane, anion-exchange membrane, and solid oxide electrolysis systems are compared in terms of operating temperature, heat generation, heat transfer medium, and integration constraints. Reported COP values for commercial high-temperature vapour-compression heat pumps range from 2.4 to 5.8, depending on operating conditions. The heat-pump technologies reviewed include vapour-compression systems with single-stage, multistage, cascade, and transcritical configurations, together with absorption and adsorption systems, with a focus on suitable working fluids and practical limitations. The review distinguishes between direct heat recovery and heat recovery assisted by heat pumps, and it identifies two main areas of application: external supply for district heating, industrial consumers, and energy communities; and internal support for feedwater preheating, water cycle integration, and steam generation. A selection framework is proposed in which source- and sink-temperature compatibility determines thermodynamic feasibility, COP characterizes heat-pump performance, and LCoH supports techno-economic comparison. Direct heat recovery should be preferred when temperatures are compatible, while heat pumps can operate as enabling technologies when temperature upgrading is required and system-level economic and environmental performance remains advantageous. Full article

Hydrogen (H₂) is a common product of carbohydrate fermentation by the intestinal microbiota, transferred to the blood along the pressure gradient, and exhaled. As H₂ is not produced or metabolized in human cells, alveolar H₂ is distributed throughout the human body, including cellular organelles such as mitochondria, owing to systemic circulation and gas exchange. The electron transport chain comprises a series of oxidation–reduction (redox) enzymes in the mitochondria of human cells that facilitate adenosine triphosphate (ATP) synthesis. The catalytic activity of electron-transport enzymes is optimized at certain electrochemical potentials, as is hydrogen ion activity (pH). However, the human body is an aqueous system that must be electrically neutral. Membrane potentials exist between the interior and exterior of human cells because of the unequal distribution of ions across the membrane. The single-electrode potential can only be assessed relative to that of another electrode (i.e., a reference electrode). The electrochemical potentials relative to a standard hydrogen electrode (SHE) were measured; it was found that H₂ partial pressure was a fundamental factor that affected the SHE, pH, and the reversible hydrogen electrode. The H₂ partial pressure is not a unit used to characterize the human body; therefore, breath H₂ is an external variable in the redox environment in the human body. Full article

Microwave-driven thermochemical cycles can split water for hydrogen production at temperatures far below those of conventional solar thermochemical routes, yet the responsible physical mechanisms remain unclear and numerical models for the coupled solar-microwave hybrid system are still scarce. Building on previous experimental work, we developed a coupled numerical model that integrates impedance matching, non-thermal enhancement, two-stage Arrhenius kinetics, and energy conservation to systematically investigate the interplay between microwave power, temperature evolution, and reaction progress. The model predictions agree well with experimental data in terms of temperature evolution trends, power threshold ranges, and reaction timescales. The results indicate that, within the present modeling framework, the effective microwave absorption efficiency increases from 1.2% at low temperatures to approximately 14% near 85 °C, with the non-thermal enhancement factor contributing as an empirical parameter. Under pure microwave mode, the required power threshold for reaction initiation is approximately 120 W; the solar-microwave synergistic mode reduces this threshold to about 70 W, a 42% reduction. At an input power of 100 W, the energy conversion efficiency reaches a maximum of 42%. Analysis of the sudden temperature change identifies 85 °C as the critical triggering temperature: below it, the system remains in a low-absorption cold state, while once crossed, a positive feedback mechanism rapidly propels the system into the high-temperature reaction regime. This study provides a numerical modeling framework for describing the coupled solar-microwave thermal behavior of the system and for guiding the optimization of its operational parameters. Since the available measurements cannot independently separate the thermal and non-thermal contributions, the non-thermal enhancement remains an empirically introduced factor rather than an experimentally established physical effect. Full article

Hydrogen appears in almost every maritime decarbonization roadmap, yet it barely sails: 19 vessels against more than 1600 powered by liquefied natural gas. This study combines an analysis of the global alternative-fueled fleet and orderbook (3094 fuel-vessel records) with an assessment of 26 international and European governance units, two of which are documented absences of an instrument, grouped into the seven conditions a hydrogen vessel needs to be built, certified, crewed, insured, financed and refueled. None of the 26 fully covers the condition it addresses, and the deficits run across all seven domains. Nor does binding demand regulation reach hydrogen where it is currently viable: the tonnage threshold it applies makes eligible nine out of ten LNG or methanol vessels and only two out of ten hydrogen vessels, a difference that narrows but does not close once adjusted for vessel segment. We term this pattern governance misalignment: conditions that must be met simultaneously but advance at different speeds, with unequal legal force, with ambiguous signals, and across fleet segments that do not coincide. The obstacle is not the absence of regulation but its lack of synchronization, a diagnosis replicable for any fuel whose deployment depends on several regulatory domains. Full article