Fabrication and characterisation of nanostructured palladium hydride microelectrode pH sensors

Nanostructured Pd films were electrodeposited on Pt microdisc electrodes using a molecular template created by a hexagonal lyotropic crystalline phase. While retaining micrometer dimensions these electrodes possess huge electroactive surface areas; i.e. roughness factors of H1-e Pd films were estimated to be typically 300. Hydrogen absorption was shown to be very fast for H1-e Pd films and the films were readily loaded with hydrogen to β phase. It was found to be possible to obtain the Pd-hydride with the desired H/Pd ratio by controlling the potential and the time of electrolysis. The nanostructured Pd-hydride microelectrodes showed excellent potentiometric response over a wide range of pH (2 ~ 12). The potentiometric pH response was rapid, stable, reproducible and almost theoretical in deaerated solutions. These properties, clearly superior to conventional micro pH sensors, were thought to be achieved by the combination of (1) the α+β Pd-hydride phase which was known to show almost theoretical potential with normal size electrodes and (2) a nanostructured film with a huge surface area and a rapid potential determining process (H+ + e ⇄ H(Pd-H)). H1-e Pd-hydride microelectrodes had limited lifetime (typically 1 ~ 3 hours in deaerated solutions) because of the continuous removal of hydrogen under open circuit conditions. Although the lifetime is too short for continuous process monitoring, it is sufficient for many analytical applications. The electrode can be reloaded with hydrogen quickly, which makes it possible to perform pH measurements repeatedly.