In this work, the effect of pH and potential is examined for the oxidation of carbon cloth electrodes used in capacitive deionization (CDI) processes. The degree of oxidation of the electrode surface, examined using the electrode's potential of zero charge (E_pzc) and measured using chronoamperometry and cyclic voltammetry, is found to be strongly correlated to the pH of the solution at the interface. Local pH measurements are examined at anodes and cathodes in full CDI and membrane-assisted capacitive deionization (MCDI) cells at cell voltages ranging from 0.3–1.2 V. The cathode is shown to be basic under charging potentials while the anode is found to be acidic. This local pH is found to be highly transient during charging and discharging in CDI cells while the pH is found to be relatively static in the MCDI cells, maintaining a basic pH at the cathode and an acidic pH at the anode even when the cell is discharged. Ion exchange membranes (IEM) are found to have two functions: (1) limiting co-ion expulsion that results from specific ion adsorption and (2) limiting the effects of parasitic Faradaic reactions on the separation process by stabilizing the local pH thereby mitigating dissolved oxygen reduction at the cathode and lessening carbon oxidation at the anode. Performance comparisons including the salt adsorption capacity and charge efficiency are also compared for these systems.

Capacitive deionization (CDI) operated under inverted mode involves electronic charging and discharge steps with corresponding ion concentration and desalting coupled with simultaneous energy storage. In this work, an energy recovery system derived from a Ćuk dc–dc converter is explored to transfer the energy stored from one inverted capacitive deionization (i-CDI) cell during the electronic discharge step to another during the charge step, decreasing the overall energy requirement for capacitive water desalination. The i-CDI cell, a subset of CDI architecture operated in an inverted mode, is improved by incorporating ion-selective membranes to allow inverted membrane capacitive deionization (i-MCDI), leading to enhanced charge storage achieved with reduced energy input. For example, in comparison to i-CDI that requires ∼12 J g⁻¹ of energy input, the i-MCDI cell requires only 8 J g⁻¹. By incorporating the recovery system, the energy penalty can be reduced to only require ∼8 and 4 J g⁻¹ for i-CDI and i-MCDI cells, respectively. Improvement in energy recovery was shown to be achieved by reducing charge leakage, with the i-MCDI cell showing up to 3 times the leakage resistance of the i-CDI cell.

A commercially available carbon cloth electrode is repetitively oxidized using a wetting–drying procedure in order to enhance its negative chemical surface charge, consequently creating scenarios of symmetric electrode pairs with different potentials of zero charge (E_PZC) versus potential at short circuit for ion desorption (E_o) in a potential distribution. In literature, this study of capacitive deionization (CDI) for the first time provides a new analytical approach that is capable of estimating the E_PZC and potential distribution diagram. The approach leverages the modified Donnan model with chemical surface charge, in which the values of the chemical surface charge are quantified by investigating the effluent pH at steady state with and without carbon electrode pairs in a flow cell. Results from constant-voltage CDI tests show that, for a symmetric pair of carbon electrodes, the ion adsorption–desorption performance becomes efficient when a carbon electrode possesses equal positive and negative chemical surface charges or its net chemical surface charge is zero. By mapping E_PZC and E_o in a potential distribution diagram, ineffective ion adsorption–desorption results from electronic charge consumed to repel co-ions at the intermediate charge storage stage. Additionally, Faradaic reactions at the electrodes are discussed based upon pH responses during CDI charging and discharging.

The pH swings from water electrolysis are leveraged to condition OH ⁻ -based facile CO ₂ capture solvents using an electrochemical flow cell for direct air capture (DAC). Besides demonstrating the DAC using a membrane contactor, promoting CO ₂ release from a CO ₃ ²⁻ solution at the anode is specifically studied by adjusting the volumetric flow rate, anode chamber volume, residence time, and K ₂ CO ₃ concentration. Through case-by-case comparisons coupled with modeled results, increasing current, reducing volumetric flow rate, and/or reducing CO ₃ ²⁻ concentration are the effective methods to promote CO ₂ release from a CO ₃ ²⁻ -containing solvent, whereas enlarging the anode chamber volume poses a minor effect. Moreover, the discrepancies between the experimental and modeled results may be caused by H ⁺ crossover rather than K ⁺ transport through the Nafion membrane during water electrolysis based upon the total alkalinity measurements for the K ₂ CO ₃ solutions gleaned from the anode. It is believed that such results will provide guidance to design and operate an electrochemical flow cell for electrochemistry-assisted DAC and point source CO ₂ capture.

Brewer’s spent grain (BSG) as the major byproduct in the brewing industry is a promising feedstock to produce value-added products such as volatile fatty acids (VFAs). Synergistic ball mill–enzymatic hydrolysis (BM-EH) process is an environmentally friendly pretreatment method for lignocellulosic materials before bioprocessing. This study investigated the potential of raw and BM-EH pretreated BSG feedstocks to produce VFAs through a direct thermophilic anaerobic fermentation process without introducing a methanogen inhibitor. The highest VFA concentration of over 30 g/L was achieved under the high-solid loading fermentation (HS) of raw BSG. The synergistic BM-EH pretreatment helps to increase the cellulose conversion to 70%. Under conventional low TS fermentation conditions, compared to the controlled sample, prolonged pretreatment of the BSG substrate resulted in increased VFA yields from 0.25 to 0.33 g/g_VS, and butyric acid became dominant instead of acetic acid.