Electroreduction of CO2/CO to C2 products: process modeling, downstream separation, system integration, and economic analysis.

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 <$20/MWh will yield a positive net present value and payback times of less than 15 years. Overall, the high temperature (SOEC-based) two-step conversion process has a greater potential for scale-up than the direct electrochemical conversion route. Strategies for integrating the electrochemical CO2/CO conversion process into the existing gas and oil infrastructure are outlined. Current barriers for industrialization of CO2 electrolyzers and possible solutions are discussed as well

High pressure electrochemical reduction of CO2 to formic acid/formate: a comparison between bipolar membranes and cation exchange membranes

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

High-pressure electrochemical reduction of CO2 to formic acid/formate: effect of pH on the downstream separation process and economics

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

On the binding of calcium by micelles composed of carboxy-modified pluronics measured by means of differential potentiometric titration and modeled with a self-consistent-field theory

\u3cp\u3eWe perform differential potentiometric titration measurements for the binding of Ca\u3csup\u3e2+\u3c/sup\u3e ions to micelles composed of the carboxylic acid end-standing Pluronic P85 block copolymer (i.e., CAE-85 (COOH-(EO) \u3csub\u3e26\u3c/sub\u3e-(PO)\u3csub\u3e39\u3c/sub\u3e-(EO)\u3csub\u3e26\u3c/sub\u3e-COOH)). Two different ion-selective electrodes (ISEs) are used to detect the free calcium concentration; the first ISE is an indicator electrode, and the second is a reference electrode. The titration is done by adding the block copolymers to a known solution of Ca\u3csup\u3e2+\u3c/sup\u3e at neutral pH and high enough temperature (above the critical micellization temperature CMT) and various amount of added monovalent salt. By measuring the difference in the electromotive force between the two ISEs, the amount of Ca\u3csup\u3e2+\u3c/sup\u3e that is bound by the micelles is calculated. This is then used to determine the binding constant of Ca \u3csup\u3e2+\u3c/sup\u3e with the micelles, which is a missing parameter needed to perform molecular realistic self-consistent-field (SCF) calculations. It turns out that the micelles from block copolymer CAE-85 bind Ca\u3csup\u3e2+\u3c/sup\u3e ions both electrostatically and specifically. The specific binding between Ca\u3csup\u3e2+\u3c/sup\u3e and carboxylic groups in the corona of the micelles is modeled through the reaction equilibrium -COOCa\u3csup\u3e+\u3c/sup\u3e ⇄ -COO\u3csup\u3e-\u3c/sup\u3e + Ca \u3csup\u3e2+\u3c/sup\u3e with pK\u3csub\u3eCa\u3c/sub\u3e = 1.7 ± 0.06.\u3c/p\u3

Solar thermochemical conversion of CO2 into synthetic fuels via ferrite based redox reactions

In this contribution, we report the synthesis NiFe2O4 and CoFe2O4 redox materials via sol-gel method. For the synthesis of these materials via sol-gel approach, the Ni, Co, and Fe precursors were first dissolved in ethanol with the help of sonic energy. Once the metal precursors were dissolved, propylene oxide (PO) was added dropwise to the well mixed solution as a gelation agent to achieve gel formation. As-prepared gels were aged, dried and subsequently calcined in presence air. The calcined powder obtained was characterized towards its phase/chemical composition, particle morphology, and specific surface area (SSA). Derived NiFe2O4 and CoFe2O4 redox materials were further investigated towards thermochemical splitting of CO2 into solar fuels by performing several reduction/re-oxidation cycles using a thermogravimetric analyzer (TGA).Scopu