Catalytic Activity of Doped Praseodymium and Samarium Based Perovskites in Hydrogen and Oxygen Evolution Reactions

Over the last decades, the ABO₃ perovskites have been extensively studied as active catalyst materials for various low and high temperature redox reactions; e.g. hydrogen or oxygen evolution reactions and internal and biomass reforming. However, the mechanisms of their catalytic/electrochemical activity and stability vs. A- and B-site doping especially in aqueous solutions have not been clarified yet. Furthermore, it is not clear how the oxygen deficiency created in doped perovskites (e.g. AA’BB’O_3-δ) by partial substitution of less valent cations on A- and/or B-sites contributes to their oxygen exchange redox behavior at ambient conditions. In this regard, a comparative study of the PrNi_xCo_1-xO_3-δ and SmNi_xCo_1-xO_3-δ catalytic behavior was performed considering possible applications related to electrochemical and thermochemical hydrogen/oxygen evolution

PrNi_xCo_1-xO_3-δ and SmNi_xCo_1-xO_3-δ, where x= 0.1, 0.5, 0.9, were synthesized using a modified nitrate-glycine Pechini method (1) followed by a heat-treatment in air at 700^oC, 900°C and 1200°C. X-ray diffraction patterns reveal that the formation and chemical stability of the perovskite phase depends on the nature of the lanthanide element (A), relative composition of Ni to Co, and the heat-treatment temperature. For example, praseodymium based perovskites are more stable at higher heat-treatment temperatures (1200^oC) than SmNi_xCo_1-xO_3-δ. Furthermore, formation of the perovskite-phase (Fig. 1) is favorable at low Ni/Co ratios (x=0.1and 0.5) resulting in rhombohedral single-phase crystal structure. On contrary, high Ni/Co ratio (x=0.9) do not yield the perovskites phase.

The SEM and BET single point specific surface area (SSA) approaches were applied to determine the materials morphology. The HER and OER electrochemical performance was evaluated from cyclic voltammetry. A combination of Temperature Programmed Reduction/Oxidation (TPRO) and Thermo-Gravimetric (TG) analysis was further employed to understand their catalytic activity towards thermochemical hydrogen evolution.

In order to improve the electronic conductivity, the perovskite-graphene composites were synthesized by mixing the perovskites with graphene platelets (90 wt. %). The HER and OER electrochemical performance of PrNi_xCo_1-xO_3-δ and SmNi_xCo_1-xO_3-δ was studied in three-electrode configuration in alkaline medium. The structure-dependent electrochemical behavior of these nanocomposites as a function of nickel to cobalt molar ratio (x= 0.1, 0.5) and the heat-treatment temperature will be discussed in terms of Tafel plots, chemical stability, and potential applications in comparison to the state-of-the-art catalysts.

Acknoledgements: The authors gratefully acknowledge the financial support from the ACS Petroleum Research Fund (Project number 53614-UR10) and the NSF EPSCoR support (Awards IIA-1330840 and IIA-1330842).

References: 1.         M. C. Schrandt, P. Kolla and A. Smirnova, MRS Online Proceedings Library, 1542 (2013).