Phase Segregation in Cobalt Iron Oxide Nanowires toward Enhanced Oxygen Evolution Reaction Activity.

Eko Budiyanto,Harun Tüysüz,Heiko Wende,Soma Salamon,Yue Wang

doi:10.1021/jacsau.1c00561

Abstract

The impact of reduction post-treatment and phase segregation of cobalt iron oxide nanowires on their electrochemical oxygen evolution reaction (OER) activity is investigated. A series of cobalt iron oxide spinel nanowires are prepared via the nanocasting route using ordered mesoporous silica as a hard template. The replicated oxides are selectively reduced through a mild reduction that results in phase transformation as well as the formation of grain boundaries. The detailed structural analyses, including the 57Fe isotope-enriched Mössbauer study, validated the formation of iron oxide clusters supported by ordered mesoporous CoO nanowires after the reduction process. This affects the OER activity significantly, whereby the overpotential at 10 mA/cm2 decreases from 378 to 339 mV and the current density at 1.7 V vs RHE increases by twofold from 150 to 315 mA/cm2. In situ Raman microscopy revealed that the surfaces of reduced CoO were oxidized to cobalt with a higher oxidation state upon solvation in the KOH electrolyte. The implementation of external potential bias led to the formation of an oxyhydroxide intermediate and a disordered-spinel phase. The interactions of iron clusters with cobalt oxide at the phase boundaries were found to be beneficial to enhance the charge transfer of the cobalt oxide and boost the overall OER activity by reaching a Faradaic efficiency of up to 96%. All in all, the post-reduction and phase segregation of cobalt iron oxide play an important role as a precatalyst for the OER.

Highlights

Water electrolysis plays an important role in green hydrogen production and future clean energy technology
3) were prepared via the nanocasting method using SBA-15 as a hard template. These two ratios were chosen to examine the impact of the dilute amount and a large amount of iron substitution on the oxygen evolution reaction (OER) activity
A highly active OER catalyst has been prepared through a mild reduction of cobalt iron oxide spinel nanowires

Summary

■ INTRODUCTION

Water electrolysis plays an important role in green hydrogen production and future clean energy technology. Our group applied the PLFL on mesostructured cobalt oxides in a similar direction.[22] The PLFL could lead to particle fragmentation that significantly increases the surface area and the formation of structural defects (Co2+ tetrahedral defect and oxygen vacancies) This laser post-treatment yielded the fragmented cobalt oxide that has superior OER activity compared to the starting material and ordered mesoporous Co3O4. Suryanto et al investigated the strong electronic coupling effect between iron oxide and nickel at the interface to enhance the overall water splitting activity.[24] Even though both post-treatment methods were proven to enhance the catalytic activity of the starting materials, the formation of a spinel and rock-salt biphase is unavoidable due to high-temperature local heating by laser irradiation. The Raman spectra were recorded in situ in a chronoamperometric (CA) mode with potential held for 1 min at OCP and 0.1 V gradual potential bias from 1.0 to 1.5 V vs RHE

■ RESULTS AND DISCUSSION

■ CONCLUSIONS

■ ACKNOWLEDGMENTS

■ REFERENCES