Competing Effect of Co³⁺Reducibility and Oxygen-Deficient Defects Toward High Oxygen Evolution Activity in Co₃O₄Systems in Alkaline Medium

Abstract

In Co 3 O 4 systems, the oxygen vacancy is reported to improve the oxygen evolution reaction (OER) activity because of higher Co 2+ /Co 3+ surface ratio. In situ studies have revealed Co 3+ site reducibility as the key factor for OER activity of cobalt oxide-based systems. In this context, we have synthesized and analyzed OER activity of two Co 3 O 4 systems; c-Co 3 O 4 with higher oxygen defects or Co 2+ /Co 3+ ratio and n-Co 3 O 4 with relatively less Co 2+ /Co 3+ ratio but more Co 3+ reducibility. The systems, n-and c-Co 3 O 4 show overpotential of 380 and 440 mV at 10 mA/cm 2 and Tafel slope of 153 and 53 mV/dec, respectively, for OER. Electrochemical characterization reveals that the lowering of OER onset potential is influenced by Co 3+ reducibility rather than defects in Co 3 O 4 systems while adsorption capacitance arising from surface irregularities, pores and their geometry, and Co 3+ -O h sites cause an increase in the Tafel slope values or decrease in OER kinetics. The correlation of the key factors such as Co 3+ reducibility and oxygen defects of two different Co 3 O 4 systems toward OER activity can aid the designing of highly efficient cobalt oxide-based OER catalysts. KEYWORDS: nanocrystalline cobalt oxide, Co 2+ /Co 3+ ratio, Co 3+ -O h reducibility, OER kinetics, impedance study

Flame Spray Pyrolysis Co3O4/CoO as Highly-Efficient Nanocatalyst for Oxygen Reduction Reaction

“…In accordance with literature XPS data [64,65], the Co2 p 3/2 and Co2 p 1/2 peaks are detected in Figure 3A-C, with the expected spin-orbit splitting ∆E = 15 eV. The peaks at 779.7 and 781.6 eV are assigned to Co 3+ and Co 2+ species respectively [64], as well as the peaks at 794.8 and 796.8 eV [64]. In the case of #Co3 material, the Co 2+ /Co 3+ ratio was increased (see Table 1), indicating the presence of surface-related defects and/or tetrahedrally-coordinated Co 2+ species.…”

“…X-Ray Photoelectron Spectroscopy: XPS spectra, presented in Figure 3A-C, were recorded to probe the cobalt oxidation states and the associated oxygen vacancies. In accordance with literature XPS data [64,65], the Co2 p 3/2 and Co2 p 1/2 peaks are detected in Figure 3A-C, with the expected spin-orbit splitting ∆E = 15 eV. The peaks at 779.7 and 781.6 eV are assigned to Co 3+ and Co 2+ species respectively [64], as well as the peaks at 794.8 and 796.8 eV [64].…”

“…In order to clarify the origin of Co 2+ presence, the XPS spectra for O1 s have been examined as well, see Figure 3D-F. The dominant O-XPS peaks at 529.7 (blue) and 531.3 eV (brown) in Figure 3D-F, correspond to lattice-oxygen and adsorbed-oxygen species respectively [64,65]. As expected, the increased population of surface vacancies in #Co3 material promotes the Co 2+ states, i.e., compare with #Co1 data in Figure 3D-F.…”

Incorporation of Cu/Ni in Ordered Mesoporous Co‐Based Spinels to Facilitate Oxygen Evolution and Reduction Reactions in Alkaline Media and Aprotic Li−O2 Batteries

“…Cyclic voltammograms were then analyzed to investigate the alteration on the catalyst's surface upon applied potential bias. Three redox coupled peaks obtained from CV measurements (Figure S15) in the pre‐catalytic region might be assigned to the redox processes of Co 2+ ⇄Co 3+ (A1‐C1), Co 3+ ⇄Co 4+ (A2‐C2), and Ni 2+ ⇄Ni 3+ (A3–C3), and further formation of Co 4+ and Ni 3+ −(O)OH active species [3,55] . The position and intensity of the Co and Ni redox peaks are strongly influenced by the surface reactivity of the material, which is affected by the strong electronic interactions between the different metals [44,53,56] .…”

Molybdenum‐Promoted Surface Reconstruction in Polymorphic Cobalt for Initiating Rapid Oxygen Evolution

“…[ 44 ] As demonstrated in the first CV cycle presented in Figure 4 a, the oxidation and reduction peaks at the potential below 1.3 V should be attribute to the reversible transformation between Co 2+ and Co 3+ . [ 45 ] Generally, the further transition of Co 3+ /Co 4+ redox couple often occur at about 1.4 V for cobalt‐containing catalysts. [ 46 ] For the CMN‐500 catalyst with a mixture of Co 0 /Co 2+ /Co 3+ and abundant exposed surface cobalt sites, the primary pre‐oxidation occurred at a wide region of 1.0–1.3 V, corresponding to the formation of cobalt oxyhydroxide (CoOOH).…”