Kinkajou : Are there any other any other types of Fuel Cell?
Erasmus : Air-Breathing Laminar Flow-Based Microfluidic Fuel Cell (LFFC)
Recent advances in the efficiency of this design are due to 2 factors. Firstly, the introduction of air breathing gas diffusion electrode as the cathode. Oxygen replenishment in the depletion boundary layer of the cathode is increased by up to 10,000 times compared to the diffusion of oxygen in aqueous media. Power densities of 26 mW/cm2 have been attained by air breathing systems using formic acid as fuel compared to power densities of approximately 5 mW/cm2 attained by Aqueous based systems. Also, utilisation of an air breathing gas diffusion design simplifies the structure of the LFFC and removes the need for an external reservoir for oxidant solution.
Secondly membraneless laminar flow structures have improved power output by creating a stable liquid-liquid interface between fuel and oxidant /electrolyte inputs. This further reduces critical membrane related problems such as drying out or flooding by products of combustion process, namely water.
Further developments in the chemical composition and structure of the cathode and anode may well enhance electrode performance and fuel utilisation in the future.
There is also significant potential for improving power output through adjustment of types of fuel utilised in the cell (e.g. formic acid, methanol) and types of oxidants, (such as hydrogen peroxide or sulphuric acid).
Such laminar flow designs are only possible utilising a sub microscopic structure, typically imprinted onto silicon chips, using much of the same fabrication technology as Integrated circuits do.
The implementation of Nano porous separator is (such as polycarbonate with .05 µm pore size and a pore density of 68 pores per square centimetre) has also improved power density by reducing fuel diffusion to the catalyst layer.
The design of the cathode collector is complex due to the size constraints as well as the necessity for the use of noble metals. For example, some designs have use palladium cobalt alloy electro-plated onto a thin layer of gold.
Modifications of the electrolyte laminar stream are also possible.
The laminar flow characteristic eliminates convection mixing of fuel and oxidant, thus making it possible to design a fuel cell without the use of a separating membrane. The only way the fluids mix is through diffusion, and it is restricted to the centre of the channel. This mixing area is variable, and can be adjusted by changing the channel dimensions and flow rate to optimize energy production.
The laminar flow nature also allows the anolyte and catholyte composition to be chosen independent of each other. The liquid–liquid interface of microfluidic fuel cells eliminates some of the disadvantages of the PEM-based fuel cells such as membrane degradation, water management, and fuel crossover. However, the distance from the anode to the cathode limits the performance of membraneless fuel cells since it is a longer distance for a proton to travel.
Gas Processing Fuel Cell
Laminar Flow Microfluidic Fuel Cell
Erasmus : In the design of fuel cells of this size, microfabrication technology is critical. CO2 lasers been used to engrave micro channels for the fuel stream. The nanoparticles of electro catalysts must be deposited evenly. Catalyst technology is still developing. Allowance must be made for differential expansion of the silicon and metal layers under heavy load. Cracking commonly occurs causing leakage of fuel or oxidant /electrolyte streams.
Better methods must also be developed from moving oxidation by-products such as carbon monoxide which can block the dehydrogenation chemical pathway. Control of fuel diffusion is critical in minimising the poisoning of the catalyst and the creation of mixed voltage potential is at the cathode.
The use of formic acid in some systems is interesting because it can act both as a fuel and as an electrolyte, consequently enhancing both electron and proton transport at the anode. The electrolyte stream separates the fuel from direct exposure at the cathode. Oxidation of formic acid at the cathode substantially reduces power output and performance.
At the anode, formic acid decomposes and creates electrons and protons:
HCOOH → CO2 + 2H+ + 2e− E0 = 0.22 V
Oxygen diffusing into H2SO4 solution reacts with electrons from the cathode and protons from the liquid-liquid interface to form water. The catalytic oxygen reduction reaction (ORR) is a multi-electron process with a number of elementary steps, entailing different reaction intermediates.
Microfluidic Cell Displaying Energy Production
Without considering the reaction intermediates, the oxygen reduction at cathode is considered as: O2 + 4H+ + 4e− → 2H2O E0 = 1.229 V
So, the main overpotential losses are mainly associated with activation and Ohmic losses. The Ohmic losses are mainly attributed to the electrolyte proton conductivity and external resistance of electrodes and connections.
At high power outputs (current density) carbon dioxide bubbles form over the anode due to the electro-oxidation of formic acid. Fuel cell design needs to allow these bubbles to be dissolved by the stream or washed away by the stream drag force before growing large enough to block the microchannels.
Changes in flow rate affect the mixing of the eco-laminar streams and can affect fuel cell performance. Fuel crossover is significant at lower flow rates.
Since the microfluidic fuel cell takes the advantage of co-laminar streams, changes in flow rate will affect the molecular diffusion and consequently the cell performance.
The power output of a LFFC with an air-breathing cathode is less sensitive to flow rate compared to conventional LFFCs. An air-breathing LFFC with optimized flow architecture operating in very low flow rates seems to be a suitable power source for portable applications. The anode is made of porous carbon paper which can present more active area rather than a non-porous and planar anode.
A 140% increase in power density and a sixfold increase in fuel utilization were observed by replacing commercial carbon paper with porous electrodes, in some studies.
Erasmus: Microfluidic fuel cells represent a relatively new type of microscale fuel device, which have a lower cost and satisfy the demand of power generation. In these devices, fuel and oxidant streams introduced into a microchannel, proceed in parallel laminar flow without turbulent mixing. However, diffusion happens across the interface between the two streams transverse to the flow streams. Laminar flow occurs through design with viscous forces are dominant over inertia forces. (This type of effect occurs at low Reynolds numbers).
Microfluidic Device Chip
Microfluidic devices have been used in many applications such as:
- Clinical diagnostics.
- Environmental monitoring.
- Energy generation.
Fuel cell designs may incorporate proton exchange membranes (PEM). PEMs increase complexity if produced with multiple layers to enhance their ionic, electrical, and mechanical properties. Microfluidic fuel cells have higher efficiency once running at higher temperatures. However, the PEM tends to dry out at these temperatures resulting in a reduction of protons exchanged/decreased bio electricity production. The incorporation of a PEM does improve operation simplicity. They are more likely to operate at room temperature.
PEM Microfluidic fuel cells are rarely used nowadays due to the disadvantages that they pose, such as water management and fuel cross-over.
Erasmus : Another form of laminar flow-based fuel cells (LFFCs), eliminates the PEM.
Elimination of PEMs reduces problems of fuel crossover and anode dry out. Eliminating PEMs also reduces design complexity and cost and improves turbulent mixing and power generation in optimised designs. This direction of development is currently most favoured.
The disadvantage of the membraneless microfluidic fuel cell design is the limited proton conductance. This is because of the further distance from the anode to the cathode that a proton needs to travel. It is possible to focus a stream on one side of the channel, by changing the ratio of volumetric flow rates (Q2/Q1) in a microchannel. In a microreactor with buffer and reagent streams flowing in parallel, this can be used in reversing an unwanted reaction; that is crucial for enzyme/cofactor regeneration; by focusing the reagent stream close to the electrode.
Erasmus : Microfabrication techniques allowing high power density output and the ability to burn energy dense fuels are the main promoters for fuel cells over batteries
The small size of microfluidic fuel cell reaction chambers results in a high surface to volume ratio. This improves power density, energy production start-up time and allows for faster power generation recovery.
Since microfluidic fuel cells have larger power densities, they are being considered as a potential replacement for conventional batteries.
Methanol, ethanol, hydrogen, glucose, and vanadium redox species have been used as the fuel in their microfluidic fuel cell designs. Oxygen, in both gas and liquid form, is the most commonly used oxidant, but there have also been some designs based on hydrogen peroxide, potassium permanganate, and vanadium redox species as the oxidant.
An electrolyte such as a base or a strong acid is added to support the ionic charge transportation within the fuel and oxidant.
System of Processing Fuel Cell
Kinkajou : Tell us something about the production techniques for microfluidic laminar flow fuel cells, (LFFC’s).
Erasmus :Microfluidic fuel cells are typically made using standard photolithography techniques. In soft lithography, a solid mould carrying a negative pattern of the microchannel is prepared by microfabrication techniques. A piece of solid substrate such as glass or silicon wafer is spin-coated with a photoresist. Then a desired pattern is printed on a photomask, followed by UV exposure. Then the photoresist exposed to UV light is removed and is etched down to the preferred thickness.
New fabrication techniques are being explored, one reported to employ a standard laser winter. This uses a non-photolithography process.
The microfluidic channel is printed onto commercially available thermoplastic Shrinky-Dinks, polystyrene thermoplastic sheets that shrink after being heated in an oven. This could be used as a substitute to the photolithography method of making a mould with microfluidic channels on it. It is reported that after heating the Shrinky-Dink for several minutes at approximately 160 degrees C, the printed pattern shrank equally in all directions by about 63% from the original dimensions while the height of the pattern increased significantly.
This method is significantly less expensive than conventional photolithography and micromachining methods, and can generate patterns of various heights by changing the number of prints. The main disadvantage associated with this method is that the quality of the pattern, especially at the edges, is not as precise as if it were generated by standard photolithography techniques. In a microfluidic fuel cell, electrodes are typically positioned in parallel on the side or the bottom wall of the channel for a side-by-side streaming microfluidic fuel cell.
Gold, carbon cloth, carbon paper, and graphite are all commonly used electrodes.
Erasmus :The position of the electrodes influences the Ohmic resistance in the channel; electrodes spaced close to each other allow lower internal resistance and are consequently more desirable. However, the design must be optimized to avoid the inter-diffusion area in the centre of the channel.
It was found that by decreasing the size of the channel, the current density at the same volumetric flow rate is increased.
Overall, the performance of the porous electrode fuel cell was improved significantly with power densities as high as 131 mW cm2 and nearly complete fuel utilization. ((Typical performances include power densities of 6 to 100 mW per square centimetre)).
The limitations of this fuel cell are that the concentration of the reactant is zero at the surface, and a concentration boundary layer is formed in the channel that limits the flux of the reactant to the surface depending on the flow characteristics. Additionally, to have a practical level of energy conversion efficiency, the cell must operate at high voltages.
Researchers have used a vertical stacking geometry which is volumetrically efficient with little dead volume and takes advantages of using flow-through electrodes. In this design, the electrodes are separated by the electrolyte and are arranged horizontal to each other while the fuel and oxidant flow is introduced vertical to the electrodes
Air-breathing laminar flow-based fuel cells. In an air breathing laminar flow-based fuel cell, a stream of liquid fuel such as methanol and a stream of oxidant such as hydrogen peroxide are introduced into a microchannel, where the anode and cathode form the sidewalls. This design is called multistream laminar flow-based fuel cells (LFFCs). One of the main advantages of LFFCs is that the composition of fuel and oxidant can be selected independently, allowing it to be used with a variety of media.
To provide higher oxygen concentration and consequently higher power generation rate, a porous gas diffusion electrode (GDE) as the cathode was used to allow direct oxygen transport from the air into the cathode. This makes air-breathing microfluidic fuel cells convenient in many applications, as the oxygen that is used as the oxidant in the microfluidic channel can be taken directly from the surrounding atmosphere.
A formic acid containing sulphuric acid stream entered the channel. An electrolyte stream containing sulphuric acid (without formic acid) entered the channel and prevented formic acid from reaching the GDE cathode
. The maximum current density of 130 mA cm2 and the maximum power density of 26 mW cm2 were obtained when formic acid concentration was 1 M, five times the power density that could be obtained from the same LFFC using an oxygen-saturated aqueous stream instead of an air breathing cathode. This is due to the higher diffusion coefficient of oxygen in air.
The performance and fuel utilization can be altered by adjusting electrode-to-electrode distances, electrode designs, and fuel concentrations.
Additionally, low Ohmic losses were achieved using low anode-to-cathode spacing. Finally, mass transport is more efficient in this design due to a uniform supply of fuel over the anode and bubble removal from the anode active sites.
Comparison Advantages Disadvantages to Fuel Cell Types
Microbial Fuel Cells
Kinkajou : Are there any other any other types of Fuel Cell?
Erasmus : Microbial fuel cells (MFCs) are based on either bacteria or enzyme systems derived from bacteria. Early designs of microbial fuel cells used a mediator chemical that assisted the transfer of electrons from bacteria in the fuel cell to the anode. Typical mediators are thionine, methyl blue and metal viologen, humic acid and neutral red. Mediators developed to date tend to be expensive and toxic.
Mediator free MFCs are a more recent development. In these MFC structures bacteria would typically have electrochemically active redox proteins such as cytochromes on their outer cell membranes. Fuel is oxidised by microbes generating carbon dioxide, electrons and protons. These systems are capable of transferring electrons directly to the anode.
Bioenergy Microbial Fuel Cell
Among the electrochemically active bacteria are,
- Shewanella putrefaciens
- Shewanella oneidensis) Shewanella spp. Are popular because they are very heat- tolerant and cold-tolerant)
- Aeromonas hydrophila
- Also, C. butyricum and C. beijerinckii have been reported to successfully generate electricity where starch is the fuel.
- Algae have been used as the electrogenic microbes.
- Pseudomonas, Proteobacteria, and Geobacter families are the most common electrogenic bacteria employed in microbial fuel cells
Typical bacteria used in microbial fuel cells include Pseudomonas, Geobacter, and Shewanella and Proteobacteria.
Some microbial fuel cells use pure culture, meaning only one strain of bacteria or algae is allowed to grow in the chamber. However, there are also some examples of binary or mixed-culture fuel cells that obtain results better or similar to pure-culture counterparts.
Some bacteria, which have pili on their external membrane, are able to transfer their electron production via these pili.
There has been some work using hydrogen produced by the fermentation of glucose by Clostridium butyricum as the reactant at the anode of a hydrogen and air fuel cell. Though the cell functioned, it was found to be unreliable owing to the unstable nature of hydrogen production by the micro-organisms.
One of the smallest microbial fuel cells with a total volume of 0.3 mL was reported to generate a maximum current density of 92 A m3 using Geobacter sulfurreducens and 127 A m3 using Shewanella oneidensis.
(The organisms capable of producing an electric current are termed exoelectrogens.
Microorganisms are inoculated in the anode chamber to generate electricity through their metabolism, while chemical solution in the cathode chamber acts as an electrolyte. The electrons are transferred through a proton exchange membrane between these two chambers.
The system still requires considerable research and development. Factors that affect efficiency include bacterial strains, on exchange membrane is and system conditions such as temperature and pH.
Some alternate forms of MFC also exist. Some microbial systems such as those designed to work on biomass or waste are often used to generate hydrogen which can then be used in micro-or macro electrical generating systems.
Microbial fuel cells have been very successful as lab-scale designs, but similar to microfluidic fuel cells, this type of fuel cells has not been commercially used in real world applications. An effort of making these fuel cells more practical is needed to extend this technology beyond the laboratory.
Other systems make use of photosynthetic bacteria to produce organic metabolites and to act as electron donors. They are sometimes known as biologic photovoltaic systems.
Microscale microbial fuel cells have received increasing attention because they have shorter start-up time and faster power generation recovery after refilling. They can also greatly improve the density of power generation because the density essentially depends on the surface-to-volume ratio in such reactors. Microbial fuel cells are commonly fabricated by placing electrodes and PEM between two PDMS chambers.
The choice of electrodes and biocatalysts (microorganism) are the two main factors that affect the power output.
Bacterial Fuel Cell
Electrode designs incorporate materials such as:
- Carbon cloth,
- Silver chloride
- Toray cloth,
Like microfluidic fuel cells, the chambers are fabricated using soft lithography methods.
Subtypes of microbial fuel cells include:
Mediator-less MFC: MFC's produce electric current by the bacterial decomposition of organic compounds in water
Microbial electrolysis cells (MEC). MECs partially reverse the process to generate hydrogen or methane by applying a voltage to bacteria to supplement the voltage generated by the microbial decomposition of organics sufficiently lead to the electrolysis of water or the production of methane
A complete reversal of the MFC principle is found in microbial electrosynthesis, in which carbon dioxide is reduced by bacteria using an external electric current to form multi-carbon organic compounds. The principle here is that the fuel cell produces fuel not electricity.
Soil Based Microbial Fuel Cell
Kinkajou : Are there any other types of microbial fuel cell?
Erasmus :Soil-based microbial fuel cell
A soil-based MFC is designed to sit in the soil. This cell carries inbuilt nutrient rich media called the inoculum and usually incorporates a PEM (product exchange membrane). Due to the complexity of soil microbes, the fuel usually incorporates complex sugars and other nutrients.
The anode generally is inserted deep within the soil, while the cathode is positioned on the soil surface and is exposed to the oxygen. The basic principle is that the aerobic microbes within the soil (which consume oxygen), act as an oxygen filter much like the expensive PEM materials used in laboratory MFC (microbial fuel cell) systems. This results in an increase redox potential in the soil as the depth of implementation of the anode is increased.
MFCs (microbial fuel cells) that do not use a membrane can deploy anaerobic bacteria in aerobic environments however, membrane-less MFCs will experience cathode contamination by the indigenous bacteria and the power-supplying microbe. The novel passive diffusion of nanoporous membranes can achieve the benefits of a membrane-less MFC without worry of cathode contamination.
SMFCs (soil-based MFCs) with simple structures can generate electrical energy while decontaminating wastewater through desalination. Most SMFCs used for wastewater treatment contain plants to mimic constructed wetlands.
Kinkajou : I take it that PEM is a more common in the microbial world than in other types of fuel cell?
Erasmus : Nanoporous membrane microbial fuel cells have been produced. The Nano porous (typically polymer) membrane in these types of cells allows passive diffusion within the cell, without proton exchange. (Hence non-PEM). Nano porous polymer filter membranes are typically made of nylon, cellulose or polycarbonate. These are substantially cheaper (i.e. 10%) than other more technical alternatives such as Nafion-117 acting as PEMs.
Porous membranes allow passive diffusion thereby reducing the necessary power supplied to the MFC in order to keep the PEM active and increasing the total output of energy from the cell.
In summary, comparing the two systems showed that removing the PEM enabled an increase in the amount of maximum power density and a decrease in the Coulombic efficiency of the microbial fuel cells.
Microbial Electricity Generation
Kinkajou : Describe the Electrical generation process used by microbes.
Erasmus :When microorganisms consume chemical fuel, typically sugars in aerobic conditions, they produce carbon dioxide and water. However in anaerobic conditions (oxygen is absent), they produce carbon dioxide and protons plus electrons as below:
C12H22O11 + 13H2O → 12CO2 + 48H+ + 48e−
Microbial fuel cells use inorganic mediator materials to connect to the electron transport chain existing within bacterial cells, thereby channelling produced electrons. The inorganic mediators attached through the cell walls become reduced and transfer the electrons to attached electrodes. As the electrons are removed, the inorganic mediator returns to its original oxidised state, thereby able to repeat the redox production process.
This can happen only under anaerobic conditions; if oxygen is present, it will collect all the electrons, as it has a greater electronegativity than the mediators used.
In a microbial fuel cell operation, the anode is the terminal electron acceptor recognized by bacteria in the anodic chamber. Therefore, the microbial activity is strongly dependent on the redox potential of the anode.
In fact, it was recently published that a Michaelis-Menten curve was obtained between the anodic potential and the power output of an acetate driven microbial fuel cell. A critical anodic potential seems to exist at which a maximum power output of a microbial fuel cell is achieved.
A number of mediators have been suggested for use in microbial fuel cells. These include natural red, methylene blue, thionine, or resorufin.
In the second chamber of the MFC is another solution and electrode. This electrode, called the cathode is positively charged and is the equivalent of the oxygen sink at the end of the electron transport chain, only now it is external to the biological cell. The solution is an oxidizing agent that picks up the electrons at the cathode. The most convenient option is to use a solution of a solid oxidizing agent, rather than to use oxygen is the oxidising agent.
Connecting the two electrodes is a wire (or other electrically conductive path, this path would typically include either a battery or an energy using “device”.
Kinkajou : Where would you use microbial fuel cells for Power generation?
Erasmus : Microbial fuel cells have a number of potential uses, generally focusing on harvesting electricity for use as a power source.
The use of MFCs is attractive for applications that require only low power but where replacing batteries may be time-consuming and expensive such as wireless sensor networks.
Virtually any organic material could be used to feed the fuel cell, including coupling cells to wastewater treatment facilities.
Bacteria would consume waste material from the water and produce supplementary power for the plant. The gains to be made from doing this are that MFCs are a very clean and efficient method of energy production.
MFCs also use energy much more efficiently than standard combustion engines, which are limited by the Carnot Cycle. In theory, an MFC is capable of energy efficiency far beyond 50%. However, using the new microbial fuel cells, conversion of the energy to hydrogen is 8 times as high as conventional hydrogen production technologies.
However, MFCs do not have to be used on a large scale, as the electrodes in some cases need only be 7 μm thick by 2 cm long. The advantages to using an MFC in this situation as opposed to a normal battery is that it uses a renewable form of energy and would not need to be recharged like a standard battery would. In addition to this, they could operate well in mild conditions, 20 °C to 40 °C and also at pH of around 7.
Although more powerful than metal catalysts, they are currently too unstable for long-term medical applications such as in pacemakers.
Uses for Microbial Power Cells
Kinkajou : Can you think of any other uses for microbial fuel cells?
Erasmus :Biosensors is another application.
The use of microbial fuel cells to power environmental sensors would be beneficial because they would be able to sustain power for a longer amount of time and enable the collection and retrieval of undersea data without using a wire infrastructure. The energy created by these fuel cells was enough to sustain sensors after an initial start-up time in research to demonstrate the effectiveness of the fuel cell as a power source for such sensors.
Kinkajou : So comment on some of the advantages of microbial fuel cells.
Erasmus : Compared with enzymatic fuel cells, microbial fuel cells are often more desirable due to the fact that enzymes existing in living cells perform more stably and at a lower cost than purified enzymes.
Multiple cultures is used in wastewater based microbial fuel cells
Bacterial reactions have been observed in a wide range of temperatures. Microbial fuel cells can operate under mild conditions (room temperature and neutral pH value) with a considerably high power density, long running period without recharging. They are generally regarded as having environmentally friendly reaction processes, and low cost. It is demonstrated that this type of fuel cell could be miniaturized to the microscale, which is highly useful for medical applications.
Microorganisms and medium: The nutrition provided to the bacteria in microbial fuel cells is an important factor that influences the power output. Most microbial fuel cells require their fuel to be low molecular. This pre-requirement puts macromolecular compounds, such as complex carbohydrates, out of the list of usable biofuels.
Fuel cells have been designed to use complex fuel such as natural carbohydrates like starch. C. butyricum and C. beijerinckii were used as the biocatalysts to accelerate the reaction due to their ability to digest various substrates and their high hydrogen production rate. The versatility of these bacteria makes them excellent choices for biocatalysts in microbial fuel cells. In this particular work, starch had been used as the fuel for the device.
Kinkajou : So can we use the enzymes to catalyse fuel cell activity independent of the presence of the actual bacteria?
Erasmus : An important variant of the microbial fuel cell is the enzymatic fuel cell. In enzymatic fuel cells, enzymes are used as catalysts to produce electricity. Enzymes derived from biological systems are used to convert chemical energy (from a fuel) into electricity. Currently, the systems have low power output and poor long-term stability.
The enzymatic fuel cell is however easy to miniaturize. They operate by the same principle as other fuel cells. That is, fuel is oxidised at the anode; causing electrons to move from the anode to the lower potential cathode. They typically utilise small biomolecules such as glucose as fuel. PEMs are generally not necessary for most enzymatic biofuel cells because the enzymes involved are capable of selectively utilising substrates, eliminating the need for a membrane separator.
Compared to a conventional fuel cell, an enzymatic biofuel cell has a simpler design and is more cost-efficient.
As the enzymes are generally derived from biological organisms, most reactions happen under temperatures between 25 and 40°C and at neutral pH. However, to optimise energy production, in somatic fuel cells must maintain homeostasis to stay within the operating parameters of the enzyme. Bacterial cells would of course maintain their own internal homeostasis.
Choice of substrate (e.g. glucose versus fructose versus alcohol), Substrate concentrations, temperature, and pH are critical to the power rating of these fuel cells. Increasing the area of the electrodes by changing from planar to spherical microporous configurations also increases the power rating.
Different enzymes have different stabilities over different timescales. Systems used include alcohol dehydrogenase enzyme, glucose oxidase as well as bacterial lactase enzymes. Surprisingly these enzyme systems are also capable of being used in conjunction with noble metal catalysts such as iridium osmium platinum or palladium.
Perhaps in somatic fuel cells may well be the power source of choice for implanted devices such as pacemakers. They are inherently safer than the use of toxic metals or potentially disseminatable microbes.
Nano – scaling of enzymatic fuel cells through the use of carbon nanotubes or engraved silicon wafers also suggests possible power rating improvements.
Erasmus : But the disadvantages of low voltage, low current, and lower power density in enzymatic fuel cells limit their application areas for now. Since enzymes are not used for electricity generation in nature, it is difficult to establish electrical communication between proteins and electron surfaces. Biological electricity is carried in chemical packets such as ATP (adenosine triphosphate).
The conversion of chemical energy into wire born electrical energy is currently difficult and has stability problems (due to connection problems amongst others), at nano scales.
Implanted biofuel cells in vivo have been a focus of some developments to date. If the fuel cell is able to utilise available glucose within the body, the lifetime of the implants can be substantially enhanced. Currently designs have been tested within human serum but not within the human body.
Some researchers have demonstrated that fuel cells implanted into clams can generate electrical power using biological glucose as fuel. Power outputs of between 5 mW and 40 mW have been achieved. Other researchers have demonstrated power generation from the implementation of bio- electro-catalyst coated carbon fibres within grapes.
Outputs of approximately 2 mW mm-2 were achieved at voltages of .5 V. Another group of researchers have implanted biofuel cells utilising glucose into the abdomen of a cockroach. A maximum power density of 55 mW cm-2 at 0.2 V was achieved. In enzymatic fuel cell implanted into the retroperitoneal area of a rat achieved power output of 6.5 mW but this reduced with time. Glucose seems to be the fuel of choice in many research projects.
For biofuel cells to be viable constant and effective power needs to be generated over extended periods of time. Today we do not have the technological capacity to achieve these goals.
Living batteries such as this one could one day be used in environmental monitoring as well as several security applications.
Glucose-based microbial or enzymatic fuel cells could make them a viable solution to eliminate the need for replacement surgeries for this and other medical devices.
Human cells are also being used as biocatalysts. Some researchers have shown the possibility of using human macrophages in a biofuel cells application. NADPH oxidase is an enzyme that enables electron transport across the plasma membrane. Human macrophages were employed in their fuel cells to activate NADPH oxidase and generate current.
Fuel Cell Systems
Kinkajou : So what you think the future holds?
Erasmus : In the future, biological microfluidic fuel cells can be designed to be completely biodegradable so they could be used as non-permanent power sources in the environment without pollution.
Microbial fuel cells also offer a means of extracting energy from the otherwise untapped source of wastewater.
Microbial fuel cells that are inoculated with appropriate bacteria can oxidize the particles in wastewater and produce green energy and clean water without contaminants simultaneously
At this time, the amount of power supplied by fuel cells of this size is not adequate to satisfy realistic energy demands. This is primarily limited by the anodic surface area, and requires further research in device architecture and novel catalysts. Miniature biofuel cells also struggle with long term and consistent power supply, which would require the addition of capacitors to store charge or further
Solar Battery Storage
Kinkajou : So what you see is a likely possibility is the fuel cell development?
Erasmus : I think the biggest barrier to the development of fuel cells is the financial impediment to the development of a technological solution for niche environments. The likely uses will not generate enough profit to cover the cost of innovation. This means much innovation and development will have to be funded in a much longer term view than is possible by technology companies. Governments are likely to be the major players.
The major factor likely to drive long-term development is the ability to power small devices such as mobile phones for much longer than is capable with battery technology. Also it would be faster to refuel a fuel cell rather than to recharge a battery.
Surprisingly there are many places on this planet where power supply does not exist. Choices in the sites include photovoltaic solar panels generating electricity or installation of generators burning chemical fuel. The problem is that where power does not exist in remote locations, the need for power eclipses the capacity of fuel cells to deliver power. Most likely power will be required in such places in the multi- kilowatt range not in the hundred Watt range.
Solid oxide fuel cells perhaps show the most promise for future development. However there are obvious applications for microbial or enzymatic fuel cells, or for the development of bio fuel cells.
Solar Voltaic Battery Systems
Kinkajou : Any comments Goo?
Goo : There are a number of frontiers for improvements in today’s world for small scale energy generation and energy storage technologies.
The biggest problems that I can see is what do you do with it, when you don’t use it regularly. Gadgets like inverters and generators may have moving parts that have a specific life. Inverters are set to dominate with the growth of technology, but I can see long term role rationalizations between non-moving part inverters and inverter generators. Power purity is becoming a critical issue in today’s electronic world.
Normal battery technologies are very good at what they do. However, secondary batteries require maintenance. I think the degree of maintenance that ensures longevity is beyond the ken on the individual. Perhaps in the long term all secondary batteries will need ID chips with memories to enable “maintenance systems to be run to maintain these batteries in peak performance. I think most people would not be aware that a Lithium battery left to go flat is stuffed.
NiCad is still a good long term home intermittent use option. Chipping secondary batteries will need to be a system issue unless we want a proliferation of incompatible propriety maintenance technologies to proliferate.
The fuel cells are an interesting area of innovation. But, they suffer from a tough competition: (long -lasting cheap primary batteries). There also needs to be substantial investment into these technologies to allow them to get to the point where they can compete with existing cheap technologies.
As usual, it is likely that development will be driven by critical use / critical situation issues where “money is no object”. The most obvious here are military applications. And in man’s journey into outer space, it is weight that is a much more important issue than is cost.