Mechanistic Studies of the Oxygen Evolution Reaction by a Cobalt-Phosphate Catalyst at Neutral pH
ABSTRACT The mechanism of the oxygen evolution reaction (OER) by catalysts prepared by electrodepositions from Co(2+) solutions in phosphate electrolytes (Co-Pi) was studied at neutral pH by electrokinetic and (18)O isotope experiments. Low-potential electrodepositions enabled the controlled preparation of ultrathin Co-Pi catalyst films (<100 nm) that could be studied kinetically in the absence of mass transport and charge transport limitations to the OER. The Co-Pi catalysts exhibit a Tafel slope approximately equal to 2.3 × RT/F for the production of oxygen from water in neutral solutions. The electrochemical rate law exhibits an inverse first order dependence on proton activity and a zeroth order dependence on phosphate for [Pi] ≥ 0.03 M. In the absence of phosphate buffer, the Tafel slope is increased ∼3-fold and the overall activity is greatly diminished. Together, these electrokinetic studies suggest a mechanism involving a rapid, one electron, one proton equilibrium between Co(III)-OH and Co(IV)-O in which a phosphate species is the proton acceptor, followed by a chemical turnover-limiting process involving oxygen-oxygen bond coupling.
SourceAvailable from: Ulrich Hintermair[Show abstract] [Hide abstract]
ABSTRACT: Molecular catalysts are known for their high activity and tunability, but their solubility and limited stability often restrict their use in practical applications. Here we describe how a molecular iridium catalyst for water oxidation directly and robustly binds to oxide surfaces without the need for any external stimulus or additional linking groups. On conductive electrode surfaces, this heterogenized molecular catalyst oxidizes water with low overpotential, high turnover frequency and minimal degradation. Spectroscopic and electrochemical studies show that it does not decompose into iridium oxide, thus preserving its molecular identity, and that it is capable of sustaining high activity towards water oxidation with stability comparable to state-of-the-art bulk metal oxide catalysts.Nature Communications 03/2015; 6:6469. DOI:10.1038/ncomms7469 · 10.74 Impact Factor
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ABSTRACT: The development of artificial photosynthesis systems that can efficiently catalyze water oxidation to generate oxygen remains one of the most important challenges in solar energy conversion to chemical energy. In photosystem II (PSII), the Mn4CaO5 cluster adopts a distorted coordination geometry and every two octahedra are linked by di-μ-oxo (edge-shared) or mono-μ-oxo (corner-shared) bridges, which is recognized as a critical structure motif for catalytic water oxidation. These structural features provide guidance on the design and synthesis of new water oxidation catalysts. Herein we synthesized a new layered organic cobalt phosphonate crystal, Co3(O3PCH2–NC4H7–CO2)2·4H2O (1) and demonstrate it as a heterogeneous catalyst for water oxidation. Its catalytic activity was compared to those of cobalt phosphonates with different structures (2–4) in terms of O2 evolution rate and O2 yield under the same reaction conditions. The compound with both mono- and di-μ-oxo bridged octahedral cobalt displays superior catalytic activity. In contrast, the presence of only mono-μ-oxo bridged cobalt in the structure results in lower O2 yield and O2 evolution rate. Further structural analysis reveals that the presence of a longer Co–N bond induces a distorted dissymmetry coordination geometry, and consequently facilitates water oxidation. These results provide important insight into the design of water oxidation catalysts.Energy & Environmental Science 02/2015; 8(2). DOI:10.1039/C4EE03234A · 15.49 Impact Factor
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ABSTRACT: This work is intended to define a new possible methodology for the TiO2 doping through the use of an electrochemical deposition of cobalt directly on the titanium nanotubes obtained by a previous galvanostatic anodization treatment in an ethylene glycol solution. This method does not seem to cause any influence on the nanotube structure, showing final products with news and interesting features with respect to the unmodified sample. Together with an unmodified photoconversion efficiency under UV light, the cobalt doped specimen reports an increase of the electrocatalytic efficiency for the oxygen evolution reaction (OER).