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Proton-exchange membrane fuel cells (PEMFCs) are a promising new energy-conversion technology due to their safe and environmentally friendly characteristics in a wide range of operating conditions. However, catalysis of the acidic oxygen reduction reaction (ORR) at the cathode is the main obstacle. In this regard, platinum-free single-atomic-site catalysts (SACs) with designable active sites are a potential solution. Unlike catalysts for the ORR in alkaline media that have been widely explored, there is still a gap between the performance of SACs and Pt-based catalysts in acidic media. In this review, we introduce the latest progress of SACs in acidic ORR. SACs consist of metal centers and substrates. We conclude controllable synthetic strategies for SACs from single-atomic sites and substrate design, discuss the relationship between the structure of single-atom sites and acidic ORR performance of SACs at the atomic level, and propose challenges and perspectives of SACs in acidic ORR.

Atomic-level insight into the unique catalytic capability of single-atom catalysts that distinguished from nanometer-sized counterparts is highly desirable for catalyst design and catalysis research. By synthesizing single Pd atoms supported on TiO2 as a catalyst, here we demonstrate a steric hindrance effect of single atoms induced by the unique isolation of single-atom active sites to achieve a remarkable enhancement on catalytic performance in the synthesis of dimethyl carbonate. Experimental results and density functional theory calculations reveal that such steric hindrance effect of single atoms favors the yield of the desired product dimethyl carbonate against further reacting with intermediates to form byproduct, because no extra Pd species around single Pd atoms provide active sites to further adsorb and activate substrates directly. The discovery of such steric hindrance effect is a valuable supplement to single-atom catalysis, and may promote single-atom catalysts to be widely applied in selective catalytic reactions.

The performance and the cost of electrocatalysts play the two most vital roles in the development and application of energy conversion technologies. Single-atom catalysts (SACs) are recently emerging as a new frontier in catalysis science. With maximum atom-utilization efficiency and unique properties, SACs exhibit great potential for enabling reasonable use of metal resources and achieving atomic economy. However, fabricating SACs and maintaining the metal centers as atomically dispersed sites under synthesis and catalysis conditions are challenging. Here, we highlight and summarize recent advances in wet-chemistry synthetic methods for SACs with special emphasis on how to achieve the stabilization of single metal atoms against migration and agglomeration. Moreover, we summarize and discuss the electrochemical applications of SACs with a focus on the oxygen reduction reaction (ORR), hydrogen evolution reaction (HER), and CO2 reduction reaction (CO2RR). At last, the current issues and the prospects for the development of this field are discussed.

Dynamic response analysis of a nonlinear train-track-bridge system under earthquakes is a time-consuming task. In this paper, a fast computing strategy is proposed to reduce the simulation time by solving the shortcomings of classical time integration algorithms in the computation of the coupled system. The core of the strategy focuses on completely explicit computational processes and unconditional stability. Based on the loosely coupling scheme, the model of the train-track-bridge system is divided into two parts, namely, the train substructure established using multibody dynamics and the track-bridge substructure established using the finite element method. Track-bridge substructure and train substructure are separately integrated by the unconditionally stable explicit algorithm, CQ3, and its degenerate form, respectively. The interaction of the two substructures depends on wheel-rail forces. The track-bridge substructure is further decoupled to describe its nonlinear behavior and to improve the speed of solving algebraic equations. The proposed strategy not only avoids the complex iterative process of the nonlinear train-track-bridge system but also eliminates the limitation of the integration time step caused by the minimum natural period of the track-bridge substructure. This paper first simulates the dynamic response of a train under a sinusoidal displacement excitation and extends the simulation strategy to analyze the vibration of the coupling system under an earthquake. The applicability, efficiency, and accuracy of the strategy are illustrated through three numerical analyses. The strategy proposed in this paper provides a convenient and greatly computationally efficient option for simulating the nonlinear dynamic response of a large-scale train-track-bridge system.

The train-bridge system is complex and presents many random factors, including stochastic track irregularities as well as random parameters of bridge and train. Moreover, the entire train-bridge system may contain more than one hundred random variables. Herein, we proposed a new random analysis method that comprehensively considers two random sources of the system. Irregularities are represented using the Karhunen–Loeve expansion and the response of the system is then calculated using the point estimate method based on a two-dimensional reduction technique. The solution of the Monte Carlo simulation is regarded as the exact solution to verify the proposed method. Comparing to the Monte Carlo simulation method, the proposed method demonstrates high precision and efficiency. Moreover, sensitivities of random irregularities and parameters of the bridge and train to the system responses are analyzed. The bridge response demonstrates the highest sensitivity to Young’s modulus and mass of the car-body, whereas the responses of the train exhibit the highest sensitivity to stochastic irregularities. Further, the influence of different random source combinations on the system response at different speeds is analyzed. The results suggest that it is sufficient to consider only random track irregularities for the train response, whereas random sources from both the bridge and train must be considered for the bridge response.

This paper highlights an integration of physical and numerical methods in evaluating a soil–breakwater system in coastal engineering. Centrifuge modelling is used to reproduce field phenomena and confirm the effectiveness of numerical methods. Numerical simulation incorporating finite element and limit equilibrium analysis is employed to explain the test results quantitatively and evaluate the deformation and stability of the complex soil–structure system. The results of physical and numerical simulation show that the proposed approaches work well and provide useful basis for potential applications to other coastal structures.

This paper presents safety evaluations of jetties found on soft clays at Huanghua Port to protect the channel from deposition due to long shore transport. It is likely to be the most critical for the jetties during construction, as far as excess pore water pressure and its effects on the embankment stability are concerned. A rubble mound jetty and a caisson jetty are considered as design alternatives for comparison. Finite element analysis and limit equilibrium analysis are carried out to evaluate the stability of the jetties during construction. For the rubble mound jetty, effective stresses in the soft clays are calculated associated with consolidation, so as to estimate the shear strength increased thereby. With the effective stresses and shear strength, the critical situation of the rubble mound jetty is revealed by the limit equilibrium analysis. For the caisson jetty, wave action is concerned, which is transformed into distributed forces on the caisson. Both the finite element analysis and the limit equilibrium analysis indicate a potential general shear failure of the caisson jetty and the foundation.

By depositing Mg vapor generated via carbothermal reduction on boron powder in the presence of different additives, dumbbell-like MgO nanofibers, NaCl particle-decorated MgO nanofibers and camphor tree stalk-like MgO fibers were produced. SEM, TEM and EDS analysis showed that the additives of Si and NaCl deposited in the sites of stacking faults in fiber, which resulted in the growth of different morphologies of decorated fibers.