Influence of Surface Oxygen Vacancies and Ruthenium Valence State on the Catalysis of Pyrochlore Oxides

Abstract

Proton exchange membrane (PEM) water electrolysis is a promising energy storage solution by electrochemically splitting water into hydrogen fuel and oxygen. However, the sluggish kinetics, high operating potential, and corrosive acidic environment during the oxygen evolution reaction (OER) require the use of scarce and costly Ir-based oxides, tremendously hampering its large-scale commercialization. Hence, developing active and stable anode catalysts with reduced precious-metal usage is desperately essential. For the first time, we report a group of Y 2−x Ba x Ru 2 O 7 pyrochlore oxides and employ them in acid OER and PEM electrolyzers. We reveal the mechanism for the promoted OER performance of Ba-doped Y 2 Ru 2 O 7 in which partially replacing Y 3+ by Ba 2+ in Y 2 Ru 2 O 7 greatly facilitates the hole-doping effect, which generates massive oxygen vacancy and multivalence of Ru 5+ / Ru 4+ , thus boosting the OER performance of Y 2−x Ba x Ru 2 O 7 . This work provides an effective method and paradigm for improving the electrocatalytic property of pyrochlore oxides.

Recent Development of Oxygen Evolution Electrocatalysts in Acidic Environment

“…Engineering defects (including anion defects and cation defects) at molecular or atomic levels, such as lattice distortion, [ 79 ] vacancies, [ 105,106 ] and unsaturated coordination sites, [ 74 ] to tune electronic structure have attracted great attention. Experimental approaches of engineering defects include heteroatoms doping, [ 74 ] foreign elements substitution, [ 68 ] element leaching, [ 50,104 ] atomic‐etching, [ 105 ] and constructing heterogeneous interfaces, [ 107–112 ] which have guided the discovery of some highly efficient catalysts.…”

“…[ 74 ] Wang and co‐workers demonstrated the crucial effects of massive oxygen vacancy and multivalence Ru 5+ /Ru 4+ on OER performance of Y 2− x Ba x Ru 2 O 7 . [ 106 ] Higher vacancies concentration and oxidation of Ru can result in stronger electrophilicity of catalysts and thus enhance the deprotonation rate to accelerate OER reaction. Similarly, cation vacancies can also improve OER activity.…”

Novel engineering of ruthenium‐based electrocatalysts for acidic water oxidation: A mini review

“…Generally, as shown in Figure 7(A), pyrochlore oxides can be denoted as A 2 B 2 O 6 O′ or [A 2 O′][B 2 O 6 ], which consists of a network of corner‐sharing BO 6 octahedra with interstitial sites being occupied by O′ and A atoms (A 2 O′) 109 . The Ru‐based pyrochlore oxides, in which the B sites are dominated by Ru atoms have also drawn a great deal of notice because of the lowered usage of Ru, while maintaining the excellent catalytic activity and stability toward OER 49,50,52–54,110–113 . For examples, Kim et al 49 used a sol–gel method to synthesize Y 2 Ru 2 O 7−δ, in which all diffraction of Y 2 Ru 2 O 7−δ was consistent with cubic phase ( Fd3m ) pyrochlore‐type Y 2 Ru 2 O 7 (YRO) without any unpurified phase.…”

“…(A) Crystalline structure of pyrochlore oxides (A 2 B 2 O 7 ). (Reproduced from Reference 50 with permission from American Chemical Society). (B) Normalized XANES spectra of Ru K‐edge with different absorption energy (E 0 ) for Y 2 Ru 2 O 7−δ , RuO 2 , and Ru foil, respectively.…”

“…The A site in Ru‐based pyrochlore oxides can also be partially replaced with other metal ions and such a replacement does not alter the crystal structure due to the flexible structure of A 2 B 2 O 6 O′ 109 . Feng et al 50 have used a sol–gel method to synthesize a series of Y 2−x Ba x Ru 2 O 7 (YBRO‐x, x = 0, 0.1, 0.15, 0.25, and 0.4) compounds. There is a slight negative shift of the main diffraction peak in the XRD patterns of YBRO‐x, indicating the occurrence of crystal structure expansion, presumably due to Ba 2+ possessing a larger ionic radius than that of Y 3+ .…”