PhD Defence Dalia Leon Chaparro | Ultrathin Alumina Overlayers at Electrocatalytic Interfaces - Selective hydrogen evolution and suppression of oxygen reduction

Ultrathin Alumina Overlayers at Electrocatalytic Interfaces - SELECTIVE HYDROGEN EVOLUTION AND SUPPRESSION OF OXYGEN REDUCTION

Dalia Leon Chaparro is a PhD student in the Department of PhotoCatalytic Synthesis. (Co)Promotors are prof.dr. G. Mul and dr.ir. G. Katsoukis from the Faculty of Science & Technology.

The development of efficient and durable electrocatalysts is essential for advancing sustainable energy technologies. A key challenge is achieving selective electrochemical reactions while minimising catalyst degradation and unwanted side reactions. This thesis investigates ultrathin alumina overlayers as a strategy to improve electrocatalytic interfaces for selective hydrogen evolution while suppressing the oxygen reduction reaction.

A reproducible methodology was established for the fabrication of ultrathin alumina and aluminosilicate overlayers on platinum electrodes using pulsed laser deposition (PLD) and atomic layer deposition (ALD). Their structural, chemical, and electrochemical properties were characterised using electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV), in situ FT-IRRAS and EC-IRRAS. The results show that oxygen partial pressure is a key parameter controlling film growth, morphology, and density, while sufficiently thick alumina layers effectively prevent CO adsorption, demonstrating their protective properties.

The electrochemical performance of ultrathin oxide overlayers was found to depend strongly on both thickness and composition. Whereas aluminosilicate films allow both proton and oxygen transport, amorphous alumina selectively suppresses oxygen reduction while maintaining proton conductivity. By combining EIS with a generalized finite-length Warburg model and in situ EC-IRRAS, proton transport through ultrathin alumina was quantified, revealing diffusion coefficients that evolve over time due to structural relaxation within the oxide layer.

Finally, the functionality of ALD-grown alumina was enhanced through surface modification with PEG12-thiol molecular wires. This functionalization improved proton accessibility and reduced film resistance while preserving the oxygen-blocking capability of the alumina overlayer, although at the expense of reduced platinum activity. Overall, this thesis demonstrates that ultrathin alumina overlayers provide an effective platform for tailoring electrochemical interfaces by balancing proton transport, oxygen selectivity, and electrocatalytic activity. These findings establish design principles for protective oxide coatings that enable selective hydrogen evolution and contribute to the development of more efficient and durable electrochemical energy conversion systems.