Understanding the Behaviour of Real Metaborates in Solution

Alkali metal borohydrides are promising candidates for large-scale hydrogen storage. They react spontaneously with water, generating dihydrogen and metaborate salts. While sodium borohydride is the most studied, potassium has the best chance of commercial application. Here we examine the physical and chemical properties of such self-hydrolysis solutions. We do this by following the hydrogen evolution, the pH changes, and monitoring the reaction intermediates using NMR. Most studies on such systems are done using dilute solutions, but real-life applications require high concentrations. We show that increasing the borohydride concentration radically changes the system's microstructure and rheology. The changes are seen already at concentrations as low as 5 w/w%, and are critical above 10 w/w%. While dilute solutions are Newtonian, concentrated reaction solutions display non-Newtonian behaviour, that we attribute to the formation and (dis)entanglement of metaborate oligomers. The implications of these findings towards using borohydride salts for hydrogen storage are discussed.

Multifunctional Surface Construction for Long-Term Cycling Stability of Li-Rich Mn-Based Layered Oxide Cathode for Li-Ion Batteries

Li-rich Mn-based layered oxides (LMLOs) are promising cathode material candidate for the next-generation Li-ion batteries (LIBs) of high energy density. However, the fast capacity fading and voltage decay as well as low Coulombic efficiency caused by irreversible oxygen release and phase transition during the electrochemical process hinder their practical application. To solve these problems, in the present study, a multifunctional surface construction involving a coating layer, spinel-layered heterostructure, and rich-in oxygen vacancies is successfully conducted by a facile thermal reduction of the LMLO particles with potassium borohydride (KBH₄ ) as the reducing agent. The multifunctional surface structure plays synergistic effects on suppressing the interface side reaction, reducing the dissolution of transition metal, increasing electron conductivity and lithium diffusion rate. As a result, electrochemical performances of the LMLO cathode are effectively enhanced. With optimization of the addition of KBH₄ , the electrode delivers a reversible capacity of 280 mAh g^-1 at 0.1 C, which maintains after 100 cycles. The capacity retention with respect to the initial capacity is as high as 98% at 1 C after 400 cycles. The present work provides insights into designing a highly effective functional surface structure of LMLO cathode materials for high-performance LIBs.

Facile Conversion of the Quinone-Semicarbazone Chromophore of Naftazone into a Fluorescent Quinol-Semicarbazide: Kinetic Study and Analysis of Naftazone in Pharmaceuticals and Human Serum

Naftazone is a quinone-semi carbazone drug that possesses a strong orange color, and hence it was usually analyzed colorimetrically or by HPLC-UV. However, these methods are not sensitive enough to determine naftazone in biological samples. Naftazone lacks intrinsic fluorescence and does not possess easily derivatizable functional groups. In this contribution, we introduced the first spectrofluorimetric method for naftazone assay through reduction-elicited fluorogenic derivatization through the reduction of its quinone-semicarbazone moiety to the corresponding quinol-semicarbazide derivative by potassium borohydride as a reduction probe. The solvent-dependent fluorescence of the reaction product was studied in various protic and aprotic solvents. Eventually, the fluorescence of the reduced naftazone was measured in 2-propanol at λ_emission of 350 nm after excitation at λ_ecxitation of 295 nm. The relative fluorescence intensity was linearly correlated to the drug concentration (r = 0.9995) from 10.0 to 500 ng/mL with high sensitivity, where the lower detection limit was 2.9 ng/mL. Hence, the method was effectively applied for naftazone tablets quality control with a mean %recovery of 100.3 ± 1.5, and the results agreed with those of the comparison HPLC-UV method. Furthermore, a new salting-out assisted liquid-liquid extraction (SALLE) method was established for naftazone extraction from human serum, followed by its determination using the developed reduction-based fluorogenic method. The developed SALLE method showed excellent recovery for naftazone from human serum (92.3-106.5%) with good precision (RSD ≤ 6.8%). Additionally, the reaction of naftazone with potassium borohydride was kinetically monitored, and it was found to follow pseudo-first-order kinetics with an activation energy of 43.8 kcal/mol. The developed method's greenness was approved using three green analytical chemistry metrics.