Enhanced Long-Term Stability of Crystalline Nickel-Boride (Ni23B6) Electrocatalyst by Encapsulation with Hexagonal Boron Nitride.

Nickel boride catalysts show great potential as low-cost and efficient alternatives to noble-metal catalysts in acidic media; however, synthesizing and isolating a specific phase and composition of nickel boride is nontrivial, and issues persist in their long-term stability as electrocatalysts. Here, a single-crystal nickel boride, Ni23B6, is reported which exhibits high electrocatalytic activity for the hydrogen evolution reaction (HER) in an acidic solution, and that its poor long-term stability can be overcome via encapsulation by single-crystal trilayer hexagonal boron nitride (hBN) film. Interestingly, hBN-covered Ni23B6 on a Ni substrate shows an identical overpotential of 52 mV at a current density of 10 mA cm-2 to that of bare Ni23B6. This phenomenon indicates that the single-crystalline hBN layer is catalytically transparent and does not obstruct HER activation. The hBN/Ni23B6/Ni has remarkable long-term stability with negligible changes to its polarization curves for 2000 cycles, whereas the Ni23B6/Ni shows significant degradation after 650 cycles. Furthermore, chronoamperometric measurements indicate that stability is preserved for >20 h. Long-term stability tests also reveal that the surface morphology and chemical structure of the hBN/Ni23B6/Ni electrode remain preserved. This work provides a model for the practical design of robust and durable electrochemical catalysts through the use of hBN encapsulation.

Phase-Engineered WS2 Monolayer Quantum Dots by Rhenium Doping.

Transition metal dichalcogenides (TMDs) occur in the thermodynamically stable trigonal prismatic (2H) phase or the metastable octahedral (1T) phase. Phase engineering of TMDs has proven to be a powerful tool for applications in energy storage devices as well as in electrocatalysis. However, the mechanism of the phase transition in TMDs and the synthesis of phase-controlled TMDs remain challenging. Here we report the synthesis of Re-doped WS2 monolayer quantum dots (MQDs) using a simple colloidal chemical process. We find that the incorporation of a small amount of electron-rich Re atoms in WS2 changes the metal-metal distance in the 2H phase initially, which introduces strain in the structure (strained 2H (S2H) phase). Increasing the concentration of Re atoms sequentially transforms the S2H phase into the 1T and 1T' phases to release the strain. In addition, we performed controlled experiments by doping MoS2 with Re to distinguish between Re and Mo atoms in scanning transmission electron microscopy images and quantified the concentration range of Re atoms in each phase of MoS2, indicating that phase engineering of WS2 or MoS2 is possible by doping with different amounts of Re atoms. We demonstrate that the 1T' WS2 MQDs with 49 at. % Re show superior catalytic performance (a low Tafel slope of 44 mV/dec, a low overpotential of 158 mV at a current density of 10 mA/cm2, and long-term durability up to 5000 cycles) for the hydrogen evolution reaction. Our findings provide understanding and control of the phase transitions in TMDs, which will allow for the efficient manufacturing and translation of phase-engineered TMDs.

Langmuir-Blodgett Monolayer of Cobalt Phthalocyanine as Ultralow Loading Single-Atom Catalyst for Highly Efficient H2O2 Production.

The electrochemical production of H2O2 via the two-electron oxygen-reduction reaction (2e- ORR) has been actively studied using systems with atomically dispersed metal-nitrogen-carbon (M-N-C) structures. However, the development of well-defined M-N-C structures that restrict the migration and agglomeration of single-metal sites remains elusive. Herein, we demonstrate a Langmuir-Blodgett (LB) monolayer of cobalt phthalocyanine (CoPc) on monolayer graphene (LB CoPc/G) as a single-metal catalyst for the 2e- ORR. The as-prepared CoPc LB monolayer has a ß-form crystalline structure with a lattice space for the facile adsorption of oxygen molecules on the cobalt active sites. The CoPc LB monolayer system provides highly exposed Co atoms in a well-defined structure without agglomeration, resulting in significantly improved catalytic activity, which is manifested by a very high H2O2 production rate per catalyst (31.04 mol gcat-1 h-1) and TOF (36.5 s-1) with constant production stability for 24 hours. To the best of our knowledge, the CoPc LB monolayer system exhibits the highest H2O2 production rate per active site. This fundamental study suggests that an LB monolayer of molecules with single-metal atoms as a well-defined structure works for single-atom catalysts.

Synthesis of 1T WSe2 on an Oxygen-Containing Substrate Using a Single Precursor.

The metallic property of metastable 1T' WSe2 and its promising catalytic performance have attracted considerable interest. A hot injection method has been used to synthesize 1T' WSe2 with a three-dimensional morphology; however, this method requires two or more precursors and long-chain ligands, which inhibit the catalytic performance. Here, we demonstrate the synthesis of 1T' WSe2 on a substrate by a simple heating-up method using a single precursor, tetraethylammonium tetraselenotungstate [(Et4N)2WSe4]. The triethylamine produced after the reaction is an electron donor that yields negatively charged WSe2, which is stabilized by triethylammonium cations as intercalants between layers and induces 1T' WSe2. The purity of 1T' WSe2 is higher on oxygen-containing crystalline substrates than amorphous substrates because the strong adhesion between WSe2 and the substrate can produce sufficient triethylammonium (TEA) intercalation. Among the oxygen-containing crystal substrates, the substrate with a lower lattice mismatch with 1T' WSe2 showed higher 1T' purity due to the uniform TEA intercalation. Furthermore, 1T' WSe2 on carbon cloth exhibited a more enhanced catalytic performance in the hydrogen evolution reaction (197 mV at 10 mA/cm2) than has been reported previously.