Perovskite-Based Multifunctional Cathode with Simultaneous Supplementation of Substrates and Electrons for Enhanced Microbial Electrosynthesis of Organics

Enhanced microbial electrosynthesis performance with 3-D algal electrodes under high CO2 sparging: Superior biofilm stability and biocathode-plankton interactions

Microbial electrosynthesis (MES) shows great promise for converting CO2 into high-value chemicals. However, cathode biofilm erosion by high CO2 sparging and the unclear role of plankton in MES hinders the continuous improvement of its performance. This study aims to enhance biofilm resistance and improve interactions between bio-cathode and plankton by upgrading waste algal biomass into 3-D porous algal electrode (PAE) with rough surface. Results showed that the acetate synthesis of PAE under 20 mL/min CO2 sparging (PAE-20) was up to 3330.61 mol/m3, 4.63 times that of carbon felt under the same conditions (CF-20). The microbial loading of PAE-20 biofilm was twice that of CF-20. Furthermore, higher cumulative abundance of functional microorganisms was observed in plankton of PAE-20 (55 %), compared to plankton of CF-20 (14 %), and enhanced biocathode-plankton interactions significantly suppressed acetate consumption. Thus, this efficient and sustainable 3-D electrode advances MES technology and offers new perspectives for waste biomass recycling.

Electrocatalytic Properties of Quasi-2D Oxides LaSrMn0.5M0.5O4 (M = Co, Ni, Cu, and Zn) for Hydrogen and Oxygen Evolution Reactions

Designing cost-effective and highly efficient electrocatalysts for water splitting is a significant challenge. We have systematically investigated a series of quasi-2D oxides, LaSrMn0.5M0.5O4 (M = Co, Ni, Cu, Zn), to enhance the electrocatalytic properties of the two half-reactions of water-splitting, namely oxygen and hydrogen evolution reactions (OER and HER). The four materials are isostructural, as confirmed by Rietveld refinements with X-ray diffraction. The oxygen contents and metal valence states were determined by iodometric titrations and X-ray photoelectron spectroscopy. Electrical conductivity measurements in a wide range of temperatures revealed semiconducting behavior for all four materials. Electrocatalytic properties were studied for both half-reactions of water-splitting, namely, oxygen-evolution and hydrogen-evolution reactions (OER and HER). For the four materials, the trends in both OER and HER were the same, which also matched the trend in electrical conductivities. Among them, LaSrMn0.5Co0.5O4 showed the best bifunctional electrocatalytic activity for both OER and HER, which may be attributed to its higher electrical conductivity and favorable electron configuration.

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.

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

Large emissions of atmospheric carbon dioxide (CO2) are causing climatic and environmental problems. It is crucial to capture and utilize the excess CO2 through diverse methods, among which the microbial electrosynthesis (MES) system has become an attractive and promising technology to mitigate greenhouse effects while reducing CO2 to high-value chemicals. However, the biological conversion and metabolic pathways through microbial catalysis have not been clearly elucidated. This review first introduces the main acetogenic bacteria for CO2 reduction and extracellular electron transfer mechanisms in MES. It then intensively analyzes the CO2 bioconversion pathways and carbon chain elongation processes in MES, together with energy supply and utilization. The factors affecting MES performance, including physical, chemical, and biological aspects, are summarized, and the strategies to promote and regulate bioconversion in MES are explored. Finally, challenges and perspectives concerning microbial electrochemical carbon sequestration are proposed, and suggestions for future research are also provided. This review provides theoretical foundation and technical support for further development and industrial application of MES for CO2 reduction.

Enhancement of acetate production in hydrogen-mediated microbial electrosynthesis reactors by addition of silica nanoparticles

Microbial electrosynthesis (MES) is a promising technology for CO2 fixation and electrical energy storage. Currently, the low current density of MES limits its practical application. The H2-mediated and non-biofilm-driven MES could work under higher current density, but it is difficult to achieve high coulombic efficiency (CE) due to low H2 solubility and poor mass transfer. Here, we proposed to enhance the hydrogen mass transfer by adding silica nanoparticles to the reactor. At pH 7, 35 ℃ and 39 A·m- 2 current density, with the addition of 0.3wt% silica nanoparticles, the volumetric mass transfer coefficient (kLa) of H2 in the reactor increased by 32.4% (from 0.37 h- 1 to 0.49 h- 1), thereby increasing the acetate production rate and CE of the reactor by 69.8% and 69.2%, respectively. The titer of acetate in the reactor with silica nanoparticles (18.5 g·L- 1) was 56.9% higher than that of the reactor without silica nanoparticles (11.8 g·L- 1). Moreover, the average acetate production rate of the reactor with silica nanoparticles was up to 2.14 g·L- 1·d- 1 in the stable increment phase, which was much higher than the other reported reactors. These results demonstrated that the addition of silica nanoparticles is an effective approach to enhancing the performance of H2-mediated MES reactors.

CuO/g-C3N4 heterojunction photocathode enhances the microbial electrosynthesis of acetate through CO2 reduction

Microbial electrosynthesis (MES) is a novel CO₂ utilization technology. Biocatalysts in this process may use electrons obtained from a photovoltaic system to reduce CO₂ to chemicals and realize energy conversion from solar energy to chemical energy. The photoelectric material CuO/g-C₃N₄ was directly introduced into the MES system using mixed culture as biocatalyst in this study. CuO/g-C₃N₄ can effectively absorb light and presents satisfactory electron and hole separation ability. Photogenerated electrons from CuO/g-C₃N₄ enhanced the electron transfer rate and reduced cathodic charge transfer resistance. CuO/g-C₃N₄ mainly improved the electron supply of electroautotrophic microorganisms through direct electron transfer rather than indirect electron transfer via hydrogen. Photogenerated holes can combine electrons from anode and provide extra driving force to improve the MES performance. Furthermore, the CuO/g-C₃N₄ photocathode also improved the biocatalytic activity by increasing the total amount of biocatalyst and regulating cathodic microbial community composition. Acetate production rate in MES with the CuO/g-C₃N₄ photocathode was 2.6 times higher than that of the control group. This study provides a new strategy for semiconductor photocathodes to improve the MES performance with mixed culture.

Cathodic biofilms - A prerequisite for microbial electrosynthesis

Cathodic biofilms have an important role in CO₂ bio-reduction to carboxylic acids and biofuels in microbial electrosynthesis (MES) cells. However, robust and resilient electroactive biofilms for an efficient CO₂ conversion are difficult to achieve. In this review, the fundamentals of cathodic biofilm formation, including energy conservation, electron transfer and development of catalytic biofilms, are presented. In addition, strategies for improving cathodic biofilm formation, such as the selection of electrode and carrier materials, cell design and operational conditions, are described. The knowledge gaps are individuated, and possible solutions are proposed to achieve stable and productive biofilms in MES cathodes.

Recent developments and key barriers to microbial CO2 electrobiorefinery

The electrochemical conversion of CO₂ can include renewable surplus electricity storage and CO₂ utilisation. This review focuses on the microbial CO₂ electrobiorefinery based on microbial electrosynthesis (MES) which merges electrochemical and microbial conversion to produce biofuels and higher-value chemicals. In this review, recent developments are discussed about bioelectrochemical conversion of CO₂ into biofuels and chemicals in MES via microbial CO₂-fixation and electricity utilisation reactions. In addition, this review examines technical approaches to overcome the current limitations of MES including the following: engineering of the biocathode, application of electron mediators, and reactor optimisation, among others. An in-depth discussion of strategies for the CO₂ electrobiorefinery is presented, including the integration of the biocathode with inorganic catalysts, screening of novel electroactive microorganisms, and metabolic engineering to improve target productivity from CO₂.