CO Current Density Research Articles

Electrochemical reduction of CO2 to CO on bimetallic CoCu–N–C catalyst

Electrochemical reduction of CO2 to valuable chemicals using electricity from renewable sources is a promising approach to realize the carbon cycle. Here, a novel bimetallic nitrogen-doped carbon catalyst (M-N-C, where M denotes the metal) CoCu–N–C electrocatalyst has been synthesized by pyrolyzing a copper-doped cobalt(II) acetate–phenanthroline complex formed from the metal acetates and 1,10-phenanthroline dissolved in ethanol. The optimal CoCu–N–C catalyst with molar ratio of Cu/(Co + Cu) = 0.15 displayed superior performance, enabling a faradaic efficiency of CO production of 76.5% at an overpotential of 0.57 V as well as an increased CO current density of 8.48 mA cm−2 at −0.98 V versus reversible hydrogen electrode (far superior than the Co–N–C of 4.20 mA cm−2 and Cu–N–C of 0.07 mA cm−2) and a decreased charge-transfer resistance compared with Co–N–C and Cu–N–C catalysts. The addition of Cu, which serves as a structure promoter to improve the dispersion of Co, thereby providing sufficient Co-Nx active sites for the electrochemical CO2 reduction reaction (CRR). Moreover, doping Cu changes the electronic environment around Co slightly and promotes the charge transfer capability so that reduces the energy barrier of potential-limiting energy, thereby improving the CRR performance of catalyst.

Redox‐Driven Cu─Pd Bond Formation to Enhance the Efficiency for Electroreduction of CO2 to CO

The redox reaction as the driving force induces the dynamic structure of the electrocatalyst, which enhances the performance of the catalysis. Yet, it lacks the understanding of the influence of redox driving force on the transforming process, inhibiting obtaining the actual factor that determines the catalytic performance. Resorting to the pulse and in situ methodology, herein, the effect of spontaneous reoxidation on the structural transformation of the Cu–Pd bimetallic catalyst for CO2 electroreduction is investigated. During the pulse program, applying the open‐circuit voltage and cathodic potential intermittently oxidizes and reduces the Cu–Pd bimetallic catalyst, which systematically reveals the catalyst redox reaction imparting the increase in CO2‐to‐CO selectivity. Through in situ X‐ray absorption spectroscopy (XAS), the dynamic structure induced by the spontaneous reoxidation is unveiled, suggesting the Cu─Pd bond formation for the bimetallic catalyst with a higher Pd content. Such a reoxidation‐induced Cu─Pd bond enhances the CO faradaic efficiency to 88% with the CO current density of −132 mA cm−2. By the methodologies, a new perspective is provided to understand the redox‐driven dynamic structure and the corresponding effect on the product selectivity, which is a vital concept to design the new catalysts in the future.

(Keynote) Deciphering Electrocatalytic Reactions with Theory and Computation: The Case of CO2 Reduction

The interfacial region between metal surface and aqueous electrolyte is of central importance for electrochemical reactions. The need to understand the properties of this electrochemical double layer (EDL) region drives extensive research.1,2 A foremost goal of research in electrocatalysis is the development of a theoretical-computational framework. The fundamental task for such a framework is to unravel the complex interplay of electronic structure effects of the catalyst material, potential-induced variations of the chemisorption state, local reaction conditions on the electrolyte side, and electrochemical kinetics of reactions of interest. A recently developed theoretical approach revealed the non-monotonic charging behaviour of a Pt electrode3 and it was thereafter used to decipher the oxygen reduction reaction.4 In the present work, we adapt this approach to study how the local reaction environment dictates the mechanism and kinetics of reduction to CO at an Ag electrode. Our hierarchical model accounts for the multistep reaction kinetics of surface reactions, local chemical equilibria (involving CO2, HCO3 -, CO3 2-, OH-, H+), the specific surface charging state at a given electrode potential, solvent polarization and ion density variations, and reactant/product transport. It integrates interface and pore-level models to account for this interplay. The combined approach rationalizes the impact of the considered effects on the kinetics of the reaction, manifested experimentally in changes of the effective Tafel slope as a function of electrode potential. Lateral interactions between chemisorbed species are seen to contribute to the decrease of the CO current density at high electrode potentials, in addition to mass transport effects, surface charging effects and pH increase. We will conclude with a discussion of the parameters that allow tuning the local reaction environment and thus the electrocatalytic activity and selectivity of the electrocatalyst. 1O.M. Magnussen and A. Gross, Toward an Atomic-Scale Understanding of Electrochemical Interface Structure and Dynamics, J. Am. Chem. Soc. 141, 4777–4790 (2019). 2M.J. Eslamibidgoli and M.H. Eikerling, Approaching the Self-consistency Challenge of Electrocatalysis with Theory and Computation, Current Opinion in Electrochemistry 9, 189-197 (2018). 3J. Huang, A. Malek, J. Zhang and M.H. Eikerling, Non-monotonic Surface Charging Behavior of Platinum: A Paradigm Change, J. Phys. Chem. C 120, 13587-13595 (2016). 4J. Huang, J. Zhang and M. Eikerling, Unifying Theoretical Framework for Deciphering the Oxygen Reduction Reaction on Platinum, Phys. Chem. Chem. Phys. 20, 11776-11786 (2018).

Pyrolysed Co-N4 Macrocycles on Carbon Supports for the Efficient Electroreduction of CO2

Converting CO2 into materials such as chemicals or synfuels is an attractive way to mitigate greenhouse gas emissions while producing value-added products1-4. Even though the intrinsic chemical stability of CO2 significantly limits its reactivity, the use of electrocatalysts allows its efficient reduction in a mild environment and at relatively low overpotentials5,6. Cobalt phthalocyanine (CoPC), cobalt tetra-phenyl porphyrin (CoTPP) and Vitamin B12 (VB12) are well-defined Co-N4 macrocycles whose incorporation into heterogeneous catalyst setups have led to materials with outstanding CO2 electroreduction reaction (CO2ERR) properties, most notably in the selective formation of CO at high current densities7-11.These Co-N4 macrocycles have also been used to manufacture catalysts efficient for the oxygen reduction reaction (ORR). Notably, the pyrolysis of the macrocyclic precursor supported by carbon materials allowed the fabrication of heterogeneous ORR catalysts with high loadings of catalytically active components, controlled local environments and enhanced stabilities12,13. While these properties could lead to these materials also being efficient catalysts for CO2ERR, to the best of our knowledge, such pyrolyzed catalysts have not been investigated for the CO2 reduction reaction.In this work, we report materials based on CoPC, CoTPP and VB12, supported by carbon black and pyrolyzed under argon, as efficient catalysts for the CO2ERR. The catalytic precursors were dispersed on carbon powders with ball-milling before being annealed at different temperatures chosen from the positions of the significant steps and plateaus observed by thermogravimetric analysis of the pure precursors. The structures of the resulting materials were analyzed by powder X-ray diffraction and X-ray absorption spectroscopy. The electrochemical behaviour of these materials for the CO2ERR were characterized in CO2-saturated potassium bicarbonate (KHCO3) electrolyte using a custom-made flow cell at a range of potentials, and the products of the reaction were analyzed with gas chromatography, 1H NMR spectroscopy and high-performance liquid chromatography.The high reactivity of the catalysts for the reduction of CO2 into CO was maintained for all materials treated at temperatures as high as 700 °C, even though the structures of their active sites were drastically different from that of the precursor molecules. The pyrolyzed materials also exhibited a change in correlation between the CO current densities and the KHCO3 concentration of the electrolyte, indicating that the reaction mechanism had changed compared to that of the pristine materials.1 Peter, S. C. Reduction of CO2 to Chemicals and Fuels: A Solution to Global Warming and Energy Crisis. ACS Energy Letters 3, 1557-1561, doi:10.1021/acsenergylett.8b00878 (2018).2 Lee, Z. H., Sethupathi, S., Lee, K. T., Bhatia, S. & Mohamed, A. R. An overview on global warming in Southeast Asia: CO2 emission status, efforts done, and barriers. Renewable and Sustainable Energy Reviews 28, 71-81, doi:10.1016/j.rser.2013.07.055 (2013).3 Saeidi, S., Amin, N. A. S. & Rahimpour, M. R. Hydrogenation of CO2 to value-added products - A review and potential future developments. Journal of CO2 Utilization 5, 66-81, doi:10.1016/j.jcou.2013.12.005 (2014).4 Zhao, G., Huang, X., Wang, X. X. & Wang, X. X. Progress in catalyst exploration for heterogeneous CO2 reduction and utilization: A critical review. Journal of Materials Chemistry A 5, 21625-21649, doi:10.1039/c7ta07290b (2017).5 Finn, C., Schnittger, S., Yellowlees, L. J. & Love, J. B. Molecular approaches to the electrochemical reduction of carbon dioxide. Chemical Communications 48, 1392-1399, doi:10.1039/c1cc15393e (2012).6 Torbensen, K., Boudy, B., Joulié, D., von Wolff, N. & Robert, M. Emergence of CO2 electrolyzers including supported molecular catalysts. Current Opinion in Electrochemistry 24, 49-55, doi:10.1016/j.coelec.2020.07.001 (2020).7 Wang, M. et al. CO2 electrochemical catalytic reduction with a highly active cobalt phthalocyanine. Nature Communications 10, doi:10.1038/s41467-019-11542-w (2019).8 Ni, W. et al. Dual single-cobalt atom-based carbon electrocatalysts for efficient CO2-to-syngas conversion with industrial current densities. Applied Catalysis B: Environmental 291, doi:10.1016/j.apcatb.2021.120092 (2021).9 Jia, C. et al. Vitamin B12 on Graphene for Highly Efficient CO2 Electroreduction. ACS Appl Mater Interfaces 12, 41288-41293, doi:10.1021/acsami.0c10125 (2020).10 Hu, B. et al. How does the ligands structure surrounding metal-N4 of Co-based macrocyclic compounds affect electrochemical reduction of CO2 performance? Electrochimica Acta 331, 135283, doi:10.1016/j.electacta.2019.135283 (2020).11 Hu, X. M., Ronne, M. H., Pedersen, S. U., Skrydstrup, T. & Daasbjerg, K. Enhanced Catalytic Activity of Cobalt Porphyrin in CO2 Electroreduction upon Immobilization on Carbon Materials. Angew Chem Int Ed Engl 56, 6468-6472, doi:10.1002/anie.201701104 (2017).12 Wan, G. et al. Tuning the Performance of Single-Atom Electrocatalysts: Support-Induced Structural Reconstruction. Chemistry of Materials 30, 7494-7502, doi:10.1021/acs.chemmater.8b02315 (2018).13 Wan, G. et al. Anion-Regulated Selective Generation of Cobalt Sites in Carbon: Toward Superior Bifunctional Electrocatalysis. Adv Mater 29, doi:10.1002/adma.201703436 (2017).

(Invited) N-Doped Carbon Catalysts in Carbon-Dioxide Electroreduction: Effect of the Chemical Composition and Morphology

N-doped carbon (N-C) materials are increasingly popular in different electrochemical and catalytic applications. Due to the structural and stoichiometric diversity of these materials, the role of different functional moieties is still controversial. We have synthesized a set of N-C catalysts, with identical morphology (~27 nm pore size). By systematically changing the precursors, we have varied the amount and chemical nature of N-functions on the catalyst surface. The CO2 reduction (CO2R) properties of these catalysts were tested in electrochemical experiments, where CO was the major product in all cases, while CH4 appeared as minor product. Importantly, the CO2R activity changed with the chemical composition. The activity was correlated with the amount of different N-functions, and correlation was found only for the -NOx species. Interestingly, the amount of this species decreased radically during CO2R, which was coupled with the performance decrease. The observations were rationalized by the adsorption/desorption properties of the samples.In the second part of my presentation I will shed light on the importance of catalyst morphology, using N–C catalysts as a model system. We found that CO2R activity, selectivity, and stability of N–C electrodes are highly dependent on their porosity. The presence of mesopores was demonstrated to be beneficial in achieving high CO selectivity and current density, with an optimal pore size around 27 nm. Even after convoluting factors other than morphology (e.g., surface chemistry, level of graphitization, surface area), the reasons behind the observed trends are complex. CO2 adsorption properties, wetting characteristics, and geometric effects are jointly responsible for the massive difference in the CO2R performance. All these properties must be taken into consideration when we aim to understand the reduction mechanism on different catalysts and while improving the performance further to a technologically relevant level (as alternatives to precious metal catalysts).

Boosting Electrocatalytic CO 2 Reduction with Conjugated Bimetallic Co/Zn Polyphthalocyanine Frameworks

Boosting Electrocatalytic CO ₂ Reduction with Conjugated Bimetallic Co/Zn Polyphthalocyanine Frameworks

Conversion of Reactive Carbon Solutions into CO at Low Voltage and High Carbon Efficiency.

Electrolyzers are now capable of reducing carbon dioxide (CO2) into products at high reaction rates but are often characterized by low energy efficiencies and low CO2 utilization efficiencies. We report here an electrolyzer that reduces 3.0 M KHCO3(aq) into CO(g) at a high rate (partial current density for CO of 220 mA cm–2) and a CO2 utilization efficiency of 40%, at a voltage of merely 2.3 V. These results were made possible by using: (i) a reactive carbon solution enriched in KHCO3 as the feedstock instead of gaseous CO2; (ii) a cation exchange membrane instead of an anion exchange membrane, which is common to the field; and (iii) the hydrogen oxidation reaction (HOR) at the anode instead of the oxygen evolution reaction (OER). The voltage reported here is the lowest reported for any CO2 to CO electrolyzer that operates at high current densities (i.e., a partial current density for CO greater than 200 mA cm–2) with a CO2 utilization efficiency of greater than 20%. This study highlights how the choice of feedstock, membrane, and anode chemistries affects the rate and efficiency at which CO2 is converted into products.

Electronic Regulation of Nickel Single Atoms by Confined Nickel Nanoparticles for Energy‐Efficient CO2 Electroreduction

AbstractModulating the electronic structure of atomically dispersed active sites is promising to boost catalytic activity but is challenging to achieve. Here we show a cooperative Ni single‐atom‐on‐nanoparticle catalyst (NiSA/NP) prepared via direct solid‐state pyrolysis, where Ni nanoparticles donate electrons to Ni(i)−N−C sites via a network of carbon nanotubes, achieving a high CO current density of 346 mA cm−2 at −0.5 V vs RHE in an alkaline flow cell. When coupled with a NiFe‐based anode in a zero‐gap membrane electrolyzer, the catalyst delivers an industrially relevant CO current density of 310 mA cm−2 at a low cell voltage of −2.3 V, corresponding to an overall energy efficiency of 57 %. The superior CO2 electroreduction performance is attributed to the enhanced adsorption of key intermediate COOH* on the electron‐rich Ni single atoms, as well as a high density of active sites.

Electronic Regulation of Nickel Single Atoms by Confined Nickel Nanoparticles for Energy-Efficient CO2 Electroreduction.

Modulating the electronic structure of atomically dispersed active sites is promising to boost catalytic activity but is challenging to achieve. Here we show a cooperative Ni single-atom-on-nanoparticle catalyst (NiSA/NP) prepared via direct solid-state pyrolysis, where Ni nanoparticles donate electrons to Ni(i)-N-C sites via a network of carbon nanotubes, achieving a high CO current density of 346 mA cm-2 at -0.5 V vs RHE in an alkaline flow cell. When coupled with a NiFe-based anode in a zero-gap membrane electrolyzer, the catalyst delivers an industrially relevant CO current density of 310 mA cm-2 at a low cell voltage of -2.3 V, corresponding to an overall energy efficiency of 57 %. The superior CO2 electroreduction performance is attributed to the enhanced adsorption of key intermediate COOH* on the electron-rich Ni single atoms, as well as a high density of active sites.

Modeling the Local Environment within Porous Electrode during Electrochemical Reduction of Bicarbonate

The electrochemical reduction of bicarbonate to renewable chemicals without external gaseous CO2 supply has been motivated as a means of integrating conversion with upstream CO2 capture. The way that CO2 is formed and transported during CO2-mediated bicarbonate reduction in flow cells is profoundly different from conventional CO2 saturated and gas-fed systems and a thorough understanding of the process would allow further advancements. Here, we report a comprehensive two-phase mass transport model to estimate the local concentration of species in the porous electrode resultant from homogeneous and electrochemical reactions of (bi)carbonate and CO2. The model indicates that significant CO2 is generated in the porous electrode during electrochemical reduction, even though the starting bicarbonate solution contains negligible CO2. However, the in situ formation of CO2 and subsequent reduction to CO exhibits a plateau at high potentials due to neutralization of the protons by the alkaline reaction products, acting as the limiting step toward higher CO current densities. Nevertheless, the pH in the catalyst layer exhibits a relatively smaller rise, compared to conventional electrochemical CO2 reduction cells, because of the reaction between protons and CO32– and OH– that is confined to a relatively small volume. A large fraction of the CL exhibits a mildly alkaline environment at high current densities, while an appreciable amount of carbonic acid (0.1–1 mM) and a lower pH exist adjacent to the membrane, which locally favor hydrogen evolution, especially at low electrolyte concentrations. The results presented here provide insights into local cathodic conditions for both bicarbonate cells and direct-CO2 reduction membrane electrode assembly cells utilizing cation exchange membranes facing the cathode.

Few-walled carbon nanotubes derived from shoe waste plastics: Effect of feedstock composition on synthesis, properties and application as CO2 reduction electrodes

This work investigates the chemical recycling of shoe waste plastics, i.e. polyurethane (PU) and ethylene-vinyl acetate (EVA) using a pyrolysis-chemical vapor deposition process over Fe/MgO catalyst. Results suggested PU facilitated highly selective production of few-walled CNTs with small diameters and narrow size distribution while EVA tended to yield multi-walled CNTs. The synergistic interactions between pyrolysis gases components from PU and EVA allowed facile tailoring of CNT size distribution by using either sorted PU and EVA or mixture (EVA/PU) in appropriate ratios. CNTs derived from pure PU or EVA/PU were more suitable in designing effective electrocatalysts for CO2 reduction with Faradaic efficiency of 85–95% and CO current density of >9 mA cm−2. This was because the presence of few-walled CNTs could enhance electrical conductivity and specific surface area without compromising the loading of active materials.

A Heteroleptic Gold Hydride Nanocluster for Efficient and Selective Electrocatalytic Reduction of CO2 to CO.

It has been a long-standing challenge to create and identify the active sites of heterogeneous catalysts, because it is difficult to precisely control the interfacial chemistry at the molecular level. Here we report the synthesis and catalysis of a heteroleptic gold trihydride nanocluster, [Au22H3(dppe)3(PPh3)8]3+ [dppe = 1,2-bis(diphenylphosphino)ethane, PPh3 = triphenylphosphine]. The Au22H3 core consists of two Au11 units bonded via six uncoordinated Au sites. The three H atoms bridge the six uncoordinated Au atoms and are found to play a key role in catalyzing electrochemical reduction of CO2 to CO with a 92.7% Faradaic efficiency (FE) at -0.6 V (vs RHE) and high reaction activity (134 A/gAu mass activity). The CO current density and FECO remained nearly constant for the CO2 reduction reaction for more than 10 h, indicating remarkable stability of the Au22H3 catalyst. The Au22H3 catalytic performance is among the best Au-based catalysts reported thus far for electrochemical reduction of CO2. Density functional theory (DFT) calculations suggest that the hydride coordinated Au sites are the active centers, which facilitate the formation of the key *COOH intermediate.

Diminishing the Uncoordinated N Species in Co-N-C Catalysts toward Highly Efficient Electrochemical CO2 Reduction

Atomically dispersed metal and nitrogen codoped carbon (M-N-C) catalysts with N-coordinated metal (MNx) sites have shown compelling performance in electrochemical CO2 reduction (ECR). However, extra uncoordinated N species commonly coexist with MNx sites in M-N-C, which are impossible to ignore due to their inevitable interference in catalytic performance. Considering this, we developed high-performance Co-N-C for ECR by diminishing the uncoordinated N species. The resulting electrocatalyst displays a CO faradic efficiency (FECO) of 99.4% with a CO current density of −24.8 mA·cm–2 at a low overpotential of 0.49 V in an H-type cell, and a high FECO over 90% is obtained in a flow cell within a wide current density window (50–600 mA·cm–2), exceeding all reported Co-N-C catalysts. Density functional theory calculations reveal that isolated CoN4 sites can reduce the energy barrier required for the formation of COOH* and suppress the occurrence of hydrogen evolution compared with CoN4 sites with extra uncoordinated N species, thus resulting in enhanced activity and selectivity in CO production.

Bimetallic Gold-Silver Nanostructures Drive Low Overpotentials for Electrochemical Carbon Dioxide Reduction.

Alloy formation is an advanced approach to improve desired properties that the monoelements cannot achieve. Alloys are usually designed to tailor intrinsic natures or induce synergistic effects by combining materials with distinct properties. Indeed, unprecedented properties have emerged in many cases, superior to a simple sum of pure elements. Here, we present Au-Ag alloy nanostructures with prominent catalytic properties in an electrochemical carbon dioxide reduction reaction (eCO2RR). The Au-Ag hollow nanocubes are prepared by galvanic replacement of Au on Ag nanocubes. When the Au-to-Ag ratio is 1:1 (Au1Ag1), the alloy hollow nanocubes exhibit maximum Faradaic efficiencies of CO production in a wide potential range and high mass activity and CO current density superior to those of the bare metals. In particular, overpotentials are estimated to be similar to or lower than that of the Au catalyst under various standard metrics. Density functional theory calculations, machine learning, and a statistical consideration demonstrate that the optimal configuration of the *COOH intermediate is a bidentate coordination structure where C binds to Au and O binds to Ag. This active Au-Ag neighboring configuration has a maximum population and enhanced intrinsic catalytic activity on the Au1Ag1 surface among other Au-to-Ag compositions, in good agreement with the experimental results. Further application of Au1Ag1 to a membrane electrode assembly cell at neutral conditions shows enhanced CO Faradaic efficiency and current densities compared to Au or Ag nanocubes, indicating the possible extension of Au-Ag alloys to larger electrochemical systems. These results give a new insight into the synergistic roles of Au and Ag in the eCO2RR and offer a fresh direction toward a rational design of bimetallic catalysts at a practical scale.

A promising single-atom Co-N-C catalyst for efficient CO2 electroreduction and high-current solar conversion of CO2 to CO

Excavating highly efficient M-N-C electrocatalysts for electrochemical CO2 reduction (ECR) is of paramount importance. Herein, we report a single-atom Co-N-C catalyst (CoN4-CB) with a high CO Faradaic efficiency (FECO, 98.7%) in ECR comparable to that of a similarly prepared NiN4-CB catalyst. Impressively, CoN4-CB demonstrates a CO turnover frequency of 27173 h−1 and a CO current density of −33.6 mA∙cm−2 at −0.76 V, 20.2 and 6.8 times higher than that of NiN4-CB, respectively. In a solar-driven ECR system composed of a Si solar cell and a flow cell, CoN4-CB shows a remarkably large current density of 98.3 mA∙cm−2 with an average FECO of 92.1%. Theoretical calculations suggest that the energy barrier for *COOH formation largely decreases on CoN4 sites compared with NiN4 sites, leading to a low onset potential and high activity for CO production. This work will boost the development of efficient M-N-C electrocatalysts and further practical application of solar-driven ECR system.

Electrochemical CO2 Reduction at Silver from a Local Perspective

The electrochemical reduction of CO2 to chemical fuels and value-added chemicals is a viable pathway to store renewably generated electrical energy and to mitigate the negative impact of anthropogenic CO2 production. Herein, we study how the local reaction environment dictates the mechanism and kinetics of CO2 reduction to CO at an Ag electrode. The local reaction environment is determined using a hierarchical model that accounts for multistep reaction kinetics, specific surface charging state at a given electrode potential, and mass transport phenomena. The model reveals vital mechanistic insights into the reaction behavior. The increasing Tafel slope with overpotential is seen to be influenced by the surface charging relation and mass transport effects. In addition, the decrease of the CO current density at high overpotentials is found to be caused not only by the decrease in CO2 concentration due to mass transport, surface charge effects, and pH increase but also by lateral interactions between HCOOad, COOHad, and Had. Moreover, we explore how the electrolyte properties, including bicarbonate concentration, solvated cation size, and CO2 partial pressure, tune the local reaction environment.

Temperature‐Dependent CO2 Electroreduction over Fe‐N‐C and Ni‐N‐C Single‐Atom Catalysts

AbstractReaction temperature is an important parameter to tune the selectivity and activity of electrochemical CO2 reduction reaction (CO2RR) due to different thermodynamics of CO2RR and competitive hydrogen evolution reaction (HER). In this work, temperature‐dependent CO2RR over Fe‐N‐C and Ni‐N‐C single‐atom catalysts are investigated from 303 to 343 K. Increasing the reaction temperature improves and decreases CO Faradaic efficiency over Fe‐N‐C and Ni‐N‐C catalysts at high overpotentials, respectively. CO current density over Fe‐N‐C catalyst increases with temperature, then gets into a plateau at 323 K, finally reaches the maximum value of 185.8 mA cm−2 at 343 K. While CO current density over Ni‐N‐C catalyst achieves the maximum value of 252.5 mA cm−2 at 323 K, and then drops significantly to 202.9 mA cm−2 at 343 K. Temperature programmed desorption results and density functional theory calculations reveal that the difference of temperature‐dependent variation on CO Faradaic efficiency and current density between Fe‐N‐C and Ni‐N‐C catalysts results from the varied adsorption strength of key reaction intermediates during CO2RR.

Temperature-Dependent CO2 Electroreduction over Fe-N-C and Ni-N-C Single-Atom Catalysts.

Reaction temperature is an important parameter to tune the selectivity and activity of electrochemical CO2 reduction reaction (CO2 RR) due to different thermodynamics of CO2 RR and competitive hydrogen evolution reaction (HER). In this work, temperature-dependent CO2 RR over Fe-N-C and Ni-N-C single-atom catalysts are investigated from 303 to 343 K. Increasing the reaction temperature improves and decreases CO Faradaic efficiency over Fe-N-C and Ni-N-C catalysts at high overpotentials, respectively. CO current density over Fe-N-C catalyst increases with temperature, then gets into a plateau at 323 K, finally reaches the maximum value of 185.8 mA cm-2 at 343 K. While CO current density over Ni-N-C catalyst achieves the maximum value of 252.5 mA cm-2 at 323 K, and then drops significantly to 202.9 mA cm-2 at 343 K. Temperature programmed desorption results and density functional theory calculations reveal that the difference of temperature-dependent variation on CO Faradaic efficiency and current density between Fe-N-C and Ni-N-C catalysts results from the varied adsorption strength of key reaction intermediates during CO2 RR.

Non-peripheral octamethyl-substituted cobalt phthalocyanine nanorods supported on N-doped reduced graphene oxide achieve efficient electrocatalytic CO2 reduction to CO

Developing electrocatalysts that exhibit highly-efficient CO2reduction is crucial for a healthier environment. This report demonstrates that the eight methyl groups on cobalt phthalocyanine significantly improved its catalytic CO2 reduction performance. Theoretical computations confirmed that the non-peripheral octamethyl-substituted cobalt(II) phthalocyanine (N-CoMe2Pc) catalyst showed enhanced CO2 adsorption and activation on the Co surface at low overpotentials relative to pristine cobalt(II) phthalocyanine (CoPc). The N-CoMe2Pc nanorods and nitrogen-doped reduced graphene oxide (NRGO) nanocomposites were successfully prepared by a facile, non-covalent immobilization strategy and their electrocatalytic CO2 reduction activity was explored in both H-type and flow cell configurations. It was thus verified that immobilizing N-CoMe2Pc nanorods onto NRGO surfaces can lead to enhanced electroactivity and selectivity in terms of reducing CO2 to CO. Among the various tested nanocomposites, N-CoMe2Pc/NRGO with a catalyst: NRGO ratio of 6:10 showed superior CO faradaic efficiency/selectivity (90.0 %) and a total current density reaching 9.7 mA cm−2 at –0.8 V vs. RHE in an H-type cell containing a neutral electrolyte. A CO faradaic efficiency of 90.4% and a current density of 14.8 mA cm−2 were achieved at the same overpotential using a flow cell. The selectivity of this system was further improved to 94.1%, with an accompanying CO current density of 56.4 mA cm−2 and an impressive turnover frequency of 6.2 s−1 at lower potential (–0.6 V vs. RHE) in alkaline solution. This work provides new insights relevant for developing low-cost, effective, phthalocyanine-based catalysts for electrochemical CO2 reduction.

Efficient Electrochemical Reduction of CO2 to CO in Ionic Liquids

AbstractElectrochemical reduction of carbon dioxide (CO2) to CO is a promising strategy. However, achieving high Faradaic efficiency with high current density using ionic liquid (IL) electrolyte remains a challenge. In this study, the new IL N‐octyltrimethyl ammonium 1,2,4‐triazole ([N1118][TRIZ]) shows outstanding performance for electrochemical reduction of CO2 to CO on the commercial Ag electrode, and the current density can be up to 50.8 mAcm−2 with a Faradaic efficiency of 90.6 %. The current density of CO is much higher than those reported in the IL electrolyte. In addition, the density functional theory (DFT) calculation further proved that [N1118][TRIZ] interact with CO2 to form [N1118]+[TRIZ‐CO2]− complex which played a key role in reducing the activation energy of CO2. The formation of [N1118]+[TRIZ‐CO2]− complex was verified by Fourier Transform Infrared (FTIR) spectroscopy. According to the molecular orbital theory, the electrons obtained from IL was filled in the anti‐bonding orbit (π*) of the CO2, resulting in reducing the C=O bond energy. This work provides a new strategy to design novel ILs for high efficiency electrochemical reduction of CO2 to CO.