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Concentrating active Pt atoms in the outer layers of electrocatalysts is a very effective approach to greatly reduce the Pt loading without compromising the electrocatalytic performance and the total electrochemically active surface area (ECSA) for the oxygen reduction reaction (ORR) in hydrogen-based proton-exchange membrane fuel cells. Accordingly, a facile, low-cost, and hydrogen-assisted two-step method is developed in this work, to massively prepare carbon-supported uniform, small-sized, and surfactant-free Pd nanoparticles (NPs) with ultrathin ∼3-atomic-layer Pt shells (Pd@Pt 3L NPs/C). Comprehensive physicochemical characterizations, electrochemical analyses, fuel cell tests, and density functional theory calculations reveal that, benefiting from the ultrathin Pt-shell nanostructure as well as the resulting ligand and geometric effects, Pd@Pt 3L NPs/C exhibits not only significantly enhanced ECSA, electrocatalytic activity, and noble-metal (NM) utilization compared to commercial Pt/C, showing 81.24 m 2 /g Pt , 0.710 mA/cm 2 , and 352/577 mA/mg NM/Pt in ECSA, area-, and NM-/Pt-mass-specific activity, respectively; but also a much better electrochemical stability during the 10,000-cycle accelerated degradation test. More importantly, the corresponding 25-cm 2 H 2 -air/O 2 fuel cell with the low cathodic Pt loading of ∼ 0.152 mg Pt /cm 2 geo achieves the high power density of 0.962/1.261 W/cm 2 geo at the current density of only 1,600 mA/cm 2 geo , which is much higher than that for the commercial Pt/C. This work not only develops a high-performance and practical Pt-based ORR electrocatalyst, but also provides a scalable preparation method for fabricating the ultrathin Pt-shell nanostructure, which can be further expanded to other metal shells for other energy-conversion applications.

Pt-rare earth metal (Pt-RE) alloys are considered to be one of the most promising electrocatalysts for producing oxygen reduction reactions (ORRs) due to their compressively strained Pt overlayer and their exceptional negative-alloy formation energies, which result in excellent activity and stability. However, there are still great challenges in the chemical synthesis of Pt-RE nanoalloys. Herein, we report a simple method employing the nanopores of porous carbon as nanoreactors to synthesize a Pt5La nanoalloy. The Pt5La alloy nanoparticles are embedded in porous carbon (Pt5La@C) with a particle size of around 1–3 nm and also exhibit a very narrow size distribution because of the confined-space effect. The as-prepared Pt5La@C nanoalloy exhibits highly efficient ORR performance with a half-wave potential of 0.912 V in 0.1 M HClO4, which is 56 mV higher than that of a commercial Pt/C catalyst. Moreover, it achieves an improved intrinsic activity of 0.69 mA cm−2 and, a mass activity of 0.42 A mgPt−1 at 0.90 V. In addition, it also delivers a very stable lifespan performance, with negligible decay in half-wave potential after accelerated stress testing for 10,000 cycles. This work also provides a new method for the development of promising Pt-RE nanoalloys with ultrasmall nanoparticles with a very narrow size distribution for various efficient energy-conversion devices.

Electrochemical nitrate reduction to ammonia is an efficient strategy for nitrate removal and ammonia production in ambient conditions. TiO2 is a promising electrocatalyst for such a reaction, but chemical doping is still needed to further improve the electrocatalytic properties of TiO2. Here, we investigated the effect of Zr-doping on the nitrate reduction reaction processes on the (101) surface of anatase TiO2 using first-principles calculations. Two models with different Zr-doping levels were built. The reaction pathways and the potential-determining steps were established based on a thorough investigation of the variation in Gibbs free energy of each possible elementary step. The results show that a high level of Zr doping was effective to lower the Gibbs free energy for nitrate adsorption; however, Zr doping may promote the competing hydrogen evolution reaction (HER) by reducing the adsorption Gibbs free energy of H. Moreover, Zr doping also increases the adsorption Gibbs free energies for the intermediate products NO2 and NO, which may result in an earlier termination of the reaction, by releasing the intermediates as the final products without producing ammonia. Therefore, Zr doping may decrease the Faradaic efficiency and selectivity of TiO2 for the reaction and should be treated with caution experimentally.

High cost has undoubtedly become the biggest obstacle to the commercialization of proton exchange membrane fuel cells (PEMFCs), in which Pt-based catalysts employed in the cathodic catalyst layer (CCL) account for the major portion of the cost. Although non-precious metal catalysts (NPMCs) show appreciable activity and stability in the oxygen reduction reaction (ORR), the performance of fuel cells based on NPMCs remains unsatisfactory compared to those using Pt-based CCL. Therefore, most studies on NPMC-based fuel cells focus on developing highly active catalysts rather than facilitating oxygen transport. In this work, the oxygen transport behavior in CCLs based on highly active Fe-N-C catalysts is comprehensively explored through the elaborate design of two types of membrane electrode structures, one containing low-Pt-based CCL and NPMC-based dummy catalyst layer (DCL) and the other containing only the NPMC-based CCL. Using Zn-N-C based DCLs of different thickness, the bulk oxygen transport resistance at the unit thickness in NPMC-based CCL was quantified via the limiting current method combined with linear fitting analysis. Then, the local and bulk resistances in NPMC-based CCLs were quantified via the limiting current method and scanning electron microscopy, respectively. Results show that the ratios of local and bulk oxygen transport resistances in NPMC-based CCL are 80% and 20%, respectively, and that an enhancement of local oxygen transport is critical to greatly improve the performance of NPMC-based PEMFCs. Furthermore, the activity of active sites per unit in NPMC-based CCLs was determined to be lower than that in the Pt-based CCL, thus explaining worse cell performance of NPMC-based membrane electrode assemblys (MEAs). It is believed that the development of NPMC-based PEMFCs should proceed not only through the design of catalysts with higher activity but also through the improvement of oxygen transport in the CCL.

The advantages of zero emission and high energy efficiency make proton exchange membrane fuel cells (PEMFCs) promising options for future energy conversion devices. To address the cost issue associated with Pt-based electrocatalysts, considerable effort over the past several years has been devoted to catalyst surface modification by means of novel electrocatalysts, such as solid catalysts with an ionic liquid layer (SCILL), which improves both the oxygen reduction reaction (ORR) activity and durability. However, despite numerous reports of dramatically enhanced ORR activity, as determined via the rotating disk electrode (RDE) method, few studies on the application of SCILLs in membrane electrode assembly (MEA) have been reported. The underlying reason lies in the well-acknowledged technological gap between half-cells and full-cells, which originates from the disparate microenvironments for three phase boundaries. Therefore, the objective of this review is to compare the detailed information about improvements in fuel cell performance in both half- and full-cells, thus increasing the fundamental understanding of the mechanism of SCILL. In this review, the concept of SCILL and its origin are introduced, the outstanding electrochemical performance of SCILL catalysts in both RDE and MEA measurements is summarized, and the durability of SCILL catalysts is analysed. Subsequently, proposed mechanisms for the enhanced ORR activity in half-cells, the improved oxygen transport in full-cells and the boosted stability of SCILL catalysts are discussed, while the effects of the IL chemical structure, IL loading as well as the operating conditions on the performance and lifetime of SCILL catalysts are assessed. Finally, comprehensive conclusions are presented, and perspectives are proposed in the last section. It is believed that the new insight presented in this review could provide guidance for the further development of SCILLs in low-Pt PEMFCs. Graphical Abstract

Electrochemical nitrate reduction to ammonia is an efficient strategy for nitrate removal and ammonia production in ambient conditions. TiO[sub.2] is a promising electrocatalyst for such a reaction, but chemical doping is still needed to further improve the electrocatalytic properties of TiO[sub.2]. Here, we investigated the effect of Zr-doping on the nitrate reduction reaction processes on the (101) surface of anatase TiO[sub.2] using first-principles calculations. Two models with different Zr-doping levels were built. The reaction pathways and the potential-determining steps were established based on a thorough investigation of the variation in Gibbs free energy of each possible elementary step. The results show that a high level of Zr doping was effective to lower the Gibbs free energy for nitrate adsorption; however, Zr doping may promote the competing hydrogen evolution reaction (HER) by reducing the adsorption Gibbs free energy of H. Moreover, Zr doping also increases the adsorption Gibbs free energies for the intermediate products NO[sub.2] and NO, which may result in an earlier termination of the reaction, by releasing the intermediates as the final products without producing ammonia. Therefore, Zr doping may decrease the Faradaic efficiency and selectivity of TiO[sub.2] for the reaction and should be treated with caution experimentally.

Pt-rare earth metal (Pt-RE) alloys are considered to be one of the most promising electrocatalysts for producing oxygen reduction reactions (ORRs) due to their compressively strained Pt overlayer and their exceptional negative-alloy formation energies, which result in excellent activity and stability. However, there are still great challenges in the chemical synthesis of Pt-RE nanoalloys. Herein, we report a simple method employing the nanopores of porous carbon as nanoreactors to synthesize a Pt[sub.5]La nanoalloy. The Pt[sub.5]La alloy nanoparticles are embedded in porous carbon (Pt[sub.5]La@C) with a particle size of around 1–3 nm and also exhibit a very narrow size distribution because of the confined-space effect. The as-prepared Pt[sub.5]La@C nanoalloy exhibits highly efficient ORR performance with a half-wave potential of 0.912 V in 0.1 M HClO[sub.4], which is 56 mV higher than that of a commercial Pt/C catalyst. Moreover, it achieves an improved intrinsic activity of 0.69 mA cm[sup.−2] and, a mass activity of 0.42 A mg[sub.Pt] [sup.−1] at 0.90 V. In addition, it also delivers a very stable lifespan performance, with negligible decay in half-wave potential after accelerated stress testing for 10,000 cycles. This work also provides a new method for the development of promising Pt-RE nanoalloys with ultrasmall nanoparticles with a very narrow size distribution for various efficient energy-conversion devices.

Compared with internal combustion engine vehicles and electric vehicles, the advantages of fuel cell vehicles are still in a qualitative stage, and there is no clear quantitative analysis. In this paper, the energy consumption and emission results of twelve different hydrogen paths are calculated through the secondary modeling in GREET, and the total energy consumption and emissions of fuel cell vehicles, internal combustion engine vehicles, and electric vehicles are obtained. The results show that electric vehicles have the lowest energy consumption throughout their life cycle, and fuel cell vehicles are only about 22% higher than electric vehicles. At the same time, based on the analysis of multi-factor environmental indicators such as global warming potential, human toxicity potential, photochemical smog potential, aerosol potential and acidification potential, fuel cell vehicles have the lowest life cycle emissions. Finally, a comparison of the total cost of ownership of the three representative vehicles shows that the cost of fuel cell vehicles is the highest under current production. However, combined with the analysis of the cost reduction space of the main components of the fuel cell system, the results show that when the mileage is higher than 56,000 kilometers, The total cost of ownership of fuel cell vehicles will be lower than internal combustion engine vehicles.

Undoubtedly, there remains an urgent prerequisite to achieve significant advances in both the specific capacity and cyclability of Li-O 2 batteries for their practical application. In this work, a series of unique three-dimensional (3D) α-MnO 2 /MWCNTs hybrids are successfully prepared using a facile lyophilization method and investigated as the cathode of Li-O 2 batteries. Thereinto, cross-linked α-MnO 2 /MWCNTs nanocomposites are first synthesized via a modified chemical route. Results demonstrate that MnO 2 nanorods in the nanocomposites have a length of 100–400 nm and a diameter ranging from 5 to 10 nm, and more attractively, the as-lyophilized 3D MnO 2 /MWCNTs hybrids is uniquely constructed with large amounts of interconnected macroporous channels. The Li-O 2 battery with the 3D macroporous hybrid cathode that has a mass percentage of 50% of α-MnO 2 delivers a high discharge specific capacity of 8,643 mAh·g −1 at 100 mA·g −1 , and maintains over 90 cycles before the discharge voltage drops to 2.0 V under a controlled specific capacity of 1,000 mAh·g −1 . It is observed that when being recharged, the product of toroidal Li 2 O 2 particles disappears and electrode surfaces are well recovered, thus confirming a good reversibility. The excellent performance of Li-O 2 battery with the 3D α-MnO 2 /MWCNTs macroporous hybrid cathode is ascribed to a synergistic combination between the unique macroporous architecture and highly efficient bi-functional α-MnO 2 /MWCNTs electrocatalyst.

To reduce the adverse effects of stern-shaft system vibration on ship performance, this work combined hydrodynamic excitations calculated for a semi-submerged propeller and established a multibody dynamics (MBDs) model of the stern shaft system that included a flexible shaft, propeller, and elastically damped support bearings. The MBDs model’s accuracy was verified through comparison between experimentally identified modal parameters and those computed by the model. It was found that the bearing stiffness and the hydrodynamic excitation frequency collectively determine the vibration amplitude and modal shape of the shaft system, based on an analysis of varied bearing stiffness and damping. Bearing displacement had a significant impact on shafting vibration. And the tie rod with a stiffness of 2.5 × 107 N/m provided a noticeable vibration damping effect. The findings offered theoretical support for mitigating stern-shaft vibration in high-speed vessels subjected to hydrodynamic excitation from semi-submerged propellers.