Microbial Electrosynthesis: The Future of Next-Generation Biofuel Production—A Review

Abstract

Microbial electrosynthesis (MES) has emerged as a promising bio-electrochemical technology for sustainable CO₂ conversion into valuable organic compounds since it uses living electroactive microbes to directly convert CO₂ into value-added products. This review synthesizes advancements in MES from 2010 to 2025, focusing on the electrode materials, microbial communities, reactor engineering, performance trends, techno-economic evaluations, and future challenges, especially on the results reported between 2020 and 2025, thus highlighting that MES technology is now a technology to be reckoned with in the spectrum of biofuel technology production. While the current productivity and scalability of microbial electrochemical systems (MESs) remain limited compared to conventional CO₂ conversion technologies, MES offers distinct advantages, including process simplicity, as it operates under ambient conditions without the need for high pressures or temperatures; modularity, allowing reactors to be stacked or scaled incrementally to match varying throughput requirements; and seamless integration with circular economy strategies, enabling the direct valorization of waste streams, wastewater, or renewable electricity into valuable multi-carbon products. These features position MES as a promising platform for sustainable and adaptable CO₂ utilization, particularly in decentralized or resource-constrained settings. Recent innovations in electrode materials, such as conductive polymers and metal–organic frameworks, have enhanced electron transfer efficiency and microbial attachment, leading to improved MES performance. The development of diverse microbial consortia has expanded the range of products achievable through MES, with studies highlighting the importance of microbial interactions and metabolic pathways in product formation. Advancements in reactor design, including continuous-flow systems and membrane-less configurations, have addressed scalability issues, enhancing mass transfer and system stability. Performance metrics, such as the current densities and product yields, have improved due to exceptionally high product selectivity and surface-area-normalized production compared to abiotic systems, demonstrating the potential of MES for industrial applications. Techno-economic analyses indicate that while MES offers promising economic prospects, challenges related to cost-effective electrode materials and system integration remain. Future research should focus on optimizing microbial communities, developing advanced electrode materials, and designing scalable reactors to overcome the existing limitations. Addressing these challenges will be crucial for the commercialization of MES as a viable technology for sustainable chemical production. Microbial electrosynthesis (MES) offers a novel route to biofuels by directly converting CO₂ and renewable electricity into energy carriers, bypassing the costly biomass feedstocks required in conventional pathways. With advances in electrode materials, reactor engineering, and microbial performance, MES could achieve cost-competitive, carbon-neutral fuels, positioning it as a critical complement to future biofuel technologies.

1. Introduction

Microbial electrosynthesis (MES) is a bio-electrochemical approach that uses microorganisms as biocatalysts to convert CO₂ and electrons (from a cathode) into multi-carbon organic compounds such as acetate, ethanol, butyrate and medium-chain fatty acids. The process was introduced and demonstrated by Nevin et al. in 2010, who showed Sporomusa ovata converting CO₂ into acetate on a cathode with high coulombic efficiency, thereby establishing the core MES paradigm: a polarized cathode supplies electrons, electro-trophic microbes fix CO₂ (commonly via the Wood–Ljungdahl pathway), and products are excreted to the bulk medium [1].

In the broader landscape of renewable energy technologies, microbial electrosynthesis (MES) represents a fundamentally different paradigm from conventional biofuel production. Traditional first-generation biofuels, such as bioethanol and biodiesel, rely on sugar- or lipid-rich feedstocks (e.g., corn, sugarcane, soy, palm), raising concerns about land-use competition, food security, and biodiversity impacts. Second-generation biofuels, derived from lignocellulosic residues, alleviate some of these issues but require energy-intensive pre-treatments and enzymatic hydrolysis, which add to both the cost and process complexity. More advanced platforms, such as algal biofuels, promise higher yields but are still hampered by the cultivation costs, harvesting challenges, and scale-up uncertainties. In contrast, MES directly converts CO₂—a non-food, abundant carbon source—into energy carriers and bio-based chemicals, coupling electrochemical energy with microbial metabolism under mild conditions. This provides MES with unique positioning as a “power-to-fuels” and carbon recycling technology, rather than a biomass-dependent route.

Between 2010 and about 2019, MES developed gradually—moving from a proof of concept toward improved electrode designs and mixed-culture demonstrations (notable step increases in productivity were already reported around 2015)—but the field’s expansion accelerated markedly after 2020 as innovations in electrode materials, sequencing-enabled microbiome analyses, reactor engineering, and early techno-economic modeling converged. The period 2020–2025 therefore represents an intensely productive window for MES research and is the focus of the sections that follow [2,3].

At its core, MES relies on three interlinked elements.

• Electrode–microbe electron transfer. Electrons originate at a cathode (driven by an external power source, preferably renewable). Electron transfer mechanisms include (a) direct extracellular electron transfer (EET) via conductive biofilms or redox proteins, and (b) indirect hydrogen-mediated transfer, wherein electrochemically produced H₂ is consumed by hydrogenotrophic acetogens. Distinguishing between these pathways remains a major experimental and conceptual theme across the 2010–2025 studies [1,4].
• Electro-trophic metabolism. Many acetogens (e.g., Sporomusa, Acetobacterium, Clostridium) deploy the Wood–Ljungdahl pathway to fix CO₂ to acetyl-CoA and then excrete acetate or reduce further to ethanol/butanol under specified conditions. Advances in metabolic understanding and synthetic biology approaches to expand the product scope intensified after 2020 [5,6].
• System design. The reactor architecture, cathode material and geometry, operational potential, pH, mass transfer and anode chemistry jointly determine the rates, titers and selectivity. From the initial graphite electrodes (2010) to engineered 3D-printed cathodes, materials and reactor choices have been central to MES progress [1,7].

Figure 1 shows a schematic image of the MES fundamentals, which include the cathode, microbes (biofilm), CO₂ feed, products, anode, and possible electron pathways.

Microbial electrosynthesis (MES) is a bio-electrochemical process that uses a cathode as an electron donor and an anode as the counter electrode to drive CO₂ reduction into value-added products. At the cathode, electrons are supplied either directly to electroactive microbes within a biofilm or indirectly via in situ hydrogen evolution, while the anode typically supports water oxidation to release protons and maintain the charge balance. A dense, conductive biofilm of acetogenic or methanogenic microorganisms plays a central role, mediating electron uptake and channeling the reducing power into metabolic pathways such as the Wood–Ljungdahl pathway. Fed with CO₂ as the sole carbon source, MES can generate acetate, ethanol, methane, and medium-chain fatty acids, with electron transfer occurring either through direct microbe–electrode interactions, hydrogen-mediated transfer, or formate intermediates. This integration of electrochemistry and microbial metabolism enables sustainable conversion of renewable electricity and CO₂ into fuels and chemicals.

There are several complementary drivers that produced an acceleration of MES research, production and performance gains post-2020. The most important is materials and manufacturing advancements for the electrodes (graphene, 3D printing, composites). Carbon nanomaterials, conductive polymers and additive manufacturing enabled cathodes with a higher surface area, better mass transfer and improved microbial colonization, but electrode design has advanced from plain graphite and carbon felts to sophisticated, engineered cathodes. Thus, the early era (2010–2016) was mostly graphite plates, carbon felt, and reticulated vitreous carbon [1,2]. Then, transitional improvements in 2017–2019 by incorporating carbon nanotubes and hierarchical porous coatings improved the areal productivity and biofilm development (benchmarks established prior to the 2020 surge) [8]. Finally, rapid innovation was reached in 2020–2025 by achieving complex architecture like graphene/rGO and carbon-based 3D architectures. Multiple reports [7,9], but especially [7], detail how graphene derivatives and 3D-printed carbon aerogels increase interface conductivity and biofilm compatibility (improving both the startup and steady-state rates). Research undertaken mainly in 2023–2024 consolidated these developments and highlighted reproducible productivity increases tied to advanced cathode materials [7,10], especially due to the fact that metal oxide coatings emerged when catalyst layering was introduced (including Ni, MoS₂, CoP, and oxide coatings) to tailor hydrogen evolution and lower overpotentials, balancing microbe-friendly H₂ production with direct electron transfer. Recent catalyst-assisted studies [11,12] evaluated Ni- and transition-metal-based modifications for higher acetate yields.

Nowadays, there are biocompatible bimetallic oxide coatings such as MnFe₂O₄ coatings that substantially increase acetate areal productivity and support Acetobacterium enrichment—one of the most significant material-driven performance jumps in the 2020s—and additive manufacturing/3D-printed electrodes, which enabled bespoke lattice structures that optimize mass transfer, conductivity and biofilm penetration, showing dramatic performance improvements in scaled lab reactors [12].

Microbiome and sequencing: Paper [13] is a meta-analysis that synthesized 22 MES studies and revealed a conserved cathodic core microbiome (e.g., Desulfovibrio, Acetobacterium, Methanobacterium), showing that operational levers (inoculum pre-treatment, potential, pH) systematically shape community outcomes and product selectivity. These insights shifted attention to strategies for community steering and bioaugmentation [13].

Systems and TEA: As electrode and microbiome science matured, investigators began publishing more techno-economic and modelling work—mapping the cost drivers, scale-up bottlenecks and niche markets (e.g., specialty chemicals, point-source CO₂ valorization), making MES of greater interest to engineering and policy communities [10,11].

As papers [8,14] show, organizations like American Society for Testing and Materials (ASTM)and International Organization for Standardization (ISO) are actively developing standards for additive manufacturing, which can be applied to MES components. Moreover, even though the papers do not provide specific CFD simulation parameters for MES, general parameters commonly considered in CFD simulations include the Reynolds number, flow rate, pressure drop, temperature and concentration gradients, mesh type and resolution.

Figure 2 shows the growth in published papers on MES while highlighting the key milestone years.

Figure 3 shows a comparative chart concerning different cathode types, from graphite to an Fe-Mn-coated one.

As can be assessed from Figure 3, carbonaceous electrodes are stable but limited in conductivity and catalytic activity. Graphene-based electrodes enhance electron transfer and biofilm growth, though scale-up remains challenging. Composite electrodes improve hydrogen management and productivity but require complex fabrication, while bio-metallic oxides lower overpotentials and enhance stability yet face long-term durability issues [15,16]. The comparison underscores the need for hybrid designs to balance cost, performance, and scalability.

Moreover, intensive research has been conducted toward understanding and engineering the cathodic microbiome, which is central to selectivity and stability. Thus, Ref. [17] synthesized several studies and found an ~80% overlap in the cathodic core community across acetogenic and methanogenic systems—an observation that provided a predictable foundation for community manipulation (bioaugmentation, pretreatment, potential control) [9]. Since 2020, multiple groups have reported that seeding with acetogens (e.g., Acetobacterium, Clostridium spp.) or using selective pre-treatments (heat, chemical inhibitors) can bias communities toward acetogenesis and reduce methane formation. Operational variables (pH, potential, H₂ partial pressure) remain powerful levers for maintaining acetogen dominance. Refs. [3,4] and recent perspectives advocate and have begun to demonstrate the use of engineered pure cultures and edited pathways to expand products beyond acetate (toward butyrate, caproate, alcohols), while improving EET capabilities and tolerance to electrochemical conditions [5,6].

Another important aspect related to MES is reactor engineering and system configurations because it sums system-level engineering mass transfer, electrode spacing, gas handling, and product separation. There are several critical aspects that must be taken into consideration when designing an MES reactor: flow electrodes and stacked reactors demonstrate higher titers (e.g., reports of several g L⁻¹ acetate in flow systems), suggesting scale-friendly concepts that improve gas and liquid mass transfer [18]. Alternative cell designs that improve CO₂ availability to biofilms (gas diffusion cathodes) or use salinity gradients to drive the current have been explored, showing system gains in productivity and coulombic efficiency. Ref. [8] details mathematical models and control strategies developed between 2021 and 2025 that now allow optimization of potential, flow, and H₂ evolution rates to predict productivity and scale behavior, illuminating pathways for pilot reactors [19].

Recent techno-economic assessments suggest that the economic bottleneck of conventional biofuels lies in feedstock logistics and pretreatment, which can account for 40–60% of production costs, as study [20] suggests. MES bypasses these constraints by eliminating the dependence on complex biomass hydrolysis and instead utilizes renewable electricity and CO₂ as its primary inputs. Although MES is currently at low technology readiness levels (TRL 2–3) and suffers from modest productivities compared to fermentation benchmarks, its process simplicity, modularity, and compatibility with intermittent renewable power suggest that the costs could fall significantly as electrode materials, reactor engineering, and microbial chassis improve. Comparative techno-economic models indicate that if MES systems achieve >100 mA·cm⁻² current density, >70% coulombic efficiency, and continuous operation for >3000 h, the levelized cost of acetate and higher-value biofuels could become competitive with lignocellulosic ethanol and algal biodiesel pathways under scenarios of declining renewable electricity costs.

Moreover, compared with other CO₂ usage technologies such as abiotic electrochemical reduction and photobiological systems, MES leverages electroactive microorganisms to catalyze the reduction of CO₂ at the cathode, converting it into chemicals such as acetate, methane, or biofuel. The CO₂ conversion efficiency in MES is strongly influenced by the microbial metabolism, electrode materials, and reactor design. While MES generally operates under ambient conditions and requires lower overpotentials compared to abiotic systems, the CO₂ fixation rates are limited by microbial growth kinetics and mass transport limitations. MESs is particularly advantageous for integrating renewable electricity with biochemical production, offering high selectivity for multi-carbon compounds.

On the other hand, abiotic electrochemical systems employ inorganic catalysts, such as copper, silver, or tin, to reduce CO₂ into products like carbon monoxide, format, or hydrocarbons. AER systems can achieve higher reaction rates than MES due to faster electron transfer and catalyst-driven kinetics. However, the CO₂ utilization efficiency is often challenged by competing side reactions, such as hydrogen evolution, and the need for elevated overpotentials, which can increase energy consumption. On the positive side, AER allows for precise control over reaction conditions and product selectivity, enabling rapid CO₂ conversion under optimized electrochemical parameters.

Comparatively, photobiological systems rely on photosynthetic microorganisms, such as cyanobacteria or microalgae, to capture CO₂ using light energy, converting it into biomass, lipids, or other bio-products. PBSs directly integrate solar energy, which reduces the need for external electrical inputs. However, the CO₂ fixation rates in PBSs are constrained by light penetration, photoinhibition, and nutrient availability. While PBSs provide a sustainable and low-energy pathway for CO₂ assimilation, the product specificity is lower compared to MES or AER, as photosynthetic metabolism often produces a complex mixture of biomolecules.

Table 1. Table shows the differences between the 3 technologies.

System	CO2 Conversion Mechanism	Reaction Rate	Product Specificity	Energy Requirement	Advantages	Limitations
MES	Microbial catalysis at electrodes	Moderate	High for targeted compounds (e.g., acetate, methane)	Low to moderate (ambient conditions, requires applied voltage)	High selectivity, integrates with waste streams	Limited by microbial kinetics and mass transfer
AER	Inorganic electrocatalysis	High	Variable, depends on catalyst (CO, formate, hydrocarbons)	High (overpotentials, electricity input)	Fast conversion, tunable product selectivity	Competing side reactions, high energy input
PBS	Photosynthetic CO2 fixation	Low to moderate	Low to moderate (biomass, lipids)	Low (light-driven)	Uses solar energy directly, sustainable	Low product specificity, light and nutrient limitations

is summarizing the main differences between the three technologies.

2. Evolution of Electrode Materials for Microbial Electrosynthesis

2.1. Carbonaceous Materials and Graphene-Based Electrodes

From 2010 to 2015, MES cathodes were dominated by graphite plates, carbon felt, reticulated vitreous carbon, and carbon papers because they were inexpensive, chemically stable, and biocompatible for acetogenic biofilms. The performance increases in this period mainly came from enlarging the surface area and improving the roughness to favor biofilm attachment and reduce ohmic losses. Subsequent reviews and historical perspectives document this early reliance on porous carbons and the pivotal role of surface morphology in accelerating start-up and boosting electron transfer to acetogens [20,21].

From ~2016 onward, graphene and graphene-derivative cathodes—reduced graphene oxide (rGO), graphene aerogels, and graphene–polymer blends—rapidly entered MESs. Graphene’s high conductivity, tunable surface chemistry, and ability to form hierarchical 3D networks improved both direct extracellular electron transfer and controlled H₂-mediated routes by enhancing the local electron density and biofilm density. Dedicated reviews synthesize dozens of reports showing that graphene coatings and graphene-based composites systematically increase the current density, coulombic efficiency, and acetate/MCFA productivities compared with unmodified felt or plate electrodes [22].

A parallel trend in the 2020s was circular-carbon electrodes—biochar and waste-derived carbons (including algal-derived frameworks)—which deliver high porosity, wettability, and microbe-compatible functionalities at low cost. Recent studies show that tailoring the pore architecture and oxygen-containing groups in such carbons improves colonization and durability, while maintaining performance near that of engineered graphene—an attractive direction for scale-up [7,23].

2.1.1. Advantages and Disadvantages on Using Carbonaceous and Graphene-Based Electrodes in MES

• Advantages

• Carbonaceous electrodes (graphite, felt, RVC, paper): Low cost, scalable, chemically stable, and relatively easy to modify.
• Graphene-based electrodes: Outstanding conductivity, hierarchical porosity, tunable surface chemistry, and superior performance in terms of electron transfer and product selectivity.
• Circular-carbon electrodes: Sustainable, low-cost, derived from biomass or waste, high porosity and surface functionalization enable strong microbial adhesion and long-term durability.

• Disadvantages

• Plain carbon electrodes: Limited conductivity compared to advanced materials, poor catalytic activity toward hydrogen evolution requires high applied potentials.
• Graphene-based: High cost of synthesis, difficulties of large-scale and reproducible fabrication, potential instability under prolonged operation.
• Circular carbons: High variability in structure and performance depending on feedstock and processing conditions, often lower conductivity than engineered graphene-based.

2.1.2. Gaps and Challenges in Using Carbonaceous and Graphene-Based Electrodes in MES

Despite the clear advances, several challenges remain: (i) scalability: transitioning from lab-scale felts and coatings to industrial-scale electrodes remains uncertain, particularly for graphene-based systems, (ii) durability: not having standardized electrode characterization (like surface area, porosity, and conductivity) makes cross-comparison less useful, (iii) mechanistic understanding: the interplay between electrode surface chemistry, electron transfer pathways, and microbial community dynamics is not fully resolved, and (iv) circular carbon standardization: lack of standardized processing and characterization methods makes cross-comparison between studies difficult.

2.1.3. Future Perspectives on Using Carbonaceous and Graphene-Based Electrodes in MES

• Hybrid electrodes: Combining biochar/circular carbons with thin graphene or catalytic skins may achieve both cost-effectiveness and high performance.
• Scalable manufacturing: Techniques such as 3D printing and roll-to-roll coating can bridge lab innovation with industrial feasibility.
• Functionalized surfaces: Rational tuning of oxygen groups, nitrogen doping, or heteroatom functionalization could steer microbial adhesion and selectivity.
• Integration with techno-economics: Future studies should assess not only performance metrics (current density, coulombic efficiency) but also life-cycle impacts and cost per unit product.

  Table 2. Table shows the above-mentioned aspects in an easier to read way.

  Electrode Type	Advantages	Disadvantages	Gaps/Challenges	Future Perspectives
  Carbonaceous (graphite, felt, paper, RVC)	- Low cost, abundant, scalable - Chemically stable - High surface roughness → good biofilm attachment - Easy to modify with catalysts	- Limited conductivity - Low intrinsic catalytic activity (HER) - Requires high applied potentials	- Needs improved catalytic function - Limited long-term durability data - Scaling issues for high productivity	- Surface functionalization (doping, coatings) - Integration with conductive polymers - Engineering 3D-structured carbons
  Graphene-based (rGO, aerogels, composites)	- Exceptional conductivity - Hierarchical porosity improves mass transfer - Tunable chemistry (functional groups) - Enhanced electron transfer and product selectivity	- High synthesis cost - Scale-up remains difficult - Potential instability during long-term runs	- Lack of standardized fabrication methods - Limited understanding of graphene–microbe interactions - Reproducibility concerns across labs	- Hybrid electrodes (graphene + biochar) - Roll-to-roll or scalable coating methods - Use in advanced 3D-printed cathodes
  Circular carbons (biochar, waste-derived)	- Sustainable and low-cost - Derived from biomass or waste (circular economy) - High porosity and wettability - Comparable biofilm colonization to graphene	- Variability in properties (feedstock dependent) - Often lower conductivity than graphene - Inconsistent performance across studies	- Lack of standardization in feedstock processing - Limited comparative benchmarks with engineered electrodes	- Standardized production methods - Hybridization with conductive nanomaterials - Scalable sustainable electrodes for industrial MES

  is summarizing the advantages, disadvantages, gaps and future perspectives for carbonaceous electrode materials.

2.2. Composite Electrodes (Carbon + Polymers/Metals/Catalysts)

As the field matured, researchers shifted from “plain carbon” to composite cathodes that combine a conductive carbon backbone with electrocatalysts or conductive polymers to manage interfacial kinetics and in situ H₂ evolution. Since ~2020, three families have stood out. (i) Three-dimensional-printed carbon lattices (often Ni/Mo-modified) that fine-tune H₂ delivery and mass transfer. Carefully engineered lattice topologies (cubic, diamond, Schwarz, etc.) and catalytic skins improve local H₂ availability to acetogens and dramatically raise the volumetric production rates in bio-electrochemical CO₂-to-organics systems [17,24]. (ii) Conductive polymer composites—for example, commercially available conductive ABS/PLA lattices or poly-pyrrole/polyaniline coatings—used either as the whole scaffold or as surface modifiers. Studies with electrically conductive polymer lattices (sometimes further Ni-functionalized) report stable acetate and methane production while offering low-cost, manufacturable structures; they also reveal trade-offs between polymer conductivity, mechanical integrity, and long-term biocompatibility [25]. (iii) Catalyst-assisted carbon cathodes (Ni, Cu, Fe, Co, Mo, perovskites). These composites lower the overpotentials for HER, enabling a microbe-friendly H₂ flux without damaging the local pH, and improve the coulombic efficiencies for acetogenesis when the potential is carefully regulated. Newer work evaluates Fe, Cu, and Ni surfaces in side-by-side tests for CO₂-to-acetate MES, clarifying how the catalyst identity steers the selectivity and biofilm composition [26].

2.2.1. Advantages and Disadvantages of Using Composite Electrodes

• Advantages

• Three-dimensional-printed carbon lattices (Ni/Mo-modified): Their engineered porosity and topology provide enhanced mass transfer and fine-tuned H₂ delivery to microbial biofilms. Studies show significantly increased volumetric productivities, particularly for acetate, due to the localized control of the hydrogen flux near acetogens [12,27].
• Conductive polymer composites (poly-pyrrole, polyaniline, conductive PLA/ABS): These materials are low-cost, easily manufacturable, and mechanically flexible, enabling scalable electrode designs. Polymers also offer a tunable surface chemistry that supports microbial adhesion. Hybrid polymer–metal composites have shown stable long-term operation for acetate and methane production [19,21,25].
• Catalyst-assisted cathodes (Ni, Cu, Fe, Co, Mo, perovskites): The incorporation of catalytic coatings lowers the HER (hydrogen evolution reaction) overpotentials and provides a controlled, microbe-friendly H₂ flux, avoiding damaging the local alkalinization. This leads to higher coulombic efficiencies and improved selectivity toward acetate and other reduced compounds [14,19,20,21].

• Disadvantages

• Three-dimensional-printed lattices require specialized manufacturing and may suffer from structural brittleness after prolonged use.
• Conductive polymers often face trade-offs between electrical conductivity and mechanical stability; their long-term bio-compatibility in harsh electrochemical environments is still under debate.
• Metal catalysts risk ion leaching, which can be toxic to microbial communities, and can increase system costs.

2.2.2. Gaps and Challenges in Using Composite Electrodes

Several unresolved issues persist in the development of composite electrodes. (i) Standardization: Results vary widely due to differences in synthesis methods, surface treatments, and testing protocols. (ii) Durability: Few studies evaluate electrode performance over thousands of operational hours, a requirement for industrial adoption. (iii) Biofilm–electrode interactions: The impact of the catalyst type or polymer chemistry on microbial community assembly and metabolic activity remains underexplored. (iv) Economic scaling: While high-performing in the lab, composites often involve costly metals or polymers that raise questions of economic feasibility.

2.2.3. Future Perspectives on Using Composite Electrodes

To move MES closer to commercialization, future research should prioritize the following:

• Hybrid electrodes: Combining biochar or other circular carbons with thin catalytic coatings (Ni, Fe, Co) to balance performance and cost.
• Advanced manufacturing: Adoption of 3D printing, laser etching, and roll-to-roll coating for scalable, reproducible electrodes.
• Eco-friendly catalysts: Replacement of noble or heavy metals with earth-abundant, biodegradable alternatives (e.g., Fe- or Mn-based composites).
• Integrated techno-economic analysis: Studies should pair electrode innovations with LCA (life-cycle assessment) and cost modeling to assess the true scalability.

Table 3. Table shows the above-mentioned aspects in an easier to read way.

Composite Electrode Type	Advantages	Disadvantages	Gaps/Challenges	Future Perspectives
Three-dimensional-Printed Carbon Lattices (Ni/Mo, doped carbons)	- Tailored 3D geometry for high surface area and mass transfer - Enhanced localized H2 delivery to biofilms - High productivities for acetate and other products	- Requires specialized equipment - Mechanical brittleness under long-term use - Fabrication costs still high	- Limited durability testing (>1000 h) - Lack of standard print protocols - Scale-up feasibility unproven	- Scale-up via industrial 3D printing - Hybridization with low-cost biochars - Integration into modular reactor stacks
Conductive Polymers (poly-pyrrole, polyaniline, PLA/ABS blends)	- Low-cost and lightweight - Flexible, scalable manufacturing - Tunable chemistry enhances microbial adhesion - Good long-term stability in some studies	- Moderate conductivity compared to metals - Mechanical degradation in harsh electrochemical environments - Potential bio-compatibility concerns	- Limited reproducibility between labs - Unknown long-term chemical resistance - Need more data on polymer–biofilm interactions	- Polymer–metal or polymer–carbon hybrids - Development of biodegradable conductive polymers - Use in flexible/portable MES reactors
Catalyst-Coated Cathodes (Ni, Fe, Cu, Co, Mo, perovskites)	- Lower HER overpotentials - Controlled H2 flux prevents pH shocks - Increased coulombic efficiency and selectivity	- Risk of metal ion leaching (microbial toxicity) - Added costs (especially noble/perovskite catalysts) - Complex synthesis routes	- Few studies on biofilm community shifts under catalysts - Insufficient life-cycle and cost analyses - Variability in coating adhesion and durability	- Earth-abundant catalysts (Fe, Mn) - Thin catalytic coatings over low-cost supports - Techno-economic integration with renewable energy systems

is summarizing the advantages, disadvantages, gaps and future perspectives for composite electrodes.

2.3. Bimetallic Oxide Cathodes

The latest wave (2023–2025) introduces bimetallic oxides on carbon supports to couple high conductivity with redox-active, microbe-compatible surfaces. A notable example is the Fe-Mn spinel-type oxides (e.g., MnFe₂O₄) deposited onto carbon felt: reports in 2024 describe substantially higher acetate areal productivities, faster start-up, and an enrichment of acetogenic taxa (e.g., Acetobacterium) compared with uncoated controls—attributed to balanced HER kinetics, improved charge transfer, and favorable surface chemistry for biofilm nucleation [27].

These oxide coatings are attractive because they stabilize the interfacial pH and buffer redox conditions, mitigating local extremes that can detach biofilms. Their modularity also allows tuning of the oxygen vacancies and metal ratios to match the desired HER rates for acetogen uptake. Early demonstrations rely on lab-scale felts, but the chemistry is compatible with roll-to-roll coating—suggesting a path to durable, scalable cathodes if adhesion and fouling can be controlled over thousands of hours [16].

2.3.1. Advantages and Disadvantages of Using Bimetallic Oxide Cathodes

• Advantages

• Synergistic catalysis: Combining two or more metals enhances electron transfer kinetics and lowers overpotentials compared to monometallic oxides.
• Abundance and sustainability: Many BMOs (Fe-, Mn-, Ni-based) are earth-abundant, low-cost, and environmentally benign compared to noble metals.
• Enhanced biofilm performance: Oxide surfaces offer hydrophilicity, tunable porosity, and surface oxygen groups that promote microbial adhesion and stable electro-trophic growth.
• Selectivity: BMOs can regulate H₂ evolution to match microbial uptake rates, avoiding accumulation and energy losses.

• Disadvantages

• Conductivity issues: Many oxides have lower intrinsic conductivity compared to carbon or graphene, requiring support materials or dopants.
• Metal leaching: Prolonged use may release ions (e.g., Ni²⁺, Co²⁺) potentially toxic to microbial communities.
• Complex synthesis: Hydrothermal, sol–gel, or electrodeposition methods can be costly and difficult to scale.
• Variability: Performance is highly dependent on metal ratios, synthesis conditions, and electrode architecture.

2.3.2. Gaps and Challenges in Using Bimetallic Oxide Cathodes

Despite the clear advances over the previous two types, several challenges remain. (i) Mechanistic clarity: The precise role of BMOs in balancing direct electron transfer vs. H₂-mediated pathways remains underexplored. (ii) Durability: Long-term stability (>3000 h) under continuous operation is rarely demonstrated. (iii) Standardization: Few comparative studies exist across different BMO compositions under identical MES conditions. (iv) Integration: Limited techno-economic assessments exist for industrial application of BMOs.

2.3.3. Future Perspectives on Using Bimetallic Oxide Cathodes

• Low-cost synthesis: Emphasis on scalable, eco-friendly methods (e.g., electrodeposition on felts, waste-derived metal oxides).
• Hybrid electrodes: Combination of BMOs with graphene or biochar supports to overcome conductivity limitations.
• Mechanistic studies: In situ spectroscopy and omics approaches could resolve how BMOs interact with electro-trophic consortia.
• Circular economy approaches: Utilizing waste streams (steel slag, mine tailings) as oxide precursors to reduce costs.

Table 4. Table is showing the above-mentioned aspects in an easier to read way.

Electrode Type	Advantages	Disadvantages	Gaps/Challenges	Future Perspectives
Bio-metallic Oxides (Fe-Mn, Ni-Co, Cu-Fe, perovskites)	- Synergistic catalytic activity - Earth-abundant, low-cost metals - Hydrophilic, biofilm-friendly surfaces - Tunable HER selectivity	- Low intrinsic conductivity - Risk of metal ion leaching (toxicity) - Complex and costly synthesis - Performance highly condition-dependent	- Limited long-term stability data - Lack of standardized testing - Poor understanding of electron transfer mechanisms - Few techno-economic studies	- Hybrid electrodes with carbon/graphene supports - Scalable electrodeposition or waste-derived oxides - Mechanistic in situ studies - Integration with circular economy feedstocks

is summarizing the advantages, disadvantages, gaps and future perspectives for bimetallic electrodes.

2.4. Cross-Cutting Insights on the Development of MES Electrodes Resulting from the 2010–2025 Literature

Across these families, two principles recur. First, hierarchical porosity + high conductivity (graphene, aerogels, printed lattices) correlate with faster colonization and higher steady-state currents. Second, catalytic moderation of the HER (metals/oxides) is essential to couple electrochemistry with microbial uptake: too little H₂ limits rates; too much shifts communities toward methanogenesis or causes pH spikes. Recent studies formalize this by voltage/H₂-flux control, stabilizing acetate production over long runs [4].

Looking ahead, the most promising cathodes combine sustainable carbons (biochar/graphene hybrids) with thin catalytic skins (Ni, Fe-Mn spinel, perovskite) on engineered 3D architectures. Such designs aim to deliver low-cost, high-durability, and field-scale manufacturability while preserving the microenvironments mesophilic acetogens require [7,17].

2.5. Anode and Anodic Reactions

While most research on microbial electrosynthesis (MES) emphasizes cathodic CO₂ reduction, the anode reaction is equally critical in defining the system’s overall energy demand and efficiency. In conventional MES, the anode typically catalyzes the oxygen evolution reaction (OER), oxidizing water into O₂ and protons. However, the OER is kinetically sluggish and requires high overpotentials, which significantly increases the cell voltage and overall energy input. Moreover, oxygen crossover into the cathode chamber can inhibit anaerobic acetogens or methanogens, destabilizing microbial communities. These limitations have prompted growing interest in alternative anodic reactions that consume lower energy and generate valuable by-products. Examples include the oxidation of organics (e.g., acetate, glycerol, wastewater organics), sulfide, or ammonia, which proceeds at lower potentials than the OER and thus reduces the voltage required to drive MES. Such strategies can simultaneously improve system efficiency, valorize waste streams, and mitigate O₂ intrusion risks. Importantly, pairing CO₂-reducing cathodes with value-generating anodic processes could transform MES from a single-product to a co-production platform, enhancing both techno-economic feasibility and sustainability. Overall, engineering the anode reaction is a key but often overlooked lever to reduce energy costs, stabilize biocathode operation, and broaden the impact of MES within integrated biofuel and biorefinery systems.

Table 5. Table shows the main characteristics related to anodes and the anode reaction.

Anode Reaction Type	Advantages	Disadvantages	Gaps/Challenges	Future Perspectives
Oxygen evolution reaction (OER, water oxidation)	Simple, well-studied reaction - Produces protons to balance cathodic CO2 reduction - No external feed required	- High overpotential → high energy demand - Generates O2, which can inhibit anaerobic microbes if it crosses over - No value-added product	- Need for robust, O2-tight separators - Lack of efficient low-cost OER catalysts compatible with MES	- Development of selective membranes preventing O2 crossover - Design of durable, low-overpotential OER catalysts (e.g., non-precious metal oxides)
Organic oxidation (e.g., acetate, glycerol, wastewater organics)	- Lower overpotential than the OER - Potential to co-produce value-added chemicals - Enables wastewater valorization	- Requires continuous organic feed - Possible fouling or toxicity from complex waste streams - Added complexity in feedstock logistics	- Limited studies on long-term operation - Incomplete understanding of anodic microbiome dynamics	- Integration of MES with wastewater treatment - Tailoring anodic catalysts for selective oxidation of targeted substrates
Sulfide oxidation	- Lower thermodynamic potential than the OER - Useful for treating sulfide-rich effluents - Produces elemental sulfur or sulfate as by-products	- Limited substrate availability (industrial niche) - Possible corrosion issues from sulfur species	- Few pilot-scale demonstrations - Need to control sulfur deposition on electrodes	- Coupling with biogas desulfurization - Development of sulfur-tolerant electrode materials
Ammonia oxidation	- Potential to co-produce nitrate/nitrite as fertilizers - Lower potential than OER - Useful in nitrogen-rich waste valorization	- Formation of undesirable intermediates (e.g., N2O, a greenhouse gas) - Limited microbial compatibility data	- Stability and selectivity of anodic catalysts not well explored - Environmental risks of incomplete oxidation	- Design of selective catalysts to minimize N2O - Integration with agricultural wastewater treatment for circular nutrient recovery

is summarizing the main characteristics of anode reaction.

3. Evolution of Microbial Communities and Biofilm Engineering

Microbial electrosynthesis (MES) relies on cathodic biofilms that reduce CO₂ to organics using electrons supplied by a solid electrode, either directly (DET) or indirectly via H₂/format (MET). Since 2010, cathode material design and operating strategies have progressively shifted from inert carbons toward graphene-modified carbons, composites, and bi-metallic oxides, each reshaping the biofilm structure, community assembly, and electron-transfer modes. Early MES studies on graphite felt/paper and carbon cloth consistently enriched acetogenic biofilms (e.g., Acetobacterium, Sporomusa) and co-occurring hydrogenogenic partners such as Desulfovibrio, establishing the canonical view that H₂-mediated CO₂ reduction dominates under most operating regimes. Recent meta-analyses quantify this picture, showing that acetogenic and methanogenic MES share ~80% of a “cathodic core microbiome,” with inoculum pre-treatments and set potentials steering the community outcomes and the (re)emergence of methanogens over time. These data also implicate Desulfovibrio as an early H₂ initiator that supports Acetobacterium or Methanobacterium depending on the conditions [9,27].

3.1. Carbonaceous Materials and Graphene

Unmodified carbon felts/RVC remain workhorses because of their biocompatibility, roughness, and cost; they reliably host thick acetogenic biofilms and tolerate the alkaline microenvironments associated with vigorous HERs. Yet their limited intrinsic catalytic activity means higher overpotentials and slower start-up when the H₂ supply is rate-limiting. Surface engineering with graphene and reduced graphene oxide (rGO) has therefore become a central biofilm-engineering strategy. Graphene coatings/aerogels raise conductivity, provide hierarchical porosity for CO₂ delivery, and present functional groups that improve attachment, collectively boosting the electron transfer, coulombic efficiency, and product selectivity (e.g., acetate) compared with bare carbon felts. Recent focused reviews emphasize that graphene-modified cathodes increase bacterial loading and biofilm continuity, while also noting scale-up and cost challenges (agglomeration, reproducibility, fabrication). Multiple bench studies show rGO-modified felts producing denser, more electroactive biofilms and better selectivity [22,28].

The mechanism of interaction has several steps. (i) Direct electron transfer (DET): Some electroactive microbes, like Sporomusa ovata and Clostridium spp., can attach directly to the carbon surface. They use outer-membrane cytochromes or conductive pili (nanowires) to accept electrons from the cathode. (ii) Biofilm formation: Carbonaceous electrodes promote the formation of dense biofilms because of their rough surfaces and hydrophilic properties, which enhance microbial adhesion. (iii) Indirect electron transfer (IET): Carbon electrodes can mediate electron transfer via redox mediators, either naturally secreted by microbes (e.g., flavins) or externally added compounds.

3.2. Composite Electrodes

From ~2020 onward, composites—carbon scaffolds integrated with metals, alloys, or conductive polymers—have accelerated progress by engineering the H₂ micro-niche at the biofilm–electrode interface. Three-dimensional-printed carbon aerogels plated with Ni-Mo exemplify this shift: By distributing large currents over a high surface area at low local current density, they stabilize start-up, minimize bubble disruption, and sustain near-quantitative conversion in electro-methanogenesis models; analogous principles are now applied for acetogenic MES. These electrodes effectively tune H₂ generation to match the microbial uptake, which promotes stable acetogenic or methanogenic biofilms with high coulombic efficiency. Parallel work with conductive polymer composites and nickel-foam-decorated felts similarly reports higher titers via improved in situ H₂ delivery and durable biofilm architectures [24].

Community-wise, composites tend to narrow the functional guilds near the cathode by providing steadier electron donors (H₂) and more uniform shear/porosity, enriching the Acetobacterium in acetogenic cells or Methanococcus/Methanobacterium in electro-methanogenesis, while maintaining supportive fermenters and sulfur cycle shuttles (e.g., Desulfovibrio). Reviews of cathode materials underscore that such material-driven biogeography is as decisive as the inoculum choice for steering communities away from parasitic methanogenesis when acetate or longer-chain acids are targeted [29].

The mechanism of interaction has several steps. (i) Enhanced DET and IET: The conductive polymer matrix or graphene sheets increase the electron transfer efficiency, providing more active sites for microbial attachment. (ii) Mechanical stability: Composites provide robust support for biofilms, which is especially important under long-term MES operation. (iii) Surface functionalization: Many composites can be chemically modified to introduce functional groups (–OH, –COOH, –NH₂) that enhance microbial adhesion and electroactivity.

3.3. Bi-Metallic Oxides (BMOs)

Bi-metallic oxides (e.g., Fe-Mn, Co-Ni, spinel/perovskites) emerged strongly after 2020 as earth-abundant HER catalysts that are biofilm-friendly. Recent studies show Fe-Mn oxide-modified cathodes are biocompatible and electroactive, enabling efficient CO₂-to-acetate conversion while supporting stable acetogenic biofilms; Co-Ni on carbon felt has achieved high faradaic efficiencies to acetate/ethanol with robust biofilm formation. Mechanistically, BMOs lower the HER overpotentials, moderate the local pH, and present hydrophilic oxide surfaces that can boost extracellular polymeric substance (EPS) formation and cell adhesion, thereby balancing the DET/MET pathways in mixed cultures. Earlier work with Fe₃O₄/GAC 3D cathodes already highlighted the benefit of magnetite nanoparticles for acetate productivity and biofilm enrichment [16,30,31].

The mechanism of interaction has several steps. (i) Electrocatalytic electron transfer: Bimetallic surfaces facilitate faster electron transfer to microbes or mediators by lowering the overpotential for key reactions like CO₂ reduction. (ii) Synergistic effects: One metal can improve electron conductivity, while the other promotes surface reactions that generate reduced species available for microbial uptake. (iii) Microbe–metal surface interaction: Certain microbes form strong biofilms on metallic surfaces, sometimes incorporating extracellular polymeric substances (EPSs) that interact with metal atoms to enhance electron uptake.

Figure 4 shows the microbial attachment and electron transfer mechanisms for these three electrode types.

3.4. Gaps and Challenges in Co-Shape Communities

Across materials, the set potential, CO₂ supply, and current distribution remain dominant levers: more negative potentials accelerate the HER but can favor methanogens, whereas milder potentials and periodic interventions (e.g., headspace flushing, selective inhibitors) help maintain acetogenic dominance. Flow-through architectures and “zero-gap” designs that retain H₂ or distribute it evenly reduce competitive losses and select for target guilds; recent reports demonstrate leaps forward in areal productivity via improved gas retention and tailored flow paths [27,32].

3.5. Future Perspectives on Biofilm Engineering

The field is converging on material–microbiome co-design: (i) hierarchical carbons/graphene to maximize attachment and electron access; (ii) composites that attach H₂ to biofilms with printable, scalable architectures; and (iii) BMOs to deliver low-overpotential, biocompatible HERs on earth-abundant chemistries. Meta-analysis suggests that sustained suppression of methanogenesis will require repeated operational nudges alongside material design, because community assembly trends stochastically move toward methanogens over time. Future priorities include standardized comparative testing, omics-resolved mechanism mapping of DET vs. MET across surfaces, and scale-up studies that couple biofilm durability (≥1000–3000 h) with techno-economics [9].

Table 6. Table showd the different types of microorganisms and their main characteristics.

Microorganism	Type/Classification	Electron Transfer Mechanism	Typical Substrates	Main Products	Key Features in MES
Geobacter sulfurreducens	Gram-negative δ-proteobacteria	Direct electron transfer via pili (“nanowires”)	Acetate, lactate	Current, CO2 reduction products	Highly efficient anode-respiring bacteria, strong biofilm formation
Shewanella oneidensis	Gram-negative γ-proteobacteria	Direct electron transfer + soluble mediators (flavins)	Lactate, pyruvate	Current, H2	Versatile metabolism, can transfer electrons to electrodes and metals
Clostridium spp.	Gram-positive anaerobes	Indirect electron transfer (mediators)	Sugars, organic acids	Acetate, butyrate, ethanol	Strong fermentative activity, useful in cathodic CO2 reduction
Methanogens (e.g., Methanococcus maripaludis)	Archaea	Direct or mediated electron uptake	H2, CO2	Methane	Essential for microbial electrosynthesis of methane from CO2
Acetobacterium woodii	Gram-positive anaerobe	Indirect via electron carriers	H2, CO2	Acetate	Efficient CO2-to-acetate conversion in microbial electrosynthesis
Desulfuromonas acetexigens	Gram-negative δ-proteobacteria	Direct electron transfer	Acetate, lactate	Current	Strong anode respiration, used in high-current MES
Anaerobium acetethylicum	Gram-positive anaerobe	Mediated electron transfer	Sugars, pyruvate	Hydrogen, ethanol	Useful in biohydrogen production and MES cathodes

is summarizing the main types of microorganisms and their characteristics.

4. Evolution of Reactor Configurations and Process Engineering

The 2010s began with simple H-cell or dual-chamber reactors (anion/cation exchange membranes, carbon-felt cathodes), later accompanied by membrane-less designs to reduce the ohmic losses and parts count. Proof-of-concept studies showed acetate production in membrane-less cells but also highlighted cross-over, oxygen intrusion, and biofilm instability at scale [33]. As the field matured, engineering priorities shifted toward gas management, mass transfer, and current distribution. Since 2022, it has been clear that single-chamber membrane-less reactors are generally unsuitable for high-rate MES unless gas separation and pH control are carefully addressed, steering research toward flat-plate, directed-flow, and compartmentalized stacks [34].

A notable step was the emergence of directed flow-through/plate reactors that spread the current over large, thin electrodes and orchestrate CO₂ delivery across the biofilm. Recent plate-reactor studies report productivities competitive with established biotechnologies, clarifying the hydrodynamics, pressure drop, and flow-path design as primary levers [35,36]. In parallel, zero-gap configurations with extended flow paths have been introduced to retain and consume in situ H₂ before it escapes, markedly improving the conversion to biomethane and offering design lessons that translate to acetogenic MES [32].

4.1. Carbonaceous and Graphene Cathodes: Surface Area, Gas Handling, and GDEs

Reactor advances with carbon felt/paper focused on increasing the specific area, lowering the ohmic losses, and improving the CO₂ supply to thick biofilms. Gas-management features—gas diffusion layers (GDLs), microporous layers (MPLs), and macroporous substrates (MPSs)—migrated from electrochemical CO₂-RR into MES, enabling stable gas–liquid–solid contact while limiting electrolyte flooding [35]. Incorporating graphene/rGO coatings on felt and papers increased the conductivity and wettability, enabling denser, more continuous biofilms that tolerate higher areal currents in flow-through plates or stacked cells; comprehensive reactor-design reviews since 2022 capture these material–hydrodynamic interactions [36]. As a bridge to scale, GDE-assisted MES has been repeatedly highlighted for raising areal productivity by reducing boundary-layer limitations and improving CO₂ transfer into the cathode [37].

4.2. Composite Electrodes: Engineering the H₂ Micro-Niche

From ~2020, composite cathodes—carbon scaffolds integrated with metals/alloys or conductive polymers—have been paired with 3D flow architectures to manage where and how fast H₂ is generated. Custom 3D-printed carbon aerogels plated with Ni-Mo demonstrated unprecedented volumetric rates in electro-methanogenesis by tuning the H₂ flux via topology and current distribution, an approach now informing acetogenic MES stack design (low local j to prevent bubble detachment; high total j for throughput) [24]. Beyond printing, flow-electrode MES—pumping conductive carbon slurries through the cathode compartment—improves the electrode wetting and mass transfer while maintaining reasonable energy efficiency, illustrating a different scale-up route [18]. New biocathode constructs that combine 3D frameworks with catalytic skins report >90% total Faradaic efficiency for C₂ products (acetate/ethanol) by enhancing the electron availability and minimizing the diffusional dead zones [31].

4.3. Bimetallic Oxides: Low-Overpotential HER and Biofilm Compatibility

A major post-2020 trend is coating carbon scaffolds with earth-abundant bimetallic oxides (BMOs) (e.g., Fe-Mn, Co-Ni) to lower the HER overpotentials and stabilize the local pH near the biofilm. This process–electrode co-design lets reactors run at milder potentials/voltages while sustaining high areal currents without stripping the biofilms via violent gas evolution. Recent studies with Fe-Mn oxides show higher acetate productivities and robust community assembly on flat-plate and felt cathodes; Co-Ni on felt likewise improves the conversion and Faradaic efficiency for C₂ products [16]. Reactor reviews now treat catalyst selection and placement as integral to configuration—e.g., thin catalytic skins on flow-through plates vs. zero-gap cells—because the H₂ residence time and bubble size strongly influence the selectivity and coulombic efficiency [36].

4.4. Scale-Up Lessons: Hydrodynamics, Compartmentalization, and Control

System-level analyses show that scaling MES demands predictable flow regimes, gas retention, and robust compartments that prevent cross-over while minimizing resistances. The pilot-scale BES literature identifies module-based stacks, short ionic paths, and balanced gas management as universal success factors, aligning with new MES plate and zero-gap designs [38,39]. Finally, process control—regulating the potential/voltage and H₂ availability—has emerged as a decisive tool to stabilize acetate synthesis and suppress methanogenesis over long runs, tying operation tightly to reactor hardware [4].

4.5. Gaps and Limitations in Reactor Configurations and Process Engineering

Despite the creative designs (H-cell, membrane-separated flow cells, trickle beds, tubular/plate stacks), insufficient delivery of CO₂ and reducing equivalents to deep biofilm layers caps space–time yields. Even recent productivity gains with compact plate/tubular designs still report gradients of pH, H₂ and CO₂ within porous cathodes that sustain unwanted methanogenesis or limit carboxylate formation, indicating persistent transport non-uniformity at scale [34,40]. Most reactors rely on in situ H₂ as the actual electron carrier; however, H₂ slip to the headspace, short residence times, and poor retention across electrode channels depress the coulombic efficiency and selectivity. Zero-gap cells improve the ohmic losses but still struggle with H₂ retention and flow distribution unless specifically engineered (extended flow paths, porous flow fields), and sensitivity to membrane hydration/ohmic rises remains [32,41]. Traditional centimeter-scale gaps inflate cell voltages. Zero-gap/BPM configurations lower resistance but introduce new constraints—water management, carbonate crossover, and mechanical compression tolerances—making stable long-term operation nontrivial, especially with real waste streams [41]. Gas-diffusion electrodes (GDEs) and hierarchical carbons enable high interfacial areas, yet they are susceptible to flooding, biofouling, and loss of three-phase contact over time; conducting-polymer retrofits remain promising but not field-proven in long campaigns [37]. Aside from a few studies on stacked/flow-electrode MES and recent plate/tubular prototypes, the literature still lacks month-scale (>1000 h) runs with stable product slates, full energy balances, and validated cleaning/maintenance cycles. Most techno-economic inferences rest on short-run lab data [18,35]. pH control can suppress methanogenesis, but micro-gradients inside thick felts maintain hydrogenotrophic methanogens; bioaugmentation often fails to persist without matching hydrodynamics and selective pressures [40].

While MES offers potential for sustainable chemical production, one of the key technical challenges lies in in situ product separation and recovery from dilute aqueous solutions. MES processes often generate products in very low concentrations, typically in the range of milligrams to grams per liter. Such dilute solutions pose significant challenges for downstream processing. Conventional separation techniques, such as distillation or solvent extraction, become energy-intensive and economically unfeasible at these concentrations because the energy cost per unit of product recovered increases dramatically. Many MES products, including organic acids, alcohols, and other bio-based chemicals, can be toxic to the microbial community at higher concentrations. This limits the achievable product titer in the reactor. As a result, continuous removal of the product is desirable, but performing efficient in situ recovery without harming the microbes requires carefully tuned separation strategies. In situ separation methods must be highly selective, as MES media often contain complex mixtures of salts, nutrients, and by-products. Techniques such as pervaporation, membrane extraction, or adsorption need to preferentially target the product without removing essential nutrients or damaging the microbial biofilm/electrodes. Achieving this balance is technically challenging and requires specialized materials and configurations. MES is inherently coupled with an electrochemical system, where microbial activity occurs on electrodes. Any in situ separation method must therefore be compatible with the electrochemical environment, including the pH, ionic strength, and potential ranges. For instance, membrane-based extraction may introduce electrical resistance or fouling, while adsorbents may interfere with electrode performance. For in situ recovery to be viable, it must minimize additional energy input and costs. Dilute solutions make conventional recovery techniques costly, so novel low-energy approaches such as electrochemical product separation, selective membrane extraction, or reactive adsorption are being explored. However, scaling these approaches while maintaining microbial activity and system stability remains a major hurdle. MES systems often produce multiple products simultaneously, sometimes with changing ratios over time. This dynamic product profile complicates in situ separation because the separation system must handle variable chemical compositions without loss of efficiency or selectivity.

4.6. Future Perspectives on Reactor Configurations and Process Engineering

Plate-type and tubular “zero-dead-volume” reactors with engineered flow fields should be prioritized, since they minimize diffusion distances and enable precise residence-time control. New channel topologies (serpentine, interdigitated) designed by CFD specifically for H₂ retention and uniform shear show clear promise for stabilizing selectivity and raising current density [36]. Treat H₂ as a managed intermediate: (i) design extended/porous flow paths for in-plane retention; (ii) implement headspace recirculation and controlled pressurization; and (iii) integrate micro-sparging or catalytic “H₂ polishing” zones to keep H₂ near the biofilm. Early zero-gap demonstrations indicate that such tactics can sharply improve the methane or acetate productivities when coupled with proper flow distribution [32,41]. Pair GDEs with anti-flooding microstructures and breathable binders; explore binder-less, mechanically robust GDEs and conducting-polymer interlayers to maintain three-phase contact and reduce start-up times. Material advances must be co-designed with cleaning strategies (back-pulsing, periodic polarity tweaks) to extend the runtime [38]. Slurry/flow electrodes (e.g., PAC-amended) increase the effective surface area and homogenize the current distribution in stacks, doubling acetate productivities and improving energy efficiency; translating this into hygienic, low-shedding industrial modules is a near-term engineering task [18]. The highest reported throughputs increasingly pair MES with gas fermentation or chain-elongation steps and employ intensified separation (e.g., electrodialysis) to avoid end-product inhibition. Designing reactors for in-line product extraction and modular coupling will be key to reproducible >g·L⁻¹ titers and to reaching TRL 4–5 [42]. Community shifts, foulants, and inorganic scaling in real matrices require reactors that accommodate online cleaning, membrane re-wetting in zero-gap stacks, and replaceable cathode cassettes. Long-horizon field trials with water/waste CO₂ are urgently needed [6]. Pair continuous sensors (pH, redox, gas composition) with digital twins that couple biofilm growth, electrochemistry, and hydrodynamics; optimize for the cost per kg product rather than the current density alone. Reviews emphasize the importance of moving beyond H-cells toward controlled, instrumented pilots [7,34].

5. Performance Trends and Techno-Economic Signals

5.1. Performance Trends

Early 2010s—proof of concept. Dual-chamber “H-cell” and membrane-less systems with carbon felt/paper cathodes achieved low areal productivities and modest coulombic efficiencies (CEs), limited by mass transfer, ohmic losses, and uncontrolled H₂ evolution. Reviews through 2023–2024, such as [3], summarize that these reactors established feasibility but rarely exceeded tens of g·m⁻²·d⁻¹ acetate productivities [3].

Between 2016 and 2020—materials and gas management emerged on the scene of MES. Graphene-modified/porous carbons and early gas-diffusion approaches pushed the current density and CE upward by improving the CO₂ supply and biofilm continuity. Empirical thresholds for economic relevance began to appear in TEAs, indicating MES would likely require 50–100 mA·cm⁻² class cathode current densities to pay off at realistic electricity prices [43].

In early 2020 until 2023—flow-electrode MES with PAC slurries roughly doubled acetate production rates versus static packed felts while keeping energy consumption near 0.02 kWh·g⁻¹ (acetate), a meaningful step toward process intensification [8]. Nickel-foam-decorated felts showcased the value of in situ H₂ delivery, achieving high acetate titers with mixed communities [44].

In the later years (2023–2025)—plate/zero-gap reactors and catalytic skins, a new generation of thin plate MES reactors, reported productivities competitive with established biotechnologies, signaling a break from the H-cell era [36]. In parallel, zero-gap cells with extended flow paths improved H₂ retention and markedly enhanced biomethane conversion, highlighting the centrality of hydrogen management to CE and energy efficiency [33]. Bimetallic-oxide or low-cost metal catalysts integrated on carbon scaffolds sustained higher rates by lowering the HER overpotential without resorting to precious metals, improving stability in long runs [45].

Record and representative metrics: A synthesis of MES cathode materials collates the best-in-class numbers—including |j| ≈ 200 A·m⁻² and acetate ≈ 1330 g·m⁻²·d⁻¹ on advanced biocathodes under optimized conditions—while also noting that these are not yet routine at pilot scale [30]. Single-strain platforms (e.g., Clostridium ljungdahlii) demonstrated controllable, high-rate acetate production in well-managed reactors, demonstrating how operation can narrow product variance [5]. Catalyst-assisted MES further tied current density → productivity scaling to controlled H₂ flux and community shifts [26].

Stability via operational control: Voltage/H₂-flux regulation strategies published in 2024–2025 show stabilization of acetate synthesis and suppression of methanogenesis by matching electron delivery with microbial uptake—an operational complement to hardware gains [4].

State of play (2024–2025 perspectives): Field overviews position CO₂-to-carboxylate MES around TRL 3, with the throughputs approaching those in syngas/gas-fermentation trains in the best lab-scale plate/zero-gap systems, especially when integrated with downstream bioprocessing [42].

Recent studies from 2023 and 2024 have focused on the long-term stability of both bimetallic composites. Their main conclusion was that while improving the optimal content of MXene, the stability of Ru-based catalysts was significantly improved [46], which demonstrated that the NiMn/Ti-1 electrocatalyst exhibited superior stability, maintaining a constant potential with only a marginal increase [47]. This stability is crucial for MES applications where sustained electrochemical performance is required.

While specific studies on the large-scale manufacturing of composite electrodes for MES are limited, advancements in electrode manufacturing techniques can be applied to MES systems. For instance, a review paper [48] from 2025 discussed the use of electromagnetic techniques in carbon fiber and composite manufacturing. These techniques promise to significantly reduce industrial energy consumption and emissions, which could be beneficial when scaling up MES systems that utilize composite electrodes.

Additionally, research in additive manufacturing has explored the fabrication of composite electrodes. Study [49] published in 2024 investigated the manufacturing of shape-controllable flexible PEDOT/graphene oxide composite electrodes using laser scribing. This method offers a quick, effective, and environmentally friendly approach to creating composite electrodes, which could be adapted for large-scale MES applications.

5.2. Techno-Economic Signals

Foundational TEAs indicate that MES viability hinges on low carbon intensity power and high areal current density (≈50–100 mA·cm⁻²), with the stack voltage and CE serving as the dominant levers; these targets have guided reactor/material design since 2020 [43]. Recent plate and zero-gap demonstrations report lower ohmic losses and better H₂ utilization, which directly reduce the specific energy consumption and capex per m² of electrode—improving the modeled cost per kg product [32,35]. Long-term operation with non-precious HER catalysts in zero-gap cells suggests a path to durable performance without noble metals, improving capex/opex assumptions and de-risking supply chains [45]. TEAs for electro-microbial production (EMP)—though not MES-only—show that coupling DAC/CO₂ capture + electrobiology can approach competitiveness for energy-dense fuels (e.g., n-butanol), if renewable power is cheap and the conversion steps are intensified; the same cost drivers (electricity price, current density, productivity) map onto MES [10]. Reviews and perspectives [30,42] emphasize movement toward modular stacks and process intensification (gas-diffusion layers, integrated separations) as prerequisites for bankable pilots; the key risks remain flooding/biofouling, H₂ slip, and operational durability beyond 1000 h.

Crucially, MES also integrates seamlessly with circular economy and decarbonization strategies. While biomass-based biofuels recycle biogenic carbon, MES enables direct valorization of industrial CO₂ streams (e.g., from cement, steel, or biogas upgrading), transforming a liability into a resource. Moreover, the product spectrum of MES is broader than conventional biofuels: in addition to acetate and ethanol, MES can yield butyrate, propionate, and medium-chain fatty acids, which serve as precursors for sustainable aviation fuels and advanced drop-in hydrocarbons. Such versatility strengthens the case for MES as a future-oriented complement—not merely an alternative—to existing biofuel platforms.

Another important aspect related to the techno-economic study of MES is the life-cycle assessment.

Even though MESs offers promising pathways for sustainable energy generation and resource recovery, in order to evaluate their environmental sustainability, a comprehensive life-cycle assessment (LCA) is essential, encompassing all the stages from electrode manufacturing to reactor operation and product separation. This LCA can be split into three main directions.

• Electrode Manufacturing and Carbon Footprint

Electrodes are central to MES performance, yet their production can significantly contribute to the overall carbon footprint. Materials such as graphite, carbon cloth, or carbon-based nanomaterials often require energy-intensive processes, including high-temperature pyrolysis or chemical treatments. These processes result in considerable greenhouse gas (GHG) emissions, primarily CO₂, associated with raw material extraction, processing, and transport. Optimizing electrode materials with lower embodied energy and longer lifespans can substantially reduce the system’s carbon footprint.

2.

Reactor Operation: Energy and Water Consumption

The operational phase of MES involves maintaining optimal conditions for microbial activity, including the temperature, pH, and redox potential. Energy consumption arises from pumping, aeration, mixing, and electrical input to sustain electron flow between the anode and the cathode. Depending on the scale, energy requirements can dominate the environmental impact of MES, particularly when non-renewable electricity is used. Additionally, water is consumed both as a reaction medium and to maintain the hydraulic flow, especially in continuous-flow reactors. Efficient reactor design, such as minimizing ohmic losses and enhancing mass transfer, can reduce both the energy and water footprints.

3.

Product Separation and Resource Recovery

After microbial conversion, products such as biofuels, hydrogen, or chemicals require separation and purification. These processes—distillation, filtration, or electrochemical extraction—often demand significant energy and water input, further contributing to the system’s environmental impact. The choice of separation technology and its integration with the reactor can markedly influence MES sustainability. For instance, in situ product recovery can minimize downstream processing energy and reduce water consumption.

Table 7. Table shows a comparative assessment of the above-mentioned aspects.

MES Stage	Key Activities/Processes	Carbon Footprint (CO2 eq)	Energy Consumption	Water Consumption	Notes/Mitigation Strategies
Electrode Manufacturing	Production of graphite, carbon cloth, nanomaterials; chemical/thermal treatments	High (due to raw material extraction and high-temp processes)	Moderate to high (pyrolysis, synthesis)	Low to moderate	Use low-carbon materials, recycle electrodes, extend electrode lifespan
Reactor Operation	Pumping, aeration, mixing, electrical input; maintaining microbial environment	Moderate (depends on electricity source)	High (continuous operation, electrical input)	Moderate to high (reaction medium, flow maintenance)	Optimize reactor design, reduce ohmic losses, use renewable electricity
Product Separation	Filtration, distillation, electrochemical extraction	Moderate (energy-intensive processes contribute indirectly)	High (heating, separation processes)	High (cooling, washing, extraction)	Implement in situ recovery, integrate energy-efficient separation techniques
Overall MES System	Combined impact of all stages	High to moderate (dominated by electrode and energy use)	High (operation + separation)	Moderate to high	Holistic optimization of materials, reactor, and separation reduces environmental footprint

summarizes the main key features on circular economy and decarbonization strategies.

6. Conclusions

• Microbial electrosynthesis (MES) has progressed remarkably since its inception, moving from proof-of-concept studies in H-type reactors toward increasingly sophisticated systems that integrate novel electrode materials, microbial community engineering, and process optimization. The literature consistently highlights that electrode design and material innovation—ranging from carbonaceous substrates and graphene-based coatings to composite cathodes and bio-metallic oxides—are central drivers of improved performance, as they directly impact the electron transfer efficiency, biofilm formation, and hydrogen evolution dynamics.
• Parallel to these advances, microbial community engineering has transitioned from reliance on mixed consortia to more controlled and, in some cases, genetically optimized strains, offering opportunities to fine-tune product selectivity and stability. Similarly, reactor configurations have evolved from simple two-chamber setups to plate, tubular, and zero-gap flow designs, significantly reducing ohmic losses and improving mass transfer.
• Despite these advances, several challenges remain. MES still faces critical barriers to scale-up, including insufficient long-term stability, hydrogen management inefficiencies, incomplete understanding of microbe–electrode interactions, and economic constraints tied to the electricity demand and reactor capital costs. Current techno-economic assessments emphasize the need for higher current densities, improved Faradaic efficiencies, and durable low-cost materials to render MES competitive with conventional biotechnologies and power-to-X alternatives.
• Looking forward, the field is poised to benefit from integrative approaches—combining advanced material science, systems biology, and process engineering with data-driven modeling and techno-economic analysis. In particular, hybrid electrode architectures (graphene–oxide composites, catalytic coatings), biofilm engineering strategies, and intensified flow-through reactor designs hold promise for achieving the productivity thresholds required for industrial relevance. Furthermore, the alignment of MES research with renewable electricity integration and CO₂ circular economy initiatives underscores its potential role in decarbonization strategies.
• Taken together, MES stands at the convergence of bioelectrochemistry, renewable energy integration, and carbon capture utilization (CCU). Its potential lies not in competing head-to-head with ethanol or biodiesel at current scales but in providing a new route to carbon-neutral liquid fuels that leverages cheap renewable electricity, reduces pressure on agricultural land, and transforms waste CO₂ into valuable products. This positions MES as a critical component of the next generation of biofuel technologies, aligning with global priorities concerning carbon neutrality, energy security, and industrial decarbonization.
• In conclusion, while MES is not yet a mature industrial technology, the trajectory of scientific progress between 2010 and 2025 signals growing momentum toward practical application. Success will depend on bridging laboratory innovations with scalable, robust, and economically viable process configurations.