MgO/CuO Modified Stainless Steel Mesh Cathode for Enhanced CO 2 Reduction to Biomethane in Microbial Electrosynthesis System

Microbial electrosynthesis (MES) systems aim to use electroactive microorganisms (EAMs) to achieve electricity-driven CO2 fixation for biosynthesis of multicarbon chemicals. However, the low efficiencies of extracellular electron transfer (EET) and CO2 assimilation of EAMs remain the essential limiting factors that restrict performance of MES systems. Herein, we developed an electrosynthetic biohybrid system to synergistically supply electrons and CO2 to Rhodopseudomonas palustris (an EAM) for lycopene biosynthesis. Intracellular carbon and energy fluxes were redirected by strengthening the lycopene biosynthesis pathway and blocking the nitrogen-fixation pathway, resulting in 23-fold increase in lycopene yield than that of the wild-type R. palustris. To enhance extracellular transfer of CO2 and electrons to R. palustris, metal-organic frameworks (MOFs) with high CO2 adsorption capacity were assembled with polydopamine on cell membrane to construct a biohybrid MES system, which produced 3.55mg/L lycopene in two consecutive MES cycles, the highest lycopene production from CO2. Electrochemical and transcriptomic analyses revealed that the biohybrid MES system stimulated microbial metabolism including EET, the Calvin-Benson-Bassham cycle and lycopene biosynthesis, thereby improving CO2-to-chemical conversion. This study demonstrated directional supply of electrons and CO2 to EAMs enabled high-performance MES systems, which also offered insights into the mechanisms underlying efficient CO2 fixation and carbon-negative biomanufacturing.

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

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

Sustainable vision toward development of microbial electrosynthesis for diverse resource recovery: Circular economy

Efficient production of lycopene from CO2 via microbial electrosynthesis

Microbial electrosynthesis (MES) systems aim to use electroactive microorganisms (EAMs) to achieve electricity‐driven CO 2 fixation for biosynthesis of multicarbon chemicals. However, the low efficiencies of extracellular electron transfer (EET) and CO 2 assimilation of EAMs remain the essential limiting factors that restrict performance of MES systems. Herein, we developed an electrosynthetic biohybrid system to synergistically supply electrons and CO 2 to Rhodopseudomonas palustris (an EAM) for lycopene biosynthesis. Intracellular carbon and energy fluxes were redirected by strengthening the lycopene biosynthesis pathway and blocking the nitrogen‐fixation pathway, resulting in 23‐fold increase in lycopene yield than that of the wild‐type R. palustris . To enhance extracellular transfer of CO 2 and electrons to R. palustris , metal‐organic frameworks (MOFs) with high CO 2 adsorption capacity were assembled with polydopamine on cell membrane to construct a biohybrid MES system, which produced 3.55 mg/L lycopene in two consecutive MES cycles, the highest lycopene production from CO 2 . Electrochemical and transcriptomic analyses revealed that the biohybrid MES system stimulated microbial metabolism including EET, the Calvin‐Benson‐Bassham cycle and lycopene biosynthesis, thereby improving CO 2 ‐to‐chemical conversion. This study demonstrated directional supply of electrons and CO 2 to EAMs enabled high‐performance MES systems, which also offered insights into the mechanisms underlying efficient CO 2 fixation and carbon‐negative biomanufacturing.

Nano-magnetite effects on CO2 reduction of homoacetogens with different extracellular electron transfer pathways

Anaerobic digestion (AD) is a widely used technique to treat organic waste and produce biogas. This article presents a practical approach to increase biogas yield of an AD system using a microbial electrosynthesis system (MES). The biocathode in MES reduces carbon dioxide with the supplied electrons and protons (H+) to form methane. We demonstrate that the MES is able to produce biogas with over 90% methane when fed with reject water obtained from a local wastewater treatment plant. The optimised cathode potential was observed in the range of −0.70 V to −0.60 V and optimised feed pH was around 7.0. With autoclaved feed, these conditions allowed methane yields of about 9.05 mmol/L(reactor)-day. A control experiment was then carried out to make a comparison between open circuit and MES methanogenesis. The highest methane yield of about 22.1 mmol/L(reactor)-day was obtained during MES operation that performed 10–15% better than the open circuit mode of operation. We suggest and describe an integrated AD-MES system, by installing MES in the reject water loop, as a novel approach to improve the efficiency and productivity of existing waste/wastewater treatment plants.

The cathode material is one of the key factors in enhancing the overall performance of microbial electrosynthesis system (MES). Nickel-based materials are the best option for cathodes in MES due to their excellent catalytic activity. This study aims to evaluate the performance of nickel foam (NF) as self-cathode material in MES for acetate production from CO2. A biocatalyst at the cathode was provided using a mixed-culture of anaerobic sludge from palm oil mill effluent (POME). The field emission scanning electron microscopy (FE-SEM) was used to analyze the cathode surface morphology, while high-performance liquid chromatography (HPLC) was used to quantify the volatile fatty acids (VFAs) in the effluent. The results indicate that the self-cathode NF exhibited excellent performance, achieving an acetate production rate (QAcetate) of 46.0 mM/d, compared to 41.7 mM/d with a graphite felt (GF) cathode at a cathode potential of -0.8 V. Additionally, the self-cathode NF in the MES system demonstrated a coulombic efficiency (CE) of approximately 22.9%. Moreover, the type of cathode material and the microbial community attached to the cathode surface significantly influenced MES 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

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.

Sodium limitation stimulates Acetobacterium woodii enrichment and boosts CO2 conversion to acetate in self-controlled microbial electrosynthesis systems.

Direct CO2 delivery with hollow stainless steel/graphene foam electrode for enhanced methane production in microbial electrosynthesis

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.

Synergetic interaction of magnetic field and loaded magnetite for enhanced acetate production in biocathode of microbial electrosynthesis system