Membrane Fuel Cell Bipolar Plates Research Articles

Thermally nitrided stainless steels for polymer electrolyte membrane fuel cell bipolar plates

Thermal nitridation of a model Ni–50Cr alloy at 1100 °C for 2 h in pure nitrogen resulted in the formation of a continuous, protective CrN/Cr2N surface layer with a low interfacial contact resistance. Application of similar nitridation parameters to an austenitic stainless steel, 349™, however, resulted in a discontinuous mixture of discrete CrN, Cr2N and (Cr,Fe)2N1−x (x = 0–0.5) phase surface particles overlying an exposed γ austenite-based matrix, rather than a continuous nitride surface layer. The interfacial contact resistance of the 349™ was reduced significantly by the nitridation treatment. However, in the simulated PEMFC environments (1 M H2SO4 + 2 ppm F− solutions at 70 °C sparged with either hydrogen or air), very high corrosion currents were observed under both anodic and cathodic conditions. This poor behavior was linked to the lack of continuity of the Cr-rich nitride surface formed on 349™. Issues regarding achieving continuous, protective Cr-nitride surface layers on stainless steel alloys are discussed.

Thermally nitrided stainless steels for polymer electrolyte membrane fuel cell bipolar plates

Thermal nitridation of AISI446 mod-1 superferritic stainless steel for 24 h at 1100 ◦ C resulted in an adherent, inward growing surface layer based on (Cr, Fe)2N1−x (x = 0–0.5). The layer was not continuous, and although it resulted in low interfacial contact resistance (ICR) and good corrosion resistance under simulated polymer electrolyte membrane fuel cell (PEMFC) cathodic conditions; poor corrosion resistance was observed under simulated anodic conditions. Nitridation for 2 h at 1100 ◦ C resulted in little nitrogen uptake and a tinted surface. Analysis by SEM, XPS, and AES suggested a complex heterogeneous modification of the native passive oxide film by nitrogen rather than the desired microns-thick exclusive Cr-rich nitride layer. Surprisingly, this modification resulted in both good corrosion resistance under simulated cathodic and anodic conditions and low ICR, well over an order of magnitude lower than the untreated alloy. Further, little increase in ICR was observed under passivating polarization conditions. The potential of this phenomenon for PEMFC bipolar plates is discussed. © 2004 Elsevier B.V. All rights reserved.

Alloys that form conductive and passivating oxides for proton exchange membrane fuel cell bipolar plates

During the operation of proton exchange membrane (PEM) fuel cells, a high-resistance oxide is often formed on the cathode surface of base metal bipolar plates. Over time, this corrosion mechanism leads to a drop in fuel cell efficiency and potentially to complete failure. To address this problem, we have developed alloys capable of forming oxides that are both conductive and chemically stable under PEM fuel cell operating conditions. Five alloys of titanium with tantalum or niobium were investigated. The oxides were formed on the alloys by cyclic voltammetry in solutions mimicking the cathode- and anode-side environment of a PEM fuel cell. The oxides of all tested alloys had lower surface resistance than the oxide of pure titanium. We also investigated the chemical durability of Ti–Nb and Ti–Ta alloys in more concentrated solutions beyond those typically found in PEM fuel cells. The oxide films formed on Ti–Nb and Ti–Ta alloys remained conductive and chemically stable in these concentrated solutions. The stability of the oxide films was evaluated; Ti alloys having 3% Ta and Nb were identified as potential candidates for bipolar plate materials.