Potential For Oxygen Reduction Reaction Research Articles

Weakening Intermediate Bindings on CuPd/Pd Core/shell Nanoparticles to Achieve Pt‐Like Bifunctional Activity for Hydrogen Evolution and Oxygen Reduction Reactions

AbstractAlthough Pd is a potential substitution of Pt‐based catalysts for the hydrogen evolution reaction (HER) and oxygen reduction reaction (ORR), the binding of *H and oxygenated (*O, *OOH, *OH) intermediates on Pd are stronger than on Pt, leading to its inferior activity for HER and ORR. In this work, CuPd/Pd core/shell nanoparticles with an ultrathin Pd shell (0.5 nm) are developed, which demonstrate the Pt‐like bifunctional activity for HER and ORR in acid electrolytes. The overpotential at 350 mA cm−2 for HER and the half‐wave potential for ORR on the optimal CuPd/Pd core/shell NPs are 76 mV and 0.854 V versus reversible hydrogen electrode (RHE), respectively, which are comparable to that of Pt and among the best of the reported Pd‐based catalysts. Density functional theory calculations indicate that the significantly enhanced HER/ORR activity on CuPd/Pd core/shell NPs with 0.5 nm Pd shell stem from the compressive strain induced downshift of d‐band center for Pd (by 2.0%), which weakens the binding strength of *H and oxygenated intermediates and promotes the reaction kinetics.

Atomic layer deposited nickel sulfide for bifunctional oxygen evolution/reduction electrocatalysis and zinc–air batteries

Rechargeable Zn−air batteries are a promising type of metal-air batteries for high-density energy storage. However, their practical use is limited by the use of costly noble-metal electrocatalysts for the sluggish kinetics of the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) occurred at the air electrode of the Zn−air batteries. This work reports a new non-precious bifunctional OER/ORR electrocatalyst of NiS x /carbon nanotubes (CNTs), which is made by atomic layer deposition (ALD) of nickel sulfide (NiS x ) on CNTs, for the applications for the air electrode of the Zn−air batteries. The NiS x /CNT electrocatalyst on a carbon cloth electrode exhibits a low OER overpotential of 288 mV to reach 10 mA cm−2 in current density, and the electrocatalyst on a rotating disk electrode exhibits a half-wave ORR potential of 0.81 V in alkaline electrolyte. With the use of the NiS x /CNT electrocatalyst for the air electrode, the fabricated aqueous rechargeable Zn−air batteries show a fairly good maximum output power density of 110 mW cm−2, which highlights the great promise of the ALD NiS x /CNT electrocatalyst for Zn−air batteries.

Synthesis of Ag-Ni-Fe-P Multielemental Nanoparticles as Bifunctional Oxygen Reduction/Evolution Reaction Electrocatalysts.

Multielemental nanoparticles (MENPs) provide the possibility to integrate multiple catalytic functions from different elements into one nanoparticle. However, it is difficult to synthesize Ag-based MENPs with transition metals such as Ni and Fe because of the strong phase segregation between Ag and the other metals. Here, we show that nonmetal element P can help the amalgamation of Ag with other metals. Ag-Ni-Fe-P MENPs are successfully synthesized by a solution-phase chemistry, and they demonstrate excellent bifunctional oxygen reduction reaction (ORR)/oxygen evolution reaction (OER) catalytic activities (the potential gap of the potential at 10 mA·cm-2 for OER and half-wave potential for ORR is 630 mV). More important, the synergistic effect from the MENPs endows them with even higher ORR or OER activity than the Ag or NiFeP nanoparticles. A rechargeable Zn-air battery is fabricated by using the Ag-Ni-Fe-P MENPs as the air electrode. The battery has an energy efficiency of ∼60% at 10 mA cm-2. Its performance is almost unchanged during a working period of 250 h, surpassing the Pt/C+IrO2-based battery. These results suggest that the rationally designed MENPs can integrate multiple catalytic functions together and achieve a synergistic effect, which can be used as high-performance multifunctional catalysts.

DFT study of C2N-supported Ag3M (M = Cu, Pd, and Pt) clusters as potential oxygen reduction reaction catalysts

The performance of C2N-supported Ag3M clusters (Ag3M/C2N, M = Cu, Pd, and Pt) as oxygen reduction reaction (ORR) catalysts is investigated systematically by density functional theory methods. All the studied Ag3M can stably anchored on C2N based on the calculated binding energy. The investigated binding strength of the ORR species (*O, *OOH, and *OH) on Ag3M/C2N predicts that the ORR catalytic activity of some Ag3M/C2N (e.g. Ag3Cu-B/C2N) may similar or even better than that of Pt(111). Further analysis of calculated adsorption free energy (ΔG*species) demonstrates that the ΔG*OH exhibits good linear relationship with ΔG*OOH and ΔG*O. Based on the free energy diagrams, the ORR catalytic activity of Ag4 and Ag3M is significantly improved after they supported on C2N. The obtained results of ORR overpotential (ηORR) demonstrate that two catalysts possess higher ORR activity than Pt(111), namely Ag4/C2N and Ag3Cu-B/C2N, with the ηORR of 0.31 and 0.33 V, respectively. Meanwhile, the ηORR on Ag3Pt-B/C2N, Ag3Pd-T/C2N, and Ag3Pd-B/C2N is also comparable to Pt(111).

Encapsulated NiCo2S4‐based straight bamboo‐shaped N‐CNT as efficient and stable oxygen electrocatalysts

AbstractDeveloping highly effective and stable non‐precious bifunctional oxygen electrocatalysts is crucial and challenging. Herein, we report nano‐mediated straight bamboo‐shaped nitrogen‐doped carbon nanotubes modified with encapsulated NiCo2S4‐based nanoparticles (E‐NiCo2S4/SBS‐NCNTs) by electrodeposition and procedural calcination strategy, in which nitrogen‐doped carbon nanotubes with straight bamboo shape (SBS‐NCNTs) show more average diameter and encapsulated NiCo2S4‐based nanoparticles (E‐NiCo2S4) are coated by carbon, and what makes us even more excited is that they are evenly dispersed on SBS‐NCNTs surface, not just at the tip or inside. When applied for oxygen evolution reaction (OER) and oxygen reduction reaction (ORR), E‐NiCo2S4/SBS‐NCNTs show excellent catalytic activity, and onset potential (Eonset) in OER and ORR is only 1.52 V and 0.95 V (vs. RHE) in 0.1 M KOH, respectively. More importantly, the hybrids exhibit good methanol tolerance, with almost unchanged for ORR performance after adding 0.5 M methanol, and it has highly stability, the current density dropped by 22% after 7 h of reaction. The interaction of NiCo2S4 with the external carbon layer can greatly enhance the electrocatalytic activity toward the OER and it has a complete 4e– reaction process toward the ORR. The excellent electrocatalytic activity is mainly due to synergistic enhancement effect, in which SBS‐NCNTs with highly catalytic activity provide good conductivity, expose more active sites and promotes fast mass transfer; Carbon‐coated NiCo2S4‐based nanoparticles can improve the local work function of SBS‐NCNT through synergistic electronic interaction, promoting O2 adsorption, ensure rapid electron transport, and enable SBS‐NCNT to improve its electrocatalytic activity. Because of the weakness of Ostwald effect and synergistic enhancement, the stability and catalytic activity of NiCo2S4‐based nanoparticles can be greatly improved. All these advantages can synergistically improve the electrocatalytic efficiency and make it become one of the most promising bifunctional oxygen electrocatalysts.

An efficient combination strategy for high-performance asymmetric-electrolyte metal–air batteries

Asymmetric-electrolyte metal–air batteries (AMABs) are the most promising technologies for realizing efficient energy conversion. However, the issues in cathode and separator parts seriously impede their practical applications. This article previews the latest finding on high-voltage AMABs with long cycle stability. Impressively, the generality of the developed approach is demonstrated by the AZnABs, ASiABs and ASnABs, exhibiting the enhanced operating voltage, power densities, and excellent rate performance.

Hierarchical N,P co-doped graphene aerogels framework assembling vertically grown CoMn-LDH nanosheets as efficient bifunctional electrocatalyst for rechargeable Zinc-air battery

Exploiting the low-cost and high-efficiency bifunctional oxygen electrocatalysts to substitute platinum-group metals is highly desirable but challenging for energy storage/conversion technologies. Herein, we develop a combined gelation/self-assemble/freeze drying process to fabricate a free-standing porous architectures through vertical anchoring two-dimensional (2D) CoMn-LDH nanosheets on three-dimensional (3D) hierarchical N,P co-doped graphene aerogels (NPGA) framework. This unique configuration endows CoMn-LDH/NPGA outstanding catalytic activity toward both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) with a potential difference of ca. 0.72 V between the OER potential at 10 mA cm−2 and the ORR potential at −3 mA cm−2, which is comparable to commercial Pt/C + IrO2 benchmarks, and therefore renders the CoMn-LDH/NPGA assembled zinc-air battery a superior rechargeable performance and cycling stability. In-depth structure-to-property correlation indicates that the prominent bifunctional activity of CoMn-LDH/NPGA are ascribed to large electrochemical active surface area, the rapid mass/charge transfers, the increased exposure and full utilization of active sites originated from the synergistic effect between the uniformly dispersed 2D CoMn-LDH nanosheets and the 3D hierarchical porous NPGA framework.

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.

Operando X-ray Absorption Spectroscopic Study on the Influence of Specific Adsorption of the Sulfo Group in the Perfluorosulfonic Acid Ionomer on the Oxygen Reduction Reaction Activity of the Pt/C Catalyst

The influence of specific adsorption of the sulfo group in the perfluorosulfonic acid ionomer, Nafion, on the oxygen reduction reaction (ORR) of a carbon-supported Pt/C catalyst using a thin-film rotating disk electrode was investigated. The relationship between the catalyst activity and coating of the Pt/C catalyst with Nafion was quantitatively evaluated through electrochemical measurements, operando X-ray absorption spectroscopy (XAS), and CO stripping voltammetry. Activity of the Pt/C catalyst decreased with increasing ionomer-to-carbon weight ratios. To date, the effect of specific adsorption on a catalyst has been investigated using CO stripping voltammetry. However, quantitative evaluation of specific adsorption at the ORR potential (0.50–1.0 V) has not been performed yet. We quantitatively evaluated the specific adsorption during the ORR by measuring the 5d orbital vacancy of the Pt/C catalyst using operando XAS. The difference in the electronic structures of Pt in the high potential range, with and without the ionomer, was successfully established.

Theoretical Study on a Potential Oxygen Reduction Reaction Electrocatalyst: Single Fe Atoms Supported on Graphite Carbonitride

In recent years, one of the research directions of proton-exchange membrane fuel cells (PEMFCs) was to exploit efficient electrocatalysts for oxygen reduction reaction (ORR) instead of precious metals. In this study, on the basis of the density-functional theory (DFT) calculations, we designed a new type of single-atom ORR electrocatalyst by doping single iron atoms into the N-coordination cavity of the substrate graphite carbonitride (Fe/g-C3N4). The adsorption site and the adsorption energy of all the intermediates, the reaction energy barriers, potential energy surface, and Mulliken charges have been analyzed. The feasible ORR reaction paths and the most favorable ORR reaction mechanism were performed. Our calculation results prove that Fe/g-C3N4 is a potential electrocatalyst toward ORR. This work proposes a novel notion for the development of cathode materials in PEMFCs.

Stability of TiO2-Based Materials under PEFC Operation Condition for Non-Platinum Cathodes

1. Background Pt/C catalysts used for cathodes of polymer electrolyte fuel cells (PEFCs) have problems such as high cost and insufficient durability against potential change [1]. Therefore, we have developed a non-platinum catalyst based on group 4 and 5 transition metal oxides because they are inexpensive and stable in acidic solution [2]. We successfully demonstrated that Nb-doped TiOx catalyst showed high onset potential for oxygen reduction reaction (ORR) and higher electrochemical durability than platinum [3]. However, the ORR current was much smaller than that of platinum catalyst. An increase in the surface area of oxides is effective to increase the ORR current. Nano-sizing of oxide particles is one of the solutions to increase the surface area of oxides. In general, the solubilities of nanoparticles are larger than those of large particles. Therefore, we need to evaluate the stability of nanoparticles in acidic solution. Some investigations regarding the stability of titanium oxide nanoparticle which is one candidate of group 4 oxide cathodes [4, 5, 6] have been already performed. However, the stability in PEFC operating condition has not been evaluated in detail. Therefore, in this study, the chemical stability was evaluated by measuring the dissolution behavior of titanium oxide nanoparticles in an acidic electrolyte simulating the operating environment of PEFC. 2. ExperimentalA 600 mg of TiO2 nanopowder (US Research Nanomaterials, mean particle size: 6.0 nm, Anatase) was immersed in 1.0 M (pH = 0.25), or 0.32 M (pH = 0.66), or 0.10 M HClO4 (pH = 1.06) and stirred. The temperature was controlled by using a water or an oil bath in the temperature range from 30 to 90 ℃. At the desired time, the stirring was stopped, then a small amount of the solution (2 mL) was sampled and filtered through a 0.22 µm syringe filter. The sample solution was immediately diluted. The remaining solution was stirred again. This procedure was repeated. The concentration of titanium in the sample was measured by ICP-AES (Seiko Instruments).3. Results and DiscussionFigure 1 shows the concentration of titanium of TiO2 nanopowder with diameter of 6.0 nm as a function of immersion time in 0.1 M HClO4 under air at 30, 50, 70, and 90 ℃. The concentration of titanium became constant after 800, 500, 300, and 150 h immersion at 30, 50, 70 and 90 ℃, respectively, meaning that dissolution reached equilibrium. The solubility of TiO2 nanopowder in 0.1 M HClO4 were 16, 6.2, 3.2, and 2.2 µmol dm-3 at 30, 50, 70, and 90 ℃, respectively. The solubility of TiO2 nanopowder decreases with increase in temperature and becomes chemically stable, which indicates that TiO2 nanoparticles are more suitable for PEFC operating condition at high temperature.Figure 2 shows the van't Hoff plots of the solubility of the TiO2 nanopowder in HClO4 and the solubility of platinum black powder in 0.1 M HClO4 [7] is also plotted. The solubility of TiO2 decreases with increase in temperature, while the solubility of platinum black increases with increase in temperature in acidic electrolytes. From the slope of the approximate line of the van't Hoff plot of the solubility of TiO2 nanopowder, the apparent standard enthalpy change of dissolution of TiO2 nanoparticles, ΔHº , in 0.10 M HClO4 is estimated to be -29.1 kJ mol-1, and the dissolution reaction of TiO2 nanoparticles in an acidic electrolytes is exothermal. Furthermore, from the value of the standard Gibbs energy change of the dissolution reaction, ΔGº , calculated from the solubility of TiO2 nanopowder at 30 ℃, the apparent standard dissolution entropy change, ΔSº , in 0.10 M HClO4 was calculated to be -151 J mol-1 K-1.AcknowledgmentsThis research is conducted with the support of the National Research and Development Corporation New Energy and Industrial Technology Development Organization (NEDO). 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.Reference[1] S. Kawahara, S. Mitsushima, K. Ota, N. Kamiya, ECS Trans., 3(1), 625(2006).[2] A. Ishihara, Y. Ohgi, K. Matsuzawa, S. Mitsushima, and K. Ota, Electrochim. Acta, 55, 8005 (2010).[3] M. Hamazaki, A. Ishihara, Y. Kohno, K. Matsuzawa, S. Mitsushima, K. Ota, Electrochemistry, 83(10), 817(2015).[4] J. Schmidt, W. Vogelsberger, J. Solution Chem., 38, 1267 (2009).[5] J. Schmidt, W. Vogelsberger, J. Phys. Chem., 110, 3955(2006).[6] S. E. Ziemniak, M. E. Jones, K. E. S. Combs, J. Solution Chem., 22(7), 601(1993).[7] S. Mitsushima, Y. Koizumi, S. Uzuka, K. Ota, Electrochim. Acta, 54, 455 (2008). Figure 1

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).

(Invited) Thermoplasmonic Generation of Carbon Nanohydrids for Enhanced Electrocatalysis

Local heating from plasmonic nanostructures has shown great potential in photothermal therapy, optical trapping, heterogeneous catalysis, and synthesis of nanostructures. Titanium nitride (TiN) is one the most promising plasmonic materials for thermal applications (thermoplasmonics) due to its high thermostability (i.e.refractory), CMOS compatibility, and resistance to chemically harsh environments. On the other hand, hydrogen peroxide (H2O2) has a large annual production approaching 4 ×106 tons worldwide and the selective reduction of O2 via electrochemical oxygen reduction reaction (ORR) using renewable electricity is a promising way for H2O2 production. Carbon-based materials have shown great potential for the two-electron ORR to hydrogen peroxide. In this contribution, I will show a general approach to use the highly intense local temperature generated from plasmonic TiN nanocrystals for generating TiN/carbon nanohybrids that show enhanced efficiency and selectivity toward the ORR. The general applicability of plasmon-driven synthesis of nanocarbons to electrocatalysis will be discussed.

Modeling Temperature- and Humidity-Dependent Kinetics of Oxygen Reduction Reaction

Designing active catalysts for oxygen reduction reaction (ORR) is one of the major challenges for polymer electrolyte fuel cells. Density functional theory (DFT) calculations have made significant contributions on understanding ORR pathways, rate-limiting step and catalytic activity on various transition metal surfaces 1-5. However, all these theoretical studies are for reactions at room temperature in aqueous liquid electrolytes. The condition is significantly different from actual operation conditions of fuel cells [60 to 80 °C and 30 to 100 % relative humidity (RH)]. Although the DFT models reasonably explain experimental results obtained by the rotating disk electrode (RDE) at room temperature in aqueous liquid electrolytes, the reaction mechanism and catalytic activity at the operating conditions can be significantly different. Here, we examine effects of temperature and RH on the ORR by using a micro-kinetic model. The examination indicates that both the rate-limiting step and material-dependence of the catalytic activity are strongly influenced by both temperature and RH.The theoretical analyses were carried out by extending a micro-kinetic model proposed by Hansen et al5 to higher temperature and lower RH conditions. For the extension, following assumptions were adopted. Free energies for reaction intermediates (adsorbed O, OH and HO2) are independent of temperature and relative humidity (RH) and fixed at the values in the original model.An additional reaction barrier due to the solvent reorganization, which was introduced to explain the experimentally observed prefactor in the original model, is entirely attributed to be enthalpic. The assumption (1) can be partially validated by relatively small entropies of the adsorbed species comparing with gaseous species. Because the assumption (2) is not verified, we also carried out the same simulation assuming that the solvent reorganization barrier is fully entropic [we define this assumption as (3)].Figures 1 (a) and (c) show the calculated ORR current density at 0.9V vs. reversible hydrogen electrode (RHE) under 1 atm O2 under the assumption (2) as a function of the binding energy of OH adsorbate vs. that of Pt (111). Figures 1 (b) and (d) show the calculated ORR current density in the same way as Figures 1 (a) and (c) under the assumption (3). In all conditions, in the right leg of the volcano-plot, where the OH binding energy is larger than the optimal point, the reaction is limited by the formation of the reaction intermediates. On the other hand, in the left leg of the volcano, the reaction is limited by the removal of the OH adsorbate. Regardless the type of the assumption [(2) or (3)], both the optimal catalytic activity and the optimal OH binding energy decrease with the rise in the temperature from 25 to 80 °C [see Figs. 1 (a) and (b)]. The low catalytic activity at high temperature is attributed to the drop in the reversible potential of ORR at high temperature. The shift in the optimal OH binding energy is attributed to the destabilization of OH adsorbate relative to the non-adsorbed species, which hold larger entropies than the adsorbates. The drop in RH enhances the optimal catalytic activity, while the opposite shift is observed for the optimal OH binding energy [see Figs. 1 (c) and (d)]. The both trends stem from the lowering of the water activity, which promotes the OH removal from the surface. All in all, our theoretical analyses indicate that the optimal OH binding energy strongly depends on both temperature and RH, indicating that the optimal catalyst at low temperature and in aqueous liquid electrolytes may not be optimal anymore at high temperature and low RH. Although our theoretical analyses strongly rely on non-verified assumptions (1)-(3) and the DFT model adopting the ice-like water,1 which is unlikely a proper model of the electrolyte at high temperature and low RH, the observation indicates the significance of more precise understanding of temperature-, RH- and material-dependence of ORR.In the presentation, we will present experimental data on temperature- and RH-dependent ORR activity and discuss the validity of the theoretical model. References J. K. Nørskov et al., J. Phys. Chem. B, 108, 17886 (2004).J. Greeley et al., Nat. Chem., 1, 552 (2009).J. X. Wang et al., J. Phs. Chem. A, 111, 12702 (2007).R. Jinnouch et al., Phys. Chem. Chem. Phys., 13, 21070 (2011).H. A. Hansen et al., J. Phys. Chem. C, 118, 6706 (2014). Figure 1

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.

Electrocatalysts Based on Cobalt- and Nitrogen-Doped Nanocarbon Composites for Oxygen Reduction Reaction and Anion Exchange Membrane Fuel Cells

With an increasing energy demand, there is a need for renewable energy conversion devices. Among the possible options are low temperature polymer electrolyte fuel cells, in which electrocatalysts play vital roles, especially on the cathode, where the oxygen reduction reaction (ORR) occurs. The most efficient electrocatalysts for the ORR are based on noble metals, mainly platinum, which however have various disadvantages including their high price and scarcity. As a replacement, different transition metal-nitrogen-carbon (M−N−C) catalyst materials have shown great promise.1,2 Herein, a composite of nanocarbons, namely carbide-derived carbon/carbon nanotube (CDC/CNT), is employed as a support to produce the M−N−C type of catalysts. This composite offers a novel catalyst structure in which both micro- and mesopores are present that can be beneficial for the anion exchange membrane fuel cell (AEMFC) application. The doping was done via pyrolysis at 800 °C in the presence of a cobalt salt and a nitrogen precursor (either dicyandiamide, urea or melamine).3 The catalysts were characterized using different physico-chemical methods (e.g. SEM, TEM, XPS, XRD, Raman spectroscopy, and N2 adsorption), which proved the success of doping as well as the feasible micro- and mesoporous structure with defects present. Both the RDE and RRDE methods were employed for the electrochemical testing and in alkaline medium all three catalyst materials exhibited similar and good electrocatalytic activity for the ORR. The half-wave potential for ORR of Co-N-CDC/CNT catalysts was close to that of a Pt/C catalyst (Figure 1, left). The possible application of the Co-N-CDC/CNT material was tested out by employing it as a cathode catalyst in AEMFC. The Co-N-CDC/CNT catalyst together with a novel HMT-PMBI4 membrane exhibited excellent performance with peak power density of 577 mW cm–2, which was significantly higher than that obtained with a Pt/C cathode (Figure 1, right). This shows that the Co-N-CDC/CNT materials are promising cathode catalysts for the AEMFC application.3 References A. Sarapuu, E. Kibena-Põldsepp, M. Borghei, and K. Tammeveski, J. Mater. Chem. A, 6, 776-804 (2018).G. Wu, A. Santandreu, W. Kellogg, S. Gupta, O. Ogoke, H. Zhang, H. L. Wang, and L. Dai, Nano Energy, 29, 83−110 (2016).J. Lilloja, E. Kibena-Põldsepp, A. Sarapuu, M. Kodali, Y. Chen, T. Asset, M. Käärik, M. Merisalu, P. Paiste, J. Aruväli, A. Treshchalov, M. Rähn, J. Leis, V. Sammelselg, S. Holdcroft, P. Atanassov, and K. Tammeveski, ACS Appl. Energy Mater., 2020, (DOI: 10.1021/acsaem.0c00381)A. G. Wright, J. T. Fan, B. Britton, T. Weissbach, H. F. Lee, E. A. Kitching, T. J. Peckham, and S. Holdcroft, Energy Environ. Sci., 9, 2130-2142 (2016). Figure 1

Amino functionalized carbon nanotubes supported CoNi@CoO–NiO core/shell nanoparticles as highly efficient bifunctional catalyst for rechargeable Zn-air batteries

Oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are the core reaction processes of rechargeable Zn-air battery (ZAB) cathode. Therefore, exploring a bifunctional catalyst with excellent electrochemical performance, high durability, and low cost is essential for rechargeable ZAB. In this work, amino functionalized carbon nanotubes supported core/shell nanoparticles composed of CoNi alloy core and CoO–NiO shell (CoNi@CoO–NiO/NH2-CNTs-1) is synthesized through a simple and efficient hydrothermal reaction and calcination method, which shows higher ORR/OER bifunctional catalytic performance than the single metal-based catalyst, such as Ni@NiO/NH2-CNTs and Co@CoO/NH2-CNTs. The fabricated bimetallic alloy based catalyst CoNi@CoO–NiO/NH2-CNTs-3 with the optimized loading content of CoNi@CoO–NiO core/shell nanoparticles, presents the best bifunctional catalytic performance for ORR/OER. Experimental studies reveal that CoNi@CoO–NiO/NH2-CNTs-3 exhibits the onset potential of 0.956 V and 1.423 V vs. RHE for ORR and OER, respectively. It also exhibits a low overpotential of 377 mV to achieve a 10 mA cm−2 current density for OER, and positive half-wave potentials of 0.794 V for ORR. And the potential difference between half-wave potential of ORR (E1/2) and the potential at 10 mA cm−2 for OER (Ej10) is 0.813 V. In addition, when CoNi@CoO–NiO/NH2-CNTs-3 is used as an air electrode catalyst of rechargeable ZAB, its maximum power density and open circuit voltage (OCV) can reach 128.7 mW cm−2 and 1.458 V (The commercially available catalyst of Pt/C–RuO2 is 88.1 mW cm−2), which strongly demonstrates that the fabricated catalyst CoNi@CoO–NiO/NH2-CNTs-3 can be used as a highly efficient bifunctional catalyst for ZABs, and is expected to replace those expensive precious metal electrocatalysts to meet the growing demand for new energy devices.

Enhanced Oxygen Reduction Reaction Performance Using Intermolecular Forces Coupled with More Exposed Molecular Orbitals of Triphenylamine in Co-porphyrin Electrocatalysts.

Triphenylamine (TPA) has often been used as a building block to construct functional organic materials yet is rarely employed in oxygen reduction reaction (ORR) due to its strong electron-donating ability. This versatile segment bears a three-dimensional spatial structure whose effect has not been fully explored in catalytic systems. To this end, five symmetric cobalt porphyrins with carbazole and TPA derivatives have been synthesized and their ORR performance has been evaluated in acid medium. It was found that all compounds produced mainly hydrogen peroxide in oxygen reduction, with CP1 attaching benzyl derivatives and XCP4 possessing TPA-carbazole substituents at the meso-position of porphyrin, showing similar but more positive ORR potential as compared to the other analogues. Importantly, XCP4 achieved the greatest response current and the largest electron transfer numbers and H2O2 yields among the investigated molecules. Detailed electrochemical measurements suggested that the dipole-induced partial charges on the porphyrin in tandem with the more exposed molecular orbitals on TPA contributed to this enhancement, with the former attracting more protons to the affinity of reactive sites and the latter increasing the collision frequency between the electrocatalyst and H+ in solution. This is the first attempt to integrate the intermolecular forces with more exposed molecular orbitals in altering the electrochemical process.

Boosting the Performance of Nitrogen‐Doped Mesoporous Carbon Oxygen Electrode with Ultrathin 2D Iron/Cobalt Selenides

AbstractHigh‐performance and low‐cost oxygen electrode catalysts for rechargeable metal–air battery have been regarded as one pivotal issue for the ever‐increasing energy‐demanding devices like electric vehicles, domestic appliances, and consumer electronics. To develop an effective substitute of noble‐metal‐based catalysts, the modified nitrogen‐doped mesoporous carbon (NMC) is proposed as composite catalysts with a series of 2D transition metal selenides (MSe, M = Fe and/or Co) by liquid exfoliation, anchoring, and thermal activation. By varying the transition metal elements, CoSe@NMC is confirmed as the competitive alternative relative to the coupled commercial catalysts (IrO2 and Pt/C), with a comparable overpotential for oxygen evolution reaction at 10 mA cm−2 and positively shifted half‐wave potential for oxygen reduction reaction even after accelerated durability tests for 5000 cycles. Therefore, this postmodification strategy can promote further design toward the advanced electrode materials for other energy conversion and storage techniques.

Coupling of iron phthalocyanine at carbon defect site via π-π stacking for enhanced oxygen reduction reaction

The intrinsic activity of transition metal catalytic centers for oxygen reduction reaction (ORR) depends heavily on its electronic structure, which with an electron-rich environment will boost the ORR performance. In this work, we firstly revealed the defective graphene (DG) substrate with 585 defects could efficiently mediate charge redistribution of the attached exfoliated monolayer Iron Phthalocyanine (FePc) by using density functional theory (DFT) calculation. The electrons transfer to FePc from 585 defects forms an electron-rich region on Fe atom, and high-density electrons further raise the d-band center of Fe atom. Apparently, this adjustment of electronic structure for Fe atoms is beneficial to the adsorption and reaction of O2 molecules, inducing more positive initial potential and larger current density for ORR. Based on this finding, DG obtained by the heat treatment was prepared to couple exfoliated monolayer FePc through stable π-π stacking. As expected, FePc/DG hybrid exhibits outstanding electrocatalytic ORR performance with a positive initial potential (0.98 V vs. RHE) and a high current density (5.45 mA·cm−2) in 0.1 M KOH electrolytes. In addition, the FePc/DG hybrid was utilized to assemble a zinc-air battery device, which reveals the power density of 190 mW·cm−2.