Recent progress on mechanisms,principles,and strategies for high-activity and high-stability non-PGM fuel cell catalyst design

The commercialization of a polymer membrane H2-O2 fuel cell and its widespread use call for the development of cost-effective oxygen reduction reaction(ORR)nonplatinum group metal(NPGM)catalysts.Nevertheless,to meet the requests for the real-world fuel cell application and replacing platinum catalysts,it still needs to address some challenges for NPGM catalysts regarding the sluggish ORR kinetics in the cathode and their poor durability in acidic environment.In response to these issues,numerous efforts have been made to study NPGM catalysts both theoretically and experimentally,developed these into the atomically dispersed coordinated metal-nitrogen-carbon(M-N-C)form over the past decades.In this review,we present a comprehensive summary of recent advancements on NPGM catalysts with high activity and durability.Catalyst design strategies in terms of optimizing active-site density and enhancing catalyst stability against demetalization and carbon corrosion are highlighted.It is also emphasized the importance of understanding the mechanisms and principles behind those strategies through a combination of theoretical modeling and experimental work.Especially,further understanding the mechanisms related to the active-site structure and the formation process of the single-atom active site under pyrolysis conditions is critical for active-site engineering.Optimizing the active-site distance is the basic principle for improving catalyst activity through increasing the catalyst active-site density.Theoretical studies for the catalyst deactivation mechanism and modeling stable active-site structures provide both mechanisms and principles to improve the NPGM catalyst durability.Finally,currently remained challenges and perspectives in the future on designing high-performance atomically dispersed NPGM catalysts toward fuel cell application are discussed.

Holistic Approach to Safety and Operational Stability: Analyzing VVER-1200 Reactor Dynamics in SGTR and AC Power Loss

VVER-1200 (Water-Water Energetic Reactor) represents a significant advancement in nuclear power generation, emphasizing the continuous analysis and enhancement of safety systems for reliable operation. The proposed study focuses on simulating combined scenarios involving steam generator tube rupture (SGTR) and AC power loss using core algorithms and models within personal computer transient analyzer (PCTRAN). Reactor kinetic equations, thermal-hydraulic balance, and safety system models are discussed to elucidate their role in simulating SGTR and AC power loss. Safety criteria, boundaries and initial conditions are outlined to provide a comprehensive understanding of the simulation framework. The analysis delves into dynamic behavior of VVER-1200, placing emphasis on thermal-hydraulic implications, essential reactor parameters, and radiation monitoring to facilitate impact evaluation. Continuous monitoring and maintenance of safety systems are underscored to ensure stable core cooling, particularly during proposed transient conditions. Through meticulous analysis and comparison with established benchmarks, this study contributes to bolstering the safety and reliability of VVER-1200 reactors by identifying vulnerabilities, assessing mitigation strategies, and refining emergency response protocols. Practical implications of this study offer a crucial understanding of reactor behavior, safety system performance, and emergency response strategies, thereby improving safety, optimizing operational practices, and reducing risks in nuclear reactor accidents.

Mechanisms of reactive element Y on the purification of K4169 superalloy during vacuum induction melting

The effects of rare earth element Y on the purification of K4169 superalloy during vacuum induction melting were investigated at different superheating temperatures. The corresponding interaction mechanisms were also clarified. Results showed that the addition of Y remarkably promoted the purification effect on the K4169 melt. The contents of O and S in the K4169 as-cast alloy ingots after purification were 3–4 and 8–10 ppm, respectively. The degrees of deoxidation and desulfurization increased to 50% and 57%, respectively, upon the addition of 0.1 wt% Y. The yttrium-rich phase that precipitated at the grain boundary blocked the diffusion of C and the accumulation of S, thereby contributing to the purification of the alloy.

Mechanisms of yttrium on the wettability,surface tension and interactions between Ni-20Co-20Cr-10Al-ξY alloys and MgO ceramics

The mechanisms of Y on the wettability,surface tension,and interactions between the Ni-20 Co-20 Cr-10 Al-ξY alloys and MgO ceramics at 1873 K were investigated by sessile drop experiments.The results of nonlinear fitting showed that the equilibrium contact angles and Y concentrations were approximately in accord with the log-normal distribution law.The equilibrium contact angles changed from 101.5°to 140.5°with Y increasing from 0 wt.%to 1.23 wt.%.Cross-sectional microstructure observations revealed that the thermal dissociation of ceramics occurred and the released[O]atoms can react with Y to produce Y_(2)O_(3) reaction layer along three-phase interphase area.Wetting kinetics analyses indicated that surface tension of the melt droplets had been positively correlated with the Y concentrations,and it increased from 737.8–1045.1 mN/m.Meanwhile,the pinning effect of the rough substrate surface on the three-phase line hindered the spreading of the liquid on ceramics.The change in total free energy of the alloys/ceramics system was considered as the key factor affecting the wettability.Moreover,the surface morphology and thermodynamic stability of ceramics also had some influence on the wettability.

On the transformation textures influenced by deformation in electrical steels,high manganese steels and pure titanium sheets

Transformation texture is normally different to deformation and recrystallization textures,thus influencing materials properties differently.As deformation and recrystallization are often inseparable to transformation in materials which shows a variety in types such as diffusional or non-diffusional transformations,different phenomena or rules of strengthening transformation textures occur.This paper summarizes the complicated phenomena and rules by comparison of a lot of authors’published and unpublished data collected from mainly electrical steels,high manganese steels and pure titanium sheets.Three kinds of influencing deformation are identified,namely the dynamic transformation with concurrent deformation and transformation,the transformation preceded by deformation and recrystallization and the surface effect induced transformation,and the textures related with them develop in different mechanisms.It is stressed that surface effect induced transformation is particularly effective to enhance transformation texture.It is also shown that the materials properties are also improved by controlled transformation textures,in particular in electrical steels.It is hoped that these phenomena and processing techniques are beneficial to the establishment of transformation texture theory and property improvement in practice.