Preparation and characterization of platinum and iridium electrocatalysts on titanium-based supports for the hydrogen evolution, oxygen evolution and oxygen reduction reactions

In the current PhD thesis, platinum (Pt) and iridium (Ir) electrocatalysts were prepared on Ti-based supports via the galvanic replacement/ deposition process and tested for the hydrogen evolution (HER), oxygen evolution (OER) and oxygen reduction (ORR) reactions in acidic medium. First, Pt electrodes on metallic Ti foil, Ti felt and black titania nanotubes (bTNTs) were prepared via the galvanic deposition process. These electrodes, characterized by relatively low Pt content (19 – 215 μgPt cm-2) and thin deposit of thickly distributed Pt particles covering almost the whole substrates, exhibited high electrocatalytic performance towards HER, with the Pt/bTNT catalyst be intrinsically the most active. Also, electrodes of continuous Ir(Ni) film on Ti foil were prepared via the galvanic replacement method, upon partial replacement of surface layers of pre-electrodeposited Ni film by Ir, resulting in electrodes composed of Ir-shell on (Ir-Ni)-core. Their electrocatalytic activity towards OER was higher in comparison with the Ir/Ti catalysts (prepared via the galvanic deposition method), highlighting the beneficial effect/ contribution of Ni on IrO2 activity for OER, as also supported in the literature. Ru was added in these electrocatalysts via the same process and the mixed IrO2-RuO2/(Ni) coated Ti electrocatalysts (55 – 175 μgPGM cm-2) exhibited indeed more enhanced activity thanks to Ru, maintaining high stability thanks to Ir towards OER. Moreover, Ir was deposited on bTNTs, upon fully replacing distributed pre-electrodeposited Ni particles on bTNTs by Ir. The resulting Ir/bTNTs catalysts (351 μgIr cm-2) exhibited similar intrinsic electrocatalytic activity towards OER in comparison with Ir/Ti foil electrodes. Finally, Ir-Pt were co-deposited on Ti foil via the galvanic deposition process, resulting in mixed precious metals deposit that fully covered the Ti substrate (120 – 150 μgPGM cm-2). The bifunctionality of the bimetallic IrO2-Pt catalysts for OER/ORR was studied, exhibiting higher electrocatalytic activity than IrO2/Ti electrodes and other IrO2-Pt in the literature towards OER, and comparable activity with the Pt/Ti catalyst (prepared via the galvanic deposition process) towards ORR.All the electrocatalysts of the current PhD thesis, were studied both for their electrocatalytic activity and for their stability in acidic medium. These studies, combined with the low precious metal content of these electrodes, could be the basis for the preparation of electrocatalysts in water electrolysis cells and fuel cells processes, as well as auxiliary electrodes in electrochemical industrial processes, such as wastewater treatment, electroplating and seawater electrodialysis.Στην παρούσα διδακτορική διατριβή παρασκευάστηκαν με τη μέθοδο της γαλβανικής αντικατάστασης/ απόθεσης ηλεκτροκαταλύτες λευκοχρύσου (Pt) και ιριδίου (Ir) σε υποστρώματα με βάση το τιτάνιο και μελετήθηκαν για τις δράσεις έκλυσης υδρογόνου (HER), έκλυσης οξυγόνου (OER) και αναγωγής οξυγόνου (ORR) σε όξινο περιβάλλον. Παρασκευάστηκαν ηλεκτρόδια Pt σε μεταλλικό φύλλο Ti, ύφασμα Ti και νανοσωλήνες μέλανος διοξειδίου του τιτανίου (bTNT) με τη μέθοδο της γαλβανικής απόθεσης. Τα ηλεκτρόδια αυτά που χαρακτηρίστηκαν από σχετικά χαμηλή ποσότητα Pt (19 – 215 μgPt cm-2) και λεπτό απόθεμα πυκνά διασπαρμένων σωματιδίων Pt καλύπτοντας σχεδόν πλήρως τα υποστρώματα, επέδειξαν υψηλή ηλεκτροκαταλυτική ικανότητα για τη HER, με τον καταλύτη Pt/bTNT να είναι εγγενώς ο πιο ενεργός. Ακόμη, με τη μέθοδο της γαλβανικής αντικατάστασης παρασκευάστηκαν ηλεκτρόδια συνεχούς υμενίου Ir-Ni σε φύλλο Ti, έπειτα από μερική αντικατάσταση επιφανειακών στοιβάδων προ-ηλεκτροαποτεθέντος υμενίου Ni από Ir, καταλήγοντας σε ηλεκτρόδια κελύφους Ir και πυρήνα (Ir-Ni). Η ηλεκτροκαταλυτική τους δράση για την OER, συγκρινόμενη με τον απλό καταλύτη Ir/Ti (παρασκευασθέντος με γαλβανική απόθεση), ήταν υψηλότερη, αναδεικνύοντας την ευεργετική συνεισφορά του Ni στην ηλεκτροκαταλυτική ενεργότητα του IrO2 για την OER. Σε αυτούς τους καταλύτες προστέθηκε Ru με την ίδια μέθοδο και πράγματι οι μικτοί ηλεκτροκαταλύτες υμενίου IrO2-RuO2/(Ni) (μάζας 55 – 175 μgPGM cm-2) επέδειξαν πιο βελτιωμένη ενεργότητα χάρη στο Ru και παράλληλα διατήρησαν υψηλή σταθερότητα χάρη στο Ir για την OER. Επιπρόσθετα, αποτέθηκε Ir σε υπόστρωμα bTNTs αντικαθιστώντας πλήρως διασπαρμένα προ-αποτεθέντα σωματίδια Ni. Οι προκύπτοντες καταλύτες Ir/bTNTs (μάζας 351 μgIr cm-2) επέδειξαν παρόμοια εγγενή ηλεκτροκαταλυτική ικανότητα για την OER συγκριτικά με ηλεκτρόδια Ir σε μεταλλικό φύλλο Ti. Τέλος, με τη μέθοδο της γαλβανικής απόθεσης, συναποτέθηκαν Ir-Pt σε φύλλο Ti, καταλήγοντας σε μικτό απόθεμα ευγενών μετάλλων που κάλυψε πλήρως την επιφάνεια του Ti (μάζας 120 – 150 μgPGM cm-2). Οι διμεταλλικοί καταλύτες IrO2-Pt μελετήθηκαν ως διλειτουργικοί για τις OER/ORR, επιδεικνύοντας ηλεκτροκαταλυτική ικανότητα για την OER υψηλότερη από ηλεκτρόδια IrO2/Ti και αντίστοιχους διμεταλλικούς καταλύτες της βιβλιογραφίας, ενώ για την ORR συγκρίσιμη με τον Pt/Ti (παρασκευασθέντος με γαλβανική απόθεση).Οι ηλεκτροκαταλύτες της παρούσας διδακτορικής διατριβής, μελετήθηκαν τόσο για την ηλεκτροκαταλυτική τους ενεργότητα, όσο για τη σταθερότητα σε όξινο περιβάλλον. Σε συνδυασμό με τη χαμηλή ποσότητα ευγενών μετάλλων που βρέθηκε ότι περιέχουν, μπορούν να αποτελέσουν τη βάση για τη δημιουργία ηλεκτροδίων για κυψέλες ηλεκτρόλυσης του νερού και κυψέλες καυσίμου, καθώς επίσης ως βοηθητικά ηλεκτρόδια σε πληθώρα ηλεκτροχημικών βιομηχανικών διεργασιών, όπως κατεργασία αποβλήτων, επιμεταλλώσεις, ηλεκτροδιαπίδυση θαλασσινού νερού

Oxygen Evolution Reaction at IrO2/Ir(Ni) Film Electrodes Prepared by Galvanic Replacement and Anodization: Effect of Precursor Ni Film Thickness

IrO2/Ir(Ni) film electrodes of variable Ni content have been prepared via a galvanic replacement method, whereby surface layers of pre-deposited Ni are replaced by Ir, followed by electrochemical anodization. Electrodeposition of Ni on a glassy carbon electrode support has been carried out at constant potential and the charge of electrodeposited Ni controlled so as to investigate the effect of precursor Ni layer thickness on the electrocatalytic activity of the corresponding IrO2/Ir(Ni)/GC electrodes for the oxygen evolution reaction (OER). After their preparation, these electrodes were characterized by microscopic (SEM) and spectroscopic (EDS, XPS) techniques, revealing the formation of Ir deposits on the Ni support and a thin IrO2 layer on their surfaces. To determine the electroactive surface area of the IrO2 coatings, cyclic voltammograms were recorded in the potential range between hydrogen and oxygen evolution and the charge under the anodic part of the curves, corresponding to Ir surface oxide formation, served as an indicator of the quantity of active IrO2 in the film. The electrocatalytic activity of the coatings for OER was investigated by current–potential curves under steady state conditions, revealing that the catalysts prepared from thinner Ni films exhibited enhanced electrocatalytic performance

Methanol Oxidation at Platinum Coated Black Titania Nanotubes and Titanium Felt Electrodes

Optimized Pt-based methanol oxidation reaction (MOR) anodes are essential for commercial direct methanol fuel cells (DMFCs) and methanol electrolyzers for hydrogen production. High surface area Ti supports are known to increase Pt catalytic activity and utilization. Pt has been deposited on black titania nanotubes (bTNTs), Ti felts and, for comparison, Ti foils by a galvanic deposition process, whereby Pt(IV) from a chloroplatinate solution is spontaneously reduced to metallic Pt (at 65 °C) onto chemically reduced (by CaH2) TNTs (resulting in bTNTs), chemically etched (HCl + NaF) Ti felts and grinded Ti foils. All Pt/Ti-based electrodes prepared by this method showed enhanced intrinsic catalytic activity towards MOR when compared to Pt and other Pt/Ti-based catalysts. The very high/high mass specific activity of Pt/bTNTs (ca 700 mA mgPt−1 at the voltammetric peak of 5 mV s−1 in 0.5 M MeOH) and of Pt/Ti-felt (ca 60 mA mgPt−1, accordingly) make these electrodes good candidates for MOR anodes and/or reactive Gas Diffusion Layer Electrodes (GDLEs) in DMFCs and/or methanol electrolysis cells

Galvanic Deposition of Pt Nanoparticles on Black TiO2 Nanotubes for Hydrogen Evolving Cathodes

A galvanic deposition method for the in-situ formation of Pt nanoparticles (NPs) on top and inner surfaces of high-aspect-ratio black TiO2-nanotube electrodes (bTNTs) for true utilization of their total surface area has been developed. Density functional theory calculations indicated that the deposition of Pt NPs was favored on bTNTs with a preferred [004] orientation and a deposition mechanism occurring via oxygen vacancies, where electrons were localized. High-resolution transmission electron microscopy images revealed a graded deposition of Pt NPs with an average diameter of around 2.5 nm along the complete nanotube axis (length/pore diameter of 130 : 1). Hydrogen evolution reaction (HER) studies in acidic electrolytes showed comparable results to bulk Pt (per geometric area) and Pt/C commercial catalysts (per mg of Pt). The presented novel HER cathodes of minimal engineering and low noble metal loadings (μg cm−2 range) achieved low Tafel slopes (30–34 mV dec−1) and high stability in acidic conditions. This study provides important insights for the in-situ formation and deposition of NPs in high-aspect-ratio structures for energy applications

Galvanic Deposition of Pt Nanoparticles on Black TiO2 Nanotubes for Hydrogen Evolving Cathodes

A galvanic deposition method for the in-situ formation of Pt nanoparticles (NPs) on top and inner surfaces of high-aspect-ratio black TiO2-nanotube electrodes (bTNTs) for true utilization of their total surface area has been developed. Density functional theory calculations indicated that the deposition of Pt NPs was favored on bTNTs with a preferred [004] orientation and a deposition mechanism occurring via oxygen vacancies, where electrons were localized. High-resolution transmission electron microscopy images revealed a graded deposition of Pt NPs with an average diameter of around 2.5 nm along the complete nanotube axis (length/pore diameter of 130 : 1). Hydrogen evolution reaction (HER) studies in acidic electrolytes showed comparable results to bulk Pt (per geometric area) and Pt/C commercial catalysts (per mg of Pt). The presented novel HER cathodes of minimal engineering and low noble metal loadings (μg cm−2 range) achieved low Tafel slopes (30–34 mV dec−1) and high stability in acidic conditions. This study provides important insights for the in-situ formation and deposition of NPs in high-aspect-ratio structures for energy applications