Toward efficient catalysts for electrochemical CO2 conversion to C2 products

With impressive progress in carbon capture and renewable energy, carbon dioxide (CO₂) conversion into useful chemicals has become a potential tool against climate change. Electrochemical CO₂ conversion into C₂ products (ethylene and ethanol) is an especially economically promising approach and an active research area. Nonetheless, catalyst layer design for CO₂ conversion is challenging because of the complex CO₂-to-C₂ reaction pathways. In this review, we highlight key ideas in catalyst layer design for CO₂ conversion to C₂ in the past few years. We identify three fundamental principles to control catalyst selectivity—local CO₂ and CO concentration, local pH, and intermediate–catalyst interaction. To achieve these goals, we introduce design strategies for both catalytic materials and overall catalyst layer morphology.     

A new Na+ glass-dispersed Na2CO3 composite for a solid state electrochemical CO2 gas sensor

The electrical conductivity of the 40Na₂O:50SiO₂:10B₂O₃ glass-dispersed Na₂CO₃ composite solid electrolyte system, prepared by liquid phase sintering, is systematically investigated using complex impedance spectroscopy. The unreacted glass glues the Na₂CO₃ grains together, which not only reduces the micropores but also improves the ionic conductivity and mechanical strength of the pellet. The conductivity enhancement in such a composite solid electrolyte system is discussed in the light of the increased concentration of charge carriers in a diffuse space charge layer formed at the crystalline-glass interface. A galvanic CO₂ gas sensor using an optimised composite electrolyte (50 wt% glass-dispersed Na₂CO₃) is found to be more stable against thermal cycles (heating and cooling) vis-à-vis the sensor based on a pure crystalline solid electrolyte. Received: 12 June 1998 / Accepted: 21 October 1998     

The interpretation of solution electrolyte vibrational bands in potential-difference infrared spectroscopy

The influence of ionic migration to and from the surrounding solution reservoir upon potential-difference infrared (PDIR) spectra is examined for some cases involving anionic adsorption in order to elucidate its consequences upon the net potential-induced compositional changes in the thin-layer solution. Representative PDIR spectra for the adsorption of azide anions on gold are compared in the absence and presence of excess alkali perchlorate supporting electrolyte. In the latter, the loss of solution azide in the spectral thin layer upon stepping to a more positive potential, resulting from increased azide adsorption, is accompanied by extensive migration of perchlorate into the thin layer. The form of the spectra induced by potential-dependent azide specific adsorption differs in these two circumstances since in the former the ionic migration between the thin-layer cavity and the solution reservoir necessary for charge compensation is provided by the azide electrolyte itself, whereas in the latter case migration of the supporting electrolyte yields a fixed quantity of azide in the thin layer. The intensities and sign of the PDIR bands arising from solution-phase azide and perchlorate enable the extent of the potential-dependent anionic redistribution in the thin-layer cavity to be quantified. In the absence of added perchlorate, the magnitude of the solution azide band is diminished substantially, inferring that replenishment of the thin-layer solution concentration occurs predominantly via N⁻₃ migration from the surrounding solution reservoir. Similar results were also obtained for cyanate adsorption on gold. The influence of cation as well as anion migration on this thin-layer charge redistribution was examined by employing an infrared-active cation, NH⁺₄, as well as from the addition of H₃O⁺. While the results indicate that cation migration can contribute substantially to this charge redistribution, anion migration typically appears to predominate when specific anion adsorption is encountered. Some general consequences of such ion migration effects to the interpretation of PDIR spectra are noted.     

Quasi-square-shaped cadmium hydroxide nanocatalysts for electrochemical CO2 reduction with high efficiency

Powered by a renewable electricity source, electrochemical CO₂ reduction reaction is a promising solution to facilitate the carbon balance. However, it is still a challenge to achieve a desired product with commercial current density and high efficiency. Herein we designed quasi-square-shaped cadmium hydroxide nanocatalysts for CO₂ electroreduction to CO. It was discovered that the catalyst is very active and selective for the reaction. The current density could be as high as 200 mA cm⁻² with a nearly 100% selectivity in a commonly used H-type cell using the ionic liquid-based electrolyte. In addition, the faradaic efficiency of CO could reach 90% at a very low overpotential of 100 mV. Density functional theory studies and control experiments reveal that the outstanding performance of the catalyst was attributed to its unique structure. It not only provides low Cd–O coordination, but also exposes high activity (002) facet, which requires lower energy for the formation of CO. Besides, the high concentration of CO can be achieved from the low concentration CO₂via an adsorption-electrolysis device.

Quasi-square cadmium hydroxide nanocrystals (Cdhy-QS) showed outstanding performance for electroreduction CO₂ to CO.     

Further Study of CO2 Electrochemical Reduction on Palladium Modified BDD Electrode: Influence of Electrolyte

The study of CO₂ electrochemical reduction to useful compounds using bare or modified BDD electrode attracts numerous attentions. Meanwhile, the efficiency of products obtained from CO₂ electrochemical reduction is known to be determined by the electrode material and the electrolyte. Formic acid as main product and CO as a minor product, have also been known on the CO₂ reduction using BDD electrode. Recently, we reported the successful improvement of CO production from the reduction of CO₂ by decorating the surface of BDD electrode with palladium particles. Following this, herein, we present further investigation on electrolyte dependence, including cation and anion dependence and also concentration effect in order to understand deeply the CO₂ reduction on surface of palladium modified BDD electrode. The results suggest the use of NaCl and KCl as a catholyte for optimum performance, in addition to the improvement of CO₂ reduction product in higher electrolyte concentration.     

红外光谱电化学研究金电极表面上CO2的还原

以碳酸丙烯酯(PrC)为溶剂,高氯酸四丁基胺(TBAP)为电解质,利用电化学及红外光谱电化学开展了金电极上二氧化碳的还原研究。运用现场红外光谱跟踪电化学还原过程反应物及产物的生成和消失。红外光谱电化学循环伏吸法表明,在消耗CO2的同时,金电极上有CO的产生,且伴随有碳酸根的形成。结合电化学和光谱电化学结果,提出了一种电还原机理:在非水介质中,CO2电还原过程中生成了中间体CO2.-,随后CO2.-分别以两个途径进行还原,其一是直接被还原成CO,其二是与CO2结合生成C2O4.-而后歧化成CO以及CO32-。两个反应同时进行,且第一个反应是可逆过程。     

Comparing interfacial cation hydration at catalytic active sites and spectator sites on gold electrodes: understanding structure sensitive CO2 reduction kinetics

Hydrated cations present in the electrochemical double layer (EDL) are known to play a crucial role in electrocatalytic CO₂ reduction (CO₂R), and numerous studies have attempted to explain how the cation effect contributes to the complex CO₂R mechanism. CO₂R is a structure sensitive reaction, indicating that a small fraction of total surface sites may account for the majority of catalytic turnover. Despite intense interest in specific cation effects, probing site-specific, cation-dependent solvation structures remains a significant challenge. In this work, CO adsorbed on Au is used as a vibrational Stark reporter to indirectly probe solvation structure using vibrational sum frequency generation (VSFG) spectroscopy. Two modes corresponding to atop adsorption of CO are observed with unique frequency shifts and potential-dependent intensity profiles, corresponding to direct adsorption of CO to inactive surface sites, and in situ generated CO produced at catalytic active sites. Analysis of the cation-dependent Stark tuning slopes for each of these species provides estimates of the hydrated cation radius upon adsorption to active and inactive sites on the Au electrode. While cations are found to retain their bulk hydration shell upon adsorption at inactive sites, catalytic active sites are characterized by a single layer of water between the Au surface and the electrolyte cation. We propose that the drastic increase in catalytic performance at active sites stems from this unique solvation structure at the Au/electrolyte interface. Building on this evidence of a site-specific EDL structure will be critical to understand the connection between cation-dependent interfacial solvation and CO₂R performance.

Site-specific vibrational probes were used to elucidate the interfacial solvation structure between catalytic active sites and inactive sites on a Au electrode to reveal a unique, opposing cation-dependent double layer structure at active sites.     

2-Aminobenzenethiol-Functionalized Silver-Decorated Nanoporous Silicon Photoelectrodes for Selective CO2 Reduction

A molecularly thin layer of 2-aminobenzenethiol (2-ABT) was adsorbed onto nanoporous p-type silicon (b-Si) photocathodes decorated with Ag nanoparticles (Ag NPs). The addition of 2-ABT alters the balance of the CO₂ reduction and hydrogen evolution reactions, resulting in more selective and efficient reduction of CO₂ to CO. The 2-ABT adsorbate layer was characterized by Fourier transform infrared (FTIR) spectroscopy and modeled by density functional theory calculations. Ex situ X-ray photoelectron spectroscopy (XPS) of the 2-ABT modified electrodes suggests that surface Ag atoms are in the +1 oxidation state and coordinated to 2-ABT via Ag−S bonds. Under visible light illumination, the onset potential for CO₂ reduction was −50 mV vs. RHE, an anodic shift of about 150 mV relative to a sample without 2-ABT. The adsorption of 2-ABT lowers the overpotentials for both CO₂ reduction and hydrogen evolution. A comparison of electrodes functionalized with different aromatic thiols and amines suggests that the primary role of the thiol group in 2-ABT is to anchor the NH₂ group near the Ag surface, where it serves to bind CO₂ and also to assist in proton transfer.     

The effect of specific adsorption of halide ions on electrochemical CO2 reduction

In the electrochemical CO₂ reduction reaction (CO₂RR), halide ions could impose a significant effect on multi-carbon (C₂₊) product production for Cu-based catalysts by a combined contribution from various mechanisms. However, the nature of specific adsorption of halide ions remains elusive due to the difficulty in decoupling different effects. This paper describes a facile method to actively immobilize the morphology of Cu-based catalysts during the CO₂RR, which makes it possible to reveal the fundamental mechanism of specific adsorption of halide ions. A stable morphology is obtained by pre-reduction in aqueous KX (X = Cl, Br, I) electrolytes followed by conducting the CO₂RR using non-buffered and non-specifically adsorbed K₂SO₄ as the supporting electrolyte, by which the change of local pH and cation concentration is also maintained during the CO₂RR. In situ spectroscopy revealed that the specific adsorption of halide ions enhances the adsorption of *CO intermediates, which enables a high selectivity of 84.5% for C₂₊ products in 1.0 M KI.

Reconstruction of catalyst morphology induced by halide ions over of Cu-based catalysts during CO₂ electroreduction is suppressed using K₂SO₄ as supporting electrolyte. Adsorption of halide ions is an enabling factor to enhance CO₂ electroreduction.     

2‐Aminobenzenethiol‐Functionalized Silver‐Decorated Nanoporous Silicon Photoelectrodes for Selective CO2 Reduction

A molecularly thin layer of 2‐aminobenzenethiol (2‐ABT) was adsorbed onto nanoporous p‐type silicon (b‐Si) photocathodes decorated with Ag nanoparticles (Ag NPs). The addition of 2‐ABT alters the balance of the CO₂ reduction and hydrogen evolution reactions, resulting in more selective and efficient reduction of CO₂ to CO. The 2‐ABT adsorbate layer was characterized by Fourier transform infrared (FTIR) spectroscopy and modeled by density functional theory calculations. Ex situ X‐ray photoelectron spectroscopy (XPS) of the 2‐ABT modified electrodes suggests that surface Ag atoms are in the +1 oxidation state and coordinated to 2‐ABT via Ag?S bonds. Under visible light illumination, the onset potential for CO₂ reduction was ?50 mV vs. RHE, an anodic shift of about 150 mV relative to a sample without 2‐ABT. The adsorption of 2‐ABT lowers the overpotentials for both CO₂ reduction and hydrogen evolution. A comparison of electrodes functionalized with different aromatic thiols and amines suggests that the primary role of the thiol group in 2‐ABT is to anchor the NH₂ group near the Ag surface, where it serves to bind CO₂ and also to assist in proton transfer.     

Electrode Potentials of Cells with an Electrolyte Based on Zirconium Dioxide in Gas Mixtures N2+ O2+ CO2+ CO: A Platinum Electrode

Steady-state potentials of various platinum electrodes are measured in cells containing electrolyte ZrO₂+ Y₂O₃(10 mol %) in the temperature range 673–773 K in binary equilibrium gas mixtures N₂+ O₂and CO + CO₂, as well as in four-component nonequilibrium gas mixtures N₂+ O₂+ CO₂+ CO containing 0–3 vol % CO and 0–10 vol % O₂. Adding CO to a gas mixture makes the electrode potential deviate from equilibrium, which is explained by chemisorption of CO on the electrode. The oxygen, which is adsorbed on platinum, interacts with CO; as a result, CO₂undergoes desorption and the surface concentration of CO drops.     

Hydride Formation Diminishes CO2 Reduction Rate on Palladium

The catalytic hydrogenation of CO₂ includes the dissociation of hydrogen and further reaction with CO₂ and intermediates. We investigate how the amount of hydrogen in the bulk of the catalyst affects the hydrogenation reaction taking place at the surface. For this, we developed an experimental setup described herein, based on a magnetic suspension balance and an infrared spectrometer, and measured pressure-composition isotherms of the Pd−H system under conditions relevant for CO₂ reduction. The addition of CO₂ has no influence on the measured hydrogen absorption isotherms. The pressure dependence of the CO formation rate changes suddenly upon formation of the β-PdH phase. This effect is attributed to a smaller surface coverage of hydrogen due to repulsive electronic interactions affecting both bulk and surface hydrogen.     

Ag@Au Core‐Shell Nanowires for Nearly 100 % CO2‐to‐CO Electroreduction

Electrochemical reduction of carbon dioxide (CO₂) to CO is regarded as an efficient method to utilize the greenhouse gas CO₂, because the CO product can be further converted into high value‐added chemicals via the Fisher–Tropsch process. Among all electrocatalysts used for CO₂‐to‐CO reduction, Au‐based catalysts have been demonstrated to possess high selectivity, but their precious price limits their future large‐scale applications. Thus, simultaneously achieving high selectivity and reasonable price is of great importance for the development of Au‐based catalysts. Here, we report Ag@Au core–shell nanowires as electrocatalyst for CO₂ reduction, in which a nanometer‐thick Au film is uniformly deposited on the core Ag nanowire. Importantly, the Ag@Au catalyst with a relative low Au content can drive CO generation with nearly 100 % Faraday efficiency in 0.1 m KCl electrolyte at an overpotential of ca. ?1.0 V. This high selectivity of CO₂ reduction could be attributed to a suitable adsorption strength for the key intermediate on Au film together with the synergistic effects between the Au shell and Ag core and the strong interaction between CO₂ and Cl^? ions in the electrolyte, which may further pave the way for the development of high‐efficiency electrocatalysts for CO₂ reduction.     

The effect of Al2O3-coating coverage on the electrochemical properties in LiCoO2 thin films

The electrochemical properties of nanoscale Al₂O₃-coated LiCoO₂ thin films were examined as a function of the coating coverage. Al₂O₃-coated LiCoO₂ films showed enhanced cycle-life performance with increasing degree of coating coverage, which was attributed to the suppression of Co dissolution and F⁻ concentration in the electrolyte. Moreover, an Al₂O₃-coating layer with partial coverage clearly improved the electrochemical properties, even at 60 °C or with a water-contaminated electrolyte. Even though metal-oxide coating on LiCoO₂ has been actively investigated, the mechanisms of nanoscale coating have yet to be clearly identified. In this article, surface analysis suggested that the Al₂O₃-coating layer had transformed to an AlF₃ ∙3H₂O layer during cycling, which inhibited the generation of HF by scavenging H₂O molecules present in the electrolyte.     

Efficient CO2 Removal for Ultra‐Pure CO Production by Two Hybrid Ultramicroporous Materials

Removal of CO₂ from CO gas mixtures is a necessary but challenging step during production of ultra‐pure CO as processed from either steam reforming of hydrocarbons or CO₂ reduction. Herein, two hybrid ultramicroporous materials (HUMs), SIFSIX‐3‐Ni and TIFSIX‐2‐Cu‐i , which are known to exhibit strong affinity for CO₂, were examined with respect to their performance for this separation. The single‐gas CO sorption isotherms of these HUMs were measured for the first time and are indicative of weak affinity for CO and benchmark CO₂/CO selectivity (>4000 for SIFSIX‐3‐Ni ). This prompted us to conduct dynamic breakthrough experiments and compare performance with other porous materials. Ultra‐pure CO (99.99 %) was thereby obtained from CO gas mixtures containing both trace (1 %) and bulk (50 %) levels of CO₂ in a one‐step physisorption‐based separation process.     

Optical and morphological characterization of bispyrazole thin films for gas sensing applications

The optical gas recognition capabilities of thin film layer of 4-[bis[(3,5-dimethyl-1H-pyrazol-1-yl)methyl]-amino]phenol deposed on quartz substrates were studied. The dynamic gas responses to the following analytes have been investigated as air pollutants (SO₂, NO₂, CO, CH₄ and NH₃). The spin-coated bispyrazole layer appears to have reversible response towards SO₂ and a very low and irreversible response to NO₂. The selectivity of the thin film based on bispyrazole layer with respect to other analytes was also examined and the present data show that the thin sensing layer in the presence of CO, CH₄ and NH₃ in low concentration does not influence its optical properties.     

Highly Efficient Porous Carbon Electrocatalyst with Controllable N-Species Content for Selective CO2 Reduction

We report a straightforward strategy to design efficient N doped porous carbon (NPC) electrocatalyst that has a high concentration of easily accessible active sites for the CO₂ reduction reaction (CO₂RR). The NPC with large amounts of active N (pyridinic and graphitic N) and highly porous structure is prepared by using an oxygen-rich metal–organic framework (Zn-MOF-74) precursor. The amount of active N species can be tuned by optimizing the calcination temperature and time. Owing to the large pore sizes, the active sites are well exposed to electrolyte for CO₂RR. The NPC exhibits superior CO₂RR activity with a small onset potential of −0.35 V and a high faradaic efficiency (FE) of 98.4 % towards CO at −0.55 V vs. RHE, one of the highest values among NPC-based CO₂RR electrocatalysts. This work advances an effective and facile way towards highly active and cost-effective alternatives to noble-metal CO₂RR electrocatalysts for practical applications.     

Pebax/TSIL blend thin film composite membranes for CO_2 separation

In this study a thin film composite (TFC) membrane with a Pebax/Task-specific ionic liquid (TSIL) blend selective layer was prepared. Defect-free Pebax/TSIL layers were coated successfully on a polysulfone ultrafiltration porous support with a polydimethylsiloxane (PDMS) gutter layer. Different parameters in the membrane preparation (e.g. concentration, coating time) were investigated and optimized. The morphology of the membranes was studied by scanning electron microscopy (SEM), while the thermal properties and chemical structures of the membrane materials were investigated by thermo-gravimetric analyzer (TGA), differential scanning calorimetry (DSC) and Fourier transform infrared spectroscopy (FTIR). The CO₂ separation performance of the membrane was evaluated using a mixed gas permeation test. Experimental results show that the incorporation of TSIL into the Pebax matrix can significantly increase both CO₂ permeance and CO₂/N₂ selectivity. With the presence of water vapor, the membrane exhibits the best CO₂/N₂ selectivity at a relative humidity of around 75%, where a CO₂ permeance of around 500 GPU and a CO₂/N₂ selectivity of 46 were documented. A further increase in the relative humidity resulted in higher CO₂ permeance but decreased CO₂/N₂ selectivity. Experiments also show that CO₂ permeance decreases with a CO₂ partial pressure increase, which is considered a characteristic in facilitated transport membranes.     

Effects of thermal activation on the oxidation pathways of methanol at bulk Pt–Ru alloy electrodes

The competition between pathways that lead to adsorbed CO and CO₂ during the electrochemical oxidation of 1.0 M methanol in 0.1 M HClO₄ on two bulk Pt–Ru alloys (10 at.% Ru (X_Ru≈0.1) and 90 at.% Ru (X_Ru≈0.9)) was investigated for temperatures in the range of 25–80°C. On the high Ru content alloy studied (X_Ru≈0.9), the dissociative chemisorption of methanol was inhibited below 70°C; the faradaic current for methanol oxidation was low, and only small quantities of adsorbed CO and CO₂ were detected with infrared spectroscopy between 0.2–0.8 V (vs. RHE). At 80°C, strong infrared bands from CO₂ and adsorbed, atop coordinated CO were observed over the potential ranges of 0.4–0.8 V and 0.2–0.8 V, respectively. The infrared measurements are consistent with the observation that bulk, high Ru content alloy electrodes appear passivated toward methanol oxidation below 70°C. On the low Ru content alloy studied (X_Ru≈0.1), the methanol surface chemistry was similar to that of pure, polycrystalline Pt, but the electrode was more poison resistant than Pt. For both alloys, the persistence of strong adsorbed CO bands and rapid CO₂ production between 0.4–0.8 V suggests CO functions as a reactive species with high steady-state coverages at these potentials.     

Synergistic Interplay of Dual-Active-Sites on Metallic Ni-MOFs Loaded with Pt for Thermal-Photocatalytic Conversion of Atmospheric CO2 under Infrared Light Irradiation

Infrared light driven photocatalytic reduction of atmospheric CO₂ is challenging due to the ultralow concentration of CO₂ (0.04 %) and the low energy of infrared light. Herein, we develop a metallic nickel-based metal–organic framework loaded with Pt (Pt/Ni-MOF), which shows excellent activity for thermal-photocatalytic conversion of atmospheric CO₂ with H₂ even under infrared light irradiation. The open Ni sites are beneficial to capture and activate atmospheric CO₂, while the photogenerated electrons dominate H₂ dissociation on the Pt sites. Simultaneously, thermal energy results in spilling of the dissociated H₂ to Ni sites, where the adsorbed CO₂ is thermally reduced to CO and CH₄. The synergistic interplay of dual-active-sites renders Pt/Ni-MOF a record efficiency of 9.57 % at 940 nm for converting atmospheric CO₂, enables the procurement of CO₂ to be independent of the emission sources, and improves the energy efficiency for trace CO₂ conversion by eliminating the capture media regeneration and molecular CO₂ release.