Platinum-Nickel Nanotubes Array as Cathode for PEMFC
Authors: Lagrichi Othman , Morin Arnaud , Buttard Denis 
Published: February 2020

Keywords: Electrocatalyst, PEMFC, PtNi alloy, Oxygen Reduction Reaction, Electrode
The technology of polymer electrolyte membrane fuel cells (PEMFCs) have recently attracted much attention from both fundamental and applied point of views for their future potential as clean and mobile power sources. One of the barriers of their commercialization is the high cost of the catalyst [1]. 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.
Platinum nanoparticles supported on carbon is used due to its high surface area but has some limitations, namely insufficient activity and durability particularly at the cathode where the oxygen reduction reaction (ORR) occurs. The latter is 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 [2]. This architecture can hardly be more optimized, it is for this reason that many researches have been lead to design new architectures of electrodes.
Previous studies have demonstrated the benefits of Pt nanostructured electrodes showing high performances for low Pt loadings and stability such as NanoStructured Thin Film (NSTF) by 3M [3], Pt nanowires [4], unsupported Pt Aerogels [5], PtCu nanotubes [6].These architectures are highly desired to meet desirable performance to substitute the state-of-the-art Pt/C electrocatalysts architecture.
The aim of this work is to develop and control a Pt-Based tubular nanostructure made only with catalyst without a carbon support. The vertically aligned tubes exibits less O2 loss because they are oriented parallel to the reactants flux [7]. Stamenkovic and co-workers reported that alloying Pt with Ni exhibits a higher ORR activity than with other transition metals [8]. Accordingly, combining the tubular structure and the higher activity of PtNi catalyst has the potential for overcoming the limitations of conventional Pt/C electrodes.

In order to elaborate the PtNi nanotubes, porous anodic alumina template (PAAT) is used as a sacrificial mold in which Ni nanowires were electrodeposited. The PAAT allows the control of the geometry of the desired nanotubes. In terms of comparison, the theoretical Pt loading is fixed at 0,15 mg/cm² assuming that the tubes are made with Pt nanoparticles of 5 nm in diameter. Afterwards, Pt is deposited through spontaneous galvanic displacement process. The latter involves the control of a large set of parameters (Cation (K2PtCl6 and H2PtCl6), pH, annealing at different temperatures, concentrations, HCl addition at different concentrations, time, temperature) in order to transform NiNWs to PtNiNTs. Later, using the hot pressing process of Nafion® membrane on the synthetized nanotubes lead to their integration in a complete Membrane Electrode Assembly (MEA) in order to characterize their electrochemical and transport limitations in real operating conditions. Also, to the extent of optimizing the activity of the PtNiNTs, further studies to be done regarding the entire structure of nanotubes including their diameter, length, density and the introduction of Nafion® ionomer.

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Figure 1