Electrochemical Reduction Of CO2 Research Articles

Direct quantification of electrochemical CO2 reduction products with an improved DEMS setup

Direct quantification of electrochemical CO2 reduction products with an improved DEMS setup

Maximizing CO2 reduction efficiency: surface‐regulated highly porous Ag‐Cu alloy aerogel inserted with multi‐wall carbon nanotube achieving nearly complete CO selectivity

The electrochemical reduction of CO2 (eCO2RR) holds promise in mitigating atmospheric greenhouse gas levels but is hindered by low reaction kinetics, high energy barriers, and poor selectivity. To address these challenges, we developed a novel series of cost‐effective AgCu‐containing metal‐aerogel catalysts with high electrochemical surface areas (ECSA) using a top‐down reduction approach. The Ag85Cu15 aerogel, with an ECSA of 27.41 cm², achieved a Faraday efficiency (FE%) of 89.3% for CO production at ‐0.9 V vs. RHE. Increasing the Cu content to over 50% in the aerogel matrix produced small amounts of C2H4, with a maximum FE% of 12.9% at ‐1.0 V vs. RHE. To further enhance CO2 reduction efficiency, multi‐walled carbon nanotubes (CNT) were incorporated into the Ag85Cu15 alloy aerogel via a hydrothermal treatment. The highly dispersed CNTs within the aerogel matrix increased the ECSA to 57.00 cm² by forming a well‐defined porous structure through van‐der Waals interactions, improving CO selectivity, and achieving a FE% of 98.6% at ‐0.7 V vs. RHE and a partial current density of 9.6 mA/cm² in an H‐cell. 86% of the initial FECO% was maintained during an 18 h test with continuous electrolysis.

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.

A First-Principles Study on the Reaction Mechanisms of Electrochemical CO2 Reduction to C1 and C2 Products on Cu(110)

The mechanism of the electrochemical CO2 reduction reaction on a Cu(110) surface has yet to be fully revealed. In this work, based on first-principles calculations, we investigate the mechanisms of the CO2 reduction reaction to produce C1 (including one C atom) and C2 (including two C atoms) products on a Cu(110) surface. The results show that CH4 and C2H5OH are the main C1 and C2 products on the Cu(110) surface, respectively. CH4 is produced along the pathway CO2 → COOH* → CO* → CHO* → CH2O* → CH3O* → CH4. C2H5OH is produced via the C-C coupling pathway between CO* and CH2O* intermediates, which is the key reaction step. This is because CO* and CH2O* coupling to CO-CH2O* has the lowest barrier among the CHxO* (x = 0–2) coupling pathways. Therefore, it is the most likely C-C coupling pathway. Further, CO-CH2O* is gradually hydrogenated to C2H5OH along the following pathway: CO-CH2O* → CHO-CH2O* → CHOH-CH2* → CH2OH-CH2* → CH2OH-CH3* → C2H5OH.

Improving the roughness of copper working electrode through electroless deposition for carbon dioxide reduction reaction

Electrocatalytic CO2 conversion into usable fuels (CO, CH4, C2H4, and HCOOH) is a promising approach to mitigate the greenhouse effect and the energy deficit. Herein, we employ the wet etching process to significantly enhance the surface roughness of the copper foil, followed by deposition of Cu particles on the surface-etched copper foil (Cu NPs/Cu foil) for electrochemical CO2 reduction. FESEM and AFM results confirms the morphology and roughness changes. Introducing glycine to the electrolyte increase the Faraday efficiency (FE) of the resulting products. Upon adding 5 × 10−5 mM glycine significantly improves the FEs 10.1 % CO, 8.7 % CH4, 11.7 % C2H4, and 44.5 % HCOOH, with 75.0 % as C1 and C2 compounds. Concurrently, hydrogen production decreases to 20.2 % at a potential of –1.4 V utilizing roughened Cu NPs/Cu foil as a catalyst. These results shed light on the factors contributing to increased hydrocarbons and formic acid production with enhanced surface roughness of Cu foil.

Electrochemical reduction of CO2 on a CoTPP/MWCNT composite: Investigation of operation parameters influence on CH3OH production by differential electrochemical mass spectrometry (DEMS)

Renewable electricity-driven electrochemical production of small organic molecules, such as CH3OH, from chemical industry waste CO2 feedstock is highly desirable for circular economy. These reactions proceed via multiple intermediate steps which causes high overpotential and poor selectivity imposing a challenge for designing techno-economically viable systems. Proper understanding of the reaction mechanism is essential to overcome those challenges. Herein, we present a simple qualitative analysis to understand the reaction mechanism during electrochemical reduction of CO2 (eCO2R) on a cobalt tetraphenylporphyrin / multiwalled carbon nanotube (CoTPP/MWCNT) composite in the temperature range of 20–50 °C by employing differential electrochemical mass spectrometry (DEMS) in 0.1 M and 0.5 M KHCO3 electrolytes. Interestingly, temperature is observed to strongly affect the onset potentials for product generation in such a way that with the increase of temperature from 20 °C to 50 °C a decrease in the onset potential specifically for methanol formation is observed. Moreover, formaldehyde (HCHO) formation appears to occur at lower overpotentials before the formation of CH3OH which suggests that on the composite electrocatalyst, HCHO is an important intermediate on a route to CH3OH. This work offers valuable information on reaction routes to CH3OH and temperature effects on the eCO2R selectivity on molecular catalysts.

Electrochemical co-reduction of N2 and CO2 to urea using In2S3 anchored on S-doped reduced graphene oxide

The generation of urea by electrocatalysis using N2 and CO2 as feedstock is considered as a promising and green sustainable alternative to conventional energy-consuming industrial processes. Based on the existing studies, it was found that the effective adsorption and activation of N2 and CO2 on the catalyst surface is the primary condition for electrocatalytic generation, while the main by-product hydrogen evolution reaction (HER). Should be avoided as much as possible in order to improve the yield. In this paper, an In2S3-loaded electrocatalyst on sulfur-doped reduced graphene oxide for urea synthesis was designed and prepared with an average Faraday efficiency (FE) of 37.17 % and a urea yield of 7.24 mmol h−1 g−1, while maintaining long-term stability. In2S3@S-RGO has a unique porous structure of doped graphene, with continuous micron-sized pores that facilitate fast mass transfer, improve kinetic efficiency, and expand electroactive sites, thus enhancing the urea synthesis performance. Interestingly, the In2S3@S-RGO electrocatalyst not only exhibits excellent electrochemical performance, but also facilitates the adsorption and activation of reactants (N2 and CO2) due to the inhibitory effect of the indium-based material on HER. This work may provide new ideas for advanced catalysts to improve the efficiency of electrochemical synthesis of urea Faraday and to facilitate CN coupling applications.

Effects of electrochemical active surface area of Cu on electrochemical CO2 reduction in acidic electrolyte using Cu nanoparticles on surfactant-treated carbon

Electrochemical carbon dioxide reduction reaction (CO2RR) presents a viable approach for transforming CO2 into valuable products. Traditional employment of neutral or alkaline electrolytes in CO2RR aims to avoid the hydrogen evolution reaction (HER). However, the usage of such electrolytes has intrinsic limitations on the availability of CO2 due to homogeneous equilibrium reactions, resulting in high energy costs for electrolyte regeneration. Lowering the pH of electrolytes can mitigate this concern and facilitate industrially relevant efficiencies. While recent studies show promising results for CO2RR in acidic environments using flow cells, the translation of conventional electrocatalysts from neutral or alkaline conditions to acidic conditions remains unclear. Hence, our investigation focuses on assessing the impact of surfactant-modified synthesis methods on CO2RR performance in acidic electrolytes.We first synthesized Cu nanoparticles on surfactant-treated carbon black by employing various surfactants: hexadecyltrimethylammonium bromide (CTAB), sodium dodecyl sulfate, and Triton X-100, representing cationic, anionic, and nonionic surfactants, respectively. The objective of this synthesis was to investigate how surfactants influence the size and distribution of Cu nanoparticles (Cu NPs), the electrochemical active surface area (ECSA) of Cu, and, consequently, the performance of CO2RR in acidic electrolytes. The findings reveal that Cu NPs on carbon black show a different profile of size and distribution depending on the surfactant used for synthesis, resulting in varied ECSA. In addition, ECSA plays a crucial role in the production of C2H4 in an acidic medium, determining the onset potential and production rate of C2H4 while maintaining original Cu activity. In summary, surfactant treatment not only controls the size but also enables the uniform distribution of Cu NPs on carbon black. Consequently, Cu NPs exhibiting larger ECSA demonstrate enhanced efficiency in the production of C2H4 compared to counterparts with smaller ECSA under acidic environment.

High-purity ethylene production via indirect carbon dioxide electrochemical reduction

High-purity ethylene production from CO2 electroreduction (CO2RR) is a coveted, yet arduous feat because the product stream comprises a blend of unreacted CO2, H2, and other off-target CO2 reduction products. Here we present an indirect reduction strategy for CO2-to-ethylene conversion, one that employs 2-bromoethanol (Br-EO) as a mediator. Br-EO is initially generated from CO2RR and subsequently undergoes reduction to ethylene without the need for energy-intensive separation steps. The optimized AC-Ag/C catalyst with Cl incorporation reduces the energy barrier of the debromination step during Br-EO reduction, and accelerates the mass-transfer process, delivering a 4-fold decrease of the relaxation time constant. Resultantly, AC-Ag/C achieved a FEethylene of over 95.0 ± 0.36% at a low potential of −0.08 V versus reversible hydrogen electrode (RHE) in an H-type cell with 0.5 M KCl electrolyte, alongside a near 100% selectivity within the range of −0.38 to −0.58 V versus RHE. Through this indirect strategy, the average ethylene purity within 6-hour electrolysis was 98.00 ± 1.45 wt%, at −0.48 V (vs RHE) from the neutralized electrolyte after CO2 reduction over the Cu/Cu2O catalyst in a flow-cell.

Decoupling and quantifying the mass transfer resistance of the gas diffusion electrode for CO2 electrochemical reduction reaction

Mass transfer capacity ultimately dominates and limits the performance of CO2 electrochemical reduction reaction (CO2RR), which is closely related to industrial applications. Hence, the quantification of CO2 transport resistance is urgent and necessary. In addition, the transport of CO2 involves the entire process from the gas phase source to the catalytic site, resulting in the superposition of transport resistance from each component of the electrolyzer, which makes the transport resistance decoupling challenging. Based on the limiting current method, an experimental method is developed here to quantify the transport resistance of the complex porous electrode of the carbon fiber substrate (CFS), microporous layer (MPL) and catalyst layer (CL). Additionally, a dummy catalyst layer (DCL) is designed, which has a similar pore structure to CL but no catalytic sites for CO2RR, to further quantify the resistance in CL formed from pore structures and ionomer properties. The experimental results can provide the optimization direction and experimental basis for the subsequent electrode design.

Multiscale Modeling of CO2 Electrochemical Reduction on Copper Electrocatalysts: A Review of Advancements, Challenges, and Future Directions.

Although CO2 contributes to global warming, it also offers potential as a raw material for the production of hydrocarbons (CH4, C2H4 and CH3OH). Electrochemical CO2 reduction reaction (eCO2RR) is an emerging technology that utilizes renewable energy to convert CO2 into valuable fuels, solving environmental and energy problems simultaneously. Insights gained at any individual scale can only provide a limited view of that specific scale. Multiscale modeling, which involves coupling atomistic-level insights (DFT) and (MD), with mesoscale (KMCand MK) and macroscale (CFD) simulations, has received significant attention recently. While multiscale modeling of eCO2RR on electrocatalysts across all scales is limited due to its complexity, this review offers an overview of recent works on single scales and the coupling of two and three scales, such as "DFT+MD", "DFT+KMC", "DFT+MK", "KMC/MK+CFD" and "DFT+MK/KMC+CFD", focusing particularly on Cu-based electrocatalysts. This sets it apart from other reviews that solely focus exclusively on a single scale or only on a combination of DFT and MK/KMC scales. Furthermore, this review offers a concise overview of machine learning (ML) applications for eCO2RR, an emerging approach that has not yet been reviewed. Finally, this review highlights the key challenges, research gaps and perspectives of multiscale modeling for eCO2RR.

Constructing Favorable Microenvironment on Copper Grain Boundaries for CO2 Electro-conversion to Multicarbon Products.

The electrochemical CO2 reduction reaction (eCO2RR) to multicarbon chemicals provides a promising avenue for storing renewable energy. Herein, we synthesized small Cu nanoparticles featuring enriched tiny grain boundaries (RGBs-Cu) through spatial confinement and in situ electroreduction. In-situ spectroscopy and theoretical calculations demonstrate that small-sized Cu grain boundaries significantly enhance the adsorption of the *CO intermediate, owing to the presence of abundant low-coordinated and disordered atoms. Furthermore, these grain boundaries, generated in situ under high current conditions, exhibit excellent stability during the eCO2RR process, thereby creating a stable *CO-rich microenvironment. This high local *CO concentration around the catalyst surface can reduce the energy barrier for C-C coupling and significantly increase the Faradaic efficiency (FE) for multicarbon products across both neutral and alkaline electrolytes. Specifically, the developed RGBs-Cu electrocatalyst achieved a peak FE of 77.3% for multicarbon products and maintained more than 134 h stability at a constant current density of -500 mA cm-2.

Distinct CO2-run-out regime from steric effect of electric double layer in electrochemical CO2 reduction

The field of electrochemical CO2 reduction reaction (eCO2RR) is pursuing high operating current densities, eventually controlled by CO2 transport. Here, we develop a new multiscale modeling approach that is able to more generally describe the effects of the electric double layer (EDL) on CO2 transport over a wide potential window extending to utmost potentials. By leveraging it, we identify a distinct CO2-run-out regime where the supply of CO2 runs out due to the EDL steric effect from a dense layer of solvated cations with the maximum layer thickness equal to the solvated cation size. Consequently, CO2RR current density drops at a relatively negative transition potential generating a bell-shaped polarization curve, which is in contrast to the CO2-transport-limited regime where the current density reaches a plateau. Furthermore, we develop a graphical method, verified by experimental data, to generally predict the transition to the CO2-run-out regime. This work sheds new light on the EDL effects for catalyst design and electrolyzer engineering.

Electrochemical CO2 Reduction on Cu-Based Monatomic Alloys: A DFT Study.

In recent years, single-atom alloy catalysts (SAAs) have received much attention due to the combination of structural features of both single-atom and alloy catalysts, as well as their efficient catalytic activity, high selectivity, and high stability in various chemical reactions. In this work, we designed a series of Cu-based SAAs by doping isolated 3d transition metal (TM1) atoms on the surface of Cu(111) (TM1 = Fe, Co, Ru, Rh, Os and Ir), in which Ir1/Cu(111) SAAs are considered to be the most stable among 3d-series SAAs due to their optimal binding energy (Eb). The density of states of SAAs have been systematically investigated to further discuss structural properties. Based on density functional theory calculations, the activity and selectivity of Ir1/Cu(111) SAAs are investigated for electrocatalytic CO2 reduction reaction (CO2RR). The initial hydrogenation of CO2 on Ir1/Cu(111) SAAs can form *CO intermediates, which will be further to CH4 production by the pathway of *CO → *CHO → *CHOH → *CH2OH → *CH2 → *CH3 → CH4. This study provides theoretical insights for the rational design of selective Cu-based monatomic alloy catalysts.

CO2 electroreduction on single atom catalysts: Role of the local coordination

Single Atom Catalysts (SACs) are a new frontier in catalysis, merging the positive aspects of both homogeneous and heterogenous catalysts. CO2 electrochemical reduction is a key reaction for the energy transition. In this work, we investigate the role of the local coordination on the activation of CO2 on SACs, by means of density functional theory (DFT) calculations. We scrutinize 20 transitional metal atoms embedded in a graphene support. The results indicate that the local coordination has a critical role both to the binding energy of the metal to the support and the activation of CO2. The role of the environment around the active site is as important as the nature of the metal atom. At the same time, the stability of reaction intermediates strongly depends on the local coordination. In some cases, the relative stability of the intermediates changes as function of the environment, affecting the selectivity of the reaction. This study provides evidence of the importance of the local coordination in single-atom catalysis, a crucial aspect if one is interested in providing a rationale to the reactivity, or in predicting novel catalysts.

Confined CuⅠ sites in partially electro-reduced 2D conductive Cu-MOF film for boosting CO2 electrocatalysis to C2 products

Cu-based catalysts, especially co-sites of Cu0 and CuⅠ, are the best competitors for electrochemical CO2 fixation to produce multi-carbon (C2 + ) products. However, it is challenging to construct highly stable CuⅠ sites due to the changeability of its valence state and electronic structure in the electroreduction process, and the mechanism is also elusive. Herein, a 2D conductive CuⅡ-MOF film (CuⅡHHTP@Cu0) is prepared on Cu (100) foil by in-situ electrochemical assembly. After pre-reduction, the in-situ produced Cu2O clusters are confined in the cavities of the film created by the local breaking of the MOF framework to construct a unique CuⅠ-CuⅡHHTP@Cu0 catalyst, so that a novel strategy is proposed to stablize highly dispersed CuⅠ sites during electrochemical reduction, thus protecting the Cu2O clusters from further reduction and ensuring the coexistence of CuⅠ (111)-Cu0 (100) sites. As such, the current density of CO2 electrochemical reduction catalyzed by this catalyst exceeds 300 mA cm−2, the duration overs 50 h, and the C2 selectivity achieves 72.6 %. In-situ spectroscopy and theoretical calculations reveal that the optimized microenvironment of the CuⅠ (111) sites reduces the energy barrier of *OCCOH coupling, and Cu0 (100) sites increase the coverage of *CO, thus boosting the production of C2 products.

Selective and stable CO2 electroreduction at high rates via control of local H2O/CO2 ratio

Controlling the concentrations of H2O and CO2 at the reaction interface is crucial for achieving efficient electrochemical CO2 reduction. However, precise control of these variables during catalysis remains challenging, and the underlying mechanisms are not fully understood. Herein, guided by a multi-physics model, we demonstrate that tuning the local H2O/CO2 concentrations is achievable by thin polymer coatings on the catalyst surface. Beyond the often-explored hydrophobicity, polymer properties of gas permeability and water-uptake ability are even more critical for this purpose. With these insights, we achieve CO2 reduction on copper with Faradaic efficiency exceeding 87% towards multi-carbon products at a high current density of −2 A cm−2. Encouraging cathodic energy efficiency (>50%) is also observed at this high current density due to the substantially reduced cathodic potential. Additionally, we demonstrate stable CO2 reduction for over 150 h at practically relevant current densities owning to the robust reaction interface. Moreover, this strategy has been extended to membrane electrode assemblies and other catalysts for CO2 reduction. Our findings underscore the significance of fine-tuning the local H2O/CO2 balance for future CO2 reduction applications.

Cu Based Dilute Alloys for Tuning the C2+ Selectivity of Electrochemical CO2 Reduction.

Electrochemical CO2 reduction is a promising technology for replacing fossil fuel feedstocks in the chemical industry but further improvements in catalyst selectivity need to be made. So far, only copper-based catalysts have shown efficient conversion of CO2 into the desired multi-carbon (C2+) products. This work explores Cu-based dilute alloys to systematically tune the energy landscape of CO2 electrolysis toward C2+ products. Selection of the dilute alloy components is guided by grand canonical density functional theory simulations using the calculated binding energies of the reaction intermediates CO*, CHO*, and OCCO* dimer as descriptors for the selectivity toward C2+ products. A physical vapor deposition catalyst testing platform is employed to isolate the effect of alloy composition on the C2+/C1 product branching ratio without interference from catalyst morphology or catalyst integration. Six dilute alloy catalysts are prepared and tested with respect to their C2+/C1 product ratio using different electrolyzer environments including selected tests in a 100-cm2 electrolyzer. Consistent with theory, CuAl, CuB, CuGa and especially CuSc show increased selectivity toward C2+ products by making CO dimerization energetically more favorable on the dominant Cu facets, demonstrating the power of using the dilute alloy approach to tune the selectivity of CO2 electrolysis.

Sensitivity Analysis of One-Dimensional Multiphysics Simulation of CO2 Electrolysis Cell.

Electrochemical (EC) carbon dioxide (CO2) reduction, where CO2 is converted to value-added products such as fuel precursors, plays a key role in helping the world's energy system reach net-zero carbon emissions. Simulations of EC cells provide valuable insight into their operation since detailed experimental results on short length and time scales are difficult to obtain. In this work, we construct a 1D simulation of a membrane-electrode-assembly EC cell for CO2 reduction, using a porous silver gas diffusion cathode. We run the simulation under different electrolyte conditions, showing how the cell performance is affected. We then perform a sensitivity analysis of all input parameters to the simulation, which has not been presented before in the literature. We show that the CO partial current density (i CO) is significantly affected by each input parameter of the simulation. i CO is most sensitive to EC kinetic parameters (i 0/α) of all EC reactions, with a 1% change in α resulting in up to 6% change in i CO. Since there is uncertainty associated with the value of each input parameter, this indicates that infidelity between experiment and simulation is likely, and thus, caution should be practiced when comparing experimental results to simulation results. Further, we show that the large range of conditions simulated in literature helps to explain the large variance in reported values of i 0 and α. The results of this paper demonstrate the potential of sensitivity analysis methods to quickly optimize aspects of cell performance (CO2 utilization, Faradaic efficiency, etc.).

ICP-MS Assisted EDX Tomography: A Robust Method for Studying Electrolyte Penetration Phenomena in Gas Diffusion Electrodes Applied to CO2 Electrolysis.

A carbon paper-based gas diffusion electrode (GDE) is used with a bismuth(III) subcarbonate active catalyst phase for the electrochemical reduction of CO2 in a gas/electrolyte flow-by configuration electrolyser at high current density. It is demonstrated that in this configuration, the gas and catholyte phases recombine to form K2CO3/KHCO3 precipitates to an extent that after electrolyses, vast amount of K+ ions is found by EDX mapping in the entire GDE structure. The fact that the entirety of the GDE gets wetted during electrolysis should, however, not be interpreted as a sign of flooding of the catalyst layer, since electrolyte perspiring through the GDE can largely be removed with the outflow gas, and the efficiency of electrolysis (toward the selective production of formate) can thus be maintained high for several hours. For a full spatial scale quantitative monitoring of electrolyte penetration into the GDE, (relying on K+ ions as tracer) the method of inductively coupled plasma-mass spectrometry (ICP-MS) assisted energy dispersive X-ray (EDX) tomography is introduced. This new, cheap and robust tomography of non-uniform aspect ratio has a large planar span that comprises the entire GDE surface area and a submicrometer depth resolution, hence it can provide quantitative information about the amount and distribution of K+ remnants inside the GDE structure, in threedimensions.