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Salt Bridge for Microbial Fuel Cell: Full or Partly Full?

Salt Bridge for Microbial Fuel Cell: Full or Partly Full?


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I'm making a microbial fuel cell out of benthic mud and salt water in containers. My salt bridge will be made of agar and table salt solidified in PVC pipes. Does the salt bridge connecting the anode (benthic mud) and cathode (salt water) have to be full of the agar-salt solution, or can it be partly full?


Main function of the salt bridge is to conduct electrons. It should be in such a way that the agar within the PVC pipe from both end should be in contact with anode and cathode. so I would say keep it completely filled for better conduction.


Salt Bridge Definition

A salt bridge is a connection containing a weak electrolyte between the oxidation and reduction half-cells in a galvanic cell (e.g., voltaic cell, Daniell cell). Its purpose is to keep the electrochemical reaction from reaching equilibrium too quickly. If a cell is constructed without a salt bridge, one solution would quickly accumulate positive charge while the other would accumulate negative charge. This would halt the reaction and thus the generation of electricity.


How to make your own microbial fuel cell

This article was taken from the November 2014 issue of WIRED magazine. Be the first to read WIRED's articles in print before they're posted online, and get your hands on loads of additional content by subscribing online.

For Bruce Logan, microbes are an electrifying subject. The Kappe professor of environmental engineering at Pennsylvania State University works on creating alternative-fuel cells powered by anaerobic bacteria. "Because these bacteria breathe without oxygen they will transfer electrons from their food outside their bodies to an electrode," he says. Enough bacteria doing this at the same time will generate a current. At just a few millivolts It won't charge a phone, but it's enough to get a reading.

  • 2x 1l plastic containers
  • 15cm plastic pipe with cap
  • 3g agar
  • 11.8g salt, dissolved in 150ml of distilled water
  • 2x pencil leads
  • Coated copper
  • Glue gun
  • Aquarium air pump
  • Voltmeter
  • Sediment from the bottom of a lake

Get drilling

Drill a hole for the copper wire in the lid of each container. In one lid drill two more holes for the air pump and for ventilation this is your cathode container, the other container is the anode.

Drill a further hole in the side of each container the same diameter as your plastic pipe.

Coil the graphite

Strip the ends of two pieces of copper wire and wrap one around each pencil lead. "Any graphite will work, but you don't want to use metals as they corrode," says Logan. Insert one electrode into each hole in the container lids and the air-pump tube into the other, then seal with hot glue.

Mix the solution

Dissolve 100g per litre of agar in boiling water and mix in the salt. Seal one end of the PVC pipe and pour in the mixture. Allow this to cool and solidify, then remove the seal from the end, place the ends of the pipe into the holes in each container and again use hot glue to seal.

Squeeze the oxygen

Fill the cathode container with salt water and the anode with sediment, ensuring both electrodes are submerged, and the latter is full. "If there's oxygen, the bacteria will breathe it," says Logan. "Then they won't generate electricity because they'll give electrons to the oxygen instead."


How do Microbial Fuel Cells Work?

Microbial fuel cells work by allowing bacteria to do what they do best, oxidize and reduce organic molecules. Bacterial respiration is basically one big redox reaction in which electrons are being moved around. Whenever you have moving electrons, the potential exists for harnessing an electromotive force to perform useful work. A MFC consists of an anode and a cathode separated by a cation specific membrane. Microbes at the anode oxidize the organic fuel generating protons which pass through the membrane to the cathode, and electrons which pass through the anode to an external circuit to generate a current. The trick of course is collecting the electrons released by bacteria as they respire. This leads to two types of MFCs: mediator and mediatorless.


Discussion

Our work shows, for the first time, the utility of using biologically relevant redox molecules in translating electronic signals to changes in engineered bacterial gene expression. Our system is based on coupling Pyo-driven SoxR activation 31,35 with electronic control of Fcn(O/R) redox form 18,41,42 . This integration allows us to open a new communication pathway and develop a novel framework to connect electronic signals to gene expression. We present in this paper robust evidence and thorough characterization of a functioning bacterial electrogenetic device. To our knowledge, our work is first in demonstrating and characterizing an electrode-based system for reversible and specific redox-driven genetic control in bacteria.

Our applications of this system to genetically induce bacterial motility and cell-to-cell communication highlight its versatility in that it builds upon advances in using electronic control of behaviours that are naturally redox-dependent. Additionally, although we highlight the dynamic gene-actuation capabilities of our system, our aims differ from those of other synthetic biology efforts that enlist non-native components to recognize alternative input signals for precise genetic control using light 47,48,49,50 or magnetic and radio waves 51,52,53 . Instead, we minimally rewire the cells to take advantage of native redox interactions, and provide insights into their developing role as mediators for bio-electronic communication.

We foresee that our system can be tailored to produce a variety of responses, guide various behaviours, and further the use of other electronic 28,31 and redox-based systems to access and affect biomolecular information transfer, perhaps as part of MFC or BES systems for gene expression based on potential, current or electron acceptor availability. We show preliminary results that expand on the redox mediators, cell genetic background and oxygen levels that are used. Our approach may prove useful for spatio-temporal control of cells immobilized at or near electrode surfaces, for metabolic engineering applications, gut-on-a-chip systems, and other bio-hybrid devices where precise cellular spatio-temporal control is desirable. Additionally, our system offers an additional mode (in addition to light, magnetic, and radio) of relaying electronic signals to cells. Such cells can be programmed to respond to an unprecedentedly wide array of biological and non-biological information and make ‘smarter’ decisions than previously possible. Our electrogenetic device additionally offers electronic interrogation of SoxRS-specific targets and electron-flow-dependent processes. In sum, our work to translate electronic signals to bacterial gene expression represents a new way of using redox molecules and electron flow for guiding biological function.


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Abstract

Scaling up microbial fuel cells (MFCs) is inevitable when power outputs have to be obtained that can power electrical devices other than small sensors. This research has used a bipolar plate MFC stack of four cells with a total working volume of 20 L and a total membrane surface area of 2 m 2 . The cathode limited MFC performance due to oxygen reduction rate and cell reversal. Furthermore, residence time distribution curves showed that bending membranes resulted in flow paths through which the catholyte could flow from inlet to outlet, while leaving the reactants unconverted. The cathode was improved by decreasing the pH, purging pure oxygen, and increasing the flow rate, which resulted in a 13-fold power density increase to 144 W m −3 and a volumetric resistivity of only 1.2 mΩ m 3 per cell. Both results are major achievements compared to results currently published for laboratory and scaled-up MFCs. When designing a scaled-up MFC, it is important to ensure optimal contact between electrodes and substrate and to minimize the distances between electrodes.


A-level chemistry salt bridge questions (how do they work?)

In a Galvanic cell, how exactly does the salt bridge work? I've been told that it completes the circuit by allowing ions to flow between the half cells and balance out the charges as the system has to be electrically neutral for a voltage to exist.

1.) Why does it have to be electrically neutral for a voltage to exist?
2.) From my GCSE physics knowledge (I don't do A-level physics) current is the flow of electrons. If electrons are not flowing between the salt bridge, how is current maintained?
3.) Does the salt bridge not dry out after a while when ions are transferred to each half cell? Is it therefore replaced? Would ions from the solution in one half cell not transfer to the other by diffusion?

I would honestly appreciate any help with any of these questions as I've spent about 2 hours trying to figure it out myself with minimal success

Not what you're looking for? Try&hellip

(Original post by aque1408)
In a Galvanic cell, how exactly does the salt bridge work? I've been told that it completes the circuit by allowing ions to flow between the half cells and balance out the charges as the system has to be electrically neutral for a voltage to exist.

1.) Why does it have to be electrically neutral for a voltage to exist?
2.) From my GCSE physics knowledge (I don't do A-level physics) current is the flow of electrons. If electrons are not flowing between the salt bridge, how is current maintained?
3.) Does the salt bridge not dry out after a while when ions are transferred to each half cell? Is it therefore replaced? Would ions from the solution in one half cell not transfer to the other by diffusion?

I would honestly appreciate any help with any of these questions as I've spent about 2 hours trying to figure it out myself with minimal success

1.) Why does it have to be electrically neutral for a voltage to exist?

If a charge were to develp in the half-cell it would oppose the forces pulling the electrons. For example when an electron moves from the half-cell around the external circuit the half-cell would develop a positive charge. This would prevent the loss of a further electron by electrostatic attraction. (simplistic, but it works)

2.) From my GCSE physics knowledge (I don't do A-level physics) current is the flow of electrons. If electrons are not flowing between the salt bridge, how is current maintained?

Current is actually a flow of charge, not necessarily electrons. The movement of ions constitutes a current.

3.) Does the salt bridge not dry out after a while when ions are transferred to each half cell? Is it therefore replaced? Would ions from the solution in one half cell not transfer to the other by diffusion?


The ions are not "wet", they are solute particles in the solvent. The solvent remains in the salt bridge and is unaffected by the flow of ions. Ions are prevented from moving in the opposite direction by the same forces that cause them to move in the first place, electrostatic attraction.


Abstract

The performance of sediment microbial fuel cells (SMFCs) is usually limited by the structure, moisture, and salt content of the soil where they are allocated. Despite the influence of soil, so far most of efforts to improve SMFCs have been limited to the hardware design of the bioelectrochemical device. Our main objective was to enhance performance of SMFCs by stimulating the in situ formation of silica colloids in a low conductivity rice paddy soil. Our results have revealed that the presence of a silica colloid network, described by cryo-SEM analysis, reduced soil resistivity, enhanced ion mobility and consequently enhanced the power production by a factor of 10. Furthermore, our silica-supplemented soil showed better utilization of the electron donor, either acetate or natural rice root exudates, by electrogenic microbial populations. Sustainable manipulation of soil micromorphology using environmentally friendly reagents such as silica offers a novel approach for enhancing the performance of in situ microbial electrochemical applications in low conductivity soils, thus silica colloid geoengineering should be considered as part of future applications of SMFCs.


Fuel Cell Basics

Fuel cells can provide heat and electricity for buildings and electrical power for vehicles and electronic devices.

How Fuel Cells Work

Fuel cells work like batteries, but they do not run down or need recharging. They produce electricity and heat as long as fuel is supplied. A fuel cell consists of two electrodes—a negative electrode (or anode) and a positive electrode (or cathode)—sandwiched around an electrolyte. A fuel, such as hydrogen, is fed to the anode, and air is fed to the cathode. In a polymer electrolyte membrane fuel cell, a catalyst separates hydrogen atoms into protons and electrons, which take different paths to the cathode. The electrons go through an external circuit, creating a flow of electricity. The protons migrate through the electrolyte to the cathode, where they reunite with oxygen and the electrons to produce water and heat

Types of Fuel Cells

Although the basic operations of all fuel cells are the same, special varieties have been developed to take advantage of different electrolytes and serve different application needs. The fuel and the charged species migrating through the electrolyte may be different, but the principle is the same. An oxidation occurs at the anode, while a reduction occurs at the cathode. The two reactions are connected by a charged species that migrates through the electrolyte and electrons that flow through the external circuit.

Polymer Electrolyte Membrane Fuel Cells

Polymer electrolyte membrane (PEM) fuel cells, also called proton exchange membrane fuel cells, use a proton-conducting polymer membrane as the electrolyte. Hydrogen is typically used as the fuel. These cells operate at relatively low temperatures and can quickly vary their output to meet shifting power demands. PEM fuel cells are the best candidates for powering automobiles. They can also be used for stationary power production. However, due to their low operating temperature, they cannot directly use hydrocarbon fuels, such as natural gas, liquefied natural gas, or ethanol. These fuels must be converted to hydrogen in a fuel reformer to be able to be used by a PEM fuel cell.

Direct-Methanol Fuel Cells

The direct-methanol fuel cell (DMFC) is similar to the PEM cell in that it uses a proton conducting polymer membrane as an electrolyte. However, DMFCs use methanol directly on the anode, which eliminates the need for a fuel reformer. DMFCs are of interest for powering portable electronic devices, such as laptop computers and battery rechargers. Methanol provides a higher energy density than hydrogen, which makes it an attractive fuel for portable devices.

Alkaline Fuel Cells

Alkaline fuel cells use an alkaline electrolyte such as potassium hydroxide or an alkaline membrane that conducts hydroxide ions rather than protons. Originally used by the National Aeronautics and Space Administration (NASA) on space missions, alkaline fuel cells are now finding new applications, such as in portable power.

Phosphoric Acid Fuel Cells

Phosphoric acid fuel cells use a phosphoric acid electrolyte that conducts protons held inside a porous matrix, and operate at about 200°C. They are typically used in modules of 400 kW or greater and are being used for stationary power production in hotels, hospitals, grocery stores, and office buildings, where waste heat can also be used. Phosphoric acid can also be immobilized in polymer membranes, and fuel cells using these membranes are of interest for a variety of stationary power applications.

Molten Carbonate Fuel Cells

Molten carbonate fuel cells use a molten carbonate salt immobilized in a porous matrix that conducts carbonate ions as their electrolyte. They are already being used in a variety of medium-to-large-scale stationary applications, where their high efficiency produces net energy savings. Their high-temperature operation (approximately 600°C) enables them to internally reform fuels such as natural gas and biogas.

Solid Oxide Fuel Cells

Solid oxide fuel cells use a thin layer of ceramic as a solid electrolyte that conducts oxide ions. They are being developed for use in a variety of stationary power applications, as well as in auxiliary power devices for heavy-duty trucks. Operating at 700°C–1,000°C with zirconia-based electrolytes, and as low as 500°C with ceria-based electrolytes, these fuel cells can internally reform natural gas and biogas, and can be combined with a gas turbine to produce electrical efficiencies as high as 75%.

Combined Heat and Power Fuel Cells

In addition to electricity, fuel cells produce heat. This heat can be used to fulfill heating needs, including hot water and space heating. Combined heat and power fuel cells are of interest for powering houses and buildings, where total efficiency as high as 90% is achievable. This high-efficiency operation saves money, saves energy, and reduces greenhouse gas emissions.

Regenerative or Reversible Fuel Cells

This special class of fuel cells produces electricity from hydrogen and oxygen, but can be reversed and powered with electricity to produce hydrogen and oxygen. This emerging technology could provide storage of excess energy produced by intermittent renewable energy sources, such as wind and solar power stations, releasing this energy during times of low power production.


Watch the video: DIY Microbial Fuel Cell easy (September 2022).


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