| Proton exchange membrance fuel cell (PEMFC) are typical fuel cells working on the relatively low temperature (<100℃). They are suitable for requirements of powe sources from protable devices, electric vehicle and distributed power station with the advantages of low working temperature, rapid start-up and zero emission. State of the art anode and cathode electrocatalysts used in these fuel cells are generally Pt-based noble metal catalysts. One of the key problems for these low temperature fuel cells is to keep the electrocatalyst with high activity,long lifetime and low Pt loading, which directly determines the commercialization schedule of fuel cells.The noble electrodes such as Pt and Pd are most important components in the cathode electrocatalysts. In order to evaluate the electrocatalytic activity of Pt-based electrocatalysts and to improve the catalytic performance, knowledge of the real surface area of such electrodes is extremely important, because it is required to calculate the specific activity of a given electrochemical process. In this thesis, the main contents contain two parts. The first part: Double layer capacitance measurements and adsorption of probe molecules or metal atoms have been applied to evaluate the real surface area of polycrystalline Pd/Pt and rough Pt film electrodes. Furthermore, the values determined from different approaches are carefully compared, the applicability and limitations associated with different approaches will be discussed. The second part:These results are briefly summarized below.一,Determination of the real surface area of polycrystalline Pd/Pt and rough Pt film electrodes.1)Four different methods have been exploited to determine the real surface area of a pc-Pd electrode. The surface area determined from oxygen adsorption or double layer capacitance agrees well with each other, while the method with Cu upd gives a smaller value (ca. 5% ). Our results reveal that the saturation coverage of CO at pc-Pd with CO doesed 0.35 V is ca. 0.66 ML, which is much smaller than 0.75 ML as reported earlier. These results reveals that all these four methods can be used for the determination of the real surface area of smooth pc-Pd electrodes.2)Four different methods, i.e., underpotential deposition (UPD) of Cu, under potential deposition of H atoms, surface oxides reduction and voltammetric measurement of double layer capacitance, are carried out in 0.5 M H2SO4 to evaluate the real surface area of smooth pc-Pt electrode. By assuming that the charge for the formation of a full monolayer of Cu is 420μC/cm2 and the charge in the potential region from 0.05~0.5 V is ca. 220μC/cm2, at smooth pc-Pt the real surface area derived from the stripping charge of under potential deposited Cu and H atoms are in well consistent with each other. From the charge for PtOH and PtO formation/reduction, it is found that in 0.5 M H2SO4 the upper potential limit for the formation of a monolayer of PtOH and PtO are 1.18 and 1.52 V at a scan rate of 20 mV/s. Furthermore, the saturated CO coverage is also estimated using the real surface area as determined by the four methods, which is found to be ca. 0.67 ML with CO dosing at 0.05 V.3)The same methods and processes as smooth pc-Pt electrode are carried out evaluate the real surface area of the rough Pt film electrode. From the charge for PtOH and PtO formation/reduction, it is found that in 0.5 M H2SO4 the upper potential limit for the formation of a monolayer of PtOH and PtO are 1.19 and 1.53 V at a scan rate of 20 mV/s. Furthermore, the saturated CO coverage is also estimated using the real surface area as determined by the four methods, which is found to be ca. 0.60 ML with CO dosing at 0.05 V.二,The preparation of Pd @ x ML Pt and Pd @ x sub-ML Pt bimetallic electrodes and the activity for oxygen reduction reaction(ORR).1 ) Bimetallic electrodes with one to five monolayers of Pt on pc-Pd substrate(denoted at Pd @ x ML Pt) are prepared using under-potential deposition (UPD) of Cu following with galvanic displacement of Cu by Pt. The half wave potential for ORR at Pd@1ML Pt is about 30mV more positive than that at pc-Pt electrode.2)Bimetallic electrodes with one to five sub-monolayers of Pt on pc-Pd substrate(denoted at Pd @ x sub-ML Pt) are prepared using the same approach as above. The ORR activity is found to increase monotonically with the layer thickness of Pt overlayers up to four monolayers, and it remains unchanged with further increase in the Pt overlayers to 5 sub-ML. The ORR activity is higher than that of pc-Pt when the thickness of Pt overlayers is larger than two snb-MLs. Under otherwise identical condition, the half wave potential for ORR at Pd@4 sub-ML Pt is about 25mV more positive than that at pc-Pt electrode.
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