Single Cell Performance of PEFC Using the Surface-Treated Stainless Steel Bipolar Plate

Abstract

1 .Introduction Polymer electrolyte fuel cells (PEFCs) have been attracting much attention in recent years. Bipolar plate is one of the most important components in PEFCs and requires many criteria. For example, sufficient corrosion resistance especially in case of metals, good mechanical property, processibility, gas-impermeability and low cost. Stainless steels are regarded as the most promising material because it could greatly reduce the production cost. On use of stainless steels as bipolar plate material, one faces intrinsic problem of corrosion. Better corrosion resistance would be achieved by the formation of passive films enriched with chromium. However, passive films, which are dielectric by nature, could deteriorate the electric contact to carbon gas diffusion layer (GDL). Many surface treatments have been proposed so far to improve the contact resistance. The authors have proposed electrophoretic deposition (EPD) of nanosized TiN-SBR composite on a stainless steel substrate1). In this study, the method is applied to type 316L stainless steel bipolar plates to evaluate single cell performance. 2 .Experimental details Commercially available type 316L stainless steel plate was finished with a diamond paste (6 μm) and washed ultrasonically in hexane. TiN–SBR coating on the stainless steel surface was carried out by EPD process. The suspension was composed by 0.064 mass% of TiN (50 nm, Wako), 0.24mass% of emulsified SBR (NIHON Zeon) and 2-propanol. The prepared suspension was ultrasonically homogenized for 10 min. EPD treatments under a constant voltage (500 V) were controlled using a DC power supply. After the EPD, the stainless steel plates were vacuum dried. Interfacial contact resistance (ICR) of the treated stainless steel against carbon paper was evaluated as a function of compressive force. Polarization tests are carried out in a 0.05 M SO4 2-+ 2 ppm F- solution (pH: 3.3) at 353 K using a saturated calomel electrode (SCE) as a reference electrode. Bipolar plates after the design by JARI2) with serpentine flow field were machined with the type 316L stainless steel. Graphite bipolar plates were also used for comparison. A single cell was assembled from the bipolar plates and a commercially available MEA employing carbon paper GDL. The single cell was operated at 348K under an ambient pressure for 500 h. The applied current densities were 0 and 0.5 A cm−2. After the cell operation, the surfaces of the bipolar plates were analyzed by SEM and XPS. The contamination level of MEA was evaluated by XRF analysis. 3 .Experimental results and discussion After the EPD treatment, ICR between the stainless steel and carbon paper was improved from 63.2 mΩ cm2 to 10.7 mΩ cm2 (at 100 N cm-2), while the ICR between graphite and carbon paper was 4.5 mΩ cm2. The initial i-V characteristic for cells employing different bipolar plates (untreated 316L, treated 316L and graphite) indicated comparable voltage for treated 316L to graphite, while untreated 316L showed the lowest voltage due to iR drop. The difference expanded as the cell operation proceeded. The cell voltage under applied current density of 0.5 A cm−2 after 500 h operation were 0.648 V for graphite, 0.633 V for treated 316L and 0.616 V for untreated 316L. The change in open circuit voltage (OCV) was also evaluated for 500 h. The OCVs after the cells were left under open circuit condition for 500 h were 0.810 V for graphite, 0.807 V for treated 316L, 0.633 V for untreated 316L. The XRF analysis of MEA after the cell operations indicated highest concentration of iron for the cell with untreated 316L, while the contamination was successfully suppressed for the treated 316L. References 1) M. Kumagai et al., Electrochem. Acta, 54 574(2008). 2) Japan Automobile Research Institute, NEDO Report (12-17-013-2-0002-2), 2001, p. 21.