Modeling photoelectrolysis: hydrogen evolution overpotential on platinum nanoparticles

Erno Kemppainen

Research output: ThesisDoctoral ThesisCollection of Articles

Abstract

In photoelectrochemical (PEC) water splitting, or photoelectrolysis of water, solar irradiation is converted to hydrogen (H2). The reaction consists of two half-reactions, the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER). Direct solar-to-fuel conversion overcomes key problems such as the variability of the sunlight. The physical phenomena of PEC H2 production can roughly be divided into light absorption, charge transport, electrochemistry, and product collection. Since in steady-state operation these processes affect each other quite statically, they could in first approximation be modeled separately, but a detailed analysis requires consideration of their interactions. A model coupling mass transport and reaction kinetics was developed to study their effects on the HER on platinum (Pt) nanoparticles. The model was applied to minimize the catalyst consumption, while maintaining HER performance sufficient for the photoelectrolysis. Catalysts, such as Pt, are needed to reduce voltage losses, but they often are expensive. A major result found through simulations and experiments was that the amount of Pt needed for good HER performance could be significantly reduced, because the HER kinetics on Pt is faster than typically thought.It was observed that the mass transport losses were independent of the Pt loading, suggesting that mass transport in the electrolyte is one-dimensional (1D). Simulations that included the dynamics of H2 transfer between liquid electrolyte and small gas bubbles showed that this process could significantly affect the H2 transport. A comparison of 1D and 2D models indicated that mass transport in the electrolyte can be described accurately with a 1D-model, because the contribution of the mass transport near the nanoparticles is often negligible. The importance of the mass transport of especially H2 on the HER overpotential was demonstrated. Since Pt is an even better catalyst than commonly thought, an electrode optimized for the photoelectrolysis may need only a small fraction of the amount of Pt typically used. Although a comprehensive analysis of the electrode operation may require a significantly more detailed model, for most purposes an analytical 1D-model should be sufficiently accurate.
Translated title of the contributionFotoelektrolyysin mallinnus: vedyntuoton ylipotentiaali platinananopartikkelien pinnalla
Original languageEnglish
QualificationDoctor's degree
Awarding Institution
  • Aalto University
Supervisors/Advisors
  • Lund, Peter, Supervising Professor
  • Halme, Janne, Thesis Advisor
Publisher
Print ISBNs978-952-60-7135-0
Electronic ISBNs978-952-60-7134-3
Publication statusPublished - 2016
MoE publication typeG5 Doctoral dissertation (article)

Keywords

  • photoelectrolysis
  • hydrogen evolution reaction
  • photoelectrochemistry
  • platinum
  • reaction kinetics
  • mass transport

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