Electroreduction of oxygen on nanostructured palladium catalysts

Abstract

Doktoritöö eesmärk oli valmistada ja uurida erinevaid nanostruktuurseid Pd/C katalüsaatoreid hapniku elektrokeemilise redutseerumisreaktsiooni jaoks. Hapniku redutseerumisreaktsioon on oluline, kuna see toimub madalatemperatuursete kütuseelementide katoodil. Pd on Pt järel aktiivsuselt teine metall sellele reaktsioonile. Seetõttu on hapniku redutseerumist pallaadiumil küllaltki palju uuritud. Selles töös kasutati Pd/C katalüsaatorite valmistamiseks nii keemilisi, füüsikalisi kui elektrokeemilisi meetodeid. Valmistatud katalüsaatormaterjale karakteriseeriti pinnaanalüüsi (skaneeriv ja läbistuselektronmikroskoopia, röntgenfotoelektronspektroskoopia) ning elektrokeemiliste meetoditega (tsükliline voltamperomeetria ja pöörleva ketaselektroodi meetod). Elektrokeemilised mõõtmised tehti väävel- ja perkloorhappe ning kaaliumhüdroksiidi lahuses. Alusmaterjalide võrdluses andsid parimaid tulemusi grafeenile sadestatud Pd osakesed. Läbivalt saadi parem osakeste jaotus lämmastikuga dopeeritud süsinikmaterjalidel. neil katalüsaatormaterjalidel toimus hapniku neljaelektroniline redutseerumine kõigis lahustes. See on ka eelistatud reaktsioonitee madalatemperatuursetes kütuseelementides. Katalüsaatorite elektrokatalüütilist aktiivsust hapniku redutseerumisel hinnati eriaktiivsuse alusel, mis oli eriti vajalik kuubikujuliste Pd nanoosakeste võrdlemisel sfääriliste Pd katalüsaatoritega, kus kuubikujulistel katalüsaatoritel oli tunduvalt kõrgem eriaktiivsus võrreldes sfääriliste osakestega Pd/C katalüsaatoriga. Kõigi katalüsaatormaterjalide puhul määrati ka Tafeli tõusu väärtused, mille põhjal järeldati et hapniku elektroredutseerumine nanostruktuursetel Pd/C katalüsaatoritel toimub sarnaselt kompaktsete Pd elektroodidega.

The purpose of this PhD thesis was to prepare various Pd/C nanostructured catalyst materials. These nanostructured materials were studied in order to evaluate their use for oxygen reduction reaction (ORR) in low temperature fuel cells. Pd is in second place in activity when compared to Pt and for this reason Pd has often been studied as a replacement to Pt in low-temperature fuel-cell applications. In this work Pd nanostructures were prepared using chemical, physical and electrochemical methods. These catalyst materials were investigated using various surface analysis methods (scanning electron microscopy, transmission electron microscopy and X-ray photoelectron spectroscopy) and electrochemically using cyclic voltammetry and rotating disc electrode methods. The electrochemical measurements were carried out in either perchloric acid, sulphuric acid or potassium hydroxide solutions. As a support material graphene showed good results in this work, providing good dispersion and small Pd nanoparticles. Furthermore, during this study some nitrogen-doped carbon materials were evaluated as support materials and showed good dispersion of Pd nanoparticles. The oxygen reduction reaction on all of the prepared catalysts proceeded through four-electron pathway, which is the preferred pathway for fuel-cell applications. The electrocatalytic activity of catalyst materials towards the ORR was evaluated by determining specific activities, which was especially useful when comparing the cubic and spherical Pd nanoparticles, where higher specific activities were noted in cuboid structured Pd nanoparticles in comparison to spherical Pd nanoparticles supported on Vulcan carbon. These materials were also evaluated using Tafel plots from which it was concluded that the oxygen electroreduction proceeds similarly to polycrystalline Pd electrodes.