CO2 In Aqueous Solutions Research Articles

Converting Energy with Glycerol and CO2 in a Microfluidic Fuel Cell Equipped with CuBiO4/CuO Photocathode: Bypassing Bubbles Challenge of Concurrent Water Splitting.

The imperative to address CO2 emissions has prompted the search for alternative approaches to capture this gas with minimal energy consumption. In this context, leveraging the CO2 reduction reaction (CO2RR) as an oxidant in fuel cells has emerged as a sophisticated strategy to convert this gas into usable energy. This study introduces a hybrid microfluidic photo fuel cell (μPFC) designed for the efficient conversion of CO2 and glycerol into electrical energy. The prototype integrates 3D-printed components with glass sealing, enabling precise control over the reactant flow and the use of light-sensitive catalysts. The anodic glycerol electrooxidation was investigated on Pt/C dispersed on carbon paper (CP), while the CO2RR was carried out on CuBiO4/CP and CuBiO4/CuO/CP in the presence of solar light. Half-cell measurements demonstrate the photoactivity of CuBiO4/CuO/CP and CuBiO4/CP electrodes for the CO2RR under light exposure at low onset potential in a neutral pH solution, generating a positive theoretical open-circuit voltage of 0.89-0.91 V when coupled to glycerol electrooxidation in an alkaline medium. The use of the mixed medium in the membraneless system equipped with the photosensitive catalysts allowed the building of this galvanic cell. However, the feasibility of using CuBiO4/CP is hindered by the disruption of the colaminar channel caused by hydrogen bubbles produced during concurrent water splitting. In contrast, the μPFC equipped with a CuBiO4/CuO/CP photocathode demonstrates a stable and reproducible performance, delivering a maximum power density of 0.9 mW cm-2. The formation of the CuBiO4/CuO heterojunction effectively suppresses photocatalytic water splitting, allowing for efficient CO2 conversion without disruption of the laminar flow channel. This innovative approach highlights the potential of μPFCs as sustainable energy converters for the utilization of CO2 in aqueous solutions, offering a pathway toward carbon-neutral energy production.

Review on the Potential Co-Application of Aqueous CO2 Solution in Cementitious Material

The escalating concerns over climate change and the urgent demand for sustainable development have propelled significant efforts to mitigate carbon footprints by sequestering carbon dioxide (CO<sub>2</sub>) into cementitious materials. One promising approach involves the incorporation of calcium-rich materials along with super-saturated aqueous solutions in cement, a method demonstrated to be effective in various studies. However, further research is necessary to address technical challenges and optimize the process. This paper presents a comprehensive review of the existing knowledge on the impact of different calcium-rich materials and curing conditions on the workability and compressive strength of concrete, with a particular focus on aqueous CO<sub>2</sub>. Utilizing the Systematic Literature Review (SLR) method, this study identifies the problem, selects the subject of investigation, and gathers pertinent information from suitable databases. The selected documents were analyzed, evaluated, and synthesized to combine the findings. Additionally, the parameters of carbonated concrete utilized in recent studies were analyzed and discussed, providing a thorough understanding of the current advancements and areas requiring further investigation.

Covalent Functionalization of Silicon with Plasma-Grown "Fuzzy" Graphene: Robust Aqueous Photoelectrodes for CO2 Reduction by Molecular Catalysts.

Carbon electrodes are ideal for electrochemistry with molecular catalysts, exhibiting facile charge transfer and good stability. Yet for solar-driven catalysis with semiconductor light absorbers, stable semiconductor/carbon interfaces can be difficult to achieve, and carbon's high optical extinction means it can only be used in ultrathin layers. Here, we demonstrate a plasma-enhanced chemical vapor deposition process that achieves well-controlled deposition of out-of-plane "fuzzy" graphene (FG) on thermally oxidized Si substrates. The resulting Si|FG interfaces possess a silicon oxycarbide (SiOC) interfacial layer, implying covalent bonding between Si and the FG film that is consistent with the mechanical robustness observed from the films. The FG layer is uniform and tunable in thickness and optical transparency by deposition time. Using p-type Si|FG substrates, noncovalent immobilization of cobalt phthalocyanine (CoPc) molecular catalysts was employed for the photoelectrochemical reduction of CO2 in aqueous solution. The Si|FG|CoPc photocathodes exhibited good catalytic activity, yielding a current density of ∼1 mA/cm2, Faradaic efficiency for CO of ∼70% (balance H2), and stable photocurrent for at least 30 h at -1.5 V vs Ag/AgCl under 1-sun illumination. The results suggest that plasma-deposited FG is a robust carbon electrode for molecular catalysts and suitable for further development of aqueous-stable Si photocathodes for CO2 reduction.

Measurement of the reaction enthalpy of CO2 in aqueous solutions with thermographic and gravimetric methods

In this work, a new concept for the approximate determination of the reaction enthalpy of the reaction between CO2 and monoethanolamine (MEA) in aqueous solution was developed. For this purpose, a CO2 gas stream was flowed into aqueous MEA solutions with different concentrations of 1 wt%, 2.5 wt% and 7.5 wt%. The weight difference ∆T, which is based on the increase in CO2 bound by the MEA over time, was documented using a thermographic camera. The mass difference ∆m, which is also based on the increase in CO2 bound by the MEA over time, was determined using a balance. By determining ∆T and ∆m, an approximate calculation of the reaction enthalpy is possible. The deviation from the values from the data known from the literature was less than 5% in all experiments.

Designing multifunctional basalt-CeO2@C3N4/epoxy novolac composite coating with outstanding corrosion resistance and CO2 gas barrier properties

The greenhouse effect caused by CO2 emissions is becoming more and more serious, and the carbon capture, utilization and storage (CCUS) is a feasible strategy to reduce emissions. However, the corrosion problem brought by CCUS is unprecedentedly serious. In this paper, we present a composite epoxy novolac (EN) system for resisting corrosion of metals in CCUS environments. In this system, nanorod CeO2 and nanosheet C3N4 are simultaneously grown on the surface of the micron sheet basalt (Bt), which are used as effective additives to improve the corrosion resistance and CO2 gas barrier properties of EN. Only with 3 % of nanoparticles addition, a significant reduction of the gas diffusion of composite film to dry CO2 has been observed, up to 5.88 × 10−12 m2/s. This is associated with the synergistic enhancement of the physical shielding effect by the nano and micron sheets. Additionally, we found that adsorption of oxygen vacancy of CeO2 material with CO2 enhances the stability of corrosion resistance. Therefore, the |Z|0.01 Hz of the Bt-CeO2@C3N4/EN coating is above 106 Ω cm2 higher than that of the EN coating after 25 days of immersion in a 3.0 MPa CO2 aqueous solution at 70 °C. Overall, the anti-corrosion coating system combines long-lasting water resistance and low CO2 gas transmission in harsh corrosive environments, and thus has a high potential for CCUS applications.

Plastron effect enhanced electrochemical CO2 reduction activity over hydrophobic three-dimensional nanoporous silver.

The electrochemical reduction of CO2 on catalyst surfaces is hindered by the inefficient mass transfer of CO2 in aqueous solutions. In this study, we employed an electrochemical reduction approach to fabricate a hydrophobic three-dimensional nanoporous silver catalyst with a plastron effect, aiming to enhance the CO2 diffusion. The resulting catalyst exhibited an exceptional performance with the FECO peaking at 95% at -0.65 V (vs. RHE) and demonstrated remarkable stability during continuous electrolysis for 48 hours. Control experiments, together with Tafel analysis, EIS measurements, and contact angle results, confirmed that the notable enhancement of performance was attributed to the hydrophobic porous structure that facilitated efficient storage and rapid mass transfer of low-solubility CO2 gas reactants.

Stability and dynamics of bubble comprising carbon dioxide and air

The stability and dynamics of a single partial-equilibrium bubble comprising carbon dioxide (CO2) gas and air were theoretically investigated. The partial equilibrium bubbles, which are typically generated by bubbling in CO2 aqueous solutions initiated by pre-existing fine air bubbles, were assumed to be in equilibrium with the surrounding liquids with respect to the pressure and chemical potential of CO2 but not with respect to the chemical potential of air. The stability of the bubble was formulated using thermodynamic inequalities and two types of potential energy that became minimum and maximum at stable and unstable bubble radii, respectively. The dynamics of the bubble were studied by numerically solving the time evolution equation for the bubble radius. In addition, the stability and dynamics of a single complete-equilibrium bubble comprising CO2 gas and air are discussed.

Organic Polymer Dots PhotocatalyzeCO2 Reduction in Aqueous Solution.

Developing low-cost and efficient photocatalysts to convert CO2 into valuable fuels is desirable to realize a carbon-neutral society. In this work, we report that polymer dots (Pdots) of poly[(9,9'-dioctylfluorenyl-2,7-diyl)-co-(1,4-benzo-thiadiazole)] (PFBT) without adding any extra co-catalyst can photocatalytic reduction of CO2 into CO in aqueous solution, rendering a CO production rate of 57 μmol g-1 h-1 with a detectable selectivity of up to 100%. 5 cycles of CO2 re-purging experiments show no distinct decline in CO amount and reaction rate, indicating the promising photocatalytic stability of PFBT Pdots in photocatalytic CO2 reduction reaction. Mechanistic study reveals that photo-excited PFBT Pdots are reduced by TEOA first, then the reduced PFBT Pdots can bind CO2 and reduce it into CO via their intrinsic active sites. This work highlights the application of organic Pdots for CO2 reduction in the aqueous solution, which therefore provides a strategy to develop highly efficient and environmentally friendly nanoparticular photocatalysts for CO2 reduction.

Organic Polymer Dots Photocatalyze CO2 Reduction in Aqueous Solution

AbstractDeveloping low‐cost and efficient photocatalysts to convert CO2 into valuable fuels is desirable to realize a carbon‐neutral society. In this work, we report that polymer dots (Pdots) of poly[(9,9′‐dioctylfluorenyl‐2,7‐diyl)‐co‐(1,4‐benzo‐thiadiazole)] (PFBT), without adding any extra co‐catalyst, can photocatalyze reduction of CO2 into CO in aqueous solution, rendering a CO production rate of 57 μmol g−1 h−1 with a detectable selectivity of up to 100 %. After 5 cycles of CO2 re‐purging experiments, no distinct decline in CO amount and reaction rate was observed, indicating the promising photocatalytic stability of PFBT Pdots in the photocatalytic CO2 reduction reaction. A mechanistic study reveals that photoexcited PFBT Pdots are reduced by sacrificial donor first, then the reduced PFBT Pdots can bind CO2 and reduce it into CO via their intrinsic active sites. This work highlights the application of organic Pdots for CO2 reduction in aqueous solution, which therefore provides a strategy to develop highly efficient and environmentally friendly nanoparticulate photocatalysts for CO2 reduction.

Production of renewable fuels by the photocatalytic reduction of CO2 using magnesium doped natural ilmenite

The photo-reductive performance of natural ilmenite was boosted and the production of renewable fuels from the reduction of CO2 was enhanced by doping the natural mineral with magnesium. The doping was achieved by high energy ball milling in the presence of MgO and Mg(NO3)2. The photo-reduction of CO2 in aqueous solution led to the evolution of H2, CH4, C2H4, and C2H6, and the insertion of Mg in the structure of ilmenite enabled increases of up to 1245% in the fuel production yield, reaching total production of 210.9 µmol h−1 gcat−1. Displacements of the conduction band to more negative potentials were evidenced for the samples doped with magnesium. Indirect effects such as increases in the valence band maximum, and the introduction of intermediate energy levels were also evidenced through the measurement of the crystallite size and the determination of the band structure of the materials. Mott-Schottky analyses of the samples showed the n-type nature of the semiconductor materials and enabled the estimation of the density of charge carriers, which strongly influenced the photocatalytic performance. The strong potential of the application of natural ilmenite in gas phase artificial photosynthesis was proved by the evaluation of CO2 reduction in gas conditions, which allowed the enhancement in the selectivity and significantly increased the production of CH4 as compared to aqueous solution, reaching an important yield of CH4 of 16.1 µmol h−1 gcat−1.

Electrochemical Reduction of Amine-Captured CO2 in Aqueous Solutions

Technologies that can capture CO2 and enable conversion into value-adding chemicals and fuels or stable minerals for sequestration are vital for transitioning towards net zero or even negative greenhouse gas emissions. Conventional approaches for electrochemically converting CO2 have utilized a decoupled approach of first capturing and concentrating CO2, and then using the concentrated CO2 as a feedstock for conventional electrochemical processes. Direct electrochemical reduction of amine-captured CO2 1,2 can potentially offer advantages by removing the need to thermally regenerate the amine capture solution, which can be energy intensive and typically uses thermal energy from nonrenewable sources. In this talk, we share our recent work on the electrochemical reduction of amine-captured CO2 to value-adding products like CO and stable minerals like carbonates. We discuss the influence of the capture environment on the resulting capture solution chemistry, and how to alter the capture solution speciation through electrolyte design. We further consider the detailed CO2 reduction mechanisms in these amine-containing solutions and provide design strategies for increasing the Faradaic efficiency of CO2 reduction vs. the competitive hydrogen evolution reaction (HER), as well as decreasing the overpotential of CO2 reduction.

Speeds of Sound in Binary Mixtures of Water and Carbon Dioxide at Temperatures from 273 K to 313 K and at Pressures up to 50 MPa

Knowledge of thermodynamic properties of aqueous solutions of CO2 is crucial for various applications including climate science, carbon capture, utilisation and storage (CCUS), and seawater desalination. However, there is a lack of reliable experimental data, and the equation of state (EOS) predictions are not reliable, particularly for sound speeds in low CO2 concentrations typical of water resources. For this reason, we have measured speeds of sound in three different aqueous solutions containing CO2. We report speeds of sound in the single-phase liquid region for binary mixtures of water and CO2 for mole fractions of CO2 of 0.0118, 0.0066 and 0.0015 at temperatures from 273.15 K to 313.15 K and at pressures up to 50 MPa, measured using a dual-path pulse-echo apparatus. The relative standard uncertainties of the sound speeds are 0.05 %, 0.03 % and 0.01 % at 0.0118, 0.0066 and 0.0015 CO2 mole fractions, respectively. The change in sound speeds as functions of composition, pressure and temperature are analysed in this study. We find that dissolution of CO2 in water increases its sound speeds at all conditions, with the greatest increase occurring at the highest mole fractions of CO2. Our sound speed data agree well with the limited available experimental data in the literature but deviate from the EOS-CG of Gernert and Span by up to 7 % at the lowest temperatures, highest pressures, and highest CO2 mole fraction. The new low-uncertainty sound speed data presented in this work could provide a basis for development of an improved EOS and in establishing reliable predictions of the change in thermodynamic properties of seawater-like mixtures due to absorption of CO2 gas.

Equilibrium absorption of CO2 in aqueous solution of N-dimethylamino ethanol and 2-(ethylamino)ethanol, measuring and thermodynamic modeling

The equilibrium carbon dioxide solubility in aqueous solutions of N-dimethylaminoethanol (DMAE) 40 wt% + 2-(ethylamino) ethanol (EAE) 5 wt% and DMAE 30 wt% + EAE 5 wt% was measured at various temperatures (313.15, 328.15, 343.15, 358.15 K) in the range of pressures from 6.5 to 236 kPa. The enthalpy of absorption of carbon dioxide was measured for the mixture of DMAE (40 wt%) + EAE (5 wt%), at 328.15 K. The Deshmukh-Mather model was used to correlate the equilibrium solubility of carbon dioxide data, and the model parameters were optimized. The optimized model could correlate the vapor-liquid equilibrium with the 9.4 average absolute percentage deviation (AAD%), and the obtained AAD% for enthalpy of absorption in aqueous solution of DMAE (40 wt%) + EAE (5 wt%) was 24.0.

Boosting C–C coupling to multicarbon products via high-pressure CO electroreduction

Electrochemical CO reduction reaction (CORR) provides a promising approach for producing valuable multicarbon products (C2+), while the low solubility of CO in aqueous solution and high energy barrier of C–C coupling as well as the competing hydrogen evolution reaction (HER) largely limit the efficiency for C2+ production in CORR. Here we report an overturn on the Faradaic efficiency of CORR from being HER-dominant to C2+ formation-dominant over a wide potential window, accompanied by a significant activity enhancement over a Moss-like Cu catalyst via pressuring CO. With the CO pressure rising from 1 to 40 atm, the C2+ Faradaic efficiency and partial current density remarkably increase from 22.8% and 18.9 mA cm−2 to 89.7% and 116.7 mA cm−2, respectively. Experimental and theoretical investigations reveal that high pressure-induced high CO coverage on metallic Cu surface weakens the Cu–C bond via reducing electron transfer from Cu to adsorbed CO and restrains hydrogen adsorption, which significantly facilitates the C–C coupling while suppressing HER on the predominant Cu(111) surface, thereby boosting the CO electroreduction to C2+ activity.

Metalation of a Hierarchical Self-Assembly Consisting of π-Stacked Cubes through Single-Crystal-to-Single-Crystal Transformation

Single-crystal-to-single-crystal metalation of organic ligands represents a novel method to prepare metal–organic complexes, but remains challenging. Herein, a hierarchical self-assembly {(H12L8)·([N(C2H5)4]+)3·(ClO4−)15·(H2O)32} (1) (L = tris(2-benzimidazolylmethyl) amine) consisting of π-stacked cubes which are assembled from eight partially protonated L ligands is obtained. By soaking the crystals of compound 1 in the aqueous solution of Co(SCN)2, the ligands coordinate with Co2+ ions stoichiometrically and ClO4− exchange with SCN− via single-crystal-to-single-crystal transformation, leading to {([CoSCNL]+)8·([NC8H20]+)3·(SCN)11·(H2O)13} (2).

Solubility of carbon dioxide in water: Some useful results for hydrate nucleation.

In this paper, the solubility of carbon dioxide (CO2) in water along the isobar of 400 bar is determined by computer simulations using the well-known TIP4P/Ice force field for water and the TraPPE model for CO2. In particular, the solubility of CO2 in water when in contact with the CO2 liquid phase and the solubility of CO2 in water when in contact with the hydrate have been determined. The solubility of CO2 in a liquid-liquid system decreases as the temperature increases. The solubility of CO2 in a hydrate-liquid system increases with temperature. The two curves intersect at a certain temperature that determines the dissociation temperature of the hydrate at 400bar (T3). We compare the predictions with T3 obtained using the direct coexistence technique in a previous work. The results of both methods agree, and we suggest 290(2)K as the value of T3 for this system using the same cutoff distance for dispersive interactions. We also propose a novel and alternative route to evaluate the change in chemical potential for the formation of hydrates along the isobar. The new approach is based on the use of the solubility curve of CO2 when the aqueous solution is in contact with the hydrate phase. It considers rigorously the non-ideality of the aqueous solution of CO2, providing reliable values for the driving force for nucleation of hydrates in good agreement with other thermodynamic routes used. It is shown that the driving force for hydrate nucleation at 400bar is larger for the methane hydrate than for the carbon dioxide hydrate when compared at the same supercooling. We have also analyzed and discussed the effect of the cutoff distance of dispersive interactions and the occupancy of CO2 on the driving force for nucleation of the hydrate.

Probing Solubility and pH of CO2 in aqueous solutions: Implications for CO2 injection into oceans

CO2 sequestration is among the most anticipated methods to mitigate the already detrimental concentrations of CO2 in the atmosphere. Among sequestration methods, CO2 injection into oceans is of great significance due to the oceans’ large sequestration capacity. However, there are concerns about the changes in water pH as CO2 is injected into oceans. Previous studies in conditions representative of CCS in the ocean are scarce. In the current study, we experimentally measure the pH and solubility at pressures up to 400 atm, temperatures between 283 and 298 K, and different aqueous solutions in a high-pressure autoclave reactor. The results indicated that increasing pressure increases the solubility of CO2 in aqueous solutions, resulting in lower pH values. In contrast, increasing salinity and temperature lowers the solubility and, as a result, increases the system's pH. Among all the tested aqueous solutions, the synthetic seawater mimicked that of a potential injection point in the South China sea, exhibiting the highest salting-out effect and, therefore, the lowest solubility (i.e., the highest pH). The experimental dataset of this study was fed to a machine learning algorithm, Group modeling data handling(GMDH), to develop an explainable, white-box solubility model. The model could predict the pH as a function of solubility, temperature, pressure, and salinity with an accuracy of 0.87. The pH values from the model were compared to those from previous studies, and a good agreement among the values was found. Lastly, a parameter importance analysis was conducted to shed further light on the model's performance. Pressure and temperature were found to be the most and the least influential factors, respectively. As the implantation of the technology is currently being considered in China, the current study can pave the way to better understand the interactions and mechanisms involved in conditions representative of ocean sequestration before large-scale operations.

Direct methylation of benzene with methane over Co/MFI catalysts generated by self-dispersion of Co(OH)2

A methane–benzene reaction was performed over Co/MFI prepared with an aqueous solution of Co(OAc)2 and MFI zeolite.

Direct observation of bicarbonate and water reduction on gold: understanding the potential dependent proton source during hydrogen evolution.

The electrochemical conversion of CO2 represents a promising way to simultaneously reduce CO2 emissions and store chemical energy. However, the competition between CO2 reduction (CO2R) and the H2 evolution reaction (HER) hinders the efficient conversion of CO2 in aqueous solution. In water, CO2 is in dynamic equilibrium with H2CO3, HCO3 -, and CO3 2-. While CO2 and its associated carbonate species represent carbon sources for CO2R, recent studies by Koper and co-workers indicate that H2CO3 and HCO3 - also act as proton sources during HER (J. Am. Chem. Soc. 2020, 142, 4154-4161, ACS Catal. 2021, 11, 4936-4945, J. Catal. 2022, 405, 346-354), which can favorably compete with water at certain potentials. However, accurately distinguishing between competing reaction mechanisms as a function of potential requires direct observation of the non-equilibrium product distribution present at the electrode/electrolyte interface. In this study, we employ vibrational sum frequency generation (VSFG) spectroscopy to directly probe the interfacial species produced during competing HER/CO2R on Au electrodes. The vibrational spectra at the Ar-purged Na2SO4 solution/Au interface, where only HER occurs, show a strong peak around 3650 cm-1, which appears at the HER onset potential and is assigned to OH-. Notably, this species is absent for the CO2-purged Na2SO4 solution/gold interface; instead, a peak around 3400 cm-1 appears at catalytic potential, which is assigned to CO3 2- in the electrochemical double layer. These spectral reporters allow us to differentiate between HER mechanisms based on water reduction (OH- product) and HCO3 - reduction (CO3 2- product). Monitoring the relative intensities of these features as a function of potential in NaHCO3 electrolyte reveals that the proton donor switches from HCO3 - at low overpotential to H2O at higher overpotential. This work represents the first direct detection of OH- on a metal electrode produced during HER and provides important insights into the surface reactions that mediate selectivity between HER and CO2R in aqueous solution.

Calculation of Cathodic Limiting Current Density in Weak Acids: Part I. Aqueous CO2 Solutions

The limiting current density for the hydrogen evolution reaction in aqueous saturated carbon dioxide solutions needed revisiting, as the basic understanding of the underlying reaction mechanism in weak acids has changed over the past few years. We now know that the direct reduction of undissociated carbonic acid on a metal surface in aqueous carbon dioxide solutions is not significant, as was thought before, and that there is only a single dominant pathway for hydrogen evolution: reduction of free hydrogen ions. The main role of weak carbonic acid is to provide additional hydrogen ions via buffering. Therefore, a new mathematical model was needed for calculation of the limiting current density for hydrogen ion reduction, that accounts for both hydrogen ion diffusion in the boundary layer and simultaneous buffering provided by dissociation of weak carbonic acid. The new model relies on analytically solving the co-diffusion of hydrogen ions and carbonic acid in the mass transfer boundary layer, with simultaneous homogenous chemical reactions. The new expression for the limiting current density takes the form: The performance of the new model was successfully validated by comparing it with experimental data over a broad range of conditions.