•Rh porphyrins on carbon catalyze CO oxidation by water-soluble electron acceptors.•The dependence of the rate on the redox potential of the acceptors was examined.•The reaction was used to remove CO from CO-contaminated H2 gas.•Rh porphyrins with common ligands achieved complete conversion of CO to CO2.•Rh porphyrins on carbon give higher conversion than that dissolved in solution.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.

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.

•A Rh phthalocyanine catalyzed the electro-oxidation of glucose at lower potentials than conventional catalysts using Co phthalocyanine.•This catalyst also catalyzed the oxidation of gluconate, a product of 2-electron oxidation of glucose.•Coulometry and product analysis demonstrate that multi-electron oxidation of glucose occurs.•Substituents effects on Rh phthalocyanine were examined.•The exposure to negative potentials is needed to activate this Rh phthalocyanine catalyst.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.

•We prepared cobalt octaethylporphyrin (Co-OEP)-modified perovskite/carbon catalysts.•ORR activity of perovskite/carbon was enhanced by Co-OEP-modification.•RRDE measurements suggested that the 2 + 2 electron reduction of O2 is promoted.•The porphyrin plays a role as a two-electron O2 reduction catalyst to give HO2−.•HO2− is further reduced to OH− by the perovskite-type oxide.Perovskite-type oxide-carbon (Vulcan XC72) mixture (La0.6Sr0.4Mn0.6Fe0.4O3/C) was modified by a metalloporphyrin (cobalt octaethylporphyrin: Co-OEP) having two-electron O2 reduction activity, and its electrochemical reduction activity for O2 (ORR) was investigated in an alkaline solution by rotating ring disk electrode (RRDE) voltammetry. The Co-OEP/La0.6Sr0.4Mn0.6Fe0.4O3/C catalyst showed improved ORR activity, with a positive shift of the onset potential. In addition, a decreased ring current compared to Co-OEP/C suggested that the quasi-four-electron reduction of O2 was also enhanced. Further experiments showed that ORR activity was also enhanced by Co-OEP-modification of other types of carbon (Ketjenblack EC600JD, Denka Black) or perovskite-type oxide (La0.6Ca0.4Mn0.6Fe0.4O3, La0.8Sr0.2Co0.6Fe0.4O3). In the case of the addition of other porphyrin complexes (cobalt tetraphenylporphyrin (Co-TPP), iron octaethylporphyrin (Fe-OEP)) to a La0.6Sr0.4Mn0.6Fe0.4O3/C catalyst, the onset potential did not shift to the positive side due to the lower activity compared to Co-OEP.

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.

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.Highlights► We demonstrated that a Co porphyrin can catalyze the reduction of quinones by CO. ► The reaction was confirmed by amperometry, UV-spectroscopy, and gas chromatography. ► The rate of this reaction significantly depended on the redox potentials of quinones. ► This reaction can be regarded as the regeneration of fuel for an indirect CO fuel cell system.

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.

A Rh porphyrin on carbon black was shown to catalyze the electro-oxidation of several aliphatic alcohols (ethanol, 1-propanol, and 2-propanol) and benzyl alcohols. The overpotentials for alcohol oxidation were very low. The reaction mechanism and substrate specificity are discussed.

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.Graphical abstractHighlights► We investigated electro-oxidation of hydrazine derivatives by metalloporphyrins. ► The hydrazine derivatives are difficult to be oxidized by conventional catalysts. ► A Co porphyrin exhibited the strongest activity toward carbohydrazide. ► Co porphyrins have higher activity toward carbohydrazide than hydrazine.

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.Graphical abstractHighlights► We investigated electrochemical CO oxidation activity of Rh tetraphenylporphyrins. ► Ortho-methyl groups on meso phenyl groups strongly inhibit CO oxidation activity. ► The effect of ortho-methyl groups was also examined by spectroscopy. ► O2 reduction is not strongly inhibited by ortho-methyl groups. ► The results suggest that steric hindrance of ortho-methyl groups is important.

In this communication, we demonstrate that certain kinds of Rh porphyrins on carbon black can electrochemically oxidize aldose at low potentials. The onset potential was much lower than those with the other complex-based catalysts. A product analysis suggested that this reaction involves 2-electron oxidation of the aldehyde group.

A one-compartment membrane-less electrochemical H2 generator from borohydride was realized using a Rh porphyrin and RuO2 as the anode and cathode, respectively. H2 generation from this cell was successfully controlled electrochemically by varying the potential applied. The regulation of H2 generation was based on the selectivity of the anode and cathode. We found that RuO2 exhibits H2O electro-reduction activity without electro-oxidation or chemical decomposition of borohydride, and used the catalyst as a selective cathode in the electrochemical H2 generator. Anode and cathode potentials of the electrochemical H2 generator were measured separately. The both potentials were discussed in terms of the catalytic activities of a Rh porphyrin and RuO2.

Rhodium phthalocyanin was shown to exhibit high catalytic activity toward oxalic acid oxidation in terms of the onset potential and current density. The onset potential for the oxidation of oxalic acid is much lower than those of the previous electrodes. The substrate specificity and the effect of anion were also examined. The results suggest that oxalic acid would first coordinate to the Rh center in the catalytic cycle. An amperometric sensor for oxalic acid was constructed using Rh phthalocyanin. This sensor worked at a low applied potential (0.75 V vs. a reversible hydrogen electrode). The limit of detection of the sensor reached 1 μM.

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 pophyrins 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 °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 H2 oxidation activity, in contrast to Pt-based 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 electrocatalyst. Rh porphyrins, which can catalyze electrochemical CO oxidation in low potential regions, were adsorbed on a carbon-supported PtRu catalyst (PtRu/C) to increase its CO tolerance. The combined electrocatalysts (Rh(porphyrin)−PtRu/C) exhibited significantly higher activity toward CO(2%)-contaminated H2 than the PtRu/C alone; the onset potential for H2 oxidation reaches 0.2 V (vs a reversible hydrogen electrode) on the combined catalyst. In contrast, the composite of PtRu black and Rh(porphyrin) scarcely exhibited CO tolerance. This suggests that the enhancement of CO tolerance in Rh(porphyrin)−PtRu/C is mainly caused by Rh porphyrins on carbon. The significance of Rh porphyrins on carbon was shown by the findings that a mixed catalyst composed of carbon-supported Rh(porphyrin) (Rh(porphyrin)/C) and PtRu/C gave high oxidation activity toward CO(2%)/H2. Rh porphyrin on carbon would decrease the CO concentration around PtRu particles and thus reduce the CO poisoning of PtRu catalysts.

l-Ascorbic acid (AA), also known as vitamin C, is an environmentally-benign and biologically-friendly compound that can be used as an alternative fuel for direct oxidation fuel cells. While direct ascorbic acid fuel cells (DAAFCs) have been studied experimentally, modelling and simulation of these devices have been overlooked. In this work, we develop a mathematical model to describe a DAAFC and validate it with experimental data. The model is formulated by integrating the mass and charge balances, and model parameters are estimated by best-fitting to experimental data of current–voltage curves. By comparing the transient voltage curves predicted by dynamic simulation and experiments, the model is further validated. Various parameters that affect the power generation are studied by simulation. The cathodic reaction is found to be the most significant determinant of power generation, followed by fuel feed concentration and the mass-transfer coefficient of ascorbic acid. These studies also reveal that the power density steadily increases with respect to the fuel feed concentration. The results may guide future development and operation of a more efficient DAAFC.

We have realized a novel hydrogen peroxide fuel cell that uses hydrogen peroxide (H2O2) as both an electron acceptor (oxidant) and a fuel. H2O2 is oxidized at the anode and reduced at the cathode. Power generation is based on the difference in catalysis toward H2O2 between the anode and cathode. The anode catalyst oxidizes H2O2 at a more negative potential than that at which the cathode catalyst reduces H2O2. We found that Ag is suitable for use as a cathode catalyst, and that Au, Pt, Pd, and Ni are desirable for use as anode catalysts. Alkaline electrolyte is necessary for power generation. The performance of this cell is clearly explained by cyclic voltammograms of H2O2 at these electrodes. This cell does not require a membrane to separate the anode and cathode compartments. Furthermore, separate paths are not needed for the fuel and electron acceptor (oxidant). These properties make it possible to construct fuel cells with a one-compartment structure.

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.

A Rh porphyrin on carbon black was shown to catalyze the electro-oxidation of several aliphatic alcohols (ethanol, 1-propanol, and 2-propanol) and benzyl alcohols. The overpotentials for alcohol oxidation were very low. The reaction mechanism and substrate specificity are discussed.

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 pophyrins 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 °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 H2 oxidation activity, in contrast to Pt-based catalysts.

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.

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.

In this communication, we demonstrate that certain kinds of Rh porphyrins on carbon black can electrochemically oxidize aldose at low potentials. The onset potential was much lower than those with the other complex-based catalysts. A product analysis suggested that this reaction involves 2-electron oxidation of the aldehyde group.