H2 Flow Research Articles

Optimal design of supercritical He–H2 PCHE in SABER system by multi-objective genetic algorithm

A double-circle straight-channel printed circuit heat exchanger (PCHE) was first employed in a synergistic air-breathing rocket engine (SABER) system for supercritical He and H2 heat transfer. The heat transfer mechanism of the supercritical flow in the PCHE channel was numerically analyzed. For considered the pressure drop, heat transfer efficiency, and ratio of heat flux to weight, simultaneously, the design of the PCHE is multi-objective genetic optimized. The results shown that the supercritical H2 flow is chaotic near the pseudo-critical point, which is a coupled effect of buoyancy and gravity. Chaotic flow leads to an asymmetrical temperature distribution, which deteriorates the heat transfer performance. For 27 numerical experimental cases designed using the center composite surface method, the determine factors of the regression models of the three objectives for both cold and hot sides were all above 92 %. The Pareto optimal solutions for the supercritical He - H2 PCHE design and performance were obtained based on the nondominated sorting genetic algorithm II. From a comprehensive view of the three targets, the optimal designs were the A-4 and B-4 solutions for the cold and hot sides, respectively.

Effects of plasma spraying process on microstructure and mechanical properties of Cr2AlC/410 composite coatings

To investigate the influences of Cr2AlC mass fraction and supersonic plasma spraying process on the microstructure and mechanical properties of Cr2AlC reinforced 410 stainless steel composite coatings, the coatings containing different mass fractions of Cr2AlC were prepared and investigated. The composite coating exhibited low porosity and high adhesion strength. The addition of Cr2AlC significantly enhanced the hardness of the composite coatings through particle strengthening. However, when the mass fraction of Cr2AlC was 20%, the aggregation of Cr2AlC resulted in a strong decrease in the coating preparation efficiency, as well as a decline in adhesion strength. In the supersonic plasma spraying process, the Ar flow rate mainly influenced the flight velocity of the particles, while the H2 flow rate and the current mainly affected the temperature of the plasma torch. Consequently, all of them influenced the melting degree of particles and the quality of the coating. The lowest porosity and the highest hardness and adhesion strength could be obtained when the Ar flow rate is 125 L/min, the H2 flow rate is 25 L/min, and the current is 385 A.

Impact of Carrier Gas Flow Rate on the Synthesis of Monolayer WSe2 via Hydrogen-Assisted Chemical Vapor Deposition.

Transition metal dichalcogenides (TMDs), particularly monolayer TMDs with direct bandgap properties, are key to advancing optoelectronic device technology. WSe2 stands out due to its adjustable carrier transport, making it a prime candidate for optoelectronic applications. This study explores monolayer WSe2 synthesis via H2-assisted CVD, focusing on how carrier gas flow rate affects WSe2 quality. A comprehensive characterization of monolayer WSe2 was conducted using OM (optical microscope), Raman spectroscopy, PL spectroscopy, AFM, SEM, XPS, HRTEM, and XRD. It was found that H2 incorporation and flow rate critically influence WSe2's growth and structural integrity, with low flow rates favoring precursor concentration for product formation and high rates causing disintegration of existing structures. This research accentuates the significance of fine-tuning the carrier gas flow rate for optimizing monolayer WSe2 synthesis, offering insights for fabricating monolayer TMDs like WS2, MoSe2, and MoS2, and facilitating their broader integration into optoelectronic devices.

Cost-effectiveness and economic growth potential evaluation of olive pomace gasification process to sustainable fuel: Comparison study of different gasifying agent

This research addresses the growing global demand for sustainable and renewable energy solutions, focusing on biomass gasification (BG) as a promising avenue. Olive pomace is explored as a primary feedstock, assessing its potential as a plentiful and renewable energy source. The study aims to bridge existing gaps in the literature, particularly in the simultaneous and comparative examination of CO2, steam, and air as traditional gasifying agents. Utilizing Aspen Plus, a validated simulation tool, the research models the gasification process (GP) while considering key parameters such as gasifying agent, feed composition, and temperature. Through meticulous simulation and validation against experimental data, the goal is to develop a robust model tailored for gasification. A thorough sensitivity analysis evaluates critical parameters' impact on exergy efficiency, levelized cost of hydrogen (LCOH), and molar flow of hydrogen (H2). Parameters include temperature variations (600°C to 1000°C), equivalence ratio (ER) for the air agent (0.15–0.3), C/CO2 ratio for the CO2 agent (1–2.5), and SBR for the steam agent (0.5–2). The aim is to provide a nuanced understanding of these parameters and their influence on system performance, offering valuable insights for process optimization. The response surface methodology is employed to optimize critical parameters for distinct gasification agents, aiming to identify the most economically efficient agent. The study's findings are crucial for enhancing BG efficiency and developing sustainable energy systems. An economic evaluation underscores the cost-effectiveness of the GP, emphasizing its potential for economic growth in the renewable energy sector and long-term savings. In the concluding phase, optimization of the GP aimed at minimizing LCOH and maximizing molar flow of H2 and exergy efficiency was conducted. Upon scrutinizing the optimization results, a compelling conclusion arose: The steam agent emerged as the most suitable, with 48% exergy efficiency, 1.77 €/kg LCOH, and an optimal temperature of 884°C. Notably, steam showcased remarkable exergy efficiency, even when subjected to lower temperatures, positioning it as a superior choice.

Numerical and experimental investigation of performance and emission characteristics of hydrogen enrichment with prosopis juliflora biodiesel in CI engine using RSM and ANN optimization

IC engines need renewable fuel due to environmental degradation and fossil fuel depletion. Biodiesel was a popular alternative fuel. However, hydrogen has zero emissions and a higher auto ignition temperature. This study examines diesel engine operation with hydrogen-enriched juliflora biodiesel/diesel blend (B20). The 4-Stroke, direct injection diesel engine (KIRLOSKAR-AV1) produces 5.2 kW at 1500rpm. A B20 compound was injected into the combustion cylinder via the intake manifold during testing. Hydrogen was supplied through the intake manifold. Diesel (D0), B20, and B20 with hydrogen are tested on 6 lpm, 9 lpm, and 12 lpm respectively. Increase in hydrogen concentration with B20 blend will increased brake thermal efficiency. The mixture B20 at a flow rate of 12 L per minute increased the BTE by 27.51%, which is a 4.88% improvement compared to the original B20. Similarly, this concentration exhibit an 18.75% reduced Brake Specific Fuel Consumption (BSFC) compared to the B20 combination. CO and HC emissions decreases and increased in NOx emissions while increasing H2 flow. Further, to develop the RSM and ANN predicted model for optimization of CI engine characteristics responses. The RSM model analyzed the lower mean square error (MSE) and actual relative error (ARE) for the engine responses compared to the ANN model.

Precious metal-based Catalytic Membrane Reactors for continuous flow catalytic hydrodechlorination

This work is focused on the development of Catalytic Membrane Reactors (CMRs) comprising precious metals as active phases for a comparative assessment in continuous-flow catalytic hydrodechlorination (HDC). HDC has proved its remarkable potential for application as polishing step in drinking water treatment plants, but studies operating in continuous mode are scarce. Preliminary experiments were conducted in batch operation using Pd/Al2O3, Rh/Al2O3 and Pt/Al2O3 powder catalysts to evaluate the influence of the active phase on the removal of prochloraz (PCZ) (100 μg L−1), a pesticide listed on the EU Watch List (2022/1307), by HDC. PCZ removal was successfully described by a pseudo-first order kinetic equation and reaction pathways were proposed. Among the catalysts tested, Pt-based suffered a significant deactivation, not warranting the elimination of this micropollutant. Pd/Al2O3 exhibited a faster removal of PCZ than Rh/Al2O3, while this catalyst resulted in further hydrogenation of the non-chlorinated reaction product. Accordingly, different CMRs were developed by decorating cylindrical inert porous alumina membranes with Pd, Rh, and a combination of Pd-Rh as active phases (∼1% wt.). All CMRs showed a remarkable stability along 100 h on stream, being Pd/CMR be the most effective, with a pseudo-first order rate constant value of 0.062 min−1. An assessment of the impact of operating conditions (aqueous flow rate, PCZ initial concentration, temperature and H2 flow rate) was conducted using the Pd/CMR, which notably remained stable for 450 h on stream. The versatility of the system was finally demonstrated in tap water, achieving a steady-state PCZ conversion close to 95 %.

Hydrogen-included plasma-assisted reactive sputtering for conductivity control of ultra-wide bandgap amorphous gallium oxide

Conductivity control of a-Ga2Ox films by cation/anion off-stoichiometry such as oxygen vacancy formation and hydrogen doping have been achieved by hydrogen-included plasma-assisted reactive sputter deposition system and physical and electrical properties of a-Ga2Ox films formed by this system have been investigated. The change in resistivity of a-Ga2Ox thin films deposited by the hydrogen-included plasma-assisted reactive sputtering was then investigated by changing the H2 flow rate ratio H2/(Ar + H2). The a-Ga2Ox thin films with semiconducting properties with a resistivity as low as 102 Ωcm was demonstrated using the plasma-assisted reactive sputtering system with addition to H2. Along with the low resistivity, the a-GaOx thin films with high film density and band gap energy of 5.2 g cm−3 and 4.8 eV were realized. The electrical resistivity of the a-Ga2Ox thin films can be controlled from 102 Ωcm to 105 Ωcm by appropriately controlling the amount of hydrogen introduced from the plasma. The results indicate that the hydrogen acts as a shallow donor, which increases the carrier concentration, can be efficiently introduced by using the plasma-assisted reactive sputtering system with addition to H2.

Hydrogen production from biomass steam gasification: Experiment and simulation

In pine chips gasification experiment, at 700 °C and 15 % H2O, with the mass ratio of Ca90Fe10/B to 1.5, the H2 yield is 210.50 mL/g. The H2 yield improves by 2.39 times and the tar yield decreases by 16 % compared to without CaO. In the pine chips gasification simulation with the system's own thermal equilibrium, at 700 °C, the heat release of Ca90Fe10 adsorption of CO2 is 56.91 MJ/h for Ca90Fe10/B mass ratio of 1.5, and the required O2 inflow is 1.9 kg/h, the molar flow of H2 is 0.58 kmol/h. To achieve material circulation, 0.5 kg/h of CH4 is added to the calciner for combustion to satisfy heat absorption of CaCO3 to regenerate CaO, which consumes 2.0 kg/h of O2. The whole cycle O2 consumption is 3.90 kg/h, which is 1.02 kg/h less than without CaO, and the H2 yield has increased by 1.35 times.

A combined approach-based techno-economic-environmental multi-optimization of a hydrogen generation system through waste biomass air-steam gasification

In this study, a biomass air-steam gasification system for hydrogen production was simulated in Aspen Plus and validated using experimental studies. Sewage sludge was considered as the biomass feed. A combined approach including the combination of the NSGA-II algorithm, the response surface methodology (RSM) and TOPSIS decision-making method, was used to multi-optimize this hydrogen production system. To optimize the performance of the system, first, mathematical equations between equivalence ratio (ER), steam to biomass ratio (SBR), and gasifier temperature as the decision variables and the mole flow of H2 and CO2 and the levelized cost of hydrogen (LCOH) as the objective functions were obtained by the RSM technique. To maximize the mole flow of H2 and minimize CO2 emission and LCOH, a multi-objective optimization using NSGA-II was conducted. The ER, SBR and gasifier temperature were analyzed in the range of 0.1–0.4, 0.2 to 2 and 600 °C–1000 °C, respectively. By employing the TOPSIS decision-making method, several optimum points were obtained by giving each of the decision variables a specific weight. The lowest LCOH was found to be 1.5 €/kg H2 at the ER of 0.103, SBR of 0.83, and gasification temperature of 956 °C.

Subsurface hydrogen storage controlled by small-scale rock heterogeneities

Subsurface porous rocks hold significant hydrogen (H2) storage potential to support an H2-based energy future. Understanding H2 flow and trapping in subsurface rocks is crucial to reliably evaluate their storage efficiency. In this work, we perform cyclic H2 flow visualization experiments on a layered rock sample with varying pore and throat sizes. During drainage, H2 follows a path consisting of large pores and throats, through a low permeability rock layer, substantially reducing H2 storage capacity. Moreover, due to the rock heterogeneity and depending on the experimental flow strategy, imbibition unexpectedly results in higher H2 saturation compared to drainage. These results emphasize that small-scale rock heterogeneity, which is often unaccounted for in reservoir-scale models, plays a vital role in H2 displacement and trapping in subsurface porous media, with implications for efficient storage strategies.

Reactive sputtering of ferroelectric AlScN films with H2 gas flow for endurance improvement

The impact of H2 gas flow in the reactive sputtering process to 60 nm-thick ferroelectric Al1−x Sc x N films is investigated with x of 0.26 (high-Sc) and 0.12 (low-Sc). Al1−x Sc x N films exhibit clear ferroelectric switching, confirming the robustness against reducing ambient. The dielectric constants (ε i) as well as the leakage current decrease, and the breakdown field (E BD) increases with H2 flow. Although the remanent polarization (P r) decreases with H2 flow, the wake-up effect is suppressed for the high-Sc film, and the fatigue effect is weakened for the low-Sc film. By probing the change in the coercive field (E c) after the switching cycle test, we anticipate oxygen impurities bonded to Sc and Al atoms are the source of wake-up and fatigue effects, respectively. As a result, a high endurance cycle of 2 × 107 times was achieved for low-Sc films with H2 flow.

Influence of deposition temperature and hydrogen on sustainable and transfer-free graphene transparent electrode for organic solar cells

The transfer-free graphene transparent conducting electrode (TCE) is a promising alternative to indium tin oxide (ITO) for organic solar cells (OSCs). In the present work, a comprehensive investigation on how deposition temperature and H2 flow rates affect the growth, structural, optical, and electrical properties of graphene produced by RF plasma enhanced chemical vapor deposition using sustainable sources was conducted. Inverted-geometry OSCs with P3HT: PCBM photoactive layer were fabricated on transfer-free graphene TCEs developed under different conditions. Moreover, the coupling of silver nanowires (AgNWs) with different graphene films was studied for hybrid graphene-AgNWs TCEs for OSCs. Devices based on graphene TCEs prepared at low or zero H2 flow have shown better performances than those at high flow of H2. Similarly, graphene TCEs prepared at high temperature (>700 °C, on quartz) led to a deteriorated device performance due to the highly increased growth of vertically oriented graphene nanosheets, which dramatically reduced film transmittance and increased surface roughness. The present work provides solid understanding of the growth mechanism of RF-PECVD graphene on glass from a sustainable carbon source. More importantly, the sustainable, ecofriendly, cost- and time-effective production of scalable transfer-free graphene TCEs for OSCs is optimized which paves the way towards ITO-free optoelectronics.

Vapor-phase deoxydehydration of 1,2-propanediol to form propylene over Cu-loaded H3PO4/SiO2-Al2O3 catalyst

Vapor-phase catalytic deoxydehydration of 1,2-propanediol (1,2-PDO) to form propylene was performed over various metal-loaded catalysts in an H2 flow at an ambient pressure. Cu-based catalysts exhibited a relatively high activity for the formation of propylene, while other metal-based catalysts, such as Pt-, Ru-, Rh-, Ni-, and Co-loaded SiO2-Al2O3, primarily produced C-C bond cleavage products. Cu-loaded H3PO4/SiO2-Al2O3 catalysts showed the best catalytic performance, and the catalyst with a Cu and H3PO4 loading of 8 and 10 wt%, respectively, could provide a propylene selectivity of 95.0% with a complete conversion of 1,2-PDO at 280 °C. The Cu-loaded H3PO4/SiO2-Al2O3 catalysts primarily contained Lewis acid sites, and the Cu particles were well dispersed on the catalyst surface with an averaged particle size of 3.1 nm. It is proposed that the vapor-phase deoxydehydration of 1,2-PDO to propylene undergoes multi-step reactions with propanal and 1-propanol as the reaction intermediates.

Hydrogenolysis of biomass‐derived levulinic acid to produce 1,4‐pentanediol over Co/ZrO2 catalyst

AbstractBACKGROUNDLevulinic acid is an important platform chemical derivable from lignocellulosic biomass. In this work, the hydrogenolysis of levulinic acid to produce 1,4‐pentanediol was performed over some Co‐based non‐noble catalysts. The relationship between the catalyst properties and the catalytic activity was investigated, and the effects of the reaction conditions on the catalytic activity were studied.RESULTSThe Co/ZrO2 catalyst prepared by a coprecipitation method showed a high catalytic performance in the hydrogenolysis of levulinic acid to form 1,4‐pentanediol. The calcination of Co/ZrO2 significantly affected the catalytic activity, and a high calcination temperature of 750 °C could promote the formation of 1,4‐pentanediol. Meanwhile, the reduction of Co/ZrO2 at 400 °C in an H2 flow was beneficial for producing 1,4‐pentanediol, and the Co species after the reduction existed as the forms of Co2+ and Co0. A high H2 pressure and a moderate reaction temperature of 200 °C were beneficial for producing 1,4‐pentanediol, and a high 1,4‐pentanediol selectivity of 85.6% with a complete conversion of levulinic acid was obtained over the Co/ZrO2 catalyst at 200 °C under an H2 pressure of 7 MPa.CONCLUSIONThe Co species in the 400 °C‐reduced Co/ZrO2 catalyst existed as the forms of Co2+ and Co0. It was proposed that the synergistic effect of Co2+ and Co0 contributed to the effective formation of 1,4‐pentanediol from levulinic acid. © 2024 Society of Chemical Industry (SCI).

Alleviating anode flooding by mesoporous carbon supports for alkaline polymer electrolyte fuel cells

Water management is one of the critical issues affecting the performance and stability of alkaline polymer electrolyte fuel cells (APEFCs). This study focuses on the effect of carbon supports on the water management of APEFCs. A series of carbon-supported Ru catalysts (Ru/MCP-x) were prepared using mesoporous carbon powder (MCP-x) with three-dimensional through-nanopore structures as supports and compared with Ru/XC72. The results show that although the catalysts’ hydrogen oxidation reaction (HOR) catalytic activities are similar in KOH solutions, the APEFCs performance can be severely different. APEFCs assembled with Ru/MCP-x and Ru/XC72 (noted as Ru/MCP-x cell and Ru/XC72 cell) have little performance difference at high inlet H2 flow (1000 mL/min), but have a noticeable difference at low inlet H2 flow (200 mL/min). By combining the electrochemical AC impedance and distribution of relaxation time (DRT) method, we quantitatively identify that severe gas transfer resistance occurred in the anode catalyst layer of the Ru/XC72 cell under low inlet H2 flow, which is more in line with the actual APEFCs operating conditions, led to considerable degradation of the Ru/XC72 cell performance. The high gas transfer resistance is later ascribed to the anode flooding by combining the voltammetry curves and DRT plots. In contrast, for Ru/MCP-x cells, increasing the carbon supports’ mesopore diameter dramatically reduced the gas transfer resistance and mitigated the anode flooding. This study shows a quantitative comparison of the impacts of different carbon supports on APEFCs performance and gas transfer resistance, indicating that mesoporous carbon materials have the potential to be supports for APEFCs anode catalysts to alleviate the anode flooding.

Unraveling the phylogenomic diversity of Methanomassiliicoccales and implications for mitigating ruminant methane emissions

BackgroundMethanomassiliicoccales are a recently identified order of methanogens that are diverse across global environments particularly the gastrointestinal tracts of animals; however, their metabolic capacities are defined via a limited number of cultured strains.ResultsHere, we profile and analyze 243 Methanomassiliicoccales genomes assembled from cultured representatives and uncultured metagenomes recovered from various biomes, including the gastrointestinal tracts of different animal species. Our analyses reveal the presence of numerous undefined genera and genetic variability in metabolic capabilities within Methanomassiliicoccales lineages, which is essential for adaptation to their ecological niches. In particular, gastrointestinal tract Methanomassiliicoccales demonstrate the presence of co-diversified members with their hosts over evolutionary timescales and likely originated in the natural environment. We highlight the presence of diverse clades of vitamin transporter BtuC proteins that distinguish Methanomassiliicoccales from other archaeal orders and likely provide a competitive advantage in efficiently handling B12. Furthermore, genome-centric metatranscriptomic analysis of ruminants with varying methane yields reveal elevated expression of select Methanomassiliicoccales genera in low methane animals and suggest that B12 exchanges could enable them to occupy ecological niches that possibly alter the direction of H2 utilization.ConclusionsWe provide a comprehensive and updated account of divergent Methanomassiliicoccales lineages, drawing from numerous uncultured genomes obtained from various habitats. We also highlight their unique metabolic capabilities involving B12, which could serve as promising targets for mitigating ruminant methane emissions by altering H2 flow.

Optimization of ex-situ biomethanation process in trickle bed reactor: The impact of slight H2/CO2 ratio adjustments and different packing materials

This study aimed to evaluate and optimize trickle bed reactor (TBR) performance for biological biogas upgrading at different gas loading rates (10-35 m3/m3d) by adjusting H2 flow (H2/CO2 ratio 4-3.7) and utilizing various packing materials. The three TBR reactors operated at thermophilic conditions (50○C) with different packing materials under same gas loading rate. Obtained results indicated that optimal performance was achieved at a gas loading rate of 14.3 m3/m3d and H2/CO2 ratio of 3.7, with average CH4 concentrations in the effluent gas from 90.8 % to 91.5 %, regardless of the packing material employed. Increasing the gas loading rate resulted in decreased CH4 content (<90 %), indicating limited treatment capacity at higher loading rates. The studied packing materials had a slight impact on reactor performance indicating that the shape of the making material has a greater influence of the reactor performance. Microbial communities analyses revealed dominance of hydrogenotrophic methanogens (Methanobacterium, Methanothermobacter, and Methanoculleus). This study highlights the importance of optimizing the H2/CO2 and considering packing materials for TBR performance.

Low-temperature oxidative removal of benzene from the air using titanium carbide (MXene)-Supported platinum catalysts

MXenes are an emerging class of two-dimensional (2D) inorganic materials with great potential for versatile applications such as adsorption and catalysis. Here, we describe the synthesis of a platinized titanium carbide MXene (Pt@Ti3C2) catalyst with varying amounts of platinum (0.1%–2 wt.%) for the low-temperature oxidation of benzene, an aromatic volatile organic compound often found in industrial flue gas. A 1% formulation of Pt@Ti3C2–R allowed near-complete (97%) oxidation of benzene to CO2 at 225 °C with a steady-state reaction rate (r) of 0.119 mol g−1·h−1. This low-temperature catalytic oxidation reaction was promoted by an increase in the lattice oxygen (O*)/Pt2+ species (active sites) of 1%Pt@Ti3C2–R from 45.3/34.6% to 71.0/61.1% through pre-thermal reduction under H2 flow, as revealed by X-ray photoelectron spectroscopy, temperature-programmed reduction, and in situ diffuse reflectance infrared Fourier transform spectroscopy analyses. The cataltyic activity of 1% Pt@Ti3C2–R against benzene was assessed under the control of the key process variables (e.g., catalyst mass, flow rate, benzene concentration, relative humidity, and time-on-stream) to help optimize the oxidation reaction process. The results provide new insights into the use of platinum-based 2D MXene catalysts for low-temperature oxidative removal of benzene from the air.

Photovoltaic and mechanical properties of boron carbide films prepared by magnetron sputtering

In this study, amorphous boron carbide (B x C) and hydrogenated amorphous B x C (B x C:H) films were deposited via magnetron sputtering to investigate the effects of hydrogen on the film properties. The critical load decreased with increasing H2 flow ratio, probably due to an increase in the internal stress. In addition, the optical bandgap increased with the H2 flow ratio increased. The bandgap was controlled by the chemical bonding of carbon atoms and the chemical composition of films. The resistivity of the films increased with the H2 flow ratio and bandgap energy. The current–voltage characteristics of B x C(:H)/n-type Si heterojunctions under white light illumination showed that the short-circuit current density and open-circuit voltage were higher than the previously reported values. Results revealed that the introduction of hydrogen during deposition reduced the short-circuit current density, fill factor, and conversion efficiency, whereas the open-circuit voltage remained almost unchanged.

Sublimation-based wafer-scale monolayer WS2 formation via self-limited thinning of few-layer WS2.

Atomically-thin monolayer WS2 is a promising channel material for next-generation Moore's nanoelectronics owing to its high theoretical room temperature electron mobility and immunity to short channel effect. The high photoluminescence (PL) quantum yield of the monolayer WS2 also makes it highly promising for future high-performance optoelectronics. However, the difficulty in strictly growing monolayer WS2, due to its non-self-limiting growth mechanism, may hinder its industrial development because of the uncontrollable growth kinetics in attaining the high uniformity in thickness and property on the wafer-scale. In this study, we report a scalable process to achieve a 4 inch wafer-scale fully-covered strictly monolayer WS2 by applying the in situ self-limited thinning of multilayer WS2 formed by sulfurization of WOx films. Through a pulsed supply of sulfur precursor vapor under a continuous H2 flow, the self-limited thinning process can effectively trim down the overgrown multilayer WS2 to the monolayer limit without damaging the remaining bottom WS2 monolayer. Density functional theory (DFT) calculations reveal that the self-limited thinning arises from the thermodynamic instability of the WS2 top layers as opposed to a stable bottom monolayer WS2 on sapphire above a vacuum sublimation temperature of WS2. The self-limited thinning approach overcomes the intrinsic limitation of conventional vapor-based growth methods in preventing the 2nd layer WS2 domain nucleation/growth. It also offers additional advantages, such as scalability, simplicity, and possibility for batch processing, thus opening up a new avenue to develop a manufacturing-viable growth technology for the preparation of a strictly-monolayer WS2 on the wafer-scale.