Hydrofluoroolefin-based mixed refrigerant for enhanced performance of hydrogen liquefaction process

Hydrogen has the highest gravimetric energy density of all fuels; however, it has a low volumetric energy density, unfavorable for storage and transportation. Hydrogen is usually liquefied to meet the bulk transportation needs. The exothermic interconversion of its spin isomers is an additional activity to an already energy-intensive process. The most significant temperature drop occurs in the precooling cycle (between ?150 °C and up to ?180 °C) and consumes more than 50% of the required energy. To reduce the energy consumption and improve the exergy efficiency of the hydrogen liquefaction process, a new high-boiling component, Hydrofluoroolefin (HFO-1234yf), is added to the precooled mixed refrigerant. As a result, the specific energy consumption of precooling cycle reduces by 41.8%, from 10.15 kWh/kg_LH2 to 5.90 kWh/kg_LH2, for the overall process. The exergy efficiency of the proposed case increases by 43.7%; however, the total equipment cost is also the highest. The inflated cost is primarily due to the added ortho-to-para hydrogen conversion reactor, boosting the para-hydrogen concentration. From the perspective of bulk storage and transportation of liquid hydrogen, the simplicity of design and low energy consumption build a convincing case for considering the commercialization of the process.     

Expected impacts on greenhouse gas and air pollutant emissions due to a possible transition towards a hydrogen economy in German road transport

Transitioning German road transport partially to hydrogen energy is among the possibilities being discussed to help meet national climate targets. This study investigates impacts of a hypothetical, complete transition from conventionally-fueled to hydrogen-powered German transport through representative scenarios. Our results show that German emissions change between ?179 and +95 MtCO₂eq annually, depending on the scenario, with renewable-powered electrolysis leading to the greatest emissions reduction, while electrolysis using the fossil-intense current electricity mix leads to the greatest increase. German energy emissions of regulated pollutants decrease significantly, indicating the potential for simultaneous air quality improvements. Vehicular hydrogen demand is 1000 PJ annually, requiring 446–525 TWh for electrolysis, hydrogen transport and storage, which could be supplied by future German renewable generation, supporting the potential for CO₂-free hydrogen traffic and increased energy security. Thus hydrogen-powered transport could contribute significantly to climate and air quality goals, warranting further research and political discussion about this possibility.     

A review on alloyed quantum dots and their applications as photocatalysts

Semiconductor driven artificial photocatalysis is the most sustainable technology towards addressing the growing energy and environmental pollution issues. In this context, alloyed quantum dots (QDs) are an emerging class of promising nanomaterials gathering tremendous attention in this area due to several beneficial features. Compared to other bulk semiconductors, alloyed QDs are cost-effective, stable, less-toxic with superior optoelectronic features, which significantly enhances their solar energy conversion efficiency. Herein, the present review summarizes the fundamentals of alloyed QDs, various synthesis techniques, and discusses optical as well as structural properties from data interpretation point of view taking suitably reported literature. Moreover, we have provided a comprehensive summary of recent state of art metal chalcogenides based alloyed QD systems towards H₂ evolution, CO₂ reduction, and pollutant degradation. Finally, the review discusses the associated challenges and future prospects of alloyed QDs with a special focus on preparation, property engineering, theoretical aspect, stability and other field application. Additionally, the overarching aim is to provide researchers an in-depth understanding in the field of alloyed QDs relating to synthesis, characterisation, and promotes their photocatalytic applications, and can foster as a manual to future researchers.     

Potential of renewable surplus electricity for power-to-gas and geo-methanation in Switzerland

Energy systems are increasingly exposed to variable surplus electricity from renewable sources, particularly photovoltaics. This study estimates the potential to use surplus electricity for power-to-gas with geo-methanation for Switzerland by integrated energy system and power-to-gas modelling. Various CO₂ point sources are assessed concerning exploitable emissions for power-to-gas, which were found to be abundantly available such that 60 TWh surplus electricity could be converted to methane, which is the equivalent of the current annual Swiss natural gas demand. However, the maximum available surplus electricity is only 19 TWh even in a scenario with high photovoltaic expansion. Moreover, making this surplus electricity available for power-to-gas requires an ideal load shifting capacity of up to 10 times the currently installed pumped-hydro capacity. Considering also geological and economic boundary conditions for geo-methanation at run-of-river and municipal waste incinerator sites with nearby CO₂ sources reduces the exploitable surplus electricity from 19 to 2 TWh.     

The production and application of hydrogen in steel industry

Due to the increasingly serious environmental issues and continuous depletion of fossil resources, the steel industry is facing unprecedented pressure to reduce CO₂ emissions and achieve the sustainable energy development. Hydrogen is considered as the most promising clean energy in the 21st century due to the diverse sources, high calorific value, good thermal conductivity and high reaction rate, making hydrogen have great potential to apply in the steel industry. In this review, different hydrogen production technologies which have potential to provide hydrogen or hydrogen-rich gas for the great demand of steel plants are described. The applications of hydrogen in the blast furnace (BF) production process, direct reduction iron (DRI) process and smelting reduction iron process are summarized. Furthermore, the functions of hydrogen or hydrogen-rich gas as fuels are also discussed. In addition, some suggestions and outlooks are provided for future development of steel industry in China.     

Titanate quantum dots-sensitized Cu2S nanocomposites for superficial H2 production via photocatalytic water splitting

TiO₂ quantum dots-sensitized Cu₂S (Cu₂S/TiO₂) nanocomposites with varying concentration of TiO₂ QDs are synthesized via a facile two-stage hydrothermal-wet impregnation method. X-ray diffraction analysis confirms the formation of Cu₂S and TiO₂with chalcocite and anatase phases, respectively. The observed shoulder-like absorption peaks indicate the UV–visible light-driven properties of the composite. Morphological analysis reveals that the fabricated Cu₂S/TiO₂ composite consists of Cu₂S with a nano rod-like shape (average length and width of ～856 and ～213 nm, respectively) and nanosheets-like structures (average length and width of ～283 and ～289 nm, respectively), whereas the TiO₂ is formed as quantum dots with a size range of 8.2 ± 0.4 nm. Chemical state analysis shows the presence of Cu⁺, S^2?, Ni²⁺, and O^2? in the nanocomposite. The H₂ evolution rate over the optimized photocatalyst is found to be ～45.6 mmol h^?1g^?1_cat under simulated solar irradiation, which is around 5 and 2.4-fold higher than that of the pristine TiO₂ and Cu₂S, respectively. Continuous H₂ production for 30 h is achieved during time-on-stream experiments, demonstrating the excellent stability and durability of the Cu₂S/TiO₂ photocatalyst for large-scale applications.     

Natural aloe vera derived Pt supported N-doped porous carbon: A highly durable cathode catalyst of PEM fuel cell

Evolution of highly durable electrocatalyst for oxygen reduction reaction (ORR) is the most critical barrier in commercializing polymer electrolyte membrane fuel cell (PEMFC). In this work, Pt deposited N-doped mesoporous carbon derived from Aloe Vera is developed as an efficient and robust electro catalyst for ORR. Due to its high mesoporous nature, the aloe vera derived carbon (AVC) play a very vital role in supporting Pt nanoparticles (NPs) with N-doping. After doping N into AVC, more defects are created which facilitates uniform distribution of Pt NPs leading to more active sites towards ORR. Pt/N-AVC shows excellent ORR activity when compared with commercial Pt/C and showing a half wave potential (E_1/2–0.87 V Vs. RHE) and reduction potential (E_red ~ 0.72 V Vs. RHE) towards ORR. Even after 30,000 potential cycles, Pt/N-AVC shows in its E_1/2 only ~5 mV negative shift and lesser agglomeration of Pt NPs is seen in the catalyst. In membrane electrode assembly (MEA) fabrication, Pt/N-AVC as a cathode catalyst in a PEMFC fixture and performance were studied. The Pt/N-AVC shows good performance, which proves the potential application of this naturally available bio derived carbon, which serves as an excellent high durable support material in PEMFC. All these features show that the Pt/N-AVC is the most stable, efficient and suitable candidate for ORR catalyst.     

Facile modified polyol synthesis of FeCo nanoparticles with oxyhydroxide surface layer as efficient oxygen evolution reaction electrocatalysts

The oxygen evolution reaction (OER) at anode requires high overpotential and is still challenging. The metallic core-oxyhydroxide layer structure is an efficient method to lower an overpotential. We synthesized Fe rich FeCo core-Co rich FeCo oxyhydroxide layer with a different particle size of 173 nm, 225 nm, and 387 nm (FeCo 173, 225, 387) through a difference in the reduction rate of Fe/Co precursors using facile modified polyol synthesis. To investigate the effect of conductivity, CoFe₂O₄ nanoparticles of 80–130 nm were synthesized. Among samples, FeCo 173 showed remarkable catalytic performance of 316 mV at a current density of 10 mA/cm² in 0.1 M KOH compared to RuO₂ (408 mV), FeCo 225 (323 mV), FeCo 387 (334 mV), CoFe₂O₄ (382 mV). Moreover, FeCo 173 showed good stability for 60,000 s while RuO₂ showed a gradual increase in overpotential to maintain 10 mA/cm² after 15,000 s in chronopotentiometry. The excellent performance was attributed to Fe-rich metallic core, a small amount of Fe doping into CoOOH, and the synergic effect between the active site of Co rich FeCoOOH and conductive Fe rich metallic core. Following this result, it shows that the use of such FeCo electrodes has advantages in the production of hydrogen via electrochemical water oxidation.     

Simultaneous biohydrogen (H2) and bioplastic (poly-β-hydroxybutyrate-PHB) productions under dark,photo, and subsequent dark and photo fermentation utilizing various wastes

The present study is focused on bio hydrogen (H₂) and bioplastic (i.e., poly-β-hydroxybutyrate; PHB) productions utilizing various wastes under dark fermentation, photo fermentation and subsequent dark-photo fermentation. Potential bio H₂ and PHB producing microbes were enriched and isolated. The effects of substrate (rice husk hydrolysate, rice straw hydrolysate, dairy industry wastewater, and rice mill wastewater) concentration (10–100%) and pH (5.5–8.0) were examined in the batch mode under the dark and photo fermentation conditions. Using 100% rice straw hydrolysate at pH 7, the maximum bio H₂ (1.53 ± 0.04 mol H₂/mol glucose) and PHB (9.8 ± 0.14 g/L) were produced under dark fermentation condition by Bacillus cereus. In the subsequent dark-photo fermentation, the highest amounts of bio H₂ and PHB were recorded utilizing 100% rice straw hydrolysate (1.82 ± 0.01 mol H₂/mol glucose and 19.15 ± 0.25 g/L PHB) at a pH of 7.0 using Bacillus cereus (KR809374) and Rhodopseudomonas rutila. The subsequent dark-photo fermentative bio H₂ and PHB productions obtained using renewable biomass (i.e., rice husk hydrolysate and rice straw hydrolysate) can be considered with respect to the sustainable management of global energy sources and environmental issues.     

CoMo heterohierarchical foam-structured cathode for anion exchange membrane water electrolyzer

It is important to consider the synergy of heterostructures to improve the slow kinetics of water dissociation in the alkaline hydrogen evolution reaction (HER). Herein, we report a simple method to design a heterohierarchical CoMo catalyst. The CoMo catalyst was prepared by simple one-pot electrodeposition on carbon paper (CP). The CoMo/CP catalyst was optimized for the alkaline HER by controlling the electrodeposition bath conditions, potential, and time. The optimized catalyst shows the heterohierarchical structure containing the electrically conductive metallic Co in the bulk and Mo-incorporated Co containing Mo⁴⁺ at the surface. It exhibited a lower HER overpotential of 0.11 V at ?20 mA/cm² compared to those of the others owing to the synergetic effect of the between the Co and Mo incorporated Co. The results highlight the advantages of the simple method developed herein for the design of heterohierarchical catalysts.     

Effect of organic fraction of municipal solid waste addition to high rate activated sludge system for hydrogen production from carbon rich waste sludge

Municipal solid waste has been used for bio-methane production for many years. However, both methane and carbon dioxide that is produced during bio-methanization increases the greenhouse gas emissions; therefore, hydrogen production can be one of the alternatives for energy production from waste. Hydrogen production from the organic substance was studied in this study with the waste activated sludge from the municipal wastewater treatment. High rated activated sludge (HRAS) process was applied for the treatment to reduce energy consumption and enhance the organic composition of WAS. The highest COD removal (76%) occurred with the 12 g/L organic fraction of municipal solid waste (OFMSW) addition at a retention time of 120 min. The maximum hydrogen and methane yields for the WAS was 18.9 mL/g VS and 410 mL/g VS respectively. Total carbon emission per g VS of the substrate (OFMSW + waste activated sludge) was found as 0.087 mmol CO₂ and 28.16 mmol CO₂ for dark fermentation and bio-methanization respectively. These kinds of treatment technologies required for the wastewater treatment plantcompensate it some of the energy needs in a renewable source. In this way, the HRAS process decreases the energy requirement of wastewater treatment plant, and carbon-rich waste sludge enables green energy production via lower carbon emissions.     

High-efficiency of novel hierarchical 3D macroporous g-C3N4 material on solar-driven photocatalytic water-splitting for hydrogen evolution

The use of multi-pore nanostructured g-C₃N₄ photocatalysts is an efficient approach to separate photogenerated charge carriers and increase visible light photocatalytic performance. Recent progress has yielded nanostructured material through hard templating, which limits potential applications. Integrating the 2D building block into multidimensional porous structures remains a significant challenge in scalable production. Herein, a novel technique based on P407 bubble clusters templating and fixation by freezing is described for the first time to fabricate a 3D opened-up macroporous g-C₃N₄ nanostructures for photocatalytic H₂ evolution. Three-dimensional hierarchical nanostructures provide more contact active sites and synergistically promote the creation of heterogeneous catalytic interfaces. This feature is very useful for understanding the transfer path of photoinduced charges as well as the origins of the high charge separation efficiency in photocatalytic reactions, thus yielding a remarkable visible light-induced H₂ evolution rate of 4213.6 μmol h⁻¹ g⁻¹, which is nearly 5.6 times (716 μmol h⁻¹ g⁻¹) higher than that of lamellar bulk g-C₃N₄. This newly developed approach offers a promising alternative to synthesize broad-spectral response 3D hierarchal g-C₃N₄ nanostructures and can be extended to assemble other functional nanomaterials as building blocks into macroscopic configurations coupled with electronic modulation strategy simultaneously.     

Polyaniline sensitized Pt@TiO2 for visible-light-driven H2 generation

Polyaniline is a typical conducting polymer with high migration electron rate, good stability, eco-friendly properties, and high absorption coefficients for visible light. In the present study, polyaniline decorated Pt@TiO₂ for visible light-driven H₂ generation is reported for the first time. The above-mentioned nanocomposite is prepared through a simple oxidative-polymerization and characterized by infrared spectroscopy, transmission electron microscopy, X–ray diffraction, thermogravimetric analysis, and ultraviolet–visible diffuse reflectance spectra. Polyaniline modification improves the absorption of the nanocomposite in visible light region via a photosensitization effect similar to dye–sensitization but does not influence the crystal structure and size of Pt@TiO₂. The polyaniline modified Pt@TiO₂ exhibits a remarkable visible light activity (61.8 μmol h⁻¹ g⁻¹) and good stability for H₂ generation (with an average apparent quantum yield of 10.1%) with thioglycolic acid as an electron donor. This work provides new insights into using conducting polymers, including polyaniline, as a sensitizer to modify Pt@TiO₂ for visible-light hydrogen generation.     

Enhanced hydrogen storage performance of Cu3(BTC)2 in situ inserted with few-layer silicon-based nanosheets

Silicon-based nanosheets (SNS) were synthesised via a mild (60 °C) and time-saving (8 h) modified topochemical method. Then, Cu₃(BTC)₂ and SNS@Cu₃(BTC)₂ were successfully synthesised by microwave irradiation, and their characteristics and hydrogen storage performance were analysed by multiple techniques. The accordion-like SNS exhibited void spaces, a unique low buckled structure, and ultrathin, almost transparent, loosely stacked layers with a high specific surface area (362 m²/g). After in-situ synthesis with Cu₃(BTC)₂, the SNS compound achieved a high specific surface area (1526 m²/g), outstanding hydrogen storage performance (5.6 wt%), and a desirable hydrogen diffusion coefficient (10^?7). Thus, SNS doping improved the hydrogen storage performance of Cu₃(BTC)₂ by 64% through electron transfer reactions with Cu enabled by the unique composite nanostructure of SNS@Cu₃(BTC)₂. This study presents a promising method of synthesising SNS and porous composite materials for hydrogen storage.     

A strategy of mid-temperature natural gas based chemical looping reforming for hydrogen production

Steam methane reforming (SMR) needs the reaction heat at a temperature above 800 °C provided by the combustion of natural gas and suffers from adverse environmental impact and the hydrogen separated from other chemicals needs extra energy penalty. In order to avoid the expensive cost and high power consumption caused by capturing CO₂ after combustion in SMR, natural gas Chemical Looping Reforming (CLR) is proposed, where the chemical looping combustion of metal oxides replaced the direct combustion of NG to convert natural gas to hydrogen and carbon dioxide. Although CO₂ can be separated with less energy penalty when combustion, CLR still require higher temperature heat for the hydrogen production and cause the poor sintering of oxygen carriers (OC). Here, we report a high-rate hydrogen production and low-energy penalty of strategy by natural gas chemical-looping process with both metallic oxide reduction and metal oxidation coupled with steam. Fe₃O₄ is employed as an oxygen carrier. Different from the common chemical looping reforming, the double side reactions of both the reduction and oxidization enable to provide the hydrogen in the range of 500–600 °C under the atmospheric pressure. Furthermore, the CO₂ is absorbed and captured with reduction reaction simultaneously.Through the thermodynamic analysis and irreversibility analysis of hydrogen production by natural gas via chemical looping reforming at atmospheric pressure, we provide a possibility of hydrogen production from methane at moderate temperature. The reported results in this paper should be viewed as optimistic due to several idealized assumptions: Considering that the chemical looping reaction is carried out at the equilibrium temperature of 500 °C, and complete CO₂ capture can be achieved. It is assumed that the unreacted methane and hydrogen are completely separated by physical adsorption. This paper may have the potential of saving the natural gas consumption required to produce 1 m³ H₂ and reducing the cost of hydrogen production.     

Assessment of ammonia as energy carrier in the use with reversible solid oxide cells

Ammonia represents one of the most promising potential solutions as energy vector and hydrogen carrier, having a higher potential to transport energy than hydrogen itself in a pressurized form. Furthermore, solid oxide fuel cells (SOFCs) can directly be fed with ammonia, thus allowing for immediate electrical power and heat generation. This paper deals with the analysis of the dynamic behavior of commercial SOFCs when fueled with ammonia. Several measurements at different temperatures have been performed and performances are compared with hydrogen and a stoichiometrically equivalent mixture of H₂ and N₂ (3:1 M ratio). Higher temperature led to smaller drops in voltage for both fuels, thus providing higher efficiencies. Ammonia resulted slightly more performant (48% at 760 °C) than hydrogen (45% at 760 °C), in short stack tests. Moreover, different ammonia-to-air ratios have been investigated and the stack area-specific resistance has been studied in detail by comparing numerical modeling predictions and experimental values.     

Kinetics of formic acid decomposition in subcritical and supercritical water – a Raman spectroscopic study

The decomposition of formic acid is studied in a continuous sub- or supercritical water reactor at temperatures between 300 and 430 °C, a pressure of 25 MPa, residence times between 4 and 65 s, and a feedstock concentration of 3.6 wt%. In situ Raman spectroscopy is used to produce real-time data and accurately quantify decomposition product yields of H₂, CO₂, and CO. Collected spectra are used to determine global decomposition rates and kinetic rates for individual reaction pathways. First-order global Arrhenius parameters are determined as log A (s⁻¹) = 1.6 ± 0.20 and E_A = 9.5 ± 0.55 kcal/mol for subcritical decomposition, and log A (s⁻¹) = 12.56 ± 1.96 and E_A = 41.90 ± 6.08 kcal/mol for supercritical decomposition. Subcritical and supercritical Arrhenius parameters for individual pathways are proposed. The variance in rate parameters is likely due to changing thermophysical properties of water across the critical point. There is strong evidence for a surface catalyzed free-radical mechanism responsible for rapid decomposition above the critical point, facilitated by low density at supercritical conditions.     

Determination of the potential of cyanobacterial strains for hydrogen production

Hydrogen (H₂) is a renewable, abundant, and nonpolluting source of energy. Photosynthetic organisms capture sunlight very efficiently and convert it into organic molecules. Cyanobacteria produce H₂ by breaking down organic compounds and water. In this study, biological H₂ was produced from various strains of cyanobacteria. Moreover, H₂ accumulation by Synechocystis sp. PCC 6803 was as high as 0.037 μmol/mg Chl/h within 120 h in the dark. The wild-type, filamentous, non-heterocystous cyanobacterium Desertifilum sp. IPPAS B-1220 was found to produce a maximum of 0.229 μmol/mg Chl/h in the gas phase within 166 h in the light, which was on par with the maximum yield reported in the literature. DCMU at 10 μM increased H₂ production by Desertifilum sp. IPPAS B-1220 by 1.5-fold to 0.348 μmol H₂/mg Chl/h. This is the first report on the capability of Desertifilum cyanobacterium to produce H₂.     

Dark fermentative biohydrogen production using pretreated Scenedesmus obliquus biomass under an integrated paradigm of biorefinery

In recent times, biohydrogen production from microalgal feedstock has garnered considerable research interests to sustainably replace the fossil fuels. The present work adapted an integrated approach of utilizing deoiled Scenedesmus obliquus biomass as feedstock for biohydrogen production and valorization of dark fermentation (DF) effluent via biomethanation. The microalgae was cultivated under different CO₂ concentration. CO₂-air sparging of 5% v/v supported maximum microalgal growth and carbohydrate production with CO₂ fixation ability of 727.7 mg L^?1 d^?1. Thereafter, lipid present in microalgae was extracted for biodiesel production and the deoiled microalgal biomass (DMB) was subjected to different pretreatment techniques to maximize the carbohydrate recovery and biohydrogen yield. Steam heating (121 °C) in coherence with H₂SO₄ (0.5 N) documented highest carbohydrate recovery of 87.5%. DF of acid-thermal pretreated DMB resulted in maximum H₂ yield of 97.6 mL g^?1 VS which was almost 10 times higher as compared to untreated DMB (9.8 mL g^?1 VS). Subsequent utilization of DF effluent in biomethanation process resulted in cumulative methane production of 1060 mL L^?1. The total substrate energy recovered from integrated biofuel production system was 30%. The present study envisages a microalgal biorefinery to produce biohydrogen via DF coupled with concomitant CO₂ sequestration.