Li-Mei Zhang

A newly-designed sandwich-structured graphene–Pt–graphene catalyst with improved electrocatalytic performance for fuel cells

A novel sandwich-structured graphene–Pt–graphene (G–P–G) catalyst has been synthesized by a convenient approach. The obtained G–P–G catalyst has been characterized by X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, transmission electron microscopy, high resolution transmission electron microscopy, and electrochemical measurements. Structural characterization shows that the G–P–G catalyst has a well-defined sandwich-like morphology. The results of electrochemical measurements indicate that the G–P–G exhibits 1.27 times higher activity for methanol electrooxidation than that of the Pt/graphene catalyst. Importantly, the results of the accelerated potential cycling test demonstrate that the G–P–G catalyst possesses 1.7 times higher stability than that of Pt/graphene. The significantly enhanced electrochemical performance is ascribed to its unique sandwich-like structure. Pt nanoparticles are anchored between the two adjacent graphene sheets, substantially enhancing the metal–support interaction, and graphene could act as a “mesh bag” to prevent the Pt species from leaking into the electrolyte, so its stability has considerably been enhanced. The effect of composited graphene amount on the stability of the hybrid has also been systematically studied. The stability of the catalyst increases with the increase of the introduced GO amount and the G–P–G50 shows optimized electrocatalytic performance. These findings suggest that the sandwich-structured G–P–G catalyst holds tremendous promise for fuel cells.

Graphitic carbon nitride nanosheet coated carbon black as a high-performance PtRu catalyst support material for methanol electrooxidation

PtRu supported on a C@g-C3N4 NS (g-C3N4 nanosheet coated Vulcan XC-72 carbon black) catalyst has been prepared by a microwave-assisted polyol process (MAPP). The results of electrochemical measurements show that the PtRu/C@g-C3N4 NS catalyst has excellent activity due to more uniform dispersion and smaller size of PtRu nanoparticles (PtRu NPs), and higher stability ascribed to the stronger metal–support interaction (SMSI) between PtRu NPs and the composite support. Physical characterisation using techniques such as X-ray diffraction (XRD), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) has indicated that the bulk g-C3N4 shell outside of the as-prepared C@bulk g-C3N4 (bulk g-C3N4 coated Vulcan XC-72 carbon black, C@bulk g-C3N4) indeed exfoliated to layered g-C3N4 nanosheets and formed a composite material of Vulcan XC-72 coated with g-C3N4 nanosheets. Furthermore, the results indicate that the mass catalytic activity of the PtRu/C@g-C3N4 NS catalyst substantially enhanced, which is a factor of 2.1 times higher than that of the PtRu/C catalyst prepared by the same procedure and the accelerated potential cycling tests (APCTs) show that the PtRu/C@g-C3N4 NS catalyst possesses 14% higher stability and much greater poison tolerance than as-prepared PtRu/C. The significantly enhanced performance of the PtRu/C@g-C3N4 NS catalyst is ascribed to the following reasons: the inherently excellent mechanical resistance and stability of g-C3N4 nanosheets in acidic and oxidative environments; the increased electron conductivity of the support by forming a core–shell structure of C@g-C3N4 NS; SMSI between metal NPs and the composite support. Based on this novel approach to fabricate a C@g-C3N4 NS hybrid nanostructure, many other interesting applications might also be discovered.

3D Hierarchical Pt-Nitrogen-Doped-Graphene-Carbonized Commercially Available Sponge as a Superior Electrocatalyst for Low-Temperature Fuel Cells

Three-dimensional hierarchical nitrogen-doped graphene (3D-NG) frameworks were successfully fabricated through a feasible solution dip-coating method with commercially available sponges as the initial backbone. A spongy template can help hinder the graphene plates restacking in the period of the annealing process. The Pt/3D-NG catalyst was synthesized employing a polyol reduction process. The resultant Pt/3D-NG exhibits 2.3 times higher activity for methanol electro-oxidation along with the improvement in stability as compared with Pt/G owing to their favorable features including large specific surface area, high pore volume, high N doping level, and the homogeneous dispersion of Pt nanoparticles. Besides, Pt/3D-NG also presents high oxygen reduction reaction (ORR) performance in acid media when compared with Pt/3D-G and Pt/G. This work raises a valid solution for the fabrication of 3D functional freestanding graphene-based composites for a variety of applications in fuel cell catalysis, energy storage, and conversion.

Graphitic-C3N4 quantum dots modified carbon nanotubes as a novel support material for a low Pt loading fuel cell catalyst

For developing low Pt loading and high performance catalysts for direct methanol fuel cells, a graphitic-C3N4 quantum dots (g-C3N4 QDs) modified CNT (CNT-QDs) composite material was constructed via a π–π stacking method, in which the g-C3N4 QDs act a bridged unit between the CNTs and metal nanoparticles (NPs). Compared to conventional acid-functionalized CNTs used for supporting novel metal NPs, the CNT-QDs have less structural damage leading to their high stability. Moreover, the electrochemical test results indicate that the mass catalytic activity of the PtRu/CNT-QDs catalyst is 2.3 times higher than that of PtRu/CNT, leading to a 56.5% reduction of novel metal loading. The accelerated potential cycling tests (APCTs vs. RHE 0–1.2 V) show that the PtRu/CNT-QDs catalyst possesses 15.1% higher stability than that of the conventional acid-functionalized CNTs supported PtRu (PtRu/T-CNT) catalyst. The significantly enhanced performance obtained for the PtRu/CNT-QDs catalyst was ascribed to the homogeneous dispersion of PtRu NPs on the composite support due to its abundant Lewis acid sites for anchoring the PtRu NPs and the excellent mechanical resistance and stability of the g-C3N4/CNT composite materials in acidic and oxidative environments, as well as the strong metal–support interaction (SMSI) between the metal NPs and g-C3N4.

One-pot synthesis of a three-dimensional graphene aerogel supported Pt catalyst for methanol electrooxidation

A three-dimensional (3D) structured Pt/graphene aerogel has been synthesized by a facile one-pot solvothermal process. The as-synthesized catalyst is characterized by X-ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, transmission electron microscopy, and electrochemical tests. It has been found that the Pt/graphene aerogel catalyst exhibits a well-developed 3D interconnected porous graphene framework with Pt nanoparticles (NPs) decorated on the surface of the graphene aerogel. More importantly, the as-made Pt/graphene aerogel catalyst exhibits a much higher electrocatalytic activity and stability than the Pt/graphene for methanol electrooxidation. The enhancement may result from the unique 3D graphene architecture, and the efficient assembly between the Pt NPs and graphene aerogel. These outstanding properties suggest that the Pt/graphene aerogel catalyst holds tremendous potential for fuel cell applications.

Three-dimensional TiO2@C nano-network with high porosity as a highly efficient Pt-based catalyst support for methanol electrooxidation

The development of highly active and durable Pt-based catalysts is an important issue for methanol electrooxidation. Research into highly efficient supports provides a feasible method to achieve such catalysts. In this paper, carbon-coated TiO2 nanowires with a unique three-dimensional network structure are prepared as a Pt-based catalyst support by the carbonization of a resorcinol–formaldehyde polymer. Physical characterization confirms that this unique structure can provide a large specific surface area, high porosity and efficient transport channels. The abundant heterogeneous interfaces between the TiO2 nanowires and carbon provide numerous Pt loading sites, which greatly improves the utilization of Pt. Strikingly, electrochemical measurements show that the as-prepared catalyst has much better activity and durability than commercial Pt/C for methanol electrooxidation. Furthermore, the single direct methanol fuel cell test demonstrates that the as-prepared catalyst has higher power density and polarization current. We attribute this excellent catalytic performance to the unique nano-network structure, the numerous Pt anchoring sites, and the synergetic effect of the different components.

Nitrogen-doped carbon with mesoporous structure as high surface area catalyst support for methanol oxidation reaction

Mesoporous nitrogen-doped carbon (MNC) with a high surface area has been synthesized via carbonizing polyaniline using silica nanoparticles as template. The more silica nanoparticles, the smaller the micropore surface area is and the larger the mesoporous surface area is. Moreover, with an increase in the amount of silica nanoparticles, the electrocatalytic activity of Pt/MNC catalysts shows a downward trend after an intimal increase, and the Pt/MNC-1/6 (with the weight ratio of aniline monomer to silica nanoparticles of 1/6) catalyst has the highest activity, ascribed to the optimal Pt nanoparticles size, which is closely related to the pore structure of the support. In addition, the electrocatalytic activity and stability of Pt/MNC-1/6 catalyst are significantly superior to that of Pt/nitrogen-doped carbon (Pt/CNx) catalyst. For the same electrocatalytic activity, the Pt loading of Pt/MNC-1/6 catalyst is reduced by 33.3% compared to the Pt/CNx catalyst. The high electrocatalytic activity originates from the introduction of mesoporous structures that can facilitate mass transfer and improve the dispersion of Pt nanoparticles. Furthermore, the Ostwald ripening behavior of Pt nanoparticles is limited in the mesoporous structure of MNC-1/6, which weakens the aggregation effect of Pt nanoparticles during the electrocatalytic processes, thus enhancing the electrocatalytic stability of the catalyst.

Clustered-Microcapsule-Shaped Microporous Carbon-Coated Sulfur Composite Synthesized via in Situ Oxidation

Hollow materials as sulfur hosts have been intensively investigated to address the poor cycling stabilities of Li-S batteries. Herein, we report an enhanced hollow framework to improve the applicability of the sulfur confinement. A clustered-microcapsule-shaped microporous carbon coated sulfur (CM-S@MPC) composite is prepared from the clustered zinc sulfide precursor, through an in situ oxidation process. The high specific surface area and the in situ preparation guarantee the uniform distribution of sulfur inside the carbon microcapsule, even under a higher sulfur content of 83 wt %. In addition, the interconnected frame constructed by the stacking of carbon microcapsules also mitigates the lithium polysulfide loss by setting interlayered hurdles on their pathway along the outward diffusion. Hence, these enable a full demonstration of excellent cycling stability, compared to the control sample obtained via physical sulfur infiltration. The outstanding decay rate of 0.039% per cycle is achieved during 700 cycles at 1 C, even under high sulfur loading.

Effect of N-doped carbon quantum dots/multiwall-carbon nanotube composite support on Pt catalytic performance for methanol electrooxidation

N-Doped carbon quantum dots (NCQDs)/multiwall-carbon nanotube (MWCNT) supports are synthesized by a one pot hydrothermal treatment process at different contents of precursor. NCQDs–MWCNT as support can be widely used in the process of electrocatalysis. In this paper, the Pt/NCQDs–MWCNT catalysts are prepared by a microwave-assisted polyol process (MAPP) method and the effects of NCQDs with different contents on the performance of Pt-based anode catalysts for methanol oxidation reaction (MOR) are systematically demonstrated. The electrochemical tests reveal that the Pt/NCQDs–MWCNT catalyst exhibits the best performance for MOR when precursor content is 3 g. In terms of the electrochemical and characterization results, the moderate content of precursor for NCQDs plays multiple roles in the electrocatalytic performance: promoting the dispersion of untreated MWCNT significantly in solution; providing the plentiful oxygen-containing groups to deposit Pt nanoparticles; and facilitating the formation of homogeneous Pt nanoparticles.