Thesis presented December 13, 2022

Abstract:
The PhD project focussed on the application of a cobalt tetraazamacrocyclic complex, in the literature commonly referred to as [Co(CR)Cl₂]⁺ as a molecular catalyst for the hydrogen evolution reaction (HER). This was within the broader scope of the EU MSCA H2020 ITN ‘eSCALED’ project, which had as one of its central aims the objective to create an artificial leaf for the storage of solar energy in chemical fuels and, as part of this, sought the development of novel bio-inspired and scalable materials for the various components of the selected Polymer Electrolyte Membrane (PEM) electrolyser device architecture. This included researching molecular catalysts based on Earth-abundant first-row transition metals to replace the scarce noble metal-based materials currently used commercially in such technology.
In this context, a number of projects were pursued: firstly, studies of the mechanism of the catalyst itself under organic electrocatalytic conditions were carried out, generating catalytic intermediates by controlled reduction and/or protonation of the catalyst and identifying them using spectroscopy (UV vis, NMR, EPR) and following the catalytic behaviour with electrochemical techniques. From this, an ECEC mechanism with a rate-determining second protonation step associated with the release of H₂ was identified, noting in particular an initial protonation step on the macrocycle at the Co(II) state that was hypothesised to involve the macrocycle amine group acting as a proton relay under the investigated conditions.
Secondly, a new synthetic strategy towards novel derivatives of [Co(CR)Cl₂]⁺ modified at the para-position of the macrocycle pyridine group was developed, which enabled the synthesis of a pyrene-functionalised derivative which could be anchored onto sp2-carbon surfaces, such as multi-walled carbon nanotubes (MWCNTs), by π-stacking interactions. The behaviour and catalytic properties of the immobilised catalyst were studied by electrochemical methods and compared with an analogous derivative functionalised at the macrocycle amine from collaborators at ICIQ, showing that both derivatives work as heterogenised electrocatalysts for the HER with high faradaic efficiencies and good stability over one hour at pH 2 and particularly pH 7, but the derivative functionalised on the pyridine displays higher current densities and greater stability, invoking some consideration of rational design principles for modifying molecular catalysts.
Thirdly, studies of a photocatalytic system made up of copper indium sulfide quantum dots (CuInS₂ QDs) as a photosensitiser with either [Co(CR)Cl₂]⁺ or its benzoic acid-functionalised derivative were carried out in ascorbate buffer, focussing on the photocatalytic performance and electron transfer (ET) processes between the CuInS₂ QDs and the catalyst to find explanations for the remarkable activity and robustness reported in literature for closely related systems. CuInS₂ QDs modified to have a ‘hybrid-passivation’ ligand system making them suitable for immobilisation with NiO films were used, since this would be a requirement for future preparation of photoelectrodes with these materials. Photocatalytic hydrogen measurements, steady state fluorescence quenching, and ultrafast transient absorption spectroscopy indicated extremely rapid QD-catalyst ET processes for both the derivative and unmodified catalyst. To understand this behaviour, a static binding model with a strong binding equilibrium was adapted for the system. Considering the ability of QD particles to bind multiple catalyst molecules, the high efficiency of quenching followed a Poisson distribution up to about two equivalents of catalyst per QD. This prompts a reconsideration of the importance of anchoring groups for QD-catalyst ET efficiency in solution.
Additionally, the development of printable MWCNT-based inks, immobilisation of catalysts on scalable printed electrodes and catalyst-functionalised conducting polymers were studied.

Keywords:
Electrochemical mechanism, hydrogen, artificial photosynthesis, molecular catalyst, hydrogen evolution, cobalt

On-line thesis. ​