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Effect of Manganese Porphyrin Covalent Immobilization on Electrocatalytic Water Oxidation and Oxygen Reduction Reactions
journal contribution
posted on 2019-01-21, 00:00 authored by Aleksei
N. Marianov, Yijiao JiangComplexes
of first row transition metals are a promising class
of tunable and inexpensive catalysts for electrochemical energy applications.
Although considerable efforts have been devoted to the activity studies,
little attention has been paid to the effects of different immobilization
modes on reaction mechanisms. In this work, we studied the influence
of covalent immobilization on the performance of Mn tetraphenylporphyrin
in oxygen evolution (OER) and oxygen reduction (ORR) reactions. Ligation
of the complex to carbon surface was attained via potentiostatic electroreduction
of porphyrin diazonium salt with the following metalation and electrodeposition
time was found to be a convenient tool to control the amount of electrochemically
active catalyst on the electrode. Cyclic voltammetry suggests that
the increase of porphyrin surface concentration upon prolonged electrodeposition
shortens average Mn–Mn distance and proportionally enhances
probability of at least two metal atoms simultaneously participating
in a catalytic process. Optimization of organic layer density has
profound effect on the catalyst performance in ORR in alkaline medium.
5 min electrodeposition furnishes the best catalyst, which features
the 4-electron pathway being predominant at low overpotentials where
the noncovalent counterpart shows selectivity to H2O of
∼50%. What is more, overall catalytic current at −0.79
V vs NHE was 2.4 times higher for covalently immobilized porphyrinate.
Electrokinetic measurements and impedance spectroscopy suggest that
the reaction proceeds via formation of MnII intermediate
with stepwise O2 reduction to H2O2 and then to H2O. Similar effects were observed in acidic
electrolyte. The OER rate is less sensitive to immobilization mode
and mainly depends on the amount of accessible catalyst.
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Keywords
Cyclic voltammetrynoncovalent counterpartreaction mechanismsporphyrin surface concentration2.4 timeselectrochemical energy applications5 min electrodepositionreaction proceedsElectrokinetic measurementsporphyrin diazonium saltmetal atomscarbon surfaceMn IIelectrodeposition shortensO 2 reductionimmobilization modeelectrodeposition timerow transition metalscovalent immobilizationOER rateimpedance spectroscopyMn tetraphenylporphyrinoxygen reduction4- electron pathwayactivity studiescatalyst performancelayer densityElectrocatalytic Water OxidationSimilar effectsORRH 2 O 2H 2 Oacidic electrolytepotentiostatic electroreductionNHEOxygen Reduction Reactions ComplexesManganese Porphyrin Covalent Immobilizationimmobilization modesoxygen evolution
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