Enhancement of bio-hydrogen generation by spirulina via an electrochemical photo-bioreactor (EPBR)

The biological production of hydrogen by microalgae is considered as an advantageous process. However, its yields are sometimes limited. To go beyond this limit, the improvement of the H₂ generation rate by Spirulina was studied via an electrochemical photo-bioreactor (EPBR). This EPBR led to hydrogen evolution rates of up to 27.49 and 13.37 mol of H₂.d⁻¹.m⁻³ for the anode and cathode chambers, respectively, under 0.3 V voltage and ~2.5 mA current. These results represent about a 4-fold increase compared to the H₂ production rate recorded without the application of a voltage. This increase in bio-hydrogen production is correlated with a drop in the concentration of NADPH. The Electrochemical Sequential Batch Reactor (ESRB) provided a more interesting total production rate which was 2.65 m³ m⁻³ d⁻¹, compared to the batch mode, which gave 1.2 m³ m⁻³.d⁻¹. The results show, for the first time, the boosting effect of the voltage on the metabolism of H₂ production by the Spirulina strain.     

The dual capacity of the NiSn alloy/MWCNT nanocomposite for sodium and hydrogen ions storage using porous Cu foam as a current collector

A nanostructured NiSn alloy/multi-walled carbon nanotube (MWCNT) composite was successfully synthesized for highly reversible sodium and hydrogen ions storage by using an electrochemical deposition process on porous Cu foam. The surface morphology of the resulting NiSn alloy/MWCNT nanocomposite was characterized using a field-emission scanning electron microscope, indicating the formation of sphere-like NiSn alloy nanoparticles with an average size of 190 nm. On the other hand, X-ray diffraction analysis, energy dispersive and Fourier transform infrared spectroscopies were employed to determine the crystalline structure, elemental surface and chemical composition of the nanocomposite electrode. The initial sodium discharge capacity of the electrode was maximized at ∼550 mAh g⁻¹ under the current density of 1000 mA g⁻¹, and a high hydrogen discharge capacity of 5200 mAh g⁻¹ was obtained at 1100 mA g⁻¹ after 20 cycles. A comprehensive comparison between the sodium and hydrogen ions capacities in this study and those of the literature for different materials and structures was also performed. Accordingly, the resulting nanocomposite electrode with dual capacity may offer promising applications in both sodium-ion battery and hydrogen storage.     

Comparative study on the electrochemical performance of coals and coal chars in a molten carbonate direct carbon fuel cell with a novel anode structure

Molten carbonate direct carbon fuel cells (MC-DCFCs) allow the efficient and clean use of coal. In this study, a novel anode structure is designed, and the performances of six coal-based fuels are investigated in MC-DCFC. The mechanisms of performance differences are investigated, as well as the effect of operating temperature on performance. The results reveal the fuel cell performance in the following order: meagre coal (109.8) ≈ bituminous coal (108.7) > bituminous coal char (98.1) > lignite coal (83.7) > lignite coal char (71.3) > meagre coal char (53.2) in mW cm^?2. Coal performs better because of its high carbon content, high volatile content, rich oxygen-containing functional groups, larger specific surface area, stronger thermal reactivity, and other factors. The electrochemical reactivity of coal fuel increased with higher reaction temperatures and varied throughout the temperature ranges. This study implies that using coal fuel to commercialize MC-DCFC could be a realistic alternative.     

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.     

HRT control as a strategy to enhance continuous hydrogen production from sugarcane juice under mesophilic and thermophilic conditions in AFBRs

The objective of this study was to evaluate the effects of hydraulic retention time (HRT) (8–1 h) on H₂ production from sugarcane juice (5000 mg COD L⁻¹) in mesophilic (30 °C, AFBR-30) and thermophilic (55 °C, AFBR-55) anaerobic fluidized bed reactors (AFBRs). At HRTs of 8 and 1 h in AFBR-30, the H₂ production rates were 60 and 116 mL H₂ h⁻¹ L⁻¹, the hydrogen yields were 0.60 and 0.10 mol H₂ mol⁻¹ hexose, and the highest bacterial diversities were 2.47 and 2.34, respectively. In AFBR-55, the decrease in the HRT from 8 to 1 h increased the hydrogen production rate to 501 mL H₂ h⁻¹ L⁻¹ at the HRT of 1 h. The maximum hydrogen yield of 1.52 mol H₂ mol⁻¹ hexose was observed at the HRT of 2 h and was associated with the lowest bacterial diversity (0.92) and highest bacterial dominance (0.52).     

Polyaniline@N-doped macroporous carbon foam as self-supporting anodes for microbial fuel cells

The inefficient extracellular electron transfer (EET) is detrimental to power generation and waste degradation in microbial fuel cells (MFCs). Herein, we report a self-supporting anode for MFCs prepared by graphitization of steamed bread slices followed by in-situ polymerization to fabricate polyaniline@N-doped macroporous carbon foam (PANI@NMCF). The natural nitrogen-containing wheat flour was fermented and carbonized to form NMCF with a high specific surface area of 818.1 m² g^?1. After the NMCF surface modified by PANI, the enhanced hydrophilicity and conductivity of the PANI@NMCF anode would facilitate microbial adhesion, biofilm formation, and electron transfer. The surface improvements enhance the EET process for high-performance MFCs, including a short startup time of 21.7 h, high maximum output power density of 1160 ± 17 mW m^?2, and decolourisation efficiency of 88.6 ± 1.2% for 36 h. The chemical oxygen demand removal efficiency was about 84.6 ± 1.1% at end of the operating cycles. This work provides a good foundation for our future development of carbon-based electrode materials for energy conversion and storage devices.     

Improvement of biohydrogen production from brewery wastewater: Evaluation of inocula,support and reactor

This work explores the production of biohydrogen from brewery wastewater using as inoculum a culture produced by natural fermentation of synthetic wastewater and Klebsiella pneumoniae isolated from the environment. Klebsiella pneumoniae showed good performance as inoculum, as evaluated using assays of between 9 and 16 cycles, with durations of 12 and 24 h, carbohydrate concentrations from 2.79 to 7.22 g L⁻¹, and applied volumetric organic loads from 2.6 to 12.6 g carbohydrate L⁻¹ day⁻¹. The best results were achieved with applied volumetric organic loads of 12.6 g carbohydrate L⁻¹ day⁻¹ and cycle length of 12 h, resulting in mean volumetric productivity of 0.88 L H₂ L⁻¹ day⁻¹, maximum molar flow of 10.80 mmol H₂ h⁻¹, and mean yield of 0.70 mol H₂ mol⁻¹ glucose consumed. The biogas H₂ content was between 18 and 42%, while the mean organic compounds removal and carbohydrate conversion efficiencies were 23 and 81%, respectively. The inoculum produced by natural fermentation was not viable.     

Impact of operational conditions on development of the hydrogen-producing microbial consortium in an AnSBBR from cassava wastewater rich in lactic acid

Biohydrogen production from cassava starch wastewater was evaluated in anaerobic sequencing batch biofilm reactor (AnSBBR) using different inoculum (mixed cultures from naturally fermented wastewater and anaerobic sludge thermally treated) and feeding strategies (batch and fed-batch). The highest hydrogen productivity (2.4 LH₂ L⁻¹ d⁻¹) and yield (11.7 molH₂ kg⁻¹Carbohydrates) were verified in low and intermediate organic load rates (12 and 14 g L⁻¹ d⁻¹) and longer cycle time (4 h), respectively. The productivity was favored by fed-batch strategy, and yield by batch. The hydrogen production was verified in both inoculum sources. However, in the assays inoculated from naturally fermented wastewater, with higher organic load rate (18 g L⁻¹ d⁻¹) and intermediate cycle time (3 h) no hydrogen was observed, regardless the feeding strategy, indicating that the inhibitory effects of the indigenous microorganisms present in cassava starch wastewater were more expressive in these conditions. The operational conditions applied to hydrogen production in AnSBBR from cassava starch wastewater may influence the microflora development in the reactor. In this study three possible scenarios were verified: hydrogen-producing bacteria (HPB) growth; hydrogen-producing bacteria inhibition or coexistence between ones and lactic acid bacteria (LAB), which are autochthones of this wastewater.     

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.     

Transition metal/carbon nanoparticle composite catalysts as platinum substitutes for bioelectrochemical hydrogen production using microbial electrolysis cells

Various metal nanoparticle catalysts supported on Vulcan XC-72 and carbon-nanomaterial-based catalysts were fabricated and compared and assessed as substitutes of platinum in microbial electrolysis cells (MECs). The metal-nanoparticle-loaded cathodes exhibited relatively better hydrogen production and electrochemical properties than cathodes coated with carbon nanoparticles (CNPs) and carbon nanotubes (CNTs) did. Catalysts containing Pt (alone or mixed with other metals) most effectively produced hydrogen in terms of overall conversion efficiency, followed by Ni alone or combined with other metals in the order: Pt/C (80.6%) > PtNi/C (76.8%) > PtCu/C (72.6%) > Ni/C (73.0%) > Cu/C (65.8%) > CNPs (47.0%) > CNTs (38.9%) > plain carbon felt (38.7%). Further, in terms of long-term catalytic stability, Ni-based catalysts degraded to a lesser extent over time than did the Cu/C catalyst (which showed the maximum degradation). Overall, the hydrogen generation efficiency, catalyst stability, and current density of the Ni-based catalysts were almost comparable to those of Pt catalysts. Thus, Ni is an effective and inexpensive alternative to Pt catalysts for hydrogen production by MECs.     

Facial synthesis of p-p heterojunction composites: Evaluation of their electrochemical properties with photovoltaics-electrolyzer water splitting using two-electrode system

Mixed transition metal oxides have garnered widespread interest as alternative electrocatalysts for the oxygen and hydrogen generation reactions; however, they tend to require extended synthetic routes, in addition to possessing limited electrocatalytic activities and stabilities. Herein, we report the observation of a synergistic effect between the non-precious metal oxides Mn₃O₄ and Co₃O₄ with CuO and NiO, wherein the resulting composites exhibit promising properties as catalysts for the alkaline water electrolysis process. The activities of these composites in both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) were improved compared to their counterparts, and the dynamic potentials of these processes were reduced. Importantly, low overpotentials of 202 and 380 mV were found for the CuO–Mn₃O₄ composite catalysts for the OER and the HER at 10 mA/cm², respectively. In addition, electrochemical impedance spectroscopy measurements showed a reduced impedance response for the composites, which was dominated by the relaxation of the intermediate frequency associated with the adsorption of the intermediate. Furthermore, the superior catalytic activities of the composites were attributed to their structural properties, high electroactive surface areas, fast electron transport kinetics, and good chemo-electrical bonding between Mn₃O₄ and CuO. Importantly, merging with a marketable silicon-based solar cells, the accumulated PV-EC water splitting device obtains greater hydrogen production under stimulated solar light irradiation. This work offers a typical demonstration and respected strategies for practical large-scale solar H₂ production via an economical PV-EC technology.     

Turning glycerol surplus into renewable syngas through glycerol steam reforming over a sol-gel Ni–Mo2C-Al2O3 catalyst

In this work, a sol-gel Ni–Mo₂C–Al₂O₃ catalyst is employed for the first time in the glycerol steam reforming for syngas production. Catalyst stability and activity are investigated in the temperature range of 550 °C–700 °C and time on stream up to 30 h. As reaction temperature increases, from 550 °C to 700 °C, H₂ yield boosts from 22% to 60%. The stability test, carried out at milder conditions (600 °C and Gas-Hourly Space-Velocity (GHSV) of 50,000 mL h⁻¹.g_cat⁻¹), shows high catalyst stability, up to 30 h, with final conversion, H₂ yield, and H₂/CO ratio of 95%, 53% and 1.95, respectively. Both virgin and spent catalysts have been characterized by a multitude of techniques, e.g., Atomic-Absorption spectroscopy, Raman spectroscopy, N₂-adsorption-desorption, and Transmission Electron Microscopy (TEM), among others. Regarding the spent catalysts, carbon deposits’ morphology becomes more graphitic as the reaction temperature increases, and the total coke formation is mitigated by increasing reaction temperature and lowering GHSV.     

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.     

Novel sonochemical synthesis of Zn2V2O7 nanostructures for electrochemical hydrogen storage

Although utilization of diverse classes of metal oxides as hydrogen storage materials has been reported, but there is still a major need to introduce efficient materials. Herein, mesoporous Zn₂V₂O₇ nanostructures were produced by a new sonochemical method using hydrazine, zinc nitrate, and ammonium vanadate as the starting reagents and then annealed at 700 °C. Prior to annealing, Zn₃V₃O₈ was produced in the presence of ultrasonic waves, whereas in the absence of ultrasonic waves, Zn₂(VO₄)₂ was the major product. In fact, ultrasonic waves interfered with the reaction mechanism and reduced V⁵⁺ to V⁴⁺ and V³⁺. Because of the proper composition and structure of these nanostructures, they were used for electrochemical storage of hydrogen. Storage of over 2899 mAh/g after 20 cycles by flower-like nanostructures revealed their high capability. The results also showed that morphology affects efficiency such that three-dimensional spherical nanostructures had a storage capacity of 2247 mAh/g after 20 cycles.     

Synergistic effect of titanium oxide underlayer and interlayer on zirconium-doped zinc ferrite photoanode for photoelectrochemical water splitting

Here, we report the synergistic effect of dual TiO₂ layers to enhance the PEC performance of Zirconium-doped zinc ferrite (ZZFO) photoanode by improving the charge carrier density and suppressing the photogenerated charge recombination. The TiO₂ underlayer works as a blocking layer to remarkably suppress the back-injection of electrons from the fluorine-doped tin oxide (FTO) leading to reducing the bulk charge recombination. While interlayer TiO₂ improves the bulk charge transfer property of ZZFO photoanodes. The optimal TiO₂ double-layer modified ZZFO photoanode exhibits an enhanced photocurrent of 0.435 mA/cm² at 1.23 V vs. reversible hydrogen electrode (RHE), which is 2.5 times higher than that of the ZZFO photoanode. The effect of each layer was deeply investigated by electrochemical impedance spectroscopy (EIS), intensity-modulated photocurrent spectroscopy (IMPS) and time-resolved photoluminescence studies (TRPL) with the aim of gaining a clear picture of the interface modifications and their impact on the efficiency of the ZZFO photoanode.     

Preparation and improvement of nickel catalyst supported ordered mesoporous spherical silica for thermocatalytic decomposition of methane

The thermocatalytic alteration of CH₄ into highly pure hydrogen and filaments of carbon was investigated on a series of Ni-catalysts with various contents (25, 40, 55, and 70 wt%) supported mesoporous spherical SiO₂. The silica with ordered structure and high specific surface area (1136 m²/g) was synthesized using the Stöber technique with TEOS as a silica precursor and CTAB as the template in a simple synthesis system of aqueous-phase. This technique led to the preparation of mesoporous spherical silica. The prepared samples were characterized using BET, TPR, XRD, TPO, and SEM analyses. The prepared catalysts with different nickel loading showed the BET surface area ranging from 225.0 to 725.7 m²/g. These results indicated that an increase in nickel content decreases the surface area and leads to a subsequent collapse of a pore structure. SEM analysis confirmed a spherical nanostructure of catalysts and revealed that with the increase in loading of Ni, the particle size enlarged, because of the agglomeration of the particles. The results implied that the high methane conversion of 54% obtained over the 55 wt% Ni/SiO₂ at 575 °C and this sample had higher stability at lower reaction temperature than the other prepared catalysts, slowly deactivation was observed for this catalyst at a period of 300 min of time on stream.     

Screening and optimisation of hydrogen production by newly isolated nitrogen-fixing cyanobacterial strains

Recently, there has been a propensity to postpone dealing with the world's climate concerns until later, resulting in a 1.5 °C rise in temperature over the last century. Therefore, interest in biologically derived, inexhaustible energy sources based on solar energy is growing. Cyanobacteria have the potential to produce clean, renewable fuels in the form of hydrogen (H₂) gas, derived from solar energy and water. The current study reports the screening 11 cyanobacterial strains isolated from rice paddies and hotsprings for efficient H₂ producers. According to our findings, H₂ concentrations in the species ranged from 3.6 to 48.9 μmol mg⁻¹ Chl a h⁻¹. H₂ production by isolated species was shown to have a 2% positive influence on oxygen (O₂) and carbon dioxide (CO₂) concentrations and a 2% negative effect on all nitrogen gas (N₂) concentrations. It was discovered that at high CO₂ concentrations, photosynthesis is enhanced but H₂ production is suppressed. Anabaena variabilis BTA-1047 was found to be the most active H₂-producing species, with an H₂ production activity of 21.3 μmol mg⁻¹ Chl a h⁻¹. Moreover, a 1% O₂: 2% CO₂ gas mixture doubled the strain activity of H₂ production. The findings of the study called into the question the notion that only an anaerobic environment is required for H₂ production by N₂-fixing cyanobacterial species and explored whether H₂ productivity can be increased by stimulating the micro-anaerobic environment with a carbon source.     

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.     

Electrochemically activated carbon micro-electrode arrays for electrochemical micro-capacitors

Interdigitated carbon micro-electrode arrays for micro-capacitors are fabricated through the carbon microelectromechanical systems (C-MEMS) technique which is based on the carbonization of patterned photoresist. To improve the capacitive behavior, electrochemical activation is performed on carbon micro-electrode arrays. Cyclic voltammetry (CV) and galvanostatic charge-discharge results demonstrate that the electrochemical activation effectively increases the capacitance of the micro-electrode arrays by three orders of magnitude. Although the charge-discharge experiments show the non-ideal behavior of micro-capacitors, the specific geometric capacitance reaches as high as 75 mF cm⁻² at a scan rate of 5 mV s⁻¹ after electrochemical activation for 30 min. The capacitance loss is less than 13% after 1000 CV cycles. These results indicate that electrochemically activated C-MEMS micro-electrode arrays are promising candidates for on-chip electrochemical micro-capacitor application.     

Hierarchically porous carbon from biomass tar as sustainable electrode material for high-performance supercapacitors

The byproduct tar from biomass gasification process had seriously impeded development and applications of this technology, thus novel path for biomass tar valorization is had been continuously pursued. Given its high carbon content, this work attempted to convert biomass tar into hierarchically porous carbon by thermal activation with acetate potassium. The optimal product produced with mass ratio of biomass tar to acetate potassium of 1:3 and activation temperature at 800 °C was revealed as excellent electrode material for high-performance supercapacitor, which demonstrated electrochemical capacitance up to 310.4 F/g at 0.2 A/g, whilst preserved 91% of initial capacitance after 5000 charge-discharge circles at current density of 5 A/g. These excellent properties had arisen from the open and hierarchical porosity and large surface area. This work disclosed the great potential of biomass tar as sustainable and competent candidate for fabricating high-performance electrode material for electrochemical energy devices, and may bring up new opportunities to development of biomass gasification technologies.