Oxygen Reduction Reaction Onset Potential Research Articles

Nitrogen-doped carbon nanowalls/diamond films as efficient electrocatalysts toward oxygen reduction reaction

Substitution of commercial Pt/C electrocatalysts with efficient carbon-based ones for oxygen reduction reaction (ORR) still remains a huge challenge. For practical ORR applications it is significant to design robust 3D network nanostructures in that they do not require polymer binders. For conventional powder catalysts, they must be combined with substrate, leading to their shedding and degradation. In this work, vertically-aligned N-doped carbon nanowalls/diamond (N-CNWs/D) films are synthesized by means of a microwave plasma chemical vapor deposition technique, where nitrogen doping is conducted during the growth process and a subsequent facile annealing treatment under Ar atmosphere. The obtained Ar treated N-CNWs/D film exhibits an ORR onset potential of 835 mV (versus reversible hydrogen electrode) in 0.1 mol l−1 KOH solution in a four-electron reaction pathway. It also displays excellent tolerance toward methanol crossover and long-term stability (e.g. a current density loss of only 10% even after 16 h measurement). The boosting ORR performance can be attributed to the activated pyridinic N dopant at abundant edge sites and enlarged electrochemical surface areas of N-CNWs/D films. This work not only develops a controllable strategy to fabricate binder-free carbon-based ORR electrocatalysts, but also paves a way to in-depth understand actual active sites in terms of ORR pathway mechanisms.

Heteroatom-Doped Graphites Oxygen Reduction Catalysts for Anion Exchange Membrane Fuel Cells

Catalysts for the oxygen reduction reaction (ORR) are crucial for electrochemical energy conversion and storage devices. The development of active, ultra-low-cost, critical raw materials-free catalysts is vital for the success of the anion exchange membrane fuel cell (AEMFC) technology [1]. In this work, we present metal-free oxygen reduction catalysts based on heteroatoms (I, N, S or B)-doped graphites by a scalable and reproducible one-step planetary ball milling protocol [2]. The distribution and the morphology of these ultra-low cost catalysts are characterized by advanced electron microscopy showing uniform distribution of heteroatoms in the graphite matrix. We have tested the catalysts for ORR in 0.1 M KOH solution in a rotating ring disk electrode (RRDE) and doped-graphites showed enhanced ORR performance relative to the pristine graphite. Among these, the N-graphite exhibiting the highest ORR onset potential of 0.87 V with a low peroxide yield of 15%@0.1 V. Good ORR kinetic (jk,s ) and mass (ik,m ) activities were also obtained. The catalysts also show remarkable stability in both RDE stress and corrosion tests. Finally, we showed good performance in operando H2-O2 AEMFC, reaching ~350 mW cm‒ 2 at 60 oC with a high voltage efficiency. These results encourage further development of metal-free ultra-low-cost and stable catalysts, easily scalable for AEMFC cathode production.

Noble Metal-Added Titanate Nanosheets for PEFC Cathode

A large overpotential of oxygen reduction reaction (ORR) at a cathode in polymer electrolyte fuel cells (PEFCs) is one of the serious problems to decrease the energy conversion efficiency. Nørskov’s group reported that even Pt which is currently used as an ORR catalyst cannot reach the theoretical oxygen electrode potential of 1.23 V at 298 K [1]. On the other hand, Yamamoto et al. recently suggested that noble metal doped titanium dioxides (TiO2) can reach the theoretical potential by changing the absorption energies of the ORR intermediates according to the first-principle calculations [2]. However, the first-principle calculations did not consider the continuous electron supply to proceed the ORR. It is difficult to form the sufficient electron conduction path from supports to active sites on the TiO2 because TiO2 has low conductivity due to a large band gap. Therefore, we focused on the formation of the active sites on the TiO2 surface by doping noble metal such as Rh and Pd and the electron conduction path from carbon supports to the active sites by preparing the titanate nanosheets.Na2Ti3O7 was synthesized as precursor of anionic titanate nanosheets([Ti3O7]2- ;TiNS) according to the previous paper [3]. Na2Ti3O7 was treated through ion-exchange reaction with hydrochloric acid and tetramethylammonium aqueous solution, finally TiNS were obtained. A TEM observation revealed that the formation of very large sheet-like materials with a size of approximately upwards of ten micrometers, resulting that TiNS were completely synthesized.Next, we focused on the formation of the electron conduction path through nanosheets from supports using the electron tunneling effect. When the nanosheets were directly supported on the glassy carbon electrode, the ORR activity was the same as that of GC, and almost no ORR current flowed even low potential region. Then, vapor grown carbon fibers (VGCFs) as purchased, with immersion of mixed acid (MA; H2SO4 and HNO3) at 170 oC for 24 h and PDDA (Poly(diallyl dimethylammonium chloride)) as polycation were attempted to use as supports. The TiNS were immersed in the distilled water dispersed with the treated VGCFs to be supported by them. These catalysts were designated as TiNS/VGCF, TiNS/VGCF(MA), and TiNS/VGCF(MA-PDDA).The ORR current of TiNS/VGCF(MA-PDDA) was larger than that of TiNS/VGCF, TiNS/VGCF(MA). In addition, the ORR onset potential of the TiNS/VGCF(MA-PDDA) increased by 0.3 V compared to that of TiNS/VGCF. These results suggest that surface treatment of VGCFs was effective to form the electron conduction path from VGCF to active sites of nanosheet surface. The electrostatic interaction between VGCF surface positively charged by PDDA and anionic TiNS contributed the formation of the electron conduction path. However, according to the TEM observation, TiNS were partially contacted with VGCF and some TiNS were agglomerated. It is necessary to obtain higher ORR activity that a control of size of TiNS and contact with supports such as VGCF.On the other hand, we attempted to dope Rh and Pd to TiNS through solid-state reaction. Rh-,Pd-doped TiNS (designated as Rh-TiNS and Pd-TiNS) were synthesized the same way as previous paper [3]. XRD patterns of the precursors revealed that Rh2O3 or PdO were existed with Na2Ti3O7 above the addition of 5 atomic %. Rh-TiNS and Pd-TiNS were supported by VGCF(MA-PDDA) after Rh-TiNS and Pd-TiNS were treated TMAOH aqueous solution. The ORR activities of the samples with the addition of 5at% Rh or 10at% Pd were higher than those of the samples which Rh2O3 or PdO were not observed in XRD patterns. The ORR active sites were estimated by Rh2O3 or PdO deposited on the surface of the TiNS. This result showed that an amount of doped Rh or Pd was limited to Na2Ti3O7. The limitation of doped Rh or Pd was 5at% or 10at%, respectively. Therefore, the doped amounts of noble metals such as Rh or Pd to TiNS should be appropriately controlled.AcknowledgementThe authors thank the New Energy and Industrial Technology Development Organization (NEDO) for financial support. This work was partly supported by TOKYO CITY UNIVERSITY Interdisciplinary Research Center for Nano Science and Technology for instrumental analysis.Reference(1) A. Kulkarni et al., Chem. Rev., 118, 2302 (2018).(2) Y. Yamamoto et al., J. Phys. Chem. C., 123, 19486 (2019).(3) W. Soontornchaiyakul et al., RSC Adv., 7, 21790 (2017). Figure 1

CoNi Nanoparticles Supported on N-Doped Bifunctional Hollow Carbon Composites as High-Performance ORR/OER Catalysts for Rechargeable Zn-Air Batteries.

Searching for high-quality air electrode catalysts is the long-term goal for the practical application of Zn-air batteries. Here, a series of coexistent composite materials (CoNi/NHCS-TUC-x) of cobalt-nickel supported on nitrogen-doped hollow spherical carbon and tubular carbon are obtained using a simple pyrolysis strategy. Co and Ni in the composites are mainly present in the form of alloy nanoparticles, M-Nx and M-Cx (M = Co or Ni) species, with high oxygen reduction reaction (ORR)/oxygen evolution reaction (OER) electroactivity. The materials containing different proportions of spherical carbon and tubular carbon obtained by simply adjusting the raw materials for generating tubular carbon exhibit interesting bifunctional performance: samples with an abundant tubular content have the highest ORR onset potential (0.91 V vs reversible hydrogen electrode), while those with a rich spherical content have the highest ORR current density (5.13 mA·cm-2). Furthermore, CoNi/NHCS-TUC-3 provides the lowest potential difference (ΔE = Ej=10 - E1/2) of 0.806 V. We then test the potential possibility of CoNi/NHCS-TUC-3 as an air electrode for primary and rechargeable Zn-air batteries. The primary battery delivers an open-circuit potential of 1.59 V, a peak power density of 361.8 mA·cm-2, and a specific capacity of 756.5 mA h·gZn-1. The rechargeable battery could be cycled stably for more than 55 h at 10 mA·cm-2. These characteristics make CoNi/NHCS-TUC-3 a superior electrocatalyst for both the ORR and OER, as well as a suitable bifunctional electrode applied to a rechargeable Zn-air battery.

Bifunctional Oxygen Electrocatalysis on Mixed Metal Phthalocyanine-Modified Carbon Nanotubes Prepared via Pyrolysis.

Non-precious-metal catalysts are promising alternatives for Pt-based cathode materials in low-temperature fuel cells, which is of great environmental importance. Here, we have investigated the bifunctional electrocatalytic activity toward the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) of mixed metal (FeNi; FeMn; FeCo) phthalocyanine-modified multiwalled carbon nanotubes (MWCNTs) prepared by a simple pyrolysis method. Among the bimetallic catalysts containing nitrogen derived from corresponding metal phthalocyanines, we report the excellent ORR activity of FeCoN-MWCNT and FeMnN-MWCNT catalysts with the ORR onset potential of 0.93 V and FeNiN-MWCNT catalyst for the OER having EOER = 1.58 V at 10 mA cm–2. The surface morphology, structure, and elemental composition of the prepared catalysts were examined with scanning electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy. The FeCoN-MWCNT and FeMnN-MWCNT catalysts were prepared as cathodes and tested in anion-exchange membrane fuel cells (AEMFCs). Both catalysts displayed remarkable AEMFC performance with a peak power density as high as 692 mW cm–2 for FeCoN-MWCNT.

Silver Nanowire‐Based Catalysts for Oxygen Reduction Reaction in Alkaline Solution

AbstractThe electrocatalytic activity of commercial silver nanowires (AgNWs) with different diameters (35, 90, and 120 nm) was investigated towards the oxygen reduction reaction (ORR) in 0.1 M KOH solution. The rotating disc electrode (RDE) data analysis revealed that the ORR onset potential and half‐wave potential shift positively for AgNWs as compared to a polycrystalline Ag electrode. The enhanced ORR performance of AgNWs with a diameter of 35 nm was attributed to the elongated porous structure of the catalyst material, which facilitated the mass transport in the catalyst layer. The Koutecky‐Levich analysis was used to determine the kinetic parameters of O2 reduction and to confirm that the ORR proceeds via 4‐electron pathway on AgNW‐modified electrodes. Stability tests showed that 35 nm thick AgNWs had high stability during long‐term potential cycling. Based on the obtained results, the AgNWs have great potential as efficient catalysts for alkaline membrane fuel cell technologies.

Red Bean Pod Derived Heterostructure Carbon Decorated with Hollow Mixed Transition Metals as a Bifunctional Catalyst in Zn-Air Batteries.

Design and synthesis of low-cost and efficient bifunctional catalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in Zn-air batteries are essential and challenging. We report a facile method to synthesize heterostructure carbon consisting of graphitic and amorphous carbon derived from the agricultural waste of red bean pods. The heterostructure carbon possesses a large surface area of 625.5 m2 g-1 , showing ORR onset potential of 0.89 V vs. RHE and OER overpotential of 470 mV at 5 mA cm-2 . Introducing hollow FeCo nanoparticles and nitrogen dopant improves the bifunctional catalytic activity of the carbon, delivering ORR onset potential of 0.93 V vs. RHE and OER overpotential of 360 mV. Electron energy-loss spectroscopy (EELS) O K-edge map suggests the presence of localized oxygen on the FeCo nanoparticles, suggesting the oxidation of the nanoparticles. Zn-air battery with these carbon-based catalysts exhibits a peak power density as high as 116.2 mW cm-2 and stable cycling performance over 210 discharge/charge cycles. This work contributes to the advancement of bifunctional oxygen electrocatalysts while converting agricultural waste into value-added material.

ORR performance evaluation of Al-substituted MnFe2O4/ reduced graphene oxide nanocomposite

Active and durable oxygen reduction reaction (ORR) catalysts are of utmost importance to realize the commercialization of hydrogen fuel cells and metal-air batteries. Al-substituted MnFe2O4-based ternary oxide and reduced graphene oxide (MAF-RGO) nanocomposite is synthesized using an in-situ co-precipitation followed by a hydrothermal process and verified for ORR electrocatalysis in the alkaline electrolyte (0.1 M KOH). MAF-RGO is first analyzed using physicochemical characterization tools including X-ray diffraction, Raman spectroscopy, sorption studies, electron microscopy, X-ray photoelectron spectroscopy, etc. Further, the characteristic ORR peak centered at 0.56 V vs. reversible hydrogen electrode (RHE) in cyclic voltammetry (CV) studies confirms the electrocatalytic performance of MAF-RGO. The ORR onset potential of 0.92 V vs. RHE is obtained in linear sweep voltammetry (LSV) measurement at 1600 rpm in O2-saturated electrolyte exhibiting an improved ORR performance as compared to the commercial electrocatalyst. The reduction kinetics is observed to follow the desirable near 4-e- mechanism. In addition, the electrocatalyst exhibits improved relative current stability of 86% and methanol poisoning resistance of 82%, which is better in comparison to the standard Pt/C. The observed electrochemical performance results from the synergism between the oxygen vacancy-rich Al-substituted metallic oxide active species and the functional group enriched predominantly mesoporous RGO sheets with excellent electrical conductivity. The introduction of metallic species enhanced the inter-planar spacing between graphitic sheets easing the maneuver of reactant species through the electrocatalyst and accessing more ORR-active sites. This study establishes the potency of mixed transition metal oxide/nanocarbon composites as durable high-performance ORR-active systems.

Molecular M-N4 macrocycles in a nitrogen-carbon matrix as a highly durable oxygen reduction reaction (ORR) electrocatalysts in acid media

Proton electrolyte membrane fuel cell (PEMFC) is one of the most popular clean energy systems, which has attracted attention in a few recent years. However, due to the sluggish oxygen reduction reaction (ORR) in acid media, platinum-based catalysts are highly required in the-state-of-the-art PEMFC. Also, the low durability and scarcity of novel metals significantly limit its wide application. This study applied a novel plasma engineering route to fabricate molecular transition metal macrocycles (M-N4 where M = Fe, Co, Mn) in situ into nitrogen-carbon support. CoN4 showed the highest ORR onset (Eonset) and halfwave potential (E1/2) of 0.78 and 0.67 V vs. RHE, respectively, which was only slightly inferior by 100 mV in ORR onset compared to the benchmark 20 wt.% Pt/C. On the other hand, CoN4 presented extremely high durability (E1/2 = −15 mV after 3000 cycles) and outperformed 20 wt.% Pt/C (E1/2 = −140 mV after 3000 cycles). This study presented a novel plasma engineering to fabricate molecular transitional metal-N4 doped carbon as a highly durable and economic ORR electrocatalyst to replace current benchmark Pt-based metal for PEMFC.

Pyrolysis-Derived Carbon Auto-Coated Co–Ni Oxide-based Nanoparticles on Graphene-like Nanosheets for High-Performance Oxygen Electrocatalysis

Engineering transition-metal-based bifunctional oxygen electrocatalysts with high activity and stability has always been a problem. Traditional bare transition-metal-based nanoparticles supported on the outer surface of carbon materials are susceptible to the Ostwald-ripening effect in a catalytic process, consequently suffering from poor long-term operation. However, constructing a highly efficient and stable bifunctional oxygen electrocatalyst is still challenging. Here, we report graphene-like carbon nanosheets modified with carbon shell-coated binary Co–Ni oxide-based nanoparticles (CoNiOx@C/G-NSs) by one-step pyrolysis method, in which the hybrids show a three-dimensional fluffy and hierarchical porous nanostructure and large numbers of carbon shell-coated binary Co–Ni oxide-based nanoparticles (CoNiOx@C) dispersed on graphene-like carbon nanosheets evenly. More excitingly, the carbon nanosheets are nitrogen-doped. When applied for oxygen evolution reaction (OER) and oxygen reduction reaction (ORR), the onset potential (Eonset) of OER reached 1.26 V, and onset potential of ORR and half-wave potential (E1/2) reached 0.90 and 0.78 V, respectively. The hybrids showed enhanced methanol tolerance, with few changes in the ORR performance with or without adding methanol, and high stability, with 84% activity retention after 10 h continuous reaction. The productivity of H2O2 is about 5% and the number of electron transfer (n) is about 3.9 in the process of catalyzing ORR; density functional theory calculation reveals its high selectivity in the 4e– pathway. Its excellent performance is mainly attributed to the synergistic enhancement effect: three-dimensional fluffy and hierarchical porous nanostructure provided a large catalytic activity area, which enabled the effective participation of species and facilitated rapid mass transfer; the rapid adsorption of O2 by N-doped graphene-like carbon nanosheets ensured fast electron transport; synergistic reactivity between carbon shell and CoNiOx enhanced the activity, and numerous CoNiOx@C nanoparticles reconciled the electron density of the carbon shell, which introduced more active sites; and the carbon shell weakened the Ostwald-ripening effect and ensured the stability of the nanoparticle. All advantages synergistically enhance the catalytic efficiency and make it one of the best bifunctional oxygen electrocatalysts.

Green synthesis of N, P-co doped porous reduced graphene oxide as an active metal-free electrocatalyst toward oxygen reduction reaction

Developing high-efficient and price-friendly novel cathode catalysts to substitute commercial Pt/C catalyst is extremely vital for the future of fuel cells system, because the oxygen reduction reaction (ORR) is the cornerstone of diverse sustainable energy-conversion technologies. Metal-free heteroatom-doped carbons as ORR catalysts have attracted worldwide attentions. Herein, low-cost and easily available inorganic fertilizers are utilized as heteroatom precursors, successfully fabricating the N, P-co doped porous reduced graphene oxide (NP-PrGO) with the help of ascorbic acid, and the ORR performance of NP-PrGO samples at diverse pyrolysis temperatures are further studied in detail. The physical characterizations illustrate that all products exhibit loose and porous three-dimensional structures, large surface area, abundant defects and excellent electrocatalytic activity. Particularly, the optimal NP-PrGO-1100 (the pyrolysis temperature is 1100 °C) displays a great positive ORR onset potential (0.913 V vs. RHE) and half-wave potential (0.819 V vs. RHE), as well as a large current density at 0.2 V (vs. RHE, −5.48 mA cm−2). Furthermore, it also shows superb methanol tolerance and extraordinary electrochemical stability, which is better than that of Pt/C. The origin of enhanced ORR performance for NP-PrGO-1100 can be attributed to its porous structure, large surface area (202.02 m2 g−1) and highest amount of active N species (pyridinic N and graphitic N, 82%). These glorious properties make the NP-PrGO-1100 a promising metal-free ORR catalyst for replacing Pt/C in fuel cells.

Shape-tuned, surface-active and support-free silver oxygen reduction electrocatalyst enabled high performance fully non-PGM alkaline fuel cell.

Exploring non-platinum group metal (n-PGM) based efficient oxygen reduction reaction (ORR) electro-catalysts is highly important for realizing advancement in sustainable next generation-alkaline anion exchange membrane fuel cells (AAEMFCs). Herein, we demonstrate a new “hierarchical shape tuning approach” for the synthesis of controlled sized and shaped non-PGM based Ag ORR electro-catalysts with surface active nano-islands. Hierarchical shapes ranging from spherical (S-AgNs), worm-in-sphere, sphere-in-worm and vermiform (worm-like) Ag nanostructures (V-AgNs) were obtained by precisely varying the ratios of capping agent to dual reducing agents in water at ambient conditions. Compared to S-AgNs, V-AgNs revealed a higher mass normalized ORR Tafel activity (0.303 A mgAg−1 at 0.9 V), onset (1.06 V) and half wave (0.78 V) potentials and higher retention of limiting current density (>88%) after 5000 cycles in 0.5 M potassium hydroxide (KOH) solution attributable to their unique worm like morphology with surface active nano-islands and support free-nature enabled better catalyst utilization. In a fully “non-PGM AAEMFC” (n-PAAEMFC), V-AgNs exhibited the highest fuel cell activity of 115.6 mW cm−2 and stable short-term durability (∼240 h) compared to S-AgNs (41.3 mW cm−2) and previously reported fully n-PAAEMFCs indicating their potential use in next-generation alkaline fuel cells.

Ceria Nanodots Anchored on ZIF67-Derived Framework By Atomic Layer Deposition for Efficient Bifunctional Oxygen Electrocatalysis

The oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), which are the two most important reactions for electrocatalytic activities, has been widely studied for sustainable and renewable electrochemical applications, including fuel cells[1] , [2], metal-air batteries[3] and water electrolysis[4]. For the development of new catalysts alternative to noble metal-based materials, metal organic frameworks (MOFs) are being actively used as the precursor structure. ZIF-67, a kind of MOF structure made of cobalt nitrate and 2-methylimidazole ligands, has proved to be a good candidate for ORR catalysis due to their rich of Co-N-C active sites. However, ZIF-67 tends to aggregate during the pyrolysis process by breaking C-H or C-O bonds.In this study, atomic layer deposition (ALD) was applied to coat an atomically thin cerium oxide (CeO2, ceria) coatings the MOF structure, which is expected to incur synergistic catalytic activity and suppress the thermal agglomeration. ALD is a self-limiting surface chemistry process, and is able to create highly conformally layer-by-layer thin film on any complex 3D structure with controllable thickness at an atomic level.[5] Ceria has shown good synergistic effect with other metals for OER performance due to its ability for electron exchange and ion reduction.[6] We demonstrates a facile synthesis of Co/Ce co-doped porous carbon nanomaterials by pyrolyzing the ceria-coated ZIF structure. The resultant samples showed uniform crystal structure which are in well accordance with ZIF-67 precursors. The as-prepared samples also showed enhanced electrocatalytic activities for both ORR and OER comparable to that of noble metal benchmarks (Pt/C and IrO2) and in terms of onset potential, half-wave potential, and current density. A sample prepared by performing 15 cycles of ceria ALD followed a pyrolysis at 700 °C shows the highest onset potential (0.95 V versus RHE) for ORR and superior stability in comparison to commercialized Pt/C. Meanwhile, a sample with 5 cycles of ceria ALD exhibits an excellent OER performance (1.48 V at 10 mA cm-2) and maintained a good OER durability performance (from 1.48 V to 1.50 V after 2,000 potentiodynamic cycles).

P-Doped SnO2 Powder as a Support for PEFC Cathode

Polymer electrolyte fuel cells (PEFCs) are used as power sources for residential and transportation use. In the present PEFCs, a large amount of platinum supported carbon is used as a catalyst in the cathode electrode. However, a platinum catalyst has some problems such as high cost, small resources, low durability, and large overvoltage for oxygen reduction reaction (ORR)considering the future widespread use of PEFC2. In addition, the durability of carbon support has become a serious problem especially under future driving conditions3 (high temperature and high voltage) for automobiles. In order to solve the problem, there is a strong demand for substitution of highly durable materials, that is, a non-platinum catalyst and a non-carbon support. Therefore, we have developed platinum substitution catalysts using group 4,5 metal oxides4.Regarding non-carbon supports, we have focused on Nb-doped TiO2 because of its high stability in acidic media. However, the specific surface area and the electro-conductivity of Nb-doped TiO2 are insufficient to support an oxide catalyst. Therefore, in this study, we focused on P-doped SnO2 (Mitsubishi Materials), which has excellent specific surface area, electronic conductivity, and stability under oxidative atmosphere, and examined its applicability as a cathode support.On the other hand, regarding the active sites, even carbon was used as a support, the oxygen reduction activities of group 4 and 5 oxide cathodes were much lower than that of platinum. This is because both quality and quantity of the active sites of oxide-based cathodes are lower than that of platinum. Recently, according to the first-principles calculation, Sugino et al, found that when Pd is doped into TiO2, the onset potential for ORR exceeds platinum (Because the adsorption Gibbs energies of the reaction intermediates of ORR are adjusted, the energy barrier along the reaction path become lower. The quality of active sites is improved by Pd doping into TiO26). Although no calculations have been performed for tin oxide, we focused on Pd doping into SnO2 to form the active sites in this study. Therefore, the purpose of this study was to modify P-doped SnO2 with Pd and investigate its catalytic activity.A 2 mM H2PdCl4 aqueous solution was mixed with ethanol, and P-doped SnO2 (Mitsubishi Materials) as a support was further added. An electron beam irradiation was carried out at room temperature. The conditions of the electron beam irradiation were as follows; electron beam energy of 2 MGy, absorbed dose to water of 500 kGy, and irradiation time of 50 min. Sample A was prepared by mixing 20 mL of a 2 mM H2PdCl4 aqueous solution and 1 mL of ethanol, that is, adjusting the volume ratio of (ethanol) / (H2PdCl4) = 0.05. Sample B was prepared by mixing 3.4 mL of 2 mM H2PdCl4 aqueous solution and 17 mL of ethanol, that is, adjusting the volume ratio of (ethanol) / (H2PdCl4) = 5. A loading Pd was adjusted to 1 wt% with respect to P-doped SnO2 in both samples.Figure 1 shows the comparison of potential-current curves for the ORR of Pd-modified P-doped SnO2. Sample A had higher catalytic activity than sample B. It was found that the ORR activity was improved by reducing the volume ratio of (ethanol) / (H2PdCl4) at the catalyst preparation.AcknowledgmentThis research is supported by the New Energy and Industrial Technology Development Organization (NEDO) and the Strategic International Joint Research Program (SICORP) of the Japan Science and Technology Agency (JST). In addition, the authors wish to thank the support of JSPS grants-in-aid for scientific research, Suzuki Foundation and Tonen General Sekiyu Research / Development Encouragement & Scholarship Foundation.

A pyridinic Fe-N4 macrocycle models the active sites in Fe/N-doped carbon electrocatalysts

Iron- and nitrogen-doped carbon (Fe-N-C) materials are leading candidates to replace platinum catalysts for the oxygen reduction reaction (ORR) in fuel cells; however, their active site structures remain poorly understood. A leading postulate is that the iron-containing active sites exist primarily in a pyridinic Fe-N4 ligation environment, yet, molecular model catalysts generally feature pyrrolic coordination. Herein, we report a molecular pyridinic hexaazacyclophane macrocycle, (phen2N2)Fe, and compare its spectroscopic, electrochemical, and catalytic properties for ORR to a typical Fe-N-C material and prototypical pyrrolic iron macrocycles. N 1s XPS and XAS signatures for (phen2N2)Fe are remarkably similar to those of Fe-N-C. Electrochemical studies reveal that (phen2N2)Fe has a relatively high Fe(III/II) potential with a correlated ORR onset potential within 150 mV of Fe-N-C. Unlike the pyrrolic macrocycles, (phen2N2)Fe displays excellent selectivity for four-electron ORR, comparable to Fe-N-C materials. The aggregate spectroscopic and electrochemical data demonstrate that (phen2N2)Fe is a more effective model of Fe-N-C active sites relative to the pyrrolic iron macrocycles, thereby establishing a new molecular platform that can aid understanding of this important class of catalytic materials.

Modulation of Single Atomic Co and Fe Sites on Hollow Carbon Nanospheres as Oxygen Electrodes for Rechargeable Zn-Air Batteries.

Efficient bifunctional electrocatalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are required for metal air batteries, to replace costly metals, such as Pt and Ir/Ru based compounds, which are typically used as benchmarks for ORR and OER, respectively. Isolated single atomic sites coordinated with nitrogen on carbon supports (M-N-C) have promising performance for replacement of precious metal catalysts. However, most of monometallic M-N-C catalysts demonstrate unsatisfactory bifunctional performance. Herein, a facile way of preparing bimetallic Fe and Co sites entrapped in nitrogen-doped hollow carbon nanospheres (Fe,Co-SA/CS) is explored, drawing on the unique structure and pore characteristics of Zeolitic imidazole frameworks and molecular size of Ferrocene, an Fe containing species. Fe,Co-SA/CS showed an ORR onset potential and half wave potential of 0.96 and 0.86 V, respectively. For OER, (Fe,Co)-SA/CS attained its anodic current density of 10 mA cm-2 at an overpotential of 360 mV. Interestingly, the oxygen electrode activity (ΔE) for (Fe,Co)-SA/CS and commercial Pt/C-RuO2 is calculated to be 0.73 V, exhibiting the bifunctional catalytic activity of (Fe,Co)-SA/CS. (Fe,Co)-SA/CS evidenced desirable specific capacity and cyclic stability than Pt/C-RuO2 mixture when utilized as an air cathode in a homemade Zinc-air battery.

Metal (Mn, Fe, Co, Ni, Cu, and Zn) Phthalocyanine-Immobilized Mesoporous Carbon Nitride Materials as Durable Electrode Modifiers for the Oxygen Reduction Reaction.

In the search for alternative sources to replace fossil fuels, carbon nitride materials can be used in a variety of ways. In the present work, porosity is introduced to the carbon nitride material using mesoporous silica material, MCM-41, as a hard template, and a mesoporous carbon nitride (MCN) material is synthesized. Further, the MCN is modified by immobilizing metal phthalocyanine (MPc, where M = Mn, Fe, Co, Ni, Cu, and Zn). The resulting MPc-incorporated MCN materials (MPc@MCN) were tested for the electrocatalytic oxygen reduction reaction (ORR) in acidic and basic media. Detailed studies reveal that the FePc@MCN and CoPc@MCN materials exhibit higher ORR activity than the other composites in 0.1 M KOH. FePc@MCN follows a direct four-electron oxygen reduction mechanism and shows ORR onset potential (vs RHE) at 0.93 V (in 0.1 M KOH), which is very close to the onset potential exhibited by the state-of-the-art material, Pt-C (1.0 V), and higher than several similar composites of MPc with carbon supports tested in similar environments. Besides, due to the inherent property of coordination through nitrogen present on the MCN, FePc@MCN shows excellent stability even after 3000 cyclic voltammetry (CV) cycles. FePc@MCN was found to have a better methanol tolerance in comparison to Pt-C in basic medium. CoPc@MCN shows a highly selective two-electron reduction reaction in both acidic and basic media at lower overpotential than many of the reported catalysts for the two-electron oxygen reduction. Therefore, these materials (FePc@MCN and CoPc@MCN) can be used as suitable alternatives to replace Pt and other expensive materials in ORR and related applications.

Heteropolytungstate-assisted fabrication and deposition of catalytic silver nanoparticles on different reduced graphene oxide supports: Electroreduction of oxygen in alkaline electrolyte

Abstract The unique feature of Keggin-type dodecatungstosilicate (SiW12O404−) to undergo adsorption on silver surfaces is explored here to fabricate, modify and stabilize silver nanoparticles in acid medium, as well as to link them to different reduced graphene oxide (rGO) substrates: SiO2-doted rGO, carboxylate-derivatized rGO, and conventional rGO (obtained through the chemical reduction method). Upon medium transfer to alkaline electrolyte, the inorganic polyoxometalate-capping-ligand disappears but silver nanostructures remain on rGO supports. The electrocatalytic activity of the resulting systems are probed during the oxygen reduction reaction (ORR). It is apparent from XRD data that silver is crystalline in nature and, particularly in a case of SiO2-doted rGO supports, metallic silver nanocrystals have face-centered-cubic structures enriched with high-index Ag(111) facets known to facilitate ORR. Functionalization of graphene with SiO2 seems to provide an efficient way to keep the nanosheets of graphene exfoliated thus facilitating unimpeded flux of reactants and making the system easily accessible for intercalation with catalytic Ag. Silver nanoparticles supported onto SiO2-doted rGO are relatively the most active toward ORR when examined under rotating ring disk conditions in terms of the observed highest catalytic current density, the most positive ORR onset potential, and the lowest percent formation of the H2O2 reaction intermediate.

Unveiling the Potential of an Fe Bis(terpyridine) Complex for Precise Development of an Fe-N-C Electrocatalyst to Promote the Oxygen Reduction Reaction.

Improvements in highly efficient precious-metal-free electrocatalysts for the oxygen reduction reaction (ORR) are extremely important but still a significant challenge. Herein, we report a novel catalyst design strategy integrating a bis(terpyridine) (hexadentate chelating ligand) with Fe which acts as nitrogen, a self-supporting carbon source, and a potent metal-ligand active site binding structure (Fe-btpy) and promotes the formation of Fe-Nx/C active sites, bypassing the complications induced during Fe-N-C catalyst synthesis. The resulting Fe-N/C(H,P) electrocatalyst shows a very high ORR onset (Eonset) and half-wave potential (E1/2) of 1.05 and 0.89 V (vs reversible hydrogen electrode), respectively, outperforming the commercial Pt/C catalyst in alkaline medium. Most importantly, the Fe-N/C(H,P) catalyst displays decent stability and remarkable methanol tolerance in comparison to the Pt/C catalyst. A fabricated rechargeable zinc-air battery with an Fe-N/C(H,P) cathode catalyst demonstrated an excellent peak power density of 225 mW cm-2 at a current density of 240 mA cm-2, in comparison to the Pt/C cathode catalyst. This work illuminates blueprints utilizing a new long-chain one-dimensional macromolecule that could be viable to produce Fe-N/C-based carbon electrocatalysts toward energy conversion applications.

Carbon-supported cobalt (III) complex for direct reduction of oxygen in alkaline medium

Transition metal-nitrogen-carbon catalysts obtained from pyrolysis of simple sources of metal and nitrogen or metal-organic frameworks are envisioned as promising replacement of Pt-based catalysts for oxygen reduction reaction (ORR) in fuel cells and metal-air batteries. However, the lack of clarity on the active site structure is a fundamental problem in developing efficient catalysts for the complete reduction of oxygen with maximum active site density. In this study, we have synthesized a simple metal-organic complex, [Co(bpy)2CO3]NO3 ⋅5H2O, 1 (bpy = 2, 2′-bipyridine) and studied its ORR activity in 0.1 M KOH supporting on Ketjenblack EC-600JD, 1/C. The crystal structure of 1 was unambiguously determined using single-crystal X-ray diffraction. The onset potential of ORR, Tafel slopes, kinetic current density, the fraction of oxygens reduced to peroxyl ion (χHO2−) and the electron transfer number (n) were compared with relevant literature and commercial 20 wt % Pt/C. Primarily, 1/C reduces O2 through the direct 4-electron pathway. ‘n’ and χHO2−on 1/C were found to be 3.87 and 7% at 0.7 ViR-freevs. RHE, respectively. The turn-over frequencies for the 4- and 2-electron process were found to be 0.124 and 0.001 electrons [Co]−1 s−1 at 0.7 ViR-freevs. RHE, respectively. Density-functional theory analysis reveals that the preferential activation of the side-on mode of adsorption on 1 is responsible for high selectivity in the direct 4-electron pathway.