Energy from CO2 using capacitive electrodes - a model for energy extraction cycles

Bifunctional Ionic Deep Eutectic Electrolytes for CO2 Electroreduction

CO₂ is a low-cost monomer capable of promoting industrially scalable carboxylation reactions. Sustainable activation of CO₂ through electroreduction process (ECO₂R) can be achieved in stable electrolyte media. This study synthesized and characterized novel diethyl ammonium chloride-diethanolamine bifunctional ionic deep eutectic electrolyte (DEACl-DEA), using diethanolamine (DEA) as hydrogen bond donors (HBD) and diethyl ammonium chloride (DEACl) as hydrogen bond acceptors (HBA). The DEACl-DEA has -69.78 °C deep eutectic point and cathodic electrochemical stability limit of -1.7 V versus Ag/AgCl. In the DEACl-DEA (1:3) electrolyte, electroreduction of CO₂ to CO₂ ^•- was achieved at -1.5 V versus Ag/AgCl, recording a faradaic efficiency (FE) of 94%. After 350 s of continuous CO₂ sparging, an asymptotic current response is reached, and DEACl-DEA (1:3) has an ambient CO₂ capture capacity of 52.71 mol/L. However, DEACl-DEA has a low faradaic efficiency <94% and behaves like a regular amine during the CO₂ electroreduction process when mole ratios of HBA-HBD are greater than 1:3. The electrochemical impedance spectroscopy (EIS) and COSMO-RS analyses confirmed that the bifunctional CO₂ sorption by the DEACl-DEA (1:3) electrolyte promote the ECO₂R process. According to the EIS, high CO₂ coverage on the DEACl-DEA/Ag-electrode surface induces an electrochemical double layer capacitance (EDCL) of 3.15 × 10^-9 F, which is lower than the 8.76 × 10^-9 F for the ordinary DEACl-DEA/Ag-electrode. COSMO-RS analysis shows that the decrease in EDCL arises due to the interaction of CO₂ non-polar sites (0.314, 0.097, and 0.779 e/nm²) with that of DEACl (0.013, 0.567 e/nm²) and DEA (0.115, 0.396 e/nm²). These results establish for the first time that a higher cathodic limit beyond the typical CO₂ reduction potential is a criterion for using any deep eutectic electrolytes for sustainable CO₂ electroreduction process.

Electroreduction of CO2 and Quantification in New Transition-Metal-Based Deep Eutectic Solvents Using Single-Atom Ag Electrocatalyst

Deep eutectic solvents (DESs) are efficient media for CO2 capture, and an electroreduction process using the deterministic surface of single-atom electrocatalysts is a facile way to screen gas absorption capacities of novel DESs. Using newly prepared transition-metal-based DESs indexed as TDESs, the interfacial mechanism, detection, quantification, and coordination modes of CO2 were determined for the first time. The CO2 has a minimum detection time of 300 s, whereas 500 s of continous ambient CO2 saturation provided ZnCl2/ethanolamine (EA) (1:4) and CoCl2/EA (1:4) TDESs with a maximum CO2 absorption capacity of 0.2259 and 0.1440 mmol/L, respectively. The results indicated that CO2 coordination modes of η1 (C) and η2 (O, O) with Zn in ZnCl2/EA (1:4) TDESs are conceivable. We found that the transition metals in TDESs form an interface at the compact layer of the electrocatalyst, while CO2 •-/CO2 reside in the diffuse layer. These findings are important because they provide reliable inferences about interfacial phenomena for facile screening of CO2 capture capacity of DESs or other green solvents.

The Green Lean Amine Machine: Harvesting Electric Power While Capturing Carbon Dioxide from Breath

As wearable technologies redefine the way people exchange information, receive entertainment, and monitor health, the development of sustainable power sources that capture energy from the user's everyday activities garners increasing interest. Electric fishes, such as the electric eel and the torpedo ray, provide inspiration for such a power source with their ability to generate massive discharges of electricity solely from the metabolic processes within their bodies. Inspired by their example, the device presented in this work harnesses electric power from ion gradients established by capturing the carbon dioxide (CO2 ) from human breath. Upon localized exposure to CO2 , this novel adaptation of reverse electrodialysis chemically generates ion gradients from a single initial solution uniformly distributed throughout the device instead of requiring the active circulation of two different external solutions. A thorough analysis of the relationship between electrical output and the concentration of carbon capture agent (monoethanolamine, MEA), the amount of CO2 captured, and the device geometry informs device design. The prototype device presented here harvests enough energy from a breath-generated ion gradient to power small electronic devices, such as a light-emitting diode (LED).

Periodic energy conversion in an electric-double-layer capacitor

Electrostatic conversion devices operate through periodic modulation of capacitance. Such devices have a wide range of configurations, involving either changes in permittivity, electrode-plate spacing or wetting area. The presented study examines, theoretically, a potential configuration of an electric-double-layer capacitor (EDLC)-based transducer, as it converts concentration and temperature oscillations into an electric alternating current. A constant voltage applied at the EDLC electrodes results in the formation of two opposite-sign EDLs, and an electric current is generated when ionic charges pass from one EDL to the other. In the examined configuration, this ionic charge transfer is induced by boundary modulation of temperature and/or concentration. To capture the oscillating dynamics of the ion distribution and ion flux, we solve the full set of Poisson-Nernst-Planck (PNP) equations coupled with the energy equation. We find that the transducer's optimal conditions for conversion, for which the device's frequency response is maximized, are governed by three main factors: low irreversible Joule heating, confined geometry, where the capacitor thickness is a close as possible to the EDL's characteristic screening length, and, most importantly, 'tuning' the system to a resonance frequency dictated by the interplay between geometry and characteristic time scales for mass and heat diffusion.

Theory of pH changes in water desalination by capacitive deionization

In electrochemical water desalination, a large difference in pH can develop between feed and effluent water. These pH changes can affect the long-term stability of membranes and electrodes. Often Faradaic reactions are implicated to explain these pH changes. However, quantitative theory has not been developed yet to underpin these considerations. We develop a theory for electrochemical water desalination which includes not only Faradaic reactions but also the fact that all ions in the water have different mobilities (diffusion coefficients). We quantify the latter effect by microscopic physics-based modeling of pH changes in Membrane Capacitive Deionization (MCDI), a water desalination technology employing porous carbon electrodes and ion-exchange membranes. We derive a dynamic model and include the following phenomena: I) different mobilities of various ions, combined with acid-base equilibrium reactions; II) chemical surface charge groups in the micropores of the porous carbon electrodes, where electrical double layers are formed; and III) Faradaic reactions in the micropores. The theory predicts small pH changes during desalination cycles in MCDI if we only consider phenomena I) and II), but predicts that these pH changes can be much stronger if we consider phenomenon III) as well, which is in line with earlier statements in the literature on the relevance of Faradaic reactions to explain pH fluctuations.

Design, construction, and testing of a supercapacitive swing adsorption module for CO2 separation

We present a device which is able to separate gases from a gas stream using supercapacitive energy.

Resistance identification and rational process design in Capacitive Deionization

Capacitive Deionization (CDI) is an electrochemical method for water desalination employing porous carbon electrodes. To enhance the performance of CDI, identification of electronic and ionic resistances in the CDI cell is important. In this work, we outline a method to identify these resistances. We illustrate our method by calculating the resistances in a CDI cell with membranes (MCDI) and by using this knowledge to improve the cell design. To identify the resistances, we derive a full-scale MCDI model. This model is validated against experimental data and used to calculate the ionic resistances across the MCDI cell. We present a novel way to measure the electronic resistances in a CDI cell, as well as the spacer channel thickness and porosity after assembly of the MCDI cell. We identify that for inflow salt concentrations of 20 mM the resistance is mainly located in the spacer channel and the external electrical circuit, not in the electrodes. Based on these findings, we show that the carbon electrode thickness can be increased without significantly increasing the energy consumption per mol salt removed, which has the advantage that the desalination time can be lengthened significantly.

Ion transport through electrolyte/polyelectrolyte multi-layers

Ion transport of multi-ionic solutions through layered electrolyte and polyelectrolyte structures are relevant in a large variety of technical systems such as micro and nanofluidic devices, sensors, batteries and large desalination process systems. We report a new direct numerical simulation model coined E_nPE_n: it allows to solve a set of first principle equations to predict for multiple ions their concentration and electrical potential profiles in electro-chemically complex architectures of n layered electrolytes E and n polyelectrolytes PE. E_nPE_n can robustly capture ion transport in sub-millimeter architectures with submicron polyelectrolyte layers. We proof the strength of E_nPE_n for three yet unsolved architectures: (a) selective Na over Ca transport in surface modified ion selective membranes, (b) ion transport and water splitting in bipolar membranes and (c) transport of weak electrolytes.