Little by little, it’s one revolution at a time ecological and energy approaching. And, this time, no big project that mobilizes square kilometers or a new type of factory. No, it is an innocuous object, anchored in your daily life: the battery. Around the world, various teams are working to make them biodegradable, which are easy to produce and made from common materials, rather than rare metals. Even better, the batteries of tomorrow will produce green energy, through the joint action of microorganisms and their environment, be it the sun’s rays, sewage or even our sweat.
Biopiles: and it works!
In these two cases, the microorganisms chosen make it possible to eliminate platinum, a rare metal that is often used as a catalyst for the chemical reaction, and lithium, a contaminating component that is extracted and forms the anode. Another advantage is that the manufacture of these biofuel cells is quite simple. At least in principle. “It is enough to let the bacterial population colonize the anode by cultivating it in the future conditions of use, for example, in an environment poor in oxygen and rich in organic effluents”, highlights Frédéric Barrière, professor at the Department of Chemistry at the University of Rennes. Ecological, easy to produce… Too good to be true?
In fact, there is a major problem: the current delivered by these electroactive microorganisms is still very low, especially compared to that of “artificial” systems such as photovoltaics.
“Biofuel cells just don’t have the potential to power a large-scale electrical grid. admits Seokheun Choi, a researcher in electrical and computer engineering at Binghamton University. Nor are they capable of powering even more modest devices, such as simple smartphone batteries. But there is no reason to abandon them: “The objective was simply poorly thought out,” says the researcher, a fervent promoter of biofuel cells.
I live it as a catalyst
Like their classic counterparts, these different biofuel cells are based on an oxidation-reduction reaction. A fuel is “oxidized” at the anode by a catalyst, here microorganisms or enzymes. This reaction releases electrons which in turn reduce oxygen, most often at the cathode. Which electrons are then harvested to deliver a current. Please note that the biophotovoltaic battery also requires exposure to the sun.
Biofuel cells: key dates
English biologist MC Potter discovers that baker’s yeast emits electricity.
Discovery of exo-electrogenic bacteria capable of breaking down a substrate to emit electrons outside its membrane.
Development of the first bacterial biofuel cells, using sewage sludge as fuel.
A biophotovoltaic biofuel cell powers a microprocessor for 6 months using solar radiation and water.
“BIOPHOTOVOLTAIC” ALGAE FOR OUTDOORS
At the University of Cambridge, Paolo Bombelli’s British team is betting on solar energy. The anode of his biofuel cell, presented in 2022, is covered by a cyanobacteria -photosynthetic bacteria-, of the type synecocystis, that captures sunlight and, submerged in water, delivers electrons outside its membrane. It is a biophotovoltaic device: “The electric current is based on the electrons released by the water during photosynthesis”, explains Paolo Bombelli.
The system worked for more than 6 months without problems, feeding an ARM Cortex-type microprocessor, one of the most used in the Internet of things. All in realistic conditions, without auxiliary lighting or external assistance! It remains to feed, in addition, a wireless communication system. A real challenge because “Photosynthetic bacteria like synecocystis are less electrogenic than the type geobacter, that supply conventional biofuel cells, acknowledges the scientist. That said, it just takes a bigger system to compensate.” So it’s time for a balance between miniaturization and power.
To increase the delivered current density, one approach is to make several bacterial populations work together by successive stage in a matrix that protects and isolates them. The hypothesis has also been demonstrated by Seokheun Choi, from Binghamton University, in another series of works: associating cyanobacteria synecocystis with other bacteria that emit riboflavins, molecules that optimize electron transfer, the current density increases.
“The idea is promising, but even more complex to implement than conventional biofuel cells,” analyzes Frédéric Barrière, from the University of Rennes.
BIOCELLS THAT ACTIVATE BODY FLUIDS
A pile of paper that lights up thanks to… saliva. This is the device that Seokheun Choi’s team at Binghamton University introduced in 2016 and continues to refine. Its nickel-carbon anode is covered in freeze-dried bacteria. It is separated from the cathode, made of carbon, by a wax-based proton exchange membrane. Everything is printed on an easily foldable paper backing, to create the necessary interfaces. The saliva is used to resuscitate the lyophilized bacteria and trigger their electrical production.
In fact, “The necessary nutrients can come from any human fluid, such as sweat or sewage”, specifies the researcher, who managed to power an LED for 20 minutes by stacking 16 microbatteries of this type. These modest returns are likely to increase in the future: “Currently, we are limiting at 100 µW/cm², but playing with bacterial selection and genetic engineering we can reach levels of the order of mW. Power that will make possible the transfer of wireless signals”, adds Seo-kheun Choi. The other challenge will be to better preserve the bacterial cells before use.
In a study published earlier this year in nanoenergy, the researcher proposes an alternative to freeze-drying: placed in a nutrient-deficient environment, the bacteria can form spores, germinate and generate electrical energy again once fed. To provide very soon, according to him, devices in isolated places and with limited resources.
But, once again, it is the industrial scaling that worries us: “The real difficulty lies in terms of engineering, says Alain Bergel, a researcher at the University of Toulouse. For to make large paper surfaces conductive, it is necessary to apply stainless steel grids, whose printing processes are costly. Very often, the game is not worth it. ” Until the miraculous application.
ENZYMATIC BATTERIES FOR THE HUMAN BODY
For its part, the French start-up BEFC is also designing a paper biobattery… but with a remarkable subtlety: as a catalyst, it does not use bacteria but directly enzymes. These proteins, found in our cells or in microorganisms, are responsible for breaking down organic molecules.
“An enzyme very selectively attacks a particular type of compound, such as glucose oxidase that breaks down glucose in the blood, or lactate oxidase that targets lactate in sweat.” explains Serge Cosnier, CNRS researcher and BEFC scientific adviser. Thanks to this high specificity, enzyme biofuel cells deliver 100 to 1000 times more energy per cm² of anode than a microbial biofuel cell, allowing for miniaturized devices…
But this efficiency has a drawback:
The enzymes only remain active for about 1 month before degrading, where bacteria, the living organisms, can reproduce and repopulate the anode. A limited useful life but that could be enough for specific applications. BEFC is therefore experimenting with early applications, particularly in pregnancy tests.
Ultimately, researchers dream of supplying more ambitious, skin-attached or even implantable sensors. “The levels of energy produced do not allow to power a pacemaker, but it could be enough for a variety of very localized measurements: temperature, glucose level, etc. ”, imagine Serge Cosnier. In this sense, enzymes also have a clear advantage over bacteria: they are biocompatible.
“These are the support materials that remain to be certified”, adds the researcher. Even so, for some teams, the short life of this enzymatic battery is not enough: therefore, the search for a biobattery that is both bacterial and intracorporeal is not abandoned. “We have identified bacteria that are both electroactive and compatible with the human body: they thrive in particular in the digestive system, at the level of the microbiota, says Frédéric Barrière, from the University of Rennes. Unfortunately, at the moment, none have been studied in sufficient depth. ” What if our microbiota one day started to produce electricity?
If interest in this energy has resurfaced in recent years, it is because we would be at the dawn of a new industrial revolution: that of the Internet of Things, those small connected everyday tools that require minimal power: cameras, doorbells, clocks, pillows. , shower heads… Studies estimate that there could be 50 billion of them by 2030, two or three times more than in 2021. And, throughout this beautiful world, the arrival of the “internet of disposable objects” is expected. These are single-use devices with modest power requirements: to track packages or containers, provide small medical diagnostics, carry out military or environmental surveillance. ..
However, powering these devices with traditional button cell batteries is unthinkable. Not only because of induced contamination, “but also because it would require multiplying by 3 the current extraction of lithium, already energized, in particular for the production of car batteries”, abounds Paolo Bombelli, a biochemist at the University of Cambridge. This is precisely where this new generation of batteries would come in. “A little The sensor that emits small volumes of data episodically requires a minimum power supply, of the order of a milliwatt, which can be achieved with a biobattery”, Says Seokheun Choi.
The future of the biofuel cell seems to be mapped out, there is only one crucial step left before its transition to industrial scale, scheduled for 2030: “They work well in the lab, but the real challenge will be getting them to work in real conditions, warns Alain Bergel, from the chemical engineering laboratory in Toulouse.
Weather and environmental hazards can destabilize microbial populations and their electricity production. ” A final barrier that will soon fall under the effect of an electrified quest.