Pengju Ren

Enhanced Electron Penetration through an Ultrathin Graphene Layer for Highly Efficient Catalysis of the Hydrogen Evolution Reaction

Major challenges encountered when trying to replace precious-metal-based electrocatalysts of the hydrogen evolution reaction (HER) in acidic media are related to the low efficiency and stability of non-precious-metal compounds. Therefore, new concepts and strategies have to be devised to develop electrocatalysts that are based on earth-abundant materials. Herein, we report a hierarchical architecture that consists of ultrathin graphene shells (only 1-3 layers) that encapsulate a uniform CoNi nanoalloy to enhance its HER performance in acidic media. The optimized catalyst exhibits high stability and activity with an onset overpotential of almost zero versus the reversible hydrogen electrode (RHE) and an overpotential of only 142 mV at 10 mA cm(-2) , which is quite close to that of commercial 40 % Pt/C catalysts. Density functional theory (DFT) calculations indicate that the ultrathin graphene shells strongly promote electron penetration from the CoNi nanoalloy to the graphene surface. With nitrogen dopants, they synergistically increase the electron density on the graphene surface, which results in superior HER activity on the graphene shells.

Highly active and durable non-precious-metal catalysts encapsulated in carbon nanotubes for hydrogen evolution reaction

Employing a low-cost and highly efficient electrocatalyst to replace Pt-based catalysts for hydrogen evolution reaction (HER) has attracted increasing interest in renewable energy research. Earth-abundant transition metals such as Fe, Co and Ni have been investigated as promising alternatives in alkaline electrolytes. However, these non-precious-metal catalysts are not stable in acids, excluding their application in the acidic solid polymer electrolyte (SPE). Herein, we report a strategy to encapsulate 3d transition metals Fe, Co and the FeCo alloy into nitrogen-doped carbon nanotubes (CNTs) and investigated their HER activity in acidic electrolytes. The optimized catalysts exhibited long-term durability and high activity with only an ∼70 mV onset overpotential vs. RHE which is quite close to that of the commercial 40% Pt/C catalyst, demonstrating the potential for the replacement of Pt-based catalysts. Density functional theory (DFT) calculations indicated that the introduction of metal and nitrogen dopants can synergistically optimize the electronic structure of the CNTs and the adsorption free energy of H atoms on CNTs, and therefore promote the HER with a Volmer–Heyrovsky mechanism.

Triggering the electrocatalytic hydrogen evolution activity of the inert two-dimensional MoS2 surface via single-atom metal doping

Electrocatalytic splitting of water is one of the most efficient technologies for hydrogen production, and two-dimensional (2D) MoS2 has been considered as a potential alternative to Pt-based catalysts in the hydrogen evolution reaction (HER). However, the catalytic activity of 2D MoS2 is always contributed from its edge sites, leaving a large number of in-plane domains useless. Herein, we for the first time demonstrated that the catalytic activity of in-plane S atoms of MoS2 can be triggered via single-atom metal doping in HER. In experiments, single Pt atom-doped, few-layer MoS2 nanosheets (Pt–MoS2) showed a significantly enhanced HER activity compared with pure MoS2, originating from the tuned adsorption behavior of H atoms on the in-plane S sites neighboring the doped Pt atoms, according to the density functional theory (DFT) calculations. Furthermore, the HER activity of MoS2 doped with a number of transition metals was screened by virtue of DFT calculations, resulting in a volcano curve along the adsorption free energy of H atoms , which was further confirmed in experiment by using non-precious metals such as Co and Ni atoms doping 2D MoS2 as the catalysts.

Single layer graphene encapsulating non-precious metals as high-performance electrocatalysts for water oxidation

The oxygen evolution reaction (OER) is recognized as a key half-reaction in water electrolysis for clean hydrogen energy, which is kinetically not favored and usually requires precious metal catalysts such as IrO2 and RuO2 to reduce the overpotential. The major challenge in using non-precious metals in place of these precious metal catalysts for OER is their low efficiency and poor stability, which urgently demand new concepts and strategies to tackle this issue. Herein, we report a universal strategy to directly synthesize single layer graphene encapsulating uniform earth-abundant 3d transition-metal nanoparticles such as Fe, Co, Ni and their alloys in a confined channel of mesoporous silica. The single atomic thickness of the graphene shell immensely promotes the electron transfer from the encapsulated metals to the graphene surface, which efficiently optimizes the electronic structure of the graphene surface and thereby triggers the OER activity of the inert graphene surface. We investigated a series of non-precious 3d metals encapsulated within single layer graphene, and found that the encapsulated FeNi alloy showed the best OER activity in alkaline solutions with only 280 mV overpotential at 10 mA cm−2, and also possessed a high durability after 10 000 cycles. Both the activity and durability of the non-precious catalyst even exceed that of the commercial IrO2 catalyst, showing great potential to replace precious metal catalysts in the OER.

Silicon carbide-derived carbon nanocomposite as a substitute for mercury in the catalytic hydrochlorination of acetylene

Acetylene hydrochlorination is an important coal-based technology for the industrial production of vinyl chloride, however it is plagued by the toxicity of the mercury chloride catalyst. Therefore extensive efforts have been made to explore alternative catalysts with various metals. Here we report that a nanocomposite of nitrogen-doped carbon derived from silicon carbide activates acetylene directly for hydrochlorination in the absence of additional metal species. The catalyst delivers stable performance during a 150 hour test with acetylene conversion reaching 80% and vinyl chloride selectivity over 98% at 200 °C. Experimental studies and theoretical simulations reveal that the carbon atoms bonded with pyrrolic nitrogen atoms are the active sites. This proof-of-concept study demonstrates that such a nanocomposite is a potential substitute for mercury while further work is still necessary to bring this to the industrial stage. Furthermore, the finding also provides guidance for design of carbon-based catalysts for activation of other alkynes.

Toward fundamentals of confined catalysis in carbon nanotubes.

An increasing number of experimental studies have demonstrated that metal or metal oxide nanoparticles confined inside carbon nanotubes (CNTs) exhibit different catalytic activities with respect to the same metals deposited on the CNT exterior walls, with some reactions enhanced and others hindered. In this article, we describe the concept of confinement energy, which enables prediction of confinement effects on catalytic activities in different reactions. Combining density functional theory calculations and experiments by taking typical transition metals such as Fe, FeCo, RhMn, and Ru as models, we observed stronger strains and deformations within the CNT channels due to different electronic structures and spatial confinement. This leads to downshifted d-band states, and consequently the adsorption of molecules such as CO, N2, and O2 is weakened. Thus, the confined space of CNTs provides essentially a unique microenvironment due to the electronic effects, which shifts the volcano curve of the catalytic activities toward the metals with higher binding energies. The extent of the shift depends on the specific metals and the CNT diameters. This concept generalizes the diverse effects observed in experiments for different reactions, and it is anticipated to be applicable to an even broader range of reactions other than redox of metal species, CO hydrogenation, ammonia synthesis and decomposition discussed here.

High-performance hydrogen evolution electrocatalysis by layer-controlled MoS2 nanosheets

Hydrogen is considered as an important clean energy carrier for the future, and electrocatalytic splitting of water is one of the most efficient technologies for hydrogen production. As a potential alternative to Pt-based catalysts in hydrogen evolution reaction (HER), two-dimensional (2D) molybdenum sulfide (MoS2) nanomaterials have attracted enormous research interest, while the structure control for high-performance HER electrocatalysis remains a considerable challenge due to the lack of efficient preparation techniques. Herein, we reported a one-pot chemical method to directly synthesize 2D MoS2 with controllable layers. Multiple-layer MoS2 (ML-MoS2), few-layer MoS2 (FL-MoS2) and single-layer MoS2 coating on carbon nanotubes (SL-MoS2-CNTs) can be efficiently prepared through the modulation of experimental conditions. The enhanced catalytic activity in HER is demonstrated by reducing the layer number of MoS2 nanosheets. Remarkably, the optimized SL-MoS2-CNTs sample showed long-term durability with an accelerated degradation experiment even after more than 10 000 recycles, and high HER activity with an onset overpotential of only ∼40 mV vs. RHE. This study introduces a novel, cheap and facile strategy to prepare layer-controlled 2D MoS2 nanosheets in a large quantity, and is expected to broaden the already wide range energy applications of 2D MoS2 nanosheets.

Aberration-corrected STEM of Four-atom Rhenium Nanowires Confined within Carbon Nanotubes

Due to the spatial confinement effect nanowires (NWs) of just a few atoms thick in diameter can possess novel quantum properties. These ultrathin NWs, however, tend to be both chemically and structurally unstable. Carbon nanotubes (CNTs), on the other hand, can be used as templates to synthesize nanostructures that remain stable [1-5]. For example, nanofilling of Eu can produce single-atom chains in double-wall CNTs (DW-CNTs) [3]. Self-assemble of graphene nanoribbons within single-wall CNTs and the helical twist and screw-like motion of the carbon nanoribbons have been observed [4]. Monocrystalline FeCo NWs inside CNTs with unique magnetic properties have been synthesized [5]. The strong metal-CNT interaction and the confinement effects of small diameter CNTs can induce the formation of novel metal phases that are generally not stable. We report here the discovery of selfassembled, ultra-long and atomically thin Re NWs with an unusual fcc-stacking pattern along the length of the CNTs. Re usually possesses a hcp structure which is extremely stable and no phase transition occurs under pressures to 216 GPa and temperatures up to its melting point [6].