Chemical Kinetic Method for Active-Site Quantification in Fe-N-C Catalysts and Correlation with Molecular Probe and Spectroscopic Site-Counting Methods.

Mononuclear Fe ions ligated by nitrogen (FeNx) dispersed on nitrogen-doped carbon (Fe-N-C) serve as active centers for electrocatalytic O2 reduction and thermocatalytic aerobic oxidations. Despite their promise as replacements for precious metals in a variety of practical applications, such as fuel cells, the discovery of new Fe-N-C catalysts has relied primarily on empirical approaches. In this context, the development of quantitative structure-reactivity relationships and benchmarking of catalysts prepared by different synthetic routes and by different laboratories would be facilitated by the broader adoption of methods to quantify atomically dispersed FeNx active centers. In this study, we develop a kinetic probe reaction method that uses the aerobic oxidation of a model hydroquinone substrate to quantify the density of FeNx centers in Fe-N-C catalysts. The kinetic method is compared with low-temperature Mössbauer spectroscopy, CO pulse chemisorption, and electrochemical reductive stripping of NO derived from NO2- on a suite of Fe-N-C catalysts prepared by diverse routes and featuring either the exclusive presence of Fe as FeNx sites or the coexistence of aggregated Fe species in addition to FeNx. The FeNx site densities derived from the kinetic method correlate well with those obtained from CO pulse chemisorption and Mössbauer spectroscopy. The broad survey of Fe-N-C materials also reveals the presence of outliers and challenges associated with each site quantification approach. The kinetic method developed here does not require pretreatments that may alter active-site distributions or specialized equipment beyond reaction vessels and standard analytical instrumentation.

Regulating Catalytic Properties and Thermal Stability of Pt and PtCo Intermetallic Fuel-Cell Catalysts via Strong Coupling Effects between Single-Metal Site-Rich Carbon and Pt.

Developing low platinum-group-metal (PGM) catalysts for the oxygen reduction reaction (ORR) in proton-exchange membrane fuel cells (PEMFCs) for heavy-duty vehicles (HDVs) remains a great challenge due to the highly demanded power density and long-term durability. This work explores the possible synergistic effect between single Mn site-rich carbon (MnSA-NC) and Pt nanoparticles, aiming to improve intrinsic activity and stability of PGM catalysts. Density functional theory (DFT) calculations predicted a strong coupling effect between Pt and MnN4 sites in the carbon support, strengthening their interactions to immobilize Pt nanoparticles during the ORR. The adjacent MnN4 sites weaken oxygen adsorption at Pt to enhance intrinsic activity. Well-dispersed Pt (2.1 nm) and ordered L12-Pt3Co nanoparticles (3.3 nm) were retained on the MnSA-NC support after indispensable high-temperature annealing up to 800 °C, suggesting enhanced thermal stability. Both PGM catalysts were thoroughly studied in membrane electrode assemblies (MEAs), showing compelling performance and durability. The Pt@MnSA-NC catalyst achieved a mass activity (MA) of 0.63 A mgPt-1 at 0.9 ViR-free and maintained 78% of its initial performance after a 30,000-cycle accelerated stress test (AST). The L12-Pt3Co@MnSA-NC catalyst accomplished a much higher MA of 0.91 A mgPt-1 and a current density of 1.63 A cm-2 at 0.7 V under traditional light-duty vehicle (LDV) H2-air conditions (150 kPaabs and 0.10 mgPt cm-2). Furthermore, the same catalyst in an HDV MEA (250 kPaabs and 0.20 mgPt cm-2) delivered 1.75 A cm-2 at 0.7 V, only losing 18% performance after 90,000 cycles of the AST, demonstrating great potential to meet the DOE targets.

Engineering Atomic Single Metal-FeN4Cl Sites with Enhanced Oxygen-Reduction Activity for High-Performance Proton Exchange Membrane Fuel Cells.

Fe-N-C single-atomic metal site catalysts (SACs) have garnered tremendous interest in the oxygen reduction reaction (ORR) to substitute Pt-based catalysts in proton exchange membrane fuel cells. Nowadays, efforts have been devoted to modulating the electronic structure of metal single-atomic sites for enhancing the catalytic activities of Fe-N-C SACs, like doping heteroatoms to modulate the electronic structure of the Fe-Nx active center. However, most strategies use uncontrolled long-range interactions with heteroatoms on the Fe-Nx substrate, and thus the effect may not precisely control near-range coordinated interactions. Herein, the chlorine (Cl) is used to adjust the Fe-Nx active center via a near-range coordinated interaction. The synthesized FeN4Cl SAC likely contains the FeN4Cl active sites in the carbon matrix. The additional Fe-Cl coordination improves the instrinsic ORR activity compared with normal FeNx SAC, evidenced by density functional theory calculations, the measured ORR half-wave potential (E1/2, 0.818 V), and excellent membrane electrode assembly performance.

Tuning Two-Electron Oxygen-Reduction Pathways for H2 O2 Electrosynthesis via Engineering Atomically Dispersed Single Metal Site Catalysts.

The hydrogen peroxide (H2 O2 ) generation via the electrochemical oxygen reduction reaction (ORR) under ambient conditions is emerging as an alternative and green strategy to the traditional energy-intensive anthraquinone process and unsafe direct synthesis using H2 and O2 . It enables on-site and decentralized H2 O2 production using air and renewable electricity for various applications. Currently, atomically dispersed single metal site catalysts have emerged as the most promising platinum group metal (PGM)-free electrocatalysts for the ORR. Further tuning their central metal sites, coordination environments, and local structures can be highly active and selective for H2 O2 production via the 2e- ORR. Herein, recent methodologies and achievements on developing single metal site catalysts for selective O2 to H2 O2 reduction are summarized. Combined with theoretical computation and advanced characterization, a structure-property correlation to guide rational catalyst design with a favorable 2e- ORR process is aimed to provide. Due to the oxidative nature of H2 O2 and the derived free radicals, catalyst stability and effective solutions to improve catalyst tolerance to H2 O2 are emphasized. Transferring intrinsic catalyst properties to electrode performance for viable applications always remains a grand challenge. The key performance metrics and knowledge during the electrolyzer development are, therefore, highlighted.

Atomic Structure Evolution of Pt-Co Binary Catalysts: Single Metal Sites versus Intermetallic Nanocrystals.

Due to their exceptional catalytic properties for the oxygen reduction reaction (ORR) and other crucial electrochemical reactions, PtCo intermetallic nanoparticle (NP) and single atomic (SA) Pt metal site catalysts have received considerable attention. However, their formation mechanisms at the atomic level during high-temperature annealing processes remain elusive. Here, the thermally driven structure evolution of Pt-Co binary catalyst systems is investigated using advanced in situ electron microscopy, including PtCo intermetallic alloys and single Pt/Co metal sites. The pre-doping of CoN4 sites in carbon supports and the initial Pt NP sizes play essential roles in forming either Pt3 Co intermetallics or single Pt/Co metal sites. Importantly, the initial Pt NP loadings against the carbon support are critical to whether alloying to L12 -ordered Pt3 Co NPs or atomizing to SA Pt sites at high temperatures. High Pt NP loadings (e.g., 20%) tend to lead to the formation of highly ordered Pt3 Co intermetallic NPs with excellent activity and enhanced stability toward the ORR. In contrast, at a relatively low Pt loading (<6 wt%), the formation of single Pt sites in the form of PtC3 N is thermodynamically favorable, in which a synergy between the PtC3 N and the CoN4 sites could enhance the catalytic activity for the ORR, but showing insufficient stability.

Advanced Nanocarbons for Enhanced Performance and Durability of Platinum Catalysts in Proton Exchange Membrane Fuel Cells.

Insufficient stability of current carbon supported Pt and Pt alloy catalysts is a significant barrier for proton-exchange membrane fuel cells (PEMFCs). As a primary degradation cause to trigger Pt nanoparticle migration, dissolution, and aggregation, carbon corrosion remains a significant challenge. Compared with enhancing Pt and PtM alloy particle stability, improving support stability is rather challenging due to carbon's thermodynamic instability under fuel cell operation. In recent years, significant efforts have been made to develop highly durable carbon-based supports concerning innovative nanostructure design and synthesis along with mechanistic understanding. This review critically discusses recent progress in developing carbon-based materials for Pt catalysts and provides synthesis-structure-performance correlations to elucidate underlying stability enhancement mechanisms. The mechanisms and impacts of carbon support degradation on Pt catalyst performance are first discussed. The general strategies are summarized to tailor the carbon structures and strengthen the metal-support interactions, followed by discussions on how these designs lead to enhanced support stability. Based on current experimental and theoretical studies, the critical features of carbon supports are analyzed concerning their impacts on the performance and durability of Pt catalysts in fuel cells. Finally, the perspectives are shared on future directions to develop advanced carbon materials with favorable morphologies and nanostructures to increase Pt utilization, strengthen metal-support interactions, facilitate mass/charge transfer, and enhance corrosion resistance.

Advanced Electrocatalysts with Single-Metal-Atom Active Sites.

Electrocatalysts with single metal atoms as active sites have received increasing attention owing to their high atomic utilization efficiency and exotic catalytic activity and selectivity. This review aims to provide a comprehensive summary on the recent development of such single-atom electrocatalysts (SAECs) for various energy-conversion reactions. The discussion starts with an introduction of the different types of SAECs, followed by an overview of the synthetic methodologies to control the atomic dispersion of metal sites and atomically resolved characterization using state-of-the-art microscopic and spectroscopic techniques. In recognition of the extensive applications of SAECs, the electrocatalytic studies are dissected in terms of various important electrochemical reactions, including hydrogen evolution reaction (HER), oxygen evolution reaction (OER), oxygen reduction reaction (ORR), carbon dioxide reduction reaction (CO2RR), and nitrogen reduction reaction (NRR). Examples of SAECs are deliberated in each case in terms of their catalytic performance, structure-property relationships, and catalytic enhancement mechanisms. A perspective is provided at the end of each section about remaining challenges and opportunities for the development of SAECs for the targeted reaction.

Shock Exfoliation of Graphene Fluoride in Microwave.

An unprecedented microwave-based strategy is developed to facilitate solid-phase, instantaneous delamination and decomposition of graphite fluoride (GF) into few-layer, partially fluorinated graphene. The shock reaction occurs (and completes in few seconds) under microwave irradiation upon exposing GF to either "microwave-induced plasma" generated in vacuum or "catalyst effect" caused by intense sparking of graphite at ambient conditions. A detailed analysis of the structural and compositional transformations in these processes indicates that the GF experiences considerable exfoliation and defluorination, during which sp2 -bonded carbon is partially recovered despite significant structural defects being introduced. The exfoliated fluorinated graphene shows excellent electrochemical performance as anode materials in potassium ion batteries and as catalysts for the conversion of O2 to H2 O2 . This simple and scalable method requires minimal energy input and does not involve the use of other chemicals, which is attractive for extensive research in fluorine-containing graphene and its derivatives in laboratories and industrial applications.

Mathematical simulation of temperature distribution in tumor tissue and surrounding healthy tissue treated by laser combined with indocyanine green.

BACKGROUND: Photothermal therapy is a local treatment method for cancer and the heat energy generated from it could destroy the tumor cells. This study is aimed to investigate the temperature distribution in tumor tissue and surrounding health tissue of tumor bearing mice applying mathematical simulation model. Tumor bearing mice treated by laser combined with or without indocyanine green. Monte Carlo method and the Pennes bio-heat equation were used to calculate the light distribution and heat energy. COMSOL Multiphysic was adopted to construct three dimensional temperature distribution model. RESULTS: This study revealed that the data calculated by simulation model is in good agreement with the surface temperature monitored by infrared thermometer. Effected by the optical parameters and boundary conditions of tissue, the highest temperature of tissue treated by laser combined with indocyanine green was about 65 °C which located in tumor tissue and the highest temperature of tissue treated by laser was about 43 °C which located under the tumor tissue. The temperature difference was about 20 °C. Temperature distribution in tissue was not uniform. The temperature difference in different parts of tumor tissue raised up to 15 °C. The temperature of tumor tissue treated by laser combined with indocyanine green was about 20 °C higher than that of the surrounding healthy tissue. CONCLUSIONS: Reasonably good matching between the calculated temperature and the measured temperature was achieved, thus demonstrated great utility of our modeling method and approaches for deepening understand in the temperature distribution in tumor tissue and surrounding healthy tissue during the laser combined with photosensitizer. The simulation model could provide guidance and reference function for the effect of photothermal therapy.

In Situ and Operando Characterization of Proton Exchange Membrane Fuel Cells.

For proton exchange membrane fuel cells (PEMFCs) to become a mainstream energy source, significant improvements in their performance, durability, and efficiency are necessary. To improve their durability, there must be a solid understanding of how the structural and electrochemical processes are affected during operation to propose mitigation strategies. To this aim, in situ and operando characterization techniques can locally identify structural and electrochemical processes, which cannot be captured using conventional techniques. Linking these properties in the same geometric area has been challenging due to its inherent limitations, such as sample size and imaging resolution. This has created a knowledge gap in structure-to-electrochemical performance relationships as operation and degradation unevenly affect different areas of the cell. In the recent past, catalyst layer degradation, hot spots, and water management have been structurally and electrochemically visualized in the same geometric area, revealing new interactions. To further the research in this direction, these interconnected fields are reviewed, followed by a roadmap for in situ characterization of PEMFCs, treating structural and electrochemical processes as a unified subject. With this approach, the knowledge of the degradation of PEMFCs will be significantly improved.

Investigation of water transport in fuel cells using water transport plates and solid plates.

Water management of proton exchange membrane fuel cells (PEMFCs) is of vital importance to achieve better performance and durability. In this study, porous hydrophilic water transport plates (WTPs) with different pore structures were prepared and employed to improve water management in PEMFCs. Polarization curves, electrochemical impedance spectroscopy (EIS) and water balance were tested to investigate the effect of pore structure on cell performance and water transport process. The results show that pore structure has little effect on drainage function due to excess liquid water flux of WTPs, while the membrane hydration is improved with increased surface evaporation rate of WTPs, resulting in better cell performance. The favorable cell performance shows that WTP is a promising technique to improve water management in PEMFCs.

3D Pd/Co core-shell nanoneedle arrays as a high-performance cathode catalyst layer for AAEMFCs.

A novel cathode architecture using vertically aligned Co nanoneedle arrays as an ordered support for application in alkaline anion-exchange membrane fuel cells (AAEMFCs) has been developed. The Co nanoneedle arrays were directly grown on a stainless steel sheet via a hydrothermal reaction and then a Pd layer was deposited on the surface of the Co nanoneedle arrays using a vacuum sputter-deposition method to form Pd/Co nanoneedle arrays. After transferring the Pd/Co nanoneedle arrays to an AAEM, a cathode catalyst layer was formed. Without the use of an alkaline ionomer, the AAEMFC with the prepared cathode catalyst layer showed an enhanced performance with ultra-low Pd loading of down to 33.5 µg cm-2, which is much higher than the conventionally used cathode electrode with a Pt loading of 100 µg cm-2. This is the first report where three-dimensional Co nanoneedle arrays have been used as the cathode support in an AAEMFC, which is able to deliver a higher power density without an alkaline ionomer than that of conventional membrane electrode assembly (MEA).

Highly stable nanostructured membrane electrode assembly based on Pt/Nb2O5 nanobelts with reduced platinum loading for proton exchange membrane fuel cells.

Proton exchange membrane fuel cells are promising candidates for the next-generation power sources; however, poor durability and high cost impede their widespread application. To address this dilemma, a nanostructured membrane electrode assembly (MEA) based on Pt/Nb2O5 nanobelts (NBs) was constructed through hydrothermal synthesis and the physical vapour deposition method. Pt/Nb2O5 NBs were directly aligned with Nafion membrane without ionomer as a binder. The prepared catalyst layer is ultrathin and has ultralow Pt loading. A single cell performance of 5.80 kW gPt-1 (cathode) and 12.03 kW gPt-1 (anode) was achieved by the Pt/Nb2O5 NBs-based MEA (66.0 µgPt cm-2). The accelerated durability test indicates that the Pt/Nb2O5 NBs-based MEA is far more stable than conventional Pt/C-based MEA.