Intermolecular Energy Gap‐Induced Formation of High‐Valent Cobalt Species in CoOOH Surface Layer on Cobalt Sulfides for Efficient Water Oxidation

Abstract

Transition metal-based electrocatalysts will undergo surface reconstruction to form active oxyhydroxide-based hybrids, which are regarded as the "truecatalysts" for the oxygen evolution reaction (OER). Much effort has been devoted to understanding the surface reconstruction, but little on identifying the origin of the enhanced performance derived from the substrate effect. Herein, we report the electrochemical synthesis of amorphous CoOOH layers on the surface of various cobalt sulfides (CoS α ), and identify that the reduced intermolecular energy gap (Δ inter ) between the valence band maximum (VBM) of CoOOH and the conduction band minimum (CBM) of CoS α can accelerate the formation of OER-active high-valent Co 4 + species. The combination of electrochemical and in situ spectroscopic approaches, including cyclic voltammetry (CV), operando electron paramagnetic resonance (EPR) and Raman, reveals that Co species in the CoOOH/Co 9 S 8 are more readily oxidized to CoO 2 /Co 9 S 8 than in CoOOH and other CoOOH/CoS α . This work provides a new design principle for transition metal-based OER electrocatalysts.

Identifying the geometric catalytic active sites of crystalline cobalt oxyhydroxides for oxygen evolution reaction

“…The Co atoms on lateral facets exhibit higher d-band centers compared with that on the basal (0001) facet (Supplementary Fig. 20 ), implying their favorable absorption of reaction intermediates 58 , 59 . The oxygen 2 p -band center was calculated (O 1c of lateral facets and O 3c of basal facets) because it reflects the metal d-character more accurately and has a direct relation to the catalytic reactivity 60 .…”

Direct Dioxygen Radical Coupling Driven by Octahedral Ruthenium–Oxygen–Cobalt Collaborative Coordination for Acidic Oxygen Evolution Reaction

“…The above results confirm the structural integrity of active Ru oct –O–Co oct collaborative coordination, which can also be reflected by comparing the CV curves of the three catalysts (Figure d and Figure S40). Three pairs of obvious redox couple at around 1.29, 1.45, and 1.56 V vs RHE for the Co 3 O 4 are observed before OER currents, which can be assigned to the Co II Co III /Co III Co III , Co III Co III /Co III Co IV , and Co III Co IV /Co IV Co IV redox reactions, respectively. , It is noted that though CoO 2 has been regarded as the active OER species in alkaline electrolytes for the majority of Co-based catalysts, − it is highly dissolvable in acid, and thus the formation of CoO 2 would significantly degrade the OER stability (Figure d). For the RuO 2 , we also observed a weak reduction peak around 1.41 V vs RHE assigned to RuO 4 /RuO 2 redox reaction that is detrimental to the acidic OER durability .…”

“…Besides the emerged Raman signal assigned to dioxygen radical coupling and the peak at 936 cm −1 originating from ClO 4 − in the electrolyte, the characteristic Raman peaks of the RuCoO x do not change or shift as the potential increases (Figure 3f), suggesting the high structural stability of the RuCoO x under positive bias. In sharp contrast, an extra Raman band at around 580 cm −1 that is raised from the acidic soluble CoO 2 species 50,62,64 is observed at ≥1.36 V vs RHE for the Co 3 O 4 (Figure 5e). This result is almost consistent with the CV measurement (Figure 5d) and well explains the different acidic OER stability of the Co 3 O 4 and RuCoO x (Figure 4d).…”

Zeolitic Imidazolate Framework-Derived Co3S4@NiFe-LDH Core–Shell Heterostructure as Efficient Bifunctional Electrocatalyst for Water Splitting

“…According to prior literature, − we modeled Co–S–Fe–O–Ni units to analyze the electronic interactions at the interface (Figure g). The Co 3+ 3d 6 configuration with fully occupied π-symmetry (t 2g ) 3d orbitals primarily undergoes electron–electron repulsion when interacting with bridging S 2– . However, Co 2+ with one unpaired electron in the π-symmetry (t 2g ) 3d orbital can interact by donating π-electrons to bridging S 2– .…”