Application of a proton exchange membrane fuel cell (PEMFC) in a planar, open-cathode configuration can significantly reduce system weight and complexity compared to a conventional fuel cell stack with closed cathodes. This decrease in device weight makes an array of open-cathode fuel cells a compelling power source for unmanned aerial vehicles because vehicle performance is highly dependent on weight. Forfeiting the ability to precondition incoming air to the cathodes leaves the fuel cells vulnerable to dry-out. Our previous studies on open-cathode fuel cells demonstrate that low porosity gas diffusion media facilitate a lower cell temperature and improve cell hydration but cell performance is still limited by significant ohmic resistance related to dehydration by evaporation at moderate cell temperatures (~40°C). Short-side-chain (SSC) ionomers with high ion exchange capacity have shown promise in closed cathode fuel cells by improving proton conductivity, especially during low RH operation, compared to their long-side-chain counterpart, Nafion. In this work, we study the relationship between cathode catalyst layer ionomer and cell polarization behavior in ambient conditions that have been observed to exacerbate dry-out and flooding in open-cathode fuel cells. Catalyst-coated membranes (CCMs) are prepared in-house with an automated, ultrasonic spray, layer-by-layer deposition method. Catalyst layers are formed with ionomers of variable equivalent weight and pendant side chain length and the catalyst layer proton conductivity is measured as a function of these properties. We use cyclic voltammetry and electrochemical impedance spectroscopy (EIS) to characterize the catalyst layers and performance losses in various ambient conditions.