CO2 Diffusion Research Articles

Three-dimensional core reconstruction and performance evaluation of CO2 displacement in a tight oil reservoir

In this study, a three-dimensional tight sandstone core was reconstructed digitally using serials thin sections cut from a core sample used in laboratory carbon dioxide (CO2) flooding experiments. Based on the digital core, multiphase flow simulations were carried out to evaluate the performance of three prior laboratory-tested injection strategies: CO2 continuous injection, water alternative gas (WAG: CO2) injection, and cyclic CO2 injection. The chosen image-based reconstruction led to a close porosity and permeability match with the sampled core in an economical way. The numerical evaluation closely replicated the laboratory-tested procedures, thus providing realistic fluid distributions before CO2 injection. To determine saturation functions for CO2 injection after water flooding, sensitivities of endpoint relative permeability and capillary pressure to CO2 continuous injection were quantified. Water relative permeability was identified as a key factor. The saturation functions were used in a history match between simulations and laboratory data where close agreements were observed for all three CO2 injection strategies. Then critical enhanced oil recovery (EOR) operational parameters (soaking time, slug ratio, cycle size, and test duration) were optimized to maximize CO2 displacement performance, which led to the following observations: (1) While extended soaking after CO2 continuous injection increased incremental oil recovery, such increase was limited due to concurrent soaking during low-rate CO2 injection. (2) In WAG injection, CO2 and brine slug sizes and ratios were optimized: increasing the cycle size performed better than adjusting the slug ratio for favorable mobility control and efficient CO2 EOR. (3) For cyclic CO2 injection, the total CO2 injection volume was the most important factor, followed by the number of cycles, to allow CO2 diffusion into tight core to maximize the swept pore volumes. The history-matched simulations suggest that both WAG and cyclic CO2 injection can achieve high displacement effectiveness for the tight reservoir, compared with that achieved with slight soaking after CO2 continuous injection. The injected brine slugs in the WAG injection reduced the CO2 needed at the expense of reduced storage capacity. Compared to WAG, cyclic CO2 injection, though realizing the highest oil recovery and gas storage, requires more CO2.

Synthesis and properties of La-Fe-Si/La-Co magnetocaloric composites

LaFe11.8Si1.2/16wt%La65Co35 samples were prepared by spark plasma sintering (SPS) and subsequent annealing. The La65Co35 binder increased the desired 1:13 phase content in these samples during annealing at 1323 K. Interestingly, the α-Fe phase formed between the LaFe11.8Si1.2 particles and the La65Co35 binder and then disappeared as the annealing time extended. The sample annealed for 0.5 h had a distinct core-shell structure with Co in the shell, a desirable table-like (−∆SM)−T curve was obtained and its RC (153 J·kg−1) was 107% higher compared to the sample before annealing. The diffusion of Co from La3Co to the 1:13 phase was hampered by the growth of La5Si3 phase. Relatively large (−∆SM)max of 8.51 J·kg−1·K−1 (Δμ0H=2 T) and (σbc)max of 374 MPa for the sample annealed for 24 h were obtained. These results indicated that the method combining SPS and diffusion annealing is promising to prepare La-Fe-Si based magnetocaloric composites with good properties.

Mesophyll conductance and N allocation co-explained the variation in photosynthesis in two canola genotypes under contrasting nitrogen supply.

The application of nitrogen fertilizer within a normal range has been found to increase the leaf nitrogen content and photosynthetic rate of canola plants (Brassica napus L.). Despite numerous studies on the separate effects of CO2 diffusion limitation and nitrogen allocation trade-off on photosynthetic rate, few have examined both these factors in relation to the photosynthetic rate of canola. In this study, two genotypes of canola with varying leaf nitrogen content were analyzed to determine the impact of nitrogen supply on leaf photosynthesis, mesophyll conductance, and nitrogen partitioning. The results showed that the CO2 assimilation rate (A), mesophyll conductance (g m), and photosynthetic nitrogen content (N psn) increased with an increase in nitrogen supply in both genotypes. The relationship between nitrogen content and A followed a linear-plateau regression, while A had linear relationships with both photosynthetic nitrogen content and g m, indicating that the key to enhancing A is increasing the distribution of leaf nitrogen into the photosynthetic apparatus and g m, rather than just increasing nitrogen content. Under high nitrogen treatment, the genotype (QZ) with high nitrogen content had 50.7% more nitrogen than the other genotype (ZY21), but had similar A, which was primarily due to ZY21's higher photosynthetic nitrogen distribution ratio and stomatal conductance (g sw). On the other hand, QZ showed a higher A than ZY21 under low nitrogen treatment as QZ had higher N psn and g m compared to ZY21. Our results indicate that, in selecting high PNUE rapeseed varieties, it is important to consider the higher photosynthetic nitrogen distribution ratio and CO2 diffusion conductance.

Mixed matrix membranes (MMMs) comprising the CO2-Philic wavelike layer of hydrogen-bonded organic frameworks (HOFs) for highly efficient blood oxygenation

Oxygenation efficiency from extracorporeal membrane oxygenation (ECMO) is hindered by membrane resistance and CO2/O2 selectivity in the process of gas transport through membrane. Herein, we designed a mixed matrix membrane (MMM) as an artificial lung membrane, which was prepared by blending polyphenylene oxide (PPO) with hydrogen-bonded organic frameworks (HOFs) named PFC-11. The interface interactions showed that PFC-11 and PPO had good interfacial compatibility and formed a uniform structure with great mechanical properties. The addition of PFC-11 provided additional gas transport channels for MMMs, and the special groups and atoms in the channels promoted the diffusion of CO2. The gas permeance and CO2/O2 selectivity of MMMs were effectively improved. The oxygenation performance test and liquid entry pressure measurement proved that PPO/PFC-11 MMMs had sufficient oxygenation capacity and excellent wetting-resistant ability to meet the actual ECMO application. The hemocompatibility of MMMs was evaluated by hemolysis rate and blood cell adhesion. The PPO/PFC-11 MMMs exhibited excellent performance for ECMO and provided a novel direction for the research and development of efficient artificial lung membranes.

Bi2MoO6 Embedded in 3D Porous N,O-Doped Carbon Nanosheets for Photocatalytic CO2 Reduction.

Artificial photosynthesis is promising to convert solar energy and CO2 into valuable chemicals, and to alleviate the problems of the greenhouse effect and the climate change crisis. Here, we fabricated a novel photocatalyst by directly growing Bi2MoO6 nanosheets on three-dimensional (3D) N,O-doped carbon (NO-C). Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) show that the designed photocatalyst ensured the close contact between Bi2MoO6 and NO-C, and reduced the stacking of the NO-C layers to provide abundant channels for the diffusion of CO2, while NO-C can allow for fast electron transfer. The charge transfer in this composite was determined to follow a step-scheme mechanism, which not only facilitates the separation of charge carriers but also retains a strong redox capability. Benefiting from this unique 3D structure and the synergistic effect, BMO/NO-C showed excellent performance in photocatalytic CO2 reductions. The yields of the best BMO/NO-C catalysts for CH4 and CO were 9.14 and 14.49 μmol g-1 h-1, respectively. This work provides new insights into constructing step-scheme photocatalytic systems with the 3D nanostructures.

A multiphysical‐geochemical coupling model for caprock sealing efficiency in CO2 geosequestration

AbstractPrecipitation or dissolution due to geochemical reactions has been observed in the caprocks for CO2 geosequestration. Geochemical reactions modify the caprock sealing efficiency with self‐limiting or self‐enhancement. However, the effect of this modification on the caprock sealing efficiency has not been fully investigated through multiphysical‐geochemical coupling analysis. In this study, a multiphysical‐geochemical coupling model was proposed to analyze caprock sealing efficiency. This coupling model considered the full couplings of caprock deformation, two‐phase flow, CO2 concentration diffusion, geochemical reaction, and CO2 sorption. The two‐phase flow only occurs in the fracture network and the CO2 may partially dissolve into water and diffuse through the concentration difference. The dissolved CO2 has geochemical reactions with some critical minerals, thus altering flow channels. The CO2 in the fracture network diffuses into matrix, causing the matrix swelling. This fully coupling model was validated with a penetration experiment on a cement cube and compared with two other models for CO2 storage plumes. Finally, the effects of geochemical reactions on penetration depth and pore pressure were studied through parametric study. The numerical simulations reveal that the coupling of geochemical reactions and matrix diffusion significantly affect the caprock sealing efficiency. Geochemical reactions occur at a short time after the arrival of CO2 concentration and modify the fracture porosity. The CO2 diffusion into the matrix requires a much longer time and mainly induces matrix swelling. These effects may produce self‐enhancement or self‐limiting depending on the flow rate in the fracture network, thus significantly modifying caprock sealing efficiency.

Variation in leaf mesophyll anatomy of fern species imposes significant effects on leaf gas exchange, light capture, and leaf hydraulic conductance

The mesophyll anatomical traits are essential factors for efficient light capture, CO2 diffusion, and hydraulics in leaves. At the same time, leaf hydraulics are governed by the xylem anatomical traits. Thus, simultaneous analyses of the mesophyll and xylem anatomy will clarify the links among light capture, CO2 capture, and water use. However, such simultaneous analyses have been scarcely performed, particularly on non-seed plants. Using seven fern species, we first showed that fern species with a large mesophyll thickness had a high photosynthetic rate related to high light capture, high drought tolerance, and low leaf hydraulic conductance. The chloroplast surface area (Sc) per mesophyll thickness significantly decreased with an increase in mesophyll thickness, which may increase light diffusion and absorption efficiency in each chloroplast. The photosynthetic rate per Sc was almost constant with mesophyll thickness, which suggests that ferns enhance their light capture ability via the regulation of chloroplast density.

From tetraalkylphosphonium ionic liquids to phosphonium ylides: How the ionic sizes influence carbon dioxide capture?

Carbon dioxide (CO2) capture via the phosphonium ylide mechanism represents an interesting alternative to alkylamine-based carboxamidation. The tertiary cation-based CO2 sorption supplements the nitrogen-containing anion-based CO2 sorption which ultimately results in very competitive aggregate capacities of such ionic compounds. In the present work, we reveal the impact of the cation and anion sizes on the deprotonation and subsequent carboxylation of the α-methylene carbon of the phosphonium-based cations. In particular, we computationally investigate the role of the alkyl chains and rationalize the previously published experimental results. Since the anions strongly influence the kinetics and thermodynamics of the processes involving their counterions, we herein supplemented the triethyl-butyl phosphonium [P2224] and trihexyl-tetradecyl phosphonium [P66614] cations in our models with the benzimidazolide [BENZIM], and 2–methylthio-benzimidazolide aprotic anions [2MTBENZIM]. By having determined the energetic effects, activation barriers, and steric hindrances faced by the above-referenced processes through potential energy surface scans empowered by electronic-structure simulations, we formulated lucid guidelines for the molecular design of the CO2 scavengers of this class. We found that the alkyl chain length plays a marginal role in the reaction profiles for CO2 capture. In turn, an identity of the anion was found to impact the phosphonium ylide formation stages by 20 % by adjusting local intermolecular interactions. By analyzing molecular geometries in their initial, transition, and product states along with the atomic hydrophobicities, we explained the stepwise energetics of the CO2 chemisorption by the phosphonium-based sorbents of complicated structure and two-stage performance. As we did not detect differences of commensurate magnitude between the [P2224] and [P66614] cations paired with both [BENZIM] and [2MTBENZIM], we linked a somewhat deteriorated experimentally detected performance (∼10 %) of [P66614] to differing shear viscosities and, thus, slowed down the diffusion of CO2. For the sake of practical applications, the [P2224]-based ionic liquids look preferable as long as their physicochemical properties meet the criteria of a specific technological task and the costs of their synthesis are comparable to those of the [P66614]-based ones.

Diffusion coefficient and the volume swelling of CO2/light oil systems: Insights from dynamic volume analysis and molecular dynamics simulation

The oil volume expansion and CO2 diffusion are one of the main mechanism of CO2-enhanced oil recovery (CO2-EOR). This paper established a series of experiments, numerical simulations, and molecular dynamics (MD) simulations to describe mass transfer behaviors between oil–gas phases. Expressly, the CO2 diffusion coefficient and light oil swelling factor are signification parameters to quantify and analyze these behaviors. In detail, the CO2 diffusion coefficient and the oil swelling factor were obtained from the traditional pressure decay method and the advanced MD simulation. Synthetically, the pressure decay method and MD simulation results were mutually verified. The results showed that the equilibrium pressure was proportional to the mole fraction of CO2 and inversely proportional to the mole fraction of light oil. The equilibrium time was proportional to the mole fraction of CO2 and light oil. At the temperature of 333.15 K and the initial pressure of 7.5 MPa, the CO2 diffusion coefficient in light oil was positively correlated with the relative molar proportion of CO2, while the light oil swelling factor was vice versa. In addition, the closer to the CO2-light oil interface, the greater the light oil's potential energy and self-diffusion coefficient, and the stronger the transport ability.

Mechanism of CO2 capture by ethanol-biochar functionalized cross-linking enhanced ammonia

CCUS technology is an important means to achieve carbon neutralization of energy utilization, in which the ammonia CO2-capture method is a relatively mature technology, with some problems (such as low CO2 absorption efficiency and serious ammonia escape). An ethanol/biochar synergistic ammonia carbon capture technology is proposed, and the mass transfer coefficient is studied by wetted-wall column, so as to assist in proving that biochar and ethanol have synergistic effect on the absorption of CO2 by ammonia. The simulation shows that the micropores in biochar adsorb and enrich CO2, the mesopores and macropores diffuse and transport CO2, and dynamically retain free NH3, thereby enhancing the mass transfer capacity of CO2 in the carbon capture system. And the experiment shows that the mass transfer efficiency of biochar-ethanol to the reaction solution is increased by 15.4%, which is higher than that of ethanol (7.1%),but biochar can make ammonia escape seriously. If the ammonia concentration can be maintained constant, the enhancement effect of the pore structure of biochar on the diffusion of CO2 into ammonia will provide a new direction for the improvement of CO2 capture by ammonia method.

Contrasting leaf intercellular space development in sorghum and maize modulates different tolerance capacity to water limitation.

The objective of this study was to evaluate the relationship between intercellular spaces and leaf gas exchange and the effect of total intercellular space on the growth of maize and sorghum under water restriction. The experiments were conducted in a greenhouse in a 2 × 3 factorial arrangement (two plant types and three water conditions: field capacity (FC = 100%), 75%FC, and 50%FC) with 10 replicates. The lack of water was a limiting factor for maize because it showed reductions in leaf area, leaf thickness, biomass, and gas exchange parameters, while sorghum remained unchanged, maintaining its water-use efficiency. This maintenance was correlated with the growth of intercellular spaces in sorghum leaves because the increased internal volume led to better CO2 control and prevented excessive water loss under drought stress. In addition, sorghum had more stomata than maize. These characteristics contributed to the drought tolerance of sorghum, while maize could not make the same adjustments. Therefore, changes in intercellular spaces promoted adjustments to avoid water loss and may have improved CO2 diffusion, characteristics that are important for drought-tolerant plants.

Nanoconfined Molecular Catalysts in Integrated Gas Diffusion Electrodes for High‐Current‐Density CO2 Electroreduction

AbstractMolecular catalysts are promising catalysts to electrochemically convert CO2 into CO with high selectivity. However, achieving industrial‐level current density remains challenging due to the limitation of charge‐ and mass‐transport in gas diffusion electrode. Herein, a novel gas diffusion electrode architecture by confining highly dispersed cobalt(II) phthalocyanine (CoPc) molecules into ‐graphene oxide (GO) nanosheets (denoted as CoPc@GO) is designed. Benefiting from the accelerated CO2 diffusion and charge transport in the nanoconfined structure, the designed electrode achieves a high CO partial current density of 481.65 ± 12.50 mA cm−2 and a cathode energy efficiency over 64% for CO. The experimentally measured CO2 transport dynamics and molecular dynamics simulation confirm the accelerated CO2 diffusion, while theoretical calculations reveal the decreased energy barrier of the CO2 activation in the confined space. This study paves a new way for electrode architecture design that would accelerate the implementation of CO2 electrolysis technology.

Enhancement of carbon sequestration via MIL-100(Fe)@PUS in bacterial-algal symbiosis treating municipal wastewater

Bacterial-algal symbiosis (BAS) is a promising carbon neutrality technology to treat municipal wastewater. However, there are still non-trivial CO2 emissions in BAS due to the slow diffusion and biosorption of CO2. Aiming to reduce CO2 emissions, the inoculation ratio of aerobic sludge to algae was further optimized at 4:1 on the base of favorable carbon conversion. MIL-100(Fe) served as CO2 adsorbents was immobilized on polyurethane sponge (PUS) to increase the interaction with microbes. When MIL-100(Fe)@PUS was added to BAS in the treatment of municipal wastewater, zero CO2 emission was achieved and the carbon sequestration efficiency was increased from 79.9% to 89.0%. Most genes related to metabolic function were derived from Proteobacteria and Chlorophyta. The mechanism of enhanced carbon sequestration in BAS could be attributed to both enrichment of algae (Chlorella and Micractinium) and increased abundance of functional genes related to PS I, PS II and Calvin cycle in photosynthesis.

Investigation on the dynamic behaviors of single surface CO nanobubbles during CO2 electroreduction in ionic liquids

CO nanobubbles generated on the electrode surface affects inevitably the reaction performance during the electroreduction of CO2. Understanding the dynamic behaviors of single surface CO nanobubbles, especially on the onset stage (i.e., nanobubble nucleation), has been received wide attentions in recent years. However, it is a great challenge to detect the nanobubble nucleation and evolution behaviors in-situ due to the transitory and nanoscale nature of nanobubble. In this work, for the first time, a single-entity electrochemistry method was developed to generate the single CO nanobubbles exclusively and stably in ionic liquids (ILs) by CO2 electroreduction. The unique dynamic behaviors of CO nanobubble and special regulations were revealed by the recorded voltammograms, where the critical supersaturation and the energy barrier for the CO nanobubble nucleation are much lower than that of other gases in aqueous solutions, thus CO nanobubbles are easy formed in ILs systems. It is proved through the classic nucleation theory that the main control factor of CO nanobubble nucleation is the relatively low surface free energy in ILs systems. Furthermore, experiments and simulations simultaneously revealed that the nanoelectrode surface is partially covered by the CO nanobubble due to the slower reaction kinetics instead of CO2 diffusion rate. The insights gained have relevant implications in regulating the behavior of nanobubbles as well as the performance of CO2 electroreduction to CO.

CO2 Adsorption on N-Doped Porous Biocarbon Synthesized from Biomass Corncobs in Simulated Flue Gas

This study was to develop a low-cost N-doped porous biocarbon adsorbent that can directly adsorb CO2 in high-temperature flue gas from fossil fuel combustion. The porous biocarbon was prepared by nitrogen doping and nitrogen-oxygen codoping through K2CO3 activation. Results showed that these samples exhibited a high specific surface area of 1209-2307 m2/g with a pore volume of 0.492-0.868 cm3/g and a nitrogen content of 0.41-3.3 wt %. The optimized sample CNNK-1 exhibited a high adsorption capacity of 1.30 and 0.27 mmol/g in the simulated flue gas (14.4 vol % CO2 + 85.6 vol % N2) and a high CO2/N2 selectivity of 80 and 20 at 25 and 100 °C and 1 bar, respectively. Studies revealed that too many microporous pores could hinder CO2 diffusion and adsorption due to the decrease of CO2 partial pressure and thermodynamic driving force in the simulated flue gas. The CO2 adsorption of the samples was mainly chemical adsorption at 100 °C, which depended on the surface nitrogen functional groups. Nitrogen functional groups (pyridinic-N and primary and secondary amines) reacted chemically with CO2 to produce graphitic-N, pyrrolic-like structures, and carboxyl functional groups (-N-COOH). Nitrogen and oxygen codoping increased the amount of nitrogen doping content in the sample, but acidic oxygen functional groups (carboxyl groups, lactones, and phenols) were introduced, which weakened the acid-base interactions between the sample and CO2 molecules. It was demonstrated that SO2 and water vapor had inhibition effects on CO2 adsorption, while NO nearly has no effect on the complex flue gas. Cyclic regenerative adsorption showed that CNNK-1 possessed excellent regeneration and stabilization ability in complex flue gases, indicating that corncob-derived biocarbon had excellent CO2 adsorption in high-temperature flue gas.

Novel titanium matrix composites reinforced by NiCoCr medium entropy particles

Titanium matrix composites (TMCs) reinforced with 2–8.3 wt.% NiCoCr medium-entropy alloy particles (NiCoCrp/Ti) were synthesized through an in-situ reaction between NiCoCr and Ti powders during a spark plasma sintering process at 850 °C and 1000 °C. At 850 °C, there are obvious interdiffusion layers with Ti3(Ni, Co) intermetallic around NiCoCr particles. At 1000 °C, the sufficient diffusion of Ni, Co and Cr away from the prior NiCoCr particles results in clusters of granular intermetallic (Ti3(Ni, Co) and Ti2(Ni, Co)) in the Ti matrix. Besides, the addition of NiCoCr particles leads to a significant improvement in tensile strength and hardness. Ti-2wt.%NiCoCr sintered at 1000 °C has a combination of high tensile strength (788 MPa) and good ductility (23.4%). Furthermore, the mechanical properties are mainly correlated to the amount and distribution of the intermetallic compounds in TMCs.

A new method for carbonating magnesia and its possible application to soil stabilization and CO2 capture

Traditional MgO carbonation (MgO and gaseous CO2) is a promising and sustainable technique in construction fields. However, using this technique to treat in-field soils on a large scale presents problems associated with CO2 diffusion, carbonation conditions, and mixing processes. To address such problems, this study proposed a new method that used NaHCO3 as a CO2 carrier to carbonate MgO, and then the carbonated MgO is applied to treat soils. The engineering properties of the stabilized soil were investigated. A discussion on the technical and economic feasibility of this new method was followed. The results showed that the soil with MgO + NaHCO3 had better mechanical performance than that with OPC due to the formation of various magnesium carbonates. The new method would be more convenient to operate on-site than the traditional carbonation method and presented a strong economic potential for the soda industry. Overall, this method could be an upgrade to MgO carbonation.

Characteristics of CO2 hydrate formation and dissociation at different CO2-water ratios in a porous medium

Hydrate-based CO2 sequestration (HBCS) is a new concept to store a huge amount of CO2 in geological sediments in the form of crystalline solids. This technique can address several environmental challenges of the current geologic carbon sequestration approaches. This study aims to evaluate and compare the characteristics and kinetics of CO2 hydrate formation and dissociation in the porous medium (three-phase) and bulk (two-phase) conditions at different CO2-water ratios. The experimental results indicate that the pressure drops and consequently hydrate nucleation and growth are limited in the porous medium due to the presence of solid particles, limited CO2 mass transfer, and heat transfer. Calculation of CO2 mass consumption and rate of gas uptake during hydrate formation also support the limited CO2 consumption and gas uptake rate in the porous medium, compared to the bulk condition. The calculated driving force for each test condition suggests its critical importance in the kinetics of CO2 hydrate formation. On the other hand, limited CO2 diffusion to the aqueous phase, hydrate nucleation at the solid-CO2-water interface, and better protection of newly formed hydrates are among the main reasons for the success of hydrate formation at the CO2-limited ratio only in the porous medium. Lastly, three steps during hydrate dissociation via thermal stimulation, such as heat transfer driving, kinetic driving, and fluid flow driving, are no longer discernable in the porous medium due to the limited heat and mass transfer during the process.

CO2 Diffusion and Carbonation in OPC/γ-2CaO·SiO2 Composite

Gamma dicalcium silicate (γ-2CaO∙SiO2, abbreviated as γ-C2S) is considered a potential candidate as a construction material owing to its high carbonation reactivity and consequent CO2 absorption. This study investigates the diffusion of CO2, a physical process, into hardened cement paste and the resulting carbonation, a chemical process. CO2 diffuses from a region of high concentration to one of a lower concentration, which is the inner core of the hardened cement. This study aimed to examine whether the diffusion of CO2 into the ordinary Portland cement (OPC)/γ-C2S composite paste followed the conventional laws of diffusion. We also studied the diffusion of CaCO3 to determine if carbonation products were formed in the pores and examined the capture of CO2. The paste specimens were prepared and subjected to CO2 in the carbonation chambers for varying periods. The results showed that the CaCO3 deposited in the pores affected the rate of diffusion of CO2 in the mortars and pastes, resulting in the densification of such bodies and a decreased rate of diffusion, leading to the shutdown of diffusion. The diffusion of CO2 in hardened cement pastes made from OPC and γ-C2S follows Fick’s second law, wherein there is a change in the concentration of CO2 diffusing at a particular distance with time.

Nanoscale Management of CO Transport in CO2 Electroreduction: Boosting Faradaic Efficiency to Multicarbon Products via Nanostructured Tandem Electrocatalysts

AbstractTandem catalysis presents a promising strategy to improve the selectivity toward multicarbon products in the electrocatalytic carbon dioxide reduction reaction (CO2RR). For CO2RR, CO is a critical intermediate for producing multicarbon products. However, the management of CO localization and CO diffusion remains underexplored despite its critical role. Herein, a 3D tandem catalyst electrode with silver nanoparticles (Ag NPs) is designed to generate CO as an intermediate product within a copper (Cu) nanoneedle array. Via this nanostructured design, CO2 forms C2+ products with a high Faradaic efficiency (FEC2+) of 64% in an H‐cell and 70% in a flow cell with a current density of 350 mA cm−2. These figures‐of‐merit are currently among the top literature reports. More importantly, in situ Raman spectroscopy and finite‐element method calculations are employed to elucidate the origins of enhanced selectivity. These approaches reveal the crucial role of prolonging the CO diffusion path length for improving CO utilization during CO2 conversion with tandem catalyst systems. The favorable CO2RR FEC2+ in two distinct environments (H‐cell and flow cell) further corroborates that this effect is not limited to a particular reactor environment. Overall, this study provides new insights for designing tandem catalysts for improved CO2RR selectivity to C2+ products.