Recent advances in biocathode materials and configurations for reactor applications in microbial electrosynthesis of CO2.

Carbon dioxide reduction to high–value chemicals in microbial electrosynthesis system: Biological conversion and regulation strategies

Bioelectrochemical systems (BESs) is a term that encompasses a group of novel technologies able to interconvert electrical energy and chemical energy by means of a bioelectroactive biofilm. Microbial electrosynthesis (MES) systems, which branch off from BESs, are able to convert CO2 into valuable organic chemicals and fuels. This study demonstrates that CO2 reduction in MES systems can be enhanced by enriching the inoculum and improving CO2 availability to the biofilm. The proposed system is proven to be a repetitive, efficient, and selective way of consuming CO2 for the production of acetic acid, showing cathodic efficiencies of over 55% and CO2 conversions of over 80%. Continuous recirculation of the gas headspace through the catholyte allowed for a 44% improvement in performance, achieving CO2 fixation rates of 171 mL CO2 L−1·d−1, a maximum daily acetate production rate of 261 mg HAc·L−1·d−1, and a maximum acetate titer of 1957 mg·L−1. High-throughput sequencing revealed that CO2 reduction was mainly driven by a mixed-culture biocathode, in which Sporomusa and Clostridium, both bioelectrochemical acetogenic bacteria, were identified together with other species such as Desulfovibrio, Pseudomonas, Arcobacter, Acinetobacter or Sulfurospirillum, which are usually found in cathodic biofilms. Moreover, results suggest that these communities are responsible of maintaining a stable reactor performance.

Enrichment of salt-tolerant CO2–fixing communities in microbial electrosynthesis systems using porous ceramic hollow tube wrapped with carbon cloth as cathode and for CO2 supply

The recent concept of microbial electrosynthesis (MES) has evolved as an electricity-driven production technology for chemicals from low-value carbon dioxide (CO2) using micro-organisms as biocatalysts. MES from CO2 comprises bioelectrochemical reduction of CO2 to multi-carbon organic compounds using the reducing equivalents produced at the electrically-polarized cathode. The use of CO2 as a feedstock for chemicals is gaining much attention, since CO2 is abundantly available and its use is independent of the food supply chain. MES based on CO2 reduction produces acetate as a primary product. In order to elucidate the performance of the bioelectrochemical CO2 reduction process using different operation modes (batch vs. continuous), an investigation was carried out using a MES system with a flow-through biocathode supplied with 20 : 80 (v/v) or 80 : 20 (v/v) CO2 : N2 gas. The highest acetate production rate of 149 mg L-1 d-1 was observed with a 3.1 V applied cell-voltage under batch mode. While running in continuous mode, high acetate production was achieved with a maximum rate of 100 mg L-1 d-1. In the continuous mode, the acetate production was not sustained over long-term operation, likely due to insufficient microbial biocatalyst retention within the biocathode compartment (i.e. suspended micro-organisms were washed out of the system). Restarting batch mode operations resulted in a renewed production of acetate. This showed an apparent domination of suspended biocatalysts over the attached (biofilm forming) biocatalysts. Long term CO2 reduction at the biocathode resulted in the accumulation of acetate, and more reduced compounds like ethanol and butyrate were also formed. Improvements in the production rate and different biomass retention strategies (e.g. selecting for biofilm forming micro-organisms) should be investigated to enable continuous biochemical production from CO2 using MES. Certainly, other process optimizations will be required to establish MES as an innovative sustainable technology for manufacturing biochemicals from CO2 as a next generation feedstock.

Efficient production of lycopene from CO2 via microbial electrosynthesis

Power-to-gas (P2G) technologies convert electrical energy into renewable gaseous fuels or value-added molecules to be used for transportation, heat production, or as a feedstock for the chemical industry, respectively. Microbial Electrosynthesis Systems (MES) use electricity and microorganisms to produce such energy carriers, such as hydrogen (H2) or methane (CH4), Figure 1a. Also known as Bioelectrochemical power-to-gas (BEP2G), MES combines CO2 sequestration, renewable energy carrier production, and energy storage. The BEP2G technology presents several advantages such as i) single-step CO2 conversion into CH4, so that no separate process is needed to generate H2 for the reduction of CO2, ii) bioelectrochemical reactions take place at ambient conditions, which translates into lower costs of materials and of energy consumption, and iii) higher CH4 yields can be achieved, when compared to conventional (electro)catalytic reactions.In this study, a continuous conversion of CO2 to CH4 in a laboratory scale membraneless Microbial Electrosynthesis System (MES) is achieved. The MES cell (Figure 1b) for CH4 production was built using Plexiglass plates assembled to create two compartments divided by two layers of electrically insulating Nylon cloth, with a total area of 96 cm2 (anode-cathode interface area). The cathode compartment consisted of a stainless steel mesh (current collector) and it was filled with 45 g of carbon felt while the anode consisted of a Ti/IrO2 mesh current collector and it was filled with 20 g of carbon felt. The cathode compartment was maintained at a constant temperature of 30°C and continuously supplied with CO2 and a solution of nutrients. The cathode compartment was initially inoculated with 20 mL of homogenized anaerobic sludge. To further increase CH4 production, nickel was (in-situ) electrodeposited on the cathode carbon felt.The introduction of Ni at 0.2 g L-1 into the influent exhibited a remarkable improvement in CO2 conversion. At an applied voltage of 2.8 V, the MES with the Ni-modified carbon-felt cathode accomplished a CH4 production rate of 2.3 L d-1 which was 30% higher than that of this system before Ni was introduced to the influent stream (1.6 L d-1), Figure 1c. Similar improvements were noted with other performance parameters, such as the Coulombic efficiency and the energy consumption, which considerably improved from 56% to 73% and 11.5 Wh LH 2 −1 to 7.4 Wh LH 2 −1, respectively. Furthermore, once dissolved Ni was removed from the influent solution, the performance remained unchanged suggesting its successful in-situ electrodeposition on carbon felt. Electrochemical characterization using EIS and cyclic voltammetry techniques confirmed Ni electrodeposition. These results demonstrated a great potential of using in-situ Ni electrodeposition to improve catalytic properties of the MES biocathode. Also, with respect to the energy storage capacity of the system, a progressive increase of the MES internal capacitance from 2.6 F to 3.7 F was observed, suggesting that electroactive biofilms can be used to develop bioelectrochemical supercapacitors.It can be concluded that bioelectrochemical conversion of CO2 to CH4 can be exploited as a potential technology for long-term (power-to-gas conversion) and short term (bio-capacitor) energy storage combined with CO2 sequestration. This approach could be seen as a pathway for MES integration with the existing industrial processes producing significant amounts of CO2 as well as with new technologies based on the biorefinery concept.Figure 1: a) The concept of Microbial Electrosynthesis System (MES) for CH4 production; b) schematic representation of a MES cell for CH4 production from CO2, and c) summary of the results obtained before and after the in situ electrodeposition of nickel at the cathode. Figure 1

Microbial electrosynthesis (MES) systems can convert CO2 to acetate and other value-added chemicals using electricity as the reducing power. Several electrochemically active redox mediators can enhance interfacial electron transport between bacteria and the electrode in MES systems. In this study, different redox mediators, such as neutral red (NR), 2-hydroxy-1,4-naphthoquinone (HNQ), and hydroquinone (HQ), were compared to facilitate an MES-based CO2 reduction reaction on the cathode. The mediators, NR and HNQ, improved acetate production from CO2 (165 mM and 161 mM, respectively) compared to the control (without a mediator = 149 mM), whereas HQ showed lower acetate production (115 mM). On the other hand, when mediators were used, the electron and carbon recovery efficiency decreased because of the presence of bioelectrochemical reduction pathways other than acetate production. Cyclic voltammetry of an MES with such mediators revealed CO2 reduction to acetate on the cathode surface. These results suggest that the addition of mediators to MES can improve CO2 conversion to acetate with further optimization in an operating strategy of electrosynthesis processes.

BackgroundAs outlined by the Intergovernmental Panel on Climate Change, we need to approach global net zero CO2 emissions by approximately 2050 to prevent warming beyond 1.5 °C and the associated environmental tipping points. Future microbial electrosynthesis (MES) systems could decrease net CO2 emissions by capturing it from industrial sources. MES is a process where electroactive microorganisms convert the carbon from CO2 and reduction power from a cathode into reduced organic compounds. However, no MES system has attained an efficiency compatible with a financially feasible scale-up. To improve MES efficiency, we need to consider the energetic constraints of extracellular electron uptake (EEU) from an electrode to cytoplasmic electron carriers like NAD+. In many microbes, EEU to the cytoplasm must pass through the respiratory quinone pool (Q-pool). However, electron transfer from the Q-pool to cytoplasmic NAD+ is thermodynamically unfavorable. Here, we model the thermodynamic barrier for Q-pool dependent EEU using the well-characterized bidirectional electron transfer pathway of Shewanella oneidensis, which has NADH dehydrogenases that are energetically coupled to proton-motive force (PMF), sodium-motive force (SMF), or uncoupled. We also tested our hypothesis that Q-pool dependent EEU to NAD+ is ion-motive force (IMF)-limited in S. oneidensis expressing butanediol dehydrogenase (Bdh), a heterologous NADH-dependent enzyme. We assessed membrane potential changes in S. oneidensis + Bdh on a cathode at the single-cell level pre to post injection with acetoin, the substrate of Bdh.ResultsWe modeled the Gibbs free energy change for electron transfer from respiratory quinones to NADH under conditions reflecting changes in membrane potential, pH, reactant to product ratio, and energetically coupled IMF. Of the 40 conditions modeled for each method of energetic coupling (PMF, SMF, and uncoupled), none were thermodynamically favorable without PMF or SMF. We also found that membrane potential decreased upon initiation of EEU to NAD+ for S. oneidensis on a cathode.ConclusionsOur results suggest that Q-pool-dependent EEU is both IMF-dependent and is IMF-limited in a proof-of-concept system. Because microbes that rely on Q-pool-dependent EEU are among the most genetically tractable and metabolically flexible options for MES systems, it is important that we account for this thermodynamic bottleneck in future MES platform designs.

A review of microbial electrosynthesis applied to carbon dioxide capture and conversion: The basic principles, electrode materials, and bioproducts

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.

Effect of different hydrogen evolution rates at cathode on bioelectrochemical reduction of CO2 to acetate

Microbial photo electrosynthesis for efficient CO2 conversion using MXenes: Materials, mechanisms, and applications

Chain elongation in continuous microbial electrosynthesis cells: The effect of pH and precursors supply

The present energy demand of the world is increasing but the fossil fuels are gradually depleting. As a result, the need for alternative fuels and energy sources is growing. Fuel cells could be one alternative to address the challenge. The fuel cell can convert CO2 to value-added chemicals. The potential of bio-fuel cells, specifically enzymatic fuel cells and microbial fuel cells, and the importance of immobilization technology in bio-fuel cells are highlighted. The review paper also includes a detailed explanation of the microbial electrosynthesis system to reduce CO2 and the value-added products during microbial electrosynthesis. Future research in bio-electrochemical synthesis for CO2 conversion is expected to prioritize enhancing biocatalyst efficiency, refining reactor design, exploring novel electrode materials, understanding microbial interactions, integrating renewable energy sources, and investigating electrochemical processes for carbon capture and selective CO2 reduction. The challenges and perspectives of bio-electrochemical systems in the application of CO2 conversion are also discussed. Overall, this review paper provides valuable insights into the latest developments and criteria for effective research and implementation in bio-fuel cells, immobilization technology, and microbial electro-synthesis systems.

Stoichiometric theory in aquatic carbon sequestration under elevated carbon dioxide