Main Transport Mechanism Research Articles

The increasing influence of atmospheric moisture transport on hydrometeorological extremes in the Euromediterranean region with global warming

The Euromediterranean area is a key region in which the link between atmospheric moisture transport and hydrometeorological extremes concerns. Atmospheric rivers, the main moisture transport mechanism affecting the region, play a notable role in extreme precipitation there. Moreover, moisture transport deficits from two of the major oceanic moisture sources of the planet, the North Atlantic Ocean and the Mediterranean Sea, are strongly related to drought occurrence in that region. Here, we examine the projected changes in these relationships with global warming, under a high-emission scenario (shared socioeconomic pathway 5–8.5). Here we show that, for the mid-21st century, a moderate increase in the influence of moisture transport on winter precipitation maxima is projected, in line with its increasing concurrence with atmospheric rivers. A stronger increase is estimated for the relationship between moisture transport deficits and drought occurrence, for which probabilities between two and three times greater than those observed in the present climate are obtained for the mid- and end-21st century. This highlights the increasing importance of moisture transport from the ocean in future droughts in the region, especially in the context of reduced local moisture inputs from terrestrial evaporation as a consequence of drier soil.

Modeling Water Transport in Polymer Electrolyte Membrane Electrolyzers Using a One-Dimensional Transport Model

In this work, we present a water transport model to quantify the movement of water across Nafion® membranes in a proton exchange membrane electrolyzer as a function of varying operating conditions and membrane parameters. This physics-based model is based on the three main water transport mechanisms: diffusion, electro-osmotic drag, and pressure-driven flow. Three sets of equations are obtained to model the movement of water on the cathode side – I. Material balances for hydrogen and water in the flow channel (z-direction), II. Water movement across the membrane in the x-direction, and III. Expressions for variable membrane properties to serve as model inputs. The condensation of water at the cathode is also modeled to understand the respective transport contributions from the vapor and liquid phases. The coupled equation sets are solved numerically with appropriate boundary conditions. An analytical solution is also obtained for the governing differential equation for the mole fraction of water in the vapor phase. This study is perhaps the first effort for a detailed physics-based transport model to predict the water transport in the electrolyzer in one dimension using the actual measured values for the physical parameters of the system. The model results are compared with the experimental data available for water transport, and a good agreement is observed over the wide range of current, temperature and pressure differentials. Further, with the help of this simple transport model, the numerical analysis is performed to delineate the effect of electrolyzer operating conditions on the net water transport across the membrane, water condensation at the cathode, individual contribution of the transport fluxes, and electrolyzer design. Finally, the model is exercised to simulate the dependence of water transport as a function of membrane thickness. This confirms the validity of the current approach of using thin reinforced membranes by electrolyzer fabricators. Figure 1

Nonlinear Charge Transport and Excitable Phenomena in Semiconductor Superlattices.

Semiconductor superlattices are periodic nanostructures consisting of epitaxially grown quantum wells and barriers. For thick barriers, the quantum wells are weakly coupled and the main transport mechanism is a sequential resonant tunneling of electrons between wells. We review quantum transport in these materials, and the rate equations for electron densities, currents, and the self-consistent electric potential or field. Depending on superlattice configuration, doping density, temperature, voltage bias, and other parameters, superlattices behave as excitable systems, and can respond to abrupt dc bias changes by large transients involving charge density waves before arriving at a stable stationary state. For other parameters, the superlattices may have self-sustained oscillations of the current through them. These oscillations are due to repeated triggering and recycling of charge density waves, and can be periodic in time, quasiperiodic, and chaotic. Modifying the superlattice configuration, it is possible to attain robust chaos due to wave dynamics. External noise of appropriate strength can generate time-periodic current oscillations when the superlattice is in a stable stationary state without noise, which is called the coherence resonance. In turn, these oscillations can resonate with a periodic signal in the presence of sufficient noise, thereby displaying a stochastic resonance. These properties can be exploited to design and build many devices. Here, we describe detectors of weak signals by using coherence and stochastic resonance and fast generators of true random sequences useful for safe communications and storage.

Advective and diffusive gas phase transport in vadose zones: Importance for defining vapour risks and natural source zone depletion of petroleum hydrocarbons

Quantifying the interlinked behaviour of the soil microbiome, fluid flow, multi-component transport and partitioning, and biodegradation is key to characterising vapour risks and natural source zone depletion (NSZD) of light non-aqueous phase liquid (LNAPL) petroleum hydrocarbons. Critical to vapour transport and NSZD is transport of gases through the vadose zone (oxygen from the atmosphere, volatile organic compounds (VOCs), methane and carbon dioxide from the zone of LNAPL biodegradation). Volatilisation of VOCs from LNAPL, aerobic biodegradation, methanogenesis and heat production all generate gas pressure changes that may lead to enhanced gas fluxes apart from diffusion. Despite the importance of the gaseous phase dynamics in the vadose zone processes, the relative pressure changes and consequent scales of advective (buoyancy and pressure driven) / diffusive transport is less studied. We use a validated multi-phase multi-component non-isothermal modelling framework to differentiate gas transport mechanisms. We simulate a multicomponent unweathered gasoline LNAPL with high VOC content to maximise the potential for pressure changes due to volatilisation and to enable the joint effects of methanogenesis and shallower aerobic biodegradation of vapours to be assessed, along with heat production. Considering a uniform fine sand profile with LNAPL resident in the water table capillary zone, results suggest that biodegradation plays the key role in gas phase formation and consequent pressure build-up. Results suggest that advection is the main transport mechanism over a thin zone inside the LNAPL/capillary region, where the effective gaseous diffusion is very low. In the bulk of the vadose zone above the LNAPL region, the pressure change is minimal, and gaseous diffusion is dominant. Even for high biodegradation rate cases, pressure build-up due to heat generation (inducing buoyancy effects) is smaller than the contribution of gas formation due to biodegradation. The findings are critical to support broader assumptions of diffusive transport being dominant in vapour transport and NSZD assessments.

Modeling water transport in polymer electrolyte membrane electrolyzers using a one-dimensional transport model

In this paper, we present a one-dimensional transport model for water transport across Nafion® membranes in a proton exchange membrane electrolyzer. This physics-based model is based on the three main water transport mechanisms: diffusion, electro-osmotic drag, and pressure-driven flow. A good agreement is obtained between the model predictions using the measured values of the physical parameters as inputs and the previously reported experimental data. Further, with the help of this simple transport model, the numerical analysis is performed to delineate the effect of electrolyzer operating conditions on the net water transport across the membrane, water condensation at the cathode, individual contribution of the transport fluxes, and electrolyzer design. Finally, the model is exercised to simulate the dependence of water transport as a function of membrane thickness. This confirms the validity of the current approach of using thin reinforced membranes by electrolyzer fabricators.

Soil type regulates the divergent loss characteristics of sediment associated carbon and nitrogen in different size classes during rainfall erosion on cultivated lands

Sediment associated carbon and nitrogen loss under rainfall, an important cause of soil quality degradation and water eutrophication, strongly depends on the intrinsic properties of original soil types. Relative to total loss, the transport behaviors of organic carbon and nitrogen among sediment size classes and response to soil types remain poorly understood. The concentrations of organic carbon (OC) and total nitrogen (TN) in different sediment size classes (>1, 0.25–1, 0.10–0.25, and <0.10 mm) and their contributions to total sediment load during rainfall erosion were determined under field plot rainfall simulation (at 90 mm h−1) on three contrasting soil types (Luvisol, Alisol and Ferralsol) with increased aggregate stability. During rainfall erosion, the concentrations of OC and TN in total and different sized sediments decreased first and then reached a steady state. The variability of OC and TN concentrations (coefficient of variations in 4.2–53.1% and 6.6–41.9%) among sediment size classes decreased from Luvisol to Ferralsol. Compared to original soils, sediments exhibited larger C/N ratios for Luvisol, and smaller values for Alisol, indicating the more selective transport of labile organic matter for weaker aggregated soils. Among sediment size classes, fine particles (<0.10 mm) accounted 69–88% of total OC and TN losses for Luvisol, and decreased to 30–39% for Ferralsol; and the main transport mechanisms of sediment associated OC and TN shifting from suspension-saltation (<0.10 mm) to rolling (>0.25 mm) with increased aggregate stability. Among original soil properties, inorganic cementing agents (including amorphous iron oxides and clay minerals) showed closer relationships with sediment OC and TN losses (|r| = 0.61–0.89, p < 0.001) than organic matter properties (|r| = 0.55–0.87, p < 0.001), further implying the important role of soil aggregate stability across soil types. This study provides an in-depth understanding on soil carbon and nitrogen losses and their divergent characteristics among soil types deserves consideration in the development of erosion model and land management in agricultural systems.

Unravelling the origin of the atmospheric moisture deficit that leads to droughts

AbstractDrought is one of the most catastrophic natural hazards, and precipitation plays a major role in the development and intensification of drought events. The amount of precipitation resulting from humidity transported from a given moisture source can be key in revealing the origin of the atmospheric moisture deficit underlying drought occurrence. Here this study demonstrates, for the first time, the predominant role of moisture transport deficit in drought genesis. In most land areas, the estimated conditional probability of drought given an equivalent moisture deficit received either from the ocean or from the continents is higher than 10%. This probability is over 15% in the regions where the main atmospheric moisture transport mechanisms are active and over 20% in some hotspot regions, such as central-east North America, south-east South America and east Europe, where lower incoming moisture is almost synonymous with drought occurrence. Our results indicated that the contribution deficit of the dominant moisture source to the precipitation of a region could improve the predictability of droughts, with enormous hydrological, socioeconomic and environmental implications.

Theoretical insights into He/CH4 separation by MXene nanopore

Natural gas is currently the main industrial source of helium, thus He/CH4 separation has become increasing important. Nanopores based on two-dimensional materials have been considered as very promising for He/CH4 separation, but the transport mechanism through them is yet unclear. Here, both non-equilibrium molecular dynamics (NEMD) and equilibrium MD (EMD) were employed to study the transport of He/CH4 through nanopores of different MXenes with various pore diameters (d), pore density and functional groups, which showed two main transport mechanisms, molecular sieving and Knudsen diffusion. Small d was found to be crucial for the molecular sieving and consequently, high He/CH4 selectivity (SHe/CH4). The density profiles of gas molecules in EMD simulations were used to predict SHe/CH4 and the permeance for gas passing through different nanopores, yielding results well agreeing with NEMD simulations. This work provides theoretical insights for gas transport through MXene nanopore and may help designing nanopores in the future.

Characterizing Isotopic Composition and Trajectories of Atmospheric River Events

Landfalling atmospheric rivers (LARs) are important drivers of mid-latitude climate; however, our understanding of the water vapour sources, storm trajectories, and receiving waters of ARs is limited. This study aims to characterize LARs in southwest British Columbia by their isotopic composition and storm track trajectories and to better understand how AR-derived precipitation is manifested in watershed waters. ARs were depleted (−11.71‰ δ18O, −85.80‰ δ2H, n = 19) compared to non-ARs (−9.47‰ δ18O, −69.58‰ δ2H, n = 32) (p = 0.03); however, the difference is minimal. LAR storm tracks did not show any obvious correlation to their isotopic composition, despite the large variability in their source regions across the Pacific Ocean. The lack of correlation is attributed to mixing air parcels, thereby incorporating moisture with different isotopic compositions into the main transport mechanism. D-excess values for ARs and non-ARs were statistically similar, although seasonal differences were observed. ARs with higher d-excess were sourced from the central Pacific, whereas ARs with lower d-excess had storm tracks through the northern Pacific. Watershed water d-excess values (mean = 8.58 ± 2.97‰) were more similar to winter precipitation (mean = 10.1 ± 5.1‰), compared to summer (mean = 2.8 ± 4.3‰), likely due to their source of winter precipitation at high elevation. A greater range in AR d-excess winter values relative to summer values (3.6–16.6‰, −0.3–6.0‰, respectively) is attributed to storm track variability.

Sediment leakage on the beach and upper shoreface due to extreme storms

The quantification of sediment exchanges between the beach and the lower shoreface, although being still poorly understood, is required to adequately forecast long-term coastal evolution. The effect of extreme storms on the morphodynamics of the nearshore and the sediment budget of the coastal zone seems to be strongly conditioned by local parameters, which should be incorporated into studies of medium- and long-term coastal evolution. In the upper shoreface, the dynamics of shore-parallel bars in sandy coasts have been extensively studied, but their implications for the sediment budget remain controversial as it is unclear whether their evolution simply represents a morphological change or denotes an escape of sediment from the upper shoreface. In order to provide new knowledge to address these questions, fifteen topo-bathymetric surveys that extend from the emerged beach to 15 m water depth were carried out along 1 km of Castelldefels beach (Llobregat Delta, Barcelona, western Mediterranean Sea) during the period 2011–2022. Morphological variations on the shoreface are studied, volumetric changes are estimated and implications on sedimentary exchanges in the littoral profile are inferred. The largest morphological (volumetric) changes are caused by cross-shore displacements of the nearshore bars, usually onshore during fair-weather conditions and offshore during storms. Volumetric changes suggest that shoreline retreat and the erosion of the shallow section of the profile during the last decade provides sediment to both the distal upper shoreface and the lower shoreface. The resulting sediment budget in the profile is almost zero in absence of extreme storms, but becomes highly negative when including extreme storms events that cause a net offshore transfer of sediment from the upper to the lower shoreface and, probably, the inner shelf. The characteristics of the study area (morphology, sediments, waves, tides) and in-situ hydrodynamic measurements at the lower shoreface suggest that the main potential onshore transport mechanisms (velocity and acceleration skewness during low-mid energy event) are small, making a significant transfer of sediment from the lower shoreface towards the shallow beach area unlikely at interannual scales. Thus, the study area is an example of low-lying coastal stretch in which the potential long-term shoreline retreat caused by sea level rise cannot be fully compensated by the sediment supplied from the lower shoreface. Other mechanisms, such as sediment supplied by longshore transport, are needed to counteract this trend.

Temperature-dependent dynamics of electrokinetic conservative and reactive transport in porous media: A model-based analysis

Electrokinetic techniques employ direct current electric fields to enhance the transport of amendments in low permeability porous media and have been demonstrated effective for in situ remediation of both organic contaminants and heavy metals. The application of electric potential gradients give rise to coupled chemical, hydraulic and electric fluxes, which are at the basis of the main transport mechanisms: electromigration and electroosmosis. Previous research has highlighted the significant impacts of charge interactions and fluid composition, including temperature-dependent properties such as electrolyte conductivity and density, on these transport phenomena. However, current models of electrokinetic applications often assume isothermal conditions and overlook the production of heat resulting from Joule heating.This study provides a detailed model-based investigation, systematically exploring the effects of temperature on electrokinetic conservative and reactive transport in porous media. By incorporating temperature-dependent material properties and progressively investigating the impact of temperature on each transport mechanism, we analyze the effects of temperature variations in both 1D and 2D systems. The study reveals how temperature dynamically influences the physical, chemical and electrostatic processes controlling electrokinetic transport. A temperature increase results in a higher speed of amendments delivery by both electromigration and electroosmosis and increases the kinetics of degradation reactions. The simulations also reveal a feedback mechanism in which higher aqueous conductivity results in increased Joule heating, leading to a faster temperature rise and, subsequently, to higher electrolyte conductivity. Finally, we estimate the electric energy requirements of the system at varying temperatures and show how these changes impact the rate of contaminant removal.

A comprehensive experimental and modelling approach for the evaluation of cross-over fluxes in Vanadium Redox Flow Battery

Vanadium cross-over is a critical issue in Vanadium Redox Flow Battery consisting in a complex interplay of different mechanisms of which a complete comprehension has not been reached yet. Due to the complexity of the involved phenomena, several models have been developed in literature to investigate vanadium cross-over. However, the conventional approaches for model calibration present a limited set of experiments for the validation preventing a complete understanding of cross-over phenomena. In this work a new and comprehensive approach is proposed. It is based on charge-discharge cycles with fixed exchanged capacity, able to isolate the capacity loss induced by cross-over fluxes, and on the measure of the self-discharge of the single electrolyte solutions by exploiting through-plate reference electrodes. Moreover, a 1D physically-based model of the operation of the battery is developed and calibrated on the data of electrolyte imbalance during charge-discharge cycles at three different current densities to obtain model parameters able to accurately describe the involved physics in different operating conditions. The model is then exploited to investigate the main vanadium transport mechanisms through the membrane and to evaluate the influence of the current density on the vanadium cross-over fluxes, net vanadium transport and self-discharge rate of the electrolyte.

A Physiology-Based Mathematical Model to Understand Drug Delivery from Contact Lenses to the Back of the Eye.

Therapeutic contact lenses, able to store drug and deliver it to the eye surface in a sustained fashion, gained interest as an effective and patient-friendly alternative to eye drops. Recent animal studies also demonstrated the presence of therapeutic drug levels in the back of the eye after wearing drug-loaded contact lenses, thus opening the possibility of treating the posterior segment without need of invasive intraocular injections. The drug pathways from contact lenses to the back of the eye require further investigation. A mechanistic mathematical model was developed to evaluate the drug concentration over time in the tears, sclera and choroid, retina, aqueous humor and vitreous humor after the application of a therapeutic contact lens. The main drug transport mechanisms of the eye and the barrier properties of the different tissues were included in the model. Validation was performed by comparison with experimental data in literature. The model predictions of drug concentration over time reflected the experimental data both in the anterior and posterior segment of the eye. The model can differentiate between contributions to transport from different pathways. The model constitutes a first step towards the possibility of predicting the ocular drug distribution and the treatment efficacy in the early stage of contact lens development, and it may help reduce both the need for in vivo tests (with ethical and economic advantages) and the gap between the lens design and clinical application. It also allows for an improved understanding of drug transport in the eye.

A Novel Extraction Procedure of Contact Characteristic Parameters from Current-Voltage Curves in CdZnTe and CdTe Detectors.

The estimation of the characteristic parameters of the electrical contacts in CdZnTe and CdTe detectors is related to the identification of the main transport mechanisms dominating the currents. These investigations are typically approached by modelling the current-voltage (I-V) curves with the interfacial layer-thermionic-diffusion (ITD) theory, which incorporates the thermionic emission, diffusion and interfacial layer theories into a single theory. The implementation of the ITD model in measured I-V curves is a critical procedure, requiring dedicated simplifications, several best fitting parameters and the identification of the voltage range where each transport mechanism dominates. In this work, we will present a novel method allowing through a simple procedure the estimation of some characteristic parameters of the metal-semiconductor interface in CdZnTe and CdTe detectors. The barrier height and the effects of the interfacial layer will be evaluated through the application of a new function related to the differentiation of the experimental I-V curves.

Protein Orientation Defines Rectification of Electronic Current via Solid-State Junction of Entire Photosystem-1 Complex.

We demonstrate that the direction of current rectification via one of nature's most efficient light-harvesting systems, the photosystem 1 complex (PS1), can be controlled by its orientation on Au substrates. Molecular self-assembly of the PS1 complex using four different linkers with distinct functional head groups that interact by electrostatic and hydrogen bonds with different surface parts of the entire protein PS1 complex was used to tailor the PS1 orientation. We observe an orientation-dependent rectification in the current-voltage characteristics for linker/PS1 molecule junctions. Results of an earlier study using a surface two-site PS1 mutant complex having its orientation set by covalent binding to the Au substrate supports our conclusion. Current-voltage-temperature measurements on the linker/PS1 complex indicate off-resonant tunneling as the main electron transport mechanism. Our ultraviolet photoemission spectroscopy results highlight the importance of the protein orientation for the energy level alignment and provide insight into the charge transport mechanism via the PS1 transport chain.

Identifying spatio-temporal climatic characteristics and events of the South-Asian aerosol pollution transport to the Tibetan Plateau

South Asian aerosol pollutants can be transported to the Tibetan Plateau (TP) and affect the local atmospheric environment; however, few studies have produced statistical results for the long term. In this study, 41 years of pollution transport events from South Asia to the TP were screened based on Modern-Era Retrospective Analysis for Research and Applications, Version 2 (MERRA-2) reanalysis data using objective methods, and the main pollution transport mechanisms were revealed. We used the empirical orthogonal functions (EOF) and the rotated empirical orthogonal functions (REOF) methods to obtain the spatiotemporal variation characteristics of the aerosol optical depth (AOD) over the TP and the large loading regions of AOD variation from 1980 to 2020 and combined them with the backward trajectory to screen for severe pollution events in the TP influenced by transport from South Asia. The three regions (eastern, central and western regions) with large AOD loading variations in the southern TP are shown by the REOF. Based on the objectively defined AOD indices for the three regions, 2058 days of strong pollution were identified, of which 1702 days (82.7%) were affected by pollution transport from South Asia. Of these 1702 days, the central region had the highest number of days at 661 (38.8%), followed by the western and eastern regions with 544 (32%) and 497 (29.2%) days, respectively. The results obtained from the transport events in the three regions suggest that South Asian pollutants can be transported across the Himalayas to the TP by valley winds, north-south valley passages, and cyclonic uplift. These findings deepen the understanding of the aerosol pollution of the TP affected by pollution transport from South Asia.

Potential distribution over temperature sensors of p-n junction diodes with arbitrary doping of the base region

In p-n-junction temperature sensors connected in the forwardbiased, the temperature dependence of the built-in potential is important, while in the reverse biased p-n-junction temperature sensors, it is necessary to study the temperature dependence of the built-in potential and space-charge region width.For this case, as well as for homogeneous and gradient alloyed cases, the temperature dependence of built-in potential and space-charge region widthare studied and mathematical analysis is presented for these cases.Based on these mathematical analysis, the results are obtained for cases where the base region of p-n-junction temperature sensors is doped at different concentrations with a homogeneous or inhomogeneous distributions of impurities. It is well known that in conventional temperature sensors, when the main current transport mechanism is determined by generation-recombination processes in space charge region, the dependence of the space charge region width on the temperature can affect the linearity of temperature response curve of sensor, it is desirable to increase the doping rate of the base region to weaken this effect, or it is necessary to use p-n junction.

Two-dimensional model of grain boundary diffusion and oxidation

The grinding of the structure of materials is accompanied by a change in the physical and mechanical properties. This occurs largely due to the accumulation of energy and defects in the structure, which activates the diffusion of impurities contained in the material. The increase in the number of grain boundaries and joints can cause the inelastic behavior of the material, its additional chemical activation. For some metals and alloys this leads to strengthening, while for others it leads to rapid degradation of mechanical properties. Grain boundary diffusion in such materials is the main mechanism of transport of alloying components or harmful impurities, so its study is important. This paper presents a two-dimensional model of grain boundary diffusion in a material with an explicit structure assignment. The model takes into account the presence of chemical transformations that can determine the corrosion mechanisms under operating conditions. For simplicity of calculation the material structure is taken symmetrical and contains two phases: grains and a boundary phase. Diffusion and kinetic parameters of the phases may differ. The model is represented in dimensionless form so that the distances between neighboring grains or the widths of the boundary phase are the same and the grain sizes can vary. Depending on the ratio of phase sizes we can speak about micro- and nanocrystalline structure. The problem was solved numerically using implicit difference scheme and coordinate splitting. Diffusion and kinetic parameters, which are close to the parameters of oxygen grain boundary diffusion in titanium and titanium oxidation, respectively, were taken for the calculations. Integral concentrations reflect the dynamics (kinetics) of oxygen and oxides accumulation over the calculation area. Results showing the role of changes in the oxidation kinetics due to changes in the reaction constants in the phases and the phase size ratio are presented. The model can be useful for assessing the degree of influence of grain boundary diffusion on the oxidation process and the accompanying change in properties, as well as for setting up appropriate experiments.

Where does the link between atmospheric moisture transport and extreme precipitation matter?

Atmospheric moisture transport is the primary component of the atmospheric branch of the water cycle, and its anomalies strongly influence drought and precipitation extremes. We utilised the full geographical and temporal spectrum of the ERA-5 reanalysis data and extreme value theory to identify regions where the atmospheric moisture transport, quantified as local integrated moisture vertical transport (IVT), influences daily extreme precipitation, and where this influence has a relevant dynamic component, which may alter the dependency between IVT and extreme precipitation as temperatures increase with climate change. We showed that this dependency is weak or negligible in tropical regions and strong but nonuniform in extratropical regions. Its influence is much greater in areas where the main moisture transport mechanisms occur, namely, atmospheric rivers, low-level jets, and tropical cyclones. The dynamic component of IVT, linked to wind, is highly consequential in regions with landfalling atmospheric rivers, landfalling tropical cyclones, or moisture-transporting low-level jets.

Hydrogen Diffusion in Organic-Rich Porous Media: Implications for Hydrogen Geo-storage

Subsurface hydrogen storage for large-scale energy supply to decarbonize a variety of greenhouse gas-emitting activities is gaining momentum worldwide to combat climate change. Hydrogen’s extensive range of flammable concentrations and low minimum ignition energy in air combined with its highly diffusive nature mandate implementation of comprehensive risk management protocols to maintain storage safety. In particular, the role of hydrogen diffusion process (self-diffusion) needs further investigation to understand H2 transport behavior and elude the associated risk of leakage for the long-term safety of the underground hydrogen storage process. Recent molecular simulation studies suggest that, compared to other subsurface storage options, depleted shale reservoirs may offer preferential storage characteristics due to excellent sealing and adsorption capabilities of shale. Data on hydrogen diffusion in organic (kerogen)-rich shale is scarce. Here, we applied molecular dynamics to compute hydrogen self-diffusivity in kerogen systems with different structures at various pressures, ranging from 3 to 41 MPa, and under an isothermal condition of 360 K. Two kerogen types of varying maturity (II-A and II-C) were utilized to create slit nanopores of 0.5 and 2 nm in size. The Knudsen number was calculated based on the mean free path and determined by the density values obtained from simulation to improve diffusion coefficient estimates. Results obtained indicate the Kn data in the range from 0.64 to 9.72, demonstrating that the main transport mechanism was transitional in nature. With increasing pressure, diffusivity declined regardless of the slit pore size or kerogen type. H2 diffusion was greater for the 2 nm pore system compared to both the 0.5 nm in II-A (0.004–0.02 cm2/s) and II-C (0.0037 to 0.019 cm2/s). In addition, for a fixed nanopore size, thermal maturity does not seem to impact diffusivity. Finally, simulation results were regressed to delineate a continuous description for diffusivity as a function of pressure. For curve fitting, R2 values were found to be 98 and 92% for 2 nm pore size of II-A and II-C, respectively, while 97 and 79% for 0.5 nm pore size of II-A and II-C, respectively. The results obtained are not only of interest for storage of hydrogen in shale but also for any subsurface hydrogen storage approach where a shale layer acts as a seal to form a trap.