Mn2O3, TiO2 and NiO based electrocatalysts with Ni and Pt for oxygen reduction and evolution reaction

[D. D. Mladenović]

IX, 111 str.

Fizička hemija - Fizička hemija materijala i Fizička hemija - elektrohemija / Physical Chemistry - Physical Chemistry of Materials and Physical Chemistry - Electrochemistry

Konstantan porast potreba čovečanstva za energijom doveo je do pojačane potrošnje fosilnih goriva koja će po nekim proračunima biti potpuno potrošena do 2050. godine. Sa druge strane, povećana potrošnja fosilnih goriva dovodi do emisije niza štetnih jedinjenja u atmosferu što pak drastično narušava kvalitet života ljudi i ostavlja ogromne posledice po živi svet. Zbog toga, naučnici širom sveta ubrzano rade kako bi pronašli neki novi izvor energije koji bi bio obnovljiv i ekološki prihvatljiv. Jedna od alternativa fosilnim gorivim je tzv. „vodonična ekonomija“ koja se bazira na korišćenju vodonika kao medijuma za skladištenje i transport energije. Po ovom modelu energija bi se skladištila u obliku vodonika procesom elektrolize kada postoji dotok energije iz prirodnih obnovljivih izvora kao što su solarna energija, energija vetra, vode i nuklearna energija, dok bi se u trenucima nedostatka dotoka energije iz primarnog izvora uskladišteni vodonik konvertovao nazad u električnu energiju u gorivnim ćelijama. Gorivne ćelije su sistemi koji hemijsku energiju reaktanata pretvaraju u električnu energiju a kao nusproizvod reakcije dobija se i voda. Ukoliko je u gorivnoj ćelije moguće odigravanje reakcije u suprotnom smeru, tj. da se iz vode i uz utrošak električne energije proizvode reaktanti (vodonik i kiseonik), onda se takva ćelija naziva objedinjena regenerativna gorivna ćelija. Glavni problem prilikom dizajniranja i konstrukcije jedne ovakve ćelije predstavljaju kinetički spore reakcije redukcije i izdvajanja kiseonika koje se odigravaju na fizički istoj elektrodi. Zbog toga je dizajniranje i ispitivanje efikasnih bifunkcionalnih katalizatora koji bi se koristili za katalizu ovih reakcija od ključnog značaja za širu implementaciju ovih sistema. U okviru ove disertacije sintetisan je niz katalizatora koji za osnovu imaju okside prelaznih metala Mn, Ni i Ti, i dekorisani su nanočesticama Pt i Ni, a koji bi se potencijalno upotrebljavali kao bifunkcionalni katalizatori za katalizu kiseoničnih reakcija u regenerativnim gorivnim ćelijama. Na početku su sintetisana tri oksida, Mn2O3, Mn2O3-TiO2 i Mn2O3-NiO koji su metodom mikrotalansog ozračivanja dekorisani nanočesticama legure PtNi i testirani su u 0,1 M KOH. Od devet sintetisanih katalizatora najbolje performanse pokazao je Mn2O3-NiO materijal dekorisan nanočesticama PtNi. Ovaj katalizator pokazao je najveće vrednosti jd na 1800 rpm (-4,48 mA cm-2), najmanje vrednosti parametra b od 62 i 109 mV dec-1, a Kutečki-Levič analizom zaključeno je da se na ovom katalizatoru reakcija redukcije kiseonika odigrava kinetički povoljnijim četvoro-elektronskim mehanizmom. Sa druge strane, u procesu izdvajanja kiseonika, ovaj katalizator je pokazao jednu od najmanjih vrednosti nadnapona potrebnog za dostizanje gustine struje od 10 mA cm-2 što apostrofira njegovu potencijalnu bifunkcionalnu primenu. Kako bi se dalje unapredio PtNi/Mn2O3-NiO katalizator, oksidna osnova je sintetisana iznova, delimično izmenjenom metodom, i dopirana je azotom u različitim odnosima kako bi se dobili PtNi/Mn2O3-NiO-N katalizatori sa tri različita odnosa N:Mn2O3-NiO. Ispitivanjem drugog seta sintetisanih katalizatora utvrđeno je da katalizator PtNi/Mn2O3-NiO-N (1:2) pokazuje najbolje performanse sa gustinom granične difuzione struje na 1800 rpm prilikom redukcije kiseonika od - 2,94 mA cm-2 i Tafelovim nagibom od 67 mV dec-1. Kutečki-Levič analizom utvrđeno je da se na Doktorska disertacija Dušan Mladenović ovom katalizatoru redukcija kiseonika takođe pretežno odvija kinetički povoljnijim četvoroelektronskim mehanizmom redukcije sa brojem izmenjenih elektrona od 3,38. Prilikom ispitivanja reakcije izdvajanja kiseonika nađeno je da PtNi/Mn2O3-NiO-N (1:2) dostiže gustinu struje od 10 mA cm-2 pri nadnaponu od 0,50 V dok je potencijal početka reakcije iznosio 1,61 V vs. RHE. Katalizatori sintetisani i testirani u okviru ove doktorske disertacije pokazali su vrlo dobre bifunkcionalne performanse u pogledu katalize ispitivanih reakcija koje su uporedive sa komercijalnim katalizatorima Pt/C i IrO2 koji se smatraju trenutno najboljim katalizatorima reakcije redukcije i izdvajanja kiseonika, respektivno. Pored dobrih katalitičkih performansi pokazali su takođe i dobru stabilnost pri dugotrajnoj eksploataciji u realnoj regenerativnoj gorivnoj ćeliji (simulirano tzv. svič testom), te je po mišljenu autora potencijal komercijalne primene sintetisanih katalizatora izuzetno veliki. Dalja ispitivanja bi trebalo usmeriti ka daljoj optimizaciji sinteze i pripreme katalitičkih mastila kao i ka dizajniranju realne regenerativne gorivne ćelije i ispitivanju performansi katalizatora u njoj.

The constant increase in humanity's energy needs has led to increased consumption of fossil fuels, which according to some calculations will be completely consumed by the year 2050. On the other hand, the increased consumption of fossil fuels leads to the emission of a number of harmful compounds into the atmosphere, which in turn drastically impairs the quality of life of people and leaves enormous consequences for the living world. Therefore, scientists around the world are working rapidly to find a new source of energy that would be renewable and environmentally friendly. One of the alternatives to fossil fuels is the so-called "hydrogen economy" which is based on the use of hydrogen as a medium for storing and transporting energy. According to this model, energy would be stored in the form of hydrogen through the process of electrolysis when there is an inflow of energy from natural renewable sources such as solar energy, wind energy, water energy and nuclear energy, while in moments of lack of inflow of energy from the primary source, the stored hydrogen would be converted back into electricity in fuel cells. Fuel cells are systems that convert the chemical energy of reactants into electrical energy and water. If in the fuel cell it is possible for the reaction to take place in the opposite direction, i.e., to produce reactants (hydrogen and oxygen) from water and with the consumption of electricity, then such a cell is called a unitized regenerative fuel cell. The main problem during the design and construction of such a cell is the kinetically slow reactions of reduction and evolution of oxygen that take place on the physically identical electrode. Therefore, the design and testing of efficient bifunctional catalysts that would be used to catalyse these reactions is crucial for the wider implementation of these systems. Within this dissertation, a series of catalysts, based on transition metal oxides Mn, Ni and Ti, decorated with Pt and Ni nanoparticles, were synthesized which could potentially be used as bifunctional catalysts for the catalysis of oxygen reactions in regenerative fuel cells. At the beginning, three oxides were synthesized, Mn2O3, Mn2O3-TiO2 and Mn2O3-NiO, which were decorated with PtNi alloy nanoparticles by microwave irradiation and tested in 0.1 M KOH. Of the nine synthesized catalysts, the Mn2O3-NiO material decorated with PtNi nanoparticles showed the best catalytic performance. This catalyst showed the highest values of jd at 1800 rpm (-4.48 mA cm-2), the lowest values of the parameter b of 62 and 109 mV dec-1, and the Koutecky-Levich analysis concluded that the oxygen reduction reaction takes place on this catalyst by a kinetically more favourable fourelectron mechanism. On the other hand, in the process of oxygen evolution, this catalyst showed one of the lowest values of the overpotential required to reach a current density of 10 mA cm-2, which apostrophizes its potential bifunctional application. In order to further improve the PtNi/Mn2O3-NiO catalyst, the oxide base was synthesized again by a partially modified method and doped with nitrogen in different ratios to obtain PtNi/Mn2O3- NiO-N catalysts with three different N:Mn2O3-NiO ratios. By testing the second set of synthesized catalysts, it was determined that the PtNi/Mn2O3-NiO-N (1:2) catalyst shows the best performance with a limiting diffusion current density at 1800 rpm during oxygen reduction of -2.94 mA cm-2 and a Tafel slope of 67 mV dec-1. The Koutecky-Levich analysis determined that on this catalyst the reduction of oxygen also predominantly takes place by a kinetically more favourable four-electron reduction mechanism with the number of exchanged electrons of 3.38. When examining the oxygen evolution reaction, it was found that PtNi/Mn2O3-NiO-N (1:2) reaches a current density of 10 mA cm-2 at an overvoltage of 0.50 V, while the reaction onset potential was 1.61 V vs. RHE. Catalysts synthesized and tested within this doctoral dissertation showed very good bifunctional performance in terms of the catalysis of the investigated reactions, comparable to the commercial catalysts Pt/C and IrO2, which are considered currently as the best catalysts for the oxygen reduction and evolution reactions, respectively. In addition to good catalytic performance, they also showed good stability during long-term exploitation in a real regenerative fuel cell (simulated by the so-called switch test), so in the opinion of the author, the potential for commercial application of the synthesized catalysts is very high. Further tests should be directed towards further optimization of the synthesis and preparation of catalytic inks as well as towards designing a realistic regenerative fuel cell and testing the performance of the catalyst in it.

vodonična ekonomija; objedinjene regenerativne gorivne ćelije; reakcija redukcije kiseonika; reakcija izdvajanja kiseonika; oksidi prelaznih metala

hydrogen economy; unitized regenerative fuel cell; oxygen reduction reaction; oxygen evolution reaction; transition metal oxides

544.473:546.30(043.3)

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