Microbial Fuel Cells (MFC’s) – Producing Electricity While Treating Water Waste


Microbial Fuel

Study: Self-supporting nitrogen-doped reduced graphene Oxide@Carbon nanofiber hybrid membranes as high-performance integrated air cathodes in Microbial fuel cells. Image Credit: Peddalanka Ramesh Babu/Shutterstock.com

In an article available as a pre-proof in the journal Carbon, researchers used electrospinning methodologies to develop an air cathode built of self-sustaining nitrogen-doped reduced graphene oxide@carbon nanofiber (N rGO@CNF) hybrid sheets suitable for microbial fuel cells.

Microbial Fuel Cells for Bioenergy Production

It is critical to develop eco-friendly and sustainable technology in light of rising climate change consequences and global energy demand.

Microbial fuel cells (MFCs), a developing biological electrolytic system with good prospects as a maintainable bioenergy generation system, have piqued scientists’ curiosity for the past few years since they can concurrently produce electricity as well as treat water waste by transforming chemical energy contained in organic material to electricity with the help of microbes and fuel (usually wastewater).

MFC outperforms alternative methods for producing energy from biological material in terms of operating and functional characteristics, such as excellent direct effectiveness, ambient temperature functioning, and no need for supplementary energy or gas treatment.

Composition of a Typical MFC

The organic materials undergo oxidation in the anode compartment, generating protons and electrons. The electrons then move via an exterior circuit, yielding electrical energy, whereas the protons move to the cathode compartment via the electrolyte, in which they interact with the electron acceptors (O2). This results in the oxygen reduction reaction (ORR), which produces water using a two-electron or four-electron mechanism.

The electron receptors in the cathodic chamber have a critical role in energy production via microbial fuel cells; oxygen from the air is the best electron recipient because it is readily available and inexpensive. Since the slow ORR conducted in the cathodic chamber is considered the main hurdle, and improving ORR may considerably boost the total MFC effectiveness, MFC output is highly reliant on electrode performance, particularly that of the cathode.

** Graphical Abstract

MFC

How to Improve Performance of Air Cathode in MFCs

In a singular chambered microbial fuel cell, the typical air cathode comprises of three parts: the catalytic layer (CL), the substrate or the supporting layer (SL), and the conducting gas diffusion layer (GDL). Since the effectiveness of the air cathode is mostly determined by the catalytic layer, substantial research into catalyst designing and development has been carried out to enhance ORR taking place in the air cathode.

Thanks to their high catalysis performance, composites based on platinum (Pt) are currently the most widely utilized catalytic materials, but their industrial applications have been restricted by their significant prices, limited availability, and vulnerability to deactivation induced by biofouling and poisons in MFC settings.

Carbonaceous materials have come to the fore as excellent air cathode catalytic materials for microbial fuel cells as compared to platinum and other metallic catalysts, owing to their inexpensive prices, great stability, toxin tolerance, and excellent catalysis performance in ORR, making them viable substitutes to Pt-based catalysts.

Influence of Heteroatom Doping

One of the most successful ways for improving the ORR performance of carbonaceous materials has been established to be heteroatom doping. Injecting nitrogen (N) into the carbon framework activates electrons by creating charge spots, resulting in increased ORR catalysis performance.

Owing to the ease of agglomeration of carbon-based nanomaterials, which can obstruct catalytically active spots, the ORR effectiveness of carbonaceous composites doped with heteroatoms is still not optimal. Reduced graphene oxide (rGO) is presently utilized as an alternative form of carbon-based material to produce carbon-carbon hybrids for ORR usage. The blend of rGO and N-injected nanocarbons has a higher conductance, meaning more active spots for ORR are available.

Key Findings of the Study

In this paper, self-sustaining N-injected rGO@CNF hybridized membranes were effectively constructed using an electrospinning approach involving the addition of graphene oxide to a polyacrylonitrile (PAN) mixture followed by thermal processing in an NH3 setting.

The constructed rGO@CNFs can be used as embedded cathodes in microbial fuel cells directly. Their architectures, make-up, and texture were studied, as well as their electrolytic characteristics and MFC effectiveness, which were examined against pure NCNF and CAC electrodes.

The test results showed that rGO@CNFs outperformed the pure NCNF and CAC in terms of MFC effectiveness and ORR activation. In addition, the quantity of rGO incorporated in CNF had a significant impact on ORR activity and MFC effectiveness. On the basis of these findings, electrospun self-sustaining rGO@CNF hybridized membranes are suggested to be viable direct cathode options in MFCs.

Reference

Xu, M., Wu, L., Zhu, M., Wang, Z., Huang, Z.-H., & Wang, M.-X. (2022). Self-supporting nitrogen-doped reduced graphene Oxide@Carbon nanofiber hybrid membranes as high-performance integrated air cathodes in Microbial fuel cells. Carbon. Available at: https://www.sciencedirect.com/science/article/pii/S0008622322001968?via%3Dihub

Cleaning Waste Water and Salt Water with a Solar Heater Inspired by the Lotus Flower: KAUST


KAUST Sunlight Steam untitledChemical tricks improve the efficiency and durability of photothermal membranes that use sunlight to turn water into steam.

A point-of-use solar distillation device that can clean up saltwater and wastewater without producing greenhouse gases has been constructed by a research team from King Abdullah University of Science and Technology (KAUST)1.

The key to the new technology is a floating membrane coated with a special light-absorbing polymer that repairs its hydrophobic “skin” when damaged.

For centuries, attempts have been made to use the sun’s heat to distill clean water from polluted sources. Simple solar stills, such as a glass plate placed over a water-filled box, are inexpensive to operate but are notoriously inefficient. This is because water is a poor light absorber, and any captured heat tends to distribute uniformly through the still instead of localizing at surfaces where evaporation occurs.

To combat these problems, researchers are developing floating “solar generator” materials that heat up quickly in sunlight and then trap heat at air–water interfaces for steam production.

KAUST Sunlight Steam untitled

A polypyrrole (PPy)-coated device that absorbs sunlight and releases it as heat can rapidly purify water through distillation  Reproduced with permission © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

These devices are usually coated with water-repellant waxy molecules, such as fluorinated alkyl chains, for better floating. However, damage from ultraviolet rays and oxidative chemicals can degrade the hydrophobic layers, causing the generator to sink.

Inspired by the lotus flower, a plant that restores damage to its hydrophobic leaves through the migration of waxy molecules, KAUST Associate Professor Peng Wang and colleagues from the University’s Biological and Environmental Science and Engineering Division developed a self-healing solar generator.

The researchers coated a tightly woven stainless steel mesh with polypyrrole (PPy), a light-absorbing polymer with high photothermal conversion efficiency and bumpy surface microstructures. The team modified the PPy film with fluoroalkylsilane chains, enabling it to act as a reservoir that supplies additional hydrophobic chains to damaged regions through biomimetic self-migration.

The new device nearly tripled the output of freshwater from typical solar stills, thanks to a significant jump in temperature at the air–water interface and a conversion efficiency of close to 60 percent. It also exhibited remarkable damage resistance: after the team used a plasma source to oxidize the mesh and make it sink to the bottom of a beaker, they found a simple one-hour treatment in sunlight was sufficient to restore its self-floating capability.

The team’s first prototype — a transparent plastic condensing chamber and solar fan mounted on top of a PPy-coated mesh — floats lightly on the surface of seawater and distills a steady stream of water for more than 100 consecutive hours.

“Careful material selection allowed us to integrate two types of functions into one distillation device,” Wang said. “This has great potential to be employed in point-of-use potable water production.”

Reference

  1. Zhang, L., Tang, B., Wu, J., Li, R. & Wang, P.  Hydrophobic light-to-heat conversion membranes with self-healing ability for interfacial solar heating. Advanced Materials advance online publication, 17 July 2015 doi: 10.1002/adma.201502362 | article

Using Platinum-nickel Nanoalloys and Microwaves for Catalytic Water Treatment


Water Treatment Catalyst Microwavesid37351Water treatment technologies to remove contaminants from waste water can be made more efficient by incorporating nanomembranes or catalytic nanoparticles (get more insights into how nanotechnology is applied to water treatment). Compared to conventional treatment techniques, the use of catalysts, especially nanoparticle catalysts, can shorten treatment time, target recalcitrant substances, and selectively transform wastes into valuable products for instance by recovering carbon, nitrogen and phosphorus.   By . Copyright © Nanowerk

An issue with these systems is the expense associated with the initial investment and subsequent replenishment of catalysts. The reason for the high cost of catalytic water treatment is the use of expensive noble metals such as platinum and palladium for catalyzing the degradation of environmental contaminants. In the quest to find equally effective – in some cases even more effective – yet less expensive catalyst alternatives, researchers have developed bimetallic alloys by blending a noble metal nanoparticles with cheaper promoter metals such as copper and nickel. The blending ratio of these metals is an important parameter that controls the reactivity of alloy nanocatalysts. The challenge with this approach is how to find the composition of alloy nanoparticles that show the greatest catalytic reactivity for a contaminant of interest. Doing this requires the synthesis of a series of different nanoparticles which then each needs to be screened for their catalytic activity. Researchers at the University of Notre Dame now have successfully synthesized suspended platinum/nickel nanoalloys using a cycle-controlled microwave-assisted polyol reduction method.

The metal alloy nanoparticles synthesized by this method have a dynamic structure. For example, in the synthesis of platinum (Pt) and nickel (Ni) nanoalloy, a Pt core forms first, which then catalyzes the reduction of Ni2+. Ni then blends with Pt, giving a Pt/Ni alloy shell. After platinum is exhausted, the new shell is completely made of nickel. The team, led by Chongzheng Na, an Assistant Professor in the Department of Civil and Environmental Engineering and Earth Sciences, reported their findings in the August 7, 2014 online edition of Applied Catalysis B: Environmental (“Microwave-assisted optimization of platinum-nickel nanoalloys for catalytic water treatment”).

formation of nanoalloyFormation of the dynamic Pt/Ni nanoalloy under microwave irradiation and the volcano plot of the catalytic rate and surface compositions. (Image: Na group, University of Notre Dame)

“Our one-pot method creates nanoparticles with a range of surface compositions without much change of the particle size,” Hanyu Ma, a Ph.D. student in Na’s group, tells Nanowerk. “The varied surface compositions permit the rapid determination of the optimal Pt/Ni composition to be used as an effective nanoalloy for reducing the model water contaminant p-nitrophenol.” As Na and his team point out, the adoption of this facile synthesis method in catalyst designs may permit the rapid screening of nanoalloys for other water contaminants. Given the compositional dynamics of this technique, a series of nanoalloys with different surface compositions can be quickly synthesized using a single starting solution and the optimal metal ratio experimentally determined to find the best catalytic reactivity for degrading the pollutant. Ma explains that the structure-activity relationship of alloys is often linked to the averaged composition of an entire particle.

“As we have shown in our paper, the average composition could misrepresent the real composition on surface, where reactions occur,” he says. “With active control of the surface composition, we now can ensure that the reactivity is linked to the correct composition of a nanoalloy.” The researchers note that when the precursors of a noble metal and a transition metal react with a mild reactant such as polyol solvents, their difference in redox potential plays an important role, controlling which metal is reduced and which is not. “As far as we know, this has not been discussed explicitly in the past, particularly when microwave irradiation is used,” adds Ma. “At the beginning of the synthesis, the solvent absorbs microwaves and thus is heated to an above-ambient temperature.

At this temperature, polyol can only reduce noble metal cations so noble metal nanoparticles are formed. Once the nanoparticles are formed, they absorbs microwaves themselves – more than the solvent does – giving to a localized elevated temperature and forming nano hot spots around the nanoparticles. At the elevated temperature, transition metal cations can now be reduced so an alloy mixture is deposited on surface.” The surface composition is controlled by the availability of noble and transition metal precursors in solution.

According to the researchers, two factors play critical roles in establishing the compositional dynamics: the difference of redox potentials between noble and transition metals; and the difference of microwave absorptivity between metal nanoparticles and polyol solvent. Whereas in this present paper Na’s team demonstrated the usefulness of microwave-assisted polyol reduction for synthesizing binary nanoalloys with varied surface compositions, they hope to extend its application to the synthesis of ternary, quaternary, and even more sophisticated nanoalloys.