Zhao Cai

Ultrathin dendritic Pt3Cu triangular pyramid caps with enhanced electrocatalytic activity.

Here we report on the synthesis of novel dendritic Pt3Cu triangular pyramid caps via a solvothermal coreduction method. These caps had three-dimensional caved structures with ultrathin branches, as evidenced by high-resolution transmission electron microscopy (HRTEM) and HAADF-STEM characterization. Tuning the reduction kinetics of two metal precursors by an iodide ion was believed to be the key for the formation of an alloyed nanostructure. Electro-oxidation of methanol and formic acid showed dramatically improved electrocatalytic activities and poison-tolerance for these nanoalloys as compared to commercial Pt/C catalysts, which was attributed to their unique open porous structure with interconnected network, ultrahigh surface areas, as well as synergetic effect of the two metallic components.

Ultrathin branched PtFe and PtRuFe nanodendrites with enhanced electrocatalytic activity

PtFe and PtRuFe nanodendrites with highly branched structure were obtained through a facile one-pot strategy. Time dependent experiments revealed that the kinetic control of the reduction process of the metal precursors played a key role in the formation of such open porous structure. Owing to its ultrathin branches, open porous but interconnected structure and synergetic effect of multicomponents, the PtRuFe nanodendrite turned out to be a high-performance electrocatalyst for methanol oxidation. It has been demonstrated that PtRuFe nanodendrites had a methanol oxidation mass activity of 1.14 A mg−1 Pt and a specific activity of 2.03 mA cm−2, which were far better than those of PtFe and commercial Pt/C catalyst.

Phosphorus oxoanion-intercalated layered double hydroxides for high-performance oxygen evolution

Rational design and controlled fabrication of efficient and cost-effective electrodes for the oxygen evolution reaction (OER) are critical for addressing the unprecedented energy crisis. Nickel–iron layered double hydroxides (NiFe-LDHs) with specific interlayer anions (i.e. phosphate, phosphite, and hypophosphite) were fabricated by a co-precipitation method and investigated as oxygen evolution electrocatalysts. Intercalation of the phosphorus oxoanion enhanced the OER activity in an alkaline solution; the optimal performance (i.e., a low onset potential of 215 mV, a small Tafel slope of 37.7 mV/dec, and stable electrochemical behavior) was achieved with the hypophosphite-intercalated NiFe-LDH catalyst, demonstrating dramatic enhancement over the traditional carbonate-intercalated NiFe-LDH in terms of activity and durability. This enhanced performance is attributed to the interaction between the intercalated phosphorous oxoanions and the edge-sharing MO6 (M = Ni, Fe) layers, which modifies the surface electronic structure of the Ni sites. This concept should be inspiring for the design of more effective LDH-based oxygen evolution electrocatalysts.

Effects of redox-active interlayer anions on the oxygen evolution reactivity of NiFe-layered double hydroxide nanosheets

Nickel-iron layered double hydroxide (NiFe-LDH) nanosheets have shown optimal oxygen evolution reaction (OER) performance; however, the role of the intercalated ions in the OER activity remains unclear. In this work, we show that the activity of the NiFe-LDHs can be tailored by the intercalated anions with different redox potentials. The intercalation of anions with low redox potential (high reducing ability), such as hypophosphites, leads to NiFe-LDHs with low OER overpotential of 240 mV and a small Tafel slope of 36.9 mV/dec, whereas NiFe-LDHs intercalated with anions of high redox potential (low reducing ability), such as fluorion, show a high overpotential of 370 mV and a Tafel slope of 80.8 mV/dec. The OER activity shows a surprising linear correlation with the standard redox potential. Density functional theory calculations and X-ray photoelectron spectroscopy analysis indicate that the intercalated anions alter the electronic structure of metal atoms which exposed at the surface. Anions with low standard redox potential and strong reducing ability transfer more electrons to the hydroxide layers. This increases the electron density of the surface metal sites and stabilizes their high-valence states, whose formation is known as the critical step prior to the OER process.

Topotactic reduction of layered double hydroxides for atomically thick two-dimensional non-noble-metal alloy

Layered double hydroxides (LDHs) have been widely used as catalysts owing to their tunable structure and atomic dispersion of high-valence metal ions; however, limited tunability of electronic structure and valence states have hindered further improvement in their catalytic performance. Herein, we reduced ultrathin LDH precursors in situ and topotactically converted them to atomically thick (~2 nm) two-dimensional (2D) multi-metallic, single crystalline alloy nanosheets with highly tunable metallic compositions. The as-obtained alloy nanosheets not only maintained the vertically aligned ultrathin 2D structure, but also inherited the atomic dispersion of the minor metallic compositions of the LDH precursors, even though the atomic percentage was higher than 20%, which is far beyond the reported percentages for single-atom dispersions (usually less than 0.1%). Besides, surface engineering of the alloy nanosheets can finely tune the surface electronic structure for catalytic applications. Such in situ topotactic conversion strategy has introduced a novel approach for atomically dispersed alloy nanostructures and reinforced the synthetic methodology for ultrathin 2D metal-based catalysts.

Solvent switching and purification of colloidal nanoparticles through water/oil Interfaces within a density gradient

AbstractTraditional post-treatment of colloidal nanoparticles (NPs) usually involves repeated centrifugation-wash-sonication processes to separate NPs from the original synthetic environment; however, such separation processes have either high energy cost or low efficiency and tend to cause aggregation. Here we show a general and scalable colloid post-processing technique based on density gradient centrifugation through water/oil interfaces. Such a one-step technique can switch the solvent in a colloid at almost any concentration without aggregation, and meanwhile purify colloidal nanoparticles by separating them from by-products and environmental impurities. Droplet sedimentation was shown to be the mechanism of this one-step concentration/purification process, and mathematical modeling was established to quantify the accumulation and sedimentation velocities of different NPs.

Aligned N-doped carbon nanotube bundles with interconnected hierarchical structure as an efficient bi-functional oxygen electrocatalyst

The fabrication of cost effective and efficient electrocatalysts with functional building blocks to replace noble metal ones is of great importance for energy related applications yet remains a great challenge. Herein, we report the fabrication of a hierarchical structure containing CNTs/graphene/transition-metal hybrids (h-NCNTs/Gr/TM) with excellent bifunctional oxygen electrocatalytic activity. The synthesis was rationally designed by the growth of shorter nitrogen-doped CNTs (S-NCNTs) on longer NCNTs arrays (L-NCNTs), while graphene layers were in situ generated at their interconnecting sites. The hybrid material shows excellent OER and ORR performance, and was also demonstrated to be a highly active bifunctional catalyst for Zn–air batteries, which could be due to rapid electron transport and full exposure of active sites in the hierarchical structure.

Application of Nanoseparation in Reaction Mechanism Analysis

Density gradient centrifugation has been established to obtain monodisperse nanoparticles with strictly uniform size and morphology, which are usually hard to be obtained by synthetic optimization. Previous chapters have demonstrated the versatility and universality of such separation method, by which nearly all kinds of nanostructures can be separated, including particles, clusters, and assemblies. Further, reaction mechanism, as well as structure–property relationship, can also be investigated based on the separated fractions. The focus of this chapter is the reaction mechanism analysis using density gradient centrifugation, namely by introducing a distinctive functional gradient layer, such as reaction zone and assembly zone, reaction mechanisms can be therefore studied since the reaction time can be pre-designed and the reaction environment can be switched extremely fast in a centrifugal force field. In a word, “lab in a tube” based on nanoseparation opens a new door for the investigation of synthetic optimization, assembly behavior, and surface reaction of various nanostructures.

Activating basal plane in NiFe layered double hydroxide by Mn2+ doping for efficient and durable oxygen evolution reaction

Foreign metal ions with reducing ability were doped into NiFe layered double hydroxides (NiFe-LDHs) to activate the basal plane in NiFe-LDHs for oxygen evolution reaction (OER). Mn2+-Doped NiFe-LDH array electrode yields a low onset potential of 1.41 V and exhibits outstanding stability. The study herein illustrates a new dimension of electronic structure regulation and promises further optimization of highly efficient electrocatalysts.

Introducing Fe2+ into Nickel–Iron Layered Double Hydroxide: Local Structure Modulated Water Oxidation Activity

Exploring materials with regulated local structures and understanding how the atomic motifs govern the reactivity and durability of catalysts are a critical challenge for designing advanced catalysts. Herein we report the tuning of the local atomic structure of nickel-iron layered double hydroxides (NiFe-LDHs) by partially substituting Ni2+ with Fe2+ to introduce Fe-O-Fe moieties. These Fe2+ -containing NiFe-LDHs exhibit enhanced oxygen evolution reaction (OER) activity with an ultralow overpotential of 195 mV at the current density of 10 mA cm-2 , which is among the best OER catalytic performance to date. In-situ X-ray absorption, Raman, and electrochemical analysis jointly reveal that the Fe-O-Fe motifs could stabilize high-valent metal sites at low overpotentials, thereby enhancing the OER activity. These results reveal the importance of tuning the local atomic structure for designing high efficiency electrocatalysts.