Water oxidation by amorphous cobalt-based oxides: in situ tracking of redox transitions and mode of catalysis

Abstract

Water oxidation by amorphous oxides is of high interest in artificial photosynthesis and other routes towards non-fossil fuels, but the mode of catalysis in these materials is insufficiently understood. We tracked mechanistically relevant oxidation-state and structural changes of an amorphous Co-based catalyst film by in-situ experiments combining directly synchrotron-based X-ray absorption spectroscopy (XAS) with electrocatalysis. Unlike a classical solid-state material, the bulk material is found to undergo chemical changes. Two redox transitions at midpoints potentials of about 1.0 V (Co II 0.4Co III 0.6  all-Co III ) and 1.2 V (all-Co III  Co III 0.8Co IV 0.2) vs. NHE at pH 7 are coupled to structural changes. These redox transitions can be induced by variation of either electric potential or pH; they are broader than predicted by a simple Nernstian model, suggesting interacting bridged cobalt ions. Tracking reaction kinetics by UV-Vis-absorption and time-resolved mass spectroscopy reveal that accumulated oxidizing equivalents facilitate dioxygen formation. On these grounds, a new framework model of catalysis in the amorphous, hydrated and volume-active oxide is proposed: Within the oxide film, cobalt ions at the margins of Co-oxo fragments undergo Co II Co III Co IV oxidationstate changes coupled to structural modification and deprotonation of Co-oxo bridges. By encounter of two (or more) Co IV ions, an active site is formed at which the O-O bondformation step can take place. The Tafel slope is determined by both the interaction between cobalt ions (width of the redox transition) and their encounter probability. Our results represent a first step toward development of new concepts that address the solid-molecular Janus nature of the amorphous oxide. Insights and concepts described herein for the Co-based catalyst film may be of general relevance also for other amorphous oxides with water-oxidation activity. EXPERIMENTAL XAS measurements -in-situ experimentCoCat-coated electrodes were prepared by electrodeposition in a separate electrochemical setup before start of the in-situ XAS measurements from a solution of 0.5 mM Co 2+ ions in 0.1 M KPi at pH 7 (deposition of about 50 nmol cm -1 of Co ions, see ESI for further details). The in-situ XAS measurements were performed at the SuperXAS beamline of the Swiss Light Source (SLS) in Villigen, Switzerland. The excitation energy was selected by a double-crystal monochromator (Si-111, detuning to 50 % intensity, scan range of 7650-8400 eV) and used to irradiate the backside of the ITO/PET electrode at an angle of 45°. The spot size of the X-ray beam on the sample was 5 mm × 1 mm. Due to employment of a large spot size (defocussed beam) and a rapid-scanning mode, the influence of radiation-induced modifications was negligible, as verified in control experiments. The cobalt K-edge fluorescence was monitored perpendicular to the incident beam by a scintillation detector (19.6 cm 2 active area, 51BMI/2E1-YAP-Neg, Scionix), which was shielded by a 25 μm iron foil agai...

An Operando Investigation of (Ni–Fe–Co–Ce)Ox System as Highly Efficient Electrocatalyst for Oxygen Evolution Reaction

“…In addition, a similar Co XANES shift has been observed in different cobalt-based electrocatalysts in literature between OCP and oxygen evolution conditions. 56,[82][83][84][85] This shift has been interpreted as the formation of a Co(IV) fraction at catalytic conditions, following a Co(II)  Co(III)  Co(IV) redox scheme. The EXAFS spectrum collected under the catalytic condition (SI, Figure S3a) also resembles the one reported by Kanan et al 56 Therefore, we hypothesize the presence of a mixed configuration at the surface at high current densities (~ 8 mA cm -2 ), where we observe the partial oxidative conversion to cobalt oxyhydroxide (Co(II)  Co(III)) and the presence of Co(IV) (Co(III)  Co(IV)) as an active species in the catalytic cycle.…”

Understanding the Oxygen Evolution Reaction Mechanism on CoOx using Operando Ambient-Pressure X-ray Photoelectron Spectroscopy

“…This has led to novel insights into the nature of catalytically active sites at the atomic scale (19,25,27,28), as well as the role of structural transformations from the resting to the catalytic state (29,30,31,32). Recently, observations that highly disordered or amorphous materials can exhibit enhanced catalytic activity relative to their crystalline counterparts (28,32,33,34,35,36) has ignited significant interest in developing and characterizing such systems. For example, several recent studies used electrochemical methods to show that the bulk region of CoO x -based catalysts can be active for OER owing to the formation of a highly accessible CoO(OH) layered structure, in which water and electrolyte can efficiently intercalate (12,19,25,31,36,37).…”

“…Substantial work has been aimed at understanding relationships between structure, mechanism, and activity (19,20,21,22,23,24,25,26). This has led to novel insights into the nature of catalytically active sites at the atomic scale (19,25,27,28), as well as the role of structural transformations from the resting to the catalytic state (29,30,31,32). Recently, observations that highly disordered or amorphous materials can exhibit enhanced catalytic activity relative to their crystalline counterparts (28,32,33,34,35,36) has ignited significant interest in developing and characterizing such systems.…”

In Situ Observations of Oxygen Evolution Cocatalysts on Photoelectrodes by X-ray Absorption Spectroscopy: Comparison between Cobalt-Phosphate and Cobalt-Borate

“…9,15,[21][22][23][24][25][26][27] Among these, cobalt-phosphate (Co-P i ) and cobalt-borate (Co-B i ) cocatalysts electrodeposited from a dilute Co 2+ solution containing phosphate and borate electrolytes, respectively, are known to generate highly active OER cocatalysts. [28][29][30][31][32][33][34][35][36][37][38][39][40][41] As an example, Nocera et al reported that a Si photoelectrode modified with a Co-P i cocatalyst exhibited high activity, with a solar energy conversion efficiency of 4.7%. 21 Domen et al also discovered that a Co-P i -modified Ba-doped Ta 3 N 5 photoelectrode had an efficiency of 1.5% using single-photon photoanodes system.…”

“…In the present work, we focused on the differences in the reaction activities of Co-P i and Co-B i cocatalysts on photoelectrodes. In spite of many previous studies, [28][29][30][31][32][33][34][35][36][37][38][39][40][41] during photoelectrochemical reactions, using in situ Co-K edge XAFS, and subsequently discuss the functioning of these OER cocatalysts.…”