Several postmortem analyses were conducted to investigate the change of Pt distribution in the active components in a phosphoric acid fuel cell (PAFC) during a long‐term operation. Quantitative analysis of an electron probe microanalyzer (EPMA) indicates not only the loss of Pt in a cathode but also Pt migration to an anode during operation and identifies the Pt in a matrix. It was found that the behavior of Pt during cell operation is associated with operation mode and presumably subsequent singular distribution of phosphoric acid. Observation by transmission electron microscopy (TEM) demonstrates that Pt in an electrode near a matrix grows earlier than that of the center and backing paper side of an electrode. In addition to the results of Pt dissolution, a morphological consideration through TEM gives clear evidence that particle coalescence is also involved in particle growth as sintering mechanism.

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In the in situ cyclic voltammetry that we have developed, the anode potential was shifted, and hysteresis of the anode potential appeared during the scanning of the cathode potential when the hydrogen partial pressure was decreased to ca. 5% at the anode. It was concluded that proton deficiency was responsible for the hysteresis at the anode. Further, at this hydrogen pressure in real single-cell operation, the cathode potential (iR-free) deviated from Nernst's law, which showed the interaction between the anode potential and the cathode potential. This means that anode polarization influenced cathode polarization under a low hydrogen partial pressure where the proton deficiency occurred.

The effective platinum metal surface area in a real phosphoric acid fuel cell was evaluated by in situ cyclic voltammetry which has been developed in our laboratory. It was found that the in situ electrochemical platinum surface area of a phosphoric acid fuel cell under real operational conditions was around 25 m~/g. It was concluded that this low value came from the isolation of the platinum particles from a carbon support, which was revealed by transmission micrography and the distortion of an in situ cyclic voltammogram. Furthermore, x-ray photoelectron spectroscopy analyses of long-term operated electrodes showed that two or three fluorines were detached from the polytetrafluoroethylene (PTFE) surface. The timing of PTFE degradation was in good agreement with the change in the in situ EC-MSA.Recently, the development of phosphoric acid fuel cells (PAFC) has been accelerated owing to their advantages in environmental preservation. Accordingly, there have been many technical developments followed by successful demonstrations through feasibility studies. 1' To cut down the cost of PAFCs and to distribute them to areas of demand, cell performance and reliability must be improved. Because cathode polarization was considered to be one of the predominant factors in cell performance, this has been the focus of attempts to improve efficiency. Roughly speaking, the cathodic polarization can be improved by taking three courses of action. The first is to design and develop a more active platinum alloy catalyst. 3 The second is to increase the effective metal surface area by optimizing the impregnation process of the electrolyte in electrodes. The third is to optimize the structure of Teflon-bonded electrodes. This optimization is necessary for obtaining the foremost gas diffusion electrode.Platinum catalysts including some platinum alloy catalysts are used in PAFC systems. The size of the platinum metal particles is from around 20 to 50 A, so that the relative metal surface area results in more than 100 m2/g in some cases. However, the platinum metal surface, due to the structure of the Teflon-bonded electrode, has not been fully utilized in electrode reactions. Furthermore, the stability of the metal surface area should be considered as being linked to the degree of electrolyte absorption in the electrode. The stability of the support should be also considered. 4 Thus, the optimization of the electrode and electrolyte management for a higher effective metal surface area is one of the most significant features in the improvement of cell performance and the stability of real fuel cell systems. In this context, we have developed a new method of in situ cyclic voltammetry to evaluate the effective platinum metal surface area.In this paper, we describe a new method, in situ cyclic voltammetry, and elucidate the effective metal surface area [in situ metal surface area (MSA)] in PAFC operational conditions. Furthermore, as to the change of spectroscopic points, Alderucci reported the decrease in the PTFE peak as being a deplet...