Advancements in Noble Metal-Decorated Porous Carbon Nanoarchitectures: Key Catalysts for Direct Liquid Fuel Cells.

Noble-metal nanocrystals have emerged as essential electrode materials for catalytic oxidation of organic small molecule fuels in direct liquid fuel cells (DLFCs). However, for large-scale commercialization of DLFCs, adopting cost-effective techniques and optimizing their structures using advanced matrices are crucial. Notably, noble metal-decorated porous carbon nanoarchitectures exhibit exceptional electrocatalytic performances owing to their three-dimensional cross-linked porous networks, large accessible surface areas, homogeneous dispersion (of noble metals), reliable structural stability, and outstanding electrical conductivity. Consequently, they can be utilized to develop next-generation anode catalysts for DLFCs. Considering the recent expeditious advancements in this field, this comprehensive review provides an overview of the current progress in noble metal-decorated porous carbon nanoarchitectures. This paper meticulously outlines the associated synthetic strategies, precise microstructure regulation techniques, and their application in electrooxidation of small organic molecules. Furthermore, the review highlights the research challenges and future opportunities in this prospective research field, offering valuable insights for both researchers and industry experts.

A 2D/2D heterojunction of ultrathin Pd nanosheet/MXene towards highly efficient methanol oxidation reaction: the significance of 2D material nanoarchitectonics.

Two-dimensional (2D) Pd nanosheet-based catalysts have recently garnered widespread attention due to their high atom utilization efficiency. However, their catalytic ability and structural stability still require significant enhancement before they can be widely applied. In this study, we presented the rational design and controllable fabrication of a novel 2D/2D heterojunction, which consists of ultrathin Pd nanosheets (NSs) grown on the Ti3C2Tx MXene surface (Pd NSs/MXene). This heterostructure was achieved through a robust and convenient stereo-assembly strategy. The newly developed Pd NSs/MXene heterojunction not only provides numerous exposed active Pd atoms with an optimized electronic structure but also enables an intimate Pd/MXene interfacial interaction, ensuring a stable hybrid configuration. Consequently, the resulting Pd NSs/MXene heterojunction exhibits exceptional methanol oxidation properties. It possesses a large electrochemically active surface area, high mass and specific activities, and a long operating life, which are significantly superior to those of traditional Pd nanoparticle/carbon and Pd nanosheet/carbon catalysts. Theoretical simulations further reveal strong electronic interactions between the Pd nanosheet and MXene, which dramatically enhance the adsorption energy of the Pd component and simultaneously lower its d-band center. As a result, the Pd NSs/MXene heterojunction is less susceptible to CO poisoning. This work introduces a new 2D/2D heterojunction based on MXene and noble metallic materials and holds significance for the development of other novel heterojunctions, particularly within the realm of 2D material nanoarchitectonics.

Ultrafine cobalt selenide nanowires tangled with MXene nanosheets as highly efficient electrocatalysts toward the hydrogen evolution reaction.

Hydrogen energy has attracted sustainable attention in the exploitation and application of advanced power-generator devices, and electrocatalysts for the hydrogen evolution reaction (HER) have been regarded as one of the core components in the current electrochemical hydrogen production systems. In this work, a facile and cost-effective bottom-up strategy is developed for the construction of 1D ultrafine cobalt selenide nanowires tangled with 2D Ti3C2Tx MXene nanosheets (CoSe NW/Ti3C2Tx) through an in situ stereo-assembly process. Such an architectural design endows the hybrid system not only with a large accessible surface for the rapid transportation of reactants, but also with numerous exposed CoSe edge sites, thereby generating substantial synergic coupling effects. The as-derived CoSe NW/Ti3C2Tx hybrid demonstrates competitive electrocatalytic properties toward the HER with a small onset potential of 84 mV, a low Tafel slope of 56 mV dec-1 and exceptional cycling performance, which are superior to those of bare CoSe and Ti3C2Tx materials. It is believed this promising nanoarchitecture may provide new possibilities for the design and construction of precious-metal-free electrocatalysts with high efficiency and great stability in the energy-conversion field.

Interfacial engineering of worm-shaped palladium nanocrystals anchored on polyelectrolyte-modified MXene nanosheets for highly efficient methanol oxidation.

The development of high-efficiency methanol oxidation electrocatalysts with acceptable costs is central to the practical use of direct methanol fuel cell. In this work, a convenient interfacial engineering strategy is developed to the design and construction of quasi-one-dimensional worm-shaped palladium nanocrystals strongly coupled with positively-charged polyelectrolyte-modified Ti3C2Tx MXene (Pd NWs/PDDA-MX) via the direct electrostatic attractions. Because of the intriguing structural features including ultrathin-sheet nature, homogeneous Pd dispersion, numerous grain boundaries, strong electronic interaction, and high metallic conductivity, the as-fabricated Pd NWs/PDDA-MX hybrid shows superior electrocatalytic performance with a large electrochemically active surface area of 105.3 m2 g-1, a high mass activity of 1526.5 mA mg-1, and reliable long-term durability towards alkaline methanol oxidation reaction, far outperforming the commercial Pd nanoparticle/carbon catalysts. Density functional theory calculation further demonstrate that there are strong electronic interactions in the Pd nanoworm/Ti3C2Tx model with a depressed CO adsorption energy, thereby guaranteeing a stable interfacial contact as well as strong antitoxic ability.

Polyelectrolyte-Induced Stereoassembly of Grain Boundary-Enriched Platinum Nanoworms on Ti3C2Tx MXene Nanosheets for Efficient Methanol Oxidation.

Direct methanol fuel cells with high energy conversion efficiency and low hazard emissions have aroused great attention from both academic and industrial communities, but their large-scale commercial application has been blocked by high costs as well as short lifespan of the anode Pt catalysts. Here, we demonstrate a simple and scalable noncovalent strategy for the synthesis of quasi-one-dimensional (1D) Pt nanoworms grown on poly(diallyldimethyl-ammonium chloride) (PDDA)-functionalized Ti3C2Tx nanosheets as anode catalysts for methanol electrooxidation. Interestingly, the introduction of PDDA on Ti3C2Tx nanosheets can not only effectively adjust their surface charge property to strengthen the electrostatic interaction between metal and support but also induce the stereoassembly of worm-shaped Pt nanocrystals with abundant catalytically active grain boundaries, which enable the resulting hybrid to express high electrocatalytic activity, remarkable durability, and strong antipoisoning ability for methanol electrooxidation, which are better than those of the traditional Pt nanoparticle electrocatalysts loaded on carbon black, carbon nanotubes, reduced graphene oxide, and MXene matrixes. Theoretical simulations disclose that the more stable worm-shaped Pt configuration with an optimized electronic structure on the Ti3C2Tx surface possesses a weaker CO adsorption ability than that of the Pt nanoclusters, thereby providing a dramatically enhanced and sustainable electrocatalytic performance.

Graphene Nanoarchitectonics: Recent Advances in Graphene-Based Electrocatalysts for Hydrogen Evolution Reaction.

Under the double pressures of both the energy crisis and environmental pollution, the exploitation and utilization of hydrogen, a clean and renewable power resource, has become an important trend in the development of sustainable energy-production and energy-consumption systems. In this regard, the electrocatalytic hydrogen evolution reaction (HER) provides an efficient and clean pathway for the mass production of hydrogen fuel and has motivated the design and construction of highly active HER electrocatalysts of an acceptable cost. In particular, graphene-based electrocatalysts commonly exhibit an enhanced HER performance owing to their distinctive structural merits, including a large surface area, high electrical conductivity, and good chemical stability. Considering the rapidly growing research enthusiasm for this topic over the last several years, herein, a panoramic review of recent advances in graphene-based electrocatalysts is presented, covering various advanced synthetic strategies, microstructural characterizations, and the applications of such materials in HER electrocatalysis. Lastly, future perspectives on the challenges and opportunities awaiting this emerging field are proposed and discussed.

First Principles Study on the CO Oxidation on Mn-Embedded Divacancy Graphene.

The CO oxidation mechanism on graphene with divacancy (DG) embedded with transition metal from Sc to Zn has been studied by using first principles calculations. The results indicate that O2 molecule is preferentially adsorbed on Sc, Ti, V, Cr, Mn, and Fe-DG, which can avoid the CO poisoning problem that many catalysts facing and is beneficial to the CO oxidation progress. Further study indicates that Mn-DG shows the best catalytic properties for CO oxidation with consideration of both Langmuir-Hinshelwood (LH) and Eley-Rideal (ER) oxidation mechanisms. Along the ER mechanism, the reaction energy barrier for the first step (CO free + O2pre-adsorbed → OOCO) is 0.96 eV. Along the LH mechanism, the energy barrier for the rate limiting step (CO adsorbed + O2adsorbed → OOCO) is only 0.41 eV, indicating that the CO oxidation on Mn-DG will occur along LH mechanism. The Hirshfeld charge distributions of O2 and CO molecules is tuned by the embedded Mn atom, and the charge transfer from the embedded Mn atom to the adsorbed molecules plays an important role for the CO oxidation. The result shows that the Mn-embedded divacancy graphene is a noble-metal free and efficient catalyst for CO oxidation at low temperature.

Few-Layered Trigonal WS2 Nanosheet-Coated Graphite Foam as an Efficient Free-Standing Electrode for a Hydrogen Evolution Reaction.

Few-layered tungsten disulfide (WS2) with a controlled-phase ratio (the highest trigonal-phase ratio being 67%) was exfoliated via lithium insertion. The exfoliated WS2 nanosheets were then anchored onto three-dimensional (3D) graphite foam (GF) to fabricate free-standing binder-free electrodes. The 3D GF can increase the interfacial contact between the WS2 nanosheets and the electrolyte and facilitate ion transfer. Without the nonconductive binder, an intimate contact between the WS2 and GF interface can be created, leading to the improvement of electrical conductivity. In comparison to the pure WS2 nanosheets, the overpotential for a hydrogen evolution reaction is significantly decreased from 350 mV to 190 mV at 10 mA/cm2, and no deactivation occurs after 1000 cycles. The density functional theory computations reveal that the efficient catalytic activity of the trigonal-phase WS2/GF electrode is attributed to the lower Gibbs free energy for H* adsorption and higher electrical conductivity.

Worm-Shape Pt Nanocrystals Grown on Nitrogen-Doped Low-Defect Graphene Sheets: Highly Efficient Electrocatalysts for Methanol Oxidation Reaction.

Although direct methanol fuel cell offers high energy use efficiency and low pollution emission, the lack of suitable electrode materials poses a great challenge to its commercial application. Herein, a facile and scalable approach is developed to fabricate a hybrid electrocatalyst consisting of strongly coupled worm-shape Pt nanocrystals and nitrogen-doped low-defect graphene (N-LDG) sheets. Interestingly, it is found that the formation of Pt nanoworms (NWs) is induced by the N atoms in the high-quality carbon matrix, which also allows the integration of their respective structural advantages and leads to a strong synergetic coupling effect. As a result, the obtained Pt NW/N-LDG catalyst exhibits an extremely high mass activity of 1283.1 mA mg-1 toward methanol oxidation reaction, accompanied by reliable long-term stability and good antipoisoning ability, which are dramatically enhanced as compared with conventional Pt nanoparticle catalysts dispersed on undoped LDG, reduced graphene oxide, and commercial carbon black supports.

Enhancement of the Stability of Fluorine Atoms on Defective Graphene and at Graphene/Fluorographene Interface.

Fluorinated graphene is one of the most important derivatives of graphene and has been found to have great potential in optoelectronic and photonic nanodevices. However, the stability of F atoms on fluorinated graphene under different conditions, which is essential to maintain the desired properties of fluorinated graphene, is still unclear. In this work, we investigate the diffusion of F atoms on pristine graphene, graphene with defects, and at graphene/fluorographene interfaces by using density functional theory calculations. We find that an isolated F atom diffuses easily on graphene, but those F atoms can be localized by inducing vacancies or absorbates in graphene and by creating graphene/fluorographene interfaces, which would strengthen the binding energy of F atoms on graphene and increase the diffusion energy barrier of F atoms remarkably.