This dissertation work studies the fundamental aspects of the electrocatalysis of the hydrogen electrode reaction (HER) and oxygen electrode reaction (OER) on platinum over a wide temperature range from ambient up to 220°C. Previously, the majority of the work reported was restricted to temperatures below 70°C due to apparatus constraints, whereas the current operation temperature for proton exchange membrane fuel cells is around 100oC and is envisioned to operate at even higher temperatures. In this work, a special apparatus for controlled hydrodynamic study was constructed, which can keep the system in a single aqueous phase at elevated temperatures.
The growth kinetics and mechanism of the anodic oxide film on platinum are studied under potential sweep conditions. By fitting the current equation derived based on the framework of the point defect model (PDM) on the linear polarization curves, the kinetic parameters for film growth and dissolution are extracted, which agree well with other findings.
The kinetics and mechanism of the HER are investigated both at ambient temperature with a rotating ring disk electrode and at elevated temperatures with a platinized nickel electrode. Ambient results by micropolarization analysis agree well with findings in literature, and yield an exchange current density on the order of mA/cm2. An activation energy of 17.3kJ/mol is determined. This is comparable with that of a bulk platinum electrode, and is lower than sputtered platinum and single crystal platinum electrodes in alkaline solutions. Surprisingly, the apparent Tafel slope of the hydrogen evolution reaction is almost temperature independent. The most probable reason is that two parallel reactions with different activation energy and transfer coefficients are occurring at the interface.
The OER on platinum is also studied by potential sweep method and potentiostatic polarization method. The sluggish nature of this reaction is postulated to be due to the existence of a thin oxide layer on the electrode surface so that the electrons resides in the metal have to quantum mechanically tunnel (QMT) through this layer in order to reach the oxygen species in the solution or adsorbed on the surface.
The exponential decay of the current with potential (the inverse Tafel's law) upon the formation of an oxide film can be accounted for by combining QMT theory for charge transfer across an interface and the PDM for film growth and dissolution. A new method for extremely thin oxide film thickness measurement is also developed by combining these two theories. This method employs the tunneling current of the hydrogen oxidation reaction as a probe and is demonstrated to be a very sensitive and convenient in situ technique. However, currently the thickness range that can be measured is limited to 1-2nm, since the passive current of platinum imposing a lower limit of the tunneling current that can be measured.
The tunneling constant, which defines the blocking character of the film, is measured to be (0.57 ± 0.035) x 108cm-1, and is temperature independent. A barrier height of 0.31eV at the film/solution interface is then yielded. Theoretical interpretation is implemented by developing a model for the electronic structure of the metal/barrier layer/solution interphase utilizing the Boltzmann's distribution law for the distribution of the energy states of the redox species in the solution phase and by noting that the barrier layer is a highly doped (and probably degenerate) defect (oxygen vacancy) semiconductor, having a Debye length that is the same order as the thickness of the film.