Can Finite Element Method Obtain SVET Current Densities Closer to True Localized Corrosion Rates?

In this paper, the finite element method was used to simulate the response of the scanning vibrating electrode technique (SVET) across an iron-zinc cut-edge sample in order to provide a deeper understanding of the localized corrosion rates measured using SVET. It was found that, if the diffusion layer was neglected, the simulated current density using the Laplace equation fitted the experimental SVET current density perfectly. However, the electrolyte was not perturbed by a vibrating SVET probe in the field, so a diffusion layer existed. Therefore, the SVET current densities obtained from the local conductivity of the electrolyte would likely be more representative of the true corrosion rates than the SVET current densities obtained from the bulk conductivity. To help overcome this difference between natural conditions and those imposed by the SVET experiment, a local electrolyte corrected conductivity SVET (LECC-SVET) current density was introduced, which was obtained by replacing the bulk electrolyte conductivity measured experimentally by the local electrolyte conductivity simulated using the Nernst-Einstein equation. Although the LECC-SVET current density did not fit the experimental SVET current density as perfectly as that obtained from the Laplace equation, it likely represents current densities closer to the true, unperturbed corrosion conditions than the SVET data from the bulk conductivity.

Ladder Mechanisms of Ion Transport in Prussian Blue Analogues.

Prussian blue (PB) and its analogues (PBAs) are drawing attention as promising materials for sodium-ion batteries and other applications, such as desalination of water. Because of the possibilities to explore many analogous materials with engineered, defect-rich environments, computational optimization of ion-transport mechanisms that are key to the device performance could facilitate real-world applications. In this work, we have applied a multiscale approach involving quantum chemistry, self-consistent mean-field theory, and finite-element modeling to investigate ion transport in PBAs. We identify a cyanide-mediated ladder mechanism as the primary process of ion transport. Defects are found to be impermissible to diffusion, and a random distribution model accurately predicts the impact of defect concentrations. Notably, the inclusion of intermediary local minima in the models is key for predicting a realistic diffusion constant. Furthermore, the intermediary landscape is found to be an essential difference between both the intercalating species and the type of cation doping in PBAs. We also show that the ladder mechanism, when employed in multiscale computations, properly predicts the macroscopic charging performance based on atomistic results. In conclusion, the findings in this work may suggest the guiding principles for the design of new and effective PBAs for different applications.

An energy efficient bi-functional electrode for continuous cation-selective capacitive deionization.

Effective ion intercalation nanomaterials provide tremendous opportunities to various deionization systems such as capacitive deionization (CDI) to significantly improve the removal capacity of brackish water desalination. However, the asymmetric design of CDI devices causes a low removal rate due to the indispensable regeneration half-cycle. Furthermore, choices of chloride selective electrodes for such devices are limited. This imposes a big challenge on further improvement of CDI systems. Herein, we report a cation-selective CDI system using a single bi-functional Na2VTi(PO4)3@carbon nanomaterial with redox couples of V4+/V3+ and Ti3+/Ti4+ as an advanced symmetric electrode. The as-prepared continuous desalination set-up shows a superior removal rate of 0.022 mg g-1 s-1 (1.32 mg g-1 min-1) with a high half-cycle removal capacity of 35 mg g-1, and extremely low energy consumption of 0.14 W h g-1 (at a current density of 100 mA g-1). In addition, an extremely high cycle-stability of at least 50 cycles is achieved. The bi-functional intercalation mechanism is investigated by in situ XRD and ex situ XPS. The symmetric device yields a simplified and low-cost configuration with improved energy efficiency and high removal capacity. This opens a new horizon towards the commercialization of CDI technologies.

Efficient Sodium-Ion Intercalation into the Freestanding Prussian Blue/Graphene Aerogel Anode in a Hybrid Capacitive Deionization System.

In this study, we introduced an efficient hybrid capacitive deionization (HCDI) system for removal of NaCl from brackish water, in which Prussian blue nanocubes embedded in a highly conductive reduced graphene oxide aerogel have been used as a binderfree intercalation anode to remove Na+ ions. The combination of redox-active nanocubes and the three-dimensional porous graphene network yielded a high salt removal capacity of 130 mg g-1 at the current density of 100 mA g-1. Moreover, energy recovery and energy consumption upon different desorption voltages of the HCDI system were investigated and the result showed a notably low energy consumption of 0.23 Wh g-1 and a high energy recovery of 39%. Furthermore, the real-time intercalation process was verified by in situ X-ray powder diffraction measurements, which confirmed the intercalation and deintercalation processes during charging and discharging, respectively. Eventually, a perfect stability of the desalination unit was confirmed through the steady performance of 100 cycles. The improved efficiency as well as ease of fabrication opens a shiny horizon for our HCDI system toward commercialization of such technology for brackish water desalination.

3D printed electrodes for efficient membrane capacitive deionization.

There is increasing interests in cost-effective and energy-efficient technologies for the desalination of salt water. However, the challenge in the scalability of the suitable compositions of electrodes has significantly hindered the development of capacitive deionization (CDI) as a promising technology for the desalination of brackish water. Herein, we introduced a 3D printing technology as a new route to fabricate electrodes with adjustable composition, which exhibited large-scale applications as free-standing, binder-free, and robust electrodes. The 3D printed electrodes were designed with ordered macro-channels that facilitated effective ion diffusion. The high salt removal capacity of 75 mg g-1 was achieved for membrane capacitive deionization (MCDI) using 3D printed nitrogen-doped graphene oxide/carbon nanotube electrodes with the total electrode mass of 20 mg. The improved mechanical stability and strong bonding of the chemical components in the electrodes allowed a long cycle lifetime for the MCDI devices. The adjusted operational mode (current density) enabled a low energy consumption of 0.331 W h g-1 and high energy recovery of ∼27%. Furthermore, the results obtained from the finite element simulations of the ion diffusion behavior quantified the structure-function relationships of the MCDI electrodes.

Activated Luffa derived biowaste carbon for enhanced desalination performance in brackish water.

Membrane capacitive deionization (MCDI) is an effective process to remove salt ions from brackish water. In this work, a systematic investigation was carried out to study the effects of applied potential and salt concentration on salt adsorption capacity (SAC), charge efficiency (Λ) and energy consumption in an MCDI system using Luffa biowaste derived carbon as electrodes. We studied the comparative MCDI performance of Luffa derived carbon as electrodes before and after activation. Furthermore, the desalination capacities of the electrodes were quantified by batch-mode experiments in a 2500 mg L-1 NaCl solution at 0.8-1.2 V. Activated Luffa carbon showed a high SAC of 38 mg g-1 at 1.2 V in a 2500 mg L-1 NaCl solution with a low energy consumption of 132 kJ mol-1 salt as compared to non-activated samples (22 mg g-1, 143 kJ mol-1). The adsorption mechanisms were investigated using kinetic models and isotherms under various applied potentials. Consequently, the excellent SAC of activated Luffa carbon can be attributed to the presence of micro/mesoporous network structure formed due to the activation process for the propagation of the salt ions.