The deposition of platinum by the galvanic replacement of electrosorbed hydrogen

• Main Author: Rawlings, Benjamin Cyril
• Published: University of Bristol 2015
• Subjects: 621.31
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.681732

The combustion engine and fossil fuels have transformed human civilization through access to high densities of energy for transport and electricity. However, as fossil fuel resources decline and issues of climate change grow, alternative sources of energy are required. Fuel cells provide the possibility of clean energy through the use of a variety of fuels. Before widespread use of fuel cells is possible, obstacles in the design must be overcome, the greatest of these is the electrode design. Electrodes require a high activity for the electrocatalytic reactions and a high stability during operation. Typically Pt group metals are required to meet these targets. Control of the deposition of these metals, particularly Pt, is essential for the design of the next generation of fuel cell electrodes. In this project a protocol is developed for the epitaxial deposition of Pt using the galvanic replacement of controlled amounts of electrosorbed H. Control was achieved through the selection of the galvanic replacement potential limits and the use of Pd films (1-10 ML thick) on Au as the substrate. Pd thin films exhibit a well known interaction with H characterised by energetically separated processes: H adsorption and H absorption at lower underpotentials. The galvanic replacement kinetics and the reaction stoichiometry were investigated with electrochemical and the quartz crystal microbalance (QCM) techniques. The deposition on the 2 ML Pd/ Au films showed the expected deposition efficiency (2:1 H to Pt). On 10 ML Pd/Au samples the galvanic replacement of the absorbed H resulted in a higher amount of Pt deposition per growth cycle. Pt film surface composition has been confirmed by X-ray photo emission spectroscopy (XPS) , while scanning tunnelling microscopy (STM) results showed quasi-2D Pt film growth. Surprisingly the Pt-10 ML Pd/ Au films showed more reversible absorption/ desorption behaviour than on the corresponding 10 ML Pd/ Au films. A novel method of retrieving H desorption curves from the combination of open circuit potential(OCP) transients and QCM data was developed. This provided the equivalent of continuous H UPD sweeps during the deposition of Pt, characterising changes in the Pt surface as it forms. From this it was observed that the absorbed H peak remained constant during deposition while the adsorbed H peak increased by 5-7% per replacement cycle after the first replacement. The increase in H adsorbed to the surface was interpreted as an increase in surface area due to nano scale roughening of the surface as Pt was deposited.