ABSTRACT
Phase engineering is promising to increase the intrinsic activity of the catalyst toward hydrogen evolution reaction (HER). However, the polymorphism interface is unstable due to the presence of metastable phases. Herein, phase engineering and dispersion stabilization are applied simultaneously to boost the HER activity of cobalt without sacrificing the stability. A fast and facile approach (plasma cathodic electro deposition) is developed to prepare cobalt film with a hetero-phase structure. The polymorphs of cobalt are realized through reduced stacking fault energy due to the doping of Mo, and the high temperature treatment resulted from the plasma discharge. Meanwhile, homogeneously dispersed oxide/carbide nanoparticles are produced from the reaction of plasma-induced oxygen/carbon atoms with electro-deposited metal. The existence of rich polymorphism interface and oxide/carbide help to facilitate H2 production by the tuning of electronic structure and the increase of active sites. Furthermore, oxide/carbide dispersoid effectively prevents the phase transition through a pinning effect on the grain boundary. As-prepared Co-hybrid/CoO_MoC exhibits both high HER activity and robust stability (44 mV at 10 mA cm-2, Tafel slope of 53.2 mV dec-1, no degradation after 100 h test). The work reported here provides an alternate approach to the design of advanced HER catalysts for real application.
ABSTRACT
Cerium conversion coating (CeCC) and Ce-Mo conversion coating (CeMCC) were prepared on aluminum alloy 6063 (AA6063) by immersion in alkaline conversion baths. Surface morphology and composition of the conversion coatings were characterized by scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). And electrochemical measurements were used to assess corrosion performance of the coatings. The SEM observations showed that CeMCC possessed a smoother and more uniform structure than CeCC, and the thickness of CeCC and CeMCC was about 0.8 and 1.2 µm respectively. The XPS depth analysis indicated that CeMCC contained a considerable amount of molybdenum and the cerium content was higher than that of CeCC at all coating depths. CeCC comprised of Al2O3, Ce2O3, CeO2, and cerium hydroxides, and the composition of CeMCC also included MoO2, MoO3, Al2(MoO4)3 and Na2MoO4 besides the above mentioned components. A potentiodynamic polarization (PDP) test revealed that the corrosion current density (i corr) values for bare alloy and CeCC were 13.36 and 4.38 µA cm-2 respectively in 3.5 wt% NaCl solution, while CeMCC exhibited the lowest i corr value of 0.24 µA cm-2, about two orders of magnitude lower than that of the substrate. Furthermore, the results obtained from both a cupric sulfate drop test and electrochemical impedance spectroscopy (EIS) characterization suggested that CeMCC possessed higher corrosion resistance in comparison with CeCC.
