Mechanistic Studies of the Oxygen Evolution Reaction by a Cobalt-Phosphate Catalyst at Neutral pH
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.
- "The lower limit of TOF for oxygen evolution for the present Co 3 O 4 nanorods is calculated as 4.15×10 −3 s −1 at 410 mV overpotential which is larger than the TOF reported for the Co-Pi catalyst (~2 × 10 −3 s −1 at 410 mV overpotential). Similarly, the present value is also larger than that reported (8×10 −4 s −1 ) for Co 3 O 4 nanorods, prepared in mesoporous silica as a template, under photochemical conditions . The enhanced catalytic activity of porous Co 3 O 4 nanorods towards the oxygen evolution reaction may be attributed to the combination of several factors including the high porosity, higher surface area, enrichment of Co 3+ ions on the surface of the nanorods apart from the (110) orientation of the rods. "
[Show abstract] [Hide abstract] 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).0Comments 2Citations
- "Deposition of IrO í µí±¥ , CoO í µí±¥ , and RuO í µí±¥ cocatalysts on n-semiconductors seems all to enhance the activity for O 2 evolution and CoO í µí±¥ was found to be the best one. As example, Surendranath et al. described the self-assembly of a highly active cobalt-based oxygen evolving catalyst that forms as a thin film on inert electrodes when aqueous solutions of Co 2+ salts are electrolyzed in presence of phosphate or borate . These authors evidenced that this catalyst can be interfaced with light absorbing and charge separating materials to affect photoelectrochemical water-splitting. "
[Show abstract] [Hide abstract] ABSTRACT: Efficient and stable photocatalysts for water oxidation are highly sought after in the field of photoelectrochemical (PEC) water splitting. Herein, a new type of tantalum and aluminum co-doped iron oxide (Ta/Al-Fe2O3) material was fabricated by a simple drop coating method. XPS analysis suggests that Ta and Al were successfully co-doped into Fe2O3 and Ta can greatly influence the chemical environment of Al and O on the surface of catalyst. The resultant optimum (0.25%)Ta/(10%)Al-Fe2O3 film presented excellent enhanced PEC activity and photostability. A 15 times higher photocurrent density as well as two times higher incident-photon-to-current efficiency (IPCE, 430 nm) can be clearly observed relative to (10%)Al-Fe2O3 at 0.35 V vs. Ag/AgCl. The dramatic enhanced PEC and IPCE performance are attributed to mixed effects induced by tantalum doping, such as positive shift of flat band potential (ca. 50 mV), a reduction in anodic overpotential for water oxidation and greatly reduced charge transfer resistance, which eventually facilitate more efficient separation and easier transfer of photogenerated electron-hole pairs. The highly improved visible light activity and film stability indicate that tantalum and aluminum co-doped iron oxide will be a promising semiconductor for water oxidation.0Comments 12Citations
- "Incorporating metal cations such as Si, Ti, Al, Mo, Cr, Nb141516171819202122 has also been proven to be effective methods for increasing carrier concentrations and hencing conductivity. Meanwhile, surface modifications with water oxidation catalysts such as IrO 2 , Co-phosphate or Co nanoparticles (NPs)232425262728 would improve reaction kinetics and reduce overpotentials. However, previous researches concern much on single substrate modification, little attention has been paid to photoelectrodes with simultaneously two substances modification , which were found to exhibit potentially more excellent PEC performance . "