Ternary Ni-Co-Fe oxyhydroxide oxygen evolution catalysts: Intrinsic activity trends, electrical conductivity, and electronic band structure
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Nickel-, cobalt-, and iron-based (oxy)hydroxides comprise the most-commonly studied electrocatalysts for the oxygen-evolution reaction (OER) in alkaline solution. A fundamental understanding of composition-structure-activity relationships for mixed-metal Ni-Co and Ni-Co-Fe (oxy)hydroxides is important to guide the design of advanced OER catalysts. Here we use cyclic voltammetry, chronopotentiometry, inductively-coupled plasma-optical emission spectroscopy, and in situ electrical conductivity measurements to characterize the properties and activity of various compositions of Ni-Co-Fe (oxy)hydroxides prepared by cathodic co-electrodeposition. Consistent with previous studies, we find Fe is essential for the mixed-metal (oxy)hydroxides to achieve high OER activity. In the rigorous absence of Fe (achieved by using specially cleaned electrolytes), the most-active Ni-Co (oxy)hydroxide composition has an OER turn-over frequency only twice that of pure Co (oxy)hydroxide, suggesting minimal synergism between the two metals. The addition of Co to Ni-Fe (oxy)hydroxides shifts the onset of electrical conductivity to lower potentials, but has little effect on the intrinsic OER activity, with the most-active Ni-Co-Fe (oxy)hydroxide having an OER turn-over frequency only ~ 1.5 times that of the Ni-Fe (oxy)hydroxides. The magnitudes of the electrical conductivities are similar for all the compositions measured. Density-functional-theory-calculated projected density of states show a significant contribution of all chemical elements at the valence band edge of the mixed-metal oxyhydroxide electronic structure, demonstrating significant electronic hybridization between the elements. The calculations suggest the involvement of all the elements in modulating the electronic structure at putative Fe-based active sites that are probably located at edges or defects in the two-dimensional oxyhydroxide sheets.
Keywordselectrocatalysis heterogeneous catalysis water electrolysis oxygen evolution density functional theory
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This work was primarily supported by the National Science Foundation Chemical Catalysis program under Grant CHE-1566348. The computational work was supported by the Nancy and Stephen Grand Technion Energy Program (GTEP) and a grant from the Ministry of Science and Technology (MOST), Israel. The project made use of CAMCOR facilities supported by grants from the W. M. Keck Foundation, the M. J. Murdock Charitable Trust, ONAMI, the Air Force Research Laboratory (No. FA8650-05-1-5041), the National Science Foundation (Nos. 0923577 and 0421086), and the University of Oregon. ICP-OES was performed at the W. M. Keck Collaboratory for Plasma Spectrometry at Oregon State University and we acknowledge Andy Ungerer for help with data acquisition and interpretation. S. W. B. further acknowledges support from the Sloan and Dreyfus Foundations. The students of the UO 2015 CH399 “Research Immersion” course are acknowledged for preliminary data collection. The authors thank Adam Batchellor for insightful discussion.
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