Four-electron Pathway Research Articles

Controlling N-Doping Nature at Carbon Aerogels from Biomass for Enhanced Oxygen Reduction in Seawater Batteries.

Pyridinic N-type doped at carbon has been known to have better electrocatalytic activity toward the oxygen reduction reaction (ORR) than the others. Herein, we proposed to prepare pyridinic N doped at carbon aerogels (CaA) derived from biomass, i.e., coir fiber (CF) and palm empty fruit bunches (PEFBs), by adjusting the pyrolysis temperature during carbonization of the biomass-based-cellulose aerogels. The cellulose aerogels were prepared by the ammonia-urea system as the cellulose solvent, in which ammonia also acted as a N source for doping and urea as the cellulose cross-linker. The as-prepared cellulose aerogels were directly pyrolyzed to produce N-doped CaA. It was found that the type of N doping is dominated by pyrrolic N at pyrolysis temperature of 600 °C, pyridinic N at 700 °C, and graphitic N at 800 °C. The pyridinic N exhibited better performance as an electrocatalyst for the ORR than pyrrolic N and graphitic N. The ORR using pyridinic N follows the four-electron pathway, which quantitatively implies a more electrochemically stable process. When used as a cathode for the Mg-air battery using a 3.5% NaCl electrolyte, the pyridinic N CaA exhibited excellent performance by giving a cell voltage of approximately 1.1 V and delivered a high discharge capacity of 411.64 mA h g-1 for CF and 492.64 mA h g-1 for PEFB corresponding to an energy density of 464.23 and 529.49 mW h g-1, respectively.

Interfacial engineering of high-performance Fe2P2O7-based electrocatalysts for alkaline exchange membrane fuel cells

Fuel cells promise high energy density and energy conversion efficiency but are plagued by the scarcity of platinum for facilitating the oxygen reduction reaction (ORR). Herein, a facile synthesis of a heterojunction electrocatalyst of ZIF-8 derived carbon (Z8C) supported Fe2P2O7 (Fe2P2O7@Z8C) is reported. The electrocatalyst exhibited higher half-wave potential than the state-of-the-art Pt/C catalyst by more than 33 mV and achieved a peak power density of 152.47 mW cm−2, outperforming the Pt/C-driven cell by ca. 14.5 mW cm−2 in alkaline membrane electrode assemblies under similar operating conditions. X-ray photoelectron and X-ray absorption spectroscopic studies suggested a spontaneous electron redistribution across the heterojunction interface in Fe2P2O7@Z8C. Density functional theory calculations indicated that the electron redistribution may remarkably promote the intrinsic activity of Fe2P2O7@Z8C toward the four-electron ORR pathway. These findings offer an indispensable strategy for rational design of highly efficient and durable non-noble electrocatalysts.

Nickel nanoparticles supported on doped graphene-based materials for the ORR and HER in alkaline medium

In this work, Ni nanoparticles (NPs), with nominal metallic loading of 20 wt%, were deposited on graphene-based materials (GMs) doped with nitrogen and sulphur, in order to study their electrocatalytic activity towards the oxygen reduction reaction (ORR) and hydrogen evolution reaction (HER) in alkaline medium. Different chemical reducing agents were employed to synthesize the catalysts. Results indicate that the presence of heteroatoms in the GMs influences the activity of both the GMs and the Ni-based catalysts. Thus, best results were obtained for catalyst containing nitrogen and sulphur for the ORR which favours the direct four-electron pathway, whereas for the HER, the highest performance was observed for N-doped materials.

Engineering Co-N-Cr Cross-Interfacial Electron Bridges to Break Activity-Stability Trade-Off for Superdurable Bifunctional Single Atom Oxygen Electrocatalysts.

Atomically dispersed metal-nitrogen-carbon (M-N-C) catalysts have exhibited encouraging oxygen reduction reaction (ORR) activity. Nevertheless, the insufficient long-term stability remains a widespread concern owing to the inevitable 2-electron byproducts, H2O2. Here, we construct Co-N-Cr cross-interfacial electron bridges (CIEBs) via the interfacial electronic coupling between Cr2O3 and Co-N-C, breaking the activity-stability trade-off. The partially occupied Cr 3d-orbitals of Co-N-Cr CIEBs induce the electron rearrangement of CoN4 sites, lowering the Co-OOH* antibonding orbital occupancy and accelerating the adsorption of intermediates. Consequently, the Co-N-Cr CIEBs suppress the two-electron ORR process and approach the apex of Sabatier volcano plot for four-electron pathway simultaneously. As a proof-of-concept, the Co-N-Cr CIEBs is synthesized by the molten salt template method, exhibiting dominant 4-electron selectively and extremely low H2O2 yield confirmed by Damjanovic kinetic analysis. The Co-N-Cr CIEBs demonstrates impressive bifunctional oxygen catalytic activity (▵E=0.70 V) and breakthrough durability including 100 % current retention after 10 h continuous operation and cycling performance over 1500 h for Zn-air battery. The hybrid interfacial configuration and the understanding of the electronic coupling mechanism reported here could shed new light on the design of superdurable M-N-C catalysts.

Electroless Deposition of Palladium Nanoparticles on Graphdiyne Boosts Electrochemiluminescence.

Modulating the electronic structure of metal nanoparticles via metal-support interaction has attracted intense interest in the field of catalytic science. However, the roles of supporting substrates in regulating the catalytic properties of electrochemiluminescence (ECL) remain elusive. Here, we find that the use of graphdiyne (GDY) as the substrate for electroless deposition of Pd nanoparticles (Pd/GDY) produces the most pronounced anodic signal enhancement in luminol-dissolved oxygen (O2) ECL system as co-reactant accelerator over other carbon-based Pd composite nanomaterials. Pd/GDY exhibits electrocatalytic activity for the reduction of O2 through a four-electron pathway at approximately -0.059 V (vs Ag/AgCl) in neutral solution forming reactive oxygen species (ROS) as intermediates. The study shows that the interaction of Pd and GDY increases the amount and stability of ROS on the Pd/GDY electrode surface and promotes the reaction of ROS and luminol anion radical to generate excited luminol, which significantly boosts the luminol anodic ECL emission. Based on quenching of luminol ECL through the consumption of ROS by antioxidants, we develop a platform for the detection of intracellular antioxidants. This study provides an avenue for the development of efficient luminol ECL systems in neutral media and expands the biological application of ECL systems.

D-Orbital steered FeN4 moiety through N, S dual-site adjustation for zinc-air flow battery

The implementation of pristine covalent organic polymer (COP) with well-defined structure as air electrode may spark fresh vitality to rechargeable zinc-air flow batteries (ZAFBs), but it still remains challenges in synergistically regulating their electronic states and structural porosity for the great device performance. Here, we conquer these issues by exploiting N and S co-doped graphene with COP rich in metal–ligand nitrogen to synergistically construct an effective catalyst for oxygen reduction reaction (ORR). Among them, the N and S co-doped sites with high electronegativity properties alter the number of electron occupations in the d orbital of the iron centre and form electron-transfer bridges, thereby boosting the selectivity of the ORR-catalysed four-electron pathway. Meanwhile, the introduction of COP materials aids the formation of pore interstices in the graphene lamellae, which both adequately expose the active sites and facilitate the transport of reactive substances. Benefiting from the synergistic effect, as-prepared catalyst exhibits excellent half-wave potentials (E1/2 = 912 mV) and stability (merely 8.8% drop after a long-term durability test of 50000 s). Further, ZAFBs assembled with the N/SG@COP catalyst demonstrate exceptional power density (163.8 mW cm−2) and continuous charge and discharge for approximately 140 h at 10 mA cm−2, outperforming the noble-metal benchmarks.

Insights on MOF-derived metal-carbon nanostructures for oxygen evolution.

Electrochemical water splitting has been regarded a promising method for the production of green hydrogen, addressing the need for efficient energy conversion and storage. However, it is severely hindered by the oxygen evolution reaction (OER) because of its multi-step four-electron transfer pathway with sluggish reaction kinetics. Microporous metal-organic-frameworks (MOFs), by virtue of large specific surface area, high porosity, tunable composition and morphology, find widespread use as precursors of metal-carbon nanostructures. The resulting carbon nanomaterials can well inherit the characteristics and advantages of the crystalline MOF precursors, and exhibit versatile application prospects in the fields of environment and energy, particularly in OER. Herein, a meticulous overview of the synthesis strategy for MOF-derived metal-carbon nanostructures and the origins of their enhanced OER properties has been demonstrated. We comprehensively illustrate these aspects across three dimensions: MOF selection, metal introduction, and carbon structures. Finally, the challenges and future prospects for this emerging field will be presented.

Nanowires-based MnO2-Ru/rGO: An efficient oxygen reduction reaction electrocatalyst

Oxygen Reduction Reaction (ORR) for clean energy is hindered by expensive Pt-based electrocatalysts, prompting efforts to replace it with alternative electrocatalysts. Thus, we started by synthesizing MnO2 nanowires through a hydrothermal approach, followed by the growth of ruthenium nanoparticles (Ru NPs) without surface modification, using just 2.0 wt% of the noble metal (MnO2-Ru). However, to further enhance the electrocatalyst's performance and reduce costs, we combined different ratios of reduced graphene oxide (rGO) with the electrocatalyst. X-ray photoelectron spectroscopy, scanning electron microscopy, transmission electron microscopy, and energy dispersive X-ray spectroscopy were employed to characterize the chemical composition and morphological properties of MnO2-Ru. These analyses identified the presence of the compounds during synthesis and confirmed the deposition of Ru NPs on the surface of MnO2 nanowires. The optimized MnO2-Ru/rGO demonstrated superior ORR activity than rGO, MnO2, and MnO2-Ru individually, with more positive onset potential (−0.054 V) and half-wave potential of −0.173 V. Notably, MnO2-Ru/rGO reduced oxygen via the four-electron transfer pathway. Furthermore, the higher stability and excellent methanol tolerance of MnO2-Ru/rGO compared to the commercial 20 wt% Pt/C indicates its suitability for fuel cells, maintaining approximately 70 % of its initial current after 8000 s.

A New Catalyst System for the Oxygen Reduction Reaction (ORR) at Fuel Cell Cathodes: P-Block Precious Group Metal-Free Tin and Nitrogen-Doped Carbon (SnNC) Catalyst

The sluggish kinetics of the Oxygen Reduction Reaction (ORR) and the consequently large amount of platinum catalyst required for cathode material in Proton Exchange Membrane Fuel Cell (PEMFC) is today’s primary hurdle for commercializing fuel cell technology in automotive application.1 A class of carbon-doped with metal and nitrogen electrocatalysts (labelled MNC) is promising substitute for platinum-group metals (PGMs) in oxygen reduction reaction (ORR) in PEMFC.2 Particular attention was placed on a specific family of MNC catalysts derived from 3d transition metal precursors with FeNC and CoNC being the most promising for PEMFC applications requiring high power density.3-5 In the present contribution, we report on a new MNC catalyst system, different by its chemical nature from all others catalysts previously reported as ORR-active in acidic medium. The highlights of this work are the successful design and synthesis of SnNC as a non-noble metal catalyst for the first time, and the finding that SnNC hosts Sn-based active sites that can achieve the same turnover frequency as Fe-based active sites in FeNC, and much higher turnover frequency than Co-based active sites in CoNC. FeNC and CoNC materials have, until now, been the state-of-art PGM-free catalysts for ORR in acidic medium. The present SnNC material shows also high and similar selectivity for the four-electron ORR pathway as FeNC, much higher than CoNC materials. The prepared SnNC catalysts meet and exceed the FeNC catalysts in terms of hydrogen-air fuel cell power density. The SnNC-NH3 catalysts displayed a 40-50% higher current density than FeNC-NH3 at cell voltages below 0.7 V. Added benefits include a Fenton-inactive character of Sn. Among other techniques, density functional theory and 119Sn Mössbauer spectroscopy were used in combination to investigate SnNC, providing for the first time insights on the structure of its active sites, their rate-determining step for ORR and selectivity for the four-electron reduction pathway. References Holton, O. T.; Stevenson, J. W., The Role of Platinum in Proton Exchange Membrane Fuel Cells. Platinum Metals Review 2013, 57 (4), 259-271.Luo, F.; Wagner, S.; Ju, W.; Primbs, M.; Li, S.; Wang, H.; Kramm, U. I.; Strasser, P., Kinetic Diagnostics and Synthetic Design of Platinum Group Metal-Free Electrocatalysts for the Oxygen Reduction Reaction Using Reactivity Maps and Site Utilization Descriptors. Journal of the American Chemical Society 2022, 144 (30), 13487-13498.Zitolo, A.; Ranjbar-Sahraie, N.; Mineva, T.; Li, J.; Jia, Q.; Stamatin, S.; Harrington, G. F.; Lyth, S. M.; Krtil, P.; Mukerjee, S.; Fonda, E.; Jaouen, F., Identification of catalytic sites in cobalt-nitrogen-carbon materials for the oxygen reduction reaction. Nature Communications 2017, 8 (1), 957.Zitolo, A.; Goellner, V.; Armel, V.; Sougrati, M.-T.; Mineva, T.; Stievano, L.; Fonda, E.; Jaouen, F., Identification of catalytic sites for oxygen reduction in iron- and nitrogen-doped graphene materials. Nature materials 2015, 14 (9), 937-942.Luo, F.; Roy, A.; Silvioli, L.; Cullen, D. A.; Zitolo, A.; Sougrati, M. T.; Oguz, I. C.; Mineva, T.; Teschner, D.; Wagner, S.; Wen, J.; Dionigi, F.; Kramm, U. I.; Rossmeisl, J.; Jaouen, F.; Strasser, P., P-block single-metal-site tin/nitrogen-doped carbon fuel cell cathode catalyst for oxygen reduction reaction. Nature materials 2020, 19 (11), 1215-1223.

Palladium phosphoselenide (PdPSe) – Reduced graphene oxide composite: A tri-functional electrocatalyst and a cathode catalyst for alkaline fuel cells

Developing a new and efficient electrocatalyst is highly desirable for fundamental electrochemical energy applications such as fuel cells. In this work, we have introduced palladium phosphoselenide (PdPSe), a layer-type ternary material, as a tri-functional electrocatalyst in the three most fundamental electrochemical reactions, such as oxygen reduction reaction (ORR), hydrogen evolution reaction (HER), and oxygen evolution reaction (OER). To enhance the electrocatalytic activity and stability of the electrode material, reduced graphene oxide (rGO) is incorporated into the system by making a composite (rGO-PdPSe), which effectively reduces the overpotentials and increases the current densities for HER and OER. Moreover, in ORR, it turns into a four-electron pathway with comparable kinetics. Further, the composite material is applied in anion exchange membrane fuel cell application as a cathode catalyst. This composite's facile kinetics and robust performance suggests that ternary phosphoselenides based trifunctional electrocatalysts would be a promising material for large-scale application of clean energy conversion devices.

(Invited) Metal Oxide Mixed Sulphide of Controlled Crystal Structure As Catalysts for Two-Electron Water Oxidation

Conventional catalysts based on noble metals (Pt, Ru) are often used for production of oxygen during water electrolysis in four-electron pathway as they can decrease gas evolution overpotential. Another possible valuable product from water oxidation is hydrogen peroxide obtained via two-electron process. Currently, transition metal oxides, are readily employed to catalyse this reaction. Although they display certain activity, it is still not satisfactory for mass production of H2O2, therefore further optimisations are needed. We suggest in this work, in addition to typical nanostructure modifications of metal oxides (MoO2) such as doping or structural engineering, an application of transition metal dichalcogenides (MoS2). Consequently, the oxide is partially replaced by sulphur. The selection of molybdenum as a centre atom ensures sufficient electrical conductivity of material as an electrode active layer. This combination is intended to provide the benefits thanks to layered materials of tuned interlayer spacing as well as controlled number of active sites. It enables selective adsorption of intermediate products of water oxidation (O∗, OH∗, and OOH∗), and therefore increased preference towards two-electron pathway.Various chemical approaches are considered to synthesize this material, especially hydrothermal treatment in autoclave and high-temperature sintering of precursors. The catalytic activity of the nanostructured metal oxide is further enhanced by its deposition on conductive high surface area support made of carbon/graphite particles of strictly defined textural parameters and surface chemistry. The final composite is obtained using hydrophobic binders (PVDF and PTFE) to form the electrode and improve the catalyst stability. The measurements are carried out in flow electrolyser unit with an alkaline electrolyte. The amount of produced H2O2, H2 as well as quantity and quality of evolved gases are assessed in terms of faradaic efficiency, pressure and overvoltage measurements.

New Materials for Alkaline Fuel Cells from Electroplating Industries Waste Solution Oxygen Reduction Reaction Catalyzed by Pd on Carbon Nanotubes, Graphene and Carbon Black

Issues concerning the energy sector are the focus of international discussions; the progressive depletion of fossil reserves and the increase in energy demand have led to the rise in fuel and electricity prices. In this context, fuel cells are promising candidates for a sustainable future. Oxygen Reduction Reaction (ORR) is the bottle-neck strategic reaction ruling the fuel cell efficiency process. This reaction allows the release of chemical energy in the form of electricity without producing any waste. However, the slow kinetics of ORR require highly effective electrocatalysts for proper boosting. It is well known that best available catalysts, in terms of raw bulk elements, belong to the so-called platinum group metals (PGMs). PGMs are very expensive, hence lowering the PGM content of ORR catalysts is among the most prevalent research focuses [1,2].We propose two different ways to obtain carbon-based ORR catalyst with a low continent of PMGs. The first approach involves the use of Single-Ion catalysts designed by carbon nanotubes functionalized with palladium(II) complexes show [3]. The second approach is totally innovative and aims at recovering precious metals to reduce industrial waste and promote the development of a circular economy. Large quantities of wastewater are produced in electroplating industries. This wastewater not only has to be properly processed before it is released into the environment, but also contains metals like palladium [4]. Using carbon materials and an electroless deposition, palladium can be recovered from the wastewater of the electroplating industry. We focus on carbon black (CB) and two different types of graphene-type materials named as (G4) and (G7). It was found that CB and G4 can recover up to 97% palladium. In this way we obtained hybrid materials containing palladium that could be promising candidates for use as catalysts for the ORR in alkaline fuel cells.New catalysts were treated by preparing a nafion polymeric dispersion and tested with electroanalytical techniques. Rotating ring-disk electrode (RRDE) experiments were performed to investigate the catalytic pathway of ORR and asses the number of exchanged electrons per oxygen molecule. Results confirm that tested materials promote the four-electron reaction pathway and reduce the formation of hydrogen peroxide. These approaches provide the basis for further developments to improve the efficiency of future low-cost catalysts for alkaline fuel cells.[1] Passaponti, M.; Lari, L.; Bonechi, M.; Bruni, F.; Giurlani, W.; Sciortino, G.; Rosi, L.; Fabbri, L.; Vizza, M.; Lazarov, V.K.; et al. Optimisation Study of Co Deposition on Chars from MAP of Waste Tyres as Green Electrodes in ORR for Alkaline Fuel Cells. Energies 2020, 13, 5646, doi:10.3390/en13215646.[2] Bonechi, M.; Giurlani, W.; Vizza, M.; Savastano, M.; Stefani, A.; Bianchi, A.; Fontanesi, C.; Innocenti, M. On the Oxygen Reduction Reaction Mechanism Catalyzed by Pd Complexes on 2D Carbon. A Theoretical Study. Catalysts 2021, 11, 764, doi:10.3390/catal11070764.[3] Savastano, M.; Passaponti, M.; Giurlani, W.; Lari, L.; Calisi, N.; Delgado-Pinar, E.; Serrano, E.S.; Garcia-España, E.; Innocenti, M.; Lazarov, V.K.; et al. Linear, Tripodal, Macrocyclic: Ligand Geometry and ORR Activity of Supported Pd(II) Complexes. Inorganica Chim. Acta 2021, 518, 120250, doi:10.1016/j.ica.2021.120250.[4] De Beni, E.; Giurlani, W.; Fabbri, L.; Emanuele, R.; Santini, S.; Sarti, C.; Martellini, T.; Piciollo, E.; Cincinelli, A.; Innocenti, M. Graphene-Based Nanomaterials in the Electroplating Industry: A Suitable Choice for Heavy Metal Removal from Wastewater. Chemosphere 2022, 292, 133448, doi:10.1016/j.chemosphere.2021.133448.

4-6 carbophene: A two-dimensional carbon material with Dirac cone and superior activity for electrochemical oxygen reduction reactions

The positive exploration of efficient and cost-effective electrocatalysts for the oxygen reduction reaction (ORR) is crucial for the sustainable development of energy conversion and storage technologies. Recently, a new type of two-dimensional (2D) carbon crystals, called 4-6 carbophene, has been successfully synthesized. It consists of a four-carbon ring and a six-carbon ring. In this work, we have discovered that 4-6 carbophene is a two-dimensional Dirac cone material through density-functional theory (DFT) calculations. Therefore, we further investigated the oxygen reduction reaction on 4-6 carbophene to understand its catalytic properties. The results demonstrate that on 4-6 carbophene, the ORR prefers a direct four-electron pathway. Under acidic conditions, the ORR overpotential of 4-6 carbophene is as low as 0.59 V, which is comparable to some metal benchmark catalysts, demonstrating that 4-6 carbophene is a metal-free ORR candidate catalyst. In addition, the richly charged carbon atoms on 4-6 carbophene exhibit remarkable charge transfer when binding to the adsorbed intermediates. This results in a strong binding strength. Thus, 4-6 carbophene has favorable ORR catalytic activity. This work not only provides promising low-cost catalysts in pure carbon materials for ORR, promoting the development of metal-free ORR catalysts, but also enhances the understanding of Dirac cone two-dimensional materials for applications in catalysis.

Reduced graphene oxide-supported palladium oxide-MOx for improving the performance of air-cathode microbial fuel cells: Influence of the Sn, Ce, Zn, and Fe precursors

Over the past few decades, microbial fuel cells (MFCs) have attracted significant interest within the realm of renewable energy technologies owing to their capability to generate electricity from low-grade substrates. However, their commercialization and scaling-up are impeded by significant obstacles, including high capital costs, limited durability, and sluggish oxygen reduction reaction (ORR). Here, we demonstrate an environmentally-friendly approach for developing highly efficient electrocatalysts consisting of palladium oxide (PdO) and different transition metal oxides (i.e., tin (IV) oxide (SnO2), ceric oxide (CeO2), iron (III) oxide (Fe2O3), and zinc oxide (ZnO)) embedded within reduced graphene oxide (rGO) framework for improving the efficiency of ORR in neutral medium and MFCs. Among the as-prepared electrocatalysts, the PdO–SnO2/rGO possesses the highest ORR electrochemical catalytic activity in a neutral medium (pH ∼ 7.2) following a four-electron ORR pathway, with a current density slightly lower than that of benchmark Pt/C electrocatalyst. The assessment of MFC performance is conducted by employing activated sludge as the inoculum and glucose as the sole electron donor in the anode chamber of single-chamber, air-cathode MFCs. Consistent with electrochemical performance, the maximum power output of an MFC equipped with PdO–SnO2/rGO as an air cathode is significantly higher than other tested MFCs, reaching 84.6 ± 2.8 mW m−2, which represents ∼ 91 % of the power output of an MFC with Pt/C air cathode. Our results reveal that the incorporation of PdO into transition metal reduced graphene oxide framework has the potential to facilitate their long-term application as air cathodes in MFCs that possess easy fabrication, relatively affordable cost, and outstanding electrochemical catalytic oxygen reduction capability, for enhancing electricity generation from wastewater.

Sustainable Synthesis of Metal-Doped Lignin-Derived Electrospun Carbon Fibers for the Development of ORR Electrocatalysts.

The aim of this work is to establish the Oxygen Reduction Reaction (ORR) activity of self-standing electrospun carbon fiber catalysts obtained from different metallic salt/lignin solutions. Through a single-step electrospinning technique, freestanding carbon fiber (CF) electrodes embedded with various metal nanoparticles (Co, Fe, Pt, and Pd), with 8-16 wt% loadings, were prepared using organosolv lignin as the initial material. These fibers were formed from a solution of lignin and ethanol, into which the metallic salt precursors were introduced, without additives or the use of toxic reagents. The resulting non-woven cloths were thermostabilized in air and then carbonized at 900 °C. The presence of metals led to varying degrees of porosity development during carbonization, improving the accessibility of the electrolyte to active sites. The obtained Pt and Pd metal-loaded carbon fibers showed high nanoparticle dispersion. The performance of the electrocatalyst for the oxygen reduction reaction was assessed in alkaline and acidic electrolytes and compared to establish which metals were the most suitable for producing carbon fibers with the highest electrocatalytic activity. In accordance with their superior dispersion and balanced pore size distribution, the carbon fibers loaded with 8 wt% palladium showed the best ORR activity, with onset potentials of 0.97 and 0.95 V in alkaline and acid media, respectively. In addition, this electrocatalyst exhibits good stability and selectivity for the four-electron energy pathway while using lower metal loadings compared to commercial catalysts.

Fe&Fe2O3 Dual-Decorated on N-Doped Porous Carbon for Oxygen Reduction Reaction

Catalysts for the cathodic oxygen reduction reaction (ORR) are important in fuel cells. Herein, a class of non-precious metal ORR catalyst, namely Fe&Fe2O3 dual-decorated on N-doped porous carbon (Fe&Fe2O3/NC), is prepared from the pyrolysis of a mixture containing Fe2O3/NC, Fe salt and melamine, followed by acid-leaching, where Fe2O3/NC is firstly synthesized from one-step pyrolyzing the precursors of coal, KOH, melamine and Fe salt. As a result, the Fe&Fe2O3/NC provides a much better ORR catalytic activity with the peak potential (Ep) and half-wave potential (E1/2) of 902 mV and 852 mV, respectively, than these of Fe2O3/NC (824 mV and 768 mV) and close to these of 20 wt% commercial Pt/C (918 mV and 863 mV). Furthermore, the ORR occurred on Fe&Fe2O3/NC follows near a four-electron transfer pathway. It also has a good stability and methanol-resistance. By comparing the structures of Fe2O3/NC and Fe&Fe2O3/NC, the essential roles on improving the ORR catalytic activity are discussed: both the abundant graphitic N active sites and the synergistic effect of Fe and Fe2O3 play important roles. This work not only provides a general guideline for the design and development of non-precious metal-based catalysts for ORR but also enriches the application scope of coal.

MXene induced two-electron oxygen reduction of Pd for H2O2 generation

The production of H2O2 by oxygen reduction reaction (ORR) is low energy consumption and environmentally friendly. Pd/C is an effective electrocatalyst for oxygen reduction, while its selectivity is low due to a tendentious four-electron pathway ORR process. The limited reserves of palladium and inevitable corrosion issue of carbon support add extra obstacles toward practical application. In this work, Pd nanoparticles with 3–5 nm were synthesized on MXene sheets. Results indicated that Pd/MXene exhibited 3.5∼4.2 times ring current than Pd/C and much higher H2O2 selectivity. The H2O2 yield of Pd/MXene was measured to be 1.43 mol g−1 h−1. The excellence in electrochemistry was from the synergistic effect of Pd and MXene. The existence of MXene increases metallic Pd on the surface. This study on MXenes as supports of Pd nanoparticles for two-electron oxygen reduction provides extensive reference significance to H2O2 generation.

A High-Nuclearity Copper Sulfide Nanocluster [S-Cu50] Featuring a Double-Shell Structure Configuration with Cu(II)/Cu(I) Valences.

This work represents an important step in the quest for creating atomically precise binary semiconductor nanoclusters (BS-NCs). Compared with coinage metal NCs, the preparation of BS-NCs requires strict control of the reaction kinetics to guarantee the formation of an atomically precise single phase under mild conditions, which otherwise could lead to the generation of multiple phases. Herein, we developed an acid-assisted thiolate dissociation approach that employs suitable acid to induce cleavage of the S-C bonds in the Cu-S-R (R = alkyl) precursor, spontaneously fostering the formation of the [Cu-S-Cu] skeleton upon the addition of extra Cu sources. Through this method, a high-nuclearity copper sulfide nanocluster, Cu50S12(SC(CH3)3)20(CF3COO)12 (abbreviated as [S-Cu50] hereafter), has been successfully prepared in high yield, and its atomic structure was accurately modeled through single-crystal X-ray diffraction. It was revealed that [S-Cu50] exhibits a unique double-shell structural configuration of [Cu14S12]@[Cu36S20], and the innermost [Cu14] moiety displays a rhombic dodecahedron geometry, which has never been observed in previously synthesized Cu metal, hydride, or chalcogenide NCs. Importantly, [S-Cu50] represents the first example incorporating mixed Cu(II)/Cu(I) valences in reported atomically precise copper sulfide NCs, which was unambiguously confirmed by XPS, EPR, and XANES. In addition, the electronic structure of [S-Cu50] was established by a variety of optical investigations, including absorption, photoluminescence, and ultrafast transient absorption spectroscopies, as well as theoretical calculations. Moreover, [S-Cu50] is air-stable and demonstrates electrocatalytic activity in ORR with a four-electron pathway.

Heme-Based Composites as Cathode Catalysts for Lithium-Oxygen Batteries and Analysis of the Reaction Mechanism

Despite the high theoretical energy density of lithium-oxygen (Li-O2) batteries, the charge/discharge efficiency is unsatisfactory. To overcome this critical problem, we synthesized heme-graphene composites (HEME-GO) as catalysts for Li-O2 batteries. The introduction of graphene produced π-π interactions with the heme matrix, resulting in a composite with enhanced ORR/OER catalytic activity. The free energy diagram of the redox reaction was calculated using density functional theory (DFT) for HEME-GO based on the four-electron reaction pathway, and it was demonstrated that HEME-GO has the lowest ORR overpotential (0.67 V) and OER overpotential (0.53 V). The catalytic mechanism of HEME-GO was also quantitatively described by calculating the adsorption energy of intermediates in the rate determining step (RDS). In addition, the Li-O2 batteries with the composite catalyst exhibited better cycling performance, discharge capacity (7770 mAh g−1), and lower overpotential due to the ability of heme to scavenge superoxide radicals and thus protect the electrode. The results in this paper contribute to the understanding of the redox process of Li-O2 batteries for organic systems and suggest innovative ideas for the design of environmentally friendly batteries.

Density functional theory investigation of the role of Fe doped in boron carbide nanotube as an electro-catalyst for oxygen reduction reaction in fuel cells

A critical issue in enhancing the performance of polymer electrolyte membrane fuel cells (FCs) is the slow kinetics of the cathodic oxygen reduction reaction (ORR). The development of electrocatalysts with selectivity toward the four-electron (4e) pathway and high electrochemical activity to ORR reaction is important for fuel cell applications. Within the present study, it was found that boron carbide nanotube (BC3NT) is an encouraging ORR-EC based on density functional theory computations. In the pristine BC3NT, the neighboring B atoms with positive charges on the surface of the material surface were incapable of providing active sites for the dissociation of O. However, the ORR catalytic activity of BC3NT improved under the ligand effect due to the replacement of Fe atom, where there was a slight over-potential that was similar or lower than that of platinum (111), which demonstrated its superior ORR activity. The results suggest that BC-based materials are considered promising for ORR catalysis and for designing highly efficient ORR-ECs as alternatives to platinum-based catalysts.