Carbon-based bifunctional electrocatalysts for oxygen reduction and oxygen evolution reactions:Optimization strategies and mechanistic analysis

Electrocatalysts are one of the essential components for the devices of high-efficiency green energy storage and conversion,such as metal-air cells,fuel cells,and water electrolysis systems.While catalysts made from noble metals possess high catalytic performance in both oxygen reduction reaction(ORR)and oxygen evolution reaction(OER),their scarcity and expensiveness significantly limit large-scale applications.In this regard,metal-free/non-noble metal carbon-based catalysts have become competitive alternatives to replace catalysts made of noble metals.Nevertheless,low catalytic ORR/OER performance is the challenge of carbon-based catalysts for the commercial applications of metal-air batteries.To solve the problem of poor catalytic performance,two strategies have been proposed:(1)controlling the microstructure of the catalysts to expose more active sites as the channels of rapid mass and electron transfer;and(2)reducing the reaction energy barrier by optimizing the electronic structures of the catalysts via surface engineering.Here,we review different types of bifunctional ORR/OER electrocatalysts with the activated surface sites.We focus on how the challenge can be overcome with different methods of material synthesis,structural and surface characterization,performance validation/optimization,to outline the principles of surface modifications behind catalyst designs.In particular,we provide critical analysis in the challenges that we are facing in structural design and surface engineering of bifunctional ORR/OER catalysts and indicate the possible solution for these problems,providing the society with clearer ideas on the practical prospects of noble-metal-free electrocatalysts for their future applications.

Additive manufacturing of bulk metallic glasses:Fundamental principle,current/future developments and applications

As an advanced manufacturing technique,the advent of additive manufacturing(AM) has opened a new horizon of alternative ways to tackle the challenge of fundamental limits for manufacturing bulk metallic glasses(BMGs).In particular,selective laser melting(SLM),direct metal deposition(DMD),electron beam melting(EBM),and laser foil printing(LFP) have been used for producing BMGs with dimensions larger than what is possible using conventional techniques such as melt-spinning,suction-casting,die-casting,etc.In this review,we analyzed the current status,issues,structural evolution,and key properties of BMGs based on these emerging AM technologies.The aim is to outline a direction for the development of BMGs using AM technology,establishing a fundamental principle to optimize processing parameters for designing alloy compositions with the high glass-forming ability(GFA),and thermal stability against crystallization.This will provide the fundamental science underpinning the future development of AM technology in the fabrication of high-density,defect-free,and completely amorphous alloy components and devices.

Advanced Strategies for Stabilizing Single‑Atom Catalysts for Energy Storage and Conversion

Well-defined atomically dispersed metal catalysts(or single-atom catalysts)have been widely studied to fundamentally under-stand their catalytic mechanisms,improve the catalytic efficiency,increase the abundance of active components,enhance the catalyst utilization,and develop cost-effective catalysts to effectively reduce the usage of noble metals.Such single-atom cata-lysts have relatively higher selectivity and catalytic activity with maximum atom utilization due to their unique characteristics of high metal dispersion and a low-coordination environment.However,freestanding single atoms are thermodynamically unstable,such that during synthesis and catalytic reactions,they inevitably tend to agglomerate to reduce the system energy associated with their large surface areas.Therefore,developing innovative strategies to stabilize single-atom catalysts,including mass-separated soft landing,one-pot pyrolysis,co-precipitation,impregnation,atomic layer deposition,and organometallic complexation,is critically needed.Many types of supporting materials,including polymers,have been commonly used to stabilize single atoms in these fabrication techniques.Herein,we review the stabilization strategies of single-atom catalyst,including different synthesis methods,specific metals and carriers,specific catalytic reactions,and their advantages and disadvantages.In particular,this review focuses on the application of polymers in the synthesis and stabilization of single-atom catalysts,including their functions as carriers for metal single atoms,synthetic templates,encapsulation agents,and protection agents during the fabrication process.The technical challenges that are currently faced by single-atom catalysts are summarized,and perspectives related to future research directions including catalytic mechanisms,enhancement of the catalyst loading content,and large-scale implementation are proposed to realize their practical applications.

Molten salt assisted self-activated carbon with controllable architecture for aqueous supercapacitor

Activated carbons have been widely employed as electrode materials of aqueous supercapacitors but the use of hazardous and corrosive activating agents challenges conventional activation procedures.Here,using a unique molten salt assisted self-activation technique,we have devised an eco-friendly and simple method to synthesize oxygen-rich hierarchical porous carbon with controllable architecture.Mixture of sodium carboxymethylcellulose and NaCl was pyrolyzed in one step,creating in-situ produced Na_(2)CO_(3)-NaCl molten salt that carried out the activation work.Na2 CO3 acts as the activating agent in the reaction media of NaCl during the self-activation process.The obtained carbon exhibited a remarkable specific capacitance of 278 F g^(−1) at 0.5 A g^(−1) and retained 76%capacitance at 50 A g^(−1) in a three-electrode cell.The fabricated aqueous coin cell achieved 81%capacitance retention at 50 A g^(−1) and the highest specific energy density of 12.8 Wh kg^(−1) at 214.6 W kg^(−1),which are superior compared to the commercial activated carbon(64%at 50 A g^(−1) and 8.4 Wh kg^(−1) at 194.8 W kg^(−1)).Moreover,capacitance fading was not observed after 10000 cycles at 5 A g^(−1).Considering the species diversity and low cost of self-salt polymers on the market,this strategy will expect to become a scalable approach for synthesizing high-performance capacitive carbons.

Thermoelectric performance enhancement by manipulation of Sr/Ti doping in two sublayers of Ca_(3)Co_(4)O_(9)

Thermoelectric(TE)performance of Ca_(3)Co_(4)O_(9)(CCO)has been investigated extensively via a doping strategy in the past decades.However,the doping sites of different sublayers in CCO and their contributions to the TE performance remain unrevealed because of its strong correlated electronic system.In this work,Sr and Ti are chosen to realize doping at the[Ca_(2)CoO_(3)]and[CoO_(2)]sublayers in CCO.It was found that figure of merit(ZT)at 957 K of Ti-doped CCO was improved 30% than that of undoped CCO whereas 1 at% Sr doping brought about a 150% increase in ZT as compared to undoped CCO.The significant increase in electronic conductivity and the Seebeck coefficient are attributed to the enhanced carrier concentration and spin-entropy of Co^(4+) originating from the Sr doping effects in[Ca_(2)CoO_(3)]sublayer,which are evidenced by the scanning electron microscope(SEM),Raman,Hall,and X-ray photoelectron spectroscopy(XPS)analysis.Furthermore,the reduced thermal conductivity is attributed to the improved phonon scattering from heavier Sr doped Ca site in[Ca_(2)CoO_(3)]sublayer.Our findings demonstrate that doping at Ca sites of[Ca_(2)CoO_(3)]layer is a feasible pathway to boost TE performance of CCO material through promoting the electronic conductivity and the Seebeck coefficient,and reducing the thermal conductivity simultaneously.This work provides a deep understanding of the current limited ZT enhancement on CCO material and provides an approach to enhance the TE performance of other layered structure materials.