Activity, stability and degradation of multi walled carbon nanotube (MWCNT) supported Pt fuel cell electrocatalysts

Abstract

Understanding and improving durability of fuel cell catalysts are currently one of the major goals in fuel cell research. Here, we present a comparative stability study of multi walled carbon nanotube (MWCNT) and conventional carbon supported platinum nanoparticle electrocatalysts for the oxygen reduction reaction (ORR). The aim of this study was to obtain insight into the mechanisms controlling degradation, in particular the role of nanoparticle coarsening and support corrosion effects. A MWCNT-supported 20 wt.% Pt catalyst and a Vulcan XC 72R-supported 20 wt.% Pt catalyst with a BET surface area of around 150 m(2) g(-1) and with a comparable Pt mean particle size were subjected to electrode potential cycling in a "lifetime" stability regime (voltage cycles between 0.5 to 1.0 V vs. RHE) and a "start-up" stability regime (cycles between 0.5 to 1.5 V vs. RHE). Before, during and after potential cycling, the ORR activity and structural/morphological (XRD, TEM) characteristics were recorded and analyzed. Our results did not indicate any activity benefit of MWCNT support for the kinetic rate of ORR. In the "lifetime" regime, the MWCNT supported Pt catalyst showed clearly smaller electrochemically active surface area (ECSA) and mass activity losses compared to the Vulcan XC 72R supported Pt catalyst. In the "start-up" regime, Pt on MWCNT exhibited a reduced relative ECSA loss compared to Pt on Vulcan XC 72R. We directly imaged the trace of a migrating platinum particle inside a MWCNT suggesting enhanced adhesion between Pt atoms and the graphene tube walls. Our data suggests that the ECSA loss differences between the two catalysts are not controlled by particle growth. We rather conclude that over the time scale of our stability tests (10,000 potential cycles and beyond), the macroscopic ECSA loss is primarily controlled by carbon corrosion associated with Pt particle detachment and loss of electrical contact.

Nitrogen-doped coatings on carbon nanotubes and their stabilizing effect on Pt nanoparticles

“…6 This shows that the N-C coatings had no significantly detrimental effect on the electrocatalytic ORR activity. Cyclic voltammetry (CV) (Fig.…”

“…The growth rate was found to be dependent on the surface area of the carbon support with high surface area carbons showing a slower Pt coarsening rate than low surface area supports. 5,6 In parallel to the particle coarsening, carbon supports corrode and degrade due to the high cathodic electrode potentials. Much has been learned about the molecularscale degradation mechanisms of fuel cell cathodes, 8 however, to date, robust and practical material-design strategies to mitigate the structural degradation of carbon-supported Pt nanoparticle catalysts has remained scarce.…”

Monometallic Palladium for Oxygen Reduction in PEM Fuel Cells: Particle-Size Effect, Reaction Mechanism, and Voltage Cycling Stability

“…In this experimental study, we examine the following issues: i) mass-based and specific surface area based ORR activities as function of the ECSA, which can be assumed to be approximately inversely proportional to the particle size; 24 ii) stability of Pd-based electrocatalysts toward voltage cycling in 0.1 M HClO 4 , which is commonly used to simulate the PEMFC environment; 25 and, iii) insight into the oxygen reduction reaction mechanism on Pd catalysts.…”

What Can We Learn in Electrocatalysis, from Nanoparticulated Precious and/or Non-Precious Catalytic Centers Interacting with Their Support?

“…High graphitic carbon materials, such as carbon nanotubes (CNT) and graphene oxides (GO) are widely used as supports for both precious and non-precious catalytic centers [104,171,177,[231][232][233][234][235][236][237][238][239][240][241] The high graphitic carbons are less corrosive than low graphitic carbon as shown in Figure 27a [164]. Taking CNT as an example, the Raman spectra show that the oxidized multi-walled CNT (ox.MWCNT) has dominant graphitic sp 2 domains, as compared to carbon (Vulcan XC-72) which shows highly disordered sp 3 domains, Figure 27b When a catalytic center, e.g., Pt, is chemically deposited onto highly graphitic carbon the interaction takes place with both sp 3 and sp 2 domains, Figure 28a,b, shows the close Pt particle size and morphology [201].…”

“…Low and high graphitic carbon are, e.g., carbon black (CB), single-walled or multi-walled carbon nanotube (CNT), graphene oxide (GO) [55,[163][164][165][166][167][168][169][170][171][172][173][174][175][176][177][178][179][180][181][182] and carbon nitride (CN) [183][184][185][186] among others. The semi-conducting oxides, such as TiO 2 , doped-TiO 2 and doped-SnO 2 , have been reported as support for catalytic centers [187][188][189][190][191][192][193].…”