An Efficient Bio‐inspired Oxygen Reduction Reaction Catalyst: MnOx Nanosheets Incorporated Iron Phthalocyanine Functionalized Graphene

The rapidly increasing energy demand for human activities stimulates the lasting research interests to develop renewable energy alternatives worldwide. The electrochemical reduction of oxygen is one of key steps in controlling the performance of various next-generation energy conversion and storage devices, such as fuel cells, metal-air batteries. The electrochemical reduction of oxygen is one of key steps in controlling the performance of various next-generation energy conversion and storage devices, such as fuel cells, metal-air batteries. The commercialization of these technologies prominently depends on the development of low-cost high-performance electrocatalysts for oxygen reduction reaction (ORR) to replace the precious metal-based catalysts. In this presentation, several reasonable ways for designing new low-cost nanocatalysts with high electrocatalytic activities and superior stability for ORR will be discussed. By focusing on the creation and the enrichment of highly active sites for ORR and simultaneously considering the mass transfer and electron transportation, we have developed several efficient ORR nanocatalysts.1-7 The further improvement of the performance can be achieved by introducing transition metal or nanostructures into these nanocatalysts. Furthermore, understanding the origin of high activity of these electrocatalysts in ORR is also critical for developing efficient non-precious metal catalysts but still challenging. We developed a new highly active Fe-N-C ORR catalyst containing Fe-Nx coordination sites and Fe/Fe3C nanocrystals, and revealed the origin of its activity by intensively investigating the composition and the structure of the catalyst and their correlations with the electrochemical performance. Based on our experimental and theoretical results, it can be concluded that the high ORR activity in this type of Fe-N-C catalysts should be ascribed to that Fe/Fe3C nanocrystals boost the activity of Fe-Nx. These new findings open an avenue for the rational design and bottom-up synthesis of low-cost highly active ORR electrocatalysts.

The development of energy storage and conversion devices provides a beneficial approach for the renewable energy application, which can help relieve the severe reliance on fossil fuels and also address problems related to global climate change. Currently, the efficiencies of energy conversion devices such as the metal–air batteries and fuel cells are mainly limited by the sluggish kinetics of the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) processes. Developing catalytically active and cost effective catalyst for the ORR and OER is of prime importance. So far, noble metals and their alloys, such as Pt and Pt-Pd, have been exclusively used as electrochemical bifunctional catalyst in ORR and OER due to their superior catalytic activity. However, the high cost, limited availability, poor durability and sluggish electron transfer kinetics of noble metal based bifunctional catalysts have impeded the practical application of metal-air batteries. Therefore, the discovery of cost effective, catalytically active alternative bifunctional catalyst such as non-precious metals, carbonaceous materials, and transition metal oxides is highly desirable. Perovskite oxide (ABO3) possesses a unique electronic structure and chemical defect properties, and has been demonstrated to be a promising non-precious metal catalyst among the transition metal oxides in the application of metal air batteries and alkaline fuel cells. It is known that the intrinsic electrochemical catalytic activity is mainly determined by the B site cation in perovskite oxide. Extensive research conducted by Yang et al. demonstrated that the ORR and OER catalytic activities of perovskite oxides follow a volcanic relationship with the filling of electrons in antibonding orbitals [1]. Among the typical LaMO3 (M= Mn, Co, Fe, Ni, Cr), LaMnO3 and LaCoO3 have exhibited highest ORR and OER catalytic activities, respectively. In addition, due to the desirable electronic properties of the perovskite oxides, strategies such as cation partial substitution and oxygen non-stoichiometry formation could, therefore, be utilized as the effective approaches to fine-tune the catalytic properties and to achieve a better bifunctional activity [2,3]. To enhance the bifunctional catalytic performance, improved specific surface area as well as enhanced oxygen channel are desired. Researchers have found that electrochemical catalysts with porous nanofiber structures could favor the ORR and OER pathways by providing uniform O2 electrolyte distribution, and beneficial oxygen diffusion channels. Zhao et al. have synthesized mesoporous La0.5Sr0.5CoO2.91 nanowires through the multistep microemulsion method, showing significant enhanced catalytic performance [4]. Xu et al. have demonstrated that the porous La0.75Sr0.25MnO3 nanotube catalyst fabricated through facile electrospinning technologies provides favorable advantages to the availability of the catalytic active sites in the organic solvent electrolyte in Li-O2 batteries [5].Herein, we developed a bifunctional perovskite catalyst towards both ORR and OER in alkaline solution. Cobalt cations were doped into Pr0.5Ba0.5MnO3-δ perovskite to achieve the higher intrinsic ORR and OER activities by engineering the structure symmetry, electronic and the oxygen vacancy defects of the material. The bifunctional catalyst with a unique porous nanofiber structure was fabricated by electrospinning technology (Figure.1). A single phase perovskite oxide Co doped Pr0.5Ba0.5MnO3-δ was obtained after sintering the electrospun precursor at high temperature. The ORR and OER activities of the composite catalysts consisting 50 wt% of the as-synthesized perovskite oxides and 50 wt% carbon black were investigated in 0.1 KOH with rotating disk electrode (RDE). Significant enhancement in ORR and OER performance was achieved via using the composite catalyst with Co doped Pr0.5Ba0.5MnO3-δ nanofiber/carbon black with respect to the reduced overpotential and improved current density. In particular, the Co doped Pr0.5Ba0.5MnO3-δ nanofiber/carbon black composite demonstrated enhanced electron transfer number in the ORR process, indicating preferable dominance of four electron transfer pathway. Moreover, the Co doped Pr0.5Ba0.5MnO3-δ composite exhibited high stability during the cycling tests, indicating its promising applications in metal-air batteries.

Fabricating Pt-decorated three dimensional N-doped carbon porous microspherical cavity catalyst for advanced oxygen reduction reaction

Bimetallic niobium-iron oxynitride as a highly active catalyst towards the oxygen reduction reaction in acidic media

Robust and efficient catalysts for the oxygen reduction reaction (ORR) are required for the development of various energy storage and conversion devices. In this study, a durable and high-performance Fe3 C@graphene ORR catalyst has been developed by the carbonization of urea- and agar-modified Fe2 O3 nanorods. The influence of the carbonization temperature and annealing time on the activity and stability of the resulting Fe/C catalyst was studied in detail. The Fe/C catalyst synthesized at a temperature of 700 °C (holding time: 60 min) showed better ORR activity and improved stability compared to a commercial Pt/C catalyst. The improved ORR catalytic activity of the catalyst is due to its high Fe3 C content and its good durability results from the unique microstructure of the Fe3 C@graphene hybrid.

The oxygen reduction reaction (ORR) has long been of interest in relation to its many energy applications and interesting multi-pathway mechanisms. The ORR is a key reaction in a range of electrochemical energy conversion and storage devices, such as hydrogen fuel cells and metal-air batteries, respectively. These devices are expected to play an ever-increasing role in the global transition to net zero emissions. Metal-air batteries, such as Zn/air batteries, can operate at room temperature, use recyclable materials, are environmentally friendly, and are preferred in relation to consumer safety than batteries relying on organic solvents and reactive electrode species.1 High rates of the ORR are crucial for the development of high performance Zn/air batteries and catalysts play a significant role, with lowering of the catalyst cost also of increasing importance.The costs of conventional Pt-based ORR catalysts are high. Therefore, metal-free carbon-based ORR electrocatalysts are viewed as increasingly promising alternatives, especially as they are lower in cost due to the availability of the precursor materials. Carbon is also a good electrical conductor and support material, is chemically stable, and can have large surface areas. However, the ORR kinetics are sluggish on carbon and chemical and physical modifications are required to enhance its activity. In typical Zn/air batteries, the ORR occurs at the three-phase boundary (TPB) formed between the solid electrode, liquid electrolyte, and gaseous oxygen. The porosity of the catalyst layer, the wettability of the catalyst/electrolyte interface, and the gas permeability and hydrophobicity of the gas diffusion layer (GDL) are thus also important, significantly influencing the cathode performance and durability. The catalyst layer (CL) must therefore be constructed with both a high-performance catalyst and an optimized TPB length to provide high performance without compromising durability.In the current work, we have doped nitrogen into the lattice of a family of nanoporous colloid imprinted carbon (CIC) powders to increase its ORR activity. The CICs are unique for their versatility in terms of pore size control and ease of surface functionalization.2 Pore sizes in the range of 12 to 100 nm were examined and their effect on the ORR activity and mass transport limitations were investigated. To carry out N-doping, the CICs were exposed to ammonia at 800 ˚C for 7 hr. Catalyst inks were then prepared by mixing the CICs with a binder in an isopropyl alcohol/water solution. Aliquots of the ink were drop-casted on the disc of an RRDE system, or were spray coated or drop-casted on a GDL to determine the ORR performance in an in-house Zn/air battery testing cell, with the N-doped CIC catalyst layer sandwiched between an electrolyte chamber and a graphite current collector. In this setup, O2 gas was flowed through the pores in the GDL to the catalyst/electrolyte interface, a Zn wire installed in the electrolyte chamber was used as the reference electrode, and a Ni sponge was used as the counter electrode.The RRDE experiments showed that, after N doping of the CIC powders, the production of peroxide decreased significantly and the ORR onset potential increased to a very respectable value of ca. 0.9 V vs RHE, indicating the successful activation of the ORR. Electron transfer numbers were found to be greater than 3.5, indicating that either a direct or pseudo- 4 electron transfer ORR pathway is dominant. In agreement with the literature, the ORR currents in the kinetic regions increased linearly with mass loading, expected to be proportional to the total active N-doped CIC surface area.3 The N-doped CIC samples retained excellent performance up to a loading of 0.350 μg/cm2 without losing mechanical stability.Similar N-doped CICs and binders of different hydrophobicity were tested in the Zn/air battery testing system. Electrodes made with a hydrophobic NCS microporous layer (MPL) showed much better ORR performance and durability than hydrophilic NCS MPLs. Although electrodes made using hydrophobic polytetrafluoroethylene (PTFE) as the binder in the catalyst layer showed a similar initial performance to those made using hydrophilic Nafion binders, the PTFE based electrodes exhibited better durability. Furthermore, the temperature and pressure used during electrode fabrication were also found to have a significant impact on the binder distribution and ORR performance. Once optimized, a very good correlation was obtained between the N-doped CIC catalyst performance in the RRDE setup and in the Zn/air battery testing system. References J. Pan et al., Adv. Sci., 5, 1700691 (2018).X. Li et al., ACS Appl. Mater. Interfaces, 10, 2130–2142 (2018).N. Gavrilov et al., J. Power Sources, 220, 306–316 (2012).

Development of cathode catalysts with high oxygen reduction reaction (ORR) activity and high durability is essential for the large-scale commercialization of polymer electrolyte fuel cells (PEFCs), which are applied to fuel cell vehicles (FCVs) and residential co-generation systems. At the present stage, costly Pt or its alloys have been employed as the cathode catalyst, having appreciable ORR activity and durability in strong acidic electrolyte at low operating temperatures <100 °C. The mass activity MA of Pt-based catalysts is defined as MA (A gPt −1) = j S (A m−2) × ECA (m2 gPt −1). To increase the area-specific activity j S (current density per active surface area), Pt-M alloys (Pt-Fe, Pt-Co, Pt-Ni, Pt-Cr, etc.) have been examined. Assuming spherical catalyst particles, the electrochemically active surface area ECA is inversely proportional to the particle diameter. To increase the ECA, it is effective to disperse Pt or Pt-alloy nanoparticles on high-surface-area carbon supports (Pt/C or Pt-M/C). However, for developing such catalysts having both high ORR activity and high durability, there has been long-standing controversy surrounding important issues such as the crystal structures (ordered and disordered) and the chemical composition of Pt-M alloys, as well as their optimum particle size. Furthermore, because the mechanism of enhancement of the ORR activities at Pt-M alloys is still unclear, the strategy for designing new potential catalysts has not yet been established. This presentation focuses on the following topics for the cathode catalysts from bulk single crystals to practical nanoparticle catalysts. 1) Pt particle-size effect on the durability for an accelerated test, which simulates load-cycles for FCVsSo far, much research has suffered from trade-offs in determining the optimal particle-size of the catalysts, due to the different trends of Pt particle-size effects on the MA and durability. However, by the use of our n-Pt/C catalysts with a very narrow size distribution (σd ≤ 10%), the most durable catalyst with the highest MA over the whole test period (65°C, 0.6 ↔ 1.0 V, up to 30,000 cycles) was found to be n-Pt2 nm/C.1 2) Enhanced ORR activities at Pt-M alloys2, 3 2-1) Enormously enhanced ORR activity at Pt-skin layer formed on Pt3Co(111) single crystal electrode Pt-skin/Pt–Co(111) single crystals exhibited extremely high ORR activity, j S. The j S value at 0.9 V reached a maximum at Pt73Co27(111), the value of which is ca. 27 times higher than that on a pure Pt(111) electrode.4 By the use of in situ STM, in situ SXS, ex situ XPS, and DFT calculation, we have succeeded in correlating such a high j S with the specific surface structure. 2-2) ORR activity and durability of ordered- and disordered-Pt3Co/C5 We have recently clarified the effect of the crystal structure of Pt3Co alloy nanoparticles on the ORR activity, H2O2 yield, and durability, for the first time, by the use of the ordered- and disordered-Pt3Co/C catalysts with the nearly identical average particle size, size distribution, and composition.5 3) Enhancement in the ORR activity and durability at stabilized Pt-skin–PtCo alloy catalysts6, 7 We have successfully prepared PtCo alloy nanoparticles, having a stabilized Pt skin (one to two atomic layers: xAL), supported on carbon black or graphitized carbon black (PtxAL–PtCo/C or PtxAL–PtCo/GCB). These new catalysts exhibited high MA for the ORR, together with superlative durability. This work was supported by funds for the ‘‘Superlative, Stable, and Scalable Performance Fuel Cell (SPer-FC)’’ project and “High Performance Fuel Cell (HiPer-FC)” project from the NEDO of Japan. References H. Yano, M. Watanabe, A. Iiyama, and H. Uchida, Nano Energy, 8, 13893 (2016).H. Uchida, H. Yano, M. Wakisaka, and M. Watanabe, Electrochemistry, 79, 303 (2011).M. Watanabe, D. A. Tryk, M. Wakisaka, H. Yano, and H. Uchida, Electrochim. Acta, 84, 187 (2012).S. Kobayashi, M. Wakisaka, D. A. Tryk, A. Iiyama, and H. Uchida, J. Phys. Chem. C. 121, 11234 (2017).H. Yano, I. Arima, M. Watanabe, A. Iiyama, and H. Uchida, J. Electrochem. Soc., 164, F966 (2017).M. Watanabe, H. Yano, D. A. Tryk, and H. Uchida, J. Electrochem. Soc., 163, F455 (2016).M. Chiwata, H. Yano, S. Ogawa, M. Watanabe, A. Iiyama, H. Uchida, Electrochemistry, 84, 133 (2016).

Promotion of oxygen reduction performance by Fe3O4 nanoparticles support nitrogen-doped three dimensional meso/macroporous carbon based electrocatalyst

Hierarchically Porous Co3C/Co-N-C/G Modified Graphitic Carbon: A Trifunctional Corrosion-Resistant Electrode for Oxygen Reduction, Hydrogen Evolution and Oxygen Evolution Reactions

Advanced Hierarchically 2D and 3D Nanostructured Materials for Electrochemical Clean Energy Conversion

Non-precious-metal catalysts (NPMCs) for oxygen reduction reaction (ORR) have been of tremendous interests in energy conversion and storage devices. Among several classes of NPMCs, iron and nitrogen supported on carbon (Fe-N/C) have shown the most promising ORR activity. It has been widely suggested that an active site structure for Fe-N/C catalysts contains Fe-Nx coordination. However, the preparation of Fe-N/C catalysts mostly involves a high-temperature pyrolysis step, which generates not only Fe-Nx sites, but also a significant portion of less active large iron-based particles. This poses a great challenge to rational design of Fe-N/C catalysts with abundant Fe-Nx species. We developed “silica-protective-layer-assisted” synthetic approach that can preferentially generate the catalytically active Fe-Nx sites in Fe-N/C catalysts while suppressing the formation of large Fe-based particles. The catalyst preparation consisted of an adsorption of Fe porphyrinic precursor on carbon nanotubes (CNTs), silica layer overcoating, high-temperature pyrolysis, and silica layer etching, which yielded CNTs coated with thin layer of porphyrinic carbon (CNT/PC) catalysts. In situ X-ray absorption spectroscopy during the preparation of CNT/PC catalyst revealed that the interaction between the silica layer and Fe-N4 in a porphyrin precursor appears to protect the Fe-N4 site and to prevent the formation of large Fe-based particles. The CNT/PC catalyst showed very high ORR activity and remarkable stability in alkaline media. Importantly, an alkaline anion exchange membrane fuel cell (AEMFC) with a CNT/PC-based cathode exhibited record high current and power densities among NPMC-based AEMFCs. In addition, a CNT/PC-based cathode exhibited a high volumetric current density of 320 A cm-3 in acidic proton exchange membrane fuel cell, comparable with 2020 DOE target (300 A cm-3). We further demonstrated the generality of this synthetic strategy to other carbon supports including reduced graphene oxides and carbon blacks.

A bifunctional catalyst for both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) could be employed in a unitized regenerative fuel cell, an energy storage device that can be coupled to intermittent renewable energy such as wind or solar to peak-shift electricity to the grid. Though significant work has been done over the past few decades, no catalyst for both ORR and OER with high activity for both reactions have been discovered. It is well know that platinum or platinum based alloys exhibit the highest activities for ORR among all other catalyst, however they display poor activity for OER. In contrary, transition metal (hydro)oxides, such as iron, nickel, or cobalt (hydro)oxides, always possess highly activities for OER but poor activity for ORR. Due to “migration-effect” of transition metal in ORR, in which the free transition metal (hydro)oxides will move to the surfaces of platinum during electrochemical process and results in the dramatically decrease of the ORR activity of Pt eventually, it is impossible to achieve high activity for both ORR and OER by simply mix platinum or platinum based alloy with transition metal (hydro)oxides together. Inspired by this, we developed a novel method to prepare 7nm platinum nanoparticles doped with atomic sized cobalt oxides for the applications of ORR and OER. Our primary results showed that these hetero-structured materials exhibited higher activity than state-of-the-art commercial platinum for ORR, but also 10 times higher activity for OER than Co3O4. Most importantly, our catalysts also demonstrated excellent stability. Apparently, the interfaces must play a dominated role in both ORR and OER. Investigation of the interfaces between platinum and Co3O4 of our catalyst in atomic scale is crucial to understand how the interfaces worked and what kinds of interfaces are better for both ORR and OER. Our work will advanced the fundamental understanding of the metal-oxide interface in enhancing the ORR and OER, which is envisioned to shed light on the design of advanced catalysts for catalyzing complex chemical processes.

Development of high‐efficiency and low‐cost electrocatalysts as the platinum substitute for the oxygen reduction reaction (ORR) is of significance for electrochemical energy conversion and storage devices, such as fuel cells and rechargeable batteries. Here we report graphene‐based nitrogen‐coordinated Fe−Co dual‐atom catalysts (referred to as FeCoN6) with markedly enhanced ORR activity by ligand‐modification. By density functional theory calculations, we thoroughly investigated the ORR activity of three preferred FeCoN6 isomers (denoted as FeCoN6‐I, FeCoN6‐II, and FeCoN6‐III) and their complexes with the ligands of ORR intermediates such as *O, *OH, and *O2. Our results reveal that these ligands cause the apparent shift of d‐orbitals of metal atoms toward the Fermi level to modulate the adsorption strength for reaction intermediates, thereby significantly improving the ORR activity of FeCoN6 complexes. Notably, FeCoN6‐I(OH) and FeCoN6‐I(O2) with the lowest overpotential of ∼0.23 V possessed the best ORR activity, which are superior to pristine FeCoN6 and available Pt catalysts. These results not only build on the fundamental understanding of the catalytic mechanism of ligand modified dual‐atom catalysts but also provide a new strategy to develop highly efficient ORR electrocatalysts by ligand‐modification engineering.

Nitrogen and fluorine co-doped graphite nanofibers (N/F-GNF) and their cumulative effect with Fe and Co have been developed as an alternative non-precious metal catalyst for efficient oxygen reduction reaction (ORR) in acidic media. The synergistic effect between the doped hetero atoms and the co-ordinated Fe and Co towards ORR activity and durability of the catalyst is deeply investigated. A high ORR onset potential comparable with commercial Pt/C catalyst is observed with the Fe-Co/NF-GNF catalyst, which indicates that this catalyst is a potential alternative to Pt/C. A fivefold increase in mass activity is achieved by the Fe-Co/NF-GNF catalyst compared to the simple N/F-GNF catalyst, which endorses the significant role of transition metal atoms in enhancing ORR activity. The advanced Fe-Co/NF-GNF catalyst also exhibits complete tolerance to CH3OH and CO. The Fe-Co/NF-GNF catalyst also exhibits excellent durability towards the ORR with only a 10 mV negative shift in its half wave potential after a 10 000 repeated potential cycling test, whereas in the case of a commercial Pt/C catalyst there was an ∼110 mV negative shift under similar environmental conditions. More stringent corrosive test cycles were also performed by maintaining the cell as high as 1.4 V with a later decrease to 0.6 V vs. RHE for 300 cycles, which showed the excellent durability of the Fe-Co/NF-GNF catalyst in comparison with the Pt/C catalyst. XPS analysis of the Fe-Co/NF-GNF catalyst presents the ORR active chemical states of N (pyridinic-N and graphitic-N) and F (semi-ionic-F) and the co-ordinated sites of Fe and Co species with the dopants. The excellent performance and durability of the Fe-Co/NF-GNF catalyst is due to the synergistic effect between the hetero atoms dopants (N and F) and strong co-ordinating bonds of M-N-C, which protect the graphene layers around the metallic species and greatly mitigates the leaching of Co and Fe during the long term cycling test. The high activity and long term durability of the Fe-Co/NF-GNF catalyst make it a promising ORR electrocatalyst for the fuel cell cathode reaction.

The oxygen reduction reaction (ORR) is one of the most important reactions in renewable energy conversion and storage devices. The full deployment of these devices depends on the development of highly active, stable, and low‐cost catalysts. Herein, a new hybrid material consisting of Na2Ta8O21−x/Ta2O5/Ta3N5 nanocrystals grown on N‐doped reduced graphene oxide is reported. This catalyst shows a significantly enhanced ORR activity by ≈4 orders of magnitude in acidic media and by ≈2 orders of magnitude in alkaline media compared to individual Na2Ta8O21−x on graphene. Moreover, it has excellent stability in both acid and alkaline media. It also has much better methanol tolerance than the commercial Pt/C, which is relevant to methanol fuel cells. The high ORR activity arises not only from the synergistic effect among the three Ta phases, but also from the concomitant nitrogen doping of the reduced graphene oxide nanosheets. A correlation between ORR activity and nitrogen content is demonstrated. Deep insights into the mechanism of the synergistic effect among these three Ta‐based phases, which boosts the ORR's kinetics, are acquired by combining specific experiments and density functional theory calculations.