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Green hydrogen, proposed as a sustainable alternative for conventional fuels, has gained utmost importance due to its reduced carbon footprint and potential application in maintaining balance between energy generation and demand. Utilization of hydrogen-based systems are challenged by cost effective storage methods. Bio-hydrogen storage technologies using cryogenic and adsorptive methods are discussed in this review paper, along with their operating conditions and storage capacities. An analysis of operational challenges and recent advancements in hydrogen storage techniques is presented. With a storage pressure of 70 MP, cryogenic hydrogen is almost twice as dense as compressed hydrogen. Technical challenges such as material cost and explosion risk can be addressed by hydrogen adsorption on activated carbon active sites. Maximum hydrogen storage capacity are reported by activated carbon obtained from Chitosan (6.77 wt%), Bamboo (6.6 wt%) and African palm shell (6.5 wt%) at 77 K and 4–20 Mpa. Further, nanotechnology approaches are presented in detail for increasing hydrogen storage capacity in pristine Si and Mg. The future perspectives in terms of technological, and economic aspects are discussed. • Detailed review on hydrogen storage methods is presented. • Role of cryogenics and adsorptive media are discussed. • The comparative performance of various methods is tabulated. • Existing research gap and future perspectives are analyzed. [ABSTRACT FROM AUTHOR]

The pressure on reducing greenhouse gas emissions has increased since the focus on climate change mitigation has grown. As the human development index is directly related to per capita energy consumption, sustainable energy production with reduced carbon footprint is the essential pathway for cleaner environment. Over numerous existing biological processes, hydrogen production via photo fermentation and photolysis of water is considered a more sustainable and economical method of hydrogen production. While both biological modes suffer from certain limitations, such as low yields and low production rates, practical implementation is still a long way off due to these limitations. The unique properties of nanomaterials (NMs) can help increase the efficiency of the biomass to biohydrogen process in significant ways. Applications of organic and inorganic nanoparticles in photocatalytic and photo fermentative hydrogen production is discussed in this paper. This review reports a wide range of applications of nanomaterials including the details of photo fermentative microorganisms and their mechanism of action. The maximum photo fermentative bio-H 2 production was reported in the presence of SiC–Fe 3 O 4 (3.02 mol H 2 /mol acetate), Ni (41 mol H 2 /mol sugar), graphite-C 3 N 4 (64.2 mol H 2 /mol sugar) whereas photocatalytic bio-H 2 yield was highest when Pd–Ni/CdS (54 mmol/g.h), Ni-MO/g-C 3 N 4 (1785 mmol/g.h) and CuO/TiO 2 (20.3 mmol/g.h) nanoparticles were employed. To design photocatalytic systems for practical and scalable use, the future research should be focused on deeper theoretical knowledge of mechanisms of light harvesting, ± charge separation and transport, surface biochemical reactions, semiconductor–liquid interactions, and reactor dynamics. [Display omitted] • Recent developments for improving water photolysis and photo-fermentation are reviewed. • Applications of organic and inorganic nanoparticles in hydrogen production are discussed. • The working principle and mechanism of hydrogen production are discussed. • The influence of key operating factors are presented. • Scope for future research was presented. [ABSTRACT FROM AUTHOR]

Environmental contamination caused by oxyanions like nitrate, selenate, chromate, and other related functional groups in water and wastewater is identified as a serious threat to human health. Among the different techniques attempted to remove or convert these compounds, reduction with the use of microorganisms has gained importance due to high selectivity and efficiency. Hydrogen-based membrane bioreactor (HMBR) is identified as a suitable method due to the non-toxic properties of hydrogen as an electron-donor, reduced secondary pollution, and easy generation of hydrogen through cost-effective novel methods. This research article is focused on the principle and mechanism of removal of anions, organic compounds, heavy toxic polyatomic metal ions, and antibiotics using hydrogen mediated microbial reduction process. Denitrification studies in HMBR at different operating conditions, namely membrane type, pH, nitrate concentration, and hydrogen partial pressure, are reviewed and the commonly reported efficiency ranges between 70 and 100%. The removal efficiencies attained by HMBR are as follows: perchlorate 30–100%, selenate 92–100%, phenol 87–99%, arsenate 60–68% and halogenoalkanes 45–99%. The mechanism of reduction in HMBR was explained with reference to nitrate, arsenate, chlorophenol, and chromate. [Display omitted] • Mechanism of Hydrogen based Membrane Reactor and applications for removal of pollutants are reviewed. • The working principle is discussed with schematic illustration. • The influence of key operating variables is discussed. • Scope for future research and research gap is presented. [ABSTRACT FROM AUTHOR]

• Biological sequestration techniques for CO 2 reduction are described. • Bioenergy with carbon capture and storage concept is reviewed. • The economic feasibility and environmental outlook are reviewed. • A novel illustration of the artificial photosynthesis process is addressed. Drastic changes in climatic conditions accelerate the need for the implementation of emission reduction techniques to restrict global warming. Conventional technologies do face severe drawbacks in reducing carbon emissions. Carbon neutrality is considered as the most essential concept as it can alleviate the issues of greenhouse gas emissions while deriving energy and fuels from fossil origin. Several reduction technologies are proposed in the recent past, in which the integration of renewable energy resources such as solar and wind power are signified. The review aims to provide an understanding of the technology transition to attain carbon neutrality through production of carbon-free energies such as hydrogen and bioenergy. Despite the chemical and physical sequestration methods, the biological sequestration technique is given special attention for the mitigation of CO 2 emissions by utilizing photosynthetic microalgae. Of numerous microalgae, Chlorella vulgaris , a common eukaryotic microalga found in freshwater showed a maximum CO 2 fixation rate of about 3.35 g/L/d with 10 % CO 2 input. Apart from capturing and storing carbon in the geological site, the concept of utilizing and converting carbon into valuable byproducts is described elaborately. The possibilities for utilizing renewable energy sources for carbon-free energy production through unique prototypes are reviewed and elaborated. It mainly aids to utilize renewable resources more than conventional, which significantly minimizes the carbon footprint. The integration of bioenergy, carbon capture and storage concepts with renewable energy sources extends opportunities to minimize emissions and transition towards carbon neutrality and indeed can aid in achieving carbon negativity. The aforementioned concept was reported to produce about 12.5 million tons ton of hydrogen with 133 million tons of CO 2 recovery. An innovative prototype to resolve emission issues other than CO 2 was also addressed. Illustrations of the artificial photosynthesis process, convective vortex system and integrated multi energy system are disc proposed innovative carbon reduction technologies' economic feasibility and environmental outlook technologies are reviewed and presented. [ABSTRACT FROM AUTHOR]