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Summary: Hollow cobalt nitrogen‐doped carbon (H‐CoNC) is suggested for use in the membraneless hydrogen peroxide fuel cell (HPFC) and enzymatic biofuel cell (EBC) as anodic catalyst boosting hydrogen peroxide oxidation reaction (HPOR). For fabricating H‐CoNCs, a facile spray pyrolysis‐assisted process is used, and such produced H‐CoNCs show a porous and hollow‐shell structure, while they include a large amount of isolated Co atoms and coordinate bonds with Co and nitrogen (CoN4). This structure promotes mass transfer to the active site and excellent catalytic activity for HPOR. With these benefits of H‐CoNCs, the current density of the bioanode consisting of H‐CoNC, carbon nanotube, and glucose oxidase (H‐CoNC/CNT/GOx) observed at 0.3 V under 150 mM glucose is 315.5 μA cm−2, which is 2.1 times higher than that of a conventionally synthesized catalyst using Co‐doped carbon nanoparticles (CoNC‐NPs) (CoNC‐NPs/CNT/GOx, 146.2 μA cm−2). With this superior catalytic activity for HPOR, maximum power density (MPD) of membraneless EBC using H‐CoNC/CNT/GOx is 33.8 ± 4.52 μW cm−2, which is 52% higher than that of CoNP‐NPs/CNT/GOx. In addition, a membraneless HPFC using H‐CoNC/CNT demonstrates 4.87 times higher MPD (231.3 ± 11.3 μW cm−2) than that using CoNC‐NPs/CNT, proving that H‐CoNC improves the performance of fuel cells by its excellent catalytic activity. [ABSTRACT FROM AUTHOR]

Summary: Platinum‐free electrocatalysts for oxygen–reduction reactions in acidic medium are challenging as the cathode of PEMFC. Here, we synthesized core‐shell‐type leaf‐shaped CoFe‐NC (L‐CoFe‐NC) catalysts using 2D ZIFs as cores and 3D ZIFs as shells. The amounts of Co and Fe were carefully controlled to obtain highly dispersed CoFe‐doped carbon. This strategy made it easy to tune the catalyst while maintaining the advantages of the 2D structures. The prepared L‐CoFe‐NC showed a half‐wave potential (E1/2) of 0.77 V and an onset potential (Eonset) of 0.88 V in 0.5 M H2SO4. In addition, the results confirmed high durability even after 10 000 tests while retaining excellent performance. This outstanding stability was achieved owing to the high dispersion of Co and Fe in the 2D carbon frame. Furthermore, PEMFC was evaluated using an MEA containing L‐CoFe‐NC catalysts as a cathode. [ABSTRACT FROM AUTHOR]

[Display omitted] • M–N–C catalyst with Single-atoms (Fe, Co) and Co nanoparticles were synthesized. • Optimizing PPy concentration enhanced the active site for ORR in alkaline media. • The prepared catalyst showed excellent ORR performance, stability, and durability. • The prepared catalyst based-zinc-air batteries achieved high performance. Non-platinum electrocatalyst development for improving the oxygen reduction reaction (ORR) is critical in electrochemical energy conversion applications such as zinc-air batteries (ZABs). Herein, we developed dual Fe-Co single atoms and Co nanoparticles embedded N-doped hollow carbon (Fe-Co/N-HCS-x) catalysts via the carbonization of polypyrrole (PPy)@zeolitic imidazolate framework (ZIF) core–shell particles, with various PPy concentrations. PPy serves as a secondary nitrogen source that enhances nitrogen amount and promotes metal dispersion to increase active sites density. The Fe-Co/N-HCS-x catalysts with optimized PPy concentration control demonstrated improved ORR performance in alkaline media, with a half-wave potential (E 1/2) of 0.921 V and an onset potential (E onset) of 1.019 V. Fe-Co/N-HCS-x also exhibited high ZABs performance, with a power density of 244.6 mW cm−2 and an operating time of more than 100 h. Our research contributes to the development of optimized active sites for ORR, which can be applied to new electrochemical storage and conversion systems. [ABSTRACT FROM AUTHOR]

Oxygen reduction reaction (ORR), one of the key reactions for fuel cells and zinc-air batteries, should be improved for higher performance. Herein, we fabricated hollow Fe, Co, and nitrogen co-doped carbon (H-FeCo-NC) catalyst, which was prepared by carbonization of core-shell particles made of polypyrrole (PPy)-coated polystyrene (PS) spheres as cores and (Zn, Co) bimetallic-zeolitic imidazolate frameworks (ZnCoBZIFs) as shells. PPy was used as a nitrogen and a carbon source. The H-FeCo-NC catalyst had a high surface area of 324.08 m2 g−1 with uniformly distributed Fe and Co species, and excellent ORR performance with the half-wave potential of 0.888 V vs. reversible hydrogen electrode in alkaline media. Furthermore, the H-FeCo-NC catalyst demonstrated exceptional stability, durability, and tolerance to methanol crossover. • Hollow Fe, Co, and N-doped carbon was prepared using polypyrrole and MOFs. • Polypyrrole served as the source of carbon and nitrogen. • Hollow FeCo-NC catalyst showed high surface area with high metal dispersion. • The ORR activity, stability, and durability of the H-FeCo-NC catalyst were excellent. [ABSTRACT FROM AUTHOR]

Governing the fundamental reaction in lithium–oxygen batteries is vital to realizing their potentially high energy density. Here, novel oxygen reduction reaction (ORR) catalysts capable of mediating the lithium and oxygen reaction within a solution‐driven discharge, which promotes the solution‐phase formation of lithium peroxide (Li2O2), are reported, thus enhancing the discharge capacity. The new catalysts are derived from mimicking the biological redox mediation in the electron transport chain in Escherichia coli, where vitamin K2 mediates the oxidation of flavin mononucleotide and the reduction of cytochrome b in the cell membrane. The redox potential of vitamin K2 is demonstrated to coincide with the suitable ORR potential range of lithium–oxygen batteries in aprotic solvent, thereby enabling its successful functioning as a redox mediator (RM) triggering the solution‐based discharge. The use of vitamin K2 prevents the growth of film‐like Li2O2 even in an ether‐based electrolyte, which has been reported to induce surface‐driven discharge and early passivation of the electrode, thus boosting the discharge capacity by ≈30 times. The similarity of the redox mediation in the biological cell and lithium–oxygen "cell" inspires the exploration of redox active bio‐organic compounds for potential high‐performance RMs toward achieving high specific energies for lithium–oxygen batteries. A new redox mediator (RM), vitamin K2, for oxygen reduction reaction of lithium–oxygen batteries is developed, which is enabled by the inspiration from biological respiration system. Vitamin K2 surprisingly enhances the discharge capacity of lithium–oxygen cell by converting the reaction pathway of oxygen reduction. Successeful utilization of vitamin K2 shows validity of exploiting biological knowledge to discover high‐performance RM. [ABSTRACT FROM AUTHOR]

Developments in fuel cell technology have led to the design of new efficient electrocatalysts for the oxygen reduction reaction (ORR). In particular, metal-nitrogen-carbon (M-N-C) catalysts are promising alternatives to conventional noble metal ORR catalysts. Since the M-N-C catalyst has lower intrinsic activity than the noble metal catalyst, M-N-C catalysts should feature large surface areas, accessibility of active sites, short mass/charge transfer lengths, and surface permeability; these properties can lead to excellent catalytic activities. In addition, carbon-based materials, such as carbon nanotubes (CNTs), can be used to effectively improve the conductivity and stability of M-N-C catalysts. However, synthesizing carbon-based metal electrocatalysts with homogeneously distributed active sites and well-controlled structures remains challenging. In this study, we designed a carbon nanotube-encapsulated hollow Co-Fe-NC electrocatalyst by combining ultrasonic spray pyrolysis and a pseudomorphic transformation to a metal-organic framework, followed by carbonization. Spray pyrolysis formed hollow spheres without additional templates, requiring no further acidic leaching steps. Pseudomorphic transformation strategies can preserve the original morphology even after transforming the material to a metal-organic framework. The prepared CoFe-NC-CNT electrocatalyst exhibited high catalytic activity (half-wave potential of 0.924 V) and excellent 4-electron ORR selectivity. [Display omitted] • CoFe-NC-CNT was prepared by combining spray pyrolysis, MOF conversion, carbonization. • Hollow structure of CoFe-NC-CNT provides high electrochemical surface area. • CNTs in CoFe-NC-CNTs increase active site density by improving metal dispersion. • CoFe-NC-CNT exhibits excellent oxygen reduction activity and durability. [ABSTRACT FROM AUTHOR]

[Display omitted] • W-doped TiO 2 was synthesized by a simple solvothermal method. • Doping TiO 2 with W created more defective sites than undoped TiO 2. • Pt/W-doped TiO 2 catalysts exhibited high ORR activity and durability in acidic media. The carbon-supported platinum electrocatalyst's severe carbon corrosion issue in acidic media limits the widespread application of proton exchange membrane fuel cells (PEMFCs). Therefore, exploring novel noncarbon supports with improved corrosion resistance is needed to resolve this problem. This research presented tungsten-modified titanium dioxide (W-doped TiO 2) support for the Pt catalyst toward the oxygen reduction reaction (ORR). W-doped TiO 2 showed a higher surface area and more Ti3+ defect sites and oxygen vacancies than the undoped TiO 2 support, which enhanced strong metal support interaction for the deposited Pt catalyst. Furthermore, the high ORR activity of the Pt/W-doped TiO 2 catalyst was demonstrated using a high electrochemical surface area of 44.1 m2/g Pt and mass activity value of 41.22 A/g Pt for electrochemical performance in 0.1 M HClO 4. Moreover, the rotating ring-disk electrode measurement of the Pt/W-doped TiO 2 catalyst under acidic conditions showed a four-electron pathway with an ultrasmall peroxide yield of 0.42 %. Particularly, the durability test showed that the Pt/W-doped TiO 2 had a remarkable stability compared to commercial catalysts. Therefore, this study provides a promising pathway to improve the stability of Pt-based catalysts for PEMFCs. [ABSTRACT FROM AUTHOR]

[Display omitted] • Ultra-low loading of Pt deposited on a hollow cobalt-nitrogen-rich carbon support. • Improved utilization of ultra-low-loading Pt through a hollow carbon support. • The catalyst showed higher mass activity compared to the commercial catalyst. • The prepared catalyst was applied to the cathode in a single cell. • The prepared catalyst showed higher durability than the commercial catalyst. The oxygen reduction reaction (ORR) at the cathode of proton exchange membrane fuel cells is very slow. Therefore, platinum (Pt)-based catalysts are only used in practical applications. However, due to the high cost of Pt-based noble electrocatalysts, research is underway to reduce the use of noble metals or replace Pt-based noble catalysts. To reduce the total production cost of catalysts in fuel cells, a simple mass production process is required and the use of noble metals should be reduced. Metal–organic framework (MOF)-derived non-noble metal electrocatalysts have been extensively investigated as ORR catalysts due to their large surface area and open pore structure. However, MOF-based non-noble metal catalysts exhibit low catalytic activity and stability compared to commercial Pt-based catalysts. To address this problem, a small amount of precious metal is added to MOFs to enhance the performance of MOF-based electrocatalysts. In this study, we synthesized an ultra-low-loading Pt electrocatalyst (2.2- wt% Pt) supported on MOF-derived hollow Co, nitrogen-doped carbon (h-Co-NC). h-Co-NC was prepared by carbonizing the MOF@MOF core–shell structure. After carbonization, Pt was loaded using the polyol method, and a hollow and single-atomic Pt/h-Co-NC electrocatalyst was successfully synthesized by annealing. The synthesized hollow Pt/h-Co-NC electrocatalyst exhibited superior ORR performance and stability compared with non-hollow Pt/Co-NC. [ABSTRACT FROM AUTHOR]

[Display omitted] • TiONC electrocatalyst was synthesized via pyrolysis of MIL125(Ti)–NH 2 and melamine. • The effects of melamine and pyrolysis temperature were investigated. • TiONC-MA-1000 exhibits a superior production rate of 6.99 mol⋅g cat −1⋅h−1 by H-cell. • The defective Ti3+ is proposed as the primary active site. The 2-electron electrochemical oxygen reduction reaction (ORR) is considered a viable method for synthesizing H 2 O 2 because of its efficiency and environmental friendliness. However, conventional catalysts for this reaction predominantly rely on high-cost metals and require complex preparation procedures. Herein, we synthesized an electrocatalyst containing TiN and N-doped carbon via the pyrolysis of melamine and MIL125(Ti)–NH 2. The effects of melamine and pyrolysis temperature on the chemical composition and defect site formation were investigated. Importantly, the obtained results suggest that the generated Ti3+ sites are more important in enhancing the performance of the 2-electron ORR, surpassing defective carbon and oxygen functional groups. In particular, the resulting TiONC-MA-1000 catalyst prepared through pyrolysis at 1,000 °C contained abundant Ti3+ sites, resulting in high 2-electron ORR activity with a molar selectivity exceeding 91 % and high stability over a 12 h testing period. The electrochemical H 2 O 2 generation experiments conducted in an H-cell configuration employing TiONC-MA-1000 as the electrocatalyst yielded 6.99 mol⋅g catalyst −1⋅h−1 H 2 O 2. Therefore, the present study proposes a facile approach for fabricating highly active electrocatalysts suitable for large-scale H 2 O 2 production. [ABSTRACT FROM AUTHOR]

[Display omitted] • MOF-PPy composites were synthesized in a one-pot reaction. • The CoFe-NC-PPy with single-atom of Fe and Co was successfully synthesized. • PPy served as secondary nitrogen sources for enhanced ORR activity. • The CoFe-NC-PPy showed excellent ORR activity and durability in alkaline media. • The AEMFC performance with CoFe-NC-PPy exhibited remarkable efficiency. Metal-nitrogen-doped carbon (M−N−C) catalysts have emerged as highly active noble-metal-free catalysts for oxygen reduction reactions (ORR) in anion-exchange membrane fuel cells (AEMFCs). However, the performance of existing M−N−C catalysts is limited by the inaccessibility of internal M−N x active sites and loss of nitrogen doping after the carbonization process. Here, a single-atom Co, Fe and N-doped porous carbon catalyst (CoFe-NC-PPy) were constructed using a complex of metal–organic frameworks (MOFs) and polypyrrole (PPy). As a new approach, we synthesized MOF and PPy composites by combining in situ MOF synthesis and pyrrole polymerization in a one-pot reaction. Through pyrolysis, the PPy and MOF composites demonstrated an enhanced specific surface area, a high active-site density, and a high concentration of nitrogen species in the M−N−C catalyst. These contributed to a superior ORR performance (E 1/2 of 0.915 V) and high durability in alkaline media. Additionally, the CoFe-NC-PPy material studied in an AEMFC enabled high performance with a current density of 550 mA cm−2 at 0.6 V and a peak power density of 352 mW cm−2. This study suggests potential in combining polymers and MOFs in the synthesis of ORR catalysts for fuel cells. [ABSTRACT FROM AUTHOR]

[Display omitted] • Cobalt-based tofu was prepared by chelating metal cation and proteins. • Cobalt-Nitrogen-Phosphorous-doped carbon was prepared by carbonization of tofu. • Developed catalyst exhibits excellent oxygen reduction activity and durability. The oxygen reduction reaction at the cathode of an anion exchange membrane–fuel cell progresses very slowly. Therefore, only platinum-based catalysts with adequate adsorption/desorption binding energy for oxygen are used in practical applications. Because noble metal–based electrocatalysts are very expensive, research is underway to reduce the amount of noble metals used or to replace platinum-based catalysts. Further, to reduce the production cost of fuel cells, a simple fabrication process enabling mass productivity and replacement of noble metals must be developed. Transition metal catalysts with metal–nitrogen-doped carbon have been investigated as an alternative to the noble metal catalysts for oxygen reduction reaction at the cathode of a fuel cell. In this study, to generate metal–nitrogen–carbon sites, protein precipitation was performed by chelating metal cations with the functional groups of proteins. This approach employed a traditional method that uses tofu (made from soy milk) to intake proteins. To separate the proteins dissolved in soy milk in the solid state, a coagulant containing transition metals was added. Tofu containing bimetallic zinc and cobalt ions was prepared and carbonized to synthesize nitrogen- and phosphorus-doped carbon structures and single-atomic cobalt active sites. The prepared tofu-derived catalyst exhibited excellent electrocatalytic performance with a half-wave potential of 0.86 V, an onset potential of 0.981 V, and high durability. [ABSTRACT FROM AUTHOR]

Fe-N-C catalysts are promising alternatives to the platinum-group catalysts for use in oxygen reduction reactions of proton exchange membrane fuel cells. However, Fe-N-C catalysts suffer from poor durability, compared to non-precious metal catalysts, because of their accelerated demetallation by the Fenton reaction. In this study, we report the synthesis of a melamine-encapsulated Co-ZnO-C composite as a precursor and template for zeolite-imidazole-frameworks (ZIF-8). This approach allows formation of Co-N-C for constructing unique structures at meso- and macropore scales, while maintaining microporosity. Density functional theory analysis confirms the superior stability of the Co-N-C catalyst over other M-N-C catalysts (M = Fe, Ni, Cr, and Mn). Furthermore, it reveals that a closed interaction between the Co-N 4 moiety and organic adducts enhances oxophilicity, which prefers a 4-electron ORR activity. The Co-NC catalyst with a developed pore structure shows remarkable durability (6.7% performance degradation for 100 h) and full cell performance in H 2 /O 2 under 1 bar of backpressure (723 mW/cm2 of maximum power density). Consequently, the unique structure of the synthesized catalyst successfully translates to the computationally-established ORR activity in the half-cell; superior durability is seen in the real device operation and stability analysis. This work is expected to support next-generation fuel cell development. [Display omitted] • Melamine encapsulated Co-ZnO-C was prepared as precursor and template for atomic-cluster and porous Co-N-C catalyst. • ma-Co-NC exhibited excellent proton exchange membrane fuel cell performance and stability. • This approach allows formation of M-N-C for constructing unique structures at meso- and macropore scales, while maintaining microporosity. [ABSTRACT FROM AUTHOR]

• Hollow structure of Co-NC was synthesized by spray pyrolysis. • Co-doped ZnO act as both template and precursor for MOF synthesis. • The Co-NC hollow sphere showed outstanding half-cell and single-cell performance. • This strategy is applicable to other metal M-NC catalysts. Metal–nitrogen–carbon (M-NC) catalysts have been reported as promising electrocatalysts to replace noble-metal catalysts (Pt/C, Au/C, Ru/C etc.) in many industrial processes. In this regard, carbonized zeolite imidazole frameworks (ZIFs), as precursors of M-NC catalysts, have been extensively studied because of their porosity and ligand composition containing nitrogen and carbon. In this study, we synthesize Co-doped ZnO@ZIF-8 particles from a Co-doped ZnO sphere via the spray pyrolysis method and pseudomorphic replication. Ultrasonic spray pyrolysis is an attractive approach to operate mass production with high-purity homogeneous structures. Further, pseudomorphic replication makes it possible to control the morphology of the metal–organic framework (MOF) particles and easily prepare MOF composite particles. In the study, the pyrolyzed ZIF particles are applied to oxygen reduction reactions in alkaline media, and our results indicate that particles show a high half-wave potential of 0.904 V, which makes them suitable for diverse electrochemical applications. [ABSTRACT FROM AUTHOR]