On the Degradation of Pt Nanocatalysts Supported on Carbon Nanotubes in High Temperature PEM Fuel Cells

Abstract

Carbon nanotubes are recognized as a promising alternative support material for fuel cell electrocatalysts due to their higher electrical conductivity and higher durability, when compared to carbon black. In fact, using Pt nanoparticles supported on carbon nanotubes (CNTs) wrapped in poly (vinylphosphonic acid)-doped polybenzimidazole (PVPA-PBI) makes a remarkably high durable fuel cell, thus opening the door for the next generation of these devices [1]. In this work, the degradation mechanism of 3.4 nm Pt nanoparticle catalysts supported on PVPA-PBI wrapped CNTs was investigated by aberration corrected TEM, before and after voltage cycling. In order to carry this experiment, Pt/PVPA-PBI/CNT powder was deposited on a gold grid attached to a gold plate, which was used as a working electrode in a three electrode electrochemical cell. To simulate the effect of fuel cell cycling, the TEM grid was cycled between 1 and 1.5 V RHE in N2 saturated 0.1 HClO4liquid electrolyte for 1000, 1500, and 2000 cycles. In this fashion, pre-defined locations of the electrocatalyst on the TEM grid were observed before and after cycling, by an aberration-corrected JEOL ARM 200F. In the first 1000 cycles, the main mechanism for the loss of electrochemical active surface area was found to be particle motion, followed by coalescence. Severe structural deformation of the carbon nanotubes during voltage cycling is another source of degradation. The wave-like structure of the carbon formed after voltage cycling is the result of the appearance of the defect sites on the carbon nanotubes which convert the flat hexagon structure to curved heptagon and pentagon carbon rings. In order to understand if there is any correlation between carbon degradation and particle movement on the carbon support, carbon degradation was accelerated under the electron beam, while the behavior of the nanoparticles was observed. It is shown that the particles start to move as soon as carbon atoms at the interface of the carbon/particle interface are removed. During voltage cycling, carbon corrosion occurs through a series of reactions at the carbon/ionomer, ionomer/particle, and carbon/particle interfaces. These reactions result in the formation of carbon dioxide and removal of carbon atoms at the particle/carbon interface. When carbon atoms at the carbon/particle interface are removed from the surface of carbon, leaving a carbon defect site, the particle tends to make new bonds with the next carbon atoms to possibly decrease its free surface energy. In the next 1000 cycles, single atoms and atomic clusters appears on the ionomer phase at the surface of carbon nanotubes. These single atoms and atomic clusters move subsequently toward large particles and deposit on their surfaces. The atomic clusters will either dissolve again to single atoms or ions and redeposit on the large particles or move toward particles and redeposit between them with consequent bridging. Reference: [1] M. Berber, T. Fujigaya, N. Nakashima, Chemcatchem, 6 (2) 567-572 (2014)