The effect of ionomer distribution on the oxygen mass transport resistance, the proton resistivity of the cathode catalyst layer, and the H 2 /air fuel cell performance was investigated for catalysts with surface modified carbon supports. By introducing nitrogen containing surface groups, it was shown that the ionomer distribution in the cathodic electrode can be optimized to decrease mass transport related voltage losses at high current density. The in house prepared catalysts were fully characterized by TEM, TGA, elemental analysis, and XPS. Thin-film rotating disk electrode measurements showed that the carbon support modification did not affect the oxygen reduction activity of the catalysts, but exclusively affects the ionomer distribution in the electrode during electrode preparation. Limiting current measurements were used to determine the pressure independent oxygen transport resistance -primarily attributed to oxygen transport in the ionomer film -which decreases for catalysts with surface modified carbon support. Systematically lowering the ionomer to carbon ratio (I/C) from 0.65 to 0.25 revealed a maximum performance at I/C = 0.4, where an optimum between ionomer thickness and proton conductivity within the catalyst layer is obtained. From this work, it can be concluded that not only ionomer film thickness, but more importantly ionomer distribution is the key to high performance low Pt loaded electrodes.
This study focuses on voltage cycling induced degradation of cathodes with different loading (0.4 and 0.1 mg Pt /cm 2) when applying square wave or triangular wave based accelerated stress tests (ASTs) between 0.6 and 1.0 V RHE. The degradation of the H 2 /O 2 and H 2 /air performance upon extended voltage cycling (up to 30000 cycles) was analyzed in terms of the voltage loss contributions from ORR kinetics, O 2 mass transport resistances and proton conduction resistances in the cathode. The extent of cathode thinning due to carbon support corrosion was determined by post mortem electrode thickness measurements. Square waves were found to cause a more rapid loss of ECSA and mass activity compared to triangular waves, which was shown to be due to the longer hold periods at high potentials rather than to the rate of the potential transient. The observed increase of the O 2 mass transport resistance with voltage cycling was found to mainly depend on the available Pt surface area, while mass transport resistances due to carbon corrosion were found to be insignificant. Finally, it was shown that by lowering the upper potential limit to 0.85 V RHE , low-loaded catalyst layers can sustain 30000 potential cycles without degradation of the H 2 /air performance.
The effect of the catalyst synthesis method on the location of platinum nanoparticles on a high surface area Ketjenblack is investigated with respect to the high current density performance in low loaded proton exchange membrane fuel cells (PEMFC). Catalysts were prepared using various synthetic methods to deposit platinum nanoparticles at different locations on the carbon surface, e.g. inside or outside the pores of the primary particle. Transmission electron microscopy (TEM) suggested, that the Pt-particle deposition can be controlled to be preferentially on the outer carbon surface or within the pores. Electrochemical characterization was performed in thin-film rotating disk electrode (RDE) setup as well as in 5 cm 2 single cell MEA tests. Although the carbon support was identical for all catalysts, the one with more Pt particles deposited on the outer carbon surface performed superior at high current which was attributed to a lower oxygen mass transport resistance. From the presented data, it can be concluded that not only the type or the surface area of the carbon black support affects the fuel cell performance, but that the synthesis approach is an additional parameter to tune the fuel cell performance at high current density.
Bimetallic alloys based on Pt and Y are potential cathode catalysts for proton exchange membrane fuel cells (PEMFCs) due to their high oxygen reduction reaction (ORR) activity. Nevertheless, the synthesis of carbon supported Pt x Y catalysts is challenging due to the low standard reduction potential of yttrium compared to platinum. Hence, extended electrochemical testing in actual PEMFCs remains elusive, especially with respect to catalyst degradation upon voltage-cycling. In this publication, we present the synthesis of a bimetallic Pt x Y/C catalyst via impregnation of commercial Pt/C with an yttrium halide precursor and subsequent heat-treatment in H 2 at 1200 • C. This catalyst showed a high specific ORR activity, at a mass activity similar to Pt/C due to its comparably low electrochemical surface area (ECSA). On the other hand, the large particle size of the here synthesized Pt x Y/C catalyst (≈10 nm) resulted in a significantly enhanced stability versus degradation in an accelerated stress test (AST) based on voltage-cycling between 0.6 and 1.0 V RHE at 50 mV s −1 , showing a superior ECSA, ORR activity and H 2 /air performance after 30000 cycles compared to a standard Pt/C catalyst.
The poor discharge and recharge efficiency demonstrated by lithium–air batteries renders the search for highly active and inexpensive oxygen reduction and evolution catalysts crucial to the development of these energy‐storage and conversion devices. Previous works have shown that manganese oxides are promising lithium–oxygen cathode catalysts, which is in agreement with their remarkable activities for the reduction and evolution of oxygen in aqueous media. Motivated by these resembling catalytic behaviors, we prepared and characterized a number of manganese oxide modifications directly on carbon black and attempted to correlate their oxygen reduction and evolution activities in aprotic and aqueous electrolytes. Although our results cannot confirm this correlation, they provide valuable insight into the reaction mechanisms at play in each medium. More precisely, in 0.1 M potassium hydroxide, the reduction of oxygen is related to the reduction of a manganese(III) intermediate whereas the oxidation of hydrogen peroxide (which was regarded as a mimic of the lithium peroxide produced upon lithium–oxygen battery discharge) correlates with the transition between manganese(II) and manganese(III) phases. In the aprotic medium, manganese oxide cathodes prefilled with lithium peroxide showed a strong catalytic effect but were not active in the oxidation of lithium peroxide produced in the previous discharge. This discrepancy is thought to arise from the stark differences in the sizes and morphologies of the lithium peroxide involved in each test, which implies that the catalytic activity of a material for the oxidation of lithium peroxide prefilled on electrodes is not indicative of its behavior in the charging of a real lithium–oxygen cell.
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