•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.

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.Graphical abstractHighlights► Corrosion-resistant Pt–Ti alloy/sub-stoichiometric Ti oxide catalysts were prepared. ► Pt–Ti/TiOx MEA shows a mass activity comparable to that of conventional Pt/XC72. ► Performance loss of Pt–Ti/TiOx MEA was very limited even after potential cycling to 1.5 V. ► Superior stability of Pt–Ti/TiOx catalyst under high potential is due to the robustness of the TiOx support.

Nano-submicron particles of sub-stoichiometric titanium oxide (TiOx) were synthesized by irradiation of TiO2 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 TiO2 largely depended on the liquid: acetonitrile most strongly promoted the reduction of TiO2 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 Magnéli phase, Ti4O7. While the electrical conductivity of as-washed TiOx was significantly low, annealing at 900 °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.

Gas diffusion backings (GDBs) with various PTFE loadings for unitized regenerative polymer fuel cells (URFCs) were prepared and the relations between the PTFE loading amount and the URFC performance were examined. As for the GDB of the hydrogen electrode, both the fuel cell and water electrolysis performances were not affected by the amount of PTFE loading on the hydrogen side GDB. However, the URFC performances significantly depended on the PTFE loading amount of the GDB for the oxygen electrode; during the fuel cell and water electrolysis operations, URFC showed higher performances with smaller PTFE loadings but the cell with no PTFE-coated GDB showed a very deteriorated fuel cell performance. Cycle properties of the URFC revealed that the efficiency of the URFC decreased with the increasing cycles when the PTFE loading on oxygen side GDB was too low, however, a stable operation can be achieved with the appropriate PTFE loading on the GDB.

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 <1/3 by using the electrode prepared by the paste method compared to the conventional one without degrading the URFC performance.

Nano-submicron particles of sub-stoichiometric titanium oxide (TiOx) were synthesized by irradiation of TiO2 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 TiO2 largely depended on the liquid: acetonitrile most strongly promoted the reduction of TiO2 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 Magnéli phase, Ti4O7. While the electrical conductivity of as-washed TiOx was significantly low, annealing at 900 °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.