Electrochemical Corrosion Study of Metallic Materials in Phosphoric Acid as Bipolar Plates for HT-PEFCs

Abstract

This work investigates various commercially available austenitic stainless steels and Ni-based alloys as possible metallic bipolar plates for high-temperature polymer electrolyte fuel cells (HT-PEFCs). The dynamic formation and depletion of passivation layers that depends strongly on applied potential was analyzed in 85 wt% phosphoric acid at RT and 130 • C by means of cyclic voltammetry and steady-state polarization. All materials showed a beneficial passivation in the potential window 0.3-1.2 V, which is mainly based on a stable inner Cr 2 O 3 layer, and a reduction of the passive layers at cathodic polarization. Alloy 2.4869 (Cronix 80) with 80 wt% Ni and 20 wt% Cr reveals the lowest corrosion rates of 16.1 μA cm −2 at 130 • C in the passive region at 0.6 V and a free corrosion potential E cor of 235 mV. The improvement of passivity was achieved by the dominant superficial Ni 3 (PO 4 ) 2 layer. Alloying additions Mo and Ti revealed a characteristic increase of passive current densities due to instability of these passivation components at 130 • C. Passivation based on Fe oxides and phosphates in stainless steels shows to be beneficial at RT, but less efficient at 130 • C.High-temperature polymer electrolyte fuel cells (HT-PEFCs) operate in the range of 120-180 • C and currently employ phosphoric acid doped polybenzimidazole membranes. This in turn represents a quite aggressive environment for the components of a fuel cell. Presently, bipolar plates are made of graphitic composite materials. Significant drawbacks are the apparent corrosion rates, a high effort of manufacturing and the uptake of phosphoric acid. Previous studies have demonstrated a wide range of corrosion current densities for different types of graphite up to the level of 1 mA cm −2 in ∼100 wt% H 3 PO 4 at 185-200 • C depending strongly on polarization, amount of binders or resins and heat-treatment.

Metallic Bipolar Plates for High Temperature Polymer Electrolyte Membrane Fuel Cells

“…First, the cathode side sample might be considered to be passivated and protected but the anode side samples not. This is in agreement with the findings from the electrochemical studies on a range of austenitic steels and nickel‐based alloys done by Weissbecker et al 5, in 85% H 3 PO 4 at 130 °C. They found that passivation (formation of protective oxide) was more prone to occur at cathodic potential (0.6 V) than at anodic potential (0 V).…”

“…HTPEMFCs have therefore lower risk of cathode side flooding and allow the utilization of reformate fuels 4. At present HTPEM stacks are usually constructed with graphite bipolar plates and the use of metallic (steel) bipolar plates (BPPs) in contrast has only been recently discussed 5. Metallic bipolar plates offer the advantages of lower weight and volume, lower costs and better protection against phosphoric acid migration.…”

“…Weissbecker et al. have tested the suitability of different alloys to the exposure to phosphoric acid using polarization methods 5. They concluded that for the category of stainless steel, alloys with no additions of titanium, manganese and molybdenum proved to be most corrosion resistant.…”

Assessment of the durability of low-cost Al bipolar plates for High Temperature PEM fuel cells

“…However, metallic BPPs have also been considered, particularly for LT-PEMFCs, due to their lower cost, lower weight and volume (especially when thin films are used), and an easier manufacture than graphitebased ones [31,32]. For HT-PEMFCs, the use of metallic BPPs has only been recently considered [18,[33][34][35][36][37][38]. HT-PEMFCs present a more corrosive environment in comparison to LT-PEMFCs, due to the higher temperature of operation, that enhances corrosion processes in acidic environments.…”

“…Metallic BPPs are considered a promising alternative for HT-PEMFCs, in order to minimize the effects of phosphoric acid up-take, as well as to reduce the total weight and cost of the stack [34,38]. Some authors have studied stainless steel BPPs, coated with different materials to increase their resistance to corrosion like Au, Ni, etc [30,34,35].…”