The effect of electrolyte composition on the electroreduction of CO2 to CO on Ag based gas diffusion electrodes

Carbon Flow in Acidic CO2 Electroreduction

Electrochemical CO2 reduction in acidic media attracts extensive research attention due to its potential in increasing carbon efficiency. In most reports, alkali cations are introduced to suppress hydrogen evolution and to promote CO2 reduction. However, the mass transport of alkali cations through cation exchange membrane induces the change of electrolyte compositions. Herein, the variation of electrolyte compositions and the flow of carbon during CO2 reduction are analyzed quantitatively by simulation and experiments. If the initial amount of alkali cations in the anolyte is higher than the initial amount of H+ in the catholyte, the pH of the catholyte increases remarkably in long-term CO2 reduction electrolysis, resulting in the decrease of carbon efficiency. Bicarbonate salt precipitation on the cathode with alkali cation-containing catholyte is another origin of the decrease of CO2 reduction Faradaic efficiency and carbon efficiency. To maintain high carbon efficiency, the electrolyte should contain low concentration of alkali cations or even be free of alkali cations. Decorating the catalyst of cathode with ionomer with high density of cation sites enables CO2 reduction in pure acid solution, achieving 30-h stable carbon efficiency.

Syngas Production Improvement from CO2RR Using Cu-Sn Electrodeposited Catalysts

Electrochemical reduction of CO2 is an efficient and novel strategy to reduce the amount of this greenhouse-effect pollutant gas in the atmosphere while synthesizing value-added products, all of it with an easy synergy with intermittent renewable energies. This study investigates the influence of different ways of combining electrodeposited Cu and Sn as metallic elements in the electrocatalyst. From there, the use of Sn alone or with a small amount of Cu beneath is investigated, and finally, the best catalyst obtained, which has Sn over a slight Cu layer, is evaluated in consecutive cycles to make an initial exploration of the catalyst durability. As a result of this work, after optimization of the Sn and Cu-based catalysts, it is possible to obtain more than 60% of the organic products of interest, predominantly CO, the main component of syngas. Finally, this great amount of CO is obtained under low cell potential (below 3 V), which is a remarkable result in terms of the cost of the process.

Towards the Use of Low-Concentration CO2 Sources by Direct Selective Electrocatalytic Reduction

The direct CO2 reduction reaction (CO2RR) from simulated flue gas of various CO2 concentrations could minimize extra energy for pre-concentration processes to highly concentrated CO2 as a feedstock. We investigate the challenges for CO2RR caused by low CO2 concentrations and provide strategies concerning the impact of the chosen electrocatalyst material and the selection of the electrolyte to attain high CO selectivity. We continuously feed CO2 mixed with N2 (the typical dilutant in flue gas) in various ratios to gas diffusion electrodes in a model flow-through electrolyzer. Operating the CO2RR at lower CO2 concentrations results in an overpotential shift to more cathodic values. We show that higher active catalysts can maintain high CO selectivity down to 5 % CO2 by using NiCu-based catalysts. NiCu reached its limit when the CO2 concentration was lowered to 2 %, due to low CO2 availability and competition of carbonate formation. Employing near-neutral electrolytes with buffering capacity, we maintained high Faradaic efficiency at low overpotentials and higher CO2 utilization at low CO2 concentration.

Unfolding Electrolyzer Characteristics to Reveal Solar-to-Chemical Efficiency Potential: Rapid Analysis Method Bridging Electrochemistry and Photovoltaics

Development of photovoltaic-electrochemical (PV-EC) systems for energy storage and industry decarbonization requires multidisciplinary collaborative efforts of different research groups from both photovoltaic and electrochemical research communities. Consequently, the evaluation of the solar-to-chemical or solar-to-fuel efficiency of a new electrolyzer (EC) as a part of a PV-EC system is a time-consuming task that is challenging in a routine optimization loop. To address this issue, a new rapid assessment method is proposed. This method employs power balance requirements to unfold the input EC characteristics into the parameter space of PV-EC systems. The system parameters, composed with the EC output characteristics, yield the solar-to-chemical efficiency attainable by the electrolyzer in combination with any PV device under any irradiance at any relative PV-to-EC scaling and any mode of power coupling. This comprehensive overview is achieved via a mathematically simple conversion of the EC characteristics in any spreadsheet software. The method, designed to streamline the development and minimize the efforts of both the photovoltaic and electrochemical communities, is demonstrated via the analysis of CO2-reduction electrolyzer characteristics and verified with dedicated PV-EC experiments.

Electrochemical CO2 Conversion Commercialization Pathways: A Concise Review on Experimental Frontiers and Technoeconomic Analysis

Technoeconomic analysis (TEA) studies are vital for formulating guidelines that drive the commercialization of electrochemical CO2 reduction (eCO2R) technologies. In this review, we first discuss the progress in the field of eCO2R processes by providing current state-of-the-art metrices (e.g., faradic efficiency, current density) based on the recent heterogeneous catalysts' discovery, electrolytes, electrolyzers configuration, and electrolysis process designs. Next, we assessed the TEA studies for a wide range of eCO2R final products, different modes of eCO2R systems/processes, and discussed their relative competitiveness with relevant commercial products. Finally, we discuss challenges and future directions essential for eCO2R commercialization by linking suggestions from TEA studies. We believe that this review will catalyze innovation in formulating advanced eCO2R strategies to meet the TEA benchmarks for the conversion of CO2 into valuable chemicals at the industrial scale.

Effect of Halide Anions on Electrochemical CO2 Reduction in Non-Aqueous Choline Solutions using Ag and Au Electrodes

In this study, the effect of halide anions on the selectivity of the CO2 reduction reaction to CO was investigated in choline-based ethylene glycol solutions containing different halides (ChCl : EG, ChBr : EG, ChI : EG). The CO2RR was studied using silver (Ag) and gold (Au) electrodes in a compact H-cell. Our findings reveal that chloride effectively suppresses the hydrogen evolution reaction and enhances the selectivity of carbon monoxide production on both Ag and Au electrodes, with relatively high selectivity values of 84 % and 62 %, respectively. Additionally, the effect of varying ethylene glycol content in the choline chloride-containing electrolyte (ChCl : EG 1 : X, X=2, 3, 4) was investigated to improve the current density during CO2RR on the Ag electrode. We observed that a mole ratio of 1 : 4 exhibited the highest current density with a comparable faradaic efficiency toward CO. Notably, an evident surface reconstruction process took place on the Ag surface in the presence of Cl- ions, whereas on Au, this phenomenon was less pronounced. Overall, this study provides new insights into anion-induced surface restructuring of Ag and Au electrodes during CO2RR, and its consequences on the reduction performance on such surfaces in non-aqueous electrolytes.

Alkali Metal Cations Induce Structural Evolution on Au(111) During Cathodic Polarization

The activity of the electrocatalytic CO2 reduction reaction (CO2RR) is substantially affected by alkali metal cations (AM+) in electrolytes, yet the underlying mechanism is still controversial. Here, we employed electrochemical scanning tunneling microscopy and in situ observed Au(111) surface roughening in AM+ electrolytes during cathodic polarization. The roughened surface is highly active for catalyzing the CO2RR due to the formation of surface low-coordinated Au atoms. The critical potential for surface roughening follows the order Cs+ > Rb+ > K+ > Na+ > Li+, and the surface proportion of roughened area decreases in the order of Cs+ > Rb+ > K+ > Na+ > Li+. Electrochemical CO2RR measurements demonstrate that the catalytic activity strongly correlates with the surface roughness. Furthermore, we found that AM+ is critical for surface roughening to occur. The results unveil the unrecognized effect of AM+ on the surface structural evolution and elucidate that the AM+-induced formation of surface high-activity sites contributes to the enhanced CO2RR in large AM+ electrolytes.

CO2 Valorization in Deep Eutectic Solvents

The deep eutectic solvent (DES) has emerged in recent years as a valuable medium for converting CO2 into valuable chemicals because of its easy availability, stability, and safety, and its capability to dissolve carbon dioxide. CO2 valorization in DES has evolved rapidly over the past 20 years. As well as being used as solvents for acid/base-promoted CO2 conversion for the production of cyclic carbonates and carbamates, DESs can be used as reaction media for electrochemical CO2 reduction for formic acid and CO. Among these products, cyclic carbonates can be used as solvents and electrolytes, carbamate derivatives include the core structure of many herbicides and pesticides, and formic acid and carbon monoxide, the C1 electrochemical products, are essential raw materials in the chemical industries. An overview of the application of DESs for CO2 valorization in recent years is presented in this review, followed by a compilation and comparison of product types and reaction mechanisms within the different types of DESs, and an outlook on how CO2 valorization will be developed in the future.

Scalable Low-Temperature CO2 Electrolysis: Current Status and Outlook

The electrochemical CO2 reduction (eCO2R) in membrane electrode assemblies (MEAs) has brought e-chemical production one step closer to commercialization because of its advantages of minimized ohmic resistance and stackability. However, the current performance of reported eCO2R in MEAs is still far below the threshold for economic feasibility where low overall cell voltage (<2 V) and extensive stability (>5 years) are required. Furthermore, while the production cost of e-chemicals heavily relies on the carbon capture and product separation processes, these areas have received much less attention compared to CO2 electrolysis, itself. In this perspective, we examine the current status of eCO2R technologies from both academic and industrial points of view. We highlight the gap between current capabilities and commercialization standards and offer future research directions for eCO2R technologies with the hope of achieving industrially viable e-chemical production.

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.

Reactive capture and electrochemical conversion of CO2 with ionic liquids and deep eutectic solvents

Ionic liquids (ILs) and deep eutectic solvents (DESs) have tremendous potential for reactive capture and conversion (RCC) of CO2 due to their wide electrochemical stability window, low volatility, and high CO2 solubility. There is environmental and economic interest in the direct utilization of the captured CO2 using electrified and modular processes that forgo the thermal- or pressure-swing regeneration steps to concentrate CO2, eliminating the need to compress, transport, or store the gas. The conventional electrochemical conversion of CO2 with aqueous electrolytes presents limited CO2 solubility and high energy requirement to achieve industrially relevant products. Additionally, aqueous systems have competitive hydrogen evolution. In the past decade, there has been significant progress toward the design of ILs and DESs, and their composites to separate CO2 from dilute streams. In parallel, but not necessarily in synergy, there have been studies focused on a few select ILs and DESs for electrochemical reduction of CO2, often diluting them with aqueous or non-aqueous solvents. The resulting electrode-electrolyte interfaces present a complex speciation for RCC. In this review, we describe how the ILs and DESs are tuned for RCC and specifically address the CO2 chemisorption and electroreduction mechanisms. Critical bulk and interfacial properties of ILs and DESs are discussed in the context of RCC, and the potential of these electrolytes are presented through a techno-economic evaluation.

Dynamics of the Boundary Layer in Pulsed CO2 Electrolysis

Electrochemical reduction of CO2 poses a vast potential to contribute to a defossilized industry. Despite tremendous developments within the field, mass transport limitations, carbonate salt formation, and electrode degradation mechanisms still hamper the process performance. One promising approach to tweak CO2 electrolysis beyond today's limitations is pulsed electrolysis with potential cycling between an operating and a regeneration mode. Here, we rigorously model the boundary layer at a silver electrode in pulsed operation to get profound insights into the dynamic reorganization of the electrode microenvironment. In our simulation, pulsed electrolysis leads to a significant improvement of up to six times higher CO current density and 20 times higher cathodic energy efficiency when pulsing between -1.85 and -1.05 V vs SHE compared to constant potential operation. We found that elevated reactant availability in pulsed electrolysis originates from alternating replenishment of CO2 by diffusion and not from pH-induced carbonate and bicarbonate conversion. Moreover, pulsed electrolysis substantially promotes carbonate removal from the electrode by up to 83 % compared to constant potential operation, thus reducing the risk of salt formation. Therefore, this model lays the groundwork for an accurate simulation of the dynamic boundary layer modulation, which can provide insights into manifold electrochemical conversions.

Turning copper into an efficient and stable CO evolution catalyst beyond noble metals

Using renewable electricity to convert CO2 into CO offers a sustainable route to produce a versatile intermediate to synthesize various chemicals and fuels. For economic CO2-to-CO conversion at scale, however, there exists a trade-off between selectivity and activity, necessitating the delicate design of efficient catalysts to hit the sweet spot. We demonstrate here that copper co-alloyed with isolated antimony and palladium atoms can efficiently activate and convert CO2 molecules into CO. This trimetallic single-atom alloy catalyst (Cu92Sb5Pd3) achieves an outstanding CO selectivity of 100% (±1.5%) at -402 mA cm-2 and a high activity up to -1 A cm-2 in a neutral electrolyte, surpassing numerous state-of-the-art noble metal catalysts. Moreover, it exhibits long-term stability over 528 h at -100 mA cm-2 with an FECO above 95%. Operando spectroscopy and theoretical simulation provide explicit evidence for the charge redistribution between Sb/Pd additions and Cu base, demonstrating that Sb and Pd single atoms synergistically shift the electronic structure of Cu for CO production and suppress hydrogen evolution. Additionally, the collaborative interactions enhance the overall stability of the catalyst. These results showcase that Sb/Pd-doped Cu can steadily carry out efficient CO2 electrolysis under mild conditions, challenging the monopoly of noble metals in large-scale CO2-to-CO conversion.

Solar-Driven Sustainability: III-V Semiconductor for Green Energy Production Technologies

Long-term societal prosperity depends on addressing the world's energy and environmental problems, and photocatalysis has emerged as a viable remedy. Improving the efficiency of photocatalytic processes is fundamentally achieved by optimizing the effective utilization of solar energy and enhancing the efficient separation of photogenerated charges. It has been demonstrated that the fabrication of III-V semiconductor-based photocatalysts is effective in increasing solar light absorption, long-term stability, large-scale production and promoting charge transfer. This focused review explores on the current developments in III-V semiconductor materials for solar-powered photocatalytic systems. The review explores on various subjects, including the advancement of III-V semiconductors, photocatalytic mechanisms, and their uses in H2 conversion, CO2 reduction, environmental remediation, and photocatalytic oxidation and reduction reactions. In order to design heterostructures, the review delves into basic concepts including solar light absorption and effective charge separation. It also highlights significant advancements in green energy systems for water splitting, emphasizing the significance of establishing eco-friendly systems for CO2 reduction and hydrogen production. The main purpose is to produce hydrogen through sustainable and ecologically friendly energy conversion. The review intends to foster the development of greener and more sustainable energy source by encouraging researchers and developers to focus on practical applications and advancements in solar-powered photocatalysis.

Probabilistic Techno-Economic Assessment of Medium-Scale Photoelectrochemical Fuel Generation Plants

Photoelectrochemical (PEC) systems are promising approaches for sustainable fuel processing. PEC devices, like conventional photovoltaic-electrolyzer (PV-EC) systems, utilize solar energy for splitting water into hydrogen and oxygen. Contrary to PV-EC systems, PEC devices integrate the photoabsorber, the ionic membrane, and the catalysts into a single reactor. This integration of elements potentially makes PEC systems simpler in design, increases efficiency, offers a cost advantage, and allows for implementation with higher flexibility in use. We present a detailed techno-economic evaluation of PEC systems with three different device designs. We combine a system-level techno-economic analysis based on physical performance models (including degradation) with stochastic methods for uncertainty assessments, also considering the use of PV and EC learning curves for future cost scenarios. For hydrogen, we assess different PEC device design options (utilizing liquid or water vapor as reactant) and compare them to conventional PV-EC systems (anion or cation exchange). We show that in the current scenario, PEC systems (with a levelized cost of hydrogen of 6.32 $/kgH2 ) located in southern Spain are not yet competitive, operating at 64% higher costs than the PV-driven anion exchange EC systems. Our analysis indicates that PEC plants' material and size are the most significant factors affecting hydrogen costs. PEC designs operating with water vapor are the most economical designs, with the potential to cost about 10% less than PV-EC systems and to reach a 2 $/kgH2 target by 2040. If a sunlight concentrator is incorporated, the PEC-produced hydrogen cost is significantly lower (3.59 $/kgH2 in the current scenario). Versions of the concentrated PEC system that incorporate reversible operation and CO2 reduction indicate a levelized cost of storage of 0.2803 $/kWh for the former and a levelized cost of CO of 0.546 $/kgCO for the latter. These findings demonstrate the competitiveness and viability of (concentrated) PEC systems and their versatile use cases. Our study shows the potential of PEC devices and systems for hydrogen production (current and future potential), storage applications, and CO production, thereby highlighting the importance of sustainable and cost-effective design considerations for future advancements in technology development in this field.

Electrocatalysis in deep eutectic solvents: from fundamental properties to applications

Electrocatalysis stands out as a promising avenue for synthesizing high-value products with minimal environmental footprint, aligning with the imperative for sustainable energy solutions. Deep eutectic solvents (DESs), renowned for their eco-friendly, safe, and cost-effective nature, present myriad advantages, including extensive opportunities for material innovation and utilization as reaction media in electrocatalysis. This review initiates with an exposition on the distinctive features of DESs, progressing to explore their applications as solvents in electrocatalyst synthesis and electrocatalysis. Additionally, it offers an insightful analysis of the challenges and prospects inherent in electrocatalysis within DESs. By delving into these aspects comprehensively, this review aims to furnish a nuanced understanding of DESs, thus broadening their horizons in the realm of electrocatalysis and facilitating their expanded application.

Revealing the structural evolution of CuAg composites during electrochemical carbon monoxide reduction

Comprehending the catalyst structural evolution during the electrocatalytic process is crucial for establishing robust structure/performance correlations for future catalysts design. Herein, we interrogate the structural evolution of a promising Cu-Ag oxide catalyst precursor during electrochemical carbon monoxide reduction. By using extensive in situ and ex situ characterization techniques, we reveal that the homogenous oxide precursors undergo a transformation to a bimetallic composite consisting of small Ag nanoparticles enveloped by thin layers of amorphous Cu. We believe that the amorphous Cu layer with undercoordinated nature is responsible for the enhanced catalytic performance of the current catalyst composite. By tuning the Cu/Ag ratio in the oxide precursor, we find that increasing the Ag concentration greatly promotes liquid products formation while suppressing the byproduct hydrogen. CO2/CO co-feeding electrolysis and isotopic labelling experiments suggest that high CO concentrations in the feed favor the formation of multi-carbon products. Overall, we anticipate the insights obtained for Cu-Ag bimetallic systems for CO electroreduction in this study may guide future catalyst design with improved performance.

Ionic Liquids in Metal, Photo-, Electro-, and (Bio) Catalysis

Ionic liquids (ILs) have unique physicochemical properties that make them advantageous for catalysis, such as low vapor pressure, non-flammability, high thermal and chemical stabilities, and the ability to enhance the activity and stability of (bio)catalysts. ILs can improve the efficiency, selectivity, and sustainability of bio(transformations) by acting as activators of enzymes, selectively dissolving substrates and products, and reducing toxicity. They can also be recycled and reused multiple times without losing their effectiveness. ILs based on imidazolium cation are preferred for structural organization aspects, with a semiorganized layer surrounding the catalyst. ILs act as a container, providing a confined space that allows modulation of electronic and geometric effects, miscibility of reactants and products, and residence time of species. ILs can stabilize ionic and radical species and control the catalytic activity of dynamic processes. Supported IL phase (SILP) derivatives and polymeric ILs (PILs) are good options for molecular engineering of greener catalytic processes. The major factors governing metal, photo-, electro-, and biocatalysts in ILs are discussed in detail based on the vast literature available over the past two and a half decades. Catalytic reactions, ranging from hydrogenation and cross-coupling to oxidations, promoted by homogeneous and heterogeneous catalysts in both single and multiphase conditions, are extensively reviewed and discussed considering the knowledge accumulated until now.

Three-dimensional N-doped carbon nanosheets loaded with heterostructured Ni/Ni3ZnC0.7 nanoparticles for selective and efficient CO2 reduction

Electrocatalytic CO2 reduction (CO2RR) has emerged as a promising approach for converting CO2 into valuable chemicals and fuels to achieve a sustainable carbon cycle. However, the development of efficient electrocatalysts with high current densities and superior product selectivity remains a significant challenge. In this study, we present the synthesis of a porous nitrogen-doped carbon nanosheet loaded with heterostructured Ni/Ni3ZnC0.7 nanoparticles through a facile hydrothermal-calcination method (Ni/Ni3ZnC0.7-NC). Remarkably, the Ni/Ni3ZnC0.7-NC catalyst exhibits outstanding performance towards CO2RR in an H-cell, demonstrating a high CO faradaic efficiency of 92.47% and a current density (jCO) of 15.77 mA cm-2 at 0.87 V vs. RHE. To further explore its potential industrial applications, we constructed a flow cell and a rechargeable Zn-CO2 flow cell utilizing the Ni/Ni3ZnC0.7-NC catalyst as the cathode. Impressively, not only does the Ni/Ni3ZnC0.7-NC catalyst achieve an industrial high current density of 254 mA cm-2 at a voltage of -1.19 V vs. RHE in the flow cell, but it also exhibits a maximum power density of 4.2 mW cm-2 at 22 mA cm-2 in the Zn-CO2 flow cell, while maintaining excellent rechargeability. Density functional theory (DFT) calculations indicate that Ni/Ni3ZnC0.7-NC possesses more spontaneous reaction pathways for CO2 reduction to CO, owing to its heterogeneous structure in contrast to Ni3ZnC0.7-NC and Ni-NC. Consequently, Ni/Ni3ZnC0.7-NC demonstrates accelerated CO2RR reaction kinetics, resulting in improved catalytic activity and selectivity for CO2RR.

CO2 Electrolyzers

CO2 electrolyzers have progressed rapidly in energy efficiency and catalyst selectivity toward valuable chemical feedstocks and fuels, such as syngas, ethylene, ethanol, and methane. However, each component within these complex systems influences the overall performance, and the further advances needed to realize commercialization will require an approach that considers the whole process, with the electrochemical cell at the center. Beyond the cell boundaries, the electrolyzer must integrate with upstream CO2 feeds and downstream separation processes in a way that minimizes overall product energy intensity and presents viable use cases. Here we begin by describing upstream CO2 sources, their energy intensities, and impurities. We then focus on the cell, the most common CO2 electrolyzer system architectures, and each component within these systems. We evaluate the energy savings and the feasibility of alternative approaches including integration with CO2 capture, direct conversion of flue gas and two-step conversion via carbon monoxide. We evaluate pathways that minimize downstream separations and produce concentrated streams compatible with existing sectors. Applying this comprehensive upstream-to-downstream approach, we highlight the most promising routes, and outlook, for electrochemical CO2 reduction.

Boosting CO2 electrocatalysis through electrical double layer regulations

Interfacial investigation for fine-tuning microenvironment has recently emerged as a promising method to optimize the electrochemical CO2 reduction system. The electrical double layer located at the electrode-electrolyte interface presents a particularly significant impact on electrochemical reactions. However, its effect on the activity and selectivity of CO2 electrocatalysis remains poorly understood. Here, we utilized two-dimensional mica flakes, a material with a high dielectric constant, to modify the electrical double layer of Ag nanoparticles. This modification resulted in a significant enhancement of current densities for CO2 reduction and an impressive Faradaic efficiency of 98% for CO production. Our mechanistic investigations suggest that the enhancement of the electrical double layer capacitance through mica modification enriched local CO2 concentration near the reaction interface, thus facilitating CO2 electroreduction.

Quantifying mass transport limitations in a microfluidic CO2 electrolyzer with a gas diffusion cathode

A gas diffusion electrode (GDE) based CO2 electrolyzer shows enhanced CO2 transport to the catalyst surface, significantly increasing current density compared to traditional planar immersed electrodes. A two-dimensional model for the cathode side of a microfluidic CO2 to CO electrolysis device with a GDE is developed. The model, validated against experimental data, examines key operational parameters and electrode materials. It predicts an initial rise in CO partial current density (PCD), peaking at 75 mA cm-2 at -1.3 V vs RHE for a fully flooded catalyst layer, then declining due to continuous decrease in CO2 availability near the catalyst surface. Factors like electrolyte flow rate and CO2 gas mass flow rate influence PCD, with a trade-off between high CO PCD and CO2 conversion efficiency observed with increased CO2 gas flow. We observe that a significant portion of the catalyst layer remains underutilized, and suggest improvements like varying electrode porosity and anisotropic layers to enhance mass transport and CO PCD. This research offers insights into optimizing CO2 electrolysis device performance.

Electrocatalytic CO2 Reduction to C2+ Products in Flow Cells

Electrocatalytic CO2 reduction into value-added fuels and chemicals by renewable electric energy is one of the important strategies to address global energy shortage and carbon emission. Though the classical H-type electrolytic cell can quickly screen high-efficiency catalysts, the low current density and limited CO2 mass transfer process essentially impede its industrial applications. The electrolytic cells based on electrolyte flow system (flow cells) have shown great potential for industrial devices, due to higher current density, improved local CO2 concentration, and better mass transfer efficiency. The design and optimization of flow cells are of great significance to further accelerate the industrialization of electrocatalytic CO2 reduction reaction (CO2 RR). In this review, the progress of flow cells for CO2 RR to C2+ products is concerned. Firstly, the main events in the development of the flow cells for CO2 RR are outlined. Second, the main design principles of CO2 RR to C2+ products, the architectures, and types of flow cells are summarized. Third, the main strategies for optimizing flow cells to generate C2+ products are reviewed in detail, including cathode, anode, ion exchange membrane, and electrolyte. Finally, the preliminary attempts, challenges, and the research prospects of flow cells for industrial CO2 RR toward C2+ products are discussed.

Cu-Based Materials for Enhanced C2+ Product Selectivity in Photo-/Electro-Catalytic CO2 Reduction: Challenges and Prospects

Carbon dioxide conversion into valuable products using photocatalysis and electrocatalysis is an effective approach to mitigate global environmental issues and the energy shortages. Among the materials utilized for catalytic reduction of CO2, Cu-based materials are highly advantageous owing to their widespread availability, cost-effectiveness, and environmental sustainability. Furthermore, Cu-based materials demonstrate interesting abilities in the adsorption and activation of carbon dioxide, allowing the formation of C2+ compounds through C-C coupling process. Herein, the basic principles of photocatalytic CO2 reduction reactions (PCO2RR) and electrocatalytic CO2 reduction reaction (ECO2RR) and the pathways for the generation C2+ products are introduced. This review categorizes Cu-based materials into different groups including Cu metal, Cu oxides, Cu alloys, and Cu SACs, Cu heterojunctions based on their catalytic applications. The relationship between the Cu surfaces and their efficiency in both PCO2RR and ECO2RR is emphasized. Through a review of recent studies on PCO2RR and ECO2RR using Cu-based catalysts, the focus is on understanding the underlying reasons for the enhanced selectivity toward C2+ products. Finally, the opportunities and challenges associated with Cu-based materials in the CO2 catalytic reduction applications are presented, along with research directions that can guide for the design of highly active and selective Cu-based materials for CO2 reduction processes in the future.

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.

Enhanced performance of molecular electrocatalysts for CO2 reduction in a flow cell following K+ addition

Electrocatalytic CO2 reduction is a key aspect of artificial photosynthesis systems designed to produce fuels. Although some molecular catalysts have good performance for CO2 reduction, these compounds also suffer from poor durability and energy efficiency. The present work demonstrates the improved CO2 reduction activity exhibited by molecular catalysts in a flow cell. These catalysts were composed of a cobalt-tetrapyridino-porphyrazine complex supported on carbon black together with potassium salt and were both stable and efficient. These systems were found to promote electrocatalytic CO2 reduction with a current density of 100 mA/cm2 and generated CO over at least 1 week with a selectivity of approximately 95%. The optimal catalyst gave a turnover number of 3,800,000 and an energy conversion efficiency of more than 62% even at 200 mA/cm2.

Enhancing C≥2 product selectivity in electrochemical CO2 reduction by controlling the microstructure of gas diffusion electrodes

We fabricate polymer-based gas diffusion electrodes with controllable microstructure for the electrochemical reduction of CO2, by means of electrospinning and physical vapor deposition. We show that the microstructure of the electrospun substrate is affecting the selectivity of a Cu catalyst, steering it from H2 to C2H4 and other multicarbon products. Specifically, we demonstrate that gas diffusion electrodes with small pores (e.g. mean pore size 0.2 μm) and strong hydrophobicity (e.g. water entry pressure >1 bar) are necessary for achieving a remarkable faradaic efficiency of ∼50% for C2H4 and ∼75% for C≥2 products in neutral 1M KCl electrolyte at 200 mA cm-2. We observe a gradual shift from C2H4 to CH4 to H2 during long-term electrochemical reduction of CO2, which we ascribe to hygroscopic carbonate precipitation in the gas diffusion electrode resulting in flooding of the Cu catalyst by the electrolyte. We demonstrate that even with minimal electrolyte overpressure of 50 mbar, gas diffusion electrodes with large pores (mean pore size 1.1 μm) lose selectivity to carbon products completely, suddenly, and irreversibly in favor of H2. In contrast, we find that gas diffusion electrodes with small pore size (mean pore size 0.2 μm) and strong hydrophobicity (water entry pressure ∼5 bar) are capable of resisting up to 1 bar of electrolyte overpressure during CO2RR without loss of selectivity. We rationalize these experimental results in the context of a double phase boundary reactivity, where an electrolyte layer covers the Cu catalyst and thus governs local CO2 availability. Our results emphasize the pivotal role of microstructure and hydrophobicity in promoting high C≥2 product selectivity and long-term stability in CO2RR flow cells.

Pathways to enhance electrochemical CO2 reduction identified through direct pore-level modeling

Electrochemical conversion of CO2 to fuels and valuable products is one pathway to reduce CO2 emissions. Electrolyzers using gas diffusion electrodes (GDEs) show much higher current densities than aqueous phase electrolyzers, yet models for multi-physical transport remain relatively undeveloped, often relying on volume-averaged approximations. Many physical phenomena interact inside the GDE, which is a multiphase environment (gaseous reactants and products, liquid electrolyte, and solid catalyst), and a multiscale problem, where "pore-scale" phenomena affect observations at the "macro-scale". We present a direct (not volume-averaged) pore-level transport model featuring a liquid electrolyte domain and a gaseous domain coupled at the liquid-gas interface. Transport is resolved, in 2D, around individual nanoparticles comprising the catalyst layer, including the electric double layer and steric effects. The GDE behavior at the pore-level is studied in detail under various idealized catalyst geometries configurations, showing how the catalyst layer thickness, roughness, and liquid wetting behavior all contribute to (or restrict) the transport necessary for CO2 reduction. The analysis identifies several pathways to enhance GDE performance, opening the possibility for increasing the current density by an order of magnitude or more. The results also suggest that the typical liquid-gas interface in the GDE of experimental demonstrations form a filled front rather than a wetting film, the electrochemical reaction is not taking place at a triple-phase boundary but rather a thicker zone around the triple-phase boundary, the solubility reduction at high electrolyte concentrations is an important contributor to transport limitations, and there is considerable heterogeneity in the use of the catalyst. The model allows unprecedented visualization of the transport dynamics inside the GDE across multiple length scales, making it a key step forward on the path to understanding and enhancing GDEs for electrochemical CO2 reduction.

Integrated CO2 capture and electrochemical upgradation: the underpinning mechanism and techno-chemical analysis

Coupling post-combustion CO2 capture with electrochemical utilization (CCU) is a quantum leap in renewable energy science since it eliminates the cost and energy involved in the transport and storage of CO2. However, the major challenges involved in industrial scale implementation are selecting an appropriate solvent/electrolyte for CO2 capture, modeling an appropriate infrastructure by coupling an electrolyser with a CO2 point source and a separator to isolate CO2 reduction reaction (CO2RR) products, and finally selection of an appropriate electrocatalyst. In this review, we highlight the major difficulties with detailed mechanistic interpretation in each step, to find out the underpinning mechanism involved in the integration of electrochemical CCU to achieve higher-value products. In the past decades, most of the studies dealt with individual parts of the integration process, i.e., either selecting a solvent for CO2 capture, designing an electrocatalyst, or choosing an ideal electrolyte. In this context, it is important to note that solvents such as monoethanolamine, bicarbonate, and ionic liquids are often used as electrolytes in CO2 capture media. Therefore, it is essential to fabricate a cost-effective electrolyser that should function as a reversible binder with CO2 and an electron pool capable of recovering the solvent to electrolyte reversibly. For example, reversible ionic liquids, which are non-ionic in their normal forms, but produce ionic forms after CO2 capture, can be further reverted back to their original non-ionic forms after CO2 release with almost 100% efficiency through the chemical or thermal modulations. This review also sheds light on a focused techno-economic evolution for converting the electrochemically integrated CCU process from a pilot-scale project to industrial-scale implementation. In brief, this review article will summarize a state-of-the-art argumentation of challenges and outcomes over the different segments involved in electrochemically integrated CCU to stimulate urgent progress in the field.

Highly Durable Nanoporous Cu2-xS Films for Efficient Hydrogen Evolution Electrocatalysis under Mild pH Conditions

Copper-based hydrogen evolution electrocatalysts are promising materials to scale-up hydrogen production due to their reported high current densities; however, electrode durability remains a challenge. Here, we report a facile, cost-effective, and scalable synthetic route to produce Cu2-xS electrocatalysts, exhibiting hydrogen evolution rates that increase for ∼1 month of operation. Our Cu2-xS electrodes reach a state-of-the-art performance of ∼400 mA cm-2 at -1 V vs RHE under mild conditions (pH 8.6), with almost 100% Faradaic efficiency for hydrogen evolution. The rise in current density was found to scale with the electrode electrochemically active surface area. The increased performance of our Cu2-xS electrodes correlates with a decrease in the Tafel slope, while analyses by X-ray photoemission spectroscopy, operando X-ray diffraction, and in situ spectroelectrochemistry cooperatively revealed the Cu-centered nature of the catalytically active species. These results allowed us to increase fundamental understanding of heterogeneous electrocatalyst transformation and consequent structure-activity relationship. This facile synthesis of highly durable and efficient Cu2-xS electrocatalysts enables the development of competitive electrodes for hydrogen evolution under mild pH conditions.

Performance and Stability of Aemion and Aemion+ Membranes in Zero-Gap CO2 Electrolyzers with Mild Anolyte Solutions

The dependence of performance and stability of a zero-gap CO2 electrolyzer on the properties of the anion exchange membrane (AEM) is examined. This work firstly assesses the influence of the anolyte when using an Aemion membrane and then shows that when using 10 mM KHCO3 , a CO2 electrolyzer using a next-generation Aemion+ membrane can achieve lower cell voltages and longer lifetimes due to increased water permeation. The impact of lower permselectivity of Aemion+ on water transport is also discussed. Using Aemion+, a cell voltage of 3.17 V at 200 mA cm-2 is achieved at room temperature, with a faradaic efficiency of >90 %. Stable CO2 electrolysis at 100 mA cm-2 is demonstrated for 100 h, but with reduced lifetime at 300 mA cm-2 . However, the lifetime of the cell at high current densities is shown to be increased by improving water transport characteristics of the AEM and reducing dimensional swelling, as well as by improving cathode design to reduce localized dehydration of the membrane.

Effects of Iron Species on Low Temperature CO2 Electrolyzers

Electrochemical energy conversion devices are considered key in reducing CO2 emissions and significant efforts are being applied to accelerate device development. Unlike other technologies, low temperature electrolyzers have the ability to directly convert CO2 into a range of value-added chemicals. To make them commercially viable, however, device efficiency and durability must be increased. Although their design is similar to more mature water electrolyzers and fuel cells, new cell concepts and components are needed. Due to the complexity of the system, singular component optimization is common. As a result, the component interplay is often overlooked. The influence of Fe-species clearly shows that the cell must be considered holistically during optimization, to avoid future issues due to component interference or cross-contamination. Fe-impurities are ubiquitous, and their influence on single components is well-researched. The activity of non-noble anodes has been increased through the deliberate addition of iron. At the same time, however, Fe-species accelerate cathode and membrane degradation. Here, we interpret literature on single components to gain an understanding of how Fe-species influence low temperature CO2 electrolyzers holistically. The role of Fe-species serves to highlight the need for considerations regarding component interplay in general.

Direct Imaging of Local pH Reveals Bubble-Induced Mixing in a CO2 Electrolyzer

Electrochemical CO2 reduction poses a promising pathway to produce hydrocarbon chemicals and fuels without relying on fossil fuels. Gas diffusion electrodes allow high selectivity for desired carbon products at high current density by ensuring a sufficient CO2 mass transfer rate to the catalyst layer. In addition to CO2 mass transfer, the product selectivity also strongly depends on the local pH at the catalyst surface. In this work, we directly visualize for the first time the two-dimensional (2D) pH profile in the catholyte channel of a gas-fed CO2 electrolyzer equipped with a bipolar membrane. The pH profile is imaged with operando fluorescence lifetime imaging microscopy (FLIM) using a pH-sensitive quinolinium-based dye. We demonstrate that bubble-induced mixing plays an important role in the Faradaic efficiency. Our concentration measurements show that the pH at the catalyst remains lower at -100 mA cm-2 than at -10 mA cm-2, implying that bubble-induced advection outweighs the additional OH- flux at these current densities. We also prove that the pH buffering effect of CO2 from the gas feed and dissolved CO2 in the catholyte prevents the gas diffusion electrode from becoming strongly alkaline. Our findings suggest that gas-fed CO2 electrolyzers with a bipolar membrane and a flowing catholyte are promising designs for scale-up and high-current-density operation because they are able to avoid extreme pH values in the catalyst layer.

Confinement of an alkaline environment for electrocatalytic CO2 reduction in acidic electrolytes

Acidic electrochemical CO₂ reduction reaction (CO₂RR) can minimize carbonate formation and eliminate CO₂ crossover, thereby improving long-term stability and enhancing single-pass carbon efficiency (SPCE). However, the kinetically favored hydrogen evolution reaction (HER) is generally predominant under acidic conditions. This paper describes the confinement of a local alkaline environment for efficient CO₂RR in a strongly acidic electrolyte through the manipulation of mass transfer processes in well-designed hollow-structured Ag@C electrocatalysts. A high faradaic efficiency of over 95% at a current density of 300 mA cm^-2 and an SPCE of 46.2% at a CO₂ flow rate of 2 standard cubic centimeters per minute are achieved in the acidic electrolyte, with enhanced stability compared to that under alkaline conditions. Computational modeling results reveal that the unique structure of Ag@C could regulate the diffusion process of OH^- and H⁺, confining a high-pH local reaction environment for the promoted activity. This work presents a promising route to engineer the microenvironment through the regulation of mass transport that permits the CO₂RR in acidic electrolytes with high performance.

Converting CO2 into Value-Added Products by Cu2 O-Based Catalysts: From Photocatalysis, Electrocatalysis to Photoelectrocatalysis

Converting CO₂ into value-added products by photocatalysis, electrocatalysis, and photoelectrocatalysis is a promising method to alleviate the global environmental problems and energy crisis. Among the semiconductor materials applied in CO₂ catalytic reduction, Cu₂ O has the advantages of abundant reserves, low price and environmental friendliness. Moreover, Cu₂ O has unique adsorption and activation properties for CO₂ , which is conducive to the generation of C₂₊ products through CC coupling. This review introduces the basic principles of CO₂ reduction and summarizes the pathways for the generation of C₁ , C₂ , and C₂₊ products. The factors affecting CO₂ reduction performance are further discussed from the perspective of the reaction environment, medium, and novel reactor design. Then, the properties of Cu₂ O-based catalysts in CO₂ reduction are summarized and several optimization strategies to enhance their stability and redox capacity are discussed. Subsequently, the application of Cu₂ O-based catalysts in photocatalytic, electrocatalytic, and photoelectrocatalytic CO₂ reduction is described. Finally, the opportunities, challenges and several research directions of Cu₂ O-based catalysts in the field of CO₂ catalytic reduction are presented, which is guidance for its wide application in the energy and environmental fields is provided.

Electrochemical C-N coupling of CO2 and nitrogenous small molecules for the electrosynthesis of organonitrogen compounds

Electrochemical C-N coupling reactions based on abundant small molecules (such as CO₂ and N₂) have attracted increasing attention as a new "green synthetic strategy" for the synthesis of organonitrogen compounds, which have been widely used in organic synthesis, materials chemistry, and biochemistry. The traditional technology employed for the synthesis of organonitrogen compounds containing C-N bonds often requires the addition of metal reagents or oxidants under harsh conditions with high energy consumption and environmental concerns. By contrast, electrosynthesis avoids the use of other reducing agents or oxidants by utilizing "electrons", which are the cleanest "reagent" and can reduce the generation of by-products, consistent with the atomic economy and green chemistry. In this study, we present a comprehensive review on the electrosynthesis of high value-added organonitrogens from the abundant CO₂ and nitrogenous small molecules (N₂, NO, NO₂^-, NO₃^-, NH₃, etc.) via the C-N coupling reaction. The associated fundamental concepts, theoretical models, emerging electrocatalysts, and value-added target products, together with the current challenges and future opportunities are discussed. This critical review will greatly increase the understanding of electrochemical C-N coupling reactions, and thus attract research interest in the fixation of carbon and nitrogen.

Nanoscale Electron Transfer Variations at Electrocatalyst-Electrolyte Interfaces Resolved by in Situ Conductive Atomic Force Microscopy

Rational innovation of electrocatalysts requires detailed knowledge of spatial property variations across the solid-electrolyte interface. We introduce correlative atomic force microscopy (AFM) to simultaneously probe, in situ and at the nanoscale, electrical conductivity, chemical-frictional, and morphological properties of a bimetallic copper-gold system for CO2 electroreduction. In air, water, and bicarbonate electrolyte, current-voltage curves reveal resistive CuOx islands in line with local current contrasts, while frictional imaging indicates qualitative variations in the hydration layer molecular ordering upon change from water to electrolyte. Nanoscale current contrast on polycrystalline Au shows resistive grain boundaries and electrocatalytically passive adlayer regions. In situ conductive AFM imaging in water shows mesoscale regions of low current and reveals that reduced interfacial electric currents are accompanied by increased friction forces, thus indicating variations in the interfacial molecular ordering affected by the electrolyte composition and ionic species. These findings provide insights into how local electrochemical environments and adsorbed species affect interfacial charge transfer processes and support building in situ structure-property relationships in catalysis and energy conversion research.

Inhomogeneities in the Catholyte Channel Limit the Upscaling of CO2 Flow Electrolysers

The use of gas diffusion electrodes that supply gaseous CO₂ directly to the catalyst layer has greatly improved the performance of electrochemical CO₂ conversion. However, reports of high current densities and Faradaic efficiencies primarily come from small lab scale electrolysers. Such electrolysers typically have a geometric area of 5 cm², while an industrial electrolyser would require an area closer to 1 m². The difference in scales means that many limitations that manifest only for larger electrolysers are not captured in lab scale setups. We develop a 2D computational model of both a lab scale and upscaled CO₂ electrolyser to determine performance limitations at larger scales and how they compare to the performance limitations observed at the lab scale. We find that for the same current density larger electrolysers exhibit much greater reaction and local environment inhomogeneity. Increasing catalyst layer pH and widening concentration boundary layers of the KHCO₃ buffer in the electrolyte channel lead to higher activation overpotential and increased parasitic loss of reactant CO₂ to the electrolyte solution. We show that a variable catalyst loading along the direction of the flow channel may improve the economics of a large scale CO₂ electrolyser.

Strategies to Enhance CO2 Electrochemical Reduction from Reactive Carbon Solutions

CO₂ electrochemical reduction (CO₂ ER) from (bi)carbonate feed presents an opportunity to efficiently couple this process to alkaline-based carbon capture systems. Likewise, while this method of reducing CO₂ currently lags behind CO₂ gas-fed electrolysers in certain performance metrics, it offers a significant improvement in CO₂ utilization which makes the method worth exploring. This paper presents two simple modifications to a bicarbonate-fed CO₂ ER system that enhance the selectivity towards CO. Specifically, a modified hydrophilic cathode with Ag catalyst loaded through electrodeposition and the addition of dodecyltrimethylammonium bromide (DTAB), a low-cost surfactant, to the catholyte enabled the system to achieve a FE_CO of 85% and 73% at 100 and 200 mA·cm^-2, respectively. The modifications were tested in 4 h long experiments where DTAB helped maintain FE_CO stable even when the pH of the catholyte became more alkaline, and it improved the CO² utilization compared to a system without DTAB.

Operando characterization of continuous flow CO2 electrolyzers: current status and future prospects

The performance of continuous-flow CO₂ electrolyzers has substantially increased in recent years, achieving current density and selectivity (particularly for CO production) meeting the industrial targets. Further improvement is, however, necessary in terms of stability and energy efficiency, as well as in high-value multicarbon product formation. Accelerating this process requires deeper understanding of the complex interplay of chemical-physical processes taking place in CO₂ electrolyzer cells. Operando characterization can provide these insights under working conditions, helping to identify the reasons for performance losses. Despite this fact, only relatively few studies have taken advantage of such methods up to now, applying operando techniques to characterize practically relevant CO₂ electrolyzers. These studies include X-ray absorption- and Raman spectroscopy, fluorescent microscopy, scanning probe techniques, mass spectrometry, and radiography. Their objective was to characterize the catalyst structure, its microenviroment, membrane properties, etc., and relate them to the device performance (reaction rates and product distribution). Here we review the current state-of-the-art of operando methods, associated challenges, and also their future potential. We aim to motivate researchers to perform operando characterization in continuous-flow CO₂ electrolyzers, to understand the reaction mechanism and device operation under practically relevant conditions, thereby advancing the field towards industrialization.

When Flooding Is Not Catastrophic-Woven Gas Diffusion Electrodes Enable Stable CO2 Electrolysis

Electrochemical CO₂ reduction has the potential to use excess renewable electricity to produce hydrocarbon chemicals and fuels. Gas diffusion electrodes (GDEs) allow overcoming the limitations of CO₂ mass transfer but are sensitive to flooding from (hydrostatic) pressure differences, which inhibits upscaling. We investigate the effect of the flooding behavior on the CO₂ reduction performance. Our study includes six commercial gas diffusion layer materials with different microstructures (carbon cloth and carbon paper) and thicknesses coated with a Ag catalyst and exposed to differential pressures corresponding to different flow regimes (gas breakthrough, flow-by, and liquid breakthrough). We show that physical electrowetting further limits the flow-by regime at commercially relevant current densities (≥200 mA cm^-2), which reduces the Faradaic efficiency for CO (FE_CO) for most carbon papers. However, the carbon cloth GDE maintains its high CO₂ reduction performance despite being flooded with the electrolyte due to its bimodal pore structure. Exposed to pressure differences equivalent to 100 cm height, the carbon cloth is able to sustain an average FE_CO of 69% at 200 mA cm^-2 even when the liquid continuously breaks through. CO₂ electrolyzers with carbon cloth GDEs are therefore promising for scale-up because they enable high CO₂ reduction efficiency while tolerating a broad range of flow regimes.

Toward Effective CO2 Reduction in an Acid Medium: Electrocatalysis at Cu2O-Derived Polycrystalline Cu Sites Immobilized within the Network of WO3 Nanowires

A hybrid catalytic system composed of copper (I)-oxide-derived copper nanocenters immobilized within the network of tungsten oxide nanowires has exhibited electrocatalytic activity toward CO₂ reduction in an acid medium (0.5 mol dm^-3 H₂SO₄). The catalytic system facilitates conversion of CO₂ to methanol and is fairly selective with respect to the competing hydrogen evolution. The preparative procedure has involved voltammetric electroreduction of Cu₂O toward the formation and immobilization of catalytic Cu sites within the hexagonal structures of WO₃ nanowires which are simultaneously partially reduced to mixed-valence hydrogen tungsten (VI, V) oxide bronzes, H _x WO₃, coexisting with sub-stoichiometric tungsten (VI, IV) oxides, WO_3-y . After the initial loss of Cu through its dissolution to Cu²⁺ during positive potential scanning up to 1 V (vs RHE), the remaining copper is not electroactive and seems to be trapped within in the network of hexagonal WO₃. Using the ultramicroelectrode-based probe, evidence has also been provided that partially reduced nonstoichiometric tungsten oxides induce reduction of CO₂ to the CO-type reaction intermediates. The chronocoulometric data are consistent with the view that existence of copper sites dispersed in WO₃ improves electron transfers and charge propagation within the hybrid catalytic layer. The enhanced tolerance of the catalyst to the competitive hydrogen evolution during CO₂R should be explained in terms of the ability of H _x WO₃ to consume protons and absorb hydrogen as well as to shift the proton discharge at Cu toward more negative potentials. However, the capacity of WO₃ to interact with catalytic copper and to adsorb CO-type reaction intermediates is expected to facilitate removal of the poisoning CO-type adsorbates from Cu sites.

Electrocatalytic Conversion of CO2 to Formate at Low Overpotential by Electrolyte Engineering in Model Molecular Catalysis

An electrolyte engineering strategy was developed for CO₂ reduction into formate with a model molecular catalyst, [Rh(bpy)(Cp*)Cl]Cl, by modifying the solvent (organic or aqueous), the proton source (H₂ O or acetic acid), and the electrode/solution interface with imidazolium- and pyrrolidinium-based ionic liquids (ILs). Experimental and theoretical density functional theory investigations suggested that π⁺ -π interactions between the imidazolium-based IL cation and the reduced bipyridine ligand of the catalyst improved the efficiency of the CO₂ reduction reaction (CO₂ RR) by lowering the overpotential, while granting partial suppression of the hydrogen evolution reaction. This allowed tuning the selectivity towards formate, reaching for this catalyst an unprecedented faradaic efficiency (FE_HCOO -) ≥90 % and energy efficiency of 66 % in acetonitrile solution. For the first time, relevant CO₂ conversion to formic acid/formate was reached at low overpotential (0.28 V) using a homogeneous catalyst in acidic aqueous solution (pH=3.8). These results open up a new strategy based on electrolyte engineering for enhancing carbon balance in CO₂ RR.

Functionalized Ag with Thiol Ligand to Promote Effective CO2 Electroreduction

It is challenging while critical to develop efficient catalysts that can achieve both high current density and high energy efficiency for electrocatalytic CO₂ reduction (CO₂R). Herein, we report a strategy of tailoring the surface electronic structure of an Ag catalyst via thiol ligand modification to improve its intrinsic activity, selectivity, and further energy efficiency toward CO₂R. Specifically, interconnected Ag nanoparticles with residual thiol ligands on the surface were prepared through electrochemical activation of a thiol-ligand-based Ag complex. When it was used as a catalyst for CO₂R, the thiol-ligand modified Ag exhibited high CO selectivity (>90%) throughout a wide electrode-potential range; furthermore, high cathodic energy efficiencies of >90% and >70% were obtained for CO formation at high current densities of 150 and 750 mA cm^-2, respectively, outperforming the state-of-the-art Ag-based electrocatalysts for CO₂ to CO conversion. The first-principle calculations on the reaction energetics suggest that the binding energies of the key intermediate -*COOH on Ag are optimized by the adsorbed thiol ligand, thus favoring CO formation while suppressing the competing H₂ evolution. Our findings provide a rational design strategy for CO₂ reduction electrocatalyst by electronic modulation through surface-adsorbed ligands.

Electrochemical Reduction of CO2 in Tubular Flow Cells under Gas-Liquid Taylor Flow

Electrochemical reduction of CO₂ using renewable energy is a promising avenue for sustainable production of bulk chemicals. However, CO₂ electrolysis in aqueous systems is severely limited by mass transfer, leading to low reactor performance insufficient for industrial application. This paper shows that structured reactors operated under gas-liquid Taylor flow can overcome these limitations and significantly improve the reactor performance. This is achieved by reducing the boundary layer for mass transfer to the thin liquid film between the CO₂ bubbles and the electrode. This work aims to understand the relationship between process conditions, mass transfer, and reactor performance by developing an easy-to-use analytical model. We find that the film thickness and the volume ratio of CO₂/electrolyte fed to the reactor significantly affect the current density and the faradaic efficiency. Additionally, we find industrially relevant performance when operating the reactor at an elevated pressure beyond 5 bar. We compare our predictions with numerical simulations based on the unit cell approach, showing good agreement for a large window of operating parameters, illustrating when the easy-to-use predictive expressions for the current density and faradaic efficiency can be applied.

Cracks as Efficient Tools to Mitigate Flooding in Gas Diffusion Electrodes Used for the Electrochemical Reduction of Carbon Dioxide

The advantage of employing gas diffusion electrodes (GDEs) in carbon dioxide reduction electrolyzers is that they allow CO₂ to reach the catalyst in gaseous state, enabling current densities that are orders of magnitude larger than what is achievable in standard H-type cells. The gain in the reaction rate comes, however, at the cost of stability issues related to flooding that occurs when excess electrolyte permeates the micropores of the GDE, effectively blocking the access of CO₂ to the catalyst. For electrolyzers operated with alkaline electrolytes, flooding leaves clear traces within the GDE in the form of precipitated potassium (hydrogen)carbonates. By analyzing the amount and distribution of precipitates, and by quantifying potassium salts transported through the GDE during operation (electrolyte perspiration), important information can be gained with regard to the extent and means of flooding. In this work, a novel combination of energy dispersive X-ray and inductively coupled plasma mass spectrometry based methods is employed to study flooding-related phenomena in GDEs differing in the abundance of cracks in the microporous layer. It is concluded that cracks play an important role in the electrolyte management of CO₂ electrolyzers, and that electrolyte perspiration through cracks is paramount in avoiding flooding-related performance drops.