Oxygen Reduction Catalysts Research Articles

High-performance intermetallic PtCo oxygen reduction catalyst promoted by molybdenum

We report the top-down modification of intermetallic platinum-cobalt (Pt-Co) nanoparticle catalyst with molybdenum (Mo) promoters that significantly enhance its oxygen reduction activity. With an optimized coverage of the Mo promoters, X-ray absorption spectroscopy reveals the cluster-like Mo species effectively weakened the binding affinity of Pt surfaces towards oxygen. The Mo-promoted intermetallic Pt-Co catalyst thus exhibits high ORR mass activity of 0.89 A/mgPt compared to 0.22 A/mgPt and 0.62 A/mgPt for pure Pt and Pt-Co respectively under rotating disk electrode testing conditions. The high ORR activity further enabled the operation of the fuel cell membrane electrodes under low cathode relative humidities with minimal penalties in ionic transport resistance at low current regions. Density functional theory calculations reveal the beneficial role of molybdenum oxide cluster species in lowering the d-band energy surface Pt atoms, particularly at Pt segregated regions with no Pt-Co phase in proximity to induce the strain or ligand effects.

A co-doped oxygen reduction catalyst with FeCu promotes the stability of microbial fuel cells

Air cathode microbial fuel cell (AC-MFC) cannot be used on a large scale because of its low oxygen reduction reaction (ORR) efficiency. Despite the fact that bimetallic catalysts can greatly enhance the oxygen reduction rate by regulating the electronic structure of the active site, the flaws of insufficient exposure of the active site and easy metal agglomeration limit its catalytic activity. Herein, we report on the preparation of a stable heteroatomic substrate using a copper material organic framework as a precursor, covered by Fe-based active sites. As a result of dipole-dipole interactions, the reduced product Fe2+ forms a weak Fe-O surface that is conducive to the adsorption of active substances. The presence of Fe0 enhances the electrical conductivity of the catalytic, thus promoting ORR efficiency. Through redox coupling, the D-band center of Fe at FeCu@CN is optimized and brought close to the Fermi level to facilitate electron transfer. Notably, FeCu@CN demonstrates a superior power density of 2796.23 ± 278.58 mW m−3, far exceeding that of Pt/C (1363.93 ± 102.56 mW m−3), in the application of microbial fuel cells (MFCs). Meanwhile, the MFC-loaded FeCu@CN maintains excellent stability and outstanding output voltage after 1000 h, which provides feasibility for large-scale application.

(Invited) Atomically Dispersed M-N-C Catalysts for Oxygen Reduction Reactions: Understanding Degradation and Improving Durability

In the past decade, there has been significant progress in improving activity of atomically dispersed MN4 (M=Fe, Co, Mn, etc.) type catalysts for oxygen reduction. However, there is a significant challenge related to its poor durability. In this presentation, we will discuss our recent work on synthesizing highly active MN4 catalysts, understanding their degradation mechanisms. We will present our technical solutions to address the durability challenge.

Stable Co/N-Doped Carbon Nanotubes as Catalysts for Oxygen Reduction

It is an urgent and difficult task to develop low-cost, high-performance catalysts for the oxygen reduction reaction (ORR) to overcome the inherent defects of platinum-based catalysts, including low abundance, high cost, poor stability, and poor methanol tolerance. In this study, N-doped carbon nanotubes (N-CNTs) and Co/N-CNT catalysts are synthesized by grinding and subsequent calcination, and the whole process does not require strict control of the reaction conditions, thus greatly simplifying the synthesis route of the catalysts. The unique structure, in which Co nanoparticles are evenly embedded in the tube and tip of CNTs, exhibits outstanding ORR performance and electrochemical stability as a result of the Co–CNT interactions. Furthermore, due to the catalytic graphitization ability of Co nanoparticles, a high graphitization degree, large specific surface area, and abundant pore structure are achieved by adjusting the calcination temperature, thus accelerating electron transfer and oxygen diffusion. The optimized catalyst, calcined at 750 °C (Co/N-CNT-750), exhibits better ORR activity and excellent methanol tolerance in alkaline media, indicating its great potential for large-scale application in storage technologies.

(Digital Presentation) Effects of Foreign Elements Added Oxide-Based Electrocatalyst for Oxygen Reduction Reaction as Non-Precious Metel Cathodes

On Dec. 2020, new MIRAI of fuel cell vehicle (FCV) from Toyota started to sell in Japan. However, the price of new model is almost same as previous model. For wide commercialization of FCV, it should be required for more development. One of the barriers of innovative system for the FCV is conventional platinum-based electrocatalyst. The cost of Pt is very expensive due to its very poor resources. Moreover, the conventional Pt-based electrocatalyst cannot reach enough performance for next generation FCV that is proposed in new roadmap released from NEDO, Japan. From this point of view, the non-precious metal electrocatalyst should be required for the breakthrough against above issue. We have focused and studied group 4 and 5 metal oxide-based electrocatalyst as non-platinum catalysts for the oxygen reduction reaction (ORR) because of low-cost, abundant reserves, and high stability in acidic electrolytes [1-2]. We found that titanium oxide prepared from TiOTPyzPz supported on multi-walled carbon nanotubes had superior ORR activity [3]. On the other hand, it was published as an international patent that the addition of other elements such as Fe and Ni is affected to enhance the ORR activity of Ti oxide-based electrocatalyst [4]. In this study, we have investigated to apply for the TiOTPyzPz as a starting material with and without Fe, Ni, and Zn addition to enhance the ORR activity of Ti oxide-based electrocatalyst.2,3-Dicyanopyrazine, urea, and Ti isopropoxide were dissolved in quinoline and refluxed to synthesize TiOTPyzPz. Iron acetate, Nickel acetate, and Zinc acetate were also added to dissolve in quinoline to obtain the Fe, Ni, and Zn-added TiOTPyzPz as a starting material. The molar ratio of Ti:Fe:Ni:Zn was set to constant. These starting materials were mixed with carbon nanotube by ball-milling to prepare the precursors. These precursors were heat-treated under low oxygen partial pressure for 3 h to obtain titanium oxide-based catalysts. The catalyst powder was dispersed into 1-propanol with Nafion solution to prepare a catalyst ink. The ink was dropped on a glassy carbon rod, and dried for an hour to use as a working electrode in electrochemical measurement.Electrochemical measurements were performed in 0.5 mol dm-3 H2SO4 at 30 oC with a conventional 3-electrode cell. A reversible hydrogen electrode (RHE) and a glassy carbon plate were used as used as a reference and counter electrode, respectively. Slow scan voltammetry (SSV) was performed at a scan rate of 5 mV s-1 from 0.2 V to 1.2 V vs. RHE under O2 and N2. The ORR current (i ORR) was determined by calculating the difference between the current under O2 and N2.Figure 1 shows the effect of Fe, Ni, and Zn addition to TiOTPyzPz as a starting material on the ORR polarization curves of the titanium oxide-based catalysts. The vertical axis is based on catalyst weight. Ti oxid-based electrocatalyst prepared from Fe, Ni, and Zn addition to TiOTPyzPz (TiOx-Fe, Ni, Zn) resulted in the increase in the onset potential for the ORR and ORR current compared to that without addition (TiOx), revealing that the addition of Fe, Ni, and Zn was found to be effective in improving the ORR activity. XRD pattern of TiOx shows several peaks identified TiO2-Rutile, TiO2-Anatanse and TiO2-Brookite while the XRD pattern of TiOx-Fe, Ni, Zn also shows several peaks identified similar to TiOx. In the case of XRD pattern of TiOx-Fe, Ni, Zn, the peak identified compounds relating to additional elements such as Fe, Ni and Zn does not detect strongly. It is suggested that the addition of Fe, Ni and Zn to TiOTPyzPz conduce to the distortion of crystal phase of TiO2 [5] and it affected to suppress the crystal growth of Ti oxide-based electrocatalyst. These facts are possibly contributed to enhance the ORR activity of TiOx-Fe, Ni, Zn compared to that of TiOx.Acknowledgement: The authors thank New Energy and Industrial Technology Development Organization (NEDO) and ENEOS Tonen General Research / Development Encouragement & Scholarship Foundation for financial support.Reference[1] A. Ishihara, Y. Ohgi, K. Matsuzawa, S. Mitsushima, and K. Ota, Electrochim. Acta, 55, 8005 (2010).[2] A. Ishihara, S. Tominaka, S. Mitsushima, H. Imai, H. Imai, O. Sugino, and K. Ota, Curr. Opin. Electrochem., 21 , 234 (2020).[3] S. Tominaka, A. Ishihara, T. Nagai, and K. Ota, ACS Omega, 2, 5209 (2017).[4] K. Takahashi, T. Imai, R. Monden, Y. Wakisaka, and S. Sato, Oxygen Reduction Catalyst, Process for Producing Same, and Polymer Electrolyte Membrane Fuel Cell, WO/2013/008501.[5] Y. Yamamoto, S. Kasamatsu, and O. Sugino, J. Phys. Chem. C, 123, 19486 (2019). Figure 1

Journey of Fe-N4 Sites from Birth in Furnace till Death in Cell

One central topic in the Fe-N-C electrocatalysis for the oxygen reduction reaction (ORR) is the nature of the active site(s) responsible for the superior ORR activity of Fe-N-C electrocatalysts in acidic media. Significant progress has been made in this regard in the past decade, thanks to the employment of advanced in situ/operando characterization techniques including Mossbauer and X-ray absorption spectroscopy (XAS). The key findings are that the local structure of at least one type of Fe-N4 sites changes dramatically from produced in the furnace, exposed to air, and immersed in electrochemical media in aqueous solution or proton exchange membrane fuel cells (PEMFCs) with applied potentials. Despite these new findings, what the local structure of Fe-N4 sites is when they are catalyzing the ORR in acidic media, assuming Fe-N4 sites as the primary ORR-active centers of Fe-N-C electrocatalysts, remains unclear. Another mystery is the cause(s) of the rapid degradation of Fe-N4 sites during PEMFC operation.Herein, we walk through a possible journey of Fe-N4 sites from being produced by pyrolysis till decomposition in PEMFCs. In particular, we propose a new local structure of the exposed Fe-N4 site at lower potentials in acidic media: (H2O)2-Fe(II)-N4 with an octahedral structure subject to Jahn-Teller distortions. Specifically, in-plane Fe(II)-N4 sites are formed upon high-temperature (> 600 ℃) pyrolysis. Those Fe(II)-N4 sites accessible by air get spontaneously oxidized forming (O)2-Fe(III)-N4 ((O)2 represents the two axial oxygen ligands rather than one oxygen molecule) when exposed to air. The (O)2-Fe(III)-N4 structure is most likely also the form of the exposed Fe-N4 sites in acidic media at elevated potentials ( > 0.8 V versus reversible hydrogen electrode (RHE). As the potential scans cathodically across the Fe(III)/Fe(II) redox potential, the (O)2-Fe(III)-N4 structure transforms to the (H2O)2-Fe(II)-N4 structure with the axial ligands of (O)2 replaced by water molecules with elongated Fe-OH2 bonds due to the Jahn-Teller effect on high-spin Fe(II) octahedral complexes. This high-spin (O)2-Fe(III)-N4 structure was inferred from the combination of the presence of two Fourier-Transform of extended X-ray absorption fine structure (FT-EXAFS) peaks of several Fe-N-C catalysts1,2 at lower potentials; the low intensity of the pre-edge peak in the X-ray absorption near edge structure (XANES) spectra; with in situ Mossbauer results.3 In contrast, the Jahn-Teller effect is absent on high-spin Fe(III) octahedral complexes such as (O)2-Fe(III)-N4, correspondingly only one Fe-N/O FT-EXAFS peak was observed at elevated potentials or ex situ. The H2O molecule(s) in the (H2O)2-Fe(II)-N4 is replaceable by O2 molecules during the ORR. The exposed Fe-N4 sites transform to inorganic iron species such as iron oxides during PEMFC operation according to the recent operando Mossbauer results,4 which accounts partly for the degradation of Fe-N-C catalysts during the ORR. Acknowledgements This work was supported by the US Department of Energy under award number DE-EE0008416. We acknowledge the support from the DOE, Energy Efficiency and Renewable Energy, Hydrogen and Fuel Cell Technologies Office through the Electrocatalysis Consortium (ElectroCat) and the DOE programme and technology managers, D. Papageorgopoulos, D. Peterson and N. Garland. This research used beamline 7-BM and 8-ID of the National Synchrotron Light Source II, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Brookhaven National Laboratory under Contract No. DE-SC0012704. In-temperature XAS experiments were performed at the MRCAT in Advanced Photon Source (APS), a DOE Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357 by UChicago Argonne, LLC. Jiao, L. et al. Chemical vapour deposition of Fe–N–C oxygen reduction catalysts with full utilization of dense Fe–N4 sites. Mater. 20, 1385-1391 (2021).Deng, Y. et al. g-C3N4 promoted MOF derived hollow carbon nanopolyhedra doped with high density/fraction of single Fe atoms as an ultra-high performance non-precious catalyst towards acidic ORR and PEM fuel cells. Journal of Materials Chemistry A 7, 5020-5030 (2019).Zelenay, P. ElectroCat (Electrocatalysis Consortium). Report No. LA-UR-20-24045 United States 10.2172/1631569 LANL English, Medium: ED; Size: 67 p. (; Los Alamos National Lab. (LANL), Los Alamos, NM (United States), 2020)Li, J. et al. Identification of durable and non-durable FeNx sites in Fe–N–C materials for proton exchange membrane fuel cells. Catal. 4, 10-19 (2021). Figure 1

Deconvoluting Transport and Kinetics on Ionic Liquid-Modified Fe Catalysts for Oxygen Reduction

Introduction Hydrogen fuel cells could play a key role in the decarbonization of the energy sector. However, their commercialization is hindered by the sluggish kinetics of the oxygen reduction reaction (ORR) and by the requirement of platinum, which is expensive and unsustainable. Alternative catalysts based on transition metals have proven promising, but their activity and durability is still far from that of platinum.Recently, thin layers of ionic liquids have been successfully used to improve both the durability and activity of noble-metal free ORR catalysts[1]. As shown in Figure 1, the presence of the ionic liquid layer can influence the reaction kinetics through multiple effects: (i) O2 transport, (ii) water transport (iii) proton transport and (iv) binding to the reaction intermediate.These competing effects are generally convoluted. To that end, we have recently developed models to quantify the effect of the ionic liquid on both oxygen transport and reaction kinetics. Effect of Ionic liquids on reaction kinetics The ionic liquids tested for oxygen reduction are hydrophobic and their degree of water uptake is largely controlled by the cation. By decreasing the concentration of water at the active sites, they can cause the dehydration of the *OH intermediate, ultimately weakening the *OH bond[2]. For those catalysts such as Pt, which sit on the strong binding side of the volcano, this weakening of OH causes an increase in activity[3].To the contrary, we have now shown that ionic liquid layers decrease the activity of catalyst on the weak binding side of the volcano, such as iron phtalocyanine (see Figure 2). By changing ionic liquid, we can control hydrophobicity and by monitoring the position of the voltammetric peak for *OH adsorption we can probe the strength of *OH binding. In this way, we are able to confirm the earlier hypothesis that hydrophobicity controls *OH binding[2]. We hence provide a new lever for tailoring the reaction kinetics of oxygen reduction for any transition metal catalyst using ionic liquids. Oxygen Transport in ionic liquid layers The ideal ionic liquid should transport oxygen quickly, while featuring high oxygen solubility. However, it has been shown that O2 solubility and diffusivity cannot be independently tuned, as, for example, fluorination of the anion improves oxygen concentration, at the expense of diffusivity[4]. The optimum balance between these two parameters has been so far unclear. To that end, we have developed a way to deconvolute the effect of oxygen solubility and diffusivity.Using gravimetric, volumetric and electrochemical techniques, we reliably characterized oxygen transport in ionic liquids, and we were able to experimentally distinguish between oxygen activity and concentration. By using Fick’s diffusion and the Faraday law, we obtained a prediction for the diffusion-limited current in a rotating disc-electrode, as a function of the nature of the ionic liquid, its thickness and the rotational speed. As shown in in Figure 3, the diffusion-limited current density correlates to the permeability of oxygen in the ionic liquid layer; with our experimental data confirming the model.When looking at the diffusion-limited region, this model leads to the trivial conclusion that oxygen transport can be improved by maximizing oxygen permeability in the ionic liquid and increasing the thickness to that of the diffusion layer. However, real fuel cell devices are operated at a potential where both oxygen transport and kinetics are rate-limiting.In this case, the reaction rate is proportional to oxygen activity in the ionic liquid. Using the model developed, we were able to conclude that oxygen activity should be maximised over transport and that a single monolayer is the ideal ionic liquid coverage.Finally, we used a sorption analyser to study the ionic liquid distribution on the surface of the catalyst. We observed that rather than forming the ideal monolayer, the ionic liquid fills up the smallest pores first and tends to block bigger pores. Our current work is focusing on improving catalyst wettability by the ionic liquid to approach the ideal distribution. Conclusion In conclusion, the cation and anion of ionic liquids can be independently tailored to control hydrophobicity on one side and O2 activity on the other. This can ultimately allow to optimize the binding energy of key intermediates and maximize the reactant concentration at the active site. Our approach provides a rational method to improve the activity of noble-metal-free oxygen reduction catalysts. [1] Favero et al., Adv. Energy Sustainability Res. 2021, 2, 2000062 [2] Casalongue et al., Nat. Commun., 2013, 4, 2817 [3] Huang et al., J. Electrochem. Soc., 2017, 164, F1448 [4] Vanhouette et al., RCS Adv., 2018, 8, 4525 Figure 1

Novel Way to Turn Spent Li-Ion Battery Graphite into Valuable and Active Catalyst for Electrochemical Oxygen Reduction

Battery technology has been recognized as one of the main driving forces and key technologies required to create a carbon-neutral economy and, therefore, batteries are currently one of the most rapidly growing technology sectors due to the rapid rise in the number of electric vehicles (EV´s). Additionally, the employment of Li-ion batteries (LIBs) in portable electronics has become a necessity in the modern world. This produces spend Li-ion batteries (SLIBs) in substantial volumes, it was estimated that in 2019 188,000 tonnes of SLIBs were available for recycling and it is expected that this number will rise to 125 million in 2030.1,2 To obtain the full benefits of electrification the value chain associated with batteries needs to become more sustainable, which strongly depends on the ability of SLIBs recycling to maximize the components return back to materials circulation. However, current SLIBs recycling strategies has been focused on cathode materials alone while largely overlooking the anode of the battery. In turn, anode recycling has so far concentrated mostly on the metal in the electrode and only very recently has the focus shifted towards the recycling of graphite powders.In this work, a novel strategy for recycling SLIB graphite and reforming it as a valuable catalyst material for electrochemical oxygen reduction reaction (ORR) is proposed. A modified Hummers method was applied to fabricate graphene oxide (GO) from SLIB graphite powder. The GO reduction and doping with nitrogen was achieved by pyrolyzing sample at 800 °C in the presence of DCDA. The prepared nitrogen-doped graphene (NG-Bat) was characterised by various physicochemical (SEM, TEM, XRD, Raman and XPS) and electrochemical characterization methods (LSV, EIS, chronoamperometry). For electrochemical characterisation RDE setup with the glassy carbon working electrode, modified with studied catalyst, in 0.1 M KOH solution was used. The physical and electrochemical characteristics of NG-Bat were compared with commercially available nitrogen doped graphene (NG-Com) sample.Based on XRD and Raman data, SLIB-derived graphite maintains its high quality, even after intense exploitation in the batteries and is therefore a valuable material that is worth recycling.3 Homogeneous distribution of carbon, nitrogen, and oxygen in NG-Bat sample was achieved and the overall nitrogen content was found to be 18.4 at%, based on XPS analysis. The onset potential of the ORR for the NG-Bat catalyst material is 0.867 V, and for the NG-Com catalyst 0.797 V. The NG-Bat shows a larger oxygen reduction current and higher electrocatalytic activity than the catalyst based on NG-Com (Figure 1). Based on EIS fitting very different charge transfer resistance and double-layer capacitance values are found, probably caused by different conductivities of the catalyst materials. NG-Bat showed much high electrocatalytic activity towards the ORR than commercial NG-Com based-catalyst material, caused probably by a higher content of active nitrogen species and the presence of carbon vacancies on the surface of graphene, as confirmed by XRD, Raman and XPS experiments.The findings demonstrate that SLIB graphite is still a valuable material and a promising precursor for GO fabrication. One of the potential applications of the SLIB-derived graphene could be the production of catalyst material for the oxygen reduction reaction (ORR), thereby open new avenues to the fuel cell, metal-air battery, and sensor industries.

Tunning the Activity of Silver Alloy Electrocatalysts for the Oxygen Reduction Reaction in Alkaline Media

A bottleneck to achieve efficient and stable electrocatalysts for Anion Exchange Membrane Fuel Cells (AEMFCs) is the relatively slow kinetics of the oxygen reduction reaction (ORR) taking place at the cathode. The rather mild alkaline conditions in the AEMFC enable the utilization of various PGM-free catalysts, which brings up new possibilities of finding abundant and inexpensive electrocatalysts without compromising the power density. From theory and previous experiments, it is known that Au and Ag bind oxygen weakly, whereas other transition metals (e.g., Pd, Cu, Fe, Ni) bind strongly to oxygenated species1. Therefore, a suitable geometric arrangement between Ag, an abundant metal with relatively high specific activity, and other metals may give rise to optimum binding of oxygen and thus a high activity. A good strategy to improve the activity of Ag-based catalysts could be a rational design of alloys that can enhance the intrinsic activity of Ag by both ensemble and electronic effects2–4. The former is based on the combination of active sites that facilitate the initial oxygen binding together with Ag active sites that help desorb hydroxide anions, whereas the latter is induced by the alloying metal (which binds oxygenated species strongly) on the electronic structure of the overlaying Ag that tunes its binding to oxygen5. Herein, we report the fabrication and characterization of Ag thin-films alloyed with Pd, Ni, Cu and Mo as a tool to investigate the ORR reaction in terms of ensemble and electronic effects and thus relate alloy composition and geometric arrangement with ORR activity. Ag thin-film model catalysts are fabricated by means of physical vapor deposition methods (PVD) with high control over the amount of material deposited. By varying alloy composition and evaluating electrochemical activity and stability, together with detailed physical characterization, the degree and extent of electronic effects are elucidated. In the same way, sputtering of the same amount of Ag on thin-films of different metals evidences the role of the alloying metal on tuning the activity of Ag through an adjustment of its d-band that changes the binding of oxygenated species, as well as through an ensemble effect that provides active sites for the first elementary steps of the reaction. These results provide insights for more tailored design of electrocatalysts in alkaline media by shedding new light on the mechanisms through which the ORR kinetics are improved in alkaline media. Nørskov, J. K. et al. Origin of the overpotential for oxygen reduction at a fuel-cell cathode. J. Phys. Chem. B 108, 17886–17892 (2004).Betancourt, L. E. et al. Enhancing ORR Performance of Bimetallic PdAg Electrocatalysts by Designing Interactions between Pd and Ag. ACS Appl. Energy Mater. 3, 2342–2349 (2020).Slanac, D. A., Hardin, W. G., Johnston, K. P. & Stevenson, K. J. Atomic ensemble and electronic effects in Ag-rich AgPd nanoalloy catalysts for oxygen reduction in alkaline media. J. Am. Chem. Soc. 134, 9812–9819 (2012).Zamora Zeledón, J. A. et al. Tuning the electronic structure of Ag-Pd alloys to enhance performance for alkaline oxygen reduction. Nat. Commun. 12, 1–9 (2021).Stamenkovic, V. et al. Changing the Activity of Electrocatalysts for Oxygen Reduction by Tuning the Surface Electronic Structure. Angew. Chemie 118, 2963–2967 (2006).

Highly Stable Pt-Zn Intermetallic Oxygen Reduction Catalysts Supported on MOF-Derived Carbon for Proton Exchange Membrane Fuel Cells

Commercialization of polymer electrolyte membrane fuel cells (PEMFCs) requires a dramatic reduction in the price of catalysts, which account for a significant portion of the total cost. Therefore, since most catalysts in PEMFCs are present in the cathode where the oxygen reduction reaction (ORR) occurs, it is necessary to ensure sufficient durability after the reaction and activity of the catalysts for the ORR. In this work, using metal-organic framework (MOF)-based carbon supports, platinum (Pt) catalysts were converted into platinum-zinc (Pt-Zn) intermetallic catalysts to enhance the durability of the catalysts, moreover, the additional interaction was introduced between nanoparticles and supports, improving the stability of the catalyst. High-magnification transmission electron microscopy and X-ray diffraction analysis confirmed the ordered structure of intermetallic Pt-Zn nanoparticles. In the half-cell test, the mass activity and the specific activity of the L10-PtZn/Zn-NC catalyst were 3.8 and 5.7 times as great as that of commercial Pt/C, respectively. Furthermore, to evaluate the stability of the catalysts while the ORR condition, we conducted the durability cycles from 0.6 V to 1.0 V versus RHE. After the durability cycles of 30k, the electrochemically active surface areas (ECSAs) of the L10-PtZn/Zn-NC(103.1% of the initial ECSA) showed significantly enhanced stability compared to that of the commercial Pt/C(66.2% of the initial ECSA). This work provides us a new strategy to obtain the high stability of the intermetallic catalysts as well as the enhanced activity using MOF-derived carbon support.

(Keynote) Identification of Active Species and Mechanisms in Non-Precious Metal Oxygen Reduction Catalysts By Reductive Treatments and Magnetic Measurements

This talk addresses the Oxygen Reduction Reaction (ORR) utilizing Fe- and Co-based non precious metal (NPM) catalysts. There has long been uncertainty as to the active site for ORR in these materials due to the heterogeneity of the as-prepared catalyst. We report on two innovations directed at understanding the active sites in these materials. In the first, we interrogate the ORR activity in acid of a Fe porphyrin on different supports. While the activity is high when the Fe porphyrin is adsorbed on XC72 (a graphitic carbon), this activity is much lower when the porphyrin is adsorbed on either MoS2 or g-C3N4. Numerous physical characterization techniques show the electronic structure around the Fe center is the same for all three supports. Only the Fe porphyrin supported on XC72 exhibits a pH dependence in its ORR activity. This observation, coupled with the increased hydrophilicity of XC72 relative to the other supports, suggests that the support-electrolyte interaction controls the ORR activity. Modification of MoS2 to increase its hydrophilicity results in a more active ORR catalyst. In the second area, we use magnetometry and other techniques to examine the effect of two different reductive gas treatments on pyrolyzed ORR catalysts. We find that reductive gas treatments yield more superparamagnetic/ferromagnetic Fe species, with specific preparations yielding materials that consist almost entirely of reduced Fe (particularly Fe4N). Interestingly, catalysts exhibiting large percentages of superparamagnetic/ferromagnetic materials exhibit high activities for ORR catalysis as high as those reported in the literature.

Alloyed Pt–Zn Oxygen Reduction Catalysts for Proton Exchange Membrane Fuel Cells

Better oxygen reduction catalysts are needed to appreciably increase the efficiency of proton exchange membrane fuel cells. In this work, high-activity Pt–Zn alloy catalysts were prepared using two distinct synthetic approaches. The first─where Zn was introduced to Pt nanoparticles via atomic layer deposition─led to a 30% increase in Pt-mass-normalized fuel cell activity. Density functional theory calculations elucidated the origin of this enhancement and motivated the preparation of an alloy with increased Zn content. To this end, a fabrication technique leveraging Zn electroplating was employed to prototype a second Pt–Zn alloy with a rotating disk electrode (RDE). The higher-dosed structure (∼19 at. % Zn) delivered exemplary activity (MA = ∼3 A/mgPt), but the synthetic approach was not amenable to membrane electrode assembly (MEA) fabrication. An Arrhenius analysis was carried out to project the hypothetical catalytic enhancement under fuel cell operating conditions. This exercise introduced a discussion on the reduced activity observed in MEAs relative to the benchtop RDE setups used more ubiquitously for catalyst evaluation.

2D Single-Atom Fe–N–C Catalyst Derived from a Layered Complex as an Oxygen Reduction Catalyst for PEMFCs

Fe single-atom catalysts of oxygen reduction reaction (ORR) are restricted by the agglomeration during the synthesis process and inferior stability, especially in acidic conditions. An efficient synthesis strategy is urgently needed to alleviate these disadvantages. In this work, a two-dimensional (2D) single-atom Fe–N–C catalyst derived from a layered complex was designed and synthesized for the ORR. Fe single atoms dispersed on 2D hierarchical porous N-doped carbon nanosheets (Fe–N–C) were derived from a layered complex through the coordination of Fe3+ and benzidine hydrochloride. The unique 2D hierarchical porous nanosheets with a special edge effect can not only provide a large specific surface area and promote the mass transfer of ORR but also facilitate the affinity of Fe single atoms. Furthermore, the well-distributed Fe single atoms and Fe–Nx–C structure can increase the utilization rate of metal atoms and enhance the catalytic activity of materials. As expected, the catalyst shows superior ORR performance and excellent electrochemical stability.

Atomically Dispersed CoN3C1‐TeN1C3 Diatomic Sites Anchored in N‐Doped Carbon as Efficient Bifunctional Catalyst for Synergistic Electrocatalytic Hydrogen Evolution and Oxygen Reduction (Small 29/2022)

Atomically Dispersed CoN₃C₁‐TeN₁C₃ Diatomic Sites Anchored in N‐Doped Carbon as Efficient Bifunctional Catalyst for Synergistic Electrocatalytic Hydrogen Evolution and Oxygen Reduction (Small 29/2022)

Atomically dispersed iron sites with a nitrogen–carbon coating as highly active and durable oxygen reduction catalysts for fuel cells

Atomically dispersed iron sites with a nitrogen–carbon coating as highly active and durable oxygen reduction catalysts for fuel cells

Hexamethylenetetramine-derived pyridinic N abundant porous carbon-supported Co/Co-Nx nanoparticles as highly efficient oxygen reduction catalyst and zinc-air battery cathode

Fuel cells and metal-air batteries attracted extensive research attention owing to high power density, renewability, and environmental friendliness. Hence, developing heteroatom-doped pyridinic N abundant porous carbon materials with large surface area have an extensive effect on oxygen reduction reaction (ORR) for fuel cell and metal-air battery. Herein, high content of pyridinic N porous carbon-supported Co/Co-Nx nanoparticles were synthesized as ORR catalysts by carbonizing complexes of hexamethylenetetramine (HMT) and Co(NO3)2·6H2O with silica template. The HMT-Co@SiO2-900 with high content of pyridinic N, large specific surface area, and porous structure exhibits a more positive half-wave potential (0.85 V) than 20% Pt/C and the outstanding performance oxygen evolution reaction behavior. The low Tafel slope of HMT-Co@SiO2-900 revealed the faster reaction kinetics during ORR. The HMT-Co@SiO2-900 zinc-air battery showed a high excellent cyclic stability in continuous charge/discharge cycles for 420 times at 10 mA/cm2. This research provides an efficient strategy for the preparation of pyridinic N porous electrocatalyst by the reduction of diffusion restriction and exposure of active sites.

Gram-Scale Synthesis of Carbon-Supported Sub-5 nm PtNi Nanocrystals for Efficient Oxygen Reduction

The preparation of a high performance and durability with low-platinum (Pt) loading oxygen reduction catalysts remains a challenge for the practical application of fuel cells. Alloying Pt with a transition metal can greatly improve the activity and durability for oxygen reduction reaction (ORR). In this work, we present a one-pot wet-chemical strategy to controllably synthesize carbon supported sub-5 nm PtNi nanocrystals with a ~3% Pt loading. The as-prepared PtNi/C-200 catalyst with a Pt/Ni atomic ratio of 2:3 shows a high oxygen reduction activity of 0.66 A mgpt−1 and outstanding durability over 10,000 potential cycles in 0.1 M KOH in a half-cell condition. The PtNi/C-200 catalyst exhibits the highest ORR activity, with an onset potential (Eonset) of 0.98 V and a half-wave potential (E1/2) of 0.84 V. The mass activity and specific activity are 3.89 times and 9.16 times those of 5% commercial Pt/C. More importantly, this strategy can be applied to the gram-scale synthesis of high-efficiency electrocatalysts. As a result, this effective synthesis strategy has a significant meaning in practical applications of full cells.

Construction of N, P Co-Doped Carbon Frames Anchored with Fe Single Atoms and Fe2 P Nanoparticles as a Robust Coupling Catalyst for Electrocatalytic Oxygen Reduction.

A coupling catalyst of highly dispersed N, P co-doped carbon frames (NPCFs) anchored with Fe single atoms (SAs) and Fe2 P nanoparticles (NPs) is synthesized by a novel in situ doping-adsorption-phosphatization strategy for the electrocatalytic oxygen reduction reaction (ORR). The optimized Fe SAs-Fe2 P NPs/NPCFs-2.5 catalyst shows a superior ORR activity and stability in 0.5 m H2 SO4 and 0.1 m KOH, respectively. Theoretical calculations reveal a synergistic effect, in that the existence of Fe2 P weakens the adsorption of ORR intermediates on active sites and lowers the reaction free energy. The doped P atoms with a strong electron-donating ability elevate the energy level of Fe-3d orbitals and facilitate the adsorption of O2 . The active Fe atoms exist in a low oxidation state and are less positively charged, and they serve as an electron reservoir capable of donating and releasing electrons, thus improving the ORR activity. Operando and in situ characterization results indicate that the atomically dispersed FeN4 /FeP coupled active centers in the Fe SAs-Fe2 P NPs/NPCFs-2.5 catalyst are characteristic of the different catalytic mechanisms in acidic and alkaline media. This work proposes a novel idea for constructing coupling catalysts with atomic-level precision and provides a strong reference for the development of high-efficiency ORR electrocatalysts for practical application.

Preparation and Electrocatalytic Properties of CuS/NCNT Composites

Copper sulfide was prepared successfully by reflux method in the research. To improve the dispersion and conductivity of copper sulfide, CuS/NCNTs composites were prepared by one-step hydrothermal method using nitrogen doped carbon nanotubes (NCNTs) and copper sulfide. The conductivity and dispersion of the composites were improved, and the catalytic effect of oxygen reduction was significantly increased. The precursor of CuS/NCNTs oxygen reduction catalyst was calcined to obtain CuS/NCNTs oxygen reduction catalyst with good stability and methanol resistance. The onset potential, current density and half-wave potential were significantly improved. In the process of composite materials calcination, the oxygen reduction catalyst with high catalytic rate was obtained by adjusting the proportion of CuS, CNTs and nitrogen sources, which provided a practical experimental method for sulfide oxidant catalysis. This cheap catalytic material was expected to replace noble metal catalyst like Pt/C, and be widely used in the field of oxygen reduction catalyst.

Recycling synthesis of single-atom Zn-nitrogen-carbon catalyst for electrocatalytic reduction of O2 to H2O2

Portable electrochemical synthesis of H2O2, which is widely used for disinfection, sterilization, and waste treatment, has attracted increasing attention. However, low atomic utilization efficiency and by-product disposal during the synthesis of electrocatalysts are inevitable. Based on the advantages of single-atom catalysts (SACs) towards H2O2 synthesis via a 2e− transfer oxygen reduction reaction, this work presents the synthesis of ZnNC SACs as excellent oxygen reduction catalysts with high atomic use efficiency and almost no generated waste by using formamide and metal powder as precursors. The ZnNC SACs exhibit H2O2 selectivity exceeding 80% when working continuously for 10,000 s. The synthetic strategy described in this paper is intended to be used as a supplement to the synthesis of metal-nitrogen-carbon SACs with high atomic utilization efficiency and low waste generation for environmental and chemical applications.