Electrochemical oxidation of maricite NaFePO4 in mild aqueous solutions as a way to boost its charge storage capacity

Lithium has a low abundance in the Earth's crust, which in a few years will lead to difficult lithium production, and therefore difficult production of lithium-ion batteries. Sodium-ion batteries, on the other hand, have been proven to be a good replacement. The material obtained from iron combined with the phosphate and pyrophosphate compounds of sodium has attracted attention as a possible cathode material for sodium-ion batteries. NaFePO4 exists in two polymorphic structures (triphylite and maricite). Maricite NaFePO4 is a more thermodynamically stable structure than triphylite NaFePO4 but doesn’t have channels for Na+ movement and electrochemical performance of this structure is low. In comparison to maricite NaFePO4, triphylite NaFePO4 (structural analogue to LiFePO4) has one-dimensional channels for Na+-ions movement and better electrochemical activity but it is not stable and is difficult to synthesize. Herein, the maricite NaFePO4 can be obtained by sintering a polyanionic compound, Na4Fe3(PO4)2P2O7, at temperatures above 600 °C, as shown by XRD. Na4Fe3(PO4)2P2O7 is synthesized by the glycine-nitrate process after which it was sintered at temperatures above 500 °C. The glycine-nitrate process was found to catalyze the decomposition of the sintered Na4Fe3(PO4)2P2O7 to the NaFePO4 maricite. The electrochemical characterization of the sintered material, evaluated in aqueous NaNO3 and LiNO3 electrolyte by cyclic voltammetry, showed poor electrochemical activity of maricite NaFePO4. By exposing the sintered material to high anodic potentials, the electrochemical activity and specific capacity of the material were increased by 50% in case of NaNO3 and 80% in case of LiNO3 relative to the pristine with low activity. After electrochemical measurements, residual powder was characterized by FTIR and Raman spectroscopy. It was shown that high anodic polarization of the material tested in LiNO3 causes the formation of triphylite LiFePO4. Similarly, it is assumed that the electrochemical activity obtained by deep anodic polarization of the material in NaNO3 electrolyte originates from the formed triphylite NaFePO4. The obtained results open novel directions regarding the use of NaFePO4 in metal-ion rechargeable batteries

The mechanism behind Pd(II) and carbofuran-induced change of graphene quantum dots photoluminescence intensity

The increasing presence of pesticides and heavy metals in the environment and their negative impact on human, animal, and plant health, demand green, low-cost, and simple methods for their monitoring. Due to photoluminescence (PL) in the visible part of the spectrum, biocompatibility, and ecological acceptability, graphene quantum dots (GQDs) are at the center of attention in the field of optical sensing. GQDs show great potential as PL sensors for Pd(II) ions and insecticide carbofuran. In this work, FTIR spectroscopy and Density Functional Theory (DFT) calculation were used to resolve the mechanism of PL change in the presence of these analytes

Surface chemistry, thermal stability and structural properties of graphene oxide/12-tungstophosphoric acid nanocomposite

In recent years the nanocomposites of graphene oxide (GO) and different inorganic and organic compounds have shown great potential for charge storage applications. In present work we have investigated the influence of 12-tungstophosphoric acid (WPA) on surface chemistry of graphene oxide and thermal stability of nanocomposite. For this purpose nanocomposites with different mass ratios of GO and WPA were prepared. The thermal stability of nanocomposites was investigated by thermogravimetric and differential thermal analysis (TGA-DTA) while changes in surface chemistry of GO and structural properties of WPA were investigated by Fourier transform infrared spectroscopy (FTIR) and temperature programmed desorption (TPD) method. The TGA-DTA measurements of composites have shown that the major mass loss, due to carbon combustion, is shifted to higher temperatures (~500 °C vs. 380 °C of pure GO). Furthermore, when the amount of WPA is higher than 25 mass percent the nanocomposites start to act like individual components, which was also confirmed by FTIR analysis. The amount of surface oxygen groups, monitored by both TPD and FTIR methods, showed ˝V˝ shaped dependence from the quantity of WPA with minimum at about 12 mass percent of WPA. At the same time, the FTIR spectra revealed the structural changes of WPA, displayed as shifting and splitting of characteristic bands of Keggin anion structure

Simply Prepared Magnesium Vanadium Oxides as Cathode Materials for Rechargeable Aqueous Magnesium Ion Batteries

Vanadium-oxide-based materials exist with various vanadium oxidation states having rich chemistry and ability to form layered structures. These properties make them suitable for different applications, including energy conversion and storage. Magnesium vanadium oxide materials obtained using simple preparation route were studied as potential cathodes for rechargeable aqueous magnesium ion batteries. Structural characterization of the synthesized materials was performed using XRD and vibrational spectroscopy techniques (FTIR and Raman spectroscopy). Electrochemical behavior of the materials, observed by cyclic voltammetry, was further explained by BVS calculations. Sluggish Mg2+ ion kinetics in MgV2O6 was shown as a result of poor electronic and ionic wiring. Complex redox behavior of the studied materials is dependent on phase composition and metal ion inserted/deinserted into/from the material. Among the studied magnesium vanadium oxides, the multiphase oxide systems exhibited better Mg2+ insertion/deinsertion performances than the single-phase ones. Carbon addition was found to be an effective dual strategy for enhancing the charge storage behavior of MgV2O6. © 2022 by the authors

Layered CaV2O6 as promising electrode material for multivalent storage

While the world is facing a higher demand for lithium, its limited resources associated with the high price, are becoming problematic. Other crucial drawbacks of Li-ion batteries are their toxicity and safety concerns. Therefore, researchers are oriented towards development of non-Li batteries based on eco-friendly and earth-abundant materials to overcome drawbacks of Li-ion technology. Alternative abundant metals and their ions such as Mg and Ca could be a good choice for rechargeable batteries in terms of cost and eco-friendliness. Mg2+ and Ca2+ ions could transfer two electrons per redox process which theoretically has a positive effect on battery performance. The materials upon which multivalent ions will intercalate with fast diffusion rate are hard to find. Metal vanadium oxide (MxVyOz) materials become promising materials for rechargeable batteries, so herein, a standard sol-gel combustion route was used for the preparation of the CaV2O6 layered precursor. Two samples are synthesized from the vanadate precursor, the first when it was heated at 400 °C (CaVO) and the second when CaVO was integrated with 10 wt % of sucrose under thermal treatment at 400 °C, in Ar atmosphere (CaVO/C). Obtained CaVO and CaVO/C powders were thoroughly characterized by XRD, TG-DTA, FTIR, and Raman spectroscopy. The electrochemical performance of the obtained samples was evaluated for multivalent-ion storage in saturated aqueous electrolytic solutions of Mg (NO3)2 and Ca (NO3)2 by cyclic voltammetry and chronopotentiometry. For comparison, measurements were also done in saturated LiNO3. Results indicated that CaVO can store more Li+ ions than Mg2+ and Ca2+ ions, but CaVO in LiNO3 shows a substantial loss of capacity upon cycling, which is not observed in the case of Mg (NO3)2 and Ca (NO3)2. On the other hand, CaVO/C composite showed a significant improvement for Ca an Mg storage capacity, which exceeded capacity storage of Li+ ions. The high and stable discharge capacity of CaVO/C, amounting to 89.3 mA h g−1 at 0.5 A g-1, was obtained in Ca (NO3)2. Obtained results are promising and open novel directions regarding the use of CaV2O6 for multivalent rechargeable batteries, especially for Ca-ion batteries

The Mechanism Behind Pd(II) and Carbofuran-Induced Change of Graphene Quantum Dots Photoluminescence Intensity

The increasing presence of pesticides and heavy metals in the environment and their negative impact on human, animal, and plant health, demand green, low-cost, and simple methods for their monitoring. Due to photoluminescence (PL) in the visible part of the spectrum, biocompatibility, and ecological acceptability, graphene quantum dots (GQDs) are at the center of attention in the field of optical sensing. GQDs show great potential as PL sensors for Pd(II) ions and insecticide carbofuran. In this work, FTIR spectroscopy and Density Functional Theory (DFT) calculation were used to resolve the mechanism of PL change in the presence of these analytes

Study of the interaction between graphene oxide and 12-tungstophosphoric acid in their nanocomposite

The rich surface chemistry and large surface area of graphene oxide (GO) provide a platform for various functional materials that synergistically enhance charge storage properties of the composite. In present work we have investigated interaction between GO and 12- thungstophosphoric acid (WPA) in their nanocomposites as a function of different mass ratio of constituents. For this purpose, the Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectrometry (XPS), temperature programmed desorption method (TPD) and thermogravimetric/differential thermal analysis (TGA-DTA) methods were used. FTIR spectra have shown shifts and splitting of characteristic bands of WPA as a result of interactions with GO. Both XPS and TPD methods have shown an initial decrease of the total amount of surface oxygen groups of GO, with a minimum at around 10 wt.% of WPA, above which a restoration of the amount of surface oxygen groups was noticed. TGA-DTA analysis revealed an improved thermal stability of the material up to 25 wt.% of WPA; at higher loading of WPA the thermal properties of nanocomposite became alike to the ones of individual components. The obtained results suggest optimal conditions for preparation of GO-WPA nanocomposites for electrochemical charge storage applications

Simply prepared Mg-V-O as potential cathode material for rechargeable aqueous magnesium ion batteries

Although today widely used in electronic devices and electric vehicles, lithium ion batteries encounter problem of future application, resulting from limited Li resources, relatively high costs and operational safety problems. Rechargeable magnesium batteries as a potential alternative to the Li-ion ones stand out because of their high theoretical specific capacity, high abundance of Mg resources, atmospheric stability, safety of handling, eco friendliness and low cost. Layered materials including oxides, sulphides and selenides are promising candidates for host materials for Mg2+ storage in rechargeable magnesium batteries. Slow migration of Mg2+ in the layered oxides, ascribed to the strong interaction between Mg and neighbouring O atoms, inspires researchers to look for the ways of improving their electrochemical performance. In this work, Mg-V-O material was synthesized by simple precipitation method, followed by thermal annealing. The obtained material is single-phase material consisted of MgV2O6 phase, according to the results of XRD, FTIR and Raman spectroscopy. Electrochemical test by cyclic voltammetry in aqueous solution revealed redox peaks corresponding to the insertion/deinsertion of Mg2+ ions into/from the material, but with poor current densities. In order to improve the electrochemical performance of the simply prepared Mg-V-O material, carbon was integrated with the Mg-V-O by sucrose-assisted thermal treatment. Although composed of several phases, the obtained Mg-V-O/C material exhibited around 40 times higher maximal specific current values of Mg2+ insertion/deinsertion than the Mg-V-O. Also, the electrochemical performance of the Mg-VO/ C for the insertion/deinsertion of Mg2+ ions was better than those of Al3+ and Li+ ions

Biliverdin-copper complex at the physiological pH

Biliverdin (BV) is a degradation product of heme catabolism, which is rapidly converted to bilirubin (BR) by BV reductase 1. Biliverdin and unconjugated BR, commonly named bile pigments, have important function in biochemical processes. The presence of copper and other biological and toxic transitional metals at significant concentrations in bile implies the possibility that metal complexes with bile pigments can be formed 2. Consequently, our interest was to study the complex of BV with copper in physiological conditions – phosphate buffer with pH 7.4. UV-Vis spectrophotometry was applied to investigate formation/degradation of complex of BV with copper ions and to check stoichiometry by titration, showing that BV interacted with Cu2+ in 1:1 stoichiometry. Mass spectroscopy analysis confirmed this – ion at m/z 643.36 was detected. The results of Raman spectroscopy of BV were in good agreement with previous reports 3. Comparing spectra of BV and BV-Cu complex, the following differences were observed: a new band at low wave number is emerged for the complex may be attributed to Cu-N bond vibration; the band which was shifted to lower energies implicates increased stability of BV in the complex; intensity changes imply a more planar structure of BV in the complex, while stronger bands in complex imply higher delocalization of π-electrons and consequently a higher stability of the BV structure. Pertinent to this, it has been proposed that complexes of BV model compounds with Cu2+ may show unusual electronic structures that exhibit a significant ligand radical character. 1H NMR spectrum of BV in phosphate buffer had a poor resolution of signals, which may originate from aggregation, but this was of little relevance here, since the addition of copper ions led to very strong effect - the complete loss of almost all lines. The loss of signals represents the result of strong paramagnetic effects that may come from an unpaired e- that is delocalized in pπ orbitalss of the ring/ligand influencing all protons in the complex. The EPR spectrum of Cu2+ (S = 1/2; I = 3/2) in phosphate buffer shows that Cu2+ is weakly coordinated in an axial symmetry with one gr line and four lines coming from hyperfine coupling along gs. The addition of BV in equimolar concentration led to the loss of Cu2+ signal. The remaining signal in the [BV]/[Cu2+] = 1 system was broad, and did not show hyperfine structure. The g-value of the isotropic signal of BV-Cu complex was significantly lower than the average g-value of Cu2+ in the phosphate buffer indicating delocalization of the spin away from the metal nucleus. Similar EPR signals have been reported previously 4. Parallel-mode EPR showed no signal. Furthermore, the spectra were run over a wide field range and no half field lines were observed, either in parallel or in perpendicular mode. These results are consistent with S = 0 for the copper center. Further, redox properties of the complex were examined. BV showed a well-defined anodic peak. The [BV]/[Cu2+] = 2 system showed two additional oxidation peaks at much lower potentials than BV. The former potential corresponds to the oxidation of Cu1+, as we have shown previously 5. There was a slight consumption of O2 in [BV]/[Cu2+] = 1 system, which may be explained by traces of ‘free’ copper. However, in the presence of an excess of copper ([BV]/[Cu2+] = 0.5), the consumption of O2 was significant. This implies that ‘free’ Cu2+ reacts with the complex and ‘shuttles’ an e- to O2. The complex was susceptible to oxidizing agents but not to reducing agents. Considering the results obtained we conclude that, at physiological pH, BV builds a complex with copper ions in 1:1 stoichiometry. The formation of complex involves the rearrangement of electronic structure which provides increased energetic stability and strong paramagnetic effects. We believe that a complex with a highly delocalized unpaired e- and the formal BV•+-Cu1+ character best suites the outlined properties, but other structures of the complex cannot be completely ruled out. The presented results may shed new light on long-standing issues of BV chemistry and catalysis in biological systems

Biliverdin-copper complex at the physiological pH

Biliverdin (BV) is a degradation product of heme catabolism, which is rapidly converted to bilirubin (BR) by BV reductase 1. Biliverdin and unconjugated BR, commonly named bile pigments, have important function in biochemical processes. The presence of copper and other biological and toxic transitional metals at significant concentrations in bile implies the possibility that metal complexes with bile pigments can be formed 2. Consequently, our interest was to study the complex of BV with copper in physiological conditions – phosphate buffer with pH 7.4. UV-Vis spectrophotometry was applied to investigate formation/degradation of complex of BV with copper ions and to check stoichiometry by titration, showing that BV interacted with Cu2+ in 1:1 stoichiometry. Mass spectroscopy analysis confirmed this – ion at m/z 643.36 was detected. The results of Raman spectroscopy of BV were in good agreement with previous reports 3. Comparing spectra of BV and BV-Cu complex, the following differences were observed: a new band at low wave number is emerged for the complex may be attributed to Cu-N bond vibration; the band which was shifted to lower energies implicates increased stability of BV in the complex; intensity changes imply a more planar structure of BV in the complex, while stronger bands in complex imply higher delocalization of π-electrons and consequently a higher stability of the BV structure. Pertinent to this, it has been proposed that complexes of BV model compounds with Cu2+ may show unusual electronic structures that exhibit a significant ligand radical character. 1H NMR spectrum of BV in phosphate buffer had a poor resolution of signals, which may originate from aggregation, but this was of little relevance here, since the addition of copper ions led to very strong effect - the complete loss of almost all lines. The loss of signals represents the result of strong paramagnetic effects that may come from an unpaired e- that is delocalized in pπ orbitalss of the ring/ligand influencing all protons in the complex. The EPR spectrum of Cu2+ (S = 1/2; I = 3/2) in phosphate buffer shows that Cu2+ is weakly coordinated in an axial symmetry with one gr line and four lines coming from hyperfine coupling along gs. The addition of BV in equimolar concentration led to the loss of Cu2+ signal. The remaining signal in the [BV]/[Cu2+] = 1 system was broad, and did not show hyperfine structure. The g-value of the isotropic signal of BV-Cu complex was significantly lower than the average g-value of Cu2+ in the phosphate buffer indicating delocalization of the spin away from the metal nucleus. Similar EPR signals have been reported previously 4. Parallel-mode EPR showed no signal. Furthermore, the spectra were run over a wide field range and no half field lines were observed, either in parallel or in perpendicular mode. These results are consistent with S = 0 for the copper center. Further, redox properties of the complex were examined. BV showed a well-defined anodic peak. The [BV]/[Cu2+] = 2 system showed two additional oxidation peaks at much lower potentials than BV. The former potential corresponds to the oxidation of Cu1+, as we have shown previously 5. There was a slight consumption of O2 in [BV]/[Cu2+] = 1 system, which may be explained by traces of ‘free’ copper. However, in the presence of an excess of copper ([BV]/[Cu2+] = 0.5), the consumption of O2 was significant. This implies that ‘free’ Cu2+ reacts with the complex and ‘shuttles’ an e- to O2. The complex was susceptible to oxidizing agents but not to reducing agents. Considering the results obtained we conclude that, at physiological pH, BV builds a complex with copper ions in 1:1 stoichiometry. The formation of complex involves the rearrangement of electronic structure which provides increased energetic stability and strong paramagnetic effects. We believe that a complex with a highly delocalized unpaired e- and the formal BV•+-Cu1+ character best suites the outlined properties, but other structures of the complex cannot be completely ruled out. The presented results may shed new light on long-standing issues of BV chemistry and catalysis in biological systems