Continuous H2 Production Research Articles

H2Bro – a Novel Decoupled Electrocatalytic-Catalytic Water Splitting Process for Green Hydrogen Production

Green hydrogen produced by water splitting using renewable electricity is essential to achieve net-zero carbon emissions. Present water electrolysis technologies are uncompetitive with low-cost grey hydrogen produced from fossil fuels, limiting their scale-up potential. Disruptive processes that decouple the hydrogen and oxygen evolution reactions and produce them in separate cells or different stages emerge as a prospective route to reduce system cost by enabling operation without expensive membranes and sealing components.An elegant and innovative electrocatalytic-catalytic process was suggested for continuous high-current H2 production via membraneless decoupled water splitting (Schematic 1). H2 as the desired product is evolved on the cathode of the electrolytic compartment in near-neutral conditions, Br- is electrochemically oxidized on the anode side to Br2, which reacts in the bulk of the aqueous electrolyte to form bromate anions (with hypobromous acid and hypobromite anions as the intermediate products), whereas a catalytic transformation of the soluble redox couple (BrO3 -/Br-) mediates O2 evolution in the catalytic compartment.Bromate-mediated decoupled water splitting system offers decoupling of the H2 and O2 gases as a safety measure, a membraneless architecture as a cell design simplification, continuous operation mode at low temperature of 60 °C and less corrosive environment along with the potential use of abundant (electro)catalytic materials. The use of the redox mediator also allows bypassing the energetically unfavorable oxygen evolution reaction and, by doing so, decreasing the cell voltage.The concept of the new electrocatalytic-catalytic system was verified experimentally in batch-to-batch mode, resulting in long-term Faradaic efficiency of 98±2% and voltage efficiency of 62% at 1 A cm-2 already at its infancy stage [1]. Nevertheless, the merging of the electrolytic and catalytic parts into an integrated continuous flow system is still in process.In this work, a variety of possible system part designs were suggested and discussed, with an emphasis on integrating the electrolytic and catalytic parts into a continuous flow system. The first iteration of the laboratory scale experimental setup was designed in a continuous stirred reactor mode, using Pt as the cathode material for HER, DSA as the anode material for BER, and RuO2 Adams powder as the catalyst for bromate decomposition, and aiming for a high bromide utilization (BrO3 -/Br- ratio) level at the cell voltage of 1.6 V and the current density values above 0.2 A cm-2. The system was designed to automatically control the electrochemical parameters of the electrolytic process, to adjust the electrolyte flow rate through the electrolytic cell and the bromate/bromide mixture residence time in the catalytic cell, to control the H2 evolution rate, and to detect undesired O2 evolution (if any) in the electrolytic cell, and to optimize and maintain the temperature and pH values. A set of the key parameters affecting Faradaic efficiency, cell voltage and voltage efficiency are analyzed in detail.Some light was also shed on the development of a new type of solid-buffer PGM-free electrocatalysts for hydrogen evolution reaction (HER) in near-neutral conditions, as well as on deciphering the mechanism of bromate decomposition process and the development of novel PGM-free catalyst materials as more affordable alternatives for RuO2 in the catalytic cell.References Slobodkin, E. Davydova, M. Sananis, A. Breytus, A. Rothschild “Electrochemical and chemical cycle for high-efficiency decoupled water splitting in a near-neutral electrolyte", accepted for publication in Nature Materials Funded by the European Union (ERC), Project #101097966 “Continuous electrolytic-catalytic decoupled water electrolysis for green hydrogen production” (H2Bro). Views and opinions expressed are however those of the author(s) only and do not necessarily reflect those of the European Union or the European Research Council Executive Agency. Neither the European Union nor the granting authority can be held responsible for them.” Figure 1

CO2 Fixation to Prebiotic Intermediates over Heterogeneous Catalysts.

ConspectusThe study of the origin of life requires a multifaceted approach to understanding where and how life arose on Earth. One of the most compelling hypotheses is the chemosynthetic origin of life at hydrothermal vents, as this condition has been considered viable for early forms of life. The continuous production of H2 and heat by serpentinization generates reductive conditions at hydrothermal vents, in which CO2 can be used to build large biomolecules. Although this involves surface catalysis and an autocatalytic process, in which solid minerals act as catalysts in the conversion of CO2 to metabolically important organic molecules, the systematic investigation of heterogeneous catalysis to comprehend prebiotic chemistry at hydrothermal vents has not been undertaken.In this Account, we discuss geochemical CO2 fixation to metabolic intermediates by synthetic minerals at hydrothermal vents from the perspective of heterogeneous catalysis. Ni and Fe are the most abundant transition metals at hydrothermal vents and occur in the active site of the enzymes carbon monoxide dehydrogenases/acetyl coenzyme A synthases (CODH/ACS). Synthetic free-standing NiFe alloy nanoparticles can convert CO2 to acetyl coenzyme A pathway intermediates such as formate, acetate, and pyruvate. The same alloy can further convert pyruvate to citramalate, which is essential in the biological citramalate pathway. Thermal treatment of Ni3Fe nanoparticles under NH3, which can occur in hydrothermal vents, results in Ni3FeN/Ni3Fe heterostructures. This catalyst has been demonstrated to produce prebiotic formamide and acetamide from CO2 and H2O using Ni3FeN/Ni3Fe as both substrate and catalyst. In the process of serpentinization, Co can be reduced in the vicinity of olivine, a Mg-Fe silicate mineral. This produces CoFe and CoFe2 with serpentine in nature, representing SiO2-supported CoFe alloys. In mimicking these natural minerals, synthetic SiO2-supported CoFe alloys demonstrate the same liquid products as NiFe alloys, namely, formate, acetate, and pyruvate under mild hydrothermal vent conditions. In contrast to the NiFe system, hydrocarbons up to C6 were detected in the gas phase, which is also present in hydrothermal vents. The addition of alkali and alkaline-earth metals to the catalysts results in enhanced formate concentration, playing a promotional role in CO2 reduction. Finally, Co was loaded onto ordered mesoporous SiO2 after modification with cations to simulate the minerals found in hydrothermal vents. These catalysts were then investigated under diminished H2O concentration, revealing the conversion of CO2 to CO, CH4, methanol, and acetate. Notably, the selectivity to metabolically relevant methanol was enhanced in the presence of cations that could generate and stabilize the methoxy intermediate. Calculation using the machine learning approach revealed the possibility of predicting the selectivity of CO2 fixation when modifying mesoporous SiO2 supports with heterocations. Our research demonstrates that minerals at hydrothermal vents can convert CO2 into metabolites under a variety of prebiotic conditions, potentially paving the way for modern biological CO2 fixation processes.

Cu-mediated electron-rich Ru centers on metal carbides for highly efficient hydrogen production from seawater

Seawater splitting for hydrogen production provides a promising pathway for green energy systems. However, the sluggish kinetics, corrosive ions, and the calcareous deposit seriously impede the wide-scale application of seawater electrolysis. Here, we report a trace amount of Cu-mediated anchoring electron-rich Ru centers on metal carbides (Cu-Ru/WC@C) for efficient hydrogen evolution from corrosive seawater. Benefiting from the conductive, electron-loss, and protophilic capacities of Cu atoms, the created Cu-Ru/WC@C catalyst possesses an electron-rich, anti-corrosive, and low oxophilic microenvironments, which eventually delivers excellent hydrogen evolution activities and anti-corrosion properties in seawater electrolysis. It requires a low overpotential of 392 mV to reach 1 A cm−2 and good long-term stability in alkaline seawater. Notably, the membrane electrolyzer with Cu-Ru/WC@C cathode exhibits outstanding performances for continuous H2 production under 1.875 V with 250 mA cm−2. This approach provides essential insights into the seawater corrosion resistance for cathode materials that match the electrochemical hydrogen production industry.

A Dual-Cation Exchange Membrane Electrolyzer for Continuous H2 Production from Seawater.

Direct seawater splitting (DSS) offers an aspirational route toward green hydrogen (H2) production but remains challenging when operating in a practically continuous manner, mainly due to the difficulty in establishing the water supply-consumption balance under the interference from impurity ions. A DSS system is reported for continuous ampere-level H2 production by coupling a dual-cation exchange membrane (CEM) three-compartment architecture with a circulatory electrolyte design. Monovalent-selective CEMs decouple the transmembrane water migration from interferences of Mg2+, Ca2+, and Cl- ions while maintaining ionic neutrality during electrolysis; the self-loop concentrated alkaline electrolyte ensures the constant gradient of water chemical potential, allowing a specific water supply-consumption balance relationship in a seawater-electrolyte-H2 sequence to be built among an expanded current range. Even paired with commercialized Ni foams, this electrolyzer (model size: 2 × 2 cm2) continuously produces H2 from flowing seawater with a rate of 7.5 mL min-1 at an industrially relevant current of 1.0 A over 100 h. More importantly, the energy consumption can be further reduced by coupling more efficient NiMo/NiFe foams (≈6.2 kWh Nm-3 H2 at 1.0 A), demonstrating the potential to further optimize the continuous DSS electrolyzer for practical applications.

Temporally Decoupled Ammonia Splitting by a Zn-NH3 Battery with an Ammonia Oxidation/Hydrogen Evolution Bifunctional Electrocatalyst as a Cathode.

Ammonia splitting to hydrogen is a decisive route for hydrogen economy but is seriously limited by the complex device and low efficiency. Here, we design and propose a new rechargeable Zn-NH3 battery based on temporally decoupled ammonia splitting to achieve efficient NH3-to-H2 conversion. In this system, ammonia is oxidized into nitrogen during cathodic charging (2NH3 + 6OH- → N2 + 6H2O + 6e-) with external electrical energy conversion and storage, while during cathodic discharging, water is reduced to hydrogen (2H2O + 2e- → H2 + 2OH-) with electrical energy generation. In this loop, continuous and efficient H2 production without separation and purification is achieved. With the help of the ammonia oxidation reaction (AOR) and hydrogen evolution reaction (HER) bifunctional catalyst of Mo2C/NiCu@C, a rechargeable Zn-NH3 battery is realized that exhibits a high NH3-to-H2 FE of 91.6% with outstanding durability for 900 cycles (300 h) at 20 mA/cm2, enabling efficient and continuous NH3-to-H2 conversion.

Photofermentative Biohydrogen Production Enhancement with Nanoparticle

This study will explore the use of a biological pathway known as photo fermentation for the production of biohydrogen under anoxic and nitrogen deficient conditions due to its high hydrogen production, and to emphasize the role of a dominating enzyme, the nitrogenase enzyme, in catalyzing the process of hydrogen evolution from a medium composed of Purple Non-Sulfur Bacteria (Rhodopseudomonas Palustris) with glycerol as the substrate. For Purple Non-Sulphur Bacteria, the optimal H2 production temperature ranges between 30 °C to 36 °C. Simultaneously, illumination is applied by LED lights and is varied from no string LED illumination, to single string illumination (240 W/m2), and double string illumination (480 W/m2) to simulate and examine performance in outdoor conditions and achieved a maximum of 428% increase in hydrogen productivity compared to previous studies. Consequently, this suggests that it is economically appealing to use the non-growing R. palustris bacterium as the chosen biocatalyst for continuous hydrogen production. These preliminary results constitute the development of continuous H2 production which evolves into the green fuel of the future production.

Continuous dark-photo fermentative H2 production from synthetic lignocellulose hydrolysate with different photoheterotrophic cultures: Sequential vs. co-culture processes

Integration of dark and photo-fermentation is a promising strategy to efficiently produce renewable hydrogen from different types of waste biomass. In this study, sequential and co-culture dark-photo fermentations were evaluated and compared during continuous operation using synthetic lignocellulose hydrolysate as a substrate. Mixed dark-fermentative bacterial consortium was combined with two different photo-fermentative bacterial cultures: pure culture of Rhodobacter sphaeroides (RSC) and mixed photoheterotrophic consortium (MPC). The sequential process showed higher total H2 yields compared to co-culture fermentation for both RSC and MPC. The highest yield of 4.15 mol H2/molsugar was obtained for the sequential process with RSC at an HRT of 3.5 days and pH 7.5. For the co-culture process, the yield reached 2.88 mol H2/molsugar at pH 7.0 and an HRT of 3.5 days. Complete sugar utilization was observed for dark fermentation. RSC preferred acetate as a carbon source, while MPC preferred butyrate. Photoheterotrophic bacteria, Rhodopseudomonas pseudopalustris and Rhodoplanes piscinae, present in the MPC inoculum were outcompeted by undesirable microorganisms during the continuous process, resulting in low HPR. On the other hand, in both sequential and co-culture processes, a high relative abundance of R. sphaeroides (>40%) was observed in the RSC, indicating effective cooperation between dark and photo bacteria.

Hydrogen production by aqueous phase reforming over stable La-promoted Ni-based hydrotalcite catalysts

Methanol, a common product of biomass pyrolysis or gasification, contains a high hydrogen content and serves as a valuable feedstock for hydrogen production through catalytic aqueous phase reforming (APR). However, poor hydrothermal stability of metal oxides support of the APR catalyst in liquid phase presents a challenge for the continuous and effective production of H2 over Ni-based hydrotalcite catalysts. A series of La-promoted NiMgAl hydrotalcite catalysts were prepared and their hydrogen production performances during methanol APR were evaluated. Promoted 5La–NiMgAl catalyst shows superior methanol APR reactivity due to the higher concentration of medium-strength basic sites than unpromoted catalyst. The La species adsorbs CO2 to form LaCO3OH and maintains La–NiMgAl catalyst structural integrity, which limits Ni leaching during reaction and promotes catalyst stability.

Continuous dark-fermentative H2 production using carbon components of lignocellulose hydrolysates: Insight into the difference between mixed and single substrates

Hydrogen produced from lignocellulosic biomass via dark fermentation is a promising alternative to fossil fuels. This study assessed fermentative H2 production from single sugars and sugar mixtures mimicking real lignocellulose hydrolysates during a continuous process. The highest hydrogen production rate (HPR) was obtained with pentose sugars, reaching 8.80 and 8.09 L/L-d for arabinose and xylose, respectively. Glucose showed the lowest HPR (6.19 L/L-d), while in the case of sugar mixtures (glucose, cellobiose, xylose, arabinose), HPR was approximately 7.3 L/L-d and was lower than expected from calculations. Thus, cofermentation reduced the H2 production potential. It was shown that pentose sugars are preferred for hydrogen production, but their utilization is limited by the presence of glucose, which is utilized preferentially in sugar mixtures. Xylose, arabinose and sugar mixtures showed similar H2 yields (approximately 0.29 mol H2/mol C), so the efficiency of substrate conversion into H2 was comparable for these substrates. The microbial community distribution showed that the Clostridium genus was dominant regardless of the substrate, with Clostridium beijerinckii and Clostridium guangxiense being the main bacterial species. Different substrates did not significantly affect the type of microorganisms present in the microbiome and changed only their relative abundance.

3D printing of cubic zirconia lattice supports for hydrogen production

The demand for hydrogen has extraordinarily grown during the last years, being one of the most attractive forms of fuels to produce green energy. Cubic zirconia ceramics are considered promising catalytic supports, and the additive manufacturing of porous 3D structures based on these ceramics could enhance their catalytic performance. Herein, lightweight highly porous (up to 88%) 3D patterned 8 mol% yttria-stabilized cubic zirconia (8YSZ) scaffolds are manufactured by robocasting from pseudoplastic aqueous-based inks to produce catalytic supports for the hydrogen (H2) production. These scaffolds are thermally treated at temperatures ranging between 1000 and 1400 °C and, hence, mechanically and electrically characterized. 3D 8YSZ structures sintered at 1200 °C, with an appropriate balance between high porosity (86%) and compressive strength (3.7 MPa), are impregnated with palladium (Pd) catalytic nanoparticles and employed in the catalytic dehydrogenation of renewable formic acid (FA) using a fixed-bed reactor. 3D Pd/8YSZ catalyst leads to the continuous production of CO-free H2 with a FA conversion of 32% at T = 55 °C.

Vacancy and doping engineering of Ni-based charge-buffer electrode for highly-efficient membrane-free and decoupled hydrogen/oxygen evolution

The realization of the membrane-free two-step water electrolysis is particularly important yet challenging for the low-cost and large-scale supply of hydrogen energy. In this effort, Co-doped Ni(OH)2 nanosheets were successfully anchored onto the nickel foam (NF) substrate through the in-situ growth of metal-organic frame material and the subsequent alkali-etching technique. Using the well-regulated Co-doping Ni(OH)2@NF electrodes as a charge mediator, electrochemical hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) were decoupled on time scales, thus affording a membrane-free two-step route for H2 and O2 productions. In this architecture, the first HER process on the cathode could be maintained for 1300 s at a current of 100 mA, while the corresponding Ni(OH)2 charge mediator was simultaneously oxidized to NiOOH, with a decent cell voltage of 1.542 V. The subsequent OER process involved a reduction/regeneration of Ni(OH)2 (from NiOOH to Ni(OH)2) and an anodic O2-production, with an operating voltage of 0.291 V. Moreover, the Ni-Zn battery assembled through the combination of NiOOH and Zn sheet could replace the second step of OER to achieve the coupling of continuous H2-production and battery discharge, thus also providing a new way for hydrogen production without an external power supply. Experiment and theoretical calculations have shown that the cobalt-doping not only improved the conductivity of the charge-buffer electrode, but also shifted its redox potential cathodically and boosted the adsorption affinity of the buffer medium to OH– ions, both contributing to promoted HER and OER activity. Therefore, this decoupled water electrolysis device affords a promising pathway to support the efficient conversion of renewables to hydrogen.

Three-dimensional ordered macroporous molybdenum doped NiCoP honeycomb electrode for two-step water electrolysis

Two-step alkaline water electrolysis is considered a safe and efficient method for producing hydrogen from renewable energy. Reversal of the current polarity in a bifunctional electrocatalyst used as a gas evolution electrode (GEE) in two-step water electrolysis can generate H2/O2 at different times and in different spaces. The design of a bifunctional electrocatalyst with high durability and excellent activity is imperative to achieving continuous, safe, and pure H2 generation via two-step alkaline water electrolysis. Here, we present for the first time a novel 3D Mo-doped NiCo phosphide honeycomb electrocatalyst that was grown on nickel foam (3D Mo-NiCoP/NF) and fabricated using polystyrene as a template. The electrocatalyst exhibited extremely low overpotentials in both the hydrogen evolution reaction (HER; 117 mV at 10 mA/cm2) and the oxygen evolution reaction (OER; 344 mV at 100 mA/cm2). As a bifunctional electrocatalyst for two-step alkaline water electrolysis, the device had a 1.784 V cell voltage at 10 mA/cm2, 95% decoupling efficiency, and ∼83% energy conversion efficiency. Taken together, the use of 3D Mo-NiCoP/NF as a GEE reduced the complexity and lowered the cost of the electrolyzer. The latter could be used to construct highly competitive water-splitting systems for continuous H2 production and green energy harvesting.

Flexible BiOI thin films photocatalysts toward renewable solar fuels production

Bismuth oxyiodide (BiOI) is an emerging photocatalyst material with remarkable electrical and optical properties, bringing it to the forefront of materials research. Herein, thin films of BiOI photocatalysts have been successfully synthesized by a one-pot microwave-hydrothermal method. The BiOI films were deposited on earth-abundant mica substrates, which provide a flexible and effective surface for the controlled growth of well-dispersed flowers of high surface area and enhanced light absorption. The films were evaluated as photocatalyst to generate alternative and renewable solar fuels from CO2 reduction, H2O splitting, and N2 photofixation under visible light. The results demonstrated the ability of the flower-shaped BiOI films to generate various value-added compounds, such as HCOOH (4.8 mmoles g-1 h-1), CH3OH (9.5 mmoles g-1 h-1), H2 (0.6 mmoles g-1 h-1), and NH3 (0.11 mmoles g-1 h-1) under visible light irradiation. Mainly, the H2 evolution was promoted (x3) after adding 3% wt. of calcium gluconate as a sacrificial agent, reaching a continuous H2 production (1.8 mmoles g-1 h-1). This enhancement effect was related to the oxidation of the sacrificial agent, favoring a high number of electrons available to reduce H+ to H2. In addition, the recycling of the photocatalyst was evidenced after four cycles, which activity was reactivated after the fifth cycle with a thermal treatment at 160 °C under vacuum conditions.

Scalable synthesis of BiVO4 thin films via anodic plating and thermal calcination

Fabrication of high-quality semiconductor thin films has long been a subject of keen interest in the photocatalytic field. Here, we report a facile, solution-based anodic plating and calcination for large-scale synthesis of BiVO4 thin films on indium tin oxide coated glass for use as photoanodes in solar water splitting. Using Na2SO3 as a sacrificial reagent, continuous solar H2 production with 94% Faradaic efficiency was obtained over 6 h of photoelectrochemical water splitting.

An NIR-Driven Upconversion/C3N4/CoP Photocatalyst for Efficient Hydrogen Production by Inhibiting Electron-Hole Pair Recombination for Alzheimer's Disease Therapy.

Redox imbalance and abnormal amyloid protein (Aβ) buildup are key factors in the etiology of Alzheimer's disease (AD). As an antioxidant, the hydrogen molecule (H2) has the potential to cure AD by specifically scavenging highly harmful reactive oxygen species (ROS) such as •OH. However, due to the low solubility of H2 (1.6 ppm), the traditional H2 administration pathway cannot easily achieve long-term and effective accumulation of H2 in the foci. Therefore, how to achieve the continuous release of H2 in situ is the key to improve the therapeutic effect on AD. As a corollary, we designed a rare earth ion doped g-C3N4 upconversion photocatalyst, which can respond to NIR and realize the continuous production of H2 by photocatalytic decomposition of H2O in biological tissue, which avoids the problem of the poor penetration of visible light. The introduction of CoP cocatalyst accelerates the separation and transfer of photogenerated electrons in g-C3N4, thus improving the photocatalytic activity of hydrogen evolution reaction. The morphology of the composite photocatalyst was shown by transmission electron microscopy, and the crystal structure was studied by X-ray diffractometry and Raman analysis. In addition, the ability of g-C3N4 to chelate metal ions and the photothermal properties of CoP can inhibit Aβ and reduce the deposition of Aβ in the brain. Efficient in situ hydrogen production therapy combined with multitarget synergism solves the problem of a poor therapeutic effect of a single target. In vivo studies have shown that UCNP@CoP@g-C3N4 can reduce Aβ deposition, improve memory impairment, and reduce neuroinflammation in AD mice.

Facile Functionalization of Carbon Electrodes for Efficient Electroenzymatic Hydrogen Production.

Enzymatic electrocatalysis holds promise for new biotechnological approaches to produce chemical commodities such as molecular hydrogen (H2). However, typical inhibitory limitations include low stability and/or low electrocatalytic currents (low product yields). Here we report a facile single-step electrode preparation procedure using indium-tin oxide nanoparticles on carbon electrodes. The subsequent immobilization of a model [FeFe]-hydrogenase from Clostridium pasteurianum ("CpI") on the functionalized carbon electrode permits comparatively large quantities of H2 to be produced in a stable manner. Specifically, we observe current densities of >8 mA/cm2 at -0.8 V vs the standard hydrogen electrode (SHE) by direct electron transfer (DET) from cyclic voltammetry, with an onset potential for H2 production close to its standard potential at pH 7 (approximately -0.4 V vs. SHE). Importantly, hydrogenase-modified electrodes show high stability retaining ∼92% of their electrocatalytic current after 120 h of continuous potentiostatic H2 production at -0.6 V vs. SHE; gas chromatography confirmed ∼100% Faradaic efficiency. As the bioelectrode preparation method balances simplicity, performance, and stability, it paves the way for DET on other electroenzymatic reactions as well as semiartificial photosynthesis.

Reactivity and Stability of Different Red Muds as Oxygen Carriers for Chemical Looping Combustion of Methane and Biomass in a Fluidized Bed

The reactivity and stability of two low-cost red muds (LuShan and XinFa) as oxygen carriers for chemical looping combustion (CLC) were experimentally investigated in a fluidized bed, with methane and biomass char, respectively. CH4 conversions using the present red muds were quite low, <20% after stable cycles. The XinFa red mud had better reactivity and stability in comparison with the LuShan red mud, with its CO2 selectivity maintained at about 80% during 50 cycles. Due to the high content of Ca in the LuShan red mud, its promoting effect on the water–gas shift reaction led to continuous high H2 production during biomass char-fueled experiments. The XinFa red mud achieved a complete conversion of char with only CO2 as the gaseous product. Further, it showed better resistance to attrition. The agglomeration affinity in the case of using biomass char as fuel was stronger than that when using methane as fuel. NaAlSiO4 with a high melting point was detected in both fresh and cycled oxygen carriers. Ca2Al2SiO7 was found in the LuShan red mud due to the interactions among Ca, Al, and Si at elevated temperatures.

Continuous H2 production during fermentation of the carbon components of lignocellulose hydrolysates: Insight into the influence of pH conditions on the conversion efficiency of individual sugars

Sugars released from lignocellulose biomass are a promising substrate for biohydrogen production. This study evaluates the effect of pH controlled between 4.0 and 7.5 on continuous dark-fermentative H2 production from the mixture of cellobiose, xylose and arabinose. High hydrogen production rate was obtained for pH values between 6.0 and 7.0 with a maximum of 7.41 ± 0.16 L/L-d at pH 7.0. On the other hand, the highest H2 yields of around 1.74 ± 0.02 mol/molconsumed were obtained at pH 4.5, 5.0 and 6.0. Cellobiose was completely utilized in nearly the entire pH range, while the highest consumption of xylose and arabinose was obtained at pH 6.0 and 7.0, respectively. It shows the challenges in selecting optimum pH for fermentation of mixed sugars. Significant impact of pH conditions on the microbial structure was observed. Between pH 4.0 and 7.0 Clostridium genus dominated the consortium, while above pH 7.0 relative abundance of Bacillus genus increased significantly.

Factor-based assessment of continuous bio-H2 production from cheese whey

Despite having been widely investigated, dark fermentative H2 production from organic residues is still limited by process-related issues which may hamper the perspectives of full-scale process implementation. Such constraints are mainly due to the process complexity, which is largely affected by multiple and often mutually interacting factors. In the present work, the results of continuous fermentative H2 production experiments using synthetic cheese whey as the input substrate were used to gain detailed knowledge of the process features and identify suitable and critical operating conditions. Specifically, innovative process interpretation involved a combination of analytical characterization of the fermentation broth, mass balance calculations and statistical methods (correlation and principal component analyses) to derive systematic considerations for process characterization and scale-up. The metabolic products mainly included acetate and butyrate, which however were likely to derive (in different proportions depending on the operating conditions) from both hydrogenogenic and competing pathways. For some tests, lactate and succinate were also found to have been formed. It was observed that the main features of the process (H2 yield and rate, stability condition) were correlated with the operational and analytical parameters. The first three principal components identified by the statistical analysis were able to account for: 1) the effect of retention time and total metabolites produced; 2) biogas (H2 and CO2) generation, butyrate production and stability condition; and 3) organic loading rate and propionate production. The results suggested that the main features of hydrogenogenic fermentation can be described by a reduced set of factors that may be usefully adopted for both process monitoring and prediction purposes.

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+, S2−, Ni2+, and O2− in the nanocomposite. The H2 evolution rate over the optimized photocatalyst is found to be ∼45.6 mmol h−1g−1cat 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.