Effect of cationic and anionic solid polymer electrolyte on direct electrochemical reduction of gaseous CO2 to fuel

Advanced Catalyst Design and Reactor Configuration Upgrade in Electrochemical Carbon Dioxide Conversion

Electrochemical carbon dioxide reduction reaction (CO2 RR) driven by renewable energy shows great promise in mitigating and potentially reversing the devastating effects of anthropogenic climate change and environmental degradation. The simultaneous synthesis of energy-dense chemicals can meet global energy demand while decoupling emissions from economic growth. However, the development of CO2 RR technology faces challenges in catalyst discovery and device optimization that hinder their industrial implementation. In this contribution, a comprehensive overview of the current state of CO2 RR research is provided, starting with the background and motivation for this technology, followed by the fundamentals and evaluated metrics. Then the underlying design principles of electrocatalysts are discussed, emphasizing their structure-performance correlations and advanced electrochemical assembly cells that can increase CO2 RR selectivity and throughput. Finally, the review looks to the future and identifies opportunities for innovation in mechanism discovery, material screening strategies, and device assemblies to move toward a carbon-neutral society.

Use of Chitosan as Copper Binder in the Continuous Electrochemical Reduction of CO2 to Ethylene in Alkaline Medium

This work explores the potential of novel renewable materials in electrode fabrication for the electrochemical conversion of carbon dioxide (CO₂) to ethylene in alkaline media. In this regard, the use of the renewable chitosan (CS) biopolymer as ion-exchange binder of the copper (Cu) electrocatalyst nanoparticles (NPs) is compared with commercial anion-exchange binders Sustainion and Fumion on the fabrication of gas diffusion electrodes (GDEs) for the electrochemical reduction of carbon dioxide (CO₂R) in an alkaline medium. They were tested in membrane electrode assemblies (MEAs), where selectivity to ethylene (C₂H₄) increased when using the Cu:CS GDE compared to the Cu:Sustainion and Cu:Fumion GDEs, respectively, with a Faradaic efficiency (FE) of 93.7% at 10 mA cm^-2 and a cell potential of -1.9 V, with a C₂H₄ production rate of 420 µmol m^-2 s^-1 for the Cu:CS GDE. Upon increasing current density to 90 mA cm^-2, however, the production rate of the Cu:CS GDE rose to 509 µmol/m²s but the FE dropped to 69% due to increasing hydrogen evolution reaction (HER) competition. The control of mass transport limitations by tuning up the membrane overlayer properties in membrane coated electrodes (MCE) prepared by coating a CS-based membrane over the Cu:CS GDE enhanced its selectivity to C₂H₄ to a FE of 98% at 10 mA cm^-2 with negligible competing HER. The concentration of carbon monoxide was below the experimental detection limit irrespective of the current density, with no CO₂ crossover to the anodic compartment. This study suggests there may be potential in sustainable alernatives to fossil-based or perfluorinated materials in ion-exchange membrane and electrode fabrication, which constitute a step forward towards decarbonization in the circular economy perspective.

Low coordination number copper catalysts for electrochemical CO2 methanation in a membrane electrode assembly

The electrochemical conversion of CO2 to methane provides a means to store intermittent renewable electricity in the form of a carbon-neutral hydrocarbon fuel that benefits from an established global distribution network. The stability and selectivity of reported approaches reside below technoeconomic-related requirements. Membrane electrode assembly-based reactors offer a known path to stability; however, highly alkaline conditions on the cathode favour C-C coupling and multi-carbon products. In computational studies herein, we find that copper in a low coordination number favours methane even under highly alkaline conditions. Experimentally, we develop a carbon nanoparticle moderator strategy that confines a copper-complex catalyst when employed in a membrane electrode assembly. In-situ XAS measurements confirm that increased carbon nanoparticle loadings can reduce the metallic copper coordination number. At a copper coordination number of 4.2 we demonstrate a CO2-to-methane selectivity of 62%, a methane partial current density of 136 mA cm-2, and > 110 hours of stable operation.

Imidazolium functionalized polymers for effective electrochemical reduction of CO2

Imidazolium functionalized polymer electrolytes for the electrochemical reduction of gaseous CO2 (ERGC) were studied for the first time in a developed reactor at room temperature and atmospheric pressure. It was found that reaction environment favors the CO2 reduction reaction by overcoming the mass transfer of CO2 with the use of imidazolium fixed functional groups. The selectivity and Faradaic efficiency of products formed during ERGC is enhanced due to the modified functional groups in the solid polymer matrix. This work may open up new research opportunities for the conversion of gaseous CO2 to green fuels.

Electrocatalysis for CO2 conversion: from fundamentals to value-added products

This timely and comprehensive review mainly summarizes advances in heterogeneous electroreduction of CO₂: from fundamentals to value-added products.

Hydrophobic thiol coatings to facilitate a triphasic interface for carbon dioxide reduction to ethylene at gas diffusion electrodes

The electrochemical reduction of CO2 continues to see significant interest as a viable means of both producing important chemical materials and lowering carbon emissions. The primary challenge to making this process economically viable is the design of catalyst, electrode and reactor components that can selectively produce just one of the many possible CO2 reduction products. In this work, we report the use of hydrophobic 1-octadecanethiol coatings at copper coated gas diffusion electrodes to enhance the production of ethylene. This thiol coating gives a substantial increase in the production of ethylene at low current densities as well as a change in the rate determining step, as indicated by the substantial reduction in the Tafel slope. The observed changes to the CO2 reduction reaction indicate that the thiol layer provides a triphasic interface within the gas diffusion electrode catalyst layer.

An Analysis of Research on Membrane-Coated Electrodes in the 2001–2019 Period: Potential Application to CO2 Capture and Utilization

The chemistry and electrochemistry basic fields have been active for the last two decades of the past century studying how the modification of the electrodes’ surface by coating with conductive thin films enhances their electrocatalytic activity and sensitivity. In light of the development of alternative sustainable ways of energy storage and carbon dioxide conversion by electrochemical reduction, these research studies are starting to jump into the 21st century to more applied fields such as chemical engineering, energy and environmental science, and engineering. The huge amount of literature on experimental works dealing with the development of CO2 electroreduction processes addresses electrocatalyst development and reactor configurations. Membranes can help with understanding and controlling the mass transport limitations of current electrodes as well as leading to novel reactor designs. The present work makes use of a bibliometric analysis directed to the papers published in the 21st century on membrane-coated electrodes and electrocatalysts to enhance the electrochemical reactor performance and their potential in the urgent issue of carbon dioxide capture and utilization.

Pushing the activity of CO2 electroreduction by system engineering

As a promising technology that may solve global environmental challenges and enable intermittent renewable energy storage as well as zero-carbon-emission energy cycling, the carbon dioxide reduction reaction has been extensively studied in the past several years. Beyond the fruitful progresses and innovations in catalysts, the system engineering-based research on the full carbon dioxide reduction reaction is urgently needed toward the industrial application. In this review, we summarize and discuss recent works on the innovations in the reactor architectures and optimizations based on system engineering in carbon dioxide reduction reaction. Some challenges and future trends in this field are further discussed, especially on the system engineering factors.

Power to methanol technologies via CO2 recovery: CO2 hydrogenation and electrocatalytic routes

Various strategies are proposed to date in order to convert CO2 to large diversity of useful chemicals. The following review discusses two important approaches that produce methanol from CO2. These two includes CO2 hydrogenation and electrocatalytic routes. These processes could recycle CO2, permitting a carbon neutral, closed loop of fuel combustion and CO2 reduction to prevent a rising concentration of this greenhouse gas in the atmosphere. Besides, intermittent electricity generation can be stored in an energy-dense, portable form in chemical bonds. The present review reports more recent findings and drawbacks of these two processes. The present review study revealed that the hydrogenation process could become readily operational in comparison to electrocatalytic process. The electrocatalytic approach still has serious technical issues in terms of kinetically sluggish multi-electron transfer process during CO2 reduction reaction that requires excessive over-potential, relatively poor selectivity, poor durability in the long term, and the absence of the optimized standard experimental and commercial systems.

Electrochemical CO2 Reduction into Chemical Feedstocks: From Mechanistic Electrocatalysis Models to System Design

The electrochemical reduction of CO₂ is a promising route to convert intermittent renewable energy to storable fuels and valuable chemical feedstocks. To scale this technology for industrial implementation, a deepened understanding of how the CO₂ reduction reaction (CO₂ RR) proceeds will help converge on optimal operating parameters. Here, a techno-economic analysis is presented with the goal of identifying maximally profitable products and the performance targets that must be met to ensure economic viability-metrics that include current density, Faradaic efficiency, energy efficiency, and stability. The latest computational understanding of the CO₂ RR is discussed along with how this can contribute to the rational design of efficient, selective, and stable electrocatalysts. Catalyst materials are classified according to their selectivity for products of interest and their potential to achieve performance targets is assessed. The recent progress and opportunities in system design for CO₂ electroreduction are described. To conclude, the remaining technological challenges are highlighted, suggesting full-cell energy efficiency as a guiding performance metric for industrial impact.

Towards Higher Rate Electrochemical CO2 Conversion: From Liquid-Phase to Gas-Phase Systems

Electrochemical CO2 conversion offers a promising route for value-added products such as formate, carbon monoxide, and hydrocarbons. As a result of the highly required overpotential for CO2 reduction, researchers have extensively studied the development of catalyst materials in a typical H-type cell, utilizing a dissolved CO2 reactant in the liquid phase. However, the low CO2 solubility in an aqueous solution has critically limited productivity, thereby hindering its practical application. In efforts to realize commercially available CO2 conversion, gas-phase reactor systems have recently attracted considerable attention. Although the achieved performance to date reflects a high feasibility, further development is still required in order for a well-established technology. Accordingly, this review aims to promote the further study of gas-phase systems for CO2 reduction, by generally examining some previous approaches from liquid-phase to gas-phase systems. Finally, we outline major challenges, with significant lessons for practical CO2 conversion systems.

Promoter Effects of Functional Groups of Hydroxide-Conductive Membranes on Advanced CO2 Electroreduction to Formate

The electrochemical reduction of CO₂ at ambient conditions provides a latent solution of turning waste greenhouse gases into commodity chemicals or fuels; however, a satisfactory ion-conducting membrane for maximizing the performance of a CO₂ electrolyzer has not been developed. Here, we report the synthesis of a sequence of hydroxide-conductive polymer membranes, which are based on polymer composites of poly(vinyl alcohol)/Guar hydroxypropyltrimonium chloride, for use in CO₂ electrolysis. The effect of different membrane functional groups, including thiophene, hydroxybenzyl, and dimethyloctanal, on the efficiency and selectivity of CO₂ electroreduction to formate is thoroughly evaluated. The membrane incorporating thiophene groups exhibits the highest Faradaic efficiency of 71.5% at an applied potential of -1.64 V versus saturated calomel electrode (SCE) for formate. In comparison, membranes containing hydroxybenzyl and dimethyloctanal groups produced lower efficiencies of 67.6 and 68.6%, respectively, whereas the commercial Nafion 212 membrane was only 57.6% efficient. The improved efficiency and selectivity of membranes containing thiophene groups are attributed to a significantly increased hydroxide conductivity (0.105 S cm^-1), excellent physicochemical properties, and the simultaneous attenuation of formate product crossover.

Tailoring gas-phase CO2 electroreduction selectivity to hydrocarbons at Cu nanoparticles

Copper-based surfaces appear as the most active catalysts for CO₂ electroreduction to hydrocarbons, even though formation rates and efficiencies still need to be improved. The aim of the present work is to evaluate the continuous gas-phase CO₂ electroreduction to hydrocarbons (i.e. ethylene and methane) at copper nanoparticulated-based surfaces, paying attention to particle size influence (ranging from 25-80 nm) on reaction productivity, selectivity, and Faraday efficiency (FE) for CO₂ conversion. The effect of the current density and the presence of a microporous layer within the working electrode are then evaluated. Copper-based gas diffusion electrodes are prepared by airbrushing the catalytic ink onto carbon supports, which are then coupled to a cation exchange membrane (Nafion) in a membrane electrode assembly. The results show that the use of smaller copper nanoparticles (25 nm) leads to a higher ethylene production (1148 μmol m^-2 s^-1) with a remarkable high FE (92.8%), at the same time, diminishing the competitive hydrogen evolution reaction in terms of FE. This work demonstrates the importance of nanoparticle size on reaction selectivity, which may be of help to design enhanced electrocatalytic materials for CO₂ valorization to hydrocarbons.

Copper-Based Metal-Organic Porous Materials for CO2 Electrocatalytic Reduction to Alcohols

The electrocatalytic reduction of CO₂ has been investigated using four Cu-based metal-organic porous materials supported on gas diffusion electrodes, namely, (1) HKUST-1 metal-organic framework (MOF), [Cu₃ (μ₆ -C₉ H₃ O₆ )₂ ]_n ; (2) CuAdeAce MOF, [Cu₃ (μ₃ -C₅ H₄ N₅ )₂ ]_n ; (3) CuDTA mesoporous metal-organic aerogel (MOA), [Cu(μ-C₂ H₂ N₂ S₂ )]_n ; and (4) CuZnDTA MOA, [Cu_0.6 Zn_0.4 (μ-C₂ H₂ N₂ S₂ )]_n . The electrodes show relatively high surface areas, accessibilities, and exposure of the Cu catalytic centers as well as favorable electrocatalytic CO₂ reduction performance, that is, they have a high efficiency for the production of methanol and ethanol in the liquid phase. The maximum cumulative Faradaic efficiencies for CO₂ conversion at HKUST-1-, CuAdeAce-, CuDTA-, and CuZnDTA-based electrodes are 15.9, 1.2, 6, and 9.9 %, respectively, at a current density of 10 mA cm^-2 , an electrolyte-flow/area ratio of 3 mL min cm^-2 , and a gas-flow/area ratio of 20 mL min cm^-2 . We can correlate these observations with the structural features of the electrodes. Furthermore, HKUST-1- and CuZnDTA-based electrodes show stable electrocatalytic performance for 17 and 12 h, respectively.

Understanding Ammonium Transport in Bioelectrochemical Systems towards its Recovery

We report an integrated experimental and simulation study of ammonia recovery using microbial electrolysis cells (MECs). The transport of various species during the batch-mode operation of an MEC was examined experimentally and the results were used to validate the mathematical model for such an operation. It was found that, while the generated electrical current through the system tends to acidify (or basify) the anolyte (or catholyte), their effects are buffered by a cascade of chemical groups such as the NH₃/NH₄⁺ group, leading to relatively stable pH values in both anolyte and catholyte. The transport of NH₄⁺ ions accounts for ~90% of the total current, thus quantitatively confirming that the NH₄⁺ ions serve as effective proton shuttles during MEC operations. Analysis further indicated that, because of the Donnan equilibrium at cation exchange membrane-anolyte/catholyte interfaces, the Na⁺ ion in the anolyte actually facilitates the transport of NH₄⁺ ions during the early stage of a batch cycle and they compete with the NH₄⁺ ions weakly at later time. These insights, along with a new and simple method for predicting the strength of ammonia diffusion from the catholyte toward the anolyte, will help effective design and operation of bioeletrochemical system-based ammonia recovery systems.

Salen ligand complexes as electrocatalysts for direct electrochemical reduction of gaseous carbon dioxide to value added products

CO₂, being a linear and centrosymmetric molecule, is very stable, and the electrochemical reduction of CO₂ requires energy. However, the salen complexes are found to be very efficient to minimize overpotential as compared to their metal counterparts.

Emerging electrochemical energy conversion and storage technologies

Electrochemical cells and systems play a key role in a wide range of industry sectors. These devices are critical enabling technologies for renewable energy; energy management, conservation, and storage; pollution control/monitoring; and greenhouse gas reduction. A large number of electrochemical energy technologies have been developed in the past. These systems continue to be optimized in terms of cost, life time, and performance, leading to their continued expansion into existing and emerging market sectors. The more established technologies such as deep-cycle batteries and sensors are being joined by emerging technologies such as fuel cells, large format lithium-ion batteries, electrochemical reactors; ion transport membranes and supercapacitors. This growing demand (multi billion dollars) for electrochemical energy systems along with the increasing maturity of a number of technologies is having a significant effect on the global research and development effort which is increasing in both in size and depth. A number of new technologies, which will have substantial impact on the environment and the way we produce and utilize energy, are under development. This paper presents an overview of several emerging electrochemical energy technologies along with a discussion some of the key technical challenges.

Electrochemical conversion of CO2 to fuels: tuning of the reaction zone using suitable functional groups in a solid polymer electrolyte

The electrochemical reduction of gaseous CO₂ is studied for the first time using sterically hindered bulky quaternary ammonium ions in a solid polymer matrix at room temperature and atmospheric pressure in a developed electrochemical reactor.