Public Thesis defense - IMMC

SST

20 janvier 2020

16h15

Louvain-la-Neuve

Place Croix du Sud - Auditoire SUD 09

Electrocatalytic activity of nanostructured nickel thin film electrodes during oxygen and hydrogen evolution reactions by Adeline DELVAUX

Pour l’obtention du grade de Docteur en sciences de l’ingénieur et technologie

With the increasing energy demand and the resulting risk of global warming, conventional polluting hydrocarbon fuels need to be replaced by greener ones. In this context, hydrogen produced by water electrolysis from renewable electricity seems to be one of the most promising candidates as carbon-neutral energy carrier. Nevertheless, research efforts continue to be directed towards further improving the electrochemical kinetics and the overall cell efficiency.

The first objective of this thesis was to study the possibility of reducing the oxygen evolution reaction (OER) overpotential by de-alloying Ni thin film electrodes in order to favor bubble detachment. Ni(Al) alloys were deposited by magnetron co-sputtering, and were then chemically leached in a concentrated hydroxide solution with the aim of dissolving out the aluminum. Electrochemical testing showed that our de-alloyed electrodes present significantly lower overpotentials for the OER than untreated pure Ni electrodes. This result was attributed to a higher surface roughness, leading to a better bubble detachment at the electrode surface.

The second objective of this work was to study an often overlooked parasitic reactions step during the hydrogen evolution reaction (HER), namely the diffusion and absorption of hydrogen adatoms from the electrode surface into the bulk. Our Ni thin film electrodes were obtained by magnetron sputtering using several deposition pressures in order to create different internal stresses and microstructures in the films. Electrochemical testing was then coupled to a multi-beam optical sensor in order to obtain high resolution in-situ curvature measurements during the HER. Indeed, hydrogen absorption results in a constrained volume expansion into the Ni thin film, leading to internal stress generation which can be monitored in-situ by means of curvature measurement. Results showed that the fraction of hydrogen being absorbed into the electrode is high at low overpotential, while the rate of absorption is low. This was attributed to the fact that, in these conditions, the hydrogen adatoms have sufficient time to diffuse into the Ni film. Moreover, this thesis highlights the fact that the absolute amount of hydrogen being absorbed into the Ni increases when the grain size is reduced, due to a higher grain boundaries density which are favorite absorption sites for hydrogen.

Jury members :

  • Prof. Joris Proost (UCLouvain), supervisor
  • Prof. Hervé Jeanmart (UCLouvain), chairperson
  • Prof. Sophie Hermans (UCLouvain), secretary
  • Prof. Hosni Idrissi (UCLouvain)
  • Dr. Renaud Delmelle (ZHAW, Switzerland)
  • Dr. Asif Ansar (DLR, Germany)

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