Enhancing performance of microbial fuel cells by using novel double-layer-capacitor-materials modified anodes

Super-capacitor (SC) activated-carbon (AC) carbon-nanotubes (CNTs) (SC-AC-CNTs) is a kind of AC-based composite material and it combines the advantages of carbon nanotubes and activated carbon, including a series of peculiar properties such as low charge transmission resistance, super large specific area and excellent power characteristic. In this study, SC-AC-CNTs are first used to modify the carbon cloth (CC) anodes of microbial fuel cells (MFCs) and compared with that of SC-AC and CC. The measurements show that the specific surface area is increased from 219.519 m² g^?1 to 283.643 m² g^?1 after modification. The new anode is assembled in a urine-powered MFC (UMFC) to test its effectiveness. It is found that the amount of microorganisms attached on the new anode is much larger than that on the blank anode in UMFC. The maximum power densities of the UMFC assembled with SC-AC-CNTs and SC-AC modified anodes are 899.52 mW m^?2 and 555.10 mW m^?2, which are 2.9 and 1.8 times of that of the blank UMFC, respectively. The tests also shows that the UMFC with SC-AC-CNTs-modified anode creates a much longer duration of 105 h at high-voltage plateau in a single cycle that is about 2–3 times of the other two groups. These findings demonstrate that these two double layer capacitor materials can effectively boost overall MFC performance.     

Polyaniline/reduced graphene oxide-modified carbon fiber brush anode for high-performance microbial fuel cells

Polyaniline (PANI)/reduced graphene oxide (rGO) were synthesized by in-situ polymerization and were decorated on mesophase pitch-based carbon fiber brush (Pitch-CB) anode to promote microbial fuel cells (MFCs) power production. Mesophase pitch-based carbon fiber brush (Pitch-CB) becomes one of the most important research objects in MFCs. The mesophase pitch-based carbon fiber (CF) has excellent conductivity (about 2.0 μΩ m) compared with PAN-based CF (about 30 μΩ m). But the high conductive CF's surfaces have strong inert, and they are relatively smooth, which make it difficult to be adhered and enriched by microbes. By applying the PANI/rGO composite anode, the maximum power density (MPD) was increased to 862 mW m^?2, which was approximately 1.21 times higher than that of the Pitch-CB. The PANI can improve the surface roughness and surface potential of CFs, thus enhancing the adhesion of microbes and electrogenic performance of MFC. After the rGO was doped, the electrogenic performance of MFC was further improved. This study introduces a promising modifying method for the fabrication of high-performance anodes from simple, environment-friendly materials.     

Cathode modification with peptide nanotubes (PNTs) incorporating redox mediators for azo dyes decolorization enhancement in microbial fuel cells

To enhance azo dye reduction in cathode of microbial fuel cells (MFCs) and power generation, a novel cathode modification method was developed on carbon paper (CP) through immobilization of redox mediators (RMs) with self-assembled peptide nanotubes (PNTs) as the carrier. Results showed that the optimum peptide concentration for PNT self-assembly on electrode and Orange II decolorization in MFCs was 2 mg mL^?1. The PNT/RMs/CP electrodes exhibited higher electrocatalytic activities than PNT or RM solely modified electrodes and raw carbon paper electrode. MFCs loaded with the riboflavin (RF)/PNT modified cathode (PNT/RF/CP) or anthraquinone-2, 6-disulfonate (AQDS)/PNT modified cathode (PNT/AQDS/CP) showed an enhanced decolorization rate to Orange II compared to that with the control electrode, with the reduction kinetic constants increased by 1.3 and 1.2 folds, respectively. Furthermore, the MFCs with the PNT/AQDS/CP cathode and PNT/RF/CP cathode generated a higher maximum power density of 55.5 mW m^?2 and 72.6 mW m^?2, respectively, compared to the control (15.5 mW m^?2). The PNT/RMs modification could reduce cathode total internal resistance and accelerate electron transfer from electrodes to dyes, which may result in the enhanced performance of MFCs.     

Electrochemical performance of different carbon fuels on a hybrid direct carbon fuel cell

In this work, three processed carbon fuels including activated carbon, carbon black and graphite have been employed to investigate influence of the chemical and physical properties of carbon on the HDCFC performance in different anode atmospheres at 650–800 °C. The results reveal that the electrochemical activity is strongly dependent on crystalline structure, thermal stability and textural properties of carbon fuels. The activated carbon samples demonstrate a better performance with a peak power density of 326 mW cm^?2 in CO₂ at 750 °C, compared to 147 and 59 mW cm^?2 with carbon black and graphite samples, respectively. Compared to the ohmic resistance, the polarization resistance plays a more dominated role in the cell performance. When replacing N₂ by CO₂ purge gas, the power density is the strongly temperature dependent due to the Boudouard reaction.     

Ni-samaria-doped ceria (Ni-SDC) anode-supported solid oxide fuel cell (SOFC) operating with CO

The performance of nickel-samaria-doped ceria (Ni-SDC) anode-supported cell with CO-CO₂ feed was evaluated. The aim of this work is to examine carbon formation on the Ni-SDC anode when feeding with CO under conditions when carbon deposition is thermodynamically favoured. Electrochemical tests were conducted at intermediate temperatures (550–700 °C) using 20 and 40% CO concentrations. Cell operating with 40% CO at 600–700 °C provided maximum power densities of 239–270 mW cm^?2, 1.5 times smaller than that achieved with humidified H₂. Much lower maximum power densities were attained with 20% CO (50–88 mW cm^?2). Some degradation was observed during the 6 h galvanostatic operation at 0.1 A cm^?2 with 40% CO fuel at 550 °C which is believed due to the accumulation of carbon at the anode. The degradation in cell potential occurred at a rate of 4.5 mV h^?1, but it did not lead to cell collapse. EDX mapping at the cross-section of the anode revealed that carbon formed in the Ni-SDC cell was primarily deposited in the anode section close to the fuel entry point. Carbon was not detected at the electrolyte-anode interface and the middle of the anode, allowing the cell to continue operation with CO fuel without a catastrophic failure.     

A novel anode modified by 1,5‐dihydroxyanthraquinone/multiwalled carbon nanotubes composite in marine sediment microbial fuel cell and its electrochemical performance

A novel anode modified by 1,5‐dihydroxyanthraquinone/multiwalled carbon nanotubes (DAQ/MWCNTs) composite was fabricated in order to improve its electrochemical performance in marine sediment microbial fuel cell (MSMFC). The number of microbes on the DAQ/MWCNTs modified anode is 18.02 times of blank anode, and it has the best electrochemical performance compared with other anodes. Its exchange current density and biofilm capacitance are 1472.15 times and 18.66 times higher than that of blank anode, respectively. The maximum power density of composite modified MSMFC (890.12 mW·m^?2) is 3.01‐fold of blank cell (296.11 mW·m^?2). The mechanism analysis of DAQ/MWCNTs shows that DAQ, as an electron‐transfer mediator, can accelerate electron transfer to benefit the electrochemical performance of anode. This provides a new anode modification method for the development of high power MSMFC as long‐term power source to drive monitoring instruments.     

Simultaneous degradation of P-nitroaniline and electricity generation by using a microfiltration membrane dual-chamber microbial fuel cell

Microfiltration membrane, a potential alternative for traditional proton exchange membrane (PEM) due to its strong ability of proton transfer, cost-effectiveness, sustainability and high anti-pollution capability in microbial fuel cell (MFC). In this study, a novel MFC using bilayer microfiltration membrane as separator, inoculated sludge as biocatalyst and P-nitroaniline (PNA) as electron donor was successfully constructed to evaluate its performance. Furthermore, we also investigated the effects of initial PNA concentration, co-substrate (acetate) and cultivated microorganisms on MFC performance. Results showed that the maximum power density of 4.43, 3.05, 2.62 and 2.18 mW m^?2 was acquired with 50, 100, 150 and 300 mg L^?1 of PNA as substrate, respectively. However, with the addition of 500 mg L^?1 of acetate into reaction system contained 100 mg L^?1 of PNA, the higher power production of 6.24 mW m^?2 was obtained, which was 2.05 times higher than that using 100 mg L^?1 of PNA as the sole substrate. Meanwhile, the MFC working on cultivated microorganisms displayed a maximal power density of 7.32 mW m^?2 and a maximum PNA degradation efficiency of 54.75%. And after an electricity production cycle, the number of microbes in the anode chamber significantly increased. This study provides a promising technology for bioelectricity generation by biodegrading biorefractory pollutants in wastewater.     

Hydrogen production rates with closely-spaced felt anodes and cathodes compared to brush anodes in two-chamber microbial electrolysis cells

Flat anodes placed close to the cathode or membrane to reduce distances between electrodes in microbial electrolysis cells (MECs) could be used to develop compact reactors, in contrast to microbial fuel cells (MFCs) where electrodes cannot be too close due to oxygen crossover from the cathode to the anode that reduces performance. Graphite fiber brush anodes are often used in MECs due to their proven performance in MFCs. However, brush anodes have not been directly compared to flat anodes in MECs, which are completely anaerobic, and therefore oxygen crossover is not a factor for felt or brush anodes. MEC performance was compared using flat felt or brush anodes in two-chamber, cubic type MECs operated in fed-batch mode, using acetate in a 50 mM phosphate buffer. Despite placement of felt anodes next to the membrane, MECs with felt anodes had a lower hydrogen gas production rate of 0.32 ± 0.02 m³-H₂/m³-d than brush anodes (0.38 ± 0.02 m³-H₂/m³-d). The main reason for the reduced performance was substrate-limited mass transfer to the felt anodes. To reduce mass transfer limitations, the felt anode electrolyte was stirred, which increased the hydrogen gas production rate to 0.41 ± 0.04 m³-H₂/m³-d. These results demonstrate brush electrodes can improve performance of bioelectrochemical reactors even under fully anaerobic conditions.     

Facile preparation of binder-free NiO/MnO2-carbon felt anode to enhance electricity generation and dye wastewater degradation performances of microbial fuel cell

Binder-free NiO/MnO₂-carbon felt electrode is prepared with a facile two-step hydrothermal method. The NiO self-grown on the carbon felt is used as the skeleton structure to support the in-situ growth of MnO₂. Both the core and shell materials are excellent pseudocapacitance materials. The compositing of such pseudocapacitance metal oxides can produce synergistic effects, so that the modified electrode has a high capacitance. NiO/MnO₂-carbon felt electrode also possesses a high specific surface area, super hydrophilicity and good biocompatibility, which are conducive to the enrichment of typical exoelectrogen Geobacter. As the anode, NiO/MnO₂-carbon felt electrode can effectively improve the electricity generation and methyl orange (MO) wastewater degradation performances of microbial fuel cell (MFC). The highest output voltage and the maximum power density of MFC with NiO/MnO₂-carbon felt anode are respectively 652 mV and 628 mW m^?2, which are much higher than those of MFC with MnO₂-carbon felt anode (613 mV, 544 mW m^?2), NiO-carbon felt anode (504 mV, 197 mW m^?2) and unmodified carbon felt anode (423 mV, 162 mW m^?2). The decolourization efficiency and the chemical oxygen demand (COD) removal rate of MO for MFC with NiO/MnO₂-carbon felt anode are respectively 92.5% and 58.2% at 48 h.     

Metal-supported solid oxide fuel cells operated in direct-flame configuration

Metal-supported solid oxide fuel cells (MS-SOFC) with infiltrated catalysts on both anode and cathode side are operated in direct-flame configuration, with a propane flame impinging on the anode. Placing thermal insulation on the cathode dramatically increases cell temperature and performance. The optimum burner-to-cell gap height is a strong function of flame conditions. Cell performance at the optimum gap is determined within the region of stable non-coking conditions, with equivalence ratio from 1 to 1.9 and flow velocity from 100 to 300 cm s^?1. In this region, performance is most strongly correlated to flow velocity and open circuit voltage. The highest peak power density achieved is 633 mW cm^?2 at 833 °C, for equivalence ratio of 1.8 and flow velocity of 300 cm s^?1. The cell starts to produce power within 10 s of being placed in the flame, and displays stable performance over 10 extremely rapid thermal cycles. The cell provides stable performance for >20 h of semi-continuous operation.     

Landfill leachate: A promising substrate for microbial fuel cells

Landfill leachate emerges as a promising feedstock for microbial fuel cells (MFCs). In the present investigation, direct air-breathing cathode-based MFCs are fabricated to investigate the maximum open circuit potential from landfill leachate. Three MFCs that have different cathode areas are fabricated and studied for 17 days under open circuit conditions. The maximum open circuit voltage (OCV) of the cell is observed to be as high as 1.29 V which is the highest OCV ever reported in the literature using landfill leachate. The maximum cathode area specific power density achieved in the reactor is 1513 mW m^?2. Further studies are under progress to understand the origin of high OCV obtained from landfill leachate-based MFCs.     

Physical characterization and electrochemical performance of copper–iron–ceria–YSZ anode‐based SOFCs in H2 and methane fuels

Thermal instability and poor electrochemical activity of copper‐ceria‐YSZ anodes at the solid oxide fuel cells (SOFCs) operation temperature (>700 °C) necessitates the use of new strategy to improve the performance of respective anodes for direct hydrocarbon SOFCs. In the present study, iron is incorporated into copper–ceria–YSZ anodes in order to investigate the structural, morphological, and electrochemical properties by using various techniques such as X‐ray diffraction, elemental mapping, current–voltage testing, and electrochemical impedance spectroscopy. X‐ray diffraction shows that copper promotes the reduction of iron oxide, and formation of cubic phase of copper–iron metals is observed after reduction in H₂ at 800 °C. Elemental mapping shows better distribution of metal catalyst inside the pores of copper–ceria–YSZ anodes at 800 °C in the presence of iron. The maximum power densities of copper–ceria–YSZ anodes and copper–iron–ceria–YSZ anodes are observed to be 140 and 195 mW cm^?2 in H₂ fuel and 70 and 90 mW cm^?2 in CH₄ fuel at 800 °C. The maximum power density increases with the increase in Cu–Fe metal loading, temperature and with the addition of 1‐wt% Pd in copper–iron–ceria–YSZ anodes. The decrease in performance from 125 to 100 mW cm^?2 is observed during the exposure of CH₄ fuel for 46 h. Electrochemical impedance spectra show an increase in ohmic and total resistance of cell because of sintering and carbon formation, which affects the catalytic activity of anode lowering the performance of SOFC as suggested by post SEM analysis. Copyright © 2015 John Wiley & Sons, Ltd.     

Hydrothermally synthesized NiO-samarium doped ceria nano-composite as an anode material for intermediate-temperature solid oxide fuel cells

NiO-Ce_0.8Sm_0.2O_1.9 (Ni-SDC) composite anode powders are synthesized with a hydrothermal technique. The average size of the particles in the anode powder is about 10 nm. Different phases distribute uniformly in the composite. The anode sintered at 700 °C exhibits an electrical conductivity of above 100 S cm^?1, three orders of magnitude higher than that of a similar solid-mixed composite anode with the same composition. The anode synthesized through the hydrothermal process also possesses a higher catalytic activity. An SDC-carbonate composite electrolyte-supported single cell with the composite anode exhibits a maximum power density of 738 mW cm^?2 at 700 °C with H₂ as fuel, much higher than that of a similar cell with a solid-mixed anode. The cell also exhibits a promising stability with methanol as fuel.     

Performance of a direct formic acid fuel cell fabricated by ultrasonic spraying

The performances of a direct formic acid fuel cells (DFAFCs) comprising anode catalyst layers prepared via the following three different coating techniques are tested: direct paint (DP), ultrasonic spraying on the diffusion layer (US-D), and ultrasonic spraying directly on the membrane (US-M). These tests confirm that the ultrasonic spraying is a suitable method for the fabricating DFAFC anodes. Palladium black was used for the anode catalyst and a commercially available Pt/C cathode electrode was used for all tests. Scanning electron microscopy (SEM) revealed deep cracks caused by the porous substrate in the catalyst layers prepared by DP and by ultrasonic spraying on the diffusion layer. However, catalyst layers prepared by ultrasonic spraying directly on the membrane were less cracked and less porous, with small Pd particles. The catalyst layer prepared by ultrasonic spraying directly on the membrane showed the highest electrochemical surface area (ECSA) among the three anodes. In performance tests, ultrasonic spraying on the membrane yielded the highest power output because it produces the lowest ohmic resistance, the lowest anode potential, and the highest ECSA. By coating the catalyst membrane directly with ultrasonic spraying, we prepared a DFAFC with maximum power density as high as 245 mW cm^?2 using 5 M formic acid with 2 mg cm^?2 of catalyst loading.     

Effect of anode thickness and Cu content on consolidation and performance of planar copper-based anode-supported SOFC

Planar, Cu-containing Gadolinia-doped ceria anode-supported solid oxide fuel cells to be used at intermediate temperature (500–750 °C) were produced in the present work. The Intermediate temperature solid oxide fuel cells were fabricated using Li₂O as sintering aid for Gadolinia-doped ceria, varying the anode-to-electrolyte thickness ratio (r) from 2 to 10 and the CuO content in the anode from 45 vol% to 55 vol%. Co-sintering of the thermo-pressed green cells was carried out at 900 °C for 3 h. The electrolyte densification was favoured by increasing the r value, this being accounted for the enhanced compressive stresses induced by the supporting anode on the electrolyte upon sintering. Larger CuO content positively influences the overall cell performance, due to the improved electronic conductivity of the anode. Nevertheless, CuO concentration cannot exceed 50 vol% because of the tensile stresses (and corresponding flaws) generated in the electrolyte for larger amount. IT-SOFC containing 50 vol% CuO was characterized by an Open Circuit Voltage ≈0.82 V and a maximum power density of 200 mW cm^?2 at 700 °C.     

Development and evaluation of a 3-cell stack of metal-based solid oxide fuel cells fabricated via a sinter-joining method for auxiliary power unit applications

Recently, metal-based solid oxide fuel cells (SOFCs) receive much attention for potential application in auxiliary power units (APUs). In this study, a sinter-joining method with a silver bonding layer is proposed. This method enables the fabrication of metal-based SOFCs by joining metal plates and conventional ceramic cells using a silver bonding layer. This sinter-joining method has the advantage of full-sintering of the cathode at 1100 °C, which facilitates a lower area specific resistance (ASR) of the cathode. Furthermore, the entire manufacturing process is conducted under air atmosphere. A 5 × 5 cm² metal-based cell is successfully fabricated by the sinter-joining method, and a maximum power density of 433 mW cm^?2 and a low polarization resistance of 0.12 Ω cm² is obtained. Using the metal-based cells, a prototype 3-cell SOFC stack is developed considering mechanical robustness and diesel reformate fuel supply for future APU system applications. The stack exhibits a maximum power density of 100 mW cm^?2 and is tested for 120 h. After the test, a post-mortem analysis is conducted, and the causes of the low electrochemical performance and degradation issue are investigated. In the conclusion, the sinter-joining method is considered as one of the methods for metal-based SOFCs.     

Catalytic activity of infiltrated La0.3Sr0.7Ti0.3Fe0.7O3−δ–CeO2 as a composite SOFC anode material for H2 and CO oxidation

Finding cost-effective and efficient anode materials for solid oxide fuel cells (SOFCs) is of prime importance to develop renewable energy technologies. In this paper, La and Fe co-doped SrTiO₃ perovskite oxide, La_0.3Sr_0.7Ti_0.3Fe_0.7O_3?δ (LSTF0.7) composited with CeO₂ is prepared as a composite anode by solution infiltration method. The H₂ and CO oxidation behavior and the electrochemical performance (electrochemical impedance spectra, I–V and I–P curves) of the scandia-stabilized zirconia (ScSZ) electrolyte supported cells fabricated by tape casting with the LSTF0.7–CeO₂ composite anode are subsequently measured at various temperatures (700–850 °C). Electrochemical impedance spectra (EIS) of the prepared cells with the LSTF0.7–CeO₂|ScSZ|La_0.8Sr_0.2MnO₃ (LSM)–ScSZ configuration illustrate that the anode polarization resistance distinguished from the whole cell is 0.072 Ω cm² in H₂, whereas 0.151 Ω cm² in CO at 850 °C. The maximal power densities (MPDs) of the cell at 700, 750, 800 and 850 °C are 217, 462, 612, 815 mW cm^?2 in H₂ and 145, 349, 508, 721 mW cm^?2 in CO, respectively. Moreover, a significant decrease of anode activation energy towards H₂ oxidation is clearly demonstrated, indicating a better electrochemical performance in H₂ than in CO. These results demonstrate an alternative composite anode with high electrocatalytic activity for SOFC practical applications.     

Enhanced performance of direct peroxide–peroxide fuel cells by employing three-dimensional Ni and Co@TiC nanoarrays anodes

Thorn-like Ni@TiC NAs and flake-like Co@TiC NAs electrodes without any conductive agent and binder are simply fabricated by the potentiostatic electrodeposition of Ni and Co catalysts on the TiC nanowire arrays (NAs). The electrocatalytic activity of H₂O₂ oxidation on the Ni@TiC NAs electrodes is better than that on the Co@TiC NAs electrodes. The Ni@TiC NAs electrodes demonstrate a rough surface and have many nano-needles on the rod edges, which assures the high utilized efficiency of Ni catalysts. These particular three-dimensional structures may be very suitable for H₂O₂ electrooxidation. The anodic current of Ni@TiC NAs anode reaches 0.32 A cm^?2 at 0.3 V in 1.0 M H₂O₂ + 4 M KOH solution. The DPFCs employing Ni@TiC NAs anodes display the peak power density of 30.2 mW cm^?2 and open circuit voltage of 0.90 V at 85.1 mA cm^?2 with desirable cell stability at 10 mL min^?1 flow rate and 20 °C, which is much higher than those previously reported.     

Co-extruded dual-layer hollow fiber with different electrolyte structure for a high temperature micro-tubular solid oxide fuel cell

In this study, the phase inversion-based co-extrusion method was employed to fabricate a structural-improved electrolyte/anode dual-layer hollow fiber (HF) precursor, which was then co-sintered at 1450 °C. The electrolyte structures were thoroughly investigated by varying the loading of electrolyte material (i.e. Yttria-stabilized zirconia, YSZ) with differing particle sizes (i.e. micron, sub-micron, and nano-sized) during suspension preparation. The results showed that the most promising electrolyte layer with thin, dense, gas-tight, and defect-free properties was obtained by mixing 70% submicron-YSZ and 30% nano-YSZ in electrolyte suspension (E-0.7sub0.3nano). This electrolyte formulation co-extruded with a thick nickel-oxide-YSZ (NiO-YSZ) anode layer yielded the highest bending strength of 85 MPa, providing major mechanical strength to the HF. Besides that, the nitrogen permeability value at 2.87 × 10^?6 mol m^?2 s^?1 Pa^?1 suggested that the electrolyte was gas-tight, preventing fuel and oxidant transport. The fiber was then reduced to nickel (Ni)-cermet anode. It was developed to be a complete micro-tubular solid oxide fuel cell (MT-SOFC) by depositing the lanthanum strontium cobalt ferrite (LSCF)/YSZ cathode via brush painting on the dual-layer HF. The cell was fed with hydrogen gas and yielded an open-circuit voltage (OCV) as high as 1.06 V with maximum power density of 0.243 W cm^?2, at 875 °C. Based on this test, it was found that the electrolyte structural-modified dual-layer hollow fiber-based MT-SOFC using mixed particle sizes may result in a promising OCV. However, the relatively low value for power density may be due to a less porous anode; thus, improvements in the anode's structure are required in future research.     

Rare-earth oxide–Li0.3Ni0.9Cu0.07Sr0.03O2-δ composites for advanced fuel cells

Recent development on electrolyte-free fuel cell (EFFC) holding the same function with the traditional solid oxide fuel cell (SOFC) but with a much simpler structure has drawn increasing attention. Herein, we report a composite of industrial grade rare-earth precursor for agriculture and Li_0.3Ni_0.9Cu_0.07Sr_0.03O_2-δ (RE–LNCS) for EFFCs. Both structural and electrical properties are investigated on the composite. It reveals that the RE–LNCS possesses a comparable ionic and an electronic conductivities, 0.11 S cm^?1 and 0.20 S cm^?1 at 550 °C, respectively. An excellent power output of 1180 mW cm^?2 has been achieved at 550 °C, which is much better than that of the conventional anode/electrolyte/cathode based SOFCs, only around 360 mW cm^?2 by using ionic conducting rare-earth material as the electrolyte. Engineering large size cells with active area of 25 cm² prepared by tape-casting and hot-pressing gave a power output up to 12 W. This work develops a new functional single layer composite material for EFFCs and further explores the device functions.