Intermetallic Ni2Ta Electrocatalyst for the Oxygen Evolution Reaction in Highly Acidic Electrolytes.

The identification of materials capable of catalyzing the oxygen evolution reaction (OER) in highly acidic electrolytes is a critical bottleneck in the development of many water-splitting technologies. Bulk-scale solid-state compounds can be readily produced using high-temperature reactions and therefore used to expand the scope of earth-abundant OER catalysts capable of operating under strongly acidic conditions. Here, we show that high temperature arc melting and powder metallurgy reactions can be used to synthesize electrodes consisting of intermetallic Ni2Ta that can catalyze the OER in 0.50 M H2SO4. Arc melted Ni2Ta electrodes evolve oxygen at a current density of 10 mA/cm2 for >66 h with corrosion rates 2 orders of magnitude lower than that of pure Ni. The overpotential required for pellets of polycrystalline Ni2Ta to produce a current density of 10 mA/cm2 is 570 mV. This strategy can be generalized to include other first-row transition metals, including intermetallic Fe2Ta and Co2Ta systems.

Low-Temperature Solution Synthesis of Transition Metal Dichalcogenide Alloys with Tunable Optical Properties.

Nanostructures of layered transition metal dichalcogenide (TMD) alloys with tunable compositions are promising candidates for a broad scope of applications in electronics, optoelectronics, topological devices, and catalysis. Most TMD alloy nanostructures are synthesized as films on substrates using gas-phase methods at high temperatures. However, lower temperature solution routes present an attractive alternative with the potential for larger-scale, higher-yield syntheses of freestanding, higher surface area materials. Here, we report the direct solution synthesis of colloidal few-layer TMD alloys, MoxW1-xSe2 and WS2ySe2(1-y), exhibiting fully tunable metal and chalcogen compositions that span the MoSe2-WSe2 and WS2-WSe2 solid solutions, respectively. Chemical guidelines for achieving the targeted compounds are presented, along with comprehensive structural characterizations (X-ray diffraction, electron microscopy, Raman, and UV-visible spectroscopies). High-resolution microscopic imaging confirms the formation of TMD alloys and identifies a random distribution of the alloyed elements. Analysis of the tilt-angle dependency of the intensities associated with atomic-resolution annular dark field imaging line scans reveals the types of point vacancies present in the samples, thus providing atomic-level insights into the structures of colloidal TMD alloy nanostructures that were previously only accessible for substrate-confined films. The A excitonic transition of the TMD alloy nanostructures can be readily adjusted between 1.51 and 1.93 eV through metal and chalcogen alloying, correlating the compositional modulation to the realization of tunable optical properties.

Stability and Activity of Non-Noble-Metal-Based Catalysts Toward the Hydrogen Evolution Reaction.

A fundamental understanding of the behavior of non-noble based materials toward the hydrogen evolution reaction is crucial for the successful implementation into practical devices. Through the implementation of a highly sensitive inductively coupled plasma mass spectrometer coupled to a scanning flow cell, the activity and stability of non-noble electrocatalysts is presented. The studied catalysts comprise a range of compositions, including metal carbides (WC), sulfides (MoS2 ), phosphides (Ni5 P4 , Co2 P), and their base metals (W, Ni, Mo, Co); their activity, stability, and degradation behavior was elaborated and compared to the state-of-the-art catalyst platinum. The non-noble materials are stable at HER potentials but dissolve substantially when no current is flowing. Through pre- and post-characterization of the catalysts, explanations of their stability (thermodynamics and kinetics) are discussed, challenges for the application in real devices are analyzed, and strategies for circumventing dissolution are suggested. The precise correlation of metal dissolution with applied potential/current density allows for narrowing down suitable material choices as replacement for precious group metals as for example, platinum and opens up new ways in finding cost-efficient, active, and stable new-generation electrocatalysts.

Metal Ruthenate Perovskites as Heterogeneous Catalysts for the Hydrolysis of Ammonia Borane.

Ammonia borane (NH3-BH3) is of interest as a hydrogen storage material because of its ease of use and its ability to release three molar equivalents of H2(g) via catalytic hydrolysis. Most heterogeneous catalysts for ammonia borane hydrolysis are nanoparticles containing expensive noble metals. Here, we show that metal ruthenate perovskites function as active and durable catalysts for ammonia borane hydrolysis. As a bulk powder, CaRuO3 catalyzes the hydrolysis of ammonia borane at room temperature and is recyclable and reusable. CaRuO3 facilitates the release of H2(g) from aqueous ammonia borane solutions at comparable rates to some other heterogeneous catalyst systems while having a low noble metal content. Other ruthenium-based perovskites, including SrRuO3, Ca2LaRuO6, Sr2CoRuO6, and SrLaCoRuO6, are similarly active catalysts for room-temperature ammonia borane hydrolysis.