Effect of Addition of Carbon Nanotubes on the Performance of a Low Pt Loading Membrane-Electrode-Assembly in Proton Exchange Membrane Fuel Cells

The cell performance and Pt utilization of low-Pt proton exchange membrane fuel cells (PEMFCs) have been significantly improved through incorporating carbon materials into the conventional Pt/C catalytic layer of the membrane electrode assembly (MEA). However, the introduction methods for the carbon materials have not been investigated. In this work, carbon nanotube (CNT) as an additive was added to the low-Pt loading catalytic layer (0.1 mg(Pt).cm(-2)) by two methods: a separated CNT layer deposited on the top of the conventional Pt/C layer (CCM-1) and a mixture layer by blending CNT and Pt/C catalyst (CCM-2). The conventional low-Pt loading catalytic layer was employed as control group (CCM-0). The microstructure of the catalytic layers was characterized by scanning electron microscopy, transmission electron microscopy and nitrogen sorption isotherms method. The electrochemical properties of the catalytic layer and membrane electrode were evaluated by cyclic voltammetry (CV), electrochemical impedance (EIS) and linear scanning voltammetry. The results indicated that the cell performance of the conventional low-Pt loading catalyst coated membrane was improved by the introduction of CNTs in both CCM-1 and CCM-2. Compared to the conventional CCM (CCM-0) with a peak power density of 0.522 W.cm(-2) at 70 degrees C and 100% relative humidity (RH) without backpressure, the maximum power densities of CCM-1 and CCM-2 have been improved by 22.4% and 60.0% under the same test conditions, respectively. The increased performance of CCM-1 is believed to result from the enhancement of contact interface between the catalytic layer and the gas diffusion layer in CCM-1 and consequent decrease of the contact resistance. Furthermore, the outstanding power density of CCM-2 is not only owing to the decreased interface contact resistance between the CCM and the gas diffusion layer, but also due to the significant improvement of gas transmission in the catalytic layer, which leads to the decrease of electrochemical reactant resistance and then improvement of the Pt utilization. That has been confirmed by the Pt utilization of 34.4%, 35.6% and 44.7% for CCM-0, CCM-1 and CCM-2. In addition, it also was confirmed by the extremely low power output (2.9 mW.cm(-2)) of a CCM with only CNT in the catalytic layer when the fuel cell was tested at 70 degrees C and 100% RH without back pressure. In addition, the optimum loading of CNT in the mixed catalytic layer is 37.5 mu g.cm(-2) with the peak power density of 0.91 W.cm(-2). This work shows that mixing of CNT and Pt/C catalyst into a catalytic layer is an effective method for improving the Pt utilization and reducing the loading of Pt catalyst.