Thermal Activiation of Sr3Fe2O7 Electrocatalysts for Water Oxidation at Neutral pH

One of the great challenges in artificial photosynthesis is the development of efficient catalysts for oxidation of water to molecular oxygen. To date, the most extensively investigated catalysts for O₂ evolution are nanostructured IrO₂ and RuO₂.¹ They are robust and efficient water oxidation catalysts that exhibit high turnover frequencies under mild conditions. However, the high cost and scarcity of noble metals severely limit the widespread use of these catalysts for solar fuel production. Thus, considerable effort has been devoted for development of efficient catalysts based on more abundant materials. Recently, we demonstrated that stabilization of the Mn³⁺ species relative to charge disproportionation (2Mn³⁺ → Mn²⁺ + Mn⁴⁺) is an effective strategy to lower the overpotential under neutral pH.^2,3 In this study, we examined O₂ evolution activity of Sr₃Fe₂O_7-δ at various temperature conditions, as Sr₃Fe₂O_7-δ is known to show the charge disproportionation (2Fe⁴⁺ → Fe³⁺ + Fe⁵⁺) below 70 ^oC.⁴

A Sr₃Fe₂O_7-δ film electrode was prepared by a spray drying method. Sr₃Fe₂O_7-δ polycrystalline was synthesized by solid state reaction of SrCO₃ and Fe₂O₃ and then suspended in ethanol. The suspension was deposited on a fluorine-doped tin oxide (FTO) electrode held on a 170 ^oC hotplate. The concentration of dissolved oxygen was monitored simultaneously with current density versus potential measurements using a needle-type oxygen microsensor.

Figure 1 shows polarization curves of a Sr₃Fe₂O_7-δ electrode at 30 ^oC and 70 ^oC. Upon sweeping the electrode potential at 30 ^oC, an increase in both anodic current and dissolved O₂ concentration was observed at an onset potential of approximately 1.1 V. This result indicates that the observed anodic current was attributed to water oxidation. It should be noted that when the solution temperature was raised to 70 ^oC, the onset potentials for O₂ production showed a negative shift from 1.1 V to 0.9 V. In this presentation, we will discuss the origin of the observed difference in O₂ evolution activity at between 30 ^oC and 70 ^oC as well as temperature dependent efficiency of charge disproportionation of Fe⁴⁺.

References

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2. T. Takashima, K. Hashimoto, and R. Nakamura, J. Am. Chem. Soc., 134, 18153 (2012).

3. T. Takashima, K. Hashimoto, and R. Nakamura, J. Am. Chem. Soc., 134, 1519 (2012).

4. K. Kuzushita, S. Morimoto, S. Nasu, and S. Nakamura, J. Phys. Soc. Jpn., 69, 2767 (2000).