The oxygen evolution reaction (OER) plays a critical role in numerous energy conversion devices such as water splitting cells, metal-air batteries, and fuel cells. However, the high overpotential and sluggish kinetics associated with the OER greatly reduces the efficiency of these devices. To date, ruthenium oxide is considered the most active catalyst for OER, but its high cost and low abundance has prevented its large-scale application as an OER catalyst.[1] Thus, active, stable, and affordable OER catalysts are highly sought after. Among the various non-noble metal catalysts, transition metal oxyhydroxides such as NiOOH and Ni-doped FeOOH have recently attracted much attention owing to their high OER activity.[2] In addition, based on recent calculations and in situ studies, oxyhydroxides have been proposed to be the stable phase for cobalt-based oxide catalysts in alkaline conditions under OER-relevant potentials.[3,4] As a result, understanding the activity and stability of oxyhydroxides in alkaline conditions is important to aid in the design and optimization of OER catalysts. In this study, we investigate the chemical composition and structure of pure and modified CoOOH and their correlation to the activity and stability of the OER.

Pure CoOOH samples were prepared using a chemical bath deposition technique followed by chemical oxidation. [5] To modify CoOOH with Ni and Mn, wet impregnation was carried out prior to chemical oxidation. All the CoOOH samples were grown on a stainless steel mesh substrate and have a nanowire morphology (Figure 1). The nanowire-on-stainless steel mesh design serves two purposes: it ensures excellent electrical contact and improves the active surface area of the electrode. All electrodes prepared by this method were characterized by using electron microscopy, X-ray diffraction, Raman spectroscopy, and X-ray photoelectron spectroscopy prior to and after electrochemical testing.

OER activity was investigated in a three electrode cell in 0.1 M KOH. Based on steady-state linear sweep voltammetry, Ni-modified CoOOH samples showed greatly improved OER activity, and the increase in performance is correlated with higher Ni content (Figure 1). On the other hand, Mn modification showed no effect on the OER activity of CoOOH. Using electrochemical impedance spectroscopy, Ni modification of CoOOH lowered the charge transfer resistance, whereas Mn modification did not. Ni-modified CoOOH samples exhibited lower Tafel slopes compared to pure CoOOH samples. This suggests a difference in the reaction rate-determining step between pure and Ni-modified CoOOH. X-ray photoelectron spectroscopy of the CoOOH samples before and after stability testing revealed lower OH content in the surface region after testing and the loss of surface OH increased with higher applied potential.

Acknowledgment This material is based upon work supported by the National Science Foundation under Grant No. CHE-1465082. ZC acknowledges support by the Natural Sciences and Engineering Research Council of Canada (NSERC) Postgraduate Scholarship.

References

[1] Y. Lee, J. Suntivich, K. J. May, E. E. Perry, Y. Shao-Horn, J. Phys. Chem. Lett., 2012, 3, 399-404.

[2] M. W. Louie, A. T. Bell, J. Am. Chem. Soc., 2013, 135, 12329-12337.

[3] M. Bajdich, M. Garcia-Mota, A. Vojvodic, J. K. Norskov and A. T. Bell, J. Am. Chem. Soc., 2013, 135, 13521-13530.

[4] B. S. Yeo, A. T. Bell, J. Am. Chem. Soc. 2011, 133, 5587-5593.

[5] D. Lee, J.-Y. Choi, K. Feng, H. W. Park, Z. Chen, Adv. Energy Mater., 2014, 4, 1301389.

Figure 1 (Top) SEM image of a CoOOH sample and (Bottom) linear sweep voltammetry of pure and Ni-modified CoOOH. The inset in the bottom panel shows Nyquist plots for pure and Ni-modified CoOOH.