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Platinum group metal‐free catalysts have been considered the most promising alternative for platinum‐based catalysts for the oxygen reduction reaction in fuel cells. Despite the significant advancement made in activity, their viability as fuel cell catalysts is still questionable due to their low durability. So far, deciphering the degradation mechanisms of this class of catalysts has been hampered by their undefined structure. Herein, we used a molecular model catalyst, iron‐phthalocyanine, featuring Fe−N4 active sites with resemblance to those in the more active Fe−N−C catalysts, and studied their degradation mechanisms. Based on X‐ray photoelectron spectroscopy and the electrochemical measurements, three main demetallation processes were identified: at potentials higher than 0.65 V vs. RHE, where the metal center is Fe3+, an electrochemical oxidation of the ligand ring is occurring, between 0.6 and 0.2 V vs. RHE, Fenton reagents are produced and attack the catalyst and support, and at lower voltages, where peroxide is produced by the catalyst and the carbon support. The combination of the different iron oxidation states together with the oxygen species directs to different degradation mechanisms. The decay rates obtained in the stability measurements establish what is mainly causing the loss of activity. Thereby, this model molecule can aid in understanding the degradation mechanisms of other platinum group metal‐free oxygen reduction reaction catalysts. [ABSTRACT FROM AUTHOR]

Exploring low‐cost and high‐efficient electrocatalyst is an exigent task in developing novel sustainable energy conversion systems, such as fuel cells and electrocatalytic fuel generations. 2D materials, specifically 2D superlattice materials focused here, featured highly accessible active areas, high density of active sites, and high compatibility with property‐complementary materials to form heterostructures with desired synergetic effects, have demonstrated to be promising electrocatalysts for boosting the performance of sustainable energy conversion and storage devices. Nevertheless, the reaction kinetics, and in particular, the functional mechanisms of the 2D superlattice‐based catalysts yet remain ambiguous. In this review, based on the recent progress of 2D superlattice materials in electrocatalysis applications, the rational design and fabrication of 2D superlattices are first summarized and the application of 2D superlattices in electrocatalysis is then specifically discussed. Finally, perspectives on the current challenges and the strategies for the future design of 2D superlattice materials are outlined. This review attempts to establish an intrinsic correlation between the 2D superlattice heterostructures and the catalytic properties, so as to provide some insights into developing high‐performance electrocatalysts for next‐generation sustainable energy conversion and storage. [ABSTRACT FROM AUTHOR]

The replacement of Pt/C catalysts with iron−nitrogen−carbon (Fe−N−C) nanomaterials in the oxygen reduction reaction (ORR) is appealing for the large‐scale application of fuel cells. Herein, an extended Stöber method was used to in situ synthesize Fe−N co‐doped porous carbon spheres (Fe−N−Cs) with large surface areas and impressive ORR performance. Fe−N−Cs were synthesized using 3‐aminophenol as N precursor, ferrocenealdehyde (Fc−CHO) as Fe source and crosslinker, and Pluronic F127 as soft template. Among all prepared catalysts, Fe−N−Cs‐1.0 exhibits much enhanced ORR activities. The initial potential and half‐wave potential are 0.992 V, 0.873 V (vs. RHE) and 0.981 V, 0.871 V in alkaline and acidic media, respectively, which are higher than that of commercial Pt/C catalyst (0.986 V, 0.868 V and 0.979 V, 0.843 V). Meanwhile, the methanol tolerance and durability of Fe−N−Cs‐1.0 were far better than Pt/C. This work manifests the Fe−N−Cs electrocatalysts are expected to replace the commercial Pt/C catalyst in the area of ORR. [ABSTRACT FROM AUTHOR]

The highly efficient energy conversion of the polymer‐electrolyte‐membrane fuel cell (PEMFC) is extremely limited by the sluggish oxygen reduction reaction (ORR) kinetics and poor electrochemical stability of catalysts. Hitherto, to replace costly Pt‐based catalysts, non‐noble‐metal ORR catalysts are developed, among which transition metal–heteroatoms–carbon (TM–H–C) materials present great potential for industrial applications due to their outstanding catalytic activity and low expense. However, their poor stability during testing in a two‐electrode system and their high complexity have become a big barrier for commercial applications. Thus, herein, to simplify the research, the typical Fe–N–C material with the relatively simple constitution and structure, is selected as a model catalyst for TM–H–C to explore and improve the stability of such a kind of catalysts. Then, different types of active sites (centers) and coordination in Fe–N–C are systematically summarized and discussed, and the possible attenuation mechanism and strategies are analyzed. Finally, some challenges faced by such catalysts and their prospects are proposed to shed some light on the future development trend of TM–H–C materials for advanced ORR catalysis. [ABSTRACT FROM AUTHOR]

Efficient electrocatalysis of the oxygen reduction (ORR) and evolution (OER) reactions is essential in numerous renewable energy conversion systems, such as fuel cells, metal‐air batteries, and water electrolyzers. Design and optimization of electrocatalytic materials for such systems primarily rely on understanding the nature of active centers on the catalyst surface. This review focuses on several important aspects of the experimental identification of active sites on various model bifunctional ORR/OER electrocatalytic surfaces. Applications of the state‐of‐the‐art experimental techniques are analyzed. In addition, approaches to investigate and understand the influence of some supporting electrolyte components on the ORR and OER activities are discussed. [ABSTRACT FROM AUTHOR]

Ligand and strain effects can tune the adsorption energy of key reaction intermediates on a catalyst surface to speed up rate‐limiting steps of the reaction. As novel fields like high‐entropy alloys emerge, understanding these effects on the atomic structure level is paramount: What atoms near the binding site determine the reactivity of the alloy surface? By statistical analysis of 2000 density functional theory calculations and subsequent host/guest calculations, it is shown that three atomic positions in the third layer of an fcc(111) metallic structure fourth‐nearest to the adsorption site display significantly increased influence on reactivity over any second or third nearest atomic positions. Subsequently observed in multiple facets and host metals, the effect cannot be explained simply through the d‐band model or a valence configuration model but rather by favorable directions of interaction determined by lattice geometry and the valence difference between host and guest elements. These results advance the general understanding of how the electronic interaction of different elements affect adsorbate–surface interactions and will contribute to design principles for rational catalyst discovery of better, more stable and energy efficient catalysts to be employed in energy conversion, fuel cell technologies, and industrial processes. [ABSTRACT FROM AUTHOR]

Atomically dispersed transition metal‐Nx sites have emerged as a frontier for electrocatalysis because of the maximized atom utilization. However, there is still the problem that the reactant is difficult to reach active sites inside the catalytic layer in the practical proton exchange membrane fuel cell (PEMFC) testing, resulting in the ineffective utilization of the deeply hided active sites. In the device manner, the favorite structure of electrocatalysts for good mass transfer is vital for PEMFC. Herein, a facile one‐step approach to synthesize atomically dispersed Fe‐Nx species on hierarchically porous carbon nanostructures as a high‐efficient and stable atomically dispersed catalyst for oxygen reduction in acidic media is reported, which is achieved by a predesigned hierarchical covalent organic polymer (COP) with iron anchored. COP materials with well‐defined building blocks can stabilize the dopants and provide efficient mass transport. The appropriate hierarchical pore structure is proved to facilitate the mass transport of reactants to the active sites, ensuring the utilization of active sites in devices. Particularly, the structurally optimized HSAC/Fe‐3 displays a maximum power density of up to 824 mW cm−2, higher than other samples with fewer mesopores. Accordingly, this work will offer inspirations for designing efficient atomically dispersed electrocatalyst in PEMFC device. [ABSTRACT FROM AUTHOR]

The development of stable, cost‐efficient and active materials is one of the main challenges in catalysis. The utilization of platinum in the electroreduction of oxygen is a salient example where the development of new material combinations has led to a drastic increase in specific activity compared to bare platinum. These material classes comprise nanostructured thin films, platinum alloys, shape‐controlled nanostructures and core–shell architectures. Excessive platinum substitution, however, leads to structural and catalytic instabilities. Herein, we introduce a catalyst concept that comprises the use of an atomically thin platinum film deposited on a potential‐triggered passivating support. The model catalyst exhibits an equal specific activity with higher atom utilization compared to bulk platinum. By using potential‐triggered passivation of titanium carbide, irregularities in the Pt film heal out via the formation of insoluble oxide species at the solid/liquid interface. The adaptation of the described catalyst design to the nanoscale and to high‐surface‐area structures highlight the potential for stable, passivating catalyst systems for various electrocatalytic reactions such as the oxygen reduction reaction. [ABSTRACT FROM AUTHOR]

Electrochemical scanning tunneling microscopy (EC‐STM) has become a reference among in operando techniques for probing the surface at molecular and atomic levels, allowing the fundamental processes occurring at the microscopic scale to be understood while an electrochemical reaction is occurring. Metal‐porphyrins and metal‐phthalocyanines (MN4) are synthons in biological molecules and have been successfully employed in the homogeneous as well as heterogeneous catalysis of many reactions. Therefore, studying the behavior in situ at the solid/liquid interface of these molecules allows information on their electronic and topographic structures to be obtained. Herein, Fe(II)‐ and Fe(III)‐coordinated phthalocyanines were examined by using the EC‐STM technique. An Au(111) single crystal was used as substrate to be functionalized with Fe‐phthalocyanines. These molecules were characterized in their ability to catalyze the oxygen reduction reaction, which is of great interest, especially in fuel cell technology. The FeN4 molecules in the form of an adlayer represent the catalyst material, which is probed under potential control in 0.1 M HClO4 by EC‐STM, with the aim to obtain information about the O2 adsorption/reduction process. [ABSTRACT FROM AUTHOR]

The design and synthesis of highly stable, low‐cost electrocatalysts for the oxygen reduction reaction to supersede noble‐metal catalysts is critical for their application in the field of fuel cells. Herein, we report a facile approach for the synthesis of CeO2 encapsulated by iron, sulfur, and nitrogen‐doped carbon nanoparticles (CeO2@Fe,S,N−C NPs) by pyrolyzing CeO2@polypyrrole, which is obtained in the presence of sodium dodecylbenzene sulfonate as a surfactant and FeCl3 as an oxidant during the polymerization of pyrrole on the CeO2 surface. As Fe and S are effectively doped into the N‐doped carbon derived from polypyrrole, a large number of active sites are created, which greatly enhances the catalytic activity of the sample. The achieved sample displays excellent catalytic activity with an onset and halfwave potential of 0.924 V and 0.835 V, respectively, and a limiting current density of 5.45 mA cm−2 at the potential of 0.4 V vs. RHE. Moreover, it illustrates a high durability and strong methanol tolerance, endowing it with the prospect of applications in energy conversion. [ABSTRACT FROM AUTHOR]

Efficient yet low‐cost electrocatalysts for the oxygen reduction reaction (ORR) are highly desirable for energy systems such as metal‐air batteries and fuel cells. Herein, we report the synthesis of efficient cobalt‐modified nitrogen‐doped carbon (N−C/Co) electrocatalysts by using a one‐pot pyrolysis. The abundant source of N in g‐C3N4 contributes to the in situ N doping in the carbon matrix, which favors the formation of Co‐related catalytic active sites. The N−C/Co‐1 (prepared with 1 mmol Co salt) sample exhibits the best performance towards the ORR, with a positive half‐wave potential of 0.86 V (vs. the reversible hydrogen electrode, RHE), high stability and excellent methanol tolerance, which outperforms the commercial Pt/C catalyst. We find that the high performance is dependent on the optimal content of Co that can achieve the maximum amount of Co−Nx catalytic species. In addition, we reveal that the size effect of Co nanoparticles is not the determining factor for the ORR activity in the current system, as the kinetics of the ORR reaction is dominantly determined by the active sites derived from Co−Nx species. By exploiting the N−C/Co‐1 sample as an air cathode catalyst, the assembled Zn‐air battery presents a maximal power density of 153 mW cm−2 as well as good durability during continuous cycle tests. [ABSTRACT FROM AUTHOR]

The performance of the oxygen reduction reaction (ORR) is a core issue in of fuel cells. ORR is a slow kinetic process; platinum normally is used to accelerate the reaction. This is a bottleneck for further development of fuel cells. Transition metal and heteroatom co-doped carbon materials have great potential in ORR catalysis. However, one-step synthesis of high activity and stability transition metal and heteroatom co-doped electrocatalysts is still a challenge. In this work, nitrogen, fluorine, and cobalt co-doped carbon catalyst, MPCo-950-5, is prepared from melamine, polytetrafluoroethylene, and cobalt acetate by a one-step method. MPCo-950-5 with a graphene-like structure and its specific surface area is 642 m2g-1. The ORR half-wave potential of MPCo-950-5 is positively shifted by 35 mV compared to that of commercial Pt/C. In addition, MPCo-950-5 also shows excellent stability and methanol tolerance. This work serves as a simple and feasible method for preparing high ORR active transition metal and heteroatom co-doped carbon materials. [ABSTRACT FROM AUTHOR]

Further improving the oxygen reduction reaction (ORR) catalytic activity of carbon catalysts is highly challenging and remains a key issue to push forward their commercial progress as Pt alternatives in fuel cells and metal‐air batteries. Herein, a facile and economic strategy was proposed to maximize the active sites exposed in porous carbon catalysts by growing dense N‐doped carbon nanonodules firmly located within carbon fiber networks (NCN@CF) via the in situ polymerization of dopamine on bacterial cellulose fibers and a subsequent carbonization process. This metal‐free hybrid catalyst exhibits enhanced ORR activity and better stabilities against long duration and the methanol crossover effect than the commercial Pt/C catalyst in 0.1 M KOH. Notably, when the NCN@CF hybrid was used as an air electrode catalyst in a Zn‐air battery, the as‐fabricated battery presented a larger peak power density of 168 mW cm−2, higher specific capacity of 720.5 mA h g−1 and energy density of 900.0 Wh kg−1 compared to those based on commercial Pt/C and most N‐doped carbon catalysts ever reported, evidencing its highly promising use as a Pt‐alternative catalyst for the ORR. [ABSTRACT FROM AUTHOR]

Proton/anion‐exchange membrane fuel cells (PEMFC and AEMFC), generating electricity from the H2 energy with zero‐carbon emission, have become very promising energy conversion devices to meet the increasing energy demand and reduce the dependence on fossil fuels. Currently, platinum (Pt) based materials have proven to be the most efficient electrocatalysts for the oxygen reduction reaction (ORR) at the cathode of the PEMFC and AEMFC. However, the high price and the limited reserve of Pt greatly hinder their industrial applications. Recently, transition metal‐nitrogen‐carbon (M−N−C) based material, as an efficient alternative to replace the precious metal Pt‐based material, has attracted researchers' attention. Great contributions have been devoted to develop M−N−C based electrocatalysts. This review summarizes recent advances on M−N−C based electrocatalysts used for ORR from the point view of morphology. One‐dimensional (1D), two‐dimensional (2D), three‐dimensional (3D) and multi‐dimensional (MD) M−N−C materials were discussed thoroughly with particular attention on the relationship between the structure and the catalytic activity. [ABSTRACT FROM AUTHOR]

One of the challenging goals in the large‐scale realization of hydrogen‐based fuel cells is to replace or minimize the use of expensive Pt‐based electrocatalysts to reduce molecular oxygen at the cathode. Recently, non‐metal doped nanosheets of transition metal dichalcogenides (TMDs) have emerged as a promising Pt‐free electrocatalyst for hydrogen evolution reaction. Herein, we investigate the potential of nitrogen‐ and phosphorus‐ (N‐ and P‐) doped TMDs (MoS2, MoSe2, WS2, and WSe2) as high‐performing electrocatalysts for ORR using first principle calculations, motivated by recent experimental advances to incorporate single S/Se vacancies on the surface of TMD monolayers using electrochemical methods. We observed a strong hybridization of p‐orbitals of N/P atom with the p‐ and d‐orbitals of neighbouring S/Se and Mo/W atoms, respectively, activating pristine TMDs by increasing density of states near Fermi‐level. Based on the free energy profiles of the four‐electron reduction process, we expect that N‐TMDs to be efficient in catalyzing ORR, whereas, too strong binding of reaction intermediates prevents ORR on P‐TMDs. Among the candidates considered, N‐WS2 is found to be a promising TMD as ORR catalyst with the overpotential as low as 0.31 V, stimulating further experimental studies. The potential of N‐ and P‐doped transition metal transition metal dichalcogenides (TMDs) as electrocatalysts for oxygen reduction is investigated using first principle calculations. Based on the free energy profiles of the four‐electron reduction process, it is expected that N‐TMDs to be efficient in catalyzing ORR, whereas, too strong binding of reaction intermediates prevents ORR on P‐TMDs, stimulating further experimental studies. [ABSTRACT FROM AUTHOR]

Abstract: Coral‐like, clean PtPd bimetallic nanochains with controlled composition are successfully synthesized by using carbon monoxide as reducing agent and soft‐template. The Pt74Pd26 nanochain catalyst exhibits a much more positive half‐wave potential than pure Pt nanochains (∼35 mV) and commercial Pt/C (∼30 mV) toward the oxygen reduction reaction. Meanwhile, oxygen reduction on the Pt74Pd26 electrode has only a 4 mV degradation in half‐wave potential over 5,000 cycling period, showing superior electrochemical stability. The enhanced performance could be due to the modified electronic structure (such as downshifted d‐band center and weakened bonding to oxygenated species) and the coral‐like nanochains with mesoporous structure and clean surface. This work may demonstrate a universal approach to facilitate screening of high‐efficiency catalysts for broad energy conversion/storage systems and sensing devices. [ABSTRACT FROM AUTHOR]

Abstract: Achieving active and sustainable electrocatalysts for the oxygen reduction reaction with low cost and high performance attracts great interest in the field of fuel cells. Here, we report a facile route to prepare hollow CeO2/CePO4@N,P co‐doped carbon by pyrolyzing CeO2@polyaniline obtained in the presence of phytic acid. By virtue of the co‐doping of N and P elements as well as the existence of oxygen vacancies and the formation of a void, the product illustrated excellent catalytic activity towards the oxygen reduction reaction with onset and half‐wave potentials of 0.918 and 0.822 V, respectively, and a current density of 4.38 mA cm−2 at 0.565 V vs. RHE. Furthermore, it presented high durability and strong methanol tolerance, revealing its great potential for applications in energy conversion. [ABSTRACT FROM AUTHOR]

Developing new non-precious-metal catalysts for the cathodic oxygen reduction reaction (ORR) in low-temperature fuel cells is highly desirable for replacing Pt-based noble-metal electrocatalysts. Herein, we report a two-step method to prepare Fe, Cu, and N-doped reduced graphene oxide/carbon nanotube (rGO/CNT) composites through the low-temperature hydrothermal treatment of iron-porphyrin-modified graphene and copper-chloride-loaded CNTs followed by high-temperature calcination, for use as efficient and stable electrocatalysts for the ORR. The optimized Fe/Cu−N-rGO/CNT catalyst exhibited higher ORR activity and superior stability, as compared to a commercial Pt/C catalyst in 0.1 mol ⋅ L−1 KOH solutions, and also demonstrates good stability in acidic electrolytes. These results suggest that additional doping of Cu ions into the Fe/N/C catalyst significantly promotes the catalyst ORR performance. [ABSTRACT FROM AUTHOR]

The use of rotating ring-disk electrodes has become a general practice for evaluating the activity of the oxygen reduction reaction (ORR) performed with electrocatalysts. This technique typically employs simple equations based on the charge/mass balance to calculate the average number of electrons transferred and the percentage of peroxide production. However, the underlying mechanistic assumptions of this analysis and the uncertainty involved when considering the catalytic decomposition of HO2− have not been well elucidated. This work demonstrates through electrokinetic derivations that the conventional formalism presumes a stepwise ORR pathway and four reaction parameters in total are provided for indexing ORR activity. Furthermore, we provide a method of current density deconvolution to separate the individual reaction components and exemplify its use for Tafel analysis. Finally, we provide analytical solutions to the reaction parameters for the ORR when heterogeneous HO2− decomposition is involved. [ABSTRACT FROM AUTHOR]

Keywords: electrocatalysis; fuel cells; nanostructures; oxygen reduction reaction; self-healing EN electrocatalysis fuel cells nanostructures oxygen reduction reaction self-healing 2343 2343 1 06/18/20 20200602 NES 200602 B The Cover Feature b illustrates the potential triggered stabilization of platinum thin-film/core-shell architectures during electrocatalysis. Electrocatalysis, fuel cells, nanostructures, oxygen reduction reaction, self-healing. [Extracted from the article]

Fe‐N‐doped graphitic carbon materials exhibit high efficiency and durability for oxygen reduction reaction (ORR). Although iron has relatively low price, the precursors for carbon and nitrogen used in previous studies have relatively high cost. Here reported is the preparation of highly efficient Fe‐N/C‐based ORR electrocatalysts by use of low‐cost urea as the precursors. Fe‐N/C‐based hybrids are prepared through a two‐step pyrolysis. During the first‐step pyrolysis, the precursors convert into g‐C3N4 with Fe located into the sixfold cavities, which ensures the relatively uniform distribution of Fe. The second‐step pyrolysis converts Fe‐g‐C3N4 into Fe‐N/C‐based hybrids which contain multiple types of active components, Fe moieties (FeCxNy or FeNx), Fe and Fe3N nanoparticles, for ORR. The obtained Fe‐N/C‐based hybrids display a superior electrocatalytic performance for ORR with an onset potential of 0.940 V and half‐wave potential of 0.810 V versus reversible hydrogen electrode, which are comparable to those of Pt/C at the same catalyst loading. The hybrids show higher tolerance to methanol and much greater long‐term stability than commercial Pt/C. The findings provide a cost‐effective approach for the preparation of high efficient and stable electrocatalysts for ORR and will be very helpful to the development of electrochemical energy storage and conversion. [ABSTRACT FROM AUTHOR]