Catalyst For Reaction Research Articles

Origin of Stability and Activity Enhancements in Pt-Based Oxygen Reduction Reaction Catalysts Via Defect-Mediated Dopant Adsorption

Platinum alloys are highly efficient electrocatalysts for the oxygen reduction reaction (ORR) in acidic conditions. However, these alloys are susceptible to metal loss through leaching and degradation, leading to reduced catalyst stability and activity. Recently, it has been shown that doping with oxophilic elements can significantly alleviate these problems, with a prominent example being Mo-doped Pt alloys. Here, to achieve atomic scale understanding of the exceptional activity and stability of these alloys, we present a detailed density functional theory description of the dopants’ structures and impact on electrocatalyst properties. Beginning with the Mo/Pt system, we demonstrate that Mo can be stabilized in the form of low-dimensional oxyhydroxide moieties on Pt defects. The resulting structures enhance stability and activity via distinct physical processes, with the Mo moieties both directly inhibiting Pt dissolution at defects and indirectly enhancing ORR activity by generation of strain fields on surrounding Pt terraces. We then generalize these analyses to other metal dopant elements, and we demonstrate that similar low-dimensional oxyhydroxide structures control the electrocatalytic properties through an intricate interplay of the structures’ acid stability, intrinsic activity for the ORR, and ability to induce ORR-promoting strain fields on Pt.Reference:Sawant, K., Zeng, Z. & Greeley, J., Angew. Chem., Int. Ed., 10.1002/anie.202312747

Benefits and Limitations of Encapsulating Platinum Electrocatalysts with Niobium Oxide

The development of energy infrastructure based on renewable sources is vital to reduce our reliance on fossil fuels. Technologies such as hydrogen fuel cells and water electrolyzers are able to bridge the gap between the intermittency issues associated with green energy sources such as wind and solar, and the demand for reliable electricity by being able to convert between electrical and chemical energy. Nanostructured and mesoporous platinum (Pt) prepared by electrodeposition or sputter coating have been shown to have increased performance in hydrogen fuel cells compared to the industry standard of Pt nanoparticles supported on carbon.1,2 Over the lifetime of a fuel cell Pt nanostructures and nanoparticles are, however, known to restructure and lose their surface area by mechanisms such as Ostwald ripening, dissolution, and coalescence. Previously, thin film Pt catalysts have been shown to be stabilized by a nanoscale film of niobium oxide.3 Herein, nanoscale thin films of niobium oxide are investigated as a stabilizing material through use by encapsulating nanostructured Pt catalysts.Nanostructured Pt catalysts were electrodeposited onto Pt-coated Si wafer substrates. Subsequently, niobium oxide films with thicknesses of 0.5 nm, 3 nm, and 4.5 nm were deposited onto the Pt catalyst using atomic layer deposition. X-ray fluorescence and conductive atomic force microscopy techniques were used to confirm the presence and coverage of the niobium oxide layer on the Pt catalyst. The performance of the encapsulated and un-encapsulated Pt catalysts as oxygen reduction reaction catalysts over the course of up to a thousand degradation cycles were evaluated using electrochemical techniques. The morphology of the catalyst before and after the degradation testing was also assessed using scanning electron microscopy techniques. Electrochemical impedance spectroscopy indicated that increasing the thickness of the niobium oxide layer increased the resistance of the catalyst towards the oxygen reduction reaction. The combined results of the analyses performed by electrochemical techniques and electron microscopy measurements revealed a range of film thicknesses for the niobium oxide that successfully stabilized the electrochemically active surface area and surface morphology of the Pt catalyst. Paul, M. T. Y.; Gates, B. D. Mesoporous Platinum Prepared by Electrodeposition for Ultralow Loading Proton Exchange Membrane Fuel Cells. Sci Rep 2019, 9, 4161.Debe, M. K. Tutorial on the Fundamental Characteristics and Practical Properties of Nanostructured Thin Film (NSTF) Catalysts. Electrochem. Soc. 2013, 160, F522.Eastcott, J.; Gates, B. D. Nanoscale Thin Films of Niobium Oxide on Platinum Surfaces: Creating a Platform for Optimizing Material Composition and Electrochemical Stability. Can. J. Chem. 2017, 96, 260–266.

Controlled Catalyst Layer Architectures for Fuel Cell Applications

The catalyst layer composition and architecture play a pivotal role in bridging the gaps between ex situ and in situ catalyst activities in fuel cells.1 The catalyst-support architecture strongly influences the catalyst distribution, ionomer distribution and mass transport losses, occurring at high current density.2 Herein, a novel architecture based on self-standing fiber-network has been fabricated and characterized.3 The resulting porous structure of such catalyst layer is expected to control mass transport limitation across both the interface of catalyst layer by increasing the number of Triple Phase Boundaries. Further platinum catalyst is deposited onto this fibrous support as well as the ionomer. The full electrode is characterized for its morphology and electrochemically verified in both ex situ as well as in situ configurations. References (1) Jiantao Fan, Ming Chen, Zhiliang Zhao, Zhen Zhang, Siyu Ye, Shaoyi Xu, Haijiang Wang & Hui Li, “Bridging the Gap between Highly Active Oxygen Reduction Reaction Catalysts and Effective Catalyst Layers for Proton Exchange Membrane Fuel Cells”, Nature Energy, 6, 475–486, (2021).(2) Nagappan Ramaswamy, Wenbin Gu, Joseph M. Ziegelbauer and Swami Kumaraguru, “Carbon Support Microstructure Impact on High Current Density Transport Resistances in PEMFC Cathode”, Journal of The Electrochemical Society ,167, 064515, 2020.(3) Giorgio Ercolano, Filippo Farina, Sara Cavaliere, Deborah J. Jones and Jacques Rozière, “Towards Ultrathin Pt Films on Nanofibers by Surface-Limited Electrodeposition for Electrocatalytic Applications”, Journal of Materials Chemistry A , 5, 3974–3980, 2017.

Understanding Activity and Stability of Ru1-XMxO2 Oxygen Evolution Reaction Catalyst Surfaces (M = Ti, Zr): Theory and Experiments

Proton-exchange membrane water electrolyzers, pivotal for sustainable hydrogen production, encounter challenges in the oxygen evolution reaction (OER) marked by high overpotential and limited electrocatalyst stability. This study delves into the computational exploration of two metal substitutions, titanium (Ti) and zirconium (Zr), within ruthenium oxide (RuO2), aiming to enhance catalytic stability. The investigation encompasses nanoscale titanium-substituted and zirconium-substituted rutile RuO2, (Ru1-xMxO2) and evaluating their atomic and electronic structures, OER activity, and dissolution behavior.In the case of Ti substitution, varying concentrations (x = 0–50 at %) were systematically investigated. Our experiments and theoretical analyses elucidate concentration-dependent effects on the electronic structure, showcasing shifts in the d-band and O2p band centers. Titanium substitution induces electron accumulation and depletion regions on the surface, influencing OER activity and stability. Electrochemical testing corroborates the theory, demonstrating that lower Ti concentrations (12.5 and 20 at %) enhance catalyst stability and mitigate ruthenium dissolution. Specifically, Ti substitution influences reaction pathways, with penta-coordinated Ru sites exhibiting the lowest barriers for OER, while hexa-coordinated Ru sites facilitate lower energetic barriers for dissolution.Simultaneously, the study extends to the substitution of Zr within rutile RuO2. Computational and experimental analyses reveal anisotropic shifts in the rutile unit cell, dependent on Zr concentration. Zr substitution induces alterations in the electronic structure, impacting the surface density of states and electron density distribution. Experimentally, lower OER activity is observed for Zr-substituted RuO2 compared to pure RuO2, consistent with computational averages across all sites. However, specific sites with Zr substitution exhibit higher theoretical OER activity. Dissolution analyses underscore the complexity of interactions, with Zr affecting activation barriers for Ru dissolution, thus influencing OER stability.Notably, the dissolution behavior emerges as a critical aspect of this investigation. In Ti-substituted RuO2, experimental accelerated durability testing showcases enhanced stability and reduced Ru dissolution at low and intermediate Ti concentrations. Computational analyses corroborate these findings, indicating site-specific effects on both OER activity and stability. Conversely, Zr-substituted RuO2 exhibits lower Ru dissolution compared to commercial RuO2, emphasizing the role of Zr in impeding Ru dissolution while contributing to surface restructuring.In conclusion, this comprehensive exploration of Ti and Zr substitutions within ruthenium oxide advances our understanding of their effects on atomic and electronic structures, OER activity, and dissolution dynamics. The intricate interplay between specific sites, metal concentrations, and substituents provides valuable insights for designing high-performance, cost-effective OER nanoscale electrocatalysts. The findings underscore the importance of considering both activity and stability in the pursuit of efficient and durable electrocatalysts for sustainable hydrogen production.

Quantification of O2 Crossover and Concurrent Faradaic Efficiency Loss of H2 Production Via PEM Water Electrolysis

PEM water electrolyzers (PEMWEs) are considered to be a key technology for the decarbonization of major sectors by being able to store fluctuating renewable energy as chemical energy in the form of hydrogen gas. While the scarcity of the necessary iridium catalyst on the anode limits widespread application minding the high loadings still needed today,1 there are additional efficiency losses that are still to be fully understood.By employing a proton exchange membrane, PEMWEs are able to produce hydrogen at elevated pressure on the cathode while keeping the balance of plant costs low as the anode can still be operated with liquid water at close to ambient pressure. Hydrogen crossover to the anode compartment through the membrane is significantly increased at elevated cathode pressures, which is majorly considered a safety issue. As the anode catalyst does not serve as a good hydrogen oxidation reaction catalyst, explosive mixtures of hydrogen and oxygen can form at the anode (and its exhaust) depending on the membrane type and thickness, current density, temperature, hydrogen partial pressure at the cathode, cathode catalyst layer structure etc.2-4 Aside of safety concerns – which can be bypassed by the use of recombination interlayers in the membrane5 – gas crossover also reduces the Faradaic efficiency of the system as parts of the hydrogen produced cannot be used further because it does not reach the cathode exhaust.A so far not majorly considered additional Faradaic efficiency loss stems from oxygen crossover from the anode to the cathode. As the platinum based cathode catalyst is very active for the oxygen reduction reaction, basically all of the oxygen crossing over to the cathode compartment recombines with the hydrogen present, forming liquid water.6 The permeability of oxygen through common PEM materials is significantly lower than that of hydrogen.7 But due to the osmotic drag from the anode to the cathode as well as possibly high bubble pressures of oxygen at the anode catalyst layer / membrane interface,8 oxygen crossover deserves closer attention.In this study we show quantification of the oxygen crossover in operating PEMWE single cells by a full balance of the gases detected at both the anode and cathode exhaust using an online mass spectrometer. A direct correlation of Faradaic efficiency of the oxygen and hydrogen produced matches the expected chemical recombination on the cathode, which serves as a direct measure of the oxygen crossover to the cathode compartment during PEMWE operation. Depending on the exact operating conditions the Faradaic efficiency loss caused by oxygen crossover can easily be in the percentage range, stressing the importance of having a closer look at the oxygen crossover characteristics of PEMWE cells in order to improve the Faradaic efficiency of the system. Taie, Z.; Peng, X.; Kulkarni, D.; Zenyuk, I. V.; Weber, A. Z.; Hagen, C.; Danilovic, N., Pathway to Complete Energy Sector Decarbonization with Available Iridium Resources using Ultralow Loaded Water Electrolyzers. ACS Appl Mater Interfaces 2020, 12, 52701-52712.Bernt, M.; Schröter, J.; Möckl, M.; Gasteiger, H. A., Analysis of Gas Permeation Phenomena in a PEM Water Electrolyzer Operated at High Pressure and High Current Density. J. Electrochem. Soc. 2020, 167.Ito, H.; Miyazaki, N.; Ishida, M.; Nakano, A., Cross-permeation and consumption of hydrogen during proton exchange membrane electrolysis. Int. J. Hydrogen Energy 2016, 41, 20439-20446.Trinke, P.; Keeley, G. P.; Carmo, M.; Bensmann, B.; Hanke-Rauschenbach, R., Elucidating the Effect of Mass Transport Resistances on Hydrogen Crossover and Cell Performance in PEM Water Electrolyzers by Varying the Cathode Ionomer Content. J. Electrochem. Soc. 2019, 166, F465-F471.Zhang, Z.; Han, Z.; Testino, A.; Gubler, L., Platinum and Cerium-Zirconium Oxide Co-Doped Membrane for Mitigated H2 Crossover and Ionomer Degradation in PEWE. J. Electrochem. Soc. 2022, 169.Schalenbach, M.; Carmo, M.; Fritz, D. L.; Mergel, J.; Stolten, D., Pressurized PEM water electrolysis: Efficiency and gas crossover. Int. J. Hydrogen Energy 2013, 38, 14921-14933.Schalenbach, M.; Hoefner, T.; Paciok, P.; Carmo, M.; Lueke, W.; Stolten, D., Gas Permeation through Nafion. Part 1: Measurements. The Journal of Physical Chemistry C 2015, 119, 25145-25155.Weber, C. C.; Wrubel, J. A.; Gubler, L.; Bender, G.; De Angelis, S.; Buchi, F. N., How the Porous Transport Layer Interface Affects Catalyst Utilization and Performance in Polymer Electrolyte Water Electrolysis. ACS Appl Mater Interfaces 2023, 15, 34750-34763.

Enhancing Anode Reversal Tolerance by Low-Carbon and Carbon-Free Anode Catalyst Layers for Proton Exchange Membrane Fuel Cells

Proton exchange membrane fuel cells (PEMFCs), as the main hydrogen utilization device, are considered a powerful tool for accelerating a carbon-neutral society. However, durability and cost are the two main obstacles to the commercialization of PEMFCs. Specifically, the anode reversal has been extensively studied as a severe issue that deteriorates the performance and durability of PEMFCs and leads to cell failure. Commercially, iridium oxide (IrOx) or iridium black is intensively utilized as oxygen evolution reaction catalysts to fabricate reversal tolerant anodes (RTAs) despite the slight performance loss. However, previous studies concentrated on investigating the origin or phenomena during the cell-reversal, the key factors and mechanisms affecting the anti-reversal performance, and the interaction among the anode catalyst layer components are still yet to be explored.Hence, we systematically studied the performance using three typical carbon supports with three metal contents and different metal deposition orders by experiments and model calculations. First, we synthesized Ir outside Ir/Pt/C and Pt outside Pt/It/C with uniform nanoparticle size and distribution and characterized them comprehensively. A new parameter, “relative metal coverage”, is put forward to evaluate and predict the anti-reversal ability, showing that higher relative metal coverage results in a longer reversal time and lower polarization performance degradation rate. In addition, Ir outside Ir/Pt/C shows better reversal tolerance than Pt outside Pt/It/C, which has been rarely mentioned in previous studies. Further, the experimental results show that even though the metal coverage is high, the performance degradation caused by carbon corrosion is still inevitable. On this basis, we synthesized a high specific surface area conductive oxide Ti4O7 as the anode catalyst support to eliminate the adverse effects of carbon corrosion. Previously, the initial performance of Ti4O7-supported catalyst was obstructed by the disadvantaged electrical conductivity of Ti4O7. We obtain a comparable polarization performance after optimizing the Ti4O7-supported anode catalyst layer parameters by shortening the electron transfer pathway and increasing metal coverage. Typically, the Ir@IrOx/Pt/Ti4O7-fabricated RTA with low Ir loading displays approximately ten times longer reversal time (6 hours) and two orders of magnitude lower degradation rate than a conventional carbon-supported counterpart. The degradation origin of the Ti4O7-supported anodes is also studied by postmortem characterizations, pointing to the Pt oxidation caused by the formation of TiOx thin layers on the Pt surface. These two works aim to promote the development of highly durable and reversal tolerance anode and provide a bright perspective for practical high-performance, low-degradation, and cost-friendly RTA fabrication.

Elucidating Fundamental Structure-Function Relationships for Iridium Oxide Catalysts in PEM Water Electrolyzers

Proton exchange membrane water electrolysis (PEMWE) will contribute substantially towards decarbonization efforts and is expected to make up 40% of the green hydrogen market.1 A primary barrier for PEMWE scale-up is the detrimentally high usage of iridium for the oxygen evolution reaction catalyst. To achieve large-scale PEMWE deployment, loading reductions from 2-3 mgIr cm-2 to 0.10 mgIr cm-2 must occur.2 Such a feat can only be achieved through significant improvements in activity and durability. However, there exists a lack of fundamental understanding in how catalyst properties correlate to performance, thus hindering optimization efforts. The phenomena of catalyst activity/durability are frequently treated with purely empirical approaches, and thus the foundational knowledge needed for improved catalyst design—such as understanding how crystallinity and oxidation states impact performance in a membrane electrode assembly (MEA)—has not been well investigated.3 This work serves to bridge this gap in knowledge by correlating extensive structural characterization data with MEA activity and durability for six commercial IrOx catalysts. Properties such as oxidation states, chemical composition, local atomic/electronic configuration, crystallinity, surface area, and particle size distribution were elucidated for each catalyst in powder and/or MEA form. Following MEA conditioning, catalyst activity was benchmarked via polarization curves and Tafel analysis. Lastly, durability was evaluated by performing a 500h accelerated stress test (adapted from the H2New consortium), with electrochemical characterization conducted every 100h. The presence of amorphous character and the Ir3+ oxidation state was the best determinant of activity. Durability, on the other hand, was associated with greater crystalline character and more redox-stable cyclic voltammogram profiles. Correlations regarding chemical composition, local atomic/electronic configurations, particle size, and surface area will be discussed as well. Smolinka, T.; Wiebe, N.; Sterchele, P.; Palzer, A.; Lehner, F.; Jansen, M.; Kiemel, S.; Miehe, R.; Wahren, S.; Zimmermann, F. Studie IndWEDe: Industrialisierung Der Wasserelektrolyse in Deutschland: Chancen Und Herausforderungen Für Nachhaltigen Wasserstoff Für Verkehr, Strom Und Wärme.; 2018.Clapp, M.; Zalitis, C. M.; Ryan, M. Perspectives on Current and Future Iridium Demand and Iridium Oxide Catalysts for PEM Water Electrolysis. Catal Today 2023, 420 (March), 114140. https://doi.org/10.1016/j.cattod.2023.114140.Lazaridis, T.; Stühmeier, B. M.; Gasteiger, H. A.; El-Sayed, H. A. Capabilities and Limitations of Rotating Disk Electrodes versus Membrane Electrode Assemblies in the Investigation of Electrocatalysts. Nat Catal 2022, 5 (5), 363–373. https://doi.org/10.1038/s41929-022-00776-5.

PGM-Free Oxygen Evolution Reaction Catalyst for PEM Water Electrolysis – a Mechanistic Study on Activity and Durability for Practical Applications

Low temperature proton exchange membrane water electrolyzer (PEMWE) represents a critical technology for green hydrogen production. It offers the advantages of significantly higher current density and higher H2 purity, rendering it a preferred technology when high energy efficiency and low footprint are essential. Working in the oxidative and acidic environment under high polarization voltage, however, adds substantial demand to the electrode catalyst and the support. This is particularly the case at anode where the oxygen evolution reaction (OER) takes place. At present, the platinum group metal (PGM) materials such as Ir black or Ir oxide are catalysts of choice. Their high cost and limited reserve add a significant cost barrier to PEM electrolyzer, which contributes to the overall expense of hydrogen production next only to the cost of electricity. Replacing Ir with earth-abundant transition metal oxides could help to reduce the electrolyzer system cost.For PEMWE application, a PGM-free anodic catalyst must be highly active to match the performance of Ir. Furthermore, it must be stable against oxidative acid corrosion. At Argonne National Laboratory, we recently developed a new PGM-free OER of a nanofibrous cobalt spinel catalyst co-doped with lanthanum and manganese (LMCF). [1] The catalyst was synthesized from the precursor of cobalt zeolitic methyl-imidazolate framework (Co-ZIF) doped with manganese and lanthanum. The Co-ZIF was mixed with a polymer solution and electrospun into nanofiber before being converted to fibrous cobalt spinel under low temperature oxidation to remove all the organics and carbon. The new PGM-free catalyst demonstrated a low overpotential of 353 mV (RHE) at 10 mA/cm2 and a low OER degradation over 360 hours in acidic electrolyte. A PEMWE containing this catalyst at its anode at ~2.0 mg/cm2 demonstrated a current density of 2000 mA/cm2 at 2.47 volts (Nafion® 115 membrane) or 4000 mA/cm2 at 3.00 volt (Nafion® 212 membrane). The membrane electrode was also subjected to accelerated-stress-test (AST) in both voltage cycling and galvanostatic run at different current densities. Low degradation rates under both ASTs were observed.In addition to catalyst design and synthesis, we also conducted extensive structural characterizations, stability measurement, as well as computational modeling. Our in situ study found that the OER process involves significant lattice oxygen movement, leading to lowering instead of raising the cobalt oxidation state during the electrolysis. The measurement of the stability number, combined with computational Pourbaix diagram, elucidated the importance of the operating potential in reducing catalyst dissolution. These findings offer in-depth understanding and direction of future development for PGM-free OER catalyst in PEM water electrolyzer. Acknowledgement: This work is supported by the U.S. Department of Energy, Office of Energy Efficiency and Renewal Energy, Hydrogen and Fuel Cell Technologies Office, and by Laboratory Directed Research and Development funding of Argonne National Laboratory, provided by the Director, Office of Science, of the U.S. Department of Energy under Contract No. DEAC02-06CH11357. Work performed at the Center for Nanoscale Materials and Advanced Photon Source, both U.S. Department of Energy Office of Science User Facilities, was supported by the U.S. DOE, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.[1] “La- and Mn-doped cobalt spinel oxygen evolution catalyst for proton exchange membrane electrolysis” Lina Chong, Guoping Gao, Jianguo Wen, Haixia Li, Haiping Xu, Zach Green, Joshua D. Sugar, A. Jeremy Kropf, Wenqian Xu, Xiao-Min Lin, Hui Xu, Lin-Wang Wang, Di-Jia Liu, Science 380, 609–616 (2023)

Electrodes Modified with Overlayers for Enhanced CO2 Reduction Selectivity

Like many electrocatalytic reactions involving the transfer of multiple protons and electrons, the CO2 reduction reaction frequently exhibits poor selectivity among many possible products. In this presentation, I will describe our laboratory’s efforts to design electrochemical platforms that enable the control over proton transfer dynamics to electrocatalysts using molecular and polymeric overlayers with the goal of increasing product selectivity and understanding reaction mechanism. This platform is generally applicable to both molecular and metallic systems, and we demonstrate how it can be used to improve the selectivity of nonprecious metal O2 reduction and NO3 - reduction catalysts.For the CO2 reduction reaction, this strategy as applied to metal electrodes is also broadly germane and is used to modulate catalyst reactivity such that it results in the selective production of ethylene, ethanol, methanol, carbon monoxide, and methane, with examples for each with Faradaic efficiencies in excess of 70%. A common theme is that the hydrophobicity of the metal-overlayer interface frequently controls product selectivity and can steer the site of protonation events at intermediates. In this manner, for example, the selectivity of catalysts between the C2 products ethanol and ethylene can be dictated. To aid in mechanistic understanding and catalyst characterization, the majority of these studies are conducted on nominally flat metal electrodes. However, we will also describe ongoing efforts to nanostructure these CO2 reduction systems to develop selective catalysts that operate at high current densities.

Novel Insight into the Synergistic Mechanism for Pd and Rh Promoting the Hydro-Defluorination of 4-Fluorophenol over Bimetallic Rh-Pd/C Catalysts.

This study explores the synergistic effect between the Rh and Pd of bimetallic Rh-Pd/C catalysts for the catalytic hydro-defluorination (HDF) of 4-fluorophenol (4-FP). It was found that 4-FP could not be efficiently hydro-defluorinated over 6% Pd/C and 6% Rh/C due to the inherent properties of Pd and Rh species in the dissociation of H2 and the activation of C-F bonds. Compared with 6% Pd/C and 6% Rh/C, bimetallic Rh-Pd/C catalysts, especially 1% Rh-5% Pd/C, exhibited much higher catalytic activity in the HDF of 4-FP, suggesting that the synergistic effect between the Rh and Pd of the catalyst was much more positive. Catalyst characterizations (BET, XRD, TEM, and XPS) were introduced to clarify the mechanism for the synergistic effect between the Rh and Pd of the catalyst in the HDF reaction and revealed that it was mainly attributed to the bifunctional mechanism: Pd species were favorable for the dissociation of H2, and Rh species were beneficial to the activation of C-F bonds in the HDF reaction. Meanwhile, the interaction between Rh and Pd species enabled Rh and Pd to exhibit a more positive synergistic effect, which promoted the migration of atomic H* from Pd to Rh species and thus enhanced the HDF of 4-FP. Furthermore, 1% Rh-5% Pd/C prepared using 20-40 equiv NaBH4 exhibited the best performance in the catalytic HDF of 4-FP. Catalysis characterizations suggested that appropriate Rh3+/Rh0 and Pd2+/Pd0 ratios were beneficial to the dissociation of H2 and the activation of C-F bonds, which caused the more positive synergistic effect between the Rh and Pd of Rh-Pd/C in the HDF reaction. This work offers a valuable strategy for enhancing the performance of catalytic HDF catalysts via promoting synergistic effects.

Interface engineering of the dz2 electrons mobility for single atom catalytic activity and selectivity

Transition metal singlet embedded in nitrogen-doped carbon material (M-N-C) has been demonstrated as a promising electrochemical oxygen reduction reaction (ORR) catalyst; however, the unsatisfying activity and production selectivity have hampered its widespread applications in energy storage and conversion technologies. Herein, interface engineering by facilitating M-N-C catalysts (M from 3d to 4d electron-containing elements) with MXene has been utilized to regulate their ORR performance. It is found that the charge transfer occurring within the interface not only tunes the electron occupancy of the 3d/4d orbitals of the metal site, but also delocalizes the population of the dz2 states. This alternation enhances the mobility of the electrons and promotes the 4e catalytic process thermodynamically. Meanwhile, the formation of ∗HOOH, the key reaction intermediate for 2e reaction, is hindered due to the alleviation of the binding capacity, which is beneficial to improve production selectivity. This study provides foundational understanding for the ORR catalytic mechanism at the atomic level and opens up new avenues for designing high-demanded electrocatalysts.

Oxophilic gallium single atoms bridged ruthenium clusters for practical anion-exchange membrane electrolyzer

The development of highly efficient and durable alkaline hydrogen evolution reaction (HER) catalysts is crucial for achieving high-performance practical anion exchange membrane water electrolyzer (AEMWE) at ampere-level current density. Herein, we report a design concept by employing Ga single atoms as an electronic bridge to stabilize the Ru clusters for boosting alkaline HER performance in practical AEMWE. Experimental and theoretical results collectively reveal that the bridged Ga sites trigger strong metal-support interaction for the homogeneous distribution of Ru clusters with high density, as well as optimize the Ru–H bond strength due to the electron transfer between Ru and Ga for enhanced intrinsic HER activity. Moreover, the oxophilic Ga sites near the Ru clusters tend to adsorb the hydroxyl species and accelerate the water dissociation for sufficient proton supplement in an alkaline medium. The Ru–GaSA/N–C catalyst exhibits a low overpotential of 4 ± 1 mV (10 mA cm−2) and high mass activity of 9.3 ± 0.5 A mg−1Ru at −0.05 V vs RHE. In particular, the Ru–GaSA/N–C-based AEMWE in 1 M KOH delivers a voltage of only 1.74 V to reach an industrial current density of 1 A cm−2, and can steadily operate at 1 A cm−2 for more than 170 h.

Exploring Catalytic Intermediates in Pd-Catalyzed Aerobic Oxidative Amination of 1,3-Dienes: Multiple Metal Interactions of the Palladium Nanoclusters.

Metal nanoclusters (NCs) have unique properties because of their small size, which makes them useful as catalysts in reactions like cross-coupling. Pd-catalyzed oxidative amination, which involves dehydrogenative C-N bond formation, uses Pd complexes as the active species. It is known that the catalytic conditions involve the formation of Pd(0) species from Pd NCs, but the precise role of Pd NCs in the transformations has not been established. In this study, we investigated the characteristic properties of Pd NCs in oxidative amination of 1,3-dienes. The reaction achieved direct amination of commercially accessible 1,3-dienes with secondary aromatic amines, providing a variety of nitrogen containing 1,3-dienes. The compound was applicable to radical polymerization to provide the nitrogen-fabricated 1,3-diene-based polymer, which exhibited a different thermal stability compared to aliphatic nitrogen-fabricated diene polymers. In addition to the synthetic utility, by combining X-ray absorption fine structure and small-angle X-ray scattering analysis, we revealed amines and 1,3-dienes affected metal leaching from the Pd nanoparticles and stabilization of Pd NCs in the catalytic reaction. Additionally, DFT calculation suggested that the catalytic intermediate contained multiple adjacent Pd atoms and was responsible for formation of an σ-allylic intermediate that is difficult to form with the use of Pd complexes. These results could be used to understand the underlying phenomenon in the oxidative coupling reaction and develop Pd NCs-based dehydrogenation.

Regulating Fe-spin state by doping P heteroatom in Fe-N-C catalyst derived from colloidal nanoparticles encapsulated ZIF-8 to boost oxygen reduction activity

Over the past decades, Fe-N-C carbon materials have been considered as one of the most ideal alternatives to the noble metal catalysts (e.g. Pt/C) for the sluggish oxygen reduction reaction (ORR) in zinc-air batteries and fuel cells. In this research, ZIF-8 cage encapsulation confinement strategy coupled with post P-doping has been employed to design porous P, N and Fe co-doped carbon (FeNC-P) for efficient oxygen reduction reaction (ORR). The zero-field cooling (ZFC) temperature-dependent magnetic susceptibility measurement revealed that incorporation of P in the Fe-N-C catalysts can induce the transition of spin polarization configuration, which promotes the desorption of *OH in the crucial step of the ORR reaction process. The best-performing FeNC-P catalyst displays a 1.00 V (vs RHE) of Eonset (onset potential) and a 0.90 V (vs RHE) of E1/2 (half-wave potential) in an alkaline solution, which exceeds the benchmark Pt/C catalyst. Moreover, it also delivers a high power density of 130 mW·cm−2 in the self-made zinc-air battery, surpassing the benchmark Pt/C catalyst (90 mW·cm−2).

Revealing the Role of Mo Leaching in the Structural Transformation of NiMo Thin Film Catalysts upon Hydrogen Evolution Reaction.

NiMo alloys are considered highly promising non-noble Hydrogen Evolution Reaction (HER) catalysts. Besides the synergistic effect of alloying elements, recent attention is drawn to the Mo leaching from the catalyst. This work investigates the role of Mo in NiMo alloys during HER, aiming to understand the interplay between compositional, structural, and electronic factors on the activity, and their effects on the electrode material and catalyst properties. For this purpose, sputter-deposited low roughness NixMo100-x thin films are produced. The investigation of catalyst performance depending on their chemical composition shows a volcano-shaped plot, peaking for the Ni65Mo35 alloy with the highest intrinsic activity in alkaline HER. A comprehensive electrode surface analysis combining transmission electron microscopy, X-ray photoelectron spectroscopy and atomic force microscopy identifies the leaching of Mo on a structural level and indicates the formation of a Ni(OH)2-rich surface area. The ultimate surface characteristics of the NiMo catalysts depend on the initial composition and the electrochemical procedure. Based on the findings, it conclude that the observed catalytic properties of NiMo alloys in HER are determined by a complex interplay of increasing roughness, available surface species and their synergies. The leaching of Mo has a proven structural effect and is considered one of several factors contributing to the enhanced catalyst activity.

Rational integration of MoSe2 and BN with TiO2 to design nanoengineered ternary heterojunctions for sustainable hydrogen energy: Experimental evidences and theoretical anticipations

Ternary MoSe2-BN/TiO2 and binary MoSe2–TiO2 heterostructures are epitomized as Tortoise and Hare nanocatalysts to outline the synergism between bilayered MoSe2-BN over pristine MoSe2 for their applicability in the photochemical and photo/-electrochemical water splitting operations. We report here the fabrication of MoSe2-BN integrated TiO2 catalytic systems and ascertained experimental and theoretical investigations of MoSe2-BN/TiO2 and MoSe2–TiO2 heterostructures that are directly proportional to their long-term applicability as the hydrogen evolution reaction (HER) catalysts. Optimized 5 wt% MoSe2-BN/TiO2 and 7.5 wt% MoSe2–TiO2 exhibited nearly 32-fold and 18-fold higher photochemical hydrogen evolution efficiency than bare TiO2 with 43.1% and 25.08% apparent quantum yield, respectively. Photo/-electrochemical studies verified the diversity of MoSe2-BN/TiO2 and MoSe2–TiO2 catalysts as applied potential values diminished drastically to attain benchmark 10 mA/cm2 current density for 5 wt % MoSe2-BN/TiO2 (0.31 V) and 7.5 wt% MoSe2–TiO2 (0.61 V). This work showcases the augmented HER activity of MoSe2-BN/TiO2 due to the shielding of MoSe2 grain boundaries via BN-encapsulation that impeded the degradation of MoSe2 in aqueous condition ascribed to its wide band energy in UV region making it the advanced tunnel barrier.

Development of Dual Cooperative Multi‐Urea Catalysts for Henry Reaction

AbstractA dual cooperative multi‐urea catalyst for Henry reaction was developed in which four urea groups were introduced into a rigid and rotatable framework. The tetraurea catalyst 1 showed higher activity for Henry reaction than its analogues, close‐bisurea 2 or distal‐bisurea 3 with two urea units, demonstrating additional cooperativity between the two substrate‐pinching catalytic sites of tetraurea 1. Addition of the reaction product to the reaction system resulted in a small amount of further activation. This work would provide a basis for new cooperative molecular catalysts, leading to a high degree of regulation based on allosteric cooperativity, e.g. with a distinct ON/OFF threshold for reactivity, or feedback activation by using the product as a reaction template.

Theoretical Insights of Curvature Effects of FeN4-Doped Carbon Nanotubes on ORR Activity.

The advancement of metal-air batteries is critically contingent on the progression of efficient catalysts for the oxygen reduction reaction (ORR). The potential applications of a series of FeN4-doped carbon nanotubes (Fe-N4CNTs) of varying diameters as ORR catalysts were examined using density functional theory. We explored the stability and electronic properties of Fe-N4CNTs by analyzing the energy and examining the density of states. A marked dependence of the catalytic performance on the nanotube diameter was observed: as the transition from (5, 5) to (10, 10) Fe-N4CNTs occurred, the catalytic activity on the outer surface of the carbon tubes enhanced progressively, with the overpotential reducing from 0.94 to 0.86 V. Conversely, the catalytic activity on the inner surface of the carbon tubes decreased progressively with the overpotential also increasing from 0.62 to 1.04 V. In addition, we found that curvature significantly affected the electronic structure and charge transfer at the FeN4 site, regulating the adsorption and desorption of reactants, intermediates, and products during electrocatalysis and thus influencing the catalytic activity of the Fe-N4CNTs. This investigation offers valuable guidance for the design of Fe-based single-atom catalysts and the practical application of Fe-N-C materials.

Facet-Dependent Evolution of Active Components on Spinel Co3O4 for Electrochemical Ammonia Synthesis.

Spinel cobalt oxides (Co3O4) have emerged as a promising class of catalysts for the electrochemical nitrate reduction reaction (eNO3RR) to ammonia, offering advantages such as low cost, high activity, and selectivity. However, the specific role of crystallographic facets in determining the catalysts' performance remains elusive, impeding the development of efficient catalysts. In this study, we have synthesized various Co3O4 nanostructures with exposed facets of {100}, {111}, {110}, and {112}, aiming to investigate the dependence of the eNO3RR activity on the crystallographic facets. Among the catalysts tested, Co3O4 {111} shows the best performance, achieving an ammonia Faradaic efficiency of 99.1 ± 1.8% with a yield rate of 35.2 ± 0.6 mg h-1 cm-2 at -0.6 V vs RHE. Experimental and theoretical results reveal a transformation process in which the active phases evolve from Co3O4 to Co3O4-x with oxygen vacancy (Ov), followed by a Co3O4-x-Ov/Co(OH)2 hybrid, and finally Co(OH)2. This process is observed for all facets, but the formation of Ov and Co(OH)2 is the most rapid on the (111) surface. The presence of Ov significantly reduces the free energy of the *NH2 intermediate formation from 1.81 to -0.53 eV, and plentiful active sites on the densely reconstructed Co(OH)2 make Co3O4 {111} an ideal catalyst for ammonia synthesis via eNO3RR. This work provides insights into the understanding of the realistic active components, offers a strategy for developing highly efficient Co-based spinel catalysts for ammonia synthesis through tuning the exposed facets, and helps further advance the design and optimization of catalysts in the field of eNO3RR.

High performance bagasse based iron–nitrogen-carbon catalyst for oxygen reduction reaction

The design and synthesis of highly stable and low-cost for the oxygen reduction reactions (ORR) catalysts to replace non-noble metal catalysts is crucial for their applications in fuel cells. In this work, bagasse activated carbon (BAC) was used as the carbon source to prepare the high specific area and multi-porous carbon. The BAC@FeN catalysts derived from BAC, FeCl2 and melamine were prepared by pre-activation treatment and high-temperature pyrolysis. The BAC@FeN-900–30 catalyst encapsulated with Fe3C and Fe3O4 exhibited an outstanding catalytic activity (Eonset = 1.01 V and E1/2 = 0.87 V) and superior operational stability of 90.50 % compared to Pt/C (20 wt%) catalyst. The results demonstrated that iron, carbon and nitrogen piggybacked BAC could generate abundant Fe-Nx active sites, which greatly improved the catalytic activity of ORR and had a broad application prospect in energy conversion.