Probing CO2 Reduction Pathways for Copper Catalysis Using an Ionic Liquid as a Chemical Trapping Agent

Crystalline-Amorphous Interfaces Engineering of CoO-InOx for Highly Efficient CO2 Electroreduction to CO

As a fundamental product of CO2 conversion through two-electron transfer, CO is used to produce numerous chemicals and fuels with high efficiency, which has broad application prospects. In this work, it has successfully optimized catalytic activity by fabricating an electrocatalyst featuring crystalline-amorphous CoO-InOx interfaces, thereby significantly expediting CO production. The 1.21%CoO-InOx consists of randomly dispersed CoO crystalline particles among amorphous InOx nanoribbons. In contrast to the same-phase structure, the unique CoO-InOx heterostructure provides plentiful reactive crystalline-amorphous interfacial sites. The Faradaic efficiency of CO (FECO) can reach up to 95.67% with a current density of 61.72 mA cm-2 in a typical H-cell using MeCN containing 0.5 M 1-Butyl-3-methylimidazolium hexafluorophosphate ([Bmim]PF6) as the electrolyte. Comprehensive experiments indicate that CoO-InOx interfaces with optimization of charge transfer enhance the double-layer capacitance and CO2 adsorption capacity. Theoretical calculations further reveal that the regulating of the electronic structure at interfacial sites not only optimizes the Gibbs free energy of *COOH intermediate formation but also inhibits HER, resulting in high selectivity toward CO.

Ionic Liquids Modulating Local Microenvironment of Ni-Fe Binary Single Atom Catalyst for Efficient Electrochemical CO2 Reduction

The Ni and Fe dual-atom catalysts still undergo strikingly attenuation under high current density and high overpotential. To ameliorate the issue, the ionic liquids with different cations or anions are used in this work to regulate the micro-surface of nitrogen-doped carbon supported Ni and Fe dual-atom sites catalyst (NiFe-N-C) by an impregnation method. The experimental data reveals the dual function of ionic liquids, which enhances CO2 adsorption ability and modulates electronic structure, facilitating CO2 anion radical (CO2 •¯) stabilization and decreasing onset potential. The theoretical calculation results prove that the attachment of ionic liquids modulates electronic structure, reduces energy barrier of CO2 •¯ formation, and enhances overall ECR performance. Based on these merits, BMImPF6 modified NiFe-N-C (NiFe-N-C/BMImPF6) achieves the high CO faradaic efficiency of 91.9% with a CO partial current density of -120 mA cm-2 at -1.0 V. When the NiFe-N-C/BMImPF6 is assembled as cathode of Zn-CO2 battery, it delivers the highest power density of 2.61 mW cm-2 at 2.57 mA cm-2 and superior cycling stability. This work will afford a direction to modify the microenvironment of other dual-atom catalysts for high-performance CO2 electroreduction.

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.

High-throughput Compositional Screening of PdxTi1-xHy and PdxNb1-xHy Hydrides for CO2 Reduction

Electrochemical experiments and theoretical calculations have shown that Pd-based metal hydrides can perform well for the CO2 reduction reaction (CO2RR). Our previous work on doped-PdH showed that doping Ti and Nb into PdH can improve the CO2RR activity, suggesting that the Pd alloy hydrides with better performance are likely to be found in the PdxTi1-xHy and PdxNb1-xHy phase space. However, the vast compositional and structural space with different alloy hydride compositions and surface adsorbates, makes it intractable to screen out the stable and active PdxM1-xHy catalysts using density functional theory calculations. Herein, an active learning cluster expansion (ALCE) surrogate model equipped with Monte Carlo simulated annealing (MCSA), a CO* binding energy filter and a kinetic model are used to identify promising PdxTi1-xHy and PdxNb1-xHy catalysts with high stability and superior activity. Using our approach, we identify 24 stable and active candidates of PdxTi1-xHy and 5 active candidates of PdxNb1-xHy. Among these candidates, the Pd0.23Ti0.77H, Pd0.19Ti0.81H0.94, and Pd0.17Nb0.83H0.25 are predicted to display current densities of approximately 5.1, 5.1 and 4.6 μA cm-2 at -0.5 V overpotential, respectively, which are significantly higher than that of PdH at 3.7 μA cm-2.

Multiscale CO2 Electrocatalysis to C2+ Products: Reaction Mechanisms, Catalyst Design, and Device Fabrication

Electrosynthesis of value-added chemicals, directly from CO2, could foster achievement of carbon neutral through an alternative electrical approach to the energy-intensive thermochemical industry for carbon utilization. Progress in this area, based on electrogeneration of multicarbon products through CO2 electroreduction, however, lags far behind that for C1 products. Reaction routes are complicated and kinetics are slow with scale up to the high levels required for commercialization, posing significant problems. In this review, we identify and summarize state-of-art progress in multicarbon synthesis with a multiscale perspective and discuss current hurdles to be resolved for multicarbon generation from CO2 reduction including atomistic mechanisms, nanoscale electrocatalysts, microscale electrodes, and macroscale electrolyzers with guidelines for future research. The review ends with a cross-scale perspective that links discrepancies between different approaches with extensions to performance and stability issues that arise from extensions to an industrial environment.

Surface Hydrophobicity Engineering of Pt-Based Noble Metal Aerogels by Ionic Liquids toward Enhanced Electrocatalytic Oxygen Reduction

Modulating the surface properties of electrocatalysts with ligands could effectively regulate their catalytic properties, while limited in-depth understanding of the surface ligands restricted their rational combination. Herein, ionic liquids (ILs) with different lengths of hydrophobic side chains were employed to regulate the surface hydrophobicity of noble metal aerogels, for comprehending the relationship between surface hydrophobicity and oxygen reduction reaction (ORR) activity and enhancing electrocatalytic ORR. The volcano-like trends between the hydrophobicity and the ORR activity for various Pt-based aerogels indicated that a suitable hydrophobic surface constructed by ILs was most favorable for contacting with oxygen molecules and the desorption of oxygen intermediates. Typically, the PtPd aerogel modified by ILs (PtPd aer-[MTBD][PFSI]) exhibited an inspiring ORR activity, with a 70 mV increase in half-wave potential and a 7.1-fold mass activity compared to the commercial Pt/C. Therefore, the regularity between the surface hydrophobicity and ORR activity of noble metal aerogels was uncovered and will facilitate the modulation of electrocatalysts for practical applications.

Increasing Accessible Active Site Density of Non-Precious Metal Oxygen Reduction Reaction Catalysts through Ionic Liquid Modification

Non-precious metal catalysts show great promise to replace the state-of-the-art Pt-based catalysts for catalyzing the oxygen reduction reaction (ORR), while their catalytic activity still needs to be greatly improved before their broad-based application. Here, we report a facile approach to improving the performance of zeolitic imidazolate framework-derived carbon (ZDC) toward the ORR by incorporating a small amount of ionic liquid (IL). The IL would preferentially fill the micropores of ZDC and greatly enhance the utilization of the active sites within the micropores, which are initially not accessible due to insufficient surface wetting. It is also disclosed that the ORR activity in terms of kinetic current at 0.85 V depends on the loading amount of the IL, and the maximum activity is obtained at a mass ratio of IL to ZDC at 1.2. The optimum stems from the counterbalance between the enhanced utilization of the active sites within the micropores and the retarded diffusion of the reactants within the IL phase due to its high viscosity.

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.

Superbase and Hydrophobic Ionic Liquid Confined within Ni Foams as a Free-Standing Catalyst for CO2 Electroreduction

Access to high-performance and cost-effective catalyst materials is one of the crucial preconditions for the industrial application of electrochemical CO₂ reduction (ECR). In this work, a facile and simple strategy is proposed for the construction of a free-standing electrocatalyst via confining a superbase and hydrophobic ionic liquid (IL, [P₆₆₆₁₄][triz]) into Ni foam pores, denoted as [P₆₆₆₁₄][triz]@Ni foam. These ILs can modulate the surface of Ni foam and create a microenvironment with high CO₂ concentration around the electrode/electrolyte interface, which successfully suppresses the hydrogen evolution reaction (HER) of Ni foam. Consequently, the synthesized [P₆₆₆₁₄][triz]@Ni foam sample can obtain a CO product with 63% Faradaic efficiency from the ECR procedure, while no detectable CO can be found on pristine Ni foam. Owing to the superbase IL, the valency of Ni species retains Ni(I)/Ni(0) during electrolysis. Furthermore, the strikingly high CO₂ capacity by [P₆₆₆₁₄][Triz] (0.91 mol CO₂ per mole of IL) offers a high CO₂ local concentration in the reaction region. Theoretical calculations indicated that the neutral CO₂ molecule turned to be negatively charged with -0.546 e and changed into a bent geometry, thus rendering CO₂ activation and reduction in a low-energy pathway. This study provides a new method of electrode interface modification for the design of efficient ECR catalysts.

CO2 Electroreduction on Silver Foams Modified by Ionic Liquids with Different Cation Side Chain Length

Ionic liquids (ILs) are capable of tuning the kinetics of electroreduction processes by modifying a catalyst interface. In this work, a group of hydrophobic imidazolium-based ILs were immobilized on Ag foams by using a procedure known as "solid catalyst with ionic liquid layer" (SCILL). The derived electrocatalysts demonstrated altered selectivity and CO production rates for the electrochemical reduction of CO₂ compared to the unmodified Ag foam. The activity change caused by the IL was dependent on the length of the N-alkyl substituent. The rate of CO production is optimized at moderate chain length and IL loadings. The observed trends are attributed to a local enrichment of CO₂-based species in the proximity of the catalyst and a modification of the environment of its active sites. On the contrary, high loadings or long IL chains render the surface inaccessible and favor the hydrogen evolution reaction.

"Giant" Nitrogen Uptake in Ionic Liquids Confined in Carbon Pores

Ionic liquids are well known for their high gas absorption capacity. It is shown that this is not a solvent constant, but can be enhanced by another factor of 10 by pore confinement, here of the ionic liquid (IL) 1-ethyl-3-methylimidazolium acetate (EmimOAc) in the pores of carbon materials. A matrix of four different carbon compounds with micro- and mesopores as well as with and without nitrogen doping is utilized to investigate the influence of the carbons structure on the nitrogen uptake in the pore-confined EmimOAc. In general, the absorption is most improved for IL in micropores and in nitrogen-doped carbon. This effect is so large that it is already seen in TGA and DSC experiments. Due to the low vapor pressure of the IL, standard volumetric sorption experiments can be used to quantify details of this effect. It is reasoned that it is the change of the molecular arrangement of the ions in the restricted space of the pores that creates additional free volume to host molecular nitrogen.

Copper Nanocrystal Morphology Determines the Viability of Molecular Surface Functionalization in Tuning Electrocatalytic Behavior in CO2 Reduction

Molecular surface functionalization of metallic catalysts is emerging as an ever-developing approach to tuning their catalytic performance. Here, we report the synthesis of hybrid catalysts comprising copper nanocrystals (CuNCs) and an imidazolium ligand for the electrochemical CO₂ reduction reaction (CO₂RR). We show that this organic modifier steers the selectivity of cubic CuNCs toward liquid products. A comparison between cubic and spherical CuNCs reveals the impact of surface reconstruction on the viability of surface functionalization schemes. Indeed, the intrinsic instability of spherical CuNCs leads to ejection of the functionalized surface atoms. Finally, we also demonstrate that the more stable hybrid nanocrystal catalysts, which include cubic CuNCs, can be transferred into gas-flow CO₂RR cells for testing under more industrially relevant conditions.

Optimization Strategies for Selective CO2 Electroreduction to Fuels

Capturing CO2 from the atmosphere and converting it into fuels are an efficient strategy to stop the deteriorating greenhouse effect and alleviate the energy crisis. Among various CO2 conversion approaches, electrocatalytic CO2 reduction reaction (CO2RR) has received extensive attention because of its mild operating conditions. However, the high onset potential, low selectivity toward multi-carbon products and poor cruising ability of CO2RR impede its development. To regulate product distribution, previous studies performed electrocatalyst modification using several universal methods, including composition manipulation, morphology control, surface modification, and defect engineering. Recent studies have revealed that the cathode and electrolytes influence the selectivity of CO2RR via pH changes and ionic effects, or by directly participating in the reduction pathway as cocatalysts. This review summarizes the state-of-the-art optimization strategies to efficiently enhance CO2RR selectivity from two main aspects, namely the cathode electrocatalyst and the electrolyte.