High-performance Oxygen Reduction Reaction Research Articles

Etching engineering on controllable synthesis of etched N-doped hierarchical porous carbon toward efficient oxygen reduction reaction in zinc–air batteries

Low-cost, high-performance oxygen reduction reaction (ORR) catalysts greatly determine the practical application of advanced energy storage and conversion systems, including fuel cells and metal–air batteries. Precise design and controllable preparation of cutting-edge ORR catalysts demonstrate much meaningful to their electrochemical performance. Herein, etching engineering is applied on polypyrrole-derived carbon to controllably prepare etched N-doped hierarchical porous carbon (ENHPC–t, t means etching time) by using ammonia as an etching source. By systematical characterization and analysis, we find altering etching time exerts a profound effect on the morphology, structure, and composition of ENHPC–t. In spite of relatively lower defect density than the counterparts, the obtained ENHPC–5 with unique hierarchical porous structure, much larger specific surface area, and more optimized N configurations, display significantly improved ORR performance in both basic and acid media. When constructed as a cathode catalyst for zinc–air batteries, it also presents competitive performance to commercial Pt/C catalysts. Undoubtedly, our adopted etching strategy provides good guidance on a controllable synthesis of low-price but high-performance carbon-based ORR catalysts.

Oxygen Reduction Reaction: MnN4 Oxygen Reduction Electrocatalyst: Operando Investigation of Active Sites and High Performance in Zinc–Air Battery (Adv. Energy Mater. 6/2021)

In article number 2002753, Lirong Zheng, Aijuan Han, Junfeng Liu and co-workers report the development of a manganese single-atomic-site catalyst anchored on porous nitrogen-doped carbon with Mn-N4 structure, which exhibits superior electrocatalytic activity and stability for the alkaline oxygen reduction reaction (ORR) and in zinc-air batteries. Furthermore, operando XAFS is applied to uncover the mechanism under realistic conditions. This work paves the way to construct high-performance ORR electrocatalysts through rational design.

Nanostructured tubular carbon materials doped with cobalt as electrocatalyst for efficient oxygen reduction reaction

The development of non-precious metal catalysts with high efficiency and superior reaction durability for oxygen reduction reaction (ORR) is of great importance for construction of next-generation fuel cells. Herein, we report the preparation of in-situ nitrogen-doped carbon nanotube electrocatalysts (PPy-NCNTs) which were prepared by the pyrolysis of one-dimensional nanotube-like polypyrrole conjugated polymers precursor (PPy-Tubes) synthesized by oxidative polymerization and self-assembly method. The as-prepared PPy-NCNTs show good ORR performance, e.g., its half-wave potential reaches 0.826 V. In order to further improve the ORR performance of PPy-NCNTs, we prepared the Co-N co-doped carbon nanotube materials (Co-N-PPy-NCNTs) with different Co contents by adding cobalt nitrate into the polymerization solution during synthesis of PPy nanotube followed by a pyrolysis treatment. Comparatively, the Co-N-PPy-NCNTs have better ORR performance than that of PPy-NCNTs with a half-wave potential of 0.830 V, a biased 4e− reaction pathway, and better stability and methanol tolerance (high than Pt/C catalysts in the methanol tolerance test and the durability test). Taking advantages of its simple and cost-efficient preparation, high ORR performance and superior stability and methanol tolerance, the Co-N-PPy-NCNTs should be a kind of ideal candidate as an alternative to the precious metal platinum-based catalysts for fuel cells.

Advanced Platinum-Based Oxygen Reduction Electrocatalysts for Fuel Cells.

ConspectusFuel cells are among the cutting-edge energy technologies. Their commercial development is still hindered by noble platinum (Pt) catalysts for the oxygen reduction reaction (ORR) at the cathode, which not only determine the energy conversion efficiency and service life but also are closely related to the cost and broad application of fuel cells. Given the bright and enormous future of fuel cells, ORR catalysts should possess highly efficient performance yet meet the acceptable Pt costs for large-scale application. Extensive efforts are concentrated on the optimization of Pt-based nanostructures and upgradation of functional carriers to achieve the low-cost and high-activity Pt-based catalysts. By improving the Pt utilization and accessible surface, reducing Pt consumption and catalyst costs, accelerating mass exchange and electron transfer, alleviating the corrosion and agglomeration of carriers and Pt, accompanying with the assistance of robust yet effective functional supports, the service level and life of Pt-based electrocatalysts would be significantly improved and fuel cells could get into commercial market covering broader applications.In this Account, we focus on the recent development of Pt-based catalysts to figure out the problems associated with ORR catalysts in fuel cells. Recent development of Pt-based catalysts is discussed in different stages: (1) multiscale development of Pt-based nanostructures; (2) multielement regulation over Pt-based alloy composition; (3) upgradation of carbon and noncarbon support architectures; (4) development of integrated Pt-based catalysts for fuel cells. Finally, we propose some future issues (such as reaction mechanism, dynamic evolutions, and structure-activity relationship) for Pt-based catalysts, which mainly involve the preparation strategy of Pt-integrated catalysts (combination of Pt nanostructures with nanocarbons), performance evaluation (standard measurement protocols, laboratory-level rotating disk electrode (RDE) measurements, application-level membrane electrode assembly (MEA) service test), advanced interpretation techniques (spectroscopy, electron microscopy, and in situ monitoring), and cutting-edge simulation/calculations and artificial intelligence (simulation, calculations, machine learning, big data screening). This Account calls for the comprehensive development of multiscale, multicomponent, and high-entropy Pt-based alloy nanostructures, and novel and stable carriers, which provide more available options for rational design of low-cost and high-performance Pt-integrated ORR catalysts. More importantly, it will give an in-depth understanding of the reaction mechanism, dynamic development, and structure-performance relationship for Pt-based catalysts in fuel cells and related energy technologies.

Self-templated formation of cobalt-embedded hollow N-doped carbon spheres for efficient oxygen reduction

The slow kinetics at the cathode of oxygen reduction reaction (ORR) seriously limits the efficiencies of fuel cells and metal-air batteries. Pt, the state-of-the-art ORR electrocatalyst, suffers from high cost, low earth abundance, and poor stability. Here a self-templated strategy based on metal-organic frameworks (MOFs) is proposed for the fabrication of hollow nitrogen-doped carbon spheres that are embedded with cobalt nanoparticles (Co/HNC). The Co/HNC manifests better ORR activities, methanol tolerance, and stability than commercial Pt/C. The high ORR performance of Co/NHC can be attributed to the hollow structure which provides enlarged electrochemically active surface area, the formation of more Co-N species, and the introduction of defects. This work highlights the significance of rational engineering of MOFs for enhanced ORR activity and stability and offers new routes to the design and synthesis of high-performance electrocatalysts.

Tri-atomic Pt clusters induce effective pathways in a Cocore-Pdshell nanocatalyst surface for a high-performance oxygen reduction reaction.

The crux of the hot topic concerning the widespread replacement of fuel cells (FCs) with traditional petrochemical energy is to balance improving the oxygen reduction reaction (ORR) and reducing the cost. The present study employs density functional theory (DFT) to investigate the effect of Pt ensemble size regulation from a single atom to full coverage on the physio-chemical properties, oxygen adsorption energies and overall ORR efficiency of bimetallic nanocatalysts (NCs) with a Cocore-Pdshell structure. Our results reveal that the electronegativity difference and lattice strain between neighboring heteroatoms are enhanced to trigger a synergetic effect in local domains, with the Pt cluster size reduced from nanometers to subnanometers. They induce a directed and tunable charge relocation mechanism from deep Co to topmost Pt to optimize the adsorption energies of O2/O* and achieve excellent ORR kinetics performance with minimum Pt usage but maximum Pt atom utilization (i.e., Pt1 to Pt3) compared with benchmark Pt(111). Such a dependency between the cluster size and corresponding ORR performance for the established Co@Pd-Ptn system can be applied to accurately guide the experimental synthesis of ordered heterogeneous catalysts (e.g., other core@shell-clusters structures) toward low Pt, high efficiency and green economy.

Metal- and Nonmetal-Incorporated Vitamin B <sub>12</sub> on Graphene as a Bioderived Electrocatalyst for High-Performance Oxygen Reduction Reaction in Acidic Media

Metal- and Nonmetal-Incorporated Vitamin B <sub>12</sub> on Graphene as a Bioderived Electrocatalyst for High-Performance Oxygen Reduction Reaction in Acidic Media

FeCo Nanoparticle-Loaded Nutshell-Derived Porous Carbon as Sustainable Catalyst in Al-Air Batteries

Developing a high-performance ORR (oxygen reduction reaction) catalyst at low cost has been a challenge for the commercialization of high-energy density and low production cost aluminium-air batteries. Herein, we report a catalyst, prepared by pyrolyzing the shell waste of peanut or pistachio, followed by concurrent nitrogen-doping and FeCo alloy nanoparticle loading. Large surface area (1246.4 m 2 g -1 ) of pistachio shell-derived carbon can be obtained by combining physical and chemical treatments of the biomass. Such a large surface area carbon eases nitrogen doping and provides more nucleation sites for FeCo alloy growth, furnishing the resultant catalyst (FeCo/N-C-Pistachio) with higher content of N, Fe, and Co with a larger electrochemically active surface area as compared to its peanut shell counterpart (FeCo/N-C-Peanut). The FeCo/N-C-Pistachio displays a promising onset potential of 0.93 V vs. RHE and a high saturating current density of 4.49 mA cm -2 , suggesting its high ORR activity. An aluminium-air battery, with FeCo/N-C-Pistachio catalyst on the cathode and coupled with a commercial aluminium 1100 anode, delivers a power density of 99.7 mW cm -2 and a stable discharge voltage at 1.37 V over 5 h of operation. This high-performance, low-cost, and environmentally sustainable electrocatalyst shows potential for large-scale adoption of aluminium-air batteries.

MOF-derived PtCo/Co3O4 nanocomposites in carbonaceous matrices as high-performance ORR electrocatalysts synthesized via laser ablation techniques

ZIF-67-derived carbon-based bimetallic nanocomposites with reduced Pt-loading via laser ablation synthesis in solution (LASiS) as a superior electrocatalyst for oxygen reduction reaction (ORR).

Fe7C3 nanoparticles with in situ grown CNT on nitrogen doped hollow carbon cube with greatly enhanced conductivity and ORR performance for alkaline fuel cell

Reasonable design of the porous metal-organic frameworks (MOFs) to convert a burgeoning carbon-based catalysts with high oxygen reduction reaction (ORR) activity are still challenging in energy conversion, storage and transport. Herein, Fe7C3-doped in-situ grown carbon nanotubes and N-doped hollow carbon (Fex-CNT@NHC) are prepared by using a simple and robust preparation method, which is used cubic ZIF-8-derived zinc oxide cubes as a template for secondary MOFs re-growth followed by the final carbonization. In the 0.1 M KOH, the as-pyrolyzed Fe0.1-CNT@NHC electrocatalyst displays the value of half-wave potential is 0.92 V and the value of diffusion-limited current density is 6.08 mA cm−2, respectively, which are close to the corresponding electrochemical values of the standard commercial Pt/C (0.89 V and 5.89 mA cm−2). Meanwhile, the material has passed relevant tests on its long-term stability and methanol tolerance in alkaline media, showing that it has excellent ORR activity and efficient stability under electrocatalysis. Furthermore, the Fe0.1-CNT@NHC materials catalyze a Zn-air battery that delivers a performed peak power density of 105.9 mW cm−2. The impressive catalytic activity of Fe0.1-CNT@NHC stems from the effective synergy between efficient Fe and N co-doping, large specific surface area, and high electrical conductivity. This preparation route for carbon nanomaterials will provide a new synthetic strategy to synthesize high-performance non-noble metal carbon-based ORR catalysts for practical energy-related applications.

Pomegranate-like C60@cobalt/nitrogen-codoped porous carbon for high-performance oxygen reduction reaction and lithium-sulfur battery

Porous carbon materials play essential roles in electrocatalysis and electrochemical energy storage. It is of significant importance to rationally design and tune their porous structure and active sites for achieving high electrochemical activity and stability. Herein, we develop a novel approach to tune the morphology of porous carbon materials (PCM) by embedding fullerene C60, achieving improved performance of oxygen reduction reaction (ORR) and lithium-sulfur (Li-S) battery. Owing to the strong interaction between C60 and imidazole moieties, pomegranate-like hybrid of C60-embedded zeolitic imidazolate framework (ZIF-67) precursor is synthesized, which is further pyrolyzed to form C60-embedded cobalt/nitrogen-codoped porous carbon materials (abbreviated as C60@Co-N-PCM). Remarkably, the unique structure of C60@Co-N-PCM offers excellent ORR electrocatalytic activity and stability in alkaline solutions, outperforming the commercial Pt/C (20 wt.%) catalyst. Besides, C60@Co-N-PCM as a novel cathode delivers a high specific capacity of ∼ 900 mAh·g−1 at 0.2 C rate in Li-S batteries, which is superior to the pristine ZIF-67-derived PCM without embedding C60.

Nitrogen-Doped Porous Graphene-like Carbon Nanosheets as Efficient Oxygen Reduction Reaction Catalysts under Alkaline and Acidic Conditions

Electrocatalysts are vulnerable to be influenced during the oxygen reduction reaction (ORR) process on the cathodes of direct methanol fuel cells, mainly by the reaction products and air-containing traces of acidic gas carbon dioxide (CO₂), leading to low activity and durability. Herein, an ORR catalyst was derived from furfural, nitrogen carbide (g-C₃N₄), and calcium chloride (CaCl₂) via a dual-template method, resulting in two-dimensional graphene-like carbon nanosheets with a micro–mesoporous architecture, large Brunauer–Emmett–Teller surface areas (748 m²/g), an ultrahigh pyridinic nitrogen-doping level (13.6 wt %), and high degree of graphitization. The as-synthesized sample shows high ORR performance and robust durability in both alkaline and acidic electrolytes undergoing a gas mixture of O₂ and CO₂ (95:5), demonstrating a strong tolerance of CO₂ during the ORR process. Besides, the as-prepared material also exhibits competitive properties in the selective capture of CO₂ gas versus N₂. Thus, this work provides a facile route of preparing high-performance bifunctional carbonaceous materials in both ORR and CO₂ gas capture.

N,S-Co-Doped Porous Carbon Nanofiber Films Derived from Fullerenes (C60 ) as Efficient Electrocatalysts for Oxygen Reduction and a Zn-Air Battery.

The development of highly efficient metal-free electrocatalysts for the oxygen reduction reaction (ORR) has attracted great attention for the creation of electrochemical energy devices. In this study, one-dimensional (1 D) fullerene nanofibers prepared from liquid-liquid interfacial precipitation are first fabricated into fullerene-derived carbon nanofiber films (FCNFs) through a simple filtration procedure. Then, pyrolysis of the FCNFs in the presence of ammonia and sulfur produces N- and S-co-doped porous carbon nanofiber films (N,S-PCNFs). As excellent metal-free electrocatalysts for the ORR, N,S-PCNFs exhibit remarkable catalytic activity, superior stability, and excellent methanol tolerance in both alkaline and acidic solution. Such a high ORR performance benefits from the robust porous nanofiber network structure with high concentrations of active N- and S- groups and abundant defects. Notably, upon practical use of N,S-PCNFs as catalysts in Zn-air batteries, a high power density and a large operating voltage are achieved, with a performance comparable to that of the commercial Pt/C catalyst. This work presents a facile strategy for the creation of a new class of energy nanomaterials based on fullerenes, demonstrating their practical uses in electrocatalytic ORR processes and Zn-air batteries.

In situ surface-confined fabrication of single atomic Fe-N4 on N-doped carbon nanoleaves for oxygen reduction reaction

An efficient and robust in situ surface-confined strategy was demonstrated for the fabrication of single-atom Fe-N 4 on N-doped carbon nanoleaves (L-FeNC). Benefiting from abundant Fe-N 4 active sites, enhanced mass and charge transfer, L-FeNC delivered superior performance for ORR and Zn-air battery to commercial Pt/C. • In situ surface-confined strategy was demonstrated to fabricate L-FeNC. • L-FeNC showed an E 1/2 of 0.89 V and good stability for ORR in 0.1 M KOH. • High ORR performance is owing to abundant Fe-N 4 , favored mass and charge transfer. Controllable fabrication of Fe-N-C based single-atom catalysts (SACs) for enhanced electrocatalytic performance is highly desirable but still challenging. Here, an in situ surface-confined strategy was demonstrated for the synthesis of single atomic Fe-N 4 on N-doped carbon nanoleaves (L-FeNC). The in situ generated Zn 3 [Fe(CN) 6 ] 2 could not only serve as a protection layer against collapse of nanoleaves but also provide abundant Fe source for the formation of Fe-N moieties during pyrolysis, leading to high surface area and high graphitization degree of L-FeNC simultaneously. Benefiting from abundant Fe-N 4 active sites, enhanced mass and charge transfer, the as-prepared L-FeNC manifested a half-wave potential of 0.89 V for oxygen reduction reaction (ORR) in 0.1 M KOH. A maximum power density of 140 mW cm −2 and stable discharge voltage even after operation for 50,000 s have been demonstrated when the L-FeNC was used as air cathode for Zn-air battery. This work not only provided a unique surface-confined strategy for the synthesis of two-dimensional nanocarbons, but also demonstrated the significant benefit from rational design and engineering of Fe-N-C SACs, thus offering great opportunities for fabrication of efficient energy conversion and storage devices.

Self-templating construction of N, P-co-doped carbon nanosheets for efficient eletreocatalytic oxygen reduction reaction

The performance of metal-free oxygen reduction reaction (ORR) electrocatalysts are far from satisfactory, thus impeding their real world application. Herein, we present a C3N4 self-templating method to construct a P and N co-doped porous carbon nanosheets with high ORR performance. The conbination of the templating effect and the synergy effect direved from N, P co-doping is revealed of utmost importance towards ORR. The optimized catalyst represents an outstanding ORR performance with an onset potential of 0.93 V and half-wave potential of 0.85 V, comparable to those of commercial Pt/C electrocatalysts (1.04 V, 0.84 V).

Preparation of iron and nitrogen co-doped carbon material Fe/N-CCM-T for oxygen reduction reaction

In this paper, iron and nitrogen co-doped carbon material with nanotube structure (Fe/N-CCM-T) was synthesized by pyrolyzing a mixture of Fe salt, chitosan and melamine and displayed high electrocatalytic performance for oxygen reduction reaction (ORR). The structure of the Fe/N-CCM-T was characterized and their ORR performance in alkaline media was investigated by linear sweep voltammetry, cyclic voltammetry and chronoamperometry. Fe/N-CCM-T displayed better ORR performance than other carbon materials like Fe/N-CC-800. The Fe/N-CCM-800 with a large surface area (302.5 m2/g) especially exhibited the best ORR electrocatalytic performance among the prepared carbon materials, which was also proved by its similar Tafel slope (76 mV decade−1) to Pt/C catalyst (74 mV decade−1). Fe/N-CCM-800 showed similar ORR activity as commercial Pt/C catalyst, but superior tolerance to methanol and stability. Such high ORR performance of the Fe/N-CCM-T can be attributed to its nanotube structure, high specific surface area (SSA), high graphitic-N and pyridinic-N contents.

Electrospun Polyacrylonitrile, Styrene–Acrylonitrile Copolymer and Carbon Nanotube Composite Fiber Based Oxygen Reduction Catalysts

An ever increasing worldwide energy demand drives the need for the development of more efficient and greener energy production and storage technologies.1 In recent years, one-dimensional nanofibrous materials (NFMs) have drawn a lot of attention and been widely studied to address these topical problems. For example, the electrospun NFMs can be employed as an oxygen reduction reaction (ORR) catalyst at the fuel cell cathode. The NFMs are appealing because of their attractive properties e.g. high specific surface area, high length/diameter ratio, specific porosities and multiple functionalities. For the preparation of NFMs, a simple and versatile technique called electrospinning can be applied.2 In our previous study of NFMs, we investigated pyrolyzed electrospun multiwall carbon nanotube (MWCNT) and styrene-acrylonitrile copolymer (SAN) fiber materials as ORR catalysts in alkaline solution. It was found that during the thermal decomposition of SAN, the ORR-active N-functionalities can be embedded into the carbon nanomaterial.3 For further optimization of the catalyst material ORR activity, another nitrogen-containing polymer (polyacrylonitrile, PAN) was incorporated into the NFM composition to study the ORR performance of three-component composite material (SAN/PAN/MWCNT) in our latest work regarding NFMs.4 The ORR performance of SAN/PAN/MWCNT-based catalysts was evaluated in 0.1 M KOH solution by rotating disk electrode and linear sweep voltammetry methods. The catalyst with the highest activity toward the ORR was prepared via pyrolysis at 1100 °C (SAN/PAN/MWCNT-1100) and this material outperformed the best electrospun two-component, SAN and MWCNT, composite fiber-based catalyst from our previous study.3 For physical characterization of the materials, scanning electron microscopy (SEM), X–ray photoelectron spectroscopy (XPS), Raman spectroscopy and N2 adsorption-desorption studies were performed. The structure of the catalysts is nanofibrous with visible MWCNTs according to the SEM images (Figure 1a). The high ORR activity of the catalysts prepared in this work is most likely attributed to the nitrogen species as determined by XPS. The catalyst SAN/PAN/MWCNT-1100 also showed good stability during long-term testing in 0.1 M KOH solution (Figure 1b).4 The material with the highest ORR performance used in the present study (SAN/PAN/MWCNT-1100) is a promising catalyst for the development of efficient cathode catalysts for low-temperature fuel cells.4

Room Temperature Synthesis of Ordered Intermetallic Electrocatalysts

The conversion of O2 to H2O is an important fuel cell reaction for the recovery of renewable electricity from chemical fuels. Alloys of Pt-group metals (PGMs) with transition metals have emerged as catalysts with enhanced activity and durability for the oxygen reduction reaction (ORR). To date, most studies focus exclusively on alloys that adopt a Face-centered cubic (FCC) or Face-centered tetragonal (FCT) crystal lattice. Under ORR operating conditions, these materials evolve from alloys to Pt core-alloy shell materials, in this case both the alloy core and Pt shell retains the FCC or FCT type crystal structure; indicating that materials can dynamically change during catalysis.Here we present the synthesis of Pd-Bi based ordered intermetallics by partial dealloying of a Bi rich alloy. We found that under ORR operation conditions, β-PdBi2 (I4/mmm space group) which adopts a tetragonal lattice, transforms to α-Pd3Bi (Pmma space group) which adopts an orthorhombic crystal structure; this transformation is mediated by oxidative Bi corrosion under potential cycling, and reconstruction of the atoms in the bulk. To the best of our knowledge, this is the first report of a material that exhibits an electrochemically driven, non-congruent phase transition among materials with different crystal structures. The resulting phase-converted Pd3Bi is stable and exhibits high performance for ORR. Phase-converted Pd3Bi outperform Pt and Pd metal, reaching mass activities of 2.6 A/mgPd, which is nearly 10X higher than Pt/C (~0.3 A/mgPt) and Pd/C (~0.2 A/mg) at 0.9 V vs the Real Hydrogen Electrode (RHE). The mass activity of Pd3Bi decreases by ~38% after 10,000 cycles, indicating that it is stable. Activation energy (Ea) determined by temperature dependent electro-kinetic measurements indicates that the activity enhancement on phase-converted Pd3Bi originates from anion-poisoning resistant behavior. These results establish that PGMs-based ordered intermetallics with low-symmetry crystal structures can be highly active and durable electrocatalysts.

Carbon-Free Connected Platinum–Iron Catalysts with Enhanced Chemically Ordered Structures As Durable Oxygen Reduction Electrocatalysts for PEFCs

To realize the widespread uses of polymer electrolyte fuel cells (PEFCs), the improvement of the activity and durability of Pt-based electrocatalysts for oxygen reduction reaction (ORR) is essential. In our group, a carbon-free connected Pt–Fe catalyst with a porous hollow capsule structure has been developed. [1–3] This catalyst consists of a nanosized beaded network by the connection of Pt–Fe nanoparticles. The beaded metal network is electrically conductive, enabling the removal of carbon supports form a catalyst layer. The carbon-free cathode catalyst layer using a connected Pt–Fe catalyst exhibits high durability against start/stop cycles, because of the elimination of carbon corrosion problems. In addition, the ORR specific activity of a connected Pt–Fe catalyst is about 9 times higher than that of a commercial Pt nanoparticle catalyst supported on carbon black (Pt/C). Thus, a connected Pt–Fe catalyst provides a high ORR activity and excellent start/stop durability. On the other hand, the dissolution of catalyst metals during a load cycle operation is one of critical issues for Pt-based ORR catalysts. Our previous studies using Pt-alloy nanoparticle catalysts on carbon black demonstrate that chemical ordered (superlattice) structures improve load cycle durability due to the suppression of metal dissolution. [4–8] Therefore, as shown in Figure 1A, this study developed a carbon-free connected Pt–Fe catalyst with a high chemical ordering structure to achieve high durability against both start/stop and load cycle operations.In this study, we proposed a new synthesis method using the combination of a SiO2 coating and an annealing treatment (Figure 1B) to control chemically-ordered (face centered tetragonal: fct) structures in a connected Pt–Fe catalyst. The SiO2 layer coated on catalyst surfaces prevents the detachment of Pt–Fe nanoparticles and the large catalyst agglomeration and coalescence during the high-temperature annealing. Using this method, the connected Pt–Fe catalyst having a nano-sized network and a high fct degree (70–80%) was successfully prepared. This catalyst exhibited ca. ten times higher ORR specific activity, compared with that of the Pt/C. Furthermore, as shown in Fig. 1C, the connected Pt–Fe catalyst with the high fct degree (76%) showed the higher retention of the ORR specific activity after 10000 load cycles (0.6 V for 3 s and 1.0 V for 3 s in N2-saturated 0.1 M HClO4 solution at 60 °C), compared to the catalyst with the low fct degree (47%). The results of the STEM-EDX line mapping indicated that a highly ordered fct structure greatly contributes to the suppression of metal dissolution from the catalyst.This study demonstrated, for the first time, the synthesis method as shown in Figure 1B to prepare a connected Pt–Fe catalyst with a nanosized network and a high chemical ordering structure. The carbon-free connected Pt–Fe catalyst with an enhanced chemically ordered structure exhibited a high ORR activity as well as improved durability against both start/stop and load cycle operations. The results obtained in this study provide useful guidelines for the design of high-performance ORR catalysts for PEFCs. Acknowledgements This work was financially supported by Kanagawa Institute of Industrial Science and Technology (KISTEC) and Core Research for Evolutionary Science and Technology at the Japan Science and Technology Agency (JST-CREST, JPMJCR1543).

Coral‐like nitrogen doped carbon derived from polyaniline‐silicon nitride hybrid for highly active oxygen reduction electrocatalysis

AbstractIn this work, a coral‐like nitrogen‐doped carbon electrocatalyst has been prepared via self‐assembly oxidative polymerization of aniline monomers onto silicon nitride (Si3N4) nanospheres, followed by carbonization. The prepared catalyst at 900°C (denoted as NC‐900) shows excellent oxygen reduction reaction (ORR) activity by achieving an onset potential (η) of 0.90 V versus reversible hydrogen electrode (RHE) and half wave potential (E1/2) of 0.82 V versus RHE comparable to that of the commercial Pt/C (η = 0.99 V vs. RHE and E1/2 = 0.87 V vs RHE) and even outperforming many carbon‐based electrocatalysts recently reported in the literature. The catalyst NC‐900 also displays better stability and resistance toward methanol crossover effect than Pt/C in alkaline solution. The high ORR performance of NC‐900 catalyst was associated with high content of nitrogen species and hierarchical mesoporous structure that facilitate high electron and mass transport.