FeN4 Active Sites Electronically Coupled with PtFe Alloys for Ultralow Pt Loading Hybrid Electrocatalysts in Proton Exchange Membrane Fuel Cells

The exorbitant cost of Pt-based electrocatalysts and the poor durability of non-noble metal electrocatalysts for proton exchange membrane fuel cells limited their practical application. Here, FeN4 active sites electronically coupled with PtFe alloys (PtFe-FeNC) were successfully prepared by a vapor deposition strategy as an ultralow Pt loading (0.64 wt %) hybrid electrocatalyst. The FeN4 sites on the FeNC matrix are able to effectively anchor the PtFe alloys, thus inhibiting their aggregation during long-life cycling. These PtFe alloys, in turn, can efficiently restrain the leaching of the FeN4 sites from the FeNC matrix. Thus, the PtFe-FeNC demonstrated an improved Pt mass activity of 2.33 A mgPt-1 at 0.9 V toward oxygen reduction reaction, which is 12.9 times higher than that of commercial Pt/C (0.18 A mgPt-1). It demonstrated great stability, with the Pt mass activity decreasing by only 9.4% after 70,000 cycles. Importantly, the fuel cell with an ultralow Pt loading in the cathode (0.012 mgPt cm-2) displays a high Pt mass activity of 1.75 A mgPt-1 at 0.9 ViR-free, which is significantly better than commercial MEA (0.25 A mgPt-1). Interestingly, PtFe-FeNC catalysts possess enhanced durability, exhibiting a 12.5% decrease in peak power density compared to the 51.7% decrease of FeNC.

Neighboring Platinum Atomic Sites Activate Platinum-Cobalt Nanoclusters as High-Performance ORR/OER/HER Electrocatalysts

The rational design of efficient and multifunctional electrocatalysts for energy conversion devices is one of the major challenges for clean and renewable energy transition. Herein, the local electronic structure of cobalt-platinum nanoclusters is regulated by adjacent platinum atomic site encapsulated in N-doped hollow carbon nanotubes (PtSA -PtCo NCs/N-CNTs) by pyrolysis of melamine-orientation-induced zeolite imidazole metal-organic frameworks (ZIF-67) with thimbleful platinum doping. The introduction of melamine can reactivate adjacent carbon atoms and initiate the oriented growth of nitrogen-doped carbon nanotubes. The systematic analysis suggests the significant role of thimbleful neighboring low-coordinated Pt─N2 in altering the localized electronic structure of PtCo nanoclusters. The optimized PtSA -PtCo NCs/N-CNTs-900 exhibit excellent hydrogen evolution reaction (HER)/oxygen evolution reaction (OER)/oxygen reduction reaction (ORR)/ catalytic performance reaching the current density of 10 mA cm-2 in 1 m KOH under the low 47 (HER) and 252 mV (OER) overpotentials, and a high half-wave potential of 0.86 and 0.89 V (ORR) in 0.1 m KOH and 0.1 m HClO4 , respectively. Remarkably, the PtSA -PtCo NC/N-CNT-900 also presents outstanding catalytic performances toward water splitting and rechargeable Zn-air batteries. The theoretical calculations reveal that optimal regulation of the electronic structure of PtCo nanoclusters by thimbleful neighboring Pt atomic reduces the reaction energy barrier in electrochemical process, facilitating the ORR/OER/HER performance.

Spontaneous Internal Electric Field in Heterojunction Boosts Bifunctional Oxygen Electrocatalysts for Zinc-Air Batteries: Theory, Experiment, and Application

Heterojunctions are a promising class of materials for high-efficiency bifunctional oxygen electrocatalysts in both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). However, the conventional theories fail to explain why many catalysts behave differently in ORR and OER, despite a reversible path (* O2 ⇋* OOH⇋* O⇋* OH). This study proposes the electron-/hole-rich catalytic center theory (e/h-CCT) to supplement the existing theories, it suggests that the Fermi level of catalysts determines the direction of electron transfer, which affects the direction of the oxidation/reduction reaction, and the density of states (DOS) near the Fermi level determines the accessibility for injecting electrons and holes. Additionally, heterojunctions with different Fermi levels form electron-/hole-rich catalytic centers near the Fermi levels to promote ORR/OER, respectively. To verify the universality of the e/h-CCT theory, this study reveals the randomly synthesized heterostructural Fe3 N-FeN0.0324 (Fex N@PC with DFT calculations and electrochemical tests. The results show that the heterostructural F3 N-FeN0.0324 facilitates the catalytic activities for ORR and OER simultaneously by forming an internal electron-/hole-rich interface. The rechargeable ZABs with Fex N@PC cathode display a high open circuit potential of 1.504 V, high power density of 223.67 mW cm-2 , high specific capacity of 766.20 mAh g-1 at 5 mA cm-2 , and excellent stability for over 300 h.

Controlled Synthesis of Carbon-Supported Pt-Based Electrocatalysts for Proton Exchange Membrane Fuel Cells

Proton exchange membrane fuel cells are playing an increasing role in postpandemic economic recovery and climate action plans. However, their performance, cost, and durability are significantly related to Pt-based electrocatalysts, hampering their large-scale commercial application. Hence, considerable efforts have been devoted to improving the activity and durability of Pt-based electrocatalysts by controlled synthesis in recent years as an effective method for decreasing Pt use, and consequently, the cost. Therefore, this review article focuses on the synthesis processes of carbon-supported Pt-based electrocatalysts, which significantly affect the nanoparticle size, shape, and dispersion on supports and thus the activity and durability of the prepared electrocatalysts. The reviewed processes include (i) the functionalization of a commercial carbon support for enhanced catalyst–support interaction and additional catalytic effects, (ii) the methods for loading Pt-based electrocatalysts onto a carbon support that impact the manufacturing costs of electrocatalysts, (iii) the preparation of spherical and nonspherical Pt-based electrocatalysts (polyhedrons, nanocages, nanoframes, one- and two-dimensional nanostructures), and (iv) the postsynthesis treatments of supported electrocatalysts. The influences of the supports, key experimental parameters, and postsynthesis treatments on Pt-based electrocatalysts are scrutinized in detail. Future research directions are outlined, including (i) the full exploitation of the potential functionalization of commercial carbon supports, (ii) scaled-up one-pot synthesis of carbon-supported Pt-based electrocatalysts, and (iii) simplification of postsynthesis treatments. One-pot synthesis in aqueous instead of organic reaction systems and the minimal use of organic ligands are preferred to simplify the synthesis and postsynthesis treatment processes and to promote the mass production of commercial carbon-supported Pt-based electrocatalysts.

Graphical Abstract

This review focuses on the synthesis process of Pt-based electrocatalysts/C to develop aqueous one-pot synthesis at large-scale production for PEMFC stack application.

Designing Sites in Heterogeneous Catalysis: Are We Reaching Selectivities Competitive With Those of Homogeneous Catalysts?

A critical review of different prominent nanotechnologies adapted to catalysis is provided, with focus on how they contribute to the improvement of selectivity in heterogeneous catalysis. Ways to modify catalytic sites range from the use of the reversible or irreversible adsorption of molecular modifiers to the immobilization or tethering of homogeneous catalysts and the development of well-defined catalytic sites on solid surfaces. The latter covers methods for the dispersion of single-atom sites within solid supports as well as the use of complex nanostructures, and it includes the post-modification of materials via processes such as silylation and atomic layer deposition. All these methodologies exhibit both advantages and limitations, but all offer new avenues for the design of catalysts for specific applications. Because of the high cost of most nanotechnologies and the fact that the resulting materials may exhibit limited thermal or chemical stability, they may be best aimed at improving the selective synthesis of high value-added chemicals, to be incorporated in organic synthesis schemes, but other applications are being explored as well to address problems in energy production, for instance, and to design greener chemical processes. The details of each of these approaches are discussed, and representative examples are provided. We conclude with some general remarks on the future of this field.

Core-shell and heterostructured silver-nickel nanocatalysts fabricated by γ-radiation induced synthesis for oxygen reduction in alkaline media

To reach commercial viability for fuel cells, one needs to develop active and robust Pt-free electrocatalysts. Silver has great potential to replace Pt as the catalyst for the oxygen reduction reaction (ORR) in alkaline media due to its low cost and superior stability. However, its catalytic activity needs to be improved. One possible solution is to fabricate bimetallic nanostructures, which demonstrate a bifunctional enhancement in the electrochemical performance. Here, two types of bimetallic silver-nickel nanocatalysts, core-shells (Ag@NiO) and heterostructures (Ag/Ni), are fabricated using γ-radiation induced synthesis. The Ag@NiO nanoparticles consist of an amorphous, NiO layer as a shell and a facetted crystalline Ag particle as a core. Meanwhile, the Ag/Ni heterostructures comprise Ag particles decorated with Ni/Ni(oxy-hydro)-oxide clusters. Both materials demonstrate similar and increased alkaline ORR activity as compared to monometallic catalysts. It was revealed that the enhanced catalytic activity of the core-shells is mainly attributed to the electronic ligand effect. While in the Ag/Ni heterostructures, a lattice mismatch between the Ni-based clusters and Ag implies a significant lattice strain, which, in turn, is responsible for the increased activity of the catalyst. Also, the Ag/Ni samples exhibit good stability under operating conditions due to the existence of stable Ni³⁺ compounds on the surface.

Ultrathin Twisty PdNi Alloy Nanowires as Highly Active ORR Electrocatalysts Exhibiting Morphology-Induced Durability over 200 K Cycles

Even though the anion exchange membrane fuel cells have many advantages, the stability of their electrocatalysts for oxygen reduction reaction (ORR) has remained remarkably poor. We report here on the ultrathin twisty PdNi-alloy nanowires (NWs) exhibiting a very low reaction overpotential with an E_1/2 ∼ 0.95 V versus RHE in alkaline media maintained over 200 K cycles, the highest ever recorded for an electrocatalyst. The mass activity of the used NWs is >10 times higher than fresh commercial Pt/C. Therein, Ni improves the Pd d-band center for a more efficient ORR, and its leaching continuously regenerates the surface active sites. The twisty nanowire morphology imparts multiple anchor points on the electrode surface to arrest their detachment or coalescence and extra stability from self-entanglement. The significance of the NW morphology was further confirmed from the high-temperature durability studies. The study demonstrates that tailoring the number of contact points to the electrode-surface may help realize commercial-grade stability in the highly active electrocatalysts.

Organocatalyzed Heterocyclic Transformations In Green Media: A Review

Background:

Since the discovery of metal-free catalysts or organocatalysts about twenty years ago, a number of small molecules with different structures have been used to accelerate organic transformations. With the development of environmental awareness, to obtain highly efficient scaffolds, scientists have directed their studies towards synthetic methodologies that minimize or preferably eliminate the formation of waste, avoid toxic solvents and reagents and use renewable starting materials as far as possible.

Methods:

In this connection, the organocatalytic reactions providing efficiency and selectivity for most of the transformations have become an endless topic in organic chemistry since several advantages from both practical and environmental standpoints. Organocatalysts contributing to the transformation of reactants into products with the least possible waste production, have been serving the concept of green chemistry.

Results and Conclusion:

Organocatalysts have been classified based on their binding capacity to the substrate with covalent or noncovalent interactions involving hydrogen bonding and electrostatic interaction. Diverse types of small organic compounds including proline and its derivatives, phase-transfer catalysts, (thio)urease, phosphoric acids, sulfones, N-oxides, guanidines, cinchona derivatives, aminoindanol, and amino acids have been utilized as hydrogen bonding organocatalysts in different chemical transformations.

Dual Single-Atomic Ni-N4 and Fe-N4 Sites Constructing Janus Hollow Graphene for Selective Oxygen Electrocatalysis

Nitrogen-coordinated metal single atoms in carbon have aroused extensive interest recently and have been growing as an active research frontier in a wide range of key renewable energy reactions and devices. Herein, a step-by-step self-assembly strategy is developed to allocate nickel (Ni) and iron (Fe) single atoms respectively on the inner and outer walls of graphene hollow nanospheres (GHSs), realizing separate-sided different single-atom functionalization of hollow graphene. The Ni or Fe single atom is demonstrated to be coordinated with four N atoms via the formation of a Ni-N₄ or Fe-N₄ planar configuration. The developed Ni-N₄ /GHSs/Fe-N₄ Janus material exhibits excellent bifunctional electrocatalytic performance, in which the outer Fe-N₄ clusters dominantly contribute to high activity toward the oxygen reduction reaction (ORR), while the inner Ni-N₄ clusters are responsible for excellent activity toward the oxygen evolution reaction (OER). Density functional theory calculations demonstrate the structures and reactivities of Fe-N₄ and Ni-N₄ for the ORR and OER. The Ni-N₄ /GHSs/Fe-N₄ endows a rechargeable Zn-air battery with excellent energy efficiency and cycling stability as an air-cathode, outperforming that of the benchmark Pt/C+RuO₂ air-cathode. The current work paves a new avenue for precise control of single-atom sites on carbon surface for the high-performance and selective electrocatalysts.

Segmentation and Re-encapsulation of Porous PtCu Nanoparticles by Generated Carbon Shell for Enhanced Ethylene Glycol Oxidation and Oxygen-Reduction Reaction

Hierarchical porous carbon-encapsulated ultrasmall PtCu (UsPtCu@C) nanoparticles (NPs) were constructed based on segmentation and re-encapsulation of porous PtCu NPs by using glucose as a green biomass carbon source. The synergistic electronic effect from the bimetallic elements can enhance the catalytic activity by adjusting the surface electronic structure of Pt. Most importantly, the generated porous carbon shell provided a large contact surface area, excellent electrical conductivity, and structural stability, and the ultrasmall PtCu NPs exhibited an increased electrochemical performance compared with their PtCu matrix because of the exposure of more catalytically active centers. This synergistic relationship between the components resulted in enhanced catalytic activity and better stability of the obtained UsPtCu@C for ethylene glycol oxidation reaction and the oxygen-reduction reaction in alkaline electrolyte, which was higher than the PtCu NPs and commercial Pt/C (20 wt % Pt on Vulcan XC-72). The electrochemically active surface areas of the UsPtCu@C, PtCu NPs, and commercial Pt/C were calculated to be approximately 230.2, 32.8, and 64.0 m²/g_Pt, respectively; the mass activity of the UsPtCu@C for the ethylene glycol oxidation reaction was 8.5 A/mg_Pt, which was 14.2 and 8.5 times that of PtCu NPs and commercial Pt/C, respectively. The specific activity of UsPtCu@C was 3.7 mA/cm_pt², which was 2.1 and 2.3 times that of PtCu NPs and commercial Pt/C, respectively. The onset potential (E_on-set) of UsPtCu@C for the oxygen-reduction reaction was 0.96 V (vs reversible hydrogen electrode, RHE), which was 110 and 60 mV higher than PtCu and commercial Pt/C, respectively. The half-wave potentials (E_1/2) of UsPtCu@C, PtCu, and Pt/C were 0.88, 0.56, and 0.82 V (vs RHE), respectively, which indicated that the UsPtCu@C catalyst had an excellent bifunctional electrocatalytic activity.

Paraffin Removal in the Synthesis of Novel Janus Carbon Nano-Onions

Recent research has demonstrated that Janus nanoparticles provide a novel strategy to prepare catalysts and biomaterials. This is because of the versatility of being able to modify both sides of a particle with different properties. Carbon nano-onions are an excellent material as a support for different applications such as metal nanoparticles due to their unique structure and good conductivity. Because of their physical and chemical properties, carbon nano-onions are an ideal material to create Janus nanoparticles for further amphiphillic modifications. This article aims to show a preparatory process to ensure the removal of paraffin efficiently. The main method to be able to create these particles is employing the Pickering emulsion process. Paraffin wax is used as the hydrophobic part of this mixture and serves to block one side of the CNOs to facilitate their modification only on one side. Therefore, its removal is essential to obtain this catalytic nanoparticle.

Emerging Materials in Heterogeneous Electrocatalysis Involving Oxygen for Energy Harvesting

Water-based renewable energy cycle involved in water splitting, fuel cells, and metal-air batteries has been gaining increasing attention for sustainable generation and storage of energy. The major challenges in these technologies arise due to the poor kinetics of the oxygen reduction reaction (ORR) and the oxygen evolution reactions (OER), besides the high cost of the catalysts. Attempts to address these issues have led to the development of many novel and inexpensive catalysts as well as newer mechanistic insights, particularly so in the last three-four years when more catalysts have been investigated than ever before. With the growing emphasis on bifunctionality, that is, materials that can facilitate both reduction and evolution of oxygen, this review is intended to discuss all major families of ORR, OER, and bifunctional catalysts such as metals, alloys, oxides, other chalcogenides, pnictides, and metal-free materials developed during this period in a single platform, while also directing the readers to specific and detailed review articles dealing with each family. In addition, each section highlights the latest theoretical and experimental insights that may further improve ORR/OER performances. The bifunctional catalysts being sufficiently new, no consensus appears to have emerged about the efficiencies. Therefore, a statistical analysis of their performances by considering nearly all literature reports that have appeared in this period is presented. The current challenges in rational design of these catalysts as well as probable strategies to improve their performances are presented.

Ordered Pt3Co Intermetallic Nanoparticles Derived from Metal-Organic Frameworks for Oxygen Reduction

Highly ordered Pt alloy structures are proven effective to improve their catalytic activity and stability for the oxygen reduction reaction (ORR) for proton exchange membrane fuel cells. Here, we report a new approach to preparing ordered Pt₃Co intermetallic nanoparticles through a facile thermal treatment of Pt nanoparticles supported on Co-doped metal-organic-framework (MOF)-derived carbon. In particular, the atomically dispersed Co sites, which are originally embedded into MOF-derived carbon, diffuse into Pt nanocrystals and form ordered Pt₃Co structures. It is very crucial for the formation of the ordered Pt₃Co to carefully control the doping content of Co into the MOFs and the heating temperatures for Co diffusion. The optimal Pt₃Co nanoparticle catalyst has achieved significantly enhanced activity and stability, exhibiting a half-wave potential up to 0.92 V vs reversible hydrogen electrode (RHE) and only losing 12 mV after 30 000 potential cycling between 0.6 and 1.0 V. The highly ordered intermetallic structure was retained after the accelerated stress tests made evident by atomic-scale elemental mapping. Fuel cell tests further verified the high intrinsic activity of the ordered Pt₃Co catalysts. Unlike the direct use of MOF-derived carbon supports for depositing Pt, we utilized MOF-derived carbon containing atomically dispersed Co sites as Co sources to prepare ordered Pt₃Co intermetallic catalysts. The new synthesis approach provides an effective strategy to develop active and stable Pt alloy catalysts by leveraging the unique properties of MOFs such as 3D structures, high surface areas, and controlled nitrogen and transition metal dopings.

Template-Free Preparation of 3D Porous Co-Doped VN Nanosheet-Assembled Microflowers with Enhanced Oxygen Reduction Activity

Developing cheap and stable electrocatalysts is considered the key factor to achieve the large-scale application of fuel cells. In this paper, three-dimensional (3D) porous Co-doped vanadium nitride (VN) nanosheet-assembled microflowers are prepared with a facile solvothermal approach followed by nitridation at 500 °C in NH₃. It is found that the microflower morphology and the Co doping both significantly enhance the oxygen reduction reaction (ORR) performance of the materials. Because the unique 3D porous structure provides higher specific surface area and more active sites as well as enriching the d electrons of V via doping, Co also improves the intrinsic activity of VN. Our optimal V_0.95Co_0.05N microflowers achieve a half-wave potential for the ORR of up to 0.80 V in 0.1 M KOH solution, which is almost comparable to that of commercial 20% Pt/C. More importantly, the catalysts show superior durability with little current decline (less than 12%) during chronoamperometric evaluation for over 25 000 s. These features make the V_0.95Co_0.05N microflowers attractive for fuel cell applications.

In Situ Fabrication of Defective CoNx Single Clusters on Reduced Graphene Oxide Sheets with Excellent Electrocatalytic Activity for Oxygen Reduction

A facile one-step strategy for anchoring defective CoN_x single clusters on partly reduced graphene oxide (RGO) is constructed to significantly improve the catalytic performance of non-noble metal complexes toward oxygen reduction reaction (ORR). Sequent loading with trace amounts of metal-free porphyrin and Co²⁺ in RGO can dramatically enhance both the half-wave potential and the peak current density. Intriguingly, the RGO/P/2Co single cluster exhibits the best ORR catalytic performance with the half-wave potential of 0.834 V, extremely approaching that of commercial Pt/C (0.836 V). This half-wave potential surpasses most of the reported half-wave potentials of RGO supported non-noble metal ORR catalysts through low-temperature synthesis. Furthermore, the as-prepared RGO/P/2Co delivers a peak current density of 1.3 times higher than that of Pt/C at the same loading, together with a high mass activity of 2.76 A mg_Co^-1. During the durability test, a cathodic current loss less than 10% is recorded after 8000 continuous potential cycles. Insights into this successful example will be conducive to the development of elegant routes for constructing metal nitrogen (MN)-based ORR catalysts with high efficiency, outstanding stability, and excellent selectivity.