Size effects and active state formation of cobalt oxide nanoparticles during the oxygen evolution reaction

Abstract

Water electrolysis is a key technology to establish CO2-neutral hydrogen production. Nonetheless, the near-surface structure of electrocatalysts during the anodic oxygen evolution reaction (OER) is still largely unknown, which hampers knowledge-driven optimization. Here using operando X-ray absorption spectroscopy and density functional theory calculations, we provide quantitative near-surface structural insights into oxygen-evolving CoOx(OH)y nanoparticles by tracking their size-dependent catalytic activity down to 1 nm and their structural adaptation to OER conditions. We uncover a superior intrinsic OER activity of sub-5 nm nanoparticles and a size-dependent oxidation leading to a near-surface Co–O bond contraction during OER. We find that accumulation of oxidative charge within the surface Co3+O6 units triggers an electron redistribution and an oxyl radical as predominant surface-terminating motif. This contrasts the long-standing view of high-valent metal ions driving the OER, and thus, our advanced operando spectroscopy study provides much needed fundamental understanding of the oxygen-evolving near-surface chemistry.

Atomic-Scale Insights into Morphological, Structural, and Compositional Evolution of CoOOH during Oxygen Evolution Reaction

“…Two pairs of redox peaks were observed in Figure a, that is, A1/C1 and A2/C2 at ∼1.1 and ∼1.5 V versus RHE, respectively, which would correspond to Co­(+II) ↔ Co­(+III) and Co­(+III) ↔ Co­(+IV) . However, such conventional assignment neglects associated changes in the oxygen chemical and electronic structure . In particular, a recent study revised the redox couples of Co hydroxide by operando scanning transmission X-ray microscopy and found that the peaks at ∼1.2 and ∼1.55 V versus RHE indicate Co­(+II) ↔ Co­(+III, +II) and Co­(+III, +II) ↔ Co­(+III) transition.…”

“…28 However, such conventional assignment neglects associated changes in the oxygen chemical and electronic structure. 29 In particular, a recent study 15 revised the redox couples of Co hydroxide by operando scanning transmission X-ray microscopy and found that the peaks at ∼1. The area inside the closed CV curve, shown in Figure 2a, increases during the first 30 cycles, while it remains unchanged after 50 cycles.…”

Molecule-Enhanced Electrocatalysis of Sustainable Oxygen Evolution Using Organoselenium Functionalized Metal–Organic Nanosheets

“…Accordingly, it is of great significance to develop earth-abundant catalysts with both high activity and electrochemical tolerance for OER. Transition metal-based catalysts are typical representatives; however, they still face the challenge of improving overall performance to meet the industrial demand. − Therefore, remolding the reactivity of metal active sites (MASs) is of vital importance to realize large-scale hydrogen production via water electrolysis. − One of the commonly implemented and powerful approaches is to introduce heteroatoms, such as selenium (Se), into a metal crystal lattice for efficiently modulating the electronic structure of MASs. − Particularly, the Se element doped into the metal lattice usually results in metal selenide, which can significantly change the adsorption of reactive intermediates as well as reduce the OER energy barrier. − Nevertheless, such catalysts easily undergo structural reconstruction and are transformed into metal oxides/hydroxides/oxyhydroxides during OER. − At the same time, the instability of lattice Se leads to its massive or even complete dissolution in the electrolyte under the oxidation bias potential, thus substantially weakening the modulation effect on MASs, that is, the activation energy barrier will increase with the dissolution of functional Se (Scheme ). − Moreover, large quantities of highly toxic Se-containing species dissolved from the catalysts will pollute the environment.…”

High-Performance Oxygen Evolution Reaction Electrocatalysts Discovered via High-Throughput Aerogel Synthesis

“…XPS results of Fe 2 Co 2 Ce ternary aerogels as prepared for the stability test and after the stability test: (a 1–2 ) High-resolution Co 2p: ca.782.4 and 798 eV are assigned to Co 2+ in Co 2p 3/2 and Co 2p 1/2 ; ca.780.5 and 796 eV are assigned to Co 3+ in Co 2p 3/2 and Co 2p 1/2 ; 789 and 805 eV are assigned to Co 2+ satellite peaks; and 786 and 802 eV are assigned to Co 3+ satellite peaks; ,, (b 1–2 ) High-resolution Fe 2p spectra: ca.709.7 and 723.4 eV are assigned to Fe 2+ in Fe 2p 3/2 and Fe 2p 1/2 ; ca.709.7 and 723.4 eV are assigned to Fe 3+ in Fe 2p 3/2 and Fe 2p 1/2 ; 716 and 727 eV are assigned to Fe 2+ satellite peaks; and 718 and 729 eV are assigned to Fe 3+ satellite peaks. − (c 1–2 ) High-resolution O 1s spectra: ca. 535.5, 532.1, 531.3, and 530 eV, which can be assigned to water that was adsorbed (H 2 O adsorbed ), oxygen in hydroxide and oxyhydroxide­(O M–OH ), defective oxygen site (O defective ), and oxygen in metal oxide­(O M–O ), respectively. ,− , …”