Bimetallic zeolitic imidazole framework derived Co@NC materials as oxygen reduction reaction catalysts application for microbial fuel cells

Microbial fuel cells (MFCs), a promising future energy conversion technology, play a significant role in the area of sustainable and renewable energy. In air-cathode MFCs, the catalytic activity for oxygen reduction reaction (ORR) of cathode electrocatalyst is the key factor to the performance of MFCs. Development of efficient and economical ORR electrocatalysts is an important step for the wide application of MFCs. Herein, Co wrapped carbon nanotubes (CNTs) N-doped nanoporous carbon materials (Co@NC-Co_xZn_y) are constructed via a facile zinc-assisted growth pyrolytic approach of bimetallic zeolitic imidazole frameworks (BMZIFs)-derived strategy. They are directly prepared via carbonization of the precursor Co_xZn_y-BMZIFs. During the pyrolysis process, the evaporation of zinc plays critical role in the in-situ growth of CNTs. For instance, the optimal catalyst, Co@NC-Co₁Zn₃, exhibits excellent ORR performance activity and stability with on-set potential (E_on-set) of 0.830 V (vs. RHE) and diffusion-limited current density (j_L) of 6.706 mA cm^?2, which is superior to the benchmark catalyst of commercial 20 wt% Pt/C. Additionally, Co@NC-Co₁Zn₃ displays four-electron pathway, long-term stability and better resistance to methanol tolerance. The MFC with Co@NC-Co₁Zn₃ cathode shows a maximum power density of 1039 mW m^?2, and outperforms the MFC with commercial 20 wt% Pt/C catalyst (678 mW m^?2). This work paved the way for exploring cost-effective, superior performance non-precious metal-based catalysts for air-cathode MFCs.     

Continuous photosynthetic biohydrogen production from acetate-rich wastewater: Influence of light intensity

Photo-biohydrogen by microalgae is attractive sustainable energy caused by the utilization of solar energy and water. However, due to oxygen (O₂) sensitive hydrogenase (HydA) activity, effective control of O₂ and light intensity is critical for achieving sustainable photosynthetic hydrogen (H₂) production. Here we demonstrate continuous algal H₂ production using acetate-enriched fermenter effluent, achieving the complete O₂ cessation without sulfur depletion. Average H₂ production of 108 ± 4 μmol L^?1 for Chlamydomonas reinhardtii and 88 ± 7 μmol L^?1 for Chlorella sorokiniana at 100 μmol m^?2 s^?1 were observed for 15 days, respectively. The highest light energy to H₂ energy conversion efficiency (LHCE) of 1.61% for C. reinhardtii and 1.06% for C. Sorokiniana was obtained under low light intensity (50 μmol m^?2 s^?1) but the LHCE decreased with the increase of light intensity followed by photoinhibition, which led to a decrease of HydA activity and H₂ production. Low H₂ production was observed at 50 μmol m^?2 s^?1 under the highest LHCE, in which microalgae exhibited photoinhibition biomass growth kinetics to produce chlorophyll a (Chl a) for electron generation. These results demonstrate that light is a feasible strategy for producing electron for H₂ production under anoxygenic photosynthesis.     

Power generation using an activated carbon fiber felt cathode in an upflow microbial fuel cell

An activated carbon fiber felt (ACFF) cathode lacking metal catalysts is used in an upflow microbial fuel cell (UMFC). The maximum power density with the ACFF cathode is 315 mW m⁻², compared to lower values with cathodes made of plain carbon paper (67 mW m⁻²), carbon felt (77 mW m⁻²), or platinum-coated carbon paper (124 mW m⁻², 0.2 mg-Pt cm⁻²). The addition of platinum to the ACFF cathode (0.2 mg-Pt cm⁻²) increases the maximum power density to 391 mW m⁻². Power production is further increased to 784 mW m⁻² by increasing the cathode surface area and shaping it into a tubular form. With ACFF cutting into granules, the maximum power is 481 mW m⁻² (0.5 cm granules), and 667 mW m⁻² (1.0 cm granules). These results show that ACFF cathodes lacking metal catalysts can be used to substantially increase power production in UMFC compared to traditional materials lacking a precious metal catalyst.     

Enhanced bioelectricity generation and cathodic oxygen reduction of air breathing microbial fuel cells based on MoS2 decorated carbon nanotube

Increasing efforts have been devoted to enhancing the cathode activity towards oxygen reduction and improve power generation of air breathing microbial fuel cells. Exploring non-precious metal and highly active cathodic catalyst plays a key role in improving cathode performance. Our work aims to investigate the electrocatalyst behavior and power output of the single-chamber MFC equipped with carbon nanotubes hybridized molybdenum disulfide nanocomposites (CNT/MoS₂) cathode. MoS₂ nanosheets embedded into the CNTs network structure is synthesized by a facile hydrothermal method. The CNT/MoS₂-MFC achieves a maximum power density of 53.0 mW m⁻², which is much higher than those MFCs with pure CNTs (21.4 mW m⁻²) or solely MoS₂ (14.4 mW m⁻²) cathode. The oxygen reduction reaction (ORR) test also demonstrates a promoted electrocatalytic activity of synthesized material, which may be attributed to the special interlaced structure and abundant oxygen chemisorption sites of CNT/MoS₂. Such CNTs-based noble-metal-free catalyst presents a new approach to the application of MFCs cathode materials.     

Relating biomass composition and the distribution of metabolic functions in the co-fermentation of sugarcane vinasse and glycerol

This study aimed to determine the effect of increasing the organic loading rate (OLR) from 60 to 90 and 120 kg COD m^?3 d^?1 in the co-fermentation of glycerol and sugarcane vinasse (50%: 50% proportion on a COD basis) in a thermophilic anaerobic fluidized bed reactor (55 °C) at the fixed hydraulic retention time of 4 h. The highest values of hydrogen production rate (1851 mL H₂ d^?1 L^?1_bed) and yield (0.29 mmol H₂ g^?1 COD_added) were found at 120 kg COD m^?3 d^?1 and coincided with butanoate as a major liquid metabolite (2620 mg L^?1). The reverse β-oxidation of lactate into butanoate contributed to its synthesis and was linked to synergism between Clostridium (relative abundance of 77.8%) and Lacticaseibacillus (7.2%) in the reactor. The identification of the butyryl-CoA/acetate-CoA transferase gene, which may have catalyzed the conversion of butyryl-CoA into butanoate using acetate as an acceptor, also supported this.     

Chemically and thermally reduced graphene oxide supported Pt catalysts prepared by supercritical deposition

Reduced graphene oxide (RGO) is used in many energy applications, especially in Polymer Electrolyte Membrane (PEM) fuel cells, as carbon sourced catalyst support materials. In this study, thermally (T-RGO) and chemically (C-RGO) reduced GO support materials were synthesized for utilization in PEM fuel cells. Pt catalysts were synthesized using supercritical carbon dioxide (SCCO₂) deposition technique over synthesized support materials. Physical (BET, SEM-EDX, FTIR, RAMAN, XRD, TEM, ICP-MS and optical tensiometer) and electrochemical (CV, PEM fuel cell test) characterizations of synthesized support materials and corresponding Pt catalysts were performed. The differences between the structures of thermally and chemically reduced graphene oxide supports and their Pt catalysts were investigated. The ECSA values of the Pt/T-RGO and Pt/C-RGO catalysts are 19.86 m² g^?1 and 6.31 m² g^?1, respectively. The current and power density values of the Pt/T-RGO and Pt/C-RGO catalysts at 0.6 V are 84 mA cm^?2, 80 mA cm^?2 and 50 mW cm^?2, 45 mW cm^?2, respectively. Pt/T-RGO and Pt/C-RGO catalysts showed similar trend in PEMFC environment.     

Biomass pectin-derived N,S-enriched carbon with hierarchical porous structure as a metal-free catalyst for enhancing bio-electricity generation

Heteroatoms-doped carbon-based materials (with non-precious metals or no metals) with porous structure have already shown high catalytic activities for oxygen reduction reaction (ORR), especially in microbial fuel cells (MFCs). Here, we use pectin extracted from pomelo peels as carbon source to prepare metal-free and sulphur/nitrogen co-doped partially-graphitized carbon (HP-SN-PGCs) by using silica nanospheres as sacrificial templates. Single-chamber MFC (SC-MFC) with HP-SN-PGC-0.5 (0.5 g of silica) cathode has the shortest start-up time (45 h) and lowest charge transfer resistance (19.3 Ω). The maximum power density of HP-SN-PGC-0.5 (1161.34 mW m⁻²) cathode is higher than that of Pt/C (1116.90 mW m⁻²) at the initial cycle. After 75 d operation, power density of HP-SN-PGC-0.5 cathode only declines 4.6%, which is more stable than that of Pt/C (37.69%). HP-SN-PGC-0.5 has a highly porous structure (869.25 m² g⁻¹) by removal of templates and Fe species (as the graphitization catalyst) to facilitate exposure of active sites and diffusion of ORR-related intermediates (OH⁻ and HO₂⁻, etc) to accessible active sites. N and S species provide highly active sites to enhance OH⁻ generation to conduct the 4e⁻ ORR process. Thus, this study presents a viable ORR catalyst with high activity and long-term stability for bio-electricity generation from organic wastewater in SC-MFCs.     

FeCo alloys in-situ formed in Co/Co2P/N-doped carbon as a durable catalyst for boosting bio-electrons-driven oxygen reduction in microbial fuel cells

Non-noble metal catalyst with high catalytic activity and stability towards oxygen reduction reaction (ORR) is critical for durable bioelectricity generation in air-cathode microbial fuel cells (MFCs). Herein, nitrogen-doped (iron-cobalt alloy)/cobalt/cobalt phosphide/partly-graphitized carbon ((FeCo)/Co/Co₂P/NPGC) catalysts are prepared by using cornstalks via a facile method. Carbonization temperature exerts a great effect on catalyst structure and ORR activity. FeCo alloys are in-situ formed in the catalysts above 900 °C, which are considered as the highly-active component in catalyzing ORR. AC-MFC with FeCo/Co/Co₂P/NPGC (950 °C) cathode shows the highest power density of 997.74 ± 5 mW m^?2, which only declines 8.65% after 90 d operation. The highest Coulombic efficiency (23.3%) and the lowest charge transfer resistance (22.89 Ω) are obtained by FeCo/Co/Co₂P/NPGC (950 °C) cathode, indicating that it has a high bio-electrons recycling rate. Highly porous structure (539.50 m² g^?1) can provide the interconnected channels to facilitate the transport of O₂. FeCo alloys promote charge transfer and catalytic decomposition of H₂O₂ to ?OH and ?O₂^?, which inhibits cathodic biofilm growth to improve ORR durability. Synergies between metallic components (FeCo/Co/Co₂P) and N-doped carbon energetically improve the ORR catalytic activity of (FeCo)/Co/Co₂P/NPGC catalysts, which have the potential to be widely used as catalysts in MFCs.     

Cobalt-impregnated La0.75Sr0.25Cr0.5Mn0.5O3−δ anodes for solid oxide fuel cells

In this study, we will report our investigation for La_0.75Sr_0.25Cr_0.5Mn_0.5O_3−δ (LSCrM) based anodes impregnated with solutions of cobalt (Co) nitrate. A YSZ supported SOFC with pure LSCrM anode and La_0.7Sr_0.3MnO₃ (LSM) cathode exhibits the maximum power density (P_max) of 58.7 and 5.2 mW cm⁻² at 850 °C in dry H₂ and dry CH₄. After the modification of anode with Co nitrate, the P_max reaches 196.2 mW cm⁻² in dry H₂ and 28.5 mW cm⁻² in dry CH₄, about 3.34 times and 5.48 times increase, respectively. These results indicate that Co is also a potential catalyst for LSCrM anode. Moreover, the effect of impregnation amount of catalyst on the cell performance is also evaluated in this study.     

Investigation on sulphonated zinc oxide nanorod incorporated sulphonated poly (1,4-phenylene ether ether sulfone) nanocomposite membranes for improved performance of microbial fuel cell

Hydrothermally prepared zinc oxide nanorods are sulphonated (S–ZnO NR) and incorporated into 15% Sulphonated Poly (1,4-Phenylene Ether Ether Sulfone) (SPEES) to improve the hydrophilicity, water uptake and ion transfer capacity. Water uptake and ion transfer capacity increased to 34.6 ± 0.6% and 2.0 ± 0.05 meq g^?1 from 29.8 ± 0.3% and 1.4 ± 0.04 meq g^?1 by adding 7.5 wt% S–ZnO NR to SPEES. Morphological studies show the prepared S–ZnO NR is well dispersed in the polymer matrix. SPEES +7.5 wt% S–ZnO NR membrane exhibits optimum performance after three-weeks of continual operation in a fabricated microbial fuel cell (MFC) to produce a maximum power density of 142 ± 1.2 mW m^?2 with a reduced biofilm compared to plain SPEES (59 ± 0.8 mW m^?2), unsulphonated filler incorporated SPEES (SPEES + 7.5 wt% ZnO, 68 ± 1.1 mW m^?2) and Nafion (130 ± 1.5 mW m^?2) thereby suggesting its suitability as a sustainable and improved cation exchange membrane (CEM) for MFCs.     

Study of the influence of Nafion/C composition on electrochemical performance of PEM single cells with ultra-low platinum load

The electrochemical performance of membrane electrode assemblies (MEAs) with ultra-low platinum load (0.02 mg_Pt cm^?2) and different compositions of Nafion/C in the catalytic layer have been investigated. The electrodes were fabricated depositing the catalytic ink, prepared with commercial catalyst (HiSPEC 2000), onto the gas diffusion layers by wet powder spraying. The MEAs were electrochemically tested using current-voltage curves and electrochemical impedance spectroscopy measurements. The experiments were carried out at 70 °C in H₂/O₂ and H₂/air as reactant gases at 1 and 2 bar pressure and 100% of relative humidity. For all MEAs tested, power density increases when the gasses pressure is increased from 1 to 2 bar. On the other hand, power density also increased when oxygen is used instead of air as oxidant gas in cathode. The lower power density (34 mW cm^?2) and power per Pt loading (0.86 kW g_Pt^?1) corresponds to the MEA prepared without Nafion in anode and cathode catalytic layers working with hydrogen and air at 1 bar pressure as reactants gas. The MEA with 30% wt Nafion/C reached the highest power density (422 mW cm^?2) and power per Pt loading (10.60 kW g_Pt^?1) using hydrogen and oxygen at 2 bar pressure. Finally, electrode surface microstructure and cross sections of MEAs were analyzed by Scanning Electron Microscopy (SEM). Examination of the electrodes, revealed that the most uniform ionomer network surface corresponds to the electrode with 40 wt% Nafion/C, and MEA ionomer-free catalytic layer shows delamination, it leads to low electrochemical performance.     

Transitional metal alloyed nanoparticles entrapped into the highly porous N-doped 3D honeycombed carbon: A high-efficiency bifunctional oxygen electrocatalyst for boosting rechargeable Zn-air batteries

Rational development of low-cost, durable and high-performance bifunctional oxygen catalysts is highly crucial for metal-air batteries. Herein, transition metal alloyed FeCo nanoparticles (NPs) embedded into N-doped honeycombed carbon (FeCo@N-HC) was efficiently prepared by a one-step carbonization method in the existence of NH₄Cl and citric acid. Benefiting from the honeycomb-like architectures and the synergistic effects of the FeCo alloy with the doped-carbon matrix, the as-synthesized FeCo@N-HC exhibited outstanding oxygen reduction reaction (ORR) with the more positive onset potential (E_onset = 0.98 V vs. RHE) and half-wave potential (E_1/2 = 0.85 V vs. RHE), coupled with outstanding oxygen evolution reaction (OER) with the lower overpotential (318 mV) at 10 mA cm^?2. Besides, the home-made Zn-air battery has the larger power density of 144 mW cm^?2 than Pt/C + RuO₂ (80 mW cm^?2). This research offers some valuable guidelines for constructing robust oxygen catalysts in clean energy storage and conversion technologies.     

Effects of humidity on SrTi0.05Co0.95O3-δ cathode properties and durability of solid oxide fuel cells

Degradation suffering from ambient water vapor in air is harmful for cathodes of solid oxide fuel cells (SOFCs), it is necessary and important to research its attenuation mechanism and related preventive methods. Here, effects of humidity on SrTi_0.05Co_0.95O_3-δ (STC) cathode have been discussed in detailed. X-ray diffraction analysis reveals that STC is relatively stable in wet air with a little crystal contraction, but without obvious impurities. Because active sites of STC for oxygen reduction reaction (ORR) are covered by water vapor, the electrical conductivity decreases while the polarization resistance (R_p) increases with the increasing of the water vapor concentration in air. Compared with R_p measured in dry air, the enhanced R_p value is 0.0347 Ω cm² and 0.0493 Ω cm² in containing 3% and 5% H₂O air after a continuous testing for 96 h at 800 °C, respectively. Furthermore, a maxium power densities of 832 mW cm^?2 and 814 mW cm^?2 can be obtained for STC/LSGM/STC in dry air and containing 5% H₂O air after 96 h operating at 800 °C, respecctively. Interestingly, the degradation coming from humidity can be recovered partially by switching the feedgas back to dry air again. The R_p of STC becomes ~0.0402 Ω cm² again after switching the operating atmosphere from 5% H₂O air to dry air for 5 h.     

Influence of biomass addition on electricity harvesting from solid phase microbial fuel cells

In this study, two types of biomass (Acorus calamus leaves and wheat straw) were added to a matrix of sediment and soil inside the anode of solid phase microbial fuel cells (SMFCs) in order to increase their output power. SMFC containing 3% leaves in their sediment had a maximum power density of 195 mW m⁻² in contrast to 4.6 mW m⁻² of that SMFC without leaves. Similarly, SMFC containing 1% wheat straw in their soil environment had a maximum power density of 167 mW m⁻². It suggests that the addition of biomass in appropriate proportions increases contact opportunities between the matrix, the anode and the added biomass, increases organic matter content, and enhances cellulase activity, thus serving as an important method for enhancing output power in SMFCs.     

Enhanced photocatalytic performance of NiFe2O4 nanoparticle spinel for hydrogen production

The spinel NiFe₂O₄, prepared from nitrates precursors, was characterized by thermal analyses, X-Ray Diffraction, UV-Vis diffuse reflectance, Scanning electron microscopy, X-Ray Fluorescence spectrometry, X-ray photoelectron spectroscopy and photo-electrochemistry measurements. The X-ray diffrcation analysis of the powder indicates a cubic phase with a lattice constant of 8.327(8) Å and crystallite size of 19 nm. The X-Ray Fluorescence spectrometry indicates a stoichiometry, very close to NiFe₂O₄ catalyst calcined at 900 °C The X-ray photoelectron spectroscopy analysis confirmed the valences and crystallographic sites of the transition elements. The direct optical gap of NiFe₂O₄ (1.78 eV), due to the crystal field splitting of the 3d orbital in the octahedral site, is well suited for the solar spectrum and attractive for photo-electrochemical H₂ production. The flat band potential (E_fb = 0.47 V_SCE) was obtained from the capacitance-potential (C^?2 - E) characteristic in NaOH (0.1 M) electrolyte. A conduction band of ?1.11 V_SCE, more cathodic than the H₂ level (?0.8 V_SCE), enabled the use of NiFe₂O₄ for the water reduction into hydrogen. The H₂ evolution rate of 46.5 μmol g^?1 min^?1 was obtained under optimal conditions (1 mg of catalyst/mL, NaOH and 50 °C) in the presence of SO₃^2? (10^?3 M) as hole scavenger under visible light flux of 23 mW cm^?2. A deactivation effect of only 1% was obtained.     

Preparation and characterization of activated carbon from rubber-seed shell by physical activation with steam

The use of rubber-seed shell as a raw material for the production of activated carbon with physical activation was investigated. The produced activated carbons were characterized by Nitrogen adsorption isotherms, Scanning electron microscope, Thermo-gravimetric and Differential scanning calorimetric in order to understand the rubber-seed shell activated carbon. The results showed that rubber-seed shell is a good precursor for activated carbon. The optimal activation condition is: temperature 880 °C, steam flow 6 kg h^?1, residence time 60 min. Characteristics of activated carbon with a high yield (30.5%) are: specific surface area (S_BET) 948 m² g^?1, total volume 0.988 m³ kg^?1, iodine number of adsorbent (q_iodine) 1.326 g g^?1, amount of methylene blue adsorption of adsorbent (q_mb) 265 mg g^?1, hardness 94.7%. It is demonstrated that rubber-seed shell is an attractive source of raw material for producing high capacity activated carbon by physical activation with steam.     

Sustained photobiological hydrogen production by Chlorella vulgaris without nutrient starvation

This article describes the ability of the Chlorella vulgaris BEIJ strain G-120 to produce hydrogen (H₂) via both direct and indirect pathways without the use of nutrient starvation. Photobiological H₂ production reached a maximum rate of 12 mL H₂ L^?1 h^?1, corresponding to a light conversion efficiency (light to H₂) of 7.7% (average 3.2%, over the 8-day period) of PAR, (photosynthetically active irradiance). Cells presented a maximum in vivo hydrogenase activity of 25.5 ± 0.2 nmoles H₂ μgChl^?1 h^?1 and the calculated in vitro hydrogenase activity was 830 ± 61 nmoles H₂ μgChl^?1 h^?1. The strain is able to grow either heterotrophically or photo autotrophically. The total output of 896 mL of H₂ was attained for illuminated culture and 405 mL for dark cultures. The average H₂ production rate was 4.98 mL L^?1 h^?1 for the illuminated culture and 2.08 mL L^?1 h^?1 for the one maintained in the dark.     

Ion transport resistance in Microbial Electrolysis Cells with anion and cation exchange membranes

Previous studies have shown that Microbial Electrolysis Cells (MECs) perform better when an anion exchange membrane (AEM) than when a cation exchange membrane (CEM) separates the electrode chambers. Here, we have further studied this phenomenon by comparing two analysis methods for bio-electrochemical systems, based on potential losses and partial system resistances. Our study reconfirmed the large difference in performance between the AEM configuration (2.1 m³ H₂ m^?3 d^?1) and CEM configuration (0.4 m³ H₂ m^?3 d^?1) at 1 V. This better performance was caused mainly by the much lower internal resistance of the AEM configuration (192 mΩ m²) compared to the CEM configuration (435 mΩ m²). This lower internal resistance could be attributed to the lower transport resistance of ions through the AEM compared to the CEM caused by the properties of both membranes. By analyzing the changes in resistances the limitations in an MEC can be identified which can lead to improved cell design and higher hydrogen production rates.     

Enzymatic routes to hydrogen and organic acids production from banana waste fermentation by autochthonous bacteria: Optimization of pH and temperature

The Central Composite Rotational Design (CCRD) was employed to find the optimum pH (5.09–7.91) and temperature (27.1–46,9 °C) for hydrogen production in banana waste (BW) fermentation by autochthonous microbial biomass. The P and Rm ranged between 6.06 and 62.43 mL H₂ and 1.13–12.56 mL H₂.h^?1, respectively. The temperature 37 °C and pH 7.0 were the optimum conditions for P (70.19 mL H₂) and Rm (12.43 mL H₂.h^?1) as predicted by the mathematical model. Fructose and glucose are the primary alternative carbon sources in banana waste-fed batch reactors. The high concentration of lactic acid and H₂ production was associated to Lactobacillus (52–81%) and Clostridium (14–35%). However, the most important finding was about butyric acid (HBu). This acid is the better indicator of hydrogen production than acetic acid (HAc). The pH effected carbohydrates fermentation and organic acids production. The genes encoding the enzymes related to galactose, sucrose, fructose, arabinose and xylose metabolism were predominant.     

Fe2O3-encapsulated and Fe-Nx-containing hierarchical porous carbon spheres as efficient electrocatalyst for oxygen reduction reaction

It is highly desirable to develop high-efficiency non-precious electrocatalysts toward oxygen reduction reaction (ORR). In this work, Fe₂O₃-encapsulated and Fe-Nx-containing porous carbon spheres (Fe₂O₃/N-MCCS) with unique multi-cage structures and high specific surface area (1360 m² g^?1) are fabricated. The unique porous structure of Fe₂O₃/N-MCCS ensures fast transportation of oxygen during ORR. The combined effect of Fe₂O₃ nanoparticles and Fe-Nx configurations endows Fe₂O₃/N-MCCS (E_1/2 = 0.837 V vs. RHE) with superior ORR activity and methanol tolerance to Pt/C. And, Fe₂O₃/N-MCCS exhibits better stability than nitrogen-modified carbon. The characterization results of Fe₂O₃/N-MCCS after long-term test reveals its excellent structural stability. Impressively, zinc-air battery based on Fe₂O₃/N-MCCS showed a peak power density of 132.4 mW cm^?2 and a specific capacity of 797 mAh g^?1, respectively.