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Direct electrochemical reduction of CO2 to C2 products such as ethylene is more efficient in alkaline media, but it suffers from parasitic loss of reactants due to (bi)carbonate formation. A two-step process where the CO2 is first electrochemically reduced to CO and subsequently converted to desired C2 products has the potential to overcome the limitations posed by direct CO2 electroreduction. In this study, we investigated the technical and economic feasibility of the direct and indirect CO2 conversion routes to C2 products. For the indirect route, CO2 to CO conversion in a high temperature solid oxide electrolysis cell (SOEC) or a low temperature electrolyzer has been considered. The product distribution, conversion, selectivities, current densities, and cell potentials are different for both CO2 conversion routes, which affects the downstream processing and the economics. A detailed process design and techno-economic analysis of both CO2 conversion pathways are presented, which includes CO2 capture, CO2 (and CO) conversion, CO2 (and CO) recycling, and product separation. Our economic analysis shows that both conversion routes are not profitable under the base case scenario, but the economics can be improved significantly by reducing the cell voltage, the capital cost of the electrolyzers, and the electricity price. For both routes, a cell voltage of 2.5 V, a capital cost of $10,000/m2, and an electricity price of

[Image: see text] Force field-based molecular simulations were used to calculate thermal expansivities, heat capacities, and Joule–Thomson coefficients of binary (standard) hydrogen–water mixtures for temperatures between 366.15 and 423.15 K and pressures between 50 and 1000 bar. The mole fraction of water in saturated hydrogen–water mixtures in the gas phase ranges from 0.004 to 0.138. The same properties were calculated for pure hydrogen at 323.15 K and pressures between 100 and 1000 bar. Simulations were performed using the TIP3P and a modified TIP4P force field for water and the Marx, Vrabec, Cracknell, Buch, and Hirschfelder force fields for hydrogen. The vapor–liquid equilibria of hydrogen–water mixtures were calculated along the melting line of ice Ih, corresponding to temperatures between 264.21 and 272.4 K, using the TIP3P force field for water and the Marx force field for hydrogen. In this temperature range, the solubilities and the chemical potentials of hydrogen and water were obtained. Based on the computed solubility data of hydrogen in water, the freezing-point depression of water was computed ranging from 264.21 to 272.4 K. The modified TIP4P and Marx force fields were used to improve the solubility calculations of hydrogen–water mixtures reported in our previous study [ A. Rahbari;J. Chem. Eng. Data2019, 64, 4103−4115] for temperatures between 323 and 423 K and pressures ranging from 100 to 1000 bar. The chemical potentials of ice Ih were calculated as a function of pressure between 100 and 1000 bar, along the melting line for temperatures between 264.21 and 272.4 K, using the IAPWS equation of state for ice Ih. We show that at low pressures, the presence of water has a large effect on the thermodynamic properties of compressed hydrogen. Our conclusions may have consequences for the energetics of a hydrogen refueling station using electrochemical hydrogen compressors.

Formic acid (FA) is an interesting hydrogen (H2) and carbon monoxide (CO) carrier that can be produced by the electrochemical reduction of carbon dioxide (CO2) using renewable energy. The separation of FA from water is challenging due to the strong (cross)association of the components and the presence of a high boiling azeotrope. For the separation of dilute FA solutions, liquid-liquid extraction is preferred over conventional distillation because distilling large amounts of water is very energy-intensive. In this study, we use 2-methyltetrahydrofuran (2-MTHF) to extract FA from the CO2 electrolysis process, which typically contains

According to the ISO 14687-2:2019 standard, the water content of H2 fuel for transportation and stationary applications should not exceed 5 ppm (molar). To achieve this water content, zeolites can be used as a selective adsorbent for water. In this work, a computational screening study is carried out for the first time to identify potential zeolite frameworks for the drying of high-pressure H2 gas using Monte Carlo (MC) simulations. We show that the Si/Al ratio and adsorption selectivity have a negative correlation. 218 zeolites available in the database of the International Zeolite Association are considered in the screening. We computed the adsorption selectivity of each zeolite for water from the high-pressure H2 gas having water content relevant to vehicular applications and near saturation. It is shown that due to the formation of water clusters, the water content in the H2 gas has a significant effect on the selectivity of zeolites with a helium void fraction larger than 0.1. Under each operating condition, five most promising zeolites are identified based on the adsorption selectivity, the pore limiting diameter, and the volume of H2 gas that can be dried by 1 dm3 of zeolite. It is shown that at 12.3 ppm (molar) water content, structures with helium void fractions smaller than 0.07 are preferred. The structures identified for 478 ppm (molar) water content have helium void fractions larger than 0.26. The proposed zeolites can be used to dry 400−8000 times their own volume of H2 gas depending on the operating conditions. Our findings strongly indicate that zeolites are potential candidates for the drying of high-pressure H2 gas.

Carbon dioxide (CO2) can electrochemically be converted to a range of products including formic acid (HCOOH) and acetic acid (CH3COOH). The yield of the products in an electrolysis cell depends on the solubility of CO2 in the (aqueous) mixture. In absence of experimental data, Monte Carlo simulations in the Gibbs ensemble are used to compute the VLE of the binary systems, CO2-H2O, CO2-HCOOH and CO2-CH3COOH, and the ternary systems, CO2-HCOOH-H2O and CO2-CH3COOH-H2O. In addition, the PC-SAFT equation of state (EoS) is used to model the VLE of these strongly associating mixtures. Both methods correctly predicts the liquid-phase compositions, but the gas-phase compositions are less accurately described. The challenges to model these systems are related to the simultaneous formation of dimers, rings, and chains, which requires accurate force fields and advanced biasing schemes in MC simulations, and association theories that can account for this effect.

We use a high-pressure semicontinuous batch electrochemical reactor with a tin-based cathode to demonstrate that it is possible to efficiently convert CO2 to formic acid (FA) in low-pH (i.e., pH < pKa) electrolyte solutions. The effects of CO2 pressure (up to 50 bar), bipolar membranes, and electrolyte (K2SO4) concentration on the current density (CD) and the Faraday efficiency (FE) of formic acid were investigated. The highest FE (?80%) of FA was achieved at a pressure of around 50 bar at a cell potential of 3.5 V and a CD of ?30 mA/cm2. To suppress the hydrogen evolution reaction (HER), the electrochemical reduction of CO2 in aqueous media is typically performed at alkaline conditions. The consequence of this is that products like formic acid, which has a pKa of 3.75, will almost completely dissociate into the formate form. The pH of the electrolyte solution has a strong influence not only on the electrochemical reduction process of CO2 but also on the downstream separation of (dilute) acid products like formic acid. The selection of separation processes depends on the dissociation state of the acids. A review of separation technologies for formic acid/formate removal from aqueous dilute streams is provided. By applying common separation heuristics, we have selected liquid-liquid extraction and electrodialysis for formic acid and formate separation, respectively. An economic evaluation of both separation processes shows that the formic acid route is more attractive than the formate one. These results urge for a better design of (1) CO2 electrocatalysts that can operate at low pH without affecting the selectivity of the desired products and (2) technologies for efficient separation of dilute products from (photo)electrochemical reactors.

Hydrogen is one of the most popular alternatives for energy storage. Because of its low volumetric energy density, hydrogen should be compressed for practical storage and transportation purposes. Recently, electrochemical hydrogen compressors (EHCs) have been developed that are capable of compressing hydrogen up to P = 1000 bar, and have the potential of reducing compression costs to 3 kWh/kg. As EHC compressed hydrogen is saturated with water, the maximum water content in gaseous hydrogen should meet the fuel requirements issued by the International Organization for Standardization (ISO) when refuelling fuel cell electric vehicles. The ISO 14687-2:2012 standard has limited the water concentration in hydrogen gas to 5 μmol water per mol hydrogen fuel mixture. Knowledge on the vapor liquid equilibrium of H2O-H2 mixtures is crucial for designing a method to remove H2O from compressed H2. To the best of our knowledge, the only experimental high pressure data (P > 300 bar) for the H2O-H2 phase coexistence is from 1927 [J. Am. Chem. Soc., 1927, 49, 65-78]. In this paper, we have used molecular simulation and thermodynamic modeling to study the phase coexistence of the H2O-H2 system for temperatures between T = 283 K and T = 423 K and pressures between P = 10 bar and P = 1000 bar. It is shown that the Peng-Robinson equation of state and the Soave Redlich-Kwong equation of state with van der Waals mixing rules fail to accurately predict the equilibrium coexistence compositions of the liquid and gas phase, with or without fitted binary interaction parameters. We have shown that the solubility of water in compressed hydrogen is adequately predicted using force-field-based molecular simulations. The modeling of phase coexistence of H2O-H2 mixtures will be improved by using polarizable models for water. In the Supporting Information, we present a detailed overview of available experimental vapor-liquid equilibrium and solubility data for the H2O-H2 system at high pressures.

In this work a model of an elevated pressure CO2 electrolyzer producing primarily formate or formic acid is presented. It consists of three parts: A model of the bulk electrolyte, the diffusion layer, and the electrode surface. Data from the literature was used to validate both the bulk portion of the model, as well as the overall model. Results from the literature were further explored and explained by reference to the model and faradaic efficiency is predicted very well (R-Square of 0.99 for the fitted data, and 0.98 for the non-fitted data). The primary effect of increasing the pressure on a CO2 electrolyzer is seen to be increasing the maximum attainable partial current density, while the faradaic efficiency and specific energy of formation plateau at pressures above 10-20 bar, at 95% and of 3.7 kWh/kg, respectively. Unlike the efficiencies, the profitability of running a reactor increases with pressure, following a similar trend as partial current density, showing the importance of this quantity as a performance metric of a CO2 electrolyzer. In general this work shows the utility of a model of this sort in the design, evaluation and operation of CO2 electrolyzers.

A high pressure semicontinuous batch electrolyzer is used to convert CO2 to formic acid/formate on a tin-based cathode using bipolar membranes (BPMs) and cation exchange membranes (CEMs). The effects of CO2 pressure up to 50 bar, electrolyte concentration, flow rate, cell potential, and the two types of membranes on the current density (CD) and Faraday efficiency (FE) for formic acid/formate are investigated. Increasing the CO2 pressure yields a high FE up to 90% at a cell potential of 3.5 V and a CD of ∼30 mA/cm2. The FE decreases significantly at higher cell potentials and current densities, and lower pressures. Up to 2 wt % formate was produced at a cell potential of 4 V, a CD of ∼100 mA/cm2, and a FE of 65%. The advantages and disadvantages of using BPMs and CEMs in electrochemical cells for CO2 conversion to formic acid/formate are discussed.

Syngas is an important intermediate in the chemical process industry. It is used for the production of hydrocarbons, acetic acid, oxo-alcohols, and other chemicals. Depending on the target product and stoichiometry of the reaction, an optimum (molar) ratio between hydrogen and carbon monoxide (H2:CO) in the syngas is required. Different technologies are available to control the H2:CO molar ratio in the syngas. The combination of steam reforming of methane (SRM) and the water-gas shift (WGS) reaction is the most established approach for syngas production. In this work, to adjust the H2:CO ratio, we have considered formic acid (FA) as a source for both hydrogen and carbon monoxide. Using thermochemical equilibrium calculations, we show that the syngas composition can be controlled by cofeeding formic acid into the SRM process. The H2:CO molar ratio can be adjusted to a value between one and three by adjusting the concentration of FA in the reaction feed. At steam reforming conditions, typically above 900 K, FA can decompose to water and carbon monoxide and/or to hydrogen and carbon dioxide. Our results show that cofeeding FA into the SRM process can adjust the H2:CO molar ratio in a single step. This can potentially be an alternative to the WGS process.

Permeation of binary gaseous mixtures through a silicalite-1 membrane was studied. Results are reported for the fluxes and separation factors as a function of the feed composition, the pressure and the temperature. For the various cases a comparison is made between the binary and the single-component permeation behavior. For the permeation of the mixtures the stronger adsorbed component is little affected by the presence of the weaker adsorbed component. On the other hand, for the weaker adsorbed component a clear reduction in the flux is observed. As a consequence, the separation factor for mixtures differs considerably from the so-called ideal separation factor, which is the ratio of the one-component permeances. As expected, the separation factor depends on the temperature. The separation factor is also a function of the composition and the feed pressure. This means that the binary equilibrium adsorption cannot be described with the extended Langmuir model. Binary permeation and separation behavior is described using the ideal adsorption solution theory for equilibrium adsorption.

The one-component steady-state permeation of gases through a silicalite-1 zeolite composite membrane as a function of the temperature is studied from 190 to 680 K for light hydrocarbons, noble gases, and some inorganic gases. In general, with increasing temperature the permeance shows a maximum followed by a minimum. For gases weakly adsorbed the permeance has only a minimum and for gases strongly adsorbed only a maximum is observed in the permeance. The permeance for various gases, for a feed pressure of 101 kPa, span four orders of magnitude. The lowest permeation is for i-butane at 300 K: a permeance of 0.07 × 10−8 mol. m−2.s−1.Pa−1. The highest value is observed for methane: a permeance of 70 × 10−8 mol. m−2.s−1.Pa−1 at about 240 K. A comparison between the isobars and the temperature dependence of the steady-state permeance, both at 101 kPa, shows that at the temperature where the amount adsorbed vanishes the permeance starts to increase. The temperature dependence of the steady-state fluxes through the silicalite-1 membrane can be described only if two diffusion mechanisms are taken into account. For high occupancies the mass transport can be described by equilibrium adsorption followed by surface diffusion and for low occupancies the mass transport can be described by activated gaseous diffusion. With increasing temperature the mass-transport mechanism shifts from the surface diffusion regime to the activated gaseous diffusion regime. With these two diffusivities modeling results agree well with experimental results for the one-component flux through the silicalite-1 zeolite membrane.

Using Maxwell-Stefan equations, experimental and computational results of binary diffusion in pore- and cage-type zeolitic structures are described. In the generalized Maxwell-Stefan (GMS) formulation, the Fick diffusivity is written as the product of two separate contributions, the GMS or corrected diffusivity and the thermodynamic factor. The concentration dependence of the GMS diffusivity for one- and two-component diffusion in zeolitic structures is investigated. From the Maxwell - Stefan equations, different models for the Fick diffusion coefficient matrix for the description of binary mass transport in molecular sieve materials are derived. Various models used predict binary diffusion in zeolitic structures. First, theoretical predictions of binary apparent diffusivities as a function of the occupancy are compared to results from Monte Carlo simulations. Second, theoretical results of binary uptake profiles are compared to experimental results for the system ethylbenzene/benzene/ZSM-5. For different zeolitic structures, that is, pore- and cage-type structures, results of the Monte Carlo simulations agree well with the theoretical predictions. In cage-type structures, the effect of counterexchange between sorbed molecules is demonstrated. Experimental results of transient uptake profiles of a mixture of benzene and ethylbenzene in ZSM-5 follow predictions of the theoretical single-file diffusion model.