Carbon-based products are essential to society, yet producing them from fossil fuels is unsustainable. Microorganisms have the ability to take up electrons from solid electrodes and convert carbon dioxide (CO₂) to valuable carbon-based chemicals. However, higher productivities and energy efficiencies are needed to reach a viability that can make the technology transformative. Here, we show how a biofilm-based microbial porous cathode in a directed flow-through electrochemical system can continuously reduce CO₂ to even-chain C2–C6 carboxylic acids over 248 days. We demonstrate a threefold higher biofilm concentration, volumetric current density, and productivity compared with the state of the art. Most notably, the volumetric productivity (VP) resembles those achieved in laboratory-scale and industrial syngas (CO-H₂-CO₂) fermentation and chain elongation fermentation. This work highlights key design parameters for efficient electricity-driven microbial CO₂ reduction. There is need and room to improve the rates of electrode colonization and microbe-specific kinetics to scale up the technology.

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The 27th edition of the European Fuel Cell Forum with a focus on Low Temperature Electrolyzers, Fuel Cells, and H2 Processing saw the return to the normal in-person conference modus. With the hitherto highest number of participants of the low-temperature conference branch, an excellent line-up of oral and poster presentations, and an overall positive vibe in conversations, the conference asserted the standing of the EFCF as the prime forum for scientific-technical exchanges on electrochemical hydrogen technologies in Europe. [...]@en

Soil moisture monitoring is essential for a variety of applications including agriculture, forestry, and environmental monitoring. However, soil moisture sensors may be expensive and require batteries or other energy sources, making them unsuitable for remote or off-grid locations and farmers. Improper e-waste management of short-lived sensing components can reveal the contradictions of solutions aimed at environmental sustainability, which also degrade environmental health. Therefore, the development of low-cost, off-grid, biodegradable in-situ soil moisture sensing system (SMSS) is necessary for these regions. This article provides an overview of the current state-of-the-art in low-cost, off-grid, and biodegradable in-situ soil moisture sensing. It highlights low-cost SMSS components including hardware (microcontrollers and communication modules), software, and off-grid ambient energy sources. It also highlights the current research in biodegradable polymers used for moisture sensing. The challenges in combining low-cost, off-grid, and biodegradable soil moisture sensing are identified as a research gap. Finally, the underlining question of the “perfect” choice of SMSS is explored based on the trade-offs of performance, operational feasibility, and the newly proposed aspect of biodegradability, consequently suggesting context-specific decisions by consciously managing these tradeoffs.

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Microbial electrochemical technologies (METs) employ microorganisms utilizing solid-state electrodes as either electron sink or electron source, such as in microbial electrosynthesis (MES). METs reaction rate is traditionally normalized to the electrode dimensions or to the electrolyte volume, but should also be normalized to biomass amount present in the system at any given time. In biofilm-based systems, a major challenge is to determine the biomass amount in a non-destructive manner, especially in systems operated in continuous mode and using 3D electrodes. We developed a simple method using a nitrogen balance and optical density to determine the amount of microorganisms in biofilm and in suspension at any given time. For four MES reactors converting CO₂ to carboxylates, >99% of the biomass was present as biofilm after 69 days of reactor operation. After a lag phase, the biomass-specific growth rate had increased to 0.12–0.16 days⁻¹. After 100 days of operation, growth became insignificant. Biomass-specific production rates of carboxylates varied between 0.08–0.37 mol_C mol_X⁻¹d⁻¹. Using biomass-specific rates, one can more effectively assess the performance of MES, identify its limitations, and compare it to other fermentation technologies.

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In microbial electrosynthesis (MES), microorganisms grow on a cathode electrode as a biofilm, or in the catholyte as planktonic biomass, and utilize CO₂for their growth and metabolism. Modification of the cathode with metals can improve MES performance, due to their catalytic activity for H2 production, which can be consumed by microorganisms, or via modifying the cathode properties. On the other hand, metals can have an inhibiting effect on MES. While these single roles of metals and their oxides have been identified, an investigation of the simultaneous effects on MES is still lacking. Here, we modify activated carbon (AC) electrodes with nickel (Ni) at high (5%) and low (0.01%) loadings, to investigate its combined effects on MES. Upon Ni impregnation, multiple factors explained the MES performance, including electrocatalytic H2 production, trace element availability, metal toxicity, Ni leaching and redeposition/bio-crystalization. Instead, the electrode surface properties (i.e., surface area and pore structure) were not affected by Ni addition. Compared to unmodified AC, low Ni loading did not improve abiotic H2 production, whereas at high Ni loading a 6-fold increase was observed. During biological experiments, low Ni loading resulted in over a 3-fold increase of acetate production and 35% higher planktonic growth, compared to unmodified AC. Instead, high Ni loading resulted in 25-fold increase of acetate production, 21% decrease of planktonic growth, and improved biofilm growth. Unmodified AC, and low and high Ni loading each resulted in unique microbial community composition. The effect of Ni on MES is therefore concentration-dependent, with apparently different mechanisms of interaction being prevalent at low or high Ni loadings.

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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.

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Electrocatalytic metals and microorganisms can be combined for CO2 conversion in microbial electrosynthesis (MES). However, a systematic investigation on the nature of interactions between metals and MES is still lacking. To investigate this nature, we integrated a copper electrocatalyst, converting CO2 to formate, with microorganisms, converting CO2 to acetate. A co-catalytic (i. e. metabolic) relationship was evident, as up to 140 mg L-1 of formate was produced solely by copper oxide, while formate was also evidently produced by copper and consumed by microorganisms producing acetate. Due to non-metabolic interactions, current density decreased by over 4 times, though acetate yield increased by 3.3 times. Despite the antimicrobial role of copper, biofilm formation was possible on a pure copper surface. Overall, we show for the first time that a CO2 -reducing copper electrocatalyst can be combined with MES under biological conditions, resulting in metabolic and non-metabolic interactions.

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Carbon dioxide (CO₂) can be converted to valuable products using different catalysts, including metal or biological catalysts (e. g. microorganisms). Some products formed by metal electrocatalysts can be further utilized by microorganisms, and therefore catalytic cooperation can be envisioned. To prevent cumbersome separations, it is beneficial when both catalyst work under the same conditions, or at least in the same reaction medium. Here, we will show that a formate-producing copper electrocatalyst can function in a biological medium. Furthermore, we will show that the effluent of the copper-containing reactor can be used without purification as the sole medium for a bio-reactor, inoculated with a mixed culture of microorganisms. In that second reactor, formate, H₂ and CO₂ are consumed by the microorganisms, forming acetate and methane. Compared to simple buffer electrolyte, catalytic activity of copper was improved in the presence of microbial growth medium, likely due to EDTA (Ethylenediaminetetraacetic acid) present in the latter.

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Microbial electrosynthesis (MES) allows carbon-waste and renewable electricity valorization into industrially-relevant chemicals. MES has received much attention in laboratory-scale research, although a techno-economic-driven roadmap towards validation and large-scale demonstration of the technology is lacking. In this work, two main integrated systems were modelled, centered on (1) MES-from-CO₂ and (2) MES from short-chain carboxylates, both for the production of pure, or mixture of, acetate, n–butyrate, and n–caproate. Twenty eight key parameters were identified, and their impact on techno-economic feasibility of the systems assessed. The main capital and operating costs were found to be the anode material cost (59%) and the electricity consumption (up to 69%), respectively. Under current state-of-the-art MES performance and economic conditions, these systems were found non-viable. However, it was demonstrated that sole improvement of MES performance, independent of improvement of non-technological parameters, would result in profitability. In otherwise state-of-the-art conditions, an improved electron selectivity (≥36%) towards n-caproate, especially at the expense of acetate, was showed to result in positive net present values (i.e. profitability; NPV). Cell voltage, faradaic efficiency, and current density also have significant impact on both the capital and operating costs. Variation in electricity cost on overall process feasibility was also investigated, with a cost lower than 0.045 € kWh⁻¹ resulting in positive NPV of the state-of-the-art scenario. Maximum purification costs were also determined to assess the integration of a product's separation unit, which was showed possible at positive NPV. Finally, we briefly discuss CO₂ electroreduction versus MES, and their potential market complementarities.

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CONSPECTUS:Carbon-based products are crucial to our society,but their production from fossil-based carbon is unsustainable.Production pathways based on the reuse of CO2will achieve ultimatesustainability. Furthermore, the costs of renewable electricityproduction are decreasing at such a high rate, that electricity isexpected to be the main energy carrier from 2040 onward. Electricity-driven novel processes that convert CO2into chemicals need to befurther developed. Microbial electrosynthesis is a biocathode-drivenprocess in which electroactive microorganisms derive electrons fromsolid-state electrodes to catalyze the reduction of CO2or organics andgenerate valuable extracellular multicarbon reduced products. Micro-organisms can be tuned to high-rate and selective product formation.Optimization and upscaling of microbial electrosynthesis to practical,real life applications is dependent upon performance improvementwhile maintaining low cost. Extensive biofilm development, enhanced electron transfer rate from solid-state electrodes tomicroorganisms and increased chemical production rate require optimized microbial consortia, efficient reactor designs, andimproved cathode materials.This Account is about the development of different electrode materials purposely designed for improved microbial electrosynthesis:NanoWeb-RVC and EPD-3D. Both types of electrodes are biocompatible, highly conductive three-dimensional hierarchical porousstructures. Both chemical vapor deposition (CVD) and electrophoretic deposition were used to grow homogeneous and uniformcarbon nanotube layers on the honeycomb structure of reticulated vitreous carbon. The high surface area to volume ratio of theseelectrodes maximizes the available surface area for biofilm development, i.e., enabling an increased catalyst loading. Simultaneously,the nanostructure makes it possible for a continuous electroactive biofilm to be formed, with increased electron transfer rate and highCoulombic efficiencies. Fully autotrophic biofilms from mixed cultures developed on both types of electrodes rely on CO2as the solecarbon source and the solid-state electrode as the unique energy supply.We presentfirst the synthesis and characteristics of the bare electrodes. We then report the outstanding performance indicators ofthese novel biocathodes: current densities up to−200 A m−2and acetate production rates up to 1330 g m−2day−1, with electron andCO2recoveries into acetate being very close to 100% for mature biofilms. The performance indicators are still among the highestreported by either purposely designed or commercially available biocathodes. Finally, we made use of the titration and off-gasanalysis sensor (TOGA) to elucidate the electron transfer mechanism in these efficient biocathodes. Planktonic cells in the catholytewere found irrelevant for acetate production. We identified the electron transfer to be mediated by biologically induced H2.H2is notdetected in the headspace of the reactors, unless CO2feeding is interrupted or the cathodes sterilized. Thus, the biofilm is extremelyefficient in consuming the generated H2. Finally, we successfully demonstrated the use of a synthetic biogas mixture as a CO2source.We thus proved the potential of microbial electrosynthesis for the simultaneous upgrading of biogas, whilefixating CO2via theproduction of acetate.@en

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.

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The selective and high-rate microbial electrosynthesis of a valuable molecule is favoured from an economic perspective. We demonstrate here that increasing selectivity towards n-butyrate and n-caproate over acetate, while maintaining high production rate and electron recovery, is achievable by adjusting the CO2 feeding strategy and hydraulic retention time. We show that high CO2 loading rate (173 L d−1) and long hydraulic retention time (14 days) triggers bioelectrochemical chain elongation to n-butyrate and n-caproate
(53.6 ± 4.1%C), whereas lower CO2 loading rate (8.6 L d−1) does not, even with increasing HRT (96.4 ± 2.4%C into acetate). Moreover, temporarily detaching and re-attaching the biofilm allowed to obtain high-rate n-caproate production of 2.0 ± 0.1 g L−1 d−1 (max 3.1 g L−1), while producing 3.3 ± 0.2 g L−1 d−1n-butyrate (max 9.3 g L−1), at an increased 63.6 ± 5.0%C into both. This was achieved with 69.8 ± 2.8% total electron recovery at −100.8 Am−2.@en