Towards rational electrode design: quantifying the triple-phase boundary activity of Pt in solid acid fuel cell anodes by electrochemical impedance spectroscopy

Abstract

Solid acid fuel cells based on CsH2PO4 as the electrolyte and Pt as the electrocatalyst are a promising intermediate temperature energy conversion technology. However, improving the electrode microstructure to achieve an optimal area-normalized resistance, while keeping or even lowering the Pt catalyst loading is particularly challenging due to the solid nature of the electrode components. Several architectures have been empirically developed, such as CsH2PO4 micro- or nanoparticles mixed with Pt nanoparticles or covered with thin-film Pt. For an optimal electrode design, a quantitative measurement of the fundamental parameter, namely the specific triple-phase boundary activity of Pt at the fuel cell operating conditions, is needed. Geometrically simple, well-controlled electrodes are typically fabricated for this purpose via lithography techniques. This approach however is not suitable for solid acids due to the water solubility of the electrolyte. Here we present a simple, water-free fabrication scheme to create a controlled electrode geometry consisting of a hole-patterned Pt thin film that allows measurements of the specific triple-phase boundary activity of Pt in an anodic environment. Based on electrochemical impedance spectroscopy measurements in a symmetric cell configuration, the triple-phase boundary activity of Pt is determined to be on the order of 1.3 kΩ m. This information is critical for the rational design of a solid acid fuel cell electrode without tedious empirical optimization.