Heterogenous porphyrin-based catalysts for electrochemical energy applications

Theoretical thesis.%%%%%%%%%%%%1. Introduction  – 2. Literature review  – 3. Experimental methods  – 4. Covalent immobilisation and electrocatalytic activity of manganese porphyrin  – 5. Covalent immobilisation and electrocatalytic activity of cobalt porphyrin in CO2ERR and ORR  – 6. Rational design of stable cobalt porphyrin-based catalysts for CO2ERR  – 7. Conclusions and outlook  – Appendix.%%%%Complexes of the 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 structure-activity relationships, little attention has been paid to the effects of immobilisation mode on their performance. This thesis shows that covalent grafting of porphyrin-based catalysts to the surface of carbon electrodes could be used as an efficient and simple method that allows to significantly improve rates and selectivities of electrochemical reactions relevant for electrochemical energy applications. In the first part of this dissertation a reliable procedure for covalent immobilisation of metalloporphyrins onto the surface of carbon electrodes was established. To achieve this, electroreduction of corresponding diazonium salts was chosen as it allows to create an extremely durable C-C bond of a complex with the supporting electrode. Indeed, the reduction of tetraphenylporphyrin diazonium salt under mild conditions on carbon electrodes followed by treatment with hot solution of Mn(OAc)2 in DMF/CH3COOH proved to be a reliable tool for immobilisation of MnTPP on carbon cloth. The resulting hybrid materials were studied using CV and Raman spectroscopy and the resulting layer of complex was found to possess all signature CV and spectral features characteristic for MnTPP while showing complete lack of solubility signifying the success of covalent immobilisation. Variation of electrodeposition time was found to be a convenient tool to control the density of organometallic layer which in turn allows to shorten the Mn‧‧‧Mn distance and thus increase the probability of two Mn atoms taking part in a concerted electrochemical process. The use of covalent immobilisation proved to be highly beneficial for ORR in which we achieved significantly higher reduction current density and nearly 100 % selectivity towards 4e- pathway under low overpotentials after 5 min-long TPP electrodeposition. This feature could be explained by the stepwise reduction of O2 to H2O2 and then to H2O. At the same time the rate of OER appears to be independent of the immobilisation mode and proportional to the amount of electrochemically active complex on the surface. The study of MnTPP-modified electrodes in CO2ERR did not result in a significant CO2 reduction current due to inherently low activity of the catalyst itself. At the same time, significant suppression of the hydrogen evolution upon covalent immobilisation of MnTPP was observed. This phenomenon was ascribed to the blocking effect of well-formed organic layer and much better surface coverage…