Probing dissolved CO2(aq) in aqueous solutions for CO2 electroreduction and storage

Pressure-Boosted C-C Coupling for CO2 Electroreduction to Ethanol over the Cascade Sites of Cu Single-Atom and Cu Cluster

Tandem catalysis of the CO2 electroreduction reaction (CO2RR) offers a promising way to obtain multicarbon products (C2+) with high Faradaic efficiency (FE). However, the synchronously changed CO supply and consumption with potentials and the mismatch between the two potential windows of the different components in tandem catalysts impede further understanding and development of tandem catalysis. Herein, a novel strategy of pressure regulating to separately promote the CO supply and consumption is reported on the cascade sites of Cu single-atom and Cu cluster, where a near-linear relationship between CO2 pressure, CO supply, and ethanol FE is well established. Benefiting from the above strategy, the ethanol FE of 41.2% is achieved under 26 bar CO2, which increases by ≈3 times that under ambient pressure. This work provides a new method of CO supply regulation and gives more mechanism insights into the tandem catalysis of CO2 electroreduction toward multicarbon products.

Evaluation of Aliphatic Alcohols for CO2 Capture Using the Characteristic Carbonate Frequency

Control over CO2 capture and utilization are important scientific and technological challenges. Although a variety of amine absorbents are used for capture, releasing the captured CO2 is often difficult and limits their recyclability. Therefore, it is crucial to control the strength of the CO2 bond with the absorbent. Furthermore, it is desirable to use a method that can conveniently report the strength of this bond. This motivates exploring adducts of CO2 with alcohols in the presence of a base, using vibrational spectroscopy to report on the bond strength. Although reactions of alcohols with CO2 to form alkyl carbonates are known, a systematic study of these adducts has not been conducted. Here we show formation of alkyl carbonates by a series of alcohols spanning the pKa range 9.5 to 16.8. We show experimental and computational results for the frequency of the characteristic asymmetric stretch of the carbonate and demonstrate that it correlates inversely with the pKa of the alcohol. Based on computations of the bond lengths and previous work, we propose that this frequency also correlates inversely with the adduct strength. This work extends the scope of CO2 capture reagents and inspires further research in tuning alcohols as reversible absorbents.

CuPt Alloy Enabling the Tandem Catalysis for Reduction of HCOOH and NO3 - to Urea at High Current Density

The formation of urea by electrocatalytic reduction of C1-reactants and NO3 - is an attractive way to store renewable electricity, close the carbon cycle, and eliminate nitrate contaminants from wastewater. Involving insufficient supply of C1 reactants and multiple electron transfers makes the reaction difficult to achieve high Faraday efficiency and high yield at high current density. Here, a urea synthesis approach is presented via electrocatalytic reductive coupling between liquid HCOOH and NO3 - on copper foam (CF) loaded Cu4Pt catalyst with optimized ratios. A urea yield of 40.08 mg h-1 cm-2 is achieved with FE up to 58.1% at a current density of -502.3 mA cm-2, superior to the productivity of previously reported catalysts. No degradation is observed over 120-h continuous operation at such a high yield rate. The highly efficient activity of Cu4Pt/CF can be attributed to the synergetic effect between Pt and Cu sites via tandem catalysis, in which the doped Pt sites enrich liquid HCOOH reactants, promote HCOOH intermolecular dehydration, and form and adsorb large amounts of *CO key intermediates. The Cu sites can generate large quantities of the key intermediate *NH2. The Cu4Pt/CF adsorbed intermediates *CO and *NH2 are the basis for subsequent thermodynamic spontaneous C─N coupling.

Monitoring chalcogenide ions-guided in situ transform active sites of tailored bismuth electrocatalysts for CO2 reduction to formate

Although bismuth catalysts enable accelerated electrochemical CO2-to-formate conversion, the intrinsic active sites and forming mechanisms under operating conditions remain elusive. Herein, we prepared Bi2O2NCN, Bi2O3, and Bi2O2S as precatalysts. Among them, Bi2O2NCN-derived catalyst possesses optimum performance of electrochemical CO2-to-formate, exhibiting an upsurge of Faradaic efficiency to 98.3% at -0.6 V vs. reversible hydrogen electrodes. In-situ infrared and electrochemical impedance spectra trace and interpret the superior performance. Multimodal structural analyses utilizing quasi-in-situ X-ray diffraction, in-situ X-ray absorption near edge structure and in-situ Raman spectra provide powerful support to monitoring the catalysts' in-situ transforms to metallic Bi, identifying the formation of the active sites influenced by the chalcogenide ions-guided: Carbodiimide promotes to form of the dominant Bi(003) facet exposure, which distinguishes from sulfide- and oxide-preferred dominant Bi(012) facets exposure. Concurrently, theoretical insights garnered from multiscale/multilevel computational analyses harmoniously corroborate the experimental findings. These findings show the pivotal role of chalcogenide in tailoring bismuth electrocatalysts for selective CO2 reduction to formate, illuminating the significance of controlling structural chemistry in designing catalysts toward high-efficiency renewable energy conversion.

Endoscopic Fourier-transform infrared spectroscopy through a fiber microprobe

Fourier-transform infrared spectroscopy (FTIR) is a powerful analytical method not only for the chemical identification of solid, liquid, and gas species but also for the quantification of their concentration. However, the chemical quantification capability of FTIR is significantly hindered when the analyte is surrounded by a strong IR absorbing medium, such as liquid solutions. To overcome this limit, here we develop an IR fiber microprobe that can be inserted into a liquid medium and obtain full FTIR spectra at points of interest. To benchmark this endoscopic FTIR method, we insert the microprobe into bulk water covering a ZnSe substrate and measure the IR transmittance of water as a function of the probe-substrate distance. The obtained vibrational modes, overall transmittance vs z profiles, quantitative absorption coefficients, and micro z-section IR transmittance spectra are all consistent with the standard IR absorption properties of water. The results pave the way for endoscopic chemical profiling inside bulk liquid solutions, promising for applications in many biological, chemical, and electrochemical systems.

Direct observation of carbon dioxide adsorption and binding at the air/aqueous interface

Carbon dioxide removal (CDR) involves reducing carbon dioxide (CO₂) concentrations. Developing new technologies and enhancing existing ones for extracting and converting CO₂ are ongoing areas of research. In all these technologies, the movement of CO2 molecules through an interface is a common process. At liquid surfaces, the nanometer-thick interfacial region is expected to play a fundamental role in enhancing or hindering the process. The interface can have significantly different conditions, such as pH, ion concentration, and ion speciation, compared with the bulk. Despite this, our knowledge of the molecular-scale details of CO2 capture and conversion at liquid interfaces is limited. Here, we report direct observation of CO2 surface adsorption and conversion to bicarbonate at the air/aqueous interface of potassium orthovanadate solutions using vibrational sum frequency generation spectroscopy. We show that orthovanadate ions enhance the hydrated CO2 population at the interface, indicated by a strong peak at 2,336 cm-1. DFT calculations suggest that CO2 molecules are bent with respect to their original linear structure, demonstrating the initiation of CO2 toHCO 3 - conversion. With increasing orthovanadate concentration and/or time of exposure, the CO2 peak disappears, and (bi)carbonate peaks appear. The characterization of the bulk solutions as well as the precipitated products suggests that the observed interfacial species are transient, different from the final products. This study provides a better understanding of CO2 transport into aqueous media, not only for CDR technologies but also for environmental and atmospheric chemistry in general.

Pressure regulated CO2 electrolysis on two-dimensional Bi2O2Se

The electrochemical reduction of carbon dioxide (CO2RR) offers potential for sustainable production and greenhouse gas mitigation, particularly with renewable energy integration. However, its widespread application is hindered by expensive catalysts, low selectivity, and limited current density. This study addresses these challenges by developing a low-mass-loading two-dimensional (2D) Bi2O2Se catalyst via chemical vapor deposition (CVD). The catalyst achieves a formate faradaic efficiency (FE) of 47.1% with a high current density of 4649 mA mg-1 at -1.15 V (vs. RHE), significantly outperforming bulk Bi2O2Se. Pressurizing CO2, a condition commonly encountered in industrial processes, further enhances formate selectivity and current density, increasing from 2189 mA mg-1 at ambient pressure (1.01 bar) to 7457 mA mg-1 at 40 bar. In situ Raman spectroscopy and DFT calculations reveal the intermediates and pathways involved, underscoring the critical role of pressure in regulating CO2RR pathways. These findings highlight the potential of 2D catalysts for sustainable and industrially relevant CO2 conversion under high pressure.

The Electrode/Electrolyte Interface Study during the Electrochemical CO2 Reduction in Acidic Electrolytes

Electrochemical CO2 Reduction (CO2R) in acidic electrolytes has gained significant attention owing to higher carbon efficiency and stability than in alkaline counterparts. However, the proton source and the role of alkali cations for CO2R are still under debate. By using rotating ring disk electrode and surface-enhanced infrared absorption spectroscopy, we find that a neutral/alkaline environment at the interface is necessary for CO2R even in acidic electrolytes. We also confirm that water molecules, rather than protons serve as the proton source for CO2R. Alkali cations in the outer Helmholtz plane activate H2O and promote the desorption of adsorbed carbon monoxide. Additionally, the solvated CO2, or CO2(aq), is the actual reactant for CO2R. This study provides a deeper understanding of the electrode/electrolyte interface during CO2R in acidic electrolytes and sheds light on further performance improvement of this system.

Sun-simulated-driven production of high-purity methanol from carbon dioxide

CO2 conversion to CH3OH under mild conditions is of particular interest yet rather challenging. Both electro- and thermo-catalytic CO2 reduction to CH3OH can only produce CH3OH in low concentration (typically mixed with water), requiring energy-intensive purification processes. Here we design a sun-simulated-driven tandem catalytic system comprising CO2 electroreduction to syngas, and further photothermal conversion into high-purity CH3OH (volume fraction > 97%). We construct a self-supporting electrocatalyst featuring dual active sites of Ni single atoms and encapsulated Co nanoparticles, which could produce syngas with a constant H2:CO ratio of ~2 via solar-powered CO2 electroreduction. The generated syngas is subsequently fed into the photothermal module, which could produce high-purity CH3OH under 1 sun-light irradiation, with a rate of 0.238 gCH3OH gcat-1 h-1. This work demonstrates a feasible and sustainable route for directly converting CO2 into high-purity CH3OH.

Facile synthesis of a 3D magnetic graphene oxide/Fe3O4/banana peel-derived cellulose composite aerogel for the efficient removal of tetracycline

Many initiatives have incorporated graphene oxide (GO) and biomass into aerogels for wastewater treatment. We report on the facile fabrication of a magnetic GO/Fe3O4/banana peel-derived cellulose (bio-cellulose) aerogel using an ultrasound-assisted mechanical mixing method and freeze-drying technique for the removal of tetracycline (TC). The component materials and composite aerogel were characterized using Fourier-transform infrared spectroscopy (FTIR), X-ray powder diffraction (XRD), Raman spectroscopy, field-emission scanning electron microscopy (FE-SEM), energy-dispersive X-ray spectroscopy (EDX), nitrogen adsorption-desorption analysis, and vibrating sample magnetometry (VSM). The effects of solution pH and adsorbent dose on the adsorption performance of the synthesized adsorbents were investigated. The adsorption behavior at the equilibrium of the GO/Fe3O4/bio-cellulose aerogel was studied and analyzed using four well-known non-linear models: Langmuir, Freundlich, Sips, and Temkin. The results showed that the experimental data fitted well with the Freundlich and Sips isotherm models. The maximum adsorption capacity achieved from the Sips model was 238.7 mg g-1. The adsorption kinetics were studied and proved to follow the Elovich kinetic model with an initial rate of 0.89 g g-1 min-1. These results confirm the favorable adsorption of TC on the heterogeneous surface that exhibits a wide range distribution of adsorption energies of the desired GO/Fe3O4/bio-cellulose aerogel. The experimental findings demonstrate that the aerogel possesses the notable features of environmental friendliness, cost-effectiveness, and comparatively high TC adsorption capacity. Therefore, utilizing biomass to develop the structure of the magnetic GO-based composite aerogel is significantly promising for antibiotic-containing wastewater treatments.

Cathodic Conversion of Pressurized CO2 at Silver Cathodes: What are the Optimal Values of Pressure and Current Density?

The cathodic reduction of pressurized CO2 (PrCO2CR) at suitable cathodes can allow to produce various chemicals, such as formic acid/formate (FA) and carbon monoxide or synthesis gas, with high faradic efficiencies (FEs) and productivities. Here, we have performed the conversion of CO2 in an undivided pressurized electrochemical reactor using silver cathode in order to determine the optimal values of CO2 pressure and current density. It was found that the plot FE vs. pressure resulted in a curve with a maximum. Similarly, an optimal value of current density can be selected for the PrCO2CR. The competition between the production of carbon monoxide and formic acid/formate is strongly affected by both the pressure and the current density. Eventually the effect of pressure and current density on the economic figures of the process was evaluated.

Catalyst-Free Transformation of Carbon Dioxide to Small Organic Compounds in Water Microdroplets Nebulized by Different Gases

A straightforward nebulized spray system is designed to explore the hydrogenation of carbon dioxide (CO2) within water microdroplets surrounded by different gases such as carbon dioxide, nitrogen, oxygen, and compressed air. The collected droplets are analyzed using water-suppressed nuclear magnetic resonance (NMR). Formate anion (HCOO-), acetate anion (CH3COO-), ethylene glycol (HOCH2CH2OH), and methane (CH4) are detected when water is nebulized. This pattern persisted when the water is saturated with CO2, indicating that CO2 in the nebulizing gas triggers the formation of these small organics. In a pure CO2 atmosphere, the formate anion concentration is determined to be ≈70 µm, referenced to dimethyl sulfoxide, which has been introduced as an internal standard in the collected water droplets. This study highlights the power of water microdroplets to initiate unexpected chemistry for the transformation of CO2 to small organic compounds.

Pressure-Dependent CO2 Electroreduction to Methane over Asymmetric Cu-N2 Single-Atom Sites

Single-atom catalysts (SACs) with unitary active sites hold great promise for realizing high selectivity toward a single product in the CO2 electroreduction reaction (CO2RR). However, achieving high Faradaic efficiency (FE) of multielectron products like methane on SACs is still challenging. Herein, we report a pressure-regulating strategy that achieves 83.5 ± 4% FE for the CO2-to-CH4 conversion on the asymmetric Cu-N2 sites, representing one of the best CO2-to-CH4 performances. Elevated CO2 pressure was demonstrated as an efficient way to inhibit the hydrogen evolution reaction via promoting the competing adsorption of reactant CO2, regardless of the nature of the active sites. Meanwhile, the asymmetric Cu-N2 structure could endow the Cu sites with stronger electronic coupling with *CO, thus suppressing the desorption of *CO and facilitating the following hydrogenation of *CO to *CHO. This work provides a synergetic strategy of the pressure-induced reaction environment regulating and the electronic structure modulating for selective CO2RR toward targeted products.

Oxide-Encapsulated Silver Electrocatalysts for Selective and Stable Syngas Production from Reactive Carbon Capture Solutions

Electrolysis of bicarbonate-containing CO2 capture solutions is a promising approach towards achieving low-cost carbon-neutral chemicals production. However, the parasitic bicarbonate-mediated hydrogen evolution reaction (HER) and electrode instability in the presence of trace impurities remain major obstacles to overcome. This work demonstrates that the combined use of titanium dioxide (TiO2) overlayers with the chelating agent ethylene diamine tetra-acetic acid (EDTA) significantly enhances the selectivity and stability of Ag-based electrocatalysts for bicarbonate electrolysis. The amorphous TiO2 overlayers suppress the HER by over 50 % at potentials more negative than -0.7 V vs. RHE, increasing the CO faradaic efficiency (FE) by 33 % (relative). In situ surface-enhanced Raman spectroscopy (SERS) measurements reveal the absence of near-surface bicarbonate species and an abundance of CO2 reduction intermediates at the Ag|TiO2 buried interface, suggesting that the overlayers suppress HER by blocking bicarbonate ions from reaching the buried active sites. In accelerated degradation tests with 5 ppm of Fe(III) impurity, the addition of EDTA allows stable CO production with >47 % FE, while the electrodes rapidly deactivate in the absence of EDTA. This work highlights the use of TiO2 overlayers for enhancing the CO : H2 ratio while simultaneously protecting electrocatalysts from impurities likely to be present in "open" carbon capture systems.

Unlock flow-type reversible aqueous Zn-CO2 batteries

Metal-CO2 batteries, which use CO2 as the active species at cathodes, are particularly promising, but device design for mass-producible CO2 reduction and energetic power supply lag behind, limiting their potential benefits. In this study, an aqueous reversible flow-type Zn-CO2 battery using a Pd/SnO2@C cathode catalyst has been assembled and demonstrates an ultra-high discharge voltage of 1.38 V, a peak power density of 4.29 mW cm-2, high-energy efficiency of 95.64% and remarkable theoretical energy density (827.3 W h kg-1). In the meantime, this optimized system achieves a high formate faradaic efficiency of 95.86% during the discharge process at a high rate of 4.0 mA cm-2. This energy- and chemical-conversion technology could store and provide electricity, eliminate CO2 and produce valuable chemicals, addressing current energy and environment issues simultaneously.

Addressing the Effectiveness and Molecular Mechanism of the Catalytic CO2 Hydration in Aqueous Solutions by Nickel Nanoparticles

Hydration of carbon dioxide in water solution is the rate limiting step for the CO2 mineralization process, a process which is at the base of many carbon capture and utilization (CCU) technologies aiming to convert carbon dioxide to added-value products and mitigate climate change. Here, we present a combined experimental and computational study to clarify the effectiveness and molecular mechanism by which nickel nanoparticles, NiNPs, may enhance CO2 hydration in aqueous solutions. Contrary to previous literature, our kinetic experiments recording changes of pHs, conductivity, and dissolved carbon dioxide in solution reveal a minimal effect of the NiNPs in catalyzing CO2 hydration. Our atomistic simulations indicate that the Ni metal surface can coordinate only a limited number of water molecules, leaving uncoordinated metal sites for the binding of carbon dioxide or other cations in solution. This deactivates the catalyst and limits the continuous re-formation of a hydroxyl-decorated surface, which was a key chemical step in the previously suggested Ni-catalyzed hydration mechanism of carbon dioxide in aqueous solutions. At our experimental conditions, which expand the investigation of NiNP applicability toward a wider range of scenarios for CCU, NiNPs show a limited catalytic effect on the rate of CO2 hydration. Our study also highlights the importance of the solvation regime: while Ni surfaces may accelerate carbon dioxide hydration in water restricted environments, it may not be the case in fully hydrated conditions.

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.

Pressure dependence in aqueous-based electrochemical CO2 reduction

Electrochemical CO₂ reduction (CO₂R) is an approach to closing the carbon cycle for chemical synthesis. To date, the field has focused on the electrolysis of ambient pressure CO₂. However, industrial CO₂ is pressurized-in capture, transport and storage-and is often in dissolved form. Here, we find that pressurization to 50 bar steers CO₂R pathways toward formate, something seen across widely-employed CO₂R catalysts. By developing operando methods compatible with high pressures, including quantitative operando Raman spectroscopy, we link the high formate selectivity to increased CO₂ coverage on the cathode surface. The interplay of theory and experiments validates the mechanism, and guides us to functionalize the surface of a Cu cathode with a proton-resistant layer to further the pressure-mediated selectivity effect. This work illustrates the value of industrial CO₂ sources as the starting feedstock for sustainable chemical synthesis.