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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...

Development of highly active bifunctional electrocatalysts for oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) is required for air electrodes of zinc-air secondary batteries (ZAB). In this study, we synthesize spinel-type MnCo2O4 (MCO) nanoparticles on highly graphitized platelet-type carbon nanofibers (pCNF) via a solvothermal method. The pCNF is selected as carbon support in this study because of the excellent stability against anodic degradation under the OER condition. The MCO nanoparticles of 2-5 nm in diameter are uniformly dispersed on pCNF and the catalyst exhibits high activities for ORR due to strong interaction pCNF and MCO, in addition to the improvement of OER activities. The MCO/carbon hybrids show comparable electrocatalytic performances to state-of-the-art bifunctional electrodes for OER and ORR. (C) The Author(s) 2020. Published by ECSJ.

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.

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.

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.

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.

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

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.

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.

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.

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.

As a factor that may affect the accuracy of the measurement of electrode potentials in proton-exchange membrane fuel cells, the potential difference in a strip of Nafion ® 117 membrane induced by a humidity or temperature gradient was examined. The sample was placed in a cell with two chambers, where the temperature and humidity could be controlled individually. Hydrogen gas was fed to both chambers, and the voltage between two electrodes set at the ends of the sample, which acts as a reversible hydrogen electrode, was measured. A higher potential of the electrode was observed when humidity or temperature was set lower than the other electrode.

The proton conductivity of recast Nafion® thin films in the lateral direction (parallel to the interface) was measured in humidified atmospheres. The conductivity decreased with a decrease in the thickness of the film: e.g., the conductivity of a film with a thickness of about 100 nm was about an order of magnitude less than that of the bulk material. The dependence of the conductivity on temperature was also measured, and a thinner film showed a higher apparent activation energy for conduction. Since both the conductivity and the apparent activation energy for conduction were affected by the thickness, these phenomena may be due to an intrinsic change in the material. Based on the fact that the apparent activation energy for conduction in the bulk membrane under dry conditions is high, the high apparent activation energy for conduction in thin films may be due to the hindrance of water adsorption.

Direct ethanol fuel cells (DEFCs) with a PtRu anode and a Pt cathode were prepared using an anion exchange membrane (AEM) as an electrolyte instead of a cation exchange membrane (CEM), as in conventional polymer electrolyte fuel cells. The maximum power density of DEFCs significantly increased from 6 mW cm−2 to 58 mW cm−2 at room temperature and atmospheric pressure when the electrolyte membrane was changed from CEM to AEM. The anode and cathode polarization curves showed a decrease in the anode potential and an increase in the cathode potential for AEM-type DEFCs compared to CEM-type. This suggests that AEM-type DEFCs have superior catalytic activity toward both ethanol oxidation and oxygen reduction in alkaline medium than in acidic medium. The product species from the exhausted liquid from DEFCs operated at a constant current density were identified by enzymatic analysis. The main product was confirmed to be acetic acid in AEM-type, while both acetaldehyde and acetic acid were detected in 1:1 ratio in CEM-type. The anodic reaction of AEM-type DEFCs can be estimated to be the oxidation of ethanol to acetic acid via a four-electron process under these experimental conditions.

An easy and sensitive electrochemical method for determining the number of carboxyl groups on carbon black (Vulcan XC 72R) was developed. This involves the modification of carboxyl groups on Vulcan XC 72R with an electrochemically active reagent (3,4-dihydroxybenzylamine) in the presence of a carbodiimide. The amount of reagent immobilized on a carboxyl group of Vulcan XC 72R was determined by cyclic voltammetry. Separation of the covalently bonded and physically adsorbed 3,4-dihydroxybenzylamine molecules is important. For this purpose, treatment with 3,4-dihydroxybenzylamine but without carbodiimide was performed, and the results were compared with those with the carbodiimide. This comparison can be used to assess the influence of the physical adsorption of 3,4-dihydroxylbenzylamine, and the number of molecules covalently bonded to carboxyl group can be determined quantitatively. Unoxidized and air-oxidized Vulcan XC 72R were evaluated with regard to the number of carboxyl groups per surface area. The effect of oxidation on the number of carboxyl groups is discussed.

l -Ascorbic acid (AA) was directly supplied to polymer electrolyte fuel cells (PEFCs) as an alternative fuel. Only dehydroascorbic acid (DHAA) was detected as a product released by the electrochemical oxidation of AA via a two-electron transfer process regardless of the anode catalyst used. The ionomer in the anode may inhibit the mass transfer of AA to the reaction sites by electrostatic repulsion. In addition, polymer resins without an ionic group such as poly(vinylidene fluoride) and poly(vinyl butyral) were also useful for reducing the contact resistance between Nafion membrane and carbon black used as an anode, although an ionomer like Nafion is needed for typical PEFCs. A reaction mechanism at the two-phase boundaries between AA and carbon black was proposed for the anode structure of DAAFCs, since lack of the proton conductivity was compensated by AA. There was too little crossover of AA through a Nafion membrane to cause a serious technical problem. The best performance (maximum power density of 16 mW cm −2 ) was attained with a Vulcan XC72 anode that included 5 wt.% Nafion at room temperature, which was about one-third of that for a DMFC with a PtRu anode.

We have demonstrated the electrocatalytic oxidation of oxalic acid by carbon-supported Rh octaethylporphyrin at low overpotential in acidic solutions. As a result of C–C bond cleavage, CO2 generation from oxalic acid was clearly verified. The onset potential of oxalic acid oxidation was much lower than those for noble-metals and Co macrocycles. Repeated scans in cyclic voltammetry indicated that oxalic acid oxidation by Rh porphyrins is a stable reaction. Oxalic acid oxidation was suppressed by the presence of halides. The suppression effect of halides increases with increasing the atomic number in order: Br− > Cl− > F−. The reaction rates drastically decreased with an increase in pH of the test solutions. The suppression effect of halides and pH dependence are best explained on the basis of the competitive adsorption of oxalic acid and other anions on Rh(III) octaethylporphyrin.

Electrochemical oxidation of fuel compounds in acidic media was examined on eight electrodes (Pt, Ru, PtRu, Rh, Ir, Pd, Au, and glassy carbon) simultaneously by multiple cyclic voltammetry (CV) with an electrochemical cell equipped with an eight-electrode configuration. Direct-type polymer electrolyte fuel cells (PEFCs), in which aqueous solutions of the fuel compounds are directly supplied to the anode, were also evaluated. The performances of direct PEFCs with various anode catalysts could be roughly estimated from the results obtained with multiple CV. This multiple evaluation may be useful for identifying novel fuels or electrocatalysts. Methanol, ethanol, ethylene glycol, 2-propanol, and d -glucose were oxidized selectively on Pt or PtRu, as reported previously. However, several compounds that are often used as reducing agents show electrochemical oxidation with unique characteristics. Large current was obtained for the oxidation of formic acid, hypophosphorous acid, and phosphorous acid on a Pd electrode. l -Ascorbic acid and sulfurous acid were oxidized on all of the electrodes used in the present study.

An electrochemical method for determining the number of carboxyl groups on glassy carbon (GC) has been developed. In this method, a carboxyl group is modified with 3,4-dihydroxybenzylamine (DHBA) by the action of carbodiimide. The number of carboxyl groups can be determined from the peak area of DHBA in cyclic voltammograms of GC. The physical adsorption of DHBA on GC is not desirable for this quantification. Thus, physical adsorption is minimized by thorough washing and optimization of the modification conditions. Physical adsorption was assessed by experiments on DHBA-treatment without carbodiimide. The minimization and assessment of the physical adsorption of DHBA enables the use of this modification to determine the number of carboxyl groups on GC. The method is quantitative and much more rapid than other methods that have been used to evaluate surface oxides. Using this method, the numbers of carboxyl groups on several electrochemically oxidized GCs were determined. The results revealed that strong electrochemical oxidation of GC increases not only the total number of carboxyl groups but also the surface area of the GC. Strong oxidation does not greatly increase the number of carboxyl groups per surface area.

The structure of the catalyst layer of PEFCs has a considerable influence on the cell performance. The catalyst layer is manufactured by coating a suspension consisting of Pt/C catalyst particles and a Nafion solution. In a catalyst layer, the decrease in the Pt/C particle agglomeration controlled by the coating condition was reported to improve the performance; however, the influence that the coating condition exerts on the structure and then performance of the catalyst layer when the compositions of suspension are different has not yet been researched well. In this study, we researched a method to control the structure of a catalyst layer by the coating condition and suspension composition. The surface structure, which is well correlated with the predicted number of agglomerated Pt/C particles, showed good agreement with the performance. Under the condition where the Pt/C particles are not dispersed well, the performance was improved with an increase in the coating speed and Nafion content. However, that improvement became negligible at the optimum Nafion content providing maximal cell performance. Further, at a higher Nafion content, the excessive dispersion of Pt/C particles in the catalyst layer deteriorated the performance drastically. These results imply that the performance improvement by coating is effective when the suspension shows fairly high viscosity. Further, a little addition of solvent to decrease the viscosity also improves the performance; however it may be deteriorated by an excessive addition.

The CO-tolerance of a carbon-supported Pt/Mo-oxide (Pt/MoOx/C) anode in proton-exchange membrane fuel cells was examined by electrochemical impedance spectroscopy. A pseudo-inductive loop was clearly observed only for a Pt/MoOx/C anode at low anodic polarizations, which indicates the onset of the electro-oxidation of adsorbed CO (COad) on the Pt/MoOx/C catalyst. The CO-tolerance of Pt/MoOx/C at low anodic polarizations is mainly ascribed to the combination of the electro-oxidation of COad and the removal of CO by the water-gas shift reaction depending on the operating conditions.

To analyse the electrocatalytic oxidation of carbon monoxide by Rh porphyrins, we isolated a CO-adduct of Rh octaethylporphyrin, and examined its properties and reactivity by IR, NMR, and X-ray crystallographic analyses. The results indicate that the CO adduct of Rh octaethylporphyrin is vulnerable to nucleophilic attack by H2O. The CO-adduct was easily oxidized by an electron acceptor (1,4-naphthoquinone) to generate CO2. This indicates that CO is sufficiently activated in the CO complex of Rh octaethylporphyrin to reduce an electron acceptor. This mechanism is in contrast to that for the CO oxidation by Pt-based electrocatalysts.

A nanosized perovskite-type oxide supported on carbon black (perovskite/C) was prepared by the polyvinyl pyrrolidone (PVP) addition method and subsequent mechanical milling with carbon black. Transmission electron microscope (TEM) observation and scanning transmission electron microscope (STEM)–energy dispersive X-ray (EDX) mapping clearly revealed that the oxide prepared by the PVP method was small (ca. 20 nm) and highly dispersed on the carbon support. The oxygen reduction reaction (ORR) activity on the perovskite/C catalyst was investigated with rotating ring-disk electrode (RRDE) measurement in an alkaline solution at 25 °C. The prepared perovskite/C catalyst showed enhanced activity compared to catalysts obtained by the conventional solid state reaction and citrate process; i.e., a positive shift of the onset potential and increased ORR current at a disk electrode. The present catalyst was also associated with decreased ring current.

Carbon-supported Pt/Mo-oxide catalysts were prepared, and the reformate tolerances of Pt/MoOx/C and conventional PtRu/C anodes were examined to clarify the features and differences between these catalysts. Fuel cell performance was evaluated under various reformate compositions and operating conditions, and the CO concentrations at the anode outlet were analyzed simultaneously using on-line gas chromatography. Pt/MoOx showed better CO tolerance than PtRu with CO(80 ppm)/H 2 mixtures, especially at higher fuel utilization conditions, which is mainly due to the higher catalytic activity of Pt/MoOx for the water-gas shift (WGS) reaction and electro-oxidation of CO. In contrast, the CO 2 tolerance of Pt/MoOx was much worse than that of PtRu with a CO 2 (20%)/H 2 mixture. The results of voltammetry indicated that the coverage of adsorbates generated by CO 2 reduction on Pt/MoOx was higher than that on PtRu, and therefore, the electro-oxidation of H 2 is partly inhibited on Pt/MoOx in the presence of 20% CO 2 . With CO(80 ppm)/CO 2 (20%)/H 2 , the voltage losses of Pt/MoOx and PtRu are almost equal to the sum of the losses with each contaminant component. Although the adsorbate coverage on Pt/MoOx increases in the presence of 20% CO 2 , CO molecules in the gas phase could still adsorb on Pt through an adsorbate ‘hole’ to promote WGS or electro-oxidation reactions, which leads to a reduction in the CO concentration under CO/CO 2 /H 2 feeding conditions.

Electrochemical oxidation of l-ascorbic acid (AA) on carbon electrodes in acidic media was investigated to use AA as a fuel for direct polymer electrolyte fuel cells (PEFCs). A higher current was obtained for the oxidation of AA by modifying glassy carbon electrodes with various carbon blacks. The anodic current density depended on the electrical double-layer capacitance formed by carbon black, Nafion ionomer, and electrolyte solution. Direct-type PEFCs, in which an aqueous solution of AA was used as a fuel, were fabricated with carbon black as anodes. A maximum power density of 15 mW cm−2 was attained at room temperature without any precious metal as an anode catalyst. This value is more than twice that obtained previously with palladium as an anode catalyst. Keywords: l-Ascorbic acid, Electrochemical oxidation, Polymer electrolyte fuel cell, Carbon black

The equilibrium adsorption amount of Co porphyrins possessing various meso-substituents on Vulcan XC 72R was determined. The adsorption amount differs significantly among the Co porphyrins, and the difference can be clearly explained by the bulkiness and orientation of the meso-substituents. These results indicate the existence of a specific interaction between the porphyrin and carbon black. The electrocatalytic O2 reduction activity by Vulcan XC 72R-supported Co porphyrins was also determined. A significant difference in the onset potential was observed. A comparison with the results of Co porphyrins on glassy carbon as well as the redox potential of Co porphyrins in an organic solvent revealed that the porphyrin–Vulcan interaction affects the onset potential of the electrocatalytic O2 reduction by Co porphyrins.

In order to estimate the durability of Nafion ® membrane as an electrolyte for direct methanol fuel cells (DMFCs), the degree of dissolution of Nafion ® membranes in mixtures of methanol and water at various temperatures up to 80 °C was examined. At 80 °C, more than 30% of the membrane was dissolved in mixed solvents with methanol concentrations of higher than 80%. Dissolution of recast films made from Nafion ® solution was also examined, because it is an important component of the catalyst layers of DMFCs. The effects of heat treatment on the durability of the recast films were also examined. Although high temperature (160 °C for 1 min) or long time (120 °C for 1 h) heat treatment improved significantly the durability at room temperature, the films were dissolved at 80 °C and the amounts of dissolution were larger than that of Nafion ® 117 membranes.

In order to survey new CO-tolerant anode for the PEFC application, the addition of TaOx and NbOx to the Pt catalyst was examined in the electrochemical oxidation of CO in a sulfuric acid solution. Voltammetric peak potentials for the oxidation of CO pre-adsorbed on the Pt surface shifted to lower potentials by these additives, indicating an enhancement of electro-catalysis of Pt for the CO oxidation. Both oxides of Ta or Nb also bring about an inhibition of the CO adsorption rate onto the Pt surface. Concerted effect of these oxides with Ru is discussed for the CO oxidation over the PtRu-TaOx and the PtRu-NbOx anodes.

Hydrazine was examined as a fuel in a direct-liquid-fueled fuel cell employing proton exchange membrane (PEM) for the electrolyte. Hydrazine showed better performance than methanol in the direct fuel cell; the cell using hydrazine gave voltage twice as high as that using methanol in the low-current density region. The I – V characteristics were drastically changed depending on the surface area of the anode catalyst. Compositions of the exhaust materials from each electrode were analyzed in order to investigate the reaction that occurred at the electrodes. The analysis revealed that the catalytic decomposition reaction of hydrazine proceeded further than the electro-oxidation reaction on the anode side using a high specific surface area catalyst. The crossover of hydrazine and ammonia through the PEM was confirmed and the reduction of the hydrazine crossover is important in developing further high performance.

Thin film electrocatalyst layers with various PTFE and Nafion contents for unitized regenerative polymer electrolyte fuel cells (URFCs) were prepared by the paste method and the performance as URFC electrodes was examined. Comparing the terminal voltage versus current density curves of the URFC, it was found that the PTFE content in the electrocatalyst layer affected only the fuel cell performance; the electrode containing 5–7 wt.% PTFE was appropriate for the URFC. The Nafion content in the electrode affected both the fuel cell and water electrolysis performance; the electrode containing 7–9 wt.% Nafion showed good performance. The addition of a small amount of iridium catalyst (about 10 at.%) to the oxygen electrode layer significantly improved the URFC performance. Catalyst loadings can be reduced to

Carbon-supported Pt/MoOx catalysts for use in PEFC anodes were prepared and their catalytic activity for the oxidation of CO-contaminated H2 was examined based on the fuel cell performance in PEFC single cell arrangements. Based on the XRD pattern and XPS measurements of the prepared Pt/MoOx/C catalysts, it was found that the deposited MoOx exists as an amorphous oxide phase. The MoOx phase shows a redox peak at around 0.45 V, which was revealed by the cyclic voltammogram of the Pt/MoOx/C in sulfuric acid solution. The PEFC performance of the cell with Pt/MoOx/C was improved under 100 ppm CO-contaminated H2 conditions compared to the Pt/C catalyst, and was almost comparable to the PtRu(1:1)/C catalyst. Keywords: Polymer electrolyte fuel cell (PEFC), Molybdenum oxide, Electrocatalyst, Carbon monoxide

The oxidation of carbon monoxide adsorbed on a supported Pt (30 wt.% and 60 wt.% Pt/C) or PtRu (30 wt.% and 45 wt.% PtRu/C) gas diffusion electrode at various humidities and temperatures was investigated in an actual PEFC configuration. It is shown that the adsorbed CO-stripping potential on Pt/C and PtRu/C decreased with an increase in the humidity of the fuel until the dew point of the fuel reached the cell temperature. The CO-stripping potential of the 45 wt.% PtRu/C is lower and less sensitive to the humidity of the fuel than that of the 30 wt.% PtRu, which suggested that the PtRu/C with a higher catalyst density adsorbs water more strongly, even under lower humidity conditions.

Nonenzymatic glucose fuel cells were prepared by using a polymer electrolyte membrane and Pt-based metal catalysts. A fuel cell with a cation exchange membrane (CEM), which is often used for conventional polymer electrolyte fuel cells, shows an open circuit voltage (OCV) of 0.86 V and a maximum power density (Pmax) of 1.5 mW cm−2 with 0.5 M d-glucose and humidified O2 at room temperature. The performance significantly increased to show an OCV of 0.97 V and Pmax of 20 mW cm−2 with 0.5 M d-glucose in 0.5 M KOH solution when the electrolyte membrane was changed from a CEM to an anion exchange membrane (AEM). This is due to the superior catalytic activity for both glucose oxidation and oxygen reduction in alkaline medium than in acidic medium. The anodic reaction of the fuel cell can be estimated to be the oxidation of glucose to gluconic acid via a two-electron process under these experimental conditions. The crossover of glucose through an electrolyte membrane was negligibly small compared with methanol and may not represent a serious technical problem due to the cross-reaction. Keywords: Polymer electrolyte fuel cell, Glucose, Anion exchange membrane, Direct oxidation, Alkaline fuel cell

Correction for ‘Effects of p-substituents on electrochemical CO oxidation by Rh porphyrin-based catalysts’ by Shin-ichi Yamazaki et al., Phys. Chem. Chem. Phys., 2010, 12, 8968–8976.

Using a newly developed substrate with microelectrodes for the four-point method, the proton conductivity of a cast thin film of the Nafion® along the lateral (parallel to the interface) direction in a humidified atmosphere was measured. It was found to be much smaller than that of the as-received Nafion membrane. This difference should affect the simulation to design the gas diffusion electrodes for proton exchange membrane fuel cells (PEMFCs), because the ionic conductivity is one of the fundamental data for it. Keywords: Ionic conductivity, Thin film, Ionomer, Four-point method, Proton exchange membrane fuel cell

Nano-submicron particles of sub-stoichiometric titanium oxide (TiOx) were synthesized by irradiation of TiO(2) particles dispersed in liquid with a Nd:YAG pulsed UV laser, and their physicochemical and electrochemical properties were examined. After laser irradiation for 1 h, spherical oxide particles of up to ca. 300 nm in diameter were formed regardless of the liquid used, however the reduction of TiO(2) largely depended on the liquid: acetonitrile most strongly promoted the reduction of TiO(2) by UV laser irradiation. The mean valence of titanium in TiOx synthesized in acetonitrile was ca. 3.5, which is comparable to that of the most-reduced Magneli phase, Ti(4)O(7). While the electrical conductivity of as-washed TiOx was significantly low, annealing at 900 degrees C in hydrogen dramatically improved conductivity. The oxidation resistance of TiOx was examined by cyclic voltammetry to a high potential (1.5 V) using a MEA under PEMFC operating conditions. TiOx showed a much lower anodic corrosion current at >1.0 V than XC-72R carbon, which suggests that TiOx may exhibit superior oxidation resistance as a catalyst support material at high potentials.

In‐situ X‐ray absorption fine structure (XAFS) is one of powerful spectroscopic techniques to analyze valence state and chemical bonding in Pt and Pt‐Ru electrocatalysts under fuel cell operating conditions. We measured Pt L3 and Ru K‐XAFS of Pt and Pt‐Ru electrocatalysts in fluorescence mode under different operation voltages using pure H2 or 97 ppm CO containing H2 as fuel gases that were fed to the anode. The radial structure functions that derived from the obtained XAFS spectra showed that the local structure around Pt and Ru atoms in the electrocatalysts changes depending on the operation voltage and shows difference with the existence of CO in the fuel gas.