Active Sites For Oxygen Reduction Reaction Research Articles

Monolayer Ta2Se: A low-cost catalyst with enhanced stability and high basal plane activity for oxygen reduction reaction

The search for cost-effective two-dimensional (2D) electrocatalysts is accelerating as these materials emerge as pivotal alternatives to traditional energy technologies for fuel cells. In this work, we report a low-cost, high-performance Ta2Se monolayer as an electrocatalyst for the oxygen reduction reaction (ORR) through first-principles calculations. The Ta2Se monolayer exhibits notable stability and low cleavage energy, underscoring its practical potential for experimental synthesis. Our computations show that the Ta2Se monolayer has a metallic band structure with substantial electronic states at the Fermi level, which suggests the favorable electron transport properties. Remarkably, the outer Ta atoms in the basal plane of Ta2Se monolayer are identified as ORR active sites, exhibiting a competitive selection for dissociative and associative four-electron pathways alongside outstanding catalytic activity. Through examining the surface coverage impacts on ORR, we disclose that at a moderate oxygen coverage, the Ta2Se monolayer's limiting potential reaches approximately 0.9 eV, outstripping the conventional Pt electrodes, thus confirming its superior ORR catalytic capability. The low cost, active basal plane sites, and high ORR activity make the Ta2Se monolayer a competitive candidate for advanced fuel cell components.

Chitosan-covered spherical natural graphite: A perfect carbon source for efficient non-PGM catalysts for electrocatalysis

Developing non-precious metal catalysts to replace Pt-based catalysts for oxygen reduction reaction (ORR) is a hotspot presently, and transition metal-nitrogen co-doped carbon (TMNC) materials have been deemed as the most promising candidates. In this work, chitosan-derived-curved-carbon covered natural spherical graphite (SG) with a well-defined core-shell structure were prepared via hydrothermal carbonization (HTC) process followed by activation under NH3 atmosphere. A series of active transition metals (M=Fe, Co, Mn) and nitrogen atom were embedded into the curved carbon to form various MNC moieties on SG surface as active sites for ORR. Among the atomic precise MNC@NSG hybrid catalysts, the FeNC@NSG shows the best ORR performance and the half-wave potential is 0.95 V (vs. RHE), which are 108 % as high as that obtained by commercial Pt/C in alkaline environment. Besides, the electrochemical stability and methanol tolerance of the catalyst is also superior to commercial Pt/C. DFT calculations reveals that the high-performance stems from the curved FeNC units in the shell and high electrical conductivity of natural SG in the core for the typical core-shell structures. Moreover, the sluggish ORR kinetics can be efficiently accelerated by the increased FeN bond length, more positive-shifted d-band center and more positive surface charge of active Fe atoms in the curved FeNC units on spherical SG. This provides an enlightenment for developing efficient non-platinum group metal (PGM) catalysts using cheap and volume natural spherical graphite as building brick for energy storage and conversion devices.

Molecular architectures of iron complexes for oxygen reduction catalysis—Activity enhancement by hydroxide ions coupling

Developing cost-effective and high-performance electrocatalysts for oxygen reduction reaction (ORR) is critical for clean energy generation. Here, we propose an approach to the synthesis of iron phthalocyanine nanotubes (FePc NTs) as a highly active and selective electrocatalyst for ORR. The performance is significantly superior to FePc in randomly aggregated and molecularly dispersed states, as well as the commercial Pt/C catalyst. When FePc NTs are anchored on graphene, the resulting architecture shifts the ORR potentials above the redox potentials of Fe2+/3+ sites. This does not obey the redox-mediated mechanism operative on conventional FePc with a Fe2+-N moiety serving as the active sites. Pourbaix analysis shows that the redox of Fe2+/3+ sites couples with HO- ions transfer, forming a HO-Fe3+-N moiety serving as the ORR active sites under the turnover condition. The chemisorption of ORR intermediates is appropriately weakened on the HO-Fe3+-N moiety compared to the Fe2+-N state and thus is intrinsically more ORR active.

Boron-intercalation-triggered crystalline transition of Pd nanosheet assemblies for an enhanced oxygen reduction reaction

The development of effective and stable cathode electrocatalysts is highly desired for fuel cells. Controlling the composition and morphology of Pd-based materials can provide a great opportunity to improve their oxygen reduction reaction (ORR) performance. Here, we report the synthesis of hexagonal close-packed (hcp) Pd2B nanosheet assemblies (Pd2B NAs) via the boronation reaction between as-synthesized Pd NAs and N,N-dimethylformamide. The hcp Pd2B NAs with uniform pore distribution can provide sufficient active sites for ORRs. The insertion of B atoms can induce the phase transition from face-centered cubic structure to hcp structure, as the most thermodynamically stable phase in the Pd-B alloy, which is beneficial for enhancing the ORR stability and toxicity resistance. Therefore, the hcp Pd2B NAs exhibit superior mass activity, specific activity and excellent stability for ORR. The present strategy of boron-intercalation-triggered crystalline transition of Pd-based nanomaterials is valuable for the design of metal–nonmetal catalysts with enhanced performance.

Pre-implanting metal oxides to endow the N-doped carbon with boosted bifunctional catalytic activities towards oxygen reduction and oxygen evolution reactions

Nitrogen-doped carbon-based materials exhibit promising prospect as the catalysts to oxygen reduction reaction (ORR). Incorporating transition metal oxides with the catalysts can endow them with oxygen evolution reaction (OER) activity to construct bifunctional catalysts for rechargeable zinc-air batteries. In this work, a catalyst (named as 300NiFe-Mi-C) was prepared by a metal oxide-implanting strategy. The mixed metal salts of Ni and Fe were first heated to produce metal oxides, and then blended with 2-methylimidazole and carbon black, and subsequently pyrolyzed at a high temperature. In the pyrolysis, a part of metal oxides was reduced to metallic state to facilitate the doping of nitrogen atoms into carbon to form the ORR active sites while a part of metal oxides was retained to afford OER activity. Benefiting from the pre-implanting strategy of metal oxides, the resultant 300NiFe-Mi-C presents enhanced OER performance with 1.56 V of OER potential at 10 mA cm−2, outperforming the 1.68 V of the controlled sample NiFe-Mi-C (without pre-implanting) and 1.70 V of RuO2. The 0.83 V of ORR half-wave potential of 300NiFe-Mi-C is also comparable to the 0.82 V of NiFe-Mi-C and 0.86 V of Pt/C, revealing satisfactory bifunctional catalytic activities. The rechargeable zinc-air battery equipped with 300NiFe-Mi-C can stably operate at ∼1.25 V with 10 mA cm−2, being higher than ∼1.21 V of Pt/C+RuO2. The battery also presents outstanding durability and rechargeability, demonstrating the bifunctional activities of 300NiFe-Mi-C can be realized in practical applications.

Sulfur-Doped Fe-N-C Cathode in Electro-Fenton System to Produce Hydrogen Peroxide and Degrade Azithromycin

Electro-Fenton is a promising process for pollutant removal. The production rate of H2O2 electrosynthesis and the accumulation concentration are crucial, but the performance of existing electrocatalysts is insufficient to achieve industry and academia application wishes. Doping electronegative elements into the metallic nitrogen is expected to produce more H2O2 and achieved higher selectivity. Herein, we rationally designed a ZIF-8 derived Fe-N/S-C catalyst with an electronegative Sulfur which exhibited excellent 2e− pathway oxygen reduction reaction (ORR) activity under alkaline conditions and efficiently degraded azithromycin (AZI). We demonstrate ZIF-8 derived pores in the specific-designed Fe-N/S-C structure significantly increase the accessible ORR active site, and the combination of Fe and S-doping in this catalyst creates the optimal three-phase interface pathways for oxygen transport. The results showed that the Fe-N/S-C-3 exhibited excellent H2O2 selectivity (76%) by changing its electronic structure, with 96% removal of AZI at alkaline conditions (pH = 13) within 210 min. Also, a comprehensive analysis of the key factors affecting H2O2 electrosynthesis is presented, considering aspects related to reactions, catalysts, electrodes, and devices. The spin density and coordinated charge redistribution occurred after S-doping has been demonstrated to be significant and practical for developing electro-Fenton technique to remove bio-refractory contaminants.

Facile and Low Temperature Synthesis of Nd1.8Sr0.2NiO4-δ Cathode Nanofibers for Intermediate Temperature Solid Oxide Fuel Cells

Solid oxide fuel cells (SOFCs) are usually operated at high temperatures (800-1000 °C). As the overall efficiency of SOFC is governed by thermodynamics and kinetics during operation, reducing the operating temperature to 500-650 °C causes significant increase in the electrode polarization losses especially at cathode due to high activation energy towards oxygen reduction reaction (ORR) at cathode, which eventually reduces the overall cell performance. Addressing increased polarization losses at relatively low temperatures (500-650 °C) has been the key issue and a focus of many groups over past couple of decades. Conventional cathode materials lose their ORR activity and so discarded for intermediate temperature (IT)-SOFC application. In this regard, mixed ionic-electronic conductors (MIECs) offer high thermal and chemical stability along with high oxygen diffusion and good electronic conductivity at IT [1,2]. In addition, Ruddlesden-Popper (RP) type MIECs with A2BO4 structure have been demonstrated as promising cathode for IT- SOFCs due to their unique structure, rapid oxygen surface exchange kinetics, and excellent stability in atmosphere. Particularly Nd2NiO4+δ oxides offer an excellent oxygen diffusion coupled with high surface exchange coefficients and comparable thermal expansion coefficients with those of oxygen-ion conductors (YSZ and CGO) [2]. Enhanced electronic conductivity and electrochemical performance have been demonstrated for partial replacement of Nd with Sr in our previous reports [3].Albeit, the intrinsic material properties such as electrochemical catalytic activity and electrical conductivity are of primary importance, an actual electrode performance strongly depends on its microstructure. Therefore, in addition to using high catalytic and conductive materials, achieving an extremely high-power density, thus, requires a finely controlled electrode microstructure, to expand the electrochemically active triple-phase boundary (TPB). Recently, electrospinning has been considered as an efficient, cost effective and promising method for synthesis of ceramic nanofibers. Pertinent literature suggests that SOFC with cathode nanofiber has superior/better performance (nearly doubled power density and significant reduced polarization resistance) compared to powder type electrodes. Selection of suitable cathode materials with modified microstructure (nanofibers) would facilitate more electrochemically active sites for the oxygen reduction at relatively low temperatures <650 °C.In this paper we synthesized Nd1.8Sr0.2NiO4-δ (NSNO) cathode nanofibers using electrospinning technique and compared its electrochemical performance with NSNO nanopowder. Practically, 10 wt.% Polyvinylpyrrolidone (PVP) solution was prepared by dissolving it in ethanol, to which stoichiometric amounts of Nd-nitrate, Sr-nitrate, Ni-nitrate (total weight percentage of metal salts- 3 wt%) were added, together with some deionized water. The mixture was then stirred for 10 h at room temperature on a magnetic stirrer to obtain a homogeneous sol. The sol was loaded into the electrospinning syringe. Distance between the spinneret and the collector was fixed at 25 cm and the high-voltage supply was maintained at 30 kV. The spinning rate was controlled at 5 ml h- 1 by a syringe pump. The obtained nanofibers were dried at 80 °C for 12 h, then sintered at 900 °C for 2 h in air to obtain single-phase NSNO.The obtained nanofibers were characterized using room temperature XRD, XPS, SEM and dc electrical conductivity (400-650 °C). In addition, electrochemical impedance spectroscopy studies on symmetric cells were carried out as a function of temperature (400-650 °C). Improved electrochemical performance is attributed to enhanced electrochemical active sites for ORR due to nanofibers and optimum porosity.

Elucidation of the Interplay between the Features of the Precursor and the Physicochemical Properties of “Core-Shell” Hierarchical Carbon Nitride Electrocatalysts for the Oxygen Reduction Reaction

The oxygen reduction reaction (ORR) is a major bottleneck in the operation of proton-exchange membrane fuel cells (PEMFCs) since it introduces large overpotentials (no less than 250-300 mV), which degrade significantly the energy conversion efficiency of the device. In addition, the electrocatalysts (ECs) yielding the lowest ORR overpotentials in the acidic environment found at the PEMFC cathode are based on Pt, a very scarce critical raw material (CRM). Pt raises concerns as it might trigger serious supply bottlenecks in the event of a practical widespread implementation of PEMFCs in the framework of today’s energy transition. Hence, the development of high-performing and durable ECs for the ORR is a major stepping stone towards the large-scale rollout of PEMFCs.In our research group a new breakthrough approach has been devised for the synthesis of ORR ECs, that is completely different form the preparation routes that are commonly adopted in the state of the art [1, 2]. Briefly, the approach consists in the pyrolysis and subsequent treatment of a precursor obtained by coupling suitable carbon species/sacrificial supports with a zeolitic inorganic-organic polymer electrolyte (Z-IOPE). The latter comprises anionic complexes including the Pt “catalyst” and other “co-catalysts”, which are bridged by organic binders [1,2].On one hand, the proposed approach has already shown its potential to yield ECs exhibiting a performance and durability improved with respect to Pt/C benchmark ECs. On the other hand, the proposed approach is extremely flexible, especially with respect to the synthesis of the initial EC precursor. In principle, it is possible to modulate a number of crucial features of such EC precursor, with a particular reference to: (i) the chemical composition (e.g., in terms of Pt, other “co-catalysts” and N; the latter plays a crucial role to stabilize the active sites into “coordination nests” on the EC surface, raising durability); and (ii) the morphology, that can be “templated” on suitable species (e.g., carbon species, sacrificial supports). The very flexibility of the proposed synthetic process needs to be leveraged appropriately to maximize the performance of the final ORR ECs, at the same time minimizing the efforts wasted on approaches with a low development potential.Herein it is elucidated how the modulation in the parameters involved in the synthesis of the EC precursor (e.g., the binder and the approaches to couple the Z-IOPE with the support) impact the physicochemical and electrochemical properties of the final ECs, with a particular reference to the chemical composition, morphology and distribution of the ORR active sites. An extensive characterization of the final ECs is carried out. Inductively-coupled plasma atomic emission spectroscopy (ICP-AES) and microanalysis are adopted to determine the bulk chemical composition. Near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) is used to elucidate the surface chemical composition and the chemical states of the surface elements. Ultra-high resolution scanning electron microscopy (UHR-SEM) and transmission electron microscopy (TEM) allow for the elucidation of EC morphology. The porosimetric features are probed by automatic gas adsorption instrumentations. Wide-angle X-ray diffraction (WAXD) is crucial to resolve the crystal structure. Cyclic voltammetry with the rotating ring-disk electrode method (CV-TF-RRDE) is used to clarify the “ex-situ” electrochemical performance and ORR reaction mechanism. Finally, the most promising ECs are used in the fabrication of single PEMFCs, that are tested as a function of the operating conditions.It is revealed that, in comparison with the Pt/C benchamark, some ECs are characterized by a much higher intrinsic kinetic performance due to the electronic and bifunctional effects bestowed by the carbon nitride support and the «co-catalysts». Finally, the binder has a marked effect on the stoichiometry, size, and distribution of the PtMx aggregates bearing the active sites.

Cobalt atom-cluster interactions synergistically enhance the activity of oxygen reduction reaction in seawater

Seawater electrocatalysis is highly desired for a variety of energy storage and conversion systems, such as water splitting and metal fuel cells that directly using seawater as electrolyte. However, the adsorption of chloride ions (Cl−) on active sites of cathodes would worsen the oxygen reduction reaction (ORR) activity and stability, thus lowering the battery performance. In this work, we firstly designed an electrocatalyst model, in which the Co atomic clusters were closely surrounded by satellite Co single atoms on N-doped carbon substrates, designated as Co-ACSAs. The theoretical calculation results suggested that the Co clusters possess stronger Cl− binding energy and acted as pre-adsorption group for Cl−, thus fully exposed Co single atoms as ORR active sites with stronger O2 adsorption energy to promote the oxygen reduction reaction process. Then, we developed an ultrafast high temperature shock strategy to synthesize the Co-ACSAs by controlling the N concentration in carbon substrates. Benefiting from the moderate interacting distance between Co clusters and satellite Co single atoms in Co-ACSAs, the electronic structure of Co clusters and Co single atoms was optimized through the modulation of the interconnected hexatomic ring. As a result, Co-ACSAs exhibit superior ORR activity and durability with on-set and half-wave potentials of 0.885 V and 0.782 V, respectively, and continuously catalyzing for 485 h in seawater electrolyte. The Co-ACSAs-based seawater battery exhibits a discharge voltage of 1.41 V at 5 mA cm−2 and realized stable energy supply for more than 390 h.

Design and synthesis of self-supporting FeCoNi- and N-doped carbon fibers/nanotubes as oxygen bifunctional catalysts for solid-state flexible Zn-air batteries

Developing effective bifunctional electrocatalysts with efficient oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) capabilities is critical to advancing rechargeable zinc-air batteries. This study harnessed the properties of organic salts in polymer solutions via electrospinning to synthesize self-supporting flexible bifunctional electrocatalysts, termed FeCoNi flexible carbon nanofibers (FeCoNi FCNFs). These catalysts benefitted from the secondary growth of FeCoNi- and N-doped carbon nanotubes on fiber surfaces, generating numerous metal-nitrogen-carbon (M−N−C) ORR active sites and FeCoNi alloy-based OER active sites. At 10 mA/cm2, FeCoNi FCNFs exhibited a half-wave potential of 0.92 V for ORR and 1.64 V for OER. Density functional theory calculations based on first principles demonstrated that FeCoNi FCNFs possessed superior bifunctional activity. Moreover, rechargeable liquid Zn-air batteries constructed with this catalyst achieved a high peak power density and exceptional cycling stability lasting over 630 h. Additionally, a solid-state wearable battery using this catalyst displayed outstanding cycle stability, continuously cycling for over 27 h. This research offers both insight and a fresh approach for designing efficient, cost-effective bifunctional solid-state or liquid zinc-air batteries.

In situ Growth of Zeolite Imidazole Frameworks (ZIF-67) on Carbon Cloth for the Application of Oxygen Reduction Reactions and Microbial Fuel Cells.

Developing high surface area catalysts is an effective strategy to enhance the oxygen reduction reaction (ORR) in the application of microbial fuel cells (MFCs). This can be achieved by developing a catalyst based on metal-organic frameworks (MOFs) because they offer a porous active site for ORR. In this work, a novel in situ growth of 2D shell nanowires of ZIF-67 as a template for N-doped carbon (Co/NC) via a carbonization route was developed to enhance the ORR performance. The effects of different reaction times and different annealing temperatures were studied for a better ORR activity. The growth of the MOF template on the carbon cloth was confirmed using scanning electron microscopy, field emission scanning electron microscopy, transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and Fourier transform infrared. The Co/NC-800 exhibited an enhancement in the ORR activity as evidenced by an onset potential and half-wave potential of 0.0 vs V Ag/AgCl and -0.1 vs V Ag/AgCl, respectively, with a limited current density exceeding the commercial Pt/C. Operating Co/NC-800 on MFC revealed a maximum power density of 30 ± 2.5 mW/m2, a maximum current density of 180 ± 2.5 mA/m2.

Bifunctional catalysts of modified doped small-sized ZIF and CNTs with entangled structures

For optimizing the lifetime and power of Zn-Air batteries (ZABs), we develop Fe, Co bimetallic doped zeolite imidazolate framework (M@CoFe-ZIF-8) from the perspective of catalytic reaction. After the pyrolysis treatment, the catalyst shows an entangled structure of 100 nm-sized ZIF-8 particles and carbon nanotubes (CNTs). The experimental result shows that the 100 nm-sized CoFe@MNC-CNTs (MNC: metal and nitrogen doped carbon) have better ORR (oxygen reduction reaction) and OER (oxygen evolution reaction) catalytic activities compared with the 1 μm-sized CoFe@BMNC-CNTs structures with the assistance of melamine. On the one hand, it is closely related to the larger specific surface area and mesoporous structure of CoFe@MNC-CNTs. On the other hand, uniformly distributed metal nitrogen (M-Nx) sites and carbon nanotube-wrapped metal nanoparticles may both be effective catalytic active sites for ORR and OER. The CoFe@MNC-CNTs exhibits remarkable catalytic performance, outperforming the commercially available 20 % Pt/C catalyst with a higher half-wave potential (0.865 V vs 0.838 V) in alkaline electrolyte. Meanwhile, the overpotential (Eover) also rivals that of the commercially available RuO2 catalyst. The Zn-air batterry based on CoFe@MNC-CNTs exhibits excellent power density, which is significantly better than Pt/C catalyst (112.1 mW·cm−2 vs 98.2 mW·cm−2). The CoFe@MNC-CNTs also exhibits over 65 h of cycling stability (10 mA·cm−2). In addition, the open circuit voltage (OCV) and cycle time of the flexible battery based on the catalyst of CoFe@MNC-CNTs remain satisfactory (1.525 V, 18 h).

Atomic-Level Regulation of Cu-Based Electrocatalyst for Enhancing Oxygen Reduction Reaction: From Single Atoms to Polymetallic Active Sites.

The slow kinetics of cathodic oxygen reduction reactions (ORR) in fuel cells and the high cost of commercial Pt-based catalysts limit their large-scale application. Cu-based single-atom catalysts (SACs) have received increasing attention as a promising ORR catalyst due to their high atom utilization, high thermodynamic activity, adjustable electronic structure, and low cost. Herein, the recent research progress of Cu-based catalysts is reviewed from single atom to polymetallic active sites for ORR. First, the design and synthesis method of Cu-based SACs are summarized. Then the atomic-level structure regulation strategy of Cu-based catalyst is proposed to improve the ORR performance. The different ORR catalytic mechanism based on the different Cu active sites is further revealed. Finally, the design principle of high-performance Cu-based SACs is proposed for ORR and the opportunities and challenges are further prospected.

The synthesis of MNC5 active site for electrochemical catalysis

Metal-nitrogen-carbon (M-N-C) catalysts with M-Nx (x = 2 or 4) sites on graphene substrate are researched intensively nowadays. However, the simple and repetitive hexatomic ring structure of graphene limits the exploration of novel efficient active sites for catalysis. Herein, a new carbon material, hydrogen-substituted graphdiyne (HsGDY), is designed as the substrate to develop new active sites. Benefiting from the unique sp and sp2 hybridized carbon atoms of HsGDY, aromatic rings in HsGDY can fix Fe single atoms, creating novel FeC6 and FeNC5 active sites. The as-synthesized Fe-N-HsGDY catalyst displays much higher activity than commercial Pt/C and is found to be one of the highest activities among the reported non-noble metal catalysts for oxygen reduction reaction (ORR). FeNC5 site with graphitic-N has been verified experimentally and theoretically as an efficient active site for ORR. Those results indicate that the rational choice of specific substrate materials with novel structures will broaden the path of creating new active sites with efficient activity.

(Keynote) Interplay between Physicochemical Properties of Precursors and “Core-Shell” Hierarchical Carbon Nitride Electrocatalysts for the Oxygen Reduction Reaction

The oxygen reduction reaction (ORR) is a major bottleneck in the operation of proton-exchange membrane fuel cells (PEMFCs) since it introduces large overpotentials (no less than 250-300 mV), which degrade significantly the energy conversion efficiency of the device. In addition, the electrocatalysts (ECs) yielding the lowest ORR overpotentials in the acidic environment found at the PEMFC cathode are based on Pt, a very scarce critical raw material (CRM). Pt raises concerns as it might trigger serious supply bottlenecks in the event of a practical widespread implementation of PEMFCs in the framework of today’s energy transition. Hence, the development of high-performing and durable ECs for the ORR is a major stepping stone towards the large-scale rollout of PEMFCs.In our research group a new breakthrough approach has been devised for the synthesis of ORR ECs, that is completely different form the preparation routes that are commonly adopted in the state of the art [1, 2]. Briefly, the approach consists in the pyrolysis and subsequent treatment of a precursor obtained by coupling suitable carbon species/sacrificial supports with a zeolitic inorganic-organic polymer electrolyte (Z-IOPE). The latter comprises anionic complexes including the Pt “catalyst” and other “co-catalysts”, which are bridged by organic binders [1,2].On one hand, the proposed approach has already shown its potential to yield ECs exhibiting a performance and durability improved with respect to Pt/C benchmark ECs. On the other hand, the proposed approach is extremely flexible, especially with respect to the synthesis of the initial EC precursor. In principle, it is possible to modulate a number of crucial features of such EC precursor, with a particular reference to: (i) the chemical composition (e.g., in terms of Pt, other “co-catalysts” and N; the latter plays a crucial role to stabilize the active sites into “coordination nests” on the EC surface, raising durability); and (ii) the morphology, that can be “templated” on suitable species (e.g., carbon species, sacrificial supports). The very flexibility of the proposed synthetic process needs to be leveraged appropriately to maximize the performance of the final ORR ECs, at the same time minimizing the efforts wasted on approaches with a low development potential.Herein it is elucidated how the modulation in the parameters involved in the synthesis of the EC precursor (e.g., the binder and the approaches to couple the Z-IOPE with the support) impact the physicochemical and electrochemical properties of the final ECs, with a particular reference to the chemical composition, morphology and distribution of the ORR active sites. An extensive characterization of the final ECs is carried out. Inductively-coupled plasma atomic emission spectroscopy (ICP-AES) and microanalysis are adopted to determine the bulk chemical composition. Near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) is used to elucidate the surface chemical composition and the chemical states of the surface elements. Ultra-high resolution scanning electron microscopy (UHR-SEM) and transmission electron microscopy (TEM) allow for the elucidation of EC morphology. The porosimetric features are probed by automatic gas adsorption instrumentations. Wide-angle X-ray diffraction (WAXD) is crucial to resolve the crystal structure. Cyclic voltammetry with the rotating ring-disk electrode method (CV-TF-RRDE) is used to clarify the “ex-situ” electrochemical performance and ORR reaction mechanism. Finally, the most promising ECs are used in the fabrication of single PEMFCs, that are tested as a function of the operating conditions.It is revealed that, in comparison with the Pt/C benchamark, some ECs are characterized by a much higher intrinsic kinetic performance due to the electronic and bifunctional effects bestowed by the carbon nitride support and the «co-catalysts». Finally, the binder has a marked effect on the stoichiometry, size, and distribution of the PtMx aggregates bearing the active sites.

Effect of Gas-Phase Nitric Oxide Adsorption on ORR Activity of Fe-N-C Catalysts

Significant progress has been made in recent years in developing platinum group metal-free catalysts, particularly so-called Fe-N-C materials, for the oxygen reduction reaction (ORR) in polymer electrolyte fuel cells.1 However, further improvement in both activity and, especially, durability are still necessary for practical applications. One of the major factors that hinder the further enhancement of the catalysts is the lack of clear understanding of the nature of the active sites in the Fe-N-C catalysts due to the complexity of the Fe-N-C catalysts’ composition and structure. Gaseous nitric oxide was reported to be a suitable probe molecule that can bond to Fe and impede the ORR of the Fe-N-C catalysts.2,3 In this work, the effect of nitric oxide gas-phase adsorption on Fe-N-C catalyst redox feature and ORR activity for a variety of Fe-N-C catalysts from different origin will be investigated. The implication of these results on the nature of the ORR active sites of the Fe-N-C catalysts will be discussed.AcknowledgementsThis work was supported by the U.S. Department of Energy (DOE), Energy Efficiency and Renewable Energy, Hydrogen and Fuel Cell Technologies Office (HFTO) under the auspices of the Electrocatalysis Consortium (ElectroCat 2.0). This work was partially authored by Argonne, a U.S. Department of Energy (DOE) Office of Science laboratory operated for DOE by UChicago Argonne, LLC under contract no. DE-AC02-06CH11357.References X.X. Wang, M.T. Swihart, and G. Wu, "Achievements, challenges, and perspectives on cathode catalysts in proton exchange membrane fuel cells for transportation", Nature Catalysis, 2 (2019) 578-589.J.L. Kneebone, et al., "A Combined Probe-Molecule, Mossbauer, Nuclear Resonance Vibrational Spectroscopy, and Density Functional Theory Approach for Evaluation of Potential Iron Active Sites in an Oxygen Reduction Reaction Catalyst", J. Phys. Chem. C 121 (2017) 16283-16290.P. Boldrin, D. Malko, A. Mehmood, U.I. Kramm, S. Wagner, S. Paul, N. Weidler, A. Kucernak, “Deactivation, reactivation and super-activation of Fe-N/C oxygen reduction electrocatalysts: Gas sorption, physical and electrochemical investigation using NO and O2”, Applied Catalysis B: Environmental 292 (2021) 120169.

Understanding the oxygen reduction reaction in the hierarchically oriented composite cathode with open, straight pores

The cathode-supported solid oxide fuel cells (SOFCs) have recently attracted great attentions due to the unique advantages of excellent fuel gas utilization and superior thermal reduction stability, but the commercialization process is limited by the poor oxygen reduction reaction (ORR) catalytic performance of low porosities and reduced three phase boundaries (TPBs) cathode substrate. In this case, a novel phase inversion tape casting technology (PITC), have been successfully applied and fabricated an advanced full ceramic bilayer composite cathode support (Sr2Fe1.5Mo0.4Ni0.1O6-δ (SFMNi) – Gd0.2Ce0.8O1.9 (GDC) support/ functional layer) with open, straight pores. The hierarchically oriented pores can be well-kept after high temperature sintering, which significantly enhance the gas (O2) diffusion process in the porous electrode, thus greatly minimizing even eliminating the concentration resistance. Moreover, the introduction of cathode functional layer remarkably enlarges the TPBs density, which is the active sites for ORR, thereby dramatically reducing the electrochemical activation polarization impedance. The full ceramic cathode support exhibits enhanced ORR kinetics activity with a favorable long-term operational stability, indicating that it is an optimized potential cathode support with great application prospects for SOFCs.

Cobalt Nanoparticle-integrated Nitrogen-doped Carbon Nanotube as an Efficient Bifunctional Electrocatalyst for Direct Methanol Fuel Cells

Catalysts are key ingredients for direct methanol fuel cells (DMFCs). Platinum (Pt) and Pt-based catalysts are commonly used in DMFCs. The present study demonstrates highly active noble metal-free cobalt nanoparticle (NP)-integrated nitrogen-doped carbon nanotubes (Co@NC) as an efficient catalyst for the oxygen reduction reaction (ORR). The formation of the Co-Nx bond in the vicinity of Co@NC created a higher number of active sites for ORR. The Co@NC catalyst showed a half-wave potential of 0.86 V and an onset potential of 0.91 V for ORR. The Pt NP-decorated Co@NC (Pt-Co@NC) displayed high mass activity of 5.39 A/mg toward methanol oxidation reaction activity, which is 3.6-fold higher than that of the conventional Pt/C catalyst, due to the synergistic effect of Co@NC. The Co@NC cathode catalyst coupled with a Pt-Co@NC anode catalyst exhibited significantly improved performance, with a peak power density of 42.6 mW/cm2 at a discharge current density of 220 mA/cm2. The Co@NC noble metal-free bifunctional catalyst broadens the range of DMFC application, as demonstrated in a polyvinyl alcohol/potassium hydroxide (PVA/KOH) gel membrane-based room-temperature DMFC, using a synthesized cathode and anode. The PVA/KOH gel-membrane-based DMFC cell delivered a power density of 8.5 mW/cm2. Thus, the results from this study demonstrate the real applicability of the as-synthesized catalyst-integrated PVA/KOH gel membrane-based DMFC for portable electronics.

Printing specific active sites for ORR and hydrazine oxidation on N-doped carbon

Among the prime challenges in clean energy research, the development of simple, cost-effective, and feasible strategies to prepare catalysts can be highlighted. As an alternative, herein N-doped carbonaceous catalysts were synthesized by the same cost-effective method, which can be used to produce both the anode and cathode of the fuel cell. The catalysts were obtained by pyrolysis of melamine-formaldehyde resins containing ions Cu2+, Co2+, or Ni2+ and used for oxygen reduction reaction (ORR) and hydrazine oxidation reaction (HzOR). Although the results prove that the activities for both reactions are derived exclusively from N-doped carbon moieties, the presence of some metal in the pyrolysis process is primordial to printing active sites on carbon structures. The catalyst obtained from Cu-based resin was superior regarding ORR, whilst that yielded from Co-based resin showed the best results for HzOR, indicating that the active sites generated for electroreduction and electrooxidation in each carbon structure are different. The best catalysts demonstrate ORR and HzOR activities very akin to the golden standard 20% Pt/C, presenting a ηonset values of 90 mV and 170 mV.

FeNC Catalysts Decorated with NiFe2O4 to Enhance Bifunctional Activity for Zn–Air Batteries

Rechargeable Zn–air battery is a promising next-generation energy storage device attributed to its high energy density, excellent safety, and low cost. However, its commercialization is hampered by sluggish kinetics of the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) at air electrodes. Herein, we have designed, fabricated, and demonstrated a highly efficient ORR/OER electrocatalyst, NiFe2O4/FeNC, using low-cost materials via a facile synthesis route. NiFe2O4 is successfully loaded on Fe/N-doped carbon (FeNC) through bonding to Fe3C in FeNC. Due to the existence of high ORR active sites such as FeN4 and Fe and N-doped carbon moieties, the half-wave potential of the ORR reaches a high value of 0.83 V. While benefited from NiFe2O4 with high OER activity and the synergistic effect between NiFe2O4 and FeNC, the overpotential is 310 mV at 10 mA cm–2 in the OER. The voltage difference between charging–discharging operations in the Zn–air battery employing the NiFe2O4/FeNC electrocatalyst only increases by 0.16 V after cycling for 100 h (600 cycles) at 10 mA cm–2, which is much lower than 1.28 V using the best commercial Pt/C and RuO2 catalysts.