Recent Progress in Precious Metal‐Free Carbon‐Based Materials towards the Oxygen Reduction Reaction: Activity, Stability, and Anti‐Poisoning

Pentagon-Rich Caged Carbon Catalyst for the Oxygen Reduction Reaction in Acidic Electrolytes

The interaction between electron spin and oxygen molecules in non-platinum catalysts, particularly carbon catalysts, significantly influences the catalytic performance of the oxygen reduction reaction (ORR). A promising approach to developing high-performance catalysts involves introducing five-membered ring structures with spin into graphitic carbons. In this study, we present the successful synthesis of cage-like cubic carbon catalysts enriched with pentagon structures using pentagon ring-containing C60 and a NaCl template. The number of pentagons contained in the structure was increased by doping with nitrogen and annealing, and the number of electron spins also increased, thereby improving catalytic activity. The prepared catalyst exhibits remarkable activity in ORR under acidic electrolytes. Furthermore, we elucidate the correlation between the pentagon structure, the number of spin, and catalytic activity, demonstrating that enhanced activity is contingent upon the presence of spin. Density functional theory (DFT) calculations support the role of spin in improving activity. The concept of spin and the introduction of pentagon structures provide new design principles for carbon catalysts.

Metal-Free Covalent Organic Frameworks for the Oxygen Reduction Reaction

The oxygen reduction reaction (ORR) is the key reaction in metal air and fuel cells. Among the catalysts that promote ORR, carbon-based metal-free catalysts are getting more attention because of their maximum atom utilization, effective active sites and satisfactory catalytic activity and stability. However, the pyrolysis synthesis of these carbons resulted in disordered porosities and uncontrolled catalytic sites, which hindered us in realizing the catalysts' design, the optimization of catalyst performance and the elucidation of structure-property relationship at the molecular level. Covalent organic frameworks (COFs) constructed with designable building blocks have been employed as metal-free electrocatalysts for the ORR due to their controlled skeletons, tailored pores size and environments, as well as well-defined location and kinds of catalytic sites. In this Concept article, the development of metal-free COFs for the ORR is summarized, and different strategies including skeletons regulation, linkages engineering and edge-sites modulation to improve the catalytic selectivity and activity are discussed. Furthermore, this Concept provides prospectives for designing and constructing powerful electrocatalysts based on the catalytic COFs.

Enhancing Polymer Electrolyte Membrane Fuel Cells with Ionic Liquids: A Review

Polymer electrolyte membrane fuel cells (PEMFCs) represent a promising clean energy solution. However, their widespread adoption faces hurdles related to component optimization. This review explores the pivotal role of ionic liquids (ILs) in enhancing PEMFC performance, focusing on their role in polymer electrolyte membranes, catalyst modification, and other components. By addressing key obstacles, including proton conductivity, catalyst stability, and fuel crossover, ILs provide a pathway towards the widespread commercialization of PEMFCs. In the realm of PEMFC membranes, ILs have shown great potential in improving proton conductivity, mechanical strength, and thermal stability. Additionally, the utilization of ILs as catalyst modifiers has shown promise in enhancing the electrocatalytic activity of electrodes by serving as an effective stabilizer to promote the dispersion of metal nanoparticles, and reduce their agglomeration, thereby augmenting catalytic performance. Furthermore, ILs can be tailored to optimize the catalyst-support interaction, ultimately enhancing the overall fuel cell efficiency. Their unique properties, such as high oxygen solubility and low volatility, offer advantages in terms of reducing mass transport and water management issues. This review not only underscores the promising advancements achieved thus far but also outlines the challenges that must be addressed to unlock the full potential of ILs in PEMFC technology, offering a valuable resource for researchers and engineers working toward the realization of efficient and durable PEMFCs.

Metal-Free Molecular Catalysts for the Oxygen Reduction Reaction: Electron Affinity as an Activity Descriptor

Heteroatom-doped polyaromatic hydrocarbons (or nanographenes) are promising molecular electrocatalysts for the oxygen reduction reaction (ORR). Here, we use density functional theory to investigate the first step of the ORR pathway (chemisorption) for a set of molecules with experimentally determined catalytic activities. Weak chemisorption is found for only negatively charged catalysts, and a strong correlation is observed between the computed electron affinities and experimental catalytic activities for a range of B- and B,N-doped polyaromatic hydrocarbons. The electron affinity is put forward as a simple activity descriptor of charged (activated) catalysts on an electrode.

3D porous carbon conductive network with highly dispersed Fe-Nxsites catalysts for oxygen reduction reaction

Intrinsic activity and reactive numbers are considered two important factors in oxygen reduction reaction (ORR) catalysts. Herein, we report the rational design and synthesis of a strongly coupled hybrid material comprising of FeZn nanoparticles (FeZn NPs) supported by a three-dimensional carbon conductive network (FeZn NPs@3D-CN) for increased ORR performance. Fe-N-C sites can offer high intrinsic activity owing to the unique bonding and oxygen vacancies, and the carbon conductive network facilitating the exposure to active sites, and increasing electron transport. Because of the synergetic effect of the conductive networks containing Fe-N-C and polyaniline, the catalysts exhibited ORR activity in an alkaline medium via a four-electron transfer process. FeZn NPs@3D-CN exhibited outstanding performance with a limited current density (6.2 mA cm^-2), the Tafel slope (81.19 mV dec^-1), and stability (23 mV negative shift after 2000 cycles), which were superior to those of 20% Pt/C (5.7 mA cm^-2, 75.1 mV dec^-1, 36 mV negative shift after 2000 cycles). This research highlights the effect of conductive networks expanding pathways and reducing the resistance of mass transport, which is a facile method to generate superior ORR electrocatalysts.

Coordinated single-molecule micelles: a self-template approach for preparing mesoporous doped carbons

Porous carbons prepared using a self-template approach inherit the pore features of template, but they exhibit almost no evenly dispersed mesopores, which is significant for diffusion-limited applications. Herein, N-doped hierarchically porous carbons (NHPCs) with uniform mesopores are prepared using a self-template method. The spherical single-molecule micelle of polystyrene-b-poly(4-vinyl pyridine) (PS-b-P4VP) is turned into a Zn²⁺-coordinated PS-b-P4VP micelle (CPM) by coordination of Zn²⁺ with the P4VP shell. Then, the self-template of the CPM is carbonized into a hollow carbon nanosphere. During carbonization, the PS core is decomposed to generate the central mesopore, whereas the Zn²⁺-coordinated P4VP shell is transformed into a carbonaceous shell. These even hollow carbon nanospheres aggregate to form uniformly mesoporous carbon lumps. Simultaneously, the coordinated Zn²⁺ of the CPM is reduced to metal zinc at high temperatures and then it is evaporated, thus creating numerous micropores in the carbonaceous shell. These NHPCs with uniform mesopores display a high specific surface area. As a demonstration in diffusion-limited applications, their catalytic performances for the oxygen reduction reaction (ORR) are investigated. Strikingly, NHPCs exhibit outstanding catalytic performances for the ORR. This self-template method paves a facile approach for preparing mesoporous carbons with high performances.

Proton Exchange Membrane Fuel Cells (PEMFCs): Advances and Challenges

The study of the electrochemical catalyst conversion of renewable electricity and carbon oxides into chemical fuels attracts a great deal of attention by different researchers. The main role of this process is in mitigating the worldwide energy crisis through a closed technological carbon cycle, where chemical fuels, such as hydrogen, are stored and reconverted to electricity via electrochemical reaction processes in fuel cells. The scientific community focuses its efforts on the development of high-performance polymeric membranes together with nanomaterials with high catalytic activity and stability in order to reduce the platinum group metal applied as a cathode to build stacks of proton exchange membrane fuel cells (PEMFCs) to work at low and moderate temperatures. The design of new conductive membranes and nanoparticles (NPs) whose morphology directly affects their catalytic properties is of utmost importance. Nanoparticle morphologies, like cubes, octahedrons, icosahedrons, bipyramids, plates, and polyhedrons, among others, are widely studied for catalysis applications. The recent progress around the high catalytic activity has focused on the stabilizing agents and their potential impact on nanomaterial synthesis to induce changes in the morphology of NPs.

Synergistic Modulation of Carbon-Based, Precious-Metal-Free Electrocatalysts for Oxygen Reduction Reaction

Developing alternatives to noble-metal-based catalysts toward the oxygen reduction reaction (ORR) process plays a key role in the application of low-temperature fuel cells. Carbon-based, precious-metal-free electrocatalysts are of great interest due to their low cost, abundant sources, active catalytic performance, and long-term stability. They are also supposed to feature intrinsically high activity and highly dense catalytic sites along with their sufficient exposure, high conductivity, and high chemical stability, as well as effective mass transfer pathways. In this Review, we focus on carbon-based, precious-metal-free nanocatalysts with synergistic modulation of active-site species and their exposure, mass transfer, and charge transport during the electrochemical process. With this knowledge, perspectives on synergistic modulation strategies are proposed to push forward the development of Pt-free ORR catalysts and the wide application of fuel cells.

Incorporation of Pt-Cr nanoparticles into highly porous MOF-5 as efficient oxygen reduction electrocatalysts

Developing new materials that can enhance the efficiency of energy conversion and storage systems is critical to meeting the rising energy demand of low-carbon economies. Mesoporous materials have the advantages of large specific surface area and multiple channels, which can increase efficiency and flexibility in terms of energy and power density. An active catalyst for oxygen reduction reaction (ORR) based on Pt-Cr nanoparticles with ultralow Pt content (0.90 wt%) has been studied in this paper. In contrast, electrocatalyst Pt/Cr/NPC-900 exhibited an ORR activity with onset potential (E _o) of 1.01 V vs. RHE in an alkaline solution that was superior to commercial Pt/C (20 wt%) (0.96 V vs. RHE). The presence of metal oxides and optimal Pt content enhanced the ORR activity. Therefore, the synergistic effect of the high surface area increased charge transfer, and excellent structural stability can achieve significant ORR efficiency, which is conducive to excellent activity. These findings provide a new perspective for economical and practical ORR electrocatalysts to be designed and synthesized rationally.