Water Oxidation by Co‐Based Oxides with Molecular Properties

Risch, Marcel; Klingan, Katharina; Zaharieva, Ivelina; Dau, Holger

doi:10.1002/9781118698648.ch9

Cited by 7 publications

(9 citation statements)

References 144 publications

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“…The Tafel slope of crystalline Co 3 O 4 was found to be very similar compared with the one of electrodeposited Co oxide films, which exhibit ≥60 mV dec −1 in 0.1 M KP i . (refs 26 , 30 , 45 ). We note that catalytic activity and Co oxide redox behaviour showed very good stability ( Supplementary Figs 3–9 ).…”

Section: Resultsmentioning

confidence: 99%

“…A well-known X-ray amorphous OER electrocatalyst is the so-called cobalt-phosphate catalyst, herein denoted as CoCat 2 23 24 25 26 27 28 29 30 , which is catalytically active in neutral phosphate-containing electrolyte and excels by self-repair properties 29 . The CoCat films consist of layer fragments of ∼12–14 octahedrally coordinated (mainly) Co 3+ ions connected via di-μ 2/3 -oxo bridges 23 24 25 .…”

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Reversible amorphization and the catalytically active state of crystalline Co3O4 during oxygen evolution

et al. 2015

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Water splitting catalysed by earth-abundant materials is pivotal for global-scale production of non-fossil fuels, yet our understanding of the active catalyst structure and reactivity is still insufficient. Here we report on the structurally reversible evolution of crystalline Co3O4 electrocatalysts during oxygen evolution reaction identified using advanced in situ X-ray techniques. At electrode potentials facilitating oxygen evolution, a sub-nanometre shell of the Co3O4 is transformed into an X-ray amorphous CoOx(OH)y which comprises di-μ-oxo-bridged Co3+/4+ ions. Unlike irreversible amorphizations, here, the formation of the catalytically-active layer is reversed by re-crystallization upon return to non-catalytic electrode conditions. The Co3O4 material thus combines the stability advantages of a controlled, stable crystalline material with high catalytic activity, thanks to the structural flexibility of its active amorphous oxides. We propose that crystalline oxides may be tailored for generating reactive amorphous surface layers at catalytic potentials, just to return to their stable crystalline state under rest conditions.

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Section: Resultsmentioning

confidence: 99%

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Reversible amorphization and the catalytically active state of crystalline Co3O4 during oxygen evolution

et al. 2015

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“…36,38 CoCat has been the subject of numerous mechanistic studies. 39,40 Bediako et al studied the dependence of current on overpotential (Tafel plots) with different buffer concentrations and conditions, decoupling the catalytic reaction from the proton−electron hopping inside the film. 13 A subsequent work focused on CV analysis to estimate a lower limit for the catalytic rate and proposed that catalysis is limited by the diffusion of the proton accepting base.…”

Section: ■ Introductionmentioning

confidence: 99%

“…Amorphous Co oxide films prepared by electrodeposition in neutral solution (CoCat) were introduced by Kanan and Nocera . The catalyst resting state is composed of a Co II –Co III mixture, with Co IV species appearing at potentials relevant to catalytic water oxidation. ,, The Co oxidation state change is coupled to release of a proton to the buffer species in solution and represents a pre-equilibrium preceding the turnover limiting step, which has been proposed to be the formation of an oxygen–oxygen bond. ,, The catalyst is formed by molecular fragments containing 14–19 Co atoms; , water oxidation takes place in the bulk of the film , at the periphery of the CoOx fragments. , CoCat has been the subject of numerous mechanistic studies. , Bediako et al studied the dependence of current on overpotential (Tafel plots) with different buffer concentrations and conditions, decoupling the catalytic reaction from the proton–electron hopping inside the film . A subsequent work focused on CV analysis to estimate a lower limit for the catalytic rate and proposed that catalysis is limited by the diffusion of the proton accepting base .…”

Section: Introductionmentioning

confidence: 99%

H/D Isotope Effects Reveal Factors Controlling Catalytic Activity in Co-Based Oxides for Water Oxidation

et al. 2019

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Understanding the mechanism for electrochemical water oxidation is important for the development of more efficient catalysts for artificial photosynthesis. A basic step is the proton-coupled electron transfer, which enables accumulation of oxidizing equivalents without buildup of a charge. We find that substituting deuterium for hydrogen resulted in an 87% decrease in the catalytic activity for water oxidation on Co-based amorphous-oxide catalysts at neutral pH, while 16O-to-18O substitution lead to a 10% decrease. In situ visible and quasi-in situ X-ray absorption spectroscopy reveal that the hydrogen-to-deuterium isotopic substitution induces an equilibrium isotope effect that shifts the oxidation potentials positively by approximately 60 mV for the proton coupled CoII/III and CoIII/IV electron transfer processes. Time-resolved spectroelectrochemical measurements indicate the absence of a kinetic isotope effect, implying that the precatalytic proton-coupled electron transfer happens through a stepwise mechanism in which electron transfer is rate-determining. An observed correlation between Co oxidation states and catalytic current for both isotopic conditions indicates that the applied potential has no direct effect on the catalytic rate, which instead depends exponentially on the average Co oxidation state. These combined results provide evidence that neither proton nor electron transfer is involved in the catalytic rate-determining step. We propose a mechanism with an active species composed by two adjacent CoIV atoms and a rate-determining step that involves oxygen–oxygen bond formation and compare it with models proposed in the literature.

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“…Here, we investigate the role of potassium ions in a cobalt-based, volumeactive [8] catalyst material which contains K + as well as phosphate ions. [22] This oxyhydroxide material, in the following denoted as CoCat, has been especially well investigated [23][24][25][26][27][28][29][30] and can serve as a model system in mechanistic investigations on water oxidation (OER) by non-noble metal materials in the environmentally friendly neutral-pH regime. [7,[31][32][33] Central open questions regarding the role of potassium ions are:…”

Section: Introductionmentioning

confidence: 99%

Role of Potassium in Electrocatalytic Water Oxidation Investigated in a Volume‐Active Cobalt Material at Neutral pH

Liu

Farhoosh

Beyer

et al. 2023

Advanced Sustainable Systems

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The oxygen evolution reaction (OER) is crucial in systems for sustainable production of hydrogen and other fuels. Catalytic OER materials often undergo potential‐induced redox transitions localized at metal sites. For volume‐active catalyst‐materials, these are necessarily coupled to charge‐compensating relocation of ions entering or leaving the material, which is insufficiently understood. The binding mode and mechanistic role of redox‐inert ions for a cobalt‐based oxyhydroxide material (CoCat) when operated at neutral pH in potassium‐phosphate (KPi) electrolyte are investigated by i) determination of K:Co and P:Co stoichiometries for various KPi‐concentrations and electrode potentials, ii) operando X‐ray spectroscopy at the potassium and cobalt K‐edges, and iii) novel time‐resolved X‐ray experiments facilitating comparison of K‐release and Co‐oxidation kinetics. Potassium likely binds non‐specifically within water layers interfacing Co‐oxyhydoxide fragments involving potassium–phosphate ion pairs. The potassium‐release kinetics are potential‐independent with a fast‐phase time‐constant of about 5 s and thus clearly slower than the potential‐induced Co oxidation of about 300 ms. It is concluded that the charge‐compensating ion flow is realized neither by potassium nor by phosphate ions, but by protons. The results reported here are likely relevant also for a broader class of volume‐active OER catalyst materials and for the amorphized near‐surface regions of microcrystalline materials.

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Water Oxidation by Co‐Based Oxides with Molecular Properties

Cited by 7 publications

References 144 publications

Reversible amorphization and the catalytically active state of crystalline Co3O4 during oxygen evolution

Reversible amorphization and the catalytically active state of crystalline Co3O4 during oxygen evolution

H/D Isotope Effects Reveal Factors Controlling Catalytic Activity in Co-Based Oxides for Water Oxidation

Role of Potassium in Electrocatalytic Water Oxidation Investigated in a Volume‐Active Cobalt Material at Neutral pH

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