Carbon-coated Na2FePO4F and Na2Fe0.5Mn0.5PO4F are successfully prepared by a simple solid-state method with ascorbic acid as carbon source. Crystal structure of Na2Fe0.5Mn0.5PO4F is found to be isostructural with Na2MnPO4F by an X-ray diffraction method, which has a three dimensional fluorophosphate framework. Scanning/transmission electron microscopy and Raman spectroscopy reveal that the addition of ascorbic acid effectively suppresses the particle growth of the samples, forming the nano-sized carbon coated materials. Electrode performance of Na2FePO4F is compared with that of Na2Fe0.5Mn0.5PO4F. The carbon-coated Na2FePO4F prepared with 2 wt% ascorbic acid delivers discharge capacity of 100–110 mAh g−1 at a rate of 1/20 C (6.2 mA g−1) with well-defined voltage plateaus at 3.06 and 2.91 V vs. Na metal. In contrast, Na2Fe0.5Mn0.5PO4F is less electrochemically active. The higher content of carbon by the adding ascorbic acid (6 wt%) and ball-milling treatment are necessary to achieve high reversible capacity. A well-optimized Na2Fe0.5Mn0.5PO4F sample delivers the discharge capacity of 110 mAh g−1 at a rate of 1/20 C (6.2 mA g−1), and it is first demonstrated that average operating voltage is higher than that of Na2FePO4F based on the possible Mn2+/Mn3+ redox couple centered at 3.53 V.
A novel fabrication approach for electrochemical sensing of nicotinamide adenine dinucleotide (NADH) using neutral red (NR) functinalized carbon nanotube/plasma-polymerized film (PPF) composite electrode is reported. The configuration of sensing electrode was NR-functionalized CNTs sandwiched between two acetonitrile PPFs on sputtered gold thin film. The NR as an electron transfer mediator shuttles the electron from CNT to gold electrode. Due to the synergistic effect between NR and CNT, the resulting electrode showed the lower detection potential and the larger sensitivity (current) than that with NR or CNT alone. As a result, the electrode showed the selectivity against the interference ascorbic acid.