Effect of Substitution in Pb2Ru2O7-δ� on the Oxygen Evolution Reaction

The identification of economically feasible and highly efficient materials supporting the oxygen evolution reaction (OER) in alkaline media will constitute a major advancement in a variety of technologies such as water splitting, elctrowinning, fuel cells, and metal-air batteries. OER has proven to be a catalytic dependent electrochemical process, and thus, the catalytic material at the oxygen electrode plays an important role in order to improve the efficiency of such technologies. Numerous research studies have been carried out on the synthesis of new electrode materials in order to reduce the voltage loss associated with the oxygen reactions.

Pyrochlores containing ruthenium as the transition metal ion possess high area specific activity [1]. Pyrochlores also demonstrate bifunctional characteristics in organic environments [2]. Among the features that make pyrochlores attractive as oxygen electrodes is the unusual framework containing a high density of oxygen vacancies and their apparent ordering, which is believed to aid in the oxygen anion mobility. However, pyrochlore structures are not as widely studied as perovskite structures, in part due to the few elements that can be substituted in the structure without changing the pyrochlore configuration and preserving the electrochemical performance. Previous reports have shown the effect of partial substitution in lead ruthenium pyrochlores (Pb_1-xA_xRu_1-yB_yO_7-δ, 0<x,y<1) for tuning its electronic properties [3]. Moreover, PbBiRu₂O_7-δ showed an encouraging performance and negligible capacity loss over 85 cycles in an iron-air battery configuration in strong alkaline media [4].  In the present work, partial substitutions of Pb (La and Bi) and Ru (Fe and Co) were performed and the effect on oxygen evolution in alkaline media was investigated. All pyrochlores were prepared by a combustion method which produced pyrochlore oxides with BET area in the range of 6 to 8 m²/g. The catalyst powders were dispersed in a Nafion containing solution to form a catalyst ink. All the samples were characterized by XRD (for phase analysis), XPS (for valency), and SEM (for surface morphology). The OER electrochemical performance has been investigated by means of quasi-steady state current measurements. In addition, kinetic parameters such as Tafel slope and electrochemical reaction order towards OH^- were determined. Tafel slope values range from 47 to 75 mV/dec and reaction order values were near unity for most of the compounds. OER activity of selected compounds is presented in Fig. 1. The OER activity for all other compositions will also be presented.

Fig. 1. OER activity at 0.5mV/s for Pb₂Ru₂O_7-δ, Pb_1.8La_0.2Ru₂O₇, and Pb_1.5Bi_0.5Ru₂O_7.

Acknowledgements

This work is supported by the IGERT-NSF fellowship and partially funded by the US Department of Energy/NETL, Oak Ridge Institute for Science and Education (ORISE) fellowship.

References

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