Efficient Hydrogen Oxidation Catalyzed by Strain-Engineered Nickel Nanoparticles

The hydroxide-exchange membrane fuel cell (HEMFC) is a promising energy conversion device. However, the development of HEMFC is hampered by the lack of platinum-group-metal-free (PGM-free) electrocatalysts for the hydrogen oxidation reaction (HOR). Now, a Ni catalyst is reported that exhibits the highest mass activity in HOR for a PGM-free catalyst as well as excellent activity in the hydrogen evolution reaction (HER). This catalyst, Ni-H-2-2 %, was optimized through pyrolysis of a Ni-containing metal-organic framework precursor under a mixed N-2/H-2 atmosphere, which yielded carbon-supported Ni nanoparticles with different levels of strains. The Ni-H-2-2 % catalyst has an optimal level of strain, which leads to an optimal hydrogen binding energy and a high number of active sites

Nuclear Magnetic Resonance Study of Atomic Motion in Bimetallic Perovskite-Type Borohydrides ACa(BH₄)₃ (A = K, Rb, or Cs)

To study the dynamical properties of the novel series of bimetallic perovskite-type borohydrides ACa(BH4)3 (A = K, Rb, or Cs), we have measured the 1H and 11B nuclear magnetic resonance spectra and spin–lattice relaxation rates in these compounds over broad temperature ranges (5–560 K) and resonance frequency ranges (14–90 MHz). Our measurements have revealed several jump processes with different characteristic rates. For all the compounds studied, the spin–lattice relaxation rates at low temperatures (T < 340 K) are governed by fast reorientations of BH4 groups. However, the experimental data in this range cannot be described in terms of a single reorientational process; this suggests a coexistence of at least two types of BH4 reorientations. Taking into account the linear coordination of each BH4 group in ACa(BH4)3 by two Ca atoms, we can attribute different reorientational processes to BH4 rotations around inequivalent symmetry axes. The parameters of reorientational motion in ACa(BH4)3 have been evaluated using the model with a two-peak distribution of activation energies. It has been found that the transitions from the low-temperature phases to the high-temperature phases of ACa(BH4)3 are accompanied by the onset of translational diffusion of intact BH4 groups. However, the jump rates for these diffusive processes remain below 108 s–1 up to the high-T limits of our experimental ranges

Essential role of oxygen vacancies of Cu-Al and Co-Al spinel oxides in their catalytic activity for the reverse water gas shift reaction

CO2 catalytic hydrogenation to CO will likely be an important part of CO2 mitigation and valorization processes. Recently, we have developed materials containing surface-formed Cu-Al spinel that act as active and stable catalysts for this reaction. Here, the fundamental characteristics of Cu-Al and Co-Al spinel catalysts were studied to understand the role of the spinel structure in its catalytic activity for the reverse water gas shift reaction. Based on the catalytic tests, Cu-Al spinel was found to have a higher catalytic activity. By means of catalysts characterization combined with theoretical studies, this increased activity was attributed to the higher number of oxygen vacancies on its surface which were formed because of the higher inversion degree of Cu-Al spinel, which in turn, resulted in the formation of a more disordered structure with cation substitution. We found that the oxygen vacancies were vital for CO2 adsorption and activation on the spinel surfaces

Unblocking Ion-occluded Pore Channels in Poly(triazine imide) Framework for Proton Conduction

Poly(triazine imide) or PTI is an ordered graphitic carbon nitride hosting angstrom-scale pores attractive for selective molecular transport. AA'-stacked PTI layers are synthesized by ionothermal route during which ions occupy the framework and occlude the pores. Synthesis of ion-free PTI hosting AB-stacked layers has been reported, however, pores in this configuration are blocked by the neighboring layer. The unavailability of open pore limits application of PTI in molecular transport. Herein, we demonstrate acid treatment for ion depletion which maintains AA' stacking and results in open pore structure. We provide first direct evidence of ion-depleted open pores by imaging with the atomic resolution using integrated differential phase-contrast scanning transmission electron microscopy. Depending on the extent of ion-exchange, AA' stacking with open channels and AB stacking with closed channels are obtained and imaged for the first time. The accessibility of open channels is demonstrated by enhanced proton transport through ion depleted PTI.LA

Infrared spectroscopy, Raman spectroscopy and x-ray diffraction of lithium purple bronze Li0.9Mo6O17 for pressure between 0 and 15 GPa

AbstractOpen data to the publication “Pressure-induced structural transitions triggering dimensional crossover in lithium purple bronze Li0:9Mo6O17

Universal approach toward high-efficiency two-dimensional perovskite solar cells via a vertical-rotation process

The emerging 2D perovskites exhibit superior stability and similar optoelectronic attributes compared to the 3D analogues, but their strong exciton-binding energy and inferior interlayer charge-transport reduce dramatically the device performance. Herein, we report a universal approach towards high-efficiency 2D perovskite solar cells (PSCs) by using the synergistic effect of NH4Cl and H2O to rotate the crystallographic orientation of 2D systems. The preferential adsorption of NH4Cl to the (202) crystal plane and the accelerated deprotonation of NH(4)(+)by H2O guide the crystal growth of the 2D framework towards vertical out-of-plane orientation, which strongly improves the 2D crystallinity, charge mobility, and carrier lifetime. As a representative, (PEA)(2)(MA)(3)Pb4I13-based PSCs (n <= 4) preparedviathe vertical-rotation process achieve a champion power conversion efficiency (PCE) of 17.03%, among the best PCEs reported for 2D PSCs. These findings offer a universal approach to rotate the orientation of 2D perovskites for efficient photovoltaics regardless of the perovskite composition

Revealing Weak Dimensional Confinement Effects in Excitonic Silver/Bismuth Double Perovskites

: Lead-free perovskites are attracting increasing interest as nontoxic materials for advanced optoelectronic applications. Here, we report on a family of silver/bismuth bromide double perovskites with lower dimensionality obtained by incorporating phenethylammonium (PEA) as an organic spacer, leading to the realization of two-dimensional double perovskites in the form of (PEA)4AgBiBr8 (n = 1) and the first reported (PEA)2CsAgBiBr7 (n = 2). In contrast to the situation prevailing in lead halide perovskites, we find a rather weak influence of electronic and dielectric confinement on the photophysics of the lead-free double perovskites, with both the 3D Cs2AgBiBr6 and the 2D n = 1 and n = 2 materials being dominated by strong excitonic effects. The large measured Stokes shift is explained by the inherent soft character of the double-perovskite lattices, rather than by the often-invoked band to band indirect recombination. We discuss the implications of these results for the use of double perovskites in light-emitting applications

Selective growth of layered perovskites for stable and efficient photovoltaics

Perovskite solar cells (PSCs) are promising alternatives toward clean energy because of their high-power conversion efficiency (PCE) and low materials and processing cost. However, their poor stability under operation still limits their practical applications. Here we design an innovative approach to control the surface growth of a low dimensional perovskite layer on top of a bulk three-dimensional (3D) perovskite film. This results in a structured perovskite interface where a distinct layered low dimensional perovskite is engineered on top of the 3D film. Structural and optical properties of the stack are investigated and solar cells are realized. When embodying the low dimensional perovskite layer, the photovoltaic cells exhibit an enhanced PCE of 20.1% on average, when compared to pristine 3D perovskite. In addition, superior stability is observed: the devices retain 85% of the initial PCE stressed under one sun illumination for 800 hours at 50 °C in an ambient environment