Heterogenous porphyrin-based catalysts for electrochemical energy applications
thesisposted on 29.03.2022, 01:49 by Aleksei N. Marianov
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 compared to analogous electrode prepared via drop-casting. Following the assessment of MnTPP-based design the electrodeposition technique was used for the synthesis of covalently immobilised CoTPP which is known for its excellent activity in electrocatalytic reduction reactions. This material showed 2.4 times higher density of electrochemically active species compared to the noncovalently immobilised analogue and the activity of the resulting electrodes shows dramatic improvement of CO formation rate during CO2ERR in neutral electrolyte under an overpotential of 500 mV. Indeed, a TOF of 8.3 s-1 was achieved contrary to the drop-cast analogue which exhibited TOF of 4.5 s-1 only. Furthermore, in full agreement with the previous results, the maximum average FE(CO) increased from 50 % to 67 % upon introduction of a covalent link with the surface. Also, the optimum potential corresponding to the highest FE(CO) was achieved under 50 mV less negative potential compared to noncovalent immobilisation. The catalyst exhibited excellent cumulative TON of 3.9‧105 in a 24 h long electrolysis surpassing performance of the drop-cast counterpart by the factor of 3. We must note here that the TON and TOF values measured in our study are among the highest to date surpassing those reported for Fe hydroxyporphyrins and Co porphyrin-based covalent organic frameworks. Apparently, the kinetics of CO2ERR under low overpotentials is highly dependent on the rate of the electron transfer between the electrode surface and the complex while the resulting phenylene linker is playing the role of a "molecular wire" within the catalytic layer.Further, the activity of covalently immobilised Co tetraphenylporphyrin in ORR was evaluated. The assessment showed that the observations made for covalently immobilised MnTPP are applicable to CoTPP as covalent ligation improves selectivity to 4e- reduction pathway from 0 % for drop-cast complex to 55 % for a material after 10 min-long electrodeposition. Also, covalent immobilisation significantly increases the rate of H2O2 reduction, and the effect is more pronounced with the electrochemical immobilisation times longer than 5 min. This change is also believed to take place due to the participation of multicentred reduction reaction.Considering its outmost importance, in the final part of this dissertation the problem of catalyst stability in CO2ERR in aqueous electrolyte was studied. For this work CoTPP was chosen as one of the most active catalysts available. In strong contrast to earlier reports, the results show that the leaching, demetallation, poisoning by CO and reduction to chlorins are not responsible for the deactivation process. Moreover, recyclability was found to be independent of the heterogenisation mode. Surprisingly, it is uptake of two oxygen atoms from the CO2 molecule that renders the porphyrin catalytically inactive. Based on the insight into the degradation mechanism a strategy for the development of more stable porphyrin-based catalysts established. The most successful approach is based on kinetic suppression of unwanted oxygen insertion reaction. Indeed, introduction of steric protection in the form of eight bulky -OMe groups into the porphyrin core furnished 100 % recyclable heterogeneous molecular catalyst. Furthermore, lateral proton donors also significantly improve catalyst longevity due to favourable proton delivery to the CO2-complex adduct and thus lower probability of oxygen uptake by the macrocyclic core. In strong contrast to Fe analogues, thermodynamic stabilisation of CoI active form by electronegative substituents or additional axial ligands such as pyridine renders the catalyst almost inactive. Thus, it was proven that the careful analysis of degradation pathway is a crucial step in the rational development of industrially viable electrocatalysts.In summary, covalent immobilisation of molecular catalysts on conductive electrodes provides higher amount of the catalytically active complex, better rate of interfacial electron transfer and shortened M‧‧‧M distance. These effects are quite general and provide numerous advantages for energy-related applications. In turn, the longevity of molecular catalysts is defined by their structure rather than heterogenisation technique. Hence, careful design of a ligand can be used to significantly enhance the lifetime of an electrocatalyst. The results described below provide solid background for the future development of not only highly active, but also more durable molecular electrocatalysts which is of paramount significance for the field of electrocatalysis.