Electrochemical adsorption of SO(2) on platinum is complicated by the change in sulfur oxidation state with potential. Here, we attempt to identify SO(2) adsorption products on catalyst coated membranes (CCMs) at different electrode potentials using a combination of in situ sulfur K-edge XANES (X-ray absorption near-edge structure) spectroscopy and electrochemical techniques. CCMs employed platinum nanoparticles supported on Vulcan carbon (Pt/VC). SO(2) was adsorbed from a SO(2)/N(2) gas mixture while holding the Pt/VC-electrode potential at 0.1, 0.5, 0.7, and 0.9 V vs a reversible hydrogen electrode (RHE). Sulfur adatoms (S(0)) are identified as the SO(2) adsorption products at 0.1 V, while mixtures of S(0), SO(2), and sulfate/bisulfate ((bi)sulfate) ions are suggested as SO(2) adsorption products at 0.5 and 0.7 V. At 0.9 V, SO(2) is completely oxidized to (bi)sulfate ions. The identity of adsorbed SO(2) species on Pt/VC catalysts at different electrode potentials is confirmed by modeling of XANES spectra using FEFF8 and a linear combination of experimental spectra from sulfur standards. Results on SO(2) speciation gained from XANES are used to compare platinum-sulfur electronic interactions for Pt(3)Co/VC versus Pt/VC catalysts in order to understand the difference between the two catalysts in terms of SO(2) contamination.

We detail the relationships between operating environment and performance of a thin and lightweight open-cathode fuel cell based on flexible circuits that may be advantageous for unmanned air vehicle (UAV) propulsion. The open-cathode fuel cell performance is studied in a broad range of realistic atmospheric flight conditions by mounting it in a wind tunnel within an environmental chamber. The relationships between the operating environment and performance are quantitated in terms of polarization behavior and the underlying loss mechanisms are discussed qualitatively in the context of electrochemical impedance spectroscopy (EIS), DC resistance and infrared temperature measurements. The wind tunnel experiments demonstrate that open cathode operation is possible over wide ranges of ambient temperature (5–55°C), relative humidity (22–90% RH), air speed (0–15.4 m s−1) and altitude (240–3240 m). Forced airflow is shown to improve mass transport, waste heat rejection, and water management, broadening the effective operating envelope of this open-cathode fuel cell over others that rely on free convection. Sacrificing air preconditioning renders open-cathode fuel cell performance sensitive to the conditions of the surrounding environment. The lightweight open-cathode fuel cell in this study has high specific power, exceeding 1.3 kW/kgcell in favorable ambient conditions of 5°C, 50% RH at sea level.

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Hydrogen fuel cells are demonstrated as the propulsion system for long-endurance, small, electric unmanned air vehicles (UAVs). Flight times of >24 hours were demonstrated for the 35-lb Ion Tiger fuel cell UAV while carrying a 5-lb payload. This paper describes the design criteria and development process used to meet these performance goals, including setting the specifications for the vehicle, fuel cell, cooling, and fueling systems.

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