Platinum-Nickel Nanotubes Array As Cathode for PEM Fuel Cell

Keywords: PEMFC, Anodic Alumina Oxide, Ni Nanowires, PtNi Alloy, Oxygen Reduction Reaction, Carbon-free electrode. Fuel cells technology have attracted great attention for future energy conversion and storage applications (1). The commercialization of polymer electrolyte membrane fuel cells (PEMFC) technologies is still a challenge due to durability and cost limitations of the catalyst (2). In order to enhance their achievement on the market, it is mandatory to optimize the high costly catalyst and improve its durability and stability or to innovate a new structure of electrodes. Commercial Pt/C catalysts (nanoparticles 3–5 nm) supported carbon black are considered as the current standard catalyst offering high surface areas and high specific activity. Unfortunately, they are limited by corrosion of the C and Pt dissolution/agglomeration through Electrochemical Ostwald ripening mechanism, which is reflected by a fast and significant loss of electrochemical surface area (ECSA) over time during fuel cell operation(3). This work focuses on reducing the amount of platinum and improving the performance and durability of membrane electrode assemblies (MEAs), aiming to reduce cost and thus encouraging the development of PEMFCs. Pt-based catalysts such as Pt-Co, Pt-Cu, and Pt-Ni etc. have shown to exhibit higher oxygen reduction reaction (ORR) activity than platinum for PEMFC. Accordingly, we aspire to develop and control a new carbon-free architecture of the cathode, made only with vertically aligned PtNi nanotubes supported onto a Nafion® membrane. Previous studies have demonstrated the benefits of platinum carbon-free nanostructures such as nanostructured thin films (NSTF) by 3M Company, USA (4), platinum opal nanostructures (5), or vertically aligned platinum nanotubes (6). They have shown very promising performance and stability for low Pt loading. A new carbon-free structure of electrodes made of vertically aligned PtNi nanotubes has been developed to improve performance and durability of PEMFC at low noble metal catalyst content. The process is based on an electrochemistry route of Ni electrodeposition inside anodic alumina oxide (AAO) mold, followed by a step of controlled spontaneous galvanic displacement (Figure 1), thermal treatment and acid treatment leading to the formation of nanotubes (~50nm in diameter and ~400nm in length). Ultraviolet spectrophotometric determination of platinum content showed a loading of ca. 0,1mg/cm². The latter are transferred onto a Nafion® membrane using an optimized hot pressing process. Finally, electrodes were integrated in a complete Membrane Electrode Assembly (MEA) in order to characterize their electrochemical and transport limitations in real operating conditions. The catalyst specific area estimated form the integration of Hupd desorption peaks at 30°C 100%RH in complete MEA, is approximately 6,7 cm²Pt/cm²geo. For comparison, we have elaborated a Pt/C standard cathode (Tanaka 50 wt.% catalyst onto Vulcan support) with low Pt loading (~20µg/cm²) which exhibits a close catalyst surface area of ~6,5 cm²Pt/cm²géo. The performance of the MEA was quantified by recording polarization curve under H2/O2 80°C (Figure 2 below). Promising results are obtained at a relative humidity of 80% at the anode side and 100% at the anode side, under a pressure of 1,38 bar / 1,5 bar respectively. Indeed, fully humified conditions at both anode and cathode lead to severe decay in performance as current density increases, ascribed to flooding. In addition, as shown in Figure 2, higher activities, estimated from the value of current density at 0.8V extracted from the polarization curve corrected from ohmic drop, are achieved with PtNiNTs comparing to Pt/C 20µg/cm². Figure 1