Structure and transport properties of triple-conducting BaxSr1−xTi1−yFeyO3−δ oxides

In this work, Ba_xSr_1−xTi_1−yFe_yO_3−δ perovskite-based mixed conducting ceramics (for x = 0, 0.2, 0.5 and y = 0.1, 0.8) were synthesized and studied. The structural analysis based on the X-ray diffraction results showed significant changes in the unit cell volume and Fe(Ti)–O distance as a function of Ba content. The morphology of the synthesized samples studied by means of scanning electron microscopy has shown different microstructures for different contents of barium and iron. Electrochemical impedance spectroscopy studies of transport properties in a wide temperature range in the dry- and wet air confirmed the influence of barium cations on charge transport in the studied samples. The total conductivity values were in the range of 10⁻³ to 10⁰ S cm⁻¹ at 600 °C. Depending on the barium and iron content, the observed change of conductivity either increases or decreases in humidified air. Thermogravimetric measurements have shown the existence of proton defects in some of the analysed materials. The highest observed molar proton concentration, equal to 5.0 × 10⁻² mol mol⁻¹ at 300 °C, was obtained for Ba_0.2Sr_0.8Ti_0.9Fe_0.1O_2.95. The relations between the structure, morphology and electrical conductivity were discussed.

Ba_xSr_1−xTi_1−yFe_yO_3−δ-based perovskite materials with different barium and iron contents are reported as triple conducting oxides (TCOs), which may conduct three charge carriers: oxygen ions, protons and electrons/holes.     

A simple synthesis of transparent and highly conducting p-type CuxAl1−xSy nanocomposite thin films as the hole transporting layer for organic solar cells

Inorganic p-type films with high mobility are very important for opto-electronic applications. It is very difficult to synthesize p-type films with a wider, tunable band gap energy and suitable band energy levels. In this research, p-type copper aluminum sulfide (Cu_xAl_1−xS_y) films with tunable optical band gap, carrier density, hole mobility and conductivity were first synthesized using a simple, low cost and low temperature chemical bath deposition method. These in situ fabricated Cu_xAl_1−xS_y films were deposited at 60 °C using an aqueous solution of copper(ii) chloride dihydrate (CuCl₂·2H₂O), aluminium nitrate nonohydrate [Al(NO₃)₃·9H₂O], thiourea [(NH₂)₂CS], and ammonium hydroxide, with citric acid as the complexing agent. Upon varying the ratio of the precursor, the band gap of the Cu_xAl_1−xS_y films can be tuned from 2.63 eV to 4.01 eV. The highest hole mobility obtained was 1.52 cm² V⁻¹ s⁻¹ and the best conductivity obtained was 546 S cm⁻¹. The Cu_xAl_1−xS_y films were used as a hole transporting layer (HTL) in organic solar cells (OSCs), and a good performance of the OSCs was demonstrated using the Cu_xAl_1−xS_y films as the HTL. These results demonstrate the remarkable potential of Cu_xAl_1−xS_y as hole transport material for opto-electronic devices.

Inorganic p-type films with high mobility are very important for opto-electronic applications.     

Gd3+ and Bi3+ co-substituted cubic zirconia; (Zr1−x−yGdxBiyO2−δ): a novel high κ relaxor dielectric and superior oxide-ion conductor

Solid oxide fuel cells (SOFCs) offer several advantages over lower temperature polymeric membrane fuels cells (PMFCs) due to their multiple fuel flexibility and requirement of low purity hydrogen. In order to decrease the operating temperature of SOFCs and to overcome the high operating cost and materials degradation challenges, the Cubic phase of ZrO₂ was stabilized with simultaneous substitution of Bi and Gd and the effect of co-doping on the oxide-ion conductivity of Zr_1−x−yBi_xGd_yO_2−δ was studied to develop a superior electrolyte separator for SOFCs. Up to 30% Gd and 20% Bi were simultaneously substituted in the cubic ZrO₂ lattice (Zr_1−x−yGd_xBi_yO_2−δ, x + y ≤ 0.4, x ≤ 0.3 and y ≤ 0.2) by employing a solution combustion method followed by multiple calcinations at 900 °C. Phase purity and composition of the material is confirmed by powder XRD and EDX measurements. The formation of an oxygen vacant Gd/Bi co-doped cubic zirconia lattice was also confirmed by Raman spectroscopy study. With the incorporation of Bi³⁺ and Gd³⁺ ions, the cubic Zr_1−x−yBi_xGd_yO_2−δ phase showed relaxor type high κ dielectric behaviour (ε′ = 9725 at 600 °C at applied frequency 20 kHz for Zr_0.6Bi_0.2Gd_0.2O_1.8) with T_m approaching 600 °C. The high polarizability of the Bi³⁺ ion coupled with synergistic interaction of Bi and Gd in the host ZrO₂ lattice seems to create the more labile oxide ion vacancies that enable superior oxide-ion transport resulting in high oxide ion conductivity (σ_o > 10⁻² S cm⁻¹, T > 500 °C for Zr_0.6Bi_0.2Gd_0.2O_1.8) at relatively lower temperatures.

The high polarizability of the Bi³⁺ ion coupled with synergistic interaction of Bi and Gd in the host ZrO₂ lattice seems to create the more labile oxide ion vacancies that enable high oxide ion conductivity at lower temperatures.     

The rapid microwave-assisted hydrothermal synthesis of NASICON-structured Na3V2O2x(PO4)2F3−2x (0 < x ≤ 1) cathode materials for Na-ion batteries

NASICON-structured Na₃V₂O_2x(PO₄)₂F_3−2x (0 < x ≤ 1) solid solutions have been prepared using a microwave-assisted hydrothermal (MW-HT) technique. Well-crystallized phases were obtained for x = 1 and 0.4 by reacting V₂O₅, NH₄H₂PO₄, and NaF precursors at temperatures as low as 180–200 °C for less than 15 min. Various available and inexpensive reducing agents were used to control the vanadium oxidation state and final product morphology. The vanadium oxidation state and O/F ratios were assessed using electron energy loss spectroscopy and infrared spectroscopy. According to electron diffraction and powder X-ray diffraction, the Na₃V₂O_2x(PO₄)₂F_3−2x solid solutions crystallized in a metastable disordered I4/mmm structure (a = 6.38643(4) Å, c = 10.62375(8) Å for Na₃V₂O₂(PO₄)₂F and a = 6.39455(5) Å, c = 10.6988(2) Å for Na₃V₂O_0.8(PO₄)₂F_2.2). With respect to electrochemical Na⁺ (de)insertion as positive electrodes (cathodes) for Na-ion batteries, the as-synthesized materials displayed two sloping plateaus upon charge and discharge, centered near 3.5–3.6 V and 4.0–4.1 V vs. Na⁺/Na, respectively, with a reversible capacity of ∼110 mA h g⁻¹. The application of a conducting carbon coating through the surface polymerization of dopamine with subsequent annealing at 500 °C improved both the rate capability (∼55 mA h g⁻¹ at a discharge rate of 10C) and capacity retention (∼93% after 50 cycles at a discharge rate of C/2).

NASICON-structured Na₃V₂O_2x(PO₄)₂F_3−2x (0 < x ≤ 1) solid solutions have been prepared using a microwave-assisted hydrothermal (MW-HT) technique.     

MoO3−x-deposited TiO2 nanotubes for stable and high-capacitance supercapacitor electrodes

Here we report the supercapacitive properties of a novel MoO_3−x/TiO₂ nanotube composite prepared by a facile galvanostatic deposition technique and subsequently thermal treatment in an argon atmosphere between 350 °C and 550 °C. X-ray diffraction and X-ray photoelectron spectroscopy confirm the existence of MoO_3−x. The MoO_3−x/TiO₂ electrode prepared at 550 °C exhibits a high specific capacitance of 23.69 mF cm⁻² at a scan rate of 10 mV s⁻¹ and good cycling stability with capacitance retention of 86.6% after 1000 cycles in 1 M Na₂SO₄ aqueous solution. Our study reveals a feasible method for the fabrication of TiO₂ nanotubes modified with electroactive MoO_3−x as high-performance electrode materials for supercapacitors.

Here we report the supercapacitive properties of a novel MoO_3−x/TiO₂ nanotube composite prepared by a facile galvanostatic deposition technique and subsequently thermal treatment in an argon atmosphere between 350 °C and 550 °C.     

Surface coating of a LiNixCoyAl1−x−yO2 (x > 0.85) cathode with Li3PO4 for applying a water-based hybrid polymer binder during Li-ion battery preparation

To produce water-stable Ni-rich lithium nickel cobalt aluminum oxides (LiNi_xCo_yAl_1−x−yO₂, x > 0.85, NCAs), the formation of trilithium phosphate (Li₃PO₄)-coated layers on the NCA surfaces was attempted through the use of a surface reaction in a mixture of ethanol and water and a post-heat treatment at 350 and 400 °C. Based on the results of X-ray photoelectron spectroscopy (XPS), the coated layers consisted of nickel phosphate (Ni₃(PO₄)₂) and Li₃PO₄. The coated NCA surface could have sufficient water stability to maintain the cathode performance in a water slurry for 1 day. In addition, the coated layers formed on the NCA surfaces did not block Li⁺-ion transfer through the Ni₃(PO₄)₂/Li₃PO₄-coating layers and enhanced the high-rate discharge performance.

To produce water-stable Ni-rich lithium nickel cobalt aluminum oxides, the formation of trilithium phosphate coated layers on the NCA surfaces was attempted through the use of a surface reaction in a mixture of ethanol and water and a post-heat treatment at 350 and 400 °C.     

Synthesis and performance of solid proton conductor molybdovanadosilicic acid

A molybdovanadosilicic acid H₅SiMo₁₁VO₄₀·8H₂O was synthesized and investigated in this work. The structure features and hydration degree of this acid were characterized by IR, UV, XRD and TG-DTA. Its proton conductivity was studied by electrochemical impedance spectroscopy (EIS). The EIS measurements demonstrated that H₅SiMo₁₁VO₄₀·8H₂O showed excellent proton conduction performance with proton conductivity reaching 5.70 × 10⁻³ S cm⁻¹ at 26 °C and 70% relative humidity. So, it is a new solid high proton conductor. The conductivity enhances with the increase of temperature, and it exhibits Arrhenius behavior. The activation energy value for proton conduction is 21.4 kJ mol⁻¹, suggesting that the proton transfer in this solid acid is dominated by Vehicle mechanism.

A novel Keggin-type proton conductor shows high proton conductivity, reaching 5.70 × 10⁻³ S cm⁻¹ at room conditions.     

Evaluation of thin film fuel cells with Zr-rich BaZrxCe0.8−xY0.2O3−δ electrolytes (x ≥ 0.4) fabricated by a single-step reactive sintering method

This paper reports a survey of power generation characteristics of anode-supported thin film fuel cells with Zr-rich BaZr_xCe_0.8−xY_0.2O_3−δ (x = 0.4, 0.6, 0.7, and 0.8) proton-conducting electrolytes, which were fabricated by single step co-firing with Zn(NO₃)₂ additives at a relatively low temperature (1400 °C). The grain sizes significantly increased to several μm for x = 0.4 and 0.6, whereas the grain sizes remained in the sub-μm ranges for x = 0.7 and 0.8, which resulted in large gaps of the fuel cell performances at x over and below 0.6. The cells for x = 0.4 and 0.6 exhibited efficient power generation, yielding peak powers of 279 and 336 mW cm⁻² at 600 °C, respectively, which were higher than those of the corresponding cells previously reported. However, the performances abruptly deteriorated with the increasing x to more than 0.7 because the electrolyte films were highly resistive due to the coarse-grained microstructures. Impedance spectroscopy for the dense sintered BaZr_xCe_0.8−xY_0.2O_3−δ discs confirmed that the total proton conductivity of BaZr_0.6Ce_0.2Y_0.2O_3−δ was higher than that of BaZr_0.4Ce_0.4Y_0.2O_3−δ at temperatures above 500 °C despite relatively small grain sizes. In addition, BaZr_0.6Ce_0.2Y_0.2O_3−δ cells could gain a stable current throughout a continuous run for a few days under CO₂-containing fuel supply, which was due to high fraction of thermodynamically stable BaZrO₃ matrices. It was demonstrated that BaZr_0.6Ce_0.2Y_0.2O_3−δ is a promising electrolyte for proton-conducting ceramic fuel cells with excellent proton conductivity and CO₂ tolerance at intermediate temperatures.

This paper reports on the survey of power generation characteristics of anode-supported thin film fuel cells with Zr-rich side BaZr_xCe_0.8–xY_0.2O_3–δ (x = 0.4, 0.6, 0.7, and 0.8) proton conducting electrolytes.     

Thermodynamics of the double sulfates Na2M2+(SO4)2·nH2O (M = Mg,Mn, Co,Ni, Cu,Zn, n = 2 or 4) of the blödite–kröhnkite family

The double sulfates with the general formula Na₂M²⁺(SO₄)₂·nH₂O (M = Mg, Mn, Co, Ni, Cu, Zn, n = 2 or 4) are being considered as materials for electrodes in sodium-based batteries or as precursors for such materials. These sulfates belong structurally to the blödite (n = 4) and kröhnkite (n = 2) family and the M cations considered in this work were Mg, Mn, Co, Ni, Cu, Zn. Using a combination of calorimetric methods, we have measured enthalpies of formation and entropies of these phases, calculated their Gibbs free energies (Δ_fG°) of formation and evaluated their stability with respect to Na₂SO₄, simple sulfates MSO₄·xH₂O, and liquid water, if appropriate. The Δ_fG° values (all data in kJ mol⁻¹) are: Na₂Ni(SO₄)₂·4H₂O: −3032.4 ± 1.9, Na₂Mg(SO₄)₂·4H₂O: −3432.3 ± 1.7, Na₂Co(SO₄)₂·4H₂O: −3034.4 ± 1.9, Na₂Zn(SO₄)₂·4H₂O: −3132.6 ± 1.9, Na₂Mn(SO₄)₂·2H₂O: −2727.3 ± 1.8. The data allow the stability of these phases to be assessed with respect to Na₂SO₄, MSO₄·mH₂O and H₂O(l). Na₂Ni(SO₄)₂·4H₂O is stable with respect to Na₂SO₄, NiSO₄ and H₂O(l) by a significant amount of ≈50 kJ mol⁻¹ whereas Na₂Mn(SO₄)₂·2H₂O is stable with respect to Na₂SO₄, MnSO₄ and H₂O(l) only by ≈25 kJ mol⁻¹. The values for the other blödite–kröhnkite phases lie in between. When considering the stability with respect to higher hydrates, the stability margin decreases; for example, Na₂Ni(SO₄)₂·4H₂O is still stable with respect to Na₂SO₄, NiSO₄·4H₂O and H₂O(l), but only by ≈20 kJ mol⁻¹. Among the phases studied and chemical reactions considered, the Na–Ni phase is the most stable one, and the Na–Mn, Na–Co, and Na–Cu phases show lower stability.

The double sulfates with the general formula Na₂M²⁺(SO₄)₂·nH₂O (M = Mg, Mn, Co, Ni, Cu, Zn, n = 2 or 4) are being considered as materials for electrodes in sodium-based batteries or as precursors for such materials.     

Theoretical hydrogen bonding calculations and proton conduction for Eu(iii)-based metal–organic framework

A water-mediated proton-conducting Eu(iii)-MOF has been synthesized, which provides a stable proton transport channel that was confirmed by theoretical calculation. The investigation of proton conduction shows that the conductivity of Eu(iii)-MOF obtained at 353 K and 98% RH is 3.5 × 10⁻³ S cm⁻¹, comparable to most of the Ln(iii)-MOF based proton conductors.

A water-mediated proton-conducting Eu(iii)-MOF has been synthesized, which provides a stable proton transport channel that was confirmed by theoretical calculation.

In recent years, with the aggravation of environmental pollution and growing depletion of petroleum, coal and other traditional fossil energy, the demand to exploit alternative cleaner energy is increasingly urgent. Compared to the dispersion of several developed new energy sources, such as solar energy, wind, geothermal heat, and so on,¹ the proton exchange membrane fuel cell (PEMFC) is recognized as a promising energy conversion system.² As an important component in PEMFC, the proton exchange membrane (PEM) directly affects the transmission efficiency of protons between electrodes.³ Currently, Nafion has been widely used as a PEM in commerce, and shows a conductivity higher than 10⁻¹ S cm⁻¹.⁴ However, the large-scale applications of Nafion are limited due to their high costs, narrow working conditions (low temperature and high relative humidity), amorphous nature, etc.⁵ To overcome these limitations, several types of proton-conducting materials have been explored over the past decade.⁶ Among them, MOF materials were employed as ideal platforms to regulate proton conductivity owing to their high crystallinity, tunable structure and tailorable functionality. The crystallographically defined structure is also conductive to the deeply analysis of proton transport path and mechanism,⁷ furthermore, due to the visual structure of MOF, Density Functional Theory (DFT) is recently used to analyse the factors that affect proton conduction from a theoretical perspective, thus providing strong support for the experimental results.⁸ Multi-carboxylate ligands usually exhibit versatile coordination modes and strong complexing ability to metal ions. Moreover, the hydrophilic –COOH groups not only donate protons but also facilitate the formation of continuous hydrogen bond channel with water molecules. Simultaneously, selecting lanthanide metal ions as nodes in the construction of MOF, more water molecules tend to be bound by Ln(iii) ions, leading to an increase in the concentration of proton carrier, which would be beneficial for the effective proton transport. Therefore, the carboxylate-bridged Ln(iii)-MOF are good candidates for proton conduction.⁹ Currently, there are several proton conductive MOF materials, such as {H[(N(Me)₄)₂][Gd₃(NIPA)₆]}·3H₂O (σ = 7.17 × 10⁻² S cm⁻¹, 75 °C, 98% RH),¹⁰ Na₂[Eu(SBBA)₂(FA)]·0.375DMF·0.4H₂O (σ = 2.91 × 10⁻² S cm⁻¹, 90 °C, 90% RH)¹¹ and {[Tb₄(TTHA)₂(H₂O)₄]·7H₂O}_n (σ = 2.57 × 10⁻² S cm⁻¹, 60 °C, 98% RH)¹² that showing ultra-high conductivities (>10⁻² S cm⁻¹). These superprotonic conductors provided advantageous supports for the assembly strategies involved Ln(iii) ion and carboxylate ligand. In this work, 1,3,5-triazine-2,4,6-triamine hexaacetic acid (H₆TTHA) and Eu(NO₃)₃·6H₂O were assembled at 140 °C for 72 h through solvothermal reaction to afforded colourless crystals, namely {[Eu₂(TTHA)(H₂O)₄]·9H₂O}_n (1). This complex has been previously reported by Wu and co-workers.¹³ In their work, the thermal stability and fluorescence properties of 1 were mainly focused. Research suggested that the complex 1 maintained structural stability until 400 °C and demonstrated strong fluorescent emission with high quantum yields (Φ > 70%), treating as a good candidate for light applications. To the best of our knowledge, MOFs usually exhibit a variety of potential applications for their structural diversity.¹⁴ For different researchers, their concerns about the applications of MOF may vary, but it is the continuously exploration and excavation of different performance that will enrich their potentials and meet them in different fields of the applications. Through careful structural analysis, we found that there is a rare infinite water cluster ((H₂O)_n) existing in the crystal structure of 1 (Fig. S1^†), (H₂O)_n further interacts with –COO⁻ groups to form an abundant hydrogen bond network (Fig. S2 and Table S1^†). The stability of (H₂O)_n as well as more complex hydrogen bond formed between (H₂O)_n and –COO⁻ groups has been confirmed by the density functional theory (DFT) calculations. The advantageous structural features including high concentration of water molecules and stable hydration channel provide the possibility to realize high proton conductivity of 1. Therefore, the proton conductivities of 1 under varying conditions were investigated in detail.Complex 1 crystallizes in the monoclinic space group C2/c, with the asymmetric building unit composed of two Eu(iii) ions, one [TTHA]⁶⁻ anion, four coordination water molecules and nine lattice water molecules. The Eu(iii) atom is distorted enneahedron coordinated by seven carboxylate oxygen atoms and two water molecules (Fig. 1a and Table S2^†). The bond length of Eu–O is in the range of 2.374(5)–2.606(5) Å (Table S3^†), comparable to that of the Eu(iii) complex reported in the literature.¹⁵ The coordination mode of [TTHA]⁶⁻ can be described as μ₆-η²η¹η¹η¹η¹η¹η¹η¹η¹η¹η¹η². In the complex 1, the adjacent metal ions were connected through O–C–O and μ–O bridging, forming a dimer, [Eu1]₂. The dimer acts as a linker and connects with four [TTHA]⁶⁻ (Fig. 1b). Furthermore, the [TTHA]⁶⁻ anions coordinate with [Eu1]₂ through six flexible arms in different directions, leading to the formation of a three-dimensional network structure, where the cavities with regular size of 8.356 × 10.678 Ų are left (Fig. 1c and Fig. S3^†). The topological representation of the network of 1 was analysed by using TOPOS software.¹⁶ As shown in Fig. 1d, the Eu(iii) ions are connected to four [TTHA]⁶⁻, which can be considered as 4-connected nodes. And the [TTHA]⁶⁻ anions were also viewed as 4-connected nodes for their connections with four Eu(iii) ions. So, the whole 3D structure was described as a 4,4-c net with an extended Schläfli symbol of {4²,8⁴}.Open in a separate windowFig. 1Coordination mode of the [TTHA]⁶⁻ in 1 showing [EuO₉] enneahedron (a). The dimer, [Eu1]₂, formed by O–C–O and μ–O bridging, connects with four [TTHA]⁶⁻ (b). The 3D structure of 1 formed by the coordination of Eu(iii) and [TTHA]⁶⁻ as well as water molecules (c). Topological representation of the network of 1 (d). Symmetry codes (i: 1.5 − x, 1.5 − y, 1 − z; ii: 0.5 + x, 1.5 − y, −0.5 + z; iii: x, 2 − y, −0.5 + z; iv: 2 − x, 2 − y, 1 − z; v: 2 − x, y, 0.5 − z; vi: 1 − x, y, 1.5 − z; vii: 0.5 + x, 0.5 + y, z).In 1, the theoretical hydrogen bonding calculations of (H₂O)_n and complex cluster were performed using the Gaussian 09 program. All the structures were obtained from the analysis of XRD results and the hydrogen atoms are optimized. We calculated the energy at DFT level by means of B3LYP-D3.^17a As polarity of molecule has great influence on intermolecular hydrogen bonding,^17b hydrogen bond-forming orbitals require larger space occupation.^17c Thus, diffuse and polarization functions augmented split valence 6-311+G(d,p) basis set is used. The binding energy (E_binding) is calculated as the difference between the energy of hydrogen-bonded cluster and the summation of the energies of each component monomer: E_binding = E_tol − ∑N_iE_iE_tol and E_i are energy of hydrogen-bonded cluster and each individual component monomer, respectively. A hydrogen-bonded cluster is more stable if interaction energy is more negative compared to other hydrogen-bonded configurations. With the help of density functional theory (DFT), we calculate the binding energies (E_binding) to compare the stability of systems. The binding energy of water cluster and complex cluster is −619.65 and −710.34 kcal mol⁻¹ (Fig. 2), respectively, indicating the complex cluster system is more stable.Open in a separate windowFig. 2The structures of water cluster and complex cluster. Oxygen, hydrogen, carbon, nitrogen atoms are marked by red, white, cyan, blue, respectively.The PXRD patterns of 1 were shown in Fig. S4.^† It was found that the diffraction peaks of powder sample are in good agreement with the simulated data from single-crystal diffraction, showing the high purity of the synthesized sample. The IR spectrum of 1 exhibits a strong peak at 3422 cm⁻¹, which corresponds to the stretching vibration of water molecules.^18a The absorption peaks appeared at 1551 cm⁻¹ and 1400 cm⁻¹ are attributed to the antisymmetric stretching of –COO⁻ groups^18b (Fig. S5^†). The water adsorption property of 1 was investigated at 25 °C by DVS Intrinsic Plus. Before the measurement, the sample was treated under 0% RH for 6 h (Fig. S6^†). Water adsorption and desorption isotherms of the fully dehydrated sample were shown in Fig. S7.^† The adsorption process in the RH range of 0–95% can be divided into three stages. In the initial stage (0–10%), the adsorption of water molecules increased rapidly, which can be attributed to the hydrogen bond interaction between carboxylic acid oxygen atoms and water molecules. Then the water adsorption increased slowly at 10–70% RH, corresponding to the formation of water clusters. Another abrupt increase of water adsorption was found when the RH is above 70%, illustrating that enough energy is needed for the water clusters to exist in the cavity of the crystal.¹⁹ Clearly, large hysteresis was observed in the adsorption–desorption isotherms, this phenomenon was caused by the strong hydrophilic of –COO⁻ groups in 1.²⁰ Furthermore, the structural integrity of the sample after adsorption/desorption cycle was confirmed by PXRD (Fig. S4^†).Based on the previous structural analysis, the proton conduction of 1 was evaluated by the alternating-current (AC) impedance analyses. The Nyquist plots of 1 obtained at different temperature and relative humidity are shown in Fig. 3a and b and Fig. 3d. The resistance is estimated from the intercept of spikes or arcs on the Z′ axis, and the conductivity (σ) is calculated by the equation of σ = l/(A·R), where l, A and R represent the sample thickness, surface area and resistance, respectively. It was found that there are two different modes observed from the impedance spectroscopies under lower relative humidity (60–90% RH), a partial arc at high frequency component can be attributed to the grain interior contribution, while a characteristic spur at low frequency component illustrates that partial-blocking electrode response allows limited diffusion.²¹ So, the only spikes displayed in the Nyquist spectra at 98% RH and 293–353 K suggest that high temperature and high relative humidity are more favourable for the proton conduction. From the temperature-dependent measurements under 98% RH, significantly, the conductivity of 1 increases gradually from 1.34 × 10⁻⁴ S cm⁻¹ at 293 K to 3.5 × 10⁻³ S cm⁻¹ at 353 K (Fig. 3c and Table S4^†). The increasing conductivity can be attributed to the important role of water molecule. The high concentration of water molecules act as carriers and transmit in the form of H⁺(H₂O)_n, and the mobility of H⁺(H₂O)_n accelerates with the rising temperature. Moreover, the higher acidity of water molecules at higher temperature is more conducive to the improvement of proton conductivity. The relative humidity dependence measured at 298 K indicated that the conductivity of 1 presented significant positive correlations with the humidity changes. The conductivity is 1.42 × 10⁻⁵ S cm⁻¹ at 60% RH and increases to be 1.63 × 10⁻⁴ S cm⁻¹ at 98% RH (Fig. 3e and Table S5^†). This can be explained by the ability of (H₂O)_n to bind water molecules and strong hydrophilic of –COO⁻ group that has been confirmed by the water adsorption process, especially when the RH is above 60%. For water-mediated proton conductors, the lower RH usually results in the insufficient of transport media and further affects the diffusion of protons. At present, the theoretical simulations (e.g. aMS-EVB3)²² and activation energy (E_a)^23–27 are the main methods to analysis the proton conduction mechanism. Compared with the theoretical calculations, the judgment rule with E_a is more straightforward. Here, the E_a of 1 determined from the linear fit of ln(σT) vs. 1000/T is 0.44 eV (Fig. 3f), which reveals that the proton transfer in 1 follows a typical vehicle mechanism.¹² Further evaluate the long-term stability of 1, the time-dependent proton conductivity has been conducted, indicating negligible decline of proton conductivities even lasted 12 h (Fig. 4, S8 and Table S6^†). The sample of 1 after property measurements was collected and characterized by PXRD to examine any structural change, and the PXRD spectrum shows structural integrity even at high temperature and high relative humidity environment (Fig. S4^†). The long-term stable proton conductivities of 1 can be attributed to the robust hydrogen bonding channel that has been confirmed by the DFT calculations. In recent years, the proton conductive carboxylate-based MOF have been systematic reviewed by G. Li’ group,⁹ it was found that the complex 1 shows higher conductivity of 3.5 × 10⁻³ S cm⁻¹ under 353 K and 98% RH when compared to the Ln(iii)-MOF materials, such as [Me₂NH₂][Eu(ox)₂(H₂O)]·3H₂O (σ = 2.73 × 10⁻³ S cm⁻¹, 95% RH, 55 °C),²³ {[Gd(ma)(ox)(H₂O)]_n·3H₂O} (σ = 4.7 × 10⁻⁴ S cm⁻¹, 95% RH, 80 °C),²⁴ (N₂H₅)[Nd₂(ox)₄(N₂H₅)]·4H₂O (σ = 2.7 × 10⁻³ S cm⁻¹, 100% RH, 25 °C),²⁵ {[SmK(BPDSDC)(DMF)(H₂O)]·x(solvent)}_n (σ = 1.11 × 10⁻³ S cm⁻¹, 98% RH, 80 °C),²⁶ [Nd(mpca)₂Nd(H₂O)₆Mo(CN)₈]·nH₂O (σ = 2.8 × 10⁻³ S cm⁻¹, 98% RH, 80 °C),²⁷ MFM-550(M) and MFM-555(M) (M = La, Ce, Nd, Sm, Gd, Ho) (σ = 1.46 × 10⁻⁶ to 2.97 × 10⁻⁴ S cm⁻¹, 99% RH, 20 °C)²⁸ as well as other conductive materials showing lower conductivities in the range of 10⁻⁹ to 10⁻⁵ S cm⁻¹.⁹ However, the conductivity of 1 is inferior to those Ln(iii)-MOFs with conductivities higher than 10⁻² S cm⁻¹ × ^10–12 In recent years, another two H₆TTHA-derived MOF and CP, {[Tb₄(TTHA)₂(H₂O)₄]·7H₂O}_n¹² and {[Co₃(H₃TTHA)₂(4,4′-bipy)₅(H₂O)₈]·12H₂O}_n^19b have been previously reported by our group, which show highest proton conductivities of 2.57 × 10⁻² S cm⁻¹ at 60 °C and 8.79 × 10⁻⁴ S cm⁻¹ at 80 °C under 98% RH, respectively. The noticeable performance difference between these two complexes and 1 was analysed based on the visual structures. The higher conductivity of 1 when compared to the Co(ii) complex can be attributed to the concentration of water molecules, 23.25% for 1 and 15.92% for the Co(ii) complex. The high concentration of proton carrier in 1 promotes the transfer of protons. Although the water molecular concentration of 1 is higher than that of the Tb(iii) complex, however, the coordination numbers of Ln(iii) ions in the two compounds are different, eight for the Tb(iii) ion and nine for the Eu(iii) ion, respectively. The coordination sites are obviously not satiated, especially for the Tb(iii) complex, the lower coordination number may prone to chelate more water molecules under high relative humidity, leading to the formation of more consecutive hydration channel with TTHA⁶⁻ anions and water molecules, thus accelerating the proton transport. In contrast, the molecular structure of the Eu(iii) compound contains nearly a quarter of water molecules, these water molecules have almost filled the pores, so the smaller pore structure is difficult to accommodate more adsorbed water molecules.Open in a separate windowFig. 3Nyquist plots for proton conductivity of 1 (98% RH) at 293–313 K (a) and 318–353 K (b). Plot of log(σ) vs. T for 1 in the temperature range of 293–353 K (c). Plots of the impedance plane for 1 at different relative humidities and 298 K (d). Humidity dependence of the proton conductivity at 298 K (e). Arrhenius plot of 1 at 98% RH (f).Open in a separate windowFig. 4Time-dependent proton conductivity of 1 at 343 K and 98% RH.In conclusions, a water-mediated proton-conducting Eu(iii)-MOF has been synthesized, displaying a 3D network structure with high concentration water molecules and –COO⁻ groups as well as abundant H-bond networks. Interesting, there is an infinite water cluster of (H₂O)_n existing in the crystal structure of Eu(iii)-MOF, which is rare in the H₆TTHA-derived complexes and even other reported MOF/CPs. Based on this, the density functional theory was conducted to evaluate the stability of water cluster and complex cluster. As expected, the calculated binding energies indicate that the more stable system was formed by (H₂O)_n and –COO⁻ groups, which provides a favourable guarantee for proton conduction. The advantageous structural features of Eu(iii)-MOF result in the realization of comparable proton conductivity of 3.5 × 10⁻³ S cm⁻¹ at 353 K and 98% RH and long-term stability at least 12 h. Additionally, the factors affecting the electrical conductivity of several H₆TTHA-derived MOF/CPs have been compared and analysed from the visual structures, and the structure-activity relationship of such compounds was also summarized, which will provide guidance to design novel crystalline superprotonic conductors assembled from multi-carboxylate.     

A proton conductor electrolyte based on molten CsH5(PO4)2 for intermediate-temperature fuel cells

Molten carbonate fuel cells have been commercialized as a mature technology. Due to the liquid electrolyte in molten carbonate fuel cells, gas seal and low contact resistance are easier to achieve than in other fuel cells. Herein, we report an investigation of the viability of a molten oxoacid salt as a novel type of fuel cell electrolyte. In comparison with molten carbonate electrolytes for MCFCs that operate at 500–700 °C, for which a ceramic support matrix is required, the molten proton conductor electrolyte has a lower working temperature range of 150–250 °C. The present study has shown that an electrolyte membrane, in which molten CsH₅(PO₄)₂ is held in a matrix made of PBI polymer and SiO₂ powder, has excellent thermal stability, good mechanical properties, and high proton conductivity. In addition, a molten proton conductor fuel cell equipped with such an electrolyte membrane operating at 200 °C showed an open-circuit voltage of 1.08 V, and a stable output voltage during continuous measurement for 150 h at a constant output current density of 100 mA cm⁻².

An proton conductor electrolyte membrane, in which molten CsH₅(PO₄)₂ is held in a matrix made of PBI polymer and SiO₂ powder, is prepared for intermediate-temperature fuel cells.     

Evaluation of Nd0.8−xSr0.2CaxCoO3−δ(x = 0, 0.05, 0.1, 0.15, 0.2) as a cathode material for intermediate-temperature solid oxide fuel cells

A series of Nd_0.8−xSr_0.2Ca_xCoO_3−δ(x = 0, 0.05, 0.1, 0.15, 0.2) cathode materials was synthesized by sol–gel method. The effect of Ca doping amount on the structure was examined by scanning electron microscopy (SEM), X-ray diffraction (XRD), thermal expansion, and X-ray photoelectron spectroscopy (XPS). Electrochemical properties were evaluated for possible application in solid oxide fuel cell (SOFC) cathodes. Results showed that second phase NdCaCoO_4+δ is generated when the Ca doping amount is higher than 0.1. The increase in Ca limits the electronic compensation capacity of the material, resulting in a decrease in thermal expansion coefficient (TEC). With the increase of Ca content, the conductivity increases at first and then decreases, and the highest value of 443 S cm⁻¹ is at x = 0.1 and T = 800 °C. Nd_0.7Sr_0.2Ca_0.1CoO_3−δ exhibits the lowest area specific resistance of 0.0976 Ω cm² at 800 °C. The maximum power density of Nd_0.7Sr_0.2Ca_0.1CoO_3−δ at 800 °C is 409.31 mW cm⁻². The Ca-doped material maintains good electrochemical properties under the coefficient of thermal expansion (CTE) reduction and thus can be used as an intermediate-temperature SOFC (IT-SOFC) cathode.

The increase in Ca for Nd_0.8−xSr_0.2Ca_xCoO_3−δ limits the electronic compensation capacity, resulting in a decrease in CTE. The Ca-doped material maintains good electrochemical properties under CTE reduction and thus can be used as an IT-SOFC cathode.     

Photocatalytic properties of a new Z-scheme system BaTiO3/In2S3 with a core–shell structure

It''s highly desired to design and fabricate an effective Z-scheme photo-catalyst with excellent charge transfer and separation, and a more negative conduction band edge (E_CB) than O₂/·O₂⁻ (−0.33 eV) and a more positive valence band edge (E_VB) than ·OH/OH⁻ (+2.27 eV) which provides high-energy redox radicals. Herein, we firstly designed and synthesized a core–shell-heterojunction-structured Z-scheme system BaTiO₃@In₂S₃ (BT@IS, labelled as BTIS) through a hydrothermal method, where commercial BT was used as the core and In(NO₃)₃·xH₂O together with thioacetamide as the precursor of IS was utilized as the shell material. In this system, the shell IS possesses a E_CB of −0.76 eV and visible-light-response E_g of 1.92 eV, while the core BT possesses a E_VB of 3.38 eV, which is well suited for a Z-scheme. It was found that the as-prepared BTIS possesses a higher photocatalytic degradation ability for methyl orange (MO) than commercial BT and the as-prepared IS fabricated by the same processing parameters as those of BTIS. Holes (h⁺) and superoxide radicals (·O₂⁻) were found to be the dominant active species for BTIS. In this work, the core–shell structure has inhibited the production of ·OH because the shell IS has shielded the OH⁻ from h⁺. It is assumed that if the structure of BTIS is a composite, not a core–shell structure, ·OH could be produced during photocatalysis, and therefore a higher photocatalytic efficiency would be obtained. This current work opens a new pathway for designing Z-scheme photocatalysts and offers new insight into the Z-scheme mechanism for applications in the field of photocatalysis.

It''s highly desired to design an effective Z-scheme photocatalyst with excellent charge transfer and separation, a more negative conduction band edge (E_CB) than O₂/·O₂⁻ (−0.33 eV) and a more positive valence band edge (E_VB) than ·OH/OH⁻ (+2.27 eV).     

Structural and thermal properties of Na2Mn(SO4)2·4H2O and Na2Ni(SO4)2·10H2O

The title compounds were prepared via a wet chemistry route and their crystal structures were determined from single crystal X-ray diffraction data. Na₂Mn(SO₄)₂·4H₂O crystallizes with a monoclinic symmetry, space group P2₁/c, with a = 5.5415(2), b = 8.3447(3), c = 11.2281(3) Å, β = 100.172(1)°, V = 511.05(3) ų and Z = 2. Na₂Ni(SO₄)₂·10H₂O also crystallizes with a monoclinic symmetry, space group P2₁/c, with a = 12.5050(8), b = 6.4812(4), c = 10.0210(6) Å, β = 106.138(2)°, V = 780.17(8) ų and Z = 2. Na₂Mn(SO₄)₂·4H₂O is a new member of the blödite family of compounds, whereas Na₂Ni(SO₄)₂·10H₂O is isostructural with Na₂Mg(SO₄)₂·10H₂O. The structure of Na₂Mn(SO₄)₂·4H₂O is built up of [Mn(SO₄)₂(H₂O)₄]²⁻ building blocks connected through moderate O–H⋯O hydrogen bonds with the sodium atoms occupying the large tunnels along the a axis and the manganese atom lying on an inversion center, whereas the structure of Na₂Ni(SO₄)₂·10H₂O is built up of [Ni(H₂O)₆]²⁺ and [Na₂(SO₄)₂(H₂O)₄]²⁻ layers. These layers which are parallel to the (100) plane are interconnected through moderate O–H⋯O hydrogen bonds. The thermal gravimetric- and the powder X-ray diffraction-analyzes showed that only the nickel phase was almost pure. At a temperature above 300 °C, all the water molecules evaporated and a structural phase transition from P2₁/c-Na₂Ni(SO₄)₂·10H₂O to C2/c-Na₂Ni(SO₄)₂ was observed. C2/c-Na₂Ni(SO₄)₂ is thermally more stable than Na₂Fe(SO₄)₂ and therefore it would be suitable as the positive electrode for sodium ion batteries if a stable electrolyte at high voltage is developed.

Two compounds were prepared via a supersaturation method. Their crystal structures were solved and compared to Na₂M(SO₄)₂·nH₂O (M = Mn, Ni and n = 0, 1, 2, 3, 4, 5, 6, 10, 16). Furthermore, phase transitions as a function of temperature were observed.     

An in situ DRIFTS study on nitrogen electrochemical reduction over an Fe/BaZr0.8Y0.2O3−δ-Ru catalyst at 220 °C in an electrolysis cell using a CsH2PO4/SiP2O7 electrolyte

In situ DRIFTS measurements of an Fe/BZY-Ru cathode catalyst in an electrolysis cell using a CsH₂PO₄/SiP₂O₇ electrolyte were carried out in a mixed N₂–H₂ gas flow under polarization. The formation of N₂H_x species was confirmed under polarization, and an associative mechanism in the electrochemical NRR process was verified.

In situ DRIFTS measurements of an Fe/BZY-Ru cathode catalyst in an electrolysis cell using a CsH₂PO₄/SiP₂O₇ electrolyte were carried out in a mixed N₂–H₂ gas flow under polarization.

NH₃ production contributes about 2% of the world''s energy consumption annually, and most of the consumption is from the strongly endothermic steam reforming of methane (SRM) at 800–1000 °C in the Haber–Bosch process. Other major energy consuming processes include CO₂ removal, reactant gas purification, reactant gas compression for NH₃ synthesis, and NH₃ separation.¹ Due to its high energy density, NH₃ was expected to be a promising energy carrier in recent years.² NH₃ produced by renewable energy sources can be used as a carbon-free energy carrier. One of the promising methods to utilize renewable energy sources for NH₃ production is electrochemical N₂ reduction. Various electrolysis cells with solid and liquid electrolytes have been reported for electrochemical N₂ reduction over a wide temperature range. For the electrochemical N₂ reduction reaction (NRR) at low temperatures (T < 100 °C), the main limitation is the difficulty of N₂ activation and the low solubility of N₂ in aqueous media. High temperatures (T > 500 °C) can lead to decomposition of the produced NH₃. Hence, an electrochemical NRR at intermediate temperatures is desirable. Phosphates such as CsH₂PO₄ and CsH₅(PO₄)₂ are typically used as electrolytes at intermediate temperatures. These inorganic oxyacid salts have high proton conductivity and stability. CsH₂PO₄ mixed with SiP₂O₇ as a matrix exhibits a proton conductivity of ca. 1 × 10⁻² S cm⁻¹ at 220 °C.³ In our previous work on electrochemical NH₃ synthesis using an Fe/BZY-RuO₂ catalyst and CsH₂PO₄/SiP₂O₇ electrolyte at 220 °C and ambient pressure, the highest current efficiency of 7.1% and the highest NH₃ yield rate of 4.5 × 10⁻¹⁰ mol (s⁻¹ cm²) were achieved at −0.4 V (vs. open circuit voltage (OCV)) and −1.5 V, respectively. In addition, N₂H₄ was successfully detected at −0.2 V (vs. OCV), which indicated an associative mechanism,⁴ which is one of the two main reaction mechanisms in NH₃ synthesis, namely, associative and dissociative mechanisms. In the associative mechanism the N N bond in a N₂ molecule adsorbed on the catalyst surface is cleaved after an H atom attaches to the N atom of the adsorbed N₂ molecule, whereas in the dissociative mechanism the N N bond is broken on the catalyst surface before an H atom attaches to the N₂ molecule.Typical electrochemical characterizations such as impedance spectroscopy,^5,6 current–voltage curve (IV curve) testing,^7,8 cyclic voltammetry,⁹ and potentiostatic pulse experiment¹⁰ can only provide indirect information on surface reactions at the electrode. It is indispensable to analyse directly adsorbed species on the electrode catalysts for the understanding of the reaction mechanism. In situ spectroscopy is making rapid progress and has already been applied to electrochemical devices, providing valuable information of the chemical species during the reactions.¹¹In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) for electrolysis cells attracts much attention recently.^12,13In situ DRIFTS has recently become a powerful tool for investigating the reaction pathway in electrochemical N₂ reduction reaction.^14–16 In DRIFTS, infrared (IR) beam irradiates the sample disk, and then can be reflected at or transmitted through the sample disk. The scattered IR beam is collected by the spherical focusing mirrors and finally converted by a detector.In this work, a commercial DRIFTS setup was modified and applied to electrochemical N₂ reduction as shown in Fig. 1. To enable an operation temperature higher than 200 °C that is requisite for proton conduction in the electrolyte, an electrical heating wire was placed in the base near the micro-cup. The temperature was measured by a thermocouple located next to the heating wire. An electrolysis cell was set on the micro-cup of the metal base. For protection of the ZnSe window from oxidation at the high operating temperature, a water-cooling system was attached to the metal base near the dome to keep the temperature of the window below 200 °C. The specific size of the apparatus is shown in the Fig. 2. An O-ring fits to the dome can ensure gas tightness of the small space in the dome. There are three pathways which connect the inside space of the dome and atmosphere. Two of them are used for the inlet and outlet gas flows, and the one connecting directly with the micro-cup was used for Pt lead wires. To avoid a short-circuit between the two Pt lead wires, a thin polyimide film (12.5 μm thick, Kapton, Dupont, Delaware, United States) which is stable from −269 to +400 °C was used to cover the Pt wires.Open in a separate windowFig. 1A schematic of modified DRIFTS setup applied to electrochemical N₂ reduction. The temperature of the electrolysis cell is measured at the same place where it is heated.Open in a separate windowFig. 2(a) The bottom-up view of the dome. (b) The vertical view of the metal base.In our previous study of the electrochemical NH₃ synthesis process at 220 °C, a ø 10 mm carbon paper was used on the cathode side to increase the current collection area.⁴ However, in the in situ DRIFTS tests, the cathode catalysts must be exposed to the IR beam, hence a carbon ring with an inside diameter of 7 mm was used instead as shown in Fig. 3. On the anode side, a ø 10 mm Pt/C loaded on carbon paper was used for current collection. The electrolysis cell consists of 0.1 g SiP₂O₇/CsH₂PO₄ electrolyte (mix ratio of SiP₂O₇ : CsH₂PO₄ was 1 : 1) compressed with 0.035 g Fe/BZY-Ru (mix ratio of Fe/BZY : RuO₂ was 1 : 1) catalyst on the top layer. In our previous work of the electrochemical NH₃ synthesis using Fe/BZY-RuO₂, we have successfully detected N₂H₄ as well as NH₃, which indicated the triple bond of N₂ was broken simultaneously with the addition of H. Fe/BZY was prepared in the same way as in the previous work.⁴ The as-prepared Fe/BZY powder was mixed with RuO₂ powder, and then the mixture was reduced in H₂ flow at 220 °C for 1 h. To fix the electrolysis cell, the current collection materials and the Pt lead wires, PTFE sheets (Gore Hyper-Sheet Gasket, W. L. Gore & Associate, Inc., Delaware, USA) formed into rings were used as the support. The experimental conditions in the dome are similar to those in a single-chamber reactor. The inlet gas is a mixture of N₂ and H₂, which is different from the two-chamber reactor in electrolysis tests in the previous work. A background spectrum was measured under H₂ gas flow of 8 mL min⁻¹ at 160 °C with 100 scans at OCV. All DRIFTS spectra are displayed as log(I₀/I) where I₀/I is the relative reflectance (I₀ is the background reflectance). Then the measurements were carried out under N₂ gas flow of 8 mL min⁻¹ at 300 °C at OCV and under mixed N₂ and H₂ gas flow (both are 8 mL min⁻¹) at 250 °C at OCV and various applied biases.Open in a separate windowFig. 3A schematic of electrolysis cell on the micro-cup.According to the results in Fig. 4, three kinds of sharp peaks at 1050, 1300 and 1540 cm⁻¹, and a broad peak at 3300 cm⁻¹ were observed. Peak at 1050 cm⁻¹ is attributed to N–N stretching (reported at 1106 cm⁻¹),¹⁷ which appeared in all experimental conditions even when only N₂ gas was supplied. This implies that the N₂ adsorption sites on the catalyst surface are highly active. Peaks at 1300, 1540 and 3300 cm⁻¹ are assigned to H–N–H wagging, H–N–H bending and N–H stretching (reported at 1270, 1461 and 3235 cm⁻¹).¹⁷ These three kinds of peaks appeared only when both N₂ and H₂ were supplied, and seem to become stronger with applied bias. It is obvious that N₂H_x species, which can be intermediate species in the associative mechanism, were formed on the surface of Fe/BZY-Ru. The peak at 900 cm⁻¹ is probably attributed to NH₃ gas.¹⁸ It is likely that the peaks at 1400 cm⁻¹ and 2800 cm⁻¹ are assigned to NH₄⁺.¹⁹ The strong signal at 2360 cm⁻¹ is associative with gas-phase CO₂ that exists in IR beam path outside the dome.Open in a separate windowFig. 4Electrochemical in situ-FTIR spectra of the NRR on the Fe/BZY-Ru electrode at various electrochemical potentials. Then the measurements were carried out under N₂ at 300 °C at OCV and under mixed N₂ and H₂ at 250 °C at OCV, −0.2, −0.4, −1.5, −3.2 and −4.0 V.Only quite low current densities were able to be loaded as shown in Fig. 5. Reactant gases were introduced to the system without humidification, which might have led to low proton conductivity and electrolyte decomposition. In addition, current collection area of the carbon paper ring on the cathode side is small.Open in a separate windowFig. 5Currents for applied voltages of −0.2 V, −0.4 V, −1.5 V, −3.2 V, and −4.0 V. In situ DRIFTS measurements were carried out in a N₂–H₂ gas mixture under polarization, which corresponds to a situation in a single-chamber reactor. The background was measured in H₂ flow, and the sample measurements were carried out in a mixed N₂–H₂ gas flow. In the obtained spectra, a peak at 1100 cm⁻¹ was assigned to N–N stretching, and those at 1301, 1600, and 3300 cm⁻¹ were assigned to –NH₂ wagging, H–N–H bending, and N–H stretching. The intensity of the H–N–H wagging peak was enhanced by increasing the applied voltage. Appearance of these peaks confirmed the formation of N₂H_x (1 ≤ x ≤ 4) species in the NRR process and consequently demonstrated that NRR proceeded via an associative mechanism over Fe/BZY-Ru cathode catalyst on a SiP₂O₇/CsH₂PO₄ electrolyte.     

Proton conductivity resulting from different triazole-based ligands in two new bifunctional decavanadates

Triazole, similarly to imidazole, makes a prominent contribution to the proton conductivity of porous materials. To investigate the effects of triazole-based ligands in polyoxovanadates (POVs) on proton conduction, we designed and synthesized two decavanadate-based POVs, [Zn₃(C₂H₄N₄)₆(H₂O)₆](V₁₀O₂₈)·14H₂O (1) and [Zn₃(C₂H₃N₃)₈(H₂O)₄](V₁₀O₂₈)·8H₂O (2) constructed from the ligands 3-amino-1,2,4-triazole and 1H-1,2,4-triazole, respectively, via an aqueous solution evaporation method. Surprisingly, complex 1 obtained a superior proton conductivity of 1.24 × 10⁻² S cm⁻¹ under 60 °C and 98% RH, which is much higher than that of complex 2. Furthermore, due to the contribution of the conjugate properties of the ligands to the third-order nonlinear optical (NLO) properties, we also studied its two-photon responses and achieved satisfactory results.

To investigate the effects of triazole-based ligands in polyoxovanadates (POVs) on proton conductivity, we designed and synthesized two decavanadate-based POVs.     

Highly enhanced electrical properties of lanthanum-silicate-oxide-based SOFC electrolytes with co-doped tin and bismuth in La9.33−xBixSi6−ySnyO26

Solid oxide fuel cells (SOFCs) are one of the most promising clean energy sources to be developed. However, the operating temperature of SOFCs is currently still very high, ranging between 1073 and 1273 K. Reducing the operating temperature of SOFCs to intermediate temperatures in between 773 and 1073 K without decreasing the conductivity value is a challenging research topic and has received much attention from researchers. The electrolyte is one of the components in SOFCs which has an important role in reducing the operating temperature of the SOFC compared to the other two fuel cell components, namely the anode and cathode. Therefore, an electrolyte that has high conductivity at moderate operating temperature is needed to obtain SOFC with medium operating temperature as well. La_9.33Si₆O₂₆ (LSO) is a potential electrolyte that has high conductivity at moderate operating temperatures when this material is modified by doping with metal ions. Here, we report a modification of the structure of the LSO by partial substitution of La with Bi³⁺ ions and Si with Sn⁴⁺, which forms La_9.33−xBi_xSi_6−ySn_yO₂₆ with x = 0.5, 1.0, 1.5, and y = 0.1, 0.3, 0.5, in order to obtain an electrolyte of LSO with high conductivity at moderate operating temperatures. The addition of Bi and Sn as dopants has increased the conductivity of the LSO. Our work indicated highly enhanced electrical properties of La_7.83Bi_1.5Si_5.7Sn_0.3O₂₆ at 873 K (1.84 × 10⁻² S cm⁻¹) with considerably low activation energy (E_a) of 0.80 eV comparing to pristine La_9.33Si₆O₂₆ (0.08 × 10⁻² S cm⁻¹).

Structure modification of La_9.33Si₆O₂₆ (LSO) as SOFC electrolyte via a bi-doping mechanism provides enhanced electrical properties of La_7.83Bi_1.5Si_5.7Sn_0.3O₂₆ at 873 K (1.84 × 10⁻² S cm⁻¹) with low activation energy of 0.80 eV compared to pristine LSO (0.08 × 10⁻² S cm⁻¹).     

Off-stoichiometric Na3−3xV2+x(PO4)3/C nanocomposites as cathode materials for high-performance sodium-ion batteries prepared by high-energy ball milling

Na₃V₂(PO₄)₃ (NVP) is regarded as a promising cathode material for sustainable energy storage applications. Here we present an efficient method to synthesize off-stoichiometric Na_3−3xV_2+x(PO₄)₃/C (x = 0–0.10) nanocomposites with excellent high-rate and long-life performance for sodium-ion batteries by high-energy ball milling. It is found that Na_3−3xV_2+x(PO₄)₃/C nanocomposites with x = 0.05 (NVP-0.05) exhibit the most excellent performance. When cycled at a rate of 1C in the range of 2.3–3.9 V, the initial discharge capacity of NVP-0.05 is 112.4 mA h g⁻¹, which is about 96% of its theoretical value (117.6 mA h g⁻¹). Even at 20C, it still delivers a discharge capacity of 92.3 mA h g⁻¹ (79% of the theoretical capacity). The specific capacity of NVP-0.05 is as high as 100.7 mA h g⁻¹ after 500 cycles at 5C, which maintains 95% of its initial value (106 mA h g⁻¹). The significantly improved electrochemical performance of NVP-0.05 is attributed to the decrease of internal resistance and increase of the Na⁺ ion diffusion coefficient.

Na₃V₂(PO₄)₃ (NVP) is regarded as a promising cathode material for sustainable energy storage applications.     

A novel MnO–CrN nanocomposite based non-enzymatic hydrogen peroxide sensor

A MnO–CrN composite was obtained via the ammonolysis of the low-cost nitride precursors Cr(NO₃)₃·9H₂O and Mn(NO₃)₂·4H₂O at 800 °C for 8 h using a sol–gel method. The specific surface area of the synthesized powder was measured via BET analysis and it was found to be 262 m² g⁻¹. Regarding its application, the electrochemical sensing performance toward hydrogen peroxide (H₂O₂) was studied via applying cyclic voltammetry (CV) and amperometry (i–t) analysis. The linear response range was 0.33–15 000 μM with a correlation coefficient (R²) value of 0.995. Excellent performance toward H₂O₂ was observed with a limit of detection of 0.059 μM, a limit of quantification of 0.199 μM, and sensitivity of 2156.25 μA mM⁻¹ cm⁻². A short response time of within 2 s was achieved. Hence, we develop and offer an efficient approach for synthesizing a new cost-efficient material for H₂O₂ sensing.

A MnO–CrN composite was obtained via the ammonolysis of the low-cost nitride precursors Cr(NO₃)₃·9H₂O and Mn(NO₃)₂·4H₂O at 800 °C for 8 h using a sol–gel method.