Surface X-ray Diffraction Studies of the FCC(111) Electrochemical Interface

Interfacial electrochemistry, the study of the chemical properties of the interface
between a charged electrode and an ion-conducting electrolyte, is fundamental to the
development of electrocatalytic renewable energy technologies such as fuel cells,
batteries and carbon capture systems. Although vast amounts of resources are applied
to the fabrication and characterisation of electrocatalysts, for technologies such as
these to be viable on a large scale and mitigate the effects of anthropogenic climate
change, a better fundamental understanding of the atomic and electronic structure of
the electrochemical interface is required.
Recent decades have seen the development of powerful in-situ measurement techniques
that can probe the electrochemical interface under reaction conditions (e.g. when the
metal electrode surface is in contact with a liquid electrolyte, and the voltage for a
given electrochemical process is applied). This thesis presents the results of a series of
experiments employing surface X-ray diffraction (SXRD) of the electrochemical
interface of FCC(111) metal surfaces.
First presented is a study of the cation contribution to the structure of the
electrochemical interface, consisting of measurements of Pt(111) in aqueous CsOH
electrolyte. The study employs cyclic voltammetry and SXRD measurements to
construct a model of the surface metal layers and near-surface Cs layers, combined
with resonant SXRD and complimentary X-ray adsorption near-edge spectroscopy
(XANES) measurements to probe the charge distribution in the electrochemical
interface. Preliminary results support recent studies of the same system which suggest
a semi-adsorbed Cs layer with high disorder in the plane of the surface.
An SXRD study of Cu(111) and Ag(111) surfaces in varying concentrations of aqueous
acetonitrile-containing electrolyte is also presented. Non-aqueous electrolytes such as
acetonitrile have attracted attention for their high solubility of CO2 and broad potential
window of stability, and have been shown to have favourable electrocatalytic
properties when combined with aqueous electrolyte. The study shows evidence of a
combination of potential-induced dissolution and re-adsorption of surface metal layers
1
and formation of surface metal oxides, and a roughening of the metal surface with
increasing concentrations of acetonitrile.
Lastly a study of thin Co films electrodeposited on Au(111) surfaces is presented, with
SXRD measurements taken both in-plane and out of the plane of the surface suggesting
a 3D restructuring of the Co crystal structure, with the Co layers displaying an FCC
structure for thinner films and an hexagonal close-packed structure for thicker films.
These results have possible connotations for the use of similar Au(111)/Co systems in
microelectronics.
The experimental work detailed in this thesis, performed over a series of experimental
sessions at synchrotron facilities (the i07 beam line at Diamond Light Source, Didcot,
UK, the XMaS beamline at the ESRF, Grenoble, and the Advanced Photon Source,
Illinois), represent a significant contribution to our understanding of the
electrochemical interface, presenting measurements of systems never before analysed
using SXRD, such as Cu(111) in high concentrations of aqueous acetonitrile, and the
use of advanced analysis techniques, such as calculation of anomalous scattering
factors from XANES measurements being used to model the energy-dependent
scattering intensity from ordered Cs layers. This thesis demonstrates the power and
versatility of SXRD and provides foundational information for the development and
refinement of vital electrochemical technologies.