Structurally-New Hexadecanuclear Ni-Containing Silicotungstate with Catalytic Hydrogen Generation Activity

A structurally-new, carbon-free hexadecanuclear Ni-containing silicotungstate, [Ni₁₆(H₂O)₁₅(OH)₉(PO₄)₄(SiW₉O₃₄)₃]^19-, has been facilely synthesized using a one-pot, solution-based synthetic method systematically characterized by single-crystal X-ray diffraction and several other techniques. The resulting complex works as a noble-metal-free catalyst for visible-light-driven catalytic generation of hydrogen, by coupling with a [Ir(coumarin)₂(dtbbpy)][PF₆] photosensitizer and a triethanolamine (TEOA) sacrificial electron donor. Under minimally optimized conditions, a turnover number (TON) of 842 was achieved for TBA-Ni₁₆P₄(SiW₉)₃-catalyzed hydrogen evolution system. The structural stability of TBA-Ni₁₆P₄(SiW₉)₃ catalyst under photocatalytic conditions was evaluated by the mercury-poisoning test, FT-IR, and DLS measurements. The photocatalytic mechanism was elucidated by both time-solved luminescence decay and static emission quenching measurements.

First-Principles Study of the Effect of Ni-Doped on the Spinel-Type Mn-Based Cathode Discharge

Spinel-type manganese oxide is considered as a typical cobalt-free high-voltage cathode material for lithium-ion battery applications because of its low cost, non-toxicity, and easy preparation. Nevertheless, severe capacity fading during charge and discharge limits its commercialization. Therefore, understanding the electrochemical properties and its modification mechanism of spinel-type manganese oxide for a lithium-ion battery is of great research interest. Herein, we presented a theoretical study regarding the discharge process of LiMn₂O₄ and LiNi_0.5Mn_1.5O₄ using first-principles calculations based on density functional theory. We found that the discharge process is accompanied by an increase in unit cell volume and lattice distortion. Moreover, 25% Ni-substitution increases the average calculated voltage of LiMn₂O₄ from 3.83 to 4.61 V, which is very close to the experimental value. The electronic structure is further discussed to understand the mechanism of voltage increase. In addition, the Ni element also reduces the Li-ion diffusion barrier by 0.06 eV, which helps to improve the intrinsic rate performance of LiMn₂O₄. Our research can provide insight into how Ni-substitution influences the voltage and diffusion barrier of LiMn₂O₄ and pave the way for other spinel-type manganese oxide electrode applications.

Electrochemical removal of NOx by La0.8Sr0.2Mn1-xNixO3 electrodes in solid electrolyte cells: Role of Ni substitution

Electrochemical removal of nitrogen oxides (NO_x) by solid electrolyte cells (SECs) is a promising technology due to no required reductant. Herein, a series of La_0.8Sr_0.2Mn_1-xNi_xO₃ (0 ≤ x ≤ 0.5) perovskites were first synthesized and utilized as the electrode materials of SECs. The role of Ni substitution in electrode performance and NO_x reduction mechanism were revealed by various experimental characterization and first-principle calculations. The results indicate that the moderate Ni substitution (x ≤ 0.3) increased the NO_x conversion of electrodes while reduced the polarization resistance. The further investigation shows that this improvement was attributed to the more surface oxygen vacancies, better reducibility and higher Mn⁴⁺ proportion of the Ni-substituted perovskites. The electrochemical impedance spectroscopy (EIS) shows that these changes facilitated the NO_x adsorption and dissociation processes on the electrode. According to first-principle calculations, the Ni-substituted perovskite had a lower formation energy of surface oxygen vacancy, while the NO molecule adsorbed on defect surface gained more electrons thus was easier to be reduced and dissociated. Finally, the electrode performance at different operating temperatures and the operational stability were verified.

Superior Na-Storage Properties of Nickel-Substituted Na2FeSiO4@C Microspheres Encapsulated with the In Situ-Synthesized Alveolation-like Carbon Matrix

The poor electronic conductivity of Na₂FeSiO₄ always limits its electrochemical reactivities and no effective solution has been found to date. Herein, the novel Ni-substituted Na₂Fe_1-xNi_xSiO₄@C nanospheres (50-100 nm) encapsulated with a 3D hierarchical porous skeleton (named as alveolation-like configuration) constructed using in situ carbon are first synthesized via a facile sol-gel method, and the effects of Ni substitution combined with the design of a unique carbon network on Na-storage properties are assessed systematically, focusing on alleviating the inherent defects of the Na₂FeSiO₄ cathode material. A series of characterization technologies such as X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy and so forth, coupled with the electrochemical measurements and first-principles calculations, are used to explore the structure, morphology and electrochemical behaviors of the as-prepared materials. The results show that the synergism between Ni substitution and the special alveolation-like configuration enables fast Na ions mobility (from 10^-14 to 10^-12 cm² s^-1), reduces band gap energy (from 2.82 to 1.79 eV) and lowers Na-ion diffusion barriers, finally reciprocating the vigorous electrochemical kinetics of the electrode. Especially, the elaborately designed material-Na₂Fe_0.97Ni_0.03SiO₄@C-displays superior Na-storage properties of around 197.51 mA h g^-1 (corresponding to 1.43 Na⁺ intercalation) at 0.1 C within 1.5-4.5 V along with desirable capacity retention (84.44% after 100 cycles), and the rate capability is also markedly enhanced (a capacity of 133.62 mA h g^-1 at 2 C). Such the effective methodology employed in this work opens a potential pathway to synthesize the silicate cathode material with excellent electrochemical properties.

Tuneable Giant Magnetocaloric Effect in (Mn,Fe)₂(P,Si) Materials by Co-B and Ni-B Co-Doping

The influence of Co (Ni) and B co-doping on the structural, magnetic and magnetocaloric properties of (Mn,Fe) 2 (P,Si) compounds is investigated by X-ray diffraction (XRD), differential scanning calorimetry, magnetic and direct temperature change measurements. It is found that Co (Ni) and B co-doping is an effective approach to tune both the Curie temperature and the thermal hysteresis of (Mn,Fe) 2 (P,Si) materials without losing either the giant magnetocaloric effect or the positive effect of the B substitution on the mechanical stability. An increase in B concentration leads to a rapid decrease in thermal hysteresis, while an increase in the Co or Ni concentration hardly changes the thermal hysteresis of the (Mn,Fe) 2 (P,Si) compounds. However, the Curie temperature decreases slowly as a function of the Co or Ni content, while it increases dramatically for increasing B concentration. Hence, the co-substitution of Fe and P by Co (Ni) and B, respectively, offers a new control parameter to adjust the Curie temperature and reduce the thermal hysteresis of the (Mn,Fe) 2 (P,Si) materials.