The electrode in a proton exchange membrane (PEM) fuel cell is composed of a carbon-supported Pt catalyst coated with a thin layer of ionomer. At the cathode, where the oxygen reduction reaction occurs, protons arrive at the catalyst sites through the thin ionomer layer. The resistance to this protonic conduction (RnormalH+,cath) through the entire thickness of the electrode can cause significant voltage losses, especially under dry conditions. The RnormalH+,cath in the cathode with various ionomer/carbon weight ratios (I/C ratios) was characterized in a normalH2/normalN2 cell using ac impedance under various operating conditions. AC impedance data were analyzed by fitting RnormalH+,cath , cathode capacitance (Ccath) , and high frequency resistance to a simplified transmission-line model with the assumption that the proton resistance and the pseudocapacitance are distributed uniformly throughout the electrode. The proton conductivity in the given types of electrode starts to drop at I/C ratios of approximately <0.6/1 or an ionomer volume fraction of ∼13% in the electrode. The comparison to normalH2/normalO2 fuel cell performance shows that the ohmic loss in the electrode can be quantified by this technique. The cell voltage corrected for ohmic losses is independent of relative humidity (RH) and the electrode’s I/C ratio, which indicates that electrode proton resistivity ρnormalH+,cath (ratio of RnormalH+,cath over cathode thickness) is indeed an intrinsic RH-dependent electrode property. The effect of RH on the ORR kinetics was further identified to be rather small for the range of RH studied ( ⩾35% RH).

The effects of the porosity and pore size distribution of catalyst carbon support on the proton conduction and electrode performance have been studied by comparing two carbon supports [Vulcan XC 72 (V) and Ketjenblack (KB)]. Differences in the H2/O2 cell performance of the two catalysts are shown to be consistent with differences in the electrodes’ proton resistivity and the area of the Pt catalysts available for the oxygen reduction reaction (ORR) at a similar area specific activity. For the same ionomer-to-carbon weight ratio (I/C) and relative humidity (RH), the electrode with KB-supported catalyst exhibits higher proton resistivity than the one with Vulcan-supported catalyst. The cumulative surface area and cumulative volume distributions of the two catalysts were obtained from N2 BET measurements. It is concluded that the difference in surface area and pore volume for these two catalysts comes from pores smaller than 4 nm. The extent of ionomer captured within these small pores is substantial for Pt/KB and minor for Pt/V. It is shown that the ionomer captured within these small pores of the primary carbon particles does not contribute to the proton conductivity across the electrode. If the total electrode I/C-ratio is corrected for the amount of ionomer captured within the primary carbon particles, the dependence of the proton resistivity of Pt/KB and Pt/V on the effective I/C-ratio becomes identical. Since Pt/KB contains more pores smaller than 4 nm, higher total I/C-ratio is needed for Pt/KB to achieve the same proton resistivity as Pt/V based electrodes. Pt particles are more uniformly dispersed on higher surface-area carbon supports, resulting in a higher Pt area available to ORR. The selection of the proper carbon support depends on the porosity and its effect on cathode proton conduction, ORR kinetics and electrode durability.

We report a method for the fabrication of nanoporous nanowires with high surface area and well-defined pore morphology. The nanoporous nanowires are formed in a two-step process involving electrochemical deposition of a single-phase, two-component AxB1−x alloy into a nanoporous template, and subsequent chemical etching of one component from the alloy after removal from the template. Here, we demonstrate the fabrication of nanoporous gold nanowires and also show that nonporous segments can be incorporated into multisegment nanowires.

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Proton conduction resistance in the cathode electrode with various ionomer/carbon weight ratios (I/C-ratios) was characterized using AC impedance at different relative humidities (RH) in a H 2 /N 2 cell. AC impedance data were analyzed by fitting sheet resistance, R sheet , sheet capacitance, C sheet , and the high-frequency resistance, HFR, to a simplified transmission-line model with the assumption that the proton resistance and the pseudo-capacitance are distributed uniformly throughout the electrode. For given types of model membrane electrode assemblies (MEAs), a percolation threshold for proton conductivity in the electrode was observed at I/C-ratios of approximately <0.6/1. The comparison to H 2 /O 2 fuel cell performance shows that the ohmic loss in the catalyst layer can be quantified by this technique, and the parameter R sheet extracted from the AC impedance spectra is indeed an intrinsic RHdependent electrode property.

The dependence of electrode proton resistivity on electrode thickness, Pt loading, ionomer loading, and ionomer equivalent weight ͑EW͒ in proton exchange membrane ͑PEM͒ fuel cell cathodes was investigated using a Pt/Vulcan catalyst. For uniform electrodes, the electrode proton resistivity is independent of the electrode thickness and Pt loading but depends on the ionomer loading and ionomer EW. There is a strong dependence on the ionomer EW when the ionomer/carbon weight ͑I/C͒ ratio is lower than 0.8. The electrode proton resistivity strongly depends on relative humidity ͑RH͒ and the density of -SO 3 H groups in the electrode. The electrode proton resistivity becomes nearly independent of ionomer EW in electrodes when high I/C ratios are used. At low I/C ratios and low RH levels, electrodes with 850 EW ionomer exhibit better performance than those with 1050 EW. On the contrary, 850 EW electrodes give lower performance under overhumidified conditions due to electrode flooding.The proton resistivity within the electrode of a H 2 /O 2 proton exchange membrane fuel cell ͑PEMFC͒ ͑ H + ,electrode ͒ is an important design parameter for membrane electrode assembly ͑MEA͒ as it controls catalyst utilization, particularly under low relative humidity ͑RH͒ conditions. 1 When using pure H 2 ͑not reformate͒, the extent of catalyst utilization is not very critical for the H 2 oxidation reaction at the anode due to its fast kinetics; 2 however, it is highly critical for the cathode O 2 reduction reaction ͑ORR͒ with its 7 orders of magnitude slower kinetics. 3 The resistivity to proton conduction in the cathode ͑ H + ,cath ͒ is generally an MEA performance controlling factor, whereas the influence of H + ,an is usually negligible except when drawing large current densities at very low RH or at low temperature during freeze startup. 4,5 Therefore, in the present work, we have focused on evaluating the proton resistivity of various proton exchange membrane cathodes; nevertheless, the methodology and the resulting proton resistivity relationships are valid for both the cathode and the anode.In our previous work, the cathode proton resistivity was shown to depend on the electrode's ionomer to carbon weight ͑I/C͒ ratio as well as on the RH. 6,7 Furthermore, it was shown that the measured H + ,cath could be used to predict MEA performance over a wide range of I/C ratios and RH, which led to the conclusion that H + ,cath is indeed an intrinsic electrode property. This implies that for uniform electrodes ͑the ionomer uniformly covers the catalyst throughout the electrode͒, H + ,cath should be independent of electrode thickness and of the Pt weight percentage on the carbon support ͑wt % Pt/C͒ as long as the same electrode components ͑ionomer and carbon-support type͒ and I/C ratio are used. The present study further verifies that the experimental method for measuring the electrode proton resistance and its use for predicting electrode performance 6,7 is applicable for a wide range of electrode types and electrode thicknesses. Because the eq...

Understanding interactions between Nafion (perfluorosulfonic acid) and Pt catalysts is important for the development and deployment of proton exchange membrane fuel cells. However, study of such interactions is challenging and Nafion/Pt interfacial structure remains elusive. In this study, adsorption of Nafion ionomer on Au and Pt surfaces was investigated for the first time by in situ surface-enhanced Raman spectroscopy. The study is made possible by the use of uniform SiO(2)@Au core-shell particle arrays which provides very strong enhancement of Raman scattering. The high surface sensitivity offered by this approach yields insightful information on interfacial Nafion structure. Through spectral comparison of several model compounds, vibration assignments of SERS bands were made. The SER spectra suggest the direct interaction of sulfonate group with the metal surfaces, in accord with cyclic voltammetric results. Comparison of present SERS results with previous IR spectra was briefly made.

In this paper, we report on the energetics and kinetics of copper deposition on n-type Si(100), Si(111), and miscut Si(111) surfaces from 1 mM CuSO4+0.1MH2SO4 solution. Electrochemical impedance spectroscopy showed that the position of the bandedges at the surface is very similar for n-Si(100), n-Si(111), and miscut n-Si(111) surfaces. At more positive potentials, the presence of electrically active surface states was observed. Cyclic voltammetry and current transients showed that copper deposition occurs by progressive nucleation and diffusion-limited growth for the three surfaces, which was confirmed by ex situ atomic force microscopy experiments. The rates for copper nucleation on Si(100) and Si(111) are essentially the same, while miscut Si(111) shows a slightly higher nucleation rate. These results indicate that the energetics and kinetics of copper deposition from acidic sulfate solution on silicon are essentially independent of the silicon surface orientation. © 2001 The Electrochemical Society. All rights reserved.