Facet-Dependent Surface Restructuring on Nickel (Oxy)hydroxides: A Self-Activation Process for Enhanced Oxygen Evolution Reaction.

Unraveling the catalyst surface structure and behavior during reactions is essential for both mechanistic understanding and performance optimization. Here we report a phenomenon of facet-dependent surface restructuring intrinsic to ß-Ni(OH)2 catalysts during oxygen evolution reaction (OER), discovered by the correlative ex situ and operando characterization. The ex situ study after OER reveals ß-Ni(OH)2 restructuring at the edge facets to form nanoporous Ni1-xO, which is Ni deficient containing Ni3+ species. Operando liquid transmission electron microscopy (TEM) and Raman spectroscopy further identify the active role of the intermediate ß-NiOOH phase in both the OER catalysis and Ni1-xO formation, pinpointing the complete surface restructuring pathway. Such surface restructuring is shown to effectively increase the exposed active sites, accelerate Ni oxidation kinetics, and optimize *OH intermediate bonding energy toward fast OER kinetics, which leads to an extraordinary activity enhancement of ∼16-fold. Facilitated by such a self-activation process, the specially prepared ß-Ni(OH)2 with larger edge facets exhibits a 470-fold current enhancement than that of the benchmark IrO2, demonstrating a promising way to optimize metal-(oxy)hydroxide-based catalysts.

Correlation between Electrolyte Chemistry and Solid Electrolyte Interphase for Reversible Ca Metal Anodes.

The development of rechargeable Ca metal batteries (RCMBs) is hindered by the Ca2+ passivating solid electrolyte interphases (SEIs). The cation solvation structure dictated by electrolyte chemistry plays a critical role in the SEIs properties. While a relatively weak cation-solvent binding is preferred in Li metal anodes to promote anion-derived SEIs, we demonstrate an enhanced Ca deposition/stripping reversibility under a strong cation-solvent interaction, which is materialized in strongly-solvating solvent and highly-dissociated salt combinations. Such electrolyte formulations benefit the formation of solvent-occupied solvation structure and minimize the anion reduction, resulting in organic-rich/CaF2 -poor SEIs for reversible Ca metal anodes. Furthermore, RCMBs paired with an organic cathode using the optimized electrolytes are demonstrated as a proof-of-concept. Our work reveals the paradigm shift in SEIs design for Ca metal anodes, opening up new opportunities for emerging RCMBs.

Robust and Flexible Random Lasers Using Perovskite Quantum Dots Coated Nickel Foam for Speckle-Free Laser Imaging.

The advantage of using flexible metallic structures as the substrate of flexible lasers over plastic materials is its strong mechanical strength and high thermal conductivity. Here, it is proposed to deposit CsPbBr3 perovskite quantum dots onto Ni porous foam for the realization of flexible lasers. Under two-photon 800 nm excitation at room temperature, incoherent random lasing emission is observed at ≈537 nm. By external deformation of the Ni porous foam, incoherent random lasing can be tuned to amplified spontaneous emission as well as the corresponding lasing threshold be controlled. More importantly, it is demonstrated that the laser is robust to intensive bending (>1000 bending cycles) with minimum effect on the lasing intensity. This flexible laser is also shown to be an ideal light source to produce a "speckle" free micro-image.

Direct Observation of Oxygen Evolution and Surface Restructuring on Mn2O3 Nanocatalysts Using In Situ and Ex Situ Transmission Electron Microscopy.

Direct observation of oxygen evolution reaction (OER) on catalyst surface may significantly advance the mechanistic understanding of OER catalysis. Here, we report the first real-time nanoscale observation of chemical OER on Mn2O3 nanocatalyst surface using an in situ liquid holder in a transmission electron microscope (TEM). The oxygen evolution process can be directly visualized from the development of oxygen nanobubbles around nanocatalysts. The high spatial and temporal resolution further enables us to unravel the real-time formation of a surface layer on Mn2O3, whose thickness oscillation reflects a partially reversible surface restructuring relevant to OER catalysis. Ex situ atomic-resolution TEM on the residual surface layer after OER reveals its amorphous nature with reduced Mn valence and oxygen coordination. Besides shedding light on the dynamic OER catalysis, our results also demonstrate a powerful strategy combining in situ and ex situ TEM for investigating various chemical reaction mechanisms in liquid.

Breaking Trade-Off between Selectivity and Activity of Nickel-Based Hydrogenation Catalysts by Tuning Both Steric Effect and d-Band Center.

For selective hydrogenation of chemicals the high selectivity is always at the expense of activity and improving both selectivity and activity is challenging. Here, by chelating with p-fluorothiophenol (SPhF)-arrays, both steric and electronic effects are created to boost the performance of cheap nickel-based catalysts. Compared with dinickel phosphide, the SPhF-chelated one exhibits nearly 12 times higher activity and especially its selectivity is increased from 38.1% and 21.3% to nearly 100% in hydrogenations of 3-nitrostyrene and cinnamaldehyde. Commercial catalysts like Raney Ni chelating with SPhF-array also exhibits an enhanced selectivity from 20.5% and 23.4% to ≈100% along with doubled activity. Both experimental and density functional theory (DFT) calculation prove that the superior performance is attributed to the confined flat adsorption by ordered SPhF-arrays and downshifted d-band center of catalysts, leading to prohibited hydrogenation of the vinyl group and accelerative H2 activation. Such a surface modification can provide an easily-realized and low-cost way to design catalysts for the selective hydrogenation.

Iron phosphide encapsulated in P-doped graphitic carbon as efficient and stable electrocatalyst for hydrogen and oxygen evolution reactions.

The development of durable and efficient non-noble electrocatalysts for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is highly desirable but challenging for the commercialization of renewable energy systems. Herein, a facile strategy is developed for the synthesis of iron phosphide (FeP) nanoparticles protected with an overcoat of "multifunctional" P-doped graphitic carbon as a cost-effective electrocatalyst. The key point is the utilization of MOF-derived iron nanoparticles embedded in graphitic carbon (Fe@GC), which are synthesized via the pyrolysis of the Fe-MIL-88 template and subsequent phosphorization of Fe and simultaneous doping of P in carbon. Compared to the direct phosphorization of Fe-MIL-88, resulting in Fe2P on amorphous carbon (Fe2P@APC), this strategy gives easier access to phosphorization and P doping through pyrolysis temperature regulation. High-temperature pyrolysis can also yield the graphitic carbon encapsulated nanoparticle structure (FeP@GPC), which increases conductivity and prevents agglomeration as well as dissolution under harsh operating conditions, and thus contributes to enhanced activity and long-time stability. The optimized FeP@GPC exhibits superior activity compared to Fe2P/FeP@GPC and Fe2P@APC, which is attributed to the modified electronic structure of FeP due to its greater P proportion than Fe2P together with the strong synergy between the nanoparticles and graphitic carbon. In detail, FeP@GPC exhibits an ultralow overpotential of 72 mV and 278 mV to achieve the current density of 10 mA cm-2 for the HER in acid and OER in alkaline media, respectively, together with negligible degradation after 20 h, which ranks among the best Fe-based electrocatalysts.

Electrocatalysts for Hydrogen Evolution in Alkaline Electrolytes: Mechanisms, Challenges, and Prospective Solutions.

Hydrogen evolution reaction (HER) in alkaline medium is currently a point of focus for sustainable development of hydrogen as an alternative clean fuel for various energy systems, but suffers from sluggish reaction kinetics due to additional water dissociation step. So, the state-of-the-art catalysts performing well in acidic media lose considerable catalytic performance in alkaline media. This review summarizes the recent developments to overcome the kinetics issues of alkaline HER, synthesis of materials with modified morphologies, and electronic structures to tune the active sites and their applications as efficient catalysts for HER. It first explains the fundamentals and electrochemistry of HER and then outlines the requirements for an efficient and stable catalyst in alkaline medium. The challenges with alkaline HER and limitation with the electrocatalysts along with prospective solutions are then highlighted. It further describes the synthesis methods of advanced nanostructures based on carbon, noble, and inexpensive metals and their heterogeneous structures. These heterogeneous structures provide some ideal systems for analyzing the role of structure and synergy on alkaline HER catalysis. At the end, it provides the concluding remarks and future perspectives that can be helpful for tuning the catalysts active-sites with improved electrochemical efficiencies in future.