Durability of Pt Catalysts Supported on Graphitized Carbon-Black during Gas-Exchange Start-Up Operation Similar to That Used for Fuel Cell Vehicles

Abstract

We investigated the durability of cathode catalysts during a start-up (SU) process similar to that used in actual polymer electrolyte fuel cell vehicle operation. The durability of Pt supported on graphitized carbon black (Pt/GCB) catalysts was evaluated in the practical SU process, i.e., the anode gas was successively cycled between air, hydrogen, and nitrogen. The effect of the SU process on the cell performance was evaluated using two types of catalysts (commercial Pt/GCB, and that prepared in house by the "nanocapsulemethod," n-Pt/GCB). The polarization curves and cyclic voltammetry were evaluated before and after the SU evaluation. The degradation of Pt nanoparticles and carbon supports was analyzed by transmission electron microscopy, scanning transmission electron microscopy, and micro-Raman spectroscopy. We also applied glancing incidence X-ray diffraction in order to observe the depth profiles of the Pt crystallite sizes in various interfacial regions. From these analyses, we found that the degradation of the Pt catalyst occurred not only in the gas outlet region but also the gas inlet region of the cathode. The degradation in the inlet region is ascribed to both the interim electrochemical evaluations and the potential fluctuations, which cause a dissolution of Pt nanoparticles used during the SU process. Polymer electrolyte fuel cells (PEFCs) are able to convert chemical energy directly to electrical energy with high efficiency and have shown promise to be an eco-friendly power source for residential cogeneration systems and fuel cell vehicles (FCV).1-3 Nevertheless, the widespread commercialization of PEFCs has been impeded because of the large amount of the platinum catalyst required, which must be minimized to reduce the system cost. 4 The minimization can be achieved, for example, by the improvement of utilization of Pt nanoparticles dispersed on carbon black supports (Pt/CB) with high surface area used as the cathode catalyst for PEFCs as well as the improvement of the durability. During the start-up and shutdown (SU/SD) cycles of the PEFCs, air and H 2 coexist transiently in the anode until the replacement of the former with the latter (or vice versa) is completed. Reiser et al. showed that this situation causes the cathode potential to climb to more than 1.5 V due to the so-called "reverse current mechanism", 5 which significantly accelerates the degradation of the catalyst due to the oxidation of the CB and the agglomeration or dissolution of the Pt nanoparticles. 6 In our previous research, we have developed a graphitized carbon black (GCB) supported Pt catalyst prepared by the "nanocapsule method" (n-Pt/GCB) to improve the performance of Pt catalysts. 7-9The high dispersion of the Pt nanoparticle catalyst particles on the GCB support prepared by this method provides enhanced oxygen reduction reaction (ORR) activity. It was also found that the durability of the n-Pt/GCB catalyst was improved during a potential cycling evaluation that simulated the variation of the cathode potential during the...

Effect of Electronic Conductivities of Iridium Oxide/Doped SnO₂ Oxygen-Evolving Catalysts on the Polarization Properties in Proton Exchange Membrane Water Electrolysis

“…The Rohm, cell of the conventional cell in Table 2 was just mΩ cm 2 , which is assumed to include Rohm, anode (IrO2 + Pt black) and Rohm, cathode (Pt black), together with contact resistances with the gas diffusion layers (Pt/Ti mesh and carbon paper, see Materials and Methods). This value of Rohm, cell agrees precisely with those of polymer electrolyte fuel cells (PEFCs) with Nafion ® membrane of the identical thickness and Pt/C catalysts for the anode and cathode [39][40][41]. Thus, by adding the Rohm, anode of IrOx/M-SnO2 to 75 mΩ cm 2 stated above, we calculated the Rohm, cell, calc values to be 78, 143, and 1408 mΩ cm 2 , for the cells with IrOx/Sb-SnO2, IrOx/Ta-SnO2, and IrOx/Nb-SnO2, respectively.…”

Degradation Mechanisms of Carbon Supports under Hydrogen Passivation Startup and Shutdown Process for PEFCs

“…Conditions of anode and cathode gas for the air-SU/SD cycle at 50 • C and 30% RH under ambient pressure. 32 Anode gas ml/min Cathode gas ml/min Time s…”

“…However, it was found that the extents of carbon corrosion in the inlet and outlet regions were nearly the same, by using Raman spectroscopy, as reported in our previous work. 31,32 It is considered that the difference of carbon corrosion between the inlet and outlet regions was not so large under the present H 2 -SU/SD process, because the time for the gas exchange was short (1.2 s).…”

“…In order to improve the durability of PEFCs in FCVs, the use of higher stability support materials, such as graphitized CB 32 or metal oxides [16][17][18][19][20][21][22][23][24][25][26] is an effective approach to inhibit the COR during the H 2 -SU/SD process. Based on the COR mechanisms (i), (ii) and (iii), the uniform distribution of the ionomer in the cathode CL prevents both the inhibition of H 2 supply and the shortage of H + which cause the COR associated with the reduction of the Pt oxide and the ORR.…”

“…We were able to improve not only the cell performance but also the durability under air-SU/SD conditions by use of a graphitized CB (GCB)-supported Pt catalyst prepared by the "nanocapsule method" (n-Pt/GCB) as the cathode material, compared with those for a cathode prepared with a commercial Pt/GCB. 32 However the ECSA for nPt/GCB still decreased by more than 50% after 1000 cycles of air-SU/SD, compared with that before cycling.In order to improve the SU/SD durability of PEFCs in FCVs, the hydrogen passivation process (H 2 -SU/SD) has been proposed. 33 In the SD portion of this process, the O 2 in the cathode is consumed by the H 2 permeated from the anode after the closing of the cathode gas valves at both the inlet and outlet, and then the supply of H 2 is also stopped.…”