Thermodynamic and Kinetic Analyses of Ion Intercalation/Deintercalation Using Different Temperatures on NiHCF Electrodes for Battery Electrode Deionization

Adsorption selectivity of nickel hexacyanoferrate foam electrodes and influencing factors: extraction of a 98 % potassium fraction solution from pseudo-seawater

Metal hexacyanoferrates are promising adsorbents for desalination and concentration of seawater and wastewater, because of a high capacity for selective cation intercalation into their three-dimensional lattice through redox reactions. The ratio between fractional quantities of cations removed from a solution is a metric commonly used to evaluate adsorption selectivity. However, this metric also depends directly on cation concentrations in the adsorption solution, thus on the electrode potential through the reaction yield. Here, we analyzed the adsorption selectivity of nickel hexacyanoferrate (NiHCF) foam electrodes, characterized by high porosity and excellent ion diffusivity, using only the selectivity coefficient for ion exchange, i.e., independently of the electrode potential. We conducted a potassium extraction experiment with three consecutive stages, each comprising first an adsorption then a desorption process. From pseudo-seawater (K+ = 10 mmol/L, Na+ = 495 mmol/L) as first adsorption solution, we obtained a final desorption solution with a high potassium fraction (98 %; K+ = 123 mmol/L, Na+ = 3 mmol/L). Temporal concentration variations illustrated the close agreement between measurements and values calculated using only the selectivity coefficient for ion exchange, demonstrating that the adsorption selectivity of NiHCF foam electrodes was primarily influenced by ion exchange reactions, and did not depend on the electrode potential. We also demonstrated the usefulness of our foam electrodes for industrial application through a cyclability assessment and a detailed K+ adsorption selectivity evaluation in synthetic seawater, a more realistic seawater analogue containing not only Na+ and K+, but also divalent cations (Mg2+, Ca2+).

Enhanced hybrid capacitive performance for efficient and selective potassium extraction from wastewater: Insights from regulating electrode potential

Prussian blue analogues hold great promise for directly extracting potassium resource from wastewater via hybrid capacitive deionization (HCDI). However, there remain unresolved scientific issues regarding low efficiency and selectivity arising from asymmetric potential distribution induced by spontaneous charge matching. This work systematically investigated the underlying mechanisms for enhancing the storage capacity and specific affinity of representative Berlin Green towards K+ through precise regulation of insertion potential during HCDI operation. Empowered by controlling electrochemical intercalation behaviors, the compatibility between ionic and electronic kinetics was significantly enhanced. Impressive values of 160.12 mg/g, 61.27 %, and 0.07 kWh/mol were achieved under potentiostatic mode (0.1 V vs. Ag/AgCl) for insertion capacity, charge efficiency, and energy consumption, respectively. These results significantly outperformed the optimal levels obtained under constant cell voltage (0.9 V), which were 128.52 mg/g, 47.50 %, and 0.12 kWh/mol, respectively. In both aqueous solution with binary components and urine, the results emphasized the potential of the synergy effect between lattice hindrance and insertion chemistry in promoting intercalation selectivity, with the highest selectivity coefficients of 28.35 (K+/Na+), 76.22 (K+/Ca2+) and 175.12 (K+/Mg2+), respectively. The presented concept-to-proof offers a versatile approach for the advancement of high-performance HCDI and paves the way towards its sustainable application in nutrient recycling from natural waters or wastewaters.

Electrochemical kinetic energy harvesting mediated by ion solvation switching in two-immiscible liquid electrolyte

Kinetic energy harvesting has significant potential, but current methods, such as friction and deformation-based systems, require high-frequency inputs and highly durable materials. We report an electrochemical system using a two-phase immiscible liquid electrolyte and Prussian blue analogue electrodes for harvesting low-frequency kinetic energy. This system converts translational kinetic energy from the displacement of electrodes between electrolyte phases into electrical energy, achieving a peak power of 6.4 ± 0.08 μW cm-2, with a peak voltage of 96 mV and peak current density of 183 μA cm-2 using a 300 Ω load. This load is several thousand times smaller than those typically employed in conventional methods. The charge density reaches 2.73 mC cm-2, while the energy density is 116 μJ cm-2 during a harvesting cycle. Also, the system provides a continuous current flow of approximately 5 μA cm-2 at 0.005 Hz for 23 cycles without performance decay. The driving force behind voltage generation is the difference in solvation Gibbs free energy between the two electrolyte phases. Additionally, we demonstrate the system's functionality in a microfluidic harvester, generating a maximum power density of 200 nW cm-2 by converting the kinetic energy to propel the electrolyte through the microfluidic channel into electricity.

Self-reporting electroswitchable colorimetric platform for smart ammonium recovery from wastewater

Recovery of ammonium from wastewater represents a sustainable strategy within the context of global resource depletion, environmental pollution and carbon neutralization. The present study developed an advanced self-reporting electroswitchable colorimetric platform (SECP) to realize smart ammonium recovery based on the electrically stimulated transformation of Prussian blue/Prussian white (PB/PW) redox couple. The key to SECP was the selectivity of ammonium adsorption, sensitivity of desorption to electric signals and visualability of color change during switchable adsorption/desorption transformation. The results demonstrated the electrochemical intercalation-induced selective adsorption of NH4+ (selectivity coefficient of 3-19 versus other cations) and deintercalation-induced desorption on the PB-film electrode. At applied voltage of 1.2 V for 20 min, the negatively charged PB-film electrode achieved the maximum adsorption capacity of 3.2 mmol g-1. Reversing voltage to -0.2 V for 20 min resulted in desorption efficiency as high as 99%, indicating high adsorption/desorption reversibility and cyclic stability. The Fe(III)/Fe(II) redox dynamics were responsible for PB/PW transformation during reversible intercalation/deintercalation of NH4+. Based on the blue/transparence color change of PB/PW, the quantitative relationship was established between amounts of NH4+ adsorbed and extracted RGB values by multiple linear regression (R2 = 0.986, RMSE = 0.095). Then, the SECP was created upon the unique capability of real-time monitoring and feedback of color change of electrode to realize the automatic control of NH4+ adsorption/desorption. During five cycles of tests, the adsorption process consistently peaked at an average value of 3.15±0.04 mmol g-1, while desorption reliably approached the near-zero average of 0.06±0.04 mmol g-1. The average time of duration was 19.6±1.67 min for adsorption and 18.8±1.10 min for desorption, respectively. With electroswitchability, selectivity and self-reporting functionalities, the SECP represents a paradigm shift in smart ammonium recovery from wastewater, making wastewater treatment and resource recovery more efficient, more intelligent and more sustainable.