Improved Water Management with Thermally Sprayed Coatings on Stainless Steel Bipolar Plates of PEMFC

Abstract

Proton exchange membrane fuel cells (PEMFC) systems are efficient generators to supply electricity from the electrochemical reaction between hydrogen and oxygen. These devices use bipolar plates (BPP) that serve to distribute the reactive gases, manage the produced water and collect the generated electrons [1]. The BPPs of a PEMFC stack are usually made of composite graphitic carbon. However the plates are thick, brittle, and corrode at high potentials. Stainless steel (ss) can be used as alternative, but it has to be protected with a highly conductive and corrosion resistant coating [2]. Vacuum plasma spraying (VPS) is a suitable technique for producing dense Ti coatings for BPPs of PEM electrolyzers [3]. In this work we coat stainless steel PEMFC electrode holders (EH) with Ti by VPS technique. Several parameters such as the type of plasma torch nozzle, the powder feeding rate, and the plasma gas flow rates of Ar, N2 and H2 are varied. The surface of the VPS coating is subsequently modified with Au by electrodeposition to reduce passivation. The same layer is applied directly on stainless steel substrates for comparison purposes. The coatings were characterized by scanning electron microscopy (SEM), x-ray diffraction (XRD) and x-ray photoelectron spectroscopy (XPS) techniques. SEM images show that the thickness of the Au/Ti coating is 62 µm with a roughness factor of 0.7. The presence of TiO2 between Ti and Au is negligible as confirmed by XRD and XPS techniques. Potentiodynamic characteristics in O2-saturated 0.5 M H2SO4 show that Au/Ti/ss has comparable corrosion currents to Au/ss but higher corrosion potentials. The interfacial contact resistance (ICR) of Au/Ti/ss is 8.6 mΩ cm2 under a compaction pressure of 140 N cm-2, thus meeting the US Department of Energy (DOE) requirement for this parameter. The cell voltage-current characteristics of PEMFCs with Au and Au/Ti coated EHs are shown in Fig. 1a. The performances of the PEMFCs above 0.75 V are almost identical. However, at 0.6 V the PEMFC with the Au/Ti coatings produces almost 30% more current. By keeping the cell voltage (Ecell) constant at 0.6 V, the Au/Ti coating allows reducing the current instabilities of the PEMFC operating under strong flooding conditions, see Fig. 1b. The periodic spikes and oscillations in the generated current of the PEMFC with the Au coating are produced as result of water droplets that find difficulty to flow through channels of the flow field [4]. Finally, the same phenomenon occurs in the cathode pressure, confirming the unique property of the Au/Ti coating for reducing flooding. Thus, the thermally sprayed coatings protect the stainless bipolar plates from corrosion, increase the ICR and improve the water management in a PEMFC.