Fabrication and characterization of the ternary composite catalyst system of ZnGA/RET/DMC for the terpolymerization of CO2, propylene oxide and trimellitic anhydride

To achieve the poly(propylene carbonate trimellitic anhydride) (PPCTMA) with excellent performance, high molecular weight, enhanced yield and good thermal stability, the ternary composite catalyst system of zinc glutarate/rare earth ternary complex/double metal cyanide (ZnGA/RET/DMC) was proposed to perform the terpolymerization of CO₂, propylene oxide and trimellitic anhydride. Since the crystallinity and surface activity point of Zn–Co DMC could significantly influence the catalytic ability, mechanical ball milling was applied to increase the surface area of the Zn–Co DMC catalyst with better surface activity point. Moreover, the ZnGA/RET/DMC composite catalytic system and polycarbonate products were comparatively evaluated by XRD, SEM, FT-IR, TGA, NMR, XPS and TEM. Experimental results showed that the ZnGA/RET/DMC composite catalyst system displayed outstanding synergistic effect on the terpolymerization of CO₂, PO and TMA with better selectivity, activity, and higher molecular weight (M_w) tercopolymer than those of the individual catalyst. According to optimum reaction conditions, the M_w of PPCTMA could be up to 8.29 × 10⁴ g mol⁻¹, and the yield could be up to 66 g_polym/g_cat. The alternating tercopolymer, PPCTMA, showed wonderful thermal stability and high decomposition temperature (TGA_10% = 313 °C). A possible synergistic catalytic mechanism of the ZnGA/RET/DMC ternary composite catalyst system was also conjectured.

For poly(propylene carbonate trimellitic anhydride) with good yield, thermal stability and high molecular weight, a catalyst of zinc glutarate/rare earth ternary complex/double metal cyanide was used for terpolymerization of CO₂, propylene oxide and trimellitic anhydride.     

One-dimensional polymer-derived ceramic nanowires with electrocatalytically active metallic silicide tips as cathode catalysts for Zn–air batteries

New metallic nickel/cobalt/iron silicide droplets at the tips of polymer-derived ceramic (PDC) nanowires have been identified as stable and efficient cathode catalysts for Zn–air batteries. The as-prepared catalyst having a unique one-dimensional (1D) PDC nanowire structure with the presence of metallic silicide tips of 1D-PDC plays a crucial role in facilitating oxygen reduction/evolution reaction kinetics. The Zn–air battery was designed using Ni/PDC, Co/PDC and Fe/PDC as air electrode catalysts. In electrochemical half-cell tests, it was observed that the catalysts have a good bifunctional electrocatalytic activity. The efficiency of the catalysts to function as a cathode catalyst in real-time primary and mechanically rechargeable Zn–air battery configurations was determined. The primary battery testing results revealed that Ni/PDC and Co/PDC exhibited a stable discharge voltage plateau up to 29 h. The Fe/PDC sample, on the other hand, performed up to 23 h with an operating potential of 1.20 V at the discharge current density of 5 mA cm⁻² after which the battery ceased to perform. The Ni/PDC, Co/PDC, and Fe/PDC cathode catalysts performed galvanostatic 1200 charge–discharge cycles in a mechanically rechargeable secondary Zn–air battery configuration. The results demonstrate that the Ni/PDC, Co/PDC, and Fe/PDC materials serve as excellent and durable bifunctional cathode electrocatalysts for Zn–air batteries.

New intermetallic silicide catalysts for Zn–air batteries facilitate ORR/OER kinetics and deliver peak power densities of 59 mW cm⁻² and 1200 cycles.     

Solid base catalysts derived from Ca–Al–X (X = F−, Cl− and Br−) layered double hydroxides for methanolysis of propylene carbonate

The Ca–Al and Ca–Al–X (X = F⁻, Cl⁻ and Br⁻) catalysts were prepared via thermal decomposition of Ca–Al layered double hydroxides (LDHs), and tested for methanolysis of propylene carbonate (PC) to produce dimethyl carbonate (DMC). The catalytic performance of these catalysts increased in the order of Ca–Al–Br⁻ < Ca–Al < Ca–Al–Cl⁻ < Ca–Al–F⁻, which was consistent with the strong basicity of these materials. The recyclability test results showed that the addition of Al and halogens (F⁻, Cl⁻ and Br⁻) not only stabilized the CaO but also improved the recyclability of the catalysts. Particularly, the Ca–Al–F⁻ catalyst exerted the highest stability after 10 recycles. These catalysts have an important value for the exploitation of DMC synthesis by transesterification of PC with methanol.

The CA-F⁻ catalyst modified with Al³⁺ and F⁻ was highly active and recyclable for dimethyl carbonate synthesis.     

Recyclable iron(ii) caffeine-derived ionic salt catalyst in the Diels–Alder reaction of cyclopentadiene and α,β-unsaturated N-acyl-oxazolidinones in dimethyl carbonate

Iron(ii) triflate was used in combination with caffeine-derived salts as recyclable catalysts for the Diels–Alder reaction run in dimethyl carbonate (DMC) as a green solvent. The catalyst was prepared as an ionic salt from a xanthinium salt and Fe(OTf)₂. Various substrates including α,β-unsaturated carbonyl and N-acyloxazolidinone derivatives were reacted with cyclopentadiene using this recyclable catalyst. The use of a low catalyst loading (1 mol%) afforded high yields (up to 99%) of the corresponding cycloadducts. The recycling and the efficiency of the catalyst were demonstrated for several runs.

Iron(ii) triflate was used in combination with caffeine-derived salts as recyclable catalysts for the Diels–Alder reaction run in dimethyl carbonate (DMC) as a green solvent.     

Proton–deuterium exchange of acetone catalyzed in imidazolium-based ionic liquid–D2O mixtures

The reaction of the proton–deuterium exchange of acetone in imidazolium-based ionic liquid (IL)–deuterium oxide mixtures was studied in detail via NMR spectroscopy. Certain ILs exhibit considerable catalytic properties and contribute to the course of reaction up to the complete deuteration. The efficiency of deuterium exchange crucially depends on the features of ILs; the type of anion and chain length of cation. The linear secondary isotope effects on the NMR chemical shifts of the ¹³C atoms in acetone were observed depending on the deuteration level of the molecule.

The reaction of the proton–deuterium exchange of acetone in imidazolium-based ionic liquid (IL)–deuterium oxide mixtures was studied in detail via NMR spectroscopy.     

Silica-immobilized ionic liquid Brønsted acids as highly effective heterogeneous catalysts for the isomerization of n-heptane and n-octane

Metal-free imidazolium-based ionic liquid (IL) Brønsted acids 1-methyl imidazolium hydrogen sulphate [HMIM]HSO₄ and 1-methyl benzimidazolium hydrogen sulphate [HMBIM]HSO₄ were synthesized. Their physicochemical properties were investigated using spectroscopic and thermal techniques, including UV-Vis, FT-IR, ¹H NMR, ¹³C-NMR, mass spectrometry, and TGA. The ILs were immobilized on mesoporous silica gel and characterized by FT-IR spectroscopy, scanning electron microscopy, Brunauer–Emmett–Teller analysis, ammonia temperature-programmed desorption, and thermogravimetric analysis. [HMIM]HSO₄@silica and [HMBIM]HSO₄@silica have been successfully applied as promising replacements for conventional catalysts for alkane isomerization reactions at room temperature. Isomerization of n-heptane and n-octane was achieved with both catalysts. In addition to promoting the isomerization of n-heptane and n-octane (a quintessential reaction for petroleum refineries), these immobilized catalysts are non-hazardous and save energy.

Metal-free imidazolium-based ionic liquid (IL) Brønsted acids 1-methyl imidazolium hydrogen sulphate [HMIM]HSO₄ and 1-methyl benzimidazolium hydrogen sulphate [HMBIM]HSO₄ were synthesized.     

Performance of butanol separation from ABE mixtures by pervaporation using silicone-coated ionic liquid gel membranes

This work aims at the separation of n-butanol from aqueous solutions by means of pervaporation using membranes based on gelled ionic liquids (IL). These membranes were mechanically stabilized with a double silicone coating using two polydimethylsiloxane (PDMS) films. The first step of the membrane preparation considered the formation of a gelled ionic liquid layer, which was formed using two different imidazolium-based ionic liquids: [omim][Tf₂N] and [bmim][Tf₂N], and two different phosphonium-based ionic liquids: [P_6,6,6,14][Tf₂N] and [P_6,6,6,14][DCA]. The gelation procedure was carried out on a porous paper support using a low molecular weight gelator. The membranes obtained from this method were tested in pervaporation assays to separate butanol from model ABE (Acetone–Butanol–Ethanol) fermentation solutions. These assays were done in an experimental setup especially built for this purpose. The pervaporation performance of these ionic liquid-based membranes was compared to that obtained with a single PDMS layer membrane. From these experimental results, butanol/water selectivity for [P_6,6,6,14][Tf₂N]-based membranes reached a value equal to 892, which is 150 times higher than the value obtained for a single PDMS layer membrane. Simultaneously, for the same IL, the transmembrane fluxes (kg h⁻¹ m⁻²) of butanol and water were 37% and 99.6% lower than the values obtained using a single PDMS layer membrane, respectively. The hydrophobic character of the selected ionic liquid and its relatively high values for the transport parameters can explain this experimental response.

This work aims at the separation of n-butanol from aqueous solutions by means of pervaporation using membranes based on gelled ionic liquids (IL).     

Co/N-Doped hierarchical porous carbon as an efficient oxygen electrocatalyst for rechargeable Zn–air battery

Developing efficient electrocatalysts for ORR/OER is the key issue for the large-scale application of rechargeable Zn–air batteries. The design of Co and N co-doped carbon matrices has become a promising strategy for the fabrication of bi-functional electrocatalysts. Herein, the surface-oxidized Co nanoparticles (NPs) encapsulated into N-doped hierarchically porous carbon materials (Co/NHPC) are designed as ORR/OER catalysts for rechargeable Zn–air batteries via dual-templating strategy and pyrolysis process containing Co²⁺. The fabricated electrocatalyst displays a core–shell structure with the surface-oxidized Co nanoparticles anchored on hierarchically porous carbon sheets. The carbon shells prevent Co NP cores from aggregating, ensuring excellent electrocatalytic properties for ORR with a half-wave potential of 0.82 V and a moderate OER performance. Notably, the obtained Co/NHPC as a cathode was further assembled in a zinc–air battery that delivered an open-circuit potential of 1.50 V, even superior to that of Pt/C (1.46 V vs. RHE), a low charge–discharge voltage gap, and long cycle life. All these results demonstrate that this study provides a simple, scalable, and efficient approach to fabricate cost-effective high-performance ORR/OER catalysts for rechargeable Zn–air batteries.

The surface-oxidized Co nanoparticles incorporated in N-doped hierarchically porous carbon materials are designed as ORR/OER catalyst for rechargeable Zn–air batteries via dual-templating strategy and pyrolysis process.     

Multiblock copolymers of PPC with oligomeric PBS: with low brittle–toughness transition temperature

In order to decrease the brittle–toughness transition temperature and increase the mechanical strength of poly(propylene carbonate) (PPC), a series of multiblock copolymers of poly(propylene carbonate)-multiblock-poly(butylene succinate) (PPC-mb-PBS) are designed and synthesized. ¹H-NMR, DOSY and GPC results demonstrate the successful synthesis of PPC-mb-PBSs with designed multiblock sequence. The thermal, crystalline and mechanical properties of these PPC-mb-PBSs are evaluated by DSC, TGA, POM, tensile and tearing testing. Experiment results demonstrate that crystallinity, thermal and mechanical properties of PPC-mb-PBSs can be readily modulated by changing the composition and block length of PPC and PBS moieties. It is found that all the prepared PPC-mb-PBSs are semi-crystalline polymers with a melting temperature at 93–109 °C and a T_g at around −40 °C. Both crystallization rate and crystallinity of the multiblock copolymers increase with increasing both PBS content and PBS block length. As a consequent, the tensile strength increases with increasing PBS/PPC block ratios at room and lower temperatures. In conclusion, the amorphous PBS phase in the block copolymers acts as soft segment, endowing PPC-mb-PBS copolymers with much better flexibility than PPC at low temperature of 273 K when PPC segments are frozen.

In present work, biodegradable multiblock copolymers from oligomeric PPC and PBS with low brittle–toughness transition temperature and superior mechanical properties was synthesized, making it more potential candidate as packaging materials.     

Rechargeable aluminum batteries: effects of cations in ionic liquid electrolytes

Room temperature ionic liquids (RTILs) are solvent-free liquids comprised of densely packed cations and anions. The low vapor pressure and low flammability make ILs interesting for electrolytes in batteries. In this work, a new class of ionic liquids were formed for rechargeable aluminum/graphite battery electrolytes by mixing 1-methyl-1-propylpyrrolidinium chloride (Py13Cl) with various ratios of aluminum chloride (AlCl₃) (AlCl₃/Py13Cl molar ratio = 1.4 to 1.7). Fundamental properties of the ionic liquids, including density, viscosity, conductivity, anion concentrations and electrolyte ion percent were investigated and compared with the previously investigated 1-ethyl-3-methylimidazolium chloride (EMIC-AlCl₃) ionic liquids. The results showed that the Py13Cl–AlCl₃ ionic liquid exhibited lower density, higher viscosity and lower conductivity than its EMIC-AlCl₃ counterpart. We devised a Raman scattering spectroscopy method probing ILs over a Si substrate, and by using the Si Raman scattering peak for normalization, we quantified speciation including AlCl₄⁻, Al₂Cl₇⁻, and larger AlCl₃ related species with the general formula (AlCl₃)_n in different IL electrolytes. We found that larger (AlCl₃)_n species existed only in the Py13Cl–AlCl₃ system. We propose that the larger cationic size of Py13⁺ (142 ų) versus EMI⁺ (118 ų) dictated the differences in the chemical and physical properties of the two ionic liquids. Both ionic liquids were used as electrolytes for aluminum–graphite batteries, with the performances of batteries compared. The chloroaluminate anion-graphite charging capacity and cycling stability of the two batteries were similar. The Py13Cl–AlCl₃ based battery showed a slightly larger overpotential than EMIC-AlCl₃, leading to lower energy efficiency resulting from higher viscosity and lower conductivity. The results here provide fundamental insights into ionic liquid electrolyte design for optimal battery performance.

Room temperature ionic liquids (RTILs) are solvent-free liquids comprised of densely packed cations and anions. Properties of Py13Cl–AlCl₃ ILs were studied and compared with EMIC-AlCl₃ ILs for use as electrolyte in Al–graphite battery.     

Combining amino acids and carbohydrates into readily biodegradable,task specific ionic liquids

The growing interest in the application of ionic liquids (ILs) with simultaneous sustainability draws attention to their environmental impact in general and their biodegradability in particular. Considering this, we designed a series of novel bio-ionic liquids based on natural, abundant compounds: a carbohydrate [Carb], as the cation, and amino acids [AA], as the anions; these ILs can serve as viable alternatives to the well-known and utile cholinium AAILs. Several [Carb][AA] ILs were characterized by ¹H and ¹³C NMR, mass spectrometry, thermogravimetry (TGA) and differential scanning calorimetry (DSC). The biodegradability properties of the [Carb][AA] ILs were elucidated as well and showed biodegradation readily occurred, decomposing within 5–6 days. These novel materials were successfully utilized as catalysts for the Knoevenagel condensation reaction, where conversion values of 67–94% were achieved under exceptionally mild conditions using water as the solvent and reaction times as short as 15 minutes. These sugar based ILs were easily separated and recycled.

Combining amino acids and carbohydrates yields readily biodegradable ionic liquids with a hydrogen-bond-rich structure.     

Amino acid ionic liquids as catalysts in a solvent-free Morita–Baylis–Hillman reaction

In the present work, we describe the preparation of ten amino acid ionic liquids (AAILs) formed from ammonium salts as cations, derivatives of glycerol, and natural amino acids as anions. All of them are viscous oils, colorless or pale yellow, and hygroscopic at room temperature. They have appreciable solubility in many protic and aprotic polar solvents. The AAILs were used as catalysts in a Morita–Baylis–Hillman (MBH) reaction. The ionic liquids derivative from l-proline and l-histidine demonstrated the ability to catalyze the reaction between methyl vinyl ketone and aromatic aldehydes differently substituted in the absence of an additional co-catalyst under organic solvent-free conditions. The AAIL derivatives from l-valine, l-leucine, and l-tyrosine catalyzed the MBH reaction only in the presence of imidazole. The MBH adducts were obtained in moderate to good yields. Although the catalytic site in the ILs was in its enantiomerically pure form, all the MBH adducts were obtained in their racemic form.

In this work, we describe the preparation of ten amino acid ionic liquids (AAILs). The AAILs were used as catalysts in a Morita–Baylis–Hillman (MBH) reaction. The MBH adducts were obtained from moderate to good yields and in their racemic form.     

(La0.65Sr0.3)0.95FeO3−δ perovskite with high oxygen vacancy as efficient bifunctional electrocatalysts for Zn–air batteries

Developing low-cost, highly efficient electrocatalysts for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is desirable for rechargeable metal–air batteries. Herein, a series of perovskite structured (La_0.65Sr_0.3)_0.95FeO_3−δ catalysts with A-site deficiency were synthesized through a scalable solid state synthesis method at different calcination temperatures. The electrocatalytic activities of these catalysts were investigated by thin-film RDE technique. The catalyst calcined at 1000 °C exhibits an outstanding bi-functional activity towards the ORR and OER in alkaline electrolyte, and it also exhibits an outstanding performance in primary and rechargeable Zn–air batteries, which is comparable with the commercial noble metals Pt/C and RuO₂.

Developing low-cost, highly efficient electrocatalysts for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is desirable for rechargeable metal–air batteries.

Metal–air batteries have attracted great attention because of their high energy and power densities.^1–3 Of all the metal–air batteries, the Zn–air battery is particularly attractive and considered to be an alternative battery owing to its long cycle life, low cost and the global abundance of zinc.^4–7 Furthermore, the specific energy of the Zn–air battery is 1084 W h kg⁻¹, which is quite attractive for applications. However, to achieve a Zn–air battery with high performance and long cycle life, an efficient and highly stable bifunctional air electrode is essential, because the sluggish kinetics of the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) during discharge and charge have limited the practical application of Zn–air batteries.^8,9 To solve this problem, a variety of highly active ORR and OER electrocatalysts have been investigated, including noble metals,^10,11 carbon materials,^12–14 transition metal selenide,¹⁵ and transition metal oxides.^16–18 Noble metals exhibit high performance, but their low abundance and high cost prohibit their particle use. The carbon materials possess larger surface areas and high conductivities, but poor durability.Transition metal oxides, especially the perovskite structured oxides have been intensively investigated as bifunctional electrocatalysts in Zn–air batteries, owing to their low-cost, abundant varieties, structural stability and great potentials.^19–21 As far as we know, in perovskite catalysts, the A-site deficiency and substitute A-site metal with low valence metals can manipulate the B-site element valence, and which has a critical influence on the catalytic behaviours of the materials. Herein, a series of perovskite structured catalysts are designed with same A-site stoichiometry and same content of doping metals, but calcined at different temperatures. The relationship between the preparation temperature and the oxidation state of B-site metal is investigated; and it obviously affect their catalytic activities towards ORR and OER in alkaline medium.^22–24 The selected catalyst can be used as bifunctional electrocatalyst and is comparable with commercial noble metals.The LSF catalysts prepared at different temperature are characterized by X-ray diffraction (XRD), and the results are shown in Fig. 1(a). Compared with the standard PDF pattern (PDF: 01-089-1269), all the characteristic peaks can be well indexed as a perovskite phase with an orthorhombic structure. But the sample prepared at 900 °C exhibits two small peaks around 30° correspond to the secondary phase of La₂O₃. To avoid the effect of La₂O₃ impurity phase, the sample calcined at 900 °C is not selected for further investigated. To confirm the chemical compositions of the LSF0.95 catalysts, inductively coupled plasma with optical emission spectroscopy (ICP-OES) is used and the results are shown in Table S1.^† As shown in Table S1,^† the chemical compositions of the LSF0.95 catalysts agree well with the designed compositions. SEM images in Fig. 1(d) show the microstructure and the surface morphology of LSF perovskite catalysts calcined at different temperatures. The low magnification SEM images reveal that the LSF particles are homogeneously distributed for all LSF catalysts. The high magnification SEM images and TEM image (Fig. 1(f)) clearly exhibit agglomeration of the particles with irregularly shapes, and with increasing of the calcination temperature from 1000 °C to 1200 °C, a gradual increase of the particle size can be observed, and the surface of the agglomerated particles becomes smoother. The particle size for the LSF0.95-1000 °C, LSF0.95-1100 °C and LSF0.95-1200 °C is ∼0.8–2.5 μm, ∼0.8–3 μm, and ∼1–3.4 μm, respectively. These values are close to the results of particle size distribution (PSD) shown in Fig. S1.^† Energy dispersive X-ray (EDX) mapping in Fig. 1(e) represent that La, Sr, Fe and O are homogeneous distributed in the LSF0.95 catalysts. The high-resolution transmission electron microscopy (HRTEM) image in Fig. 1(g) and corresponding SAED pattern in Fig. 1(h) clearly exhibit the 020 crystal phase of LSF0.95-1000 °C. The specific surface areas of LSF0.95 catalysts are determined from the nitrogen adsorption/desorption isotherms. As shown in Fig. S2,^† the adsorption and desorption isotherms are almost coincided with each other, indicating no mesopores in these LSF catalysts. The specific surface area of LSF catalysts are calculated by BET model (Table S2^†), the values are 1.441 m² g⁻¹ (LSF0.95-1000 °C), 1.070 m² g⁻¹ (LSF0.95-1100 °C) and 0.816 m² g⁻¹ (LSF0.95-1200 °C), respectively. These values represent that the higher calcination temperature leads to the smaller specific surface area. Because the particles are easier to agglomerate at higher calcination temperature, resulting in the bigger particle size, which has been discussed in SEM and PSD. The surface element compositions of the LSF catalysts are analyzed by X-ray photoelectron spectroscopy (XPS). The survey XPS spectrum in Fig. S3(a)^† confirms the existence of La, Sr, Fe and O in the LSF0.95 catalysts. In Fig. S3(b),^† the peaks centered at 710.3 eV and 723.5 eV are corresponding to the binding energy of Fe 2p_3/2 and Fe 2p_1/2, respectively;^25,26 and the weak Fe-2p satellite peak indicates the multiple oxidation states of Fe. High-resolution XPS spectra of Fe 2p_3/2 and O 1s are analyzed in Fig. 1 and the chemical compositions are summarized in Table S4.^† Fe 2p_3/2 in Fig. 1(b) reveal that the oxidation state of Fe in LSF catalysts are mixed by Fe³⁺ (709–711 eV) and Fe⁴⁺ (711–713.6 eV), and the content of Fe⁴⁺ increases as the temperature increasing from 1000 °C to 1200 °C. The peaks at 528.5 eV, 529.9 eV and 531.5 eV (Fig. 1(c)) in O 1s are corresponding to the crystal lattice oxygen species, adsorbed oxygen, and adsorbed water, respectively.^27–30Open in a separate windowFig. 1XRD patterns (a) and XPS spectra of Fe 2p species (b) and O 1s species (c) of the LSF0.95-1000 °C, LSF0.95-1100 °C and LSF0.95-1200 °C. SEM images (d) and EDX maps (e) of the LSF0.95 catalysts. (f) TEM image, (g) TRTEM image and (h) SAED pattern of LSF0.95-1000 °C.Electrocatalytic activities of the LSF0.95 catalysts towards ORR are investigated by thin-film RDE technique. Fig. S4^† exhibits the comparison of cyclic voltammetry (CV) curves obtained in O₂ and N₂ saturated KOH. Two obvious cathodic peaks centered at 0.34 V and −0.02 V (vs. RHE) only can be observed in CV curve tested in O₂ saturated KOH, indicating the electrocatalytic activity of LSF catalyst towards ORR in KOH. LSV curves of the LSF catalysts with different preparation temperatures measured at 1600 rpm are compared in Fig. 2(a), and all the LSV curves possess two ORR limiting current plateaus, which are in accordance with the two reduction peaks in the CV curves. This result indicates that there are two potential ranges for ORR catalytic by the LSF catalysts, the first one is from 0.8 V to 0.37 V and the second one is 0.37–0.4 V. In the first ORR range, the onset potential of LSF0.95-1000 °C is 0.69 V, which is 26 mV positive than that of LSF0.95-1100 °C (0.67 V), and 44 mV positive than that of LSF0.95-1200 °C (0.65 V), indicating a higher ORR catalytic activity of LSF0.954-1000 °C. The half wave potential values of the LSF0.95 catalysts are 0.57 V (LSF0.95-1000 °C), 0.56 V (LSF0.951100 °C) and 0.55 V (LSF0.95-1200 °C), respectively. The positive half-wave potential of LSF0.95-1000 °C further implies the better ORR activity in the first ORR range. The onset potential and half-wave potential are also compared in the second ORR range. LSF0.95-1000 °C exhibits an onset potential of 0.36 V, and a half-wave potential of 0.17 V, which are larger than 0.34 V and 0.16 V of LSF0.95-1100 °C, and 0.33 V and 0.15 V of LSF0.95-1200 °C; suggesting a better ORR activity of LSF0.95-1000 °C in the second oxygen reduction potential range. Tafel plots of the LSF0.95 catalysts in two ORR ranges are compared in Fig. 2(b) and (c). In both ORR potential ranges, the smaller Tafel slops of LSF0.95-1000 °C (74.1 mV dec⁻¹, 65.5 mV dec⁻¹) indicates more favorable ORR kinetics relative to those of LSF0.95-1100 °C (78.4 mV dec⁻¹, 98.7 mV dec⁻¹) and LSF0.95-1200 °C (87.6 mV dec⁻¹, 96.6 mV dec⁻¹). Fig. 2(d) exhibits the electrochemical impedance spectroscopy (EIS) of the LSF0.95 catalysts. Compared with the other LSF0.95 catalysts, the semicircle diameter of LSF0.95-1000 °C is smaller, revealing a lower electronic resistance and faster electron transfer.Open in a separate windowFig. 2(a) LSV curves of LSF0.95 catalysts for the ORR at 1600 rpm in O₂ saturated 0.1 M KOH with a scan rate of 10 mV s⁻¹. (b) and (c) Tafel plots of LSF0.95 catalysts for two oxygen reduction ranges. (d) EIS of LSF0.95 catalysts for the ORR at 1600 rpm in O₂ saturated 0.1 M KOH. (e) LSV curves of LSF0.95 catalysts for the OER at 1600 rpm. (f) Tafel plots of LSF0.95 catalysts for the OER.To investigate the ORR mechanisms in these two ORR ranges, electron transfer numbers in these two ORR potential ranges are calculated from the Koutecky-Levich (K-L) plots (Fig. S5^†). In the first ORR potential range, the average electron transfer number is ∼2.0, indicating an indirect 2 × 2e⁻ ORR process, and the reaction steps are shown as follow:³¹O₂ + H₂O +2e⁻ → HO₂⁻ + OH⁻1HO₂⁻ + H₂O + 2e⁻ → 3OH⁻22HO₂⁻ → 2OH⁻ + O₂3The adsorbed O₂ is reduced to the intermediate HO₂⁻ by accepting 2e⁻ in step (1). However, the formed intermediate HO₂⁻ is not stable, it can be further reduced to the final produce OH⁻ by accepting another 2e⁻ (2), and it also can be chemical decomposed to O₂ again (3). The electron transfer number of LSF0.95 catalysts is ∼3.8 in the second ORR potential range, implying a direct 4e⁻ dominant ORR process. In this ORR process, the adsorbed O₂ can be directly reduced to the OH⁻ without intermediate HO₂⁻ generation, which is shown in eqn (4):O₂ + H₂O + 4e⁻ → 4OH⁻4The electrocatalytic activities of LSF0.95 catalysts for OER are also investigated by RDE in N₂-saturated 0.1 M KOH. Fig. 2(e) shows the comparison of OER LSV curves and all of them exhibit the similar features. As shown in Fig. 2(e), as the preparation temperature increases from 1000 °C to 1200 °C, the potential values at 6 mA cm⁻² increase from 1.74 V to 1.77 V and 1.79 V; while the current densities at 1.65 V decrease gradually from 2.65 mA cm⁻² to 1.95 mA cm⁻² and 2.46 mA cm⁻². These analyses indicate a higher electrocatalytic activity and a better conductivity of LSF0.95-1000 °C than the other two catalysts towards OER in 0.1 M KOH. To gain deep insight into the OER catalytic kinetics, Tafel plots are obtained from LSV curves. As shown in Fig. 2(f), the Tafel slop of LSF0.95-1000 °C is 71.9 mV dec⁻¹, and it is obviously smaller than 85.7 mV dec⁻¹ of LSF0.95-1100 °C and 102.1 mV dec⁻¹ of LSF0.95-1200 °C, indicating a faster OER catalytic kinetics of the LSF0.95-1000 °C. EIS of the LSF0.95 catalysts are measured at 0.8 V in 0.1 M KOH and compared in Fig. S6.^† The diameter of EIS semicircle increases gradually from LSF0.95-1000 °C to LSF0.95-1200 °C, which reveals a fastest charge transfer resistance of LSF0.95-1000 °C for the OER.All the analysis above proves the best electrocatalytic activity of LSF0.95-1000 °C towards both ORR and OER in 0.1 M KOH. To investigate the influence of BET surface areas on electrocatalytic activities of LSF0.95 catalysts towards ORR and OER, LSV curves of ORR and OER per BET surface areas are compared in Fig. S7 and S8.^† The LSV curves per BET surface areas exhibit the similar features with those per geometric surface areas, but the distances between LSV curves are smaller. LSF0.95-1000 °C still exhibits the best catalytic activity towards ORR and OER among three catalysts. These results imply that the BET surface areas can affect the electrocatalytic activities of the LSF0.95, but not the determined factor. The electrochemical active surface areas of LSF0.95 are obtained by measuring the electrochemical capacitance of the electrode–electrolyte interface in the non-faradaic region of cyclic voltammetry. As shown in Fig. S9,^† the average value of the capacitance for LSF0.95-1000 °C, LSF0.95-1100 °C and LSF0.95-1200 °C is 0.0184 mF, 0.0132 mF and 0.0116 mF respectively. LSF0.95-1000 °C exhibits the largest reaction interface, which causes a higher ORR and OER catalytic activity. Another important reason for the outstanding catalytic activity of LSF0.95-1000 °C is the content of active site. The reaction active site in perovskite catalyst is reported to be M³⁺,^31–33 and the higher content of M³⁺ means the better electrocatalytic activity. As discussed in XPS, LSF0.95-1000 °C possesses a larger content of Fe³⁺ than the other LSF0.95 catalysts, which can provide more reaction active sites for ORR and OER. Moreover, the content of oxygen vacancy in LSF0.95 catalysts also affects the electrocatalytic activities by influencing their conductivity and the lattice oxygen mobility.^27,34–36 As shown in Table S4,^† with the increasing of the preparation temperature, the content of the oxygen vacancy decreases, which also explains the better electrocatalytic activity of LSF0.95-1000 °C.By comparing the LSV curves and Tafel plots of LSF0.95-1000 °C/C (20 wt% Vulcan XC-72 carbon) with commercial 20 wt% Pt/C and RuO₂/C (20 wt% Vulcan XC-72 carbon) (Fig. S10^†), the electrocatalytic activity of LSF0.95/C for the ORR is proved to be better than RuO₂/C, but worse than 20 wt% Pt/C; while the OER activity is better than 20 wt% Pt/C, but worse than RuO₂/C. Chronoamperometric (CA) measurements are used to investigate the electrochemical stability of LSF0.95/C for the ORR and OER, and the results are shown in Fig. S11.^† After 30 hours test at 0.18 V for the ORR, the current density of LSF0.95/C keeps 99% with a negligible decrease, which is much better than that of 20 wt% Pt/C (93%). For the OER at 1.63 V, the current densities of LSF0.95/C almost keep 85% after 40 hours, and it is much larger than 64% of RuO₂/C. These results reveal an excellent electrochemical durability of LSF0.95/C. The XRD, XPS and HRTEM results of LSF0.95 in Fig. S12 and S13^† after durability measurements further identify the excellent stability. The equation ΔE = E_OER − E_ORR was used to calculate the overpotential between the OER at the current density of 10 mA cm⁻² and the ORR at the current density of −1 mA cm⁻², which is used to access the bifunctionality of the catalysts.^37,38 The overpotential values of LSF0.95/C are compared with the reported ones in Table S5.^† The ΔE values for LSF0.95/C, 20 wt% Pt/C and RuO₂/C are 1.04 V, 1.10 V and 1.12 V, respectively. Obviously, the ΔE of LSF0.95/C is smaller than commercial noble-metal based Pt/C, RuO₂/C and some other reported excellent perovskite-based bifunctional catalysts. The results further demonstrate the high bifunctional electrocatalytic activity of LSF0.95 in alkaline electrolyte.The superior bifunctional ORR and OER activity and durability of LSF0.95/C motivated us to investigate its performance in realistic primary and rechargeable Zn–air batteries. Fig. 3(b) presents discharge polarization and power density curves for Zn–air batteries based on LSF0.95-1000 °C and commercial 20 wt% Pt/C catalysts. When the current density < 70 mA cm⁻², the voltage and power density of the battery based on perovskite LSF0.95-1000 °C are very close to those of the commercial Pt/C. But when the current density > 70 mA cm⁻², LSF0.95-1000 °C exhibits a higher voltage and power density compared with commercial Pt/C, and the peak power density could be as high as 94 mW cm⁻² at 0.63 V, which is superior to the commercial Pt/C. The galvanostatic discharge curves shown in Fig. 3(c) clearly exhibit that the voltage of LSF0.95-1000 °C cathode (∼1.23 V) is similar to that of Pt/C cathode (∼1.24 V) in primary Zn–air battery at a current density of 10 mA cm⁻², after 10 h discharge, no obvious voltage drop was observed until all of the Zn metal was consumed. The specific discharge capacities at different current density are shown in Fig. 3(d), at 10 mA cm⁻², the specific capacity of LSF0.95-1000 °C based battery normalized to the mass of consumed Zn is 755 mA h g_Zn⁻¹, corresponding to a gravimetric energy density of 928 W h kg⁻¹. At 30 mA cm⁻², the specific capacity normalized to the mass of consumed Zn is ∼732 mA h g_Zn⁻¹, corresponding to a high gravimetric energy density >768 mA h g_Zn⁻¹. The rechargeable Zn–air batteries with short cycles (10 min per cycle) are performed to explore the bi-functional properties of LSF0.95-1000 °C. The batteries are running in the air at room temperature, no further oxygen or air is bubbled into the system. At 10 mA cm⁻², the charge–discharge voltage gap for LSF0.95-1000 °C is ∼0.74 V, which is very close to voltage gap between the commercial 20 wt% Pt/C and RuO₂/C (∼0.66 V), and it can keep this value after 10 h running (Fig. S14^†). This result of rechargeable Zn–air batteries also implies a superior bifunctional electrocatalytic activity of the LSF0.95 for the ORR and OER.Open in a separate windowFig. 3(a) Image of the assembled Zn–air battery. (b) Polarization curves and corresponding power density plot of the primary Zn–air batteries using LSF0.95-1000 °C and 20% Pt/C based cathodes in 6 M KOH. (c) The galvanostatic discharge curves of the primary Zn–air batteries using LSF0.95-1000 °C and 20% Pt/C based cathodes at the current density of 10 mA cm⁻². (d) Specific capacities of the primary Zn–air batteries using LSF0.95-1000 °C based cathodes at the current density of 10 mA cm⁻² and 30 mA cm⁻².     

Understanding the active sites of Fe–N–C materials and their properties in the ORR catalysis system

Metal–N–C-based catalysts prepared by pyrolysis are frequently used in the oxygen reduction reaction (ORR). Zeolitic imidazolate frameworks (ZIFs), a type of metal organic framework (MOF), are selected as precursors due to their special structure and proper pore sizes. A series of Fe–N–C catalysts with different concentrations of 2-methylimidazole were prepared with a simple solvothermal-pyrolysis method, and the transformation productivity, morphology and ORR activity were investigated. It was found that the Fe–N–C catalyst with a 2-methylimidazole concentration of 0.53 mol L⁻¹ had the best performance. In 0.1 M KOH solution, the half-wave potential was 0.852 V (vs. RHE), with the highest electrochemically active surface area (ECSA) of 94.1 cm², and the ORR reaction was dominated by a 4-electron process. The current only decreased by 10.5% after 50 000 s of chronoamperometry (CA), while the half-wave potential only decreased 20 mV in 3 M methanol. Additionally, this catalyst cannot be poisoned by Cl⁻ and SO₃²⁻ ions in the ORR process. Finally, some typical ions including SCN⁻, Fe(CN)₆³⁻ and Fe(CN)₆⁴⁻ were used to inhibit the active sites, and it was determined that Fe(ii) is the real active species. The series of synthesis and testing experiments has significance in guiding optimization of the synthesis conditions and analysis of the mechanism of active sites in Fe–N–C materials.

Metal–N–C-based catalysts prepared by pyrolysis are frequently used in the oxygen reduction reaction (ORR).     

Hot electron transfer in Zn–Ag–In–Te nanocrystal–methyl viologen complexes enhanced with higher-energy photon excitation

The dynamics of hot electron transfer from Zn–Ag–In–Te (ZAITe) nanocrystals (NCs) to adsorbed methyl viologen (MV²⁺) were investigated by transient absorption spectroscopy. The bleaching of the exciton peak in the ZAITe NC–MV²⁺ complexes evolved faster than that of ZAITe NCs. The hot electron transfer efficiency increased from 45% to 72% with increasing excitation photon energy.

Zn–Ag–In–Te nanocrystals exhibited hot electron transfer to adsorbed methyl viologen, the efficiency being enhanced from 45% to 72% with an increase in the excitation photon energy.     

The application of amino acid ionic liquids as additives in the ultrasound-assisted extraction of plant material

The aim of the present study was to determine the antioxidant activity of the aqueous extracts from Lycopodium clavatum, Cetraria islandica and Dipsacus fullonum obtained using aqueous solutions of ionic liquids by the ultrasound-assisted extraction (IL-UAE) method. Triethanolammonium salts [TEAH]⁺[AA]⁻ of four amino acids of different hydrophobicity – isoleucine – Ile, methionine – Met, threonine – Thr and arginine – Arg, were chosen as ionic liquids, because they are based on natural, bio-renewable raw materials, such as amino acids and contain a pharmaceutically and cosmetically acceptable counterion of triethanolamine. Triethanolammonium salts were synthesized, identified by spectroscopic methods (NMR and FT-IR) and characterized by thermal methods (DSC and TGA). The 2.5% w/v aqueous solutions of triethanolammonium amino acid salts were used as the solvents in combination with ultrasound assisted extraction (UAE). The estimation of antioxidant properties was carried out using the DPPH, FRAP and CUPRAC assays. Total polyphenol content was measured using the reagent Folin–Ciocalteu. The results showed that the use of [TEAH]⁺[Thr]⁻ or [TEAH]⁺[Met]⁻ aqueous solutions increased the antioxidant activity of extracts in comparison to that achieved for extracts with pure water. The use of [TEAH]⁺[Thr]⁻ as an additive for ultrasound-assisted extraction was characterized by obtaining plant extracts with the highest antioxidant potential, even 2.4-fold. The use of the AAIL-UAE method allowed obtaining higher amounts of polyphenols compared to pure water extracts, even 5.5-fold. The used method allowed the extraction of thermosensitive natural compounds, shortened the extraction time and lowered energy consumption.

The antioxidant activity of the aqueous extracts from Lycopodium clavatum, Cetraria islandica and Dipsacus fullonum obtained by ionic liquids and ultrasound-assisted extraction (IL-UAE) method was determined.     

Rational design of M–N4–Gr/V2C heterostructures as highly active ORR catalysts: a density functional theory study

Inspired by the composites of N-doped graphene and transition metal-based materials as well as MXene-based materials, heterostructures (M–N₄–Gr/V₂C) of eight different transition metals (M = Ti, Cr, Mn, Fe, Co, Ni, Cu, and Zn) doped with nitrogen-coordinated graphene and V₂C as potential catalysts for the oxygen reduction reaction (ORR) using density functional theory (DFT) were designed and are described herein. The calculations showed that the heterostructure catalysts (except for Zn–N₄–Gr/V₂C) were thermodynamically stable. Ni–N₄–Gr/V₂C and Co–N₄–Gr/V₂C showed higher activities towards the ORR, with overpotentials as low as 0.32 and 0.45 V, respectively. Excellent catalytic performance results were observed from the change in electronic structure caused by the strong interaction between V₂C and the graphene layers as well as the synergistic effect between the MN₄ groups and the graphene layers. This study further provides insights into the practical application of ORR catalysts for MXene systems through the modulation of the electronic structure of two-dimensional materials.

Heterostructures (M-N₄-Gr/V₂C) of eight different transition metals (M = Ti, Cr, Mn, Fe, Co, Ni, Cu and Zn) were designed as potential catalysts for oxygen reduction reactions (ORR).     

Excess chemical potential of thiophene in [C4MIM] [BF4, Cl,Br, CH3COO] ionic liquids,determined by molecular simulations

The excess chemical potential of thiophene in imidazolium-based ionic liquids [C₄mim][BF₄], [C₄mim][Cl], [C₄mim][Br], and [C₄mim][CH₃COO] were determined by means of molecular dynamics in conjunction with free energy perturbation techniques employing non-polarizable force fields at 300 K and 343.15 K. In addition, energetic and structural analysis were performed such as: interaction energies, averaged noncovalent interactions, radial, and combined distribution functions. The results from this work revealed that the ionic liquids (ILs) presenting the most favorable excess chemical potentials ([C₄mim][BF₄], [C₄mim][CH₃COO]) are associated with the strongest energetic interaction between the thiophene molecule and the ionic liquid anion, and with the weakest energetic interaction between the thiophene molecule and the ionic liquid cation.

Excess chemical potential of thiophene in imidazolium-based ionic liquids [C₄mim][BF₄], [C₄mim][Cl], [C₄mim][Br], and [C₄mim][CH₃COO] determined by molecular simulations.

In order to consider polarizability effects, not included in the classical forcefields, ab initio molecular dynamics (AIMD) were carried to elucidate a more representative molecular environment. The radial distribution functions (RDF) obtained from the AIMD indicated that the thiophene molecule finds the IL anions at closer distances than the imidazolium ring cation; also, the ionic liquids [C₄mim][BF₄] and [C₄mim][CH₃COO] presented more defined RDF peaks for the sulfur atom paired with hydrogen atoms within the imidazolium ring, in comparison with the thiophene–anion pair distributions, and the inverse RDF phenomena were observed in the other two ILs. Furthermore, the combined distribution functions signaled a series of interactions between thiophene and IL cation, including π–π thiophene–cation stacking (face to face, offset and edge to face), thiophene-alkyl chain interactions and hydrogen bonding between thiophene and the IL anion.The averaged noncovalent interactions determined from ab initio molecular dynamic trajectories showed that most of the interactions between the thiophene and IL ions are not strong; nevertheless, these interactions, according to the thermal fluctuation index, are stable throughout the entire simulation time.     

One-pot synthesis of symmetric imidazolium ionic liquids N,N-disubstituted with long alkyl chains

The Debus–Radziszewski imidazole synthesis was adapted to directly yield long-chain imidazolium ionic liquids. Imidazolium acetate ionic liquids with side-chains up to sixteen carbon atoms were synthesised in excellent yields via an on-water, one-pot reaction. The imidazolium acetate ILs acted as surfactants when dissolved in various solvents. The imidazolium acetate ionic liquids were also derivatised via an acid metathesis to the chloride, nitrate, and hydrogen oxalate derivatives. The thermal behaviour of all the ionic liquids was determined via thermogravimetric and calorimetric analysis.

The modified Debus–Radziszewski reaction was used as a one-pot on-water reaction to allow a greener synthesis of long-chain 1,3-dialkylimidazolium acetate ionic liquids in high yield from long-chain linear amines.     

Novel polymeric acidic ionic liquids as green catalysts for the preparation of polyoxymethylene dimethyl ethers from the acetalation of methylal with trioxane

A series of micro–mesoporous polymeric acidic ionic liquids (PAILs) have been successfully synthesized and subsequently characterized using Fourier transform-infrared spectroscopy, N₂ adsorption–desorption isotherms, scanning electron microscopy and thermogravimetry. Furthermore, the catalytic performance of the synthesized PAILs was investigated for the acetalation of methylal (DMM₁) with 1,3,5-trioxane (TOX), micro–mesoporous PAILs copolymerized by divinylbenzene with cations and anions exhibited moderate to excellent catalytic activities for the acetalation. In particular, VIMBs–AMPs–DVB, with higher specific surface area (25.51 m² g⁻¹) and total pore volume (0.15 cm³ g⁻¹) displayed an elevated conversion of formaldehyde (82.2%) and selectivity for polyoxymethylene dimethyl ethers (CH₃O(CH₂O)_nCH₃; PODE_n or DMM_n) n = 3–8 (52.6%) at 130 °C, 3.0 MPa for 8 h. Moreover, the influence of various reaction parameters was investigated by employing VIMBs–AMPs–DVB as the catalyst and it demonstrated high thermal stability and easy recovery.

Polyoxymethylene dimethyl ethers were successfully synthesized from acetalation under the catalysis of novel polymeric acidic ionic liquids (PAILs). PAILs copolymerized by divinylbenzene with ILs displayed exceptional catalytic efficiencies.