Monitoring the Chemical State of Catalysts for CO2 Electroreduction: An In Operando Study

Electrochemical Reduction of CO2 by SnOx Nanosheets Anchored on Multiwalled Carbon Nanotubes with Tunable Functional Groups

Sn-based electrocatalysts are promising for the electrochemical CO₂ reduction reaction (CO2RR), but suffer from poor activity and selectivity. A hierarchical structure composed of ultrathin SnO_x nanosheets anchored on the surface of the commercial multiwalled carbon nanotubes (MWCNTs) is synthesized by a simple hydrothermal process. The electrocatalytic performance can be further tuned by functionalization of the MWCNTs with COOH, NH₂ , and OH groups. Both SnO_x @MWCNTs-COOH and SnO_x @MWCNTs-NH₂ show excellent catalytic activity for CO₂ RR with nearly 100 % selectivity for C₁ products (formate and CO). SnO_x @MWCNTs-COOH has favorable formate selectivity with a remarkably high faradaic efficiency (FE) of 77 % at -1.25 V versus standard hydrogen electrode (SHE) and a low overpotential of 246 mV. However, SnO_x @MWCNTs-NH₂ manifests increased selectivity for CO with higher current density. Density functional theory calculations and experimental studies demonstrate that the interaction between Sn species and functional groups play an important role in the tuning of the catalytic activity and selectivity of these functionalized electrocatalysts. SnO_x @MWCNTs-COOH and SnO_x @MWCNTs-NH₂ both effectively inhibit the hydrogen evolution reaction and prove stable without any significant degradation over 20 h of continuous electrolysis at -1.25 V versus SHE.

Enhanced Stability and CO/Formate Selectivity of Plasma-Treated SnO x/AgO x Catalysts during CO2 Electroreduction

CO₂ electroreduction into useful chemicals and fuels is a promising technology that might be used to minimize the impact that the increasing industrial CO₂ emissions are having on the environment. Although plasma-oxidized silver surfaces were found to display a considerably decreased overpotential for the production of CO, the hydrogen evolution reaction (HER), a competing reaction against CO₂ reduction, was found to increase over time. More stable and C1-product-selective SnO_x/AgO_x catalysts were obtained by electrodepositing Sn on O₂-plasma-pretreated Ag surfaces. In particular, a strong suppression of HER (below 5% Faradaic efficiency (FE) at −0.8 V vs the reversible hydrogen electrode, RHE) during 20 h was observed. Ex situ scanning electron microscopy (SEM) combined with energy-dispersive X-ray spectroscopy (EDS), quasi in situ X-ray photoelectron spectroscopy (XPS), and operando X-ray absorption near-edge structure spectroscopy (XANES) measurements showed that our synthesis led to a highly roughened surface containing stable Sn^δ+/Sn species that were found to be key in the enhanced activity and stable CO/formate (HCOO^–) selectivity. Our study highlights the importance of roughness, composition, and chemical state effects in CO₂ electrocatalysis.

Structure- and Electrolyte-Sensitivity in CO2 Electroreduction

The utilization of fossil fuels (i.e., coal, petroleum, and natural gas) as the main energy source gives rise to serious environmental issues, including global warming caused by the continuously increasing level of atmospheric CO₂. To deal with this challenge, fossil fuels are being partially replaced by renewable energy such as solar and wind. However, such energy sources are usually intermittent and currently constitute a very low portion of the overall energy consumption. Recently, the electrochemical conversion of CO₂ to chemicals and fuels with high energy density driven by electricity derived from renewable energy has been recognized as a promising strategy toward sustainable energy. The activation and reduction of CO₂, which is a thermodynamically stable and kinetically inert molecule, is extremely challenging. Although the participation of protons in the CO₂ electroreduction reaction (CO₂RR) helps lower the energy barrier, high overpotentials are still needed to efficiently drive the process. On the other hand, the concurrent hydrogen evolution reaction (HER) under CO₂RR conditions leads to lower selectivity toward CO₂RR products. Electrocatalysts that are highly active and selective for multicarbon products are urgently needed to improve the energy efficiency of CO₂RR. The reduction of CO₂ involves multiple proton-electron transfers and has many complex intermediates. Recent reports have shown that the relative stability of the intermediates on the surface of catalysts determines final reaction pathways as well as the product selectivity. Furthermore, this reaction displays a strong structure-sensitivity. The atomic arrangement, electronic structure, chemical composition, and oxidation state of the catalysts significantly influence catalyst performance. Fundamental understanding of the dependence of the reaction mechanisms on the catalyst structure would guide the rational design of new nanostructured CO₂RR catalysts. As a reaction proceeding in a complex environment containing gas/liquid/solid interfaces, CO₂RR is also intensively affected by the electrolyte. The electrolyte composition in the near surface region of the electrode where the reaction takes place plays a vital role in the reactivity. However, the former might also be indirectly determined by the bulk electrolyte composition via diffusion. Adding to the complexity, the structure, chemical state and surface composition of the catalysts under reaction conditions usually undergo dynamic changes, especially when adsorbed ions are considered. Therefore, in addition to tuning the structure of the electrocatalysts, being able to also modify the electrolyte provides an alternative method to tune the activity and selectivity of CO₂RR. In situ and operando characterization methods must be employed to gain in depth understanding on the structure- and electrolyte-sensitivity of real CO₂RR catalysts under working conditions. This Account provides examples of recent advances in the development of nanostructured catalysts and mechanistic understanding of CO₂RR. It discusses how the structure of a catalyst (crystal orientation, oxidation state, atomic arrangement, defects, size, surface composition, segregation, etc.) influences the activity and selectivity, and how the electrolyte also plays a determining role in the reaction activity and selectivity. Finally, the importance of in situ and operando characterization methods to understand the structure- and electrolyte-sensitivity of the CO₂RR is discussed.

Polyoxometalate-Promoted Electrocatalytic CO2 Reduction at Nanostructured Silver in Dimethylformamide

Electrochemical reduction of CO₂ is a promising method to convert CO₂ into fuels or useful chemicals, such as carbon monoxide (CO), hydrocarbons, and alcohols. In this study, nanostructured Ag was obtained by electrodeposition of Ag in the presence of a Keggin type polyoxometalate, [PMo₁₂O₄₀]^3- (PMo). Metallic Ag is formed upon reduction of Ag⁺. Adsorption of PMo on the surface of the newly formed Ag lowers its surface energy thus stabilizes the nanostructure. The electrocatalytic performance of this Ag-PMo nanocomposite for CO₂ reduction was evaluated in a CO₂ saturated dimethylformamide medium containing 0.1 M [ n-Bu₄N]PF₆ and 0.5% (v/v) added H₂O. The results show that this Ag-PMo nanocomposite can catalyze the reduction of CO₂ to CO with an onset potential of -1.70 V versus Fc^0/+, which is only 0.29 V more negative than the estimated reversible potential (-1.41 V) for this process and 0.70 V more positive than that on bulk Ag metal. High faradaic efficiencies of about 90% were obtained over a wide range of applied potentials. A Tafel slope of 60 mV dec^-1 suggests that rapid formation of *CO₂^•- is followed by the rate-determining protonation step. This is consistent with the voltammetric data which suggest that the reduced PMo interacts strongly with CO₂ (and presumably CO₂^•-) and hence promotes the formation of CO₂^•-.

Recent advances in the nanoengineering of electrocatalysts for CO2 reduction

Emissions of CO2 from fossil fuel combustion and industrial processes have been regarded as the dominant cause of global warming. Electrochemical CO2 reduction (ECR), ideally in aqueous media, could potentially solve this problem by the storage of energy from renewable sources in the form of chemical energy in fuels or value-added chemicals in a sustainable manner. However, because of the sluggish reaction kinetics of the ECR, efficient, selective, and durable electrocatalysts are required to increase the rate this reaction. Despite considerable progress in using bulk metallic electrodes for catalyzing the ECR, greater efforts are still needed to tackle this grand challenge. In this Review, we highlight recent progress in using nanoengineering strategies to promote the electrocatalysts for the ECR. Through these approaches, considerable improvements in catalytic performance have been achieved. An outlook of future developments in applying and optimizing these strategies is also proposed.

Investigating the Role of Copper Oxide in Electrochemical CO2 Reduction in Real Time

Copper oxides have been of considerable interest as electrocatalysts for CO₂ reduction (CO2R) in aqueous electrolytes. However, their role as an active catalyst in reducing the required overpotential and improving the selectivity of reaction compared with that of polycrystalline copper remains controversial. Here, we introduce the use of selected-ion flow tube mass spectrometry, in concert with chronopotentiometry, in situ Raman spectroscopy, and computational modeling, to investigate CO2R on Cu₂O nanoneedles, Cu₂O nanocrystals, and Cu₂O nanoparticles. We show experimentally that the selective formation of gaseous C₂ products (i.e., ethylene) in CO2R is preceded by the reduction of the copper oxide (Cu₂OR) surface to metallic copper. On the basis of density functional theory modeling, CO2R products are not formed as long as Cu₂O is present at the surface because Cu₂OR is kinetically and energetically more favorable than CO2R.

Metal-based heterogeneous electrocatalysts for reduction of carbon dioxide and nitrogen: mechanisms, recent advances and perspective

Recent progress in the development of metal-based heterogeneous electrocatalysts which have been used in the electrochemical reduction of carbon dioxide and nitrogen with superior performance is comprehensively and critically reviewed.

Ambient NH3 synthesis via electrochemical reduction of N2 over cubic sub-micron SnO2 particles

Cubic sub-micron SnO₂ particles on carbon cloth (SnO₂/CC) are active for electrocatalytic N₂ reduction, with a large NH₃ yield of 1.47 × 10⁻¹⁰ mol s⁻¹ cm⁻² and a high Faradaic efficiency of 2.17%.

Bronze alloys with tin surface sites for selective electrochemical reduction of CO2

Low concentration tin bronze alloys show high selectivity for CO₂ electroreduction to CO, while high concentration tin bronze alloys show high selectivity for formate.

Liquid Hydrocarbon Production from CO2 : Recent Development in Metal-Based Electrocatalysis

Rising levels of CO₂ accumulation in the atmosphere have attracted considerable interest in technologies capable of CO₂ capture, storage and conversion. The electrochemical reduction of CO₂ into high-value liquid organic products could be of vital importance to mitigate this issue. The conversion of CO₂ into liquid fuels by using photovoltaic cells, which can readily be integrated in the current infrastructure, will help realize the creation of a sustainable cycle of carbon-based fuel that will promote zero net CO₂ emissions. Despite promising findings, significant challenges still persist that must be circumvented to make the technology profitable for large-scale utilization. With such possibilities, this Minireview presents the current high-performing catalysts for the electrochemical reduction of CO₂ to liquid hydrocarbons, address the limitations and unify the current understanding of the different reaction mechanisms. The Minireview also explores current research directions to improve process efficiencies and production rate and discusses the scope of using photo-assisted electrochemical reduction systems to find stable, highly efficient catalysts that can harvest solar energy directly to convert CO₂ into liquid hydrocarbons.

Transport Matters: Boosting CO2 Electroreduction in Mixtures of [BMIm][BF4 ]/Water by Enhanced Diffusion

Room-temperature ionic liquids (RTILs) are promising new electrolytes for efficient carbon dioxide reduction. However, due to their high viscosity, the mass transport of CO₂ in RTILs is typically slow, at least one order of magnitude slower than in aqueous systems. One possibility to improve mass transport in RTILs is to decrease their viscosity through dilution with water. Herein, defined amounts of water are added to 1-butyl-3methylimidazolium tetrafluoroborate ([BMIm][BF₄ ]), which is a hydrophilic RTIL. Electrochemical measurements on quiescent and hydrodynamic systems both indicate enhanced CO₂ electroreduction. This enhancement has its origin in thermodynamic/kinetic effects (the addition of water increases the availability of H⁺ , which is a reaction partner of CO₂ electroreduction) and in an increased rate of transport due to lower viscosity. Electrochemically determined diffusion coefficients for CO₂ in [BMIm][BF₄ ]/water systems agree well with values determined by NMR spectroscopy.

Building Blocks for High Performance in Electrocatalytic CO2 Reduction: Materials, Optimization Strategies, and Device Engineering

In recent years, screening of materials has yielded large gains in catalytic performance for the electroreduction of CO₂. However, the diversity of approaches and a still immature mechanistic understanding make it challenging to assess the real potential of each concept. In addition, achieving high performance in CO₂ (photo)electrolyzers requires not only favorable electrokinetics but also precise device engineering. In this Perspective, we analyze a broad set of literature reports to construct a set of design-performance maps that suggest patterns between performance figures and different classes of materials and optimization strategies. These maps facilitate the screening of different approaches to electrocatalyst design and the identification of promising avenues for future developments. At the device level, analysis of the network of limiting phenomena in (photo)electrochemical cells leads us to propose a straightforward performance metric based on the concepts of maximum energy efficiency and maximum product formation rate, enabling the comparison of different technologies.

Li Electrochemical Tuning of Metal Oxide for Highly Selective CO2 Reduction

Engineering active grain boundaries (GBs) in oxide-derived (OD) electrocatalysts is critical to improve the selectivity in CO₂ reduction reaction (CO₂RR), which is becoming an increasingly important pathway for renewable energy storage and usage. Different from traditional in situ electrochemical reduction under CO₂RR conditions, where some metal oxides are converted into active metallic phases but with decreased GB densities, here we introduce the Li electrochemical tuning (LiET) method to controllably reduce the oxide precursors into interconnected ultrasmall metal nanoparticles with enriched GBs. By using ZnO as a case study, we demonstrate that the LiET-Zn with freshly exposed GBs exhibits a CO₂-to-CO partial current of ∼23 mA cm^-2 at an overpotential of -948 mV, representing a 5-fold improvement from the OD-Zn with GBs eliminated during the in situ electro-reduction process. A maximal CO Faradaic efficiency of ∼91.1% is obtained by LiET-Zn on glassy carbon substrate. The CO₂-to-CO mechanism and interfacial chemistry are further probed at the molecular level by advanced in situ spectroelectrochemical technique, where the reaction intermediate of carboxyl species adsorbed on LiET-Zn surface is revealed.

Surface engineered tin foil for electrocatalytic reduction of carbon dioxide to formate

Commercially available Sn foil was anodized in organic solvents to fabricate stable and cost-effective electrode that is demonstrated to convert CO₂to formate with high selectivity.

Highly selective plasma-activated copper catalysts for carbon dioxide reduction to ethylene

There is an urgent need to develop technologies that use renewable energy to convert waste products such as carbon dioxide into hydrocarbon fuels. Carbon dioxide can be electrochemically reduced to hydrocarbons over copper catalysts, although higher efficiency is required. We have developed oxidized copper catalysts displaying lower overpotentials for carbon dioxide electroreduction and record selectivity towards ethylene (60%) through facile and tunable plasma treatments. Herein we provide insight into the improved performance of these catalysts by combining electrochemical measurements with microscopic and spectroscopic characterization techniques. Operando X-ray absorption spectroscopy and cross-sectional scanning transmission electron microscopy show that copper oxides are surprisingly resistant to reduction and copper(+) species remain on the surface during the reaction. Our results demonstrate that the roughness of oxide-derived copper catalysts plays only a partial role in determining the catalytic performance, while the presence of copper(+) is key for lowering the onset potential and enhancing ethylene selectivity.