Hierarchically porous Co@N-doped carbon fiber assembled by MOF-derived hollow polyhedrons enables effective electronic/mass transport:An advanced 1D oxygen reduction catalyst for Zn-air battery

Developing advanced oxygen reduction reaction(ORR)electrocatalysts with rapid mass/electron transport as well as conducting relevant kinetics investigations is essential for energy technologies,but both still face ongoing challenges.Herein,a facile approach was reported for achieving the highly dispersed Co nanoparticles anchored hierarchically porous N-doped carbon fibers(Co@N-HPCFs),which were assembled by core-shell MOFs-derived hollow polyhedrons.Notably,the unique one-dimensional(1D)carbon fibers with hierarchical porosity can effectively improve the exposure of active sites and facilitate the electron transfer and mass transfer,resulting in the enhanced reaction kinetics.As a result,the ORR performance of the optimal Co@N-HPCF catalysts remarkably outperforms that of commercial Pt/C in alkaline solution,reaching a limited diffusion current density(J)of 5.85 m A cm^(-2)and a half-wave potential(E_(1/2))of 0.831 V.Particularly,the prepared Co@N-HPCF catalysts can be used as an excellent air-cathode for liquid/solid-state Zn-air batteries,exhibiting great potentiality in portable/wearable energy devices.Furthermore,the reaction kinetic during ORR process is deeply explored by finite element simulation,so as to intuitively grasp the kinetic control region,diffusion control region,and mixing control region of the ORR process,and accurately obtain the relevant kinetic parameters.This work offers an effective strategy and a reliable theoretical basis for the engineering of first-class ORR electrocatalysts with fast electronic/mass transport.

Natural tea-leaf-derived, ternary-doped 3D porous carbon as a high-performance electrocatalyst for the oxygen reduction reaction

To commercialize fuel cells and metal-air batteries, cost-effective, highly active catalysts for the oxygen reduction reaction （ORR） must be developed. Herein, we describe the development of low-cost, heteroatom （N, P, Fe） ternary-doped, porous carbons （HDPC）. These materials are prepared by one-step pyrolysis of natural tea leaves treated with an iron salt, without any chemical and physical activation. The natural structure of the tea leaves provide a 3D hierarchical porous structure after carbonization. Moreover, heteroatom containing organic compounds in tea leaves act as precursors to functionalize the resultant carbon frameworks. In addition, we found that the polyphenols present in tea leaves act as ligands, reacting with Fe ions to form coordination compounds; these complexes acted as the precursors for Fe and N active sites. After pyrolysis, the as-prepared HDPC electrocatalysts, especially HDPC-800 （pyrolyzed at 800℃）, had more positive onsets, half-wave potentials, and higher catalytic activities for the ORR, which proceeds via a direct four-electron reaction pathway in alkaline media, similar to commercial Pt/C catalysts. Furthermore, HDPC-X also showed enhanced durability and better tolerance to methanol crossover and CO poisoning effects in comparison to commercial Pt/C, making them promising alternatives for state-of-the-art ORR electrocatalysts for electrochemical energy conversion. The method used here provides valuable guidelines for the design of high-performance ORR electrocatalysts from natural sources at the industrial scale.

Transition metal-nitrogen-carbon nanostructured catalysts for the oxygen reduction reaction： From mechanistic insights to structural optimization

Accelerating the rate-limiting oxygen reduction reaction （ORR） at the cathode remains the foremost issue for the commercialization of fuel cells. Transition metal-nitrogen-carbon （M-N/C, M = Fe, Co, etc.） nanostructures are the most promising class of non-precious metal catalysts （NPMCs） with satisfactory activities and stabilities in practical fuel cell applications. However, the long-debated nature of the active sites and the elusive structure-performance correlation impede further developments of M-N/C materials. In this review, we present recent endeavors to elucidate the actual structures of active sites by adopting a variety of physicochemical techniques that may provide a profound mechanistic understanding of M-N/C catalysts. Then, we focus on the spectacular progress in structural optimization strategies for M-N/C materials with tailored precursor architectures and modified synthetic routes for controlling the structural uniformity and maximizing the number of active sites in catalytic materials. The recognition of the right active centers and site-specific engineering of the nanostructures provides future directions for designing advantageous M-N/C catalysts.

Pt‑Based Intermetallic Compound Catalysts for the Oxygen Reduction Reaction:Structural Control at the Atomic Scale to Achieve a Win–Win Situation Between Catalytic Activity and Stability

The development of ordered Pt-based intermetallic compounds is an effective way to optimize the electronic characteristics of Pt and its disordered alloys,inhibit the loss of transition metal elements,and prepare fuel cell catalysts with high activity and long-term durability for the oxygen reduction reaction(ORR).This paper reviews the structure–activity characteristics,research advances,problems,and improvements in Pt-based intermetallic compound fuel cell catalysts for the ORR.First,the structural characteristics and performance advantages of Pt-based intermetallic compounds are analyzed and explained.Second,starting with 3d transition metals such as Fe,Co,and Ni,whose research achievements are common,the preparation process and properties of Pt-based intermetallic compound catalysts for the ORR are introduced in detail according to element types.Third,in view of preparation problems,improvements in the preparation processes of Pt-based intermetallic compounds are also summarized in regard to four aspects:coating to control the crystal size,doping to promote ordering transformation,constructing a“Pt skin”to improve performance,and anchoring and confinement to enhance the interaction between the crystal and support.Finally,by analyzing the research status of Pt-based intermetallic compound catalysts for the ORR,prospective research directions are suggested.

A non-precious metal catalyst for oxygen reduction prepared by heat-treating a mechanical mixture of carbon black,melamine and cobalt chloride

A non-precious metal catalyst CoMe]C for the oxygen reduction reaction is prepared by heat-treating a mechanical mixture of carbon black, melamine and cobalt chloride at 600 under nitrogen atmosphere for 2 h. The catalytic activity of CoMe/C is characterized by the electrochemical linear sweep voltammetry technique. The onset reduction potential of the catalyst is 0.55 V （vs. SCE） at a scanning rate of 5 mV/s in 0.5 mol/L H2SO4 solution. The formation of the ORR activity sites of CoMe/C is facilitated by metallic β- cobalt.

Electrocatalytic oxygen reduction performances of surface Ag granular packs electrodeposited from dual-phase Ag_(35.5)Zn_(64.5) precursor alloys by triangle wave potential cycling

Surface Ag granular packs(SAgPs) have been fabricated from dual-phase Ag_(35.5)Zn_(64.5) precursor alloy consisting of both e and c phases by using a facile one-step triangle wave potential cycling in 0.5 mol·L^(-1) KOH.During the continuous potential cyclic sweeping, the c phases preferentially dissolve during the anodic scan and dominant reduction reactions of Ag cations lead to redeposition and accumulation of Ag atoms together to form SAg Ps during cathodic scan. The e phases stay inactive to form a continuous skeleton in the inner regions. SAg Ps with an average particle size of 94-129 nm can be obtained at scan rates of 25, 50 and 100 mV·s^(-1) for 100 triangle wave potential cycles. SAgPs formed at a scan rate of 50 mV·s^(-1) exhibit superior oxygen reduction reaction performances with the onset potential of 0.93 V, half-wave potential of 0.72 V and an electron transfer number of 4.0.The above-mentioned SAgPs have superior stabilities as ORR catalysts.