Biobattery

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A biobattery generates electricity using biological/organic compound materials and processes. Biofuel cells (BFCs) produce electrical energy through oxidation and reduction reactions at two electrodes using substances like glucose, bacteria, and enzymes. These enzymes present like they do in the human body and which break down glucose, then break down into electrons and protons which are released. Although the batteries have never been commercially sold, they are still being tested, and several research teams and engineers are working to further advance the development of these batteries. Related energy-harvesting devices that can aid in the creation of bio batteries development. For example, thermoelectric generators are not traditional chemical batteries but devices that convert fluctuations in temperature directly into electricity.^[1] Triboelectric nanogenerators and piezoelectric generators^[2] are devices that convert mechanical motion, pressure, or friction into electrical energy. These devices are starting to be researched by researchers for wearable devices and implanted sensors.

Biobatteries origins stem from the earliest experiments with bioelectricity, when scientists first began to wonder whether living organisms could produce electricity. In the late 1700s, Italian scientist Luigi Galvani observed that frog legs would twitch when they came into contact with different metals, which led him to believe in the idea of animal electricity. This curiously eventually inspired the development of the frog battery, where multiple frog muscles were connected in series to create a small but measurable electric current. In 1837, Carlo Matteucci improved this setup into a more reliable device by arranging frog tissues in a way that acted like a chain of electrochemical cells.^[3] Researchers also experimented with other animal tissues, including the ox-head battery, where freshly severed ox heads were used to demonstrate that biological tissue could carry and produce electrical signals. Although these experiments were later understood to rely on chemical reactions rather than a life supplying the energy, it allowed the scientists to gain a perspective on how electricity can come from living systems that follow the same principles of electrochemical that's found in commercial batteries. As the understanding of these systems improved, the experiments developed into simplest and more ethical demonstrations like the lemon battery.^[4] It is set up with two different metal electrodes, usually copper inserted into a lemon, and the fruit’s acidic base acts as an electrolyte that allows electrons to flow through a circuit. The energy actually comes from the oxidation of zinc and reduction for hydrogen ions, not from the lemon itself which the fruit provides the medium as the place the reaction occurs. This does not only work for lemons, but a wide variety of different acidic fruits like oranges, grapefruits that have the same properties that can function as electrolytes in simple electrochemical cells.

Like any battery, bio-batteries consist of an anode, cathode, separator, and electrolyte with each component layered on top of another. Anodes and cathodes are the positive and negative areas on a battery that allow electrons to flow in and out. The anode is located at the top of the battery and the cathode is located at the bottom of the battery. Anodes allow conventional current to flow in from outside the battery, whereas cathodes allow conventional current to flow out from the battery. As conventional current is opposite to electron flow, this means that cathodes allow electrons to flow into the battery and anodes allow electrons to flow out of the battery.

Between the anode and the cathode lies the electrolyte which contains a separator. The main function of the separator is to keep the cathode and anode separated, to avoid electrical short circuits. This system as a whole, allows for a flow of protons (H⁺) and electrons (e⁻) which ultimately generates electricity.^[5]

Biobatteries utilize a wide range of chemistries, and use many different methods to produce current. The efficiencies of these batteries are commonly measured as the voltage and current output, performance stability, and sustainability. Different biobatteries have different functions, with some biobatteries being studied as a replacement for lithium ion batteries and therefore needing a high voltage, while others are intended for use inside humans and therefore need a very low voltage. Because of this, the performance of biobatteries are very application, and chemistry, specific. While there have been studies done on the efficiency of biobatteries, because of the many different types of biobatteries, there are only a few studies covering any one chemistry.

Compared to conventional batteries, such as lithium batteries, bio-batteries are less likely to retain most of their energy.^[6] This causes a problem when it comes to long term usage and storage of energy for these batteries. However, researchers are continuing to develop the battery in order to make it a more practical replacement for current batteries and sources of energy.^[6] In general biobatteries tend to record a much lower voltage than lithium ion batteries as well. Although many biobatteries cannot operate for longer than a few hours, other biobatteries are capable of running off of sweat and mechanical energy allowing them to run for years, and potentially decades.^[7] Because biobatteries are made from biological agents and/or harvest their energy from biological agents, across the board, they also tend to be more sustainable than their metal counterparts. They are currently used in low power technologies like pacemakers and insulin pumps, but are generally insufficient for technologies with significant power consumption like smartphones.^[8] While biobatteries have been studied for over a decade—and will likely take a significant amount of time to scale to the efficiency needed for applications like smartphones—biobatteries are proficient at operating in room temperature, which makes them useful tools for communities that need to power small medical devices but don’t have access to constant refrigeration like developing countries.^[8]

Although biobatteries are not ready for more energy demanding devices, several research teams and engineers are working to further advance the development of these batteries.^[9] Sony has created a bio battery that gives an output power of 50 mW (milliwatts). This output is enough to power approximately one MP3 player.^[5] In the coming years, Sony plans to take bio batteries to market, starting with toys and devices that require a small amount of energy.^[6] Several other research facilities, such as Stanford and Northeastern, are also in the process of researching and experimenting with bio batteries as an alternative source of energy.

These batteries are being studied for use in biomedical applications, specifically for testing the pH of the stomach, and after further development, for diagnosing and treating gastrointestinal tract diseases.^[8][10] The advantage of these batteries is that they run on low voltages. Inside the body there is a risk of the battery breaking down and releasing its toxic materials. This is particularly risky above around 1.2 V. The safety of edible batteries, compared to other commercial batteries, also opens up possible applications in monitoring food quality and in edible soft robotics.

The efficiency of edible batteries should not be measured by the maximum voltage they can generate as they are made to generate voltages under around 1.2 V. While edible batteries might be capable of producing a higher voltage, a low voltage is necessary to prevent tissue damage, and worse side effects such as death, post-ingestion. The capacity for edible batteries ranges from 10 micro Ah to over 20 micro Ah. Some edible batteries have also been made to be rechargeable, indicating that they can be used multiple times.

Electric eel batteries are flexible and biocompatible, making them potentially usable for biomedical applications.^[11] These batteries are also environmentally stable, and with more development, would also ideally be able to recharge in their environments.^[11] This makes this potentially suitable for biological applications.

Electric eels are capable of generating over 600 V of electricity in one burst, and are capable of very high power densities. Multiple studies have found that battery cells made by electric eel electrolytes and hydrogels can produce over 100 mV of potential and maximum power densities ranging from 0.001 kW/m³ to 1500 kW/³.^[12] These cells can be linked together to make cells with large voltage outputs, with some studies reporting voltages of over 100 V.^[13][14]

There has been an interest in using bacteria to generate and store electricity. In 2013, researchers found that E. coli is a good candidate for a living biobattery because its metabolism may sufficiently convert glucose into energy thus produce electricity.^[15] Through the combination of differing genes it is possible to optimise efficient electrical production of the organism. Bacterial bio-batteries have great potential in that they can generate electricity rather than just storing it and also that they may contain less toxic or corrosive substances than hydrochloric acid, and sulfuric acid.

Another bacteria of interest is Shewanella oneidensis (formerly Alteromonas putrefaciens, first described in 1988,^[16] is dubbed "electric bacteria" due to its ability to reduce toxic manganese ions and turn them into food.^[17] In the process it also generates electrical current, and this current is carried along tiny wires made of bacterial appendages called bacterial nano-wires. This network of bacteria and interconnected wires creates a vast bacterial biocircuit unlike anything previously known to science. Besides generating electricity it also has the ability to store electric charge.^[18]

In 2015, researchers showed that iron-oxidising and iron-reducing bacteria could load electrons onto and discharge electrons from nanoparticles of magnetite. In their research, co-cultures of iron-reducing and iron-oxidizing bacteria were exposed to simulated day-night cycles. When exposed to light, the phototrophic Fe(II)-oxidizing bacteria, Rhodopseudomonas palustris, were able to remove electrons from the magnetite thereby discharging it. In dark conditions, the anaerobic Fe(III)-reducing bacterium Geobacter sulfurreducens were able to reverse the process, putting electrons back onto the magnetite thereby recharging it.^[19][20] The researchers concluded that iron ions in magnetite minerals are bioavailable as electron sinks and electron sources under varying environmental conditions, and could effectively function as a naturally occurring battery.^[19]

MFCs have potential applications in wastewater treatment because of their ability to convert organic matter into electricity and remove pollutants. Tested microbial fuel cells have been able to remove between 30% and 95% of incoming effluent from domestic and/ or synthetic wastewater depending on the structure of the battery as well as the anode, cathode, and membrane used.^[21][22] MFCs can also potentially be used in remote areas to produce electricity using microbes specific to that area, or in environment-specific robotics.^[23]

To find the efficiency of microbial fuel cells, it is important to look at the anode, cathode, and membrane. Some combinations of anode, cathode, and membrane record power densities of less than 0.5 mW/m², while others record over 100 mW/m².^[24][25][26] Carbon and platinum are two of the most widely used cathodes for these types of cells. While platinum is a common material, researchers are looking for more cost efficient cathodes, as there are other materials that can potentially offer similar efficiencies with lower costs. The main goal of the cathode is to facilitate the oxygen reduction reaction, so choice in cathode significantly impacts the power density of the battery. Different microorganisms have vastly different power densities as well, with some microorganisms having a power density of just over 1 mW/m², and others having power densities of over 4000 mW/m².^[27]

At the anode, the sugar is oxidized, producing both electrons and protons.

Glucose → gluconolactone + 2 H⁺ + 2 e⁻

These electrons and protons now play an important role in the release of stored chemical energy. The electrons travel from the surface of the anode through an external circuit to get to the cathode.^[28] On the other hand, the protons are transferred via the electrolyte through the separator to the cathode side of the battery.^[5]

The cathode then carries out a reduction half-reaction, combining the protons and electrons with the addition of oxygen gas to produce water.

O₂ + 4 H⁺ + 4 e⁻ → 2 H₂O

Sugar batteries are being looked at for use in implantable devices due to their biocompatibility and the body’s abundant glucose that could be used to fuel the battery. In some examples, glucose in the bloodstream is oxidized to generate power. This type of battery was demonstrated in the stomach of a rat and was able to produce enough power to operate an LED or digital thermometer.^[29] This technology could go on to be used in pacemakers, biosensors, and drug delivery systems. Because fuel is being drawn from the body, it reduces the need for surgery or invasive methods for battery changes or replacement. Sugar batteries are also being looked at for portable and large-scale energy applications, meaning that they could theoretically be used as replacements for commercial batteries.^[30]

One study says that sugar biobatteries have an energy storage density capable of reaching 596 Ah kg⁻¹, a value over an order of magnitude higher than that of lithium-ion batteries.^[31] Additionally, while this battery produced a current far under that of a lithium-ion battery, it far surpassed that of common household rechargeable batteries. While these batteries do use plastics, they are regarded as being sustainable compared to regularly sold batteries.

Organic biobatteries like the lemon battery use the acidity in foods as electrolytes for the battery. These batteries can use the entirety of the food, or certain parts of the food, like the peel, and utilize reduction-oxidation chemistry to produce a charge. Unlike MFCs, a form of bacteria biobatteries that can also use foods in fuel cells, organic biobatteries use acidic foods as one of the electrodes, while in MFCs they are used to form a biofilm on a metal electrode.^[32][33]

Research on biobatteries from organic waste is limited, but one study mentions using batteries from fruit peels for low energy applications like LEDs, wall clocks, and other small household devices.^[32]

Current organic biobatteries produce much lower efficiencies than lithium-ion batteries.^[32] The batteries can only last between 10-20 hours, and their maximum voltage and current output are around 1 V and .5 A, but they are made from household organic waste, making it significantly more sustainable than other batteries.^[32][34]

Piezoelectric and triboelectric generators create electrical energy from mechanical energy. Piezoelectric generators operate from the piezoelectric effect, wherein crystals in the material are sensitive to mechanical energy, and when subjected to pressure, an electric polarization is induced, causing an electric charge. Many bio-inspired piezoelectric generators have been studied in recent years, including eggshell, onion skin, fish bladder, and spider silk piezoelectric generators. Triboelectric generators, on the other hand, work from the triboelectric effect, where there is an electric charge transfer between two materials that have come in contact. A common example of this can be seen in the children's science experiment where a balloon is rubbed on their hair, and in turn, makes their hair stand, i.e. static electricity.

Piezoelectric and triboelectric generators have been studied for use in animal tracking.^[35] Piezoelectric generators offer the advantage of being chargeable through mechanical motion which circumvents the problem of batteries dying while on the animal, ending tracking unexpectedly. Both batteries are also capable of being environment resistant, meaning that they can withstand high humidity and other environmental factors.^[36] Both have also been looked at for various wearable, implantable, and portable devices, and even for energy generating tiles or sidewalks.^{[37][38][39][36][40]}

The voltage and current outputs from batteries of this type depend on the amount and force of the movements the battery is subjected to, as well as the frequency of periodic movements. Some methods like fractal design based switched-capacitor-converters, sliding, and freestanding mode in triboelectric generators can achieve energy transfer efficiencies in the 80s up to nearly 100%.^[41][42] Piezoelectric generators achieve a range of energy transfer efficiencies from 0.3% in fish bladder generators to 60% eggshell generators.^[43][44][45]