Paper-Based Colorimetric Sensor for Hydrogen Peroxide Based Upon a Graphene Oxide/Platinum-Cobalt Nanocomposite

The peroxidase-like properties of a graphene oxide/platinum-cobalt nanocomposite and the rapid color development mechanism of 3,3′,5,5′-tetramethylbenzidine (TMB) through the decomposition of ·OH from H2O2 were utilized to prepare a nonlabelled, simple, and sensitive paper-based colorimetric sensor. This sensor allows visualization of the results and instantaneous detection, enabling quantitative measurement of H2O2. The graphene oxide/platinum cobalt composite was synthesized using a two-step procedure. Its properties were investigated using transmission electron microscopy (TEM), Raman spectroscopy, and energy-dispersive X-ray spectroscopy (EDS). The composite was subsequently transferred onto a paper substrate to create the colorimetric sensor. The optimal catalytic conditions were a composite concentration of 201.17 µg/mL, a color development time of 3 min, a TMB concentration of 2 mmol·L−1, and a pH of 4. Using the optimal conditions, the paper-based colorimetric sensor has a linear range for H2O2 from 1.0 × 10−5 to 0.1 mol·L−1, with a limit of detection of 1.0 × 10−6 mol·L−1 which is comparable or better than comparable methods. This paper-based colorimetric sensor has potential applications for the rapid determination of hydrogen peroxide.</p

What Happens to LiMnPO₄ upon Chemical Delithiation?

Olivine MnPO₄ is the delithiated phase of the lithium-ion-battery cathode (positive electrode) material LiMnPO₄, which is formed at the end of charge. This phase is metastable under ambient conditions and can only be produced by delithiation of LiMnPO₄. We have revealed the manganese dissolution phenomenon during chemical delithiation of LiMnPO₄, which causes amorphization of olivine MnPO₄. The properties of crystalline MnPO₄ obtained from carbon-coated LiMnPO₄ and of the amorphous product resulting from delithiation of pure LiMnPO₄ were studied and compared. The phosphorus-rich amorphous phases in the latter are considered to be MnHP₂O₇ and MnH₂P₂O₇ from NMR, X-ray absorption spectroscopy, and X-ray photoelectron spectroscopy analysis. The thermal stability of MnPO₄ is significantly higher under high vacuum than at ambient condition, which is shown to be related to surface water removal

Electrochemical Performance of Nanosized Disordered LiVOPO₄

ε-LiVOPO₄ is a promising multielectron cathode material for Li-ion batteries that can accommodate two electrons per vanadium, leading to higher energy densities. However, poor electronic conductivity and low lithium ion diffusivity currently result in low rate capability and poor cycle life. To enhance the electrochemical performance of ε-LiVOPO₄, in this work, we optimized its solid-state synthesis route using in situ synchrotron X-ray diffraction and applied a combination of high-energy ball-milling with electronically and ionically conductive coatings aiming to improve bulk and surface Li diffusion. We show that high-energy ball-milling, while reducing the particle size also introduces structural disorder, as evidenced by ⁷Li and ³¹P NMR and X-ray absorption spectroscopy. We also show that a combination of electronically and ionically conductive coatings helps to utilize close to theoretical capacity for ε-LiVOPO₄ at C/50 (1 C = 153 mA h g^–1) and to enhance rate performance and capacity retention. The optimized ε-LiVOPO₄/Li₃VO₄/acetylene black composite yields the high cycling capacity of 250 mA h g^–1 at C/5 for over 70 cycles