Search

A Pt/Pd/C core–shell-structured catalyst with an extraordinarily high activity of 3625 A g–1-Pt at 0.9 V versus a reversible hydrogen electrode (RHE) for the oxygen reduction reaction (ORR) was cre...

Introduction Nanoparticles of Pt as well as Pt-based alloys are widely used as cathode catalyst materials for proton exchange membrane fuel cells (PEMFC). However, electrochemical stability of the materials is rather low under practical operating conditions of PEMFC cathode, resulting in severe deactivation of oxygen reduction reaction (ORR). Therefore, further material’s developments are required for next-generation PEMFC cathode catalysts, i.e., more enhanced ORR durability with low platinum group metal (PGM) usage. High entropy alloys (HEAs), defined as single phase solid solutions of five or more elements in equal composition ratios, are known as thermodynamically stable, in comparison to conventional binary alloys. Furthermore, complex atomic-level local structures bring about unique electronic as well as (electro-)chemical properties that originating from lattice strains induced by specific local structures and/or so-called sluggish diffusion of the constituent elements. [1] However, to our best knowledge, no study has been made for ORR properties of Pt alloying with non-PGM Cantor alloy (fcc structure HEA with equi-atomic ratio of Cr-Mn-Fe-Co-Ni [2]) elements in strong acid condition. In this study, we synthesized lattice stacking structures of Pt/HEA(hkl) (hkl = (111), (110), (100)) through arc-plasma deposition (APD) of the Cantor alloy layer on Pt(hkl) substrate surfaces, followed by deposition of the surface Pt layer in ~10-7 Pa. Then, we performed cross-sectional STEM-EDS observations for Pt/HEA/Pt(hkl) stacking structures with atomic-level resolution and evaluated the ORR properties (initial activity and structural stability). Experimental An APD target of Cr-Mn-Fe-Co-Ni (Cantor alloy) was fabricated by sintering of equal quantity corresponding elements. 10 ML(monolayer)-thick (1 ML = ca. 0.3 nm) Cantor alloy layer (as HEA) was vacuum-deposited by APD on surface cleaned Pt(hkl) substrate surfaces at 300 K, and subsequently annealed in vacuum at 773 K for 30 minutes. Then, 4ML-thick Pt layer was deposited on the pre-deposited Cantor alloy layer at 300 K and annealed at 623 K. The samples thus prepared are designated as Pt/HEA/Pt(hkl). The atomic-level micro-structures and chemical bonding states of Pt/HEA/Pt(hkl) surfaces were characterized by cross-sectional STEM-EDS, RHEED, XPS etc. CV and LSV with the RDE method were conducted in N2-purged and O2-saturated 0.1 M HClO4. ORR activity was evaluated from j k values at 0.9 V vs. RHE by using Koutecky-Levich equation and structural stability (ORR durability) was discussed based upon activity transitions during applying the potential cycles (PCs) of 0.6(3s)‐1.0(3s) V vs. RHE in O2 saturated 0.1 M HClO4 at room temperature. Results and Discussion Atomically-resolved, cross-sectional HAADF-STEM images for Pt/HEA/Pt(hkl) (a) and EDS line profiles of elemental distributions at corresponding yellow arrows (b) are presented in Figure 1. As clearly shown in (a), irrespective of the Pt surface indices, (hkl), HEA (Cantor alloy) and surface Pt layers are epitaxially grown on the substrates. By contrast, elemental distributions in each surface normal (b) seriously depend on the substrate Pt lattice indices. Notably, severe thermal diffusion of the constituent elements including Pt is confirmed by (a) and (b) for both Pt/HEA/Pt(110) and (100). Figure 2 summarizes electrochemical results. 4ML-thick-Pt/10ML-thick-Co/Pt(hkl) that prepared under the same preparation condition of Pt/HEA/Pt(hkl), and clean Pt(hkl) (light blue and gray, respectively) are also shown as references. As shown in the figure, the Pt/HEA and Pt/Co fabricated on Pt(111) substrate (top) show quite similar CV characteristics (shrink in hydrogen adsorption charges (0 – 0.3 V) and higher potential shifts in oxygen-related species adsorption (0.6 – 1.0 V)), in comparison to clean Pt(111), and almost the same initial ORR activity. Meanwhile, the Pt/HEA fabricated on Pt(110) and (100) substrates show more reduced hydrogen absorption charges, compared with corresponding Pt/Co samples. Particularly, distinctive redox features for clean Pt(110) at 0.12 V and for Pt(100) at 0.35 V are absent for corresponding CVs, suggesting specific topmost surface atomic-structures might be formed in the electrolyte, that probably resulting from significant electronic interactions between surface Pt and HEA constituent elements (Cr, Mn, Fe, Co, Ni) and/or local strain of the surface Pt layer induced by distorted Pt-HEA lattices located nearby. One might notice that Pt/HEA/Pt(110) reveals remarkable ORR activity enhancement compared with corresponding Pt/Co/Pt(110), while the activities for Pt/HEA and corresponding Pt/Co surfaces fabricated on Pt(100) are almost the same value. At the meeting, correlations between surface atomic-level micro-structures of Pt/HEA/Pt(hkl) and ORR properties will be discussed in detail. Acknowledgement This study was supported by new energy and industrial technology development organization (NEDO) of Japan and JST SPRING, Grant Number JPMJJSP2114. Reference [1] J. Yeh, JOM, 65, 1759 (2013). [2] B. Cantor et al., Mater. Sci. Eng. A, 375, 213 (2004). Figure 1

An indirect fuel cell system is constructed. The system is composed of a redox flow battery (RFB) to extract electrical energy and two chemical reactors (anolyte and catholyte regenerators). A quinone as a redox mediator is reduced by a mixture of hydrogen and carbon monoxide in the anolyte regenerator, whereas a polyoxometalate as another redox mediator is oxidized in the catholyte regenerator, followed by a steady-state power generation at the RFB using the two redox mediators as active materials. This system demonstrates how to reduce the amount of platinum required in a proton-exchange membrane fuel cell (PEMFC), especially when using a fuel other than pure hydrogen. The RFB in our system contains two gas-diffusion electrodes (GDEs) with a platinum electrocatalyst to insert a “pure hydrogen gas phase” between the anolyte and catholyte to avoid cross-contamination. These two GDEs participate in the hydrogen evolution reaction and hydrogen oxidation reaction, respectively, and require only a small amount of platinum. In addition, the catalysts used in the anolyte regenerator are rhodium complexes. However, these catalysts are in a dissolved state (molecular catalysts) with micromolar-order concentrations, and very little noble metal is used. A carbonaceous catalyst without platinum is used in the catholyte regenerator. This eliminates the need for a noble metal for the oxygen reduction reaction, which is the main reason why platinum is used in a large amount in a conventional PEMFC. Steady-state operations of the anode side, the cathode side, and the total system are demonstrated in this work. Although a small amount of noble metal is still required at this stage, this work may contribute to the complete elimination of noble metals from a PEMFC.

We investigated the microstructures of Au-deposited PtCo@Pt core-shell nanoparticles (NPs) and discussed enhancement of the oxygen reduction reaction (ORR) properties by Au termination of low-coordination sites of the Pt-shell. The Au-deposited PtCo@Pt NPs showed an improved electrochemical structural stability, together with slight increment in increased initial, pristine area-specific activity relative to the non-Au-deposited PtCo@Pt NPs. Atomic-level microstructural characterization was performed by a back-side illumination fluorescence X-ray absorption fine structure (BI-FXAFS) method and scanning transmission electron microscopy with energy dispersive spectroscopy (STEM-EDS). The BI-FXAFS results indicated that compressive lattice strain in the Pt-shells of the PtCo@Pt NPs was almost unchanged by the subsequent Au deposition. Furthermore, STEM-EDS mapping of Pt, Co, and Au clearly showed that the deposited Au tended to localize at low-coordination sites of the Pt-shell surface, e.g., edges and corners. The atomic-level microstructural characterization conducted in this study demonstrated that effective Au surface terminations of the Pt-shell enhance the ORR durability of Pt-based core-shell type NP catalysts.

The adsorption of a porphyrazine (a tetraazaporphyrin) on the surfaces of Pt nanoparticles enhanced the activity of the oxygen reduction reaction (ORR) of the Pt catalysts for use in a polymer electrolyte fuel cell (PEFC). The apparent electrochemically active surface area was significantly decreased upon adsorption of the organic macrocycle. The promotion of the ORR was ascribed to the enhanced activity of the residual Pt surface area. Cyclic voltammograms under argon atmosphere and CO stripping voltammetry indicate that the porphyrazine impedes the generation of OH adsorbed on the surface of Pt. Other π-conjugated aromatic compounds tested did not increase (but rather, inhibited) the ORR activity. It should be noted that the porphyrazine, comparably large in size relative to the Pt nanoparticles, enhanced the catalytic activity of the Pt surface.

Reversible air electrodes with high activity and durability during the discharge/charge processes are required for rechargeable zinc-air batteries. Although carbon materials are widely used as catalyst supports and conductive additives for metal oxide catalysts in the catalyst layer (CL) and as the main active constituent in the gas-diffusion layer (GDL), the carbon is oxidized during the charge step, causing degradation of the air electrode. In this study, carbon-free air electrodes were prepared using electro-conductive oxides, namely, indium tin oxide or antimony-doped tin oxide, as an alternative to carbon materials in both the CL and GDL to achieve reversibility and durability of the air electrode in zinc-air batteries. While the initial performance of the carbon-free air electrodes was inferior to that of the carbon-based air electrodes, the lifetime of the carbon-free air electrodes was remarkably longer. These results indicate a critical role of electro-conductive oxides in both the CL and GDL for durable air electrodes.

To discuss the relationship between the structure and the oxygen reduction reaction (ORR) activity of pyridylmethylamine copper complexes containing carboxy groups, CuII complexes of a N,N-bis(2-pyridylmethyl)phenylmethylamine tridentate ligand (L1) or its carboxy derivatives (2-{[bis(2-pyridylmethyl)amino]methyl}benzoic acid: L2, 3-{[bis(2-pyridylmethyl)amino]methyl}benzoic acid: L3, or 4-{[bis(2-pyridylmethyl)amino]methyl}benzoic acid: L4) were synthesized and characterized. The X-ray crystal structures of mononuclear CuII complexes ligated by L2–L4 were obtained as dichloride complexes, and a crystallographic analysis revealed that all of them have a square pyramidal geometry. The ESR spectroscopy showed that these complexes also had a square pyramidal geometry in a neutral aqueous solution. Cyclic voltammetry analysis in a neutral aqueous solution suggested that only the CuII complex of L2 shows intramolecular interaction between the carboxy group and copper ion in the aqueous solution. The introduction of a carboxy group dramatically increased the amount of the CuII complexes adsorbed on carbon black, which can slightly increase the ORR activity.

Here, we report remarkable oxygen evolution reaction (OER) catalytic activity of brownmillerite (BM)-type Ca2 FeCoO5 . The OER activity of this oxide is comparable to or beyond those of the state-of-the-art perovskite (PV)-catalyst Ba0.5 Sr0.5 Co0.8 Fe0.2 O3-δ (BSCF) and a precious-metal catalyst RuO2 , emphasizing the importance of the characteristic BM structure with multiple coordination environments of transition metal (TM) species. Also, Ca2 FeCoO5 is clearly advantageous in terms of expense/laboriousness of the material synthesis. These facts make this oxide a promising OER catalyst used in many energy conversion technologies such as metal-air secondary batteries and hydrogen production from electrochemical/photocatalytic water splitting.

A carbon-supported Rh phthalocyanine catalyzed the electro-oxidation of glucose in basic solution. The overpotentials for the oxidation are lower than those with other conventional Co phthalocyanine-based catalysts. This electrocatalytic oxidation is coupled with the redox potential of Rh phthalocyanine. The introduction of an electron-donating group causes negative shift in the redox potential, and decreases the onset potential for the oxidation of glucose. The Rh phthalocyanine catalyst needs activation before the catalytic oxidation of glucose. This activation is achieved by the exposure of the catalyst to negative potentials. This catalyst also oxidizes gluconate, which is a possible 2-electron oxidation product of glucose. Electrolysis of glucose solution shows that glucose undergoes multi-electron (more than 2) oxidation by the Rh phthalocyanine.

A new type of transmission-line model (TLM) as an equivalent circuit for the electrochemical impedance is developed to describe a porous electrode impregnated with an electrolyte solution. Two kinds of TLMs, i.e., one for diffusion-migration and the other for a porous electrode, are combined. To describe the double-layer capacitance strictly, the interfacial impedance is explicitly divided into the faradaic impedance and double-layer capacitance, and the latter is distributed into contributions of each ionic species. The obtained model describes diffusion, migration, and concentration changes of all ionic species within a porous electrode. In combination with a TLM for a bulk electrolyte solution, an equivalent circuit for a whole cell is also made. This should be a powerful tool for analyzing the impedances of devices with porous electrodes, such as batteries and electric double-layer capacitors, especially in the low-frequency region.

Perovskite-type oxide-based La0.6Sr0.4Mn(1-x)Co x O3/carbon (x = 0, 0.1, 0.2, 0.3, 1) catalysts were prepared by calcination of a mixture of metal nitrates and PVP at 600 °C, followed by milling of the calcined powder with a carbon support. The catalytic activities for the oxygen reduction reaction (ORR) were tested in alkaline electrolyte by using a rotating ring-disk electrode (RRDE) at 25 °C. With an increase in the doping amount of Co, the onset potential of the La0.6Sr0.4Mn(1-x)Co x O3/carbon catalyst was found to be shifted in a positive direction and the ring current during ORR decreased. Especially, the La0.6Sr0.4Mn0.7Co0.3O3/carbon (x = 0.3) catalyst showed the highest ORR activity among the prepared catalysts. The La0.6Sr0.4Mn0.7Co0.3O3/carbon was tested as the cathode of an alkaline membrane-type direct hydrazine fuel cell (DHFC), and a maximum power density of 133 mW cm−2, which exceeded that for the MEA using Pt/C (73.4 mW cm−2), was obtained at 50 °C. In addition, the open circuit voltage (OCV) for the DHFC (0.846 V) was greater than that of a commercial Pt/C cathode (0.788 V). Thus, the perovskite-type La0.6Sr0.4Mn0.7Co0.3O3 catalyst is expected to be a good candidate for use as a catalytic material for a DHFC cathode as an alternative to Pt.

Zinc-air secondary battery (ZAB) is a promising candidate for the development of new generation of energy conversion systems due to its relatively high theoretical energy density of 1370 Wh kg-1. The decrease of overpotential for oxygen-reduction-reaction (ORR) and oxygen-evolution-reaction (OER) in air electrode, together with providing of high durability, are essential for commercialization of zinc air battery. As precious-metal-free electrocatalysts, transition metal oxide nanoparticles supported on nanocarbon materials are promising for highly active ORR/OER catalysts. The carbon has an important role to provide enough electrical conductivity, but the carbon degradation at high OER potential is a key issue for practical application. Very recently, we reported that platelet-type carbon nanofibers (pCNFs) with high graphitization degree are very resistive in the OER environments in the highly concentrated alkaline media [1]. Surprisingly, carbon edge plane at the sidewall of the carbon nanofibers is exposed to electrolyte, but negligible oxidative dissolution occurs at the OER condition for 1 month. Thus, pCNFs have high potential as the durable carbon material for the air electrode of ZAB. In the present study, novel hybrid electrocatalyst consisting of pCNFs and spinel-type oxide nanoparticles are prepared by a solvothermal method as a durable and active ORR/OER electrocatalyst. The commercially available pCNFs (Sigma-Aldrich) were used in this study. Commercial carbon black (Denka black, DB) were also used for comparison. Each carbon support was treated in concentrated HNO3 aqueous solution at 110℃ for the surface activation. MnCo2O4 nanoparticles (MCO) were loaded on each carbon support by means of solvothermal synthesis; an ethanol solution containing manganese acetate and cobalt acetate was heated at 150℃ for 3 h. The electrochemical measurements were carried out with the carbon counter electrode and Hg/HgO/KOH aq. reference electrode in 4 and 8 mol dm-3 KOH aqueous solution, for the electrocatalytic activity evaluation and for the durability tests, respectively. HRTEM image of the MCO/pCNF hybrid (Fig. 1) reveals the uniform distribution of MCO nanoparticles, which are 5 nm in diameter on average, on the looped carbon planes of the pCNFs. It is likely that the MCO is strongly coupled with pCNF. The Mn/Co ratio obtained by TEM-EDS analysis of MCO nanoparticles loaded on pCNFs was 1/2 in agreement with the target composition. The electrochemical measurements demonstrated markedly high activity of MCO/carbon hybrid towards ORR with the onset potential as high as 0.90 V vs RHE. High catalytic performance probably results from the good interface formation between MCO and carbon due to the solvothermal synthesis. The durability test using gas-diffusion electrodes was carried out under repeated galvanostatic ORR/OER polarization at ±20 mA cm-2 for 1 h. Although the MCO/DB electrode shows the overpotential increase after 30 cycles, the overpotential for MCO/pCNF is stable even after 80 cycles. References [1] Y. Sato, D. Kowalski, Y. Aoki and H. Habazaki, Appl. Cat. A, 597 (2020) 117555. Acknowledgment This work is based on results obtained from a project, "Research and Development Initiative for Scientific Innovation of New Generation Batteries (RISING2)", JPNP16001, commissioned by the New Energy and Industrial Technology Development Organization (NEDO), Japan. Figure 1 HRTEM image of MCO/pCNF hybrid. Figure 1

Carbon-supported Rh porphyrins catalyze the oxidation of carbon monoxide by water-soluble electron acceptors. The rate of this reaction is plotted as a function of the redox potential of the electron acceptor. The rate increases with an increase in the redox potential until it reaches a plateau. This profile can be explained in terms of the electrocatalytic CO oxidation activity of the Rh porphyrin. The removal of CO from CO(2%)/H2 by a solution containing a carbon-supported Rh porphyrin and an electron acceptor is examined. The complete conversion of CO to CO2 is achieved with only a slight amount of Rh porphyrins. Rh porphyrin on carbon black gives higher conversion than that dissolved in solution. This reaction can be used not only to remove CO in anode gas of stationary polymer electrolyte fuel cells but also to regenerate a reductant in indirect CO fuel cell systems.

The electrochemical behaviors of a tris(2-pyridylmethyl)amine copper complex (Cu-tmpa) in aqueous buffer solutions were examined. Acid-base and redox equilibria were clarified using cyclic voltammetry and ESR spectra. The pKa values of both CuII-tmpa and CuI-tmpa were determined based on the pH-E°′ diagram of Cu-tmpa. The slopes in the diagram strongly suggest the presence of hydroxo-bridged dimer complexes of CuII-tmpa. This notion is supported by ESR spectra. The oxygen reduction reaction (ORR) by Cu-tmpa was also examined over a wide range of pHs. The ORR current of Cu-tmpa decreased significantly above pH 12.0. This pH value coincides with the pKa value of CuI-tmpa. The decrease in ORR current in basic solution is attributed to the formation of a hydroxo complex of CuI. These findings are important for the application of Cu complexes as cathodes in fuel cells.

The ionic and electronic effective conductivities of an electrode mixture layers for all-solid-state lithium-ion batteries containing Li 2 S P 2 S 5 as a solid electrolyte were investigated by AC impedance measurements and analysis using a transmission-line model (TLM). Samples containing graphite (graphite electrodes) or LiNi 0.5 Co 0.2 Mn 0.3 O 2 (NCM electrodes) as the active material were measured under a “substrate | sample | bulk electrolyte | sample | substrate” configuration (ion-electron connection) and a “substrate | sample | substrate” configuration (electron-electron connection). Theoretically, if the electronic resistance is negligibly small, which is the case with our graphite electrodes, measurement with the ion-electron connection should be effective for evaluating ionic conductivity. However, if the electronic resistance is comparable to the ionic resistance, which is the case with our NCM electrodes, the results with the ion-electron connection may contain some inherent inaccuracy. In this report, we theoretically and practically demonstrate the advantage of analyzing the results with the electron-electron connection, which gives both the ionic and electronic conductivities. The similarity of the behavior of ionic conductivity with the graphite and NCM electrodes confirms the reliability of this analysis.

Bifunctional electrocatalysts for both the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) are developed with perovskite-type oxide catalyst and antimony-doped tin oxide (ATO) as an oxidation-resistant conductive additive to increase the stability under an oxidizing atmosphere. In conventional carbon-supported catalysts, carbon plays an important role not only in increasing electric conductivity but also in catalyzing the first 2-electron reaction of the ORR. The ATO-mixed catalyst itself requires a perovskite-type oxide (ABO3) catalyst that is active in the first step of the ORR, such as La0.6Ca0.4MnO3 (LCMO), because of the poor ORR activity of ATO. On the other hand, OER performance strongly depends on the composition of the perovskite oxide and La0.6Ca0.4CoO3 (LCCO) shows high OER activity. Here, La0.6Ca0.4Co0.7Mn0.3O3 (LCCMO), in which Co and Mn are mixed at the B site of the perovskite-type oxide, mixed with ATO is proposed as a carbon-free bifunctional catalyst that shows activity for both the ORR and OER. The activity and durability of gas-diffusion air electrodes with the bifunctional catalysts are evaluated in zinc–air batteries, and the results suggest that the catalyst with ATO achieved similar charge-discharge performance and a longer lifetime compared to the conventional catalyst with carbon.

Gross fuel starvation can seriously damage the anode of polymer electrolyte fuel cells (PEFCs) due to the exceptionally high anodic potential induced by oxygen evolution and carbon corrosion reactions to generate protons, instead of hydrogen oxidation. In this report, we describe a highly effective reversal-tolerant anode catalyst (RTA) based on Pt/Ti4O7 with a very small amount of Ir. A membrane electrode assembly (MEA) using a Pt/Ti4O7 RTA including 0.04 mgIr cm−2 shows almost no performance loss even after severe cell reversal. This is in clear contrast with the results obtained with an Pt/C RTA including the same amount of Ir, for which a significant voltage loss is observed after a cell reversal test. Moreover, the reversal-tolerance of Pt/Ti4O7 RTA is considerably high even with a very small amount of Ir, as low as 0.014 mgIr cm−2. An Ir/Ti4O7 based anode also shows superior RTA property as well as the mitigation of start-up/shut-down degradation due to the limited activity of Ir for the oxygen reduction reaction. A post-mortem analysis of the tested MEA confirmed that Pt/Ti4O7 RTA alleviates damage to not only the catalyst layer but also the micro porous layer, which leads to a better RTA property for the Pt/Ti4O7.

Mathematical solutions were obtained for the impedance of one-dimensional transmission-line (TML) models with all possible variations of the boundary conditions and connections to an external circuit. First, three types of connection (Z-type, T-type, and E-type) were considered. A more generalized connection that includes the characteristics of both Z-type and T-type (universal-type) was also considered. All four of the resulting general solutions were versatile, but quite complicated. Next, comprehensive variations of the general solutions depending on specific boundary conditions, which are generally simpler, were also considered. Finally, an equivalent transform from a three-terminal TML model to a simple Y-circuit was calculated. This enables calculation of the serial connection of multiple TML models with different parameters, which represents a multi-layered electrode.

A new easy-to-use process for obtaining fine powders of lanthanum-based perovskite-type oxides (La1−xAexMO3: Ae=Ca, Sr, M=Mn, Co, Fe) with a high specific surface area was successfully developed by the use of polyvinyl pyrrolidone (PVP) as a dispersant. This approach made it possible to prepare a single phase of perovskite by calcining at a lower temperature (600°C) compared to conventional synthetic processes such as a solid state reaction and citrate process. SEM observations revealed that the oxide prepared with PVP was ca. 20–30nm in size, while those prepared by the citrate process and solid state reaction were ca. 50nm and 1μm, respectively. Furthermore, the results of a thermal analysis in which we heated the precursors for the different reactions support the idea that PVP plays an important role in the formation of the perovskite phase at low temperature.

We present a carbon-supported iron phthalocyanine catalyst for the electro-oxidation of N,N-diaminourea (DAU), a hydrazine derivative. This catalyst can oxidize DAU, which is scarcely oxidized by conventional metal electrodes. The onset potential reaches 0.3 V vs. a reversible hydrogen electrode, and the oxidation current of DAU is comparable to that of hydrazine. The activity of Co phthalocyanine was also examined; the Fe phthalocyanine catalyst exhibited a higher oxidation current for DAU than the Co phthalocyanine catalyst. The dependence of the activity on the concentration of DAU suggests that DAU forms a complex with Fe phthalocyanine. Based on this concentration-dependence and an HPLC (high performance liquid chromatography) analysis of DAU hydrolysis, it was shown that hydrolysis of DAU in an electrolyte solution does not significantly affect the electro-oxidation of DAU by a carbon-supported Fe phthalocyanine. We made an anion-exchange membrane fuel cell that incorporates Fe phthalocyanine as an anode catalyst. This cell delivered considerable electricity when DAU was supplied as a fuel (open circuit potential: 0.59 V, short

Here, we report remarkable oxygen evolution reaction (OER) catalytic activity of brownmillerite (BM)-type Ca

The low activity and durability of noble-metal-free catalysts in acidic media are major problems that hinder their practical use. This study reports significantly enhanced activity of a cobalt-containing carbonaceous catalyst used for the hydrogen-evolution reaction in a water electrolyzer by using a proton-exchange membrane (PEM), which is an efficient energy-conversion device that is suitable for intermittent renewable energy sources. On the basis of X-ray absorption near-edge structure, the enhancement in activity was attributed to a change in the local structure surrounding the Co atom. The PEM water electrolyzer formed by using the catalyst with an optimized structure and Co surface concentration showed an enthalpy efficiency of 80 % at 1 A cm−2.

The electrochemical behaviors of a cobalt porphyrin (Co octaethylporphyrin (Co-OEP)) adsorbed on highly oriented pyrolytic graphite (HOPG) were examined. Co-OEP on HOPG exhibits highly reversible voltammograms with a peak separation of almost zero. The redox potential of Co-OEP on HOPG in sulfuric acid was much higher than a previously reported value of Co-OEP on an edge plane electrode in perchloric acid. The high potential would be due to the instability of cationic trivalent state of Co on HOPG; hydrophilic trivalent state is not familiar with a hydrophobic surface of HOPG, and the interaction between cationic molecules causes positive shift in the redox potential. Certain anions (Cl−, Br−and ClO4−) cause a negative shift of midpoint potential of Co-OEP, since they counteract the positive charge of trivalent Co in Co-OEP. The dissociation constants of Co-OEP-Cl and Co-OEP-Br were determined from the potential shift. The constants indicate that Br−coordinates on Co-OEP more strongly than Cl−. ClO4−has a different effect on the redox behaviors of Co-OEP than halides. While the oxidation of CO oxidation by Co-OEP was suppressed by the coordination of these halides, ClO4−did not significantly decrease the CO oxidation current, but rather decreased the overpotential for CO oxidation. These results suggest that halides coordinate on trivalent Co-OEP, while ClO4−interacts with positive charge of trivalent Co-OEP without coordination.

The enhancement of the oxygen reduction reaction (ORR) activity of Pt catalysts by porphyrazines (tetraazaporphyrin and phthalocyanine) is analyzed electrochemically. While tetra(t-butyl)tetraazaporphyrin significantly enhances the ORR activity, the enhancement by dilithium phthalocyanine is limited; this molecule increases the ORR at high potentials but decreases it at low potentials. The cyclic voltammograms (CVs) under an Ar atmosphere and a Tafel analysis reveal that the phthalocyanine promotes the ORR by destabilization of OHad, an intermediate of the ORR, at high potentials. However, this positive effect of the phthalocyanine is counteracted by its inhibitory effects at low potentials. Thus, the tetra(t-butyl)tetraazaporphyrin, which does not have a significant inhibitory effect, would be a better enhancer. The method for preparing tetra(t-butyl)tetraazaporphyrin-adsorbed Pt catalysts was investigated. This molecule is not only mainly adsorbed on a carbon black support, but also adsorbed on the surface of Pt nanoparticles. Tetra(t-butyl)tetraazaporphyrin-adsorbed carbon-supported Pt nanoparticles are prepared by an equilibrium adsorption method and an evaporation-to-dryness method. The optimized preparation method is discussed. Using the evaporation-to-dryness method, we also examined the effects of several porphyrins on the ORR of Pt catalysts.

Gross fuel starvation (cell reversal) event are known to damage the anode of PEFC seriously due to exceptionally high anodic potential (> 2 V vs RHE) when the anode gas channel or manifolds are clogged with water or ice. This high potential is due to oxygen evolution and/or carbon corrosion reactions (OER & COR) at anode to generate protons, instead of hydrogen oxidation reaction. Especially, COR may lead to rapid destruction of the anode structure so that promotion of OER instead of COR would be better way to mitigate serious damages by the fuel starvation. Thus, reversal tolerant anode (RTA), which includes water splitting catalyst to promote OER is one of the solution from material side to mitigate these serious damages. So far, water splitting catalyst (iridium and/or ruthenium) mixed Pt/C has been proposed as RTA, however a significant amount of iridium is indispensable to mitigate the damages. Here we will present oxidation-resistant and electro-conductive titanium oxide (Ti4O7) supported Pt (Pt/Ti4O7) catalyst-based RTA with very small amount of Ir without compromising RTA properties. For example, MEAs using a conventional Pt/C RTA including 0.04 mgIr cm-2 showed significant voltage loss after severe cell reversal test (0.2 A cm-2 load @H2-starved anode for 7,200 seconds), however, almost no performance loss was observed for the Pt/Ti4O7 RTA including the same amount of Ir. Moreover this superior reversal tolerant property of Pt/Ti4O7 RTA was found to be kept at as low as 0.014 mgIr cm-2. Detailed data about electrochemical and post mortem analyses of the tested MEAs will be discussed at the Meeting.

A new design of a redox flow battery (RFB), which is composed of two subcells separated by a gas phase of hydrogen, is proposed to eliminate the crossover of ionic species between the anolyte and catholyte. This idea not only increases the possible combinations of the two electrolytes, but also opens up the prospect of a revival of the old idea of an indirect fuel cell, which is composed of an RFB and two chemical reactors to regenerate the electrolytes using a fuel and oxygen. This paper describes the operation of a subcell as a component of an indirect fuel cell system. In the cycling test, oxidation/reduction of the electroactive species in each electrolyte were repeated with a hydrogen electrode as the counter electrode. This result demonstrates the possibility of this newly proposed RFB without crossover. In the operation of the subcell with a chemical reactor, a molecular catalyst (a rhodium porphyrin) was dissolved in the anolyte, and then a fuel was bubbled in the anolyte reservoir. As the electroactive species was reduced by the fuel, a steady-state oxidation current was observed at the cell. This demonstrates the negative half of the newly proposed indirect fuel cell.

We found that a water-soluble cobalt tetraphenylporphyrin tetrasulfonic acid catalyzes the reduction of redox mediators (quinones and indigo carmine) by CO. The reduction was analyzed by constant-potential amperometry, UV-spectroscopy, and gas chromatography. The rate of this reaction was quantitatively determined by monitoring the reduced forms of these compounds by amperometry. The reduction rate significantly depended on the redox potentials of these compounds. Since the reduced forms of indigo carmine and quinones can act as a good fuel for fuel cells, this reaction can be regarded as the conversion of the fuel from CO to other redox active species.

The switchable generation of hydrogen from a hydrazine solution was achieved with an electrochemical cell that has a cobalt anode and a platinum cathode. The H2 generator is based on the combination of hydrazine electro-oxidation on cobalt anode and H2O electro-reduction on platinum cathode. The rate of H2 generation was regulated by switching with no power supply. Slight electricity was generated from this cell along with H2. This cell uses selective anode and cathode catalysts, and hence a membraneless (one-compartment) structure could be realized for the regulation of H2 generation.

The robustness of the cathode catalysts used in polymer electrolyte fuel cells (PEFCs) is one of the major factors that determines their durability. In this work, a new class of corrosion-resistant catalyst, Pt–Ti alloy nanoparticles deposited on nano-sized sub-stoichiometric titanium oxide (TiOx), was prepared, and the durability of a Pt–Ti/TiOx cathode under conditions of fuel cell operation was evaluated. Cell performance under a constant current density for >900 h was examined to demonstrate the practical stability of the Pt–Ti/TiOx MEA under PEFC operating conditions. To investigate the effect of high potentials on cathode catalyst activity, a potential cycling test between 1.0 V and 1.5 V vs. a hydrogen anode was applied to the MEA. The results indicated that the electrochemical surface area (ECA) of the Pt–Ti/TiOx MEA is much more stable than that of a conventional Pt/XC72 MEA, and there is almost no loss of ECA even after 10,000 potential cycles. In addition, there was almost no change in the internal resistance of the MEA. TEM analyses of the potential-cycled MEA clearly revealed the excellent stability of Pt nanoparticles supported on TiOx particles.

Hydrazine hydrate has been studied as a fuel for use in anion exchange membrane fuel cells. To overcome its toxicity, derivatives of hydrazine have also been studied as possible fuels. However, conventional electrocatalysts show only weak activity in the electro-oxidation of hydrazine derivatives. In this study, we report that carbon-supported cobalt porphyrin catalysts can catalyze the electro-oxidation of certain hydrazine derivatives as well as hydrazine. The dependence of this activity on the concentration suggests that the carbohydrazide strongly interacts with the Co porphyrin. High activity of a Co porphyrin in the oxidation of certain kinds of hydrazine derivatives opens a big possibility to application including a vehicle that safer hydrazine derivatives can also be oxidized in fuel cells.

The ortho-methyl groups on meso-phenyl substituents were shown to strongly inhibit CO oxidation by Rh tetraphenylporphyrins. When both of the ortho-positions were occupied by methyl groups, CO oxidation was dramatically inhibited. The CO adduct was characterized by spectroscopy. O2 reduction by Rh tetraphenylporphyrins with ortho-methyl groups on meso phenyl substituents was also examined. The effects of ortho-methyl groups on CO oxidation are discussed in terms of steric hindrance.

We have found that certain kind of complexes on carbon black catalyze borohydride electro-oxidation. All of the Rh porphyrins tested in this study exhibited high activity regardless of the structure of the porphyrin ligands. While other metalloporphyrins also catalyzed borohydride electro-oxidation, the activities were much lower than those of Rh porphyrins. Even Rh tetrakis(4-methylphenyl)porphyrin, which exhibited the highest activity, scarcely catalyzed the chemical decomposition of borohydride under open-circuit conditions. These metalloporphyrin-based catalysts tested do not promote H2 electro-oxidation. Other Rh complexes (Rh2Cl2(CO)4 and Rh phthalocyanin) also catalyze borohydride electro-oxidation. Rh2Cl2(CO)4 oxidizes borohydride at lower potentials than Rh porphyrins; it can catalyze borohydride even below 0 V vs. RHE. A one-compartment H2-generator that also generates electric power was constructed using Rh2Cl2(CO)4 as an anode catalyst.

Reversible air electrodes integrated with a polymer electrolyte membrane have been proposed for use in rechargeable metal–air batteries or unitized regenerative fuel cells to reduce the impact of atmospheric carbon dioxide. Reversible air electrodes were prepared with an anion-exchange membrane (AEM) as a polymer electrolyte membrane and platinum-based catalysts. The AEM at the interface between the alkaline electrolyte and the air electrode layer plays major roles in AEM-type air electrodes as follows: it blocks (a) the permeation of cations in the alkaline electrolyte into the air electrode layer to prevent carbonate precipitation, (b) penetration of the alkaline solution itself, and (c) neutralization of the alkaline electrolyte by carbon dioxide, all of which prevent performance degradation of oxygen reactions. Catalysts for decreasing the overvoltage of oxygen reactions were also investigated with the AEM-type air electrode, and the overall efficiency was improved due to a remarkable decrease in the potential for the oxygen evolution reaction with Pt–Ir catalysts.

CO poisoning of the Pt-based anodes (PtRu) of stationary proton exchange membrane fuel cells remains a severe problem for their realization. We tried to counteract this problem using a CO oxidation...

Electrochemical CO oxidation by several carbon-supported rhodium tetraphenylporphyrins with systematically varied meso-substituents was investigated. A quantitative analysis revealed that the p-substituents on the meso-phenyl groups significantly affected CO oxidation activity. The electrocatalytic reaction was characterized in detail based on the spectroscopic and X-ray structural results as well as electrochemical analyses. The difference in the activity among Rh porphyrins is discussed in terms of the properties of p-substituents along with a proposed reaction mechanism. Rhodium tetrakis(4-carboxyphenyl)porphyrin (Rh(TCPP)), which exhibited the highest activity among the porphyrins tested, oxidized CO at a high rate at much lower potentials (0.1 V vs. a reversible hydrogen electrode, at 60 degrees C) than the present PtRu catalysts. This means that CO is electrochemically oxidized by this catalyst when a slight overpotential is applied during the operation of a proton exchange membrane fuel cell. This catalyst exhibited little H(2) oxidation activity, in contrast to Pt-based catalysts.