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Carbon corrosion properties and performance of multi-walled carbon nanotube support with and without nitrogen-functionalization in fuel cell electrodes
KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Chemical Engineering, Applied Electrochemistry.
KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Chemical Engineering, Applied Electrochemistry.ORCID iD: 0000-0003-4770-9554
Aalto University.
Aalto University.
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

Pt-supported on multi-walled carbon nanotubes (MWCNT) and N-modified MWCNT (N-MWCNT) catalysts are synthesized by pyrolysis from emeraldine solution and microemulsion. Their electrochemical properties and carbon corrosion resistance in a Proton Exchange Membrane Fuel Cell (PEMFC) are compared with a commercial Pt/Vulcan catalyst through I-V curves, cyclic voltammetry and CO stripping. The initial fuel cell performances of the Pt/(N-)MWCNT catalysts are superior to Pt/Vulcan. The corrosion of the catalysts is quantified by the continuous measure of the CO2 release by online-mass spectrometry during potentiodynamic cycling between 0.1 and 1.6 V at 80°C. The results show that Pt/MWCNT (with the lowest double-layer capacity) is the most stable catalyst followed by Pt/N-MWCNT and Pt/Vulcan, initially losing carbon at a rate of 1.1, 3.4 and 4.7 µgC (mg Ctot)−1 cycle−1 , respectively. After about 30 % carbon loss (50-70 cycles) all catalysts corrode at an approximate rate of 5.5 µgC mg−1 cycle−1. At this stage, all show similar electrochemical surface area and double-layer capacity. However, the substantial diminution of the initially very thick and porous Pt/(N-)MWCNT catalyst layers after corrosion consequences in lower fuel cell performance compared to the structurally less affected Pt/Vulcan electrode. The results clearly reveal that CNT-based catalyst supports are more corrosion resistant compared to state-of-the-art Vulcan. Moreover, the performance of the corroded electrodes envisages the importance of electrode porosity. 

Keywords [en]
Carbon nanotubes, oxygen reduction, PEMFC, carbon corrosion, mass spectrometry
National Category
Chemical Engineering
Identifiers
URN: urn:nbn:se:kth:diva-261219OAI: oai:DiVA.org:kth-261219DiVA, id: diva2:1357322
Note

QC 20191004

Available from: 2019-10-03 Created: 2019-10-03 Last updated: 2019-10-04Bibliographically approved
In thesis
1. Electrochemical evaluation of new materials in polymer electrolyte fuel cells
Open this publication in new window or tab >>Electrochemical evaluation of new materials in polymer electrolyte fuel cells
2019 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Polymer electrolyte fuel cells (PEFC) convert the chemical energy in hydrogen to electrical energy and heat, with the only exhaust being water. Fuel cells are considered key in achieving a sustainable energy sector. The main obstacles to wide scale commercialization are cost and durability. The aim of this thesis is to evaluate new materials for PEFC to potentially lower cost and increase durability. To lower the amount of expensive platinum catalyst in the fuel cell, the activities of Pt-rare earth metal (REM) alloy catalysts have been tested. To improve the lifetime of the carbon support, the carbon corrosion properties of multi walled carbon nanotubes have been evaluated. To reduce the overall cost of fuel cell stacks, carbon coated and metal coated bipolar plates have been tested. To increase the performance and lifetime of anion exchange membranes, the water transport has been studied.

The results show that the Pt-REM catalysts had at least two times higher specific activity than pure platinum, and even higher activities should be obtainable if the surface structures are further refined.

Multi-walled carbon nanotubes had lower carbon corrosion than conventional carbon Vulcan XC-72. However, once severely corroded their porous structure collapsed, causing major performance losses.

The carbon coated metallic bipolar plates showed no significant increase of internal contact resistance (ICR) by cycling, suggesting that these coatings are stable in fuel cells. The NiMo- and NiMoP coated bipolar plates showed low ICR, however, presence of the coated bipolar plates caused secondary harmful effects on the polymer membrane and ionomer.

Considering the water transport through anion exchange membranes it was found that most membranes showed very similar water transport properties, with more water detected at both the anode and cathode when a current was applied. The most significant factor governing the water transport properties was the membrane thickness, with thicker membranes reducing the backflow of water from anode to cathode.

The results indicate that all of the new tested materials have the capability to improve the lifetime and reduce cost and thereby improve the overall performance of PEFC.

Abstract [sv]

Polymerelektrolytbränsleceller (PEFC) omvandlar den kemiskt bundna energin i vätgas till elektrisk energi och värme, med endast vatten som utsläpp. Bränsleceller ses som en viktig del i att skapa en hållbar energisektor. Det största hindret för kommersialisering är kostnaden och den begränsande livslängden. Syftet med denna avhandling är att utvärdera nya material som skulle kunna sänka kostnaden och öka hållbarheten av PEFC. För att minska mängden dyr platinakatalysator i bränslecellen har aktiviteten av legerade katalysatorer av platina och sällsynta jordartsmetaller testats. För att öka livslängden av bränslecellen har kolkorrosionsegenskaperna av flerväggade kolnanorör (MWCNT) utvärderats. För att kunna minska den totala kostnaden på bränslecellsstacken har kol- och metallbelagda bipolära plattor undersökts. För att öka livslängden och öka prestandan av anjonledande membran har vattentransportegenskaperna av dessa membran studerats.

Resultaten visar att de legerade katalysatorerna hade mer än två gånger högre elektrokemisk aktivitet än ren platina. Ännu högre elektrokemiska aktiviteter bör kunna erhållas om ytstrukturen kan förbättra ytterligare.

För MWCNT var kolkorrosionen lägre än för de konventionella kolpartiklarna av Vulcan XC-72. Efter mycket korrosion, kollapsade dock den porösa strukturen, vilket ledde till stora förluster i prestanda.

De kolbelagda bipolära plattorna uppvisade inga signifikanta ändringar i kontaktmotstånd (ICR) efter de elektrokemiska testerna. Detta betyder att de är stabila i bränsleceller. De NiMo- och NiMoP-belagda bipolära plattorna hade låga ICR-värden, dock ledde beläggningens närvaro till försämringar av membran- och elektrodegenskaper.

Alla testade anjonledande membran uppvisade liknande vattentransportegenskaper, med ökning av vatten på både anoden och katoden under drift. Membranens tjocklek visade sig ha störst påverkan på vattentransporten. Med tjockare membran detekterades mindre vatten på katoden, vilket betyder att tillbakaflödet av vatten hämmas av membranets tjocklek.

Sammanfattningsvis visar resultaten att alla nya testade material i alla fall till viss del kan lösa problemen med den höga kostnaden och korta livslängden och därmed öka den totala prestandan av PEFC.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2019. p. 66
Series
TRITA-CBH-FOU ; 2019:50
Keywords
Fuel cell, Pt-REM, Alloy catalyst, Multi walled carbon nanotubes, Bipolar plates, Water transport, Bränslecell, Pt-REM, Legerad katalysator, Flerväggade kolnanorör, Bipolära plattor, Vattentransport
National Category
Chemical Engineering
Research subject
Chemical Engineering
Identifiers
urn:nbn:se:kth:diva-261102 (URN)978-91-7873-326-2 (ISBN)
Public defence
2019-11-05, F3, Lindstedtsvägen 26, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

QC 2019-10-04

Available from: 2019-10-04 Created: 2019-10-03 Last updated: 2019-10-04Bibliographically approved

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