Towards long-term operation of flow-electrode capacitive deionization (FCDI): Optimization of operating parameters and regeneration of flow-electrode

Fluorine recovery from low-concentration fluorine wastewater by flow-electrode capacitive deionization and fluid bed crystallization (FCDI-FBC): Preconcentration and high-quality fluorite pellets formation

Fluorine (F), critical for various industries, faces resource scarcity due to limited reserves of its primary source, fluorite (CaF₂). While fluorine-containing wastewater from industrial processes represents a valuable potential resource, recovering fluorine from low-concentration wastewater remains challenging. This study introduces a cyclic "preconcentration + recovery" system combining flow-electrode capacitive deionization (FCDI) and fluidized bed crystallization (FBC) to address this gap. FCDI preconcentrates fluorine ions into high-concentration brine, and FBC facilitates the formation of high-purity fluorite crystals. Experimental parameters influencing FCDI efficiency - such as influent fluoride concentration, electrode solution composition, and flow rate - were systematically evaluated. Additionally, the cyclic operation was modeled to enhance the whole recovery rate across multiple cycles. The experimental results demonstrated that FCDI achieves an 83.90% fluoride removal rate under optimal conditions with energy-efficient operation. FBC produces fluorite crystals of up to 97.20% purity, classified as acid-grade. The integrated FCDI-FBC system achieves a fluoride recovery rate of 64.40% in single operation mode, with further improvements in cyclic mode. The proposed system offers a sustainable and economically feasible solution to fluorine recovery from low-concentration wastewater, representing a significant step toward the sustainable utilization of non-renewable fluorite resources.

Practical potential of suspension electrodes for enhanced limiting currents in electrochemical CO2 reduction

CO2 conversion is an important part of the transition towards clean fuels and chemicals. However, low solubility of CO2 in water and its slow diffusion cause mass transfer limitations in aqueous electrochemical CO2 reduction. This significantly limits the partial current densities towards any desired CO2-reduction product. We propose using flowable suspension electrodes to spread the current over a larger volume and alleviate mass transfer limitations, which could allow high partial current densities for CO2 conversion even in aqueous environments. To identify the requirements for a well-performing suspension electrode, we use a transmission line model to simulate the local electric and ionic current distributions throughout a channel and show that the electrocatalysis is best distributed over the catholyte volume when the electric, ionic and charge transfer resistances are balanced. In addition, we used electrochemical impedance spectroscopy to measure the different resistance contributions and correlated the results with rheology measurements to show that particle size and shape impact the ever-present trade-off between conductivity and flowability. We combine the modelling and experimental results to evaluate which carbon type is most suitable for use in a suspension electrode for CO2 reduction, and predict a good reaction distribution throughout activated carbon and carbon black suspensions. Finally, we tested several suspension electrodes in a CO2 electrolyzer. Even though mass transport limitations should be reduced, the CO partial current densities are capped at 2.8 mA cm-2, which may be due to engineering limitations. We conclude that using suspension electrodes is challenging for sensitive reactions like CO2 reduction, and may be more suitable for use in other electrochemical conversion reactions suffering from mass transfer limitations that are less affected by competing reactions and contaminations.