Project Se(L)ect(i)vity (PTDC/EQU-EQU/6193/2020)

Flow electrode capacitive deionization (FCDI) is a desalination technology employing flowable carbon slurries to remove salt from an influent through the electro-sorption of ions at the surface of pores of activated carbon particles. This study presents an extensive morphological, electrochemical and rheological analysis of flow electrodes prepared using commercial (YP50F, YP80F, Norit, PAC) and lab-synthesized (KUA, PAC-OX) activated carbons. Simultaneous optimization of particle size, surface area, and surface chemistry of activated carbons is essential to enhance desalination efficiency in FCDI applications. The lab-made highly microporous activated carbon (KUA), prepared from a Spanish anthracite, exhibited a remarkably high specific surface area ( 2800 m2/g) but required first a particle size reduction through ball milling (from 56 μm to 12 μm) for the respective flow electrodes to achieve flowability. The slurry of milled fine KUA (designated as KUAF) shows a specific capacitance of 55 F/g, a 38-fold increase compared to its pristine form. The KUA-F flow electrode also achieved a maximum salt adsorption capacity of 185 mg/g, outperforming the leading commercial alternative (YP80F) by 26 %. The FCDI cell with the KUA-F flow electrode exhibited a desalination efficiency of 79 % at 15 wt% loading, surpassing YP80F by 29 %. In contrast, using PAC-OX (oxidized form of PAC), despite increasing oxygen functional groups and with relatively higher specific surface area, led only to a 2 % improvement in desalination performance, highlighting that oxidation alone at larger particle sizes and broader distribution is insufficient.

As billions of people suffer from water scarcity, finding sustainable water resources is imperative. Flow capacitive deionization (FCDI) is a highly promising desalination process that can produce clean water from saline streams such as brackish and seawater. Conventional FCDI systems employ Computerised Numerical Control (CNC)-milled graphite plates that serve as current collectors and flow electrode channels. However, they have drawbacks such as high manufacturing costs, waste generation, and the difficulty of producing complex geometries required for efficient flow electrode mixing. Here, we successfully demonstrate that 3D-printed flow electrode gaskets, made of non-conductive polyethylene terephthalate glycol (PET-G) or a carbon black-infused conductive polylactic acid (PLA), are viable alternatives to traditional graphite plates. In specific cases, the desalination and energy efficiency in FCDI cells with 3D-printed conductive gaskets were even 25 % and 10 % higher, respectively, compared to traditional CNC-milled current collectors. The transition to 3D printing offers notable benefits, such as the competence to fabricate complex designs that enhance internal mixing and charge percolation. This innovation represents a change of paradigm in the way FCDI cells should be designed and manufactured, using additive manufacturing, which represents an efficient, scalable, and cost-effective substitute for the conventional approach, contributing therefore for the advancement of FCDI desalination technology.

Flow electrode capacitive deionization (FCDI) is an emerging desalination technology that utilises flowable electrodes and can be operated in diverse configuration modes. This study provides a systematic assessment of the three main configuration arrangements under a voltage range between 0.8 and 2.0 V: isolated closed-cycle (ICC), short-circuited closed-cycle (SCC), and single-cycle with separate concentrate chamber (SCSC). The ICC mode shows the highest specific energy consumption (up to 72.02 Wh/mol of NaCl at 2.0 V) and low operational stability manifested by extreme alteration of pH in the electrode compartments (anode compartment pH down to 2.17; cathode compartment pH up to 12.08), which leads to the need for frequent electrode regeneration or replacement. In comparison to the ICC mode, the SCC mode exhibited superior performance, with a 44.3 % increase in salt removal and up to 3.95 % higher current efficiency at 2.0 V, due to the regeneration of electrodes through short-circuiting, as it reduces the electrical resistance and minimises the side reactions. The SCSC mode emerged as the most stable and reliable among the three, with uniform current and conductivity profiles, as well as minimal pH fluctuations, which is critical to produce treated water within desired quality standards. These findings highlight the promising potential of SCSC mode as an optimal configuration for scalable, continuous and energy-efficient FCDI systems, providing a balanced solution for long-term desalination with reduced operational complexity and costs.

The recent growing demand for lithium worldwide, led by the Li-ion battery market, has sparked research into alternative sources of this material. In this context, selective lithium recovery from concentrated brines represents a sustainable and economical alternative to lithium mining activities. In this work, we developed a mathematical model of the recently implemented Lithium Membrane Flow Electrode Capacitive De-Ionization (Li-MFCDI) process, used to selectively extract lithium from a synthetic geothermal brine. The model was validated against the available experimental data and was used to perform a comprehensive parametric analysis. The model predicts the effects of the applied voltage, flow rates, and the adopted membranes on the process performance. These findings highlight the importance of the membrane conductivity-selectivity trade-off for process productivity. Furthermore, this simulation tool will substantially contribute to the development of this novel technology.

Flow capacitive deionization (FCDI) is an emerging desalination technology at which flow electrodes (shear-thinning flowable carbon slurries) are used to remove ions from saline water. The geometry of flow electrode channels, which provide the path and ensure the distribution and mixing of the flow electrodes, is one of the most important aspects to be optimized. This work presents experimental and computational fluid dynamics (CFD) modelling analysis of the influence of the geometry of flow electrode channels on FCDI performance. Flow electrode gaskets (with open, serpentine (short) horizontal and serpentine (long) vertical channels) were 3D printed using a polyethylene terephthalate glycol (PET-G) filament. The FCDI cell with a vertical serpentine flow electrode channel exhibited the poorest performance due to channel blockage by carbon particles, while the best results were achieved with a horizontal serpentine flow electrode channel. CFD simulations aided in understanding this behaviour by showing that the channel geometry strongly affects the local shear rate, and thus the local viscosity of flow electrodes. Thus, it is recommended to design channels that induce flow disturbance aiming for increasing the shear rate and hence reducing flow electrode viscosity, therefore promoting their flowability and reducing clogging chances.