Microbial Electrosynthesis System Research Articles

Microbial electrosynthesis of valuable chemicals from the reduction of CO2: a review.

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.

In-situ biogenic FeS boosted acetate accumulation through CO2 capture and valorization using microbial electrosynthesis (MES)

Biosynthetic strategy of ferrous sulfide (FeS) through Shewanella oneidensis MR-1 in bioelectrochemical systems has been extensively explored to maintain bettered electron transfer performance. In this study, the simultaneous FeS biosynthesis and CO2 reduction were conducted in the microbial electrosynthesis (MES) system, aiming at improving electron transfer efficiency as well as acetate production. It was shown that the FeS was successfully biomineralized and then loaded onto the sludge in the Fe3+/S2O32−/MR-1 group, of which the acetate accumulation increased by 87.50%, 64.63% and 33.66% than the control, MR-1 and Fe3+/S2O32− groups. Also, electrochemical performance revealed by the CV analysis was enhanced, whereas the increased EPS accumulation and intracellular electron transfer activity by the biosynthesized FeS could also be proved. Moreover, the biogenic FeS enriched the abundances of acetogenic and sulfur-utilizing species as Acetobacterium, Clostridium_sensu_stricto_13, Sulfuricurvum and Desulfovibrio. Meanwhile, strongly positive correlations between Shewanella, the sulfur utilizers and acetogens were confirmed in the Fe3+/S2O32−/MR-1 group. With improved electron transfer efficiency and the boosted acetate accumulation, the in-situ biogenic FeS might be promising for CO2 capture and utilization.

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

Microbial electrosynthesis (MES) enables the production of carbon-neutral chemicals using CO2 as a carbon source. Acetic acid is the main MES product, but recent studies show the direct production of elongated carboxylic acids, e.g., butyric and caproic acid. However, the production of elongated acids in MES systems is still inefficient due to the low growth rates of acetogenic bacteria and to limited solventogenic rates. Subsequently, researchers have produced elongated carboxylic acids directly from acetic acid or have operated MES systems at low pH to favor solventogenesis. However, the effect the addition of different chain elongation precursors and the operation pH exerts in the bioelectrochemical production of elongated acids remains unclear. To investigate this, three pH-controlled MES systems were operated in this study with continuous liquid and gas supply. MES systems elongating acetic acid at pH 6 achieved higher butyric (0.71 vs. 0.42 g L−1) and caproic acid (0.71 vs. 0.42 g L−1) titers in the absence of CO2 sparging. Additionally, lowering the pH to 5 in the MES systems fed with CO2 and acetic acid improved the elongated acids titers, reaching 0.72 g L−1 butyric and 0.33 g L−1 caproic acid. The 16 S rRNA analysis showed the community was dominated by Oscillibacter at pH 6, and by Clostridium at pH 5. Furthermore, the first scanning electron microscopy pictures revealing biofilm stratification in MES cathodes were taken in this study, where homogeneous rod-shaped bacteria biofilm layers, in contact with the graphite cathode, were covered by heterogeneous biofilm layers.

Shewanella oneidensis MR-1 coupled with biomass-derived carbon dots for promoted bioelectrochemical CO2 reduction: Mechanism elucidation of intensified energy metabolism

Bioelectrochemical CO2 reduction reaction (CO2RR) has been touted as one of the promising strategies to convert CO2 to high-value chemicals via the efficient microbial electrosynthesis (MES). However, the sluggish electron transfer efficiency between microbes and cathodes hampers the CO2 reduction efficiency. Herein, biomass-derived carbon dots (CDs) with high electrical conductivity were coupled with Shewanella oneidensis (S. oneidensis) MR-1 to construct a nanomaterial-bacterial biohybrid system to promote CO2-to-formate formation. Specifically, the corncob-derived CDs (cc-CDs) were highly-biocompatible with S. oneidensis MR-1, with such a constructed biohybrid system showing a marked formate titer of 0.50 mmol/L. Upon doping with Fe element, the resultant Fe-cc-CDs exhibited a considerably boosted formate titer of 1.22 mmol/L, which was 7.7 times higher than the pure S. oneidensis MR-1 system. Such significant CO2-to-formate conversion enhancement was contributed by the internalized CDs in S. oneidensis MR-1 stimulating extracellular electron transfer efficiency, therefore accelerating energy metabolism for formate bioelectrosynthesis. The intensified energy metabolism in S. oneidensis MR-1 was corroborated by the increased concentrations of ATP, NADH and overexpression of energy-related genes. This work demonstrates an intensified energy metabolism exemplification for CO2 reduction, which can be expanded to a host of different nanomaterials-internalized MES systems for bioelectrochemical implications.

Selective colonization of multifunctional microbes that facilitates caproate production in microbial electrosynthesis system

As promising and value-added platform chemicals, the biosynthesis of medium-chain fatty acids (MCFAs, C6-C12) via chain elongation (CE) in microbial electrosynthesis systems (MES) has recently garnered significant attentions due to the unique bioelectrocatalytic behaviors and the reduction of exogenous electron donors (EDs). Caproate-synthesizing bacteria (CSB) played a crucial role in the generation of acetyl-CoA, which subsequently underwent reverse β oxidation to synthesize MCFAs. However, the presence of electrochemically active bacteria (EAB) was also pivotal for MES as they generated hydrogen in the cathodic electron transfer pathways, the specific colonization patterns of key symbionts remained unknown. The aim of this study was to investigate the selective colonization of EAB and CSB on the performance of caproate production in MES. The reactors colonized EAB and CSB achieved the maximum caproate concentration of 5332.5 mg COD/L, which was 7.9 and 1.3 times higher compared to that inoculated with either sole EAB or CSB, respectively. Moreover, the reactors colonized with mixed culture exhibited a clear redox peak and preferable electron transfer efficiency. CSB and EAB mutually interacted in metabolic activities, thereby resulting in increased substrate utilization and caproate production. The findings of this study provide a new strategy and insights for value-added chemicals recovery in microbial electrosynthesis platform.

Establishing CRISPRi for Programmable Gene Repression and Genome Evolution in Cupriavidus necator.

Cupriavidus necator H16 is a "Knallgas" bacterium with the ability to utilize various carbon sources and has been employed as a versatile microbial cell factory to produce a wide range of value-added compounds. However, limited genome engineering, especially gene regulation methods, has constrained its full potential as a microbial production platform. The advent of CRISPR/Cas9 technology has shown promise in addressing this limitation. Here, we developed an optimized CRISPR interference (CRISPRi) system for gene repression in C. necator by expressing a codon-optimized deactivated Cas9 (dCas9) and appropriate single guide RNAs (sgRNAs). CRISPRi was proven to be a programmable and controllable tool and could successfully repress both exogenous and endogenous genes. As a case study, we decreased the accumulation of polyhydroxyalkanoate (PHB) via CRISPRi and rewired the carbon fluxes to the synthesis of lycopene. Additionally, by disturbing the expression of DNA mismatch repair gene mutS with CRISPRi, we established CRISPRi-Mutator for genome evolution, rapidly generating mutant strains with enhanced hydrogen peroxide tolerance and robustness in microbial electrosynthesis (MES) system. Our work provides an efficient CRISPRi toolkit for advanced genetic manipulation and optimization of C. necator cell factories for diverse biotechnology applications.

Integrated dark-fermentation-microbial electrosynthesis for efficient wastewater treatment and bioenergy recovery

High ammonia concentration in wastewater can hinder methane production rate in anaerobic digestion (AD)-microbial electrosynthesis systems (ADMES). To address this issue, a dual-chamber reactor was fabricated using an anion exchange membrane (AEM) to separate the dark-fermentation (DF) and ADMES process, preventing ammonia migration from the DF chamber to the ADMES chamber. As a result, the DF-ADMES achieved a high methane yield based on chemical oxygen demand (COD) of 0.35 L CH4/gCOD compared to control operation AD (0.23 L CH4/gCOD) and ADMES (0.30 L CH4/gCOD). Additionally, hydrogen could be recovered from the DF chamber which improved the energy efficiency of the DF-ADMES reactor (91.7 %) as compared to control AD (53.4 %) and ADMES (71.9 %). Thus, a dual-chamber DF-ADMES with an AEM separator could be a feasible design for scalable treatment of high nitrogen-containing wastewater and high bioenergy recovery.

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

Microbial electrosynthesis (MES) offers a promising approach for converting CO2 into valuable chemicals such as acetate. However, the relative low conversion rate severely limits its practical application. This study investigated the impact of different hydrogen evolution rates on the conversion rate of CO2 to acetate in the MES system. Three potentials (−0.8 V, −0.9 V and −1.0 V) corresponding to various hydrogen evolution rates were set and analyzed, revealing an optimal hydrogen evolution rate, yielding a maximum acetate formation rate of 1410.9 mg/L and 73.5 % coulomb efficiency. The electrochemical findings revealed that an optimal hydrogen evolution rate facilitated the formation of an electroactive biofilm. The microbial community of the cathode biofilm highlighted key genera, including Clostridium and Acetobacterium, which played essential roles in electrosynthesis within the MES system. Notably, a low hydrogen evolution rate failed to provide sufficient energy for the electrochemical reduction of CO2 to acetate, while a high rate led to cathode alkalinization, impeding the reaction and causing significant energy wastage. Therefore, maintaining an appropriate hydrogen evolution rate is crucial for the development of mature electroactive biofilms and achieving optimal performance in the MES system.

Biogas-Upgrading of Anaerobic Digestion Effluent CO2 into CH4 Using a Microbial Electrosynthesis Cell

Upgrading anaerobic digestion (AD) biogas has been highlighted to secure an alternative renewable energy source. The CO2 content of AD effluent is over 40%, thus it is necessary to separate and/or increase CH4 content up to 95%. This study examines a microbial electrosynthesis (MES) to directly convert CO2 into CH4 by cathode attached cell. The MES with −1.0V (vs. Ag/AgCl) applied cathodic potential presented a maximum rate of 10.55L CH4/ m2 cat/ day and achieve 96% of final CH4 content. The next-generation sequencing indicates the most methanogens attached to the cathode surface rather than suspension. These results suggest that cathode attached cells play a major role in the biogas-upgrading of CO2 into CH4 in the MES system.

Exploring Acetogenesis in Firmicutes: From Phylogenetic Analysis to Solid Medium Cultivation with Solid-Phase Electrochemical Isolation Equipments.

This study introduces a groundbreaking approach for the exploration and utilization of electrotrophic acetogens, essential for advancing microbial electrosynthesis systems (MES). Our initial focus was the development of Solid-Phase Electrochemical Isolation Equipment (SPECIEs), a novel cultivation method for isolating electrotrophic acetogens directly from environmental samples on a solid medium. SPECIEs uses electrotrophy as a selection pressure, successfully overcoming the traditional cultivation method limitations and enabling the cultivation of diverse microbial communities with enhanced specificity towards acetogens. Following the establishment of SPECIEs, we conducted a genome-based phylogenetic analysis using the Genome Taxonomy Database (GTDB) to identify potential electrotrophic acetogens within the Firmicutes phylum and its related lineages. Subsequently, we validated the electrotrophic capabilities of selected strains under electrode-oxidizing conditions in a liquid medium. This sequential approach, integrating innovative cultivation techniques with detailed phylogenetic analysis, paves the way for further advances in microbial cultivation and the identification of new biocatalysts for sustainable energy applications.

Conducting polymer as potential retrofitting material for gas diffusion electrode to enhance microbial electrosynthesis: State-of-the-art review

Microbial electrosynthesis (MES) have been proven effective at reducing carbon dioxide (CO2) and synthesizing valuable organic commodities with the aid of electrical energy. The development of highly productive MES is challenging due to low bacterial loading, low electron transfer rate, and low solubility of CO2, which can decrease the production of relevant chemicals and further limit the future potential of upscaling. Many innovations have been established to upscale the system including the application of gas diffusion electrodes (GDEs) in a three-chambered MES system. To date, two types of commercially available GDEs have been employed in MES: polytetrafluoroethylene (PTFE) and carbon-based GDEs. The process of bacterial adhesion on the electrolyte-facing side of the GDE is influenced by material surface properties, such as surface charge, wettability, roughness, and area. Thus, a suitable material is required to modify the aforementioned GDE surfaces. Recently, researchers have been keen on modifying bio-electrodes with conducting polymers in microbial fuel cells and MES as they show fascinating outcomes. Moreover, modifying GDEs using conducting polymers (CPs) is well-established in fuel cells but highly lacking in MES. Several modification strategies can be adopted in MES, such as the microporous layer (MPL) coating, CP MPL, and CP-based MPL. Last, the present review features possible modifications of carbon-based GDE using CPs and its challenges.

Cathode materials in microbial electrosynthesis systems for carbon dioxide reduction: recent progress and perspectives

Microbial electrosynthesis (MES) is an emerging technology that enables the synthesis of value-added chemicals from carbon dioxide (CO2) or inorganic carbon compounds by coupling renewable electricity to microbial metabolism. However, MES still faces challenges in achieving high production of value-added chemicals due to the limited extracellular electron transfer efficiency at the biotic-abiotic interfaces. To overcome this bottleneck, it is crucial to develop novel cathodes and modified materials. This review systematically summarizes recent advancements in cathode materials in the field of electrocatalyst-assisted and photocatalyst-assisted MES. The effects of various material types are further investigated by comparing metal-free and metal materials and photocatalyst materials of different semiconductor types. Additionally, the review introduces the maximum production rate of value-added chemicals and conversion efficiency achieved by these cathode materials while highlighting the advantages and disadvantages of different material types. To the best of our knowledge, in electrocatalyst-assisted systems, the maximum CH4 yield on graphene aerogel/polypyrrole cathode achieved 1,672 mmol m-2 d-1, and the maximum Faraday efficiency (FE) of CH4 reached up to 97.5% on graphite plate. Meanwhile, the maximum acetate yield achieved 1,330 g m-2 d-1 with CO2 conversion efficiency into acetate close to 100% on carbon nanotube cathodes. In photocatalyst-assisted systems, the maximum acetate yield could reach 0.51 g L-1 d-1 with the coulombic efficiency of 96% on the MnFe2O4/g-C3N4 photocathode. Finally, prospects for future development and practical applications of MES are discussed, offering theoretical guidance for the fabrication of cathode materials that can improve production efficiency and reduce energy input.

Microbially catalyzed anode and cathode microbial electrosynthesis system for efficient metformin removal and volatile fatty acid production

The removal of different pharmaceuticals and personal care products from surface water is crucial. This study focused on the removal and transformation of metformin (MTF), an emerging contaminant in aqueous solutions using a dual biocatalyzed microbial electrosynthesis system (MES). Successful biodegradation of MTF (91 %) was achieved within 120 h with improved bioelectrochemical performance. The current density (−849 mA/m2) with drug loading at 0.5 ppm was 10.3, 7.6, and 2.4 times higher than that of the control, 0.1 ppm, and 0.3, ppm, respectively. Volatile fatty acids (VFAs) production also improved with excellent acetate, propionate, and butyrate production at dual biocatalyzed MES. Liquid chromatography mass spectrometry studies indicated improved mineralization with more MTF bioproducts. MTF regulated the microbial flora through enrichment of the electroactive phyla Proteobacteria and Bacteroidetes. This study provides a new perspective for the use of dual biocatalyzed microbial electrosynthesis systems in bioremediation research.

Autoinducer-2 quorum sensing regulates biofilm formation and chain elongation metabolic pathways to enhance caproate synthesis in microbial electrochemical system

Quorum sensing (QS) have been explored extensively. However, most studies focused on N-acyl homoserine lactones (AHLs) participating in intraspecies QS. In this study, autoinducer-2 (AI-2, participating in interspecies QS) with different concentration was investigated for chain elongation in microbial electrosynthesis (MES). The results demonstrated that the R3 treatment, which involved adding 10 μM of 4,5-dihydroxy-2,3-pentanedione (DPD) in the reactor, exhibited the best performance. The concentration of caproate was increased by 66.88% and the redox activity of cathodic electroactive biofilms (EABs) was enhanced. Meanwhile, microbial community data indicated that Negativicutes relative abundance was increased obviously in R3 treatment. In this study, the transcriptome Kyoto Encyclopedia of Genes and Genomes (KEGG) and Gene Ontology (GO) databases were used to analyze the metabolic pathway of chain elongation involving fatty acid biosynthesis (FAB) pathway and reverse β-oxidization (RBO) pathway. KEGG analysis revealed that fatty acid elongation metabolism (p < 0.001), tryptophan metabolism (p < 0.01), arginine and proline metabolism (p < 0.05) were significantly improved in R3 treatment. GO analysis suggested that R3 treatment mainly upregulated significantly transmembrane signaling receptor activity (p < 0.01), oxidoreductase activity (p < 0.05), and phosphorelay signal transduction (p < 0.05). Moreover, metatranscriptomic analyses also showed that R3 treatment could upregulate the LuxP extracellular receptor, LuxO transcriptional activator, LsrB periplasmic protein, and were beneficial to both FAB and RBO pathways. These findings provided a new insight into chain elongation in MES system.

H2 mediated mixed culture microbial electrosynthesis for high titer acetate production from CO2

Microbial electrosynthesis (MES) converts CO2 into value-added products such as volatile fatty acids (VFAs) with minimal energy use, but low production titer has limited scale-up and commercialization. Mediated electron transfer via H2 on the MES cathode has shown a higher conversion rate than the direct biofilm-based approach, as it is tunable via cathode potential control and accelerates electrosynthesis from CO2. Here we report high acetate titers can be achieved via improved in situ H2 supply by nickel foam decorated carbon felt cathode in mixed community MES systems. Acetate concentration of 12.5 g L−1 was observed in 14 days with nickel-carbon cathode at a poised potential of −0.89 V (vs. standard hydrogen electrode, SHE), which was much higher than cathodes using stainless steel (5.2 g L−1) or carbon felt alone (1.7 g L−1) with the same projected surface area. A higher acetate concentration of 16.0 g L−1 in the cathode was achieved over long-term operation for 32 days, but crossover was observed in batch operation, as additional acetate (5.8 g L−1) was also found in the abiotic anode chamber. We observed the low Faradaic efficiencies in acetate production, attributed to partial H2 utilization for electrosynthesis. The selective acetate production with high titer demonstrated in this study shows the H2-mediated electron transfer with common cathode materials carries good promise in MES development.

Microbially catalyzed enhanced bioelectrochemical performance using covalent organic framework–modified cathode in a microbial electrosynthesis system

Electrode modification plays a critical role in enhancing the bioelectrochemical performance of a microbial electrosynthesis system (MES). This study involved the modification of the conventional carbon felt (CF) electrode through in situ covalent grafting with a covalent organic framework (TpPa-COF). Subsequently, the performance of this modified electrode was assessed as a cathode in MES. Various physical and bioelectrochemical techniques, such as chronoamperometry, cyclic voltammetry, and electrochemical spectroscopy, demonstrated the remarkable stability, reduced electrode resistance, increased current density, and superior bioelectrochemical activity of the modified electrode. The application of COF@CF caused a 3.2-fold improvement in current density, leading to an enhanced production of volatile fatty acids. The rough surface of the COF@CF electrode and its abundant catalytically active sites facilitated the growth of microorganisms, particularly exoelectrogenic and fermentative genera such as Desulfitobacterium, Clostridium, and Desulfovibrio. These findings highlight the promising potential of COF@CF in various bioelectrochemical applications.

Facial Synthesis, Stability, and Interaction of Ti3C2Tx@PC Composites for High-Performance Biocathode Microbial Electrosynthesis Systems.

Developing high-performance biocathodes remain one of the most challenging aspects of the microbial electrosynthesis (MES) system and the primary factor limiting its output. Herein, a hollow porous carbon (PC) fabricated with MXenes coated over an electrode was developed for MES systems to facilitate the direct delivery of CO2 to microorganisms colonized. The result highlighted that MXene@PC (Ti3C2Tx@PC) has a surface area of 434 m2/g. The Ti3C2Tx@PC MES cycle shows that in cycle 4 and cycle 5, the values are -309.2 and -352.3. Cyclic voltammetry showed that the coated electrode current response (mA) increased from -4.5 to -20.2. The substantial redox peaks of Ti3C2Tx@PC biofilms are displayed at -741, -516, and -427 mV vs Ag/AgCl, suggesting an enhanced electron transfer owing to the Ti3C2Tx@PC complex coating. Additionally, more active sites enhanced mass transfer and microbial development, resulting in a 46% rise in butyrate compared to the uncoated control. These findings demonstrate the value of PC modification as a method for MES-based product selection.

Visible light-driven dual photoelectrode microbial electrosynthesis using BiVO4-RuO2-IrO2 on Ti mesh photoanode and ZIF-67/g-C3N4 on carbon felt photocathode for the efficient reduction of CO2 into acetate

Microbial electrosynthesis (MES) systems integrated with solar energy (photoelectrode-assisted MES) represent an attractive method for the fixation of CO2 through microbial electrochemical processes. In this study, BiVO4-RuO2-IrO2/Ti with long-term stability and corrosion resistance was used as photoanode through the electrophoretic deposition of BiVO4 on RuO2-IrO2/Ti mesh, significantly reducing the charge recombination rate and enhancing the oxygen evolution reaction (OER) ability. Simultaneously, the ZIF-67/g-C3N4 photocathode can significantly increase CO2 absorption and hydrogen production by enhancing electron–hole separation ability under visible light. Consequently, the MES system with the BiVO4-RuO2-IrO2/Ti anode and ZIF-67/g-C3N4 photocathode obtained an acetate yield of 0.46 g/L/d at −0.9 V versus Ag/AgCl within 14 days. This value was 4.6 times that under dark conditions. The solar-to-acetate conversion efficiency was determined to be 1.52%. Furthermore, the MES with dual photoelectrode still obtained an acetate yield of 0.09 g/L/d at −0.6 V versus Ag/AgCl. These results showed that the MES with the BiVO4-RuO2-IrO2/Ti photoanode and ZIF-67/g-C3N4 photocathode reduced OER overpotential, increased electron transport rate, and improved CO2 supply for the simultaneous supplementation of substrates and electrons in MES. This work provides a new strategy for constructing efficient photoelectrodes in MES to achieve highly efficient chemical generation for CO2 transformation under solar light.

Role of microbial electrosynthesis system in CO2 capture and conversion: a recent advancement toward cathode development.

Microbial electrosynthesis (MES) is an emerging electrochemical technology currently being researched as a CO2 sequestration method to address climate change. MES can convert CO2 from pollution or waste materials into various carbon compounds with low energy requirements using electrogenic microbes as biocatalysts. However, the critical component in this technology, the cathode, still needs to perform more effectively than other conventional CO2 reduction methods because of poor selectivity, complex metabolism pathways of microbes, and high material cost. These characteristics lead to the weak interactions of microbes and cathode electrocatalytic activities. These approaches range from cathode modification using conventional engineering approaches to new fabrication methods. Aside from cathode development, the operating procedure also plays a critical function and strategy to optimize electrosynthesis production in reducing operating costs, such as hybridization and integration of MES. If this technology could be realized, it would offer a new way to utilize excess CO2 from industries and generate profitable commodities in the future to replace fossil fuel-derived products. In recent years, several potential approaches have been tested and studied to boost the capabilities of CO2-reducing bio-cathodes regarding surface morphology, current density, and biocompatibility, which would be further elaborated. This compilation aims to showcase that the achievements of MES have significantly improved and the future direction this is going with some recommendations. Highlights - MES approach in carbon sequestration using the biotic component.- The role of microbes as biocatalysts in MES and their metabolic pathways are discussed.- Methods and materials used to modify biocathode for enhancing CO2 reduction are presented.

Highly efficient and photostable conversion of bicarbonate to acetate by a pyrrolic nitrogen enriched Ag3PO4/g-C3N4 photocathode with an intermediate band level in photo-assisted microbial electrosynthesis systems

A highly efficient and photostable acetate production from bicarbonate (HCO3−) reduction is demonstrated in a photo-assisted microbial electrosynthesis system (MES) using a pyrrolic nitrogen enriched Ag3PO4/g-C3N4 photocathode. Urea-treatment of the Ag3PO4/g-C3N4 photocathode synergistically improved the MES performance by up to 1.8 folds under visible light illumination, achieving acetate production of 9.6 mM/d with coulombic efficiency of 95%, negligible catalyst leaching, and solar-to-acetate efficiency of 1.15% over 12 days continuous operation. The photocorrosion of Ag3PO4 was efficiently inhibited by a two-step charge transfer process mediated by an intermediate band level introduced by oxygen-vacancies in the Ag3PO4 structure, which also facilitated light absorption and charge transfer through a Z-scheme mechanism, whereas the increased pyrrole N in the g-C3N4 provided supplementary active sites for direct and indirect (via H2) electron transfer to Serratia marcescens electrotroph. This study demonstrates the ingenious use of photocatalytic heterojunctions for achieving step-changes in the MES performance.