Searching General Sufficient‐and‐Necessary Conditions for Ultrafast Hydrogen‐Evolving Electrocatalysis

Abstract

The development of cost-effective and high-performance electrocatalysts for the hydrogen evolution reaction (HER) is one critical step toward successful transition into a sustainable green energy era. Different from previous design strategies based on single parameter, here the necessary and sufficient conditions are proposed to develop bulk non-noble metal oxides which are generally considered inactive toward HER in alkaline solutions: i) multiple active sites for different reaction intermediates and ii) a short reaction path created by ordered distribution and appropriate numbers of these active sites. Computational studies predict that a synergistic interplay between the ordered oxygen vacancies (at pyramidal high-spin Co 3+ sites) and the O 2p ligand holes (OLH; at metallic octahedral intermediate-spin Co 4+ sites) in RBaCo 2 O 5.5+δ (δ = 1/4; R = lanthanides) can produce a near-ideal HER reaction path to adsorb H 2 O and release H 2 , respectively. Experimentally, the as-synthesized (Gd 0.5 La 0.5 )BaCo 2 O 5.75 outperforms the state-of-the-art Pt/C catalyst in many aspects. The proof-of-concept results reveal that the simultaneous possession of ordered oxygen vacancies and an appropriate number of OLH can realize a near-optimal synergistic catalytic effect, which is pivotal for rational design of oxygen-containing materials.

Direct evidence of boosted oxygen evolution over perovskite by enhanced lattice oxygen participation

“…For example, only a very small shift toward lower photon energies was found at the Co L 3 -edge as Si 4+ is incorporated into SCO (Fig. 4b), indicative of an insignificant decline in the surface Co valence 52 . Considering the inertness of silicon towards electrocatalysis, the OER activity of SCO and Si-SCO based on the AEM pathway should be primarily determined by the valence of B-site surface cobalt ions 7 .…”

Bulk and Surface Properties Regulation of Single/Double Perovskites to Realize Enhanced Oxygen Evolution Reactivity

“…Similarly, the Mo‐ L 2,3 edge spectra before and after OER catalysis also suggest a high stability of the valence state (Figure S5 b). After OER catalysis the Co‐ L 2,3 and Fe‐ L 2,3 sXAS spectra of the material (Figure b and c) exhibits features only slightly higher in energy, indicating an increase of the valence state of Co and Fe ions in agreement with our hard X‐ray XAS result . By comparing the Co‐ L 3 edge peak position with the given reference samples, the average Co valence state of the SP/DP after OER catalysis was calculated to be +3.49, close to the value of +3.4 in the highly efficient SrCoO 2.7 electrocatalyst .…”

Self‐Assembled Ruddlesden–Popper/Perovskite Hybrid with Lattice‐Oxygen Activation as a Superior Oxygen Evolution Electrocatalyst

“…Based on the well‐proven charge transfer models, the metal–oxygen covalency increases with the increased metal valence. [ 41,47–49 ] For example, an increase of the Co valence from Co 2+ /Co 3+ to Co 4+ results in a decrease in the charge transfer energy (Δ) from positive (6–7 eV for Co 2+ and 2–3 eV for Co 3+ ) to negative (≈−2 eV for Co 4+ ), which is accompanied by enhanced Co 3d‐O 2p covalency. [ 48,49 ] Accordingly, higher Co/Fe valence state with higher content of Co 4+ /Fe 4+ endows RP/P‐LSCF as a stronger covalent oxide.…”

“…[ 41,47–49 ] For example, an increase of the Co valence from Co 2+ /Co 3+ to Co 4+ results in a decrease in the charge transfer energy (Δ) from positive (6–7 eV for Co 2+ and 2–3 eV for Co 3+ ) to negative (≈−2 eV for Co 4+ ), which is accompanied by enhanced Co 3d‐O 2p covalency. [ 48,49 ] Accordingly, higher Co/Fe valence state with higher content of Co 4+ /Fe 4+ endows RP/P‐LSCF as a stronger covalent oxide. The metal 3d‐oxygen 2p covalency can be further directly explored by the O K ‐edge XAS spectra, considering that the pre‐edge peak below ≈532 eV in the O‐ K XAS spectra corresponds to the unoccupied O 2p orbitals hybridized with transition metal 3d orbitals.…”