Electrochemical Reduction of Carbon Dioxide to Value‐Added Products: The Electrocatalyst and Microbial Electrosynthesis

C-H bond cyanation: electrochemical synthesis of phenylbenzimidoyl cyanide derivatives

The application of electricity in chemical processes represents a sustainable technology for the future. This green activation mode derives from renewable energy sources (such as solar, wind, and hydropower), safeguarding resources by being less polluting and utilizing less materials. C-H bond functionalisation is one of the most powerful synthetic methods for forging molecular complexity to access valuable chemicals in a single step transformation. Herein, an electrochemical C-H bond cyanation of imine derivatives under electrochemical reaction conditions has been developed. This is a new, simple, fast and non-toxic way for the direct cyanation of imine derivatives. Acetonitrile was found to be a new and effective cyanation reagent under catalyst-free electrochemical conditions. The cyanation protocol can be applied to diverse substrates including substituted and unsubstituted imine derivatives. The electrochemical method has been carried out in an undivided cell at constant current at 0 °C for 1 h using a Carbon rod as cathode and a magnesium plate as anode.

Potential and challenge in accelerating high-value conversion of CO2 in microbial electrosynthesis system via data-driven approach

Microbial electrosynthesis for CO2 utilization (MESCU) producing valuable chemicals with high energy density has garnered attention due to its long-term stability and high coulombic efficiency. The data-driven approaches offer a promising avenue by leveraging existing data to uncover the underlying patterns. This comprehensive review firstly uncovered the potentials of utilizing data-driven approaches to enhance high-value conversion of CO2 via MESCU. Firstly, critical challenges of MESCU advancing have been identified, including reactor configuration, cathode design, and microbial analysis. Subsequently, the potential of data-driven approaches to tackle the corresponding challenges, encompassing the identification of pivotal parameters governing reactor setup and cathode design, alongside the decipheration of omics data derived from microbial communities, have been discussed. Correspondingly, the future direction of data-driven approaches in assisting the application of MESCU has been addressed. This review offers guidance and theoretical support for future data-driven applications to accelerate MESCU research and potential industrialization.

Improved electrochemical reduction of CO2 to syngas with a highly exfoliated Ti3C2Tx MXene-gold composite

Transforming carbon dioxide (CO2) into valuable chemicals via electroreduction presents a sustainable and viable approach to mitigating excess CO2 in the atmosphere. This report provides fresh insights into the design of a new titanium-based MXene composite as a catalyst for the efficient conversion of CO2 in a safe aqueous medium. Despite its excellent electrocatalytic activity towards CO2 reduction and high selectivity for CO production, the high cost of Au and the decline in catalytic activity on a larger scale hinder its large-scale CO2 conversion applications. In this research, we have successfully prepared an Au/Ti3C2Tx composite and tested its catalytic activity in the electrochemical CO2 reduction reaction (ECRR). The as-prepared composite features strong interactions between gold atoms and the MXene support, achieved through the formation of metal-oxygen/carbon bonds. The Au/Ti3C2Tx electrode demonstrated a significant current density of 17.3 mA cm-2 at a potential of -0.42 V vs. RHE, in a CO2 saturated atmosphere (faradaic efficiency: CO = 48.3% and H2 = 25.6%). Nyquist plots further indicated a reduction in the charge-transfer resistance of the Au/Ti3C2Tx layer, signifying rapid charge transfer between the Au and Ti3C2Tx. Furthermore, it is known that liquid crossover through the Gas Diffusion Electrode (GDE) significantly improves CO2 diffusion to catalyst active sites, thereby enhancing CO2 conversion efficiency. The goal of this work is to design an interface between metal and MXene so that CO2 can be electroreduced to fuels and other useful chemical compounds with great selectivity.

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.

Electrocatalytic Reactions for Converting CO2 to Value-Added Products: Recent Progress and Emerging Trends

Carbon dioxide (CO₂) emissions are an important environmental issue that causes greenhouse and climate change effects on the earth. Nowadays, CO₂ has various conversion methods to be a potential carbon resource, such as photocatalytic, electrocatalytic, and photo-electrocatalytic. CO₂ conversion into value-added products has many advantages, including facile control of the reaction rate by adjusting the applied voltage and minimal environmental pollution. The development of efficient electrocatalysts and improving their viability with appropriate reactor designs is essential for the commercialization of this environmentally friendly method. In addition, microbial electrosynthesis which utilizes an electroactive bio-film electrode as a catalyst can be considered as another option to reduce CO₂. This review highlights the methods which can contribute to the increase in efficiency of carbon dioxide reduction (CO₂R) processes through electrode structure with the introduction of various electrolytes such as ionic liquid, sulfate, and bicarbonate electrolytes, with the control of pH and with the control of the operating pressure and temperature of the electrolyzer. It also presents the research status, a fundamental understanding of carbon dioxide reduction reaction (CO₂RR) mechanisms, the development of electrochemical CO₂R technologies, and challenges and opportunities for future research.

Enhanced carbon dioxide biomethanation with hydrogen using anaerobic granular sludge and metal-organic frameworks: Microbial community response and energy metabolism analysis

In this work, metal-organic frameworks (MOFs) were prepared to evaluate its impact on carbon dioxide (CO₂) biomethanization during anaerobic degradation (AD). The results showed that MOFs significantly improved the CO₂ biomethanation efficiency, especially in the AD reactors using a concentration of 1.0 g/L MOFs. Furthermore, MOFs promoted direct interspecific electron transfer and alleviated the hydrogen competition of bacteria. Meanwhile, hydrogenotrophic methanogens were enriched in the AD reactors with MOFs. After the addition of MOFs, there was 3.28 times and 3.41 times increase in the abundance of metabolic functions related to methanogenesis by CO₂ reduction with hydrogen and dark hydrogen oxidation, respectively. There was an increased abundance of all genes that encode the key enzymes used in methane metabolism. However, functional genes involved in nitrate reduction had their expressions inhibited. The work may offer a contribution to helping the industry achieve the carbon capture and utilization policy.

The Route from Green H2 Production through Bioethanol Reforming to CO2 Catalytic Conversion: A Review

Currently, a progressively different approach to the generation of power and the production of fuels for the automotive sector as well as for domestic applications is being taken. As a result, research on the feasibility of applying renewable energy sources to the present energy scenario has been progressively growing, aiming to reduce greenhouse gas emissions. Following more than one approach, the integration of renewables mainly involves the utilization of biomass-derived raw material and the combination of power generated via clean sources with conventional power generation systems. The aim of this review article is to provide a satisfactory overview of the most recent progress in the catalysis of hydrogen production through sustainable reforming and CO2 utilization. In particular, attention is focused on the route that, starting from bioethanol reforming for H2 production, leads to the use of the produced CO2 for different purposes and by means of different catalytic processes, passing through the water–gas shift stage. The newest approaches reported in the literature are reviewed, showing that it is possible to successfully produce “green” and sustainable hydrogen, which can represent a power storage technology, and its utilization is a strategy for the integration of renewables into the power generation scenario. Moreover, this hydrogen may be used for CO2 catalytic conversion to hydrocarbons, thus giving CO2 added value.

Recent Advances in Synthesis and Applications of Single-Atom Catalysts for Rechargeable Batteries

The rapid development of flexible and wearable optoelectronic devices, demanding the superior, reliable, and ultra-long cycling energy storage systems. But poor performances of electrode materials used in energy devices are main obstacles. Recently, single-atom catalysts (SACs) are considered as emerging and potential candidates as electrode materials for battery devices. Herein, we have discussed the recent methods for the fabrication of SACs for rechargeable metal-air batteries, metal-CO₂ batteries, metal-sulfur batteries, and other batteries, following the recent advances in assembling and performance of these batteries by using SACs as electrode materials. The role of SACs to solve the bottle-neck problems of these energy storage devices and future perspectives are also discussed.

Strategies for Improved Electrochemical CO2 Reduction to Value-added Products by Highly Anticipated Copper-based Nanoarchitectures

Uncontrolled CO₂ emission from various industrial and domestic sources is a considerable threat to environmental sustainability. Scientists are trying to develop multiple approaches to not only reduce CO₂ emissions but also utilize this potent pollutant to get economically feasible products. The electrochemical reduction of CO₂ (ERC) is one way to effectively convert CO₂ to more useful products (ranging from C1 to C5). Nevertheless, this process is kinetically hindered and less selective towards a specific product and, consequently, requires an efficient electrocatalyst with characteristics like selectivity, stability, reusability, low cost, and environmentally benign. Owing to specified commercial features, copper (Cu)-based materials are highly anticipated and widely investigated for the last two decades. However, their non-modified polycrystalline Cu forms usually lack selectivity and lower overpotential of CO₂ reduction. Therefore, extensive research is in progress to induce various alterations ranging from morphological and surface chemistry tuning to structural and optoelectrical characteristics modifications. This review provides an overview of those strategies to improve the CO₂ conversion efficiency through Cu-based ERC into valuable C1, C2_, and higher molecular weight hydrocarbons. The thermodynamics and kinetics of CO₂ reduction via Cu-based electrocatalysts are discussed in detail with the support of the first principle DFT-based models. In the last portion of the review, the reported mechanisms for various products are summarized, with a short overview of the outlook. This review is expected to provide important basics as well as advanced information for experienced as well as new researchers to develop various strategies for Cu and related materials to achieve improved ERC.

Purposely Designed Hierarchical Porous Electrodes for High Rate Microbial Electrosynthesis of Acetate from Carbon Dioxide

Carbon-based products are crucial to our society, but their production from fossil-based carbon is unsustainable. Production pathways based on the reuse of CO₂ will achieve ultimate sustainability. Furthermore, the costs of renewable electricity production are decreasing at such a high rate, that electricity is expected to be the main energy carrier from 2040 onward. Electricity-driven novel processes that convert CO₂ into chemicals need to be further developed. Microbial electrosynthesis is a biocathode-driven process in which electroactive microorganisms derive electrons from solid-state electrodes to catalyze the reduction of CO₂ or organics and generate valuable extracellular multicarbon reduced products. Microorganisms 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 improvement while maintaining low cost. Extensive biofilm development, enhanced electron transfer rate from solid-state electrodes to microorganisms and increased chemical production rate require optimized microbial consortia, efficient reactor designs, and improved 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 porous structures. Both chemical vapor deposition (CVD) and electrophoretic deposition were used to grow homogeneous and uniform carbon nanotube layers on the honeycomb structure of reticulated vitreous carbon. The high surface area to volume ratio of these electrodes 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 high Coulombic efficiencies. Fully autotrophic biofilms from mixed cultures developed on both types of electrodes rely on CO₂ as the sole carbon source and the solid-state electrode as the unique energy supply. We present first the synthesis and characteristics of the bare electrodes. We then report the outstanding performance indicators of these novel biocathodes: current densities up to -200 A m^-2 and acetate production rates up to 1330 g m^-2 day^-1, with electron and CO₂ recoveries into acetate being very close to 100% for mature biofilms. The performance indicators are still among the highest reported by either purposely designed or commercially available biocathodes. Finally, we made use of the titration and off-gas analysis sensor (TOGA) to elucidate the electron transfer mechanism in these efficient biocathodes. Planktonic cells in the catholyte were found irrelevant for acetate production. We identified the electron transfer to be mediated by biologically induced H₂. H₂ is not detected in the headspace of the reactors, unless CO₂ feeding is interrupted or the cathodes sterilized. Thus, the biofilm is extremely efficient in consuming the generated H₂. Finally, we successfully demonstrated the use of a synthetic biogas mixture as a CO₂ source. We thus proved the potential of microbial electrosynthesis for the simultaneous upgrading of biogas, while fixating CO₂ via the production of acetate.

Multiphase Methods in Organic Electrosynthesis

With water providing a highly favored solution environment for industrial processes (and in biological processes), it is interesting to develop water-based electrolysis processes for the synthesis and conversion of organic and biomass-based molecules. Molecules with low solubility in aqueous media can be dispersed/solubilized (i) by physical dispersion tools (e.g., milling, power ultrasound, or high-shear ultraturrax processing), (ii) in some cases by pressurization/supersaturation (e.g., for gases), (iii) by adding cosolvents or "carriers" such as chremophor EL, or (iv) by adding surfactants to generate micelles, microemulsions, and/or stabilized biphasic conditions. This Account examines and compares methodologies to bring the dispersed or multiphase system into contact with an electrode. Both the microscopic process based on individual particle impact and the overall electro-organic transformation are of interest. Distinct mechanistic cases for multiphase redox processes are considered. Most traditional electro-organic transformations are performed in homogeneous solution with reagents, products, electrolyte, and possibly mediators or redox catalysts all in the same (usually organic) solution phase. This may lead to challenges in the product separation step and in the reuse of solvents and electrolytes. When aqueous electrolyte media are used, reagents and products (or even the electrolyte) may be present as microdroplets or nanoparticles. Redox transformations then occur during interfacial "collisions" under multiphase conditions or within a reaction layer when a redox mediator is present. Benefits of this approach can be (i) the use of a highly conducting aqueous electrolyte, (ii) simple separation of products and reuse of the electrolyte, (iii) phase-transfer conditions in redox catalysis, (iv) new reaction pathways, and (v) improved sustainability. In some cases, a surface phase or phase boundary processes can lead to interesting changes in reaction pathways. Controlling the reaction zone within the multiphase redox system poses a challenge, and methods based on microchannel flow reactors have been developed to provide a higher degree of control. However, detrimental effects in microchannel systems are also observed, in particular for limited current densities (which can be very low in microchannel multiphase flow) or in the development of technical solutions for scale-up of multiphase redox transformations. This Account describes physical approaches (and reactor designs) to bring multiphase redox systems into effective contact with the electrode surface as well as cases of important electro-organic multiphase transformations. Mechanistic cases considered are "impacts" by microdroplets or particles at the electrode, effects of dissolved intermediates or redox mediators, and effects of dissolved redox catalysts. These mechanistic cases are discussed for important multiphase transformations for gaseous, liquid, and solid dispersed phases. Processes based on mesoporous membranes and hydrogen-permeable palladium membranes are discussed.

Zinc: A promising material for electrocatalyst-assisted microbial electrosynthesis of carboxylic acids from carbon dioxide

Microbial electrosynthesis (MES) has been proposed as a sustainable platform to simultaneously achieve wastewater treatment, renewable energy generation and chemicals production. Currently, the CO₂ valorization via MES is restricted by the low production rate, while that via electrochemical reduction is limited by the production of C1 products with high efficiency and selectivity. The electrocatalyst-assisted MES could potentially solve these bottlenecks of both MES and electrochemical reduction technology by increasing the production rate and expanding the product range. Here, four types of metals were evaluated for mixed culture-based, electrocatalyst-assisted MES with the fabrication of electrical-biological hybrid cathodes. Cathodes based on In, Zn, Ti and Cu showed high parallelism at 30 A/m². However, no parallelism was observed at 50 A/m², and only Zn experienced a further increase of the maximum acetic acid production rate (1.23 ± 0.02 g/L/d, 313 ± 5 g/m²/d) and titer (9.2 ± 0.1 g/L), with the highest value of the production rate normalized to the project area of the fiber cathodes. Other volatile fatty acids and ethanol were below 0.5 g/L. Moreover, it was the sharp H₂ generation, which mainly caused the fluctuation of coulombic efficiency. The application of such Zn-based electrical-biological hybrid system shall provide a more efficient route for CO₂ valorization.