The application of porous silicon (PSi) for gas sensing devices has gained a considerable attention in the last decade. This work considers the electrical features of PSi layers prepared by electrochemical etching. We find that in order to get a better understanding of the absorption properties of PSi surface, it is necessary to know how the PSi morphology depends on the etching parameters. The physical structure of PSi, i.e., porosity, and pore size distribution can be controlled by changing the Hydrofluoric Acid concentration, current density, anodizing length and etching time in anodizing procedure. We describe our test system for gas sensors and investigate on the electrical behavior of PSi layers (p-type) in N 2 gas for various fabrication conditions. The results show that the current density increases significantly as N 2 gas is adsorbed. The measurements of the I-V characteristics were carried out at atmospheric pressure, room temperature, and with N 2 gas as well.

Studies on the hydration properties, proton conductivity, and water content of perfluorinated ionomer thin films at temperatures relevant to fuel cell operation temperatures (around 80 °C) and the effect of ionomer chemistry are scarce. In this work, we report the water content and proton conductivity properties of thin-film ionomers (30 nm) at 80 °C over a wide range of relative humidity (0−90%) for seven different ionomers differing in the side-chain structure, including the number of protogenic groups, with the equivalent weight ranging from 620 to 1100 g/mol of sulfonic acid. The results show that the acid content or equivalent weight of the ionomer is the strongest determinant of both the swelling and the proton conductivity of ionomer films at a given relative humidity. The molar water content (λ) of ionomer films normalized to the molar protogenic group is observed to be equivalentweight-dependent, implying that the affinity for water is acid-content-dependent. At high relative humidity conditions (>70%) pertinent to fuel cell operations, the proton conductivity of low-equivalent-weight ionomers was higher than that of higher-equivalent-weight ionomers. However, upon correlating the proton conductivity with molar water content (λ), the differences reduce dramatically, highlighting that water content is the controlling factor for proton conduction. Significantly higher values of both water content and proton conductivity are observed at 80 °C compared to those at 30 °C, implying that room temperature data are not reliable for estimating ionomer properties in the fuel cell catalyst layer.

The replacement of pure Pt catalysts with Pt–Co and Pt–Ni alloy catalysts has helped reduce the cost of polymer electrolyte fuel cells but has also introduced long-term performance degradation due to leaching out of cobalt and nickel into the catalyst layer ionomer and polymer electrolyte membrane. This work reports the impact of exchange of protons with cobalt ions on the humidity-dependent (0–90% RH) hydration and conductivity of ∼30 nm thin ionomer films at a fuel cell-relevant temperature (80 °C) for seven different ionomers varying in equivalent weight from 620 to 1100 g/mol of sulfonic acid. A significant suppression (up to 2 orders of magnitude at low RH) in ionic conductivity was observed for all ionomers upon exchange of protons with cobalt ions, consistent with prior studies. From a corresponding measurement of film swelling characteristics of H+-form and Co2+-exchanged ionomers, not reported earlier, the present study provides a clear piece of evidence that the water content of the ionomer films decreases upon Co2+ exchange. This suppression in water uptake is the dominant cause of the reduction in ionic conductivity of ionomer films upon Co2+ exchange. The most interesting finding of the study is that a large variation in conductivity between the H+ form and Co2+ form of ionomer films at a given RH is significantly minimized when conductivity is correlated with the water content (λ = number of water molecules per protogenic group). Accordingly, the work introduces a new universal relationship for ionic conductivity and water content of the ionomer films.

Near ambient pressure X-ray photoelectron spectroscopy (nAP-XPS) affords unparalleled insight into the physicochemical processes that drive electrocatalytic devices.1 Studies featuring nAP-XPS span a broad range of materials and reactions, with many focused on thin films or other well-defined materials. Experiments featuring a water vapor atmosphere, or a humidified atmosphere (often with O2 or H2 as the other vapor component) have been effectively used to study the fundamentals of the electrocatalytic reactions occurring in polymer electrolyte membrane fuel cells and water electrolyzers (PEMFCs and PEMWEs).2,3 However, many of these studies have solely focused on the interaction between gas and catalyst, even if ionomer was also present within the catalyst layer. This talk focuses on the investigation of the evolution of ionomer species, and the catalyst-ionomer interface under conditions relevant to the operation of PEM devices. Analyzing the catalytically relevant surfaces and interfaces present in PEMFC and PEMWE electrodes with nAP-XPS is a complex challenge, due in part to the subtlety of the changes induced in nAP-XP spectra by interactions between the catalyst, ionomer, and gas. Adsorption of a gaseous reactant species onto a catalyst’s surface results in a weak interaction and a small chemical shift in the adsorbent species,5 while ionomer may undergo protonation, re-orientation or degradation upon exposure to reactants, also altering the spectra. Indeed, few studies feature XPS measurements of Nafion, as it has been shown to degrade over the course of standard XPS operation.6 This issue is likely exacerbated through the use of much more intense synchrotron radiation sources most commonly used for nAP-XPS experiments in the literature. Within this work, we present an evaluation of the degree of Nafion degradation during our measurements using a unique Scienta Omicron HiPP-3 nAP-XPS system, and the results of an approach to data collection developed to mitigate the induced damage. This enables the reliable study of both Nafion films and Nafion present within platinum-catalyst based PEMFC cathodes in a water vapor atmosphere using a laboratory-based, commercially available nAP-XPS system. This work lays the foundation for future study of different classes of electrocatalysts at the electrode scale, with the goal of deconvoluting complex changes at the electrode surfaces and interfaces, both in inert and catalytically relevant environments. (1) Starr, D. E.; Liu, Z.; Hävecker, M.; Knop-Gericke, A.; Bluhm, H. Investigation of Solid/Vapor Interfaces Using Ambient Pressure X-Ray Photoelectron Spectroscopy. Chem. Soc. Rev. 2013, 42, 5833–5857. (2) Yamamoto, S.; Bluhm, H.; Andersson, K.; Ketteler, G.; Ogasawara, H.; Salmeron, M.; Nilsson, A. In Situ X-Ray Photoelectron Spectroscopy Studies of Water on Metals and Oxides at Ambient Conditions. J. Phys. Condens. Matter 2008, 20 (18). (3) Casalongue, H. S.; Kaya, S.; Viswanathan, V.; Miller, D. J.; Friebel, D.; Hansen, H. A.; Nørskov, J. K.; Nilsson, A.; Ogasawara, H. Direct Observation of the Oxygenated Species during Oxygen Reduction on a Platinum Fuel Cell Cathode. Nat. Commun. 2013, 4. (4) Saveleva, V. A.; Savinova, E. R. Insights into Electrocatalysis from Ambient Pressure Photoelectron Spectroscopy. Curr. Opin. Electrochem. 2019, 17, 79–89. (5) Dzara, M. J.; Artyushkova, K.; Shulda, S.; Strand, M. B.; Ngo, C.; Crumlin, E. J.; Gennett, T.; Pylypenko, S. Characterization of Complex Interactions at the Gas − Solid Interface with in Situ Spectroscopy : The Case of Nitrogen-Functionalized Carbon. J. Phys. Chem. C 2019, 123 (14), 9074–9086. (6) Paul, D. K.; Giorgi, J. B.; Karan, K. Chemical and Ionic Conductivity Degradation of Ultra-Thin Ionomer Film by X-Ray Beam Exposure. J. Electrochem. Soc. 2013, 160 (4), F464–F469.