Abstract
An energy economy based on H2 from water electrolysis is perhaps the most promising answer to manmade climate change. So far an incomplete understanding of the interfacial electrochemistry of water electrolysis prevents us from building the best water splitting devices possible. In particular the catalysts on which the involved reactions occur offer room for improvement. In this thesis, I adress urgent open questions regarding both the reductive and oxidative half-cell reactions of water electrolysis, the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER), respectively, on Pt and Au. Prior work clarifies that the rate of these reactions depends on all aspects of the interface: electrolyte and electrode composition and structure. Pt was chosen for investigating the HER because of its unique combination of activity and stability; Au is a useful model system for OER electrocatalysis.
On Pt single crystals and a microelectrode, I have investigated the HER and adsorbed H, Pt-H, electrochemically and by sum frequency generation (SFG) spectroscopy in the presence and absence of a pulsed, ultrashort near-IR perturbation that triggers charge transfer and the HER on a femtosecond timescale. The HER reaction intermediate, monocoordinated Pt-H, is concluded to exist in an equilibrium with two- and threefold coordinated Pt-H and a solvated interfacial proton (possibly an oxonium ion) that is in contact with the Pt surface. I find that charge transfer from Pt to the liquid side is only possible on femtosecond timescales if interfacial electrolyte and adsorbates are in a particular goldilocks structure. It is further concluded that the HER on Pt is enabled when approaching its onset potential by a change in the interphase that increases the mobility of adsorbed H on subpicosecond timescales, possibly by a change in H adsorption energies in different sites and in interfacial solvent structure.
Surface oxidation is a prerequisite for the OER and a function of surface structure. Here I investigate the spatial heterogeneity of surface electrooxidation and OER activity on poly- and nanocrystalline Au electrodes by second harmonic (SH) microscopy. I show that the SH signal of the unoxidized surface can be used to predict patterns in surface reconstruction that occurs during potential cycling. I find that OER activity is highly local and bound to two distinctive types of defects. The first is stable with respect to surface atom reconstruction and extends into the bulk, while the second, a pure surface defect, is not. From work on the OER on Pt conducted within the scope of this thesis, I conclude that higher oxidation states of Pt (> +II) are not OER active.
Finally, the adsorption of H2SO4 and HClO4 derived anions to a Pt(111) electrode was studied by SFG, as anion adsorption influences HER and OER. SO4_2- (and not HSO4_- ) was identified as the adsorbate in H2SO4. The potential dependence of the optical response of SO4_2- adsorbed on Pt(111) was quantitatively explained by a microscopic model. ClO4_- adsorption was only observed after Pt(111) had been covered by OH at higher potentials, but not on the bare metal. Also, two distinct ClO4_- subpopulations, compared to only one for SO4_2-, were found.
The insights gained within the scope of this thesis and the employed methods may assist us in building better catalysts for water electrolysis in the future.
