Harvest of electrical energy from fermented microalgal residue using a microbial fuel cell

The application of microalgal biomass for fermentation has been highlighted as a means of producing a range of value-added biofuels and chemicals. On the other hand, the microalgal residue from the fermentation process still contains as much as 50% organic contaminants, which can be a valuable substrate for further bioenergy recovery. In this study, a microbial fuel cell and automatic external load control by maximum power point tracking (MPPT) were implemented to harvest the electrical energy from waste fermented microalgal residue (FMR). The MFC with MPPT produced the highest amount of energy (1.82 kJ/L) compared to the other MFCs with fixed resistances: 0.98 (1000 Ω), 1.16 (500 Ω), and 1.17 kJ/L (300 Ω). The MFC with MPPT also showed the highest maximum power density (88.6 mW/m²) and COD removal efficiency (620.0 mg COD/L removal with 85% removal efficiency). The implementation of MPPT gained an approximate 12.9% energy yield compared to the previous fermentation stage. These results suggest that FMR can be an appropriate feedstock for electrical energy recovery using MFCs, and the combined fermentation and MFC system improves significantly the energy recovery and treatment efficiency from FMR.     

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.     

Graphite nanopowder functionalized 3-D acrylamide polymeric anode for enhanced performance of microbial fuel cell

3-D highly conductive polyvinyl formaldehyde sponges functionalized with acrylamide are fabricated using polyvinyl alcohol with varying concentrations of graphite nanopowder. The properties of the fabricated anodes are analyzed and its application in microbial fuel cells is evaluated. A comparative study with Graphite felt is also performed to evaluate its commercial viability. The presence of Hydroxyl and Amine functional groups enhanced the hydrophilic and biocompatible nature of the synthesized anodes. The phylogenetic analysis substantiated the biocompatible nature and mercury porosimetry showed macroporous nature of the fabricated anode. The highest power density of ~8 W/m² is recorded for C10 establishing solid biofilm formation. A ~94% COD removal revealed the versatility of the anode for MFC based wastewater treatment. The MFC performance was twice than that of control and was also highest among the most reported modified 3-D anodes. The durability study displayed the commercial opportunity of the anode for real-time MFC operation.     

Metal zeolite imidazole skeleton-based phosphate derivatives as bioanodes for enhanced microbial fuel cell energy output

Microbial fuel cells (MFCs) as green and sustainable technology for power generation still face various obstacles such as the lower power generation. In this work, a kind of phosphate-derived composites using ZIF-67 (Co–Ni-ZIF/P) as framework was prepared and employed it as anode in MFC. Results demonstrated that the Co–Ni-ZIF/P anode could reduce the electron transfer resistance and simultaneously improve the bioelectrocatalytic activity of the anode and microbial colonization. The maximum energy output of MFC with Co–Ni-ZIF/P anode reached 4.511 W m^?3, which was 2.18 times larger than that of MFC with Co–Ni-ZIF anode (2.07 W m^?3). Microbial community structure analysis proved that more electrochemically active microbial species were attracted on the proposed anodes, and their synergistic effect could facilitate the extracellular electron transfer (EET) process. In particular, Geobacter was the most typical exogenous electron-producing enriched on the Co–Ni-ZIF/P anode with a relative abundance of up to 60.83%. The present work offers a promising prospect for the application of bio-functional ZIF for the high-performance anode construction of MFCs.     

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.     

A new side-looking at the dark fermentation of sugarcane vinasse: Improving the carboxylates production in mesophilic EGSB by selection of the hydraulic retention time and substrate concentration

In recent years, a lot of scientific effort has been put into reusing the energy potential of sugarcane vinasse by dark fermentation. However, the findings so far indicate that new pathways need to be followed. In this context, this study assessed the effect of hydraulic retention time (HRT, from 24 to 1 h) on vinasse fermentation (10, 20, and 30 g COD L^?1) in three mesophilic expanded granular sludge bed reactors (EGSB). The carbohydrate conversion remained above 60% at all organic loading rates applied. The maximum hydrogen production rate (8.77 L day^?1 L^?1) was obtained for 720 kg COD m^?3 day^?1 and associated to the lactate-acetate pathway. The highest productivities of propionic, acetic, and butyric acids were 3.11, 1.68, and 2.45 g L^?1 h^?1, respectively, at a HRT of 1 h. At this HRT, the degrees of acidification remained between 54% and 76% in all EGSB reactors. This research provides insights for carboxylate production from sugarcane vinasse and suggests applying the EGSB setup in the acidogenic stage of two-stage processes.     

Co-generation of hydrogen and electricity from biodiesel process effluents

In this study, we apply a short-term voltage (0.2–0.8 V) to both crude glycerol (CG) and an anaerobic digestion (AD) effluent in a single-chamber microbial fuel cell (MFC) for power production. This improves the bioelectrogenesis in both CG (in MFC-1) and the AD effluent (in MFC-2), but higher power generation is attained in MFC-2. The use of domestic and synthetic wastewaters in the AD process leads to the generation of 195 and 350 mL H₂/L-medium, respectively. MFC-2 performs better than MFC-1 in terms of both voltage generation and chemical oxygen demand (COD) reduction. The application of 0.8 V yields a power density of 311 mW/m² (1.94 times higher than that of the control (160 mW/m²)). In addition, MFC-2 exhibits a 70% COD removal at 0.8 V, which decreases to 56% at 0.2 V. Thus, the application of a short-term voltage in MFC can stimulate both bioelectrogenesis and COD removal.     

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.     

Electrocatalytic studies of siloxene sheets encrusted with cobalt chalcogenides (S,Se) for water splitting

Two-dimensional siloxene sheets were superficially coated with cobalt chalcogenides to optimize interfacial properties for broad applications in the field of catalysis. These catalytic composites were investigated for electrochemical water splitting in an alkaline electrolyte medium. The synthesis of siloxene sheet-cobalt chalcogenides composites was confirmed by X-ray diffraction, field emission scanning electron microscopy, high-resolution transmission electron microscopy, adsorption studies, and X-ray photoelectron spectroscopy analyses. Potentiometric and impedimetric experiments were performed to understand the inherent electrocatalytic activity of the developed catalysts. Variations in the onset potential and overpotential at a constant current density of ±10 mA/cm² for hydrogen and oxygen evolution reactions—HER and OER, respectively—were evaluated with respect to a reversible hydrogen electrode (RHE). The catalysts exhibit superior current and catalytic activity due to interfacial kinetics, retaining lower Tafel slopes of ～30 mV/dec for the OER and HER; they also exhibited improved, long-term stability for 12 h, indicating potential utility in commercial applications.     

Enhanced methane production using pretreated sludge in MEC-AD system: Performance,microbial activity,and implications at different applied voltages

Although various pretreatment methods are employed to promote sludge hydrolysis and thereby promoting methane production in the subsequent microbial electrolysis cell assisted anaerobic digestion (MEC-AD) system, the questions arise are, “which pretreatment method on waste activated sludge (WAS) maximises the sludge hydrolysis and what is the optimal applied voltage on anaerobic digestion (AD) to stimulates the direct interspecies electron transfer (DIET) performance and thereby accelerating the methane production fed with pretreated WAS?” was still unanswered. Herein, firstly, a series of pretreatment methods to hydrolyse and mineralise the organic matter of WAS was performed to evaluate solubilization efficiency and thereafter, the influence of different applied voltages (0.3 V, 0.6 V, and 0.9 V) on coupled MEC-AD reactors fed with pretreated WAS was investigated to apprehend the DIET promotion for methane production. The results indicated that in MEC-AD reactors, the methane yield increased by 27.2%, 44.8%, and 37.3% when the applied voltages were 0.3 V, 0.6 V, and 0.9 V, respectively. Therefore, the alkaline-thermal pretreatment (ATP) enhanced the sludge hydrolysis in WAS, followed by an applied voltage of 0.6 V in the MEC-AD reactor fed with pretreated WAS, enhanced methane production under DIET stimulation induced by the increased abundance of electroactive microorganisms (EAM) and the advanced electron transfer. Besides, the energy balance estimation validates that with an applied voltage of 0.6 V in MEC-AD could achieve higher net energy input.     

Effect of physical activation/surface functional groups on wettability and electrochemical performance of carbon/activated carbon aerogels based electrode materials for electrochemical capacitors

Polymeric carbon/activated carbon aerogels were synthesized through sol-gel polycondensation reaction followed by the carbonization at 800 °C under Argon (Ar) atmosphere and subsequent physical activation under CO₂ environment at different temperatures with different degrees of burn-off. Significant increase in BET specific surface area (SSA) from 537 to 1775 m²g⁻¹ and pore volume from 0.24 to 0.94 cm³g⁻¹ was observed after physical activation while the pore size remained constant (around 2 nm). Morphological characterization of the carbon and activated carbons was conducted using X-ray diffraction (XRD) and Raman spectroscopy. Fourier-transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) were used to investigate the effect of thermal treatment (surface cleaning) on the chemical composition of carbon samples.Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were used to analyse the capacitive and resistive behaviour of non-activated/activated/and surface cleaned activated carbons employed as electroactive material in a two electrode symmetrical electrochemical capacitor (EC) cell with 6 M KOH solution used as the electrolyte.CV measurements showed improved specific capacitance (SC) of 197 Fg⁻¹ for activated carbon as compared to the SC of 136 Fg⁻¹ when non-activated carbon was used as electroactive material at a scan rate of 5 mVs⁻¹. Reduction in SC from 197 Fg⁻¹ to 163 Fg⁻¹ was witnessed after surface cleaning at elevated temperatures due to the reduction of surface oxygen function groups.The result of EIS measurements showed low internal resistance for all carbon samples indicating that the polymeric carbons possess a highly conductive three dimensional crosslinked structure. Because of their preferred properties such as controlled porosity, exceptionally high specific surface area, high conductivity and desirable capacitive behaviour, these materials have shown potential to be adopted as electrode materials in electrochemical capacitors.     

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.     

Wind-powered 250 kW electrolyzer for dynamic hydrogen production: A pilot study

Alkaline water electrolysis is the most promising approach for the industrial production of green hydrogen. This study investigates the dynamic operational characteristics of an industrial-scale alkaline electrolyzer with a rated hydrogen production of 50 m³/h. Strategies for system control and equipment improvement in dynamic-mode alkaline electrolytic hydrogen production are discussed. The electrolyzer can operate over a 30%–100% rated power load, thereby facilitating high-purity (>99.5%) H₂ production, competitive DC energy efficiency (4.01–4.51 kW h/Nm³ H₂, i.e., 73.1%–65.0% LHV), and good gas–liquid fluid balance. A safe H₂ content of 2% in O₂ (50% LFL) can be guaranteed by adjusting the system pressure. In transient operation, the electrolyzer can realize minute-level power and pressure modulation with high accuracy. The results confirm that the proposed alkaline electrolyzer can absorb highly fluctuating energy output from renewables because of its capability to operate in a dynamic mode.     

Multilayer additive manufacturing of catalyst-coated membranes for polymer electrolyte membrane fuel cells by inkjet printing

Inkjet printing is a versatile, contactless and accurate material deposition technology. The present work is focused on developing innovative strategies for inkjet printing of Catalyst-Coated Membranes (CCM) by performing Additive Manufacturing (AM) applied to Polymer Electrolyte Membrane Fuel Cells (PEMFC), without resorting to intermediate substrates. Three different approaches for AM are presented and discussed: a) inkjet-printing of the membrane ionomer layer and the top catalyst layer; b) inkjet-printing of both catalyst layers onto a membrane; c) inkjet-printing of the ionomer layer as well as the catalyst layers onto the reinforcement layer of the membrane. The produced catalyst and membrane layers were characterized and proved uniform in terms of catalyst loading (0.2–0.4 and 0.08 mg_Pt cm^?2 for cathode and anode, respectively), ionomer distribution and thickness homogeneity (4 μm for catalyst layers). The fully inkjet-printed CCM outperformed conventionally made assemblies in electrochemical-performance testing, even reaching 15% higher power density.     

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.     

Avenues to the financial viability of microbial electrolysis cells [MEC] for domestic wastewater treatment and hydrogen production

We propose targets, based on real world data, necessary to design a financially viable microbial electrolysis cell (MEC) for the treatment of domestic wastewater. By reducing the cost of the anode and current collecting materials by 90%, a viable organic loading rate would be between 800 and 1,400g-COD/m³/d (2–3A/m²). The anode and current collector materials account for 94% of the total material costs; consequently, cost savings in any other material are moot. If the bioanode can be reused after 20 years, further, significant savings could be achieved. To develop targets we used real world data, for the first time, to evaluate the financial viability of MECs against the current predominant method of wastewater treatment: activated sludge. We modelled net present values for eight potential scenarios and the performances required for MECs to break-even.     

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.     

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.     

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).     

Occurrence of power overshoot for two-chambered MFC at nearly steady-state operation

At high electrical current regime power overshoot may occur in microbial fuel cell (MFC) operation with both cell voltage and electrical current declining at reduced external loads. This study demonstrated the power overshoot in a two-chamber MFC using acetate and oxygen respectively as anodic and cathodic fuels. The cell worked well until reaching 0.51 V and 790 mA/m² at power density of 400 mW/m²; further reducing external load leads to decrease in both cell voltage and generated current. During the noted power overshoot regime the internal resistance of MFC increased monotonically with decreasing external load. Based on the electrochemical analysis of anodic and cathodic losses, the occurrence of power overshoot is proposed to be determined by the combined resistance of intracellular loss and of extracellular electron transfer (EET) loss.