Impact analysis of operating conditions on carbon dioxide reduction in microbial electrosynthesis: Insight into the substance utilization and microbial response

Microbial electrosynthesis (MES) is facing a series of problems including low energy utilization and production efficiency of high value-added products, which seriously hinder its practical application. In this study, a more practical direct current power source was used and the anaerobic activated sludge from wastewater treatment plants was inoculated to construct the acetic acid-producing MES. The operating conditions of acetic acid production were further optimized and the specific mechanisms involving the substance utilization and microbial response were revealed. The optimum conditions were the potential of 3.0 V and pH 6.0. Under these conditions, highly electroactive biofilms formed and all kinds of substances were effectively utilized. In addition, dominant bacteria (Acetobacterium, Desulfovibrio, Sulfuricurvum, Sulfurospirillum, and Fusibacter) had high abundances. Under optimal conditions, acetic acid-forming characteristic genera (Acetobacterium) had the highest relative abundance (Biocathode-25.82 % and Suspension-17.24 %). This study provided references for the optimal operating conditions of MES and revealed the corresponding mechanisms.

Enhanced bio-electrochemical performance of microbially catalysed anode and cathode in a microbial electrosynthesis system

Most bio-electrochemical systems (BESs) use biotic/abiotic electrode combinations, with platinum-based abiotic electrodes being the most common. However, the non-renewability, cost, and poisonous nature of such electrode systems based on noble metals are major bottlenecks in BES commercialisation. Microbial electrosynthesis (MES), which is a sustainable energy platform that simultaneously treats wastewater and produces chemical commodities, also faces the same problem. In this study, a dual bio-catalysed MES system with a biotic anode and cathode (MES-D) was tested and compared with a biotic cathode/abiotic anode system (MES-S). Different bio-electrochemical tests revealed improved BES performance in MES-D, with a 3.9-fold improvement in current density compared to that of MES-S. Volatile fatty acid (VFA) generation also increased 3.2-, 4.1-, and 1.8-fold in MES-D compared with that in MES-S for acetate, propionate, and butyrate, respectively. The improved performance of MES-D could be attributed to the microbial metabolism at the bioanode, which generated additional electrons, as well as accumulative VFA production by both the bioanode and biocathode chambers. Microbial community analysis revealed the enrichment of electroactive bacteria such as Proteobacteria (60%), Bacteroidetes (67%), and Firmicutes + Proteobacteria + Bacteroidetes (75%) on the MES-S cathode and MES-D cathode and anode, respectively. These results signify the potential of combined bioanode/biocathode BESs such as MES for application in improving energy and chemical commodity production.

Microbial electrosynthesis: carbonaceous electrode materials for CO2 conversion

Microbial electrosynthesis (MES) is a sustainable approach to address greenhouse gas (GHG) emissions using anthropogenic carbon dioxide (CO₂) as a building block to create clean fuels and highly valuable chemicals. The efficiency of MES-based CO₂ conversion is closely related to the performance of electrode material and, in particular, the cathode for which carbonaceous materials are frequently used. Compared to expensive metal electrodes, carbonaceous materials are biocompatible with a high specific surface area, wide range of possible morphologies, and excellent chemical stability, and their use can maximize the growth of bacteria and enhance electron transfer rates. Examples include MES cathodes based on carbon nanotubes, graphene, graphene oxide, graphite, graphite felt, graphitic carbon nitride (g-C₃N₄), activated carbon, carbon felt, carbon dots, carbon fibers, carbon brushes, carbon cloth, reticulated vitreous carbon foam, MXenes, and biochar. Herein, we review the state-of-the-art MES, including thermodynamic and kinetic processes that underpin MES-based CO₂ conversion, as well as the impact of reactor type and configuration, selection of biocompatible electrolytes, product selectivity, and the use of novel methods for stimulating biomass accumulation. Specific emphasis is placed on carbonaceous electrode materials, their 3D bioprinting and surface features, and the use of waste-derived carbon or biochar as an outstanding material for further improving the environmental conditions of CO₂ conversion using carbon-hungry microbes and as a step toward the circular economy. MES would be an outstanding technique to develop rocket fuels and bioderived products using CO₂ in the atmosphere for the Mars mission.

Model development of bioelectrochemical systems: A critical review from the perspective of physiochemical principles and mathematical methods

Bioelectrochemical systems (BESs) are promising devices for wastewater treatment and bio-energy production. Since various processes are interacted and affect the overall performance of the device, the development of theoretical modeling is an efficient approach to understand the fundamental mechanisms that govern the performance of the BES. This review aims to summarize the physiochemical principle and mathematical method in BES models, which is of great importance for the establishment of an accurate model while has received little attention in previous reviews. In this review, we begin with a classification of existing models including bioelectrochemical models, electronic models, and machine learning models. Subsequently, physiochemical principles and mathematical methods in models are discussed from two aspects: one is the description of methodology how to build a framework for models, and the other is to further review additional methods that can enrich model functions. Finally, the advantages/disadvantages, extended applications, and perspectives of models are discussed. It is expected that this review can provide a viewpoint from methodologies to understand BES models.

Osmotically assisted reverse osmosis, simulated to achieve high solute concentrations, at low energy consumption

Microbial electrosynthesis (MES), is an emerging technology, for sustainable wastewater treatment. The dilute acetate solution, produced via MES, must be recovered, as dilute solutions can be expensive to store and transport. The acetate is expensive and environmentally damaging to recover by heat-intensive evaporative methods, such as distillation. In pursuit of a better energy economy, a membrane separation system is simulated to raise the concentration from 1 to 30 wt%, at a hydraulic pressure of approximately 50 bar. The concentrate is then simulated to be heat dried. Reverse osmosis (RO) could rase the acetate concentration to 8 wt%. A novel adaptation of osmotically assisted reverse osmosis (OARO) is then simulated to increase the concentration from 8 to 30 wt%. The inclusion of OARO, rather than a standalone RO unit, reduces the total heat and electric power requirement by a factor of 4.3. It adds to the membrane area requirement by a factor of 6. The OARO simulations are conducted by the internal concentration polarisation (ICP) model. Before the model is used, it is fitted to OARO experimental data, obtained from the literature. Membrane structure number of 701 µm and permeability coefficient of 2.51 L/m²/h/bar are ascertained from this model fitting exercise.

A General Model for Biofilm-Driven Microbial Electrosynthesis of Carboxylates From CO2

Up to now, computational modeling of microbial electrosynthesis (MES) has been underexplored, but is necessary to achieve breakthrough understanding of the process-limiting steps. Here, a general framework for modeling microbial kinetics in a MES reactor is presented. A thermodynamic approach is used to link microbial metabolism to the electrochemical reduction of an intracellular mediator, allowing to predict cellular growth and current consumption. The model accounts for CO₂ reduction to acetate, and further elongation to n-butyrate and n-caproate. Simulation results were compared with experimental data obtained from different sources and proved the model is able to successfully describe microbial kinetics (growth, chain elongation, and product inhibition) and reactor performance (current density, organics titer). The capacity of the model to simulate different system configurations is also shown. Model results suggest CO₂ dissolved concentration might be limiting existing MES systems, and highlight the importance of the delivery method utilized to supply it. Simulation results also indicate that for biofilm-driven reactors, continuous mode significantly enhances microbial growth and might allow denser biofilms to be formed and higher current densities to be achieved.

Microbial electrosynthesis: is it sustainable for bioproduction of acetic acid?

Microbial electrosynthesis (MES) is an innovative technology for electricity driven microbial reduction of carbon dioxide (CO2) to useful multi-carbon compounds. This study assesses the cradle-to-gate environmental burdens associated with acetic acid (AA) production via MES using graphene functionalized carbon felt cathode. The analysis shows that, though the environmental impact for the production of the functionalized cathode is substantially higher when compared to carbon felt with no modification, the improved productivity of the process helps in reducing the overall impact. It is also shown that, while energy used for extraction of AA is the key environmental hotspot, ion-exchange membrane and reactor medium (catholyte & anolyte) are other important contributors. A sensitivity analysis, describing four different scenarios, considering either continuous or fed-batch operation, is also described. Results show that even if MES productivity can be theoretically increased to match the highest space time yield reported for acetogenic bacteria in a continuous gas fermenter (148 g L-1 d-1), the environmental impact of AA produced using MES systems would still be significantly higher than that produced using a fossil-based process. Use of fed-batch operation and renewable (solar) energy sources do help in reducing the impact, however, the low production rates and overall high energy requirement makes large-scale implementation of such systems impractical. The analysis suggests a minimum threshold production rate of 4100 g m-2 d-1, that needs to be achieved, before MES could be seen as a sustainable alternative to fossil-based AA production.

Magnetite nanoparticle anchored graphene cathode enhances microbial electrosynthesis of polyhydroxybutyrate by Rhodopseudomonas palustris TIE-1

Microbial electrosynthesis (MES) is an emerging technology that can convert carbon dioxide (CO₂) into value-added organic carbon compounds using electrons supplied from a cathode. However, MES is affected by low product formation due to limited extracellular electron uptake by microbes. Herein, a novel cathode was developed from chemically synthesized magnetite nanoparticles and reduced graphene oxide nanocomposite (rGO-MNPs). This nanocomposite was electrochemically deposited on carbon felt (CF/rGO-MNPs), and the modified material was used as a cathode for MES production. The bioplastic, polyhydroxybutyrate (PHB) produced by Rhodopseudomonas palustris TIE-1 (TIE-1), was measured from reactors with modified and unmodified cathodes. Results demonstrate that the magnetite nanoparticle anchored graphene cathode (CF/rGO-MNPs) exhibited higher PHB production (91.31 ± 0.9 mg l^-1). This is ∼4.2 times higher than unmodified carbon felt (CF), and 20 times higher than previously reported using graphite. This modified cathode enhanced electron uptake to -11.7 ± 0.1 μA cm^-2, ∼5 times higher than CF cathode (-2.3 ± 0.08 μA cm^-2). The faradaic efficiency of the modified cathode was ∼2 times higher than the unmodified cathode. Electrochemical analysis and scanning electron microscopy suggest that rGO-MNPs facilitated electron uptake and improved PHB production by TIE-1. Overall, the nanocomposite (rGO-MNPs) cathode modification enhances MES efficiency.

A Comprehensive Modeling Analysis of Formate-Mediated Microbial Electrosynthesis*

Mediated microbial electrosynthesis (MES) represents a promising strategy for the capture and conversion of CO₂ into carbon-based products. We describe the development and application of a comprehensive multiphysics model to analyze a formate-mediated MES reactor. The model shows that this system can achieve a biomass productivity of ∼1.7 g L^-1  h^-1 but is limited by a competitive trade-off between O₂ gas/liquid mass transfer and CO₂ transport to the cathode. Synthetic metabolic strategies are evaluated for formatotrophic growth, which can enable an energy efficiency of ∼21 %, a 30 % improvement over the Calvin cycle. However, carbon utilization efficiency is only ∼10 % in the best cases due to a futile CO₂ cycle, so gas recycling will be necessary for greater efficiency. Finally, separating electrochemical and microbial processes into separate reactors enables a higher biomass productivity of ∼2.4 g L^-1  h^-1 . The mediated MES model and analysis presented here can guide process design for conversion of CO₂ into renewable chemical feedstocks.

Microbial Electrosynthesis: Where Do We Go from Here?

The valorization of CO₂ to valuable products via microbial electrosynthesis (MES) is a technology transcending the disciplines of microbiology, (electro)chemistry, and engineering, bringing opportunities and challenges. As the field looks to the future, further emphasis is expected to be placed on engineering efficient reactors for biocatalysts, to thrive and overcome factors which may be limiting performance. Meanwhile, ample opportunities exist to take the lessons learned in traditional and adjacent electrochemical fields to shortcut learning curves. As the technology transitions into the next decade, research into robust and adaptable biocatalysts will then be necessary as reactors shape into larger and more efficient configurations, as well as presenting more extreme temperature, salinity, and pressure conditions.