Electrocatalytic O2 Reduction Properties of Pyrochlore-Type Oxides for Alkaline DAFCs

Abstract

To explore new electrocatalysts for alkaline direct alcohol fuel cells (ADAFCs), oxygen reduction reaction (ORR) activity of the pyrochlores Ln 2 Ru 2 O 7-(LnR, Ln=Pr,Nd,Sm,Gd,Dy,Yb) and Ln 2 Ru 2-x Mn x O 7-(LnRMn x , Ln=Pr,Nd) were examined in 0.1 M KOH solution at 70 o C. The onset potential (E on ) of the oxygen reduction current and the efficiency (Eff 4 ) of 4-electron reduction of oxygen were evaluated by semi-steady state voltammetry with rotating ring-disk electrodes. In the LnR series, PrR with the highest ORR activity showed E on = ~ 0.85 V vs. reversible hydrogen electrode and a Eff 4 value above 90 %. Their E on and Eff 4 values revealed that LnR with a smaller atomic number had a higher ORR activity. This trend was in good agreement with that of electrical conductivities of the LnR. In addition, effects of Mn substitution for the Ru-site were confirmed in both PrRMn x and NdRMn x series. Their ORR activities increased with an increase in the amount of Mn incorporation. The NdRMn 0.25 exhibited the highest ORR activity (E on = 0.95 V and Eff 4 > 90%) in the formation range of the pyrochlore single phase. Moreover, the ORR selectivity was much higher than that of a conventional 20 mass% Pt/C catalyst: the E on value was 0.95 V even in 0.1 KOH containing 1 M methanol, ethanol, ethylene glycol and 2-propernol, whereas the E on the value was 0.7 V for the Pt/C catalyst.

Catalysis Meets Spintronics; Spin Potentials Associated with Open-Shell Orbital Configurations Enhance the Activity of Pt3Co Nanostructures for Oxygen Reduction: A Density Functional Theory Study

“…This effect resembles closely the Ruderman-Kittel-Kasuya-Yosida (RKKY) interactions. Closed-shell LaNiO 3 and Pt metal improve drastically their ORR activities after doping with magnetic Fe atoms. , The same phenomenon occurs for magnetic graphene compositions − via RKKY interactions and for Mn-doped pyrochlores . Magnetic Pt 3 M (111) alloys (where M = Fe, Ni, Co) with Pt-skin surfaces and M-enriched subsurface are the most active and stable ORR catalysts to date. ,− Previous studies evidence that Pt–Co alloys are the most accomplished solid catalysts for ORR. − They are suitable for the manufacturing of cathodes in fuel cells (Figure ).…”

“…19,20 The same phenomenon occurs for magnetic graphene compositions 21−23 via RKKY interactions and for Mn-doped pyrochlores. 24 Magnetic Pt 3 M (111) alloys (where M = Fe, Ni, Co) with Pt-skin surfaces and M-enriched subsurface are the most active and stable ORR catalysts to date. 20,25−27 Previous studies evidence that Pt−Co alloys are the most accomplished solid catalysts for ORR.…”

Review—Recent Progress in Electrocatalysts for Oxygen Reduction Suitable for Alkaline Anion Exchange Membrane Fuel Cells

“…122 Instead, they are typically used to form alloys with noble metals to enhance ORR kinetics by changing the crystal lattice structure, electron density, surface morphology, ensemble effects, and synergetic effects, etc. Recently, some novel carbon-supported (with or without doping elements of N, S, P and B) first row transition metal and metal oxides, [123][124][125][126] chalcogenides, 127-129 spinels, [130][131][132] pyrochlores 115,133,134 have been reported to show some activity for ORR in alkaline media. Among them, similar and even faster ORR kinetics than Pt based electrocatalysts have been widely seen on this class of materials by stabilizing the metal or metal oxides with surface nitrogen (or other elements) functionalities on graphitic surfaces, which are the main focus of this section due to their direct applicability in AAEMFCs.…”

Effect of B Site Coordination Environment in the ORR Activity in Disordered Brownmillerites Ba2In2–xCexO5+δ

“…Sluggish kinetics of oxygen reduction reaction (ORR) in the cathode side and the consequent necessity for high concentrations of Pt in the catalyst is the primary bottleneck for extensive utilization of fuel cells in small domestic or automobile applications. − Hence, there is an urgent need to limit the usage of Pt and promote low cost, non-Pt catalysts with comparable catalytic properties. , So far, a large variety of materials have been explored as possible electrocatalysts for ORR and nitrogen doped carbon nanostructures, N4-macrocycles, oxides such as perovskites, brownmillerites, pyrochlores, and so on have been proposed to be alternative ORR catalysts. − Recent progress in the electrocatalyst development for fuel cells has revealed that stable structured oxides can open up many new avenues, especially in alkaline fuel cells. The characteristic features that make these classes of materials as ideal candidates for electrochemical applications are mixed ionic and electronic conduction (MIEC), favorable microstructure and good chemical/thermal/mechanical stability along with significant catalytic activity. − Brownmillerite-type compounds that exhibit MIEC property can be considered as potential candidates in these classes of materials.…”