Carbon Surface Oxidation by Short-Term Ozone Treatment for Modeling Long-Term Degradation of Fuel Cell Cathodes

Abstract

In regard to degradation phenomena in a polymer electrolyte fuel cell ͑PEFC͒, there is little information on voltage loss due to the excess accumulation of liquid water blocking pores for the gas supply in the cathode ͑flooding͒ observed in a long-term operation over 5000-10,000 h. In this study, modeling of this degradation phenomenon was attempted using ozone treatment of the cathode catalyst layer ͑CL͒. The hydrophilicity of the layer was increased by the treatment, which was attributed to oxidation of the carbon surface of the catalyst ͑Pt-loaded carbon black, Pt/C͒ exposed to the gas phase in the CL. The flooding was observed at the PEFC with the cathode CL formed using 10 wt % Pt/C and treated with ozone for 10 h, and the extent of the flooding was enlarged by the ozone treatment of 15 h. The flooding was caused by the 5 h ozone treatment when the Pt content was 20 or 50 wt %. These results indicated that modeling of the long-term degradation was achieved by the short-term ozone treatment, and that the excessive hydrophilicity of the carbon surface of Pt/C was one of the highly potent candidates for causes of the degradation.Polymer electrolyte fuel cells ͑PEFCs͒ have recently been in practical use in power sources of electric vehicles and cogeneration systems for domestic electricity and heating, although the scale of PEFC usage is very limited. 1 Because the PEFC converts chemical energy to electricity by H 2 oxidation at the anode and O 2 reduction at the cathode in high efficiency with only water exhausted, 2-4 the substitution of conventional energy systems by the PEFC would help to improve the global environment. 5 In the next stage of the realization of the practical use of the PEFC, it is necessary to increase the number of the PEFC systems by gaining reliability and reducing the cost of the system. Durability of the PEFC closely relates to the reliability and the cost reduction. To obtain information about avoiding degradation of the PEFC and producing durable PEFCs, there have been an increased number of studies on the degradation mechanism at the components of the PEFC. 6 Nevertheless, several degradation phenomena occurring during a long-term operation have not clarified yet. One of those is the voltage loss at the cathode due to blocking of pathways for O 2 transfer by liquid water ͑flooding͒, which often begins to be observed for the operation over 5000-10,000 h in the state-of-the-art PEFC, 7 less than the PEFC-lifetime target of 40,000-90,000 h for the stationary PEFCs. Few studies have been reported so far on the mechanism of this degradation phenomenon and on the place causing the degradation.The O 2 transfer pathways are pores inside the cathode. They are faced with the surface of the catalyst particles consisting of Pt or Pt alloy supported on carbon black ͑hereafter called Pt/C͒ and the surface of the polymer electrolyte covering Pt/C in the catalyst layer ͑CL͒, and the surface of carbon and the fluorinated resins in the gas-diffusion layer. In the case of the PEFC degradation that give...

Fabrication of N-doped and shape-controlled porous monolithic carbons from polyacrylonitrile for supercapacitors

“…3f). 42,43 These peaks were assigned to C]O groups (group 1), carbonyl oxygen atoms in esters, amides, anhydrides and carbonyl oxygen atoms in hydroxyls or ethers (group 2), the ether oxygen atoms in esters and anhydrides (group 3), and the oxygen atoms in carboxyl groups (group 4). The atomic contents of C, N and O species of PMC were calculated quantitatively as 86, 2.6, and 11 at%, respectively.…”

Carbonaceous Oxygen Reduction Catalyst Formed from Phthalonitrile Derivatives Using Cobalt Chloride as Template Source

“…The performance improvement was due to the lower cell resistance of 3,4-DCPy(1.84:1)CoFe700900 (0.13-0.16 cm 2 ) than that of 3,4-DCPy(1.84:1)CoFe700 (0.22-0.23 cm 2 ) and possibly due to the improved O 2 transfer inside the cathode catalyst layer, which was attributed to improved removal of liquid water caused by the alteration of the surface properties of the catalyst particles. 35 The additional heat-treatment at 900 • C was effective for reducing the resistance of the catalyst particles.…”

Establishment of Accelerated Degradation Method of PEFC Stacks (NEDO Project: Fundamental Research of Degradation of PEFC Stacks)

“…As to the degradation of cathode, it was observed that the catalyst layer became thinner and Pt concentration took place from the analysis of the MEA that the O 2 gain was increased by long-term operation, and it was indicated that the carbon corrosion progressed. Moreover, it was found that the catalyst layer that the gas diffusivity of cathode was degraded by long-term operation became hydrophilic from the result of steam adsorption measurement 5) . Additionally, the carbon corrosion took place by the high potential holding (~1 V) without Pt 6) , and it was accelerated by coexistence of the Pt on the carbon 7) .…”