Engineering Surface Structure of Spinel Oxides via High-Valent Vanadium Doping for Remarkably Enhanced Electrocatalytic Oxygen Evolution Reaction

Abstract

Spinel oxides (AB 2 O 4 ) with unique crystal structures have been widely explored as promising alternative catalysts for efficient oxygen evolution reactions; however, developing novel methods to fabricate robust, cost-effective, and high-performance spinel oxide based electrocatalysts is still a great challenge. Here, utilizing a complementary experimental and theoretical approach, pentavalent vanadium doping in the spinel oxides (i.e., Co 3 O 4 and NiFe 2 O 4 ) has been thoroughly investigated to engineer their surface structures for the enhanced electrocatalytic oxygen evolution reaction. Specifically, when the optimal concentration of vanadium (ca. 7.7 at. %) is incorporated into Co 3 O 4 , the required overpotential to reach a certain j GEOM and j ECSA decreases dramatically for oxygen evolution reactions in alkaline media. Even after 30 h of chronopotentiometry, the required potential for V-doped Co 3 O 4 just increases by 16.3 mV, being much lower than that of the undoped one. It is observed that the pentavalent vanadium doping introduces lattice distortions and defects on the surface, which in turn exposes more active sites for reactions. DFT calculations further reveal the rate-determining step changing from the step of *-O to *-OOH to the step of *-OH to *-O, while the corresponding energy barriers decrease from 1.73 to 1.57 eV accordingly after high-valent V doping. Moreover, the oxygen intermediate probing method using methanol as a probing reagent also demonstrates a stronger OH* adsorption on the surface after V doping. When vanadium doping is performed in the inverse spinel matrix of NiFe 2 O 4 , impressive performance enhancement in the oxygen evolution reaction is as well witnessed. All these results clearly illustrate that the V doping process can not only efficiently improve the electrochemical properties of spinel transition metal oxides but also provide new insights into the design of high-performance water oxidation electrocatalysts.

A CoV2O4 precatalyst for the oxygen evolution reaction: highlighting the importance of postmortem electrocatalyst characterization

“…[1][2][3][4] V-Doped Co oxide materials have recently emerged as a promising class of alkaline OER catalysts. Previous studies of V-doped Co oxide materials show increased activity for the OER at low overpotentials, [5][6][7][8][9][10][11][12][13][14] and typically suggest that this increased activity is a result of changes to the catalyst's electronic structure and/or in increased stabilization of adsorbed OER intermediates. [5][6][7][8] In this study, we report a CoV 2 O 4 material that shows exceptional specific activity per BET surface area for the OER based on as-synthesized characterization data.…”

“…This XPS characterization is consistent with other synthesized CoV 2 O 4 materials. 5,19 OER activity measurements were performed using previously reported protocols. 20,21 Catalyst inks were prepared by mixing the as-synthesized catalyst particles and Nafion in a water-isopropanol solution, and the resulting inks were dropcast onto polished glassy carbon electrode surfaces (0.196 cm 2 ) resulting in films with mass loadings of 0.84 mg cm À2 .…”

“…Previous studies have reported that V-doped Co oxide materials show higher activity for the OER than their parent oxide materials. [5][6][7][8][9][10][11][12][13] In many cases, this observed higher OER activity compared to the V-free parent materials is attributed to specific changes in the physical and/or electronic structure upon V incorporation resulting in changes to material conductivity, adsorption energy of OER intermediates, and/or the rate determining step in the catalytic mechanism. In the case of V-doped amorphous CoO x films, 7,9 crystalline V-doped CoOOH 8 and CoFe-based 12 nanoparticles, and Co-V hydroxide nanostructures, 13 postmortem characterization shows these materials retain significant concentrations of V after OER stability measurements, consistent with the assertion that the continued presence of V is an important component of the OER mechanism for these materials.…”

“…For example, in previous studies of OER by CoV 2Àx Fe x O 4 and Co 3Àx V x O 4 nanoparticles, comprehensive mechanistic and computational analysis was conducted with the assumption that V was an important component in the active catalyst species. 5,6 However, in these studies, either postmortem analysis was not reported to confirm the presence of V in the material after the OER, 6 or reported postmortem characterization showing the loss of V during the OER was not considered in the mechanistic analysis. 5 Similarly, studies of V-doped CoP materials and Co-Mo-V catalysts both attribute increased catalytic activity to the presence of V in the materials, but did not include postmortem characterization showing the continued presence of V after the OER.…”

“…5,6 However, in these studies, either postmortem analysis was not reported to confirm the presence of V in the material after the OER, 6 or reported postmortem characterization showing the loss of V during the OER was not considered in the mechanistic analysis. 5 Similarly, studies of V-doped CoP materials and Co-Mo-V catalysts both attribute increased catalytic activity to the presence of V in the materials, but did not include postmortem characterization showing the continued presence of V after the OER. 10,14 This is not to say that the mechanistic arguments in these previous studies are incorrect, but rather suggests that the composition of the post-OER catalyst should be reported and discussed when considering possible catalytic mechanisms.…”