Progress and Challenges in Biohydrogen Production

According to data from the International Energy Agency (IEA), in order to achieve net-zero CO2 emissions by 2050, approximately 306 million tonnes of green hydrogen (H2) must be produced annually [...]


Background
According to data from the International Energy Agency (IEA), in order to achieve net-zero CO 2 emissions by 2050, approximately 306 million tonnes of green hydrogen (H 2 ) must be produced annually [1].Green H 2 is produced by the electrolysis of water, which requires a high amount of electricity.In order to be carbon-neutral, the electricity used to power the process must come from renewable sources such as solar or wind power.Therefore, only 4% of the world's H 2 production is currently produced by this method.This direction, although ecologically justified, is a more expensive and more complicated method of obtaining H 2 than, for example, steam methane reforming (SMR), which is currently used to produce most of the H 2 used in industry.The blue (with CO 2 capture) and gray (without CO 2 capture) hydrogen produced in the SMR technology constitute approximately 95% of all currently produced types of H 2 .The IEA reports that H 2 production will have to grow at a rate of 66% in 2020-2030 and then 23% in 2030-2050 [1].
Although the combustion of hydrogen fuel does not emit CO 2 and other greenhouse gases, taking the above into account, its carbon footprint is still significant and depends on the technology of its production.Therefore, there is a justified need to search for methods of H 2 production that will be both technologically and economically competitive for the current solutions, in addition to being environmentally friendly.An alternative may be biological methods that fit directly into the principles of circular economy, material and energy recycling and support the idea of reducing greenhouse gas emissions to the atmosphere.Such technologies include anaerobic dark fermentation and biophotolysis.

Overview of Undertaken Research Issues
The investigation on dark fermentation methods for the H 2 production focuses primarily on the selection of the suitable organic substrate, effective pre-treatment techniques and the determination of the technological parameters used.The methods based on direct biophotolysis require the determination of the most efficient conditions for microalgae biomass cultivations and the course of the H 2 production phase.An important aspect of the biological methods of H 2 production is their optimization and assessment of technological, economic and environmental effectiveness.

Dark Fermentation
Liu et al., 2020 [2], verified the efficiency of biohydrogen production from co-fermentation of cattle manure and food waste.In order to maximize the process, the proportions of the tested substrates, their initial concentration in the reactors and the hydraulic retention time were determined.The results showed that the co-fermentation of cattle manure and food waste is an economically promising approach to biohydrogen production [2].The solid and liquid fractions of food waste were also tested by Hovorukha et al., 2021 [3].In order to intensify of the dark fermentation, a granular microbial preparation (GMP) method was used, which allowed obtaining 102 dm 3 H 2 /kg solid waste and 2.3 dm 3 H 2 /dm 3 of liquid fraction.Anaerobic digestion led to a 91-fold reduction in the mass of the substrate and a 3-fold reduction in organic pollutant concentrations in the wastewater [3].GMP was also used in the studies of Tashyrev et al., 2022 [4], to intensify H 2 production with food waste coupled with liquid organic leachate purification.The four-module direct-flow installation was used in this study and efficiently joined anaerobic and aerobic bacteria with other micro-and macroorganisms to simultaneously recycle organic waste and remediate the resulting leachate and H 2 production [4].Another paper presents the optimal conditions for the production of H 2 from microalgae biomass (Chlorella sp.) using enriched anaerobic mixed cultures as an inoculum [5].It has been proved that pH and temperature had the main influence on the efficiency of H 2 production.The highest production was observed at pH 7 and 35 • C [5].Cárdenas et al., 2018, demonstrated the possibility of producing H 2 from coffee mucilage combined with organic wastes (wholesale market garbage) in a dark fermentation process.The highest cumulative volume of H 2 , 25.9 dm 3 , was obtained using the 8:2 ratio (coffee mucilage: organic waste) after 72 h, which corresponded to 1.295 dm 3 H 2 /dm 3 of substrates [6].The same authors performed the analysis and estimation of the H 2 production from coffee mucilage mixed with organic wastes by dark anaerobic fermentation in a co-digestion system using an artificial neural network and fuzzy logic model [7].

Pre-Treatment Methods
The analysis of the available methods of pre-treatment of organic substrates before H 2 anaerobic digestion was presented by Singh et al., 2022 [8].The authors conclude that for cost-effective biohydrogen production, the substrate should be cheap and renewable.Substrates including algal biomass, agriculture residue and wastewaters are readily available.Moreover, substrates rich in starch and cellulose such as plant stalks or agricultural waste or food industry waste such as cheese whey are reported to support dark fermentation.However, their direct utilization as a substrate is not recommended due to their complex nature.Therefore, they must be pre-treated before being used to release fermentable sugars [8].In other studies [9], simultaneous saccharification and fermentation (SSF) and pre-hydrolysis with SSF (PSSF) were used to produce H 2 from the biomass of Chlorella sp.This yielded 170 cm 3 H 2 /g VS (volatile solids) .Pre-hydrolyzing the biomass at 50 • C for 12 h resulted in the production of 1.8 g/dm 3 of reducing sugars, leading to a H 2 yield of 172 cm 3 H 2 /g VS [9].Other authors have studied a simultaneous processing of sugar beet pulp (SBP) for H 2 production using various pre-treatment methods [10].The experiments showed that pre-treatment in 140 • C resulted in the highest H 2 yields of 113 dm 3 /kg VS [10].

Direct Biophotolysis
Touloupakis et al., 2021 [11] summarized the current state of knowledge related to the production of H 2 in the biophotolysis process.The article presents the direct and indirect biochemical mechanisms of the processes of H 2 production by using microalgae biomass.The obtained efficiency of producing this gas is also presented depending on the algae species, type of photobioreactors used and technological conditions and the culture medium.The presented analyses proved that the production of H 2 in this manner is not economically justified, and there is a need for further optimization related to the selection of algae strains and technological parameters [11].Dębowski et al., 2021 [12], presented the photobiological production of H 2 by microalgae compared to other technologies presented in the literature.The authors concluded that the use of microalgae is a highly promising method of producing H 2 via biological processes.There is sufficient data from laboratoryscale studies to start experiments in pilot systems on a semi-industrial scale.However, validation tests must be carried out to verify whether natural water or wastewater can be used as nutrient sources.Other technological issues to be addressed are methods to enrich, refine, store and use biohydrogen [12].

Summary
Considering the need to reduce CO 2 emissions by implementing clean-energy systems, an important research issue is the development of biohydrogen production.An important step in the progress of any technology based on biological processes is to thoroughly understand the mechanisms and conditions for obtaining efficient process results.Research on a laboratory scale and reviews summarizing the achievements to date are very important due to the cognitive aspect.They make it possible to gather the appropriate knowledge and background to carry out the appropriate optimization procedures, increase the scale of experiments, enable solutions to be raised to a higher technical readiness level (TRL) and the final implementation on a large scale.The mentioned works published in Energies fit perfectly into the current and innovative research trends developed by scientists around the world.