The historical arc of the technology stretches back to 1911, when M.C. Potter first demonstrated that microbial metabolism could produce electricity using yeast and bacteria. B. Cohen advanced the concept in 1931 by building microbial half fuel cell stacks that generated measurable voltages. But the decisive turning point came late in the twentieth century with the discovery of electroactive bacteria such as Shewanella oneidensis and Geobacter sulfurreducens. These organisms shuttle electrons outside their cells without the need for synthetic mediators, a capability that transformed fuel cell design. Geobacter species, in particular, form conductive biofilms threaded with cytochrome-based nanowires, dramatically improving electron transfer and, with it, power output. Those early findings laid the groundwork for today’s materials research, including polypyrrole-coated anodes and nitrogen-doped carbon nanotubes that boost bacterial adhesion and electron transfer rates.

At its heart, a microbial fuel cell is a two-part electrochemical machine. In the anodic chamber, electroactive microorganisms oxidize organic or inorganic substrates, releasing protons and electrons as by-products of their metabolism. The electrons travel through an external circuit from anode to cathode, generating usable current, while protons migrate across a proton exchange membrane to the cathodic chamber. There, they combine with a terminal electron acceptor, most often oxygen, completing the circuit. Oxygen is the ideal acceptor because it leaves no toxic residue, and platinum is commonly used to catalyze the cathode reaction. But platinum is expensive, which undermines the economics of the whole system. Researchers are therefore testing cheaper alternatives, including manganese dioxide and iron- and cobalt-based catalysts, as well as biocathodes in which microorganisms themselves catalyze the electron transfer, reducing reliance on noble metals.

The microbiology inside the anode is where the real chemistry happens. Substrates such as glucose and acetate are broken down through glycolysis, the tricarboxylic acid cycle and respiratory electron transport chains, producing the reduced cofactors NADH and FADH2 that ultimately donate electrons to the anode. The efficiency of this process is measured as Coulombic efficiency, the fraction of electrons from the oxidized substrate that are captured as electrical current, and losses occur whenever microbes divert electrons toward biomass growth, methanogenesis or competing respiratory pathways. Extracellular electron transfer, the defining trait of electrogenic microbes, occurs through direct contact, through soluble mediators, or through conductive nanowires. Acetate, with its simple molecular structure, remains the benchmark substrate, delivering some of the highest electron recovery rates observed.

Design choices matter enormously. Dual-chambered fuel cells separate oxidation and reduction into distinct compartments divided by a proton exchange membrane, maximizing stability and electron transfer efficiency, but the membrane adds cost and operational complexity. Single-chambered designs merge the anode and cathode into one vessel, cutting internal resistance and simplifying construction, often using an air cathode that draws oxygen passively from the atmosphere. The trade-off is that oxygen can leak into the anodic chamber, disrupting the anaerobic metabolism the electroactive bacteria depend on. Temperature is another critical variable: performance peaks between 20 and 40 degrees Celsius and falls off outside that window as microbial activity slows or proteins denature, a serious concern for outdoor deployments subject to seasonal swings.

What sets microbial fuel cells apart from other renewable technologies is the extraordinary range of waste streams they can consume. Carbohydrates from wastewater and biomass are the most widely used substrates, prized for their biodegradability, though complex sugars require enzymatic hydrolysis and can cause biofouling. Organic acids, particularly acetate, feed directly into microbial electron transport and avoid the fouling problems of sugars. Alcohols such as ethanol from bioethanol production waste can be oxidized to acetate by acetogenic bacteria, while inorganic compounds like ammonia, hydrogen and sulfate extend the technology’s reach into nitrogen removal and sulfur cycling. Power outputs vary widely depending on the feedstock, ranging from roughly 1 to 3600 milliwatts per square meter, with most systems falling between 10 and 1000.

The review highlights striking demonstrations across agro-industrial waste. Cassava starch wastewater, laden with organic matter and cyanide precursors, was treated in a fuel cell system that removed 88 percent of the chemical oxygen demand within 120 hours while producing a maximum power density of 1771 milliwatts per square meter. Adjusting anode pH from 5.0 to 9.0 in a single-chambered system boosted power density to 22.19 watts per cubic meter, underscoring pH as a master variable. Cellulose-degrading consortia, chicken feathers processed by Pseudomonas aeruginosa at 1206.78 milliwatts per square meter, rice straw hydrolysate, brewery wastewater with over 91 percent COD removal, and slaughterhouse effluent at 578 milliwatts per square meter all confirm the breadth of usable feedstocks. Fish market wastewater achieved 85 percent COD removal at 350 milliwatts per square meter, and fish scales have even been repurposed as biocompatible anode material.

Household and xenobiotic wastes round out the portfolio. Food waste leachate in a dual-chamber system generated 482 milliwatts per square meter with 85 percent COD removal, while blackwater treatment achieved roughly 80 percent COD removal with stable voltage production. Azo dyes, the synthetic colorants behind much textile pollution, can serve as electron acceptors at the cathode, where the azo bond is reductively cleaved into aromatic amines while the anode simultaneously generates current. Studies with Klebsiella pneumoniae, Geobacter and Shewanella showed that replacing ferricyanide with azo dyes as the cathodic acceptor increased power output by 40 percent. Selenium-contaminated wastewater can be reduced to less toxic elemental selenium, and nitrate removal efficiencies of up to 95 percent have been reported, though high selenium concentrations above 50 milligrams per liter begin to inhibit microbial activity.

The economics remain the technology’s Achilles heel. Platinum catalysts can cost as much as 3000 dollars per kilogram, and Nafion proton exchange membranes run between 500 and 1000 dollars per square meter. Activated carbon electrodes and ceramic membranes offer cheaper alternatives without sacrificing performance. Operational costs are low because the fuel is waste that often carries a disposal charge, and the systems need little external energy, avoiding the aeration expenses of activated sludge treatment. Yet power densities generally remain below 2 watts per square meter, well short of solar panels exceeding 20 percent efficiency, and scaling from laboratory reactors exceeding 1000 watts per cubic meter at 20-milliliter volumes to a 500-milliliter reactor producing just 20 watts per cubic meter illustrates the persistent scale-up penalty.

Real-world evidence is nonetheless accumulating. A liter-scale, single-chamber air-cathode system operating on primary effluent at a municipal wastewater treatment plant ran continuously for more than 400 days, achieving regular COD removal of up to 70 percent and power densities of 150 to 200 milliwatts per square meter of cathode area, all without external aeration or chemical supplements. The exoelectrogenic communities, dominated by Geobacter and Shewanella, adapted to shifting influent conditions over the seasons. The authors of the review argue that the path forward lies in high-performance electrode materials such as metal-organic frameworks and carbon nanomaterials, engineered microbial strains with enhanced electron transfer, modular and stackable reactor designs, and hybrid systems coupling fuel cells with anaerobic digestion or constructed wetlands. For off-grid communities and developing regions, where low-cost electricity and sanitation are equally scarce, microbial fuel cells may find their first foothold, converting the world’s waste into a modest but meaningful current.

Subject of Research: Microbial fuel cell technology for bioelectricity generation and waste utilization

Article Title: Bio-electrochemical energy recovery: the role of microbial fuel cells in waste utilization

Article References: Kuity, N. T., & Sambrani, S. A. (2025). Bio-electrochemical energy recovery: the role of microbial fuel cells in waste utilization. Discover Biotechnology, 2(1), Article 17. https://doi.org/10.1007/s44340-025-00025-y

Image Credits: AI Generated

DOI: 10.1007/s44340-025-00025-y

Keywords: microbial fuel cells, bioelectricity, wastewater treatment, electroactive bacteria, Geobacter sulfurreducens, Shewanella oneidensis, bioremediation, extracellular electron transfer, azo dyes, agro-waste, proton exchange membrane, sustainable energy

The stakes could hardly be higher. Primary platinum group metal production is geographically precarious, with South Africa’s Bushveld Igneous Complex supplying roughly 88.7 percent of the world’s platinum group metals and Russia’s palladium-rich Norilsk-Talnakh region alone accounting for about 43.8 percent of global palladium output. Life cycle assessments place the carbon footprint of producing a single kilogram of refined metal at between 28 and 42 tonnes of carbon dioxide equivalent, while the classic aqua regia leaching route releases toxic nitrogen oxide and chlorine-bearing gases. As millions of converters are scrapped each year, spent automotive catalyst has become the obvious secondary resource, but the industry has lacked a clean, low-energy route that works on the real, unprocessed solid material rather than pre-dissolved metal salts.

The Nottingham team, led by Christine Paul with Frankie Rawson, Katalin Kovács and Helena I. Gomes, turned to Cupriavidus metallidurans CH34, a bacterium with almost legendary resistance to toxic metals. Its defensive arsenal sits on two megaplasmids carrying metal-specific gene clusters including cop, czc and cnr, allowing it to shrug off millimolar concentrations of nickel, zinc, copper and cadmium. Crucially, the organism is also electrogenic: it forms biofilms on graphite electrodes and generates current densities of 15 to 150 milliamperes per square metre. Previous bioelectrochemical studies of precious metals had only ever worked with dissolved metal ions in model solutions, or with real catalyst material that had first been chemically leached. This new work, published in Cleaner Engineering and Technology, is the first to put an electroactive culture in direct contact with untreated spent catalyst solids.

The experimental design was elegantly simple. Dual-chamber H-type microbial fuel cells were built from borosilicate glass, with carbon felt electrodes separated by a perfluorinated sulfonic acid membrane and a ferricyanide catholyte chosen specifically so that the cathode would never limit the measured response. The anode chambers received untreated spent catalyst powder at loadings of 10,000, 50,000 and 100,000 parts per million, corresponding to pulp densities of 1, 5 and 10 percent, alongside catalyst-free controls. Eight reactor types, each run in biological triplicate, allowed the team to separate purely chemical effects from genuinely biological ones. After 168 hours of operation, inductively coupled plasma mass spectrometry tracked exactly where the platinum, palladium and rhodium had ended up across the liquid, biomass, electrode and residual solid fractions.

The first surprise came from the biomass data. At the lowest loading of 10,000 parts per million, the bacteria actually grew better than in the catalyst-free control, reaching 221.4 micrograms per millilitre of planktonic protein at 96 hours compared with 201.3 in the control, suggesting the moderate metal challenge stimulated rather than suppressed the culture. But at 50,000 and 100,000 parts per million, growth collapsed to just 60 to 75 percent below the control level. Scanning electron microscopy told part of the story: at the higher loadings, aggregated catalyst particles blanketed the carbon felt electrodes, masking the underlying fibre structure and leaving little visible biological material. The electrode-attached population fared better than the free-floating cells, but the overall picture was one of progressive physical and chemical suffocation.

That suffocation had two distinct sources. The first was acidity. Even in completely abiotic reactors, adding catalyst drove the anolyte pH down in a dose-dependent fashion, from 5.95 in the catalyst-free control to 4.87, 4.06 and 3.93 at the three loadings respectively. This chemical acid load occurred entirely without microbial help and, crucially, preceded the decline in bacterial growth, establishing that acidification was a cause rather than a consequence of the biological collapse. The second source was electrical. Open-circuit voltage peaked at an impressive 0.442 volts at 10,000 parts per million, actually exceeding the catalyst-free control, but fell to around 0.12 to 0.15 volts at higher loadings. Internal resistance told the same story, with activation resistance jumping from 142 ohms in the clean biotic reactor to 3688 ohms at low loading and over 7000 ohms at the highest concentrations, while power density plummeted by more than two orders of magnitude across the range.

Cyclic voltammetry added a further layer of insight. The anodic peak current at 10,000 parts per million reached 688.89 microamperes, more than double the 327.65 microamperes of the catalyst-free control, and the Randles-Ševčík slope, a measure of diffusion-coupled electron transfer, was likewise highest at moderate loading. But the weaker linearity of that fit hinted at extra processes beyond simple diffusion control, and at 50,000 and 100,000 parts per million the peak currents fell to 278.25 and 96.24 microamperes respectively, with increasingly non-Nernstian behaviour indicating quasi-reversible to irreversible electron transfer governed by kinetic limitations. In plain terms, the electrochemical conversation between bacteria and electrode was sharpest at moderate catalyst loading and progressively garbled as particles piled up on the surface.

The metal redistribution results were the most striking of all. Rhodium proved to be the star performer: at 10,000 parts per million, the biotic reactors mobilised 17.93 percent of the rhodium input, compared with just 0.87 percent of platinum and 1.20 percent of palladium. Moreover, 10.3 percent of the rhodium ended up associated with the bacterial biomass, a fraction that was below detection in the abiotic controls, pointing to a genuinely selective biological interaction. The explanation likely lies in catalyst chemistry: road-aged three-way catalysts contain rhodium predominantly in oxidised form, whereas platinum sits mostly as metal. Dissolving metallic platinum group metals requires both a high oxidation potential and a complexing ligand such as chloride, neither of which was present in the anolyte, but rhodium that is already oxidised can bypass that demanding first step. The biological contribution, however, was confined to biosorption and accumulation on the biomass rather than true dissolution of the solid matrix, and the authors are careful to note that no oxidation-state analysis confirmed reduction.

Perhaps the most counterintuitive finding was that mobilisation per gram of catalyst was highest at the lowest loading, despite the more favourable pH there. Platinum recovery per unit mass fell from 2.01 hundredths of a milligram per gram at 10,000 parts per million to 0.332 hundredths at 100,000 parts per million, with palladium and rhodium showing the same pattern. This points to accessible surface area, not acidity, as the true bottleneck: at high solids loadings, particles aggregate, the reactive surface shrinks, and the sheer mass of solid available for re-adsorption pulls dissolved species back out of solution. The same pulp-density penalty has been observed in conventional hydrometallurgical processing, where palladium and platinum recovery drops from above 90 percent at 5 to 8 percent solids to below 70 percent at 20 percent.

The honest conclusion is that this system, as it stands, cannot yet compete with industrial recovery: even at the optimum loading, less than 18 percent of rhodium and under 2 percent of platinum and palladium were mobilised. But the study does something arguably more valuable than demonstrating a working process. It maps the operational window, showing that the transition between sustained and suppressed performance lies somewhere between 10,000 and 50,000 parts per million, and it identifies the two mechanisms, chemical acidification and particulate fouling of the electrode, that constrain it. The authors sketch a roadmap of possible fixes: pH control to decouple acidification from surface effects, pre-washing the catalyst to reduce the acid load at source, fed-batch dosing to keep instantaneous loading within the functional window, or a two-stage configuration that separates mild chemical solubilisation from the bioelectrochemical step that the biology performs best, namely selectively capturing mobilised rhodium. Achieving process-relevant yields, they conclude, will require intervention at the solid-liquid interface rather than tinkering with the microbe itself. For a field desperate to break its dependence on South African ore bodies and 1200-degree furnaces, that is a map worth having.

Subject of Research: Bioelectrochemical recovery of platinum group metals from untreated spent automotive catalyst using Cupriavidus metallidurans

Article Title: Bioelectrochemical treatment of untreated spent car catalyst: Effects of solids loading on platinum group metal redistribution and process limitations

Article References: Paul, C., Rawson, F., Kovács, K., & Gomes, H. I. (2026). Bioelectrochemical treatment of untreated spent car catalyst: Effects of solids loading on platinum group metal redistribution and process limitations. Cleaner Engineering and Technology, 34, Article 101322. https://doi.org/10.1016/j.clet.2026.101322

Image Credits: AI Generated

DOI: 10.1016/j.clet.2026.101322

Keywords: platinum group metals, spent car catalyst, microbial fuel cell, Cupriavidus metallidurans, rhodium recovery, bioelectrochemical systems, urban mining, circular economy, biomining, hydrometallurgy, electroactive bacteria, metal recycling

The answer that MES offers is deceptively simple in concept. A typical system consists of two chambers separated by an ion-exchange membrane. In the anodic compartment, water or organic substrates are oxidized to release electrons. In the cathodic compartment, electroactive microorganisms intercept those electrons either directly from the cathode surface, where they form biofilms, or indirectly via hydrogen gas and redox mediators generated at the electrode. Once inside the cell, the electrons enter microbial metabolism and serve as reducing power for fixing carbon dioxide. Because the microbial catalysts are alive, they replicate themselves, operate under mild near-ambient conditions, and tolerate feedstock variability in a way that expensive metal catalysts cannot. And because microbial metabolism is enormously diverse, MES can in principle reach C3 and longer-chain molecules that remain stubbornly out of reach for conventional electrochemistry.

The contrast with existing CO2 conversion technologies is stark. Thermocatalytic routes such as the Sabatier reaction and reverse water-gas shift chemistry require high temperatures, high-purity hydrogen as a reductant, and durable metal catalysts that suffer from carbon deposition and poisoning. Electrochemical CO2 reduction on copper and other catalysts can make carbon monoxide, formate, and C2 products under ambient conditions, but selectivity for C3 and more complex molecules remains poor, catalysts deactivate, and long-term stability is inadequate. Meanwhile, green hydrogen produced by water electrolysis currently costs between 4.5 and 6.0 US dollars per kilogram, meaning that simply reducing CO2 with hydrogen often yields chemicals worth less than the hydrogen consumed. Microbial catalysts sidestep many of these constraints, using self-assembled enzymatic pathways such as the Wood-Ljungdahl route to weave carbon dioxide into multi-carbon products with remarkable specificity.

The performance numbers reported in recent studies are striking. Acetate, the workhorse product of MES, is routinely produced with coulombic efficiencies exceeding 90 percent, meaning that more than nine out of every ten electrons supplied by the circuit end up stored in the target molecule. Most impressively, a continuous thermophilic hydrogen-mediated system using the acetogenic bacterium Thermoanaerobacter kivui has achieved acetate concentrations of up to 29.4 grams per liter, roughly 490 millimolar, from carbon dioxide. That is an order of magnitude beyond typical laboratory titers and begins to approach concentrations relevant to industrial separation. For context, commercial acetic acid is produced today by methanol carbonylation in plants rated at 200,000 to 650,000 tonnes per year, so MES still has far to travel in scale, current density, and process intensification, but the trajectory of improvement is unmistakable.

Beyond acetate, the product spectrum widens considerably. When acetate and ethanol accumulate in the reactor, chain-elongating microbes take over, running reverse beta-oxidation pathways that stitch short-chain intermediates into C4 through C8 medium-chain fatty acids such as butyrate and caproate, which command far higher market prices as feed and chemical precursors. Solventogenic metabolism can be triggered by tuning reactor operating conditions, reducing accumulated organic acids to ethanol, butanol, and 2,3-butanediol. Reductive branches of the tricarboxylic acid cycle yield lactate and succinate. Photo-bioelectrochemical systems using the purple bacterium Rhodobacter sphaeroides go further still, simultaneously converting CO2 into biomass and hydrogen gas, while MES-integrated setups direct CO2-derived carbon into intracellular polyhydroxybutyrate, a biodegradable plastic. In each case, the electron source is the electrode rather than sugar, decoupling production from agricultural feedstocks.

One of the most commercially mature applications is biogas upgrading. Anaerobic digestion plants in Germany, Denmark, and the Netherlands already supply 10 to 20 percent of renewable power in parts of the European Union, but raw biogas contains only 50 to 70 percent methane, with the remainder mostly CO2 plus troublesome impurities such as siloxanes and sulfur compounds. Conventional pressure swing adsorption can polish biogas to roughly 97 percent methane for pipeline injection, but contaminants foul the adsorbents and raise costs. MES offers an elegant pre-treatment: raw biogas is sparged into the cathode chamber, where methanogenic archaea electrochemically reduce the CO2 fraction to additional methane. Recent work with biogas from an operating anaerobic digestion plant achieved 95 percent methane in the upgraded gas at a methane production rate of 8.8 liters of CH4 per square meter of catalyst per day. Because gaseous impurities dissolve into the liquid phase during this step, the downstream PSA unit faces a lighter, cleaner load, and the captured CO2 is not merely discarded but converted into fuel.

The review also highlights an emerging application with obvious public appeal: power-to-protein. In these schemes, renewable electricity splits water to generate hydrogen, formate, or methanol, which feed bioreactors cultivating protein-rich microorganisms for food and feed. Single-cell protein is not new; companies such as Unibio and Calysta have commercialized fermentation-based production, but their processes traditionally rely on sugar substrates that compete with food supply. A techno-economic assessment of solar-driven microbial protein production found that photovoltaic-powered systems could achieve protein yields per unit of land up to an order of magnitude higher than conventional agriculture, and that estimate assumed conservative solar-to-electricity and power-to-chemical conversion efficiencies of 5 percent or less. Because electricity delivers reducing power to microbes far more efficiently than photosynthesis delivers it to crops, the land-use arithmetic of protein production could be transformed, freeing farmland while feeding a growing population.

None of this means MES is ready for prime time, and the authors are candid about the obstacles. Most laboratory studies still rely on small H-type reactors whose distant electrodes and ion-exchange membranes impose severe ohmic resistance and overpotentials, often pushing cell voltages above 3 volts and crushing energy efficiency. The oxygen evolution reaction at the anode is kinetically sluggish, particularly on carbon-based electrodes, and acts as a bottleneck for the entire system. Mixed microbial consortia, while robust, tend to foul membranes and default to acetate rather than more valuable products, whereas pure cultures of Shewanella, Sporomusa, Geobacter, or Rhodobacter offer precision but demand sterility. The field is responding with nanostructured and conductive-polymer-coated cathodes such as polyaniline-deposited graphite felt, synthetic-biology strains with engineered electron-transfer and metabolic pathways, artificial redox mediators like neutral red, and scalable reactor geometries including bubble columns and 3D-printed electrodes designed to improve hydrogen delivery.

The most pragmatic near-term strategy may be integration rather than replacement. Because MES alone is unlikely to reach commercial viability at current productivities, the authors describe two-stage processes in which MES first converts CO2 to acetate, which is then recovered and fed to a second, optimized bioprocess that converts it into long-chain alkyl esters or high-value isoprenoids. Such hybrid configurations sidestep the selectivity limits of mixed-culture MES while still exploiting its unique ability to fix carbon with electricity. Coupled with direct air capture and low-carbon power, an integrated MES platform could even operate as a carbon-negative factory, drawing down atmospheric CO2 while selling chemicals, fuel, and protein. The remaining challenges, from current density to product recovery costs, are formidable but increasingly quantified, and for the first time the road from laboratory biofilm to industrial biorefinery looks less like a leap of faith and more like an engineering schedule.

Subject of Research: Microbial electrosynthesis using renewable electricity to convert CO2 into value-added chemicals, biogas, and protein

Article Title: Renewable electricity–driven microbial electrosynthesis for high-value CO2 valorization: recent trends

Article References: Lee, C. H., Kim, M., Kong, D. S., Son, H., & Kim, J. R. (2026). Renewable electricity–driven microbial electrosynthesis for high-value CO2 valorization: recent trends. Advances in Industrial and Engineering Chemistry, 2(1), Article 4. https://doi.org/10.1007/s44405-026-00044-1

Image Credits: AI Generated

DOI: 10.1007/s44405-026-00044-1

Keywords: microbial electrosynthesis, CO2 valorization, renewable electricity, acetate production, biogas upgrading, single-cell protein, polyhydroxybutyrate, bioelectrochemical systems, Wood-Ljungdahl pathway, power-to-protein, carbon capture and utilization, electroactive bacteria