Ionic liquids and organic ionic plastic crystals utilizing small phosphonium cations

The development of new liquid and solid state electrolytes is paramount for the advancement of electrochemical devices such as lithium batteries and solar cells. Ionic liquids have shown great promise in both these applications. Here we demonstrate the use of phosphonium cations with small alkyl chain substituents, in combination with a range of different anions, to produce a variety of new halide free ionic liquids that are fluid, conductive and with sufficient thermal stability for a range of electrochemical applications. Walden plot analysis of the new phosphonium ionic liquids shows that these can be classed as &quot;good&quot; ionic liquids, with low degrees of ion pairing and/or aggregation, and the lithium deposition and stripping from one of these ionic liquids has been demonstrated. Furthermore, for the first time phosphonium cations have been used to form a range of organic ionic plastic crystals. These materials can show significant ionic conductivity in the solid state and thus are of great interest as potential solid-state electrolyte materials. <br /

The evolving SARS-CoV-2 epidemic in Africa: Insights from rapidly expanding genomic surveillance

INTRODUCTION Investment in Africa over the past year with regard to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) sequencing has led to a massive increase in the number of sequences, which, to date, exceeds 100,000 sequences generated to track the pandemic on the continent. These sequences have profoundly affected how public health officials in Africa have navigated the COVID-19 pandemic. RATIONALE We demonstrate how the first 100,000 SARS-CoV-2 sequences from Africa have helped monitor the epidemic on the continent, how genomic surveillance expanded over the course of the pandemic, and how we adapted our sequencing methods to deal with an evolving virus. Finally, we also examine how viral lineages have spread across the continent in a phylogeographic framework to gain insights into the underlying temporal and spatial transmission dynamics for several variants of concern (VOCs). RESULTS Our results indicate that the number of countries in Africa that can sequence the virus within their own borders is growing and that this is coupled with a shorter turnaround time from the time of sampling to sequence submission. Ongoing evolution necessitated the continual updating of primer sets, and, as a result, eight primer sets were designed in tandem with viral evolution and used to ensure effective sequencing of the virus. The pandemic unfolded through multiple waves of infection that were each driven by distinct genetic lineages, with B.1-like ancestral strains associated with the first pandemic wave of infections in 2020. Successive waves on the continent were fueled by different VOCs, with Alpha and Beta cocirculating in distinct spatial patterns during the second wave and Delta and Omicron affecting the whole continent during the third and fourth waves, respectively. Phylogeographic reconstruction points toward distinct differences in viral importation and exportation patterns associated with the Alpha, Beta, Delta, and Omicron variants and subvariants, when considering both Africa versus the rest of the world and viral dissemination within the continent. Our epidemiological and phylogenetic inferences therefore underscore the heterogeneous nature of the pandemic on the continent and highlight key insights and challenges, for instance, recognizing the limitations of low testing proportions. We also highlight the early warning capacity that genomic surveillance in Africa has had for the rest of the world with the detection of new lineages and variants, the most recent being the characterization of various Omicron subvariants. CONCLUSION Sustained investment for diagnostics and genomic surveillance in Africa is needed as the virus continues to evolve. This is important not only to help combat SARS-CoV-2 on the continent but also because it can be used as a platform to help address the many emerging and reemerging infectious disease threats in Africa. In particular, capacity building for local sequencing within countries or within the continent should be prioritized because this is generally associated with shorter turnaround times, providing the most benefit to local public health authorities tasked with pandemic response and mitigation and allowing for the fastest reaction to localized outbreaks. These investments are crucial for pandemic preparedness and response and will serve the health of the continent well into the 21st century

Effect of ZIF-8 Crystal Size on the O2 Electro-Reduction Performance of Pyrolyzed Fe–N–C Catalysts

The effect of ZIF-8 crystal size on the morphology and performance of Fe–N–C catalysts synthesized via the pyrolysis of a ferrous salt, phenanthroline and the metal-organic framework ZIF-8 is investigated in detail. Various ZIF-8 samples with average crystal size ranging from 100 to 1600 nm were prepared. The process parameters allowing a templating effect after argon pyrolysis were investigated. It is shown that the milling speed, used to prepare catalyst precursors, and the heating mode, used for pyrolysis, are critical factors for templating nano-ZIFs into nano-sized Fe–N–C particles with open porosity. Templating could be achieved when combining a reduced milling speed with a ramped heating mode. For templated Fe–N–C materials, the performance and activity improved with decreased ZIF-8 crystal size. With the Fe–N–C catalyst templated from the smallest ZIF-8 crystals, the current densities in H2/O2 polymer electrolyte fuel cell at 0.5 V reached ca. 900 mA cm−2, compared to only ca. 450 mA cm−2 with our previous approach. This templating process opens the path to a morphological control of Fe–N–C catalysts derived from metal-organic frameworks which, when combined with the versatility of the coordination chemistry of such materials, offers a platform for the rational design of optimized Metal–N–C catalysts

Ionic material for photoelectrochemical solar cells

The pressure on governments to find alternative energy sources which are cheaper, sustainable and also less polluting, has triggered a significant amount of research into solar cells. Conventional solar cells are mainly based on silicon, but the cost of production and materials of these types of devices is high. Dye sensitised solar cells (DSSCs) are an alternative type of solar cell that uses cheaper components. Since their discovery in 1991, many research groups across the world have investigated this type of device in an attempt to understand and optimise their operation. There are several factors that limit the performance of these devices, such as corrosion of the counter electrode, leakage of the commonly used molecular liquid electrolyte and insufficient light absorption by the sensitisers. The aim of this research was to synthesise and characterise novel ionic liquids for use as the electrolyte in dye sensitised solar cells, and also understand the behaviour of commonly used ionic liquids, such as the imidazolium-based family, when in contact with the TiO2 photoanode layer. New phosphonium ionic liquids were synthesised for use as electrolytes in DSSCs. Different anions such as tetrafluoroborate, hexafluorophosphate, dicyanamide, bis(trifluoromethanesulfonyl)amide, thiocyanate and bis(fluorosulfonyl)imide, combined with different asymmetric phosphonium cations, were used to prepare either ionic liquids, or solids that show plastic crystal behaviour. The chemical and physical properties of these phosphonium ionic liquids were measured and they all show relatively good thermal stability, except for the bis(fluorosulfonyl)imide series. They also exhibit good ionic conductivity and are considered to be “good” ionic liquids according to their position on the Walden plot. These new phosphonium ionic liquids were utilised as electrolytes in dye sensitised solar cells using dithienothiophene organic dyes, and the results compared to those obtained using the common ruthenium-based dyes. The effect of film thickness, scattering layer and particle size were investigated with the organic sensitisers. With these new dithienothiophene sensitisers, a thin (2 μm) transparent TiO2 layer with a scattering layer of ~ 6 μm is the optimum for the device to perform well. The addition of chenodeoxycholic acid in the dye bath showed an improvement in device efficiency. Addition of small amounts of solvent such as water, valeronitrile or tetraglyme are also discussed. These novel phosphonium ionic liquids show promising behaviour in these cells. The best device performance was achieved with the least viscous phosphonium ionic liquids containing the methoxy group on one of the alkyl chains. The power conversion efficiencies using these dithienothiophene dyes were all > 5 % under full sun and > 6 % at low sun intensity. In order to understand the behaviour of ionic liquids in dye sensitised solar cells, the effect of this class of material in contact with the semiconductor was investigated by measurement of flatband potentials. Depending on the “acidity” or “basicity” of the neat ionic liquids, the position of the conduction band edge of the semiconductor is shifted. The purity of the ionic liquid is also a very important factor as it can affect the position of the flatband potential of the TiO2-ionic liquid junction. For example, C2mimBF4 gives two different flatband potentials depending on the quality of the ionic liquid from the supplier. The effects of acid treatment and the addition of additives such as lithium iodide and N-methylbenzimidazole to the ionic liquid were also investigated. As expected, addition of small ions such as Li+ or H+ shift the flatband potential towards more positive values, whereas the presence of basic materials such as 4-tert-butylpyridine or N-methylbenzimidazole gives more negative potentials. These additives as well as the ionic liquids play an important role in dye sensitised solar cells, especially on the open circuit voltage. The use of imidazolium, ammonium and phosphonium based ionic liquid electrolytes was also investigated with porphyrin sensitisers and transient spectroscopy measurements were performed to elucidate the influence of the ionic liquid on the device performance. Another interesting class of materials, which are related to the family of ionic liquids studied here, are the organic ionic plastic crystals. These were investigated as potential solid electrolytes in dye sensitised solar cells. It is the first time that these materials have been successfully used in DSSCs. Relatively good performance was obtained with an electrolyte based on the organic ionic plastic crystal, C1mpyrN(CN)2. Performance of > 5 % was obtained with Ruthenium as the sensitiser under full sun intensity. Finally, the use of another type of solid electrolyte based on succinonitrile was investigated with both the porphyrin sensitiser (P159) and with a ruthenium based sensitiser (N719). Over 5 % efficiency was achieved at low sun intensity with the porphyrin dye. This is the first time that such a good performance has been obtained with a solid electrolyte in porphyrin based DSSCs

Ionic material for photoelectrochemical solar cells

The pressure on governments to find alternative energy sources which are cheaper, sustainable and also less polluting, has triggered a significant amount of research into solar cells. Conventional solar cells are mainly based on silicon, but the cost of production and materials of these types of devices is high. Dye sensitised solar cells (DSSCs) are an alternative type of solar cell that uses cheaper components. Since their discovery in 1991, many research groups across the world have investigated this type of device in an attempt to understand and optimise their operation. There are several factors that limit the performance of these devices, such as corrosion of the counter electrode, leakage of the commonly used molecular liquid electrolyte and insufficient light absorption by the sensitisers. The aim of this research was to synthesise and characterise novel ionic liquids for use as the electrolyte in dye sensitised solar cells, and also understand the behaviour of commonly used ionic liquids, such as the imidazolium-based family, when in contact with the TiO2 photoanode layer. New phosphonium ionic liquids were synthesised for use as electrolytes in DSSCs. Different anions such as tetrafluoroborate, hexafluorophosphate, dicyanamide, bis(trifluoromethanesulfonyl)amide, thiocyanate and bis(fluorosulfonyl)imide, combined with different asymmetric phosphonium cations, were used to prepare either ionic liquids, or solids that show plastic crystal behaviour. The chemical and physical properties of these phosphonium ionic liquids were measured and they all show relatively good thermal stability, except for the bis(fluorosulfonyl)imide series. They also exhibit good ionic conductivity and are considered to be “good” ionic liquids according to their position on the Walden plot. These new phosphonium ionic liquids were utilised as electrolytes in dye sensitised solar cells using dithienothiophene organic dyes, and the results compared to those obtained using the common ruthenium-based dyes. The effect of film thickness, scattering layer and particle size were investigated with the organic sensitisers. With these new dithienothiophene sensitisers, a thin (2 μm) transparent TiO2 layer with a scattering layer of ~ 6 μm is the optimum for the device to perform well. The addition of chenodeoxycholic acid in the dye bath showed an improvement in device efficiency. Addition of small amounts of solvent such as water, valeronitrile or tetraglyme are also discussed. These novel phosphonium ionic liquids show promising behaviour in these cells. The best device performance was achieved with the least viscous phosphonium ionic liquids containing the methoxy group on one of the alkyl chains. The power conversion efficiencies using these dithienothiophene dyes were all > 5 % under full sun and > 6 % at low sun intensity. In order to understand the behaviour of ionic liquids in dye sensitised solar cells, the effect of this class of material in contact with the semiconductor was investigated by measurement of flatband potentials. Depending on the “acidity” or “basicity” of the neat ionic liquids, the position of the conduction band edge of the semiconductor is shifted. The purity of the ionic liquid is also a very important factor as it can affect the position of the flatband potential of the TiO2-ionic liquid junction. For example, C2mimBF4 gives two different flatband potentials depending on the quality of the ionic liquid from the supplier. The effects of acid treatment and the addition of additives such as lithium iodide and N-methylbenzimidazole to the ionic liquid were also investigated. As expected, addition of small ions such as Li+ or H+ shift the flatband potential towards more positive values, whereas the presence of basic materials such as 4-tert-butylpyridine or N-methylbenzimidazole gives more negative potentials. These additives as well as the ionic liquids play an important role in dye sensitised solar cells, especially on the open circuit voltage. The use of imidazolium, ammonium and phosphonium based ionic liquid electrolytes was also investigated with porphyrin sensitisers and transient spectroscopy measurements were performed to elucidate the influence of the ionic liquid on the device performance. Another interesting class of materials, which are related to the family of ionic liquids studied here, are the organic ionic plastic crystals. These were investigated as potential solid electrolytes in dye sensitised solar cells. It is the first time that these materials have been successfully used in DSSCs. Relatively good performance was obtained with an electrolyte based on the organic ionic plastic crystal, C1mpyrN(CN)2. Performance of > 5 % was obtained with Ruthenium as the sensitiser under full sun intensity. Finally, the use of another type of solid electrolyte based on succinonitrile was investigated with both the porphyrin sensitiser (P159) and with a ruthenium based sensitiser (N719). Over 5 % efficiency was achieved at low sun intensity with the porphyrin dye. This is the first time that such a good performance has been obtained with a solid electrolyte in porphyrin based DSSCs

Effect of ZIF-8 Crystal Size on the O2 Electro-Reduction Performance of Pyrolyzed Fe–N–C Catalysts

International audienceThe effect of ZIF-8 crystal size on the morphology and performance of Fe–N–C catalysts synthesized via the pyrolysis of a ferrous salt, phenanthroline and the metal-organic framework ZIF-8 is investigated in detail. Various ZIF-8 samples with average crystal size ranging from 100 to 1600 nm were prepared. The process parameters allowing a templating effect after argon pyrolysis were investigated. It is shown that the milling speed, used to prepare catalyst precursors, and the heating mode, used for pyrolysis, are critical factors for templating nano-ZIFs into nano-sized Fe–N–C particles with open porosity. Templating could be achieved when combining a reduced milling speed with a ramped heating mode. For templated Fe–N–C materials, the performance and activity improved with decreased ZIF-8 crystal size. With the Fe–N–C catalyst templated from the smallest ZIF-8 crystals, the current densities in H2/O2 polymer electrolyte fuel cell at 0.5 V reached ca. 900 mA cm −2 , compared to only ca. 450 mA cm −2 with our previous approach. This templating process opens the path to a morphological control of Fe–N–C catalysts derived from metal-organic frameworks which, when combined with the versatility of the coordination chemistry of such materials, offers a platform for the rational design of optimized Metal–N–C catalysts

Degradation by Hydrogen Peroxide of Metal-Nitrogen-Carbon Catalysts for Oxygen Reduction

International audience7 8 Fe-N-C and CoN -C materials are promising catalysts for reducing oxygen in fuel cells. The degradation of such catalysts induced by H 2 O 2 was investigated by contacting them ex situ with various amounts of H 2 O 2. The degradation increased with increasing amounts of H 2 O 2. The effect was most severe for Cr-N-C followed by Fe-N-C and last by CoN -C. Treatment with H 2 O 2 leads to diminished oxygen reduction activity at high potential and/or reduced transport properties at high current density in fuel cell. From spectroscopic characterisation, it was found that 66 and 80% of the CoN x C y and FeN x C y moieties present in pristine catalysts survived the extensive H 2 O 2 treatment, respectively. In parallel, the activity for oxygen reduction was divided by ca 6–10 for Fe-N-C and by ca 3 for CoN -C. The results suggest that the main degradation mechanism in fuel cell for such catalysts is due to a chemical reaction with H 2 O 2 that is generated during operation. The super-proportional decrease of the oxygen reduction activity with loss of FeN x C y and CoN x C y moieties suggests either that only a small fraction of such moieties are initially located on the top surface, or that their turnover frequency for oxygen reduction was drastically reduced due to surface oxidation by H 2 O 2

Organic ionic plastic crystal electrolytes; a new class of electrolyte for high efficiency solid state dye-sensitized solar cells

Dye-sensitized solar cells are an increasingly promising alternative to conventional silicon solar cells as a method of converting solar energy to electricity and thus providing an effectively inexhaustible energy source. However, the most efficient of these devices currently utilize liquid electrolytes, which suffer from the associated problems of leakage and evaporation. Hence, significant research is currently focused on the development of solid state alternatives. Here we report a new class of solid state electrolyte for these devices, organic ionic plastic crystal electrolytes, that allow relatively rapid diffusion of the redox couple through the matrix, which is critical to the cell performance. A range of different organic ionic plastic crystal materials, utilizing different cation and anion structures, have been investigated and the conductivities, diffusion rates and photovoltaic performance of the electrolytes are reported. The best material, utilizing the dicyanamide anion, achieves efficiencies of more than 5%