Active Sites For Oxygen Reduction Reaction Research Articles

Pt/C protection by Fe-N-C via Fenton reaction for direct methanol fuel cells operated under high concentrated methanol with high performance

In this paper, a hierarchical cathode catalytic layer (CCL) composed of Pt/C and Fe-N-C was proposed to mitigate severe methanol crossover in direct methanol fuel cells (DMFCs). Several validation experiments and electrochemical analysis were conducted to reveal that the reactive oxygen species (ROS) produced by Fe-N-C via Fenton reaction in an incomplete four-electron transfer in oxygen reduction reaction (ORR) can be utilized to oxidize methanol nearby Pt/C efficiently, thus protecting ORR active sites from poisoning in concentrated methanol solution. Based on this strategy, the optimized DMFC outputs a high peak power density of 22.5 mWcm−2 at 10 M methanol, which is among the highest performance achieved merely by CCL design. Additionally, the reaction of ROS with methanol is confirmed to protect the Fe-N-C CCL from the potential attack of ROS, leading to a great stability of the DMFC. This work is of great practical significance for the design of DMFCs with high specific energy density and good stability for portable application.

Deepening the Understanding of Carbon Active Sites for ORR Using Electrochemical and Spectrochemical Techniques.

Defect-containing carbon nanotube materials were prepared by subjecting two commercial multiwalled carbon nanotubes (MWCNTs) of different purities to purification (HCl) and oxidative conditions (HNO3) and further heat treatment to remove surface oxygen groups. The as-prepared carbon materials were physicochemically characterized to observe changes in their properties after the different treatments. TEM microscopy shows morphological modifications in the MWCNTs after the treatments such as broken walls and carbon defects including topological defects. This leads to both higher surface areas and active sites. The carbon defects were analysed by Raman spectroscopy, but the active surface area (ASA) and the electrochemical active surface area (EASA) values showed that not all the defects are equally active for oxygen reduction reactions (ORRs). This suggests the importance of calculating either ASA or EASA in carbon materials with different structures to determine the activity of these defects. The as-prepared defect-containing multiwalled carbon nanotubes exhibit good catalytic performance due to the formation of carbon defects active for ORR such as edge sites and topological defects. Moreover, they exhibit good stability and methanol tolerances. The as-prepared MWCNTs sample with the highest purity is a promising defective carbon material for ORR because its activity is only related to high concentrations of active carbon defects including edge sites and topological defects.

Gaseous Nitric Oxide As Probe Molecule for Fe-N-C ORR Electrocatalysts

Iron nitrogen carbon (Fe-N-C) electrocatalysts remain the most promising platinum group metal-free (PGM-free) catalysts for the oxygen reduction reaction (ORR) in polymer electrolyte fuel cells (PEFCs). Although significant progress has been made in recent years in improving both the ORR activity and stability of the Fe-N-C catalysts, further enhancement in the catalyst durability while maintaining the activity is imperative for the practical applications. One of the major factors that hinders the further enhancement of the catalysts is the lack of clear understanding of the nature of the active sites and density 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 proven to be a suitable probe molecule that can bond to Fe and impede the ORR of the Fe-N-C catalysts. In recent years, we have studied the effects of gas-phase adsorption and desorption of nitric oxide on the redox features and ORR activity on Fe-N-C catalysts synthesized by different methods. We have used various characterization tools to quantify the amount of nitric oxide adsorbed and to identify the adsorption sites and geometry, including electrochemical stripping, temperature programmed desorption, and in situ X-ray techniques. The corresponding results obtained will be summarized in this talk. 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 utilized the resources of the Advanced Photon Source, a U.S. DOE Office of Science user facility operated by Argonne National Laboratory for DOE Office. 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.

The Impact of Carbon Corrosion Towards Oxygen Reduction Reaction Activity on Atomically Dispersed M-N-C Catalysts

Atomically dispersed Pt group metal free (PGM free) M-N-C catalysts offer a promising pathway towards a clean energy future.1 This class of materials has proven to be a viable substitute for commercial Pt/C to catalyze oxygen reduction reaction (ORR) with similar activity at only a fraction of the raw material cost.2 To gain a wider market adoption, however, M-N-C catalysts need to significantly improve its durability compared to Pt and its alloys in a real-world operating condition. Towards this end, substantial efforts have focused on making more durable M-N-C catalysts while simultaneously maintaining and improving its ORR activity.3,4 As part of the effort to understand the fundamental aspects that control M-N-C catalysts’ durability during ORR condition, we employ a combined approach using first principles density functional theory (DFT) and density functional tight binding theory (DFTB) to identify the impact of carbon corrosion occurring on M-N-C catalyst towards ORR activity. Our starting atomic representation of the ORR active site is the “C10” type local structure composed of FeN4C10 local defect5 embedded in a monolayer graphene support of varying cell size. As carbon corrosion occurs and consequently creating local carbon vacancies surrounding the active site, we then obtain a family of FeN4C11 and FeN4C12 type of local structures. Our DFT calculations of OH binding energy on these corrosion derived structures suggest that the presence of C vacancies and the subsequent H passivation on dangling C introduces elastic structural deformation to various degree that decreases the theoretical ORR limiting potential. This decrease in DFT calculated ORR limiting potential across a family of FeN4C10, FeN4C11, and FeN4C12 local structures can be attributed to a mechanical energy arising from the surface reconstructions. Reference D. A. Cullen et al., Nat Energy, 6, 462–474 (2021).G. Wu, K. L. More, C. M. Johnston, and P. Zelenay, Science, 332, 443–447 (2011).E. F. Holby, G. Wang, and P. Zelenay, ACS Catal., 10, 14527–14539 (2020).T. Patniboon and H. A. Hansen, ACS Catal., 11, 13102–13118 (2021).H. T. Chung et al., Science, 357, 479–484 (2017).

Influence of Nb Doping on the Performance of Oxygen Reduction Reaction on TiO2 in Acidic Environmental

Polymer electrolyte fuel cells (PEFCs) have attracted attention as one of the next-generation power sources that are environmentally friendly. However, the high manufacturer's costs, short service life, and insufficient cell performance are solved to realize widespread commercialization. A non-platinum group metal catalyst is relatively low in cost and highly durable. In particular, the oxides of group 4 and 5 elements were reported to show high oxygen reduction reaction (ORR) activity and durability. Their oxides are considered dominant candidates for the non-platinum group metal catalysts, although they have a fateful flaw that their ORR activity is clearly poorer than traditional platinum-supported carbon catalysts. It was reported that introducing oxygen vacancies and other transition metals into the crystal structure of 4- and 5-group metal oxides effectively enhanced the ORR activity. The active site of 4- and 5-group metal oxides is unclear despite it being well-known that the oxygen vacancies and dopant atoms are related to its ORR activity. It is now indicated that candidates of active site were the oxygen vacancies around the dopant atom and the crystal distortion. However, it is hard to separate their effects because both dopant atoms and oxygen vacancies distort the local crystal structure. On the other hand, identifying the ORR active site leads to obtaining the catalyst design with high ORR activity. We focused on the oxide nanosheet, the simplest structure formed by a six-coordinate octahedral, as the model electrode. The thickness of oxide nanosheets is about 1 nm, and forming a single layer on a substrate is relatively easy. Thus, it was expected that the electron conductivity was guaranteed enough by the tunnel effect. This study employed titanium oxide nanosheets (TiO2ns) as the model catalyst and niobium as a dopant to investigate the effect of dopant atoms on ORR activity. The ratio of Nb/Ti in the prepared model electrode was 0/1, 1/1, and 2/1. In addition, the oxygen vacancies were introduced into all prepared electrodes to study the impact of oxygen vacancies on ORR activity at each Nb/Ti ratio. The thickness of Nb-doped TiO2ns (Nb-TiO2ns) was about 1.2 nm for the 1/1 ratio and slightly thicker for the 0/1 ratio. In contrast, the thickness of the 2/1 ratio of Nb-TiO2ns was about two times thicker than the 1/1 ratio. The thicknesses of Nb-TiO2ns in each ratio were increased by the reduction treatment to introduce the oxygen vacancies. One reason for this thickness increase may be related to the phase transition into the anatase structure. ORR activity was evaluated from the on-set potential of ORR (E ORR) measured at the step scan with 3 minutes step width. E ORR was slightly elevated with Nb doping. The ion radii of Nb and Ti atoms are 3 pm different when it is assumed that both ions are six-coordinated and their valence is Nb5+ and Ti4+. These results showed that the crystal structure distortion probably has a slight but positive effect on ORR activity.After the reduction treatments, the E ORR of the 0/1 ratio of Nb-TiO2ns increased and was the same as that of the 2/1 ratio. The introduction of oxygen vacancies certainly enhanced the ORR activity of the 0/1 ratio of Nb-TiO2ns. The oxygen vacancies are thought to be necessary to enhance the ORR activity, although the reduction condition wasn’t optimized for the Nb-doped samples.From the above results, we consider that the active site is the oxygen vacancy site rather than the crystal distortion site at this time.

Novel coal-based carbon encapsulating Co-N-C derived from ZIF-67 composite as an efficient chainmail electrocatalyst for zinc-air batteries

Energy shortages and the global climate crisis have led researchers to develop renewable clean energy sources. Zinc-air batteries have attracted much attention due to their high theoretical energy density, safety, and conversion efficiency. However, the cathodic oxygen reduction reaction (ORR) exhibits slow reaction kinetics and high reaction energy barriers, which require catalysts with high activity, selectivity, and stability to improve the reaction rate. Chainmail catalysts have attracted attention in the field of energy conversion and storage due to their structural stability. In this paper, coal-based carbon encapsulating Co-N-C chainmail electrocatalysts are designed and constructed, which can effectively reduce the thermal crumpling phenomenon of the framework structure, enhance the defect content and provide active sites for ORR. The results show that G/Co-N-C-0.025 exhibits a higher half-wave potential than that of Pt/C, with a high ultimate current density of 4.6 mA cm−2, good cycling stability, and high methanol resistance. A primary zinc-air battery is assembled, with an open-circuit voltage of 1.417 V and a maximum power density of 82 mW cm−2. The specific capacity is 789.2 mAh g−1 at a current of 20 mA cm−2. This work provides new ideas for the design and application of high-performance chainmail electrocatalysts.

Phosphorus and nitrogen co-doped-graphene: Stability and catalytic activity in oxygen reduction reaction

This study systematically investigated the stable configurations and oxygen reduction reaction (ORR) catalytic activity of PN co-doped graphene using first-principles methods. We found that PN co-doped graphene substrates are generally highly stable. The adsorption energy of adsorbates is linearly positively correlated with the number of electrons obtained from the substrate. The P atoms serve as catalytic activity sites, the co-doping of N significantly enhances the adsorption energies of intermediate species in the ORR process, facilitating the direct dissociation of O2 and O2H. The solvation effect has a non-negligible impact on the adsorption energy of adsorbates, especially for O2. Due to the excessive adsorption of O, it poisons and inhibits the catalytic activity of P active sites for ORR. However, after O adsorption, the C atoms neighboring the PN impurity atoms in the P-Nn-Gra (n=2,3) substrates exhibit better catalytic activity than that of graphene doped with P/N alone. The P-Nn-defect-Gra (n=2,3,4) substrates are potential catalysts with good HER catalytic activity.

Role of Vacancy Defects and Nitrogen Dopants for the Reduction of Oxygen on Graphene.

Disentangling the roles of nitrogen dopants and vacancy defects (VG) in metal-free carbon catalysts for the oxygen reduction reaction (ORR) ideally requires studying both the dopants and defects separately. Here, we systematically introduced nitrogen dopants and VGs via plasma treatment into the basal plane of monolayer graphene as a model carbon catalyst to investigate their specific roles in ORR catalysis. An increased defect density including dopants is positively associated with boosted ORR activity. Nitrogen dopants are responsible for an improved current via a 2e- pathway generating hydroperoxide, while VGs result in enhanced kinetics and water production. We therefore infer that VGs in graphene are responsible for the improved ORR kinetics, while nitrogen dopants majorly influence the selectivity of ORR reaction products. The nitrogen dopants without VGs lead to a higher overpotential compared with the pristine graphene. Instead of the attribution of the ORR active site to only nitrogen species in carbon materials, the improved ORR activity in nitrogen-doped carbon materials should be attributed to the active sites constituted of VGs, oxygen dopants, and nitrogen dopants. Through this work, we provide important insights into the intertwined roles of nitrogen and VGs as well as oxygen dopants in nitrogen-doped metal-free catalysts for a more efficient ORR.

Single atoms/metal nanoparticles-decorated hollow carbon spheres as oxygen reduction catalyst for zinc-air batteries

Non-platinum electrocatalyst development for improving the oxygen reduction reaction (ORR) is critical in electrochemical energy conversion applications such as zinc-air batteries (ZABs). Herein, we developed dual Fe-Co single atoms and Co nanoparticles embedded N-doped hollow carbon (Fe-Co/N-HCS-x) catalysts via the carbonization of polypyrrole (PPy)@zeolitic imidazolate framework (ZIF) core–shell particles, with various PPy concentrations. PPy serves as a secondary nitrogen source that enhances nitrogen amount and promotes metal dispersion to increase active sites density. The Fe-Co/N-HCS-x catalysts with optimized PPy concentration control demonstrated improved ORR performance in alkaline media, with a half-wave potential (E1/2) of 0.921 V and an onset potential (Eonset) of 1.019 V. Fe-Co/N-HCS-x also exhibited high ZABs performance, with a power density of 244.6 mW cm−2 and an operating time of more than 100 h. Our research contributes to the development of optimized active sites for ORR, which can be applied to new electrochemical storage and conversion systems.

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.

Catalytic effects of graphene structures on Pt/graphene catalysts.

Pt/C catalysts have been considered the ideal cathodic catalyst for proton exchange membrane fuel cells (PEMFCs) due to their superior oxygen reduction reaction (ORR) catalytic activity at low temperatures. However, oxidation and corrosion of the carbon black support at the cathode result in the agglomeration of Pt particles, which reduces the active sites in the Pt/C catalyst. Graphene supports have shown great promise to address this issue, and therefore, finding out the main structural features of the graphene support is of great significance for guiding the rational construction of graphene-based Pt (Pt/graphene) catalysts for optimized ORR catalysts. In order to systematically study the influence of the structural features of the graphene support on the electro-catalytic properties of Pt/graphene catalysts, we prepared porous nitrogen-doped reduced graphene oxide (P-NRGO), nitrogen-doped reduced graphene oxide (NRGO), treated P-NRGO (TP-NRGO) and reduced graphene oxide (RGO) with different nitrogen species contents (7.76, 7.54, 3.24, and 0.14 at%), oxygen species contents (18.68, 18.12, 6.34 and 21.12 at%), specific surface areas (370.4, 70.6, 347.7 and 276.2 m2 g-1) and pore volumes (1.366, 0.1424, 1.3299 and 1.0414 cm3 g-1). The ORR activity of the four Pt/graphene catalysts when listed in the order of their half-wave potentials (E 1/2) and peak power densities was found to be as Pt/P-NRGO > Pt/NRGO > Pt/TP-NRGO > Pt/RGO. The long-term durability of Pt/P-NRGO for the operation of H2-air PEMFCs is better than that of commercial Pt/C catalysts. The excellent ORR catalytic performance of Pt/P-NRGO compared to that of the other three Pt/graphene catalysts is ascribed to the high nitrogen species content of P-NRGO that can facilitate the uniform dispersion of Pt particles and provide accessible active sites for ORR. The results indicate that the specific surface area (SSA) and heteroatom dopants have strong influence on the Pt particle size, and that the nitrogen species of graphene supports play a more important role than the oxygen species, specific surface area and pore volume for the Pt/graphene catalysts in providing accessible active sites.

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.