Linking Composition, Structure and Thickness of CoOOH layers to Oxygen Evolution Reaction Activity by Correlative Microscopy.

The role of ß-CoOOH crystallographic orientations in catalytic activity for the oxygen evolution reaction (OER) remains elusive. We combine correlative electron backscatter diffraction/scanning electrochemical cell microscopy with X-ray photoelectron spectroscopy, transmission electron microscopy, and atom probe tomography to establish the structure-activity relationships of various faceted ß-CoOOH formed on a Co microelectrode under OER conditions. We reveal that ≈6 nm ß-CoOOH(01 1 ‾ ${\bar{1}}$ 0), grown on [ 1 ‾ 2 1 ‾ ${\bar{1}2\bar{1}}$ 0]-oriented Co, exhibits higher OER activity than ≈3 nm ß-CoOOH(10 1 ‾ ${\bar{1}}$ 3) or ≈6 nm ß-CoOOH(0006) formed on [02 2 ‾ 1 ] ${\bar{2}1]}$ - and [0001]-oriented Co, respectively. This arises from higher amounts of incorporated hydroxyl ions and more easily reducible CoIII -O sites present in ß-CoOOH(01 1 ‾ ${\bar{1}}$ 0) than those in the latter two oxyhydroxide facets. Our correlative multimodal approach shows great promise in linking local activity with atomic-scale details of structure, thickness and composition of active species, which opens opportunities to design pre-catalysts with preferred defects that promote the formation of the most active OER species.

Electro-optic modulation aberration correction algorithm based on phase difference compensation.

In order to achieve wide field-of-view, high-resolution LIDAR, a gating imaging structure combining an electro-optic crystal and an electron multiplication CCD is constructed. According to the index ellipsoid theory, a 3D ray tracing model is established to explore the principle of electro-optic modulation. The field-of-view and interference intensity distribution of the LiNbO3(LN) crystal electro-optic modulation are studied by using the proposed model. In order to solve the problem that the interference light intensity at the edge of the field-of-view of crystal electro-optic modulation is not homogeneous, we study and propose an electro-optic modulation aberration correction algorithm based on phase difference compensation. The corrected interference light intensity at the edge of the field-of-view increased from 0.87 to 0.94. Finally, a LIDAR imaging simulation system is established to image the target at a distance of 2000 m. The results show that under the condition of a 1° wide field-of-view, the imaging accuracy of the system is 5 mm, and the average imaging error introduced by the crystal electro-optic modulation is less than 2 cm.

In situ fabrication of dynamic self-optimizing Ni3S2 nanosheets as an efficient catalyst for the oxygen evolution reaction.

In this work, a dynamic self-optimizing material consisting of nickel-sulfide nanosheets anchored onto Ni foam (DSO-Ni3S2-NF) as the model material was constructed using a hydrothermal method, and its electrocatalytic performance for oxygen evolution was evaluated. It was found that the electrocatalytic activity of the dynamic self-optimizing (DSO) 25 h-Ni3S2-NF for oxygen evolution is significantly enhanced compared with that of pristine 0 h-Ni3S2-NF since the formed oxide layer evolves into new active sites and the specific process of activity optimization was explored dynamically. The best oxygen evolution reaction (OER) performance was achieved by 25 h-Ni3S2-NF catalyst, which required merely 241 mV overpotential to deliver a current density of 20 mA cm-2, and its Tafel slope was as low as ∼40 mV dec-1, which was superior to most nickel-based catalysts, in 1 M KOH electrolyte. The current density was found to be increased gradually at the same potential and the stability test curves were steady with ignorable decline, showing that the promising strategy of the preparation of a dynamic self-optimizing pre-catalyst may open a new pathway to prepare low-cost, high-performance and stable water splitting catalysts.

The in situ etching assisted synthesis of Pt-Fe-Mn ternary alloys with high-index facets as efficient catalysts for electro-oxidation reactions.

Pt-Based alloys enclosed with high-index facets (HIFs) generally show much higher specific catalytic activities than their counterparts with low-index facets in electro-catalytic reactions. However, the exposure of a certain Pt surface would require a well-defined nanostructure, which usually can only be obtained at larger sizes. Therefore, a low dispersion of Pt atoms in Pt-based alloys with HIFs would affect the atomic utilization of Pt, resulting in most of these Pt-based alloys exhibiting lower mass activity than commercial Pt/C and Pt black catalysts for electro-catalytic reactions. Herein, we address a novel strategy to divide the surface areas of larger sized nanocrystals into small surface area nanocrystals by in situ etching Pt-Fe-Mn concave cubes (CNCs) while maintaining the morphology of the Pt-Fe-Mn alloys to improve the utilization of Pt atoms and thus increase the mass activity. Remarkably, the Pt-Fe-Mn unique concave cube (UCNC) nanocrystals (NCs) showed much higher specific and mass activities toward the methanol oxidation reaction (MOR) than the Pt-Fe-Mn CNCs, commercial Pt black and Pt/C. The kinetic analysis from Tafel plots indicated that UCNC Pt-Fe-Mn NCs had the lowest Tafel slope at whole potentials and the splitting of the first C-H bond of a CH3OH molecule with the first electron transfer was the rate-determining step at high potentials (above 0.45 V). In situ Fourier transform infrared reflection (FTIR) spectroscopic investigation at the molecular level indicated that methanol chemical absorption took place at a low potential of -0.2 V at the UCNC NC electrode. Meanwhile, much higher CO2 productivity was observed at the UCNC NC electrode, indicating the strong anti-poisoning ability of the UCNC Pt-Fe-Mn NCs during methanol electrooxidation. Furthermore, in the formic acid oxidation (FAOR) test, the activity and long-term durability of the Pt-Fe-Mn UCNC NCs were also found to be superior to those of the Pt-Fe-Mn CNCs, commercial Pt black and Pt/C. The enhanced catalytic performance in both the MOR and FAOR is most probably due to the unique HIF structure consisting of small sized particles, enhanced Pt utilization, the richness of crystalline defects and synergetic effects of Pt, Fe, and Mn metals. Our present work provides an insight into the rational design of Pt based alloys with HIFs to improve the catalytic performance of electro-catalytic reactions for fundamental study.

Cobalt/Molybdenum Phosphide and Oxide Heterostructures Encapsulated in N-Doped Carbon Nanocomposite for Overall Water Splitting in Alkaline Media.

The development of designing and searching inexpensive electrocatalysts with high activity for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is significant to enable water splitting as a future renewable energy source. Herein, we synthesize a new CoP(MoP)-CoMoO3 heterostructure coated by a N-doped carbon shell [CoP(MoP)-CoMoO3@CN] via thermal decomposition and phosphatizing of the CoMoO4·0.9H2O nanowires encapsulated in N-doped carbon. At 10 mA·cm-2, this CoP(MoP)-CoMoO3@CN nanocomposite exhibits superior electrocatalytic activity at low overpotentials of 296 mV for OER and 198 mV for HER in alkaline media. More importantly, we achieve a current density of 10 mA·cm-2 at 1.55 V by using this CoP(MoP)-CoMoO3@CN as both cathode and anode for overall water splitting. This promising performance could be due to the high activity of CoP(MoP)-CoMoO3 and the good conductivity of the external mesoporous N-carbon shell, which makes the CoP(MoP)-CoMoO3@CN nanowires a competitive alternative to noble-metal-based catalysts for water splitting.

Promoting effect of nickel hydroxide on the electrocatalytic performance of Pt in alkaline solution.

Despite intense research in the past decades, the lack of high-performance catalysts for fuel cell reactions remains a challenge in realizing fuel cell applications. Herein, we report a novel hybrid nanomaterial of platinum-nickel hydroxide-nanotubes (Pt/Ni(OH)2/CNTs) for improving electrocatalytic performance in alkaline environments. Ni(OH)2 was directly grown on functionalized nanotubes and then, Pt nanoparticles were in situ immobilized by the microwave synthesis method. Due to electronic and synergistic effects, 10 : 2-Pt/Ni(OH)2/N-CNT catalyst exhibited 2.77 times specific activity and 6.27 times mass activity toward methanol oxidation reaction (MOR), which were higher than those of commercial Pt/C in alkaline solution. The CO-stripping experiments and hydrogen evolution reaction (HER) further demonstrated that Ni(OH)2 could promote oxidation removal of carbonaceous poison for MOR via accelerating water dissociation: (i) Ni(OH)2 acted on an H2O molecule, leading to the formation of OHad; (ii) OHad oxidized the intermediate COad to CO2. Furthermore, the 10 : 2-Pt/Ni(OH)2/N-CNT catalyst also exhibited 2.07 times specific activity and 1.67 times mass activity toward oxygen reduction reaction (ORR), which were higher than those of commercial Pt/C in alkaline solution and Pt/N-CNT catalysts. Thus, the preparation of this hybrid nanomaterial provides a new direction for catalyst performance optimization towards next-generation fuel cells in alkaline environments.

Structure Effects of 2D Materials on α-Nickel Hydroxide for Oxygen Evolution Reaction.

To engineer low-cost, high-efficiency, and stable oxygen evolution reaction (OER) catalysts, structure effects should be primarily understood. Focusing on this, we systematically investigated the relationship between structures of materials and their OER performances by taking four 2D α-Ni(OH)2 as model materials, including layer-stacked bud-like Ni(OH)2-NB, flower-like Ni(OH)2-NF, and petal-like Ni(OH)2-NP as well as the ultralarge sheet-like Ni(OH)2-NS. For the first three (layer-stacking) catalysts, with the decrease of stacked layers, their accessible surface areas, abilities to adsorb OH-, diffusion properties, and the intrinsic activities of active sites increase, which accounts for their steadily enhanced activity. As expected, Ni(OH)2-NP shows the lowest overpotential (260 mV at 10 mA cm-2) and Tafel slope (78.6 mV dec-1) with a robust stability over 10 h among the samples, which also outperforms the benchmark IrO2 (360 mV and 115.8 mV dec-1) catalyst. Interestingly, Ni(OH)2-NS relative to Ni(OH)2-NP exhibits even faster substance diffusion due to the sheet-like structure, but shows inferior OER activity, which is mainly because the Ni(OH)2-NP with a smaller size possesses more active boundary sites (higher reactivity of active sites) than Ni(OH)2-NS, considering the adsorption properties and accessible surface areas of the two samples are quite similar. By comparing the different structures and their OER behaviors of four α-Ni(OH)2 samples, our work may shed some light on the structure effect of 2D materials and accelerate the development of efficient OER catalysts.

A General Strategy Assisted with Dual Reductants and Dual Protecting Agents for Preparing Pt-Based Alloys with High-Index Facets and Excellent Electrocatalytic Performance.

Synthesizing noble metallic nanoparticles (NPs) enclosed by high-index facets (HIFs) is challenged as it involves the tuning of growth kinetics, the selective adsorption of certain chemical species, and the epitaxial growth from HIF enclosed seeds. Herein, a simple and general strategy is reported by using dual reduction agents and dual capping agents to prepare Pt-based alloy NPs with HIFs, in which both glycine and poly(vinylpyrrolidone) serve as the reductants and capping agents. Due to the facilely tunable growth/nucleation rates and protecting abilities of the reductants and capping agents, Pt concave nanocube (CNC), binary Pt-Ni CNC, ternary Pt-Mn-Cu CNC, and Pt-Mn-Cu ramiform polyhedron alloy NPs terminated by HIFs as well as other NPs with well-defined morphologies such as Pt-Mn-Cu nanocube and Pt-Mn-Cu nanoflower are obtained with this approach. Owing to the high density of low-coordinated Pt sites (HIF structure) and the unique electronic effect of Pt-Mn-Cu ternary alloys, the as-prepared Pt-Mn-Cu NPs show enhanced catalytic activity toward methanol and formic acid electro-oxidation reactions with excellent stability. This work provides a promising methodology for designing and fabricating Pt-based alloy NPs as efficient fuel cell catalyst.