Electrocatalysis of oxygen reduction on non-precious metal catalysts
Date
2020-12-11
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Abstract
Loodust saastava süsinikul põhineva energiatsükli asendamine on üks kõige kriitilisemaid 21. sajandi probleeme. Paraku tuleb sisuliselt kogu transpordiks kasutatav energia endiselt fossiilsetest kütustest. Üheks alternatiiviks sisepõlemismootorile on polümeer-elektrolüüt kütuseelemendid (PEFC). Kütuseelement on olemuselt seade, milles keemiline energia (konkreetsel juhul vesiniku ja hapniku vahelisel reaktsioonil tekkiv energia) muundatakse kasulikuks tööks. PEFC puhul on lõppsaaduseks puhas vesi, mis tähendab et sisepõlemismootori asendamisel selliste kütuseelementidega on võimalik vältida CO2 teket. Põhiline probleem PEFC tehnoloogia juures on katoodil toimuva hapniku redutseerumisreaktsiooni katalüsaatorite vähene aktiivsus ja stabiilsus. Siiani kasutatakse madalatemperatuursetes kütuseelementides selle reaktsiooni katalüsaatorina plaatinal põhinevaid materjale, aga plaatina on äärmiselt kallis ning vähese levimusega metall. Selle doktoritöö käigus arendati plaatina asendamiseks süsiniknanomaterjalidest uudseid elektrokatalüsaatoreid. Grafeeni, süsiniknanotorude ja karbiidset päritolu süsiniku dopeerimisel saavutati atomaarselt jaotunud aktiivtsentritega, mis pidasid sammu või isegi suurendasid kütuseelemendi võimsustihedust võrreldes kommertsiaalsete Pt/C materjalidega. Töö käigus demonstreeriti uusi strateegiaid süsiniknanomaterjalidest komposiitide valmistamiseks ning poorse struktuuri optimeerimiseks, mis suurendas samuti kütuseelemendi võimsustihedust. Käesoleva doktoritöö tulemused näitavad, et mitte-väärismetallkatalüsaatoritega on võimalik asendada kütuseelemendis plaatina-põhiseid katoodimaterjale ning seeläbi oluliselt vähendada süsteemi maksumust.
Moving on from the carbon energy cycle that is destroying our environment and deteriorating living conditions is one of the priorities for humanity in the 21st century. However, nearly all of the energy used for transportation is still being produced by burning fossil fuels. Polymer electrolyte fuel cells (PEFC) have emerged as one of the pathways towards reducing the CO2 emissions from automotive applications and have already seen wide-scale use in fuel cell electric vehicles. The main remaining problems towards replacement of internal combustion engines by PEFCs are cost and durability. These issues stem from the slow oxygen reduction reaction (ORR), which takes place at the fuel cell cathode. So far, nearly all commercially available fuel cell systems have used platinum-based catalysts on the cathode to achieve this, but it has become apparent that the long-term activity and stability targets cannot be met by Pt-based materials while keeping costs low enough. This PhD thesis focuses on developing novel electrocatalysts for this reaction by rationally designing synthesis strategies for carbon nanomaterials. By using graphene, carbon nanotubes and carbide-derived carbon (CDC) and combining them with methods such as ball-milling and pyrolysis, new catalysts with atomically dispersed catalytic centres were developed. The dispersion of catalytic sites on such a level enhanced the activity of the catalyst material and thereby the power output of the fuel cell. Composite materials from carbon nanotubes and graphene or CDC were created to optimise the porous structure of the fuel cell electrode, which further raised the power density of the cell. New strategies for the optimisation of porosity in catalyst materials are also proposed in this thesis. In the end, multiple catalysts rivalling the activity of commercially used Pt/C materials are presented in this thesis, showing a great promise of non-precious metal catalysts for replacing Pt in automotive fuel cells.
Moving on from the carbon energy cycle that is destroying our environment and deteriorating living conditions is one of the priorities for humanity in the 21st century. However, nearly all of the energy used for transportation is still being produced by burning fossil fuels. Polymer electrolyte fuel cells (PEFC) have emerged as one of the pathways towards reducing the CO2 emissions from automotive applications and have already seen wide-scale use in fuel cell electric vehicles. The main remaining problems towards replacement of internal combustion engines by PEFCs are cost and durability. These issues stem from the slow oxygen reduction reaction (ORR), which takes place at the fuel cell cathode. So far, nearly all commercially available fuel cell systems have used platinum-based catalysts on the cathode to achieve this, but it has become apparent that the long-term activity and stability targets cannot be met by Pt-based materials while keeping costs low enough. This PhD thesis focuses on developing novel electrocatalysts for this reaction by rationally designing synthesis strategies for carbon nanomaterials. By using graphene, carbon nanotubes and carbide-derived carbon (CDC) and combining them with methods such as ball-milling and pyrolysis, new catalysts with atomically dispersed catalytic centres were developed. The dispersion of catalytic sites on such a level enhanced the activity of the catalyst material and thereby the power output of the fuel cell. Composite materials from carbon nanotubes and graphene or CDC were created to optimise the porous structure of the fuel cell electrode, which further raised the power density of the cell. New strategies for the optimisation of porosity in catalyst materials are also proposed in this thesis. In the end, multiple catalysts rivalling the activity of commercially used Pt/C materials are presented in this thesis, showing a great promise of non-precious metal catalysts for replacing Pt in automotive fuel cells.
Description
Väitekirja elektrooniline versioon ei sisalda publikatsioone
Keywords
fuel cells, electrochemical reduction, oxygen, non-precious metals, carbon nanotubes, catalysts