doctoral dissertation
Tina Đukić (Author), Nejc Hodnik (Mentor), Boštjan Genorio (Thesis defence commission member), Dušan Strmčnik (Thesis defence commission member), Igor A. Pašti (Thesis defence commission member)

Abstract

The presented doctoral dissertation highlights the importance of the durability of state-of-theart carbon-supported binary platinum (Pt)-nanoalloy electrocatalysts, used to accelerate a generally sluggish cathodic oxygen reduction reaction (ORR) in a low-temperature proton exchange membrane fuel cell (PEMFC), which is particularly used for the operation of hydrogen-powered vehicles and energy devices. The main focus of the dissertation is the stability of metallic (alloy) nanoparticles (NPs), specifically the stability of Pt and less noble 3d transition metal (M). Using advanced electrochemical characterisation techniques, such as an electrochemical flow cell system coupled to an inductively coupled plasma mass spectrometry (EFC-ICP-MS) and a high-temperature disc-electrode (HT-DE) system, special attention was paid to a fundamental understanding of the metal dissolution phenomena and correlated mechanisms (such as Pt oxide formation/reduction and Pt redeposition) depending on both the operating conditions of the PEMFC and the intrinsic properties of the electrocatalyst itself. By simulating operating conditions of the PEMFC (i.e. using a temperature of 75 ℃ and potential windows that simulate both ‘operating’ voltage window as well as less usual – wider – voltage window of the PEMFC), we first have demonstrated that both temperature and potential/voltage window have a significant impact on the stability of metal NPs of Ptnanoalloy-based electrocatalysts. In particular, we have shown that the dissolution of the M from Pt-alloy NPs increases with increasing temperature. In addition, we have made an important observation that the rate of Pt redeposition back to the electrocatalyst layer also increases with increasing temperature, thus concealing the actual amount of dissolved Pt. Furthermore, it has been shown that the stability of metal NPs deteriorates more significantly with the widening of the potential window at the close-to-real operating temperature, due to an increased dissolution of both Pt and M. These results have prompted further research aimed at improving the stability of Pt-nanoalloy electrocatalysts under aggressive PEMFC operating conditions by adjusting (i) the operating conditions of the PEMFC as well as (ii) the intrinsic properties, particularly composition and structure of the Pt-nanoalloy electrocatalyst. Therefore, in continuation, by simulating typical operation of the PEMFC, it has been demonstrated that by narrowing the potential window from 0.6–0.95 VRHE (still mostly used potential/voltage window for the simulation of the electrocatalyst degradation during PEMFC operation) to 0.7–0.85 VRHE, i.e. by adjusting both potential limits, the losses of electrochemically active surface area and specific activity of the electrocatalyst can be significantly reduced. Additionally, whereas the metal dissolution mechanism at both potential windows remained unchanged, narrowing potential window also resulted in a decrease in dissolution of both metals (Pt and M). Furthermore, it has also been demonstrated that the hold time at the upper potential limit (UPL) also has a significant impact on the dissolution of metals from Pt nanoalloys. Namely, it has been shown that a longer hold time at the UPL increases the dissolution of Pt and M due to prolonged formation of Pt oxide, i.e. due to a larger amount of formed oxide. On the other hand, the hold time at the lower potential limit (LPL) has proved to have no significant impact on the dissolution mechanisms of both metals. Finally, emphasis has been placed on the design of state-of-the-art carbon-supported binary Pt-nanoalloy electrocatalysts. In particular, the effect of crystal structure in correlation with the chemical composition (atomic ratio of Pt to M) on the stability of metallic NPs has been investigated. In this regard, it has been confirmed that enrichment of the crystal structure with an M can improve the electrocatalyst properties (i.e. its intrinsic activity towards ORR) at the beginning of its life, whereas the properties of such an electrocatalyst significantly deteriorate after an accelerated degradation test that simulates PEMFC operating conditions. Nevertheless, it has been further observed that a compromise between activity and stability of a Pt-nanoalloy electrocatalyst rich in an M can be achieved by adjusting the crystal structure. In other words, we have demonstrated that an optimal Pt-to-Co composition, with the presence of a specific ordered intermetallic phase, particularly tetragonal L10–Pt-Co, can improve the stability of a cobalt-rich Pt-nanoalloy electrocatalyst.

Keywords

proton exchange membrane fuel cell;oxygen reduction reaction;durability of platinum nanoalloys;operating conditions;intrinsic properties;

Data

Language: English
Year of publishing:
Typology: 2.08 - Doctoral Dissertation
Organization: UL FKKT - Faculty of Chemistry and Chemical Technology
Publisher: [T. Đukić]
UDC: 621.352.6:620.3(043.3)
COBISS: 243784451 Link will open in a new window
Views: 173
Downloads: 56
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Other data

Secondary language: Slovenian
Secondary title: Stabilnost binarnih platinskih nanozlitin na ogljikovem nosilcu kot katalizatorjev za nizkotemperaturne gorivne celice
Secondary abstract: Pričajoča doktorska disertacija poudarja pomen stabilnosti najsodobnejših binarnih elektrokatalizatorjev iz nanozlitin platine na ogljikovem nosilcu, ki se uporabljajo za pospeševanje običajno počasne katodne reakcije redukcije kisika (angl. oxygen reduction reaction, ORR) v nizkotemperaturni gorivni celici s protonsko izmenjevalno membrano (angl. proton exchange membrane fuel cell, PEMFC), ki se uporablja predvsem v vozilih in energetskih napravah na vodikov pogon. Glavni poudarek disertacije je na stabilnosti kovinskih (zlitinskih) nanodelcev, natančneje na stabilnosti platine in manj plemenite 3d prehodne kovine. Z uporabo naprednih tehnik elektrokemijske karakterizacije, kot je sistem elektrokemijske pretočne celice, povezane z masno spektrometrijo z induktivno sklopljeno plazmo (angl. electrochemical flow cell coupled to an inductively coupled plasma mass spectrometry, EFC-ICP-MS) in sistem visokotemperaturne disk-elektrode (angl. hightemperature disc electrode, HT-DE), je bila posebna pozornost namenjena temeljnemu razumevanju pojavov raztapljanja kovin in povezanih mehanizmov (kot je tvorba/redukcija platinskega oksida in ponovna redepozicija platine), odvisno od pogojev delovanja PEMFC in intrinzičninh lastnosti samega elektrokatalizatorja. S simulacijo delovnih pogojev PEMFC (tj. z uporabo temperature 75 ℃ in potencialnih oken, ki simulirajo tako "delovno" napetostno okno kot tudi manj običajno - širše - napetostno okno PEMFC) smo najprej dokazali, da temperatura in potencialno/napetostno okno pomembno vplivata na stabilnost kovinskih nanodelcev elektrokatalizatorjev na osnovi nanozlitin platine. Zlasti smo pokazali, da se raztapljanje manj plemenite 3d prehodne kovine iz nanodelcev platinske zlitine povečuje z naraščajočo temperaturo. Poleg tega smo opazili, da se hitrost ponovne redepozicije platine nazaj v plast elektrokatalizatorja prav tako povečuje z naraščajočo temperaturo, s čimer se prikrije dejanska količina raztopljene platine. Nadalje smo pokazali, da se stabilnost kovinskih nanodelcev dodatno poslabša s širjenjem potencialnega okna pri temperaturi, ki simulira dejansko temperaturo PEMFC, zaradi povečanega raztapljanja tako platine kot manj plemenite 3d prehodne kovine. Ti rezultati so bili motivacija za nadaljnje raziskave, namenjene izboljšanju stabilnosti elektrokatalizatorjev iz nanozlitin platine v agresivnih delovnih pogojih PEMFC, in sicer s prilagoditvijo (i) delovnih pogojev PEMFC kot tudi (ii) intrinzičnih lastnosti, zlasti sestave in strukture elektrokatalizatorja iz nanozlitin platine. Zato je bilo v nadaljevanju s simulacijo tipičnega obratovanja PEMFC dokazano, da se z zoženjem potencialnega okna z 0,6–0,95 VRHE (še vedno večinoma uporabljeno potencialno/napetostno okno za simulacijo degradacije elektrokatalizatorja med obratovanjem PEMFC) na 0,7–0,85 VRHE, oz. s prilagoditvijo obeh potencialnih meja, izguba elektrokemijsko aktivne površine in izguba specifične aktivnosti elektrokatalizatorja lahko znatno zmanjšata. Poleg tega, medtem ko je mehanizem raztapljanja kovin pri obeh potencialnih oknih ostal nespremenjen, je zoženje potencialnega okna povzročilo tudi zmanjšanje raztapljanja obeh kovin (platine in legirne, manj plemenite 3d prehodne kovine). Poleg tega je bilo tudi dokazano, da ima čas zadrževanja na zgornji potencialni meji (angl. upper potential limit, UPL) tudi pomemben vpliv na raztapljanje kovin iz platinskih nanozlitin. Dokazano je namreč, da daljši čas zadrževanja na UPL poveča raztapljanje platine in manj plemenite 3d prehodne kovine zaradi podaljšanega časa tvorbe platinskega oksida ter posledično večje količine nastalega oksida. Po drugi strani pa se je izkazalo, da zadrževalni čas na spodnji potencialni meji (angl. lower potential limit, LPL) nima pomembnega vpliva na mehanizme raztapljanja obeh kovin, ampak vpliva le na hitrost kinetike redukcije površine platinskega oksida. Končno je podan poudarek na oblikovanju najsodobnejših binarnih elektrokatalizatorjev iz nanozlitin platine na ogljikovem nosilcu. Zlasti je bil raziskan vpliv kristalne strukture v povezavi s kemijsko sestavo (atomsko razmerje med platino in manj plemenito 3d prehodno kovino) na stabilnost kovinskih nanodelcev. V zvezi s tem je bilo potrjeno, da lahko obogatitev kristalne strukture z manj plemenito 3d prehodno kovino izboljša lastnosti elektrokatalizatorja (tj. njegovo intrinzično aktivnost za ORR) na začetku njegove življenjske dobe, medtem ko se lastnosti takšnega elektrokatalizatorja bistveno poslabšajo po pospešenem testu degradacije, ki simulira pogoje obratovanja PEMFC. Vendar, je bilo nadalje ugotovljeno, da je mogoče kompromis med aktivnostjo in stabilnostjo elektrokatalizatorja iz platinske nanozlitine bogate z manj plemenito 3d prehodno kovino, doseči s prilagoditvijo kristalne strukture. Z drugimi besedami, dokazali smo, da lahko optimalna sestava Pt-Co s prisotnostjo specifične urejene intermetalne faze, zlasti tetragonalne L10–Pt-Co, izboljša stabilnost elektrokatalizatorja iz platinske nanozlitine bogate s kobaltom.
Secondary keywords: gorivna celica s protonsko izmenjevalno membrano;reakcija redukcije kisika;nanozlitine;stabilnost platinskih nanozlitin;katalizatorji;obratovalni pogoji;intrinzične lastnosti;doktorske disertacije;Elektrokemija;Elektrokatalizatorji;Disertacije;Nanostrukturni materiali;Univerzitetna in visokošolska dela;
Type (COBISS): Doctoral dissertation
Study programme: 1001051
Thesis comment: Univ. v Ljubljani, Fak. za kemijo in kemijsko tehnologijo
Pages: 127 str.
ID: 26719457